The density of a material is the mass divided by the volume of a sample of the material; it is often expressed in gram per cubic centimetre or gr/cc. The density of most plastics is about the same as the density of water, which is 1 gr/cc. The density of various plastics at room temperature is shown in this table.
PLASTICS DENSITY IN GR/CC
LDPE 0.92
HDPE 0.95
PVC 1.40
ABS 1.02
PP 0.91
NYLON-6 1.13
PETP 1.35
PS 1.06
FEP 2.15
The density can also be described by the term specific volume. The specific volume is the volume divided by the mass of a sample of a material; it is the reciprocal of the density and can be expressed in cc/gr, The density or the specific volume is affected by temperature and pressure. The mobility of the plastics molecules increases with higher temperatures. As a result, the specific volume increases with increasing temperature, as illustrated in this figure (vol_t.cvs) for HDPE.
The specific volume increases rapidly as the plastics approaches the melting point. Beyond the melting point, the slope changes abruptly and the volume increases more slowly. A diagram that shows the effect of pressure and temperature on the specific volume of a plastics is called a P-V-T diagram.
MELTING POINT
The melting point is the temperature at which the crystallites melt. Since amorphous plastics do not have crystallites, there is no melting point for amorphous plastics, only for semi-crystalline plastics. Semi-crystalline plastics are usually processed about 50 degrees C above the melting point. If the plastics is susceptible to degradation, the processing temperature should be as low as possible. When the plastics has a high viscosity, the processing should be as high as possible, without degrading the plastics.
GLASS TRANSITION TEMPERATURE
Plastics at low temperature are rigid and stiff. At higher temperatures, plastics become soft and ductile because the molecules become flexible and can change conformation. The transition between the two states is called the glass transition temperature.
When the glass transition temperature is above room temperature, the plastics will be hard and brittle at room temperature - for example, polystyrene.
When the glass transition temperature is below room temperature, the plastics will be soft and flexible at room temperature. Melt and glass transition temperatures for some plastics are shown in this table:
Plastics Glass Transition
Temperature [° C]
Melting Point
polystyrene (PS) 101 -
polyvinylchloride (PVC) 80 -
polymethyl methacrylate (PMMA) 105 -
acrylonitrile butadiene styrene (ABS) 115 -
polycarbonate (PC) 115 -
low density polyethylene (LDPE) -120/-90 120
high density polyethylene (HDPE) -120/-90 130
polypropylene (PP) -10 175
nylon-6 (PA-6) 50 225
polyethylene terephthalate (PET) 70 275
polybutylene terephthalate (PBT) 45 250
polyvinylidene fluoride (PVDF) -40 170
tetrafluoroethylene/
hexafluoropropylene copolymer (FEP) 70 275
Sunday, November 1, 2009
THERMAL PROPERTIES
The thermal properties of a plastics are important in understanding how a particular material behaves in an extruder Knowledge of the thermal properties allows the selection of the appropriate machine, setting of correct process conditions, and in analysing process problems. The most important thermal properties are: thermal conductivity, specific heat, thermal stability and induction time, density, melting point, and glass transition temperature. We will discuss these next.
Thermal Conductivity
Thermal conductivity is probably the most important thermal property. Thermal conductivity is the ability of a material to conduct heat. Plastics have a low thermal conductivity - they are considered to be thermal insulators. This means that heating and cooling plastics by conduction is a slow process. Heating occurs in the extruder and cooling occurs after the die. The low thermal conductivity often determines how fast a plastics can be processed. This is true not only in extrusion but also in injection molding and, in fact, most plastics processing operations.
Another aspect of the low thermal conductivity of plastics is that non-uniform plastics temperatures are likely to occur. For instance, if a plastics melt is introduced to an extrusion die with a high temperature region on one side of the channel, it will take considerable time for the melt temperatures to equalise by conduction. If the channel is 20 mm in diameter, it may take 5 to 10 minutes for the temperatures to equalise. A typical residence time in an extrusion die is only about 30 seconds. As a result, the residence is too short for the melt temperatures to equalise by conduction.
This means that the high temperature melt stream will persist all through the die and will cause non-uniform flow at the die exit; typically, this will result in a local thick spot in the extruded product .
Specific Heat and Enthalpy
The specific heat is the amount of heat necessary to increase the temperature of a material by one degree . In most cases, the specific heat of semi-crystalline plastics is higher than amorphous plastics.
The amount of heat necessary to raise the temperature of a material from a base temperature to a higher temperature is determined by the enthalpy difference between the two temperatures . If we use room temperature as the base temperature the enthalpy of different plastics can be plotted against temperature.
The enthalpy is expressed in kW.hr/kg or HP.hr/lb; it is a specific energy, in other words, energy per unit mass. Most of the energy required in the processing of plastics is needed to increase the temperature of the plastics. If we know the starting temperature, usually room temperature, and the discharge temperature, we can determine the minimum energy required to process the plastics. For instance, if we look at curve for PVC then we can see that the specific energy required to raise the temperature from room temperature to 150C is about 0.05 kW.hr/kg. Thus, for each kg/hr we require 0.05 kW. If we process PVC starting at room temperature up to 150C at 100 kg/hr (220 lbs/hr) the minimum power requirement is 5 kW (6.7 HP).
If we compare low density polyethylene (LDPE) to PVC, we see that LDPE requires about 0.15 kW.hr/kg to go from room temperature to 150C. Thus, the specific energy requirement for LDPE is much higher than for PVC. In general, semi-crystalline plastics have higher specific energy requirement than amorphous plastics. Obviously, this affects the cooling as well. It means that to cool LDPE from 150C to room temperature much more heat has to be removed than in cooling the same mass of PVC at 150C down to room temperature.
Thermal Stability and Induction Time
Plastics can degrade in the extrusion process. The main variables involved in degradation are temperature and the length of time that a plastics is subjected to high temperatures. Plastics degrade when exposed to high temperatures; the higher the temperature, the more rapid the degradation. Degradation can result in loss of mechanical properties, optical properties, appearance problems, degassing, burning, etc. Other variables can affect degradation, for instance the presence of oxygen.
The induction time is a measure of the thermal stability of a plastics; it is the time at elevated temperature that the plastics can survive without measurable degradation. The longer the induction time at a certain temperature, the better the thermal stability of the plastics. The induction time can be measured using various instruments, such as a TGA (thermogravimetric analyser), TMA (thermomechanical analyser), cone-and-plate rheometer, and other instruments.
When the induction time is measured at several temperatures, the induction time can be plotted against temperature as shown here (inductio.cvs) for HDPE, high density polyethylene and EAA, ethylene-acrylic acid. Two things are clear from the figure. One, the induction time reduces exponentially with temperature. Two, the induction time for HDPE is much longer than for EAA. The thermal stability of one plastics can be much different from another plastics.
The thermal stability and induction time of a plastics can be improved by adding thermal stabilisers [slide]. In fact, most plastics contain thermal stabilisers. Some plastics have such poor thermal stability that they would not be melt processable without thermal stabilisers; an example is rigid PVC.
Thermal Conductivity
Thermal conductivity is probably the most important thermal property. Thermal conductivity is the ability of a material to conduct heat. Plastics have a low thermal conductivity - they are considered to be thermal insulators. This means that heating and cooling plastics by conduction is a slow process. Heating occurs in the extruder and cooling occurs after the die. The low thermal conductivity often determines how fast a plastics can be processed. This is true not only in extrusion but also in injection molding and, in fact, most plastics processing operations.
Another aspect of the low thermal conductivity of plastics is that non-uniform plastics temperatures are likely to occur. For instance, if a plastics melt is introduced to an extrusion die with a high temperature region on one side of the channel, it will take considerable time for the melt temperatures to equalise by conduction. If the channel is 20 mm in diameter, it may take 5 to 10 minutes for the temperatures to equalise. A typical residence time in an extrusion die is only about 30 seconds. As a result, the residence is too short for the melt temperatures to equalise by conduction.
This means that the high temperature melt stream will persist all through the die and will cause non-uniform flow at the die exit; typically, this will result in a local thick spot in the extruded product .
Specific Heat and Enthalpy
The specific heat is the amount of heat necessary to increase the temperature of a material by one degree . In most cases, the specific heat of semi-crystalline plastics is higher than amorphous plastics.
The amount of heat necessary to raise the temperature of a material from a base temperature to a higher temperature is determined by the enthalpy difference between the two temperatures . If we use room temperature as the base temperature the enthalpy of different plastics can be plotted against temperature.
The enthalpy is expressed in kW.hr/kg or HP.hr/lb; it is a specific energy, in other words, energy per unit mass. Most of the energy required in the processing of plastics is needed to increase the temperature of the plastics. If we know the starting temperature, usually room temperature, and the discharge temperature, we can determine the minimum energy required to process the plastics. For instance, if we look at curve for PVC then we can see that the specific energy required to raise the temperature from room temperature to 150C is about 0.05 kW.hr/kg. Thus, for each kg/hr we require 0.05 kW. If we process PVC starting at room temperature up to 150C at 100 kg/hr (220 lbs/hr) the minimum power requirement is 5 kW (6.7 HP).
If we compare low density polyethylene (LDPE) to PVC, we see that LDPE requires about 0.15 kW.hr/kg to go from room temperature to 150C. Thus, the specific energy requirement for LDPE is much higher than for PVC. In general, semi-crystalline plastics have higher specific energy requirement than amorphous plastics. Obviously, this affects the cooling as well. It means that to cool LDPE from 150C to room temperature much more heat has to be removed than in cooling the same mass of PVC at 150C down to room temperature.
Thermal Stability and Induction Time
Plastics can degrade in the extrusion process. The main variables involved in degradation are temperature and the length of time that a plastics is subjected to high temperatures. Plastics degrade when exposed to high temperatures; the higher the temperature, the more rapid the degradation. Degradation can result in loss of mechanical properties, optical properties, appearance problems, degassing, burning, etc. Other variables can affect degradation, for instance the presence of oxygen.
The induction time is a measure of the thermal stability of a plastics; it is the time at elevated temperature that the plastics can survive without measurable degradation. The longer the induction time at a certain temperature, the better the thermal stability of the plastics. The induction time can be measured using various instruments, such as a TGA (thermogravimetric analyser), TMA (thermomechanical analyser), cone-and-plate rheometer, and other instruments.
When the induction time is measured at several temperatures, the induction time can be plotted against temperature as shown here (inductio.cvs) for HDPE, high density polyethylene and EAA, ethylene-acrylic acid. Two things are clear from the figure. One, the induction time reduces exponentially with temperature. Two, the induction time for HDPE is much longer than for EAA. The thermal stability of one plastics can be much different from another plastics.
The thermal stability and induction time of a plastics can be improved by adding thermal stabilisers [slide]. In fact, most plastics contain thermal stabilisers. Some plastics have such poor thermal stability that they would not be melt processable without thermal stabilisers; an example is rigid PVC.
FLOW BEHAVIOUR OF PLASTICS MELTS
In order to understand how a plastics behaves during processing we need to know how the plastics melt flows. One of the most important flow properties is the viscosity of a fluid. Viscosity is the resistance to flow. A low viscosity fluid, such as water, flows easily. A high viscosity fluid, such as honey, flows less easily [slide]. The viscosity in shearing flow is the shear stress acting on the fluid divided by the shear rate [slide], this will be explained later.
Since plastics are made up of very long molecules, they have high melt viscosities. The viscosity is often expressed in the units Pascal.second; the units Poise are also used. It is easy to convert from Poise to Pa.sec: ten Poise equals one Pa.sec. This table shows the approximate viscosity of various fluids, expressed in Pa.sec.
MATERIAL VISCOSITY IN PASCAL.SECOND
AIR 0.00001
WATER 0.001
OLIVE OIL 0.1
PLASTICS MELTS 100 to 1,000,000
PITCH 1,000,000,000
It is clear from the table that the viscosity of plastics is much higher than the viscosity of water, by at least five orders of magnitude! With a higher plastics viscosity, more torque is required on the extruder and more pressure is necessary to force the plastics melt through the die. The viscosity of a plastics is strongly dependent on the molecular weight of the plastics; the higher the molecular weight, the higher the viscosity. Since for one plastics, for example polyethylene (PE), there are many grades with different molecular weights, the viscosities can vary substantially from one polyethylene to another.
Melt Index
The ability of a plastics melt to flow is often measured in a melt index tester. The melt index machine is a simple ram extruder.
Plastics is placed in the reservoir and heated to the appropriate temperature. A weight is placed on top of the ram and this causes the plastics melt to be extruded out of the melt index die located at the bottom of the reservoir. The melt index (MI), sometimes called the melt flow index (MFI), is the amount of plastics extruded in grams in a certain time period, usually ten minutes.
A low viscosity plastics will flow out faster than a high viscosity plastics. Thus, a high MI is indicates a low viscosity plastics and a low MI a high viscosity plastics. The term fractional melt plastics is often used; this means that the plastics has a melt index less than one. A melt index less than one is considered low and, thus, fractional melt plastics have high viscosity.
The Effects of Shearing
When a is processed it is usually exposed to shearing flow. This is due to different layers of the plastics moving at different velocities. The rate of shearing that occurs in a fluid is called the shear rate; it is the difference in velocity between two fluid elements divided by the normal distance between the elements.
The shear rate is determined by the flow rate and the geometry of the flow channel. When the flow rate of the plastics is high, the shear rates will be high. Also, when the flow channel is small, the shear rate will be high.
Shear Thinning or Pseudoplastics Behaviour
In plastics, the viscosity changes when the shear rate change. A fluid that behaves that way is called a non-Newtonian or non-linear fluid.
The viscosity of plastics melts reduces with increasing shear rate; this is called shear thinning or pseudoplastics behaviour. This behaviour is due to the fact that the plastics molecules are very long and entangled. The entanglements of the molecules determine the viscosity of a plastics. When a plastics is exposed to a high shear rate, the number of entanglements of the molecules reduce and with it the viscosity.
When the shear rate reduces, the viscosity increases again. This behaviour is called shear thinning behaviour, it is also called pseudoplastics behaviour. This behaviour is very important in extrusion.
If we plot the viscosity vs. shear rate for a fluid and it forms a straight line on a log-log plot, we call the fluid a power law fluid [slide] , see figure 4 (vis_srt1.cvs). If the actual viscosity-shear rate curve is close to a straight line, the actual behaviour can be approximated with a power law expression.
The power law equation has two important parameters: the consistency index and the power law index. The consistency index is the value of the viscosity at a shear rate of one. The power law index is a measure of the degree of shear thinning behaviour; for plastics it varies between zero and one. The closer the power law index is to zero, the more strongly shear thinning the plastics. When the power law index is close to one, the plastics is only slightly shear thinning. When the power law index equals one the viscosity is not affected by shear rate; a fluid that behaves this way is called Newtonian.
Effect of Temperature on Viscosity
When the temperature of a plastics melt is increased the viscosity reduces.
The effect of temperature on viscosity varies from one plastics to another. In general, amorphous plastics have a high temperature sensitivity relative to semi-crystalline plastics.
The temperature coefficient for amorphous plastics ranges from about 5 to 20 percent. This means that the viscosity changes from 5 to 20 percent for each degree Centigrade change in temperature.
For semi-crystalline plastics the temperature coefficient of the viscosity is about 2 to 3 percent. A change in the extruder barrel is going to have a larger effect on an amorphous plastics than on a semi-crystalline plastics. Good temperature control, therefore, is even more critical in amorphous plastics than in semi-crystalline plastics.
Viscous Heat Generation
When a plastics melt is sheared, heat is being generated in the plastics; this is called viscous heat generation. The viscous heat generation is determined by the product of viscosity and shear rate squared. Thus, the higher the viscosity of the plastics, the higher the viscous heat generation. The same is true for the shear rate, however, the shear rate has a stronger effect since the viscous heating increases with the shear rate squared.
As a result of the high viscosity of plastics, in extrusion most of the heating of the plastics comes from viscous heat generation. In fact, in some cases too much viscous heat generation occurs in the extruder and the machine has to be cooled to maintain the desired melt temperatures.
Since plastics are made up of very long molecules, they have high melt viscosities. The viscosity is often expressed in the units Pascal.second; the units Poise are also used. It is easy to convert from Poise to Pa.sec: ten Poise equals one Pa.sec. This table shows the approximate viscosity of various fluids, expressed in Pa.sec.
MATERIAL VISCOSITY IN PASCAL.SECOND
AIR 0.00001
WATER 0.001
OLIVE OIL 0.1
PLASTICS MELTS 100 to 1,000,000
PITCH 1,000,000,000
It is clear from the table that the viscosity of plastics is much higher than the viscosity of water, by at least five orders of magnitude! With a higher plastics viscosity, more torque is required on the extruder and more pressure is necessary to force the plastics melt through the die. The viscosity of a plastics is strongly dependent on the molecular weight of the plastics; the higher the molecular weight, the higher the viscosity. Since for one plastics, for example polyethylene (PE), there are many grades with different molecular weights, the viscosities can vary substantially from one polyethylene to another.
Melt Index
The ability of a plastics melt to flow is often measured in a melt index tester. The melt index machine is a simple ram extruder.
Plastics is placed in the reservoir and heated to the appropriate temperature. A weight is placed on top of the ram and this causes the plastics melt to be extruded out of the melt index die located at the bottom of the reservoir. The melt index (MI), sometimes called the melt flow index (MFI), is the amount of plastics extruded in grams in a certain time period, usually ten minutes.
A low viscosity plastics will flow out faster than a high viscosity plastics. Thus, a high MI is indicates a low viscosity plastics and a low MI a high viscosity plastics. The term fractional melt plastics is often used; this means that the plastics has a melt index less than one. A melt index less than one is considered low and, thus, fractional melt plastics have high viscosity.
The Effects of Shearing
When a is processed it is usually exposed to shearing flow. This is due to different layers of the plastics moving at different velocities. The rate of shearing that occurs in a fluid is called the shear rate; it is the difference in velocity between two fluid elements divided by the normal distance between the elements.
The shear rate is determined by the flow rate and the geometry of the flow channel. When the flow rate of the plastics is high, the shear rates will be high. Also, when the flow channel is small, the shear rate will be high.
Shear Thinning or Pseudoplastics Behaviour
In plastics, the viscosity changes when the shear rate change. A fluid that behaves that way is called a non-Newtonian or non-linear fluid.
The viscosity of plastics melts reduces with increasing shear rate; this is called shear thinning or pseudoplastics behaviour. This behaviour is due to the fact that the plastics molecules are very long and entangled. The entanglements of the molecules determine the viscosity of a plastics. When a plastics is exposed to a high shear rate, the number of entanglements of the molecules reduce and with it the viscosity.
When the shear rate reduces, the viscosity increases again. This behaviour is called shear thinning behaviour, it is also called pseudoplastics behaviour. This behaviour is very important in extrusion.
If we plot the viscosity vs. shear rate for a fluid and it forms a straight line on a log-log plot, we call the fluid a power law fluid [slide] , see figure 4 (vis_srt1.cvs). If the actual viscosity-shear rate curve is close to a straight line, the actual behaviour can be approximated with a power law expression.
The power law equation has two important parameters: the consistency index and the power law index. The consistency index is the value of the viscosity at a shear rate of one. The power law index is a measure of the degree of shear thinning behaviour; for plastics it varies between zero and one. The closer the power law index is to zero, the more strongly shear thinning the plastics. When the power law index is close to one, the plastics is only slightly shear thinning. When the power law index equals one the viscosity is not affected by shear rate; a fluid that behaves this way is called Newtonian.
Effect of Temperature on Viscosity
When the temperature of a plastics melt is increased the viscosity reduces.
The effect of temperature on viscosity varies from one plastics to another. In general, amorphous plastics have a high temperature sensitivity relative to semi-crystalline plastics.
The temperature coefficient for amorphous plastics ranges from about 5 to 20 percent. This means that the viscosity changes from 5 to 20 percent for each degree Centigrade change in temperature.
For semi-crystalline plastics the temperature coefficient of the viscosity is about 2 to 3 percent. A change in the extruder barrel is going to have a larger effect on an amorphous plastics than on a semi-crystalline plastics. Good temperature control, therefore, is even more critical in amorphous plastics than in semi-crystalline plastics.
Viscous Heat Generation
When a plastics melt is sheared, heat is being generated in the plastics; this is called viscous heat generation. The viscous heat generation is determined by the product of viscosity and shear rate squared. Thus, the higher the viscosity of the plastics, the higher the viscous heat generation. The same is true for the shear rate, however, the shear rate has a stronger effect since the viscous heating increases with the shear rate squared.
As a result of the high viscosity of plastics, in extrusion most of the heating of the plastics comes from viscous heat generation. In fact, in some cases too much viscous heat generation occurs in the extruder and the machine has to be cooled to maintain the desired melt temperatures.
PLASTICS AND PROPERTIES IMPORTANT TO EXTRUSION
THERMOPLASTICS AND THERMOSETS
Plastics are carbon based materials made up of very long molecules. Plastics are also called polymers; they are manufactured by modification of natural products or by synthesis from intermediates. Plastics can be divided into thermoplastics and thermosets.
Thermoplastics are plastics that soften or melt and flow as a thick fluid when heated above a certain temperature. In this state, the material is often referred to as a plastics melt. It is also in this state that the material is usually formed or shaped into a product. Upon cooling thermoplastics harden and behave as a solid. After a thermoplastics product has been formed, it can be reheated and softened to be shaped again. Thus, thermoplastics can be processed several times and this is what makes them suitable for recycling.
Thermosets are plastics that harden when heated above a certain temperature. The hardening is due to a curing or crosslinking reaction that connects the individual molecules and causes the formation of a three-dimensional molecular network. The shaping of thermosets usually occurs before the crosslinking sets in, thus, at a temperature below the curing temperature. The crosslinking reaction is not reversible; a thermoset cannot be softened again like a thermoplastics. It is more difficult, therefore, to recycle a thermoset than a thermoplastics. Examples of thermosets are phenolics, ureas, certain polyesters, melamines, and epoxies.
Amorphous and Semi-Crystalline Plastics
Thermoplastics can be further divided into amorphous and semi-crystalline plastics.
Amorphous plastics have a random, irregular molecular structure without crystalline regions. Examples of amorphous plastics are polystyrene (PS), polycarbonate (PC), acrylic (PMMA), acrylonitirile butadiene styrene (ABS), and polyvinylchloride (PVC).
Semi-crystalline plastics can form highly regular regions where the molecules form crystals; these crystalline regions are referred to as crystallites. The ability to form crystals is determined to a large extent by the shape of the plastics molecule. Plastics that have linear molecules without large sidegroups usually have the ability to form crystallites. An example is high density polyethylene (HDPE), which can achieve high levels of crystallinity, as high as ninety percent. Other polymers that can form crystalline regions are acetal (POM), nylon (PA), polyester terephthalate (PETP), low density polyethylene (LDPE), and polypropylene (PP).
Plastics with bulky side groups often cannot form crystallites and, therefore, are amorphous. An example is polystyrene.
The crystalline regions in thermoplastics have different properties than the amorphous regions, for instance the density and the optical properties are different. As a result, the light transmission through a plastics changes when crystallites are present; the crystallites act as a filler and make the material opaque or translucent below the melting point [slide].
Above the melting point, the crystallites disappear and the material is transparent. Since amorphous plastics have no crystallites, they are often transparent [slide] - unless of course they contain fillers or other materials that alter the optical properties. It is interesting to note that “crystal polystyrene” is an amorphous plastics. It is called “crystal” because it is transparent, not because it is crystalline.
Semi-crystalline plastics are never completely crystalline; the highest level of crystallinity occurs in high density polyethylene with a crystallinity of up to 90 percent [slide]. Despite this, semi-crystalline plastics are often referred to as crystalline material. It should be remembered though, that the term semi-crystalline is more appropriate. Some plastics crystallise rapidly, e.g. high density polyethylene, others crystallise slowly, e.g. polyethylene terephthalate (PET). In fact, if PET is quenched rapidly after melt forming, it may cool down in completely amorphous state. In general, the morphology that develops in a plastics will depend on how fast it is cooled during and after the shaping process [slide]. The morphology is also affected by the stresses exerted on the plastics during and after the shaping process.
Thus, the flow and temperatures in the die and downstream of the die play an important role in the morphology that ultimately develops in the plastics part. The part properties are strongly determined by the morphology of the part. Therefore, the extruded product properties are affected by the flow and temperatures in the die and downstream of the die.
3.1.2 Liquid Crystalline Polymers
Next, we will discuss liquid crystalline plastics or LCPs. The molecules of LCPs are rod-like structures organised in large parallel domains; this is true not only in the solid state but also in the melt state [slide]. The large, ordered domains give LCPs unique characteristics compared to amorphous and semi-crystalline plastics.
Differences in mechanical and physical properties between plastics can often be attributed to their structure. The order in semi-crystalline plastics and LCPs make them stiffer, stronger, and less resistant to impact than amorphous plastics. Semi-crystalline and liquid crystalline plastics tend to be more resistant to creep, heat, and chemicals; however, they tend to require higher melt temperatures in processing.
Despite the higher processing temperatures, LCPs shrink less during cooling than amorphous plastics.
When amorphous plastics are heated they soften gradually, while semi-crystalline plastics tend to soften more abruptly. In melt processing, amorphous plastics usually do not flow as easily as semi-crystalline plastics. LCPs have the high melt temperature of semi-crystalline plastics, but they soften gradually like amorphous plastics. LCPs have the lowest viscosity and shrinkage of all thermoplastics.
Elastomers
Elastomers are materials capable of large elastic deformations. There are three types of elastomers: conventional (vulcanizable) elastomers, reactive system elastomers, and thermoplastics elastomers.
Conventional elastomers become elastic by creating a three-dimensional network of cross-links between the molecules. The formation of chemical cross-links is called “vulcanisation” or “curing.” Examples of conventional elastomers are polyisoprene and polybutadiene. Elastomers can also be produced from low-molecular-weight reactive chemicals. Some polyurethane and silicone elastomers fall into this category. In thermoplastics elastomers or TPEs there is no chemical crosslinking. The links between the molecules are formed by physical links rather than chemical links. Examples are urethane TPE and styrenic TPE.
Plastics are carbon based materials made up of very long molecules. Plastics are also called polymers; they are manufactured by modification of natural products or by synthesis from intermediates. Plastics can be divided into thermoplastics and thermosets.
Thermoplastics are plastics that soften or melt and flow as a thick fluid when heated above a certain temperature. In this state, the material is often referred to as a plastics melt. It is also in this state that the material is usually formed or shaped into a product. Upon cooling thermoplastics harden and behave as a solid. After a thermoplastics product has been formed, it can be reheated and softened to be shaped again. Thus, thermoplastics can be processed several times and this is what makes them suitable for recycling.
Thermosets are plastics that harden when heated above a certain temperature. The hardening is due to a curing or crosslinking reaction that connects the individual molecules and causes the formation of a three-dimensional molecular network. The shaping of thermosets usually occurs before the crosslinking sets in, thus, at a temperature below the curing temperature. The crosslinking reaction is not reversible; a thermoset cannot be softened again like a thermoplastics. It is more difficult, therefore, to recycle a thermoset than a thermoplastics. Examples of thermosets are phenolics, ureas, certain polyesters, melamines, and epoxies.
Amorphous and Semi-Crystalline Plastics
Thermoplastics can be further divided into amorphous and semi-crystalline plastics.
Amorphous plastics have a random, irregular molecular structure without crystalline regions. Examples of amorphous plastics are polystyrene (PS), polycarbonate (PC), acrylic (PMMA), acrylonitirile butadiene styrene (ABS), and polyvinylchloride (PVC).
Semi-crystalline plastics can form highly regular regions where the molecules form crystals; these crystalline regions are referred to as crystallites. The ability to form crystals is determined to a large extent by the shape of the plastics molecule. Plastics that have linear molecules without large sidegroups usually have the ability to form crystallites. An example is high density polyethylene (HDPE), which can achieve high levels of crystallinity, as high as ninety percent. Other polymers that can form crystalline regions are acetal (POM), nylon (PA), polyester terephthalate (PETP), low density polyethylene (LDPE), and polypropylene (PP).
Plastics with bulky side groups often cannot form crystallites and, therefore, are amorphous. An example is polystyrene.
The crystalline regions in thermoplastics have different properties than the amorphous regions, for instance the density and the optical properties are different. As a result, the light transmission through a plastics changes when crystallites are present; the crystallites act as a filler and make the material opaque or translucent below the melting point [slide].
Above the melting point, the crystallites disappear and the material is transparent. Since amorphous plastics have no crystallites, they are often transparent [slide] - unless of course they contain fillers or other materials that alter the optical properties. It is interesting to note that “crystal polystyrene” is an amorphous plastics. It is called “crystal” because it is transparent, not because it is crystalline.
Semi-crystalline plastics are never completely crystalline; the highest level of crystallinity occurs in high density polyethylene with a crystallinity of up to 90 percent [slide]. Despite this, semi-crystalline plastics are often referred to as crystalline material. It should be remembered though, that the term semi-crystalline is more appropriate. Some plastics crystallise rapidly, e.g. high density polyethylene, others crystallise slowly, e.g. polyethylene terephthalate (PET). In fact, if PET is quenched rapidly after melt forming, it may cool down in completely amorphous state. In general, the morphology that develops in a plastics will depend on how fast it is cooled during and after the shaping process [slide]. The morphology is also affected by the stresses exerted on the plastics during and after the shaping process.
Thus, the flow and temperatures in the die and downstream of the die play an important role in the morphology that ultimately develops in the plastics part. The part properties are strongly determined by the morphology of the part. Therefore, the extruded product properties are affected by the flow and temperatures in the die and downstream of the die.
3.1.2 Liquid Crystalline Polymers
Next, we will discuss liquid crystalline plastics or LCPs. The molecules of LCPs are rod-like structures organised in large parallel domains; this is true not only in the solid state but also in the melt state [slide]. The large, ordered domains give LCPs unique characteristics compared to amorphous and semi-crystalline plastics.
Differences in mechanical and physical properties between plastics can often be attributed to their structure. The order in semi-crystalline plastics and LCPs make them stiffer, stronger, and less resistant to impact than amorphous plastics. Semi-crystalline and liquid crystalline plastics tend to be more resistant to creep, heat, and chemicals; however, they tend to require higher melt temperatures in processing.
Despite the higher processing temperatures, LCPs shrink less during cooling than amorphous plastics.
When amorphous plastics are heated they soften gradually, while semi-crystalline plastics tend to soften more abruptly. In melt processing, amorphous plastics usually do not flow as easily as semi-crystalline plastics. LCPs have the high melt temperature of semi-crystalline plastics, but they soften gradually like amorphous plastics. LCPs have the lowest viscosity and shrinkage of all thermoplastics.
Elastomers
Elastomers are materials capable of large elastic deformations. There are three types of elastomers: conventional (vulcanizable) elastomers, reactive system elastomers, and thermoplastics elastomers.
Conventional elastomers become elastic by creating a three-dimensional network of cross-links between the molecules. The formation of chemical cross-links is called “vulcanisation” or “curing.” Examples of conventional elastomers are polyisoprene and polybutadiene. Elastomers can also be produced from low-molecular-weight reactive chemicals. Some polyurethane and silicone elastomers fall into this category. In thermoplastics elastomers or TPEs there is no chemical crosslinking. The links between the molecules are formed by physical links rather than chemical links. Examples are urethane TPE and styrenic TPE.
BLOWN FILM LINES
A blown film line is quite different from a flat film line. In a blown film line a tubular film is extruded vertically upwards as shown here. The tube expands due to internal air pressure. The ratio of the bubble diameter and the die diameter is called the blow-up ratio. Typical blow-up ratios used in LDPE film extrusion for packaging are in the range of 2.0:1 to 2.5:1. When the bubble has cooled sufficiently, the bubble is flattened in a collapsing frame and pulled through a set of nip rolls at the top of the collapsing frame. From there, the layflat is guided over several idler rollers to the winder where the film is rolled up over a core. Some lines perforate and heat seal the layflat prior to wind-up.
One advantage of the blown film process is that it can produce not only tubular products (bags) but also flat film, simply by slitting open the tube. In some blown film processes, the plastics is extruded downwards to produce films with special properties.
EXTRUSION COMPOUNDING LINES
Compounding lines come in many shapes and sizes. Compounding can be done on single screw extruders, twin screw extruders, reciprocating single screw compounders, batch internal mixers, and continuous internal mixers. The configuration of the line will be determined by the ingredients that have to be combined in the compounding extruder. The downstream equipment typically consists of a pelletising system. Some pelletisers cut extruded strands that are cooled in a water bath; these are called strand pelletisers and form cylindrical pellets. Dicers cut an extruded sheet rather than strands. The pellets from a dicer have a uniform cubic or octahedral shape. Other pelletisers cut the material right at the die exit; these are called die face pelletisers. The pellet shape from a die face pelletiser is often more spherical than cylindrical.
Different pelletisers:
• Strand pelletisers
• Dicers
• Water ring pelletisers
• Under water pelletisers
Compounding extruders can also be combined with direct forming systems downstream. In some lines, a gear pump is placed at the discharge end of the extruder to generate the diehead pressure and to control the throughput. An example of a combination compounding/sheet extrusion line is shown here.
The plastics is introduced to the first feed port of the compounding extruder, the filler is introduced to the second feed port, and the volatiles and air entrapment are removed from the vent port. A gear pump is placed between the compounding extruder and the sheet die. The sheet is fed to a roll stack, from there it is handled as in a normal sheet line as discussed earlier.
PROFILE EXTRUSION LINES
Many extrusion lines are used for the production of profiles. Profile lines also come in many shapes and forms. A typical extrusion line consists of an extruder, a calibrating unit, a cooling unit, a measurement device, a haul-off, and a coiler, cutter, or saw.
On some profile lines, a film or foil is laminated to the extruded profile. The number of profiles that are extruded is enormous; some examples of extruded profiles are shown here.
One advantage of the blown film process is that it can produce not only tubular products (bags) but also flat film, simply by slitting open the tube. In some blown film processes, the plastics is extruded downwards to produce films with special properties.
EXTRUSION COMPOUNDING LINES
Compounding lines come in many shapes and sizes. Compounding can be done on single screw extruders, twin screw extruders, reciprocating single screw compounders, batch internal mixers, and continuous internal mixers. The configuration of the line will be determined by the ingredients that have to be combined in the compounding extruder. The downstream equipment typically consists of a pelletising system. Some pelletisers cut extruded strands that are cooled in a water bath; these are called strand pelletisers and form cylindrical pellets. Dicers cut an extruded sheet rather than strands. The pellets from a dicer have a uniform cubic or octahedral shape. Other pelletisers cut the material right at the die exit; these are called die face pelletisers. The pellet shape from a die face pelletiser is often more spherical than cylindrical.
Different pelletisers:
• Strand pelletisers
• Dicers
• Water ring pelletisers
• Under water pelletisers
Compounding extruders can also be combined with direct forming systems downstream. In some lines, a gear pump is placed at the discharge end of the extruder to generate the diehead pressure and to control the throughput. An example of a combination compounding/sheet extrusion line is shown here.
The plastics is introduced to the first feed port of the compounding extruder, the filler is introduced to the second feed port, and the volatiles and air entrapment are removed from the vent port. A gear pump is placed between the compounding extruder and the sheet die. The sheet is fed to a roll stack, from there it is handled as in a normal sheet line as discussed earlier.
PROFILE EXTRUSION LINES
Many extrusion lines are used for the production of profiles. Profile lines also come in many shapes and forms. A typical extrusion line consists of an extruder, a calibrating unit, a cooling unit, a measurement device, a haul-off, and a coiler, cutter, or saw.
On some profile lines, a film or foil is laminated to the extruded profile. The number of profiles that are extruded is enormous; some examples of extruded profiles are shown here.
COMBINATION OF MATERIALS
The requirements of many products, particularly in packaging applications, are so stringent that they cannot be met by a single plastics. In order to meet the requirements, often two or more materials have to be combined. There are a number of techniques to combine different materials; some of the more important ones are: co extrusion, coating, and lamination. Co extrusion was covered in session 1; we will now discuss coating and lamination in more detail.
Extrusion Coating
In extrusion coating, a molten layer of plastics film is combined with a moving solid web or substrate. The substrate can be paper, paperboard, foil, plastics film, or fabrics; the substrate can also be a multi-layer product.
Extrusion Lamination
Extrusion lamination involves two or more substrates, for instance paper and aluminium foil, combined by using a plastics film as the adhesive between the two substrates. The extruded sheet or film can be laminated with a film on one side or both sides. The laminate can be paper, foil, mesh, or a number of other materials. With lamination, many different structures of sheet or film products can be made. The laminate is unrolled from a payoff and combined with the film and immediately led into a set of nip rolls. After lamination, the film is handled as a regular film.
Extrusion Coating
In extrusion coating, a molten layer of plastics film is combined with a moving solid web or substrate. The substrate can be paper, paperboard, foil, plastics film, or fabrics; the substrate can also be a multi-layer product.
Extrusion Lamination
Extrusion lamination involves two or more substrates, for instance paper and aluminium foil, combined by using a plastics film as the adhesive between the two substrates. The extruded sheet or film can be laminated with a film on one side or both sides. The laminate can be paper, foil, mesh, or a number of other materials. With lamination, many different structures of sheet or film products can be made. The laminate is unrolled from a payoff and combined with the film and immediately led into a set of nip rolls. After lamination, the film is handled as a regular film.
COMPLETE EXTRUSION LINES
In this section we will discuss:
• Typical components of an extrusion line
• Tubing and pipe extrusion lines
• Sheet and film lines
• Extrusion compounding lines
• Profile extrusion lines
It is obvious that the extruder alone is not sufficient to make extruded product. In addition to the extruder, we need upstream and downstream equipment to produce a useful product.
Typical elements of an extrusion line are:
• Resin handling system
• Drying system
• Extruder
• Post-shaping or calibrating device
• Cooling device
• Take-up device
• Cutter or saw
Besides the four main types listed earlier, there are quite a few more lines, such as fiber spinning lines, extrusion blowmolding machines, integrated sheet and thermoforming lines, and others.
Tubing and Pipe Extrusion Line
Dies for tubing and pipe were discussed earlier. Small diameter tubing, less than about 10 mm, is usually made without a calibrator - this is called “free extrusion.” Large diameter tubing and pipe is made with a calibrator or sizing device just downstream of the die.
The purpose of the sizing unit is to solidify the plastics in the calibrating section to a thickness sufficient to transfer the stresses acting on the product, while maintaining the desired shape and dimension. The main components of a typical tubing extrusion line are shown here.
A gear pump may or may not be used, depending on the precision that is required in the extrusion process. The internal air pressure of the tubing is controlled to achieve the correct values for the outside diameter and wall thickness. The diameter is often measured with a laser gage to allow close monitoring and control of the diameter. The diameter and the wall thickness are determined mostly by the extruder output, the puller speed, and the internal air pressure. Closed loop control systems are available that automatically set the appropriate values of gear pump speed, puller speed, and internal air pressure. After the puller, the tubing may be cut or it may be reeled up on a spool.
Film and Sheet Lines Using The Roll Stack
There are no major differences between the extrusion of flat film and sheet. The main components of a sheet line are the extruder, the roll stack, the cooling section, the nip roll section, and the winder. The roll stack contains three rolls that are often referred to as polishing rolls. They are used to exert pressure on the sheet and to impart the surface conditions of the rolls to the plastics sheet. If a smooth surface is required, smooth rolls will be used. If a textured surface is needed, a textured surface is used on the roll.
The rolls are normally cored so that the temperature of the rolls can be controlled. This is usually done with circulating oil. The temperature of each roll can be adjusted separately. The rolls can be in a vertical position as shown or they can be at an angle.
The cooling section consists of a number of rolls positioned in a frame; the sheet is guided over and under the rolls to keep the sheet flat.
At the end of the cooling section are the pull rolls or nip rolls; these are rubber rolls that pull the sheet from the roll stack to maintain a certain tension in the sheet.
After the nip rolls, the sheet is led to the winder that rolls the sheet on a core. Many different winders are available; some winders automatically transfer the sheet to a new core when one package is full.
Film Lines Using Chill Roll Casting
With thin film, the film is often cast on a chill roll rather than extruded into a roll stack. The main components of a cast film line are the, the film die, the chill roll unit, the thickness gauging system, the surface treatment unit, and the winder. The film is extruded downward onto the chill roll. The initial contact between the film and the chill roll is established by the use of an air knife. The air knife produces a thin stream of high velocity air across the width of the chill roll, the air stream pushes the film against the roll surface.
From the chill roll unit, the film is lead to a thickness gauging unit where the thickness of the film is measured across the width of the film. Next, the film passes through a surface treatment unit and then to the winder. A surface treatment unit may be incorporated to improve adhesion, for instance for a subsequent printing or laminating operation.
The most important adhesion promoters are:
• Flame treatment
• Corona discharge treatment
• Ozone treatment
• primers
• Typical components of an extrusion line
• Tubing and pipe extrusion lines
• Sheet and film lines
• Extrusion compounding lines
• Profile extrusion lines
It is obvious that the extruder alone is not sufficient to make extruded product. In addition to the extruder, we need upstream and downstream equipment to produce a useful product.
Typical elements of an extrusion line are:
• Resin handling system
• Drying system
• Extruder
• Post-shaping or calibrating device
• Cooling device
• Take-up device
• Cutter or saw
Besides the four main types listed earlier, there are quite a few more lines, such as fiber spinning lines, extrusion blowmolding machines, integrated sheet and thermoforming lines, and others.
Tubing and Pipe Extrusion Line
Dies for tubing and pipe were discussed earlier. Small diameter tubing, less than about 10 mm, is usually made without a calibrator - this is called “free extrusion.” Large diameter tubing and pipe is made with a calibrator or sizing device just downstream of the die.
The purpose of the sizing unit is to solidify the plastics in the calibrating section to a thickness sufficient to transfer the stresses acting on the product, while maintaining the desired shape and dimension. The main components of a typical tubing extrusion line are shown here.
A gear pump may or may not be used, depending on the precision that is required in the extrusion process. The internal air pressure of the tubing is controlled to achieve the correct values for the outside diameter and wall thickness. The diameter is often measured with a laser gage to allow close monitoring and control of the diameter. The diameter and the wall thickness are determined mostly by the extruder output, the puller speed, and the internal air pressure. Closed loop control systems are available that automatically set the appropriate values of gear pump speed, puller speed, and internal air pressure. After the puller, the tubing may be cut or it may be reeled up on a spool.
Film and Sheet Lines Using The Roll Stack
There are no major differences between the extrusion of flat film and sheet. The main components of a sheet line are the extruder, the roll stack, the cooling section, the nip roll section, and the winder. The roll stack contains three rolls that are often referred to as polishing rolls. They are used to exert pressure on the sheet and to impart the surface conditions of the rolls to the plastics sheet. If a smooth surface is required, smooth rolls will be used. If a textured surface is needed, a textured surface is used on the roll.
The rolls are normally cored so that the temperature of the rolls can be controlled. This is usually done with circulating oil. The temperature of each roll can be adjusted separately. The rolls can be in a vertical position as shown or they can be at an angle.
The cooling section consists of a number of rolls positioned in a frame; the sheet is guided over and under the rolls to keep the sheet flat.
At the end of the cooling section are the pull rolls or nip rolls; these are rubber rolls that pull the sheet from the roll stack to maintain a certain tension in the sheet.
After the nip rolls, the sheet is led to the winder that rolls the sheet on a core. Many different winders are available; some winders automatically transfer the sheet to a new core when one package is full.
Film Lines Using Chill Roll Casting
With thin film, the film is often cast on a chill roll rather than extruded into a roll stack. The main components of a cast film line are the, the film die, the chill roll unit, the thickness gauging system, the surface treatment unit, and the winder. The film is extruded downward onto the chill roll. The initial contact between the film and the chill roll is established by the use of an air knife. The air knife produces a thin stream of high velocity air across the width of the chill roll, the air stream pushes the film against the roll surface.
From the chill roll unit, the film is lead to a thickness gauging unit where the thickness of the film is measured across the width of the film. Next, the film passes through a surface treatment unit and then to the winder. A surface treatment unit may be incorporated to improve adhesion, for instance for a subsequent printing or laminating operation.
The most important adhesion promoters are:
• Flame treatment
• Corona discharge treatment
• Ozone treatment
• primers
PRESSURE FEEDBACK CONTROL
As we discussed in session 1, in many extruders a screen pack is used to trap contaminants in the plastics. When contaminants build up the flow resistance of the screen pack will increase, reducing the die inlet pressure. As a result, the extruder output will reduce and the dimensions of the extruded product will reduce as well. This is of course undesirable.
This problem can be avoided by using pressure feedback control. This involves measuring the die inlet pressure and feeding this signal back to the extruder drive. When the diehead pressure changes, the screw speed is adjusted automatically to maintain the diehead pressure to a constant value. Pressure feedback allows more precise control of the diehead pressure and improves extrusion stability. Pressure feedback control is also used with gear pumps to control the inlet or suction pressure of the gear pump.
When the diehead pressure changes, it may take a minute or longer before the pressure feedback control takes effect and brings the pressure back to the setpoint. As a result, pressure feedback control will only work with slow, gradual changes in pressure. It will not work with rapid pressure fluctuations occurring over 15 to 30 seconds or less.
MELT TEMPERATURE FEEDBACK CONTROL
Constant output from the extruder requires that both the die inlet pressure and melt temperature in the die remain constant. Pressure feedback control can be used to make sure that the diehead pressure stays the same. A similar control can be used for the melt temperature. Not surprisingly, this is called melt temperature feedback
The signal from a melt temperature probe at the end of the extruder is fed back to the last one or two barrel temperature zones. If the measured melt temperature deviates from the setpoint, the setpoint of the last one or two barrel temperature zones is changed to bring the melt temperature back to the setpoint.
If the melt temperature changes, it will generally take several minutes before a change in the setpoint of the barrel temperature zone will bring the melt temperature back to the required level. As a result, melt temperature feedback control will only work on slow, gradual changes in melt temperature occurring over at least several minutes. It will not work on rapid temperature fluctuations.
CONTROL OF THE EXTRUSION LINE
Instrumentation is important not only for the extruder itself but also for upstream and downstream equipment. Since there are many different types of extrusion lines, it will not be possible to discuss all of them. We will discuss some of the main control issues in extrusion lines and then discuss the main types of extrusion lines.
One of the main issues in control of an extrusion line is the coordination of the extruder with the puller. In course 1 we discussed the importance of controlling the screw speed. It is equally important to control the puller speed. To keep dimensional variation to a minimum it is important to keep both the extruder screw speed and the puller speed constant. When the screw drive and puller drive have digital control it is possible to link the two drives digitally so that the ratio of the screw and puller speed can be controlled absolutely steady. It also allows an increase in screw speed with an automatic increase in the puller speed.
Control of the dimensions of the extruded product is generally obtained by adjusting the ratio of screw to puller speed; either the screw speed or the puller speed can be changed. It is often easier and quicker to change the puller speed than the screw speed. When the screw speed is changed, it can take 30 seconds or longer before the extruder reaches steady conditions.
Dimensional control obviously requires measurement of the extruded product. In circular extrusions, such as tubing, wire coating, and pipe, the diameter is usually measured with a laser gage. It is good practice to measure the diameter in both the vertical and horizontal direction - this can be done with a dual axis laser gage. The diameter of circular products can be measured with a laser gage. The diameter should be measured along two perpendicular axes; this can be done with a dual-axis laser gage.
Wall thickness can be measured using ultrasonic transducers. The transducer can be made to travel around the circumference of the pipe or tube or multiple stationary transducers can be used. Usually there are two transducers in the horizontal and two in the vertical plane.
In film and sheet extrusion the web thickness is often measured using a scanning thickness gage. The measuring head moves across the moving web so that both transverse and longitudinal thickness variation can be measured. The measurement is often made using a nuclear gage.
This problem can be avoided by using pressure feedback control. This involves measuring the die inlet pressure and feeding this signal back to the extruder drive. When the diehead pressure changes, the screw speed is adjusted automatically to maintain the diehead pressure to a constant value. Pressure feedback allows more precise control of the diehead pressure and improves extrusion stability. Pressure feedback control is also used with gear pumps to control the inlet or suction pressure of the gear pump.
When the diehead pressure changes, it may take a minute or longer before the pressure feedback control takes effect and brings the pressure back to the setpoint. As a result, pressure feedback control will only work with slow, gradual changes in pressure. It will not work with rapid pressure fluctuations occurring over 15 to 30 seconds or less.
MELT TEMPERATURE FEEDBACK CONTROL
Constant output from the extruder requires that both the die inlet pressure and melt temperature in the die remain constant. Pressure feedback control can be used to make sure that the diehead pressure stays the same. A similar control can be used for the melt temperature. Not surprisingly, this is called melt temperature feedback
The signal from a melt temperature probe at the end of the extruder is fed back to the last one or two barrel temperature zones. If the measured melt temperature deviates from the setpoint, the setpoint of the last one or two barrel temperature zones is changed to bring the melt temperature back to the setpoint.
If the melt temperature changes, it will generally take several minutes before a change in the setpoint of the barrel temperature zone will bring the melt temperature back to the required level. As a result, melt temperature feedback control will only work on slow, gradual changes in melt temperature occurring over at least several minutes. It will not work on rapid temperature fluctuations.
CONTROL OF THE EXTRUSION LINE
Instrumentation is important not only for the extruder itself but also for upstream and downstream equipment. Since there are many different types of extrusion lines, it will not be possible to discuss all of them. We will discuss some of the main control issues in extrusion lines and then discuss the main types of extrusion lines.
One of the main issues in control of an extrusion line is the coordination of the extruder with the puller. In course 1 we discussed the importance of controlling the screw speed. It is equally important to control the puller speed. To keep dimensional variation to a minimum it is important to keep both the extruder screw speed and the puller speed constant. When the screw drive and puller drive have digital control it is possible to link the two drives digitally so that the ratio of the screw and puller speed can be controlled absolutely steady. It also allows an increase in screw speed with an automatic increase in the puller speed.
Control of the dimensions of the extruded product is generally obtained by adjusting the ratio of screw to puller speed; either the screw speed or the puller speed can be changed. It is often easier and quicker to change the puller speed than the screw speed. When the screw speed is changed, it can take 30 seconds or longer before the extruder reaches steady conditions.
Dimensional control obviously requires measurement of the extruded product. In circular extrusions, such as tubing, wire coating, and pipe, the diameter is usually measured with a laser gage. It is good practice to measure the diameter in both the vertical and horizontal direction - this can be done with a dual axis laser gage. The diameter of circular products can be measured with a laser gage. The diameter should be measured along two perpendicular axes; this can be done with a dual-axis laser gage.
Wall thickness can be measured using ultrasonic transducers. The transducer can be made to travel around the circumference of the pipe or tube or multiple stationary transducers can be used. Usually there are two transducers in the horizontal and two in the vertical plane.
In film and sheet extrusion the web thickness is often measured using a scanning thickness gage. The measuring head moves across the moving web so that both transverse and longitudinal thickness variation can be measured. The measurement is often made using a nuclear gage.
TEMPERATURE CONTROL
In the extrusion process, good temperature control is important to achieve good process stability. There are two main types of temperature control: on-off control and proportional control. In on-off control the heater power is either full on or completely off.
The temperature versus time for on-off control of heating is shown here; the power vs. time is shown as well. When the measured temperature is below the setpoint, the power is full on. When it reaches the setpoint the power shuts off. After the initial increase from room temperature, the temperature will vary in a cyclic manner with a corresponding on-off cycling of the power.
The advantage of on-off control is that it is simple and the average temperature is right at the setpoint. The disadvantage is that the actual temperature always cycles and the actual temperature variation can be quite large, as much as 10-20 degrees C. The larger the extruder, the greater the temperature variation tends to get. Because of this, on-off control is not recommended in extrusion, except for non-critical processes.
Advantage on-off control:
• Simple
• Average temperature right at set-point
Disadvantage on-off control:
• Actual temperature cycles
• Temperature variation is quite large
In proportional control, the power is proportional to the temperature within a certain temperature region; this region is called the proportional band. The temperature versus time for proportional control is show here; the power vs. time is shown as well.
The advantage of proportional control is that the temperature can reach a steady value, as opposed to on-off control. The power level can adjust itself exactly to the level that is required to maintain the correct temperature. For good temperature control in extruders it is important to have proportional control of both heating and cooling.
Some commercial extruders combine proportional heating with on-off cooling - this results in poor temperature control when cooling is activated.
Advantage of proportional control:
• Temperature can reach a steady value
• Power level adjusted to correct level automatically
A limitation of simple proportional control or P-control is that the temperature can remain steady only as long as the thermal conditions around the extruder are constant. When there is an upset in the thermal conditions, such as a change in ambient temperature, the actual temperature will change and the P-control will not be able to correct it. In other words, in P-control there is no reset capability.
Disadvantages of proportional only control:
• Temperature will change when thermal conditions change
• There is no reset capability
In proportional control with integrating action, also called PI-control, there is reset capability.
The controller integrates the difference between actual temperature and setpoint and continues to act on the process until the difference is zero. When there is an upset in the process there will be a temporary deviation from the setpoint, but eventually the actual temperature will go to the setpoint again.
Controllers with derivative action react to the rate of temperature change. This allows the controller to react to a process upset more quickly. Controllers with proportional and derivative action are called PD controllers, controllers with proportional and integrating action are called PI controllers, and controllers with proportional and integral and derivative action are called PID controllers. PID control is commonly used in extruders.
For a control action, the controller has to be tuned to the characteristics of the extruder at typical process conditions. Tuning of a PID controller involves determining 1) the correct width of the proportional band and 2) the time constant for integrating and 3) the time constant for derivative action. Even a good controller will give poor control if it is not properly tuned.
As a result, careful attention should be paid to tuning controllers that require manual tuning. Nowadays, there are number of controllers that tune themselves automatically; these are called “self-tuning” or “auto-tuning” controllers. With these controllers one does not have to worry about manually tuning the controllers.
A relatively new method of control is fuzzy logic control or FLC. FLC is an artificial intelligence based technology, designed to simulate human decision making. It can be used in systems that use many variables to enhance process control. Developing a fuzzy logic application requires the generation of a knowledge base; this can be a time consuming process.
Generation of a knowledge base for FLC involves identifying:
• Process variables that are important in control
• Membership functions for each variable, such as high, low, and medium
• Fuzzy rules which define the knowledge of what to do about an observation, based on previous operating experience
• FLC is slowly starting to be used in the plastics processing industry. It has already been applied a number of times in injection moulding, fewer applications have been reported in extrusion. It has been shown, however, that FLC can outperform conventional PID control if the knowledge base is sufficiently developed.
The temperature versus time for on-off control of heating is shown here; the power vs. time is shown as well. When the measured temperature is below the setpoint, the power is full on. When it reaches the setpoint the power shuts off. After the initial increase from room temperature, the temperature will vary in a cyclic manner with a corresponding on-off cycling of the power.
The advantage of on-off control is that it is simple and the average temperature is right at the setpoint. The disadvantage is that the actual temperature always cycles and the actual temperature variation can be quite large, as much as 10-20 degrees C. The larger the extruder, the greater the temperature variation tends to get. Because of this, on-off control is not recommended in extrusion, except for non-critical processes.
Advantage on-off control:
• Simple
• Average temperature right at set-point
Disadvantage on-off control:
• Actual temperature cycles
• Temperature variation is quite large
In proportional control, the power is proportional to the temperature within a certain temperature region; this region is called the proportional band. The temperature versus time for proportional control is show here; the power vs. time is shown as well.
The advantage of proportional control is that the temperature can reach a steady value, as opposed to on-off control. The power level can adjust itself exactly to the level that is required to maintain the correct temperature. For good temperature control in extruders it is important to have proportional control of both heating and cooling.
Some commercial extruders combine proportional heating with on-off cooling - this results in poor temperature control when cooling is activated.
Advantage of proportional control:
• Temperature can reach a steady value
• Power level adjusted to correct level automatically
A limitation of simple proportional control or P-control is that the temperature can remain steady only as long as the thermal conditions around the extruder are constant. When there is an upset in the thermal conditions, such as a change in ambient temperature, the actual temperature will change and the P-control will not be able to correct it. In other words, in P-control there is no reset capability.
Disadvantages of proportional only control:
• Temperature will change when thermal conditions change
• There is no reset capability
In proportional control with integrating action, also called PI-control, there is reset capability.
The controller integrates the difference between actual temperature and setpoint and continues to act on the process until the difference is zero. When there is an upset in the process there will be a temporary deviation from the setpoint, but eventually the actual temperature will go to the setpoint again.
Controllers with derivative action react to the rate of temperature change. This allows the controller to react to a process upset more quickly. Controllers with proportional and derivative action are called PD controllers, controllers with proportional and integrating action are called PI controllers, and controllers with proportional and integral and derivative action are called PID controllers. PID control is commonly used in extruders.
For a control action, the controller has to be tuned to the characteristics of the extruder at typical process conditions. Tuning of a PID controller involves determining 1) the correct width of the proportional band and 2) the time constant for integrating and 3) the time constant for derivative action. Even a good controller will give poor control if it is not properly tuned.
As a result, careful attention should be paid to tuning controllers that require manual tuning. Nowadays, there are number of controllers that tune themselves automatically; these are called “self-tuning” or “auto-tuning” controllers. With these controllers one does not have to worry about manually tuning the controllers.
A relatively new method of control is fuzzy logic control or FLC. FLC is an artificial intelligence based technology, designed to simulate human decision making. It can be used in systems that use many variables to enhance process control. Developing a fuzzy logic application requires the generation of a knowledge base; this can be a time consuming process.
Generation of a knowledge base for FLC involves identifying:
• Process variables that are important in control
• Membership functions for each variable, such as high, low, and medium
• Fuzzy rules which define the knowledge of what to do about an observation, based on previous operating experience
• FLC is slowly starting to be used in the plastics processing industry. It has already been applied a number of times in injection moulding, fewer applications have been reported in extrusion. It has been shown, however, that FLC can outperform conventional PID control if the knowledge base is sufficiently developed.
Melt Temperature Measurement
The temperature of the plastics melt is often measured with an immersion TC. The probe protrudes into the melt and reads the temperature at the point of the TC junction. To avoid conduction errors the junction should be thermally insulated from the base of the probe. The immersion TC is inexpensive, good for point measurement, and available with adjustable depth. One drawback is that the immersion probe changes the velocities in the channel and, thus, the melt temperatures. Also, dead spots may occur behind the immersion probe; this can lead to degradation in plastics with limited thermal stability.
Advantages of immersion melt temperature probe:
• Inexpensive
• Point measurement
• Available with adjustable depth
Disadvantages of immersion melt temperature probe:
• Changes the velocities in the channel
• Changes the melt temperatures in the channel
• Dead spots can occur behind the probe
A flush mounted probe can be used. This measures the melt temperature at the wall, which is usually the same as the metal wall temperature. As a result, this melt temperature measurement is not the most useful. Another probe has a straight protruding design; these probes are also available with adjustable depth so that temperatures at different positions in the channel can be measured. Yet another design has a tip on the probe that points upstream. The TC junction is located in the tip. The benefit of this design is that there is minimal disturbance of flow where the temperature is measured. This probe is also available with adjustable depth.
It is also possible to run a bridge across the channel with several probes attached to it. This allows simultaneous melt temperature measurement at several locations.
Barrel Temperature Measurement
The temperature of the barrel is usually measured with TC or RTD sensors pressed into the barrel; the sensors are generally spring loaded. Many temperature sensors are constructed with a metal sheath to obtain sufficient mechanical strength. As a result, significant conduction errors can occur in the measurement. This can be minimised by using a sensor whose temperature sensing element is thermally insulated from the rest of the probe. The accuracy of the measurement is strongly dependent on the depth of the well, the type of sensor, and the air velocity. For accurate temperature measurement the well depth should be at least 25 mm (1 inch) and the extruder barrel and die should be shielded from air currents.
Measurement error can be reduced by thermally insulated sensing tip
Measurement accuracy is dependent on:
• Depth of the well
• Type of temperature sensor
• Air velocity
Advantages of immersion melt temperature probe:
• Inexpensive
• Point measurement
• Available with adjustable depth
Disadvantages of immersion melt temperature probe:
• Changes the velocities in the channel
• Changes the melt temperatures in the channel
• Dead spots can occur behind the probe
A flush mounted probe can be used. This measures the melt temperature at the wall, which is usually the same as the metal wall temperature. As a result, this melt temperature measurement is not the most useful. Another probe has a straight protruding design; these probes are also available with adjustable depth so that temperatures at different positions in the channel can be measured. Yet another design has a tip on the probe that points upstream. The TC junction is located in the tip. The benefit of this design is that there is minimal disturbance of flow where the temperature is measured. This probe is also available with adjustable depth.
It is also possible to run a bridge across the channel with several probes attached to it. This allows simultaneous melt temperature measurement at several locations.
Barrel Temperature Measurement
The temperature of the barrel is usually measured with TC or RTD sensors pressed into the barrel; the sensors are generally spring loaded. Many temperature sensors are constructed with a metal sheath to obtain sufficient mechanical strength. As a result, significant conduction errors can occur in the measurement. This can be minimised by using a sensor whose temperature sensing element is thermally insulated from the rest of the probe. The accuracy of the measurement is strongly dependent on the depth of the well, the type of sensor, and the air velocity. For accurate temperature measurement the well depth should be at least 25 mm (1 inch) and the extruder barrel and die should be shielded from air currents.
Measurement error can be reduced by thermally insulated sensing tip
Measurement accuracy is dependent on:
• Depth of the well
• Type of temperature sensor
• Air velocity
MOST IMPORTANT PROCESS PARAMETERS
The most important process parameters are melt pressure and melt temperature. They are generally the best indicators of how well or how poorly an extruder functions. Process problems, in most cases, first become obvious from melt pressure and/or temperature readings. Just as a doctor first measures blood pressure and temperature of a patient, a process engineer should measure melt pressure and temperature of an extruder to obtain the most telling (revealing) information from the process.
Other important process parameters are:
• Screw speed
• Motor load
• Barrel temperatures
• Die temperatures
• Power draw of the various heaters
• Cooling rate of the various cooling units
• Vacuum level in vented extrusion
These parameters relate just to the extruder. However, there are many more process parameters for the entire extrusion line depending on the design of the extrusion line.
Important parameters for any extrusion line are:
• Line speed
• Dimensions of the extruded product
• Cooling rate or cooling water temperature
• Line tension
Many other factors can influence the extrusion process, such as ambient temperature, relative humidity, air currents around the extruder, and plant voltage variations.
Measurement of melt pressure is important for two reasons, one: process monitoring and control and two: safety. The diehead pressure in the extruder determines the output from the extruder. It is the pressure necessary to overcome the flow resistance of the die. When the diehead pressure changes with time, the extruder output will change correspondingly and, with it, the dimensions of the extruded product. As a result, when we monitor how the pressure varies with time, we can see exactly the stability or lack of stability of the extrusion process It is best, therefore, to plot pressure with a chart recorder or, better, to monitor the variation of pressure with a data acquisition system. A simple analog or digital display of pressure is much less useful.
It is also critically important to measure pressure in the extruder to prevent serious accidents that can happen when excessively high pressures occur. Under some circumstances, very high pressures can be generated in the extruder, causing the extruder to explode. The barrel can crack open under excessive pressure or the die may explode off the extruder. Either situation is extremely dangerous and should be avoided if at all possible. All extruders should have an over-pressure safety device, such as a rupture disk or a shear pin in the head clamp. Even with such an over-pressure safety device, the extruder should have at least one melt pressure measurement because over-pressure devices may not work properly or may have been disabled. Pressure can build up very quickly without a warning and cause a catastrophic explosion. For that reason an over-pressure shutdown should be used; this automatically turns off the extruder drive when the pressure exceeds a critical value.
Pressure Transducers
The most common pressure transducers in extrusion are the strain gage transducer and the piezo-electric transducer. The strain gage transducer can be either a capillary or a pushrod transducer. In these transducers there are two diaphragms, one in contact with the plastics melt and one some distance away from the hot plastics melt. In the capillary type the two diaphragms are linked hydraulically, while in the pushrod type there is a mechanical link. A strain gage is attached to the second diaphragm to measure the deflection. This deflection can be related to the pressure at the first diaphragm.
Most capillary transducers are filled with Mercury. Since the diaphragm of the transducer is quite thin, there is a danger of rupture of the diaphragm and leakage of Mercury into the plastics and into the workplace. Since the type of liquid fill is often not shown on the transducer label, Mercury contamination may occur unknowingly!
Another transducer is the pneumatic pressure transducer. It has good robustness, but poor temperature sensitivity, poor dynamic response, and average measurement error.
Pneumatic transducer:
• Good robustness
• Poor temperature sensitivity
• Poor dynamic response
• Average measurement error
The capillary transducer has fair robustness, fair temperature sensitivity, and fair dynamic response. The total measurement error varies from 0.5 to 3% dependent on the quality of the transducer.
Capillary transducer:
• Fair robustness
• Fair temperature sensitivity
• Fair dynamic response
• Measurement error range from fair to poor
The pushrod is similar to the capillary transducer, except that is has poor temperature sensitivity and poor total error. The piezo-resistive transducer has good robustness due to its relatively thick diaphragm, good temperature sensitivity, good dynamic response, and the lowest measurement error.
Piezo-resistive transducer:
• Good robustness
• Good temperature sensitivity
• Good dynamic response
• Lowest measurement error
When pressure is measured at the end of the screw, the pressure will show a cyclic variation in sync with the rotation of the screw - this is called “screw beat.” This variation is due to the pressure at the leading side of the flight being higher than at the trailing side. When the pressure is plotted against time, the pressure profile shows a sawtooth pattern]. The time between pressure peaks is exactly the time for one revolution of the screw with a single flighted screw. After the breaker plate the pressure variation is much lower. In fact, this is one of the benefits of using a breaker plate; it dampens the pressure fluctuations at the end of the screw.
Temperature Measurement
There are three main types of temperature measurement: thermocouple, RTD, and infrared. Temperature is usually measured with thermocouple (TC) type temperature sensors. The principle f the TC is that when two dissimilar metals are connected and the temperature T of the junction is different from a reference junction at T0, there will be a voltage generated at the output end that is related to the temperature difference T-T0. Since the temperature measurement is determined by the exact combination of metal wires, it is important that the correct wires are used when wiring changes are made.
Advantage thermocouples:
• Wide temperature range
• Good response time
• Good for point sensing
• Inexpensive
Disadvantages thermocouples:
• Require special lead wire
• Require a reference junction
• Low signal output
• Limited stability
Another temperature sensor that is used in extrusion is the resistance temperature detector or RTD. The principle of the RTD is that the resistance of metals changes with temperature, so that by measuring resistance, the temperature can be determined. RTDs use a pure platinum resistance element to achieve high accuracy; platinum also has a linear relationship between resistance and temperature. Advantages of RTDs over TCs are higher output signal, better stability and accuracy, also, they do not require special lead wires or a reference junction. On the other hand, TCs are less expensive and are better for point sensing because the sensing element is a point.
Advantages of RTD over TC:
• Higher signal output
• Better stability and accuracy
• Good for area sensing
• No need for special lead wires
• No need for reference junction
Disadvantages of RTD:
• More expensive than thermocouples
• Not good for point sensing
A third type of temperature measurement uses infrared (IR) detectors. The IR detector is based on the fact that objects emit radiation that changes with temperature. Thus, by measuring the radiation emitted by an object, the surface temperature of the object can be determined. IR temperature probes are useful because they allow non-contact temperature measurement. For instance, the temperature distribution across an extruded sheet can be measured using an IR probe without leaving any marks on the extruded product.
There are also IR probes that are mounted in the extruder to measure melt temperature in the machine. These probes have a sapphire window and measure the radiation coming off the plastics melt. If the melt is opaque the temperature measured is the surface temperature, which is the wall temperature. If the melt is transparent, the radiation from inner layers will also be measured, so that the temperature will be stock temperature averaged over a certain distance. The advantage of IR measurement is that the response is very rapid, in the range of milliseconds. A drawback of the IR measurement is its relatively high cost.
Other important process parameters are:
• Screw speed
• Motor load
• Barrel temperatures
• Die temperatures
• Power draw of the various heaters
• Cooling rate of the various cooling units
• Vacuum level in vented extrusion
These parameters relate just to the extruder. However, there are many more process parameters for the entire extrusion line depending on the design of the extrusion line.
Important parameters for any extrusion line are:
• Line speed
• Dimensions of the extruded product
• Cooling rate or cooling water temperature
• Line tension
Many other factors can influence the extrusion process, such as ambient temperature, relative humidity, air currents around the extruder, and plant voltage variations.
Measurement of melt pressure is important for two reasons, one: process monitoring and control and two: safety. The diehead pressure in the extruder determines the output from the extruder. It is the pressure necessary to overcome the flow resistance of the die. When the diehead pressure changes with time, the extruder output will change correspondingly and, with it, the dimensions of the extruded product. As a result, when we monitor how the pressure varies with time, we can see exactly the stability or lack of stability of the extrusion process It is best, therefore, to plot pressure with a chart recorder or, better, to monitor the variation of pressure with a data acquisition system. A simple analog or digital display of pressure is much less useful.
It is also critically important to measure pressure in the extruder to prevent serious accidents that can happen when excessively high pressures occur. Under some circumstances, very high pressures can be generated in the extruder, causing the extruder to explode. The barrel can crack open under excessive pressure or the die may explode off the extruder. Either situation is extremely dangerous and should be avoided if at all possible. All extruders should have an over-pressure safety device, such as a rupture disk or a shear pin in the head clamp. Even with such an over-pressure safety device, the extruder should have at least one melt pressure measurement because over-pressure devices may not work properly or may have been disabled. Pressure can build up very quickly without a warning and cause a catastrophic explosion. For that reason an over-pressure shutdown should be used; this automatically turns off the extruder drive when the pressure exceeds a critical value.
Pressure Transducers
The most common pressure transducers in extrusion are the strain gage transducer and the piezo-electric transducer. The strain gage transducer can be either a capillary or a pushrod transducer. In these transducers there are two diaphragms, one in contact with the plastics melt and one some distance away from the hot plastics melt. In the capillary type the two diaphragms are linked hydraulically, while in the pushrod type there is a mechanical link. A strain gage is attached to the second diaphragm to measure the deflection. This deflection can be related to the pressure at the first diaphragm.
Most capillary transducers are filled with Mercury. Since the diaphragm of the transducer is quite thin, there is a danger of rupture of the diaphragm and leakage of Mercury into the plastics and into the workplace. Since the type of liquid fill is often not shown on the transducer label, Mercury contamination may occur unknowingly!
Another transducer is the pneumatic pressure transducer. It has good robustness, but poor temperature sensitivity, poor dynamic response, and average measurement error.
Pneumatic transducer:
• Good robustness
• Poor temperature sensitivity
• Poor dynamic response
• Average measurement error
The capillary transducer has fair robustness, fair temperature sensitivity, and fair dynamic response. The total measurement error varies from 0.5 to 3% dependent on the quality of the transducer.
Capillary transducer:
• Fair robustness
• Fair temperature sensitivity
• Fair dynamic response
• Measurement error range from fair to poor
The pushrod is similar to the capillary transducer, except that is has poor temperature sensitivity and poor total error. The piezo-resistive transducer has good robustness due to its relatively thick diaphragm, good temperature sensitivity, good dynamic response, and the lowest measurement error.
Piezo-resistive transducer:
• Good robustness
• Good temperature sensitivity
• Good dynamic response
• Lowest measurement error
When pressure is measured at the end of the screw, the pressure will show a cyclic variation in sync with the rotation of the screw - this is called “screw beat.” This variation is due to the pressure at the leading side of the flight being higher than at the trailing side. When the pressure is plotted against time, the pressure profile shows a sawtooth pattern]. The time between pressure peaks is exactly the time for one revolution of the screw with a single flighted screw. After the breaker plate the pressure variation is much lower. In fact, this is one of the benefits of using a breaker plate; it dampens the pressure fluctuations at the end of the screw.
Temperature Measurement
There are three main types of temperature measurement: thermocouple, RTD, and infrared. Temperature is usually measured with thermocouple (TC) type temperature sensors. The principle f the TC is that when two dissimilar metals are connected and the temperature T of the junction is different from a reference junction at T0, there will be a voltage generated at the output end that is related to the temperature difference T-T0. Since the temperature measurement is determined by the exact combination of metal wires, it is important that the correct wires are used when wiring changes are made.
Advantage thermocouples:
• Wide temperature range
• Good response time
• Good for point sensing
• Inexpensive
Disadvantages thermocouples:
• Require special lead wire
• Require a reference junction
• Low signal output
• Limited stability
Another temperature sensor that is used in extrusion is the resistance temperature detector or RTD. The principle of the RTD is that the resistance of metals changes with temperature, so that by measuring resistance, the temperature can be determined. RTDs use a pure platinum resistance element to achieve high accuracy; platinum also has a linear relationship between resistance and temperature. Advantages of RTDs over TCs are higher output signal, better stability and accuracy, also, they do not require special lead wires or a reference junction. On the other hand, TCs are less expensive and are better for point sensing because the sensing element is a point.
Advantages of RTD over TC:
• Higher signal output
• Better stability and accuracy
• Good for area sensing
• No need for special lead wires
• No need for reference junction
Disadvantages of RTD:
• More expensive than thermocouples
• Not good for point sensing
A third type of temperature measurement uses infrared (IR) detectors. The IR detector is based on the fact that objects emit radiation that changes with temperature. Thus, by measuring the radiation emitted by an object, the surface temperature of the object can be determined. IR temperature probes are useful because they allow non-contact temperature measurement. For instance, the temperature distribution across an extruded sheet can be measured using an IR probe without leaving any marks on the extruded product.
There are also IR probes that are mounted in the extruder to measure melt temperature in the machine. These probes have a sapphire window and measure the radiation coming off the plastics melt. If the melt is opaque the temperature measured is the surface temperature, which is the wall temperature. If the melt is transparent, the radiation from inner layers will also be measured, so that the temperature will be stock temperature averaged over a certain distance. The advantage of IR measurement is that the response is very rapid, in the range of milliseconds. A drawback of the IR measurement is its relatively high cost.
Extrusion Compounding Lines
Compounding lines come in many shapes and sizes. Compounding can be done on single screw extruders, twin screw extruders, reciprocating single screw compounders, batch internal mixers, and continuous internal mixers. The configuration of the line will be determined by the ingredients that have to be combined in the compounding extruder. The downstream equipment typically consists of a pelletising system.
Some pelletisers cut extruded strands that are cooled in a water bath; these are called strand pelletisers. Dicers cut an extruded sheet rather than strands. The pellets from a dicer have a uniform cubic or octahedral shape. Other pelletisers cut the material right at the die exit; these are called die face pelletisers.
Compounding extruders can also be combined with direct forming systems downstream. In many cases a gear pump is placed at the discharge end of the extruder to generate the diehead pressure and to control the throughput. An example of a combination compounding/sheet extrusion line is shown here:
The plastics is introduced to the first feed port of the compounding extruder, the filler is introduced to the second feed port, and the volatiles and air entrapment are removed from the vent port. A gear pump is placed between the compounding extruder and the sheet die.
The sheet is fed to a roll stack, from there it is handled as in a normal sheet line as discussed earlier. Compounding lines will be covered in much more detail in a later session.
Profile Extrusion Lines
Many extrusion lines are used for the production of profiles. Profile lines also come in many shapes and forms. A typical extrusion line consists of an extruder, a calibrating unit, a cooling unit, a measurement device, a haul-off, and a coiler or cutter or saw.
On some profile lines a film or foil is laminated to the extruded profile. The number of profiles that are extruded is enormous; some examples of extruded profiles are shown here.
Some pelletisers cut extruded strands that are cooled in a water bath; these are called strand pelletisers. Dicers cut an extruded sheet rather than strands. The pellets from a dicer have a uniform cubic or octahedral shape. Other pelletisers cut the material right at the die exit; these are called die face pelletisers.
Compounding extruders can also be combined with direct forming systems downstream. In many cases a gear pump is placed at the discharge end of the extruder to generate the diehead pressure and to control the throughput. An example of a combination compounding/sheet extrusion line is shown here:
The plastics is introduced to the first feed port of the compounding extruder, the filler is introduced to the second feed port, and the volatiles and air entrapment are removed from the vent port. A gear pump is placed between the compounding extruder and the sheet die.
The sheet is fed to a roll stack, from there it is handled as in a normal sheet line as discussed earlier. Compounding lines will be covered in much more detail in a later session.
Profile Extrusion Lines
Many extrusion lines are used for the production of profiles. Profile lines also come in many shapes and forms. A typical extrusion line consists of an extruder, a calibrating unit, a cooling unit, a measurement device, a haul-off, and a coiler or cutter or saw.
On some profile lines a film or foil is laminated to the extruded profile. The number of profiles that are extruded is enormous; some examples of extruded profiles are shown here.
Combination of Materials
The requirements of many products, particularly in packaging applications, are such that they cannot be met by a single plastics. In order to meet the requirements often two or more materials have to be combined. There are a number of techniques to combine different materials; some of the more important ones are: co extrusion, coating, and lamination. We will discuss these in more detail.
Co Extrusion
Co Extrusion is a commonly used technique to combine two or more plastics passing through a single extrusion die. There are two major co extrusion techniques: the feed block system and the multi-manifold system. In the feed block system the different plastics are combined in the feed block module and then enter into a regular extrusion die with a single inlet, manifold, and outlet.
The advantage of the feed block system is that it is simple, inexpensive, and allows many layers to be combined. The main drawback of the feed block system is that the flow properties of the different plastics have to be quite close to avoid interface distortion. This limits the choice of materials that can be combined through feed block co extrusion.
In the multi-manifold system each plastics has its own entrance and manifold in the extrusion die. The different melt streams are combined just before the exit of the die, so that minimum interface distortion can occur.
The advantage of the multi-manifold system is that plastics with widely different flow properties can be combined. As a result, there is a wide choice of materials that can be combined through this extrusion technique. The disadvantage is that the design of the die is more complicated and, therefore, the die is more expensive.
Many multi-manifold dies are possible: flat film and sheet dies, tubing and pipe dies, blown film dies, and profile dies.
Extrusion Coating
In extrusion coating a molten layer of plastics film is combined with a moving solid web or substrate. The substrate can be paper, paperboard, foil, plastics film, or fabrics; the substrate can also be a multi-layer product.
Extrusion Lamination
Extrusion lamination involves two or more substrates, for instance paper and aluminium foil, combined by using a plastics film as the adhesive between the two substrates.
The extruded sheet or film can be laminated with a film on one side or both sides. The laminate can be paper, foil, mesh, or a number of other materials. With lamination many different structures of sheet or film products can be made. The laminate is unrolled from a payoff and combined with the film and immediately led into a set of nip rolls. After lamination the film is handled as a regular film
Blown Film Lines
A blown film line is quite different from a flat film line. In a blown film line a tubular film is extruded vertically upwards as shown here:
Air is introduced to the inside of the tube, as a result, the tube expands to a bubble with a diameter larger than the diameter of the die. The ratio of the bubble diameter and the die diameter is called the blow up ratio. Typical blow-up ratios used in LDPE film extrusion for packaging are in the range of 2.0 to 2.5:1. When the bubble has cooled sufficiently, the bubble is flattened in a collapsing frame and pulled through a set of nip rolls at the top of the collapsing frame. From there the layflat is guided over several idler rollers to the winder where the film is rolled up over a core.
One advantage of the blown film process is that it can produce not only tubular products (bags) but also flat film, simply by slitting open the tube. In some blown film processes the plastics is extruded downwards to produce films with special properties.
Co Extrusion
Co Extrusion is a commonly used technique to combine two or more plastics passing through a single extrusion die. There are two major co extrusion techniques: the feed block system and the multi-manifold system. In the feed block system the different plastics are combined in the feed block module and then enter into a regular extrusion die with a single inlet, manifold, and outlet.
The advantage of the feed block system is that it is simple, inexpensive, and allows many layers to be combined. The main drawback of the feed block system is that the flow properties of the different plastics have to be quite close to avoid interface distortion. This limits the choice of materials that can be combined through feed block co extrusion.
In the multi-manifold system each plastics has its own entrance and manifold in the extrusion die. The different melt streams are combined just before the exit of the die, so that minimum interface distortion can occur.
The advantage of the multi-manifold system is that plastics with widely different flow properties can be combined. As a result, there is a wide choice of materials that can be combined through this extrusion technique. The disadvantage is that the design of the die is more complicated and, therefore, the die is more expensive.
Many multi-manifold dies are possible: flat film and sheet dies, tubing and pipe dies, blown film dies, and profile dies.
Extrusion Coating
In extrusion coating a molten layer of plastics film is combined with a moving solid web or substrate. The substrate can be paper, paperboard, foil, plastics film, or fabrics; the substrate can also be a multi-layer product.
Extrusion Lamination
Extrusion lamination involves two or more substrates, for instance paper and aluminium foil, combined by using a plastics film as the adhesive between the two substrates.
The extruded sheet or film can be laminated with a film on one side or both sides. The laminate can be paper, foil, mesh, or a number of other materials. With lamination many different structures of sheet or film products can be made. The laminate is unrolled from a payoff and combined with the film and immediately led into a set of nip rolls. After lamination the film is handled as a regular film
Blown Film Lines
A blown film line is quite different from a flat film line. In a blown film line a tubular film is extruded vertically upwards as shown here:
Air is introduced to the inside of the tube, as a result, the tube expands to a bubble with a diameter larger than the diameter of the die. The ratio of the bubble diameter and the die diameter is called the blow up ratio. Typical blow-up ratios used in LDPE film extrusion for packaging are in the range of 2.0 to 2.5:1. When the bubble has cooled sufficiently, the bubble is flattened in a collapsing frame and pulled through a set of nip rolls at the top of the collapsing frame. From there the layflat is guided over several idler rollers to the winder where the film is rolled up over a core.
One advantage of the blown film process is that it can produce not only tubular products (bags) but also flat film, simply by slitting open the tube. In some blown film processes the plastics is extruded downwards to produce films with special properties.
COMPLETE EXTRUSION LINES
It is obvious that the extruder alone is not sufficient to make extruded product. In addition to the extruder we need upstream and downstream equipment to produce a useful product.
The main elements of an extrusion line are:
• Resin handling system
• Drying system
• Extruder
• Post-shaping or calibrating device
• Cooling device
• Take-up device
• Cutter or saw
There are many different types of extrusion lines, the main types are:
• Tubing and pipe extrusion lines
• Film and sheet extrusion lines
• Extrusion compounding lines
• Profile extrusion lines
Besides these four main types there are quite a few more, such as fibre spinning lines, extrusion blowmoulding machines, integrated sheet and thermoforming lines, etc.
Tubing and Pipe Extrusion Lines
Dies for tubing and pipe were discussed earlier already. Small diameter tubing (less than about 10 mm) is usually made with a free extrusion process; this is a process without a sizing or calibrating unit. Large diameter tubing and pipe is made with a sizing device just downstream of the die.
The purpose of the sizing unit is to solidify the plastics in the calibrating section to a thickness sufficient to transfer the stresses acting on the product, while maintaining the desired shape and dimension. The main components of a typical tubing extrusion line are shown here.
This line does not use a sizing unit and, thus, would be used for small diameter tubing. The gear pump may or may not be used depending on the precision that is required in the extrusion process. The internal air pressure of the tubing is controlled to achieve the correct values for the outside diameter and wall thickness. The diameter is often measured with a laser gage to allow close monitoring and control of the diameter. The diameter and the wall thickness are determined mostly by the extruder output, the puller speed, and the internal air pressure. Closed loop control systems are available that automatically set the appropriate values screw or gear pump speed, the puller speed, and internal air pressure. After the puller the tubing may be cut or it may be reeled up on a spool. Tubing and pipe lines will be discussed in more detail in a later session.
Film and Sheet Lines Using the Roll Stack Process
There are no major differences between the extrusion of flat film and sheet.
The main components of a sheet line are the extruder, the roll stack, the cooling section, the nip roll section, and the winder (show figure). The roll stack contains three roll that are often referred to as polishing rolls. They are used to exert pressure on the sheet and to impart the surface conditions of the rolls to the plastics sheet. If a smooth surface is required, smooth rolls will be used. If a texture surface is needed, a textured surface is used on the roll. It is possible to have one textured surface and one smooth surface by having a smooth and textured roll next to each other.
The rolls are normally cored so that the temperature of the rolls can be controlled. This is usually done with circulation hot oil. The temperature of each roll can be adjusted separately. The rolls can be in a vertical position as shown or they can be at an angle. The cooling section consists of a number of roll positioned in a frame; the sheet is over and under the roll to keep the sheet flat.
At the end of the cooling section are the pull rolls or nip rolls; these are rubber rolls that pull the sheet from the roll stack to maintain a certain tension in the sheet. After the nip rolls, the sheet is led to the winder that rolls the sheet on a core. Many different winders are available; some winders automatically transfer the sheet to a new core when one package is full. Sheet lines will be covered in more detail in a later session.
Film Lines Using Chill Roll Casting
With thin film, the film is often cast on a chill roll rather than extruded into a roll stack. The main components of a cast film line are the extruder, the film die, the chill roll unit, the thickness gauging system, the surface treatment unit, and the winder.
The film is extruded downward onto the chill roll. The initial contact between the film and the chill roll is established by the use of an air knife. The air knife produces a thin stream of high velocity air across the width of the chill roll, the air stream pushes the film against the roll surface.
From the chill roll unit the film is lead to a thickness gauging unit where the thickness of the sheet is measured across the width of the film. Most thickness gages for film and sheet have a scanning measuring head that traverses the film back and forth to measure thickness both along the length and across the width of the film.
After the thickness gauging unit the film passes through a surface treatment unit. Such a unit is incorporated if a surface treatment of the film is required. This is usually done to improve adhesion, for instance for a subsequent printing or laminating operation.
The most important adhesion promoters are:
• Flame treatment
• Corona discharge treatment
• Ozone treatment
• Primers
From the treatment unit the film is led to the winder unit. Just as with sheet extrusion, many different types of winders are available. Cast film lines will be covered in more detail in a later session.
The main elements of an extrusion line are:
• Resin handling system
• Drying system
• Extruder
• Post-shaping or calibrating device
• Cooling device
• Take-up device
• Cutter or saw
There are many different types of extrusion lines, the main types are:
• Tubing and pipe extrusion lines
• Film and sheet extrusion lines
• Extrusion compounding lines
• Profile extrusion lines
Besides these four main types there are quite a few more, such as fibre spinning lines, extrusion blowmoulding machines, integrated sheet and thermoforming lines, etc.
Tubing and Pipe Extrusion Lines
Dies for tubing and pipe were discussed earlier already. Small diameter tubing (less than about 10 mm) is usually made with a free extrusion process; this is a process without a sizing or calibrating unit. Large diameter tubing and pipe is made with a sizing device just downstream of the die.
The purpose of the sizing unit is to solidify the plastics in the calibrating section to a thickness sufficient to transfer the stresses acting on the product, while maintaining the desired shape and dimension. The main components of a typical tubing extrusion line are shown here.
This line does not use a sizing unit and, thus, would be used for small diameter tubing. The gear pump may or may not be used depending on the precision that is required in the extrusion process. The internal air pressure of the tubing is controlled to achieve the correct values for the outside diameter and wall thickness. The diameter is often measured with a laser gage to allow close monitoring and control of the diameter. The diameter and the wall thickness are determined mostly by the extruder output, the puller speed, and the internal air pressure. Closed loop control systems are available that automatically set the appropriate values screw or gear pump speed, the puller speed, and internal air pressure. After the puller the tubing may be cut or it may be reeled up on a spool. Tubing and pipe lines will be discussed in more detail in a later session.
Film and Sheet Lines Using the Roll Stack Process
There are no major differences between the extrusion of flat film and sheet.
The main components of a sheet line are the extruder, the roll stack, the cooling section, the nip roll section, and the winder (show figure). The roll stack contains three roll that are often referred to as polishing rolls. They are used to exert pressure on the sheet and to impart the surface conditions of the rolls to the plastics sheet. If a smooth surface is required, smooth rolls will be used. If a texture surface is needed, a textured surface is used on the roll. It is possible to have one textured surface and one smooth surface by having a smooth and textured roll next to each other.
The rolls are normally cored so that the temperature of the rolls can be controlled. This is usually done with circulation hot oil. The temperature of each roll can be adjusted separately. The rolls can be in a vertical position as shown or they can be at an angle. The cooling section consists of a number of roll positioned in a frame; the sheet is over and under the roll to keep the sheet flat.
At the end of the cooling section are the pull rolls or nip rolls; these are rubber rolls that pull the sheet from the roll stack to maintain a certain tension in the sheet. After the nip rolls, the sheet is led to the winder that rolls the sheet on a core. Many different winders are available; some winders automatically transfer the sheet to a new core when one package is full. Sheet lines will be covered in more detail in a later session.
Film Lines Using Chill Roll Casting
With thin film, the film is often cast on a chill roll rather than extruded into a roll stack. The main components of a cast film line are the extruder, the film die, the chill roll unit, the thickness gauging system, the surface treatment unit, and the winder.
The film is extruded downward onto the chill roll. The initial contact between the film and the chill roll is established by the use of an air knife. The air knife produces a thin stream of high velocity air across the width of the chill roll, the air stream pushes the film against the roll surface.
From the chill roll unit the film is lead to a thickness gauging unit where the thickness of the sheet is measured across the width of the film. Most thickness gages for film and sheet have a scanning measuring head that traverses the film back and forth to measure thickness both along the length and across the width of the film.
After the thickness gauging unit the film passes through a surface treatment unit. Such a unit is incorporated if a surface treatment of the film is required. This is usually done to improve adhesion, for instance for a subsequent printing or laminating operation.
The most important adhesion promoters are:
• Flame treatment
• Corona discharge treatment
• Ozone treatment
• Primers
From the treatment unit the film is led to the winder unit. Just as with sheet extrusion, many different types of winders are available. Cast film lines will be covered in more detail in a later session.
Temperature Measurement
Temperature is usually measured with thermocouple (TC) type temperature sensors. The principle of the TC is that when two dissimilar metals are connected and the temperature T of the junction is different from a reference junction at T0, there will be a voltage generated at the output end that is related to the temperature difference T-T0.
Since the temperature measurement is determined by the exact combination of metal wires, it is important that the correct wires are used when wiring changes are made.
Another temperature sensor that is used in extrusion is the resistance temperature detector or RTD. The principle of the RTD is that the resistance of metals changes with temperature, so that by measuring resistance, the temperature can be determined. RTDs use a pure platinum resistance element to achieve high accuracy; platinum also has a linear relationship between resistance and temperature. Advantages of RTDs over TCs are higher output signal, better stability and accuracy, also, they do not require special lead wires or a reference junction. On the other hand, TCs are less expensive and are better for point sensing.
A third type of temperature measurement uses infrared (IR) detectors. The IR detector is based on the fact that objects emit radiation that changes with temperature. Thus, by measuring the radiation emitted by an object, the surface temperature of the object can be determined. IR temperature probes are useful because they allow non-contact temperature measurement. For instance, the temperature distribution across an extruded sheet can be measured using an IR probe without leaving any marks on the extruded product.
IR probes are also made that are mounted in the extruder to measure melt temperature in the machine. These probes have a sapphire window and measure the radiation coming off the plastics melt. If the melt is opaque the temperature measured is the surface temperature, which is the wall temperature. If the melt is transparent, the radiation from inner layers will also be measured, so that the temperature will be stock temperature averaged over a certain distance. The advantage of IR measurement is that the response is very rapid, in the range of milliseconds. A drawback of the IR measurement is its relatively high cost.
Melt Temperature Measurement
The temperature of the plastics melt is often measured with an immersion TC [show immersion TC]. The probe protrudes into the melt and reads the temperature at the point of the TC junction. To avoid conduction errors the junction should be thermally insulated from the base of the probe. One drawback of an immersion probe is that it changes the velocities in the channel. Since the melt temperatures are determined by the velocities, the immersion probe will influence the melt temperatures. As a result, the measured temperature will be different from the melt temperature at the same point without the immersion probe. Another drawback is that dead spots may occur behind the immersion probe; this can be detrimental in plastics that are susceptible to degradation.
A number of different melt temperature probes can be used as shown here.
A flush mounted probe can be used. This measures the melt temperature at the wall, which is usually the same as the metal wall temperature. As a result, this melt temperature measurement is not the most useful. Another probe has a straight protruding design; these probes are also available with adjustable depth so that temperatures at different positions in the channel can be measured. Yet another design has a tip on the probe that points upstream. The TC junction is located in the tip. The benefit of this design is that there is minimal disturbance of flow where the temperature is measured. This probe is also available with adjustable depth. It is also possible to run a bridge across the channel with several probes attached to it. This allows simultaneous melt temperature measurement at several locations.
Barrel Temperature Measurement
The temperature of the barrel is usually measured with TC or RTD sensors pressed into the barrel; the sensors are generally spring loaded. Many temperature sensors are constructed with a metal sheath to obtain sufficient mechanical strength. As a result, significant conduction errors can occur in the measurement. The accuracy of the measurement is strongly dependent on the depth of the well, the type of sensor, and the air velocity.
The effect of the depth of the TC well is shown in this figure.
The actual temperature is 185 C. When the depth of the well is less than about 30 mm (about 1 inch) the indicated temperature is considerably below the actual temperature. When the well depth is more than 30 mm the measurement error with the insulated TC. With the non-insulated TC the indicated temperature is even further below the actual temperature; even with a well depth of 60 mm the indicated temperature is still several degrees below actual temperature.
When the air velocity increases, the indicated temperature drops as much as 10 to 15 degree C. The drop is larger with the conventional TC compared to the insulated TC. The practical result of this is that drafts around the extruder can cause substantial temperature measurement errors.
Temperature Control
In the extrusion process good temperature control is important to achieve good process stability. There are two main types of temperature control: on-off control and proportional control. In on-off control the power is either full on or completely off.
The temperature versus time for on-off control is show here [show figure]; the power vs. time is shown as well. When the measured temperature is below the set point, the power is full on. As a result, the temperature will rise. When it reaches the set point the power shuts off, however, the temperature will continue to increase for some time - this can be several minutes. When eventually the temperature drops below the set point, the power will turn on again.
After the initial increase from room temperature, the temperature will vary in a cyclic manner with a corresponding on-off cycling of the power.
The advantage of on-off control is that it is simple and the average temperature is right at the set point. The disadvantage is that the actual temperature always cycles with the temperature variation can be quite large, as much as 10-20 degrees C. The larger the extruder, the greater the temperature variation tends to get. Because of this, on-off control is not recommended in extrusion, except for very non-critical processes.
In proportional control, the power is proportional to the temperature within a certain temperature region; this region is called the proportional band. The temperature versus time for proportional control is show here [show figure]; the power vs. time is shown as well.
Initially, when the machine heats up from room temperature the power will be full on until the temperature reaches the proportional band. Within the proportional band the power reduces as the temperature increases. If there is an overshoot where the temperature exceeds the proportional band, the power will be completely off. When the temperature reduces in the proportional band, the power increases. The amplitude of the oscillations will gradually reduce and eventually the temperature will reach a steady value; the power will also reach a steady value.
The advantage of proportional control is that the temperature can be steady, as opposed to on-off control. The power level can adjust itself exactly to the level that is required to maintain the correct temperature. A limitation of simple proportional control or P-control is that the temperature can be steady only as long as the thermal conditions around the extruder are constant. When there is an upset in the thermal conditions, such as a change in ambient temperature, the actual temperature will change and the P-control will not be able to correct it. In other words, in P-control there is no reset capability.
In proportional control with integrating action, PI-control, there is reset capability. The controller integrates the difference between actual temperature and setpoint and continues to act on the process until the difference is zero. When there is an upset in the process there will be a temporary deviation from the setpoint, but eventually the actual temperature will go to the setpoint again.
Proportional controllers can also have derivative action. This means that the controller reacts to changes in the rate of temperature change. The rate of temperature change is determined by the derivative of the temperature-time curve; that is why this is called derivative action.
Proportional control with derivative action is called a PD-control and with both integrating and derivative action PID-control. PID-control is commonly used on extruders.
For a controller to work properly on an extruder, the controller has to be tuned to the characteristics of the extruder. Tuning of a PID controller involves determining the correct width of the proportional band and the time constants for integrating and derivative action. Even the best controller that is not properly tuned will give very poor control. As a result, careful attention should be paid to tuning controllers that require manual tuning. Nowadays, there are number of controllers that tune themselves automatically, so-called “self-tuning” controllers. With these controller one does not have to worry about manually tuning the controllers.
A relatively new method of control is fuzzy logic control or FLC. FLC is an artificial intelligence based technology, designed to simulate human decision making. It can be used in systems that use many variables to enhance process control. Developing a fuzzy logic application requires the generation of a knowledge base; this can be a time consuming process.
It involves identifying:
• Process variables that are important in control
• Membership functions for each variable, such as high, low, and medium
• Fuzzy rules which define the knowledge what to do about an observation, based on previous operating experience
FLC is slowly starting to be used in the plastics processing industry. It has already been applied a number of times in injection moulding, fewer applications have been reported in extrusion.
It has been shown, however, that FLC can outperform conventional PID control if the knowledge base is sufficiently developed.
Since the temperature measurement is determined by the exact combination of metal wires, it is important that the correct wires are used when wiring changes are made.
Another temperature sensor that is used in extrusion is the resistance temperature detector or RTD. The principle of the RTD is that the resistance of metals changes with temperature, so that by measuring resistance, the temperature can be determined. RTDs use a pure platinum resistance element to achieve high accuracy; platinum also has a linear relationship between resistance and temperature. Advantages of RTDs over TCs are higher output signal, better stability and accuracy, also, they do not require special lead wires or a reference junction. On the other hand, TCs are less expensive and are better for point sensing.
A third type of temperature measurement uses infrared (IR) detectors. The IR detector is based on the fact that objects emit radiation that changes with temperature. Thus, by measuring the radiation emitted by an object, the surface temperature of the object can be determined. IR temperature probes are useful because they allow non-contact temperature measurement. For instance, the temperature distribution across an extruded sheet can be measured using an IR probe without leaving any marks on the extruded product.
IR probes are also made that are mounted in the extruder to measure melt temperature in the machine. These probes have a sapphire window and measure the radiation coming off the plastics melt. If the melt is opaque the temperature measured is the surface temperature, which is the wall temperature. If the melt is transparent, the radiation from inner layers will also be measured, so that the temperature will be stock temperature averaged over a certain distance. The advantage of IR measurement is that the response is very rapid, in the range of milliseconds. A drawback of the IR measurement is its relatively high cost.
Melt Temperature Measurement
The temperature of the plastics melt is often measured with an immersion TC [show immersion TC]. The probe protrudes into the melt and reads the temperature at the point of the TC junction. To avoid conduction errors the junction should be thermally insulated from the base of the probe. One drawback of an immersion probe is that it changes the velocities in the channel. Since the melt temperatures are determined by the velocities, the immersion probe will influence the melt temperatures. As a result, the measured temperature will be different from the melt temperature at the same point without the immersion probe. Another drawback is that dead spots may occur behind the immersion probe; this can be detrimental in plastics that are susceptible to degradation.
A number of different melt temperature probes can be used as shown here.
A flush mounted probe can be used. This measures the melt temperature at the wall, which is usually the same as the metal wall temperature. As a result, this melt temperature measurement is not the most useful. Another probe has a straight protruding design; these probes are also available with adjustable depth so that temperatures at different positions in the channel can be measured. Yet another design has a tip on the probe that points upstream. The TC junction is located in the tip. The benefit of this design is that there is minimal disturbance of flow where the temperature is measured. This probe is also available with adjustable depth. It is also possible to run a bridge across the channel with several probes attached to it. This allows simultaneous melt temperature measurement at several locations.
Barrel Temperature Measurement
The temperature of the barrel is usually measured with TC or RTD sensors pressed into the barrel; the sensors are generally spring loaded. Many temperature sensors are constructed with a metal sheath to obtain sufficient mechanical strength. As a result, significant conduction errors can occur in the measurement. The accuracy of the measurement is strongly dependent on the depth of the well, the type of sensor, and the air velocity.
The effect of the depth of the TC well is shown in this figure.
The actual temperature is 185 C. When the depth of the well is less than about 30 mm (about 1 inch) the indicated temperature is considerably below the actual temperature. When the well depth is more than 30 mm the measurement error with the insulated TC. With the non-insulated TC the indicated temperature is even further below the actual temperature; even with a well depth of 60 mm the indicated temperature is still several degrees below actual temperature.
When the air velocity increases, the indicated temperature drops as much as 10 to 15 degree C. The drop is larger with the conventional TC compared to the insulated TC. The practical result of this is that drafts around the extruder can cause substantial temperature measurement errors.
Temperature Control
In the extrusion process good temperature control is important to achieve good process stability. There are two main types of temperature control: on-off control and proportional control. In on-off control the power is either full on or completely off.
The temperature versus time for on-off control is show here [show figure]; the power vs. time is shown as well. When the measured temperature is below the set point, the power is full on. As a result, the temperature will rise. When it reaches the set point the power shuts off, however, the temperature will continue to increase for some time - this can be several minutes. When eventually the temperature drops below the set point, the power will turn on again.
After the initial increase from room temperature, the temperature will vary in a cyclic manner with a corresponding on-off cycling of the power.
The advantage of on-off control is that it is simple and the average temperature is right at the set point. The disadvantage is that the actual temperature always cycles with the temperature variation can be quite large, as much as 10-20 degrees C. The larger the extruder, the greater the temperature variation tends to get. Because of this, on-off control is not recommended in extrusion, except for very non-critical processes.
In proportional control, the power is proportional to the temperature within a certain temperature region; this region is called the proportional band. The temperature versus time for proportional control is show here [show figure]; the power vs. time is shown as well.
Initially, when the machine heats up from room temperature the power will be full on until the temperature reaches the proportional band. Within the proportional band the power reduces as the temperature increases. If there is an overshoot where the temperature exceeds the proportional band, the power will be completely off. When the temperature reduces in the proportional band, the power increases. The amplitude of the oscillations will gradually reduce and eventually the temperature will reach a steady value; the power will also reach a steady value.
The advantage of proportional control is that the temperature can be steady, as opposed to on-off control. The power level can adjust itself exactly to the level that is required to maintain the correct temperature. A limitation of simple proportional control or P-control is that the temperature can be steady only as long as the thermal conditions around the extruder are constant. When there is an upset in the thermal conditions, such as a change in ambient temperature, the actual temperature will change and the P-control will not be able to correct it. In other words, in P-control there is no reset capability.
In proportional control with integrating action, PI-control, there is reset capability. The controller integrates the difference between actual temperature and setpoint and continues to act on the process until the difference is zero. When there is an upset in the process there will be a temporary deviation from the setpoint, but eventually the actual temperature will go to the setpoint again.
Proportional controllers can also have derivative action. This means that the controller reacts to changes in the rate of temperature change. The rate of temperature change is determined by the derivative of the temperature-time curve; that is why this is called derivative action.
Proportional control with derivative action is called a PD-control and with both integrating and derivative action PID-control. PID-control is commonly used on extruders.
For a controller to work properly on an extruder, the controller has to be tuned to the characteristics of the extruder. Tuning of a PID controller involves determining the correct width of the proportional band and the time constants for integrating and derivative action. Even the best controller that is not properly tuned will give very poor control. As a result, careful attention should be paid to tuning controllers that require manual tuning. Nowadays, there are number of controllers that tune themselves automatically, so-called “self-tuning” controllers. With these controller one does not have to worry about manually tuning the controllers.
A relatively new method of control is fuzzy logic control or FLC. FLC is an artificial intelligence based technology, designed to simulate human decision making. It can be used in systems that use many variables to enhance process control. Developing a fuzzy logic application requires the generation of a knowledge base; this can be a time consuming process.
It involves identifying:
• Process variables that are important in control
• Membership functions for each variable, such as high, low, and medium
• Fuzzy rules which define the knowledge what to do about an observation, based on previous operating experience
FLC is slowly starting to be used in the plastics processing industry. It has already been applied a number of times in injection moulding, fewer applications have been reported in extrusion.
It has been shown, however, that FLC can outperform conventional PID control if the knowledge base is sufficiently developed.
INSTRUMENTATION
Instrumentation is one of the most essential elements of an extruder. It is necessary to measure important process parameters to know what is going on in the extruder and to be able to control the process. Clearly, if plastics melt temperature is not measured, it is impossible to control the melt temperature. One reason that instrumentation is so important is that it is generally not possible to observe what happens inside an extruder. Without instrumentation on the extruder, we would be almost completely ignorant about the inner workings of the extruder. Instrumentation, therefore, can be considered as the “window to the process.”
When the extruder develops a problem, we are almost completely dependent on the instrumentation to determine what is happening inside the extruder. As result, good instrumentation is critically important when we trouble shoot extrusion problems.
Most Important Process Parameters
The most important process parameters are melt pressure and melt temperature. They are the best indicators of how well or how poorly an extruder functions. Process problems, in most cases, first become obvious from melt pressure and/or temperature readings. Just think what a doctor does when a patient comes into the office with a problem. Usually, the first check of the patient’s condition will be made by taking blood pressure and body temperature. These are two good indicators of the functioning of the human body. In the same fashion, melt pressure and temperature are good indicators of the functioning of an extruder.
Other important process parameters are:
• Screw speed
• Motor load
• Barrel temperatures
• Die temperatures
• Power draw of the various heaters
• Cooling rate of the various cooling units
• Vacuum level in vented extrusion
These parameters relate just to the extruder. However, there are many more process parameters for the entire extrusion line and this, of course, depends on the details of the extrusion line.
Important parameters for any extrusion line are:
• Line speed
• Dimensions of the extruded product
• Cooling rate or cooling water temperature
• Line tension
Many other factors can influence the extrusion process, such as ambient temperature, relative humidity, air currents around the extruder, plant voltage variations, etc.
Melt Pressure
Measurement of melt pressure is important for two reasons, one: process monitoring and control and two: safety. The diehead pressure in the extruder determines the output from the extruder. It is the pressure necessary to overcome the resistance of the die. When the diehead pressure changes with time, the extruder output will correspondingly and so will the dimensions of the extruded product.
As a result, when we monitor how the pressure varies with time, we can see exactly the stability or lack of stability of the extrusion process.
It is best, therefore, to plot pressure with a chart recorder or, better, to monitor the variation of pressure with a computer data acquisition system. A simple analog or digital display of pressure is much less useful.
It is also critically important to measure pressure in the extruder to prevent serious accidents that can happen when excessively high pressures are generated in the extruder.
Under some circumstances very high pressures can be generated in the extruder, causing the extruder to explode. The barrel can crack open under excessive pressure or the die may be blown from the extruder. Either situation is extremely dangerous and should be avoided if at all possible. All extruders should have an over-pressure safety device, such as a rupture disk or a shear pin in the clamp holding the die against the extruder barrel. Even with such an over-pressure safety device, the extruder should have at least one melt pressure measurement. The reason for this is that sometimes over-pressure devices do not work properly or are disabled. Pressure can build up very quickly without a warning and cause a catastrophic explosion. With a pressure measurement it is a good idea to use an automatic shutoff when the pressure reaches a critical value.
Pressure Transducers
There are a number of different pressure transducers. The most common ones in extrusion are the strain gage transducer and the piezo-electric transducer. The strain gage transducer can be either a capillary or a pushrod transducer. In these transducers there are two diaphragms, one in contact with the plastics melt and one some distance away from the hot plastics melt. There is a connection between the first and second diaphragm, a hydraulic connection in the capillary type and a pushrod in the pushrod type. A strain gage is attached to the second diaphragm to measure the deflection. This deflection can be related to the pressure at the first diaphragm.
Most capillary transducers are filled with Mercury. Since the diaphragm of the transducer is quite thin, there is a danger of rupture of the diaphragm and leakage of Mercury into the plastics and into the workplace.
Unfortunately, sometimes people using transducers are unaware of the fact that they are filled with Mercury because many transducers do not carry a label indicating that the transducer is filled with Mercury.
Another type of transducer is the pneumatic pressure transducer. It has good robustness, but poor temperature sensitivity, poor dynamic response, and average measurement error. The capillary transducer has fair robustness, fair temperature sensitivity, and fair dynamic response. The total measurement error varies from 0.5 to 3% dependent on the quality of the transducer. The pushrod is similar to the capillary transducer, except that is has poor temperature sensitivity and poor total error. The piezo-electric transducer has good robustness due to its relatively thick diaphragm, good temperature sensitivity, good dynamic response, and low measurement error.
When the extruder develops a problem, we are almost completely dependent on the instrumentation to determine what is happening inside the extruder. As result, good instrumentation is critically important when we trouble shoot extrusion problems.
Most Important Process Parameters
The most important process parameters are melt pressure and melt temperature. They are the best indicators of how well or how poorly an extruder functions. Process problems, in most cases, first become obvious from melt pressure and/or temperature readings. Just think what a doctor does when a patient comes into the office with a problem. Usually, the first check of the patient’s condition will be made by taking blood pressure and body temperature. These are two good indicators of the functioning of the human body. In the same fashion, melt pressure and temperature are good indicators of the functioning of an extruder.
Other important process parameters are:
• Screw speed
• Motor load
• Barrel temperatures
• Die temperatures
• Power draw of the various heaters
• Cooling rate of the various cooling units
• Vacuum level in vented extrusion
These parameters relate just to the extruder. However, there are many more process parameters for the entire extrusion line and this, of course, depends on the details of the extrusion line.
Important parameters for any extrusion line are:
• Line speed
• Dimensions of the extruded product
• Cooling rate or cooling water temperature
• Line tension
Many other factors can influence the extrusion process, such as ambient temperature, relative humidity, air currents around the extruder, plant voltage variations, etc.
Melt Pressure
Measurement of melt pressure is important for two reasons, one: process monitoring and control and two: safety. The diehead pressure in the extruder determines the output from the extruder. It is the pressure necessary to overcome the resistance of the die. When the diehead pressure changes with time, the extruder output will correspondingly and so will the dimensions of the extruded product.
As a result, when we monitor how the pressure varies with time, we can see exactly the stability or lack of stability of the extrusion process.
It is best, therefore, to plot pressure with a chart recorder or, better, to monitor the variation of pressure with a computer data acquisition system. A simple analog or digital display of pressure is much less useful.
It is also critically important to measure pressure in the extruder to prevent serious accidents that can happen when excessively high pressures are generated in the extruder.
Under some circumstances very high pressures can be generated in the extruder, causing the extruder to explode. The barrel can crack open under excessive pressure or the die may be blown from the extruder. Either situation is extremely dangerous and should be avoided if at all possible. All extruders should have an over-pressure safety device, such as a rupture disk or a shear pin in the clamp holding the die against the extruder barrel. Even with such an over-pressure safety device, the extruder should have at least one melt pressure measurement. The reason for this is that sometimes over-pressure devices do not work properly or are disabled. Pressure can build up very quickly without a warning and cause a catastrophic explosion. With a pressure measurement it is a good idea to use an automatic shutoff when the pressure reaches a critical value.
Pressure Transducers
There are a number of different pressure transducers. The most common ones in extrusion are the strain gage transducer and the piezo-electric transducer. The strain gage transducer can be either a capillary or a pushrod transducer. In these transducers there are two diaphragms, one in contact with the plastics melt and one some distance away from the hot plastics melt. There is a connection between the first and second diaphragm, a hydraulic connection in the capillary type and a pushrod in the pushrod type. A strain gage is attached to the second diaphragm to measure the deflection. This deflection can be related to the pressure at the first diaphragm.
Most capillary transducers are filled with Mercury. Since the diaphragm of the transducer is quite thin, there is a danger of rupture of the diaphragm and leakage of Mercury into the plastics and into the workplace.
Unfortunately, sometimes people using transducers are unaware of the fact that they are filled with Mercury because many transducers do not carry a label indicating that the transducer is filled with Mercury.
Another type of transducer is the pneumatic pressure transducer. It has good robustness, but poor temperature sensitivity, poor dynamic response, and average measurement error. The capillary transducer has fair robustness, fair temperature sensitivity, and fair dynamic response. The total measurement error varies from 0.5 to 3% dependent on the quality of the transducer. The pushrod is similar to the capillary transducer, except that is has poor temperature sensitivity and poor total error. The piezo-electric transducer has good robustness due to its relatively thick diaphragm, good temperature sensitivity, good dynamic response, and low measurement error.
Saturday, October 31, 2009
Extrusion
Extrusion Lamination
Extrusion lamination involves two or more substrates, for instance paper and aluminium foil, combined by using a plastics film as the adhesive between the two substrates.
The extruded sheet or film can be laminated with a film on one side or both sides. The laminate can be paper, foil, mesh, or a number of other materials. With lamination many different structures of sheet or film products can be made. The laminate is unrolled from a payoff and combined with the film and immediately led into a set of nip rolls. After lamination the film is handled as a regular film
1.13.8 Blown Film Lines
A blown film line is quite different from a flat film line. In a blown film line a tubular film is extruded vertically upwards.
Air is introduced to the inside of the tube, as a result, the tube expands to a bubble with a diameter larger than the diameter of the die. The ratio of the bubble diameter and the die diameter is called the blow up ratio. Typical blow-up ratios used in LDPE film extrusion for packaging are in the range of 2.0 to 2.5:1. When the bubble has cooled sufficiently, the bubble is flattened in a collapsing frame and pulled through a set of nip rolls at the top of the collapsing frame. From there the layflat is guided over several idler rollers to the winder where the film is rolled up over a core.
One advantage of the blown film process is that it can produce not only tubular products (bags) but also flat film, simply by slitting open the tube. In some blown film processes the plastics is extruded downwards to produce films with special properties.
1.13.9 Extrusion Compounding Lines
Compounding lines come in many shapes and sizes. Compounding can be done on single screw extruders, twin screw extruders, reciprocating single screw compounders, batch internal mixers, and continuous internal mixers. The configuration of the line will be determined by the ingredients that have to be combined in the compounding extruder. The downstream equipment typically consists of a pelletising system. Some pelletisers cut extruded strands that are cooled in a water bath; these are called strand pelletisers. Dicers cut an extruded sheet rather than strands. The pellets from a dicer have a uniform cubic or octahedral shape. Other pelletisers cut the material right at the die exit; these are called die face pelletisers.
Compounding extruders can also be combined with direct forming systems downstream. In many cases a gear pump is placed at the discharge end of the extruder to generate the diehead pressure and to control the throughput. An example of a combination compounding/sheet extrusion line is shown here:
The plastics is introduced to the first feed port of the compounding extruder, the filler is introduced to the second feed port, and the volatiles and air entrapment are removed from the vent port. A gear pump is placed between the compounding extruder and the sheet die.
The sheet is fed to a roll stack, from there it is handled as in a normal sheet line as discussed earlier. Compounding lines will be covered in much more detail in a later session.
1.1.13.10 Profile Extrusion Lines
Many extrusion lines are used for the production of profiles. Profile lines also come in many shapes and forms. A typical extrusion line consists of an extruder, a calibrating unit, a cooling unit, a measurement device, a haul-off, and a coiler or cutter or saw.
On some profile lines a film or foil is laminated to the extruded profile. The number of profiles that are extruded is enormous; some examples of extruded profiles are shown here.
Extrusion lamination involves two or more substrates, for instance paper and aluminium foil, combined by using a plastics film as the adhesive between the two substrates.
The extruded sheet or film can be laminated with a film on one side or both sides. The laminate can be paper, foil, mesh, or a number of other materials. With lamination many different structures of sheet or film products can be made. The laminate is unrolled from a payoff and combined with the film and immediately led into a set of nip rolls. After lamination the film is handled as a regular film
1.13.8 Blown Film Lines
A blown film line is quite different from a flat film line. In a blown film line a tubular film is extruded vertically upwards.
Air is introduced to the inside of the tube, as a result, the tube expands to a bubble with a diameter larger than the diameter of the die. The ratio of the bubble diameter and the die diameter is called the blow up ratio. Typical blow-up ratios used in LDPE film extrusion for packaging are in the range of 2.0 to 2.5:1. When the bubble has cooled sufficiently, the bubble is flattened in a collapsing frame and pulled through a set of nip rolls at the top of the collapsing frame. From there the layflat is guided over several idler rollers to the winder where the film is rolled up over a core.
One advantage of the blown film process is that it can produce not only tubular products (bags) but also flat film, simply by slitting open the tube. In some blown film processes the plastics is extruded downwards to produce films with special properties.
1.13.9 Extrusion Compounding Lines
Compounding lines come in many shapes and sizes. Compounding can be done on single screw extruders, twin screw extruders, reciprocating single screw compounders, batch internal mixers, and continuous internal mixers. The configuration of the line will be determined by the ingredients that have to be combined in the compounding extruder. The downstream equipment typically consists of a pelletising system. Some pelletisers cut extruded strands that are cooled in a water bath; these are called strand pelletisers. Dicers cut an extruded sheet rather than strands. The pellets from a dicer have a uniform cubic or octahedral shape. Other pelletisers cut the material right at the die exit; these are called die face pelletisers.
Compounding extruders can also be combined with direct forming systems downstream. In many cases a gear pump is placed at the discharge end of the extruder to generate the diehead pressure and to control the throughput. An example of a combination compounding/sheet extrusion line is shown here:
The plastics is introduced to the first feed port of the compounding extruder, the filler is introduced to the second feed port, and the volatiles and air entrapment are removed from the vent port. A gear pump is placed between the compounding extruder and the sheet die.
The sheet is fed to a roll stack, from there it is handled as in a normal sheet line as discussed earlier. Compounding lines will be covered in much more detail in a later session.
1.1.13.10 Profile Extrusion Lines
Many extrusion lines are used for the production of profiles. Profile lines also come in many shapes and forms. A typical extrusion line consists of an extruder, a calibrating unit, a cooling unit, a measurement device, a haul-off, and a coiler or cutter or saw.
On some profile lines a film or foil is laminated to the extruded profile. The number of profiles that are extruded is enormous; some examples of extruded profiles are shown here.
Complete Extrusion Lines
It is obvious that the extruder alone is not sufficient to make extruded product. In addition to the extruder we need upstream and downstream equipment to produce a useful product.
The main elements of an extrusion line are:
• Resin handling system
• Drying system
• Extruder
• Post-shaping or calibrating device
• Cooling device
• Take-up device
• Cutter or saw
There are many different types of extrusion lines, the main types are:
• Tubing and pipe extrusion lines
• Film and sheet extrusion lines
• Extrusion compounding lines
• Profile extrusion lines
Besides these four main types there are quite a few more, such as fibre spinning lines, extrusion blowmoulding machines, integrated sheet and thermoforming lines, etc.
1.13.1 Tubing and Pipe Extrusion Lines
Dies for tubing and pipe were discussed earlier already. Small diameter tubing (less than about 10 mm) is usually made with a free extrusion process; this is a process without a sizing or calibrating unit. Large diameter tubing and pipe is made with a sizing device just downstream of the die.
The purpose of the sizing unit is to solidify the plastics in the calibrating section to a thickness sufficient to transfer the stresses acting on the product, while maintaining the desired shape and dimension. The main components of a typical tubing extrusion line are shown here.
This line does not use a sizing unit and, thus, would be used for small diameter tubing. The gear pump may or may not be used depending on the precision that is required in the extrusion process. The internal air pressure of the tubing is controlled to achieve the correct values for the outside diameter and wall thickness. The diameter is often measured with a laser gage to allow close monitoring and control of the diameter. The diameter and the wall thickness are determined mostly by the extruder output, the puller speed, and the internal air pressure. Closed loop control systems are available that automatically set the appropriate values screw or gear pump speed, the puller speed, and internal air pressure. After the puller the tubing may be cut or it may be reeled up on a spool. Tubing and pipe lines will be discussed in more detail in a later session.
1.13.2 Film and Sheet Lines Using the Roll Stack Process
There are no major differences between the extrusion of flat film and sheet.
The main components of a sheet line are the extruder, the roll stack, the cooling section, the nip roll section, and the winder (show figure). The roll stack contains three roll that are often referred to as polishing rolls. They are used to exert pressure on the sheet and to impart the surface conditions of the rolls to the plastics sheet. If a smooth surface is required, smooth rolls will be used. If a texture surface is needed, a textured surface is used on the roll. It is possible to have one textured surface and one smooth surface by having a smooth and textured roll next to each other.
The rolls are normally cored so that the temperature of the rolls can be controlled. This is usually done with circulation hot oil. The temperature of each roll can be adjusted separately. The rolls can be in a vertical position as shown or they can be at an angle. The cooling section consists of a number of roll positioned in a frame; the sheet is over and under the roll to keep the sheet flat.
At the end of the cooling section are the pull rolls or nip rolls; these are rubber rolls that pull the sheet from the roll stack to maintain a certain tension in the sheet. After the nip rolls, the sheet is led to the winder that rolls the sheet on a core. Many different winders are available; some winders automatically transfer the sheet to a new core when one package is full. Sheet lines will be covered in more detail in a later session.
1.13.3 Film Lines Using Chill Roll Casting
With thin film, the film is often cast on a chill roll rather than extruded into a roll stack. The main components of a cast film line are the extruder, the film die, the chill roll unit, the thickness gauging system, the surface treatment unit, and the winder.
The film is extruded downward onto the chill roll. The initial contact between the film and the chill roll is established by the use of an air knife. The air knife produces a thin stream of high velocity air across the width of the chill roll, the air stream pushes the film against the roll surface.
From the chill roll unit the film is lead to a thickness gauging unit where the thickness of the sheet is measured across the width of the film. Most thickness gages for film and sheet have a scanning measuring head that traverses the film back and forth to measure thickness both along the length and across the width of the film.
After the thickness gauging unit the film passes through a surface treatment unit. Such a unit is incorporated if a surface treatment of the film is required. This is usually done to improve adhesion, for instance for a subsequent printing or laminating operation.
The most important adhesion promoters are:
• Flame treatment
• Corona discharge treatment
• Ozone treatment
• Primers
From the treatment unit the film is led to the winder unit. Just as with sheet extrusion, many different types of winders are available. Cast film lines will be covered in more detail in a later session.
1.13.4 Combination of Materials
The requirements of many products, particularly in packaging applications, are such that they cannot be met by a single plastics. In order to meet the requirements often two or more materials have to be combined. There are a number of techniques to combine different materials; some of the more important ones are: co extrusion, coating, and lamination. We will discuss these in more detail.
The main elements of an extrusion line are:
• Resin handling system
• Drying system
• Extruder
• Post-shaping or calibrating device
• Cooling device
• Take-up device
• Cutter or saw
There are many different types of extrusion lines, the main types are:
• Tubing and pipe extrusion lines
• Film and sheet extrusion lines
• Extrusion compounding lines
• Profile extrusion lines
Besides these four main types there are quite a few more, such as fibre spinning lines, extrusion blowmoulding machines, integrated sheet and thermoforming lines, etc.
1.13.1 Tubing and Pipe Extrusion Lines
Dies for tubing and pipe were discussed earlier already. Small diameter tubing (less than about 10 mm) is usually made with a free extrusion process; this is a process without a sizing or calibrating unit. Large diameter tubing and pipe is made with a sizing device just downstream of the die.
The purpose of the sizing unit is to solidify the plastics in the calibrating section to a thickness sufficient to transfer the stresses acting on the product, while maintaining the desired shape and dimension. The main components of a typical tubing extrusion line are shown here.
This line does not use a sizing unit and, thus, would be used for small diameter tubing. The gear pump may or may not be used depending on the precision that is required in the extrusion process. The internal air pressure of the tubing is controlled to achieve the correct values for the outside diameter and wall thickness. The diameter is often measured with a laser gage to allow close monitoring and control of the diameter. The diameter and the wall thickness are determined mostly by the extruder output, the puller speed, and the internal air pressure. Closed loop control systems are available that automatically set the appropriate values screw or gear pump speed, the puller speed, and internal air pressure. After the puller the tubing may be cut or it may be reeled up on a spool. Tubing and pipe lines will be discussed in more detail in a later session.
1.13.2 Film and Sheet Lines Using the Roll Stack Process
There are no major differences between the extrusion of flat film and sheet.
The main components of a sheet line are the extruder, the roll stack, the cooling section, the nip roll section, and the winder (show figure). The roll stack contains three roll that are often referred to as polishing rolls. They are used to exert pressure on the sheet and to impart the surface conditions of the rolls to the plastics sheet. If a smooth surface is required, smooth rolls will be used. If a texture surface is needed, a textured surface is used on the roll. It is possible to have one textured surface and one smooth surface by having a smooth and textured roll next to each other.
The rolls are normally cored so that the temperature of the rolls can be controlled. This is usually done with circulation hot oil. The temperature of each roll can be adjusted separately. The rolls can be in a vertical position as shown or they can be at an angle. The cooling section consists of a number of roll positioned in a frame; the sheet is over and under the roll to keep the sheet flat.
At the end of the cooling section are the pull rolls or nip rolls; these are rubber rolls that pull the sheet from the roll stack to maintain a certain tension in the sheet. After the nip rolls, the sheet is led to the winder that rolls the sheet on a core. Many different winders are available; some winders automatically transfer the sheet to a new core when one package is full. Sheet lines will be covered in more detail in a later session.
1.13.3 Film Lines Using Chill Roll Casting
With thin film, the film is often cast on a chill roll rather than extruded into a roll stack. The main components of a cast film line are the extruder, the film die, the chill roll unit, the thickness gauging system, the surface treatment unit, and the winder.
The film is extruded downward onto the chill roll. The initial contact between the film and the chill roll is established by the use of an air knife. The air knife produces a thin stream of high velocity air across the width of the chill roll, the air stream pushes the film against the roll surface.
From the chill roll unit the film is lead to a thickness gauging unit where the thickness of the sheet is measured across the width of the film. Most thickness gages for film and sheet have a scanning measuring head that traverses the film back and forth to measure thickness both along the length and across the width of the film.
After the thickness gauging unit the film passes through a surface treatment unit. Such a unit is incorporated if a surface treatment of the film is required. This is usually done to improve adhesion, for instance for a subsequent printing or laminating operation.
The most important adhesion promoters are:
• Flame treatment
• Corona discharge treatment
• Ozone treatment
• Primers
From the treatment unit the film is led to the winder unit. Just as with sheet extrusion, many different types of winders are available. Cast film lines will be covered in more detail in a later session.
1.13.4 Combination of Materials
The requirements of many products, particularly in packaging applications, are such that they cannot be met by a single plastics. In order to meet the requirements often two or more materials have to be combined. There are a number of techniques to combine different materials; some of the more important ones are: co extrusion, coating, and lamination. We will discuss these in more detail.
The Extrusion Die
The die is placed at the discharge end of the extruder. Its function is to shape the flowing plastics into the desired shape of the extruded product. Dies can be categorized by the shape of the product that they produce. Annular dies are used to make tubing, pipe, and wire coating. Slit dies are used to make flat film and sheet. Circular dies are used to make fibre and rod. Profile dies are used to make shapes other annular, circular or rectangular. Dies are also named by the product that they produce. So, we talk about tubing dies, flat film dies, blown film dies, etc.
The inlet channel of the die is usually designed to match the exit of the extruder. If the die entrance does not match the extruder exit, an adapter can be used between the extruder and the die. The three main elements of the die flow channel are the inlet channel, the manifold, and the land region. The flow channel of the die should be designed such that the plastics melt achieve a uniform velocity across the die exit. Many different dies can be used.
Dies can be categorized by the shape that they produce; we have tubing and pipe dies, film and sheet dies, wire coating dies, and profile die. Profiles are extruded shapes other than circular, annular, or rectangular.
The shape of the land region of the die will correspond to the shape of the extruded product. An example of an inline tube or pipe die is shown here. The material flow into the die from the extruder, then it flows around a torpedo. The torpedo is supported by spider legs that have a streamlined shape to achieve smooth flow around the support legs. From the torpedo, the plastics melt flows to the tip and die where it is shaped into an annulus, so that a tube shaped product emerges from the die.
The size and shape of the land region are not exactly the same as the extruded product, there are several reasons for this: draw down, cooling, swelling, and relaxation. This is discussed in more detail in a later session. Because of the several variables affecting the size and the shape of the extruded plastics, it is often difficult to predict how exactly the size and shape of the plastics will change as it leaves the die. As a result, it is also difficult to predict how the die flow channel should be shaped to achieve the desired shape of the extruded product. This is an important reason why die design is sometimes still largely based on experience rather than on engineering calculations.
With the advent of improved numerical techniques and commercial die flow analysis software, this situation is improving, however, die design is still often involves a trial and error process.
1.10.1 Co Extrusion Dies
Another type of die that is used in the extrusion industry is the co extrusion die. This type of die is used to make a multi-layered product in one step. There are two main [list] co extrusion systems: the feed block system and the multi-manifold system. In the feed block system the different plastics melt streams are combined in a feed block and then fed into a regular single manifold extrusion die . In the multi-manifold system the different plastics melt streams enter the die separately and each material has its own manifold. The different melt streams combines close to the die exit to make the multi-layered product. Co extrusion dies will be discussed in more detail in a later session.
The inlet channel of the die is usually designed to match the exit of the extruder. If the die entrance does not match the extruder exit, an adapter can be used between the extruder and the die. The three main elements of the die flow channel are the inlet channel, the manifold, and the land region. The flow channel of the die should be designed such that the plastics melt achieve a uniform velocity across the die exit. Many different dies can be used.
Dies can be categorized by the shape that they produce; we have tubing and pipe dies, film and sheet dies, wire coating dies, and profile die. Profiles are extruded shapes other than circular, annular, or rectangular.
The shape of the land region of the die will correspond to the shape of the extruded product. An example of an inline tube or pipe die is shown here. The material flow into the die from the extruder, then it flows around a torpedo. The torpedo is supported by spider legs that have a streamlined shape to achieve smooth flow around the support legs. From the torpedo, the plastics melt flows to the tip and die where it is shaped into an annulus, so that a tube shaped product emerges from the die.
The size and shape of the land region are not exactly the same as the extruded product, there are several reasons for this: draw down, cooling, swelling, and relaxation. This is discussed in more detail in a later session. Because of the several variables affecting the size and the shape of the extruded plastics, it is often difficult to predict how exactly the size and shape of the plastics will change as it leaves the die. As a result, it is also difficult to predict how the die flow channel should be shaped to achieve the desired shape of the extruded product. This is an important reason why die design is sometimes still largely based on experience rather than on engineering calculations.
With the advent of improved numerical techniques and commercial die flow analysis software, this situation is improving, however, die design is still often involves a trial and error process.
1.10.1 Co Extrusion Dies
Another type of die that is used in the extrusion industry is the co extrusion die. This type of die is used to make a multi-layered product in one step. There are two main [list] co extrusion systems: the feed block system and the multi-manifold system. In the feed block system the different plastics melt streams are combined in a feed block and then fed into a regular single manifold extrusion die . In the multi-manifold system the different plastics melt streams enter the die separately and each material has its own manifold. The different melt streams combines close to the die exit to make the multi-layered product. Co extrusion dies will be discussed in more detail in a later session.
WHAT IS AN EXTRUDER
To explain what an extruder is, we will define some of the related terms. First of all, to extrude is to push out. When a material is extruded it is forced through an opening; the opening is called the die. For instance, when we squeeze toothpaste from a tube, we extrude tooth paste. As the material flows through the die it acquires the shape of the die flow channel. A machine that is used to extrude a material is called an extruder. Many different materials can be extruded, for instance clays, ceramics, food, metals, and of course plastics.
The main function of an extruder is to develop sufficient pressure in the material to force the material through the die. The pressure necessary to force a material through the die depends on the geometry of the die, the flow properties of the material, and the flow rate. So, basically, an extruder is a machine capable of developing pressure. In other words, an extruder is a pump. A plastics extruder is a pump for plastics materials. This is not to be confused with a plasticating extruder; this is a machine that not only extrudes but also plasticates or melts the material. A plasticating extruder is fed with solid plastics particles and delivers a completely molten plastics to the die.
Extruders are the most common machines in the plastics processing industry. Extruders are not only used in extrusion operations, most moulding operations also use an extruder, for instance injection moulding and blow moulding. Essentially every plastics part will have gone through an extruder at one point or another; in many cases, more than once!
1.2 DIFFERENT TYPES OF EXTRUDERS
There are many different types of extruder. In the plastics industry, there are three main types: the screw extruder, which is the most common, the ram extruder, and the drum or disk extruder, which is the least common.
In a screw extruder a screw rotates in a cylinder; the rotation of the screw creates a pumping action. A screw extruder can have one screw or more than one screw.
An extruder with one screw is called a single screw extruder; it is the most common machine in the plastics processing industry. An extruder with more than one screw is called a multi-screw extruder. The most common multi-screw extruder is the twin-screw extruder; it has two screws.
There are several types of twin screw extruder. In most twin screw extruders the screws are located side by side. If both screws rotate in the same direction, the extruder is called a co-rotating twin screw extruder.
If the screws rotate in opposite direction, we call it a counter-rotating twin screw extruder. Twin screw extruders can run at high speed or at low speed, depending on the application. High speed extruders run at around 200 to 500 rpm and even higher; they are primarily used in compounding. Low speed extruders run at about 10 to 40 rpm and used mostly in profile extrusion applications.
Most twin screw extruders for profile extrusion are counter-rotating extruders. This is because counter-rotating extruders tend to have better conveying characteristics than co-rotating extruders.
Most twin screw extruders have parallel screws, but some extruders have conical screws where the screws are not parallel.
Another distinguishing feature of twin screw extruders is the extent of intermeshing of the screws. The screws can be fully intermeshing, partially. Most twin screw extruders are intermeshing. The advantage of non-intermeshing twin screw extruders is that they can be made with very long length without concern of metal-to-metal contact between the screws. The L/D ratio can be as high as 100:1 and higher. The disadvantage of non-intermeshing twin screws is that they have poor dispersive mixing capability.
Ram extruders use a reciprocating piston to force the material forward and through the die. Ram extruders have very good conveying characteristics and can develop very high pressures.
The drawback of ram extruders is that they have low melting capacity. Therefore, they are not used very often for normal plastics. There are some unusual plastics, however, that are often processed on a ram extruder. These are the so called “intractable” plastics that cannot be processed on normal extruders. Examples of such plastics are PTFE (poly-tetra-fluoro-ethylene) and ultra high molecular weight poly-ethylene. These plastics do not melt like normal plastics and are formed by sintering. Continuous products can be made on a ram extruder; the line speed is quite low though, in the range of 25 to 75 cm per hour (10 to 30 inch per hour).
The main function of an extruder is to develop sufficient pressure in the material to force the material through the die. The pressure necessary to force a material through the die depends on the geometry of the die, the flow properties of the material, and the flow rate. So, basically, an extruder is a machine capable of developing pressure. In other words, an extruder is a pump. A plastics extruder is a pump for plastics materials. This is not to be confused with a plasticating extruder; this is a machine that not only extrudes but also plasticates or melts the material. A plasticating extruder is fed with solid plastics particles and delivers a completely molten plastics to the die.
Extruders are the most common machines in the plastics processing industry. Extruders are not only used in extrusion operations, most moulding operations also use an extruder, for instance injection moulding and blow moulding. Essentially every plastics part will have gone through an extruder at one point or another; in many cases, more than once!
1.2 DIFFERENT TYPES OF EXTRUDERS
There are many different types of extruder. In the plastics industry, there are three main types: the screw extruder, which is the most common, the ram extruder, and the drum or disk extruder, which is the least common.
In a screw extruder a screw rotates in a cylinder; the rotation of the screw creates a pumping action. A screw extruder can have one screw or more than one screw.
An extruder with one screw is called a single screw extruder; it is the most common machine in the plastics processing industry. An extruder with more than one screw is called a multi-screw extruder. The most common multi-screw extruder is the twin-screw extruder; it has two screws.
There are several types of twin screw extruder. In most twin screw extruders the screws are located side by side. If both screws rotate in the same direction, the extruder is called a co-rotating twin screw extruder.
If the screws rotate in opposite direction, we call it a counter-rotating twin screw extruder. Twin screw extruders can run at high speed or at low speed, depending on the application. High speed extruders run at around 200 to 500 rpm and even higher; they are primarily used in compounding. Low speed extruders run at about 10 to 40 rpm and used mostly in profile extrusion applications.
Most twin screw extruders for profile extrusion are counter-rotating extruders. This is because counter-rotating extruders tend to have better conveying characteristics than co-rotating extruders.
Most twin screw extruders have parallel screws, but some extruders have conical screws where the screws are not parallel.
Another distinguishing feature of twin screw extruders is the extent of intermeshing of the screws. The screws can be fully intermeshing, partially. Most twin screw extruders are intermeshing. The advantage of non-intermeshing twin screw extruders is that they can be made with very long length without concern of metal-to-metal contact between the screws. The L/D ratio can be as high as 100:1 and higher. The disadvantage of non-intermeshing twin screws is that they have poor dispersive mixing capability.
Ram extruders use a reciprocating piston to force the material forward and through the die. Ram extruders have very good conveying characteristics and can develop very high pressures.
The drawback of ram extruders is that they have low melting capacity. Therefore, they are not used very often for normal plastics. There are some unusual plastics, however, that are often processed on a ram extruder. These are the so called “intractable” plastics that cannot be processed on normal extruders. Examples of such plastics are PTFE (poly-tetra-fluoro-ethylene) and ultra high molecular weight poly-ethylene. These plastics do not melt like normal plastics and are formed by sintering. Continuous products can be made on a ram extruder; the line speed is quite low though, in the range of 25 to 75 cm per hour (10 to 30 inch per hour).
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