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.
Saturday, October 31, 2009
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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