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.
