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.
