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.
