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.
