Definition: The control of temperature via a measurement device so as to maintain a set variation i.e. +/- to a certain degree. Being in control.
Temperature control is a key factor within the plastics community. Whether this is material temperature, barrel temperature, mold, water bath or air temperature it is critical to the processing of material no mater which process is being used.
What is happening in a plastics manufacturer's process is a thermal process. To put it simply as a good friend states it is like making toast. We have time and temperature which are both used in conjunction to make or produce toast to the desired level of doneness. Within Plastics manufacturing we have heat going into the process and than we have to take the heat back out, but key to both is how do you control this temperature or heat so that the process is maintained and in control. We also can mention cooling the same way.
The typical process would involve the use of thermocouples. These than would be the monitoring device that creates a signal and than forwards it back to the controller. Thermocouples come in various types and configurations. Basically it is two dissimilar materials which are combined into a probe type arrangement which when heat /cold is applied create voltage and send this signal back to a sensor or reading unit. Be sure that it is always a continuous strand as a junction or joint within the wire length can create a point of resistance thus alternating the signal. Also make sure it is of the correct type, as there are various types.
The reading unit, controller, PID than receives the imputes (electrical current) data received and will than send out its signal to either increase temperature or turn down temperature (off) so as to maintain the temperature within the range of the unit. It really is a pretty simple arrangement in terms of what is happening. But the range that is set for when the unit has to increase temperature and decrease temperature is critical. What is meant is it 15 degrees or 5 degrees? The other issue is where is the thermocouple placed? That is the real question that needs to be answered. It must be remembered that location has to be known especially in a hot runner mold. Are you controlling the tips or actually the back of the drop to the tip?
The heat impute into the machine device has to be correct also. In using a heater is the wattage correct for the amount of heat that needs to be imputed. If the heater is always in the on position (drawing amperage) this may indicate that the wattage is undersized for heater band or probe.
So basically what are had are two functions, one that senses the temperature and one that will supply the temperature. On the supply side think of it as both heat and cold as in some system we may actually have cooling involved such as in a mold and or on the barrel of the machine or air temperature for a blown film line.
So what is critical is that the operator or set up person is in control of the process:
For temperature:
This means that the unit is turning on and off within its range and maintaining the set point.
A way to check the above is that the heaters or coolers on the process being controlled will cycle: turn on and off. If in fact the controller never turns on or off than the process is not in control. The process is controlling itself. For example this may be that there is a worn screw so the heats on the barrel don't come on and when checking to see what the temperature is, it appears above the temperature set point. The process is not in control. This can also occurs in the mold, with miss-placed water lines or no water lines in that the steel /mold will seek its own equilibrium. When first injecting plastic into the mold it is cool but after running a while it warms up and the parts themselves start to look better and better, yet the processor has saved and packaged all those parts. Are they all the same, NO! So when investigating breakage warpage etc. this needs to be taken into consideration. It has been stated in some publications / papers that to get a thermally stable process it can take a few hundred shots/cycles.
Within the plastics industry the following are some of the temperature concerns that should be monitored / controlled:
1- melt temperature
2- mould temperature
3- die temperature
4- water bath temperature
5- air temperature ( blown film)
6- drying temperature
7- part temperature upon exit
Not only should these be checked but checked via an independent temperature sensor so that there is something to compare the settings on the machine with.
