Automatic Measurement and Control of Melt Temperature in Injection Moulding Machines

Abstract

Temperature measurement in the nozzle of an injection moulding machine with a simple thermocouple probe is not accurate due to heat losses along the probe. A thermal guard ring temperature system has been employed to eliminate this heat loss and provide an accurate temperature signal. This signal is used to automatically control the heat input into the polymer to provide the desired melt temperature. Accurate melt temperature measurement of thermoplastics in injection moulding machines is of the utmost importance. Accurate automatic monitoring of temperatures makes possible the optimisation of operating temperatures for commercial production machines, and greatly simplifies the setting up and changing over procedures. In addition, in experimental work, the study of the effects of melt temperature and other machine parameters on product quality (dimensions, surface finish etc.) becomes a possibility. A step further ahead, continuous measurement makes feasible the automatic control of melt temperature in an injection machine, as against the indirect and therefore only approximate temperature control system based on individual zone heater controllers, generally used at present. To our knowledge, no method has been developed and used successfully in practice to continuously measure melt temperature on injection moulding machines during production runs. Because of the mechanical robustness required to withstand high pressures, simple thermocouple probes do not give accurate results due to heat losses along their body. Several types of probe have been designed in the past attempting to reduce this heat loss, but without great success. At Sira Institute during an investigation aimed at improved control of injection moulding machines, a new probe has been developed. A thermal guard ring (TGR) control system is used to isolate the melt thermocouple from the outside environment.