Hints on Mold Design for Plastics with High Cavity Surface Temperatures

There are more and more engineering plastics and super engineering plastics for which the cavity surface temperatures exceeds 100°C.
When the surface temperature of the cavity exceeds around 90°C, normally, raising the temperature becomes difficult when the temperature control is done using water (hot water).
Generally, the following methods are used.

(1) Oil temperature control

In temperature control using oil, the temperature is maintained constant by re-circulating oil from a circulating pump by passing it through joints and hoses and a flow path provided in the mold plate or cavity. Once the temperature rises up to the preset temperature, it is possible to maintain a relatively stable temperature.
However, there is the drawback that raising the temperature takes time.
In addition, there is the danger of burns in handling the oil, and also the after-treatment of oil is cumbersome.

（2）Electric heater control

In temperature control using an electric heater (cartridge heater), the temperature is maintained constant by using the heater in combination with a temperature sensor (thermo couple, etc.). Since the thermal capacity is high, there is the advantage that raising the temperature is fast.
However, the temperature in the vicinity of the heater is high and the temperature becomes low at locations distant from the heater, and it is difficult to maintain a uniform temperature distribution.
In addition, the life of the heater is limited and it needs to be replaced periodically.
The mounting holes and the clearance are very important in installing a heater. If the clearance is large, the heater gets heated excessively and hence its life gets reduced. The Misumi M-HTM3021 (Mold temperature controller for cartridge heater) can control the heater temperature using the PID control method, and extremely stable temperature control becomes possible more than an ON-OFF control type controller,
In order to carry out temperature control more accurately, it is recommended to install a thermal insulating plate between the platen of the injection molding machine and the installation plate of the mold. Or else, it is also possible to install thermal insulating plates in the periphery of the mold plate.

Table: Examples of molding materials with high cavity surface temperatures

Name of plastic Cavity surface temperature (°C) PPS with 30% glass fiber (Polyphenylene sulfide) 130 to 150 LCP with 40% glass (Liquid crystal polymer) 70 to 110 PET (Polyethylene terephthalate) 130 to 150 PA46 (Polyimide / Nylon-46) 80 to 120 PC (Polycarbonate) 80 to 120