Plastics Molding & Manufacturing/Temperature
Temperature control[edit | edit source]
High temperature increases plastics flow rate to fill in the cavity. However, to prevent materials from degrade, the high temperature needed to be controlled.
Beside the mould temperature, the plastic feeded into hopper are needed for pre-drying. Each plastic resin has its own drying temperature and its own drying time. Also the drying is also largely depending on the ambient air humidity.
The pre-drying temperature control is important because if pre-drying temperature is too high, it will causes strength degradation and discoloration. If too low, it causes drooling nozzle and silver streak.
As seen from the picture below, the plastic polypropylene undergoes different temperature changes when it is in different parts of machine In hopper feeder, the plastic is at 20–30 °C , then in the injection machine which are at high temperature of 240–260 °C. Finally the plastics cools down to 20–80 °C.
In general :
If temperature is ↑ , the plastic will fill the cavity more faster but it will introduce flashes, shrinkage and internal stress
If temperature is ↓ , the plastic will have lesser flashes and lower internal stress but will introduce short mould and higher parting line