Characteristics of transparent plastics:
PMMA has high viscosity and poor fluidity, so PMMA injection molding must be carried out with high material temperature and high injection pressure. The influence of injection temperature is greater than that of injection pressure, but the increase of injection pressure is conducive to improving the shrinkage rate of products.
The injection temperature range is wide, the melting temperature is 160℃, and the decomposition temperature is 270℃, so the material temperature adjustment range is wide and the manufacturability is good. Therefore, the improvement of fluidity can be started from the injection temperature. Poor impact, poor wear resistance, easy to scratch, easy to crack, so the mold temperature should be increased and the condensation process should be improved to overcome these defects.
PC has high viscosity, high melting temperature and poor fluidity. Therefore, Poly-carbonate injection molding must be carried out at a higher temperature (270-320℃). Relatively speaking, the range of material temperature adjustment is narrow and the manufacturability is not as good as PMMA. Injection pressure has little effect on fluidity, but due to high viscosity, the injection pressure should still be high. Accordingly, in order to prevent internal stress, the holding time should be as short as possible.
The shrinkage rate is large and the size is stable, but the product has large internal stress and is easy to crack, so it is appropriate to improve the fluidity by increasing the temperature instead of the pressure, and reduce the possibility of cracking by increasing the mold temperature, improving the mold structure and post-treatment. When the injection speed is low, the gate is prone to defects such as ripples, the temperature of the radiation nozzle should be controlled separately, the mold temperature should be high, and the resistance of the runner and gate should be small.
PET has high molding temperature and narrow adjustment range of material temperature (260-300℃), but after melting, it has good fluidity, so its manufacturability is poor, and anti-flow device is often added in the nozzle. The mechanical strength and performance are not high after injection, and the performance can be improved only through stretching process and modification.
Accurate control of mold temperature is to prevent warpage. As an important factor of deformation, hot runner mold is recommended. The mold temperature should be high, otherwise it will cause poor surface gloss and difficulty in demoulding.