How to avoid internal stress for PC injection molding

If want to reduce or eliminate cracking of injection molded poly-carbonate, it can take two measures:

  • Reduce internal stress;
  • Or improve the cracking resistance of plastics.

What can do to reduce the internal stress of the PC injection molding process ?

  • Mold Temperature

Increasing the mold temperature can reduce the internal stress. When the mold temperature is controlled between 80 ~ 110 degrees, the internal stress is small.

  • Barrel Temperature

Increasing the  barrel temperature can reduce the internal stress. For the PC part injection molding, the barrel temperature can be controlled between 260 ~ 330℃, and the higher the temperature is, the lower the internal stress will be.

  • Ejection Pressure

The viscosity of PC material is high, which  requires high pressure in theory, but high pressure will greatly increase internal stress, so it is inappropriate to use high pressure here. The pressure should be kept between 800 ~ 1200 kg.

  • Ejection speed

High ejection speed will increase the internal stress, and the ejection speed should not be very high in the occasion of low internal stress need. The injection speed should be 50~ ~70%, which is 45~ ~60mm/ s for normal injection molding machines.

  • Holding time

Longer holding time will increase internal stress, and the holding time should be between 2 ~ 3 seconds.

  • Melting speed

Too high melting speed will cause the molecular chain to tear excessively and reduce the molecular weight, thus reducing the strength and impact modulus, resulting in the product cracking. The linear speed of  melting should be controlled at 400 ~ 450 mm/s.

  • Annealing treatment

In order to reduce the internal stress, it is necessary to anneal. The annealing setting are 85 ~ 100℃ and the annealing time is 1 ~ 2 hours. The annealing time should be done quickly after injection molding.

  • Mold design

For the PC injection molding, the product breaks at the corner, where radii is small. it is known that stress is easy to occur where R is small
It is necessary to design with uniform thickness and large R. The external R should be between 2.5 and 3 mm, and the internal R should be between 1.5 and 2 mm.

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