Cracking is a common defect of plastic products, and the main reason is due to stress deformation. There are mainly residual stresses, external stresses, and stress deformations caused by the external environment.
(1) Crack caused by residual stress
Residual stress is mainly caused by the following three conditions, namely, overfilling, demolding, and metal inserts. As a crack generated in the case of excessive filling, the solution can be mainly started in the following aspects:
1) Since the pressure loss of the sprue is the smallest, if the crack is mainly generated near the sprue, it may be considered to use a multi-point distribution point gate, a side gate and a shank gate.
2) Under the premise of ensuring that the resin does not decompose or deteriorate, the proper increase of the resin temperature can lower the melt viscosity and improve the fluidity, and at the same time, the injection pressure can be lowered to reduce the stress.
3) Under normal circumstances, stress is easy to occur when the mold temperature is low, and the temperature should be appropriately increased. However, when the injection speed is high, even if the mold temperature is lower, the stress can be reduced.
4) If the injection and holding time are too long, stress will also be generated, and the effect of appropriately shortening or performing Th holding pressure switching is better.
5) Non-crystalline resins, such as AS resin, ABS resin, PMMA resin and other relatively crystalline resins such as polyethylene, polyoxymethylene, etc., tend to cause residual stress, which should be noted.
(2) Crack caused by external stress
The external stress here is mainly due to the unreasonable design of the stress concentration, especially at the sharp corners.
(3) Cracks caused by the external environment
Chemicals, water degradation due to moisture absorption, and excessive use of recycled materials can degrade physical properties and cause cracks.