Cracking is one of the most common problems in plastic products, the main factor being due to stress deformation. There are residual stresses, external stresses, and stress deformations caused by the external environment.
(-) crack caused by residual stress
Residual stress is mainly caused by the following three conditions, namely, overfilling, demolding, and metal inserts. If the filling is excessive and the plastic product is cracked, it can be solved from these directions:
(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 gate, a side gate and a handle gate.
(2) In the case where the resin is not decomposed or deteriorated, appropriately increasing 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 likely 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 maintaining pressure switching is better.
(5) Non-crystalline resins, such as AS resin, ABS resin, PMMA resin, etc., which are more likely to cause residual stress than crystalline resins such as polyethylene and polyoxymethylene, should be noted.
(2) Cracks 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.