Cracking is a common defect of plastic products. The main reason is due to stress deformation. It mainly includes residual stress, external stress and stress deformation caused by external environment.

1. Cracking caused by residual stress
The residual stress is mainly caused by the following three conditions, namely, overfilling, demoulding and metal inlay. As the crack caused by overfilling, the solution can be mainly in the following aspects:
(1) Since the pressure loss of the straight gate is the smallest, if the crack is mainly generated near the straight gate, the multi-point distribution gate, side gate and handle gate can be considered.
(2) On the premise of ensuring that the resin does not decompose and deteriorate, properly increasing the resin temperature can reduce the melt viscosity, improve the fluidity, and also reduce the injection pressure to reduce the stress.
(3) Generally, when the mold temperature is low, it is easy to produce stress, and the temperature should be appropriately increased. However, when the injection speed is higher, even if the mold temperature is lower, the stress generation can be reduced.
(4) Too long injection and pressure holding time will also produce stress, so it is better to shorten it properly or carry out Th pressure holding switching.
(5) Non-crystallic resins, such as AS resin, ABS resin, PMMA resin, etc., are more likely to produce residual stress than crystalline resins, such as polyethylene and polyformaldehyde, which should be noted.

During demoulding and ejection, due to the small demoulding slope, rough mold glue and punch, the ejection force is too large, resulting in stress, and sometimes even whitening or cracking around the ejection rod. The cause can be determined by carefully observing the location of cracking. When inserting metal parts during injection molding, stress is most likely to occur, and it is easy to crack after a period of time, which is extremely harmful. This is mainly due to the great difference in the thermal expansion coefficient of metal and resin, which causes stress, and over time, the stress exceeds the strength of the gradually deteriorated resin material and causes cracks. In order to prevent the resulting cracking, as an experience, the universal polystyrene with the wall thickness of 7 "and the outer diameter of the embedded metal parts is basically not suitable for adding inserts, and the insert has the smallest impact on nylon. Because the thermal expansion coefficient of glass fiber reinforced resin material is small, it is suitable for the embedded parts. In addition, preheating the metal inserts before molding also has a good effect.

2.Cracking caused by external stress
The external stress here is mainly caused by stress concentration due to unreasonable design, especially at sharp corners.

3. Cracking caused by external environment
Water degradation caused by chemicals, moisture absorption, and excessive use of recycled materials will lead to deterioration of physical properties and cracking.