Cracking is one of the most common defects in plastic products, and the main cause of this is due to stress deformation. There are mainly residual stresses, external stresses and stress deformations caused by the external environment.

(1) Cracking caused by residual stress

Residual stresses are mainly caused by the following three conditions, namely overfilling, demoulding and metal inserts. As a crack in the case of overfilling, its solution can be mainly started in the following aspects:

(1) Since the pressure loss of the gate is the smallest, if the crack is mainly generated near the gate, it can be considered to use the multi-point distribution point gate, side gate and shank gate method instead.
(2) Under the premise of ensuring that the resin does not decompose and deteriorate, appropriately increasing the resin temperature can reduce the melt viscosity, improve the fluidity, and also reduce the injection pressure to reduce the stress.
(3) In general, stress is easy to occur when the mold temperature is low, and the temperature should be increased appropriately. However, when the injection rate is high, even if the mold temperature is lower, the generation of stress can be reduced.
(4) Excessive injection and holding time will also produce stress, so it is better to shorten it appropriately or switch multiple pressure holdings.
(5) Amorphous resins, such as AS resin, ABS resin, PMMA resin, etc., are more prone to residual stress than crystalline resins such as polyethylene and polyoxymethylene, and should be noted.

When the demoulding is pushed, due to the small demoulding slope and the rough mold cavity and punch, the push-out force is too large, resulting in stress, and sometimes even whitening or cracking around the push-out rod. Just look closely at where the crack is occurring to determine the cause.

When metal parts are embedded in injection molding at the same time, 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 stress caused by the large difference in the coefficient of thermal expansion between the metal and the resin, and over time, the stress exceeds the strength of the deteriorating resin material and cracks occur.

In order to prevent the resulting cracking, general-purpose polystyrene with wall thickness and outer diameter of the embedded metal parts is basically not suitable for inserts, and the inserts have the least impact on nylon. Due to the small coefficient of thermal expansion of glass fiber reinforced resin materials, they are more suitable for inserts. In addition, preheating the metal insert before molding also has a good effect.

(2) Cracking caused by external stress

The external stress here is mainly caused by the stress concentration caused by the unreasonable design, especially at the sharp corners.

(3) Cracks caused by the external environment

Water degradation caused by chemicals, moisture absorption, and excessive use of recycled materials can deteriorate the physical properties and cause cracks.