Why do plastic products crack and do you know the reason?

Cracking, including the surface of the parts filamentary cracks, microcrack, top white, cracking and due to the parts sticky mold, runner sticky mold and caused or trauma crisis, according to the cracking time divided into demold cracking and application cracking. These are mainly caused by the following aspects.

Injection Processing aspect

Processing pressure is too large, speed is too fast, the more material filling, injection, holding pressure time is too long, which will cause excessive internal stress and cracking.

Adjust the mold opening speed and pressure to prevent the quick and strong pulling of the parts from causing the demold cracking.

Properly adjust the mold temperature to make the parts easy to release, and properly adjust the material temperature to prevent decomposition.

Prevent the cracking due to the degradation of the plastic caused by the fusion trace and the mechanical strength.

Appropriate use of mold release agent, pay attention to the frequent elimination of mold surface attached to the aerosol and other substances.

Residual stress in the molded parts can be reduced by annealing heat treatment immediately after molding to eliminate internal stress and reduce the generation of cracks.
 

Injection Mold aspect

The ejection should be balanced, such as the number and cross-sectional area of the ejector bar should be sufficient, the slope of the mold release should be sufficient, and the cavity surface should be smooth enough, to prevent cracking due to the concentration of residual stress in the ejection caused by external forces.

The structure of the parts should not be too thin, the transition part should try to use rounded transition, avoid sharp corners, chamfering caused by stress concentration.

Use metal inserts as little as possible to prevent the inserts and the shrinkage rate of the parts caused by different internal stress increases.

For deep bottom parts, appropriate air inlet channels should be set to prevent the formation of negative vacuum pressure.

The main runner should be large enough to release the mold before the sprues have time to cure so that it is easy to release the mold.

The main runner bushing and nozzle joint should prevent the cold hard material from dragging and making the part stick to the mold.
 

Injection Material 

The recycled material content is too high, resulting in low strength parts.

The humidity is too high, causing some plastics to react chemically with water vapor, reducing the strength and causing ejection cracking.

The material itself is not suitable for the environment being processed or the quality is not good, and contamination can cause cracking.
 

Injection Machine aspect

The plasticizing capacity of the injection molding machine should be appropriate, too small plasticizing is not fully mixed and become brittle, too large will degrade.