What are the causes of cracking of injection molded products?

Plastic injection molding products cracking, including the surface of the parts filamentary cracks, micro-crack, top white, cracking and due to the parts sticky mold, runner sticky mold or trauma crisis, according to the cracking time is divided into demold cracking and application cracking. These are mainly caused by the following aspects.

1. Processing aspects

(1) processing pressure is too large, speed is too fast, the more material filling, injection, holding pressure time is too long, will cause excessive internal stress and cracking.

(2) Adjust the mold opening speed and pressure to prevent fast and strong pulling parts from cracking.

(3) Appropriately adjust the mold temperature to make the parts easy to release, and appropriately adjust the material temperature to prevent decomposition.

(4) Prevent cracking due to fusion marks and low mechanical strength caused by plastic degradation.

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

(6) Residual stress in the part can be reduced by annealing heat treatment immediately after molding to eliminate internal stress and reduce the generation of cracks.

2. Mold

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

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

(3)Use metal inserts as little as possible to prevent different shrinkage rates of the inserts and parts caused by increased internal stress.

(4) The deep bottom parts should be set up with an appropriate air intake orifice to prevent the formation of negative vacuum pressure.

(5) The main channel is large enough to release the mold when the gate material has not had time to cure so that it is easy to release the mold.

(6) Mainstream channel bushing and nozzle joint should prevent the dragging of cold hard material and make the parts stick to the fixed mold.

3. Material

(1)The content of recycled material is too high, causing the strength of the parts to be too low.

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

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

4. Machine aspect

Injection molding machine plasticizing capacity should be appropriate, too small plasticization is not fully mixed and become brittle, too large will degrade.

5. Process

Process selection is not appropriate, such as too high material temperature, back pressure is too large, melt too fast, etc., resulting in lower physical properties.