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Analysis of causes of cracking of injection molded products and improvement methods

March 11, 2024

Cracking of injection molded products includes filamentous cracks, micro-cracks, top whitening, cracking on the surface of the part, and trauma risks caused by the mold sticking of the part and the runner sticking. According to the cracking time, it is divided into demoulding cracking and application cracking. Mainly due to the following reasons:


1. Processing

1) Excessive processing pressure, too fast speed, more filling, injection, and too long pressure holding time will cause excessive internal stress and cracking.

2) Adjust the mold opening speed and pressure to prevent demolding and cracking caused by rapid forced drawing of parts.

3) Increase the mold temperature appropriately to make the parts easy to demould, and lower the material temperature appropriately to prevent decomposition.

4) Prevent cracking due to lower mechanical strength due to welding marks and plastic degradation.

5) Use release agent appropriately and pay attention to frequently eliminate aerosol and other substances attached to the mold surface.

6) The residual stress of the part can be eliminated by annealing heat treatment immediately after molding to reduce the generation of cracks.


2. Mold

1) Ejection must be balanced. For example, the number of ejector pins and cross-sectional area must be sufficient, the demoulding slope must be sufficient, and the cavity surface must be smooth enough to prevent cracking due to the concentration of residual stress in ejection caused by external forces.

2) The structure of the parts should not be too thin, and arc transitions should be used as much as possible in the transition parts to avoid stress concentration caused by sharp corners and chamfers.

3) Use as few metal inserts as possible to prevent internal stress from increasing due to different shrinkage rates between the insert and the finished product.

4) Appropriate demoulding air inlets should be provided for deep-bottomed parts to prevent the formation of vacuum negative pressure.

5) The sprue is large enough to allow the gate material to be demoulded before it has time to solidify, making it easy to demould.

6) The connection between the main flow bushing and the nozzle should prevent the cold hard material from being dragged and causing the part to stick to the fixed mold.


3. Materials

1) The content of recycled materials is too high, resulting in low strength of the parts.

2) Excessive humidity causes chemical reactions between some plastics and water vapor, reducing their strength and causing ejection and cracking.

3) The material itself is not suitable for the environment in which it is being processed or is of poor quality. Contamination will cause cracking.


4. Machine

1) The plasticizing capacity of the injection molding machine must be appropriate. If it is too small, the plasticization will be insufficient and it will not be fully mixed and it will become brittle. If it is too large, it will degrade.