Reasons For Cracking Of Injection Molded Products

Cracking, including filament cracks on the surface of the part, microcracking, whitening, cracking, and damage due to the sticking of the part, the sticking of the flow path, or the traumatic failure, splitting and cracking according to the cracking time. There are mainly the following reasons.

1. Processing

(1) If the processing pressure is too large, the speed is too fast, the more filling, the injection and the holding time are too long, the internal stress will be too large and crack.

(2) Adjusting the mold opening speed and pressure to prevent demoulding cracking caused by rapid drawing of the workpiece.

(3) Appropriately increase the mold temperature, make the parts easy to demould, and appropriately reduce the material temperature to prevent decomposition.

(4) Prevention of cracking due to weld line degradation due to plastic degradation caused by plastic degradation.

(5) Appropriate use of the release agent, and attention should be paid to the elimination of substances such as aerosols adhering to the mold surface.

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

2. Materials

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

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

(3) The material itself is not suitable for the environment or quality that is being processed, and the pollution will cause cracking.

3. Mold aspect

(1) The balance should be balanced. For example, if the number of ejector pins and the cross-sectional area are sufficient, the draft angle should be sufficient, and the cavity surface should be sufficiently smooth to prevent cracking due to external force causing residual stress concentration.

(2) The structure of the workpiece should not be too thin, and the transition part should adopt the arc transition as much as possible to avoid stress concentration caused by sharp corners and chamfers.

(3) Minimize the use of metal inserts to prevent the internal stress from increasing due to the difference in shrinkage between the insert and the part.

(4) Appropriate stripping intake passages shall be provided for deep-bottomed parts to prevent vacuum underpressure.

(5) The main channel is large enough to release the gate material in the future when it is cured, so that it is easy to demould.

(6) The main channel bushing and the nozzle joint should prevent the chilled material from being dragged to make the workpiece stick to the fixed mold.

4. Machine side

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

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