Analysis of injection molding problems - The reason for the crack in injection mold products
Jan 22 , 2021

Cracking includes filament cracks, microcracks, whitening, cracking, and damage caused by adhesion of parts and flow paths on the surface of parts.

The main reasons are as follows:

1 Processing:

1) If the processing pressure is too high, the speed is too fast, the filling is too much, the injection and holding time is too much, all of these will lead to excessive internal stress and fracture.

2) Adjust the speed and pressure of mold opening to prevent mold release cracks caused by rapid drawing of the workpiece.

3) Properly increase the mold temperature to make the parts easy to demold, and appropriately reduce the material temperature to prevent decomposition.

4) Prevent the cracking caused by the weld line, and the degradation of the plastic will reduce the mechanical strength.

5) Appropriate use of mold release agents, attention should be paid to eliminate aerosols and other substances attached to the mold surface.

6) By performing annealing heat treatment immediately after forming, the residual stress and internal stress of the workpiece can be eliminated, thereby reducing the occurrence of cracks.

2 Mould aspect:

1) The ejection should be balanced. For example, the number and cross-sectional area of ejector pins should be sufficient, the draft angle should be sufficient, and the surface of the mold cavity should be smooth enough to prevent cracking due to residual stress concentration caused by external forces.

2) The structure of the workpiece should not be too thin, and the transition part should use 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 internal stress from increasing due to the difference in shrinkage between inserts and parts.

4) For components with deep bottoms, appropriate stripping inlet channels should be provided to prevent vacuum negative pressure.

5) The sprue is large enough to demold the gate material when it is not cured, so it is easy to demold.

6) The sprue bushing and nozzle joints should prevent the cooling material from being dragged to make the workpiece stick to the fixed mold.

3 Material:

1) The content of recycled materials is too high, the strength of the part is too low because of this.

2) Too much humidity causes some plastics to react with water vapor, reduce strength and cause cracking.

3) The material itself is not suitable for the processing environment or the environment of poor quality. If it is contaminated, it will cause rupture.

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