Cracks of Injection Molded Parts

Causes of molded parts cracks

• Residual stress caused by forming condition (satiety mould)
• Residual stress occurs when stripping
• Stress concentration caused by the shape of injection moulded parts
• Error caused by insert thermal expansion
• Materials deterioration

Associate knowledge of cracks
The residual stress within the product. Moulding conditions should be set towards to elimination of stress. That is to increase cylinder and mould temperature, and when the flow ability of the material becomes a good state, the injection pressure should not be too high.

Pre-heated metal insert can ease the residual stress during injection moulding. In the same principle, annealing after forming the insert is also a method.

When setting the moulding condition, reduce the injection pressure is the most effective.
If the ejector pins are balanced, release more force when demoulding.

Solutions
Instant: Increase cylinder temperature, reduce injection pressure and release rate.
Short-term: Improve the smooth finish of the injection mould cavity.
Long-term: Stripping slope should be sufficient, release pin should be balance distribution, the product design should not have acute angle, as well as to try to avoid thickness differences of the injection molded parts.

Differences in materials
Pay attention that the PC, PVC and other materials are tends to stick to the mould chrome layer, especially in the corners.
PC material prone to produce crack in the mould insert, it is better to add some reinforced glass fiber.

References
Due to solvent, oil and medicine adhesion, the injection molded parts may have cracks (stress cracking). A little oil may cause a lot of cracks.