Inward sinking, or dents, in injection molded parts result from local shrinkage that occurs after the gate is sealed or due to insufficient material injection. This is a common issue in the surface of injection molded products.
Causes of Dents:
- Localized Shrinkage: Dents usually occur when increased wall thickness leads to a higher localized shrinkage rate, often near external corners or abrupt changes in wall thickness (e.g., behind protrusions, ribs, or supports).
- Thermal Expansion: The primary cause is the thermal expansion and contraction of materials. Thermoplastics have high thermal expansion coefficients, leading to significant changes during cooling.
- Cooling Dynamics: Cooling rates vary across the part. Corners cool fastest and solidify earlier, while thicker central sections cool more slowly, causing further shrinkage and resulting in dents.
- Residual Stress: Residual stresses can reduce impact strength and temperature resistance, contributing to warping if shrinkage rates differ significantly across the part.
Factors Influencing Shrinkage:
- Material Properties: Different materials have varying thermal expansion coefficients. Semi-crystalline plastics have higher shrinkage rates, while amorphous materials exhibit lower shrinkage, reducing dent formation.
- Molding Conditions: Key factors include the maximum and minimum temperature ranges, holding pressure, dimensions, shape, and cooling speed/uniformity.
Solutions to Prevent Dents:
- Adjust Process Conditions: Modify parameters during the holding pressure phase, such as injecting additional material to compensate for shrinkage.
- Design Considerations: Avoid thick-walled components when possible. If thick walls are unavoidable, consider hollow designs with smooth transitions and large radii to minimize dents at corners.
By addressing these factors, the occurrence of inward sinking in injection molded parts can be significantly reduced.