How to solve the problem of shrinkage deformation in injection-molded plastic boxes

Shrinkage deformation of injection-molded plastic boxes is a key defect affecting product qualification rates. To address it, targeted solutions must be developed based on its root causes.

I. Four Core Causes

Material Differences: Crystalline plastics (PE, PP) have a shrinkage rate of 1.5%-3.0%, while amorphous plastics (PS, PC) only have 0.5%-1.0%. High-shrinkage materials are prone to causing shrinkage. Poor material fluidity leads to "material shortage shrinkage" in thick-walled or complex structures.

Mold Defects: Improper gate size (insufficient material supply for too-small gates, uneven cooling for too-large gates); cooling channel spacing from the cavity deviates from the 10-20mm standard, resulting in uneven cooling rates and inducing shrinkage.

Process Deviations: Excessively high injection temperature prolongs cooling time, while excessively low temperature affects filling; improper injection pressure easily causes material shortage or internal stress; holding pressure < 60% of injection pressure or holding time < 10 seconds fails to compensate for shrinkage.

Structural Issues: Large wall thickness differences (thicker walls have significantly higher shrinkage); structures like slender cantilevers and thin ribs are prone to shrinkage due to uneven melt flow or cooling.

II. Targeted Solutions

Material Optimization: Prioritize low-shrinkage materials (e.g., PC/ABS for electronic boxes); add 30% glass fiber to enhance strength, reducing shrinkage rate by 30%-40%. Hygroscopic materials (e.g., PA) need drying at 80-120℃ for 4-6 hours to control moisture ≤ 0.2% and avoid affecting fluidity.

Mold Improvement: Optimize gates via CAE mold flow analysis (multi-point gates for large boxes, appropriately enlarged gates for thin-walled boxes); strictly control cooling channel spacing at 10-20mm, increase pipe density in thick-walled areas, and regularly clean scale to ensure uniform cooling.

Process Adjustment: Set temperatures based on material properties (180-220℃ for PP injection, 40-80℃ mold temperature for crystalline plastics); set holding pressure to 60%-80% of injection pressure, extend holding time to 30 seconds for thick-walled products; cooling time is 20-60 seconds (extended for thick-walled products). Adopt multi-stage injection ("fast filling in early stage, slow holding in later stage") to reduce internal stress.

Structural Optimization: Ensure wall thickness difference ≤ 2mm in design, with gradual transition for thick-walled areas; ribs follow the principle of "thickness = 0.5-0.7x main wall thickness, height ≤ 3x main wall thickness, spacing = 2-3x rib height" and match with fillet transitions to reduce stress concentration.

III. Conclusion

Resolving shrinkage deformation requires comprehensive optimization from four dimensions: material, mold, process, and structure. Verifying and adjusting solutions through small-batch trial production can effectively reduce shrinkage rate and improve product quality and production efficiency.