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Causes and Solutions of Shrinkage Dent Defects on Plastic Parts

2026-06-30 11:22:15 Injection Mold
Shrinkage dent refers to sunken uneven surfaces on thick plastic sections including ribs, screw posts and large flat panels. It originates from insufficient melt feeding during cooling shrinkage, affected by raw materials, mold design, molding parameters and equipment conditions.
Material-Related Causes and Improvements

Crystalline plastics like PP, PE and POM have shrinkage rates from 1.5% to 3%, far higher than amorphous ABS and PC at 0.4%–0.8%, easily forming obvious dents on thick walls. Unfinished raw material drying generates internal water vapor to destroy melt compactness, while excessive regrind reduces fluidity and blocks pressure transmission. Solutions include adding glass fiber or mineral fillers to cut shrinkage, fully drying hygroscopic materials such as PC and PA for over 4 hours, and limiting regrind proportion below 20% with uniform particle size.

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Mold Structural Defects and Optimization Plans
Uneven wall thickness is the core structural trigger; wall thickness difference over double creates severe feeding shortage at thick positions. Small gates, long runners and far gate-to-thick-wall distance cause large pressure loss during holding. Insufficient exhaust traps air to block melt filling. Optimizations include unifying wall thickness and hollowing thick posts and ribs, widening gates and runners, placing gates near maximum-thickness areas, and adding 0.01–0.02mm exhaust slots at thick-wall corresponding mold positions. Hot runner systems can also sustain continuous melt supply to supplement shrinkage.
Improper Molding Parameters and Adjustment Methods

Low injection and holding pressure, short holding time and fast filling speed all lead to early gate solidification without adequate feeding. Low barrel and mold temperature speed up cooling, while insufficient backpressure causes loose melt density. Adjustment steps: raise holding pressure to 60%–80% of injection pressure and extend holding time by 3–8 seconds for thick-wall goods; slow initial filling speed to delay surface skin formation; moderately lift barrel and mold temperature, and set backpressure between 0.6 and 1.2MPa for full plasticization.

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Equipment and On-Site Management Countermeasures
Insufficient clamping force causes slight mold expansion under high holding pressure, while inadequate screw buffer lacks spare melt for shrinkage compensation. Unreasonable cooling circuits lead to uneven local cooling, and over-sprayed release agent creates isolation layers between melt and mold. Solutions: increase clamping tonnage, reserve 5–10mm screw buffer stroke, add spiral cooling inserts at thick ribs and posts, and reduce release agent dosage only to sticky positions.
On-Site Troubleshooting Logic
Adjust holding pressure and time first for low-cost quick improvements; if ineffective, inspect raw material drying and regrind ratio; then optimize mold gate, exhaust and cooling structures; finally verify equipment clamping force and screw storage volume. Minor dents can be eliminated via parameter adjustment, while deep large-area shrinkage requires product thickness reduction or mold gate modification for fundamental improvement.

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