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How to Solve Trapped Gas Problems in Thick-Walled Injection Molded Parts

2026-05-13 10:41:20 Injection Mold
Thick-walled injection molded parts are prone to trapped gas due to slow melt flow, restricted cavity vent paths, and air trapped in thick-wall dead zones. This issue leads to surface burns, bubbles, sink marks, dimensional instability, and even reduced mechanical strength. Solving trapped gas requires systematic improvements across mold design, process parameters, and product structure to ensure complete air evacuation during filling.
1. Mold Design Modifications

Optimize venting systems: add vent grooves (0.02–0.03mm deep) at melt flow ends, thick-wall intersections, and rib roots to release trapped air. Use vent pins or inserts at deep dead zones for targeted air evacuation. Adjust gate locations to direct melt flow away from thick-wall dead zones, ensuring uniform filling without air entrapment. Modify cooling circuits to reduce melt viscosity and improve flow, allowing air to escape more easily.

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2. Process Parameter Adjustments

Reduce injection speed to minimize air entrapment during filling, especially in thick-wall sections. Increase mold and melt temperatures to improve melt flowability, reducing the likelihood of air being trapped in high-viscosity regions. Apply multi-stage injection: slow initial fill to displace air, followed by controlled fast fill, then holding pressure to compress residual air. Increase back pressure to improve melt homogeneity and reduce air entrapment from resin moisture or additives.
3. Product Structure Optimization

Minimize thickness variations and eliminate sudden transitions that create enclosed air pockets. Add draft angles to thick-wall sections to facilitate air movement toward vents. Integrate flow leaders or rib modifications to direct melt flow toward vents, preventing stagnation in dead zones. For extremely thick sections, consider hollow structures or material reductions to reduce the volume of trapped air.

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4. Material and Pre-Processing Improvements

Dry hygroscopic materials thoroughly to eliminate moisture-related gas formation. Use low-viscosity grades or add lubricants to improve melt flow and air release. Avoid excessive use of additives that generate gas during molding, or switch to low-gas alternatives. Purge the barrel regularly to remove degraded material that causes gas buildup.

Defect PhenomenonCauses of Trapped GasMold Optimization MeasuresProcess Adjustment Scheme
Surface Burning & Sink MarkPoor venting at thick wall junction; rapid melt filling compresses airAdd 0.02~0.03mm vent grooves at melt terminal positionReduce injection speed; properly increase mold temperature
Internal Bubble & VoidMoisture in raw material; enclosed dead zone inside thick wallAdopt insert venting and vent pin structureStrengthen material drying; raise back pressure
Weld Line & Weak StrengthAir accumulation at melt convergence positionOptimize gate position to change melt flow directionMulti-stage slow injection; extend holding time
Deformation & Dimensional DeviationUneven gas escape leads to inconsistent cooling shrinkageOptimize cooling circuit layout for uniform temperatureStabilize injection pressure and cooling time

5. Post-Mold Inspection and Validation

Inspect parts for surface discoloration, bubbles, or voids to identify trapped gas locations. Use mold flow analysis to simulate melt flow and air evacuation, optimizing vent and gate locations before production. Test modified parameters with short shots to observe filling patterns and air entrapment, refining adjustments until issues are resolved.
Addressing trapped gas in thick-walled parts requires coordinated changes to mold design, process parameters, product structure, and material handling. By systematically improving venting, flowability, and air evacuation, manufacturers can eliminate defects, improve part quality, and ensure consistent production of thick-walled components.

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