Common Defects and Solutions in ABS Plastic Injection Molding
ABS material balances rigidity, toughness and processing performance, widely adopted for home appliance housings, electronic components and daily consumer goods. Nevertheless, ABS is sensitive to temperature and shear force. During mass production, manufacturers frequently encounter splay, sink marks, obvious weld lines, warpage and burning defects. Proper control of forming conditions helps stabilize surface appearance and dimensional precision of ABS products.
1. Splay, Bubbles and Water Mark Optimization
ABS absorbs moisture easily. Water inside raw materials vaporizes inside high-temperature barrels and forms silvery streaks on product surfaces. Operators must dry pellets at 80–85°C for 3–4 hours, and seal dried materials to prevent re-absorbing moisture. When barrel temperature exceeds 260°C, molecular decomposition generates gas and also creates splay.
Reduce injection velocity to avoid entrapping air during filling. Improve mold venting at melt terminals and weld positions. Set up cold slug wells near gates to intercept degraded low-temperature melt. Clean accumulated carbon on screws and barrels regularly to eliminate persistent surface defects.
2. Sink Marks and Internal Voids Improvement
Sink marks usually emerge on thick walls and rib junctions. Outer layers solidify ahead of inner sections, and volume contraction creates surface depressions or hidden vacuum voids. Increase holding pressure and extend holding time to provide continuous feeding and compensate shrinkage. Moderately raise injection pressure to achieve full cavity filling. Control mold temperature to prevent rapid surface solidification.
Widen gate dimensions to prolong gate freeze time and maintain pressure transmission toward thick sections. Optimize part wall thickness and avoid abrupt thickness transitions. Install overflow wells for supplementary feeding if product modification cannot be realized. Avoid simply lifting barrel temperature, as overheating accelerates degradation and embrittlement.
3. Prominent Weld Lines and Low Joint Strength Solutions
Weld lines form when two melt fronts converge, weakening product strength and ruining appearance. Raise melt and mold temperature to improve molecular fusion quality. Slow down injection speed to prevent early cooling of melt fronts. Enhance venting at weld zones; trapped air directly leads to concave and dark weld marks.
Adjust gate positions to transfer weld lines to non-stressed and invisible surfaces. Add auxiliary gates to change melt merging angles. Appropriately increase holding pressure to compact weld interfaces. Avoid excessive injection speed, because intensive shear heat may trigger burning.
4. Product Warpage and Unstable Dimensional Tolerance
Differential shrinkage in all directions and uneven cooling create warpage for ABS molded parts. Balance cooling water flow between moving and fixed mold halves to narrow temperature difference. Extend cooling cycle to ensure full shaping before ejection. Lower injection and holding pressure to reduce residual forming stress. Fixture cooling after ejection helps slow deformation.
Arrange ejector pins evenly to prevent distortion caused by uneven ejection force. Keep mold temperature fluctuation within ±5℃ during continuous production, as unstable mold temperature causes repeated dimensional deviation.
5. Burning, Black Streaks and Yellow Discoloration Control
Long melt residence time and local over-shear cause ABS carbonization. Avoid setting barrel temperature at upper limits; control nozzle temperature strictly. Reduce unnecessary high injection speed and high pressure to cut shear heat. Prevent air trapping by clearing blocked mold vents. Reduce screw retraction distance to avoid long-term melt retention inside nozzles. Empty barrels or lower temperature during long shutdown periods.
Summary
Most ABS molding defects relate to raw material drying, temperature control, venting and residual stress. Follow the troubleshooting sequence: manage raw material pretreatment first, then optimize temperature, pressure and velocity parameters, and finally modify mold structure. Appearance defects such as splay mainly rely on sufficient drying and improved venting. Warpage and strength issues require balanced cooling and optimized holding conditions. Matching stable process parameters with reasonable mold design lowers reject rates and ensures consistent appearance, size and mechanical properties for mass production.
