How to Improve White Marks on Injection Molded Parts

White marks (ejector white marks, stress whitening) are common appearance defects in injection molding, especially on ribs, bosses, deep cavities and thin walls. The root cause is excessive local stress during ejection, which stretches the material and causes whitening. Improvements can be made from part design, mold structure, process parameters, materials and production management.

Part Structure Optimization

Stress concentration areas such as ribs, bosses and corners must use rounded transitions with a minimum radius of 0.5 mm. The ratio of rib height to wall thickness should be controlled within 3:1. Wall thickness should be as uniform as possible to avoid uneven cooling.Sufficient draft angle must be applied: 1°–3° for general parts, 2°–5° for deep cavities and brittle materials. Uniform wall thickness and proper structure reduce ejection resistance and prevent stress whitening.

Mold Structure Improvement

The ejection system should be optimized to distribute force evenly. Increase the number of ejector pins or use blade ejectors, ejector blocks or stripper plates to enlarge contact area. Ejector pins should avoid sharp corners, thin ribs and abrupt thickness changes.The cavity, core, ribs and bosses should be polished to Ra ≤ 0.2 μm to reduce friction. Cooling channels should be arranged evenly, especially near thick sections and bosses, to ensure uniform cooling. Proper venting prevents vacuum adhesion and excessive mold sticking. For deep-cavity parts, stripper plates or sleeve ejectors are recommended for balanced ejection.

Injection Process Adjustment

Cooling time should be extended to ensure the part is fully hardened before ejection. Lower mold temperature accelerates surface hardening and improves rigidity during ejection.Injection speed and pressure can be moderately reduced to lower internal stress. Over-packing should be avoided to prevent excessive mold sticking. Multi-stage packing is helpful, with lower pressure in the final stage. Melt temperature should be set properly to ensure flow without over-softening the part. Ejection speed should be stable and moderate to avoid impact stress.

Material Selection and Treatment

Materials with better toughness are less likely to show white marks. Toughened grades or alloy materials can be used for high-appearance parts. Raw materials must be properly dried to prevent brittleness caused by moisture.For slight whitening, low-temperature baking can help relieve internal stress, but it is only a temporary remedy. The fundamental solution lies in mold and process optimization.

Production Control and Quick Troubleshooting

Regular mold maintenance includes checking ejector pins, cleaning cavities, maintaining smooth cooling channels and ensuring stable movement. Standardized process settings help maintain consistent quality.Single-point white marks are usually caused by undersized ejector pins or poor positioning. White marks on ribs or bosses often result from insufficient cooling, lack of radii or insufficient draft. Large-area whitening is generally caused by high wrapping force, insufficient cooling or unbalanced ejection. Through systematic optimization, most white mark defects can be effectively eliminated.