Practical Solutions for Surface Defect Treatment of Injection Molded Parts

Common surface defects of injection products include sink mark, bubble, silver streak, fusion line, flash and pit, induced by raw material, mold, molding process and field operation. Practical improvement measures covering emergency rework and process prevention reduce scrap loss based on mass production experience and classified defect analysis.

1. Improvement Measures for Sink Mark and Shrinkage Dent

Shrinkage dent concentrates on uneven wall thickness, back of rib and outer wall of boss. Temporary solution is raising corresponding packing pressure and extending packing time by 0.3~1 second to fill shrinkage gap via continuous melt feeding; meanwhile barrel temperature is lowered by 5~10℃ to reduce cooling shrinkage rate of polymer. Long-term permanent solution is optimizing product wall thickness with thickness ratio controlled within 1.5 times; local enlarged gate or submarine gate is added on mold opposite to concave position to shorten feeding distance; cooling channel is arranged near thick-wall area to accelerate surface solidification and restrain inward collapse of outer layer. Minor dent can be repaired by hot melt filling and polishing while deep penetrating dent is unrepairable requiring adjustment of mold and process parameters.

2. Elimination Plan for Bubble and Internal Cavity

Superficial tiny bubble originates from wet raw material and trapped cavity air: drying duration is extended immediately and moisture content of PC and PMMA is controlled below 0.02%; vent groove depth is increased to 0.01mm at bubble gathering area with breathable steel embedded at rib and blind corner to discharge sealed trapped gas. Internal hollow cavity is caused by insufficient packing or excessive injection speed: mid-to-late stage injection velocity is reduced segmentally and secondary packing pressure is lifted. Shallow surface bubble can be removed via grinding and polishing while deep internal hollow belongs to structural defect impossible for post-repair and only adjustable by production parameter modification.

3. Countermeasures against Silver Streak and Water Mark

Silver streak mainly comes from excessive raw material moisture, overheated barrel degradation and condensed water on feed throat. Residual melt inside barrel is emptied during shutdown with heating zone temperature decreased and raw material re-dried thoroughly; thermal insulation sleeve is equipped at feed throat to avoid condensed water drop. Dead corner of runner causing detained carbonized material is polished into smooth arc to eliminate strip silver mark from degraded plastic. Shallow silver streak on finished product can be removed by surface grinding and polishing while large-area through silver mark leads to direct scrapping.

4. Optimization Methods for Fusion Line Defect

Fusion line forms where two melt streams converge bringing surface indentation and poor mechanical strength. Process improvement includes raising melt and mold temperature to enhance melt fusion activity and slowing down injection speed at convergence zone. Extra vent slot and overflow well are set at fusion line position on mold to discharge low-temperature trapped gas with waste slug; gate location is revised to shift fusion trace away from visible appearance surface. Fine shallow fusion line is eliminated via polishing while deep penetrating mark demands mold and process modification for reproduction.

5. Control Solutions for Flash and Burr

Flash arises from insufficient clamping force, excessive barrel temperature and enlarged mold clearance. Temporary on-site improvement is increasing clamping tonnage, lowering barrel temperature and reducing injection & packing pressure; worn clearance at parting line, insert and ejector pin is trimmed during mold stop with ejector pin fitting gap adjusted below 0.01mm. Tiny flash of small-size part is removed by low-temperature ultrasonic deburring; large workpiece is trimmed by dedicated hand tool; ultra-thin appearance component adopts cryogenic deflashing to prevent surface scratch.

6. Treatment for Pit and Black Speck Defect

Black speck is generated from carbonized residual material, mold rust debris and raw material impurity: nozzle carbon deposit is cleaned periodically and magnetic iron remover is installed at raw material feeding port to filter metallic scrap; mirror mold cavity is wiped regularly and vent groove cleared from dirt before production with idle purging shot for pre-production cleaning. Superficial tiny black dot is eliminated by fine grinding and polishing while deeply embedded speck results in finished product rejection directly.

Minor surface defects are repaired by post-processing to cut production waste; repeated batch defects must be solved fundamentally via raw material, mold and process optimization to reduce bad rate and balance production efficiency and finished appearance quality.