Common Injection Molding Defects and Solutions

Injection molding is a core process in plastic product manufacturing and a key focus of quality control. In actual production, various defects are hard to completely avoid; the key lies in accurately identifying root causes and taking effective corrective measures. Based on years of injection molding experience, Haipenghui Precision Industry has summarized six common injection molding defects and their solutions.

1. Sink Marks and Dents Phenomenon: Localized depressions on the product surface, commonly found in thick-wall areas, on the back of ribs, or where wall thickness changes abruptly. Root causes: Insufficient holding pressure or too short holding time; unreasonable gate location with poor feeding channels; uneven cooling resulting in large local shrinkage differences. Solutions: Process adjustment: increase holding pressure and extend holding time. Mold optimization: adjust gate location or add additional feed gates; optimize cooling channel layout. Product design: reduce abrupt wall-thickness changes and add fillets at rib roots.

2. Weld Lines Phenomenon: Visible lines or color differences where two or more melt fronts meet, affecting appearance and structural strength. Root causes: Melt fronts meeting at too low a temperature in multi-gate feeding; poor mold venting blocking melt fusion; insufficient material flowability. Solutions: Process adjustment: raise barrel and mold temperatures; increase injection speed. Mold optimization: add vent slots; adjust gate locations to move weld lines to non-appearance or low-stress areas. Material selection: use materials with better flowability or add suitable additives.

3. Flash (Burns) Phenomenon: Excess thin sheets of plastic overflowing from parting lines, ejector pin holes, or slider fits. Root causes: Insufficient clamping force causing the mold to open; worn or damaged mold parting surfaces; excessively high injection pressure or speed. Solutions: Equipment adjustment: increase clamping force. Mold maintenance: repair parting surfaces to ensure mold closing accuracy. Process optimization: reduce injection pressure and speed; optimize the holding pressure switch-over point.

4. Flow Marks and Jetting Phenomenon: Wavy or snake-like patterns on the product surface, especially near the gate. Root causes: Melt jetting caused by excessive speed through the gate; mold temperature too low causing premature cooling of the melt front; gate too small or improperly positioned. Solutions: Process adjustment: reduce injection speed and adopt multi-stage injection; raise mold temperature. Mold optimization: enlarge the gate; use fan gates or tab gates for smooth filling. Position adjustment: optimize gate location to avoid direct impact on the core.

5. Warpage Phenomenon: Twisting, bending, or lifting of the product after ejection, affecting dimensional accuracy and assembly. Root causes: Uneven cooling causing large shrinkage differences across areas; uneven holding pressure distribution; asymmetric product structure or large wall-thickness differences. Solutions: Mold optimization: optimize cooling channel layout for balanced cooling; adjust the ejection system to ensure uniform ejection force. Process adjustment: adjust the holding pressure curve; extend cooling time appropriately. Product design: add reinforcing ribs to increase rigidity; reduce wall-thickness differences.

6. Dimensional Deviation Phenomenon: Key product dimensions exceeding drawing tolerances, affecting assembly and function. Root causes: Inaccurate calculation of molding shrinkage; fluctuations in process parameters (temperature, pressure, time); insufficient mold machining precision or wear. Solutions: Upfront mold flow analysis: use simulation during mold design to accurately predict shrinkage. Process stabilization: implement standardized operations and monitor key process parameters. Mold correction: correct dimensions based on trial molding results (welding, insert replacement, etc.). Environment control: keep workshop temperature and humidity stable to reduce material moisture variation.