Key facts
Injection molding defects and how to fix them: flash, sink marks, warpage, short shots, weld lines and burn marks - root causes and proven solutions from our molding engineers.
Most molding defects trace back to three roots: the mold design, the process window or the material. Here is how our engineers diagnose the six most common ones.
The Six Usual Suspects
- Flash: material escapes at the parting line – causes: clamp force too low, venting too deep, worn parting surfaces. Fix: process first, then re-land the parting line
- Sink marks: local depressions over thick sections – causes: insufficient packing, ribs too thick. Fix: longer/earlier hold, core out thick areas, ribs at 50-60% wall
- Warpage: part twists after ejection – causes: uneven shrinkage from differential cooling or fiber orientation. Fix: balance cooling, symmetric gating, adjust pack profile
- Short shots: part not filled – causes: trapped air, low injection speed, thin flow path. Fix: add venting, raise speed/temperature, reposition gate
- Weld lines: weak visible seams where flows meet – causes: multiple gates or holes splitting flow. Fix: relocate gates, raise melt temperature, vent at junction
- Burn marks: black streaks at fill ends – causes: compressed air ignition. Fix: vent at flow ends, reduce injection speed
Prevention Beats Correction
Our DFM review flags sink and warp risks before tooling; mold flow analysis predicts fill and weld lines before steel is cut. That is why our T1 trials pass the first time far more often than the industry average.
The tuning order that fixes defects fastest
When a trial part shows a defect, resist the urge to change everything at once. The working order is: check material drying first (moisture causes splay and bubbles that mimic other defects), then verify melt and mold temperatures against the datasheet, then adjust injection speed and packing profile, and only then touch the mold – venting, gate size, cooling. Each step is logged, so if the defect moves rather than disappears, you know which variable caused it.
Cosmetic defects on textured or polished surfaces follow the same logic, plus one extra check: mold surface condition. A polished cavity that lost its polish in a high-wear zone produces gloss variation that no process setting will fix. Our maintenance schedule exists precisely to keep surface condition inside the process window, and the flow analysis stage catches fill-related causes before steel is cut.