Common Injection Molding Defects & How to Fix Them
Most injection molding defects trace back to a few causes: non-uniform wall thickness, wrong temperature or pressure, poor venting, or moisture in the resin. Sink marks and warping come from thick/uneven walls; short shots and flow lines from low pressure/temperature; flash from too much; burn marks from trapped air. Here's a defect-by-defect guide with causes and fixes.
See our rapid tooling & injection molding service and the design guide.
Key takeaways
- Sink marks / warping → uneven or thick walls; fix with uniform thin walls, correct ribs, balanced cooling.
- Short shots → not enough material fills the cavity; raise injection pressure/speed/temperature, improve venting.
- Flash → material seeps at the parting line; lower pressure, check clamp force and mold fit.
- Weld/flow lines → flow fronts meet or cool unevenly; adjust gate location, temperature and speed.
- Burn marks / voids → trapped air or moisture; add venting, dry the resin, slow the fill.
- Good part design prevents most defects — get a free DFM review with your quote.
Injection molding defects, causes & fixes
| Defect | Typical cause | How to fix |
|---|---|---|
| Sink marks | Thick sections/ribs cool & shrink | Uniform thin walls; ribs ≤60% wall; adjust packing |
| Warping | Uneven cooling / internal stress | Uniform walls; balanced cooling; correct gate |
| Short shot | Cavity not fully filled | ↑ pressure/speed/temp; improve venting; larger gate |
| Flash | Material escapes the parting line | ↓ pressure; check clamp force & mold fit |
| Flow lines | Uneven flow/cooling near gate | ↑ temp/speed; relocate/resize gate |
| Weld (knit) lines | Two flow fronts meet & weld weakly | Move gate; ↑ temp; add flow leaders/vents |
| Burn marks | Trapped air overheated (diesel effect) | Add venting; slow fill; lower temp |
| Voids / bubbles | Trapped gas or moisture | Dry resin; ↑ packing; add venting |
Design vs process vs material
Defects come from three levers, and it's worth checking them in order:
- Design: uniform walls, correct ribs/bosses, draft and radii prevent sink, warp and short shots before they start — see the design guide.
- Process: injection pressure, speed, melt and mold temperature, packing and cooling time are dialed in to eliminate flow, fill and flash issues.
- Material: the right resin, properly dried, avoids splay, brittleness and voids. Moisture-sensitive resins (nylon, PC, ABS) must be dried.
The most common defects
Sink marks appear over thick ribs/bosses — fix with the wall-thickness rules. Warping is uneven shrinkage — balance walls and cooling. Short shots leave the part incomplete — add pressure, heat or venting. Flash is excess at the split line — reduce pressure and check the tool. Flash, weld lines and burn marks each have a clear cause in the table above.
Frequently asked questions
What are the most common injection molding defects?
How do you fix sink marks?
What causes a short shot?
What is a weld line and is it a problem?
Are molding defects a design or process problem?
Sources & further reading: MatWeb — plastic material properties · ISO 2768 general tolerances.
Manufacturing next step
Apply this guidance to your part
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