Common Injection Molding Defects & How to Fix Them

Mr. Liu· Engineering DirectorJuly 11, 2026

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?

Sink marks, warping, short shots, flash, flow lines, weld (knit) lines, burn marks and voids. Most trace back to non-uniform wall thickness, incorrect temperature/pressure, poor venting, or moisture in the resin.

How do you fix sink marks?

Sink marks come from thick sections cooling and shrinking. Fix them by keeping walls uniform and thin, sizing ribs to 40–60% of the wall, coring out thick areas, and increasing packing pressure/time.

What causes a short shot?

The cavity doesn't fill completely — usually from too little injection pressure, speed or melt temperature, an undersized gate, or trapped air (poor venting). Increase fill parameters and improve venting.

What is a weld line and is it a problem?

A weld (knit) line forms where two flow fronts meet, e.g. around a hole. It can be a cosmetic mark and a weak spot. Relocating the gate, raising melt temperature and adding venting reduce it.

Are molding defects a design or process problem?

Both. Many defects (sink, warp, short shot) are prevented by good part design — uniform walls, correct ribs and draft — while others are tuned out with process settings (pressure, temperature, cooling) and correct, dried material.

Sources & further reading: MatWeb — plastic material properties · ISO 2768 general tolerances.

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