Injection Molding Defects: Sink Marks, Warping, Flash and Weld Lines

Injection molding can turn out thousands of identical plastic parts an hour — or thousands of identical defects if the part and process aren’t right. Sink marks, warping, flash, weld lines and short shots are the defects that show up most, and almost all of them trace back to a handful of causes: uneven wall thickness, bad gating, or process settings that are off. The good news is that most are designed out long before the mould is cut. This guide covers the common injection molding defects, what causes them, and how to prevent them.

Sink Marks

Sink marks are shallow depressions on the surface, usually opposite a thick feature like a rib or boss. As the thick section cools and shrinks, it pulls the surface inward. The cure is design: keep walls uniform and make ribs thinner than the wall they join (around 50–60% of wall thickness). Thick bosses should be cored out. Uniform wall thickness is the master rule that prevents most sink.

Warping

Warping is a part that twists or bows out of shape after ejection, caused by uneven cooling and uneven shrinkage across the part. Thick-and-thin sections cool at different rates and pull the part out of flat. Prevent it with uniform walls, balanced cooling, gate placement that fills the part evenly, and by allowing enough cooling time. Fibre-filled materials shrink directionally, which can add to warp — see engineering plastics.

Flash

Flash is a thin film of plastic that squeezes out along the parting line or around ejector pins, leaving a fin that must be trimmed. It comes from too much injection pressure, a worn or poorly clamped mould, or melt that is too hot and runny. Correct clamp force, mould maintenance and dialled-in pressure and temperature keep flash under control.

Weld (Knit) Lines

Weld lines form where two flow fronts meet — for example after the melt splits around a hole — and rejoin slightly cooled, leaving a visible line that is also a weak point. They can’t always be eliminated, but you can move them to non-critical areas with gate placement, raise melt and mould temperature so the fronts fuse better, and avoid unnecessary holes and obstructions in the flow path.

Short Shots and Flow Lines

A short shot is an incompletely filled part — the melt froze before filling the cavity, from low pressure, a cold melt, thin sections or trapped air. Flow lines are wavy patterns or streaks showing how the plastic flowed, usually from uneven speed or thin walls. Both improve with higher melt temperature and pressure, better venting, and thicker or more uniform sections.

Defect, Cause and Fix at a Glance

Designing Moulded Parts That Run Clean

• Uniform wall thickness is the foundation — it prevents sink, warp and short shots all at once.
• Ribs thinner than walls, bosses cored, generous fillets at corners to keep flow and cooling even — the core rules in our injection molding design guide.
• Add draft so parts eject cleanly without drag marks or sticking.
• Plan gate and weld-line locations with your moulder so visible faces and stressed areas stay clean.
• Choose the right resin and account for its shrinkage and fillers — see our materials guide.

The Bottom Line

Injection molding defects look like process problems but usually start on the drawing. Sink, warp and short shots come from uneven walls; flash from pressure and tooling; weld lines from flow that splits and rejoins. Keep walls uniform, make ribs and bosses behave, add draft and fillets, and place gates deliberately, and the same mould that could stamp out thousands of defects instead stamps out thousands of good parts. Most of this is decided before the steel is ever cut — which is why a design review pays for itself.

MechPart Pro molds to clean, repeatable quality — reviewing wall thickness, ribs, gates and draft, and tuning the process to kill sink, warp, flash and weld lines — as part of our free design-for-manufacturability review. Compare moldable resins in our engineering plastics guide and the wider materials guide.