Twelve injection molding defects and what actually causes them

The part fills incompletely. A thin unfilled section, typically at the end of the flow path or in a thick section where flow is harder to reach.

• Insufficient melt temperature — the most common cause. The material is too viscous to complete the flow.
• Insufficient injection pressure or speed, or a slow profile that lets the skin freeze before the flow completes.
• Insufficient packing, so thick sections have not been filled and pressurised before freeze-off.
• Blocked or restricted gate or runner, or a nozzle not fully open.
• Vent blocked at the end of the flow path, so air cannot escape and the material stops.
• Damp material — moisture raises the effective viscosity and requires more pressure to flow.

Raise melt temperature in steps of 5°C and re-test — this fixes most short shots on its own. If it does not, increase packing pressure and time. Check the last cavity to fill: if air is trapped there, the vent is blocked and the mould needs clearing, not the settings changing.

A faint line on the surface, usually around a hole, an insert or behind an obstacle, where two flow fronts met and did not fully fuse.

• Flow front splitting around an obstacle — this is the cause; weld lines are geometry, not a mistake.
• Trapped air at the meeting point, which prevents proper fusion.
• Low melt temperature or low packing pressure — the fronts meet at low temperature and do not fuse.

In order of effectiveness: relocate the gate so the fronts meet where the line is least visible; add a cold slug well at that position so air is displaced properly; raise melt temperature slightly and increase packing pressure to improve fusion. Where appearance is critical, consider relocating the feature rather than fighting the weld line.

Alternating matte and glossy bands running along the flow direction, like fingerprints or the surface of a rippled material.

• Poor plasticising — the melt is not homogeneous, so different regions have different viscosities.
• Wrong screw design for the material, or a screw with worn flights that cannot convey and mix properly.
• Melt temperature too low, or a barrel temperature profile that is uneven along its length.

Improve plasticising: raise back-barrel zone temperatures and hold them there, increase screw speed, and verify the screw is right for the resin. If a worn screw is the cause, no temperature change will fix it permanently — the screw needs rebuilding or replacing.

A tail, string or ripple extending from the gate area, usually with a small pimple at the gate itself.

• Material injected too fast for the gate area, so it squirts before it can spread.
• Gate too small relative to the shot size.
• Melt temperature too high, making the material too fluid at the gate.

Reduce the initial injection speed, use a slower first stage to fill the gate, and lower the melt temperature slightly. Where the gate position is fixed by geometry, a gate insert with a smaller or differently shaped gate is the permanent fix.

Visible bubbles on the surface or voids inside the wall. Sometimes a bubble with a tail of material behind it.

• Damp material — by far the most common cause. Water vapour forms a bubble.
• Trapped air from poor venting, compressed by the packing phase.
• Gas from degradation if the material has been held too long or the barrel is too hot.

Fix the drying first — correct temperature, time and dew point. If bubbles persist and the bubble has a tail behind it, it is air from a venting problem, and the mould needs vent improvement rather than setting changes.

Silvery or pearlescent streaks along the flow direction, sometimes described as looking like the surface of a rippled material.

• Moisture in the melt — the dominant cause in almost every case. Water vapourises and forms micro-bubbles that reflect light.
• Drying time or temperature insufficient, or the material being re-moistened after drying.
• Hopper lid not sealed, letting humid air into dried material.
• Dryer not actually holding temperature, or dew point too high.

Correct drying — this is the whole fix. See our material drying guide for temperatures and times by resin: PP 80°C for 2–4 hours, PC 120°C for 4 hours, PA 80°C for 4–6 hours, PET 120–150°C. If streaks persist after correct drying, check the hopper seal and confirm the dryer reaches and holds temperature. Raising melt temperature will not fix this and can make it worse.

Black or dark specks on or in the part, sometimes with a burnt smell. They may appear only after a long idle period.

• Degraded material trapped in dead corners of the screw, barrel or nozzle. This is the most common cause.
• Worn screw or barrel, where dead space has opened up and material sits and degrades.
• Incoming material already contaminated — a supply problem, not a machine problem.
• Barrel temperature too high, or residence time too long.

Mould a shot immediately after a full purge. If it is clean, the contamination is coming from material sitting in the machine — which means purging, dead-corner cleaning, or a screw problem. If it is still contaminated, the problem is incoming material or the barrel temperature. This single test separates the two causes in five minutes.

Localised depressions on the surface, typically in a thick section, around a boss, or where a heavy section meets a thin wall.

• Uneven shrinkage — a thick section shrinks more than a thin one over a longer time.
• Insufficient packing — not enough material held under pressure to compensate.
• Mould too hot, extending the cooling window.
• Design issue — a sharp thick-to-thin transition the material cannot fill.

Process: increase packing pressure and packing time, and lower the mould temperature. Design, which is the more permanent fix: make thick-to-thin transitions gradual, add a rib at the base of a boss rather than a solid thick section, and core out thick areas so the wall thickness is uniform. A rib of 60% of the wall thickness will do the structural work of a solid section while avoiding the sink.

Brown or scorched streaks, typically at the end of the flow path or where the material stops moving.

• Trapped air compressed by the packing phase — the classic cause. Air heats rapidly under compression.
• Blocked vent at the end of the flow path.
• Melt temperature too high for the material.
• Injection speed too slow, so the material is dwelling and degrading in the barrel.

Improve venting at the last cavity to fill — this fixes most cases. Then reduce back pressure and slow injection so the material does not dwell in the barrel. Lowering melt temperature also helps. Check where on the part the burn appears: at the very end of flow means venting, elsewhere means something else.

The whole part is bent, twisted or dished rather than flat. Usually more visible after the part has cooled fully.

• Uneven shrinkage through the thickness — the part cools unevenly, and different regions shrink by different amounts.
• Unbalanced mould temperature across the cavity — one side hotter than the other.
• Asymmetric design — uneven wall thickness, or material concentrated on one side.
• Excessive packing pressure or time, forcing too much material into one region.
• Residual stress from fibre orientation in filled grades.

Check mould temperature balance across all zones first — an unbalanced cooling circuit is the most common cause and the easiest to check. Then reduce packing pressure and time. Design fixes that genuinely help: uniform wall thickness, balanced material distribution, and ribs rather than thick sections. For fibre-filled grades, orientation is unavoidable, so the fix is in the design geometry, not the settings.

A thin lip of extra material around the edge of the part, usually along the parting line, or around a pin.

• Excessive clamping force relative to the flash gap — the pressure pushing material out exceeds what the clamp can resist.
• Worn or damaged mould — worn guide pins, a worn parting face, or flash gap opened up.
• Mould not properly aligned, or clamping not square.
• Excessive packing pressure, or too much material in a small shot size.
• Melt temperature too high, making the material too fluid to stay in the cavity.

Reduce packing pressure, packing time and melt temperature in that order. If flash persists, it is a mould or machine issue rather than a setting: check guide pin wear and platen alignment. On a used machine, platen parallelism is worth checking specifically — it is a common issue and produces flash that no setting change will fix.

Parts that meet tolerance early in the run and drift out of specification as production continues. Or inconsistency from shot to shot.

• Damp material — moisture varies between batches, changing shrinkage and melt viscosity.
• Worn screw — unstable shot weight, so packing pressure varies part to part.
• Failing thermocouple, so barrel temperature drifts through the run.
• Mould temperature not controlled — the most commonly overlooked cause.
• Nozzle or hopper temperature not set correctly, so the melt temperature changes with ambient conditions.

Work through the four that change over time: drying consistency, thermocouple function, screw condition and mould temperature control. Take a reading from the actual nozzle rather than trusting the set value, because the nozzle heater is the one most often wrong. Shot-to-shot variation points to screw or nozzle temperature; drift over hours points to drying or thermocouples.