Injection Moulding Defects: 19 Problems, Causes and Fixes

Most injection moulding defects come from four root domains: part design, tooling, material and process settings. A repeatable troubleshooting method is to identify the symptom, confirm whether it follows the cavity or the machine, then work the cheapest root domain first — material and process before steel.

The table below lists 19 defects with their usual causes and first fixes. The sections after it cover the highest-impact defects in detail. Nordmould screens for the design-driven members of this list in every free DFM review.

What are the injection moulding defects, at a glance?

Defect	What it looks like	Most likely causes	First fixes
Short shot	Incomplete part, missing edges	Low melt temperature, poor venting, small gate, insufficient shot	Raise melt/mould temperature, clean vents, confirm shot size
Flash	Thin fin at parting line or pins	Over-packing, worn parting line, low clamp force	Reduce hold pressure, verify clamp tonnage, inspect shut-offs
Sink marks	Dimples opposite ribs/bosses	Thick local sections, under-packing, early gate freeze	Thinner ribs (40-60% of wall), more packing, longer hold
Voids / bubbles	Internal cavities	Thick sections, volatiles, under-packing	Core out thick zones, dry material, increase hold
Warpage	Bowed, twisted or cupped part	Differential cooling, wall variation, fibre orientation	Balance cooling, uniform walls, review gate location
Weld / knit lines	Line where flow fronts meet	Cold fronts, poor venting, gate location	Raise melt temperature, improve venting, move gate
Burn marks / dieseling	Black/brown scorch at end of fill	Trapped air, blocked vents, excessive speed	Slow final fill, clean/deepen vents
Splay / silver streaks	Silvery surface streaks	Moisture in resin, shear heat, contamination	Dry resin to spec, reduce shear, check regrind
Jetting	Snake-like flow pattern from gate	Poor gate landing, high initial speed	Reduce initial fill speed, redirect gate onto a wall
Flow lines / marks	Visible rings or streaks	Hesitation, cold mould, wall transitions	Raise mould temperature, smooth transitions (3:1 taper)
Drag marks / scuffing	Scratches in ejection direction	Insufficient draft, rough cavity surface	Add draft (1°+), polish cavity in draw direction
Ejector pin marks	Dents or whitening at pins	Part too soft at ejection, too few pins	Longer cooling, more/larger pins, add draft
Gloss variation	Patchy shine	Mould temperature swings, uneven polish	Stabilise mould temperature, repolish, steady packing
Colour streaks (marbling)	Swirled, uneven colour	Poor masterbatch dispersion	Increase back pressure/mixing, review colourant carrier
Black specks	Dark particles in part	Degraded resin in barrel/hot runner	Purge, check residence time and hot-runner dead spots
Delamination	Flaking layers	Contamination, incompatible regrind, moisture	Purge, control regrind source and ratio, dry material
Brittleness / cracking	Snaps, crazing, stress whitening	Degradation, residual stress, over-packing	Lower melt temperature/residence, anneal, review packing
Stringing / drooling	Strings or leaks at gate	Nozzle temperature too high, poor decompression	Lower nozzle temperature, add suck-back, check valve
Gate blush	Hazy or frosted gate area	High shear at gate, small gate	Enlarge gate, reduce fill speed at gate

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Sink marks — why do they appear opposite ribs and bosses?

A sink mark is a surface depression caused by local volumetric shrinkage: the skin solidifies first, then the thicker core beneath it keeps shrinking and pulls the skin inward. The classic trigger geometry is a rib or boss whose thickness exceeds roughly 60% of the wall it sits on.

Design fixes come first: keep ribs at 40-60% of nominal wall, bosses at 40-60%, and core out any local mass above 4-5 mm. Process fixes second: increase hold pressure and hold time until the part weight stops rising, and confirm the gate has not frozen early. If sink persists at full packing on a correct design, the gate or cooling layout needs review — that is a tooling conversation.

Warpage — why does the part not keep its shape?

Warpage is differential shrinkage made visible. One region cools or crystallises faster than another, or glass fibres align with flow and shrink less in one direction than the other. The result is bow, twist or cup that shows up in assembly, sealing or flatness checks.

The investigation order that works: first confirm the pattern is stable (same distortion, same direction, every shot). Then try controlled changes to cooling time, mould temperature and packing — one at a time. If the pattern does not move, the cause is structural: wall-thickness distribution, gate location, cooling-channel balance or datum design. Nordmould reviews these four at DFM stage because steel changes after T1 sampling cost from hundreds to thousands of euros.

Flash — excess material at the parting line

Flash appears where the cavity is not sealed against the injection pressure: at the parting line, around ejector pins, at shut-offs and at vents. Over-packing is the most common process cause; worn shut-offs and insufficient clamp force are the common tooling causes.

A useful discrimination test: reduce hold pressure in controlled steps. If the flash shrinks, it was process-driven. If it stays fixed in the same place, inspect the steel — parting-line fit, shut-off wear, vent depth and mould support. Chronic flash at one edge of a multi-cavity tool points to that cavity's sealing, not the machine.

Weld lines — cosmetic seam or structural crack?

A weld line forms wherever two melt fronts meet — around every hole, boss and insert. It is unavoidable; the question is whether it is strong and whether it is visible. Strength drops when the fronts meet cold, at a shallow angle, or when glass fibre does not knit across the seam.

Keep weld lines away from snap hooks, sealing beads, load paths and cosmetic A-surfaces by choosing gate location deliberately. Raise melt temperature and fill speed if the seam is weak; improve venting at the meeting point. Where a weld line would cross a critical feature, relocating the gate — decided at DFM, not after sampling — is the real fix.

Short shots — when the cavity does not fill

A short shot means the melt front stopped before the cavity was full. The usual suspects, in order of checking cost: blocked or undersized vents at the last-to-fill region, low melt or mould temperature, a restrictive gate, insufficient shot size or cushion, and a flow length that exceeds the material's flow-length-to-thickness ratio.

Run a short-shot sequence (progressively fuller shots) to find where the front stops. If it always stops at the same region, venting and gating at that region are the target — not more injection pressure, which typically trades a short shot for flash or burns.

Burn marks and splay — the two that blame the material

Burn marks (dieseling) are scorched resin caused by trapped air compressing and igniting at the end of fill — a venting problem first, a fill-speed problem second. Splay (silver streaks) is moisture flashing to steam at the melt front — a drying problem first.

Hygroscopic resins such as PA, PBT, PC, PET, POM and TPU must be dried to their specified dew point and time; splay on these materials means check the dryer log before touching the tool. Burn marks locked to the same end-of-fill corner mean clean or deepen the vent at that corner.

When do you stop tuning the machine and review the tool?

The decision rule used by experienced process engineers: if a defect repeats in the same cavity, at the same location, across controlled changes to temperature, pressure and speed, it is a tooling or design signal. Machine settings can suppress a steel problem for a shift; they cannot remove it.

Escalate to a tooling review when you see: flash fixed to one shut-off edge, burns fixed to one vent, short shots fixed to one last-to-fill region, warpage that ignores cooling changes, or cavity-to-cavity dimensional drift in a multi-cavity tool. Nordmould's sampling reviews follow the same rule — T1 parts are read cavity by cavity, not as one averaged sample.