7 Most Common Injection Moulding Defects (and How to Fix Them)

1. Sink marks

What it looks like. A small depression or dimple on the surface of the part, usually opposite a thick section, rib, or boss. Looks like someone pushed a fingertip into warm clay.

What causes it. Sink marks are caused by differential shrinkage. Thick sections cool slower than surrounding thin walls. As the thick section continues to shrink after the surface has solidified, it pulls the surface inward — creating the sink.

Design fixes. Reduce wall thickness at the thick section. Core out thick areas to make them hollow. Reduce rib/boss thickness to 60% of the adjacent wall. Add opposite-side ribs to mask the sink visually.

Process fixes. Increase packing pressure (forces more plastic into the cavity during cooling). Increase packing time. Reduce melt temperature (faster solidification, less shrinkage window). Increase cooling time. Switch gate location so the thick section is fed last and packed longest.

Reality check. Sink marks are the single most common injection moulding defect. They are almost always a design issue (thick section somewhere) that no amount of process tuning fully fixes. The real fix is always in the part or mould design.

2. Short shot

What it looks like. The part is incomplete — plastic didn't fill the entire cavity. One corner or the far end from the gate is missing material. The part looks like it ran out of plastic.

What causes it. The molten plastic froze before reaching the end of the cavity. This happens when: the injection pressure or speed is too low, the melt temperature is too low, the mould temperature is too cold, the gate is too small (restricts flow), the wall section is too thin for the flow length, or the part has a trapped air pocket that the plastic can't push past.

Design fixes. Increase wall thickness in the thin section that's not filling. Add flow leaders (slightly thicker channels on the back side that guide flow to distant areas). Relocate the gate closer to the problem area. Add vents at the last-to-fill area so trapped air can escape.

Process fixes. Increase injection speed (faster fill before freeze-off). Increase melt temperature (keeps the plastic fluid longer). Increase mould temperature (slows freeze-off at the wall). Increase injection pressure. Check shot size — ensure enough plastic is being injected.

3. Flash

What it looks like. A thin fin of excess plastic extending out from the parting line or around ejector pins. Feels sharp to the touch. On cosmetic parts, flash is an automatic reject.

What causes it. The mould halves are not sealing perfectly at the parting line, and molten plastic is leaking into the gap. This happens when: clamp force is too low (the mould opens slightly under injection pressure), the mould faces are damaged, worn or contaminated (debris on the parting surface), injection pressure is too high, or the mould is over-packed.

Design fixes. Ensure the parting line is on a flat, machined surface (avoid parting on curved or stepped faces where possible). Minimise projected area (smaller area = less force trying to open the mould).

Process fixes. Increase clamp force. Reduce injection pressure. Reduce packing pressure. Clean the mould faces. Check mould alignment — platens may have shifted.

Mould fixes. Re-machine the parting line surface. Add crush ribs around the cavity to concentrate sealing force. Replace worn leader pins/bushings that allow mould misalignment.

4. Warping

What it looks like. The part is dimensionally correct but twisted, bowed or bent — it doesn't sit flat. A box lid that should be flat has a banana curve. A panel that should be straight has a twist.

What causes it. Warping is caused by uneven shrinkage across the part as it cools. One side cools faster than the other → the fast side shrinks first → the part curves toward the fast-cooling side. Causes include: non-uniform wall thickness (thick sections shrink more), asymmetric cooling (one mould half hotter than the other), incorrect gate location (flow-induced orientation), or a semi-crystalline material with high anisotropic shrinkage.

Design fixes. Uniform wall thickness (Rule 1 — the most common cause of warping). Symmetric part design where possible. Add stiffening ribs to resist the bending.

Process fixes. Equalise mould cooling on both halves. Increase cooling time (the part exits more rigid and resists warping as it air-cools). Reduce packing pressure if the part is over-packed on one side. Use a fixture or jig to hold the part in shape as it cools post-ejection.

Material note. Semi-crystalline plastics (PP, Nylon, POM) warp more than amorphous plastics (ABS, PC, PS) because crystallisation produces higher, more directional shrinkage. If warping is persistent, switching from PP to ABS (where functionally acceptable) can eliminate it.

5. Weld lines (knit lines)

What it looks like. A visible line on the part surface where two flow fronts met during filling. May also appear as a faint V-shaped notch. Cosmetically objectionable on glossy parts. Structurally weak — weld-line strength can be 10–50% lower than the base material.

What causes it. When molten plastic flows around an obstacle (a hole, a boss, a core pin) or enters from multiple gates, the two flow fronts converge and weld together. If the fronts are still hot and under pressure, the weld is strong and nearly invisible. If one or both fronts have started to cool, the weld is weak and visible.

Design fixes. Relocate the gate so flow fronts meet in a non-critical area. Reduce the number of gates (fewer gates = fewer weld lines). Eliminate unnecessary holes or obstacles that split the flow. Add overflow tabs at the weld-line location — the first cold/dirty plastic at the flow front exits through the tab, leaving clean hot plastic to form the weld.

Process fixes. Increase melt temperature (hotter fronts weld better). Increase injection speed (fronts arrive hotter). Increase mould temperature at the weld area. Increase packing pressure (forces the fronts together harder).

6. Burn marks

What it looks like. Black or brown discolouration, usually at the last-to-fill area of the part or at the end of a flow path. May also appear as a rough, degraded surface texture.

What causes it. Trapped air in the cavity is compressed by the advancing plastic to the point where it ignites (diesel effect) and chars the plastic. The air has nowhere to go because the vents are clogged, missing or too small.

Design/mould fixes. Add or enlarge vents at the last-to-fill area. Clean existing vents (they clog with off-gassing over time). Relocate the gate to change the fill pattern and move the last-to-fill area to a vent-able location.

Process fixes. Reduce injection speed (slower fill = less compression of trapped air). Reduce clamp force slightly (allows the parting line to 'breathe' and vent — but too much = flash). Use vacuum venting if standard vents are insufficient.

7. Jetting

What it looks like. A snake-like, squiggly pattern on the part surface near the gate, as if a stream of toothpaste was squeezed onto the surface and then pressed flat.

What causes it. Jetting occurs when the molten plastic enters the cavity as a free jet — it squirts through the gate and hits the opposite wall before the cavity fills from the gate outward. The initial jet cools on the surface before the rest of the cavity fills, leaving a visible frozen squiggle.

Design fixes. Redesign the gate to enter into a wall (so the plastic impinges against a surface immediately and flows outward) rather than jetting across an open cavity. Use a fan gate or tab gate instead of a pin gate.

Process fixes. Reduce injection speed at the beginning of fill (so the plastic enters slowly and doesn't jet). Increase melt temperature (lower viscosity, less tendency to jet). Increase gate size (wider gate = lower velocity = less jetting).

Prevention beats troubleshooting

Every defect in this list is easier and cheaper to prevent at the design stage than to fix on the production floor. A thorough DFM review (covered in our separate DFM guide) catches the design-side causes of all seven defects before any steel is cut.

At Rehman Industry, DFM review is the first step on every job. We flag thick sections that will sink, thin sections that won't fill, parting-line risks that will flash, and gate locations that will cause weld lines — all before tooling starts. That upfront attention is what keeps defect rates low in production.

Running into one of these defects right now? Send us photos of the reject and the process settings you're running. We'll help diagnose it — even if we didn't build the mould.