Diagnosing the seven most common injection molding defects

Troubleshoot in the right order

Injection molding defects almost always have multiple plausible causes, and the fastest diagnosis comes from testing the cheapest variables first: process settings before tooling changes, tooling before material changes. Document the baseline before adjusting anything — a defect you can't reproduce on demand is a defect you can't verify you've fixed.

Fill-related defects: short shots, weld lines, jetting

Short shots stem from insufficient material reaching the cavity extremities: check shot size and cushion first, then injection speed and pressure limits, then melt and mold temperature, and only then venting and gate size. Weld lines are inevitable wherever flow fronts meet — the goal is strength and invisibility, achieved through higher melt/mold temperatures, faster fill, and relocating gates so welds form in non-critical areas.

Jetting — the snake-like surface defect near the gate — is a flow instability from material entering an open cavity at high velocity. Slower initial injection speed, larger gates, or gate relocation against a wall resolves most cases.

Pressure-related defects: sink, flash

Sink marks form where thick sections cool last and pull surface material inward. Raise holding pressure and time, verify the gate hasn't frozen before packing completes, and reduce melt temperature moderately. If the geometry allows, coring out thick sections is the permanent fix — no process window fully compensates for a rib-to-wall ratio above roughly 0.6.

Flash is excess material escaping the parting line: either clamp force is insufficient for the projected area at the pressures being run, or the tool no longer seals. Distinguish by checking whether flash appears at consistent locations (tool wear or damage) or varies shot to shot (process/clamp).

Thermal defects: warpage, burn marks

Warpage is differential shrinkage — from asymmetric cooling, fiber orientation in reinforced grades, or non-uniform wall thickness. Balance cooling circuits first and verify actual cavity surface temperatures rather than trusting controller setpoints. For glass-filled materials, gate placement governs fiber orientation and therefore warp direction; process adjustments only fine-tune what the gate layout has determined.

Burn marks at end-of-fill are compressed, overheated air — the diesel effect. Improve venting, reduce injection speed in the final fill stage, and check for blocked or peened-over vents before touching temperatures.