Injection molding defects are not random. Each one is the visible signature of a specific imbalance (heat, pressure, flow or time), and each has a fix that costs minutes once the cause is named. What makes defects expensive is not their complexity; it is the habit of changing parameters at random until the symptom disappears, which makes good results unreproducible and bad results undiagnosable.
This guide covers the six defects behind most injection molding scrap.
1. The Defect Map
| Defect | Root Cause Family | First Lever to Pull |
|---|---|---|
| Short shot | Incomplete fill | Melt temperature / injection speed |
| Sink marks | Thick-section shrinkage | Wall thickness design (then holding pressure) |
| Warpage | Internal stress + uneven cooling | Cooling balance (then mold temperature) |
| Weld lines | Melt fronts meeting without full fusion | Gate position (then melt/mold temperature) |
| Burn marks | Trapped air igniting (diesel effect) | Venting (then final-phase speed) |
| Jetting | Free jetting melt, poor flow front | Gate geometry / injection speed profile |
2. Short Shots, The Cavity Did Not Fill
What it looks like: the part is incomplete, a missing edge, an unfilled rib, a short far wall.
What causes it: melt temperature too low; injection pressure or speed too low; flow length too long for the wall thickness; blocked or undersized vents creating back pressure; machine shot capacity or tonnage at its limit; cold slug at the gate blocking initial flow.
What fixes it: raise melt temperature within the resin window; increase injection pressure and fill speed; verify vents are open and correctly sized; rebalance gates to shorten flow paths; confirm machine capacity against part volume. If the short shot moves around between shots, suspect a machine or material issue; if it sits in the same location every shot, suspect the tool, gate position, venting or a flow restriction.
3. Sink Marks, The Surface Pulled Inward
What it looks like: localized depressions on the surface, almost always opposite an internal thick section, a boss, a rib intersection, a mounting pad.
What causes it: thick sections cool and shrink after the surface has already solidified. The molten material beneath has no feed path left, so the surface collapses inward. Classic generators: thick bosses behind cosmetic surfaces, rib intersections, and non-uniform walls.
What fixes it, design first: keep walls within ±25% of nominal; use ribs at roughly 60% of adjacent wall thickness instead of thick walls; core out unavoidable thick sections; position gates so the thickest sections fill last, under full holding pressure. Then process: increase holding pressure and time; improve cooling so the section solidifies uniformly; reduce melt temperature slightly to cut overall shrinkage.
4. Warpage, The Part Will Not Sit Flat
What it looks like: twisting, bowing or cupping, the part does not sit flat or mate with its assembly.
What causes it: internal stress plus uneven cooling. When walls vary significantly, thicker areas retain heat longer, creating differential shrinkage across the part. Glass-filled resins add anisotropic shrinkage (different along versus across fiber flow), which warps flat plates even with uniform walls. Ejection too early, before the part is rigid enough, adds handling stress.
What fixes it: balance the cooling circuit so both mold halves remove heat at the same rate; raise mold temperature to reduce residual stress; extend cooling time before ejection; for filled resins, manage flow direction and gate position to balance fiber orientation; where the part is a large flat plate, consider a slight crown or rib structure to resist bowing.
5. Weld Lines, Where Two Fronts Met
What it looks like: a visible line or notch on the surface where two melt fronts met and fused imperfectly. Mechanically, the weld is the weakest section of the part.
What causes it: geometry forces the flow to split around a hole or rib and recombine; the meeting fronts have cooled slightly and do not fully fuse. Severity depends on melt temperature, fill speed, mold temperature and trapped air at the meeting point.
What fixes it: design levers first, gate position to move weld lines off cosmetic and load-bearing surfaces; wall uniformity; overflow wells positioned at the meeting point so the cold front is captured outside the part. Then process, higher melt and mold temperature, faster fill to improve fusion, venting so trapped air does not chill the fronts. Where a weld line must carry load, redesign so it does not.
6. Burn Marks, The Diesel Effect
What it looks like: scorched or discolored spots, usually at the last-fill location, sometimes with a silver or matte streak.
What causes it: trapped air at the end of fill compresses adiabatically and ignites, the diesel effect. The root cause is venting, not temperature: air has nowhere to go, so it heats until it burns the resin.
What fixes it: improve venting at last-fill locations (depth and land area both matter); reduce injection speed in the final fill phase; enlarge gates or runners if pressure drop is excessive; add overflow wells. Burn marks usually solve with vents alone, slowing the cycle is the second choice, not the first.
7. Jetting, The Worm-Look Defect
What it looks like: a worm-like or snake-like freeze-off pattern on the surface, usually near the gate.
What causes it: the melt jet shoots across the cavity as a free stream instead of forming a progressive flow front, then solidifies in that shape before the cavity fills. Typical causes: gate too small or poorly positioned, injection speed too high at start of fill, or low melt temperature.
What fixes it: enlarge or reposition the gate so the jet impinges on a cavity wall or core pin and forms a front immediately; use a speed profile that starts slow and accelerates; raise melt temperature. Jetting is a gate design problem first, process adjustments compensate, but the gate geometry is the cure.
8. The Troubleshooting Order That Actually Works
Work from the material outward, changing one variable at a time:
- Material: correct resin and grade? Dried to spec (dew point and time)? Correct masterbatch ratio? Contamination from regrind?
- Temperatures: barrel profile, nozzle, mold temperature, measured, not setpoint
- Pressures: injection, holding, back pressure
- Speeds and times: injection profile, holding time, cooling time
- Mold condition: vents clean, cooling circuits flowing, slides and ejectors functioning, no damage at parting line
Every defect in this guide is diagnosable in one disciplined cycle if you change one thing at a time and record it. If your line is fighting one of these defects right now, send us the defect photos, the part geometry and the current process sheet, the root cause is usually visible before anything gets cut.