Quality
Avoiding common molding defects
Most molding defects have a small number of causes. Knowing whether a given defect is a design problem, a tooling problem, or a process problem is what makes it fixable.
1. Sink marks
A depression in the surface, usually opposite a rib, boss, or thick section. The thick area continues to contract after the surface has frozen and pulls it inward.
Fix in the design. Reduce rib thickness to 50 to 60 percent of the nominal wall, support bosses with gussets instead of mass, and keep walls uniform. Process changes such as raising holding pressure only mask a marginal case.
Most defects are geometry or process, not material. Sink and voids come from thick sections; weld lines from flow splitting around a feature and meeting cold; short shots from insufficient pressure, gate size or venting; flash from clamp force or a worn parting face; warp from unequal cooling or fiber orientation. Each has a first place to look.
2. Weld and knit lines
A visible line where two flow fronts meet, typically behind a hole or around a boss. It is also the weakest region of the part.
Fix in the tool. Move the gate so the fronts merge in a structural rather than a cosmetic area. Raising melt and tool temperature improves the bond but will not move the line.
3. Short shots
The cavity does not fill completely. Usually caused by a wall too thin for the flow length, a gate too small, inadequate venting, or a resin whose viscosity was underestimated.
Fix in the design first. Check wall thickness against the resin window. Then check venting and gate size before assuming an injection pressure problem.
4. Flash
Thin webs of plastic escaping along the parting line. Caused by clamp force too low for the projected area, plate deflection, worn shut-off faces, or a parting line running across a curved surface.
Fix in the tool. Place parting lines on sharp edges rather than filleted surfaces, and confirm the machine tonnage is right for the projected area.
5. Warp
The part distorts after ejection. It is a shrinkage problem: different regions cooled at different rates or shrank by different amounts.
Fix in the design and the cooling. Uniform walls, symmetric geometry, and a cooling circuit that removes heat evenly. Filled resins warp more because their shrinkage is directional, so a glass-filled grade needs the flow direction considered explicitly.
6. Voids and bubbles
Internal cavities in thick sections. As the outside freezes, the still-molten core contracts and pulls itself apart. Bubbles from trapped moisture look similar but come from undried resin.
Fix in the design for voids, in the process for moisture. Thin the section for voids. Dry hygroscopic resins such as PC, PA, and PBT properly for bubbles.
7. Burn marks and drag marks
Burns are scorched patches at the last area to fill, caused by air compressed rather than vented. Drag marks are scuffs from the part sliding against steel on ejection.
Fix in the tool. Add or clean vents for burns. For drag marks, increase draft and check the tool finish direction.
Where to look first
| If the defect is | Look at |
|---|---|
| Sink, void, warp | Part design: wall thickness and uniformity |
| Weld line, burn, flash | Tool design: gate, vents, parting line |
| Short shot | Design first, then gate and venting, then process |
| Bubbles, brittleness | Process: material drying and melt temperature |
| Drag marks | Draft and tool surface |
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