Flow marks, weld lines, sink marks: which moulding defects are normal and which are rejects
Every injection-moulded part carries traces of how it was made. The useful question is not whether a mark exists, but whether it is a normal trace of the process or a defect that should have been caught.
The six defects and where they come from
| Defect | What you see | Usual cause |
|---|---|---|
| Flow marks | Wave-like ripples, strongest near the gate | Melt or mould too cold, unsteady injection speed, sudden wall-thickness change, small gate |
| Weld lines | A fine line where two flows of plastic met | Unavoidable around holes and on parts with several gates; the part is weaker along the line |
| Sink marks | Shallow dents over thick ribs and bosses | Not enough holding pressure, local wall too thick, cooling too short |
| Gas marks (burns) | White or dark streaks | Air trapped in the cavity, compressed and heated until it scorches the surface; poor venting or injection too fast |
| Silver streaks | Silvery lines fanning out across the surface | Moisture in the resin turning to steam — almost always resin that was not dried properly |
| Flash | Thin plastic fins at the parting line or ejector pins | Clamp force too low, melt temperature or pressure too high, worn mould, debris on the parting face |
Normal within limits: weld lines, flow marks, sink marks
These three cannot always be eliminated by the process. They are accepted across the industry as long as they stay inside limits — and the limits depend on which face of the part they are on.
- Weld lines: acceptable on hidden faces, and on visible faces if thin, flush and not noticeable at 30 cm. A reject if thick, white, sunken or cracked.
- Flow marks: acceptable off the main face if shallow and not felt by hand. A reject if they cover the front in waves you can feel.
- Sink marks: acceptable as slight dents on the back or hidden areas that do not affect assembly. A reject if clearly visible on the front, or anywhere the drawing forbids them.
Always a reject: gas marks, silver streaks, flash
These are not traces of the process; they are faults, and a factory that ships them is shipping its own mistakes.
- Gas marks are burns from poor venting. If you can see one on a visible face, the part fails and the mould or settings get fixed.
- Silver streaks mean the resin was not dried. They are rejected without exception, because the same moisture that marks the surface also weakens the part.
- Flash is not allowed in principle. A very thin fin under 0.05 mm on a hidden face can be trimmed by hand; anything sharp enough to cut a finger, or on a visible face, is a reject and does not ship.
The 30 cm rule
Our internal standard is the one most of the industry uses. Hold the part 30 cm from the eye, under a standard light source or daylight, and look at it straight on — no raking light, no magnifier, no hunting. If you cannot see the mark, it passes. If you can see it clearly, it fails.
The rule is applied in grades. The main face — the side a customer sees first — gets the strictest reading. Sides, the back and faces hidden after assembly are allowed more.
Above all of this sits the signed approval sample and the agreed AQL. When the two sides disagree about a mark, the sample decides, not the 30 cm rule.
Clear parts are judged far more strictly
On a transparent part in PC, PCTG or PS, weld lines, flow marks, faint gas marks and silver streaks are all obvious, so almost none of them are tolerated — even a fine weld line is usually a reject.
On a solid colour, especially a dark one, a slight weld line, light flow marks or a small sink on the back can pass if they are not noticeable and do not affect use. If a clear bottle is your product, say so at the drawing stage: it changes where the gates go.
When a buyer rejects a mark that is within limits
It happens. We handle it in the same order every time:
- Put the defect next to the signed sample, the inspection standard and the AQL, and check whether it is inside what was agreed. Photograph both.
- If it is a process trace the sample allows, we offer a discount so the buyer keeps the goods. If the buyer still wants to return them, the signed sample is the reference; the usual outcome is a partial replacement rather than a full return.
- If the defect is outside what the sample allows, it is our fault, and we pay for the rework, replacement or return.
Every step depends on one thing agreed in advance: a physical approval sample that both sides signed. Without it, the conversation is opinion against opinion.