Troubleshooting in injection molding — from shrinkage hole to flash
How to read a faulty plastic part and find the right cause in machine, mould, or material
A defective subject tells a story if you know what to look for. The shape of the defect — whether it is an indentation, a burr or a crooked part — often points directly back to whether the problem is in the material, in the machine's settings or in the mould itself. Being able to read that story is one of the most important skills you develop as a plastic moulder, because a machine in full operation repeats the same error again and again until someone fixes the cause.
§The most common error types
| Error type | How it looks | Typical cause |
|---|---|---|
| Sink hole / indentation | Small pit or dent in the surface, often over thick areas | For too little follow-up pressure too fast cooling or uneven wall thickness |
| Flash (burr) | Thin plastic edge along the dividing line | For low closing force worn tools or too high spray pressure |
| Warpage (skew twisting) | The part is bent or twisted after molding | Uneven cooling or different shrinkage in the material. |
| Weldline (flow line) | Visible, faint line where two melt fronts meet | For low melting temperature or incorrect inlet placement |
| Burnt stripes | Dark or black stripes, often at the last filled area | Indicators for scoring |
| Underfilling (short shot). | The part is not completely filled | For too little material too low pressure or too cold melt |
§Sink holes and indentations
Sink holes occur because plastic takes up less space when it cools and shrinks. In thin items, the shrinkage rarely causes problems, but in thick sections — for example at ribs or reinforcements — the inner core may still be fluid while the surface has already solidified. The result is a small hole or dent where the material has pulled in from inside. The solution is usually to extend the holding time so more material is pressed in while the part is still cooling, or to redesign the wall thickness so it's more uniform.
§Flash — when plastic escapes from the mold
Flash is the thin unwanted plastic edge that is pressed out along the parting line where the two mold halves meet. This typically happens because the closing force is too low relative to the injection pressure because the mold is worn or doesn't close completely tight or because the injection pressure is simply set too high for the current mold. Flash is not just a cosmetic problem — it can also mean the mold takes damage over time if it continues to be pressed unevenly.
§Warpage and internal stress
Warpage — the workpiece twisting or bending after it comes out of the mould — is often related to different parts of the workpiece cooling and shrinking at different rates. If there are fibres in the material, they can also settle in a specific direction during filling, giving different shrinkage along and across. Uniform, controlled cooling of the entire mould is the most important line of defence against warpage.
Weldlines and burnt stripes
When the melt fills a mold with multiple inlets, or when it must run around a hole or core, two melt fronts meet somewhere in the mold. That meeting often leaves a weak, visible line—a weldline—because the two fronts don't melt fully together. Burn marks arise for a different reason: air that can't escape the mold in time gets compressed so fast that it literally ignites the material locally, called the diesel effect. Better venting in the mold is often the solution.
- 01Always check the material's dryness and storage first — moisture causes several different error types at the same time.
- 02Change only one setting at a time (temperature, pressure, or time), so you can see what actually worked.
- 03Note cycle time and settings when a batch runs well – it is your reference next time.
- 04Is the error sudden after a break, so suspect first heating and material humidity before changing the mould.
“An error repeated a thousand times an hour is not bad luck — it is a cause waiting to be found.”
— Professional rule of thumb in plastic production.