Flash is a thin, unwanted fin or skin of excess material on a molded or cast part, formed when molten plastic, rubber or metal is forced into a gap in the mold. It appears most often along the parting line and around ejector pins, and it is either trimmed off or prevented at the tool.
In short: flash is material that escaped the cavity through a gap it should not have found.
What Causes Flash?
A mold is two or more pieces of steel or aluminum clamped together while plastic is injected under high pressure. Anywhere those pieces meet, at the parting line, around ejector pins and their holes, at slides and inserts, there is a potential gap. Flash forms when the melt pressure pushes plastic into that gap.
There are three broad causes:
- The mold does not seal. Worn or damaged parting surfaces, poorly fitted shut-offs and loose ejector pins leave gaps the melt can enter.
- The clamp is not strong enough. If the clamping force is lower than the force the injection pressure exerts across the projected area of the part, the mold halves are pushed slightly apart.
- The melt is too aggressive. Excessive injection pressure or speed, or a melt that is too hot and runny, finds gaps a normal shot would not.
In everyday usage "flash" has other meanings, in photography, software and memory chips, but in manufacturing it always means this excess material. It is not the same as the parting line itself: the parting line is where the mold splits and every part has one; flash is the defect that can form along it.
Flash vs Short Shot
Flash and short shot are opposite ends of the same process window, which is why fixing one carelessly can cause the other.
| Flash | Short shot | |
|---|---|---|
| What happens | Too much material escapes the cavity | Too little material reaches the end of the cavity |
| How it looks | A thin fin or skin along a seam or pin | An incomplete part, missing an edge or feature |
| Typical process cause | Injection pressure too high, clamping force too low | Shot volume or injection pressure too low, melt freezing too early |
| Typical mold cause | Worn or poorly fitted parting surfaces, pins and holes | Blocked or missing vents, poor gate position |
| Usual fix | Improve mold fit, increase clamp force, slow injection | Raise pressure or shot size, add venting, move the gate |
How Is Flash Prevented and Removed?
Prevention starts with the tool: accurately machined, well-fitted parting surfaces and pins, and a press with enough clamping force for the part's projected area. On the process side, injection speed and pressure are set high enough to fill the part but no higher.
In some processes a little flash is expected and removed afterwards. In compression molding, flash is taken off after demolding by trimming, tumbling or cryogenic deflashing. Blow molding produces pinch-off flash that is trimmed from each part. LSR molding can avoid it at the source with self-sealing tooling, as NICE Rapid's LSR molding service does.
Frequently Asked Questions
What is flash in injection molding?
It is a thin layer of excess plastic that forms on a molded part where molten material leaks into a gap in the mold, usually at the parting line or around ejector pins. It is caused by poor mold fit, too little clamping force or too much injection pressure.
What does flash mean in manufacturing?
In molding and casting, flash means the thin excess material that squeezes out between mold components and solidifies on the part. It is trimmed off after molding or prevented with a well-fitted mold and correct process settings.
How do you get rid of flash?
On the part, flash is removed by trimming, tumbling or cryogenic deflashing. To stop it forming, the mold's parting surfaces and pins are refitted or repaired, clamping force is increased, and injection pressure or speed is reduced.
Related Services at NICE Rapid
- Plastic Injection Molding — molds CNC machined in-house, with first-article trials (T1, T2) before production.
- LSR Molding — self-sealing cavity geometry that eliminates flash without secondary deflashing.
- Request a Quote — share your model and note edges where flash or a trim mark is not acceptable.
Sources
- MIT OpenCourseWare, 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — flashes "develop at the mold parting line or ejector pin installation point"; causes: poor mold quality, injection pressure too high or clamping force too weak; solutions: slow injection, increase clamping force, improve parting line, ejector pin and hole surfaces; short shot causes and solutions — ocw.mit.edu
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