Flash – also called burr or spew – is the thin fin of plastic that escapes from the cavity at the parting line, around ejector pins, or through vents. A little flash is normal and gets trimmed. A lot of flash means something is wrong, and if it keeps growing the tool is being damaged every shot.
The physics: clamp force vs cavity pressure
Flash appears when the separating force from the melt inside the cavity exceeds the clamp force holding the tool shut. The separating force is the projected area of the part (including runners) multiplied by the average cavity pressure.
Rule of thumb for required clamp force:
| Material | Flow type | Cavity pressure (bar) | Typical clamp (t) per 100 cm2 |
|---|---|---|---|
| PP / HDPE | Easy flow | 250-350 | 2.5-3.5 |
| ABS | Medium | 300-400 | 3.0-4.0 |
| PC | Stiff flow | 400-600 | 4.0-6.0 |
| PA6 + 30% GF | Stiff flow | 400-600 | 4.0-6.0 |
If you are running a part on a machine at 90% of its rated clamp force and see flash at the parting line, the first question is whether you are simply under-clamped. See how to calculate clamp tonnage for the full method.
Causes, most likely first
| Symptom | Cause | Fix |
|---|---|---|
| Flash along whole parting line | Clamp force too low | Move to a larger machine, or reduce projected area |
| Flash on one side only | Tool not parallel / platen deflecting | Check tool seating and machine platen parallelism |
| Flash at vents | Vent too deep | Re-machine vent to 0.02-0.03 mm (polyolefin) |
| Flash around ejector pins | Pin clearance worn | Replace pins / re-fit bushings |
| Flash appears after long run | Parting line wear, or debris on the line | Clean and inspect; re-grind the parting line if scored |
| Flash only at high speed | Pressure spike at end of fill | Profile the screw; slow the last 10% |
| Flash after material change | Lower viscosity resin | Re-check clamp requirement for the new grade |
Process fixes before you touch the tool
- Reduce injection speed at the end of fill – most flash comes from a pressure spike in the last few percent, not from steady pressure
- Reduce holding pressure – if flash appears during hold, you are over-packing
- Reduce melt temperature – hotter melt is thinner and escapes more easily; 10-15 C can be enough
- Check the switchover point – switching from injection to holding too late overfills the cavity
- Increase clamp force – if the machine has headroom
Note the trade-off: several of these (lower speed, lower temperature) push you toward weld lines and short shots. Change one variable at a time and re-check the part, or you will chase the defect around the machine.
When the tool is the problem
Flash that appears suddenly on a tool that has run cleanly for a long time is almost always mechanical:
- Debris on the parting line – a small piece of solidified plastic holds the tool open a few hundredths. Clean it before anything else; this costs nothing and fixes most sudden flash.
- Worn parting line – repeated closing wears the land. Needs re-grinding and re-fitting.
- Platen or tie-bar stretch – on an older machine the platens deflect under load, opening the tool at the centre. Check with a dial indicator at full clamp.
- Cracked cavity – a hairline crack grows with every shot and will eventually fail. Stop and repair.
What is acceptable
Zero flash is not a realistic specification. Working practice:
- Flash thickness up to 0.05 mm on non-mating surfaces: usually accepted as-is
- Flash on a mating or sealing face: not acceptable – it changes the stack height
- Flash that requires manual trimming: cost it, because trimming is labour and it is a hidden cost per part
If your supplier quotes a part and the price assumes no deflashing, ask how long the operator takes per part. On a 10,000-piece order, 5 seconds of trimming per part is 14 hours of labour – that belongs in the unit price, not in a surprise.
Related: injection moulding hub, cost per part calculator.
Related guide: knit lines and weld lines in injection molding.
Flash on a part you are sourcing? Send a photo and the part dimensions and we will tell you whether it is clamp force, venting or tool wear.
