Flash
Plastic escapes through the parting line or around the ejector pins.
Symptoms
- Thin plastic film along the parting line
- Ring-shaped flash around ejector pins
- Part weight higher than normal
Tip: Flash is more often caused by the V/P changeover and holding phase than by insufficient clamping force.
Order of solutions
- Clean the mold and check the parting surface with blue marking compound
- Move the V/P changeover earlier; reduce holding pressure and time
- Reduce melt temperature and speed
- Calculate the required clamping force and compare it with the machine capacity
- Check the vent depth
- Have the toolmaker check the mold surface and the platen parallelism
Likely causes
Process setting
- Holding pressure too high or holding time too long
- V/P changeover too late; the cavity is overfilled and produces a pressure peak
- Melt temperature too high, viscosity too low
- Injection speed too high
Machine
- Clamping force insufficient or set incorrectly
- Platen parallelism out of tolerance
- Tie bars or toggle mechanism worn
Mold
- Parting surface worn, crushed or dirty
- Vent channel too deep
- Foreign matter on the mold contact surfaces
Part design
- Projected area large compared with the machine clamping force
- Thin sharp edges on the parting surface
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Related articles
Why does flash occur? A checklist from clamping force to mold wear
Plastic flowing out from the parting line is often not simply a matter of "not enough tonnage." How to tell the causes apart.
CalculationHow is clamping force calculated? Step by step, with an example
Finding the required clamping force from projected area and cavity pressure, safety margin and common mistakes.
Other defects
Short shot (incomplete fill)
The cavity does not fill completely; the farthest or thinnest area of the part stays incomplete.
Sink mark
A dent on the surface opposite a thick section; the surface is drawn inwards.
Void (internal vacuum void)
Internal cavity in a thick section of the part; the outer surface may look perfect.
Weld line (knit line)
A line and a weak zone where two melt fronts meet.