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.
Flash is plastic escaping from the mold's parting line or gaps. It occurs when the force trying to open the mold exceeds the force holding it closed. So the two sides need to be considered separately.
The force trying to open the mold
Multiplying the pressure inside the cavity by the part's projected area onto the parting plane gives the opening force:
Opening force = projected area × cavity pressure
Things that increase this force:
- Excessive holding pressure or too long a holding time.
- A late V/P changeover: the cavity becomes nearly full under velocity control, then spikes in pressure.
- Melt that's too hot: viscosity drops, and it flows even through the thinnest gap.
- Injection speed that's too high: creates a pressure spike inside the cavity.
The force holding the mold closed
- Is the machine's clamping tonnage enough for the opening force (plus a safety margin)? If not, you need a bigger machine or lower pressure.
- Parallelism of the mold plates and toggle adjustment: if clamping isn't distributed evenly, you get flash at one corner even if the total tonnage is sufficient.
- Parting-line wear, denting, foreign matter: a plastic chip or an oil film can leave the mold open by microns.
Vent depth
If the vent channel is too deep, the melt enters it. Roughly speaking, free-flowing materials (e.g. PA) need a very shallow channel, while viscous materials (PC, ABS) use a slightly deeper one. Get the exact values from your mold maker and the material data sheet; "let's make it a bit deeper" can bring on the flash problem.
Solution order
- Clean the mold, inspect the parting line (contact check with blue dye).
- Move the V/P changeover earlier, reduce holding pressure and time.
- Lower melt temperature and injection speed.
- Check the tonnage: calculate the required tonnage and compare it with the machine's capacity.
- If it still continues, check the mold surface and plate parallelism.
See the clamping force calculation article for how to find the required clamping tonnage. The calculation tool estimates this based on the part, cavity count and material.
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