Burn mark (dieseling)
Usually a brown-black burn at the last point to fill; trapped air heats up and burns the plastic.
Symptoms
- Dark mark at a corner, rib base or end of flow
- Gets worse when the speed is increased
- May occur together with a short shot
Tip: Confirm the diagnosis by reducing the speed first; if it gets better, the problem is venting.
Order of solutions
- Reduce the speed stage in the last-filled zone
- Clean the vent channels
- Add venting to the mold or use a porous steel insert
- Do not keep the clamping force unnecessarily high
- Reduce the melt temperature
Likely causes
Process setting
- Injection speed too high in the last zone
- Melt temperature too high
- Mold clamping force too high (closes off the venting)
Mold
- Venting insufficient or dirty
- No air escape through the ejector pin clearance
Part design
- Geometry that forms an air pocket (blind rib)
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Related articles
Other defects
Short shot (incomplete fill)
The cavity does not fill completely; the farthest or thinnest area of the part stays incomplete.
Flash
Plastic escapes through the parting line or around the ejector pins.
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.