Brittleness / low impact strength
The part breaks more easily than expected; sometimes the surface looks fine.
Symptoms
- Breaks during assembly or when dropped
- Fracture surface smooth and glossy
- Low strength despite the right grade
Tip: Hydrolysis damage is not visible to the eye; if in doubt, have the viscosity (MFI) measured.
Order of solutions
- Check the drying conditions and the residence time
- Limit the regrind ratio; use it in a controlled way
- Optimize the melt temperature
- Add radii at the corners and move the weld line away from the critical area
- If necessary, talk to the grade supplier/manufacturer
Likely causes
Process setting
- Excessive melt temperature or long residence time
- Internal stress due to low melt/mold temperature
- Weld line area
Raw material
- Drop in molecular weight due to moisture (PC, PET, PA, PBT)
- High regrind ratio or repeatedly processed material
- Wrong grade
Part design
- Sharp corners (stress concentration)
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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.