Stress cracking
The part cracks during assembly, on contact with oil/solvent or over time; the crack usually starts at stressed areas.
Symptoms
- Fine cracks around the gate or screw boss
- Cracking after cleaning or contact with oil
- Whitening on the fracture surface
Tip: You can check whether there is stress in the part with a suitable solvent test (using a material-specific procedure).
Order of solutions
- Reduce the holding pressure; reduce the stress by raising the mold temperature
- Subject the part to a stress-relief treatment (annealing) at a temperature and time suitable for the material
- Make a list of chemical contacts and replace incompatible products
- Add radii at the corners and review the thickness and ratio at the screw bosses
Likely causes
Process setting
- High internal stress: excessive holding, low mold temperature
- High speed and shear
- Stress build-up around the gate
Raw material
- Amorphous materials (PC, ABS, PS) are sensitive to chemicals
- Incompatible oil, solvent, cleaner
Part design
- Sharp corners, screw bosses
- Press-fit (interference fit) stress
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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.