Sink mark
A dent on the surface opposite a thick section; the surface is drawn inwards.
Symptoms
- Dent on the back of a rib, boss (screw post) or thick section
- Clearly visible difference in light reflection on glossy surfaces
Tip: Change only one variable at a time so that you know which one worked.
Order of solutions
- Determine the holding time with a gate freeze study
- Increase the holding pressure in steps
- Bring the melt temperature down to the lower-middle part of the range
- Add extra cooling or a copper-alloy insert at the thick section
- In the design, reduce the rib thickness to roughly half to two thirds of the nominal wall; add radii at transitions
Likely causes
Process setting
- Holding pressure too low or holding time ends before the gate freezes
- Melt temperature too high (more shrinkage)
- Mold temperature too high, the thick section cools too slowly
- Metering/cushion insufficient
Mold
- Gate too small and freezes early
- Cooling insufficient at the thick section
Part design
- Rib/boss thickness far above half of the nominal wall
- Abrupt thickness transition
- Gate far from the thick section
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Related articles
Sink marks and voids: thick regions, ribs and holding
Two faces of the same problem: a dent on the surface or a void inside. Clues to tell them apart and the lasting design fix.
Process settingV/P changeover and holding setup: which values to start the machine with?
The path to getting injection speed, speed profile, V/P changeover, holding pressure and holding time right from the start; the "gate freeze" study.
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.
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.