Injection molding sink marks are visible depressions caused when material in a thicker region shrinks after the outer skin has solidified and is not sufficiently compensated. The defect is often seen opposite ribs, bosses, internal fillets, or other heavy sections.
Sink is not solved reliably by increasing pressure alone. The correct response depends on geometry, gate freeze, material shrinkage, cooling, mold temperature, packing, and the appearance requirement.
Where sink marks come from
| Contributor | Mechanism | First check |
|---|---|---|
| Heavy wall, rib, or boss | Core material contracts after the skin freezes | Wall map and section thickness ratio |
| Early gate freeze | Packing cannot compensate for late shrinkage | Gate size, land, freeze time, and pressure trace |
| Insufficient packing or cooling | Material is not adequately compensated or solidified | Transfer, holding profile, cooling time, and part weight |
| High shrinkage or temperature | More volumetric contraction becomes visible | Grade, melt/mold temperature, and cooling balance |
Design solutions before tooling
- Core out solid bosses and heavy sections while preserving function.
- Use ribs and gussets for stiffness instead of adding mass to the main wall.
- Keep wall transitions gradual and avoid abrupt thickness changes beneath cosmetic surfaces.
- Review boss and rib thickness relative to the nominal wall and check texture shadowing on visible faces.
- Use a gate location that allows packing to reach the heavy region before the gate freezes.
Process troubleshooting sequence
- Confirm the mark is sink rather than void, blush, warpage, or texture shadowing.
- Run a short-shot and fill/pack review to locate the heavy region and gate-freeze constraint.
- Measure part weight as packing time or pressure changes, then identify when the gate stops accepting additional material.
- Check cooling-channel balance and the part ejection temperature.
- Validate appearance, dimensions, flatness, and cycle time after each controlled change.
The engineering team can evaluate geometry and gate location before steel changes. The injection molding team can establish the process window, while the mold-making team can correct cooling or insert geometry. Contact Cavity Mold with photos and a wall-thickness map.
Technical references
The Autodesk sink-mark reference explains shrinkage, packing, gate freeze, and geometry remedies. Xometry’s sink-design guide shows how ribs, bosses, and surface finish influence visible sink.