A sink mark is a local depression on the surface of a molded part, and it usually points to something specific underneath: a thick section that cooled more slowly than the material around it. Reading the position of the mark is often enough to identify the cause.
What a sink mark is, and why it appears
Plastic shrinks as it cools, and thicker regions cool more slowly than thin ones. While the outer skin has already solidified, the thicker interior is still contracting, and that contraction pulls the surface inwards. The result is a shallow dish on the visible face, most often opposite a rib, a boss, a mounting point or a thick section. It is a shrinkage effect rather than a moulding defect in the sense of a scratch or a burn, which is why it is fixed by changing geometry or process rather than by polishing the tool.
| Where the mark appears | Usual cause | First countermeasure |
|---|---|---|
| Opposite a rib | Rib thicker than the nominal wall | Reduce rib thickness to around half the wall |
| Opposite a boss | Local mass of material | Thin the boss wall; support it with ribs instead of a solid core |
| Near the gate | Packing pressure or gate size | Adjust gate dimensions and holding pressure profile |
| Across a whole face | Wall thickness variation | Redesign to a more uniform thickness |
How to solve sink marks in a running tool
Process adjustments come first because they cost nothing. Lengthening the hold time and adjusting the holding pressure profile lets more material pack into the cavity as the part cools, which compensates for some of the shrinkage. Raising the melt temperature improves flow but also increases shrinkage, so it is usually a balance rather than a fix. Increasing the gate size helps where the gate froze before packing finished, but it is a tool modification.
Where the process cannot solve it, the tool is next. Enlarging the gate and runner improves packing; adding local cooling to the thick region speeds its solidification so it shrinks less; and moving the gate so it fills the thin sections first can reduce the depression. Each change has a trade-off: stronger cooling in one area changes the shrinkage balance elsewhere, which can introduce warpage that was not previously a problem.
Design changes that remove the cause
The most reliable fix is geometric, because it addresses the reason the part shrinks unevenly. Ribs should be thinner than the wall they stiffen, commonly around half its thickness, so that the rib cools at a similar rate to the surface. Bosses should be hollow with a thin wall rather than solid, and where a boss meets a wall the junction can be relieved so material does not accumulate at the intersection. Stepping a thick section down gradually is better than changing thickness abruptly.
Two further details help. A textured surface hides shallow sink marks effectively, which is why a part with a matte or grained finish often looks acceptable while the same geometry in a polished finish does not. And where appearance is critical, the design can shift the thick features to the non-visible face, so the mark forms where it will not be seen. Material property references, including shrinkage behaviour, are published by ASM International.
How do ejector pin marks differ from sink marks?
Raised or impressed at one point, not a gradual dish.
An ejector pin mark appears where the pin pushed the part out of the tool, so it is localised, often circular, and sits on the face that the pin contacted. A sink mark is a diffuse depression caused by shrinkage, usually on the opposite face from the feature that caused it. The two are frequently confused on inspection reports, and they have entirely different remedies: an ejector mark is a tool and process issue, while a sink mark is usually a geometry issue.
Two other surface features are worth distinguishing at the same time. A flow mark appears as a ripple near the gate and reflects how the melt filled the cavity, while a weld line appears where two flow fronts met and is a line rather than a depression. Naming the defect precisely in an inspection report is what allows the right countermeasure to be applied on the first attempt rather than the third.
How to brief a molder about a sink problem
The useful information is the position of the mark, the face it appears on, whether it follows a rib or boss, the material, and whether the part already runs in production or is being sampled. A photograph with a scale helps more than a description, and a sample part with the mark circled removes ambiguity entirely. Where the part has a cosmetic specification, saying so is essential, because a mark acceptable on an internal face may be a reject on a visible one.
The response to expect is a diagnosis separating the geometric cause from the process contribution, followed by a recommendation that states its trade-off. Any fix applied to the tool changes something else, so a molder who can explain what a change will do to warpage, cycle time or gate vestige is more useful than one who promises the mark will disappear. Process practice is described by the NIST Manufacturing Extension Partnership, and inspection reporting conventions follow ASME standards, with material handling obligations set out by the US EPA. Related defect discussion sits under injection molding and prototype injection molding.
Where the defect persists after both geometry and process are addressed, an acceptable fallback is to texture the affected face, since a grained surface hides shallow depressions that a polished face reveals. In inspection terms this must be agreed in writing rather than assumed, because it changes the cosmetic specification, and the appearance criteria for a textured surface are usually agreed against a physical sample rather than a written description, following the surface-condition terminology published by ASTM Committee B08.
Send the part model with the visible faces marked and a photograph of the defect, and request a molding review covering rib and boss geometry.
FAQ
What is a sink mark in injection molding?
A shallow surface depression caused by uneven shrinkage, where a thicker region cools and contracts after the surrounding skin has solidified. It usually forms opposite a rib, boss or other thick feature rather than randomly across the part.
How do you solve sink marks?
Start with process adjustments to holding pressure and time, then consider gate changes, and address the geometry if the mark persists. Reducing rib thickness to about half the wall and hollowing bosses removes the cause rather than compensating for it.
How do ejector pin marks differ from sink marks?
An ejector pin mark is localised and appears where the pin contacted the part, while a sink mark is a gradual depression caused by shrinkage, usually opposite a thick feature. The two have different remedies and should be named separately in inspection reports.