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What causes sink marks in injection moulding?
Sink marks are caused by thick sections, usually where a rib or boss meets the wall, that keep shrinking after the gate has frozen and can no longer be packed with more plastic, as Komaspec's defects guide explains.
A sink mark is a shallow dip in the surface, typically on the face opposite a rib, boss or other thick spot. It is often easiest to see on glossy or light-coloured parts under raking light. For a first part, sink is mostly a design question, and it is far cheaper to solve on the CAD model than on the mould.
How a sink mark forms
Plastic shrinks as it cools. The outer skin against the mould freezes first, while the core of a thick section stays molten longer. During moulding the press keeps pushing in extra plastic (the packing or holding phase) to make up for that shrinkage, but only until the gate freezes. Komaspec describes what happens next: the still-shrinking core of the thick section either pulls the surface inward, which is a sink mark, or tears apart inside, which is a void. It treats the two as the same defect showing in opposite directions.
That is why sink marks appear over ribs, bosses and thick sections, which Komaspec's defects table lists as the typical locations. Wherever a rib joins a wall, the junction is thicker than either the rib or the wall alone, so it cools last.
Design causes and fixes
The durable fix is in the geometry. Komaspec's defects guide says the lasting cure is to keep rib bases at 50 to 60 % of the wall they attach to and to core out thick features so the wall stays uniform.
| Cause | Design fix |
|---|---|
| Rib too thick at the base | Make the rib base thinner than the wall (see the table below) |
| Solid boss or thick pad | Core it out so its wall stays close to the nominal wall |
| Thick section kept for stiffness | Core it out from the back and add ribs to restore stiffness |
| Abrupt change in wall thickness | Taper the transition gradually |
| Boss merged into a side wall | Separate it and connect it with a rib |
On the last row, Komaspec's design guide says plainly never to fuse a boss directly into a side wall.
Published rib proportions
Several design references give starting points for ribs. Their figures overlap but are not identical.
| Rule | Figure | Source |
|---|---|---|
| Rib base thickness | 50 to 60 % of the adjoining wall | Wise Plastics; Komaspec defects guide |
| Rib base thickness | 40 to 60 % of the nominal wall | Komaspec design guide; Wikipedia |
| Rib thickness | 50 to 70 % of the nominal wall | Aprios |
| Ribs, bosses and gussets | about 40 to 60 % of the adjacent wall | Protolabs |
| Semi-crystalline resins | 40 to 50 % of the wall | Komaspec design guide |
| Amorphous resins (ABS, PC, PC/ABS) | up to 60 % of the wall | Komaspec design guide |
| Rib height | no more than about 3 times the nominal wall | Wise Plastics; Aprios |
| Rib height | 2.5 to 3 times the nominal wall | Wikipedia |
| Rib spacing | at least 2 times the wall | Wise Plastics; Komaspec; Wikipedia |
| Rib base radius | about 25 % of the wall | Wise Plastics |
| Rib draft | 0.5 to 1° per side | Wise Plastics |
Note that rib height is measured against the nominal wall thickness, not against the rib's own thickness. Komaspec's design guide is the exception: it states its 3 times height limit against the rib thickness, which gives a much shorter rib. Wise Plastics, Aprios and Wikipedia all measure against the wall, so this article follows them; agree the limit with your toolmaker. Komaspec says semi-crystalline resins such as polypropylene, polyethylene, nylon, acetal and PBT shrink more, which is why it asks for thinner ribs on them; it allows up to 60 % for amorphous resins such as ABS and polycarbonate. Komaspec's design guide also explains the mechanism: if the rib base goes above 60 %, the extra material cools after the visible surface has already set and leaves a sink.
The sources disagree on the upper limit for rib thickness (60 % versus 70 %), so treat these as ranges to agree with the toolmaker, not fixed limits.
Bosses
Bosses for screws and inserts are a classic sink location because they put a cylinder of plastic onto the wall. Published guidance:
- Komaspec: keep the boss wall near 60 % of the nominal wall, and size the boss outside diameter at roughly 2 to 2.4 times the screw's major diameter.
- Wikipedia: keep the boss wall at about 0.6 times the nominal wall, or less, to limit sink; it notes a thicker wall may be accepted on hidden areas that take self-tapping screws.
- Protolabs: bosses should have walls of about 40 to 60 % of the surrounding area to avoid sink.
For the screw itself, follow the fastener maker's boss recommendation for your resin.
What the moulder can do with the process
Packing harder and longer can hide a sink mark, and Komaspec says packing pressure and time can help. It also warns that this does not fix the underlying defect: it adds cycle time and moulded-in stress, and can leave a process window so narrow that it fails when the shop temperature changes. Komaspec also notes that sink marks cannot be fixed after moulding. Process changes are a fair way to tune a borderline part, but they are not a substitute for correct wall and rib proportions.
Other levers the toolmaker may raise, such as gate size and position or cooling near thick areas, depend on the specific mould; ask for them to be reviewed in the design stage.
A step-by-step sink check before cutting steel
- Identify the cosmetic faces: where would a dip be unacceptable?
- Find every rib, boss and thick pad behind those faces.
- Check each rib base against the percentage for your resin family; use the lower end for semi-crystalline resins.
- Check rib height against the nominal wall, and spacing between ribs.
- Core out solid bosses and thick sections; replace lost stiffness with ribs.
- Taper any changes in wall thickness instead of stepping them.
- For a part that must look perfect on the opposite face, ask the toolmaker whether a mould flow analysis is worthwhile before steel is cut.
These proportions are planning guidance from published design guides. The resin supplier's data and the toolmaker's review should confirm them for your grade and geometry.
Questions buyers ask
Can sink marks be fixed after the mould is made?
The process can sometimes reduce them, but Komaspec warns that packing harder and longer leaves a fragile process and extra stress. A lasting fix usually means changing steel, for example thinning a rib, which is harder once the mould is built. That is why it pays to check the design first.
Will a texture hide sink marks?
A texture can make a shallow sink less noticeable than a high polish does, but it does not remove it. Texture also needs more draft, which is covered in the article on how much draft a part needs. Check the finish and the rib proportions together.
Why is my part sinking even though the wall is uniform?
Look at the junctions. Even with a uniform wall, a rib or boss that is too thick at its base creates a thick spot where it joins. Komaspec's guide also ties sink over thick bosses to packing that ends before the core has finished shrinking.
Do some plastics sink more than others?
Yes. Komaspec groups polypropylene, polyethylene, nylon, acetal and PBT as semi-crystalline resins that shrink more, and recommends thinner ribs (40 to 50 % of the wall) for them. Amorphous resins like ABS and polycarbonate shrink less, so slightly thicker ribs are acceptable.
Sources
- Komaspec, Injection molding defects: https://www.komaspec.com/about-us/blog/injection-molding-defects/
- Komaspec, Plastic part design for injection molding: https://www.komaspec.com/about-us/blog/plastic-part-design-for-injection-molding/
- Wise Plastics, DFM 101: wall thickness and ribs: https://wiseplastics.com/blog/dfm-101-wall-thickness-and-ribs
- Aprios, Optimizing wall thickness in injection molding for part success: https://www.aprios.com/insights/optimizing-wall-thickness-in-injection-molding-for-part-success
- Protolabs, Improving part design with uniform wall thickness: https://www.protolabs.com/resources/design-tips/improving-part-design-with-uniform-wall-thickness/
- Wikipedia, Design of plastic components: https://en.wikipedia.org/wiki/Design_of_plastic_components