DefinitionSink marks are shallow depressions on the surface of a molded part, usually directly opposite a thick section such as a rib, boss or wall junction.
Explained
What Are Sink Marks?
Sink marks are shallow depressions on the surface of a molded part, usually directly opposite a thick section such as a rib, boss or wall junction. They form when the thicker material inside keeps shrinking after the surface has cooled, pulling that surface inward.
In short: a sink mark is the visible dent left where a thick section shrank more than the wall around it.
What Causes Sink Marks in Injection Molding?
All plastics shrink as they cool from melt to solid. In a part of even thickness that shrinkage is uniform and the mold is sized to allow for it. Where the part is locally thicker, the core of that section stays molten longer. As it finally cools and contracts, it draws the already-set skin down with it, and a dent appears on the surface.
That is why sink marks cluster in predictable places: opposite ribs, at the base of bosses, at thick corners and wherever the wall thickness jumps. One US patent on molded thermoplastic articles describes the rib-to-wall thickness ratio as one of the largest contributing factors, with sink forming once a rib exceeds about 25% of the wall thickness in thermoplastic olefins and about 40% in many other plastics.
Process settings play a part as well. Packing pressure and packing time push extra material into the cavity to make up for shrinkage; if either is too low, or the gate freezes before the thick section is packed, sink follows. A runner that is too small can starve the part in the same way.
Sink Marks vs Voids
Both come from the same shrinkage in thick sections; what differs is where it ends up.
| Sink mark | Void | |
|---|---|---|
| Where it appears | On the surface, as a dent | Inside the part, as an internal cavity |
| When it forms | The skin is still soft enough to be pulled inward | The skin has set rigid, so the shrinkage opens a hollow inside instead |
| How it is found | Visual inspection, especially under raking light | Sectioning, X-ray or a weak spot under load |
| Main concern | Cosmetic, on show surfaces | Strength and sealing |
Fixing one by process alone can simply turn it into the other, which is why the lasting fix is usually in the geometry.
How to Prevent Sink Marks
- Keep the nominal wall uniform and core out thick areas such as solid bosses.
- Design ribs noticeably thinner than the wall they sit on.
- Place the gate at, or feed toward, the thickest section so it can be packed before the gate freezes.
- Tune packing pressure, packing time and cooling so the thick section is fed as it shrinks.
- On cosmetic surfaces, a texture helps hide slight sink that cannot be designed out.
Frequently Asked Questions
What are sink marks in injection molding? They are shallow dents on the surface of a molded plastic part, typically opposite a rib, boss or other thick section. They appear because the thick section keeps shrinking after the surface has set, pulling the surface inward.
What causes sink marks? The main cause is uneven wall thickness: thick sections cool more slowly and shrink more than the surrounding wall. Too little packing pressure or packing time, a gate that freezes too early, or an undersized runner makes them worse.
How do you fix sink marks? The lasting fix is design: keep walls uniform, core out thick areas and make ribs thinner than the wall. On the molding side, increasing packing pressure and time, gating into the thick section and adjusting cooling can reduce sink on an existing tool.
Key takeaways
- Sink marks are shallow surface dents that typically appear opposite ribs, bosses or wall junctions.
- They are caused by thicker sections cooling and shrinking more than the surrounding wall after the surface has set.
- Rib-to-wall thickness ratios above about 25% for thermoplastic olefins or 40% for many other plastics promote sink marks.
- Insufficient packing pressure or time, early gate freeze and undersized runners can make sink marks worse.
- Sink marks form on the surface when the skin is still soft; voids form inside when the skin is rigid.
- The lasting fix is usually geometric: uniform walls, cored-out thick sections, thinner ribs and proper gating.
Related terms
References
- US Patent 7,846,533 — patents.google.com
- 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — MIT OpenCourseWare
- Mitigation of Sink Voids in Thick-Walled Thermoplastic Components via Integrated Taguchi DOE and CAE Simulations — Polymers (PMC)
- European Patent Application EP0495614A2 — patents.google.com