What Is a 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.

Sink-prone features are flagged during the DFM review that starts every plastic injection molding project, before the mold is cut.

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.

Related Services at NICE Rapid

• Plastic Injection Molding — DFM review first, then T1 and T2 mold trials before pilot production, so sink is caught before volume parts ship.
• Rapid Tooling — aluminum bridge tooling to prove out rib and boss geometry on real molded parts before production steel.
• Request a Quote — send a STEP file and mark any cosmetic faces where sink is not acceptable.

Sources

1. US Patent 7,846,533, Background — sink mark defined as a "divot or indentation in the surface of the article" created by uneven material shrinkage; projection-to-wall thickness ratio as a major factor, with sink forming above about 25% (thermoplastic olefin) or at 40% (other thermoplastics) — patents.google.com
2. MIT OpenCourseWare, 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — gate at the thickest area to avoid sink marks from part shrinkage; undersized runners cause sink; "equal cooling from the surface" as the solution — ocw.mit.edu
3. Mitigation of Sink Voids in Thick-Walled Thermoplastic Components via Integrated Taguchi DOE and CAE Simulations, Polymers (PMC) — sink marks as surface defects vs sink voids as internal cavities; packing pressure, packing time, mold and melt temperature and cooling time as key shrinkage parameters — pmc.ncbi.nlm.nih.gov
4. European Patent Application EP0495614A2, Background — with rapid skin solidification, voids form in the thick wall portion instead of sink marks on the surface — patents.google.com