Injection Moulding Headaches Part 1: Sink Marks (and How to Fix Them) // Alliance Tooling Blog

What’s your biggest annoyance when it comes to injection moulded parts? If you ask a production manager or a design engineer, sink marks will usually be near the top of the list. Precision is in the details, but as we all know, so are the defects.

This is the first in Alliance Tooling’s new weekly series, where it will be breaking down common moulding annoyances and looking at the engineering reality of how to fix them. Starting with the subtle, frustrating, and often costly issue of sink marks.

The Problem: Sink Marks

Sink marks are those subtle (or sometimes very visible) depressions on the surface of a moulded part. They usually show up over thick sections or internal features like ribs.

While they might look like minor blemishes, they cause real issues:

• Ruin cosmetic finishes: A dealbreaker for high-visibility or “A-surface” parts.
• Signal internal stress: They are a visual warning of poor packing or uneven heat retention.
• Lead to rejected parts: Resulting in wasted material and unhappy customers.

Why do sink marks happen?

Sink marks are typically caused by uneven cooling and material shrinkage. Thicker areas of plastic take longer to cool; as they shrink, they pull the surface inward.

Common culprits include:

• Overly thick wall sections
• Poor rib-to-wall ratios
• Inadequate packing pressure
• Incorrect gate placement

How to design them out (the smart way)

At Alliance Tooling, we aim to solve these problems before they happen. It’s much easier to fix geometry in CAD than it is to modify hardened steel once the tool is cut.

Here’s how sink marks can often be eliminated at the design stage:

• Maintain uniform wall thickness wherever possible.
• Keep ribs to around 50–70% of the wall thickness.
• Use coring to reduce thick sections.
• Optimise gate location to improve material flow and packing.

Already in production? Here’s how to fix it

If sink marks are already showing up in your parts, there are still ways to address the issue on the shop floor:

• Increase packing pressure or time to force more material into the shrinking areas.
• Adjust mould temperature for more even cooling.
• Improve venting and flow paths so the cavity packs correctly.
• Tool modifications may be required if process adjustments aren’t enough.

Where we come in

We focus on helping you avoid these costly issues from the outset. We bring practical, real-world tooling experience to every project to solve production problems properly.

Whether you are:

• Designing a new component
• Struggling with an existing tool
• Or trying to improve part quality…

Got a part with sink issues (or want to avoid them altogether)? Book a quick call with Chris Rossell (Technical Director) here.

Next week: Flash – why it happens and how to stop it for good.

Read more news from Alliance Tooling here.

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