What Causes Sink Marks in Injection Molding and How to Reduce Them

Sink marks are a common surface defect in injection molded plastic parts. They usually appear as slight depressions or shallow marks on the product surface, often in areas with thicker wall sections, ribs, bosses, or uneven cooling. Although sink marks may look like a simple appearance issue, they are often related to part design, material shrinkage behavior, mold structure, gate design, and processing conditions.

In actual production, sink marks may be caused by one or more factors working together. These factors can include processing methods, part geometry, material selection, and mold design. Some conditions, such as the basic material choice or product structure, may be difficult to change once the project is already defined. However, there are still many practical ways to reduce sink marks through mold optimization and process adjustment.

Main Causes of Sink Marks in Injection Molding

Sink marks are generally caused by insufficient compensation of material shrinkage during cooling. As the molten plastic cools and contracts inside the cavity, the surface may collapse slightly if there is not enough packing pressure or enough material feeding into the part during the holding phase. This is especially common in thicker sections where the interior cools more slowly than the surface.

Part geometry is one of the key factors. If the wall thickness is uneven, or if ribs, bosses, and reinforced sections are too thick compared with the surrounding wall, the shrinkage behavior becomes less uniform and sink marks become more likely. Material type also plays a role because different plastics have different shrinkage rates and flow behavior.

How Mold Design Affects Sink Marks

From the mold manufacturing point of view, mold design has a strong influence on shrink mark formation. Cooling channel layout, runner design, gate type, and gate size can all affect how the material fills the cavity and how shrinkage is compensated during packing and cooling.

For example, smaller gates such as pin or tunnel-type gates may cool and freeze more quickly than larger tapered gates. If the gate freezes too early, the cavity can no longer receive additional material during the holding stage. This reduces shrinkage compensation and increases the chance of sink marks. In contrast, a better gate design can help keep the material flow path open longer and improve packing effectiveness.

How Processing Conditions Influence Shrinkage

Processing conditions are another important factor in sink mark control. Filling pressure, holding pressure, and holding time all affect how much additional material enters the cavity after the initial filling stage. Once the cavity is filled, extra material is still needed to compensate for volume shrinkage during cooling.

If the packing pressure is too low, or if the holding time is too short, the cavity will not receive enough compensation material. As a result, shrinkage increases and sink marks may become larger or more visible. Increasing or optimizing these conditions can often improve the situation, although process adjustment alone may not always eliminate sink marks completely.

Common Methods to Reduce Sink Marks

• Optimize Part Geometry
  Reduce excessive wall thickness differences and avoid overly thick ribs, bosses, or local sections where possible.
• Improve Gate Design
  Select a suitable gate type and gate size so that the cavity can be packed effectively before the gate freezes.
• Optimize Cooling Design
  Improve cooling channel layout to promote more uniform cooling and reduce localized shrinkage differences.
• Adjust Packing Conditions
  Set appropriate holding pressure and holding time to improve shrinkage compensation after cavity filling.
• Review Material Selection
  Different materials have different shrinkage characteristics, so material behavior should be considered during product and mold development.

Why Experience Matters in Solving Sink Marks

In many cases, sink marks cannot be solved by a single adjustment. A satisfactory result often requires combined evaluation of product design, mold structure, gate location, cooling efficiency, and molding parameters. Experienced mold makers and injection molding suppliers can often reduce shrinkage defects significantly by adjusting both tooling and process conditions together.

Even if sink marks cannot be completely removed in the first trial, practical engineering improvements can often reduce them to an acceptable level for appearance and function.

Injection Mold and Production Support from FITMOLD

FITMOLD supports custom injection mold development and plastic part production for projects that require practical mold optimization, better manufacturability, and stable production quality. For products with shrinkage risk, we help evaluate part structure, gate design, cooling layout, and molding conditions in order to reduce surface defects and improve production consistency.

If you are looking for a manufacturing partner for custom injection molds and plastic part production, FITMOLD can support your project from design review to tooling and production.

Contact us: sales@fitmold.com