Sink marks in injection molding are one of the most common surface defects in plastic parts. They appear as small dents or depressions, usually on the surface above thick sections or ribs.

In many cases, sink marks in injection molding do not appear immediately after ejection. Instead, they develop during cooling as the internal material continues to shrink.

Recently, we worked on a structural housing project for a European customer. The part looked perfect right after molding. However, after full cooling, sink marks appeared above rib areas.

This is a typical example of how sink marks in injection molding are related to uneven shrinkage and cooling behavior.

Understanding how to fix sink marks in injection molding is essential for improving product quality and reducing scrap rate.

What Causes Sink Marks in Injection Molding?

Sink marks in injection molding are mainly caused by uneven shrinkage during the cooling stage.

When the inner material shrinks more than the outer surface, the surface is pulled inward, creating visible defects.

Uneven Wall Thickness (Main Cause)

Uneven wall thickness is the most common cause of sink marks in injection molding.

Thicker areas cool more slowly than thin areas. As a result, the internal material continues shrinking while the outer surface has already solidified.

This imbalance creates sink marks on the surface.

To reduce sink marks in injection molding, designers should always keep wall thickness as uniform as possible.

Improper Rib Design

Ribs are necessary to strengthen plastic parts, but poor rib design often leads to sink marks in injection molding.

If rib thickness is too large, shrinkage becomes uneven between rib and wall areas.

This causes surface dents directly above the rib.

A proper rib design should follow engineering rules, such as keeping rib thickness around 50–60% of wall thickness.

Material Shrinkage Behavior

Different materials show different shrinkage rates in injection molding.

Common materials include:

  • PP (Polypropylene)
  • PE (Polyethylene)
  • PA (Nylon)
  • POM

These materials tend to have higher shrinkage, which increases the risk of sink marks in injection molding.

Engineering plastics like ABS and PC are more dimensionally stable and help reduce defects.

Low Packing Pressure

Packing pressure plays an important role in preventing sink marks in injection molding.

If packing pressure is too low, the cavity is not fully compensated during shrinkage.

As a result, internal shrinkage pulls the surface inward and creates sink marks.

Poor Cooling Design

Cooling imbalance is another key factor that leads to sink marks in injection molding.

If some areas cool faster than others, shrinkage becomes uneven across the part.

This is especially critical in thick sections and rib structures.

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https://www.fentormold.com/

How to Fix Sink Marks in Injection Molding

There are several effective ways to fix sink marks in injection molding, including design improvement, material selection, and process optimization.

Improve Part Design

Good design is the first step to reduce sink marks in injection molding.

Key principles include:

  • Maintain uniform wall thickness
  • Avoid overly thick ribs
  • Use smooth transitions instead of sharp thickness changes

Better design helps reduce shrinkage imbalance from the beginning.

Optimize Packing Pressure

Packing pressure must be carefully controlled in injection molding.

If pressure is too low, sink marks will appear.

If pressure is too high, internal stress may increase.

A balanced setting helps stabilize part quality and reduce defects.

Improve Cooling System

A well-designed cooling system reduces temperature differences inside the mold.

This helps ensure uniform shrinkage during cooling.

As a result, sink marks in injection molding can be significantly reduced.

Select the Right Material

Material selection has a direct impact on sink marks in injection molding.

Engineers should consider:

  • Shrinkage rate
  • Flow behavior
  • Fiber reinforcement

Choosing the right material improves dimensional stability and reduces defects.

Use Mold Flow Analysis

Mold flow simulation helps predict sink marks in injection molding before production.

It allows engineers to optimize:

  • Wall thickness
  • Rib design
  • Gate location
  • Cooling system

According to Autodesk Moldflow, sink marks are strongly related to uneven shrinkage behavior:
https://www.autodesk.com/products/moldflow

A Real Project Example

We recently worked on an industrial housing project with sink marks in injection molding issues.

The part included several rib structures on the backside.

During production, sink marks appeared on the outer surface above ribs.

After analysis, we identified two main causes:

  • Rib thickness was too large
  • Packing pressure was too low

We fixed the issue by optimizing rib design and adjusting process parameters.

You can learn more about our injection molding capability here:
https://fentormold.com/Injection-Mold/

After optimization, sink marks were eliminated within acceptable tolerance.

Common Engineering Mistakes

Many sink marks in injection molding problems come from simple mistakes:

  • Over-thick rib design
  • Uneven wall thickness
  • Low packing pressure
  • Poor cooling layout
  • Skipping mold flow analysis

Avoiding these mistakes significantly improves product quality.

FAQ

Why do sink marks in injection molding appear?

Because internal material shrinks more than the outer surface during cooling.

How to prevent sink marks in injection molding?

By improving part design, cooling system, and packing pressure control.

Are sink marks in injection molding related to material?

Yes. Different materials have different shrinkage rates.

Can mold design fix sink marks in injection molding?

Yes. Proper wall thickness and rib design are critical.

Final Thoughts

Sink marks in injection molding are a common but controllable defect.

They are mainly caused by uneven shrinkage during cooling.

With proper design, material selection, and process optimization, sink marks in injection molding can be significantly reduced.

Better engineering leads to better product quality and more stable production