Sink Marks in Injection Molding

Introduction

Sink marks are one of the most common cosmetic defects encountered in injection molding. They appear as shallow depressions or dimples on the surface of a molded plastic part, usually near areas where the internal section is thicker than the surrounding wall.

Although a sink mark may initially look like a surface-quality problem, its root cause is often related to part geometry, material shrinkage, cooling behavior, or molding conditions.

For engineers, understanding why sink marks occur is important because correcting them after tooling is complete can require process changes, mold modifications, or even part redesign.

This guide explains the main causes of sink marks, the design features most likely to create them, and practical strategies for reducing sink risk before production.

What Are Sink Marks in Injection Molding?

A sink mark is a localized depression that forms on the surface of an injection molded part as the plastic cools and contracts.

During molding, molten plastic fills the cavity and begins cooling from the mold surface inward. The outer material solidifies first, while thicker internal sections remain hot for longer.

As the material inside these thick sections continues to cool and shrink, it can pull the already solidified outer surface inward. The result is a visible surface depression.

Sink marks commonly occur around:

• Thick wall sections
• Ribs
• Bosses
• Mounting features
• Internal intersections
• Reinforcement structures
• Sudden wall-thickness transitions

The more concentrated the material is in one area, the greater the potential for uneven cooling and shrinkage.

Why Do Sink Marks Occur?

Sink marks are generally caused by a combination of material shrinkage, insufficient packing, and non-uniform cooling.

Several factors may contribute at the same time.

1. Excessive Wall Thickness

Thick sections take longer to cool than thin sections.

If a component contains localized areas with significantly more material, the center of those areas may continue shrinking after the exterior surface has already solidified.

This makes excessive or uneven wall thickness one of the most important sink-mark risks to address during part design.

2. Thick Ribs and Bosses

Ribs and bosses are necessary in many injection molded components, but they can create localized material buildup when they are too thick.

For example, a thick rib connected directly to a nominal wall effectively creates a much thicker cross-section at the intersection.

The same problem can occur at the base of a solid or oversized boss.

These regions cool more slowly and are common locations for visible sink marks on the opposite cosmetic surface.

3. Insufficient Packing Pressure

After the cavity fills, additional material is typically packed into the cavity to compensate for material shrinkage during cooling.

If packing pressure is too low or packing time is too short, there may not be enough material available to compensate for volumetric shrinkage.

This can increase the likelihood of sink marks, particularly in thicker sections.

4. Gate Freezing Too Early

Packing pressure can only remain effective while the gate is still open enough for material to enter the cavity.

If the gate freezes too early, additional material can no longer reach the shrinking region.

Gate size, gate location, material flow, and section thickness therefore all influence how effectively the cavity can be packed.

5. Uneven Cooling

Different regions of a molded part may cool at different rates.

Thick sections naturally cool more slowly, but mold temperature distribution and cooling-channel layout can also contribute to uneven cooling.

When one area remains hot considerably longer than surrounding areas, localized shrinkage becomes more difficult to control.

6. Material Shrinkage Characteristics

Different thermoplastics have different shrinkage behaviors.

Material type, grade, fillers, processing temperature, fiber content, and part geometry can all influence dimensional shrinkage.

This means a geometry that performs well with one material may show significantly different surface behavior when molded in another.

Material selection should therefore be considered together with part design rather than as an isolated decision.

Where are Sink Marks Most Likely to Appear?

Certain part features deserve particular attention during DFM review.

Rib-to-Wall Intersections

Ribs improve stiffness without requiring the entire part wall to become thicker. However, excessively thick ribs create concentrated material at their connection with the main wall.

This can cause sink marks directly opposite the rib.

The goal should be to achieve stiffness through geometry while limiting unnecessary material buildup.

Bosses

Screw bosses and mounting bosses are another common source of sink marks.

Large solid bosses contain substantial material and cool slowly. Connecting a heavy boss directly to a cosmetic wall can make the defect particularly visible.

Coring the boss and using appropriately designed supporting ribs can often reduce this risk.

Thick-to-Thin Transitions

Sudden changes in wall thickness can cause both flow and cooling problems.

Instead of changing immediately from a thin wall to a heavy section, smoother transitions can help reduce localized thermal differences and improve molding consistency.

Intersecting Structural Features

Several ribs, bosses, walls, or gussets meeting at the same location can unintentionally create a large mass of plastic.

Each individual feature may appear acceptable when reviewed alone, while their combined intersection creates a much thicker effective section.

This is why part geometry should be evaluated as a complete system during DFM.

How to Prevent Sink Marks Through Part Design

Preventing sink marks during the design stage is usually more effective than attempting to compensate for poor geometry through molding parameters alone.

Maintain More Uniform Wall Thickness

Uniform wall thickness promotes more consistent filling, packing, and cooling.

Where different section thicknesses are functionally required, use gradual transitions whenever possible rather than abrupt changes.

Removing unnecessary material can also reduce cooling time and improve dimensional stability.

Optimize Rib Geometry

Ribs should provide structural reinforcement without creating excessively thick intersections.

Instead of making ribs heavier to increase stiffness, engineers can evaluate:

• Rib thickness
• Rib height
• Rib spacing
• Draft
• Fillet geometry
• Connection to surrounding walls

Well-designed ribs can improve stiffness while maintaining a more moldable material distribution.

Core Out Thick Bosses

Where possible, avoid large solid bosses.

Coring removes unnecessary internal material while maintaining the functional external geometry of the feature.

Bosses can also be supported with ribs or gussets rather than simply increasing boss thickness.

This reduces material concentration and can improve cooling behavior.

Process Adjustments for Reducing Sink Marks

Not every sink problem requires a design change. When the part geometry is fundamentally sound, process optimization may help.

Potential adjustments include:

Increase Packing Pressure

Additional packing pressure can help compensate for shrinkage as the material cools.

However, excessive packing may introduce other issues, such as internal stress, flash, or dimensional variation. The process should therefore be optimized rather than simply maximizing pressure.

Optimize Packing Time

Increasing packing time may allow additional material to compensate for shrinkage until the gate freezes.

Once the gate has frozen, additional hold time generally cannot add more material to the cavity.

Review Melt and Mold Temperature

Temperature affects flow, cooling, solidification, and shrinkage behavior.

Processing conditions should be optimized for the specific material and part geometry rather than adjusted independently.

Improve Cooling

More balanced mold cooling can reduce temperature variation across the part and improve cycle consistency.

For parts with heavy localized sections, cooling-channel design may need additional consideration during tooling development.

Design Problem or Process Problem?

One of the most important questions when troubleshooting sink marks is whether the root cause comes primarily from the part geometry or the molding process.

If sink marks consistently appear opposite a thick boss, heavy rib, or material intersection, geometry is often a significant contributor.

If the design already has relatively uniform sections but sink severity changes considerably with packing or cooling conditions, process optimization may provide greater improvement.

In many real projects, both design and processing contribute.

A DFM review before tooling can identify high-risk geometry early and reduce reliance on aggressive processing adjustments later.

Why Sink Marks Should Be Addressed Before Tooling

A small geometry adjustment in CAD is usually easier than modifying a hardened production mold.

If sink-prone areas are identified before tooling begins, engineers may be able to:

• Reduce local wall thickness
• Core heavy sections
• Modify rib geometry
• Redesign boss connections
• Improve thickness transitions
• Reconsider gate strategy
• Improve cooling planning

Once tooling is complete, correcting the same problem may involve mold rework, welding, machining, process compromises, or part-design revisions.

Early DFM therefore helps reduce both technical risk and unnecessary tooling changes.

Sink Marks and Cosmetic Surfaces

Sink marks are especially important for components with visible Class-A or customer-facing surfaces.

Examples include:

• Consumer electronics housings
• Medical device enclosures
• Automotive interior components
• Control panels
• Equipment covers
• Precision instrument housings

When structural features must be positioned behind a cosmetic surface, their geometry should be evaluated carefully.

A feature that is mechanically acceptable may still create unacceptable surface read-through after molding.

Cosmetic requirements should therefore be defined during the design and tooling stages rather than evaluated only after the first molded samples arrive.

A Practical Sink Mark Prevention Checklist

Before releasing a plastic part for tooling, review the following:

• Are wall sections reasonably uniform?
• Are there unnecessary thick areas?
• Are ribs appropriately proportioned?
• Are large bosses cored?
• Do multiple features intersect in one location?
• Are wall-thickness transitions gradual?
• Are critical cosmetic surfaces located opposite structural features?
• Can the gate effectively pack thicker regions?
• Has mold cooling been considered around heavy sections?
• Has the design received a DFM review?

These checks can help identify sink-mark risks while changes are still relatively easy to implement.

Need Engineering Support?

Sink marks are often easier to prevent through good part design and DFM than to correct after the mold has been manufactured.

AccuMolds supports product designers and manufacturing teams with DFM analysis, plastic part design review, material selection, mold-flow considerations, precision tooling, prototype development, and injection molding production.

Our engineering team can review wall thickness, ribs, bosses, material intersections, gate strategy, cooling requirements, and cosmetic surfaces to identify potential sink-mark risks before production.

Whether you are developing a new injection molded component or troubleshooting an existing part, early engineering review can help improve surface quality, production stability, and manufacturability.

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