How to Get Rid of Sink Marks in Injection Molding

Injection molding is one of the most widely used manufacturing processes for producing plastic parts with high precision and efficiency. However, one common defect that continues to challenge engineers and mold designers is the formation of sink marks. These small, often shallow depressions on the surface of a molded part can compromise both aesthetics and mechanical integrity, especially in consumer-facing or high-performance components.

In this article, we'll explore what causes sink marks, why they occur, and most importantly, how to prevent or eliminate them through design, processing, and material strategies.

What Are Sink Marks?

Sink marks are small depressions or indentations that appear on the surface of a molded plastic part, usually directly above thick internal sections such as ribs, bosses, or localized areas of extra material.

While they may not always affect the functionality of a part, sink marks can severely impact its appearance, surface uniformity, and perceived quality — which is critical for visible consumer products, automotive parts, and medical devices.

What Causes Sink Marks in Injection Molding?

Sink marks are primarily caused by uneven cooling and shrinkage of the molten plastic during the injection molding process. Here's how it happens:

1. Material is injected into the mold cavity.

2. Thicker sections take longer to cool and solidify than thinner areas.

3. As the thick section cools, it shrinks inward, pulling the external surface along with it.

4. If the outer skin has already solidified, the inward pulling causes a visible dimple or depression — the sink mark.

Common Contributors to Sink Marks:

• Excessive wall thickness

• Improper rib/boss design

• Insufficient packing pressure

• Premature gate freezing

• Low melt or mold temperature

• Incompatible material selection

Where Do Sink Marks Commonly Occur?

Sink marks are frequently found in areas such as:

• Rib intersections

• Boss bases (for screws or inserts)

• Thick wall transitions

• Localized structural reinforcements

• Behind logos or text embossed into parts

Understanding where they occur is the first step in redesigning or adjusting your molding parameters to prevent them.

How to Eliminate or Prevent Sink Marks

Eliminating sink marks requires a combination of part design optimization, mold design best practices, and processing adjustments. Below is a breakdown of methods you can use.

1. Optimize Part Design

Design is the most critical and effective area for preventing sink marks.

A. Uniform Wall Thickness

• Rule: Always aim for uniform wall thickness throughout the part.

• Thick areas cool slower than thin ones, causing uneven shrinkage.

• Use coring (hollowing out) to reduce mass without sacrificing strength.

B. Rib Design Guidelines

• Ribs should be no more than 50-70% of the adjacent wall thickness.

• Tall, thick ribs create hotspots that cool slowly, leading to sinks.

• Add fillets or tapered ribs to reduce shrinkage stress.

C. Boss Design Optimization

• Bosses used for screws or alignment often create sink marks.

• Reduce boss wall thickness or connect bosses to sidewalls via ribs to spread out the mass.

D. Use Coring Instead of Solid Mass

• Replace thick solid features with cored-out sections to maintain structure while minimizing material.

• For example, a thick block can be designed as a shell with internal ribs instead of a solid chunk.

2. Adjust Mold Design

If design changes aren't feasible or sufficient, look to the mold tooling.

A. Gate Location and Size

• Place gates close to thick sections to maintain pressure and fill.

• Undersized gates can freeze too soon, preventing proper packing.

• Optimize gate size and shape for balanced flow and pressure.

B. Use of Venting and Ejector Pins

• Poor venting can trap air and cause uneven filling or shrinkage.

• Strategically placed vents and ejector pins help reduce cooling stress.

C. Cooling Channel Placement

• Add cooling lines near thick sections to even out cooling rates.

• Uniform cooling prevents hot spots that cause sink marks.

3. Modify Processing Parameters

Fine-tuning your injection molding settings can also reduce sink marks significantly.

A. Increase Packing Pressure and Time

• Packing pressure compensates for material shrinkage as the part cools.

• Higher packing pressure fills in low-pressure areas and reduces sinks.

• Extend the packing time until the gate is fully frozen.

B. Raise Mold Temperature Slightly

• Warmer mold surfaces allow more time for material to pack before solidifying.

• Avoid excessive temperature rise, as this may introduce warping or flash.

C. Adjust Melt Temperature

• Higher melt temperatures allow the plastic to flow better into features and thin sections.

• Prevents early skin formation that can trap shrinkage inside.

D. Slower Injection Speed

• Fast injection may trap air or create uneven flow.

• Slowing the injection allows the material to fill and cool more evenly.

4. Material Selection Matters

Different polymers shrink at different rates. Consider the material's shrinkage properties when designing for injection molding.

Low-Shrink Materials (Less prone to sink marks):

• ABS

• PC (Polycarbonate)

• Filled polymers (e.g., glass-filled nylon)

High-Shrink Materials (More prone to sink marks):

• Polypropylene (PP)

• Polyethylene (PE)

• Nylon (unfilled)

Using glass-filled or mineral-filled resins can reduce shrinkage and minimize sink marks, especially for structural parts.

Simulation and Analysis Tools

Before tooling or production, use mold flow analysis software to simulate:

• Cooling rates

• Shrinkage patterns

• Packing pressure distribution

• Potential sink mark locations

Tools like Autodesk Moldflow, Moldex3D, or SolidWorks Plastics can identify and fix issues virtually, saving time and money in tooling revisions.

Cosmetic Fixes (If Sink Marks Can't Be Avoided)

Sometimes, despite all efforts, minor sink marks still occur. In such cases:

A. Texture the Surface

• Applying matte or textured finishes can hide sink marks from the eye.

B. Post-Processing

• Light sanding, filling, or painting may disguise minor sinks on visible surfaces.

C. Redesign for Acceptable Aesthetics

• Place sink-prone features on non-visible surfaces.

• Modify parting lines or label placement to draw attention away.

Conclusion

Sink marks are a common but preventable defect in injection molding. By addressing root causes through thoughtful part design, mold configuration, and processing control, you can significantly reduce or eliminate them. The goal is to balance aesthetics and function while maintaining production efficiency.

Whether you're designing a housing for a consumer product or a technical part for automotive or electronics, knowing how to handle sink marks will help you produce higher-quality, more reliable, and visually appealing molded parts.

Below is a video on how to repair Sink Marks in Injection Molding, which is also of good reference value.

Video link:https://www.youtube.com/watch?v=BW6WN3sk7BI