Sink marks are one of the most common and frustrating cosmetic defects in plastic injection molding. These small depressions or dimples on the surface of a molded part can reduce perceived product quality—even when the part functions perfectly. Whether you’re designing a new part or troubleshooting an existing issue, understanding the causes of sink marks and how to prevent them is key to producing consistent, high-quality components.
In this blog, we’ll break down the causes of sink marks in injection molding and share practical strategies to control or eliminate them during the design and manufacturing process.
What Are Sink Marks in Injection Molding?
Sink marks appear as small, sunken areas on the surface of molded plastic parts, typically in thicker regions or around structural features like ribs and bosses. These depressions occur when the inner portion of the part cools and shrinks more than the outer surface, causing material to pull inward.
While sink marks rarely affect structural integrity, they can make products appear defective or poorly made—especially in consumer-facing applications or in precision parts with tight visual requirements.
What Causes Sink Marks?
Several factors can lead to sink marks during the injection molding process. Understanding the root cause helps determine the best corrective action:
- Excessive Wall Thickness – Thicker areas take longer to cool and shrink more, pulling the surface inward.
- Inadequate Packing Pressure – If not enough pressure is applied during the holding phase, molten plastic can’t fully compensate for shrinkage.
- Insufficient Cooling Time – Removing the part from the mold too early may cause the surface to collapse inward as the core continues to cool.
- Poor Mold Venting – Air trapped in the cavity prevents material from filling completely, contributing to weak packing and uneven surfaces.
- Material Properties – Resins with high shrinkage rates or poor thermal conductivity are more prone to sink marks.
7 Design and Process Changes to Make to Reduce Sink Marks
Part design is the first line of defense against sink marks, with process adjustments following closely behind. Minor adjustments made during these first stages can prevent major issues later in production:
1. Optimize Wall Thickness
Avoid significant differences in wall thickness across the part. Uniform walls help ensure even cooling and shrinkage. For thicker sections, consider coring out material to reduce bulk and maintain surface integrity.
2. Ribs and Bosses
Use design guidelines for ribs and bosses to avoid excessive material buildup. As a rule of thumb, rib thickness should be no more than 60% of the adjoining wall thickness. When ribs or bosses are too thick, sink marks are much more likely to appear on the opposite surface.
3. Smooth Transitions
Incorporate generous radii and smooth transitions between features to promote consistent flow and minimize stress concentrations. Abrupt changes in thickness often lead to uneven cooling and localized sinking.
4. Increase Packing Pressure and Time
Use higher packing pressure during the holding phase to force additional material into the mold as the part cools. This helps counteract the volumetric shrinkage that causes sink marks.
5. Extend Cooling Time
Allow the part to remain in the mold longer to ensure the interior cools and solidifies completely before ejection. This can prevent the surface from collapsing inward after removal.
6. Adjust Mold and Melt Temperatures
Higher mold temperatures may increase sink risk by slowing the cooling rate. On the flip side, excessively low melt temperatures can cause poor flow and incomplete packing. Work with your process tech to balance these variables.
7. Fine-Tune Injection Speed
Slower injection speeds may reduce shear and improve fill consistency, thereby helping reduce localized shrinking. However, every mold is different—trial and error may be needed to find the sweet spot.
Injection Molding Design Strategies
If you’re experiencing persistent sink marks, it may be time to review your mold design:
- Gate Placement – Position gates to optimize flow and packing in thicker areas.
- Cooling Channel Layout –Ensure cooling lines are placed adequately around thicker sections to promote even solidification.
- Vent Locations – Good venting prevents trapped gas from interfering with material flow, which supports better packing.
Material Selection Matters When It Comes to Sink Marks
Different materials exhibit different shrinkage behaviors. When surface quality is a priority, choosing the right resin can make all the difference. Amorphous resins like ABS and polycarbonate tend to shrink less and cool more evenly, reducing the chance of sink. Semi-crystalline materials such as nylon and polypropylene shrink more and may require design and process adjustments to avoid defects. Additives and fillers (e.g., glass fibers) can also reduce shrinkage and improve dimensional stability.
Consult your material supplier or molding partner to determine the best resin for your specific part geometry and cosmetic requirements.
How Molding Dynamics Helps Prevent Sink Marks
At Molding Dynamics, we treat every new project as a collaboration. Our team of experienced engineers and process technicians work closely with customers to:
- Review part designs for potential sink-prone features
- Use mold flow analysis to simulate material behavior and identify risk areas
- Recommend design, process, or material adjustments to reduce sink marks
- Ensure molds are optimized for even cooling and efficient packing
- Validate every part through rigorous quality control processes
By addressing sink marks from every angle—design, process, mold, and material—we help our customers achieve consistent, high-quality molded parts that look as good as they perform.
Let’s Talk About Your Next Project
Whether you’re launching a new part or troubleshooting issues with an existing mold, we’re here to help you get it right. Contact us to request a quote, review your design, or speak with one of our experts about eliminating sink marks and other surface defects from your plastic parts.
