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One of the most frequent and visually disruptive defects in plastic injection molded parts is the sink mark—an indentation or depression on the surface that undermines both appearance and functional quality. Rather than addressing sink marks reactively through redesigns or mold adjustments, it is far more cost-effective and efficient to prevent them proactively. Correcting these flaws later often requires changes to the part design, gate locations, or runner systems, leading to increased costs and production delays—and even then, total elimination of sink marks is not guaranteed.
By making smart choices in material selection and part design during the early stages of product development, engineers can significantly reduce the likelihood of sink marks, leading to improved product quality and greater manufacturing efficiency.
Understand Material Behavior During Shrinkage
All plastics shrink as they cool, but the extent of shrinkage varies depending on the material. Semi-crystalline plastics like PBT and PP are more prone to sink marks due to their higher shrinkage rates. In contrast, amorphous materials such as PMMA and ABS tend to shrink less, thereby reducing the risk of surface defects.
Reinforced plastics and structural foams typically exhibit lower shrinkage compared to their non-reinforced counterparts. Choosing such materials can help minimize the appearance of sink marks. Additionally, materials with low melt flow—such as PC—may lead to incomplete filling and uneven shrinkage. While selecting low-shrink materials may seem like a logical solution, shrinkage rate is only one of many considerations. Always balance shrinkage performance with other part and material requirements.
Optimize Part Design to Prevent Sink Marks
Use Thinner Walls Where Possible
Shrinkage is directly proportional to wall thickness. Thicker walls tend to shrink more, which makes parts susceptible to sink marks. Moreover, thicker sections increase material usage and extend cycle times. Designing parts with thinner walls not only mitigates sink risks but also improves production efficiency. Refer to standard wall thickness guidelines based on part size and material type.
① Recommended wall thickness for small-size molded parts
② Recommended wall thickness for mid-size molded parts
③ Recommended wall thickness for large-size molded parts
Ensure Uniform Wall Thickness
In parts with inconsistent wall thickness, thinner areas cool and solidify faster than thicker regions. This results in internal stress and unbalanced shrinkage, increasing the chances of sink marks. Designing parts with uniform wall thickness promotes even cooling and reduces defect rates. If uniformity is not feasible, ensure the plastic flows from thick to thin sections to reduce stress buildup.
Properly Design Rib Features
Rib structures often cause localized thick sections that are prone to shrinkage. To address this, incorporate rib relief features such as U-shaped grooves, serrated breaks, or stepped profiles to redistribute material and improve cooling consistency.
Follow Design Guidelines for Bosses
Bosses, often used for fasteners or inserts, can create thick sections if not properly designed. Use standard boss design rules to maintain structural strength while avoiding sink-causing thickness.
Work with a Qualified Manufacturing Partner
Experienced engineers can predict and resolve sink mark issues using design experience or simulation tools. Even when the design is finalized, molding experts can optimize the runner system, gate positioning, and processing parameters to control material flow and shrinkage behavior. Partnering with a skilled injection molding supplier ensures the final product meets quality expectations without costly trial-and-error corrections.
At Zonze, we draw on nearly two decades of precision manufacturing expertise in injection molding and CNC machining. Our engineers are available to provide free design-for-manufacturing (DFM) analysis to help you prevent sink marks before they ever appear. Reach out to discuss your project and achieve better results from the start.