Preventing Sink Marks Before They Appear

One common defect in plastic injection molded parts is the sink mark, which appears as a dimple or shallow depression on the surface, affecting both quality and visual appeal.

As a manufacturing supplier, we recommend addressing sink marks before they occur, rather than scrambling for solutions once they appear. Addressing sink marks after the fact often requires redesigning the part or adjusting mold gates or runners, which not only increases production costs but also extends the product development cycle. Worse still, despite all efforts, you may still have to compromise and accept some presence of sink marks.

To prevent sink marks from the outset, consider factors such as selecting proper materials and improving product design. By focusing on these areas, you can eliminate part defects early on, enhancing product quality and boosting production efficiency.

All resins shrink as they cool, but the shrinkage rate varies among different plastics. Semi-crystalline plastics, like PBT and PP, are more prone to sink marks on part surfaces. In contrast, amorphous plastics such as PMMA and ABS are less susceptible to sink, thus mitigating shrinking risks. Reinforced plastics and structural foam have lower shrinkage rates compared to non-reinforced thermoplastics and non-structural foam, thereby reducing shrinkage defects to some extent. Refer to the chart below for the shrinkage rates of various plastic materials.

Additionally, plastic materials with lower fluidity, such as PC, are more likely to cause sink marks because the melted materials do not adequately fill the part as they cool and shrink. However, this doesn't mean you should only choose materials with a lower shrinkage rate to mitigate shrinking risks. Shrinkage is just one factor in material selection; many other factors need consideration. Opt for materials with lower shrinkage rates only if other requirements are also met.

Avoid sink marks from the part design

1. The wall thickness should be thin rather than thick

The degree of shrinkage in plastic molded parts is directly related to the part's wall thickness. Thicker walls can lead to excessive shrinkage, making the parts more susceptible to sink marks. Additionally, overly thick walls not only waste material but also extend the injection molding cycle, thereby increasing manufacturing costs. The chart below illustrates the minimum and recommended wall thicknesses for various common materials.

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

2. Ensure the uniform wall thickness

When injection molding a part with non-uniform wall thickness, the hot plastic in thinner sections cools faster than in thicker sections, leading to uneven cooling. This disparity causes more shrinkage and increases the likelihood of sink marks. To minimize these risks, ensuring uniform wall thickness is crucial. If non-uniformity is unavoidable, the mold should be designed to allow the material to flow from thick to thin areas.

Plastic in thin wall thickness cools and solidifies fast, while plastic in thick cools slow, making it possible to generate sink marks.

3. Design the reasonable ribs thickness

Shrinkage often happens in areas with thick ribs. To minimize this, design the ribs with specific features such as U grooves, height differences, or serrated grooves.

4. Follow the design guidelines for boss

A boss is a mounting feature intended to hold a screw or thread-forming screw. If the boss design does not adhere to specific guidelines, it can thicken the wall and lead to sink marks. Following these design guidelines will help you achieve high-quality parts.

Collaborating with a Reliable Partner

Experienced engineers can easily analyze shrinking risks in product design through extensive experience or simulation software. However, if your product design is already finalized, expert engineers can still prevent shrinking issues. They can relocate gates to direct the melt plastic flow from thick to thin, design the mold's cold runner system effectively, and adjust injection molding parameters to meet your expectations. Collaborating with an experienced manufacturing supplier is crucial to avoid receiving defective products. Zonze, with nearly 20 years of expertise in injection molding and CNC machining, is committed to advanced manufacturing technologies. If you want to discuss your project with our engineers, contact us for a free DFM analysis of your part design.