Troubleshooting Injection Molding Sink Marks

Sink marks are unwanted depressions, or dimples, on the surface of an injection molded part. Generally, these defects do not affect the part’s strength or function; however, designers and injection molders still want to avoid cosmetic blemishes. Proper part design can help, but the design of the mold and the skill of the injection molder are also factors.

What are injection molding sink marks and what causes them?

Sink marks are shallow depressions on the surface of an injection-molded part. Because they reflect light in different directions, however, these cosmetic defects can be quite visible. Sink marks also detract from perceptions of product quality, even if they don’t affect a part’s strength or function. Sink marks are caused by thicker wall sections, and these thicker sections in parts are typically found at the bases of ribs and bosses.

In plastic injection molding, ribs run perpendicular to a wall and provide strength and support. Bosses provide channels for screws and aid in product assembly. The nominal wall is a structural element that determines the part’s overall strength and provides a mounting place for features such as latches, hinges, or threads.

During the injection molding process, plastic is heated and then shrinks as it cools and hardens. When a part’s wall sections are thick, they tend to cool more slowly and shrink more. This shrinking creates internal stress, which is relieved when the surface of the plastic contracts upon itself, creating a divot-like sink mark.

How do you prevent sink marks and voids?

Sink marks are commonplace molding defects, but there are things that part designers can do to prevent them. There are recommended practices to follow, but proper mold design and plastic processing also have key roles to play.

Part Design

Most sink marks occur where ribs or bosses intersect with the nominal wall. By following ABBYLEE’s design for manufacturability (DFM) guidelines, however, you can minimize sink marks.

Ribs

• Thickness – Make the ribs 50% to 60% (0.5 to 0.6 T) as thick as the nominal wall.
• Height – Make the ribs 3 times the height of the nominal intersecting wall (3T).
• Draft – Add 0.5° draft to each side of the rib.

Bosses

• Thickness – Make the bottom of the boss 60% (0.6T) as thick as the nominal wall.
• Fillets – Make the fillets 25% (0.25 T) of the radius of the nominal intersecting wall.
• Draft – Add 0.5° draft to each side, both inside and outside of the boss.

For DFM assistance, use ABBYLEE’s injection molding DFM tools to analyze your part design and check for wall thickness and potential sink marks.

Surface Finish and Part Color

Glossy surfaces and darker colors make sink marks more pronounced. Textured surfaces and lighter colors help to minimize sink mark visibility.

Mold Design

When designing an injection mold, there are three main considerations for avoiding sink marks: Gates, Cooling, and Venting.

Gates & Cooling

Gates are openings in a mold through which molten plastic is injected. Cooling lines support the uniform cooling of injection molds. By using metals with high thermal conductivity, such as beryllium copper, it’s easier to maintain even cooling across the mold.

Processing

Plastic processing has three key aspects that affect sink marks: Plastic melt temperature, Mold temperature, and Insufficient packing/holding pressure.