Common Causes of Sink Marks in Plastic Injection Molding

Sink marks are one of the most common appearance defects in plastic injection molding. Many customers notice local depressions, uneven surfaces, or wave-like marks on plastic parts when receiving samples, especially around ribs, screw bosses, and thick wall sections. Although sink marks may not always affect product functionality, they can significantly reduce appearance quality, particularly for automotive, home appliance, and consumer electronic products.

Sink marks are mainly caused by uneven cooling and shrinkage of plastic material. After molten plastic is injected into the mold, the surface layer cools and solidifies first, while thicker internal areas cool more slowly. As the internal material continues shrinking, insufficient packing pressure may pull the outer surface inward, creating visible depressions.

In many plastic products, rib design is one of the most common causes of sink marks. If ribs are too thick, the internal area becomes significantly thicker than surrounding walls, leading to uneven cooling. In most cases, rib thickness is recommended to stay around 50% to 70% of the main wall thickness to balance structural strength and appearance quality. Similar issues may also occur around screw bosses and snap-fit areas, which is why structural optimization during plastic injection mold design is important.

Material selection also affects shrinkage behavior. Different plastic materials have very different shrinkage rates. For example, PP material is generally more prone to sink marks compared to ABS, while glass fiber reinforced materials tend to provide better dimensional stability. Selecting suitable materials early in the project can help improve appearance quality.

Besides product structure, injection molding process parameters are equally important. Insufficient holding pressure, short packing time, or undersized gates may prevent enough material from compensating shrinkage during cooling. Uneven mold cooling can also worsen local sink mark issues. Experienced injection molding engineers often reduce sink marks by adjusting packing parameters, optimizing gate positions, and improving cooling system layouts.

The plastic injection mold design itself also plays a major role. If gate locations are too far from thick sections, packing efficiency may decrease. Poor cooling channel design can create localized hot spots inside the mold. For products with high appearance requirements, many mold manufacturers perform mold flow analysis during the early development stage to predict shrinkage risks in advance.

At QINGDAO HOPE MOULD, potential sink mark areas are carefully evaluated based on product structure and material properties during plastic injection mold development. In many cases, a small structural modification can greatly improve product appearance and reduce future production problems.

As market expectations for product appearance continue increasing, sink marks are no longer just a production issue. They directly affect brand image and customer experience. For companies developing new plastic products, considering sink mark control solutions early in the plastic injection molding project can help reduce future modification costs and improve mass production stability.