In high-precision injection molding, achieving a flawless surface finish is often as important as meeting tight dimensional tolerances. One of the most common defects that can compromise a part's aesthetic and structural quality is the sink mark. These subtle depressions on the surface of a molded part are not just cosmetic flaws; they can indicate areas of high stress and may affect the performance of the component. Sink marks are caused by the natural behavior of plastics as they cool and solidify. Fortunately, with careful attention to Plastic Part Design Optimization, injection molding tooling, and process control, they are almost entirely preventable. This article explores what sink marks are, the factors that cause them, and the best practices for preventing them.
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A sink mark is a localized depression or dimple on the surface of an injection molded part. It is not a loss of material but rather the result of the material shrinking unevenly as it cools. Sink marks typically appear on the surface of a part opposite thick features like ribs, bosses, or gussets. While they are most noticeable on flat, glossy surfaces, they can occur on any part where wall thicknesses are not uniform. In some cases, if the surface skin of the part is strong enough to resist being pulled inward, the shrinkage may instead form an internal bubble known as a void. A sink mark and a void are essentially two outcomes of the same root cause: volumetric shrinkage.
The formation of sink marks is directly linked to the physics of how thermoplastic materials cool. When plastic is injected into a mold, it is in a hot, molten state. As it cools and transitions to a solid, it shrinks in volume. If a part has sections of varying thickness, those sections will cool and shrink at different rates, which is the primary driver of sink marks.
The single most significant cause of sink marks is poor part design, specifically non-uniform wall thickness. Thicker sections of a part cool much more slowly than thinner sections. The outer surfaces of the part cool first by coming into contact with the mold walls, forming a solid skin while the core remains molten. As this thick, molten core finally cools and contracts, it pulls the already-solidified surface skin inward, causing the visible depression.
This is why sink marks are most common opposite features like ribs and bosses. These features create localized thick sections where they join the main wall of the part. A fundamental rule in Design for Manufacturing (DFM) for injection molding is to keep wall thicknesses as uniform as possible. As a guideline, the thickness of a rib should be no more than 40-60% of the thickness of the wall to which it is attached. This minimizes the mass of material at the intersection, allowing it to cool at a rate closer to the surrounding wall.
While part design is the primary factor, molding process parameters can either help mitigate or worsen the tendency for a part to sink. The most important process setting in this context is the packing or holding pressure. After the initial injection fills the cavity, a secondary packing pressure is applied to force more material into the mold. This extra material compensates for the shrinkage that occurs during cooling. If the packing pressure is too low, or if the holding time is too short, there will not be enough material packed into the cavity to offset the natural shrinkage, and sink marks will likely form.
Other parameters, such as melt temperature, can also have an effect. A higher melt temperature means the plastic will shrink more as it cools, increasing the potential for sink. Similarly, a short cooling time can result in the part being ejected from the mold before it has fully solidified, allowing the still-soft surface to be pulled inward as the core continues to cool.
The design of the mold itself can play a role. The gate, which is the opening through which plastic enters the part cavity, is particularly important. If the gate is too small, it can freeze and solidify before the packing phase is complete. Once the gate is frozen, no more material can be forced into the part, and packing pressure becomes ineffective, making sink marks in thick sections almost inevitable. In addition, the layout of cooling channels within the mold can contribute to sink if they do not provide even and efficient cooling, leading to "hot spots" that shrink differently than the rest of the part.
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Preventing sink marks is best accomplished proactively, with a focus on good design from the very beginning.
The most effective way to prevent sink is to address it at the source: the part design. Adhering to DFM development services and Design for Injection Molding (DfIM) principles is paramount. This involves designing parts with a nominal wall thickness that is as uniform as possible. For areas that require extra support, it is better to use multiple, smaller ribs rather than one large one. Thick sections that cannot be avoided should be "cored out," meaning material is removed from the center to create a more uniform wall, without sacrificing structural integrity.
If a design is already fixed, process adjustments can help reduce or eliminate sink marks. The first step is typically to increase the packing pressure or extend the holding time to force more material into the mold. If the sink persists, it is a sign that the gate may be freezing off too early. Other adjustments include lowering the melt and mold temperatures to reduce overall shrinkage, or increasing the overall cooling time to ensure the part is stable before being ejected.
Some materials naturally shrink more than others. Semi-crystalline materials like Polypropylene and Nylon generally have higher shrinkage rates than amorphous materials like ABS or Polycarbonate. Switching to a material with a lower shrinkage rate, or using a grade filled with glass or other fibers that limit shrinkage, can be an effective solution. On the tooling side, if process changes are not enough, the gate size may need to be increased to allow for a longer packing time. Additionally, working with an expert tooling company that offers precise tooling solutions and plastic injection mould tooling services can help.
Sink marks are a direct consequence of thermal shrinkage, but they are largely a preventable design flaw. While an expert process engineer can make adjustments to mitigate the issue, process changes can only do so much to compensate for a part that is not designed for manufacturability. The best strategy is a proactive one, focusing on robust part design from the outset. By collaborating with a design and manufacturing company that offers design for manufacturing services, secondary operations, and Rapid Prototyping Services like 3D Printed Prototypes, companies can ensure their components are optimized for the injection molding process, leading to dimensionally stable, aesthetically flawless parts right from the first shot.
If you're finding sink marks on your molded parts and it’s impacting the quality of your product, reach out to Aprios! We specialize in resolving these types of defects and ensuring your parts look and perform as intended.