Ever noticed those small, dimple-like depressions on a plastic surface that just shouldn’t be there? Those are sink marks, a common defect in injection molded parts that can quietly wreck your product’s look, integrity, and performance. While they might seem like a minor cosmetic issue, sink marks can signal deeper problems in the molding process.
Industries like automotive, consumer electronics, medical devices, and appliances are particularly sensitive to these flaws, where precision, durability, and a flawless finish are non-negotiable.
In this article, we’ll break down what causes sink marks, why they matter, and how you can prevent them to produce better, stronger, and more reliable molded parts.
- What Are Sink Marks?
- How Do Sink Marks Form?
- Common Causes of Sink Marks in Injection Molding
- How To Avoid Sink Marks
- Partner With an Experienced Product Development Firm
What Are Sink Marks?
Sink marks are small, shallow depressions or indentations that appear on the surface of injection molded plastic parts. These imperfections are typically found in areas where the material is thicker, causing an uneven surface that can affect both the appearance and structural integrity of the part.
Sink marks in injection molding may seem minor, but they can compromise the overall quality and aesthetic appeal of a product, making it essential to address and prevent them during the injection molding process.
The Cost of Sink Marks
Sink marks aren’t just a cosmetic issue — they can have a real impact on your bottom line. Here’s how they drive up manufacturing costs and slow down production:
- Rework costs: Fixing sink marks often means extra labor, secondary operations, or even manual touch-ups — all of which add time and expense to your process. In some cases, reworking parts can cost more than producing them correctly in the first place.
- Scrap material: If a part is too damaged to rework, it gets tossed — wasting not only the raw materials but also the energy and machine time that went into producing it.
- Extended manufacturing cycles: Detecting and correcting sink marks slows everything down. It can lead to bottlenecks on the line, delayed shipments, and missed deadlines, especially when mold adjustments or process changes are needed mid-run.
How Do Sink Marks Form?
Sink marks form when there is uneven cooling and shrinking of the material, typically in thicker sections of the part. As the plastic cools and solidifies, it contracts, causing the surface to sink inward, creating a noticeable blemish.
Some materials are more prone to sink marks than others. Sink marks in plastics are especially susceptible, particularly when parts have thick walls or uneven cooling. While sink marks can also occur in aluminum die casting, the metal is generally more forgiving.
Minor imperfections in aluminum are less noticeable, and the material tends to cool more uniformly, reducing the risk. Still, thick sections in metal parts can sink if not designed or cooled properly.
Common Causes of Sink Marks in Injection Molding
Sink marks in injection molding are caused by various factors. Understanding these root causes is essential for implementing effective prevention strategies and ensuring high-quality plastic parts.
Material Shrinkage and Cooling
Sink marks in injection molding often stem from uneven cooling and shrinkage of plastic material. This differential shrinkage results in internal stresses that pull the surface inward, forming sink marks. Different plastics exhibit varying shrinkage rates and cooling behaviors, influencing the severity of sink marks.
Proper mold design, precise processing conditions, and careful material selection are critical factors in managing shrinkage effectively to minimize sink marks and ensure consistent part quality.
Part Design and Thickness
When designing parts, particularly in areas where two outer walls intersect or angled surfaces meet (commonly referred to as “baloney cuts”), it’s important to carefully consider the wall thickness.
Thicker sections of the part cool at a slower rate during the molding process, leading to uneven shrinkage compared to thinner sections. Following recommended guidelines for internal wall and rib thickness — typically around 50% to 60% of the nominal outside wall thickness — helps minimize variations in cooling rates and reduce the likelihood of sink marks.
Insufficient Packing Pressure
During the injection molding process, after the molten plastic is injected into the mold cavity, it undergoes a phase where it is packed or compressed to ensure it completely fills the mold and compensates for shrinkage as it cools.
When the packing pressure is inadequate, several issues arise:
- Incomplete filling: The molten plastic may not fully reach all areas of the mold cavity, especially in thicker sections or intricate geometries.
- Air traps: Insufficient pressure can trap air pockets within the material, leading to voids or inconsistent material density.
- Uneven distribution: The plastic material may not uniformly distribute across the mold cavity, resulting in variations in wall thickness.
These factors contribute to uneven cooling and shrinkage during the solidification phase. As thicker sections cool slower than thinner areas, they are more prone to sink inward due to internal stresses.
Inadequate Cooling Time
Inadequate cooling time during injection molding is a key factor that can lead to sink marks on plastic parts. After the molten plastic fills the mold cavity, it needs sufficient time to cool and solidify before ejection.
When cooling time is insufficient:
- Parts may not fully solidify, leading to deformations or sink marks.
- Different sections cool at different rates, with thicker areas particularly prone to uneven cooling.
- Insufficient cooling can leave residual stresses in the material, causing it to contract unevenly and form sink marks.
To mitigate these issues, it’s essential to optimize the cooling time based on the material properties, part design, and molding conditions. Proper cooling allows for uniform solidification of the plastic material, minimizing internal stresses and reducing the likelihood of sink marks.
Mold and Gate Design
In injection molding, the design of molds and gates is crucial for preventing sink marks. A gate refers to the point where molten plastic enters the mold cavity. Proper gate location ensures even distribution of plastic material, minimizing uneven cooling that can cause sink marks on the part surface. Incorrect gate placement can lead to areas of the part cooling at different rates, resulting in depressions or blemishes.
Effective venting within the mold is also essential to expel air or gasses trapped during injection. Inadequate venting can create voids or air pockets in the plastic part, disrupting uniform cooling and contributing to sink marks. Well-designed cooling channels in the mold promote consistent cooling across the entire cavity, minimizing shrinkage disparities that lead to surface imperfections like sink marks.
How To Avoid Sink Marks
To steer clear of sink marks and other cosmetic flaws in injection molded parts, it’s all about solid injection mold engineering and harnessing 3D CAD tools effectively. Collaborating with molders and toolmakers can also allow you to continuously refine part designs.
- Optimize mold design: Make sure the mold design encourages smooth plastic flow and even cooling, preventing uneven shrinkage and sink marks on your parts.
- Increase holding pressure: Use enough pressure during injection to fill the mold completely, minimizing air pockets and ensuring consistent part density, which helps avoid sink marks.
- Select appropriate materials: Choose materials that shrink evenly and flow well, reducing the chances of uneven cooling and shrinkage that cause sink marks.
- Maintain consistent mold temperature: Keep the mold temperature stable throughout the process to ensure parts cool uniformly, preventing variations in shrinkage that can lead to sink marks.
- Use textured surfaces: Applying a heavier surface texture to the outer part can help disguise minor sink marks, making them less visible without affecting part performance.
Partner With an Experienced Product Development Firm
At StudioRed, we’ve spent over 40 years mastering the complexities of molded plastics, helping companies design smarter and manufacture better. Our founder, Phil, even made waves in the early days of computer design — using his plastics expertise to help revolutionize how the Osborne computer was assembled, a story we’ve shared on the StudioRed blog.
Today, we bring that same depth of knowledge to every project. Our team combines advanced CAD tools, in-depth mold analysis, and material know-how to prevent issues like sink marks before they happen. When you work with StudioRed, you’re not just getting a product — you’re getting a partner committed to building it right.
Contact us to discover how we can help you achieve manufacturing excellence.
