What Causes Sink Marks? A Guide to Prevention
There are two ways to approach product development: you can either fix problems as they appear or design them out from the very beginning. When it comes to flaws like sink marks, the proactive approach is always better. These frustrating surface depressions are a classic example of an issue that’s far easier to prevent in the design phase than it is to fix on the factory floor. Waiting until the mold is already made can lead to expensive rework, production delays, and a compromised final product. By understanding the root causes, you can make smart decisions early on that guarantee a flawless result and turn a great concept into a perfect physical product without the last-minute drama.
Key Takeaways
Design is your best defense against sink marks. The most effective way to get a flawless product is by addressing potential issues like uneven wall thickness before the mold is ever made. This proactive approach saves time, money, and prevents last-minute production headaches.
Manufacturing adjustments can only do so much. While your production partner can fine-tune settings like pressure and temperature, these are finishing touches, not fixes for core design problems. A well-engineered part is the foundation for a smooth and predictable manufacturing run.
Cosmetic flaws undermine the brand experience. A sink mark is more than a small dent; it's a detail that can make a premium product feel cheap and rushed. For any physical campaign asset or merchandise, ensuring a perfect surface finish is essential to protecting your client's brand.
What Is a Sink Mark?
When you’re creating a physical product for a campaign, the last thing you want is a last-minute surprise that compromises the look and feel. Sink marks are one of those common, frustrating flaws that can pop up in plastic injection molding. Think of them as small dents or dips that appear on the surface of a finished part. While they might seem minor, they can make a high-end product feel cheap and unfinished, undermining the premium experience you’re trying to deliver for your client.
Understanding what causes these imperfections is the first step to preventing them. It all comes down to how the plastic behaves as it cools inside the mold. A great design and engineering partner knows how to anticipate these issues from the very beginning, ensuring the final product is as flawless as the initial concept.
How Sink Marks Form
Sink marks happen because plastic shrinks as it cools—it’s a natural part of the process. The problem arises when a part cools unevenly. In areas where the plastic is thicker, the outer surface cools and solidifies first while the inside is still molten. As that inner material finally cools and shrinks, it pulls the already-solid surface inward, creating a visible dip. This process is called thermal contraction.
More often than not, the root cause of sink marks can be traced back to the initial part design. An ideal plastic part has consistent, uniform wall thickness throughout. When one section is significantly thicker than an adjacent one, it creates the perfect conditions for uneven cooling and, ultimately, a sink mark.
What Sink Marks Look Like
Visually, sink marks are exactly what they sound like: shallow depressions or craters on the surface of a plastic part. You’ll most often find them in thicker sections, like over a rib or a boss designed for a screw. They can be subtle or very noticeable, depending on the material, color, and surface finish of the product. A glossy finish, for example, will make sink marks much more obvious than a textured one.
While these marks usually don't affect the part's strength or function, they are considered a major cosmetic defect. For any brand-focused project, aesthetics are everything. A product riddled with sink marks looks rushed and poorly made, which is why identifying them as a quality flaw is standard practice. Preventing them is key to delivering a product that looks and feels premium.
What Causes Sink Marks?
Sink marks aren't random flaws; they're predictable outcomes of specific issues in either the product's design or the manufacturing process. Think of them as symptoms of an underlying problem. When you see a dip or depression on the surface of a molded part, it's a sign that something went wrong as the plastic cooled and solidified. Understanding the root causes is the first step to creating flawless products that live up to your agency's creative vision. Most sink marks can be traced back to four main culprits: inconsistent wall thickness, insufficient pressure, high-shrink materials, or incorrect temperatures. Let's break down what each of these means for your project.
Uneven Wall Thickness
This is the most common cause of sink marks. When a part has both thick and thin sections, the plastic cools at different rates. The thin areas solidify quickly, while the thicker, more insulated sections take longer to cool. As that thick area cools, it shrinks and pulls the still-soft surface material inward, creating a visible depression. To prevent this, the goal is to design for manufacturability by maintaining a uniform wall thickness throughout the part. If a thicker section is unavoidable for structural reasons, we can core it out or use design features like ribs to add strength without adding mass.
Low Packing and Hold Pressure
During injection molding, after the initial injection fills the mold cavity, a "packing" or "hold" pressure is applied. This phase forces more molten plastic into the mold to compensate for the material shrinkage that naturally occurs during cooling. If the packing pressure is too low, it can't push enough material into the thicker sections to fill the internal voids left by shrinkage. The result is a sink mark on the surface. While a skilled machine operator can adjust these settings, relying on high pressure to fix a design flaw can introduce other problems, like stress marks or flash.
High Material Shrinkage
Every plastic has a unique shrinkage rate—it's just a fundamental property of the material. Some materials, like polypropylene or nylon, shrink significantly more than others as they transition from a molten to a solid state. When you use a high-shrink material, you have a much smaller margin for error in your part design and process settings. Areas with a high concentration of material, like the base of a boss or a thick rib, will experience more volumetric shrinkage, making them prime locations for sink marks. Choosing a low-shrink polymer early in the development process is one of the most effective ways to prevent this issue from the start.
Incorrect Processing Temperatures
Temperature control is everything in injection molding. This includes both the temperature of the molten plastic (melt temp) and the temperature of the steel mold itself. If the melt temperature is too high, the plastic will be less viscous and shrink more dramatically upon cooling. Similarly, if the mold temperature is too high, it will slow down the cooling process, giving the part more time to shrink unevenly. Finding the perfect balance is key. An experienced engineering team will work with the manufacturer to establish a precise processing window that minimizes defects and ensures consistent, high-quality parts every time.
Why Do Sink Marks Matter?
At first glance, a tiny dip on the surface of a plastic part might seem like a minor imperfection. But in the world of product development, sink marks are a red flag. For agencies launching a physical product, whether it's branded merchandise or a key component of an experiential campaign, these flaws can undermine the entire project. They aren't just cosmetic issues; they are indicators of underlying problems in the design or manufacturing process that can impact how people perceive a brand.
Think of it this way: you wouldn't deliver a final campaign video with a rendering glitch, or a print ad with a typo. A sink mark is the physical equivalent. It signals a lack of attention to detail and can make a premium product feel cheap. Beyond the surface-level impression, sink marks can also point to more serious issues with the part's structural stability and can lead to unexpected costs and delays. Understanding why they matter is the first step in ensuring the physical products you create for your clients are flawless, durable, and delivered on budget.
Compromised Aesthetics
For any branded product, the look and feel are non-negotiable. Sink marks are small depressions or craters on a part's surface that disrupt the intended finish, whether it's high-gloss, matte, or textured. They catch the light in unflattering ways and can make a smooth, sleek surface look wavy and uneven. This immediately lowers the perceived quality of the product.
When you’re creating an influencer kit or a piece of high-end merchandise, the goal is to create a premium experience. A visible flaw like a sink mark can instantly break that illusion, making the product feel rushed or poorly executed. It’s a small detail that has a huge impact on brand perception, turning what should be a "wow" moment into a disappointment.
Reduced Structural Integrity
While sink marks are primarily a cosmetic defect, they can sometimes be a symptom of a weaker part. These marks form because of internal stresses created when plastic cools and shrinks unevenly. This same internal stress can create weak points within the part, making it more susceptible to cracking or breaking under pressure. If the product is something that will be handled frequently, dropped, or put under any kind of load, this becomes a serious functional risk.
For example, if a sink mark appears near a snap-fit feature or a screw boss, it could indicate that the area is not as strong as it was designed to be. This could lead to product failure down the line, resulting in unhappy customers and a damaged brand reputation.
Higher Manufacturing Costs
Ignoring sink marks during the design phase almost always costs more money later. Fixing them once a mold has already been produced is an expensive and time-consuming process. It can involve complex modifications to the steel tool, extensive trial-and-error on the molding machine, or even a complete redesign of the part. These fixes lead to production delays and budget overruns—two things that can derail any campaign launch.
The most effective way to deal with sink marks is to design them out from the very beginning. By working with an experienced engineering partner, you can address potential issues like wall thickness and gate location before any steel is cut. This proactive approach is far more efficient and ensures you avoid the hidden costs of rework, saving both time and money.
How to Design Out Sink Marks
The best way to fix sink marks is to prevent them from ever happening. While your manufacturing partner can make adjustments on the production floor, the most impactful changes happen long before that—right in the CAD file. Addressing potential sink issues during the industrial design phase is the most efficient and cost-effective strategy. It ensures your final product is both visually flawless and structurally sound, saving you from expensive tooling changes and production delays down the line.
This proactive approach, known as Design for Manufacturing (DFM), is at the core of how we work. It’s about making smart, engineering-led decisions early on to guarantee a smooth path to production and a perfect final product. Here are the key design strategies we use to eliminate sink marks from the start.
Maintain Uniform Wall Thickness
One of the golden rules of injection molding is to keep wall thickness consistent throughout your part. When some sections are thick and others are thin, the plastic cools at different rates. The thicker areas cool slower, causing the material to shrink inward and pull the surface down, creating a sink mark. As one plastics expert notes, this "leads to uneven cooling rates, resulting in warping, sink marks, and other defects." We aim for a single, uniform thickness to ensure the part cools and solidifies evenly, resulting in a clean, consistent surface finish that looks and feels premium.
Add Ribs for Support
Sometimes, a design needs extra strength and rigidity that a thin wall can’t provide on its own. Instead of just making the entire wall thicker (which would invite sink marks), we use ribs. Ribs are thin, wall-like features designed to add structural support without adding a lot of mass. By placing them on the non-visible side of a part, we can reinforce the design and prevent warping. This technique allows thicker areas to cool more evenly and keeps the part strong, effectively preventing sink marks while maintaining the intended form. It’s a strategic way to achieve strength without compromising aesthetics.
Optimize Gate Location
A gate is the opening where molten plastic enters the mold cavity. Its location is critical because it dictates how the plastic flows and fills the part. Poor gate placement can create issues with pressure and cooling, especially in complex geometries. We strategically position gates to ensure the plastic can reach every part of the mold, including the inner portions of any thicker sections. Proper gate design helps maintain consistent packing pressure as the part cools, which is essential for preventing the material from shrinking away from the mold wall and creating a sink.
Select Low-Shrink Materials
Not all plastics are created equal. Different materials shrink at different rates as they cool, and some are more prone to sinking than others. For example, crystalline plastics like polypropylene tend to shrink more than amorphous plastics like ABS. Part of our engineering process involves helping you select the right material for your product’s specific needs. If a design requires thicker sections that can’t be avoided, choosing a low-shrink, glass-filled, or fiber-reinforced resin can significantly reduce the risk of sink marks, ensuring the final product holds its shape and looks exactly as intended.
Fixing Sink Marks in the Molding Process
Even a perfectly designed part can develop sink marks if the injection molding process isn't dialed in correctly. When your agency hands off a design for manufacturing, the work isn't over. The next phase involves a close partnership with your molding supplier to fine-tune the machine settings. Think of it as the final polish that brings your creative vision to life. The good news is that these cosmetic flaws can often be resolved with on-the-fly adjustments right at the press, without needing to go back and modify the expensive steel tooling. Fine-tuning the molding parameters is a critical step in achieving a flawless surface finish and ensuring your final product is something everyone can be proud of. It’s a collaborative effort where small changes to pressure, temperature, and timing can make a huge difference in the final quality. This is where having a strong engineering partner becomes invaluable, as they can translate design intent into specific machine instructions for the technicians on the factory floor. They act as the bridge between your creative brief and the physical realities of production. Here are the most common process adjustments used to eliminate sink marks and produce high-quality parts that will impress your clients and their customers.
Increase Packing and Hold Pressure
Once the initial injection fills the mold, the packing and hold phase begins. This is where pressure is maintained to push more molten plastic into the cavity, compensating for the material that shrinks as it cools. If this pressure is too low, thicker sections of the part won't get enough material to offset the volume loss, causing the surface to sink inward. A skilled machine operator can incrementally increase the packing pressure to force-feed the thicker areas until they are properly filled. It’s a delicate balance—too much pressure can cause other issues like flash—but getting it right is one of the most effective ways to combat sink marks.
Adjust Mold and Material Temperatures
Temperature control is everything in injection molding. If the plastic melt temperature is too high, the material stays molten for longer, increasing the overall cooling time and giving it more opportunity to shrink unevenly. This can directly lead to sink marks. Similarly, the temperature of the mold itself plays a huge role. A mold that’s too hot can prolong the cooling cycle, while one that’s too cold can cause the material to solidify too quickly at the surface, trapping molten plastic underneath. The goal is to find the sweet spot where the material flows properly but cools at a controlled, uniform rate. Adjusting these processing temperatures is a standard procedure for optimizing part quality.
Modify Injection Speed
The speed at which molten plastic is injected into the mold can also influence the formation of sink marks. A faster injection speed helps ensure the cavity is filled completely and uniformly before the plastic has a chance to start cooling and solidifying. This is especially important for parts with complex geometries or long flow paths, where the material might otherwise hesitate or cool prematurely. By increasing the injection speed, the material can pack into every corner of the mold more effectively, reducing the likelihood of under-filled areas that later turn into sinks. This adjustment helps create a more consistent density throughout the part, leading to a better surface finish.
Extend Cooling Time
Sometimes, the simplest solution is to give the part more time to cool inside the mold. Extending the cooling time allows the thicker sections, which naturally hold heat longer, to solidify more completely before the part is ejected. If a part is removed from the mold while its core is still molten or soft, the outer surface can be pulled inward as the core finally cools and shrinks, creating a sink mark. By keeping the part in the mold under pressure for a longer duration, you ensure it’s stable enough to hold its shape. While this can slightly increase cycle time, the improvement in part quality is often well worth it.
How to Inspect for Sink Marks
Catching sink marks is about more than just a visual check under bright lights. A rigorous inspection process gives you the data needed to trace the problem back to its source, whether it's a design flaw or a process issue. For agencies creating physical products for a campaign, this step is non-negotiable—it ensures the final piece feels premium and polished, not cheap. A subtle dip in a surface can completely undermine the perceived quality of a product, turning a high-impact brand moment into a disappointment.
Effective inspection relies on having the right tools and, just as importantly, the right standards. While a quick glance might spot a major sink mark, minor depressions that affect the product's feel or how light reflects off its surface require more precise methods. The goal isn't just to pass or fail a part; it's to understand the exact depth and location of any imperfection. This information becomes a critical feedback loop that helps engineers fine-tune the molding process or even revisit the part design, saving time and money on future production runs. Different tools offer different levels of detail, from traditional touch-probes to advanced non-contact scanners, and choosing the right one depends on your quality goals.
Use 3D Optical Scanners
For a truly comprehensive look at a part’s surface, nothing beats a 3D optical scanner. These non-contact systems, like a 3D optical profilometer, can scan an entire surface in seconds, capturing hundreds of thousands of measurement points. Instead of just checking a few spots, this technology creates a complete 3D map of the part’s topography. This allows you to see the highest and lowest points across the whole area, making it easy to detect even the most subtle sink marks that would be invisible to the naked eye. It’s the most reliable way to get a clear, data-rich picture of surface quality and ensure every part is perfect.
Check Key Dimensions with a CMM
A more traditional tool for part inspection is the Coordinate Measuring Machine, or CMM. A CMM uses a physical probe to touch the part at specific, pre-programmed points to measure its dimensions. While CMMs are excellent for verifying critical dimensions like hole diameters or overall length, they can struggle with surface defects like sink marks. Because the probe only gathers a limited number of points, it can easily miss the true depth of a depression, especially if the sink mark is small. It provides an accurate measurement of the points it touches, but it doesn't give you a complete picture of the surface between those points, which is where sink marks live.
Define Your Quality Control Standards
The best inspection tools in the world are only useful if you know what you’re looking for. Before production begins, it’s essential to establish clear quality control standards that define what constitutes an acceptable part. How deep can a sink mark be before the part is rejected? Are they acceptable on non-cosmetic surfaces? Answering these questions upfront prevents subjective judgment calls on the factory floor. By measuring and quantifying sink marks, you can pinpoint their root causes—like inconsistent molding pressure or temperature—and make targeted adjustments. This turns inspection from a simple quality check into a powerful process improvement tool, ensuring every product that reaches your audience is flawless.
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Frequently Asked Questions
What's the single most important thing I can do to prevent sink marks? Hands down, the most effective way to prevent sink marks is to maintain a consistent wall thickness throughout your part design. This is the golden rule of injection molding. When a part is designed with uniform walls, the plastic cools and shrinks at an even rate, which eliminates the internal stresses that pull the surface inward. While other factors matter, getting the wall thickness right from the very beginning in the design phase solves the problem before it even starts.
Is fixing a sink mark a design problem or a manufacturing problem? It can be both, but it’s most effectively solved in the design phase. Think of it as a shared responsibility. A great design makes it easy to produce a flawless part, while a skilled manufacturing team can adjust their process to correct for minor issues. However, relying solely on manufacturing adjustments to fix a fundamental design flaw can lead to other problems. The ideal approach is to have your design and engineering partner address it from the start, ensuring the part is optimized for production long before it hits the factory floor.
My product has a textured surface. Do I still need to worry about sink marks? Yes, you do. While a textured finish is great at hiding minor imperfections that would be obvious on a glossy surface, the underlying cause of the sink mark is still there. This means the part may have internal stresses that could compromise its strength, even if you can't see the flaw. For a purely cosmetic part, you might get away with it, but if the product needs to be durable, it's important to address the root cause of the sink mark regardless of the surface finish.
Can a sink mark ever be acceptable? This really depends on the product and your quality standards. On a highly visible, cosmetic surface of a premium product, any sink mark is usually a dealbreaker. However, on an internal component that will never be seen by the customer, a minor sink mark that doesn't affect the part's function might be perfectly acceptable. The key is to define these quality standards with your engineering partner before production begins, so everyone is aligned on what constitutes a pass or a fail.
Why can't we just rely on the manufacturer to fix sink marks during production? While manufacturers can make adjustments to pressure, temperature, and timing to reduce sink marks, these are often just workarounds for a design issue. Pushing the molding process to its limits to compensate for a flaw can introduce new problems, like internal stress, warping, or flash. The most reliable and cost-effective way to get a perfect part is to design it correctly from the start. Process adjustments should be for fine-tuning, not for fixing fundamental problems.
