All is Not Sunk: How to Deal with Sink Marks in an Injection-Molded Part
There are many things that can affect a plastic part’s design. Many factors can affect how strong the part will be or how thoroughly it is formed. But many other factors can also affect how the part looks. Function over form? Well, not always. Never before in history have we gotten so good at producing parts that the function is not really an issue.
Nowadays, most parts are at least up to snuff enough for the real differentiator to be appearance and there are many factors that influence a part’s appearance. For the sake of this article, we’ll assume that a part’s functional design is sound. We’ll also assume that the aesthetic design is reasonable. We will instead address a make-or-break problem found in many lower-quality parts: sink marks.
Sink marks: you may not have known what they were, but you’ve seen them.
If you are unfamiliar with plastic part design, you may not be aware of the sink mark. You’ve more than likely seen them, though. You may not know what they are, but once you do, you’ll see them everywhere. A sink occurs when a given plastic part has a variation in wall thickness.
When plastic is injected into a mold, one of the first things that happens is that the material contacts the mold. Almost without fail, the mold will be cooler than the plastic. At the point of contact, the plastic instantly begins to cool. As the mold fills, the plastic has less room to move. It then cools and solidifies. But in areas where there are wider spaces, the plastic stays molten longer. This allows its material to cool at different rates (see figure below) based on thickness.
A sink may be identified as a dip in the surface of a plastic part that generally coincides with an internal structure. There are many ways to get sinks, just like there are many ways to deal with them. You’ll note that the previous sentence didn’t indicate that we can get rid of them altogether. Believe it or not, there are some sinks that just cannot be avoided. So when you find yourself in a bind, there are things you can do to cope with sinks.
Why do sinks happen? It’s all about shrinkage. Sinks happen because thicker plastic cools slower than thinner plastic. That means thicker plastic will retain heat longer than thinner plastic. It’s all about thermal mass.
If your wall thickness suddenly changes, say, at a corner, the thicker cross-section where the two walls meet will continue to cool and shrink long after the thinner walls. This will cause the material in the thicker area to draw into where the plastic is still warm. Depending on the severity of the difference in thicknesses, the sink could be fairly minor or it could produce a gaping hole in your part.
A thicker cross-section will always cool more slowly and therefore shrink more than the thinner walls around it.
If you discover a sink in your part, don’t worry. All is not lost. Depending on the reason for the sink, you have many options. (For the sake of discussion, the following recommendations appear in the order of ease of implementation.)
Option 1: Ignore it. (Cue scary, shocking music and a scream!) If the sink shows in an area that will be hidden from sight, chances are you can just leave it there. Most often, sinks don’t affect structural integrity or strength. They are merely unsightly. Look at where the sink is. If the sink will be covered by another part or is inside an assembly where no one will see it, then no harm, no foul.
You’ll know it’s there, but your end user may never see it. Disaster averted. If, however, the sink shows up somewhere visible, you’ll need to take a hard look at what is causing it.
Option 2: Camouflage it. Labels are often applied to parts for no other reason than to cover a sink. They provide a visual mask behind which anything can lurk. It is very common to find screw holes or snap features under a label. The long and short of it is that if you don’t want to see something, cover it up. (This also works in politics.)
Of course, labels aren’t the only form of camouflage. Texture can be incredibly effective at hiding minor flaws on a surface. If the sink isn’t very noticeable, a texture applied over it may be enough to hide it from sight. Obviously, the more pronounced the sink, the more aggressive the texture will need to be. Unfortunately, at some point you’ll no longer be able to hide the sink behind a texture. Then you’ll have to take off the kid gloves.
Features can provide wonderful camouflage. By changing the topology of your model, you can hide a multitude of design sins. Say, for instance, you have a screw boss on the back side of your model’s face. The base of the boss will have a certain wall thickness. Where it meets the wall, the cross-section will be thicker; therefore, a sink will likely form on the opposite side of the wall from the boss. You must have the boss, so you look for ways to reduce or eliminate the sink.
Screw bosses are a major source of sink marks.
The first thing to look for is a thinner wall. As with all things “design,” there is a formula for this. For any given wall thickness, an intersecting interior wall should be from 50 to 80 percent of the main wall. A good rule of thumb is for the interior wall to be 75 percent of the main wall’s thickness. Try reducing your boss wall to 75 percent of the main wall and see if that helps. It may not. Don’t worry. There are still more tricks up your sleeve.
Don’t forget to account for draft angle. If your boss (the feature, not the guy hanging over your shoulder wondering why your part has sink marks) is too tall, by the time you get to the base you could have excessive thickness.
If you are going to have a sink mark anyway, you can try to make it look like you intended for it to be there.
Exaggeration is a good way of camouflaging a sink. If you cannot get rid of the aforementioned screw boss sink mark, try putting in a feature that exaggerates it.
Option 3: Design the sinks out. If the main problem is thicker sections, just avoid those, right? It’s often easier said than done, but you can generally find a way to reduce the thermal mass without changing major chunks of your design. For instance, you could make the hole in your screw boss deeper. Or you could put in a moat (your design is your castle?) around your screw boss. Or you could just reduce the wall thickness locally from the other side.
There are any number of ways you can reduce the uneven thermal mass.
3D modeling programs provide some powerful plastic part design capabilities. For example, in SOLIDWORKS, you can use Thickness Analysis to assess your part. Just identify the face you want to look at and the software will calculate where a sink could end up. In fact, the company now offers its SOLIDWORKS Plastics simulation software, which is meant to optimize and assist in plastic parts design. Its suite of powerful features lets users analyze designs and locate potential problems long before they shell out for tooling.
Thickness Analysis can show you where a sink might occur.
Not only can you find problems but you can also explore solutions. The Plastics software lets you play with packing pressure (essentially how much material fills the mold), feature modification, and even gate location and size to eliminate sinks.
In the long run, you need to remember that plastic part design can present you with some fairly interesting challenges. While most can be designed out or at least reduced, some aren’t going away. You just have to turn them to your advantage or learn to live with them and make the best of it. (Sounds a lot like some coworkers.) Regardless, plastic part design is fun and produces many worthwhile and enjoyable products that we find hard to live without. If you just keep your attention on working with the problems you encounter, you will end up with a higher profit margin and better customer satisfaction. And that’s what this is all about, right?
For more information about plastic part design, you can check with your local college or university. Centers of higher learning commonly offer classes and degree programs on the topic. For more information on how SOLIDWORKS can help you in your designs, visit www.solidworks.com.
About the Author
Michael Hudspeth has been a designer for two decades, a lifelong artist, an avid model builder and author (specializing in science fiction). He, his wife, two daughters and one too many cats thrive in the great American heartland, just outside of St. Louis, Missouri.