Textured plastics are everywhere. Just looking across my desk, I see molded texture on my monitor stand, the wireless USB holder, the remote controls, the keyboard, my water bottle…
It’s easy to take these surfaces for granted because they are so common. But in reality, a LOT of engineering goes into making these common surfaces look good and feel good.
Textured plastic surfaces are everywhere
How do I control plastic textures?
Most likely, these textures were controlled in production by comparing them (either by sight or touch) to a mold texture standard. The method is exactly as you’d guess: an inspector holds a sample next to the production plastic or mold and then determines whether the two match, sufficiently.
A VDI 3400 texture card (Xometry).
The texture card above includes samples based on the VDI 3400 standard (from the Verein Deutscher Ingenieur, the Society of German Engineers). Other common standards include SPI (Society of the Plastics Industry) and Mold Tech.
Visual/tactile comparison gives you a fast and straightforward assessment of a molded plastic texture. For many surfaces, this method is wholly adequate. For precision surfaces, however, which may be functional as well as aesthetic, visual comparison may not provide enough information to control a surface.
Which surface did I make?
One of the first challenges with visual comparison is that it’s a very subjective inspection method. Some surfaces are quite distinct—a VDI39 finish has a “crystal-like” finish, for example, while other textures are more “pebbled” or “veiny,” like various leathers. The fact that we have to use these kinds of words to describe the surfaces should give you a clue that controlling them will be a challenge!
What’s more, telling these surfaces apart gets harder as the textures get finer. Can you distinguish between these samples from the VDI comparator above?
It’s not always easy to distinguish between comparator plaques!
So, what can a manufacturer do when a surface starts to look a little less “right” than it did before? How do we know whether the mold has worn out or the materials have changed?
Did I hit the number?
In recent years a lot of plastics manufacturers have realized that they’ll need to move beyond visual comparison and begin describing their surfaces using numerical values (similar to what’s happened in metals machining). A manufacturer may begin tracking an average roughness In recent years a lot of plastics manufacturers have realized that they’ll need to move beyond visual comparison and begin describing their surfaces using numerical values (similar to what’s happened in metals machining). For example, a manufacturer may begin tracking average roughness (Ra, or Sa), which can indicate if a process has changed in some way (maybe the mold has worn or the material is flowing differently). Parameters like Ra give a manufacturer traceable values that can show trends over time—and that’s a big step forward.
But, while a simple surface parameter like Ra can give a general sense of a surface, it can’t describe all of the subtle aspects that our eye uses to tell “pebbly” from “veiny.” Ra can’t tell you about the density or direction of swirls, or even the frequency of bumps in a pebbled texture.
To complicate the matter, very different surfaces can produce the same Ra value. These textures, for example, are difficult to distinguish using standard height parameters…yet our eyes would know immediately if one versus the other showed up on our dashboards…
Very different surfaces can produce the same Ra value!
To add to the challenge, many textures will produce wildly different Ra values depending on the location of the measurement. Even trying to state the measurement location can be subjective (start from the largest swirl and measure in the direction of…?).
Moving beyond the numbers
When we need to appeal to people’s visual assessment of “quality,” we may need to go beyond the numbers. A combination of parameters may be the right way to control a particular texture (perhaps, say, tracking average roughness, summit curvature, and pit/porosity parameters).
Gathering data: measuring a plastic texture and VDI comparator plaques using a Zygo Zegage
Software that lets you visualize surfaces while also generating numerical values can be the bridge to determining these kinds of controls. The image below show the surface of the remote control (above) in OmniSurf3D surface texture software. Software like this can help you visualize and explore surface texture while also calculating surface texture parameters.
A stitched image of the surface texture on the remote control shown above.
In many cases, software like OmniSurf and OmniSurf3D can help you to determine the best way to control a surface. Once you’ve figured out an inspection recipe, a simple production measurement may be all that’s required to track the values and control the surface.
Advanced analyses, like this Pit/Porosity analysis in OmniSurf3D, can help differentiate and control complex surfaces.
In the coming years, we may find that artificial intelligence may prove helpful in correlating certain parameters to human assessments of a surface. It’s another case where surface texture software can provide the bridge between subjective viewing and objective parameter values.
If you’re curious how it all works, visit our Surface Library, where you can download 3D measurement data of all of the surfaces on the VDI 3400 comparator card above.
Need some help figuring out how to control your surfaces? Contact us to set up a time to talk about your surfaces. We have a lot of expertise in this area, and we’d be glad to help out.