Crisp emboss/deboss colors with Stratasys polyjet
Recently I was printing some board game tokens on our Stratasys J750, and I noticed the colors on my debossed labels looked blurry. At first I thought there was something wrong with the printer, but I soon realised the problem was in my design! I changed my design slightly to get crisp colors. If you've had similar problems with embossed or debossed text or logos, here's how to get them sharp.
I've included instructions for Solidworks, but the same idea applies to any CAD tool or mesh editor.
Step 1: Why does it look blurry?
Have a closer look at the part on the left in this photo:
It's not a blurry photo: the part actually looks like that, almost dirty or like the colored resins have mixed during the print. But the problem is with how the part was designed in Solidworks. When I added the black color to the deboss, I drag-and-dropped the black material onto the "cut" feature, creating what Solidworks calls a feature color. This means the vertical sides of the deboss also get the black color. Why does that matter? To understand that, we need to look through the part, so I made a diagram with a cross-section through the deboss:
Like in the photos, the problem part is the one on the left. As you can see, it has a "shell" of yellow around a core, which I've drawn as gray. (The actual material used in the core depends on print settings, but it doesn't matter for us: only the shell matters.)
The colored Vero resins used by Stratasys polyjet printers are somewhat transparent - especially the Vero Vivid colors, which offer the widest color gamut. This is why the slicer adds this core, which provides an opaque backing to achieve the full depth of color you expect. But that transparency can be a problem, as in this case. Look at where the black and the yellow meet: the yellow shell gets thinner and thinner as the black meets it at the corner. When you look through that corner (which is the edge of the deboss), you're looking through a tiny layer of yellow, so you can see the black behind it. That's what gives the blurry appearance.
Step 2: Fix the problem
Now we understand that the problem's caused by looking through a thin layer of yellow at the black color, let's look at the diagram again, and this time compare the version on the right.
As you can see, if only the bottom face of the deboss were black, not the sides, then there wouldn't be a problem. There's no way you can look through the yellow and get less than the normal shell thickness before you see black.
It's a subtle change in Solidworks but it's easy to do. When assigning the materials/appearances to parts, just be sure to drag the material onto the bottom face of the deboss, not onto the feature in the feature tree. (If the deboss is a label with lots of text or a more complex logo, it might be several faces, so be careful to get all of them.) That's all you need to do!
Other CAD tools have the same concepts of applying materials to faces, you just access them in a different way. I'm sure you can work it out. If you're using a vertex paint tool inside a mesh editor (such as Blender or 3ds Max) instead of a CAD tool, you're probably deciding for yourself which vertices or triangles to paint on, so just make sure you put the boundary between the colors on an interior corner (like in the right-hand side of the diagram).
Step 3: Comparing emboss and deboss
That's debossing solved, but what if your labels are embossed instead? Well, let's look at another cross-section:
In this case the situation is reversed. Having the sides black puts the boundary on an inside corner, which is fine: you can't see through the yellow to the black. But if you put the black only on the outer face, then when you look through the side of the emboss, you'll see the same color-bleed effect.
A key thing to note here is that you don't have to use feature colors to get the crisp colors in this case. It's fine to use face colors as long as you apply the colors to the side faces as well as the top.
What's in common between the two cases is the inside and outside corners. If you want a sharp boundary between contrasting colors, it needs to be on an inside corner. If you put the boundary on an outside corner, it'll show the same color bleeding you saw in the first picture. If you remember this rule, you can apply it to any kind of part geometry, not just embosses and debosses.
This is the same whether it's a black feature on a light-colored body or a dark-colored feature on a white body, but different colors will show the effect differently, just because the different colored resins have different levels of translucency. Vero White and Vero Black (and the newer "Pure" versions) are very opaque, so the effect will be the worst when one of the colors is white or black.
One last thing is that I'd like to find a better name for this than "color bleed". Although that's what it looks like, the name suggests that it's the colored resins bleeding into each other, but that isn't what's happening. It's really an artefact of how the slicer turns the 2D appearance information it gets from the CAD file or mesh into solid 3D regions of color. Maybe when full-color 3D printing is more widespread, CAD tools will automatically help to put the boundaries in the right place to get the best 3D prints. Until then, you'll get the best results if you learn these little rules about how the slicing and printing process works. Just like how you design your part geometry for the specific manufacturing process (whether additive or something else), and just like how you might design appearance for overmoulding or painting, you can design for polyjet full-color and get crisp, sharp colors every time.