Printing with multiple materials unlocks a huge and very fun design space, but multi-material printing requires hundreds of dollars of cost premium over a low-end off the shelf FDM printer. Your options are 1) a super expensive dual-extruder setup, 2) a ~$300 MMU (and only if you own a Prusa), or 3) a $600 or $800 Palette 2 (which I’ll probably get before too long, anyway). But it turns out there’s quite a bit you can do
By the end of this article, you might be thinking “I’ve seen this before…” and that’s because Make Anything has done a whole bunch of videos exploring this very technique (which he has coined Multipass 3D printing… I like it), and much further than I have. I’m claiming independent co-development though – I first made business cards using this technique (which I LOVE, and suggest you try) at the beginning of November 2018, which I exchanged with a few people at Supercon last year Nov 2-4 (see here: https://twitter.com/szeloof/status/1070664272651452416). Make Anything’s first video was later that month, so I think it was truly independent work.
Basically, with multipass 3d printing you can make 2-d objects that look like these:
Make Anything actually takes this a step further with folded flat-pattern designs to get simple 3D shapes, which I think is tremendously cool: Single Extruder Multicolor in Cura Tutorial // Multipass 3D Printing (Youtube)
Technique: ColorPrint (color-by-layer)
ColorPrint is the easiest color technique, and has been around for quite a while. At least PrusaSlicer supports it in-app now, so you can easily add color changes at whatever layer you like without having to use a fiddly web tool to process pre=sliced g-code. Basically, extrude areas you want printed in a different color to a different height, and insert a filament change at the layer boundary.
The disadvantage is that you have no control over surface thickness or object uniformity, and stuff just doesn’t come out looking quite as good if it’s not somewhat predisposed to the color geometry in the first place.
Take the earring above that we whipped up the morning of the 4th of July this year. It was easy to put together and prints fast, but the plume is rough around the edges and since the colorful surface is the top side, it looks pretty rough.
Here’s another ColorPrint design I did a while back – a fridge magnet based on a decal my girlfriend cut based on a photo she took of her dog, Astro.
ColorPrint is a great option here because the topmost layer can be black, which only requires one layer to get good contrast, and therefore doesn’t make the surface too uneven. Plus this one was sliced in Cura with ironing enabled, so it looks a bit better than if it were just a raw top surface.
Technique: Multipass 3D printing
The idea here is to do all your color work on the first layer only, and just make a handful of .STLs that you print one on top of the other. When I first had this idea, I was trying to make a business card in the style of a front panel, with a paint inlay to get the color. It turns out, this is pretty hard to do.
You can see in this rough-draft that the paint I was using is thin enough to flow into the layer line gaps, and doesn’t fill in the letters very well. I think ideally you’d goop on colored epoxy then wipe it off, but I never could get that process to work cleanly with a 3D-printed substrate. A milled part would have worked much better, but even still, just getting the paint right turns out to be pretty hard.
Then it occurred to me that I could probably do this with filament itself – just print the letters, then print the rest of the business card face-down right over the top of it.
This was my favorite result of that experiment. It puts more limitations on minimum feature size of the letters you use, because they have to be printed now with the positive of a .4mm nozzle, rather than just where the nozzle avoids.
I think the biggest disadvantage of this method is that it works best with a dark positive color (or colors) on a lighter negative space, since the positive space can only be one layer thick. For example, if I wanted white letters on a dark card like my paint attempt before, the single-layer white letters would be partially translucent, and take on some of the color and darkness of the background printed over them.
This rough-draft SpaceX drone ship coaster shows a bit of what I mean
The yellow is a pretty translucent filament to begin with, so it gets really pulled down by the black behind it. The white is much thicker in color, but even it gets a little blunted.
I printed this from the following 3 .STLs, in order although facing down on the bed as you look at them in these images:
These were each sliced at .3mm layer height to get as much thickness as possible on that first layer with the colors. In reality, only the first layer needs to be .3mm thick, but then the rest is just for rigidity anyway.
The problem here is that we can’t make the white and yellow any thicker. If we try to print a two-layer chunk instead of one, but we still do the same color-by-color print order, the nozzle begins to hit and smear the existing plastic as it tries to lay down an adjacent line of a later color. This is actually ok to some extent, but totally nonfunctional past a 2 layer differential, and results in slightly less crispy edges.
Instead, for the next pass of this particular project, I tried combining colorprint and multipass. The issue is that the dark color of the black brings down the thin white and yellow layers below it. If those white and yellow layers are instead backed by white, the black won’t change, but the white and yellow will brighten up considerably.
To get this effect, I printed the yellow and white STLs the exact same, but added a color change for just the second layer of the “black” section, so that it’d print black>white (1 layer)>black for the rest of the body. The disadvantage of doing this is a 1-layer white ring around the outside of the coaster, but I don’t much mind that. Unfortunately, to get any more aggressive with manual filament changes IS possible without substantially increasing them or needing a purge block, but would require the slicer being able to print in free air above the bed with no part of the model in contact, which is a whole other challenge.
Technique: Multipass light pipe
One thing I haven’t seen done yet is using clear filament as a sort of light pipe, in this same setting. What inspired me to try this was actually this Ahmsville Dial on Tindie, from Ahmsville (Youtube). I’m pretty sure he used Multipass to get the logo inlay on the top surface, but to look at the pictures, I thought it actually lit up like the ring around the bottom does. That product actually doesn’t behave like this, but I thought it’d be a great effect for front panel indication, so I put together this first pass at it.
This part has 3 numbers printed in clear PETG as the first part of the first layer, then a box structure printed around those that provides baffles to prevent light leak between elements. It’s printed with the right spacing to fit over some neopixel addressable LED strip I had laying around.
In this case, I printed a relatively thin 0.2mm first layer, when I probably should have used the max recommended for a 0.4mm nozzle of 0.3 or 0.35. Given that you can only print one layer, results would probably be even better with a fatter nozzle that can print a thicker first layer, though that would not be without compromises for the rest of the print.
Here’s a video so you can see how the light plays. I can imagine some cool front panels done this way, maybe even cooler with white instead of clear.