I played with FreeCAD in the past, but hadn’t put much effort into it beyond creating some basic shapes. I was recently motivated to learn to do more in CAD in relation to the NES Cartridge Display project. I needed to modify the existing design so that I could implement a two color process.
I found this project through Hackaday: https://hackaday.com/2024/02/20/multicolor-resin-prints-give-it-a-shot/
The author demonstrates how to “inject” resin onto a print to create a two color finish: https://www.instructables.com/Writing-High-contrast-Text-on-a-Resin-3D-Print/
I wanted to try it right away, but the current base design had holes in the center of the letters which would not do.
Learning to CAD
I was fortunate enough to have access to Autodesk Inventor. The tutorials are fantastic, and within a couple of days I finished a design for the PS Display base:
Using what I learned through that project, the next step for the NES base was to get the existing .stl file into a format that I could modify with Inventor. Pulling the raw .stl into Inventor appeared to work at first, but when I exported the file only my extrusions were present. The solution is a free program called MeshMixer. It converts .stl into .stp, .obj, and many other formats. The dimensions were blown up almost 100% from MeshMixer (I resized it in the slicer at the end to compensate). EDIT: I later found a better, more reliable method here: https://forums.autodesk.com/t5/inventor-forum/how-do-i-convert-mesh-part-from-stl-file-to-solid-in-inventor/td-p/10984036 It uses an add-on to Inventor that does the conversion perfectly. After that, it was just a matter of adding a few extrusions and exporting back out to .stl format.
Adapting the Method
I found blunt syringes with a short needle (specifically designed for non-medical) online. The other change I made was to mix UV curing red nail polish with translucent red resin (about a 50/50 mix). It was a bit difficult to draw the mixture into the syringe. A few cycles of drawing then clearing the air out of the syringe and I had most of the mixture in the syringe.
The best method for application seems to be dripping the solution into place and allowing it to flow and self-level. After the first pass on the letters, I went back and ‘topped off’ any that needed additional mixture to bring it level. For the longer oval around the outside, I kept the syringe moving while slowly depressing the stopper. Again, going around for a second pass to level out the mixture. Once the mix is in place, I put it into the UV curing station for 5 minutes. The result was relatively hard; I could make a mark, but not dent, it if I pushed my finger nail in.
Conclusions
All in all, this method is a relatively easy way to add multi-color detail to resin prints. It results in a much cleaner look than can be achieved (by me) with paint and brush. The time savings with this method versus painting 2 coats by hand was substantial as well.