Finally an Excuse to Use the Fancy 3D Printers! | ENGI 210: Prototyping and Fabrication

This week’s assignment entailed 3D printing an impossible object only made possible by additive manufacturing. I found this sphere trapped in a box on Thingiverse and decided I wanted to recreate it. The assignment required printing five copies of the same impossible object using at least two different printing methods.

While I had 3D printed a lot for my ENGI 120 and 200 projects last year, as well as for my job, I had only ever done FDM printing. I was excited to finally have a legitimate reason to use the SLA printers at the OEDK!

The first step of the project was to download the Thingiverse STL file and scale it in MeshMixer. The object was supposed to fit in a gumball capsule so, after a few quick seconds of “measuring”, I determined a cube side length of 3.5cm would fit within the capsule.

I saved this new scaled STL file and uploaded it to 3DPrinterOS, where I selected the Layout and Slicing settings. I FDM printed on the Prusas with Layer Height 0.35mm, Wall Thickness 1 mm, and Infill Density 25%. I generated supports and added a brim for better build plate adhesion. I would have added a raft as the Thingiverse description suggested, but the raft option in 3DPrinterOS was glitching and resulting in 60+ hour print times, so brim it was.

Previewing supports in 3DPrinterOS

The print was a success, but the supports were incredibly difficult to remove. After about 10 minutes tearing off tiny bits of support with needle-nose pliers and wire cutters, I finally managed to have a somewhat clean final product. After the supports were removed, the surface of the print was now very rough, so I tried to sand it a bit with sandpaper, but I noticed my attempts at sanding simply left scratch marks on the surface of the part instead of removing the bumps.

Before and after removing supports with pliers and wire-cutters

Another issue I noticed was that the print was rather… large. After having spent so long removing the supports, I was horrified to realize that I had misjudged the size of the gumball capsule – my cube was far too large. So I returned to MeshMixer and scaled it down some more.

I printed five copies of my piece on the Prusas and was happily surprised to find that the supports were much easier to remove at the smaller scale.

The next step was to SLA print on the Form printers. I uploaded my scaled STL file to PreForm and kept the default settings but made sure to generate support. I had some issues connecting the desktop to the printer, as no printers would be listed as available. I eventually got around that issue by manually typing in the printer’s IP address.

I SLA printed two copies of my impossible object: the first I did on the Form 2 with rigid resin and the second I did on the Form 3 with tough resin. The post-printing steps varied slightly because the rigid resin required a different sequence of baths than the tough resin, but I followed the listed guidelines: 20 minutes for the tough resin bath and 6 minutes for the first rigid resin bath and 9 minutes for the second. After that, I fully cured my cubes: the rigid resin cube required 15 minutes at 80C and the tough resin required 120 minutes at 60C.

SLA printing support generation preview

SLA print straight out of the Form, still adhered to bed

One issue I ran into when SLA printing was deciding when to remove the supports (before curing or after?) I sought the advice of the Lab Assistants, but they weren’t sure either, so I decided to try it both ways, one for each print. (This was actually my motivation for SLA printing two copies instead of just the one required one).

Removing supports from resin print with needle nose pliers

I found the supports were much easier to remove before curing since they were still flexible; however, because the part hadn’t fully cured and thus hardened yet either, it was easy to deform the actual part itself. On the other hand, the cube that had the support removed after curing was left with lots of tiny bumps where the supports had been. I found the former issue more tolerable than the latter, so in the future, I would continue to remove the supports before curing.