For my impossible object, I wanted to create an optimal illusion, something that looked like it would have to be connected from multiple parts to be assembled. The design I ultimately decided upon can be found here on GrabCAD. This is what it looks like:
CAD file of impossible block.
This block is just a loop, but it looks “impossible” at first glance. It would be difficult to make this with the molding techniques we’ve learned, but it is very doable (with the correct structural supports) using 3D printing.
The two 3D printing techniques I used were (1) The Prusa printer, and (2) the Formlabs resin printer. I have used a Prusa before, but I had never done 3D resin printing, so this was completely new to me.
Setting up both files to print was very easy. I resized my block to 20% its original size to fit into the gum-ball containers. Both the Prusa and Formlabs software had automatic structural support generation that would allow for the best print possible. After slicing the files, I sent them to print on their respective printers. Both printers took about the same printing time: about 2 hours and 15 minutes for the Prusa, and 2 hours and 30 minutes on the Formlabs printer (to print 3 on each printer).
Prusa printing 3 of the blocks
After the Prusa printer finished, post-processing was easy; I simply ripped out the structural support, and all of it came out easily. The pos-processing for the Formlabs prints was a little more involved. I first had to wash the pieces, cut out the structural supports (that didn’t come our nearly as cleanly as the Prusa pieces), and cure the pieces. Even after this, the resin pieces still had hardened reside left over from the structural supports. I did not have time to sand these pieces down, but I would have given I had the time.
I washed the pieces for 15 minutes, then cured them for 15 minutes at 60 degrees per the instructions on the table.
With this print that required lots of structural support on both printers, it seems like the Prusa printer results in a more aesthetic piece. I would be interested in testing the mechanical strength of each of these pieces to see if they significantly differ, since the Prusa pieces are anisotropic. Overall, seeing the difference between the two 3D printing methods was really interesting. I should use them more!
Cost Analysis:
| Cost Type | Cost | Price | Source | Quantity | Total |
| Materials | SLA resin | $149/L | Formlabs | ~6.7mL/hour | $2.50 |
| White PLA filament | $30/kg | Prusa | ~320g/hour | $24 | |
| Labor | 3D printing technician | $21/hour | ZipRecruiter | 1 | $21 |
| Design Engineer | $38/hour | Indeed | 1 | $38 | |
| Overhead | Prusa SLA printer | $0.21/hour | Prusa Forum | 2.5 | $0.53 |
| Formlabs SLA printer | $10/hour | FormLabs Forum | 2.5 | $25 | |
| TOTAL | $90.03 |
