This week, our assignment was to 3D print an impossible object.

Procedure:

We started off by looking on Thingiverse to find an object that would be impossible to create using any technique other than 3D printing. I chose to make gyro rotating rings that are kind of like a fidget toy. This object is “impossible” because it is made up of rings that are inside of each other which can typically only be done by having an opening for the inner rings to be inserted.

We also needed to use two different printing processes for this assignment, and I chose to do fused deposition modeling (FDM) and stereolithography (SLA). FDM works by extruding fiber through an extruder with a hot end. SLA works by using a laser to photopolymerize material and it creates its own supports.

Since I had previous experience with the FDM printers, I decided to start by printing the rings exactly as the file was on Thingiverse to identify what changes I needed to make to the file for the object to be correct. I was really pleased with the results, but it was a little too big to fit into the gumball capsule we were given. To account for this, I tried again, but scaled the object down to 90%. After doing this, it fit perfectly, maximizing the space that it took up in the capsule.

Since this worked well and I was starting early in the assignment period, I decided to print four of these at once to have the three needed ones and to have two extra to choose from in case any of them had any issues. This took 8 hours, so I decided to start it at night and come back for it in the morning. Even though there are often problems with long prints like this one, I did not have any problems and was ready to move onto the SLA prints.

For the SLA prints, I used the FormLabs printers. I saw that printing one would take 6 hours and two would take 8 hours, so I decided to print two at once. When I came back, I washed the pieces, removed the supports, and cured them. I realized after that the rings were very difficult to take apart because there were very small supports between them that I was not able to reach. I used a box cutter and was able to get between the rings, but I ended up breaking the resin which ruined the object. I also was only able to separate the inner ring on one (the outer two were still together) and the outer ring on the other (the inner two were still together).

I decided to rotate the middle ring by 90° so that there  were less supports between the parts to break off. I also printed these at the same time overnight, and when I came back in the morning, I followed the same procedure. This time though, I was not able to even get any of the rings apart at all. This was pretty discouraging, but I had one more idea that I wanted to try out – I wanted to find a way to edit the stl file and increase the space between the rings so that the supports would be easier to break. I was also planning on trying to use the other SLA  printer since I was told that it was more accurate. This way, I could also compare the two and better understand what was going wrong.

After working on the file, I went in the morning to start the print, but was surprised to see that it said out of order 🙁 So I decided to try one last time on the FormLabs printer and see how it came out. I was also surprised that this print took a lot less than the others, about 5 hours. When I came back, I washed the pieces and then tried to break the supports off. I was so happy that it actually worked!! I cured the pieces and sanded off the remaining pieces of resin that made the surface rough. One of the pieces did have a cut going through it, and I think that could be due to an error in the file (not likely since I just copied and pasted the same object within the file), or possibly something going wrong in the printer in that location. I decided not to print it again though since the piece is functional and I did not want to use up more time or resources as other people were trying to print as well.

Overall, this was a great learning experience, and I am very happy with how the results turned out!

Cost Analysis:

Raw material cost

Material	Ticket Price	Spent
PLA	$49.95 (750 g)	$9.99 (about 150 g)
Grey Pro Resin	$199 (1 L)	$31.44 (158 mL)
Total		$41.43

Labor cost

Assuming minimum wage in Texas of $7.25

Task	Time	Spent
Training	1 hour	$7.25
Prep/Design	4 hours	$29.00
Printing/post-processing*	6 hours	$43.50
Total		$79.75

* Does not include actual printing times since I was not working at these times

Machine time/machine hour cost

Machine hour cost can be calculated by dividing the cost of the machine by the total hours it should function, and multiplying that result by the time that the machine was in use.

The Original Prusa i3 printer at the OEDK costs about $1.099, and is expected to last for about 800 print-hours before maintenance. For this case of 800 hours, and since I used the printer for about 12 hours, the total machine time cost was $16.49.

$1,099/(800 hours) = ($1.37/hour)×12 hours = $16.49

The Formlabs 2 printer at the OEDK costs about $2,400, and is expected to last for no less than 36 months of regular operation. For this case of 26,280 hours, and since I used the printer for about 20 hours, the total machine time cost was $1.83.

$2,400/(26,280 hours) = ($0.09/hour)×20 hours = $1.83

Project total: $139.502 (for 11 rings, five of which were completely post-processed)

Overall, I really enjoyed this project and hope to use the plasma cutter more in the future!