Hi guys, welcome back to another prototyping session with Ravi. This time, we’re making five “impossible” 3-D printed parts that can fit in a gumball using two different manufacturing methods (SLA & FDM).

To start, I first designed a part on SolidWorks. I designed it specifically for 3-D printing and it’s pretty simple as well. To summarize, it’s a 1”x1” cube with a sphere cut out in the middle. There are square cutouts on each face to visualize the sphere inside. Some intentional designs include rotating the square cutouts by 45 degrees to prevent overhangs and fillet all around for smoother prints – here are some images.

After designing it in SolidWorks, I exported it as a .STL file. From here, I loaded the file into PrusaSlicer and printed it using 0.10 detail and default settings. I determined that there was enough surface area for good bed adhesion so there was no need for brims or rafts. Here are the settings I used and the final print quality:

Lastly, I printed using FormLabs “AmicableKitten”. The thought process here is super different and while I don’t completely understand, I’ve had past iterative experience that makes sense. PreForm was a new software for me, I’ve used PrusaSlicer in the past for SLA printers as well. Instead of printing to maximize SA on the build plate, it’s better to print “diagonally” and utilize supports. Having a lower SA with tank lowers forces with each layer resulting in less supports and print time. The part ended up pretty good – I’m kind of bummed about supports entering the internal geometry of my part. I couldn’t figure out how to prevent this in PreForm but this is a little more straightforward in PrusaSlicer. Here are the results of my print:

If I had to go back, I’d try to make a more complicated part. A sphere cutout from a cube is still technically considered impossible with current manufacturing processes. Also, the SLA print left “dots” from the supports, so I would also go back and reduce support contacts. Here’s an example of what I mean by thick contact points – I had to go back and use some sand paper post-processing.

I’d probably explore more complex internal geometry. For example, something I’ve always wanted to do is create countercurrent pathways modeled with gyroids to maximize SA contact – this has applications in heat exchangers. Also, I’d like to try and use Form3 (SLS) – definitely looking forward to that.

Here’s the final cost chart:

Cost	Price	Source	Quantity	Total
fdm filament	$30/1 kg	prusa3d	~30 g	$0.9
formlabs “Rigid 10k” resin	$300/1 L	dynamism	~10 mL	$0.3
operator cost (me)	$20	me, the operator	2 hrs.	$40
			TOTAL:	~$41.2

https://www.thingiverse.com/thing:6296969