I can't take credit for coming up with this idea (I saw it at a conference) but I thought it was super cool. If you're looking for a quick, simple and cool project using your 3D printer - this one might be for you.
I've only tried it with PLA since it melts at a relatively low temperature and it does not outgas noxious fumes like other materials.
Step 1: Design
Use whatever design software you're comfortable with to create an STL file for your 3D printer. I won't go into it but you can literally find dozens of software packages to make these, from SolidWorks to TinkerCAD and everything in between.
After a couple of tries we settled on a 1mm thick design but you might want to try different thicknesses. The key to making the PLA stick to the shirt are the little "nubs" on the back. We made these 2mm in diameter and .75mm tall. These will melt onto the material and hold the applique in place. We also tried to stay roughly within a 3" boundary.
Step 2: Iron On!
Turn your shirt inside out and place your design with the nubs touching the material. Iron on high for approximately 10 seconds. It doesn't take much! I ironed on a flat table instead of an ironing board to keep the applique flat.
Let the shirt and design cool in place. Don't move it too much while it is still hot, it may warp, wrinkle or come off while it is still hot.
Placement is critical and kind of tough with the shirt inside out!
Step 4: Enjoy!
You can machine wash the garment, just don't put it in the dryer!
Step 5: Sky Is the Limit
We only had black and white filament on hand but you can of course do it with whatever color you'd like. We also tried hot gluing them onto backpacks and that works too!
If you like this, post pics of your own designs!