Finding our knight in shining plastic
For this assignment, Allison and I chose to cast a knight as our chosen chess piece. We browed Thingiverse for a suitable model with a few criteria in mind:
- No holes to ensure it is removable from the mold
- Appropriate size (<3 in)
- Aesthetically pleasing
- Symmetrical across one plane
We ended up selecting the knight found at this link.
Preparing and printing our knight in shining plastic
Once our knight was safe and sound in our Downloads folder, he was ready for processing. First, we split him in half in MeshMixer right between the eyes and kept both halves. We then exported both halves to Fusion360 to then design the bases for each mold negative. We created a rectangular base with the base of the knight piece lined up with the edge to create a pour hole for the mold. We then added four pins to one half with a 4mm diameter and four holes to the other with 3mm diameter. These values were chosen so that the mold halves would accommodate each other with a bit of tolerance. These were lined up so that the molds we created next would have pins and holes that lined up with each other.
Finally, it was time to print. We exported our mold negatives as STLs and loaded them to the queue on 3DPrinterOS. Once a Prusa printer was free (which was definitely a challenge to find), we started the print of one half, then the other.
Molding our knight into shining plastic
With our 3D prints fresh off the Prusa, we then used cardboard to make an indeterminate box frame around our knight halves using hot glue as our adhesive. We then mixed two-part silicone and filled each negative with about one inch of silicone. We let these set overnight and then ripped open the cardboard perimeter to find our very well-detailed mold halves.
With our mold ready to go, we locked them in place using rubber bands and carefully lined up the seam between the halves to reduce its visibility in our final knights. Once everything was in place we then mixed two-part polyurethane quickly together and poured it into our mold, ensuring that any bubbles that were trapped in our knight’s snout had the opportunity to escape. We then marveled almost every time as the mixture went from clear to white like crystalizing bubbles. After 15 minutes, the first of our fleet was complete! We then repeated the molding process to create an 8-strong militia.
Since we were really happy with the detail in our knights making each look like it was printed and how clean the white knights looked, the only post-processing we deemed necessary was sanding the bases with 800-grit sandpaper to ensure they were level.
lessons learned
- Remember that the 3D print is the negative of the mold, therefore the holes should be smaller than the pins
- Rubber bands work better than duct tape to hold the mold halves together
- Allow air bubbles to be released from areas that can capture air by rolling the mold (see below)
Overall this was a really fun project for both of us and we were really happy with the outcomes!
Cost analysis
- Labor: 15 hr x $20/hr = $300
- Materials
- Overhead: $20
Total: $332.70
