If you missed the first half of our work, you can find that here in KJ’s post on our final. Once we finished making the positive molds using 3D-printing and CNC milling, it was time to make the negative molds that we would actually pour our plastic into. These negatives were made of a two part mixture of Platsil silicone rubber. When making this, we simply followed the instructions given on the containers and thoroughly and carefully mixed equal parts of each by weight. The wooden CNC milled half was made deep enough that we could simply pour in the silicone rubber straight into the positive mold (after spraying the part with mold release to prevent the rubber from sticking the the wood).
For the 3D printed positive, we constructed a small box out of foam board and glued the positive inside of that before spraying with mold release and filling with the rubber silicone mixture.
Once both halves were set and cured, they looked like this:
Left: negative of CNC milled half Right: negative of 3D printed half
After creating the negative molds, it was time to actually create our eight actual pieces. To do this, we put the halves together using the ridges in the molds as alignment guides before holding the whole thing closed with rubber bands. Then, we estimated the volume of plastic we would need by pouring in some water into the mold. Once we had that done, we mixed up the appropriate amount of EasyFlo 60 liquid plastic according the the directions (equal parts by volume) and poured it into our mold. It took a few tries to get the pouring process right to minimize air bubbles (basically just pour slowly down the sides) but once we got it down, we split up the work (4 each) and came in and poured when we had time.
For each piece, we cut away the excess plastic using a knife and a hacksaw
Then, we cleaned up the sides where the two halves meet to give the piece a cleaner look. I used a dremel with a sanding bit for this part while KJ used an ultrasonic knife. Either way, all the pieces came our looking quite nice and clean.
Cost Analysis:
Raw Materials Cost:
2×4 wood piece: $3.89
3D printer plastic (total): ~$5.00
Platsil 73-25 Gallon Kit: $249.00
EasyFlo 60 Casting Resin (Gallon Kit): $109.00
Mold Release Spray: $6.00
Total raw materials cost: $372.89
Labor Cost:
KJ: 3 hours (not including waiting time)
Me: 2 hours (not including waiting time)
Minimum wage in Texas: $7.25
5 * 7.25 = $36.25
Total labor cost: $36.25
Machine Cost:
Monthly membership to makerspace with 3D printers and CNC machine: $50
Total machine cost: $50
Total cost: $459.14
