Milling the Positive Mold. To create the initial positive mold I modeled both sides in Rhino and then used the Shopbot to mill them out of 1.5″ foam. For the roughing pass I used a 1/4″ ballnose endmill, and an 1/8″ flat endmill for the finishing pass, both at 9000 RPMs with feed rates of 6/6 and 10/6 respectively. In the photo above you can see the machine halfway through the finishing pass, with the left side still a bit rough.
Gessoing the Positive Mold. Once the mold was finished milling and had been sanded down, I applied a layer of Gesso with a small brush to fill in the remaining porous areas in the foam.
Risers and Sprue. While milling the foam my original risers and sprue had been torn off by the router – I had made them only 1/4″ in diameter or less, so they were extremely fragile – so I improvised by using some nails for the risers and glueing a piece of dowel onto the mold to form the sprue.
Pouring the Oomoo. Since I wanted to cast the final piece out of a hard plastic, I created the negative mold out of a flexible material. Once the Gesso had dried I brushed all the molds with some dish soap to ease the demolding process. Using 3/4 of a plastic cup of both part A and part B, I mixed up a batch of Oomoo and poured it into the three molds, trying not to get too much on the risers and sprue.
The Negative Mold. After letting the Oomoo set for about 2-2.5 hours I removed the negative mold from the foam and washed off the soapy residue.
Readying the Negative Mold. Similar to the previous pour, I applied a thin coating of dish soap to all parts of the mold using my finger, and then inset the three smaller pieces into the bottom mold.
Strapping the Mold. After aligning the top piece with the rest of the mold I strapped everything together using 8 or 9 rubber bands. I tried also placing a heavy object on top of the mold to help keep the plastic from escaping but that proved less useful than the rubber bands.
Pouring the Hard Plastic. Donning safety gear I mixed up some Smooth-Cast 300, which has a pot-life of 3 minutes and sets in about 7-10 minutes, and poured that very carefully into the sprue. After pouring about half the contents of my cup I would shift the mold to assist air in escaping from some of the smaller spaces. When the plastic begins to appear at the tops of the risers I would press down on the center to make sure the mold was completely filled, and then place it under the ventilation hood to cure.
Demolding the Plastic Fastener. The process of demolding was very simple, but since my mold was poured horizontally there was a substantial amount of material that seeped into the spaces between the top and bottom pieces. However, this excess material was easily removed from the pieces.
Before Sanding. This was the piece before being cleaned up. To remove the excess plastic I used an xacto knife and a medium grit sandpaper.
The Plastic Fastener. The final piece after post-production.
Replication. I made a number of the fasteners, six of which are displayed here. Some broke during early tests of of the snap fit, and others while being cleaned up.
Fastener Trifecta. Here three fasteners are connected to each other as they would be in the screen. In the next iteration I would try casting in a plastic that is slightly more flexible to facilitate an easier connection between the pieces, and work out a set of tolerances to incorporate into the design itself.
COMPLETE FABRICATION 