# Week 8: Molding and Casting, or Much Ado About Oomoo <a href="../index.html">Back to Home</a> In this week we learned the ancient art of molding and casting! There were three distinct steps to the process: * Cut a positive mold out of wax * Cast a negative mold out of Oomoo * Cast the final plaster piece Files for download: * <a href="files/Katara_necklace_shallower.stl">STL file for milling the wax mold</a> ## Design I wanted to finally make something that worked towards my final project, so I thought I'd make the pendant of my glowing necklace. I rather quickly realized that I could only do a prototype, because there were lots of components of the necklace I couldn't figure out in time, namely: * A translucent plastic material (we only had on hand plaster, drystone and hydrostone), and I wanted to make sure it worked before spending money on different material * I have no idea how to cast something that could work as a hinge/clasp mechanism The key challenge for this was that I needed the pendant to be hollow. To achieve this through casting, I decided to make the two sides of the pendant separately as shells. Initially, I was going to do it by having a top concave mold suspended over the bottom convex mold and leaving a hole for the plaster to be poured through. But then I realized that, since I was making holes in the shells anyway so that I could attach the two sides together with string, that the top mold could just sit on the hole-making studs in the bottom mold. This also meant that I didn't need any keys and sprues. For the actual design, my boyfriend gave me the excellent idea to do Katara's Water Tribe necklace from Avatar: The Last Airbender. It's a simple design and lends itself well to a large-diameter but thin pendant. <img src="images/necklace_picture.jpg"/> As always, I used vectorizer.io to convert this to svg, then CloudConvert to convert it to pdf to load into Rhino. I had to clean up some of the angular curves by filleting or drawing curves across angles and trimming. I then converted the curves to surfaces with PlanarSrf. I made the half-pendant polysurface base by BooleanUnion-ing a torus and a cylinder and trimming it in half. I then made a slightly scaled-down copy and used BooleanDifference to carve out the inside of the shell. I then extruded the design surfaces into the pendant. I measured my block of machineable wax with a caliper and CADed it, setting both mold pieces into the block. I tried to make sure there was enough space around the patterned mold for the mill bit to mill all the way through. I had some issues with unclosed polysurfaces after doing BooleanDifference, but what's going on in the vertical sides luckily doesn't matter. <img src="images/necklace_cad.jpg"/> ## Milling the positive mold I used the Roland with the 1/8" ball-end mill with the following settings: * DPI: 250 * Speed: 20 * Overlap Percent: 50% for roughing cut and 90% for finishing cut The roughing took about 40 min; the finishing took about 100 min. The finishing would have been shorter if the toolpaths Fab Modules calculated for the column cuts didn't for some reason extend beyond the block: <img src="images/toolpath.jpg"/> The mold broke on a particular brittle side when I was too hasty in trying to get it off the bed, but it bit of tape was sufficient to patch it up. <img src="images/wax_mold.jpg"/> ## Casting the Oomoo negative mold I measured the two components into separate cups (separately they have long "pot" lives), guestimating equal volumes. When I was ready, I mixed them together, trying to avoid bubbles, rather unsuccessfully (mixed, it becomes to thick to pour after about 15 min). I poured the Oomoo in and tapped the block for a few minutes to shake out the bubbles. It cured fully (no tacky feeling) after about 2 hours. <img src="images/oomoo_in_wax.jpg"/> Things were a little bubbly, but nothing too severe. <img src="images/oomoo_mold.jpg"/> ## Casting the final piece My classmates had already made some plaster of paris pieces, and they were all powdery and brittle. Hydrostone is supposed to be much harder, so I decided to try that out instead. It's supposed to cure in 2 hours, but I just left it to cure overnight. <img src="images/hydrostone_curing.jpg"/> The holes alas didn't come out, and the thickness was constant, but it kind of worked! The hydrostone is indeed less powdery than plaster of paris, and has a nice stone-like look. <img src="images/hydrostone_piece_up.jpg"/> <img src="images/hydrostone_piece_down.jpg"/> Some things I should have done: * Made the well with the bowl mold shallower, to decrease roughing cut time * Made the holes, and thus stud supports, larger, and shift them so that they're closer to the center (definitely not on the sloped sides) * Made the pattern a little wider - some parts were shallower than they should've been because they were too narrow for the 1/8" mill * To ensure that the holes would come out and that the thickness was consistent, I should have had made indents in the top concave mold that fit into the studes in the convex mold.