# Week 8: Molding and Casting, or Much Ado About Oomoo
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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:
* STL file for milling the wax mold
## 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.
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.
## 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:
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.
## 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.
Things were a little bubbly, but nothing too severe.
## 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.
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.
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.