Molding & Casting

Casting Chocolate Doges and Other Things

Overview

For this week of How to Make, our assignment was to CAD a part to cast, machine a positive of that part out of machinable wax, cast the negative mold from OOMOO resin in the positive wax plug, then to take that negative mold and make a cast of our part using hydrostone, drystone, metals, or some other casting material. This week was fairly intense, in that there were many parts to accomplish in a very limited timeframe. However, despite all of the challenges, and there were many, I enjoyed this week a lot. It was back to working with my hands a lot, and I really liked that. I was able to cast the spinner totem from Inception, a Doge, and a copy of my Brass Rat, all in hydrostone. They turned out surprisingly well. I attempted to cast in drystone but ran into some difficulties. I will be attempting to cast the totem in the low melting temp metal alloy in the near future, as well as making a food safe mold of the Doge and casting chocolate Doges - because why not!

Above: Some of the CAD files I used this week, including the totem piece, split into two sections and attached to the mold plug base, as well as the design files for the Doge that I would 3D print and later cast.

Designing the Positive Mold Plug

The first step to this week was to design the parts that we wanted to cast, and to place them into a mold design somehow. I did this in Solidworks. I first designed the totem fairly quickly, then sat there for around 2 hours figuring out how to split the stupid totem in half to attach to the mold. Finally, I figured out that I needed to sketch a line through the centerline of the part, then use the Split command, select that line and the curved surface above that, and it would split the piece in two.

Once this was done, I needed to design the mold to place them in. I designed it as a simple box, with some insets to place the parts in, and some cylindrical connecting pieces to connect the two pieces of the mold together after they were demolded. Once this was done, it was time to export the stl and get machining! The mold box containing the totem pieces was sent to be machined in the wax, while the stl containing the Doge (which was obtained online) was sent to the 3D printer to be printed quickly.

Above: The cut files from Neil's mod software is shown. On the left, we can see the wax rough cut being sent to the Roland SRM Mill. On the right, we can see the STL being processes for the wax fine cut, which is done via rastering the mold. The fine cut is done after the rough cut.

Using the Mod Software to Machine the Wax

The mold then needed to be machined! I took a wax block (3" x 3.5" x 1.5") and taped it solidly down to the milling bed. I went through the mod software, uploaded my STL, chose the wax rough cut, positioned the zero point on the wax block, calculated the toolpath for the rough cut, and sent it to the printer. It worked the first time I sent it thankfully! I would periodically pause the process and vacuum the wax shavings out as the mill was running.

Once the rough cut had been finished, I proceeded to calculate the wax fine cut. This is a very similar process, but does rastering along all 3 axis at once, instead of just an xy cut, then z step down. The wax fine cut was then sent to the mill, without changing the mill bit, wax position, or the zero position. This process can be seen in the pictures below as well.

Above: The Roland SRM mill doing the wax rough cut and wax fine cut, cutting out the positive mold/plug for the two halves of the Inception totem.

Making the Negative Mold with OOMOO Silocone Resin

The mold was then taken out of the mill and cleaned off to get rid of extra wax shavings. The OOMOO 2 part solution was then weighed out and mixed together. The mixing was done slowly and up/down to minimize bubble formation. When it was poured, it was also poured slowly and in a thin stream to minimize bubble formation. It was then poured into the wax positive mold up until the brim of the mold. Before it was poured, the positive section of the mold was painted with OOMOO using a small brush to minimize bubbles after the bulk resin was poured in.

Once it was poured in, the wax mold was shaken and tapped to get rid of as many bubbles as possible from the resin. It was then allowed to set for around 2 hours before it was taken out of the mold.

Above: Shown on the left is the wax positve mold/plug. On the right is the OOMOO being mixed. You can see the different resin part colors mixing together.

Casting the Totem using Hydrostone and Demolding Afterwards

The mold was then used to cast the totem in Hydrostone. The Hydrostone was mixed together, powder into water, and pressed into the two mold pieces, which were then clamped together. I waited a few hours for the Hydrostone to cure, then took the totem out. There were a few issues with one of the ends of the totem not going quite far enough into the mold so it looks cut off. It still spins pretty well though.

Above: The process for making the Hydrostone mold is shown, including mixing the Hydrostone, pouring into the negative mold, demolding, and spinning the top.

Making Molds for the Doge and Brass Rat

The next thing I tried to do was to skip the wax milling step and cast over some actual objects. For this, I first 3D printed a Doge design that I found online. It printed quite well. I then made a container with two seperate compartments - one for the doge and one for my Brass Rat. I then mixed some OOMOO together and poured it into the two compartments, making sure to avoid as many bubbles as possible.

Once the OOMOO 25 was set, I very carefully cut into the molds using a utility knife. By cutting part way into the mold, along the axis of the Doge's back, I was able to pull the 3D printed Doge out of the mold. Same for the Brass Rat. I now had super detailed molds for both.

Above: The process of creating the negative molds for the Doge and Brass Rat is shown.

Casting the Doge and Brass Rat

The next step was to cast both using the Hydrostone. I initially tried the Drystone, but it was too thick to force into the mold cavities. After mixing the Hydrostone, and creating holes to pour it into in the top of the molds, I proceeded to pour it in, then pressed or clamped the molds shut, and waited for them to dry.

Once they were dry, I carefully opened up the mold and pulled them out. I had some issues with the Doge's legs not being molded correctly (so he's a little stunted... :( ), but other that that, both parts looked great. There was a surprisng amount of detail for the Brass Rat. If the lower ring section had been slightly thicker, it would have been almost identical. There is a slight parting line on it where the mold was cut, but it's hard to notice on the ring, and virtually nonexistant on the Doge.

Above: Some images of the Hydrostone Doge and Hydrostone Brass Rat. It gave a surpsiing amount of detail. The 3D printing layers are even visible on the Doge.

Casting the rings in metal

Later, I cast the rings in a low temperature melting alloy in the Oomoo mold as well as in bronze using the Oomoo mold to make a wax model and then lost wax castin the ring in bronze.