Zijun Wei - How to Make Almost Anything
Week 8.
Casting and Molding
1. Making the Replica mold (convex in my case, transferring from design file to physical mold)
This week we were shown how to make a mold for casting liquid and metal. This technique is useful for making multiple copies of a part in a batch fashion, and a particular time-saver for making plastic components. Metal takes a bit more time, as it takes time to melt and then solidify the material, but it is still a very efficient approach.
I first made a wax mold out of an stl model using the shopbot desktop, a much smaller version of the shopbot router used previously on OSB. I used the 1/8" endmill, although later I found out that there is a 1/16" endmill, which could have given me more details.
Since the router cannot untercut, a 3D model can only be resolved from one side, I only cut out half of the alien head in the machinable wax.
2. Making the Cast mold (concave in my case, flipping the polarity and make a mold that will form the shape in the target material)
This is the mold used in molding the target material. Since I am using the white liquid plastic, I used the Smooth-on Oomoo.
After spraying a layer of EaseExpo onto the Wax mold features to facilite releasing, I pour in the Oomoo mixture of 1:1 (A:B) by volume. The first trial took me longer than expected, and probably close to the end of the pot life, and as a result, even though negative pressure could pop the most of the bubbles in the mixture, the plastic had become so viscous that the surface could not go flat again. Good news is the inside features are ok.
I did a second trial, and by working faster and pouring only small streams of material, the surface looked better.
1:1 by volume of liquid plastic, and pouring into the Cast mold within the 2-min pot life, the half head came out pretty nice.
Later, I decided to make a full head. I was able to get a dirty wax that no one needed from the staff and made the other half into a wax mold. Following the same procedure, I made mirror oomoo cast mold to the one made above, and I bundled them together with tapes, as I did not make post for alignment in the original wax mold, then poured in the liquid plastic.
The original cast had some defect because some parts in the cast mold did not have air outlet and the air bubble stuck their kept the plastic liquid away. By cutting an outlet and tilting the mold during the casting, I was able to get rid of most of the trapped air and get a nice full head, after some polishing.
Xenomorph party!
