Overview

This week, I had to design a 3D mold, machine it, and cast parts from it. I wanted to make something that would serve a real purpose and not be merely an artistic piece. Moreover, I wanted my design to consist of more than one part. Thus, I chose to make a caster assembly. The wheel of the caster was made using the molding and casting process, and the remaining parts of the assembly were 3D printed. The process I followed to make the 2 part mold and then cast the wheel consisted of the following major steps:

1. Make a 3D model of the part to created.
2. Split the part model into 2 CNC-machinable components.
3. Design a 2-part positive mold for each of the 2 components.
• Add "inverted tabs" to the positive mold, to ensure that the 2 parts of the negative mold will be properly aligned during the casting process.
• Add an "inverted hole" to the positive mold, such that the negatvie mold will have a hole for pouring material into it.
• Add a small "inverted hole" to the positive mold, such that the negative mold will have a hole for air to come out when you are pouring the material
4. Machine the 2-part positive mold on eithet the Desktop ShopBot or the Modella.
5. Fill with OOMOO® Silicone Rubber each part of the positive mold to create the 2-part negative mold.
6. Fill the 2-part negative mold with Hydrostone or Drystone to cast the desired part.
7. Sand off any undesirable surface features to achieve the desired finishing.

Making the 3D model

I based my caster wheel design on this model from GrabCad. Since the GrabCad model came as a part file rather than assembly, I first had to learn how to separate the different components of the model in SolidWorks. Hence I learned about and mastered the SolidWorks split features, and was able to separate the model into three parts as shown below.

The wheel was the part I wanted to mold and cast, but because of its geometry, and surface characteristics, I needed to use a 2-part mold process. Thus, I first had to split the wheel into 2 identcal pieces. The obvious choice was to split the wheel along plane of the circle as it is normally done. However, I decided to be explorative this time and make the nonintuitive choice. I wanted to see what would happen if I were to split the wheel along the plane parallel to the rotational axis. I was curious to see whether this choice would work or not.

The next step was to design the actual mold which would encorporate the wheel.There were several things to keep in mind and incorporate into the desing. Those included tabs into which the 2 peices of the mold would fit into one another and an inverted hole through which material would be pored. Designing the tabs was the most difficult part because they had to be design in the inverted way - that is when WOOMOO is poored into the mold, the pieces made out of WOOMOO would have to fit into those tab. In additiona, I also had to account - in an inverted way - for the fact the interior tab (inverted exterior tab) needs to be smaller than the exterior tab (inverted interior tab), and that the base of the tabs should have larger area than the top of the tabs.

Onece the 2-part mold models were designed I milled them out of wax using on the Desktop ShopBot. I hotglued a block of wax to a board, and then I mounted the board to the sacrificial layer of the Desktop shopbot with four screws.

The video below shows the milling process from start to finish for one of the pieces.

And here are the 2 mold pieces after the milling:

Next, I filled the molds with OOMOO and placed them in the vacuum chamber until air bubbles trapped in the material had escaped. Then I left the OOMOO cure overnight.

After a night of curing, the negative molds were ready.

I then poured hydrostone into the 2 piece negative mold and let that cure overnight as well.

And here is the final result.