Week 7: Molding and Casting
October 27 - November 3
[0] Overview
With a large variety of materials avaiable to one's fingertips, it's no surprise that our lab has so many options for casting materials. Practically every material that we have in the lab is being used for real world applications.
This week's assignments:
(1.) design a mold around the stock and tooling that you'll be using
(2.) mill it (rough cut + (at least) three-axis finish cut)
(3.) and use it to cast parts
[1] Group Assignment
Attached is the group assignent
here
.
We tested a variety of differnet molds and materials such as EcoFlex 30, Smooth Sil 945, Oomoo 30, Hydro-stone, Task-9 urethene resin, and Foam-it. One of my favorites was testing the chocolate and a nice, shimmering layer of edible refractive paper that added texture.
I chose Oomoo for my molds since they were heat resistant, which is required for casting metal at around 300°C. I was surprised about the foaming (which makes sense since it also releases heat). Based on the different mold material, the quality of the mold impacted the final casting (can't be higher quality than the mold itself).
[2] CAD Modeling: Inverted Mold
The goal for this week will be to create a metal gear - one of several I hope to make. I first started with an inverted mold from fusion. My goal is to slowly work into gears and cast them to make a fully funcitonal gearbox. This would take a couple of extra days, but I spent the majority of this week learning how to make the molds as well as the initial casting.
First, I created the spur gear using the Fusion 360 Python Tool, which allowed me to specify the pitch, the gear size, etc. Then I created an inverse mold of the object, since I had to make the mold to cast the metal in. The file for the gear can be found
here (.OBJ)
[3] Milling the Mold
Before I had to cut, I used the ASPIRE software to draw toolpaths for the 3D milling. Using wax, I was able to configure it and set the polished cutting using the 1/8th toolbit. The rough toolbit wasn't thin enough to go through with an initial cut, so I had to use the fine drill bit from the start. This is likely due to the space between the gear teeth being too small for the larger drill bit to mill. There were suspicious after effects on the mold, with a thin square layer taken off even though my file has no trace of a square object, even as a sketch in fusion.
[4] Setting the Mold and the Metal
As mentioned previously, I used Ooomoo because silicone has good heat resistant properties, and can withstand up to roughly 500°C - 800°C. So I mixed half of each mix A and B together in a 1:1 ratio to create the purple color solution. I used roughly 35mL of solution, which was 1/5 of a cup. This takes up to 24 hours to set, so I left it overnight. Attached is a picture of a mixture of both parts of Ooomoo 30.
While not the most beautiful mold to look at, I didn't see any bubble formation which means that the mold would be able to have a smooth texture and finish.
Once I got the mold, I heated up the alloy using the only functional setting: the pizza setting. After waiting for fifteen minutes thinking it wasn't working, I was somewhat surprised to see that it had melted the majority of the alloy into the correct gear shape.
Since I did not have enough time to reheat it, I left it like this as I found it interesting how the metal melted away to form these random cracks and patterns. The benefit of this design is that the circle in the center was able to form perfectly. This means that more gears like this would be able to fit into a proper gearbox in the future, something I intend on making entirely from this material. Half the gears did not come out well enough to be used, which should be resolved with a better heating system. Overall, I thoroughly enjoyed this week, as it was a quick break from the more intensive programming and development of PCB boards.