10. Molding and casting

This week's topic and assignment:

For the molding and casting assignment, I made a hollow egg-like enclosure. This is the first enclosure prototype for my final project, a wearable temperature sensor.

Designing:

I designed the mold using Fusion360. Instead of creating the design for the final product, I decided to design the mold itself. It was a fun exercise for me to think through the positive and negative space elements of the design and how it would all come together. I found an short online tutorial for a hollow mold that was particulalry helpful. DIY Moldmaking Skills: How to Reproduce a Solid Part as a Thin-Walled, Hollow Cast Part

I ended up needing to invert my design because I had initially designed it to be a two-step project (mill mold, use mold to cast result), but then Henk said assignment was for it to be three-step (mill mold, make mold from milled mold, then use the 2nd mold for casting).

Machining

I machined the design in blue machinable wax on a ShopBot. I set up the toolpaths with Partworks3D. I did a single-axis rough cut and a dual-axis finishing job. I used a long 1/8th inch ball-end mill. The rough cut toolpath used a 48% overlap, 20mm pass depth, 6000rpm spindle speed, 20mm/second feed rate, and 25mm/sec plunge rate. The finishing toolpath was the same except I changed the stepover to be 20% instead of 48%, and selected dual axis.

The machining time was close to calculated estimate of 1 hour 30 minutes and I was pretty happy with the end result. If I could improve the design, I think I would make the registrations larger next time. Aside from that, I think the design was milled as I expected it to be.

Casting

I used PMC 121/30 Pourable Urethane rubber with a 30 minute pot life and 16 hour cure time. I roughly measured the amount I needed by filling the mold with water and pouring it into a cup. The cup overflowed, so I decided to pour the mix into two cups so I could mix and pour it more easily. I gently poured the two parts of the urethane mix into the cups while the cups were on a scale so I was able to make a ratio of 1:1 mix. I stirred each cup for 5 minutes with a timer, and I spent around 10 minutes pouring the mix into the mold as slowly as possilbe so I could reduce bubbles. After I filled one side of the mold, I realized that I had forgotten to add mold release, so I added it to the second side of the mold before pouring in the rest of the urethane.

24 hour later, the cast had not set. It was still very liquidy and messy. My suspicion is that I did not mix the urethane parts together well enough. With Anne's help, I decided to remoe the messy urethane that had not set yet, clean the mold, and add a new mix of urethane. I poured the original unset cast into a glass jar for disposal. I cleaned the mold with paper towels as well as I could. I created a new PMC121/30 mixture from the same bottle, stirred it vigorously, and poured it into the mold.

I still need to add: - Photos - Fusion design file - Group assignment info - Lecture video links - Make cast from mold and write about results of cast