I was interested in metal casting as I had never worked with metal before.
Pretty early, and I don't remember where the flash of inspiration came from, I got the idea of making challenge coins for the EECS section.
As EECS had recently changed to a new logo, it was as good of a time as any.
Although I suspected that most of these types of coins are actually pressure cast in the real world, I wanted to give it a shot nonetheless with our techniques.
On the front side, I CADed the the EECS logo using the SVG data directly.
I went through several iterations of which course code to use for the rim, until I found out that not everyone in our section was registered for the Course 6 code.
On the back, I paid tribute to our favorite section instructor, Anthony, with a quote and a shrug that he's known for.
Design for Manufacture & Wax Milling
As a two-part mould, I cut my wax block down in half.
Rather than the big CNC router, I would be using the Othermill to machine my wax because of the level of fine detail.
This further required cutting my wax block down to fit in it!
Each half was also designed with registration posts for the two pieces of the mould to fit together.
Oomoo Woes & Mould-making
Since I knew I needed to cast around 20 coins, I decided to cast multiple Oomoo moulds to parallelize that later step.
Owing to a bad batch of Oomoo that had been left out for too long, I faced numerous issues, but eventually managed to cast a set of four (out of eight) that were reasonably good and without too many bubbles (using both the vaccuum chamber and lots of table banging!)
Casting
The metal we worked with was Roto 281, a low-melt, 58% Bismuth and 42% tin alloy.
The process was fairly straight forward - load metal chunks into a crucible, let sit in oven, melt, and pour into mould.
The mould halves had to first be treated with talc powder to help break surface tension, however, and were held together with spare Oomoo mould halves and rubber bands.
To ensure that we had enough pressure for the fine features to be cast, the mould was held and poured into vertically.
In addition to casting a double-sided coin, I tested out casting a single-sided coin to check the viability of doing single-sided coins in the event that we were starting to run out of Roto 281.
Final Casts
The double-sided coin came out relatively good, although it was elongated vertically.
We suspect it was due to the weight of the Roto 281 pulling it vertically, as well as the side holder pieces not being rigid structures.
The coins additionally had a streak down the middle, and it's uncertain if that's where impurities in the metal are first deposited during the pour, or if the motion of the liquid Roto 281 removes the talc that was there.
One pleasant surprise I found was that the smallest bubbles in the Oomoo mould ended up either full of talc, or were too small for the Roto 281 to cast.
As I suspected, the single-sided coins were not viable due to their flat pour, which meant that we couldn't build enough height to create enough pressure for casting.
Group Casting
With the assistance of Solomon Chen and Olivia Seow (and a personal order of more Roto 281),
we returned the week after to cast the coins for the section.
At someone else's suggestion, I 3D printed jigs for holding the Oomoo moulds together.
Unfortunately, I didn't take into account that the Oomoo moulds were unevenly poured, so most of them were thicker than the tolerance I left on the jigs.
We ended up bandsawing them open, and using them as rigid jigs in combination with rubber bands for the Oomoo moulds.
Interestingly, after many, many, uses, they too started deforming from the heat!
The end result was quite satisfactory, with every coin coming out somewhat unique, but all having the fine features from the moulds.
Everyone in our section who was left by the end of HTMaA received one, as did Anthony, Harrison, and Neil - accounting for 19 coins total (ft. Olivia's death metal cat).
