Challenge Coins - Part 2
Well it was weeks of work, loads of setbacks, tons of trial and error and both heartbreak and pure joy, but I am very pleased to say I have successfully created custom challenge coins! I covered how the coins were designed and prototyped in a previous post, this time around it’s about how they were molded and cast in pewter. You’ll find the part two video at the bottom of this post. The plan was simple enough, design a mold form, or mold of a mold, in Fusion360, print both halves, make silicone molds from those, cast pewter in those molds. While it seemed simple enough, the actual task took quite a bit of time and a whole lot of research and experimentation to make happen.
Luckily, with my 3D modeling and animation background, combined with my creature effects background (yeah, I’ve done both) the mold design was fairly straight forward. I made a new design and imported the coin model, then I modeled a top mold and bottom model, both intersecting the coin half way. I also modeled both positive and negative keys, vents and the pour spout. Then I exported stl files to print on my Elegoo Mars resin printer. The first orientation I chose, with the mold angled front to back, resulted in a funky offset line through the bottom. At first I thought it was just a print anomaly, like I didn’t tighten the build plate enough or something, but after wasting the time and resin to print it again, I realized it was a suction problem with the orientation (oh well, test molds!)
While the new molds were printing I took advantage of the new test molds to do a quick experiment with the Smooth-On Mold Max 60 high temp silicone. Silicone can be kinda tricky and temperamental, and I wanted to make sure it would cure in contact with the Elegoo ABS-like resin I used. So I mixed up a partial batch and poured a test. The next day I pulled out a perfectly cured and really detailed mold, with no air bubbles! Since I have neither a vacuum chamber to degass the silicone or a pressure pot to cure in, I had to rely on careful mixing and pouring techniques to hopefully get a bubble-free pour, thankfully it worked.
Flash forward to about a week later, it was time to pour up the actual molds. I started with a good coat of Universal Mold Release, concentrating on the walls and the keys. Pulling the test pour out was a little tricky so I wanted to make sure there was no sticking. I poured up two molds and let them cure overnight. The plan was to use a scalpel to carefully loosen the silicone around the walls and gently pry up an edge and pull them out. After all, it worked on the test pour. Well, one thing I didn’t take into consideration was how thin and far more flexible the test pour was. It didn’t need to be thick to tell me what I needed to know, in fact it would have been a waste of silicone. I also didn’t consider just how brittle resin prints are, and let’s just say it didn’t go well.
In addition to breaking both mold forms, I also discovered a major problem with my stl export from weeks ago. Somehow I managed to not select one of the keys when I exported in Fusion360. I suspect it was because it wasn’t visible in the “home” view, but I have no idea really. All I knew was I was pretty upset with past me, and it was back to the drawing board.
So I went back to Fusion360 and added “wild walls” to the design, little removable panels. As you can see the Elegoo Mars print volume is a little small for the design. Optimally I’d like to print these at 45º, but that just won’t fit. But the new design printed out great, and the walls actually fit. The plan was to tape them on, lube up the mold with vaseline (no longer messing around) and pour up two new molds.
The plan worked perfectly, or would have if I had enough silicone. Back to Amazon, and wait another week. I should also mention that about this time I decided to completely remodel my studio, or at least change everything around. Since LEGO related animation projects aren’t really happening anymore at my studio and I was crammed into that little space in the back I decided to switch spaces. This was no small task, and was days of work clearing shelves, moving crates, shoving around furniture and generally wrecking the joint. It was like a game of Tetris, that I was losing big time.
Once a new pack of Mold Max 60 came in I could pour up the other mold, and it was time to bring on the heat. I purchased a Hot Pot 2, which is designed for melting lead (for casting bullets) and built a little stand for it with some wood I had laying around and my new bandsaw. The little stand that came with it was a molten metal accident waiting to happen, so I quickly cobbled together something a little more stable. I also ordered my first ingot of britannia pewter, which surprisingly came in bar form. Now with some metal, a way to melt it and two proper mold halves it was time to start casting.
But since this was my first time doing this, I decided to do some experimenting with the first botched mold. I cut the extra positive key off so it could close all the way and cut some aluminum squares to use as backing plates (mmmm, mold sandwich!). The new bandsaw was coming in handy.
Anxious to try everything out I did a quick test with pretty much no prep and was surprised just how well it came out. It was slightly misaligned, but had no flashing and all the detail was there. I had been fretting for weeks that when it came time to pour things just weren’t going to work. I was doing simple gravity casting, no pressure, no centrifuge, just dumping hot metal into a mold. I had done enough molding and casting in the past with other materials that I was fully expecting some real problems, but it was fine! And this was without preheating, dusting with talc and even proper mold alignment. Needless to say I was excited.
I did a few more test pours, experimenting with preheating, clamping techniques and how fast I dumped the metal. I quickly discovered that pouring fast was far superior, that I needed a better and faster way to clamp than clumsily attaching two C-clamps and that my Hot Pot was capable of way higher heat than my molds could take. On the fourth pour I actually singed the test mold, darkening it and making it smoke a bit. I overcame this with monitoring the temp with a laser thermometer and unplugging it when it was too hot. The great thing about metal casting is when it doesn’t work out, you just dump it back in the pot and try again! The pour spout is pretty easily popped off with a little flexing back and forth, and the vents snip off easily and go right back in the pot as well. This was followed with a little filing on the top of the coin for a smooth finish.
I continued to test until I was comfortable and then switched to the good mold. Before I knew it, I had a good handful of coins, eleven in fact. Now it was time to experiment with blackening. For this I used Jax Pewter Black. Now I must stress at this point that this stuff is not to be messed with. It contains hydrochloric acid, selenious acid and copper sulphate, it’s dangerous. I used a respirator, goggles and double gloves. Take this stuff seriously please. Using an acid brush and a small plastic container I gave each coin a bath which quickly tarnished them. Then it was time to break out the steel wool.
My original plan was to use a wire wheel to burnish the coins, but the one I picked up proved to be too abrasive and didn’t give a finish I liked. Luckily I also picked up some steel wool and the finish it leaves is pretty awesome. I had been shooting video the entire process, and of course I shot the step where I buffed it for the first time. You can see my reaction of total joy in that clip, I was absolutely giddy. I then proceeded to buff the rest of the coins. It’s labor intensive, but the results are so nice. In the image below you can see every stage of the metal coins: raw cast, cleaned up, blackened and buffed.
A week later, with another hero mold made and a fresh pewter ingot, I set out to make a larger run. I did a totally of 35 coins on that day, I haven’t calculated the price per coin, but since they have no monetary value they’re both worthless and priceless at the same time. Next I needed a way to store them. Initially I was just putting them in a small cardboard box, but they’re heavy and that box wasn’t going to last long. I quickly designed a tray to store them, and by quickly I mean in like 15 minutes. I sliced it and kicked off a print on my Prusa i3 Mk3 and they were ready the next morning. They work great, and show off the coins wonderfully.
I’ve already given out a few, and the reaction has been great. They seem to make everyone happy, including me, so I consider it to be a successful project. I figure I’ll make another run in the future, all I need is some pewter and some time, I have everything else I need on hand.