It’s official, I can now make jewellery. After some experiments with investment casting in tin, I moved on to bronze, just to see what it’s like. While the results of the first casting run in bronze could have been better, I know exactly what I need to do for future runs to improve metal flow and get better castings. If you are curious about the technical details of this process, keep reading. Much of this article repeats what I had written in my introductory foundry post, but there are proper conclusions this time.
My original intent was to make a video showing off the process of making some miniature pewter chalices, but the first few casting runs in lead-free crown pewter (an alloy which is mostly tin) were disasters. The first time, I made the mistake of using pure plaster for the mould, since plaster is cheap and easy to find. I’ve seen the effects of heat on plaster, so I expected there to be cracks running through the mould; I didn’t expect the plaster to crack to the point of having molten metal going straight through and getting all over the place. Cleaning that up was something of a chore. As I have since been informed, a 1:1:1 mix of plaster, diatomaceous earth, and water is best for diy investment mixtures. After my disastrous first experiment (no injuries, luckily), I decided to wait until getting a delivery of Ransom & Randolph Plasticast before proceeding with the next experiment.
The first run using proper investment didn’t turn out so well, but at least it provided some valuable information. To begin, you know how you can trap air inside a cup if you plunge it upside-down into any liquid? Well, given the orientation of the chalices on my part tree, that was, more or less, unavoidable.
I attempted to alleviate this problem by tilting and rotating the part tree as I lowered it into the liquid investment, but to no avail; I failed to release the trapped air bubbles, as I discovered after pulling the casting out.
There were also lots of little beads all over the surface of the castings, indicating smaller bubbles throughout the liquid investment, creating voids in the mould. This was easily fixed by increasing the de-gassing time from one minute to eight (giving me a full minute to pour the liquid investment into the flask). However, that wasn’t the only fix that needed to be made. You may have noticed that the tiny cups weren’t the only items I attempted to cast; the fifth item, and first on the part tree, is a pattern for a zipper pull that I designed many years ago, and I had two printed at Shapeways, one cast in sterling silver, and the other printed directly in aluminium via SLS. When I attempted to make a third (I’d like to have one for every quarter-zip jumper that I own) myself, however, it acted like a slag trap, and the main body looked absolutely horrid.
Therefore, I made it a point for future part trees to always have a slag trap directly below the pouring cone, though I already had another part tree made, and I didn’t feel like taking it apart. For the next casting run, in which I attempted to make another cup along with a mushroom that I found on Thingiverse back when I was still a novice playing around with FDM printers. This is when I made another critical error, purely out of laxity (fancy synonym for laziness). My kiln is non-programmable, so in order to properly ramp up the temperature in accordance with the burnout schedule for Formlabs castable wax resin, I have to babysit the kiln for hours at a time, increasing the temperature by 40 degrees every ten minutes for the first ramp, then by 36 degrees every ten minutes for the second ramp. When I tried to make the mushroom, I said “screw this,” set the kiln to 700 degrees, left it for two hours, then came back, increased the temperature to 1350 (all these temperatures are in Farenheit, by the way, since that’s the scale used on my kiln), and left it for another two hours before turning it down to 300, and making sure to hold it for about an hour after I saw that the internal temperature had, indeed, reached 300 degrees. As I had feared, the mould had cracked, though not as severely as the pure plaster mould (which I ramped properly, for the record), and I got flash on my castings.
Flash is perfectly normal for moulds with seams, such as two-piece sand moulds or steel dies. However, this isn’t supposed to occur with investment casting. Furthermore, the mushroom came out in two pieces, one of which was an unrecognisable lump with much of it missing. Right then, the burnout cycle is definitely not a corner I can afford to cut! It was some time before I could make another mould then, since I needed to two two-hour periods free in order to manually ramp the kiln. When I finally did, I chose a far better design for the part tree, incorporating a slag trap, as I had printed a batch of five.
I didn’t intend for the cups to tilt, that’s just what ended up happening, thanks to my not-so-steady hands, which I’m trying to fix by practising miniature painting. Regardless, this time, the casting came out quite well; the slag trap worked exactly as intended, and the main sprue below was quite clean.
There is still room for improvement, as I noticed a few beads, indicative of voids in the mould. There were also some voids in the cups themselves, though I suspect that’s simply because the walls are so thin, and tin does some weird things anyway. Perhaps, one day, I’ll perfect this process to the point where I could make miniature chalices that are worthy of selling as dollhouse accessories on Etsy. In the mean time, the result of this particular casting run was satisfactory, so I moved on to bronze. I used an earlier part tree that was lacking a slag trap, so there were some problems with the casting, but as I mentioned at the beginning of this article, I know exactly how to fix the problems that I had.
I ended up having to bend the branches of this part tree in order to fit it into the flask, as you can see in the video. One of the rings did not fully form, and what I pulled from the quenching bucket the morning after casting indicated to me that the sprue became blocked in the middle of the pour, so that particular mould cavity didn’t fill completely. No matter, I managed to get two halfway decent rings out of this casting run. The next time I do this, who knows when, I’ll try a better part tree design with a wider variety of shapes and sizes. If I can get some stones, I’ll be able to complete the two rings that I have now, though finishing the setting is going to be much tougher with bronze than with gold, since the latter is quite soft, but the former is tougher than mild steel. In the mean time, I need ideas; there isn’t a lot of demand for alto clefs and stars of chaos, after all. Perhaps it’s time to revive the Cooperative Artisan’s Guild, as I’ve seen some rather impressive jewellery designs that have never been brought to life. I could fix that; after all, I can do this:
Let me know what you think – especially you, Corinne, if you read this. I hope you find both this post and my video to be informative.