Making a mould frame
Vulcanization not only requires even heat but generates a lot of pressure. This is due to the material expanding as it cures. If you think about it, this is a good thing because it allows the silicone will fill out all the nice details you have spent so much time sculpting. But you don’t want it to expand too much because that would make the miniatures noticeably smaller than the green stuff master. As such, silicone manufacturers usually list shrinkage on their website. Econosil, for instance, has a shrinkage of 1.1%. We therefore need to counteract this expansion by using a robust metal frame made out of aluminium or steel. A second advantage of the thick metal is that it more evenly distributes heat (think of the differences between a cheap thin-bottomed cooking pot and a more expensive, heavy one). We should try to keep the material thickness even, however: if one side is more thin than the other, this would imply an uneven vulcanization rate – this can result in more pronounced mould lines when one mould half cures before the other.
If you live in such a forsaken part of the world as I do (for mould making at least), chances are that mould frames are either prohibitively expensive or connected to import from abroad and the high shipping and tax rates that comes with. I was not willing to spend 50 euros or more on a piece of machined aluminium after having spend already so much on raw materials and other parts for this project. It’s just a square hole in a block of 30 mm thick aluminium, right? So enthusiastically, I bought some cheap scrap aluminium at different thicknesses (20, 25 and 30 mm) and started drilling holes into the thickest one (I bought a used drill stand along as well). Well, I soon did not feel too optimistic about my consumer-grade drill surviving this if I were to drill holes around the whole perimeter (it’s 30 mm thick metal after all).
So I settled on a minimum amount of holes and used two hand saws to remove the rest (first a fret saw to connect the holes and then a 300 mm metal saw). This took a few hours in total but I did a bit at a time, so I was finished after a week or so. The result is a rather crude but hopefully functional mould frame. I could not be bothered to make it pretty. I made sure to deburr all the sharp edges such that there is no risk of accidentally cutting myself and that the surfaces where the holes are were planar. I will use C clamps to attach the 8 mm thick aluminium plates on both sides, hopefully creating a more or less sealed enclosure (I think heat transfer is more important than an air-tight seal here).
I also bought some square tubing I intend to test at a later point (I will post a picture at a later point). If the wall thickness is enough (around 5 mm I believe), this would be a far easier way of creating mould frames of different sizes. However, I did not want to introduce more potential failure points right in the beginning which is why I took the more labour-intensive approach described above.
If mould frames are affordable at your place, good for you – then I would suggest you buy them instead. A note on commercial mould frames. Some models feature a small channel along the sides to allow for excess silicone to ooze out. The presence or absence of these channels seemed to be a rather controversial one in the past if they are necessary or not. Quoting a Castaldo representative, the industry generally does not use them but some jewelry makers like them because silicone oozing out of the mould frame looks rather satisfactory. The general consensus seems that there is no clear argument for or against these holes, so they seem to bring little if any benefit. I therefore did not add them for the simple two reasons that (i) you probably don’t want silicone potentially messing up your oven and (ii) I was frankly too lazy to drill a single further hole.
Next, I’m going to do some vulcanization experiments in a toaster oven.