A first foray into mould making
For a first test I figured that my anatomically not quite correct arms may be good objects of study. First, they are relatively planar avoiding advanced mould line engineering. Second, they are made out of green stuff, my putty of choice these days. I want to see how green stuff and its details survive under 165-177 degrees C (320-350 degrees F) – the vulcanization temperature of Econosil. Experiments I did a few years back with green stuff under a FIMO coat showed that it seems to partially degas leaving a crumbly dark green residue. This was several tens of degrees lower, though (I believe you bake FIMO at around 110 degrees C). Third, the test also contains a weapon made out of polystyrene, another material I want to test how it behaves under heat.
If these tests are successful I would move on to a big hat (a hat that is, not the dwarf). This test piece introduces a new challenge and a new material: slight undercuts in the top section where the feathers are – and the material is a mix of green stuff and Procreate, a putty supposedly more stable under vulcanization temperatures. We will put this to the test. Here I need to be more clever about my mould line placement and slightly angle the piece.
But on with the first test. I placed the mould frame onto a 8 mm plate, generously coating everything with baby powder (talcum). This will hopefully prevent the silicone from sticking to the crudely worked mould frame. Then I applied the first layer of Econosil – you can simply cut the strips with a scraper, I used a piece of styrene sheet for a lack of a better tool. This stuff has a consistency comparable to freshly mixed putty, I would say slightly more stiff then Green Stuff – in line with Procreate. I made sure not to get baby powder onto the first layer. Then a second and a half layer to fill the mould frame half-way. I pressed everything down to fill all the gaps. Then I placed the masters in a way that they would fill without leaving cavities (that is the top to bottom direction in the picture).
Perhaps I need to add a vent or two to the weapon’s arm. Vents are there to avoid creating said cavities – the air can escape through the vent instead allowing pewter to fill this section. I then placed several so-called mould locks across the mould half, in my case IKEA-like shelf retainers (these pins you put into Billy-like furniture). They force the mould to lock into a specific state. The only thing worse than pronounced mold lines are shifted mould halves – that’s pretty hard to correct (looking at you 90s GW plastic minis). People also use acorn nuts but these pins were significantly cheaper for me and narrower as well, so I bought them instead – the thing you should note, they remain permanently attached to one mould half. After having added the mould locks, I gave everything a dusting with baby powder and added the remaining two-and-a-half layers of silicone. Once again, try to avoid getting powder in between the layers. Then I pressed everything gently down again such that all gaps where filled. The nice thing about Econosil is that you can simply add a bit more at a location that does not have enough material. When everything was roughly flush with the mould frame, I powdered everything again and placed the second plate onto the frame. Lastly I tightened the clamps.
I had already pre-heated the toaster oven to 170 degrees with air circulation. Castaldo advises to factor in 15 min vulcanization time for each 6 mm, that makes 75 min for 30 mm. It took 20 min to reach the terminal temperature again (the cold metal apparently cooled down the oven quite a bit). Then I left it in there for about two hours after that to account for a small delay in reaching the same temperature inside the mould. A note on oven temperatures – reading online reviews, some models are apparently rather unreliable. I would suggest to buy a (good!) oven thermometer as well. Those are well-spend euros – nothing would be worse than a failure at this point because the toaster oven temperature control decided to wreak havoc. I was fortunate enough that my oven was bang on. Also: Factor in the size of the clamps when you buy a model. After around 30 minutes after putting the mould in, some silicone starting oozing out at the only corner where I had not placed a clamp. I am glad I did not use less clamps as this showed me how much pressure was building up in there. After two hours, the oven rang a satisfactorily sounding bell – I did not immediately take it out but let it cool down by itself. No idea if this is necessary but I figured that preventing thermal shock at this stage cannot be wrong. At the time of this writing, I am still waiting for the mould to cool down (and the kitchen to become less smelly), so I am excited about the outcome.
After 2 hours the mould frame was cold enough to carefully remove it from the oven. The clamps had decided to become permanently attached to the aluminium plate – likely the lacquer melting – so I left them on there for the time being. The rubber easily removed from the frame and after some well-placed surgical cuts with a scalpel I was able to carefully tear open the mould. The result looks for the lack of a better word… amazing. The rubber looks cured and preserved the details as good as I was hoping. The green stuff survived much better than I anticipated, in fact, it looks exactly as before albeit being much darker. I cannot tell any degradation and it is not crumbly. This is great news because that means I can potentially use my green stuff masters more than once! The styrene is a different story. It partially melted / deformed and broke when I lifted it up. Lesson learned: avoid styrene in sculpts for moulds that are going to get vulcanized. It’s not too bad though and I need to make some test casts before I allow myself a final verdict. As for the mould line, the masters submerged more than I was intending into bottom half – a lesson from this could be that I should not press them in as much as I did. Some talcum had collected in spots such as the weapon arm (close to the shoulder and at the tip of the molten axe head)-- fortunately only in uncritical areas but something to look out for – it can potentially create bad mould lines. There are some alternatives to talcum such as mould release spray which I may order and test at some point.
Next, I should probably cut sprues, vents and do some test castings…