Lessons Learned:
- Glue your work down in the mould box. Didn't matter much with heavier originals - they just sat there. Medium weight/size originals moved a bit when the airy RTV glooped around them, but no catastrophes. Polystyrene... floats! Whatever the aeration process is for the RTV, it got the polystyrene lightweight original parts floating. I actually had to use a brass rod to hold one of the pieces on the bottom of the mould box. Remains to be seen if that mould is FUBAR.
- Have enough RTV. I used all I had, 2 lbs, and I ran short by (I figure) about another pound or at least 3/4 of one. The only serious part of the shortage is a pour mould with incomplete coverage of one high protrusion and another where I can see a shadow through the mould to a higher piece. Both, I'm hoping, can be salvaged by a subsequent pour on top of more RTV, once I have some. It's amazing how much of that stuff you'll go through, even when you try to keep space around the parts as small as you can while retaining wall strength. Odd shaped parts is obviously a factor - height seems to be critical - if you've got a big box and one part makes you pour more, it is a lot more!
- Mix, mix, mix the RTV. I thougth I did that. But the last part of the RTV, even after I scraped the walls during mixing repeatedly, always seemed to come out whitish rather than yellow. I think the only way to beat this is a) have a little extra of the additive on hand in case you are short a bit and b) buy a silicon scraper to scrape the inside of the jar during mixing and dumping out. It'll end up being a sacrificial tool, but if it makes for better pours, that's okay.
- Two part moulds are tricky. I still have to see if the second half of the cacti mould will work out right. You have to align any and all pieces on an axis that you want to be the mould centerline and glue them there and that's a feat. Then you have to pour in such a way as to get under your pieces but not up the sides or overtop. Sounds easy but a wide mouth 1 pound jar of RTV is no super accurate pouring tool. I have a lot of respect for the effort at building multi-part (more than 2) moulds that people use for spin-casting complex metal figure.
- The plastic blisters for miniatures are an excellent source of small mould boxes. Any other reasonably leak proof plastic container is another good choice - I've repurposed a plastic box from a 12 of Ferro Rocher chocolates as well as two old disposable tuperware containers. Only one warning: Rounded corners and trying to get pieces close to the edge to use minimal RTV without tearing are mutually oppositional.
We'll see how this batch turns out. I've got 5 pounds of Quick Set from Alumalite coming and another pound of HS-III for some stuff I have with undercuts.
Once I finish the current batch of moulds, I'll try to pour and see what sorts of results I get. One of the items is a thin modern desk... I look forward to trying to get casts of that out of the mould without damaging the piece or the mould....
I also need to get some more dental plaster. My last set of casting experiments in plaster (some successful, some not - the tinfoil over a die-cast car to make a mould shell might work in Hot Wheels scale, but not in 28mm... the plaster is heavy enough to distory the tinfoil shell even when supported by sand or ballast. But Plaster works great as a replacement for resin for vehicles, especially if you can create mould voids using blocks. It's strong enough you don't need to mould an entire brick out of it (a shell will do) and takes detail just about as well as resin.
More as the next set of experiments are attempted....
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