I have a couple ideas for how I want to do castings of Jack’s paw. One involves a negative mold, the other a positive.
There’s a saying, “only silicone sticks to silicone” which is pretty much true. Silicone will cross-link right into a cured piece and it’s as if it’s welded. So, you need release.
And, you need not just any release – you need a wax or petroleum release, not a silicone release! Most of the time when you are casting urethanes in silicone molds, you want a silicone mist between the mold and the urethane, so it’ll be less likely to do any damage to the rubber. However, only silicone sticks to silicone – a silicone mist can still bond to the fresh silicone, and you get patchy de-molding, tearing, and a ruined mold.
A few years ago I used this brand, which is different: it’s wax-based. I can tell because it has the distinctive smell of butcher’s wax (beeswax and turpentine) it’s getting atomized into micro-particles and driven into the cracks so the mold won’t bind. That’s the principle, anyway. You really need to know your materials if you’re working with glues, resins, or silicones; just knowing what’s compatible and what resists UV is a huge topic.
I absolutely love it when a situation works out such that I understand it, understand the engineering or chemistry or physics behind it, and predict the outcome based on that, and I’m right. Sometimes I’m so confident in my analysis that I charge forward fearlessly and don’t bother with experiments to confirm my expectations. This was one of those. I mixed a batch of purple silicone (left over from another batch that had different colored activator) and degassed it, then sprayed the inside of the mold with the wax release, poured it in, and let it sit.
Hours later, I examined the block of silicone and determined that the entire mass was welded together. I began to tug on one edge and the purple silicone started to stretch as though it was glued there, which makes sense, because silicone is also useful as glue. Uh-oh.
I was ready to throw the whole thing away in disgust and re-cast the mold from the original plaster, but I kind of twisted it in frustration, and the center popped cleanly out. That was when I realized that the casting was vacuum attached to the mold; it was not silicone-to-silicone bonded. Relief! Silicone is not cheap stuff.
Now I have a purple silicone positive paw, and a green silicone negative. The plans are to cast some polyurethane plastic and powdered steel or bronze into the negative mold. That’ll give a pretty cool desktop plaque-oid thingie once it’s buffed with steel wool to bring the metallicity out. I’ll do that next week then the polyurethane plastic arrives in the mail.
The positive silicone mold is going to be more complicated. I have some polypropylene sheet, 1/8″ thick that I am going to bandsaw to shape so I can make a mold box that I can tape together around the silicone positive. Then I plan to mix some beach sand and resin and pour a layer of that in and let it cure. When I break the mold down, that should give me a paw-print in beach sand. After that, it’ll go back into the same mold box, so I can put some sea-glass that Voyager collected, and maybe a pebble or two, and I’ll do an over-pour of water clear resin on top of that. If it works out and is clear and bubble free then it’s just “a simple matter of” some sanding and polishing and I’ll have, in theory, a dog’s footprint left over from an eternal beach frolic.
I love this stuff, because it’s all about figuring out a process that will get a desired result with a minimum of opportunity for failure. Really, though, the horizon of failure is pretty far away, once I have the plaster original safely stored, I can always re-wind and go back to a point where things were non-fail.
I love the idea – the beach footprint. What a cool thing that will be.
I’ve had trouble with chemistry and the interaction of various plastics together. Usually it starts out fine and you think everything will be OK, and then a year later it is completely and utterly not OK and you have to sit down and re-think the whole project. I’m thinking in particular of the way paints react with finishes. Also, I now grasp why my mold release (which was made for rubber) did not work so well in my silicone mold. LOL.
kestrel@#1:
I’ve had trouble with chemistry and the interaction of various plastics together. Usually it starts out fine and you think everything will be OK, and then a year later it is completely and utterly not OK and you have to sit down and re-think the whole project.
Yes, for example some silicones cure with a metal catalyst (tin, platinum) and there’s something about the catalyst that cures urethane (some kind of sulfur compound, going from memory) that makes the urethane turn to goo after a couple years, in a slow reaction. There are some epoxies that are great together, where others make eachother, or one, crumble. There are some formulations of polyurethane paints that will melt polyurethane glues over years. I don’t know what goes in paints and glues to make them UV stable, but that’s another potential source of incompatibility – as are water, and UV. For example, I love “gorilla glue” (polyurethane foam) but if it’s someplace damp it turns into something gross in 10 years – but if you clamp it between wood or whatever so moisture won’t get at it, it’s OK for 20 or 30. And that’s time. When I was studying “fine art” black and white photography in the late 80s, my teacher was an “archival processes” fan, which meant that every bit of fixer had to be washed out in multiple water baths, all papers and glues had to be compatible and time-tested, etc. Light Impressions seemed to be the only people who actually understood and tested their products (mat board that wouldn’t turn brown in the presence of an ambrotype with a hint of prussic acid in it…) etc. It was fascinating and infuriating and WWDD? What would Da Vinci do? Because it seemed like the renaissance Italians had hit upon a pretty good mix of process materials. The other folks who appear to “get it” are West Marine which makes sense since boats have to deal with fuels, UV, salt, and abrasives.
Over on some of the knifemaker forums I see really cringy stuff like people putting a handle together using bakelite (epoxy resin and fabric) and urethane glue. Maybe if it’s the right epoxy and the right urethane it won’t disintegrate in 10 years but I want to know they’ve done the testing.
That reminds me, I have a piece of carbon fiber rope holding a resin handle slab to the side of my porch. It’s been 2 years and if it’s got anything but mold growing on it (and the wrong kind of mold could indicate that it’s “food”!) I want to know.
In one of Larry Niven’s Ringworld books, a starfaring civilization is collapsed when a bacterium gets loose that loves to eat their equivalent of PVC electrical wire insulation. Nom nom bzzt all your civilization come crashing down. I always wondered if the designers of glues and paints take into consideration the energy value of the food they are brewing – what if some extremophile decides resin is yummy? Oh, was that a boat?
Is the title a nod to the Sisters Of Mercy?
People have discussed the idea of plastic-eating bacteria as a way of dealing with ocean pollution and spills for years. Niven probably heard about that and made the next obvious jump from it, because even the people talking about such things realized it would be bad if they got out uncontrolled.
djudge@#3:
Is the title a nod to the Sisters Of Mercy?
I’m a huge fan. I mean, they’re terrible but strangely great.