I've been animating for a while, but have recently started exploring silicone puppets, which is a whole world out of my comfort zone.
One thing I keep coming up against when casting with Platsil 10, is the high viscosity causing a lot of air bubbles when pouring into smaller moulds.
For more detail: I'm using a 2 part plaster mould taken from a clay sculpt. I paint in the first layer of silicone, lay in a wooden skull/block with rod for the neck, before closing the mould and pour the rest through the neckline.
I've seen people using lighter fluid for thinning platsil to paint with, but am I right in thinking this would be a bad technique for using with a mould as the lighter fluid would have nowhere to evaporate too, and possibly interfere with the initial skin layer that's been painted on first?
Any help or advice would be much appreciated!
My plastic 10 got thicker with age, or more accurately it got kind of "stringy". I had the same problem pouring it through a skinny neck, with a bit of foam around the neck wire reducing the space to pour through even more. I continued to use it for painting the skin layers on, but switched to a softer silicone, Ecoflex 00-30, for pouring inside. The Ecoflex was a bit thinner and easier to pour, as well. There is a softer Platsil as well, I have a small container but haven't tried it yet, so I don't know if it has a lower viscosity than the Gel-10. By now it would be a couple of years old and possibly not as good.
I think you are right about the lighter fluid thinner, I use it for painting the outside but would not use it for filling the mould. You could try adding a little Deadener, which makes the silicone softer, but also makes it tend to be tacky on the surface. Not so bad if there is a skin painted on without it, beforehand. But if you use too much, I have had it leach oil right through the skin surface. I wanted it softer but didn't want the oiliness, that's why I switched to a softer silicone and stopped using it.
Small bubbles might not be a big problem, if you have 2 or 3 layers of skin paint on. Big voids below the surface can be bad, if they affect the way the silicone bends, but little bubbles aren't such an issue. I got a big void in the back of the head on one, but filled it with cut foam and silicone, and since it wasn't in the face, I got away with it.
Filling the head mould with the neck hole pointing up risks getting air pockets in the nose and lip, so I part fill the face before joining the two halves. I put the front half of the mould on the bench, face down, and pour silicone in, with the armature positioned so it will get locked into place when the silicone sets. That way, there are no areas that trap big pockets of air, and bubbles can rise. I use a block to locate the armature, which can be removed when I join the 2 mould halves together so the neck is clear to pour through. Easiest to show what I mean:
Nick has covered a lot of it. I prefer Platsil gel00 for heads, but it too is quite thick to pour. My solution is a little deadener to thin it, and some retarder so it takes longer to set and allows air bubbles to rise.
But I also use Nick's technique of painting in the surface layers then part-filling with the face down and the armature placed in, making sure the cavities are well filled before adding a nice big blob into the back part of the mould and pressing it together. I clamp the mould neck upwards and can usually see any air bubbles.
But retarder is the thing that allows you time to get them out.
Interesting to see Aardman injecting into moulds, so the syringe injects into a tube in the mould casing which reaches the sculpted cavity at the bottom, so it fills from the bottom and drives the air upwards. I did this on a full-size Frankenstein head (effectively a silicone bathing cap for the actor, about 6mm at its thickest point), and while it was pretty wasteful of silicone, I can confirm that it filled the mould really well. Have a look at this video, and you will see a mould constructed for injection, and a neat circular one also for injecting and making several identical things at once, although I think the mould is silicone and fast cast resin is injected, in that case.
Thanks so much for all this guys. Really useful information.
Nick, I watched your video when I was first looking into silicone, but I'd totally forgotten about deadener. I've got some ordered now, so will see if that solves some of my problems when it arrives. I'm actually getting the worst bubble in a similar place to Lovecraft, just at the back of the skull, but there's also a few other cavities popping up in other places. I think filling the face more before closing the mould will help with that too. Failing that I'll have a look at the Ecoflex as an alternative.
I'd actually considered using a syringe, but more with the idea it might help sort of... jet the liquid in, with more force. But actually the way they seem to be doing it in that Aardman video is really interesting. I wonder if you could create a contraption with a straw at the tip of a syringe in order to fill from the bottom up. Maybe all a bit Heath Robinson.
Retarder is a good shout too. I'll have a little experiment and let you know how it goes!
I used a syringe to make latex dinosaur teeth. If you just pour liquid latex into the mould, it seals over and traps air in the tip, so I thinned the latex a little and placed the tip of the needle right down into the tip of the mould. That gave me teeth without the tips missing. (Had trouble buying the syringes though, they thought I was a druggie, when I said it was for injecting latex they got even more concerned. Went back with my plaster moulds and a faulty cast.) But that might not help if there is an actual undercut where the rising air has nowhere to go.
It isn't always possible to fill from the lowest point with a puppet head mould, and pouring through a thin neck is alway a challenge. I think the straw idea sounds like it could help if it fits on a syringe so you can add some pressure. Now I want to try it! I have some large syringes from crop feeding a sick chicken, which would hold pretty much all the silicone you need for a head.
I have only done a couple of silicone bodies, both in fibreglass moulds. I made them with necks, and the heads separate, so they joined just under the chin and where hair could also hide the joint around the back. I filled them through the neck, standing, with arms down enough so that the fingertips were always downhill. (I can't recommend silicone for thick bodies like this one unless you can fill with silicone foam, needs heavy armature and still too hard to bend and too heavy.)
For filing the head mould I actually used a funnel and a piece of dowel that would then force the silicone into the mould. Obviously the top of the syringe/funnel/tube has to be higher than the highest point of the item.
There is no way that would have been possible without the retarder - it was in Platsil gel 10 - and I think I got about 30 mins of time for the silicone to remain liquid, so air bubbles had time to escape.
With heads and bodies I find that overfilling both halves of the mould which it is open, and then putting the halves together fairly quickly is a good way to exclude air, as the silicone does not have time to slump away and leave a gap. But it is a messy process!