The basic "grate" over this issue, is the prices of the one pack silicon rubbers (guttering / fish tanks etc) that set by humidity catalyising the reaction, vs., the 2 pack silicons, that are used for casting thick solid objeccts in one go.
The humidity catalysing (where I live) costs about $5 or $6 at the best prices for 300ml tubes, while the 2 packs cost wayyyyyyyyyy more than that, are hard to get, and cost heaps in shipping........
So I have figured out a slowish way to make thick castings, using the humidity setting silicon.
First off, I use the neutral cure silicon rubbers, as I just don't like the stink of the accetic acid setting ones.
The neutral cure fumes are not that healthy to breath in either.
But anyway - the secret to my succcess is the fact that I use high humidity to set the silicon in layers, of about 6 to 8mm thick at a time.
This is not the ideal thickness, in terms of the speed of setting, it's a convenient thickness. It's the ratio of speed of setting and the time between reapplication, the ease of application, and my general absent mindedness.
Many thin layers is a lot of fiddling around, and going over 6mm to 8mm takes too long to set, as the water vapor has to penetrate that depth of silicon rubber - from the outside in... and THICK layers, tend to take far, far longer to set through the mass, than several thinner layers.
Ideally I suppose that one could apply the sillicone rubber in 50 layers one millimeter thick, every 2 hours on the hour... or that one could do each 6 to 8 mm thick layer, every day or three, over a week or two.
How does this all work.
I want thick hard silicon castings, for machine isolation bearings, to isolate vibrating motors etc., from their physical surrounds - by the use of soft-ish rubber bearings.
The cost of the single component silicon rubbers, comes to about $18 for 900ml of silicon, to cast the 8 bearings, where as the 2 pack silicons??? I think the asking price is about $80 for 1/2 a liter of rubber. It might be even $120? Don't know - I looked at the prices a long time ago and after passing out on the price list - I ruled them out for anything except absolutely must have circumstances.
The secret to this is that the silicon rubbers are non permeable to water, but they are porous to water VAPOUR.
One of the riches sources of water vapour, is WATER!!!!! In fact water is composed of 100% water vapour.
Now here is where it gets interesting.
IF you immerse your freshly extruded silicon layer of some 6 to 8 mm thick, in COLD water for a day or two or even three, it will set HARD, and into a good rubber.
Dropping the silicon into hot to boiling water, ruins it's setting and it becomes a "set" rubber, but it is soft and more jelly like - which is not good - unless you want it like that.
I think this creates a shorter chain polymer.
Anyway this picture shows how to build up the layering of the silicon rubber, starting with the bottom layer, then toss it un a bucket of water for a few days, then then next layer, and the bucket for a few days, and then the third layer etc...
FRESHLY set, wet plaster molds work well to, as oddly enough the silicon rubber does not STICK to the mould..... It bonds to the mold surface, but when kept wet, in the bucket of water, the two cleanly seperate - Though a hammer on the plaster is a good help.
Step 1: The Casting
I lined some angle iron with aluminum foil and then filled it up with a layer of silicon, then a soak in the sink for a few days, then a quick wipe, then another layer, then another soak etc., etc., etc..
And now I have a solid casting that is about 55mm thick along the edges.
This will have some finishing touches applied later, and then it will be cut up into 40mm lenths, and slung under the stand of the machinery in question.
Bingo! Elastomer bearings!
Step 2: Other Castings
The material cost is about $18Au.
Time - about 2 to 3 days per layer of silicon rubber (sealant / guttering / windows / fish tanks), of soaking in cold water.
Step 3: The plaster molds
Perhaps casting the plaster onto a beeswax coated pattern may have some thing to do with it.... and a slight trace of wax was on the surface of the plaster?
But this Vibration Isolation Elsatomer Bearing, was cast using the layering / bucket of cold water method.
It turned out pretty good.
Remember - thicker layers is not better, abouut 6 to 8mm thick seems about optimum, on a 2 or 3 day water soak cycle. Probably about 4 or 5mm thick per layer seems better on a 2 day soak cycle.
If you have put it on a bit too thick, you can "feel" when the silicon, under the hard / hardening skin has not properly set, as it feels gooey - kind of like pressing on a reasonablly sized blister, under the skin,
Overall, this is an excellent method to make siulicon rubber castings, from the single pack silicons, when you have the time, and the ability to make molds, and everything can be done well in advance.
This method is very useful for the times that you don't like shelling out big dollars, for a couple of small containers of "proper casting" silicon - the 2 pack material.
I have not bothere to look up the pricing of the silicons in the other parts of the world, but;
a) When living in Australia, and facing what I call, "The Great Australian Gouge" - where many imported things cost like 3 to 5 times that of thier retail prices in the country of origin,
b) Aas well as our thieving Australia Post - where it costs less to send it from the other side of the planet than it does to post it down the street, and the amount of stuff that gets stolen in transit, then
c) The local trader with the tubes of guttering / fish tank / window / general purpose silicon sealant, is both convenient and attractive, under some circumstances, the layering process of silicon casting is ideal for small scale runs of specialist items.
Good luck with your projects.