Quite often I find myself making small block moulds – simply building some walls around the piece I want to mould and then flooding the whole box with silicone. This procedure is extremely quick and most of the times suitable for reproducing a small number of items. The great advantages are time and – depending on the object – hidden seamlines. Sometimes I cut the silicone open to make demoulding easier. Sounds a bit viscous, especially on small pieces, but actually the more ragged the cut is, the better the mould will close again.
Now, with intricate little b*rds, you really want to be able to see whether a) your piece is not hugging airbubbles while moulding and b) the material you are casting in gets into every cavity. There are a variety of good prototyping silicones for that purpose, but I want to talk about two of them in detail.
Rhodorsil 3040
Rhodorsil3040 from Rhodia is a shore A35 platinum/addition cure RTV silicone which is extremely translucent – in fact, it is the clearest I know of. The other advantage of Rhodorsil 3040 is its very slow curing. Normally it takes around 24h for it to cure fully, which allows even microbubbles to rise to the surface and pop, giving you a flawless mould with silicone reaching into the smallest cavitiy.
What I do is after mixing and degassing (although with this stuff not as necessary) is fill the mould and let it sit for an hour or so, after which I can see wheter any bubbles need ‘assistance’ in rising. After I am sure that the mould will be fine, I place the mould somewhere warm overnight, max. 50°C (make sure that you are not melting hot glue or your plastiline scultpure or anything!)
That should speed up the cure and allow you to continue working the next morning
When moulding, I recommend placing your object in a translucent box (acrylic or polystyrene sheets glued together, your old fishtank…) at least something with highly polished surfaces especially the base plate, because once you are filling the mould, the surface of your base-plate is where you are trying to see your mouldsurface through. Do not use superglue to stick your box together as it willcause inhibition to addition cure silicone
If you use say MDF, you will get a very dull surface and even the clearest silicone will be opaque.
If you do everything right, your mould should look like this (not necessarily a frog, though)
A variation of this cut open or half mould is making it two-part. A little more complex, but will look a lot more professional and may be a little more accurate in terms of locating the seamline. It is
also suitable for very hard materials that you wouldn’t be able to cut with a knife.
In this case, embed your piece in clay or plastiline and create a VERY neat seamline. Add keys to taste and accurately insert rod or brass tubing at least the diameter of the bolts you intend to use to close the mould later. Like before, build a box around your piece and flood with your moulding material. Once set, flip the mould upside down, remove the clay and push both the rods and the box up, so you can pour the other side. When finished, clean up the mould and if you are using a rubbery material, its a great idea to wedge the actual mould between two pieces of acrylic or board (if you don’t need to see the inside of the mould – like when using opaque material)
This makes a sly little mould for producing bigger ammounts of the same thing, they are pretty durable and you can mould several pieces at once (which can speed things up severely!)
common mistakes:
bad release (especially with PU), airbubbles, loss of transparency due to bad release/choice of housing, object moving/floating, bad seamlines, overtightening, airbubbles in the cast
