January mouldmaking exercises – ‘Two legs good..’ Part 1

But before tackling the drawings .. and already digressing ..

‘Two legs good..’ but four legs, a little harder! Here’s a method I’ve been rehearsing for a ‘simple’ 2-piece mould for a complete four-legged animal. Since I have no small four- legged forms of my own at the moment I’m using a toy rhino.

toy rhino

There are two main challenges with forms like this. The first is the fact that the area between the four legs needs to be, somehow, a separate mould piece. The second, especially in the case of this rhino, is ensuring (since the feet will becoming the pouring holes) that all the details of the head will be filled. There are fairly simple ways of managing both.

making a temporary mould plug -1

Here, above, I am starting to make a temporary ‘plug‘, filling the whole area under the belly out to a halfway mark along the thickness of each leg. I have used standard plasticine (‘Newplast’ in the UK) warmed a little with a hair-dryer, but any soft material can be used which is later easily removable from the prototype and is compatible in contact with standard RTV silicone. The plug is almost finished below.

making a temporary mould plug -2This is the easiest point (before securing to a baseboard) to press in or cut locating ‘keys’ (otherwise known as studs or ‘natches‘) which will later be translated into silicone and help those parts of the mould to stay in place.

making a temporary mould plug -3Below, the plasticine plug has been neatened at the bottom, securing the whole thing to a baseboard (a small nail or two driven in from the other side may help if needed, or gluing the feet temporarily with UHU). Next the whole form is covered with a silicone skin (for this size 5-10mm thick will do, though I have gone a bit thicker). Usually one would convert a standard silicone rubber (normally pourable) to a ‘thixotropic‘ non-slump paste by adding a small amount of thickening additive. Here I am using a special silicone rubber which is already like a thick gel ( Rhodorsil V1069) partly because I need to use it up, but also because I’ve become ‘fond’ of the smell of the catalyst which gives the air a definite Vim-like and minty ‘zing’! The silicone must be brushed on thinly and carefully at first to ensure that all surface details are captured. Brushes can be cleaned straight after use with white spirit.

applying siliconeThis first coat needs to be achieved fairly quickly while the silicone paste is at it’s most spreadable. Usually after about 30mins there is a slight but noticeable thickening but this helps because the silicone can then be more liberally ‘trowelled’ on to complete the thickness. Below, the silicone has already set (on average overnight) and I have trimmed the edge on the baseboard. For this mouldmaking method, sometimes known as ‘split skin‘, everything is covered and the silicone is split or divided afterwards.

finished silicone skinUnless it is very thick (which would be uneconomic since silicone rubber is expensive) the silicone mould needs some help in keeping its shape later. Obviously this has to be done while the prototype is still inside so all is left exactly as it is for the moment. Common practice with larger forms is to make a rigid casing directly on the silicone using fibreglass. I am using plaster here, to make a casing in two removable halves. The best way to do this is firstly to build a smooth dividing wall of plasticine which bisects the form (omitted in the following photos). What is shown below is the plaster jacket completed on one side, made by partly pouring/partly sculpting the plaster on up to the plasticine divider (plaster goes through a very brief paste-like stage during setting when this can be done quickly). Once set the edge of the first plaster half must be thoroughly Vaselined to prevent the second plaster half from fusing to it. This half is then done in the same way.

first half of plaster jacket

The two plaster casing halves shown have been cleaned up a little (with rasps or coarse sandpaper) because even ragged plaster edges can occasionally cut.

plaster casing completed

Part 2 describing the making of the silicone ‘plug’ part and the completion of the mould will be added soon.


6 thoughts on “January mouldmaking exercises – ‘Two legs good..’ Part 1

    • Hello Angela,
      I don’t quite understand. If you mean the prototype form is attached to a base and not removable I don’t see why the steps I’ve described can’t be followed anyway. If you mean you want to attach the cast form to a permanent base, how to do it would depend on what you’re using to cast and what the base will be made of. Perhaps you can explain a bit more?

    • Actually, yes I see what you mean now .. having had a bit more time to visualize. You mean, how do you pour the silicone in for the ‘plug’ if the base is in the way? The solution is fairly simple. You follow the same steps; making the plasticine plug, making the overall silicone skin etc. The only difference is that you need to cut a hole through the silicone skin at the side to access the void and pour the plug from there.

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