Saturday, 19 January 2008

Mould Box

I'm going to be making moulds for drop casting these masters, its a process you can do at home, without the use of a centrifuge.

The first thing you will need to make such Moulds, is a Mould Box. The base of mine is made up of a couple of pieces of plywood about 7-8mm thick. The larger section on the base is about 9.5cm square, with a smaller piece on top of this about 6.5cm square. On top of this is some modelling putty (Plasticine), this will be used to form the "bed" for the miniature.

For the sides of the Mould Box, I'd recommend Lego, its ideal as it is sufficiently "watertight" when constructed to hold the liquid rubber of the mould, before its set, but also very flexible enabling you to remove and adjust in height etc. Dig some out of the loft if you have any, if not, you can buy small quantities on ebay for a few pounds.

For a 25mm foot figure, which this mould box is made for, its a 12x12 lego box, which will have 4 layers. This size works well in my experience for this size of figure.

In this photo I have added the first two layers of side walls of lego and smoothed out the plasticine to this level, this is effectively half of the mould.

We now need to bed the miniature into the material, this is a tricky process to get right, to some extent its trial and error, and with practice, you'll get a feel for it. I'll use the same tools to do this as I used for sculpting, also using a light smear of Petroleum Jelly (vaseline) on them to stop sticking and for smoothing the surface. Roughly you want about half the figure embedded in the material, but you must follow some basic rules:

1. Don't lock bits of the figure in, if you completely enclose an arm or leg say in one half of the mould, how will it subsequently come out?

2. Try to avoid rapid changes in height or thickness on each mould half, if you do have such areas, they will be weaknesses in the mould and vulnerable to "tearing out" or failure, ruining your mould and any subsequent castings.

3. Try to get a nice clean hard edge between the figure and the plasticine, roughly flat and level. Its worth spending a bit of time on this going round the figure carefully. Do this bit well, and the mould will last a long time, and you'll have little or no flash or mould lines.

4. Ultimately you want a line of mould running around the figure, up and down each leg etc, it therefore won't be a level mould in the centre, it will gently undulate round the figure, however bring the edges of the mould back to the same height by the time you get to the edges, as you do want each half to be roughly even overall.

5. Try to avoid areas where one half of the mould "cuts back on itself" i.e. a big undercut, this is for the same reason as 1. its an area that's likely to fail when the mould is in use, and also may make it hard to get the figure out, once cast. In this example, I have broken this rule, at least to some extent, as the turnbacks of the coat, will suffer this problem. Hopefully it won't ruin the mould, but it's possible, let's hope it works out!

I've carried this process out with the basic armature in this photo. You'll note that I've added a small wooden block to the base (this will be the opening down which the metal will be poured). I've also made some holes or dents in the plasticine around the mould, these will act as locating plugs for the two mould halves (one male one female). Note in regard to this I have not put any in the bottom quarter of the mould. We need to leave this clear to cut out vent channels etc when the mould has been made.

Its now ready to have the rest of the lego bricks built up, and the mould poured. We'll do this next time.


MurdocK said...

Interesting, why the 'middle' block of wood?

In my mind this simply makes the mould thicker, much thicker than needed for usual RTV.

Also, what is your source of RTV?

Here in western Canada the Dow Corning material has dried up due to distribution issues. Some small kits can still be found but larger (more economical) amounts are just not available unless directly ordered from the company in really VAST quantities.

John D said...

I find that 4 "lego" blocks deep for the Mould works well. I suppose you could get away with something thinner, it depends on the orientation of the figure as well, but they do get very hot when in use. A mould done on this format uses about 200g of rubber in total.

I'm based in the UK, where supplies of RTV are available from Alec Tiranti, I'll detail in subsequent posts.

Stagonian Jeff said...

Please keep these posts coming. I don't personally expect to try duplicating what you do -- but it is very interesting and helps me to appreciate what goes into sculpting and molding figures.

-- Jeff

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