Reusable rubber moulds

As getting a mould made professionally was proving too costly for me, I took a gamble and signed up for the 'reusable rubber mould making' course at the Art Academy. Doing it myself might not have the same results, but at least I could move the project on. Clay Fred had been languishing in the basement for couple of months now! See my notes made during the process below.

Materials needed for making a reusable rubber mould

Mix 10% of catalyst with the silicon rubber (in this instance Silastic, c £12 a kg). You need more catalyst when it's cold, less when it's warm.

Process for making the rubber jacket

Use pre-cast seams (one example shown below has keys integrated) or pour some rubber into home-made troughs to create seams for later

Build up the layers of rubber on the model:

• 1st layer: rubber + catalyst mixture. You can pour this over the model quite easily; blow, or use compressed air to get into all the nooks and crannies; good coverage of all the surface texture at the point is vital as this the impressions made here are what you cast into when the mould is finished. Wait for this to dry until tacky to the touch.
  
  
• 2nd layer: rubber + catalyst + thixo. This needs to be applied either with (gloved) hands or a spatula; you can see the difference in texture now; without the thixo, the model was still quite visible. Wait for this to dry until tacky to the touch.
  
  
• 3rd layer: as above only making the surface as smooth as possible; this is so it fits snuggly into the hard shell. If the surface is too rough you can apply white spirit to smooth it out. Wait for this to dry until tacky to the touch.
  
  
• 4th layer: pin the pre-made seams to where you want the joins to go onto your mould. Affix using more rubber mixture. Remove pins!
  
  Bear in mind the mould needs to come away from the model (or cast) in a direct line -  ie no twists or angles - so use this as a guide to seam placement.
  
  Fred was large so needed to have a multi-part mould. Ideally his head would have been made from yet another section so we could run the seam over his ears, but we had limited time.
  
  Cut keys in the seams, or attach keys to the body at this point to prevent the mould shifting in the hard shell. Leave to dry.

Shims
Cut along the seams to a depth of half way (ie, not all the way down to the clay) and wedge pieces of plastic or card joined with tape to create a barrier between pieces of the mould. This is so that when you make the hard shell, you maintain the multi-parts and don't end up sticking the whole thing together.

Spray with release agent (a kind of wax) to prevent the rubber from sticking the hard shell. Try not to get this on the floor; once it gets onto the soles of your shoes it makes everything very slippery!

Process for making the hard shell

In this instance we used chopped strand fibreglass sheets and resin to make the shell. It's a lot more durable and lighter to transport than plaster, but - to my mind - it's horrible stuff to work with. Even with health and safety precautions in place I came home with very itchy skin, and the smell of the mould curing over the next week or so was overpowering. If I were make another mould myself, I'd use plaster.

Once the jacket has dried, use an angle grinder to remove the sharp edges from the seam and drill holes where you will bolt the shell back together.

Remove the mould - ta da! The clay model wasn't much to look at at the end of it.