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RTV Silicone Rubber Closed Pour (Cavity Mould)

RTV Silicone Rubber Closed Pour Method

This is quite a complex method, but is used by Fibrous Plasterers very regularly to make moulds, with what is a pre made supportive case. We do not have any pictures, but the text should guide you well!

First lay down you former as if you were making a block mould. Ensure there are no places where air will entrap, overhangs on verticals as you look from the side are bad for this. Cover the entire model in cling film to act as a barrier.

Next, roll out clay or use a clay cutter to regulate thickness and layer this all over the model, ensuring a constant thickness as best you can. Keyways MUST be added here. You will be pouring silicone into the case so you will need to make the shapes in the clay now.

Varnish the clay, or shellac if you prefer, and dust down well with french chalk. Now begin to make the support case. You can make it from plaster or grp. See the guide on lamination or support cases for more help with that part. Dont forget some form of release agent on the bench! Vaseline, grease, tallow, lard etc!

Assuming you have completed the case, trim it all up neatly on the bench and then draw around the base, you need to know where to put it back. Now carefully lift off the former (help may be needed!) and clean out any clay stuck on. Make three holes for wide mouthed funnels (minimum 1"!!) down the length. One at each end stop and one in middle!

Remove clay and cling film from the former and replace the case. Its likely if its plaster it will not need fixing, but i usually do in case of knocks! Line up with your drawn lines as best you can, that goes without saying! Put a bead of clay all around the mating faces after securing down to prevent rtv escaping during pour!

Now calculate your silicone requirement in litres. How do it do that? Simple. Take the clay you removed, and measure its volume....bucket of water, graduated bucket etc and work back to a weight using the SG or Specific Gravity. Also include the volume of one entire funnell. For example if you have 2 litres clay (plus the funnell!) and are using an RTV with an SG of 1.23, you will be needed (2 x 1.23)= 2.46 kg of silicone. I always work on just the base, then the activator acts as a contingency volume.

Mix your RTV as per the guides and begin to pour in slowly at ONE END. Fill the funnel once the end is filled, which gives pressure and forces a wall of silicone down the apeture you made earlier with the clay......

Watch for the RTV in the centre, re-charge the first funnell.

Eventually what you will see is a small amount of RTV in all three funnells. They will self level and at this point the pour is complete. Wait for the cure and then deconstruct your Closed Pour Assembly. You will need to cut off the rtv in the funnels, use them as extra keyways.

Many choose to make funnels from soft metals like zinc, as they can be undone and removed and reused.

Before removing the case, consider mounting a couple of straight edges to the bottom so it will sit flat for use. 2x1 works well. Plaster wads or resin and matting will stick them well.

Remove the case, then demould the rtv from the former and replace to the case. One skin mould with no thixo and no air bubbles in sight

(in theory!! it depends on technique AND former architecture)

Smooth-on have made an excellent video which is on you tube. We have embedded that here for you!