In the previous part of this article we discussed some of the materials that we would use and some of the issues involved in creating masters. We will now proceed to create a pour box i.e. a container to hold the RTV silicone around your masters while it sets.
I build my pour boxes from 3mm MDF, foamboard offcuts, masking tape and Klean Klay modelling clay. MDF is good for the base because it's stiff and flat. As the base will form the open face of the final mold it needs to be as flat as possible.
The first step is to arrange your masters on the MDF. RTV silicone is expensive so you want to arrange your masters with minimal wasted space. However you also need to leave at least 4-5mm (3/16") between each piece inside the mold and a little more for the external walls, say 5-6mm (1/4"). Cured RTV silicone is quite flexible, so you need relatively thick internal and external walls to ensure your cast pieces aren't deformed by the walls bowing when you pour resin or plaster into the mold.
Obviously the best thickness to use varies depending on the set strength of the RTV product you're using and these estimates may be a little generous. I notice molds from professionals like Bruce Hirst have 2-4mm internal mold walls however I figure it makes sense to be a little over generous (and use a bit more silicone that is absolutely necessary), than skimp on it and end up with a mold that tears after the first casting.
Once you're happy with the arrangement glue each master down with a small dab of superglue. It's important that the masters are firmly fixed to the MDF base. When poured, the silicone is a dense liquid, and light masters can float up before the mold sets.
If you ever want to remold or modify your masters chances are you'll want to get them off the base. A small dab of superglue can be broken by twisting the master off the MDF. However if you're confident you won't rework the masters, glue them down solidly because this will ensure that no silicone is able to seep underneath.
Foamboard is ideal for the construction of the box walls as you can cut through one layer of card and some of the foam while leaving the other layer of card intact and then fold it to form watertight corners.
Take a continuous strip of foamboard that's long enough to entirely surround your masters. Stand it on the base and cut the outer side of the foamboard and fold it in towards the masters. Repeat for as many corners as you need. Don't feel obliged to make square or rectangular pour boxes but consider the method you intend to use for flattening the top surface when casting i.e. will you be using the 'smooth cover' technique or using a scraper?
Once you've cut the foam card into a shape you're happy with, tape it down to the MDF using strips of masking tape. I usually also tape up the final corner of the foam wall too. While enough masking tape would probably be sufficient to stop the silicone from escaping the pour box, it's pretty viscous stuff and can flow out or under the smallest gap so I also run a bead of Klean Klay around the entire pour box, making doubly sure the edge of the foam card wall is nicely sealed down to the MDF base, and the final open edge is completely sealed.
Klean Klay is a reusable modelling clay for use with RTV molding. Plasticene and some other modelling clays contain sulphur which will stop RTV products from curing properly. You can create temporary masters from Klean Klay. It's handy stuff to have around, pretty cheap, and you should be able to pick it up from any supplier that stocks RTV molding products.
Right! That's our pour box made so in the next part of this article we will work out how much silicone we'll need to mix.