Tutorial: Silicon Rubber Moulding & Casting

Silicon rubber moulding is an extremely effective process, allowing one to create exact reproductions of an item even with the finest of details.

• Plastic Solvent
• Small piece of dowel (approximately 1cm)
• Rigid material to make the mould box (e.g MDF or sheet of plastic)
• Glue gun
• Mixing pot
• Mixing stick
• Condensation cure rubber
• Hardener (condensation cure rubber catalyst)
• Polyurethane fast cast resin (parts A & B)
• Polyurethane pigments/metal powder (optional)
• Scalpel
• Tape
• Item to be moulded
• Appropriate PPE

Creating a mould:

To begin with, take the object in which you want to make a cast of, when casting the whole object (rather than just one side) you require a hole to pour the resin for the next half of the tutorial, so using your glue gun, glue a small piece of dowel to the top of your item and then again to a square sheet of plastic, roughly 2cm+ larger than the item. Then begin to build a box (taller and wider than the object), around the object leaving a 5mm-10mm gap. If you have chosen plastic to construct your box out of, use the plastic solvent to glue the edges of the walls together to form a solid piece. If you can see any gaps between these walls go along each join with a glue gun.

With a clean mixing pot, mix 10% of the hardener (a green/blue liquid) with the condensation cure rubber, check the manufacturers guide first though, as each may vary. Mix the two thoroughly until all traces of marbling has disappeared. Then put the pot into the vacuum degassing machine, designed to either extract air or add it - in this case you want to extract. At this point make sure that you have not overfilled your container with too much rubber, as it will expand under the pressure of drawing the bubbles out, if so put into a larger container before continuing. You will find that the rubber tends to look as though there is more air contained within it now, though this is just the air rising to the top, when it looks as if it is boiling take it out by gently pulling the leaver back, slowly drawing out the pressurized air.

To remove the bubbles that remain on the surface, begin to roll the rubber around the sides of the pot until all are burst. Pour this into one side of the mould, tilting it slightly, letting it slowly push itself into the detail - whatever you do DON'T pour directly over the object, this will create more bubbles!

Leave this to go off. Once it is dry remove from it's casing, and cut away any excess rubber. If you have cast a whole object with a scalpel cut a small slit down one of the sides of the rubber mould (it is better to cut with a wavy line). Do not cut in half as this will make the mould unusable or at least very difficult to mould again with! If you would like to reuse the mould, it is imperative that a small v-shaped vent is created at the top so that the air is displaced when you pour in the resin.

Pouring into the mould:

When you have removed your object used to create the mould, a resin must be made, but first off it is essential that the mould is sealed enough so that the resin mixture doesn't leak out. To do this simply wrap with tape, making sure not to distort the shape. The resin consists of part A and Part B, the Polyurethane is a fast-drying resin (about 5 minutes) so when mixed be sure to work quickly. The portions for this is 50:50 so shake well and weigh up the same amount of each and stir. The size of your cast determines how much you need - it is always best to make more than you need rather than less. At this stage you can add metal powders or pigments if you wish to have your cast coloured, although you can paint it afterwards if you give it a good scrub with detergent and plastic primer.

Pour into the mould, giving it a tap to make sure there's no air. If you need a flat backed cast, pour into an open mould, and with a larger piece of flat material, such as plastic, lay on the exposed resin and place a weight on top. Wait until it is cold to remove from the rubber mould, cut off and sand the nodule where the dowel once was. And voilà!