How to Make a Silicone Mould

Silicone moulds are much more forgiving with undercuts than fibreglass because they are so flexible. Silicone moulds are used more for casting urethane foams and resins to make solid objects. They are excellent for making bottles and glasses and usually have a cut down one side to make it easier to remove the cast. A wooden box or tube is used around the mould to support it when it is filled
with casting material. The picture at left shows one with a fibreglass support jacket.
These moulds are easy to make but the material is expensive. You need to balance the size of your object with the size of your wallet. That being said they are robust moulds and can give many impressions. For the purpose of description we will make a mould of a aeroplane engine cylinder head.

Difficulty: Moderate

Instructions

Things You'll Need:

• Silicone mould material
• Small set of scales
• Scrap wood
• Wood tools

1. 
    
   MDF copy of cylinder head

   You usually don't need a release agent with silicone rubber moulds as the silicone peels off the object your copying. If the object is porous it is wise to seal it with shellac just to be on the safe side. The object to be moulded is a cylinder head for a 9 cylinder Wright Cyclone aeroplane engine. It was made out of MDF from a CAD drawing of the engine.

2. 
    
   Plug mounted inside box

   Build a box around the head as shown in the photo so you can pour in the silicone rubber. Notice the blocks of wood in the corner of the box? This is to reduce the volume of silicone required and hence the cost. Fill the box up with water and then empty it into a measuring container to measure the volume of the mould. This will be the amount of silicone rubber you need to mix. Dry the box out before proceeding. Mix up the silicone rubber in the correct proportions by weight and mix thoroughly. Use the correct ratio is important with silicone rubber because of its narrow tolerance range. Pour gently so you get a thin dribble of silicone to fill up box. This minimises any bubbles getting trapped in the mould. You can use a vacuum chamber to remove all the
   bubbles from the mix but that is pretty high end stuff. Leave it for a day to cure.

3. 
    
   Finished mould of a tappet cover

   Remove the base of the box and remove the cylinder head plug by peeling the silicone back. The mould is ready for casting. The picture show a tappet cover mould that was made for the cylinder head.

4. 
    
   Sanding a cast cylinder head

   Polyurethane foam is poured into the mould to make the casts. It cures very quickly and you can remove them after about half an hour. A little light sanding and the head is ready to have the fins attached.

5. You can make moulds of bottles or glasses but use a PVC tube instead of a box. You can then make hollow bottles by melting breakable glass resin and pouring it into the mould and roll the mould around by hand lining the inside of the mould with resin. It is a tricky technique that needs a bit of practice but it's a good way to make breakable bottles and glasses. Be careful removing the resin bottle when it has cooled as it is very fragile.

Tips & Warnings

• Only mix the amount of silicone you need

Resources

• DecoWorks Pty Ltd
• Prop making tips