A silicone mould is much more forgiving with undercuts than one made from fibreglass because they are so flexible. A silicone mould is 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 the silicone rubber when it is filled with casting material.
A silicone mould is easy to make but the materials are 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 engine cylinder head for a full-size prop replica of a Wright Cyclone radial engine.
Cylinder head silicone mould
The following steps describe the construction of a silicone mould and casting of a prop cylinder head for an aeroplane engine. More construction details for the engine construction can be found here.
You usually don’t need a release agent with a silicone mould as it easily 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 prop cylinder head for a 9 cylinder Wright Cyclone aeroplane engine. It was made from MDF from a CAD drawing of the engine.
Build a box around the head as shown in the photo so you can pour in the silicone rubber. Make the base removable to enable removal of the wooden form. 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.
After the silicone has cured remove the base of the box and carefully remove the wooden cylinder head plug by peeling the silicone back. Because of its flexibility you can have small undercuts with silicone but be careful removing casts as it can tear.
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 MDF cooling fins attached.
You can make moulds of bottles or glasses. It is easier to use a PVC tube around the object instead of making a box to contain the silicone. 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.