I will show how to make a simple silicone matrix/cavity mould, single piece, the simplest form of matrix mould, it is possible to do multipart matrix moulds but I wont be covering that in this.
I needed to make a silicone mould of a part and I didn’t want to use a load of silicone by doing a block mould for such a large part so I thought I would give matrix moulding a go.
there are a few methods for creating a thin silicone mould with a fibreglass jacket, one method is to add a thickener to the silicone paint it on until you think its thick enough then let it set, and build a fibreglass jacket on it. problem with this method is that you can end up with thin spots on silicone mould which would be prone to tearing.
A matrix mould allows a uniform thickness of mould over the whole part you want to cast and it uses significantly less silicone than a block mould so it works out slightly cheaper. the down side is that it requires a little more time for the setup.
This kind of mould is also referred to as a cavity mould as you are creating a cavity between the master (the part you want to mould) and the jacket.
Full details on how to do this are on the instructables page