With the mould inverted the cut should be as shown.
There are various well-publicised methods of making rubber split moulds which all involve pouring a layer of rubber, waiting for it to solidify and then pouring a subsequent layer, at least part of which will be in contact with the first layer. When adopting this type of process it is essential that any cured rubber that will have an uncured rubber in contact with it is covered with a thin film of petroleum jelly thinned with white spirits to ensure that they do not stick together.
Skin moulds are usually used to reproduce objects that are highly detailed and comparatively tall and thin rather than flat or squat. They are advantageous as there is no split mark but a disadvantage is that they usually require a split support shell.
1. Stick original pattern down on a baseboard that is at least 25mm larger all around than the pattern.
2. Fill in any gaps between patterns and baseboard with plasticine or plaster.
3. Calculate the amount of rubber needed to make the mould by working out the mould surface area in square centimetres, and allowing one gram of rubber to 3 square centimetres of mould surface for patterns up to 150mm high, and one gram of rubber to 2 square centimetres above 150mm.
4. Mix together RTV 25C, Flexil S and TIXO TA1 Thixotropic additive.
5. Degass if possible.
6. Apply the mixed rubber to the pattern surface using a brush to stipple into fine detail and a trowel or pallet knife for the bulk. The average mould thickness is 3-4mm for small moulds and 5-6mm for larger ones. A 5mm layer of rubber can be applied quite readily. If a second layer is required it should be added when the first is cured but still sticky.
7. Small moulds may be self supporting or may be supported in sand but for larger moulds the best support method is normally to make a split shell mould in glassfibre or possibly plaster of paris, A split mould has to have its split or splits on a line that will enable a rigid mould to be removed without being affected by undercuts. An obvious example is a sphere where the split would have to be on the centre line to avoid either half being undercut. To make a split shell form a flange on the pattern using hardboard, cardboard, plasticine or almost any self-supporting material and make a GRP laminate around the first part of the mould and the flange. Allow the first laminate to cure, remove flange-making material and make a second GRP laminate around the second part of the mould and pre-formed flange. When the second laminate has cured drill boltholes through the flanges and strip the separate split shells from the rubber mould. N.B. When making a GRP shell on a silicone rubber mould it is not necessary to use a release agent but it is essential that some form of release is applied both to the original flange material and the moulded GRP flange before making the second shell. When the rubber mould is removed from the pattern the shell is assembled around it and bolted together using the previously drilled holes.
8. To remove the rubber mould from its pattern or castings made in it simply lubricate the outside using French chalk or a mixture of detergent and water and turn the mould inside-out over and off the object as if removing a kitchen glove.