RTV Silicone Mould

Introduction
Silicone rubber is an excellent moulding material used mainly where minute surface detail is required usually in conjunction with deep undercuts. There is virtually no shrinkage and is recommended where a larger number of casts are required.

As there are many types of silicone rubber available this information has been designed to give a general introduction to its use.

Details such as cure time and percentage of curing agent to add will be stated on the containers.

Mixing, Mould Preparation
As resins used in silicone rubber moulds can reach a temperature in excess of 160ºC it is recommended that the rubber be allowed to cure for at least 24hrs.

Post curing of the cured silicone mould for a further several hours at 10ºC higher than the curing temperature of the resin may also prolong mould life.

Master patterns can be made from any material, which is firm enough to be painted with Silicone. The master must be prepared with great care, as all surface defects will reproduce on the finished mould. Master patterns made of porous material such as wood, plaster or stone can be sealed with G4 Sealer. If this surface is then polished with mould release wax, a high gloss will be imparted on to the silicone mould. Other release agents that can be used are petroleum jelly and polyvinyl alcohol.

The silicone rubber is activated by the addition of the curing agent. Using disposable or reusable polypropylene type cups stir in the curing agent by hand using a flat stirrer.

Care must be taken not to stir in air, although stirring must be thorough to ensure a complete mix of the two components. It is recommended that after mixing, the material is “degassed” by using a vacuum chamber, as it is very difficult to remove air by any other method. At room temperature 20ºC-25ºC a realistic working time would be about 30-40 minutes. But this time may vary. After this the rubber will be too thick to flow easily.

Mould Making: Simple Open Mould
A mould box will be necessary which should be a little larger than the master and can be made from a variety of material, e.g. wood or cardboard, etc. Any gaps should be sealed with Newplast or Newclay; the mould box should then be coated with a fine layer of petroleum jelly. Mix the silicone rubber and if a vacuum is not available paint over the surface of the master pattern with a thin coat. Allow to stand for approximately 10 minutes before pouring the bulk, hold the container as low as possible and pour in a thin stream. Leave to cure for 24 hours before removing the mould.

Step 1	Step 2	Step 3
Fix pattern to mould box case	Mix silicone rubber following mixing instructions	Pour rubber over pattern from one point

Step 4	Step 5
Demould after 24 hours	Mould ready for production

Mould With Plaster Support case
If the master is very irregular then a mould with support case should be considered. This will also reduce the amount of silicone rubber needed thus reducing cost.

The prepared master is covered with aluminium foil and fitted onto the mould box base. Cover the aluminium foil with a layer of Newplast or clay at least 1cm thick. Assemble the box. Casting plaster is then poured over the covered master and left to set. Disassemble the box and drill pour holes.

Remove Newplast/Newclay and aluminium foil from the master.

Reassemble the box and position the plaster over the master. Prepare and pour the silicone rubber slowly into the filling hole until it comes up the air escape holes. Leave to cure for 24 hours then remove mould from the master and store for 7 days.

Step 1	Step 2	Step 3	Step 4
Fix pattern to mould base	Embed pattern with 10mm coat of plasticene or clay	Pour plaster over pattern to form case and allow to set	Lift off plaster case and remove all traces of plasticene or clay
Step 5	Step 6	Step 7	Step 8
Drill pour holes in case	Mix silicone rubber following mixing instructions	Replace plaster case Pour rubber evenly through holes in case	Demould after 24hours

Two Part Mould
A part mould (split mould) is necessary where it is difficult to remove either the master or the castings from an open mould. A mould box will again be needed and should be prepared as already described.

Wrap half the master in aluminium foil (the half that will be at the bottom when positioned in the box) and position in the box resting on 1cm blocks. Next fill with clay up to the level of the aluminium foil wrapping. Suitable indentations or location pegs should then be inserted into the clay. Prepare, paint and pour the silicone rubber as before and allow to cure. After the 24 hours invert the box and dismantle, removing the wax/clay, blocks, location pegs and aluminium foil.

Reassemble the box and apply suitable release agents. Prepare, paint and pour as before and allow to cure. After 24 hours dismantle and remove both parts of the mould from master pattern. Drill two or more vent holes and pouring holes.

Step 1	Step 2	Step 3	Step 4
Select pattern	Embed half pattern laterally in plasticene and mould box with locating dowels	Mix silicone rubber following mixing instructions	Pour rubber over pattern on to plasticene base
Step 5	Step 6	Step 7	Step 8
Allow rubber to cure. Remove all plasticene and return to mould box	Apply AFH release agent to mating surface and pour second half of mould	Demould after 24hours	Clamp two halves of mould together.

Two Part Mould With Support Case
This is a combination of a two-part mould and a mould with support case, for irregular shapes that cannot be released from an open mould. This will also reduce the amount of silicone rubber needed thus reducing cost.

Follow the instructions for making two part moulds up to and including filling with clay up to level of aluminium wrapping. Then cover the visible half of the master with aluminium foil, this in turn is covered with a layer of Newplast or Newclay at least 1cm thick. Inset location pegs into the wax/clay.

Next pour casting plaster over the master and leave to harden.

Disassemble box and remove plaster, clay, location pegs and aluminium foil. Drill two or more vent holes and pouring holes.

Reassemble the box and position plaster case over the master.

Prepare and pour silicone rubber slowly into filling hole until it comes up air escape holes.

Leave to cure for 24 hours.

Disassemble box and remove the clay filling, half the box, aluminium foil and location pegs.

Reassemble box and apply release agent to the inside.

Cover master and repeat the process. Leave to cure for 24 hours. Remove mould.

Making A Skin Mould With Silicone And Thixotropic Agent
Prepare master pattern and apply release agent. Mix silicone rubber base and curing agent thoroughly and then add the required amount of thixotropic agent to the mixture and again mix thoroughly. Using either a brush or spatula cover the master pattern with the thixotropic coating to a depth of at least 1cm.

If the master pattern is large it is advisable to prepare the thixotropic mixes in several batches, covering a section at a time. Leave to cure for 24 hours, then before releasing the mould cover with a support case. This can be made of plaster or fibreglass, the latter being the better choice if the mould is large and a lightweight case is required.

Once the support case has cured, carefully remove and then peel off the rubber mould from the master pattern.

Finally reassemble the silicone mould into the support case.

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