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Silicone Rubber RTV 25C & Flexil S


RTV 25C with Flexil S and TIXO T1A can be used to produce flexible but tough moulds from almost any stable surfaced material, without the use of a release agent.

RTV 25C should be carefully mixed with Flexil S in the proportions of 20:1 by weight. It is suggested that to ensure perfect mixing the 2 materials are firstly mixed in one container, then transferred to a second container and remixed, with any unmixed material in the first container being scraped from sides or base and transferred with a pallet knife.

  A Guide to Cure Times

  MATERIAL                             PROPORTION    POT LIFE       DEMOULD
  RTV 25C+FLEXIL S                     20:1                75 mins           16 hours
  RTV25C+FLEXILS+TIXOT1A     20:1+2%        45 mins           16 hours

   Note 1.
  20:1 = 100g to 5g.
  20:1 + 2% = 100g to 5grms + 2.1grms.
  Note 2.
  Complete cure to full Shore A hardness will take 24 hours.
  Note 3.
  Above times are approximate and are determined at a room temperature of  20C (68F).


To ensure the production of void-free moulds RTV 25C+FLEXIL S should be degassed under vacuum after mixing. Using a vacuum of 29"Hg. the material will expand to 4-5 times its original volume, crest and recede approximately to the initial level: a film coating along container sides will be evident above the original volume. After an additional 1-2 minutes the deaeration cycle is complete regardless of any bubbles that may be still breaking on the surface.
If a vacuum degassing system is not available, satisfactory quality can often be produced by placing the poured mould on a vibrating table or painting a thin layer of rubber on to a pattern and 'working' it with a brush for several minutes before pouring the bulk of the rubber.

Conventional Moulds

1. Stick original pattern down on baseboard.

2. Fill in any gaps between pattern and baseboard with plasticine or plaster type product.

3. Build a liquid-tight wall around pattern approx. 10-12mm away from pattern and 12-15mm higher than the highest point of the pattern to form a casting box.

4. Calculate the amount of rubber required to surround the pattern and completely fill the casting box cavity. A cavity of 100ml will require 130grms of rubber to fill it. A cavity of 20 cubic inches will require 330grms (approx. 3/41b) to fill it.

5. Carefully and thoroughly mix together the required amounts of RTV 25C and Flexil S. Degass under vacuum or stand on a vibrating table for 10 minutes.

6. If degassing is not possible brush a thin layer of RTV 25C/Flexil S over the mould surface and pierce or brush away any air bubbles that appear over a 10 minute period.

7. Pour the remainder of the rubber into the casting box ensuring that the highest point of the pattern is covered. To minimise any air entrapment the rubber should be poured slowly in one corner of the casting box and allowed to gradually spread and flow over the pattern.

8. When the rubber has hardened (16 hours) it may be removed from the casting box by removing the walls and lifting pattern and mould from the base. The mould should then be carefully peeled back and the pattern removed.

9. The mould life will be increased if it can be left to age for 24 hours before the first casting is made in it.

10. With some large and irregularly shaped moulds it will be advantageous to build a wooden casting box that can be dissembled to remove the mould and then re-assembled to act as a frame and support for the mould.

Some objects, such as a model horse standing on its four legs can be particularly difficult regarding mould making and casting. A suggested method is as follows:

A. Stick the horse down on its 4 legs using glue or plasticine. A 'light' pattern must be fastened down very securely as it will tend to float out of the liquid silicone rubber).

B. Form a casting box and proceed as numbers 3-7 above.

C. When the rubber has hardened remove casting box and mould from base and turn upside down.

D. With a very sharp knife make a central cut from horses head down to the body and to the base of tail. Then cut from centre line down to body and out to each leg. This will enable the mould to be opened up and the pattern and subsequent castings be removed with minimal difficulty.


                       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 Moulding
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.