RTV Silicone for Resin & Plaster-of-Paris, Guidance for Casting & Moulding Materials....

MARAGON - ARTS & CRAFTS

Casting, Moulding & Modelling - Materials & Kits

All About RTV Silicone

RTV (Room Temperature Vulcanising) silicone is often used for making moulds from non-flexible originals whose surface detail would prevent separation from a mould made of a rigid material such as plaster-of-Paris. (RTV silicone rubber should not be confused with the silicone mastic used around the home and as an industrial sealant). Although casting plaster can cope with extremely ornate and intricate detail, plaster moulds cannot be separated from an original that has any undercut detail unless that undercutting is temporarily filled. Silicone is therefore very widely used in model-making and many technical situations. What isn’t generally known is that most silicones can also be used for casting tin and lead objects. As well as being very heat-resistant, silicone will withstand contact with a vast range of chemicals; silicone has the further advantage of often not requiring any release agent either at the mould-making stage or in subsequent casting work, and is better able to reproduce sharp angles and perfectly-flat surfaces than other flexible materials such as alginate or latex. (Although there are rapid-setting varieties - down to a few minutes - because of the relatively high cost of silicone and unless repeated use is required, alginate is usually preferred when making moulds from the body, etc., and for most 'one-off' situations). RTV silicone is activated by combining it with an additive, which is normally used in a ratio between 1:10 and 1:50, depending on the particular product. The additives are sometimes coloured which helps in recognising proper mixing, although white silicone is often preferred (see below for ways of colouring silicone). Unlike casting resin, etc., however, the specified mixing ratios for RTV silicones generally need to be adhered to quite strictly. Provided the mould is allowed to thoroughly cure in position, there is usually negligible shrinkage on release. Silicone rubber liquid is available in a range of viscosities measured against a standard scale, as are the elasticity and hardness of the set rubber. Rapid-setting or viscous types (above 30 Shore A) may need de-gassing (removing bubbles in a vacuum-chamber) after mixing.

There are occasions when the topology of the original would make it impracticable to de-mould the silicone in one piece; however, by slicing through the set silicone (or even carefully tearing it where a sharp blade is not appropriate) - then matching up and strapping the sections together, it is possible to make moulds and produce castings from the most complicated originals; seam lines in the finishing castings can be easily be smoothed over later. An alternative is to mould and cast projections, extended limbs, etc., separately - which makes it easier to introduce reinforcement (dowels or wire) into slender areas or vulnerable points.

Preparation
This section deals with the basic method of forming a block-mould, i.e. pouring the silicone into a simple container with the original fixed down or suspended within it. Two other ways of making a silicone mould are described lower down the page.

Making a block-mould using RTV silicone

A box or container is required in which the original will be securely positioned or suspended. All surfaces and joints in the box, and between the box and the original, and any open joints and points in the original itself must all be thoroughly sealed and stopped. As can be seen, this process is virtually the same as for making plaster moulds - but there are some exceptions: firstly, RTV silicone is relatively expensive, so it is a good idea to ensure that the shape and size of the container ensures the most economic use of the silicone: suitable filling within the box (e.g. plasticine, plaster) is also often used to save on material - the only limiting criteria being an adequate covering over the original (relative to its size and detailing) - bearing in mind also that the thicker or more massive the mould, the more inflexible it will be when it comes to releasing the casts that are produced from it. Depending on the type of casting that it will eventually be used to make, consideration should be given at this stage to any special adaptations to the exterior shape of the mould e.g. incorporating a lip so that the mould can be properly suspended, for instance, if a heat-sink or weight-relief arrangement is required, however an alternative, more-efficient method of forming larger moulds, or moulds that need support during subsequent casting work, is illustrated lower down the page.

Silicone moulds can usually be persuaded to separate from most smooth originals once fully set, including plaster-of-Paris, china, glass, plastics, good quality paper, unglazed pottery/clayware, latex, most metals and most painted surfaces. There are, in fact, very few surface-types that consistently need a release agent - most notable amongst them are rough timber or end-grain, the rough or broken surfaces of ceramics and plaster, cloth and fabrics, etc. - and originals which are themselves made of silicone* and certain other rubbers (some alloys and even some types of stainless steel can react unfavourably with silicone and either not release or impair the curing process; this is also true of some organic - fruit/vegetable/leaf - materials and a few paints and plasticines). If in doubt, do a surface test and leave it for 24 - 36 hours (use a 6 - 10mm deep patch of silicone for the test, rather than a thin smear). Besides the chore of having to apply it, the disadvantage of a release agent is that there is always some loss of detail - although this may not matter, or may even be desirable. If a release agent is necessary, this can either be brushed, sprayed or rubbed on; most polishes, waxes, WD40, baby oils, etc. but vaseline is ideal in most situations, and is also effective in sealing joint-lines, small cracks around screwheads, etc. Shellac can be used to seal particularly porous originals beforehand. The same criteria apply to all the other surfaces within the mould-box. Note that until it has set, silicone is sticky and difficult to remove from tools and skin (turps can be used for cleaning up, but it is usually easier to peel it off after it has set - except on fabrics). *Moulds can be made in sections, in which case the cured portion(s) must be coated with a total barrier such as spray paint, nail varnish etc, to avoid unwanted adhesion.

Before mixing the silicone, take all the usual precautions with sheeting or newspaper on floor and work surfaces, with the right tools to hand, and wear overalls or old clothes, and have a good supply of paper towels on hand. Try to work out exactly what volume of silicone you are going to need bearing in mind that you will also be using a certain volume of additive - but you should still allow an extra 5 - 10% more more than you have calculated because the pouring process should be as continuous as possible. 1kg of silicone is approximately 920ml. (The volume of silicone needed can be gauged by measuring the volume of dry sand or rice needed to fill the mould space, but do this before applying any release agent). Depending on the size and complexity of the project, it is often a good idea to have an assistant who can move or hold the box at a particular angle, particularly if there is any undercut detailing which may require turning the mould-box itself.

Mixing and Pouring
Make sure the mould container or box and the original or all in position and prepared as above. There should be no interruptions while mixing up and pouring, so make sure that someone else can answer the ‘phone, deal with callers, etc. The mixed silicone is quite thin and pourable, so an open plastic jug of about twice the volume that you are mixing is ideal. Pour in the silicone and then immediately reseal the silicone container. Use the additive* at the specified ratio and mix thoroughly with a large open spoon, etc. Remember to scrape down the sides of the jug also. Slowly pour the mix as near as possible to the base of the original - rather than over the object - preferably working from one direction only - this will normally ensure that air or air bubbles do not get trapped. if there is any undercut detail that faces down (and which can’t be avoided when devising the mould box) the whole container may have to be manoeuvred and turned - possibly by an assistant - so that the silicone can be induced to flow into these areas while you continue adding the mixture. In some instances it may be necessary to stipple the silicone on with a paint brush prior to pouring. Avoid feathering out the silicone or making it thinner on the open surface, and generally ensure there is at least a 6mm covering on even the lightest objects - thin areas can take a lot longer to cure. *Small amounts (up to 3%) of artists’ oil paints can also be added to colour the silicone.

Curing
Once pouring is complete, avoid moving the mould during the curing period, preferably keeping the container at a steady temperature, out of direct sunlight, and away from any obvious source of heat. Separation - usually at 24 - 36 hours - should be a slow peeling-back* process, bearing in mind that all the contact surfaces still need special care for a while longer, as these areas will not have cured at the same rate as the main mass of the mould. *Leave the mould in a warm area if tackiness persists.

Use and Care
Allow a further 24 - 36 hours after separation then, before using for the first time, wash the mould thoroughly in warm water with a mild detergent using a soft sponge or cloth, and allow to drain and dry thoroughly. When in use, depending on the material of the castings, the mould will need regular inspection and cleaning as above. Larger moulds will need support to avoid any stretching and distortion from the weight of the wet casting plaster; tape or light strapping is sometimes sufficient, or the mould can be gently nestled into a box of dry plaster or sand, etc., or suspended in water; moulds for deep/heavy objects may need a rigid jacket or casing of plaster-of-Paris, possibly made in two halves strapped together. Expanding foam contained between the mould and an outer box can be used to support surprisingly heavy castings, but note that a barrier such as petroleum jelly or plastic film is needed to prevent the uncured foam adhering to non-plastic surfaces.

When not in use, make sure the mould is completely clean and dry and is stored in a cool, dry situation, out of direct sunlight - and not positioned or poised at any angle that might cause temporary or permanent distortion.

Two-Stage Mould-Forming
For a more efficient use of material, to preserve flexibility and give support during subsequent casting work, a two-stage process is normally used when making larger moulds out of RTV silicone: a layer of clay or plasticine is spread over the original; this is then placed in a suitable mould-box and plaster-of-Paris poured in. Once set, the plaster case is lifted off and the clay filling removed; when the plaster case is put back over the original there will be a void where the clay was; silicone can now be poured into this cavity via holes that have been either cast or drilled through the plaster case; the silicone liquid should just reach into these holes when pouring is complete, then allowed to cure for 24 - 36 hours before demoulding. Plastic film can be used to keep the original clean; the clay layer should be at least 10mm thick and can also be covered with film. For best results - and particularly if it is going to be used to support the finished silicone mould in subsequent casting operations - the inside of the plaster case should be smooth, have a positive draught (taper towards the open end) and be sealed with shellac or varnish after being allowed to dry out for a few days. Herculite No.2 is a suitable plaster for mould cases as it is extremely durable.

Making a silicone in 2 stages using plaster-of-Paris

If a suitable mould-box can be devised, this procedure can be adapted for making moulds of wall fixtures, plaques, etc. - the whole assembly being effectively turned on its side, with the filling and vents holes repositioned as necessary.

Butter-on Application
There are situations where a short-cut can be taken to the whole mould-making process. A thickening agent - technically referred to as thixotropic additive, or just 'thixo' for short - can be mixed into the silicone after the catalyst has been added*. This converts the silicone into a paste of about the same consistency as a soft sandwich spread. This can be buttered directly on to the original using the back of a table knife, etc. Because of the risk of trapping air and bubbles within the silicone and at the contact surface, the butter-on method is only suited to straightforward originals where there are no undercuts or recesses. A few minutes should be allowed for the thixo to take effect after mixing in - the silicone will then just about hold a peak in the mixing pot. The original should be firmly positioned on a base-board and the silicone pasted on, starting at the bottom and working up - with upward strokes of the knife, etc. - preferably applying a thin film all over first so that visible bubbles can be eliminated. It is usually best done in two or more coats - perhaps inverting the piece once the previous coat has set. Inevitably, the outer surface of the newly-created mould will only be as good as the finish that can be given to it with the tool used - although it may be possible to fix a sheet against, or pass a profile over the outside to achieve a more regular surface or outline. Note:- thixo usually significantly shortens both working time (pot-life), as well as curing time. *Thixo mixing ratios vary between 1 and 3% (some thixo varieties solidify when cold, but should only be used at room temperature).

Colouring Standard RTV Silicone
Although plain, white silicone is often preferred, coloured silicone can give a useful contrast with the other materials being used in the casting sequence. It is also often the case that a pigmented rubber is an artistic or technical requirement in its own right.

Standard RTV silicone can be coloured in several ways (in combination, if desired), using artists’ oil paints.

i). By adding the oil paint into the catalysed mixture; 4% by weight will yield a very strongly- coloured rubber, so add it it very gradually until the required shade is achieved; avoid using more than 5% by weight.
ii). Because uncured silicone adheres readily to set silicone - and very little else! - a thin skin of coloured silicone can be applied to unpigmented, cured rubber (using the same guidelines for mixing).
iii). Mixing the oil-paint directly into the catalyst; this method has the advantage of making it easier to recognise when the catalyst has been thoroughly mixed with the rubber base. Avoid using more than one gram (or millilitre) of oil paint for every 10ml of the catalyst used - even half this amount will produce a strong shade in the mixed silicone; To ensure that no clumps of undiluted paint sink to the bottom, start off by thoroughly mixing a very small amount of the catalyst with the oil paste - then add the remainder gradually, stirring continuously.