Silicone RTV Rubber Moulds And Mould Making Compounds

RTV-2 silicone rubbers are pourable, spreadable or kneadable compounds, which will crosslink in the presence of a second component to form flexible rubbers. They are outstanding as mould-making materials because of their excellent properties:

· Easy to use

· Excellent release properties

· Accurate reproduction of surface details

· Good heat resistance

· Little or no shrinkage

RTV-2 silicone rubbers can also be use for encapsulation and sealing, or for making flexible parts.

RTV-2 silicone rubbers always consist of two components, namely the rubber base itself and a catalyst, or the components A and B. They are cured or vulcanised at room temperature (RTV= room temperature vulcanising).

Different grades are available, with different properties suitable for a wide range of applications. They can be classified according to the following criteria.

1. Vulcanising system (condensation-curing or addition-curing)

2. Consistency (pourable, spreadable or kneadable)

3. Mechanical properties of the cured rubbers (e.g. tear resistance and elasticity)

Condensation-curing RTV-2 silicone rubbers

These cure at room temperature after addition of catalyst, volatile alcohols being liberated during the reaction. Shrinkage occurs, which is completed after about a week. Although not very effective, the vulcanising process can be speeded up by heating. The temperature should under no circumstances exceed 70oC. It is also most important that freshly made moulds, i.e. which still contain the alcohol produced during the curing reaction, are not subjected to temperatures higher than 80oC, since this would cause a loss of elasticity and softening or stickiness of the silicone rubber (see also Mould pre-treatment).

Addition-curing RTV-2 silicone rubbers

These vulcanise without the formation of reaction products, so that there is practically no shrinkage. The cured rubbers can therefore be used immediately after they are fully cured. Vulcanisation can also take place at higher temperatures (up to 200oC) which greatly reduces the required curing time. In this case however, one must take the high coefficient of thermal expansion of silicone rubber into account.

Cleanliness is especially important when processing addition-curing RTV-2 rubbers, since the platinum catalyst can lose its power through contamination. Catalysts used for condensation-curing silicone rubbers, for example, can interfere with vulcanisation already in small amounts. Furthermore, vulcanisation of addition-curing rubbers can be delayed or completely prevented if they are in contact with certain materials such as natural and synthetic rubbers, PVC, amine-cured epoxy resins, plasticine, adhesives, soldering tin etc. Preliminary tests are therefore vitally important.

Storage stability

RTV2 silicone rubbers will remain in usable condition for at least 12 months, provided they have been stored cool and dry, in tightly closed containers. The T series of catalysts have a shelf life of at least 6 months in closed containers, and should be protected from heat and moisture. All containers should be tightly re-sealed every time some of their components have been removed.

RTV-2 silicone rubbers and food regulations

Finished products made from addition-curing silicone rubbers are suitable for food contact applications in compliance with food regulations.

Condensation-curing RTV-2 silicone rubbers

These are vulcanised by adding a liquid or paste-like catalyst. The pot life of the resultant mix, as well as its vulcanising time, will vary between a few minutes and several hours, depending on the type of catalyst and the amount used, the ambient temperature and relative humidity, as well as the amount of movement in the air. Optimum characteristics of the cured rubber can only be achieved if a suitable catalyst is used in the prescribed amounts.

Addition-curing RTV-2 silicone rubbers

Here the mixing ratio for components A and B is fixed and must be adhered to precisely in order to achieve the desired end product properties.

Preparation of components

All pourable components should be stirred thoroughly before use to ensure even distribution of the filler. Stirring also improves flow in higher viscosity systems.

Mixing of components

Even mixing of the rubber base and catalyst (in the case of condensation-curing rubbers) or the components A and B (in the case of addition-curing rubbers) is easily achieved for pourable and spreadable compounds, using a spatula. Larger batches are best mixed with a mechanical stirrer. Kneadable compounds can be mixed by hand, on the rolls or in a compounder, depending on the size of the batch.

Removal of entrapped air

To ensure bubble-free moulds, pourable mixes should be deaerated under vacuum. The catalysed mix is poured into a vessel so that it is no more than a quarter full. A vacuum of 10 – 20 mbar is then applied which initially causes the mix to rise, usually collapsing before the vessel’s rim has been reached. If, however, there are signs that the mix will run over, a small amount of air is introduced and the process is repeated until the mix collapses. Evacuation is discontinued immediately afterwards, in other words the mix should under no circumstances be evacuated until no more bubbles form, since certain substances essential for vulcanisation to take place could escape under these conditions.

Application to the pattern

A thickening of the catalysed mix marks the start of vulcanisation. The material should be used well before the pot life has expired, i.e. whilst it is still in a free-flowing state which makes application that much easier. Every increase in temperature reduces the pot life.

After evacuation, the mix is poured a thin stream, from a low height, on to the object of which a mould is to be made. The point where the stream hits the object should not be changed if possible.

If there are no possibilities of evacuating the mix, or if a spreadable compound is involved, a bubble-free mould surface can be achieved by first applying a thin coat of the catalysed mix to the pattern, using a stiff, short-bristled brush, allowing the rubber to partly vulcanise and, whilst it is still tacky, pouring the rest of the rubber on to the pattern in a thin stream, and from a great height until desired wall thickness has been attained. In the case of a spreadable compound, this may be applied with a brush or spatula.

Production of moulds

Models and their pre-treatment

Models made of standard materials such as wood, plastic, wax, metal, plaster, glass, porcelain and stone are well suited for the production of RTV-2 silicone rubber moulds, although in certain cases it is necessary to prepare the surface, as described below.

· Loose particles should be removed

· Porous surfaces must be sealed to prevent the rubber sticking to the model through absorption. This can be done using any of the following:

Ø Water soluble synthetic resins such as methyl cellulose (wallpaper adhesive) or polyvinyl alcohol

Ø Concentrated soap or detergent solutions

Ø Vaseline

Ø Wax or paraffin in liquid form or as 5 – 10% solutions in surgical spirit.

· Glass, porcelain and ceramic surfaces can form a chemical bond with silicone rubbers and should therefore first be coated with detergent solution, vaseline or wax.

· Even smooth, non-absorbent surfaces, which do not normally require the application of a release agent, may become siliconised after repeated contact with silicone rubbers. This tends to make the rubber stick to the surface. Here, too, applying an extremely thin film of detergent, vaseline or wax can solve the problem.

It is important to remember that pre-treatment of the model surfaces can change their appearance, e.g. darkening of wood, discolouration of natural and artificial stone. Furthermore, certain materials used to make models and patterns, some of the above mentioned agents used to treat surfaces, and models from which moulds have already been made with condensation-curing silicone rubbers. This causes the surface in immediate contact with the model to remain sticky. In such cases, the model surface should be treated with a suitable release agent or vaseline (pharmaceutical quality).

To get an idea of the possible changes resulting from surface treatment, it is essential to carry out preliminary tests on an unimportant area of the model or one that is not exposed to view. This also enables one to check that the cured rubber does not adhere to the treated surface.

Single part moulds

When a mould is made from a flat-backed relief, a frame first surrounds this, which should be higher than the model. Wax and wooden models should be firmly anchored to the base because their lightweight would cause them to float to the surface of the liquid silicone rubber. The edges of the models should be sealed, e.g. with a safe plasticene, to prevent the silicone rubber getting underneath. The requisite amount of liquid silicone rubber is then catalysed and, if possible, evacuated. The mixture is then carefully poured over the model. Small moulds are sufficiently stable to ensure dimensional stability of the cured silicone rubber. Moulds of raised or vertical relief’s are made by brushing a non-sag silicone rubber onto the surface to give a film thickness of 1 – 2mm. This procedure also prevents the formation of air bubbles. After this first coat has been vulcanised, a further coat, 1 – 1.5mm thick, is applied. A plaster or polyester laminate support should be applied to the silicone rubber skin in order to give it the necessary stability.

Large moulds with uniform wall thickness can be made as follows. The original is covered with a layer of safe plasticine and surrounded by a frame. Plaster of Paris is then poured on top of the plasticene-covered model. When the plaster has set, the plasticene-covered model is removed from the plaster mould and the plasticene removed. Pouring holes are then drilled into the plaster mould, which is then placed on top of the model. Silicone rubber is poured through these holes into the space formerly occupied by the plasticene. A thin silicone rubber skin is thus produced which faithfully reproduces the surface of the original and is supported by the plaster backing.

Two-part moulds

Here, the model is embedded in safe plasticene to about half its depth. The plasticene surface adjacent to the model is smoothed and provided with “locks”, i.e. trapezoidal recesses are cut into the plasticene, which are reproduced in the silicone rubber mould. These prevent the two mould halves becoming displaced in relation to each other. The model is now surrounded by a frame and the silicone rubber poured evenly on top of the protruding model. After vulcanisation of the first mould half, the model is turned round and the plasticene removed. The first mould half remains on the model. The surface of the mould is then carefully brushed with a mixture of wax in white spirit to prevent the two mould halves sticking together during vulcanisation of the second mould half. The wax solution consists of 1 part by weight of wax in 10 – 15 parts by weight of surgical spirit. Other suitable release agents include nitrocellulose lacquer. After the solvent has evaporated, the second half of the mould is made as described above.

Moulds of large objects are made by brushing the silicone rubber onto the surface of the model protruding from the plasticene.

After vulcanisation, a plaster or polyester laminate support is applied. The plasticene is then removed and the now visible edge of the mould is coated with one of the above-mentioned release agents. The other half of the mould is made in exactly the same way. Pouring holes are either cut subsequently, or are made at the same time as the mould by inserting a suitable material.

Mould pre-treatment

To achieve optimum properties, moulds made from condensation-curing RTV-2 silicone rubbers must be exposed to the atmosphere for 48 – 72 hours at room temperature – depending on cure time and wall thickness – in order to completely remove the reaction products formed during vulcanisation. Alternatively, moulds can be exposed to the atmosphere for 24 hours at room temperature, followed by conditioning at 70 – 80oC in a well ventilated drying oven. As a guide, heat treatment should be about 6 hours per cm wall thickness.

Moulds made from addition-curing RTV-2 silicone rubbers can be used immediately after being stripped from the model.

The largest number of castings taken from a mould can, however, be achieved if it is heated for a few hours to its maximum service temperature not exceeding 200oC, before being used for the first time.

Wax and plaster

Castings made of wax or plaster put but little stress on the moulds. When casting wax, it is advisable to preheat the moulds to the temperature of the molten wax to ensure good flow into the mould. Moulds that have been used repeatedly for casting wax show a deposit of wax scales on the surface. These should be removed with white spirit, which is then allowed to evaporate, making the mould once more suitable for use. Another way of removing such wax deposits is to heat the mould in a drying oven to 120 – 150oC, placing it upside down on a suitable tray to catch the molten wax as it drips down.

Silicone rubber

It is possible to cast silicone rubber objects in RTV-2 silicone rubber moulds. The mould cavity should first be treated with a wax solution in white spirit or oter suitable release agent, to prevent the casting sticking to the mould surface.

Polyesters, epoxies and polyurethane’s

Polyester, epoxy and polyurethane casting resins can also be used to produce castings from silicone rubber moulds. No heat curing resins should be used, especially on the case of polyesters, since this would greatly limit the number of castings that can be taken from a mould. It is generally advisable to heat the moulds to 120 – 150oC between castings, in order to remove any volatile casting resin constituents, which may have penetrated into the rubber. Special moulds made from addition curing RTV-2 silicone rubber may be used for casting polyurethane. The use of a barrier coat is not absolutely essential, although this will definitely prolong the working life of the mould.

Condensation-curing RTV-2 silicone rubbers can, however, also be used, provided a suitable barrier coat is applied. This is a special coating compound, which prolongs the life of the mould and is applied by brushing or spraying.

During cure, the PUR bonds firmly to the barrier coat which thus becomes the outer skin of the casting.

If other synthetic resins are to be cast in the silicone rubber moulds it is advisable to carry out preliminary tests.

Metal alloys

There are silicone rubbers that are recommended for low-melting metal alloys. The temperature of the molten metal at the moment of casting should not exceed 300oC. To ensure even wetting of the mould surface, it is coated with graphite powder, acetylene black or silicon carbide. Thin-walled moulds should be used and be placed on a sheet of aluminium or other material with good thermal conductivity before pouring in the metal. Since the mould expands considerably at high temperatures, we would recommend working with a number of moulds to give each one a chance to cool down before it is used again. The first castings are generally unusable since the mould still gives off gases, giving the surface of the castings a pockmarked appearance.

Electrotypes

Silicone rubber moulds can be used for making electrotypes provided of course that the electrolyte does not attack the rubber. Slightly acid or alkaline electroplating solutions may be used and an electrically conductive film of graphite or silver coats the mould surface.

Repair of damaged moulds

Damaged moulds made of RTV-2 silicone rubbers can be easily repaired, using freshly mixed compound of the same type as was used for making the mould. Areas to be repaired must be clean, or be cleaned with acetone. Cracks should be carefully pulled apart and both sides coated with catalysed silicone rubber. The crack is then pushed together and excess silicone rubber removed. The mould is then vulcanised to complete the repair.

Damaged moulds can also be repaired with acetic acid cures single part silicone sealants.

Joining silicone rubber to various materials

How to achieve a good bond between RTV-2 silicone rubber and other materials

Because of the non-stick properties of RTV-2 silicone rubbers, special measures have to be taken to make these materials stick to others, e.g. by applying a suitable primer to promote adhesion. Primers are low-viscosity solutions of reactive silanes or silicone resins. Evaporation of the solvent leaves behind a film, which cures at room temperature through atmospheric humidity, or by application of heat. This film causes adhesion between the substrate and the subsequently applied RTV-2 silicone rubber.

The primer used will depend on whether the surface is absorbent or non-absorbent.

To improve adhesion, it is best to roughen the surface with sandpaper. Metals should be sandblasted if possible. Removing the abrasive dust and thorough degreasing with acetone or white spirit follows this. Plastics can also be cleaned with acetone provided; they are not attacked by it. White spirit can also be used for this purpose. In the case of wood, all that is normally necessary is to ensure that it is clean. If necessary, it can be planned or sanded down with emery paper.

Brushing, spraying or dipping may apply primers. Drying times vary between 30 and 60 minutes at room temperature. Adhesion may be improved by baking the resin film for 10 minutes at 130 – 150oC after it has air-dried.

Maximum adhesion to the primed surface will develop about 48 hours after vulcanisation at the earliest.

How to achieve a good bond between fresh and cured RTV-2 silicone rubbers

In this case primers are not normally suitable. Condensation-curing rubbers normally adhere well if vulcanised on to similar rubbers, provided these are not too old. In the case of addition-curing rubbers, however, special measures have to be taken to achieve adhesion, details of which we will gladly supply on request.

Bonding cured RTV-2 silicone rubbers to other materials

If the moulds have to be stuck to a surface, using a single part acid cure RTV-1 silicone rubber, can do this. This one-part material is applied to the pre-treated and primed substrate as well as to the clean silicone rubber mould, both parts being brought together immediately. By moving the mould about a bit, one ensures that it is positioned properly. Excess material is removed. Smaller moulds will have bonded satisfactorily already after 1 – 2 hours, but this figure should only be taken as a guide, since relative humidity, mould size and ambient temperature are all relevant.

Bonding cured RTV-2 silicone rubber to cured silicone rubber

Here, the surfaces to be joined are thinly coated with an RTV-1 silicone rubber All other details are exactly the same as for bonding cured silicone rubber to other materials.

Removal of silicone rubbers

Uncured silicone rubber residues can be removed from containers or clothing by dissolving them in hydrocarbons such as white spirit. Unused material is best left to vulcanise inside the mixing vessel, after which it can be removed quite easily. Cured material can only be removed mechanically from clothes, after first swelling it in white spirit. Suitable protective clothing should therefore be worn when working with RTV-2 silicone rubber.

Safety precautions when working with RTV-2 silicone rubber

According to experience so far, RTV-2 silicone rubbers are non-toxic as supplied. In some cases, however, contact with the skin and, particularly, the eyes can cause irritation, so that appropriate safety precautions should be taken. If the catalyst gets in contact with the skin, it should be washed off with plenty of soap and water. If there are signs of irritation, a doctor should be consulted. If catalyst has splashed into the eyes, these should be rinsed with plenty of water and medical advice sought without delay. Adequate ventilation of the work place should be ensured when handling solvents and normal safety precautions should be observed.