HOW TO USE GLOBALSIL AD SILICONE ELASTOMERS (polymerisation by addition).

Preparation of the model and polymerisation inhibition.
Though the Globalsil AD silicone elastomers have been formulated for the reproduction of models made with the most varied materials, vulcanization may be inhibited if placed in contact with certain chemical products.
Wood with a high resin and tar content, plasticine containing sulphur, neoprene glues, condensation silicone rubbers, isocyanates, ammines, polyester resins, natural rubber, tin, sulphured products in general, can dramatically interfere with the hardening process of addition silicone rubbers. This condition is characterised by incomplete vulcanization or a very sticky contact surface.
In any case, it is always advisable to make a preliminary test to see whether treatment of the model with special barrier products or the adoption of other measures to prevent the inhibition of vulcanization is necessary.
If this test indicates the possible incompatibility between the silicone rubber and the model, the model will need to be coated with a layer of polyvinyl alcohol separator applied by brush, dipping or pad (eg. VPA SEPARATOR) followed by a very fine layer of vaseline oil, after evaporation of the volatile stage.

MIXING.
A temperature above 38°C in the working environment should be avoided to prevent premature vulcanization of the base+hardener mixture.
The surfaces to be reproduced and the containers must be thoroughly cleaned and dried.
The capacity of the containers in which mixing of the two components takes place must be at least four times the volume occupied by the mixture itself in order to aid mixing and degassing operations.
Component B (hardener) should be mixed before use. The base (component A ) should be mixed with the hardener (component B) in the ratio of 10 : 1 parts in weight.
The two products are supplied in pre-weighed packages in the correct ratio, but if a lower quantity is used than the contents of the A+B kit, the two components must be weighed accurately using scales with a deviation of no more than 1 gram.
Mixing can be done by hand, with special batching machines, or by a variable speed drill with helical rotor (like those used for mixing paints).
Only and always use clean spatulas and tools, avoid prolonged mixing at high speed, always scrape the container edges carefully to disperse the material gathered in the reaction mix.
After a first mix, it is advisable to transfer the Base+hardener mixture into another clean container and briefly mix the two components together again. This is to avoid transferring any product that has not been blended well onto the model and mould.
The use of batching machines is highly recommended if the work volume justifies the expense.

DEGASSING.
The air trapped during mixing must be removed in order to avoid the formation of bubbles in the hardened product. Mixture degassing can be obtained by using a special, sealed dome and applying a vacuum of about 20mm of mercury, bearing in mind that, at this stage, the mixture volume can increase by 4-5 times, therefore mixing must take place in containers of appropriate capacity. After reaching its maximum expansion, the mixture volume starts to decrease and at this point the vacuum can be applied for another 1-2 minutes.

POURING.
Pour the degassed silicone rubber from a height of about 8 cm from the surface of the model to be reproduced. In the case of complex models, the product must be poured continually until the whole surface of the model is covered with a thin layer. Then, wait about 5 minutes before pouring the remaining material; during this waiting time it will be possible to eliminate any bubbles forming on the undercuts that rise to the surface, using compressed air or working gently on the film with a spatula or brush.
It is possible to block the hardening process of any leftover product storing it in the freezer for re-use the day after. In this case the mixture workability time will be reduced proportionately with the time it remains at room temperature after each freezer storage cycle.

HARDENING.
The hardening of Globalsil AD silicone rubbers starts the moment mixing is started of component A with component B, i.e. base with hardener.
Regardless of the thickness applied, at room temperature “delivery” of the mould will be possible after 18-24 hours. “Delivery” times can be reduced increasing the temperature during hardening; this procedure can, however, cause a little linear shrinkage.

“DELIVERY”.
Before removing the silicone rubber mould from the model, it is firstly necessary to remove all the support equipment, eg: container caisson, etc.
This operation should be carried out with a continuous, gentle movement, avoiding the use of sharp-edged tools.

MOULD LIFE.
The life of a silicone rubber mould depends on the nature of the material placed in contact with it and the actual contact time.
Products like epoxy, polyurethane and polyester resins contain highly aggressive hardeners, catalysts and plasticizers which are gradually absorbed by the mould surface. Over time, this can cause the formation of chemical bonds that tend to make “delivery” operations difficult.
The use of special barrier-paints or release waxes (eg. GLOBALWAX 200) is helpful in minimising the problem and considerably lengthening (up to four times..) mould life. These products should be applied before every pour.

PRECAUTIONS.
On contact with acids, the hardener of Globalsil AD silicone rubber (component B) can cause strong bases and oxidising agents, hydrogen, a highly inflammable gas; therefore contact with these materials should be avoided. Keep the hardener in well-closed containers away from damp and heat.