A model is always reproduced using a mould. The aim of the impression is to obtain a medium that can be burned without leaving any residue, such as wax or certain plastics, which then allow for a final reproduction in metal.
No temperature and pressure are required to create the silicone mould. Firstly, this results in almost no shrinkage of the later casting and secondly, materials such as wax, plastic, polymers from 3D printing, natural products, etc. can also be moulded.
During production, the model is poured into liquid silicone and air bubbles contained in the mixed silicone are sucked out under vacuum. The curing process takes several hours. The mould is separated according to the "cutting technique" (see section on rubber mould). Silicone moulds now have a higher hardness than rubber moulds. This results in advantages with regard to the seam, surface accuracy, stability in narrow places or large areas, as well as the mould’s life and durability.
Due to the advantages of silicone moulds described above, more than 90% of the impressions made in our company today are made of silicone.
In some cases, rubber moulds allow for the air to escape better during wax spraying and are somewhat more elastic, for example, when reproducing hollow bodies.
To put it simply, in the production of rubber moulds, several layers of rubber are glued on top of each other, the model is placed on top of this and then several layers of rubber are glued on top of this again. This "sandwich" is now heated at approx. 150 °C in a metal press.
The rubber liquefies, but cannot expand inside the press, which creates considerable pressure. A forming and curing process is initiated by the heat-induced transformation and pressure. The rubber mould created after pressing must now be cut open to remove the enclosed metal model. Generally, a distinction is made between two processes. If the contact surfaces are impregnated with a release agent before pressing, they will not bond completely. The pressed form can therefore be torn apart in the middle (tearing technique).
With more complex models the use of the "cutting technique" is recommended. In the cutting technique, a mould is cut open using a surgeon's scalpel along the model’s previously marked parting line. Apart from the restriction in the model materials suitable for use (must withstand heat and pressure – no hollow parts!), the rubber mould has the crucial disadvantage that there is approx. 3.5% total shrinkage, which can even exceed 5% in the case of solid parts.
Metal moulds are used exclusively for the production of large quantities. The reason for this is the high production costs, which often require a very large order quantity for them to be financially absorbed into the individual part’s unit costs. This results in high precision and lower production costs of the raw castings, as machine-made plastic parts can be cast instead of wax.