When it comes to deciding between silicone mold rubber and polyurethane mold rubber, the decision will be influenced by the medium of casting to be used later by the artist among other factors.
One of the first things to consider when planning to make a mold is which mold making material to choose. The choice depends on various factors ranging from the structure and material of the model and the method of making the mold to the casting material and the number of castings to be made to even the personal comfort/skills of the artist.
While there are varied material options for making molds, let us consider between the choice of polyurethane and silicone rubber. It should be kept in mind that silicone rubber does not require a release agent, is easy to use and yields excellent chemical resistance. On the other hand, the polyurethane counterpart will make stronger molds and offers a range of hardness.
And the considerations are:
Generally speaking, both polyurethane and silicone rubbers can be used on any surface and will work on any kind of non-living model. However, if using a platinum-cured silicone, keep in mind that sulfur can inhibit the curing of the mold and it will fail to cure properly in the recommended cure time. Therefore, it should not be applied on sulfur-containing clay apart from some plastics, polyester resins and epoxy resins. However, these curing issues can be overcome by sealing the model properly with a sealing agent.
When it comes to the casting medium, silicone rubber is the material of choice when the subsequent casts are to be made with polyurethane resin, polyester resin, epoxy resin or even polyurethane foam. As the rubber negates the use of release agent, the mold making process will not end up damaging the model either.
When wax, plaster or concrete will be cast in the mold, the molds are usually made with polyurethane rubber. But it can work well with epoxy resins too. Silicone rubber can cause efflorescence when making a concrete casting.
Considering the number of castings to be made, well-designed polyurethane molds can be used again and again to make any number of castings. The same goes for silicone rubber except when casting materials like epoxy resin as the corrosive action can wear down silicone molds, thus yielding only a limited number of castings. Apart from this, the cost factor also comes into play and artists prefer to use polyurethane mold rubber when they are looking to make few castings only as the silicone variants are priced higher and work out more expensive in terms of per casting cost.
To be fair, both polyurethane and silicone rubbers have their own specific properties and advantages and disadvantages. However, the materials will work excellently as mold making compounds and can be used with any type of model to make almost any kind of casting. The final choice will obviously depend on the preference and dexterity of the artist.