The castings used in Schomberg Scale Models kits are made from either Hydrocal Gypsum Cement or urethane resin.
Hydrocal is a brand name for a very hard setting plaster. Unlike Plaster of Paris, it can withstand fairly rough handling without breaking. That's the primary reason why we use it to cast the walls of our kits as well as for many of the detail parts. It may be quite easily cut and sanded and it takes most types of paint very well. Some kit manufacturers recommend that the plaster parts in their kits be primed (sealed) before painting. We believe that the natural porosity of the hydrocal helps to absorb the paint, imparting a very low shine to the painted parts. A technique that can give very beautiful and realistic effects to a hydrocal casting is painting with washes of watercolour, where the colour may be built-up in semi-transparent layers. This technique works particularly well on stone castings where a subtle variation in colour is desired. However, plaster also has its faults. The weight of a hydrocal casting is considerably more than that of an equivalent resin casting. The main reasons that we use hydrocal for walls is the cost, with the price of hydrocal being much less than urethane resin.
Urethane resin has revolutionized the 'cottage' casting industry. Unlike thermal plastic casting (styrene, for example), the resin does not need to be heated to a high temperature and injected into the moulds under great pressure. Working with small batches of resin, we hand pour only a few moulds at a time. This takes longer than injection moulding but we don't have to spend tens of thousands of dollars on dies and machinery. It is this single fact that allows us to produce fine kits at a reasonable price. Resin is preferable to hydrocal where cross sections of parts may be very thin or fine. Such detail would not survive removal from the mould if cast in plaster. As well, resin is the choice material when the moulds have significant 'undercuts' or areas of extreme overhang or recession.
Although much more expensive than hydrocal, resin is much less prone to breakage if mishandled and will take paint extremely well if pre-washed in liquid dish washing detergent and warm water. An old toothbrush helps in this operation.
Of course none of the casting methods discussed above would be possible without a good mould. Schomberg Scale Models uses premium quality RTV (room temperature vulcanizing) silicone rubber for all moulds. This material hardens or 'cures' without the external application of heat making it ideal for small batch use. Most of the moulds we use are 'open face' or one piece, which are filled from the back with casting resin or hydrocal. Occasionally, we make multi-part moulds if the patterns to be copied are of a particularly complex shape. RTV moulds used in resin casting have a fairly short working life due to the heat generated by the curing resin and by the chemical reaction of the resin with the mould. These moulds must be replaced at regular intervals to maintain casting quality. In contrast, moulds used for hydrocal casting have an indefinite life due to the relatively gentle reaction of the curing plaster. That is a good thing since a set of wall moulds for a large O scale building kit may have several hundred dollars worth of RTV rubber in them.
Finally, we use Mt. Albert Scale lumber almost exclusively in our kits. The dimensioning and finish of this stripwood is without equal in our hobby.