 Smaller and cheaper runs can be done with cast copper molds, and some companies do even smaller runs using cast resin or rubber molds, but the durability is of a lower standard than carbon steel and labour costs are higher.Specialized kits cast in resin are available from companies such as Anigrand, Collect Aire, CMK, CMR, and Unicraft, made in molds similar to those used for limited run plastic kits, but usually not as durable, hence the much smaller numbers of each kit that are made, and their higher price.Vacuum forming is another common alternative to injection molded kits but require more skill to assemble, and usually lack detail parts that must be supplied by the modeller.