These alloys are more commonly known as ‘Certo alloys, . When subject to shock loading they are quite brittle, but under constant loading conditions they exhibit some plasticity, Unusually the mechanical strength of these alloys increases with ageing. Even so their moderate mechanical strength makes them suitable only for prototype moldings,
Bismuth-tin alloys have low melting points, ranging from 40 °C to 180 °C. They are well suited to conventional casting techniques and for die and vacuum casting, Frequently they are used with a special-purpose spray gun, which is used to spray a coating of the alloy onto a master to form a cavity that can be inserted into a bolster.
This material is frequently used for making cavity forms because of the ease with which it can be done. A resin and hardener are mixed together and then a finely divided aluminum powder is added to the mixture. The aluminum is an essential additive to help with heat conduction, as the resin itself is a poor conductor of heat.
The resulting mixture is thoroughly mixed under a high vacuum to obviate the inclusion of air bubbles. Once a homogeneous mixture is achieved, the resin may be carefully cast over a master to achieve a cavity insert, which is then loaded into a bolster (usually steel).
Owing to the low mechanical strength of the material and its poor thermal conductivity, its use is limited to prototype work only. However, many very intricate prototypes have been successfully molded from molds with epoxy resin inserts. Several major car manufacturers use this technique to obtain good-quality small plastic moldings for evaluation, it is a cheap and quick method of making cavity inserts from suitable master forms.