The primary slurry has two main functions: to capture the detail of the wax pattern and to provide the surface that the alloy will be cast against.

The backup slurry is used to create the bulk of the shell. It will provide the strength for the shell to survive the dewax and casting process. The backup shell coats will also determine the dimensional and heat transfer properties of the shell.

The primary slurry formulation is chosen based on the parts being cast, the alloy used and the process. A good starting point can be found on R&R's primary binder application instructions or by contacting R&R’s technical team at 800.253.4502.

This depends on the slurry used, the surface requirements and the parts being dipped. In general, primaries containing high percentages of zircon are used for ferrous and other reactive alloys. One or two coats would be required with zircon stucco. For non-ferrous applications, slurries with lesser amounts or even no zircon could be used. The number of primary coats required is typically two; however, parts with a lot of detail and with critical geometry may require more primary coats to fill tight areas.

This depends on the metal being cast. When casting a ferrous metal, a zircon slurry can be used for primary coats. In some cases, fused silica can be blended in as well. For non-ferrous metals, fused silica can be used in primary slurries, although even in these cases some zircon may be blended in as the density of the zircon will help the flow properties of the slurry. For backups, the typical choices are fused silica (Ranco-Sil™ fused silica) and aluminosilicates (Mulgrain 47, 60, 70).

Fused silica has a low density (about 2.2 g/ml) and pound for pound will go a longer way. Shells made with fused silica are lighter. Fused silica is very rigid at casting temperatures and is not prone to shell bulging during casting. Fused silica is also much easier to knock out as it goes through a volume/phase transformation during the casting process.

The primary advantage of using aluminosilicates is that it is very abundant and is more economical than fused silica.

The benefit of a two slurry system is that you can formulate each slurry to optimize the process of shell building. The goal of the primary slurry is to capture the detail of the pattern and provide a protective layer that the metal can be cast against with minimal reaction. Backup slurries are formulated to provide bulk and strength to the shell, typically consist of larger refractory particles and are formulated to have a lower viscosity.