Brushing helps to wedge the fine refractory fillers in the relatively big pores between the Quartz grains of a silicate mould, hence representing a very effective application method for coating of pockets and inaccessible angles. Nevertheless, brushing strongly depends on the operator’s skills and promotes uneven wet film thicknesses. In the worst case, brush marks may be visible on the casting surface. Usually, hot spots are pre-coated by brushing. Coating consumption can be controlled very easily, resulting in a very good transfer efficiency.
Core packages with low complexity (i.e. few cavities) are usually dipped into a coating dip tank. The application is very well suited for automatic applications and commonly found in automotive foundries where extreme high production rates and high transfer efficiencies (99%) are required. Usually coatings that are optimised for dipping performances have a very short flow, limiting the formation of drips and runs on the mould surface. The special additive system demands for a high dilution rate with a small parameter window, requiring a continuous checking of the application state. For non-automated dipping stations the operators should note that with a faster withdrawal speed a fast interruption of the contact between the substrate and the coating tank less material flows back, ultimately increasing the wet film thickness.
Flow coating is a very fast and reliable application method for large and complex moulds and core packages that are impossible to dip. For a good application, the piece is put into an angle of approx. 20 to 40 ° (relative to vertical). The operator then applies the coating using lateral movements from top to bottom. Flow coating requires a medium flowing coating system, hence the coated piece must be turned or rotated several times to remove unnecessary material. In some foundries, compressed air is used to remove drips and runs from the casting surface. Although flow coating is a fast and easy application method, it requires the installation of a flow tank and to increase the transfer efficiency, a closed circuit for the ready mixed coating. Usually refractory coatings with optimized flow coating performance also penetrates the surface of a mould or core, applying further protection against metal penetration.
Flat surfaces can be coated very effectively by spraying from a vertical angle. The application with spraying requires especially designed guns to transform the usually thick coating into a fine spray. This leads to very low transfer efficiencies (approx. 30%), as most coating material does not reach the substrate at all. Furthermore, coatings based on organic solvents cause hazardous spray mists. Like with brushing, deep cavities cannot be coated properly using conventional spray guns. Additionally, spray guns that are not put into solvent when not in use, the nozzle usually tends to block. To overcome the negative side effects of the back pressure of air in conventional spray systems, airless spraying instruments were introduced into the market some years ago.
The spray coating application during centrifugal casting processes is an exception, as the hot coquille is sprayed from inside, allowing the coating to quickly form a homogeneous dry film and maintaining a very high transfer efficiency.