Refractory coatings exist to improve casting quality, but when not properly applied, they can cause unwanted characteristics in the final casting.
By Jay Morrison, Carpenter Brothers (Mequon, Wisconsin)
Brushing coating on molds is useful when extra care needs to be given to certain areas.
Applying a refractory coating to molds and cores can be a useful step to improve surface finish, control the heat transfer characteristics and thus the microstructure in the casting, improve core venting and prevent certain defects in the casting. As with any process step, proper techniques in applying the coating are paramount in achieving the desired effects and avoiding the cause of new quality issues. Metalcasters can apply coatings to their sand molds in four main ways, all with varying advantages and disadvantages.
Brushing/swabbing: Brushing or swabbing coating on molds provides good, controlled coverage. This method makes it easy to apply multiple coats and can be used with multiple types of coatings. The method works the coating into the refractory surface well, which can be helpful when applying to corners or walls. However, it is a slow process that is labor-intensive and the application can vary depending on the operator.
Spraying: 'This method is faster than brushing or swabbing and covers a large area readily. It also doesn't leave brush marks. However, like brushing, the application of the coating can vary by operator. The operator may miss some areas with the spray or find it difficult to properly coat deep pockets. Over-spraying may also occur. Finally, spraying a solvent-based coating releases fugitive emissions.
Flow coating: Flow coating is fast, less labor-intensive then brushing or spraying, and easily automated. It applies good coverage and is optimal for large cores and molds. However, the system used to apply the coating brings a level of equipment upkeep that requires regular maintenance.
Dipping: Some foundries opt to dip their cores or molds in the coating. This process provides good coating coverage and can be mechanized. Automated dipping gives an even, consistent coating and is well suited for high production. Manual dipping is labor intensive and slower.
No matter the method, after the refractory coating is applied, a period of drying must occur. Coatings need to dry because the water and solvents expand to many times the volume when heated. The methods of drying used depend on whether the coating is water or solvent-based. Some solvent-based coatings are designed to be lit with a flame, which then activates the clements within the coating.
Metalcasters have a wide choice of useful refractory coatings, but they are designed with different chemistries and formulas that will all require specific process parameters to do their job correctly. This includes the optimum thickness of application, drying time, viscosity, and dispersion of particles within the solution. Metalcasters should consult the manufacturer for recommendations on each coatings' use and follow them closely.
A common method of drying the refractory coating is to light it on fire, which also activates the binders in the coating.
Components of a Refractory Coating
Carrier: The refractory is carried in a liquid where solids arc suspended, typically water or a solvent blend.
Refractory: Coatings contain a refractory mineral or combination of minerals, such as graphite, zircon and chromite.
Suspension agents/rheological modifiers: These are responsible for how the material flows and hangs on a mold or core.
Dispersants: The viscosity of a coating is stabilized with dispersants that change the surface chemistry of the particles so they don't stick to one another.
Binders or film former: Binders are responsible for adhesion of particles to the surface and cohesion to other particles within the coating.
Surfactants: Anything that changes the surface tension of the carrier is a surfactant, and they can be used to increase coating penetration on sand cores and molds.
Preservatives: Bacteria in coatings cause issues with suspension and viscosity, and preservatives prevent micro-organism growth.
Dyes/pigments: These are added for contrast in molding and arc typically iron containing or organic pigment.
(We'll continue next week with a review of other parts of the April 2019 article in MODERN CASTING Magazine. "Refractory coatings exist to improve casting quality. But when not properly applied, they can cause unwanted characteristics in the final casting. Seven main coating-relating defects, their likely causes and corrections arc explained here." If you can't wait, here's a link.)
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