Choice of the right coating
The type of casting and surrounding parameters play the major role in selecting basic properties like the powder or ready mixed supply, carrier liquid (water or alcohol) and the refractory type (carbon, oxides or silicates). Centrifugal castings have different requirements than other permanent mould castings and these of course have requirements very different from those of classical sand castings. The latter include many different parameters like mineralogical composition (e.g. Quartz, Chromite) of the mould material, its type and rate of reclaim, type and amount of organic compounds (e.g. binder) and last but not least the settling type that also affects the applicability of a coating system.
The casted alloy and the casting temperatures usually predetermine the exact composition of the refractory fillers. As experience teaches, particle size, density, thermal conductivity and expansion as well as the sintering and melting point (or range) help to evaluate possible reactions of the refractories with the alloy and mould material. In the best case, the refractory filler does not show any wettability with the molten metal. Of course price also matters in most cases, resulting in a trade-off between technical benefits and cost savings which, and that should not be underestimated, can be attained by a better coating-dilution ratio for the product.
But not all refractory fillers can be applied with the same performance with all different application methods. The shape of Zircon grains allows such coatings to be applied via brushing, flow-coating or by dipping. Tabular alumina that mainly consists of the mineral Corundum may provide the same high refractoriness, but due to the commonly angular grain shape the application may be not as good as with Zircon. Finally the rheological properties of a refractory coating are predefined by special additives. Hence, one single refractory coating cannot be applied by all methods (brushing, flow coating, dipping and spraying) in equal quality. Even in those rare cases, at least different dilutions have to be used.
Of course you would not unquestioningly order a full truck load of coating and try your luck, but will be guided by our capable technical service staff serving you with the full service from process analysis over coating trial planning until supply of the needed technical equipment.
Example for finding the right coating
Many factors influence the technical requirements towards a coating system. For example in a foundry that produces ductile iron castings at 1450 °C from small and medium size walled automotive moulds and cores in cold box where veining is a big issue and there is enough hot-air dying capacity, a water-based but fast drying, ready mixed coating with highly refractory Mullite and close-fitting phyllosilicates offers good prerequisites for initial trials. RWB 8361-FD would be your coating of choice. But this coating is also suited for flow coating application of bigger pieces as the following pictures demonstrate:
Know-how since 1863
It should not be disregarded that it is of high benefit to know that a specific coating worked well in a different foundry with similar requirements and surrounding parameters. The James Durrans Group supplies many top-class foundries around the world. Out of this pool of synergies we can easily find coatings for successful trials.