CPT International 02/2019

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COATINGS Figure 6: Schematic illustration of the Leidenfrost effect. Figure 7: The formation of different structures is controlled, among other things, by the coating. Figure 8: When drawing the tubes, the coating should adhere to the cast part and be completely removed. with defined grain sizes and shapes, water, binder, dispersion agent and surfactants to control surface tension. Figure 6 shows the various processes that occur when an aqueous coating is sprayed onto the hot centrifugal casting mold. If drops of the coating hit the mold, the coating is accelerated by the rotary motion of the mold and pressed against the hot mold as a result of the centrifugal forces. The water in the coating immediately begins to boil. Leidenfrost effects cause gas layers or gas bubbles to form. As the coating dries, channels form in the coating. Figure 7 illustrates the concept of forming different structures from the gas bubbles. Depending on the coating composition, the centrifugal forces and the mold temperature, different channels are formed, which in turn lead to different surface structures during casting. Additional coating requirements In addition to the functions of coating for centrifugal casting described above, there are a number of additional, application-specific requirements: A key characteristic with regard to the process steps is, for example, extraction behaviour: When the tube is extracted from the mold after solidification, the coating should adhere to the casting and, ideally, be completely removed from the mold with the casting (Figure 8). In terms of cleanliness in the workplace, it is also desirable that a contiguous layer of the coating remains on the casting, and that it is as dust-free as possible (Figure 9). If the next process step involves blasting, the coating should be easy to remove – in case of a textured surface it may also need to be removed from the undercuts. For health and safety reasons, the coating must not contain crystalline quartz. Consistent quality is crucial In order to gain a competitive advantage, many foundries strive to increase the productivity of their processes and/ or improve the quality of their castings. This leads to ever narrower process windows, with the risk that even the smallest defects can cause major disruption. Coatings must also meet these more demanding requirements. It is therefore becoming increasingly important to use stable coating products of uniform quality. Foundries need products with consistent properties in terms of composition, viscosity and application behaviour. However, natural raw materials are typically used 66
Figure 9: A continuous layer of coating remains on the casting, which minimizes dust. Figure 10: The uniformity of the defined surface structure is checked during the quality inspection. in the production of coatings, and these can vary in their quality. Comprehensive quality testing of the raw materials enables deviations from specified properties to be detected at an early stage. In addition, regular process checks should be carried out using suitable measuring methods. In structural casting, the uniformity of the surface structure desired by the customer is also an integral component of the quality assurance process (Figure 10). Innovation potential for foundries and coating suppliers From the automotive to the chemical and paper industries; from pipes to rolls to cylinder liners: when it comes to the production of rotationally symmetrical components, centrifugal casting is a proven, technically sophisticated casting process with enormous application and innovation potential. For foundries, it is well worth consulting an experienced coating supplier at an early stage in the development process of any new centrifugal casting applications. The foundry specialists and chemists at Hüttenes-Albertus are always happy to take on new challenges. Since there is often no “off-theshelf” product for special applications, we develop and test solutions step by step together with our customers – from initial testing all the way through to series production readiness. www.huettenes-albertus.com Dr. Klaus Seeger, Head of Research and Development Coatings, Dr. Ekaterina Potaturina, Product Manager Coatings, Hüttenes-Albertus Chemische Werke GmbH YOUR PARTNER FOR TURNKEY PROJECTS in no-bake moulding shops for: • moulding lines • continuous mixers • mechanical and thermal reclamations • chromite separations Smooth pneumatic conveying system for: • sand • bentonite • carbon • filter dust QUALITY MADE IN GERMANY CASTING PLANT & TECHNOLOGY 2/<strong>2019</strong> 67 FAT Förder- und Anlagentechnik GmbH · D-57572 Niederfischbach · Tel. +49 (0) 27 34/5 09-0 · fat.info@f-a-t.de · www.f-a-t.de
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CPT International 02/2019

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