CPT International 02/2019

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COATINGS Figure 3: A coating with good gas permeability helps to avoid gas-related casting defects such as “dents” (left) or “pinholes” (right) Figure 4: High-quality centrifugal casting products after finishing. Figure 5: Castings before further processing. meters have a significant impact on the cooling rate of the melt and consequently the mechanical properties of the casting. These, in turn, are a crucial component of customer requirements. Particularly with thin-walled pipes, insulating properties need to be specifically adjusted in order to achieve maximum workability. The selection of suitable raw materials has an impact on thermal conductivity, but also on factors such as chemical reactivity, fire resistance, hardness and application properties of the coating. The grain size distribution is also important: a fine grain size has many grain boundaries and thus slows down heat transfer. A large grain size, on the other hand, creates more cavities between the individual grains, which also leads to an insulating effect. Solids content, viscosity and rheological properties must be balanced in such a way that, on the one hand, the desired layer thickness is achieved while, at the same time, the coating is easy to spray on and allows an even application. Since the coating is a dispersion that may sediment out during storage, appropriate preparation is required before application. Before use, the coating is adjusted to the desired processing viscosity by determining the density or efflux time. Controlling surface textures Since most castings are machined after production, there are no special requirements regarding the roughness of the casting surface for many applications. For certain applications, the shape and height of any roughness are clearly defined. This applies, for example, to grey cast iron wastewater pipes: during their manufacture, the aim is to achieve smooth cast surfaces. Although this requirement has no direct influence on the functionality of the casting, some foundries regard it as an externally visible quality criterion with which they want to score points with their customers. There are, however, also customer specifications that require the production of rough surface textures: For cylinder liners, a rough surface with a precisely defined surface structure is expressly desired (Figures 4 and 5). Cylinder liners can be pressed into engine blocks. It is also possible to cast the liners into the engine block after machining. With the further development of casting processes and coatings, it is now possible to produce defined surface structures that enable the liners to be cast directly into the engine block. The molten metal flows around the structured surface, filling in recesses and undercuts so that the engine block and liner materials form an extremely strong and resilient bond. The aim of this process is to obtain a rough surface structure during casting so that the outside of the cylinder liner does not require any further processing. The depth and shape of the structure can be determined according to customer specifications. Hedgehog or mushroom-shaped elevations are possible, as are structures with undercuts. The structure depths vary between 0.3 and 1.1 mm, depending on customer requirements. The structures with undercuts can only be processed in pressure die casting, while the liners with hedgehog or mushroom structure can be processed in conventional gravity die casting. In addition to the special configuration of certain machine parameters, the coating plays a decisive role in achieving these surface structures. It can therefore be said that the coating acts as a forming factor (molding material). A modern coating for the production of structured cylinder liners is composed of special refractory materials 64
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CPT International 02/2019

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