Wall thickness
The castings and wall thickness of mineral casting components must in general be at least five times greater than the maximum pebble diameters. With a typical maximum pebble diameter of 16 mm, the castings and wall thickness must therefore be at least 80 mm. Due to the low residual stresses varying wall thicknesses and sudden changes from thin to thick wall thicknesses can be achieved without a problem in a mineral casting component. Good rigidity values are attainable with the appropriate selection of cross sections and by taking into account the greatly varying permissible tensile and compressive stresses.
It is also possible to work with thinner wall thicknesses in the non-load-bearing parts of the construction. Here mineral casting mixtures are used that have a smaller maximum pebble diameter. The first casting of a multi-level casting procedure, where first the finer mixture is poured followed by the coarser composition, is called pre-casting.
Mould release slopes are used to release the casting blank, similar to the method used for gray cast iron. The recommendation to use an approx. 5° slope for cast parts made from gray cast iron is often also used for mineral casting. However, in practice smaller casting angles are quite sufficient for mineral casting.
Whilst the mineral casting mixture is produced air enters the compound simply through the mixing process itself. Besides this, air is also entrapped inside the mould and the compound during the casting procedure. To ensure that the finished component will not exhibit any shrinkage cavities later on, after pouring in the compound the mould is vibrated until practically no more trapped air bubbles up.
For this purpose there must be a suitable venting of the mould. This can be done, if mould segments and casting parts are arranged in such a way that they do not obstruct either the incoming mineral casting or the outflowing air. Bubbles can form particularly on horizontal surfaces, which the mineral casting rises against from below. Such surfaces are to be avoided if possible or they are to have a relatively large angle, in order to make it possible for air to escape.
During the construction of mineral casting components it is essential to bear the material’s special characteristics in mind. Cast components can, for example, withstand substantially higher compressive and tensile forces.
When anchoring cast parts it is to be ensured that they are set at a sufficient distance from the component’s edges, so that they do not break off. For the minimum distance to the edges use the corresponding table of the cast part.
If screw connections are to be used within the mineral casting component, it is to be ensured that the cast material can withstand the loads created by the forces acting on the thread. If there is the risk of it shearing off, it must be treated with suitable metal threaded bushes, which are cast in the necessary places.
The notch effect is clearly reduced and the redirection of the force is improved by rounding and/or chamfering the connectors between the component units.
Likewise suitable measures must be taken for the necessary transport to the customer. So for very large components lifting points, which make it possible to load them safely with cranes and similar lifting equipment, must be designed into the structure. To prevent any transport damages, rails can also be inserted on the underside for transport by fork-lift trucks.
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