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.

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.

Ventilation

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.

Loading capacity

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.

Threaded inserts, steel plates, transportation anchors, cables and trunking, s and hollow cells can be cast directly in the mineral casting component as it is a cold casting process.

In order to ensure that it is in optimal working order later on, all mechanical components must be fastened securely as their position can no longer be corrected later on and substantial lift forces develop during the casting procedure, which could change the position if the fastening is not secure enough. It must be ensured when the component is being sesigned and more importantly later with the construction of the mould, that this insert does not obstruct the flow of material or the venting of the mold.

If the threaded holes are too close to the edge or if the mineral casting component has hole pattern with mating holes, special cast parts are moulded in. These special cast parts (mouldings, plates) are anchored to the mineral casting component with hexagonal screws.

As described in the chapter on accuracy, a maximum accuracy of approximately +/- 0.1 mm/m can be attained with good moulds. Many components however, have a mating surface for other parts, which clearly require a greater accuracy, for example for the attachment of linear guide systems, sliding or mounting surfaces. It is therefore imperative to integrate these functional surfaces in order to attain the specified tolerances. Today Schneeberger essentially uses four procedures as below, in order to create the necessary surfaces:

•          Machining of mineral casting - with this procedure the mineral casting is poured with an excess and afterwards machined to the exact size by milling or grinding.

•          Machining of cast parts - with this procedure, metal parts, such as steel or cast iron plates, are cast into the mineral casting and machined after the casting has hardened.

•          Replication - with this procedure the mineral casting is poured approximate 2 mm undersize. After removing the mould there is a second casting stage where very fine material is cast with a precise setting jig.

•          Subsequent casting in of metal parts - With this procedure high-precision cast parts are precisely cast in after removal of the mould using positioning gauges.

By using the FEM (finite element method) one can carry out calculations of different characteristics of component geometry and machine designs. A typical use is the calculation of deformations on machine bases or components and the derivative of optimized geometry from these results. In addition, temperature dynamics and vibration response can be determined very accurately nowadays.

Some practical examples, such as the decrease in the deflection of components of a textile machine (Cetex) or the reduction of the regenerative effect on turning lathes (Boehringer Werkzeugmaschinen GmbH) thanks to the better absorption behavior of mineral casting in direct comparison to welded or cast designs, or the optimization of the thermal behavior (Schneeberger AG).