The structural design of a casting is always closely linked to the casting process and the amount of work involved. Therefore, the structural design (Favorable casting design, Favorable casting engineering) of a casting is largely influenced by casting-related considerations. This is why the layout and conception of a casting depends on the following factors:
The decisions to be made by the foundry regarding the molding and demolding of a pattern are largely based on the design documents. A favorable casting engineering requires the engineer to have great knowledge and understanding of how to design a casting in order to be able to produce the workpiece in a molding and casting process which is as simple as possible.
The following lists general design rules which should be observed when designing a casting.
Design rules for castings:
- More generally, a casting should consist and be composed of geometrically simple bodies (e.g. cylinder, cone, cube, sphere, etc.).
- Plane surfaces are to be preferred, curved surfaces should follow simple curves.
- The material selection should be based on the most favorable casting material for the anticipated stresses (e.g. “brittle” flake graphite cast iron should not be used for impact- or shock-like stresses).
- With regard to stresses, the most favorable mold for the material should be chosen (e.g. when using flake graphite cast iron, tensile stresses should be avoided).
- Where possible, permanent molds should be redesigned to avoid the use of cores.
- If cores have to be included, they should have a simple design and be positioned carefully.
- Castings made of cast iron and (or) pressure die castings should be, if possible, have the same wall thicknesses.
- Changes in wall thickness are to be alleviated by means of smooth or gradual transitions (e.g. using Heuvers’ circle method).
- Sharp edges, notches and material accumulations should generally be avoided.
- The rib thickness sr should be less than the wall thickness sW (sr~0.6 · sW to sr~0.8 · sW).
- Attention should be paid to curvatures (R ~1/4 · sW to R~1/3 · sW) and mold inclinations (for further information, see DIN 250 and DIN EN 12890).
- Undercuts (recesses, retracted molds) and closed cavities in the pattern should be avoided where possible.
- Loose pieces and attachments should be avoided if possible.
- Casting-related deviations should be taken into account with dimensions for which tolerances are not indicated (general tolerances). The following standards apply to casting materials: heavy metals: DIN 1687; light metals: DIN 1688; steel castings (GE, GS, GX): DIN EN 10293; malleable cast iron (GJMW, GJMB): DIN 1684; nodular graphite cast iron (GJS): DIN 1685; flake graphite cast iron(GJL): DIN 1686
- Suitable clamping elements must be included for machining.
- The surfaces to be machined should be readily accessible and there should enough room for tool removal.
Examples and further information on strain-related and favorable casting design are provided under the following references:
Favorable casting design
Favorable casting engineering
Heuvers’ circle method