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The synthetic, spherical sand ‘MinSand’ is used to improve the standard mould material for cores and moulds and is a cost-effective alternative to zircon sand.

For a long time, Bauxite has been used in the production of shaped and unshaped products for the refractory industry. An additional application has been developed in more recent years, utilising Bauxite in the foundry industry. MinSand, which is produced from Bauxite, can be used with all types of castings.

Production of beads

After fusing and super-cooling in an electric arc furnace, air is blown into a jet of liquid bauxite to obtain the spherical shaped grains/beads of MinSand. The beads are classified into different particle sizes by screening, some examples of these are shown in table 1.

Sieve

AFS 50

AFS 65

AFS 90

425μm 11,5    
300μm 28,5 23,8 6,9
212μm 35 28,9 33,5
150μm 23,5 24,5 33,5
106μm 2,95 15,9 9,8
75μm   3,5 4,3
53μm   0,8  

Table 1: Typical weight percentage of several particle sizes of MinSand, other sizes available on request

MinSand when used as a core and moulding sand for the foundry industry
Figure 1: MinSand when used as a core and moulding sand for the foundry industry

Flowability, thermal & dimensional stability and core strength

The spherical shape of the sand-binder mixture produced with MinSand significantly improves the flowability compared with other mould materials available on the market. The rounded grain shape also creates strong binder bridges which results in the need for reduced binder additions when utilising MinSand. Studies have shown that the binder additions for core and mould production with MinSand compared to silica sand can be reduced by about 40%. The low addition of binder resin also means that MinSand can be utilised with any coating systems.

The heat resistance of a core and moulding sand plays a major role in the stainless steel and cast steel industry, where the highest casting temperatures are achieved. MinSand is suitable as a mould material for these types of casting as it has high refractoriness (sintering temperature > 1,950 °C), it is not prone to mould material-metal reactions and is resistant to metal penetration even in highly thermally stressed castings. The heat resistance of MinSand is attributed to its high alumina content.

Due to the very small linear expansion of MinSand, castings can be produced with a high level of accuracy. This is also a special advantage for the non-ferrous light metal castings in which intricate cores are produced, for example a lattice consisting of MinSand can be used in the production of water jacket cores for cylinder heads. Due to the high strength, tension cracks are avoided in the core and the mould material is even suitable for thin-walled castings.

In the casting of grey and ductile iron, veining can often arise if using quartz sand, which often succumbs to quartz inversion; this can be avoided through the use of MinSand. MinSand is extremely stable and allows the casting of complex shapes without casting defects. Moreover, the moulding sand is an alternative to cold-box mixes with gas-forming additives which helps produce heavy metal castings without any metal penetration.

Conclusion

The spherical shape of the sand-binder mixture produced with MinSand significantly improves the flowability compared with other moulded materials available on the market. Because of this, along with the excellent refractoriness, low linear thermal expansion, high dimensional stability, good gas permeability and the reduction in binder additions required, MinSand is an excellent addition to special moulding sands for the foundry industry.

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