The Influence of Moulding Sand on the Compressive Stresses

In moulding of large-size castings, the lack of uniformity in heating results in the deformation of individual mould parts. The resulting deformations cause internal stre-sses in the casting and scabbing in the mould. The main cause of deformation in the mould element is to be found in the α β transformation of the quartz grains. The allotropic modification of the quartz crystals from α into β quartzite occurs at the temperatures ranging from 575 °C to 580 °C and during this process the quartz grains increase their volume. The volumetric change causes stresses, which are transferred across the residual of the mould mixture toward the interior of the mould where they cause deformation of the mould cavity face. This transformation is reversible, which means that β α transformation occurs during the cooling of the quartz grains, and this transformation causes the shrinking of the grains. Figure 1. shows the volumetric changes of different types of quartz sand depending on the temperature. Since the ratio between the mass and the surface of the grain affects significantly the grain heating rate, the total The aim of this research is to explain how the distribution of moulding sand particles, apart from combination with other components (bentonite and water contents) affects the increase of compressive stresses in the moulding mixture. The granularity of sand and water content have great effect in the increase of compressive stresses. The regression analysis was used to obtain the mathematical model which describes the dependence of the compressive stresses on the mean size of the sand grains and water content in the mould mixture.