The topic of this work is the study of the occurrence of the scabbing phenomenon in the mould. The performed tests have shown that the granulometric composition of sand is very important in the heating process of a disposable sand mould. In casting larger moulds, substantial heating causes temperature extension of the quartz grains within the mould. The temperature extension of the quartz grains causes pressure stress in the mould which in turn results in the deformation of the mould cavity. When deformation occurs, some parts of the mould can break off. Should this happen, the consequence is a faulty casting with the defect called a scab. The mould properties are in a way pre-determined by the method of mould fabrication, composition and morphologic properties of the mould mixture. It is impossible to achieve that all the properties of the mould mixture be maximal, primarily because some of them are mutually opposed. Therefore, knowing the mechanism which leads to scabbing is of primary importance. The scabbing phenomenon may be reduced by the reduction in temperature extension of the quartz grains. The mould deformation depends on the property of flexibility which depends on the granulometric composition of the mould mixture. Knowing the laws among sizes that define the property of mould mixture flexibility, and combining with the heat exchange mechanisms during casting, may directly affect the occurrence of scabbing. By selecting a certain sand granulation and by determining the technological properties of mould flexibility the sands of different granulometric compositions can be classified according to their scabbing tendency.