The quality of a clay-bonded molding material is characterized by technological molding and casting properties. The molding technology properties are applicable in the molding process and in interaction with the mold forming process parameters they determine the quality of the mold.
Water mixed with clay binds and plastify the granular basic molding material (s. Plasticity). A large number of technological properties are crucially dependent on the interaction between clay and water. Setting and maintenance of a basic water content which is adjusted to suit the type and intensity of compaction is a basic requirement for a high molding material and mold quality. Maximum properties may be achieved with different water contents (Figure 1). Therefore, definition of the optimum water content is always a compromise and must take into account the specific requirements for the molding material.
However, equal water content in a clay-bonded mold material mixture does not mean equal molding characteristics. The absolute water content for achieving optimum properties of the mold material, is based on a number of factors. These include the properties of the basic molding material (e.g. porosity of the quartz sand grains, water absorption capacity, theoretical specific core area), the type of bentonite, intensity of conditioning, content of bonding clay and additives (s. Addition of molding material), as well as recirculation condition of used sand (oolitization, dusting).
In order to allow for comparison of mold material mixtures of different compositions and for determining optimum absolute water contents for the various molding methods and compaction processes, the concepts of “water demand” and “moisture content” have been introduced according to M. Dietert and F. Hofmann. The water demand of a molding material corresponds to the water content present in a molding material mixture in ready-to-use condition and is measured by means of the test methods for compaction and moldability.
Mold sand testing