Casting is applied for bonds with high clay content. The casting process is more expensive and therefore being replaced by molding processes [KLOC09].
3.2.2.2 Molding and Pressing
Most vitrified bonded tools are manufactured by molding. In the past, the mixture was formed into the mold manually; today fully or partially automated systems supply and disperse the mixture into steel molds. The mixture has to be filled into the mold homogeneously and without demixing, which is done by conveyor belts, vibrating trough, or other supply systems. The filling height is leveled with sheets. Molding gets more complicated for complex grinding tools such as wheels with different layers for crankshaft grinding, different hardness areas for raceway grinding, or large width for centerless grinding [TYRO03].
The pressing process defines the density distribution and thus the specific characteristics of the component [BEHR11]. The pressure is an important factor for tool quality and commonly not published. Today, computer controlled hydraulic presses between 500 kN to 45 MN are in use [TYRO03]. Commonly, the target volume, not the target pressure, controls the pressing process. Segments for superabrasive wheels are pressed with hydraulic or pneumatic presses or by hand at comparatively low pressures (max. 50 N/mm2).
Inevitable friction between the dies, molds, and powder leads to density gradients, (Fig. 3.7), which can result in sintering distortions and inhomogeneous material properties [BEHR11]. Additives like paraffin, polysaccharide, silicon oil, wax, and polyethylene emulsions lubricate the mixture and therefore support the pressing process [BOTS05, p. 25]. Oscillations of about 60 Hz can be superimposed to the pressing process and enhance the density distribution [BEHR11]. Expensive two-die presses can be substituted.
Hot pressing is used for high density tools [KLOC09, p. 59]. Hot pressing of diamonds in vitrified bond produces wheels of low porosity. Using graphite molds enables higher sintering temperatures in the hot pressing process. However, as the graphite molds are of low strength, the process has to be heated above regular sintering temperatures to limit pressing pressures [MARI04, p. 418]. As consequence, the bonds are densified with less than 2 % of open porosity.
3.2.2.3 Drying
In the drying or debinding process, the water from the temporary binders is expelled [TYRO03]. The drying process is conducted in a special drying furnace or in the sintering furnace during the general heat-up period. The drying temperature depends on tool ingredients, composition, and application and ranges usually between 200 to 600 °C.