Sequence control of pressing velocity for pressure in press casting process using greensand mould

Abstract

Recently, a press casting process using a greensand mould was actively developed by the authors' group to improve productivity and casting quality. This process consists mainly of two parts: in the first, molten metal is poured in the lower mould by a ladle, and in the second, the upper mould falls down toward the lower mould to press the molten metal. The feature based advantages of this greensand casting are an astonishing increase in casting yield (over 90%) and energy saving. A rapid pressing process should be realised as a vital task to achieve an increase in productivity. In addition, the advanced technique of this process will also suppress casting defects such as a cold shut and the inclusion of generated oxide film due to the temperature decrease in molten metal. As the technical problem on high speed pressing, the other defects are caused by increased pressure. Metal penetration, in which the solidified molten metal is soaked among the sand particles in greensand, often occurs. Hence, the authors propose controlling the pressing velocity in order to realise high production competence and to suppress the rapid increase in pressure. In this paper, a pressure suppression method in high speed pressing is proposed in which the sequential velocity control of the press can be applied to various casting moulds. The control design is conducted simply and theoretically, and a brief mathematical model of the fluid pressure is built using a complicated, exact model of CFD. Then, the permitted range of pressure fluctuation is given for the production of high quality casting. Considering the upper limit of the pressure constraint for defect free pressing, the switching velocity pattern of multisteps is derived using the proposed mathematical model without trial and error adjustment. The control performance for pressure fluctuation using the obtained velocity reference is checked by CFD simulations.