Thermally enhanced slip casting of alumina ceramics

Slip casting is a process which is very suitable for the production of larg e components and thin-walled bodies of complex shape but which is also time and energy intensive. One method of increasing the casting rate is by heat ing the slip. This has been examined via the use of both microwave and conv ectional heat energy for 72 wt% nitric acid dispersed aqueous alumina slips . It has been found to provide an increase in the casting rate over ambient temperature processing, with a gain of 55% when using microwave energy and 20% using a convection oven at 45 degrees C. The enhanced rate is apparent ly achieved via two mechanisms. At temperatures less than approximately 40 degrees C it is probably due to a reduction in the slip viscosity enabling easier water movement through the cast layer and mould. At higher temperatu res there is a change in the dispersion state of the slip. It appears that as the slip temperature increases above ambient the slip becomes increasing ly flocculated and that above about 45 degrees C this influences the castin g rate. By 65 degrees C the slip is fully flocculated. A further enhancemen t in the casting rate is achieved with microwave energy over convectional h eat energy. This is attributed to the rate of movement of water through the mould being increased more than the rate of water uptake by the mould. Thi s in turn is believed to be due to the plaster of paris being essentially t ransparent to microwave radiation so that the only significant heating that occurs in the mould is due to the presence of the water. Since the water i n the mould cannot move around as freely as it can in the slip, local tempe rature gradients can rapidly develop. This causes the water front within th e mould to move faster than with convectional heating. No evidence has been found that this accelerated rate has any detrimental effect on the cast bo dy produced. (C) 1998 Elsevier Science Limited and Techna S.r.l.