[Trade Journal]
Publication: Ceramic Industry
Chicago, IL, United States
vol. 4, no. 3, p. 205, col. 2-3
Some Observations in the Casting of Heavy Electrical Porcelains
E. H. Fritz.
SUCCESSFUL CASTING depends primarily on the proper choice of clays. There are fast and slow casting clays. Among the first class are those which build up a firm solid wall from the mold to where the slip is casting and thus shows a distinct change slip to solid material. The slow casting clays consist of those clays which build up a firm cast adjacent to the mold but gradually the wall becomes flabby and soft as the slip is approached. The Bingham Plastometer, like the settling test, will classify clays as to their casting value. Clays which show low plasticity according to this test are the best casting clays.
In heavy pieces up to 500 pounds with walls five to eight inches thick, the ball clay content should be kept as low as possible. For an eight to 12 inch wall the ball clay should be increased and also casting time and the maximum thickness of section that can be cast is decreased. For casting, all ball clays, china clays and scrap are thrown in the mill and ground for 45 minutes, the water and silicate added can be determined by the amount used in the previous charge. Then spar and flint are added and the entire charge is ground for 30 minutes. A check on the viscosity and specific gravity of the slip is then made. The viscosity desired is when 100 c. c. of slip runs through a 7/32 inch hole, in from 40 to 65. sec., the specific gravity, must be equal to 60.2 oz. per quart. The slip is then passed through a 30 mesh screen into a cistern and is continuously agitated. The slip is checked every morning. The viscosity in the cistern must lie between 35 to 55. This is lowered by agitation in the cistern. The maximum deflocculation is not reached until long after the initial blunging and agitation. The viscosity seems to be more important than the specific gravity for casting determination. The specific gravity value should be set as high as possible without exceeding the viscosity limits. The specific gravity should be reduced to bring the viscosity to the proper limits. A decrease of 0.1 to 0.2 oz. per quart is usually sufficient to lower the viscosity from 10 to 20 points. Open top molds are so superior to funnel type molds that no other choice is possible. The slip is supplied by a continuous pump circulation. The molds are released by the application of compressed air or talc. Talc cannot be used when the cores are large and when the taper is not great.
