Abstract: A fluidized bed has an aperture plate which is inclined to the horizontal, to cause excessively large or dense particles to migrate to a collection point from which they may be removed from the particulate mass. Disruption of fluid flow through the aperture plate due to settled out particles is thus avoided. Preferably the aperture plate is made of a material which has the same permeability as the fluidized mass, thus inherently providing flow regulation necessary for even fluidization above the slope plate. To make investment casting molds, a gravity "rainsander" device is coupled with a fluidized bed. When slurry used in the mold making process drips from a pattern it falls into the fluidized bed thereby converting the errant drops into particulate balls which are conveniently removed.