Abstract: A closed core box is provided with one or more inlets for receiving and passing gas fluidized resin-coated sand to fill the interior of such core box, and with one or more screened outlets for allowing egress of such gas. A closed cylindrical or angular sand-fluidizing chamber is provided for each of the inlets, the chamber having one end connectable to a pressurized gas supply and an opposite end communicating with an inlet. At least part of each chamber is filled with resin-coated sand and a non-porous piston is placed on the sand within each chamber, each piston providing an annular gap (i.e. about 1/16 of an inch) with the interior of such chamber to direct the gas supply along the periphery of the chamber interior.