Abstract: In an apparatus for the compaction of foundry sand by the impact of pressurized gas on the foundry sand as filled into a flask over a pattern plate with a pattern therein, a specially designed diaphragm is used to get an even and constant hardness of the mold in every horizontal plane thereof and/or to put an end to the formation of shear cracks in the mold between different parts of the volume of the sand. This diaphragm is placed over the so far uncompacted surface of the foundry sand and it has a permeability to gas varying with the height of the sand to be compacted so that such permeability is less in the parts with high and/or plateau-like pattern contours than in parts where there is no pattern on the pattern plate.