an apparatus for the compaction of foundry mold making material using gas under pressure , comprises a pressure vessel for the such gas , a mold flask , a filling frame , a pattern plate and a pattern thereon onto which the material is to be loosely heaped . the flask , frame and plate form a mold chamber under the pressure vessel and between them there a valve is placed , the valve having a movable valve member adapted to open up a flow cross section of the valve predominantly under the effect of gas under pressure in the pressure vessel . the valve member is guided within the pressure vessel , with the valve member being adapted to perform an opening motion , which at least mainly directed into the mold chamber .