Machine for compacting sand moulds by air impact or expansion waves

In a sand mould air impact or blast compacting machine having a compressed air drum (1) for delivering air through a diffuser (2) onto a moulding box (3) for sand (5) to be compacted, the improvements comprising:

• a peripheral passage (16) right before the diffuser (2) for the air to be delivered to the diffuser, four independent membranes (11) within the peripheral passage (16), the membranes being solidly attached by marginal areas to valve bodies (7) in order to define respective enveloping chambers (12) for pressurization to deform the membranes (11) towards respective valve seats (14) on a core (15) that defines an inner wall of the peripheral passage (16) such that, when pressure is lost in the enveloping chambers (12), most of the air is delivered at a periphery of the moulding box (3);
• wherein the membranes (11) are square or rectangular with dimensions matching the moulding box (3) and their solid attachment by marginal areas to the valve bodies (7) is through screws (10) with which the valve bodies (7) are fixed to an upper supporting frame (8) and the diffuser (2).