The machine is provided with a pressure drum (1) capable of supplying a current of blowing an through the diffuser (2) towards the moulding box (3), and its characteristics are focused on a specific valve system in which a tubular valve body (7), to which the diffuser (2) is in turn solidly attached, is fixed to the mouth of the drum (1) with the assistance of a supporting frame (8), this assembly defining, with a large central core (15), a peripheral passage (16) for the outlet of an in which an inner valve seat (14) is established for a tubular membrane (11) that is fixed through its marginal areas to the marginal areas of the valve body (7), an enveloping chamber (12) being defined outside the same which, upon being pressurised, causes the membrane (11) to be choked against the seat (14) and consequently the valve to be closed Opening of the valve (11-14) results, through the diffuser (2), in a jet of air which mainly concentrates on the perimetric area of the moulding box (3), improving sand compacting (5)
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