An assembly of a mould (10, 11) for plastic materials, and at least one pressure-actuated pneumatic device, in which the mould (10, 11) comprises a first mould member and a second movable mould member having internal facing surfaces (12, 13) defining a cavity of the mould (10, 11); in which the first one of said mould members (10, 11) comprises said at least one pressure-actuated pneumatic valve device suitable to be connected to a pressurised air source, to generate an air jet in the cavity of the mould (10, 11) for ejecting a moulded article; in which the pneumatic valve device comprises: a valve body configured with a seat for housing a movable valve member (20, 21); the valve member (20, 21) in said housing seat being axially movable between an advanced opening position and a fully retracted closing position of the pneumatic valve device; the valve member (20, 21) comprising a valve head provided with a side surface and a front shaped surface having a peripheral edge, and a rearwardly extending stem; a biasing spring to urge the valve member (20, 21) in the fully retracted closing position; and conduit means for supplying by the pneumatic valve device in the mould cavity a flow of pressurised air in the advanced opening position of the valve member (20, 21), the head of the valve member (20, 21) comprising a front shaped surface configured to provide an annular space with the internal facing surface of the movable member of the mould, along said peripheral edge; characterised by comprising: an air feeding chamber and a flow path (32, 33) for the pressurised air in the movable valve member (20, 21), extending from a rear end of the stem to at least one transversal air exit hole opening on the side surface of the valve head, the space between the at least one transversal air exit hole of the air flow path (32, 33) and the peripheral edge of the front shaped surface of the valve member (20, 21) being greater than a local depth of the cavity of the mould, to close the air flow path (32, 33) in an intermediate retracted position of the valve member (20, 21) when urged by the movable mould member at the closure of the mould. a valve body configured with a seat for housing a movable valve member (20, 21); the valve member (20, 21) in said housing seat being axially movable between an advanced opening position and a fully retracted closing position of the pneumatic valve device; the valve member (20, 21) comprising a valve head provided with a side surface and a front shaped surface having a peripheral edge, and a rearwardly extending stem; a biasing spring to urge the valve member (20, 21) in the fully retracted closing position; and conduit means for supplying by the pneumatic valve device in the mould cavity a flow of pressurised air in the advanced opening position of the valve member (20, 21), the head of the valve member (20, 21) comprising a front shaped surface configured to provide an annular space with the internal facing surface of the movable member of the mould, along said peripheral edge; characterised by comprising: an air feeding chamber and a flow path (32, 33) for the pressurised air in the movable valve member (20, 21), extending from a rear end of the stem to at least one transversal air exit hole opening on the side surface of the valve head, the space between the at least one transversal air exit hole of the air flow path (32, 33) and the peripheral edge of the front shaped surface of the valve member (20, 21) being greater than a local depth of the cavity of the mould, to close the air flow path (32, 33) in an intermediate retracted position of the valve member (20, 21) when urged by the movable mould member at the closure of the mould.