A frothing device for delivering pre-expanded polyurethane mixtures. The device comprises a tubular gate bar (16) for distributing the mixture, provided with a longitudinal slot (18) which extends transversely and in the direction of the flow, towards a diffuser (19) comprising a pre-expansion cavity for the mixture, defined by a series of differently oriented walls (20 - 25) to cause at least one flow deviation of the polyurethane mixture emerging from the slot (18) of the distribution bar (16). The slot (18) of the distribution bar (16) has moreover slanted edges (28, 29 - fig. 3) which diverge in the flow direction and towards the pre-expansion cavity of the diffuser (19).