A cleaning device for a low-pressure mixing chamber particularly for polyurethane, comprising a conical mixing screw (11), which is actuated rotationally by motorization means (12) rigidly coupled to a slider (13), which moves on an advancement guide (14); the slider (13) is associated with means (15) for positioning the screw (11) inside a mixing chamber (16) between a retracted mixing position and an advanced cleaning position. Means (17) for measuring the axial thrust of the screw (11) on the residues of reacted material that are present in the chamber (16) and/or means (24) for measuring the torque generated by the motorization means (12) are associated with the positioning means (15). Means (19) are also provided for detecting the position of the screw (11) in the point where the axial thrust of the screw (11) and/or the torque generated by the motorization means (12) are/is highest; the position detection means (19), the axial thrust measuring means (17) and the torque measuring means (24) are interfaced with means (20) for comparing the measured values of position, axial thrust and/or torque of the motorization means (12) with the preset limit values of position, axial thrust and/or torque. There are also control and feedback means (21a), which manage the retraction of the positioning means (15) if the limit values of axial thrust or torque are exceeded, so as to return the slider (13) to a position in which the axial thrust of the screw (11) and the torque generated by the motorization means (12) are lower than the respective preset limit values.