the additional function of guiding and centering the plunger 35 of the aforementioned metering unit the reciprocating movement of the plunger 35 is obtained by means of a lead screw 49 which is fixed for example screwed onto the end of the connecting element 40 opposite that of the plunger 35 the lead screw 49 is preferably of the ball bearing type being provided with suitable grooves for circulation of the balls in order to prevent the plunger 35 the element 40 and the lead screw 49 from rotating during their longitudinal movement retaining means have been provided comprising for example a pin or a screw 51 protruding from one side of the element 40 and sliding along a lateral groove 52 in the wall of the cylinder 41 it is obvious however that other solutions are possible without prejudice to the innovatory features of this invention the aforementioned device operates as follows the mixing chamber of the head 10 is initially closed the plungers 25 of the two metering units 15 16 are all pushed forward in their respective metering chambers 26 while each component is made to circulate through the system of valves and ducts 13 19 21 20 23 and 14 19 39 22 20 39 24 in the device by means of the motor 29 and the gearing 28 the screws 42 of both the metering units 15 and 16 are made to move thus causing the plungers 25 to move backwards simultaneously and the metering chambers 26 to fill up with their respective components to.