expelled in this way no mixture residues remain in and pollute the system by hardening the new apparatus for the production of a flowable reaction mixture comprises 2 a mixing chamber in which a discharging piston is mounted and into which the supply lines lead via injection nozzles said mixing chamber opening into a guide bore of said housing disposed transversely to said mixing chamber 4 an outlet channel located in said guide bore through which the reaction mixture exits said mix head and 5 a cleaning piston capable of cleaning said outlet channel wherein said throttle slide concentrically surrounds said cleaning piston the novelty of the invention resides in arranging the throttle slide in the guide bore concentrically around the cleaning piston where the guide bore extends beyond the outlet of the mixing chamber in the direction of flow of the mixture in the mixing phase the throttle slide forms an annular channel with the front end of the cleaning piston which annular channel leads into the outlet channel this ensures that the mixture stream issuing from the throttling gap formed between the mixing chamber outlet and the throttle slide cannot directly enter the outlet channel but first enters an annular channel which is formed by the walls of the guide bore and by the cleaning piston which projects beyond the throttle slide from this annular channel the reaction mixture then is discharged between the junction edge of the guide bore and the outlet channel to the end edge of the.