A mixing device for generating reaction plastic, the mixing device comprising: a) a mixing chamber for mixing reactive components to generate the reaction plastic; b) a discharge unit connected to the mixing chamber for discharging the reaction plastic generated in the mixing chamber; and c) a cleaning piston that is axially aligned with a discharge pipe of the discharge unit and is moveable into the discharge pipe to clean the discharge pipe from reaction plastic; characterized in that the mixing device comprising: d) a fluid supply device for providing a lubricant into the discharge unit via an outlet in order to lubricate the discharge unit, wherein i) the fluid supply device is configured to lubricate the movement of the cleaning piston in the discharge pipe, wherein ii) the fluid supply device comprises a gas supply device for providing gas together with the lubricant via the outlet into the discharge unit, wherein the fluid supply device is configured to provide the lubricant into the discharge unit while the cleaning piston is moved into the discharge pipe as well as while the cleaning piston is moved out of the discharge pipe, e) a control piston arranged within the mixing chamber for controlling flows of the reactive components; wherein i) an axial direction of the control piston is oblique, preferably perpendicular, to an axial direction of a discharge pipe of the discharge unit, f) wherein the fluid supply device comprises a lubricating device for providing the lubricant, the lubricating device being connected to the gas supply device and g) the lubricating device comprises a pneumatic and/or hydraulic valve, and h) wherein the cleaning piston is configured to leave open a crossover between the mixing chamber and the discharge pipe, if not used to clean the discharge pipe; and i) the outlet is located at the crossover between mixing chamber and the discharge pipe. a) a mixing chamber for mixing reactive components to generate the reaction plastic; b) a discharge unit connected to the mixing chamber for discharging the reaction plastic generated in the mixing chamber; and c) a cleaning piston that is axially aligned with a discharge pipe of the discharge unit and is moveable into the discharge pipe to clean the discharge pipe from reaction plastic; d) a fluid supply device for providing a lubricant into the discharge unit via an outlet in order to lubricate the discharge unit, wherein i) the fluid supply device is configured to lubricate the movement of the cleaning piston in the discharge pipe, wherein ii) the fluid supply device comprises a gas supply device for providing gas together with the lubricant via the outlet into the discharge unit, wherein the fluid supply device is configured to provide the lubricant into the discharge unit while the cleaning piston is moved into the discharge pipe as well as while the cleaning piston is moved out of the discharge pipe, i) the fluid supply device is configured to lubricate the movement of the cleaning piston in the discharge pipe, wherein ii) the fluid supply device comprises a gas supply device for providing gas together with the lubricant via the outlet into the discharge unit, wherein the fluid supply device is configured to provide the lubricant into the discharge unit while the cleaning piston is moved into the discharge pipe as well as while the cleaning piston is moved out of the discharge pipe, e) a control piston arranged within the mixing chamber for controlling flows of the reactive components; wherein i) an axial direction of the control piston is oblique, preferably perpendicular, to an axial direction of a discharge pipe of the discharge unit, i) an axial direction of the control piston is oblique, preferably perpendicular, to an axial direction of a discharge pipe of the discharge unit, f) wherein the fluid supply device comprises a lubricating device for providing the lubricant, the lubricating device being connected to the gas supply device and g) the lubricating device comprises a pneumatic and/or hydraulic valve, and h) wherein the cleaning piston is configured to leave open a crossover between the mixing chamber and the discharge pipe, if not used to clean the discharge pipe; and i) the outlet is located at the crossover between mixing chamber and the discharge pipe.