in the case of a mixer head for reactive mixing of two or more plastics material components in a mixing chamber , the mixture components are fed under the pressure simultaneously and conjointly into the mixing chamber of the mixer head via a dosing valve in each case during a delivery stage . during a recirculation stage , the plastics material components are pumped around in a circulatory flow flowing through the dosing valve and through a recirculating valve . to assure that even small delivery volumes per unit of time may be discharged evenly , the dosing valve and the recirculation valve are constructed as pressure valves and connected in series one behind the other , in such a manner that only the dosing valve or the recirculation valve is switched to the throughflow condition .