the recirculation stage is illustrated diagrammatically in fig1 . one plastics material component a is impelled into the dosing valve 12 by a pump 2 via the intake pipe 20 , from the tank 10 . meanwhile the pressure of the pump 2 is monitored by means of a pressure gauge 4 . the dosing valve 12 is constructed as a pressure control valve and is switched to the by - pass state , so that the fluid is fed through the passage 62 to the connector of a recirculation valve 13 at which it flows into the recirculation pipe 21 under pressure control and return to the tank 10 . in an identical manner , the plastics material component b is impelled into the dosing valve 14 by the pump 3 , via the intake pipe 22 , from the tank 11 under monitoring by means of the pressure gauge 5 , which valve is merely traversed in uncontrolled manner by this flow of fluid , whereupon the fluid enters the recirculation valve 15 , at which it reaches the recirculation pipe 23 under pressure control and returns from the latter to the tank . in the delivery stage , the dosing valves 12 , 14 and the recirculation valves 13 , 15 are switched by means of the air control 16 , 17 . to this end , the control air 16 is cut off from the dosing valves 12 , 14 so that these assume the control state and that the plastics material component under pressure control penetrates into the mixing chamber 6 via the nozzle bore 43 . an agitator 8 is present in the mixing chamber 6 and is driven via a mixing motor 7 , which mixes the two plastics material components a and b fed into it and feeds them to the processing station via the outflow nozzle 9 . it should also be noted that , during the delivery stage , when the control air 16 , 18 is cut off from the dosing valves 12 , 14 , the recirculation valves 13 , 15 are acted upon at the same time by control air 17 , 19 , so that these are blocked reliably . because the control pressure of the dosing valves 12 , 14 corresponds to that of the recirculation valves 13 , 15 , the pumps 2 , 3 do not detect any shift from the recirculation stage to the delivery stage , which assures that pressure peaks do not occur at the start of the delivery stage . even if the fluid is compressible because of inclusion of air bubbles , and is conveyed in compressible ducts , no discharge phenomena occur from charged accumulators , because the pressure in the recirculation stage and in the pressure stage , is kept identical . a particularly uncomplicated structural design derives from the fact that four identical valves 12 - 15 are utilized . in the same way , it is possible to combine the control functions of the dosing valve 12 , 14 and recirculating valve 13 , 15 in a single valve , because it is always one valve only ( either the dosing valve or the recirculation valve ) which is operative under pressure control whereas the other valve should be closed reliably . the problem posed in the foregoing may consequently also be resolved by the series connection of a pressure control valve with a 3 / 2 way valve . the 3 / 2 way valve performs the switching operation between the recirculating and delivery operations , whereas the fore - connected pressure control valve always assures a constant pressure in the system . because of the identical nature of the four valves 12 , 15 , it is sufficient according to fig2 to 7 , to describe one valve only . the dosing valve 12 shown in fig2 comprises a nozzle unit 26 which is connected in sealed manner to a back cover 27 and interposed o - rings 52 via screws not shown in particular . this delimits a central recess which is described as an air chamber 28 . in the direction towards the nozzle bore 43 , the air chamber 28 is delimited by a flexible diaphragm 39 which separates the air chamber 28 from the fluid chamber 29 . the plastics material component a or b is introduced into the fluid chamber 29 via the connecting bores 41 , flows around the needle 37 during the recirculation stage , which during this stage is seated in sealing manner in the nozzle bore 43 , and flows out of the dosing valve 12 again via the connecting bore 42 . the pressure control action is obtained by the fact that the fluid pressure acts on the diaphragm 39 from the side of the fluid chamber 29 , whereas the needle 37 is biased at the other side by the return force of a spring 57 . it is of importance in this connection that the needle 37 should be adjusted to have its tip fitting in precise sealing manner in the nozzle bore 43 , because the same is mounted in an articulated manner with respect to the spring 57 . skewing movements of the spring 57 are taken up by the articulated mounting of the needle 37 in the area of a cylinder 36 , so that this assures that the needle tip is always set centrally and sealingly in the nozzle bore 43 . according to fig6 the needle 37 is provided at its extremity close to the spring with a disc 55 acted upon according to fig5 by the tip 48 of a bolt or plunger 38 , which at its back extremity close to the spring has a central recess 46 . the plunger is fitted in sealing manner in the recess 56 of a cylinder 36 ( fig7 ) by means of an o - ring 51 , a ball 49 engaging in the recess 46 of the plunger 38 , whereof the other extremity engages in a corresponding central recess 58 in the area of a bearing sleeve 33 . the bearing sleeve has a central disc or plate whereon is seated the one extremity of the spring 57 formed as a coil spring , whereof the other extremity bears on a bearing sleeve 32 . a pin of an axially displaceable spindle 31 engages in the central bore of the bearing sleeve 32 , which is screwed into the cover 27 of the valve unit by means of a corresponding screw - thread . the twisting displacement of the spindle 31 is stopped by means of the nut 47 . the spindle 31 is sealed in the cover 27 by means of the o - ring 53 . the diaphragm 39 is thrust in sealing manner against an end - side stop of the cylinder 36 by means of a pressure balancing plate 34 and of a nut 40 . the nut 40 has its inner screw - thread engaged on a corresponding external screw - thread on the extension of the cylinder 36 . the outer periphery of the diaphragm 39 is held fixedly between the end side of a bushing 35 and the corresponding inner surface of the nozzle unit 26 . to this end , the nozzle unit 26 is connected to the cover 27 via securing plates 30 . the case member 64 of the mixer head 60 bears sealingly on the front side of the nozzle unit 26 via o - rings 54 . to this end , the nozzle unit 26 forms a tapered nozzle opening 43 , wherein the corresponding tapered tip of the needle 37 engages sealingly . if the dosing valve 12 is operating in the recirculating condition , control air 16 is fed in via the screw connector means 50 , so that the diaphragm 39 is displaced towards the right and the needle 37 is held fast in the nozzle bore 43 in sealing manner . it is essential in this connection that the cylinder 36 is mounted in axially displaceable manner in the area of a central recess 44 of the bushing 35 and that the controlling air may reach the diaphragm 39 via grooves 45 radially arranged on the outer periphery of the recess 44 . upon passing to operation of the delivery state , the controlling air 16 is cut off from the screw connection means 50 so that the dosing valve 12 shifts to the pressure control operation . the return force of the spring 57 is so adjusted that the tip of the needle 37 clears an annular cross - section in the area of the nozzle bore 43 , so that the fluid flows out of the nozzle bore 43 from the fluid chamber 29 and penetrates into the mixing chamber 6 of the mixer head 1 . a pressure rise in the fluid chamber 29 acts via diaphragm 39 on the spring 57 thereby increasing its return force , so that the same pressure is always present in the fluid chamber 29 . a complete mixer head comprising four valves 12 - 15 is illustrated in particular at this juncture in fig8 and 9 . it should also be noted that the recirculating operation may be omitted in the case of plastics material components which are easy to process , so that the recirculation valves 13 , 15 may also be omitted , and only the dosing valves 12 , 14 are present instead . this embodiment ( without recirculation valves ) should also be encompassed by the protection of the present invention . the mixer head 1 shown in fig8 and 9 comprises an upper case member 66 and a lower case member 64 which are held together by appropriate screws 83 . the upper case section 66 has present on it a central flange 69 wherein a driving spindle 76 is installed in a rotationally drivable manner for an agitator which is illustrated diagrammatically in fig1 . the driving spindle 76 comprises a top nut 87 which bears at the end side on a sleeve 68 immediately below it . within the sleeve 68 , the driving spindle 76 is provided with small radial plates 70 . the driving spindle 76 is rotatably journalled in upper and lower ball - bearings 95 , 96 and passes via a spindle seal 105 into the mixing chamber 6 of the mixer head 1 in sealed manner wherein is situated an agitator not illustrated in particular . the plastics material components which are to be processed are fed via an upper dosing unit ( not illustrated ) into the fluid connectors 98 , 99 ( fig9 ) in a manner not illustrated in particular . the fluid then penetrates in each case into a passage 61 in which it reaches the connector 41 ( see fig2 and 8 ) of the fluid passage 29 of the dosing valve 12 , 14 . the fluid is positioned therein behind the nozzle bore 43 and is fed into the mixing chamber 6 only when the controlling air has been cut off from the connector 50 during operation in the delivery mode and the needle is shifted into operation in the pressure control mode , so that the plastics material component enters the mixing chamber 6 under controlled pressure via the dosing valve 12 , 14 ( delivery operation ). the recirculation occurs as follows : the needle 37 closes the nozzle bore 43 of the dosing valve 12 , 14 in such manner that the fluid coming from the bore 41 flows around the needle 37 and passes into the connecting passage 62 via the bore 42 . the fluid coming from the passage 62 then enters the fluid chamber 29 of the recirculation valve 13 , 15 . the diaphragm 39 is loaded by the fluid pressure , in such manner that a balanced state is established with the preloaded spring 57 and that the needle 37 clears the nozzle bore 43 . the fluid then flows via the nozzle bore 43 into the recirculation pipe ( see corrected fig9 ). the construction - kit - like structure of the mixer head 1 is also apparent from fig8 and 9 , disclosing as a matter of fact that the lower case section 64 comprising its passage 62 is attached to the side of the mixer head 1 via corresponding o - rings 103 . the dosing valves 12 , 14 , together with the case section 64 , may thus be replaced by dosing valves of a different kind . the valves 12 - 15 as such are joined to the case 64 , 66 of the mixer head 1 via appropriate screws . 84 .