A printer head assembly for a 3D printer, comprising: a fluid-dispensing head (11, 111) including a manifold; first and second nozzles (14, 15) mounted to the manifold (126, 26); a coupling member (12, 112) coupling the fluid-dispensing head to a support so as to allow movement of the fluid-dispensing head by which each of the first and second nozzles (14, 15) may be moved to and from a respective nozzle-operating position, wherein with one of the first and second nozzles in its nozzle-operating position the outlet of the other of the first and second nozzles is disposed above the outlet of the one of the first and second nozzles (14, 15); wherein: each of the first and second nozzles (14, 15) has an outlet (71, 70) closed by a respective valve member; and the printer head assembly further comprises: a rocker (30, 130) mounted to oscillate about a rocker axis, the rocker having at least one cam surface (40,41) engaging a respective fixture (36, 35) for driving the movement of the fluid-dispensing head, the rocker further including first and second abutments (142, 143, 42, 43) engaging respective ones of the valve members for opening or closing the nozzle outlet of each nozzle in its nozzle-operating position, and an actuator controlling the angular position of the rocker. a fluid-dispensing head (11, 111) including a manifold; first and second nozzles (14, 15) mounted to the manifold (126, 26); a coupling member (12, 112) coupling the fluid-dispensing head to a support so as to allow movement of the fluid-dispensing head by which each of the first and second nozzles (14, 15) may be moved to and from a respective nozzle-operating position, wherein with one of the first and second nozzles in its nozzle-operating position the outlet of the other of the first and second nozzles is disposed above the outlet of the one of the first and second nozzles (14, 15); wherein: each of the first and second nozzles (14, 15) has an outlet (71, 70) closed by a respective valve member; and the printer head assembly further comprises: a rocker (30, 130) mounted to oscillate about a rocker axis, the rocker having at least one cam surface (40,41) engaging a respective fixture (36, 35) for driving the movement of the fluid-dispensing head, the rocker further including first and second abutments (142, 143, 42, 43) engaging respective ones of the valve members for opening or closing the nozzle outlet of each nozzle in its nozzle-operating position, and an actuator controlling the angular position of the rocker. each of the first and second nozzles (14, 15) has an outlet (71, 70) closed by a respective valve member; and the printer head assembly further comprises: a rocker (30, 130) mounted to oscillate about a rocker axis, the rocker having at least one cam surface (40,41) engaging a respective fixture (36, 35) for driving the movement of the fluid-dispensing head, the rocker further including first and second abutments (142, 143, 42, 43) engaging respective ones of the valve members for opening or closing the nozzle outlet of each nozzle in its nozzle-operating position, and an actuator controlling the angular position of the rocker. a rocker (30, 130) mounted to oscillate about a rocker axis, the rocker having at least one cam surface (40,41) engaging a respective fixture (36, 35) for driving the movement of the fluid-dispensing head, the rocker further including first and second abutments (142, 143, 42, 43) engaging respective ones of the valve members for opening or closing the nozzle outlet of each nozzle in its nozzle-operating position, and an actuator controlling the angular position of the rocker.