A screen printing head system, comprising: a screen printing head which is displaced in a first direction over a surface of a workpiece in a printing operation to print deposits of a print medium on the workpiece, the screen printing head comprising: a print head assembly comprising: a print unit comprising a main body including a print medium cavity, which in use contains a print medium, and a delivery aperture, which is in fluid communication with the print medium cavity and through which print medium is in use delivered from the print medium cavity to the surface of the workpiece; a rotatable unit which is disposed in the print medium cavity of the main body and rotatable to displace print medium as contained in the print medium cavity; and a drive unit for rotating the rotatable unit; the screen printing head system being characterized by further comprising control means for controlling the drive unit for rotating the rotatable unit such as to displace print medium as contained in the print medium cavity, wherein the control means are such that the drive unit is operable to rotate the rotatable unit to apply a force to the print medium at the delivery aperture in the main body in a second direction which is opposite to the first direction of displacement of the screen printing head, whereby the force as applied in the second direction counteracts a force as generated by displacement of the screen printing head in the first direction. a screen printing head which is displaced in a first direction over a surface of a workpiece in a printing operation to print deposits of a print medium on the workpiece, the screen printing head comprising: a print head assembly comprising: a print unit comprising a main body including a print medium cavity, which in use contains a print medium, and a delivery aperture, which is in fluid communication with the print medium cavity and through which print medium is in use delivered from the print medium cavity to the surface of the workpiece; a rotatable unit which is disposed in the print medium cavity of the main body and rotatable to displace print medium as contained in the print medium cavity; and a drive unit for rotating the rotatable unit; the screen printing head system being characterized by further comprising control means for controlling the drive unit for rotating the rotatable unit such as to displace print medium as contained in the print medium cavity, wherein the control means are such that the drive unit is operable to rotate the rotatable unit to apply a force to the print medium at the delivery aperture in the main body in a second direction which is opposite to the first direction of displacement of the screen printing head, whereby the force as applied in the second direction counteracts a force as generated by displacement of the screen printing head in the first direction. a print head assembly comprising: a print unit comprising a main body including a print medium cavity, which in use contains a print medium, and a delivery aperture, which is in fluid communication with the print medium cavity and through which print medium is in use delivered from the print medium cavity to the surface of the workpiece; a rotatable unit which is disposed in the print medium cavity of the main body and rotatable to displace print medium as contained in the print medium cavity; and a drive unit for rotating the rotatable unit; the screen printing head system being characterized by further comprising control means for controlling the drive unit for rotating the rotatable unit such as to displace print medium as contained in the print medium cavity, wherein the control means are such that the drive unit is operable to rotate the rotatable unit to apply a force to the print medium at the delivery aperture in the main body in a second direction which is opposite to the first direction of displacement of the screen printing head, whereby the force as applied in the second direction counteracts a force as generated by displacement of the screen printing head in the first direction. a print unit comprising a main body including a print medium cavity, which in use contains a print medium, and a delivery aperture, which is in fluid communication with the print medium cavity and through which print medium is in use delivered from the print medium cavity to the surface of the workpiece; a rotatable unit which is disposed in the print medium cavity of the main body and rotatable to displace print medium as contained in the print medium cavity; and a drive unit for rotating the rotatable unit; the screen printing head system being characterized by further comprising control means for controlling the drive unit for rotating the rotatable unit such as to displace print medium as contained in the print medium cavity, wherein the control means are such that the drive unit is operable to rotate the rotatable unit to apply a force to the print medium at the delivery aperture in the main body in a second direction which is opposite to the first direction of displacement of the screen printing head, whereby the force as applied in the second direction counteracts a force as generated by displacement of the screen printing head in the first direction.