Ink refill unit with controlled incremental ink ejection for print cartridge

An ink refill unit for a print cartridge, said unit comprising:

• a housing defining a fluid path in fluid communication with an outlet and configured to be operatively engaged with the print cartridge;
• a fluid reservoir having an ejection port arranged in fluid communication with the fluid path inside the housing;
• and
• an ejector assembly configured to incrementally eject fluid from the reservoir when the assembly is actuated by operative engagement of the print cartridge with the housing,
• wherein the ejector assembly includes a piston and cylinder ink reservoir, said piston having a thread which engages a screw thread in an internal wall of the cylinder, the piston rotationally biased by a wound spring and the ejector having a pawl-and-ratchet detent preventing the biased piston threading into the reservoir until the pawl is actuated by the print cartridge.