Ink refill unit with asymmetrically positioned ink outlet

1. An ink refill unit for replenishing an ink cartridge in an inkjet printer, the ink cartridge having an interface for engaging the ink refill unit, an ink storage compartment partially defined by flexible wall, and a constrictor mechanism for constricting the flexible wall, the ink refill unit comprising:

an ink storage compartment;

a docking portion for engaging the interface of the cartridge, the docking portion having a base plate with an ink outlet for connection to an inlet port on the interface and a constrictor actuation mechanism for actuating the constriction mechanism of the ink cartridge as the ink refill unit engages the interface and de-actuating the constriction mechanism as the ink refill unit disengages the interface; and

engagement formations for releasably engaging the interface, the engagement formations being formed on the transverse centre line of the base plate; wherein, the centre of the ink outlet is spaced from the transverse centre line.

2. An ink refill unit according to claim 1 wherein the base plate has a spigot projecting therefrom for insertion into an aperture in the interface, the spigot having a cross section that is keyed to the shape of the aperture to ensure that the refill is correctly match to the cartridge.

3. An ink refill unit according to claim 2 wherein the spigot has a tri-star cross section.

4. An ink refill unit according to claim 3 wherein the spigot actuates a valve in the cartridge when the refill engages the interface.

5. An ink refill unit according to claim 1 , wherein:

the cartridge comprises an ink feed system for connection to a printhead assembly with an array of ink ejection nozzles; and

the ink storage compartment has a variable storage volume and a displaceable wall section biased to expand the variable storage volume to generate a negative pressure therein, such that ink does not inadvertently leak from the ink ejection nozzles.

6. An ink refill unit according to claim 1 , further comprising:

a body containing a quantity of ink;

an ink outlet for engaging a complementary ink inlet in the interface; and

a spigot extending from the body for insertion in an aperture in the interface,

wherein the lateral cross section of the spigot is keyed to the shape defined by the aperture to ensure the ink refill is correctly matched to the cartridge.