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1. An ink cartridge for a printer with an inkjet printhead, the ink cartridge comprising:a rigid housing, the housing having a docking face for abutting the printer such that when installed, the docking face faces downwards;an ink storage volume;an ink outlet valve for fluid communication with the printhead, the ink outlet valve being positioned in the docking face;a variable volume structure within the ink storage volume, the variable volume structure being inflatable with air and having a section upwardly displaceable within the ink storage volume as air inflates the variable volume structure during use;an air inlet positioned in the docking face and spaced from the outlet valve is in fluid communication with the variable volume structure such that the air inlet lets air into the ink storage volume as ink is drawn out through the outlet valve; and,a conduit in the ink storage volume, one end of the conduit being connected to the ink outlet valve and other end being open to ink within the ink storage volume and positioned remote from the docking face such that the other end does not get obstructed by the variable volume structure as the variable volume structure inflates.
2. An ink cartridge according to claim 1 wherein the air inlet having a frangible seal to provide an air tight seal prior to installation of the ink cartridge.
3. An ink cartridge according to claim 2 wherein the outlet valve and the air inlet are configured for engagement with complementary formations on the printer such that the ink outlet valve is opened upon installation of the cartridge in the printer, and the frangible seal is ruptured upon installation in the printer.
4. An ink cartridge according to claim 1 wherein the variable volume structure is an air bag such that upon installation in the printer, the air inlet vents the air bag to atmosphere.
5. An ink cartridge according to claim 1 wherein during use the variable volume structure in the ink storage volume expands to keep a constant head of ink above the outlet valve.
6. An ink cartridge according to claim 1 wherein the ink outlet valve and the air inlet are recessed into the docking face.
7. An ink cartridge according to claim 3 wherein the outlet valve and the air inlet are opened simultaneously as the cartridge is installed.
8. An ink cartridge according to claim 1 wherein the docking face is substantially flat.
9. An ink cartridge according to claim 8 wherein the rigid housing is partially defined by a roof wall, the roof wall being substantially flat, parallel to, and directly opposite the docking wall such that the ink cartridges are vertically stackable on each other.
10. An ink cartridge according to claim 4 wherein the air bag has flat top and bottom sheets separated by side walls folded in a concertina fashion when the air bag is deflated.
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