Abstract: An ink cartridge for a thermal ink jet printer containing a separate collapsible ink reservoir which is kept under negative pressure by a pressure regulator to prevent ink leakage. The collapsible reservoir has a pair of flexible plastic membrane ink bag sidewalls each connected at their periphery to form an ink bag and an ink pressure regulator in the bag. The pressure regulator comprises a pair of spaced parallel side plates urged apart by a spring toward the adjacent bag sidewalls whereby the reservoir is collapsible against the spring pressure to an essentially flat shape to permit substantially complete dispensation of ink from the bag. Inadvertent puncture of the thin bag walls by the regulator is prevented by a protective edge guard in the form of a layer of tough plastic bonded to the outer surfaces of the sideplates, the protective layers each having a peripheral edge which extends beyond the edge of the side plate to prevent the edges of the sideplates from directly contacting the bag walls.