Underpressure regulating mechanism for inkjet pens

A device, applicable to an inkjet pen composed of a print head and an ink cartridge having a first and a second ports formed thereon, fluid-communicated with ambient air, for regulating underpressure in said cartridge and prevent ink from leakage, comprising:

• a seat, formed adjacent to said first port, having a vent fluid-communicated with said first port and interior of said cartridge;
• an expandable air bag, fluid-communicated with said second port, for being expanded by ambient air when said underpressure in said cartridge increases, and moving a connecting element thereby;
• a resilient element, connecting to said connecting element, for providing a pressing force to bias said connecting element against expansion direction of said air bag;
• and
• a valve element, movably mounted in said seat, for selectively sealing and opening said port corresponding to said pressing force of said resilient element and said expansion of said air bag.