Pagewidth thermal ink jet printhead

A pagewidth, thermal ink jet printhead for use in an ink jet printer and of type assembled from a plurality of fully functional printhead subunits, each subunit having an array of droplet emitting nozzles, so that when the printhead is fixedly mounted in the printer, the nozzles confront a path through which a recording medium is moved to define a printing zone having the length of at least the width of a page, the printhead comprising:

• a structural bar having an edge surface between end surfaces for mounting of roofshooter type printhead subunits thereon, the edge surface having a length at least equal to that of the printing zone, a predetermined width as measured in the direction perpendicular to the bar length and parallel to said bar edge surface, and a predetermined thickness as measured in a direction perpendicular to the bar edge surface, so that the edge surface of the bar has a surface area defined by the bar length and predetermined width, the predetermined width being a distance equal to a dimension of between one and two roofshooter type printhead subunits mounted on said bar edge surface, the predetermined bar thickness having a larger dimension than the bar width, the edge surface confronting the recording medium path when said structural bar is mounted in the printer;
• a passageway being provided within the bar and being adjacently spaced a predetermined distance from the bar edge surface;
• a plurality of openings penetrating the adjacent edge surface and communicating with the passageway;
• a plurality of roofshooter type printhead subunits being mounted on the bar edge surface, each subunit having an ink inlet aligned with a respective one of the openings in said bar edge surface and having a plurality of heating elements, each of which is aligned with a respective one of the subunit nozzles for ejection of ink droplets in a direction normal to the heating elements and towards the recording medium path;
• means for fixedly mounting the structural bar within the printer, so that the subunits confront the recording medium and are spaced predetermined distance therefrom;
• means for providing ink to the bar passageway from an ink supply;
• and
• means for selectively applying electrical signals to the heating elements of the subunits, the signals representing digitized data for the drop-on-demand ejection of ink droplets by the temporary vaporization of ink as a result of the application of the electrical signals, whereby the structural bar thickness is sufficient to provide enough mass for the bar to prevent its warping as a result of the operating temperature of the pagewidth printhead.