In the art of thermal ink jet printing, it is known to provide controlled and localized heat transfer to a defined volume of ink which is located adjacent to an ink jet orifice. This heat transfer is sufficient to vaporize the ink in such volume and cause it to expand, thereby ejecting ink from the orifice during the printing of characters on a print medium. The above predefined volume of ink is customarily provided in a so-called barrier layer which is constructed to have a plurality of ink reservoirs therein. These reservoirs are located between a corresponding plurality of heater resistor elements and a corresponding plurality of orifice segments for ejecting ink therefrom.
One purpose of these reservoirs is to contain the expanding ink bubble and pressure wave and make ink ejection more efficient. Additionally, the reservoir wall is used to slow down cavitation produced by the collapsing ink bubble. For a further discussion of this pressure wave phenomena, reference may be made to a book by F. G. Hammitt entitled Cavitation and Multiphase Flow Phenomena, McGraw-Hill 1980, page 167 et seq, incorporated herein by reference.
The useful life of these prior art ink jet print head assemblies has been limited by the cavitation-produced wear from the pressure wave created in the assembly when an ink bubble collapses upon ejection from an orifice. This pressure wave produces a significant and repeated force at the individual heater resistor elements and thus produces wear and ultimate failure of one or more of these resistor elements after a repeated number of ink jet operations. In addition to the above problem of resistor wear and failure, prior art ink jet head assemblies of the above type have been constructed using polymer materials, such as those known in the art by the trade names RISTON and VACREL.
Whereas these polymer materials have proven satisfactory in many respects, they have on occasion exhibited unacceptably high failure rates when subjected to substantial wear produced by pressure waves from the collapsing ink bubbles during ink jet printing operations. Additionally, in some printing applications wherein the printer is exposed to extreme environments and/or wear, these polymer materials have been known to swell and lift from the underlying substrate support and thereby render the print head assembly inoperative.