Design and development of a color thermal inkjet print cartridge - includes related articles on capillary forces in a foam matrix and print quality and pen development

Hewlett-Packard Journal, August, 1988 by Jeffrey P. Baker, David A. Johnson, Vyomesh Joshi, Stephen J. Nigro

In addition to the ink delivery pressure, there are numerous other design goals for the reservoir system. These include some musts--items that cannot be compromised--and wants--items that can be considered trade-offs for yield and cost considerations. Among the must items are that materials be compatible and that the reservoir be inexpensive, robust, clean, manufacturable, and able to supply bubble-free ink to the printhead under all operating conditions. Desirable but negotiable goals for the reservoir system include that it be lightweight, space efficient, and usable in a three-color pen.

Initial Pen Prototypes

Basically, three different methods were examined: a gravity system, mechanical pressure regulation, and capillary ink containment.

The gravity approach, which stores the ink below the nozzle position and "hangs" the ink from the orifice, is difficult to implement. The mechanical pressure regulator, a collapsible bladder like the ThinkJet printer's, contained too little ink (scaling up this design was risky). Also, if two or three of these bladders were placed inside a single small pen to make a multicolor pen, it would be difficult to avoid interaction between the bladders.

Most of the liquid-ink pens on the market today use a capillary ink containment system. Fiber-tipped and roller-ball pens hold the ink supply within a capillary matrix or tube. The reason for this is simple: fibers are cheap and capillary forces are reliable and naturally balanced. The reservoir can have any shape desired: only a continuous path is needed between the supply and the pen tip. If the body of a capillary pen breaks, the ink remains contained within the maxtrix and will not run out freely.

Capillary Reservoir Thermal Inkjet Pens

Our first try at using a capillary reservoir on thermal inkjet pens employed standard felt pen fibers. The pen nib has very small ink channels and therefore a strong ink pulling force. To oppose this force and to provide design margin against leakage, the reservoir of the felt-tipped pen has a capillary pressue of approximately two feet of water. That is, the bundle of fibers can pull water up two feet above its surface by capillary force. The corresponding pressure created by the meniscus at the orifice in our thermal inkjet head is approximately 9 inches of water. The felt pen type reservoir was just too powerful and emptied the drop generator region as soonas it was installed.

Glass beads can provide a capillary force because of the interstitial space between the spheres. The advantage of beads is that they come in a variety of sizes, so the force can be adjusted accurately and easily. Inkjet pens made with a glass bead reservoir worked well and showed that a capillary system could be used for thermal inkjet technology. They did not, however, meet all of the design goals. Weight, volumetric inefficiency, and material concerns convinced us that further exploration was desired.

Thicker fibers would reduce the capillary force, but they are nonstandard and therefore expensive, and their efficiency is marginal (around 40%). Other methods were explored such as rolled-up plastic film or long plastic tubing, but none met all of the requirements. The thin textile fibers would be a perfect desisgn if a method could be found to maintain a 6-fiber-diameter spacing between the strands to give a proper capillary pressure with high volumetric efficiency. Microphotographs showed that open-celled polyurethane foam looked like the solution that we had been searching for (see Fig. 12).