Ink supply system for a thermal ink-jet printer

Abstract: A system for supplying liquid ink to a thermal ink-jet printing apparatus comprises a housing (12) defining a single chamber (13) having a ventilation port (14) and an outlet port (16). A medium (18) occupies at least a portion of the chamber, the medium being adapted to retain a quantity of liquid ink. A scavenger member (20) is disposed across the outlet port, providing a capillary force greater than that of the medium.

Description: The present invention relates to a system for supplying liquid ink to a printhead in a thermal ink-jet printing apparatus. In existing thermal ink jet printing, the printhead comprises one or more ink filled channels, such as disclosed in US-A 4,463,359, communicating with a relatively small ink supply chamber, or manifold, at one end and having an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in each of the channels, a predetermined distance from the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper. Because the droplet of ink is emitted only when the resistor is actuated, this general type of thermal ink-jet printing is known as "drop-on-demand" printing. It has been found that drop-on-demand thermal ink-jet printers work most effectively when the pressure of the ink in the printhead nozzle is kept within a predetermined range of gauge pressures. Specifically, at those times during operation in which an individual nozzle or an entire printhead is not actively emitting a droplet of ink, it is important that a certain negative pressure, or "back pressure," exist in each of the nozzles and, by extension, within the ink supply manifold of the printhead. A discussion of desirable ranges for back pressure in thermal ink-jet printing is given in the "Xerox Disclosure Journal," Vol. 16, No. 4, July/August 1991, p. 233. This back pressure is important for practical applications to prevent unintended leakage, or "weeping," of liquid ink out of the nozzles onto the copy surface. Such weeping will obviously have adverse results on copy quality, as liquid ink leaks out of the printhead uncontrollably. A typical end-user product in this art is a cartridge in the form of a prepackaged, usually disposable item comprising a sealed container holding a supply of ink and, operatively attached thereto, a printhead having a linear or matrix array of channels. Generally the cartridge may include terminals to interface with the electronic control of the printer; electronic parts in the cartridge itself are associated with the ink channels in the printhead, such as the resistors and any electronic temperature sensors, as well as digital means for converting incoming signals for imagewise operation of the heaters. In one common design of printer, the cartridge is held with the printhead against the sheet on which an image is to be rendered, and is then moved across the sheet periodically, in swaths, to form the image, much like a typewriter. Full-width linear arrays, in which the sheet is moved past a linear array of channels which extends across the full width of the sheet, are also known. Typically, cartridges are purchased as needed by the consumer and used either until the supply of ink is exhausted, or, equally if not more importantly, until the amount of ink in the cartridge becomes insufficient to maintain the back pressure of ink to the printhead within the useful range. US-A-5,047,790 discloses an ink jet cartridge having controlled capillary containment of the ink, comprising an array of spaced apart capillary members positioned in the cartridge reservoir. The spacing of the capillary members provide the required capillarity to generate the necessary back pressure and to prevent ink leakage through the printhead under the force of gravity. A filter screen is mounted over a well and the lower ends of the capillary member reside perpendicularly thereon and are urged thereagainst. Ink in the well is fed to the printhead. JP-A-2,034,353, showing the preamble of claim 1, discloses an ink jet cartridge having a housing with integral printhead and ink supply tank. The tank is in communication with the atmosphere via a chamber which contains a porous member which is ink repellent but permits the passage of air. The tank is majority filled with a porous member but a separate porous member is located adjacent and covers ink feed outlet from the tank and is in contact with the porous member filling the majority of the tank. Ink passing through outlet is fed to the printhead. The porous member covering the tank outlet has higher capillarity than the other porous member filling the majority of the tank. US-A-4,771,295 discloses an ink jet cartridge having multiple ink storage compartments filled with foam and in communication with a printhead. Each compartment has an upstanding pipe covered by a screen filter. The foam is compressed against the filter-covered pipe ends to increase the capillarity of the foam in the regions above the filters. The filters provide filtering of air bubbles and debris from the ink passing therethrough. Other considerations are crucial for a practical ink supply as well. The back pressure, for instance, must be maintained at a usable level for as long as possible while there is still a supply of ink in an ink cartridge. Therefore, a cartridge must be so designed as to maintain the back pressure within the usable range for as large a proportion of the total range of ink levels in the cartridge as possible. Failure to maintain back pressure causes the ink remaining in the cartridge to leak out through the printhead or otherwise be wasted. It is an object of the present invention to enable the back pressure in an ink jet printhead to be maintained at a desired level. In accordance with the present invention, a system for supplying liquid ink to a thermal ink-jet printing apparatus, comprising: a housing defining a single chamber having an outlet port; a medium occupying at least a portion of the chamber, the medium being adapted to retain a quantity of liquid ink by a capillary force thereof; a scavenger member disposed across the outlet port, providing a capillary force greater than the capillary force of the medium; and a free space within the chamber adjacent a surface of the scavenger member, wherein ink is flowable between the scavenger member and the free space, the scavenger member being adapted to function as an ink capacitor. By way of example only, an embodiment of the invention will be described with reference to the accompanying drawings, in which: Figure 1A is a sectional, elevational view of a cartridge incorporating the present invention. Figure 1B is an exploded view of a cartridge as in Figure 1A incorporating the present invention. Figure 2A is a graph illustrating back pressure of liquid ink as a function of the amount of ink in the cartridge. Figure 2B is a detail of the graph of Figure 2A. Figure 3 is an elevational view of a thermal ink jet printing apparatus. Figure 3 is a general elevational view of a type of thermal ink-jet printer in which the printhead and the ink supply therefor are combined in a single package, referred to hereinafter as cartridge 10. The main portion of cartridge 10 is the ink supply, with another portion forming the actual printhead 100. In this embodiment of the invention, cartridge 10 is placed within a larger thermal ink jet printing apparatus in which the cartridge 10 is caused to move along carriage 200 in such a way that printhead 100, moving relative to sheet 210, may print characters on the sheet 210 as the cartridge 10 moves across the sheet, somewhat in the manner of a typewriter. In the example illustrated, printhead 100 is of such a dimension that each path of cartridge 10 along sheet 210 enables printhead 100 to print out a single line of text, although it is generally not necessary for the text lines to conform to the swaths of the copy cartridge 10. With each swath of cartridge 10, sheet 210 may be indexed (by means not shown) in the direction of the arrow 205 so that any number of passes of printhead 100 may be employed to generate text or image onto the sheet 210. Cartridge 10 also includes means, generally shown as 220, by which digital image data may be entered into the various heating elements 110 of printhead 100 to print out the desired image. These means 220 may include, for example, plug means which are incorporated in the cartridge 10 and which accept a bus or cable from the data-processing portion of the apparatus, and permit an operative connection therefrom to the heating elements in the printhead 100. Figure 1A is a sectional, elevational view of cartridge 10. The cartridge 10 has a main portion in the form of a housing 12. Housing 12 is typically made of a lightweight but durable plastic. Housing 12 defines a chamber 13 for the storage of liquid ink, and further has defined therein a ventilation port 14, open to the atmosphere, and an output port 16. At the end of the output port 16 (as shown at the broken portion of Figure 1A) is an ink jet printhead 100, and specifically the ink supply manifold thereof, substantially as described above. An ink-saturated medium, shown here as three separate portions each marked 18, which will be described in detail below, occupies most of the chamber 13 of housing 12. Figure 1B is an exploded view of cartidge 10, showing how the various elements of cartridge 10 may be formed into a compact customer-replaceable unit. Other parts of the cartridge 10 shown in Figure 1B include a heat sink 24 and cover 28 having openings 29 therein to permit ventilation of the interior of housing 12 through ventilation port 14. A practical design will typically include space for on-board circuitry for selective activation of the heating elements in the printhead 100. Also shown in Figures 1A and 1B is a tube 30 extending from ventilation port 14 toward the center of the interior of housing 12, through openings in each portion of medium 18. The tube 30 defines a channel permitting substantially direct ventilation of open space within the chamber. Preferably, medium 18 (shown as three portions of material) is in the form of a needled felt of polyester fibers. Needled felt is made of fibers physically interlocked by the action of, for example, a needle loom, although in addition the fibers may be matted together by soaking or steam heating. Preferably, the needled felt is of a density of between 0.06 and 0.13 grams per cubic centimeter and it has been found that the optimum density is 0.095 grams per cubic centimeter. This optimum density reflects the most advantageous volume efficiency, as described above, for holding liquid ink. A type of felt suitable for this purpose is manufactured by BMP of America, Medina, NY. It has been found, in order to provide the back pressure of liquid ink within the desired range, while still providing a useful volume efficiency and portability, that the polyester fibers forming the needled felt should be of two intermingled types, the first type of polyester fiber being of a greater fineness than the second type of polyester fiber. Specifically, an advantageous composition of needled felt comprises approximately equal proportions of 6 denier and 16 denier polyester fibers. Medium 18 is packed inside the enclosure of housing 12 in such a manner that the felt exerts reasonable contact and compression against the inner walls. In one commercially-practical arrangement, the medium 18 is created by stacking three layers of needled felt, each one-half inch in thickness, and packing them inside the housing 12. Also within housing 12 is a member made of a material providing a high capillary pressure, indicated as scavenger 20. Scavenger 20 is a relatively small member which serves as a porous capillary barrier between the medium 18 and the output port 16, which leads to the manifold of printhead 100. Preferably, scavenger 20 is made of an acoustic melamine foam, which is felted (compressed with heat and pressure) by 50% in the direction of intended ink flow. One suitable type of melamine foam is made by Illbruck USA, Minneapolis, MN, and sold under the trade name "Wiltec." The scavenger 20 preferably further includes a filter cloth, indicated as 22, which is attached to the melamine using a porous hot-melt laminating adhesive. In general, the preferred material for the filter cloth 22 is monofilament polyester screening fabric. This filtered cloth provides a number of practical advantages. Typically, no specific structure (such as a wire mesh) for holding the scavenger 20 against the opening into outlet port 16 is necessary. Further, there need not be any adhesive between the filter cloth 22 and the outlet port 16. The high capillary force provided by filter cloth 22 creates a film of ink between the filter cloth 22 and the outlet port 16, by virtue of the planarity (no wrinkles or bumps) of the filter cloth 22 against the scavenger 20, the compression of the scavenger 20 against the outlet port 16, and the saturation of the scavenger 20. This film serves to block out air from the outlet port 16. In Figure 1, it can be seen that one portion of the outer surface of scavenger 20 abuts the medium 18, while other portions of the surface are exposed to open space, indicated as 15, between the medium 18 and the inner walls of housing 12. The single chamber 13 is so designed that a given quantity of ink may conceivably flow to or from the medium 18, to or from the scavenger 20, or to or from the free space within the chamber 13; that is, there are no solid internal barriers to the flow of ink within chamber 13. Generally, this arrangement serves to maintain the back pressure of liquid ink within a manageable range while the copy cartridge is slowly emptied of liquid ink. Because ink transmittance through medium 18 is not rapid enough to supply ink continuously to printhead 100, and because the felt of medium 18 does not provide the necessary seal to permit continuous, air-free flow of ink through outlet port 16, scavenger 20 is intended to act as an ink capacitor, from which ink can be drawn even under conditions of a high rate of ink demand, as will be explained in detail below. In a typical commercial thermal ink jet printing apparatus, wherein the printhead is moved across a sheet in a number of swaths, the time for printing an eight-inch swath is approximately 0.5 seconds. The time in which the cartridge 10 changes direction between printing swaths is approximately 0.1 seconds. The scavenger 20 tends to desaturate during the printing of a swath, as ink is placed on the sheet; the time between printing swaths is useful as a "recovery" time in which the scavenger 20 is allowed to resaturate, thereby returning to an equilibrium back pressure. In one commercially-practical cartridge, the medium 18 is initially loaded with 68 cubic centimeters of liquid ink, of which it is desired to obtain at least 53 cubic centimeters for printing purposes while the back pressure of the cartridge is within a usable range. A typical volume of the scavenger 20 is two cubic centimeters. In printing a typical eight-inch swath in the course of printing a document, the scavenger 20 may be desaturated by up to 2.5% of the ink therein in 0.5 seconds, and this desaturation will cause an increase in back pressure at the printhead 100. This principle can best be envisioned by analogy to a common sponge: it is easier to squeeze out a quantity of liquid from a saturated sponge than it is to squeeze out the same quantity of liquid from a less-saturated sponge, even if the necessary amount of liquid is in the nearly-dry sponge. As desaturation causes an increase in back pressure with any absorbent medium, this back pressure will increase significantly in the course of printing a single swath of significant density across a sheet. However, although desaturation of scavenger 20 will cause an increase in back pressure at the printhead 100, this increased back pressure from scavenger 20 works in the other direction as well. That is, desaturation of scavenger 20 will also cause a negative pressure against the medium 18, thereby causing a quantity of liquid ink to move from medium 18 to the scavenger 20, thereby resaturating scavenger 20 and thereby lowering the back pressure thereof. In this way the combination of medium 18 and scavenger 20 acts as a system for stabilizing the back pressure at printhead 100 as the supply of ink in medium 18 decreases. Figures 2A and 2B are graphs showing the performance of a cartridge 10, showing that the back pressure maintained at the printhead 100 is kept within a usable range for a great portion of ink levels in the copy cartridge 10. In Figure 2A, the X-axis represents the volume of ink delivered through the printhead 100 (i.e., as the cartridge empties out), while the Y-axis represents the back pressure at the printhead in millimeters of water, which is, on the whole, comparable to millimeters of liquid ink. As can be seen clearly in Figure 2A, the back pressure is maintained at the best range, 12.5 mm to 125 mm, up to the point where over 55 cc's of ink are delivered. Preferably, the cartridge 10 is originally loaded with 68 cc's of ink, and so therefore, only a reasonably small amount of ink is wasted because of insufficient back pressure. In the graph of Figure 2A can be seen two lines; the solid line being the "static capillary pressure" of the cartridge at the printhead 100, while the dotted line above the solid line represents momentary back pressures created in the course of printing out individual swaths across a sheet, as in a typical context of printing documents such as the apparatus shown generally in Figure 4. Figure 2B is a detailed view of a portion of the graph of Figure 2A, showing a typical behavior of back pressure in a copy cartridge 10, in the course of continuous or substantially continuous use. In the type of thermal ink jet printing apparatus as shown in Figure 1, the copy cartridge 10 reciprocates across the copy sheet so that printhead 100 may print out an image, in a series of parallel swaths,on the copy sheet. Each swath across the copy sheet typically lasts 0.5 seconds, while the turnaround time at the end of each swath is approximately 0.1 seconds (in typical commercial embodiments, the printhead 100 ejects ink onto the copy sheet when the copy cartridge 10 is moving in either direction). As mentioned above, liquid ink is drawn out of the copy cartridge 10 in the course of printing a swath, and the scavenger 20 substantially resaturates during the momentary changes of direction of the copy cartridge 10. When the scavenger 20 (and, by extension, the entire ink supply including medium 18) desaturates even only slightly, the back pressure will increase. In substantially continuous use of the copy cartridge 10, the periodic desaturating and resaturating of scavenger 20 translates into a cyclical pattern of increasing and re-correcting back pressures, which can be seen in Figure 2B. In Figure 2B, the finely-dotted lines, forming a sawtooth pattern with increasing portions a and decreasing portions b , show the actual continuous-time behavior of the back pressures between the solid line (static capillary back pressure) and the local maxima indicated generally by the larger dotted line visible in Figure 2A. With each sawtooth, the momentary increases shown by portion a represent the increase in back pressure as the ink supply system gives up ink in the course of printing a swath; the relatively quicker down portions b of each sawtooth represent the relatively rapid resaturation of the scavenger 20 in the turnaround times. In addition to the desaturation of the medium 18, another source of back pressure in a cartridge such as 10 is the "impedance" of ink flow through the various elements of the cartridge 10, caused by various shear forces among the medium 18, scavenger 20, and other parts. There are also shear forces at the microscopic level, for example, within the felt of medium 18 and the foam of scavenger 20. In this way, it can be seen that the structure and materials of Figures 1A and 1B provide not only the desired range of back pressures toward the printhead in a consistent manner over the life of the copy cartridge, but also maintain a relatively consistent level of back pressure, even in the course of continuous use of the copy cartridge.

Claims: A system (10) for supplying liquid ink to a thermal ink-jet printing apparatus, comprising: a housing (12) defining a single chamber (13) having an outlet port (16); a medium (18) occupying at least a portion of the chamber, the medium being adapted to retain a quantity of liquid ink by a capillary force thereof; and a scavenger member (20) disposed across the outlet port, providing a capillary force greater than the capillary force of the medium; characterized by: a free space (15) within the chamber adjacent a surface of the scavenger member, wherein ink is flowable between the scavenger member and the free space, the scavenger member being adapted to function as an ink capacitor. A system as claimed in claim 1, wherein the scavenger member comprises melamine foam. A system as claimed in any one of the preceding claims, further including a filter cloth attached to the scavenger member. A system as claimed in claim 3, wherein the filter cloth comprises monofilament polyester screening fabric. A system as claimed in claim 3 or claim 4, wherein the filter cloth is hot-melted to the scavenger member. A system as claimed in any one of the preceding claims, wherein the medium is so disposed relative to the scavenger member within the chamber that a portion of the medium directly contacts a surface of the scavenger member. A system as claimed in any one of the preceding claims, wherein the medium comprises a needled felt made from at least two polyester fibers, with one of the polyester fibers being of a different fineness than the other polyester fiber. A system as claimed in claim 7, wherein the medium comprises approximately equal proportions of polyester fibers of each fineness. A system as claimed in claim 7 or claim 8, wherein the medium comprises 6 denier and 16 denier polyester fibers. A system as claimed in any one of claims 7 to 9, wherein the medium has a density within the range of from about 0.06 to about 0.13 grams per cubic centimeter.

Inventors: CARLOTTA MICHAEL, DIETL STEVEN J, CARLOTTA, MICHAEL, DIETL, STEVEN J.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1993
Main classification: B41J2
IPC codes: B41J2