Abstract:
An ink-jet printing system having a pressure regulator that changes the volume of the ink receptacle as the ink pressure changes relative to the ambient pressure so that the ink remains at a substantially constant pressure for delivery to the print head.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This is a continuation of application Ser. No. 08/549,106 filed on Oct. 27, 1998 now U.S. Pat. No. 5, 980,028. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of ink-jet printing and, more particularly, to the delivery of ink and the control of ink pressures to ink-jet print heads. 
     Ink-jet technology is relatively well developed. The basics of this technology are described by W. J. Lloyd and H. T. Taub in “Ink-Jet Devices,” Chapter 13 of  Output Hardcopy Devices  (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988) and in various articles in the  Hewlett-Packard Journal , Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No 5. (October 1988), Vol. 43, No. 4, (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45. No. 1 (February 1994). 
     The typical thermal ink-jet print head has an array of precisely formed nozzles attached to a print head substrate that incorporates an array of firing chambers that receive liquid ink (i.e., colorants dissolved or dispersed in a solvent) from an ink reservoir. Each chamber has a thin-film resistor, known as a “firing resistor”, located opposite the nozzle so ink can collect between it and the nozzle. When electric printing pulses heat the thermal inkjet firing resistor, a small portion of the ink near it vaporizes and ejects a drop of ink from the print head. The nozzles are arranged in a matrix array. Properly sequencing the operation of each nozzle causes characters or images to form on the paper as the print head moves past the paper. 
     An ink delivery system delivers ink at a slight vacuum, known as a “back pressure”, to the print head so that the ink does not leak out of the nozzles. Without such back pressure, the ink may leak or “drool” out of the nozzles and onto the printing medium or into the printer mechanism. This back pressure, however, must be small enough so that when the firing resistors are energized, the resistors can overcome the back pressure and eject ink droplets in a consistent and predictable manner. Typically, this vacuum is approximately two to three inches  0 f water below atmospheric pressure or minus two to three inches. 
     Back pressure regulation has become more critical in recent years because of the evolution in the design of print cartridges. The mass of the moving parts and the volume of ink in motion are being reduced so that simpler drive mechanisms can be used. This reduction in mass has decreased the capacity of the materials around the print head to absorb the heat generated by the firing resistors during operation. The result is that unless the transfer of heat from the firing resistors is carefully managed, the ink and the print head may be subjected to wide fluctuations in temperature. These fluctuations in temperature can also result in wide variations in back pressure as the ink heats and cools. The net result is that all of these changes have a degrading affect on print quality. 
     Accumulators are widely used in hydraulic systems to smooth out pressure fluctuations and to act as shock absorbers against propagating pressure waves. In these applications a compressible gas such as nitrogen or air is used, and the gas is alternately compressed and decompressed as needed. One such use in an ink-jet printing system is disclosed in US Pat. No. 4,223,323 by Bader et al. 
     While such accumulators work well in those pressure ranges where the gas can be alternately compressed and decompressed, these systems have little affect where the gas is not compressed. 
     SUMMARY OF THE INVENTION 
     Briefly and in general terms, an apparatus according to the present invention includes a fluid accumulator forming a portion of the ink containment for a print head. The accumulator changes the volume of the ink containment as the temperature of the ink changes so that the ink remains at substantially constant pressure for delivery to the print head. 
     In another embodiment, an apparatus according to the present invention includes an ink reservoir containing ink at a pressure P 1 , an ink-jet print head for printing on a medium with ink at a pressure P 2 , a pressure regulator connected to both the ink reservoir and the print head so that the regulator receives ink at a pressure P 1  from the reservoir and supplies ink at a pressure P 2  to the print head, where P 1  is larger than P 2 , and a fluid accumulator operatively connected to the print head so that as the temperature of the ink varies, the ink supplied to the print head remains at substantially constant pressure. 
     Other aspects and advantages of the invention will become apparent from the following detailed description, taken into conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic, perspective view of an ink-jet printer according to the present invention. 
     FIG. 2 is an exploded, perspective view of a portion of the print cartridge of FIG.  1 . 
     FIG. 3 is an exploded, perspective view of a second portion of the print cartridge of FIG.  1 . 
     FIG. 4 is a side elevation view, in cross section taken along lines  4 — 4  and  4 ′— 4 ′ in FIGS. 2 and 3 respectively, illustrating the normal operating position of the pressure regulator. 
     FIG. 5 is a side elevation view, in cross section taken along lines  4 — 4  and  4 ′— 4 ′ in FIGS. 2 and 3 respectively, illustrating the opening of the orifice of the pressure regulator to allow the entry of ink into the housing of the print cartridge. 
     FIG. 6 is a side elevation view, in cross section taken along lines  4 — 4  and  4 ′— 4 ′ in FIGS. 2 and 3 respectively, illustrating the accumulator accommodating changes in the volume of ink. 
     FIG. 7 is a side elevation view, in cross section taken along lines  4 — 4  and  4 ′— 4 ′ in FIGS. 2 and 3 respectively, illustrating the service station drawing air down the snorkel and out of the print head. 
     FIG. 8 is a side elevation view, in cross section taken along lines  4 — 4  and  4 ′— 4 ′ in FIGS. 2 and 3 respectively, illustrating the service station drawing air down the snorkel and out of the print head as the orifice of the pressure regulator opens to allow the entry of ink into the housing of the print cartridge. 
     FIG. 9 is a side elevation view, in cross section, illustrating a bellows operating as an accumulator. 
     FIG. 10 is a side elevation view, in cross section, illustrating a piston operating as an accumulator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in the drawings for the purposes of illustration, the invention is embodied in an apparatus for providing ink to an ink-jet print head at substantially constant pressure. 
     Referring to FIG. 1, reference numeral  12  generally indicates a printer including a print cartridge  14  that ejects drops  16  of ink on command. The drops form images on a printing medium  18  such as paper. The printing medium is moved laterally with respect to the print cartridge  14  by two print rollers  20 ,  20 ′ and a motor  21  that engages the printing medium. The print cartridge is moved back and forth across the printing medium by a drive belt  23  and a motor  24 . The print cartridge contains a plurality of firing resistors, not shown, that are energized on command by an electrical circuit  26 . The circuit sequentially energizes the firing resistors in a manner so that as the print cartridge  14  moves laterally across the paper and the paper moved by the rollers  20 ,  20 ′, the drops  16  form images on the printing medium  18 . 
     Referring to FIG. 1, ink is supplied to the print cartridge  14  from an ink reservoir  30 . The ink reservoir is stationary and may be either flaccid or pressurized. The ink is supplied from the reservoir by an integral connector  32  that is removably attached to a conduit  34  by a double acting valve  36 . The connector  32  allows the reservoir to be replaced when the ink supply is exhausted. The ink in the reservoir is maintained at a pressure P 1  sufficient to maintain the flow of ink through the conduit  34  necessary to meet the maximum ink flow requirements of the print cartridge (which pressure could be from −20 inches to +100 inches of water). This pressure also depends on the diameter and length of the conduit  34 . The conduit has a generally helical shape to accommodate the motion of the print cartridge  14  with respect to the ink reservoir  30 . When the connector is separated from the conduit, the double acting valve  36  simultaneously shuts both openings so that air is not ingested into the system. Likewise when the connector is fitted to the conduit, the double acting valve simultaneously opens both the connector  32  and the conduit  34  to allow fluid communication of the ink between the ink reservoir  30  and the print cartridge  14  without ingesting air into the system. 
     The conduit  34 , FIG. 1 terminates in a particle filter  37  that collects any material that could clog the print cartridge  14  during operation. The filter is located on the high pressure side of the ink pressure regulator so that if any air is ingested in the reservoir  30 , at the double acting valve  36  or in the conduit  34 , the air will flow into the print cartridge and will not block the filter or impede the ink flow. 
     The printer  12 , FIG. 1, also includes a service station  40  that can draw a vacuum on the nozzles, not shown, on the print cartridge  14 . The service station includes a deformable cup  42  that engages and seals against the nozzles. The cup is connected to a source of vacuum  44  by a valve  45 . The service station operates by directing the print cartridge  14  over the cup  42  where a vacuum is drawn on the nozzles and the ink is sucked through the nozzles and out of the cartridge. 
     The print cartridge  14  of FIG. 1 is shown in two exploded views in FIGS. 2 and 3. The print cartridge includes a top plate  47  that is formed from two contiguous, over-lapping flat panels  50 ,  50 ′. The panels form an interior hollow passage  54  for the ink within the top plate. This passage receives an intake tube  48 , terminates at an orifice  49 , FIG. 5, and distributes ink into the print cartridge. The upper panel  50  of the top plate contains a small vent  53  that communicates with the atmosphere. The lower panel  50 ′ contains a circular opening  51  of substantially larger diameter. Sandwiched and sealed between the panels  50 ,  50 ′ is a diaphragm  52  that forms a fluid tight seal across the circular opening  51 , FIG.  5 . The peripheral margin of the diaphragm  52  is thereby sealed against both air and ink. The diaphragm can be fabricated from either thin polyethylene plastic or polyvinyldene fluoride so that the diaphragm is impervious to both air and ink. The diaphragm is deformable and flexible and may be either resilient or not. When a pressure difference is developed across the surface of the diaphragm, the diaphragm expands into the print cartridge as illustrated in FIGS. 4-6. The upper side of the diaphragm is continuously exposed to atmospheric pressure through the vent  53 . 
     Referring to FIG. 2, reference numeral  60  generally indicates a pressure regulator that supports the diaphragm  52  and regulates the pressure of ink supplied into the print head  14 . The pressure regulator includes a lever  62  that rotates about an axle  64  that is supported from two supports  66 . The supports are mounted on the underside of the lower panel  50 ′ of the top plate  47 . The lever also includes an integral arm  68  that contains a valve seat  70  for the ink orifice  49 . The valve seat is a flattened, planar surface of room temperature vulcanizing silicone (RTV) and is counter sunk into the surface of the integral arm  68 . The lever is aligned so that when the lever  62  is parallel with the plane of the top plate  47 , the valve seat  70  is seated and ink orifice  49  is thereby shut as illustrated in FIG.  4 . 
     The lever  62 , FIG. 2 engages the diaphragm  52  with a piston  75  and an accumulator spring  74 . The accumulator spring  74  is mounted in a circular depression  72  in the lever so that the spring does not move off of the lever  62 . The piston is attached to the spring  74  and is held in place by a peripheral, concave engaging surface  76 . Referring to FIGS. 4,  5 , and  6 , the accumulator spring  74  is designed so that a differential pressure across the diaphragm  52  can cause the diaphragm to flex and the piston  75  to move reciprocally up and down without moving the lever  62  and opening the ink inlet valve  49 ,  70 . In FIG. 4 the diaphragm  52  is contracted slightly downward or is more concave in shape. In FIG. 6 the diaphragm is contracted slightly upward or is more planar in shape. The illustrated motion shows a portion of the wall of the ink containment moving and changing the volume of the ink containment. If the print cartridge is subjected to either heating or cooling, the diaphragm flexes to accommodate the change in volume necessary to maintain the pressure of the ink to the print head constant during the temperature transient. 
     In FIG. 5 the ink valve  49 ,  70  opens when the piston  75  is forced sufficiently downward by the diaphragm to bottom out against the lever  62  and to mechanically cause its motion. The lever  62  is supported within the print cartridge  14  by a pressure setting spring  78 . The pressure setting spring  78  is designed so that its force on the lever  62  is equal to the opening force or cracking force on the ink valve  49 ,  70 . The force of this spring is set to be equal to the area of the diaphragm  52  that is uncovered by the opening  51 , FIG. 2, multiplied by the pressure difference between atmospheric pressure and the pressure of the ink supplied to the print head  86 , FIG.  5 . Typically, this differential pressure is approximately minus three inches (−3″) of water. The pressure setting spring  78  is also preloaded so that the force on the lever  62  is essentially constant over the travel of the lever. Such a constant spring force causes the motion of the lever to be large for any given change in the cracking pressure. In other words, a small change in pressure will cause a large movement in the lever. The net result is that when the valve seat  70  is moved off the valve nozzle  49  by a distance equal to approximately the radius of the nozzle  49 , the valve will open to full flow condition. 
     Referring to FIG. 3, the print cartridge  14  further includes a housing  82  that receives the top plate  47  in a step  83  formed in the end of the side walls of the housing. The housing and the top plate together comprise the ink containment for the print head  86 . During normal printing operation this containment is the volume that is maintained at constant pressure by the pressure regulator  60 , FIG.  2 . In the bottom wall of the housing  82  are a plurality of ink feed slots  84  that allow the ink to flow to the print head  86 . The print head is a semiconductor substrate on to which are placed the firing chambers, the firing resistors, and the orifice plate in the conventional manner. The print head is mounted on a flexible conductor  87  by tab bonding and electrical signals to the firing resistors are established through the conductors  88 , FIGS. 1 and 3. 
     Referring to FIG. 3, reference numeral  90  generally indicates a primming assembly for removing air from the interior of the print cartridge  14 . The priming assembly includes four side walls  92  and a top wall  93  that form an intermediate chamber  91  around the print head  86 . These walls support the pressure setting spring  78  above the bottom wall of the housing  82  and also form a secondary differential pressurization area above the print head as described below. The top wall  93  also includes a flow orifice  94  and a snorkel  95 . The snorkel is a conduit with an inlet  96  that connects the intermediate chamber  91  with an area  98  in the print cartridge where air gathers. The print cartridge  14  is designed to entrap and to warehouse any air in the cartridge in the area  98 . Air is thus stored in an out of the way location so that air and air bubbles do not interfere with the flow of ink during printing. 
     The flow orifice  94  is sized so that during all printing operations the ink flows to the print head  86  through the orifice  94  and not through the snorkel  95 . The orifice is sized so that when printing at maximum ink flow, the orifice has a pressure drop through it that is less than the height of the snorkel  95 . 
     The priming assembly  90 , FIG. 7, also includes the service station  40  described above which can engage and seal the print head  86 . The service station draws ink out through the print head  86  at a much higher flow rate than during any printing operation. The flow orifice  94  is sized so that under this high ink flow condition, such a large pressure drop is developed across the flow orifice  94  that the ink and air in the top area  98  of the print cartridge are drawn down the snorkel  95  and out the print head  86  as illustrated in FIG.  7 . 
     In operation, the ink reservoir  30 , FIG.  1  and the print cartridge  14  are initially filled with ink and sealed. The ink conduit  34  may or may not be filled with ink. To begin, the ink reservoir  30  is connected to the ink conduit  34  by the double acting valve  36 . When the printer  12 , FIG. 1, commands the print cartridge  14  to commence ejecting drops  16 , FIG. 1, ink flows through the conduit  34  and any air in the conduit flows into the print cartridge and becomes trapped in the top area  98  of the housing. As illustrated in FIG. 4, at this point the print cartridge has a slight air bubble  98  in the top of the housing, the ink orifice  49  is shut by the lever  62 , the diaphragm  52  is slightly concave, and any ink flow to the print head  86  is passing through the flow orifice  94 . 
     As the print head  86 , FIG. 5 continues to eject drops of ink on command from the printer, the pressure of the ink in the print cartridge  14  drops. In this embodiment the differential pressure across the cartridge goes more negative than minus three inches (−3″) of water. The diaphragm  52  becomes more concave due to differential pressure between atmospheric pressure in the vent  53  and the pressure in the housing  82 . This drop in pressure continues until the piston  75 , FIG. 5, bottoms out against the lever  62  and then the diaphragm forces the piston to move the lever and to open the orifice  49  as illustrated in FIG.  5 . This is rotational motion of the lever  62  around the axle  64 , FIG.  5 . The point at which the orifice  49  opens is the “cracking pressure” and is determined by the pressure setting spring  78 . Ink then flows into the print cartridge  14 , the pressure in the print cartridge is restored, and any air is collected in the area  98 . When the differential pressure across the diaphragm  52  decreases due to the inflow of the ink, the piston  75  allows the lever to shut the orifice  49  and the flow of ink into the print cartridge stops. 
     If the temperature of the print cartridge goes up due, for example, to operation of the print head, this could cause either the pressure of the ink in the housing  82  to rise or the volume of air to increase. As discussed above, a wall portion of the ink containment moves to accommodate this increase in temperature. The diaphragm  52  flexes upward as illustrated in FIG.  6  and becomes more planar to maintain the pressure within the housing constant. If there is a decrease in temperature, the diaphragm flexes downward and becomes more concave to maintain constant pressure. This is relative motion between the piston  75  and the lever  62  and is permitted by the accumulator spring  74 . The lever  62  is remains stationary and is unaffected by such temperature excursions. 
     To remove any air from the top area  98  of the housing  82 , the print cartridge  14  is purged using the service station  40 . Referring to FIGS. 7 and 8, a vacuum  44  is applied to the nozzles of the print head  86  and a very high flow rate is induced through the print cartridge. Any air in the print cartridge is drawn down the snorkel  95  as illustrated in FIG. 7 instead of the flow orifice  94  because of the small size of the flow orifice and the large pressure drop across it. The volume of air drawn down the snorkel and out of the housing is replaced by a fluid volume of ink because the differential pressure in the housing drops and the orifice  49  opens as illustrated in FIG.  8 . The result is to rapidly prime the print cartridge with ink and to remove the air from the system. 
     Although specific embodiments of the invention have been described and illustrated, the invention is not limited to the specific forms or arrangement of parts so described and illustrated herein. Referring to FIG. 9 and 10 it is contemplated that the diaphragm  52  could be replaced by a piston  102  sliding reciprocally in a cylinder  104  or a bellows  106  urged in a direction to maintain the ink at a substantially constant pressure. The invention is limited only by the claims.