Abstract:
The present invention is an ink supply for use in an ink jet printing system of the type having an ink jet printhead spaced from the ink supply. The ink jet printing system has an ink conduit configured for connection to each of the printhead and the ink supply for providing ink to the ink jet printhead. The ink supply includes an ink container for storing ink and a fine mesh disposed in a fluid path between the ink container and the ink conduit. The fine mesh has a mesh opening size which does not permit air to pass therethrough under normal nominal air bubble pressure experienced by the ink jet printing system in normal usage and storage.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to ink jet printers, and more particularly, to printing systems which employ off axis ink supplies connected to a carriage mounted printhead via tubing.  
           [0002]    Ink jet printers are well known in the art, and many utilize a carriage which carries one or more inkjet cartridges. These carriages typically carry the printheads in a traversing or scanning movement, transverse to the printer paper path. It is also well known to provide an external stationary ink reservoir connected to the scanning cartridge via a tube. The external reservoir is typically known as an “off axis” ink reservoir. While providing increased ink capacity, these off axis systems present a number of problems. One problem is that of vapor loses from the tubing and air diffusion into the tubing system. A tube material that has been used is LDEE (low density polyethylene), since it is a low modulus material which is easy to bend. This low modulus material suffers from relatively high vapor losses and air diffusion into the tube. As a result of the vapor losses, the ink can change properties, degrading print quality and eventually causing tube or printhead clogging. Another problem with air diffusion into the tubing is that the printhead can ingest this air as ink is drawn from the external reservoir. As a result of air ingestion, the printhead can fill with air. During thermal fluctuations, the air can expand, causing printhead drool.  
           [0003]    Another problem relating to printhead air ingestion is that this ingested air in the printhead can cause printhead starvation. Printhead starvation results when air enters a bubble chamber and displacing ink, reducing the ink volume in the bubble chamber. As a heating element is heated to form a vapor bubble to eject ink from the bubble chamber the volume of ink ejected is reduced by the air in the chamber, reducing the quality of the output image. In addition, the reduced volume of ink ejected reduces the cooling of the heating element tending to reduce the lifetime of the printhead.  
           [0004]    Air enters the tube connecting the reservoir with the printhead in two predominant ways. The first is air from the external reservoir can enter the tube. Air enters the external reservoir either through diffusion into this reservoir or during the filling process of the external reservoir air may become entrapped within the reservoir. As ink is drawn from the external reservoir the entrapped air within the reservoir is drawn into the tube. A second way in which air enters the tube is through diffusion of air from outside of the tube to the inside of the tube. Once air is present within the tube any increases in air within the tube produces an increase in the diffusion rate of air through the tube material, further exacerbating the problem of air ingestion in the printhead.  
           [0005]    U.S. Pat. No. 5,426,459 to Kaplinski, assigned to the assignee of the present invention, incorporated herein by reference discloses the use of a section of finely woven stainless steel mesh as a combined filter and air check valve for use in an “on axis” type print cartridge. An on axis print cartridge makes use of a printhead which is integrated with an ink reservoir. Therefore, an external tube is not required to fluidly connect the printhead and the reservoir. The air check valve is provided in the fluid path between the printhead and the ink reservoir to prevent air bubbles from traveling from the printhead into the reservoir. The valve also serves the function of a filter to prevent particulate contaminants from flowing from the ink reservoir into the printhead and clogging the printhead nozzles. The Kaplinski reference deals with the problem of leakage of air bubbles into the ink reservoir which equalizes the pressure on the ink in the reservoir reducing the negative pressure which is required to prevent the printhead from drooling when the printhead is subject to minor shocks during handling or operation. The Kaplinski reference does not deal with an off axis type printing system and therefore does not recognize the problem of introduction of air into the printing system via an external ink supply or the problem of air diffusion into the tube connecting the external reservoir with the printhead.  
           [0006]    There is an ever present need of techniques for preventing the introduction of air into the printhead via the external reservoir in off axis printing systems. This technique should be a reliable way of preventing air ingestion by the printhead which reduces the printhead life. In addition, this technique should be relatively inexpensive and well suited to the manufacturing environment to reduce manufacturing costs of both the external reservoir as well as the off axis printing system.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is an ink supply for use in an ink jet printing system of the type having an ink jet printhead spaced from the ink supply. The ink jet printing system has an ink conduit configured for connection to each of the printhead and the ink supply for providing ink to the ink jet printhead. The ink supply includes an ink container for storing ink and a fine mesh disposed in a fluid path between the ink container and the ink conduit. The fine mesh has a mesh opening size which does not permit air to pass therethrough under normal nominal air bubble pressure experienced by the ink jet printing system in normal usage and storage. In one preferred embodiment the mesh is a wire mesh having a mesh size in the range from 10 microns to 100 microns. In this preferred embodiment the mesh is positioned within the ink container. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a simplified schematic diagram of an ink jet printhead connected via a length of tubing to an off axis ink reservoir, with an air bubble in the off axis ink reservoir to illustrate the air introduction into the printing system problem addressed by the present invention.  
         [0009]    [0009]FIG. 2 is a simplified schematic diagram of the external reservoir which includes a fitment having the bubble screen of the present invention attached thereto and with a flaccid bag partially shown with dotted lines.  
         [0010]    [0010]FIG. 3 shows the fitment of FIG. 2 in section, taken across a plane defined by A-A′, shown in perspective, with an air bubble positioned in front of the bubble screen.  
         [0011]    [0011]FIG. 4 shows a representation of the bubble screen of the present invention as viewed in the direction of fluid flow through the fitment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]    [0012]FIG. 1 is a simplified representation of an off axis printing system  10  which makes use of an off axis or external ink reservoir. The printing system  10  includes a printhead  12 , an external reservoir  14  and a tube or conduit  16  fluidically connecting the printhead  12  with the external reservoir  14 . A scanning carriage (not shown) moves the printhead  12  as ink droplets are selectively ejected from the printhead  12  onto print media such as paper. Under normal conditions the printhead  12  is under a slight negative pressure which is used to draw ink from the external reservoir through tube  16  to the printhead  12 . The external reservoir  14  includes a bubble screen  18  which is the subject of the present invention for preventing air, represented by bubble  20 , trapped within the external reservoir  14  from entering the tube  16  and printhead  12 . As air increases within the tube  16  the diffusion rate of air through the tube  16  from outside of the tube  16  tends to increase. This air within the tube  16  is drawn into the printhead  12  which can result in air ingestion in the printhead which can result in printhead overheating reducing the printhead life. In addition, air ingestion into the printhead  12  can result in printhead  12  drool due to thermal fluctuations or air pressure changes.  
         [0013]    The external reservoir  14  includes a flaccid bag  22  which is attached to a fitment  24 . The fitment  24  includes a fluid interconnect  26  for connecting to a corresponding fluid interconnect (not shown) attached to the tubing  16 . The fitment  24  and the corresponding fluid interconnects allow the external reservoir  14  to be replaced when the fluid within the external reservoir  14  is exhausted.  
         [0014]    The bubble screen  18  of the present invention prevents air within the external reservoir  14  for entering either the tubing  16  or the printhead  12 . Air which enters tubing  16  forms a bubble referred to as a “seed bubble”. The larger the seed bubble or area in contact with the tube  16  wall, the larger the diffusion rate of air into the tube  16 . Both the air entering tube  16  from the external reservoir  14  and the air which diffuses into the tube  16  from the outside is drawn into the printhead  12  which can result in drooling problems as well as a reduction in the printhead  12  life.  
         [0015]    Once air is present in the tube  16  which extends between the printhead  12  and the external reservoir  14 , then further air diffusion into the tube  16  becomes a greater problem. The pressure of the outside atmosphere (outside the tube), the total pressure within the bag  22 , and the total bubble pressure are equalized (assume they are level and static), as represented by the following equation:  
         P tot,tube =P tot,bag =P tot,outside    
         [0016]    Where P tot,tube  represents the total pressure in the tube  16 , P tot,bag  represents the total pressure in the bag  22  and P tot,outside  represents the total pressure outside the bag  22  and tube  16 . The total pressure is equal to air (primarily oxygen and nitrogen, not counting vapors) pressure plus partial pressure of vapor, as represented by the following:  
         
       P 
       tot,tube 
       =P 
       air,tube 
       +P 
       vapor,tube 
       =P 
       air,outside 
       +P 
       vapor,outside  
     
         [0017]    Where P air,tube  is the air pressure in the tube  16 , P vapor,tube  is the partial pressure of vapor in the tube  16 , P air,outside  is the pressure of air outside and P vapor,outside  is the partial pressure of vapor outside the tube  16 . Therefore, rearranging the above yields the following equation:  
         ( P   air,outside   −P   air,tube )=(P vapor,tube   −P   vapor,outside )  
         [0018]    The vapor air in the tube  16  is fully saturated. However, the pressure of vapor outside may vary. Air will tend to diffuse through the tube material toward in the direction of highest pressure of vapor. For example, in Arizona the vapor pressure may be very low. In Florida, it would be typically very high. In dry environments, such as Arizona, the diffusion rate of air from outside the tube  16  into the tube  16  can be very high.  
         [0019]    With low performance tubing materials, the diffusion rate of air into the tubing  16  is further increased. In addition, the more air within the tube  16  the greater the rate of diffusion of air into the tube  16  from outside of the tube further increasing the air entering the printhead  12 . Therefore, it is important that air be prevented from entering the tube  16  to limit air ingestion by the printhead  12 .  
         [0020]    [0020]FIG. 2 shows a greatly enlarged view of the fitment  24  having the bubble screen  18  of the present invention mounted therein with the bag  22  partially shown with dotted lines. The fitment  24  includes a fluid interconnect  26  for fluidly connecting the external reservoir  14  with a fluid interconnect (not shown) attached to the tube  16 . This fluid interconnect  26  allows the flow of fluid from bag  22  to tube  16  and then into printhead  12 . The fluid interconnect  26  allows fluid to flow from the external reservoir  14  only when properly connected to the corresponding fluid interconnect associated with tube  16 . In one preferred embodiment the fluid interconnect associated with the tube  16  is a needle valve and the fluid interconnect  26  associated with the external reservoir  14  is a septum and popit valve. The use of the fluid interconnect  26  on the external reservoir  14  allows the external reservoir  14  to be handled and stored without ink spillage as well as limit or prevent the introduction of air into the external reservoir  14 .  
         [0021]    The bag  22  is attached to the fitment to form a hermetic seal for preventing ink leakage. The hermetic seal between the fitment  24  and the bag  22  may be formed by welding, bonding with adhesives or some conventional technique.  
         [0022]    [0022]FIG. 3 is a section of the fitment  24  taken across a plane defined by A-A′, shown in perspective. The fluid interconnect  26  details are not shown in FIG. 3 for simplicity. The bubble screen  18  of the present invention is positioned on the fitment  24  to extend across the fluid path  30  within the fitment  24 . The bubble screen  18  is shown in more detail in FIG. 4. The bubble screen  18  prevents bubbles such as bubble  20  from passing through the fluid path  30  and into the tube  16 .  
         [0023]    Negative pressure on the ink within the external reservoir  14  will tend to draw ink as well as any entrapped air bubbles such as bubble  20  through the fitment  24  into the tube  16  and through the printhead  12 . The bubble screen  18  is a fine mesh having an opening size which does not permit air bubbles to pass therethrough under normal air bubble pressure experienced by the printhead  12  in the normal usage or storage.  
         [0024]    In one preferred embodiment the bubble screen  18  is a section of finely woven stainless steel mesh, the edges of which are attached to the fitment  24 . The mesh passage size is sufficiently small that while ink may pass through the passages of the mesh, air bubbles under normal atmospheric pressure will not pass through the mesh passages which are wetted by the ink. The required air bubble pressure necessary to permit bubbles to pass through the mesh, in this embodiment, about 30 inches of water, is well above that experienced by the printhead  12  under typical storage, handling or operational conditions. As a result, the mesh serves the function of a bubble screen for preventing air from entering both the tubing  16  and the printhead  12 .  
         [0025]    [0025]FIG. 4 is a view of the screen  18  as viewed from inside the bag  22  looking out through the fluid path  30 . The screen  18  is attached to the inner wall of the fitment  24  for preventing bubbles from passing around the bubble screen  18  and entering the tube  16 . The weave shown in FIG. 4 is only for illustrative purposes and is not to represent the only type of weave suitable for the bubble screen  18 . A wide variety of screen weaves may be suitable for preventing air from passing. One particular weave the was suitable is a twilled dutch weave type mesh.  
         [0026]    In general, the weave size of the screen  18  will depend on ink characteristics within the external reservoir  14 . It is the surface tension which prevents bubbles larger S than the screen mesh from breaking up and passing through the screen  18 . Therefore, changes in surface tension of the ink will require appropriate changes in the bubble screen size to ensure bubbles do not pass through the screen  18 . In addition, the weave size will be dependent on pressure differential across the screen  18 . In general, the greater the pressure differential across the screen  18  the smaller the weave or mesh size required to prevent bubble passage through the screen  18 . In one preferred embodiment the screen size is in the range from 10 microns to 100 microns.  
         [0027]    In the case of a negative pressure printhead  12 , the pressure differential drop across the bubble screen  18  is based on negative pressure created by the printhead  12 . For one type of negative pressure printhead  12  the negative pressure produced by the printhead  12  is below 30 inches of water. If the printhead  12  creates greater negative pressure or if the external reservoir  14  is pressurized, each of which may produce a pressure drop across the bubble screen  18  which is greater than 30 inches of water then a mesh size would be required to prevent bubbles from passing through the mesh and into the tube  16 .  
         [0028]    Although the bubble screen  18  is described as a mesh, a variety of other structures such as a porous material such as Gortex™ having proper hole sizes is also suitable.