Patent Publication Number: US-6908180-B2

Title: Ink delivery apparatus for inkjet printhead

Description:
FIELD OF THE INVENTION 
   The invention generally relates to inkjet printing, and more particularly to an ink delivery apparatus for an inkjet printhead. 
   BACKGROUND OF THE INVENTION 
   Inkjet printers can be divided into two major categories, commonly referred to as continuous inkjet and drop-on-demand (DOD) ink-jet. 
   In DOD inkjet printers, the ink delivery apparatus for an ink-jet printhead delivers the ink to the printhead at a slight vacuum or negative pressure known as a “back pressure”. The slight negative pressure in the printhead is desired because it prevents the ink from leaking, i.e. drooling, out of closely spaced ink discharge nozzles in the printhead, by tending to draw the ink at the nozzles back into the printhead. Moreover, it forms a slightly concave ink meniscus at each nozzle which helps to keep the nozzle clean. Typically, as stated in prior art U.S. Pat. No. 5,650,811 issued Jul. 22, 1997, the slight negative pressure in the printhead may be approximately two to three inches of water below atmospheric pressure. The patent also states that the slight negative pressure can be created by positioning an ink reservoir for the printhead below the printhead. Alternatively, the slight negative pressure can be created by using a nonlinear spring to pull a membrane bladder outward at an opening in an ink reservoir above the printhead. This latter approach is described in detail in U.S. Pat. No. 4,509,062 issued Apr. 2, 1985. 
   Today, most DOD inkjet printheads have an “onboard” ink reservoir. In other words, the ink reservoir is fixed atop the printhead and moves with it during the printing operation. Often, as disclosed in prior art U.S. Pat. No. 5,975,689 issued Nov. 2, 1999 and U.S. Pat. No. 5,872,584 issued Feb. 16, 1999, the negative pressure regulator is coupled with an ink replenishment mechanism, both of which are located inside the ink reservoir. However, a problem that can occur with locating the negative pressure regulator and ink replenishment mechanism inside the reservoir is that it is subject to corrosion and chemical attack by the ink inside the reservoir. Also, the mechanism may contaminate the ink. 
   SUMMARY OF THE INVENTION 
   An ink delivery apparatus for an inkjet printhead, comprising an ink reservoir for delivering some ink at a slight negative pressure to a printhead, and a pressure regulator and ink replenishment mechanism that maintains substantially constant negative pressure in the reservoir when some ink in the reservoir is delivered to the printhead and in response to the ink delivery replenishes someink to the reservoir from an ink supply source, is characterized in that: 
   the pressure regulator and ink replenishment mechanism includes a pressure regulator member that air-tightly covers an opening in the reservoir and is compliant to maintain substantially constant negative pressure in the reservoir, and a replenishment activator connected to the pressure regulator member outside the reservoir to avoid being exposed to the ink and which initiates ink replenishment to the reservoir when some ink in the reservoir is delivered to the printhead. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1-3  are elevation views, partly in section, of an ink delivery apparatus according to a preferred embodiment of the invention, illustrating operation of the apparatus; and 
       FIG. 4  is an elevation view, partly in section, of an alternate embodiment of the ink delivery apparatus. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention is depicted as embodied in a drop-on-demand (DOD) inkjet printer. Because the features of such a printer are generally known, the description which follows is directed in particular only to those elements forming part of or cooperating with the disclosed embodiment of the invention. It is to be understood, however, that other elements not disclosed may take various forms known to a person of ordinary skill in the art. 
     FIG. 1  shows an ink delivery apparatus  10  for an DOD ink-jet printhead  12 . The ink delivery apparatus  10  includes a closed ink reservoir or ink accumulating chamber  14  fixed atop the printhead  12 . An ink  16  in the reservoir  14  is intended to drain first through a filter  18  and then through a bottom slot  20 , and into the printhead  12 . A slight-vacuum airspace  22 , i.e. one that is slightly below atmospheric pressure, exists above the ink level  24  in the reservoir. This is consistent with the known need to deliver the ink  16  to the printhead  12  at slight negative pressure known as a “back pressure”. Typically, as stated in prior U.S. Pat. No. 5,650,811 issued Jul. 22, 1997, the slight negative pressure in the printhead  12  may be approximately two to three inches of water below atmospheric pressure. The slight negative pressure is desired because it prevents the ink  16  from leaking, i.e. drooling, out of closely spaced ink discharge nozzles (not shown) in a nozzle plate  26  in the printhead  12 , by tending to draw the ink at the nozzles back into the printhead. Moreover, it forms a slightly concave ink meniscus at each nozzle which helps to keep the nozzle clean. 
   A pressure regulator and ink replenishment mechanism  28  maintains substantially constant negative pressure in the reservoir  14  when some ink  16  in the reservoir is delivered to the printhead  12 , and in response to the ink delivery replenishes some ink to the reservoir from a pressurized ink supply source  30 . This an be seen by comparing  FIGS. 1 and 2 . 
   The pressure regulator and ink replenishment mechanism  28  includes a pressure regulator membrane or diaphragm  32  that air-tightly covers an opening  34  in the reservoir  14 . The pressure regulator membrane  32  is compliant in order to maintain substantially constant negative pressure in the reservoir  14  by deforming inwardly as shown in  FIG. 2 , when some ink  16  is delivered to the printhead, and by returning outwardly as shown in  FIG. 1 , when some ink is replenished to the reservoir. Also, there is included a replenishment activator  36  that is connected to the pressure regulator membrane  32  outside the reservoir  14  to avoid being exposed to the ink  14 . The replenishment activator  36  initiates ink replenishment to the reservoir  14  when the pressure regulator membrane  32  deforms inwardly as shown in FIG.  2  and terminates ink replenishment when the pressure regulator membrane returns outwardly as shown in FIG.  1 . 
   An ink conduit  38 , such as a tube, longitudinally extends from the pressurized ink source  30  into the reservoir  14 . The ink conduit  38  empties into the reservoir  14  as shown in FIG.  2 . 
   The replenishment activator  36  includes a valve membrane or diaphragm  40  that air-tightly covers an opening  42  in the reservoir  14 . The valve membrane  40  is compliant to normally cap or close the ink conduit  38  substantially at the opening  42  as shown in  FIG. 1 , to prevent ink replenishment to the reservoir, and to be stretched to uncap or open the ink conduit as shown in  FIG. 2 , to allow ink replenishment to the reservoir. The replenishment activator  36  includes a rocker lever  44  that is pivotally mounted via a pivot pin  46  on the reservoir  14  and has a lever portion  48  connected to the pressure regulator membrane  32  and a lever portion  50  connected to the valve membrane  40 . The rocker lever  44  thus interconnects the pressure regulator membrane  32  and the valve membrane  40  to pivot clockwise about the pivot pin  46  as shown in  FIG. 2 , to cause the valve membrane to uncap the ink conduit  38  when the pressure regulator membrane deforms inwardly, and to pivot counterclockwise about the pivot pin as shown in  FIG. 1 , to recap the ink conduit when the pressure regulator membrane returns outwardly. 
   A spring  52  applies a counterclockwise pivoting force in  FIG. 1  to the rocker lever  44  that causes the end portion  50  of the rocker lever which is connected to the valve membrane  40  to hold the valve membrane capping the ink conduit  30 . The pivoting force is light enough to be overcome when the pressure regulator membrane  32  deforms inwardly as shown in FIG.  2 . 
   The portion  48  of the rocker lever  44  that is connected to the pressure regulator membrane  32  is actually the free end of a cantilevered leaf spring  54  whose other end  56  is anchored to a rigid rocker arm  58 . The rocker arm  58  has a free end that is the portion  50  of the rocker lever which is connected to the valve diaphragm  40 . As shown in  FIG. 3 , the leaf spring  54  permits the portion  48  to be deformed away from the rocker arm  58 , without pivoting the rocker lever  44  about the pivot pin  46 , should the pressure regulator membrane  32  be further deformed outwardly (beyond its outward deformation in  FIG. 1 ) when the valve membrane  40  is capping the ink conduit  38 . The pressure regulator membrane  32  might further deform outwardly in this instance due to a slight increase in the atmospheric pressure or a heating of the ink  16  in the reservoir  14 . 
     FIG. 4  shows an embodiment of the ink delivery apparatus  10  which is a modified version of the one shown in  FIGS. 1-3 . In  FIG. 4 , elements that are the same as in  FIGS. 1-3  have the same reference numbers. One distinction however is that the spring  52  in  FIG. 4  is a helical tension spring, whereas the spring  52  in  FIGS. 1-3  is a helical compression spring. 
   The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
   Parts List 
   
       
         10 . ink delivery apparatus 
         12 . inkjet printhead 
         14 . ink reservoir 
         16 . ink 
         18 . filter 
         20 . bottom slot 
         22 . airspace 
         24 . ink level 
         26 . nozzle plate 
         28 . pressure regulator and ink replenishment mechanism 
         30 . ink supply source 
         32 . pressure regulator membrane 
         34 . opening 
         36 . replenishment activator 
         38 . ink conduit 
         40 . valve membrane 
         42 . opening 
         44 . rocker lever 
         46 . pivot pin 
         48 . lever portion 
         50 . lever portion 
         52 . spring 
         54 . leaf spring 
         56 . spring end 
         58 . rocker arm