Abstract:
An inkjet printhead squeegee is split on one side so that its wiping force on a printhead is highest on the front stroke and much lighter on the backstroke when the surface is dry. Alternatively, the split can be arranged so that the front stroke is light enough to leave the printhead wet, and then the backstroke is much firmer to completely squeegee the printhead dry. The result is high pressure strokes on a dry printhead are avoided.

Description:
BACKGROUND 
   1. Field of the Invention 
   The embodiments of the present invention relate generally to inkjet printing mechanisms, and in particular to squeegees for wiping excess ink off inkjet printheads. 
   2. Background of the Invention 
   Inkjet printheads eject controlled sprays of ink onto a page while printing. Each such printhead has very small nozzles through which drops of various colored ink are fired. To print a typical image, the printhead is moved back and forth across a page, while ejecting patterns of ink drops. Conventional printheads use piezo-electric and thermal printhead technology. For instance, thermal ink ejection mechanisms are shown in U.S. Pat. No. 5,278,584 issued to Brian J. Keefe et al on Jan. 11, 1994 and U.S. Pat. No. 4,683,481, issued to Samuel A. Johnson on Jul. 28, 1987. 
   A wiper assembly mechanism is typically mounted within the housing of the printing mechanism to clean and protect the printhead. The printhead can be moved over the assembly for maintenance, specifically for wiping off ink residues and any paper dust or other debris that have collected on the printhead. 
   A wiping sequence generally includes a forward and a backward wiping stroke. In the forward stroke, a wiper blade of the wiper assembly moves from its home position and across the printhead to scrape off ink residues from the printhead. After the forward stroke, the wiper blade moves back to its home position in the backward stroke and wipes the printhead a second time. 
   In the forward stroke, most ink residues on the printhead are wiped off, and such wets one side of the wiper blade. On the backward stroke, a dry wiping of the printhead occurs if no other fluids are used to moisten the wiper blade. Dry wiping of the printhead can damage the nozzles on the printhead and the wiper blade itself. What is needed is a squeegee and method that reduce or prevent such printhead wear and damage. 
   SUMMARY OF THE INVENTION 
   Briefly, a squeegee embodiment of the present invention includes a set of wiper blades that bend over easier to one side than the other. The wiper blades are arranged in conjunction with an inkjet printhead so that the direction that requires the higher force is the one used when the printhead is wet with excess ink. The easy-to-bend direction is used for the backstroke when the printhead is driest. 
   An advantage of embodiments of the present invention is a squeegee is provided for an inkjet printhead to clean off excess ink. 
   Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which description illustrates by way of example the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective diagram of an inkjet printer embodiment of the present invention; 
       FIG. 2  is a diagram of a wiper assembly embodiment useful in the printer embodiment of  FIG. 1 ; 
       FIGS. 3A and 3B  are side view diagrams that illustrate the wiping of a printhead during forward and back strokes by using a squeegee embodiment of the present invention in the printer of  FIGS. 1 and 2 ; and 
       FIGS. 4A and 4B  are side view diagrams that partially illustrate the wiping of a printhead during forward and back strokes by using another wiper embodiment of the present invention useful in the printer embodiment of FIG.  1 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates an inkjet printer embodiment of the present invention, and is referred to herein by the general reference numeral  100 . The printer  100  is representative of the many kinds of devices that use inkjets and spittoon reservoirs, and that can therefore benefit from embodiments of the present invention. For example, some inkjet-based fax and copier machines are included in alternative embodiments of the present invention. 
   The typical inkjet printer  100  includes a chassis  102  surrounded by a housing or casing enclosure  104 . Sheets of paper are typically fed through a print zone  106  for printing as they pass by an inkjet and carriage assembly. Printer  100  includes an inkjet cartridge  108 , a thermal printhead  110 , a servicing region  112 , a sliding carriage guide rod  114 , a scanning axis  116 , and a printer controller  118  that receives print-job instructions from a host computer. The sliding carriage guide rod  114  mounted on chassis  102  allows an inkjet carriage  120  to slide back and forth across the print zone  106 . The scanning axis  116  is defined by the guide rod  114 . Carriage-position information feedback can be provided to the printer controller  118  by an optical encoder reader mounted to the carriage  120 . Such typically reads an encoder strip that extends along the path of carriage travel. 
   The carriage  120  moves across the guide rod  114  to the servicing region  112  inside the casing  104 . Wipers and drain basins within the servicing region  112  are used to keep the inkjet cartridge clean and disposes of excess ink that is wiped off. 
   After arriving inside the print zone  106 , each sheet of paper is printed with ink squirted from the inkjet cartridge  108 . Such cartridge  108  is sometimes called a “pen” by artisans. The inkjet cartridge  108  includes a supply of ink, a printhead  110  with an orifice plate, and a plurality of nozzles. The printhead  110  illustrated in  FIG. 1  represents a thermal inkjet printhead, although other types of printheads may be used, such as piezoelectric printheads. 
   The outer surface of the orifice plate of the printhead  110  preferably lies in a common printhead plane. In one embodiment, such printhead plane extends substantially horizontally. 
   Here, only some of the pen servicing functions are discussed, e.g., wiping of the printhead  110 . The wiping can be performed by a wiper assembly incorporated in a service station like that illustrated in U.S. Pat. No. 6,132,026, issued to Bret K. Taylor et al on Oct. 17, 2000. Alternatively, the wiper assembly can be mounted independently on the chassis. 
     FIG. 2  represents a wiper assembly  200  which can be separated from a service station and mounted independently on the printer chassis. 
   The wiper assembly  200  is mounted in the servicing region beneath the printhead in a position for wiping. It includes a pair of wiper blades  202  and  203 , e.g., made of ethylene-propylene-diene-monomer (EPDM). Each wiper blade  202  and  203  acts as a squeegee to wipe excess ink off a printhead. Each wiper blade extends vertically up to the printhead plane from a platform  204 . 
   A wiping of the printhead occurs when a rack  206  connected to the platform  204  moves along a slot defined within a base frame  208  of the wiper assembly. The rack  206  is driven back and forth along the slot by a wiper gear which meshes with the rack gear  210 . The wiper gear is turned by a motor in the printer through a gear train. Both the slot and the rack  206  lie parallel to the nozzles of the printhead  110 . Such is substantially parallel to the media advancement direction  212 , in which the media sheet is advanced through the print zone  106  ( FIG. 1 ) during printing operations. 
   A pair of frames  214  and  215  are respectively located at two sides of the base  208 . They project up from and along the rack  206 . A pair of guide track slots  216  and  217  are defined by tops of frames  218  and  219 , and the bottom edges of frames  220  and  221 . On the other hand, the platform  204  has two projections  222  at two respective sides for fitting into the slots  216 ,  217 . In this way, the platform  204  is restricted to slide along the slots during the wiping process. 
   Rack  206  has a support  224 , which extends upward and is mounted on the rack  206  at an end away from the rack gear  210 . A pivot arm  226  at an end of the platform  204  fits into a pivot slot  228  at an end of the support such that the platform  204  is mounted to the support  224 . In this way, when the rack  206  slides back and forth along the slot (not shown), the platform  204  moves accordingly as driven by the support  224 . 
   Each wiper blade  202  and  203  has a slit  234  on one of its side walls  230 ,  232 . Each slit  234  extends substantially parallel to the printhead plane in a widthwise direction as shown in FIG.  2 . Slit  234  on an elastic wiper blade causes the stiffness of the wiper blade to be different during the front and back strokes of the wiping action. The change in wiper blade stiffness puts less pressure on the printhead when the previous stroke has already dried it. 
   In one embodiment, the forward stroke is defined as being the first squeegee stroke across the printhead. The backward stroke returns it to the home position. 
   In the forward stroke represented in  FIG. 3A , the wiper blade  302  exerts a force F 1  on the printhead  304 . This closes up slits  306  and increases the stiffness of the wiper blades  302 . As a result, the wiper blade  302  exerts maximum pressure and a maximum wiping force on the printhead  304 . 
   In the backward stroke represented in  FIG. 3B , the force F 2  on the wiper blade  302  exerted by the printhead  304  widens the opening  308  of each slit  306  on the back wall  310  of the respective wiper blade  302 . In this direction, the wiper blades  302  are not as stiff, so each wiper blade  302  applies a minimum of wiping force on the printhead. 
   In one embodiment of the present invention, each slit  306  runs the full end-to-end width of its respective wiper blades  302 . The depth of each slit is about three-fifths of the thickness of its respective wiper blade. The spacing between each slit  306  and the platform is about four-sevenths of the height of its respective wiper blade. Such depths and spacings control the amount of the wiping force that will be applied on the printhead during the backward stroke. The geometry and placement of the slits and the material of the wiper itself can be used to adjust such backstroke wiping force. 
     FIGS. 4A and 4B  represent another inkjet and wiper assembly embodiment of the present invention, and is referred to herein by the general reference numeral  400 . A printhead  402  moves back and forth in relation to a pair of wipers  404  and  405  mounted to a platform  406 . The wiper assembly embodiment  400  is similar to that of FIG.  2 . Each wiper blade  404  and  405  acts as a squeegee to wipe excess ink off the printhead. A pair of rigid columns or buttresses  408  and  409  are fixedly mounted to the platform  406 . Each buttress  408  and  409  extends substantially parallel to its respective wiper blade but is shorter in height than the wiper blades  404  and  405 . Furthermore, each buttress  408  and  409  is placed behind and in close proximity to its respective wiper blade  404  and  405  in the forward wiping direction of the forward stroke as illustrated by FIG.  4 A. 
   In the forward stroke of  FIG. 4A , each wiper blade  404  and  405  is bent over by the printhead  402  and comes into contact with its respective buttress  408  and  409  due to the wiping direction and the position of the buttresses. The bottom part of each wiper blade  404  and  405  is then restricted from further bent-over by the buttresses  408  and  409 . In this way, the effective stiffness of the wiper blades  404  and  405  is increased during the forward stoke. As a result, the wiper blades  404  and  405  exert a maximum pressure and wiping force on the printhead  402  during its forward stroke. 
   In the backward stroke represented in  FIG. 4B , each buttress  408  and  409  does nothing to restrict the flexing of wiper blades  404  and  405  in a direction opposite to the backward wiping direction  412 . Each wiper blade  404  and  405  flexes away from its corresponding buttress  408  and  409 . Therefore, during the backward stroke, the wiper blades have a relatively low effective stiffness as compared to the forward stroke. In this way, each wiper blade  404  and  405  is manipulated to apply a minimum of wiping force on the printhead  402  during the backward stroke. 
   Alternatives can be made. For example, the buttresses  408  and  409  can be made of flexible materials. Each buttress mainly functions to increase the effective stiffness of its respective wiper blades during the forward stroke by applying an additional force on the wiper blade.