Abstract:
A method for wiping a printhead of an inkjet printing mechanism is provided. During a wiping process, firstly, a first amount of wiping force is exerted on the printhead for wiping the printhead in a first direction. Subsequently, a smaller amount of wiping force is exerted on the printhead for wiping the printhead in a second direction opposite to the first direction. The potential damages on the printhead by a dry wiping can thus be decreased by controlling the wiping force on the printhead.

Description:
BACKGROUND 
     This invention relates generally to inkjet printing mechanisms, and in particular to techniques for maintaining inkjet printhead at its optimal conditions. 
     Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as “ink,” onto a media sheet. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the media sheet, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezoelectric or thermal printhead technology. 
     To clean and protect the printhead, typically a “wiper assembly” mechanism is mounted within the housing of the printing mechanism so 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. 
     Normally, a wiping sequence includes a forward and a backward wiping stroke. During the forward stroke, a wiper blade of the wiper assembly moves from its home position, which is in front of the printhead in a media advancement direction, towards the other end of the printhead until it has passed the other end for wiping off ink residues on the printhead. After the forward stroke, the wiper blade moves from a position behind the printhead in the media advancement direction to its home position during the backward stroke; wiping also occurs during such a backward stroke. 
     The wiper blade normally has two sides at its wiping end for wiping the printhead during the forward and backward strokes respectively. During the forward stroke, most ink residues on the printhead are wiped off, and one side of the wiper blade&#39;s wiping end becomes wet due to the ink residues. However, the other side of the wiper blade&#39;s wiping end remains dry due to the fact that it is not in contact with the inks on the printhead during the forward stroke. Therefore, when the other side of the wiping end wipes the printhead during the backward stroke, a dry wiping of the printhead occurs if no other fluids are used to moisten the wiper blade. Such a dry wiping of the printhead may not be desirable in that it may increase the risk of damaging the nozzles on the printhead and the wiper blade itself. 
     Solutions have been introduced to solve such a problem. For example, some fluids can be used to wet the wiper blade before the backward stroke starts. However, such a solution can be relatively complicated because more parts and more complicated coordination between these parts are required to wet the wiper blade. 
     Therefore, there is a need for an improved printhead wiping mechanism which reduces the risk of damaging the printhead during the backward stroke of a wiping process more conveniently. 
     SUMMARY 
     According to an aspect of the present invention, a method for wiping a printhead of an inkjet printing mechanism is provided. During a wiping process, firstly, a first amount of wiping force is exerted on the printhead for wiping the printhead in a first direction. Subsequently, a smaller amount of wiping force is exerted on the printhead for wiping the printhead in a second direction opposite to the first direction. The potential damages on the printhead by a dry wiping can thus be decreased by controlling the wiping force on the printhead. 
     According to a second aspect of the invention, a wiper assembly is provided for wiping a printhead of an inkjet printing mechanism having a chassis, with the printhead on a carriage supported by the chassis for moving to a wiping position. The wiper assembly includes a guide track mounted to the chassis, a platform movable along the track in a forward and a backward direction during a forward and a backward wiping stroke respectively, and a wiper blade mounted on the platform for wiping the printhead when the printhead is in the wiping position. A character of the wiper blade can be adjusted so that the wiper blade exerts different amounts of wiping forces on the printhead during the forward and the backward strokes. 
    
    
     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 fragmented, partially schematic, perspective view of one form of an inkjet printing mechanism in the prior art; 
     FIG. 2 is a perspective view illustrating an exemplary embodiment of the present invention of a wiper assembly that can be used in the printing mechanism of FIG. 1; and 
     FIGS. 3 and 4 are side views illustrating wiping of the printhead during different strokes by using the wiper assembly of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     For convenience, the concepts of the present invention are illustrated in the environment of an inkjet printer  100 , while it is understood that the present invention as illustrated by the exemplary embodiment can also be used in other inkjet printing mechanisms such as facsimile machines and copiers. 
     The typical inkjet printer  100  includes a chassis  102  surrounded by a housing or casing enclosure  104 . Sheets of print media (not shown) for example paper are fed through a print zone  106  within which images are imprinted onto the media sheets. 
     The printer  100  also has a printer controller, illustrated schematically as a microprocessor  120 , that receives instructions from a host device, typically a computer, such as a personal computer (not shown), and manages different operations of different components of the printer  100 . 
     A carriage guide rod  116  is supported by the chassis  102  to slidably support an inkjet carriage  122  for travel back and forth across the print zone  106  along a scanning axis  118  defined by the guide rod  116 . To provide carriage positional feedback information to printer controller  120 , an optical encoder reader (not shown) can be mounted to carriage  122  to read an encoder strip extending along the path of carriage travel. 
     The carriage  122  is also propelled along guide rod  116  into a servicing region, as indicated generally by arrow  114 , located within the interior of the casing  104 . The servicing region  114  houses a service station  112 , which may provide various conventional printhead servicing functions as generally understood in the art. 
     In the print zone  106 , the media sheet receives ink from an inkjet cartridge  108  on the carriage. The cartridge  108  is also often called a “pen” by those in the art. The illustrated pen  108  includes a reservoir (not shown) for storing a supply of ink. The pen  108  also has a printhead  110 , which has an orifice plate with a plurality of nozzles formed therethrough in a manner well-known to those skilled in the art. The illustrated printhead  110  is a thermal inkjet printhead, although other types of printheads may be used, such as piezoelectric printheads. 
     In the present application, only part of the pen servicing functions is discussed, namely, wiping of the printhead  110 . It is understood that a wiping mechanism can be incorporated in the service station  112  as illustrated in U.S. Pat. No. 6,132,026, assigned to the present assignee, Hewlett-Packard Company. Alternatively, as shown in the exemplary embodiment of the present invention illustrated by FIGS. 2-4, the wiping mechanism can be separated from the service station  112  and mounted on the chassis  102  alone. 
     A wiper assembly  200 , as illustrated in FIG. 2, is mounted on the chassis  102  and beneath the pen  108  when the pen  108  is in a position for wiping (see FIG.  1 ). The wiper assembly  200  has a pair of flexible wiper blades  204  mounted on a platform  206  for wiping a printhead, illustrated as the printhead  110  in FIG.  1 . The wiping occurs when a rack  220  connected to the platform  206  slides along a slot (not shown) defined within a base frame  210  of the wiper assembly. The rack  220  is driven back and forth along the slot by a rotatable wiper gear (not shown), which engages a plurality of engaging teeth  222  on the rack  220 . Furthermore, the wiper gear is rotated by a motor (not shown) in the printer through a gear train (not shown) therebetween as generally understood in the art. In addition, both the slot and the rack  220  extend in a direction substantially parallel to the direction in which the nozzles (not shown) of the printhead  110  are aligned. Such a direction is substantially parallel to the media advancement direction  234 , in which the media sheet is advanced through the print zone  106  (FIG. 1) during printing operations. 
     A pair of frame walls  208  respectively located at two sides of the base  210  project upward in the exemplary embodiment of the present invention and extend horizontally in a direction substantially parallel to the rack  220 . Each wall  208  has an opening or guide track  212  formed therein by an upper layer  226  and a lower layer  228  spaced from each other; each layer extends parallel to the media advancement direction  234  and has an inner surface  230 ,  232  respectively facing each other. On the other hand, the platform  206  has two projections  202  at two respective sides for fitting into the opening  212 . In this way, the platform  206  is restricted to slide along the guide tracks  212  during the wiping process. 
     Furthermore, the rack  220  has a support  216 , which extends upward and is mounted on the rack  220  at an end away from the engaging teeth  222 . A pivot arm  214  at an end of the platform  206  fits into a pivot slot  224  at an end of the support such that the platform  206  is rotatably mounted to the support  216 . In this way, when the rack  220  slides back and forth along the slot (not shown), the platform  206  moves accordingly as driven by the support  216 . 
     In addition, the openings or guide tracks  212  have a width slightly larger than the diameter of the projections  202  so that the platform is able to rotate slightly in an upward or a downward direction about an axis  218  passing through the center of the pivot arm. Such a slight rotation of the platform about the axis  218  allows the projections  202  to contact different inner surfaces of the upper and lower layers during different wiping strokes, which will be discussed in more details with reference to FIGS. 3 and 4. 
     For the purpose of this application, a forward stroke of a wiping process in the exemplary embodiment is defined as the wiping of the printhead when the wiper assembly moves from its home position in front of the printhead to a position behind the printhead in the media advance direction  24 . Such a forward stroke wiping is to wipe off ink residues on the printhead. A backward stroke occurs subsequent to the forward stroke, and during the backward stroke, the wiper assembly moves from the position behind the printhead back to its home position. 
     It is understood that the wiping force on the printhead by the wiper blade is substantially affected by the pressure on the printhead by the wiper blade. Furthermore, the pressure is mainly affected by the interference between the wiper blade and the printhead. The exemplary embodiment allows such an interference to be adjusted during the forward and backward stroke respectively so that the wiping force on the printhead can also be adjusted. 
     During the forward stroke as shown in FIG. 3, due to the interaction between the wiper blade  204  and the printhead  110 , the printhead  110  exerts a force on the wiper blade  204  in a direction opposite to the direction in which the wiper assembly moves. It is noted that the guide tracks  212  have a width slightly larger than the diameter of the projections  202  and the projections are allowed to move upward or downward slightly within the boundary of the guide tracks  212 . It is further noted that during the forward stroke, the wiper assembly moves in a direction opposite to the media advancement direction  234  and the wiper blade  204  is located behind the pivot arm  214  about which the platform rotates. Therefore during the forward stroke, the force on the wiper blade exerted by the printhead  110  drives the platform  206  upward until the projections  202  reach the inner surfaces  230  of the upper layers  226 . In this way, the wiper blade  204  interacts with the printhead  110  with maximum interference during the forward stroke. As a result, the wiper blade  204  exerts a maximum wiping force on the printhead  110  during the forward stroke. 
     During the backward stroke as shown in FIG. 4, however, the force on the wiper blade  204  exerted by the printhead  110  drives the platform  206  downward until the projections  202  reach the inner surfaces  232  of the lower layers  228 . In this way, the wiper blade  204  interacts with the printhead  110  with a minimum interference during the backward stroke and exerts a minimum wiping force on the printhead. 
     The exemplary embodiment adjusts the wiping force by mechanically adjusting the interference between the wiper blade and the printhead. Alternatives can be made. For example, it is noted that the wiper blade  204  contacts the printhead at different sides of the wiping end during the forward and backward strokes respectively. Therefore, the wiper blade can also exert different wiping forces on the printhead during different strokes if the two sides of wiping end have different friction coefficients. Such a design can be achieved by, for example, using different materials to form the different sides of the wiping end. In that case, the mechanical adjustment of the interference as discussed above is not needed.