Abstract:
A method for wiping a printhead of an inkjet printing mechanism is provided. The printhead surface defines a printhead plane. In the method, firstly a wiper is moved substantially parallel to the printhead plane for wiping a first portion of the printhead surface. After the first portion of printhead surface is wiped, the wiper is moved away from the printhead plane for wiping a second portion of the printhead surface.

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 residue as well as any paper dust or other debris that has collected on the printhead. 
     During the wiping process, there are risks that due to the wiping force, inks removed from the printhead may splash onto a pen-interconnect area where the printhead is electrically connected to the electrical components of the printing mechanism through conventional interconnects. The splash of ink onto the pen-interconnect area may cause an electrical short between the printhead and the interconnects and consequently a failure of the printhead. 
     Therefore, there is a need for an improved printhead wiping mechanism which reduces the risk of splashing ink onto undesired areas during a wiping process. 
     SUMMARY 
     According to an aspect of the present invention, a method for wiping a printhead of an inkjet printing mechanism is provided. The printhead surface defines a printhead plane. In the method, firstly a wiper is moved substantially parallel to the printhead plane for wiping a first portion of the printhead surface. After the first portion of printhead surface is wiped, the wiper is moved away from the printhead plane for wiping a second portion of the printhead surface. 
     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, and with the printhead surface defining a printhead plane. The wiper assembly has a frame which defines a first and second guide tracks connected thereto. The first track extends substantially parallel to the printhead plane, while the second track extends away from the printhead plane. The wiper assembly also has a platform movable along the tracks and a wiper mounted on the platform for wiping the printhead when the printhead is in the wiping position. 
     According to a further aspect of the invention, an inkjet printing mechanism includes a chassis and a printhead on a carriage supported by the chassis for moving to a wiping position. The printhead surface defines a printhead plane. The inkjet printing mechanism further includes a wiper assembly that has a frame which defines a first and second guide tracks connected thereto. The first track extends substantially parallel to the printhead plane, while the second track extends away from the printhead plane. The wiper assembly also has a platform movable along the tracks and a wiper mounted on the platform for wiping the printhead when it is in the wiping position. 
    
    
     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 
     FIG. 3 is a perspective view illustrating in detail a portion of 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  20 , 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. 
     While it is apparent that the printer components may vary from model to model, the typical inkjet printer  20  includes a chassis  22  surrounded by a housing or casing enclosure  24 . Sheets of print media for example paper are fed through a print zone  25  by an adaptive print media handling system  26 . The print media handling system  26  has a feed tray  28  for storing sheets of paper before printing. After printing, the sheet then lands on a pair of retractable output drying wing members  30 , shown extended to receive a printed sheet. The wings  30  momentarily hold the newly printed sheet above any previously printed sheets still drying in an output tray portion  32  before pivotally retracting to the sides, as shown by curved arrows  33 , to drop the newly printed sheet into the output tray  32 . 
     The printer  20  also has a printer controller, illustrated schematically as a microprocessor  36 , that receives instructions from a host device, typically a computer, such as a personal computer (not shown). 
     A carriage guide rod  38  is supported by the chassis  22  to slidably support an inkjet carriage  40  for travel back and forth across the print zone  25  along a scanning axis  42  defined by the guide rod  38 . To provide carriage positional feedback information to printer controller  36 , an optical encoder reader may be mounted to carriage  40  to read an encoder strip extending along the path of carriage travel. 
     The carriage  40  is also propelled along guide rod  38  into a servicing region, as indicated generally by arrow  44 , located within the interior of the casing  24 . The servicing region  44  houses a service station  45 , which may provide various conventional printhead servicing functions as generally understood in the art. 
     In the print zone  25 , the media sheet receives ink from an inkjet cartridge  50  on the carriage. The cartridge  50  is also often called a “pen” by those in the art. The illustrated pen  50  includes a reservoir for storing a supply of ink. The pen  50  also has a printhead  54 , 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  54  is a thermal inkjet printhead, although other types of printheads may be used, such as piezoelectric printheads. The printhead  54  typically includes a substrate layer having a plurality of resistors which are associated with the nozzles. Upon energizing a selected resistor, a bubble of gas is formed to eject a droplet of ink from the nozzle and onto media in the print zone  25 . The printhead resistors are selectively energized in response to enabling or firing command control signals. The control signals may be delivered by a conventional multi-conductor strip (not shown) from the controller  36  to the printhead carriage  40 , and through conventional interconnects (partially shown as  58  in FIG. 2) between the carriage and the pen  50  to the printhead  54 . In particular, the area where the interconnects between the carriage  40  and the pen  50  exist is defined as a pen-interconnect area  59  (see FIG. 2) for the purpose of this application. 
     Preferably, the outer surface of the orifice plate of the printhead  54  lies in a common printhead plane. This printhead plane may be used as a reference plane for establishing a desired media-to-printhead spacing, which is one important component of print quality. Furthermore, this printhead plane may also serve as a servicing reference plane, to which the various appliances of the service station  45  may be adjusted for optimum pen servicing. Proper pen servicing not only enhances print quality, but also prolongs pen life by maintaining the health of the printhead  54 . 
     In the present application, only part of the pen servicing functions is discussed, namely, wiping of the printhead  54 . It is understood that a wiping mechanism can be incorporated in the service station  45  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 and 3, the wiping mechanism can be separated from the service station  45  and mounted on the chassis  22  alone. 
     In FIG. 2, a wiper assembly  60  is mounted on the chassis  22  and beneath the pen  50  when the pen  50  is in a wiping region, as indicated generally by arrow  46 , located within the interior of the casing  24 . The wiper assembly  60  has a pair of flexible wiper blades  74  mounted on a platform  76  for wiping the printhead  54 . The wiping occurs when a rack  72  connected to the platform  76  slides along a slot  63  defined within a base frame  62  of the wiper assembly. The rack is driven back and forth along the slot  63  by a rotatable wiper gear  90 , which engages a plurality of engaging teeth on the rack  72 . Further, the wiper gear  90  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, the slot  63  as well as the rack  72  extends in a direction substantially parallel to the direction in which the nozzles (not shown) of the printhead  54  are aligned as shown by arrow A in FIG.  2 . Such a direction is substantially perpendicular to the scanning axis  42 . 
     A pair of frame walls  64  respectively located at two sides of the base  62  project upward in the exemplary embodiment of the present invention and extend horizontally in a direction substantially parallel to the slot  63  as well as the printhead plane. Each wall  64  has an opening or a guide track  66  formed therein, and the platform  76  has two projections (see FIG. 3) at two respective sides for fitting into the guide tracks  66 . In this way, the platform  76  is restricted to slide along the guide tracks  66  during the wiping process. 
     As shown in FIG. 2, the guide tracks  66  can be divided into two parts, a horizontal part  68  extending substantially parallel to the printhead plane and an incline part  70  extending away from the printhead plane. In the exemplary embodiment, the wiper assembly is located under the printhead  54 , the horizontal part extends horizontally, and the incline part extends downward at a predetermined degree to the horizontal part  68 . Such a degree is mainly determined by the requirements of the wiping force as well as by the availability of space in the casing  24 . 
     In the exemplary embodiment, the two parts  68 ,  70  of the guide tracks are connected at a point  78  within a horizontal coverage of the printhead  54  and close to an end of the printhead adjacent to the pen-interconnect area  59 . Furthermore, a forward stroke of a wiping process in the exemplary embodiment is generated as the wiper assembly moves from a position located at an end of the printhead away from the pen-interconnect area toward the end of the printhead adjacent to the pen-interconnect area, and vice versa. 
     Therefore, in the exemplary embodiment, the platform  76  firstly moves substantially parallel to the printhead plane and then moves downward along the incline part  70  in the forward stroke. In the backward stroke of the wiping process, the platform  76  firstly moves upward along the incline part  70  and then moves substantially parallel to the printhead plane along the horizontal part. 
     It is understood that the flexible wiper blades are bent over by the printhead during the wiping processes, and the interference between the wiper blades  74  and the printhead affects the wiping force exerted on the printhead  54 . In the exemplary embodiment, when the wiper assembly moves along the horizontal part of the guide track, the wiper blades are bent to a certain extent and exert a desired wiping force on the printhead. When the platform travels along the incline part in the forward stroke, the wiper blades remain contact with the printhead for a short period for wiping the remaining part of the printhead surface. However, since the wiper blades  74  move away from the printhead plane when the platform  76  moves along the incline part  70  in the forward stroke, the interference between the wiper blades  74  and the printhead  54  is gradually reduced accordingly. As a result, the wiper blades  74  exert a reduced amount of wiping force on the remaining part of the printhead  54  as compared to when the platform moves along the horizontal part  68 . Since the incline part  70  originates from the connection point  78  within the horizontal coverage of the printhead and is distanced from the pen-interconnect area  59 , such a reduced wiping force consequently reduces the risk of splashing ink into the pen-interconnect area  59 . In addition, the wiper blades  76  moving away from the printhead plane in the forward stroke would cause the ink removed to move in a direction away from the printhead. Such a direction of the ink movements would also help to reduce the risk discussed above. 
     The connection point  78  is designed such that in the forward stroke, the wiper blades  74  wipe a large portion of the nozzles in the printhead  54  when the platform  76  moves along the horizontal part. When the platform moves along the incline part  70  in the forward stroke, the wiper blades  74  wipe only the last few nozzles close to the pen-interconnect area  59 . Therefore, the reduced amount of wiping force is only exerted on the last few nozzles in the forward stroke, and the diminished effect on the whole wiping is relatively negligible. Furthermore, in the backward stroke, the platform  76  climbs up the incline part of the guide track, and the wiper blades  74  wipe the orifice plate of the printhead from the beginning of its edge adjacent to the pen-interconnect area. In this way, the backward stroke of the wiping process compensates the reduced wiping force on the last few nozzles in the forward stroke as described above. 
     FIG. 3 illustrates in detail part of the wiper mechanism, with the wiper gear  90  removed. As shown, the rack  72  has a support  73 , which extends upward and is mounted on the rack  72  at an end away from the wiper gear  90 . A pivot arm  84  at an end of the platform  76  fits into a pivot slot  86  at an end of the support such that the platform  76  is rotatably mounted to the support  73 . Furthermore, a pair of projections  80  at the two sides of the platform  76  fit into the guide track  66  such that the movements of the projections  80  as well as the platform  76  are restricted by the guide track  66 . 
     When the rack  72  slides back and forth along the slot  63 , the platform  76  moves accordingly as driven by the support  73 . It is noted that the platform is rotatable in an upward or a downward direction about an axis  82  passing through the center of the pivot arm. Such a rotation of the platform about the axis  82  allows the projections  80  to move downward in the forward stroke and to move upward in the backward stroke when the projections  80  are on the incline part  70 . As a result, the platform  76 , as well as the wiper blades  74  thereon, moves away from the printhead plane when the projections  86  move along the incline part  70  of the guide track  66  in the forward stroke. 
     Alternatives can be made to the exemplary embodiment. For example, the incline part  70  can be in an arc shape instead of being straight as shown in FIGS. 2 and 3.