Patent Publication Number: US-6340218-B1

Title: Single-pass wiping system for inkjet printheads

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to inkjet printing mechanisms, and more particularly to a single-pass wiper system that removes ink residue from an inkjet printhead in a more time efficient manner than earlier systems, allowing the printhead to more quickly return to printing which increases the throughput rating of the unit (measured in pages per minute). 
     BACKGROUND OF THE INVENTION 
     Inkjet printing mechanisms use cartridges, often called “pens,” which eject drops of liquid colorant, referred to generally herein as “ink,” onto a page. 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 page, ejecting 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 piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481. In a thermal system, barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). 
     To clean and protect the printhead, typically a “service station” mechanism is supported by the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which substantially seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as “spitting,” with the waste ink being collected in a “spittoon” reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead. The wiping action is usually achieved through relative motion of the printhead and wiper, for instance by moving the printhead across the wiper, by moving the wiper across the printhead, or by moving both the printhead and the wiper. 
     To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment-based inks have been developed. These pigment-based inks have a higher solid content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to form high quality images on readily available and economical plain paper, as well as on recently developed specialty coated papers, transparencies, fabric and other media. Unfortunately, the combination of small nozzles and quick drying ink leaves the printheads susceptible to clogging, not only from dried ink and minute dust particles or paper fibers, but also from the solids within the new inks themselves. Partially or completely blocked nozzles can lead to either missing or misdirected drops on the print media, either of which degrades the print quality. Thus, keeping the nozzle face plate clean becomes even more important when using pigment based inks, because they tend to accumulate more debris than the earlier dye based inks. 
     As the inkjet industry investigates new printhead designs, the tendency is toward using permanent or semi-permanent printheads in what is known in the industry as an “off-axis” printer. In an off-axis system, the printheads carry only a small ink supply across the printzone, with this supply being replenished through tubing that delivers ink from an “off-axis” stationary reservoir placed at a remote stationary location within the printer. There are a variety of advantages associated with these off-axis printing systems, but the permanent or semi-permanent nature of the printheads requires special considerations for servicing, particularly when wiping ink residue from the printheads, which must be done without any appreciable wear that could decrease printhead life. To accomplish this objective, an ink solvent has been used in an off-axis printer, specifically the DeskJet® 2000C Professional Series color inkjet printer, sold by the present assignee Hewlett-Packard Company. In this ink solvent system, a polyethylene glycol (“PEG”) compound is stored in a porous medium such as a plastic or foam block that is in intimate contact with a reservoir, with this porous block having an applicator portion exposed so the elastomeric wiper can contact the applicator. This elastomeric wiper moves across the applicator to collect PEG, which is then wiped across the printhead to dissolve accumulated ink residue and to deposit a non-stick coating of PEG on the printhead face to retard farther collection of ink residue. The PEG fluid also acts as a lubricant, so the rubbing action of the wiper does not unnecessarily wear the printhead. Other wiper systems without a solvent have also been sold by the Hewlett-Packard Company in the DeskJet® 850C, 855C, 870C and 890C models of color inkjet printers. These scraper systems used a rotary tumbler to wipe the printheads. Another solventless wiper scraper system has been sold by the present assignee, the Hewlett-Packard Company, in the DeskJet® 720C, 722C, 710C, 712C, 810C, 812C, 830C, 832C, 880C, 882C, 895C and 970C models of inkjet printers, which used a translating pallet to wipe the wipers across the printheads. 
     All of the Hewlett-Packard Company&#39;s DeskJet® printer models mentioned in the paragraph above used wiper assemblies having the cross sectional configuration shown in FIGS. 4-6 of the drawings. FIG. 4 is a side view of a wiper assembly W at rest. FIGS. 5 and 6 show side views of the wiper assembly W making a two-pass wiping stroke, first to the right in FIG. 5, then to the left in FIG. 6, removing ink residue R from an external surface of an orifice plate of printhead P. The wiper assembly W has a first elastomeric wiper blade W 1  and a second wiper blade W 2  which are mounted to a sled S which moves the blades past the stationary printhead P to wiper the ink residue, Q and other debris from the orifice plate. This earlier dual-blade wiper system is described at length in U.S. Pat. No. 5,614,930, currently assigned to the Hewlett-Packard Company, and was first used in the Hewlett-Packard Company&#39;s DeskJet® 850C color inkjet printer. The 
     DeskJet® model 850C printer employed a revolutionary rotary, orthogonal wiping scheme where the wipers ran along the length of the linear arrays, wicking ink I from one nozzle to the next. This wicked ink I acted as a solvent to break down ink residue accumulated on the nozzle plate. To facilitate this wicking action and subsequent printhead cleaning accomplish this wiping action, the wiper blades W 1  and W 2  have special contours at their tips. The blades W 1  and W 2  are mirror-images of each other, having outboard rounded edges R 1  and R 2 , respectively, and inboard angular wiping edges A 1  and A 2 , respectively. The rounded edges encounter the nozzles first and form a capillary channel between the blade and the orifice plate to wick liquid ink I from the nozzles as the wipers moved orthogonally along the length of the nozzle arrays, as shown for edge R 2  in FIG.  5  and edge R 1  in FIG.  6 . The wicked ink I is pulled by the rounded edges R 1 , R 2  of the leading wiper blade to the next nozzle in the array, where the ink I acts as a solvent to dissolve dried ink residue  9  accumulated on the printhead face plate. The angular edge of the trailing wiper blade then scraps the dissolved residue Q′ from the orifice plate, as shown for edge A 1  in FIG.  5  and edge A 2  in FIG.  6 . The black ink wiper had notches cut in the tip which served as escape passageways for balled-up ink residue to be moved away from the nozzle arrays during the wiping stroke. 
     Unfortunately, the dual bladed wiping assembly W of FIGS. 4-6 required a back-and-forth slewing motion, first in the direction of arrow D 1  (FIG.  5 ), then in the opposite direction of arrow D 2 , to wipe the ink residue and foreign debris. Q, Q′ from the printhead P. The back-and-forth wiping strokes were required to mask any defects in the wiper tip. Since the wiper tips are non-uniform, the redundancy in using two different surfaces for wiping masks the tip imperfections because there is an extremely low likelihood that both blades will have the same imperfections at the same lateral location across their tips. Thus, on each pass, a different wiping edge is used to clean the printhead, specifically, edges R 2  and A 1  when traveling in the D 1  direction in FIG. 5, and edges R 1  and A 2  when traveling in the D 2  direction in FIG.  6 . The problem with this bi-directional wiping scheme is that it severely reduced the printer&#39;s throughput, a printer rating measured in pages per minute, because of the time required to slew the wiper back-and-forth to complete a wiping routine. Especially as the length of printhead nozzle arrays increases, nearing one inch (2.54 centimeters), wiping cycles of over three seconds are anticipated when using this earlier bi-directional wiping scheme. Given the fact that it is desirable to wipe the printhead not only before a printjob, but also periodically during a printjob, this bi-directional operation began to seriously impact desired throughput goals. 
     Thus, a need exists for an inkjet printhead cleaning system which wipes ink residue and ink solvent from the printhead more quickly without impacting the throughput rating of a unit. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a single-pass wiping system is provided for cleaning ink residue from an inkjet printhead in an inkjet printing mechanism. The wiper system includes a platform moveable through a wiping stroke, along with a first wiper blade and a second wiper blade each supported by the platform. A third wiper blade is supported by the platform between the first wiper blade and the second wiper blade to wipe ink residue from the printhead during a unidirectional wiping stroke through contact first with the first wiper blade, followed by contact with the third wiper blade, followed by contact with the second wiper blade. 
     According to another aspect of the present invention, an inkjet printing mechanism is provided with a single-pass wiping system as described above. 
     According to yet another aspect of the present invention, a method is provided for cleaning ink residue from an inkjet printhead in an inkjet printing mechanism, including the step of providing a first wiper blade, a second wiper blade, and a third wiper blade, each supported by the platform, with the third wiper blade located between the first wiper blade and the second wiper blade. In a wiping step, ink residue is wiped from the printhead in a unidirectional wiping stroke by first wiping the printhead with the first wiper blade, followed by wiping the printhead with the third wiper blade, followed by wiping the printhead with the second wiper blade. 
     An overall goal of the present invention is to provide an inkjet printing mechanism which prints sharp vivid images over the life of the printhead and the printing mechanism, particularly when using fast drying pigment or dye-based inks, whether dispensed from an off-axis system or from a replaceable ink cartridge system. 
     Another important goal of the present invention is to provide a single-pass wiper system and method for wiping an inkjet printhead in an inkjet printing mechanism, without seriously impacting the throughput of a unit to provide consumers with a fast inkjet printing mechanism. 
     Still another goal of the present invention is to provide a single-pass wiper system for cleaning printhead wipers in an inkjet printing mechanism which is quieter than earlier systems, and which thus provides consumers with a reliable, quiet inkjet printing unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of one form of an inkjet printing mechanism, here, an inkjet printer, including a printhead service station having one form of a single-pass wiper system-of the present invention for cleaning an inkjet printhead wiper. 
     FIG. 2 is an enlarged side elevational view of the single-pass wiper system of FIG. 1, shown at rest. 
     FIG. 3 is an enlarged side elevational view of the single-pass wiper system of FIGS. 1 and 2, shown cleaning the printhead in a single direction of movement. 
     FIG. 4 is a side elevational view of a prior art dual-bladed, bi-directional wiper system described in the Background section above, shown at rest. 
     FIG. 5 is a side elevational view of the prior art wiper system of FIG. 4, shown cleaning an inkjet printhead in a first direction of movement. 
     FIG. 6 is a elevational view of the prior art wiper system of FIGS. 4 and 5, shown cleaning the inkjet printhead in a second direction of movement, opposite the direction shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     FIG. 1 illustrates an embodiment of an inkjet printing mechanism, here shown as an “off-axis” inkjet printer  20 , constructed in accordance with the present invention, which may be used for printing for business reports, correspondence, desktop publishing, and the like, in an industrial, office, home or other environment. A variety of inkjet printing mechanisms are commercially available. For instance, some of the printing mechanisms that may embody the present invention include plotters, portable printing units, copiers, cameras, video printers, and facsimile machines, to name a few, as well as various combination devices, such as a combination facsimile/printer. For convenience the concepts of the present invention are illustrated in the environment of an inkjet printer  20 . 
     While it is apparent that the printer components may vary from model to model, the typical inkjet printer  20  includes a frame or chassis  22  surrounded by a housing, casing or enclosure  24 , typically of a plastic material. Sheets of print media are fed through a printzone  25  by a media handling system  26 . The print media may be any type of suitable sheet material, such as paper, card-stock, transparencies, photographic paper, fabric, mylar, and the like, but for convenience, the illustrated embodiment is described using paper as the print medium. The media handling system  26  has a feed tray  28  for storing sheets of paper before printing. A series of conventional paper drive rollers driven by a stepper motor and drive gear assembly (not shown), may be used to move the print media from the input supply tray  28 , through the printzone  25 , and after printing, onto a pair of extended output drying wing members  30 , shown in a retracted or rest position in FIG.  1 . The wings  30  momentarily hold a newly printed sheet above any previously printed sheets still drying in an output tray portion  32 , then the wings  30  retract to the sides to drop the newly printed sheet into the output tray  32 . The media handling system  26  may include a series of adjustment mechanisms for accommodating different sizes of print media, including letter, legal, A-4, envelopes, etc., such as a sliding length adjustment lever  34 , a sliding width adjustment lever  36 , and an envelope feed port  38 . 
     The printer  20  also has a printer controller, illustrated schematically as a microprocessor  40 , that receives instructions from a host device, typically a computer, such as a personal computer (not shown). The printer controller  40  may also operate in response to user inputs provided through a key pad  42  located on the exterior of the casing  24 . A monitor coupled to the computer host may be used to display visual information to an operator, such as the printer status or a particular program being run on the host computer. Personal computers, their input devices, such as a keyboard and/or a mouse device, and monitors are all well known to those skilled in the art. 
     A carriage guide rod  44  is supported by the chassis  22  to slideably support an off-axis inkjet pen carriage system  45  for travel back and forth across the printzone  25  along a scanning axis  46 . The carriage  45  is also propelled along guide rod  44  into a servicing region, as indicated generally by arrow  48 , located within the interior of the housing  24 . A conventional carriage drive gear and DC (direct current) motor assembly may be coupled to drive an endless belt (not shown), which may be secured in a conventional manner to the carriage  45 , with the DC motor operating in response to control signals received from the controller  40  to incrementally advance the carriage  45  along guide rod  44  in response to rotation of the DC motor. To provide carriage positional feedback information to printer controller  40 , a conventional encoder strip may extend along the length of the printzone  25  and over the service station area  48 , with a conventional optical encoder reader being mounted on the back surface of printhead carriage  45  to read positional information provided by the encoder strip. The manner of providing positional feedback information via an encoder strip reader may be accomplished in a variety of different ways known to those skilled in the art. 
     In the printzone  25 , a sheet of print media receives ink from an inkjet cartridge, such as a black ink cartridge  50  and three monochrome color ink cartridges  52 ,  54  and  56 , shown schematically in FIG.  2 . The cartridges  50 - 56  are also often called “pens” by those in the art. The black ink pen  50  is illustrated herein as containing a pigment-based ink. While the illustrated color pens  52 - 56  may contain pigment-based inks, for the purposes of illustration, color pens  52 - 56  are described as each containing a dye-based ink of the colors cyan, magenta and yellow, respectively. It is apparent that other types of inks may also be used in pens  50 - 56 , such as paraffin-based inks, as well as hybrid or composite inks having both dye and pigment characteristics. 
     The illustrated pens  50 - 56  each include small reservoirs for storing a supply of ink in what is known as an “off-axis” ink delivery system, which is in contrast to a replaceable cartridge system where each pen has a reservoir that carries the entire ink supply as the printhead reciprocates over the printzone  25  along the scan axis  46 . Hence, the replaceable cartridge system may be considered as an “on-axis” system, whereas systems which store the main ink supply at a stationary location remote from the printzone scanning axis are called “off-axis” systems. Other hybrid systems known as “snapper systems” have replaceable ink reservoirs which snap onto permanent or semi-permanent printheads. All of these different types of printhead systems may be cleaned using the servicing system described below. 
     In the illustrated off-axis printer  20 , ink of each color for each printhead is delivered via a conduit or tubing system  58  from a group of main stationary reservoirs  60 ,  62 ,  64  and  66  to the on-board reservoirs of pens  50 ,  52 ,  54  and  56 , respectively. The stationary or main reservoirs  60 - 66  are replaceable ink supplies stored in a receptacle  68  supported by the printer chassis  22 . Each of pens  50 ,  52 ,  54  and  56  have printheads  70 ,  72 ,  74  and  76 , respectively, which selectively eject ink to from an image on a sheet of media in the printzone  25 . The concepts disclosed herein for cleaning the printheads  70 - 76  apply equally to the totally replaceable inkjet cartridges and snapper systems, as well as to the illustrated off-axis semi-permanent or permanent printheads, although the greatest benefits of the illustrated system may be realized in snapper and off-axis systems where extended printhead life is particularly desirable. 
     The printheads  70 ,  72 ,  74  and  76  each have an orifice plate with a plurality of nozzles formed therethrough in a manner well known to those skilled in the art. The nozzles of each printhead  70 - 76  are typically formed in at least one, but typically two linear arrays along the orifice plate. Thus, the term “linear” as used herein may be interpreted as “nearly linear” or substantially linear, and may include nozzle arrangements slightly offset from one another, for example, in a zigzag arrangement. Each linear array is typically aligned in a longitudinal direction perpendicular to the scanning axis  46 , with the length of each array determining the maximum image swath for a single pass of the printhead. The illustrated printheads  70 - 76  are thermal inkjet printheads, although other types of printheads may be used, such as piezoelectric printheads. The thermal printheads  70 - 76  typically include a plurality of resistors which are associated with the nozzles. Upon energizing a selected resistor, a bubble of gas is formed which ejects a droplet of ink from the nozzle and onto a sheet of paper in the printzone  25  under the nozzle. The printhead resistors are selectively energized in response to firing command control signals delivered by a multi-conductor strip  78  from the controller  40  to the printhead carriage  45 . 
     Single-Pass Wiping Service Station System 
     FIGS. 2 and 3 illustrate one form of a single pass wiper service station system  80 , constructed in accordance with the present invention, including a stationary frame  82  which is supported by the printer chassis  22  in the servicing region  48  within the printer casing  24 . To service printheads  70 - 76  of the pens  50 - 56 , the service station  80  includes a stepper motor and pinion gear assembly  84  coupled to drive a moveable platform or pallet member  85  through engagement with a rack gear  86  located along the underside of the pallet  85 . Here, as the gear of assembly  84  rotates in the direction of curved arrow  87 , the servicing platform  85  is shown as a translationally moving member, moving in a rearward direction as indicated by arrow  88  in FIG. 2, although a rotary platform, or a combination platform having both rotary and translational motion, may also be used. 
     Several wiper blade assemblies, such as wiper blade assembly  90 , may be supported along the upper surface of the pallet  85 . Indeed, preferably platform  85  supports one such wiper assembly for each printhead  70 - 76 , but for the purposes of operational illustration, only the black wiper assembly  90  is shown for cleaning the black printhead  70 . The wiper assembly  90  may be molded from a resilient, non-abrasive, elastomeric material, such as nitrile rubber, ethylene polypropylene diene monomer (EPDM), or other comparable materials known in the art. For the assembly  90 , a suitable durometer, that is, the relative hardness of the elastomer, may be selected from the range of 35-80 on the Shore A scale, or more preferably within the range of 60-80, or even more preferably at a durometer of 70+/−5, which is a standard manufacturing tolerance. 
     The wiper assembly  90  is a tri-blade design, having a pair of outboard wiper blades  92  and  94 , between which is sandwiched a medial or interior wiper blade  95 . The blades  92 ,  94  and  95  may be separately mounted to the pallet  85 , or more preferably, the blades are molded together as a single unit extending upwardly from a base portion  96 . Another desirable component of the single pass wiper service station  80  is a wiper scraper bar  98 , which extends downwardly from an upper portion of the service station frame  82  and into the path of the wiper blades  92 - 95  to scrape ink residue from the blades when passing under the scraper bar  98 . 
     Preferably, the exterior wiper blades  92  and  94  are formed as described in U.S. Pat. No. 5,614,930, which is discussed in the Background section above. The first exterior blade  92  has a rounded outboard wiping edge  100 , and an angular interior wiping edge  102 , while the second exterior blade  94  has a rounded outboard wiping edge  104 , and an angular interior wiping edge  106 . The medial blade  95  has two opposing angular wiping edges  108  and  110 . While the blades  92 - 95  may be all of the same width, one or more may be of a different width. For instance, the medial blade  95  may be wider than the printhead to remove ink and other debris, such as fibers, which may be clinging to the two cheek regions of the printhead located to each side of the nozzle orifice plate. Use of such a wider blade in conjunction with an orifice plate width blade was commercially available in the service station for the DeskJet® 2000C Professional Series color inkjet printer, sold by the present assignee, the Hewlett-Packard Company. 
     FIG. 3 shows the single-pass wiper assembly  90  in operation performing a method of cleaning the black printhead  70  in a single, unidirectional wiping stroke, shown here moving the blades  92 - 95  to the right as indicated by arrow  88 ′. Accumulated ink residue  112  is first encountered by the rounded wiping edge  104  of the leading blade  94 . As described in U.S. Pat. No. 5,614,930, the leading rounded edge  104  wicks ink  114  from the nozzle it is wiping, then moves this wicked ink  114  to the next nozzles encountered in the array. This wicked ink  114  acts as both an ink residue solvent and an orifice plate lubricant. Dissolved ink residue  116 , which passes under the rounded wiping edge  104  of the leading blade  94 , then accumulates between blades  94  and  95 , where it is encountered by the leading edge  110  of the medial blade  95 . Any remaining ink residue  118  which escapes under the medial blade  95  is then wiped away by the angular wiping edge  102  of the trailing blade  92 , leaving the printhead substantially clear of ink residue  112 . 
     Following the wiping stroke of FIG. 3, the wiper blades  92 - 95  pass under the scraper bar  98  to the right in the direction of arrow  88  (FIG. 2) in a scraping stroke. The ink residue  112 - 118  accumulated on the leading surfaces and wiper tips of the blades  92 - 95  encounters the exterior edge of the scraper bar  98  and is flicked off the blades to land within the interior of the service station frame  82 , such as shown for the scraped ink residue  120 . After all of the wiper blades  92 - 95  have passed under the scraper bar  98 , the wiper assembly  90  may be stored at rest within a wiper storage area  122  defined by the service station frame  82 . Upon exiting the storage area  122 , moving in the direction opposite to arrow  88 , any ink residue remaining on the surfaces facing toward the left in the views of FIGS. 2 and 3 of blades  92 - 95  is removed by the interior edge of the scraper bar  98 . 
     After exiting the wiper storage area  122 , the symmetrical nature of the wiper assembly  90  advantageously allows a single-pass wiping stroke in the direction opposite to arrow  88 ′, with blade  92  serving as the leading blade and blade  94  being the trailing blade. In this manner wiping edges  100 ,  108  and  106  of blades  92 ,  94  and  95  are then used to clean the printhead  70 , in the same matter as described above with respect to FIG.  3 . Ink residue collected on the blades  92 - 95  during this reverse wiping stroke is removed by passing the wiper assembly under the scraper bar  98  as described above, to ready the wiper assembly  90  for the next wiping stroke. 
     CONCLUSION 
     A variety of advantages may be realized using the single-pass wiper service station  80 . One of the main advantages of the illustrated service station  80  is the ability to perform a unidirectional wiping stroke that cleans the printheads  70 - 76  with the same efficiency as the earlier dual-bladed wiper assemblies were able to accomplish with a time-consuming bi-directional wiping stroke. It is apparent however, that in some instances, it may be desirable to perform a bi-directional wiping stroke, indeed even a series of bi-directional wiping strokes, such as after a heavy spit routine used to recover clogged or partially clogged nozzles. One such mopping type of wiping routine is described in U.S. Pat. No. 5,614,930, which also describes the earlier dual-bladed wiping system mentioned above. 
     Moreover, while the description above implies that the wiper blades  92 - 95  have wiping edges  100 - 110  which are straight across their entire width, it is apparent to those skilled art that in some implementations it may be preferable to provide the blades, such as the black blades  92 - 95 , with ink residue escape recesses as taught in U.S. Pat. No. 5,614,930. Furthermore, the wiper assembly  90  may also be used with an ink solvent application system, such as that which is commercially available in the DeskJet® 2000C Professional Series color inkjet printer as described in the Background section above. 
     It is apparent that other modifications may be made to the illustrated wiper assembly  90  while still employing the concepts described herein. For instance, while the wiper assembly  90  is shown with the medial blade  95  located centrally between the outboard blades  92  and  94 , in some implementations it may be desirable to locate the medial blade  95  closer to one outboard blade than to the other. Shifting the medial blade  95  closer to one outboard blade may change the wiping characteristics of the strokes in each direction, such as by yielding a more robust wiping stroke in one direction and a gentler stroke in the opposite direction. 
     Another significant advantage of the single-pass wiper service station  80  is that the wiping operation has the potential to be quieter than the wiping operation performed by the earlier dual-bladed wiper assemblies. For instance, in a dual bladed system, the wipers first wipe in one direction, then they must stop, and then reverse direction for the second half of the bi-directional stroke. This stopping and reversing the direction of the pallet carrying the wiper assembly generates motor and gear noises which may be undesirable to some operators. The unidirectional single pass wiping stroke accomplished with the tri-bladed wiper assembly  90  advantageously eliminates the noise associated with stopping and reversing the direction of travel of the pallet during a bi-directional wiping stroke. An additional advantage of the single-pass wiping system  80  is that the wiping control algorithms processed by the controller  40  are simplified, leaving more computational time available for other printing subroutines. 
     Finally, another main advantage of the single-pass wiper service station  80  is the ability to significantly increase the overall throughput of the printer  20  by cutting down the time require to wipe the printheads by about 20% from that required for wiping with the earlier dual-bladed wiping system described in the Background section above. Use of the single-pass wiping system  80  increases the throughput rating, measured in pages per minute, of the printer  20 , while efficiently cleaning the printheads  70 - 76  to maintain high print quality, providing consumers with a more robust, faster printing unit.