Patent Abstract:
A printhead assembly includes an elongate ink distribution assembly defining elongate ink ducts from which ink transfer ports extend. The ink distribution assembly further defines a recess in which a laminated stack structure is received in fluid communication with the ink transfer ports. The laminated stack structure has layers between which ink channels in fluid communication with the ports are interleaved. The laminated stack defines at least one cavity in which respective ink ejection print head integrated circuits (ICs) can be received in fluid communication with the ink channels. The cavity is formed in the laminated stack structure so that the ICs can be disposed at a slight angle to the longitudinal axis of the ink distribution assembly.

Full Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of U.S. application Ser. No. 12/036,910 filed Feb. 25, 2008, which is a continuation of U.S. application Ser. No. 11/707,946 filed on Feb. 20, 2007, now issued U.S. Pat. No. 7,354,208 which is a continuation of U.S. application Ser. No. 10/296,524 filed on Jul. 7, 2003, now issued U.S. Pat. No. 7,210,867, which is a 371 of PCT/AU00/00598 filed on May 24, 2000 all of which are herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The following invention relates to a paper thickness sensor in a printer. 
         [0003]    More particularly, though not exclusively, the invention relates to a paper thickness sensor used for adjusting the space between a printhead and a platen in an A4 pagewidth drop on demand printer capable of printing up to 1600 dpi photographic quality at up to 160 pages per minute. 
         [0004]    The overall design of a printer in which the paper thickness sensor can be utilized revolves around the use of replaceable printhead modules in an array approximately 8 inches (20 cm) long. An advantage of such a system is the ability to easily remove and replace any defective modules in a printhead array. This would eliminate having to scrap an entire printhead if only one chip is defective. 
         [0005]    A printhead module in such a printer can be comprised of a “Memjet” chip, being a chip having mounted thereon a vast number of thermo-actuators in micro-mechanics and micro-electromechanical systems (MEMS). Such actuators might be those as disclosed in U.S. Pat. No. 6,044,646 to the present applicant, however, there might be other MEMS print chips. 
         [0006]    The printhead, being the environment within which the paper thickness sensor of the present invention is to be situated, might typically have six ink chambers and be capable of printing four color process (CMYK) as well as infra-red ink and fixative. An air pump would supply filtered air to the printhead, which could be used to keep foreign particles away from its ink nozzles. The printhead module is typically to be connected to a replaceable cassette which contains the ink supply and an air filter. 
         [0007]    Each printhead module receives ink via a distribution molding that transfers the ink. Typically, ten modules butt together to form a complete eight inch printhead assembly suitable for printing A4 paper without the need for scanning movement of the printhead across the paper width. 
         [0008]    The printheads themselves are modular, so complete eight inch printhead arrays can be configured to form printheads of arbitrary width. 
         [0009]    Additionally, a second printhead assembly can be mounted on the opposite side of a paper feed path to enable double-sided high speed printing. 
       CO-PENDING APPLICATIONS 
       [0010]    Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications filed by the applicant or assignee of the present invention simultaneously with the present application:
       PCT/AU00/00518, PCT/AU00/00519, PCT/AU00/00520, PCT/AU00/00521, PCT/AU00/00522, PCT/AU00/00523, PCT/AU00/00524, PCT/AU00/00525, PCT/AU00/00526, PCT/AU00/00527, PCT/AU00/00528, PCT/AU00/00529, PCT/AU00/00530, PCT/AU00/00531, PCT/AU00/00532, PCT/AU00/00533, PCT/AU00/00534, PCT/AU00/00535, PCT/AU00/00536, PCT/AU00/00537, PCT/AU00/00538, PCT/AU00/00539, PCT/AU00/00540, PCT/AU00/00541, PCT/AU00/00542, PCT/AU00/00543, PCT/AU00/00544, PCT/AU00/00545, PCT/AU00/00547, PCT/AU00/00546, PCT/AU00/00554, PCT/AU00/00556, PCT/AU00/00557, PCT/AU00/00558, PCT/AU00/00559, PCT/AU00/00560, PCT/AU00/00561, PCT/AU00/00562, PCT/AU00/00563, PCT/AU00/00564, PCT/AU00/00565, PCT/AU00/00566, PCT/AU00/00567, PCT/AU00/00568, PCT/AU00/00569, PCT/AU00/00570, PCT/AU00/00571, PCT/AU00/00572, PCT/AU00/00573, PCT/AU00/00574, PCT/AU00/00575, PCT/AU00/00576, PCT/AU00/00577, PCT/AU00/00578, PCT/AU00/00579, PCT/AU00/00581, PCT/AU00/00580, PCT/AU00/00582, PCT/AU00/00587, PCT/AU00/00588, PCT/AU00/00589, PCT/AU00/00583, PCT/AU00/00593, PCT/AU00/00590, PCT/AU00/00591, PCT/AU00/00592, PCT/AU00/00584, PCT/AU00/00585, PCT/AU00/00586, PCT/AU00/00594, PCT/AU00/00595, PCT/AU00/00596, PCT/AU00/00597, PCT/AU00/00598, PCT/AU00/00516, PCT/AU00/00517, PCT/AU00/00511, PCT/AU00/00501, PCT/AU00/00502, PCT/AU00/00503, PCT/AU00/00504, PCT/AU00/00505, PCT/AU00/00506, PCT/AU00/00507, PCT/AU00/00508, PCT/AU00/00509, PCT/AU00/00510, PCT/AU00/00512, PCT/AU00/00513, PCT/AU00/00514, PCT/AU00/00515       
 
         [0012]    The disclosures of these co-pending applications are incorporated herein by cross-reference. 
       OBJECTS OF THE INVENTION 
       [0013]    It is an object of the present invention to provide a paper thickness sensor in a printer. 
         [0014]    It is another object of the present invention to provide a paper thickness sensor used for adjusting a printhead-to-platen clearance for the pagewidth printhead assembly as broadly described herein. 
         [0015]    It is another object of the present invention to provide a pagewidth printhead assembly having a paper thickness sensor therein to aid in adjusting a printhead-to-platen clearance. 
         [0016]    It is yet another object of the present invention to provide a method of adjusting the clearance between a printhead and a platen in a pagewidth printhead assembly. 
       SUMMARY OF THE INVENTION 
       [0017]    The present invention provides a pagewidth printer comprising: 
         [0018]    a printhead having an array of fixed printing nozzles thereon, 
         [0019]    a platen having a platen surface upon which a sheet rides to receive on a print surface thereof ink from said printing nozzles, 
         [0020]    a sensor to measure an offset of said print surface with respect to said printing nozzles, and 
         [0021]    means to effect movement of said platen to alter said offset. 
         [0022]    Preferably the platen is mounted so as to rotate about a longitudinal axis thereof and said platen surface extends along the platen parallel with said axis at a non-constant distance from said axis such that compensatory rotation of the platen effects the offset of said print surface with respect to said printing nozzles. 
         [0023]    Preferably the sensor is an optical sensor. 
         [0024]    Preferably the optical sensor senses the position of a pivotal sensor flag that engages the print surface. 
         [0025]    Preferably the sensor flag is mounted upon a spring-biased pivotal shaft mounted to the printhead. 
         [0026]    The present invention also provides a method of adjusting an offset between an array of printing nozzles on a printhead and a print surface of a sheet riding upon a platen, the method comprising the steps of sensing the offset between the printhead and the print surface of the sheet and moving the platen so as to make any necessary compensation to said offset. 
         [0027]    Preferably the platen includes a longitudinal axis and a platen surface parallel with said axis at a non-constant distance from said axis, the method including effecting compensatory rotation of the platen. 
         [0028]    As used herein, the term “ink” is intended to mean any fluid which flows through the printhead to be delivered to a sheet. The fluid may be one of many different coloured inks, infra-red ink, a fixative or the like. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    A preferred form of the present invention will now be described by way of example with reference to the accompanying drawings wherein: 
           [0030]      FIG. 1  is a front perspective view of a print engine assembly 
           [0031]      FIG. 2  is a rear perspective view of the print engine assembly of  FIG. 1   
           [0032]      FIG. 3  is an exploded perspective view of the print engine assembly of  FIG. 1 . 
           [0033]      FIG. 4  is a schematic front perspective view of a printhead assembly. 
           [0034]      FIG. 5  is a rear schematic perspective view of the printhead assembly of  FIG. 4 . 
           [0035]      FIG. 6  is an exploded perspective illustration of the printhead assembly. 
           [0036]      FIG. 7  is a cross-sectional end elevational view of the printhead assembly of  FIGS. 4 to 6  with the section taken through the centre of the printhead. 
           [0037]      FIG. 8  is a schematic cross-sectional end elevational view of the printhead assembly of  FIGS. 4 to 6  taken near the left end of  FIG. 4 . 
           [0038]      FIG. 9A  is a schematic end elevational view of mounting of the print chip and nozzle guard in the laminated stack structure of the printhead 
           [0039]      FIG. 9B  is an enlarged end elevational cross section of  FIG. 9A   FIG. 10  is an exploded perspective illustration of a printhead cover assembly. 
           [0040]      FIG. 11  is a schematic perspective illustration of an ink distribution molding. 
           [0041]      FIG. 12  is an exploded perspective illustration showing the layers forming part of a laminated ink distribution structure according to the present invention. 
           [0042]      FIG. 13  is a stepped sectional view from above of the structure depicted in  FIGS. 9A and 9B , 
           [0043]      FIG. 14  is a stepped sectional view from below of the structure depicted in  FIG. 13 . 
           [0044]      FIG. 15  is a schematic perspective illustration of a first laminate layer. 
           [0045]      FIG. 16  is a schematic perspective illustration of a second laminate layer. 
           [0046]      FIG. 17  is a schematic perspective illustration of a third laminate layer. 
           [0047]      FIG. 18  is a schematic perspective illustration of a fourth laminate layer. 
           [0048]      FIG. 19  is a schematic perspective illustration of a fifth laminate layer. 
           [0049]      FIG. 20  is a perspective view of the air valve molding 
           [0050]      FIG. 21  is a rear perspective view of the right hand end of the platen 
           [0051]      FIG. 22  is a rear perspective view of the left hand end of the platen 
           [0052]      FIG. 23  is an exploded view of the platen 
           [0053]      FIG. 24  is a transverse cross-sectional view of the platen 
           [0054]      FIG. 25  is a front perspective view of the optical paper sensor arrangement 
           [0055]      FIG. 26  is a schematic perspective illustration of a printhead assembly and ink lines attached to an ink reservoir cassette. 
           [0056]      FIG. 27  is a partly exploded view of  FIG. 26 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0057]    In  FIGS. 1 to 3  of the accompanying drawings there is schematically depicted the core components of a print engine assembly, showing the general environment in which the laminated ink distribution structure of the present invention can be located. The print engine assembly includes a chassis  10  fabricated from pressed steel, aluminium, plastics or other rigid material. Chassis  10  is intended to be mounted within the body of a printer and serves to mount a printhead assembly  11 , a paper feed mechanism and other related components within the external plastics casing of a printer. 
         [0058]    In general terms, the chassis  10  supports the printhead assembly  11  such that ink is ejected therefrom and onto a sheet of paper or other print medium being transported below the printhead then through exit slot  19  by the feed mechanism. The paper feed mechanism includes a feed roller  12 , feed idler rollers  13 , a platen generally designated as  14 , exit rollers  15  and a pin wheel assembly  16 , all driven by a stepper motor  17 . These paper feed components are mounted between a pair of bearing moldings  18 , which are in turn mounted to the chassis  10  at each respective end thereof. 
         [0059]    A printhead assembly  11  is mounted to the chassis  10  by means of respective printhead spacers  20  mounted to the chassis  10 . The spacer moldings  20  increase the printhead assembly length to 220 mm allowing clearance on either side of 210 mm wide paper. 
         [0060]    The printhead construction is shown generally in  FIGS. 4 to 8 . 
         [0061]    The printhead assembly  11  includes a printed circuit board (PCB)  21  having mounted thereon various electronic components including a 64 MB DRAM  22 , a PEC chip  23 , a QA chip connector  24 , a microcontroller  25 , and a dual motor driver chip  26 . The printhead is typically 203 mm long and has ten print chips  27  ( FIG. 13 ), each typically 21 mm long. These print chips  27  are each disposed at a slight angle to the longitudinal axis of the printhead (see  FIG. 12 ), with a slight overlap between each print chip which enables continuous transmission of ink over the entire length of the array. Each print chip  27  is electronically connected to an end of one of the tape automated bond (TAB) films  28 , the other end of which is maintained in electrical contact with the undersurface of the printed circuit board  21  by means of a TAB film backing pad  29 . 
         [0062]    The preferred print chip construction is as described in U.S. Pat. No. 6,044,646 by the present applicant. Each such print chip  27  is approximately 21 mm long, less than 1 mm wide and about 0.3 mm high, and has on its lower surface thousands of MEMS inkjet nozzles  30 , shown schematically in  FIGS. 9A and 9B , arranged generally in six lines—one for each ink type to be applied. Each line of nozzles may follow a staggered pattern to allow closer dot spacing. Six corresponding lines of ink passages  31  extend through from the rear of the print chip to transport ink to the rear of each nozzle. To protect the delicate nozzles on the surface of the print chip each print chip has a nozzle guard  43 , best seen in  FIG. 9A , with microapertures  44  aligned with the nozzles  30 , so that the ink drops ejected at high speed from the nozzles pass through these microapertures to be deposited on the paper passing over the platen  14 . 
         [0063]    Ink is delivered to the print chips via a distribution molding  35  and laminated stack  36  arrangement forming part of the printhead  11 . Ink from an ink cassette  37  ( FIGS. 26 and 27 ) is relayed via individual ink hoses  38  to individual ink inlet ports  34  integrally molded with a plastics duct cover  39  which forms a lid over the plastics distribution molding  35 . The distribution molding  35  includes six individual longitudinal ink ducts  40  and an air duct  41  which extend throughout the length of the array. Ink is transferred from the inlet ports  34  to respective ink ducts  40  via individual cross-flow ink channels  42 , as best seen with reference to  FIG. 7 . It should be noted in this regard that although there are six ducts depicted, a different number of ducts might be provided. Six ducts are suitable for a printer capable of printing four color process (CMYK) as well as infra-red ink and fixative. 
         [0064]    Air is delivered to the air duct  41  via an air inlet port  61 , to supply air to each print chip  27 , as described later with reference to  FIGS. 6 to 8 ,  20  and  21 . 
         [0065]    Situated within a longitudinally extending stack recess  45  formed in the underside of distribution molding  35  are a number of laminated layers forming a laminated ink distribution stack  36 . The layers of the laminate are typically formed of micro-molded plastics material. The TAB film  28  extends from the undersurface of the printhead PCB  21 , around the rear of the distribution molding  35  to be received within a respective TAB film recess  46  ( FIG. 21 ), a number of which are situated along a chip housing layer  47  of the laminated stack  36 . The TAB film relays electrical signals from the printed circuit board  21  to individual print chips  27  supported by the laminated structure. 
         [0066]    The distribution molding, laminated stack  36  and associated components are best described with reference to  FIGS. 7 to 19 . 
         [0067]      FIG. 10  depicts the distribution molding cover  39  formed as a plastics molding and including a number of positioning spigots  48  which serve to locate the upper printhead cover  49  thereon. 
         [0068]    As shown in  FIG. 7 , an ink transfer port  50  connects one of the ink ducts  39  (the fourth duct from the left) down to one of six lower ink ducts or transitional ducts  51  in the underside of the distribution molding. All of the ink ducts  40  have corresponding transfer ports  50  communicating with respective ones of the transitional ducts  51 . The transitional ducts  51  are parallel with each other but angled acutely with respect to the ink ducts  40  so as to line up with the rows of ink holes of the first layer  52  of the laminated stack  36  to be described below. 
         [0069]    The first layer  52  incorporates twenty four individual ink holes  53  for each of ten print chips  27 . That is, where ten such print chips are provided, the first layer  52  includes two hundred and forty ink holes  53 . The first layer  52  also includes a row of air holes  54  alongside one longitudinal edge thereof. 
         [0070]    The individual groups of twenty four ink holes  53  are formed generally in a rectangular array with aligned rows of ink holes. Each row of four ink holes is aligned with a transitional duct  51  and is parallel to a respective print chip. 
         [0071]    The undersurface of the first layer  52  includes underside recesses  55 . Each recess  55  communicates with one of the ink holes of the two centre-most rows of four holes  53  (considered in the direction transversely across the layer  52 ). That is, holes  53   a  ( FIG. 13 ) deliver ink to the right hand recess  55   a  shown in  FIG. 14 , whereas the holes  53   b  deliver ink to the left most underside recesses  55   b  shown in  FIG. 14 . 
         [0072]    The second layer  56  includes a pair of slots  57 , each receiving ink from one of the underside recesses  55  of the first layer. 
         [0073]    The second layer  56  also includes ink holes  53  which are aligned with the outer two sets of ink holes  53  of the first layer  52 . That is, ink passing through the outer sixteen ink holes  53  of the first layer  52  for each print chip pass directly through corresponding holes  53  passing through the second layer  56 . 
         [0074]    The underside of the second layer  56  has formed therein a number of transversely extending channels  58  to relay ink passing through ink holes  53   c  and  53   d  toward the centre. These channels extend to align with a pair of slots  59  formed through a third layer  60  of the laminate. It should be noted in this regard that the third layer  60  of the laminate includes four slots  59  corresponding with each print chip, with two inner slots being aligned with the pair of slots formed in the second layer  56  and outer slots between which the inner slots reside. 
         [0075]    The third layer  60  also includes an array of air holes  54  aligned with the corresponding air hole arrays  54  provided in the first and second layers  52  and  56 . 
         [0076]    The third layer  60  has only eight remaining ink holes  53  corresponding with each print chip. These outermost holes  53  are aligned with the outermost holes  53  provided in the first and second laminate layers. As shown in  FIGS. 9A and 9B , the third layer  60  includes in its underside surface a transversely extending channel  61  corresponding to each hole  53 . These channels  61  deliver ink from the corresponding hole  53  to a position just outside the alignment of slots  59  therethrough. 
         [0077]    As best seen in  FIGS. 9A and 9B , the top three layers of the laminated stack  36  thus serve to direct the ink (shown by broken hatched lines in  FIG. 9B ) from the more widely spaced ink ducts  40  of the distribution molding to slots aligned with the ink passages  31  through the upper surface of each print chip  27 . 
         [0078]    As shown in  FIG. 13 , which is a view from above the laminated stack, the slots  57  and  59  can in fact be comprised of discrete co-linear spaced slot segments. 
         [0079]    The fourth layer  62  of the laminated stack  36  includes an array of ten chip-slots  65  each receiving the upper portion of a respective print chip  27 . 
         [0080]    The fifth and final layer  64  also includes an array of chip-slots  65  which receive the chip and nozzle guard assembly  43 . 
         [0081]    The TAB film  28  is sandwiched between the fourth and fifth layers  62  and  64 , one or both of which can be provided with recesses to accommodate the thickness of the TAB film. 
         [0082]    The laminated stack is formed as a precision micro-molding, injection molded in an Acetal type material. It accommodates the array of print chips  27  with the TAB film already attached and mates with the cover molding  39  described earlier. 
         [0083]    Rib details in the underside of the micro-molding provides support for the TAB film when they are bonded together. The TAB film forms the underside wall of the printhead module, as there is sufficient structural integrity between the pitch of the ribs to support a flexible film. The edges of the TAB film seal on the underside wall of the cover molding  39 . The chip is bonded onto one hundred micron wide ribs that run the length of the micro-molding, providing a final ink feed to the print nozzles. 
         [0084]    The design of the micro-molding allow for a physical overlap of the print chips when they are butted in a line. Because the printhead chips now form a continuous strip with a generous tolerance, they can be adjusted digitally to produce a near perfect print pattern rather than relying on very close toleranced moldings and exotic materials to perform the same function. The pitch of the modules is typically 20.33 mm. 
         [0085]    The individual layers of the laminated stack as well as the cover molding  39  and distribution molding can be glued or otherwise bonded together to provide a sealed unit. The ink paths can be sealed by a bonded transparent plastic film serving to indicate when inks are in the ink paths, so they can be fully capped off when the upper part of the adhesive film is folded over. Ink charging is then complete. 
         [0086]    The four upper layers  52 ,  56 ,  60 ,  62  of the laminated stack  36  have aligned air holes  54  which communicate with air passages  63  formed as channels formed in the bottom surface of the fourth layer  62 , as shown in  FIGS. 9   b  and  13 . These passages provide pressurised air to the space between the print chip surface and the nozzle guard  43  whilst the printer is in operation. Air from this pressurised zone passes through the micro-apertures  44  in the nozzle guard, thus preventing the build-up of any dust or unwanted contaminants at those apertures. This supply of pressurised air can be turned off to prevent ink drying on the nozzle surfaces during periods of non-use of the printer, control of this air supply being by means of the air valve assembly shown in  FIGS. 6 to 8 ,  20  and  21 . 
         [0087]    With reference to  FIGS. 6 to 8 , within the air duct  41  of the printhead there is located an air valve molding  66  formed as a channel with a series of apertures  67  in its base. The spacing of these apertures corresponds to air passages  68  formed in the base of the air duct  41  (see  FIG. 6 ), the air valve molding being movable longitudinally within the air duct so that the apertures  67  can be brought into alignment with passages  68  to allow supply the pressurized air through the laminated stack to the cavity between the print chip and the nozzle guard, or moved out of alignment to close off the air supply. Compression springs  69  maintain a sealing inter-engagement of the bottom of the air valve molding  66  with the base of the air duct  41  to prevent leakage when the valve is closed. 
         [0088]    The air valve molding  66  has a cam follower  70  extending from one end thereof, which engages an air valve cam surface  71  on an end cap  74  of the platen  14  so as to selectively move the air valve molding longitudinally within the air duct  41  according to the rotational positional of the multi-function platen  14 , which may be rotated between printing, capping and blotting positions depending on the operational status of the printer, as will be described below in more detail with reference to  FIGS. 21 to 24 . When the platen  14  is in its rotational position for printing, the cam holds the air valve in its open position to supply air to the print chip surface, whereas when the platen is rotated to the non-printing position in which it caps off the micro-apertures of the nozzle guard, the cam moves the air valve molding to the valve closed position. 
         [0089]    With reference to  FIGS. 21 to 24 , the platen member  14  extends parallel to the printhead, supported by a rotary shaft  73  mounted in bearing molding  18  and rotatable by means of gear  79  (see  FIG. 3 ). The shaft is provided with a right hand end cap  74  and left hand end cap  75  at respective ends, having cams  76 ,  77 . 
         [0090]    The platen member  14  has a platen surface  78 , a capping portion  80  and an exposed blotting portion  81  extending along its length, each separated by 120°. During printing, the platen member is rotated so that the platen surface  78  is positioned opposite the printhead so that the platen surface acts as a support for that portion of the paper being printed at the time. When the printer is not in use, the platen member is rotated so that the capping portion  80  contacts the bottom of the printhead, sealing in a locus surrounding the microapertures  44 . This, in combination with the closure of the air valve by means of the air valve arrangement when the platen  14  is in its capping position, maintains a closed atmosphere at the print nozzle surface. This serves to reduce evaporation of the ink solvent (usually water) and thus reduce drying of ink on the print nozzles while the printer is not in use. 
         [0091]    The third function of the rotary platen member is as an ink blotter to receive ink from priming of the print nozzles at printer start up or maintenance operations of the printer. During this printer mode, the platen member  14  is rotated so that the exposed blotting portion  81  is located in the ink ejection path opposite the nozzle guard  43 . The exposed blotting portion  81  is an exposed part of a body of blotting material  82  inside the platen member  14 , so that the ink received on the exposed portion  81  is drawn into the body of the platen member. 
         [0092]    Further details of the platen member construction may be seen from  FIGS. 23 and 24 . The platen member consists generally of an extruded or molded hollow platen body  83  which forms the platen surface  78  and receives the shaped body of blotting material  82  of which a part projects through a longitudinal slot in the platen body to form the exposed blotting surface  81 . A flat portion  84  of the platen body  83  serves as a base for attachment of the capping member  80 , which consists of a capper housing  85 , a capper seal member  86  and a foam member  87  for contacting the nozzle guard  43 . 
         [0093]    With reference again to  FIG. 1 , each bearing molding  18  rides on a pair of vertical rails  101 . That is, the capping assembly is mounted to four vertical rails  101  enabling the assembly to move vertically. A spring  102  under either end of the capping assembly biases the assembly into a raised position, maintaining cams  76 , 77  in contact with the spacer projections  100 . 
         [0094]    The printhead  11  is capped when not is use by the full-width capping member  80  using the elastomeric (or similar) seal  86 . In order to rotate the platen assembly  14 , the main roller drive motor is reversed. This brings a reversing gear into contact with the gear  79  on the end of the platen assembly and rotates it into one of its three functional positions, each separated by 120°. 
         [0095]    The cams  76 ,  77  on the platen end caps  74 ,  75  co-operate with projections  100  on the respective printhead spacers  20  to control the spacing between the platen member and the printhead depending on the rotary position of the platen member. In this manner, the platen is moved away from the printhead during the transition between platen positions to provide sufficient clearance from the printhead and moved back to the appropriate distances for its respective paper support, capping and blotting functions. 
         [0096]    In addition, the cam arrangement for the rotary platen provides a mechanism for fine adjustment of the distance between the platen surface and the printer nozzles by slight rotation of the platen  14 . This allows compensation of the nozzle-platen distance in response to the thickness of the paper or other material being printed, as detected by the optical paper thickness sensor arrangement illustrated in  FIG. 25 . 
         [0097]    The optical paper sensor includes an optical sensor  88  mounted on the lower surface of the PCB  21  and a sensor flag arrangement mounted on the arms  89  protruding from the distribution molding. The flag arrangement comprises a sensor flag member  90  mounted on a shaft  91  which is biased by torsion spring  92 . As paper enters the feed rollers, the lowermost portion of the flag member contacts the paper and rotates against the bias of the spring  92  by an amount dependent on the paper thickness. The optical sensor detects this movement of the flag member and the PCB responds to the detected paper thickness by causing compensatory rotation of the platen  14  to optimize the distance between the paper surface and the nozzles. 
         [0098]      FIGS. 26 and 27  show attachment of the illustrated printhead assembly to a replaceable ink cassette  93 . Six different inks are supplied to the printhead through hoses  94  leading from an array of female ink valves  95  located inside the printer body. The replaceable cassette  93  containing a six compartment ink bladder and corresponding male valve array is inserted into the printer and mated to the valves  95 . The cassette also contains an air inlet  96  and air filter (not shown), and mates to the air intake connector  97  situated beside the ink valves, leading to the air pump  98  supplying filtered air to the printhead. A QA chip is included in the cassette. The QA chip meets with a contact  99  located between the ink valves  95  and air intake connector  96  in the printer as the cassette is inserted to provide communication to the QA chip connector  24  on the PCB.

Technology Classification (CPC): 1