Abstract:
An inkjet printer that has a printhead with an array of ink ejection nozzles, an upstream ink line for connecting the printhead to an ink supply, a downstream ink line for connecting the printhead to a sump, a pump in the downstream ink line for drawing fluid out of the printhead, a gas inlet in communication with the printhead, the gas inlet being configured to open to atmosphere during a printhead de-priming operation, and close to atmosphere during a printhead priming operation and, an accumulator positioned in the upstream ink line for generating a positive pressure pulse for priming the printhead.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/495,818 filed Jul. 31, 2006, all of which is herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of printing and in particular inkjet printing. 
     CO-PENDING APPLICATIONS 
     The following applications have been filed by the Applicant simultaneously with the present application: U.S. Pat. Nos. 7,581,812 7,641,304 Ser. Nos. 11/495,817 11/495,814 11/495,823 U.S. Pat. Nos. 7,657,128 7,523,672 Ser. Nos. 11/495,820 11/495,819 
     The disclosures of these co-pending applications are incorporated herein by reference. 
     CROSS REFERENCES TO RELATED APPLICATIONS 
     Various methods, systems and apparatus relating to the present invention are disclosed in the following U.S. patents/patent applications filed by the applicant or assignee of the present invention: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
             
             
               
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     The disclosures of these applications and patents are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Inkjet printing is a popular and versatile form of print imaging. The Assignee has developed printers that eject ink through MEMS printhead IC&#39;s. These printhead IC&#39;s (integrated circuits) are formed using lithographic etching and deposition techniques used for semiconductor fabrication. 
     The micro-scale nozzle structures in MEMS printhead IC&#39;s allow a high nozzle density (nozzles per unit of IC surface area), high print resolutions, low power consumption, self cooling operation and therefore high print speeds. Such printheads are described in detail in U.S. Pat. No. 6,746,105, filed Jun. 4, 2002 and U.S. patent application Ser. No. 10/728,804, filed 8 Dec. 2003 to the present Assignee. The disclosures of these documents are incorporated herein by reference. 
     The small nozzle structures and high nozzle densities can create difficulties with nozzle clogging, de-priming, nozzle drying (decap), color mixing, nozzle flooding, bubble contamination in the ink stream and so on. Each of these issues can produce artifacts that are detrimental to the print quality. The component parts of the printer are designed to minimize the risk that these problems will occur. The optimum situation would be printer components whose inherent function is able to preclude these problem issues from arising. In reality, the many different types of operating conditions, mishaps, unduly rough handling during transport or day to day operation, make it impossible to address the above problems via the ‘passive’ control of component design, material selection and so on. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, the present invention provides an inkjet printer comprising: 
     an ink supply; 
     an ink manifold in fluid communication with the ink supply; 
     a printhead IC with and array of ink ejection nozzles mounted to the ink manifold; 
     a pump in fluid communication with the ink manifold; and, 
     a gas inlet that can be opened to establish fluid communication between the ink manifold and a supply of gas, and can be closed to form a gas tight seal; such that, 
     the ink manifold can be primed with ink when the gas inlet is closed, and de-primed of ink when the gas inlet is open. 
     Actively priming and de-priming the ink manifold provides the user with the ability to correct many of the problems associated with MEMS printheads after they occur. In light of this, it is not as crucial that the printer components themselves safeguard against issues such as de-prime, color mixing and outgassing. An active control system for the ink flow through the printer means that the user can prime, deprime, or purge the printhead IC. Also, the upstream line can be deprimed and/or the downstream line can be deprimed (and of course subsequently re-primed). This control system allows the user to correct and print artifact causing conditions as and when they occur. 
     Preferably, the ink supply is connected to the ink manifold via an upstream ink line, and the pump is a downstream pump connected to the ink manifold via a downstream ink line. In a further preferred form, the printer further comprises an upstream pump in the upstream ink line. In a preferred embodiment, the gas inlet is an air inlet which can open to atmosphere. In preferred embodiments, the manifold has an inlet connected to the upstream ink line and an outlet connected to the downstream ink line such that when priming the ink manifold, the hydrostatic pressure in the ink at the ink ejection nozzle is less than atmospheric. 
     Preferably, the upstream and downstream pumps are independently operable. In a further preferred form, the upstream and downstream pumps are reversible for pumping ink in a reverse direction. Preferably, the downstream ink line connects the ink manifold to the ink supply via the downstream pump and the outlet of the ink manifold is in fluid communication with a gas vent for gas drawn into the ink manifold during depriming. Optionally, the gas vent is in the ink supply. 
     Preferably, the upstream and the downstream pumps are peristaltic pumps. Optionally, the upstream pump and the downstream pumps are provided by a six-way peristaltic pump head driven by a single motor. Optionally, the upstream pump and the downstream pump are driven by separate motors. If the printer only has a single pump, the pump may be a three-way peristaltic pump head. Preferably, the upstream ink line has a pressure regulator that allows ink to flow to the ink manifold at a predetermined threshold pressure difference across the pressure regulator. Preferably, the printer further comprises a capping member for sealing the array of nozzles on the printhead IC. 
     Preferably, the printer is a color printer with a separate ink supplies for each ink color, and respective inlets and outlets for each ink color in the ink manifold. 
     Preferably, the printhead IC is a pagewidth printhead and the ink manifold is an elongate structure with the inlet at one end and the outlet at the opposite end. In one preferred form, the upstream pump and the downstream pump can operate at different flow rates. Optionally, the upstream pump and the downstream pump can act as shout off valves in the upstream and down stream lines respectively. Preferably, the printer further comprises an ink filter upstream of the ink manifold for removing bubbles and contaminants from ink flowing to the manifold. 
     It will be appreciated that the term ‘ink’, when used throughout this specification, refers to all types of printable fluid and is not limited to liquid colorants. Infrared inks and other types of functionalized fluids are encompassed by the term ‘ink’ as well as the cyan, magenta, yellow and possibly black inks that are typically used by inkjet printers. 
     According to a second aspect, the present invention provides an inkjet printer comprising: 
     a printhead IC with and array of ink ejection nozzles; 
     an ink manifold for distributing ink to the printhead IC, the ink manifold having an ink inlet and an ink outlet; 
     an upstream pump in fluid communication with the ink inlet; and, 
     a downstream pump in fluid communication with the ink outlet; wherein, 
     the upstream pump and the downstream pump are independently operable. 
     With a pump at the inlet and the outlet of the manifold the user can actively control the ink flows though the printer and use this control for ink purges, de-priming, re-priming and ink pressure regulation. Actively priming and de-priming the ink manifold provides the user with the ability to correct many of the problems associated with MEMS printheads after they occur. In light of this, it is not as crucial that the printer components themselves safeguard against issues such as de-prime, color mixing and outgassing. An active control system for the ink flow through the printer means that the user can prime, deprime, or purge the printhead IC. Also, the upstream line can be deprimed and/or the downstream line can be deprimed (and of course subsequently re-primed). This control system allows the user to correct and print artifact causing conditions as and when they occur. 
     Preferably, the printer further comprises a gas inlet that can be opened to establish fluid communication between the ink manifold and a supply of gas, and can be closed to form a gas tight seal; such that, 
     the ink manifold can be primed with ink when the gas inlet is closed, and de-primed of ink when the gas inlet is open. 
     The manifold and the printhead IC can be deprimed by shutting off the upstream pump and operating the downstream pump to draw air in through the ink ejection nozzles. However, a gas inlet upstream of the manifold will allow ink to be retained in the printhead IC. This is useful for creating an ink foam on the face of the printhead IC to clean particulates from the nozzles (this is discussed further in the Detailed Description below). De-priming by drawing air in through an inlet rather than the ejection nozzles leaves more residual ink in the printhead IC for forming the ink foam. 
     Preferably, the printer further comprises an ink supply is connected to the inlet of the ink manifold via an upstream ink line, and the downstream pump connected to the ink manifold via a downstream ink line. In a preferred embodiment, the gas inlet is an air inlet which can open to atmosphere. In preferred embodiments, the hydrostatic pressure in the ink at the ink ejection nozzle is less than atmospheric. In a further preferred form, the upstream and downstream pumps are reversible for pumping ink in a reverse direction. Preferably, the downstream ink line connects the ink manifold to the ink supply via the downstream pump and the outlet of the ink manifold is in fluid communication with a gas vent for gas drawn into the ink manifold during depriming. Optionally, the gas vent is in the ink supply. 
     Preferably, the upstream and the downstream pumps are peristaltic pumps. Optionally, the upstream pump and the downstream pumps are provided by a six-way peristaltic pump head driven by a single motor. Optionally, the upstream pump and the downstream pump are driven by separate motors. If the printer only has a single pump, the pump may be a three-way peristaltic pump head. Preferably, the upstream ink line has a pressure regulator that allows ink to flow to the ink manifold at a predetermined threshold pressure difference across the pressure regulator. Preferably, the printer further comprises a capping member for sealing the array of nozzles on the printhead IC. 
     Preferably, the printer is a color printer with a separate ink supplies for each ink color, and respective inlets and outlets for each ink color in the ink manifold. 
     Preferably, the printhead IC is a pagewidth printhead and the ink manifold is an elongate structure with the inlet at one end and the outlet at the opposite end. In one preferred form, the upstream pump and the downstream pump can operate at different flow rates. Optionally, the upstream pump and the downstream pump can act as shout off valves in the upstream and down stream lines respectively. Preferably, the printer further comprises an ink filter upstream of the ink manifold for removing bubbles and contaminants from ink flowing to the manifold. 
     It will be appreciated that the term ‘ink’, when used throughout this specification, refers to all types of printable fluid and is not limited to liquid colorants. Infrared inks and other types of functionalized fluids are encompassed by the term ‘ink’ as well as the cyan, magenta, yellow and possibly black inks that are typically used by inkjet printers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: 
         FIG. 1A  is a top and side perspective of a printhead assembly using a LCP ink manifold according to the prior art; 
         FIG. 1B  is an exploded perspective of the print cartridge body components that support the printhead assembly of  FIG. 1A ; 
         FIG. 2  is an exploded perspective of the printhead assembly shown in  FIG. 1A ; 
         FIG. 3  is the exploded perspective of  FIG. 2  shown from below; 
         FIG. 4  is transverse section through the printhead assembly of  FIG. 1A ; 
         FIG. 5  shows a magnified partial perspective view of the bottom of the drop triangle end of a printhead integrated circuit module; 
         FIG. 6  shows a magnified perspective view of the join between two printhead integrated circuit modules; 
         FIG. 7  shows a magnified partial perspective view of the top of the drop triangle end of a printhead integrated circuit module; 
         FIG. 8  is a partial bottom view of the LCP manifold and the printhead IC; 
         FIG. 9  is an enlarged partial bottom view of the LCP manifold and the printhead IC; 
         FIG. 10  shows the fine conduits in the underside of the LCP manifold; 
         FIG. 11  shows the typical artifacts from outgassing bubbles forming in the LCP manifold and the printhead IC; 
         FIG. 12  is a sketch of the fluidic system for a prior art printer; 
         FIG. 13  is a sketch of a dual pump embodiment of the active fluidic system of the present invention; and, 
         FIG. 14  is a sketch of a single pump embodiment of the active fluidic system of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The printers using prior art types of fluid architecture are exemplified by the disclosure in the Assignee&#39;s co-pending U.S. Ser. No. 11/014,769, filed Dec. 20, 2004, which is incorporated herein by cross reference. For context, the printhead assembly from this printer design will be described before the embodiments of the present invention. 
     Printhead Assembly 
     The printhead assembly  22  shown in  FIGS. 1A to 4  is adapted to be attached to the underside of the main body  20  to receive ink from the outlets molding  27  (see  FIG. 1B ). 
     The printhead assembly  22  generally comprises an ink manifold that receives ink from the ink cartridges, or ink storage modules as they are referred to in U.S. Ser. No. 11/014,769, and distributes it to the printhead integrated circuits (IC&#39;s). The ink manifold is made up of an elongate upper member  62  fixed to an elongate lower member  65 . The upper member  62  is configured to extend beneath the main body  20 , between the posts  26 . A plurality of U-shaped clips  63  project from the upper member  62 . These pass through the recesses  37  provided in the rigid plate  34  and become captured by lugs (not shown) formed in the main body  20  to secure the printhead assembly  22 . 
     The upper element  62  has a plurality of feed tubes  64  that are received within the outlets in the outlet molding  27  when the printhead assembly  22  secures to the main body  20 . The feed tubes  64  may be provided with an outer coating to guard against ink leakage. 
     The upper member  62  is made from a liquid crystal polymer (LCP) which offers a number of advantages. It can be molded so that its coefficient of thermal expansion (CTE) is similar to that of silicon. It will be appreciated that any significant difference in the CTE&#39;s of the printhead integrated circuit  74  (discussed below) and the underlying moldings can cause the entire structure to bow. However, as the CTE of LCP in the mold direction is much less than that in the non-mold direction (˜5 ppm/° C. compared to ˜20 ppm/° C.), care must be take to ensure that the mold direction of the LCP moldings is unidirectional with the longitudinal extent of the printhead integrated circuit (IC)  74 . LCP also has a relatively high stiffness with a modulus that is typically 5 times that of ‘normal plastics’ such as polycarbonates, styrene, nylon, PET and polypropylene. 
     As best shown in  FIG. 2 , upper member  62  has an open channel configuration for receiving a lower member  65 , which is bonded thereto, via an adhesive film  66 . The lower member  65  is also made from an LCP and has a plurality of ink channels  67  formed along its length. Each of the ink channels  67  receive ink from one of the feed tubes  64 , and distribute the ink along the length of the printhead assembly  22 . The channels are 1 mm wide and separated by 0.75 mm thick walls. 
     In the embodiment shown, the lower member  65  has five channels  67  extending along its length. Each channel  67  receives ink from only one of the five feed tubes  64 , which in turn receives ink from one of the ink storage modules  45  (see FIG. 10 of U.S. Ser. No. 11/014,769 cross referenced above). In this regard, adhesive film  66  also acts to seal the individual ink channels  67  to prevent cross channel mixing of the ink when the lower member  65  is assembled to the upper member  62 . 
     In the bottom of each channel  67  are a series of equi-spaced holes  69  (best seen in  FIG. 3 ) to give five rows of holes  69  in the bottom surface of the lower member  65 . The middle row of holes  69  extends along the centre-line of the lower member  65 , directly above the printhead IC  74 . As best seen in  FIG. 8 , other rows of holes  69  on either side of the middle row need conduits  70  from each hole  69  to the centre so that ink can be fed to the printhead IC  74 . 
     Referring to  FIG. 4 , the printhead IC  74  is mounted to the underside of the lower member  65  by a polymer sealing film  71 . This film may be a thermoplastic film such as a PET or Polysulphone film, or it may be in the form of a thermoset film, such as those manufactured by AL technologies and Rogers Corporation. The polymer sealing film  71  is a laminate with adhesive layers on both sides of a central film, and laminated onto the underside of the lower member  65 . As shown in  FIGS. 3 ,  8  and  9 , a plurality of holes  72  are laser drilled through the adhesive film  71  to coincide with the centrally disposed ink delivery points (the middle row of holes  69  and the ends of the conduits  70 ) for fluid communication between the printhead IC  74  and the channels  67 . 
     The thickness of the polymer sealing film  71  is critical to the effectiveness of the ink seal it provides. As best seen in  FIGS. 7 and 8 , the polymer sealing film seals the etched channels  77  on the reverse side of the printhead IC  74 , as well as the conduits  70  on the other side of the film. However, as the film  71  seals across the open end of the conduits  70 , it can also bulge or sag into the conduit. The section of film that sags into a conduit  70  runs across several of the etched channels  77  in the printhead IC  74 . The sagging may cause a gap between the walls separating each of the etched channels  77 . Obviously, this breaches the seal and allows ink to leak out of the printhead IC  74  and or between etched channels  77 . 
     To guard against this, the polymer sealing film  71  should be thick enough to account for any sagging into the conduits  70  while maintaining the seal over the etched channels  77 . The minimum thickness of the polymer sealing film  71  will depend on:
         1. the width of the conduit into which it sags;   2. the thickness of the adhesive layers in the film&#39;s laminate structure;   3. the ‘stiffness’ of the adhesive layer as the printhead IC  74  is being pushed into it; and,   4. the modulus of the central film material of the laminate.       

     A polymer sealing film  71  thickness of 25 microns is adequate for the printhead assembly  22  shown. However, increasing the thickness to 50, 100 or even 200 microns will correspondingly increase the reliability of the seal provided. 
     Ink delivery inlets  73  are formed in the ‘front’ surface of a printhead IC  74 . The inlets  73  supply ink to respective nozzles (described in FIGS. 23 to 36 of U.S. Ser. No. 11/014,769 cross referenced above) positioned on the inlets. The ink must be delivered to the IC&#39;s so as to supply ink to each and every individual inlet  73 . Accordingly, the inlets  73  within an individual printhead IC  74  are physically grouped to reduce ink supply complexity and wiring complexity. They are also grouped logically to minimize power consumption and allow a variety of printing speeds. 
     Each printhead IC  74  is configured to receive and print five different colours of ink (C, M, Y, K and IR) and contains 1280 ink inlets per colour, with these nozzles being divided into even and odd nozzles (640 each). Even and odd nozzles for each colour are provided on different rows on the printhead IC  74  and are aligned vertically to perform true 1600 dpi printing, meaning that nozzles are arranged in 10 rows, as clearly shown in  FIG. 5 . The horizontal distance between two adjacent nozzles on a single row is 31.75 microns, whilst the vertical distance between rows of nozzles is based on the firing order of the nozzles, but rows are typically separated by an exact number of dot lines, plus a fraction of a dot line corresponding to the distance the paper will move between row firing times. Also, the spacing of even and odd rows of nozzles for a given colour must be such that they can share an ink channel, as will be described below. 
     As alluded to previously, the present invention is related to page-width printing and as such the printhead ICs  74  are arranged to extend horizontally across the width of the printhead assembly  22 . To achieve this, individual printhead ICs  74  are linked together in abutting arrangement across the adhesive surface of the polymer sealing film  71 , as shown in  FIGS. 2 and 3 . The printhead IC&#39;s  74  may be attached to the polymer sealing film  71  by heating the IC&#39;s above the melting point of the adhesive layer and then pressing them into the sealing film  71 , or melting the adhesive layer under the IC with a laser before pressing them into the film. Another option is to both heat the IC (not above the adhesive melting point) and the adhesive layer, before pressing it into the film  71 . 
     The length of an individual printhead IC  74  is around 20-22 mm. To print an A4/US letter sized page, 11-12 individual printhead ICs  74  are contiguously linked together. The number of individual printhead ICs  74  may be varied to accommodate sheets of other widths. 
     The printhead ICs  74  may be linked together in a variety of ways. One particular manner for linking the ICs  74  is shown in  FIG. 6 . In this arrangement, the ICs  74  are shaped at their ends to link together to form a horizontal line of ICs, with no vertical offset between neighboring ICs. A sloping join is provided between the ICs having substantially a 45° angle. The joining edge is not straight and has a sawtooth profile to facilitate positioning, and the ICs  74  are intended to be spaced about 11 microns apart, measured perpendicular to the joining edge. In this arrangement, the left most ink delivery nozzles (not shown but fabricated on the ink delivery inlets  73 ) on each row are dropped by 10 line pitches and arranged in a triangle configuration. This arrangement provides a degree of overlap of nozzles at the join and maintains the pitch of the nozzles to ensure that the drops of ink are delivered consistently along the printing zone. This arrangement also ensures that more silicon is provided at the edge of the IC  74  to ensure sufficient linkage. Whilst control of the operation of the nozzles is performed by the SoPEC device (discussed later in of U.S. Ser. No. 11/014,769 cross referenced above), compensation for the nozzles may be performed in the printhead, or may also be performed by the SoPEC device, depending on the storage requirements. In this regard it will be appreciated that the dropped triangle arrangement of nozzles disposed at one end of the IC  74  provides the minimum on-printhead storage requirements. However where storage requirements are less critical, shapes other than a triangle can be used, for example, the dropped rows may take the form of a trapezoid. 
     The upper surface of the printhead ICs have a number of bond pads  75  provided along an edge thereof which provide a means for receiving data and or power to control the operation of the nozzles from the SoPEC device. To aid in positioning the ICs  74  correctly on the adhesive surface of the polymer sealing film  71  and aligning the ICs  74  such that they correctly align with the holes  72  formed in the polymer sealing film  71 , fiducials  76  are also provided on the surface of the ICs  74 . The fiducials  76  are in the form of markers that are readily identifiable by appropriate positioning equipment to indicate the true position of the IC  74  with respect to a neighboring IC and the surface of the polymer sealing film  71 , and are strategically positioned at the edges of the ICs  74 , and along the length of the polymer sealing film  71 . 
     In order to receive the ink from the holes  72  formed in the polymer sealing film  71  and to distribute the ink to the ink inlets  73 , the underside of each printhead IC  74  is configured as shown in  FIG. 7 . A number of etched channels  77  are provided, with each channel  77  in fluid communication with a pair of rows of inlets  73  dedicated to delivering one particular colour or type of ink. The channels  77  are about 80 microns wide, which is equivalent to the width of the holes  72  in the polymer sealing film  71 , and extend the length of the IC  74 . The channels  77  are divided into sections by silicon walls  78 . Each section is directly supplied with ink, to reduce the flow path to the inlets  73  and the likelihood of ink starvation to the individual nozzles. In this regard, each section feeds approximately 128 nozzles via their respective inlets  73 . 
       FIG. 9  shows more clearly how the ink is fed to the etched channels  77  formed in the underside of the ICs  74  for supply to the nozzles. As shown, holes  72  formed through the polymer sealing film  71  are aligned with one of the channels  77  at the point where the silicon wall  78  separates the channel  77  into sections. The holes  72  are about 80 microns in width which is substantially the same width of the channels  77  such that one hole  72  supplies ink to two sections of the channel  77 . It will be appreciated that this halves the density of holes  72  required in the polymer sealing film  71 . 
     Following attachment and alignment of each of the printhead ICs  74  to the surface of the polymer sealing film  71 , a flex PCB  79  (see  FIG. 4 ) is attached along an edge of the ICs  74  so that control signals and power can be supplied to the bond pads  75  to control and operate the nozzles. As shown more clearly in  FIG. 1 , the flex PCB  79  extends from the printhead assembly  22  and folds around the printhead assembly  22 . 
     The flex PCB  79  may also have a plurality of decoupling capacitors  81  arranged along its length for controlling the power and data signals received. As best shown in  FIGS. 2 and 3 , the flex PCB  79  has a plurality of electrical contacts  180  formed along its length for receiving power and or data signals from the control circuitry of the cradle unit  12 . A plurality of holes  80  are also formed along the distal edge of the flex PCB  79  which provide a means for attaching the flex PCB to the flange portion  40  of the rigid plate  34  of the main body  20 . The manner in which the electrical contacts of the flex PCB  79  contact the power and data contacts of the cradle unit  12  will be described later. 
     As shown in  FIG. 4 , a media shield  82  protects the printhead ICs  74  from damage which may occur due to contact with the passing media. The media shield  82  is attached to the upper member  62  upstream of the printhead ICs  74  via an appropriate clip-lock arrangement or via an adhesive. When attached in this manner, the printhead ICs  74  sit below the surface of the media shield  82 , out of the path of the passing media. 
     A space  83  is provided between the media shield  82  and the upper  62  and lower  65  members which can receive pressurized air from an air compressor or the like. As this space  83  extends along the length of the printhead assembly  22 , compressed air can be supplied to the space  83  from either end of the printhead assembly  22  and be evenly distributed along the assembly. The inner surface of the media shield  82  is provided with a series of fins  84  which define a plurality of air outlets evenly distributed along the length of the media shield  82  through which the compressed air travels and is directed across the printhead ICs  74  in the direction of the media delivery. This arrangement acts to prevent dust and other particulate matter carried with the media from settling on the surface of the printhead ICs, which could cause blockage and damage to the nozzles. 
     Active Ink Flow Control System 
     The present invention gives the user a versatile control system for correcting many of the detrimental conditions that are possible during the operative life of the printer. It is also capable of preparing the printhead for transport, long term storage and re-activation. It can also allow the user to establish a desired negative pressure at the printhead IC nozzles. The control system requires easily incorporated modifications to the prior art printer designs described above.
 
Printhead Maintenance Requirements
 
The printer&#39;s maintenance system should meet several requirements:
         sealing for hydration   sealing to exclude particulates   drop ejection for hydration   drop ejection for ink purge   correction of dried nozzles   correction of flooding   correction of particulate fouling   correction of outgassing   correction of color mixing and   correction of deprime       

     Various mechanisms and components within the printer assembly are designed with a view to minimizing any problems that the printhead maintenance system will need to address. However, it is unrealistic to expect that the design of the printer assembly components can deal with all the problems that arise for the printhead maintenance system. In relation to sealing the nozzle face for hydration and sealing the nozzles to exclude particulates the maintenance system can incorporate a capping member with a perimeter seal that will achieve these two requirements. 
     Drop ejection for hydration (or keep wet drops) and drop ejection for ink purge require the print engine controller (PEC) to play a roll in the overall printhead maintenance system. 
     The particulate fouling can be dealt with using filters positioned upstream of the printhead. However, care must be taken that small sized filters do not become too much of a flow constriction. By increasing the surface area of the filter the appropriate ink supply rate to the printhead can be maintained. 
     Correcting a flooded printhead will typically involve some type of blotting or wiping mechanism to remove beads of ink on the nozzle face of the printhead. Methods and systems for removing ink flooded across an ink ejection face of a printhead are described in our earlier filed U.S. application Ser. Nos. 11/246,707 (“Printhead Maintenance Assembly with Film Transport of Ink”), 11/246,706 (“Method of Maintaining a Printhead using Film Transport of Ink”), 11/246,705 (“Method of Removing Ink from a Printhead using Film Transfer”), and 11/246,708 (“Method of Removing Particulates from a Printhead using Film Transfer”), all filed on Oct. 11, 2005. The contents of each of these US applications are incorporated herein by reference. 
     Dried nozzles, outgassing, color mixing and nozzle deprime are more difficult to correct as they typically require a strong ink purge. Purging ink is relatively wasteful and creates an ink removal problem for the capping mechanism. Again the arrangements described in the above referenced US applications incorporate an ink collection and transport to sump function. 
     Outgassing is a significant problem for printheads having micron scale fluid flow conduits. Outgassing occurs when gasses dissolved in the ink (typically nitrogen) come out of solution to form bubbles. These bubbles can lodge in the ink line or even the ink ejection chambers and prevent the downstream nozzles from ejecting. 
       FIG. 10  shows the underside of the LCP moulding  65 . Conduits  70  extend between the point where the printed IC (not shown) is mounted and the holes  69 . Bubbles from outgassing  100  form in the upstream ink line and feed down to the printed IC. 
       FIG. 11  shows the artifacts that result from outgassing bubbles. As the bubbles  100  feed into the printhead IC, the nozzles deprime and start ejecting the bubble gas rather than ink. This appears as arrow head shaped artifacts  102  in the resulting print. Hopefully pressure from upstream ink flow eventually clears the bubble from the printhead IC and the artifacts disappear. However, the bubbles  100  can have a tendency to get stuck at conduit discontinuities. Discontinuities such as the silicon wall  78  across the channel  77  in the printhead IC (see  FIG. 9 ) tend to trap some of the bubbles and effectively form an ink blockage to nozzles fed from that end of the channel  77 . These usually result in streak type artifacts  104  extending from the bottom corners of the arrow head artifact  102 . There is a significant risk that these bubbles do not eventually clear with continued printing which can result in persistent artifacts or nozzle burn out from lack of ink cooling. 
     Another problem that is difficult to address using component design is color mixing. Color mixing occurs when ink of one color establishes a fluid connection with ink of another color via the face of the nozzle plate. Ink from one ink loan can be driven into the ink loan of a different color by slightly different hydraulic pressures within each line, osmotic pressure differences and even simple diffusion. 
     Capping and wiping the nozzle plate will remove the vast majority of particulates that create the fluid flow path between nozzles. However, printhead IC&#39;s with high nozzle densities require only a single piece of paper dust or thin surface film to create significant color mixing while the printer is left idle for hours or overnight. 
     Instead of placing a heavy reliance on the design of the printhead assembly components to deal with factors that give rise to printhead maintenance issues, the present invention uses an active control system for the printhead maintenance regime to correct issues as they arise. 
       FIG. 12  is a schematic representation of the fluid architecture for the printhead shown in  FIGS. 1 to 11 . The different ink colors are fed from respective ink tanks  112  to the LCP manifold  164  via a filter  160  and pressure regulator  162 . The inlet  166  to the LCP manifold  164  is intermediate the ends of its elongate top molding to assist the ink to evenly fill the length of the channel  67  (see  FIG. 10 ). From the channels  67 , the ink is fed through holes to the smaller conduits  70  (see  FIG. 10 ) that lead to the five separate printhead IC&#39;s  74 . This architecture terminates the ink line at the printhead IC  74 . Hence any attempts to change the ink flow conditions within the printhead IC  74  need to occur by intervention upstream. 
     Actively Controlled Flow Conditions 
       FIG. 13  is a fluid architecture in which the printhead IC  74  is not the end of the ink line. The channels  67  in the LCP manifold  164  are fed with ink from the ink tank  112  via a filter and pressure regulator  162 . The inlet  166  to the LCP ink manifold  164  is at one end instead a point intermediate the ends. As with the prior art fluid system, the ink is still fed to the smaller conduits  70  (see  FIG. 10 ) and finally the printhead IC&#39;s  74 . However, the invention provides an ink outlet  172  at the opposite end of the LCP manifold  164  so that the ink line continues downstream to connect the LCP manifold back to the ink tank  112 . If necessary, the downstream ink line could lead to an ink sump (not shown) but it will be appreciated that this is an inefficient use of ink. 
     Optionally, the fluidic system can have a branched downstream ink line that can selectively feed to a sump or recirculate back to the ink tank  112 .  FIG. 14  shows a fluidic architecture with this configuration. This option is useful if the downstream ink flow is likely to be contaminated with other inks. The downstream flow can be initially diverted to the sump  184  until the LCP manifold has been flushed, and then recirculated to the ink tank  112  once again. The upstream ink line has a pump  168  driven by motor  170 . Similarly, the downstream ink line has a pump  176  driven by another motor  174 . Optionally, the upstream and downstream pumps are not two separate pumps, but rather two separate lines running through a single pump. This can be implemented with a six-way peristaltic pump head driven with a single motor. However, for the purposes of illustrating the conceptual basis of the system, the pumps  168  and  176  are shown as separate elements with individual drives  170  and  174 . 
     The downstream ink line terminates at an ink outlet  180  in the ink tank  112 . Returning the ink to the ink tank  112  is, of course, far more efficient than purging it to a waste sump. Using this system, outgassing bubbles can completely bypass the printhead IC  74  in favour of the downstream ink line. Any bubble introduced into the ink line when the ink cartridges are replaced can also be purged. Likewise, the pressure from the upstream pump  168  can be used to recover dried and or clogged nozzles. In fact, all the printhead maintenance requirements listed above can be performed automatically or user initiated with the active control system shown. 
     Controlled Printhead Assembly Deprime 
     The ink tank  112  has an air inlet  178  so that the LCP manifold can be deprimed of ink if desired. Depriming for storage or shipping guards against ink leakage or color mixing between ink lines during period of inactivity (discussed above). It also allows the user to reprime the printhead assembly to a known ‘good’ state before use or after an inadvertent deprime. Depriming the LCP manifold is also useful for cleaning particulates from the exposed face of the printhead IC&#39;s  74  by creating an ink foam. By depriming the LCP manifold  164 , residual ink remains in the small conduits  70  and the printhead IC&#39;s  74 . Pumping air with the upstream pump  168  and shutting off the downstream flow by stopping pump  176 , the air escapes through the ejection nozzles and foams the residual ink. This cleaning technique is described in detail in the Applicant&#39;s co-pending applications the contents of which are incorporated herein by reference. 
     The upstream and downstream pumps  168  and  176  can be provided by peristaltic pumps. In the printers of the type shown in the above referenced U.S. Ser. No. 11/014,769 the peristaltic pumps have a displacement resolution of 10 microliters. This equates to about 5 mm of travel on an appropriately dimensional peristaltic tube. These specifications give the most flow rate of about 3 milliliters per minute and very low pulse in the resulting flow. 
       FIG. 14  shows a single pump implementation of the fluidic control system. The upstream pump has been replaced with an impulse generator in the form of an accumulator  182 . The accumulator generates a short pressure burst to prime the fine structures (conduits  70 ) of the LCP manifold and the printhead IC  74 . In this embodiment, the downstream pump  176  sucks ink into the LCP manifold  164 . To prevent air being drawn in through the nozzles of the printhead IC&#39;s, a capping member  190  forms a perimeter seal over the nozzle array. Once the pump  176  has filled the main channels  67  of the LCP manifold, the accumulator  182  creates an impulse to prime the nozzles of the printhead IC  74 . The impulse also floods the face of the printhead IC with ink. The flooded ink may be removed with mechanisms described in the above referenced FNE27US, FNE28US and FNE29US. Once the nozzle flood has been cleaned, a brief purge print will print out any superficial mixed ink. 
     The single pump embodiment uses three valves per color a sump valve  186  for diverting flow to the sump  184 , an ink tank valve  188  and the accumulator  182  (which can be open or closed). Ideally, the valves should be zero displacement, zero leak, fast and easy to actuate. Ordinary workers in this field will readily identify a range of suitable valve mechanisms. Obviously, the accumulator will not be zero displacement but the pressure impulse is often required immediately prior to its role as a shut off valve so its displacement is not generally detrimental. For a three color printer, the fluidic system involves nine valves, three pumps and the perimeter seal on the capper. Hence the control of flow conditions within the printhead assembly is provided using relatively few active components. 
     The invention has been described herein by way of example only. Skilled workers in this field will readily recognise many variations and modifications which do not depart from the spirit and scope of the broad inventive concept.