Patent Description:
Inkjet printers employing Memjet® pagewide technology are commercially available for a number of different printing applications, including desktop printers, digital inkjet presses and wideformat printers. Memjet® printers typically comprise one or more stationary inkjet printheads having a length of at least <NUM>, which are user-replaceable. For example, a desktop label printer comprises a single user-replaceable full color, a high-speed inkjet press comprises a plurality of user-replaceable monochrome printheads aligned along a media feed direction, and a wideformat printer comprises a plurality of user-replaceable printheads in a staggered overlapping arrangement so as to span across a wideformat media feed path.

Analogue printing presses are conventionally used for relatively long print runs where the cost of producing dedicated printing plates is economically feasible. Increasingly, industrial print systems use single-pass digital inkjet printing for relatively shorter print runs. Digital inkjet printing avoids the high set-up costs of producing printing plates and allows each print job to be tailored to a particular customer. Desirably, web feed systems for existing analogue print systems should be adaptable so as to enable 'drop-in' inkjet modules in place of, for example, offset printing stations. It is therefore desirable for inkjet modules to occupy minimal space with respect to a media feed direction, whilst allowing full color printing at high speeds with optimum print quality.

Memjet® printing technology, which uses rows of print chips butted end-on-end to construct a pagewide printhead, is highly suited for reducing the overall span of the print zone along a media feed direction. Each print chip has five rows of nozzles, which may be used for 5x redundant printing in a monochrome printhead.

<CIT> describes a printing system having a configurable array of print modules, each print module having a respective monochrome printhead configured for single-pass printing. Four print modules may be arranged along a media path for full-color (CMYK) printing with 5x redundancy in each color plane. While the system described in <CIT> provides OEMs with flexibility in the design of inkjet presses, as well as high-quality and high-speed printing using 5x redundancy, the print modules must be aligned and spaced along the media feed path for full-color printing. This places demands on media feed systems, which are required to align all colors and, consequently, there are relatively high set-up costs for OEMs. Nevertheless, those costs are a still significantly less than alternative pagewide printing systems that use overlapping print chips or very large print chips to achieve single-pass printing.

<CIT> describes a full-color pagewide printhead having two rows of butting print chips receiving ink from a common manifold. The printhead has 2x redundancy for each ink color provided by four active nozzle rows in each row of print chips.

It would be desirable to provide a low-cost printing unit having multiple redundancy in each ink color, which minimizes a span of the print zone along the media feed direction for printing in four colors (CMYK). It would be further desirable to provide such a printing unit, which allows access to printhead(s) for replacement, simplifies printhead alignment and set-up procedures, and enables printing with variable printhead-paper-spacing (PPS) whilst optimizing print quality.

Documents <CIT> and <CIT> disclose inkjet printhead assemblies having a configurable array of print modules.

According to the invention, there is provided a printing unit comprising:.

The printing unit according to the first aspect advantageously provides a highly compact printing system for printing in full color, with multiple redundancy and a narrow print zone span. The printing unit is therefore well suited for use as a 'drop-in' unit in existing media feed systems currently used for analogue printing. In essence, the printing unit functions as a 'mini inkjet press' inasmuch as it achieves a similar print quality to existing inkjet presses having a series of monochrome printheads, but with a fraction of the footprint and cost.

Preferably, each compensatory set of nozzles comprises dropped nozzle rows offset from main nozzle rows in each print chip, the print chip being configured to either delay or advance firing of the dropped nozzle rows relative to the main nozzle rows.

Preferably, each printhead is asymmetrically disposed towards a front side of its respective inkjet module, and wherein the opposed inkjet modules in said forward and reverse orientations position the pair of printheads in a relatively proximal arrangement and position a pair of cappers in a relatively distal arrangement.

Preferably, a span of a print zone along the media feed direction is less than <NUM>.

Preferably, each row of print chips has at least fourfold redundancy, whereby aligned nozzles of at least four nozzle rows are capable of printing onto a same pixel position.

Preferably, each inkjet module comprises a module chassis having a base plate with rear and end walls extending upwards therefrom.

Preferably, each base plate is C-shaped in plan view having a pair of transverse arms extending parallel to the media feed direction from opposite ends of a longitudinal base member extending perpendicular to the media feed direction, each base plate defining an open longitudinal slot for receiving a respective printhead.

Preferably, the opposed inkjet modules in said forward and reverse orientations have opposed C-shaped base plates, such that the pair of open longitudinal slots are proximally positioned relative to the pair of longitudinal base members.

Preferably, a second inkjet module in the reverse orientation is rotated by <NUM> degrees relative to a first inkjet module in the forward orientation.

In a second aspect, there is provided a method of printing redundantly in four colors from first and second printheads aligned along a media feed direction, each of the first and second printheads having respective first and second rows of print chips butted end-on-end, the first and second rows of print chips having <NUM> degree rotational symmetry, said method comprising the steps of:.

wherein compensatory sets of nozzles in the first and second rows of print chips are offset, along the media feed direction, in each of the first and second printheads.

The method according to the second aspect advantageously hides print artefacts arising from compensatory sets of nozzles at chip join regions, as described below in connection with <FIG>.

Preferably, each print chip contains a respective compensatory set of nozzles at one end thereof adjacent a neighboring print chip.

Preferably, the compensatory set of nozzles comprises dropped nozzle rows offset from main nozzle rows in a respective print chip.

Preferably, firing of the dropped nozzle rows is either advanced or delayed relative to respective main nozzle rows in each print chip.

Preferably, the first row of print chips in each printhead advances firing of respective dropped nozzle rows and the second row of print chips delays firing of respective dropped nozzle rows, or vice versa.

Preferably, each compensatory set of nozzles in the first row of print chips is aligned, along the media feed direction, with main nozzle rows in the second row of print chips.

Preferably, printing of each ink is shared, at join regions between neighboring chips, between one or more nozzle rows of the compensatory set of nozzles in the first row or print chips and one or more nozzle rows of the main nozzle rows in the second row of print chips.

Preferably, print artefacts arising from the compensatory set of nozzles in the first row of print chips are masked by the main nozzle rows in the second row of print chips.

Preferably, only one ink is printed from two nozzle rows per print chip.

In one embodiment, each print chip comprises five nozzle rows, and a central nozzle row does not receive any ink.

In a related third aspect, there is provided a printing assembly comprising:.

wherein compensatory sets of nozzles in the first and second rows of print chips are offset, along the media feed direction.

Preferably, the first row of print chips advances firing of respective dropped nozzle rows and the second row of print chips delays firing of respective dropped nozzle rows, or vice versa.

It will, of course, be appreciated that preferred embodiments described above in connection with the first, second and third aspects, are equally applicable to any one of the first, second and third aspects, where relevant.

As used herein, the term "inkjet module" is taken to mean an assembly of components, which includes an inkjet printhead, such as an elongate printhead configured for single-pass printing (known in the art as a "pagewide" or "linehead" printhead). The inkjet module typically includes one or more of the following components to provide a fully integrated inkjet system: maintenance components, such as a capper and/or a wiper; mechanisms for moving the printhead and/or maintenance components; ink delivery components, such as pump(s), valve(s), ink connector(s) etc; and electronic circuitry for supplying power and/or data to the printhead.

As used herein, the term "ink" is taken to mean any printing fluid, which may be printed from an inkjet printhead. The ink may or may not contain a colorant. Accordingly, the term "ink" may include conventional dye-based or pigment based inks, infrared inks, fixatives (e.g. pre-coats and finishers), 3D printing fluids, solar inks, biological fluids, sensing fluids and the like.

As used herein, the term "mounted" includes both direct mounting and indirect mounting via an intervening part.

Specific embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:.

Referring to <FIG>, there is shown a printing unit <NUM> mounted on a support chassis <NUM> configured for feeding media past the printing unit along a media feed direction M. The printing unit <NUM>, shown in isolation in <FIG>, comprises a unit chassis <NUM> and a pair of opposed upstream and downstream inkjet modules 1A and 1B mounted in tandem on the unit chassis in forward and reverse orientations. An individual inkjet module <NUM> is described in detail below.

Each inkjet module <NUM> comprises a module chassis <NUM> pivotally mounted on chassis side bars <NUM> of the unit chassis <NUM> about a respective pair of module pivots <NUM> positioned at opposite sides of the module chassis. Accordingly, each inkjet module <NUM> is pivotable about a pivot axis perpendicular to the media feed direction M. The upstream and downstream inkjet modules 1A and 1B of the printing unit <NUM> are pivotally movable towards and away from each other, such that the printing unit may be configurable in a clamshell-closed configuration for printing (<FIG> and <FIG>) and a clamshell-open configuration (<FIG>) for printhead replacement and/or maintenance. Each module chassis <NUM> has an open respective front face, which facilitates access to internal components of each individual inkjet module <NUM> in the clamshell-open configuration by virtue of the opposed relationship of the inkjet modules in the printing unit (i.e. one inkjet module is rotated by <NUM> degrees relative to the other inkjet module). Gas struts <NUM> interconnect each module chassis <NUM> and the chassis side bars <NUM> to provide a dampened overcenter pivoting mechanism for each inkjet module <NUM>.

In the embodiment shown in <FIG>, the upstream and downstream inkjet modules 1A and 1B are independently pivotable so that one or both of the inkjet modules may be pivoted. However, the skilled person will appreciate that other pivoting mechanisms may be employed, whereby the pair of module chassis <NUM> are mechanically linked such that the inkjet modules necessarily both pivot into the clamshell-open positioned. These and other pivoting mechanisms will be readily apparent to the person skilled in art.

Each individual inkjet module <NUM> is a fully integrated unit comprising a respective printhead <NUM>, as well as a capper and a wiper for maintaining the printhead. Each printhead <NUM> is of the type described in <CIT> and comprises two rows of print chips <NUM> mounted on a unitary surface of a respective ink manifold. Each row of print chips <NUM> comprises a plurality of print chips butted end-on-end along a length of its respective printhead <NUM>. Each inkjet module <NUM> prints two colors of ink from two rows of print chips <NUM> of its respective printhead <NUM> and, furthermore, printheads <NUM> of the pair of inkjet modules <NUM> mounted in tandem on the unit chassis <NUM> are wholly aligned with respect to a media feed direction M, such that the printing unit <NUM> is configured for redundant full color printing of four ink colors (CMYK). Redundancy in each color channel is provided by multiple aligned nozzle rows (e.g. <NUM>, <NUM> or <NUM> nozzle rows) in each printhead <NUM> that are wholly aligned along the media feed direction M and print the same colored ink. Each set of aligned nozzles is, therefore, capable of printing onto a same pixel position during single-pass printing from the stationary printheads <NUM> to provide redundancy in each color.

The module chassis <NUM> of each inkjet module <NUM> has an elongate base plate <NUM> with a rear wall <NUM> and a pair of opposite end walls <NUM> extending upwards from the base plate. Each base plate is C-shaped in plan view having a pair of transverse arms <NUM> extending parallel to the media feed direction from opposite ends of a longitudinal base member <NUM> extending perpendicular to the media feed direction. Each base plate <NUM>, therefore, defines an open longitudinal slot <NUM> for receiving a respective printhead <NUM>. In <FIG>, the printheads <NUM> are raised above the base plate <NUM> for capping and/or wiping, and in <FIG> the printheads are lowered through the slots <NUM> for printing.

The opposed upstream and downstream inkjet modules 1A and 1B in respective forward and reverse orientations in the printing unit <NUM> have opposed C-shaped base plates <NUM>, such that the pair of open longitudinal slots <NUM> are proximally positioned relative to the pair of longitudinal base members <NUM>. Accordingly, the printheads <NUM>, which are received through their respective slots <NUM>, are disposed relatively proximally, thereby minimizing the total span of the print zone (indicated by double-headed arrow Z in <FIG>) along the media feed direction M. For example, the print zone Z containing both printheads <NUM> may span less than <NUM>, less than <NUM> or less than <NUM> in the printing unit <NUM>.

Referring to <FIG>, the base plates <NUM> also serve as datum plates for each inkjet module <NUM> via datuming engagement with chassis datum blocks <NUM> projecting inwardly from respective chassis side bars <NUM>. Each chassis datum block <NUM> serves as a common datum for opposed transverse arms <NUM> of the pair of base plates <NUM>. The chassis datum blocks <NUM> provide a z-datum for each inkjet module <NUM>, as well as having x- and y-datum features for gross datuming of the inkjet modules along x- and y-axes. Fine adjustment of the relative skew of printheads <NUM> about a theta z-axis is provided by a printhead nest, as will be explained in further detail below.

In <FIG> the printing unit <NUM> is shown in its printing position with both printheads <NUM> projecting through respective slots <NUM> of the base plates <NUM>. An aerosol extractor <NUM> is mounted to a rear end bar <NUM> of the unit chassis <NUM> and extends beneath the base plate <NUM> of the downstream inkjet module 1B towards the downstream printhead, relative to the media feed direction M. The aerosol extractor <NUM> is cantilevered by virtue of being pivotally mounted to the rear end bar <NUM> via a spring-loaded pivot mount <NUM>, and has a free end proximal the downstream printhead <NUM>.

The aerosol extractor <NUM> comprises a ducting arm <NUM>, which extends from a vacuum port <NUM> at one end and connected to a suction manifold <NUM> at an opposite end. The ducting arm <NUM> and the suction manifold <NUM> have a generally low height profile with a planar lower surface extending parallel with a plane of the base plates <NUM>. In its quiescent position, the aerosol extractor <NUM> is biased against the base plate <NUM> of the downstream inkjet module <NUM> and extends parallel with a media feed path so as to occupy minimal space between the base plate and, for example, a platen for supporting print media.

The suction manifold <NUM> is coextensive with the slots <NUM> and has a plurality of suction nozzles <NUM> for extracting aerosol from the vicinity of the print zone. The suction nozzles <NUM> are configured to direct an airflow through the print zone generally along the same direction as the media feed direction M. Therefore, the aerosol extractor <NUM> not only serves to remove ink mist, but also assists in stabilizing vortices associated with a stream of droplets in the print zone during printing. The base plate <NUM> of the upstream inkjet module 1A facilitates uniform airflow through the print zone, which is optimal for stabilizing vortices associated with a stream of droplets ejected from the printheads <NUM>. Airflow provided by the aerosol extractor <NUM> may be further optimized by, for example, an optional interstitial bar having a respective plate positioned between the printheads <NUM> to provide a more uniform airflow between the printheads and through the print zones (see <FIG>).

The printing unit <NUM> is configurable for printing at different throw distances relative to print media (known in the art as printhead-paper-spacing or PPS) by virtue of adjusting the heights of the printheads <NUM> using lift mechanisms in each inkjet module <NUM>. Height adjustment of printheads <NUM> typically disrupts an optimized airflow through the print zone. However, in the printing unit <NUM>, the cantilevered aerosol extractor <NUM> enables a height of the suction nozzles proximal the downstream printhead to be adjusted. In particular, and referring now to <FIG>, a leading tab portion <NUM> connected to the suction manifold <NUM> is positioned for butting engagement with a printhead nest <NUM> of supporting the downstream printhead <NUM>. Hence, when the downstream printhead <NUM> is lowered, butting engagement of the printhead nest <NUM> with the tab portion <NUM> pivots the suction manifold <NUM> against the bias of the pivot mount <NUM> and thereby lowers the height of the suction nozzles <NUM> commensurate with the height of the printhead <NUM>. When the printhead <NUM> is raised, the bias of the pivot mount <NUM> causes the suction manifold <NUM> to be raised with the printhead. In this way, the printing unit <NUM> is suitable for variable PPS printing with optimized aerosol extraction and airflow through the print zone.

In one embodiment, the printheads <NUM> may be plumbed such that each row of print chips <NUM> (with individual print chips <NUM> having multiple aligned nozzle rows for redundant printing) receives only one color of ink. With four rows of print chips <NUM> across two printheads <NUM>, full color (CMYK) redundant printing may be achieved using all nozzle rows in each print chip. In this way, the printing unit <NUM> can mimic a conventional inkjet press (e.g. FujiFilm JPress <NUM>) having monochrome inkjet print bars, albeit with a much narrower print zone (and lower cost) than conventional systems.

However, in order to maximize print quality, the printing unit <NUM> may, in an alternative embodiment shown in <FIG>, make use of the inherent architecture of each printhead <NUM> having two rows of print chips <NUM> with <NUM> degree rotational symmetry. As described in <CIT>, first and second rows of print chips 5A and 5B in each printhead <NUM> comprise four color channels whereby each individual print chip <NUM> within one row may be supplied with two colors of ink. Thus, the two printheads <NUM> in the printing unit <NUM> may be considered to have <NUM> color channels (two color channels per row of print chips <NUM>) for printing four different inks (CMYK).

A fundamental problem, which is ubiquitous in any pagewide printing system having multiple print chips, is a loss of print quality at join regions between print chips <NUM>. Inevitably, pagewide printheads require some form of compensation in order to print across chip join regions using, for example, an electronic stitching technique, mechanical positioning of chips, a dedicated chip design to enable butting chips, or combinations thereof. In Memjet® printheads <NUM> having butting chips <NUM>, print quality problems are generally minimized by virtue of the physical proximity of neighboring chips and a proprietary chip architecture having 'dropped nozzle rows' (see, for example, <CIT>). Nozzle firing in the dropped nozzle rows is either delayed or advanced relative to main nozzle rows, depending on the orientation of the print chip <NUM>, in order to provide seamless joins between neighboring print chips. Nevertheless, print artefacts may still exist as a result of the dropped nozzle rows in Memjet® printheads, especially in certain print modes, as described in <CIT>.

The printing unit <NUM> having two color channels available per ink color allows the printheads <NUM> to be plumbed so as to mask any print artefacts arising from join regions between neighboring print chips <NUM>. Essentially, each color of ink is allocated to a first color channel of a print chip <NUM> in the first row 5A in a forward orientation and a second color channel of a print chip <NUM> of the second row 5B in a reversed orientation (i.e. an orientation rotated <NUM> degrees relative to the print chip of the first row 5A). In this way, a compensatory set of nozzles 8A in the print chip of the first row 5A (e.g. dropped nozzle rows) are offset from a compensatory set of nozzles 8B in the print chip of the second row 5B. Thus, any print artefacts arising from dropped nozzle rows 8A in the first row of print chips 5A are minimized by corresponding (i.e. aligned) nozzles from a main nozzle region 7B in the second row of print chips 5B. Likewise, any print artefacts arising from dropped nozzle rows 8B in the second row of print chips 5B are minimized by corresponding (i.e. aligned) nozzle from a main nozzle region 7A in the first row of print chips 5A.

In the printhead <NUM> shown in <FIG>, the first row of print chips 5A has two cyan (C) nozzle rows (each nozzle row having 'odd' and 'even' sub-rows) and two yellow (Y) nozzle rows. The middle nozzle row (N) is unused, as described in <CIT>, to provide separation between the color channels and minimize ink mixing on the nozzle plate. Likewise, the second row of print chips 5B has two cyan (C) nozzle rows and two yellow (Y) nozzle rows. For each color (e.g. cyan) there are four nozzles aligned along the media feed direction to provide 4x redundancy. However, as shown in <FIG>, only two cyan dots originate from the dropped nozzle rows 8A of the first row of print chips 5A; the other two cyan dots originate from the main nozzle rows 7B of the second row of print chips 5B. Likwise, aligned yellow (Y) dots originate from the dropped nozzle rows 8A of the first row of print chips 5A and the main nozzle rows 7B of the second row of print chips 5B.

Accordingly, it will be appreciated that the <NUM> degree rotational symmetry of the first and second rows of print chips 5A and 5B in the same printhead <NUM> allows print artefacts originating from the dropped nozzle rows <NUM> to be hidden or at least minimized. This complementary arrangement of first and second rows of print chips 5A and 5B in each printhead <NUM>, combined with a suitable ink plumbing order, advantageously maximizes print quality in the printing unit <NUM> having two printheads. Each printhead <NUM> receives two colors of ink, but both inks are supplied to both rows of print chips <NUM> in a respective printhead.

For the sake of completeness, an individual inkjet module <NUM> used in tandem in the printing system <NUM> will now be described with reference to <FIG>.

As shown in <FIG>, the inkjet module <NUM> comprises the chassis <NUM> having the elongate base plate <NUM> with the rear wall <NUM> and a pair of opposite end walls <NUM> extending upwards from the base plate. Aside from providing the chassis <NUM> with structural rigidity, the rear wall <NUM> also serves as a support for mounting various fluidic components (e.g. pinch valves <NUM> and pumps <NUM>) and electronic components (e.g. module controller PCB <NUM>) on both its front and rear faces. Openings in the rear wall <NUM> allow fluidic connections from the rear face of the inkjet module <NUM>, without requiring overhead access. Openings may also be provided for the purpose of accessing in situ a screw adjuster of either printhead nest <NUM> in the printing unit <NUM> using a suitable tool (not shown), as will be explained in further detail below.

The base plate <NUM> is generally C-shaped in plan view having the pair of transverse arms <NUM> extending from opposite ends of the longitudinal base member <NUM> along a nominal x-axis of the inkjet module <NUM>. The open longitudinal slot <NUM>, defined between the transverse arms <NUM>, extends parallel with a longitudinal axis along a nominal y-axis of the inkjet module <NUM> and is configured for receiving the elongate printhead <NUM>. Thus, the printhead <NUM> is asymmetrically positioned in the inkjet module <NUM> towards a front side thereof, so that the printheads are positioned proximally in the printing unit <NUM>. The printhead <NUM> may be either lowered through the slot <NUM> for printing or raised above the base plate <NUM> for maintenance (e.g. capping and/or wiping).

A pair of posts <NUM> extend upwards from the transverse arms <NUM> of the base plate <NUM> at opposite ends of the open longitudinal slot <NUM>. Each post <NUM> is anchored to the base plate <NUM> at a lower end thereof and secured to a respective end wall <NUM> at an upper end thereof. A pair of brackets <NUM> are slidably engaged with the posts <NUM> via respective sleeve bushings <NUM> inserted in each bracket. Each sleeve bushing <NUM> is slidably movable relative to a respective post <NUM> allowing vertical linear movement of the brackets <NUM> towards and away from the base plate <NUM> along a nominal z-axis of the inkjet module <NUM>. A flanged portion <NUM> at a lower end of each sleeve bushing <NUM> is fastened to each bracket <NUM> and datums its respective bracket against the base plate <NUM> in the printhead lowered position (<FIG>).

An elongate printhead carrier <NUM> is fixedly supported between the brackets <NUM> and is linearly slidably movable with the brackets. The printhead carrier <NUM> comprises spaced apart front and rear carrier plates <NUM> interconnecting the brackets <NUM> and defining a cavity <NUM> therebetween for housing electronic components supplying power and data to the printhead <NUM>. A brace <NUM> interconnects upper parts of the carrier plates <NUM>, while a pair of carrier datum blocks <NUM> interconnect lower parts of the carrier plates. The carrier datum blocks <NUM> are positioned at opposite longitudinal ends of the printhead carrier <NUM> towards respective brackets <NUM>. The braced printhead carrier <NUM>, in combination with the sleeve bushings <NUM>, posts <NUM> and chassis <NUM> provide a robust support structure for the printhead <NUM>. The printhead <NUM> is itself secured within a complementary nest <NUM> to form a printhead nest assembly <NUM>, which is mounted to the carrier datum blocks <NUM> via screw fasteners <NUM> engaged with the nest.

The printhead <NUM> is linearly slidably movable towards and away from the base plate <NUM> between a printing position (<FIG>) and a maintenance position (<FIG>) by means of a lift mechanism operatively connected to each bracket <NUM>. The lift mechanism also enables the height of the printhead <NUM> to be adjusted relative to print media in the printing position. As best shown in <FIG>, the lift mechanism comprises a pair of lead screws <NUM> rotatably mounted to the base plate <NUM> and extending upwards parallel with the posts <NUM>. Each lead screw <NUM> has respective lead nut <NUM> fixedly connected to a respective bracket via a lead nut connector <NUM>. The lead screws <NUM> are rotatable by means of an interconnecting pulley belt assembly operatively <NUM> connected to a common lift motor <NUM>. Accordingly, the printhead <NUM> may be raised and lowered by actuation of the lift motor <NUM>, which rotates the leads screws <NUM> simultaneously via the pulley belt assembly <NUM>, thereby raising or lowering the printhead carrier <NUM> connected to the lead nuts <NUM> via the brackets <NUM>.

As best shown in <FIG>, the inkjet module <NUM> comprises a wiper carriage <NUM>, having a microfiber wiping web <NUM>, parked at one end of the longitudinal slot <NUM>. In the printhead raised position, the wiper carriage <NUM> is movable longitudinally along the length of printhead <NUM> by means of a wiper movement mechanism <NUM> mounted on a longitudinal wiper support <NUM> in order to wipe ink and debris from the printhead face. In the printhead lowered position (<FIG>), one of the brackets <NUM>, having a bracket roof <NUM> and bracket sidewalls <NUM>, shields the wiper carriage <NUM>. Thus, the bracket roof <NUM> and bracket sidewalls <NUM> provide at least some protection from ink mist and/or debris that may contaminate the wiper carriage <NUM> via an open front face of the inkjet module <NUM> during printing.

The inkjet module <NUM> further comprises a capping assembly <NUM> which is parked towards the rear wall <NUM> and linearly slidably movable towards and away from the printhead <NUM> along transverse capper rails <NUM> by means of rack-and-pinion mechanism <NUM>. The capping assembly comprises <NUM> a capper base <NUM> slidably engaged with the capper rails <NUM>, a perimeter printhead capper <NUM> mounted on the capper base, and cam guides <NUM> mounted fast with the capper base at opposite ends of the printhead capper. In its parked (covered) position shown in <FIG>, the printhead capper <NUM> is covered with a cap cover <NUM> pivotally mounted to the rear wall <NUM> of the chassis <NUM>. The cap cover <NUM> takes the form of a rigid plate, which seals against a perimeter seal <NUM> of the printhead capper <NUM> and maintains a humid environment within the printhead capper whenever the printhead capper is not being used for capping the printhead <NUM>. The wiper movement mechanism <NUM> is mounted on the wiper support <NUM>, which is fixedly attached to the rear wall <NUM> directly above the cap cover <NUM>.

For printhead capping, the capping assembly <NUM> is laterally moved away from the cap cover <NUM> into alignment with the printhead <NUM>, and the printhead is gently lowered onto the printhead capper <NUM> into a capped position using the lift mechanism. With the printhead raised, transverse movement of the capping assembly <NUM> back towards the rear wall <NUM> engages a rear cam surface <NUM> of the cam guides <NUM> with an engagement node <NUM> of respective rocker arms <NUM> at each end of the cap cover. The rocker arms <NUM> are pivotally mounted to the rear wall <NUM> and allow the cap cover <NUM> to pivot upwards on engagement with the cam guides <NUM>, thereby enabling the capping assembly <NUM> to slidingly traverse under the cap cover. Once the capping assembly <NUM> has reached its rearmost parked position, the cap cover <NUM> pivots back downwards, by virtue of the profile of the cam guides <NUM> and rocker arms <NUM>, into the covered position in which the printhead capper <NUM> is covered by the cap cover.

<FIG> shows the rear cam surface <NUM> of the cam guide <NUM> engaged with an engagement node <NUM> of the rocker arm <NUM> as the capping assembly <NUM> approaches the rear wall <NUM>. <FIG> shows the rocker arm <NUM> pivoted upwards as the capping assembly transitions towards its covered position. <FIG> shows the capping assembly <NUM> in its rearmost parked position with the rocker arm <NUM> pivoted back into a horizontal plane and the printhead capper <NUM> covered by the cap cover <NUM>. For printhead capping, the capping assembly <NUM> slides from its parked position shown in <FIG> towards the printhead <NUM>. A front cam surface <NUM> of the cam guide <NUM> engages with the engagement node <NUM> of the rocker arm <NUM> in order to pivot the rocker arm upwards and allow sliding movement of the capping assembly towards the printhead <NUM>.

As foreshadowed above, and referring now to <FIG>, the printhead carrier <NUM> defines a cavity <NUM> between front and rear plates <NUM> thereof. The cavity <NUM> houses a supply module <NUM>, which includes front and rear PCBs <NUM> for supplying power and/or data to the printhead <NUM>. A cooling fan <NUM> is positioned between the PCBs <NUM> for cooling electronic components with cool air drawn into the cavity <NUM> from an upper side of the printhead carrier <NUM>. The brace <NUM>, which defines a roof portion of the printhead carrier <NUM>, has an open truss structure, which allows circulation of cool air through the cavity <NUM> and between the PCBs <NUM>. The supply module <NUM> further comprises ink couplings <NUM> for engagement with complementary ink ports <NUM> at opposite ends of the printhead <NUM>. The supply module <NUM> forms ink and electrical connections with the printhead <NUM> upon installation of the printhead (secured in its printhead nest assembly <NUM>) onto the printhead carrier <NUM>, as will be explained in more detail below.

<FIG> show the printhead nest assembly <NUM> in isolation. As shown in <FIG>, the nest is in its closed position with the printhead <NUM> nestably secured within the nest <NUM> and enveloped about all sides by the nest. In <FIG>, the nest <NUM> is in its open position, which allows removal of the printhead <NUM> from the nest, but only when the printhead nest assembly <NUM> is fully detached from the printhead carrier <NUM>. In other words, the printhead <NUM> must be united with the nest <NUM> to form the printhead nest assembly <NUM> before the printhead (e.g. a replacement printhead) can be installed in the inkjet module <NUM> by fastening the nest <NUM> to the printhead carrier <NUM>, thereby to form a print module <NUM> comprising the printhead carrier <NUM>, the supply module <NUM>, the nest <NUM> and the printhead <NUM> fast with each other.

The nest <NUM> is configured for detachable fastening to the printhead carrier <NUM> via the pair of screw fasteners <NUM>, which extend vertically through a height of the printhead carrier <NUM>. Each screw fastener <NUM> has a screw lever <NUM> at one end which is user-accessible from above printhead carrier <NUM> and a screw tip projecting through a recessed opening <NUM> in a respective carrier datum block <NUM> (<FIG>). An upper surface of the nest <NUM> has a pair of datum pins <NUM> configured for complementary engagement with the recessed openings <NUM> of the carrier datum blocks <NUM>. For installation of the printhead nest assembly <NUM>, each screw fastener <NUM> is screwed through a hollowed bore <NUM> of a respective datum pin <NUM> and into a threaded nut insert <NUM> of the nest <NUM>. Thus, the printhead nest assembly <NUM> may be firmly secured to the printhead carrier <NUM> with accurate datuming controlled by complementary datuming engagement between the datums pins <NUM> and the recessed openings <NUM> in each carrier datum block <NUM>. The nest <NUM> enables the use of relatively large datum pins <NUM>, separate from the printhead <NUM>, for highly accurate and repeatable datuming between the printhead carrier <NUM> and the printhead nest assembly <NUM>.

Screw fastening of the printhead nest assembly <NUM> to the printhead carrier <NUM> via the carrier datum blocks <NUM> simultaneously forms ink and electrical connections between the printhead <NUM> and the supply module <NUM>. Ink ports <NUM> at opposite ends of the printhead <NUM> are raised into engagement with ink connectors <NUM> of the supply module <NUM>. Likewise, electrical contacts <NUM> extending along opposite longitudinal sides of the printhead <NUM> are brought into electrical contact with complementary PCB contacts <NUM> of respective PCBs <NUM> in the supply module <NUM>. Spring-biased PCB mounting plates <NUM> of the supply module <NUM> allow the PCBs <NUM> to flex laterally away from each other while the printhead <NUM> is raised between the PCBs during installation of the printhead nest assembly <NUM>. The spring bias provides reliable electrical connections, while the requisite insertion force (for both the ink and electrical connections) is provided by the screw fasteners <NUM>, which are readily operable by the user using the screw levers <NUM>. Accordingly, this arrangement obviates the movable supply assembly and two-staged ink and electrical connections, described in <CIT>.

The printhead nest assembly <NUM> may be fastened to the printhead carrier <NUM> either in the printhead lowered (<FIG>) or printhead raised position (<FIG>), depending on whichever configuration is more accessible in a particular modular set-up of the inkjet module <NUM>. As shown in <FIG>, the printhead nest assembly <NUM> has been removed in the printhead lowered position.

Referring now to <FIG> and <FIG>, the nest <NUM> is configurable in a nest open position for printhead removal and insertion. The nest <NUM> comprises first and second longitudinal side bars <NUM> and <NUM> extending parallel with opposite longitudinal sides of the printhead <NUM> and a pair of shorter transverse end bars <NUM> interconnecting each end of the longitudinal side bars to define a rectangular (oblong) nest cavity <NUM>. The first longitudinal side bar <NUM> and end bars are fixed <NUM>, while the second longitudinal side bar <NUM> is movable towards and away from the first longitudinal side bar between the open and closed positions.

Each end bar <NUM> has a dowel pin <NUM> received the movable second longitudinal side bar <NUM>. Sliding movement of the second longitudinal side bar <NUM> relative to the fixed dowel pins <NUM> provides relative linear movement of the second longitudinal side bar towards and away from the first longitudinal side bar <NUM>.

Movement of the second longitudinal side bar is <NUM> effected by means of a locking mechanism, which configures the nest <NUM> in either the closed or open positions. The locking mechanism comprises a pair of nest levers <NUM>, each nest lever being pivotally attached to a respective end bar <NUM> and having a pivot axis perpendicular to a horizontal plane of the nest (i.e. parallel to a direction of droplet ejection from the printhead <NUM>). Each nest lever <NUM> defines a cam slot <NUM> engaged with a respective follower pin <NUM> extending parallel with the pivot axis at opposite ends of the second longitudinal side bar <NUM>. Pivoting motion of each nest lever <NUM> away from its respective end bar <NUM> moves the second longitudinal side bar <NUM> linearly away from the first longitudinal side bar <NUM>, by virtue of the cam engagement between the cam slots <NUM> and follower pins <NUM>, in order to open the nest <NUM>. Conversely, pivoting motion of each nest lever <NUM> towards respective end bars <NUM> moves the second longitudinal side bar <NUM> linearly towards the first longitudinal side bar <NUM> in order to lock the nest <NUM> closed. Each nest lever <NUM> has a finger-grip portion <NUM> at an opposite end from the pivot axis for user actuation of the locking mechanism.

In its closed position, the nest <NUM> is configured to form an ink mist seal around the printhead <NUM>. The ink mist seal inhibits the ingress of ink mist into the supply module <NUM> and thereby protects sensitive electronic circuitry on the PCBs <NUM> from fouling by any ink mist generated during printing. The ink mist seal comprises a pair of opposed first and second longitudinal lips <NUM> projecting inwardly towards the printhead from respective first and second longitudinal side bars <NUM> and <NUM>. Each lip <NUM> is engaged with a longitudinal edge region <NUM> of the printhead <NUM> so as to form part of the ink mist seal.

In order to insert the printhead <NUM> into the nest <NUM>, the nest is firstly configured into its open position as shown in <FIG>. The printhead is then laterally guided into the open nest cavity <NUM> at an oblique angle (<FIG>) towards the first longitudinal side bar <NUM>. A first longitudinal flange <NUM> at one side of the printhead <NUM> is initially held at an angle below the longitudinal lip <NUM> of the first longitudinal side bar <NUM> so as to overlap with the lip, and then the printhead is rotated about its longitudinal axis into a plane parallel with a plane of the nest. Printhead datums <NUM> at opposite ends of printhead <NUM> engage with complementary nest datums <NUM> (<FIG>) to provide accurate and repeatable positioning of the printhead within the nest.

With the printhead <NUM> properly positioned inside the open nest (<FIG>), the nest levers <NUM> are pivoted inwards so as to close the second longitudinal side bar <NUM> and lock the nest <NUM> into its closed position, thereby forming the locked printhead nest assembly <NUM> (<FIG>). Closure of the nest <NUM> moves the longitudinal lip <NUM> of the second longitudinal side bar <NUM> towards the printhead <NUM> to complete the ink mist seal with each longitudinal flange <NUM> of the printhead positioned beneath and overlapping with its respective longitudinal lip.

The complete printhead nest assembly <NUM> may then be secured to the printhead carrier <NUM> using the screw fasteners <NUM> as described above. For printhead removal, the reverse procedure is followed whereby the printhead nest assembly <NUM> is detached from the printhead carrier <NUM>, the nest opened using the nest levers <NUM>, and the printhead <NUM> removed obliquely from the open nest <NUM>.

In the printing unit <NUM>, alignment of upstream and downstream printheads <NUM> is critical for ensuring optimum print quality. While the above-described datuming arrangements, both within each inkjet module <NUM> and between the pair of inkjet modules in the printing unit <NUM>, provide robust positioning of the printheads <NUM>, small misalignments between the printheads are, to some extent, inevitable in printing systems comprising multiple printheads, especially when the printheads are replaceable. Non-optimal alignment of the printheads along the x-, y- and z-axes can usually be compensated electronically, if necessary, using information harvested from test patterns during set-up of the system.

However, skew misalignments between the printheads are more difficult to compensate electronically and, therefore, print quality is usually optimized when such skew misalignments are minimized mechanically. Skew misalignment refers to a rotational misalignment of one printhead relative to the other about a z-axis, based on the nominal coordinate system shown in <FIG> and <FIG>. Ideally, of course, both printheads should be parallel.

<FIG> shows a modified printhead nest assembly <NUM> comprising a modified printhead nest <NUM> and printhead <NUM>, which is adapted for correcting skew misalignments between the pair of printheads in the printing unit <NUM>. In the modified printhead nest <NUM>, a cantilever spring <NUM> is formed at one end of the printhead nest by means of micromachined slots <NUM> defined in the first (fixed) longitudinal side bar <NUM>. A screw adjuster <NUM>, received through a screw opening in the first longitudinal side bar <NUM>, is in butting engagement with the cantilever spring <NUM> for urging the cantilever spring towards and away from the printhead <NUM>. Since the printhead <NUM> is datumed against the cantilever spring <NUM>, the screw adjuster <NUM>, when screwed along the x-axis, can impart a slight rotational movement to one end of the printhead <NUM> via movement of the cantilever spring <NUM>. Accordingly, fine skew adjustments to the printhead <NUM> can be made in situ using the screw adjuster <NUM>.

Typically, in the printing unit <NUM>, the printhead <NUM> of one inkjet module is taken to be a reference printhead, and the skew of the other printhead is adjusted relative to the reference printhead. Hence, only one of the printhead nests is required to have the cantilever spring <NUM> and screw adjuster <NUM>, although in practice it is convenient for both printhead nests to be identical.

As foreshadowed above, the screw adjuster <NUM> is preferably accessible when the printing module <NUM> is being set-up for use. Therefore, the rear wall <NUM> of each module chassis <NUM> typically has a suitable window enabling external access to the screw adjuster <NUM> (either in the printhead raised or printhead lowered position) when the printing unit <NUM> is in its clamshell-closed position, shown in <FIG>.

Optimizing airflow through print zones during high-speed printing is known to improve print quality, especially for high PPS printing - that is a printhead-paper-spacing (PPS) of greater than about <NUM> (e.g. <NUM> to <NUM> or <NUM> to <NUM>). For example, <CIT> (assigned to Hewlett-Packard Development Company, L. ) describes an inkjet printer having means for generating an airflow through the print zone in a direction of media travel. The airflow is generated using an upstream blower, downstream suction or a combination thereof. Subsequent studies by the present Applicant have confirmed the importance of controlling airflow through print zone(s) as a means for optimizing print quality. A uniform airflow creates a pressure gradient across the print zone, which tends to stabilize vortices associated with the stream of ejected ink droplets. Those vortices are generated by interaction between the stream of ink droplets and a Couette flow induced by the moving print media. In the absence of a forced airflow through the print zone producing a pressure gradient, the vortices tend to drift, resulting in unique print artefacts, known as "tiger-striping" or "woodgraining" effects.

Referring to <FIG>, there is shown a modified printing system <NUM>, which is similar to the printing system <NUM> described above in connection with <FIG>, but having an interstitial bar <NUM> positioned in a space between respective upstream and downstream printheads 3A and 3B of the upstream and downstream inkjet modules 1A and 1B. Where relevant, like reference numerals are used to indicate like features in the printing system <NUM> and the modified printing system <NUM>.

The interstitial bar <NUM> extends between opposite side bars <NUM> of the unit chassis <NUM> - that is, parallel with the end bars <NUM> and the longitudinal axes of the upstream and downstream printheads 3A and 3B. The interstitial bar <NUM> comprises a polymer plate <NUM> attached to an underside of a metal support bar <NUM>, the polymer plate defining a planar lower surface positioned at substantially a same height as a lower surface of the printheads 3A and 3B relative to print media <NUM>. In some embodiments, the interstitial bar <NUM> and/or the polymer plate <NUM> may be height-adjustable to match the relative heights of the polymer plate and the printheads 3A and 3B.

The polymer plate <NUM> has a width dimension that extends substantially entirely across a space between the upstream and downstream printheads 3A and 3B (e.g. at least <NUM>%, at least <NUM>% or at least <NUM>% across the inter-printhead space) and a length dimension at least coextensive with the printheads. By filling the inter-printhead space in this way, a relatively uniform airflow is provided from an upstream print zone <NUM>, through a downstream print zone <NUM> and towards suction nozzles <NUM> of the aerosol extractor <NUM> (<FIG>). This optimized airflow advantageously stabilizes vortices associated with the stream of ejected ink droplets ejected from the printheads 3A and 3B, thereby minimizing misplacement of stray satellite droplets and optimizing print quality. In the absence of the interstitial bar <NUM>, airflow is less uniform and the suction nozzles <NUM> have minimal effect on the upstream print zone <NUM>, instead drawing air primarily from the inter-printhead space and surrounds.

Additionally, the polymer plate <NUM> advantageously minimizes condensation of ink mist onto the interstitial bar <NUM>. Condensate on, for example, metal surfaces can undesirably drip onto print media and foul print images.

As best shown in <FIG>, an upper surface of the support bar <NUM> has a pair of recessed portions <NUM> configured for receiving complementary parts of the upstream and downstream inkjet modules 1A and 1B. Specifically, the bracket sidewalls <NUM> of each inkjet module are received in respective recessed portions <NUM> when the printing unit <NUM> is in its clamshell-closed position.

It will be appreciated that the interstitial bar <NUM> may be useful for datuming each inkjet module against the unit chassis <NUM>. However, in the embodiment shown in <FIG>, datuming of each inkjet module <NUM> is achieved via respective magnet datums <NUM>, fast with each module chassis <NUM>, engaging with complementary chassis datum blocks in the form of electromagnets <NUM>. Thus, secure datuming of each inkjet module <NUM> against the unit chassis <NUM> is achieved via magnetic attraction, as described in <CIT>). Release of the inkjet modules <NUM> from respective printing positions may be controlled by the electromagnets <NUM>.

Referring to <FIG>, there is shown a variant of the interstitial bar <NUM> in which a resiliently deformable polymer film <NUM> is attached to a lower surface of the support bar <NUM> in place of the polymer plate <NUM>. The film <NUM> has a first wing 322A and a second wing 322B extending upstream and downstream, respectively, from longitudinal edges of the support bar <NUM> relative to the media feed direction. As shown in <FIG> and <FIG>, in its non-deformed configuration, the film <NUM> is generally planar having its plane extending parallel with the print media <NUM>. However, downward movement of the print modules (having respective nests <NUM>) towards the print media <NUM> causes the upstream and downstream wings 322A and 322B of the film <NUM> to bend downwards towards the print media by virtue of engagement with respective nests of the upstream and downstream inkjet modules 1A and 1B (<FIG>). Accordingly, each of the upstream and downstream wings 322A and 322B functions as a resilient flap, which is able to bend towards the print media <NUM> via engagement with a respective nest <NUM>.

Since the film <NUM> is attached along a longitudinal mid-portion of the support bar <NUM> via retainer pins <NUM>, the film <NUM> adopts a concave profile between the upstream and downstream printheads 3A and 3B in the printing position shown in <FIG>. Engagement between the nests <NUM> and respective wings 322A and 332B forms a partial seal therebetween which is sufficient to minimize airflow through the space between the upstream and downstream printheads 3A and 3B. Thus, the film <NUM> provides a more effective seal across the space between the upstream and downstream printheads 3A and 3B than the arrangement shown in <FIG>, because the polymer plate <NUM> can only partially extend across this space depending on the height(s) of the printheads relative to the print media <NUM>. The film <NUM> can accommodate a range of different printhead heights, whilst still maintaining an effective seal and optimizing airflow through the print zones.

From the foregoing and <FIG>, it will be further appreciated that the nest <NUM> corresponding to the downstream printhead 3B engages with both the downstream wing 322B as well as the tab portion <NUM> (see <FIG>) of the aerosol extractor <NUM> simultaneously. This dual function of the nest <NUM> is particularly advantageous for optimizing airflow through the print zone(s) by controlling both the height of the suction nozzles <NUM> commensurate with the height of the printhead 3B, as well as the configuration of the film <NUM>.

Claim 1:
A printing unit comprising:
a unit chassis (<NUM>); and
a pair of opposed inkjet modules (<NUM>) mounted in tandem on the unit chassis in forward and reverse orientations, each inkjet module having one respective printhead (<NUM>),
wherein:
each printhead comprises first and second rows of print chips (<NUM>) having <NUM>-degree rotational symmetry mounted on a unitary surface of a respective ink manifold, the first and second rows of print chips comprising a plurality of print chips butted end-on-end along a length of the printhead;
the printheads are wholly aligned with respect to a media feed direction for printing in four ink colors from the pair of printheads;
the first row of print chips in one of the printheads prints two colors of ink and the second row of print chips in said printhead prints the same two colors of ink; and
compensatory sets of nozzles adjacent join regions in the first row of print chips are offset, with respect to the media feed direction, from compensatory sets of nozzles adjacent join regions in the second row of print chips.