Abstract:
In one aspect, the invention features assemblies for depositing droplets on a substrate during relative motion of the assembly and the substrate along a process direction. The assemblies include a first printhead module and a second printhead module contacting the first printhead module, each of the printhead modules including a surface that includes an array of nozzles through which the printhead modules can eject fluid droplets, wherein each nozzle in the first printhead module&#39;s nozzle array is offset with respect to a corresponding nozzle in the second printhead module&#39;s nozzle array in a direction orthogonal to the process direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 USC §119(e)(1) to U.S. patent application Ser. No. 11/118,293, entitled “DROPLET EJECTION APPARATUS ALIGNMENT,” filed on Apr. 29, 2005, which claims priority to Provisional Patent Application No. 60/566,729, filed on Apr. 30, 2004, the entire contents of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to droplet ejection devices, and more particularly to alignment of the droplet ejection devices. 
       BACKGROUND 
       [0003]    Examples of droplet ejection devices include ink jet printers. Inkjet printers typically include an ink path from an ink supply to a nozzle path in a printhead module. The nozzle path terminates in a nozzle opening in a surface of the printhead module from which ink drops are ejected. Ink drop ejection is controlled by pressurizing ink in the ink path with an actuator, which may be, for example, a piezoelectric deflector, a thermal bubble jet generator, or an electro statically deflected element. A typical printhead module has an array of ink paths with corresponding nozzle openings and associated actuators, and drop ejection from each nozzle opening can be independently controlled. In a drop-on-demand printhead module, each actuator is fired to selectively eject a drop at a specific pixel location of an image as the printhead module and a printing substrate are moved relative to one another. In high performance printhead modules, the nozzle openings typically have a diameter of 50 micron or less, e.g., around 25 microns, are separated at a pitch corresponding to 100-600 nozzles/inch or more, have a resolution of 100 to 600 dpi or more, and provide drop sizes of about 1 to 70 picoliters (pl) or less. Drop ejection frequency is typically 10 kHz or more. 
         [0004]    Hoisington et al. U.S. Pat. No. 5,265,315, the entire contents of which is hereby incorporated by reference, describes a printhead module that has a semiconductor printhead module body and a piezoelectric actuator. The printhead module body is made of silicon, which is etched to define ink chambers. Nozzle openings are defined by a separate nozzle plate, which is attached to the silicon body. The piezoelectric actuator has a layer of piezoelectric material, which changes geometry, or bends, in response to an applied voltage. The bending of the piezoelectric layer pressurizes ink in a pumping chamber located along the ink path. 
         [0005]    Printing accuracy is influenced by a number of factors, including the size and velocity uniformity of drops ejected by the nozzles in the head, as well as the alignment of the head relative to the printing substrate. In printers utilizing multiple printhead modules, head alignment accuracy is critical to printing accuracy as errors in alignment between printhead modules or between printhead modules and other components of a droplet ejection device can result in erroneous droplet placement relative to droplets from different printhead modules in addition to erroneous drop placement relative to the substrate. 
         [0006]    In many applications, particularly in droplet deposition devices utilizing multiple printhead modules, printhead modules are aligned by iteratively adjusting a printhead module&#39;s position and checking nozzle location either by direct optical inspection of the printhead module or by printing and examining a test image. This procedure is repeated whenever a printhead module is removed or replaced. 
       SUMMARY 
       [0007]    In general, in a first aspect, the invention features assemblies for mounting a printhead module in an apparatus for depositing droplets on a substrate. The assemblies include a frame having an opening extending through the frame and configured to expose a surface of the printhead module mounted in the assembly, and a spring element adapted to spring load the printhead module against an edge of the opening when the printhead module is mounted in the assembly. 
         [0008]    Embodiments of the assemblies can include one or more of the following features and/or features of other aspects of the invention. The surface of the printhead module can include an array of nozzles through which droplets are ejected and the spring element can be adapted to spring load the printhead module against the frame by applying a mechanical force to the printhead module in a direction orthogonal droplet ejection direction. The spring element can include a flexure. The frame can include a plate formed to include the opening and the flexure. The plate can be a metallic plate. The plate can be formed from stainless steel, invar, or alumina. The flexure can be attached to the plate by a fastener, such as a screw, a bolt, a pin, or a rivet. In some embodiments, the spring element includes a coiled spring. The frame can include a plate and the coiled spring can be attached to the plate. The edge of the opening in the frame can include an alignment datum for precisely positioning a droplet ejection device mounted in the assembly with respect to the assembly along an axis. The spring element can be located on the opposite side of the opening from the alignment datum. The alignment datum can include a precision surface that contacts the printhead module when the droplet ejection device is mounted in the assembly. The precision surface can be offset from other portions of the opening&#39;s edge. The frame can further include one or more additional openings extending through the frame, each opening being configured to receive a corresponding printhead module. The assembly can also include one or more additional spring elements each corresponding to the one or more additional openings and each being adapted to spring load the corresponding printhead module against an edge of the respective opening when the corresponding printhead module is mounted in the assembly. The assembly can include the printhead module. 
         [0009]    In another aspect, the invention features droplet deposition systems that include the assembly and a substrate carrier configured to position the substrate relative to the assembly so that the printhead module can deposit droplets onto the substrate. 
         [0010]    In general, in another aspect, the invention features assemblies for depositing droplets on a substrate during relative motion of the assembly and the substrate along a process direction. The assemblies include a first printhead module and a second printhead module contacting the first printhead module, each of the printhead modules including a surface that includes an array of nozzles through which the printhead modules can eject fluid droplets, wherein each nozzle in the first printhead module&#39;s nozzle array is offset with respect to a corresponding nozzle in the second printhead module&#39;s nozzle array in a direction orthogonal to the process direction. 
         [0011]    Embodiments of the assemblies can include one or more of the following features and/or features of other aspects of the invention. Each nozzle in the first printhead module&#39;s nozzle array can be offset by an amount less than the spacing of adjacent nozzles in the nozzle array. The first printhead module can include at least one alignment datum that contacts a corresponding alignment datum on the second printhead module. The alignment datum of the first printhead module can include a precision surface offset from the adjacent region of the first printhead module. The array of nozzles in the surfaces of the first and second printhead modules can each include a row of regularly spaced nozzles. The assembly can further include one or more additional printhead modules, each additional printhead module being coupled to the first and second printhead modules by the clamp. Each additional printhead module can contacts at least one other printhead module. In some embodiments, the assembly can further include a fluid supply configured to supply the first and second printhead modules with a fluid. The assembly can include a frame having an opening extending through the frame and configured to expose the surfaces of the first and second printhead modules when the printhead modules are mounted in the frame. The assembly can include a clamp securing the first printhead module to the second printhead module. 
         [0012]    In general, in another aspect, the invention features assemblies for depositing droplets on a substrate as the apparatus and the substrate move relative to each other along a process direction, the assemblies including a first printhead module and a second printhead module, each of the printhead modules including a surface that has an array of nozzles through which the printhead modules can eject droplets, the first and second printhead modules being arranged so that each nozzle in the first printhead module&#39;s nozzle array is offset with respect to a corresponding nozzle in the second printhead module&#39;s nozzle array in a direction orthogonal to the process direction, each of the printhead modules further including at least one alignment datum, wherein at least one alignment datum of the first printhead module contacts at least one alignment datum of the second printhead module. Embodiments of the assemblies can include features of other aspects of the invention. 
         [0013]    In general, in another aspect, the invention features assemblies for mounting a printhead module in an apparatus for depositing droplets on a substrate. The assemblies include a frame having an opening extending through the frame and configured to expose a surface of the printhead module mounted in the assembly, wherein the surface includes an array of nozzles through which the printhead module can eject droplets, and a clamp element attached to the frame and adapted to press the printhead module against an edge of the opening when the printhead module is mounted in the assembly. 
         [0014]    Embodiments of the assemblies can include one or more of the following features and/or features of other aspects of the invention. The clamp element can press the printhead module against the edge of the opening in the direction the nozzle array. The clamp element can press the printhead module against the edge of the opening in a direction orthogonal to the array of nozzles. The frame can include a plate formed to include the opening and the clamp element is secured to the plate by a fastener. The plate can be a metallic plate. The plate can be formed from stainless steel, invar, or alumina. The clamp element can include a mechanical actuator, wherein adjusting the mechanical actuator varies a force with which the clamping element presses the printhead module against the opening edge. The edge of the opening in the frame can include at least one alignment datum for precisely positioning the printhead module mounted in the assembly with respect to the assembly along an axis. The clamp element can be attached to the frame on the opposite side of the opening from the alignment datum. The alignment datum can include a precision surface that contacts the droplet ejection device when the droplet ejection device is mounted in the assembly. The precision surface can be offset from other portions of the opening&#39;s edge. The frame can include one or more additional openings extending through the frame, each opening being configured to receive a corresponding printhead module. The assembly can further include one or more additional clamp elements attached to the frame each corresponding to the one or more additional openings and each being adapted to press the corresponding printhead module against an edge of the respective opening when the corresponding printhead module is mounted in the assembly. 
         [0015]    In general, in a further aspect, the invention features assemblies for depositing droplets on a substrate during relative motion of the assembly and the substrate along a process direction where the assemblies include a printhead module including a surface that has a array of nozzles through which the printhead module can eject droplets, a frame having an opening extending through the frame and configured to expose the surface of the printhead module including the array of nozzles, a piezoelectric actuator mechanically coupled to the frame and the printhead module, and an electronic controller in electrical communication with the piezoelectric actuator, the electronic controller configured to cause the piezoelectric actuator to vary the position of the printhead module in the opening with respect to an axis of the apparatus. 
         [0016]    Embodiments of the assemblies can include one or more of the following features and/or features of other aspects of the invention. The axis can be orthogonal to the process direction. The axis can be parallel to the array of nozzles. The piezoelectric actuator can include a stack of layers of a piezoelectric material. 
         [0017]    In general, in another aspect, the invention features an apparatus for depositing droplets on a substrate, including a droplet ejection device including a face having a plurality of nozzles through which droplets can be ejected and a first surface non-parallel to the face, the first surface including a first alignment datum offset from a major portion of the first surface, wherein the first alignment datum aligns the nozzles relative to a first axis of the apparatus when contacting a corresponding alignment datum of the apparatus. 
         [0018]    Embodiments of the apparatus can include one or more of the following features and/or features of other aspects of the invention. The major portion of the first surface can be substantially planar. The plurality of nozzles can include an array of nozzles extending along the first axis. The apparatus can include a second surface comprising a second alignment datum offset from a major portion of the second surface, wherein the second alignment datum aligns the nozzles relative to a second axis when the printhead module is mounted with the second alignment datum contacting a corresponding alignment datum of the apparatus. The second axis can be orthogonal to the first axis. The first alignment datum can protrude from the first surface of the body. Alternatively, the first alignment datum can be recessed from the first surface of the body. The first alignment datum can include a planar surface. The planar surface can define a plane substantially orthogonal to the first axis. The planar surface can be substantially parallel to the first surface. The planar surface can have an R a  less than an R a  of the first surface of the body. The planar surface can have an R a  of about 10 micrometers or less (e.g., about eight micrometers or less, about five micrometers or less, about four micrometers or less, about three micrometers or less, about two micrometers or less). The first alignment datum can include a post. The droplet ejection device can be a printhead module (e.g., an ink jet printhead module). The printhead module can include a piezoelectric actuator and a pumping chamber in communication with one of the nozzles and the piezoelectric actuator is configured to apply pressure to ink in the pumping chamber. The apparatus can be configured to print images with a maximum resolution of about 300 dpi or more (e.g., 500 dpi or more, 600 dpi or more, 700 dpi or more, 800 dpi or more, 900 dpi or more, 1,000 dpi or more). 
         [0019]    In general, in another aspect, the invention features a frame for mounting a droplet ejection device in an apparatus for depositing droplets on a substrate, the frame including an opening extending through the frame for receiving the printhead module, and a first alignment datum offset from an edge of the opening, wherein the first alignment datum aligns the droplet ejection device relative to a first axis of the apparatus when contacting a corresponding alignment datum of the droplet ejection device. 
         [0020]    Embodiments of the frame can include one or more of the following features and/or features of other aspects of the invention. The frame can further include a second alignment datum offset from the edge of the opening, wherein the second alignment datum aligns the droplet ejection device relative to a second axis of the apparatus when contacting a corresponding alignment datum of the droplet ejection device. The first axis can be orthogonal to the second axis. The first alignment datum can protrude from the edge of the opening. The first alignment datum can include a planar surface. The planar surface can define a plane substantially orthogonal to the first axis. The planar surface has an R a  of about 10 micrometers or less (e.g., about eight micrometers or less, about five micrometers or less, about four micrometers or less, about three micrometers or less, about two micrometers or less). 
         [0021]    In general, in a further aspect, the invention features a frame for mounting a droplet ejection device in an apparatus for depositing droplets on a substrate, the frame including an opening extending through the frame for receiving the droplet ejection device, and a spring element adapted to spring load the droplet ejection device against a first portion of an edge of the opening when the droplet ejection device is mounted in the frame. 
         [0022]    Embodiments of the frame can include one or more of the following features and/or features of other aspects of the invention. The spring element can be adapted to spring load the droplet ejection device in a direction orthogonal to a direction in which the droplet ejection device ejects droplets. The first portion of the opening edge can include an alignment datum. The alignment datum can align nozzles in the droplet ejection device relative to a first axis of the apparatus when contacting a corresponding alignment datum of the droplet ejection device. The alignment datum can be offset from the first portion of the opening edge. A second portion of the opening edge different from the first portion can include the spring element. The second portion of the opening edge can be opposite the first portion. The spring element can be attached to a surface of the frame. 
         [0023]    In general, in another aspect, the invention features an apparatus for depositing droplets on a substrate, including a droplet ejection device, a frame having an opening extending through the frame for receiving the droplet ejection device, an actuator coupling the droplet ejection device to the frame, and an electronic controller coupled to the actuator, wherein during operation the electronic controller causes the actuator to vary the position of the droplet ejection device in the opening with respect to an axis of the apparatus. 
         [0024]    Embodiments of the apparatus can include one or more of the following features, and/or features of other aspects of the invention. The axis can be orthogonal to a direction in which the droplet ejection device ejects droplets. 
         [0025]    In general, in a further aspect, the invention features an apparatus, including first and second droplet ejection devices, each comprising an alignment datum offset from a surface of the respective droplet ejection device, wherein the alignment datum of the first droplet ejection device contacts the alignment datum of the second droplet ejection device. 
         [0026]    Embodiments of the apparatus can include one or more of the following features, and/or features of other aspects of other aspects of the invention. The droplets form an image on the substrate having a resolution and the dithering can have an amplitude less than a pixel size of the resolution. Ejecting can be completed in a single pass of the substrate relative to the droplet ejection device. The droplet ejection device can be coupled to a frame by an actuator which moves the droplet ejection device relative to the frame to cause the dithering. 
         [0027]    In general, in a further aspect, the invention features a method, including ejecting droplets from a droplet ejection device onto a substrate while moving the substrate relative to the droplet ejection device in a first direction, and dithering the position of the droplet ejection device in a direction orthogonal to the first direction. Embodiments of the method can include features of other aspects of the invention. 
         [0028]    Embodiments of the invention may provide one or more of the following advantages. 
         [0029]    In some embodiments, printhead modules can be mounted in a printing device with little or no adjustment required to accurately align the printhead modules. This can reduce or remove the need for iterative alignment. It can also simplify printhead module alignment, thereby reducing the need for having a skilled technician setup the printing device or realign the printhead modules during device maintenance. Subsequently, embodiments of the invention can reduce down-time in a printing device when servicing or replacing printhead modules. Some embodiments can reduce print errors associated with alignment changes due to thermal expansion of a printhead module or frame. 
         [0030]    Embodiments can provide automated and/or on-the-fly adjustment of a printhead module&#39;s position along one or more axes in a printing device. This can correct printhead module alignment errors without significant printer down time. Systematic print errors due to printhead module misalignment or due to nozzle defects within a printhead module can be reduced by varying the position of the printhead module during printing. 
         [0031]    In some embodiments, printhead modules can be compactly arranged, reducing the size of a printing device. Compact arrangements can reduce thermal variations between different printhead modules, which can in turn reduce differential thermal expansion and related print errors. 
         [0032]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0033]      FIG. 1  is a schematic diagram of a continuous web printing press. 
           [0034]      FIG. 2  is a perspective view of a print bar positioned relative to a web in a continuous web printing press. 
           [0035]      FIGS. 3A and 3B  are an exploded and perspective views of printhead modules in a print frame. 
           [0036]      FIG. 4A  is a plan view of a frame. 
           [0037]      FIG. 4B  is a perspective view of a printhead module. 
           [0038]      FIGS. 4C and 4D  are plan views of the printhead module mounted in the frame. 
           [0039]      FIG. 5A  is a plan view of another embodiment of a printhead module mounted in a frame. 
           [0040]      FIG. 5B  is a side view of a further embodiment of a printhead module mounted in a frame. 
           [0041]      FIG. 6A  is a plan view of another embodiment of a printhead module mounted in a frame. 
           [0042]      FIG. 6B  is a plan view of another embodiment of a frame. 
           [0043]      FIG. 7  is a plan view of yet a further embodiment of a printhead module mounted in a frame. 
           [0044]      FIG. 8A  is a perspective view of another embodiment of a printhead module. 
           [0045]      FIG. 8B  is a side view of the printhead module shown in  FIG. 8A  mounted in a frame. 
           [0046]      FIG. 9  is a perspective view of a frame for mounting four printhead modules. 
           [0047]      FIG. 10  is a schematic diagram of a printhead module mounted coupled to a frame with an actuator. 
           [0048]      FIG. 11A  is a schematic diagram of an assembly including multiple printhead modules. 
           [0049]      FIGS. 11B and 11C  are schematic diagrams of embodiments of alignment datums. 
           [0050]      FIG. 11D  is a diagram showing nozzle spacing in a portion of an assembly that includes multiple printhead modules. 
       
    
    
       [0051]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION  
       [0052]    Referring to  FIG. 1 , a continuous web printing press layout  10  includes a series of stations or printing towers  12  for printing different colors onto a moving web  14 . The web  14  is driven from a supply roll  15  on stand  16  onto a paper path that leads sequentially to print stations  12 . The four print stations define a print zone  18  in which ink is applied to the substrate. An optional dryer  17  may be placed after the final print station. After printing, the web is slit into sheets that are stacked at station  19 . For printing wide-format webs, such as newsprint, the print stations typically accommodates a web width of about 25-30 inches or more. A general layout for offset lithographic printing that can be adapted for ink-jet printing is further described in U.S. Pat. No. 5,365,843, the entire contents of which is hereby incorporated by reference. 
         [0053]    Referring also to  FIG. 2 , each print station includes a print bar  24 . The print bar  24  is a mounting structure for printhead modules  30  which are arranged in an array and from which ink is ejected to render a desired image on the web  14 . The printhead modules  30  are mounted in print bar receptacles  21  such that the faces (not shown in  FIG. 2 ) of the printhead modules from which ink is ejected are exposed from the lower surface of the print bar  24 . The printhead modules  30  can be arranged in an array to offset nozzle openings, thereby increasing print resolution or printing speed. In a printing condition, the print bar  24  is arranged above the web path to provide proper alignment and a uniform stand-off distance between the printhead modules  30  and the web  14 . 
         [0054]    The printhead modules  30  can be of various types, including piezoelectric drop on demand ink-jet printhead modules with arrays of small, finely spaced nozzle openings. Examples of piezoelectric ink-jet printhead modules are described in Hoisington U.S. Pat. No. 5,265,315; Fishbeck et al. U.S. Pat. No. 4,825,227; Hine U.S. Pat. No. 4,937,598; Bibl et al. U.S. patent application Ser. No. 10/189,947, entitled “PRINTHEAD,” filed Jul. 3, 2002, and Chen et al. U.S. Provisional Patent Application 60/510,459, entitled “PRINTHEAD MODULE WITH THIN MEMBRANE,” filed Oct. 10, 2003, the entire contents all of which are hereby incorporated by reference. Other types of printhead modules can be used, such as, for example, thermal ink-jet printhead modules in which heating of ink is used to effect ejection. Continuous ink-jet heads, that rely on deflection of a continuous stream of ink drops can also be used. In a typical arrangement, the stand off distance between the web path and the print bar is between about 0.5 and one millimeter. 
         [0055]    In order to minimize drop placement errors, the printhead modules are accurately aligned relative to each other and relative to the web. In addition to having appropriate angular orientation, a properly aligned printhead module  30  has nozzles appropriately located with respect to three translational degrees of freedom relative to the web. These are represented by x-, y-, and z-positions in the Cartesian co-ordinate system shown in  FIG. 2 . The web advances in the y-direction (the process direction) and the stand off distance corresponds to the nozzles&#39; location along the z-axis. 
         [0056]    Ideally, each nozzle is located at a nominal location from which a defect-free printhead module produces images with no drop placement errors. Practically, however, printhead modules can be aligned with its nozzles within some range of their nominal locations and still provide adequate drop-placement accuracy. Exact tolerances for printhead module alignment depend on the specific application, and can vary for different degrees of freedom. For example, in some embodiments, tolerances for x-axis placement should be smaller than z- and/or y-axis placement. For example, where nozzles from different printhead modules are interlaced to provide increased resolution, constraints on the relative alignment of printhead modules in the x-direction are more stringent that those in the y- and z-directions. In some embodiments, nozzles should be located within about 0.5 pixels (e.g., within about 0.2 pixels) of their nominal locations in the x-direction, while alignment of the nozzles to within about 1-2 pixels of their nominal location in the y-direction can provide sufficient drop placement accuracy. In applications having 600 dpi resolution, for example, one pixel corresponds to about 40 microns. Therefore, where an application demands alignment accuracy to within 0.5 pixels in one direction, a 600 dpi system should have its printhead modules aligned to within about 20 microns of their nominal positions. 
         [0057]    Referring to  FIG. 3A  and  FIG. 3B , in some embodiments, a print bar includes a frame  310  and other support elements  330 ,  340 , and  350 . A number of openings  360  (i.e., 12 openings in the present embodiment) are provided in frame  310  in which printhead modules  320  are mounted. Also shown in  FIGS. 3A and 3B  is inlet port  370  and outlet port  372  which couple to an ink supply (not shown). 
         [0058]    Referring also to  FIG. 4A , the edge of each opening  360  includes alignment datums  410 ,  420 , and  430 , which form planar protrusions from opening edges  401 A and  401 B. In addition, frame  310  includes alignment datums  440 ,  442 , and  444  that register frame  310  relative to neighboring frames or to other elements of the print bar. 
         [0059]    Referring additionally to  FIGS. 4B ,  4 C, and  4 D, a printhead module  450  includes a printhead module frame  451  in which is mounted a nozzle plate  470  including a row of nozzles  475 . Printhead module frame  451  includes alignment datums  455 ,  460 , and  465 , which protrude from edges of printhead module frame  451  and each include a planar surface. When printhead module  450  is properly mounted in opening  360 , the planar surface of each of alignment datums  410 ,  420 , and  430  in frame  310  contact corresponding planar surfaces of alignment datums  455 ,  465 , and  460  on the printhead module. Alignment datums  410  and  455  register printhead module  450  in the x-direction and alignment datums  420 ,  430 ,  460  and  465  register printhead module  450  in the y-direction. Accordingly, once printhead module  450  is mounted in frame  310  with corresponding alignment datum surfaces in contact with one another, the printhead module is aligned relative to the frame in the x-direction and y-direction. Assuming the frame is properly installed on the print bar, the printhead modules are ready for jetting without additional adjustment. 
         [0060]    The alignment datums provide accurate registration of the printhead module to the frame because distances between the planar surfaces of the printhead module alignment datums and the orifices are sufficiently close to a predetermined distance to accurately offset the orifices from the alignment datums of the frame. For example, referring specifically to  FIG. 4D , an orifice  475 A is a predetermined distance X 475A  from planar surface  455 A of alignment datum  455 . Similarly, orifices  475  are a predetermined distance Y 475  from a plane defined by surface  465 A of alignment datum  465 . Accordingly, when printhead module  470  is mounted in the frame, orifice  475 A is offset a distance X 475A  from surface  410 A of alignment datum  410  in the x-direction and a distance Y 475  from surface  420 A from alignment datum  420  in the y-direction. When the locations of the frame alignment datums are made to similar accuracy, they allow accurate alignment of printhead modules relative to one another in the frame. Similarly, accurate placement of the frame within the printing device aligns all the printhead modules in the frame relative to the substrate. 
         [0061]    The planar surfaces of the alignment datums (also referred to as “precision surfaces”) should be sufficiently smooth to maintain accurate registration of the printhead module to the frame along an axis regardless of which portion of the planar surfaces of the printhead module alignment datums is in contact with the planar surfaces of corresponding frame alignment datums. In other words, the planar surfaces should be sufficiently smooth so that small shifts of the printhead module position in one direction, due to, e.g., thermal expansion of the printhead module and/or frame, do not appreciably change the orientation of the nozzles or the location of the nozzles with respect to an orthogonal direction. 
         [0062]    Typically, the printhead module frame is manufactured so that the planar surface portions of the alignment datums are smoother than adjacent portions of surfaces of the printhead module frame. This can reduce manufacturing time and complexity because, for a particular surface of the printhead module frame, only the alignment datum surfaces, which form only a portion of a printhead module surface, need to be manufactured to high accuracy. For example, for a printhead module having a surface extending for several centimeters or tens of centimeters in one direction, only a small fraction (e.g., a few millimeters) of that surface needs to be precisely manufactured to provide the alignment datum. 
         [0063]    In some embodiments, the planar surfaces are prepared to have an arithmetical mean roughness (R a ) of about 20 microns or less (e.g., about 15 microns or less, about 10 microns or less, about 5 microns or less). The R a  of a surface can be measured using a profilometer, such as an optical profilometer (e.g., Wyko NT Series profilometer, commercially available from Veeco Metrology Group, Tucson, Ariz.) or a stylus profilometer (e.g., Dektak 6M profilometer, commercially available from Veeco Metrology Group, Santa Barbara, Calif.), for example. 
         [0064]    Alignment datums can be made by placing a printhead module frame blank (e.g., a monolithic printhead module frame blank) on a precision machining device (e.g., a dicing saw or a CNC mill) and removing material from the printhead module frame blank to form the alignment datum. Such manufacturing methods are particularly useful where at least one axis of the printhead module cannot easily be cost-effectively controlled using conventional manufacturing processes. Alternatively, or additionally, an attachment including a precision surface can be bonded onto the printhead module frame. 
         [0065]    The frame can also be manufactured using a precision manufacturing process, such as wire electrical discharge machining (EDM), jig grinding, laser cutting, computer numerical control (CNC) milling or chemical milling. The frame should be formed from a material that is rigid, sufficiently stable, and has a low thermal coefficient of expansion. For example, the frame can be formed from invar, stainless steel, or alumina. 
         [0066]    In the present embodiment, the jetting assemblies are aligned by slipping each into a corresponding opening such that the corresponding alignment datums contact each other. Once a printhead module is inserted into a opening, it is clamped to the frame. In general, a clamp fastens a printhead module to a frame by pressing the printhead module against the frame or against an opposing portion of the clamp. Typically, the clamp holds the printhead module in the frame until it is loosened or released. 
         [0067]    The type of clamp used to secure a printhead module can vary. One type of clamp that can be used is a c-clamp. In certain embodiments, clamps can be secured to the frame using adjustable fasteners (e.g., screws). An example of a clamp is shown in  FIG. 5A . Clamp  530  secures a printhead module  520  in a opening  501  of a frame  510 . Clamp  530  includes portions  532  which contact printhead module  520  and press the module against other portions of the clamp (not shown in  FIG. 5A ). Clamp  530  is secured to frame  510  by a fastener  531 . When secured, alignment datums  521 ,  522 , and  523  on printhead module  520  contact alignment datums  511 ,  512 , and  513  on frame  510 , respectively, registering the printhead module with respect to the frame. Frame  510  also includes openings  502 ,  503 , and  504 , which are shown in  FIG. 5A . 
         [0068]    In some embodiments, printhead modules can be clamped to the frame using one or more screws. The torque associated with screw tightening can be decoupled from the printhead module by providing an appropriate clamping element. An example of such a clamping element is a bracket as shown in  FIG. 5B . Printhead module  550  clamped to a frame  560  using a clamping bracket  570 . Printhead module  550  includes alignment datum  551  that contacts corresponding alignment datum  561  on an edge of a opening in frame  560 . Clamping bracket  570  is secured to frame  560  using a screw  575  which inserts through a hole  572  in bracket  570  into a threaded hole  565  in frame  560 . Torque applied to screw  575  during clamping is decoupled from printhead module  550  by bracket  570 , and does not substantially affect alignment of the printhead module. 
         [0069]    In some embodiments, different portions of a printhead module can be clamped with varying force. For example, were thermal stresses are significant, a point near an alignment datum can be clamped with higher force than other points. Such an arrangement can cause any induced slipped, due to thermal expansion, for example, to occur in a predictable/controllable manner, and in a manner that does not cause corresponding alignment datums to become disconnected. 
         [0070]    Alternatively, or additionally, to fastening each printhead module to the frame, each printhead module can be loaded against the frame using, e.g., one or more spring elements. A spring element refers to an element that spring loads the printhead module against the frame. Examples of spring elements include coiled springs and flexures. Referring to  FIG. 6A , an example of a flexure is shown. A frame  610  includes four openings,  601 ,  602 ,  603 , and  604 , each having two flexures (e.g., flexures  640  and  642  in opening  601 ). In this example, the flexures are cantilevers that spring load the printhead module (e.g., printhead module  620 ) in the y-direction. Flexures  640  and  642  load alignment datums  621  and  622  on printhead module  620  against frame datums  611  and  612 , respectively. Printhead module  620  also includes an alignment datum  623  which contacts frame alignment datum  613 , registering the printhead module in the x-direction. A clamp  630  secures printhead module  620  to frame  610 . 
         [0071]    Referring to  FIG. 6B , in another embodiment, a frame  710  includes openings  701 ,  702 ,  703 , and  704  that have spring elements for loading printhead modules in the x- and y-directions. For example, opening  701  includes a flexure  730  that loads a printhead module against alignment datum  713 , which registers the printhead module in the x-direction. In addition, frame  710  includes flexures  720  and  722  which load a printhead module against alignment datums  711  and  712  for y-direction registration. 
         [0072]    In the foregoing embodiments shown in  FIGS. 6A and 6B  the spring elements are incorporated in the frame. However, spring elements may also be discrete components that are attached to the frame. For example, referring to  FIG. 7 , in some embodiments, a printhead module  750  can be spring loaded against the edge of a opening  761  of a frame  760  using discrete coiled springs  770  and  772 . Coiled springs  770  and  772  are attached to frame  760  by bolts  771  and  773 , respectively, and spring load printhead module  750  in the y-direction. Each coiled spring has an arm (i.e., arms  775  and  776 ) that couple to frame  760  via holes  777  and  778 . The force each coiled spring applies to printhead module  750  can be adjusted by changing the hole to which its arm couples. A flexure  780  spring loads printhead module  750  against frame  760  in the x-direction. 
         [0073]    Mounting printhead modules in a frame using spring elements can be advantageous because the spring elements accommodate volume changes in the printhead module relative to the frame&#39;s opening, e.g., due to thermal expansion, without substantially changing the amount of force applied to the printhead module. In contrast, where a printhead module is tightly clamped to the frame, an increased clamping force that can accompany an increase in the printhead module&#39;s size due to thermal expansion can cause undesirable stress on the printhead module. 
         [0074]    In aforementioned embodiments that include alignment datums, the alignment datums are planar surfaces. However, in general, alignment datums can take other forms. In general, the alignment datum can take any form that provides sufficiently accurate registration of the printhead module to the frame in at least one degree of freedom. The alignment datums should also be sufficiently large and robust so as not to be deformed by mechanical mounting. 
         [0075]    In some embodiments, some alignment datums can be recessed (e.g., in the form of a bored hole) and can mate with corresponding protrusions. For example, referring to  FIG. 8A  and  FIG. 8B , a printhead module  800  can include alignment datums in the form of posts  830  and  832 , which insert into corresponding holes  841  and  842  in a frame  840 . These alignment datums register printhead module  800  with respect to the x-axis and y-axis. Posts  830  and  832  can be adjusted during assembly of printhead module  800  so that they are correctly oriented with respect to nozzles  820  in nozzle plate  810 . 
         [0076]    Furthermore, although the foregoing embodiments include alignment datums for registering a printhead module in the x- and y-directions, alignment datums can also be used to register a printhead module in the z-direction. Referring still to  FIG. 8B , for example, frame  840  includes alignment datums  853  and  855  which contact corresponding alignment datums  852  and  854  on printhead module  800 , respectively. These alignment datums offset the printhead module from the frame in the z-direction, positioning nozzles  820  a desired distance from a substrate (not shown). 
         [0077]    Another embodiment of a frame is shown in  FIG. 9 . In this embodiment, frame  1100  has four openings  1101 - 1104  for mounting printhead modules. Frame  1100  is a laminate structure and includes registration plates  1110  and  1130 , and a spacer  1120 . Registration plate  1110  includes alignment datums  1111 ,  1112 , and  1113  for registering a printhead inserted into opening  1101  in the x- and y-directions. In particular, alignment datums  1113  provide registration of a printhead in the x-direction, while datums  1111  and  1112  provide registration of a printhead in the y-direction. Registration plate  1110  includes corresponding alignment datums for registering printheads in the x- and y-directions in openings  1102 - 1104 . 
         [0078]    Registration plate  1130  includes alignment datum  1   114  for registering a printhead inserted into opening  1101  in the z-direction. Registration plate  1130  includes another alignment datum (not shown in  FIG. 9  due to the perspective of the figure) on the opposite side of opening  1101  from alignment datum  1114 . Furthermore, registration plate  1130  includes corresponding alignment datums for registering printheads in the z-direction in openings  1102 - 1104 . 
         [0079]    Furthermore, frame  1100  includes alignment datums for registration to other frames. Alignment datums  1131  and  1132 , on the edge of registration plate  1130 , register the frame to another frame in the y-direction, while alignment datums  1135  and  1136  register the frame to another frame in the x-direction. Registration plate  1130  also includes holes  1141 - 1143  for bolting the frame to a print bar or other structure of the printing system in which the frame is mounted. 
         [0080]    Frame  1100  can be relatively thin (i.e., in the z-direction). For example, frame  1100  can have a thickness of about 2 cm or less (e.g., about 1.5 cm or less, about 1 cm or less). 
         [0081]    In embodiments, registration plates  1110  and  1130  can be formed from a rigid material, such as materials that include one or more metals (e.g., alloys, such as invar). The material can have similar thermomechanical properties (e.g., coefficient of thermal expansion (CTE)) as the material(s) from which the printheads are formed. For example, the CTE of the material(s) from which the registration plate materials are formed can be within about 20 percent or less (e.g., about 10 percent or less, about 5 percent or less) over a range of temperatures at which the printheads usually operate (e.g., from about 20° C. to about 150° C.). 
         [0082]    Registration plates  1110  and  1130  can be formed by sheet metal processing methods, such as stamping, and/or by EDMing. The alignment datums on registration plates  1110  and  1130  can be formed by gouging and/or EDMing, for example. 
         [0083]    Spacer  1120  can be formed from a material having similar thermomechanical properties as the material(s) used to form registration plates  1110  and  1130 . In some embodiments, spacer  1120  can be formed from a material having a high thermal conductivity, and spacer  1120  can act as a thermal node. Alternatively, or additionally, the material forming spacer  1120  can exhibit relatively low thermal expansion. Furthermore, spacer  1120  can be formed from a material which has a high level of chemical inertness, to reduce any undesirable chemical reactions of the spacer with other materials in the frame and/or with the environment. In some embodiments, spacer  1120  can be formed from a material having a high electrical conductivity. High electrical conductivity can reduce build up of static charge on the frame. 
         [0084]    As an example, spacer  1120  can be formed form a liquid crystalline polymer (LCP) (e.g., CoolPoly® E2 commercially available from Cool Polymers Inc., Warwick, R.I.). 
         [0085]    In some embodiments, spacer  1120  is injection molded. Alternatively, the spacer can be machined from a blank sheet of material. 
         [0086]    Spacer  1120  can include registration features which couple to corresponding features in other layers of frame  1100  (e.g., in the registration plates), aligning the apertures in each layer to provide openings  1101 - 1104 . 
         [0087]    Registration plates  1110  and  1130  are secured (e.g., bonded or screwed) to either side of spacer  1120 . In some embodiments, an epoxy (e.g., a B-stage epoxy) is used to bond registration plates  1110  and  1130  to spacer  1120 . 
         [0088]    In some embodiments, additional layers can be included in the laminate structure of frame  1100 . As an example, frame  1100  can include a heater layer. The heater layer can be bonded to a surface of registration plate  1110  or registration plate  1130 . A heater layer can be formed from a Kapton flex circuit, for example. 
         [0089]    Although the foregoing embodiments relate to printhead modules which do not require adjustment along various degrees of freedom due to registration using alignment datums, in other embodiments printhead modules can include one or more actuators that adjust the printhead module position with respect to one or more degrees of freedom. For example, referring to  FIG. 10 , a frame  910  includes an actuator  940  that is coupled to a surface  960  of a printhead module  920  in a frame opening  901 . Printhead module  920  includes an orifice plate  925  having an array of orifices  930 . During operation, actuator  940  adjusts the position of printhead module  920  in the x-direction as necessary. Printhead module  920  also includes alignment datums  921  and  922  which contact corresponding frame alignment datums  911  and  912 . 
         [0090]    Actuator  940  can be an electromechanical actuator, such as a piezo-electric or electro static actuator. Examples of piezo-electric actuators include stacked piezo-electric actuators that include multiple layers of piezo-electric material stacked to increase the actuators dynamic range compared to a single layer of piezo-electric material. Stacked piezo-electric actuators are available commercially (e.g., from companies such as PI (Physik Instrumente) L.P., Auburn, Mass.). 
         [0091]    The actuator should have a minimum range of motion on the order of the image pixel spacing. Stacked piezo-electric actuators, for example, can have a dynamic range of about 5 to about 300 microns. 
         [0092]    Actuator  940  responds to drive signals from an electronic controller  950 . In some embodiments, controller  950  causes actuator  940  to adjust the position of printhead module  920  in the x-direction in response to a signal from a monitoring system  970  (e.g., an optical monitoring system, such as including a CCD camera). Monitoring system  970  monitors images (e.g., test images) printed using printhead module  940  for drop placement errors associated with misalignment of printhead module  940  in the x-direction. Where a drop placement error is detected, electronic controller  950  determines the magnitude and direction of printhead module misalignment that gave rise to the error. Based on this determination, the controller sends a signal to actuator  940 . The actuator changes the position of the printhead module in order to reduce or eliminate errors arising from printhead module misalignment. 
         [0093]    In some embodiments, actuator  940  can dither printhead module  920  back and forth in the x-direction during printing. This can reduce the effect of drop placement errors due to x-axis alignment on image quality by introducing controlled noise to the image which can mask the errors. Preferably, the printhead module should be dithered a fraction of a pixel (e.g., about ½ a pixel or ¼ of a pixel). Dither frequency can be variable or fixed. Preferably, dither frequency should be lower than jetting frequency (e.g., about 0.1, 0.05, 0.01 times the jetting frequency). However, in embodiments where the dither frequency is comparable or higher than jetting frequency, dither frequency should not be at the jetting frequency or its harmonics. 
         [0094]    In embodiments where multiple printhead modules are interlaced, each printhead module can be actuator adjusted. In addition, or alternatively, to adjusting the x-direction alignment of each printhead module to mitigated alignment errors, the actuators can adjust the interlace pattern of the printhead modules. The actuators allow the interlace spacing and/or pattern to be varied rapidly and reliably. Thus, the interlace pattern can be adjusted during printing (e.g., between images) without down time of the printing press. 
         [0095]    While in the foregoing embodiments the printhead module alignment datums register the printhead module directly to the frame, in other embodiments alignment datums can be used to register printhead modules directly to other printhead modules. For many applications, particularly those in which printing is completed with a single pass of the substrate relative to the jetting assembly, several printhead modules are positioned along the process direction (i.e., the y-direction) to achieve the requisite spatial density for the desired print quality. To reduce adverse effects of process variation on image quality, printhead modules should preferably placed very close together in the process direction. 
         [0096]    Referring to  FIG. 11A , in some embodiments, close printhead module spacing is achieved by stacking multiple printhead modules together to form a 2-D jetting array  1000 . While jetting array  1000  includes six printhead modules (i.e., printhead modules  1010 ,  1020 ,  1030 ,  1040 ,  1050 , and  1060 ), in general, the number of printhead modules in a jetting array can vary as desired. Adjacent printhead modules are registered in the y-direction via alignment datums. For example, printhead module  1010  has alignment datums  1013  and  1014 , which register it to printhead module  1020  via alignment datums  1021  and  1022 . In addition, printhead module  1010  includes alignment datums  1011  and  1012 , which register the printhead module in the y-direction to a frame (not shown). A clamp  1090  clamps the subassembly together once the printhead modules have been stacked with corresponding datums aligned (e.g., using a c-clamp). The printhead modules in jetting array  1000  can share a common ink supply and temperature control system. 
         [0097]    Corresponding nozzles in adjacent printhead modules can be offset along the x-axis to increase the print resolution of the jetting array. For example, referring to  FIG. 11D , a jetting array  1200  includes three printhead modules  1210 ,  1220 , and  1230  that are stacked together. Corresponding nozzles in printhead modules  1210  and  1220  are offset by an amount approximately equal to d/n, where d is the spacing between adjacent nozzles (e.g., between nozzles  1211 A and  1211 B,  1221 A and  1221 B, and  1231 A and  1231 B) in a nozzle array, and n is the number of printhead modules in stacked in the jetting array. Similarly corresponding nozzles in printhead modules  1220  and  1230  are also offset by d/n in the x-direction. Accordingly, the print resolution in the x-direction of the jetting assembly is reduced by a factor of n. As an example, a jetting array having a resolution of about 50 μm can be assembled from six printhead modules each having an individual resolution of about 300 μm. 
         [0098]    In some embodiments, the alignment datums on the printhead modules can include features that allow alignment of the printhead modules in the x-direction to provide the desired jet pitch. For example, referring to  FIG. 11B , protruding alignment datums  1050  and  1060  can each include multiple precision surfaces which register the printhead modules relative to one another in both the x- and y-directions. In the present embodiment, alignment datum  1050  includes precision surfaces  1051 ,  1052 , and  1053 . Similarly, alignment datum  1060  includes precision surfaces  1061 ,  1062 , and  1063 . Surfaces  1051  and  1061  register the printhead modules in the x-direction, while surfaces  1052 ,  1053 ,  1062 , and  1063  register the printhead modules in the y-direction. 
         [0099]    Another example of alignment datums that register printhead modules relative to two degrees of freedom are shown in  FIG. 11C . In this example, a protruding alignment datum  1070  inserts into a recessed alignment datum  1080 . Protruding alignment datum  1070  includes precision surfaces  1071  and  1072 . Surface  1071  contacts surface  1081  of alignment datum  1080 , registering the printhead module in the x-direction. Similarly, surface  1072  contacts surface  1082  of alignment datum  1080 , registering the printhead module in the y-direction. 
         [0100]    Stacking printhead modules in a compact 2-D jetting array can reduce the dimensions over which precision should be maintained in any given part. Since the arrays are modular and can share common ink ports and temperature control, the size, cost, and complexity of the system can be reduced relative to systems in which individual jetting assemblies are each served by their own ink supply, temperature controller, and/or are individually mounted. Furthermore, individual printhead modules can be replaced should they become defective instead of replacing an array. 
         [0101]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.