PATENT ABSTRACT
A print head assembly is provided. The printhead includes a substrate including a plurality of electrical contacts and an array of ejectors arranged on the substrate. Each ejector includes a chamber including a nozzle. A resistive element is associated with the chamber and is operable to eject liquid from the chamber through the nozzle of the chamber when actuated through the plurality of electrical contacts. At least one supply passage through the substrate supplies fluid to each ejector. A printhead holder includes a structure to retain the printhead in a fixed position and a manifold to supply fluid to each ejector through the at least one supply passage. A removable frame has a first position and a second position relative to the printhead holder. The frame includes a plurality of electrical contacts that provide an electrical connection to the plurality of electrical contacts on the substrate of the printhead when the frame is in the first position, and permits removal of the printhead from the retaining structure of the printhead holder when the frame is in the second position.

PATENT DESCRIPTION
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Reference is made to commonly-assigned, U.S. patent application Ser. Nos. 11/516,064 and 11/516,134, both filed Sep. 6, 2006, in the name of Stanley W. Stephenson, the disclosures of which are incorporated herein by reference. 
     
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to the field of digitally controlled printing devices, and in particular to printhead assemblies that include replaceable printheads. 
       BACKGROUND OF THE INVENTION 
       [0003]    Ink jet printing systems apply ink to a substrate. The inks are typically dyes and pigments in a fluid. The substrate can be comprised of an material or object. Most typically, the substrate is a flexible sheet that can be a paper, polymer or a composite of either type of material. The surface of the substrate and the ink are formulated to optimize the ink lay down. 
         [0004]    Ink drops can be applied to the substrate by modulated deflection of a stream of ink (continuous) or by selective ejection from a drop generator (drop-on-demand). The drop-on-demand (DOD) systems eject ink using either a thermal pulse delivered by a resistor or a mechanical deflection of a cavity wall by a piezoelectric actuator. Ejection of the droplet is synchronized to motion of the substrate by a controller, which selectively applies an electrical signal to each ejector to form an image. 
         [0005]    U.S. Pat. No. 6,491,385 to Anagnostopoulos et al., issued Dec. 10, 2002, entitled “CMOS/MEMS integrated ink jet print head with elongated bore and method of forming same,” describes a continuous ink jet head and it&#39;s operation. A silicon substrate supports layers on the front surface having a pair of resistive elements. A bore through the silicon substrate is supplied for each nozzle. A fluid, which can be ink, is forcibly ejected through the bore and through a nozzle formed in the layers on the front surface. The resistors are modulated to break the stream of fluid into discrete droplets. Asymmetric heating of the resistors can selectively direct the droplets into different pathways. A gutter can be used to filter out select droplets, providing a stream of selectable droplets useful for printing. The modulated stream printing system requires significant additional apparatus to manage fluid flow. 
         [0006]    Piezoelectric actuated heads use an electrically flexed membrane to pressurize a fluid-containing cavity. The membranes can be oriented in parallel or perpendicular to the ejection direction. U.S. Pat. No. 6,969,158 to Taira, issued Nov. 29, 2005, entitled “Ink-jet head,” describes a piezoelectric drop-on-demand ink jet head having the membrane perpendicular to the droplet ejection direction. A set of plates is stacked up and includes plate of piezoelectric which flexes a pressure chamber parallel to the direction of ink ejection. The membranes require a large amount of surface area, and multiple rows of ejectors are arrayed in depth across the head. Ejectors are arranged across the printing direction at a pitch of 50 dpi and are arrayed in the printing direction 12 ejectors deep on an angle theta to form a head having an effective pitch of 600 dpi. Such heads are complex, requiring multiple layers that must be bonded together to form passages to the nozzle. 
         [0007]    U.S. Pat. No. 6,926,284 to Hirst, issued Aug. 9, 2005, entitled “Sealing arrangements,” discloses a drop-on-demand inkjet head permitting single-pass printing. A single pass print head comprises 12 linear array module assemblies that are attached to a common manifold/orifice plate assembly. Droplets are ejected from the orifice by twelve staggered linear array assemblies that support piezoelectric body assemblies to provide drop-on-demand ejection of ink through the orifice array. The piezoelectric system cannot pitch nozzles closely together; in the example, each swath module has a pitch of 50 dpi. The twelve array assemblies are necessary to provide 600 dpi resolution in a horizontally and vertically staggered fashion. 
         [0008]    The orifice array on the plate can be a single two-dimensional array of orifices or a combination of orifices to form an array of nozzles. In the printing application, the orifices must be positioned such that the distance between orifices in adjacent line is at last an order of magnitude (more than ten times) the pitch between print lines. The assembly is quite complex, requiring many separate array assemblies to be attached to the orifice plate thorough the use of sub frames, stiffeners, clamp bar, washers and screws. 
         [0009]    It would be advantageous to provide a staggered array in a unitary assembly with an integral orifice plate. It would be useful for the spacing between nozzles to be less than an order of magnitude deeper than is disclosed in this patent. 
         [0010]    U.S. Pat. No. 6,722,759 to Torgerson et al., issued Apr. 20, 2004, entitled “Ink jet printhead,” describes a common thermal drop-on-demand inkjet head structure. The drop generator consists of ink chamber, an inlet to the ink chamber, a nozzle to direct the drop out of the cavity and a resistive element for creating an ink ejecting bubble. Linear arrays of drop generators are positioned on either side of a common ink feed slot. Two linear arrays are fed by a common ink feed slot. Ink from the slot passes through a flow restricting ink channels to the ink chamber. A heater resistor at the bottom of the ink chamber is energized to form a bubble in the chamber and eject a drop of ink through a nozzle in the top of the chamber. The ejectors are constrained to be in linear rows on either side of the ink jet supply slot. 
         [0011]    U.S. Pat. No. 6,367,903 to Gast et al., issued Apr. 9, 2002, entitled “Alignment of ink dots in an inkjet printer,” discloses a similar structure. The arrays of drop generators are not in a strictly linear fashion, but are slightly offset in groups of three and four generators. Generators in a group are displaced sequentially farther from the supply slot within a group. Adjacent nozzles between the groups have a maximum variation in distance from the common supply slot. 
         [0012]    U.S. Pat. No. 5,134,425 to Yeung, issued Jul. 28, 1992, entitled “Ohmic heating matrix,” discloses a passive two-dimensional array of heater resistors. The structure and arrangement of the droplet generators is not disclosed. The patent discloses the problem of power cross talk between resistors in two dimensional arrays of heater resistors. Voltages firing a resistor also apply partial voltages across unfired resistors. The parasitic voltage increases as the number of rows is increased to 5 rows. The patent applies partial voltages on certain lines to reduce the voltage cross talk. The partial energy does not eject a droplet, but maintains a common elevated temperature for both fired and unfired nozzles. The patent covers print head arrays having limited numbers of rows. 
         [0013]    U.S. Pat. No. 5,548,311 to Hine, issued Aug. 20, 1996, entitled “Mount for replaceable ink jet head,” discloses a piezoelectric drop-on-demand print head having a replaceable ink jet head. A set of nozzles selectively ejects ink when from electrical pulses are applied to transducers. The transducers are connected by wires to a series of spring contacts on the surface of the head that are electrically connected to a second set of contacts in a mobile carriage. The head structure uses connectors for each of 32 ink jets. The 32 contacts require 160 of clamping force to make a connection. A total of 400 grams of force needs to be applied at the connection to prevent disconnection due to g-forces when the carriage holding the head is translated during printing. It would be useful to reduce the complexity of the interconnection. 
         [0014]    U.S. Pat. No. 4,791,440 to Eldridge et al., issued Dec. 13, 1988, entitled “Thermal drop-on-demand ink jet print head,” discloses a structure for a DOD thermal inkjet head. A heater chip, nozzle plate and chip mount are combined to produce a pluggable unit which has both fluid and electrical connections. The patent describes the increase in cost and complexity of electrical fanout and electrical connection as the supporting electrical connections as nozzle count increases. The patent addresses those issues by organizing the heating means in multiple column and passing electrical connection through the substrate. Through connections are more complex and costly. The device has no internal semiconductor elements, and a dedicated connection is required for each heater. The author organizes the heating elements in two staggered rows on either side of tow large holes supplying a common 
         [0015]    As such, there is a need to provide a replaceable ink jet print head structure available at a reduced cost having a reduced number of semiconductor devices and electrical interconnections. 
       SUMMARY OF THE INVENTION 
       [0016]    It is an object of this invention to provide low-cost replaceable head element. Another object of the invention is to provide tool-less replacablility of critical portions of an inkjet head. 
         [0017]    According to one aspect of the present invention, a print head assembly is provided. The printhead includes a substrate including a plurality of electrical contacts and an array of ejectors arranged on the substrate. Each ejector includes a chamber including a nozzle. A resistive element is associated with the chamber and is operable to eject liquid from the chamber through the nozzle of the chamber when actuated through the plurality of electrical contacts. At least one supply passage through the substrate supplies fluid to each ejector. A printhead holder includes a structure to retain the printhead in a fixed position and a manifold to supply fluid to each ejector through the at least one supply passage. A removable frame has a first position and a second position relative to the printhead holder. The frame includes a plurality of electrical contacts that provide an electrical connection to the plurality of electrical contacts on the substrate of the printhead when the frame is in the first position, and permits removal of the printhead from the retaining structure of the printhead holder when the frame is in the second position. 
         [0018]    According to another aspect of the present invention, a method of printing includes providing an original printhead comprising: a substrate including a plurality of electrical contacts; and an array of ejectors arranged on the substrate, each ejector comprising: a chamber including a nozzle; a resistive element associated with the chamber operable to eject liquid from the chamber through the nozzle of the chamber when actuated through the plurality of electrical contacts; and at least one supply passage through the substrate; a printhead holder including a structure to retain the printhead in a fixed position and a manifold to supply fluid to each ejector through the at least one supply passage; and a removable frame having a first position and a second position relative to the printhead holder, the frame including a plurality of electrical contacts that provide an electrical connection to the plurality of electrical contacts on the substrate of the printhead when the frame is in the first position, and permit removal of the printhead from the retaining structure of the printhead holder when the frame is in the second position; printing using the original printhead; moving the frame to the second position; replacing the original printhead with another printhead; and moving the frame to the first position. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a top schematic view of an ejector in accordance with the present invention; 
           [0020]      FIG. 2  is a side sectional view through the ejector shown in  FIG. 1 ; 
           [0021]      FIG. 3  is a top view of an array of ink ejectors according to prior art; 
           [0022]      FIG. 4  is a top view of an inkjet print head assembly in accordance with prior art; 
           [0023]      FIG. 5  is a side sectional view of the inkjet print head assembly shown in  FIG. 4 ; 
           [0024]      FIG. 6  is a top schematic view of an ejector in accordance with the present invention; 
           [0025]      FIG. 7  is a schematic representation of an ejector array in accordance one example embodiment of the invention; 
           [0026]      FIG. 8  is an electrical schematic of an ink jet head in accordance with the present invention; 
           [0027]      FIG. 9  is a schematic view of a head assembly in accordance with the present invention; 
           [0028]      FIG. 10  is a side view of a printer using a head in accordance with the present invention; 
           [0029]      FIG. 11  is a top view of a head holder in accordance with the present invention; 
           [0030]      FIG. 12  is a side view of a head holder in accordance with the present invention; and 
           [0031]      FIG. 13  is a side view of the disassembled invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0032]      FIG. 1  is a top schematic view of an ejector  10  in accordance with the present invention.  FIG. 2  is a side sectional view through the ejector shown in  FIG. 1 . A substrate  3  supports a polymer layer  5 . Substrate  3  is most commonly a silicon wafer, however substrate  3  can be made of a glass or metal such as stainless steel, Invar, or nickel. An ink chamber  12  is formed as a cavity in polymer layer  5  to hold a printing ink. A cover  7  over ink chamber  12  can be formed directly over polymer layer  5  using a vacuum deposited ceramic or metal. Cover  7  over ink chamber  12  can also be a separate plate formed of ceramic or metal which is bonded to the polymer layer  5  defining ink chamber  12 . Cover  7  has an opening to form a nozzle  14  to direct an ejected droplet of ink in a specified direction when ink chamber  12  is pressurized. 
         [0033]    A heater resistor  20  is embedded in the substrate  3 . A pulse of electrical energy to heater resistor  20  causes ink within ink chamber  12  to momentarily be converted into a gaseous state. A gas bubble is formed over heater resistor  20  within ink chamber  12 , and pressurizes ink chamber  12 . Pressure within ink chamber  12  acts on ink within ink chamber  12  and forces a droplet of ink to be ejected through nozzle  14 . Inlet  16  supplies ink to ink chamber  12 . Restriction  18  can be formed at inlet  16  to improve firing efficiency by restricting the majority of the pressure pulse to ink chamber  12 . Restriction  18  can be in the form of one or more pillars formed within inlet  16 , or by a narrowing of the sidewalls in polymer layer  5  at inlet  16  of ink chamber  12 . 
         [0034]    Resistive heads are commonly made using silicon for substrate  3 . Heater resistor  20  and associated layers are formed over substrate  3 , followed by polymer layer  5 . Polymer layer  5  is patterned, followed by cover  7 , which is patterned to form nozzle  14 . After those layers have been formed, ink feed slot  22  is formed through substrate  3  using a reactive ion milling process. The reactive ion milling process has the characteristic of forming near-vertical walls through a silicon substrate  3 . The ion milling process has the virtue that the process is specific to silicon and can form ink feed slot  22  without damage to structures associated with ejectors  10  on substrate  3 . Substrate  3  is bonded to a structure which has one or more cavities  31  for supplying ink to some or all of ejectors  10  formed on substrate  3 . 
         [0035]      FIG. 3  is a top view of an array of ink ejectors according to prior art. Ejectors  10  must be supplied by ink from the rear side of substrate  3 . U.S. Pat. No. 6,722,759 describes a prior art thermal drop-on-demand printhead. Ejectors  10  are arranged in two closely packed rows that share a common ink feed slot  22 . Ink feed slot  22  passes through substrate  3 , which supports ejectors  10 . Arranging two linear rows of ejectors  10  on either side of ink feed slot  22  provides for a compact ink jet head. Because the nozzles are adjacent to each other, fluidic cross-talk can occur between the ejectors. Close packing of the nozzles makes the head susceptible to thermal cross-talk between adjacent nozzles. Overheating can become more pronounced if substrate  3  is not silicon, but a less thermally conductive material such as glass, ceramic or metal. 
         [0036]      FIG. 4  is a top view of an inkjet print head in accordance with prior art. The recitation again generally follows the structures found in U.S. Pat. No. 6,722,759. A print head  32  has two ink feed slots  22 , each feed slot feeding two rows of ejectors  10 . A set of ejector drivers  52  is formed adjacent to each row of ejectors  10 . Each ejector driver  52  is a semiconductor-switching elements that is attached to each heater resistor  20  within each ejector  10 . The power requirements for thermal drop on demand inkjet are high, typically over 1 watt of power for approximately 1 microsecond. Ejector drivers  52  then are formed of PMOS or NMOS transistors to selectively apply power to heater resistors  20 . Alternatively, ejector drivers  52  can be formed of thin-film-transistor elements having characteristics capable of meeting the power and switching times required to thermally eject a droplet from an ejector  10 . 
         [0037]    Power to ejector drivers  52  is provided by a conductor line  54  disposed one each side and down the center of substrate  3 . Conductor lines  54  supply power and return for ejector drivers  52 . Control logic  58  is disposed on both ends of the substrate  3  to decode data signals from printer controller  38  (not shown in figure). Data and power are delivered to control logic  58  through bond pads  60 . Wire bonds  62  provide connection between bond pads  60  on substrate  3  and flex circuit  64 . Data from controller  38  is delivered through flex circuit  64  through wire bonds  62  to control logic  58 . Control logic  58  responds to control data from printer controller  38   
         [0038]      FIG. 5  is a side sectional view of the inkjet print head assembly shown in  FIG. 4 . In accordance with current art, print head  32  is bonded on head holder  66 . Cavities  31  are formed in head holder  66  to provide ink to each ink feed slot  22  in print head  32 . Flex circuit  64  is bonded to head holder  66  and wire bonds  62  are connected between flex circuit  64  and bond pads  60  formed over substrate  3 . 
         [0039]    Silicon based print heads  32  are built on a silicon wafer that is diced into a rectangular shape. The sawing process to cut out print heads  32  varies by  50  microns, creating variability in location of bond pads  60  relative to the edges of substrate  3 . Bond pads  60  are small, typically 200 microns square, and require wire bonds  62  to connect to contacts areas on flex circuit  64 . Because of the variability in dimension and accuracy requirements, print heads  32  are permanently bonded to head holder  66 . 
         [0040]      FIG. 6  is a top schematic view of an ejector in accordance with the present invention. In the invention, an ejector  10  comprises an ink chamber  12  actuated by heater resistor  20 . Ink chamber  12  is fed by inlet  16  and ejects fluid through nozzle  14 . A restriction  18  can be formed at the inlet to improve ejector  10 &#39;s performance. A single ink feed slot  22  is dedicated to ejector  10 . In the case that substrate  3  is made of silicon, the ability of reactive ion etching process to form substantially columnar individual supply passages  22  to be formed through substrate  3 . Each ink feed slot  22  shares a common cavity  31  located at the back of substrate  3 . Ejector  10  in accordance with the invention provides a complete assembly that can be positioned at greater distance from adjacent ejectors  10  to eliminate fluidic cross talk and improve cooling efficiency. In the case that substrate  3  is not silicon, the greater distance prevents overheating that would result from closely spaced ejectors  10  on lower conductivity substrates  3 . Sufficient spacing between ejectors  10  further permits the use of anisotropic etching in non-silicon substrates. 
         [0041]    U.S. Pat. No. 5,134,425 discloses a passive two-dimensional array of heater resistors. The patent discloses the problem of power cross talk between resistors in two-dimensional arrays of heater resistors. Voltages used to fire a given resistor apply partial voltages across unfired resistors, significantly increasing the current and power demand. In  FIG. 6 , ejector  10  is connected to row conductor  26  and column conductor  28 . A diode  24  permits multiple ejectors  10  to be attached to a matrix of row conductors  26  and column conductors  28 . The diodes block current flows to parasitic elements, reducing power demand of the device. The diodes permit large number of columns to be used on the head. The larger number of columns permits heads with finer resolution and greater spacing between ejectors  10 . 
         [0042]      FIG. 7  is a schematic representation of an ejector array in accordance one example embodiment of the invention. A coordinate system is shown and includes a first direction X with X an axis of motion between the printhead and an ink-receiving surface. This is commonly referred to as a printing direction. A second direction Y is also shown with Y being a cross printing direction. A direction Z is also shown with Z being a direction perpendicular to the printhead. This is commonly referred to as the direction of ink drop ejection from the printhead. 
         [0043]    Ejectors  10  are shown schematically as a box having individual supply ports  22  and ejectors  10 . Ejectors  10  have been attached to a matrix of row conductors  26  and column conductors  28  to form laterally staggered columns of ejectors  10 . Each ejector  10  of a column of ejectors is staggered at a desired pitch, typically expressed in dpi or microns, which is finer than the pitch of the ejector columns. For example, each column can be pitched 600 microns apart due to the area required for each ejector. If the required printing pitch is 40 microns, each ejector in the column can be laterally staggered 40 microns to a depth of 15 ejectors (40×15=600) to achieve the required 40 micron printing pitch. 
         [0044]    The embodiments shown in  FIGS. 6 and 7  are particularly well suited for print heads having large area arrays, for example, print heads having a length dimension of four inches and a width dimension of one inch. However, the large area array print head can have other length and width dimensions. One (or a plurality of large area array print heads stitched together) can be used to form a pagewide print head. 
         [0045]    In a pagewide print head, the length of the printhead is preferably at least equal to the width of the receiver and does not “scan” during printing. The length of the page wide printhead is scalable depending on the specific application contemplated and, as such, can range from less than one inch to lengths exceeding twenty inches. 
         [0046]      FIG. 8  is an electrical schematic of an ink jet head in accordance with the present invention. Print head  32  has column conductors  28  connected to column driver  36 . Column driver  36  can be a ST Microelectronics STV 7612 Plasma Display Panel Diver that is connected to column conductors  28 . The chip responds to digital signals to either apply a drive voltage or ground to each column conductors. Each row conductor  26  is connected to a row driver  34 . Row driver  34  can be a ST Microelectronics L6451 28 Channel Ink Jet Driver that provides a DMOS power transistor to each row conductor  26 . Diode  24 , provided with each ejector  10 , provides logic to permit print head  32  to be logically driven in a sequential column wise fashion. 
         [0047]    Print head  32  is fired row sequentially. Row driver  34  applies a ground voltage to a written row. Digital signals apply a drive voltage (Vdd) or ground voltage to each row conductor  26 . Row conductors  26  having an applied drive voltage provide energy to the ejector attached to column conductor  28  and the grounded row conductor  26 . Row conductors  26  having a ground voltage are not fired. Only one row conductor  26  at a time has a ground voltage, the other row conductors are attached to high impedance drivers and cannot fire. Row conductors  26  are fired in a sequential manner, and column conductors  28  are set to a state that corresponds to a row of ejectors being fired or not fired. After all rows have been written, all ejectors are fired and the process is repeated to apply an image wise pattern of ink droplets from print head  32 . 
         [0048]      FIG. 9  is a schematic view of a head assembly in accordance with the present invention. Substrate  3  has been mounted to head holder  66  that provides a supply of ink behind substrate  3  to supply ink through individual ink feed slots  22  to each ejector on the front of substrate  3 . Row driver  34  and column driver  36  are attached to head holder  66 . 
         [0049]      FIG. 10  is a schematic side view of a printer using a head in accordance with the present invention. Printer controller  38  moves an ink receiver  40  using receiver driver  42 . Receiver driver  42  is a motor that operates on a plate or roller to drive ink receiver  40  under print head  32 . Printer controller  38  provides drive signals to row driver  34  and column driver  36  connected to print head  32  mounted on head holder  66  to apply an image-wise pattern of ink droplets onto ink receiver  40  in synchronization with the motion of ink receiver  40 . 
         [0050]      FIG. 11  is a top view of a head holder  66  in accordance with the present invention.  FIG. 12  is a side view of a head holder  66  in accordance with the present invention. In the invention, print head  32  is not bonded to head holder  66 . Head holder  66  has a recess  70  to receive print head  32 . Recess  70  is deep enough to provide a perimeter closely conforming to the perimeter of print head  32 . If print head is  450  microns thick, recess  70  can have the same depth. In another embodiment, recess  70  can provide predefined point contacts to the perimeter of print head  32 . Silicon print heads  32  made using semiconductor and MEMS processes will have flatness on the order of a few microns across the surface setting into the bottom of recess  70 . The bottom of recess  70  should have equivalent flatness to provide a seal for inks in cavities  31  and ink feed slot  22 . In the case of drop-on-demand heads, the ink is under less than  250  mm of water vacuum. The flatness of the two contacting surfaces on the bottom of recess  70  and the typical contact width of 1 mm are enough to provide a seal. 
         [0051]    A holding frame  72  is aligned and can be selectively connected to head holder  66 . In the example, holding frame  72  is a rectangular frame that aligns to the periphery of head holder  66 . Securing pins  76  fit into details in head holder  66  to securely attach holding frame  72  to head holder  66 . Head contacts  74  are secured to holding frame  72  and are formed to provide pressure contact to bond pads  60  when holding frame  72  is slide around the periphery of head holder  66  and securing pins  76  are locked into securing detail in head holder  66 . 
         [0052]    In the example, head contacts  74  are formed of gold plated beryllium-copper foil or wire, which have a bend that is flexed as holding frame  72  is secured to head holder  66 . The bend provides a wiping action on bond pads  60 , which provides reliable electrical connection during assembly. The gap between the ink-ejecting surface of print head  32  and an ink receiving substrate is small, typically 700 to 1,000 microns. Head contacts must fit into that space with enough clearance from the ink receiving substrate. Head contacts  74  can be formed of 75-micron thick beryllium-copper foil or wire and be bent nearly parallel to the ejecting surface of print head  32 . Head contacts  74  can be designed to project can project a total of 200 microns into the space between the front of print head  32  and the ink receiving substrate. Additional, non-conductive contacts  75  can be provided around the periphery of holding frame  72  to provide sufficient and balanced pressure to hold print head  32  into recess  70 . 
         [0053]    Flex circuits  64  provide electrical connection to each head contact  74 . Flex circuit  64  provides connection to row drivers  34  and column drivers  36  in the exemplary embodiment. Using the device structure of the examples, no control logic  58  is required; row drivers  34  and column drivers  36  provide those functions. The apparatus permits those components, as well as the manifold assembly to be reused. The life of ejectors  10  is limited, and the apparatus permits rapid, simple replacement of the ejectors without wasting other parts of the assembly. 
         [0054]    Bond pads  60  and head contacts  74  must be must be large enough to compensate for tolerance errors in the assembled components. The fit between ink jet head and the perimeter of recess  70  requires 50 microns of clearance. The fit between head holder  66  and holding frame  72  requires another 50 microns of clearance. Head contacts can be manufactured to 75 micron accuracy. The contact area required for good electrical connection is 125 microns. In practice, bond pads need to be 300 microns square for the apparatus to work. That bond pad area is not significantly larger than the bond pads used in current devices. Arranging ejectors  10  into a row-column configuration with internal control logic in the form of diodes  24  minimizes the number of contacts required for a given number of ejectors  10  on a substrate. 
         [0055]      FIG. 13  is a side view of the disassembled invention. Securing pins  76  have been disengaged from head holder  66 , releasing holding frame  72  to move upwards and off of head holder  66 . Flex circuit  64  permits holding frame  72  to be removed completely from the vicinity of head holder  66  to permit unhindered access to print head  32 . With holding frame  72  removed, print head  32  can be lifted from recess  70  in head holder  66  and be replaced with another print head  32 . Head contacts  74  move downward into their unloaded state position as holding frame  72  is removed. After a new print head  32  has been placed in recess  70 , holding frame  72  can be slide back around head holder  66  and secured by securing pins  76 . The action of positioning holding frame  72  back onto head holder  66  springs head contacts  74  nearly parallel to the front surface of print head  32 , the ends of head contacts  74  wipe across the surface of bond pads  60 . The presence of gold on the contact surface permits multiple head replacement with good electrical contact. 
         [0056]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
         [0000]    
       
         
               
             
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 3 
                 substrate 
               
               
                 5 
                 polymer layer 
               
               
                 7 
                 cover 
               
               
                 10 
                 ejector 
               
               
                 12 
                 ink chamber 
               
               
                 14 
                 nozzle 
               
               
                 16 
                 inlet 
               
               
                 18 
                 restriction 
               
               
                 20 
                 heater resistor 
               
               
                 22 
                 ink feed slot 
               
               
                 24 
                 diode 
               
               
                 26 
                 row conductor 
               
               
                 28 
                 column conductor 
               
               
                 30 
                 spacing distance 
               
               
                 31 
                 cavities 
               
               
                 32 
                 print head 
               
               
                 34 
                 row drivers 
               
               
                 36 
                 column drivers 
               
               
                 38 
                 printer controller 
               
               
                 40 
                 ink receiver 
               
               
                 42 
                 receiver driver 
               
               
                 52 
                 ejector drivers 
               
               
                 54 
                 conductor lines 
               
               
                 58 
                 control logic 
               
               
                 60 
                 bond pads 
               
               
                 62 
                 wire bonds 
               
               
                 64 
                 flex circuit 
               
               
                 66 
                 head holder 
               
               
                 70 
                 recess 
               
               
                 72 
                 holding frame 
               
               
                 74 
                 head contacts 
               
               
                 75 
                 non-conductive contacts 
               
               
                 76 
                 securing pin