Abstract:
A technique for fabricating datum surfaces on a print cartridge body. One technique includes fixing the print cartridge in a tooling fixture after the printhead has been affixed to the body, performing a position measurement on the fixed print cartridge to obtain position measurement data, and using the position measurement data, machining precisely located datum surfaces.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to ink jet printing systems, and more particularly to techniques for providing accurate datums for a print cartridge. 
     BACKGROUND OF THE INVENTION 
     Ink-jet printers are in widespread use today for printing functions in personal computers, graphics plotters, facsimile machines and other applications. Such printers typically include replaceable or semipermanent print cartridges which hold a supply of ink and carry the ink-jet printhead. The cartridge typically is secured into a printer carriage which supports one or a plurality of cartridges above the print medium, and traverses the medium in a direction transverse to the direction of medium travel through the printer. Electrical connections are made to the printhead by flexible wiring circuits attached to the outside of the cartridge. In a typical cartridge, the flexible wiring circuit is called a TAB head assembly, or THA, and the printhead is fabricated on the TAB circuit and integrated into the THA. The carriage receptacle has a corresponding electrical circuit with exposed contact pads which contact cartridge interconnect pads when the cartridge is mounted in the carriage. Each printhead includes a number of tiny nozzles defined in a substrate and nozzle plate structure which are selectively fired by electrical signals applied to the interconnect pads to eject droplets of ink in a controlled fashion onto the print medium. The cartridge may be connectable to auxiliary supplies of ink for replenishing the internal supply held in the cartridge. 
     In order to achieve accurate printing quality, each removable cartridge includes datum surfaces which engage against corresponding carriage surfaces to precisely locate the cartridge when inserted into the carriage. In this manner, when a cartridge ink supply is exhausted, the cartridge may be replaced with a fresh cartridge, and the printhead of the new cartridge will be precisely located relative to the carriage. The printer carriage receptacle and the cartridge are therefore designed together, so that the cartridge fits accurately within the carriage receptacle, the respective circuit pads and datum surfaces match up, and the cartridge can be removed and replaced with a fresh cartridge as needed. 
     For a typical inkjet printer, the media is loaded into the printer, and is advanced along a media path to a print area. A swath-type printer includes a carriage mounted for scanning movement along a swath axis, transverse to the media path at the print area. Hereafter, the media path is known as the X-axis, the scanning or swath axis is the Y-axis, and the Z-axis is mutually orthogonal to the X-axis and the Y-axis. For color printing, the carriage holds a plurality of ink-jet printheads, each for printing a different color ink, typically black, cyan, magenta and yellow. The printer can include a media drive mechanism for moving the media along the media path, and a carriage drive mechanism for scanning the carriage along the scan axis. The printer controller issues print control signals to cause the printheads to eject droplets of ink in a controlled manner to form a desired image or plot on the medium. 
     Inkjet printing is based on accurate ballistic delivery of small ink droplets to exact locations onto the paper or other media. Typically the droplet placement occurs onto a grid of different resolutions, most common grids being 300×300 dpi or 600×600 dpi, although other solutions are continuously being considered. One key factor for sharp and high quality images stems from the accuracy of the droplet placement. 
     Improvement of droplet position error during printing has been achieved by controlling drop trajectory through control of nozzle geometry, specifically controlling the angle of the nozzle axis relative to the media surface. Normal manufacturing variability requires that some variability in drop trajectory will always exist, and therefore the droplet position error improvements that can be realized by controlling drop trajectory alone are limited. 
     SUMMARY OF THE INVENTION 
     A technique for fabricating datum surfaces on a print cartridge body is described, and includes fixing the print cartridge in a tooling fixture, performing a position measurement on the fixed print cartridge to obtain position measurement data, and using the position measurement data to determine precision datum surface locations on the cartridge body in relation to the tooling fixture, conducting machining operations at one or more datum locations on the body to fabricate precision datum surfaces on the body. The datum surfaces are spaced from the nozzle array surface by a respective distance along a first direction, and determine a nozzle-array-to-print-media spacing during printing operations. The fabrication of the datum surfaces using the position measurement data provides accurate location of the datum surfaces relative to the nozzle array substrate in a direction transverse to the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other features and advantages of the present invention will become more apparent from the following detailed description of an exemplary embodiment thereof, as illustrate in the accompanying drawings, in which: 
     FIG. 1 is a flow diagram of an exemplary embodiment of a datum machining process in accordance with aspects of this invention. 
     FIG. 2 is a diagrammatic schematic of an exemplary embodiment of a process station for performing the machining process. 
     FIG. 3 is a side view of a print cartridge, illustrating the Z datums machined in accordance with an aspect of the invention. 
     FIG. 4 is a schematic block diagram of control elements of the process station illustrated in FIG.  2 . 
     FIGS. 5A-5B illustrate how lower cartridge-to-paper spacing significantly reduces the dot placement error. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In an exemplary embodiment, a process is employed to machine the Z axis datums on a printhead cartridge. The “Machine the Z Datums” (MTZ) tool and process can be integrated into a final assembly process for the cartridge. The MTZ process machines the Z datums of the print cartridge body based on the position of the THA surface after the THA has been mounted on the cartridge body. Thus, for this exemplary process, the print cartridge will have the THA attached to the cartridge body, cured, and wrapped at the time the MTZ process is performed. 
     The THA is mounted to the cartridge body using known techniques. In one exemplary technique, a one-part, thermally cured epoxy adhesive can be employed to attache the THA to the body. The adhesive is placed on the headland area of the cartridge body prior to placement of the THA. The THA is then placed on the headland region, allowing the adhesive to contact the THA. The THA is temporarily held in place by re-flowing a thermoplastic adhesive between the THA and cartridge body. The cartridge assembly proceeds into a thermal cure process, which cures the adhesive, forming a mechanical bond and fluid seal between the THA and the cartridge body. The cartridge assembly then proceeds onto further processes to wrap the remaining regions of the THA around the cartridge body, and fasten these regions of the THA to the cartridge body by re-flowing thermoplastic adhesive between the THA and cartridge body. Fabrication of the cartridge assembly is not complete at this point, as ink and additional components are added in subsequent assembly steps. 
     FIG. 1 is a simplified flow diagram of steps of an exemplary embodiment of the MTZ process  40 . FIG. 2 is a diagrammatic view of an exemplary embodiment of the MTZ process station  50 . In a first step  42 , the print cartridge  20  is fixtured in an MTZ tool  60  using the cartridge “X” datums  23 A,  23 B,  23 C and “Y” datum  24 A, and alternate “Z” datum locations  26 A,  26 B which are used only during the MTZ process, and not for carriage position registration. The tool  60  includes a fixture structure  60 A which holds the cartridge, a precision slide  66  and a position adjustment mechanism  68 . The cartridge is precisely located on the fixture structure  60 A and the slide apparatus  66  using the X, Y and alternate Z datums. In an exemplary embodiment, the fixture structure  60 A has built-in biasing features, so that the cartridge body is biased into the machine fixture, contacting the body at the datums indicated above. 
     For example, the fixture can be a three-sided structure, with the biasing features provided by air cylinders, solenoids, spring-loaded pins, or other types of biasing devices. The cartridge  20  has a THA  22  mounted at a cartridge snout region  27 , and the THA surface  22 A is positioned for probe measurements. The cartridge body  26  has two Z-axis datums  28 ,  30  in this exemplary embodiment, although fewer or more Z-axis datums can be employed. The Z-axis datums are machined in accordance with an aspect of this invention. 
     In a second step  44 , two points  22 A 1 ,  22 A 2  on the THA surface  22 A are measured in the Z axis using automated measuring probes  62 ,  64 . Data from the two measuring probes is used to establish the position of the cartridge relative to the tool reference frame. In an exemplary embodiment, a Z axis position of the physical center of the THA is derived, along with the angle of the THA on the “theta” axis, again relative to the tool reference frame. 
     In a third step  46 , based on the data from the THA surface probe measurement, the position of the print cartridge  20  is adjusted in the vertical or Z axis to bring the Z datums  28 ,  30  into position to be machined. This adjustment is accomplished by the precision slide apparatus  66  being moved along the Z axis by a drive system  68 , which can comprise a motor-driven leadscrew, for example. The appropriate Z-axis correction is made to the tool fixture to position the cartridge assembly such that the “Z 2 ” cartridge datum  30  may be machined. In an exemplary embodiment, the process station  50  further includes an X axis position adjustment mechanism  80  (FIG.  4 ), which can include an air bearing and X axis drive system to move the fixture  60 A in the X direction. 
     Machining of the cartridge datum locations is performed at step  48  using a milling cutter in a precision machining spindle under control of the process station, although alternate techniques can alternatively be employed to remove or add material from or to the cartridge datum location. For example, grinding, laser machining, chemical/dry etching, abrasive blasting, water jet machining, thermal forming (to heat, melt or reflow the material), photo-imaging to add material, and sputtering are all techniques which can be employed to remove or add material to or from the Z datum locations. 
     FIG. 3 is a simplified side view of the printhead  20 , showing the various dimensions of interest in this exemplary embodiment, and where die or substrate misalignment and the datum structures are exaggerated for clarity. A 1  indicates the surface of the nozzle array die or substrate. Data from the measurement probes is used to locate line A 2 , i.e. a line defined by two measured points on the substrate surface, approximating the Z height and theta-X orientation of the THA substrate relative to the tooling fixture, e.g., to the frame of the fixture. A 3  represents the normal distance from the desired Z 1  datum position to line A 2 . A 4  is the normal distance from the desired Z 2  datum position to line A 2 . Given the equation for line A 2 , and the desired Z axis dimensions A 3 , A 4 , a cut depth for each Z datum is calculated. A 5  indicates the material removed from the Z 1  datum to bring the datum surface to the desired position. A 6  represents the material removed from the Z 2  datum to bring the datum surface to the desired position. 
     In this exemplary embodiment, milling cutter  70  is mounted in a machining spindle  70 A, which rotates the milling cutter  70 . Milling cutter  72  is mounted in a machining spindle  72 A, which rotates the cutter  72 . The fixture  60 A is passed by the Z 2  machining cutter  70 , moving in the X axis, removing a precise amount of material from the “Z 2 ” datum location. Once the cartridge is clear of the Z 2  machining cutter  70 , a second Z axis correction is made to the cartridge fixture to position the cartridge such that the “Z 1 ” cartridge datum  28  may be machined. The fixture is passed by the Z 1  machining cutter  72 , removing a precise amount of material from the “Z 1 ” datum location. 
     The design of any given cartridge body may be such that, depending on setup of the process, material may or may not be removed from or added to one or both cartridge datum locations. 
     With the Z datums precisely machined, the print cartridge  20  is released from the tool fixture  60 , completing the MTZ process. 
     FIG. 4 shows the control system elements for the MTZ system. A process station controller  100  is programmed to control the process station, receiving the measurement information from the two probes  62 ,  64 , and calculating the necessary moves for machining the datums. The controller  100  is responsive to operator input parameters  82 , e.g. specifying the print cartridge type, spindle speeds, spindle feed rates determined by the X axis position adjustment  80 , desired or nominal Z datum positions, and the like. The controller can be implemented by a microprocessor or microcomputer, a personal computer, ASIC, or other known computing system. The calculations can be performed in real time, or pre-calculated and stored in look up tables in the controller memory. The controller then issues the necessary commands to the Z-axis position adjustment mechanism  68  to position the cartridge for the machining process, to the X-axis position adjustment mechanism  80  for moving the fixture past the cutters in a controlled movement to obtaining a desired feed rate, and controls the spindles  70 A,  72 A accordingly. 
     The MTZ process can in particular applications provide several advantages. One is that manufacturing variations of the molded print cartridge body, variations in the fabrication and assembly of the THA, and variations incurred in the final assembly of the print cartridge can all be accommodated by the MTZ process. The MTZ process is able to accommodate manufacturing variability by providing the capability of offsetting the effect of normal manufacturing variability or tolerances, by controlling the dimensions of the Z datum surfaces once assembly of the non-ink-filled cartridge has been essentially completed. For example, the assembly variation (3 sigma) in a known assembly process is +/−100 μm, as measured from the THA surface to the Z datum locations. With the MTZ process, through the modification or machining of the Z-datum locations, the assembly variation can be reduced substantially, e.g. in one example to +/−21 μm. 
     Another advantage is that the mean (machined) dimension of the Z datums relative to the THA surface can be easily controlled by setup of the process tool. Further, the process is readily adaptable to accommodate multiple cartridge types, and the tooling can readily accommodate types having the same basic external dimensions. 
     Machining of the Z datums provides an additional benefit in drop position error by controlling, and therefore minimizing, the distance traveled by the ink drop from the exit of the nozzle to the media surface. In this manner, variability in drop trajectory as the drop leaves the nozzle will produce less drop position error of the ink drop on the media. The MTZ process minimizes the dot placement error because it allows the writing system to have a lower cartridge-to-media spacing, and with less variability than in prior systems. This is due to the increased Z-axis datum accuracy, which in turn leads to increased accuracy in the positioning of the THA relative to the Z-axis datums, allowing much smaller tolerances in the cartridge-to-media spacing. A smaller cartridge-to-media spacing significantly reduces the dot placement error, as illustrated in FIGS. 5A and 5B. In FIG. 5A, a spacing S is provided between the printhead on the cartridge and the paper or other print medium. The distance between the theoretical drop placement and the actual drop placement, for a given angular drop error, is sigma. FIG. 5B illustrates the reduction in the error distance sigma&#39; when the spacing S&#39; is reduced. In an exemplary embodiment employing the MTZ process, an improved cartridge-to-media spacing of 1.2 mm+/−0.25 mm (3 sigma) is achieved, in comparison to a cartridge-to-media spacing of 1.4 mm+/−0.35 mm for a cartridge which does not employ the MTZ machining. 
     It is understood that the above-described embodiments are merely illustrative of the possible specific embodiments which may represent principles of the present invention. Other arrangements may readily be devised in accordance with these principles by those skilled in the art without departing from the scope and spirit of the invention.