PATENT ABSTRACT
The invention relates to apparatus and methods for producing three-dimensional objects and auxiliary systems used in conjunction with the aforementioned apparatus and methods. The apparatus and methods involve 3D printing and servicing of the equipment used in the associated 3D printer.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 60/612,068, filed on Sep. 21, 2004, the disclosure of which is incorporated herein by reference in its entirety. This application also incorporates herein by reference a U.S. patent application filed of even date herewith and identified by Attorney Docket No. ZCO-109B. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to apparatus and methods for servicing 3D printers, for example, for cleaning and aligning the printheads used in the 3D printers. 
       BACKGROUND 
       [0003]    Generally, 3D printing involves the use of an inkjet type printhead to deliver a liquid or colloidal binder material to layers of a powdered build material. The printing technique involves applying a layer of a powdered build material to a surface typically using a roller. After the build material is applied to the surface, the printhead delivers the liquid binder to predetermined areas of the layer of material. The binder infiltrates the material and reacts with the powder, causing the layer to solidify in the printed areas by, for example, activating an adhesive in the powder. The binder also penetrates into the underlying layers, producing interlayer bonding. After the first cross-sectional portion is formed, the previous steps are repeated, building successive cross-sectional portions until the final object is formed. See, for example, U.S. Pat. Nos. 6,375,874 and 6,416,850, the disclosures of which are incorporated herein by reference in their entireties. 
         [0004]    3D printers produce colored parts by using colored binder materials to solidify the powder. Clear binder is used to produce white part surfaces, and three primary colors are used in varying proportions to produce a gamut of colors. The printer must apply the variously colored binder droplets at precise locations to render the part surfaces in accurate color. 3D printers use a separate printhead to apply each binder color. In general, non-uniformity in printheads and mechanical variations in printhead mounting features produce inaccuracies in the positioning of binder droplets that must be characterized and corrected. 
         [0005]    Additionally, apparatus for carrying out 3D printing typically generates dust, which can detrimentally effect the operation of the printheads. For example, the dust can clog the jet nozzles that dispense the binder material, which can result in no binder material being dispensed or the binder material being dispensed inaccurately. 
         [0006]    It is, therefore, an object of the present invention to provide apparatus and methods for continuously and efficiently servicing 3D printers. 
       SUMMARY 
       [0007]    Generally, the invention relates to apparatus and methods for producing three-dimensional objects, such as casting cores, toys, bottles, cans, architectural models, automotive parts, molecular models, models of body parts, cell phone housings, and footwear, more rapidly and efficiently than heretofore achievable. Additionally, the invention relates to systems and methods for maintaining and operating the aforementioned apparatus. 
         [0008]    More specifically, the invention relates to apparatus and methods for aligning multiple printheads and apparatus and methods for cleaning the printheads. In one example, the alignment method is an automatic method of determining droplet-positioning errors that is particularly suited to 3D printing. In one example, a test pattern is printed with the printheads to be aligned, assuming that they are perfectly positioned. The resulting image is then scanned to determine the deviation of the images printed from perfect position. The information thus gained is then available to correct the identified errors. The present approach differs from the prior art in at least its use of the harmonic content of the signal obtained from scanning an alignment pattern to characterize misalignment. A scan traverses a multiplicity of nominally identical bar pairs, averaging out the irregularities inherent in an image printed in powder. Imaging optics are unnecessary since no edge detection is involved. 
         [0009]    In one aspect, the invention relates to a method of creating a test pattern with a plurality of printheads of a three-dimensional printer. The method includes the steps of defining an area on a build surface for receiving the test pattern, selecting a reference printhead capable of printing with a high contrast, printing a reference line with the reference printhead, and printing a test line proximate to the reference line with at least one of the remaining printheads. 
         [0010]    In various embodiments, the step of defining an area includes producing a contrast-enhancing sublayer on the build surface. The contrast-enhancing sublayer can be produced by printing the area in a solid, high contrast color using at least one of the printhead and overlaying the printed area with at least one unprinted layer of build material. In one embodiment, the area is printed with all of the available printheads at a maximum discharge level to saturate the area. 
         [0011]    The step of selecting a printhead includes the steps of printing a target above the contrast-enhancing sublayer with each of the printheads, comparing the targets to identify which target has a highest contrast relative to an unprinted area, and selecting a printhead associated with the highest contrast target. Further, the method can include the step of depositing a layer of a build material on the build surface prior to each printing step. The printing steps can include depositing a liquid binder in a predetermined pattern on the build material. The printheads, in one embodiment, print with a liquid binder having a color selected from the group consisting of magenta, yellow, cyan, clear and black. Other colors and combinations of colors are contemplated and within the scope of the invention. 
         [0012]    Additionally, the step of printing a test line can include printing alternating bars of color with at least two of the remaining printheads. The steps of printing a reference line and printing a test line can include printing a plurality of reference lines and printing a corresponding plurality of test lines. In one embodiment, the reference lines and the test lines can be printed in multiple passes. The step of printing a plurality of lines can include printing a plurality of horizontal lines and a plurality of vertical lines. Also, the step of printing a reference line can include printing ten horizontal reference lines and printing ten vertical reference lines proximate thereto, and the step of printing a test line can include printing ten corresponding horizontal test lines and printing ten corresponding vertical test lines. In some embodiments, two reference lines may be printed. In other embodiments, 20 reference lines may be printed. 
         [0013]    In a particular embodiment of the method, the steps of printing a reference line and printing a test line include printing a plurality of nominally identical line pairs parallel to a fast-axis travel of the printheads, each line pair comprising one reference line and one test line, and printing a plurality of nominally identical line pairs perpendicular to the fast-axis travel of the printheads, each line pair comprising one reference line and one test line. In one embodiment, each plurality of line pairs is arranged as an equally spaced linear array. Each test line can include a series of test bars, where each of the remaining printheads prints a central test bar that is nominally located at a distance from a corresponding reference line equal to ½ of a nominal array spacing of the reference lines. In one embodiment each remaining printhead prints a plurality of additional test bars that are incrementally displaced about the central test bar. 
         [0014]    In another aspect, the invention relates to a test pattern for aligning a plurality of printheads in a three-dimensional printer. The test pattern includes a plurality of substantially evenly spaced solid reference lines and a plurality of test lines disposed in an alternating pattern with the plurality of reference lines, wherein each of the test lines comprises at least one bar of a non-reference color. In one embodiment, the colors are printed in an alternating pattern. In various embodiments, the plurality of lines is oriented substantially vertically, or in a particular embodiment, parallel to a fast-axis printhead travel. Further, the test pattern can include a second test pattern disposed proximate the first test pattern. The second test pattern includes a second plurality of substantially evenly spaced solid reference lines and a second plurality of test lines disposed in an alternating pattern with the second plurality of reference lines. Each of the test lines comprises at least one bar of a non-reference color, and the second plurality of lines can be oriented substantially perpendicular to the fast-axis printhead travel. 
         [0015]    In another aspect, the invention relates to a method of determining a correction factor(s) for aligning a plurality of printheads. The printheads need to operate in concert to produce colored images. Due to printhead and mounting variations, the relative positions of the printheads need to be measured, and corrections need to be applied to the printhead drive signals to cause the various colors to be printed in the proper registration. Generally, a test pattern is printed with the printheads to be aligned, assuming that they are perfectly positioned. The resulting image is then scanned to determine the deviation of the images printed from their perfect position. The information thus gained is then available to correct the identified errors. The present approach differs from the prior art in at least its use of the harmonic content of the signal obtained from scanning the test pattern to characterize misalignment. A scan traverses a plurality of nominally identical line pairs, averaging out the irregularities inherent in an image printed in powder. Imaging optics are unnecessary, since no edge detection is involved. 
         [0016]    Specifically, the method includes the steps of printing a test pattern on a build surface, generating a set of electrical signals representative of the test pattern, analyzing the electrical signals to determine their harmonic content at least one frequency, and determining a correction factor(s) based on the harmonic content of the electrical signals. The test pattern can include a line pair array. In one embodiment, the method includes generating a plurality of electrical signals for analysis and determining a plurality of correction factors based on the harmonic content of the plurality of electrical signals. 
         [0017]    In various embodiments, the method includes generating the electrical signal by illuminating the test pattern and measuring reflectance of the test pattern at predetermined locations. In one embodiment, the step of analyzing the electrical signal includes applying an analog filter (e.g., using op-amps) to the signal. In another embodiment, the step of analyzing the electrical signal includes digitizing the signal and applying a digital filter (e.g., a Fast Fourier Transform) to the signal. In one embodiment, the correction factor can be determined from a set of third harmonic values. In another embodiment, the correction factor can be determined from a set of first harmonic values. The correction factor can be near a nominal test bar displacement for which a lowest value of the selected harmonic is determined. The correction factors can be determined by locating a minimum value of an analytical curve that has been fitted to, or representative of the set of third harmonic values. One embodiment of the method includes the steps of extracting third harmonic values from the signals acquired by scanning the sensor across the array, comparing the set of third harmonic values obtained for each color, and determining the correction factors based on the minimum third harmonic values. 
         [0018]    In another aspect, the invention relates to the servicing of a plurality of printheads in a three-dimensional printer. In general, quality of the parts produced in the 3-D printing process depends upon the reliable and accurate delivery of droplets of binder liquid from the nozzle arrays located on the faces of the printheads. To maintain high performance standards, the printheads must be serviced frequently during the 3-D printing process. The impact of droplets of binder liquid on the surface of the powder bed causes powder particles to be ejected from the surface of the bed. Some of the ejected material collects on the faces of the printheads, interfering with the delivery of binder liquid droplets. A principal purpose of the printhead servicing is to remove this accumulated debris from the printhead faces. 
         [0019]    One aspect of printhead servicing is a service station, which includes a cleaning station, a discharge station, and a capping station. In one embodiment, the printheads are disposable within a carriage capable of moving in at least two directions relative to the service station. Another aspect of printhead servicing is a software algorithm that specifies when each printhead needs to be serviced. In one embodiment, the printheads are disposable within a carriage capable moving in at least two directions relative to the service station. 
         [0020]    Various embodiments of the cleaning station include at least one receptacle for receiving a printhead, at least one nozzle for spraying a cleaning fluid towards a printhead face (or printing surface) of the printhead, and a wiper disposable in close proximity to the printhead face for removing excess cleaning fluid, in some cases without contacting the printhead face. The cleaning station can further include a splash guard for isolating the printhead face and preventing the cleaning fluid from migrating beyond the printhead face. The splash guard includes an open position and a sealed position, where the splash guard is biased open and is actuated from the open position to the sealed position by contact with a printhead. The splash guard can include a sealing lip that circumscribes the printhead face when in the sealed position. In one embodiment, the sealing lip is generally rectangular in shape. The wiper can be formed by one side of the sealing lip and can include a notched portion configured and located to correspond to a location of a jet nozzle array on the printhead face to prevent the wiper from contacting the jet nozzle array. The wiper is capable of movement relative to a printhead. 
         [0021]    Further, the cleaning station can include a fluid source for providing the cleaning fluid to the at least one nozzle under pressure. The cleaning fluid can be provided to the at least one nozzle via a manifold. In one embodiment, the at least one nozzle includes an array of nozzles. The at least one nozzle can be positioned to spray the cleaning fluid across the printhead face. In one embodiment, the printheads are disposed within a carriage capable of movement in two directions with respect to the service station. 
         [0022]    Various embodiments of the discharge station include a receptacle defining an opening that generally corresponds to a printhead face of a printhead. The receptacle defines a plurality of corresponding openings in one embodiment. The receptacle can include a tray for capturing and/or directing discharged fluids. In one embodiment, the discharge from the printheads is directed into a standing pool of waste liquid. 
         [0023]    Various embodiments of the capping station include a printhead cap carrier and at least one printhead cap disposed on the carrier for sealing a printhead face of a printhead. The cap is moved between an off position and a capped position by the printhead contacting the carrier. The capping station can include a plurality of caps disposed on the carrier. In one embodiment, the carrier is biased to maintain the at least one cap in an off position. The discharge station and the capping station can be a combined station. In such an embodiment, the discharge from the printheads can be constrained in a cavity defined by a printhead face, a printhead cap, and the standing pool of waste liquid. 
         [0024]    In another aspect, the invention relates to an apparatus for cleaning a printhead. The apparatus includes at least one nozzle for spraying a cleaning fluid towards a printhead face of the printhead and a wiper disposable in close proximity to the printhead face for removing excess cleaning fluid from the printhead face. 
         [0025]    In one embodiment, the apparatus includes a splash guard for isolating a printhead face and preventing cleaning fluid from migrating beyond the printhead face. The splash guard can include an open position and a sealed position, where the splash guard is actuated from the open position to the sealed position by contact with a printhead. In addition, the splash guard can include a sealing lip that circumscribes the printhead face when in the sealed position. The sealing lip is generally rectangular in shape. In one embodiment, the wiper is formed by one side of the sealing lip. The wiper can include a notched portion configured and located to correspond to a location of a jet nozzle array on the printhead face to prevent the wiper from contacting the jet nozzle array. The wiper is capable of movement relative to a printhead. Additionally, the apparatus can include a fluid source for providing cleaning fluid to the at least one nozzle under pressure. The at least one nozzle can an array of nozzles and can be positioned to spray the cleaning fluid across a printhead face. 
         [0026]    In another aspect, the invention relates to a method of cleaning a printhead. The method includes the steps of positioning a printhead face of the printhead relative to at least one nozzle, operating the at least one nozzle to spray cleaning fluid towards the printhead face, and causing relative movement between a wiper and the printhead to pass the wiper in close proximity to the printhead face to remove excess cleaning fluid. The wiper can include a notch configured and located on the wiper to correspond to a jet nozzle array on the printhead face to prevent the wiper from contacting the jet nozzle array. 
         [0027]    In various embodiments, the step of positioning the printhead face includes sealing the printhead face to prevent the cleaning fluid from migrating beyond the printhead face. The operating step can include spraying the cleaning fluid across the printhead face. In addition, the printhead can be operated to discharge any cleaning fluid ingested by the printhead during cleaning. In one embodiment, the at least one nozzle comprises an array of nozzles. 
         [0028]    In another aspect, the invention relates to an apparatus for cleaning a printhead used in a three-dimensional printer. The apparatus includes a sealing cap defining a cavity and capable of engagement with a printhead face of the printhead, a cleaning fluid source in communication with the cap for cleaning the printhead face, and a vacuum source in communication with the cap for removing used cleaning fluid and debris. In operation, the vacuum source creates a negative pressure within the cavity, the negative pressure preventing the cleaning fluid from entering a jet nozzle, drawing the cleaning fluid into the cavity from the cleaning fluid source, and/or drawing at least one of a binder fluid and debris from the jet nozzle. The apparatus may further include a wiper disposed proximate the cap, the wiper positioned to engage the printhead face as the printhead disengages from the cap. 
         [0029]    In another aspect, the invention relates to a method of cleaning a printhead used in a three-dimensional printer. The method includes the steps of engaging a printhead face of the printhead with a sealing cap defining a cavity, drawing a vacuum in the cavity, and introducing a cleaning fluid into the cavity and into contact with the printhead face. The method may further include the step of removing the cleaning fluid from the cavity. In one embodiment, the method includes the steps of disengaging the cap from the printhead face and wiping the printhead face with a wiper. The step of drawing a vacuum creates a negative pressure within the cavity, the negative pressure drawing the cleaning fluid into the cavity, preventing the cleaning fluid from entering a jet nozzle and/or drawing at least one of a binder fluid and debris from the jet nozzle. 
         [0030]    In still other embodiments, the invention can include alternative methods and apparatus for cleaning the printheads apparatus. Methods of cleaning the printhead can include wiping the printhead with a roller including a cleaning fluid, drawing a vibrating member across the printhead, drawing a cleaning fluid across the printhead by capillary action through a wick, and/or combinations thereof. In addition, the methods can include optionally the step of applying a vacuum to the printhead to remove debris. The apparatus for cleaning a printhead used in a 3D printer can include a wick disposed adjacent the printhead for drawing a cleaning fluid across the printhead. 
         [0031]    In another aspect, the invention relates to an apparatus for cleaning a printhead used in a 3D printer. The pressure in the interior of a printhead is typically lower than atmospheric pressure. This negative pressure is balanced by the surface tension of the meniscuses that form over the outlets of the printhead nozzles. It is desirable to flush the accumulated powder off the face of the printhead with a clean wash solution without allowing the solution to be drawn into the printhead when the meniscuses are destroyed. This goal is achieved in this apparatus by maintaining an environment outside the printhead in which the pressure is lower than the pressure inside the head. In addition, this induced pressure differential causes binder to flow out of the heads through the nozzles, flushing out any powder that may have lodged in the nozzle passageways. The apparatus includes a base, a cam track disposed within the base, a cap carrier slidably engaged with the cam track, and a sealing cap defining a cavity and disposed on the carrier. The cap being transportable into engagement with the face of the printhead by the carrier. In various embodiments, the apparatus includes a cleaning fluid source in communication with the cap for cleaning the printhead face and a vacuum source in communication with the cap for removing used wash fluid and debris. 
         [0032]    In further embodiments, the apparatus can also include a spring coupled to the carrier and the base to bias the carrier into a receiving position for receiving the printhead. In one embodiment, the carrier includes a stop disposed on a distal end of the carrier for engaging the printhead as the printhead enters the apparatus. The printhead slides the carrier rearward along the cam track after engaging the stop and until the printhead face and cap sealably engage. In a further embodiment, the apparatus includes a latch pawl coupled to the base for engaging with the carrier to prevent forward movement of the carrier and a wiper disposed on a proximal end of the carrier. The wiper is positioned to engage the printhead face as the printhead exits the apparatus. 
         [0033]    In still another aspect, the invention relates to a method of cleaning a printhead used in a 3D printer. The method includes the step of receiving the printhead within an apparatus that includes a base, a cam track disposed within the base, a cap carrier slidably engaged with the cam track, and a sealing cap defining a cavity and disposed on the carrier. Additional steps include engaging the face of the printhead with the cap, drawing a vacuum on the cavity, and introducing a cleaning fluid into the cavity and into contact with the printhead face. In one embodiment, the method includes the step of removing the cleaning fluid from the cavity. The method can further include disengaging the cap from the printing surface and wiping the printing surface with a wiper as the printhead is withdrawn from the apparatus. 
         [0034]    In another aspect, the invention relates to an apparatus for cleaning or reconditioning a printhead. The apparatus includes a nozzle array for spraying a washing solution towards a face of a printhead and a wicking member disposed in proximity to the printhead face for removing excess washing solution from the printhead face. 
         [0035]    In various embodiments, the nozzle array includes one or more individual nozzles. The wicking member and the printhead are capable of relative movement. A fluid source can also be included in the apparatus for providing washing solution to the nozzle array under pressure. In another embodiment, the wicking member includes at least one of a permeable material and an impermeable material. 
         [0036]    The nozzle array can be positioned to spray the washing solution at an angle with respect to the printhead face. In another embodiment, the wicking member is disposed in close proximity to the printhead face, without contacting print nozzles located on the printhead face. The spacing between the wicking member and the print nozzles can be automatically maintained. In one embodiment, the spacing is maintained by causing a portion of the wicking member to bear on the printhead face in a location removed from the print nozzles. The apparatus can also include a basin for collecting washing solution and debris. 
         [0037]    In another aspect, the invention relates to a method of cleaning or reconditioning a printhead. The method includes the steps of positioning a face of the printhead relative to at least one nozzle and operating the at least one nozzle to spray washing solution towards the printhead face. Excess washing solution is then removed from the printhead face by passing a wicking member in close proximity to the printhead face, without contacting the printhead face. 
         [0038]    In one embodiment, the step of operating the at least one nozzle includes spraying the washing solution at an angle to the printhead face. In another embodiment, the method can include the step of operating the printhead to expel washing solution ingested by the printhead during cleaning. The method can include automatically maintaining a space between the wicking member and print nozzles located on the printhead face by, for example, causing a portion of the wicking member to bear on the printhead face in a location removed from the print nozzles. 
         [0039]    In another aspect, the invention relates to a method of determining when a printhead needs to be serviced. Servicing is needed to maintain adequate printhead performance. Servicing is a time-consuming activity, however, and some aspects of the servicing process are damaging to the printhead. It is therefore desirable to service a printhead on a schedule that balances the positive and negative impacts of the process. 
         [0040]    One approach to identifying a printhead in need of service is to infer the state of the printhead indirectly from the information available about the ongoing printing process. It is common, for example, to perform printhead servicing at intervals based on the time elapsed since last service, the number of droplets dispensed since last service, and the number of layers printed since last service. Printhead service is performed when one or another of these indicative factors reaches a predetermined trigger value. Alternatively, service-triggering variables may be defined that are weighted functions of two or more indicative factors. In one implementation, the trigger values for one or more of the indicative factors are adjusted to match the characteristics of the powder and binder liquid materials in use. The specific factors and corresponding trigger values may be selected to suit a particular application, environment, and/or printhead. 
         [0041]    It is particularly desirable to identify characteristics of the images being printed that can be related quantitatively to the need for printhead service. One such factor is based on the observation that the impact of droplets printed on the powder bed ejects less debris when the underlying previous layer was printed. The binder printed on the previous layer tends to bind the powder in the fresh layer, resulting in less debris being ejected, and correspondingly less debris accumulating on the printhead face. Accordingly, in one implementation, printhead servicing is performed when the number of droplets printed over previously unprinted powder reaches a predetermined trigger value. Alternatively, a service interval based on the number of droplets dispensed since the last service may be modified to take into account the proportion of the droplets that were printed over previously unprinted powder. In another implementation, the underlying layer is considered to be unprinted if the pixel immediately underneath or any of its near neighbors are unprinted. 
         [0042]    In another aspect, the invention relates to a method of determining a condition of a printhead in use in a three-dimensional printer. The method includes the steps of acquiring a data value for at least one operational parameter of the printhead and comparing the data value to a threshold value, the relationship of the data value to the threshold value indicative of the condition of the printhead. In one embodiment, the method includes the step of initiating a service routine on the printhead if the data value exceeds the threshold value. The operational parameter can be selected from the group consisting of time elapsed, number of droplets dispensed by the printhead, number of layers printed, droplets dispensed over previously printed powder, droplets dispensed over previously unprinted powder, and combinations thereof. Additionally, the data value can be compensated during acquisition to account for an operational environmental factor of the three-dimensional printer, such as, for example, temperature, humidity, binder material, and/or build material. 
         [0043]    In another aspect, the invention relates to a method of determining a condition of a printhead in use in a three-dimensional printer. The method includes the steps of counting droplets dispensed by the printhead and determining a percentage of the droplets that were dispensed over previously unprinted pixels. The method can include the step of initiating a service routine on the printhead if the percentage exceeds a threshold value. 
         [0044]    These and other objects, along with advantages and features of the present invention herein disclosed, will become apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    In the drawings, like reference characters generally refer to the same parts throughout the different views. In addition, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
           [0046]      FIG. 1  is a schematic perspective view of a three dimensional printer in accordance with one embodiment of the invention; 
           [0047]      FIG. 2  is a schematic perspective view of a printhead carriage in accordance with one embodiment of the invention; 
           [0048]      FIGS. 3A and 3B  are a schematic perspective view and a schematic plan view, respectively, of a service station in accordance with one embodiment of the invention; 
           [0049]      FIG. 4  is a schematic representation of the interaction between the carriage and the service station during performance of a discharge function in accordance with one embodiment of the invention; 
           [0050]      FIGS. 5A-5D  are schematic representations of one embodiment of a printhead capping operation in accordance with one embodiment of the invention; 
           [0051]      FIGS. 6A-6D  are schematic representations of a printhead discharge and capping operation in accordance with an alternative embodiment of the invention; 
           [0052]      FIGS. 7A-7D  are schematic representations of a printhead cleaning station in accordance with one embodiment of the invention; 
           [0053]      FIGS. 8A-8H  are schematic representations of an alternative embodiment of a printhead cleaning station in accordance with the invention; 
           [0054]      FIGS. 9A and 9B  are schematic representations of another alternative embodiment of a printhead cleaning station in accordance with the invention; 
           [0055]      FIGS. 10A-10D  are schematic representations of yet another alternative embodiment of a printhead cleaning station in accordance with the invention; 
           [0056]      FIGS. 11A-11J  are schematic representations of one embodiment of an apparatus and method for cleaning a printhead in accordance with the invention; 
           [0057]      FIG. 12  is a schematic representation of a step of the method of cleaning a printhead in accordance with the embodiment of the invention depicted in  FIGS. 11A-11J ; 
           [0058]      FIG. 13  is a schematic perspective view of a printing operation in accordance with one embodiment of the invention; 
           [0059]      FIGS. 14A and 14B  are schematic representations of the impact of a liquid binder droplet on a build surface; 
           [0060]      FIG. 15  is a schematic perspective view of a printhead alignment process in accordance with one embodiment of the invention; 
           [0061]      FIGS. 16A and 16B  are schematic representations of a contrast test target and test pattern alignment method in accordance with one embodiment of the invention; 
           [0062]      FIGS. 17A-17D  are schematic representations of an alignment sensor system and associated electronics in accordance with one embodiment of the invention; 
           [0063]      FIG. 18  is a schematic representation of one step in a method of aligning color printheads in accordance with one embodiment of the invention; 
           [0064]      FIGS. 19A and 19B  are detailed schematic representations of a test pattern in accordance with one embodiment of the invention; 
           [0065]      FIGS. 20A-20D  are detailed schematic representations of the horizontal alignment process in accordance with one embodiment of the invention; and 
           [0066]      FIGS. 21A and 21B  are detailed schematic representations of the vertical alignment process in accordance with one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0067]    Embodiments of the present invention are described below. It is, however, expressly noted that the present invention is not limited to these embodiments, but rather the intention is that variations, modifications, and equivalents that are apparent to the person skilled in the art are also included. 
         [0068]    In brief overview,  FIG. 1  is a schematic representation of a 3D printer  10  for creating an object in accordance with one embodiment of the invention. The printer  10  produces three-dimensional objects by depositing alternating layers of build material and binder liquid on a build surface  165  or in a container to print multiple layers that ultimately form the three-dimensional object. In some embodiments, the build material may include a powder and the binder liquid may be incorporated into the build material. In some embodiments, the printer  10  may be used to create physical prototypes for viewing and design review. In other embodiments, the printer  10  may be used to create molds for casting operations, or prototypes that may be used to collect market feedback on a potential product. 
         [0069]    The printer  10  shown includes a gantry  12 , a carriage  14 , a service station assembly  16 , and a test pattern  18 . Typically, the gantry  12  is actuatable along the X-axis to manufacture the object layer by layer. In some embodiments a motor may be coupled to the gantry  12 . In other embodiments, the gantry  12  may be coupled to a screw, such that rotation of the screw moves the gantry  12  along the X-axis. In some embodiments, the gantry  12  may be actuatable along the vertical Z-axis. Other positioning systems may be employed, as desired. 
         [0070]    The carriage  14  typically includes printheads  20  capable of dispensing binder materials necessary for creating an object (see  FIG. 2 ). In some embodiments, as the gantry  12  moves along the X-axis, the carriage  14  moves back and forth along the Y-axis. The carriage  14  is coupled to the gantry  12 . Thus, as the carriage  14  moves along with the gantry  12  across the printer  10 , binder material may be deposited in a two dimensional pattern during travel across the surface of the printer  10  along the X-axis and the Y-axis. Then, typically, the next pass across the printer  10  will be at a different plane in the Z-axis, and material deposited in that z-plane on the Z-axis will bind with previously deposited material as part of the formation of the desired object. In one embodiment, a stepping-motor-driven piston underneath the build table provides Z-axis motion. 
         [0071]    To further improve performance, the printer  10  also includes the service station  16 . In some embodiments, the service station  16  is located at a fixed point on the printer  10 . Generally, the service station  16  services the printheads  20  carried by the carriage  14 . The service station  16  is generally the physical location where debris or excess materials that are on or about the printheads  20  are removed. In some embodiments, excess binder material is removed or discharged from the carriage  14 . Generally, the carriage  14  is actuated into the service station  16  for maintenance, storage, or preservation from damage. Typically, the service station  16  may be located at any point on the printer  10  where it is possible for the carriage  14  to be actuated to engage the service station  16 . Also included in the printer  10  is a test pattern  18 . In some embodiments, the test pattern  18  is a test area passed over by the printhead  20  to refine alignment of the carriage  14  in creation of an object. 
         [0072]    In some embodiments, the carriage  14  can be moved for diagnostic or service purposes. Moving the carriage  14  provides the user with access to the printheads  20  for maintenance purposes, such as cleaning or replacement. Printhead cleaning is described in detail with respect to  FIGS. 6A-6D ,  7 A- 7 D,  8 A- 8 J,  9 A- 9 B,  10 A- 10 D,  11 A- 11 J, and  12 . In some embodiments, the printheads  20  may be actuated to run a diagnostic routine of the printheads  20 . In an alternative embodiment, the carriage  14  can be raised from the printer  10  for service purposes. 
         [0073]    In one embodiment, the printer  10  includes an enclosure cover to contain any dust or other debris generated during a printing operation. The enclosed area can be heated to facilitate better reactions between the build material and the binder materials. Better reactions include, for example, faster reaction times and improved bonding. In one embodiment, the heating is accomplished by introducing warm air at a low velocity to the enclosed area. The flow of air is typically not directed at the build surface to prevent disturbing the build material after spreading. In one example, the enclosure temperature is maintained from about 90 degrees F. to about 150 degrees F., preferably from about 10 degrees F. to about 135 degrees F., and more preferably about 125 degrees F. 
         [0074]      FIG. 2  depicts one embodiment of the carriage  14  in more detail. The carriage  14  generally includes one or more printheads  20 . Typically, a printhead  20  is the apparatus through which binder liquid is ejected during the creation of an object.  FIG. 2  shows four printheads  20 ; however, in other embodiments there may be more or fewer printheads  20 . In some embodiments, the printheads  20  may be inserted into the carriage  14  such that they are offset from one another along the X-axis. In some embodiments, this offset is by substantially the same distance along the X-axis. In other embodiments, the printheads  20  may be staggered within the carriage  14  such that the distances between the printheads  20  vary. 
         [0075]      FIGS. 3A and 3B  depict one embodiment of the service station  16  in greater detail. The service station  16  typically includes a discharge station  22 , a printhead capping station  24 , and a printhead cleaning station  29 . In various embodiments, the carriage  14  may engage the discharge station  22 , the printhead capping station  24 , and the printhead cleaning station  29  in any order, and any number of times. In some embodiments, the carriage  14  may engage the same station, for example the discharge station  22 , multiple times consecutively. In other embodiments, the carriage  14  can alternate repeatedly between any of the discharge station  22 , the printhead capping station  24 , and the printhead cleaning station  29  in any order, any number of times. In some embodiments, the printheads  20  of the carriage  14  engage the service station  16  in order to perform maintenance upon the printheads  20  during creation of an object. 
         [0076]    Generally, the discharge station  22  includes discharge openings  28  through which the printheads  20  may discharge debris, such as, for example, contaminated binder. The number of the discharge openings  28  may vary. The discharge station  22  is typically an area where the printheads  20  may expel such material, thus preventing excess buildup of contaminants in the printheads  20  that could effect printing quality. Typically, debris entering the discharge station is contained so that it does not contaminate the printheads  20 , the carriage  14 , the service station  16 , or any other component of the printer  10 . 
         [0077]    In some embodiments, the printheads  20  may be actuated to a point immediately above the discharge openings  28 , where the printheads  20  discharge excess binding material or other waste through the discharge openings  28 . Generally, this waste is collected in a receptacle  47  (see  FIG. 4 .) In some embodiments, the carriage  14  is actuated into a position immediately above the service station  16  and the printheads  20  are positioned above the discharge openings  28  at the surface of the service station  16 . In some embodiments, the bottom surfaces of the printheads  20  may extend below the plane of the surface of the discharge openings  28 , where the printheads  20  may discharge material in order to rid the printheads  20  of contamination or excess building materials. This material then enters the receptacle  47 . In one embodiment, the discharge openings  28  are located above the receptacle  47 . Generally, the receptacle  47  is a location below the discharge openings  28  where the printheads  20  discharge their material. In some embodiments, the receptacle  47  may include a reservoir for containing the discharged material. 
         [0078]    Generally, the printhead capping station  24  is the area where the printheads  20  are capped by the printhead caps  26 . In one embodiment, there is one printhead cap  26  for each printhead  20 . Generally, as a result of the carrier  14  engaging the printhead capping station  24 , the printhead caps  26  are actuated into a position circumscribing the printheads  20 , such that the printhead caps  26  form a seal around the printhead face  54  (see  FIG. 5D ). The printhead caps  26  protect the printheads  20  against contamination, debris, and physical damage resulting from contact with the printheads  20 , deterioration, and the elements in general. Generally, the printhead capping station  24  may cap printheads  20  at any point in time relative to the printheads  20  engaging the discharge station  22  or the printhead cleaning station  29 . Generally, the printhead caps  26  enclose the printheads  20  in order to form a seal to prevent damage, such as drying out, from occurring to the printheads  20 . In some embodiments, maintenance may include cleaning on or about the printheads  20 . Only a single service station  16  is shown for descriptive purposes; however, multiple stations  16  may exist. Alternatively, a single service station  16  may service multiple printheads  20  by, for example, successively positioning the printheads  20  relative to the service station  16 . 
         [0079]    The printhead cleaning station  29  generally includes the area where the printheads  20  may be cleaned. In one embodiment, the printheads  20  may be cleaned with a pressurized washing solution  92  (see  FIG. 8E ). In some embodiments, the printheads  20  enter the printhead cleaning station  29  after the printheads  20  discharge material into the receptacle  47 . In other embodiments, the printheads  20  may enter the printhead cleaning station  29  without first discharging material into the receptacle  47 . In further embodiments, the printheads  20  may enter both the printhead cleaning station  29  and the discharge station  22  repeatedly and in any order. Typically, the cleaning station  29  cleans the printheads  20  by washing them in such a manner that any debris is removed from the printheads  20  and the pressurized washing solution  92  itself is contained so it does not contaminate the printheads  20 , or any other part of the printer  10 . For example, in one embodiment, the printheads  20  are cleaned in a sealed environment to contain any debris and cleaning materials. In another embodiment, the printheads  20  are protected during cleaning so that there is no excess debris or cleaning materials left on the printheads  20  that may later drip onto any component of the printer  10 , for example, the build surface  165 . In one embodiment, the printheads  20  are cleaned one at a time. In another embodiment, the printheads  20  may be cleaned simultaneously. In other embodiments, the printhead(s)  20  may be cleaned repeatedly, in any order, and at any time relative to engagement of the carrier  14  with any other components of the service station  16 . In one embodiment, the printer  10  includes logic for determining when to clean the printheads  20 , as discussed in greater detail hereinbelow. 
         [0080]      FIG. 3B  is a plan view of the service station  16  of  FIG. 3A . From this perspective, the carriage  14  is actuated along the X-axis such that the printheads  20  are aligned with the discharge openings  28 . In one embodiment, upon completion of this alignment, the printheads  20  discharge residual or waste material through the discharge openings  28 . In some embodiments, the discharge may include binder material or other building material. In some embodiments, after discharge, the printheads  20  are further actuated along the X-axis to the printhead capping station  24 , where the printhead caps  26  form a seal around the printheads  20 . The seal formed by the printhead caps  26  around the printheads  20  generally protects the printheads  20  from the elements, contamination from debris or left over binding material, and prevents the printheads  20  from drying out. 
         [0081]      FIG. 4  is a graphical representation of the discharge function of an embodiment of the invention, whereby binder material and debris  41  is discharged from the printhead  20 . In some embodiments, the binder debris  41  may include excess building material. In some embodiments, this discharge function is performed after every pass of the carriage  14  across the build surface  165 . In other embodiments, the discharge function may be performed periodically after any given number of passes of the carriage  14 . In still other embodiments, this function may be performed at fixed time intervals. In this illustrative embodiment, the carriage  14  is positioned above the service station  16  such that the printhead  20  is lined up over a spatial gap in between the aperture plates  40 . In some embodiments, the aperture plates  40  include the solid surface surrounding the discharge openings  28  (see  FIG. 3B ). After proper positioning of the carriage  14 , the printhead  20  discharges the debris  41  or other waste. Generally, this debris  41  includes contaminants, such as, for example, excess binder material left in the printhead  20 . In one embodiment, the debris  41  joins the waste liquid  42  in the waste liquid catch tray  43 . In some embodiments, the waste liquid  42  may include discharge from past discharges of the printheads  20 . Upon discharge, the droplets of binder liquid  41  impinge upon the surface of the standing pool of waste liquid  42 , minimizing splash and the consequent generation of undesirable waste liquid aerosols. A spillway  44  is located at a distance above the bottom of receptacle  47  sufficient to maintain the standing pool of waste liquid  42 . Generally, the waste liquid  42  then proceeds down the spillway  44  where it eventually exits the service station  16  via a drain  45 . In some embodiments, any overflowing waste liquid  46  also exits the waste liquid catch tray  43  via the drain  45 , thus preventing contamination to the service station  16 . 
         [0082]      FIG. 5A  illustrates one embodiment of the capping function of the invention, whereby each printhead  20  is sealed by a cap. In some embodiments, this capping function may be performed after any given number of passes across the printer  10 . In still other embodiments, this function may be performed at a fixed time interval or after completion of printing. In  FIG. 5B , the carriage  14  is actuated along the X-axis and positioned over the service station  16 . In this illustrative embodiment, there is a spatial gap between the printhead  20  and the printhead cap  26 . At this point, the printhead cap  26  has not yet capped the printhead  20 . Generally, the printhead cap  26  remains stationary until the printhead cap actuator  50  engages the printhead cap carrier  52 . In some embodiments, the carriage  14  has already moved beyond the aperture plate  40  and the discharge openings  28  and, thus, in some embodiments, the printhead  20  may have already expelled debris  41  into the waste liquid catch tray  43 . In some embodiments, the carriage  14  may have already actuated over the printhead cleaning station  29 . In some embodiments, as the carriage  14  continues actuation along the X-axis, the printhead cap actuator  50  engages the printhead cap carrier  52 . Generally, the printhead cap actuator  50  may include metal, plastic, or rubber appendages of sufficient rigidity to move the printhead cap carrier  52  along the X-axis along with the carriage  14 . 
         [0083]      FIGS. 5C-5D  illustrate the completion of the capping function. Typically, the printhead cap carrier  52  is a metal or other solid material fixed to the service station  16  and including a spring coefficient, such that movement of the carriage  14  and the printhead cap actuator  50  along the X-axis causes the printhead cap carrier  52  to move along the X-axis in this same direction. In some embodiments, this X-axis movement of the printhead cap carrier  52  then causes the printhead caps  26  to move along the Z-axis where they eventually cap the printheads  20 . In other embodiments, the carriage  14 , including the printhead cap actuators  50 , and the printhead cap carrier  52  cease movement in the direction of carriage motion  53 , and the printheads  20  are capped. 
         [0084]    Generally, the printhead cap actuator  50  engages the printhead cap carrier  52 , causing the printhead cap carrier  52  to move in the direction of the printhead cap actuator  50  motion. In some embodiments, the printhead cap carrier  52  includes a spring element  601 , whereby the printhead cap carrier will pivot relative to the outer wall of the service station  16  when the spring  601  element is compressed. This pivot results in an uneven actuation of the printhead cap  26  towards the printhead  20 . As a result, the edge of the printhead cap  26  farthest from the printhead cap actuator  50  will initiate contact with the printhead  20 . In other embodiments, it is the edge of the printhead cap  26  located closest to the printhead cap actuator  50  that initially contacts the printhead  20  first. In either of the above illustrative embodiments, the printhead cap  26  continues actuation towards the printhead  20  until the printhead cap  26  levels off and circumscribes the printheads  20 . In some embodiments, the printhead cap  26  forms a seal around the printheads  20 . In one embodiment, one printhead  20  is capped by one printhead cap  26 . In one embodiment multiple printhead caps  26  cap multiple printheads  20 . Generally, there is one printhead cap  26  used each printhead  20 . Generally, the printheads  20  may be capped by the printhead caps  26  any number of times and in any order relative to engagement of the carriage  14  with any other component of the printer  10 . 
         [0085]    As shown in  FIGS. 5C and 5D , the printhead cap carrier  52  includes an arm  600 , a spring element  601 , and a plate  602 . Generally, the arm  600  is engaged by the printhead cap actuator  50  and is moved in the direction of the printhead cap actuator  53  motion. This movement causes the spring element  601  to compress, resulting in a pivoting motion. This pivoting motion causes the plate  602  to move towards the printhead  20 . The printhead cap  26  is typically disposed on a top surface of the plate  602 . In one embodiment, the plate  602  is rigid and, thus, the printhead cap  26  approaches the printhead  20  on a skew, such that one edge of the printhead cap  26  engages the printhead  20  before any of the other edges of the printhead cap  26  engage the printhead  20 . In various embodiments, any edge of the printhead cap  26  may first engage the printhead  20 . Typically, after the first engagement between any edge of the printhead cap  26  and the printhead  20  the plate  602  continues its motion until the printhead cap  26  circumscribes the printhead  20 . Specifically, the plate  602  may bend or flex in response to the actuation force of the carriage  14  until the plate  602  adopts a substantially horizontal orientation. 
         [0086]      FIG. 5C  includes a cutaway cross-sectional view of the service station  16  and the carriage  14 . In this illustrative embodiment, the carriage  14  is actuated along the X-axis in the indicated direction of carriage motion (arrow  53 ). The printhead cap actuator  50  will come into contact with the printhead cap carrier  52  and both the printhead cap actuator  50  and the printhead cap carrier  52  will move in the direction of carriage motion  53 . In this illustrative embodiment, the printhead cap  26  is located upon the printhead cap carrier  52 . Thus, movement of the printhead cap carrier  52  in the direction of carriage motion  53  causes the printhead cap  26  to move along the Z-axis.  FIG. 5C  includes a cut-away graphical representation of the carriage  14  and the service station  16 .  FIG. 5C  illustrates the point of contact between the printhead cap actuator  50  and the printhead cap carrier  52  as the carriage  14  moves in the direction of carriage motion  53 . In this embodiment, at this point, there is a spatial gap between the printhead  20  and the printhead cap  26  and therefore the printhead cap  26  has not sealed the printhead  20 . 
         [0087]      FIG. 5D  is a graphical representation of the carriage  14  and the service station  16  at a point forward in time from that of  FIG. 5C , such that the printhead cap  26  has capped the printhead face  54  of the printhead  20 . Typically, the printhead face  54  includes the bottom face of the printhead  20  including and surrounding the point where the binder material is expelled from the printhead  20 . In this illustrative embodiment, the carriage motion  53  has caused the printhead cap actuator  50  to engage and move the printhead cap carrier  52  in the direction of carriage motion  53 . In this embodiment, the printhead face  54  has a protective seal formed around it by the printhead cap  26 . Generally, the cap or seal is sufficient to protect the printhead face  54  from damage or contamination. In some embodiments, the seal formed by the printhead cap may be airtight. 
         [0088]      FIG. 6A  is a partial cross sectional side view of an alternative embodiment of a service station  16  including a combined discharge and capping station. In this illustrative embodiment, the carriage  14  is actuated in the direction of carriage motion  53 , (along the X-axis) and positions itself over the service station  16 . In some embodiments, this actuation of the carriage  14  may be in preparation for discharge from the printhead  20 . In this illustrative embodiment, the waste liquid catch tray  43  includes waste liquid  42 . Generally, this waste liquid  42  was produced by previous discharges from past passes of the printhead  20  over the service station  16 . In some embodiments, the lower edge of the printhead cap  60  may extend into the area defined by the waste liquid catch tray  43 , but generally the lower edge of printhead cap  60  does not contact the bottom surface of the waste liquid catch tray  43  and, thus, waste liquid  42  flows freely and collects in waste liquid catch tray  43  until the waste liquid surface  61  rises to the top of spillway  44 . At this point, the waste liquid  42  then enters the waste liquid overflow tube  63  via overflow slot  62 . Generally, waste liquid overflow tube  63  carries the waste liquid  42  out of the service station  16 . 
         [0089]      FIGS. 6B through 6D  depict the capping and the discharge functions in greater detail. The carriage  14  is moving in the direction of carriage motion  53 , and is being positioned over the service station  16 .  FIG. 6B  illustrates an embodiment where contact has been made between the printhead cap actuator  50  and the printhead cap carrier  52 , but where the printhead cap carrier has not yet moved far enough in the direction of carriage motion  53  to lift the printhead cap  26  to a position where it caps the printhead  20 .  FIG. 6C  illustrates an embodiment of a point further in time from that of  FIG. 6B . As shown in  FIG. 6C , the printhead cap carrier  52  has moved the necessary distance in the direction of the carriage motion  53  to lift the printhead cap  26  to a point where it has formed a seal around the printhead  20 . The capping function is substantially similar to that described with respect to  FIGS. 5A-5D . In some embodiments, the printhead cap  26  includes a discharge column  67  that defines a cavity  64 . The printhead  20  discharges to the waste liquid catch tray  43  through the discharge column. As shown in  FIG. 6C , the printhead  20  expels debris  41  into the waste liquid catch tray  43 , where it mixes with any existing waste liquid  42 . In some embodiments, the collection of the waste liquid  42  will cross the spillway  44  and proceed to travel through the overflow slot  62  and down waste liquid overflow tube  63  as overflowing waste liquid  65 , where it is eventually expelled from service station  16 . Generally, this discharge procedure ensures a clean and clog free printhead  20  and printhead face  54  to maintain the highest possible quality three dimensional printing. In some embodiments, multiple printheads  20  may discharge material at substantially the same time. 
         [0090]    Referring again to  FIG. 6C , in some embodiments, a seal may be formed in the area defined by the discharge cavity  64 . Generally, the cavity  64  is bounded on the top by the printhead  20  and the printhead cap  26 , on the bottom by the waste liquid surface  61 , and on the sides by the discharge column  67 . In one embodiment, the level of the surface of the waste liquid  61  in the waste liquid catch tray  43  is sufficiently high to submerge a bottom portion of the discharge column  67 . The bottom portion of the discharge column  67  has a lowest point below the lowest point of the spillway  44 , which prevents the waste liquid  42  from dropping below the lowest portion of the discharge column. In such a case, and where the printhead cap  26  is sealed against the printhead face  54  of the printhead  20 , the cavity  64  is airtight, thereby preventing the printhead face  54  from drying out. In this embodiment, the discharge  41  is prevented from escaping the cavity  64  in any direction other than through the waste liquid overflow tube  63 , where it harmlessly exits the service station  16 . This exemplary embodiment minimizes the risk of contamination by the discharge  41  to any components of the printer  10 . 
         [0091]      FIGS. 7A-7D  depict one embodiment of a printhead cleaning station  500  in accordance with the invention. The printhead cleaning station  500  may also be mounted in the service station  16 . The printhead cleaning station  500  includes a reservoir  542  that holds a washing solution  543  and a pump  545  that delivers the washing solution  543  under pressure to at least one nozzle  540  and preferably an array of nozzles  540 . The nozzles  540  are capable of producing a high velocity stream of washing solution  543 . In operation, the nozzles  540  are directed to the printhead face  577  of the printhead  520 . When directed onto the printhead face  577 , the washing solution  543  loosens and removes contaminants, such as build material and binding material, from the printhead face  577 . The orientation of the nozzles  540  may be angled with respect to the printhead face  577 , such that a fluid flow is induced across a plane of the printhead face  577 . For example, the washing solution can contact the printhead  520  at the side nearest the nozzles  540  and drain from the side of the printhead  520  furthest from the nozzles  540 . This approach improves the efficacy of the stream of washing solution  543  by reducing the accumulation of washing solution on the printhead face  577 , as well as the amount of washing solution  543  and debris that would otherwise drain near and interfere with the nozzles  540 . A splash guard may also be included in the printhead cleaning station  500  to contain splashing resulting from the streams of liquid washing solution  543 . 
         [0092]    It is desirable to remove a large portion of the washing solution  543  that remains on the printhead face  577  after the operation of the nozzles  540  is complete. This is conventionally accomplished by drawing a wiping element across the printhead face  577 . A disadvantage of this approach is that contact between the wiping element and the printhead face  577  may degrade the performance of the printhead  520  by, for example, damaging the edges of the inkjet nozzle orifices. Accordingly, it is an object of this invention to provide a means of removing accumulated washing solution from the printhead face  577 , without contacting the delicate region around the inkjet nozzles. In one embodiment, a wicking member  544  may be disposed such that the printhead face  577  may pass one or more times over its upper surface  546  in close proximity, without contact, allowing capillary forces to draw accumulated washing solution  543  away from the printhead face  577 . The wicking member  544  may be made from rigid, semi-rigid, or compliant materials, and can be of an absorbent or impermeable nature, or any combination thereof. 
         [0093]    For the wicking member  544  to effectively remove accumulated washing solution  543  from the printhead face  577 , the gap between the upper surface  546  of the wicking member  544  and the printhead face  577  must be small, a desirable range being between about 0 inches to about 0.03 inches. A further object of this invention is to provide a means for maintaining the gap in this range without resort to precise, rigid, and costly components. 
         [0094]    In another embodiment, the wicking member  544  may consist of a compliant rubber sheet oriented approximately orthogonal to the direction of relative motion  547  between the wicking member  544  and the printhead  520  and with a portion of its upper surface  546  disposed so that it lightly contacts or interferes with the printhead face  577  only in non-critical areas away from the printhead nozzle orifices. The upper surface  546  of the wicking member  544  may include one or more notches  548  at locations where the wicking member  544  might otherwise contact delicate components of the printhead face  577 . System dimensions are selected so that the wicking member  544  always contacts the printhead face  577 , and is deflected as the printhead  520  passes over it, independent of expected variations in the relative positions of the printhead  520  and the printhead cleaning station  500 . The upper surface  546  accordingly follows the position of the printhead face  577 , maintaining by extension a substantially constant space between the printhead face  577  and the relieved surface notch  548 . To further prolong the life of the printhead  520 , a bending zone of the wicking member  544  can be of reduced cross-section to provide reliable bending behavior with little deformation of the upper surface  546  of the wicking member  544 . 
         [0095]      FIGS. 7B-7D  illustrate a reconditioning cycle in accordance with the invention.  FIG. 7B  shows the printhead  520  approaching the printhead cleaning station  500  along a path designated by arrow  547 . When the printheads  520  lightly contact the wicking member  544 , as shown in  FIG. 7C , motion stops along the path  547  and the washing solution  543  is directed at the printhead face  577  by the nozzle array  540 . When the spraying operation is complete, the printhead  520  continues to travel along the path  547 , as shown in  FIG. 7D . The wicking member  544  is further deflected to allow passage of the printhead  520 , and the accumulated washing solution  543  is wicked away from the printhead face  577 . After being sprayed and wiped, in some embodiments the printhead  520  may print a plurality of droplets to eject any washing solution that may have been ingested during the reconditioning process. 
         [0096]    Additional cleaning methods are contemplated, such as wiping the printhead face  577  with a cylindrical “paint roller” that cleans and moistens itself by rolling in a reservoir of wash fluid. In another embodiment, a cleaning system could include a continuous filament that carries wash fluid up to printhead face  577  and carries debris away to a sump. The system may include a small scraper that can be run over the filament to remove built up debris. 
         [0097]      FIG. 8A  depicts an alternative embodiment of cleaning a station  529  in accordance with the invention. Generally, the printer  10  is capable of determining when to clean the printheads  20  via the service station  16 , as will be described in greater detail hereinbelow. In some embodiments, only a single printhead  20  is cleaned by the service station  16 . In other embodiments, multiple printheads  20  are cleaned. In some embodiments, the service station  16  includes a nozzle manifold  80 . Generally, the nozzle manifold  80  includes at least one nozzle  540  and preferably and array of nozzles  540 . In some embodiments, the service station  16  includes a splash guard  81 . Generally, the splash guard  81  is included in the printhead cleaning station  529  to contain splashing resulting from the streams of the washing solution  543 . Typically, the splash guard  81  prevents contamination of powder or binding material by containing the washing solution  543 . Generally, the cleaning station  529  operates the same as the cleaning station  500  described with respect to  FIGS. 7A-7D , except for the addition of the manifold  80  and the splash guard  81 . 
         [0098]      FIG. 8B  is a graphical representation of the splash guard  81  that is located in the printhead cleaning station  529 . The splash guard  81  generally includes a notch  82 , a drain aperture  83 , an actuation face  89 , a flexure point  85 , and a sealing lip  86 .  FIGS. 8C-8H  depict the operation of the cleaning station  529 . Typically, the printhead  20  is actuated such that the printhead face  54  passes immediately over the notch  82  without contacting the surface of the notch  82 . Typically, avoiding contact between the printhead face  54  and notch  82  prevents damaging or altering the trajectory of jet nozzles on the printhead face  54 . In one embodiment, the sealing lip  86  may act as a wiper, contacting the printhead  20  adjacent to the printhead face  54  without contacting the printhead face  54  itself. Once the printheads  20  have cleared the notch  82 , they enter the space immediately above the drain aperture  83 . Generally, the drain aperture  83  is for passing the washing solution  543 . Once the printhead  20  is positioned roughly over the drain aperture  83 , the printhead  20  engages the actuation face  89 . Typically, the printhead  20  engages the actuation face  89  in such a way as to cause the splash guard  81  to flex along the flexure point  85 . In some embodiments, the flexure point  85  includes a pivot point allowing at least the portion of the splash guard  81  including the notch  82 , the drain aperture  83 , the actuation face  89 , and the sealing lip  86  to pivot in the direction of actuation of the printhead  20 . Generally, this pivot at the flexure point  85  raises the drain aperture  83  to the printhead  20  such that the sealing lip  86  contacts the printhead  20 . Generally, the sealing lip  86  is actuated into a position where it forms a seal around the printhead face  54 . Typically, the seal formed by the sealing lip  86  is watertight, thus preventing the washing solution  543  from contaminating the printer  10 . Generally, the only available outlet for used washing solution  543  is through the drain aperture  83 . 
         [0099]      FIG. 8C  includes another perspective of the printhead  20  as it approaches the service station  16 .  FIG. 8C  generally represents the starting position of the cleaning operation performed by the service station  16 . In this illustrative embodiment, the printhead  20  is actuated in the direction of the printhead motion  87  such that the printhead face  54  is brought above the service station  16 . As the printhead  20  is being actuated, the printhead side  88  will engage the actuation face  89  of the splashguard  81 . After this engagement, the printhead  20  moves the actuation face  89  such that the sealing lip  86  forms a seal around the printhead face  54  (see  FIG. 8D ). In some embodiments, the actuation face  89  pivots at the flexure point  85 . In some embodiments, the flexure joint  85  may include a spring element. Generally, this procedure results in the forming of a watertight seal by the splash guard  81  around the underside of the printhead  20  adjacent to the printhead face  54 . 
         [0100]      FIG. 8D  depicts the printhead  20  moved into its desired position over the service station  16 . Generally, this is the point at which the service station  16  will clean the printhead  20 . As illustrated in  FIG. 8D , the actuation face  89  seals the printhead  20  around part of the printhead face  54 . The seal is completed around the printhead face  54  by the splash guard lip  86 . Generally, the splash guard lip  86  is part of the splash guard  81 . In one embodiment, as the printhead  20  is actuating the splash guard  81  via its contact with the actuation face  89 , the resulting movement of the splash guard  81  also moves the sealing lips  86  into a position against the bottom of the printhead  20  and along the printhead face  54 . In some embodiments, the sealing lips  86  come to rest against the underside of the printhead  20  against the printhead face  54 . Generally, forming a seal around the printhead  54  on the underside of the printhead  20 , as opposed to along the printhead side  88 , is desired as it prevents contamination of the printhead side  88 , or any other side of the printhead  20 . For example, washing solution left on the printhead  20  can drip off during printing, thereby effecting print quality. 
         [0101]      FIG. 8E  is a partially sectioned side view of the service station  16  during cleaning of the printhead  20  by the service station  16  in accordance with one embodiment of the invention. Subsequent to the forming of a seal around the printhead face  54 , the nozzle manifold  80  sprays the washing solution streams  91 . Generally, the nozzle manifold  80  includes the pressurized washing solution  92 . In one embodiment, the pressurized washing solution  92  is sprayed onto the printhead face  54  in a single stream  91 . In other embodiments, there are multiple streams  91  of the pressurized washing solution  92 . In operation, the washing solution streams  91  are directed at the printhead face  54  of the printhead  20 . When directed onto the printhead face  54 , the washing solution streams  91  loosen and remove contaminants, such as binder material, from the printhead face  54 . The orientation of the washing solution streams  91  may be angled with respect to the printhead face  54 , such that a fluid flow is induced across a plane of the printhead face  54 . For example, in one embodiment, the washing solution stream  91  may contact the printhead  20  at the side nearest the nozzle manifold  80  and drain from the side of the printhead  20  furthest from the nozzle manifold  80 . This approach improves the effectiveness of the washing solution streams  91  by reducing the accumulation of the washing solution  92  on the printhead face  54 , as well as the amount of the pressurized washing solution  92  and debris that would otherwise drain near and interfere with the nozzle manifold  80 .  FIG. 8F  is another partially sectioned view of the invention illustrated in  FIG. 8E . The printhead face  54  is in proper position for cleaning. The sealing lips  86  have formed a seal around the printhead face  54  thus protecting the remainder of the printhead  20  from contamination. 
         [0102]      FIG. 8G  illustrates the movement of the printhead  20  out of the service station  16  after a cleaning operation has been performed. The printhead  20  now moves in the direction of printhead motion  93  away from the service station  16 . This is generally the same as the direction of carriage motion  53  that was used to enter the service station  16 . As the printhead  20  is actuating out of the service station  16 , the printhead face  54  is carried over the sealing lip  86  and the notch  82 . In some embodiments, the sealing lip  86  may act as a wiper and remove debris and washing solution  92  from the area on the bottom of the printhead  20  adjacent to the printhead face  54 ; however, the notch  82  prevents contact between the sealing lip  86  and printhead face  54  in an area corresponding to the location of the jet nozzles. Contact between the sealing lip  86  and the printhead face  54  may degrade the performance of the printhead  20  by, for example, damaging the edges of the inkjet nozzle orifices on the printhead face  54 . However, it is still desirable to remove a large portion of the washing solution  92  that remains on the printhead face  54  after the operation of the nozzle manifold  80  is complete. Accordingly, it is an object of this invention to provide a means of removing accumulated washing solution from the printhead face  54 , without contacting the delicate region around the jet nozzles on the printhead face  54 . Because the notch  82  prevents direct contact between the sealing lip  86  and the printhead face  54 , in one embodiment, a wicking member  544  (as described above) may be disposed such that the printhead face  54  may pass one or more times over the wicking member  544  in close proximity, without contact, allowing capillary forces to draw the accumulated pressurized washing solution  92  away from the printhead face  54 .  FIG. 8H  illustrates a partially sectioned bottom perspective view of the service station  16  of  FIG. 8A . Here it can be seen that the sensitive portion of the printhead face  54  passes over the notch  82  as the printhead  20  is actuated away from the service station  16  after a cleaning. Generally, the sensitive portion of the printhead face  54  includes the printhead jet nozzle array. 
         [0103]      FIGS. 9A and 9B  illustrate an alternative embodiment of the splash guard  81  of  FIG. 8B . In this embodiment, the splash guard  81  includes tapered sealing surfaces  94 . Generally, the tapered sealing surfaces  94  are shaped so that they will form a seal around the corners formed by the printhead edges  95 . Thus, the seal in this embodiment is formed by the tapered sealing surfaces  94  contacting both the printhead face  54 , and the printhead side  88  of the printhead  20 . Thus, the seal formed by this embodiment wraps around the edges of the printhead  20  to contain the washing solution  92  during the cleaning operation. The operation of the alternative splash guard  81  of  FIGS. 9A and 9B  and the associated cleaning components is substantially similar that described hereinabove. 
         [0104]      FIGS. 10A-10D  illustrate another alternative embodiment of the splash guard  81  of  FIG. 8B . In this embodiment, the splash guard  81  again forms a seal with the splash guard sealing lips  86 ; however, in this embodiment, the splash guard  81  is actuated into its sealed position around the printhead face  54  by a splashguard support spring  102 . This procedure is analogous to the procedure used to cap the printhead  20  in the capping operation. Generally, the printhead  20  is carried over the service station  16  in the direction of a first printhead motion (arrow  100 ). Once roughly positioned over the drain aperture  83 , the direction of the printhead motion changes direction to a substantially perpendicular printhead motion (arrow  101 ). In some embodiments, the direction of the printhead motion  101  is orthogonal to the previous direction of printhead motion  100 . The printhead  20  now proceeds in the second direction of the printhead motion  101  until the printhead side  88  engages the splash guard support spring  102 . (See FIG.  10 B) As  FIG. 10C  illustrates, the splash guard support spring  102  moves in the direction of the second printhead motion  101 . This movement engages the splash guard  81  with the printhead face  54 . 
         [0105]    Once the cleaning operation is performed as described above, the printhead  20  moves in a third direction of printhead motion (arrow  103 ) away from the service station  16 . Generally, the third direction of printhead motion  103  is opposite the first direction of printhead motion  100 , as the printhead  20  disengages from the service station  16 . This disengagement breaks the seal formed by the splash guard sealing lip  86 , and the printhead face  54  is carried over the sealing lip  86  where a wiper operation may be performed to remove debris or the washing solution  92  from the printhead face  54 . As described above, a wicking operation may also be performed. 
         [0106]      FIGS. 11A-11J  illustrate an alternative system  146  for cleaning the printhead  20 . The system  146  is located in the service station  16  ( FIG. 1 ). In one embodiment, the system  146  includes a cleaning station  148  made up generally of a latch pawl  152 , a spring  154 , a wiper  156 , a printhead cap  158 , a cap carrier  192 , a second spring  162 , and a cam track  164 . Only a single cleaning station  148  is shown for descriptive purposes; however, multiple stations  148  may be disposed in the service station  16 . Alternatively, a single cleaning station  148  may service multiple printheads  20  by, for example, successively positioning the printheads  20  relative to the cleaning station  148 . 
         [0107]      FIG. 11A  represents a starting position of the cleaning system  146 . As shown in  FIG. 11B , the printhead  20  approaches the cleaning station  148  and engages the latch pawl  152 . The latch pawl  152  is actuated as the printhead  20  passes over the latch pawl  152 . The printhead  20  continues to move past the latch pawl  152  and engages the wiper  156  ( FIG. 11C ). The printhead  20  passes over a wiper  156 . As shown in  FIG. 11D , the printhead  20  contacts the cap carrier  192 , which is driven along the cam track  164  and compresses the spring  162 . The printhead cap  26  is positioned against a printhead face  54  ( FIGS. 11E and 11F ). As shown in  FIG. 11F , the printhead cap  26  seals against the printhead face  54  while the face  54  is rinsed with washing solution  92  (see  FIG. 11F ). 
         [0108]    After the printhead face  54  is cleaned, the printhead  20  begins to move out of the service station  16  ( FIG. 11G ). The latch pawl  152  engages the cap carrier  192 , halting its movement. As shown in  FIG. 11H , the printhead  20  engages the wiper  156 , which wipes the printhead face  54 . In an alternative embodiment, the wiper  156  vibrates to further clean the printhead face  54 . In an alternative embodiment, the wiper  156  may be notched in an area corresponding to the location of the jet nozzles, thereby preventing contact between the wiper  156  and the printhead face  54 . The printhead  20  continues its forward movement, actuating the latch pawl  152  ( FIG. 11I ), which, in turn, releases the cap carrier  192  ( FIG. 11J ). The cap carrier  192  snaps back to the start position. After the printhead face  54  is cleaned, the printhead  20  begins to move out of service station  16  ( FIG. 11G ). The system  146  is now ready to clean another printhead  20 . 
         [0109]      FIG. 12  depicts the system  146  for cleaning a printhead  20 . ( FIG. 12  also depicts  FIG. 11F  in greater detail) The printhead  20  is positioned with the printhead face  54  against the printhead cap  26 , which in this embodiment is made of rubber. The cap includes a seal lip  172  for sealing about the printhead face  54 . The service station  16  is coupled to a wash fluid supply container  182  via a supply duct  184  and a wash fluid return container  186  via a return duct  188 . The wash fluid return container  186  is in communication with a vacuum source  180 , in this case a vacuum pump, via a vacuum duct  190 . Additionally, a valve  178  is located in the return duct  188 . The valve  178  may be manually or automatically actuated. 
         [0110]    In operation, the vacuum source  180  creates a vacuum within a cavity  174  in the printhead cap  54 . The vacuum pulls wash fluid from the supply container  182  through the supply duct  184 . The wash fluid enters the cavity  174  as a spray  176  against the printhead face  54 . The spray  176  washes debris, such as excess build material and dried binder, off the printhead face  54 . The used wash fluid and debris are drawn out of the cavity  174  by the vacuum source  180  and into the return container  186  via the return duct  188 . Additionally, the negative pressure created in the cavity  174  by the vacuum source  180  prevents the wash fluid from entering the jet nozzles and, in fact, may cause a small amount of binder to flow out of the nozzles to flush any powdered build material out of the nozzles. Blockages or obstructions in the jet nozzles can cause the jets to fire in the wrong direction. Once the operation is complete, the system  146  moves onto the step depicted in  FIG. 11G . In an alternative embodiment, printhead(s)  20  are disposed above the service station  16 . The sealing lip  86  is actuated into alignment with the printheads  20 , and the printheads  20  are wiped and lubricated from beneath to remove any accumulated grit and to improve the flow of binding material out of the printheads  20 . Specifically, a lubricator applies a lubricant to the printhead face  20  to moisten any debris on the printhead face  54 . Then, the printhead  20  is moved to pass the printhead face  54  over sealing lips  86 , which act as a wiper and wipes the printhead face  54  clean. 
         [0111]      FIG. 13  depicts a typical printing operation with a 3D printer in accordance with the invention. Only one printhead  220  is shown for clarity. The printhead  220  moves over a powder bed  200  that has been spread over a build surface of the 3D printer (se, for example,  FIG. 1 ). As previously described, the printhead  220  can move along an X-axis and a Y-axis. In the operation depicted, the printhead  220  is moving in a single direction (arrow  202 ). As the printhead  220  travels above the powder bed  200 , the printhead  220  performs A printing operation by depositing droplets  212  of liquid binder on to the powder bed  200  in a predetermined manner, thereby resulting in printed sections  204  and unprinted sections  206  in the powder bed  200 . 
         [0112]    After printing on the powder bed  200 , a new layer of powder is spread over the powder bed  200  in preparation for receiving the new printing  218 . As the printhead  220  deposits the droplets  212  onto the powder bed  200 , particles  210  of the powder are ejected by the impact of the droplets  212  on the powder bed  200  (see  FIGS. 14A and 14B ). These particles  210  may eventually contact and adhere to the printhead  220 . The resulting debris  216  degrades the quality of printing by, for example, interfering with a printhead nozzle  208 . The amount of particles  210  ejected will depend, in part, on whether the powder is “wet” or “dry.” The powder is wet if the underlying layer was previously printed (see  FIG. 14B ). The powder is dry if the underlying layer was previously unprinted (see  FIG. 14A ). 
         [0113]    As shown in  FIG. 14A , the printhead  220  is depositing droplets  212  on to a dry powder bed  200 . As the droplets  212  impact the powder bed  200 , a relatively large volume of particles  210  are displaced and a crater  214  is created in the powder bed  200 . The particles  210  are ejected upwardly towards the printhead  220 , where they may collect as debris  216  on a face of the printhead  220 . 
         [0114]    As shown in  FIG. 14B , the printhead  220  is depositing droplets  212  on to a wet powder bed  200 . As the droplets  212  impact the powder bed  200 , a relatively small volume of particles  210  are displaced and a relatively small crater  214  is created in the powder bed  200 . The binder printed on the previous layer tends to bind the powder in the fresh layer, thereby resulting in fewer particles being ejected, and correspondingly less debris accumulating on the printhead face. 
         [0115]    The 3D printer includes logic for monitoring the condition of the printhead  220  based on, at least in part, the number of droplets printed over previously printed and/or unprinted powder, since the last cleaning. Other factors include; for example, time in use, number of droplets dispensed, and number of layers printed. The 3D printer can determine the frequency and duration of any necessary cleaning routine, based on any one of the aforementioned factors or combination of factors reaching a set threshold value. For example, the printhead  220  may be cleaned after every five minutes of continuous use. The threshold values of any particular factor can be varied depending on the types of liquid binder and powder materials used and other operational environmental factors, such as temperature and humidity, that can affect printhead condition. 
         [0116]    Additionally or alternatively, the 3D printer can utilize other systems and methods for monitoring and maintaining the cleanliness of the printhead  220 . For example, in one embodiment, the 3D printer could include an imaging system for viewing the printhead face. A user could either manually determine that the printhead  220  requires cleaning or the 3D printer could include the imaging system for automatically determining the need for cleaning. In a manual system, an image of the printhead face is displayed to the user, for example on a video monitor, and the user can initiate a cleaning routine if deemed necessary. In one example of an automatic system, the actual image of the face of the printhead in service is sent to a processor for comparison to an image of a clean printhead face, (i.e., a test image). In one embodiment, the printhead face is dark and the powder is relatively light in color. If a significant portion of the printhead face is covered with debris, there will be a difference in contrast between the actual image and the test image. If the difference in contrast reaches a predetermined threshold, the system initiates the cleaning routine. 
         [0117]    In some embodiments, the cleanliness of the printhead face can be maintained by the use of an air curtain or an electro-static charge. The system can supply a low pressure curtain of air across the printhead face that would reduce or prevent debris from collecting on the printhead face. Alternatively, the printhead face could have an electro-static charged placed thereon that is the same charge that is applied to the powder, thereby resulting in the powder particles being repelled from the printhead face. 
         [0118]      FIG. 15  is a schematic representation of a printhead alignment process in accordance with one embodiment of the invention. Specifically, the printhead carriage  14  described hereinabove is depicted in relation to an alignment test pattern  129 . The test pattern  129  is printed on the build surface  165  of the three-dimensional printing system  10  (see  FIG. 1 ). The test pattern  129  includes a contrast-enhancing sublayer  130  that defines an area upon which an X-axis alignment pattern  133  and a Y-axis alignment pattern  134  are printed. The X and Y-axis alignment patterns  133 ,  134  are line pair arrays made up of alternating reference lines  135  and test lines  136 . Also included on the sublayer  130  is a contrast optimization pattern  131 , which is described in greater detail with respect to  FIGS. 16A and 16B . The carriage  14  includes an alignment sensor system  132  that is used to scan the test pattern  129 . The system  132  is described in greater detail with respect to  FIGS. 17A-17D . 
         [0119]    The pattern  129  is created by first spreading a layer of build material on the build surface  165 . The printheads  20  are then used to print the contrast-enhancing sublayer  130  on the layer of build material powder. Generally, the contrast-enhancing sublayer  130  provides a background reference to create a contrast between a printed layer and its surroundings. Generally, it is desirable to perform the alignment process (e.g., creating the test pattern  129 ) using the same binder solutions that will later be used to print the three-dimensional parts. Clear binder can present a particular problem, in that an image printed on powder with clear binder is difficult to distinguish from its unprinted surroundings. This problem can be solved by printing the contrast-enhancing sublayer  130 , though it is not required. 
         [0120]    The contrast-enhancing sublayer  130  is printed on the build surface  165  of dimensions sufficient to underlie the whole array of alignment pattern objects (e.g., the X-axis alignment pattern  133 , the Y-axis alignment pattern  134 , and the contrast optimization pattern  131 ). In some embodiments, a dark color such as magenta or cyan may be used. The area may be printed more than once to increase the darkness of the color. A layer of fresh powder is then spread over this sublayer  130 , obscuring the dark color. When an image is then printed on the fresh layer with clear binder, the powder is wetted in the printed areas and becomes somewhat transparent, revealing the dark color of the sublayer  130 . In some embodiments, the contrast-enhancing sublayer  130  and the powder spread over it may collectively be referred to as the contrast-enhancing sublayer  130 . The printed area then contrasts more clearly with its surroundings to be detected more readily by the alignment sensor system. 
         [0121]    Next, the contrast optimization pattern  131  is printed on the contrast-enhancing sublayer  130 . In some embodiments, the contrast optimization pattern  131  includes a printed area or target  143 - 146  (see  FIG. 16A ) from each of the printheads  20 . The alignment sensor system  132  then determines the area of highest contrast between the printed targets  143 - 146  that collectively form the contrast optimization pattern  131  with contrast-enhancing sublayer  130  to determine which target  143 - 146  of the contrast optimization pattern  131  (and its corresponding printhead  20 ) has the greatest contrast relative to an unprinted area  141  (see  FIG. 16A ) of the contrast-enhancing sublayer  130 . 
         [0122]    The general procedure is to adopt one of four colors as a reference standard and to characterize the positional errors of the other colors with respect to the reference color. In one embodiment, the four colors include clear (printed area  143 ), yellow (printed area  144 ), magenta (printed area  145 ), and cyan (printed area  146 ). It may be desirable to adopt as a reference the color that contrasts most with the unprinted background. To this end, a target is printed in each color and then examined with the alignment sensor system  132 . The color that produces the least photo sensor output may be selected. 
         [0123]      FIGS. 16A and 16B  further detail the contrast optimization pattern  131 .  FIG. 16A  is a graphical representation of the contrast optimization pattern  131  including the aforementioned targets  142 - 146 .  FIG. 16B  shows the relationship between light source current and photo sensor output (e.g., alignment sensor current). As the light impinging on a photo sensor increases, it will eventually reach a level where the sensor output approaches a maximum and becomes insensitive to further increases in light input. This state of insensitivity is commonly called saturation, and is indicated by the saturated region  147  in  FIG. 16B . The proportional region of the sensor output is indicated by the proportional region  148  in  FIG. 16B . To maximize the information content of the sensor output signal, it is desirable to avoid saturating the sensor under normal operating conditions. The powders used in 3D printing may vary widely in reflectivity, resulting in large variations in maximum sensor illumination. To compensate for this effect, the alignment sensor assembly is positioned over an unprinted area  142  above the build surface and senses unprinted area  142  (see  FIG. 16A ). The input current through the light source is gradually increased until diminishing sensor output indicates saturation. The light source current is then reduced to provide a safe operating margin within the proportional region  148 . Alternatively, the light source current can be gradually increased until a predetermined safe photo sensor output is reached. 
         [0124]    Referring back to  FIG. 15 , two substantially identical arrays of line pairs disposed substantially at right angles to each other make up the X-axis alignment test pattern  133  and the Y-axis alignment test pattern  134 . In one embodiment, one of the test patterns represents a slow axis printing and the other test pattern represents a fast axis printing of the three-dimensional printer  10 . Generally, the X-axis alignment test pattern  133  and Y-axis alignment test pattern  134  are processed in sequence, and the processes are identical. Generally, both the X-axis alignment pattern  133  and the Y-axis alignment pattern  134  include the reference line  135  and the test line  136 . In one embodiment, the reference line  135  is created by the printhead  20  that was determined to have the greatest contrast relative to the contrast-enhancing sublayer  130 . The line pairs are discussed in greater detail hereinbelow with respect to  FIGS. 18 ,  19 A,  19 B, and  21 A. 
         [0125]    In some embodiments, to determine the highest contrast between the contrast optimization pattern  131  and the contrast-enhancing sublayer  130 , the carriage  14  may include a light source  137 , for example a light emitting diode (LED), which produces a cone of light  138 . Alternatively, the light sources could be a laser or a lamp, and multiple light sources could be utilized. The LED light source  137  illuminates the general area under examination. In some embodiments, the LED light source  137  is a blue-green color to produce a high level of contrast between printed and unprinted areas. An optical filter passes light only in a narrow wavelength window that includes the LED output. Ambient room light contains relatively little light of the wavelength passed by the filter, so that the great majority of the light that reaches the photo sensor originates from the light source. As a result, the system is relatively insensitive to ambient room light variations. 
         [0126]    In another embodiment, ambient light insensitivity is achieved by modulating the light source  137  output at a frequency much higher than the signal of interest. The photo sensor output is filtered electronically to pass only the frequency of the modulated light. This increases the sensitivity of the system to low light levels. An optional lens can increase the sensitivity of the system to low light levels. 
         [0127]      FIGS. 17A-17D  depict the alignment sensor system  132  in greater detail. The system  132  is typically part of the printhead carriage  14 . In a particular embodiment, the system  132  is mounted on a printed circuit board  160  that includes, for example, the logic for directing the carriage  14 , firing the printheads  20 , and operating the alignment sensor system  132 . The system  132  generally includes the light source  137 , an optical filter  161 , a light entrance  162 , a photo sensor  163 , and an optional lens  164 . The light source  137  is used to illuminate a spot on the test pattern  129  that is about the same diameter as the width of the colored lines being scanned. The light source  137  and the photo sensor  163  could each be focused or unfocused.  FIGS. 17C-17D  depict different operational states of the alignment sensor system  132 .  FIG. 17C  illustrates the illumination of an illuminated area  166  on the build surface by the light cone  138 . In one embodiment, the light source floods the area of interest with light. In  FIG. 17D , a sensed area  142  on the illuminated build surface  165  reflects light back to the photo sensor  163 . Typically, the sensed area  142  corresponds to a print target  142 - 146  or a portion of the reference line  135  or test line  136  and is smaller in area than the illuminated area  166 . The tubular light entrance channel  162  restricts the field of view of the photo sensor to a spot small relative to the illuminated area. In some embodiments, the photo sensor  163  may include the capability of detecting a surface photovoltage from the illuminated area  166  of the printing surface. In other embodiments, the system  132  may include an optional lens  164  to focus the reflected light on the sensor  163 . 
         [0128]      FIG. 18  depicts the X-axis alignment pattern  133  of  FIG. 15 . The X-axis alignment pattern  133  and the Y-axis alignment pattern  134  are substantially identical, with the exception that the line pairs are oriented substantially perpendicularly, although alternative configurations are contemplated and considered within the scope of the invention. As previously described, the X-axis alignment pattern  133  includes a series of reference lines  135  and test lines  136 . Generally, each reference line  135  is printed by the printhead  20  with the highest contrast relative to the contrast-enhancing sublayer  130 , and each test line  136  is printed in an alternating pattern by at least one of the three remaining printheads  20  with lesser relative contrasts. As the number of printheads may vary in different embodiments, the number of corresponding color bars in each test line  136  also may vary. In one exemplary embodiment, the reference line  135  may be made of clear deposited material, and test line  136  may be sequentially repeating yellow, magenta, and cyan color deposits. Typically, the test pattern  129  is printed by the printheads  20  in order to determine if the printheads  20  are properly aligned. The test pattern  129  is printed assuming the printheads  20  are perfectly positioned. Once the test pattern  129  has been printed, the carriage  14  is actuated over the surface of the test pattern  129  and the alignment sensor system  132  scans at least a portion of test pattern  129  to determine the deviation of the test line  136  from the perfect position. The scanned results are then used to correct any identified errors. 
         [0129]      FIGS. 19A-19B  illustrate the scan spot travel paths  171  across a test pattern.  FIG. 19A  illustrates a nominal X-axis alignment pattern  170 . As the sensed area  142  passes over the printed lines in the direction of line pair replication direction  173 , the photo sensor  163  receives reflected light that originated from the light source  137 . The reflectances of the color bars differ from the unprinted background (in one example, the unprinted background is white), and the reflectances of the colors vary amongst themselves. As illustrated by  FIG. 19B , the basic unit of the target is a line pair, such as line pair  174 , which comprises a solid reference line  135  and a test line  136  including an array  181  of systematically varying short bars  191  including a first color bar  176 , a second color bar  177 , and a third color bar  178 . Alternative embodiments may have more or fewer color bars. Collectively, the color bars  176 ,  177 ,  178 , are components of the test line  136 . This line pair  174  is periodically repeated in the direction shown with a constant pitch (“P”)  197  between successive reference lines, for example, reference lines  135 . In the illustrative embodiment of  FIG. 19B , the line pair  174  is repeated 11 times; however, the number of line pairs will vary to suit a particular application and/or desired level of accuracy. 
         [0130]    In one embodiment, the scan spot traverses the array of line pairs  174  along travel paths perpendicular to the reference line  135 . In the embodiment illustrated by  FIG. 19B , complete examination of the target requires 33 scan spot passes. Three typical scan path travel paths  171  are indicated (see  FIG. 19A ). In one embodiment, the width of the color bars  176 ,  177 ,  178 , the minimum anticipated space between the bars, and the size of the scan spot should be substantially equal. The color bars  176 ,  177 ,  178  shown in  FIG. 19B  vary systematically around a spacing equal to about one half of the reference line pitch P  197 . An exemplary short bar is identified as short bar  191 . In one embodiment, the increment of variation, (“δ”), may typically be 2 pixels at 300 dpi or 0.007 inches. The position of the uppermost group of three short bars of the color bars  176 ,  177 ,  178  is nominally printed equidistant between two of the reference lines  135 . Progressing down the array, the groups of three color bars  176 ,  177 ,  178  diverge from the central position by increasing amounts, for example +/−nδ, where “n” is an integer (e.g., 1δ, 2δ, 3δ, etc.). The width and pitch of the reference lines  135  and test lines  136  are selected to optimize the signal contrast. The dimensions given herein are for illustrative purposes only and are in no way to be considered limiting. 
         [0131]      FIGS. 20A-20D  illustrate one embodiment of the alignment process with respect to a single scan spot travel path  171 .  FIGS. 20A and 20B  illustrate the single scan spot pass travel path  171  in the direction of carriage motion  193  across reference lines  135  and test lines  136 . As the scan spot passes over the printed color bars, the photo sensor receives reflected light that originated from the light source  137 . The reflectances of the color bars differ from the unprinted background, and the reflectances of the colors vary amongst themselves.  FIG. 20C  illustrates the sensor output signal, which represents strong periodicity related to the color bar spacing and peak amplitude variations due to different color reflectances. 
         [0132]    As shown in  FIG. 20D , any signal can be represented as the sum of an arbitrarily large number of sinusoids, each having a constant discrete frequency, a constant amplitude, and a constant phase relationship to a fixed standard. The process of extracting the sinusoidal constituents of a signal is called Fourier analysis. A common practical approach is to digitize the signal and to then employ a computational algorithm, such as a Fast Fourier Transform (“FFT”).  FIG. 20D  shows the principle harmonic constituents of the signal shown in  FIG. 20C . The frequency of these constituents is fixed by the geometric constraints placed on the test pattern  129 . The magnitude of the each constituent is affected by differences in color reflectivity and by the displacement (“E”)  183  (see  FIG. 20B ) of the adjustable color bar relative to its central position. The magnitude of the harmonic component whose frequency is three times the reference bar frequency increases with color test bar displacement from perfect alignment, and can be used to determine the magnitude of the displacement.  FIG. 20D  is a graphical representation of the sensor output indicating spatial frequency and a first harmonic peak  185 , a second harmonic peak  186 , a third harmonic peak  187 , and a fifth harmonic peak  188 . The first harmonic peak  185  may also be used as an indicator of misalignment. 
         [0133]      FIGS. 21A and 21B  illustrate an alignment pattern showing misalignment in one embodiment of a test pattern in accordance with the invention. As discussed above, the alignment pattern in  FIG. 19A  was shown as it would be printed by printheads  20  in perfect alignment.  FIG. 21A  shows an alignment pattern printed by misaligned printheads  20 . Each adjustable color bar, including second color bar  192 , is actually printed in a position displaced from its nominal true position. To determine the positional error  183  of each color using this alignment pattern, a total of eleven scans across this pattern are needed, as shown. Each scan will produce a signal of the sort shown in  FIG. 20C . For each of these signals, the magnitude of the third harmonic can be extracted by digital FFT or analog filtering. Although the magnitude of the third harmonic increases reliably with misalignment, the misalignment is only one component of the magnitude of the harmonic. A portion of the peak is constant and depends on the line width/space ratio. A portion of the peak is variable and depends on how well the color bars are centered between the reference lines  135 . 
         [0134]    Determining at which nominal color bar displacement the magnitude of the third harmonic is minimized can factor these other components out. The maximum value of the harmonic of interest, for example the third harmonic, for each scan is collected. By fitting a curve of these data points and determining the minimum point of this fitted curve (see  FIG. 21B ), it is possible to determine the misalignment to within a fraction of the alignment pattern step resolution. If, for example, the printhead under examination were perfectly aligned, the minimum point of the fitted curve would coincide with a nominal color bar displacement  175  of zero. 
         [0135]    The location of the minimum yields an accurate correction factor. In one embodiment, the correction factor is used to alter the timing of a firing signal to a printhead, thereby altering the location of the printhead output. Specifically, this actual measured misalignment can be used as a corrective, geometric offset, causing the printhead  20  to “fire” either early or late, so that the mechanical misalignment can be automatically compensated for during printing. As a result, a very high level of printing accuracy can be achieved, resulting in the production of dimensionally accurate three-dimensional articles, even when employing multiple printheads. In one embodiment, the alignment process is carried-out prior to printing any three-dimensional parts and/or after a printhead is replaced. 
         [0136]    Having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. The described embodiments are to be considered in all respects as only illustrative and not restrictive.