Abstract:
A method of forming a nip with a skewed transfix roll includes positioning a first axis of rotation of a transfix roll at a skewed angle with respect to a second axis of rotation of an image drum, forming a nip with the skewed transfix roll and the image drum, and operating the printer with the nip formed with the skewed transfix roll.

Description:
TECHNICAL FIELD 
     The method disclosed herein relates to printers and more particularly to printers incorporating a transfix roll. 
     BACKGROUND 
     The word “printer” as used herein encompasses any apparatus, such as a digital copier, book marking machine, facsimile machine, multi-function machine, etc., which performs a print outputting function for any purpose. Printers using intermediate transfer, transfix, or transfuse members are well known. In general, such printing systems typically include a printing or imaging member in combination with a printhead which is used to form an image on the imaging member. A final receiving surface or print medium is brought into contact with the imaging surface after the image has been placed thereon by the nozzles of the printhead. The image is then transferred and fixed to the print medium by the imaging member in combination with a transfix pressure member, or in other embodiments, by a separate fuser and pressure member. 
     Some printer systems which incorporate intermediate transfix members also incorporate a phase change ink. In one such printer system, the imaging process begins by applying a thin liquid, such as, for example, silicone oil, to an imaging member surface. The solid or hot melt ink is placed into a heated reservoir where it is melted into a liquid state. The highly engineered hot melt ink is formulated to meet a number of constraints, including low viscosity at jetting temperatures, specific visco-elastic properties at component-to-media transfer temperatures, and high durability at room temperatures. 
     The heated reservoir provides the liquefied ink to an associated printhead. Once within the printhead, the liquid ink flows through manifolds and is ejected from microscopic orifices through use of proprietary piezoelectric transducer (PZT) printhead technology. The duration and amplitude of the electrical pulse applied to the PZT is very accurately controlled so that a repeatable and precise pressure pulse can be applied to the ink resulting in the proper volume, velocity, and trajectory of the droplet. Several rows of jets, for example four rows, can be used, each one with a different color. The individual droplets of ink are jetted onto the liquid layer on the imaging member. The imaging member and liquid layer are held at a specific temperature at which the ink hardens to a ductile visco-elastic state. 
     In conjunction with forming the image on the imaging drum, a print medium is heated by feeding it through a preheater and into a nip formed between the imaging member and a pressure member, either or both of which can also be heated. The nip is maintained at a high pressure by forcing a high durometer synthetic transfix pressure member against the imaging member. As the imaging member rotates, the heated print medium is pulled into and through the nip and is pressed against the deposited ink image by the opposing surfaces of the transfix pressure member and the image member. 
     The high pressure conditions within the nip compresses the print medium and ink together, spreads the ink droplets, and fuses the ink droplets to the print medium. Heat from the preheated print medium heats the ink in the nip, making the ink sufficiently soft and tacky to adhere to the print medium. When the print medium leaves the nip, stripper fingers or other like members peel it from the printer member and direct it into a media exit path. 
     To optimize image resolution, the conditions within the nip must be carefully controlled. The transferred ink drops should spread out to cover a specific area to preserve image resolution. Too little spreading leaves gaps between the ink drops while too much spreading results in intermingling of the ink drops. Additionally, the nip conditions must be controlled to maximize the transfer of ink drops from the image member to the print medium without compromising the spread of the ink drops on the print medium. Moreover, the ink drops should be pressed into the paper with sufficient pressure to prevent their inadvertent removal by abrasion thereby optimizing printed image durability. Thus, the temperature and pressure conditions must be carefully controlled and must be consistent over the entire area of the nip. 
     The necessary pressure and temperature within the nip are a function not only of the particular ink, but also of the rate at which images are transferred from the imaging member to the print medium. In other words, spreading and transfer of ink is a function not only of the pressure and temperature conditions within the nip, but also of the duration that the ink is within the nip. Thus, as the process speed is increased, one or more of the pressure within the nip, the temperature within the nip, and the nip width (the in-process dimension of the nip) must increase to provide desired image quality. 
     The nip width is a function of the diameters of the image member and the transfix member. Thus, increased process speed is enabled by increased image member and transfix member diameter. Increasing the diameter of the image member and the transfix member, however, requires a larger frame. Nip width can also be increased, without increasing the diameter of the image member and the transfix member, by increasing the pressure within the nip thereby flattening the surfaces of the rolls within the nip. Accordingly, the applied load on the transfix pressure member in certain printer systems is increased from 1,100 pounds up to about 4,000 pounds to provide consistent image quality at increased speeds. 
     Accordingly, in order to achieve the uniform high pressures needed for high speed imaging, particular attention must be given to the manner in which the transfix pressure roller is manufactured. By way of example, force is applied to the imaging member and the transfix pressure roller at the outer edges of the rollers. Consequently, application of the high pressures needed for high speed imaging results in deformation of the transfix roll with the end portions of the transfix roll positioned closer to the axis of rotation of the image drum than the center portion of the transfix roll. The deformation of the transfix roll caused by application of force only at the outer ends of the transfix roll results in an undesired pressure profile for a transfix roll with a flat profile in the cross-process direction wherein the pressures at the outer edges of the process path are higher than the pressure in the middle portion of the process path. One approach to correcting this issue is to form a transfix roll with a crowned profile. 
     A “crowned profile” is a profile wherein the diameter of the transfix roll at the middle of the process path is larger than the diameter of the transfix roll at the outer portions of the process path. Transfix rolls with crowned profiles provide a desired image quality, roll life, and acceptable cost. Optimal performance of the crowned transfix pressure component, however, is achieved by adhering to carefully controlled manufacturing tolerances of small magnitude. 
     SUMMARY 
     A method of forming a nip with a skewed transfix roll includes positioning a first axis of rotation of a transfix roll at a skewed angle with respect to a second axis of rotation of an image drum, forming a nip with the skewed transfix roll and the image drum, and operating the printer with the nip formed with the skewed transfix roll. 
     In accordance with another embodiment, a method of operating a printer includes identifying a cross-process profile of a transfix roll, calculating a skew angle based upon the identified cross-process profile, positioning a first axis of rotation of the transfix roll at the calculated skew angle with respect to a second axis of rotation of the image drum, and operating the printer with the first axis of rotation skewed with respect to the second axis of rotation. 
     In a further embodiment, a method of improving a nip profile of a printer includes forming a nip with a transfix roll and an image drum, positioning a first axis of rotation of the transfix roll in a first orientation with respect to a second axis of rotation of the image drum, identifying a characteristic of the nip, and positioning the first axis of rotation in a second orientation with respect to the second axis of rotation based upon the identified characteristic, wherein the minimum distance from the second axis of rotation to a first end portion of the transfix roll at the second orientation is greater than the minimum distance from the second axis of rotation to the first end portion at the first orientation, and the nip profile with the first end portion at the second orientation is more uniform than the nip profile with the first end portion at the first orientation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a simplified side plan view of a printer with a transfix roll adjacent to an image drum and forming a nip; 
         FIG. 2  depicts a graph of different characteristics of two different nip profiles; 
         FIG. 3  depicts a graph of the effect of a change in crown profile of a transfix roll on the pressure within a nip for transfix rolls formed with different hardnesses; 
         FIG. 4  depicts a graph of the effect on nip width as a transfix roll is positioned with different amounts of skew with respect to an image drum; and 
         FIG. 5  depicts a procedure for skewing a transfix roll with respect to an image roll to modify nip characteristics in a nip formed by the transfix roll and the image roll. 
     
    
    
     DESCRIPTION 
     With initial reference to  FIG. 1 , a printer  100  includes a cylindrical image drum  102  which is driven by a motor  104 . Two printheads  106  and  108  are positioned to transfer ink to the printer image drum  102 . While two printheads  106  and  108  are shown, more or fewer printheads may be incorporated into a particular system. 
     A transfix roll  110  is maintained in position against the image drum  102  by a transfix roll support  112 . Guides  114  direct print media travelling along a process path  116  of the printer  100  into the nip  118  formed by the contact between the transfix roll  110  and the image drum  102 . 
     The transfix roll support  112  is configured to position the transfix roll  110  at a desired orientation with respect to the image drum  102  and to generate a desired pressure within the nip  118 . The transfix roll  110  has a crowned profile wherein the diameter of the transfix roll at the middle of the process path  116  is larger than the diameter of the transfix roll  110  at the outer portions of the process path  116 . When the transfix roll  110  is positioned against the image drum  102 , the nip  118  is formed with characteristics described with reference to  FIG. 2 . 
       FIG. 2  depicts a graph  120  of different normalized characteristics of the nip  118  and the transfix roll  110 . The line  122  of the graph  120  reflects the width of the nip  118  formed by pressing the crowned transfix roll against the image drum  102 . A nip “width” is the distance along an in-process axis of the process path  116  over which the transfix roll  110  is in contact with the image drum  102 . A nip “length” is the distance along a cross-process axis of the process path  116  over which the transfix roll  110  is in contact with the image drum  102 . The line  122  indicates that the nip width formed using the crowned transfix roll is very uniform at about 4 with a variance of about 0.1 (2.5%) along the length of the nip  118 . A uniform nip width reduces the potential for deformation of a print media as the print media is drawn through the nip  118 . 
     The line  124  of the graph  120  depicts the normalized pressure within the nip  118  generated by pressing the transfix roll  110  against the image drum  102 . The line  124  is relatively constant at about 7 with a variance of about 0.23 (3.3%) across the entire length of the nip  118 . Accordingly, the transfer of ink from the image drum  102  to print media travelling along the process path  116  would not be significantly adversely affected by the pressure variations along the length of the nip  118 . 
     The line  126  of the graph  120  depicts the strain energy generated at the layer interface between adjacent layers of the transfix roll  110 . The line  126  indicates a relatively uniform strain of about 4.4 with a peak of about 4.64 (105%) and a variance of about 0.4 (9%) across the entire width of the nip  118 . Accordingly, the material bonds within the transfix roll  110  are not overstressed. 
     Difficulties in achieving the nip characteristics shown in  FIG. 2  arise, however, because even slight changes in the profile of the transfix roll  110  result in significant changes in the nip profile. By way of example, flattening the profile of the transfix roll  110  by 30 microns results in the nip characteristics depicted by the line  130 ,  132 , and  134  in  FIG. 2 . 
     The line  130  of the graph  120  depicts the width of the nip  118  formed by pressing the transfix roll  110  with the flattened profile against the image drum  102 . The line  130  indicates that the nip width formed using the flattened transfix roll  110  varies by about 0.7 (17.5% of the nip width indicated by line  130 ) along the length of the nip  118 . Thus, the 30 micron difference between the profile used to generate the line  122  and the profile used to generate the line  130  significantly increases the nip width variation along the nip  118 . This significant increase in nip width variation substantially increases the potential for deformation of a print media as the print media is drawn through the nip  118 . 
     The line  132  of the graph  120  depicts the pressure within the nip  118  generated by pressing the transfix roll  110  with the flattened profile against the image drum  102 . The line  132  shows a peak pressure of about 8.6 with a large variance of over 2.4 (about 34% of the pressure indicated with the line  124 ) across the entire length of the nip  118 . Thus, the 30 micron difference between the profile used to generate the line  124  and the profile used to generate the line  132  significantly increases the pressure variation along the nip  118 . Accordingly, the transfer of ink from the image drum  102  to print media travelling along the process path  116  would be adversely affected by pressure variations along the length of the nip  118  formed with the flattened profile. 
     The line  134  of the graph  120  depicts the strain energy generated at the layer interface between adjacent layers of the transfix roll  110  with the flattened profile. The line  134  shows a large variance of about 4 (90% of the strain indicated with the line  126 ) across the entire width of the nip  118  with a peak strain of about 7 (175% of the strain indicated with the line  126 ). Accordingly, the 30 micron difference between the profile used to generate the line  126  and the profile used to generate the line  134  significantly increases both the maximum strain and the strain variation within the transfix roll  110 . Thus, the potential for shortening the life of the transfix roll  110  by overstressing material bonds between adjacent layers in the transfix roll  110  is significantly increased. 
     The variance in pressure across the length of the nip  118  may be ameliorated by changing the surface characteristics of the transfix roll  110 . The chart  140  of  FIG. 3 , for example, depicts the effects of a 30 micron change in profile on the pressure achieved within a nip. The data points  142 ,  144 , and  146  were obtained using an elastomer with a 60 D durometer hardness formed with a layer thickness of about 1.5 mm, about 3.1 mm, and about 4.6 mm, respectively. A 30 micron change in the profile for the transfix roll  110  incorporating the layer thicknesses of about 1.5 mm, about 3.1 mm, and about 4.6 mm resulted in pressure changes of about 32.5%, about 11.4%, and about 15.5%, respectively. Thus, increased layer thickness of the transfix roll  110  reduces pressure variances. Moreover, increased layer thickness reduces strain energy generated between adjacent layers. 
     The data points  148  and  150  were obtained using an elastomer with a 70 D durometer hardness formed with a layer thickness of about 1.5 mm, and about 3.1 mm, respectively. A 30 micron change in the profile for the transfix roll  110  incorporating the layer thicknesses of about 1.5 mm, about 3.1 mm, resulted in pressure changes of about 38.9% and 18.6%, respectively. For the corresponding thickness with a softer material (data points  142  and  144 ), the change was about 32.5%, and about 11.4%, respectively. Thus, increased material softness in the layer material of the transfix roll  110  reduces pressure variances. As material softness is reduced, however, strain energy generated between adjacent layers increases. 
     Accordingly, optimizing material hardness for reduction of pressure variations increases the potential for elastomer failure. Increased pressure uniformity and longer roll life can, however, be achieved by incorporating thicker layers of material a transfix roll  110 . As layer thickness is increased, however, achieving the high pressures necessary for high speed imaging becomes more difficult. For example, larger components may be needed. Thus, the potential for optimizing nip characteristics and transfix roller lifetime using only layer modification and material hardness modification is limited. Nip profile characteristics in the printer  100 , however, can be modified without requiring modification of the layer thickness or material hardness of the transfix roll  110 . 
     Specifically, the transfix roll support  112  is configured to allow the transfix roll  110  to be selectively skewed with respect to the image drum  102 . Skewing of the transfix roll  110  may be accomplished in any desired manner. For example, the transfix roll support  112  may incorporate a pivot and lock system whereby the desired skew angle is established and the transfix roll support locked. In a further embodiment, each end of the transfix roll support  112  may be independently movable along the in-process direction, thereby allowing the distance between each of the end portions of the transfix roll  110  and the axis of rotation of the image drum  102  to be changed. 
     In an exemplary case, a force of 2500 pounds was established between an image drum and a transfix roll with a flat profile along the length of the transfix roll. The transfix roll was then pivoted while maintaining a 2500 pound force on the system. The results are depicted in  FIG. 4  wherein the line  162  identifies the offset between the opposite ends of the transfix roll along the in-process direction and the line  164  identifies the nip width at the ends of the transfix roll. 
       FIG. 4  reveals that when the axis of rotation of the transfix roll is aligned parallel with the axis of rotation of the image drum (0 degrees skew), the nip width at the ends of the transfix roll is about 4.77 mm. The nip width at the middle of the transfix roll was determined to be 3.0 mm. As the transfix roll was pivoted, the nip width at the outer edges of the transfix roll decreased. In this example, the pivot axis is located at the middle of the transfix roll. Thus, both end portions of the transfix roll move away from the axis of rotation of the image drum at the same rate. 
     Accordingly, at 0.5 degrees of skew, or 1.5 mm of offset for both end portions of the transfix roll, the nip width at the edges of the transfix roll decreased to just over 4.4 mm. Therefore, since the nip width at the outer portions of the transfix roll decreased, as did the overall nip length, the width of the nip at the center of the transfix roll necessarily increased above 3 mm. 
     The results of the foregoing example show that skewing of a transfix roll with respect to an image roll can be used to modify the pressure profile and nip width within a nip. The extents of the changes that can be effected depend upon the elastomer thickness and hardness for a particular transfix roll. 
       FIG. 5  depicts a procedure  170  for skewing a transfix roll to modify nip profile characteristics. Initially, a crown profile for a transfix roll is determined such that the transfix roll and image drum form a nip with a desired nip profile when the axis of rotation of the transfix roll is parallel with the axis of rotation of the image drum (block  172 ). One such nip profile may exhibit a nip width, pressure, and strain energy similar to the nip width line  122 , the pressure line  124 , and the strain energy line  126 . 
     The transfix roll is then formed using manufacturing specifications directed to manufacturing a crown profile that is flatter than the determined crown profile (block  174 ). The difference between the manufacturing specifications and the crown profile determined at block  172  is selected to insure that the crown profile of the finished product will be at the design crown profile or flatter than the design crown profile by accounting for accuracy limitations in the manufacturing process. This ensures that a uniform pressure can be generated in a nip as described below. 
     The formed transfix roll is then installed into a printer device at location adjacent to an image drum (block  176 ). In one embodiment, the transfix roll may be initially installed such that the axis of rotation of the transfix roll is not parallel with the axis of rotation of the image drum. For example, the actual cross-process profile of a transfix roll can be accurately measured and used to calculate an estimated skew correction. The estimated skew correction may then be used to guide the initial installation. In another embodiment, the transfix roll is positioned with the axis of rotation of the transfix roll substantially parallel with the axis of rotation of the image drum. 
     Once the transfix roll is positioned, a nip is formed (block  178 ) by forcing the transfix roll and the image drum together at the pressure desired for operation of the printer. One or more nip characteristics (i.e., nip width or nip pressure) are then obtained (block  180 ). In one embodiment, the nip width is determined for both end portions of the roll and the center portion of the roll. Any variances in nip width can be reduced by selective skewing of the transfix roll. Alternatively, if a generic nip profile is available, the nip width at a single location along the transfix roll can be obtained to determine the nip profile along the entire transfix roll. 
     Once a skew correction is determined, the orientation of the transfix roll with respect to the image drum is modified (block  182 ). Pivoting of the transfix roll may be accomplished with a pivot axis located at any position along the axis of rotation of the transfix roll. Accordingly, in one embodiment the pivot axis is located at about the center of the process path. In another embodiment, the end portions of the transfix roll are separately positionable such that the pivot axis may be selected by the user to be at any location along the axis of rotation of the transfix roll. 
     The nip profile is then determined for the modified orientation (block  184 ) by obtaining one or more nip profile characteristics. If the nip width at the end of the roll is wider or narrower than the nip width at the end of the nip for the desired nip profile, the user may continue to pivot the transfix roll until the desired nip profile is realized. The printer is then placed into operation with the transfix roll in the skewed position relative to the image drum (block  186 ). 
     It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims.