Abstract:
A method for reducing artifacts on a toned sheet caused by buckling during fusing includes providing two compliant rollers that form a fusing nip for fusing the toned sheet. A control drives at least one of the rollers at a nominal speed to cause the rollers to rotate and there after increasing the roller speed to high speed prior to the sheet arriving at the fusing nip and after the sheet is in the nip decreasing the drive speed back to the nominal speed.

Description:
FIELD OF THE INVENTION 
     This invention relates to an electrophotographic print engine. In particular, this invention relates to reducing artifacts when fusing a receiver bearing a dry toner image. 
     BACKGROUND OF THE INTENTION 
     In a dry electrophotographic print engine, a photoreceptive element is initially charged uniformly using known methods such as employing a grid controlled corona charger, or a roller charger. An electrostatic latent image is then formed on the photoreceptive element by image-wise exposing the photoreceptive element using known methods such as a light emitting diode (LED) array, a laser scanner, or an optical exposure system. The electrostatic latent image is then converted into a visible image by bringing the photoreceptive element into close proximity to marking or toner particles contained in a development station and biasing the development station so that the marking particles would be preferentially attracted to the latent-image bearing portions of the photoreceptive element and repelled by the portions of the photoreceptive element that do not bear latent image information. The toner is then transferred to a receiver such as paper, generally by pressing the paper into contact with the toned photoreceptive element while exerting an electrostatic field to urge the toner to the receiver. Alternatively, the toner can first be transferred to a transfer intermediate member and then from the intermediate member to the receiver. 
     Color images are made by making electrostatic latent images corresponding to the subtractive primary colors, cyan, magenta, yellow, and black, converting the electrostatic latent images into color images corresponding to those subtractive colors, and transferring the images, in register, either directly to a receiver or to an intermediate transfer member and then onto a receiver. 
     The toner or marking particles typically consist of dry particles comprising a polymer binder such as polyester or polystyrene, pigment or other colorant, surface treatment addenda such as nanometer-size clusters of silica, titania, or charge agents. Toner particles typically are between 4 μm and 8 μm in diameter, but may be larger (up to approximately 30 μm in diameter) or between approximately 1 μm and 4 μm. For the purpose of this invention, toner diameter refers to the volume weighted median diameter, as measured with a commercially available device such as a Coulter Multisizer or equivalent. The toner particles typically have a glass transition temperature T g  between approximately 45° C. and 65° C., more typically between 50° C. and 60° C. For the purpose of this invention, toner or marking particles refer to the particles used to transform the electrostatic latent image into a toner image, often referred to as a visible image. The toner particles may contain a colorant such as a pigment or dye. Alternatively, the toner particles can be clear or absent any added colorant. 
     While monocomponent developers that do not comprise so-called carrier particles are used in dry electrophotographic print engines, it is more common to employ so called two-component developers. In this instance, the toner particles are mixed with magnetic particles, often referred to as carrier particles. The carrier particles are generally larger than the toner particles and are triboelectrically dissimilar to the toner particles so that the toner particles become electrically charged when contacting the carrier particles. The mixture of toner and carrier particles is often referred to as a two-component developer. 
     Two-component developers are used to transform the electrostatic latent image into a visible image by bringing the charged toner particles into close proximity to the electrostatic latent image bearing photoreceptive element, where the charged toner particles are attracted to the charge pattern making up the electrostatic latent image. The carrier particles are contained and transported by a development station comprising a so-called magnetic brush, as is known in the literature. 
     After transfer to the receiver, the toner image is fixed to the receiver by fusing. This is generally accomplished by subjecting the toner image bearing receiver to heat and pressure so that the toner is heated to a temperature above its T g  while subjecting the toner image to pressure. This allows the toner to flow and to become permanently fixed to the receiver. In addition, if a color image has been printed, the subtractive primary colored toners flow together to create the full-color print. Application of heat and pressure to the toner image bearing receiver is generally accomplished by passing the receiver between two heated compliant rollers. The durameters of the rollers can vary significantly or be near equal to one another. Load applied between these two compliant rollers results in a fusing nip width that provides the dwell time for melting the toner. As the receiver enters the fusing nip, an increased load to the fuser drive system is created. This can cause the fuser rollers to slow down, thereby slowing the speed of the receiver. This causes the lead edge of the receiver to travel more slowly. If the trail edge of the receiver is driven at a higher speed with a force greater than the beam strength of the receiver, the receiver will buckle. This can cause physical print artifacts or degrade image quality. Moreover, the print engine speed may be altered by the mismatched fuser speed through the receiver coupling between them. These variations can result in nonuniform image gloss, streaks, incomplete fusing, hot or cold offset whereby toner that is either heated too much or too little transfers from the receiver to the fuser roller or color-to color misregistration. These effects can cause print artifacts and can result in damage or increased maintenance to the print engine. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a method for reducing artifacts on a toned sheet caused by buckling during fusing, comprising:
         providing two compliant rollers that form a fusing nip for fusing the toned sheet;   driving at least one of the rollers at a nominal speed to cause the rollers to rotate and there after increasing the roller speed to high speed prior to the sheet arriving at the fusing nip and after the sheet is in the nip decreasing the drive speed back to the nominal speed.       

     An advantage of the present invention is the reduction of sheet buckling by controlling the roller speed. This will reduce artifacts. A feature of the invention is that it can be achieved by simple use of roller speed control without involving the complexity of additional structure. This invention can be practiced without increasing the size of the electrophotographic print engine in order to accommodate different length receivers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a typical prior art dry electrophotographic print engine in which the present invention can be employed. 
         FIG. 2  shows a fuser typically prior art that can be used in practicing this invention. 
         FIG. 3  is a graph of the velocity of the fuser drive as a function of time, showing the swallowing and degorging cycles of a single heavy weight receiver. 
         FIG. 4  is a graph of the velocity of the fuser drive as a function of time, showing the degorge, swallow, interframe, and overshoot cycles, when running multiple heavy weight receivers 
         FIG. 5  is a graph of the velocity and commanded velocity of the fuser drive as a function of time, showing the degorge, swallow, interframe, and overshoot cycles, when running multiple heavy weight receivers and speed up of the fuser drive, as practiced in this invention. 
         FIG. 6  is a graph of the velocity and commanded velocity of the fuser drive as a function of time, showing the degorge, swallow, interframe, and overshoot cycles, when running multiple heavy weight receivers and speed up and slow down of the fuser drive, as practiced in this invention. 
         FIG. 7  is a block diagram illustrating a method of practicing this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  depicts a dry, color electrophotographic print engine,  10 . There are imaging units for each color,  20 C,  20 M,  20 Y,  20 K, and  20 X. In each imaging unit, there is a photoreceptive element  22  is initially charged uniformly using known methods such as employing a grid controlled corona charger  28  or a roller charger. An electrostatic latent image is then formed on the photoreceptive element by image-wise exposing the photoreceptive element using known methods such as an LED array  26 , a laser scanner, or an optical exposure system. The electrostatic latent image is then converted into a visible image by bringing the photoreceptive element into close proximity to marking or toner particles contained in a development station  30  and biasing the development station so that the marking particles would be preferentially attracted to the latent-image bearing portions of the photoreceptive element and repelled by the portions of the photoreceptive element that do not bear latent image information. The toner may be transferred to an intermediate element  32 . The toner is then transferred from either the photoreceptive element  22  or the intermediate element  32  to a receiver such as paper, generally by pressing the paper into contact with the toned photoreceptive element while exerting an electrostatic field to urge the toner to the receiver. 
     Color images are made by making electrostatic latent images corresponding to the subtractive primary colors, cyan, magenta, yellow, and black, converting the electrostatic latent images into color images corresponding to those subtractive colors, and transferring the images, in register, either directly to a receiver or to an intermediate transfer member and then onto a receiver. In a preferred embodiment, the receiver is transported between the imaging units on an electrostatic transport web  50  and delivered to the fusing subsystem, although other transport arrangements such as drums, vacuum, or grippers, may be employed. Alternatively, the receiver may be transported directly into the fuser by a transfer nip. 
     The toner or marking particles typically consist of dry particles comprising a polymer binder such as polyester or polystyrene, pigment or other colorant, surface treatment addenda such as nanometer-size clusters of silica, titania, or charge agents. Toner particles typically are between 4 μm and 8 μm in diameter, but may be larger (up to approximately 30 μm in diameter) or between approximately 1 μm and 4 μm. For the purpose of this invention, toner diameter refers to the volume weighted median diameter, as measured with a commercially available device such as a Coulter Multisizer or equivalent. The toner particles typically have a glass transition temperature T g  between approximately 45° C. and 65° C., more typically between 50° C. and 60° C. For the purpose of this invention, toner or marking particles refer to the particles used to transform the electrostatic latent image into a toner image, often referred to as a visible image. The toner particles may contain a colorant such as a pigment or dye. Alternatively, the toner particles can be clear or absent any added colorant. 
     While monocomponent developers that do not comprise so-called carrier particles are used in dry electrophotographic print engines, it is more common to employ so called two-component developers. In this instance, the toner particles are mixed with magnetic particles, often referred to as carrier particles. The carrier particles are generally larger than the toner particles and are triboelectrically dissimilar to the toner particles so that the toner particles become electrically charged when contacting the carrier particles. The mixture of toner and carrier particles is often referred to as a two-component developer. 
     Two-component developers are used to transform the electrostatic latent image into a visible image by bringing the charged toner particles into close proximity to the electrostatic latent image bearing photoreceptive element, where the charged toner particles are attracted to the charge pattern making up the electrostatic latent image. The carrier particles are contained and transported by a development station comprising a so-called magnetic brush, as is known in the literature. 
     After transfer to the receiver, the toner image is fixed to the receiver by fusing. This is generally accomplished by subjecting the toner image bearing receiver to heat and pressure so that the toner is heated to a temperature above its T g  while subjecting the toner image to pressure. This allows the toner to flow and to become permanently fixed to the receiver. In addition, if a color image has been printed, the subtractive primary colored toners flow together to create the full-color print. This is generally accomplished by sandwiching the toner image bearing receiver between a pair of rollers that are pressed together, know as a fusing nip. A typical fuser system  42  is depicted in  FIG. 2 . The roller that contacts the freshly imaged side of the receiver is commonly referred to as the fusing roller  100 . The opposite roller functions to apply pressure and is thus referred to as the pressure roller  103 . In a preferred embodiment, this fusing roller is driven by an independent motor and controller  104  based on the speed profile calculated by the control unit  120 . The pressure roller  103  is driven by the fusing roller  100 . While this invention has been described in the context of a preferred embodiment as depicted in  FIG. 2 , it will be understood to apply to other known fusing configurations, such as belt fusers, comprised of a heated belt (or web) and a pressure roller. 
     Generally, there is limited control transport  102  such as a low pressure vacuum transport between the electrostatic transport web  50  and the fuser  42 . If this transport is longer than the maximum receiver length, the fuser  42  is decoupled from the electrostatic transport web  50 . This configuration greatly decreases the necessity to match speeds of the subsystems, but increases the overall size of the print engine. If the receiver is long enough such that it is still in control of the electrostatic transport web  50  when it enters the fuser  42  and the speed of the receiver in the fuser  42  is not exactly the same as the imaging unit transport speed at all times, the lead edge and trail edge of the receiver will be driven at different speeds. If the speed of the receiver in the fuser  42  is slower than the portion on the electrostatic transport web  50  the receiver will buckle between the fuser  42  and the electrostatic transport web  50 , if both are able to supply a drive force sufficient to buckle the receiver. If the speed of the receiver in the fuser  42  is greater than that on the electrostatic transport web  50 , the lead edge will be driven faster than the trail edge. If this differential drive is larger than the slack or buckle in the receiver, the receiver will be in tension and there will be a force transmitted by the fuser  42  on electrostatic transport web  50 . Either speed variation can result in variations of image gloss, streaks, incomplete fusing, hot or cold offset whereby toner that is either heated too much or too little transfers from the receiver to the fuser roller  100 , or cause color-to color misregistration. These effects can cause print artifacts and can result in damage to the print engine. 
       FIG. 3  depicts the typical velocity profile for a fuser roller  100  as a sheet enters and exits the fusing nip. The fuser motor and controller  104  is commanded to run at the nominal fuser motor speed  201 . This nominal speed is generally close to the speed of the electrostatic transport web  50  but may be altered slightly in order to induce a controlled buckle, or increase the gap between sheets after they are released from the electrostatic transport web  50 . This is shown as a reference along with the actual speed of the fusing roller  202  for a single sheet is shown as a function of time. The impact of the receiver entering the fuser nip can cause the fuser roller  100  to slow down for a period of time, thereby slowing the speed of the lead edge of the receiver. The speed in this time period can be in the range of 10-30% below the commanded speed. The difference between the nominal distance and the actual distance the receiver travels for this time period is defined as the swallowing loss  203 . This swallowing loss  203  is a function of many factors including the fuser roller materials, pressure between the fuser roller  100  and the pressure roller  103 , properties of the receiver such as caliper (or thickness), width (perpendicular to the transport direction), length, coating, gap between receiver sheets (interframe), the speed of the fuser roller  100 , and the drive system characteristics such as inertia, motor torque, responsiveness or drive stiffness. The high speed command can be determined by the control unit  120  based on the above factors including properties of the receiver. The high speed and low speed command signals produced by the control unit  120  include duration and amplitude. Part, but generally not all, of the swallowing loss  203  is the result of the “elasticity” or stiffness of the drive system. Some of the kinetic energy is converted into potential energy stored in the drive system. This energy is subsequently released as the fusing roller  100  and receiver accelerate to a speed greater than the nominal fuser motor speed  201  for a period of time. The overshoot  204  is defined as the difference between nominal distance and the actual distance the receiver travels for this time period. This overshoot  204  decreases the buckle. A position control algorithm is frequently used by the fuser drive motor and controller  104  in order to provide the precision of drive required. As a result, the difference between the swallowing loss  203  and the overshoot  204  will be corrected in the position compensation portion  205  of the velocity profile. Essentially, the motor and controller  104  adjusts the speed to slightly higher than the nominal commanded speed to correct for the net difference in distance traveled. As the trail edge of the sheet exits the fuser  42 , the speed of the fuser  42  and trail edge of the sheet increase. This is defined as degorging  206 . The effect of the swallowing loss  203 , overshoot  204 , and position compensation portion  205  are superimposed on any commanded speed mismatch between the fuser roller  100  and electrostatic transport web  50 .  FIG. 4  is an enlarged view depicting a typical degorge, interframe between receiver sheets, receiver swallowing, and overshoot sequence for subsequent sheets. 
     Increasing the inertia of the fuser system  42 , drive motor torque, or drive system stiffness may decrease the swallowing loss  203 . These modifications generally increase cost or power consumption. Another countermeasure is depicted in  FIG. 5 . Essentially, the control unit  120  increases the commanded speed of the fuser roller  100  to high fuser speed command  207  a speed greater than the nominal fuser motor speed  201  prior to the lead edge arriving so that the resulting minimum fuser roller speed is closer to the speed of the electrostatic transport web  50 . The arrival time of the sheet may be predicted based on nominal sheet timing. Alternatively, the accuracy of this prediction can be improved by sensing the sheet arrival at a sensor or switch proceeding, but relatively close to, the fuser nip, similar to the vacuum transport switch  101  in  FIG. 2 . If a position control algorithm is used, this commanded speed may be significantly higher than the desired speed in order to exaggerate the position error, forcing a larger actual speed increase—with the given drive system. The control unit  120  then returns the high fuser command speed  207  to nominal fuser motor speed  201  after the swallowing loss  203  occurs. In a preferred embodiment, the high fuser command speed  207  is 10 to 30% above the nominal speed, initiated 50 to 100 milliseconds prior to the expected arrival of the lead edge of the receiver, resulting in an actual fuser roller speed increase of approximately 5 to 15%, to the fuser high speed actual  208 . The resulting minimum speed of the fusing roller  100  would then be 5 to 20% lower than the nominal fuser motor speed  201 . After 100 to 200 milliseconds, the command speed is returned from high speed  207  to nominal speed  201 . This results in a reduced sheet buckle. A basic block diagram of the control unit used to accomplish these speed commands is depicted in  FIG. 7 . 
     Since the fuser motor and controller  104  is unable to fully attain the high commanded speed  207 , the motor controller calculates a large position error, resulting a larger overshoot  204  and position compensation portion  205 . If this occurs while the receiver is still adhered to the electrostatic transport web  50 , the fuser  42  will pull the imaging unit transport after the remaining buckle in the receiver is consumed. A countermeasure for this is depicted in  FIG. 6 . In a preferred embodiment, the control unit  120  reduces the fuser motor speed command from the high speed  207  to a low speed  209  substantially below the actual speed  202 . This low speed  209  may be in the range of 10-30% below the actual speed  202 . The timing of this change to the fuser speed command may occur just as the sheet enters the fuser nip or at the minimum actual speed during the swallowing loss, so as to reduce the impending overshoot  204 . The amplitude and duration of the low speed profile are chosen to balance the position error resulting from the combination of the degorging  206 , high fuser speed command  207 , swallowing loss  203 , and overshoot  204 . This will result in reduced position compensation  205  after the overshoot. Essentially, the net area between the actual speed profile and the desired speed profile for each sheet from the initiation of degorging until the completion of the overshoot is reduced. Similar to the high fuser speed command  207 , the amplitude of the low speed command  209  is exaggerated to increase the reduction in fuser motor speed and the ensuing overshoot  204 . 
     The torque and responsiveness of the drive system limit the amount the swallowing loss  203  can be reduced without increasing the overshoot  204  excessively. For this reason, it is important to balance the improvement in swallowing loss  203  with the increase in overshoot  204 . Since the swallowing loss  203  is a function of the paper parameters and fuser configuration, it is desirable to modify the compensation based on these parameters. 
       FIG. 7  depicts a typical block diagram for this process. The control unit  120  can determine adjustments to the fuser motor speed through an algorithm, a look up table, or a combination. The control unit  120  combines the configuration and set point information  305  with the receiver information  306  using the fusing motor speed adjustment algorithm  307  to determine the desired fuser motor speed profile  308 . Control unit  120  employs this profile  308  at the appropriate time based on the sheet timing  304  resulting in the fusing motor speed adjustments  310 . As will be clear from  FIGS. 5 and 6  in accordance with the invention, control unit  120  will command at least one of the rollers in the fusing nip to be driven at a nominal speed  201  to cause the rollers to rotate and there after increase the command speed to high fuser speed command  207  prior to the sheet arriving at the fusing nip and after the sheet is in the nip, reduce the command speed to low speed command  209 , preferably reducing the drive speed of the roller below the nominal fuser motor speed  201  and then increasing the drive speed back to the nominal fuser motor speed  201 . In some instance it is not necessary to reduce the drive speed of the roller below the nominal speed of the roller to reduce the image artifacts. 
     In a preferred embodiment, the amplitude or duration of the high fuser speed command  207 , will be increased for receivers having greater thickness, or caliper. For instance, no high speed command may be necessary for receivers with a caliper less than a certain caliper in the range of 150 to 300 microns. The actual caliper threshold will be dependent on the fuser system configuration and setpoints. The adjustments may be made based on receiver weight rather than caliper if the relationship is known. Similarly, the magnitude of the swallowing loss  203  is proportional to the width of the receiver, so the magnitude of the high speed command may be increased for wider receivers. The swallowing loss  203  is also affected by the timing of the sheet arrival relative to the release of the previous sheet. This timing is a function of the fuser speed, fuser nip width, and interframe spacing between the sheets, which is affected by the sheet length (parallel to feed direction). For this reason, the high speed command  207  may be modified depending on these parameters. The first sheet in the set is a special case in which the interframe is essentially infinite and may be treated differently from the other sheets in the run. 
     The swallowing loss  203  is a function of the force in the fusing nip. The higher the force for a given configuration, the greater the swallowing loss will be. To compensate for this in variable force systems, the control unit  120  may modify the fuser motor speed adjustments  310  based on the fuser nip force, with greater compensation preferred for higher fuser nip forces. Similarly, the compensation may be adjusted for different fuser roller  100  and pressure roller  103  designs, as well as different drive systems. 
     Since the magnitude of the overshoot  204  increases as the swallowing loss  203  increases, the low fuser speed command  208  may be adjusted when the high speed command  207  is modified. This may or may not be done proportionally, depending on the configuration. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention 
     Parts List 
     
         
           10  color electrophotographic print engine 
           20 C color 
           20 K color 
           20 M color 
           20 Y color 
           20 X color 
           22  photoreceptive element 
           26  led array 
           28  grid controlled corona charger 
           30  development station 
           32  intermediate element 
           42  fuser 
           50  electrostatic transport web 
           100  fuser roller 
           101  vacuum transport switch 
           102  limited control transport 
           103  pressure roller 
           104  motor and controller 
           120  control unit 
           201  nominal fuser motor speed 
           202  actual speed 
           203  swallowing loss 
           204  ensuing overshoot 
           205  position compensation portion 
           206  degorging 
           207  high fuser speed command 
           208  fuser high speed actual 
           209  low speed 
           304  sheet timing 
           305  set point information 
           306  receiver information 
           307  fusing motor speed adjustment algorithm 
           308  desired fuser motor speed profile 
           310  fuser motor speed adjustments