Patent Publication Number: US-7708377-B2

Title: Blade engagement apparatus for image forming machines

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Attention is directed to co-pending applications U.S. application Ser. No. 11/877,770 filed Oct. 24, 2007, entitled “LONG LIFE CLEANING SYSTEM WITH REPLACEMENT BLADES” and, U.S. application Ser. No. 12/201,140 filed concurrently herewith, entitled “SYSTEM AND METHOD OF ADJUSTING BLADE LOADS FOR BLADES ENGAGING IMAGE FORMING MACHINE MOVING SURFACES” the disclosure found in these co-pending applications is hereby incorporated herein by reference in its entirety. 
   BACKGROUND 
   Disclosed in embodiments herein are systems for metering and/or cleaning release agent on an image forming machine moving surface, and more specifically a release agent application apparatus utilizing a fixed rotating blade holder for moving blades between non-operational suspended positions and a common working position. 
   Image forming machines such as solid ink jet (SIJ) image forming machines generally use an electronic form of an image to distribute ink melted from a solid ink stick or pellet in a manner that reproduces the electronic image. In some solid ink jet imaging systems, the electronic image may be used to control the ejection of ink directly onto a media sheet. In other solid ink jet imaging systems, the electronic image is used to eject ink onto an intermediate imaging member. A media sheet is then brought into contact with the intermediate imaging member in a nip formed between the intermediate member and a transfer roller. The heat and pressure in the nip helps transfer the ink image from the intermediate imaging member to the media sheet. 
   One issue arising from the transfer of an ink image from an intermediate imaging member to a media sheet is the transfer of some ink to other machine components. For example, ink may be transferred from the intermediate imaging member to a transfer roller when a media sheet is not correctly registered with the image being transferred to the media sheet. The pressure and heat in the nip may cause a portion of the ink to adhere to the transfer roller, at least temporarily. The ink on the transfer roller may eventually adhere to the back side of a subsequent media sheet. If duplex printing operations are being performed, the quality of the image on the back side is degraded by the ink that is an artifact from a previous processed image. 
   To address these problems, various release agent applicators have been designed, often as part of an image drum maintenance system. These release agent applicators provide a coating of a release agent, such as silicone oil, onto the intermediate imaging member moving surface to reduce the undesired build-up of ink. It is desired to control the amount of release agent applied, since using of too much release agent causes undesirable streaks, also known as oil streaks, on the output prints. 
   The present application provides a new and improved apparatus for cleaning and/or metering a release agent onto an image forming device moving surface which overcomes these above-described problems. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a release agent application apparatus with an operational first blade disposed in retracted position as described herein; 
       FIG. 2  illustrates release agent application apparatus with an operational first blade disposed in wiper blade orientation in a working position metering a release agent on a moving surface; 
       FIG. 3  illustrates a blade undergoing overbending during a replacement operation; 
       FIG. 4  illustrates a release agent application apparatus with an operational first blade disposed in doctor blade orientation in a working position metering a release agent on a moving surface; 
       FIG. 5  illustrates a release agent application apparatus with an operational second blade disposed in doctor blade orientation in a working position metering a release agent on a moving surface; 
       FIG. 6  illustrates a release agent application apparatus with an operational second blade disposed in retracted position as described herein; 
       FIG. 7  illustrates a release agent application apparatus with an operational second blade disposed in wiper blade orientation in a working position metering a release agent on a moving surface; 
       FIG. 8  shows a graph of the ratio of median blade life over the life goal as a function of Weibull slope; 
       FIG. 9  is a graph of expected cleaning unit lives with various blade replacement strategies for a typical cleaning blade material; and 
       FIG. 10  is a graph illustrating the ratio of the run-to-failure replacement strategy life to the B5 replacement strategy life. 
   

   DETAILED DESCRIPTION 
   Referring now to  FIGS. 1-3 , an image forming machine, shown generally at  10 , includes a moving surface  12  suitable for receiving a controlled application of a release agent. In one example, the image forming machine  10  is a Solid Ink Jet (SIJ) printer including a rotating SIJ drum  11  having a cylindrical outer surface  12   a  rotating in a rotational direction of operation  14 . Other examples of applicable image forming machine moving surfaces  12  suitable for receiving application of a release agent can include flat moving surfaces  12   b  shown in  FIGS. 4 and 5 . These image forming machine moving surfaces  12   a ,  12   b  move in a direction of operation  14  and shall be referred to generally as moving surface  12 . 
   The image forming machine  10  also includes a blade engagement apparatus, also referred to as a release agent application apparatus, shown generally at  16  for applying a controlled amount (thickness) of release agent  13  to surface  12  as shown in  FIG. 2 , in a process referred to herein as metering. The blade engagement apparatus  16  can be used for cleaning oil and other contaminants from the surface  12  in a cleaning operation, or both cleaning and metering. 
   The blade engagement apparatus  16  can be contained in a removable cartridge unit  17 , if so desired, such as for example part of a maintenance unit, or drum maintenance unit (DMU). The maintenance unit  17  can be removed from the image forming machine  10  and discarded when its useful life has been depleted. 
   The blade engagement apparatus  16  includes a blade positioning mechanism  18  having a blade holder  19  with a plurality of blades extending therefrom. The blade positioning mechanism  18  rotates the blade holder  19  to move the blades into a working position engaging the surface  12  for metering the release agent  13  onto the surface, as described in further detail below. In the example provided herein, a pair of blades are used, including a first blade  20  and a second blade  40 . However it should be appreciated that more than two blades can be used, as described in further detail below. 
   The blade holder  19  is rigid, and can be formed of aluminum, a composite, or other rigid material. It extends transversely across the surface  12  with respect to the operational direction of movement  14 . It is adapted to be rotated about a pivot axis P. In one example, axis P can extend through the elongated holder  19 , along its length. The holder  19  is supported at the pivot axis P by being pivotally connected to the DMU  17 , or a support member attached to the image forming machine  10 , such that the pivot axis P is disposed a fixed, distance L D  from the surface  12 , as shown in  FIG. 3 . The pivot axis P is fixed in that it does not translate as the blade holder  19  rotates about axis P. Distance L D  is preferably the shortest distance between the pivot axis P and the moving surface  12 , such as for example extending from the pivot axis P towards the center of a drum-shaped moving surface  12   a , or at a right angle to a flat moving surface  12   b.    
   The blades  20 ,  40  extend from the holder  19  and terminate in ends  22  and  42  respectively. The blades  20 ,  40  include respective blade edges, or tips,  30  and  50  disposed a distance L B  from the pivot axis P, as shown in  FIGS. 3 and 4 . The blades  20 ,  40  extend transversely (with respect to the operational direction of movement  14 ) across the surface  12  such that the blade edges  30 ,  50  extend across the portion (or width) of surface  12  to which release agent is to be applied. 
   Distance L B  is greater than distance L D . The blades  20 ,  40  are formed of a compliant material, such as polyurethane, which bends, or deflects, as they are moved into the working position in which the blade tips  30 ,  50  are pressed against surface  12  generating a blade load at the tips against the surface, or material on the surface such as a release agent being metered. The interaction of the compliant blade  20 ,  40  in deflected engagement with the moving surface  12  in the working positions can be referred to generally as the blade interference. The blade interference can be considered a measure of how far the blade tip  30 ,  50  would extend into the surface  12  if the blade  20 ,  40  did not deflect. Moving the blade  20 ,  40  in a direction towards the surface  12 , with the blade at the working position, increases the blade deflection and interference, thereby increasing the blade load at the blade tip  30 ,  50  against the surface  12  or material thereon. Whereas, moving the blade  20 ,  40  in a direction away from the surface  12 , with the blade disposed in the working position, decreases the blade deflection and interference, thereby decreasing the blade load at the blade tip  30 ,  50 . The tips  30 ,  50  can be coated with PMMA, SureLube, toner or other initial blade lubricant to prevent blade flip as the blades  20 ,  40  are moved into the working positions. 
   The blades  20 ,  40  extend from the holder  19  in an angularly-spaced apart manner, with the angle formed between the blades depending on the number of blades used. As mentioned, more than two blades can be attached to the blade holder  19 , and each blade can be brought into a working position individually in a manner similar to that described below. The maximum number of blades that can be attached to the blade holder will be a function of the distance from the blade tip  30  to the blade holder pivot axis P, the desired blade holder angle between blades, and the diameter of the SIJ drum  12   a , if applicable. The blade positioning mechanism  18  may be constrained by the space available within the image forming machine  10  and clearance of the blades to the surface  12  during retraction and engagement, however it is contemplated that two to five, or more, blades may be used. 
   The blade engagement apparatus  16  also includes an actuator A connected to the blade positioning mechanism  18  for providing bi-directional rotational movement to the blade holder  19 . Actuator A is a connected to blade holder  19  to rotate the blade holder about axis P in a first direction R 1  and a second, opposite direction R 2 . Actuator A can be a bi-directional stepper motor, a solenoid, a linear actuator, or other actuator connected to holder  19  in a suitable manner for applying rotational forces for rotating holder in the R 1  and R 2  directions. A pair of actuators A can be used, each connected to opposite ends of holder  19 , for applying rotational forces thereto. The actuators A can be separately actuated, if so desired. 
   A controller, shown in  FIG. 1 , is used to provide control signals to the actuator A for rotating the holder in the R 1  and R 2  directions for moving the blades  20 ,  40  into and out of working positions with respect to the moving surface  12  as described in further detail below. While the blade  20  or  40  is in the working position, actuator A can rotate holder  19  to increase or decrease the blade interference, and thus the blade load, thereby increasing or decreasing the thickness of the release agent applied to surface  12 , as described in further detail below, and as described in the co-pending application U.S. application Ser. No. 12/201,140 filed concurrently herewith, entitled “SYSTEM AND METHOD OF ADJUSTING BLADE LOADS FOR BLADES ENGAGING IMAGE FORMING MACHINE MOVING SURFACES” incorporated herein by reference in its entirety. 
   Sensors can be used to monitor for defects such as streaks on output prints or on moving surface  12  and the controller can signal actuator A to provide incremental bi-directional changes in rotation to holder  19  to make small changes in the blade load to achieve a minimum blade load needed for preventing these defects during image forming. By using two actuators A it is possible to vary the blade interference, and thus the blade load, differently at each end of the blade holder  19  to further adjust the blade load across the blade  20 ,  40  occupying the working position. 
   During operation, one of the blades, such as for example blade  20  in  FIGS. 1-3 , can be designated as the operational blade while the other blades can be considered to be non-operational blades, such as blade  40  in these FIGURES. The operational blade  20  can be the blade located closest to the surface  12 . The operational blade  20  will typically be moved back and forth between a standby position in which the blade edge  30  is retracted, or suspended away from the surface  12 , such as shown in  FIG. 1 , and a working position in which the blade edge  30  engages the surface  12  for metering the release agent onto the surface in a metering operation as shown in  FIG. 2 . Actuator A can move the operational blade  20  from the standby position to the working position by rotating the blade holder  19  in the first rotational direction R 1 , and back to the standby position by rotating the blade holder in the second rotational direction R 2 . This can occur repeatedly for any operational blade throughout its life of operation. The operational blade  20  occupies the standby position of  FIG. 1  throughout much of the image forming process so as not to interfere with surface  12 . 
   During a metering operation, a release agent  13 , such as silicone oil or the like, is applied to surface  12  using an applicator  15  or in another known manner as shown in  FIG. 2 . The controller signals actuator A to rotate blade holder  19  in the first direction R 1  thereby moving the operational blade  20  in a direction towards the surface  12  and into the working position for metering the release agent onto the surface in a controlled thickness. The compliant blade  20  deflects as it is moved into the working position generating a blade load at the blade edge  30  against the surface, or against material on the surface such as the release agent  13  being metered. 
   As the first blade  20  engages the surface in the working position, a blade load is generated at the blade tip  30  against surface  12  for metering the release agent onto the surface. The blade load can be increased while the first blade  20  is in the working position by the actuator A rotating the blade holder  19  in the first direction R 1 , thereby moving the blade  20  in a direction towards the surface  12 , increasing the deflection and the interference of the compliant blade, thereby increasing the blade load at the tip  30  against the surface. Increasing the blade load meters a thinner layer of release agent  13  onto surface. While the first blade  20  is in the working position, in deflected engagement with the surface  12 , the blade load at tip  30  can be decreased to meter a thicker layer of release agent by the actuator A rotating the blade holder in the second direction R 2 . 
   The blade engagement mechanism  16  can include a blade positioning mechanism  18  having blades  20 ,  40  arranged in a wiper blade orientation when disposed in the working position, referred to herein as WP WB , an example which is shown in  FIG. 2 . In WP WB , the blade  20  (as it extends from the blade holder  19 ) forms an angle with surface  12  (or a tangent thereto)&lt;90 degrees. This angle is taken at the blade tip  30  at the upstream side of the blade  20 ′(with respect to the moving surface operational direction  14 ), described in further detail in the co-pending application U.S. application Ser. No. 12/201,140 filed concurrently herewith, entitled “SYSTEM AND METHOD OF ADJUSTING BLADE LOADS FOR BLADES ENGAGING IMAGE FORMING MACHINE MOVING SURFACES” previously incorporated herein by reference in its entirety. 
   Alternatively, the blade engagement mechanism  16  can include a blade positioning mechanism  18 ′ having blades  20 ,  40  arranged in a doctor blade orientation when disposed in the working position, referred to herein as WP DB , an example which is shown in  FIGS. 6 and 7 . The blade positioning mechanism  18 ′ includes a blade holder  19  having blades  20  and  40  extending therefrom. Though two blades  20  and  40  have been shown for the purposes of simplicity, and it is contemplated that N blades can be used as described above. The blade positioning mechanism  18 ′ operates in a manner similar to the blade positioning mechanism  18  described above, moving blades  20  and  40  into a working position WP DB , wherein the blade  20 ,  40  (as it extends from the blade holder  19 ) forms an angle with surface  12  (or a tangent thereto)&lt;90 degrees. The angle is taken at the blade tip  30 ,  50  at the downstream side of the blade  20 ″,  40 ″(with respect to the moving surface operational direction  14 ), as described in further detail in the co-pending application U.S. application Ser. No. 12/201,140 filed concurrently herewith, entitled “SYSTEM AND METHOD OF ADJUSTING BLADE LOADS FOR BLADES ENGAGING IMAGE FORMING MACHINE MOVING SURFACES” previously incorporated herein by reference in its entirety. In some example embodiments, the doctor blade orientation has a BHA ranging from about 10 degrees to about 40 degrees. In other example embodiments, the doctor blade orientation has a BHA ranging from about 18 degrees to about 28 degrees. 
   Referring now to  FIGS. 1 ,  3 ,  6  and  7 , a blade replacement operation for the blade engagement apparatus  16  shall be described. At the end of the operational life of the first blade  20 , the used blade is withdrawn from operation and the second blade  40  is placed into operation, as the operational blade, for movement into and out of the working position. The actuator A rotates the blade holder  19  in the first direction R 1  about the pivot axis P moving the first blade  20  towards the surface as shown in  FIG. 1 , and then across the surface  12  and past the working position creating a maximum amount of blade deflection (and blade interference), referred to as overbending, as shown in  FIG. 3 . Overbending is blade deflection, or blade interference, which is greater than amount of blade deflection, or blade interference, attained in the working position. The compliant blades  20 ,  40  are designed for overbending so that they do not break during blade replacement. 
   Rotation of the holder  19  is continued in first direction R 1  until the first blade  20  reaches a non-operational suspended position separated from the surface  12  as shown in  FIG. 6 . The first blade  20  can now be designated as a non-operational blade. In the non-operational position, the non-operational blade edge  30  can point away from the surface  12 . The next blade, blade  40 , is simultaneously brought into the operational standby, or retracted, position as shown in  FIG. 6  and can now be designated as the operational blade. In the operational standby (retracted) position, the operational blade edge  50  can point towards the surface  12 . The non-operational blade  20  is suspended a sufficient distance from surface  12  in the non-operational suspended position shown in  FIG. 6 , so as to not impede the flow of oil and contaminants from the operational blade  40  during use in the working position as shown in  FIG. 7 . 
   The operational, second blade  40  can be moved from the standby position, shown in  FIG. 6 , to the working position, shown in  FIG. 7 , by rotating the holder  19  in the first rotational direction R 1 . The operational second blade  40  can also be moved from the working position back to the standby position by rotating the holder in the second rotational direction R 2 . These actions can be repeated throughout the operational life of the second blade  40 , as described above in reference to the first blade  20 . Furthermore, the blade load at the second blade tip  50  can be increased and/or decreased for metering different thicknesses of release agent in a similar manner as described above in reference to the first blade  20 . 
   It is contemplated that examples of the blade engagement apparatus  16  can include N blades, with some examples having N equal 4 or 5 blades, and some examples having N equal to more than 5 blades. The number of blades N can be a function of the distance from the blade tip to the blade holder pivot L B , the desired blade holder angle, the diameter of the SIJ drum  12   a , the space available within the image forming machine  10 , and the clearance of the blades to the surface  12  during the retraction and engagement of the operational blade. In these embodiments, the other blades including the third blade to the N th  blade can be brought into the operational standby position and the working position, in a similar manner as described above. 
   A number of strategies (e.g., blade replacement schedules) are possible for determining when to replace blades within the maintenance unit. For an individual blade, the blade can be replaced upon detection of a blade replacement condition, such as blade failure, a predetermined amount of use, etc. Blade failure can be detected by the machine operator or by a sensor  128  within the machine. For example, the sensor  128  can observe failures on output prints, or on the surface  12  as described in co-pending application U.S. application Ser. No. 12/201,140 filed concurrently herewith, entitled “SYSTEM AND METHOD OF ADJUSTING BLADE LOADS FOR BLADES ENGAGING IMAGE FORMING MACHINE MOVING SURFACES” previously incorporated herein by reference in its entirety. 
   Blade replacement strategy can comprise one or more replacement schemes based on blade use, run-to-failure schemes, and the like. For example, replacement strategies based on blade use can comprise analysis of cleaning unit failure probability at end of life specified (e.g., by a customer, by design constraints, etc.) Individual blades can additionally be replaced at intervals desired to achieve a specific cleaning unit failure probability. 
   Another replacement strategy for an N-blade system includes replacing the first N−1 blades based on use and replacing the Nth blade upon failure. In such a scenario, failure at end of cleaning unit life is deemed acceptable, cleaning unit failure probability for N−1 blades can be pre-specified, and individual blade replacement can be performed at predetermined intervals to achieve a desired N−1 blade failure probability. 
   In yet another replacement strategy, all blades are permitted to run to failure. According to one example, machine sensing of cleaning failures need not be employed, such as where failure of each individual blade is acceptable. In another example, cleaning failures are sensed by the machine. For instance, failures can be detected when they are minor print defects, on the SIJ drum before they appear on prints, etc. 
   Blades may also be replaced after a predetermined number of prints, drum cycles, or accumulation of stress. This strategy is desirable when life of the blade is sufficiently predictable. If blade life is not predictable (e.g., has a Weibull slope near 1), then a run-to-failure strategy may be employed. Blade replacement at a predetermined interval can be employed in scenarios where the time between replacements is sufficiently long and the probability of failure before that interval is sufficiently small. Typically, less than 5% to 10% of the blade population fail before the replacement interval, which is the time between blade changes. The required length of the replacement interval may be chosen to be compatible with other machine components and to enable a desired service or running cost for the machine. For example, if a cartridge containing a blade needs to have a B10 life of 400,000 cycles in order to meet run cost goals, then the blade may be required to have only 5% failures at 400,000 cycles. For a blade with a near-random failure distribution, a very large median blade life is required in order to meet such a target (e.g., a B5 of 400,000 cycles and a Weibull slope of 1 implies a characteristic life of 7,798,290 cycles and a B50 of 5,405,363 cycles). For a more symmetric failure distribution (e.g., near normal), the median blade life required to meet the target can be much smaller (e.g., a B5 of 400,000 cycles and a Weibull slope of 3 implies a characteristic life of 1,076,564 cycles and a B50 of 952,756 cycles). 
     FIG. 8  shows a graph  40  of the ratio of median blade life over the life goal as a function of Weibull slope. For Weibull slopes less than approximately 2 or 3, the desired median blade life to meet the goal is more than twice the goal. As the Weibull slope becomes smaller, it becomes increasingly difficult to achieve these very high median lives. Assuming a sufficiently predictable failure distribution, blades may be replaced after a predetermined number of prints. 
   Blade replacements based on accumulated stress can have more certainty in the amount of blade use than replacements based on SIJ cycle count, since blade stress is induced by the friction force between the blade and the SIJ drum. Higher friction forces, created by low lubrication conditions, generate higher stresses in the blade. The hardness, texture and coating of the SIJ drum surface also influence the blade-to-surface friction. Blade stress can be inferred by measuring the friction force on the metering blade. A measurement of the total friction force across the full width of the blade represents an average of the locally varying friction forces acting on the blade edge. Integration of the friction force over the number of SIJ drum cycles is equivalent to the energy applied to the blade edge, which can be correlated to wear of the blade edge and failure to meter. 
   Knowledge of cross-process variations in the friction force can be utilized to further reduce uncertainty in the accumulated stress contributing to metering failures. Local regions of the blade edge can be expected to wear at higher rates than other regions. With digital printing machines, this information is available from the location of exposed pixels on the imaging surface. Counters  130  can record accumulated blade stress for each region along the blade edge. The counters  130  can be interrogated to determine whether the most highly stressed region of the blade is approaching the accumulated stress level that triggers blade replacement. When this accumulated stress level has been reached, the blade can be replaced. The accumulated stress level that triggers replacement can be selected to correspond to a predetermined probability of blade failure (e.g., 5% of blades expected to reach failure prior to this level). 
   In a maintenance unit having replacement blades, the blades may be replaced by any combination of the above-described run-to-failure (RTF) and use strategies described above. Table 1, below, lists examples of combinations of replacement strategies that can be used for a two blade maintenance unit  17 . Also listed are examples of lives expected from each blade and the combined maintenance unit life. In the presented examples, a blade with a run-to-failure replacement strategy is assumed to be replaced at the median (B50) life, although other points in the blade life cycle may be used. A blade replaced after a predetermined amount of use is assumed to be replaced at the B5 life (i.e., 5% blade population fails before this life), although other points (e.g., B10, B12, B15, etc.) may be used. Additionally, examples of probabilities of metering failures are listed. The first of the final two columns lists a probability of a metering failure before the maintenance unit has reached end of life (EOL), which is the probability of the first blade failing before EOL. The last column is the probability of a failure sometime during the life of the maintenance unit. 
   
     
       
         
             
           
             
               TABLE 1 
             
           
          
             
                 
             
             
               Two blade maintenance unit life for all blade replacement strategy 
             
             
               combinations. 
             
          
         
         
             
             
             
             
          
             
                 
               Blade 
                 
                 
             
             
                 
               Replacement 
                 
               Maintenance unit 
             
             
                 
               Strategies 
               Expected Lives 
               Failure Prob. 
             
          
         
         
             
             
             
             
             
             
             
             
          
             
                 
               Blade 
                 
                 
                 
               Maintenance 
               Before 
                 
             
             
                 
               1 
               Blade 2 
               Blade 1 
               Blade 2 
               unit 
               EOL 
               At EOL 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
             
             
             
          
             
               1 
               Use 
               Use 
               B5 
               B5 
               2 B5 
                5% 
               9.75%  
             
             
               2 
               Use 
               RTF 
               B5 
               B50 
               B5 + B50 
                5% 
               100% 
             
             
               3 
               RTF 
               Use 
               B50 
               B5 
               B5 + B50 
               100% 
               100% 
             
             
               4 
               RTF 
               RTF 
               B50 
               B50 
               2 B50 
               100% 
               100% 
             
             
                 
             
          
         
       
     
   
   Example combination 1 in Table 1 has the shortest maintenance unit life of the exemplified combinations but the lowest probability of at least one metering failure. Example combination 4 has the longest maintenance unit life but has two metering failures. Running the first blade to failure and then stopping the second blade before failure typically yields little or no advantage; therefore, example combination 2 will typically be preferred to example combination 3. In a scenario where it is acceptable to end the life of the print cartridge with a metering blade failure, then the “before EOL” maintenance unit failure probabilities can be used for comparisons. In an example where, at end of life, the maintenance unit failure probability is desired to be 5%, then the blades in example combination 1 can to be replaced at the B2.5 life. 
   For a failure distribution with a predictable, sharp failure point (e.g., a high Weibull slope) example combination 1 may be an optimal choice. Although the maintenance unit life is short, the B5 and B50 lives are not significantly different. Trading off a small increase in maintenance unit life may be worth the large reduction in the probability of a metering failure. Such a replacement scheme can be desirable for customers who do not want to experience a single failures (e.g., the other three combination examples may have at least one failure). The remaining combination examples may be desirable for customers who are willing to trade off an occasional metering failure that is quickly remedied for much longer print cartridge life and lower run costs. 
   If the failure distribution is not predictable or sharp, then example combination 4 may be an optimal replacement scheme. For machines having replaceable blades with random failure modes, run-to-failure has been the traditional blade service strategy. For maintenance cartridge machines  10 , such blades would only be used in very short-life cartridges. Because failure of the metering blade typically requires replacement of the entire print cartridge, it is desirable that blades have higher reliability in longer life cartridges. 
   Long print cartridge life can be achieved when maintenance units containing multiple blades are used, as described herein. For example, after running the first blade to failure, a controller can replace a failed blade that achieves the desired blade replacement. Additionally or alternatively, the operator can inform a machine controller of the failure and the machine controller can automatically replace the failed metering blade. In another example, the machine senses a metering failure before it is apparent to the operator, and then automatically replaces the failed blade. In higher speed and higher print volume machines, reliability and optimal duty cycle are high customer priorities and can be facilitated by the replacement schemes described herein. 
   Table 2 lists examples of replacement strategy combinations for a three-blade maintenance unit. The results for a three blade maintenance unit are similar to those for a two blade maintenance unit. 
   
     
       
         
             
           
             
               TABLE 2 
             
           
          
             
                 
             
             
               Three blade maintenance unit life for all blade replacement strategy 
             
             
               combinations. 
             
          
         
         
             
             
             
             
          
             
                 
                 
                 
               Maintenance unit 
             
             
                 
               Blade Replacement 
               Expected Lives 
               Failure Prob. 
             
          
         
         
             
             
             
             
             
             
          
             
                 
               Strategies 
                 
               Maintenance 
               Before 
                 
             
          
         
         
             
             
             
             
             
             
             
             
             
             
          
             
                 
               Blade 1 
               Blade 2 
               Blade 3 
               Blade 1 
               Blade 2 
               Blade 3 
               unit 
               EOL 
               At EOL 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
             
             
             
             
             
          
             
               1 
               Use 
               Use 
               Use 
               B5 
               B5 
               B5 
               3 B5 
               9.75%  
               14.3%  
             
             
               2 
               Use 
               Use 
               RTF 
               B5 
               B5 
               B50 
               2 B5 + 
               9.75%  
               100% 
             
             
                 
                 
                 
                 
                 
                 
                 
               B50 
             
             
               3 
               RTF 
               Use 
               Use 
               B50 
               B5 
               B5 
               2 B5 + 
               100% 
               100% 
             
             
                 
                 
                 
                 
                 
                 
                 
               B50 
             
             
               4 
               Use 
               RTF 
               Use 
               B5 
               B50 
               B5 
               2 B5 + 
               100% 
               100% 
             
             
                 
                 
                 
                 
                 
                 
                 
               B50 
             
             
               5 
               RTF 
               RTF 
               Use 
               B50 
               B50 
               B5 
               B5 + 2 
               100% 
               100% 
             
             
                 
                 
                 
                 
                 
                 
                 
               B50 
             
             
               6 
               RTF 
               Use 
               RTF 
               B50 
               B5 
               B50 
               B5 + 2 
               100% 
               100% 
             
             
                 
                 
                 
                 
                 
                 
                 
               B50 
             
             
               7 
               Use 
               RTF 
               RTF 
               B5 
               B50 
               B50 
               B5 + 2 
               100% 
               100% 
             
             
                 
                 
                 
                 
                 
                 
                 
               B50 
             
             
               8 
               RTF 
               RTF 
               RTF 
               B50 
               B50 
               B50 
               3 B50 
               100% 
               100% 
             
             
                 
             
          
         
       
     
   
   Table 3 lists the replacement strategy combinations for an N-blade maintenance unit, where N is an integer. Three examples of blade replacement strategies are shown. 
   
     
       
         
             
           
             
               TABLE 3 
             
           
          
             
                 
             
             
               Multiple blade maintenance unit life for blade replacement strategies. 
             
          
         
         
             
             
             
             
          
             
                 
               Blade Replacement 
                 
               Maintenance unit 
             
             
                 
               Strategies 
               Expected Lives 
               Failure Prob. 
             
          
         
         
             
             
             
             
             
             
             
             
          
             
                 
               Blades 1 to 
                 
               Blades 1 to 
                 
               Maintenance 
               Before 
                 
             
             
                 
               n − 1 
               Blade n 
               n − 1 
               Blade n 
               unit 
               EOL 
               At EOL 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
             
             
             
          
             
               1 
               Use 
               Use 
               B5 
               B5 
               n B5 
               1 − 
               1 − (0.95) n   
             
             
                 
                 
                 
                 
                 
                 
               (0.95) n−1   
             
             
               2 
               Use 
               RTF 
               B5 
               B50 
               (n − 1) B5 + 
               1 − 
               100% 
             
             
                 
                 
                 
                 
                 
               B50 
               (0.95) n−1   
             
             
               3 
               RTF 
               RTF 
               B50 
               B50 
               n B50 
               100% 
               100% 
             
             
                 
             
          
         
       
     
   
   Table 4 lists the three examples of blade replacement strategies of Table 3, and the impact of failure sensing on whether or not these strategies will meet exemplary design requirement. For sensors that detect failures before they appear on prints, the run-to-failure replacement strategy enables long life, low run cost and no failures experienced by the customer. 
   
     
       
         
             
           
             
               TABLE 4 
             
           
          
             
                 
             
             
               Blade replacement strategy and customer requirements. 
             
          
         
         
             
             
             
          
             
               Blade Replacement 
                 
                 
             
             
               Strategy 
               No Failure Sensing 
               Failure Sensing 
             
             
                 
             
             
               All blades at B5 
               Customer willing to 
               Some benefit 
             
             
                 
               trade long life and low 
             
             
                 
               run cost for few 
             
             
                 
               failures 
             
             
               First blades at B5 &amp; last 
               Failure acceptable on 
               Some benefit 
             
             
               blade RTF 
               last blade 
             
             
               All blades RTF 
               Customer willing to 
               Acceptable to all 
             
             
                 
               trade failures for long 
               customers - long life &amp; 
             
             
                 
               life and low run cost 
               low run cost without 
             
             
                 
                 
               failures 
             
             
                 
             
          
         
       
     
   
     FIG. 9  is a graph  150  of expected maintenance unit lives with various blade replacement strategies for a typical metering blade material. As can be seen, the run-to-failure strategy provides the longest life for respective blades, while the B5 strategy exhibits shorter blade life with improved duty cycle (e.g., blades are replaced before they fail, thereby reducing system down-time). 
     FIG. 10  is a graph  160  illustrating the ratio of the run-to-failure replacement strategy life to the B5 replacement strategy life. Relative to  FIG. 9 , the graph  60  represents the plotted triangles divided by the plotted diamonds. In  FIG. 10 , however, the ratio is shown as a function of the Weibull slope and the number of blades in the maintenance unit. As the Weibull slope increases, blade failure becomes more predictable with a sharper failure onset. As a result, the difference between run-to-failure and B5 replacement strategies becomes smaller for larger Weibull slopes. As the number of blades in the maintenance unit increases, the ratio of run-to-failure replacement lives over B5 replacement lives increases, albeit at a diminishing rate. 
   The blade engagement apparatus  16  provides a compact blade arrangement which can effectively extend the useful life of the release agent apparatus. It is configured to allow simplified replacement of blades  20 ,  40 , etc. As the end of life of an operating blade is reached, the used blade is withdrawn from contact with the moving surface  12 , placed into a suspended non-operational position, and another second blade is placed into operation. The life of the blade engagement apparatus  16  between service intervals required for replacement of used blades is therefore extended with high reliability. 
   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. Also that 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.