Patent Publication Number: US-6990308-B1

Title: Image forming device, print cartridge and doctor blade assembly that reduce vibrations at doctoring media nip

Description:
TECHNICAL FIELD 
   The present invention relates generally to a means for eliminating vibrations that reduce print quality in an image forming device such as a copier, facsimile machine or laser printer. More specifically, the invention relates to a print cartridge having a doctor blade assembly that eliminates vibrations at the nip formed between the doctoring blade and a doctoring surface. Still more specifically, the invention relates to a bracket for use in a print cartridge having support and clamping surfaces for improving tolerances at the nip. 
   BACKGROUND 
   Image forming devices including copiers, laser printers, facsimile machines, and the like, include a drum having a rigid cylindrical surface that is coated along a defined length of its outer surface with a photoconductive material. The surface of the drum is charged to a uniform electrical potential and then selectively exposed to light in a pattern corresponding to an original image. Those areas of the photoconductive surface exposed to light are discharged thus forming a latent electrostatic image on the photoconductive surface. A developer material, such as toner, having an electrical charge such that the toner is attracted to the photoconductive surface is brought into contact with the photoconductive surface. The drum then rotates past an intermediate transfer medium where the toner is transferred onto the medium. A recording sheet, such as a blank sheet of paper, is then brought into contact with the intermediate transfer medium and the toner thereon is transferred to the recording sheet in the form of the latent electrostatic image. The recording sheet is then heated thereby permanently fusing the toner to it. In preparation for the next image forming cycle, the photoconductive surface is discharged and residual toner is removed. 
   Typically, the toner is stored in a toner reservoir adjacent to the drum. A doctor blade and developer roller is positioned between the toner reservoir and drum for controlling the amount of toner passed to the drum. A nip created between the doctor blade and the developer roller controls the amount of toner transferred to the drum. It is important that the doctor blade make uniform and consistent contact across the entire length of the developer roller. If the doctor blade has inconsistent pressure with the developer roller during the transfer, uneven toner amounts will be transferred to the drum resulting in inconsistent and unacceptable print quality. If too much toner is transferred to the drum, printing errors may occur such as blurred images, poor color, and toner particles deposited on the background areas. Conversely, if not enough toner is transferred to the drum, the images will be too light and difficult to see. 
   Thus, for toner to be developed in an electro-photographic process it must be doctored out in a thin uniform layer onto a developer roll. The uniformity with which it is doctored has a direct impact on the quality of the resulting print. A problem in maintaining consistent contact and pressure is the developer roller profile may be non-uniformity requiring that the doctor blade move inward and outward to track the surface of the developer roller. Additionally, it is vital that contact be maintained across the entire length of the doctor blade to ensure even print quality across the width of the image. It is advantageous for the member doing the doctoring to be able to conform to the surface of the developer roll to reduce the effect of the geometry variations in the roll and in the doctoring member itself. It has been established that this can be accomplished using a piece of polyester coated with a conductive urethane filled with silicon carbide, attached to a foam substrate, adhered to an extruded metal bar. 
   One issue resulting from the use of an extruded metal bar is that the foam is unconstrained along its length and will shift with the friction of the surface of the rotating developer roll. The doctoring media (abrasively coated polyester) also has a propensity to vibrate audibly due to the stick-slip action of running against the roll. This stick-slip activity is facilitated by the shifting of the unconstrained foam which causes the doctor blade to move back and forth resulting in small perturbations on the developer roller which translate into small velocity variations. This may show up on a printed page as a repeating pattern of light and dark lines that extend across the printed image. 
   At least one design is known to use a conductive caulk to provide the electrical bridge between the supporting metal bar and the electrically conductive doctoring media. The caulk requires additional assembly time and curing time and suffers from a potential failure mode if applied improperly or damaged during assembly in the cartridge. The tolerances on the geometric properties of the bar limit the accuracy with which the foam can be positioned. Extensive testing has shown that positioning of the doctoring nip closer to the leading edge of the doctoring surface will result in fewer print defects such as skid marks. 
   One proposed solution is to lessen the amount of force that the doctor blade exerts on the developer roller. However, the amount of force applied by the doctor blade controls the amount of toner transferred to the developer roller. If the force is decreased to prevent or decrease jitter, toner transfer may be adversely affected. Also, it has been determined that lesser biasing force may prevent the doctor blade from sticking and skipping along the developer roller, but may not prevent vibrations that result in jitter. Further, another cause of jitter may be the electrical force between the developer roller and the drum. Lessening the amount of biasing force does not substantially reduce or eliminate this cause of jitter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements, and in which: 
       FIG. 1  is a side view illustrating an image forming apparatus constructed according to one embodiment of the present invention; 
       FIG. 2  is a side view of a print cartridge including the developer assembly and cleaning assembly along with the doctor blade assembly according to one embodiment of the present invention; 
       FIG. 3  is a perspective view of the developer assembly showing the doctor blade assembly according to one embodiment of the present invention; and 
       FIG. 4  is a detailed diagram of the doctor blade assembly in perspective view according to the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates the basic elements of an image forming device having a doctor blade assembly according to the present invention. Specifically,  FIG. 1  is a diagram representative of a four cartridge color laser printer, denoted generally as  100 , which illustrates the essential components of an electro-photographic apparatus. It should be understood, however, that those skilled in the art will understand that the present invention is applicable to other types of image forming systems using toner for printing with a photoconductor. The printer, image forming apparatus or image forming device, generally designated  100 , includes a plurality of similar toner or print cartridges  110 ,  210 ,  310 , and  410 . Each print cartridge  110 ,  210 ,  310  and  410  is of a similar construction but is distinguished by the toner color contained therein. In the embodiment shown, the device  100  includes a black (K) cartridge  110 , a magenta (M) cartridge  210 , a cyan (C) cartridge  310 , and a yellow (Y) cartridge  410 . Each different color toner forms an individual image of a single color that is combined in layered fashion to create the final multi-colored image. 
   Each of the print cartridges  110 ,  210 ,  310  and  410  is substantially identical and includes a photoconductor, a development assembly, and a cleaning assembly. As the print cartridges  110 ,  210 ,  310  and  410  are identical except for the toner color, the print cartridge  110  and elements for forming black images will be described, with the other color image forming units being omitted for simplification. 
   A photosensitive drum  114  is generally cylindrically-shaped with at least one end that intermeshes with the image forming device drive gears to provide for a rotational force. The photosensitive drum  114  has a smooth surface for receiving an electrostatic charge over the surface as the photosensitive drum  114  rotates past charging device  116 . The photosensitive drum  114  uniformly rotates past a scanning laser  120  directed onto a selective portion of the photosensitive drum  114  surface forming an electrostatically latent image across the width of the photoconductor representative of the outputted image. The drive gears rotate the photosensitive drum  114  continuously so as to advance the photosensitive drum  114  a predefined distance between laser scans. This process continues as the entire image pattern is formed on the photoconductor surface. 
   After receiving the latent image, the photosensitive drum  114  rotates to a developer which has a toner bin, illustrated generally as  122  in  FIG. 1 , for housing the toner and a developer roller  124  for uniformly transferring toner to the  114  photoconductor. The toner is transferred from the toner bin  122  to the photosensitive drum  114  through a doctor blade nip formed between the developer roller  124  and a doctor blade assembly  121 . The toner is a fine powder usually constructed of plastic granules that are attracted and cling to the areas of the photosensitive drum  114  that have been discharged by the scanning laser  120 . 
   The photosensitive drum  114  next rotates past an adjacently-positioned intermediate transfer mechanism belt  500  (hereinafter, ITM belt) to which the toner is transferred from the photosensitive drum  114 . As illustrated in  FIG. 1 , the ITM belt  500  is endless and extends around a series of rollers adjacent to photoconductors  114 ,  214 ,  314  and  414 . The ITM belt  500  and each photosensitive drum  114 ,  214 ,  314 ,  414  are synchronized providing for the toner from each photosensitive drum  114 ,  214 ,  314  and  414  to precisely align on the ITM belt  500  during a single pass. By way of example as viewed in  FIG. 1 , the yellow toner will be placed on the ITM belt  500 , followed by cyan, magenta, and black. 
   After depositing the toner on the ITM belt  500 , the photosensitive drum  114  rotates through a cleaning area where residual toner is removed from the surface via a brush or scraper  126 . The residual toner is moved along the length of the photosensitive drum  114  to a waste toner reservoir. In one embodiment, the photosensitive drum  114  further passes through a discharge area (not shown) having a lamp or other light source for exposing the entire photoconductor surface to light to remove any residual charge and image pattern formed by the scanning laser  120 . 
   As the photoconductors  114 ,  214 ,  314  and  414  are being charged and gathering toner, a recording sheet, such as a blank sheet of paper, is being routed to intercept the ITM belt  500 . The paper may be placed in one of the trays  510 , or introduced into the image forming device through a side track tray  520 . A series of rollers and belts transport the paper to point Z where the sheet contacts the ITM belt  500  and receives the toner. The sheet may receive an electrostatic charge prior to contact with the ITM belt  500  to assist in attracting the toner from the ITM belt  500 . The sheet and attached toner next travel through a fusser  530  having a pair of rollers and a heating element that heats and fuses the toner to the sheet. The paper with fused image is then transported out of the printer  100  for receipt by a user. 
     FIG. 2  is a detailed diagram of print cartridge  110  which, as described, above is essentially the same as any one of print cartridges  210 ,  310  and  410  of  FIG. 1 . In general, print cartridge  110  constitutes the image-forming section of an image forming device, such as laser printer  100 . The photosensitive drum  114  rotates and receives a uniform charge by charging device  116  which exposes a light image onto the photosensitive drum  114  via an exposure section  119  to form a latent image on the photosensitive drum  114 . A toner image corresponding to the latent image is formed by developer drum  550 , thus making the image visible. After the toner image is transferred to the recording medium, toner remaining on the photosensitive drum  114  is removed by scraper  126  or other suitable cleaning means. These elements, including the photosensitive drum  114 , are housed inside a development assembly  560  and cleaning assembly  570  which together constitute a housing that form the enclosure of the print cartridge  110 . Each part of the print cartridge  110  is provided with a sealing member for preventing the toner from leaking. 
   As shown in  FIG. 2 , the development assembly  560  includes a toner well  562  for housing toner, and a rotary paddle toner feeding member  564 . The toner feeding member  564  is provided within the toner well  562  and rotates as indicated in the direction of arrow A to circulate toner within the toner well  562 . The developer roller  124  receives a thin toner layer on its surface (referred to as “doctoring surface”) as a result of its rotation and is pressed against the photosensitive drum  114 . The doctor blade assembly  121  is disposed adjacent the developer roller  124  to regulate the thickness of the toner layer formed on the doctoring surface. The electric charge is imparted to the toner by a biasing voltage on the doctor blade assembly  121 . An important aspect of the present invention is the specific configuration of the doctor blade assembly  121  which, as described more fully below, substantially eliminates vibrations that would otherwise reduce print quality by improving tolerances at the nip formed between doctoring blade assembly  121  and a doctoring surface. 
   Cleaning assembly  570  comprises a cleaning blade  572  positioned in contact with the surface of the photosensitive drum  114  for scraping off the toner remaining on the photosensitive drum  114 , a skimming seal  574 , positioned below the cleaning blade  572  arranged in weak contact with the surface of the photosensitive drum  114 , for retaining up the toner which has been scraped off, and a waste toner well  576  for storing the scraped-off waste toner. 
     FIG. 3  shows the development assembly  560  for a print cartridge, such as print cartridge  110 , in more detail. Image development components of development assembly  560  include the doctor blade assembly  121 , developer roller  124  and photosensitive drum  114 . A key aspect of the present invention is the use of bracket-shaped doctor blade assembly  121  that substantially eliminates vibrations that occur during the operation and/or movement (such as during shipment) of print cartridge  110 . Such vibrations are well known to reduce print quality. In general, doctor blade assembly  121  provides a support and clamping surface for improving tolerances at the nip formed between doctor blade assembly  121  and the doctoring surface (not shown) of the development roller  124 . 
   Referring to  FIG. 3 , a doctor blade spring  610  is attached to the developer assembly  560  is positioned above the doctor blade assembly  121  to maintain a continuous force for biasing the doctor blade assembly  121  against the developer roller  124 . The amount of force applied by the doctor blade spring  610  also controls the amount of toner transferred to the developer roller  124 . The drawings illustrate the doctor blade assembly  121  substantially perpendicular to the developer roller  124 , however, other orientations may also provide for transfer of proper toner amounts. The doctor blade spring  610  may contact the doctor blade assembly  121  at one or more locations along the length of the doctor blade assembly  121  to ensure an even and distributed force is applied across the entire width of the developer roller  124 . In one embodiment, the doctor blade spring  610  provides about 1400 grams of force to the developer roller  124 , although other configurations and variations are disclosed in U.S. Pat. No. 6,078,771, assigned to the Lexmark International, Inc., the assignee of the present application, which discloses various structures for an image forming apparatus and is incorporated by reference herein in its entirety. 
   Doctor blade assembly  121  includes upper lip members  620  which support doctor blade spring  610 . Being integral to the main body of the doctor blade assembly, the use of upper lip members  620  eliminates the need for additional support structures for the doctor blade spring  610 . 
     FIG. 4  shows the doctor blade assembly  121  according to the present invention in more detail. As shown, doctor blade assembly comprises a bracket  622  having length (L) to which at least one piece of foam  640  is attached. Preferably, bracket  622  is made from a single stamped metal piece to reduce the cost of manufacturing the doctor blade assembly  121  while providing sufficient stiffness and strength. The foam  640  is captured in both directions by the bracket  622  which serves to precisely locate the foam  640  as well as to keep it from shifting during running. The lip  625  under which the foam  640  is slipped helps retain the foam  640  thereby reducing the strength requirement and cost of the adhesive holding the foam to the bracket  622 . The lip  625  also touches the actual doctoring media surface  642 , which creates a conductive path between the metal bracket  622  and the doctoring media and eliminates the need for conductive caulk. The lip  625  also traps and dampens the media and stops the stick-slip vibration action and the resulting noise, which would otherwise occur when the cartridge  110  is running. 
   A recess  630  in the stamping along its length (L) also provides a pocket to install vibration dampening material (not shown) if necessary. The additional lip members  620  the top of the bracket  622  serves to retain the doctor blade spring  610 . This eliminates the need for using retainers or any other similar fastening device. Because the doctor blade assembly  121  is made using a stamping process instead of an extrusion process, the straightness tolerances on the bracket  622  can be tighter without requiring special operations. The bent geometry of the stamped bracket  622  also has the advantage of creating a part with a very high stiffness. This helps reduce the flexing of the doctor blade assembly  121  during running and during shipping. Finally, because the process of stamping is highly automated and uses little material, the overall cost of the doctor blade assembly  121  is reduced. 
   It should be understood that modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.