Patent Application
20110026976
The invention relates to electrographic image development, and more particularly to an apparatus and method for developing an electrostatic image using dry powder deposition including compensation for slippage.
Processes for developing electrographic images with a magnetic brush using dry toner are well known in the art and are used in many electrographic printers and copiers. One electrographic printer technology employs a photoconductive image member to which a uniform electrostatic charge is applied. The imaging member is selectively exposed to light to produce an electrostatic image on the photoconductive image member.
Electrographic printers frequently employ a dry powder process for developing an electrographic image that utilizes a developer having at least two components including magnetic carrier particles and toner particles. The electrostatically-charged toner particles are pigmented for producing the final image, while the carrier particles are magnetic particles that allow delivery of the toner using electric and magnetic fields. In an example process, the developer is deposited on an electrically biased rotating toning shell. The toning shell rotates the developer into proximity with an imaging member that is moving in a process direction. At a location where the imaging member and the toning shell are in closest proximity, referred to as the “toning nip”, the toner is transferred onto the electrostatic image on the imaging member to form a toner image. In the toning nip, the magnetic carrier component of the developer forms a “nap” consisting of chains of developer particles rising from the surface of the toning shell under the influence of a magnetic field applied in the toning nip. The nap height is maximal when the magnetic field from either a north or south pole is perpendicular to the toning shell. A magnetic core having magnetic poles directed towards an interior surface of the toning shell and rotating relative to the toning shell can be used to generate the magnetic field outside the toning shell and in the toning nip. Typically, adjacent magnetic poles in the magnetic core have opposite polarity and, accordingly, as the magnetic core rotates, the magnetic field also rotates so that the magnetic field at the surface of the toning shell rotates from a direction perpendicular to the toning shell to parallel to the toning shell.
As the magnetic core rotates, the magnetic carrier chains appear to flip end over end and walk on the surface of the toning shell. The direction of rotation of the carrier chains is opposite in sense to the direction of rotation of the magnetic core. If the magnetic core rotates clockwise, the magnetic field at the surface of the toning shell and the carrier chains rotate counterclockwise. The agitation of the carrier chains provides energy to free the toner particles to interact with the electrostatic field of the image member.
This invention is directed to an electrostatic printing method in which the toning shell and the magnetic core each rotate in a co-current direction with the imaging member such that the portion of the toning shell adjacent to the image development area moves in a process direction, and the magnetic core rotates in the same direction as the toning shell such that a the average developer bulk velocity (ADBV) of the developer on the toning shell is in the same direction and proportional to the photoconductor velocity. The invention is also directed to apparatus for producing an image using the inventive method, including compensation for slippage of developer on the toning shell. A variety of developers can be employed using the inventive method. An exemplary method comprises moving the imaging member in a process direction, moving the toning shell with a co-direction velocity through the a toning nip formed between the imaging member and the toning shell, and providing a rotating magnetic core inside the toning shell rotating in the same direction as the toning shell where the magnetic field vector at a portion on the toning shell rotates in the opposite sense as the toning shell.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.
The imaging member 15 is illustrated as a drum, and is made of a material capable of retaining an electrostatic image. Alternatively, the imaging member may have configurations other than a drum. For example the imaging member may be a sheet like film for receiving an image. When configured as a film, the imaging member 15 is relatively resilient and is held in a desired position relative to the toning shell 16. In a photoconductive process, the imaging member 15 is initially charged to a uniform imaging potential. The uniform electrostatic charge on the imaging member 15 is then discharged by performing an image-wise exposure of the imaging member to form the electrostatic image.
The imaging member 15 drum and the toning shell 16 form an area therebetween known as a toning nip 6. Developer is delivered to the toning shell 16 upstream (relative to the process direction) of the toning nip 6 using a metering skive 28.
When the developer is delivered to the toning shell, initially, the average velocity of the developer at the delivery point is greater than that of the developer on other parts of the toning shell. As a result, compressed developer builds up immediately upstream of the toning nip 6 creating a roll back zone. The imaging member 15 drum rotates so that the surface of the imaging member moves in a process direction through the toning nip 6.
The toning shell 16 is provided with a driver for rotating the toning shell so that the outer surface of the toning shell 16 moves through the toning nip 6. In
Although described in terms of a rotating magnetic core with multiple poles, the invention can be practiced with any arrangement that subjects the carrier particles of the developer to a magnetic field vector that rotates in space. In an alternative arrangement, the magnetic core 14 can comprise an array of fixed magnets and the magnetic field generated by the magnetic core is modulated or varied by a suitable source to produce magnetic pole transitions of alternating maxima in the developer. A magnetic core with individually rotating magnetic poles can be used. These means of changing the magnetic field establish a speed and direction of rotation for the magnetic field of the magnetic core.
The magnetic brush operates according to principles described in U.S. Pat. Nos. 6,959,162, 4,473,029 and 4,546,060, the contents of which are fully incorporated by reference as if set forth herein. The developer preferably is a two component developer including carrier particles and pigmented toning particles. The magnetic developer particles comprise a magnetic material exhibiting hard magnetic properties.
The direction of rotation of the toning shell 16 is said to be co-current with the imaging member 15 when the surface of the toning shell 16 moves through the toning nip in the same direction as the imaging member 15. In
The speed of rotation of the magnetic core 14, the geometry of the toning nip, and the process speed of the imaging member determine the number of pole transitions that are applied to the toner in the toning nip. For a magnetic core having 14 alternating poles rotating at 1100 RPM, the magnetic field transitions from N to S about 257 times per second (14*1100/60) as measured in the frame of reference of a stationary observer. For a 17.49 inches per second imaging member 12 speed and a toning nip 6 width of about 0.375 inches, each point on the imaging member 12 will be exposed to approximately 5 north to south pole transitions during development in the toning nip 6, where 5 pole transitions is calculated as (257*.375/17.49).
The developer is delivered to the toning shell from a reservoir 7 in the lower area of the printer using a feed roller 8.
As shown in
Mixers 4 in the reservoir 7 agitate to produce friction between components of the developer so that the magnetic carrier particles and the toner particles develop opposite charges in a triboelectric process, and the toner is mixed with the magnetic carrier particles. The motions of the imaging member 15, the toning shell 16, and the magnetic core bring toner into a development relationship with the electrostatic image on the imaging member 15, and create an image development area within the toning nip 6. Marking particles from the developer applied to the electrostatic image in the image development area generate a transferable electrographic image on the imaging member and the developer, depleted of toner particles used to develop the image on the imaging member 15, is removed from the toning shell 16 and returned to the reservoir 7.
A voltage source 30 is provided for placing a dc bias on the toning shell 16. Biasing the toning shell 16 relative to ground creates an electric field that attracts the toner particles to the toning shell 16 or to the imaging member 15. The electric field is at a maximum strength where the toning shell 16 is adjacent and closest to the imaging member 15. For example a bias voltage of −600 volts dc may be applied to the toning shell in a printing process where the initial imaging member 15 voltage is at −750 volts dc, and the voltage of exposed portions of the electrostatic image on imaging member 15 is −150 volts dc.
In an embodiment of the invention, the imaging member 15 is rotated to produce an imaging member 15 velocity in a process direction, and the toning shell 16 is rotated to produce a toning shell 16 surface velocity adjacent to the imaging member 15. Rotating the toning shell 16 co-currently produces a toning shell 16 velocity that is co-directional with the imaging member 15 velocity in the toning nip 6. The rotation brings toner applied to the toning shell 16 into a developing relationship with the imaging member 15 in the toning nip 6.
ADBV=(1−s)*[π*D*(Srpm/60)−γ*(2h*(N/2)*((Crpm−Srpm)/60)) (Equation 1)
is approximately equal to the photoconductor velocity; where s is the fraction of slippage shown in
As illustrated in
Alternatively the machine controller is used to increase a shell speed and a core speed such that average developer bulk velocity (ADBV) is greater than the photoconductor velocity when the toner shape and printing requirements require it, such as when using toner particles with very high slippage. In one embodiment where the core speeds and greater then such that there are over 246 pole flips per second the machine controller is optimized by tuning the average developer bulk velocity (ADBV) to be within a specific range from 50-100% of photoconductor velocity.
As illustrated in
Each flip, is accompanied by a circumferential step of by each particle in the chain 50 in a direction opposite the movement of the magnetic core. As shown in
On the other hand, when the toning shell 16 and the magnetic core 14 each rotate in the co-current direction, the chains 50 walk in the opposite direction from the direction of travel of the imaging member 15.
Each pole transition of the magnetic core 14 from a N pole to S pole produces 180 degrees (or π radians) of rotation of the magnetic field at a local point on the toning shell.
Rapid pole transitions generated by the magnetic core 14 create an energetic and vigorous movement of developer as the developer moves through the development zone. This vigorous action constantly provides energy for separating toner from the carrier chains to facilitate the application of fresh toner particles to the toning shell 16 and the imaging member 15.
The free ends of the magnetic carrier chains (i.e. the ends of the carrier chains away from the toning shell 16) travel in arcs in response to the rotation of the magnetic field of the magnetic core 14. In free space, or for a low friction toning shell, the preferred rotation mode is for the carrier chains 50 to flip or pivot around the center of the chain rather than about the non-free end of the toner chain. The non-free end of the carrier chain is adjacent the toning shell 16, where the attraction of the magnetic field of the magnetic core is greatest. For a given chain length and a given angular rotation speed for the chain, rotation about the center of a carrier chain involves a rotational energy that is one-quarter of the rotational energy for a chain flipping around an end of a carrier chain. Rotation about the center of the carrier chain has lower energy than rotation about the end of the carrier chain. If there is low friction between the carrier chain and the toning shell, slippage can occur.
Friction between the toning shell 16 and the developer particles is functionally related to characteristics of the developer particles and the toning shell surface. Toner particles may be generally spherical shaped, or may have non-spherical shapes. Non-spherical toner particles include raisin-shaped toner particles. A low friction combination may be produced with a smooth toning shell and spherical toning particles. Toning shells may be treated to provide a roughened surface, however the roughening steps add complexity to the manufacturing of toning shells which in turn adds to the cost of manufacturing a printing apparatus.
During developing of an electrographic image, developer is delivered to the toning shell 16 upstream of the toning nip 6. Ideally, the developer is distributed in a uniform layer on the toning shell 16 so that a high quality toner image results from development of the electrostatic latent image. The direction of rotation of the magnetic field influences the production of a uniform layer of developer by affecting the behavior of the magnetic carrier particles at the metering skive. In a preferred embodiment, the imaging member 15 drum and the toning shell 16 and the magnetic core 14 rotate co-currently. Using a spherically shaped toner particle with a toning shell having a smooth surface can result in slipping of the carrier chains on the toning shell when delivering the developer particles to the toning nip 6. The toning shell 16 is rotated in a co-current direction to allow approximate matching of the developer velocity to the imaging member velocity. In
Further, as described in U.S. Pat. No. 6,728,503 issued to Stelter, Guth, Mutze, and Eck, the contents of which are fully incorporated by references as if set forth herein, effective developing of electrostatic images occurs when the average developer bulk velocity is within preferred ranges relative to image member velocity, and preferably the average developer bulk velocity is substantially equal to the image member velocity. By using relatively high, co-current magnetic core speeds in combination with relatively high co-current shell rotation speeds, it is possible to match developer velocity to process speed (imaging member velocity) while achieving at least 5 magnetic pole flips in a narrow development nip.
For example an experiment was conducted using a two component developer including 4 μm and 6 μm, cyan spherical marking particles. A 14 pole feed roller using 900 gauss magnets was used with a metering skive. The toning shell had a nominal diameter of 2 inches. The metering skive was set to a gap of 0.035 inches to the toning shell. The stripping skive was set to a gap of approximately 0.005 inches to the toning shell.
When applying a magnetic core and toning shell rotation of 800 revolutions per minute (RPM) counter-clockwise (counter-current) and 82 RPM clockwise (co-current), the developer flowed unevenly unto the toning skive and dumped out of the developing station.
On the other hand, by rotating the magnetic core and the toning shell in co-current directions, set points under which the developer flows smoothly onto the toning shell 16 to produce high quality prints can be obtained.
For example, using the printing apparatus and toner described above, applying a magnetic core and toning shell rotation of 1000 revolutions per minute (RPM) clockwise (co-current) and 220 RPM clockwise (co-current), the developer using the generally spherical toner flowed evenly onto the toning shell and skived evenly at the metering skive 28 and the take off skive 26.
By using co-current rotation of the toning shell and the magnetic core as in the present invention, as illustrated in
Table 1 below provides experimental data obtained for a 110 PPM (pages per minute) process running at approximately 18.56 inches per second employing generally spherical toner and a co-current magnetic core rotation. The metering skive was set to 0.035 inches, the take off skive was set to 0.005 inches, and the developer contained generally spherical toner. Examples of such toner can be found in the commonly assigned application U.S. Ser. No. 12/342,138 entitled: METHOD OF PREPARING TONER HAVING CONTROLLED MORPHOLOGY, filed on Dec. 23, 2008. In a magnetic brush development system, development efficiency in percent, as defined in U.S. Pat. No. 6,723,481, is the potential difference between the photoreceptor in developed image areas before and after development divided by the potential difference between the photoreceptor and the brush prior to development times 100. For example, in a discharged area development configuration, if the photoreceptor film voltage is −50 volts and the magnetic brush is −450 volts, the potential difference is 400 volts prior to development. If, during development by negatively-charged toner, the film voltage is increased by 200 volts to −250 volts in image areas by the deposition of negatively charged toner particles, the development efficiency is (200 volts divided by 400 volts) times 100, which gives an efficiency of development of 50 percent.
Table 2 below provides experimental data obtained for a 110 PPM process employing raisin-shaped toner and a co-current magnetic core rotation, except for the last two lines, for which counter-current magnetic core rotation was used. Countercurrent core rotation relative to shell rotation is indicated by a minus sign. The metering skive was set to 0.046 inches to obtain comparable developer flow rates at magnetic core speed of 700 RPM.
The last two lines of Table 2 represent setpoints used in commercial printers running at 70 PPM and proportional speedup of those setpoints.
The data in Table 1 for spherical toner and the corresponding data in Table 2 for raisin toner show that spherical toner can be developed with greater toning efficiency than raisin toner using co-current core rotation, despite not being able to be fed past the metering skive or developed at all with counter-current core rotation. From Tables 1 and 2, development efficiency for spherical toner is greater than development efficiency for raisin toner at the same conditions. Development efficiency for both types of toner with co-current core rotation generally increases with core speed and with developer flow rate, which are related. Assuming no slippage for raisin toner at the lowest shell speeds, for reasonable assumptions of 50% excess free volume fraction and 0.050 inch developer chain length, the average developer bulk velocity is calculated to be approximately 14.7 inches per second using Equation 1. As shell speed increases above 170 RPM, for acceptable images made at approximately the same average bulk developer velocity, approximately 30% slippage apparently occurs. At 452 RPM, approximately 60% slippage occurs. The slippage follows the behavior of Line 3 in
At steps 820-830, the toning shell is rotated in a co-current direction such that the portion of the toning shell adjacent to the imaging member moves in the same direction as the imaging member. Toner is applied to the toning shell upstream of the development area so that toning shell rotation brings the developer into a development relationship with the electrostatic image. A magnetic field is generated having a direction of rotation opposite in sense to the direction of rotation of the toning shell by rotating the magnetic core co-current with the toning shell, and with a rotation speed sufficient to generate an effective number of magnetic pole transitions (e.g. N-to S or S to N alternations) on each portion of the electrostatic image during passage of the electrostatic image through the development area. In an embodiment of the invention, the average developer bulk velocity through the development area is substantially the same as the velocity of the imaging member. A process controller can be used to change toning core and magnetic core rotational speeds to obtain acceptable image quality as represented by steps 840 and 850.
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.
