Magnetic roll having a smoothed release pole for a dual component development electrophotographic image forming device

A magnetic roll for transporting a dual component developer mix according to one embodiment includes a core having at least one permanent magnet that has a plurality of circumferentially spaced magnetic poles generating a magnetic field. A cylindrical sleeve is positioned around the core. The sleeve is rotatable relative to the core about a rotational axis in an operative rotational direction. At portions of the magnetic roll positioned axially inward from axial ends of the core, a magnitude of a total magnetic field strength of the magnetic field decreases by 1.5 mT/degree or less in the operative rotational direction at a radius of 0.5 mm beyond an outer circumferential surface of the sleeve throughout an area of ±15 degrees from an angular position of the magnetic roll at which a tangential component of the magnetic field is equal to zero at a release pole of the plurality of magnetic poles.

CROSS REFERENCES TO RELATED APPLICATIONS

BACKGROUND

1. Field of the Disclosure

The present disclosure relates generally to image forming devices and more particularly to a magnetic roll having a smoothed release pole for a dual component development electrophotographic image forming device.

2. Description of the Related Art

Dual component development electrophotographic image forming devices include one or more reservoirs that store a mixture of toner and magnetic carrier beads (the “developer mix”). Toner is electrostatically attracted to the carrier beads as a result of triboelectric interaction between the toner and the carrier beads. A magnetic roll includes a stationary core having one or more permanent magnets and a sleeve that rotates around the core. The permanent magnet(s) produce a series of magnetic poles that are circumferentially spaced around the outer surface of the sleeve. The magnetic poles attract the carrier beads in the reservoir having toner thereon to the outer surface of the sleeve, which transports the developer mix as the sleeve rotates. A photoconductive drum is charged by a charge roll to a predetermined voltage and a laser selectively discharges areas on the surface of the photoconductive drum to form a latent image on the surface of the photoconductive drum. The sleeve of the magnetic roll carries the developer mix in close proximity to the photoconductive drum. The sleeve is electrically biased to facilitate the transfer of toner from the chains of developer mix on the outer surface of the sleeve to the discharged areas on the surface of the photoconductive drum forming a toner image on the surface of the photoconductive drum. The photoconductive drum then transfers the toner image, directly or indirectly, to a media sheet forming a printed image on the media sheet. Developer mix on the outer surface of the sleeve that is not transferred to the photoconductive drum is transported by the sleeve back to the reservoir. After the remaining developer mix reenters the reservoir, the developer mix is no longer magnetically retained against the outer surface of the sleeve allowing the developer mix to release from the sleeve back into the reservoir.

It is desired for the magnetic poles to be configured to facilitate pick up of the developer mix from the reservoir, transfer of toner from the developer mix on the magnetic roll to the photoconductive drum and release of the developer mix back into the reservoir.

SUMMARY

A magnetic roll for transporting a developer mix that includes magnetic carrier beads and toner in a dual component development electrophotographic image forming device according to one example embodiment includes a core having at least one permanent magnet. The permanent magnet has a plurality of circumferentially spaced magnetic poles generating a magnetic field. The plurality of magnetic poles includes a release pole. A cylindrical sleeve is positioned around the core. The sleeve is rotatable relative to the core about an axis of rotation in an operative rotational direction. The release pole is positioned to magnetically attract developer mix to an outer circumferential surface of the sleeve and thereby transport the developer mix on the outer circumferential surface of the sleeve in the operative rotational direction when the sleeve rotates relative to the core to a point where a magnitude of a total magnetic field strength of the magnetic field falls below 15 mT. At portions of the magnetic roll positioned axially inward from axial ends of the core where an axial component of the magnetic field is <1 mT, the magnitude of the total magnetic field strength of the magnetic field decreases by 1.5 mT/degree or less in the operative rotational direction at a radius of 0.5 mm radially beyond the outer circumferential surface of the sleeve throughout an area of ±15 degrees from an angular position of the magnetic roll at which a tangential component of the magnetic field is equal to zero at the release pole. In some embodiments, the magnitude of the total magnetic field strength of the magnetic field decreases by 1.3 mT/degree or less, or 1.0 mT/degree or less.

A developer unit for dual component development electrophotographic image forming device according to one example embodiment includes a housing having a reservoir for storing a developer mix that includes toner and magnetic carrier beads. The developer unit includes a magnetic roll that includes a stationary core and a cylindrical sleeve positioned around the core. The core includes at least one permanent magnet having a plurality of circumferentially spaced magnetic poles generating a magnetic field. The plurality of magnetic poles includes a release pole. The sleeve is rotatable relative to the core about an axis of rotation in an operative rotational direction. An outer circumferential surface of the sleeve is positioned to transport developer mix magnetically attracted from the reservoir to the outer surface of the sleeve by the magnetic field in the operative rotation direction. The release pole is positioned to magnetically attract developer mix to the outer circumferential surface of the sleeve to transport the developer mix on the outer circumferential surface of the sleeve in the operative rotational direction to a point where the developer mix is released from the outer circumferential surface of the sleeve into the reservoir. At portions of the magnetic roll positioned axially inward from axial ends of the core where an axial component of the magnetic field is <1 mT, a magnitude of a total magnetic field strength of the magnetic field decreases by 1.5 mT/degree or less in the operative rotational direction at a radius of 0.5 mm radially beyond the outer circumferential surface of the sleeve throughout an area of ±15 degrees from an angular position of the magnetic roll at which a tangential component of the magnetic field is equal to zero at the release pole. In some embodiments, the magnitude of the total magnetic field strength of the magnetic field decreases by 1.3 mT/degree or less, or 1.0 mT/degree or less.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying drawings where like numerals represent like elements. The embodiments are described in sufficient detail to enable those skilled in the art to practice the present disclosure. It is to be understood that other embodiments may be utilized and that process, electrical and mechanical changes, etc., may be made without departing from the scope of the present disclosure. Examples merely typify possible variations. Portions and features of some embodiments may be included in or substituted for those of others. The following description, therefore, is not to be taken in a limiting sense and the scope of the present disclosure is defined only by the appended claims and their equivalents.

Referring now to the drawings and more particularly toFIG. 1, there is shown a block diagram depiction of an imaging system20according to one example embodiment. Imaging system20includes an image forming device100and a computer30. Image forming device100communicates with computer30via a communications link40. As used herein, the term “communications link” generally refers to any structure that facilitates electronic communication between multiple components and may operate using wired or wireless technology and may include communications over the Internet.

In the example embodiment shown inFIG. 1, image forming device100is a multifunction machine (sometimes referred to as an all-in-one (AIO) device) that includes a controller102, a print engine110, a laser scan unit (LSU)112, one or more toner bottles or cartridges200, one or more imaging units300, a fuser120, a user interface104, a media feed system130and media input tray140and a scanner system150. Image forming device100may communicate with computer30via a standard communication protocol, such as, for example, universal serial bus (USB), Ethernet or IEEE 802.xx. Image forming device100may be, for example, an electrophotographic printer/copier including an integrated scanner system150or a standalone electrophotographic printer.

Controller102includes a processor unit and associated memory103. The processor may include one or more integrated circuits in the form of a microprocessor or central processing unit and may be formed as one or more Application Specific Integrated Circuits (ASICs). Memory103may be any volatile or non-volatile memory or combination thereof, such as, for example, random access memory (RAM), read only memory (ROM), flash memory and/or non-volatile RAM (NVRAM). Alternatively, memory103may be in the form of a separate electronic memory (e.g., RAM, ROM, and/or NVRAM), a hard drive, a CD or DVD drive, or any memory device convenient for use with controller102. Controller102may be, for example, a combined printer and scanner controller.

In the example embodiment illustrated, controller102communicates with print engine110via a communications link160. Controller102communicates with imaging unit(s)300and processing circuitry301on each imaging unit300via communications link(s)161. Controller102communicates with toner cartridge(s)200and processing circuitry201on each toner cartridge200via communications link(s)162. Controller102communicates with fuser120and processing circuitry121thereon via a communications link163. Controller102communicates with media feed system130via a communications link164. Controller102communicates with scanner system150via a communications link165. User interface104is communicatively coupled to controller102via a communications link166. Processing circuitry121,201,301may include a processor and associated memory, such as RAM, ROM, and/or NVRAM, and may provide authentication functions, safety and operational interlocks, operating parameters and usage information related to fuser120, toner cartridge(s)200and imaging units300, respectively. Controller102processes print and scan data and operates print engine110during printing and scanner system150during scanning.

Computer30, which is optional, may be, for example, a personal computer, including memory32, such as RAM, ROM, and/or NVRAM, an input device34, such as a keyboard and/or a mouse, and a display monitor36. Computer30also includes a processor, input/output (I/O) interfaces, and may include at least one mass data storage device, such as a hard drive, a CD-ROM and/or a DVD unit (not shown). Computer30may also be a device capable of communicating with image forming device100other than a personal computer, such as, for example, a tablet computer, a smartphone, or other electronic device.

In the example embodiment illustrated, computer30includes in its memory a software program including program instructions that function as an imaging driver38, e.g., printer/scanner driver software, for image forming device100. Imaging driver38is in communication with controller102of image forming device100via communications link40. Imaging driver38facilitates communication between image forming device100and computer30. One aspect of imaging driver38may be, for example, to provide formatted print data to image forming device100, and more particularly to print engine110, to print an image. Another aspect of imaging driver38may be, for example, to facilitate the collection of scanned data from scanner system150.

In some circumstances, it may be desirable to operate image forming device100in a standalone mode. In the standalone mode, image forming device100is capable of functioning without computer30. Accordingly, all or a portion of imaging driver38, or a similar driver, may be located in controller102of image forming device100so as to accommodate printing and/or scanning functionality when operating in the standalone mode.

FIG. 2illustrates a schematic view of the interior of an example image forming device100. For purposes of clarity, the components of only one of the imaging units300are labeled inFIG. 2. Image forming device100includes a housing170having a top171, bottom172, front173and rear174. Housing170includes one or more media input trays140positioned therein. Trays140are sized to contain a stack of media sheets. As used herein, the term media is meant to encompass not only paper but also labels, envelopes, fabrics, photographic paper or any other desired substrate. Trays140are preferably removable for refilling. A media path180extends through image forming device100for moving the media sheets through the image transfer process. Media path180includes a simplex path181and may include a duplex path182. A media sheet is introduced into simplex path181from tray140by a pick mechanism132. In the example embodiment shown, pick mechanism132includes a roll134positioned at the end of a pivotable arm136. Roll134rotates to move the media sheet from tray140and into media path180. The media sheet is then moved along media path180by various transport rollers. Media sheets may also be introduced into media path180by a manual feed138having one or more rolls139.

In the example embodiment shown, image forming device100includes four toner cartridges200removably mounted in housing170in a mating relationship with four corresponding imaging units300, which may also be removably mounted in housing170. Each toner cartridge200includes a reservoir202for holding toner and an outlet port in communication with an inlet port of its corresponding imaging unit300for transferring toner from reservoir202to imaging unit300. Toner is transferred periodically from a respective toner cartridge200to its corresponding imaging unit300in order to replenish the imaging unit300. In the example embodiment illustrated, each toner cartridge200is substantially the same except for the color of toner contained therein. In one embodiment, the four toner cartridges200include yellow, cyan, magenta and black toner.

Image forming device100utilizes what is commonly referred to as a dual component development system. Each imaging unit300includes a reservoir302that stores a mixture of toner and magnetic carrier beads. The carrier beads may be coated with a polymeric film to provide triboelectric properties to attract toner to the carrier beads as the toner and the carrier beads are mixed in reservoir302. Reservoir302and a magnetic roll306collectively form a developer unit. Each imaging unit300also includes a charge roll308and a photoconductive (PC) drum310and a cleaner blade or roll (not shown) that collectively form a PC unit. PC drums310are mounted substantially parallel to each other when the imaging units300are installed in image forming device100. In the example embodiment illustrated, each imaging unit300is substantially the same except for the color of toner contained therein.

Each charge roll308forms a nip with the corresponding PC drum310. During a print operation, charge roll308charges the surface of PC drum310to a specified voltage, such as, for example, −1000 volts. A laser beam from LSU112is then directed to the surface of PC drum310and selectively discharges those areas it contacts to form a latent image. In one embodiment, areas on PC drum310illuminated by the laser beam are discharged to approximately −300 volts. Magnetic roll306attracts the carrier beads in reservoir302having toner thereon to magnetic roll306through the use of magnetic fields and transports the toner to the corresponding PC drum310. Electrostatic forces from the latent image on PC drum310strip the toner from the carrier beads to form a toner image on the surface of PC drum310.

An intermediate transfer mechanism (ITM)190is disposed adjacent to the PC drums310. In this embodiment, ITM190is formed as an endless belt trained about a drive roll192, a tension roll194and a back-up roll196. During image forming operations, ITM190moves past PC drums310in a clockwise direction as viewed inFIG. 2. One or more of PC drums310apply toner images in their respective colors to ITM190at a first transfer nip197. In one embodiment, a positive voltage field attracts the toner image from PC drums310to the surface of the moving ITM190. ITM190rotates and collects the one or more toner images from PC drums310and then conveys the toner images to a media sheet at a second transfer nip198formed between a transfer roll199and ITM190, which is supported by back-up roll196. The cleaner blade/roll removes any toner remnants on PC drum310so that the surface of PC drum310may be charged and developed with toner again.

A media sheet advancing through simplex path181receives the toner image from ITM190as it moves through the second transfer nip198. The media sheet with the toner image is then moved along the media path180and into fuser120. Fuser120includes fusing rolls or belts122that form a nip to adhere the toner image to the media sheet. The fused media sheet then passes through exit rolls126located downstream from fuser120. Exit rolls126may be rotated in either forward or reverse directions. In a forward direction, exit rolls126move the media sheet from simplex path181to an output area128on top171of image forming device100. In a reverse direction, exit rolls126move the media sheet into duplex path182for image formation on a second side of the media sheet.

While the example image forming device100shown inFIG. 2illustrates four toner cartridges200and four corresponding imaging units300, it will be appreciated that a monocolor image forming device100may include a single toner cartridge200and corresponding imaging unit300as compared to a color image forming device100that may include multiple toner cartridges200and imaging units300. Further, although image forming device100utilizes ITM190to transfer toner to the media, toner may be applied directly to the media by the one or more photoconductive drums310as is known in the art. In addition, toner may be transferred directly from each toner cartridge200to its corresponding imaging unit300or the toner may pass through an intermediate component, such as a chute, duct or hopper, that connects the toner cartridge200with its corresponding imaging unit300.

Imaging unit(s)300may be replaceable in any combination desired. For example, in one embodiment, the developer unit and PC unit are provided in separate replaceable units from each other. In another embodiment, the developer unit and PC unit are provided in a common replaceable unit. In another embodiment, toner reservoir202is provided with the developer unit instead of in a separate toner cartridge200. For a color image forming device100, the developer unit and PC unit of each color toner may be separately replaceable or the developer unit and/or the PC unit of all colors (or a subset of all colors) may be replaceable collectively as desired.

FIGS. 3 and 4show a developer unit320according to one example embodiment. Developer unit320includes a housing322having reservoir302therein. In some embodiments, housing322includes a lid324mounted on a base326. Lid324may be attached to base326by any suitable construction including, for example, by fasteners (e.g., screws328), adhesive and/or welding. Alternatively, lid324may be formed integrally with base326. In the example embodiment illustrated, base326includes a top portion326aattached (e.g., by fasteners, adhesive and/or welding) to a lower portion326b. Alternatively, top portion326aof base326may be formed integrally with lower portion326bof base326. Housing322extends generally along an axial dimension307of magnetic roll306from a first side330of housing322to a second side331of housing322. Side330leads during insertion of developer unit320into image forming device100and side331trails. A portion of magnetic roll306is exposed from reservoir302at a front332of housing322. A handle336is optionally positioned on a rear333of housing322to assist with separating developer unit320from the corresponding PC unit. Housing322also includes a top334and a bottom335.

Reservoir302holds the mixture of toner and magnetic carrier beads (the “developer mix”). Developer unit320includes an inlet port338in fluid communication with reservoir302and positioned to receive toner from toner cartridge200to replenish reservoir302when the toner concentration in reservoir302relative to the amount of carrier beads remaining in reservoir302gets too low as toner is consumed from reservoir302by the printing process. In the example embodiment illustrated, inlet port338is positioned on top334of housing322near side330; however, inlet port338may be positioned at any suitable location on housing322.

Reservoir302includes one or more agitators to stir and move the developer mix. For example, in the embodiment illustrated, reservoir302includes a pair of augers340a,340b. Augers340a,340bare arranged to move the developer mix in opposite directions along the axial length of magnetic roll306. For example, auger340ais positioned to incorporate toner from inlet port338and to move the developer mix away from side330and toward side331. Auger340bis positioned to move the developer mix away from side331, toward side330and in proximity to the bottom of magnetic roll306. This arrangement of augers340a,340bis sometimes informally referred to as a racetrack arrangement because of the circular path the developer mix in reservoir302takes when augers340a,340brotate.

With reference toFIG. 4, magnetic roll306includes a core342that includes one or more permanent magnets and that does not rotate relative to housing322. A cylindrical sleeve344encircles core342and extends along the axial length of magnetic roll306. In one embodiment, a shaft346passes through the center of core342and defines an axis of rotation347of magnetic roll306. Shaft346is fixed, i.e., shaft346does not rotate with sleeve344relative to housing322, and controls the position of core342relative to sleeve344and to the other components of developer unit320. With reference back toFIG. 3, a rotatable end cap345is positioned at one axial end of magnetic roll306, referred to as the drive side of magnetic roll306. End cap345is coupled to sleeve344such that rotation of end cap345causes sleeve344to rotate around core342. Sleeve344rotates in a clockwise direction as viewed inFIG. 4to transport the developer mix from reservoir302to PC drum310. A drive coupler350is operatively connected to end cap345either directly, such as on an end of a shaft349that extends axially outward from end cap345as shown in the example embodiment illustrated, or indirectly. Drive coupler350is positioned to receive rotational force from a corresponding drive coupler in image forming device100when developer unit320is installed in image forming device100. Any suitable drive coupler350may be used as desired, such as a spur gear or a drive coupler that receives rotational force at its axial end. In one embodiment, augers340a,340bare operatively connected to drive coupler350by one or more intermediate gears (not shown). Alternatively, augers340a,340bmay be driven independently of drive coupler350and sleeve344by a second drive coupler positioned to receive rotational force from a corresponding drive coupler in image forming device100when developer unit320is installed in image forming device100.

With reference toFIGS. 4 and 5, the permanent magnet(s) of core342produce a series of circumferentially spaced, alternating (south v. north) magnetic poles that facilitate the transport of developer mix to PC drum310as sleeve344rotates.FIG. 5shows the magnetic field lines generated by the magnetic poles of core342according to one example embodiment. Core342includes a pickup pole351positioned near the bottom of core342(near the 6 o'clock position of core342as viewed inFIG. 5). Pickup pole351magnetically attracts developer mix in reservoir302to the outer surface of sleeve344. The magnetic attraction from core342causes the developer mix to form cone or bristle-like chains that extend from the outer surface of sleeve344along the magnetic field lines.

After the developer mix is picked up at pickup pole351, as sleeve344rotates, the developer mix on sleeve344advances toward a trim bar312. Trim bar312is positioned in close proximity to the outer surface of sleeve344. Trim bar312trims the chains of developer mix as they pass to a predetermined average height defined by a trim bar gap314formed between trim bar312and the outer surface of sleeve344in order to control the mass of developer mix on the outer surface of sleeve344. Trim bar gap314dictates how much developer mix is allowed to pass on the outer surface of sleeve344from reservoir302toward PC drum310. Trim bar312may be magnetic or non-magnetic and may take a variety of different shapes including having a flat or rounded trimming surface. Trim bar312may be electrically biased to aid in trimming the chains of developer mix. Core342includes a trim pole352positioned at trim bar312to stand the chains of developer mix up on sleeve344in a generally radial orientation for trimming by trim bar312. As shown inFIG. 5, between pickup pole351and trim pole352, the chains of developer mix on sleeve344have a primarily tangential (as opposed to radial) orientation relative to the outer surface of sleeve344according to the magnetic field lines between pickup pole351and trim pole352.

As sleeve344rotates further, the developer mix on sleeve344passes in close proximity to the outer surface of PC drum310. As discussed above, electrostatic forces from the latent image formed on PC drum310by the laser beam from LSU112strip the toner from the carrier beads to form a toned image on the surface of PC drum310. Core342includes a developer pole353positioned at the point where the outer surface of sleeve344passes in close proximity to the outer surface of PC drum310to once again stand the chains of developer mix up on sleeve344in a generally radial orientation to promote the transfer of toner from sleeve344to PC drum310. The developer mix is less dense and less coarse when the chains of developer mix are stood up in a generally radial orientation than it is when the chains are more tangential. As a result, less wear occurs on the surface of PC drum310from contact between PC drum310and the chains of developer mix when the chains of developer mix on sleeve344are in a generally radial orientation.

As sleeve344continues to rotate, the remaining developer mix on sleeve344, including the toner not transferred to PC drum310and the carrier beads, is carried by magnetic roll306past PC drum310and back toward reservoir302. Core342includes a transport pole354positioned past the point where the outer surface of sleeve344passes in close proximity to the outer surface of PC drum310. Transport pole354magnetically attracts the remaining developer mix to sleeve344to prevent the remaining developer mix from migrating to PC drum310or otherwise releasing from sleeve344. As sleeve344rotates further, the remaining developer mix passes under lid324and is carried back to reservoir302by magnetic roll306. Core342includes a release pole355positioned near the top of core342along the direction of rotation of sleeve344. Release pole355magnetically attracts the remaining developer mix to sleeve344as the developer mix is carried the remaining distance to the point where it is released back into reservoir302. As the remaining developer mix passes the 2 o'clock position of core342as viewed inFIG. 5, the developer mix is no longer magnetically retained against sleeve344by core342allowing the developer mix to fall via gravity and centrifugal force back into reservoir302.

FIG. 6is a graph illustrating the magnetic profile of core342of magnetic roll306according to one example embodiment.FIG. 6is a graph of the magnetic field strength (in milli-tesla (mT)) versus angular position (in degrees) at an axial point on magnetic roll306positioned axially inward from the axial ends of core342, where the axial component of the magnetic field is near zero, e.g., <1 mT.FIG. 6shows the magnetic field strength values at the outer surface of sleeve344. The 0 degree position on the x-axis ofFIG. 6corresponds to the horizontal radius of core342(labeled “x” inFIG. 5) that faces away from PC drum310when developer unit320is in its operating position. The angular position values listed on the x-axis ofFIG. 6increase in a direction counter to the rotational direction of sleeve344(counter-clockwise as viewed inFIGS. 4 and 5). The poles351-355of magnetic roll306are labeled inFIG. 6with release pole355positioned to the far left, closest to 0 degrees, and pickup pole351positioned to the far right, closest to 360 degrees. Three lines are included inFIG. 6showing the radial component of the magnetic field (Br), the tangential component of the magnetic field (Bt) and the magnitude of the total magnetic field strength (|B|). As shown inFIG. 6, poles351-355alternate north and south in polarity.

The total magnetic field strength is highest near developer pole353in order to strongly attract the magnetic carrier beads to sleeve344and reduce the occurrence of magnetic carrier beads releasing from sleeve344to PC drum310during the transfer of toner from the developer mix on sleeve344to PC drum310. Generally, the magnitude of the total magnetic field strength must decrease significantly in the direction of rotation of sleeve344from developer pole353to release pole355(e.g., to below 10-15 mT) in order for the magnetic carrier beads to separate from sleeve344and release back into reservoir302.

It was observed that where the magnitude of the total magnetic field strength decreases too abruptly in the direction of rotation of sleeve344near release pole355, a magnetic gradient force is generated on the developer mix opposite in direction to the rotation of sleeve344, toward the higher magnetic field magnitude. When the magnetic field lines have a radial or mostly radial orientation, such as in the area of release pole355, the developer mix tends to form individual bristle-like chains that stand apart from each other as a result of mutual magnetic repulsion. The open spaces between the chains make it possible for the chains to undergo retrograde motion, moving opposite the rotation of sleeve344, as a result of the magnetic gradient force. Sliding friction between the developer mix and the surface of sleeve344as the developer mix moves opposite the rotation of sleeve344increases the wear on the magnetic carrier beads and toner particles which, in turn, may reduce the useful life of developer unit320. Further, movement of developer mix opposite the rotation of sleeve344can result in the accumulation of developer mix near release pole355. The accumulation of developer mix near release pole355may tend to knock the chains of developer mix near release pole355off of sleeve344causing the developer mix to spray from sleeve344against the inner surface of lid324instead of allowing the developer mix to smoothly release from sleeve344and drop back into reservoir302. The spray of developer mix against lid324increases the risk of leakage and may result in additional wear on the magnetic carrier beads and toner particles.

In contrast, it was observed that gradual and consistent reduction of the magnitude of the total magnetic field strength in the direction of rotation of sleeve344near release pole355reduces the occurrence of retrograde motion of developer mix near release pole355thereby reducing the wear on the magnetic carrier beads and toner particles and the spray of developer mix. Accordingly, at the portions of magnetic roll306positioned axially inward from the axial ends of core342where the axial component of the magnetic field is near zero, the magnitude of the total magnetic field strength of core342of the present disclosure decreases by 1.5 mT/degree or less in the direction of rotation of sleeve344at a radius of 0.5 mm radially beyond the outer surface of sleeve344throughout the area of release pole355, which includes ±15 degrees from the angular position of magnetic roll306at which the tangential component of the magnetic field is equal to zero at release pole355. In some embodiments, at the portions of magnetic roll306positioned axially inward from the axial ends of core342, the magnitude of the total magnetic field strength of core342decreases by 1.3 mT/degree or less, or 1.0 mT/degree or less, in the direction of rotation of sleeve344at a radius of 0.5 mm radially beyond the outer surface of sleeve344throughout the area of release pole355. Theoretically, there is no lower limit on the magnitude of the rate of change of the magnitude of the total magnetic field strength near release pole355; however, the magnitude of the total magnetic field strength near release pole355should decrease rapidly enough to provide a region of near zero magnetic field strength between release pole355and pickup pole351.

FIGS. 7A-14Billustrate eight example magnetic profiles (Example Magnetic Profiles #1-8) of magnetic rolls that were tested and analyzed for retrograde motion of developer mix near their release poles.FIGS. 7A, 8A, 9A, 10A, 11A, 12A, 13A and 14Aare graphs of the magnetic field strength (in mT) versus angular position (in degrees) near the release pole for each magnetic profile at a position axially inward from the axial ends of the core, where the axial component of the magnetic field is near zero.FIGS. 7A, 8A, 9A, 10A, 11A, 12A, 13A and 14Ashow the magnetic field strength values at a radius of 0.5 mm beyond the outer surface of the sleeve of the magnetic roll. Chains of developer mix formed on the outer surface of the sleeve are typically 1.0 mm-1.4 mm in length when the chains stand up radially on the outer surface of the sleeve. Accordingly, a radius of 0.5 mm beyond the outer surface of the sleeve represents roughly the middle of the chains of developer mix near the release pole. The 0 degree position on the x-axis ofFIGS. 7A, 8A, 9A, 10A, 11A, 12A, 13A and 14A(likeFIG. 6) corresponds to the horizontal radius of the core that faces away from the PC drum when the developer unit is in its operating position. The angular position values listed on the x-axis ofFIGS. 7A, 8A, 9A, 10A, 11A, 12A, 13A and 14A(likeFIG. 6) increase in a direction counter to the rotational direction of the sleeve. Nine lines are included in each ofFIGS. 7A, 8A, 9A, 10A, 11A, 12A, 13A and 14A. Three lines show the radial component of the magnetic field at a point axially in the center of the magnetic roll (Br center), at a point to the right of the center of the magnetic roll but axially inward from the right axial end of the magnetic roll (Br right) and at a point to the left of the center of the magnetic roll but axially inward from the left axial end of the magnetic roll (Br left). Three lines show the tangential component of the magnetic field at the same three axial points of the magnetic roll (Bt center, Bt right, Bt left) and three lines show the magnitude of the total magnetic field strength at the same three axial points of the magnetic roll (|B|center, |B|right, |B|left).

The tangential and radial components of the magnetic field near the outer surface of sleeve344may both be measured individually, e.g., using a Hall probe as is known in the art. Where the axial component of the magnetic field is near zero, the magnitude of the total magnetic field strength can then be calculated according to Equation 1, as is known in the art.

Alternatively, the radial component of the magnetic field may be measured and the expected tangential component may be calculated according to the Fourier series equations below, as is also known in the art. Axially inward from the axial ends of core342, where the axial component of the magnetic field is near zero, the radial and tangential components of the magnetic field can be represented as periodic functions of the angular position (θ) and the radius (r) from the axis of rotation of the magnetic roll. Once the radial component of the magnetic field has been measured at a constant radius, the equations below can be used to calculate the expected radial and tangential components of the magnetic field at any radius and angular position. Equation 1 above may then be used to calculate the magnitude of the total magnetic field strength at that radius and angular position. It is preferred to perform the initial radial component measurement at a radius that is close to the outer surface of the sleeve of the magnetic roll in order to maximize the signal to noise ratio in the measured values. In the equations below, Δθ represents the angular interval at which radial component measurements are taken. This measurement interval is preferably less than 1 degree. In the equations below, n is an integer that represents a frequency harmonic used to calculate the magnetic field and Nmaxis a user specified variable that represents the highest frequency harmonic used. Typically, 14<Nmax<30. The optimum Nmaxvalue varies with the particular shape of the magnetic profile being measured. The measured radial components may be compared with radial components calculated according to the equations below in order to determine the highest harmonic Nmaxnecessary to closely match the measured profile.

Fourier coefficients anand bnare calculated according to Equations 2 and 3 below for each n value based on the measured radial components of the magnetic field (Br) at the angular positions (θ) measured and the constant radius (r) of measurement.

Once the coefficients anand bnhave been calculated, the radial and tangential components of the magnetic field may be calculated at any radius and angular position using Equations 4 and 5 below.

Equation 1 above may then be used to calculate the magnitude of the total magnetic field strength at a particular radius and angular position.

FIGS. 7B, 8B, 9B, 10B, 11B, 12B, 13B and 14Bare graphs of the rate of change of the magnitude of the total magnetic field strength (in mT/degree) versus angular position (in degrees) for each of the magnetic profiles. Three lines (dB/dθcenter, dB/dθright, dB/dθleft) are included in each ofFIGS. 7B, 8B, 9B, 10B, 11B, 12B, 13B and 14Bdepicting the rate of change of the magnitude of the total magnetic field strength versus angular position at each of the three axial points of the magnetic roll inFIGS. 7A, 8A, 9A, 10A, 11A, 12A, 13A and 14A. In other words,FIGS. 7B, 8B, 9B, 10B, 11B, 12B, 13B and 14Bare graphs of the slope of each the three lines depicting the magnitude of the total magnetic field strength (|B|) shown inFIGS. 7A, 8A, 9A, 10A, 11A, 12A, 13A and 14Aplotted versus angular position.FIGS. 7B, 8B, 9B, 10B, 11B, 12B, 13B and 14Balso include the tangential component of the magnetic field at the point axially in the center of the magnetic roll (Bt center) versus angular position in order to show where the tangential component of the magnetic field is equal to zero at the release pole, which is indicated by the reference letter P in each ofFIGS. 7B, 8B, 9B, 10B, 11B, 12B, 13B and 14B.FIGS. 7B, 8B, 9B, 10B, 11B, 12B, 13B and 14Balso include a pair of vertical lines, which are indicated by the reference letters L1and L2, indicating the area of the release pole that is ±15 degrees from the angular position at which the tangential component of the magnetic field at the point axially in the center of the magnetic roll (Bt center) is equal to zero at the release pole.

Each of the magnetic rolls depicted inFIGS. 7A-14Bwas visually examined for retrograde motion of developer mix near its release pole. Table 1 below shows the results of those examinations.

As illustrated inFIG. 7A-14B, the magnitude of the rate of change of the magnitude of the total magnetic field strength near the release pole increases from Example Magnetic Profile #1 to Example Magnetic Profile #8. Example Magnetic Profiles #1 and #2 include a magnitude of the rate of change of the magnitude of the total magnetic field strength that remains below 1.0 mT/degree throughout the area of the release pole and exhibited no retrograde motion. Example Magnetic Profile #3 includes a magnitude of the rate of change of the magnitude of the total magnetic field strength that peaks at about 1.3 mT/degree in the area of the release pole and also exhibited no retrograde motion. Example Magnetic Profile #4 includes a magnitude of the rate of change of the magnitude of the total magnetic field strength that approaches 1.5 mT/degree in the area of the release pole at the left and the center of the magnetic roll and that exceeds 1.5 mT/degree in the area of the release pole at the right of the magnetic roll. Example Magnetic Profile #4 exhibited light retrograde motion. Example Magnetic Profiles #5 and #6 include a magnitude of the rate of change of the magnitude of the total magnetic field strength between 1.5 mT/degree and 2.0 mT/degree in the area of the release pole and exhibited moderate retrograde motion. Example Magnetic Profiles #7 and #8 include a magnitude of the rate of change of the magnitude of the total magnetic field strength that exceeds 2.0 mT/degree in the area of the release pole and exhibited strong retrograde motion.

Accordingly, it can be seen from Example Magnetic Profiles #1-8 that limiting the magnitude of the rate of change of the magnitude of the total magnetic field strength of core342throughout the area of release pole355to 1.5 mT/degree or less, and particularly to 1.3 mT/degree or 1.0 mT/degree or less, at a radius of 0.5 mm beyond the outer surface of sleeve344reduces the occurrence of retrograde motion of developer mix near release pole355. Reducing the occurrence of retrograde motion of developer mix near release pole355may reduce the wear on the magnetic carrier beads and toner particles and the spray of developer mix thereby increasing the useful life of developer unit320and reducing leakage. In contrast, if the magnitude of the rate of change of the magnitude of the total magnetic field strength of core342in the area of release pole355exceeds 1.5 mT/degree, and particularly 2.0 mT/degree, at a radius of 0.5 mm beyond the outer surface of sleeve344, magnetic roll306is at risk to retrograde motion of the developer mix on the outer surface of sleeve344in the area of release pole355.

The foregoing description illustrates various aspects and examples of the present disclosure. It is not intended to be exhaustive. Rather, it is chosen to illustrate the principles of the present disclosure and its practical application to enable one of ordinary skill in the art to utilize the present disclosure, including its various modifications that naturally follow. All modifications and variations are contemplated within the scope of the present disclosure as determined by the appended claims. Relatively apparent modifications include combining one or more features of various embodiments with features of other embodiments.