Image forming apparatus including an electric field having an oscillation component between an image carrier and a developer carrier

ABSTRACT

An image forming apparatus develops a latent image formed on an image carrier with a developer that forms a magnet brush on a developer carrier. The developer carrier is made up of a sleeve and a stationary magnet roller accommodated in the sleeve. The magnet roller includes a main pole for causing the developer to form the magnet brush and auxiliary poles for helping the main pole exert a magnetic force. An electric field including an oscillation component is formed between the image carrier and the developer carrier. A particular ratio is set up between a distance between the image carrier and the developer carrier, as measured at the boundary of a nip, and the shortest distance between them, between the above shortest distance and the shortest distance between the developer carrier and a metering member, or between the shortest distance between the image carrier and the developer carrier and the amount of developer scooped up to the image carrier.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:

FIG. 1 is a front view showing an image forming apparatus embodying the present invention;

FIG. 2 is a section showing a revolver or developing device included in the illustrative embodiment;

FIG. 3 is a chart showing the distribution and sizes of the magnetic forces of a magnet roller included in the revolver;

FIG. 4 is a view showing a positional relation between a main pole and auxiliary poles included in the magnet roller;

FIG. 5 is a view showing a structure in which a developing section included in the revolver and a toner container are connected to each other;

FIG. 6A is a perspective front view showing a mechanism for driving the revolver;

FIG. 6B is a view showing a mechanism for positioning the revolver;

FIG. 6C is a view showing a device for applying a bias for development to the revolver;

FIG. 7A is a plan view showing a motor for driving the revolver;

FIG. 7B is a front view of the motor;

FIG. 8 is a schematic block diagram showing a control system included in the illustrative embodiment;

FIG. 9 is a view showing a drum unit included in a monochromatic copier to which the illustrative embodiment is applied;

FIG. 10 is an enlarged view showing a developing device also included in the monochromatic copier;

FIG. 11 is a table listing the results of experiments conducted with the illustrative embodiment for estimating the omission of the trailing edge of an image and granularity;

FIG. 12 is a table showing a relation between AC frequency, which is applied as a bias, and granularity determined by experiments;

FIG. 13 is a table showing a relation between a duty ratio and granularity also determined by experiments; and

FIGS. 14 through 17 are tables each showing a particular relation between a development gap and a doctor gap and the granularity of a halftone image also determined by experiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the image forming apparatus in accordance with the present invention will be described hereinafter.

Referring to FIG. 1 of the drawings, an image forming apparatus embodying the present invention is shown and implemented as an electrophotographic color copier by way of example. As shown, the color copier is generally made up of a color scanner or color image reading device , a color printer or color image recording device , a sheet bank , and a control system that will be described later.

The color scanner includes a lamp for illuminating a document laid on a glass platen . The resulting reflection from the document is incident to a color image sensor via mirrors , and and a lens . The color image sensor reads color image information incident thereto color by color, e.g., red (R), green (G) and blue (B) image information while converting each of them to an electric signal. In the illustrative embodiment, the color image sensor includes R, G and B color separating means and a CCD (Charge Coupled Device) array or similar photoelectric transducer. An image processing section, not shown, transforms the resulting R, G and B image signals to black (Bk), cyan (C), magenta (M) and yellow (Y) color image data in accordance with the intensity of the signal.

More specifically, in response to a scanner start signal synchronous to the operation of the color printer , which will be described later, the optics including the lamp and mirrors through scans the document in a direction indicated by an arrow in FIG. . The color scanner outputs image data of one color every time it scans the document , i.e., outputs image data of four different colors by scanning the document four consecutive times. The color printer sequentially forms Bk, C, M and Y toner images while superposing them on each other, thereby completing a four-color or full-color toner image.

The color printer includes a photoconductive drum or image carrier , an optical writing unit and a revolver or developing device . The color printer further includes an intermediate image transferring unit and a fixing unit . The drum is rotatable counterclockwise, as indicated by an arrow in FIG. . Arranged around the drum are a drum cleaner , a discharge lamp , a charger , a potential sensor or charged potential sensing means , one of developing sections of the revolver selected, a density pattern sensor , and a belt included in the intermediate image transferring unit .

The optical writing unit converts the color image data output from the color scanner to a corresponding optical signal and scans the surface of the drum in accordance with the optical signal. As a result, a latent image is electrostatically formed on the drum . The optical writing unit includes a semiconductor laser or light source , a laser driver, not shown, a polygonal mirror , a motor for driving the mirror , an f/θ lens , and a mirror .

The revolver includes a Bk developing section K, a C developing section C, a M developing section M, a Y developing section Y, and a drive arrangement for causing the revolver to bodily rotate counterclockwise, as indicated by an arrow in FIG. . The developing sections K through Y each include a developing sleeve and a paddle or agitator. The developing sleeve rotates with a developer forming a magnet brush thereon and contacting the surface of the drum to thereby develop the latent image. The paddle scoops up the developer to the developing sleeve while agitating it. In the illustrative embodiment, the developer stored in each developing section is a toner and carrier mixture, i.e., a two-ingredient type developer. The toner is charged to negative polarity by being agitated together with the carrier. A bias power supply or bias applying means applies a bias for development to the developing sleeve. Consequently, the developing sleeve biases a metallic core layer included in the drum to a preselected potential. In the illustrative embodiment, the above bias is implemented by a negative DC voltage Vdc biased by an AC voltage Vac.

While the color copier is in a standby state, the revolver remains stationary with the Bk developing unit K facing the drum at a developing position. On the start of a copying operation, the color scanner starts reading Bk color image information at a preselected timing. A laser beam issuing from the semiconductor laser starts forming a Bk latent image in accordance with Bk color image data derived from the Bk color image information. The Bk developing sleeve included in the Bk developing unit K starts rotating before the leading edge of the Bk latent image arrives at the developing position. As a result, Bk latent image is developed by Bk toner from the leading edge to the trailing edge. As soon as the trailing edge of the Bk latent image moves away from the developing position, the revolver bodily rotates to bring the next developing section to the developing position. This rotation completes at least before the leading edge of the next latent image arrives at the developing position. The configuration and operation of the revolver will be described more specifically later.

The intermediate image transferring unit includes a belt cleaner and a corona discharger in addition to the previously mentioned belt . The belt is passed over a drive roller , a roller located at an image transferring position, a roller located at a cleaning position, and driven rollers. A motor, not shown, causes the belt to turn. In the illustrative embodiment, the belt is formed of ETFE (Ethylene TetraFluoroEthylene) and has electric resistance of 108 Ω/cm2 to 1010 Ω/cm2 in terms of surface resistance. The belt cleaner includes an inlet seal, a rubber blade, a discharge coil, and a mechanism for moving the inlet seal and rubber blade, although not shown specifically. While the transfer of images of the second to fourth colors from the drum to the belt is under way after the transfer of the image of the first color or Bk, the above mechanism maintains the inlet seal and rubber blade spaced from the belt . A DC voltage or an AC biased DC voltage is applied to the corona discharger . The corona discharger collectively transfers the full-color image completed on the belt to a paper sheet or similar recording medium.

The color printer includes a sheet cassette in addition to the sheet bank , which includes sheet cassettes , and . The sheet cassettes and through each are loaded with a stack of paper sheets of a particular size. Pickup rollers and , and are respectively associated with the sheet cassettes and , and . One of the pickup rollers through pays out the sheets from associated one of the sheet cassettes through selected toward a registration roller pair . A manual feed tray is available for feeding OHP (OverHead Projector) sheets, thick sheets and other special sheets by hand.

In operation, on the start of an image forming cycle, the drum rotates counterclockwise while the belt turns counterclockwise by being driven by the previously mentioned motor. In this condition, a Bk, a C, a M and a Y toner image are sequentially transferred from the drum to the belt one above the other, completing a full-color image.

More specifically, the charger uniformly charges the surface of the drum to a negative potential of about −700 V by corona discharge. The semiconductor laser scans the charged surface of the drum by raster scanning in accordance with a Bk color image signal. As a result, the charge of the drum is lost in the scanned portion in proportion to the quantity of incident light, forming a Bk latent image. Bk toner charged to negative polarity and forming a magnet brush on the Bk developing sleeve contacts the Bk latent image. At this instant, the Bk toner deposits only on the scanned portion of the drum where the charge is lost, thereby forming a Bk toner image. An image transferring device transfers the Bk toner image from the drum to the belt , which is turning in contact with and at the same speed as the drum . Let the image transfer from the drum to the belt be referred to as primary image transfer.

The drum cleaner removes some Bk toner left on the drum after the primary image transfer to thereby prepare the drum for the next image formation. The toner removed by the drum cleaner is collected in a waste toner tank via a piping, although not shown specifically.

The color scanner starts reading C image data at a preselected timing. A C latent image is formed on the drum in accordance with the C image data. After the trailing edge of the Bk latent image has moved away from the developing position, but before the leading edge of the C latent image arrives at the developing position, the revolver rotates to bring the C developing section C to the developing position. The C developing section C develops the C latent image with C toner for thereby producing a corresponding C toner image. After the trailing edge of the C latent image has moved away from the developing position, the revolver again rotates to bring the M developing section M to the developing position. This rotation also completes before the leading edge of the next or M latent image arrives at the developing position.

The formation of a M toner image and a Y toner image will not be described specifically because it is similar to the formation of the Bk and C toner images described above.

By the above procedure, the Bk, C, M and Y toner images are sequentially transferred from the drum to the belt one above the other. The corona discharger collectively transfers the resulting full-color toner image from the belt to the paper sheet . The transfer of the full-color toner image from the belt to the paper sheet will be referred to as secondary image transfer hereinafter.

More specifically, the paper sheet is fed from any one of the sheet cassettes and through or the manual feed tray and once stopped by the registration roller pair . The registration roller pair drives the paper sheet at such a timing that the leading edge of the paper sheet meets the trailing edge of the full-color toner image formed on the belt . The corona discharger charges the paper sheet , which is superposed on the full-color toner image, to positive polarity. As a result, the toner image is almost entirely transferred from the belt to the paper sheet . A discharger, not shown, located at the left-hand-side of the corona discharger discharges the paper sheet by AC+DC corona discharge, so that the paper sheet is separated from the belt . The paper sheet is then transferred to a conveyor implemented as a belt.

The conveyor conveys the paper sheet carrying the toner image thereon to the fixing unit . In the fixing unit , a heat roller and a press roller cooperate to fix the toner image on the paper sheet with heat and pressure. The paper sheet or full-color copy coming out of the fixing unit is driven out to a copy tray, not shown, face up.

After the secondary image transfer, the drum cleaner , which may be implemented as a brush roller or a rubber blade, cleans the surface of the drum . Subsequently, the discharge lamp uniformly discharges the surface of the drum . At the same time, the inlet seal and rubber blade of the belt cleaner are again pressed against the belt to thereby clean the surface of the belt .

In a repeat copy mode, after the formation of the first Y toner image on the drum , the color scanner and drum are operated to form the second Bk toner image. On the other hand, after the secondary transfer of the first full-color image from the belt to the paper sheet , the second Bk toner image is transferred to the area of the belt that has been cleaned by the belt cleaner .

In a bicolor or a tricolor copy mode, as distinguished from the above-described full-color copy mode, the same procedure is repeated a number of times corresponding to desired colors and a desired number of copies. Further, in a monocolor copy mode, one of the developing sections of the revolver corresponding to a desired color is held at the developing position until a desired number of copies have been output. At the same time, the inlet seal and blade of the belt cleaner are constantly held in contact with the belt .

Assume that the full-color copy mode operation is effected with paper sheets of size A3. Then, it is desirable to form a toner image of one color every time the belt makes one turn and therefore to complete a full-color image by four turns of the belt . More preferably, however, a toner image of one color should be formed during two turns of the belt . This makes the entire copier small size, i.e., reduces the circumferential length of the belt and guarantees a copy speed for relatively small sheet sizes while preventing the copy speed from decreasing for the maximum sheet sizes. In such a case, after the transfer of the Bk toner image from the drum to the belt , the belt makes one idle turn without any development or image transfer. During the next turn of the belt , the next or C toner image is formed and transferred to the belt . This is also true with the M and Y toner images. The revolver is caused to rotate during the idle turn of the belt .

Reference will be made to FIG. 2 for describing the revolver in detail. As shown, the revolver includes a developing unit including the developing sections K through Y. The developing unit includes a pair of disk-like end walls and a partition wall supported by the end walls at opposite ends thereof. The partition wall includes a hollow, cylindrical portion and four casing portions , C, M and Y extending radially outward from the cylindrical portion . The casing portions through Y divide the space around the cylindrical portion into four developing chambers, which are substantially identical in configuration, in the circumferential direction. The developing chambers each store the developer, i.e., toner and carrier mixture of a particular color. In the specific position shown in FIG. 2, the developing chamber of the Bk developing section K, which stores the black toner and carrier mixture, is located at the developing position. This developing chamber is followed by the developing chambers of the Y developing section Y, M developing section M, and C developing section C in the counterclockwise direction.

The following description will concentrate on the black developing chamber located at the developing position by way of example. In FIG. 2, the yellow, magenta and cyan developing chambers are simply distinguished from the black developing chamber by suffixes Y, M and C.

In the Bk developing section K, the casing portion is formed with an opening facing the drum . A developing roller or developer carrier is made up of the developing sleeve and a magnet roller disposed in the developing sleeve. A doctor blade or metering member regulates the amount of the developer deposited on and conveyed by the developing roller to the developing position. An upper screw conveyor conveys part of the developer removed by the doctor blade from the rear to the front in the direction perpendicular to the sheet surface of FIG. 2. A guide guides the screw conveyor . A paddle or agitator agitates the developer stored in the developing chamber. The paddle includes a hollow, cylindrical portion formed with a plurality of holes at spaced locations in the axial direction of the developing roller , and a plurality of blades extending radially outward from the cylindrical portion . A lower screw conveyor is disposed in the cylindrical portion and extends in the axial direction of the paddle . The lower screw conveyor conveys the developer in the opposite direction to the upper screw conveyor . The casing portion is additionally formed with a slot below the lower screw conveyor . The slot extends in the axial direction of the developing unit and may be used to discharge the developer deteriorated or to charge a fresh developer, as desired. A cap is fastened to the casing portion by, e.g., screws .

In the illustrative embodiment, the drum has a diameter of 90 mm and moves at a linear velocity of 200 mm/sec. The developing sleeve, i.e., the developing roller has a diameter of 30 mm and moves at a linear velocity of 260 mm/sec, which is 2.5 times as high as the linear velocity of the drum . A development gap between the drum and the developing roller is 0.35 mm or 0.4 mm. The magnet roller disposed in the developing roller causes the developer deposited on the roller to rise in the form of a magnet brush. More specifically, the carrier of the developer rises in the form of chains on the developing roller along magnetic lines of force issuing from the magnet roller. The charged toner deposit on the carrier to thereby form a magnet brush.

As shown in FIG. 4, The magnet roller has a plurality of magnetic poles or magnets Pthrough Pand P through P. The pole or main pole Pcauses the developer to rise in a developing region where the sleeve developing roller and drum face each other. The poles Pand Phelp the main pole Pexert such a magnetic force. The pole P scoops up the developer to the developing sleeve. The poles P and P convey the developer to the developing region. The poles P and P convey the developer in a region following the developing region. All of the poles of the magnet roller are oriented in the radial direction of the developing sleeve. While the magnet roller is shown as having eight poles, additional poles may be arranged between the pole P and the doctor blade in order to enhance the scoop-up of the developer and the ability to follow a black solid image. For example, two to four additional poles may be arranged between the pole P and the doctor blade .

The poles Pthrough Pare sequentially arranged from the upstream side to the downstream side in the direction of developer conveyance, and each is implemented by a magnet having a small sectional area. While such magnets are formed of a rate earth metal alloy, they may alternatively be formed of, e.g., a samarium alloy, particularly a samarium-cobalt alloy. An iron-neodium-boron alloy, which is a typical rare earth metal alloy, has the maximum energy product of 358 kJ/m3. An ion-neodium-boron alloy bond, which is another typical rare earth metal, has the maximum energy product of 80 kJ/m3 or so. Such magnets guarantee magnetic forces required of the surface of the developing roller despite their small sectional area. A ferrite magnet and a ferrite bond magnet, which are conventional, respectively have the maximum energy products of about 36 kJ/m3 and 20 kJ/m3. If the sleeve is allowed to have a greater diameter, then use may be made of ferrite magnets or ferrite bond magnets each having a relatively great size or each having a tip tapered toward the developing sleeve in order to reduce a half width.

It is to be noted that a half width refers to the angular width of a portion where the magnetic force is one half of the maximum or peak magnetic force of a magnetic force distribution curve normal to the developing sleeve. For example, if the maximum magnetic force of a N magnet in the normal direction is 120 mT, then the half width (50%) is 60 mT; if the half value is 80%, as also used in the art, then it is 96 mT. The smaller the half width, the closer the position where the magnet brush rises to the main pole, and the narrower the nip for development. The auxiliary pole is formed upstream and/or downstream of the main pole in the direction in which the developer is conveyed.

In the above specific configuration, the main pole Pand poles P, P, P and P are N poles while the poles P, Pand P are S poles. For example, the main magnet Phad a magnetic force of 85 mT or above in the normal direction, as measured on the developing roller. It was experimentally found that if the main pole Phad a magnetic force of 60 mT or above, defects including the deposition of the carrier were obviated. The deposition of the carrier occurred when the above magnetic force was less than 60 mT. The magnets Pthrough Peach had a width of 2 mm while the magnet Phad a half width of 16°. By further reducing the width of the magnet, the half value was further reduced. A magnet had a half value of 12° when the width was 1.6 mm.

FIG. 4 shows a positional relation between the main magnet Pand the auxiliary magnets Pand P. As shown, the half width of each of the auxiliary magnets Pand Pis selected to be 35° or below. This half width cannot be reduced relatively because the magnets P and P positioned outside of the magnets Pand Phave great half widths. The angle between each of the auxiliary magnets Pand Pand the main magnet Pis selected to be 30° or below. More specifically, because the half width of the main pole Pis 16°, the above angle is selected to be 22°. Further, the angle between the transition point (0 mT) between the magnets Pand P and the transition point (0 mT) between the magnets Pand P is selected to be 120° or below. The transition point refers to a point where the N pole and S pole replace each other.

The drum and developing roller facing each other form a nip for development therebetween. Toner moves between the drum and the magnet. In the case of contact development, the toner moves mainly in the nip or developing region. In the developing region, the size of the electric field differs from the point where the drum and developing roller are closest to each other to the point where they are remotest from each other, i.e., the boundary of the nip. In the illustrative embodiment, the gap between the drum and the developing roller is 0.4 mm or 0.35 mm. When the nip width is varied, the distance between the drum and the developing roller varies at each of the center and the boundary of the nip. Consequently, for a uniform developing layer, the strength of the electric field varies in inverse proportion to the ratio between the drum and the developing roller. Experiments conducted to determine the influence of the above electric field on the omission of a trailing edge will be described later.

To efficiently discharge the deteriorated developer via the slot , the following procedure is preferable. First, the developing unit is pulled out of the copier body via a base not shown. Subsequently, an input gear (see FIG. A), as well as other gears, is rotated via, e.g., a jig, so that the deteriorated developer is discharged with the upper and lower screw conveyors and and paddle being rotated. Also, a fresh developer may be charged via the slot with the screw conveyors and and paddle being rotated. This allows the fresh developer to be evenly scattered in the existing developer.

FIG. 5 is a section showing the black developing section K in a plane containing the axes of the upper and lower screw conveyors and . As shown, the front ends of the screw conveyors and extend to the outside of the effective axial range of the developing roller , i.e., to the outside of the front end wall of the developing unit in the illustrative embodiment. The developer conveyed by the screw conveyor drops onto the screw conveyor via a drop portion due to its own weight.

The front end of the screw conveyor further extends via the drop portion to a communication chamber positioned below a toner replenishing roller . The toner replenishing roller is included in a toner storing unit, not shown, assigned to each developing chamber. In this configuration, the developer removed by the doctor blade , conveyed by the screw conveyor and then dropped via the drop portion is conveyed by the screw conveyor to the effective axial range of the developing roller . The developer is then introduced into the developing chamber via the holes of the hollow, cylindrical portion of the paddle and again deposited on the developing roller . That is, the developer is agitated in the horizontal direction in the developing chamber. The paddle in rotation agitates the above developer introduced into the developing chamber with its blades in the vertical direction.

Further, the toner replenishing roller in rotation causes fresh toner to drop onto part of the screw conveyor existing in the communication chamber. The screw conveyor conveys the fresh toner to the drop portion . As a result, the fresh toner is mixed with the developer dropped from the screw conveyor and then fed to the developing chamber via the holes of the cylindrical portion of the paddle, increasing the toner content of the developer.

FIG. 6A is a perspective view of the rear end wall of the developing unit as seen from the front. As shown, a revolver input gear is affixed to the rear end wall . Various gears shown in FIG. 6A are positioned at the rear of the revolver input gear . Specifically, the shaft of the developing roller extends throughout the rear end wall to a position rearward of the revolver input gear . A developing roller gear is mounted on the rear end of the shaft of the developing roller . Likewise, the shafts of the upper and lower screw conveyors and extend throughout the end wall to a position rearward of the revolver input gear . An upper and a lower screw gear and are mounted on the rear ends of the screw conveyors and , respectively. An idle gear and a development input gear are mounted on the back of the rear end wall . The idle gear is held in mesh with the developing roller gear and lower screw gear . The development input gear is capable of meshing with a development output gear , which is mounted on a rear side wall included in the copier body. A motor causes the development output gear to rotate. As shown in FIG. 6A, when the developing unit is mounted to the previously mentioned base and then inserted into the copier body, the development input gear is brought into mesh with the development output gear . At the same time, the revolver input gear is brought into mesh with the revolver output gear .

As shown in FIGS. 7A and 7B, the revolver output gear and development output gear are mounted on the copier body in such a manner as to be retractable in the direction in which the base slides. Springs and constantly bias the gears and forward in the above direction. It follows that even when the gears and interfere with the gears and of the developing unit when the base is inserted into the printer body, the gears and retract and guarantee the complete insertion of the base. Also, when the gears and are driven, they do not interfere with the gears and . Subsequently, the gears and move toward the developing unit due to the action of the springs and and therefore accurately mesh with the gears and , respectively, as shown in FIG. A.

In the condition shown in FIG. 6A, the development output gear is driven in a direction indicated by an arrow A. The gear , in turn, causes the upper and lower screw gears and to rotate via the development input gear , thereby causing the upper and lower screw conveyors and to rotate. At the same time, the developing roller gear is rotated via the lower screw gear and idle gear with the result that the developing roller rotates.

In the illustrative embodiment, when the developing unit brings its desired developing section to the developing position, the gear of the developing unit surely meshes with the gear of the copier body before the developer on the developing roller contacts the drum . Further, when the developing unit moves the above developing section away from the developing position, the gear surely remains in mesh with the gear until the developer on the developing roller fully moves away from the drum . To realize such arrangements, the illustrative embodiment causes the gear to mesh with the gear at a position close to the axis of the developing unit .

A revolver motor , FIGS. 7A and 7B, causes the revolver output gear to rotate in a direction indicated by an arrow B in FIG. A. The revolver motor may be implemented as a stepping motor by way of example. The revolver output gear , in turn, rotates the developing unit in a direction indicated by an arrow C in order to bring a desired developing section to the developing position. At the same time, a positioning roller enters one of recesses formed in the circumference of the rear end wall at preselected locations, thereby positioning the developing unit . This kind of scheme, however, has the following problem. Assume that the rotation angle of the developing unit is short of a preselected angle due to irregularity in the revolver motor or irregularity in the load of the developing unit . Then, the positioning roller fails to enter the expected recess , i.e., to accurately position the developing unit . The resulting distance between the developing roller and the drum differs from a preselected distance. The preselected angle mentioned above is 90° in the case of the developing section located just upstream of the developing position.

In light of the above, in the illustrative embodiment, the rotation of the revolver motor is controlled by a control value corresponding to an angle slightly greater than the preselected angle, e.g., by 3°. At the same time, even when the developing unit actually rotates by more than the preselected angle due to such control, the developing unit is accurately positioned on the basis of the moment of rotation to act on the unit on the start of drive of the motor . Specifically, as shown in FIG. 6A, the development output gear meshing with the development input gear , which is included in the developing section located at the developing position, is rotated in the direction A as during ordinary development. The rotation of the development output gear applies a moment of rotation to the developing unit in a direction indicated by an outline arrow D, which is opposite to the ordinary direction of rotation. Further, an arrangement is made such that the developing unit stops rotating in the direction D and is locked in position when the positioning roller has entered the expected recess . Specifically, the positioning roller is mounted on a bracket that is, in turn, supported by a positioning pin . The positioning pin is positioned such that the bracket is counter to the above rotation of the developing unit as to direction.

Moreover, as shown in FIG. 6B, each recess should preferably be made up of a portion via which the positioning roller leaves the recess during ordinary rotation and a portion for locking the developing unit . The portion is inclined less than the portion . Assume that the positioning roller enters the recess and then leaves it due to the rotation of the developing unit exceeding the preselected angle. Then, the portion allows the positioning roller to smoothly leave the recess and thereby reduces a load on the drive mechanism.

In the specific arrangement shown in FIG. 2, part of the front end wall and part of the rear end wall supporting the developing roller Y and doctor blade Y are implemented as small wall members Y separable from the other portions of the end walls. This configuration applies to the other developing sections as well. In the event of cleaning of the developing chamber or the replacement of parts, the wall members Y supporting the developing roller Y and doctor blade Y are removed in order to promote easy access to the inside of the developing chamber.

As shown in FIG. 6C, a bracket is mounted on the rear side wall of the copier body and supports a conductive, rod-like terminal . The terminal is so positioned as to face the end of a shaft on which the developing roller of the developing section located at the developing position is mounted. The terminal is connected to a bias power supply for development and retractable in the direction in which the previously stated base is slidable (direction of thrust). A conductive spring or biasing means constantly biases the terminal forward toward the copier body. The end of the terminal is convex in a hemispherical configuration while the end of the shaft is concave in a hemispherical configuration. The concave end of the shaft has a slightly greater radius of curvature than the convex end of the terminal . This successfully reduces a load when the end of the shaft arrive at or leaves the end of the terminal , and allows the former to remain in stable contact with the latter. The terminal applies the bias for development only to the developing section located at the developing position in the same manner as during development. When the developing section is brought to the developing position, the end of the shaft surely contacts the end of the terminal before the developer on the developing roller contacts the drum . Also, when the developing section leaves the developing position, the end of the shaft surely remains in contact with the end of the terminal until the developer fully parts from the drum .

FIG. 8 shows a control system included in the illustrative embodiment. As shown, the control system includes a controller . The controller includes a CPU (Central Processing Unit) A, a ROM (Read Only Memory) B connected to the CPU A, and a RAM (Random Access Memory) also connected to the CPU A. The ROM B stores a basic program and basic data for executing the program. The RAM C stores various kinds of interim data. The potential sensor and density pattern sensor are connected to the CPU A via an I/O (Input/Output) interface D. The density pattern sensor is made up of a light emitting element and a light-sensitive element. The potential sensor senses the potential of the drum at a position upstream of the developing position. Also connected to the CPU A via the I/O interface D are a developing roller driver , a bias control driver or bias switching means , a charge control driver or charge potential switching means , a toner replenishment driver , a laser driver , and a revolver driver .

The bias control driver causes an AC-biased DC voltage for development to be applied to the rod-like terminal . The bias control driver is capable of selectively applying or stopping applying the AC voltage independently of the DC voltage in accordance with a control signal output from the controller . In addition, the bias control driver is capable of varying the DC voltage at a preselected timing in accordance with a control signal also output from the controller .

The charge control driver is connected to the charger in order to apply a bias to the charger . The charge control driver is capable of varying the above bias at a preselected timing in accordance with a control signal output from the controller .

The present invention is applicable to an electrophotographic, monochromatic copier, as will be described hereinafter. The monochromatic copier to be described includes a scanner similar to the color scanner except that it reads monochromatic image information. Further, the monochromatic copier is substantially identical with the color copier as to the sheet bank and control system. The following description will therefore concentrate on the image forming section.

As shown in FIG. 9, the monochromatic copier includes a photoconductive drum , which is a specific form of an image carrier, rotatable in a direction indicated by an arrow (counterclockwise). A charger uniformly charges the surface of the drum to a preselected potential. An exposing unit exposes the charged surface of the drum with a laser beam in accordance with image data to thereby form a latent image. A developing device develops the latent image with toner for producing a corresponding toner image. The developing device includes a casing and a developing sleeve or developer carrier. An image transferring unit transfers the toner image from the drum to a paper sheet or similar recording medium . A drum cleaner removes toner left on the drum after the image transfer. Further, a discharger discharges the surface of the drum to thereby prepare the drum for the next image formation.

In operation, the charger uniformly charges the surface of the drum with a charge roller. The exposing unit scans the charged surface of the drum to thereby form a latent image. The developing unit develops the latent image with toner. The image transferring unit , which includes a belt, transfers the resulting toner image from the drum to the paper sheet fed from a tray not shown. A peeler peels off the paper sheet electrostatically adhering to the drum . A fixing unit fixes the toner image transferred to the paper sheet . The drum cleaner removes the toner left on the drum after the image transfer and collects the toner. The discharge lamp discharges the surface of the drum .

FIG. 10 shows a specific configuration of the developing device . As shown, the developing device includes a developing roller adjoining the drum . A nip or developing region is formed between the developing roller and the drum . The developing roller includes a cylindrical sleeve formed of aluminum, brass, stainless steel, conductive resin or similar nonmagnetic material. A drive mechanism, not shown, causes the sleeve to rotate clockwise, as viewed in FIG. 10, or in a direction of developer conveyance. In the illustrative embodiment, the drum has a diameter of 30 mm to 60 mm and rotates at a linear velocity of 240 mm/sec. The developing sleeve has a diameter of 16 mm to 20 mm and rotates at a linear velocity of 600 mm/sec. A ratio of the drum linear velocity to the sleeve linear velocity is therefore 2.5. A developing gap between the drum and the developing sleeve is selected to be 0.4 mm.

A doctor blade or metering member is positioned upstream of the developing region in the direction of developer conveyance (clockwise as viewed in FIG. ). The doctor blade regulates the amount of the developer to be conveyed by the developing sleeve to the developing region, i.e., the height of a magnet brush. A doctor gap between the doctor blade and the sleeve is selected to be 0.4 mm. A screw is positioned at the opposite side to the drum with respect to the developing roller . The screw scoops up the developer stored in a casing to the developing roller while agitating it.

A magnet roller is held stationary within the sleeve for causing the developer to form a magnet brush on the sleeve . The magnet roller has the configuration described previously with reference to FIGS. 3 and 4. A relation between the nip width and the omission of the trailing edge of an image and granularity will be described hereinafter.

FIG. 11 shows Experiments No. 1 through No. 10 conducted with the color copier and monochromatic color copier in order to estimate the omission of the trailing edge of an image and granularity. To measure a nip width, while the drum and developing sleeve were held stationary, a bias for causing the toner to migrate from the sleeve toward the drum was applied. In this condition, the range of the drum over which the toner deposited on the drum was measured as a nip. The distance at the boundary of the nip was determined by calculation using the drum diameter, sleeve diameter, development gap, and development nip. As for the trailing edge omission rank, rank indicates that no omission was observed while rank indicates that omission was most conspicuous. Also, as for the granularity rank, rank indicates that no granularity was observed while rank indicates that granularity was most conspicuous. Ranks and above are desirable as to image quality.

As FIG. 11 indicates, when the ratio of the distance at the boundary of the nip to the development gap is 1.5 or less, an image free from the omission of a trailing edge is achievable. This condition, however, could not reduce granularity alone when the bias for development was implemented only by DC. When AC was superposed on DC under the conditions *1 described in Experiment No. 5, granularity was improved with the omission level being maintained. On the other hand, when the ratio of the distance at the boundary to the development gap was greater than 1.5, more specifically 1.97, even AC superposed on DC could not implement the desirable granularity level although somewhat improving it, compared to DC.

It has been known that AC-biased DC improves the granularity level more than DC, as will be seen by comparing Experiments No. 5 and No. 6. However, in a conventional magnet roller or developing roller (half width of 48°), a magnet brush has a great height or length while a nip width for development is great. Therefore, even after the magnet brush has formed a toner image with a minimum of granularity because of AC-biased DC, the brush remains in contact with a photoconductive element over a substantial period of time. As a result, the magnet brush removes toner from the toner image due to physical contact and electrostatically attracts the toner toward a carrier carrying no toner, disturbing the toner image and thereby rendering it granular. In the illustrative embodiment, the auxiliary poles adjoining the main pole, which is closest to the photoconductive element or image carrier, help the main pole exert a magnetic force. This reduces the half width to 25° or below and reduces the nip width. In this condition, the duration of contact of the magnet brush with the photoconductive element after the formation of the above toner image is reduced. Consequently, the toner image suffers from a minimum of disturbance, compared to the conventional toner image.

Experiment No. 8 shown in FIG. 11 was conducted except that a bias of DC −500 V was replaced with AC having various frequencies. Specifically, Experiment No. 8 was conducted under the following conditions:

color copier

drum linear velocity: 200 mm/sec

sleeve linear velocity: 260 mm/sec

drum diameter: 90 mm

sleeve diameter: 30 mm

development gap: 0.4 mm

nip: 4 mm

distance at nip boundary: 0.58 mm

ratio of distance at nip boundary to nip: 1.13

bias for development

fixed conditions: rectangular wave, duty of 50%,

peak-to-peak voltage of 800 V,

offset voltage of −500 V

variable condition: frequencies of 0 kHz to 0.9 kHz

FIG. 12 shows the results of the above experiment. As shown, AC reduced granularity although to some different degrees. Specifically, when the nip width is 4 mm and the drum linear velocity is 200 mm/sec, oscillation occurs ten times (0.5 kHz), twenty times (1 kHz), forty times (2 kHz) or 180 times (9 kHz) within the nip width. Further, when the nip width is 2 mm and the drum linear velocity is 230 mm/sec, oscillation occurs four point four times (0.5 kHz), eight point seven times (1 kHz), seventeen point four times (2 kHz) or seventy point three times (9 kHz) within the nip width. It will therefore be seen that when an oscillation component occurs ten times or more before a given point on the drum moves away from the brush contact region, granularity is successfully reduced, and a desirable granularity level is achieved when it occurs thirty times or more.

The above experiment was repeated except that the bias was varied to provide the oscillation component of the electric field with an asymmetric, rectangular waveform. Specifically, the fixed conditions of the bias were a peak-to-peak voltage of 800 V and a frequency of 4.5 kHz while the variable condition was duties of 10% to 60%. A particular offset voltage is assigned to each duty in order to implement an effective value of −500 V. A duty ratio is expressed as:

duty ratio=/100() (%)

where a denotes the duration of a bias applied to the developing roller or the developing sleeve for causing toner to move toward the drum, and b denotes the duration of a bias applied to the developing roller for causing toner to move toward the sleeve. FIG. 13 shows a relation between the duty ratio and granularity determined by the experiment. As shown, a desirable granularity level is achievable when the oscillation component of the electric field has an asymmetric, rectangular waveform so configured as to reduce the period of time over which toner moves toward the drum.

As stated above, in the illustrative embodiment, the ratio of the distance between the image carrier and the developer carrier, as measured at the boundary of the nip, to the shortest distance between them is selected to be 1.5 or below. Further, an electric field including an oscillation component is formed between the image carrier and the developer carrier. This is successful to satisfy both of granularity and the omission of a trailing. Granularity can be further reduced if the oscillation component is provided with an optimal frequency. This is also true when the waveform of the oscillation component is provided with an optimal value.

An alternative embodiment of the present invention will be described hereinafter. This embodiment is also practicable with the configuration of the color copier described with reference to FIGS. 1 through 8. Assume that the color copier shown in FIG. 1 forms a development gap Gp between the drum and the developing sleeve of the developing section located at the developing position, and forms a doctor gap Gd between the doctor blade of the above developing section and the developing sleeve. In the illustrative embodiment, experiments were conducted to estimate granularity and the omission of a trailing edge by varying the development gap Gp and doctor gap Gd.

As for image forming conditions, there were selected a ratio of the sleeve linear velocity to the drum linear velocity of 1.3, drum diameter of 90 mm, sleeve diameter of 30 mm, charge potential of −700 V, and bias of DC −500 V having a frequency of 4.5 kHz, an offset voltage of −500 V, a duty ratio of 50% and a peak voltage of 800 V, as stated earlier.

FIG. 14 shows granularity and the omission of the trailing edge of a halftone image estimated by varying the development gap Gp between 0.35 and 0.6 and varying the doctor gap Gd. As for granularity, the quantity of writing light was varied to form solid patterns of 256 different tones (sized 2 cm×2 cm) and then developed. The halftone portions of the resulting toner images having lightness of 50 degrees to 80 degrees were observed by eye. In FIG. , granularity rank indicates that granularity was not observed at all, while rank indicates that granularity was most conspicuous. As for the omission of a trailing edge, the trailing edges of the above toner images were observed by eye; rank indicates that no omission was observed, while rank indicates that omission was most conspicuous. Ranks and above are good, rank is average, and ranks and below are no good.

DC did not noticeably improve image quality when the ratio Gp/Gd was low. By contrast, when AC was superposed on DC under the conditions shown in FIG. 14, the granularity level was more improved with a decrease in ratio Gp/Gd. As for the omission of a trailing edge, attractive images were produced under any one of the above conditions. This is accounted for by the following presumable occurrences. When the ratio Gp/Gd is low, the developer scooped by the scooping pole and moved away from the doctor blade enters the development gap smaller than the doctor gap. Therefore, when the developer arrives at the developing position, it is packed more densely between the drum and the developing sleeve than when it is scooped up. Further, because the distribution of the magnetic force of the main pole is narrower than the convention distribution, a dense magnet brush is formed within the narrow nip width. This increases the probability that the developer contacts the drum within the nip width, and further promotes efficient migration of charge from the developing sleeve toward the drum. In this manner, the developer densely packed at the developing position effectively reduces granularity. Experiments showed that the ratio Gp/Gd should be smaller than at least 0.8.

FIG. 15 lists the results of comparative experiments similar to the experiments of FIG. 14, but conducted with a conventional magnet roller lacking auxiliary poles and having a main pole whose half width is about 48°. As shown, although AC replacing DC reduces granularity, no correlation exists between the ratio Gp/Gd and the granularity rank. Granularity decreases with a decrease in the development gap Gp, but the omission of a trailing edge is aggravated. No condition that satisfies both of the granularity level and omission level does not exist in the comparative experiments. Specifically, in the comparative experiments, the great half width increases the length of the magnet brush in the circumferential direction of the developing roller and thereby increases the width over which the magnet brush contacts the drum (nip width). A greater nip width directly translates into a longer period of time over which the magnet brush remains in contact with the drum. Such a period of time, in turn, increases the probability that the toner once deposited on the drum migrates toward the developing roller and therefore results in the omission of a trailing edge, as well known in the art.

In the comparative experiments, too, when the ratio Gp/Gd is low, the developer scooped by the scooping pole and moved away from the doctor blade enters the development gap smaller than the doctor gap. Therefore, when the developer arrives at the developing position, it is presumably packed more densely between the drum and the developing sleeve than when it is scooped up. Further, because the distribution of the magnetic force of the main pole is narrower than the convention distribution, a dense magnet brush is presumably formed within the narrow nip width. This increases the probability that the developer contacts the drum within the nip width, and further promotes efficient migration of charge from the developing sleeve toward the drum. However, the probability that toner once deposited on the drum migrates toward the developing roller increases for the same reason as discussed in relation to the omission of a trailing edge. As a result, despite that a toner image free from granularity is formed on the drum, the toner presumably again deposits on the magnet brush.

Experiments were conducted with the same color copier by varying the AC frequency and yielded results listed in FIG. . Specifically, the experiments were conducted under the following conditions:

drum linear velocity: 200 mm/sec

sleeve linear velocity: 260 mm/sec

drum diameter: 90 mm

sleeve diameter: 30 mm

development gap: 0.4 mm

doctor gap: 4 mm

bias for development

fixed conditions: rectangular wave, duty of 50%,

peak-to-peak voltage of 800 V,

offset voltage of −500 V

variable condition: frequencies of 0 kHz to 0.9 kHz

FIG. 15 shows the results of the above experiment as to granularity. As shown, AC reduced granularity although to some different degrees. Specifically, when the nip width is 4 mm and the drum linear velocity is 200 mm/sec, oscillation occurs ten times (0.5 kHz), twenty times (1 kHz), forty times (2 kHz) or 180 times (9 kHz) within the nip width. Further, when the nip width is 2 mm and the drum linear velocity is 230 mm/sec, oscillation occurs four point four times (0.5 kHz), four point seven times (1 kHz), seventeen point four times (2 kHz) or seventy-eight point three times (9 kHz) within the nip width. It will therefore be seen that when an oscillation component occurs ten times or more before a given point on the drum moves away from the brush contact region, granularity is successfully reduced, and a desirable granularity level is achieved when it occurs thirty times or more.

The above experiment was repeated except that the bias was varied to provide the oscillation component of the electric field with an asymmetric, rectangular waveform. Specifically, the fixed conditions of the bias were a peak-to-peak voltage of 800 V and a frequency of 4.5 kHz while the variable condition was a duty of 10% to 60%. A particular offset voltage is assigned to each duty in order to implement an effective value of −500 V. The duty ratio (a/100(a+b) (%)) and granularity were found to have the relation described with reference to FIG. . Specifically, a desirable granularity level is achievable when the oscillation component of the electric field has an asymmetric, rectangular waveform so configured as to reduce the period of time over which toner moves toward the drum.

Further, to estimate granularity and the omission of a trailing edge, the development gap Gp between the developing sleeve of the developing section located at the developing position and the drum was varied. Also, the amount p of the developer scooped up to the developing sleeve and then moved away from the doctor blade was varied. As for image forming conditions, use were again made of a sleeve linear velocity/drum linear velocity of 1.3, drum diameter of 90 mm, sleeve diameter of 30 mm, charge potential of −700 V, and bias of DC −500 V having the frequency of 4.5 kHz, offset voltage of −500, duty ratio of 50% and peak voltage 800 V. FIG. 16 lists the granularity of a halftone image and the omission of a trailing edge estimated by varying the development gap Gp between 0.35 and 0.6 and varying the amount p. The omission of a trailing edge was estimated by the same method as applied to the case wherein the gaps Gp and Gd were varied.

DC did not noticeably improve image quality when the ratio Gp/Gd was low. By contrast, when AC was superposed on DC under the conditions shown in FIG. 16, the granularity level was more improved with a decrease in ratio Gp/Gd. Specifically, the granularity level was improved as the developer is packed more densely in the narrow development gap, i.e., as the magnet brush becomes narrower and more dense. Experiments showed that the ratio Gp/p should be smaller than at least 10.

FIG. 17 lists the results of comparative experiments similar to the experiments of FIG. 16, but conducted with a conventional magnet roller lacking auxiliary poles and having a main pole whose half width is about 48°. Again, when the ratio Gp/p is low, the developer scooped by the scooping pole and moved away from the doctor blade enters the development gap smaller than the doctor gap. Therefore, when the developer arrives at the developing position, it is presumably packed more densely between the drum and the developing sleeve than when it is scooped up. The magnet brush is therefore more dense when the ratio Gp/p is low than when it is high. This increases the probability that the developer contacts the drum within the nip width, and further promotes efficient migration of charge from the developing sleeve toward the drum. However, the probability that toner once deposited on the drum migrates toward the developing roller increases for the same reason as discussed in relation to the omission of a trailing edge. As a result, despite that a toner image free from granularity is formed on the drum, the toner presumably again deposits on the magnet brush.

The frequency of the bias for development was varied with the development gap Gp and amount p being held at 0.35 mm and 0.065 g/cm2, respectively. This also derived the same results as obtained by varying the development gap Gp and amount ρ. This was also true when the oscillation component of the electric field had an asymmetric, rectangular waveform.

As stated above, in the illustrative embodiment, the ratio of the development gap Gp to the doctor gap Gd is selected to be smaller than 0.8, or the ratio of the gap Gp to the amount p of the developer is selected to be smaller than 10. In any case a dense magnet brush is formed at the developing position. Further, an electric field including an oscillation component is formed between the image carrier and the developer carrier. This is successful to satisfy both of granularity and the omission of a trailing edge. Granularity can be further reduced if the oscillation component is provided with an optimal frequency. This is also true when the waveform of the oscillation component is provided with an optimal value.

Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.

CLAIMS

1. In an image forming apparatus for forming a magnet brush on a developer carrier and causing said magnet brush to contact a latent image formed on an image carrier to thereby develop said latent image, said developer carrier comprises a sleeve and a stationary magnet roller accommodated in said sleeve, said magnet roller includes a main pole configured to cause the developer to rise in a form of the magnet brush and an auxiliary pole configured to help said main pole exert a magnetic force, a ratio of a distance between said image carrier and said developer carrier, as measured at a boundary of a nip for development, to a shortest distance between said image carrier and said developer carrier is 1.5 or below, and an electric field including an oscillation component is formed between said image carrier and said developer carrier.

2. The apparatus as claimed in claim 1, wherein the oscillation component comprises an asymmetric, rectangular waveform configured to reduce a period of time over which toner contained in the developer migrates toward said image carrier.

3. The apparatus as claimed in claim 2, wherein the oscillation component is configured to oscillate at least ten times before a given point on said image carrier moves away from a range in which the magnet brush remains in contact with said image carrier.

4. The apparatus as claimed in claim 1, wherein the oscillation component is configured to oscillate at least ten times before a given point on said image carrier moves away from a range in which the magnet brush remains in contact with said image carrier.

5. In an image forming apparatus for forming a magnet brush on a developer carrier and causing said magnet brush to contact a latent image formed on an image carrier to thereby develop said latent image, said developer carrier comprises a sleeve and a stationary magnet roller accommodated in said sleeve, said magnet roller includes a main pole configured to cause the developer to rise in a form of the magnet brush and an auxiliary pole configured to help said main pole exert a magnetic force, a ratio of a shortest distance between said image carrier and said developer carrier to a shortest distance between said developer carrier and a metering member, which regulates the developer, is smaller than 0.8, and an electric field including an oscillation component is formed between said image carrier and said developer carrier, wherein the oscillation component is configured to oscillate at least ten times before a given point on said image carrier moves away from a range in which the magnet brush contacts said image carrier.

6. The apparatus as claimed in claim 5, wherein the oscillation component comprises an asymmetric, rectangular waveform configured to reduce a period of time over which toner contained in the developer migrates toward said image carrier.

7. In an image forming apparatus for forming a magnet brush on a developer carrier and causing said magnet brush to contact a latent image formed on an image carrier to thereby develop said latent image, said developer carrier comprises a sleeve and a stationary magnet roller accommodated in said sleeve, said magnet roller includes a main pole configured to cause the developer to rise in a form of the magnet brush and an auxiliary pole configured to help said main pole exert a magnetic force, a ratio of a shortest distance between said image carrier and said developer carrier to an amount of the developer scooped up to said image carrier is smaller than 10 mm/(g/cm2) and an electric field including an oscillation component is formed between said image carrier and said developer carrier, wherein the oscillation component is configured to oscillate at least ten times before a given point on said image carrier moves away from a range in which the magnet brush contacts said image carrier.

8. The apparatus as claimed in claim 7, wherein the oscillation component comprises an asymmetric, rectangular waveform configured to reduce a period of time over which toner contained in the developer migrates toward said image carrier.

COPYRIGHT

User acknowledges that Fairview Research and its third party providers retain all right, title and interest in and to this xml under applicable copyright laws. User acquires no ownership rights to this xml including but not limited to its format. User hereby accepts the terms and conditions of the License Agreement.