Patent Grant
6925277
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and a developing device and a process cartridge for the same and more particularly to a developing device of the type using a developer carrier formed with a number of grooves.
It is a common practice with an image forming apparatus to use a developing device configured to develop a latent image formed on an image carrier with a developer, which is deposited on a developer carrier, in a developing region where the developer carrier and image carrier face each other. A problem with this type of developing device is that when the amount of the developer deposited on the developer carrier decreases, the resulting image appears non-smooth. Therefore, the prerequisite with this type of developing device is that the developer be scooped up to the developer carrier in an amount stable enough to insure high image quality.
It has been reported that the amount of the developer to deposit on the developer carrier is susceptible to the frictional resistance of the surface of the developer carrier, i.e., the former decreases with a decrease In the latter. In this sense, increasing the frictional resistance of the developer carrier is effective to stabilize the amount of the developer to deposit on the developer carrier. For this purpose, the surface of the developer carrier may be roughened by sandblasting, as taught in, e.g., Japanese Patent Publication No. 1-5711. However, the frictional resistance of a rough surface formed by sandblasting is apt to decrease due to wear ascribable to the developer as development is repeated. It is therefore difficult with the sandblasted surface to maintain the amount of the developer to deposit on the developer carrier stable over a long time.
In light of the above, Japanese Patent Laid-Open Publication No. 2000-321864, for example, discloses a developing roller whose surface is formed with a plurality of axially extending grooves. The grooves are configured to increase the frictional resistance of the surface of the developing roller for thereby stabilizing the amount of the developer to deposit on the surface. The grooves do not easily disappear despite aging, so that the frictional resistance of the above surface decreases little. The developing roller can therefore allow the developer to deposit thereon in a stable amount over a long time.
Japanese Patent Laid-Open Publication No. 2001-134069 also teaches a developing device using a developing sleeve or developer carrier formed with a plurality of axially extending grooves or recesses.
However, the conventional developing devices using a developer carrier provided with a rough surface, as stated above, have some problems left unsolved, as will be described hereinafter. First, stripe-like pitch irregularity or so-called banding, corresponding to the pitch of the grooves, appears in a toner image. The pitch irregularity is ascribable to the fact that an electric field or a magnetic field in the developing zone varies from a portion where the surface of the developer carrier faces the surface of the image carrier to a portion where the grooves of the former face the latter. Therefore, how the degradation of image quality ascribable to the pitch irregularity should be reduced is a problem awaiting solution. Particularly, in a color image forming apparatus capable of forming a color image, the pitch irregularity appears in each of toner images of different colors to be superposed, critically degrading image quality.
Second, it is likely that the developer adheres to the surface of the developer carrier due to an increase in developer pressure at opposite end portions of the developer carrier in the developing zone or that the developer come off from the opposite end portions of the developer carrier. Particularly, the developer adhered to the opposite end portions of the developer carrier brings about various serial problems including the peeling of the surface layer of the image carrier, an image smeared at opposite edge portions, a banding image ascribable to the increase or the variation of drive load, and defective cleaning.
Further, I experimentally found that the adhesion of the developer and other problems stated above are apt to occur when use is made of a developer having a small grain size for enhancing image quality or when a gap for development is narrowed.
It is a first object of the present invention to provide a developing device and a process cartridge capable of reducing, while insuring stable conveyance of a developer in an image forming range, the adhesion of the developer to the surface of a developer carrier ascribable to the above-described occurrence, and an image forming apparatus including the same.
It is a second object of the present invention to provide a developing device capable of insuring a high-quality image free from conspicuous pitch irregularity ascribable to the grooves.
A developing device of the present invention includes a developer carrier whose surface is movable to convey a developer deposited thereon to a developing zone where the developer carrier faces an image carrier. A magnetic field generating member is accommodated in the developer carrier for forming a magnetic field that retains the developer on the surface of the developer carrier. A metering member faces the surface or the developer carrier for regulating the amount of the developer being conveyed by the surface toward the developing zone. The surface of the developer carrier is configured such that the center portion, including an image forming range corresponding to the image forming range of the image carrier, in the direction of width perpendicular to the direction of movement of the surface has a higher developer conveying ability than opposite end portions outward of the center portion. Opposite ends of a magnetic pole provided on the magnetic field generating member in the direction of width face the opposite end portions of the developer carrier.
An image forming apparatus including the above developing device is also disclosed.
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:
Preferred embodiments of the present invention will be described hereinafter.
A first embodiment of the present invention is mainly directed toward the first object stated earlier. First, to better understand the present invention, reference will be made to
Subsequently, the developer 610 moved away from the doctor 73 again gathers toward the image forming range D, as indicated by an arrow C in
The first embodiment of the present invention will be described hereinafter and is applied to a tandem, electrophotographic color copier by way of example. Generally, a tandem, image forming apparatus includes a plurality of photoconductive drums or image carriers arranged side by side and a plurality of developing units each being assigned to a particular drum. Toner images of different colors each being formed on one of the drums are sequentially transferred to a sheet or recording medium one above the other, completing a composite color image. The tandem, image forming apparatus implements a far higher printing speed than an image forming apparatus of the type repeating image formation with a single photoconductive drum. However, the problem with the tandem image forming apparatus is bulky due to a plurality of image forming sections.
The tandem, image forming apparatus uses either one of a direct and an indirect image transfer system, as will be described hereinafter. As shown in
In the direct image transfer system, a sheet feeding device 6 and a fixing device 7 must be respectively located upstream of the image forming section, labeled T, and downstream of the same, further increasing the overall size of the apparatus in the direction of sheet conveyance. If the fixing device 7 is positioned closer to the image forming section T in order to reduce the overall size as far as possible, then a margin for the sheet S to form a loop is not available. As a result, the trailing edge of an image is apt to be defective due to, e.g., an impact to occur when the leading edge of the sheet S enters the fixing device or a difference in sheet conveying speed to occur when the leading edge of the sheet S leaves the fixing device 7.
On the other hand, in the indirect image transfer system, the secondary image transfer position can be relatively freely located. Therefore, as shown in
Referring to
An endless, intermediate image transfer belt 10 is positioned at the center of the copier body 100 and serves as an intermediate image transfer body.
Referring again to
A secondary image transferring device 22 is positioned at the opposite side to the image forming section 20 with respect to the belt 10. The secondary image transferring device 22 includes an endless, secondary image transfer belt 24 passed over two rollers 23 and pressed against a third roller 16 via the belt 10, so that a toner image can be transferred from the belt 10 to a sheet.
A fixing unit 25 is positioned at one side of the secondary image transferring device 22 for fixing the toner image carried on the sheet. The fixing unit 25 includes an endless, fixing belt 26 and a roller 27 pressed against the belt 26.
The secondary image transferring device 22 bifunctions to convey the sheet, carrying the toner image thereon, to the fixing unit 25. Although the secondary image transferring device 22 may, of course, be implemented by a transfer roller or a non-contact type charger, it is difficult to provide the transfer roller or the charger with the sheet conveying function.
A sheet turning device 28 is arranged below the secondary image transferring device 22 and fixing unit 25 in parallel to the image forming section 20. The sheet turning device 28 turns back a sheet in a duplex copy mode.
In operation, the operator of the copier stacks desired documents on a document tray 30 included in the ADF 400 or opens the ADF 400, lays a single document on a glass platen 32 included in the scanner 300, and then closes the ADF 400. Subsequently, the operator presses a start switch not shown. In response, the ADF 400 conveys one document from the document tray 30 to the glass platen 32, When a single document is laid on the glass platen 32 by hand, the scanner 300 is immediately driven to cause its first and second carriages 33 and 34 to move. While a light source mounted on the first carriage 33 illuminates the document, the resulting reflection from the document is reflected toward the second carriage 34, reflected by a mirror mounted on the second carriage 34 to an image sensor 36 via a lens 35.
When the start switch is pressed, a drive motor, not shown, causes one of the rollers 14 through 16 to rotate for thereby moving the belt 10; the other two rollers are driven by the belt 10. At the same time, photoconductive drums 40B (K), 40M (magenta), 40C (cyan) and 40Y (yellow) included in the four image forming means 18 each are rotated to form one of a black, a magenta, a cyan and a yellow toner image thereon. The black to yellow toner images are sequentially transferred from the drums 40B through 40Y to the belt 10 being moved one above the other, completing a composite color image on the belt 10.
Further, when the start switch is pressed, one of pickup rollers 200 arranged in the sheet feed table 200 is caused to rotate and pay out a sheet from associated one of sheet cassettes 44, which are stacked one upon the other in a paper bank 43. At this instant, a reverse roller 45 separates the above sheet being paid out from the underlying sheets. The sheet thus paid out is conveyed by a roller pair 47 to a path 46 and then introduced into a path 48, which is formed in the copier body 100. The sheet is then stopped by a registration roller pair 49. On the other hand, a sheet, paid out from a manual feed tray by a pickup roller 50, is conveyed via a path 53 to the registration roller pair 49 and then stopped by the roller pair 49.
Subsequently, the registration roller pair 49 conveys the sheet in synchronism with the movement of the belt 10 to thereby deliver the sheet to the nip between the belt 10 and the secondary image transferring device 22. As a result, the composite color image is transferred from the belt 10 to the sheet.
The sheet with the color image is conveyed to the fixing unit 25 by the secondary image transferring device 22, so that the color image is fixed on the sheet by heat and pressure. A path selector 55 steers the sheet, coming out of the fixing unit 25, toward an outlet roller pair 56 The outlet roller pair 56 drives the sheet out of the apparatus body 100 to a tray 57.
After the image transfer, the belt cleaner 17 removes toner left on the belt 10 to thereby prepare the belt 10 for the next image formation.
While the registration roller pair 49 is generally grounded, a bias may be applied thereto for removing paper dust.
Part of or the entire image forming means 18, including at least the drum 40, may be constructed into a process cartridge removably mounted to the copier body 100, so that the image forming means 18 can be easily maintained.
In the illustrative embodiment, the charger 60 included in the image forming means 18 is implemented as a charge roller configured to charge the drum 40 in contact therewith. Of course, the charger 60 may be implemented by a scorotron charger spaced from the drum 40.
Reference will be made to
In
The developing roller 65 faces the drum 40 via an opening formed in the case 70. As shown in
More specifically, the magnet roller 72 has seven magnetic poles P1 through P7 by way of example. The magnetic poles P1 through P7 are sequentially arranged in this order from a position facing a developing zone in the direction of rotation of the sleeve 65. The magnet roller 72 causes the developer to form a magnet brush on the sleeve 650.
The two screws 68 feed the developer to the sleeve 650 while agitating and circulating it. The magnet roller 72 magnetically scoops up the developer to the sleeve 650 with the result that the developer deposits on the sleeve 650 in the from of a magnet brush. The magnet brush is conveyed by the sleeve 65 in rotation while being metered by the doctor 73 to form a thin layer on the sleeve 65. Excess part of the developer removed by the doctor 73 is returned to the agitating section 66.
A bias for development is applied to the sleeve 650. In this condition, the toner contained in the developer 650 is transferred from the sleeve 650 to the drum 40 and develops a latent image formed on the drum 40 for thereby producing a corresponding toner image. The developer left on the sleeve 650 after the development parts from the sleeve 650 at a position where the magnetic force of the magnet roller 72 does not act, returning to the agitating section 66. When the toner content of the developer present in the agitating section 66 decreases due to repeated development, fresh toner is replenished to the agitating section 66 in accordance with the output of the toner content sensor 71.
The primary image transferring device 62 is implemented as a charge roller although it may be implemented as a conductive brush or a corona charger. The charge roller is pressed against the drum 40 via the belt 10.
The drum cleaner 63 includes a cleaning blade 75 formed of, e.g., polyurethane rubber and having an edge pressed against the drum 40. A brush, contacting the drum 40, is used in combination with the cleaning blade 75 for enhancing cleaning ability. In the illustrative embodiment, the brush is implemented as a conductive fur brush 76 held in contact with the drum 40 and rotatable in a direction indicated by an arrow in
More specifically, the fur brush 76, rotating in the direction counter to the rotation of the drum 40, removes the toner left on the drum 40. The toner thus deposited on the fur brush 76 is removed by the electric field roller 77, which is applied with a bias and rotating in contact with the fur brush 76. Subsequently, the toner deposited on the electric field roller 77 is removed by the scraper 78. The toner so collected in the drum cleaner 63 is conveyed to one side of the drum cleaner 63 by the collection screw 79 and then returned to the developing device 61 by a toner recycling device 80.
The quenching lamp 64 initializes the surface potential of the drum 40 with light.
When the drum 40 starts rotating, the charger 60 uniformly charges the surface of the drum 40. The scanner 300 scans the charged surface of the drum 40 with light L, which issues from a laser or an LED (Light Emitting Diode) array, in accordance with image data derived from the output of the scanner 300, thereby forming a latent image on the drum 40.
Subsequently, the developing device 61 develops the latent image with toner for thereby producing a corresponding toner image. The toner image is then transferred from the drum 40 to the belt 10 by the charge roller 62. After the image transfer, the drum cleaner 63 removes toner left on the drum 40, and then the quenching lamp 64 discharges the surface of the drum 40 to thereby prepare it for the next image formation.
The belt cleaner 17 includes a fur brush or cleaning member 90 to which a preselected bias is applied from a power supply not shown.
As shown in
When the collection screw 79 is rotated by a drive force transferred thereto from the outside, the screw 79 causes the toner conveying member 83 to move and convey the toner, collected by the drum cleaner 63, to the developing device 61 via the case 88. Subsequently, the screw 68 mounted on the case 88 delivers the toner into the developing device 61. Thereafter, the two screws 60 circulate the toner while agitating it together with the developer present in the developing device 61. The resulting mixture is fed to the sleeve 650, metered by the doctor 73 and then transferred to the drum 40, as stated earlier.
The toner grains and carrier grains or magnetic grains, constituting the two-ingredient type developer, will be described in detail hereinafter. To produce toner grains, a charge control agent (CCA) and a colorant are mixed with polyester, polyol, styrene-acryl or similar resin, and then silica, titanium oxide or similar substance is coated on the individual grain for enhancing chargeability and fluidity. The grain size of additives usually lies in the range of from 0.01 μm to 1.5 μm. For the colorant, use may be made of carbon black, Phthalocyanine Blue, quinacrydone or carmine by way of example. In the illustrative embodiment, the toner grains are chargeable to negative polarity.
The additives mentioned above may be coated on the toner grains in which wax, for example, is dispersed while the toner grains are assumed to be produced by pulverization, they may alternatively be produced by, e.g., polymerization. Generally, toner grains produced by, e.g., polymerization or heating can have a shape factor of 90% or above and can be coated with additives in a high ratio.
The volumetric mean grain size of toner grains should preferably be between 3 μm and 12 μm. In the illustrative embodiment, the volumetric mean grain size is selected to be 6 μm that can sufficiently cope with resolution as high as 1,200 dpi (dots per inch) or above.
The carrier grains each consist of a metal or resin core, containing ferrite or similar magnetic substance, and a silicone resin or similar surface layer coated on the core. The carrier grains should preferably have a grain size ranging from 20 μm to 50 μm and resistance ranging from 104 Ω to 106 Ω in terms of dynamic resistance. To measure the resistance, the carrier grains are deposited on a roller accommodating a magnet therein and having a diameter of 20 cm and rotated at 600 rpm (revolutions per minute), and a 60 mm wide, 1 mm long electrode is spaced from the roller by a gap of 0.9 mm. In this condition, an upper limit voltage, which is 400 V in the case of grains coated with high-resistance silicone or several volts in the case of iron-powder grains, is applied.
The grain size of the carrier should preferably be reduced to noticeably enhance image quality. For example, while a carrier grain size of 50 μm or above cannot improve granularity above 0.3 or so as for a halftone dot image having a color value of 60 to 90, a carrier grain size of about 35 μm improves granularity to 0.1, i.e., by almost three times, as shown in FIG. 13.
Also, to maintain image quality constant, it is necessary to stabilize the amount ρ by which the developer is scooped up, or conveyed via the doctor 73, and to reduce the deterioration of the developer The amount ρ and deterioration are noticeably influenced by the magnetic force distribution of the pole of the magnet roller 72 facing the doctor 73, the surface configuration of the sleeve 650, and the surface configuration of the developer. More specifically, as shown in
The surface of the sleeve 650 is usually formed with grooves extending in the axial direction of the sleeve 650 at spaced locations along the circumference of the sleeve 650 or is roughened by sandblasting. However, as shown in
Further, even if granularity is improved by using the carrier grains with a small grain size, the improvement is canceled by the non-smoothness of an image ascribable to a decrease in the amount ρ of scoop-up derived from the wear of the coating layers of the developer grains. The fall of the developer conveying ability ascribable to such wear becomes more conspicuous as the rotation speed of the sleeve 650 becomes higher, as in the illustrative embodiment, because wear is more aggravated. A solution to this problem is a key to a future high speed, high image quality machine.
In light of the above, in the illustrative embodiment, the surface of the sleeve 650 is provided with the following configuration in order to reduce the fall of the developer conveying ability stated above. Assuming that the sleeve 650 has an outside diameter of a and formed with n grooves 650a, that the drum 40 rotates at a linear velocity of Vp, and that the sleeve 650 rotates at a linear velocity of Vs, then the surface of the sleeve 650 is configured to satisfy the following relations:
pitch on image=aαVp/nVs≦0.5 (mm)
n≧aαVp/(0.5Vs) (1)
The number of grooves 650a, satisfying the above relations (1), allows the pitch on an image corresponding to the grooves 650a to be confined in a banding range of 0.5 mm or below difficult to see by eye, as determined by experiments (see FIGS. 17A and 17B). More specifically, when the outside diameter a of the sleeve 650 is 25 mm and when the linear speed ratio Vs/Vp is 2, the sleeve 650 is formed with 100 grooves 650a so as to implement the above banding range. In this specific condition, the relations (1) are satisfied as follows:
(25×n)/(100×2)≈0.39<0.5 (mm)
Further, fine pitch irregularity or banding is blurred by the width of a nip Nd (see
As shown in
The fall of developer conveying ability ascribable to the wear of the coating layers can be improved, as stated above. Further, by obviating the above wear, it is possible to realize an ideal, ultra-stable range in which the amount ρ of scoop-up does not vary at all. Carrier grains have heretofore been developed under the notion of extending the life by shaving off hard coating layers little by little. By contrast, the illustrative embodiment extends the life of the carrier grains, i.e., free the carrier grains from shave-off and spent by well balancing the following two effects (1) and (2):
(1) providing the carrier grains with elasticity to thereby absorb impacts and reduce shave-off, and using highly adhesive coating layers to thereby retain large grains; and
(2) causing carrier surfaces to contain grains larger than the coating layers to thereby protect the coating layers from impacts and remove spent substances.
The above carrier grains each consist of a ferrite core and a coating layer in which a charge control agent is contained in a resin component produced by the crosslinking of acrylic resin or similar thermoplastic resin and melamine resin. As shown in
Now, portions characterizing the illustrative embodiment will be described specifically hereinafter.
The sleeve 650 with the V-shaped grooves insures stable conveyance of the developer. However, if such stable conveyance is guaranteed even at opposite end portions of the sleeve 650 in the axial direction, then the magnetic force of the magnet roller 72 concentrated at the opposite end portions causes the developer to flow into the end portions of the image forming range of the sleeve 650. As a result, developer density at the opposite end portions of the nip for development increases, causing the pressure of the developer to increase between the surface of the drum 40 and that of the sleeve 650 at the opposite end portions. In this condition, the developer is apt to adhere to or drop from the opposite end portions of the sleeve 650. The developer adhered to the sleeve 650 critically damages the image forming apparatus by bringing about the peeling of the surface layer of the drum 40, an image smeared at opposite edge portions, a banding image ascribable to drive load, and defective cleaning. Such a phenomenon is accelerated due to the decreasing grain size and decreasing gap Gd for development.
(1) The center portion of each V-shaped groove (groove portion hereinafter) is extended over a range that guarantees the width of the image forming range D, i.e., to the outside of the image forming range D. More specifically, as shown in
Further, opposite ends of the pole P6 provided on the magnet roller 72 face the opposite non-groove portions of the sleeve 650. More specifically, as shown in
(2) A magnetic plate 730 is mounted on the upstream surface of the doctor 73 in the direction of developer conveyance and constitutes a magnetic member to be magnetized by the magnet roller 72. Opposite end portions of the magnetic plate 730 are protruded toward the sleeve 650 in correspondence to the opposite non-groove portions of the sleeve 650, thereby preventing the developer from flowing into the opposite end portions with magnetic restraint.
(3) As shown in
G>F≧E>D
With the above relation, the side walls 95 can surely regulate the scoop-up of the developer at the opposite ends.
In the illustrative embodiment, the gap for development is selected to be 0.4 mm or below in order to obviate a granular image as well as the omission of portions around characters and the trailing edge of an image. More preferably, the above gap should be 0.25 mm or above. A gap less than 0.25 mm is apt to cause the developer pressure to excessively rise at the center portion of the sleeve 650 in the developing zone I due to the error of the doctor gap and that of the amount of scoop-up, the oscillation of the sleeve surface and that of the drum surface, resulting in, e.g., the adhesion of the developer to the center portion of the sleeve 650.
It is to be noted that the shape of the grooves formed in the sleeve 650 is not limited to “V”, but may be replaced with any other shape, The illustrative embodiment is, of course, practicable with a sleeve whose center portion is roughened by sandblasting or formed with ridges extending in the axial direction.
A second embodiment of the present invention is directed mainly toward the second object stated earlier. Because
The developing device shown in
(1) stable scoop-up of the developer onto the developing roller
(2) reduction of the size of the carrier grains
(3) reduction of the deterioration of the developer
To satisfy the condition (1), the developing roller 65 should preferably be formed with a plurality of axially extending grooves, so that the frictional resistance of the roller surface is increased.
A first to a fourth specific examples of the illustrative embodiment to be described hereinafter are configured to solve the problem stated above. In the following description, structural elements identical with those shown in
<First Example>
In a first example, the developing device includes the following configuration in addition to the configurations of the developing device shown in
More specifically, in the first example, the number of grooves 13 formed in the sleeve 650 is selected to satisfy the above relation. The pitch of the pitch irregularity to appear in a toner image corresponds to the pitch of the grooves 13, as stated earlier. More specifically, the pitch Px of the pitch irregularity is expressed as:
Px=L/n×(Vp/Vs) (3)
As the above equation (3) indicates, the pitch Px decreases with an increase in the number n of grooves 13 or increases with an increase in the number n. Assuming that the number n of grooves 13 is minimum, then the relation of n≧(L·Vp)/(P·Vs) is rewritten as:
n=(L·Vp)/(P·Vs) (4)
By substituting the equation (4) for the equation (3), there is obtained:
Px=P (5)
As stated above, in the first example, even when the number n of the grooves 13 formed in the sleeve 650 is minimum, the pitch Px of the pitch irregularity to appear in a toner image is as small as P that cannot be recognized by eye, as the equation (5) indicates.
<Second Example>
A second example differs from the first example in that the maximum pitch P is selected to be 0.5 mm. In this case, the relation of n≧(L·Vp)/(P·Vs) is rewritten as:
n≧(L·Vp)/(0.5Vs)
From this relation, the banding pitch to appear in a toner image is expressed as LVp/nVs≦0.5
25×n/100×2≈0.39≦0.5
Therefore, when the sleeve 650 with the above specific configuration is used, the banding can be reduced to a level that cannot be seen by eye.
<Thrird Example>
A third example differs from the first and second examples in that the grooves 13 formed in the sleeve 650 each are selected to fall between 0.01 mm and 0.1 mm.
<Fourth Example>
As shown in
In light of the above, how the field strengths b and c vary in accordance with the depth of the V-shaped groove 13,
Further, as shown in
<Fifth Example>
This example differs from the first to fourth examples in that it includes a configuration satisfying the condition (2) stated earlier, In a developing device of the type using a two-ingredient type developer, the grain size of the carrier should preferably be reduced to noticeably enhance image quality, as known in the art. For example, while a carrier grain size of 50 μm or above cannot improve granularity above 0.3 or so as for a halftone dot image having a color value of 60 to 90, a carrier grain size of about 35 μm improves granularity to 0.1, i.e., by almost three times, as shown in FIG. 13. This successfully improves dot reproducibility. Considering this fact, this example forms a toner image with a developer containing magnetic carrier grains whose grain size is 50 μm or below. While the carrier grain size should preferably be as small as possible, the minimum grain size available with the state-of-the-art technologies is 20 μm, as generally understood.
<Sixth Example>
A sixth example differs from the fifth example in that it additionally includes a configuration satisfying the condition (3) stated earlier. In a developing device of the type described, to enhance image quality, it is necessary to stabilize the amount ρ by which the developer is scooped up, or conveyed via the doctor 73, and to reduce the deterioration of the developer. The amount ρ and deterioration are noticeably influenced by the magnetic force distribution of the pole of the magnet roller 72 facing the doctor 73, the surface configuration of the sleeve 650, and the surface configuration of the developer. More specifically, as shown in
Further, even if granularity is improved by using the carrier grains with a small grain size, a decrease in the amount of scoop-up ascribable to the wear of the coating layers renders images non-smooth. The fall of the developer conveying ability ascribable to such wear becomes more conspicuous as the rotation speed of the sleeve 650 becomes higher because wear is more aggravated. A solution to this problem is a key to a future high speed, high image quality machine. One of major factors of the wear of coating layers is that carrier grains have heretofore been developed under the notion of extending the life by shaving off hard coating layers little by little.
To solve the above problem, as shown in
<Seventh Example>
A seventh example differs from the sixth example in that it additionally includes the following configuration. In the seventh example, the carrier grains 700 each consist of the core 701 formed of ferrite and the coating layer 702 in which a charge control agent is contained in a resin component produced by the crosslinking of acrylic resin or similar thermoplastic resin and melamine resin. With this configuration, the carrier grain 700 is shaved off little.
To form the grooves 13 in the sleeve 650 in any one of the specific examples described above, a hollow cylindrical tube formed of, e.g., aluminum may be subjected to drawing. The pitch of the grooves 13 is less than the maximum pitch P stated earlier. The grooves 13 may extend in the axial direction of the sleeve 650 or extend spirally along the surface of the sleeve 650. Further, the sleeve 650 formed with the grooves 13 may have its surface sandblasted in order to improve the developer conveying ability and obviate the pitch irregularity at the same time.
Image forming apparatuses other than the apparatus shown in
The developing device 807 includes a case 808, the sleeve or developer carrier 650, a developer chamber or developer storing portion 809, a first and a second doctor 810 and 811, and a toner hopper 812. The case 808 is formed with an opening facing the drum 800 and so configured as to surround the lower portion of the sleeve 650. The sleeve 650 is rotatable around magnetic field generating means held stationary thereinside and implemented as a permanent magnet not shown. The first doctor 810 is spaced from the speed 650 by a preselected gap for regulating the thickness of the developer deposited on the sleeve 650.
The developer chamber 809 is positioned upstream of the first doctor 810 in the direction of rotation of the sleeve 650 and stores part of the developer removed by the doctor 810. The second doctor 811 is positioned at the bottom of the developer chamber 809 and spaced from the sleeve 650 by a preselected gap. When the toner content of the developer deposited on the sleeve 650 and therefore the thickness of the developer layer increases, the second doctor 811 removes the increment of the developer. The toner hopper 812, storing fresh toner 813 to be replenished, adjoins the developer chamber 809 and is constructed integrally with the case 808.
Part of the case 808 beneath the developer chamber 809 is implemented as a facing surface 808a formed with a projection 808b. The facing surface 808a extends over a preselected length while being inclined downward from the toner hopper 812 side toward the sleeve 650. The facing surface 802a and the bottom of the developer chamber 809 form a toner feed opening 814 for replenishing the fresh toner 813 from the hopper 812. An agitator or agitating member 815 is disposed in the toner hopper 812 for conveying the toner 813 toward the toner feed opening 814.
In
The developer chamber 809 is large enough to allow the developer 816 to circulate over the range in which the magnetic force of the sleeve 650 acts. In the developer chamber 809, the developer 816 present therein exerts a force that tends to obstruct the movement of the developer 816 being conveyed by the sleeve 650.
When the fresh toner 813 is replenished to the developer being conveyed by the sleeve 650 (moving developer layer) via the toner feed opening 814, the fresh toner 813 is conveyed to the interface mentioned above. As a result, the toner 813 lowers a frictional force acting between the moving developer layer and the circulating developer layer around the interface, thereby reducing the amount of the developer being conveyed around the interface.
On the other hand, the force, tending to obstruct the movement of the developer 816, does not act on part of the developer 816 positioned upstream of the joining point in the direction of rotation of the sleeve 650. Therefore, the developer 816 brought to the joining point and the developer 816 being conveyed at the interface are brought out of balance in amount. Consequently, the joining point shifts upward while the moving developer layer becomes thick until the developer accumulates at the position upstream of the second doctor 811.
When the developer accumulates at the above position until it stops the toner feed opening 814, the replenishment of the fresh toner 813 via the opening 814 ends. At this instant, the toner content and therefore the volume of the developer increases in the developer chamber 809, so that the space available in the chamber 809 decreases and stops the movement of the circulating developer layer. In this manner, the toner content of the developer deposited on the sleeve 650 is controlled to any preselected value.
Further, the developer 816 on the sleeve 650 is regulated by the first doctor 810 to adequate thickness and then conveyed to a developing zone where the sleeve 650 faces the drum 800. At the developing zone, only the toner of the developer 816 is electrostatically deposited on a latent image formed on the drum 800, thereby producing a corresponding toner image.
The toner feed roller 821 is implemented as a nonmagnetic, hollow cylinder formed of, e.g., aluminum, brass, stainless steel or conductive resin and caused to rotate by a drive mechanism not shown. A doctor 822 is positioned at the upstream portion of the toner feed roller 821 for metering the developer deposited on the roller 821. Further, a screw, paddle or similar agitator 824 is disposed in a casing 823 that stores the developer.
A color printer 832 includes an optical writing unit 833 that converts the color image data to optical signals and scans the drum 830 with each of the optical signals for thereby forming a latent image. A drum cleaner 834, including a precleaning discharger, adjoins the drum 830. Also arranged around the drum 830 are a quenching lamp 835, a charger 836, a potential sensor 837, a Bk developing device 838, a C developing device 839, an M developing device 840, a Y developing device 841, and an optical sensor 842 responsive to the density of a density pattern. An intermediate image transfer belt unit includes an intermediate image transfer belt (simply belt hereinafter) 843 and an intermediate image transfer roller (simply roller hereinafter) 844. The Bk through Y developing devices 838 through 841 each include a sleeve 650, a paddle for scooping up the developer while agitating it, and a toner content sensor.
The belt 843 is passed over a drive roller, a driven roller and a primary image transfer roller (simply roller hereinafter) 844 and driven by a motor, not shown, via the drive roller. A moving mechanism, not shown, selectively moves the belt 843 into or out of contact with the drum 830. A belt cleaner 845 adjoins the belt 843 at a preselected position. The belt cleaner 845 is released from the belt 843 from the time when printing starts to the time when belt transfer of the trailing edge of a Y toner image ends, and again brought into contact with the belt 843 at preselected timing for cleaning it.
The image transfer belt unit faces part of the belt 843 passed over the drive roller. The belt 846 is passed over the roller 847, a drive roller, a driven roller and so forth so as to directly convey a sheet from the position where the belt 846 faces the drive roller assigned to the belt 843 to a fixing unit 848.
When the apparatus is in a stand-by state, the revolver 900 remains in a halt at its home position where the Bk developing section 901 faces the drum 830 at a developing position. When a copy start key is pressed, a latent image is formed on the drum 830 in accordance with Bk image data by the procedure stated earlier. Let the latent image derived from the Bk image data be referred to as a Bk latent image. This is also true with Y, C and M.
To develop the Bk latent image from its leading edge, the sleeve 650 of the Bk developing section 901 starts being rotated before the above leading edge arrives at the developing position, thereby developing the Bk latent image with Bk toner. Subsequently the revolver 900 is rotated as soon as the trailing edge of the Bk latent image moves away from the developing position, locating the next developing section at the developing position. This rotation of the revolver 900 completes at least before the leading edge of a latent image derived from the next image data arrives at the developing position.
On the start of the image formation, the drum 830 and belt 843 start being rotated counterclockwise, as viewed in
Subsequently, the registration roller pair conveys the sheet such that the leading edge of the sheet meets the leading edge of the color image carried on the belt 843. When the sheet met the color image is being conveyed via a secondary image transfer position, the roller 847 transfers the color image from the belt 843 to the sheet. The sheet is then separated from the belt 846 and conveyed to the fixing unit 848. The fixing unit 848 fixes the color image on the sheet with heat and pressure. Thereafter, the sheet or print is driven out of the apparatus body by an outlet roller pair not shown.
On the other hand, the toner left on the drum 830 after the primary image transfer is removed by the drum cleaner 834 Also, the toner left on the belt 843 after the secondary image transfer is removed by the belt cleaner 845.
In a repeat copy mode, after the first M or fourth-color toner image has been formed, the color scanner 831 and drum 830 advance to a step of forming the second Bk or first-color toner image at preselected timing. As for the belt 843, after the secondary image transfer of the first color image to a sheet, the second Bk toner image is transferred to the area cleaned by the belt cleaner 845. This is followed by the same procedure as with the first sheet.
In a three-color or a two-color mode, as distinguished from the four-color mode, the operation described above is repeated a number of times corresponding to desired colors and the number of desired copies. In a single-color mode, only the developing section of the revolver 900 corresponding to desired color is held operative at the developing position until a desired number of copies have been output. In this mode operation, the belt cleaner 845 is continuously pressed against the belt 843.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2002-171161 | Jun 2002 | JP | national |
| 2002-179070 | Jun 2002 | JP | national |
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