JP2006154381A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of obtaining an image of stable image density without causing image failure, such as void, due to sticking of carrier, by employing the configuration in which a color located upstream in the rotating direction of an intermediate transfer body can obtain higher toner developability than that a color located downstream can obtain. <P>SOLUTION: The image forming apparatus has: a plurality of image forming sections disposed along the rotating direction of the intermediate transfer body, each image forming section including an image carrier, toner, a developer carrier for holding developer containing the toner, and a primary transfer means for primarily transferring a developed toner image on the image carrier to the intermediate transfer body; and a secondary transfer means for secondarily transferring, all at once, a plurality of toner images primarily transferred to the intermediate transfer body in order from the corresponding image carriers of the plurality of image forming sections. In the image forming apparatus, the toner development efficiency of the image forming section located upstream in the rotating direction of the intermediate transfer body is higher than the toner development efficiency of the image forming section located downstream. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus using an electrophotographic system such as a copying machine or a printer.

  As an electrophotographic process for obtaining a color image, YMCK developing devices and photosensitive drums are arranged along the rotation direction of the intermediate transfer member, and YMCK toner images are superimposed on the intermediate transfer member (primary transfer). After that, a tandem method is known in which one batch transfer (secondary transfer) is performed on paper.

In the color image forming apparatus using the tandem method, the toner image (for example, Y) firstly transferred to the intermediate transfer member first is a photosensitive drum having a color (for example, MCK) positioned downstream of the intermediate transfer member. After passing over, it is secondarily transferred onto the paper. At this time, the Y toner on the intermediate transfer belt is injected from the primary transfer roller in the M, C, K primary transfer portion, and retransferred onto the M, C, K photoconductor drums with a reverse charge. It is known that there is a phenomenon of being done. (See Patent Document 1)
For example, when the toner is negatively charged in the reversal development method, a positive voltage for attracting the negatively charged toner is applied to the primary transfer roller. At that time, if a positive voltage is continuously applied to the M, C, K primary transfer portion located downstream after the transfer onto the intermediate transfer belt like Y toner, a positive charge is injected into the toner. As a result, it is attracted to the negatively charged photosensitive drum and re-transferred onto the drum located on the downstream side. (This phenomenon may be referred to as “return” below.)
As described above, “return” occurs in the color located on the upstream side. Therefore, in order to set the toner adhesion amount on the paper to a target value (for example, 4 g / m ² ), the color located on the downstream side is different from the color located on the downstream side. It is conceivable that a larger amount of toner is put on the photosensitive drum as much as the “return”.

Conventionally, a solid patch image is formed on an intermediate transfer belt, the patch density is read by a sensor having an LED and a phototransistor, and the development DC bias is changed in accordance with the read value to adjust the image density to a target value. Concentration stabilization control means is known.
JP 2003-91130 A

  However, when the development DC bias is used as the density stabilizing unit, the upstream color toner (for example, Y) is loaded more than the downstream color toner (for example, MC), so that the upstream color development DC bias is It becomes higher than the downstream color. As a result, charges are more likely to be injected into the carrier in the upstream color, the amount of carrier adhesion is greater than in the downstream color, and image defects such as white spots frequently occur on the image.

  An object of the present invention is to provide a structure in which the color located upstream is higher in toner developability than the color located downstream, so that the image density is stable without causing white-out image defects due to carrier adhesion. An image forming apparatus capable of obtaining an image is provided.

  The object of the present invention is achieved by the following configurations.

(Claim 1)
Developing means for developing the electrostatic latent image formed on the image carrier with a developer containing toner while applying a development DC bias, and transferring the developed toner image on the image carrier to an intermediate transfer member 1 A plurality of image forming units having a secondary transfer unit are disposed along the moving direction of the intermediate transfer member, and a plurality of toner images sequentially transferred from the plurality of image forming units onto the intermediate transfer member are collectively collected on the recording material. In the image forming apparatus having the secondary transfer means for transferring,
By adopting at least one of the following (a) to (c), the developing efficiency of the toner of the first image forming unit located on the upstream side in the moving direction of the intermediate transfer member is located on the downstream side. An image forming apparatus characterized in that the developing efficiency of toner in the second image forming unit is higher.
(A) The charge amount of the toner in the first image forming unit is set lower than the charge amount of the toner in the second image forming unit.
(B) The amount of toner external additive in the first image forming unit is different from the amount of toner external additive in the second image forming unit.
(C) The composition of the resin coating the carrier of the developer in the first image forming unit is different from the composition of the resin coating the carrier of the developer in the second image forming unit.

(Claim 2)
Developing means for developing the electrostatic latent image formed on the image carrier with a developer including toner and carrier while applying a development DC bias, and transferring the developed toner image on the image carrier to the intermediate transfer member A plurality of image forming portions having a primary transfer means for moving the intermediate transfer member along the moving direction of the intermediate transfer member, and a plurality of toner images sequentially transferred from the plurality of image forming portions onto the intermediate transfer member on the recording material; In the image forming apparatus having the secondary transfer unit that collectively transfers the toner density of the developer in the first image forming unit located on the upstream side in the moving direction of the intermediate transfer member, An image forming apparatus comprising control means for making the toner density of the developer in the image forming section of 2 higher than the toner density.

(Claim 3)
A developing unit that has a developer carrying member that carries a developer containing toner, and that develops the electrostatic latent image formed on the image carrier with a developer containing toner while applying a development DC bias; A plurality of image forming units having a primary transfer means for transferring the toner image on the image carrier to the intermediate transfer member are disposed along the moving direction of the intermediate transfer member, and the plurality of image forming units are arranged on the intermediate transfer member. In the image forming apparatus having a secondary transfer unit that collectively transfers a plurality of toner images sequentially transferred onto a recording material, a first image forming unit located upstream in the moving direction of the intermediate transfer member An image forming apparatus comprising: a control unit configured to make the rotation speed of the developer carrier higher than the rotation speed of the developer carrier of the second image forming unit located on the downstream side.

  In the image forming apparatus of the present invention, the developability of the toner in the image forming unit located on the upstream side is higher than that on the downstream side, so the upstream development DC bias is excessively higher than the downstream development DC bias. Therefore, the image density can be stabilized without problems such as carrier adhesion.

  Embodiments of the present invention will be described. FIG. 1 is a diagram showing a configuration of a color image forming apparatus which is an example of an image forming apparatus according to an embodiment of the present invention.

  The color image forming apparatus includes an image forming apparatus main body GH and an automatic document feeder JG installed on the image forming apparatus main body GH.

  The automatic document feeder JG conveys the documents d placed on the document feed table 31 one by one, passes the reading position, and discharges them to the document discharge table 32. The automatic document feeder JG can be opened and closed. By opening the automatic document feeder JG, a document can be placed on the document placement unit 45.

  Reference numeral 40 denotes an image reading unit, which includes a scanning unit 41 including a light source and a mirror for illuminating a document, a scanning unit 42 including two mirrors, an imaging lens 43, an image pickup device 44 including a CCD, and a document placement unit 45. .

  In document reading using the automatic document feeder JG, the scanning units 41 and 42 are set at the positions shown in the figure, and the automatic document feeder JG is set at the positions shown in the figure to convey the document d and to deliver a large number of documents d. Are continuously read. In document reading using the document placement unit 45, the automatic document feeder JG is opened, the document d is placed on the document placement unit 45, and the scanning units 41 and 42 are moved to scan the document d. Thus, the original is read. The automatic document feeder JG includes an automatic duplex document feeder. Image data read from the document is stored in a storage unit of an image processing unit C2 (not shown), and image processing such as filter processing, gradation adjustment, and scaling processing is performed.

  The image forming apparatus main body GH includes four image forming units 10Y, 10M, 10C, and 10K, a control unit 100, a belt-shaped intermediate transfer body 6, a paper feeding / conveying unit, a fixing unit 24, and an operation input unit 9. (Not shown), and the four image forming units 10Y, 10M, 10C, and 10K move along the moving direction of the rotating belt-shaped intermediate transfer body 6 on the image forming unit side (the direction of the arrow AA in the figure). This is a so-called tandem full-color image forming apparatus in which 10Y, 10M, 10C, and 10K are arranged in parallel from the upstream side. The control means 100 is a computer system composed of a CPU (central processing circuit), storage means, arithmetic unit, input / output interface, and the like, and comprehensively controls the operation of each part.

  The image forming unit 10Y that forms a yellow image includes a charging unit 2Y, an exposure unit 3Y, a developing unit 4Y, and a first cleaning unit 8Y disposed around a photoconductor 1Y that is an image carrier. The image forming unit 10M that forms a magenta image includes the photoconductor 1M, the charging unit 2M, the exposure unit 3M, the developing unit 4M, and the first cleaning unit 8M as described above. The image forming unit 10C that forms a cyan image includes the photoconductor 1C, the charging unit 2C, the exposure unit 3C, the developing unit 4C, and the first cleaning unit 8C as described above. The image forming unit 10K that forms a black image includes the photoconductor 1K, the charging unit 2K, the exposure unit 3K, the developing unit 4K, and the first cleaning unit 8K as described above. The charging unit 2Y and the exposure unit 3Y, the charging unit 2M and the exposure unit 3M, the charging unit 2C and the exposure unit 3C, and the charging unit 2K and the exposure unit 3K constitute an electrostatic latent image forming unit. 5Y, 5M, 5C, and 5K are toner containers containing yellow toner, magenta toner, cyan toner, and black toner, respectively, and these toner containers correspond to consumption in the developing units 4Y, 4M, 4C, and 4K. Toner is supplied.

  Each of the photoreceptors 1Y, 1M, 1C, and 1K is a negatively chargeable OPC photoreceptor in which an OPC photosensitive layer is formed on a metal drum. As the photoreceptors 1Y, 1M, 1C, and 1K, those other than the OPC photoreceptor such as an aSi photoreceptor can be used, and a positively charged photoreceptor can also be used.

  As the charging means 2Y, 2M, 2C, and 2K, a corotron discharger or a scorotron discharger can be used, and a discharge wire type, a saw electrode type, or the like can be used.

  The exposure unit 3Y has a semiconductor laser as a light source, and performs dot exposure of the photoreceptor 1Y with a laser beam, and performs exposure based on yellow image data. Similarly, the exposure unit 3M exposes the photoconductor 1M based on the magenta image data, the exposure unit 3C exposes the photoconductor 1C based on the cyan image data, and the exposure unit 3K uses the black image data. To expose.

  As the exposure means 3Y, 3M, 3C, 3K, an exposure means other than a laser beam such as an LED array or a liquid crystal can be used, but those that perform dot exposure are preferable.

  The developing means 4Y, 4M, 4C, and 4K have developer carriers 46Y, 46M, 46C, and 46K that form openings in the photoreceptors 1Y, 1M, 1C, and 1K and can be rotated by driving from a motor. is doing.

  Details of the developing means 4Y, 4M, 4C, and 4K will be described later.

  As the developer, a developer described in Japanese Patent Application No. 2003-304317 can be used, and a two-component developer containing a toner and a carrier can be preferably used. The toner is preferably a polymerized toner having a volume average particle diameter of 3 to 6.5 μm. Further, external additive particles for controlling fluidity and chargeability are attached to the toner surface. As the external additive, at least one external additive selected from silica, alumina, titania and the like is externally added.

  As the carrier, a resin-coated carrier in which a resin is coated on the surface of magnetic particles can be suitably used.

  Further, the developing means 4Y, 4M, 4C, and 4K may be either a reversal developing type that attaches toner to an exposed portion or a regular developing type that attaches toner to an unexposed portion, a contact developing method or a non-contact developing method. Either of these may be used.

  The first cleaning means 8Y, 8M, 8C, 8K are rubber cleaning blades 88Y, 88M, 88C, 88K as first cleaning members formed long in the longitudinal direction of the photoreceptors 1Y, 1M, 1C, 1K. Each.

  The intermediate transfer member 6 is a seamless semiconductive resin belt, is stretched by a plurality of rollers, and is supported so as to be rotatable.

  The images of the respective colors formed by the image forming units 10Y, 10M, 10C and 10K are sequentially primary transferred and synthesized by the primary transfer means 7Y, 7M, 7C and 7K on the intermediate transfer body 6 which is rotated and moved. A color image is formed.

  The primary transfer units 7Y, 7M, 7C, and 7K are conductive foam rollers, and are pressed against the back surface of the intermediate transfer member by the pressure-bonding units 71Y, 71M, 71C, and 71K. The toner is primarily transferred to the roller by selecting and applying a predetermined current value from a current value table in which a matrix is formed by temperature and humidity and a counter.

  The color toners remaining on the surfaces of the photoreceptors 1Y, 1M, 1C, and 1K after the toner is primarily transferred to the intermediate transfer member 6 are respectively cleaned by the cleaning blades 88Y, 88M, 88C, and 88K as the first cleaning members. To be cleaned.

  The sheet P, which is a recording material accommodated in the sheet feeding cassette 20 of the sheet feeding / conveying section, is fed by the sheet feeding means 21 of the sheet feeding / conveying section, and is fed by the sheet feeding rollers 22A, 22B, 22C, 22D, and the registration roller 23. And the like, and is conveyed to the secondary transfer means 7A, and the color images are batch-transferred onto the paper P collectively. The paper P on which the color image has been transferred is fixed by the fixing unit 24, is sandwiched between the paper discharge rollers 25, and is placed on a paper discharge tray 26 outside the apparatus.

  On the other hand, after transferring the color image to the paper P by the transfer means 7A, the intermediate transfer body 6 from which the paper P has been separated is made of rubber, which is a second cleaning member formed long in the width direction of the intermediate transfer body 6. Cleaning is performed by the second cleaning means 8A having the cleaning blade 89A.

  The cleaning blade 89A is always in pressure contact with the intermediate transfer member 6, and the intermediate transfer member 6 is slid on the intermediate transfer member 6 as the intermediate transfer member 6 is rotated by the driving means of the intermediate transfer member, whereby the toner is cleaned. The

  In the photoreceptors 1Y, 1M, 1C, and 1K, in which residual toner is cleaned by the cleaning blades 88Y, 88M, 88C, and 88K, the charge remaining on the surface is erased by a neutralizing unit (not shown) in a neutralization process. Then, returning to the charging process again, the above-described series of image forming operations is repeated.

  The details of the developing means and its control will be described below.

  FIG. 2 is a central sectional view of the developing means 4 according to the embodiment of the present invention. Hereinafter, the developing means 4Y, 4M, 4C, and 4K are indicated by the developing means 4.

  The developing means 4 includes a developing means frame 103, a developer carrier 46 made up of a developing roller, a magnetic field generating means (magnet roll) 104, a regulating means 105 made up of a panicle plate, a water wheel type feed means 106, and a feed made up of a screw. Consists of a conveying means 107, a stirring / conveying means 112 comprising a screw, a conveying roller 109, a stripping plate 110, a collecting means 111 comprising a screw, a toner concentration sensor 101 as a toner concentration detecting means, and the like.

  The developer carrying member 46 is disposed to face the photosensitive member 1 carrying the electrostatic latent image and is rotatably supported. The developer carrying member 46 rotates as indicated by an arrow to convey the developer to the developing region DR. A developer layer necessary for development is formed by supporting the developer in the development region DR.

  The magnetic field generating means 104 is disposed inside the developer carrier 46 and has a plurality of magnetic poles N1, N2, S1, S2, and S3. Two magnetic poles S2 and S3 adjacent to each other among the plurality of magnetic poles of the magnetic field generating means 104 are arranged with the same polarity to form a repulsive magnetic field. The developer stripping magnetic pole S2 strips off the developer on the developer carrying member 46 and scatters it. The developer receiving magnetic pole S <b> 3 sucks the developer supplied by the supply means 106 and attaches it to the developer carrier 46.

  The supply means 106 is a rotatable water wheel type conveying means for supplying the developer to the developer carrying member 46, and the developer conveyed from the supply / conveying means 107 is used as a developer receiving magnetic pole of the developer carrying body 46. Supply uniformly to around S3. The supply means 106 may be a screw having a conveying function in the direction of the rotation axis.

  The supply / conveyance means 107 is arranged in parallel with the supply means 106 and conveys the developer conveyed from the agitation / conveyance means 112 to the supply means 106 while conveying the developer in the rotation axis direction.

  The agitating / conveying means 112 mixes the new toner to be replenished with the developer refluxed from the supplying / conveying means 107, agitates them, and conveys them to the upstream portion of the supplying / conveying means 107.

  A transport roller 109 is disposed in the vicinity of the developer stripping magnetic pole S2 of the developer carrier 46. The conveyance roller 109 includes a rotatable rotating member (sleeve) 109 </ b> A and a cylindrical magnet body 109 </ b> B housed inside the rotating member 109 </ b> A and fixed to the developing device frame 103. The magnet body 109B is composed of, for example, five magnetic poles N1, N2, S1, S2, and S3. The magnetic pole N1 is opposed to the developer stripping magnetic pole S2 of the developer carrier 46, and the adjacent two magnetic poles S2 and S3 are A repulsive magnetic field of the same polarity is formed.

  In the vicinity of the magnetic pole S2, the tip of the stripping plate 110 is in close proximity or light contact. The stripping plate 110 and the transport roller 109 form stripping means. The developer released from the surface of the developer carrying member 46 by the developer stripping magnetic pole S2 of the developer carrying member 46 is sucked by the magnetic pole N1 in the carrying roller 109, and carried and carried by the rotating rotating member 109A. In the vicinity of the magnetic pole S 2, it is peeled off by the peeling plate 110 and falls into the collection unit 403.

  The collection unit 111 rotatably disposed in the collection unit 403 receives and collects the developer that is peeled off by the conveyance roller 109 and the peeling plate 110 and collects the developer, and is downstream of the supply / conveyance unit 107 in the conveyance direction. In this case, the developer carrier 46 is transported out of the development region DR.

  The supply / conveyance means 107, the agitation / conveyance means 112, and the recovery means 111 are all formed of a spiral screw and convey the developer in the direction of the rotation axis while stirring the developer, and release the developer in a direction substantially perpendicular to the rotation axis. .

  The developer conveyance downstream side of the collection unit 403 and the developer conveyance downstream side of the supply unit 401 are communicated with each other through a first opening 406 formed in the vicinity of the end of the second partition 405.

  The developer conveyance downstream side of the supply unit 401 and the developer conveyance upstream side of the agitation / conveyance unit 402 communicate with each other by a second opening 407 (not shown) formed near one end of the first partition 404. is doing.

  The developer conveyance downstream side of the agitation / conveyance unit 402 and the developer conveyance upstream side of the supply unit 401 communicate with each other by a third opening 408 (not shown) formed near the other end of the first partition 404. is doing.

  A toner concentration sensor 101 provided at the bottom of the supply unit 401 on the downstream side of the developer conveyance detects the toner concentration of the developer conveyed by measuring the magnetic permeability of the developer. In response to the toner density detection signal, the control means 100 drives the toner replenishing means 5 so that the toner density becomes a predetermined value, and the new toner is provided in the vicinity of the developer conveyance upstream side of the agitation / conveyance unit 402. The toner supply opening 409 is supplied. It is also possible to optically detect the density of the developed image and perform toner supply control based on the detection result of the image density.

  The developer peeled off by the transport roller 109 and the stripping plate 110 is recovered in the recovery unit 403, and the recovered developer is transported to the developer transport downstream side by the recovery means 111 and is punched in the second partition 405. Then, the gas flows back from the first opening 406 into the supply unit 401.

  The developer in the supply unit 401 is conveyed into the agitation / conveyance unit 402 by the supply / conveyance means 107 from a second opening 407 (not shown) formed at one end of the first partition 404. The developer conveyed into the agitation / conveyance unit 402 is conveyed by the agitation / conveyance unit 112 while mixing and agitating the toner replenished from the toner replenishment opening 409 and the developer. It is discharged from a third opening 408 (not shown) drilled at the other end and is returned to the supply unit 401. In the supply unit 401, the developer is discharged while being supplied in the axial direction by the supply / conveyance means 107 and supplied to the supply means 106. The supply means 106 radiates the developer while conveying it in the axial direction and supplies it to the developer carrier 46.

  A first opening 406 provided in the vicinity of the end of the second partition 405 that partitions the collection unit 403 and the supply unit 401, and near both ends of the first partition 404 that partitions the supply unit 401 and the stirring / conveying unit 402. The provided second opening 407 (not shown) and third opening 408 (not shown) are both disposed outside the development region DR of the developer carrier 46.

  By making the rotation direction of the supply means 106 and the supply / conveyance means 107 the same as the rotation direction of the developer carrier 46, the efficiency of the supply of the developer to the developer carrier 46 is achieved. In addition, a stable developer can be supplied and a good image can be obtained.

  Further, the reverse flow of the developer in the recovery unit 403 can be reduced by reversing the rotation direction of the recovery unit 111 from the rotation direction of the developer carrier 46.

  The developer carrying member 46 is driven by a motor 102 as a driving unit, and the number of rotations thereof is controlled by the control unit 100 so as to be a predetermined value.

  The conveyance roller 109, the supply means 106, the supply / conveyance means 107, the agitation / conveyance means 112, and the recovery means 111 are driven by another motor (not shown).

  The electrostatic latent image is developed in the development region DR by the counterclockwise rotation indicated by the arrow of the photosensitive member 1 and the clockwise rotation indicated by the arrow of the developer carrier 46. A developing bias in which a developing DC bias having the same polarity as the charging polarity of the toner is superimposed on the AC bias is applied to the developer carrying member 46 by the developing bias applying means 108 and development is performed.

  Also, a solid patch image is formed on the intermediate transfer belt, the patch density is read by a sensor having an LED and a phototransistor, and the development DC bias is changed according to the read value so that the image density matches the target value. It is controlled by the control means 100.

  Hereinafter, an important configuration relating to the present invention will be described for the developing means.

(Toner development efficiency)
In the image forming apparatus of the present invention, the charge amount of the toner of the first image forming unit located on the upstream side in the moving direction of the intermediate transfer member is set to the charge amount of the toner of the second image forming unit located on the downstream side. Or the amount of the external additive of the toner in the first image forming unit located on the upstream side in the moving direction of the intermediate transfer member, and the toner in the second image forming unit located on the downstream side The composition of the coating resin of the carrier contained in the developer of the first image forming unit located on the upstream side in the moving direction of the intermediate transfer member, and the downstream side The composition of the coating resin of the carrier contained in the developer of the second image forming unit located in the second image forming unit is made different or at least one of them is adopted to the upstream side in the moving direction of the intermediate transfer member Development effect of toner in first image forming portion located But it is characterized in that higher than the developing efficiency of the toner of the second image forming unit located downstream.

  In order to reduce the charge amount of the toner, a low resistance material such as “titania” is used as an external additive for the toner, or the toner is not charged on the surface of the carrier resin coating material. There is a method of providing a material having characteristics. For example, when the toner is composed of a styrene resin, the toner charge amount can be lowered by increasing the mass% of styrene with respect to the total weight of the coating material on the carrier surface.

  In addition, the amount of toner external additives and the composition of the carrier coating resin are different between the upstream image forming unit and the downstream image forming unit, thereby increasing the fluidity of the toner in the upstream image forming unit. The development efficiency of the toner can be increased. The amount of the external additive is a mass percentage of the external additive with respect to the toner. When a plurality of types of external additives are added, it is necessary that at least one of the external additives is different between the upstream side and the downstream side.

(Developer toner concentration)
In the image forming apparatus of the present invention, the toner density of the developer in the first image forming unit located on the upstream side in the moving direction of the intermediate transfer member is set to the toner concentration of the developer in the second image forming unit located on the downstream side. It has a control means for making it higher than the concentration.

  When the toner is replenished according to the toner consumption amount, the control unit 100 controls the developer so that the toner concentration is higher in the upstream side according to a preset set value.

  Here, the toner concentration is the mass percentage of toner with respect to the total amount of developer.

(Number of rotation of developer carrier)
In the image forming apparatus of the present invention, the number of rotations of the developer carrying member of the first image forming unit located on the upstream side in the moving direction of the intermediate transfer member is set to the developer of the second image forming unit located on the downstream side. It has a control means for making it higher than the rotational speed of the carrier.

  In accordance with a preset value, the control unit 100 sets the rotation speed of the developer carrier of the image forming unit located on the upstream side to be higher than the rotation speed of the developer carrier of the image forming unit located on the downstream side. It is controlled to become.

  Since any one of the above three methods or a combination of two or more methods can be used, the developability of the toner in the image forming unit located on the upstream side can be higher than that on the downstream side. The developing DC bias is not excessively higher than the developing DC bias on the downstream side, and the image density can be stabilized without problems such as carrier adhesion.

  The details of the present invention will be described below with reference to examples, but the embodiments of the present invention are not limited thereto.

<Example 1>
A color image was formed under the following conditions using the color image forming apparatus shown in FIG. For each condition, it is assumed that Y, M, C, and K are common unless otherwise specified.

(Photoconductor)
Photoreceptor: A drum having a diameter of 60 mm was coated with a phthalocyanine pigment dispersed in polycarbonate as an organic semiconductor layer. The film thickness of the photoreceptor layer including the charge transport layer is 25 μm.

Non-image area potential of photoconductor: Detected by potential sensor and feedback control (controllable range is -250V to -900V)
Total exposure potential of photoreceptor: -45V to -150V
Photoconductor linear velocity: 220 mm / s
Exposure: Laser scanning method. Semiconductor laser (LD) power is 300 μW.

(Developer)
Developer: Two-component developing system Toner concentration sensor: Sensor for detecting the magnetic permeability of the carrier Developing roller (developer carrier): φ30 mm
Developing roller rotation speed: 252 rpm
Development DC bias: -200V to -700V.

Developing AC bias: 0.5KV _pp ~2.0KV _pp. 2KHz-7KHz.

  Distance between developing roller and photosensitive drum: 350 μm.

Developer transport amount: 300 g / m ²
(Developer)
Developer: Two-component developer of emulsion polymerized toner (styrene resin, negatively charged) and acrylic resin coated carrier (acrylic resin 100%, styrene resin 0%) As a means to improve the developability of upstream color, as shown below The amount of titania, which is an external additive of the toner, was increased from the downstream side toward the upstream side (K → C → M → Y).

  Further, the volume average particle diameter of the carrier and the volume average particle diameter of the toner are volume-based average particle diameters, and are “Coulter Counter TA-11” or “Coulter Multisizer” (both Coulter Counters) equipped with a wet disperser. This is a value measured by the company).

Y: 0.5 mass%
M: 0.4 mass%
C: 0.3% by mass
K: 0.2 mass%
Volume average particle diameter of carrier: 42 μm
Volume average particle diameter of toner: 6.5 μm
Toner concentration: 6.5% by mass
(Intermediate transfer member)
Intermediate transfer member: Seamless semiconductive resin belt (material is polyimide, surface resistivity 10 ¹¹ Ω / □, volume resistivity 10 ⁸ Ωcm)
Intermediate transfer belt tension: 5Kgf
Intermediate transfer belt drive roller diameter: φ30mm
(Primary transfer)
Primary transfer means: A foaming roller (diameter: φ20 mm, resistance: 10 ⁶ Ω, roller pressure: 500 gf) is installed on the back of the intermediate transfer member, and a predetermined current value is obtained from a current value table in which a matrix is formed by temperature and humidity and a counter. Select and apply.

(Secondary transfer)
Secondary transfer means: An intermediate transfer member is sandwiched between a backup roller and a secondary transfer roller (both resistance values are 10 ⁷ Ω), and a predetermined current value is obtained from a current value table in which the temperature / humidity and counter are combined. Select and apply.

Intermediate transfer member cleaning blade: Urethane blade (free length 9 mm, thickness 2 mm, contact angle 17 degrees)
(Concentration stabilization control)
Dmax sensor: An LED and a phototransistor are incorporated so that the patch image density on the intermediate transfer belt can be read.

  Density stabilizing means: a control method for feeding back to the developing DC bias condition based on the density information of the patch image read by the Dmax sensor.

<Example 2>
The amount of titania as an external additive is 0.2% by mass for all of Y, M, C, and K, and the styrene ratio of the carrier coating resin is directed from the downstream side to the upstream side as shown below. Thus, image formation was performed under the same conditions as in Example 1 except that the increased developer (K → C → M → Y) was used. The styrene ratio refers to the mass% of the styrene resin relative to the total weight of the coating material on the carrier surface (styrene resin + acrylic resin).

Y: 15% by mass
M: 10% by mass
C: 5% by mass
K: 0% by mass
<Example 3>
The amount of titania as an external additive is 0.2% by mass for all of Y, M, C, and K, and the toner concentration is changed from the downstream side to the upstream side as shown below (K → C → M → Y) Image formation was performed under the same conditions as in Example 1 except that the increased developer was used.

Y: 7.5 mass%
M: 7.2% by mass
C: 6.9% by mass
K: 6.5 mass%
<Example 4>
The amount of titania as an external additive is 0.2% by mass for all of Y, M, C, and K, and the rotation speed of the developing roller is changed from the downstream side to the upstream side as shown below. Image formation was performed under the same conditions as in Example 1 except that (K → C → M → Y).

Y: 308rpm
M; 280 rpm
C: 266 rpm
K: 252 rpm
<Comparative example>
The amount of titania as an external additive was 0.2% for all of Y, M, C, and K, and image formation was performed under the same conditions as in Example 1 except that the same developer was used.

(Evaluation methods and results)
(Measurement of development efficiency and charge amount)
For Examples 1 and 2 and Comparative Example, the developer was placed between the parallel plate (aluminum) electrodes while being slid, the gap between the electrodes was 0.5 mm, the DC bias was 1.0 KV, the AC bias was 4.0 KV, The development efficiency when the toner was developed under the condition of 2.0 KHz was measured. The development efficiency is the ratio of the developed toner amount to the toner amount supplied to the development area, and means the toner consumption efficiency.

  Further, the charge amount and mass of the developed toner were measured, and the charge amount per unit mass Q / m (μC / g) was defined as the charge amount.

(Development DC bias measurement)
The relationship between the number of prints and the development DC bias of Y, M, C, and K was examined for Examples 1 to 4 and Comparative Example.

  Table 1 shows the development efficiency and charge amount of each color of Examples 1-2 and Comparative Example.

  In Examples 1 and 2, it was confirmed that the charge amount decreased from the downstream side toward the upstream side (K → C → M → Y), and the development efficiency increased. On the other hand, in the comparative example, the charge amount was almost the same in Y, M, C, and K, and the development efficiency was slightly higher in K.

  FIG. 3 shows the relationship between the number of printed sheets and the developing DC bias for Examples 1 to 4, and FIG. 4 shows the same for the comparative example. Since Examples 1 to 4 had almost the same result, data is shown for each of Y, M, C, and K collectively.

3 and 4, the horizontal axis represents the number of printed sheets (10 ³ sheets), and the vertical axis represents the development DC bias (V). Point and line L1 indicated by ◆ in the figure are Y, point and line L2 indicated by ■ in the figure are M, point and line L3 indicated by Δ in the figure are C, and Χ in the figure The represented point and line L4 correspond to K, respectively.

  In the comparative example of FIG. 4, the development DC bias of each color has the highest return amount Y, and the DC bias value decreases toward the downstream side, and the variation for each color is large.

  In particular, the development DC bias of the upstream color Y has greatly increased to 650 V, carrier adhesion has occurred, and white image defects have occurred.

  In Embodiments 1 and 2 of FIG. 3, the development DC bias variation is reduced by using a developer in which the development efficiency of the developer is set higher for the upstream color, and the upstream color Y development DC as described above. There was no increase in bias. Further, through 100,000 prints, there was no image defect such as white spots.

  Further, in Examples 3 to 4 of FIG. 3, the same effect was obtained by increasing the toner density or increasing the rotation speed of the developing roller as means for increasing the developability of the upstream color.

1 is a diagram illustrating an overall configuration of a color image forming apparatus which is an example of an image forming apparatus according to an embodiment of the present invention. FIG. 3 is a central sectional view of developing means according to an embodiment of the present invention. It is a graph which shows the experimental data which contrasted the number of printed sheets and development DC bias concerning the Example of this invention. It is a graph which shows the experimental data which contrasted the number of printed sheets and development DC bias concerning the comparative example of this invention.

Explanation of symbols

10, 10Y, 10M, 10C, 10K Image forming unit 46, 46Y, 46M, 46C, 46K Developer carrier 1, 1Y, 1M, 1C, 1K Photoconductor 8, 8Y, 8M, 8C, 8K Cleaning means for photoconductor 7, 7Y, 7M, 7C, 7K Primary transfer means 6 Intermediate transfer body 8A Intermediate transfer body cleaning means 100 Control means 9 Operation input means 40 Image reading unit

Claims (3)

Developing means for developing the electrostatic latent image formed on the image carrier with a developer containing toner while applying a development DC bias, and transferring the developed toner image on the image carrier to an intermediate transfer member 1 A plurality of image forming units having a secondary transfer unit are disposed along the moving direction of the intermediate transfer member, and a plurality of toner images sequentially transferred from the plurality of image forming units onto the intermediate transfer member are collectively collected on the recording material. In the image forming apparatus having the secondary transfer means for transferring,
By adopting at least one of the following (a) to (c), the developing efficiency of the toner of the first image forming unit located on the upstream side in the moving direction of the intermediate transfer member is located on the downstream side. An image forming apparatus characterized in that the developing efficiency of toner in the second image forming unit is higher.
(A) The charge amount of the toner in the first image forming unit is set lower than the charge amount of the toner in the second image forming unit.
(B) The amount of toner external additive in the first image forming unit is different from the amount of toner external additive in the second image forming unit.
(C) The composition of the resin coating the carrier of the developer in the first image forming unit is different from the composition of the resin coating the carrier of the developer in the second image forming unit. Developing means for developing the electrostatic latent image formed on the image carrier with a developer including toner and carrier while applying a development DC bias, and transferring the developed toner image on the image carrier to the intermediate transfer member A plurality of image forming portions having a primary transfer means for moving the intermediate transfer member along the moving direction of the intermediate transfer member, and a plurality of toner images sequentially transferred from the plurality of image forming portions onto the intermediate transfer member on the recording material; In an image forming apparatus having a secondary transfer means for collectively transferring to
Control means for setting the toner concentration of the developer of the first image forming unit located upstream in the moving direction of the intermediate transfer member to be higher than the toner concentration of the developer of the second image forming unit located downstream. An image forming apparatus comprising: A developing unit that has a developer carrying member that carries a developer containing toner, and that develops the electrostatic latent image formed on the image carrier with a developer containing toner while applying a development DC bias; A plurality of image forming units having a primary transfer means for transferring the toner image on the image carrier to the intermediate transfer member are disposed along the moving direction of the intermediate transfer member, and the plurality of image forming units are arranged on the intermediate transfer member. In an image forming apparatus having a secondary transfer unit that collectively transfers a plurality of toner images transferred sequentially onto a recording material,
The number of rotations of the developer carrying member of the first image forming unit located on the upstream side in the moving direction of the intermediate transfer member is higher than the number of rotations of the developer carrying member of the second image forming unit located on the downstream side. An image forming apparatus comprising a control means for increasing the height.

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