JP2007108692A - Image-forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve excellent secondary transfer efficiency of an image formed by primarily transferring a dark toner image of high density and a light toner image of low density having the same hue onto an intermediate transfer body in order. <P>SOLUTION: The light toner image is primarily transferred before the dark toner image, and the light toner has a volume mean particle diameter greater than that of the dark toner. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention uses toner having the same hue and different density, such as dark magenta toner and light magenta toner, and further forms an image in which the toner image primarily transferred from the image carrier to the intermediate transfer member is further transferred to the recording material. Relates to the device.

  In recent years, in an electrophotographic image forming apparatus, in order to widen a color reproduction range, toners having the same hue and different densities such as dark magenta toner and light magenta toner are used.

In this type of image forming apparatus, a dark magenta toner image forming unit and a light toner image forming unit are provided, and a dark magenta toner image and a light magenta toner image are formed. In each image forming unit, each toner image formed on the photosensitive drum is primary-transferred electrostatically sequentially to the intermediate transfer member in the primary transfer region. The toner images of the respective colors primarily transferred to the intermediate transfer member are collectively transferred to the recording material by a secondary transfer member to which a bias is applied. On the other hand, the primary transfer residual toner remaining on the photosensitive drum without being primarily transferred to the intermediate transfer member is collected by a cleaner provided on the photosensitive drum. Further, the secondary transfer residual toner remaining on the intermediate transfer member without being transferred to the recording material is collected by a cleaner provided on the intermediate transfer member. The toner image transferred to the recording material is conveyed to a fixing device and is fixed to the recording material by being heated and pressurized.
JP 2000-231279 A JP 2002-72572 A

  However, in the image forming apparatus described above, there is a large amount of secondary transfer residual toner remaining on the intermediate transfer body during the secondary transfer, and there arises a problem that a toner image of a desired color cannot be formed on the recording material. That is, the toner image that has been primarily transferred first passes through the primary transfer region and is charged when another toner image is firstly transferred later. Due to this charging, the charge amount of the toner image that has been primarily transferred first becomes larger than the charge amount of the toner image that has been primarily transferred later. There is a difference between the charge amount of the toner image transferred first and the charge amount of the toner image transferred later. When this difference in charge amount becomes large, it becomes difficult to apply a bias suitable for both the toner image transferred first and the toner image transferred later to the secondary transfer member. In this way, the secondary transfer residual toner increases and it becomes difficult to form a toner image of a desired color on the recording material.

The present invention is Ru der been made in view of the above problems. The object of the present invention is to reduce the amount of secondary transfer residual toner and form a toner image of a desired color on a recording material in an image forming apparatus using toners having the same hue and different densities. An image forming apparatus is provided.

  In order to achieve the above object, the configuration of the present invention includes a first image carrier that carries a light toner image formed of light toner, and a light toner image on the first image carrier that is electrostatically transferred to an intermediate transfer member. A first transfer member for transferring the image, a second image carrier for carrying a dark toner image having the same hue as the light toner and a high density, and the dark toner on the second image carrier. A second transfer member that electrostatically transfers an image to the intermediate transfer member that holds the light toner image, and the dark toner image and the light toner image on the intermediate transfer member are collectively collected onto a recording material. A secondary transfer member for electrostatic secondary transfer, wherein the volume average particle size of the light toner is larger than the volume average particle size of the dark toner.

  In another configuration, an image carrier that carries a light toner image formed of light toner and a dark toner image having the same hue and high density as the light toner, and the light toner is electrostatically transferred to the intermediate transfer member. A transfer member that primarily transfers the dark toner image to the intermediate transfer member that holds the light toner, and the light toner and the dark toner on the intermediate transfer member collectively. A secondary transfer member that performs secondary transfer to the toner, and the volume average particle diameter of the light toner is larger than the volume average particle diameter of the dark toner.

  In the present invention, the toner particle diameter of the toner image that is primarily transferred first is increased. Since toner having a large particle size is difficult to be charged, the amount of charge applied when passing through the primary transfer region can be suppressed. In addition, in order to suppress a reduction in image quality due to the use of a large particle size toner, the particle size of a light toner that is less noticeable in image deterioration is increased. In this way, it was possible to reduce the secondary transfer residual toner by suppressing the difference in the charge amount of the toner image, and to form a toner image of a desired color on the recording material.

Embodiment 1
Embodiments of the present invention will be described below in detail with reference to the drawings.

{Overall configuration of image forming apparatus}
The image forming apparatus of this embodiment is a full-color image in which a toner image formed on a photosensitive drum 1 serving as an image carrier is primarily transferred to an intermediate transfer belt (intermediate transfer body) 5 and then secondarily transferred to a recording material. An image forming apparatus.

  The image forming apparatus according to the present exemplary embodiment includes one photosensitive drum 1 and six developing units (developing units and toner image forming apparatuses) 4 that develop an electrostatic image formed on the photosensitive drum 1. The six developing devices 4 are filled with light cyan, light magenta, dark magenta, dark cyan, yellow, and black toner, respectively.

  That is, the light cyan, light magenta, dark magenta, dark cyan, yellow, and black toner images formed on the photosensitive drum 1 are primarily transferred so as to overlap on the intermediate transfer belt 5. Subsequently, these toner images are collectively transferred to a recording material.

FIG. 1 is a schematic cross-sectional view of an image forming apparatus 100 of this embodiment.
The image forming apparatus 100 includes a reader unit A that reads a document and a printer unit B that forms an image based on image data.

  In the reader unit A, a document placed on a platen glass (not shown) is exposed and scanned by an exposure lamp (not shown). Then, the reflected light image from the original is condensed on a full color CCD sensor (not shown) by a lens (not shown) to obtain an image signal. The image signal is subjected to image processing by a video processing unit (not shown) through an amplifier circuit (not shown) and sent to the printer unit B.

  The printer unit B forms an image based on the image signal sent from the reader unit B. Note that the image forming apparatus 100 can perform image formation based on an image signal sent from a computer or a facsimile in addition to the image signal sent from the reader unit B.

  The image signal sent from the reader unit B is converted into a color signal corresponding to each color of light magenta, light cyan, yellow, dark magenta, dark cyan, and black by a signal converter (not shown) of the printer unit A. . Each of these color signals is stored in a storage device (not shown).

First, a light magenta toner image of the first color is formed.
Based on the light magenta color signal stored in the storage device, the laser exposure device 3 as an exposure unit irradiates the photosensitive drum 1 rotating in the direction of arrow R1 in the drawing with laser light L.

  Here, prior to the irradiation of the laser beam L, the surface of the photosensitive drum 1 is uniformly charged to a negative polarity by a primary charger (primary transfer means) 2. The surface of the photosensitive drum 1 is exposed to a pre-exposure lamp 11 before being charged by the primary charger 2 and is discharged.

  When the uniformly charged photosensitive drum 1 is irradiated with laser light L based on the light magenta color signal, a light magenta electrostatic image is formed on the photosensitive drum 1.

  The six developing devices 4 are held by a rotary developing device holder 41. When the rotary developing device holder 41 rotates in the direction of arrow R2 in FIG. 1, the developing device 4 can move to a position facing the photosensitive drum 1 (developing position D) and develop an electrostatic image. become.

  The light magenta developer 41 a moved to the developing position D by the rotation of the rotary developing device holder 41 develops the light image of light magenta, and a light magenta toner image is formed on the photosensitive drum 1. At this time, the light magenta toner is negatively charged by the developing device 4a.

  The light magenta toner image on the photosensitive drum 1 is transferred (primary transfer) to the intermediate transfer belt 51 by the primary transfer roller (primary transfer means) 51 when reaching the primary transfer region T1 where the photosensitive drum 1 and the intermediate transfer belt 5 are in contact with each other. The At this time, a primary transfer bias having a polarity (positive polarity) opposite to that of the toner is applied to the primary transfer roller 51 from a primary transfer bias power source (not shown).

  On the other hand, toner remaining on the photosensitive drum 1 without being transferred to the intermediate transfer belt 5 (primary transfer residual toner) is collected by the photosensitive drum cleaner 6.

Subsequently, a light cyan toner image of the second color is formed in the same manner as the process of forming the light magenta toner image of the first color.
An electrostatic image is formed on the photosensitive drum 1 based on the light cyan color signal.

  The light cyan developing device 4b filled with the light cyan toner moves to the developing position D by the rotation of the rotary developing device holder 41. The light cyan developing device 4 b develops the electrostatic image, and a light cyan toner image is formed on the photosensitive drum 1. The light cyan toner image is transferred by the primary transfer roller 51 so as to overlap the light magenta toner image on the intermediate transfer belt 5 in the primary transfer region T1. At this time, a primary transfer bias having a reverse polarity (positive polarity) to the toner is applied to the primary transfer roller 51.

  Similarly, a third color yellow toner image, a fourth color dark magenta toner image, a fifth color dark cyan toner image, and a sixth color black toner image are also formed on the photosensitive drum 1. Then, the images are sequentially transferred onto the intermediate transfer belt 51 so as to overlap each other on the intermediate transfer belt 51.

  The six color toner images transferred onto the intermediate transfer belt 51 are transferred by the secondary transfer roller (secondary transfer means) 52 in the secondary transfer region T2 where the secondary transfer roller 52 and the intermediate transfer belt 51 are in contact with each other. It is transferred to the recording material at once. At this time, a secondary transfer bias is applied to the secondary transfer roller 52 from a secondary transfer bias power source (not shown). The secondary transfer bias has a polarity (positive polarity) opposite to that of the toner.

  The recording material is stored in the recording material container 7 and is supplied to the secondary transfer region T2 by the supply unit 81.

  On the other hand, toner remaining on the intermediate transfer belt 5 without being transferred to the recording material (secondary transfer residual toner) is collected by an intermediate transfer belt cleaner 55 that can contact and separate from the intermediate transfer belt 5.

  The recording material to which the toner image has been transferred is conveyed to the fixing device 9 by the conveyance belt 82. The fixing device 9 includes a pair of rollers in contact with each other and a heater (not shown). When the recording material to which the toner image has been transferred passes through the fixing device 9, the recording material is heated and pressed by a roller, and the toner image is fixed to the recording material.

Here, each part which comprises an image forming apparatus is demonstrated concretely.
The photosensitive drum 1 is rotated in the direction of arrow R1 in FIG. The photosensitive drum 1 is provided with a photoconductive layer on a conductive base layer.
As the photosensitive drum 1, an organic photoreceptor or an amorphous silicon photoreceptor can be used. In this embodiment, an organic photoreceptor is used.

The primary charger 2 uniformly charges the surface of the photosensitive drum 1 to a predetermined polarity by applying a bias from a primary charging bias power source (not shown).
As the primary charger 2, a corona charger or a roller charger can be used.

In this embodiment, a contact charging type charging roller is used as the primary charger 2.
The charging roller is provided with an elastic layer made of foamed urethane in which carbon is dispersed on a metal cored bar. Furthermore, the surface of the elastic layer is coated with a fluorine resin.

  The charging roller is pressed against the photosensitive drum 1 with a predetermined pressure by a pressure unit (not shown), and rotates as the photosensitive drum 1 rotates. A bias obtained by superimposing a DC voltage (−350 v to 500 v) on an AC voltage (frequency 1000 Hz, amplitude 1400 v) is applied to the charging roller from a primary charging bias power source. By applying the primary charging bias, the primary charger 2 uniformly charges the photosensitive drum 1 to approximately −500V.

  The applied DC voltage of the bias is controlled by control means (not shown) based on the measurement result of the potential sensor 12 that measures the surface potential of the charged photosensitive drum 1, and the photosensitive drum 1 is set to a desired potential. Charged.

  The laser exposure device 3 is provided on the downstream side of the primary charger 2 in the rotation direction of the photosensitive drum 1. Based on the color signal stored in the storage device, the light emitting element 31 emits the laser light L. The laser beam L is reflected by the rotating polygon mirror 32, and is irradiated to the photosensitive drum 1 through the lens 33 and the reflection mirror 34. When the charged photosensitive drum 1 is irradiated with the laser light L, an electrostatic image according to the color signal is formed.

  Six developing devices 4 are provided, each of which is filled with light magenta, light cyan, yellow, dark magenta, dark cyan, and black toner. The six developing devices are held by the rotary developing device holding unit 41. The developing device 4 that develops the electrostatic image on the photosensitive drum 1 is moved to the developing position D by rotating the rotary developing holder 41 in the direction of the arrow R2.

  The so-called two-component developing system is used for the developing device 4 of this embodiment. In the developing device 4, the toner and the carrier are mixed, and the toner is triboelectrically charged to the negative polarity by the carrier.

  The developing device is loaded with a two-component developer in which a toner and a carrier are mixed.

  The toner includes colored resin particles containing a binder resin, a colorant, and other additives as necessary, and colored particles to which an external additive such as colloidal silica fine powder is externally added. Yes. The toner is a negatively chargeable polyester resin.

  The toner is produced by a polymerization method or a pulverization method. The volume average particle diameter of the manufactured toner is adjusted by a method of selecting using a mesh.

  The volume average particle size is preferably 5 μm or more and 8 μm or less. In this embodiment, the volume average particle size of the light magenta and light cyan toners is 7 μm, and the volume average particle size of the dark magenta, dark cyan, yellow and black toners is 5 μm. A method for measuring the volume average particle diameter of the toner will be described later.

  Light cyan and dark cyan have the same hue and different reflection densities. Light magenta and dark magenta have a similar relationship. A method for measuring the toner reflection density and hue will be described later.

  Hereinafter, light cyan and light magenta are collectively referred to as “light toner”, dark magenta, dark cyan, yellow, and black are collectively referred to as “dark toner”.

  By using light toner in addition to dark toner, it is possible to increase the types of toner image colors that can be formed.

The dark toner of this example is such that the reflection density is 1.4 when the mass per unit area of the toner on the high-quality paper (hereinafter referred to as “loading amount”) is 0.5 mg / cm ² . The amount of colorant is adjusted. Further, the amount of the colorant is adjusted so that the reflection density of the light toner is 0.7 when the loading amount on the high-quality paper is 0.5 mg / cm ² .

FIG. 2 shows a comparison of the reflection density of the toner with respect to the loading amount of the light toner and the dark toner on the high-quality paper. In both cases, the reflection density increases with an increase in the loading amount. When the applied amount is 0.5 mg / cm ² , the reflection density of the light toner is 0.7 and the reflection density of the dark toner is 1.4.

For example, surface oxidized or unoxidized iron, nickel, cobalt, manganese, chromium, rare earth metals, and alloys thereof, or oxide ferrite can be suitably used as the magnetic particles as the carrier. As a method for producing the carrier, a polymerization method or the like can be used. The carrier has a volume average particle size of 20 to 50 μm, preferably 30 to 40 μm, and a resistivity of 10 ⁷ Ωcm or more, preferably 10 ⁸ Ωcm or more. In this example, a carrier having a volume average particle size of 35 μm, a resistivity of 5 × 10 ⁹ Ωcm, and a magnetization of 200 emu / cc was used.

  In the vicinity of the laser optical system 3 of the printer unit B, toner storage units 10a to 10f for storing toners of respective colors are provided. When the toner in the developing devices 4a to 4f is consumed, the toner is appropriately replenished from the toner storage portions 10a to 10f.

  Here, a detection toner image formed on the photosensitive drum under a predetermined condition is transferred to the intermediate transfer belt 5 and detected by the detection sensor 53. Based on the detection result of the detection sensor 53, the replenishment amount of toner to the developing devices 4a to 4f is adjusted, and the weight ratio (TD ratio) of the toner and the carrier in the developing devices 4a to 4f is maintained at a predetermined value.

  When the toner weight in the developing device 4 is T (g) and the carrier weight is D (g), the TD ratio is expressed by T / (T + D). In the present embodiment, the TD ratio is maintained at about 8% in each of the developing devices 4a to 4f.

  The signal converter converts the image signal (image input signal) sent from the reader into a signal (image output signal) sent to the laser optical system 2 for forming an electrostatic image.

  In the image forming apparatus of this embodiment, the relationship between the image input signal and the image output signal differs depending on the color of the toner image to be formed.

  FIG. 3 shows the relationship between the image input signal and the image output signal in light cyan and light magenta.

  The lines in FIG. 3 indicate both light cyan and light magenta. FIG. 4 shows the relationship between the image input signal value and the image output signal value in dark cyan and dark magenta. The lines in FIG. 4 indicate both dark cyan and dark magenta. FIG. 5 shows the relationship between the image input signal value and the image output signal value in yellow and black. The lines in FIG. 5 indicate both yellow and black.

  The intermediate transfer belt 5 is an endless belt, and is supported by a driving roller 55, a plurality of driven rollers 53, and a secondary transfer inner roller 54. When the driving roller 55 is rotated by a driving unit (not shown), the intermediate transfer belt 5 rotates in the direction of arrow R3 in FIG.

  As a material constituting the intermediate transfer belt 5, a resin such as polyimide or polycarbonate can be used. In this example, polyimide was used.

  The thickness of the intermediate transfer belt 5 is preferably about 0.1 mm to 2 mm. In this example, the thickness was 0.2 mm.

The intermediate transfer belt 5 has a resistance adjusted to a desired value by a resistance adjusting agent such as carbon. As the resistance, a volume resistivity of 1 × 10 ⁶ to 1 × 10 ¹³ Ω · cm is suitable.
In this example, the volume resistivity was 1 × 10 ⁸ Ω · cm.

  The primary transfer roller 51 is disposed at a position facing the photosensitive drum 1 across the intermediate transfer belt 5 in the primary transfer region T1. The primary transfer roller 51 is rotatably supported by a support member (not shown).

  As the primary transfer roller 5, a member in which an elastic layer is provided on a metal core is preferably used. In this example, polyurethane-based foamed rubber was used as the elastic layer.

The elastic layer had a hardness (Asker C) of 10. Further, the resistance adjuster was dispersed to adjust the volume resistivity to about 1 × 10 ⁶ Ω · cm.

  When a bias (positive polarity) opposite in polarity to the toner image is applied from a primary transfer bias power source (not shown), the toner image on the photosensitive drum 1 is transferred to the recording material. A DC voltage of +500 v to +750 v is applied to the primary transfer roller 51.

  The secondary transfer roller 52 is disposed at a position facing the secondary transfer inner roller 54 across the intermediate transfer belt 8 in the secondary transfer region T2. The secondary transfer roller 52 is rotatably supported by a support member (not shown).

  As the secondary transfer roller 52, a member in which an elastic layer is provided on a metal core is preferably used. In this example, polyurethane-based foamed rubber was used as the elastic layer.

The elastic layer had a hardness (Asker C) of 10. Further, the resistance adjuster was dispersed to adjust the volume resistivity to about 1 × 10 ⁷ Ω · cm.

  When a bias (positive polarity) opposite in polarity to the toner image is applied from a secondary transfer bias power source (not shown), the toner image on the intermediate transfer belt 5 is transferred to the recording material. +1 Kv to +3 Kv are applied to the secondary transfer roller 52.

  The intermediate transfer belt cleaner 55 is provided so as to be able to contact and separate from the intermediate transfer belt 5. That is, the intermediate transfer belt cleaner 55 is separated from the intermediate transfer belt 5 while the toner image on the photosensitive drum 1 is primary transferred to the intermediate transfer belt 5. After the primary transfer is completed, the intermediate transfer belt cleaner 55 contacts the intermediate transfer belt 5.

  As the intermediate transfer belt cleaner 55, a polyurethane blade member having a durometer A hardness of 75 and a thickness of 2 mm was used.

  As the roller of the fixing device 9, a surface of a metal roller covered with a fluorine-based rubber having excellent releasability is used. In this embodiment, a roller having a metal roller coated with PTFA was used.

{Relationship between toner particle size and secondary transfer efficiency}
As described above, in this embodiment, six colors of toner are used as described above.
The volume particle size of each toner is 5 μm for the light magenta of the first color, 5 μm for the light cyan toner of the second color, and 7 μm for the dark magenta, dark cyan, yellow and black toners of the third to sixth colors.

Table 1 shows the experimental results of the relationship between the toner particle size and the secondary transfer efficiency.
“Particle size combination 1” in this example shown in Table 1 is the case where the volume average particle size of the first and second color toners is 7 μm and the volume average particle size of the third to sixth color toners is 5 μm. The secondary transfer efficiency of the toner images of the first to sixth colors.

  “Particle size combination 2” as a comparative example is the secondary conversion efficiency when the volume average particle sizes of the toners of the first to sixth colors are all 5 μm.

Here, the secondary transfer efficiency is measured by forming toner images of respective colors on the intermediate transfer belt 5 in a line along the traveling direction of the intermediate transfer belt 5, as shown in FIG. That is, the mass per unit area of the intermediate transfer belt 5 of these toner images is compared with the mass per unit area on the recording material after the secondary transfer.
The secondary transfer efficiency in Table 1 is expressed by the following equation.

(Secondary transfer efficiency)
= {(Mass per unit area of toner image on recording material) / (Mass per unit area of toner image on intermediate transfer belt)} × 100

  As shown in Table 1, in “Particle size combination 1”, a secondary transfer efficiency of 90% or more was achieved in all toner images.

  On the other hand, in the “particle size combination 2”, the secondary transfer efficiency of the first and second color toner images is less than 90%.

  Table 2 shows the charge amount per unit weight of the toner image of each color on the photosensitive drum 1 in “particle size combination 2”.

  Here, the toner image on the intermediate transfer belt 5 is given an electric charge by the primary transfer bias applied to the primary transfer roller 51 every time it passes through the primary transfer region T1. Therefore, the toner image (first color, second color) transferred first increases the number of passes through the primary transfer region T1. The charge amount at the time of secondary transfer becomes higher as the toner image is primarily transferred first.

  FIG. 7 shows the relationship between the voltage value of the secondary transfer bias and the amount of secondary transfer residual toner in “particle size combination 2”. FIG. 7 shows the relationship between the light magenta toner image of the first color and the dark magenta toner image of the third color.

  In the relationship shown in FIG. 7, the secondary transfer bias value V1 that minimizes the secondary transfer residual toner amount of the dark magenta toner image and the bias value V2 that minimizes the secondary transfer residual toner amount of the light magenta toner image. The difference is getting bigger. That is, it is difficult to improve the secondary transfer efficiency of the dark magenta toner image and the light toner image at the same time. This is considered to be due to the difference in the charge amount between the dark magenta toner and the light magenta toner at the time of secondary transfer due to the charge applied when the light magenta toner image passes through the primary transfer region T1. It is done.

  In addition, in order to suppress a reduction in image quality due to the use of a large particle size toner, the particle size of a light toner that is less noticeable in image deterioration is increased.

  Table 3 shows the charge amount per unit area of the toner image of each color on the photosensitive drum 1 in “particle size combination 1”. The charge amounts of the first and second color toner images are smaller than the charge amounts of the third to sixth color toner images. This is presumably because the larger the toner particle size, the smaller the area of contact between the toner and the carrier in the unit weight of the toner, and the smaller the amount of charge imparted by contact charging with the carrier.

  Further, it is considered that as the toner particle diameter increases, the area charged in the primary transfer region T1 in the unit weight of toner decreases, and the amount of charge applied when passing through the primary transfer region T1 decreases. .

  In the relationship shown in FIG. 8, the secondary transfer bias value V1 that minimizes the secondary transfer residual toner amount of the dark magenta toner image and the bias value V2 that minimizes the secondary transfer residual toner amount of the light magenta toner image. The difference is small.

  On the photosensitive drum 1, the charge amounts of the first and second color toner images are smaller than the third to sixth color toner amounts. However, the number of times that the first and second color toner images pass through the primary transfer region T1 is greater than the number of times that the toner images of the third to sixth colors pass through the primary transfer region T1. Therefore, when the primary transfer of the toner image of the sixth color is completed and the secondary transfer is performed, the difference in charge amount between the toners of the six colors is small. It is considered that the difference in secondary transfer bias value that minimizes the amount of residual secondary transfer toner between the light magenta toner of the first color and the black toner of the sixth color is reduced.

  Therefore, the secondary transfer residual toner of the dark magenta toner image and the light magenta toner image can be reduced at the same time, and a good secondary transfer efficiency can be achieved.

  As described above, by making the volume average particle diameter of the toner constituting the toner image to be primarily transferred first larger than the volume average particle diameter of the toner of the toner image to be primarily transferred later, good secondary transfer is achieved. Efficiency could be achieved. A toner image of a desired color could be formed on the recording material.

  Here, a method for measuring the charge amount per unit mass of the toner image of the toner image on the photosensitive drum 1 used in the present invention will be described.

  As a measuring device, E-SPART MODEL EST-II manufactured by Hosokawa Micron Corporation was used. The measurement conditions were Field Voltage 100 (V) and Particle Density 1 (g / cm 3), and the average charge amount per unit mass was defined as the charge amount per unit mass of toner in the toner image.

  Further, the volume average particle diameter of the toner used in the present invention was measured as follows.

  That is, toner having an average particle size of 3 μm or more was measured within a range of 0.4 μm to 60 μm using a laser scan type particle size distribution analyzer (manufactured by CIS-100 GALAI). The measurement sample was prepared as follows. First, a toner to be measured is added in a range of 0.5 to 2 mg in a solution obtained by adding 0.2 ml of a surfactant (alkylbenzene sulfonate) to 100 ml of water. Then, after dispersing for 2 minutes with an ultrasonic disperser, add about 80% of water to a cubic cell containing a magnetic stirrer, and add 1 or 2 drops of the above ultrasonically dispersed sample with a pipette. did.

  The toner reflection density and toner hue were measured as follows.

A toner image of each color as shown in FIG. 6 is formed on the intermediate transfer belt 5 and secondarily transferred onto CLC paper (80 g / m ² ) manufactured by Canon Sales Co., Ltd. By secondary transfer, a toner image with a predetermined loading amount is formed on CLC paper (80 g / m ² ).

Then, these toner images are fixed on CLC paper (80 g / m ² ) by the fixing device of the image forming apparatus. The fixing conditions at this time are usually conditions for image formation using CLC paper (80 g / m ² ). Subsequently, each toner image formed on CLC paper (80 g / m ² ) was measured with X-Rite 504 (optical system condition: incident 45 °, received light 0 °, filter condition: status A) manufactured by X-RITE. To do. Thereby, the toner density and the toner hue are obtained.

In particular, when comparing the optical density and toner hue of different toners, the amount of toner image formed on CLC paper (80 g / m ² ) is set to 0.5 g / cm ² and the toner The optical density of the toner and the hue of the toner are measured.

  Here, as the light toner, a toner having a volume average particle diameter of 3 μm to 9 μm can be used.

  As the dark toner, a toner having a volume average particle diameter of 5 μm to 12 μm can be used. At this time, the average particle diameter of the light toner is larger than the average particle diameter of the dark toner, and the difference is preferably 1 μm to 3 μm.

  Further, as the light toner, those having a charge amount of 10 μC / g to 30 μC / g on the photosensitive drum 1 can be used. As the dark toner, one having a charge amount on the photosensitive drum 1 of 20 μC / g to 50 μC / g can be used. At this time, the charge amount of the light toner is smaller than that of the dark toner, and the difference is preferably 1 μC / g to 20 μC / g.

[Embodiment 2]
FIG. 9 shows a second embodiment of the present invention. The image forming apparatus according to the present embodiment employs a tandem system in which the image forming units Pa to Pf including the photosensitive drum 1 are provided along the intermediate transfer belt 5 that rotates in the arrow R2 direction.

  Note that members having the same configuration and function as those used in the image forming apparatus of Embodiment 1 are denoted by the same reference numerals and description thereof is omitted. In addition, the volume average particle diameter, the toner density, the toner hue, and the charge amount per unit mass of the toner were measured using the same method as that shown in Example 1.

{Overall configuration of image forming apparatus}
In FIG. 9, light magenta, light cyan, yellow, dark magenta, dark cyan, and black toner images are formed at Pa, Pb, Pc, Pd, Pe, and Pf. The formed toner image is transferred onto the intermediate transfer belt 5 so as to be transferred (primary transfer). These toner images are collectively transferred (secondary transfer) to a recording material.

  A light magenta toner image forming portion Pa and a light cyan toner image forming portion Pb are arranged in order from the upstream side to the downstream side in the rotation direction of the intermediate transfer belt 5. Furthermore, a yellow toner image forming unit Pc, a magenta toner image forming unit Pd, a dark cyan toner image forming unit Pe, and a black toner image forming unit Pf are arranged. These have the same configuration except that the color of the toner image to be formed is different.

  In each of the image forming portions Pa, Pb, Pc, Pd, Pe, and Pf, a process unit is disposed around the photosensitive drums 1a, 1b, 1c, 1d, 1e, and 1f that are rotatably disposed. That is, primary chargers 2a, 2b, 2c, 2d, 2e, and 2f, and developing devices 4a, 4b, 4c, 4d, 4e, and 4f are arranged. Furthermore, photosensitive drum cleaners 6a, 6b, 6c, 6d, 6e, 6f and pre-exposure lamps 11a, 11b, 11c, 11d, 11e, 11f are arranged. In addition, primary transfer rollers 51 a, 51 b, 51 c, 51 d, 51 e, and 51 f are provided at the contact portions between the respective photosensitive drums 1 and the intermediate transfer belt 5.

  Here, the image forming operation will be described by exemplifying the light magenta image forming unit Pa. The rotating photosensitive drum 1a is discharged by the pre-exposure lamp 11a and then uniformly charged to the negative polarity by the primary charger 2a. The laser exposure device 3a irradiates the charged photosensitive drum 1a with laser light La in accordance with the color signal to form an electrostatic image. The two-component developing device 6a develops the electrostatic image with light magenta toner to form a light magenta toner image. Due to the rotation of the photosensitive drum 1a, the light magenta toner image on the photosensitive drum 1a reaches the primary transfer region T1a where the intermediate transfer belt 5 and the photosensitive drum 1a are in contact. In the primary transfer region T1a, when a bias having a polarity (positive polarity) opposite to that of the light magenta toner is applied to the primary transfer roller 5a, the light magenta toner image is primarily transferred to the intermediate transfer belt 5. Further, the toner remaining on the photosensitive drum 1a after the primary transfer is removed by the photosensitive drum cleaner 6a.

  Similarly, toner images are similarly formed in the other image forming portions Pb, Pc, Pd, Pe, and Pf. Similarly, these toner images are also primarily transferred to the intermediate transfer belt 5 that moves in the direction of the arrow R2.

  The recording material accommodated in the recording material accommodating portion 7 provided in the lower part of the printer portion B is supplied to the secondary transfer region T2 by the supply means 81 so as to synchronize with the image formation.

  Then, by applying a bias (positive polarity) opposite in polarity to the toner image to the secondary transfer roller 52, the toner image on the intermediate transfer belt 5 is transferred onto the supplied recording material. Further, the recording material is conveyed to the fixing device 9, and the toner image is fixed to the recording material by the fixing device 9.

  On the other hand, in the secondary transfer region T2, the toner remaining on the intermediate transfer belt 5 without being transferred to the recording material is collected by the intermediate transfer belt cleaner 55.

{Toner particle size and secondary transfer efficiency}
In this example, the same toner as in Example 1 was used. That is, the volume average particle diameter of the light magenta toner for the first color and the light cyan toner for the second color was set to 7 μm.

  The volume average particle size of the third color yellow toner, the fourth color dark magenta toner, the fifth color dark cyan toner, and the sixth color black toner was set to 5 μm.

  In the image forming apparatus of the present embodiment, the charge amount per unit area of the toner of each toner image on the photosensitive drum 1 of each image forming unit is as follows as in the first embodiment.

  Here, also in the tandem type image forming apparatus as in the present embodiment, the other primary transfer region in which the toner image that has been primarily transferred to the intermediate transfer belt 5 first is located downstream in the rotation direction of the intermediate transfer belt 5. Charge is applied when passing through T1.

  As a result, there is a problem that the charge amount of the toner image that has been primarily transferred first increases, and the charge amount at the time of secondary transfer becomes non-uniform.

  Also in this embodiment, in consideration of the charge applied when passing through the primary transfer region T1 located on the downstream side, the amount of charge on the photosensitive drum 1 of the toner image that has been primarily transferred first is later transferred to the primary transfer. Less than the charge amount of the toner image.

  Thereby, the difference in the charge amount of each toner image at the time of secondary transfer is reduced, and good secondary transfer can be performed.

  In addition, the particle size of the toner of the toner image that is primarily transferred first is larger than the toner of the toner image that is primarily transferred later. As a result, the amount of charge imparted when the toner image transferred first passes through the primary transfer region T1 is reduced, and good secondary transfer can be performed.

  As described above, a toner image of a desired color could be formed on the recording material.

1 is a schematic sectional view of an image forming apparatus according to a first embodiment. FIG. 6 is a diagram illustrating the relationship between the applied amount of dark toner and light toner and the optical density of toner. FIG. 6 is a diagram illustrating an image output signal with respect to an image input signal when a light magenta toner image and a light cyan toner image are formed. FIG. 6 is a diagram illustrating an image output signal with respect to an image input signal when forming a magenta toner image and a cyan toner image. It is a figure which shows the image output signal with respect to an image input signal at the time of forming a yellow toner image and a black toner image. FIG. 3 is a diagram schematically illustrating a toner image formed on an intermediate transfer belt when measuring secondary transfer efficiency. FIG. 5 is a diagram illustrating secondary transfer residual toner amounts with respect to a secondary transfer bias value when the volume average particle diameter of dark magenta toner and light magenta toner is 5 μm. FIG. 6 is a diagram illustrating a secondary transfer residual toner amount with respect to a secondary transfer bias value when a volume average particle size of light magenta toner is 7 μm and a volume average particle size of dark magenta toner is 5 μm. FIG. 3 is a schematic cross-sectional view of an image forming apparatus according to a second embodiment.

Explanation of symbols

1, 1a, 1b, 1c, 1d, 1e, 1f Photosensitive drum (image carrier)
4a, 4b, 4c, 4d, 4e, 4f Developing device (developing means)
5 Intermediate transfer belt (intermediate transfer member)
51, 51a, 51b, 51c, 51d, 51e, 51f Primary transfer roller (primary transfer means)
52 Secondary transfer roller (secondary transfer means)

Claims (2)

A first image carrier that carries a light toner image formed of light toner;
A first transfer member for electrostatically transferring a light toner image on the first image carrier to an intermediate transfer member;
A second image carrier that carries a dark toner image formed of a dark toner having the same hue and high density as the light toner;
A second transfer member that electrostatically transfers the dark toner image on the second image carrier to the intermediate transfer member that holds the light toner image;
A secondary transfer member that electrostatically and secondary-transfers the dark toner image and the light toner image on the intermediate transfer member collectively to a recording material;
The volume average particle diameter of the light toner is larger than the volume average particle diameter of the dark toner. 2. The charge amount per unit weight of the light toner on the first image carrier is less than the charge amount per unit weight of the dark toner on the second image carrier. The image forming apparatus described.

Images