JP2006189704A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus less liable to cause image defects due to transfer, even when an image forming apparatus adopts slip transfer used in a tandem-type color printer, and capable of stably performing high-quality printing, regardless of the environmental variations. <P>SOLUTION: The image forming apparatus comprises a photoreceptor, with a surface on which a latent image is formed and made a toner image by sticking toner according to the latent image, a rotatable image transport member, and a transfer section for transferring the toner image from the photoreceptor to the image transport member, wherein the photoreceptor is a positive-charged photoreceptor and the work function of a surface of the image transport member is set to ≤4.5 eV. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

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

  In recent years, so-called full-color tandem machines have been proposed in image forming apparatuses such as printers and copying machines for the purpose of forming color images at high speed and high image quality. As a typical tandem machine, four image forming units of yellow (Y), magenta (M), cyan (C), and black (K) are arranged in parallel with each other, and each of these image forming units. The toner images of yellow, magenta, cyan, and black that are sequentially formed in Step 1 are transferred once onto the intermediate transfer belt, which is an intermediate transfer member (primary transfer), and then collectively transferred from the intermediate transfer belt onto the transfer paper. And transferring (secondary transfer), and then fixing the toner image formed on the transfer paper to form a color image.

Here, since a single color toner layer to a maximum of three toner layers are formed on the intermediate transfer belt, it is necessary to stably transfer them, and an optimum transfer voltage or transfer current for that purpose is required. Is set. However, since the transfer voltage or transfer current required for this transfer largely depends on the charge amount, adhesion amount, and surface charge amount of the photoreceptor and intermediate transfer belt, the factors need to be stabilized or controlled. Become. ,
Further, in a conventional image forming apparatus employing such an intermediate transfer system, the intermediate transfer belt is supported by a backup roll from the inside, and a photosensitive member that is a latent image carrier and a secondary image are provided on the surface side of the intermediate transfer belt. A transfer device is provided.

  At this time, the rotational movements of the intermediate transfer belt, the photosensitive member, and the secondary transfer device are usually controlled independently, and are affected by the difference in rotational radius, eccentricity of the driving roll, etc. Then, slip transfer occurs. As a method for preventing image deterioration due to slip transfer, a method of ensuring slip transfer in a certain direction by consciously setting a speed difference has been proposed (see Patent Document 1).

JP-A-8-234532 (page 1-3, FIG. 1)

  When slip transfer as described above occurs, the surface potential (charge) fluctuates between the intermediate transfer belt and the photoreceptor due to frictional charging. As a result, when the required transfer voltage value (or current value) fluctuates and the frictional charge amount is excessive, insufficient transfer (image density decrease due to transfer efficiency decrease) or excessive transfer failure (discharge phenomenon due to excessive transfer electric field). Cause image scattering).

  The frictional charging is greatly affected by the environment, and the influence of the frictional charging at the time of the above-described slip transfer on the print image quality cannot be ignored particularly in a low humidity environment.

  An object of the present invention is to provide an image forming apparatus that is less likely to cause a transfer failure even in an image forming apparatus that employs the above-described slip transfer, and that can stably perform high-quality printing against environmental fluctuations.

  The above-described problems include a photosensitive member that forms a latent image on the surface and attaches toner according to the latent image to form a toner image, a rotatable image carrier, and a toner image transferred from the photosensitive member to the image carrier. In the image forming apparatus having the transfer section, the photoconductor is a positively chargeable photoconductor, which is solved by setting the work function of the surface of the image carrier to 4.5 eV or less.

  According to the present invention, intermediate transfer due to frictional charging and the amount of charge on the photosensitive member can be suppressed, and stable transfer conditions can be maintained. In addition, transfer efficiency fluctuations due to environmental fluctuations (particularly humidity fluctuations) are also suppressed, and high-quality printing can be maintained stably over a long period.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a diagram showing an image forming apparatus to which this embodiment is applied. The image forming apparatus shown in FIG. 1 is a so-called tandem type, so-called intermediate transfer type image forming apparatus, in which a plurality of image forming units 2 (2Y, 2M,. 2C, 2K), an intermediate transfer belt 1 for sequentially transferring (primary transfer) and holding each color component toner image formed by each image forming unit 2, and a superimposed toner image transferred onto the intermediate transfer belt 1 as a recording material. A secondary transfer roll 3 that performs batch transfer (secondary transfer) to a sheet 5 (transfer sheet) and a fixing unit 6 that fixes the second-transferred image on the sheet 5 are provided.

  In this embodiment, each of the image forming units 2 (2Y, 2M, 2C, 2K) is provided around a photosensitive drum 21 that rotates in the direction of arrow A, a charger 22 for charging the photosensitive drum 21, and a photosensitive drum. A laser exposure unit 23 in which an electrostatic latent image is written on the body drum 21 (the exposure beam is indicated by an arrow in the figure), each color component toner is accommodated, and the electrostatic latent image on the photosensitive drum 21 is visible with the toner A developing device 24 for forming an image, a primary transfer roll 4 (4Y, 4M, 4C, 4K) for transferring each color component toner image formed on the photosensitive drum 21 to the intermediate transfer belt 1, and a residual toner on the photosensitive drum 21 An electrophotographic device such as a drum cleaner 25 from which is removed is sequentially disposed. These image forming units 2 are arranged substantially linearly in the order of yellow (Y), magenta (M), cyan (C), and black (K) from the upstream side of the intermediate transfer belt 1.

The intermediate transfer belt 1 that is an intermediate transfer member is made of a resin such as polyimide or polyamide containing an appropriate amount of an antistatic agent such as carbon black, and its volume resistivity is 10 ⁶ to 10 ¹⁴ Ωcm. The thickness of the endless belt is, for example, about 0.1 mm. The intermediate transfer belt 1 is circulated and driven (rotated) at a predetermined speed in the direction A shown in the figure by various rolls. As these various rolls, a drive roll 7 that is driven by a motor (not shown) having excellent constant speed and circulates and drives the intermediate transfer belt 1, and an intermediate that extends substantially linearly along the arrangement direction of the photosensitive drums 21. A support roll 8 that supports the transfer belt 1, a tension roll 9 that functions as a correction roll that applies a constant tension to the intermediate transfer belt 1 and prevents meandering of the intermediate transfer belt 1, and a backup provided on the secondary transfer roll 3. A roll 10 and a cleaning backup roll 12 provided in a cleaning unit 11 that scrapes residual toner on the intermediate transfer belt 1 are provided.

  A voltage having a polarity opposite to the charging polarity of the toner is applied to each primary transfer roll 4 provided inside the intermediate transfer belt 1 facing the respective photosensitive drums 21 and extending substantially linearly. As a result, the toner images on the respective photosensitive drums 21 are sequentially electrostatically attracted to the intermediate transfer belt 1 so that the toner images superimposed on the intermediate transfer belt 1 are formed.

The secondary transfer portion is constituted by the secondary transfer roll 3 disposed on the toner image carrying surface side of the intermediate transfer belt 1, the backup roll 10, and the like. The backup roll 13 is a tube of EPDM and NBR blend rubber with carbon dispersed on the surface, and the inside is made of EPDM rubber so that the surface resistivity is 10 ⁷ to 10 ¹⁰ Ω / cm ² and the roll diameter is 20 mm. The hardness is set to 60 ° (Asker C), for example. The backup roll 10 is arranged on the back side of the intermediate transfer belt 1 and is arranged as a counter electrode of the secondary transfer roll 3.

  Next, a basic printing process of the image forming apparatus according to the present embodiment will be described. Image data output from a personal computer (not shown) or the like is input to an image forming apparatus as shown in FIG. The input image data is converted into color material gradation data of four colors of yellow (Y), magenta (M), cyan (C), and black (K), and is output to the laser exposure device.

  The laser exposure unit 23 irradiates each photosensitive drum 21 of the image forming units 2Y, 2M, 2C, and 2K with an exposure beam according to the input color material gradation data. In each of the photosensitive drums 21 of the image forming units 2Y, 2M, 2C, and 2K, the surface is charged by the charger 22, and then the surface is scanned and exposed by the laser exposure unit 23 to form an electrostatic latent image. The formed electrostatic latent image is developed as a toner image of each color of Y, M, C, and K in each of the image forming units 2Y, 2M, 2C, and 2K.

  The toner images formed on the photosensitive drums 21 of the image forming units 2Y, 2M, 2C, and 2K are transferred onto the intermediate transfer belt 1 at a primary transfer portion where the photosensitive drums 21 and the intermediate transfer belt 1 are in contact with each other. Transcribed. Here, a negatively charged OPC drum (film thickness: 24 μm) having a diameter of 150 mm was used as the photosensitive drum 21. At the primary transfer portion, the primary transfer roll 4 applies a voltage (+) having a polarity opposite to the toner charging polarity (−) to the base material of the intermediate transfer belt 1, and an unfixed toner image is transferred to the intermediate transfer belt 1. Primary transfer is performed by sequentially superimposing on the surface. Here, the rotational movements of the intermediate transfer belt 1 and the respective photosensitive drums 21 are controlled independently, and the feed speed of the intermediate transfer belt 1 is set to be about 1% faster. Therefore, slip transfer is always occurring in each primary transfer portion. The unfixed toner image primarily transferred as described above is conveyed to the secondary transfer portion as the intermediate transfer belt 1 rotates.

  On the other hand, in the paper transport system, the paper 5 is supplied from a paper supply device (not shown) at the timing of image formation. The supplied paper 5 is transported by a transport roll (not shown) and reaches the secondary transfer unit. In the secondary transfer portion, the unfixed toner image carried on the intermediate transfer belt 1 is electrostatically transferred to the sheet 5 at the secondary transfer position pressed by the secondary transfer roll 3 and the backup roll 10. .

  Thereafter, the sheet 5 on which the toner image has been electrostatically transferred is conveyed to the fixing device 6 by a sheet conveying device (not shown). The unfixed toner image on the paper 5 conveyed to the fixing device 6 is fixed on the paper 5 by being subjected to a fixing process with heat and pressure by the fixing device 6, and the paper 5 on which the fixed image is formed is discharged to a discharge roll. (Not shown) and discharged outside the apparatus. On the other hand, after the transfer to the paper 5 is completed, the residual toner remaining on the intermediate transfer belt 1 is conveyed to the cleaning unit as the intermediate transfer belt 1 rotates, and the cleaning backup roll 12 and the intermediate transfer belt cleaner 11 are transferred. Is removed from the intermediate transfer belt 1. A color image is formed by such a series of image forming processes.

Here, since the intermediate transfer belt 1 and the photosensitive drum 21 have a speed difference of about 1% in the primary transfer portion as described above, friction is always performed. In this embodiment, the photosensitive drum 21 is a negatively charged OPC, the surface work function is about 5.4 eV, and the intermediate transfer belt 1 contains an appropriate amount of carbon black in a polyimide resin, and has a volume resistance value of 10 ⁹ Ωcm. The surface work function was about 4.7 eV at a thickness of 100 μm, and the work function difference was about 0.7 eV. Next, the results of examination of the print image quality by conducting a continuous printing experiment using the combination of the medium-term transfer belt 1 and the photosensitive drum 21 will be described.
Each setting condition at the time of the printing experiment is as follows.

<Setting conditions>
・ Process speed: 500mm / sec
Writing light: 600 dpi, LD laser Photoconductor drum 21: OPC drum (outer diameter: φ150 mm), peripheral speed 500 mm / sec
Charging potential: about -500v
Intermediate transfer belt 1: peripheral speed 505 mm / sec
・ Developer (same conditions for Y, M, C, K)
Carrier average particle size: 60 μm,
Toner average particle size: 7 μm
Toner mixing ratio 2.5wt%
・ Print pattern: 1 on 1 off line pattern
Y, M, C, K single color printing and RGB two-color overprinting ・ Primary transfer voltage: 1000v
・ Secondary transfer voltage: 2500v
Under the above printing conditions, a continuous printing experiment of 5000 pages was performed, and the printing image quality was evaluated visually and with a resolution MTF. As a result, although the image density was obtained, toner scattering occurred at the line edge portion, and the MTF was 0.4 and the resolution was low.

  Next, a similar printing experiment was performed using the intermediate transfer belt 1 in which the surface of the intermediate transfer belt 1 was coated with a fluororesin and the surface work function was set to 5.2 eV. Under this condition, the work function difference between the intermediate transfer belt 1 and the photosensitive drum 21 is 0.2 eV.

  As a result, the print image quality after 5000 pages showed no abnormalities in visual observation, and the value of the resolution MTF was as good as 0.8, and it was confirmed that the toner was not scattered at the transfer portion.

  FIG. 2 shows the relationship between the surface work function of the intermediate transfer belt 1 when using a negatively charged photoconductor (OPC) and the resolution MTF measured in the example. From this result, it can be seen that the work function of the intermediate transfer belt 1 is suitably 5.5 or more in order to ensure MTF ≧ 75% at which good resolution can be obtained.

  Next, the photosensitive drum 21 is a positively charged Se drum (φ150 mm, film thickness: 60 μm, work function: 4.1 eV), and the unprocessed intermediate transfer belt 1 (work function 4.7 eV) used in Example 1 is used. A similar printing experiment was performed in combination with The work function difference between the intermediate transfer belt 1 and the photosensitive drum 21 at that time was 0.6 eV. The experimental result shows that the print image quality after 5000 pages and the resolution MTF is 0.7, which is an allowable value, and the toner scattering is small, but the image density of a part of the two-color superimposed R, G, B printing part is low, transfer Insufficient phenomenon occurred.

  The intermediate transfer belt 1 used in Example 3 was subjected to surface modification by plasma treatment in an Ar atmosphere, the surface work function was set to 4.4 eV, and a positively charged Se photosensitive drum (work function 4.1 eV) was obtained. A similar printing experiment was conducted with a combination of Under this condition, the work function difference between the intermediate transfer belt 1 and the photosensitive drum 21 is 0.3 eV.

  As a result, the print image quality after 5000 pages had no partial image density reduction and the resolution MTF value was as good as 0.8, and it was confirmed that partial transfer shortage did not occur.

  FIG. 3 shows the relationship between the surface work function of the intermediate transfer belt 1 and the image density measured in Example 3 when a positively charged photoconductor (Se photoconductor) is used. From this result, O.D. D. It can be seen that the work function of the intermediate transfer belt 1 is suitably 4.5 or less to ensure 1.2.

1 is a schematic diagram of an image forming apparatus to which the present invention is applied. The graph which showed the relationship between the work function of an intermediate transfer belt, and the resolution MTF of printing image quality at the time of using a negatively charged photoreceptor. The graph which showed the relationship between the work function of an intermediate transfer belt, and the image density of printing quality at the time of using a positively charged photoreceptor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Intermediate transfer belt, 2 ... Image forming unit, 3 ... Secondary transfer roll, 4 ... Primary transfer roll, 5 ... Paper, 6 ... Fixing device, 21 ... Photoconductor, 22 ... Charger, 23 ... Laser exposure device, 24 ... developing device, 25 ... cleaning brush.

Claims (3)

A photosensitive member that forms a latent image on the surface and attaches toner according to the latent image to form a toner image; a rotatable image carrier; and a transfer unit that transfers the toner image from the photosensitive member to the image carrier. The image forming apparatus according to claim 1, wherein the photoconductor is a positively chargeable photoconductor, and a work function of a surface of the image carrier is 4.5 eV or less.   The image forming apparatus according to claim 1, wherein the image carrier is used for color printing in which toner is overlapped through a plurality of transfer processes. 2. The image forming apparatus according to claim 1, wherein the process speed is 300 mm / s or more.

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