JPH11174780A - Multicolor image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multicolor image forming device capable of preventing the electrification irregularity owing to toner staining of a re-electrifying unit, at the time of performing the re-electrification in same polarity with the toner on the image carrier on which the toner image is formed by the re-electrifying unit, performing development of the succeeding toner image without disturbing the toner image being already formed, preventing the toner from entering the succeeding developing unit, and obtaining the excellent multicolor image. SOLUTION: In the image forming device provided with the developing unit 4 for a first color being the two component developing unit, the device is allowed to adopt an electrifying roller 5 held in light contact with a photoreceptor drum 1 as the electrifying unit for a second color (re-electrifying unit), and to re-electrify the photoreceptor drum 1 by applying DC voltage on the electrifying roller 5. A DC voltage of this electrifying voltage, in the same polarity with the polarity of the toner on the toner image of the first color formed on the photoreceptor drum 1, is the voltage larger than the toner layer potential.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a laser beam printer and an electrostatic recording apparatus, and more particularly to a multicolor image forming apparatus capable of performing multicolor printing.

[0002]

2. Description of the Related Art In recent years, there are many visible images of different colors and multicolor images obtained by synthesizing different information on a single sheet of paper. For this reason, a multicolor image having a plurality of developing units has been developed. Forming equipment is on the market.

In such a multi-color image forming apparatus, multi-color development is performed by two or more developing units while the image carrier rotates once, and the obtained multi-color image is transferred onto a transfer material, for example, on paper. In many cases, transfer is performed collectively. For example, US Pat. Nos. 4,572,651 and 4,416,533.
Discloses a technique in which development is performed by keeping the electric field constant with a DC bias in both developing devices. But,
These patents have a major problem in a method of forming a latent image, and do not suggest a problem during development.

On the other hand, US Pat. No. 4,349,268 and Japanese Patent Application Laid-Open No. 56-1444 published earlier in Japan.
No. 52 employs a non-contact developing method using an AC developing bias for the second color development, and JP-A-56-12650 employs a non-contact developing method using a DC developing bias for the second color development. A technique for preventing a visible image of a first color, that is, a toner image, from being disturbed by rubbing of a developer of a second color is disclosed. Incidentally, the above-mentioned Japanese Patent Application Laid-Open No. 56-144
No. 452 does not describe the potential of the first color toner image at all.

As described above, conventionally, in multi-color image formation, it has been known to develop the next color so as not to disturb the previously formed toner image. There is also known a method of increasing the level of the latent image potential of a toner image that has been developed in advance. That is, U.S. Pat.
No. 60,961 discloses that after forming a first color toner image, the same polarity as the toner is applied to the entire surface of the image carrier, and the latent image potential of the first color toner image (the portion where the first toner image is formed) (Surface potential of the photosensitive drum) is substantially the same as that of the non-developing portion (non-image portion) so that the toner image of the first color can be prevented from being significantly disturbed during the development of the second color. .
This method can be applied to a one-pass multicolor printing image forming apparatus, particularly, a negative image forming apparatus.
In recent years, a method called a negative recharging method has been actively studied.

[0006]

However, the conventional negative-negative recharging system has the following problems.

After developing the first color toner image, the image carrier is
When the recharger having the same polarity as the toner is recharged by the color charger (recharger), the toner of the first color on the image carrier scatters and adheres to the shield and grid portion of the recharger and becomes dirty. The toner stains on the shield and the grid accumulate as the copying process is repeated, and eventually the toner stains on the corona wire of the recharger, causing uneven charging due to recharging of the image carrier.

For this reason, the potential of the latent image of the second color after the recharging does not become substantially the same potential. Also 2
Adhesion of color toner occurs. Further, the first color toner is mixed into the second and subsequent color developing units, and the first color toner is used in the development by the second and subsequent color developing units in the subsequent image formation, so that an unclear image is formed. Will be done.

In particular, when the developing device for the first color is a two-component developing system, the carrier inevitably adheres to the image carrier and remains.
It may cause adverse effects by being mixed into the developing units for the second and subsequent colors, cause poor transfer of the toner image to the transfer material, and cause damage or deterioration of the image carrier.

When the one-component non-contact jumping development method is used for the development of the second color, the toner image of the first color is not disturbed.
Since the color development can be performed, the jumping development can be performed by 1
Although it is a convenient development method for a pasta multicolor print image forming apparatus, if a carrier is mixed into this jumping developing device,
The development characteristics deteriorated remarkably, causing problems such as poor coating of the toner on the developer carrying member and a decrease in the density of the developed image.

An object of the present invention is to eliminate uneven charging of the image carrier due to toner contamination of the recharger when the image carrier on which the toner image is formed is recharged by the recharger with the same polarity as the toner. It is possible to develop a later toner image without disturbing the already formed toner image, and to prevent the toner from the already formed toner image from being mixed into a later developing device and used. Accordingly, an object of the present invention is to provide a multicolor image forming apparatus capable of obtaining a high quality multicolor image.

[0012]

The above object is achieved by a multicolor image forming apparatus according to the present invention. In summary, the present invention includes a plurality of charging units, a plurality of exposing units, a plurality of developing units, and a transferring unit, and at least the first of the developing units is a two-component developing unit. In the multicolor image forming apparatus for forming a multicolor toner image by sequentially repeating charging, exposure and development to form a multicolor toner image and collectively transferring the obtained multicolor toner image onto a transfer material, The second and subsequent charging means are a multicolor image forming apparatus comprising a contact-type elastic charging member which makes light contact with the image carrier, and applies a DC voltage to the elastic charging member.

Another embodiment of the present invention comprises a plurality of charging means, a plurality of exposing means, a plurality of developing means, and a transferring means,
At least the first of the developing units is a two-component developing unit, and a multicolor toner image is formed by sequentially repeating charging, exposure and development on the image carrier, and the obtained multicolor toner image is formed. In a multicolor image forming apparatus that performs batch transfer onto a transfer material, a contact-type elastic charging member that is in light contact with the image carrier is disposed upstream of the transfer unit with respect to the rotation direction of the image carrier. A multicolor image forming apparatus characterized by applying a DC voltage to a member.

According to the present invention, the DC voltage is a voltage having the same polarity as that of the toner of the toner image formed on the image carrier by development and higher than the toner layer potential. The elastic charging member is formed of a cylindrical roller, and its surface layer is formed of a resin having the same polarity as the toner on the triboelectric charging series, a resin composition obtained by mixing a lubricant with the resin, or a resin obtained by mixing a fluororesin with the resin. Formed with any one of the compositions. The elastic charging member is driven to rotate with respect to the image carrier,
A carrier removing member was placed in contact with the elastic member.

[0015]

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

Embodiment 1 FIG. 1 is a schematic structural view showing an embodiment of a multicolor image forming apparatus of the present invention, and shows a multicolor electrophotographic apparatus.

Referring to FIG. 1, the multicolor electrophotographic apparatus has a photosensitive drum 1 as an image carrier on which an electrostatic latent image is formed, and a first primary charger 2 and a A first developing device 4, a charging roller 5 as a second primary charger (recharger), a second developing device 7, a transfer charger 8,
An exposure optical system for irradiating the second laser beams 3 and 6 is provided, and a cleaning device 11 is further provided.

In order to form an image, first, the photosensitive drum 1
Is charged to a negative potential of about -600 V by the first charger 2. Next, the first laser beam 3 from the image exposure system modulated by the image signal of the first color is applied to the photosensitive drum 1.
And the surface potential of the irradiated portion of the photosensitive drum 1 becomes about -1.
00V, and a first latent image of the first color is formed. The first developing device 4 employs a two-component developing system. The first latent image is reversely developed by the first developing device 4 using a two-component developer composed of negatively charged red toner and magnetic particles (carrier) such as ferrite, and is visualized as a red first toner image. You.

Photosensitive drum 1 on which first toner image is formed
Is negatively charged again by the charging roller 5, and the potential of the first toner image is changed from -100 V to -700 V by this charging.
To rise. Next, the second laser beam 6 from the image exposure system modulated by the image signal of the second color is applied to the photosensitive drum 1.
And the surface potential of the irradiated portion of the photosensitive drum 1 becomes about -1.
00V, and a second latent image of the second color is formed. The second developing device 7 employs a magnetic one-component developing system. The second latent image is reverse-developed by the second developing device 7 using a negatively charged one-component magnetic toner black toner by a jumping development method, and is visualized as a black second toner image. During development,
A bias in which an AC voltage and a DC voltage are superimposed between the photosensitive drum 1 and the developing sleeve 7a of the second developing device 7 is applied.

The first and second toner images formed on the photosensitive drum 1 in this manner are transferred onto a transfer material 9 supplied to a transfer section where the photosensitive drum 1 and the transfer charger 8 are opposed to each other.
The transfer is performed collectively by the operation of the transfer charger 8. The transfer material 9 on which the transfer of the toner image has been completed is sent to a fixing device (not shown) to fix the toner image. The toner remaining on the photosensitive drum 1 is removed by the cleaning device 11 to prepare for the next image formation.

The image exposure system described above comprises semiconductor lasers 12 and 13 for irradiating first and second laser beams 3 and 6 modulated by image signals of the first and second colors, and deflects the laser beams 3 and 6. Rotating polygon mirror 14,
An imaging lens 16 for imaging the deflected laser beams 3 and 6; The first laser beam 3 that has passed through the imaging lens 16 is reflected by the reflecting mirror 17 and irradiates the surface of the photosensitive drum 1, and the second laser beam 6 that has passed through the imaging lens 16 directly irradiates the surface of the photosensitive drum 1. Then, the respective surfaces are raster-scanned to form first-color and second-color latent images.

FIG. 2 shows the transition of the surface potential of the photosensitive drum 1 in the above image formation. The surface of the photosensitive drum 1 is uniformly charged to −600 V by the charging by the primary charger 2 (FIG. 2A), and the irradiation portion is attenuated to −100 V by the irradiation of the first laser beam 3, and the first latent image is formed. It is formed at a potential of -100 V (FIG. 2 (2)). After the first latent image is developed by the developing device 4 to form a first toner image ((3) in FIG. 2), the surface of the photosensitive drum 1 is again charged to the same polarity as the toner by the charging by the charging roller 5 again. is, the potential of the first toner image (toner layer potential V _T) becomes -700 V (FIG. 2 (4)). The irradiated portion is -100 V by the next irradiation of the second laser beam 6
And a second latent image is formed at a potential of -100 V (FIG. 2).
(5)), the second latent image is developed by the developing device 7 to form a second toner image (FIG. 2 (6)).

Here, a schematic mechanism of generation of toner stain in the conventional recharger will be described with reference to FIG. FIG.
FIG. 3 is a schematic diagram showing a photosensitive layer, a substrate, and a charge portion distribution of the first latent image after development of the first latent image. The recharger 18 is a corona charger as shown in FIG.

In the reversal development by the first developing device 4, the negative toner 20 is selectively adhered to the portion of the surface of the photosensitive drum 1 where the charge is small (the portion of the first latent image). The adhesion of the toner 20 on the surface of the photosensitive drum 1 is realized in a state where the electrostatic force such as the Coulomb force is extremely weak due to reversal development. In this state, the photosensitive drum 1 rotates in the direction of the arrow in FIG. 8 and reaches the charging area of the recharger 18.

In order to charge the surface of the photosensitive drum 1 corona, a high voltage is applied from the high voltage power supply 19 so that a constant current of about -700 μA flows through the corona wire 18a. The electric field generated between the recharger 18 and the photosensitive drum 1 at this time exits from the corona wire 18a, spreads loosely in the shield 18b, and reaches the surface of the photosensitive drum 1. On the other hand, since the shield 18b of the recharger 18 is grounded, an electric field opposite to the electric field by the corona wire is generated from the first latent image portion on the photosensitive drum 1 in a portion near the shield 18b.

The rotation of the photosensitive drum 1 causes the photosensitive drum 1
When the upper negative toner 20 is applied to the above-described location of the electric field in the opposite direction, the toner 20 adheres to the surface of the photosensitive drum 1 with a very weak electrostatic force.
A part of 0 is peeled off from the surface of the photosensitive drum 1 by the electric field in the opposite direction, and flies and adheres to the shield 18b.

When the photosensitive drum 1 further rotates and the negative toner 20 on the photosensitive drum 1 enters the charging area by the recharger 18, the surface of the photosensitive drum 1 at that portion receives corona charging, and the photosensitive drum 1 A positive charge is generated on the substrate. This positive charge generates a Coulomb force on the negative toner 20 on the photosensitive drum 1, so that the adhesive force with the photosensitive drum 1 rapidly increases, and the toner 20 is peeled off from the first latent image portion by the reverse electric field. Overcoming the picking force, it stays on the surface of the photosensitive drum 1.

As described above, most of the negative toner 20 on the photosensitive drum 1 stays on the surface of the photosensitive drum 1, and is removed from the photosensitive drum 1 and adhered to the shield 18 b by a part of the toner 20. However, if the image forming process is performed many times, the toner is accumulated due to the repetition of the adhesion of the toner 20 to the shield 18b, and the toner is slightly reduced in the portion of the shield 18b on the downstream side in the rotation direction of the photosensitive drum 1, Toner contamination occurs on the shield 18b, and eventually adheres to the corona wire 18a, causing toner contamination on the corona wire 18a. As a result, as described above, an image defect occurs due to uneven charging of the photosensitive drum 1.

Therefore, in the present invention, as shown in FIG. 3, a contact-type charging roller 5 is used as the recharger instead of the recharger 18 using the corona charger, so that inconvenience such as wire contamination is caused. Lost.

In this embodiment, the charging roller 5 is provided with an elastic resistance layer 5b on a metal core 5a, and a surface protective layer 5b on the elastic resistance layer 5b.
c is provided. The elastic resistance layer 5 b is a layer that maintains reliable contact between the photosensitive drum 1 and the charging roller 5 and controls a current flowing to the photosensitive drum 1. Surface protective layer 5
“c” is a layer for preventing seepage from the inside of the charging roller 5 and for preventing contamination of the charging roller 5 due to adhesion of toner or the like on the photosensitive drum 1. For example, a fluororesin having excellent lubricity such as PRFE and PFA is used. It is formed singly or by mixing it with another resin, and has a thickness of several μm or less.

A cleaning blade 21 was used as a cleaning means in contact with the charging roller 5 in order to improve prevention of contamination due to the adhesion of toner and the like.

In the present embodiment, the surface potential of the photosensitive drum 1 on which the first electrostatic latent image has been formed has a maximum of -600 V in the non-irradiated portion of the first laser, and the irradiated portion of the first laser (the first latent image) Part)
, The first developing unit develops only the first laser irradiating unit, and the toner layer potential of the toner image after the first developing operation is substantially at a minimum of −200 V.

When a DC voltage of -1550 V is applied to the charging roller 5 in this state, a surface discharge from the charging roller 5 at a portion near the photosensitive drum 1 causes the photosensitive drum 1 to be recharged.
The potential of the first laser non-irradiated portion of the photosensitive drum 1 is -95 at maximum.
0 V, potential of the first toner image portion (toner layer potential V _T )
Becomes substantially -700V.

As a result, an electric field is generated on the surface layer of the charging roller 5 from the charging roller 5 toward the recharged first latent image potential and the first toner layer potential, and the recharged negative toner on the photosensitive drum 1 is generated. As shown in FIG.
The electric field force that is pressed against acts. Further, the recharged negative toner 20 also has a Coulomb force between the negative toner 20 and the first latent image potential. Therefore, the photosensitive drum 1 passing through the charging roller 5
The upper negative toner 20 is prevented from flying to the charging roller 5 by the electric field force and the Coulomb force due to recharging.

That is, by keeping the average potential of the surface layer of the charging roller 5 higher than the first toner layer potential, the effect of pressing the toner 20 on the photosensitive drum 1 by the electric field is exerted, and the charging roller 5 of the toner 20 is charged. 5 can be prevented and toner contamination of the charging roller 5 can be prevented.

FIG. 4 shows the relationship between the DC voltage applied to the charging roller 5 and the surface potential of the first toner image portion and the non-image portion on the photosensitive drum 1. The first toner image formed on the photosensitive drum 1 is a solid black image formed by FF development. The black circle mark in FIG. 4 indicates the first toner image (solid black image) on the photosensitive drum 1.
Is the surface potential after recharging, and the white circle mark is the surface potential after recharging of the non-image portion other than the first toner image portion of the photosensitive drum 1.

As shown in FIG. 4, the higher the DC voltage applied to the charging roller 5, the smaller the potential difference between the first toner image portion where toner is present and the non-image portion where toner is not present. Would be preferred. However, if the surface potential of the photosensitive drum 1 is recharged to −1000 V or more by further increasing the DC voltage applied to the charging roller 5, the potential of the photosensitive drum is attenuated when the second laser 6 is irradiated next. The second latent image to be formed has a low latent image contrast. From now on, the photosensitive drum 1 by recharging
Is preferably −1000 V or less.

As described above, in this embodiment, by applying a DC voltage having the same polarity as the toner of the first toner image on the photosensitive drum 1 and higher than the toner layer potential to the charging roller 5, Since an electric field directed toward the photosensitive drum 1 is generated on the surface layer 5, toner can be prevented from flying from the first toner image to the charging roller 5, and the charging roller 5 is cleaned by the cleaning blade 21. The toner contamination on the charging roller 5 can be prevented.

Further, the first toner image on the photosensitive drum 1, ie, the first color toner image, is pressed against the photosensitive drum 1 by the charging roller 5, so that the attraction force of the first color toner image to the photosensitive drum 1 can be increased. This makes it possible to prevent the first color toner from entering the second color developing device 7.

Therefore, even if the image forming process is repeated many times, as in the case where a corona charger is used as the recharger, uneven charging such as charging of the photosensitive drum due to toner contamination of the corona wire is generated. Thus, the subsequent second toner image can be developed without disturbing the first toner image.

The first developing device 4 for the first color can arbitrarily select a desired color by adopting a two-component developing method. However, conventionally, there is a problem that the carrier remains on the photosensitive drum 1. There is.

However, in this embodiment, the electrostatic attraction force is generated by the direction of the electric field generated by the voltage applied to the charging roller 5 and the charging polarity of the frictional charging of the carrier, and the electrostatic adhering force adheres to the photosensitive drum 1 and remains. The removed carrier can be attached to the charging roller 5 and removed.

In this embodiment, in order to check the carrier removing effect of the charging roller 5, the contrast between the non-image area potential and the developing bias, so-called Vback, is extremely increased to 500V in the developing device 4 for the first color. A large amount of carrier from the first color developer was attached to the photosensitive drum 1 by the electric field generated between the developing sleeve and the photosensitive drum 1. Normally, this Vback is about 100 to 200 V, and a large amount of carrier cannot be adhered, so that the carrier removal effect by the charging roller 5 can hardly be quantified.

FIG. 5 shows the carrier collection ratio with respect to the voltage applied to the charging roller. As shown in FIG. 5, when the voltage applied to the charging roller 5 is in the range of -1000 to -2000 V, about 95% or more of the carrier can be collected from the photosensitive drum 1, and it is clear that the carrier can be almost completely removed. When the applied voltage is increased to -2000 V or more, the collection rate of the carrier is decreased. This is considered to be because the carrier itself is reversely charged by the charging roller.

As described above, the carrier of the developer of the first color remains on the photosensitive drum 1, and the carrier is mixed into the developing device of the second color to exert an adverse effect, or the transfer failure of the two-color toner image to the transfer material. And the carrier is not damaged or deteriorated by the carrier. Further, when non-contact jumping development is used for the development of the second color, there is also a problem that a toner coating defect on the developing sleeve 7a due to the mixed carrier and a decrease in the density of the developed image occur.
Could be prevented.

In this embodiment, an image of 100,000 sheets was formed using a normal original under the condition that a DC voltage of -1550 V was applied to the charging roller 5. As a result, charging unevenness due to toner contamination of the charging roller 5 does not occur, and conventionally, since the latent image potential after recharging does not become substantially the same, 2
There was no problem that the toner image of the first color was disturbed during the development of the color eye, and the toner of the first color adhered to portions other than the latent image of the second color. Further, there is no problem that the toner of the first color is mixed in the second developing device 5 of the second color and an unclear image is generated due to the mixed color of the developer of the developing device 5 at the time of forming an image later.

On the other hand, when an image of a normal document is formed under the condition that charging is performed by a corona charger without using the charging roller 5, the influence of the carrier attached to the photosensitive drum 1 near the cleaner 11 causes The scratches occurred on the photosensitive drum 1 during image formation of about 50,000 sheets.

As described above, in this embodiment, by using the charging roller 5 for recharging, no damage or deterioration of the photosensitive drum 1 is observed even when an image is formed on 100,000 sheets, and the non-contact jumping development of the second color is performed. It was also very stable and maintained high image quality.

Embodiment 2 As described above, in Embodiment 1, the charging roller 5 is used as a recharger, and the DC voltage is applied to the charging roller 5 while the first toner image is pressed against the photosensitive drum 1 by the charging roller 5. To prevent the toner from flying from the first toner image to the charging roller 5 by generating an electric field toward the photosensitive drum 1 on the surface layer of the charging roller 5. Since the cleaning is performed, even if the image forming process is repeated many times, uneven charging due to toner contamination of the recharger as in the case of the corona charger does not occur. Furthermore, the carrier remaining on the photosensitive drum 1 can be removed, and the adverse effect of mixing the carrier into the developing device 7 of the second color, the transfer failure of the toner image to the transfer material, and the damage and deterioration of the photosensitive drum 1 by the carrier can be obtained. In the case where non-contact jumping development is used for the second color development,
The problem of causing a poor toner coating on a and a decrease in the density of a developed image could also be prevented.

However, as the photosensitive drum 1, a
When an image of 50,000 sheets is formed on a normal original by using an a-Si drum using -Si (amorphous silicon) and using a toner having a positive charge polarity, a slight contamination due to toner adhesion to the charging roller 5 occurs. This caused uneven charging.

This is because the positively charged toner is more positively charged by the roller charging, but the fluororesin used for the surface protective layer 5c as the surface layer of the charging roller 5 is also more strongly charged by the frictional charging with the toner. This is because the negatively charged, positively charged toner flies strongly on the surface of the charging roller 5 due to the Coulomb force, and is difficult to remove by cleaning with the blade 21, and is charged by repeating the image forming process many times. It is considered that toner contamination occurred on the surface of the roller 5.

Therefore, in the second embodiment, the surface protective layer 5c as the surface layer of the charging roller 5 is made of a polyimide amide resin containing a nylon resin having the same charging polarity as the toner and excellent lubricity. . As a result, without changing the charging characteristics of the charging roller 5, the polyimide / amide resin on the surface layer of the charging roller 5 is also strongly positively charged by frictional charging with the toner. As a result, the repulsive force of Coulomb force is applied to the positively chargeable toner. By working, it became possible to prevent toner from adhering to the surface layer of the charging roller 5.

That is, by using a material having a triboelectric series having the same polarity as the toner and excellent lubricity as the material such as the resin contained in the surface protective layer 5c of the charging roller 5, the Coulomb repulsive force acts on the toner. Further, the adhesion of the toner to the surface layer of the charging roller 5 was prevented.

As the resin material which is negatively charged in the triboelectric series, a fluorine resin such as PFA or PTFE is typical.
This resin is also very excellent in lubricity. Other examples include resins such as polypropylene and polyethylene. Known resin materials that are positively charged in the triboelectric series include resins such as polyimide, polyamide, polyimide amide and acrylic. By using these resins having positive and negative charging polarities in appropriate amounts, the surface protective layer 5 of the charging roller 5 can be formed.
It is possible to impart improved toner adhesion prevention and sliding properties to c.

Incidentally, in this embodiment, an a-Si drum was used as the photosensitive drum 1, and the surface protective layer 5c was formed of a polyimide amide resin containing a nylon resin (having the same positive polarity as the toner and having high lubricity). Normally, 200,000 sheets of an image were formed on the original using the charging roller 5. However, uneven charging due to toner contamination of the charging roller 5 did not occur, and the latent image potential after recharging did not become substantially the same. In addition, there was no problem that the toner image of the first color was disturbed during the development of the second color and the toner adhered to portions other than the latent image of the second color.
Further, there is no problem that the developer of the first color is mixed into the developing device 5 of the second color, and an unclear image is generated due to the mixed color of the developer of the developing device 5 at the time of forming an image later.

On the other hand, when an image of a normal document is formed under the condition that charging is performed by a corona charger without using the charging roller 5, the influence of the carrier attached to the photosensitive drum 1 near the cleaner 11 causes The scratches occurred on the photosensitive drum 1 during image formation on about 100,000 sheets.

As described above, in the present embodiment, by using the charging roller 5 for recharging, no damage or deterioration of the photosensitive drum 1 is observed even when an image of 200,000 sheets is formed, and the non-contact jumping development of the second color is performed. It was also very stable and maintained high image quality.

Example 3 The photosensitive drum 1 is repeatedly charged, exposed and developed to form a multi-layered toner image on the photosensitive drum 1 and collectively transfer the toner image to a transfer material. Are formed with different toners, the transfer efficiency of each layer is different, which causes explosive toner scattering on the transfer material or color unevenness. Provoked.

Therefore, in the third embodiment, as shown in FIG. 6, a pre-transfer charging roller 55 is provided between the developing device 7 and the transfer charging device 8 in the last stage with respect to the photosensitive drum 1, and The charging roller 55 was lightly contacted with the multilayer toner image on No. 1 to charge the toner in each layer, thereby uniformly charging the toner and improving the transfer efficiency.

FIG. 7 shows the relationship between the voltage applied to the pre-transfer charging roller 55 and the transfer efficiency. About -300 to the charging roller 55
When a DC voltage of V was applied, the transfer efficiency of both colors was almost 95%, and a good image could be obtained. However,
If the voltage applied to the pre-transfer charging roller 55 is too high, the Coulomb force of the toner with the photosensitive drum 1 becomes too strong, and the transfer efficiency decreases.

Further, the pre-transfer charging roller 55 attracts the carrier, the fogging toner, and the reversal toner (toner charged to the opposite polarity, which causes image deterioration) attached to the photosensitive drum 1 by the electric field with the photosensitive drum 1. Then, it was confirmed that the photosensitive drum 1 was removed from the photosensitive drum 1 and easily separated by a blade due to the lubricity of the charging roller.

[0062]

As described above, according to the present invention, a charging roller is used as a recharger instead of a corona charger.
The Coulomb force due to roller charging and the electric field on the surface of the charging roller can prevent the toner on the photosensitive drum from flying to the charging roller, and the toner on the corona wire, such as when a corona charger is used. It is possible to eliminate charging unevenness due to dirt, thereby preventing image defects. Furthermore, by using a material having the same polarity as the toner and excellent lubricity on the surface of the charging roller in the triboelectric series, a strong Coulomb repulsion is exerted on the toner to prevent the toner from adhering to the surface of the charging roller. It can be further improved. Further, since the first color toner image on the image carrier is pressed against the image bearing member by the charging roller, the attraction force of the first color toner on the image carrier can be increased, and the first color toner can be reduced to two.
It is possible to prevent the color mixture from entering the developing device.

As a result, since the latent image potential after recharging does not become substantially the same, the toner image of the first color is disturbed during the development of the second color, and toner adheres to portions other than the latent image of the second color. there is no problem. Further, in a one-pass multi-color printing image forming apparatus, a negative or negative charging method is used, for example, in which a first-color toner is mixed into a second-color developing device and an unclear image is developed at a later development. The problem which arises when using was solved. In addition, the carrier remaining on the image carrier during the development of the first color is mixed into the developing device for the second and subsequent colors and exerts an adverse effect, poor transfer of the toner image to the transfer material, and further damage or deterioration of the image carrier. Could be prevented.

Further, fog toner and reversal toner adhering to the image carrier can be removed from the image carrier by adsorbing to the charging roller by the electric field force with the image carrier, and can be easily separated by the blade due to the lubricity of the charging roller. Was. The voltage applied to the charging roller is only about 2 kV at the upper limit, and the high voltage generator of the power supply is cheaper and smaller than a corona charger which needs to apply a voltage of up to about 10 kV.

Also, without disturbing the first color toner image,
In a non-contact jumping developing and developing device, which is a developing method suitable for developing a second color of a one-pass multicolor print image forming apparatus, when a carrier is mixed, developability is deteriorated and toner coating defects and A problem that causes a decrease in concentration occurs, but this can also be prevented.

Further, before the multi-color toner image on the image carrier is transferred collectively by installing a pre-transfer charging roller, the pre-transfer charging roller is brought into light contact with the image carrier, and The transfer efficiency could be improved by applying a charge to the multicolor toner image and uniformly charging the same polarity.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing one embodiment of an image forming apparatus of the present invention.

FIG. 2 is a schematic diagram showing a change in surface potential of the photosensitive drum in the image forming apparatus of FIG.

FIG. 3 is an explanatory diagram showing prevention of toner contamination of a charging roller as a recharger in the image forming apparatus of FIG. 1;

FIG. 4 is an explanatory diagram illustrating a relationship between a DC voltage applied to a charging roller and surface potentials of a first toner image portion and a non-image portion on a photosensitive drum.

FIG. 5 is an explanatory diagram showing a relationship between a DC voltage applied to a charging roller and a carrier collection rate by the charging roller.

FIG. 6 is a schematic configuration diagram showing another embodiment of the image forming apparatus of the present invention.

FIG. 7 is an explanatory diagram showing a DC voltage applied to a pre-transfer charging roller and a transfer efficiency of a toner image of each color according to still another embodiment of the present invention.

FIG. 8 is an explanatory diagram showing a mechanism of toner contamination of a recharger in a conventional image forming apparatus.

[Explanation of symbols]

 Reference Signs List 1 photosensitive drum 2 primary charging device 4 first developing device 5 charging roller for recharging 5c surface protection layer 7 second developing device 8 transfer charging device 20 first toner 21 cleaning blade 55 charging roller before transfer

Claims (8)

[Claims] A plurality of charging means, a plurality of exposing means, a plurality of developing means, and a transferring means, wherein at least the first of the developing means is a two-component developing means, Exposure and development are sequentially repeated to form a multicolor toner image, and the obtained multicolor toner image is collectively transferred onto a transfer material. The subsequent charging means comprises a contact-type elastic charging member that makes light contact with the image carrier, and applies a DC voltage to the elastic charging member. 2. The multicolor image forming apparatus according to claim 1, wherein the DC voltage has the same polarity as the toner of the toner image formed on the image carrier by development and is higher than the toner layer potential. 3. The elastic charging member is composed of a cylindrical roller, and its surface layer is formed of a resin having the same polarity as the toner, a resin composition obtained by mixing a lubricant with the resin, or a fluorine-based resin on the friction charging series. The image forming apparatus according to claim 1, wherein the image forming apparatus is formed of any one of a resin composition mixed with a resin. 4. The image forming apparatus according to claim 3, wherein the elastic charging member is driven to rotate with respect to the image carrier, and a carrier removing member is provided in contact with the elastic member. 5. An image forming apparatus comprising: a plurality of charging units, a plurality of exposing units, a plurality of developing units, and a transferring unit, wherein at least the first of the developing units is a two-component developing unit. Exposure and development are sequentially repeated to form a multicolor toner image, and the obtained multicolor toner image is collectively transferred onto a transfer material. A multicolor image forming apparatus, comprising: a contact-type elastic charging member that is in light contact with an image carrier on an upstream side; and applies a DC voltage to the elastic charging member. 6. The multicolor image forming apparatus according to claim 5, wherein the DC voltage has the same polarity as that of the toner of the toner image formed on the image carrier by development and is higher than the toner layer potential. 7. The elastic charging member is composed of a cylindrical roller, and its surface layer is formed of a resin having the same polarity as the toner, a resin composition in which a lubricant is mixed with the resin, or a fluorine-based resin in the friction charging series. The image forming apparatus according to claim 5, wherein the image forming apparatus is formed of any one of a resin composition mixed with a resin. 8. The image forming apparatus according to claim 7, wherein the elastic charging member is driven to rotate with respect to the image carrier, and a carrier removing member is provided in contact with the elastic member.