JPH03210577A - Image forming device - Google Patents

Abstract

PURPOSE:To improve the quality of an image without disturbing a toner image by impressing auxiliary bias with which electrostatic field in a direction where the toner image is electrostatically attracted on an auxiliary magnet member is weakened. CONSTITUTION:This device is an image forming device in which two component type developer consisting of toner and magnetic carrier whose density is <=4.0g/cm<2> is used in a developing means at least at second and succeeding stages, and auxiliary bias Vb is impressed on the auxiliary magnet member 6 installed at the back part of the developing nip part of the developing means 3b at the second and succeeding stages in order to weaken the electrostatic field in the direction where the toner image already formed by the developing means 3a at a preceding stage is electrostatically attracted. Thus, the deterioration of the image quality based on carry-over phenomenon is evaded and the excellent image is obtained without disturbing the toner image already formed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an image forming apparatus such as a copying machine or a printer that forms a plurality of toner images using electrostatic latent images.
The present invention relates to an improvement in an image forming apparatus that maintains good image quality without disturbing toner images already formed on a latent image carrier.

[Prior Art] Conventionally, various image forming apparatuses of this type using electrophotography have been provided.

In such an image forming apparatus, for example, monochrome units consisting of a latent image contracting means, a latent image bearing member, a developing means, and a transfer means are prepared for n colors, and a transfer sheet moves through each unit to form a single color. A so-called sequential transfer method is known in which images are sequentially transferred to finally obtain a color image.

In this type of apparatus, it is possible to make the image forming speed equivalent to that of a monochromatic image, but there is a problem in that the apparatus becomes extremely large. On the other hand, from the viewpoint of downsizing the device, a so-called overlap transfer method is applied in which latent image formation, development, and transfer are repeated for n colors using - latent image carriers and a transfer drum pressed against the latent image carriers. It is known that
The problem is that the image forming speed is reduced to 1/n.

As an image forming HAM that solves these problems, there is one that applies the overlapping development method (Japanese Patent Laid-Open No. 55-7306
(See Publication No. 3). This is an example of a two-color image forming apparatus, which forms a plurality of binary or three-valued electrostatic latent images on one latent image carrier, and has these two-valued or three-valued latent images. After the first latent image on the latent image carrier is developed by the first developing means, the second latent image on the latent image carrier is developed by the second developing means, and the finally formed toner image is A color image is obtained by simultaneously transferring the images to a transfer sheet, and this is effective because the apparatus is compact and the image forming speed is extremely high.

[Problems to be Solved by the Invention] However, in the second and subsequent development steps, in the case where such an overlapping development method is used, the toner image that has already been carried on the latent image carrier in the previous development step is removed. Since the toner image comes into sliding contact again with the developer of the second and subsequent developing means, the toner image formed in the previous developing step becomes significantly disturbed by the second and subsequent developing steps. Therefore, in an image forming apparatus using the overlapping development method, how to carry out subsequent development without disturbing the toner image in the previous stage has become an extremely important issue.

One possible way to solve these problems is to perform the subsequent development process using a one-component non-contact development method, but the -component non-contact development method suffers from poor image quality, especially gradation reproducibility. From this point of view, a developing method using a two-component developer consisting of a carrier and a toner, usually a magnetic brush developing method, must be used.

Against this background, the inventors of the present application have made extensive studies using a magnetic brush developing means using a two-component developer, and have found that the density of the carrier used in the two-component developer is We discovered that this is an important factor for disturbance, and developed a two-component developer used in the second and subsequent stages of development, which is a mixture of toner and a magnetic carrier with a density of 4.09/x3jX or lower. It has already been proposed that this is preferable (Japanese Patent Application No. 61-2727290).

In other words, if a two-component developer as described above is used, the magnetic carrier has a low density, so the formed magnetic brush itself will be soft.
The degree to which the magnetic brush disturbs the already formed toner image is reduced.

However, in the second and subsequent development steps, since the density of the magnetic carrier is low, it is inevitably impossible to ensure a sufficiently large binding force of the developing roll on the magnetic carrier in the developing means, and the magnetic carrier is The so-called carryover phenomenon, in which the carrier breaks the restraint of the developing roll and transfers to the latent image carrier side, is likely to occur, and the presence of the transferred carrier may cause white spots in the transferred image.
This causes a problem that the image quality is impaired, such as the edges of the character image becoming rough.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus that can effectively avoid the above-mentioned carryover phenomenon without disturbing the already formed toner image. It is.

[Means for Solving the Problems] That is, as shown in FIG. A contracting means 2 and a plurality of developing means 3 (for example, 3a, 3b) for making the formed latent image visible with toner.
), and a transfer means 5 that simultaneously transfers a plurality of toner images visualized by each developing means 3 onto a transfer sheet 4, and at least in the second and subsequent developing means 3b, the density of the toner is 4. Assuming that the image forming apparatus uses a two-component developer consisting of a magnetic carrier of OgZ13 or less, an auxiliary magnet member 6 is attached to the rear portion of the developing nip portion of at least the second-stage developing means 3b, and this auxiliary magnet An auxiliary bias vb is applied to the member 6 in order to weaken the electrostatic field in the direction of electrostatically attracting the toner image already formed by the preceding developing means 3a.

In such technical means, the latent image carrier 1 may be appropriately selected from a photoreceptor, dielectric, etc. as long as it can form a latent image by the latent image contracting means 2, and its specific configuration may be used as the latent image carrier 1. It does not matter whether it is drum-shaped or bell-shaped.

Also, regarding the latent image contracting means 2, if a latent image consisting of a potential of a predetermined level can be formed on the latent image carrier 1, the surface of the latent image carrier 1 is charged in advance and corresponds to the image. It is possible to appropriately select a method in which static electricity is removed from the exposed area or non-image area using light or ions.

Furthermore, regarding the plurality of developing means 3, as long as they are capable of developing corresponding to each latent image, the developer used (including the same color), the developing method, etc. may be selected as appropriate; Regarding the developing means 3b, from the viewpoint of maintaining good developing performance and effectively preventing the already formed toner image from being scraped off, at least the developer used is toner and a magnetic carrier having a density of 4 g/13 or less. It is necessary to use it. In this case, the magnetic carrier may be appropriately selected from a porous carrier, a ferrite carrier, a carrier in which magnetic powder is dispersed in a resin binder, etc.; however, a carrier in which magnetic powder is dispersed in a resin binder is , is preferable in that the density can be easily adjusted by the magnetic powder-containing layer. In addition, the developing means 3 of the second and subsequent stages
In b, from the viewpoint of more reliably preventing the toner image that has already been formed from being scraped off, the relative rotational speed of the developing roll with respect to the latent image carrier may be lowered to the extent that developing performance is not impaired, or the developing nip area may be reduced. It is preferable to make appropriate design changes such as arranging a repelling magnetic pole in a portion of the developing roll facing the developing roll.

Furthermore, the transfer means 5 may be appropriately selected, such as one utilizing an electrostatic transfer method or a thermal transfer method.

Further, regarding the auxiliary magnet member 6, the magnetic force between the auxiliary magnet member 6 and the latent image carrier 1 is adjusted so that a magnetic field sufficient to attract the magnetic carriers transferred to the latent image carrier 1 side due to the carryover phenomenon is obtained. It is necessary to adjust the gap between

Further, as the auxiliary bias vb to the auxiliary magnet member 6,
It is necessary that the electrostatic field between the already formed toner image and the auxiliary magnet member 6 is weakened to such an extent that the already formed toner image does not move toward the auxiliary magnet member 6 side due to electrostatic attraction force. . The auxiliary magnet member 6 may be provided separately from the developing means 3, but the auxiliary magnet member 6 may be incorporated into the housing of the developing means 3 and designed to efficiently recover the carried-over magnetic carrier. It is preferable to do δ1.

[Operation] According to the technical means described above, in the second and subsequent development steps, the corresponding latent image on the latent image carrier 1 is developed with a predetermined color toner.

At this time, in the second and subsequent development steps, a two-component developer consisting of toner and a magnetic carrier with a density of 49/x3 or less is used, so in the second and subsequent development steps, the previous development step is used. The sliding force between the already formed toner image and the developer can be kept small.

On the other hand, in the second and subsequent development steps, as shown in Figure 1(b)
As shown in , since the magnetic carrier C used has a low density and the restraining force of the magnetic carrier C by the developing roll 3R is weak, a carryover phenomenon occurs in which the magnetic carrier C is transferred to the latent image carrier 1 side. The transferred magnetic carrier C is magnetically attracted from the latent image carrier 1 to the auxiliary magnet member 6 side. In this case, since a predetermined auxiliary bias vb is applied to the auxiliary magnet member 6, the existing toner 11
The electrostatic field S in the direction of electrostatically attracting T toward the auxiliary magnet member 6 is weakened, and the existing toner image is not electrostatically attracted to the auxiliary magnet member 6.

[Embodiments] Hereinafter, the present invention will be described in detail based on embodiments shown in the accompanying drawings.

FIG. 2 is an explanatory diagram showing an embodiment in which the present invention is applied to a two-color color copying machine.

In the figure, 11 is a photosensitive drum as a latent image carrier;
12 is a charging corotron that charges the photosensitive drum 11 in advance;
13 is an LED array that forms a first latent image; 14 is an exposure lamp 14a that irradiates the document 16 on the platen 15; and a plurality of mirrors that guide reflected light from the document 16 to a predetermined portion of the photosensitive drum 11. Group 14b and photosensitive drum 11
A second latent image forming optical imaging system 17 includes an imaging lens 14c that forms a light image from the original 16 onto a predetermined portion of the document 16; Developing device 18 is a magnetic brush type second developing device using positively charged black toner, 19 is a second 1i! A pick-up meal as an auxiliary magnetic member disposed at the rear of the developing nip area of the imager 18; 20 a pre-transfer corotron that uniformizes the charging characteristics of the toner image on the photosensitive drum 11 before the transfer process; 21 a copying machine; Photosensitive drum 1 on sheet 22
1, a transfer corotron for transferring each color toner image on the photosensitive drum 11; 23, a stripping static eliminating corotron for peeling off the copy sheet 22 from the photosensitive drum 11 after the transfer process; 24, a static eliminating corotron for removing residual charges on the photosensitive drum 11 before the cleaning process , 25 is a cleaner for removing residual toner on the photosensitive drum 11, and 26 is a cleaner for removing residual toner on the photosensitive drum 11 before the next copying cycle.
27 is a sheet feeding tray that stores the copy sheet 22; 28 is a fixing device that fixes the toner edge on the copy sheet 22 after the transfer process; 29
30 is a discharge tray that discharges and stores the copied copy sheet 22 after the fixing process, and 30 is a discharge tray 30 that feeds the copy sheet 22 in the sheet feeding tray 27 to a transfer site at a predetermined timing, passes through the fixing device 28, and transfers it to the discharge tray 29. This is a sheet conveyance system that conveys sheets.

In this embodiment, the photosensitive drum 11 has a negatively charged organic photosensitive layer, and the process speed is 160 layers/s.
ec was used.

Further, as shown in FIG. 3, the first developing device 17 includes a rotating sleeve 32 formed of a non-magnetic material within the housing 31.
a, a developing roll 32 consisting of a magnet roll 32b fixedly disposed within the rotating sleeve 32a, an agitator 33 for stirring the developer, a conveyance paddle 34 for supplying the agitated developer to the developing roll 32 side, and a developer. mouth 1 le 32
A trimming parr 35 for regulating the trimming gap of the developer supplied around the developer and a mixing plate 36 for returning the developer scraped off by the trimming parr 35 to the agitator 33 are housed.

The parameters of the first developing device 17 are as follows:
Rotating sleeve 32a diameter is 50am, magnet roll 3
The magnetic pole of 2b is a 6-pole asymmetric type, and the magnetic force of the main pole is 65
0 Gauss, the gap between the photosensitive drum 11 and the developing roll 32 is 0.8 m++, the trimming gap is 0.6 m++, the magnet set angle indicating the deviation of the set position of the main pole with respect to the developing nip area is +5°, and the rotating sleeve 32
The speed of a is 400 am/Sec, and the developing bias v1 is -
A voltage of 450 (V) is used.

And again, this first development! As the developer of 117,
A carrier with an average particle diameter of 80 μm obtained by melt-mixing 30 parts by weight of styrene-n-butyl methacrylate copolymer and 70 parts by weight of cubic magnetite, styrene-〇-
Average particles l obtained by melt-mixing wire crushing and classification of 90 parts by weight of butyl methacrylate copolymer and 10 parts by weight of the red pigment Lysol Scalt (manufactured by BASF). A toner made of 12 μm is used.

Furthermore, the second developing device 18 has the same configuration as the first developing device 17, and as shown in FIG. Agitator 43 for stirring the agent
, a transport paddle 44 that supplies the agitated developer to the developing roll 42 side, a trimming parr 45 for regulating the trimming gap of the developer supplied around the developing roll 42, and a doming bar 45 that removes the developer scraped off again. A mixing plate 46 that returns to the agitator 43 side and a toner dispensing auger 47 that replenishes black toner are housed in the housing 41.

Each parameter of the second developing device 18 is as follows:
Rotating sleeve 42a diameter is 50mm, magnet roll 42
The magnetic pole b is a 6-integration symmetric repulsion type, the magnetic force of the main repulsion pole is 1250 Gauss, the dark gap between the photosensitive drum 11 and the developing roll 42 is 0.8, and the trimming gap is 0.6.
The magnet set angle indicating the deviation of the main pole set position with respect to the developing nip area is oo, and the rotating sleeve 42
The speed of a is 160jIII/5ec1 developing bias v1
A voltage set at -250 (V) is used.

Further, as the developer of this second developing device 18, ferrite carrier styrene-n-n-
Butyl methacrylate copolymer 90 parts and carbon 1 part
A toner with an average particle diameter of 10 μm obtained by classifying 9 parts of 99 parts of a charge control agent and 1 part of a charge control agent using a YGM kneading, kneading, and pulverizing machine is used.

Further, as shown in FIG. 3, the pickup roll 19 is housed in the housing 41 of the second developing device 18, and a bipolar symmetrical fixed magnet 51 is disposed inside the rotating sleeve 52. A predetermined auxiliary bias vb is applied to the rotating sleeve 52.

The parameters of the pickup roll 19 are as follows: the diameter of the rotating sleeve 52 is 20 mm, the main pole of the fixed magnet 51 is 600 Gauss, and the rotational speed of the rotating sleeve 52 is 16 aw/sec. Incidentally, reference numeral 53 is a scraper for peeling off the carrier collected by the pickup roll 19 from the pickup roll 19.

Next, the operation of the two-color copying machine according to this embodiment will be explained.

First, the surface of the photosensitive drum 11 is uniformly charged to about -600 (V) by the charging corotron 12 [Fig.
)], then the LED array 13 irradiates light according to the image information to form a first negative latent image Z1 (image working part potential of about -50 (V)) on the photosensitive drum 11 [ FIG. 4(b)]. After this, set the appropriate developing bias Vl.
(-450n in this embodiment) is applied to the developing roll 32 of the first developing device 17, and the first negative latent image Z
1 with negative polarity red toner to form a first toner image TI.
(approximately -100 (V) in this example) [
Figure 4(c).

Subsequently, the optical imaging system 14 forms a second positive latent image Z2 (
In this example, an image part potential of -550 (V) and a non-image part potential of -150 (V) were formed [FIG. 4(d)] I,
Appropriate developing bias v2 (-250 in this example)
V)) is applied to the developing roll 42 of the second developer 118, the second positive latent image z2 is developed with a positive black toner to form a second toner image T2 [FIG. ) ]
.

After this, each toner image TI on the photosensitive drum 11.

After aligning the polarity of T2 with the pre-transfer corotron 20, each toner image TI is transferred onto the copy sheet 22 by the transfer corotron 21.
, T2 are transferred and subjected to a predetermined fixing process to form a copy sheet 22.
The toner images 1 and T2 are fixed on the toner images 1 and T2.

In such an operation process, a carrier collection process by the pickup roll 19 is performed as a process after the second developing process.

Now, the dark gap δ between the pickup roll 19 and the photosensitive drum 11 shown in FIG. 3 is set to 0.2.0°4.0.6 (
) and set the above auxiliary bias Vt1200.100.
．． When the amount of carrier adhesion on the copied image and the amount of transfer of the first toner image to the pickup roll 19 were investigated by changing the voltage to O, -100, and -200 (V), the results shown in the table below were obtained. Obtained. In addition, in the same table, the number 1 of the carrier on the above-mentioned duplicated image! Regarding the transfer of the first toner image to the pickup roll 19, the grades are numbered from 1 to 5 in descending order of the good adhesion amount.
, Slight metastasis is indicated by △, and no metastasis is indicated by ○.

Table According to this table, when the gap δ is 0.2, the auxiliary bias v
b is required to be about -200m, and the gap δ is 0
．． It was understood that in the case of 4 am, the auxiliary bias vb is required to be about -100 m, and that when the gap δ is 0.6 jw, it is almost impossible to select the auxiliary bias vb appropriately.

Therefore, according to the same table, under the condition that the magnetic field of the pickup roll 19 and the electrostatic field due to the auxiliary bias vb act between the first toner image and the pickup roll 19, specifically, the first toner image and the pickup roll It is understood that the auxiliary bias vb needs to be set so as to weaken the electrostatic field that electrostatically attracts the first toner image under conditions in which the gap δ between the first toner image and the first toner image does not become extremely large. That is, when the gap δ is extremely small, the magnetic attraction force of the pickup roll 19 becomes large, so that the auxiliary bias vb cannot form an electrostatic field in which the electrostatic attraction force of the first toner image acts negatively. If the gap δ is large to some extent, the magnetic attraction force of the pickup roll 19 itself will not be large enough to attract the first toner. What is necessary is to form the electrostatic field between the two to approximately O.

[Effects of the Invention 1] As explained above, according to the image forming apparatus according to claims A to C, the sliding between the existing toner image and the magnetic brush forming developer is prevented in the second and subsequent development steps. The low-density magnetic carrier transferred to the latent image carrier is magnetically attracted to the auxiliary magnet member while keeping the contact force small and weakening the electrostatic field between it and the existing toner image. Image quality deterioration due to carryover phenomenon can be avoided and a good image can be obtained without disturbing the image.

Also, claim 2 &! According to the above-mentioned image forming apparatus, the magnetic carrier magnetically attracted to the auxiliary magnet member can be collected into the developing means, so that it is possible to prevent the magnetic carrier from being wasted.

Furthermore, according to the image forming apparatus of claim 3, each latent image is formed separately as a positive latent image and a negative latent image, and each developing means converts the corresponding latent image into a positive or negative latent image using toner of different polarity. Since negative development is performed, it is possible to effectively avoid the situation where the existing first toner image and the second toner image are mixed together in the second development process, and accordingly,
The image quality of the two systems can be further improved.

[Brief explanation of drawings]

FIG. 1(a) is an explanatory diagram showing a schematic configuration of an image forming apparatus according to the present invention, FIG. 1(b) is an explanatory diagram of its operation, and FIG. 2 is an illustration of a two-color color copying machine to which the present invention is applied. FIG. 3 is a detailed view of the main part thereof, and FIGS. 4(a) to 4(e) are explanatory views showing the image forming process of the two-color color copying machine according to the example. [Explanation of symbols] vb...Auxiliary biast...Latent image carrier 2...Latent image contracting means 3 (3a, 3b)...Developing means 4...Transfer sheet 5...Transfer means 6... ...Auxiliary magnet member 11...Photosensitive drum (latent image carrier) 12...Charging corotron (latent image contracting means) 13...
LED array (latent image contracting means) 14... optical imaging system (
latent image contracting means) 17... first developing device (developing means) 18... second developing device (developing means) 19... pick-up roll (auxiliary magnet member) 21.
...Transfer corotron (transfer means) 22...Copy sheet (transfer sheet)

Claims (1)

[Claims] 1) A latent image bearing member (1), a latent image contracting means (2) for forming a plurality of images on the latent image bearing member (1), and a latent image contracting means (2) for forming a plurality of images on the latent image bearing member (1); Multiple developing means (3; 3
a, 3b) and a transfer means (5) that simultaneously transfers a plurality of toner images developed by each developing means (3; 3a, 3b) onto a transfer sheet (4), at least from the second stage onwards. In the developing means (3; 3b), the toner and density are 4.0 g.
In an image forming apparatus using a two-component developer consisting of a magnetic carrier of /cm^3 or less, a latent image carrier ( An auxiliary magnet member (6) is provided which is spaced apart from the auxiliary magnet member (6).
), wherein an auxiliary bias (Vb) is applied to weaken an electrostatic field in a direction that electrostatically attracts a toner image already formed. 2) In the product according to claim 1, the auxiliary magnet member (6)
An image forming apparatus characterized in that: is incorporated into a housing of a corresponding developing means (3; 3b). 3) In the product according to claim 1, which forms two systems of images, one of the two images is a positive latent image and the other is a negative latent image, and the two developing means (3; 3a, 3b) are different from each other. An image forming apparatus characterized in that a corresponding image is developed in a positive or negative manner using polar toner.