JP2614063B2 - Image forming device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus capable of forming a two-color or multi-color image, and particularly relates to an image carrier such as a laser beam printer or an LED printer. It is preferably used in a two-color or multi-color image forming apparatus in which a latent image is formed by exposing an image with a convergent light beam, and then a corresponding latent image is visualized by a developing device having a different developer. It is realized.

2. Description of the Related Art In recent years, for example, a recorded image becomes clearer and information is more easily understood.For example, a format color and a calculated value or a data value color are represented by different colors, CA
It is desired that a part of the drawing output by D is output in another color, for example, it is desired to be distinguished by two colors or multi-colors, and color toners other than black have been increasingly used. .

Conventionally, as an electrophotographic apparatus for obtaining a two-color image by one-time transfer, as shown in JP-B-56-87058, 87059 and 87060, the charging potential is divided into two and the first and second electrostatic A method of forming an image and developing each of them has been proposed. However, recently, an image exposure method has been used because it can provide a sufficient development contrast and is advantageous for fine line reproducibility and scanning line fog. A two-color electrophotographic apparatus having an electrostatic image forming / developing process of twice primary charging → image exposure → reversal development → recharging → image exposure → reversal development, or development using toners of opposite polarities A two-color electrophotographic apparatus having an electrostatic image forming / developing process of primary charging → image exposure → reverse development → background exposure → regular development, which is advantageous in terms of color mixing, has been proposed and developed. To have.

However, when two-color printing is performed by using a currently practiced two-color electrophotographic apparatus, the developing process basically includes toners having different developing characteristics (for example, a one-component magnetic toner and a two-component non-magnetic toner). ), And are carried out by different developing methods, it has been difficult to reproduce a halftone image density composed of line widths and dots in a well-balanced manner for both colors. That is, since the non-magnetic toner does not contain a magnetic material, it has a lower specific gravity and higher resistance than the magnetic toner. Therefore, the developing sleeve,
Alternatively, the ratio Q / M of the charge (Q) on the toner surface due to frictional charging with the carrier and the mass (M) of the toner is such that non-magnetic toner> magnetic toner. Therefore, when developing an electrostatic latent image having exactly the same potential, an image developed with a developer having a magnetic toner becomes thinner than an image developed with a developer having a non-magnetic toner. Further, even when the developing process is performed using, for example, a two-component non-magnetic toner or the like having substantially the same developing characteristics for both colors, even in an electrophotographic apparatus employing a two-color electrophotographic method by one-time transfer, In particular, in an electrophotographic apparatus using an image forming process having a recharging step, it is almost impossible to equalize development conditions such as development contrast and reverse contrast between two electrostatic latent images, so that fogging is difficult. It is extremely difficult to avoid color and color mixture and to reproduce the line widths and dots of the two colors with substantially the same thickness.
It was difficult or impossible to reproduce the colors in a well-balanced manner.

An object of the present invention is to make a laser beam diameter used for an exposure unit for forming a latent image on an image carrier different according to the development characteristics of each color, and to provide a halftone by a line width and a set of dots of each color. An object of the present invention is to provide an image forming apparatus capable of improving the reproducibility of the image and obtaining an image of good quality.

Means for Solving the Problems The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides a photoconductor, a scanning unit that scans the photoconductor with laser light according to image data, and a first developing unit that develops a latent image on the photoconductor with a developer having a non-magnetic toner. A second developing means for developing with a developer having a magnetic toner, wherein the laser beam spot diameter at the time of forming a latent image for development by the second developing means is determined by the first developing means. An image forming apparatus characterized in that it is made larger than at the time of forming a latent image for development.

Next, an image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

FIG. 1 shows a main part of a two-color electrophotographic printer showing one embodiment of the present invention. In this embodiment, the image carrier 1 which is a drum-shaped electrophotographic photosensitive member rotates in the direction of the arrow.
Around the photosensitive drum 1, an image forming unit, that is, a primary charger 2 for uniformly charging the photosensitive drum 1, a first image exposure unit 3 for forming a first latent image,
Non-magnetic toner for developing the first latent image (two-component developer)
The first developing device 4 containing the carrier, the carrier repair roller 5,
Recharger 6, second image exposure means 7 for forming a second latent image, second developing device 8 containing magnetic toner (one-component developer) for visualizing the second latent image , A transfer charger 9 for transferring the first and second images to a transfer material P such as paper, a separation charger 10 for separating the transfer material P from the photosensitive drum 1, and removing residual toner on the photosensitive drum 1. And a pre-exposure lamp 12 for neutralizing the residual charge on the photosensitive drum 1. Since the configuration and operation of each image forming unit and the image forming process are well known to those skilled in the art, further detailed description will be omitted.

FIG. 3 shows the dark portion potential (Vd1) after the primary charging by the primary charger 2, the bright portion potential (Vl1) at the time of the first image formation by the first image exposing means, and the toner potential as the bright portion potential. (A in FIG. 3) of the developing bias of the first developing unit 4 for developing the image (FIG. 3A), the dark portion potential (Vd2) after recharging by the recharging unit 6, and the recharging. The potential (Vl1 ') after the recharging of the first image portion, the bright portion potential (Vl2) at the time of forming the second image by the second image exposure means, and the toner for developing the toner to the bright portion potential. The relationship with the DC component (Vdc2) of the developing bias of the second developing device 8 (FIG. 3B) is shown.

The first and second exposure means 3 and 7 have the same configuration, and have semiconductor laser units 13 and 14, polygon mirrors 15 and 16 for scanning laser light from the semiconductor laser units 13 and 14, and f-θ lens 17 , 18 and folding mirrors 19 and 20 for directing the laser beam onto the photosensitive drum 1.

The semiconductor laser units 13 and 14 are driven by a host device 21 such as a computer, an interface circuit 22, a DC controller 23, and a laser driver 24, which are well known to those skilled in the art.

FIGS. 2A, 2B and 2C are diagrams showing the light intensity distribution on the photosensitive drum surface.

Next, the above configuration is adopted, and the image forming process speed is 80 mm
Examples will be described more specifically when two-color printing is performed using a two-color electrophotographic printer having a resolution of 11.8 pel / mm and a resolution of 11.8 pel / mm.

Embodiment 1 In this embodiment, the photosensitive drum 1 serving as an image carrier is
An electrostatic latent image of a red image in this embodiment was image-exposed on the photosensitive drum 1 by the first image exposure unit 3, which was uniformly charged to −600 V by the secondary charger 2. The light intensity distribution of the laser beam from the first image exposure means 3 at the stationary spot on the drum surface has the shape shown in FIG.

The first electrostatic latent image was inverted by a magnetic brush development using a two-component developer in a first red developing device 4, and the exposed portion was visualized to form a toner image. The developing sleeve 4a of the red developing device 4 has a frequency of 1800H as a developing bias.
z, Vdc = -450V (-400 to -500V)
(Variable up to V) was applied. During this development, the carrier leaked onto the drum was collected by the carrier collection roller 5.

Next, the photosensitive drum 1 was again subjected to a uniform corona discharge by the recharger 6, and in the portion without the toner image, -730V (Vd2 in FIG. 3) due to saturation of the charging potential, and the portion with the red toner image At -700 V (Vl1 ').

Next, the photosensitive drum 1 was image-exposed to the electrostatic latent image of a black image in this embodiment by a laser beam by the second exposure unit 7. The second electrostatic latent image formed by the exposure was inverted by jumping development using a one-component magnetic toner in a black developing device 8, and the exposed portion was visualized to be a toner image. At this time, a frequency of 1800 Hz, Vp
A superposed DC component of Vdc = -550V (variable from -500 to -600V) was applied to an AC electric field of p1100V.

The two-color image formed on the photosensitive drum 1 as described above is transferred by a transfer charger 9 onto a transfer material P which is made into paper in this embodiment, and then transferred to a separation charger The static electricity is removed and separated by 10, and the image is fixed by a fixing device (not shown).
A color print was obtained.

In this embodiment, the bright portion potential at the time of forming the first latent image is-
90 V, and the bright portion potential at the time of forming the second latent image is -120 V. Therefore, the development contrast in the standard state is 360 V (−450 V−90 V) in color development (red in this case) and 430 V (= 550 V−120 V) in black development. By changing the development contrast, the line width and density can be changed, and in this embodiment, both red and black are variable within a range of ± 50 V from the standard value. It is determined by the level of reversal fog and the allowable level of color mixing that occurs during the second development. In other words, if the developing contrast is too large, the DC component of the developing bias approaches the dark part potential, so that toner adheres to the dark part.
So-called fogging occurs, and if it is too small,
Inverted toner (toner in developer is not 100% charged to the same polarity, there is a small amount of inverted toner of opposite polarity) adheres to dark areas, and fogging (inverted fog) also occurs . Therefore, the DC component of the developing bias must be sufficiently separated from the bright portion potential and the dark portion potential so that toner fogging does not occur. The variable range of the development contrast necessarily falls within the range shown in FIG.
0V ~ 410V, black is 360V ~ 460V).

FIG. 4 shows the data of the line width (halftone line printed with 2 dots and separated by 5 spaces) with respect to the development contrast using the apparatus of this embodiment. At this time,
Regarding the black line, two types of lasers (laser A and laser B in FIG. 2 (a) and FIG. 2 (b)) having different light intensity distributions, that is, spot diameters, on the photoreceptor surface are used to form a latent image. Two data were taken.

As can be understood from FIG. 4, when exposure is performed using a laser having the same spot diameter, the black line becomes thinner at the same development contrast (for example, at a development contrast of 410 V) (in contrast to the case where red is approximately 230 μm, Black is about 200
μm). Also, when compared at the center (standard state) of each of the development contrast variable areas for both red and black, the black line is still thinner (red: about 210 μm, black: about 200 μm). This is because the non-magnetic toner (red developer) does not contain a magnetic substance, has higher specific gravity and higher resistance than the magnetic toner (black developer), and has a charge (Q) on the toner surface and a mass of the toner ( This is because the ratio Q / M to M) is such that non-magnetic toner> magnetic toner. Therefore, when the spot diameter of the laser A is the same for both red and black, the change ranges of the line width with respect to the development contrast variable range are ΔWr (red) and ΔWb1 (black), respectively, and the overlapping range is small. Therefore, the area where the same line width can be obtained for both red and black is small, and the line width (density)
This makes the device difficult to use with a small adjustment range. Therefore, by changing the laser for forming a black latent image from A to B having a larger spot diameter, the line width is increased, and the black line width variable area (ΔWb2) includes the entire red line width variable area (ΔWr). , The overlapping range becomes large. In addition, the line widths at the time of standard setting (Vdc1 = −450 V, Vdc2 = 550 V) are almost the same.

FIG. 5 shows a pattern in which the laser is alternately turned on at 1 dot ON and 1 dot OFF.
6 is a graph showing the development contrast vs. density of an image when an image is formed with a laser A for a red image.

As is apparent from FIG. 5, when the laser A is used for both red and black, the change in the reflection density of red and black is non-linear, whereas when red is A and black is B, the reflection of red and black is It can be seen that the change in density is almost linear, and the density adjustment is easier when black is changed to B.

In each of the above embodiments, the present invention has been described as being embodied as a two-color electrophotographic apparatus. However, the present invention is not limited to this, and may be similarly applied to a multicolor image forming apparatus having three or more colors.

Effect of the Invention As described above, the image forming apparatus according to the present invention has
The laser beam spot diameter at the time of forming a latent image for developing with the second developing means having magnetic toner is made larger than that at the time of forming a latent image for developing with the first developing means having non-magnetic toner, and toner characteristics are improved. Despite the difference, the developing characteristics of the first developing means and the second developing means can be made substantially equal. That is, according to the present invention, the line width and the dot diameter of each color on the output image are made closer to each other by making the exposure spot diameter of the exposure unit for forming an electrostatic latent image different for each color. Therefore, it has the advantage that the reproducibility of halftone by the line width of each color and the set of dots is improved, and an image of good quality can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing only a main part of a two-color electrophotographic apparatus which is an embodiment of the image forming apparatus according to the present invention. FIG. 2 is a diagram showing a light intensity distribution of a laser for performing image exposure on an image carrier. FIG. 3 is a diagram showing a relationship among a dark portion potential, a bright portion potential, and a developing bias image component on the image carrier when performing two-color printing. FIGS. 4 and 5 show the development contrast vs. line width and the development contrast vs. 1d, respectively, in the embodiment of the present invention.
It is a figure which shows the curve of ot, 1space pattern density. 1: Image carrier 2: Primary charger 3, 7: Image exposure means 4, 8: Developer 6: Recharger

Claims (1)

(57) [Claims] A first developing means for developing a latent image on the photosensitive body with a developer containing a non-magnetic toner; a scanning means for scanning the photosensitive body with a laser beam corresponding to image data; A second developing unit for developing with a developer having a toner, wherein the laser beam spot diameter at the time of forming a latent image for development by the second developing unit is developed by the first developing unit. An image forming apparatus which is larger than a latent image for forming a latent image.