JPS6075846A - Electrophotographic method - Google Patents

Abstract

PURPOSE:To obtain a copied image having good reproducibility by performing plural times of electrifying, exposing, developing and transferring processes during one time of copying process and increasing or decreasing gradually exposure. CONSTITUTION:An electrophotographic copying machine has an exposing light source 4 which forms an electrostatic latent image by irradiating light to an original 3, a developing part 5 which develops the electrostatic latent image and an electrifying device 6 for transfer which performs corona discharge as a pretreatment for transfer to the developed toner image. Plural times of electrifying, exposing, developing and transferring processes are performed during one time of copying process and the exposure is gradually increased or decreased from the first time, by which the copy having good reproducibility is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an electrophotographic method including charging, exposure, development, transfer, and fixing processes, and in particular, to an electrophotographic method that provides excellent gradation reproducibility for original images with a wide contrast range. Regarding the electrophotographic method obtained.

Prior Art The brightness decay characteristics of conventional electrophotographic photosensitive materials (hereinafter simply referred to as photosensitive materials) are based on the relationship between the original image density (DIN) and the copy because the relationship between the original image density and the photosensitive material potential is not linear as shown in Figure 1. The relationship between the density (DOUT) also has a narrow dynamic range as shown in FIG. 2, and gradation defects occur in low and high density areas for original images with a wide contrast range such as photographs.

As a means for solving such gradation defects, for example, a halftone dot separation method is known in which halftone dot patterns are quickly aligned and exposed as an image.

However, even if this decomposition method is used, it is necessary to change the size of the halftone dots for each area in the original image with a wide contrast range, and in general, low density areas and intermediate density areas are reproduced well. However, in the high temperature range, it became brighter than the original image, and in solid black areas, except for the edges, the temperature turned white.

OBJECTS OF THE INVENTION It is an object of the present invention to provide an electrophotographic method that eliminates the above-mentioned drawbacks and can provide excellent continuous tone images even for original images with a wide contrast range.

The structure of the invention, that is, the object of the present invention is to perform charging, exposure, development, and transfer processes multiple times during one copying process, and to gradually increase or decrease the amount of exposure in the exposure process from the first time. This is achieved by an electrophotographic method characterized by:

Example An embodiment of the present invention will be described below with reference to the drawings.

The electrophotographic copying machine used in the present invention uses a transfer process.
A transfer roll is provided. This transfer roll rotates once while holding the recording paper on which the image has been transferred.

In the next transfer process, a toner image formed from the same document is accurately aligned with the previous toner image and transferred in an overlapping manner.

That is, to describe the electrophotographic copying machine and its general configuration with reference to FIG. 6, a corona charging device 2 is provided close to the surface of a drum-shaped photoreceptor 10 to uniformly charge the surface of the photoreceptor.

Furthermore, this electrophotographic copying machine includes an exposure light source 4 that irradiates the original 6 with light to form an electrostatic latent image, a developing section 5 that develops the electrostatic latent image, and a pre-transfer treatment for the developed toner image. a transfer charging device 6 that performs corona discharge as a transfer roller; the aforementioned transfer roll 7 that contacts the photoreceptor 1 and rotates in the opposite direction to the photoreceptor 1; and a cleaner device 8 that removes residual toner on the surface of the photoreceptor 1. It has become.

Further, a fixing roll 10 is also provided adjacent to the transfer roll 7 for fixing the transfer paper 9 onto which the image has been transferred under pressure.

The electrophotographic method of the present invention will be explained along the close-up process, taking as an example a case where the C transfer process is performed twice using a copying machine configured as described above. 1, an electrostatic latent image is formed by imagewise exposure using an exposure light source 4 whose exposure amount is increased by, for example, 60% (0.5 in terms of density) compared to conventional settings. Further, the electrostatic latent image is developed in a developing section 5 to form a toner image.

The charge of the toner particles of the toner image is neutralized by a transfer charging device 6, and the image is transferred onto a transfer paper 9, which is conveyed between the photoreceptor 1 and a transfer roll 7 at a temperature of .degree.

The transfer paper 9 on which the image has been transferred is fixed under pressure by the fixing roll 10, and then, in preparation for the second transfer, it is wound around the transfer roll 7, rotated by "C1", and returned to the transfer starting position where it is held.

First, after the residual toner on the surface of the photoreceptor 1 is removed by the cleaner device 8, the photoreceptor 1 is uniformly charged again and exposed to an image using the same original. In this second exposure process, the exposure amount is reduced to the conventional set value. The electrostatic latent image formed by this exposure is superimposed and transferred onto the transfer paper having the first transfer image through the same developing/transfer charging process as the previous one. At this time, accurate alignment with the first transferred image is performed. Similarly to the first time, the transfer paper 9 on which the image F has been transferred is passed through the fixing roll 10 and fixed, and then separated from the transfer roll 7 and carried out of the machine to complete the copying process.

The amount of toner transferred to the transfer paper through such two transfer processes is determined in relation to the density of the original image.

The first time with an increased exposure amount corresponds to the curve α shown in FIG. 4, and the second time with a normal exposure amount corresponds to the curve α. Further, the amounts of toner transferred in an overlapping manner in this two-direction transfer process are approximately added to form a curve C.

In other words, an image formed using only the normal exposure amount will have insufficient density in the high density area, resulting in poor reproducibility, whereas if the exposure amount is increased, the image in the low density area will disappear, but the image in the high density area will be The density area image is emphasized. Therefore, by changing the exposure amount and superimposing the images obtained at °C, as shown in Figure 5,
In a wide range of density areas of the original image, the relationship with the copy density becomes approximately proportional, the dynamic range is widened, and the single gradation property is improved.

In this embodiment, fixing is performed every time for each transfer process, but it is also possible to perform fixing once after all transfer processes are completed.

However, when configured in this way, exposing by gradually reducing the exposure amount as in this embodiment increases the amount of toner transferred to the transfer paper in stages, thereby preventing the occurrence of background stains, etc. That's good.

Further, although a pressure fixing method was used in this embodiment, other conventionally known heat fixing methods may also be considered. in this case.

When fixing is repeatedly performed for each transfer process, a fixing method that does not affect the transfer process may be considered.

Effects of the Invention As described above, the present invention is capable of reproducing the high-density region of an original image with a wide contrast range by increasing the exposure dose and exposing the image to the high-sensitivity region of the photosensitive material (original image density and photosensitive material potential). The image is formed at the part where the slope is large in the relationship between

In other words, the image is extracted from the high-density area of the original image, which has a wide contrast range, then the medium-density area, and then the low-density area, and then the exposure amount is changed stepwise for each extraction area, using methods such as halftone dot separation. (It is possible to obtain a copy image with good reproducibility in solid black and continuous gradation.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between original image density and sensitive material potential in the bright decay characteristics of electrophotographic photosensitive materials, Figure 2 is a diagram showing the relationship between original image density and copy density, and Figure 6 is a diagram showing the relationship between original image density and copy density in the bright decay characteristics of electrophotographic photosensitive materials. FIG. 4 is a diagram illustrating the schematic configuration of a photocopying machine, FIG.
The figure is a diagram illustrating the relationship between original image density and copy density to explain the present invention. 1... Photoreceptor 2... Corona charging device 6... Document 4... Exposure light source 5... Developing section 6... Transfer band device 7... Transfer roll 8... Cleaner device 9...transfer paper 10
...Fuser roll (6 other people) Figure 2 Figure 3

Claims (1)

[Claims] In electrophotography, which includes the processes of charging, exposing, developing, transferring, and fixing, the charging, exposing, developing, and transferring processes are performed multiple times during one copying process, and the amount of exposure in the exposure process is gradually decreased from the first time. An electrophotographic method characterized by an increase or decrease in