Method for electrophotographic image formation

Abstract: Disclosed is a method of electrophotographic image formation according to which images of high quality can be reproduced with always stable density. This method which includes the step of image formation which comprises forming a static latent image on a photoreceptor by carry­ ing out charging and exposing and then developing is characterized in that maximum image density is maintained constant by measuring surface potential of maximum image density portion just before development and setting developing bias potential based on the above measured surface potential so that difference between the surface potential and developing bias potential becomes constant.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of electrophotographic image formation and in particular to a method of electrophotographic image formation according to which images of high quality having stable reproducibility of a desirable density can be obtained. Hitherto, there have been well known a so-called CPC system which comprises carrying out charging and exposing to form a static latent image on a photoreceptor and then developing the latent image to directly form a toner image on the photoreceptor, and a so-called PPC system by which the toner image formed on a photoreceptor by development is transferred onto a plain paper. Furthermore, in the field of printing in which a half-tone film is made by color separation of an original and this film is used as a printing plate, or information obtained by color separation of an original is converted to digital signal, which film is directly used as a printing plate, usually various quality tests (control) are conducted using a proof print which approximates to print before printing; it has been studied to utilize rapid and inexpensive electrophotographic process for this proof. In such electrophotographic process, since the electric charge of static latent image subtlely changes depending on environmental conditions such as temperature or humidity, color reproducibility is influenced by the environmental conditions even under the same setting conditions; therefore adjustment by skilled workers in each necessary occasion and under complete air conditioning is required, or adjustment must be made by complicated control device. Thus, operation, device or apparatus become complicated. Moreover, in the field of printing, the print must be the same as or of approximate appearance as the commercial articles rather than be faithful to the original image, and there is the tendency to attach importance to artistic property, and severer accuracy is demanded also in balance of density. Therefore, in this field, in order to make the proof print more close to the print, various efforts have been made in an attempt to satisfy the severe conditions. For example, a method of control of quality called ink proofing has been employed for a long time, and thus time has been spent or expensive materials or apparatuses have been used. For obtaining images in good balance of image density without fogging or insufficient density by an electrophotographic process, a method has been known as disclosed in Japanese Patent Kokai No. 63-149659. The method disclosed in Japanese Patent Kokai No. 63-149659 is a method for formation of color images by subjecting a photoreceptor to a step including charging, exposing the photoreceptor to form an electrostatic latent image and development which is repeated a plurality of times, whereby the charging conditions are set for every said step so that the difference between the surface potential of the photoreceptor and the developing bias potential at respective development positions of respective developing devices used for a step becomes substantially constant. However, since according to the above method, the charging conditions are set and controlled for every step comprising charging, forming of an electrostatic image and developing so that the difference between the surface potential of the photoreceptor and the bias potential becomes constant, the desired control cannot be easily performed owing to the influence of environmental conditions in the period between charging and developing, or the charging conditions must be set and controlled with also sensing the change of environmental conditions and considering the change of the surface potential of the photoreceptor caused by a change of environmental conditions. Thus, because of the complication of the operability and the apparatus, there are still many points to be improved. Furthermore, since according to the above method, the charging conditions are set and controlled for every step, this method cannot be applied to a monochromatic system. Besides, since the charging conditions are controlled, it is not easy to set maximum image density for every color. Under the circumstances, there is a strong demand for a method which utilizes an electrophotographic process and is rapid, inexpensive and simple. As a result of research on electrophotographic process conducted by the inventors for a long time, it has been found that the maximum image density can be maintained constant by measuring the surface potential of a maximum image density portion just before developing and setting the developing bias potential based on the measured surface potential so that the difference between the surface potential and the set developing bias potential is constant. Thus, the present invention has been accomplished. US-A-4 390 265 discloses a method wherein a latent image potential is measured before the development step and by utilizing the thus obtained output, a developing bias potential is applied to a developing tray so that a potential difference between a light-sensitive drum and the developing tray is constant. SUMMARY OF THE INVENTION The present invention provides the following method for electrophotographic image formation. (1) A method for electrophotographic image formation which comprises the steps of forming an electrostatic latent image on a photoreceptor by charging and exposing and then developing, characterized by maintaining maximum image density constant by measuring the surface potential of a maximum image density portion just before development and setting the developing bias potential based on the above measured surface potential so that the difference between the surface potential and the set developing bias potential becomes constant. According to the method of the present invention, (1) reproducibility of stable color density can be obtained and (2) rapid and economically advantageous control of quality can be carried out by applying to simple color proofing in color printing. That is, irrespective of monocolor image or multicolor image, an image of stable color density can be rapidly and inexpensively obtained by a simple method which is also satisfactory for images in the field of printing, which require severe accuracy. DESCRIPTION OF THE INVENTION The maximum density image portion in the present invention is that electrostatic latent image formed portion on a photoreceptor which corresponds to an area which shows maximum density for each color of toner of cyan, magenta, yellow, and black used in the subtractive color process or an area called solid image density in the field of printing. This area is one which is usually required to have the density of the following range measured by a color densitometer, though it may vary depending on kind of ink, toner and printing machine: Cyan; 1.60 ± 0.05 Magenta; 1.45 ± 0.05 Yellow; 1.00 ± 0.05 Black; 2.00 ± 0.05 The area in the original which corresponds to the maximum image density portion on photoreceptor may be provided at one end of the original outside the image area in an usual reflecting type electrophotographic process, and in case of half-tone film and digital image signals in the field of printing, at one end outside the image thereof. Since the maximum image density portion just before development is used for setting the developing bias potential at the subsequent development by measuring surface potential thereof, it is preferred that the area corresponding to the maximum image density portion should be in at least a part of the end portion of the photoreceptor in the direction crossing at right angles with the moving direction of the developing device. In order to measure the surface potential of a portion of maximum image density just before development, the portion, the surface potential of which is to be measured, is provided at least as a part on the photoreceptor. For example, a suitable surface electrometer is provided at the position opposing the portion of maximum image density of the photoreceptor just before development, and when the portion of maximum image density of the photoreceptor passes the opposing surface electrometer, surface potential of said portion is measured. The thus measured surface potential of the portion of maximum image density is used for setting the developing bias potential in the subsequent developing step. That is, on the basis of the relation between the difference (V) in surface potential and developing bias potential on a photoreceptor previously obtained on respective colors and image density (D), so-called V-D characteristics, the developing bias potential is set so that the difference between the surface potential of a maximum image density portion and the developing bias potential becomes constant for obtaining maximum image density of each color. For example, in the case of producing a positive image from a positive half-tone film used in the field of printing, the surface potential of a maximum image density portion to be measured is the surface potential of un-exposed portion just before development and the developing bias potential is set so that the difference between this surface potential and the developing bias potential becomes constant for obtaining maximum image density; that is the deposition of toner of that color onto the area corresponding to a maximum image density portion of the photoreceptor becomes maximum. In the case of producing a positive image from a negative half-tone film, reversal development is usually utilized. In this case, since the surface potential of a maximum image density portion to be measured is the minimum surface potential of an exposed portion just before development, developing bias potential is also set so that the difference between the surface potential and the developing bias potential becomes constant for obtaining maximum image density of that color. The developing bias potential is set by controlling the electrometer and development electrode, for example, using a CPU or a look-up table for obtaining maximum image density for each color density portion for each color becomes constant. In this way, by setting the developing bias potential so that the difference between the surface potential of maximum image density portion and the developing bias potential becomes constant, maximum image density can be maintained constant even if original is changed and hence, reproducibility of color density is superior and image of high quality can be rapidly and inexpensively obtained with good operability. The method of the present invention can be applied to the formation of a monocolor image by using singly respective toners such as cyan, magenta, yellow, and black, but it is more effective for the formation of a multicolor image by repeating two or more times the image formation step according to subtractive color mixture process. Furthermore, the method of the present invention can be applied not only to an ordinary electrophotographic process comprising subjecting an original to scanning exposure, but also to a color proofing which includes contact exposure using half-tone film or scanning exposure with beams such as a laser beam, directly based on digital image signals. Especially it is optimum for a color proofing which requires severe accuracy in color density. Half-tone film or digital image signals used for the color proofing may be either a positive film or digital image signals corresponding to a positive film, or a negative film or a digital image signals corresponding to a negative film, and in the case of a negative film or digital image signal corresponding to a negative film, a so-called reversal development is utilized. In the method of the present invention, use of a photoreceptor comprising a photosensitive layer mainly composed of titanium dioxide is desired from the viewpoints of whiteness of background and gradation and in the reversal development process utilized when the half-tone film or the digital image signal is a negative film or a digital image signal corresponding to the negative film, because titanium dioxide has a bi-charging property, therefore charging in both polarities is possible and so the same toner can be used only with changing porality of charging. Furthermore, a dry toner may be used for development in the method of the present invention, but a liquid developer is preferred from the point of image quality such as graininess. The present invention will be explained further by the following examples. Example A photoreceptor comprising a photosensitive layer mainly composed of titanium dioxide was used. An apparatus was used which comprises an exposing stand which fixes a photoreceptor on which a half-tone film can further be fixed, a corona charger, tungsten light source for exposure, a surface electrometer, a liquid developing device, and a voltage controlling device for setting developing bias potential applied to development electrode based on surface potential measured by the surface electrometer. A photoreceptor was put on an exposing stand in the form of a hollow flat plate which was freely rotatably supported on a shaft and fixed by suction from fine halls of the exposing stand. A corona charger moving at a constant speed was passed over the exposing stand on which photoreceptor was fixed. The corona charger can apply corona voltage to corona wire so that same potential can be optionally applied to shield case and grid wire. While the corona charger passed over the exposing stand on which photoreceptor was put, the photoreceptor was subjected to positive corona discharging to apply a constant charge potential. Then, a half-tone film was provided on the photoreceptor so that the image side thereof faced the photosensitive layer, and a transparent sheet was put on the film, and pressure was applied thereto to bring the sheet into close contact with the half-tone film. This half-tone film was a half-tone negative made using a lith film by a scanner. Two punched holes were provided at given positions of the half-tone film, and the exposing stand had projections at the positions corresponding to the punched holes. Positioning was performed by inserting the projections through the holes. After the half-tone film was set on the photoreceptor, exposure was carried out with white light from tungsten light source provided above the exposing stand. Immediately after the exposure, the half-tone film was removed and the exposing stand with the photoreceptor fixed thereon was rotated by 180° on the shaft of the exposing stand so that the surface of the photosensitive layer of the photoreceptor faced the liquid developing device. Thereafter, the surface potential of the maximum image density portion was measured by the surface electrometer immediately before the developing device which was provided at one end of the original outside the image area, that is, the position opposing the maximum image density portion. Based on the surface potential measured, the developing bias potential was set so that the difference between the surface potential of a maximum image density portion necessary for obtaining image density of the maximum image density portion and the developing bias potential becomes constant and positive developing bias potential was applied to the development electrode. The liquid developing device comprises development electrodes the number of which corresponds to the necessary colors, a developer tank, a drip tray for developer, and a developer replenisher tank and is provided below the exposing stand in such a manner that it can move to the left and right directions. Respective development electrodes, the developer tank, and the drip tray for developer are provided so that they can also move up and down. Development is conducted with a positively charged liquid developer. The developer is supplied to the space between the development electrode and the photoreceptor from the side of the development electrode provided in parallel with the surface of the photosensitive layer and with a slight space therebetween. When this developing section passes below the exposing stand having thereon a photoreceptor, development is conducted. The above-mentioned three steps of charging, exposing and developing were used as one set and the same photoreceptor was subjected to the four sets of this image forming steps for each of yellow color, magenta color, cyan color, and black color in this order to obtain an excellent four color proof print image on the photoreceptor. The surface potential of maximum image density portion, the developing bias potential applied to development electrode and the difference in potential between the surface potential of maximum image density portion and the developing bias potential were as shown in the following table. A plurality of the above four color proof print image was prepared by setting the developing bias potential in the same manner as above so that the difference between the surface potential of maximum image density portion and the developing bias potential became constant. As a result, four color proof print images were obtained which were all good in reproducibility of color density, showing the same color density for the same color and the same tone. Composition of developers used above for respective colors and relation between the density (D) of maximum image density portion measured by densitometer and the difference in potential (V) between the surface potential of maximum image density portion and the developing bias potential were as shown below. (1) Yellow color: 1.00 = 70 V (2) Magenta color: 1.45 = 90 V (3) Cyan color: 1.60 = 90 V (4) Black color: 2.00 = 70 V Furthermore, under different environmental conditions, four color proof print images were produced with controlling in the same manner as above so that the difference between the potential of maximum image density portion necessary for maximum image density portion and the set developing bias potential was the same as the difference in potential shown in the above table. As a result, four color proof print images of high quality with good reproducibility of color density were obtained. According to the method for forming electrophotographic images of the present invention, stable reproduction of color density can be obtained and so this method is suitable for electrophotographic process and especially by applying it to simple color proof in color printing, control of quality can advantageously be performed rapidly and economically. Thus, this method is industrially very useful.

Claims: A method for electrophotographic image formation which comprises the steps of forming an electrostatic latent image on a photoreceptor by carrying out charging and exposing and then developing, wherein maximum image density is maintained constant by measuring the surface potential of a maximum image density portion just before development and setting the developing bias potential based on the above measured surface potential so that the difference between the surface potential and the set developing bias potential is constant. A method according to claim 1, wherein a multicolor image is formed by repeating said steps a plurality of times. A method according to claim 1 or 2, wherein exposing is carried out by contact exposure with a half-tone film. A method according to claim 1 or 2, wherein exposing is carried out by scanning exposure based on digital image signals. A method according to claim 4, wherein the scanning exposuring is carried out by a laser beam. A method according to any of claims 1 to 5, wherein the surface potential of a maximum image density portion is the surface potential of an unexposed portion just before development. A method according to any of claims 1 to 5, wherein the surface potential of a maximum image density portion is the minimum surface potential of an exposed portion just before development. A method according to any of claims 1 to 7, wherein the surface potential of a maximum image density portion just before development is measured on at least a part of the photoreceptor. A method according to any of claims 1 to 8, wherein the photoreceptor comprises a photosensitive layer mainly composed of titanium dioxide. A method according to any of claims 1 to 9, wherein development is carried out using a liquid developer. A method according to any of claims 1 to 10, which is applied to a color proof in the field of printing.

Inventors: MURAKAMI HAJIME, OKADA MIZUHO, MURAKAMI, HAJIME, OKADA, MIZUHO
Applicants: ISHIHARA SANGYO KAISHA, ISHIHARA SANGYO KAISHA, LTD.
Application year: 1990
Main classification: G03G15
IPC codes: G03G15, G03F3, G03G13