Positive/negative image forming method and apparatus for electrophotographic image forming device

Abstract: An electrophotographic image forming apparatus comprising a photoreceptor for forming an image corresponding to an original, a main charger for charging the image in a specific polarity, a developer for developing the image with toner, a transference charger for transferring the image on the surface of the photoreceptor, and switching devices for switching the utilization of the toner inside the developer and the polarities of a voltage applied to the transference charger.

Description: The present invention relates to a positive/negative image forming method and device for use in an electrographic image recording apparatus, for example, a copying machine. Conventionally, an electrographic image recording apparatus, for example, a copying machine, uses either positive image forming method or negative image forming method. In the former method, a positive image is formed, that is, the image of white parts of the original is formed as white while the image of black parts of the original is formed as black. In the latter method, a negative image is formed, that is, the image of black parts of the original is formed as white while the image of white parts of the original is formed as black. In the positive image recording apparatus, for example, a main charger and a developing bias are negatively polarized with the voltage of the developing bias being smaller than that of the main charger, while toner is positively polarized. The positively polarized toner is attracted to a charged portion of an electrostatic latent image, that is, the portion where the voltage is lower than that of the negatively polarized developing bias. Thus, a positive image corresponding to the original is formed and the positive image may be transferred to a copy paper via a transference charger, which is negatively polarized, that is, opposite to that of the toner. On the other hand, in the negative image recording device, for example, a main charger, a developing bias and toner are negatively polarized so that the negatively polarized toner is attracted to a non-charged portion of the electrostatic latent image on the surface of a photoreceptor drum. Thus, a negative image corresponding to the original is formed and the negative image may be transferred to a copy paper via a transference charger, which is positively polarized. The above conventional apparatus have disadvantages in that they are specific sole-purpose apparatus for forming either a positive image or a negative image. Depending upon the image required, both a positive image recording apparatus and a negative image recording apparatus may be required to be furnished, thus resulting in high cost. DE-A-3,006,725 describes a photographic printer that can operate in both positive and negative development modes using toner particles of a single polarity by changing the charge of the photoconductive surface between positive and negative values. JP-A-61-86 777 describes a microfilm reader/printer which can produce a positive copy of a microfilm image regardless of whether such microfilm image is in positive or negative form. This is achieved by providing the developing device in the electrophotographic printing section of the reader/printer with first and second developing units including respective oppositely chargeable toner particles. A transfer charger for transferring the toner image from a rotatable photosensitive drum is controlled according to the selected mode of operation; the voltages supplied to such transfer charger in the two modes are of opposite polarities. JP-A-60-108 875 also describes a microfilm reader/printer which can perform P-P or N-P copying of a microfilm image using oppositely chargeable toner contained in respective P-P mode and N-P mode developing devices. It is an object of the present invention to provide an improved electrographic positive/negative image recording apparatus. It is another object of the present invention to provide an improved method for switching the mode of operation of an electrographic positive/negative image recording apparatus. In accordance with the present invention there is provided an electrographic image recording apparatus according to claim 1. The polarity of a developing bias is the same as that of a main charger used for forming the electrostatic latent image, one of the developer units having positively polarized toner in it and the other having negatively polarized toner in it, with the polarity of a transference charger used for transferring the developed image changeable between positive and negative. With the above arrangement, the polarities of the toner and the transference charger can be freely combined, that is, four kinds of combinations (toner: + , charger: + / toner + , charger: - / toner: -, charger: + / toner: -, charger: -) become possible. Thus, both a positive image and a negative image can be formed. In accordance with the present invention there is also provided a method of selectively performing first or second modes of image recording, according to claim 4. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: Fig. 1 is a side view of an electrographic copying machine embodying the present invention; Fig. 2 shows the polarities of chargers, and toner of the copying machine; Fig. 3 is a plan view of the operation panel of the copying machine; and Fig. 4 is a block diagram of the controlling portion of the copying machine. Fig. 1 is a side view of an electrographic copying machine embodying a positive/negative image forming method and device according to a preferred embodiment of the present invention. Referring to Fig. 1, a photoreceptor drum 1 driven by a main motor rotates clockwise. Around the drum 1, there are provided a main charger 2, a developer 3, a transference charger 4, a charge-removal charger 5, a cleaner 6, and a charge-removal lamp 7. In this embodiment, the main charger 2 charges the surface of the drum 1, preferably at -800 V. Fig. 2 is a table showing the polarities of toner and the chargers, namely a transference charger and an optical charge-removal charger, with the main charger and the developing bias negatively polarized for both forming a positive image and forming a negative image. For example, when a negative image is formed, the toner is negatively polarized, the transference charger is positively polarized, and the charge-removal charger is negatively polarized. The charge-removal charger 5 is used and its polarity is changeable between positive and negative. The charge-removal charger is not essential to image forming; rather it only removes the charge from the surface of the drum 1 after the formed image is transferred therefrom by the transference charger. Referring back to Fig. 1, within the developer 3, a developing roller 31 and two-component developing toner 32 are provided. The developer 3 is positioned so that the developing roller 31 rotates adjacent to the photoreceptor drum 1 and the surfaces of the roller and drum may contact at a point. The developer 3 may be a unit structure and detachable from the copying machine. There are two such developers. One stores toner which is positively charged, and the other stores toner which is negatively charged. The former developer is used for forming a positive image and the latter developer is used for forming a negative image. Fig. 1 shows a copying machine with a developer for negative image forming. The two-components stored in the developer 3 is composed of toner, which is negatively polarized by friction charge, and carrier, which is positively polarized. Negative developing bias, that is, the same polarity as that of the main charger is charged to the developing roller 31 and is preferably at -400 V. A circuit is connected to the transference charger 4, comprising a switch SW1 and two power sources E1 (+) and E2 (-). Another circuit is connected to the charge-removal charger 5, comprising a switch SW2 and two power sources E3 (-) and E4 (+). The switching of the power sources of the transference charger 4 and the charge-removal charger 5 may be performed through an image switching key on the operation panel provided on the body of the copying machine. Fig. 3 is a plan view of the operation panel. On the operation panel 8, an image switching key 81 is provided as weil as a print key 82, zoom control keys 83, contrast control key 84, and other displays. Every time the image switching key 81 is pressed down, the display changes from "negative image" to "positive image", or "positive image" to "negative image". Fig. 4 is a block diagram showing the controlling portion of the copying machine. The whole system is controlled by a CPU 9. A controlling program is initially inputted and stored in a ROM 10. When this program is activated, a RAM 11 is used as a working area. The RAM 11 comprises a flag for storing a positive image mode and a negative image mode. As for the operation panel 8, signals are inputted or outputted through an I/O port 12. The transference charger 4 and the charge-removal charger 5 are driven by a high tension power circuit 13. The ON/OFF timing of the high tension power circuit 13 is controlled by control signals from the I/O port 12. In Fig. 1 which shows a copying machine set for forming a negative image, the transference charger 4 is connected to the power source E1 (+) and the charge-removal charger 5 is connected to the power source E3 (-). The charge-removal charger 5 is also provided in order to remove effectively the residual charge on the surface of the drum 1. Further, a charge-removal lamp 7 is also provided. The cleaner 6 removes toner and other particles remaining on the surface of the drum 1 by, for example, a fur brush 61. The image forming operation is described below. When a negative image is required, the image switching key 81 may firstly be pressed so that the "negative image" lamp on the switch is displayed, thus also operating the switches SW1 and SW2, the polarity of the transference charger 4 is set positive, and the polarity of the charge-removal charger 5 is set negative. Secondly, a developer unit 3 storing negatively polarized toner is placed in the copying machine. Thereafter, by pressing down the print key 82 shown in Fig. 3, the drum 1 is charged to preferably -800 V. When the drum 1 rotates and is exposed to light at an exposure point P, the negative charge on portion of the image corresponding to the white portion (portion of high light-reflectance) of the original image is reduced or cancelled. Where the intensity of light is high, the negative charge is reduced to preferably -50 V. The drum 1 further rotates and reaches the point where the surface is adjacent to that of the developing roller 31. The negatively charged toner 32a will be attracted to portion of relatively high electric potential. In this case, the charged portion of the drum 1 is -800 V, the portion where the charge is reduced or cancelled by the abovementioned light exposure is - 50V, and the developing bias is - 400V, so, the negatively charged toner 32a is attracted to the portion where the charge is reduced or cancelled and which has relatively high electrical potential, for example - 50V, that is, the portion of the image corresponding to the white portion of the original. Thus, a negative image is formed by attracting the negatively charged toner to the portion corresponding to the white portion of the original and by not attracting toner to the portion corresponding to the black portion of the original. At the transference charger 4, the negatively charged toner 32a on the surface of the drum is transferred to a copy sheet, say of paper by positive corona discharge. Furthermore, the charge-removal charger 5 removes the positive charge on the surface of the drum charged by the transference charger 4. The cleaner 6, using fur brush 61, removes the toner and other particles remaining on the surface of the drum 1. Further, the charge-removal lamp 7 removes the remaining charge on the surface of the drum. On the other hand, when a positive image is required, by pressing down the image switching key 81, the transference charger 4 is connected to the power source E2 (-), and the charge-removal charger 5 is connected to the power source E4 (+). The developer unit (3) is changed to a developer which stores positively polarized toner. In this embodiment, the only difference between a unit for forming a negative image and a unit for forming a positive image is that the toner inside the unit is charged in the opposite polarity and all other parts are the same, thus reducing cost. The drum 1 is charged to preferably -800 V by the main charger 2. When the drum 1 rotates and is exposed to light at the exposure point P, the negative charge or portion of the image corresponding to the white portion of the original image is reduced or cancelled. The positively polarized toner will be attracted to portions of lower electrical potential than the developing roller charged -400 V that is, the porions which are not exposed to light at the exposure point P (black portion or portion of low light-reflectance of the original image). Thus a positive image is formed. At the transference charger 4, the positively charged toner is transferred to a copy sheet, say of paper by negative corona discharge. Furthermore, the charge-removal charger 5 removes the negative charge on the surface of the drum. According to the present invention, one copying machine is available both for forming a positive image and for forming a negative image. The switching between forming a positive image and forming a negative image can be achieved only by pressing down the image switching key and by changing a developer unit to the other, thus simplifying the different images forming operation. The two detachable developer units may alternatively be two developers built in the copying machine, the selection of either developer unit for different image forming operations by simply switching a lever or the like. Additionally, if an automatic switch is provided for changing the polarities of the transference charger and also the charge-removal charger whenever a developer unit is changed, then the image switching key for pressing manually is no longer required and the operation is even more simplified. In this embodiment as described, the developing bias is the same for both forming a positive image and forming a negative image (-400 V), but it may be possible that the developing bias differs according to the image required. In such a case, two power sources, for example, - 400V and - 300V may be set and be switched in the same way when the image required is switched. Further, in this embodiment, the polarity of the main charger and the developing bias is set negatively, but it is possible to set the polarities positively. In such case, when forming a negative image, the toner is positively polarized, the transference charger is negatively polarized, and the charge-removal charger is positively polarized; when forming a positive image, the toner is negatively polarized, the transference charger is positively polarized, and the charge-removal charger is negatively polarized. Thus, positive and negative images forming can still be accommodated in one copying machine. It is an advantage of the present invention that the polarities of the toner and the transference charger are variable, and by varying them, both a positive image and a negative image can be formed using one image forming device and recording apparatus. It is not necessary to change the whole image forming device according to the image required. Further, all the parts inside the image forming device excluding the two developers and two power sources for transference charger (and, in this embodiment, also the two power sources for a charge-removal charger) are used in common, thus reducing costs. While only certain embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the present invention as claimed.

Claims: An electrographic image recording apparatus which is selectively operable in first and second modes for forming respectively positive and negative images of an original, said apparatus comprising an image carrier (1), means (2) for forming an electrostatic latent image on said image carrier, developer means (3) for developing said electrostatic latent image into a toner image, said developer means comprising first and second interchangeable developer units (3) for providing charged toner particles of a first polarity in said first mode and charged toner particles of a second, opposite polarity in said second mode, respectively transference charger means (4) for transferring said toner image from the surface of said image carrier to a surface of a recording sheet (A) by applying an electrostatic charge to the other surface of the recording sheet, charge removal means for removing residual charge from said image carrier after transfer of the toner image therefrom, and first switch means (SW1) for switching the polarity of a voltage applied to said transference charger means so that the charge applied by said transference charger means is opposite in polarity to the charge of the toner particles, characterised in that said charge removal means comprises a charge-removal charger means (5) arranged to remove such residual charge resulting from the operation of said transference charger means, and in that further switch means (SW2) is provided for switching the polarity of the charge applied by said charge-removal charger means (5), said first-mentioned and further switch means being arranged to operate automatically when changing between said first and second modes, and also being arranged so that the charge applied by said charge-removal charger means (5) is opposite in polarity to the charge applied by said transference charger means. An electrographic image recording apparatus according to claim 1, characterised in that said switch means (SW1, SW2) are operated automatically in response to the interchange of said developer units (3). An electrographic image recording apparatus according to any preceding claim, wherein the developer means (3) is biased at the same polarity as a main charger (2) included in the means for forming the electrostatic latent image. A method of selectively performing first or second modes of image recording for forming a positive or negative image corresponding to an original image, respectively, the method comprising the steps of: forming an electrostatic latent image corresponding to the original image on an image carrier (1), providing charged toner particles of a first polarity in said first mode and charged toner particles of a second, opposite polarity in said second mode to develop said latent image into a toner image, transferring the toner image from the image carrier to a recording sheet (A) by switching the polarity of a voltage applied to a transference charger means (4) so that the charge applied by said transference charger means is opposite in polarity to the charge of the toner particles, and removing residual charge from said image carrier after transferring the toner image therefrom, characterised in that said residual charge removing is performed using a charge-removal charger means (5) so as to remove such residual charge resulting from the operation of said transference charger means, and in that the polarity of said voltage applied to said transference charger means and the polarity of a voltage applied to said charge-removal charger means are both switched automatically in response to a change between said first and second modes, the charge applied by said charge-removal charger means being opposite in polarity to the charge applied by said transference charger means.

Inventors: ITOH IKUO, NAKAMURA MASATSUGU, ITOH, IKUO, NAKAMURA, MASATSUGU
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1987
Main classification: G03G15
IPC codes: G03G15