JP2006276764A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To successfully prevent generation of an abnormal image resulting from transfer omission, etc. while preventing disorder of the image and smears on the rear edge on a recording material P after transfer. <P>SOLUTION: This image forming apparatus has an image formation control means (main frame control part 17) for preventing the smears on the rear edge by making a transfer bias high when printing ratio obtained from image information shows low printing ratio in a rear edge area in the conveyance direction of the recording material P and on the other hand, avoiding generation of the abnormal image resulting from the transfer omission, etc. by maintaining the transfer bias approximately as it is when the printing ratio in the rear edge area of the recording material P shows high printing ratio. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus including an image formation control unit that transfers a toner image on an image carrier onto a recording material.

  In general, in various image forming apparatuses adopting an electrophotographic system such as a copying machine, a facsimile, or a printer, a uniformly charged photosensitive drum or the like as shown in FIG. The image carrier 1 is subjected to an exposure process corresponding to the image pattern, whereby an electrostatic charge is applied to the electrostatic latent image formed on the image carrier 1 in a developing device (not shown). A developer (toner), which is a fine powder of chromatic color, is supplied to develop the electrostatic latent image to be visualized. The toner image visualized and formed on the image carrier 1 is transferred onto the recording material P by an electrostatic transfer process using a transfer roller 2 as a transfer device disposed in a transfer region (transfer nip portion) N. The recording material P that has been transferred to the recording medium P is separated from the image carrier 1 and sent to the fixing device 3 side.

  However, since the recording material P after the transfer of the toner image is electrostatically attracted to the surface of the image carrier 1, the recording material P is peeled off from the surface of the image carrier 1 and separated. At that time, the rear end portion of the recording material P may be charged to the same polarity as the transfer bias by peeling charging. As a result, the rear end portion of the transfer material P is pulled according to the rotation direction of the image carrier 1 (direction R1 in the drawing), and the separation of the rear end portion of the recording material P is delayed, as indicated by the broken line in the figure. In addition, a so-called trailing edge jumping phenomenon of the recording material P that jumps up so as to approach the image carrier 1 may occur. Due to the occurrence of the trailing edge jump phenomenon, the recording material P comes into contact with the surface of the image carrier 1 or the bottom of the cleaning container (not shown), so-called trailing edge contamination occurs, and the unfixed toner image is rubbed. The image may be disturbed.

  In order to solve such a problem such as rear end contamination due to the jumping phenomenon of the rear end portion of the recording material P, conventionally, for example, as in Patent Document 1 below, the rear end portion of the recording material is more than other portions. The rear end of the recording material is transported in a state where the rear end portion of the recording material is strongly attached to the image carrier by controlling so as to apply a high transfer bias, and the rear end of the recording material is further shifted to the downstream side. Proposals have been made to eliminate the problem of contamination. However, in such a conventional proposal, when the printing rate is a high printing rate image such as solid black or halftone at the rear end portion of the recording material, it is applied to the rear end portion of the recording material. An abnormal image caused by transfer omission or the like may occur due to a high transfer bias.

Japanese Patent Laid-Open No. 2000-172089

  Accordingly, the present invention can satisfactorily prevent the occurrence of abnormal images due to transfer omissions and the like that have occurred at the time of a high printing rate while preventing image disturbance on the recording material and trailing edge contamination after transfer. It is an object of the present invention to provide a transfer device and an image forming apparatus which can be used.

  In order to achieve the above object, in the image forming apparatus according to the present invention, an image carrier on which a toner image corresponding to desired image information is formed, and appropriate transfer for transferring the toner image on the image carrier to a recording material. In the image forming apparatus including a transfer unit to which a voltage is applied and a transfer control unit that determines a transfer voltage to be applied to the transfer unit, the transfer control unit includes a trailing end in a conveyance direction of the recording material to be conveyed The transfer voltage is changed based on a printing rate of the image information corresponding to the area.

  According to the image forming apparatus of the present invention having such a configuration, the transfer voltage (transfer bias) applied to the rear end portion of the recording material can be switched corresponding to the printing rate of the image information. In the case where the rear end portion of the image information has a low printing rate as in the image forming apparatus according to Item 2, the rear end smear is prevented by switching the transfer voltage high as in the conventional proposed apparatus described above, while the image When the trailing edge of the information has a high printing rate, the transfer voltage is maintained to be substantially the same as the transfer voltage for other areas without switching, thereby preventing the occurrence of abnormal images due to missing transfer or the like. It has become so.

  As described above, the present invention is provided with the image forming control means for changing the transfer voltage for the rear end portion of the recording material based on the printing rate in the rear end region in the conveyance direction of the recording material obtained from the image information. It is possible to satisfactorily prevent the occurrence of abnormal images due to missing transfer while preventing image disturbance on the recording material and trailing edge contamination after transfer, improving the image quality of the image forming apparatus and improving reliability. Can be increased.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Prior to that, an example of a schematic structure of a laser beam printer as an image forming apparatus to which the present invention is applied will be described.

  In the laser beam printer shown in FIG. 1, a photosensitive drum 21 as an image carrier that is rotatably supported is provided in a process cartridge 12 that is detachably mounted in an apparatus main body 11. . The photosensitive drum 21 is configured to be rotationally driven in an arrow direction at an appropriate process speed by a driving unit (not shown). The photosensitive drum 21 is arranged so as to be close to the surface of the photosensitive drum 21. A charging roller 22 as a charging device for uniformly charging the surface is provided inside the process cartridge 12. Further, on the downstream side of the charging roller 22 in the rotation direction of the photosensitive drum 21, an exposure 13a from a laser scanner 13 disposed outside the process cartridge 12 is provided as an exposure unit for forming an electrostatic latent image. It is configured to be irradiated. Further, on the downstream side, a developing sleeve 23 a of a developing device 23 that visualizes the electrostatic latent image and forms a toner image is provided inside the process cartridge 12.

  Further, on the downstream side of the developing device 23 in the rotation direction of the photosensitive drum 21, a transfer roller 14 disposed outside the process cartridge 12 as a transfer device for transferring the toner image on the photosensitive drum 21 to the recording material P is provided. The cleaning device 24 is disposed so as to be in pressure contact with the surface of the photosensitive drum 21 and collects untransferred toner remaining on the photosensitive drum 21 on the downstream side of the transfer roller 14. 12 is provided.

  On the other hand, on the bottom side of the apparatus main body 11 described above, a paper feed cassette 15 containing a sheet-like recording material P such as paper is mounted so as to be detachable in a substantially horizontal direction. A paper feed roller 15a for feeding out the recording material P is provided at a front side end portion (right end portion in the drawing) of the sheet mounting direction, and a transfer region (transfer nip) where the transfer roller 14 is disposed from the paper feed roller 15a. Part) A registration roller 15b is arranged in the middle of the conveyance path leading to N. Further, a fixing device 16 for fixing an unfixed toner image on the recording material P by a heating action at an extended end portion of a conveyance path extending from a transfer region (transfer nip portion) N where the transfer roller 14 is disposed. Is arranged.

  Further, a control board (not shown) for controlling an image forming operation or the like is attached to the inside of the apparatus main body 11 described above, and a main body as an image formation control means including a transfer bias control means in the control board. A control unit (engine controller) 17 is provided. The main body control unit 17 includes a central processing unit (CPU) that controls each image forming process in the laser beam printer having the above-described configuration, and a memory that stores a program. The central processing unit (CPU) ) Reads out and executes a necessary program from the memory, whereby the following image forming operation is performed.

  That is, the surface of the photosensitive drum 21 that is rotationally driven in the direction of the arrow by a driving unit (not shown) is applied with a voltage of −670 V, for example, from the charging roller 22, whereby a predetermined polarity and potential, for example − It is uniformly charged to 650V. On the uniformly charged surface of the photosensitive drum 21, an image beam modulated with image information (image signal VDO) received from the external device 18 via a video controller is emitted from a laser scanner 13 as an exposure unit. Irradiation is performed as exposure 13 a, charges corresponding to the exposed portion of the exposure 13 a are removed, and an electrostatic latent image is formed on the surface of the photosensitive drum 21. The electrostatic latent image formed on the surface of the photosensitive drum 21 is visualized by a developing action of a developing sleeve 23a formed from an aluminum roller or the like in the developing device 23 to obtain a toner image. .

  On the other hand, a transfer bias (transfer voltage) by constant voltage control is applied to the transfer roller 14, and the toner image is transferred to the recording material P side by the transfer bias. For example, a constant current of 3.5 μA is applied during the pre-rotation by the control, and is changed according to the output voltage so as not to cause transfer failure or transfer omission, for example, 1.7 kV to 4.5 kV. The optimum transfer bias is obtained.

  At this time, as described above, image information (image signal VDO) is transmitted and received from the external device 18 also shown in FIG. 2 to the main body control unit (engine controller) 17 provided as the image formation control unit in the present embodiment. However, the printing rate obtained from the dot data of the image information (image data) at that time is calculated and temporarily stored in the RAM 17 a connected to the main body control unit 17. On the other hand, the ROM 17b connected to the main body control unit 17 stores a control program for executing the above-described image forming operation, and the control program prints image information as shown in FIG. A program for performing transfer bias control based on the rate is included.

  The transfer bias control operation (printing rate determination sequence) at this time will be described in detail. As shown in FIG. 3, first, the number of output images is set through the operation unit (see reference numeral 17c in FIG. 2). (Step 1) When the image forming operation is started, image information (image data) is read into the main body control unit (engine controller) 17 (Step 2), and after the recording material P is conveyed in the conveyance direction based on the image information. The printing rate in the edge area is calculated (step 3). The rear end area of the recording material P at this time is set to an area within 20 mm from the rear end edge of the recording material P to the front side in the transport direction.

  Then, the printing rate in the conveyance direction rear end region obtained at that time is compared with a predetermined reference value (for example, 40%) (step 4), and the printing rate in the conveyance direction rear end region is compared with the reference value. If the printing rate is lower than 40% (Y in Step 4), a switching signal is issued from the main body control unit 17 to the transfer bias driver 14a (see FIG. 2) of the transfer device 14. As a result, the strong transfer bias in which the transfer bias (transfer voltage) at the rear end portion of the recording material is set higher than the transfer bias during printing for other regions is turned on (step 5). Even when this strong transfer bias is applied, a constant current of 3.5 μA is applied during the pre-rotation, and is changed from 3.4 kV to 5 kV, for example, according to the output voltage.

  By applying such a strong transfer bias, the rear end portion P2 of the recording material P is strongly attracted to the surface of the photosensitive drum 1 after passing through the transfer portion nip portion N as shown in FIG. Accordingly, the rear end portion P2 of the recording material P moves upward as indicated by the arrow e along the rotation direction of the photosensitive drum 21 indicated by the arrow R1. As a result, the separation point or separation timing between the recording material P and the photosensitive drum 21 is further delayed as compared with the conventional case. As a result, the photosensitive drum 21 immediately after the trailing end portion P2 of the recording material P passes through the transfer portion nip portion N. This prevents the rear surface from being re-contacted and prevents the trailing edge from being soiled.

  Returning to FIG. 3 again, the toner image corresponding to the rear end portion P2 in the conveyance direction of the recording material P is a high printing rate image in which the printing rate such as solid black or halftone exceeds 40% of the reference value. In this case (N in Step 4), the transfer bias as described above is not switched, and the transfer bias substantially the same as the transfer bias during printing corresponding to other regions is maintained (Step 6). As a result, abnormal image formation due to transfer omission or the like that may occur due to application of a strong transfer bias is favorably avoided.

  This point will be described in more detail. First, the relationship between the rear end portion P2 of the recording material P and the transfer bias is as shown in FIG. 5, for example. When the rear end portion P2 of the recording material P has a low printing rate, when the transfer bias for the recording material rear end portion P2 is switched to a transfer strong bias higher than the transfer bias during printing, the recording material P is exposed to the photosensitive drum. Accordingly, the rear end portion P2 of the recording material P is not violated, and the conveyance of the recording material P becomes stable. On the other hand, if the off-state is maintained without applying such a strong transfer bias, rear end jumping as described in the prior art occurs, causing rear end smearing and character blurring.

  In addition, if the transfer strong bias as described above is applied even when the rear end portion P2 of the recording material P has a high printing rate, an abnormal image due to transfer omission or the like as described in the related art. However, if control is performed so that a strong transfer bias is not applied as in the present embodiment, toner is interposed between the photosensitive drum 21 and the recording material P, and recording from the photosensitive drum 21 side is performed. The separation of the material P is facilitated and the occurrence of rear end jumping is prevented.

  Returning to FIG. 3 again, after the above-described transfer bias control is performed, image information (image data) is output (steps 7 and 8), and whether or not the number of output images has reached the specified number. Is checked (steps 9 and 10). As a result, if the designated number has not been reached, the processing from step 5 described above is repeated, and if the designated number has been reached, the image forming operation is terminated.

  As described above, according to the present embodiment, the transfer bias applied to the rear end portion of the recording material P is switched in accordance with the print rate of the image information, and the rear end portion of the image information has a low print rate. In the same way as in the conventional apparatus, the rear end contamination is prevented by switching the transfer bias high. On the other hand, when the rear end portion of the image information has a high printing rate, the transfer to another region is performed without switching the transfer bias. By maintaining substantially the same as the bias, it is possible to avoid the occurrence of an abnormal image due to missing transfer or the like.

  On the other hand, the trailing edge stain as in the above-described embodiment occurs when the printing rate is low when plain paper or thin paper is used, and occurs regardless of the printing rate when OHP, cardboard, or postcard is used. Never do. The reason is that in the case of plain paper or thin paper, the stiffness of the recording material is weak, so that only the rear end portion in the transport direction is attracted to the photosensitive drum side by electrostatic force, resulting in rear end smearing. However, in the case of OHP, cardboard, postcards, etc., the stiffness of the recording material is strong, so that the rear end portion of the recording material is rarely attracted to the photosensitive drum side. That is, the rear end portion of the recording material does not come into contact with the photosensitive drum side, and there is little possibility that the rear end stain will occur.

  In consideration of such circumstances, in the embodiment shown in FIG. 6, image formation control is switched depending on the type of recording material to be used. That is, when the plain paper mode, thin paper mode, OHP mode, thick paper mode, and postcard mode are set, and the normal paper mode or thin paper mode is selected, the transfer based on the printing rate similar to the above-described embodiment is performed. Although the bias control operation is executed, when the OHP mode, the cardboard mode, or the postcard mode is selected, the transfer bias is kept constant regardless of the printing rate of the image information, and the above-described control for switching to the strong transfer bias is performed. Is not done.

  As mentioned above, although the embodiment of the invention made by the present inventor has been specifically described, the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the scope of the invention. Not too long.

  For example, in the above-described embodiment, the reference value of the printing rate is set to 40%, but the reference value can be appropriately set according to the apparatus conditions.

  In the above-described embodiment, the present invention is applied to a transfer device of a laser beam printer. However, the present invention is similarly applied to a transfer device of another image forming apparatus such as a copying machine or a facsimile. It is something that can be done.

  The transfer apparatus according to the present invention described above can be widely applied to a wide variety of image forming apparatuses such as copying machines as well as image forming apparatuses such as printers.

1 is a longitudinal sectional view illustrating a schematic configuration of a laser printer as an example of an image forming apparatus to which the present invention is applied. FIG. 3 is a block diagram showing the structure of image formation control means including transfer bias control means in an embodiment of the present invention. It is a flowchart showing the transfer bias control sequence in one Embodiment of this invention. FIG. 6 is a schematic side view illustrating the behavior of a recording material P in a transfer nip portion. FIG. 4 is a table showing the relationship between the transfer bias and the image printing rate for the trailing edge of the recording material in the transfer bias control sequence in one embodiment of the present invention shown in FIG. 3. 7 is a table showing transfer bias control modes in another embodiment of the present invention. FIG. 6 is a schematic side view illustrating the behavior of a recording material P in a transfer nip portion of a conventional apparatus.

Explanation of symbols

11 Device Main Body 12 Process Cartridge 13 Laser Scanner 14 Transfer Roller (Transfer Device)
DESCRIPTION OF SYMBOLS 15 Paper feed cassette 16 Fixing apparatus 17 Main body control part (Image formation control means including transfer bias control means)
18 External device 21 Photosensitive drum (image carrier)
22 Charging roller 23 Developing device 24 Cleaning device P Recording material N Transfer area (transfer nip portion)

Claims (5)

An image carrier on which a toner image corresponding to desired image information is formed;
Transfer means to which an appropriate transfer voltage for transferring the toner image on the image carrier to a recording material is applied;
Transfer control means for determining a transfer voltage applied to the transfer means;
In an image forming apparatus comprising:
The image forming apparatus according to claim 1, wherein the transfer control unit is configured to change the transfer voltage based on a printing rate of the image information corresponding to a rear end region in a conveyance direction of the recording material to be conveyed.   The transfer control unit performs transfer corresponding to the rear end region of the recording material when the print rate of the image information corresponding to the rear end region in the conveyance direction of the recording material is lower than a reference value. If the printing rate corresponding to the rear end region of the recording material is higher than the reference value while the voltage is set higher than the transfer voltage for the other region, the transfer to the rear end region of the recording material is performed. The image forming apparatus according to claim 1, further comprising a configuration that maintains a voltage substantially the same as a transfer voltage for another region.   3. The image forming apparatus according to claim 1, wherein the rear end region of the recording material is a region within 20 mm from the rear end edge of the recording material to the front side in the transport direction.   The transfer control means has a high printing rate when the printing rate obtained from the dot data of the image information corresponding to the rear end region of the recording material is 40% or more, and low when the printing rate is less than 40%. The image forming apparatus according to claim 1, wherein the image forming apparatus has a configuration for determining the printing rate. The transfer control unit is configured to change the transfer voltage according to a printing rate of the image information corresponding to a type of the recording material and a rear end region in a conveyance direction of the recording material. The image forming apparatus according to claim 4.

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