JP2007232749A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve first the separatbility of a transfer material from a latent image carrier that is carried out, after transfer in an image forming apparatus capable of printing in multicolors. <P>SOLUTION: The image forming apparatus includes an image carrier 11 holding a toner image thereon, a plurality of latent image forming means 2 and 5 for forming a latent image on the image carrier, developing means 3 and 6 for developing the latent image, and a transfer means 7 for transferring the toner image to a transfer material. In the image forming apparatus capable of selecting a mode for forming multicolor images and a mode for forming monochrome images. In the mode for forming monochrome images, the latent image forming means 5, which is chosen between the latent image forming means 2 and 5 and is not involved in latent image formation, is used as a pre-transfer exposure means for separating a transfer material S from the image carrier 11, prior to transfer. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, and more particularly, to an electrostatic recording type image forming apparatus capable of printing in a plurality of colors and a structure for separating a transfer material at the time of monochromatic print output.

One of the image forming methods in image forming apparatuses such as copiers, printers, facsimile machines, and printing machines. Visible image processing with a developer of electrostatic latent images formed on a photoreceptor used as a latent image carrier. In addition, there is an electrostatic recording method including a step of electrostatically transferring a visible image onto a transfer material.
In such an electrostatic recording type image forming apparatus, a method of electrostatically attaching toner to an electrostatic latent image on a photosensitive member by visible image processing is known, and a photosensitive member obtained by this method is known. The above toner image is transferred to a transfer material such as a sheet-like medium such as recording paper by an electrostatic transfer method. The electrostatically transferred toner image is fixed by a fixing device to be printed.

  In the step of transferring the toner image on the photoconductor to the transfer material, a transfer device is used that can apply a transfer bias by confronting the transfer material to the toner image on the photoconductor while conveying the transfer material. As a transfer device, a configuration is known in which a belt having a stretchable surface capable of transporting a transfer material is used, and a roller that applies a transfer bias having a polarity opposite to that of toner is applied to the belt.

  After the transfer, the transfer material is separated from the photoconductor and conveyed toward the fixing device. The separation of the transfer material from the photoconductor depends on the bending rigidity of the transfer material, so-called waist strength. In other words, when the photosensitive member has a curvature, the transfer material in the case of moving along with the photosensitive member moves in the direction of movement at the transfer position and the direction of movement of the latter while attached to the photosensitive member. It is peeled from the photoconductor by curvature separation using the direction corresponding to the curvature of the material and the shape restoring force due to its own waist strength, but the electrostatic adhesion to the photoconductor is greater than the waist strength If there is, it may move without being separated from the photoreceptor.

  Conventionally, as a configuration used for separating a transfer material from a photoconductor, a separation claw provided with a tip portion that contacts the peripheral surface of the photoconductor that has passed through the transfer process unit is used, or the back surface potential of the photoconductor is set before transfer. There is a configuration in which the transfer material is neutralized in a transfer device with an exposure device provided for lowering (see, for example, Patent Document 1).

As for the separation of the transfer material from the photosensitive member, the rear end portion of the transfer material becomes stationary after the rear end portion passes through the nip portion between the photosensitive member and the transfer device. There is a risk of so-called behavioral rage.
The transfer bias value to be applied is improperly set or the transfer material resistance value is set to a suitable transfer bias value, assuming that the separation behavior from the photoconductor is normally performed at the leading and trailing edges of these transfer materials. If it is not, a remarkable trace of potential fluctuation, which is called a peeling discharge trace, remains on the surface of the photoconductor, so that a linear trace may be formed in the next image at that portion of the photoconductor.

  As a means for improving the separation of the transfer material from the photoconductor, there is the use of a potential drop of the photoconductor by pre-transfer exposure, which can reduce the adhesion of the transfer material to the photoconductor, but pre-transfer exposure. Therefore, it is necessary to provide a space for arranging the pre-transfer exposure unit as well as increasing the number of parts. In addition, as an exposure unit light source for pre-transfer exposure, an LED having a lower directivity than the arrangement space, controllability, and cost is used. In the LED, an exposure timing and a control timing of applying a transfer bias to the transfer material are used. It is impossible to carry out with high accuracy. Similarly, exposure unevenness of the photoconductor occurs due to the directivity of the LED, and light fatigue is accelerated by the photoconductor directly under the LED.

Further, in many image forming apparatuses, when the pre-transfer exposure unit is arranged in the vicinity of a position where transfer onto a transfer material is performed, the light emitting surface becomes an upward surface and receives dust mainly toner in the image forming apparatus. Easily, if light emitting surface dust adheres, insufficient exposure and uneven exposure are caused, and a sufficient effect cannot be obtained.
Similarly, depending on the arrangement of the pre-transfer exposure unit, the optical path from the pre-transfer exposure unit may be blocked by the transfer material after the transfer material contacts the photoconductor. In that case, it is possible to expose only a range on the photoreceptor corresponding to the extreme end portion of the transfer material.
If functions for the purpose of protecting and cleaning the dust and the exposure range are given, the increase in the number of parts and the securing of space will cause problems such as an increase in size and complexity of the apparatus and an increase in cost.

JP 2005-202351 A

The present invention has been made in view of the above problems,
In an image forming apparatus capable of printing in multiple colors,
First, to improve the transfer material separation after the transfer from the latent image carrier,
Second, obtain stable separation regardless of differences in image formation mode and transfer material, etc.
Third, improve the separation when using the pre-transfer exposure means in the apparatus,
A fourth object is to obtain a stable image quality regardless of the difference in image forming mode or transfer material.

(1) In order to achieve the above object, an invention according to claim 1 is directed to an image carrier that carries a toner image, a plurality of latent image forming units that form a latent image on the image carrier, and a latent image. An image forming apparatus comprising: a developing unit that forms an image; and a transfer unit that transfers a toner image to a transfer material, and is capable of selecting a mode for forming a multicolor image and a mode for forming a single color image. In the mode for forming a latent image, the latent image forming means not used for latent image formation among the latent image forming means is used as pre-transfer exposure means for separating the transfer material from the image carrier before transfer.
(2) According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the latent image forming means not used for forming the latent image corresponds to a front end portion and a rear end portion of the transfer material at the time of pre-transfer exposure. The position to be exposed was decided to be exposed.
(3) According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the exposure condition of the pre-transfer exposure unit is controlled in accordance with the type of transfer material, the image formation mode, or a combination thereof. The configuration is as follows.
(4) In the invention according to claim 4, in the image forming apparatus according to claim 3, the exposure condition is an exposure output.
(5) In the invention according to claim 5, in the image forming apparatus according to claim 3, the exposure condition is an exposure frequency.
(6) According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, the transfer unit is configured to start the exposure by the pre-transfer exposure unit and start the transfer material. Before the start of contact with the image carrier, a stepwise value is set and bias application is started, and a predetermined time before the time when the rear end of the transfer material finishes contact with the image carrier. At least a stepwise value was set to end the bias application.
(7) According to a seventh aspect of the present invention, in the image forming apparatus according to the sixth aspect, the transfer unit is configured such that the front end of the transfer material comes into contact with the image carrier and the front end non-image portion of the transfer material. A bias (normal transfer bias) for transferring the toner image on the image carrier onto the transfer material is applied until the rear end reaches the transfer nip portion.
(8) According to an eighth aspect of the present invention, in the image forming apparatus according to the sixth aspect, the transfer unit is configured to transfer the transfer material on the basis of a time point when the transfer material is fed toward the latent image carrier. A point in time when the tip starts to contact the latent image carrier is specified, and after that point, when the transfer material moves a predetermined distance, a bias is set so that the value changes at least stepwise. It was decided to start.
(9) The invention according to claim 9 is the image forming apparatus according to any one of claims 6 to 8, wherein the transfer unit is based on a point in time when the transfer material is fed toward the image carrier. In consideration of the length of the transfer material in the moving direction, a time point at which the rear end of the transfer material ends contact with the latent image carrier is specified, and the value changes at least in steps before the predetermined time. The bias was set and the bias application was completed.
(10) According to a tenth aspect of the present invention, in the image forming apparatus according to any one of the sixth to ninth aspects, the transfer bias unit is configured to perform normal transfer with respect to a leading end side in the moving direction of the transfer material. After applying a bias with a bias value lower than that at the time of bias application, apply a bias using the bias value at the normal transfer bias, and then apply a normal transfer bias for the rear end side in the moving direction of the transfer material. The bias application was performed with a lower bias value.
(11) According to an eleventh aspect of the present invention, in the image forming apparatus according to any one of the sixth to tenth aspects, the transfer bias unit targets a non-image portion on the leading end side in the moving direction of the transfer material. After applying a bias with a bias value lower than that at the time of normal transfer bias application, a bias is applied using the bias value at the time of normal transfer bias on the basis of the time point when the tip of the image portion faces the image carrier, Thereafter, bias application is performed with a bias value lower than that at the time of normal transfer bias application for the non-image portion on the rear end side in the moving direction of the transfer material.

  According to the present invention, a bias that contributes to transfer material separation is applied using an existing pre-transfer exposure unit provided as a latent image forming unit, thereby separating the transfer material from the image carrier with a simple configuration. Can be improved.

FIG. 1 is a schematic layout diagram showing an example of an image forming apparatus suitable for carrying out the present invention. The image forming apparatus shown in FIG. 1 has a copying function capable of optical writing corresponding to image information. Have.
The image forming apparatus 20 includes functional units such as an automatic document reading / conveying unit (so-called ARDF unit) 21, a document scanning unit 22, an image forming unit 23, and a paper feeding unit 24 from above.

  The automatic document reading and conveying unit 21 has a scanner function for automatically conveying a sheet-like document and reading image information. The document scanning unit 22 reads image information such as a book-type document. The document information read by the automatic document reading / conveying unit 21 and the document scanning unit 22 is converted into an electrical signal and used for image formation in the image forming unit 23.

  The image forming unit 23 is a recording sheet S such as a sheet-like medium (hereinafter referred to as a transfer material) conveyed along a conveyance path indicated by a broken line from the tray of the paper feeding unit 24 or the manual paper feeding unit 27. The document information is recorded as a visible image with toner and fixed by the fixing device 28, and then sent to the tray 25 or sent to a finisher (not shown) in the direction of the arrow 26.

  The image forming apparatus 20 can select a single color printing mode for forming a single color image and a two color printing mode for forming a plurality of color images. That is, it is configured to be able to execute a single color printing mode in which a single color image is formed using only one color (for example, black) and a two color printing mode in which an image is formed of two colors (for example, black and red). These modes are configured so that the user can appropriately select these modes.

FIG. 2 is a schematic diagram showing an example of a structural member that performs an image forming process such as latent image formation, visible image processing, and transfer processing to a transfer material. This is a schematic arrangement, and corresponds to the A part in the figure.
The present invention can be applied not only to the above-described copying function for recording document information but also to an apparatus that performs various functions as a printer, a facsimile machine, or a printing machine as an image forming apparatus.

In FIG. 2, the image forming unit 23 includes a photoconductor (hereinafter referred to as a photoconductor drum) 11 as a drum-shaped image carrier, and around the photoconductor drum 11, a clockwise direction indicated by an arrow. A first color charging device (hereinafter referred to as a first charging device) 1 and a first color writing device (hereinafter referred to as a first writing device) for executing image forming processing in the order of rotation, and in FIG. Only the reflection mirror 2b is shown.) 2, a first color developing device (hereinafter referred to as a first developing device) 3, a second color charging device (hereinafter referred to as a second charging device) 4, a second color developing device. A second color writing device (hereinafter referred to as a second writing device; only the optical path 5a and the reflection mirror 5b are shown in FIG. 2) 5, and the photosensitive drum 1 depending on the presence or absence of the second color image information Second color developing device that can touch and separate Second developing device called.) 6, a transfer device 7, P sensor 8 used for image density adjustment, a photoreceptor cleaning device 9, a discharge device 10 is disposed, respectively.
The first writing device 2 and the second writing device 5 are latent image forming means having a function of exposing the charged photosensitive drum 11 to form a latent image. The first developing device 3 and the second developing device 4 are examples of developing means for developing the latent image on the photosensitive drum 11 with toner.

  As the writing device such as the first writing device 2 and the second writing device 5, for example, a laser beam is scanned and irradiated on the surface of the photosensitive member via a mirror, or a plurality of writing devices arranged side by side in the axial direction of the photosensitive member. There are those that irradiate the surface of the photoreceptor with light from the LED.

  FIG. 2 shows the developing roller 3a that is in contact with and separated from the photosensitive drum 11 in the first developing device 3, and black toner indicated by a black circle is supplied to the photosensitive member via a carrier indicated by a white circle. For the second developing device 6, a developing roller 6 a that is brought into contact with and separated from the photosensitive drum 11 is shown, and color toner indicated by a white circle is supplied to the photosensitive drum 11. A transfer belt 7a having left and right ends supported by support rollers with respect to the transfer device 7 is shown. The cleaning device 9 includes a cleaning brush roller 9a and a cleaning blade 9b. The transfer device 7 has a transfer belt 7a that is supported by rollers at its left and right ends and rotates. A bias roller 13 is attached to the transfer belt 7a as will be described later. The transfer unit 7 including the transfer belt 7a with the bias roller 13 is an example of a transfer unit that transfers a visible image (toner image) on the photosensitive drum 11 to a transfer material.

The transfer belt 7a is made of a fluorine-based material having a low surface friction coefficient and a surface specific resistance (JISK6911) of 1 × 10 ¹⁰ Ω to 1 × 10 ¹² Ω. The base layer that supports the surface layer is made of a rubber material such as chloroprene rubber, EPDM rubber, epichlorohydrin rubber, or a blended material thereof. If necessary, a conductive material such as carbon or metal oxide is blended to resist the material. The surface resistance (JISK6911) of the rubber surface is set to a medium resistance value of about 1 × 10 ⁷ Ω to 1 × 10 ⁹ Ω. In the present invention, not only the intermediate resistance but also a resistance region of 10 ¹³ or more or 10 ⁶ or less can be selected.

  The transfer belt 7 a forms a transfer nip portion NP (described later) with the photosensitive drum 11 at a position between the developing roller 6 a and the separation claw 12. The transfer material sent from the paper feed unit 24 or the manual paper feed unit 27 via the conveyance path is temporarily stopped by the pair of registration rollers 14 and sent out toward the transfer nip NP at an appropriate timing.

  In FIG. 2, in the image formation, in the single color printing mode and the two color printing mode, after the uniform charging process by the first charging device 1 in the rotation process of the photosensitive drum 11, first, image information about the first color. An electrostatic latent image is formed on the photosensitive drum 11 by optical writing according to the above, and a visible image is processed by toner (black toner) supplied from the developing roller 3a of the first developing device 3 to form a toner image. The

In the case of the two-color printing mode in which the image information of the second color is present, after the uniform charging process by the second charging device 4, the second writing device 5 performs optical writing according to the image information of the second color. An electrostatic latent image of the second color is formed on the photosensitive drum 11 and a visible image is processed by the second color toner (color toner, for example, red toner) supplied from the developing roller 6a of the second developing device 6. A second color toner image is formed.
In the case of the single-color printing mode in which there is no image information for the second color, there is no latent image forming process on the photosensitive drum 11 by writing by the second writing device 5, and a developing roller that can contact and separate from the photosensitive drum 11 6 a is in a state separated from the photosensitive drum 11.

  The toner image formed on the photosensitive drum 11 by these processes is electrostatically transferred to the transfer material S conveyed by the transfer belt 7a through the registration roller 14, and then the fixing device shown in FIG. By being conveyed to 28 and fixed, an image output product is obtained.

  In the transfer device 7, one of the extended surfaces of the transfer belt 7 a that can be conveyed while carrying the transfer material S is in contact with the photosensitive drum 11, and the region where the transfer belt 7 a is in contact with the photosensitive drum 11 is The transfer nip portion NP moves the transfer material S while pinching it. The toner image carried on the photosensitive drum 11 is electrostatically transferred to the transfer material S at the transfer nip portion NP.

  Here, electrostatic transfer will be described. As a configuration for transferring the toner image carried on the photosensitive drum 11 to the transfer material S, the transfer belt 7a is disposed so as to be in contact with the back surface of the transfer belt 7a at a position where the transfer belt 7a has passed through the transfer nip NP. A bias roller 13 which is a bias means is used.

The bias roller 7a is positioned in front of the transfer nip portion NP in the moving direction of the transfer belt 7a, thereby transferring a charge sufficient to electrostatically attract toner until the transfer material S reaches the transfer nip portion NP. The belt 7a is induced. For this reason, the transfer material S that has reached the transfer nip NP is charged to a polarity opposite to that of the toner by dielectric polarization generated between the transfer belt 7a and the toner on the photosensitive drum 11 side is electrostatically transferred. The
The transfer belt 7a that has passed through the transfer nip NP is cleaned of the toner adhering to the surface thereof by a transfer belt cleaning device (not shown) to prevent the transfer material S from being stained on the back.

  A cleaning device 9 that cleans residual toner after transfer of the photosensitive drum 11 is cleaned by a cleaning brush roller 9a or (and) a cleaning blade 9b into a unit (not shown) having an opening facing the photosensitive drum 11. to recover.

  As described above, the transfer material S that has received the transfer of the toner image from the photosensitive drum 11 has the tip of the separation claw 12 disposed at a position where the peripheral surface of the photosensitive drum 11 passes through the transfer nip NP. When it adheres to the peripheral surface of the body drum 11, it is peeled off from the photoreceptor drum 11 by the separation claw 12, and the direction of movement is changed to the transfer belt 7a side, and the rotation of the transfer belt 7a is shown in FIG. Then, it is conveyed toward the fixing device 28.

  In the present embodiment, the second charging device 4 for forming the second color image at the position after the first developing device 3 and before reaching the transfer nip NP in the rotation direction of the photosensitive drum 11 indicated by an arrow, A second writing device 5 and a second developing device 6 are provided.

In the monochrome printing mode, the second developing device 6 is in a state of being separated from the photoreceptor 11 because there is no need for toner image formation. The second writing device 5 does not function (it is not used for latent image formation) because latent image formation is not necessary in the prior art. In the present invention, the second writing device 5 is exposed to light before transfer by exposure of the second writing device 5. The surface potential of the body drum 11, particularly the surface potential at a portion corresponding to the non-image portion (the front end non-image portion of the transfer material or the rear end non-image portion of the transfer material) is uniformly lowered, and the photosensitive drum 11 and the transfer material It has a function as pre-transfer exposure means for weakening the electrostatic adhesion force of S and separating the transfer material from the photosensitive drum 11 before transfer.
The exposure conditions of the second writing device 5, such as the exposure output and the exposure frequency, are controlled by the control unit 100, for example, according to the type of transfer material, the image forming mode, or a combination thereof.

  In this embodiment, by controlling the pre-transfer exposure and the transfer bias application timing for lowering the charged potential of the photoconductor before transfer, the adhesion of the transfer material S to the photoconductor drum 11 is suppressed. Further, it is characterized in that the adhesion of the transfer material S to the transfer belt 7a is promoted.

  In FIG. 3, reference numeral 100 denotes a control unit provided for executing the image forming sequence program. As a configuration related to the present embodiment, the control unit 100 includes a registration motor M arranged at a position where the transfer material S can be fed toward the transfer nip NP via an I / O interface (not shown). Information from a start sensor 101 that detects a start state and an operation panel 102 for selecting an image forming mode and a transfer material size is input to the control device 100. On the output side of the control unit 100, a transfer bias driving circuit 103 and a classification sensor for detecting the type of transfer material fed out by detecting an ON signal of a feeding roller provided on the sheet feeding cassette outlet side of the tray paper feeding unit 24. 104 is connected. In addition, drive systems for various devices around the photosensitive drum are connected. As for the classification sensor 104, a sheet feeding cassette selection switch or the like can be used instead. However, in the case of the selection switch, the type of transfer material accommodated in the paper feed cassette, for example, the type of plain paper, OA paper, film, OHP, or the like is associated in advance as the paper feed cassette information. It is necessary.

  Further, the controller 100 is supplied with writing light corresponding to the image data when forming a latent image on the photosensitive drum, in this case, a pixel count value corresponding to the image data obtained by the light beam scanning. The pixel count value is used to determine the non-image area size from the front end of the transfer material and set the pre-transfer exposure timing (on / off timing).

Next, the timing at which these operations are turned on / off by the image forming sequence program will be described with reference to the timing chart of FIG.
Simultaneously with the main motor driving to start the operation, charging of the photosensitive drum 11 from the first charging device 1 is started.
Next, the registration roller 14 starts to rotate by driving the registration motor M, and before the transfer by the second writing device 5 uniformly from the surface of the photosensitive drum 11 facing the front end of the transfer material S that is fed out. The exposure is started, and when the position on the photosensitive drum 11 corresponding to the end of the leading end non-image portion in the transfer material S reaches the optical path 5a of the exposure light, the pre-transfer exposure by the second writing device 5 is finished. (The start timing of this pre-transfer exposure is indicated by a section P2 that can be the exposure start timing by the second writing device 5). The pre-transfer exposure is performed in order to improve the separation of the transfer material S from the photosensitive member.

The first writing device 1 performs exposure based on the image information within a range corresponding to the effective image area on the photosensitive drum 11.
The exposure by the first writing device 1 within the range corresponding to the effective image area on the photosensitive drum 11 is completed, and the photosensitive drum 11 corresponding to the rear end non-image portion in the transfer material S is moved by the second writing device 5. When the exposure light reaches the optical path 5a, exposure by the second writing device 5 is started again. When the position on the photosensitive drum 11 facing the rear end of the transfer material S reaches at least the position of the optical path 5a, the second writing is performed. The exposure by the apparatus 5 is ended (a section that can be the end timing of the exposure by the second writing apparatus 5 is indicated by a symbol P4 in FIG. 3). This exposure is performed to prevent the rear end behavior fluctuation of the transfer material.

In a section between the start of exposure by the second writing device 5 and before the contact between the photosensitive drum 11 and the front end of the transfer material (any timing within the section P1 that can be the start timing of the front end transfer bias application in FIG. 3). At an arbitrary timing, application of a transfer application bias equal to or below the transfer application bias output at the time of image portion transfer is started. The leading edge transfer bias value Ta is applied to the transfer belt 7 a via the bias roller 13.
Regarding the start timing of the normal transfer bias, the photosensitive drum is at an arbitrary timing after the leading edge of the transfer material comes into contact with the photosensitive drum 11 and until the trailing edge of the leading end non-image portion reaches the transfer nip NP. 11 is applied with an image portion transfer application bias value Tb, which is a normal transfer bias for transferring the toner image on the transfer material to the transfer material. As a result, the effective image area following the rear end of the leading non-image portion is transferred with the bias value Tb.

In this way, the transfer bias of different values such as Ta and then Tb is applied step by step as a bias whose value changes stepwise, and the charge to the transfer material conveying means (transfer belt 7a) is applied. It is possible to prevent the transfer material S from being charged by suppressing an abnormal increase in the amount, and in particular, to suppress the charge at the front end of the transfer material that is likely to adhere to the image carrier (photosensitive drum 11), and the transfer means and the transfer material are electrically connected. Without being related to each other, it is possible to promote the adsorption of the transfer material to the conveying means (transfer belt 7a) side by dielectric polarization.
Thereafter, the registration motor M is stopped when the transfer material feed from the registration roller 14 is completed.

Further, on the rear end side of the transfer material, the image portion transfer applied bias value Tb is switched from the image portion transfer applied bias value Tb to the applied bias of the rear end transfer bias (bias value is equivalent to Ta) at any timing when the leading end of the non-image portion passes the transfer nip NP. . When there is no subsequent transfer material, when the rear end of the transfer material (the rear end of the non-image portion at the rear end) is separated from the photosensitive drum 11, the application of the rear end transfer bias is terminated. In the example of FIG. 3, since there is a “subsequent transfer material”, the trailing edge transfer bias of the “preceding transfer material” (= the leading edge transfer bias of the subsequent transfer material) continues. The bias is applied in the same procedure as the bias application in the “preceding transfer material”.
By applying the transfer bias stepwise in this way, it is possible to promote the adsorption of the transfer material S to the transfer belt 7a side for the same reason as described above.

The rear end side of the transfer material is output at the time of image portion transfer from the time after the start of exposure by the second writing device 5 for separating the rear end of the transfer material and before the separation of the photosensitive drum 11 and the rear end of the transfer material. A rear end transfer bias equal to or less than the transfer application bias value Tb is applied to the transfer belt 7a through the bias roller 13 until at least the end of the rear end non-image portion reaches the transfer nip NP.
In FIG. 3, the output of the transfer bias is transferred from the image portion transfer applied bias value Tb at the time of image transfer at the timing when the end of the effective image area of the transfer material (the front end of the non-image portion at the rear end) passes through the transfer nip portion NP. The rear end transfer bias is switched to the same value as the bias application bias value Ta. The section P3 that can be the application start timing of the trailing edge transfer bias is after at least the end of the effective image area as shown in FIG.
The front end transfer bias value and the rear end transfer bias value at this time may be the same value or different values as long as they are equal to or less than the image portion transfer application bias value. FIG. 3 shows the case where the front end transfer bias and the rear end transfer bias have the same value.
This operation is repeatedly executed during continuous printing.
When there is no subsequent transfer material, the application of the rear end transfer bias is terminated when the rear end (rear end non-image portion) of the transfer material passes through the transfer nip NP and the transfer material S is separated from the photosensitive drum 11. To do. In the example of FIG. 3, since there is a succeeding transfer material, the trailing edge transfer bias of the preceding transfer material (= the leading edge transfer bias of the succeeding transfer material) continues, and thereafter, in the same manner as the bias application in the preceding transfer material. A bias is applied. In this way, bias application with stepwise values is applied on the front end side and the rear end side of the transfer material.

  Light emission start timing of the second writing device 5 (within the interval P2), leading edge transfer bias application start timing (within the interval P1), trailing edge transfer bias application start timing (within the interval P3), and light emission end of the second writing device 5 The timing (in the section P4) can be arbitrarily set and controlled according to the type of transfer material and / or the image forming mode.

  With respect to the timing setting for the start and end of application of the transfer bias Ta, Tb and the like given to the transfer belt 7a via the bias roller 13 by the transfer bias driving circuit 103, the front end of the transfer material contacts the photosensitive drum 11. The start time, the time when the trailing edge of the transfer material ends contact from the photosensitive drum 11, the time when the trailing edge of the leading non-image portion of the transfer material reaches the transfer nip NP, and the like are transferred by starting the registration roller 14. An appropriate bias application according to the transfer material can be performed by specifying the transfer material in consideration of the length of the transfer material in the moving direction on the basis of the time when the material S is fed.

  In the present embodiment, the time when the leading edge of the transfer material comes into contact with the photosensitive drum 11 is specified from the “on” timing of the registration motor M, the exposure start / end of the second writing device 5 and the transfer bias timing are specified. Yes. However, the present invention is not limited to this. For example, the point in time when the leading edge of the transfer material contacts the photosensitive drum may be specified from the signal of a paper jam detection sensor that is usually near the registration roller. Use of such a sensor output has an advantage that control can be performed with higher accuracy than the registration motor M because the sensor position passing point of the actual transfer material tip position can be known. The end of exposure of the second writing device 5 on the rear end side of the transfer material and the transfer bias timing may be performed based on the resist motor off timing, and “on” with better accuracy than when judging only from the on timing. , "Off" can be done.

  For example, when the transfer material is stiff paper, the amount of exposure before transfer may be small because the separation from the photosensitive drum 11 is good in the first place. In such a case, energy for useless exposure can be saved by reducing the exposure output or intermittently blinking the exposure.

  In the case where the exposure is intermittently blinked, as the second writing device 5, a writing device using a laser beam whose energy can be easily controlled is effective. For example, in the case of an image forming mode such as double-sided copying or composite image formation, the resistance value increases due to a decrease in the moisture content of the transfer material in the fixing process, so that the separation from the photoreceptor is good. In such a case, energy for useless exposure can be saved by reducing the exposure output or intermittently blinking the exposure.

  Although the image forming apparatus capable of two-color printing has been described above, the present invention is not limited to this form. That is, the present invention can also be applied to a four-color full-color image forming apparatus. In that case, it is necessary to arrange developing devices and writing devices for four colors around the photosensitive drum, and it is necessary to be able to select a single color mode or a full color mode.

That is, a single color mode for forming a single color image using only one color from magenta, cyan, yellow or black toner and a full color for forming a full color image by mixing four colors of magenta, cyan, yellow and black toners. It is necessary that the user can select these modes as appropriate.
In the single color mode, a writing device arranged at a position closest to the transfer device can be used as a pre-transfer exposure device.

  Further, in the image forming apparatus capable of two-color printing of the present embodiment, the apparatus is integrated around the photosensitive drum 11 (for example, the first color charging device 1, the first color writing device 2, the first color development). Device 3, second color charging device 4, second color writing device 5, second color developing device 6, P sensor 8, cleaning device 9, static eliminator 10, etc.) are arranged on the transfer device. The characteristics of the present invention can also be exhibited in a four-color full-color image forming apparatus.

As described above, according to the configurations described in (1) and (2) described in the section “Means for Solving the Problems”, the separation property of the transfer material from the photoconductor is improved, and a new pre-transfer is performed. There is no need to provide an exposure apparatus, and cost reduction and space saving can be achieved.
According to the configurations described in (3) to (5), it is possible to set or control the pre-transfer exposure output to an appropriate value according to the type of transfer material and the image forming mode, thereby saving unnecessary exposure energy. be able to.
According to the configuration described in (6) to (11), by setting a bias application timing using a pre-transfer exposure and a bias application start at a predetermined time by the transfer bias unit, particularly using at least a stepwise bias application state, An abnormal increase in the amount of charge with respect to the transfer material transporting means can be suppressed, and charging of the transfer material can be prevented. Thus, by applying the transfer bias stepwise, the charge amount with respect to the transfer material transporting means can be reduced. It is possible to prevent the transfer material from being charged by suppressing an abnormal increase, and to suppress the charge at the tip that tends to adhere to the image carrier, and to prevent dielectric repulsion without having a repulsive relationship between the transfer member and the transfer material. Thus, it is possible to promote the adsorption of the transfer material to the conveyance unit (belt unit) material side.
Further, the transfer bias means starts biasing by setting a value lower than the transfer bias value for the image portion for the front end portion of the transfer material in the moving direction, particularly the non-image portion corresponding to the portion, By reducing the charge density at the tip on the side, it becomes possible to suppress charging at the tip where the non-image part is located and avoid a repulsion state with the conveying member, and the tip is a latent image. It is possible to prevent adhesion to the carrier side.

1 is a schematic configuration diagram illustrating an example of an image forming apparatus. 1 is a schematic configuration diagram illustrating a configuration of a main part of an image forming apparatus. It is a timing chart.

Explanation of symbols

2 First color writing device (first writing device)
5 Second color writing device (second writing device)
7 Transfer Device 7a Transfer Belt 11 Photosensitive Drum

Claims (11)

An image carrier for carrying a toner image, a plurality of latent image forming means for forming a latent image on the image carrier, a developing means for developing the latent image, and a transfer means for transferring the toner image to a transfer material In an image forming apparatus capable of selecting a mode for forming a multi-color image and a mode for forming a single-color image,
In the mode to form a monochrome image,
An image forming apparatus comprising: a latent image forming unit that is not used for forming a latent image among the latent image forming units as a pre-transfer exposure unit for separating a transfer material from an image carrier before transfer.   2. The image forming apparatus according to claim 1, wherein the latent image forming means not used for forming the latent image exposes positions corresponding to a front end portion and a rear end portion of the transfer material at the time of pre-transfer exposure. Forming equipment.   3. The image forming apparatus according to claim 1, wherein an exposure condition of the pre-transfer exposure unit is controlled according to a type of a transfer material, an image forming mode, or a combination thereof. Forming equipment.   4. The image forming apparatus according to claim 3, wherein the exposure condition is an exposure output.   4. The image forming apparatus according to claim 3, wherein the exposure condition is an exposure frequency.   6. The image forming apparatus according to claim 1, wherein the transfer unit is configured to start the exposure after the pre-transfer exposure unit and before the leading edge of the transfer material starts to contact the image carrier. Bias application is started by setting a stepped value during this period, and at least a stepped value is set and biased at a predetermined time before the rear end of the transfer material ends contact with the image carrier. An image forming apparatus, wherein application is completed.   The image forming apparatus according to claim 6, wherein the transfer unit includes the transfer material after the front end of the transfer material comes into contact with the image carrier and until the rear end of the front non-image portion of the transfer material reaches the transfer nip portion. An image forming apparatus characterized by applying a bias (normal transfer bias) for transferring the toner image on the image bearing member to the transfer material during the interval.   7. The image forming apparatus according to claim 6, wherein the transfer means starts contact of the leading edge of the transfer material with the latent image carrier on the basis of a time point when the transfer material is fed toward the latent image carrier. An image forming apparatus comprising: identifying a time point to be performed, and setting a bias whose value changes stepwise at least when the transfer material moves a predetermined distance after the time point.   9. The image forming apparatus according to claim 6, wherein the transfer unit takes into account a moving direction length of the transfer material on the basis of a time point when the transfer material is fed toward the image carrier. Identifying a time point at which the trailing edge of the transfer material ends contact with the image carrier, and setting a bias whose value changes stepwise at least in a predetermined time before the time point and ending bias application. An image forming apparatus.   10. The image forming apparatus according to claim 6, wherein the transfer bias unit applies a bias with a bias value lower than a normal transfer bias applied to the front end side in the moving direction of the transfer material. 10. After that, the bias is applied using the bias value at the normal transfer bias, and then the bias is applied to the rear end side in the moving direction of the transfer material with a bias value lower than that at the normal transfer bias. An image forming apparatus.   11. The image forming apparatus according to claim 6, wherein the transfer bias unit has a lower bias than that when a normal transfer bias is applied to a non-image portion on the leading end side in the moving direction of the transfer material. After applying a bias according to the value, a bias is applied using a bias value at a normal transfer bias with reference to a point in time when the leading edge of the image portion faces the image carrier, and then the rear end side in the moving direction of the transfer material An image forming apparatus characterized in that bias application is performed with a bias value lower than that at the time of normal transfer bias application for a non-image portion in the above.

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