JP2019053175A - Image forming apparatus and transfer control method - Google Patents

Abstract

To provide an image forming apparatus and a transfer control method which can suppress the occurrence of separation failure of an image carrier and a sheet.SOLUTION: An image forming apparatus comprises: an image carrier for carrying a toner image; a transfer part for transferring the toner image on the image carrier to a sheet; a transfer control part for controlling the transfer part so as to apply to the sheet either a transfer bias for transferring the toner image to the sheet or a non-transfer bias for not transferring the toner image to the sheet; and a non-transfer area detection part for detecting a length of a non-transfer area to which the non-transfer bias is applied on the sheet. The transfer control part controls the transfer part in such a manner that: switching from the non-transfer bias to the transfer bias is performed at such a setting timing that the length of the non-transfer area becomes a predetermined target value; and at least one of the setting timing or the magnitude of the non-transfer bias is changed according to a difference between the length of the non-transfer area detected by the non-transfer area detection part and the target value.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an image forming apparatus and a transfer control method.

  In general, an image forming apparatus (printer, copying machine, facsimile, etc.) using an electrophotographic process technology irradiates (exposes) a charged photosensitive drum (image carrier) with laser light based on image data. To form an electrostatic latent image. Then, by supplying toner from the developing device to the photosensitive drum on which the electrostatic latent image is formed, the electrostatic latent image is visualized to form a toner image. Further, after the toner image is directly or indirectly transferred to the sheet, the toner image is formed on the sheet by fixing it by heating and pressing at the fixing nip.

  In such an image forming apparatus, when a toner image is transferred from an image carrier such as a photosensitive drum to a sheet, a transfer bias is applied to a transfer area portion that is an image area of the sheet at a transfer nip. The toner image is transferred to the paper. However, if the portion of the non-transfer area at the leading edge of the paper is charged with the same polarity as the transfer bias, the portion is attracted to the image carrier, and separation failure between the image carrier and the paper occurs.

  For this reason, a technique is known that improves the separation between the image carrier and the paper by applying a bias having a polarity opposite to the transfer bias to the non-transfer area at the leading edge of the paper (see, for example, Patent Document 1). )

JP-A-11-174860

  By the way, the reverse charge length, which is the length of the portion of the paper that is charged with the reverse polarity, is set so as to be adjusted to a predetermined target value by a specific paper at the time of shipment of the image forming apparatus, for example.

  However, since various types of paper are used in the market, the transfer electric field on the paper when a transfer bias is applied to the paper due to the resistance value, thickness, capacity, and surrounding environmental conditions of the paper. The spread of fluctuates. Further, the way in which the transfer electric field spreads on the paper also varies depending on the resistance value of a transfer member such as a transfer belt.

  As a result, the reverse charge length of the paper may be shorter than the target value, which may result in poor separation between the image carrier and the paper.

  An object of the present invention is to provide an image forming apparatus and a transfer control method capable of suppressing the occurrence of poor separation between an image carrier and a sheet.

An image forming apparatus according to the present invention includes:
An image carrier for carrying a toner image;
A transfer unit for transferring the toner image on the image carrier to a sheet;
A transfer control unit that controls the transfer unit so as to apply a transfer bias that transfers the toner image to the paper and a non-transfer bias that does not transfer the toner image to the paper;
A non-transfer area detecting unit for detecting a length of a non-transfer area to which the non-transfer bias is applied in the paper;
With
The transfer control unit
Switching from the non-transfer bias to the transfer bias at a set timing at which the length of the non-transfer area becomes a predetermined target value,
The transfer unit is controlled to change at least one of the setting timing and the absolute value of the non-transfer bias according to the difference between the length of the non-transfer region detected by the non-transfer region detection unit and the target value. To do.

The transfer control method according to the present invention includes:
A transfer control method for an image forming apparatus, comprising: an image carrier that carries a toner image; and a transfer unit that transfers the toner image on the image carrier to a sheet.
Controlling the transfer unit to apply to the paper either a transfer bias for transferring the toner image to the paper or a non-transfer bias for not transferring the toner image to the paper;
Detecting the length of the non-transfer area to which the non-transfer bias is applied in the paper;
Switching from the non-transfer bias to the transfer bias at a set timing at which the length of the non-transfer area becomes a predetermined target value,
The transfer unit is controlled so as to change at least one of the set timing and the absolute value of the non-transfer bias according to the detected difference between the length of the non-transfer area and the target value.

  According to the present invention, it is possible to suppress the occurrence of poor separation between the image carrier and the paper.

1 is a diagram schematically illustrating an overall configuration of an image forming apparatus according to an exemplary embodiment. FIG. 3 is a diagram illustrating a main part of a control system of the image forming apparatus according to the present embodiment. It is a figure which shows the example of the conventional control of the transfer bias with respect to time. It is a figure which shows the detection result by the electric potential detection part with respect to time in FIG. 3A. It is a figure which shows the detection result by the electric potential detection part with respect to time in FIG. 3A. It is a figure which shows the separation jam rate with respect to reverse charging length. It is a figure which shows the example of control of the transfer bias with respect to time. It is a figure which shows the detection result by the electric potential detection part with respect to time in FIG. It is a figure which shows the example of control of the transfer bias with respect to time. It is a figure which shows the detection result by the electric potential detection part with respect to time in FIG. 6 is a flowchart illustrating an example of an operation example when executing transfer control in the image forming apparatus. 10 is a flowchart illustrating an example of an operation example when executing transfer control in an image forming apparatus according to a modification.

  Hereinafter, the present embodiment will be described in detail with reference to the drawings. FIG. 1 is a diagram schematically showing an overall configuration of an image forming apparatus 1 according to the present embodiment. FIG. 2 is a diagram illustrating a main part of a control system of the image forming apparatus 1 according to the present embodiment.

  An image forming apparatus 1 shown in FIG. 1 and FIG. 2 is a direct transfer type monochrome image forming apparatus using an electrophotographic process technology. In other words, the image forming apparatus 1 forms an image by directly transferring a K component (black) toner image formed on the photosensitive drum 213 to a sheet.

  The image forming apparatus 1 includes an image reading unit 11, an operation display unit 12, an image processing unit 13, an image forming unit 20, a paper feeding unit 14, a paper transporting unit 16, and a control unit 101.

  The control unit 101 includes a CPU (Central Processing Unit) 102, a ROM (Read Only Memory) 103, a RAM (Random Access Memory) 104, and the like. The CPU 102 reads out a program corresponding to the processing content from the ROM 103 or the storage unit 182, develops the program in the RAM 104, and centrally controls the operation of each block of the image forming apparatus 1 in cooperation with the developed program.

  The communication unit 181 includes various interfaces such as a network interface card (NIC), a modem-demodulator (MODEM), and a universal serial bus (USB).

  The storage unit 182 includes, for example, a nonvolatile semiconductor memory (so-called flash memory) or a hard disk drive. The storage unit 182 stores, for example, a lookup table that is referred to when controlling the operation of each block.

  The control unit 101 transmits and receives various data to and from an external device (for example, a personal computer) connected to a communication network such as a LAN (Local Area Network) or a WAN (Wide Area Network) via the communication unit 181. Do. For example, the control unit 101 receives image data (input image data) in a page description language (PDL) transmitted from an external device, and forms an image on a sheet based on the image data.

  The image reading unit 11 includes an automatic document feeder 111 called an ADF (Auto Document Feeder), a document image scanning device 112 (scanner), and the like. The automatic document feeder 111 conveys the document placed on the document tray by the conveyance mechanism and sends it out to the document image scanning device 112. The document image scanning device 112 optically scans a document conveyed from the automatic document feeder 111, forms an image of reflected light from the document on a light receiving surface of a CCD (Charge Coupled Device) sensor, and forms a document image. read. The image reading unit 11 generates input image data based on the reading result by the document image scanning device 112. The input image data is subjected to predetermined image processing in the image processing unit 13.

  The operation display unit 12 includes, for example, a liquid crystal display (LCD) with a touch panel, and functions as the display unit 121 and the operation unit 122. The display unit 121 displays various operation screens, image status display, and operation status of each function according to a display control signal input from the control unit 101. The operation unit 122 includes various operation keys such as a numeric keypad and a start key, receives various input operations by the user, and outputs an operation signal to the control unit 101.

  The user operates the operation display unit 12 to display images such as document settings, image quality settings, magnification settings, application settings, output settings, single-sided / double-sided settings, and paper settings (including paper basis weight and glossiness). Settings related to formation can be made. The set information is stored in the storage unit 182, for example.

  The image processing unit 13 includes a circuit that performs digital image processing on input image data according to initial settings or user settings. For example, the image processing unit 13 performs gradation correction based on the gradation correction data under the control of the control unit 101. The image processing unit 13 performs various correction processes such as color correction and shading correction on the input image data. The image forming unit 20 is controlled based on the image data subjected to these processes.

  The image forming unit 20 forms a toner image with a K component toner based on the input image data, and a transfer unit 22 that transfers the toner image formed by the toner image forming unit 21 to a sheet. And a fixing unit 23 for fixing the toner image transferred onto the paper.

  The toner image forming unit 21 includes an exposure device 211, a charging device 212, a photosensitive drum 213 as an example of an image carrier, a developing device 214, a drum cleaning device 215, a first toner image detection unit 216, and the like.

  The photosensitive drum 213 has, for example, an undercoat layer (UCL), a charge generation layer (CGL), a charge transport layer (CGL) on the peripheral surface of an aluminum conductive cylinder (aluminum tube). It is a negatively charged organic photoreceptor (OPC: Organic Photo-conductor) in which CTL (Charge Transport Layer) is sequentially laminated.

  The charging device 212 is constituted by a corona discharge generator such as a scorotron charging device or a corotron charging device. The charging device 212 uniformly charges the surface of the photosensitive drum 213 to a negative polarity by corona discharge.

  The exposure device 211 irradiates the photosensitive drum 213 with light corresponding to a monochrome image. The positive charge generated in the charge generation layer of the photosensitive drum 213 upon receiving light irradiation is transported to the surface of the charge transport layer, whereby the surface charge (negative charge) of the photosensitive drum 213 is neutralized. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 213 due to a potential difference from the surroundings.

  The developing device 214 contains a K-component developer (for example, a two-component developer composed of toner and a magnetic carrier). The developing device 214 visualizes the electrostatic latent image to form a toner image by attaching a K component toner to the surface of the photosensitive drum 213.

  The drum cleaning device 215 includes a drum cleaning blade that is slidably contacted with the surface of the photosensitive drum 213 and removes transfer residual toner remaining on the surface of the photosensitive drum 213 after transfer.

  The first toner image detection unit 216 is disposed to face the photosensitive drum 213. The first toner image detection unit 216 is an optical sensor, for example, and detects a toner image on the photosensitive drum 213.

  The transfer unit 22 includes a transfer belt 221, a transfer roller 222, a plurality of support rollers 223, a potential detection unit 224, a second toner image detection unit 225, and the like.

  The transfer belt 221 is an endless belt, and is stretched around a plurality of support rollers 223 in a loop shape. At least one of the plurality of support rollers 223 is configured by a driving roller, and the other is configured by a driven roller. As the driving roller rotates, the transfer belt 221 travels and the paper is conveyed at a constant speed.

  The transfer roller 222 is disposed on the inner peripheral surface side of the transfer belt 221 so as to face the photosensitive drum 213. The transfer roller 222 is pressed against the photosensitive drum 213 with the transfer belt 221 interposed therebetween, thereby forming a transfer nip for transferring the toner image from the photosensitive drum 213 to the sheet.

  When the paper passes through the transfer nip, the toner image on the photosensitive drum 213 is transferred to the paper. Specifically, the transfer output (transfer bias) is applied to the transfer roller 222, and a charge (positive charge) having a polarity opposite to that of the toner is applied to the back side of the paper (the side in contact with the transfer belt 221). The image is electrostatically transferred to the paper. The sheet on which the toner image is transferred is conveyed toward the fixing unit 23.

  The potential detection unit 224 is opposed to the transfer belt 221 on the downstream side of the transfer nip, and detects the potential of the sheet conveyed on the transfer belt 221.

  The second toner image detection unit 225 faces the transfer belt 221 on the downstream side of the transfer nip. The second toner image detection unit 225 is an optical sensor, for example, and detects a toner image on a sheet conveyed on the transfer belt 221.

  The fixing unit 23 is arranged on the upper fixing unit 231 composed of a fixing roller arranged on the fixing surface (surface on which the toner image is formed) side of the paper, and on the back surface (surface opposite to the fixing surface) side of the paper. A lower fixing unit 232 composed of a pressure roller and a heating source 233 for heating the upper fixing unit 231.

  The sheet on which the toner image has been transferred and conveyed along the sheet passing path is heated and pressurized when passing through the fixing unit 23. As a result, the toner image is fixed on the paper.

  The paper feed unit 14 includes a paper feed tray unit 141. In the paper feed tray unit 141, sheets (standard paper, special paper) identified based on basis weight, size, and the like are stored for each preset paper type. A plurality of paper feed roller units are disposed in the paper feed tray unit 141. The paper feed unit 14 feeds the paper fed from the paper feed tray unit 141 to the paper transport unit 16.

  The paper discharge unit 15 includes a paper discharge roller unit 151 and the like, and discharges the paper sent from the paper transport unit 16 to the outside of the apparatus. The paper transport unit 16 includes a main transport unit 161 and the like.

  The main transport unit 161 transports the paper fed from the paper feed tray unit 141 and passes the paper to the image forming unit 20 (transfer unit 22, fixing unit 23) and from the image forming unit 20 (fixing unit 23). The sent paper is conveyed toward the paper discharge unit 15.

  The paper fed from the paper feeding unit 14 is conveyed to the image forming unit 20 by the main conveying unit 161. Then, when the sheet passes through the transfer nip, the toner image on the photosensitive drum 213 is collectively transferred to the first surface (front surface) of the sheet, and the fixing unit 23 performs a fixing process. The paper on which the image is formed is discharged out of the apparatus by the paper discharge unit 15.

  In such an image forming apparatus 1, when a toner image is transferred from an image carrier such as the photosensitive drum 213 to a sheet, a transfer bias is applied to a transfer area which is an image area of the sheet at the transfer nip. Thus, the toner image is transferred to the paper. However, if the portion of the non-transfer area at the leading edge of the paper is charged with the same polarity as the transfer bias, the portion is attracted to the photosensitive drum 213, and separation failure between the photosensitive drum 213 and the paper occurs. .

  Therefore, in this embodiment, the non-transfer bias having the opposite polarity to the transfer bias is applied to the non-transfer area at the leading end of the paper, thereby improving the separation between the photosensitive drum 213 and the paper. The non-transfer bias is a bias having the same polarity as the toner image, and is a bias that does not transfer the toner image to the paper. In this embodiment, the negative voltage shown in FIG. 3A and the like is exemplified as the non-transfer bias.

  Specifically, as shown in FIG. 3A, transfer is performed so that a non-transfer bias, that is, a negative voltage is applied to the transfer nip portion at time t1 when the leading edge of the paper passes through the transfer nip portion. The roller 222 is controlled. Then, at the time t2 when the transfer area of the paper reaches the transfer nip, the transfer roller 222 is controlled so as to switch from a non-transfer bias to a transfer bias, that is, a positive voltage and apply it to the transfer nip portion. .

  As a result, the potential detected on the paper in the potential detector 224 shows a time change as shown in FIG. 3B. As shown in FIG. 3B, the period from time t1 to time t21 when the detected potential on the paper rises from the negative voltage (non-transfer bias), reaches 0, and switches to the positive voltage (transfer bias). The reverse charge length on the paper is calculated based on the time L1.

  Such a reverse charging length is the length of a non-transfer area to which a non-transfer bias is applied in the paper. For example, when the image forming apparatus 1 is shipped, the reverse charging length is adjusted with a specific sheet and is set to a length near a predetermined target value. Accordingly, the control unit 101 performs control to switch from the non-transfer bias to the transfer bias at the set timing when the reverse charge length becomes the target value, thereby suppressing the separation failure between the photosensitive drum 213 and the paper.

  By the way, since various types of paper are used in the market, the transfer electric field on the paper when a transfer bias is applied to the paper due to the resistance value, thickness, capacity, and surrounding environmental conditions of the paper. The spread of fluctuates. Further, the spread of the transfer electric field on the paper also varies depending on the resistance value of the transfer belt 221.

  If the transfer electric field spreads on the paper, the reverse charge length of the paper also changes. For example, a case where a sheet whose resistance value is lower than the resistance value of a specific sheet used for adjusting the reverse charging length at the time of shipment of the image forming apparatus 1 will be described. In this case, when a transfer bias as shown in FIG. 3A is applied, a negative charge is charged in the non-transfer area and a positive charge is charged in the transfer area in the paper. The positive charge is likely to move to the negative charge side. Therefore, the negative charge amount in the portion where the negative charge of the paper is charged is reduced.

  Specifically, the detected potential on the sheet in the potential detecting unit 224 shows a time change as shown in FIG. 3C. In this case, the time L2 from the time t1 to the time t3 when the detected potential on the sheet rises from the negative voltage and reaches 0 is smaller than the negative charge amount in FIG. 3B. 3 becomes shorter than the time L1 shown in FIG. 3B. That is, the reverse charging length of the paper is shorter than a predetermined target value.

  Such a phenomenon that the reverse charging length of the paper becomes shorter than a predetermined target value causes the resistance value of the transfer belt 221 to be small, the humidity around the image forming apparatus 1 to be low, and the thickness of the paper to be thin. It can happen in some cases.

  If the reverse charging length of the paper becomes shorter than the target value, the non-transfer area portion is likely to be attracted to the photosensitive drum 213, which may cause a separation failure between the photosensitive drum 213 and the paper. Further, as shown in FIG. 4, it has been experimentally confirmed that the separation jam rate increases as the reverse charging length becomes smaller than the ideal value A. The ideal value A is, for example, a reverse charge length when no separation failure occurs and a toner image is appropriately formed in the transfer area.

  Therefore, in the present embodiment, the control unit 101 controls the transfer roller 222 so as to change the setting timing in accordance with the surface potential of the sheet detected by the potential detection unit 224. Specifically, the control unit 101 calculates the reverse charge length based on the surface potential of the paper detected by the potential detection unit 224.

  Then, the control unit 101 controls the transfer roller 222 so as to change the setting timing according to the difference between the calculated reverse charge length and the target value. The control unit 101 corresponds to the “transfer control unit” of the present invention, and the potential detection unit 224 and the control unit 101 correspond to the “non-transfer area detection unit” of the present invention.

  As shown in FIG. 5, when the reverse charging length is less than the target value when the control unit 101 switches from the negative voltage that is the non-transfer bias to the positive voltage that is the transfer bias, The setting timing for switching from a positive voltage to a positive voltage is set later than the setting timing before the change.

  In the case of FIG. 5, the setting timing before the change is time t2, but when the reverse charge length is less than the target value, for example, the setting timing is changed to time t4 later than time t2.

  As shown in FIG. 6, when the setting timing before the change is the time L2 (similar to FIG. 3C) from the time t1 to the time t3 as shown by the broken line, if the setting timing after the change is the time t1 From time t3 to time t5 later than time t3 is detected.

  That is, the time for the non-transfer bias to switch to the transfer bias after the leading edge of the paper passes through the transfer nip can be set to L3 longer than L2. As a result, the reverse charge length can be made longer than the reverse charge length at the set timing before the change, so that it is possible to suppress the occurrence of poor separation between the paper and the photosensitive drum 213.

  Further, as shown in FIG. 7, the control unit 101 may change the absolute value of the non-transfer bias according to the surface potential of the sheet detected by the potential detection unit 224. Specifically, when the reverse charging length is less than the target value, the control unit 101 makes the absolute value of the non-transfer bias (solid line) larger than the absolute value of the non-transfer bias before change (broken line). May be.

  By changing in this way, the charge amount of the negative charge in the non-transfer area can be increased, so that the positive charge moving from the transfer area can be canceled out. That is, as shown in FIG. 8, the time when the voltage becomes 0 can be set to t6 which is slower than t3 in the broken line.

  That is, the time for the non-transfer bias to switch to the transfer bias after the leading edge of the paper passes the transfer nip can be set to L4 longer than L2. As a result, the reverse charge length can be made longer than the reverse charge length before the change, so that it is possible to suppress the occurrence of poor separation between the paper and the photosensitive drum 213.

  The control unit 101 may change both the setting timing and the absolute value of the non-transfer bias according to the difference between the calculated reverse charge length and the target value.

  Further, the control unit 101 may perform control to increase the reverse charge length target value as the humidity around the image forming apparatus 1 increases. Further, the control unit 101 may perform control to increase the reverse charge length target value as the sheet becomes thinner. Further, the control unit 101 may perform control to increase the reverse charge length target value as the paper basis weight decreases.

  By doing so, the reverse charge length can be easily set to an appropriate value in advance, so that occurrence of poor separation can be suppressed in advance.

  If the reverse charge length on the paper becomes too long, the toner image will not be transferred to the paper, so the control unit 101 performs control for adjusting the reverse charge length to the ideal value A in FIG. As a result, an appropriate toner image can be formed in the transfer region while suppressing poor separation. Examples of such control include repetitive control for detecting the reverse charge length.

  Next, an operation example when executing transfer control in the image forming apparatus 1 will be described. FIG. 9 is a flowchart illustrating an example of an operation example when executing transfer control in the image forming apparatus 1. The process in FIG. 9 is executed when the control unit 101 receives an instruction to execute a printing process.

  As illustrated in FIG. 9, the control unit 101 calculates a reverse charging length based on the detection result of the potential detection unit 224 (step S <b> 101). Next, the control unit 101 determines whether or not the calculated reverse charge length is less than the target value (step S102).

  As a result of the determination, if the calculated value of the reverse charge length is not less than the target value (step S102, NO), the process transitions to step S104. On the other hand, when the calculated value of the reverse charging length is less than the target value (step S102, YES), the control unit 101 changes the setting timing for switching from the non-transfer bias to the transfer bias (step S103).

  Next, the control unit 101 determines whether the printing process has been completed (step S104). As a result of the determination, if the printing process has not ended (step S104, NO), the process returns to step S102. On the other hand, when the printing process is finished (step S104, YES), this control is finished.

  According to the present embodiment as described above, the transfer roller 222 is controlled so as to change at least one of the setting timing and the absolute value of the non-transfer bias according to the difference between the reverse charge length and the target value. Specifically, when the reverse charging length is less than the target value, the setting timing is set later than the setting timing before the change. Alternatively, when the reverse charging length is less than the target value, the absolute value of the non-transfer bias is made larger than the absolute value of the non-transfer bias before the change. Thereby, it is possible to suppress the occurrence of poor separation between the photosensitive drum 213 and the paper.

  In addition, by calculating the reverse charge length that is highly related to the separation property between the photosensitive drum 213 and the paper, the non-transfer area can be set to an appropriate condition with high accuracy and efficiency.

Next, an image forming apparatus 1 according to a modification will be described.
In the above embodiment, the reverse charging length is changed according to the detection potential of the potential detection unit 224. However, the control unit 101 according to the modified example has a first toner image detection unit 216 and a second toner image detection unit 225. The reverse charging length is changed in accordance with the detection result. The first toner image detection unit 216 and the control unit 101 or the second toner image detection unit 225 and the control unit 101 correspond to the “non-transfer area detection unit” of the present invention.

  When the first toner image detection unit 216 is used, the control unit 101 calculates the reverse charge length based on the detection result of the toner image that has not been transferred to the paper on the photosensitive drum 213. When the second toner image detection unit 225 is used, the control unit 101 calculates the reverse charge length based on the detection result of the non-transfer area on the paper.

  Further, in order to calculate the reverse charge length in this way, the control unit 101 controls the image forming unit 20 so as to form a patch image on the first sheet. Then, at least one of the setting timing and the absolute value of the non-transfer bias on the second sheet different from the first sheet is changed according to the reverse charge length calculated by transferring the patch image onto the first sheet. To do.

  In this way, when the reverse charge length is less than the target value, the reverse charge length can be increased, so that occurrence of poor separation between the paper and the photosensitive drum 213 can be suppressed. .

  Note that when the first toner image detection unit 216 and the second toner image detection unit 225 are used to perform control to change the reverse charging length of the paper, one sheet of paper is used to form a patch image on the first paper. It is necessary to use extra. Therefore, from the viewpoint of reducing the number of sheets to be used, it is preferable to perform control to change the reverse charging length using the detection result of the potential detection unit 224 as in the above embodiment.

  Next, an operation example when executing the transfer control in the image forming apparatus 1 according to the modification will be described. FIG. 10 is a flowchart illustrating an example of an operation example when executing transfer control in the image forming apparatus 1 according to the modification. The process in FIG. 10 is executed when the control unit 101 receives an instruction to execute a printing process.

  As shown in FIG. 10, the control unit 101 forms a patch image on the first sheet (step S201). Next, the control unit 101 calculates a reverse charge length based on the detection result of the first toner image detection unit 216 or the second toner image detection unit 225 (step S202). Next, the control unit 101 determines whether or not the calculated reverse charge length is less than the target value (step S203).

  As a result of the determination, when the calculated value of the reverse charge length is not less than the target value (step S203, NO), the process transitions to step S205. On the other hand, when the calculated value of the reverse charging length is less than the target value (step S203, YES), the control unit 101 changes the setting timing for switching from the non-transfer bias to the transfer bias (step S204).

  Next, the control unit 101 executes a printing process (step S205). Thereafter, this control ends.

  According to the above modification, it is possible to suppress the occurrence of poor separation between the photosensitive drum 213 and the paper, as in the above-described embodiment.

  In the above-described embodiment, the control for changing the reverse charge length is always performed. However, the present invention is not limited to this. For example, a configuration in which the control is not always performed may be used.

  For example, the control unit 101 changes the sheet type, starts the printing process, starts up the image forming apparatus 1, and the humidity around the image forming apparatus 1 is more than a predetermined humidity from the previous use. Control may be performed to change at least one of the set timing and the absolute value of the non-transfer bias at any time when it fluctuates.

  When the paper type is changed, the resistance value of the paper may change before and after the change, and the reverse charge length is likely to change. Therefore, it is highly necessary to perform the control. Further, when the printing process is started, it is highly necessary to change the reverse charging length at the start from the viewpoint of setting the same image forming conditions (transfer conditions) throughout the printing process.

  In addition, when the image forming apparatus 1 is started, the surrounding humidity conditions and the like may be changed and the reverse charge length is likely to change compared to the time when the image forming apparatus 1 was started last time. High nature. The same applies when the humidity around the image forming apparatus 1 fluctuates by more than a predetermined humidity (for example, 20%) from the previous use.

  In this way, the control can be performed only when necessary, and when the control does not need to be performed, the control is not performed. Can do. In particular, in the modified example, since the first sheet is used when calculating the reverse charge length, the sheet used for forming the patch image can be reduced as compared with the configuration in which the control is always performed.

  Further, the storage unit 182 may store the reverse charging length based on the detection result by the potential detection unit 224 and the paper type in association with each other. By doing so, the control unit 101 reads out the reverse charge length from the storage unit 182 and uses the absolute value of the set timing and the non-transfer bias when the paper on which the reverse charge length has been calculated is used. change. Therefore, it is possible to suppress performing the control wastefully.

  In the above embodiment, the photosensitive drum 213 is exemplified as the image carrier. However, the present invention is not limited to this, and may be another image carrier such as an intermediate transfer belt. .

  In the above embodiment, a monochrome image forming apparatus is exemplified as the image forming apparatus. However, the present invention is not limited to this, and may be a color image forming apparatus.

  In addition, each of the above-described embodiments is merely an example of actualization in carrying out the present invention, and the technical scope of the present invention should not be construed as being limited thereto. That is, the present invention can be implemented in various forms without departing from the gist or the main features thereof.

  Finally, an evaluation experiment of the image forming apparatus 1 according to the present embodiment will be described. The transfer belt 221 used in each of the following experiments was made of a chloroprene rubber substrate, had a thickness of 0.5 mm including the coating layer, and the coating layer was 3 μm thick with a PTFE material. The transfer belt 221 has an initial volume resistance of 9.5 log Ωcm (temperature 20 ° C., humidity 50% RH, when 500 V is applied), and a surface resistance of 10.5 log Ω / □ (temperature 20 ° C., humidity 50% RH, 500 V). At the time of application).

  First, using the image forming apparatus 1 shown in FIG. 1, the environmental conditions are a temperature of 20 ° C. and a humidity of 50% RH, and the paper is measured under a resistance of 10.2 logΩ / □ (temperature of 20 ° C. and humidity of 50% RH). ) Were measured for Examples 1-4 and Comparative Example 1. The target value of the reverse charging length in this experiment is 3 mm. The number of printed sheets is 1000.

  In the first and fourth embodiments, the reverse charge length is detected by the potential detection unit 224, and in the second embodiment, the reverse charge length is detected by the first toner image detection unit 216. Is the one in which the reverse charge length is detected by the second toner image detection unit 225. In the first, second, and third embodiments, the setting timing as shown in FIG. 5 is controlled to be delayed from the setting timing before the change. In the fourth embodiment, the negative polarity as shown in FIG. It is assumed that control is performed so that the absolute value of the voltage is larger than the absolute value of the voltage before the change.

  In the second and third embodiments, the first sheet is formed as a patch image having a certain length from the leading edge of the sheet.

  5 and 7, the negative voltage before change is −20 μA when converted to current, and the positive voltage is 100 μA when converted to current. Moreover, the negative voltage after the change in Example 4 is −40 μA in terms of current. The experimental results are shown in Table 1.

  The “shift amount” in Table 1 is the shift from the reverse charge length at the setting timing before the change to the reverse charge length after the setting timing is changed in the control for delaying the setting timing from the setting timing before the change. Indicates the amount.

  First, confirming Comparative Example 1, the reverse charge length is 1 mm before and after the change, and is 2 mm shorter than the target value of 3 mm. Therefore, the separation jam occurrence rate was 3%.

  On the other hand, in Examples 1 to 3, since the reverse charging length before the change is 1 mm, the reverse charging length after the change is 3 mm by adjusting the shift amount to 2 mm, which is adjusted to the target value. . Therefore, the separation occurrence rate was 0% in all cases. In Example 4, since the changed voltage is changed from −20 μA to −40 μA, the reverse charging length after the change is 3 mm, which is set to the target value. Therefore, the separation occurrence rate was 0%. That is, the effectiveness of the present invention was confirmed.

  Next, the separation jam occurrence rate when the paper type was changed from the conditions in Table 1 was measured for Example 5 and Comparative Example 2. The paper used had a resistance of 10.8 logΩ / □ (measured at a temperature of 20 ° C. and a humidity of 50% RH). In the fifth embodiment, the reverse charge length is detected by the potential detection unit 224, and the setting timing as shown in FIG. 5 is controlled to be delayed from the setting timing before the change. The negative voltage is −20 μA in terms of current, and other experimental conditions are the same as in Table 1. The experimental results are shown in Table 2.

  First, when Comparative Example 2 is confirmed, the reverse charging length is 2 mm before and after the change, which is 1 mm shorter than the target value of 3 mm. Therefore, the separation jam occurrence rate was 2%.

  On the other hand, in Example 5, since the reverse charge length before the change is 2 mm, the reverse charge length after the change is 3 mm by adjusting the shift amount to 1 mm, which is adjusted to the target value. Therefore, the separation jam occurrence rate was 0%. That is, the effectiveness of the present invention was confirmed.

  Finally, the separation jam occurrence rate when the environmental conditions were changed from the conditions in Table 1 was measured for Example 6 and Comparative Example 3. Environmental conditions were a temperature of 30 ° C. and a humidity of 80% RH. In the sixth embodiment, the reverse charge length is detected by the potential detection unit 224, and the setting timing as shown in FIG. 5 is controlled to be delayed from the setting timing before the change. The negative voltage is −20 μA in terms of current, the target value of the reverse charging length is 5 mm, and the other experimental conditions are the same as in Table 1. The experimental results are shown in Table 3.

  First, when Comparative Example 3 is confirmed, the reverse charging length is 3 mm before and after the change, which is 2 mm shorter than the target value of 5 mm. Therefore, the separation jam occurrence rate was 5%.

  On the other hand, in Example 6, since the reverse charge length before the change is 3 mm, the reverse charge length after the change is 5 mm by adjusting the shift amount to 2 mm, which is adjusted to the target value. Therefore, the separation jam occurrence rate was 0%. That is, the effectiveness of the present invention was confirmed.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 22 Transfer part 101 Control part 182 Storage part 213 Photosensitive drum 216 1st toner image detection part 221 Transfer belt 222 Transfer roller 224 Potential detection part 225 2nd toner image detection part

Claims (13)

An image carrier for carrying a toner image;
A transfer unit for transferring the toner image on the image carrier to a sheet;
A transfer control unit that controls the transfer unit so as to apply a transfer bias that transfers the toner image to the paper and a non-transfer bias that does not transfer the toner image to the paper;
A non-transfer area detecting unit for detecting a length of a non-transfer area to which the non-transfer bias is applied in the paper;
With
The transfer control unit
Switching from the non-transfer bias to the transfer bias at a set timing at which the length of the non-transfer area becomes a predetermined target value,
The transfer unit is controlled to change at least one of the setting timing and the absolute value of the non-transfer bias according to the difference between the length of the non-transfer region detected by the non-transfer region detection unit and the target value. To
Image forming apparatus. The transfer control unit, when the length of the non-transfer area detected by the non-transfer area detection unit is less than the target value, delay the setting timing than the setting timing before the change,
The image forming apparatus according to claim 1. When the length of the non-transfer area detected by the non-transfer area detection unit is less than the target value, the transfer control unit calculates the absolute value of the non-transfer bias as the non-transfer bias before the change. Larger than the absolute value,
The image forming apparatus according to claim 1. The non-transfer area detection unit detects a surface potential of the paper and calculates the length of the non-transfer area based on the surface potential.
The image forming apparatus according to claim 1. The non-transfer area detection unit detects the toner image that has not been transferred to the paper on the image carrier, and calculates the length of the non-transfer area based on the toner image.
The image forming apparatus according to claim 1. The non-transfer area detecting unit detects the non-transfer area on the paper and calculates the length of the non-transfer area based on the non-transfer area;
The image forming apparatus according to claim 1. When the patch image is transferred to the first sheet, the transfer control unit is configured to change the length of the non-transfer area detected by the non-transfer area detection unit and the difference between the target value and the first sheet. Change at least one of the setting timing and the absolute value of the non-transfer bias in a different second sheet,
The image forming apparatus according to claim 5. The transfer control unit performs control to increase the target value of the length of the non-transfer area as the humidity around the image forming apparatus increases.
The image forming apparatus according to claim 1. The transfer control unit performs control to increase the target value of the length of the non-transfer area as the sheet becomes thinner.
The image forming apparatus according to claim 1. The transfer control unit performs control to increase the target value of the length of the non-transfer area as the basis weight of the paper decreases.
The image forming apparatus according to claim 1. The transfer control unit is configured to change the sheet type, start a printing process, start the image forming apparatus, and the ambient humidity of the image forming apparatus is equal to or higher than a predetermined humidity from the previous use. When it fluctuates, control to change at least one of the setting timing and the absolute value of the non-transfer bias,
The image forming apparatus according to claim 1. A storage unit that associates and stores the length of the non-transfer area calculated by the non-transfer area detection unit and the type of the paper;
The image forming apparatus according to claim 1. A transfer control method for an image forming apparatus, comprising: an image carrier that carries a toner image; and a transfer unit that transfers the toner image on the image carrier to a sheet.
Controlling the transfer unit to apply to the paper either a transfer bias for transferring the toner image to the paper or a non-transfer bias for not transferring the toner image to the paper;
Detecting the length of the non-transfer area to which the non-transfer bias is applied in the paper;
Switching from the non-transfer bias to the transfer bias at a set timing at which the length of the non-transfer area becomes a predetermined target value,
A transfer control method for controlling the transfer unit so as to change at least one of the set timing and the absolute value of the non-transfer bias according to a difference between the detected length of the non-transfer area and the target value.

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