Image forming apparatus and transfer condition control method

Abstract

To prevent the occurrence of an image quality defect caused by electric discharge at a leading end of a sheet, in performing image formation by superimposing toners with different average particle diameters.SOLUTION: An image forming apparatus 1 comprises: a secondary transfer unit 133 that secondarily transfers toner images to a recording material; and a control unit 16 that changes a transfer condition according to the physical property value of the recording material in image formation in which toner layers using toners with different average particle diameters are superimposed with each other.SELECTED DRAWING: Figure 11

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus and a transfer condition control method. [Background technology]

[0002] Conventionally, electrophotographic image forming apparatuses form electrostatic latent images by irradiating a charged photosensitive drum with laser light based on image data. The image forming apparatus then visualizes the electrostatic latent image by attaching toner to the electrostatic latent image, forming a toner image. When an image is formed in an image forming apparatus by overlapping pulverized white toner and color toner, the image density at the leading edge of the paper can be reduced. This occurs because the leading edge of the paper bends when entering the transfer section, creating a gap between the paper and the toner image on the belt, resulting in discharge. When paper with high resistance or paper with a stiffness above a certain value is passed through the transfer section, the leading edge of the paper is likely to bend and cause discharge. When discharge occurs at the leading edge of the paper, the white toner and color toner mix due to differences in their average particle sizes, resulting in a reduction in image density at the leading edge of the paper.

[0003] To prevent the above-mentioned discharge at the leading edge of the paper, a technology for changing the transfer pressure on the leading edge of the paper is disclosed, for example, in Patent Document 1. Furthermore, to prevent the above-mentioned discharge at the leading edge of the paper, a technology for lowering the transfer voltage when the leading edge of the paper enters the transfer nip is disclosed, for example, in Patent Document 2.

[0004] Patent Document 1 describes that when the basis weight of the recording paper is equal to or greater than a predetermined basis weight, the control unit controls the transfer pressure applied to the recording paper so that it is greater than the default transfer pressure. It also describes that the control unit changes the transfer pressure applied to the recording paper depending on the stiffness of the recording paper.

[0005] Patent Document 2 describes that an inrush voltage that is lower than a specified voltage by a predetermined value is output from the time when the leading edge of the sheet in the conveyance direction enters the contact portion until a predetermined time has elapsed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-63035 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-208497 Summary of the Invention [Problem to be solved by the invention]

[0007] As mentioned above, in order to prevent discharge at the leading edge of paper, techniques for changing the transfer pressure applied to the leading edge of paper and techniques for lowering the transfer voltage have been disclosed. However, the techniques described in Patent Documents 1 and 2 do not mention the poor image quality caused by discharge at the leading edge of paper when forming an image by overlapping toners with different average particle sizes.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to prevent poor image quality caused by discharge at the leading edge of paper when forming an image by overlapping toner particles having different average particle sizes. [Means for solving the problem]

[0009] In order to solve the above problem, the image forming apparatus of the present invention includes a secondary transfer unit that performs a second transfer of a toner image onto a recording material based on transfer conditions, and a control unit that changes the transfer conditions in accordance with the physical properties of the recording material when forming an image by overlapping toner layers using toners with different average particle sizes. The image forming apparatus described above is one aspect of the present invention, and a transfer condition control method that reflects one aspect of the present invention is configured in the same manner as the image forming apparatus described above. [Effects of the Invention]

[0010] According to the present invention having the above configuration, when forming an image by overlapping toners having different average particle diameters, it is possible to prevent poor image quality caused by discharge at the leading edge of the paper. Problems, configurations, and effects other than those described above will become apparent from the following description of the embodiments. [Brief explanation of the drawings]

[0011] [Figure 1] 10A and 10B are diagrams for explaining discharge caused by bending of the leading edge of paper. [Figure 2] 10A and 10B are diagrams for explaining the influence of discharge at the leading edge of a sheet of paper on a toner layer using toners with different average particle diameters; [Figure 3] 10A and 10B are diagrams for explaining poor image quality caused by the effect of discharge at the leading edge of the paper on the toner layer; [Figure 4] 1 is a schematic cross-sectional view showing an example of the configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 5] 2 is a block diagram showing an example of the hardware configuration of a control unit of an image forming apparatus according to an embodiment of the present invention; FIG. [Figure 6] 1 is a diagram illustrating an example of the configuration of a paper resistance detection device according to an embodiment of the present invention. [Figure 7] 1 is a diagram illustrating an example of the configuration of a sheet stiffness detection device according to an embodiment of the present invention. [Figure 8] FIG. 6 is a diagram showing an example of transfer condition change control data in the image forming apparatus according to the embodiment of the present invention. [Figure 9] 10A and 10B are diagrams for explaining whether or not transfer condition change control is performed depending on the overlap of toner layers in an image forming apparatus according to an embodiment of the present invention. [Figure 10] 10A and 10B are diagrams for explaining whether or not transfer condition change control is performed depending on the resistance and stiffness of paper in an image forming apparatus according to an embodiment of the present invention. [Figure 11] 5 is a flowchart showing the procedure of a transfer condition control process in the image forming apparatus according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functions or configurations are designated by the same reference numerals, and redundant description will be omitted.

[0013] <One embodiment> First, discharge caused by bending of the leading edge of paper will be explained. Figure 1 is a diagram for explaining discharge caused by bending of the leading edge of paper.

[0014] FIG. 1 shows the configuration of a secondary transfer unit 133 of an electrophotographic image forming apparatus 1 (see FIG. 4) according to this embodiment. As shown in FIG. 1, the secondary transfer unit 133 includes an opposing roller 331, a secondary transfer roller 332, and an intermediate transfer belt 132. The secondary transfer unit 133 also includes a support roller 334, a pressure mechanism 335, and a transfer voltage application mechanism 336. The intermediate transfer belt 132 is sandwiched between the opposing roller 331 and the secondary transfer roller 332. The intermediate transfer belt 132 is stretched around the opposing roller 331 and the support roller 334 in a loop shape. The intermediate transfer belt 132 travels at a constant speed in the direction of arrow A due to the rotation of the support roller 334. A secondary transfer nip is formed where the opposing roller 331 and the secondary transfer roller 332 come into contact with each other via the intermediate transfer belt 132.

[0015] A transfer voltage application mechanism 336 applies a transfer voltage of opposite polarity to the toner to the secondary transfer roller 332. By applying the transfer voltage to the secondary transfer roller 332, the toner image formed on the intermediate transfer belt 132 is secondarily transferred onto the recording material. Hereinafter, the recording material will also be referred to as "paper."

[0016] The pressing mechanism 335 has a pressure spring 351 and a slide cam 352. The slide cam 352 rotates around an axis 353. As the slide cam 352 rotates, the pressure spring 351 urges the secondary transfer roller 332 in the direction of the arrow X. As the pressure spring 351 urges the secondary transfer roller 332, it presses the opposing roller 331 in the direction of the arrow X. When the paper P passes through the secondary transfer nip, a transfer pressure is applied from the secondary transfer roller 332. At this time, the leading edge of the paper P bends in the area immediately before the secondary transfer nip, creating a gap between the intermediate transfer belt 132 and the paper P, causing discharge. Furthermore, when an image is formed by overlapping toner layers using toners with different average particle sizes, the discharge at the leading edge of the paper can cause poor quality of the printed image.

[0017] Next, we will explain the effect of discharge at the leading edge of paper on a toner layer using toners with different average particle sizes. In this embodiment, we assume that image formation is performed by overlaying white toner and color toners whose average particle size difference is equal to or greater than a predetermined value. The predetermined value is, for example, 1 micron. Figure 2 is a diagram for explaining the effect of discharge at the leading edge of paper on a toner layer using toners with different average particle sizes.

[0018] The white particles in Figure 2 represent white toner with a large average particle size. The black particles represent color toner with a small average particle size. As shown on the left side of Figure 2, white toner with a large average particle size is usually used as a base layer on paper, forming the lower layer toner. Color toner with a small average particle size is layered on top of the lower layer toner, forming the upper layer toner.

[0019] Due to the discharge at the leading edge of the paper, some of the toner particles in the upper layer enter the gaps in the lower layer, causing the toner in the upper and lower layers to mix together, as shown on the right side of Figure 2. As a result, the density of the printed image at the leading edge of the paper becomes lighter, causing poor image quality. An image of this poor image quality is shown in Figure 3.

[0020] 3 is a diagram illustrating image quality defects caused by the effect of discharge at the leading edge of the paper on the toner layer. As shown in FIG. 3, the image density at the leading edge of the paper (the area surrounded by the dashed line) is lighter than the image density at the rear. This image quality defect occurs within a certain range from the leading edge of the paper. In this embodiment, this certain range is set to a range where discharge is likely to occur, for example, a range of 15 mm from the leading edge of the paper.

[0021] 2, an example has been described in which the toners having different average particle sizes are white toner and color toner, the difference in average particle size being a predetermined value or more, but the present invention is not limited to this. The toners having different average particle sizes may be, for example, toners manufactured by different manufacturing methods. Furthermore, the toners having different average particle sizes may be, for example, toners having a difference in toner circularity of a predetermined value (for example, 2%) or more. Furthermore, the toners having different average particle sizes may be, for example, toners having a specific gravity of a certain value (for example, 1.3 times) or more.

[0022] [Configuration example of image forming device] Next, a configuration example of the image forming apparatus 1 according to this embodiment will be described. Fig. 4 is a schematic cross-sectional view showing a configuration example of the image forming apparatus 1 according to this embodiment.

[0023] The image forming apparatus 1 is an example of an image forming apparatus that forms an image on a recording material by electrophotography. The image forming apparatus 1 is a multifunction digital image forming apparatus (MFP: Multifunction Peripheral) that has multiple functions such as a print function, a copy function, a facsimile function, and a scan function. The image forming apparatus 1 forms an image on a recording material based on document image data obtained by reading an image from a document or document image data received from an external terminal device.

[0024] As shown in FIG. 4, the image forming apparatus 1 includes a reading unit 12, an image forming unit 13, an operation display unit 15, a manual feed tray T1, a paper feed tray T2, and a paper discharge tray T3.

[0025] The reading unit 12 scans and exposes an image of a document placed on a document table or an automatic document feeder (ADF) (not shown) using the optical system of a scanning exposure device, reads the reflected light using a line image sensor, and outputs the read image.

[0026] The image forming unit 13 forms an image on paper according to the pixel value of each pixel in the document image data. The image forming unit 13 has five image writing units 131 for forming toner images of five colors: cyan (C), magenta (M), yellow (Y), black (K), and white (W). The image writing units 131 include image writing units 131Y, 131M, 131C, 131K, and 131W shown in FIG. 4. The image forming unit 13 also includes an intermediate transfer belt 132, a secondary transfer unit 133, a fixing unit 134, and the like. Hereinafter, the image writing units 131Y, 131M, 131C, 131K, and 131W may be collectively referred to as the image writing unit 131 when not being particularly distinguished.

[0027] The five image writing units 131Y, 131M, 131C, 131K, and 131W are arranged in series (tandem) along the belt surface of the intermediate transfer belt 132. The five image writing units 131 form images of the respective colors of C, M, Y, K, and W. The image writing units 131 have the same configuration except for the colors of the images they form. As shown in FIG. 4, the image writing unit 131 includes an exposure device 131a and a photosensitive member 131b (an example of an image carrier). The image writing unit 131 also includes a developing unit 131c, a charging unit 131d, a cleaning unit 131e, and a primary transfer roller 131f.

[0028] During image formation processing, the charging unit 131d in each image writing unit 131 charges the outer peripheral surface of the drum-shaped photoconductor 131b. Then, the charging unit 131d scans the outer peripheral surface of the photoconductor 131b with a light beam (light beam) emitted by the exposure device 131a based on the document image data, forming an electrostatic latent image. In this state, when the developing unit 131c supplies a color material such as toner and develops it, an image is formed on the outer peripheral surface of the photoconductor 131b.

[0029] The primary transfer roller 131f primarily transfers the image formed on the photosensitive member 131b onto the intermediate transfer belt 132, superimposing the image onto the intermediate transfer belt 132. As a result, an image made up of each color is formed on the intermediate transfer belt 132. The intermediate transfer belt 132 is an image carrier that is wound around multiple rollers and rotates. After the primary transfer, the cleaning unit 131e removes color material remaining on the photosensitive member 131b.

[0030] The secondary transfer unit 133 performs secondary transfer of the image onto the paper from the intermediate transfer belt 132 based on transfer conditions such as the set transfer voltage (current) and transfer pressure. Note that the configuration of the secondary transfer unit 133 has been described in detail in FIG. 1, so a duplicated description will be omitted.

[0031] The paper is then transported to the fixing unit 134, where it is subjected to a fixing process and then discharged to the paper output tray T3. The fixing process involves applying heat and pressure to the paper using the fixing roller 134a to fix the image to the paper. When forming images on both sides of the paper, the paper is transported to the reversing path 135 to reverse the paper surface, and then the paper is fed again to the secondary transfer unit 133.

[0032] The operation display unit 15 is composed of a display unit and an operation unit. The display unit is composed of a display device such as an LCD (Liquid Crystal Display) or an organic EL (Electro-luminescence) display. The operation unit is composed of a touch sensor or the like. The display unit and operation unit may be formed integrally as, for example, a touch panel.

[0033] In addition to the components described above, the image forming apparatus 1 also includes a control unit 16 that controls the operation of each component. Fig. 5 is a block diagram showing an example of the configuration of the control unit 16. As shown in Fig. 5, the control unit includes a CPU (Central Processing Unit) 16a, a ROM (Read Only Memory) 16b, a RAM (Random Access Memory) 16c, and a memory 16d. The CPU 16a, ROM 16b, RAM 16c, and memory 16d are connected via a bus B so as to be able to send and receive information data to and from each other.

[0034] The CPU 16a performs image formation processing in the image forming apparatus 1, transfer condition change control processing shown in Fig. 11 (described later), and the like, in accordance with a program read from the ROM 16b. Note that a GPU (Graphics Processing Unit) may be used instead of the CPU 16a, or a CPU and a GPU may be used together.

[0035] The ROM 16b is configured as a storage medium such as a nonvolatile memory, and stores programs and data executed and referenced by the CPU 16a. The ROM 16b is used as an example of a computer-readable non-transitory storage medium that stores programs executed by the control unit 16.

[0036] The RAM 16c is configured with a storage medium such as a volatile memory, and temporarily stores information (data) required for each process performed by the CPU 16a. The memory 16d is an image memory that temporarily stores image data to be recorded.

[0037] When the leading edge of the paper enters the secondary transfer nip, the physical properties of the paper may or may not cause the discharge described above. To prevent discharge from occurring at the leading edge of the paper, the control unit of the image forming apparatus 1 changes at least one of the transfer current and the transfer pressure according to the physical properties of the paper. Here, the physical properties of the paper include at least one of a characteristic value for evaluating the resistance of the paper (see FIG. 9 described later) and a characteristic value for evaluating the stiffness of the paper (see FIG. 9 described later).

[0038] Next, a detection device that detects the physical properties of paper will be described. FIG. 6 is a diagram showing an example of the configuration of a paper resistance detection device 20 according to this embodiment. As shown in FIG. 6, the paper resistance detection device 20 includes detection rollers 21a and 21b, each of which is made of conductive rubber. The paper resistance detection device 20 also includes an electrode roller 22 arranged in contact with the detection roller 21a, and a power supply unit 23 connected to the electrode roller 22. The paper resistance detection device 20 also includes a current detection unit 24 connected to the detection roller 21b. The power supply unit 23 supplies current to the detection roller 21a via the electrode roller 22. The current detection unit 24 is inserted between the detection roller 21b and ground and detects the current flowing through the paper sandwiched between the detection rollers 21a and 21b. The resistance of the paper can be calculated from the difference between the current value before and after the paper is sandwiched.

[0039] Fig. 7 is a diagram showing an example of the configuration of a paper stiffness detection device 30 according to this embodiment. As shown in Fig. 7, the paper stiffness detection device 30 includes opposing detection rollers 31a and 31b, a push-up member 32, a lifting mechanism 33, and a pressure sensor 34. The lifting mechanism 33 raises and lowers the push-up member 32. The push-up member 32 is raised by the lifting mechanism 33, and pushes up the paper sandwiched between the detection rollers 31a and 31b to a predetermined angle. The pressure sensor 34 is connected to the lifting mechanism 33, and detects the pressure when the paper is bent at the predetermined angle as the stiffness of the paper.

[0040] 6 and 7 are assumed to be installed in the conveyance path upstream of the secondary transfer unit 133. However, the present invention is not limited to this, and the paper resistance detection device 20 and the paper stiffness detection device 30 may be external devices connected to the image forming apparatus 1.

[0041] Next, the control data for transfer conditions for adjusting the transfer conditions according to the physical property values ​​of the paper will be described. When the leading edge of the paper contains an image pattern in which toner layers using toners with different average particle sizes are superimposed, the control unit of the image forming apparatus 1 changes the transfer conditions according to the physical property values ​​of the paper. When the physical property values ​​of the paper satisfy predetermined conditions, the control unit changes the transfer conditions. FIG. 8 is a diagram showing an example of transfer condition change control data in the image forming apparatus 1 according to this embodiment. FIG. 8 shows transfer conditions (transfer current and transfer pressure) corresponding to the cases where white toner is present and absent. Also, in each case, the transfer conditions corresponding to the leading edge of the paper and the area after the leading edge of the paper are shown.

[0042] When there is no white toner, the upper layer toner and the lower layer toner do not mix due to discharge at the leading edge of the paper. Therefore, as shown in Figure 8, the transfer conditions at the leading edge of the paper and after the leading edge are the same. In other words, in the present invention, when there is no white toner, the transfer conditions are not changed. As an example of the transfer conditions, for example, a transfer current of "-150 μA" and a transfer pressure of "100 N" are shown.

[0043] Furthermore, if white toner is present, the upper and lower layer toners will mix due to discharge at the leading edge of the paper. For this reason, as shown in FIG. 8, the transfer conditions for the leading edge of the paper and the area after the leading edge are different. If white toner is present, the control unit 16 changes the transfer conditions for the leading edge of the paper to be smaller than the transfer conditions for the area after the leading edge. This change in transfer conditions is only performed for the leading edge of the paper, i.e., within a range where discharge can occur (for example, a range of 15 mm from the leading edge of the paper). The control unit 16 changes the set values ​​of the transfer conditions for the leading edge of the paper to be smaller than the original set values, and after the leading edge of the paper has passed the secondary transfer unit 133, returns the set values ​​of the transfer conditions to the original set values.

[0044] For example, as shown in FIG. 8, when white toner is present, the transfer current at the leading edge of the paper is changed to "-140 μA," which is smaller than the transfer current "-150 μA" after the leading edge of the paper. Also, for example, as shown in FIG. 8, when white toner is present, the transfer pressure at the leading edge of the paper is changed to "80 N," which is smaller than the transfer pressure "100 N" after the leading edge of the paper. Furthermore, the transfer conditions after the leading edge of the paper are the same as the original settings, i.e., the transfer conditions when there is no white toner. Note that while FIG. 8 describes the change control of the transfer current and transfer pressure, the present invention is not limited to this. For example, the control unit 16 may change only either the transfer current or the transfer pressure. Furthermore, the transfer voltage may be changed instead of the transfer current.

[0045] Next, the presence or absence of control to change the transfer conditions depending on the overlap of toner layers will be described. FIG. 9 is a diagram for explaining the presence or absence of control to change the transfer conditions depending on the overlap of toner layers in the image forming apparatus 1 according to this embodiment. Case A shown in FIG. 9 is a case where a toner layer with a large average particle size (white toner) is overlapped with a toner layer with a small average particle size (cyan toner) at the leading edge of the paper. Case B is a case where a toner layer with a large average particle size (white toner) is overlapped with a toner layer with a small average particle size (cyan toner) at the trailing edge of the paper. Case C is a case where no overlap of toner layers exists at the leading edge of the paper (for example, a case where only white toner exists). Case D is a case where a toner layer with a small average particle size (cyan toner) is overlapped with a toner layer with a large average particle size (white toner) at the leading edge of the paper.

[0046] The problem of low image density due to discharge occurs only when an image pattern made up of overlapping toner layers using toners with different average particle sizes is present in a certain region including the leading edge of the paper. For this reason, the present invention controls the change of transfer conditions only when an image pattern made up of overlapping toner layers using toners with different average particle sizes is present in a certain region including the leading edge of the paper (leading edge of the paper). As a specific example, the present invention controls the change of transfer conditions for cases A and D shown in Figure 9, but does not control the change of transfer conditions for cases B and C.

[0047] Next, it will be explained whether or not there is transfer condition change control depending on the resistance and stiffness of the paper in the image forming apparatus 1. Fig. 10 is a diagram for explaining whether or not there is transfer condition change control depending on the resistance and stiffness of the paper in the image forming apparatus 1 according to this embodiment.

[0048] FIG. 10 shows volume resistivity (unit: log Ω) as a characteristic value for evaluating the resistance of paper, and stiffness (unit: mN) as a characteristic value for evaluating the stiffness of paper. In FIG. 10, paper that meets the volume resistivity and stiffness conditions (predetermined conditions for physical properties) corresponding to the circle marks is subject to transfer condition change control. As shown in the figure, if the volume resistivity is 9 log Ω or more and the stiffness is within the range of 5 mN or more and less than 60 mN, the corresponding paper is subject to transfer condition change control. Also, as shown in the figure, if the volume resistivity is 11 log Ω or more and the stiffness is less than 60 mN, the corresponding paper is subject to transfer condition change control. That is, the image forming apparatus 1 subjects paper whose resistance meets the predetermined resistance condition and whose stiffness meets the predetermined stiffness condition to transfer condition change control. Here, the predetermined resistance condition is, for example, 9 log Ω or more, and the predetermined stiffness condition is, for example, 5 mN or more and less than 60 mN. The predetermined resistance condition is, for example, 11 log Ω or more, and the predetermined stiffness condition is, for example, less than 60 mN.

[0049] 10, whether or not to control the change of transfer conditions is explained depending on the resistance and stiffness of the paper, but the present invention is not limited to this. For example, instead of stiffness, the basis weight of the paper may be used as a condition for determining whether or not to control the change of transfer conditions.

[0050] [Transfer condition change control process procedure] Next, a description will be given of the procedure for the transfer condition change control process in the image forming apparatus 1. Fig. 11 is a flowchart showing the procedure for the transfer condition change control process in the image forming apparatus 1 according to this embodiment. The process described below is executed in the image forming apparatus 1 at the start of the image formation process.

[0051] First, the control unit 16 of the image forming apparatus 1 determines whether or not the physical property values ​​of the paper satisfy predetermined conditions (S10). In this process, if the control unit 16 determines that the physical property values ​​satisfy the volume resistivity and stiffness conditions corresponding to the circle marks shown in Fig. 9, for example, the determination in S10 is YES. On the other hand, if the control unit 16 determines that the physical property values ​​do not satisfy the volume resistivity and stiffness conditions corresponding to the circle marks shown in Fig. 9, the determination in S10 is NO.

[0052] When the control unit determines that the physical property values ​​of the paper do not satisfy the predetermined conditions (NO in S10), the transfer condition change control process ends.

[0053] On the other hand, if the control unit 16 determines that the physical property values ​​of the paper satisfy the predetermined conditions (YES in S10), it performs the process of S11. In the process of S11, the control unit 16 determines whether the leading edge of the paper includes an image pattern in which toners with different average particle sizes are superimposed.

[0054] When the control unit 16 determines that the leading edge of the paper does not include an image pattern in which toners having different average particle diameters are superimposed (NO in S11), the transfer condition change control process ends.

[0055] On the other hand, when the control unit 16 determines that the leading edge of the paper includes an image pattern in which toners of different average particle sizes are superimposed (YES in S11), it performs control to change the transfer conditions (S12). In this process, the control unit 16 acquires adjustment values ​​for the transfer current and the transfer pressure from the transfer condition change control data (see FIG. 8). The control unit 16 also instructs the acquired transfer current to the transfer voltage application mechanism 336 and the transfer pressure to the pressing mechanism 335. The transfer voltage application mechanism 336 then adjusts the output voltage so that the instructed transfer current is output. The pressing mechanism 335 also adjusts the pressure so that the instructed transfer pressure is output.

[0056] Next, the control unit 16 determines whether the leading edge of the paper has passed through the secondary transfer unit 133 (S13).

[0057] When the control unit 16 determines that the leading edge of the paper has not passed through the secondary transfer unit 133 or is passing through the secondary transfer unit 133 (NO in S13), the control unit 16 repeatedly executes the process of S13.

[0058] On the other hand, when the control unit 16 determines that the leading edge of the paper has passed through the secondary transfer unit 133 (YES in S13), it returns the transfer conditions to the original setting values ​​(S14).

[0059] After the process of S14, if the result of S10 is NO or if the result of S11 is NO, the transfer condition change control process ends.

[0060] [effect] As described above, the image forming apparatus 1 according to this embodiment controls the change of transfer conditions for the leading edge of the paper when the leading edge of the paper contains an image pattern in which toner particles with different average particle sizes are superimposed. The control of the change of transfer conditions changes the transfer conditions, such as the transfer current and transfer pressure, for the leading edge of the paper to be smaller than the transfer conditions after the leading edge of the paper, so as to prevent discharge due to bending of the leading edge of the paper. After the leading edge of the paper passes through the secondary transfer section, the transfer conditions are returned to their original settings. Therefore, the image forming apparatus 1 according to the present invention can prevent poor image quality due to discharge at the leading edge of the paper when forming an image by superimposing toner particles with different average particle sizes.

[0061] The present invention is not limited to the above-described embodiment, and it goes without saying that various other applications and modifications are possible without departing from the gist of the present invention as set forth in the claims. For example, the above-described embodiment describes the configuration of an image forming apparatus in detail and specifically in order to clearly explain the present invention, and is not necessarily limited to having all of the described configurations. Furthermore, it is possible to replace part of the configuration of the embodiment described here with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is also possible to add, delete, or replace part of the configuration of an embodiment with other configurations. In addition, the control lines and information lines shown are those that are considered necessary for the explanation, and do not necessarily show all the control lines and information lines in the product. In reality, it can be assumed that almost all components are interconnected. [Explanation of symbols]

[0062] 1... image forming apparatus, 13... image forming section, 16... control section, 20... paper resistance detection device, 30... paper stiffness detection device, 133... secondary transfer section, 335... pressure contact mechanism, 336... transfer voltage application mechanism

Claims

1. a secondary transfer unit that secondarily transfers a toner image onto a recording material based on transfer conditions; a control unit that changes the transfer conditions in accordance with the physical property values ​​of the recording material in image formation by overlapping toner layers using toners with different average particle sizes. Image forming device.

2. When an image pattern in which toner layers using toners having different average particle diameters are superimposed on a leading edge of the recording material is included, the control unit changes the transfer conditions in accordance with the physical property values ​​of the recording material. The image forming apparatus according to claim 1 .

3. The control unit changes the transfer conditions when the physical property values ​​of the recording material satisfy predetermined conditions. The image forming apparatus according to claim 2 .

4. The control unit changes the set value of the transfer condition for the leading edge of the recording material to be smaller than the original set value, and after the leading edge of the recording material passes the secondary transfer unit, returns the set value of the transfer condition to the original set value. The image forming apparatus according to claim 3 .

5. The physical property values ​​include at least one of a characteristic value for evaluating the resistance of the recording material and a characteristic value for evaluating the stiffness of the recording material. The image forming apparatus according to claim 4 .

6. the transfer conditions include a transfer current and a transfer pressure in the secondary transfer unit; The control unit changes at least one of the transfer current and the transfer pressure. The image forming apparatus according to claim 4 .

7. The toners having different average particle diameters are white toner and color toner, the difference in average particle diameter being equal to or greater than a predetermined value. The image forming apparatus according to claim 1 .

8. The toners having different average particle sizes are toners manufactured by different manufacturing methods. The image forming apparatus according to claim 1 .

9. The toner particles having different average particle diameters are toner particles having a difference in toner circularity of a predetermined value or more. The image forming apparatus according to claim 1 .

10. The toners having different average particle diameters have a specific gravity equal to or greater than a certain value. The image forming apparatus according to claim 1 .

11. 1. A transfer condition control method for an image forming apparatus including a secondary transfer unit that secondarily transfers a toner image onto a recording material based on transfer conditions, comprising: In image formation in which toner layers using toners having different average particle sizes are superimposed, a step of changing the transfer conditions in accordance with the physical property values ​​of the recording material is included. Transfer condition control method.

Images