JP2012159790A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of preventing occurrence of toner dispersion and scumming even when forming a halftone image where electrostatic binding force between the toner and paper is weak.SOLUTION: When no halftone image is included within a predetermined area from the head of image data (S15: NO), a CPU 81 allows an application section 52 to apply a transfer voltage at a timing when paper S passes through the upstream side of a transfer position P and starts up the transfer voltage to a rated voltage immediately from starting the application. When the halftone image is included (S15: YES), the CPU 81 allows the application section 52 to apply the transfer voltage at the timing when the paper S passes through the transfer position P, and starts up the transfer voltage to the rated voltage a predetermined time after starting the application.

Description

  The present invention relates to an image forming apparatus that forms an image by transferring a toner image formed by an electrophotographic method onto a sheet.

  As an example of an image forming apparatus, an image forming process is performed on a sheet by directly transferring a toner image from a photosensitive drum carrying a toner image formed by an electrophotographic method onto a sheet conveyed on a sheet conveyance path. Yes (see Patent Document 1).

  In the image forming apparatus described in Patent Document 1, a transfer charger is disposed at a position facing a photosensitive drum across a sheet conveyance path for conveying a sheet, and a transfer bias voltage is applied to the transfer charger. When a transfer bias voltage is applied to the transfer charger, the image forming apparatus transfers the toner image from the photosensitive drum to a sheet.

JP 2003-57966 A

  In the image forming apparatus described in Patent Document 1, a transfer bias voltage is applied to the transfer charger after a predetermined time has elapsed since the leading edge of the sheet has passed between the photosensitive drum and the transfer charger. Since the image forming apparatus does not control the rise of the transfer bias voltage, it can be seen that the image forming apparatus is immediately raised to the rated voltage. For this reason, the sheet has a high surface potential near the portion passing between the photosensitive drum and the transfer charger at the timing when the transfer bias voltage is applied. When a halftone image having a weak electrostatic bonding force between the toner and the paper is transferred to the paper having a high surface potential, there is a high possibility that the toner scatters or becomes dirty.

  Accordingly, an object of the present invention is to provide an image forming apparatus capable of reducing toner scattering and background contamination even when a halftone image having a weak electrostatic coupling force between toner and paper is formed.

  The image forming apparatus according to the present invention includes an image carrier, a transfer unit, an application unit, and a transport unit. The image carrier carries a toner image formed by an electrophotographic method using image data. The transfer unit is disposed at a position facing the image carrier with a transfer position for transferring the toner image onto the paper, and a transfer bias voltage for transferring the toner image onto the paper is applied. The applying unit applies a transfer bias voltage to the transfer unit. The conveying unit conveys the sheet so as to pass the transfer position.

  The image forming apparatus further includes a determination unit, a measurement unit, and a control unit. The discriminating unit discriminates whether or not a halftone image is included within a predetermined range from the leading edge of the image data. The measuring means measures a first timing when the leading edge of the sheet passes the upstream side of the transfer position and a second timing when the leading edge of the sheet passes the transfer position. The control unit controls application of the transfer bias voltage by the applying unit according to the determination result of the determining unit. When the determination means determines that the halftone image is not included, the control means performs a first control process in which the transfer bias voltage is applied at the first timing and immediately rises to the rated voltage. If it is determined that the image is included, a transfer bias voltage is applied at the second timing, and a second control process is performed in which the voltage is raised to the rated voltage after a predetermined time has elapsed.

  In this configuration, the transfer unit is disposed at a position facing the image carrier that carries the toner image with the transfer position interposed therebetween. The transfer unit is applied with a transfer bias voltage for transferring the toner image onto the paper by the applying unit, and transfers the toner image carried by the image carrier onto the paper transported by the transport unit and passing the transfer position.

  Normally, the control means controls the application means so that the transfer bias voltage is applied before the leading edge of the paper passes the transfer position and the voltage is immediately raised to the rated voltage in consideration of the conveyance delay due to the paper conveyance error. ing. Immediately after the transfer bias voltage is applied to the transfer means, the surface potential of the paper becomes unstable. For this reason, the sheet has a high surface potential at the leading end, and the possibility of toner scattering and background contamination increases. In particular, when a halftone image with weak electrostatic binding force between toner and the paper is formed around the leading edge of the paper, toner scattering and background smearing will occur compared to when a solid image is formed. Likely to occur.

  Therefore, the control means determines whether or not a halftone image is included in a predetermined range from the start result of the determination result of the determination means, that is, image data input from another device or image data read from a document. Accordingly, application of the transfer bias voltage by the application unit is controlled. When it is determined by the determination means that the halftone image is not included, the control means applies a transfer bias voltage to the transfer means before the leading edge of the paper passes the transfer position, and immediately determines the rated voltage. The application unit is controlled so as to perform the first control process to start up. When the determining means determines that the halftone image is included, the control means applies the transfer bias voltage to the transfer means at the timing when the leading edge of the paper passes the transfer position, and sets the transfer bias voltage. The application means is controlled so as to perform the second control process for raising the voltage to the rated voltage after a predetermined time has elapsed since the application. In the second control process, since the rise of the transfer bias voltage becomes slower than in the first control process, the surface potential of the leading edge of the sheet does not increase, and the toner is less likely to scatter.

  A plurality of image carriers each corresponding to a different hue; a plurality of transfer units each disposed at a position facing the plurality of image carriers across a different transfer position; an applying unit; and a conveying unit. Adaptable to configuration. In this case, the measuring unit measures the first timing and the second timing for each transfer position. The first control process is a process of applying a transfer bias voltage to the transfer means corresponding to each transfer position at the first timing measured for each transfer position. The second control process is a process of applying a transfer bias voltage to the transfer unit corresponding to each transfer position at the second timing measured for each transfer position.

  In this configuration, a plurality of transfer units are arranged at positions facing a plurality of image carriers corresponding to different hues across the transfer position. When a transfer bias voltage is applied by the applying unit, each of the plurality of transfer units stacks and transfers a toner image from an image carrier disposed at a position facing the sheet passing through the transfer position.

  The control unit controls application of a transfer bias voltage to each transfer unit. When it is determined by the determining means that the halftone image is not included, the control means applies a transfer bias voltage to each transfer means before the leading edge of the paper passes each transfer position, and immediately The application unit is controlled to perform the first control process for raising the voltage. When it is determined by the determining means that the halftone image is included, the control means applies a transfer bias voltage to each transfer means at the timing when the leading edge of the paper passes each transfer position, and the transfer bias The application means is controlled so as to perform the second control process in which the voltage is raised to the rated voltage after a predetermined time has elapsed since the voltage was applied. In the second control process, the rise of the transfer bias voltage at each transfer position becomes slower than in the first control process, and the surface potential of the leading edge of the sheet does not increase.

  As a result, even when the image forming apparatus forms a halftone image in which the electrostatic binding force between the toner and the paper is weak, the toner is transferred when the toner image is transferred from each image carrier to the paper. Scattering and soiling can be reduced.

  The image forming apparatus according to the present invention can be applied to a configuration including a plurality of image carriers, a plurality of first transfer units, a second transfer unit, an application unit, and a transport unit. The plurality of image carriers carry toner images formed by an electrophotographic method using image data, each corresponding to a different hue. The plurality of first transfer units are arranged at positions facing each of the plurality of image carriers, and a first transfer bias voltage is applied to stack and transfer the toner images from the plurality of image carriers to the intermediate transfer material. Is done. The second transfer means is disposed at a position facing the intermediate transfer material across the transfer position for transferring the toner image to the paper, and a second transfer bias voltage for transferring the toner image stacked on the intermediate transfer material to the paper. Is applied. The applying unit applies a first transfer bias voltage to the first transfer unit, and applies a second transfer bias voltage to the second transfer unit. The conveying unit conveys the sheet so as to pass the transfer position.

  The image forming apparatus further includes a determination unit, a measurement unit, and a control unit. The discriminating unit discriminates whether or not a halftone image is included within a predetermined range from the leading edge of the image data. The measuring means measures a first timing when the leading edge of the sheet passes the upstream side of the transfer position and a second timing when the leading edge of the sheet passes the transfer position. The control unit controls the application of the second transfer bias voltage by the applying unit according to the determination result of the determining unit. When the determining unit determines that the halftone image is not included, the control unit performs the first control process in which the second transfer bias voltage is applied at the first timing and immediately rises to the rated voltage. If it is determined that a halftone image is included, the second transfer bias voltage is applied at the second timing, and a second control process is performed in which the voltage rises to the rated voltage after a predetermined time has elapsed.

  In this configuration, a plurality of first transfer units are arranged at positions facing a plurality of image carriers corresponding to different hues across an intermediate transfer material such as an intermediate transfer belt, and facing the intermediate transfer material across the transfer position. The second transfer means is disposed at a position where the movement is performed. When the first transfer bias voltage is applied by the applying unit, each of the plurality of first transfer units transfers the toner image from the image carrier while being stacked on the intermediate transfer material. The second transfer unit is applied with the second transfer bias voltage by the applying unit, and transfers the toner image laminated on the intermediate transfer material to the sheet passing through the transfer position.

  The control unit controls application of the second transfer bias voltage to the second transfer unit. When it is determined by the determining unit that the halftone image is not included, the control unit applies a second transfer bias voltage to the second transfer unit before the leading edge of the sheet passes the transfer position. The applying means is controlled so as to perform the first control process for immediately raising the voltage to the rated voltage. When it is determined by the determining unit that the halftone image is included, the control unit applies a second transfer bias voltage to the second transfer unit at a timing when the leading edge of the sheet passes the transfer position, The application unit is controlled to perform a second control process in which the voltage is raised to the rated voltage after a lapse of a predetermined time from the application of the second transfer bias voltage. In the second control process, since the rise of the second transfer bias voltage becomes slower than in the first control process, the surface potential of the leading edge of the sheet does not increase.

  As a result, even when the image forming apparatus forms a halftone image in which the electrostatic binding force between the toner and the paper is weak, the toner scattering and the toner image are transferred when the toner image is transferred from the intermediate transfer material to the paper. Soil can be reduced.

  It is preferable that the configuration further includes detection means for detecting whether the paper is thin paper or thick paper. In this case, the control means controls the application means when the paper detected by the detection means is thin paper.

  Toner scattering and background contamination occur when the electrostatic coupling between toner and paper is weak. In general, thin paper is more likely to cause toner scattering and background staining than thick paper. Therefore, in this configuration, the detection unit detects whether the paper on which the toner image is transferred is thin paper or thick paper. The control unit performs the above-described control when the paper is thin.

  Thereby, the image forming apparatus can effectively reduce toner scattering and background contamination.

  The image forming apparatus of the present invention can reduce toner scattering and background stain even when forming a halftone image having a weak electrostatic binding force between toner and paper.

1 is a schematic front sectional view showing a configuration of an image forming apparatus. It is explanatory drawing which shows the structure of an image formation part. 2 is a block diagram illustrating a configuration of a control unit of the image forming apparatus. FIG. It is a simplified diagram used for description of a solid image and a halftone image. FIG. 6 is a diagram illustrating a relationship between application timing (upstream of a transfer position) and a surface potential of a sheet. It is a figure which shows the relationship between an application timing (transfer position) and the surface potential of a paper. It is a flowchart which shows the flow of a process of a control part. It is explanatory drawing which shows the structure of another image forming part. It is a flowchart which shows the flow of a process of another control part. It is explanatory drawing which shows the structure of another image forming part.

  An image forming apparatus according to an embodiment of the present invention will be described below with reference to the drawings.

  The image forming apparatus 100 includes an image reading unit 200, an image forming unit 300, a paper feeding unit 400, and a control unit 80 (see FIG. 3).

  The image reading unit 200 is placed on a document table 201 made of transparent glass on which a document is placed, an automatic document feeder (ADF) 202 attached to the upper side of the document table 201, and a document table 201. An image reading unit 203 that reads an image of the original is provided. The ADF 202 automatically conveys a document on the document table 201. Further, the ADF 202 is configured to be rotatable, and a document can be manually placed by opening the document table 201.

  The image forming unit 300 includes a photosensitive drum 31, a charger 32, an exposure device 33, a developing device 34, a transfer belt 35, a cleaner 36, and a fixing device 37 that constitute the image forming unit 30. The image forming unit 300 forms an image on the paper S by electrophotographic image forming processing.

  The paper feed unit 400 includes paper feed cassettes 401 to 404 and a manual feed tray 405. Each of the paper feed cassettes 401 to 404 stores a plurality of sheets S of the same size. On the manual feed tray 405, paper S of a size or quality that is not frequently used is placed.

  The paper feed unit 400 feeds the paper S one by one from either the paper feed cassettes 401 to 404 or the manual feed tray 405. The sheet S fed from the sheet feeding unit 400 is transported on the sheet transport path L. The sheet conveyance path L is formed between the sheet feeding unit 400 and the sheet discharge tray 38 via the image forming unit 30.

  As shown in FIG. 2, the image forming unit 300 includes a sensor 51 in addition to the image forming unit 30. The sensor 51 is disposed on the upstream side of the photosensitive drum 31 and detects that the leading edge of the paper S has passed.

  The photosensitive drum 31 has a photosensitive layer formed on the surface thereof, and rotates in the direction of the arrow. The charger 32 uniformly charges the surface of the photosensitive drum 31 to a predetermined potential. The charger 32 may use either a non-contact type discharge wire, or a contact type roller or brush.

  The exposure device 33 irradiates the surface of the photosensitive drum 31 with light based on the image data. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 31 by the photoconductive action in the photosensitive layer. The exposure device 33 is a laser scanning unit, and scans laser light modulated based on image data in the axial direction of the photosensitive drum 31 via a polygon mirror. Alternatively, an exposure apparatus in which light emitting elements such as EL and LEDs are arranged in an array can be used.

  The developing device 34 supplies toner to the surface of the photosensitive drum 31 and visualizes the electrostatic latent image into a toner image.

The transfer belt 35 is looped between a plurality of rollers below the photosensitive drum 31, and has a resistance value of about 1 × 10 ⁹ to 1 × 10 ¹³ Ω · cm. A portion of the transfer belt 35 that faces the photosensitive drum 31 across a transfer position P at which the toner image is transferred to the paper S is substantially horizontal. The transfer belt 35 is disposed to stably transfer the toner image from the photosensitive drum 31 to the surface of the sheet S and to stably convey the sheet S on which the toner image before fixing is electrostatically adsorbed. Yes.

  Inside the loop-shaped movement path of the transfer belt 35, a transfer roller 35A that is in pressure contact with the photosensitive drum 31 with the transfer belt 35 interposed therebetween at the transfer position P is provided. The transfer roller 35A is applied with a transfer voltage having the opposite polarity (+) to the toner charge polarity (−), the sheet S passing between the transfer belt 35 and the photosensitive drum 31 is charged, and the toner image is transferred. Transferred from the photosensitive drum 31 to the paper S.

  The cleaner 35 </ b> B removes dirt on the surface of the transfer belt 35.

  The cleaner 36 removes the remaining toner from the surface of the photosensitive drum 31 after the transfer of the toner image onto the paper S.

  The fixing device 37 includes a heating roller 37A and a pressure roller 37B. The heating roller 37A is heated by an internal heater to a temperature at which the toner can be melted. The pressure roller 37B is in pressure contact with the heating roller 37A with a predetermined pressure. The fixing device 37 firmly fixes the toner image onto the paper S by heating and pressurizing the paper S passing between the heating roller 37A and the pressure roller 37B. The sheet S that has passed through the fixing device 37 is discharged to a discharge tray 38 that is mounted on one side surface of the image forming apparatus 100.

  As shown in FIG. 3, the sheet S includes a printing surface on which an image is formed and a margin formed around the image surface. For this reason, the image is not formed at the leading end of the sheet S in the transport direction of the sheet S, but is formed backward from a position away from the leading end of the sheet S by a predetermined distance (about 20 mm).

  The image formed on the paper S includes at least two types of images, ie, a solid image and a halftone image. As shown in FIG. 3A, a solid image has a large amount of toner melted together and transferred to the paper S, so that the electrostatic coupling force between the toner and the paper S is strong. On the other hand, as shown in FIG. 3B, the halftone image is a black and white binary image, and thus each toner is individually transferred onto the paper S. For this reason, the halftone image has a weaker electrostatic binding force between the toner and the paper S than the solid image.

  Next, the control unit 80 will be described. As shown in FIG. 4, the control unit 80 includes a CPU 81 having a ROM 82 and a RAM 83 and a timer 84. An operation unit 500, a paper feed unit 400, an image reading unit 200, and an image forming unit 300 are connected to the CPU 81.

  The CPU 81 reads out and executes a program recorded in the ROM 82, and controls each part in an overall manner. In addition to the program, the ROM 82 stores a first elapsed time TP1, a second elapsed time TP2, a first application time TV1, and a second application time TV2 (see FIGS. 6A and 7A). .) The first elapsed time TP1 is a time required from the time when the leading edge of the paper S is detected by the sensor 51 to the time when it passes the upstream side of the transfer position P. The second elapsed time TP <b> 2 is a time required from when the leading edge of the paper S is detected by the sensor 51 until it passes the transfer position P. The first application time TV1 and the second application time TV2 are times required to raise the transfer voltage to the rated voltage. The second application time TV2 is longer than the first application time TV1.

  The RAM 83 is used as a working area for the CPU 81. The timer 84 measures the elapsed time after the sensor 51 detects the leading edge of the paper S.

  The image forming unit 300 includes an application unit 52 in addition to the image forming unit 30 and the sensor 51. The application unit 52 is connected to a high-voltage power supply 53 and applies a transfer voltage to the transfer roller 35A.

  A processing flow at the time of image formation processing in the CPU 81 of the control unit 80 will be described with reference to the drawings. As shown in FIG. 5, when receiving a print condition setting and a print start instruction from the operation unit 500 (S11), the CPU 81 causes the image reading unit 200 to read an image from a document (S12). For example, the printing conditions include information indicating the presence / absence of layout printing, the number of printed sheets, and the like.

  The CPU 81 processes the image data read by the image reading unit 200 according to the printing conditions, and then temporarily stores it in the RAM 83 (S13). The CPU 81 analyzes the image data (S14), and determines whether or not a halftone image is included within a predetermined range from the top of the image data (S15).

  When the halftone image is not included (S15: NO), the CPU 81 sets the transfer voltage application timing after the sensor 51 detects the leading edge of the sheet S as shown in FIG. It is set after one elapsed time TP1 (S16). That is, the CPU 81 causes the applying unit 52 to apply a transfer voltage at a timing when the sheet S passes the upstream side of the transfer position P. The CPU 81 sets the time required for raising the transfer voltage to the first application time TV1 (S17). That is, the CPU 81 immediately raises the transfer voltage from the start of application to the rated voltage. The CPU 81 starts printing (S20).

  As shown in FIG. 6B, the sheet S is concentrated at the leading end portion, and the surface potential at the leading end portion is increased. When the sheet S has a high surface potential at the front end and a halftone image having a weak electrostatic binding force between the toner and the paper S is transferred, the toner is scattered toward the front end and the toner is scattered. And soiling easily occurs. However, since a solid image having a strong electrostatic bonding force between the toner and the paper S is transferred to the paper S in a state where the surface potential at the leading edge is high, the toner is scattered toward the leading edge. There is no toner scattering and scumming.

  When a halftone image is included (S15: YES), the CPU 81 determines the timing to apply the transfer voltage after the sensor 51 detects the leading edge of the paper S, as shown in FIG. It is set after the elapsed time TP2 (S18). That is, the CPU 81 causes the application unit 52 to apply a transfer voltage at the timing when the paper S passes the transfer position P. The CPU 81 sets the time required for raising the transfer voltage to the second application time TV2 (S19). That is, the CPU 81 raises the transfer voltage to the rated voltage after a lapse of a predetermined time from the start of application. The CPU 81 starts printing (S20).

  As shown in FIG. 7B, the sheet S does not concentrate charges at the leading end, and can suppress an increase in surface potential at the leading end. The sheet S has a high surface potential at the leading end, and even if a halftone image having a weak electrostatic binding force between the toner and the sheet S is transferred, the toner does not scatter toward the leading end. Difficult to scatter and dirt.

  As described above, the CPU 81 is required to raise the transfer voltage application timing and the transfer voltage to the rated voltage depending on whether or not a halftone image is included within a predetermined range from the leading edge of the image data. Control the time.

  As a result, the image forming apparatus 100 can reduce toner scattering and scumming even when a halftone image is formed on a sheet. In addition, when the halftone image is not included within a predetermined range from the leading end of the paper, that is, when the solid image is formed within the predetermined range from the leading end of the paper, the image forming apparatus 100 Even if a transport delay due to a transport error of S occurs, an image can be reliably formed on the paper S because the transfer voltage is applied on the upstream side of the transfer position and immediately started up.

  Further, the electrostatic binding force between the toner and the paper S is determined by the type of the paper S. In general, a thin paper S such as plain paper has a weaker electrostatic binding force with toner than a thick paper S such as a mount. Therefore, the image forming apparatus 100 has a thickness detection sensor (not shown) that detects the thickness of the conveyed paper S disposed on the paper transport path L. The CPU 81 may perform the process shown in FIG. 5 only for the thin paper S in accordance with the detection result of the thickness detection sensor. When the paper S is a thick paper, the CPU 81 starts applying the transfer voltage at the timing when the paper S passes the upstream side of the transfer position P regardless of the image included in the predetermined range from the leading edge of the image. The transfer voltage may be raised immediately to the rated voltage.

  In the above-described embodiment, the image forming apparatus 100 includes the single photosensitive drum 31 and forms a monochrome image. However, as shown in FIG. 8, the image forming apparatus 101 includes a plurality of photosensitive drums 31, and transfers toner images carried by the plurality of photosensitive drums 31 on the sheet S to transfer black (K). A color image using each color of cyan (C) magenta (M) and yellow (Y) may be formed.

  In this case, a plurality of transfer rollers 35A are arranged at positions facing the plurality of photosensitive drums 31 with each transfer position P interposed therebetween. Each transfer roller 35A is applied with a transfer voltage, charges the paper S passing through each transfer position P, and transfers the toner image from each photosensitive drum 31 to the paper S.

  In addition to the program, the ROM 82 stores the first elapsed time TP11, TP12, TP13, TP14, the second elapsed time TP21, TP22, TP23, TP24, the first application time TV1, and the second application time TV2. Yes. The first elapsed time TP11, TP12, TP13, TP14 is the time required from the time when the leading edge of the paper S is detected by the sensor 51 to the time when it passes the upstream side of each transfer position P. The second elapsed times TP21, TP22, TP23, and TP24 are times required from the time when the leading edge of the paper S is detected by the sensor 51 to the time when each of the transfer positions P is passed.

  The CPU 81 may perform the processing of S16 to S19 in FIG. 7 instead of the processing of S26 to S29 in FIG. As shown in FIG. 9, when the halftone image is not included in a predetermined range from the leading edge of the image data (S15: NO), the CPU 81 determines the timing of applying the transfer voltage at each transfer position P. It is set after the first elapsed time TP11, TP12, TP13, TP14 after 51 detects the leading edge of the paper S (S26). That is, the CPU 81 causes the applying unit 52 to apply a transfer voltage at the timing when the sheet S passes through the upstream side of each transfer position P. The CPU 81 sets the time required for raising the transfer voltage to the first application time TV1 (S27). That is, the CPU 81 immediately raises the transfer voltage from the start of application to the rated voltage.

  When a halftone image is included (S15: YES), the CPU 81 determines the timing of applying the transfer voltage after the second elapsed time TP21, TP22, TP23, TP24 after the sensor 51 detects the leading edge of the paper S. Set (S28). That is, the CPU 81 causes the applying unit 52 to apply a transfer voltage at the timing when the sheet S passes through each transfer position P. The CPU 81 sets the time required for raising the transfer voltage to the second application time TV2 (S29). That is, the CPU 81 raises the transfer voltage to the rated voltage after a lapse of a predetermined time from the start of application.

  As described above, the image forming apparatus 101 raises the transfer voltage application timing and the transfer voltage to the rated voltage depending on whether or not a halftone image is included within a predetermined range from the leading edge of the image data. By controlling the time required for this, even when an image including a halftone image at the front end portion is formed on the paper S, toner scattering and background contamination can be reduced. In addition, when the image forming apparatus 101 forms a solid image within a predetermined range from the leading edge of the paper S, the image forming apparatus 101 reliably forms an image on the paper S even if a conveyance delay due to a conveyance error of the paper S occurs. can do.

  Further, as shown in FIG. 10, the image forming apparatus 102 includes a plurality of photosensitive drums 31 having different hues, and the toner images carried by the plurality of photosensitive drums 31 are stacked on the intermediate transfer belt 351 and temporarily transferred. The toner image laminated on the intermediate transfer belt 351 is temporarily transferred onto the paper S to form a color image using each color of black (K) cyan (C) magenta (M) and yellow (Y). There may be.

  In this case, a plurality of primary transfer rollers 351A are arranged at positions facing the plurality of photosensitive drums 31 with the intermediate transfer belt 351 interposed therebetween. A secondary transfer roller 352A is disposed at a position facing the intermediate transfer belt 351 across the transfer position P. Each primary transfer roller 351A is applied with a transfer voltage, charges the intermediate transfer belt 351, and transfers a toner image from each photosensitive drum 31 to the intermediate transfer belt 351. The secondary transfer roller 352A is applied with a transfer voltage, charges the sheet S passing the transfer position P, and transfers the stacked toner images from the intermediate transfer belt 351 to the sheet S.

  The CPU 81 applies the transfer voltage to the secondary transfer roller 352A by the application unit 52 at the timing when the sheet S passes the upstream side of the transfer position P when the halftone image is not included within a predetermined range from the leading edge of the image data. The transfer voltage is immediately raised from the start of application to the rated voltage. When a halftone image is included, the CPU 81 applies a transfer voltage to the secondary transfer roller 352A at the timing when the paper S passes the transfer position P by the application unit 52, and after a predetermined time has elapsed since the application started. Raise the transfer voltage to the rated voltage.

  As described above, the image forming apparatus 102 raises the transfer voltage application timing and the transfer voltage to the rated voltage depending on whether or not a halftone image is included within a predetermined range from the leading edge of the image data. By controlling the time required for this, even when a halftone image is formed on the paper S, toner scattering and background contamination can be reduced. In addition, when forming a solid image within a predetermined range from the leading edge of the paper S, the image forming apparatus 101 reliably forms an image on the paper S even if a transport delay due to a transport error of the paper S occurs. be able to.

  The above description of the embodiment is to be considered in all respects as illustrative and not restrictive. The scope of the present invention is shown not by the above embodiments but by the claims. Furthermore, the scope of the present invention is intended to include all modifications within the meaning and scope equivalent to the scope of the claims.

P ... transfer position S ... paper TP1 ... first elapsed time TP2 ... second elapsed time TV1 ... first application time TV2 ... second application time 30 ... image forming unit 31 ... photosensitive drum 35 ... transfer belt 35A ... transfer roller 51 ... Sensor 52 ... Applying unit 53 ... High voltage power supply 80 ... Control unit 81 ... CPU
82 ... ROM
83 ... RAM
84 ... Timer 100 ... Image forming apparatus

Claims (4)

An image carrier that carries a toner image formed by an electrophotographic method using image data, and a position that faces the image carrier across a transfer position for transferring the toner image to a sheet. Transfer means to which a transfer bias voltage for transferring to the paper is applied; application means for applying the transfer bias voltage to the transfer means; and transport means for transporting the paper so as to pass through the transfer position; An image forming apparatus comprising:
A discriminating means for discriminating whether or not a halftone image is included within a predetermined range from the tip of the image data;
Measuring means for measuring a first timing when the leading edge of the paper passes the upstream side of the transfer position and a second timing when the leading edge of the paper passes the transfer position;
The control unit controls the application of the transfer bias voltage by the applying unit according to the determination result of the determining unit. When the determining unit determines that the halftone image is not included, the first unit When the transfer bias voltage is applied at the timing and the first control process is performed to immediately rise to the rated voltage, and the determination unit determines that the halftone image is included, the transfer bias is applied at the second timing. An image forming apparatus comprising: a control unit that applies a voltage and performs a second control process for raising the voltage to a rated voltage after a predetermined time has elapsed. A plurality of image carriers each corresponding to a different hue; a plurality of transfer units disposed at positions facing the plurality of image carriers across the different transfer positions; and the application unit; An image forming apparatus comprising the conveying unit,
The measuring means measures the first timing and the second timing for each transfer position;
The first control process is a process of applying the transfer bias voltage to the transfer unit corresponding to each transfer position at the first timing measured for each transfer position;
The image forming apparatus according to claim 1, wherein the second control process is a process of applying the transfer bias voltage to the transfer unit corresponding to each transfer position at the second timing measured for each transfer position. . A plurality of image carriers that carry toner images formed by electrophotography using image data, each of which has an image carrier corresponding to a different hue, and a position that faces each of the plurality of image carriers. A plurality of first transfer means arranged to apply a first transfer bias voltage for transferring the toner images from the plurality of image carriers to the intermediate transfer material, and transferring the toner images to a sheet; A second transfer unit disposed at a position facing the intermediate transfer material across the transfer position, to which a second transfer bias voltage is applied to transfer the toner image laminated on the intermediate transfer material to the paper; An application unit that applies the first transfer bias voltage to the first transfer unit and an application unit that applies the second transfer bias voltage to the second transfer unit; and a conveyance unit that conveys the sheet so as to pass through the transfer position. An image forming apparatus comprising: a stage, a
A discriminating means for discriminating whether or not a halftone image is included within a predetermined range from the tip of the image data;
Measuring means for measuring a first timing when the leading edge of the paper passes the upstream side of the transfer position and a second timing when the leading edge of the paper passes the transfer position;
Control means for controlling the application of the second transfer bias voltage by the applying means according to the determination result of the determining means; and when the determining means determines that the halftone image is not included, When the first control process of applying the second transfer bias voltage at the first timing and immediately raising the voltage to the rated voltage is performed, and the determination unit determines that the halftone image is included, the second timing An image forming apparatus comprising: a control unit configured to apply the second transfer bias voltage and to perform a second control process for raising the voltage to a rated voltage after a lapse of a predetermined time. Further comprising detection means for detecting whether the paper is thin paper or thick paper,
The image forming apparatus according to claim 1, wherein the control unit controls the application unit when the sheet detected by the detection unit is a thin sheet.

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