Image forming apparatus

Abstract

To provide an image forming apparatus capable of suppressing transfer deviation caused when thick paper rushes into a fixing part.SOLUTION: According to an aspect of the present disclosure, an image forming apparatus includes a transfer unit configured to transfer a toner image to a sheet, a fixing unit configured to fix the toner image to the sheet, a stiffness measurement unit configured to directly or indirectly measure stiffness of the sheet conveyed on a sheet conveyance path upstream of the transfer unit, and a control unit configured to control conveyance of the sheet. The fixing unit includes a first conveyance member, a second conveyance member, and a motor provided to rotate the first and second conveyance members, and is provided such that the sheet passes through a fixing nip between the first conveyance member and the second conveyance member, and the control unit is provided to send a control signal to the motor to change a sheet conveyance speed in the fixing unit according to a measurement result of the stiffness measurement unit.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] It is known that when printing on thick paper using an electrophotographic image forming apparatus, impact unevenness (banding, transfer misalignment) appears in the printed image (see, for example, Patent Document 1). Patent Document 1 describes that when paper enters the fixing unit, the conveying speed of the fixing unit changes depending on the paper thickness, so the paper conveying speed does not become constant, resulting in a speed difference between the paper conveying speed and the transfer speed depending on the paper thickness, resulting in transfer misalignment, and describes changing the PID control parameters of the fixing conveying speed depending on the paper thickness to prevent this transfer misalignment from occurring. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-206588 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the prior art, even if the conveyance speed of the fixing unit is changed depending on the paper thickness, transfer misalignment may occur depending on the paper. The present invention has been made in view of the above circumstances, and provides an image forming apparatus that can suppress transfer misalignment that occurs when thick paper enters a fixing unit. [Means for solving the problem]

[0005] The present invention provides an image forming apparatus comprising: a transfer unit configured to transfer a toner image onto paper; a fixing unit configured to fix the toner image onto the paper; a stiffness measuring unit configured to directly or indirectly measure the stiffness of paper transported on a paper transport path upstream of the transfer unit; and a control unit configured to control the transport of the paper, wherein the fixing unit comprises a first transport member, a second transport member, and a motor configured to rotate the first and second transport members, and is configured so that the paper passes through a fixing nip between the first transport member and the second transport member, and the control unit is configured to send a control signal to the motor to change the paper transport speed in the fixing unit depending on the measurement result of the stiffness measuring unit. [Effects of the Invention]

[0006] The inventors of the present invention conducted the experiments described below and found that even when sheets of paper have substantially the same basis weight, transfer misalignment may or may not occur depending on the stiffness of the paper. According to the present invention, by changing the paper transport speed in the fixing unit in accordance with the measurement result of the stiffness measuring unit, it is possible to suppress transfer misalignment that occurs when thick paper enters the fixing nip. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a schematic diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 2] FIG. 2 is an enlarged view of the area A enclosed by the dashed line in FIG. [Figure 3] FIG. 2 is an enlarged view of an area B surrounded by a dashed line in FIG. [Figure 4] 10(a) to 10(c) are explanatory diagrams of modified examples of the stiffness measuring unit. [Figure 5] 10(a) to 10(c) are explanatory diagrams of modified examples of the stiffness measuring unit. [Figure 6] 10 is a flowchart for controlling the paper transport speed of the fixing unit. [Figure 7] 10 is a flowchart for controlling the paper transport speed of the fixing unit. [Figure 8] 10 is a graph showing changes in the clock frequency of a control signal that gives an operation command to a motor for a fixing unit. [Figure 9] 1 is a graph showing experimental results of Clark stiffness testing and printing experiments. [Figure 10] 10 is a graph showing the measurement results of changes in the clock frequency of a control signal that gives an operation command to a motor for a fixing unit, and the measurement results of changes in the rotation speed of the motor for a fixing unit. [Figure 11] 10 is a graph showing the measurement results of changes in the clock frequency of a control signal that gives an operation command to a motor for a fixing unit, and the measurement results of changes in the rotation speed of the motor for a fixing unit. [Figure 12] 10 is a graph showing the measurement results of changes in the clock frequency of a control signal that gives an operation command to a motor for a fixing unit, and the measurement results of changes in the rotation speed of the motor for a fixing unit. [Figure 13] 10 is a graph showing the measurement results of changes in the clock frequency of a control signal that gives an operation command to a motor for a fixing unit, and the measurement results of changes in the rotation speed of the motor for a fixing unit. DETAILED DESCRIPTION OF THE INVENTION

[0008] The image forming apparatus of the present invention comprises a transfer unit configured to transfer a toner image onto paper, a fixing unit configured to fix the toner image onto the paper, a stiffness measuring unit configured to directly or indirectly measure the stiffness of paper transported on a paper transport path upstream of the transfer unit, and a control unit configured to control the transport of the paper, wherein the fixing unit comprises a first transport member, a second transport member, and a motor configured to rotate the first and second transport members, and is configured so that the paper passes through a fixing nip between the first transport member and the second transport member, and the control unit is configured to send a control signal to the motor to change (adjust) the paper transport speed in the fixing unit depending on the measurement result of the stiffness measuring unit.

[0009] Preferably, the control unit is configured to send a control signal to the motor so as to make the paper transport speed in the fixing unit faster than the paper transport speed during transfer in the transfer unit before the leading edge of the paper enters the fixing nip, and is configured to change (adjust) a first speed difference between the paper transport speed in the fixing unit after the speed increase and the paper transport speed during transfer in the transfer unit according to the measurement result of the stiffness measurement unit. The control unit is preferably configured to increase the first speed difference as the stiffness of the paper increases. Preferably, the control unit is configured to send a control signal to the motor when the basis weight of the paper is greater than a predetermined basis weight so that the paper transport speed in the fixing unit is faster than the paper transport speed during transfer in the transfer unit before the leading edge of the paper enters the fixing nip, and when the basis weight of the paper is less than the predetermined basis weight, to send a control signal to the motor so that the paper transport speed in the fixing unit is substantially the same as the paper transport speed during transfer in the transfer unit before the leading edge of the paper enters the fixing nip.

[0010] It is preferable that the control unit is configured to send a control signal to the motor so as to make the paper transport speed in the fixing unit faster than the paper transport speed during transfer in the transfer unit, and then decelerate the paper transport speed in the fixing unit to a speed substantially the same as or slower than the paper transport speed during transfer in the transfer unit at the timing when the leading edge of the paper enters the fixing nip. It is preferable that the control unit is configured to send a control signal to the motor so as to decelerate the paper transport speed in the fixing unit in stages to a speed substantially the same as or slower than the paper transport speed during transfer in the transfer unit. The control section is preferably arranged to cause the leading edge of the paper to enter the fixing nip while the paper transport speed in the fixing section is being reduced. Preferably, the control unit is configured to send a control signal to the motor so as to decelerate the paper transport speed in the fixing unit to a speed slower than the paper transport speed during transfer in the transfer unit after the leading edge of the paper enters the fixing nip, and then increase the paper transport speed in the fixing unit to a speed substantially the same as the paper transport speed during transfer in the transfer unit, and is configured to change (adjust) a second speed difference between the paper transport speed in the fixing unit after deceleration and the paper transport speed during transfer in the transfer unit according to the measurement result of the stiffness measurement unit. The control unit is preferably configured to increase the second speed difference as the stiffness of the paper increases.

[0011] Preferably, the control unit is configured to send a control signal to the motor when the basis weight of the paper is greater than a predetermined basis weight so as to slow down the paper transport speed in the fixing unit to a speed slower than the paper transport speed during transfer in the transfer unit, and when the basis weight of the paper is less than the predetermined basis weight so as to send a control signal to the motor so as to slow down the paper transport speed in the fixing unit to a speed substantially the same as the paper transport speed during transfer in the transfer unit. It is preferable that the control unit is configured to send a control signal to the motor so as to slow down the paper transport speed in the fixing unit to a speed slower than the paper transport speed during transfer in the transfer unit after the leading edge of the paper enters the fixing nip, and then, at the timing when the leading edge of the paper leaves the fixing nip, to increase the paper transport speed in the fixing unit to a speed substantially the same as the paper transport speed during transfer in the transfer unit. It is preferable that the control unit is configured to send a control signal to the motor so as to slow down the paper transport speed in the fixing unit to a speed slower than the paper transport speed during transfer in the transfer unit after the leading edge of the paper enters the fixing nip, and then gradually increase the paper transport speed in the fixing unit to a speed substantially the same as the paper transport speed during transfer in the transfer unit.

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The configurations shown in the drawings and the following description are merely examples, and the scope of the present invention is not limited to those shown in the drawings and the following description.

[0013] FIG. 1 is a schematic diagram showing the configuration of an image forming apparatus according to this embodiment, FIG. 2 is an enlarged view of an area A surrounded by a dashed line in FIG. 1, and FIG. 3 is an enlarged view of an area B surrounded by a dotted line in FIG. 1. The image forming apparatus 50 of this embodiment comprises a transfer unit 24 arranged to transfer a toner image onto the paper 3, a fixing unit 28 arranged to fix the toner image onto the paper 3, a stiffness measuring unit 6 arranged to directly or indirectly measure the stiffness of the paper 3 transported along the paper transport path 2 upstream of the transfer unit 24, and a control unit 7 arranged to control the transport of the paper 3, wherein the fixing unit 28 comprises a first transport member 25, a second transport member 26, and a motor 29 arranged to rotate the first transport member 25 and the second transport member 26, and is arranged so that the paper 3 passes through a fixing nip 27 between the first transport member 25 and the second transport member 26, and the control unit 7 is arranged to send a control signal to the motor 29 to change the paper transport speed in the fixing unit 28 depending on the measurement result of the stiffness measuring unit 6.

[0014] Image forming apparatus 50 is an electrophotographic image forming apparatus that forms images using electrophotographic technology. Image forming apparatus 50 may be a monochrome image forming apparatus capable of forming monochrome images, or an intermediate transfer color image forming apparatus capable of forming color images as shown in FIG. 1. Image forming apparatus 50 is a so-called tandem full-color image forming apparatus having a configuration in which toner image forming section 18a, which forms a toner image using black toner, toner image forming section 18b, which forms a toner image using cyan toner, toner image forming section 18c, which forms a toner image using magenta toner, and toner image forming section 18d, which forms a toner image using yellow toner, are arranged side by side in a predetermined direction (e.g., horizontally or vertically). Image forming apparatus 50 may also be another color image forming apparatus, a copier, a multifunction peripheral, or a facsimile machine.

[0015] The control unit 7 controls the image forming apparatus 50. The control unit 7 may include, for example, an arithmetic processing unit (e.g., a CPU), RAM, a storage device (e.g., an HDD), a network controller, a video controller, and a power supply unit. The control unit 7 is also configured to control the drive motors that rotate the drive rollers of the transport rollers 4a to 4j. This allows the control unit 7 to control the rotation speed, rotation direction, and rotation start / stop timing of the transport rollers 4a to 4j. The control unit 7 can also control the paper transport speed in the transfer unit 24 and the paper transport speed in the fixing unit 28. The control unit 7 can also control the paper transport speed in the fixing unit 28 by sending a control signal to the fixing unit motor 29. The control unit 7 can also control the direct or indirect measurement of the stiffness (stiffness) of the paper 3 by the stiffness measurement unit 6 and input the measurement results.

[0016] The toner image forming units 18a to 18d each include photoconductors 19a to 19d, a charger, an exposure unit 33, a developing unit, a transfer means, and a cleaning unit. The photoconductors 19a-19d are components on whose surfaces latent images and toner images are formed. As the photoconductors 19a-19d rotate, toner images of black toner, cyan toner, magenta toner, or yellow toner are successively formed based on image data. The photoconductors 19a-19d are, for example, photoconductor drums. The charger, exposure unit 33, development unit, transfer means, and cleaning unit are provided in this order along the outer circumferential surfaces of the photoconductors 19a-19d from upstream to downstream in the direction of rotation of the photoconductors 19a-19d.

[0017] The toner images of black toner, cyan toner, magenta toner, and yellow toner formed on the photoconductors 19a to 19d are transferred to the intermediate transfer belt 20 by the transfer means (primary transfer) and superimposed on each other. As a result, color toner images are formed on the intermediate transfer belt 20. The color toner images on the intermediate transfer belt 20 are transferred to the paper 3 transported through the paper transport path 2 in the transfer unit 24 (secondary transfer).

[0018] The paper transport device is a device that transports paper along the paper transport path 2. The paper transport device can include the paper transport path 2, transport rollers 4a to 4j, registration rollers 5, transfer unit 24, fixing unit 28, paper feed tray 21, manual feed tray 22, and paper output tray 23. The paper transport device can also have multiple paper sensors. Paper transport by the paper transport device is controlled by control signals sent from control unit 7 to motors that rotate each roller.

[0019] The paper transport device is configured to pick up paper 3 from paper feed tray 21 or manual feed tray 22, transfer the color toner image from intermediate transfer belt 20 to paper 3 in transfer section 24, fix the color toner image to paper 3 in fixing section 28, and discharge paper 3 with the color toner image to paper output tray 23. In addition, after forming a color toner image on the front surface of the paper 3, the paper transport device transports the paper 3 using transport rollers 4h to 4j, passes the paper 3 through the transfer section 24 and the fixing section 28 again, and transports the paper 3 so that a color toner image is also formed on the back surface of the paper 3.

[0020] The fixing unit 28 includes a first conveying member 25, a second conveying member 26, and a fixing unit motor 29 that is arranged to rotate the first conveying member 25 and the second conveying member 26, and is arranged so that the paper 3 passes through a fixing nip 27 between the first conveying member 25 and the second conveying member 26. The first conveying member 25 is, for example, a heating roller or a heating belt that is arranged to heat the toner image on the paper 3 that passes through the fixing nip 27. The second conveying member 26 is, for example, a pressure roller that is arranged to press the first conveying member 25 through the fixing nip 27. The fixing unit 28 includes the first conveying member 25 and the second conveying member 26, so that the toner image can be melted and pressed to fix it to the paper 3.

[0021] The fixing unit motor 29 is a motor configured to drive either the first conveying member 25 or the second conveying member 26. In FIG. 2, the motor 29 is configured to drive the second conveying member 26. Because the second conveying member 26 and the first conveying member 25 are in contact with each other at the fixing nip 27, when the motor 29 rotates to drive the second conveying member 26, the first conveying member 25 also rotates in response. Therefore, the rotation of the first conveying member 25 is reverse to the rotation of the second conveying member 26, and the fixing unit 28 can convey the paper 3 that has entered the fixing nip 27 through the fixing nip 27. The control unit 7 controls the rotation of the motor 29 using a control signal, thereby controlling the paper conveying speed of the fixing unit 28.

[0022] The type of fixing unit motor 29 may be, for example, a stepping motor, a brushed motor, a brushless motor, an induction motor, or a synchronous motor, but a stepping motor (VR type, PM type, or HB type) is preferable. This allows the rotation angle and rotation speed of motor 29 to be accurately controlled by a pulse signal (control signal) from control unit 7. Motor 29 may be equipped with a motor drive IC that controls the rotation of motor 29.

[0023] The rotation of the motor 29 is controlled by a control signal from the control unit 7, and the paper is normally (for example, plain paper (basis weight: 60 to 105 g / m 2), the paper transport speed in the fixing unit 28 is controlled to be substantially the same as the paper transport speed in the transfer unit 24. This stabilizes the paper transport speed in the transfer unit 24, improving print quality.

[0024] However, just like regular paper, cardboard (basis weight: 106 to 360 g / m 2 When printing on paper with a basis weight of 200 g / m², banding (horizontal streaks) may appear on the printed image. 2 When printing on cardboard with a basis weight of 300 g / m or more, banding is likely to appear in the printed image. 2 When printing on thick paper, banding is even more likely to appear in the printed image. This banding is thought to be caused by a speed fluctuation (impact) caused by a decrease in the rotational speed of the first transport member 25 and the second transport member 26 due to an increase in load when the leading edge of the paper 3 (cardboard) is bitten into the fixing nip 27. This fluctuation is transmitted through the paper 3 and affects the transfer nip of the transfer unit 24, causing transfer misalignment. The transmission of this impact is thought to depend on the stiffness (stiffness) of the paper 3. In other words, even for paper of the same basis weight, the stiffer the paper 3, the more likely banding is to occur. The reason for this is thought to be as follows: A stiff paper 3 can hardly bend in the paper transport path 2 between the transfer unit 24 and the fixing unit 28. Therefore, the impact when the leading edge of the paper 3 enters the fixing nip 27 is easily transmitted to the transfer unit 24 via the paper 3. On the other hand, a weak paper 3 can bend easily. Therefore, even if an impact occurs when the leading edge of the paper 3 enters the fixing nip 27, the impact can be absorbed by the paper 3 bending in the paper transport path 2 between the transfer section 24 and the fixing section 28, thereby reducing the impact on the transfer section 24.

[0025] Furthermore, it is conceivable that the speed fluctuation (impact) caused by the increase in the rotational speed of the first conveying member 25 and the second conveying member 26 due to the reduction in load when the leading edge of the paper 3 passes through the fixing nip 27 is transmitted through the paper 3 and affects the transfer nip of the transfer unit 24, resulting in transfer misalignment. If the paper 3 is stiff, the deflection of the paper 3 that occurs when the leading edge of the paper enters the fixing nip 27 is small, and conversely, if the paper 3 is weak, the deflection of the paper 3 that occurs when the leading edge of the paper enters the fixing nip 27 is large. Because the leading edge of the paper 3 passes through the fixing nip 27 while maintaining this deflection, stiff paper 3 that only maintains a small deflection does not have enough room to withstand the pulling of the paper 3 when the speed increases, and therefore stiffer paper 3 is more likely to transmit impact to the transfer unit 24 than weaker paper 3. Conversely, in the case of paper 3 with weak stiffness that maintains a large deflection, there is a large margin for the pulling of the paper 3 when the speed increases, so it is thought that the impact on the transfer section 24 is more mitigated for paper 3 with weak stiffness than for paper 3 with strong stiffness.

[0026] The stiffness measurement unit 6 is a unit that directly or indirectly measures the stiffness of the paper 3, and is provided to directly or indirectly measure the stiffness of the paper 3 transported on the paper transport path 2 upstream of the transfer unit 24. The stiffness measurement unit 6 can be provided, for example, to directly or indirectly measure the stiffness of the paper 3 as it travels toward the resist rollers 5 on the paper transport path 2 after the paper transport path 2 from the paper feed tray 21 toward the resist rollers 5 and the paper transport path 2 along which the paper 3, having formed a toner image on its front side, travels again toward the resist rollers 5 to form a toner image on its back side, converge. The stiffness measurement unit 6 is connected to the control unit 7, and the control unit 7 can input the measurement results from the stiffness measurement unit 6. The stiffness measurement unit 6 may also be controlled by the control unit 7.

[0027] 1 and 3, the stiffness measurement unit 6 can have a pressing force measurement sensor 15. The pressing force measurement sensor 15 can be provided so that the conveyed paper 3 collides with the pressing force measurement sensor 15, changing the direction of travel of the paper 3. The pressing force measurement sensor 15 can be placed, for example, at the part where the paper conveyance path 2 begins to curve. Stiff paper 3 has a large resistance when it is bent. Therefore, when stiff paper 3 being conveyed along paper conveyance path 2 hits pressing force measuring sensor 15 and the direction of travel of paper 3 changes, the measurement value measured by pressing force measuring sensor 15 becomes large. A weak sheet of paper 3 has little resistance when it is bent. Therefore, when the weak sheet of paper 3 being conveyed along the paper conveyance path 2 hits the pressing force measuring sensor 15 and the direction of travel of the sheet of paper 3 changes, the measurement value measured by the pressing force measuring sensor 15 becomes small. Therefore, the stiffness of the paper 3 conveyed along the paper conveying path 2 can be measured using the pressing force measuring sensor 15.

[0028] 4(a) to 4(c) are explanatory diagrams of a modified example of stiffness measuring unit 6. In this modified example, stiffness measuring unit 6 has a pressed plate 11 that is provided so as to rotate about a rotation axis 12. When no paper 3 is being transported along paper transport path 2 (for example, FIG. 4(a)), pressed plate 11 is pressed by spring pressure (for example, the spring pressure of a coil spring), and pressed plate 11 is fixed so that at least a part of pressed plate 11 is positioned to collide with paper 3 being transported along paper transport path 2.

[0029] When paper 3 being transported along paper transport path 2 hits pressed plate 11, pressed plate 11 rotates around rotation axis 12 against the spring pressure. For example, when paper 3 is transported along the paper transport path shown in FIG. 4(a) as shown in FIGS. 4(b) and 4(c), paper 3 hits pressed plate 11, and pressed plate 11 rotates around rotation axis 12 against the spring pressure. In FIG. 4(b), paper 3 with low stiffness (low resistance when bending) is being transported, and the rotation angle of pressed plate 11 is relatively small. In FIG. 4(c), paper 3 with high stiffness (high resistance when bending) is being transported, and the rotation angle of pressed plate 11 is relatively large.

[0030] The stiffness measuring unit 6 is provided to measure the rotation angle of this rotation or the travel distance of the pressed plate 11. When the stiffness measuring unit 6 measures the rotation angle, the stiffness measuring unit 6 can be equipped with a rotation angle detecting encoder connected to the rotation shaft 12. If the paper 3 being transported is stiff, the measured rotation angle will be large, and if the paper 3 being transported is weak, the measured rotation angle will be small. For this reason, the stiffness of the paper 3 transported along the paper transport path 2 can be measured using a rotation angle detecting encoder.

[0031] When the stiffness measurement unit 6 measures the distance traveled by the pressed plate 11, the stiffness measurement unit 6 can be equipped with a distance measurement sensor 14. The distance measurement sensor 14 may be provided to measure the amount of displacement of the pressed plate 11 on the paper transport path side of the rotating shaft 12 (above the rotating shaft 12 in FIG. 4(a)), or may be provided to measure the amount of displacement of the pressed plate 11 on the opposite side of the paper transport path (below the rotating shaft 12 in FIG. 4(a)). If the paper 3 being transported is stiff, the measured amount of displacement will be large, and if the paper 3 being transported is weak, the measured amount of displacement will be small. Therefore, the stiffness of the paper 3 transported along the paper transport path 2 can be measured using the distance measurement sensor 14.

[0032] 5(a) to (c) are explanatory diagrams of a modified example of stiffness measurement unit 6. In this modified example, stiffness measurement unit 6 has an elastic sheet member 13 that is positioned so that at least a portion of it will collide with paper 3 transported along paper transport path 2. Stiffness measurement unit 6 also has a distance measurement sensor 14. FIG. 5(a) shows a state in which paper 3 is not being transported along paper transport path 2.

[0033] When paper 3 conveyed along paper transport path 2 hits elastic sheet member 13, elastic sheet member 13 undergoes elastic deformation. For example, when paper 3 is conveyed along the paper transport path shown in FIG. 5(a) as shown in FIGS. 5(b) and 5(c), paper 3 hits elastic sheet member 13 and elastically deforms. Distance measurement sensor 14 is provided to measure the amount of displacement of elastic sheet member 13 accompanying this elastic deformation. In FIG. 5(b), paper 3 with low stiffness (low resistance when bending) is conveyed, and the amount of displacement of elastic sheet member 13 is relatively small. In FIG. 5(c), paper 3 with high stiffness (high resistance when bending) is conveyed, and the amount of displacement of elastic sheet member 13 is relatively large. Therefore, the stiffness of paper 3 conveyed along paper transport path 2 can be measured using distance measurement sensor 14.

[0034] The control unit 7 is configured to send a control signal to the fixing unit motor 29 so as to change the paper transport speed in the fixing unit 28 in accordance with the measurement result of the stiffness measurement unit 6. This makes it possible to change the paper transport speed in the fixing unit 28 in accordance with the stiffness of the paper 3, thereby suppressing transfer misalignment that occurs when thick paper enters the fixing nip 27. If the stiffness measurement unit 6 includes a distance measurement sensor 14, the measurement result of the stiffness measurement unit 6 may be data corresponding to the distance measured by the distance measurement sensor 14. If the stiffness measurement unit 6 includes a rotation angle detection encoder, the measurement result of the stiffness measurement unit 6 may be data corresponding to the rotation angle measured by the rotation angle detection encoder.

[0035] The control unit 7 may store a plurality of control signal data corresponding to the measurement results of the stiffness measurement unit 6. In this case, the control unit 7 can select one control signal data from the plurality of control signal data based on the measurement results of the stiffness measurement unit 6, and send a control signal to the fixing unit motor 29 based on the selected control signal data (table management). Such a plurality of control signal data can be created based on the experimental results of a preliminary experiment.

[0036] The measurement results of the stiffness measurement unit 6 used to calculate or select the control signal to be sent to the fixing unit motor 29 may be the measurement results of the paper 3 actually transported through the fixing unit 28 (measured for each sheet). This makes it possible to respond to changes in the stiffness of the paper 3 based on changes in the moisture content of the paper 3. Furthermore, when printing multiple sheets at once, the stiffness measurement unit 6 measures the first sheet, and a control signal is calculated or selected based on the measurement results, and this control signal can be sent to the fixing unit motor 29 when printing on each sheet 3. This can improve the printing processing speed. Also, when printing multiple sheets at once, the change in stiffness of the sheets 3 due to changes in the moisture content of the sheets 3 is slight, so this change in stiffness can be ignored. Furthermore, when double-sided printing is performed, measurements can be taken by stiffness measuring unit 6 before printing on the front side and before printing on the back side, and a control signal can be sent to fixing unit motor 29 when printing on the front side and back side based on the results of these measurements. When double-sided printing is performed, the paper 3 passes through fixing unit 28 when printing on the front side, causing the temperature to rise. For this reason, the moisture content of paper 3 changes between when printing on the front side and when printing on the back side, and the stiffness of paper 3 may change. By taking measurements by stiffness measuring unit 6 before printing on the front side and before printing on the back side, it is possible to respond to such changes in the stiffness of paper 3.

[0037] 6, the control unit 7 can control the paper transport speed of the transfer unit 24. For example, the control unit 7 can obtain the measurement result of the stiffness measurement unit 6 in step S1, and send a control signal to the fixing unit motor 29 in step S2 so that the paper transport speed of the fixing unit 28 becomes the paper transport speed based on the measurement result of the stiffness measurement unit 6.

[0038] The control unit 7 preferably controls the basis weight (g / m 2) is larger than the predetermined basis weight, a control signal is sent to the fixing unit motor 29 so as to change (adjust) the paper conveying speed in the fixing unit 28 in accordance with the measurement result of the stiffness measuring unit 6, and when the basis weight of the paper 3 is smaller than the predetermined basis weight, a control signal is sent to the fixing unit motor 29 so as to make the paper conveying speed in the fixing unit 28 substantially the same as the paper conveying speed during transfer in the transfer unit 24. The control unit 7 can control the paper conveyance speed of the transfer unit 24, for example, as shown in the control flowchart in FIG. 7. For example, the control unit 7 can acquire the basis weight of the paper 3 in step S11, and determine whether the basis weight of the paper 3 is equal to or greater than a predetermined basis weight in step S12. The control unit 7 can acquire the basis weight of the paper 3 based on, for example, print settings set by the user or settings of the paper feed tray being used. The control unit 7 can also calculate the basis weight of the paper 3 based on the output of a sensor provided in the paper conveyance path 2. The predetermined basis weight can be, for example, 221 g / m 2 It can be said that:

[0039] If it is determined in step S12 that the basis weight of the paper 3 is greater than the predetermined basis weight, the control unit 7 acquires the measurement results of the stiffness measurement unit 6 in step S13, and in step S14 sends a control signal to the fixing unit motor 29 so that the paper transport speed of the fixing unit 28 becomes the paper transport speed based on the measurement results of the stiffness measurement unit 6. If it is determined in step S12 that the basis weight of the paper 3 is smaller than the predetermined basis weight, then in step S15 the control unit 7 can send a control signal to the fixing unit motor 29 so that the paper transport speed of the fixing unit 28 is substantially the same as the paper transport speed during transfer in the transfer unit 24 (i.e., the normal paper transport speed).

[0040] FIG. 8 is a graph showing changes in the clock frequency of the control signal that the control unit 7 issues to the fixing unit motor 29 as an operation command when printing on thick paper. The fixing unit motor 29 is a stepping motor, and the higher the clock frequency, the faster the fixing unit motor 29 rotates, and the faster the paper transport speed in the fixing unit 28. The graph in FIG. 8 also shows the clock frequency corresponding to the normal speed, the timing at which the leading edge of the paper 3 enters the fixing nip 27, the timing at which the leading edge of the paper 3 leaves the fixing nip 27, and the timing at which the trailing edge of the paper 3 leaves the fixing nip 27. The "normal speed" shown in FIG. 8 is the speed when the paper transport speed in the fixing unit 28 is substantially the same as the paper transport speed during transfer in the transfer unit 24, and the clock frequency of the control signal corresponding to that.

[0041] The control unit 7 can send a control signal of a clock frequency such as the graph shown in FIG. 8 to the fixing unit motor 29 in accordance with the measurement results of the stiffness measurement unit 6, and can control the paper transport speed of the fixing unit 28. The control unit 7 is preferably configured to send a control signal to the fixing unit motor 29 so as to make the paper transport speed in the fixing unit 28 faster than the paper transport speed during transfer in the transfer unit 24 before the leading edge of the paper 3 enters the fixing nip 27, and is also configured to change (adjust) the first speed difference between the paper transport speed in the fixing unit 28 after the increased speed and the paper transport speed during transfer in the transfer unit 24 according to the measurement result of the stiffness measurement unit 6.

[0042] For example, as shown in the graph of FIG. 8 , the control unit 7 can send a control signal (clock frequency signal) to the fixing unit motor 29 so that the paper transport speed in the fixing unit 28 is faster than the paper transport speed during transfer in the transfer unit 24 by a first speed difference before the leading edge of the paper 3 enters the fixing nip 27. The first speed difference can be determined based on the measurement results of the stiffness measurement unit 6. Specifically, the control unit 7 can be configured to increase the first speed difference as the stiffness of the paper 3 increases. In this way, by adjusting the first speed difference based on the measurement results of the stiffness measurement unit 6, it is possible to prevent the impact caused by the leading edge of the paper 3 entering the fixing nip 27 from being transmitted to the transfer unit 24, and to prevent banding from appearing in the printed image.

[0043] For example, when conveyance of the paper sheet 3 starts and the leading edge of the paper sheet 3 approaches the fixing unit 28, the control unit 7 increases the clock frequency signal of the control signal to start rotating the fixing unit motor 29, the second conveying member 26, and the first conveying member 25. At this time, the control unit 7 adjusts the clock frequency of the control signal so that the paper conveyance speed in the fixing unit 28 is faster by a first speed difference than the paper conveyance speed (normal speed) during transfer in the transfer unit 24. For example, when the distance from the leading edge of the paper sheet 3 to the fixing nip 27 is between 3 mm and 10 mm, the control unit 7 can adjust the clock frequency of the control signal to a clock frequency corresponding to a paper conveyance speed (speed before paper entry) that is faster than the normal speed by the first speed difference.

[0044] The first speed difference can be adjusted so that the percentage (%) of the difference between the clock frequency corresponding to the pre-paper entry speed and the clock frequency corresponding to the normal speed (when the clock frequency corresponding to the normal speed is 100%) (speed increase rate) is 1% or more and 10% or less. Furthermore, the first speed difference can be adjusted within this range depending on the measurement results of stiffness measurement unit 6.

[0045] The control unit 7 can be configured to make the paper transport speed in the fixing unit 28 faster by a first speed difference than the paper transport speed during transfer in the transfer unit 24, and then send a control signal to the fixing unit motor 29 so as to decelerate the paper transport speed in the fixing unit 28 to a speed substantially the same as or slower than the paper transport speed during transfer in the transfer unit 24 at the timing when the leading edge of the paper 3 enters the fixing nip 27. In addition, the control unit 7 may be configured to send a control signal to the fixing unit motor 29 to gradually decelerate the paper transport speed in the fixing unit 28 to a speed substantially the same as or slower than the paper transport speed during transfer in the transfer unit 24. Furthermore, the control section 7 may be configured to cause the leading edge of the paper sheet 3 to enter the fixing nip 27 while the paper sheet conveyance speed in the fixing section 28 is being reduced.

[0046] By increasing the paper transport speed in fixing unit 28 by the first speed difference before the leading edge of paper 3 enters fixing nip 27, it is possible to prevent the paper transport speed in transfer unit 24 from changing when the leading edge of paper 3 enters fixing nip 27. Furthermore, by gradually reducing the paper transport speed in fixing unit 28, compared to a case where the paper transport speed in fixing unit 28 is suddenly reduced from an accelerated state to the paper transport speed during transfer in time with the leading edge of paper 3 entering fixing nip 27, even if the timing of paper 3 entering fixing nip 27 is delayed due to slippage with the transport roller, the paper 3 enters fixing nip 27 at a somewhat accelerated speed. This makes it possible to prevent the paper transport speed in fixing unit 28 from being significantly reduced from the paper transport speed during transfer, thereby preventing the paper transport speed in transfer unit 24 from changing. Furthermore, by gradually slowing down the paper transport speed in the fixing section 28, the deceleration time (deceleration period) is lengthened, and even if the timing at which the leading edge of the paper 3 enters the fixing nip 27 varies, banding can be suppressed as long as it is within the deceleration period.

[0047] For example, the control unit 7 starts to gradually decrease the clock frequency of the control signal so that the paper transport speed of the fixing unit 28 slows down from the speed before paper entry at the timing when or shortly before the leading edge of the paper 3 enters the fixing nip 27. Then, the control unit 7 causes the leading edge of the paper 3 to enter the fixing nip 27 immediately after starting to gradually decrease the clock frequency, and gradually decreases the clock frequency while the leading edge of the paper 3 is moving through the fixing nip 27. This makes it possible to suppress a decrease in the paper transport speed in the transfer unit 24 that occurs due to an increase in the rotational resistance (load) of the fixing unit 28 caused by the leading edge of the paper 3 entering the fixing nip 27, and to suppress the appearance of banding in the printed image. Furthermore, by the control unit 7 gradually decreasing the clock frequency of the control signal, it is possible to suppress the appearance of banding in the printed image even if the timing when the leading edge of the paper 3 enters the fixing nip 27 varies due to factors such as paper slippage in the paper transport path 2. Moreover, the control unit 7 may start to reduce the clock frequency of the control signal at the same time as the leading edge of the paper 3 enters the fixing nip 27 .

[0048] The time (period) over which the control unit 7 reduces the clock frequency in stages varies depending on the paper transport speed, but is, for example, 0.05 seconds or more and 0.1 seconds or less. The amount of reduction in the clock frequency is, for example, 40 Hz or more and 150 Hz or less. The number of stages over which the control unit 7 reduces the clock frequency in stages is, for example, 2 stages or more and 15 stages or less. The time from when the control unit 7 starts reducing the clock frequency until the leading edge of the paper 3 enters the fixing nip 27 is, for example, 0.0001 seconds or more and 0.01 seconds or less. The amount of reduction in each stage can be substantially the same.

[0049] Control unit 7 may reduce the clock frequency to a frequency at which the paper transport speed of fixing unit 28 is substantially the same as the paper transport speed (normal speed) during transfer in transfer unit 24, or may reduce the clock frequency to a frequency corresponding to a decelerated speed at which the paper transport speed of fixing unit 28 is slower than the paper transport speed during transfer in transfer unit 24. In the graph shown in Figure 8, control unit 7 reduces the clock frequency in stages to a frequency corresponding to a decelerated speed at which the paper transport speed of fixing unit 28 is slower than the paper transport speed (normal speed) during transfer in transfer unit 24.

[0050] The control unit 7 is preferably configured to send a control signal to the fixing unit motor 29 so as to slow down the paper transport speed in the fixing unit 28 to a speed slower than the paper transport speed during transfer in the transfer unit 24 when the basis weight of the paper 3 is greater than a predetermined basis weight, and to send a control signal to the fixing unit motor 29 so as to slow down the paper transport speed in the fixing unit 28 to a speed substantially the same as the paper transport speed during transfer in the transfer unit 24 when the basis weight of the paper 3 is less than the predetermined basis weight.

[0051] The control unit 7 is preferably configured to send a control signal to the fixing unit motor 29 so as to decelerate the paper transport speed in the fixing unit 28 to a speed slower than the paper transport speed during transfer in the transfer unit 24 after the leading edge of the paper 3 enters the fixing nip 27, and then increase the paper transport speed in the fixing unit 28 to a speed substantially equal to the paper transport speed during transfer in the transfer unit 24, and to change (adjust) a second speed difference between the paper transport speed in the fixing unit 28 after deceleration and the paper transport speed during transfer in the transfer unit 24, in accordance with the measurement results of the stiffness measurement unit 6. This makes it possible to prevent banding from appearing in the printed image. For example, the second speed difference shown in FIG. 8 can be adjusted in accordance with the measurement results of the stiffness measurement unit 6. The control unit 7 may be configured to increase the second speed difference as the stiffness of the paper increases.

[0052] The second speed difference can be adjusted so that the percentage (deceleration rate) of the difference between the clock frequency corresponding to the decelerated speed and the clock frequency corresponding to the normal speed is 1% to 6%, for example, when the clock frequency corresponding to the normal speed is 100%. Furthermore, the second speed difference can be adjusted within this range depending on the measurement results of the stiffness measurement unit 6.

[0053] When the control unit 7 reduces the clock frequency to a frequency at which the paper transport speed of the fixing unit 28 is slower than the paper transport speed (normal speed) during transfer in the transfer unit 24, the control unit 7 can, after reducing the clock frequency, gradually increase the clock frequency to a frequency at which the paper transport speed of the fixing unit 28 is the same as the paper transport speed (normal speed) during transfer in the transfer unit 24. Then, while gradually increasing the clock frequency, the control unit 7 can transport the paper 3 so that the leading edge of the paper 3 passes through the fixing nip 27. This can prevent an increase in the paper transport speed due to a reduction in load caused by the leading edge of the paper 3 passing through the fixing nip 27, thereby preventing banding from appearing in the printed image. Furthermore, by gradually increasing the clock frequency, even if variations occur in the timing at which the leading edge of the paper 3 passes through the fixing nip 27 due to slippage of the paper on the paper transport path 2, the appearance of banding in the printed image can be prevented. Furthermore, the control unit 7 can change the clock frequency of the control signal to a clock frequency corresponding to the normal speed when the leading edge of the paper 3 reaches a distance of 14 mm to 20 mm from the entrance of the fixing nip 27.

[0054] The control unit 7 can maintain the paper transport speed of the fixing unit 28 at the normal speed until the rear end of the paper 3 passes through the fixing nip 27 . If the next sheet of paper 3 is not immediately transported to the fixing section 28 after the rear end of the sheet of paper 3 passes through the fixing nip 27, the control section 7 can stop the rotation of the second conveying member 26 and the first conveying member 25 after the rear end of the sheet of paper 3 passes through the fixing nip 27. In the case where the next sheet of paper 3 is transported to the fixing unit 28 immediately after the trailing edge of the sheet of paper 3 leaves the fixing nip 27, when the leading edge of the next sheet of paper 3 approaches the fixing unit 28 after the trailing edge of the previous sheet of paper 3 has left the fixing nip 27, the control unit 7 adjusts the clock frequency of the control signal according to the measurement result of the stiffness measurement unit 6 so that the paper transport speed in the fixing unit 28 becomes faster by the first speed difference than the paper transport speed (normal speed) during transfer in the transfer unit 24. For example, when the trailing edge of the previous sheet of paper 3 has advanced 10 mm or more after leaving the fixing nip 27 and the distance from the leading edge of the next sheet of paper 3 to the fixing nip 27 is 3 mm or more and 10 mm or less, the control unit 7 can adjust the clock frequency of the control signal to become the clock frequency corresponding to the paper transport speed (speed before paper entry) that is faster than the normal speed by the first speed difference. Thereafter, the control unit 7 starts to gradually decrease the clock frequency of the control signal so that the paper transport speed of the fixing unit 28 slows down from the speed before the paper enters the fixing nip 27 at the timing or slightly before the leading edge of the next paper sheet 3 enters the fixing nip 27. Then, the control unit 7 causes the leading edge of the paper sheet 3 to enter the fixing nip 27 immediately after starting to gradually decrease the clock frequency, and gradually decreases the clock frequency while the leading edge of the paper sheet 3 is moving through the fixing nip 27. The following description will be omitted here as it will be a repetition of the above description.

[0055] Clark stiffness test and printing experiments Using a Clark stiffness tester, the Clark stiffness (resilience) of five types of paper was measured: Mondi paper (mondi350), Finch Paper (FINCH), Burgo paper (Chorus Silk), Domtar paper (cougar), and Oji Materia paper (Suncard). In addition, image patterns were printed on each of the five types of paper mentioned above using a digital full-color multifunction printer manufactured by Sharp Corporation (four different image patterns were printed using each type of paper), and scores were assigned to the banding occurrence. The banding occurrence was scored on a 5-point scale, with a score of 0 if no banding occurred and a higher score (maximum 5 points) for banding that was more pronounced. These scores were assigned to the four printed image patterns, and the sum of these scores was used to determine the banding score for that type of paper (maximum: 20 points). The paper transport speed in the fixing unit 28 during printing was substantially the same (unchanged) as the paper transport speed during transfer in the transfer unit 24. Table 1 shows the basis weight, Clark stiffness (stiffness), and banding score for each paper. Figure 9 is a graph showing the Clark stiffness and banding score for each paper.

[0056] [Table 1]

[0057] The experimental results shown in Table 1 and Figure 9 indicate that the stronger the Clark stiffness (stiffness) of the paper, the more likely banding appears in the printed image. The reason for this is thought to be that stiff paper cannot bend between the transfer and fixing units, and the impact when the paper enters the fixing nip is easily transmitted to the transfer unit. However, weak paper can bend between the transfer and fixing units, and the impact when the paper enters the fixing nip is thought to be less likely to be transmitted to the transfer unit.

[0058] Measurement of control signal clock frequency and motor rotation speed An oscilloscope was connected to a digital full-color multifunction printer manufactured by Sharp Corporation, and the changes in the clock frequency CLK of the control signal that the control unit gives to the fixing unit motor as an operation command and the motor rotation speed FG were measured when the leading edge of the cardboard passed through the fixing nip. The measurement results are shown in Figures 10 to 13. In the measurements whose results are shown in Figure 10, the printing paper used was Oji Materia paper (Suncard, Clark stiffness (stiffness): 465.60) shown in Table 1. The clock frequency of the control signal when the leading edge of the paper passes through the fixing nip was constant (approximately 692 Hz), and the clock frequency of the control signal was set so that the paper transport speed in the fixing section was the same as the paper transport speed (normal speed) during transfer in the transfer section.

[0059] As shown in the measurement results in Figure 10, when the leading edge of the paper enters the fixing nip (paper entering in Figure 10 (1)), the fixing nip bites the leading edge of the paper, increasing the rotational resistance (rotational load) of the fixing section, causing the motor rotation speed to slow down and resulting in undershoot (Figure 10 (2)). The motor rotation speed then returned to the normal speed corresponding to a constant clock frequency, but when the leading edge of the paper leaves the fixing nip, the rotational resistance (rotational load) decreases, the motor rotation speed increases, and an overshoot occurs (Figure 10 (3)). The motor rotation speed then returned to the normal speed corresponding to a constant clock frequency (Figure 10 (4)). When the clock frequency of the control signal was constant when the leading edge of the paper passed through the fixing nip, banding appeared in the printed image when the leading edge of the paper passed through the fixing nip. This banding is thought to be caused by the undershoot and overshoot.

[0060] In the measurements shown in Figure 11, the printing paper used was Oji Materia paper (Suncard, Clark stiffness (stiffness): 465.60) as shown in Table 1. The clock frequency of the control signal when the leading edge of the paper passed through the fixing nip was changed as shown in the graph in Figure 8. In this measurement, the clock frequency of the control signal that made the paper transport speed in the fixing section the same as the paper transport speed (normal speed) during transfer in the transfer section was approximately 692 Hz. The control unit increased the clock frequency of the control signal before the leading edge of the paper entered the fixing nip to 761 Hz (speed increase rate: 10%, frequency difference: 69 Hz) above 692 Hz. Then, just before the leading edge of the paper entered the fixing nip, the control unit began to decrease the clock frequency of the control signal, reducing it in six steps to approximately 658 Hz (deceleration rate: 5%, frequency difference: 34 Hz). The control unit 7 then caused the leading edge of the paper to enter the fixing nip immediately after starting to decrease the clock frequency of the control signal. After that, the control unit increased the clock frequency of the control signal to 692 Hz (normal speed) in two stages when the leading edge of the paper left the fixing nip. In this printing, no banding appeared in the printed image. This shows that banding can be suppressed by changing the clock frequency of the control signal as shown in the graph in Figure 8.

[0061] In the measurement whose results are shown in Figure 12, measurements and printing were carried out in the same manner as in the measurement whose results are shown in Figure 11, except that the printing paper used was Mondi paper (mondi350, Clark stiffness: 143.49) shown in Table 1. In this printing, banding appeared in the printed image. This banding is thought to have occurred because the paper was pulled by the fixing nip near the peak of the motor rotation speed at 14.65 seconds, causing transfer misalignment in the transfer area. For this reason, it was found that even when the clock frequency of the control signal is changed as shown in the graph in FIG. 8, banding may appear in the printed image if the stiffness of the paper is different.

[0062] In the measurements shown in Figure 13, the printing paper used was Mondi paper (Mondi 350, Clark stiffness: 143.49) as shown in Table 1. The clock frequency of the control signal when the leading edge of the paper passed through the fixing nip was changed as shown in the graph in Figure 8. In this measurement, the clock frequency of the control signal that made the paper transport speed in the fixing section the same as the paper transport speed (normal speed) during transfer in the transfer section was approximately 692 Hz. The control unit increased the clock frequency of the control signal before the leading edge of the paper entered the fixing nip to 727 Hz (speed increase rate: 5%, frequency difference: 35 Hz). Then, just before the leading edge of the paper entered the fixing nip, the control unit began to decrease the clock frequency of the control signal, reducing it in four steps to approximately 671 Hz (deceleration rate: 3%, frequency difference: 21 Hz). The control unit 7 then caused the leading edge of the paper to enter the fixing nip immediately after starting to decrease the clock frequency of the control signal. Thereafter, the control unit increased the clock frequency of the control signal to 692 Hz (normal speed) at the timing when the leading edge of the paper exited the fixing nip. In this printing, no banding appeared in the printed image. Therefore, it was found that when the paper is not stiff, the occurrence of banding can be suppressed by reducing the first speed difference and the second speed difference. From the above, it has been found that the occurrence of banding can be suppressed by adjusting the first speed difference and the second speed difference according to the stiffness of the paper. [Explanation of symbols]

[0063] 2: Paper transport path 3: Paper 4a to 4j: Transport rollers 5: Registration roller 6: Stiffness measurement unit 7: Control unit 10: Transport guide 11: Pressed plate 12: Rotating shaft 13: Elastic sheet member 14: Distance measurement sensor 15: Pressing force measurement sensor 18a, 18b, 18c, 18d: Toner image forming unit 19a, 19b, 19c, 19d: Photosensitive element 20: Intermediate transfer belt 21: Paper feed tray 22: Manual feed tray 23: Paper discharge tray 24: Transfer unit 25: First transport member 26: Second transport member 27: Fixing nip 28: Fixing unit 29: Fixing unit motor 33: Exposure unit 50: Image forming device

Claims

1. The apparatus includes a transfer unit that transfers a toner image onto a sheet of paper, a fixing unit that fixes the toner image onto the sheet of paper, a stiffness measuring unit that directly or indirectly measures the stiffness of a sheet of paper being transported on a paper transport path upstream of the transfer unit, and a control unit that controls the transport of the sheet of paper, the fixing unit includes a first conveying member, a second conveying member, and a motor configured to rotate the first and second conveying members, and is configured so that paper passes through a fixing nip between the first conveying member and the second conveying member; The image forming apparatus is characterized in that the control unit is configured to send a control signal to the motor so as to change the paper transport speed in the fixing unit in accordance with the measurement result of the stiffness measurement unit.

2. 2. The image forming apparatus according to claim 1, wherein the control unit is configured to send a control signal to the motor so as to make the paper transport speed in the fixing unit faster than the paper transport speed during transfer in the transfer unit before the leading edge of the paper enters the fixing nip, and is configured to change a first speed difference between the paper transport speed in the fixing unit after the speed increase and the paper transport speed during transfer in the transfer unit in accordance with the measurement result of the stiffness measurement unit.

3. 3. The image forming apparatus according to claim 2, wherein the control section is configured to increase the first speed difference as the stiffness of the paper increases.

4. 3. The image forming apparatus of claim 2, wherein the control unit is configured to send a control signal to the motor when the basis weight of the paper is greater than a predetermined basis weight so as to make the paper transport speed in the fixing unit faster than the paper transport speed during transfer in the transfer unit before the leading edge of the paper enters the fixing nip, and when the basis weight of the paper is less than the predetermined basis weight, send a control signal to the motor so as to make the paper transport speed in the fixing unit substantially the same as the paper transport speed during transfer in the transfer unit before the leading edge of the paper enters the fixing nip.

5. 2. The image forming apparatus according to claim 1, wherein the control unit is configured to send a control signal to the motor so as to make the paper transport speed in the fixing unit faster than the paper transport speed during transfer in the transfer unit, and then decelerate the paper transport speed in the fixing unit to a speed substantially the same as or slower than the paper transport speed during transfer in the transfer unit at the timing when the leading edge of the paper enters the fixing nip.

6. 6. The image forming apparatus according to claim 5, wherein the control unit is configured to send a control signal to the motor so as to decelerate the paper transport speed in the fixing unit in stages when the paper transport speed in the fixing unit is reduced to a speed substantially the same as or slower than the paper transport speed during transfer in the transfer unit.

7. 6. The image forming apparatus according to claim 5, wherein the control unit is configured to send a control signal to the motor so as to decelerate the paper transport speed in the fixing unit to a speed slower than the paper transport speed during transfer in the transfer unit after the leading edge of the paper enters the fixing nip, and then increase the paper transport speed in the fixing unit to a speed substantially the same as the paper transport speed during transfer in the transfer unit, and is configured to change a second speed difference between the paper transport speed in the fixing unit after deceleration and the paper transport speed during transfer in the transfer unit in accordance with the measurement result of the stiffness measurement unit.

8. 8. The image forming apparatus according to claim 7, wherein the control section is configured to increase the second speed difference as the stiffness of the paper increases.

9. 6. The image forming apparatus of claim 5, wherein the control unit is configured to send a control signal to the motor so as to slow down the paper transport speed in the fixing unit to a speed slower than the paper transport speed during transfer in the transfer unit when the paper basis weight is greater than a predetermined basis weight, and to send a control signal to the motor so as to slow down the paper transport speed in the fixing unit to a speed substantially the same as the paper transport speed during transfer in the transfer unit when the paper basis weight is less than the predetermined basis weight.

10. 6. The image forming apparatus of claim 5, wherein the control unit is configured to send a control signal to the motor so as to decelerate the paper transport speed in the fixing unit to a speed slower than the paper transport speed during transfer in the transfer unit after the leading edge of the paper enters the fixing nip, and then, at the timing when the leading edge of the paper leaves the fixing nip, gradually increase the paper transport speed in the fixing unit to a speed substantially the same as the paper transport speed during transfer in the transfer unit.

Images