Fixing device and image forming apparatus

Abstract

To effectively control a heating amount of a fixing member in an inter-paper period.SOLUTION: A fixing device 1 heats a sheet passing through a nip portion between a fixing belt 31 as a fixing member and a pressure roller 32 to fix a toner image formed on the sheet. The fixing device 1 includes a heating portion 36 for heating the fixing belt 31 and a heating adjustment portion 41 for adjusting the power supplied to the heating portion 36. The heating adjustment portion 41 performs reduction control for reducing an upper limit value of a duty ratio in an inter-paper period using a ratio of an energized state of the heating portion 36 in a predetermined cycle as a duty ratio than an upper limit value of a duty ratio in a paper-passing period continuous with the inter-paper period.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a fixing device and an image forming apparatus including the fixing device. [Background technology]

[0002] In an image forming apparatus, a toner image formed on a sheet is fixed by heating and pressing the sheet in a nip portion where a fixing member and a pressure member come into contact. If the fixing temperature of the fixing member is not controlled to an appropriate temperature, problems such as poor fixing may occur. The fixing temperature is controlled, for example, by repeatedly turning on and off a lamp heater for heating, so that the fixing temperature is maintained at a certain temperature difference from the set temperature.

[0003] For example, Patent Document 1 discloses a method of controlling power supply to the fixing unit so that it is not supplied when the fixing temperature is higher than the set temperature. However, if the power supply is stopped and fixing is performed using residual heat during standby, the fixing temperature drops rapidly when paper passes through the nip, and even if temperature control is performed again, it takes a long time for the fixing temperature to reach a temperature close to the set temperature.

[0004] Furthermore, Patent Document 2 discloses that, for the purpose of energy-saving fixing, a fixing member is formed of a fixing pad and a fixing belt, and the fixing belt is heated from the inside by a heater. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2002-304086 A [Patent Document 2] JP 2007-233011 A Summary of the Invention [Problem to be solved by the invention]

[0006] As disclosed in Patent Document 2, this type of fixing member equipped with a fixing belt and a fixing pad has the advantage of having a relatively small heat capacity and being able to shorten the warm-up time, but it has the problem that the surface temperature of the fixing belt is prone to fluctuating due to the small heat storage effect of the fixing belt.

[0007] In particular, the surface temperature is likely to fluctuate because the amount of heat supplied to the fixing belt differs between when the paper is passing through the nip and when it is not. During the paper-interval period when the paper is not present in the nip, the amount of heat taken from the fixing belt is small, and the surface temperature of the fixing belt is likely to rise. Therefore, after the paper-interval period, the image fixed on the first paper has a different fixing temperature between the image area corresponding to the first rotation of the rotating fixing belt and the image area corresponding to the second rotation and thereafter, and there is a risk of the fixability and glossiness being reduced in the image area corresponding to the second rotation. In addition, in order to suppress the surface temperature that rises during the paper-interval period, the lamp heater is turned off at the timing of the first rotation of the fixing belt. As a result, there is a problem that the temperature drops rapidly from the second rotation of the fixing belt onwards, which is likely to cause image defects such as uneven glossiness.

[0008] The present invention has been made in consideration of the above-mentioned problems, and its object is to provide a fixing device and an image forming apparatus that can effectively control the amount of heat applied to the fixing member during the inter-paper period, thereby suppressing fluctuations in the surface temperature of the fixing member and imparting a uniform glossiness to the image fixed to the paper, thereby enabling good image formation. [Means for solving the problem]

[0009] The solution of the present invention to achieve the above-mentioned object is a fixing device that heats paper passing through a nip portion between a fixing member and a pressure member to fix a toner image formed on the paper, the fixing device comprising a heating section that heats the fixing member and a heating adjustment section that adjusts the power supplied to the heating section, and the heating adjustment section is characterized in that the duty ratio is the ratio of the powered state of the heating section within a specified period, and the heating adjustment section performs reduction control to reduce the upper limit value of the duty ratio during a paper interval period when the paper is not present in the nip portion to less than the upper limit value of the duty ratio during a paper passing period that is consecutive to the paper interval period and when the paper is passing through the nip portion.

[0010] In addition, in the fixing device having the above configuration, a paper passage determination unit that determines the presence or absence of paper upstream of the nip portion in the paper transport direction is further provided, and the paper interval period is the period from the timing when the paper passage determination unit detects the presence of paper from the absence to the timing when it detects the presence of paper from the absence, and the paper passage period is the period from the timing when the paper passage determination unit detects the presence of paper from the absence to the timing when it detects the presence of paper from the presence of paper to the absence.

[0011] In the fixing device having the above-described configuration, it is preferable that the heat adjustment section starts the control of reducing the duty ratio immediately before the trailing edge of the paper passes through the nip portion.

[0012] In the fixing device having the above-described configuration, it is preferable that the heat adjustment section cancels the reduction control of the duty ratio immediately before the leading edge of the paper reaches the nip portion.

[0013] Furthermore, the fixing device having the above configuration may further include a temperature detection unit that detects a fixing temperature, which is the temperature of the fixing member, a temperature difference determination unit that calculates the difference between the fixing temperature detected by the temperature detection unit and a preset set temperature, and a gradient determination unit that determines a temperature gradient based on a change in the fixing temperature, wherein the gradient determination unit determines an upward or downward trend of the fixing temperature from a plurality of the fixing temperatures detected at predetermined comparison times, and the heating adjustment unit is configured to determine an upper limit value of the duty ratio and an amount of reduction therein, based on the determination results of the temperature difference determination unit and the gradient determination unit.

[0014] Furthermore, the fixing device having the above configuration may further include a time determination unit that determines the length of the paper-to-paper period, and based on a determination result of the time determination unit, the amount of reduction in the duty ratio may be made different when the length of the paper-to-paper period is equal to or less than a predetermined time and when it exceeds the predetermined time, and the amount of reduction may be made smaller when the length is equal to or less than the predetermined time.

[0015] Furthermore, in the fixing device having the above configuration, it is preferable that the fixing member has a fixing belt rotatable around an axis, the pressure member has a pressure roller, the fixing belt is provided with a sliding member that slides in contact with the inner surface of the belt and a fixing pad that fixes the sliding member, and the pressure roller is positioned opposite the fixing pad with the fixing belt in between.

[0016] In addition, an image forming apparatus including a fixing device according to each of the above-mentioned solving means is also included in the scope of the technical idea of ​​the present invention. This makes it possible to stabilize the fixing temperature by controlling the duty ratio during the paper interval and suppress fluctuations in the surface temperature of the fixing member. Effect of the Invention

[0017] According to the present invention, it is possible to effectively control the amount of heat applied to the fixing member during the inter-paper period, thereby suppressing fluctuations in the surface temperature of the fixing member and obtaining uniform glossiness of the image fixed to the paper. [Brief description of the drawings]

[0018] [Figure 1] 1 is a schematic cross-sectional view illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. [Diagram 2] 2 is a schematic cross-sectional view illustrating a configuration of a fixing device in the image forming apparatus. [Diagram 3] FIG. 2 is a functional block diagram showing an electrical configuration of the image forming apparatus. [Figure 4] 5A and 5B are explanatory diagrams illustrating an outline of basic control of a heating unit in the fixing device. [Diagram 5] 5 is an explanatory diagram showing an example of duty control of a heating unit in the fixing device. FIG. [Figure 6] 4 is a correspondence table showing arrival times of a sheet of paper to a nip portion of the fixing device. [Figure 7] 4 is a correspondence table showing arrival times of a sheet of paper to a nip portion of the fixing device. [Figure 8] 4 is an explanatory diagram showing a correspondence relationship between an inter-sheet period and a plurality of tables A and B in the image forming apparatus. FIG. [Figure 9] FIG. 11 is an explanatory diagram showing the correspondence relationship between the type of the image forming apparatus and a predetermined period T5. [Figure 10] 11 is an explanatory diagram showing a correspondence relationship between the type of the image forming apparatus and a plurality of tables A and B. FIG. [Figure 11] 4 is an explanatory diagram showing the correspondence relationship between an inter-sheet period and a plurality of tables A to C in the image forming apparatus. FIG. [Figure 12] FIG. 11 is an explanatory diagram showing the correspondence relationship between the type of the image forming apparatus and the predetermined periods T5 and T6. [Figure 13] FIG. 4 is an explanatory diagram showing the correspondence between the types of the image forming apparatus and a plurality of tables A to C. [Figure 14] 6 is a graph showing the relationship between the paper interval and the duty reduction amount. [Figure 15] 6 is a flowchart showing an example of a process of duty reduction control in the fixing device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, a fixing device and an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings.

[0020] (Image forming device) FIG. 1 is a schematic cross-sectional view showing a configuration of an image forming apparatus 100 according to an embodiment of the present invention.

[0021] The image forming device 100 is a multifunction device having a copy function, a scanner function, a facsimile function and a printer function, and transmits an image of an original document read by the image reading device 102 to the outside, and forms an image of the original document read by the image reading device 102 or an image received from the outside in color or monochrome on a recording medium such as paper by the image forming unit 101.

[0022] An original feeder 160 (automatic document feeder: ADF) is provided above the image reading unit 130 and is supported so as to be freely opened and closed relative to the image reading unit 130. The original feeder 160 sequentially conveys one or more originals one by one. The image reading unit 102 reads the originals conveyed by the original feeder 160 one by one. The image reading unit 102 includes an original placement table 130a on which an original is placed, and a placed original reading function for reading an original placed on the original placement table 130a. The image reading unit 130 scans the scanning optical system 130b to read an original placed on the original placement table 130a, or reads an original conveyed by the original feeder 160 to generate image data.

[0023] The image forming section 101 includes a fixing device 1, a developing device 2, a photoconductor drum 3, a drum cleaning device 4, a charger 5, an intermediate transfer belt device 7, a secondary transfer device 11, an optical scanning device 12, a paper feed section 18, and the like.

[0024] The image forming unit 101 handles image data corresponding to color images using the colors black (K), cyan (C), magenta (M), and yellow (Y), or monochrome images using a single color (e.g., black). The image transfer unit 50 is provided with four developing devices 2, four photoconductor drums 3, four drum cleaning devices 4, and four chargers 5 for forming four types of toner images, and four image stations Pa, Pb, Pc, and Pd are configured corresponding to black, cyan, magenta, and yellow, respectively.

[0025] The optical scanning device 12 exposes the surface of the photoreceptor drum 3 to light to form an electrostatic latent image. The developing device 2 develops the electrostatic latent image on the surface of the photoreceptor drum 3 to form a toner image on the surface of the photoreceptor drum 3. The drum cleaning device 4 removes and collects residual toner on the surface of the photoreceptor drum 3. The charger 5 uniformly charges the surface of the photoreceptor drum 3 to a predetermined potential. Through the series of operations described above, a toner image of each color is formed on the surface of each photoreceptor drum 3.

[0026] The intermediate transfer belt device 7 includes an intermediate transfer roller 6, an endless intermediate transfer belt 71, an intermediate transfer driving roller 72, an intermediate transfer driven roller 73, and a cleaning device 9. Four intermediate transfer rollers 6 are provided inside the intermediate transfer belt 71 so as to form four types of toner images corresponding to the respective colors. The intermediate transfer rollers 6 transfer the toner images of the respective colors formed on the surfaces of the photosensitive drums 3 onto the intermediate transfer belt 71 which moves in a circulating manner.

[0027] The intermediate transfer belt 71 is stretched around an intermediate transfer drive roller 72 and an intermediate transfer driven roller 73. In the image forming apparatus 100, the toner images of each color formed on the surface of each photoconductor drum 3 are sequentially transferred and superimposed to form a color toner image on the surface of the intermediate transfer belt 71. The cleaning device 9 removes and collects waste toner remaining on the surface of the intermediate transfer belt 71 without being transferred to paper.

[0028] The secondary transfer device 11 pinches and transports the paper transported through the paper transport path 21 to a transfer nip portion TN between the secondary transfer roller 11a and the intermediate transfer belt 71. When the paper passes through the transfer nip portion TN, the toner image on the surface of the intermediate transfer belt 71 is transferred onto the paper, and the paper is transported to the fixing device 1.

[0029] The fixing device 1 includes a fixing belt 31 that rotates around an axis and a pressure roller 32. The fixing device 1 sandwiches a sheet of paper onto which a toner image has been transferred between the fixing belt 31 and the pressure roller 32 and applies heat and pressure to fix the toner image onto the sheet. Although not shown in FIG. 1, the fixing device 1 also includes components other than the fixing belt 31 and the pressure roller 32. Details of the fixing device 1 will be described later.

[0030] The paper feed section 18 includes a paper feed cassette for loading, for example, paper as a recording medium used for image formation, and is provided below the optical scanning device 12. The paper is pulled out of the paper feed section 18 by a pickup roller 16 and transported to a paper transport path 21. The paper transported to the paper transport path 21 passes through the secondary transfer device 11 and the fixing device 1, and is discharged to a paper discharge tray 19 by a discharge roller 17.

[0031] A conveying roller 13, a registration roller 14, and a discharge roller 17 are arranged on the paper conveying path 21. The conveying roller 13 promotes the conveying of the paper. The registration roller 14 conveys the paper at a speed equal to the process speed at which the image forming unit 101 forms an image on the paper. The registration roller 14 is provided between the paper feed unit 18 and the secondary transfer device 11, and adjusts the timing of conveying the paper so that the toner image is transferred to the paper by the secondary transfer device 11. For example, the registration roller 14 waits (stops temporarily) while nipping the paper conveyed from the paper feed unit 18, and starts conveying the paper at a constant speed in synchronization with the secondary transfer device 11. At this time, the registration roller 14 is rotated at a circumferential speed equal to the circumferential speed of the intermediate transfer belt 71. The toner image on the intermediate transfer belt 71 is transferred to the paper at a transfer nip portion TN between the intermediate transfer belt 71 and the secondary transfer roller 11a.

[0032] When image forming apparatus 100 forms an image on the back side of the paper in addition to the front side, discharge rollers 17 change the conveying direction of the paper, and the paper is conveyed to reversing conveying path 22. In reversing conveying path 22, conveying rollers 15 guide the paper up to registration rollers 14 in a reversed state. Image forming apparatus 100 forms an image on the back side of the paper guided to registration rollers 14 in the same manner as on the front side, and discharges the paper to discharge tray 19.

[0033] (Fixing device) FIG. 2 is a schematic cross-sectional view showing the configuration of the fixing device 1 according to the embodiment of the present invention.

[0034] The fixing device 1 includes a fixing belt 31 and a pressure roller 32. As shown in Fig. 2, the fixing device 1 further includes, inside the fixing belt 31, a support member 33, a fixing pad 34, a sliding sheet 35, a heating unit 36, and a reflecting plate 37. The fixing device 1 also includes a fixing temperature sensor 38 as a temperature detection unit, and a peeling plate 39.

[0035] The fixing belt 31 is an endless flexible belt formed in a substantially circular shape. The fixing belt 31 has a configuration in which a release layer is provided on the surface of a band-shaped base material formed of, for example, a synthetic resin such as polyimide or a metal such as nickel. The fixing belt 31 is provided so as to be rotatable about an axis line perpendicular to the paper surface in FIG. 2. The inner diameter of the fixing belt 31 is, for example, 30 mm.

[0036] The fixing pad 34 is formed in a long plate shape extending along the axial direction of the fixing belt 31, and is made of, for example, synthetic resin. A sliding sheet 35 is provided on the outer peripheral surface (the surface on the side close to the fixing belt 31) of the fixing pad 34. The length of the fixing pad 34 is approximately equal to the length of the fixing belt 31 in the axial direction.

[0037] The sliding sheet 35 is provided so as to be in sliding contact with the inner circumferential surface of the fixing belt 31. The fixing pad 34 is arranged so that the sliding surface 35a of the sliding sheet 35 is in contact with the inner circumferential surface of the fixing belt 31. The fixing belt 31 rotates, but the fixing pad 34 and the sliding sheet 35 are fixed, and the fixing belt 31 slides on the sliding surface 35a. A lubricant may be applied to the sliding surface 35a of the sliding sheet 35 to reduce the frictional force with the fixing belt 31.

[0038] The support member 33 is a member that supports the fixing pad 34 and the sliding sheet 35 while pressing them against the inner circumferential surface of the fixing belt 31. The support member 33 has, for example, a substantially L-shaped cross section and has a long plate-like fixed portion 33a to which the fixing pad 34 is fixed, and a long plate-like standing portion 33b that stands from an end of the fixed portion 33a.

[0039] The heating section 36 is a member for heating the fixing belt 31, and is disposed so as to extend along the axial direction of the fixing belt 31 (perpendicular to the paper surface in FIG. 2). The heating section 36 is, for example, a lamp heater such as a halogen lamp. In the illustrated embodiment, the heating section 36 includes a first heater 36a for heating the central portion of the fixing belt 31 in the axial direction, and a second heater 36b for heating both end portions of the fixing belt 31 in the axial direction. The operation of each of the first heater 36a and the second heater 36b is determined according to the size of the paper P, etc. The fixing belt 31 is heated by the first heater 36a and the second heater 36b, for example, to 200° C. to 250° C.

[0040] The reflecting plate 37 is in the form of a thin plate, and is disposed so as to cover the surface of the supporting member 33 facing the heating unit 36. The fixing belt 31 is configured so as to be efficiently heated by the reflecting plate 37.

[0041] The pressure roller 32 is disposed at a position facing the fixing pad 34 with the fixing belt 31 interposed therebetween. The pressure roller 32 rotates about a rotation axis parallel to the axial direction of the fixing belt 31, and is disposed so as to extend substantially parallel to the fixing belt 31. At a nip portion N between the pressure roller 32 and the fixing belt 31, the fixing belt 31 is pressed against the pressure roller 32 by the fixing pad 34. The pressure roller 32 can be configured such that the surface of a cylindrical core material formed of a metal such as aluminum is covered with an elastic material such as rubber.

[0042] A driving force from a driving source such as a motor (not shown) is transmitted to the pressure roller 32 via gears, etc. The pressure roller 32 is rotated by receiving this driving force, and as the pressure roller 32 is rotated, the fixing belt 31 is rotated in the opposite direction to the rotation direction of the pressure roller 32. The paper P passes through the nip portion N between the pressure roller 32 and the fixing belt 31 along the paper transport direction S.

[0043] The fixing temperature sensor 38 detects the fixing temperature, which is the surface temperature of the fixing belt 31. The peeling plate 39 is disposed downstream of the nip portion N in the paper transport direction S, and prevents the paper P from wrapping around the fixing belt 31.

[0044] (Duty control of heating section) FIG. 3 is a functional block diagram showing the electrical configuration of the image forming apparatus 100. As shown in FIG.

[0045] As described above, image forming apparatus 100 is configured to transport paper P supplied from paper feed unit 18 to image forming unit 101 (particularly secondary transfer device 11 and fixing device 1) via paper transport unit 20 (paper transport path 21 and reversal transport path 22). Fixing device 1 includes fixing belt 31, pressure roller 32, heating unit 36, and fixing temperature sensor 38. Fixing temperature sensor 38 periodically detects the surface temperature of fixing belt 31.

[0046] The image forming apparatus 100 according to this embodiment performs basic control by controlling the heating unit 36 ​​so that the fixing temperature in the fixing device 1 approaches a set temperature. The fixing temperature is the surface temperature of the fixing belt 31 acquired based on an output signal of the fixing temperature sensor 38. The set temperature is a target value of the fixing temperature, and is changed appropriately according to the operation of the image forming apparatus 100. For example, the set temperature is set to a low temperature when the image forming apparatus 100 is in a standby state, and is set to a higher temperature than that in the standby state when a print job is input so as to be a temperature suitable for image formation.

[0047] 4 is an explanatory diagram showing an outline of basic control of the heating unit 36 ​​(first heater 36a and second heater 36b). The fixing temperature detected by the fixing temperature sensor 38 indicates temperature changes, such as rising and falling from the set temperature, as shown in FIG.

[0048] In the fixing device 1, the fixing temperature is adjusted by switching the power supply state of the first heater 36a and the second heater 36b of the heating unit 36 ​​between ON and OFF. In the fixing device 1 according to the exemplary embodiment, a control period (e.g., period t2) during a print job is set as one cycle, and the period during which the heater is ON during one cycle is duty controlled.

[0049] 4 shows an example of the heater ON period when the duty ratio is 40%, for example, and power is controlled to be supplied to the heating unit 36 ​​for 40% of one cycle (between time 0 and t1). In this way, the ratio of the energized state of the heating unit 36 ​​within a predetermined period is defined as the duty ratio, and this duty ratio is controlled to be variously switched to, for example, 20%, 40%, 60%, 80%, 100%, etc., to reduce the temperature difference between the fixing temperature and the set temperature.

[0050] Referring to FIG. 2, when the paper P passes through the nip portion N, the paper P absorbs heat from the nip portion N. Therefore, the surface temperature of the fixing belt 31 is likely to decrease during fixing. In particular, the surface temperature of the fixing belt 31 is likely to change between the first rotation of the fixing belt 31 that rotates while being pressed against the paper P and the second rotation and thereafter. Therefore, the fixing temperature differs between the image area corresponding to the first rotation of the rotating fixing belt and the image area corresponding to the second rotation and thereafter, and it is considered that the fixability and glossiness are reduced in the image area corresponding to the second rotation, which is a cause of image defects. Also, during the inter-paper period when the paper P is not present in the nip portion N, the amount of heat absorbed from the fixing belt 31 is small, so the surface temperature of the fixing belt 31 is likely to increase.

[0051] Here, the inter-paper period refers to the period from when the trailing edge of a preceding sheet of paper P passes a predetermined location on paper transport path 21 in the paper transport direction S during transport of the sheet of paper P to when the leading edge of the succeeding sheet of paper P passes the same location. During continuous transport of sheets of paper P, image forming apparatus 100 transports at least two sheets of paper P over a time span of a predetermined inter-paper period (e.g., inter-paper period T). Inter-paper period T is particularly the time from when the preceding sheet of paper P passes through nip portion N to when the succeeding sheet of paper P reaches nip portion N. The period between inter-paper period T and the next inter-paper period T is the paper passing period during which the sheet of paper P passes through nip portion N.

[0052] In response to such fixing temperature characteristics during the paper inter-period T and paper passing period, the fixing device 1 according to this embodiment is configured to suppress a sudden drop in the fixing temperature during the paper passing period by controlling the power supplied to the heating section 36 during the paper inter-period T.

[0053] 3, fixing device 1 includes a CPU 40. CPU 40 stores a heat adjustment unit 41, a temperature difference determination unit 42, a gradient determination unit 43, a time determination unit 44, and a paper passing determination unit 45 as pre-installed programs, and executes the stored programs to perform processes described below. Note that image forming apparatus 100 may be controlled by CPU 40. That is, image forming apparatus 100 and fixing device 1 may include a CPU as a common control unit.

[0054] The heating adjustment unit 41 controls the power supplied to the heating unit 36. The paper passing determination unit 45, which is provided in the CPU 40 together with the heating adjustment unit 41, determines the presence or absence of paper P upstream of the nip portion N in the paper transport direction S. A paper detection sensor (not shown) for detecting the passage of paper is provided on the paper transport path 21. The paper detection sensor is disposed in a position downstream of the junction of the paper transport path 21 and the reverse transport path 22 in the paper transport direction S, and upstream of the secondary transfer device 11. The paper detection sensor outputs a paper passing determination signal to the paper passing determination unit 45 that differs depending on the presence or absence of paper P at the detection point.

[0055] For example, the paper detection sensor is preferably installed near the registration rollers 14, or another sensor for adjusting the transport timing of the paper P at the registration rollers 14 may be used as the paper detection sensor. Since the paper P that has passed through the registration rollers 14 is transported at a constant speed without stopping until it reaches the nip N, the arrival time of the paper P at the nip N can be easily calculated as the time elapsed from the time the registration rollers 14 start transporting the paper P to the time obtained by dividing the distance between the nip N and the registration rollers 14 by the transport speed of the paper P.

[0056] 5 is an explanatory diagram showing an example of duty control of the heating unit 36. As shown in the upper part of FIG. 5, for the paper passing determination signal detected by the paper passing determination unit 45, "ON" indicates that paper P is "present," and "OFF" indicates that paper P is "not present." The timing at which the paper passing determination signal changes from ON to OFF is the start of the paper interval T, and the timing at which it changes from OFF to ON is the end of the paper interval T. In the illustrated embodiment, the periods T01, T12, T23, and T34 are the paper interval T. Also, the paper passing period is the period from the timing at which the paper passing determination signal changes from OFF to ON to the timing at which it changes from ON to OFF (the period between T01 and T12, between T12 and T23, and between T23 and T34).

[0057] 3, temperature difference determination unit 42 calculates the difference between the fixing temperature detected by fixing temperature sensor 38 and a preset set temperature. Gradient determination unit 43 compares a plurality of fixing temperatures detected by fixing temperature sensor 38 at preset comparison times to determine whether the fixing temperature is increasing or decreasing. The comparison time is set to a time equivalent to an integer multiple (e.g., t2×N, N is an integer) of a control period during a print job (e.g., period t2 shown in FIG. 4).

[0058] In the fixing device 1, the duty ratio of the duty control of the heating unit 36 ​​is determined based on the results of the determinations made by the temperature difference determination unit 42 and the gradient determination unit 43. For example, when the temperature difference (see FIG. 4) determined by the temperature difference determination unit 42 is between -3°C and 0°C and the gradient determination unit 43 determines that the fixing temperature is on the decline, the duty ratio of the heating unit 36 ​​is set to 100%, and when the gradient determination unit 43 determines that the fixing temperature is on the rise even with the same temperature difference, the duty ratio of the heating unit 36 ​​is suppressed to 88%. When the temperature difference determined by the temperature difference determination unit 42 is between -1°C and +2°C and the gradient determination unit 43 determines that the fixing temperature is on the rise, the duty ratio of the heating unit 36 ​​is set to 48%, and when the temperature difference is -4°C or more even if the fixing temperature is on the rise, the duty ratio is maintained at 96%. In this way, the energization state of the first heater 36a and the second heater 36b of the heating unit 36 ​​is controlled. The heater heating signal for the heating unit 36 ​​is controlled, for example, as shown in the middle part of FIG.

[0059] Based on the input print job, the image forming apparatus 100 starts conveying the paper P from the paper feed unit 18. For example, when conveying the paper P continuously, a plurality of sheets of paper P are sent to the fixing device 1 with a predetermined inter-paper period T therebetween. When spanning different print jobs, the time span between the last sheet of paper P of the preceding print job and the first sheet of paper P of the next print job becomes the inter-paper period T. The time determination unit 44 of the CPU 40 determines the length of such inter-paper period T.

[0060] 6 and 7 are correspondence tables showing the arrival time of the paper P at the nip N. As shown, based on the number of prints per unit time (CPM: Copies Per Minute) of the image forming apparatus 100, the process speed, and the distance from the paper detection sensor (RRM) to the nip N, FIG. 6 shows the time TA required from the timing when the leading edge of the paper P passes the paper detection sensor to the nip N, and FIG. 7 shows the time TB required from the timing when the trailing edge of the paper P passes the paper detection sensor to the nip N. For example, in the image forming apparatus 100 with a CPM of 26 sheets, the time TA is 809 ms, whereas in the high-speed image forming apparatus 100 with a CPM of 65 sheets, the time TA is as short as 121 ms. The time determination unit 44 can determine the inter-paper period T based on the information shown in such a correspondence table.

[0061] As described above, during the inter-paper period T when the paper P is not present in the nip portion N, the amount of heat taken from the fixing belt 31 is small, and the surface temperature of the fixing belt 31 is likely to rise. For example, the fixing temperature is likely to rise during the inter-paper period T01 from the start of a print job until the leading edge of the first paper P reaches the nip portion N. After the first paper P passes, for the second and subsequent paper sheets P, during the inter-paper periods T12, T23, and T34, if the fixing temperature rises too much relative to the set temperature, duty control is required to lower the temperature, while if it drops too much, the tracking of the fixing temperature during the paper passing period may become poor. Also, when the inter-paper period T is short, it is preferable to suppress the drop in the fixing temperature and increase the fixability of the subsequent paper sheets P, compared to when the inter-paper period T is long.

[0062] Therefore, in order to suppress the temperature ripple of the fixing temperature during the paper passing period and prevent the occurrence of image defects due to a drop in the fixing temperature, it is important to control the duty of the heating unit 36 ​​during the paper inter-period T. Therefore, in order to suppress an excessive rise in the fixing temperature during the paper inter-period T and connect it to the paper passing period, the heat adjustment unit 41 performs reduction control to reduce the duty ratio of the power supplied to the heating unit 36 ​​during the paper inter-period T.

[0063] In the illustrated embodiment, the heating adjustment unit 41 is configured to perform reduction control to reduce the upper limit of the duty ratio in the paper interval T below the upper limit of the duty ratio in the paper passing period. As shown in the lower part of Fig. 5, the heater heating signal subjected to reduction control shows a state in which the duty ratio is reduced in each paper interval T (T01, T12, T23, T34) compared to the heater heating signal (without reduction control) shown in the middle part. In order to reflect the length of the paper interval T in the reduction control of the duty ratio, the reduction value of the duty ratio is made different when the length of the paper interval T is equal to or less than a predetermined time T5 and when it exceeds the predetermined time T5 based on the judgment result of the time judgment unit 44, and the reduction amount of the duty ratio is controlled to be smaller when the length is equal to or less than the predetermined time T5.

[0064] The CPU 40 stores a plurality of tables A and B used for controlling the reduction of the duty ratio. Fig. 8 is an explanatory diagram showing the correspondence between the inter-sheet period T and the plurality of tables A and B, Fig. 9 is an explanatory diagram showing the correspondence between the type of image forming device 100 and the predetermined period T5, and Fig. 10 is an explanatory diagram showing the correspondence between the type of image forming device 100 and the tables A and B.

[0065] For the duty ratio reduction control, the reduction value is determined by referring to Table A or Table B. For example, as shown in Fig. 8, if the length of the inter-paper period T (e.g., T12, T23, T34) obtained from the time determination unit 44 is equal to or shorter than the predetermined time T5, the heat adjustment unit 41 refers to Table B. If the length of the inter-paper period T exceeds the predetermined time T5, the heat adjustment unit 41 refers to Table A. Also, for the inter-paper period T01 from the start of the print job to the time when the leading edge of the first sheet of paper P reaches the nip portion N, Table A is referred to regardless of the predetermined time T5.

[0066] 9, in the illustrated embodiment, the predetermined time T5 is set to 1800 ms for an image forming apparatus 100 with a CPM of 26 sheets, and 1200 ms for a high-speed image forming apparatus 100 with a CPM of 65 sheets. If the print mode of the image forming apparatus 100 acquired by the CPU 40 is the thick paper mode, the predetermined time T5 is set to 2000 ms. This predetermined time T5 specifies a case where the inter-sheet period T is relatively short. In other words, when the inter-sheet period T is a relatively short time equal to or shorter than the predetermined time T5, the heat adjustment unit 41 refers to table B to calculate the reduction value.

[0067] As shown in Fig. 10, for example, in the case of image forming apparatus 100 with a CPM of 26 sheets, table A is referenced for inter-sheet period T01, and the reduction value (amount of reduction) is read out as 71%. As described above, heating adjustment unit 41 reflects the read-out reduction value of 71% in the duty ratio set based on the determination results of temperature difference determination unit 42 and gradient determination unit 43. As shown in Fig. 5, duty control is performed to switch the upper limit of the duty ratio (middle part of the figure) of the power supplied to heating unit 36 ​​in inter-sheet period T01 to duty ratio D1 (lower part of the figure).

[0068] By reducing the duty ratio during the inter-paper period T, it takes some time for the fixing temperature to start rising even when power is supplied to the heating unit 36 ​​again during the paper passing period. In consideration of such a fixing temperature tracking capability, a lower limit of the duty ratio reduction control is also set in the fixing device 1 according to the present embodiment, and the lower limit of the duty ratio reduction control is set to 15%, for example. This prevents the fixing temperature from dropping too much during the inter-paper period T. For example, if the duty ratio is reduced by 71% during the inter-paper period T01, resulting in a duty ratio D1 below 15%, the heat adjustment unit 41 calculates the duty ratio D1 for this inter-paper period T01 as 15%.

[0069] 10, for example, in the image forming apparatus 100 with a CPM of 26 sheets, the reduction value (reduction amount) is read out as 59% for the inter-sheet period T12 by referring to table B for the second sheet onward. The heat adjustment unit 41 calculates the upper limit of the duty ratio D2 by reducing the duty ratio by 59% and taking into account the lower limit of the duty ratio of 15%. Similarly, the upper limit of the duty ratio D3 for the inter-sheet period T23 and the duty ratio D4 for the inter-sheet period T34 are calculated by referring to table B. As a result, the heater heating signal that has been subjected to the reduction control is controlled as shown in the lower part of FIG. 5. Such reduction control of the duty ratio is executed by the CPU 40 including the heat adjustment unit 41 when a print job for forming images on two or more sheets in succession is input.

[0070] The control of reducing the duty ratio by the heating adjustment unit 41 is not limited to being performed by referring to the two tables A and B as described above. For example, as shown in Fig. 11, a configuration having a table C in addition to tables A and B may be used.

[0071] FIG. 11 is an explanatory diagram showing the correspondence between the inter-paper period T and multiple tables A to C, FIG. 12 is an explanatory diagram showing the correspondence between the type of image forming device 100 and the specified periods T5 and T6, and FIG. 13 is an explanatory diagram showing the correspondence between the type of image forming device 100 and tables A to C.

[0072] 11, if the length of the inter-sheet period T (e.g., T12, T23, T34) obtained from the time determination unit 44 is less than or equal to the predetermined time T5, the heating adjustment unit 41 refers to table B, and if it is greater than or equal to the predetermined time T6, the heating adjustment unit 41 refers to table A. If the length of the inter-sheet period T is greater than the predetermined time T5 but less than the predetermined time T6, the heating adjustment unit 41 refers to table C. Also, for the inter-sheet period T01 from the start of the print job to when the leading edge of the first sheet of paper P reaches the nip portion N, table A is referenced regardless of the predetermined times T5 and T6.

[0073] As shown in Fig. 12, the predetermined time T5 is the same as the configuration shown in Fig. 9. The predetermined time T6 is set to 3300 ms in the image forming apparatus 100 with a CPM of 26 sheets, and 2700 ms in the high-speed image forming apparatus 100 with a CPM of 65 sheets. That is, when the inter-paper period T is relatively long, Table A is applied, and when it is short, Table B is applied. The corresponding Tables A and B are configured as shown in Fig. 13. The time exceeding the predetermined time T5 and less than the predetermined time T6 (T6-T5), for which Table C is applied, is set to, for example, 1500 ms in the image forming apparatus 100 with a CPM of 26 sheets, and 1500 ms in the high-speed image forming apparatus 100 with a CPM of 65 sheets, as shown in Fig. 12.

[0074] FIG. 14 is a graph showing the relationship between the paper interval and the duty reduction amount. FIG. 14 shows a case where the print mode of the image forming apparatus 100 is the thick paper mode. Table B is applied until the paper interval T is a relatively short time, up to a predetermined time T5, and when the paper interval T exceeds the predetermined time T5, Table C is applied, and when the paper interval T is equal to or longer than a predetermined time T6, Table A is applied to read out the reduction value of the duty ratio. In Table C, the reduction value calculated based on the following formula (1) from Tables A and B is applied to calculate the upper limit of the duty ratio. That is, if the reduction value of the duty ratio according to Table A is Da, the reduction value of the duty ratio according to Table B is Db, and the reduction value of the duty ratio at time (T5+ΔT) in Table C is Dc, then Dc=(Db-Da)×(T6-T5-ΔT) / (T6-T5)+Da…(1) It is expressed as:

[0075] The heating adjustment unit 41 reduces the duty ratio by the duty ratio reduction value Dc obtained by the above formula (1), and calculates the upper limit of the duty ratio taking into account the lower limit (e.g., 15%). The duty ratio of the power supplied to the heating unit 36 ​​is changed according to the calculated upper limit of the duty ratio. In this way, table C for the reduction control of the duty ratio can be variably provided between table B and table A, and more detailed control can be performed. This makes it possible to perform duty control that can prevent the fixing temperature from rising too high during the paper interval T, and also prevent the fixing temperature from falling too low.

[0076] Such reduction control of the duty ratio is started and released at a predetermined timing before and after the inter-paper period T. In this embodiment, the heat adjustment unit 41 starts the reduction control of the duty ratio immediately before the rear end of the paper P passes through the nip portion N, and releases the reduction control of the duty ratio immediately before the leading end of the paper P reaches the nip portion N. The timing immediately before the rear end of the paper P passes through the nip portion N is, in other words, the timing immediately before the start of the inter-paper period T, and is calculated by applying the fixed value shown in FIG. 7 to the start time of the inter-paper period T. Moreover, the timing immediately before the leading end of the paper P reaches the nip portion N is, in other words, the timing immediately before the end of the inter-paper period T, and is calculated by applying the fixed value shown in FIG. 6 to the end time of the inter-paper period T.

[0077] Therefore, the reduction control of the duty ratio is started immediately before the start of the paper interval period T, and the upper limit of the duty ratio is reduced. Also, the reduction control of the duty ratio is released immediately before the end of the paper interval period T, and the upper limit of the duty ratio is raised. This makes it possible to make the change in the fixing temperature follow the start and end of the paper interval period T, and to make the fixing temperature uniform during the paper passing period.

[0078] 15 is a flowchart showing a process example of the reduction control in the fixing device 1. The heat adjustment unit 41 performs reduction control to reduce the upper limit of the duty ratio in the paper interval T below the upper limit of the duty ratio in the paper passing period following the paper interval T, with the ratio of the energized state of the heating unit 36 ​​in a predetermined cycle being the duty ratio.

[0079] When a print job is input, the presence or absence of the first sheet P from the start of the print job is determined based on the job information of the print job (step S101). If it is determined that the first sheet P is "present" (YES in step S101), control proceeds to step S102. If not, step S101 is repeated.

[0080] In step S102, the duty ratio is reduced during the inter-paper period T01 up to the first sheet. The heat adjustment unit 41 reads the reduction value from, for example, table A for the duty ratio reduction control, calculates and outputs an upper limit for the duty ratio reduction control by reflecting the lower limit. Then, immediately before the leading edge of the first sheet P reaches the nip N, the duty ratio reduction control is released.

[0081] In step S103, it is determined whether the paper P being conveyed is the second or subsequent sheet. If it is determined that the paper P is "present" (YES in step S103), control proceeds to step S104. If not, it is determined that the print job has ended, and the control of reducing the duty ratio is ended.

[0082] In step S104, the paper interval T (for example, the paper interval T12 until the second sheet of paper P) is determined. Paper passing determination unit 45 determines the presence or absence of paper based on a paper passing determination signal from the paper detection sensor, and time determination unit 44 determines the length of the paper interval T.

[0083] Furthermore, the heat adjusting unit 41 selects and reads out a table to be referred to based on the determined inter-paper period T, and calculates a reduction value of the duty ratio (step S105). Also, based on the calculated inter-paper period T, the heat adjusting unit 41 determines the start and end timings of reduction control including a fixed value (step S106).

[0084] If it is determined that it is time to start the reduction control of the duty ratio (YES in step S106), the upper limit value of the duty ratio is output based on the calculated reduction value (step S107). If not, step S107 is repeated.

[0085] In step S108, it is determined whether or not there is paper P to be transported subsequently. If it is determined that paper P is "present" (YES in step S108), the process proceeds to step S104, and the duty ratio reduction control is performed by the processes of steps S104 to S107. If it is determined that paper P to be transported is "not present" (NO in step S108), the duty ratio reduction control ends.

[0086] When it is determined that the print job is completed, the CPU 40 stops the supply of power to the heating unit 36 ​​and ends the duty control. Note that, if the image forming apparatus 100 further receives input of the next print job before the end of printing, it is not necessary to stop the supply of power to the heating unit 36.

[0087] With the above configuration, the image forming apparatus 100 and the fixing device 1 can control the amount of heat applied to the fixing belt 31 during the paper interval T, suppressing fluctuations such as excessive increases or decreases in the surface temperature of the fixing belt 31, and stabilizing the fixing temperature during the paper passing period. This makes it possible to suppress image defects such as uneven glossiness in the image fixed to the paper P by the fixing device 1, and to perform good image formation.

[0088] It should be noted that this embodiment is merely an example and does not limit the present invention in any way. For example, the image forming apparatus 100 is not limited to a multifunction device, and can be a printer, a copier, a FAX machine, or any other device that has an image forming function using an electrophotographic method. In addition, the image forming apparatus 100 is not limited to a color device, and can be a monochrome device. In addition, the image forming apparatus 100 is not limited to a device capable of double-sided printing, and can be a device dedicated to single-sided printing. The duty ratio reduction control table shown in the above embodiment is not limited to the two or three types shown as examples, and can be further subdivided and configured in various ways.

[0089] The embodiments disclosed herein are illustrative in all respects and are not intended to be limiting. Therefore, the technical scope of the present invention is not interpreted solely by the above-described embodiments, but is defined by the claims. Also, all modifications within the scope and meaning equivalent to the claims are included. [Explanation of symbols]

[0090] 1 Fixing device 18 Paper feed section 20 Paper transport section 21 Paper transport path 22 Reversing conveyor path 31 Fixing belt (fixing member) 32 Pressure roller (pressure member) 33 Support member 34 Fixing Pad 35 Sliding sheet (sliding member) 36 Heating section 37 Reflector 38 Fixing temperature sensor (temperature detection section) 40 CPU 41 Heating adjustment section 42 Temperature difference determination section 43 Gradient determination unit 44 Time Judgment Section 45 Paper feed determination section 100 Image forming device 101 Image forming section N Nip S Paper feed direction P paper

Claims

1. A fixing device that heats a sheet passing through a nip portion between a fixing member and a pressure member to fix a toner image formed on the sheet, a heating section for heating the fixing member; A heating control unit that controls the power supplied to the heating unit; a temperature detection unit for detecting a fixing temperature, which is the temperature of the fixing member; a temperature difference determination unit that calculates a difference between the fixing temperature detected by the temperature detection unit and a preset temperature; a gradient determination unit that determines a temperature gradient based on a change in the fixing temperature, the gradient determination unit determines an increasing or decreasing trend of the fixing temperature from a plurality of the fixing temperatures detected at predetermined comparison times; The heating adjustment unit is a duty ratio is a ratio of an energized state of the heating unit within a predetermined cycle, and a reduction control is performed to reduce an upper limit of the duty ratio during an inter-paper period in which the paper is not present in the nip portion to a lower limit of the duty ratio during a paper passing period that is continuous with the inter-paper period and in which the paper is passing through the nip portion; a temperature difference determining unit that determines an upper limit of the duty ratio and an amount of reduction of the duty ratio based on the results of determination by the temperature difference determining unit and the gradient determining unit;

2. 2. The fixing device according to claim 1, a paper passing determination unit that determines the presence or absence of paper upstream of the nip portion in the paper transport direction, the inter-paper period is a period from when the paper passing determination unit detects the change from the presence of paper to the absence of paper to when the paper passing determination unit detects the change from the absence of paper to the presence of paper, The fixing device, wherein the paper passage period is a period from a timing when the paper passage determination unit detects the presence of paper from the absence to a timing when the paper passage determination unit detects the presence of paper from the absence.

3. 3. The fixing device according to claim 2, The fixing device according to claim 1, wherein the heat adjusting section starts a control to reduce the duty ratio immediately before a trailing edge of the paper passes through the nip section.

4. 4. The fixing device according to claim 2, The fixing device according to claim 1, wherein the heat adjusting section cancels the reduction control of the duty ratio immediately before a leading edge of the paper reaches the nip section.

5. In the fixing device according to any one of claims 1 to 4, A time determination unit that determines the length of the inter-paper period, based on a determination result of the time determination unit, a reduction amount of the duty ratio is made different between when the length of the inter-paper period is equal to or less than a predetermined time and when the length of the inter-paper period exceeds the predetermined time; a fixing device which reduces the reduction amount when the length of the inter-paper period is equal to or shorter than the predetermined time;

6. In the fixing device according to any one of claims 1 to 5, the fixing member has a fixing belt rotatable about an axis; The pressure member has a pressure roller, The fixing belt includes a sliding member that slides in contact with the inner surface of the belt, and a fixing pad that fixes the sliding member. The fixing device is characterized in that the pressure roller is disposed at a position facing the fixing pad with the fixing belt interposed therebetween.

7. 7. An image forming apparatus comprising the fixing device according to claim 1.

Images