Image forming apparatus

Abstract

This prevents image unevenness caused by deformation of the unfixed image on the paper in a direction closer to the fixing area. [Solution] The image forming apparatus 100 includes a fuser unit 30 for fixing a toner image formed on paper, a second sensor 60 installed downstream of the fuser nip of the fuser unit 30 in the paper transport direction for detecting the transported paper, and a control unit 90 for controlling the fuser unit 30. The control unit 90 determines a predetermined transport speed Vf based on the detection result of the first paper that has passed through the fuser unit 30 detected by the second sensor 60, so that the second paper that is transported after the first paper is transported at a predetermined transport speed for a certain period of time after it enters the fuser nip of the fuser unit 30, and controls the fuser unit 30 according to the determined predetermined transport speed Vf.

Description

【Technical Field】 【0001】 The present invention relates to an image forming apparatus. 【Background Art】 【0002】 Conventionally, an image forming apparatus has been disclosed that matches the paper feed speed between a fixing heat roller and a paper conveyance device upstream thereof in order to prevent the occurrence of paper wrinkling and transfer deviation (see Patent Document 1). 【Prior Art Documents】 【Patent Documents】 【0003】 【Patent Document 1】 Japanese Patent Application Laid-Open No. 6-230700 【Summary of the Invention】 【Problems to be Solved by the Invention】 【0004】 However, in the image forming apparatus disclosed in Patent Document 1 above, depending on the posture of the leading edge of the paper, the leading edge of the paper may not smoothly enter the fixing nip. In such a case, as shown in FIG. 7, there is a risk that the unfixed image on the paper is deformed (reverse loop) in a direction approaching the fixing unit. As a result, there is a problem that image unevenness occurs due to the unfixed image on the paper being rubbed or affected by heat before fixing. 【0005】 The present invention has been made in view of the above problems, and an object thereof is to prevent image unevenness caused by deformation of the unfixed image on the paper in a direction approaching the fixing unit. 【Means for Solving the Problems】 【0006】 To solve the above problems, an image forming apparatus according to the present invention includes: a fixing unit that fixes a toner image formed on paper; A sensor is installed downstream of the fixing nip of the fixing unit in the paper transport direction and detects the transported paper, A control unit that controls the fixing unit, Equipped with, The control unit, Based on the detection result obtained by the sensor when the first sheet of paper has passed through the fixing unit, the fixing unit is controlled so that the second sheet of paper, which is transported after the first sheet of paper, is transported at a predetermined transport speed. It is characterized by the following: [Effects of the Invention] 【0007】 According to the present invention, it is possible to prevent image unevenness caused by deformation of the unfixed image on the paper in a direction approaching the fixing area. [Brief explanation of the drawing] 【0008】 [Figure 1] This is a schematic diagram showing an example of the configuration of the image forming apparatus of the present invention. [Figure 2] This is a block diagram showing the control system of the image forming apparatus of the present invention. [Figure 3] This diagram shows the surrounding configuration of the transfer and fixing sections. [Figure 4] This flowchart shows the flow of the fixing speed control determination process. [Figure 5] This flowchart shows the flow of the second fixing speed control process. [Figure 6] This is a timing chart showing the timing of paper detection by the first or second sensor. [Figure 7] This figure shows the paper deformed in a direction that brings it closer to the fuser. [Modes for carrying out the invention] 【0009】 The configuration and operation of the image forming apparatus in the embodiments of the present invention will be described in detail with reference to the drawings. While the embodiments of the present invention will be described using a color image forming apparatus as an example, the present invention is not limited thereto. For example, it can also be applied to a monochrome image forming apparatus. 【0010】 Figure 1 is a schematic diagram showing the configuration of the image forming apparatus 100 according to this embodiment. As shown in Figure 1, the image forming apparatus 100, also known as a tandem color image forming apparatus, performs color image formation using four sets of image forming units 10. The image forming apparatus 100 forms images on a recording medium paper using an electrophotographic imaging process. The image forming apparatus 100 includes an image reading unit 9, a paper supply unit 20, image forming units 10, a transfer unit (transport means) 40, a fixing unit 30, an operation input unit 80, and a control unit 90, etc. The dashed line in the figure indicates the paper transport path. 【0011】 The image reading unit 9 scans and exposes the image of the document placed on the document table using the optical system of the scanning exposure apparatus, reads it into the line image sensor, and outputs an image information signal through photoelectric conversion. The output image signal is processed by the image processing unit (not shown) through analog processing, A / D conversion, shading correction, image compression, etc., and is input to the optical writing unit 3 of the image forming unit 10. 【0012】 An operation input unit 80 is provided near the image reading unit 9. The user can set image formation settings, paper size, number of prints, etc., via the operation input unit 80. 【0013】 Below the image reading unit 9, the paper supply unit 20, image forming unit 10, intermediate transfer belt 6, transfer unit 40, and fixing unit 30 are arranged. 【0014】 As the image forming unit 10, an image forming unit 10Y for forming a yellow (Y) color image is provided. Also, an image forming unit 10M for forming a magenta (M) color image is provided. Also, an image forming unit 10C for forming a cyan (C) color image is provided. Also, an image forming unit 10K for forming a black (K) color image is provided. That is, as the image forming unit 10, a total of four sets of image forming units, namely the image forming unit 10Y, the image forming unit 10M, the image forming unit 10C, and the image forming unit 10K, are provided. 【0015】 The image forming unit 10Y includes a photosensitive drum 1Y as an image carrier, and a charging unit 2Y, an optical writing unit 3Y, a developing unit 4Y, and a photosensitive drum cleaning unit 5Y arranged around it. Similarly, the image forming unit 10M includes a photosensitive drum 1M as an image carrier, and a charging unit 2M, an optical writing unit 3M, a developing unit 4M, and a photosensitive drum cleaning unit 5M arranged around it. The image forming unit 10C includes a photosensitive drum 1C as an image carrier, and a charging unit 2C, an optical writing unit 3C, a developing unit 4C, and a photosensitive drum cleaning unit 5C arranged around it. The image forming unit 10K includes a photosensitive drum 1K as an image carrier, and a charging unit 2K, an optical writing unit 3K, a developing unit 4K, and a photosensitive drum cleaning unit 5K arranged around it. Note that the photosensitive drums 1Y, 1M, 1C, 1K of the image forming units 10Y, 10M, 10C, 10K, the charging units 2Y, 2M, 2C, 2K, the optical writing units 3Y, 3M, 3C, 3K, and the photosensitive drum cleaning units 5Y, 5M, 5C, 2K each have the same functions. Therefore, unless there is a particular distinction, they are denoted without attaching the symbols Y, M, C, K. 【0016】 The image forming unit 10 writes an image information signal on the photosensitive drum 1 in the optical writing unit 3, and forms a latent image based on the image information signal on the photosensitive drum 1. Then, the latent image is developed by the developing unit 4, and a toner image, which is a visible image, is formed on the photosensitive drum 1. 【0017】 The intermediate transfer belt 6 is an endless belt, installed by a plurality of rollers, and supported so as to be able to run. The toner images of each color formed in the image forming units 10Y, 10M, 10C, and 10K are sequentially transferred onto the intermediate transfer belt 6 running by the primary transfer units 7Y, 7M, 7C, and 7K, and a color image (toner image) in which layers of each color are superimposed is primarily transferred onto the intermediate transfer belt 6. 【0018】 The paper supply unit 20 has a feed roller 21 and a counter roller 22. The paper supply unit 20 conveys the paper supplied from a paper feed tray (not shown) or outside the image forming apparatus 100, and supplies the paper to the transfer unit 40. 【0019】 The transfer unit 40 has a transfer roller 41 and a transfer counter roller 42. The transfer roller 41 is disposed in contact with the transfer counter roller 42 via the intermediate transfer belt 6, and when the paper passes through the transfer nip formed between the transfer roller 41 and the transfer counter roller 42, the toner image on the intermediate transfer belt 6 is secondarily transferred onto the paper. 【0020】 A fixing unit 30 is disposed on the downstream side of the paper in the transfer roller 41. The fixing unit 30 has a fixing roller 31, a heating roller 32, and the like. In this fixing unit 30, when the paper passes through the fixing nip formed between the fixing roller 31 and the heating roller 32, the paper is heated and pressurized to fix the transferred toner image onto the paper. Further, the fixing unit 30 fixes the toner image onto the paper and conveys the paper to the downstream side in the paper conveyance direction. 【0021】 As shown in Figure 2, each of the above-mentioned parts of the image forming apparatus 100 is connected to a control unit 90 and controlled as appropriate by the control unit 90. The control unit 90 includes a CPU (Central Processing Unit), RAM (Random Access Memory), and ROM (Read Only Memory), which are not shown. The control unit 90 performs various operations according to various processing programs for the image forming apparatus 100. Note that the image forming apparatus 100 may include components other than those described above, or it may not include some of the components described above. 【0022】 Next, with reference to Figure 3, the peripheral configuration of the transfer unit 40 and the fixing unit 30 will be described. Figure 3 shows the peripheral configuration of the transfer unit 40 and the fixing unit 30. As shown in Figure 3, a first sensor 50 for detecting the leading or trailing end of the paper P is provided at a predetermined location on the upstream side of the transfer unit 40 in the paper transport direction. Similarly, a second sensor 60 for detecting the leading or trailing end of the paper P is provided at a predetermined location on the downstream side of the fixing unit 30 in the paper transport direction. The first sensor 50 includes an arm (actuator) 51 and a laser sensor 52, etc. The first sensor 50 can determine that the paper P has begun to pass the first sensor 50 when the leading end of the paper P, which has been transported from the upstream side in the paper transport direction, comes into contact with the arm 51, causing the arm 51 to tilt, and this tilt is detected by the laser sensor 52. In other words, the first sensor 50 has detected the leading end of the paper P by detecting the tilt of the arm 51 with the laser sensor 52. Furthermore, the first sensor 50 can determine that the paper P has finished passing the first sensor 50 when the rear end of the paper P has passed and is no longer in contact with the arm 51, and the arm 51 returns to its original position, and the tilt of the arm 51 is no longer detected by the laser sensor 52. In other words, the first sensor 50 has detected the rear end of the paper P when the tilt of the arm 51 is no longer detected by the laser sensor 52. Note that the second sensor unit 60 has the same configuration as the first sensor unit 50, so the explanation of the second sensor unit 60 will be omitted. 【0023】 Furthermore, a loop sensor (loop detection means) 70 is provided between the transfer unit 40 and the fixing unit 30. The loop sensor 70 is a sensor for determining whether the orientation of the paper P passing between the transfer unit 40 and the fixing unit 30 is appropriate. The loop sensor 70 has an arm 71 and a laser sensor 72, etc. When the degree of the loop (curved shape) formed on the paper P exceeds a threshold, the arm 71 of the loop sensor 70 tilts, and by detecting this tilt with the laser sensor 72, it is possible to determine that the orientation of the paper P is no longer appropriate. In other words, the loop sensor 70 detects the paper P by detecting the tilt of the arm 71 with the laser sensor 72. Here, the state in which the orientation of the paper P is no longer appropriate includes the reverse loop state shown in Figure 7. On the other hand, in normal conditions when the orientation of the paper P is appropriate, the loop sensor 70 can determine that the orientation of the paper P is appropriate because the tilt of the arm 71 is not detected by the laser sensor 72. 【0024】 Next, the operation of the image forming apparatus 100 will be described with reference to Figures 4 and 5. Figure 4 is a flowchart showing the flow of the fixing speed control determination process. The fixing speed control determination process is performed in cooperation with the CPU of the control unit 90 of the image forming apparatus 100 and the fixing speed control determination program stored in ROM. The fixing speed control determination process is started when the control unit 90 receives a print start request. 【0025】 As shown in Figure 4, when the fixing speed control determination process is started, the control unit 90 first determines whether the first page of the print job has been fed (step S101). Here, if the print job is a mixed job with multiple types of paper mixed together, it is determined whether the first page has been fed for each type of paper. For example, in the case of a mixed job with thin paper and thick paper mixed together, if the first page of the mixed job is thin paper and the second page is thick paper, both the thin paper of the first page and the thick paper of the second page are determined to be the first pages to be fed. 【0026】 If, in step S101, it is determined that the first page of the print job has not been fed, i.e., that pages from the second page onwards have been fed (step S101; NO), the control unit 90 proceeds to step S110. The control unit 90 then decides to perform the normal fixing speed control process, i.e., the first fixing speed control process, for the following pages (step S110). The control unit 90 then terminates the fixing speed control determination process. 【0027】 Furthermore, if it is determined in step S101 that the first paper feed of the print job has occurred (step S101; YES), the control unit 90 determines whether or not the leading edge of the paper being fed has been detected by the first sensor 50 (step S102). 【0028】 In step S102, if it is determined that the leading edge of the paper has not been detected (step S102; NO), the control unit 90 repeats the determination process of step S102. On the other hand, if it is determined in step S102 that the leading edge of the paper has been detected (step S102; YES), the control unit 90 starts measuring the elapsed time (step S103). The elapsed time refers to the time from when the first sensor 50 detects the leading edge of the paper until the second sensor 60 detects the leading edge of the paper (see the timing chart in the 2nd to 5th rows from the top of Figure 6). 【0029】 Next, the control unit 90 determines whether or not the leading edge of the paper has been detected by the second sensor 60 (step S104). 【0030】 In step S104, if it is determined that the leading edge of the paper has not been detected (step S104; NO), the control unit 90 repeats the determination process in step S104. On the other hand, if it is determined in step S104 that the leading edge of the paper has been detected (step S104; YES), the control unit 90 proceeds to step S105. The control unit 90 then terminates the measurement of the elapsed time, retains the value of the elapsed time, and begins measuring the passage time (step S105). The passage time refers to the time it takes for the paper to pass through the second sensor 60. In other words, the passage time refers to the time from when the leading edge of the paper is detected by the second sensor 60 until the trailing edge of the paper is detected (see the timing chart in the 2nd to 5th rows from the top of Figure 6). 【0031】 Next, the control unit 90 determines whether or not the trailing edge of the paper has been detected by the second sensor 60 (step S106). 【0032】 If it is determined in step S106 that the trailing edge of the paper has not been detected (step S106; NO), the control unit 90 repeats the determination process in step S106. On the other hand, if it is determined in step S106 that the leading edge of the paper has been detected (step S106; YES), the control unit 90 proceeds to step S107. The control unit 90 then terminates the measurement of the passage time and retains the value of the passage time (step S107). 【0033】 Next, the control unit 90 compares the elapsed time value held in step S105 with the value obtained by dividing the distance between the first sensor 50 and the second sensor 60 (distance between sensors) Ls by the transport speed Vt in the transfer unit 40 (Ls / Vt) (step S108). The control unit 90 also compares the passage time value held in step S107 with the value obtained by dividing the length Lp of the paper in the paper feeding direction by the transport speed Vt (Lp / Vt) (step S108). 【0034】 In step S108, if the elapsed time is greater than Ls / Vt and the transit time is equal to Lp / Vt (step S108; YES), the control unit 90 proceeds to step S109. The control unit 90 then decides to execute the fixing speed control process of the present invention, i.e., the second fixing speed control process, in the following steps (step S109). In other words, as shown in the timing chart third from the top in Figure 6, if the arrival of the paper to the second sensor 60 is delayed and the transit time is equal to Lp / Vt, the cause is not the transport speed (Vf) in the fixing unit 30, but rather the delay in the arrival of the paper to the fixing nip (fixing unit 30) (fixing entry failure), so the control unit 90 decides to execute the second fixing speed control process. The control unit 90 then terminates the fixing speed control determination process. 【0035】 Furthermore, in step S108, if the condition that the elapsed time is greater than Ls / Vt and the passage time is equal to Lp / Vt is not met (step S108; NO), the control unit 90 proceeds to step S110. The control unit 90 then decides to execute the normal fixing speed control process, i.e., the first fixing speed control process, on the following pages (step S110). Specifically, as shown in the timing chart second from the top in Figure 6, if the elapsed time is equal to Ls / Vt and the passage time is equal to Lp / Vt (in the case of normal paper feeding), the control unit 90 decides to execute the first fixing speed control process. Also, as shown in the timing chart fourth from the top in Figure 6, if the arrival of the paper to the second sensor 60 is delayed and the passage time is greater than Lp / Vt (when the fixing speed (Vf) is slow), the control unit 90 decides to execute the first fixing speed control process. Furthermore, as shown in the fifth timing chart from the top in Figure 6, if the elapsed time is equal to Ls / Vt and the transit time is greater than Lp / Vt (i.e., the fixing speed (Vf) is slow), the control unit 90 decides to execute the first fixing speed control process. Then, the control unit 90 terminates the fixing speed control determination process. 【0036】 Here, the transport speed Vt mentioned above fluctuates depending on the pressure of the transfer nip in the transfer section 40, the outer diameters of the transfer roller 41 and the opposing transfer roller 42, and the condition of the paper (thickness, surface roughness, material), etc. Also, the distance Ls between sensors (see Figure 3) changes depending on the condition of the paper (basis weight, humidity control), etc. Therefore, in determining whether the elapsed time is greater than Ls / Vt, the accuracy of the determination can be ensured by pre-measuring the variability of these fluctuating factors and comparing it with the upper limit. Furthermore, by using past measurement values ​​obtained when paper was fed under the same paper type and mode (double-sided feeding, 1st side / 2nd side, etc.) as the above paper and comparing them with the maximum value, it becomes possible to make a more precise determination for each product according to the usage conditions. In addition, the accuracy of the determination can be further improved by setting the reference value for the above determination to a value corresponding to the paper type (basis weight, etc.). Furthermore, in determining whether the above transit time value is equivalent to Lp / Vt, the variation range of the transport speed Vt (Vt ± 0.5 to 1%), which is a variable factor, is used as a criterion, and if it falls within that range, it is judged to be equivalent (the same applies to the elapsed time). 【0037】 Figure 5 is a flowchart illustrating the flow of the second fixing speed control process described above. The second fixing speed control process is performed in cooperation with the CPU of the control unit 90 of the image forming apparatus 100 and the second fixing speed control program stored in the ROM. The second fixing speed control process is initiated when a decision to execute the second fixing speed control process is made in step S109 of the fixing speed control determination process described above, and when the feeding of subsequent pages of paper begins. 【0038】 As shown in Figure 5, when the second fixing speed control process is started, the control unit 90 first determines the fixing speed Vf (see Figure 3), which is the transport speed in the fixing unit 30 (step S201). Specifically, the control unit 90 determines the fixing speed Vf to be +1.0 to +2.0% of the transport speed Vt in the transfer unit 40. Note that this determination method is merely one example. For example, the fixing speed Vf may be determined according to the value of the elapsed time held in step S105 of the fixing speed control determination process described above. Specifically, the determination is made such that the fixing speed Vf becomes faster as the value of the elapsed time increases. This eliminates the reverse loop that occurs in the paper passing between the transfer unit 40 and the fixing unit 30, and prevents image misalignment in the transfer unit 40 due to excessive pulling of the paper. In addition to the elapsed time, the fixing speed Vf may also be determined according to the value of the passage time held in step S107 of the fixing speed control determination process. Specifically, the fixing speed Vf is increased as the elapsed time increases, and the fixing speed Vf is increased as the passage time increases. This is because a longer elapsed time means that the leading edge of the paper enters the fixing nip more slowly and the reverse loop is larger, and a longer passage time means that the transport speed at the fixing nip is slower. The more these conditions overlap, the faster the fixing speed Vf needs to be. This allows for a more effective elimination of the reverse loop that occurs in the paper passing between the transfer unit 40 and the fixing unit 30, and also more effectively prevents image misalignment in the transfer unit 40 due to excessive pulling of the paper. 【0039】 Next, the control unit 90 determines whether the current paper feeding conditions (paper feeding conditions) are the same as the paper feeding conditions at the time the decision to execute the second fixing speed control process was made in step S109 of the fixing speed control determination process described above (step S202). Paper feeding conditions include, for example, conditions related to paper information (stiffness, basis weight, volume resistance, humidity control state due to the environment, etc.), image information (image area, toner amount distribution, etc.), and mode information (single-sided / double-sided, high speed / low speed, etc.). In this embodiment, if the conditions related to at least one of the paper information, image information, and mode information are the same, it is determined that the current paper feeding conditions and the paper feeding conditions at the time the decision to execute the second fixing speed control process was made are the same. 【0040】 In step S202, if it is determined that the current paper feeding conditions and the paper feeding conditions at the time the decision to execute the second fixing speed control process was made are not the same (step S202; NO), the control unit 90 proceeds to step S206. 【0041】 Furthermore, in step S202, if it is determined that the current paper feeding conditions are the same as the paper feeding conditions when the decision to execute the second fixing speed control process was made (step S202; YES), the control unit 90 proceeds to step S203. Then, the control unit 90 starts the printing operation at the fixing speed Vf determined in step S201 (step S203). The control at fixing speed Vf is switched between the time the trailing edge of the paper fed when the decision to execute the second fixing speed control process was made (first paper) passes the fixing nip and before the leading edge of the next page of paper (second paper) reaches the fixing nip. This prevents paper feeding effects (image noise, wrinkles, etc.) when the speed is changed. 【0042】 Next, the control unit 90 determines whether or not paper has been detected by the loop sensor 70 (step S204). 【0043】 In step S204, if the loop sensor 70 determines that no paper has been detected (step S204; NO), the control unit 90 proceeds to step S206. 【0044】 Furthermore, if the loop sensor 70 determines in step S204 that paper has been detected (step S204; YES), the control unit 90 determines whether a predetermined time has elapsed since the start of the printing operation in step S203 (step S205). Here, the predetermined time is, for example, the time calculated by (Δt × Vt) ​​ / Vf. Δt is the difference between Ls / Vt and the elapsed time described above. 【0045】 In step S205, if it is determined that a predetermined amount of time has not elapsed since the start of the printing operation in step S203 (step S205; NO), the control unit 90 returns to step S204 and repeats the subsequent processing. 【0046】 Furthermore, if it is determined in step S205 that a predetermined time has elapsed since the start of the printing operation in step S203 (step S205; YES), the control unit 90 proceeds to step S206. 【0047】 Next, the control unit 90 performs a first fixing speed control process (step S206). In the first fixing speed control process, the fixing speed is controlled in accordance with the transport speed Vt in the transfer unit 40. In addition, the first fixing speed control process may also perform so-called loop control, which switches the fixing speed between high and low speed relative to the transport speed Vt in the transfer unit 40 based on the paper detection result by the loop sensor 70. 【0048】 Next, the control unit 90 determines whether or not the trailing end of the paper currently being fed has been detected by the second sensor 60 (step S207). 【0049】 In step S207, if the second sensor 60 determines that the trailing end of the paper currently being fed has not been detected (step S207; NO), the control unit 90 repeats the determination process in step S207. On the other hand, in step S207, if the second sensor 60 determines that the trailing end of the paper currently being fed has been detected (step S207; YES), the control unit 90 proceeds to step S208. 【0050】 Next, the control unit 90 determines whether or not there is a subsequent piece of paper to be fed (step S208). 【0051】 If it is determined in step S208 that there is further paper to be fed (step S208; YES), the control unit 90 returns to step S202 and repeats the subsequent processing. 【0052】 Furthermore, if it is determined in step S208 that there is no further paper to feed (step S208; NO), the control unit 90 terminates the second fixing speed control process. 【0053】 As described above, the image forming apparatus 100 includes a fuser unit 30 for fixing the toner image formed on the paper, a second sensor 60 installed downstream of the fuser nip of the fuser unit 30 in the paper transport direction for detecting the transported paper, and a control unit 90 for controlling the fuser unit 30. Based on the detection result obtained by the second sensor 60 for detecting the first sheet of paper that has passed through the fuser unit 30, the control unit 90 controls the fuser unit 30 so that the second sheet of paper, which is transported after the first sheet of paper, is transported at a predetermined transport speed. Specifically, the control unit 90 controls the fuser unit 30 so that the second sheet of paper is transported at a predetermined transport speed for a certain period of time after it enters the fuser nip of the fuser unit 30. Therefore, with the image forming apparatus 100, a paper feeding failure into the fuser nip can be predicted based on the detection result by the second sensor 60, and the fuser unit 30 can be controlled in subsequent paper feeding to eliminate the deterioration of the paper orientation (reverse loop) caused by the paper feeding failure. As a result, image unevenness caused by the deformation of the unfixed image on the fed paper in the direction approaching the fuser unit 30 can be prevented. Here, the fixed period during which the fixing unit 30 is controlled by the control unit 90 is, for example, the time it takes for the reverse loop of the paper length to be resolved, based on the delay in the time it takes for the paper to reach the second sensor 60. Alternatively, the fixed period may be the time it takes for the reverse loop to be resolved and for the loop sensor 70 to detect that there is no paper. Alternatively, the fixed period may be the same as the delay ΔT in the time it takes for the leading edge of the paper to reach the second sensor 60. Alternatively, data on predetermined times for each paper type / mode may be stored, and the fixed period may be a time selected from the above data according to the conditions. 【0054】 The image forming apparatus 100 also includes a transfer unit 40 (transport means) that transports the paper to the fixing unit 30. The control unit 90 controls the fixing unit 30 so that the second sheet of paper, which is transported after the first sheet of paper, is transported at a predetermined transport speed Vf that is faster than the transport speed Vt in the transfer unit 40 for a certain period of time after it enters the fixing nip of the fixing unit 30. According to the image forming apparatus 100, the transport speed (fixing speed) Vf in the fixing unit 30 is controlled to be faster than the transport speed Vt in the transfer unit 40. Therefore, in the feeding of the second and subsequent sheets of paper, deterioration of the paper's appearance due to poor paper entry into the fixing nip can be quickly and effectively resolved. 【0055】 Furthermore, it is preferable that the transport speed (fixing speed) Vf in the fixing unit 30 be switched to a transport speed Vf determined based on the detection result by the second sensor 60 before the leading edge of the second paper enters the fixing nip. This is because it is possible to effectively prevent problems such as a reduction in the effectiveness of the present invention due to variations in the timing of paper entry into the fixing nip. In addition, the transport speed Vf in the fixing unit 30 can be made variable according to the detection result of the second sensor 60. 【0056】 Furthermore, the image forming apparatus 100 includes an acquisition means (control unit 90) that acquires paper information, image information, and mode information for each sheet of paper being transported. The control unit 90 controls the fixing unit 30 when at least one of the paper information, image information, and mode information is the same for the second sheet of paper and the first sheet of paper. Therefore, with the image forming apparatus 100, the transport speed (fixing speed) Vf in the fixing unit 30 is not changed unnecessarily, thus preventing image misalignment caused by paper pulling in the transfer unit 40 and image unevenness due to insufficient resolution of reverse loops. 【0057】 Here, image noise caused by poor paper entry into the fuser nip is affected by the orientation and shape of the leading edge of the paper entering the fuser nip, as well as the paper feeding conditions. Specifically, the paper information (rigidity, basis weight, volume resistance, humidity control state due to the environment, etc.), image information (image area, toner amount distribution, etc.), and mode information (single-sided / double-sided, high / low speed, etc.) affect the position of the paper relative to the fuser nip, the likelihood of a reverse loop occurring when the above-mentioned paper entry failure occurs, and the degree of image unevenness when a reverse loop occurs. Therefore, in this embodiment, each of the above-mentioned pieces of information is acquired, and the transport speed (fusing speed) Vf in the fuser unit 30 is changed only when it matches the conditions under which the transport speed (fusing speed) Vf should be changed. An example of a case where at least one piece of information among the paper information, image information, and mode information is the same for the second paper and the first paper is when continuous paper feeding is performed from the same paper tray (cassette). Another example is when the images on the second paper and the first paper are of similar quality. Another example is when the conditions for the first and second sides of a double-sided printout match. 【0058】 Furthermore, the image forming apparatus 100 is equipped with a loop sensor (loop detection means) 70 positioned between the fixing unit 30 and the transfer unit 40 (transport means) to detect the curved shape (reverse loop) of the paper passing between the fixing unit 30 and the transfer unit 40. The control unit 90 controls the fixing unit 30 during the period from when the paper enters the fixing nip of the fixing unit 30 until the loop sensor 70 detects that the curvature of the paper has been resolved. Therefore, the image forming apparatus 100 can terminate paper feeding at an appropriate timing while the transport speed Vf in the transfer unit 40 is changed to a predetermined transport speed Vf that is faster than the transport speed Vt. As a result, the image forming apparatus 100 can transition to normal paper feeding control at an appropriate timing after the transport speed Vf in the transfer unit 40 is changed to a predetermined transport speed Vf that is faster than the transport speed Vt. 【0059】 Furthermore, the control unit 90 controls the fixing unit 30 to continue transporting at the set fixing speed Vf until a period of time has elapsed calculated from the detection result by the second sensor 60 downstream of the fixing nip of the fixing unit 30 and a speed faster than the transport speed Vt of the transfer unit 40 has elapsed. Therefore, the image forming apparatus 100 can terminate paper feeding at an appropriate timing while the transport speed Vf in the transfer unit 40 is changed to a predetermined transport speed Vf that is faster than the transport speed Vt. As a result, the image forming apparatus 100 can transition to normal paper feeding control at an appropriate timing after the transport speed Vf in the transfer unit 40 is changed to a predetermined transport speed Vf that is faster than the transport speed Vt. 【0060】 Furthermore, the control unit 90 determines whether to control the fuser unit 30 based on the time from when the paper passes through the transfer unit 40 until the paper arrives at the second sensor 60 downstream of the fuser nip, and the time the paper passes the second sensor 60 downstream of the fuser nip. In other words, the control unit 90 determines whether to execute the second fuser speed control process (see Figure 5). Therefore, the image forming apparatus 100 can accurately detect whether a paper entry failure into the fuser nip has occurred, based on the elapsed time from when the paper passes through the transfer section 40 until the paper arrives at the second sensor 60 downstream of the fuser nip, and the time the paper passes through the second sensor 60 downstream of the fuser nip. As a result, the image forming apparatus 100 can determine to execute the second fuser speed control process when a paper entry failure into the fuser nip has occurred, thereby appropriately controlling the transport speed (fusing speed) Vf in the fuser section 30. 【0061】 Furthermore, the control unit 90 controls the fuser unit 30 if the time from when the paper passes through the transfer unit 40 until the paper arrives at the second sensor 60 downstream of the fuser nip is longer than a predetermined time, and the time the paper passes the second sensor 60 downstream of the fuser nip is within a predetermined range (step S108; YES (see Figure 4)). In other words, the control unit 90 makes a decision to execute a second fuser speed control process (see Figure 5) (step S109 (see Figure 4)). Therefore, the image forming apparatus 100 can determine to execute a second fixing speed control process when a paper feeding failure occurs in the fixing nip, thereby enabling appropriate control of the transport speed (fixing speed) Vf in the fixing unit 30. 【0062】 The above description of the embodiment is merely an example of an image forming apparatus according to the present invention, and is not limited thereto. For example, in the above embodiment, the first sensor 50 is a so-called contact-type sensor composed of an arm portion (actuator) 51 and a laser sensor 52, but a known non-contact type sensor may also be used (the same applies to the second sensor 60). 【0063】 Furthermore, in the above embodiment, a transfer unit 40 is given as a transport means for transporting the paper according to the present invention to the fixing unit 30, but this transfer unit 40 is merely one example. 【0064】 Furthermore, in the above embodiment, the measured elapsed time value is retained in step S105 of the fixing speed control determination process (see Figure 4). However, for example, instead of the elapsed time, the average elapsed time from previous instances of feeding the same type / basis weight paper may be used. 【0065】 Furthermore, in the above embodiment, the measured passage time value is retained in step S107 of the fixing speed control determination process (see Figure 4). However, instead of this passage time, for example, the average passage time when the same type of paper / basis weight was passed through in the past may be used. Alternatively, the passage time calculated based on the peripheral speed calculated from the rotational speed of the motor driving the fixing unit 30 and the average outer diameter of the fixing drive member may be used. 【0066】 Furthermore, in the above embodiment, the method for measuring the transport speed (fixing speed) Vf in the fixing unit 30 is not particularly limited. For example, it may be measured from the time it takes for the paper to pass through the second sensor 60 (paper feeding duration), or it may be a method that uses the paper speed measured non-contact by a laser Doppler velocometer. 【0067】 Although embodiments of the present invention have been described, the scope of the present invention is not limited to the embodiments described above, but includes the scope of the invention as described in the claims and its equivalents. [Explanation of symbols] 【0068】 100 Image forming apparatus 30 Fixing section 40 Transfer section (transport means) 50 First Sensor 60. Second sensor 70 Loop Sensor (Loop Detection Means) 90 Control Unit

Claims

[Claim 1] A fuser unit that fixes the toner image formed on the paper, A sensor is installed downstream of the fixing nip of the fixing unit in the paper transport direction and detects the transported paper, A control unit that controls the fixing unit, Equipped with, The control unit, Based on the detection result obtained by the sensor when the first sheet of paper has passed through the fixing unit, the fixing unit is controlled so that the second sheet of paper, which is transported after the first sheet of paper, is transported at a predetermined transport speed. An image forming apparatus characterized by the following features. [Claim 2] The control unit, The fixing unit is controlled so that the second sheet of paper is transported at the predetermined transport speed for a certain period of time after it enters the fixing nip of the fixing unit. The image forming apparatus according to claim 1. [Claim 3] The system includes a transport means for transporting paper to the fixing unit, The control unit, The fixing unit is controlled so that the second sheet of paper, which is transported after the first sheet of paper, is transported at a predetermined transport speed faster than the transport speed of the transport means for a certain period of time after it enters the fixing nip of the fixing unit. The image forming apparatus according to claim 1. [Claim 4] The system includes means for acquiring paper information, image information, and mode information for each sheet of paper being transported. The control unit, When at least one of the paper information, image information, and mode information is the same for the second paper and the first paper, the fixing unit is controlled. The image forming apparatus according to claim 1. [Claim 5] The system includes a loop detection means positioned between the fixing unit and the transport means for detecting the curvature of the paper passing between the fixing unit and the transport means, The control unit, The fixing unit is controlled during the period from when the fixing nip of the fixing unit enters the fixing nip until the loop detection means detects that the curvature of the paper has been eliminated. The image forming apparatus according to claim 3. [Claim 6] The control unit, The fixing unit is controlled to continue transporting at the set fixing speed until a period of time has elapsed calculated from the detection result by the sensor downstream of the fixing nip and the transport speed of the transport means, which is faster than the transport speed. The image forming apparatus according to claim 3. [Claim 7] The control unit, Based on the time from when the paper passes through the transport means until the paper arrives at the sensor downstream of the fuser nip, and the time the paper passes the sensor downstream of the fuser nip, a decision is made to control the fuser unit. The image forming apparatus according to claim 3. [Claim 8] The control unit, If the time from when the paper passes through the transport means until the paper arrives at the sensor downstream of the fuser nip is longer than a predetermined time, and the time the paper is passing the sensor downstream of the fuser nip is within a predetermined range, the fuser unit is controlled. The image forming apparatus according to claim 7.

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