JP2019159184A - Image forming device - Google Patents

Abstract

To provide an image forming device with which it is possible to detect a shift amount from the threshold of a loop amount detectable by a loop amount detector and control the conveyance speed of a recording medium on the basis of the shift amount, there by suppress the occurrence of an image defect.SOLUTION: An image forming device comprises: an image forming unit equipped with a transfer nip part; a fixing unit equipped with a fixing nip part; a flag contactable with a recording medium in order to detect a loop amount in a conveyance path leading from the transfer nip part to the fixing nip part; a sensor for detecting a loop amount with which output turns on or off in accordance with the posture of the flag; time acquisition means for acquiring a time at which sensor output occurring before the tip of a recording medium enters the fixing nip part after having passed through the transfer nip part at a prescribed conveyance speed returns from one of on and off to the one via the other; and a control unit for controlling the conveyance speed of a recording medium on the basis of the output so that the loop amount when the side of a recording medium downstream of the tip is conveyed is brought closer to a prescribed loop amount.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer, and a facsimile machine that conveys a recording material in a state in which the recording material is bent and a loop is formed.

  In an image forming apparatus employing a conventional electrophotographic method, a toner image formed on a photoreceptor is transferred onto a recording material conveyed from a paper feeding unit at a transfer nip formed by a transfer roller or the like, A toner image is formed on the recording material. Then, the toner image is fixed on the recording material by applying heat and pressure at a fixing nip formed by a fixing roller or the like through a conveyance guide or the like.

  Since the unfixed toner image before fixing is transferred onto the recording material at the transfer nip portion, the unfixed toner image after transfer may be disturbed if the conveyance state of the recording material changes. Specifically, when the trailing edge of the recording material is removed from the paper feeding unit, the force for conveying the recording material is weakened. As a result, the conveying speed is reduced, and the toner image shrunk in the recording material conveying direction is transferred to the recording material. It may be done. Further, with a thick recording material or the like, the impact that has passed through the paper feeding unit is large, and the behavior of the recording material is disturbed in the transfer nip, and band-like image disturbance may occur in the toner image transferred to the recording material.

  Further, between the transfer nip portion and the fixing nip portion, the recording material is conveyed in a state where an unfixed toner image is placed on the recording material. For this reason, when the conveyance of the recording material is disturbed, the printing surface on which the unfixed toner image is placed may come into contact with a member in the image forming apparatus, and the toner image may be disturbed to cause an image defect. Therefore, in a state where the recording material is transferred and conveyed at the transfer nip portion, it is necessary to stably convey the recording material.

  Therefore, as disclosed in Patent Document 1, a recording material is bent in a conveyance path between the transfer nip portion and the fixing nip portion to form a loop, and the loop amount detector is provided between the transfer nip portion and the fixing nip portion. It is known that this is detected and reflected in the conveyance speed of the fixing nip portion. In Patent Document 1, information on the magnitude relationship of the loop amount with respect to a reference value is acquired indirectly by detecting the attitude of a detection flag that is rotatable by contact with a recording material, and based on this information, the fixing nip is acquired. Feedback control (loop amount control) is performed to accelerate or decelerate the conveyance speed of the section. In this way, the recording material has a constant loop amount, and the impact transmitted through the recording material from the fixing nip or the like can be alleviated at all times so that the recording material can be stably conveyed. Has been.

JP 07-234604 A

  However, in the conventional loop amount control, the loop amount that can be detected may vary due to variations in detection accuracy of the loop amount detector itself and variations in the position where the loop amount detector detects the loop amount. Specifically, in addition to the variation in detection accuracy caused by the loop amount detector itself, such as the crossing of detection flag parts rotating in contact with the recording material and the assembly position tolerance of the detection flag, in the image forming apparatus main body There is an influence such as assembly position tolerance of the loop amount detector itself. In particular, the assembly of the loop amount detector is often mounted on the electronic circuit board by soldering, which may cause a large variation in the loop amount that can be detected by the loop amount detector.

  In this way, it is possible to detect with the loop amount detector, but if the loop amount varies, the loop amount is controlled, and if the loop amount is made constant, the loop amount will deviate from the design value, and the above-mentioned Such a change in the conveying force and the influence of a shock are likely to cause an image defect.

  Furthermore, in recent years, image forming apparatuses have been downsized, and the conveyance distance between recording material conveyance members has been shortened. As a result, due to space restrictions, the loop amount detector is attached to a place where the recording material is difficult to loop, such as immediately after the transfer nip or immediately before the fixing nip. In such a configuration, the loop amount actually generated in the entire recording material is larger than the loop amount detected by the loop amount detector. Therefore, even if the detection accuracy (threshold value) of the loop amount detector is slightly varied, the loop amount actually generated in the entire recording material is greatly varied, which may lead to image defects.

  An object of the present invention is to detect an amount of deviation from a threshold of a loop amount that can be detected by a loop amount detector, and control the recording material conveyance speed based on the amount of deviation, thereby suppressing the occurrence of image defects. An object of the present invention is to provide an image forming apparatus capable of performing the above.

  In order to achieve the above object, an image forming apparatus according to the present invention includes an image forming unit including a transfer nip unit that transfers an image onto a recording material, and a fixing unit including a fixing nip unit that fixes the image onto the recording material. A flag that can contact the recording material in order to detect the loop amount of the recording material in the recording material conveyance path from the transfer nip portion to the fixing nip portion, and an output according to the attitude of the flag. A sensor for detecting the loop amount, which is turned on or off, and an output of the sensor which is generated after the leading edge of the recording material passes through the transfer nip portion at a predetermined conveyance speed and enters the fixing nip portion. A time acquisition means for acquiring a time for returning from one of on and off to the other via the other, and a loop when the downstream side of the recording material is conveyed based on the output of the time acquisition means The so as to approach a predetermined loop amount, and having a control unit for controlling the conveying speed of the recording material in one of said transfer nip the fixing nip portion.

  According to the present invention, the amount of deviation from the threshold of the loop amount that can be detected by the loop amount detector is detected, and the occurrence of image defects is suppressed by controlling the recording material conveyance speed based on the amount of deviation. It is possible to provide an image forming apparatus capable of performing the above.

1 is a cross-sectional view of an image forming apparatus according to an embodiment of the present invention. It is a fragmentary sectional view explaining the difference in the posture of the loop amount detection flag depending on the magnitude of the loop amount with respect to the design value (threshold), and a schematic configuration diagram of the optical sensor. It is a fragmentary sectional view of an image forming device for explaining variation correction of a loop amount detection flag. It is the figure which showed the peripheral speed of the pressure roller, and the output of an optical sensor for demonstrating the dispersion | variation correction of a loop amount detection flag. It is the figure which showed the relationship between the distance D of the slit of an optical sensor from the light-shielding surface of a loop amount detection flag, and the return time of a loop amount detection flag. Partial cross-sectional view of an image forming apparatus showing the transport position of a single recording material according to time series The figure which showed the peripheral speed of the pressure roller when the correction control of 1st Embodiment was implemented, and the output of an optical sensor The figure which showed the relationship between the value multiplied by V (Ave) and return time applicable to the correction control of 2nd Embodiment

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

<< First Embodiment >>
(Image forming device)
FIG. 1 is a schematic configuration diagram of an image forming apparatus 100 of the present embodiment, and shows a cross section of the image forming apparatus. As the image forming apparatus 100, an electrophotographic process type laser beam printer compatible with Legal size paper was used. The image forming apparatus 100 can switch the process speed to two stages depending on the type of the recording material S. When the basis weight of the recording material S is 105 g / m ² or less, the cardboard has a basis weight of 210 mm / s or more. For coated paper or the like, image formation is performed at a process speed of 70 mm / s.

  An image forming apparatus 100 shown in FIG. 1 includes a detachable process cartridge P (PY, PM, PC, PK). These four process cartridges PY, PM, PC, PK have the same structure. The difference is that an image is formed by toner colors contained in the process cartridge, that is, yellow (Y), magenta (M), cyan (C), and black (K) toners, respectively. The process cartridges PY, PM, PC, and PK have toner containers 23Y, 23M, 23C, and 23K, respectively.

  The process cartridges PY, PM, PC, and PK shown in FIG. 1 further have photosensitive drums 1Y, 1M, 1C, and 1K that are image carriers. In addition, the process cartridges PY, PM, PC, and PK include charging rollers 2Y, 2M, 2C, and 2K, developing rollers 3Y, 3M, 3C, and 3K, drum cleaning blades 4Y, 4M, 4C, and 4K, and waste toner, respectively. It has containers 24Y, 24M, 24C, 24K.

  Laser units 7Y, 7M, 7C, and 7K are disposed below the process cartridges PY, PM, PC, and PK, and exposure based on image signals is performed on the photosensitive drums 1Y, 1M, 1C, and 1K. The photosensitive drums 1Y, 1M, 1C, and 1K are charged to a predetermined negative polarity potential by applying a predetermined negative polarity voltage to the charging rollers 2Y, 2M, 2C, and 2K, and then the laser units 7Y, 7M. , 7C, and 7K form electrostatic latent images, respectively.

  The electrostatic latent image is reversely developed by applying a predetermined negative voltage to the developing rollers 3Y, 3M, 3C, and 3K, and Y, M, C, and K on the photosensitive drums 1Y, 1M, 1C, and 1K, respectively. A K toner image is formed. Note that the toner used in the present embodiment is negatively charged.

  The intermediate transfer belt unit includes an intermediate transfer belt 8, a driving roller 9, a tension roller 10 as a stretching roller, and a counter roller 28. Further, primary transfer rollers 6Y, 6M, 6C, and 6K as primary transfer units are disposed inside the intermediate transfer belt 8 so as to face the photosensitive drums 1Y, 1M, 1C, and 1K. The transfer voltage is applied by the voltage applying means.

  The photosensitive drums 1Y, 1M, 1C, and 1K on which the toner images are formed rotate in the arrow direction, and the intermediate transfer belt 8 rotates in the arrow Z direction by an intermediate transfer belt driving unit (not shown). Then, primary transfer is performed on the intermediate transfer belt 8 by applying a positive voltage to the primary transfer rollers 6Y, 6M, 6C, and 6K. That is, the toner images on the photosensitive drum 1Y are sequentially transferred onto the intermediate transfer belt 8 in order, and the four color toner images are overlapped. The color misregistration detection sensor 27, which is an optical sensor, can detect a calibration toner pattern formed on the intermediate transfer belt 8. As a result, the color misregistration detection sensor 27 overlaps the four color toner images to form a color toner image. The color misregistration detection sensor 27 is installed in the vicinity of the drive roller 9.

  In FIG. 1, the recording material is conveyed to a secondary transfer nip portion (hereinafter referred to as a transfer nip portion N1 as an image forming portion for forming an image on the recording material) formed by the secondary transfer roller 11 and the counter roller 28. Then, the toner image is secondarily transferred to the recording material. The transfer nip portion N1 can be configured as an upstream side conveyance portion located upstream in the recording material conveyance direction, while a fixing nip portion N2 described later is a downstream conveyance portion located downstream in the recording material conveyance direction. It can be configured as.

  In the present embodiment, the primary transfer rollers 6Y, 6M, 6C, and 6K can be separated from and contacted to the photosensitive drums 1Y, 1M, 1C, and 1K via the intermediate transfer belt 8. Accordingly, the intermediate transfer belt 8 can be changed from a state in which it is in contact with the photosensitive drums 1Y, 1M, 1C, and 1K to a state in which it is separated.

  The feeding / conveying device 12 includes a paper feed roller 14 having an outer diameter of 16 mm that feeds the recording material S from within a paper feed cassette 13 that stores the recording material S, and a pair of 16 mm outer diameter that transports the fed recording material S. Transport rollers 15 (the first pair of transport rollers 15 form the first transport nip portion). Then, the recording material S conveyed from the feeding / conveying device 12 is conveyed to the transfer nip portion N1 by a pair of registration rollers 16 (forming a second conveyance nip portion) having an outer diameter of 16 mm. Here, the conveyance distance from the pair of conveyance rollers 15 to the pair of registration rollers (second pair of conveyance rollers) 16 is 60 mm, and the conveyance distance from the pair of registration rollers 16 to the transfer nip portion N1 is 100 mm. The transport path was designed.

  In order to transfer the toner image from the intermediate transfer belt 8 to the recording material S, a positive voltage is applied to the secondary transfer roller 11. Thereby, the toner image on the intermediate transfer belt 8 can be secondarily transferred to the recording material S being conveyed. In this configuration, the secondary transfer roller 11 is not provided with a drive source, and the recording material S on which the toner image is transferred is driven by the conveying force of the pair of registration rollers 16 and the rotation of the intermediate transfer belt 8. Is conveyed by.

  Thus, the recording material S to which the toner image has been transferred is disposed downstream of the recording material along the post-secondary transfer conveyance guide 29 and the fixing inlet guide 30 as a conveyance guide that guides the conveyance position of the leading edge of the recording material S. It is conveyed to a fixing device (fixing unit) 17 as a side conveying member. The fixing device 17 is provided with a pair of fixing members that form the fixing nip portion N2. In the present embodiment, a pressure roller is used for one of the members, and the pressure roller is connected to a fixing motor. Here, the conveyance path is designed so that the conveyance distance from the transfer nip portion N1 to the fixing nip portion N2 is 50 mm.

  The fixing device 17 includes a fixing nip portion N2 including a fixing film 18 having an outer diameter of 18 mm and a pressure roller 19 having an outer diameter of 22 mm. In the fixing device 17, the pressure roller 19 is arranged so that a driving force is transmitted from a fixing motor M (not shown) via a gear.

  The unfixed toner image on the recording material S is heated and pressed at the fixing nip portion N2 to fix the toner image on the surface of the recording material S. The fixed recording material S is discharged by a pair of paper discharge rollers 20. .

  After the toner image is transferred to the recording material S, the primary transfer residual toner remaining on the surfaces of the photosensitive drums 1Y, 1M, 1C, and 1K is removed by the drum cleaning blades 4Y, 4M, 4C, and 4K, respectively. Recovered to 24Y, 24M, 24C, 24K.

  The secondary transfer residual toner is scraped off by a cleaning blade 21 as a cleaning member after the intermediate transfer belt 8 rotates in the arrow Z direction, and is collected in a waste toner collecting container 22.

  Further, the image forming apparatus 100 includes a control board 25 on which an electric circuit for controlling the image forming apparatus 100 is mounted. A control unit 26 such as a CPU (Central Processing Unit) is mounted on the control board 25. . The control unit 26 drives the intermediate transfer belt 8 related to the conveyance of the recording material S, the feeding / conveying device 12, the pair of registration rollers 16, the driving source (not shown) of the fixing device 17, and the process cartridges PY, PM, PC, PK. It controls the drive source (not shown), etc., and control related to image formation. Further, the control unit 26 collectively controls the operation of the image forming apparatus such as control relating to failure detection.

(Loop amount control)
Here, the loop amount in the present specification refers to the amount of bending of the recording material S. In the recording material S, the leading edge of the recording material S enters the fixing nip portion N2 after the leading edge of the recording material S passes through the transfer nip portion N1 and before the trailing edge of the recording material S passes through the transfer nip portion N1. . In this manner, since the recording material S on which the unfixed toner image is placed is transported between the transfer nip portion N1 and the fixing nip portion N2, unfixed on the recording material S caused by a change in the transport speed of the recording material S. The toner image may be disturbed.

  Therefore, the recording material S is bent in the recording material conveyance path between the transfer nip portion N1 and the fixing nip portion N2 to have a predetermined loop amount. As a result, the length of the recording material S located between the transfer nip portion N1 and the fixing nip portion N2 is made longer than the shortest distance between the transfer nip portion N1 and the fixing nip portion N2. As a result, the recording material S has a fixed loop amount so that the impact transmitted from the fixing nip N2 and the like through the recording material S can be reduced, and the toner image is applied to the recording material S at the transfer nip portion N1. It is possible to suppress the occurrence of image defects that occur when images are transferred.

  A loop for controlling the conveyance speed of the recording material S based on the detection result of the loop amount so that the loop amount of the recording material S in the recording material conveyance path between the transfer nip portion N1 and the fixing nip portion N2 approaches a constant value. By performing the quantity control, the recording material S can be stably conveyed.

(Loop amount detection)
As shown in FIGS. 1 and 2, the loop amount detector 31 detects the loop amount of the recording material S in the recording material conveyance path between the transfer nip portion N1 and the fixing nip portion N2. In the present embodiment, the loop amount detector 31 has a recording material S in a direction orthogonal to the straight path A with respect to the shortest path (straight path A) connecting the upper end of the transfer nip portion N1 and the lower end of the fixing nip portion N2. Detects the distance (loop amount) from the position where passes.

  In the present embodiment, a point that is a position 13 mm upstream from the position of the fixing nip portion N2 in the linear path A in the recording material conveyance direction and a position 5 mm away in the direction orthogonal to the linear path A (loop amount 5 mm) is recorded. A description will be given by taking as an example a configuration in which a state in which the material S passes is a design value. 2C shows a case where the loop amount is the design value (5 mm), FIG. 2A shows a case where the loop amount is larger than the design value, and FIG. 2B shows a case where the loop amount is smaller than the design value. ing.

  As shown in FIG. 2C, the loop amount detector 31 includes a flag 311 that can come into contact with the recording material S, an optical sensor 314, a swing shaft 312, a swing spring 313, and an electrical board 315. The flag 311 is held by the inlet guide 30 so as to be swingable about the swing shaft 312. Further, the flag 311 is urged toward the conveyance path side of the recording material S by the swinging spring 313 so that the flag leading end portion 311 a protrudes from the inlet guide conveyance surface 30 a that guides the recording material S of the inlet guide 30. On the other hand, the pivotable angle is restricted by a rotation stopper (not shown).

  In this way, the flag leading end 311a protruded toward the recording material transport unit by the swing spring 313 is prevented from rotating on the straight path A or at a position slightly beyond the straight path A. In this embodiment, the flag 311 is made of highly slidable PET (polyethylene terephthalate), and the distance from the swing shaft 312 to the end of the flag tip 311a is 18 mm. Further, the light shielding surface 311b that blocks light emitted from the light emitting element has a length of 18 mm from the swing shaft 312.

  The electrical board 315 is fixed to the entrance guide 30 (FIGS. 1 and 2A) with screws (not shown), and the optical sensor 314 is mounted by soldering. The optical sensor 314 uses a so-called transmissive photo interrupter that detects the amount of transmitted light. FIG. 2D is a schematic configuration diagram of the optical sensor 214 along the optical path viewed from the upper side of FIG. As shown in FIG. 2D, a light emitting element 314c (infrared light emitting diode), a light receiving element 314b (phototransistor), and a slit 314a provided near the light receiving element 314b between the slit 314a and the light receiving element 314b are provided. Have.

  The flag 311 includes a flag leading end portion 311a that comes into contact with the recording material S and a light shielding surface 311b that blocks light (infrared light), and can be swung by a swing shaft 312 (FIG. 2A). ) (B) (C)). In the loop amount detector 31, the flag 311 is urged by the swing spring 313, enters an initial position where the light emitted from the light emitting element is not blocked, or enters the slit 314 a of the light sensor 314 and is emitted from the light emitting element. It is detected whether the posture in which the emitted light is blocked.

  More specifically, when the flag 311 is not in contact with the recording material S, the flag 311 is urged by a swinging spring 313 to a positioning portion (not shown) and is positioned at the initial position. At this time, the flag 311 does not enter the slit 314a, and the light emitted from the light emitting element is not blocked by the light shielding surface 311b, and is in a detectable state (on state) by the light receiving element.

  On the other hand, when the loop amount of the recording material S increases, the flag leading end portion 311a is pushed in contact with the recording material S, and the flag 311 is rotated about the swing shaft 312 by the recording material S. Then, when the loop amount of the recording material S becomes a certain amount or more and the flag 311 is rotated by a predetermined angle, the flag 311 enters the slit 314a, and the light emitted from the light emitting element is blocked by the light shielding surface 311b. Detects that it is in the off state.

  In this way, by detecting the difference in the output voltage of the optical sensor 314 (light receiving element), the loop amount is indirectly fixed through whether or not the flag 311 is rotated by a predetermined angle or more in the recording material S. It is detected whether it is the above magnitude | size.

  In the present embodiment, as shown in FIG. 2C, when the loop amount is 5 mm, the flag 311 is rotated by the recording material S around the rocking shaft 312, the light shielding surface 311 b is the slit 314 a, and the light is received. The element 314b (the optical sensor 314) was disposed so as to overlap. In other words, the loop amount detector 31 of the present embodiment is configured so that the recording material loop amount can be detected by using a design value loop amount of 5 mm as a threshold value.

  That is, as shown in FIG. 2A, when the loop amount of the recording material S is larger than 5 mm, the flag tip 311a is pushed into the entrance guide 30 and the light shielding surface 311b blocks the light from the light emitting element 314c. Because there is no, it was detected that it is on. As shown in FIG. 2B, when the loop amount of the recording material S is smaller than 5 mm, the light shielding surface 311b blocks the light of the light emitting element 314a and detects that it is in the off state.

(Loop amount control using a fixing motor)
The magnitude information for the design value of the loop amount detected by the loop amount detector 31 is input to the control unit 26 provided in the image forming apparatus 100. The control unit 26 controls the rotational speed per unit time of the drive source (fixing motor M) according to the magnitude information with respect to the design value of the loop amount, and accelerates or decelerates the peripheral speed of the pressure roller 19 in the fixing device 17. Then, feedback control (loop amount control) for increasing or decreasing the loop amount is performed.

  That is, the pressure roller 19 rotates at the circumferential speed V (L) when the detection result of the loop amount detector 31 is in the off state, and the circumferential speed V (H) when the detection result of the loop amount detector 31 is in the on state. The fixing motor M can be driven at two rotation speeds so as to rotate at the same time.

  On the other hand, at the transfer nip portion N1, the rotation speed V (N) (reference conveyance speed) corresponding to the process speed of the image forming apparatus does not change depending on whether the detection result of the loop amount detector 31 is in the off state or the on state. The recording material S is configured to be conveyed.

Specifically, the control unit 26 determines the peripheral speed V (L) at the transfer nip portion N1 even if the peripheral speed V (L) has tolerances such as outer diameter tolerance of the fixing film 18 and the pressure roller 19, thermal expansion, and wear due to durability. The setting is always slower than (N), and control is performed so as to satisfy the relationship of Expression (1).
V (L) = V (N) × 0.97 (1)
Further, even if the peripheral speed V (H) has tolerances such as the outer diameter tolerance of the fixing film 18 and the pressure roller 19, thermal expansion, and wear due to durability, the control unit 26 also determines the peripheral speed V (N) at the transfer nip portion N1. It is set to be always faster than the above, and control is performed so as to satisfy the relationship of Expression (2).

V (H) = V (N) × 1.03 Expression (2)
That is, by using the relationship of the expression (1), when it is detected that the loop amount of the recording material S is smaller than 5 mm, the recording material S is conveyed at the fixing nip portion N2 at a slower speed than the transfer nip portion N1, and the loop Control to increase the amount. Further, by using the relationship of the expression (2), when it is detected that the loop amount of the recording material S is larger than 5 mm, the recording material S is conveyed at a faster speed than the transfer nip portion N1 in the fixing nip portion N2, and the loop Control to reduce the amount.

  While the loop amount control is being executed, the loop amount with respect to the design value is detected at a high speed, and the fixing motor speed is accelerated and decelerated at a high speed to control the loop amount to approach the set value of 5 mm. The configuration. In the present embodiment, a CPU having a system clock frequency of 20 MHz is used as the control unit 26, and the detection result of the loop amount detector 31 is taken every 10 ms in consideration of the fluctuation of the flag 311 and the followability of the motor speed. Based on the above, acceleration / deceleration control is performed.

  By performing the loop amount control as described above, the loop shape between the transfer nip portion N1 and the fixing nip portion N2 is designed even if thermal expansion of the pressure roller 19 or manufacturing variation of the outer diameter occurs. It is possible to control so that it approaches.

(Optical sensor output until the leading edge of the recording material reaches the fixing nip via the transfer nip)
With reference to FIG. 3, the relationship between the attitude of the flag 31 and the output of the optical sensor 314 until the leading edge of the recording material S passes through the transfer nip portion N1, reaches the fixing nip portion N2, and enters is described in time series. To do. FIG. 3A shows a state in which the leading end of the recording material S is positioned upstream of the flag 31 in the transport direction after passing through the transfer nip portion N1. FIG. 3B shows a state in which the leading edge of the recording material S moves while being guided along the inlet guide conveyance surface 30a and then comes into contact with the flag 311.

  In FIG. 3C, after the leading edge of the recording material S moves while rotating the flag 311 while being guided along the inlet guide conveying surface 30a, the flag leading edge 311a of the flag 311 protrudes from the inlet guide conveying surface 30a. The state when it is set to the state not to be shown is shown. FIG. 3D shows a state where the loop amount of the recording material S reaches a predetermined value (design value) after the leading end of the recording material S reaches and enters the fixing nip portion N2. That is, FIG. 3D shows a state in which the light shielding surface 311b overlaps the position of the light emitting element 314a, that is, the light receiving element (photosensor 314). FIG. 4 shows the output of the optical sensor 314 during the transition from the state of FIG. 3 (A) to the state of (D).

  The leading edge of the recording material S that has passed through the transfer nip portion N1 is transported toward the fixing nip portion N2 at the process speed in the image forming operation along the transport guide 29 and the fixing inlet guide 30 after the secondary transfer. In the state of FIG. 3A, the leading edge of the recording material S is transported along the entrance guide transport surface 30a of the fixing entrance guide 30. As shown in the period (A) of FIG. The light shielding state (off state) is shown.

  When the recording material S is conveyed, the leading edge of the recording material S comes into contact with the flag 311, the flag 311 is rotated, and the flag leading edge 311 a is guided by the leading edge of the recording material S as shown in FIG. The conveying surface 30a is pushed to a position where it does not protrude. In the process in which the flag leading end 311a is pushed to a position where the recording material S does not protrude from the entrance guide transport surface 30a, the detection result of the optical sensor 314 changes from the off state to the on state (FIG. 3B).

  Thereafter, the conveyance of the recording material S further proceeds, and when the leading end of the recording material S reaches the fixing nip portion N2 and enters (FIG. 3D), the leading end portion of the flag is urged by the urging force of the swinging spring 313. 311a returns to the state where it protrudes from the entrance guide transport surface 30a. The detection result of the optical sensor 314 in the process in which the flag tip portion 311a protrudes from the entrance guide conveying surface 30a by the swinging spring 313 until the tip of the recording material S reaches the fixing nip portion N2 and enters is shown in FIG. As shown by a point D in FIG.

  Thus, since the loop is not formed during the period from the start of conveyance of the recording material S to point D in FIG. 4, the peripheral speed of the pressure roller 19 is constant at V (L). (V (L) section). On the other hand, since the recording material S forms a loop in the recording material conveyance path after the point D in FIG. 4, the peripheral speed of the pressure roller 19 accelerates and decelerates based on the detection result of the optical sensor 314, and the loop amount. Control is performed (loop control section shown in FIG. 4).

(Determination on detection deviation of loop amount detector 31)
Hereinafter, determination regarding the detection deviation of the loop amount detector 31, which is a feature of the present embodiment, will be described.

  As described above, the image forming apparatus 100 increases or decreases the rotational speed per unit time of the fixing motor M based on the loop amount detection result of the loop amount detector 31 and increases the peripheral speed (rotational speed) of the pressure roller 19. Deceleration control is performed to change the conveyance speed of the recording material S at the fixing nip N2. As described above, the transfer speed of the recording material S in the fixing nip portion N2 is changed in the state where the transfer speed of the recording material S is constant (reference transport speed) in the transfer nip portion N1, and the loop amount of the design value is controlled. It is configured.

  That is, by controlling the loop amount, the flag 311 is converged so as to be in a posture (position) between a position where the light of the light emitting element can be detected and a position where the light is blocked, so that the flag 311 contacts the flag tip 311a. The loop amount of the recording material S to be set is set to a design value of 5 mm. By bringing the loop amount close to the design value in this way, the loop shape between the transfer nip portion N1 and the fixing nip portion N2 is set to a predetermined shape so that the recording material S is stably conveyed, and image defects occur. Is suppressed.

  However, the detection position of the loop amount detector 31 is affected by the assembly position tolerance of the optical sensor 314, the component tolerance of the flag 311 and the assembly position tolerance of the fixing nip portion N2 and the transfer nip portion N1. In particular, when the optical sensor 314 is mounted on the electrical board 315 by soldering, the attachment position tends to vary greatly, and the detection position of the loop amount detector 31 may vary. As described above, when the detection position of the loop amount detector 31 is shifted, the loop amount during the loop amount control is controlled so as to converge from the design value of 5 mm to a larger or smaller value. There was a risk of image defects.

  In this regard, the present inventor has detected the loop amount based on the positional relationship between the flag 311 and the optical sensor 314 and the output waveform of the optical sensor 314 in the process in which the leading edge of the recording material is conveyed from the transfer nip portion N1 to the fixing nip portion N2. It has been found that the amount of deviation of the detection position of the detector 31 from the design value can be determined. Furthermore, by changing the control of the peripheral speed of the pressure roller 19 according to the detected shift amount of the detection position, the loop amount of the recording material S converged by the loop amount control is made closer to the design value, and the image defect It has been found that the occurrence of can be suppressed. A specific implementation configuration will be described below.

  In the present embodiment, the recording material S is transported at a predetermined transport speed, and the point (point (D)) from the on state to the on state (point (D)) that transitions from the off state to the on state in FIG. The deviation amount with respect to the design value of the detection position of the loop amount detector 31 is determined using the time until (return time) until. Although the maximum distance D between the light shielding surface 311b and the light emitting element 314a (that is, the light receiving element) in the ON state illustrated in FIG. 3C is designed to be 1.5 mm, the above-described light There may be a variation of about 0.9 to 2.1 mm due to mounting variation of the sensor 314.

  For example, when the distance D becomes long, the optical sensor 314 is shifted to the conveyance path side with respect to the light shielding surface 311b of the flag 311. In this state, the return time when returning to the position where the light shielding surface 311b and the light emitting element 314a overlap after the flag tip 311a has been pushed to a position where it does not protrude from the transport surface 30a becomes longer. In addition, since the position (detection position) where the light blocking surface 311b of the optical sensor 314 and the light emitting element 314a overlap is shifted to the conveyance path surface side from the design value, the loop amount during the loop amount control is smaller than the designed loop amount. .

  On the contrary, when the distance D becomes short, the position of the light emitting element 314a is shifted so as to move away from the conveyance path. In this state, the return time when returning to the position where the light shielding surface 311b and the light emitting element 314a overlap after the flag tip portion 311a is pushed to the position where it does not protrude from the transport surface 30a is shortened. In addition, the position (detection position) where the light shielding surface 311b of the optical sensor 314 and the light emitting element 314a overlap is shifted away from the conveyance path surface from the design value, so the loop amount at the time of loop amount control is less than the design loop amount. growing.

  In this way, the return time of the flag 311 is measured. That is, the return of the flag 311 of the loop amount detector 31 actually assembled to the image forming apparatus 100 is compared with the return time of the flag 311 when the recording material S is transported at a predetermined transport speed and is positioned at the normal detection position. Measure the time difference. Thereby, the deviation | shift amount with respect to the design value of the detection position of the loop amount detector 31 is distinguishable.

  Therefore, in measuring the time, the samples having different distances D from the light shielding surface 311b to the light emitting element 314a when the flag tip 311a is pushed to a position where it does not protrude from the entrance guide transport surface 30a (0.9 mm, 1.5 mm and 2.1 mm) were prepared. And the output from the optical sensor 314 was measured for every sample using the memory high coder 8860 (made by Hioki Electric Co., Ltd.), and time was measured. FIG. 5 shows the results of measuring the relationship between the distance D from the light shielding surface 311b to the light emitting element 314a and the return time obtained by measurement.

In this verification, Laser Glossy Brochure Paper 200 (manufactured by hp), which is a gloss paper with a letter size and a basis weight of 200 g / m ³ , is processed at a process speed of 70 mm / s and a black halftone (50%) image is obtained. The sheet was printed and measured, and the return time was measured. The conveyance control of the recording material S is a control to be switched according to the conveyance position of the recording material S as shown in FIG.

  As a result, as shown in FIG. 5, a linear correlation (proportional relationship) was observed between the distance D and the return time, and it was confirmed that there was a time difference of about 100 ms at the upper and lower limits. It has been confirmed that there is a sufficient time difference compared to 10 ms which is the control period of the loop amount control, and it is possible to determine the deviation amount with respect to the design value of the detection position of the loop amount detector 31 from the return time. More specifically, as shown in FIG. 5, the return time was about 170 ms when the distance D was 1.5 mm. When the distance D was 2.1 mm larger than 1.5 mm, it was about 220 ms, and when the distance D was 0.9 mm smaller than 1.5 mm, it was about 110 ms.

  That is, it was found that the return time and the distance D have a positive correlation, and are 160 ms to 180 ms when the design value of the maximum distance D is about 1.5 mm. Therefore, when the return time is in the range of 160 ms to 180 ms, it can be specified that the maximum distance D is about a design value of 1.5 mm. Further, when the return time is larger than the range of 160 ms to 180 ms, the maximum distance D is larger than the design value 1.5 mm, and when the return time is smaller than the range of 160 ms to 180 ms, the maximum distance D is smaller than the design value 1.5 mm. Ui can be identified.

  In this time measurement, no image defect occurred when the distance D was the designed value of 1.5 mm. However, when the distance D is 0.9 mm and 2.1 mm, there are 10 band-like image defects at a position 100 mm from the rear end of the recording material, that is, at the timing when the rear end of the recording material is removed from the pair of registration rollers 16. Six out of ten, one out of ten.

  When the distance D = 0.9 mm, that is, when the loop amount is larger than the design value, the trailing edge of the recording material passes through the pair of registration rollers 16, and the conveying force on the trailing edge side of the recording material is lost. As a result, a force toward the upstream in the transport direction that eliminates the bending of the recording material S due to an excessive loop is generated in the transfer nip portion N1. For this reason, it is considered that the recording material slipped at the transfer nip portion N1 and led to a band-like image defect.

  Further, when the distance D = 2.1 mm, that is, when the loop amount is smaller than the design value, the distance between the recording material S and the intermediate transfer belt 8 is too short. For this reason, as a result of the vibration of the recording material due to the shock when the trailing edge of the recording material passes through the pair of registration rollers 16, the toner images before and after the secondary transfer are disturbed in the vicinity of the transfer nip portion N1, resulting in an image defect. It is thought that it was connected.

  Based on these results, in the present embodiment, the peripheral speed control of the pressure roller 19 is changed according to the measured return time, and the loop amount before the recording material trailing edge passes through the pair of registration rollers 16 is corrected. It was decided to control. Regarding the details of the control in the present embodiment, a partial cross-sectional view (FIG. 6) of the image forming apparatus showing the transport position of a single recording material S in time series and a period according to the transport position of the recording material S are sequentially provided. This will be described with reference to the relationship diagram of the speed control of the fixing motor M (FIG. 7).

  The peripheral speed of the pressure roller 19 is not changed in the gear train between the fixing motor M and the pressure roller 19, and is proportional to the rotational speed of the fixing motor M and the rotational speed per unit time. It is supposed to change. That is, when the rotation speed of the fixing motor M / the number of rotations per unit time increases, the peripheral speed of the pressure roller 19 increases and the rotation speed of the fixing motor M / the number of rotations per unit time decreases. In this case, the peripheral speed of the pressure roller 19 is configured to decrease. For this reason, as a specific means for controlling the peripheral speed of the pressure roller 19, a configuration in which the speed of the fixing motor M and the number of rotations per unit time are controlled will be described as an example.

  6A, that is, until the leading edge of the recording material is fed by the paper feed roller 14, until the state shown in FIG. 6B, that is, the position where 9 mm is conveyed downstream from the fixing nip portion N2. FIG. 7 shows (i) a fixed speed section. This (i) fixed speed section is an initial stage section as the transport timing. In the first section (period), the rotation speed of the fixing motor M is controlled so that the recording material S is transported at a predetermined transport speed of a constant speed. In this embodiment, the rotation speed of the fixing motor M is controlled so that the peripheral speed of the pressure roller 19 rotates at a constant speed V (L). Then, in the first section, the return time is measured as described above, and the return time is stored in the storage unit of the control unit 26.

  Subsequently, the state shown in FIG. 6B to the state shown in FIG. 6C, that is, the position where the leading end of the recording material S is conveyed 69 mm downstream from the fixing nip portion N2 (the recording material not shown in FIG. 6C). Until the rear end reaches the position of the pair of conveying rollers 15), (ii) a loop amount control section is shown as a second section (period) in FIG. 7.

  In the second section, the loop amount control is performed, and the fixing motor M is rotated at a rotational speed (speed) per unit time based on the detection result of the loop amount detector 31 in order to accelerate and decelerate the peripheral speed of the pressure roller 19. ). In the second section, the peripheral speed V (L) in the first section and the peripheral speed V (H) faster than the reference transport speed V (N) are appropriately switched and the speed is controlled. In FIG. 7, loop amount control centering on the reference transport speed is performed (loop amount control that maintains the reference transport speed).

  Subsequently, from the state of FIG. 6C to the state of FIG. 6D, that is, until the rear end of the recording material S reaches the position conveyed 5 mm upstream of the pair of registration rollers 16 (not shown). 7, the third section (period) is shown as (iii) correction control section. The control in the third section is a characteristic configuration of the present embodiment, and the fixing motor M is controlled to control the peripheral speed of the pressure roller 19 according to the return time read in the first section. The number of rotations per unit time is switched (select from a plurality of conveyance speeds). Accordingly, the recording material S is configured to correct the loop amount before passing through the pair of registration rollers 16.

  That is, when the return time is in the reference range of 160 to 180 ms, as shown in FIG. 5, it is determined that the distance D is about 1.5 mm and the detection position is as designed. Then, as indicated by a long broken line in the (C) correction control section of the fixing motor speed in FIG. 7, the fixing motor M is changed and the pressure roller 19 is driven so that the peripheral speed becomes V (N).

  When the return time is longer than 180 ms, the detection position is determined such that the distance D is longer than about 1.5 mm and the loop amount is smaller than the design value. Then, the rotational speed per unit time of the fixing motor in FIG. 7 is driven so that the peripheral speed of the pressure roller 19 becomes V (L) as indicated by the solid line in the (C) correction control section, and the loop amount is set. Increase the detection position so that it is closer to the design value.

  When the return time is shorter than 160 ms, it is determined that the detection position is such that the distance D is shorter than about 1.5 mm and the loop amount is larger than the design value. Then, as shown by a short broken line in the (C) correction control section of the fixing motor speed in FIG. 7, the circumferential speed of the pressure roller 19 is driven to V (H), and the loop amount is reduced from the detection position. To bring it closer to the design value.

  In this way, by correcting the loop amount before reaching the state in which the pair of registration rollers 16 is removed (FIG. 6D), the loop amount at the timing when the trailing edge of the recording material is removed from the pair of registration rollers 16 is obtained. Close to the design value to suppress image defects due to shock. Note that the transmission and light shielding states of the output of the optical sensor 314 in this section are not shown because they do not show a certain tendency due to the influence of the temperature of the fixing device 17 and the outer diameter of the pressure roller 19.

  Subsequently, from the state shown in FIG. 6D to the state shown in FIG. 6E, that is, until the rear end of the recording material reaches the position conveyed 5 mm downstream from the transfer nip portion N1, the fourth state is shown in FIG. As a section (period), (D) loop amount control section is shown. In this fourth section, loop amount control is performed, and the fixing motor M increases or decreases the motor speed based on the detection result of the loop amount detector 31.

  Subsequently, the state shown in FIG. 6E, that is, after the recording material trailing edge has reached the position conveyed 5 mm downstream from the transfer nip portion N1, is shown as a fifth section (period) in FIG. It is shown as a fixed speed section. In the fifth section, the number of rotations per unit time of the fixing motor M is constant so that the pressure roller 19 rotates at the peripheral speed V (L).

(Effect of this embodiment)
The quantitative effect of this embodiment will be described below. As a comparative example, when the loop amount control shown in FIG. 4 that does not include the (C) correction control section of FIG. 7 is performed, and the (C) correction control section of FIG. 7 according to the present embodiment (Example 1). Table 1 shows the result of confirming the number of image defects in the implementation.

  As shown in Table 1, in the present embodiment (Example 1), the number of image defects generated can be reduced by applying the correction control section (C) in FIG. 7 as compared with the comparative example. Further, the image defect occurring at D = 0.9 mm is at a level that is difficult to visually recognize, and at a level that is not problematic in practice. Note that the average rotation speed of the fixing motor M according to the peripheral speed of the pressure roller 19 in the loop amount control section of FIG. 7B at the time of the above confirmation was 608 rpm.

  When D = 0.9 mm, the number of rotations per unit time of the fixing motor M corresponding to the peripheral speed V (H) of the pressure roller 19 is 600 × 1.D in the correction control section of FIG. 03 = 618 rpm. Therefore, the recording material S is conveyed at a speed faster than the loop amount control section in FIG. For this reason, it is considered that at the timing when the trailing edge of the recording material is removed from the pair of registration rollers 16, the loop amount can be made smaller than the detection position as intended.

  When D = 2.1 mm, the rotation speed per unit time of the fixing motor M corresponding to the peripheral speed V (L) of the pressure roller 19 is 600 × in the correction control section of FIG. 0.97 = 582 rpm. Therefore, the recording material S is transported at a speed slower than the loop amount control section in FIG. For this reason, it is considered that at the timing when the trailing edge of the recording material passes through the pair of registration rollers 16, the loop amount larger than the detection position can be achieved as intended. As a result, it is considered that the loop amount between the transfer and fixing before passing through the pair of registration rollers can be corrected, and the occurrence of the image defect due to the drop shock can be reduced.

  In the present embodiment, a configuration is used in which a loop amount before passing through the pair of registration rollers is corrected for an image defect at the timing when the trailing edge of the recording material passes through the pair of registration rollers. In this case, the configuration of the present embodiment can be applied. Specifically, when an image failure occurs due to a shock that the recording material trailing edge comes off the paper feeding roller 14 and the pair of conveying rollers 15, the recording material trailing edge falls out of the paper feeding roller 14 and the pair of conveying rollers 15. It may be configured to correct the loop amount before.

  Similarly, when an image defect occurs due to a shock that the recording material front end enters (enters) the pair of paper discharge rollers 20, the loop amount is corrected before the recording material front end enters the pair of paper discharge rollers 20. Such a configuration may be adopted. That is, the effect of this configuration is not limited by the type and location of the conveying member.

Further, in the present embodiment, the effect has been described using a configuration for forming and conveying an image on a gloss paper having a basis weight of 200 g / m ³ , but the effect of the present configuration is not limited by the type of paper. . On the other hand, when images can be formed on both sides of paper with a small basis weight, the measurement of the return time may cause an error due to the curl of the recording material. It is more desirable to adopt a configuration in which the result of the measured return time is applied.

  Further, in the present embodiment, the effect has been described using a configuration in which gloss paper is formed and conveyed at a process speed of 70 mm / s. However, the effect of this configuration is not limited by the process speed. On the other hand, when the process speed is high, the absolute value of the return time becomes small, so that the influence of the measurement error on the control period is likely to occur, and the measurement accuracy of the detection position may deteriorate. In such a case, the return time measured at a slow process speed and the shift amount of the detected position are stored on the control board, and the stored values are used at the time of image formation at a fast process speed thereafter. Also good.

  In the present embodiment, the detection is always performed on all pages through which the return time passes. However, the detected return time and the detected position deviation amount are stored on the control board, and the subsequent images are stored. A configuration may be adopted in which stored values are used during formation.

  In the present embodiment, the return time is detected using the time for the flag to return from the transmission state to the light-shielding state with respect to the optical sensor. It can be configured to detect in the process. That is, the effect of this configuration is not limited by the flag shape or the detection means.

  Further, in the present embodiment, the speed of the pressure roller 19 in the correction control section of FIG. 7C, that is, the number of rotations per unit time of the fixing motor M is set to three stages according to the detection time of the return time. did. However, the same loop amount control as that in the loop amount control section in FIG. 7B may be performed according to the return time. Specifically, when the return time is longer than 180 ms, that is, on the side where the loop amount is small, the image defect occurrence rate is lower than when the loop amount is large. It is good also as a structure which continues.

  Further, in the present embodiment, a configuration is used in which the loop amount is corrected by changing the number of rotations of the fixing motor M per unit time according to the return time and increasing or decreasing the speed of the pressure roller 19. It is not limited to this configuration. That is, the present invention can be applied even when the secondary transfer roller has a separate drive source (motor).

  In such a configuration, when the return time is short, the motor may be controlled so as to reduce the rotation speed of the secondary transfer roller so that the loop amount becomes small. Conversely, when the return time is long, the motor may be controlled so as to increase the rotational speed of the secondary transfer roller so that the loop amount becomes large.

  In short, based on the return time, either the transfer nip portion or the fixing nip portion is set so that the loop amount when the downstream side from the leading edge of the recording material is conveyed approaches the predetermined loop amount (designed loop amount). Any recording material may be used as long as the conveyance speed of the recording material is controlled by the rotational speed of the motor.

<< Second Embodiment >>
7C, in this embodiment, in this embodiment, the average peripheral speed of the pressure roller 19 corresponding to the average conveyance speed of the recording material during the loop amount control section in FIG. The average rotation speed of M is acquired. Then, in the (C) correction control section of FIG. 7, the peripheral speed of the pressure roller 19, that is, the fixing motor M of the fixing motor M, in accordance with the acquired average peripheral speed of the pressure roller 19, that is, the average rotation speed of the fixing motor M. A configuration for correcting the rotational speed was used.

  In this embodiment, only the operation of (B) loop amount control section and (C) correction control section of FIG. 7 are different from those of the first embodiment. In the present embodiment, in the loop amount control section of FIG. 7B, the control board 25 continues to store the average peripheral speed of the pressure roller 19, that is, the average rotation speed (rpm) of the fixing motor M every 10 ms. Then, at the timing immediately before the shift to the correction control section in FIG. 7C, the average peripheral speed V (Ave) of the loop amount control section in FIG. (calculate.

  Subsequently, in (C) the correction control section of FIG. 7, the return time read in the fixed speed section of FIG. 7 (A) and the average peripheral speed (average rotation speed) calculated in the loop amount control section of FIG. 7 (B). Based on this, the peripheral speed of the pressure roller 19 (the number of rotations of the fixing motor M per unit time) is switched. Thus, the loop amount is corrected before passing through the pair of registration rollers 16.

  That is, when the return time read in the fixed speed section (A) in FIG. 7 is 160 to 180 ms which is the reference range, the following control is performed. That is, the distance D described above is about 1.5 mm, and it is determined that the detection position is as designed, and the fixing motor M per unit time is set so that the peripheral speed of the pressure roller 19 becomes V (Ave). Control the number of revolutions.

  When the return time is longer than 180 ms, it is determined that the detection position is such that the distance D is longer than about 1.5 mm and the loop amount is smaller than the design value, and the circumferential speed of the pressure roller 19 is V ( (Ave) × 0.97 The number of rotations per unit time of the fixing motor M is controlled to be 0.97. Thus, the loop amount is increased from the detection position so as to approach the design value.

  When the return time is shorter than 160 ms, control is performed as follows. That is, it is determined that the detection position is such that the distance D is shorter than about 1.5 mm and the loop amount is larger than the design value, and the peripheral speed of the pressure roller 19 is V (Ave) × 1.03. Thus, the rotational speed per unit time of the fixing motor M is controlled and driven. As a result, the loop amount is reduced from the detection position so as to approach the design value.

  In this embodiment, by correcting the loop amount before passing through the pair of registration rollers 16 as described above, image defects due to shock at the timing when the trailing edge of the recording material has passed through the pair of registration rollers 16 are suppressed. Can do.

  The quantitative effect of this embodiment will be described below. For comparison, as a comparative example, the case (C) of FIG. 7 according to the present embodiment in which the loop amount control shown in FIG. 4 without the correction control section is performed, and the present embodiment (Example 2) are performed. Table 2 shows the result of confirming the number of image defects when the correction control section in FIG.

  As shown in Table 2, by applying the correction control of the present embodiment, it was possible to reduce the number of image defects generated compared to the comparative example. Further, no image defect occurred even at D = 2.1 mm, and the occurrence of the image defect could be suppressed as compared with the configuration of the first embodiment. Note that the average peripheral speed of the pressure roller 19 in the loop amount control section of FIG. 7B at the time of the above confirmation, that is, the average rotation speed of the fixing motor M was 608 rpm, the same as in the first embodiment.

  In the present embodiment, since the average speed of the pressure roller 19, that is, the average rotation speed of the fixing motor M is calculated, the target loop amount can be obtained without being affected by a change in the temperature state of the fixing device 17. The correction was carried out. As a result, it is considered that the occurrence of image defects can be suppressed more than in the first embodiment.

  Specifically, in the (C) correction control section of FIG. 7 when D = 0.9 mm, the number of rotations of the fixing motor M per unit time is set to 608 × 1.03 = 626 rpm. The recording material S can be conveyed at a speed faster than the 618 rpm of the configuration. For this reason, it is considered that at the timing when the trailing edge of the recording material passes through the pair of registration rollers 16, it can be conveyed with a loop amount closer to the design value than in the first embodiment.

  Further, when D = 2.1 mm, the rotation speed per unit time of the fixing motor M is set to 608 × 0.97 = 590 rpm in the correction control section of FIG. 7C, so that FIG. The recording material S can be conveyed at a speed slower than the loop amount control section. For this reason, it is considered that at the timing when the trailing edge of the recording material passes through the pair of registration rollers 16, the loop amount larger than the detection position can be achieved as intended.

  As described above, according to the configuration of the present embodiment, the loop amount between the transfer and fixing before passing through the pair of registration rollers can be corrected, and the occurrence of image defects due to the drop shock can be reduced. In the present embodiment, the configuration in which the loop amount before passing through the pair of registration rollers is corrected for the image defect at the timing when the trailing edge of the recording material passes through the pair of registration rollers is used. In this case, the configuration of the present embodiment can be applied.

  Specifically, when an image failure occurs due to a shock that the recording material trailing edge comes off the paper feeding roller 14 and the pair of conveying rollers 15, the recording material trailing edge falls out of the paper feeding roller 14 and the pair of conveying rollers 15. It may be configured to correct the loop amount before.

  Similarly, when an image defect occurs due to a shock that the recording material front end enters the pair of paper discharge rollers 20, the loop amount is corrected before the recording material front end enters the pair of paper discharge rollers 20. What should I do? That is, the effect of this configuration is not limited by the type and location of the conveying member.

  In the present embodiment, as in the first embodiment, the loop amount is increased or decreased by increasing or decreasing the peripheral speed of the pressure roller 19, that is, the number of rotations per unit time of the fixing motor M, according to the return time. The configuration to be corrected was used. However, the present invention can be applied even when the secondary transfer roller has a separate drive source (motor). In short, based on the return time, either the transfer nip portion or the fixing nip portion is set so that the loop amount when the downstream side from the leading edge of the recording material is conveyed approaches the predetermined loop amount (designed loop amount). Any recording material may be used as long as the conveyance speed of the recording material is controlled by the rotational speed of the motor.

  In the present embodiment, the pressure roller 19 average peripheral speed V (Ave) in the loop amount control section of FIG. 7B, that is, the average rotation speed (rpm) of the fixing motor M in accordance with the detection time of the return time. ), And the value to be multiplied by 3 steps. However, a configuration in which a value that varies linearly according to the return time may be used. Specifically, as shown in FIG. 8, by linearly changing the value to be multiplied according to the return time, it is possible to perform the correction in accordance with the detection position shift steplessly, and further increase the correction effect. Can do.

<< Third Embodiment >>
With regard to (C) the correction control section in FIG. 7, in the present embodiment, a configuration is used in which loop amount control is performed by modifying the loop amount control section in (B) in FIG. 7. In the present configuration, only the correction control section in FIG. 7C is different in operation from the first embodiment. In the present embodiment, the loop amount control is performed in the correction control section of FIG. 7 in the same manner as the loop amount control section of FIGS. 7B and 7D, but in FIG. 7A the fixed speed section. At least one of the minimum value V (L) and the maximum value V (H) is changed according to the return time read in. Thus, the loop amount is corrected before passing through the pair of registration rollers 16.

  In general, in the loop amount control, the loop amount of the recording material has a certain amplitude because the difference in speed between V (L) and V (H) is used to approach the loop amount to a predetermined amount. Are controlled. Specifically, when the motor speed during loop amount control is V (L), the loop amount is slightly larger than the predetermined amount, and when V (H), the loop amount is slightly smaller than the predetermined amount. Therefore, if the loop amount control can be performed while the speed difference between V (L) and V (H) is small, the amplitude of the loop amount becomes small.

  Here, when the loop amount control is performed with both the V (L) and V (H) speeds set to be slow, the loop amount control tends to be performed with a certain amount of amplitude on the side where the loop amount is slightly larger. . Further, when the loop amount control is performed with both the V (L) and V (H) speeds set to be high, the loop amount control tends to be performed with a certain amplitude on the side where the loop amount is slightly smaller. . In this configuration, the loop amount is corrected by using this tendency in the correction control section of FIG.

  When the return time read in (A) fixed speed section in FIG. 7 is the reference range of 160 to 180 ms, it is determined that the above-mentioned distance D is about 1.5 mm and the detection position is as designed. Then, loop amount control is performed with V (L) and V (H) set in the equations (1) and (2).

If the return time is longer than 180 ms, it is determined that the detection position is such that the distance D is longer than about 1.5 mm and the loop amount is smaller than the design value, and V (H) is set to (B) in FIG. ) From V (H) in the loop amount control section, the relationship is changed so as to satisfy the relationship of Expression (3). On the other hand, V (L) is not changed from V (L) in the loop amount control section of FIG. 7B as shown in Expression (4). Thus, in this embodiment, in the (C) correction control section of FIG.
Loop amount control is performed by modifying the loop amount control section in FIG. 7B (at least one of V (H) and V (L) in the loop amount control section in FIG. 7B is changed).

In the present embodiment, the speed setting of V (H) in the (C) correction control section in FIG. 7 is slowed down so that the control is performed with a small loop amount as much as possible even under the loop amount control. Speed setting.
V (H) = V (N) × 1.01 (3)
V (L) = V (N) × 0.97 (4)
When the return time is shorter than 160 ms, it is determined that the detection position is such that the distance D is shorter than about 1.5 mm and the loop amount is larger than the design value, and the setting of V (L) is set according to Equation (5). Change to be related. Further, as shown in Expression (6), V (H) performs loop amount control without changing the speed. Aiming to control with a small loop amount as much as possible even under the loop amount control by increasing the speed setting of V (L) with respect to the speed setting of equations (1) and (2). The speed was set.
V (L) = V (N) × 0.99 (5)
V (H) = V (N) × 1.03 (6)
In this way, by correcting the loop amount before passing through the pair of registration rollers 16, it is possible to suppress image defects due to shock at the timing when the trailing edge of the recording material passes through the pair of registration rollers 16. The quantitative effect of this embodiment will be described below. As a comparative example, (C) according to the present embodiment (C) when the loop amount control shown in FIG. 4 without the correction control section is performed, and (C) of FIG. 7 according to the present embodiment (Example 3). Table 3 shows the result of confirming the number of image defects when the correction control section is executed.

  As shown in Table 3, by applying the correction control of the present embodiment, it was possible to reduce the number of image defects generated compared to the comparative example. Further, as in the first embodiment, image defects occurring at D = 0.9 mm are at a level that is difficult to visually recognize, and at a level that is not problematic in practice. Note that the average rotation speed of the fixing motor M in the loop amount control section in FIG. 7B at the time of the above confirmation was 608 rpm as in the first and second embodiments.

  Further, when D = 0.9 mm, the average rotation speed of the fixing motor M in the correction control section of FIG. 7C is 614 rpm, and the recording material S is faster than the loop amount control section of FIG. Can be transported. Thus, it is considered that the loop amount smaller than the detection position could be achieved as intended at the timing when the trailing edge of the recording material passes through the pair of registration rollers 16.

  In addition, when D = 2.1 mm, the average rotation speed of the fixing motor M in the correction control section in FIG. 7C is 595 rpm, and the recording material S is slower than the loop amount control section in FIG. Can be transported. Thus, it is considered that the loop amount larger than the detection position could be achieved as intended at the timing when the trailing edge of the recording material passes through the pair of registration rollers 16.

  As described above, according to the configuration of the present embodiment, the loop amount between the transfer and fixing before passing through the pair of registration rollers can be corrected, and the occurrence of image defects due to the drop shock can be reduced.

  In the present embodiment, the configuration in which the loop amount before passing through the pair of registration rollers is corrected for the image defect at the timing when the trailing edge of the recording material passes through the pair of registration rollers is used. In this case, the configuration of the present embodiment can be applied. Specifically, when an image failure occurs due to a shock that the recording material trailing edge comes off the paper feeding roller 14 and the pair of conveying rollers 15, the recording material trailing edge falls out of the paper feeding roller 14 and the pair of conveying rollers 15. It may be configured to correct the loop amount before.

  Similarly, when an image defect occurs due to a shock that the recording material front end enters the pair of paper discharge rollers 20, the loop amount is corrected before the recording material front end enters the pair of paper discharge rollers 20. What should I do? That is, the effect of this configuration is not limited by the type and location of the conveying member.

  Further, in this embodiment, the configuration is used in which the loop amount is corrected by changing the set speed of the loop amount control of the fixing motor M according to the return time. However, similarly to the first and second embodiments, the configuration is used. The effect of this configuration is not limited by the position of the motor that performs loop amount control. That is. The present invention can be applied even when the secondary transfer roller includes a separate drive source (motor). In short, based on the return time, either the transfer nip portion or the fixing nip portion is set so that the loop amount when the downstream side from the leading edge of the recording material is conveyed approaches the predetermined loop amount (designed loop amount). Any recording material may be used as long as the conveyance speed of the recording material is controlled by the rotational speed of the motor.

(Modification 1)
In the embodiment described above, the CPU 26 (FIG. 1) has both functions of the time acquisition means for acquiring the return time and the control unit for controlling the recording material conveyance speed so as to obtain a predetermined loop amount based on the return time. It is good also as a structure which has. Alternatively, the time acquisition unit and the control unit may be configured to share functions as separate members.

  In the above-described embodiment, the return time acquired in the (A) fixed speed section, which is the first section (period), is reflected in the (C) correction control section, which is the third section (period). A section (period) of 2 was defined as a loop amount control section. However, the present invention is not limited to this, and the second section may be the same (C) correction control section as the third section.

(Modification 2)
In the embodiment described above, the first section (period) in FIG. 7 is a “fixed speed section”, and the predetermined transport speed at which the leading edge of the recording material is transported is a constant speed. However, the present invention is not limited to this, The predetermined conveying speed may be a conveying speed having a predetermined speed distribution shape. In short, if the output of the loop amount detector when the leading edge of the recording material is conveyed at a predetermined conveyance speed can be acquired the time (return time) to return from one of on and off to the other through the other, The velocity distribution shape may be arbitrary.

(Modification 3)
In the above-described embodiment, a transmission type loop amount detector is used, and the return time is obtained from the time when the output of the loop amount detector returns from OFF (light shielding state) to ON (transmission state) through OFF (light shielding state). However, the present invention is not limited to this. That is, using a reflection type loop amount detector, the return time is obtained as the return time of the loop amount detector output from ON (with reflected light) to OFF (without reflected light) through ON (with reflected light). May be. That is, it is only necessary to acquire the time for the output of the loop amount detector to return from one of on and off to the other through the other.

(Modification 4)
In the above-described embodiment, the light amount sensor is used as the sensor in the loop amount detector 31 including the flag and the sensor. However, a sensor that detects other than the light amount may be used.

  In addition, the return time is used as an acquisition means for acquiring the deviation amount of the loop detection by the flag and sensor when the leading edge of the recording material is conveyed at the predetermined conveyance speed, that is, the loop amount detector, but other than this is used. You can also

11 .... secondary transfer roller, 18 .... fixing film, 19 .... pressure roller, 28 ... opposite roller, 26 ... CPU, 311 ... loop detection flag, 314 ... light sensor

Claims (17)

An image forming unit comprising a transfer nip for transferring an image to a recording material;
A fixing unit comprising a fixing nip for fixing the image to the recording material;
A flag capable of contacting the recording material in order to detect the loop amount of the recording material in the recording material conveyance path from the transfer nip portion to the fixing nip portion;
A sensor for detecting a loop, the output of which is turned on or off according to the attitude of the flag;
The time required for the sensor output to return from one of ON and OFF to the other through the other after the leading edge of the recording material passes through the transfer nip at a predetermined conveyance speed and enters the fixing nip. Time acquisition means for acquiring,
Based on the output of the time acquisition means, the recording in either the transfer nip portion or the fixing nip portion so that the loop amount when the downstream side from the leading edge of the recording material is conveyed approaches the predetermined loop amount. A control unit for controlling the conveying speed of the material;
An image forming apparatus comprising: When the time acquired by the time acquisition unit is in a reference range, the control unit controls the recording material conveyance speed to a reference conveyance speed,
The image forming apparatus according to claim 1, wherein the predetermined transport speed is slower than the reference transport speed. The controller is
When the time acquired by the time acquisition unit is longer than the reference range, when the time acquired by the time acquisition unit is shorter than the reference range, so as to increase the loop amount of the recording material, The image forming apparatus according to claim 1, wherein a conveyance speed of the recording material in any one of the transfer nip portion and the fixing nip portion is changed so that a loop amount of the recording material is reduced. . A first pair of transport rollers that form a first transport nip portion upstream of the transfer nip portion in the transport path of the recording material;
A second pair of transport rollers forming a second transport nip portion on the upstream side of the transfer nip portion on the recording material transport path and downstream of the first pair of transport rollers;
Have
Control based on the output of the time acquisition means of the control unit is performed from when the trailing edge of the recording material passes through the first transport nip to when it passes through the second transport nip. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. In transporting a single recording material,
A first period for conveying the recording material at the predetermined conveying speed;
A second period of controlling the recording material conveyance speed based on the output of the sensor so as to maintain a reference conveyance speed higher than the predetermined conveyance speed;
A third period in which the control unit selects from the plurality of transport speeds including the predetermined transport speed, the reference transport speed, and a third transport speed faster than the reference transport speed based on the output of the time acquisition unit;
The image forming apparatus according to claim 1, further comprising: In transporting a single recording material,
A first period for conveying the recording material at the predetermined conveying speed;
A second period for controlling the loop amount of the recording material at a reference conveyance speed faster than the predetermined conveyance speed based on the output of the sensor, and acquiring an average conveyance speed;
A third period in which the control unit corrects the average transport speed based on the output of the time acquisition unit;
The image forming apparatus according to claim 1, further comprising:   The image forming apparatus according to claim 6, wherein the average transport speed is corrected by changing a value by which the average transport speed is multiplied based on an output of the time acquisition unit. In transporting a single recording material,
A first period for conveying the recording material at the predetermined conveying speed;
A second period of controlling the recording material conveyance speed based on the output of the sensor so as to maintain a reference conveyance speed higher than the predetermined conveyance speed;
A third period in which the controller controls the speed of the conveyance speed based on the output of the sensor and the output of the time acquisition means;
The image forming apparatus according to claim 1, further comprising:   At least one of the maximum value and the minimum value in the third section in which the speed control of the conveyance speed is performed is based on the output of the time acquisition unit, the maximum value and the minimum value in the second section in which the speed control is performed. The image forming apparatus according to claim 8, wherein at least one of the values is corrected.   The image forming apparatus according to claim 1, further comprising a storage unit that stores the time acquired by the time acquisition unit. Based on the time acquired on the first side, the image can be formed on both sides of the recording material,
2. The second surface according to claim 1, wherein the control unit controls the conveyance speed of the recording material in one of the transfer nip portion and the fixing nip portion based on an output of the time acquisition unit. The image forming apparatus according to any one of 10.   The image forming apparatus according to claim 1, wherein the predetermined transport speed is a constant speed.   13. The image according to claim 1, wherein an image can be formed at a plurality of process speeds, and the predetermined conveying speed is a speed corresponding to any of the plurality of process speeds. Forming equipment. The sensor includes an optical sensor that detects a transmitted light amount,
The image forming apparatus according to claim 1, wherein the time acquisition unit acquires a time for the output of the optical sensor to return from OFF to ON through OFF. An inlet guide that guides the conveyance of the recording material to the fixing nip portion downstream of the transfer nip portion and upstream of the fixing nip portion;
The image forming apparatus according to claim 1, wherein the flag and the sensor are provided in the entrance guide.   The motor according to any one of claims 1 to 15, further comprising: a motor whose rotation speed per unit time is changed in order to control a conveyance speed of the recording material in any one of the transfer nip portion and the fixing nip portion. The image forming apparatus according to claim 1. One of the pair of fixing members forming the fixing nip portion is a pressure roller,
The fixing motor (M) whose rotation speed is changed in order to control the conveyance speed of the recording material in the fixing nip portion, and the fixing motor is connected to the pressure roller. The image forming apparatus according to 16.