Image forming apparatus

Abstract

To solve the problem in which: when a fixing speed for bringing a leading end of a sheet into a nip part of a fixing unit is made slower than a transfer speed, introduction of the leading end of the sheet to the fixing unit may become unstable.SOLUTION: An image forming apparatus comprises: a transfer unit that conveys a sheet while transferring a toner image formed on an image carrier to the sheet; a fixing unit that is provided downstream of the transfer unit, and conveys the sheet while fixing the toner image transferred to the sheet; a fixing driving unit that drives to rotate the fixing unit; loop detection means that is provided downstream of the transfer unit and upstream of the fixing unit, and detects loop of the sheet; and a control unit that controls the conveyance speed of a sheet conveyed by the fixing unit driven to be rotated by the fixing driving unit relative to the conveyance speed of a sheet conveyed by the transfer unit on the basis of a result of detection performed by the loop detection means. The control unit performs control so that the conveyance speed of a sheet conveyed by the fixing unit for bringing a leading end of the sheet conveyed from the transfer unit into a nip part of the fixing unit, becomes substantially the same as the conveyance speed of a sheet conveyed by the transfer unit.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus for forming an image on a sheet. [Background technology]

[0002] In conventional image forming apparatuses, the conveying speed of a sheet as a recording medium is controlled to be approximately the same in the transfer section and the fixing section. However, when comparing the conveying speed of the sheet in the transfer section (hereinafter referred to as the "transfer speed") with the conveying speed of the sheet in the fixing section (hereinafter referred to as the "fixing speed"), there is variation due to changes over time caused by temperature changes in the fixing roller of the fixing section. As a result, when a sheet is conveyed in both the transfer section and the fixing section, the fixing speed is controlled so that the sheet is not pulled by the transfer section and the fixing section or excessive bending does not occur.

[0003] Therefore, Patent Document 1 discloses an image forming apparatus in which a deflection (hereinafter referred to as a "loop") of the sheet is formed between the transfer unit and the fixing unit, and a sensor is disposed to detect the degree of the loop. Furthermore, it is also disclosed that acceleration / deceleration speed control of the fixing unit (hereinafter referred to as fixing loop control) is performed to keep the loop constant. Also, before performing the above-mentioned fixing loop control, the sheet is conveyed in a state in which a small loop is formed in the sheet when the leading edge of the sheet is nipped in the fixing unit. And, Patent Document 2 discloses that rotation is started in a state in which the fixing speed is slower than the transfer speed, and the sheet is conveyed to the fixing unit so that the sheets are not pulled against each other. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-196402 [Patent Document 2] JP 2014-215430 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, when the fixing speed at which the leading edge of the sheet enters the nip portion of the fixing portion as described in Patent Document 1 is made slower than the transfer speed, there is a risk that the leading edge of the sheet may be introduced into the fixing portion unstably. This is because the leading edge of the sheet may be partially or momentarily blocked or bound. Specifically, before the leading edge of the sheet reaches the fixing portion, the leading edge of the sheet is somewhat skewed in the width direction, and the amount of skew of the sheet increases when the leading edge of the sheet is introduced. As a result, when the sheet is in a skewed state, it is conveyed by the fixing portion, causing the sheet to rotate at the transfer portion, and the printing accuracy of the image on the sheet deteriorates.

[0006] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS An object of the present invention is to provide an image forming apparatus that stabilizes the attitude of a sheet when the leading edge of the sheet is nipped by a fixing section, thereby improving the printing accuracy of an image on the sheet. [Means for solving the problem]

[0007] One aspect for solving the above problem includes a transfer unit that transports a sheet while transferring a toner image formed on an image carrier to the sheet, a fixing unit that is provided downstream of the transfer unit and transports the sheet while fixing the toner image transferred to the sheet, a fixing drive unit that rotates and drives the fixing unit, a loop detection means that is provided downstream of the transfer unit and upstream of the fixing unit and detects a loop of the sheet, and a control unit that controls the fixing drive to change the sheet transport speed of the fixing unit relative to the sheet transport speed of the transfer unit based on the detection result of the loop detection means, and is characterized in that the control unit controls the sheet transport speed by the fixing unit at which the leading edge of the sheet transported from the transfer unit enters a nip portion of the fixing unit to be approximately equal to the sheet transport speed by the transfer unit. Effect of the Invention

[0008] According to the present invention, it is possible to stabilize the attitude of the sheet when the leading edge of the sheet is nipped by the fixing section, and to improve the printing accuracy of the image on the sheet. [Brief description of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view of an image forming apparatus according to an embodiment of the present invention; [Diagram 2] Schematic cross-sectional view of a transfer unit and a fixing unit in an embodiment. [Diagram 3] Control block diagram of a control unit in an embodiment [Figure 4] State diagram of fixing of the leading edge of a sheet in an embodiment [Diagram 5] FIG. 13 is a diagram illustrating rotation of a sheet that occurs in a transfer unit and a fixing unit in an embodiment. [Figure 6] Timing chart of fixing loop control in the embodiment [Figure 7] FIG. 13 is an explanatory diagram of the effect of image printing accuracy in the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] [Embodiment] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[0011] <Image forming device> FIG. 1 is a schematic cross-sectional view of an image forming apparatus according to an embodiment of the present invention. In the image forming apparatus 100, image information transmitted from an external connection cable (not shown) is processed by a control unit 201. Further, a laser beam is emitted from a laser scanner unit 152 in response to a signal based on the processing result, and an electrostatic latent image is formed on a photoconductor drum 151, which is an image carrier, and the electrostatic latent image on the photoconductor drum is developed by a developing device 153, and a toner image is formed on the photoconductor drum. Thereafter, a predetermined pressure force and an electrostatic load bias are applied by a primary transfer unit 154, and the toner image is transferred onto an intermediate transfer belt 155. In the case of FIG. 1, the image forming unit 150 is provided with four sets of yellow Y, magenta M, cyan C, and black Bk.

[0012] Next, the intermediate transfer belt 155 will be described. The intermediate transfer belt 155 is rotated in the direction of the arrow A in FIG. 1 by a transfer drive motor 157. Therefore, the above-mentioned Y, M, C, and Bk image forming units process the images in parallel. The image forming process for each color is performed at a timing when the image is superimposed on the upstream toner image that has been primarily transferred onto the intermediate transfer belt 155. As a result, a full-color toner image is finally formed on the intermediate transfer belt 155 and is transported to the secondary transfer unit 156.

[0013] Meanwhile, sheets S stacked in a cassette 111 are separated and fed one by one by a feeding section 110. The fed sheets S are conveyed by a drawing roller pair 114 and a conveying roller pair 121 toward a registration roller pair 131 disposed downstream in the sheet conveying direction (hereinafter simply referred to as "downstream"). The leading edge of the sheet S conveyed by the conveying roller pair 121 is abutted against a nip portion of the registration roller pair 131, thereby forming a flexure in the sheet. This corrects the skew of the sheet S. Thereafter, the sheet S is conveyed by the registration roller pair 131 toward a secondary transfer section 156 so as to be timed.

[0014] As described above, a full-color toner image is secondarily transferred onto the sheet S in the secondary transfer section 156 through the conveying process and image forming process of the sheet S. Thereafter, the sheet S is conveyed to the fixing section 160. The fixing section 160 applies a predetermined pressure force by a pair of substantially opposing fixing rollers 161 or a belt, etc., and generally applies a heating effect by a heat source such as a heater, etc., to melt and fix the toner on the sheet S. The sheet S having the fixed image thus obtained passes through the post-fixing conveying section 170, and is stacked in an aligned manner on the discharge tray 180 installed in the body of the image forming apparatus 100 by the discharge section 171. When double-sided image formation is required on the sheet S, a route is selected such that the leading edge of the sheet S is once placed on the discharge tray 180, and when the trailing edge of the sheet S passes a branch point, the sheet S is switched back and conveyed to the inverting conveying device 190. The sheet S conveyed to the reversing conveying device 190 is conveyed by the reversing conveying device 190 to the pull-out roller pair 115 and the conveying roller pair 121 in this order, and is conveyed toward the registration roller pair 131 arranged downstream. Thereafter, similar to forming an image on one side of a sheet, the skew of the sheet S is corrected by the registration roller pair 131, and then the sheet S is conveyed to the secondary transfer section 156, where the toner image on the second side is transferred to the sheet S. As for the path of the sheet S after the toner image is transferred to the sheet S, as described above, the sheet S passes through the fixing device 160 and the post-fixing conveying section 170, and is discharged by the discharge section 171 onto the discharge tray 180.

[0015] The sheet S can also be fed from a manual feed unit provided on the side of the image forming apparatus 100. In detail, the sheet S loaded on the manual feed tray 113 is fed and separated by a feed / separation roller pair 112, and is conveyed to a registration roller pair 131 by a downstream pull-out roller pair 115 and a conveying roller pair 121.

[0016] <Control of the fixing loop between the transfer unit and the fixing unit> FIG. 2 is a cross-sectional view of the sheet conveying path from the secondary transfer unit 156 to the pair of fixing rollers 161. First, the fixing loop control will be described. The loop detection means 162 is composed of a photointerrupter 162a, which is a transmission type sensor, and a flag 162b. It is arranged so that it can detect the flexure (loop) of the sheet conveyed by both the secondary transfer unit 156 and the pair of fixing rollers 161. In detail, when the sheet S is conveyed by both the secondary transfer unit 156 and the pair of fixing rollers 161, if the loop becomes large, the flag 162b is pushed by the surface of the sheet, so that the flag 162b rotates in the CW direction, and accordingly, the photointerrupter 162a is shielded from light. This detects that the loop formed in the sheet has become large. In this case, the rotation speed of the fixing drive motor 164, which is a fixing drive unit that drives the rotation of the pair of fixing rollers 161, is increased. This prevents the sheet from being buckled due to the excessive flexure of the sheet formed between the secondary transfer unit 156 and the pair of fixing rollers 161. On the other hand, when the sheet S is conveyed by both the secondary transfer unit 156 and the pair of fixing rollers 161, if the loop formed becomes small, the flag 162b rotates in the CCW direction, and the light shielding of the photointerrupter 162a is released. In this case, the rotation speed of the fixing drive motor 164 that drives the rotation of the pair of fixing rollers 161 is reduced. This increases the bending of the sheet formed between the secondary transfer unit 156 and the pair of fixing rollers 161, preventing the sheet from being pulled against each other. The sheet conveying speed by the secondary transfer unit 156 is set to a constant speed. This is to stabilize the image formation by the image forming unit 150, including the rotation of the intermediate transfer belt 155.

[0017] 2 indicates the shortest path between the secondary transfer unit 156 and the pair of fixing rollers 161. When the sheet S is transported by both the secondary transfer unit 156 and the pair of fixing rollers 161, the fixing loop control is performed to maintain a constant loop, and transport control is performed to prevent the sheet S from reaching the Lp state.

[0018] As described above, the fixing loop control is for making constant the loop formed between the secondary transfer unit 156 and the pair of fixing rollers 161 when a sheet is conveyed by both the secondary transfer unit 156 and the pair of fixing rollers 161. Therefore, based on the detection result by the loop detection means 162, the sheet conveying speed by the pair of fixing rollers 161 is controlled.

[0019] Fig. 3 is a block diagram of a control unit related to the secondary transfer unit 156 and the fixing unit 161 in Fig. 2. The control unit 201 controls the rotation speed of drive means (transfer drive motor 157, fixing drive motor 164) of various roller pairs arranged in the image forming unit 101. The control unit 201 also records sheet information such as the size and basis weight of the sheet input by the user via the operation unit 202, and collects information from the register detection sensor 132 and the fixing loop sensor 162 during sheet transport.

[0020] <Conveyance control of the leading edge of the sheet to the fixing unit> Next, sheet transport control for transporting the sheet S from the secondary transfer unit 156 to the pair of fixing rollers 161 will be described with reference to Figures 2 and 4. As shown in Figure 2, the leading edge of the sheet S sent out from the secondary transfer unit 156 is introduced into the pair of fixing rollers 161 while changing direction at the post-transfer transport guide 209 and the pre-fixing transport guide 210. Lt indicates the movement trajectory of the leading edge of the sheet S, and the sheet S advances into the fixing path rollers 161 in an arched shape. Also, Figure 4 is an enlarged cross-sectional view particularly showing the upstream of the fixing roller 161 in Figure 2.

[0021] This embodiment is characterized in that the fixing receiving speed VF when the leading edge of the sheet S enters the fixing roller pair 161 is set to be approximately equal to the transfer speed Vps. As shown in FIG. 4, the leading edge of the sheet S forms an abutment angle θ with the fixing roller tangent T at the point where it abuts against the fixing roller 161. The smaller the abutment angle θ, the more stable the entry of the sheet S into the fixing roller 161 becomes. However, when the size of the fixing unit and the entire image forming apparatus are taken into consideration, the abutment angle θ becomes relatively large, about 60°, especially in a small machine. In such a case, when the fixing speed Vf when the leading edge of the sheet S enters the nip portion of the fixing unit is set slower than the transfer speed Vps (about 92% of the transfer speed Vps) as in the conventional case, the introduction of the leading edge of the sheet S becomes unstable.

[0022] FIG. 5 is a schematic diagram of the sheet conveying state between the transfer section and the fixing section, viewed from the print surface side (intermediate transfer belt side). In the figure, F indicates the front side of the image forming apparatus main body, and R indicates the rear side of the image forming apparatus main body. The solid line indicates the case where the F side of the sheet S enters the fixing section slightly slanted, and the dashed line indicates the case where the F and R sides of the sheet S enter substantially parallel (without slanting). When comparing the solid line and the dashed line, the solid line is slanted on the front side, so the front side has a smaller loop than the dashed line, and the rear side has a larger loop than the dashed line. When the sheet is conveyed to the fixing section in this state, the difference in loop generates a turning force of the sheet in the transfer section, so the printing accuracy of the image on the sheet deteriorates. Therefore, in this embodiment, the fixing speed Vf when the leading edge of the sheet S enters the fixing roller pair 161 is made substantially equal to the transfer speed Vps. Specifically, the fixing speed Vf is set to 99.5% of the transfer speed Vps. By doing so, even when the contact angle θ is relatively large at 60°, the speed difference of the leading edge of the sheet S between the upstream and downstream of the pair of fixing rollers 161 when the leading edge of the sheet S enters the pair of fixing rollers 161 is small. As a result, the leading edge of the sheet S is smoothly introduced into the fixing section, so that the difference in the depth before the loop in the width direction of the sheet does not affect the sheet, and the straight-line stability of the sheet in the secondary transfer section 156 is increased. Therefore, the printing accuracy of the image on the sheet is also improved.

[0023] FIG. 7 shows the results of printing accuracy for the image on the sheet. As shown in FIG. 7(a), the image at the rear end of the sheet is made to tend to be skewed in the transport direction (= the main scanning parallelism is a negative value). The difference in image position in the width direction at the rear end of the sheet is set to t. FIG. 7(b), (c), and (d) show data when 20 sheets of plain paper A3 size are continuously passed through with the fixing speed Vf set to the following respective specified values. The Y axis of the graph is the amount of parallelism in the main scanning direction, and the X axis is the position in the sub-scanning direction, and the position X=400 mm is the value of the amount of parallelism in the main scanning direction for A3 size. It can be seen that for all plots, the deviation in the main scanning direction increases as it progresses in the sub-scanning direction.

[0024] FIG. 7(b) shows the case where the fixing speed Vf is 92% of the transfer speed, and the variation among 20 sheets is large, with Δ (maximum value-minimum value) being 0.8 mm. FIG. 7(c) shows the case where the fixing speed Vf is 99.5% of the transfer speed, which is the present embodiment, and the rotation variation between the transfer section and the fixing section is reduced, with Δ (maximum value-minimum value) being reduced to 0.55 mm. If the variation component can be reduced in this way, it will greatly contribute to improving the printing accuracy of the image on the sheet. FIG. 7(d) shows the case where the fixing speed Vf is 95.5% of the transfer speed (between 92% and 99.5%), with Δ (maximum value-minimum value) being 0.67 mm, and it was recognized that the smaller the difference between the fixing speed Vf and the transfer speed, the better Δ (maximum value-minimum value) is. From the above, it was found that there is an effect even if the fixing speed Vf is 95.5% of the transfer speed Vps, rather than 99.5% of the transfer speed Vps. Naturally, making the fixing speed Vf faster than the transfer speed Vps (making it greater than 100% of the transfer speed Vps) is not adopted in this embodiment because there is a risk of pulling the sheet between the fixing unit and the transfer unit.

[0025] FIG. 6 is a timing chart of the fixing loop control including the fixing speed at which the sheet of this embodiment enters the nip of the fixing unit. The fixing drive motor 164 is driven at the fixing speed Vf. The leading edge of the sheet S conveyed from the secondary transfer unit 156 enters the nip of the pair of fixing rollers 161 in the section Din in the figure. As described above, the conveying speed of the sheet S at this time is stably conveyed to the fixing unit 160 because the transfer speed and the fixing speed are substantially equal. Thereafter, the fixing exit sensor 163, which is a fixing exit detection means, detects that the leading edge of the sheet S has reached downstream after fixing. Then, the control unit 201 starts the fixing loop control. The target speed of the fixing drive motor 164 is selected to be a value Low (98 to 99%, which is 1 to 2% lower than the transfer speed) that is slower than the transfer speed, and a phase signal is sent to the motor driver. The motor driver drives the winding (stator) of the motor according to the phase signal. Since the conveying speed of the fixing unit is slower than the transfer speed, the sheet between the transfer unit and the fixing unit is bent (looped). When the loop becomes equal to or larger than a certain amount, the output signal of the loop detection sensor 162 is switched from "OFF" to "ON". When the control unit 201 detects that the output signal of the loop detection sensor 162 has switched from "OFF" to "ON", it selects a target speed of the fixing drive motor 164 to a value Hi (101 to 102%, which is 1 to 2% higher than the transfer speed) that is faster than the transfer speed. Then, it transmits a phase signal to the motor driver. The motor driver drives the motor winding (stator) according to the phase signal. Since the conveying speed of the fixing unit is faster than the transfer speed, the loop formed between the transfer unit and the fixing unit of the sheet gradually disappears. After that, when the loop becomes equal to or smaller than a certain amount, the output signal of the loop detection sensor 162 is switched from "ON" to "OFF". When the control unit 201 detects that the output signal of the loop detection sensor 162 has switched from "ON" to "OFF", it selects a low target speed, which is slower than the transfer speed, for the fixing drive motor 164. Then, it transmits a phase signal to the motor driver.

[0026] The control unit 201 detects the timing when the rear end of the sheet S passes through the transfer unit by counting the drive pulses that drive the transfer unit drive motor 157 with a counter inside the CPU. When the internal counter of the control unit 201 reaches a count value C1 of the timing when the rear end of the sheet passes through the transfer unit, the control unit 201 selects the transfer speed as the target speed of the fixing drive motor 164 and sends a phase signal to the motor driver.

[0027] This control is applied to each sheet being conveyed, and is performed in sequence in response to an output signal from the loop detection sensor 162, thereby controlling the fixing drive motor 164 so as to maintain the loop at a constant amount. [Explanation of symbols]

[0028] 100 Image forming device 101 Image forming section 156 Transcription section 157 Transfer drive motor 161 Fixing part 162 Loop detection means 163 Fixing exit detection means 164 Fixing drive motor (fixing drive unit) 201 Control section

Claims

1. a transfer section that transfers a toner image formed on an image carrier onto a sheet while conveying the sheet; a fixing section provided downstream of the transfer section and configured to fix the toner image transferred onto the sheet while conveying the sheet; a fixing drive unit that rotates the fixing unit; a loop detection unit that is provided downstream of the transfer unit and upstream of the fixing unit and detects a loop of the sheet; a control unit that controls the fixing drive so as to change the sheet conveying speed of the fixing unit relative to the sheet conveying speed of the transfer unit based on a detection result of the loop detection means, the control unit controls a sheet conveying speed by the fixing unit at which a leading edge of the sheet conveyed from the transfer unit enters a nip portion of the fixing unit to be substantially equal to a sheet conveying speed by the transfer unit. An image forming apparatus comprising:

2. a sheet conveying speed by the fixing unit when the sheet enters a nip portion of the fixing unit is slower than a sheet conveying speed by the transfer unit; It is characterized by:

3. a fixing outlet detection means is provided downstream of the fixing section; the control unit controls the fixing speed to be substantially equal to that of the transfer unit until the leading edge of the sheet is detected by the fixing exit detection unit; after the leading edge of the sheet is detected by the fixing exit detection means, loop control is started based on the detection result of the loop detection means; 2. The image forming apparatus according to claim 1,

Images