Image forming device

Abstract

To reduce a slip of a sheet that occurs in a double-side conveyance path while suppressing reduction in the productivity in the time of double-side printing.SOLUTION: An image formation apparatus comprises: a double-side conveyance roller pair 52 which conveys a sheet in the double-side conveyance path; a double-side sheet re-feeding roller pair 53 which performs re-supply of the sheet conveyed by the double-side conveyance roller pair 52 toward a transfer unit; a double-side conveyance assist control determination unit 151 which selects one of the first double-side conveyance control of conveying the sheet by the double-side sheet re-feeding roller pair 53 and the second double-side conveyance control of conveying the sheet by the double-side conveyance roller pair 52 and the double-side sheet re-feeding roller pair 53 when the double-side printing is executed in the two-sheets circulation mode; and a sheet conveyance control unit 150 which executes the double-side conveyance control selected by the double-side conveyance assist control determination unit 151.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus, for example, an image forming apparatus such as a copying machine or a printer that forms double-sided images. [Background technology]

[0002] One method for forming images on both sides of multiple sheets of paper (also called sheets) is to use a circulating paper transport system. In this circulating paper transport system, the paper is sent to a transfer unit where the image is transferred, and the image is printed on the first side of the paper. The paper is then sent to an inversion unit, inverted, and sent back to the transfer unit where the image is transferred to the second side. This allows images to be printed on both sides of the paper. To further reduce the gap between sheets and improve productivity, multiple sheets are fed together for double-sided printing, and the image on the first side is printed consecutively. The sheets with the image printed on the first side are then sent to the double-sided transport path. After that, the image on the first side of a newly fed sheet and the image on the second side of a sheet transported via the double-sided transport path are alternately printed (hereinafter referred to as double-sided circulation control). In this way, the number of sheets of paper present in the image forming device when double-sided printing is performed is called the double-sided circulation number. In the above example, the double-sided circulation number is two sheets.

[0003] In image forming devices with a double-sided recirculation system, it is necessary to make the double-sided conveyance path short and compact in order to make the image forming device more compact and space-saving. For example, Patent Document 1 proposes a configuration in which, in order to shorten the double-sided conveyance path, the position where paper is kept waiting in the double-sided conveyance path is located midway between the paper feed unit where an image is formed on the first side and the transfer unit. One possible method for making the double-sided conveyance path more compact is to increase the curvature of the double-sided re-feed unit that conveys paper from the double-sided conveyance path to the image forming unit. By increasing the curvature of the double-sided re-feed unit, the distance between the double-sided conveyance path and the image forming unit can be reduced, thereby making the image forming device more compact. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-012374 Summary of the Invention [Problem to be solved by the invention]

[0005] However, increasing the curvature of the duplex transport path (duplex refeed section) increases the resistance (hereinafter referred to as transport resistance) that occurs when the paper is bent along the transport path, which can cause slippage between the paper and the transport rollers and lead to paper jams. Increasing the nip pressure of the transport rollers to prevent slippage can accelerate wear on the transport rollers and can also cause deformation due to creep (hereinafter referred to as creep deformation) of the members that hold the transport rollers.

[0006] One way to avoid jams in the duplex transport path is to reduce the number of double-sided sheets circulated from two to one. This allows all of the transport rollers to be used to transport the paper in the duplex transport path, improving paper transport power compared to duplex circulation control. However, reducing the number of double-sided sheets circulated to one reduces productivity during duplex printing.

[0007] The present invention has been made under these circumstances, and has as its object to reduce paper slippage that occurs in the double-sided conveyance path while suppressing a decrease in productivity during double-sided printing. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, the present invention has the following configuration. (1) In an image forming apparatus capable of double-sided printing in one of a first mode in which an image is formed on a first side of a preceding sheet of paper that is transported in advance and then on a second side opposite to the first side, and an image is formed on a first side of a succeeding sheet of paper that is transported following the preceding sheet, and a second mode in which an image is formed on a first side of the succeeding sheet of paper between the image formation on the first side of the preceding sheet and the image formation on the second side of the preceding sheet, the apparatus includes a transfer unit that transfers a toner image to the paper, a fixing unit that fixes the unfixed toner image transferred by the transfer unit, and a fixing unit that fixes the unfixed toner image on the second side of the paper that has undergone image formation on the first side and passed through the fixing unit. a duplex conveying path along which paper is conveyed to transfer a toner image onto one side; a first conveying means for conveying paper in the duplex conveying path; a second conveying means for re-feeding the paper conveyed by the first conveying means toward the transfer unit; a selection means for selecting, when the duplex printing is performed in the second mode, either a first duplex conveying control for conveying paper by the second conveying means or a second duplex conveying control for conveying paper by the first conveying means and the second conveying means; and a control means for executing the duplex conveying control selected by the selection means. In the second mode, the control means controls an image formation interval, which is a distance between the leading edge of an image formed on the first side of the succeeding sheet and the leading edge of an image formed on the second side of the preceding sheet, so that the image formation interval when performing the second double-sided transport control is larger than the image formation interval when performing the first double-sided transport control. An image forming apparatus characterized by: [Effects of the Invention]

[0009] According to the present invention, it is possible to reduce slippage of paper that occurs in the double-sided conveying path while suppressing a decrease in productivity during double-sided printing. [Brief explanation of the drawings]

[0010] [Figure 1] Schematic cross-sectional view of an image forming apparatus according to Examples 1 to 3. [Figure 2] Block diagram of an image forming apparatus according to first to third embodiments [Figure 3] FIG. 10 shows the printing order of the circular double-sided printing method of Examples 1 to 3. [Figure 4] Schematic diagram of the main part showing the state of paper in the first double-sided transport control in the first to third embodiments. [Figure 5] Flowchart for explaining double-sided conveyance control in the first embodiment [Figure 6] Timing chart of double-sided conveyance control in the first embodiment [Figure 7] FIG. 10 is a schematic diagram illustrating the state of a sheet in the second double-sided conveying control according to the first embodiment. [Figure 8] FIG. 10 is a diagram showing image formation intervals in each double-sided conveyance control in the first embodiment. [Figure 9] Flowchart for explaining double-sided conveyance control in the second embodiment [Figure 10] Flowchart for explaining double-sided conveyance control in the third embodiment DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. However, the components described in this embodiment are merely examples, and unless otherwise specified, the scope of the present invention is not intended to be limited to them. Furthermore, a sheet that is transported first is called a preceding sheet, and a sheet that is transported after the preceding sheet is called a succeeding sheet. A surface of a sheet is called a first side, and a surface opposite to the first side is called a second side. [Example]

[0012] <Overall structure and image formation process> The overall configuration of a color electrophotographic image forming apparatus will be described with reference to FIG. 1. The image forming apparatus shown in Example 1 is a laser printer 100 (hereinafter referred to as printer 100) that utilizes an electrophotographic image forming process. Printer 100 includes process stations (process cartridges) 5Y, 5M, 5C, and 5K that are detachable from the printer 100 body. The four process stations 5Y, 5M, 5C, and 5K have the same structure but differ in that they form images using toner (developers) of different colors: yellow (Y), magenta (M), cyan (C), and black (K). The YMCK symbols will be omitted below unless a specific process station is being described. Each process station 5 includes a toner container 23, a photosensitive drum 1 (a photosensitive member), a charging roller 2, a developing roller 3, a cleaning blade 4, and a waste toner container 24. An exposure device 7 is located below each process station 5, and exposes the photosensitive drum 1 to light based on an image signal.

[0013] As the photosensitive drum 1 rotates, it is uniformly charged to a predetermined polarity and potential by the charging roller 2. Then, the photosensitive drum 1 is exposed to light by the exposure device 7, forming electrostatic latent images corresponding to the first to fourth color component images (yellow, magenta, cyan, and black component images) of the desired color image. The charging roller 2 is rotated in accordance with the rotation of the photosensitive drum 1. The exposure device 7 used in Example 1 is a scanner using a laser diode and a rotating polygonal mirror, and focuses a laser beam modulated according to image information on the photosensitive drum 1 to form an electrostatic latent image.

[0014] The electrostatic latent image formed on the photosensitive drum 1 is developed by the developing rollers 3 of the first to fourth process stations 5Y, 5M, 5C, and 5K. Toner of each color is attached to the electrostatic latent image on the photosensitive drum 1 via these developing rollers 3, developing it into a toner image. The toner in each developing unit is a negatively charged non-magnetic single-component toner, and the electrostatic latent image is developed by a non-magnetic single-component contact development method. A developing voltage is applied to the developing roller 3 by a developing voltage power supply (not shown), thereby performing development.

[0015] The intermediate transfer belt unit is composed of an intermediate transfer belt 8, a drive roller 9, and a secondary transfer opposing roller 10. Furthermore, primary transfer rollers 6 are disposed inside the intermediate transfer belt 8, facing each photosensitive drum 1, and a positive primary transfer voltage is applied to them by a primary transfer voltage power supply (not shown). When the drive roller 9 is rotated by a motor (not shown), the intermediate transfer belt 8 is rotated, and the secondary transfer opposing roller 10 is also rotated accordingly. Each photosensitive drum 1 rotates in the direction of the arrow (clockwise), and the intermediate transfer belt 8 rotates in the direction of arrow A, and a positive primary transfer voltage is applied to the primary transfer roller 6. As a result, the toner images on the photosensitive drum 1 are sequentially transferred (primary transfer) from the toner image on the photosensitive drum 1Y onto the intermediate transfer belt 8 (on the belt). The four color toner images are then transported to the secondary transfer roller 11 in a superimposed state. The cleaning blade 4 for the photosensitive drum 1 is pressed against the photosensitive drum 1 and removes toner (hereinafter referred to as residual toner) remaining on the surface of the photosensitive drum 1 without being transferred to the intermediate transfer belt 8, as well as other residues on the photosensitive drum (photoconductor). The belt cleaning blade 21 removes toner remaining on the intermediate transfer belt 8, and the removed toner is stored in a container 22.

[0016] The feeding / conveying device 12 has a paper feed roller 14 that feeds paper P from a paper feed cassette 13 that stores paper P, and a paper feed / conveying roller pair 15 that conveys the fed paper P. Then, the paper P conveyed from the feeding / conveying device 12 is conveyed to the secondary transfer roller 11 by a registration (hereinafter referred to as registration) roller pair 16. A registration sensor 25 detects the presence or absence of paper P. The registration sensor 25 functions as a detection means provided downstream in the conveying direction of the paper P from a position where the paper P conveyed by re-feeding joins a conveying path 75 (described later) and upstream from a transfer unit (described later). Here, the portion of the conveying path along which the paper P is conveyed, between the paper feed / conveying roller pair 15 and the registration roller pair 16, is referred to as the conveying path 75. The conveying path 75 is a conveying path along which the paper P, to have a toner image transferred on its first side, is fed from the paper feed unit (feeding / conveying device 12) to the transfer unit. In the transfer from the intermediate transfer belt 8 to the paper P, a positive voltage is applied to the secondary transfer roller 11, whereby the four-color toner image on the intermediate transfer belt 8 is transferred to the transported paper P (hereinafter referred to as secondary transfer). The position where the toner image is transferred to the paper P, i.e., the position of the secondary transfer roller 11, is the transfer section. Here, the side of the paper P transported from the supply / conveyance device 12 where the toner image is transferred is referred to as the first side.

[0017] After the toner image is transferred, the paper P is transported to the fuser 17. The fuser 17 is a film-heating fuser equipped with a fuser heater 30, a fuser roller 18 incorporating a temperature sensor 31 for measuring the temperature of the fuser heater 30, and a pressure roller 19 for pressing the fuser roller 18. The unfixed toner image is fixed to the paper P by heating and pressurizing it, and the paper P is then discharged outside the printer 100 (outside the machine) by discharge rollers 20 as an image-formed product (printed paper, etc.). The fuser 17 is a fuser unit in which the unfixed toner image transferred by the transfer unit is fixed to the paper. The direction in which the paper P is discharged outside the machine is the discharge direction (arrow B in FIG. 1). When double-sided printing is performed, the paper P that has undergone the fixing process is transported to a pair of reversing rollers 50 (described later) in the reversing unit direction (arrow C in FIG. 1).

[0018] When printing is to be performed on the second side of the paper P, which is the reverse side of the first side of the paper P, without ejecting the paper P that has passed the fixing unit 17 outside the machine, the paper P that has passed the fixing unit 17 is conveyed in the direction of the reversal point 201 to be conveyed to the duplex conveying path 70. The duplex conveying path 70 is a conveying path along which the paper is conveyed in order to transfer a toner image to the second side of the paper P that has passed the fixing unit. The duplex flapper 55 can switch the conveying direction of the paper P to either the ejection direction or the reversal unit direction. When duplex printing is to be performed, the duplex flapper 55 is switched to the reversal unit direction before the leading edge of the paper P, on whose first side an image has been formed (hereinafter referred to as "image-formed"), arrives at the duplex flapper 55.

[0019] After passing through reversal point 201, sheet P is temporarily transported by reversal roller pair 50 in the discharge direction outside the apparatus. After the rear end of sheet P passes reversal point 201, sheet P is present at the position of reversal roller pair 50, and reversal roller pair 50 temporarily stops. Then, reversal roller pair 50 rotates in a direction opposite to the previous rotation direction, thereby transporting sheet P toward duplex conveying path 70. Within duplex conveying path 70, duplex conveying roller pair 52 and duplex refeed roller pair 53 transport sheet P to refeed standby point 202 and junction point 200. Duplex conveying roller pair 52 functions as a first transport unit that transports sheet P in duplex conveying path 70. Duplex refeed roller pair 53 functions as a second transport unit that refeeds sheet P transported by duplex conveying roller pair 52 toward the transfer unit.

[0020] The double-sided conveying path 70 merges with a conveying path 75 between the pair of paper feed conveying rollers 15 and the pair of registration rollers 16 at a merging point 200. The paper P, which has been turned over, is conveyed to the secondary transfer roller 11 by the pair of registration rollers 16. Then, the unfixed toner images of the four colors on the intermediate transfer belt 8 are transferred onto the second side of the paper P. The fixing device 17 fixes the toner images transferred onto the second side of the paper P. When the double-sided flapper 55 is switched to the discharge direction, the paper P with images formed on both sides is discharged outside the apparatus by the discharge rollers 20.

[0021] <Control block diagram> FIG. 2 is a control block diagram of the printer 100 of the first embodiment. The printer control unit 101 has circuits such as a CPU 104, a ROM 104a, and a RAM 104b, and executes programs that control each device in the printer 100. The CPU 104 is connected to an image forming unit 110, which includes a charging voltage power supply (not shown), a motor drive unit 111 that drives each roller pair on the conveyance path, and a double-sided flapper drive unit 112 that switches the direction of the double-sided flapper 55, and issues instructions to these units to form images and convey paper. The CPU 104 also has a paper conveyance control unit 150 and a double-sided conveyance assist control determination unit 151 (hereinafter referred to as the determination unit 151), which are described below. These control units issue instructions to the double-sided conveyance motor 60 via the motor drive unit 111 to convey both sides of the paper P. The double-sided conveyance motor 60 is configured so that drive in both forward and reverse directions is transmitted to the reversing roller pair 50, and only drive in the forward direction is transmitted to the double-sided conveyance roller pair 52 via a one-way clutch 61.

[0022] The paper transport control unit 150 includes a first double-sided transport control unit 150a that performs first double-sided transport control (described later) and a second double-sided transport control unit 150b that performs second double-sided transport control (described later). The paper transport control unit 150 functions as a control unit that executes the double-sided transport control selected by the determination unit 151. The CPU 104 includes an image formation interval determination unit 152 that determines the image formation interval between sheets. Here, the "inter-sheet interval" refers to the interval between the trailing edge of a preceding sheet and the leading edge of a succeeding sheet being transported following the preceding sheet. The image formation interval refers to the interval between the leading edge of a first toner image formed on the preceding sheet and the leading edge of a second toner image formed on the succeeding sheet. The CPU 104 is also connected to a sensor unit, including the registration sensor 25 and other sensors (not shown) on the transport path, via a sensor unit 113, and can sequentially monitor the values ​​of each sensor.

[0023] The controller 102 is connected to the printer control unit 101 and issues print instructions to the printer control unit 101 in accordance with settings of a host computer 103 connected via a network, a printer cable, or the like. When the controller 102 receives image information and a print command from the host computer 103, it analyzes the received image information and converts it into bitmap data. During printing, the controller 102 transmits the bitmap data to the printer control unit 101 in synchronization with a TOP signal transmitted from the printer control unit 101. Information regarding the paper type of paper P in the paper feed cassette 13 that can be specified by the user (hereinafter referred to as paper type information) and settings for enabling or disabling duplex conveying assist control (described later) are also configured by the user on the host computer 103. The information specified by the user is transmitted to the CPU 104 via the controller 102. The duplex conveying assist control here refers to assist control that prevents improper conveyance (e.g., slippage) of the paper P in the duplex conveying path 70. For example, the host computer 103 functions as a designation unit for specifying whether or not to enable assist control. The paper type is information indicating the type of paper P, and is classified by, for example, plain paper, thick paper, thin paper, glossy paper, basis weight, etc.

[0024] The functions of the printer control unit 101 may be realized by the CPU 104 executing various control programs, or some or all of the functions may be performed by a dedicated circuit (ASIC) for a specific application. Also, the printer 100 may have an operation unit (not shown) that accepts input from the user, and the operation unit may function as a designation unit.

[0025] <Double-sided circulation control> The printing order for double-sided printing of multiple sheets of paper under double-sided circulation control will be explained using FIG. 3. When double-sided printing of multiple sheets of paper, the first side is printed first for a certain number of sheets, followed by the second side, then the first side, and so on, alternately printing the second side, followed by the first side, and so on. FIG. 3(a) shows the printing order when the first sides of two sheets are printed consecutively first, then the second and first sides are printed alternately, and finally the second sides are printed consecutively. In FIG. 3(a), the top row shows the first side, and the bottom row shows the second side. Time progresses from left to right. Furthermore, each sheet indicates its ordinal number. In this case, two sheets of paper circulate through the transport path within printer 100, so the case shown in FIG. 3(a) is referred to as the second mode, or two-sheet circulation mode. In the two-sheet circulation mode, an image is formed on the first side of a subsequent sheet between image formation on the first side of the preceding sheet and image formation on the second side of the preceding sheet. Note that while Fig. 3(a) shows an example in which two sheets of paper are circulated, three or more sheets of paper may also be circulated. In other words, there may be two or more subsequent sheets on whose first side an image is formed between image formation on the first side of the preceding sheet and image formation on the second side of the preceding sheet.

[0026] On the other hand, FIG. 3(b) shows a mode in which the second side of a sheet is printed after the first side is printed, and only one sheet of paper is circulated through the transport path within the printer 100. The case of FIG. 3(b) is called the single-sheet circulation mode, which is the first mode. In the single-sheet circulation mode, images are formed on the first and second sides of a preceding sheet of paper, and then an image is formed on the first side of a succeeding sheet of paper. The printer 100 of the first embodiment is capable of double-sided printing in either the first mode or the second mode.

[0027] Between printing on the first side of a sheet and printing on the second side of that sheet, time is required for the sheet to be transported through the duplex transport path. Therefore, the single-sheet circulation mode has lower productivity than the two-sheet circulation mode. The two-sheet circulation mode can increase productivity by performing image formation on a different sheet between image formation on the first side of a sheet and image formation on the second side of that sheet, compared to the single-sheet circulation mode. For example, in the example of Figure 3(a), printing on the first side of the second sheet is performed between printing on the first side of the first sheet and printing on the second side of the first sheet, and printing on the first side of the third sheet is performed between printing on the first side of the second sheet and printing on the second side of the second sheet.

[0028] The printer 100 shown in the first embodiment is capable of double-sided printing on paper up to 355.6 mm (equivalent to legal size) in the paper feed direction (hereinafter referred to as paper length). For letter-size (paper feed direction length = 279.4 mm) and A4-size (paper feed direction length = 297 mm) paper, productivity is maximized in two-sheet circulation mode. Furthermore, for legal-size paper, printing is possible in single-sheet circulation mode.

[0029] <Duplex conveying assist control determination unit> The following describes details of the control performed by the determination unit 151 to determine whether or not assistance is required in the first embodiment. The determination unit 151 determines whether or not to execute the second double-sided conveyance control (Yes, No) based on the paper type of the paper P (plain paper, thick paper, etc.) and the information on whether or not the user has specified double-sided conveyance assist control (Yes, No) in accordance with the table shown in Table 1.

[0030] [Table 1]

[0031] Table 1 shows the paper type in the first column, the user specification in the second column, and the second double-sided transport control in the third column. For example, if the paper P is thick paper 1, and the user specifies double-sided transport assist control (Yes), the second double-sided transport control is executed (Yes). If the paper P is thick paper 1, and the user does not specify double-sided transport assist control (No), the second double-sided transport control is not executed (No). Also, if the paper P is plain paper, the second double-sided transport control is not executed (No) regardless of whether the user specifies double-sided transport assist control (-). Furthermore, if the paper P is gloss paper 2, the second double-sided transport control is executed (Yes) regardless of whether the user specifies double-sided transport assist control (-). Here, if the second double-sided transport control is not executed, the first double-sided transport control is executed.

[0032] In this way, the determination unit 151 selects either the first double-sided conveying control or the second double-sided conveying control according to information related to the paper type of the paper P. The determination unit 151 selects the second double-sided conveying control when the double-sided conveying assist control is specified, and selects the first double-sided conveying control when the double-sided conveying assist control is not specified. Regardless of whether the double-sided conveying assist control is specified, the determination unit 151 selects the first double-sided conveying control when the paper P is plain paper, and selects the second double-sided conveying control when the paper P is gloss paper 2.

[0033] When double-sided conveyance is performed in the two-sheet circulation mode, the determination unit 151 determines (selects) whether to perform the second double-sided conveyance control according to Table 1. The determination unit 151 functions as a selection unit that selects either the first double-sided conveyance control or the second double-sided conveyance control when double-sided printing is performed in the two-sheet circulation mode. Note that in the single-sheet circulation mode, the second double-sided conveyance control is always selected in the first embodiment. In the first embodiment, the determination unit 151 always selects the first double-sided conveyance control regardless of the user's designation for plain paper, which is the first paper sheet with low paper stiffness. On the other hand, the determination unit 151 always selects the second double-sided conveyance control for cardboard 2 and glossy paper 2, which are second paper sheets with relatively high paper stiffness (heavy basis weight). Furthermore, for cardboard 1 and glossy paper 1, which are medium-stiffness paper types, the determination unit 151 selects the second double-sided conveyance control only when the user designates double-sided conveyance assist control, and selects the first double-sided conveyance control otherwise. By limiting the conditions for selecting the second double-sided conveying control in this way, it is possible to limit cases in which the image formation interval determination unit 152, which will be described later, widens the image interval and reduces the productivity of double-sided printing.

[0034] <First double-sided transport control> The first duplex transport control will be explained using the flowcharts of Figures 5(a) and (b) and Figures 4(a) to (d). The first duplex transport control is a control for transporting paper P by duplex refeed roller pair 53 when duplex printing is performed in two-sheet circulation mode. Figure 4 depicts only the essential parts of printer 100, and only the reference numerals of the main components are given. The first sheet of paper P is represented as paper P1, the second sheet of paper P transported after the first sheet is represented as paper P2, and the third sheet of paper P transported after the second sheet is represented as paper P3. The transport speed when an image is being formed on paper P is called the print speed.

[0035] When a print instruction is sent from the host computer 103 to the controller 102, the controller 102 instructs the printer control unit 101 to perform double-sided printing. The printer control unit 101 forms an image on the first side of the first sheet of paper P1 using the image formation process described above. The printer control unit 101 instructs the double-sided flapper drive unit 112 to switch the double-sided flapper 55 so as to guide the sheet of paper P1 toward the reversing section of the reversing roller pair 50. Figure 4(a) shows the sheet of paper P1 at this time. The sheet of paper P1 is transported by the double-sided flapper 55 toward the reversing section, and the leading edge of the sheet of paper P1 reaches the reversing point.

[0036] As the transport of the sheet P1 continues and the rear end of the sheet P1 reaches the reversal point 201, the printer control unit 101 starts the duplex transport control shown in Figure 5(a). In step (hereinafter abbreviated as S) S501, the printer control unit 101 drives the duplex transport motor 60 in the forward direction at a speed faster than the print speed (hereinafter referred to as acceleration drive). This causes the pair of reversal rollers 50 and the pair of duplex transport rollers 52 to start rotating in a direction that draws the sheet P1 into the duplex transport path 70. In the first embodiment, the transport speed of the sheet P during acceleration drive (hereinafter referred to as acceleration drive speed) is set to 1.5 times the print speed.

[0037] In S502, the printer control unit 101 determines whether the leading edge of the paper P1 has reached the position in front of the duplex refeed roller pair 53. If the printer control unit 101 determines in S502 that the leading edge of the paper P has not reached the position in front of the duplex refeed roller pair 53, the process returns to S502, but if it determines that the leading edge of the paper P has reached the position in front of the duplex refeed roller pair 53, the process proceeds to S503. The printer control unit 101 determines whether the leading edge of the paper P has reached the position in front of the duplex refeed roller pair 53 by referring to a timer (not shown) based on the distance along the conveyance path between the reversal point 201 and the duplex refeed roller pair 53 and the accelerated drive speed.

[0038] In S503, the printer control unit 101 starts driving the duplex refeed roller pair 53 and drives the duplex conveyance motor 60, which has been driven at an accelerated speed, in the forward direction at the print speed. At this time, the printer control unit 101 forms an image on the first side of sheet P2 when image formation interval B, which will be described later, has elapsed from the timing when image formation on sheet P1 started. Figure 4(b) shows the state when sheet P1 has reached just before the duplex refeed roller pair 53 and an image is being formed on the first side of sheet P2 by secondary transfer roller 11.

[0039] In S504, the printer control unit 101 determines whether the leading edge of the sheet P1 has reached the refeed standby point 202. If the printer control unit 101 determines in S504 that the leading edge of the sheet P1 has not reached the refeed standby point 202, the process returns to S504, but if it determines that the leading edge has reached the point, the process proceeds to S505. In S505, the printer control unit 101 stops the duplex refeed roller pair 53 and stops the duplex conveying motor 60, thereby stopping the duplex conveying roller pair 52.

[0040] In S506, the printer control unit 101 determines whether it is time to re-feed the sheet P1. The re-feed timing in S506 is the timing when an image formation interval A (described later) has elapsed since the start of image formation on the first side of the sheet P2, and is the timing to start image formation on the second side of the sheet P1. If the printer control unit 101 determines in S506 that it is not time to re-feed the sheet, it returns the process to S506. If it determines that it is time to re-feed the sheet, it proceeds to S507. In S507, the printer control unit 101 determines whether the above-mentioned determination unit 151 has selected to perform the second duplex conveyance control. If the printer control unit 101 determines in S507 that the second duplex conveyance control has not been selected, it proceeds to S520. If it determines that the second duplex conveyance control has been selected, it proceeds to S530. In S520, the printer control unit 101 executes the first duplex conveyance control by the first duplex conveyance control unit 150a, and ends the process.

[0041] Fig. 5(b) is a flowchart showing the first double-sided conveyance control of S520 in Fig. 5(a) executed by first double-sided conveyance control unit 150a. In S521, first double-sided conveyance control unit 150a starts driving double-sided refeed roller pair 53 to refeed sheet P1. Fig. 4(c) is a diagram showing a state in which sheet P1 is being refeeded by double-sided refeed roller pair 53.

[0042] In S522, the first double-sided conveying control unit 150a determines whether the trailing edge of the sheet P1 has passed the pair of double-sided re-feed rollers 53. If the first double-sided conveying control unit 150a determines in S522 that the trailing edge of the sheet P1 has not passed the pair of double-sided re-feed rollers 53, the process returns to S522, but if it determines that the trailing edge has passed the pair of double-sided re-feed rollers 53, the process proceeds to S523. In S523, the first double-sided conveying control unit 150a stops the pair of double-sided re-feed rollers 53 and ends the process.

[0043] Thereafter, the printer control unit 101 performs image formation on the second side of sheet P1 and switches the duplex flapper 55 via the duplex flapper drive unit 112 to instruct the duplex flapper drive unit 112 to guide sheet P1 in the ejection direction. At this time, the printer control unit 101 feeds sheet P3 at an image formation interval A (described later) from the timing when image formation on the second side of sheet P1 begins. FIG. 4(d) shows the state in which image formation on the second side of sheet P1 has been performed and the first sheet is being transported in the ejection direction. In FIG. 4(d), sheet P2 is transported to the duplex transport path 70 to have an image formed on its second side, and the leading edge of sheet P2 has reached the refeed standby point 202. Furthermore, sheet P3 is being transported toward the registration roller pair 16. The above control is repeated for the image formation on the first side of sheet P3 and subsequent processes, so a description thereof will be omitted.

[0044] (Timing chart of first double-sided transport control) Next, FIG. 6(a) shows a timing chart when the first duplex conveyance control is executed. (i) indicates the presence or absence of sheet P detected by the register sensor 25. When sheet P is present, it is at a high level, and when sheet P is not, it is at a low level. Also, when the high level is reached, for example, because the first side of sheet P2 has been conveyed, it is noted as "P2 (first side)." (ii) indicates the presence or absence of sheet P at the position of the duplex refeed roller pair 53. Following the sensor, when sheet P is present, it is at a high level, and when sheet P is not, it is at a low level. The corresponding sheet P is indicated as "P1." (iii) indicates the presence or absence of sheet P at the reversal point 201. Following the sensor, when sheet P is present, it is at a high level, and when sheet P is not, it is at a low level. Similarly, (ii) indicates which sheet P is present. (iv) indicates the drive of the duplex conveyance motor 60, or in other words, the reversal roller pair 50. With the line where the duplex conveyance motor 60 stops as the center, the discharge direction is indicated below the stop line, and the reversal direction is indicated above the stop line. In the direction of the reversing section above the stop line, the duplex conveying direction and duplex conveying direction (acceleration) are also shown. The constant speed on the line labeled "duplex conveying direction" corresponds to the print speed. The constant speed on the line labeled "duplex conveying direction (acceleration)" corresponds to the accelerated drive speed described below. (v) shows the drive of the duplex conveying roller pair 52, and is the same as (iv) except that there is no reverse rotation (negative side). (vi) shows the drive of the duplex refeed roller pair 53, and indicates either stop or drive.

[0045] 6(a) represents time, and T600 to T605 represent timing. T600 is the timing at which, after the rear end of sheet P1 passes reversal point 201, duplex conveying motor 60 is accelerated and driven in the forward direction by S501 in FIG. 5(a), thereby driving reversal roller pair 50 and duplex conveying roller pair 52. T601 is the timing at which sheet P1 arrives in front of duplex refeed roller pair 53, duplex refeed roller pair 53 is driven by S503 in FIG. 5(a), and duplex conveying motor 60 is driven at print speed.

[0046] T602 is the timing when sheet P1 arrives at refeed standby point 202, and duplex refeed roller pair 53 stops according to S505 in Fig. 5(a), and duplex conveying roller pair 52 stops. T603 is the timing when the first side of sheet P2 arrives at reversing point 201, and duplex conveying motor 60 is driven in the reverse direction, thereby driving reversing roller pair 50 in the discharge direction.

[0047] At T604, it is time to re-feed sheet P1, and this is the timing when duplex re-feed roller pair 53 is driven by S521 in FIG. 5(b) ("P1 re-feeding" in (vi)). At T605, sheet P1 passes through duplex re-feed roller pair 53, and duplex re-feed roller pair 53 is stopped by S523 in FIG. 5(b), completing duplex transport control for sheet P1. The same applies to sheets P2 and onward, and a description thereof will be omitted.

[0048] <Second double-sided transport control> Next, the second double-sided conveyance control will be explained using the flowcharts of FIGS. 5(a) and (c) and FIGS. 7(a) to (e). The second double-sided conveyance control is a control for conveying a sheet using the pair of double-sided conveyance rollers 52 and the pair of double-sided re-feed rollers 53. In S507 of FIG. 5(a), the printer control unit 101 determines that the second double-sided conveyance control has been selected, and in S530 the second double-sided conveyance control unit 150b executes the second double-sided conveyance control, ending the process. FIG. 5(c) is a flowchart showing the second double-sided conveyance control executed in S530 of FIG. 5(a). Note that FIG. 7 goes through the states of FIGS. 7(a) and (b) before reaching the state of FIG. 7(c). FIGS. 7(a) to (c) are similar to FIGS. 4(a) to (c), and therefore description thereof will be omitted.

[0049] The printer control unit 101 starts image formation on the second side of the sheet P1 when an image formation interval C (described later) has elapsed since the start of image formation on the first side of the sheet P2, and waits for the timing to re-feed the sheet P1 (S506 in FIG. 5A). When the timing to re-feed the sheet arrives, the printer control unit 101 determines by the aforementioned determination unit 151 that the second duplex conveyance control should be performed (S507 Y in FIG. 5A), and starts the second duplex conveyance control (S530 in FIG. 5A).

[0050] In S531, the second duplex conveying control unit 150b starts driving the duplex refeed roller pair 53 and starts driving the duplex conveying motor 60 in the forward direction at the print speed. Because the sheet P1 is refed by the two rollers, the duplex refeed roller pair 53 and the duplex conveying roller pair 52, the sheet is less likely to slip at the duplex refeed roller pair 53 than in the first duplex conveying control. In other words, the conveyance of the sheet P in the duplex conveying path 70 is "assisted" by the two rollers. In addition, the reversing roller pair 50 is also driven, so the sheet P2 is pulled into the duplex conveying path 70. Figure 7(d) is a diagram showing the state in which the sheet P1 is conveyed in the duplex conveying path 70 by the duplex refeed roller pair 53 and the duplex conveying roller pair 52.

[0051] In S532, the second double-sided conveying control unit 150b determines whether the leading edge of the sheet P1 has reached the registration sensor 25. If the second double-sided conveying control unit 150b determines in S532 that the leading edge of the sheet P1 has not reached the registration sensor 25, it returns the process to S532. If the second double-sided conveying control unit 150b determines that the leading edge of the sheet P1 has reached the registration sensor 25, it proceeds to S533. In S533, the second double-sided conveying control unit 150b stops the double-sided conveying motor 60, thereby stopping the double-sided conveying roller pair 52. In the first embodiment, the sheet P is gripped by the registration roller pair 16 when it reaches the registration sensor 25. Therefore, from this point on, the sheet P1 is conveyed by two rollers, the registration roller pair 16 and the double-sided refeed roller pair 53. FIG. 7(e) is a diagram showing a state in which the sheet P1 is conveyed by the registration roller pair 16 and the double-sided refeed roller pair 53 in preparation for image formation on the second side.

[0052] In S534, the second duplex conveying control unit 150b determines whether the trailing edge of sheet P1 has passed through the duplex refeed roller pair 53. If the second duplex conveying control unit 150b determines that the trailing edge of sheet P1 has not passed through the duplex refeed roller pair 53 in S534, it returns the process to S534. If the second duplex conveying control unit 150b determines that the trailing edge of sheet P1 has passed through the duplex refeed roller pair 53, it proceeds to S535. In S535, the second duplex conveying control unit 150b stops the duplex refeed roller pair 53 and ends the process. Thereafter, the printer control unit 101 performs image formation on the second side of sheet P1 and instructs the duplex flapper drive unit 112 to switch the duplex flapper 55 to guide sheet P1 in the ejection direction. At this time, the printer control unit 101 feeds sheet P3 at an image formation interval A (described later) from the timing when image formation on the second side of sheet P1 begins. Figure 7(f) shows a state similar to Figure 4(d). The above control is repeated for image formation on the first side of sheet P3 and subsequent processes.

[0053] (Timing chart of second double-sided transport control) Next, Fig. 6(b) shows a timing chart when the second double-sided conveying control is executed. (i) to (vi) are graphs similar to those in Fig. 6(a), and explanations of similar parts will be omitted. Also, T610 to T613 are similar to T600 to T603 in Fig. 6(a), and explanations will be omitted.

[0054] T614 is the timing for re-feeding paper P1, when the pair of double-sided paper re-feed rollers 53 is driven by S531 in Figure 5(c), and the pair of double-sided conveying rollers 52 is driven by the double-sided conveying motor 60 being driven at the print speed. In addition, the pair of reversing rollers 50 is driven, so that paper P2 is also conveyed to the double-sided conveying path 70.

[0055] At T615, sheet P1 reaches registration sensor 25, and duplex conveying motor 60 stops in accordance with S533 in FIG. 5C. This is also the timing when reversing roller pair 50 stops, halting the conveyance of sheet P2. At T616, the trailing edge of sheet P1 passes duplex refeed roller pair 53, and duplex refeed roller pair 53 stops in accordance with S535 in FIG. 5C, completing duplex conveyance control of sheet P1.

[0056] 6(b), during the section Ta (shaded section) during which the duplex conveying roller pair 52 is driven at the print speed from T614 to T616, the paper P1 is conveyed by two rollers, the duplex re-feed roller pair 53 and the duplex conveying roller pair 52. As a result, compared to the first duplex conveying control, the conveying force is increased by the amount that the duplex conveying roller pair 52 is driven, and the occurrence of jams due to slippage is reduced.

[0057] <Image formation interval determination unit> The control performed by the image formation interval determination unit 152 will be explained using FIG. 8. FIG. 8(a) is a diagram showing the image formation interval when first double-sided conveying control is executed in two-sheet circulation mode, and FIG. 8(b) is a diagram showing the image formation interval when second double-sided conveying control is executed in two-sheet circulation mode. FIG. 8(c) is a diagram showing the image formation interval in one-sheet circulation mode. The image formation interval determination unit 152 determines the image formation interval by selecting one from four image formation intervals, A, B, C, and D, which will be described later. The image formation intervals A, B, C, and D are design values ​​predetermined for each print speed and paper length in the conveying direction, and are selected from the intervals shown in FIG. 8(a), (b), and (c). Note that in FIG. 8, the "image formation interval" is referred to as "interval."

[0058] The image formation interval A is the same as the image formation interval during continuous single-sided printing, in which images are formed only on the first sides of multiple sheets P, and is a design value predetermined to achieve the target productivity. In the first embodiment, the image formation interval A for A4 size is set to, for example, 1 second. The image formation interval B is a design value predetermined for each printing speed so that the preceding and succeeding sheets can pass each other at the reversal point 201. In the duplex conveyance control of the first embodiment, the sheet P reversed by the reversal roller pair 50 is conveyed at a speed (accelerated drive speed) faster than the printing speed, thereby shortening the image formation interval B and improving productivity. The image formation interval B is longer than the image formation interval A (B > A). In the first embodiment, the image formation interval B = the image formation interval A × 2. In the example described above, the image formation interval B is 2 seconds (= 1 second × 2). In the first embodiment, as shown in FIGS. 8(a) and 8(b), the image formation interval B is also used for the image formation interval between the second sides of the last two sheets in the two-sheet circulation mode.

[0059] As shown in FIG. 8(c), image formation interval D is the interval between image formation on the first and second sides of the same sheet P in single-sheet circulation mode, and is determined by the length of the transport path within printer 100 and the time required for double-sided transport using double-sided transport control (hereinafter referred to as double-sided transport time). Image formation interval D is longer than image formation interval A (D>A). In Example 1, image formation interval D = image formation interval A x 2.5. In the example described above, image formation interval D is 2.5 seconds (= 1 second x 2.5). In Example 1, image formation interval D is longer than image formation interval B (D>B).

[0060] The image formation interval C is the image formation interval between the second side of the sheet P1 for which double-sided conveyance assist is performed and the first side of the previous sheet P2 when the second double-sided conveyance control is selected in the two-sheet circulation mode. The image formation interval C is designed such that at the time when the re-feeding of the sheet P1 is started in S531 of FIG. 5(c) of the second double-sided conveyance control, the trailing edge of the sheet P2 passes through the inversion point 201 and the double-sided conveyance motor 60 can be switched from the reverse rotation direction to the forward rotation direction. The image formation interval C is longer than the image formation interval A (C > A). In the first embodiment, the image formation interval C = the image formation interval A × 1.7. In the example described above, the image formation interval C is 1.7 seconds (= 1 second × 1.7). In the first embodiment, the image formation interval C is shorter than the image formation interval B and the image formation interval D (C < B, C < D). Incidentally, when the designed value of the image formation interval A is large and the trailing edge of the sheet P2 can pass through the inversion point 201 at the time when the re-feeding of the sheet P1 is started, there is no need to widen the image formation interval. For this reason, the image formation interval C = the image formation interval A can be set. Next, the productivity during double-sided printing for each double-sided conveyance control will be described using FIG. 8(d).

[0061] The sheet conveyance control unit 150 controls the image formation interval, which is the distance between the leading edge of the image formed on the first side of the subsequent sheet and the leading edge of the image formed on the second side of the preceding sheet, as follows. That is, the sheet conveyance control unit 150 controls the image formation interval (interval C) when performing the second double-sided conveyance control to be larger than the image formation interval (interval A) when performing the first double-sided conveyance control.

[0062] FIG. 8(d) is a graph with the number of printed sheets N [sheets] on the horizontal axis and the time required for printing that number of sheets (hereinafter referred to as the printing time) [seconds] on the vertical axis. The case of FIG. 8(a) is shown by a solid line, the case of (b) is shown by a dashed line, and the case of (c) is shown by a one-dot chain line. When using the first double-sided conveyance control in the two-sheet circulation mode of FIG. 8(a), the printing time is (2 × N - 3) × A + 2 × B. When using the second double-sided conveyance control in the two-sheet circulation mode of FIG. 8(b), the printing time is (N - 2) × A + (N - 1) × C + 2 × B. In the case of the one-sheet circulation mode of FIG. 8(c), the printing time is (N - 1) × A + N × D.

[0063] From FIG. 8(d), it can be seen that double-sided transport assist using the second double-sided transport control (b) in the two-sheet circulation mode shortens printing time and increases double-sided printing productivity, compared to double-sided transport assist in the single-sheet circulation mode (c). It can also be seen that the second double-sided transport control reduces double-sided printing productivity compared to the first double-sided transport control. In other words, when double-sided transport assist is not required, such as for plain paper, it is better to use the first double-sided transport control. Therefore, as explained in Table 1, the determination unit 151 determines whether to execute the second double-sided transport control, which performs assist control, depending on the paper type of paper P.

[0064] As described above, by applying the control of the first embodiment, if there is a risk of sheet slippage when stiff paper such as cardboard is re-fed on both sides, slippage can be prevented or reduced by supplementing the sheet conveying force with the pair of duplex conveying rollers 52. Furthermore, compared to the single-sheet circulation mode, double-sided printing productivity can be improved. In the first embodiment, control is employed to terminate conveyance assistance by the pair of duplex conveying rollers 52 when the leading edge of the sheet reaches the pair of registration rollers 16 during the second double-sided conveyance control. However, to improve conveyance performance, the pair of duplex conveying rollers 52 may be driven until the trailing edge of the sheet passes the pair of duplex conveying rollers 52. In this case, if the drive speed of the pair of duplex re-feed rollers 53 is changed, control is required to change the drive speed of the duplex conveyance motor 60 in synchronization.

[0065] Furthermore, in the first embodiment, the paper type information of the paper P uses information specified by the user, but instead, a paper type discrimination means may be added to the printer 100 and the detection results may be used. Also, in the first embodiment, the setting of the paper type information and the setting of whether or not to perform double-sided assist control are performed on the host computer 103, but instead, a control panel that can be operated by the user may be added to the printer 100 and settings may be made from the control panel. Note that in the first embodiment, a color laser beam printer has been used for explanation, but the present invention is not limited to this type of printer.

[0066] As described above, according to the first embodiment, it is possible to reduce slippage of paper that occurs in the double-sided conveying path while suppressing a decrease in productivity during double-sided printing. [Example]

[0067] In the second embodiment, a method will be described in which the determination unit 151 determines whether to perform double-sided conveyance assist control depending on the degree of wear and the re-feed registration arrival time. Here, the degree of wear is the degree of wear of a component that contributes to double-sided re-feeding (hereinafter referred to as the double-sided re-feed unit). The re-feed registration arrival time is the time from when the pair of double-sided re-feed rollers 53 starts to be driven until the leading edge of the sheet P reaches the registration sensor 25. The main parts are the same as in the first embodiment, and a description of the similar parts will be omitted, and only the parts that are different from the first embodiment will be described here.

[0068] <Duplex conveying assist control determination unit> The control of determining whether or not assistance is required, performed by the determination unit 151 in the second embodiment, will be described using the flowchart in Fig. 9. In the second embodiment, the processes other than S600 to S602, which are the process of determining whether or not assistance is required by the determination unit 151, are the same as those in the flowchart in Fig. 5 described in the first embodiment, and the same step numbers are used. The processes S600 to S602, which are different from those in the first embodiment, will be described. It is assumed that the printer control unit 101 has a counter. It is also assumed that the printer control unit 101 measures the refeed register arrival time using a timer (not shown) every time paper is re-fed, and stores the time in RAM 104b or the like.

[0069] When the printer control unit 101 determines in S506 that it is time to refeed the paper P1, it starts driving the duplex refeed roller pair 53 in S600 to refeed the paper P1 and counts up a counter that manages the number of refeeds (hereinafter referred to as the number of refeeds). The number of refeeds is cleared when it is detected that the duplex refeed unit, including the duplex refeed roller pair 53, has been replaced with a new one. For example, when a service technician replaces the duplex refeed unit with a new one, this is notified to the printer control unit 101 via the controller 102, and the printer control unit 101 resets the counter.

[0070] In S601, the printer control unit 101 determines whether the counted number of refeeds is less than a predetermined number E of refeeds. That is, the printer control unit 101 selects either the first duplex conveyance control or the second duplex conveyance control based on the number of refeeds. The number E of refeeds is preset according to the degree of wear of the duplex refeed roller pair 53, which is prone to slippage during duplex refeeds. In the second embodiment, the number E of refeeds is set to, for example, 100,000 times. If the printer control unit 101 determines in S601 that the counted number of refeeds is less than the predetermined number E, it determines that slippage during duplex refeeds is unlikely. The printer control unit 101 selects the first duplex conveyance control using the determination unit 151, and the process proceeds to S520. In this way, if the determination unit 151 determines that the number of refeeds is less than the predetermined number, the printer control unit 101 selects the first duplex conveyance control.

[0071] If the printer control unit 101 determines in S601 that the counted number of refeeds is equal to or greater than the predetermined number of refeeds E, the process proceeds to S602. In S602, the printer control unit 101 determines whether the refeed registration arrival time stored during the previous refeed is longer than the refeed registration arrival time F. That is, the printer control unit 101 determines whether assistance is required based on the slippage during the previous double-sided refeed. The refeed registration arrival time F is preset based on the distance along the conveyance path from the refeed standby point 202 to the registration sensor 25 (hereinafter referred to as the conveyance distance), the printing speed, the expected refed sheet variation time during normal operation, and other factors. Thus, when the number of refeeds reaches or exceeds the predetermined number, the determination unit 151 selects either the first duplex conveyance control or the second duplex conveyance control depending on the arrival time. The arrival time here refers to the time required for the leading edge of the sheet P to reach the registration sensor 25 after the start of refeeding during the previous refeed.

[0072] If the printer control unit 101 determines in S602 that the previous refeed register arrival time is longer than the predetermined refeed register arrival time F, it determines that slippage has occurred during double-sided refeeding, selects to perform the second double-sided conveyance control, and proceeds to S530. In this way, the printer control unit 101 selects the second double-sided conveyance control if the arrival time is longer than the predetermined time using the determination unit 151. In S602, the printer control unit 101 determines that the previous refeed register arrival time is shorter than the predetermined refeed register arrival time F, it determines that slippage has not occurred during double-sided refeeding, selects to perform the first double-sided conveyance control, and proceeds to S520. In this way, the printer control unit 101 selects the first double-sided conveyance control if the arrival time is shorter than the predetermined time using the determination unit 151, even if the number of refeeds has been performed is equal to or greater than the predetermined number.

[0073] As described above, in the second embodiment, the printer control unit 101 determines whether to perform duplex conveyance assist control depending on the wear level of the duplex refeed unit and the refeed register arrival time. This prevents or reduces slippage during duplex refeeding caused by wear of the duplex refeed unit, while improving duplex printing productivity compared to the single-sheet circulation mode.

[0074] Note that the embodiment is not limited to Example 2. In Example 2, whether or not to perform duplex conveyance assist control is determined based on the wear level of the duplex refeed unit and the refeed register arrival time, but the determination may be based on only one of them. Also, in Example 2, the wear level of the duplex refeed unit is determined based on the number of refeeds, but Example 2 can be similarly applied as long as the wear level of the duplex refeed unit can be determined, for example, based on the drive time of the duplex refeed roller pair 53.

[0075] In addition, in the second embodiment, a method for determining the slippage state during double-sided refeeding is described, based on the refeed register arrival time stored at the time of the previous refeed. For example, the second embodiment can be similarly applied as long as the slippage state during double-sided refeeding can be determined based on the refeed register arrival time not only at the previous time but also at the time of the previous several refeeds, or by determining the conveyance distance instead of the conveyance time.

[0076] As described above, according to the second embodiment, it is possible to reduce slippage of paper that occurs in the double-sided conveying path while suppressing a decrease in productivity during double-sided printing. [Example]

[0077] In the third embodiment, a method will be described in which the determination unit 151 determines whether to perform double-sided conveyance assist control depending on the history of the type of paper P conveyed by the double-sided paper re-feed unit. The main parts will be described in the same manner as in the first embodiment, and the description of the similar parts will be omitted, and only the parts different from the first embodiment will be described here.

[0078] <Duplex conveying assist control determination unit> The control of determining whether or not an assist is required, performed by the determination unit 151 in the third embodiment, will be described using the flowchart in FIG. 10. In the third embodiment, the processes other than the determination process S700 to S703 of whether or not an assist is required by the determination unit 151, are the same as those in the flowchart in FIG. 5 described in the first embodiment, and the same steps are assigned the same step numbers. Here, the processes S700 to S703, which are different from those in the first embodiment, will be described. Note that the printer control unit 101 is assumed to have a duplex refeed unit assist counter (hereinafter referred to as the assist counter) that increases or decreases depending on information about whether a sheet has been refeeded. Furthermore, the printer control unit 101 is assumed to measure the refeed register arrival time using a timer (not shown) each time assist control is performed, and to store the time in RAM 104b or the like. Furthermore, the printer control unit 101 is assumed to store information about whether first duplex conveying control or second duplex conveying control was performed during the previous refeed in, for example, RAM 104b.

[0079] When the time for refeeding arrives in S506, the printer control unit 101 starts driving the duplex refeed roller pair 53 in S700 to refeed the sheet P1 and updates the assist counter. The assist counter is used to predict the slippage status of the duplex refeed unit. In the third embodiment, the printer control unit 101 adds or subtracts from the assist counter according to the type of paper transported by the duplex refeed unit, in accordance with the table shown in Table 2, and updates the assist counter so that it does not become a negative value.

[0080] [Table 2]

[0081] In Table 2, the first column shows the paper type, and the second column shows the count value by which the assist counter is increased or decreased. For example, if the paper P is plain paper, the printer control unit 101 increments the assist counter by +1 (adds 1). Also, if the paper P is gloss paper 1 or gloss paper 2, the printer control unit 101 increments the assist counter by -5 (decrements 5). The determination unit 151 selects either the first duplex conveying control or the second duplex conveying control based on information about the paper type of the paper P that was previously re-fed.

[0082] The table in Table 2 is set in advance based on the amount of dirt on the duplex refeed roller pair 53 that occurs during duplex transport depending on the paper type. Dirt on the duplex refeed roller pair 53 can be caused by calcium carbonate, which is used as a paper tenon to improve the shelf life of paper P, and can cause slippage during duplex refeeding. In the third embodiment, the assist counter is updated in table format to predict the slippage status of the duplex refeed unit, but it can also be updated using a formula.

[0083] In S701, the printer control unit 101 determines whether second duplex conveyance control was performed during the previous duplex refeed. If the printer control unit 101 determines in S701 that first duplex conveyance control, not second duplex conveyance control, was performed during the previous duplex refeed, the process proceeds to S702. If the determination unit 151 determines that second duplex conveyance control was not selected during the previous refeed, the printer control unit 101 proceeds to S702 to select either first duplex conveyance control or second duplex conveyance control depending on the value of the assist counter. In S702, the printer control unit 101 determines whether the assist counter is smaller than a predetermined count value G. The count value G is preset depending on the amount of dirt on the duplex refeed roller pair 53, which makes slippage more likely during duplex refeeding, and in the third embodiment, the count value G is set to 100.

[0084] If the printer control unit 101 determines in S702 that the assist counter is smaller than a predetermined count value G, it determines that the possibility of slippage occurring during double-sided refeeding is low, selects to perform the first double-sided conveyance control, and proceeds to S520. In this way, the printer control unit 101 selects the first double-sided conveyance control if the assist counter is less than the first predetermined value by the determination unit 151. In S702, the printer control unit 101 determines that the assist counter is equal to or greater than the predetermined count value G, determines that the possibility of slippage occurring during double-sided refeeding is high, selects to perform the second double-sided conveyance control, and proceeds to S530. In this way, the printer control unit 101 selects the second double-sided conveyance control if the assist counter is equal to or greater than the first predetermined value by the determination unit 151.

[0085] If the printer control unit 101 determines in S701 that second duplex conveying control was performed instead of first duplex conveying control during the previous duplex refeed, the process proceeds to S703. In S703, the printer control unit 101 determines whether the assist counter is greater than zero. If the printer control unit 101 determines in S703 that the assist counter is greater than zero, it determines that slippage may occur during duplex refeed, selects to perform second duplex conveying control, and proceeds to S530.

[0086] If the printer control unit 101 determines in S703 that the assist counter is zero or less, it determines that the possibility of slippage during double-sided refeeding is low, selects to perform the first double-sided conveying control, and proceeds to S520. In this way, the printer control unit 101 selects the first double-sided conveying control if the determination unit 151 selected the second double-sided conveying control during the previous refeed and the assist counter is equal to or less than the second predetermined value. The printer control unit 101 selects the second double-sided conveying control if the determination unit 151 determines that the assist counter is greater than the second predetermined value. The second predetermined value (= 0) is smaller than the first predetermined value (= 100) (the second predetermined value < the first predetermined value). The reason for determining whether double-sided conveying assistance is required depending on whether the assist counter is zero in the third embodiment is as follows: Once a situation arises in which slippage during double-sided refeeding is likely to occur, the second double-sided conveying control is performed until the possibility of slippage during double-sided refeeding becomes low.

[0087] As described above, according to the third embodiment, whether to perform duplex conveyance assist control is determined based on the history of the type of paper conveyed by the duplex paper refeed unit. This prevents or reduces slippage during duplex refeeding caused by contamination of the duplex paper refeed unit, while improving productivity of duplex printing compared to the single-sheet circulation mode.

[0088] Note that the embodiment is not limited to Example 3. For example, Example 3 has been described as a method for predicting dirt in the double-sided paper refeed unit using an assist counter. However, Example 3 can be similarly applied if it is possible to determine dirt in the double-sided paper refeed unit that may cause slippage during double-sided paper refeeding, for example, by determining the distance paper is transported by the pair of double-sided paper refeed rollers 53.

[0089] As described above, according to the third embodiment, it is possible to reduce slippage of paper that occurs in the double-sided conveying path while suppressing a decrease in productivity during double-sided printing. [Explanation of symbols]

[0090] 52 Double-sided conveying roller pair 53 Double-sided refeed roller pair 70 Double-sided transport path 101 Printer control unit 150 Paper transport control unit 151 Double-sided conveyance assist control decision unit

Claims

1. An image forming apparatus capable of double-sided printing in one of a first mode in which an image is formed on a first side and a second side opposite to the first side of a preceding sheet that is transported in advance, and then an image is formed on the first side of a succeeding sheet that is transported following the preceding sheet, and a second mode in which an image is formed on the first side of the succeeding sheet between the image formation on the first side of the preceding sheet and the image formation on the second side of the preceding sheet, a transfer unit that transfers the toner image onto paper; a fixing unit that fixes the unfixed toner image transferred by the transfer unit; a double-sided conveying path along which the paper is conveyed so that a toner image can be transferred to a second side of the paper after an image has been formed on the first side and the paper has passed through the fixing unit; a first conveying means for conveying the paper in the double-sided conveying path; a second conveying means for re-feeding the paper conveyed by the first conveying means toward the transfer unit; a selection unit that selects, when the double-sided printing is performed in the second mode, either a first double-sided transport control in which the second transport unit transports the paper, or a second double-sided transport control in which the first transport unit and the second transport unit transport the paper; a control unit that executes the double-sided conveyance control selected by the selection unit; Equipped with In the second mode, the control means controls an image forming interval, which is a distance between the leading edge of an image formed on the first side of a succeeding sheet and the leading edge of an image formed on the second side of a preceding sheet, as follows: The image forming apparatus is characterized in that the image formation interval when the second double-sided conveyance control is performed is controlled to be larger than the image formation interval when the first double-sided conveyance control is performed.

2. 2. The image forming apparatus according to claim 1, wherein the selection unit selects either the first double-sided conveying control or the second double-sided conveying control in accordance with information about the type of paper.

3. a designation unit for designating an assist control for preventing a paper sheet from being transported improperly in the double-sided transport path; 3. The image forming apparatus according to claim 2, wherein the selection means selects the second double-sided conveying control when the assist control is specified by the specification means, and selects the first double-sided conveying control when the assist control is not specified by the specification means.

4. The image forming apparatus according to claim 3, characterized in that the selection means selects the first double-sided transport control when the paper is a first paper, regardless of whether the assist control is specified by the specification means, and selects the second double-sided transport control when the paper is a second paper that is stiffer than the first paper.

5. 2. The image forming apparatus according to claim 1, wherein the selection unit selects either the first double-sided conveying control or the second double-sided conveying control based on the number of times the paper has been re-fed.

6. 6. The image forming apparatus according to claim 5, wherein the selection unit selects the first double-sided conveyance control when the number of times the paper has been re-fed is less than a predetermined number of times.

7. a conveyance path along which a sheet of paper having a toner image transferred onto a first surface thereof is conveyed from a paper feed unit to the transfer unit; a detection unit provided downstream in a sheet transport direction from a position where the sheet transported by the re-feeding merges with the transport path and upstream from the transfer unit; Equipped with The image forming apparatus according to claim 6, characterized in that, when the number of times the paper has been re-fed reaches or exceeds the predetermined number, the selection means selects either the first double-sided transport control or the second double-sided transport control depending on the time it took for the leading edge of the paper to reach the detection means from the start of the paper re-fed the previous time the paper was re-fed.

8. The image forming apparatus according to claim 7, characterized in that the selection means selects the first double-sided conveying control when the number of times the paper has been re-fed is equal to or greater than the predetermined number and the arrival time is equal to or less than the predetermined time, and selects the second double-sided conveying control when the arrival time is longer than the predetermined time.

9. 2. The image forming apparatus according to claim 1, wherein the selection unit selects either the first double-sided conveying control or the second double-sided conveying control based on information regarding the paper type of the paper that was previously re-fed.

10. a counter that increases or decreases depending on the information that the paper has been re-fed; 10. The image forming apparatus according to claim 9, wherein the selection unit selects the first double-sided conveying control when the counter is less than a first predetermined value, and selects the second double-sided conveying control when the counter is equal to or greater than the first predetermined value.

11. 11. The image forming apparatus according to claim 10, wherein the selection means selects either the first double-sided conveying control or the second double-sided conveying control depending on the value of the counter if the second double-sided conveying control was not selected during the previous re-feed.

12. The image forming apparatus according to claim 11, characterized in that the selection means selects the first double-sided conveying control when the second double-sided conveying control was selected during the previous re-feed and when the counter is equal to or less than a second predetermined value that is smaller than the first predetermined value, and selects the second double-sided conveying control when the counter is greater than the second predetermined value.

Images