Image forming device

Abstract

To prevent a reduction in quality when forming images continuously on a plurality of media.SOLUTION: When an image forming apparatus 1 prints images continuously on a plurality of sheets P, if a set sheet length of the sheet P is longer than a slack limit sheet length LD based on a set sheet thickness of the sheet P, the image forming apparatus sets an inter-sheet distance D (n) as an extended distance Lst, and if otherwise, set as a reduced distance Lex. With this configuration, even when slack of the sheet P is eliminated in a passage part 25 due to the length or the thickness of the sheet P, the image forming apparatus 1 can appropriately eliminate the possibility that color shift occurs on the subsequent sheet P and reduces, when possible, the inter-sheet distance D (n) as much as possible, thereby can increase throughput.SELECTED DRAWING: Figure 12

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus, and is suitably applied to, for example, an electrophotographic printer. [Background technology]

[0002] In recent years, there have been image forming devices that print images by, for example, generating a toner image based on image data using toner in an image forming unit, transferring the toner image to paper (also called a medium) while transporting the paper in a transfer unit, and fixing the toner image to the paper by applying heat and pressure in a fixing unit. In these image forming devices, various parts such as the transfer unit, image forming unit, and fixing unit are arranged along the paper transport path, and various processes such as transfer and fixing of the toner image are sequentially performed on the paper transported in the transport direction.

[0003] In such image forming devices, it is required to maximize the number of printable sheets per unit time (e.g., one minute), but the driving force of the motor that transports the sheets is often limited due to constraints such as size and cost. Therefore, some image forming devices have been proposed that control the spacing between sheets to narrow when, for example, printing images continuously on multiple sheets of paper (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2013-44937 A (Fig. 4, etc.) Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the above-mentioned image forming apparatus, the paper conveying speeds in each section may not be completely consistent and may differ from each other. For example, in the image forming apparatus, the paper conveying speed may be faster (faster) in the fixing section on the downstream side of the paper conveying path than in the transfer section on the upstream side.

[0006] In this case, the image forming device attempts to transport the paper at a relatively high speed in the downstream fixing section, so depending on the length of the paper along the transport direction, part of the paper may remain in the transfer section, and friction between the paper and the belts, rollers, etc. that make up the transfer section may cause fluctuations in the transport speed of the paper in the transfer section.

[0007] This can cause problems in the image forming device, for example, when a toner image is being transferred to a subsequent sheet of paper in the transfer section, such as the transfer position of the toner image onto the subsequent sheet of paper being shifted, or the toner image expanding or contracting in the transport direction due to a change in the transport speed of the paper in the transfer section, which can degrade the quality of the image printed on the paper.

[0008] The present invention has been made in consideration of the above points, and aims to propose an image forming apparatus capable of preventing deterioration in quality when images are formed continuously on a plurality of media. [Means for solving the problem]

[0009] In order to solve such problems, the image forming apparatus of the present invention is provided with a transfer section including a plurality of image forming units arranged in parallel along the transport direction of the recording medium and forming an image on the recording medium, a belt body that transports the recording medium, and a transfer body that forms a transfer nip location for each of the plurality of image forming units, and a control section that controls the transport of the recording medium so that when the length of the recording medium along the transport direction is equal to or longer than a predetermined limit paper length, the inter-media distance from the end in the transport direction of the preceding recording medium, which is the recording medium transported first, to the start in the transport direction of the subsequent recording medium, which is the recording medium transported next, is equal to or longer than the transfer nip distance, which is the distance from the most upstream transfer nip location, which is the transfer nip location located most upstream in the media transport direction of the plurality of image forming units, to the most downstream transfer nip location, which is the transfer nip location located most downstream.

[0010] In addition, the image forming apparatus of the present invention is provided with a transfer section having a plurality of image forming units arranged in parallel along the transport direction of the recording medium and forming an image on the recording medium, a belt body for transporting the recording medium, and a tensioning body arranged respectively upstream and downstream of the image forming units in terms of the transport direction and tensioning the belt body and transporting the recording medium, and a control section for controlling the transport of the recording medium so that when the length of the recording medium along the transport direction is equal to or greater than a predetermined threshold value, the inter-media distance from the end in the transport direction of the preceding recording medium, which is the recording medium transported first, to the start in the transport direction of the succeeding recording medium, which is the recording medium transported next, is equal to or greater than the distance from the upstream tensioning body to the downstream tensioning body.

[0012] The present invention controls the conveyance of a recording medium so that the inter-medium distance is equal to or greater than the transfer nip distance if the length of the recording medium along the conveyance direction is equal to or greater than the critical paper length. Therefore, even if the traveling speed of the belt temporarily fluctuates due to the influence of a preceding recording medium, the present invention can allow the succeeding recording medium to reach the belt after the preceding recording medium leaves the belt and returns to its original traveling speed, thereby preventing deterioration of the image transferred from the image forming unit to the recording medium. Effect of the Invention

[0013] According to the present invention, it is possible to realize an image forming apparatus capable of preventing deterioration in quality when images are formed continuously on a plurality of media. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram illustrating an overall configuration of an image forming apparatus. [Diagram 2] 2 is a schematic diagram illustrating the configuration of a transfer unit, an image forming unit, a transfer unit, and a fixing unit. [Diagram 3] FIG. 2 is a block diagram showing a circuit configuration of the image forming apparatus. [Figure 4] 1 is a schematic diagram showing how a thin and short sheet of paper is conveyed. [Diagram 5] 1 is a schematic diagram showing how a thin and short sheet of paper is conveyed. [Figure 6] 1 is a schematic diagram showing how a single thin and long sheet of paper is conveyed. [Figure 7] 1 is a schematic diagram showing a state in which a plurality of thin and short sheets of paper are continuously transported; [Figure 8] 1 is a schematic diagram showing a state in which a plurality of thin and long sheets of paper are continuously transported; [Figure 9] 1 is a schematic diagram showing a state in which a plurality of thin and long sheets of paper are continuously transported; [Figure 10] 1 is a schematic diagram showing a state in which a plurality of thick sheets of paper are continuously transported; [Figure 11] 4 is a schematic diagram illustrating a configuration of a paper condition table. [Figure 12] 13 is a flowchart showing a paper interval setting process procedure. [Figure 13] 11 is a schematic diagram showing a state in which a plurality of thin and long sheets of paper are continuously transported when the paper spacing is set as an extension distance. [Figure 14] 13 is a schematic diagram illustrating a configuration of a transfer nip portion according to another embodiment. FIG. [Figure 15] 13 is a schematic diagram showing contact between a sheet and a transfer belt according to another embodiment. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, modes for carrying out the invention (hereinafter referred to as embodiments) will be described with reference to the drawings.

[0016] [1. Configuration of image forming device] 1, the image forming apparatus 1 is an electrophotographic printer that can form (i.e., print) a color image on a medium, such as paper P. Incidentally, the image forming apparatus 1 does not have an image scanner function for reading an original document or a communication function using a telephone line, and is a single-function SFP (Single Function Printer) that has only a printer function.

[0017] Various parts are arranged inside a housing 2 formed in a substantially box shape in the image forming apparatus 1. In the following description, the right end portion in Fig. 1 is regarded as the front of the image forming apparatus 1, and the up-down direction, left-right direction, and front-rear direction are defined when viewed from the front.

[0018] The image forming apparatus 1 is controlled overall by a control unit 3 (described in detail later). The control unit 3 is connected to a host device such as a computer device (not shown), and when it receives a print instruction or print data from the host device, it executes an image forming process (also called a print process) for forming a print image on the surface of a paper P.

[0019] An operation panel 4 that displays various information and accepts operational inputs is provided near the front of the top surface of the housing 2. The operation panel 4 has a touch panel that combines a display panel such as a liquid crystal panel with a touch sensor, LEDs (Light Emitting Diodes), etc., and displays various information under the control of the control unit 3 and accepts operational inputs from the user.

[0020] A paper cassette 5 that stores paper P is provided at the bottom inside the housing 2. A paper feed conveying section 6 is provided above and in front of the paper cassette 5. The paper feed conveying section 6 forms a paper feed conveying path W1, which is a route for conveying the paper P, by conveying guides 7 that face each other at a predetermined distance. In the paper feed conveying section 6, a hopping roller 8, a hopping sensor 9, a conveying roller pair 10, a paper sensor 11, a conveying roller pair 12, a writing sensor 13, and the like are sequentially arranged along the paper feed conveying path W1.

[0021] Among these, the hopping roller 8 and each roller constituting the pair of conveying rollers 10 and 12 are formed in a cylindrical shape with a central axis aligned in the left-right direction, and are each supported so as to be rotatable. In addition, a driving force is transmitted to some of the rollers from a paper feed motor (not shown).

[0022] The hopping sensor 9 is, for example, an optical sensor, and has a light emitting section that emits a predetermined detection light and a light receiving section that receives the detection light, and the optical path of the detection light crosses the paper feed conveyance path W1. Hereinafter, the point where the optical path of the detection light crosses the paper feed conveyance path W1 is also called the detection point. The hopping sensor 9 generates a detection signal that indicates the amount of detection light received by the light receiving section, and notifies the control unit 3 of this detection signal. In other words, the hopping sensor 9 can detect the presence or absence of paper P at the detection point based on the amount of detection light received. Hereinafter, a state in which the detection light is blocked by the paper P, that is, a state in which the presence of the paper P is detected, is called an ON state, and a state in which the detection light is not blocked by the paper P, that is, a state in which the presence of the paper P is not detected, is called an OFF state. The paper sensor 11 and the writing sensor 13 are both configured in the same manner as the hopping sensor 9, and can detect the presence or absence of paper P at their respective detection points.

[0023] The control unit 3 can determine the presence or absence of paper P at the detection point based on the detection signal obtained from the hopping sensor 9, etc., and can also, for example, measure the passing time required for the paper P to pass through the detection point, and calculate the length of the paper P along the conveying direction using this passing time and the conveying speed of the paper P.

[0024] The paper feed conveying unit 6 rotates each roller appropriately based on the control of the control unit 3, thereby picking up and separating the paper P stored in the paper cassette 5 one sheet at a time, and while guiding it with the conveying guide 7, moves it along the paper feed conveying path W1 by the conveying roller pairs 10 and 12, and sends it further rearward.

[0025] Behind the pair of transport rollers 12 in the paper feed transport section 6, a transfer section 15 is arranged below and an image forming section 22 is arranged above it, and a linear transfer transport path W2 extending rearward is formed between the two.

[0026] As shown in Fig. 2(A), which is an enlarged view of a portion of Fig. 1, the transfer unit 15 is configured such that a transfer belt 18 is tensioned around a drive roller 16 arranged at the rear side and a tension roller 17 arranged at the front side. The drive roller 16 and the tension roller 17 as tensioning bodies are formed in a cylindrical shape with their respective central axes 16X and 17X aligned in the left-right direction, and are supported rotatably. The transfer belt 18 as a belt body is configured as a so-called endless belt made of flexible resin or the like. The transfer belt 18 has a relatively smooth surface and is designed to generate sufficient frictional force between the transfer belt 18 and the paper P.

[0027] The upper portion of the transfer belt 18, i.e., the portion stretched between the drive roller upper end 16T, which is the upper end of the drive roller 16, and the tension roller upper end 17T, which is the upper end of the tension roller 17 (hereinafter referred to as the transfer belt upper stretching portion 18U), abuts against the transfer conveying path W2. For example, when a virtual straight line is provided from the rotation center of the drive roller 16 in a direction perpendicular to the transfer conveying path W2, the drive roller upper end 16T corresponds to the intersection of the straight line and the transfer belt 18. The same is true for the tension roller upper end 17T.

[0028] Transfer rollers 19 (19K, 19Y, 19M, and 19C) are provided at four locations spaced apart in the front-rear direction between the drive roller 16 and the tension roller 17 in the transfer unit 15. The transfer rollers 19 as transfer bodies are made of, for example, conductive rubber, formed in a cylindrical shape with a central axis aligned in the left-right direction, and supported rotatably.

[0029] When a driving force is transmitted from a belt drive motor (not shown) to the drive roller 16, the transfer unit 15 rotates the drive roller 16 in the direction of arrow R2, which in turn advances the transfer belt upper stretching portion 18U backward, thereby running the transfer belt 18. When the paper P is delivered from the paper feed transport unit 6, the transfer unit 15 advances the paper P backward, i.e., in the direction along the transfer transport path W2 (hereinafter, this is also referred to as the transport direction or medium transport direction), in conjunction with the travel of the transfer belt 18, with the paper P placed on the upper side of the transfer belt 18.

[0030] Four image forming units 23 (23K, 23C, 23M, and 23Y) are arranged in a line in the front-rear direction above the transfer section 15. Although each image forming unit 23 corresponds to a respective color of black (K), yellow (Y), magenta (M), and cyan (C), they are all configured in the same way except for the color. Hereinafter, the four image forming units 23 are collectively referred to as the image forming section 22.

[0031] The image forming unit 23 has a toner storage section that stores toner as a developer, a plurality of rollers, photoconductor drums 24 (24K, 24Y, 24M, and 24C), an LED head (not shown), etc. Of these, each roller and photoconductor drum 24 is configured to be columnar or cylindrical with a central axis aligned in the left-right direction, and is configured to be rotatable. The photoconductor drum 24 is also called an image drum (ID), and a charging layer made of a chargeable material is formed on its peripheral side surface.

[0032] In addition, the image forming unit 23 has each photoconductor drum 24 positioned directly above each transfer roller 19, and the image forming unit 23 biases each photoconductor drum 24 toward each transfer roller 19 due to its own weight or the like. Therefore, each photoconductor drum 24 and each transfer roller 19 form a transfer nip portion N15 (N15K, N15Y, N15M, and N15C) that sandwiches the transfer belt upper stretching portion 18U from above and below. Each transfer nip portion N15 has an extremely short length in the front-rear direction, and is represented as a dot in FIG. 2. For convenience of explanation, hereinafter, the transfer nip portion N15K located most upstream in the conveying direction of the paper P will also be referred to as the most upstream transfer nip portion N15K or the most upstream nip portion, and the transfer nip portion N15C located most downstream will also be referred to as the most downstream transfer nip portion N15C or the most downstream nip portion.

[0033] Therefore, when at least a part of the paper P is sandwiched by any one of the transfer nip points N15, that is, when at least a part of the paper P is located between the most upstream transfer nip point N15K and the most downstream transfer nip point N15C, the transfer section 15 hardly causes the paper P to slip relative to the transfer belt 18. At this time, the transfer section 15 can advance the paper P and the transfer belt 18 along the transfer conveying path W2 at substantially the same speed. For convenience of explanation, hereinafter, the area between the most upstream transfer nip point N15K and the most downstream transfer nip point N15C is also referred to as the transfer nip section.

[0034] This image forming unit 23 applies a predetermined high voltage to each roller, and receives a driving force from an ID motor (described later) via a predetermined gear, thereby appropriately rotating each roller and rotating the photoconductor drum 24 in the direction of arrow R1 (clockwise in the figure). Image data is also supplied to the LED head from the control unit 3.

[0035] The image forming unit 23 charges the photoconductor drum 24, exposes the photoconductor drum 24 by emitting light from an LED head in a pattern according to image data, and then adheres toner to develop a toner image on the photoconductor drum 24. For convenience of explanation, hereinafter, the toner is also referred to as a developer, and the toner image is also referred to as a developer image.

[0036] The transfer unit 15 rotates the drive roller 16 in the direction of the arrow R2 to run the transfer belt 18 and move the transfer belt upper stretching unit 18U backward. The transfer unit 15 also applies a predetermined high voltage to each transfer roller 19 and rotates them in the direction of the arrow R2.

[0037] When the transfer unit 15 conveys the paper P along the transfer conveying path W2, the transfer belt 18 and the paper P are sandwiched between the photosensitive drum 24 and the transfer roller 19 (i.e., the transfer nip location N15), and the toner image is transferred from the photosensitive drum 24 to the paper P. The transfer unit 15 can also transfer the toner image of each color from the photosensitive drum 24 to the paper P so that they are sequentially superimposed on top of each other at the transfer nip location N15 for each color.

[0038] A cleaning blade 20 and a toner box 21 are provided below the lower portion of the transfer belt 18 in the transfer unit 15. The cleaning blade 20 scrapes off and cleans the toner adhering to the surface of the transfer belt 18 when a transport failure of the paper P occurs during the image forming process. This makes it possible to prevent so-called show-through, in which toner adheres to and stains the back side of the paper P to be transported next, i.e., the side facing downward in the transport path W and to which the toner image is not transferred. The toner box 21 stores the toner scraped off by the cleaning blade 20.

[0039] A transition section 25 is disposed behind the transfer section 15. A curved transport guide 26 is provided in the transition section 25 at a position below an imaginary straight line drawn when the transfer transport path W2 is extended rearward. The curved transport guide 26 is curved in an arc shape that protrudes downward when viewed from the left-right direction, and the upper surface is formed as a smooth curved surface.

[0040] When the paper P is conveyed from the transfer section 15, the transfer section 25 guides the paper P so that it proceeds generally in the rear direction while restricting the downward movement range of the paper P with the curved conveying guide 26. Here, when the paper P is relatively thin and has a relatively low rigidity, the transfer section 25 advances the paper P along the curved curved transfer conveying path W3C while curving the paper P along the upper surface of the curved conveying guide 26, i.e., while forming a slack. When the paper P is relatively thick and has a relatively high rigidity, the transfer section 25 advances the paper P along the straight transfer conveying path W3L, which is a straight line that is an extension of the transfer conveying path W2 to the rear, without curving the paper P much. Hereinafter, the transfer section 25 is also referred to as a slack forming section.

[0041] In this way, in the transfer section 25, the path through which the paper P passes can be changed to a curved transfer path W3C, a straight transfer path W3L, or a path shaped between the two (hereinafter, these will be collectively referred to as the transfer path W3) depending on the thickness and rigidity of the paper P, or the tension applied to the paper P, etc. (details will be described later).

[0042] A fixing section 27 is disposed behind the transfer section 25. The fixing section 27 includes a heating belt 28 and a backup roller 29. The heating belt 28 is an endless belt that surrounds a heater, rollers, etc. (not shown), and is adapted to run in a circular shape when a driving force is supplied from a fixing motor (not shown). The backup roller 29 is located toward the rear of the lower side of the heating belt 28, is cylindrical with its central axis aligned in the left-right direction, and is supported rotatably. The backup roller 29 is biased toward the heating belt 28, and forms a fixing nip N27 at a location where it comes into contact with the heating belt 28.

[0043] When the fixing unit 27 operates under the control of the control unit 3, it heats an internal heater (not shown) and rotates the heating belt 28, thereby rotating the backup roller 29. When the fixing unit 27 operates in this manner and a sheet of paper P is transported from the forward transfer unit 25, the fixing unit 27 applies heat and pressure by clamping the sheet of paper P at the fixing nip location N27, thereby fixing the toner image to the sheet of paper P. In addition, since the heating belt 28 rotates, the fixing unit 27 can transport the sheet of paper P in an upward and rearward direction.

[0044] A paper discharge conveyance section 30 is disposed above and behind the fixing section 27 (FIG. 1). The paper discharge conveyance section 30 has a configuration similar to a part of the paper feed conveyance section 6, and forms a paper discharge conveyance path W4, which is a route for conveying the paper P, by conveyance guides 31 opposed to each other at a predetermined interval. In the paper discharge conveyance section 30, a discharge sensor 32, a conveyance roller pair 33, a discharge port 34, etc. are disposed in this order along the paper discharge conveyance path W4.

[0045] The paper discharge conveying section 30 rotates the pair of conveying rollers 33 according to the control of the control section 3, thereby conveying the paper P received from the fixing section 27 along the paper discharge conveying path W4 and discharging it from the discharge outlet 34, thereby placing it on the discharge tray 35 formed on the upper surface of the housing 2.

[0046] In this manner, the image forming apparatus 1 forms (i.e. prints) an image by transferring a toner image formed by the image forming unit 22 onto the paper P while sequentially transporting the paper P along the paper feed path W1, the transfer path W2, the transition path W3, and the discharge path W4, and fixing the toner image by the fixing unit 27. Hereinafter, the paper feed path W1, the transfer path W2, the transition path W3, and the discharge path W4 are collectively referred to as the paths W.

[0047] Next, the circuit configuration of the image forming apparatus 1 will be described with reference to Fig. 3. The circuit of the image forming apparatus 1 is mainly configured with a control unit 3. This control unit 3 is configured with a main control unit 41, a paper feed motor control unit 42, a belt motor control unit 43, an image drum (ID) motor control unit 44, a fixing motor control unit 45, etc.

[0048] The main control unit 41 has a CPU (Central Processing Unit) 51 that performs arithmetic processing, a ROM (Read Only Memory) 52 that stores various information and programs, a RAM (Random Access Memory) 53 that temporarily stores various information, and a timer counter 54 that measures time. Of these, the ROM 52 is, for example, a flash memory, and can store various information and retain the stored contents even when the power is cut off. This main control unit 41 can perform various processes while controlling each unit by reading various programs from the ROM 52 using the CPU 51 and executing them while using the RAM 53 as a work area.

[0049] To the main control unit 41 of the control unit 3, an interface unit 55 is connected in addition to the above-mentioned operation panel 4, hopping sensor 9, paper sensor 11, writing sensor 13, and discharge sensor 32.

[0050] The interface unit 55 is an interface for a wired LAN (Local Area Network) conforming to standards such as IEEE (Institute of Electrical and Electronics Engineers) 802.3 (IEEE802.3u / ab / an / ae) or a wireless LAN conforming to standards such as IEEE802.11 (IEEE802.11g / n / ac / ax), etc. This interface unit 55 is connected to a higher-level device (not shown) via a predetermined network and can receive print data and the like from the higher-level device.

[0051] Further, the paper feed motor control section 42, the belt motor control section 43, the image drum motor control section 44 and the fixing motor control section 45 of the control section 3 are connected to the paper feed motor 46, the belt motor 47, the image drum (ID) motor 48 and the fixing motor 49, respectively, and control the rotation of each of them. Among them, the paper feed motor 46 supplies a driving force to the conveying roller pairs 10 and 12 (FIG. 1) of the paper feed conveying section 6, etc. Furthermore, the belt motor 47 supplies a driving force to the drive roller 16 (FIGS. 1 and 2) of the transfer section 15, etc.

[0052] Incidentally, the image forming apparatus 1 allows the user to set information about the paper P stored in the paper cassette 5. For example, when the user performs a predetermined setting operation on the operation panel 4 and specifies values ​​related to the size and thickness of the paper P, the image forming apparatus 1 stores the values ​​in the ROM 52. In addition, when values ​​related to the size and thickness of the paper P are supplied from a higher-level device (not shown), the image forming apparatus 1 stores the values ​​in the ROM 52.

[0053] [2. Paper transport and spacing] Here, the transport of the paper P and the interval between the sheets of paper P in the portion from the transfer unit 15 through the transfer unit 25 to the fixing unit 27 in the image forming apparatus 1 will be described with reference to hypothetical states.

[0054] First, the length and intervals of each part in the transfer unit 15 and the image forming unit 22 are defined with reference to Fig. 2. Here, the interval between the tension roller upper end 17T and the transfer nip location N15K is length Lbk, the interval between the transfer nip locations N15K and N15Y is length Lky, the interval between the transfer nip locations N15Y and N15M is length Lym, and the interval between the transfer nip locations N15M and N15C is length Lmc. Also, the interval between the transfer nip location N15C and the drive roller upper end 16T is length Lcb.

[0055] Then, the length of the transfer nip section, that is, the transfer nip distance Lkc which is the distance between the most upstream transfer nip location N15K and the most downstream transfer nip location N15C, can be expressed by the following formula (1).

[0056] Lkc = Lky + Lym + Lmc …… (1)

[0057] Further, the roller distance Lr, which is the distance between the drive roller upper end 16T and the tension roller upper end 17T, corresponds to the distance between the central axes 16X and 17X, and can be expressed by the following formula (2).

[0058] Lr=Lbk+Lky+Lym+Lmc+Lcb ……(2)

[0059] Furthermore, in the image forming apparatus 1, the control unit 3 can adjust the distance between the sheets P when the sheets P are transported continuously. Here, the nth sheet P is referred to as sheet P(n), and the distance between the sheets P(n) and P(n+1) is referred to as the sheet-to-sheet distance D(n). For convenience of explanation, the sheet-to-sheet distance D(n) is also referred to as the medium-to-medium distance below. In addition, the preceding sheet P(n) is also referred to as the preceding recording medium, and the succeeding sheet P(n+1) is also referred to as the succeeding recording medium below. Furthermore, in the direction along the transfer transport path W2 (i.e., the backward direction or transport direction), the leading portion of the sheet P is also referred to as the leading portion in the transport direction, and the trailing portion of the sheet P is also referred to as the trailing portion in the transport direction.

[0060] In the image forming apparatus 1, when the interval between the sheets P is set to be relatively short, the sheet-to-sheet distance D(n) can be set to a predetermined shortened distance Lex as shown in Fig. 2(B). This shortened distance Lex is sufficiently short compared to the transfer nip distance Lkc.

[0061] However, in the image forming apparatus 1, heat is removed from the fixing unit 27 each time a sheet of paper P passes through, causing the temperature to drop, and so it becomes necessary to heat the fixing unit 27 with a heater (not shown) to raise the temperature to a predetermined level. In this case, if the shortened distance Lex is set to an excessively short value in the image forming apparatus 1, the amount of heat removed by the paper P per unit time increases, so it becomes necessary to increase the amount of heat generated by the heater, but there is a risk that the temperature inside the housing 2 will rise excessively, or that the temperature of the fixing unit 27 will not reach the predetermined temperature, causing fixing defects, etc. Therefore, in the image forming apparatus 1, the shortened distance Lex is set to a certain length without being excessively short.

[0062] Furthermore, in the image forming apparatus 1, when the interval between the sheets of paper P is set to be relatively long, the sheet-to-sheet distance D(n) can be set to a predetermined extension distance Lst, as shown in Fig. 2(C). This extension distance Lst is larger (longer) than the transfer nip distance Lkc, and when expressed by a formula (Lst=Lkc+α) using a constant α, for example, this constant α is 1 [mm].

[0063] In the image forming apparatus 1, when the inter-sheet distance D(n) is set to the shortened distance Lex, the number of sheets P transported per unit time can be increased compared to when the inter-sheet distance D(n) is set to the extended distance Lst, and the number of printed sheets per unit time (so-called throughput) can be increased. For this reason, the image forming apparatus 1 is basically configured so that the inter-sheet distance D(n) is set to the shortened distance Lex.

[0064] Incidentally, in the image forming apparatus 1, if the fixing conveying speed V27, which is the speed of the paper P in the fixing unit 27, and the transfer conveying speed V15, which is the speed of the paper P in the transfer unit 15, are made to match, the fixing conveying speed V27 may fluctuate due to errors in each section and thermal expansion of the backup roller 29, making adjustment and control extremely difficult. Also, in the image forming apparatus 1, if the fixing conveying speed V27 is made smaller (slower) than the transfer conveying speed V15, the amount of slack in a single sheet of paper P increases during conveyance between the transfer unit 15 and the fixing unit 27, causing the paper P to curve into various shapes, and eventually the top surface of the paper P may adhere to surrounding members, scattering the toner image before fixing.

[0065] Therefore, in the image forming apparatus 1, a transition section 25 is provided between the transfer section 15 and the fixing section 27, a curved transport guide 26 is arranged only on the lower side, and the fixing transport speed V27 is intentionally set to be larger (faster) than the transfer transport speed V15. Below, the manner in which the transfer process and the fixing process of the toner image are performed while transporting the paper P in the transfer section 15, the transition section 25, and the fixing section 27 of the image forming apparatus 1 is described for each type (thickness and length) and number of sheets of paper P.

[0066] [2-1. When printing on one thin, short sheet of paper] First, a case where an image is printed on a sheet of paper P that is relatively thin and has a relatively short length along the transport direction (hereinafter, also referred to as short thin paper) will be described. In this case, the image forming apparatus 1 transports the paper P along the transport path W by the transfer unit 15, and sequentially transfers toner images from each photoconductor drum 24 at each transfer nip location N15.

[0067] In this case, the leading edge of the paper P passes through each of the transfer nip points N15 of the transfer unit 15 and the image forming unit 22 in sequence, passes through the transfer unit 25, and reaches the fixing nip point N27 of the fixing unit 27. At this time, since the paper P has a relatively small rigidity, it sags due to the action of gravity, and as shown in FIG. 4, the paper P advances while curving along the curved transfer conveying path W3C in the transfer unit 25, and is eventually clamped at the fixing nip point N27.

[0068] Thereafter, the image forming apparatus 1 conveys the leading side of the paper P at the fixing conveying speed V27 in the fixing unit 27, while conveying the trailing side of the paper P at the transfer conveying speed V15 that is slower (lower) than the fixing conveying speed V27 in the transfer unit 15. In other words, an intentional speed difference is provided between the fixing conveying speed V27 and the transfer conveying speed V15 in the image forming apparatus 1. Therefore, in the image forming apparatus 1, the degree of slack formed in the paper P in the transfer section 25 gradually decreases, and the conveying path of the paper P in the transfer section 25 approaches the straight transfer conveying path W3L from the curved transfer conveying path W3C.

[0069] Eventually, in image forming apparatus 1, because sheet P is relatively short, the trailing edge of sheet P passes through the most downstream transfer nip location N15C with some slack remaining in transfer section 25, as shown in Fig. 5 which corresponds to Fig. 4. Therefore, in image forming apparatus 1, in transfer section 15, toner images can be stably transferred from photoconductor drum 24 of each image forming unit 23 while maintaining transfer conveying speed V15 without being affected by fixing conveying speed V27 from fixing section 27.

[0070] [2-2. Printing on a single, thin, long sheet of paper] Next, a case where an image is printed on a sheet of paper P that is relatively thin and has a relatively long length along the transport direction (hereinafter also referred to as long-thin paper) will be described. In this case, the image forming apparatus 1 transports the paper P along the transport path W by the transfer unit 15, as in the case of short-thin paper, and sequentially transfers toner images from each photoconductor drum 24 at each transfer nip location N15.

[0071] In this case, the leading edge of the paper P, like the case of short and thin paper, passes through the transfer section 15 and each transfer nip point N15 of the image forming section 22 in sequence, passes through the transfer section 25, and reaches the fixing nip point N27 of the fixing section 27. At this time, the paper P, like the case shown in Fig. 4, becomes slack due to the action of gravity, and advances while curving along the curved transfer conveying path W3C in the transfer section 25, and is eventually clamped at the fixing nip point N27.

[0072] Thereafter, in the image forming apparatus 1, due to the speed difference between the fixing conveying speed V27 of the fixing section 27 and the transfer conveying speed V15 of the transfer section 15, the degree of slack formed in the paper P at the transfer section 25 gradually decreases, and the conveying path for the paper P at the transfer section 25 approaches the straight transfer conveying path W3L from the curved transfer conveying path W3C.

[0073] 6, in the image forming apparatus 1, since the sheet P is relatively long, the trailing edge of the sheet P remains clamped by the most downstream transfer nip point N15C even after the slack in the sheet P is eliminated in the transfer section 25. That is, in the transfer section 25, the sheet P remains stretched between the most downstream transfer nip point N15C and the fixing nip point N27. In this case, the speed of the sheet P is the same in the transfer section 15, the transfer section 25, and the fixing section 27.

[0074] Here, the transport speed VP of the paper P is determined according to the balance of the force with which the fixing section 27 attempts to transport the paper P at the fixing nip location N27 at the fixing transport speed V27, the force with which the transfer section 15 attempts to transport the paper P at each transfer nip location N15 at the transfer transport speed V15, and the frictional force acting between the transfer belt 18 and the paper P, etc.

[0075] For this reason, in the image forming device 1, while there are a large number of transfer nip points N15 that clamp the paper P in the transfer section 15 and the contact area between the transfer belt 18 and the paper P is large, the transfer section 15 has a greater effect on the paper P than the fixing section 27, and the transport speed VP of the paper P becomes close to the transfer transport speed V15.

[0076] Thereafter, in image forming apparatus 1, the number of transfer nip points N15 that hold paper P in transfer unit 15 decreases, and as the contact area between transfer belt 18 and paper P also decreases, the proportion of the influence of transfer unit 15 on paper P gradually decreases and the influence of fixing unit 27 increases. As a result, in image forming apparatus 1, conveying speed VP of paper P gradually increases and approaches fixing conveying speed V27. Eventually, in image forming apparatus 1, by the time the trailing edge of paper P passes through the most downstream transfer nip point N15C, conveying speed VP of paper P becomes approximately the fixing conveying speed V27.

[0077] At this time, in the transfer section 15, the transport speed of the paper P changes gradually, so that the transfer position of the toner image onto the paper P hardly shifts, i.e., so-called color shift hardly occurs, and there is almost no degradation in the printed image quality.

[0078] [2-3. When printing continuously on multiple thin, short sheets of paper] Next, a case where images are continuously printed on a plurality of short thin sheets of paper will be described. It is assumed that the image forming apparatus 1 (FIG. 1) has a plurality of sheets of paper P stored in a stacked state in the paper cassette 5.

[0079] First, the image forming apparatus 1 (FIG. 1) rotates the hopping roller 8 etc. under the control of the main control unit 41 (FIG. 3) of the control unit 3, separates and sends out the first sheet of paper P(1) from the paper cassette 5, and causes the paper feed conveying unit 6 to convey the paper P(1) along the paper feed conveying path W1. Here, when the end of the paper P(1) passes by, the hopping sensor 9 (FIGS. 1 and 3) generates a detection signal indicating that it has detected an off state, i.e., that it has begun to receive detection light, and sends this to the main control unit 41 of the control unit 3.

[0080] In response to this, the control unit 3, when the main control unit 41 detects that the hopping sensor 9 has turned off based on the detection signal received from the hopping sensor 9 and that the end of the paper P(1) has passed the detection point, starts timing using the timer counter 54 (Figure 3) from the point at which the sensor switched to the off state (hereinafter referred to as the hopping passing point).

[0081] Next, the control unit 3 rotates the hopping roller 8 etc., via the main control unit 41, before the time equivalent to the inter-paper distance D(1) has elapsed from the time of hopping passage, separates and sends out the next paper P(2) from the paper cassette 5, and causes the paper feed conveyance unit 6 to convey the paper P(2) along the paper feed conveyance path W1. As a result, the distance between the paper P(1) and the paper P(2) on the paper feed conveyance path W1 is shorter than the original inter-paper distance D(1).

[0082] Eventually, paper P(1) is conveyed in the paper feed conveyance section 6 (FIG. 1) while being held by conveyance roller pair 10, passes the detection point of paper sensor 11, is further conveyed while being held by conveyance roller pair 12, and the tail of the paper passes the detection point of writing sensor 13. This causes writing sensor 13 to switch from an ON state to an OFF state. Hereinafter, this point in time will be referred to as the point of time when the preceding paper has passed.

[0083] Thereafter, when the control unit 3 detects, through the main control unit 41, based on the detection signal from the paper sensor 11 that the leading edge of the paper P(2) has reached the detection point of the paper sensor 11, it stops the rotation of the transport roller pairs 10 and 12 to stop the transport of the paper P(2). At this time, the leading edge of the paper P(2) protrudes rearward by a predetermined leading edge protrusion distance from the point where it is pinched by the transport roller pair 12. Next, the control unit 3 starts counting time with the timer counter 54 (FIG. 3) from the point when the writing sensor 13 turns off and detects the trailing edge of the paper P(1) (i.e., this is called the point of time when the preceding paper passes) through the main control unit 41.

[0084] Eventually, the control unit 3 causes the main control unit 41 to resume rotation of the pair of transport rollers 10 and 12 and send out the sheet P(2) when a time equivalent to the inter-sheet distance D(1) has elapsed since the preceding sheet passed (hereinafter also referred to as the transport time). As a result, in the image forming apparatus 1, as shown in FIG. 7, in the transfer transport path W2 downstream of the pair of transport rollers 12 on the transport path W, the distance between the sheets P(1) and P(2) can be adjusted to the inter-sheet distance D(1).

[0085] Here, the conveying time can be calculated using the distance from the conveying roller pair 12 to the writing sensor 13 (hereinafter referred to as the writing distance), the leading protrusion distance, the paper-to-paper distance D(1), and the running speed of the transfer belt 18 (i.e., the transfer conveying speed V15) using the formula (paper-to-paper distance D(1)-(writing distance-leading protrusion distance)) / (transfer conveying speed V15).

[0086] As an example, assume that the writing distance is 11 mm, the leading edge protrusion distance is 4 mm, the paper distance D(1) is 170 mm, and the transfer transport speed V15 is 160 mm / s. In this case, the transport time is calculated as (170-(11-4)) / 160=1019 ms according to the above formula.

[0087] [2-4. When printing continuously on multiple thin, long sheets of paper] Next, as a hypothetical example, a case will be described in which the paper-to-paper distance D(1) is set to a relatively short shortened distance Lex when images are continuously printed on multiple long and thin sheets of paper. This shortened distance Lex is sufficiently short compared to the transfer nip distance Lkc, as shown in FIG. 2B.

[0088] In this case, the image forming apparatus 1 sequentially transports each sheet of paper P while adjusting the spacing between sheets P(1) and P(2) to the inter-sheet distance D(1) using the paper feed transport section 6 (Figure 1), just as in the case of continuously printing on multiple sheets of long and thin paper.

[0089] First, in the image forming apparatus 1, similar to the case of printing an image on a single sheet of long and thin paper (FIG. 6), the toner image is sequentially transferred and fixed on the paper P(1) while the paper is transported through the transfer section 15, the transfer section 25, and the fixing section 27. At this time, in the image forming apparatus 1, as shown in FIG. 8, when the slack of the paper P(1) is eliminated in the transfer section 25, the transport speed VP of the paper P(1) gradually increases from the transfer transport speed V15 to approach the fixing transport speed V27.

[0090] As a result, in the transfer unit 15, the running speed of the transfer belt 18 transporting the paper P(1) also becomes the transport speed VP, gradually approaching the fixing transport speed V27 from the transfer transport speed V15. Also, in the image forming apparatus 1, the leading edge of the paper P(2) is located at a position that is the shortened distance Lex away from the trailing edge of the paper P(1) on the transfer transport path W2 of the transfer unit 15. Therefore, the transport speed VP of the paper P(2) also gradually increases from the transfer transport speed V15 and approaches the fixing transport speed V27.

[0091] 9, in the image forming apparatus 1, when the tail of the paper P(1) passes through the most downstream transfer nip location N15C, the effect of the conveying speed from the paper P(1) to the transfer unit 15 disappears, and the conveying speed in the transfer unit 15 immediately returns to the transfer conveying speed V15 from near the fixing conveying speed V27. Then, in the image forming apparatus 1, the conveying speed of the paper P(2) changes suddenly, and a phenomenon occurs in which the transfer position of the toner image on the paper P(2) at each transfer nip location N15 shifts from the original position (so-called color shift), resulting in a significant deterioration in image quality.

[0092] Therefore, in the actual image forming apparatus 1, when the length of the paper P in the transport direction is relatively large (long), the inter-paper distance D(n) is adjusted (described in detail later).

[0093] [2-5. When printing continuously on multiple sheets of thick paper] Next, as a hypothetical example, a case will be described in which, when images are continuously printed on a plurality of relatively thick sheets of paper P, that is, relatively rigid sheets, the inter-paper distance D(1) is set to a relatively short shortened distance Lex.

[0094] In this case, in the image forming apparatus 1, when the leading edge of the first sheet of paper P(1) is transported from the transfer section 15 through the transfer section 25 to the fixing section 27, as shown in FIG. 10, in the transfer section 25, it hardly bends due to gravity and proceeds in an almost straight line, i.e., along the straight transfer conveying path W3L or a conveying path close thereto.

[0095] Therefore, in the image forming apparatus 1, tension acts on the paper P(1) immediately after the leading edge of the paper P(1) reaches the fixing nip location N27 and is sandwiched, and the conveying speed of the paper P(1) gradually increases from the transfer conveying speed V15 to approach the fixing conveying speed V27. That is, in the image forming apparatus 1, the running speed of the transfer belt 18 conveying the paper P(1) in the transfer section 15 gradually increases from the transfer conveying speed V15 to approach the fixing conveying speed V27, similar to the case where the slack of the paper P is eliminated in the transfer section 25 while conveying a long thin paper (FIG. 7). Also, in the image forming apparatus 1, the leading edge of the paper P(2) is located at a position on the transfer conveying path W2 of the transfer section 15 that is separated by the shortened distance Lex from the trailing edge of the paper P(1). Therefore, the conveying speed of the paper P(2) also gradually increases from the transfer conveying speed V15 to approach the fixing conveying speed V27.

[0096] After that, in the image forming apparatus 1, the same phenomenon occurs as in the case of printing on multiple long and thin papers continuously (FIG. 9, etc.). That is, in the image forming apparatus 1, when the tail of the paper P(1) passes through the most downstream transfer nip point N15C of the transfer unit 15, the effect of the conveying speed from the paper P(1) to the transfer unit 15 disappears, and the conveying speed in the transfer unit 15 immediately returns from the fixing conveying speed V27 to the transfer conveying speed V15. Then, in the image forming apparatus 1, the conveying speed of the paper P(2) suddenly changes, and a phenomenon occurs in which the transfer position of the toner image on the paper P(2) at each transfer nip point N15 is shifted from the original position (so-called color shift), and image quality is significantly deteriorated.

[0097] Therefore, in the actual image forming apparatus 1, when the thickness of the paper P is relatively large (thick), the inter-paper distance D(n) is adjusted (this will be described in detail later).

[0098] [3. Adjusting the paper distance] Next, we will explain the adjustment of the distance D(n) between sheets of paper when the image forming apparatus 1 continuously prints images on multiple sheets of paper P. As described above, in the image forming apparatus 1, the fixing conveying speed V27 of the fixing section 27 is faster than the transfer conveying speed V15 of the transfer section 15. Therefore, when the sheet of paper P is relatively long and relatively thick, the slack of the sheet of paper P in the transfer section 25 becomes insufficient, causing the conveying speed of the sheet of paper P in the transfer section 15 to fluctuate, which may cause color misregistration.

[0099] In the case where the paper P is relatively long, in the image forming device 1, the paper P progresses along the curved transfer path W3C at the transfer section 25, and even if the amount of slack is sufficient, if the paper P is sufficiently long, there is a risk that the slack will be eliminated before the end of the paper P reaches the downstream transfer nip point N15C.

[0100] Generally, the higher the rigidity of the paper P, the less likely it is to sag, and the lower the rigidity, the more likely it is to sag. Furthermore, the rigidity of the paper P is generally proportional to its thickness. That is, in the image forming apparatus 1, when the paper P is relatively thick, the degree of sagging of the paper P formed in the transfer section 25 when the leading edge of the paper P reaches the fixing nip location N27 is an amount proportional to the rigidity of the paper P, i.e., an amount roughly proportional to the thickness of the paper P.

[0101] For this reason, in the image forming apparatus 1, when the thickness of the paper P is relatively small (thin) and the amount of slack in the paper P at the transfer section 25 is relatively large, it takes a relatively long time for the slack to be eliminated due to the speed difference between the transfer conveying speed V15 and the fixing conveying speed V27. That is, in the image forming apparatus 1, for a relatively thin paper P, even if it is relatively long, the state in which the slack is formed in the paper P at the transfer section 25 can be maintained, and color shift does not occur.

[0102] On the other hand, in the image forming apparatus 1, when the thickness of the sheet P is relatively large (thick) and the amount of slack in the sheet P at the transfer section 25 is relatively small, the slack is eliminated in a relatively short time. That is, in the image forming apparatus 1, for a relatively thick sheet P, even in a relatively short time, the slack in the sheet P is eliminated at the transfer section 25, which may cause color misregistration.

[0103] Considering such a relationship, in the image forming apparatus 1, the paper condition table T1 shown in Fig. 11 is set and stored in advance in the ROM 52 (Fig. 3). In this paper condition table T1, the thickness of the paper P (hereinafter referred to as paper thickness category TD) is divided into a number of stages, and for each paper thickness category TD, the limit length at which the slack of the paper P may not be maintained in the transfer section 25 (hereinafter referred to as slack limit paper length LD or limit paper length) is specified.

[0104] Here, the sagging limit paper length LD for each paper thickness category TD in paper condition table T1 can be calculated based on the length of the sagging portion formed in paper P in transfer section 25, the time required for the sagging portion to be eliminated, and the length of paper P that can be transported by transfer section 15 during this time, etc. Here, the section from the most downstream transfer nip point N15C to the fixing nip point N27 is called the extended transfer section, and the transport distance of paper P in this extended transfer section will be considered.

[0105] For example, when the paper P does not form a slack in the transfer section 25, i.e., when it progresses along the straight transfer path W3L, the transfer distance of the extended transfer section, specifically, the sum of the length of the straight transfer path W3L and the length Lcb (Figure 2(A)), is defined as the shortest transfer distance L1.

[0106] The conveying distance of the extended transfer section when the paper P sags in the transfer section 25, for example, the sum of the length of the curved transfer path W3C and the length Lcb, is defined as the conveying distance L2 when there is slack. Furthermore, the difference between the conveying distance L2 when there is slack and the shortest conveying distance L1 is defined as the differential conveying distance dL, and the difference between the fixing conveying speed V27 of the fixing section 27 and the transfer conveying speed V15 of the transfer section 15 is defined as the differential speed dV.

[0107] Then, the time (hereinafter referred to as slack limit time) until the leading edge of the paper P reaches the fixing nip location N27 while forming slack in the transfer section 25 and the limit at which the slack can be maintained without being eliminated is calculated by the formula (dL / dV). The length of the paper P that can be transported by the transfer section 15 during this slack limit time (hereinafter referred to as slack limit distance) is calculated by the formula (dL / dV×V15). Furthermore, the sum of the slack limit distance and the transport distance L2 under slack, i.e., the length calculated by the formula (dL / dV×V15+L2), is set as the calculated limit paper length.

[0108] Incidentally, the value of the conveying distance L2 when slack is greatest when the paper P advances along the curved transfer path W3C in the transfer section 25 and forms the greatest slack. In addition, when the conveying distance L2 when slack is used at this time, the differential conveying distance dL, the slack limit time, the slack limit distance, and the calculated limit paper length also each become largest.

[0109] In this way, in the image forming apparatus 1, when the length of the paper P (i.e., the paper length) is smaller (shorter) than the calculated limit paper length, the slack in the transfer section 25 can be maintained until the tail of the paper P passes through the most downstream transfer nip point N15C. This means that since the transport speed of the paper P in the transfer section 15 can be maintained at the transfer transport speed V15, there is no problem even if the inter-paper distance D(n) is set to a relatively short shortened distance Lex.

[0110] On the other hand, in the image forming apparatus 1, when the length of the sheet P (i.e., the sheet length) is greater (longer) than the calculated limit sheet length, the slack in the transfer section 25 is eliminated before the tail of the sheet P passes through the most downstream transfer nip location N15C. This means that the transport speed of the sheet P in the transfer section 15 cannot be maintained at the transfer transport speed V15, and therefore the sheet-to-sheet distance D(n) should be set to a relatively long extension distance Lst.

[0111] Therefore, in the image forming apparatus 1, the slack limit paper length LD for each paper thickness category TD is set to a value obtained by adding a predetermined margin to the calculated limit paper length, i.e., a value equal to or greater than the calculated limit paper length, and each value is stored in the paper condition table T1 (FIG. 11). This paper condition table T1 shows a relationship in which the slack limit paper length LD decreases as the paper thickness category TD value increases.

[0112] That is, in the image forming apparatus 1, when the thickness of the paper P belongs to the paper thickness category TD, if the length along the conveying direction is less than the sagging limit paper length LD associated with it in the paper condition table T1, there is no risk of color misregistration occurring even if the inter-paper distance D(n) is set to the shortened distance Lex (FIG. 2(B)). On the other hand, in the image forming apparatus 1, if the length along the conveying direction of the paper P is equal to or greater than the sagging limit paper length LD, there is a possibility of color misregistration occurring if the inter-paper distance D(n) is set to the shortened distance Lex.

[0113] Therefore, in the image forming apparatus 1, when printing images continuously on multiple sheets of paper P, the paper condition table T1 is referenced, and if the thickness of the paper P belongs to the paper thickness category TD and its length along the conveying direction is equal to or longer than the sagging limit paper length LD, the paper-to-paper distance D(n) is switched to the extended distance Lst (Figure 2(C)), which is sufficiently longer than the shortened distance Lex.

[0114] Specifically, when the main control unit 41 (FIG. 3) of the control unit 3 in the image forming apparatus 1 receives image data to be printed on paper P from a higher-level device (not shown), the main control unit 41 starts the above-mentioned print process. At this time, if the image data is to be printed continuously on multiple sheets of paper P, the main control unit 41 reads out a predetermined paper spacing setting program from the ROM 52 by the CPU 51 and executes it, thereby starting the paper spacing setting process procedure RT1 shown in FIG. 12, and proceeds to the first step SP1.

[0115] In step SP1, the main control unit 41 reads a setting value for the thickness of the paper P (hereinafter referred to as the set paper thickness) from the ROM 52, and proceeds to the next step SP2. In step SP2, the main control unit 41 reads a value for the length of the paper P in the transport direction (hereinafter referred to as the set paper length) from the ROM 52, and proceeds to the next step SP3.

[0116] In step SP3, the main control unit 41 refers to the paper condition table T1 (Figure 11) stored in ROM 52 to identify the paper thickness category TD to which the set paper thickness belongs, reads out the slack limit paper length LD associated with the paper thickness category TD, and proceeds to the next step SP4.

[0117] In step SP4, the main control unit 41 judges whether the set sheet length is less than the slack limit sheet length LD. If a positive result is obtained here, this indicates that the length along the conveying direction is relatively small (short) relative to the thickness of the sheet P, and when the tail of the sheet P passes through the most downstream transfer nip location N15C in the transfer unit 15, the sheet P can be maintained in a slack state in the transfer unit 25, and the conveying speed in the transfer unit 15 can be maintained at the transfer conveying speed V15. In other words, this indicates that even if the sheet-to-sheet distance D(n) is set to the shortened distance Lex, the conveying speed in the transfer unit 15 can be maintained at the transfer conveying speed V15, so there is no risk of color misalignment occurring. At this time, the main control unit 41 proceeds to the next step SP5, sets the sheet-to-sheet distance D(n) to the shortened distance Lex (FIG. 2B), and proceeds to the next step SP7.

[0118] On the other hand, if a negative result is obtained in step SP4, this indicates that the length along the conveying direction is relatively large (long) compared to the thickness of the sheet P. This also indicates that when the tail of the sheet P passes through the most downstream transfer nip location N15C in the transfer section 15, the slack in the sheet P is eliminated in the transfer section 25, and the conveying speed in the transfer section 15 cannot be maintained at the transfer conveying speed V15. In other words, this indicates that if the sheet-to-sheet distance D(n) is set to the shortened distance Lex, the conveying speed in the transfer section 15 will deviate from the transfer conveying speed V15, and color misregistration may occur. At this time, the main control section 41 proceeds to the next step SP6, sets the sheet-to-sheet distance D(n) to the extended distance Lst (FIG. 2(C)), and proceeds to the next step SP7.

[0119] In step SP7, the main control unit 41 ends the paper interval setting process procedure RT1. After that, the image forming apparatus 1 performs the printing process while adjusting the interval between the sheets of paper P to the inter-sheet distance D(n), and sequentially prints images.

[0120] [4. Effects, etc.] In the above configuration, the image forming apparatus 1 according to the present embodiment creates in advance a paper condition table T1 (FIG. 11) that indicates the relationship between the paper thickness category TD and the sagging limit paper length LD, and stores it in the ROM 52. Also, in the image forming apparatus 1, the distance between the sheets D(n) can be set to either a relatively short shortened distance Lex or a relatively long extended distance Lst (FIG. 2).

[0121] When the image forming apparatus 1 continuously prints images on a plurality of sheets of paper P, in principle, the distance D(n) between the sheets is set to the shortened distance Lex, as shown in Fig. 7. This enables the image forming apparatus 1 to increase the number of sheets of paper P that can be transported by the transport path W per unit time, and to increase the throughput, which is the number of printed sheets per unit time.

[0122] On the other hand, the image forming apparatus 1 executes the paper interval setting process procedure RT1 (FIG. 12), and when the set paper length of the paper P is longer than the slack limit paper length LD associated with the paper thickness category TD to which the set paper thickness of the paper P belongs in the paper condition table T1 (FIG. 11), sets the inter-paper distance D(n) to the extension distance Lst. This allows the image forming apparatus 1 to eliminate the possibility of color misalignment occurring in the subsequent paper P even when the slack of the paper P is eliminated in the transfer section 25 (FIG. 1) as shown in FIG. 13.

[0123] In other words, the image forming apparatus 1 increases the inter-sheet distance D(n) from normal only when there is a risk that the slack in the sheet P will be eliminated in the transfer section 25 and color shift will occur in the subsequent sheet P. This allows the image forming apparatus 1 to intentionally reduce throughput while still allowing the leading edge of the subsequent sheet P(n+1) to reach the most upstream transfer nip point N15K after the trailing edge of the preceding sheet P(n) has passed the most downstream transfer nip point N15C, thereby avoiding degradation of image quality due to color shift.

[0124] In an image forming apparatus, when images are printed continuously on multiple sheets of paper P, one possible measure to prevent color shift is to keep the inter-paper distance D(n) at the shortened distance Lex and adjust the fixing transport speed V27 in the fixing section 27 to be equal to the transfer transport speed V15, thereby preventing color shift.

[0125] However, if an image forming apparatus were to adopt this measure, it would be extremely difficult to control the fixing conveying speed V27 to be equal to the transfer conveying speed V15, since the rollers in the fixing section 27 would expand due to heat and the outer circumferential length would change. Also, in an image forming apparatus that adopts this measure, if the paper P is relatively thick and has high rigidity, almost no slack is formed in the transfer section 25, as in the case shown in FIG. 10, and color misalignment may occur in the succeeding paper P.

[0126] Therefore, in the image forming apparatus 1 according to the present embodiment, the fixing conveying speed V27 in the fixing section 27 is not strictly controlled, but is set to a speed slightly higher than the transfer conveying speed V15, and the paper P is allowed to have as much slack as possible in the transfer section 25.

[0127] As a result, in the image forming apparatus 1, there is no need to control the fixing conveying speed V27 with high precision in the fixing unit 27, and fluctuations in the conveying speed in the transfer unit 15 can be appropriately suppressed to a certain extent. Furthermore, in the paper spacing setting process procedure RT1 (FIG. 12), the image forming apparatus 1 extends the inter-paper distance D(n) to the extension distance Lst when the set paper length of the paper P is longer than the sagging limit paper length LD in the paper thickness category TD to which the set paper thickness of the paper P belongs, so that color misregistration can be prevented even if the conveying speed of the transfer unit 15 fluctuates.

[0128] Furthermore, in the image forming apparatus 1, the extension distance Lst (FIG. 2C) is set to a value 1 mm larger (longer) than the transfer nip distance Lkc. As a result, in the image forming apparatus 1, when the inter-paper distance D(n) is set to the extension distance Lst, the trailing paper P(n+1) can be brought into contact with the transfer belt 18 while the trailing end of the preceding paper P(n) is separated from the transfer belt 18 and traveling at the original transfer conveying speed V15, and color shift can be reliably prevented.

[0129] Furthermore, in the image forming apparatus 1, in the paper interval setting process procedure RT1 (FIG. 12), the set paper thickness and set paper length set by the user are used as the length and thickness of the paper P. Therefore, the image forming apparatus 1 does not need to provide a sensor or the like for measuring the thickness and length of the paper P, and can set the inter-paper distance D(n) suitable for the paper P without performing a process for measuring these.

[0130] According to the above configuration, when the image forming apparatus 1 prints images continuously on a plurality of sheets of paper P, if the set paper length of the sheet P is longer than the sagging limit paper length LD based on the set paper thickness of the sheet P, the sheet-to-sheet distance D(n) is set to the extended distance Lst, and otherwise the sheet-to-sheet distance D(n) is set to the shortened distance Lex. As a result, even if the sagging of the sheet P is eliminated in the transfer section 25 due to the length or thickness of the sheet P, the image forming apparatus 1 can appropriately eliminate the possibility of color shift occurring in the subsequent sheet P, and increase throughput by narrowing the sheet-to-sheet distance D(n) as much as possible when possible.

[0131] 5. Other embodiments In the above embodiment, the extension distance Lst (FIG. 2C) is 1 mm larger (longer) than the transfer nip distance Lkc. However, the present invention is not limited to this, and may be various other values, such as 5 mm larger than the transfer nip distance Lkc. For example, when a relatively high frictional force acts between the paper P and the transfer belt 18 in the transfer section 15 (FIG. 2), the paper P can be considered to move along the transfer conveying path W2 at almost the same speed without slipping against the transfer belt 18 while at least a part of the paper P is in contact with the transfer belt 18. In this case, the extension distance Lst may be a value obtained by adding a constant α to the roller distance Lr, which is the distance between the drive roller upper end 16T and the tension roller upper end 17T, or a value obtained by subtracting the constant α from the roller distance Lr.

[0132] In the above embodiment, the shortest conveying distance L1 and the conveying distance L2 under slack are set as the conveying distance of the paper P between the most downstream transfer nip location N15C and the fixing nip location N27, and the slack limit time and the slack limit distance are calculated based on these. However, the present invention is not limited to this. For example, the shortest conveying distance L1 and the conveying distance L2 under slack may be set as the conveying distance of the paper P between the upper end 16T of the drive roller and the fixing nip location N27, and the slack limit time and the slack limit distance may be calculated based on these. Specifically, the length Lcb may be subtracted from the shortest conveying distance L1 and the conveying distance L2 under slack in the embodiment. However, the difference between the two, that is, the differential conveying distance dL, is the same value as in the embodiment, because the subtracted length Lcb is offset.

[0133] Furthermore, in the above-described embodiment, each transfer nip location N15 of the transfer unit 15 (FIG. 2) is considered to be extremely short in the front-rear direction and is represented as a "point" in FIG. 2 and the like when viewed from the left-right direction. However, the present invention is not limited to this, and each transfer nip location N15 may be considered to have a certain distance LN in the front-rear direction, as shown in FIG. 14, for example. In this case, the transfer nip distance Lkc is the distance from the most upstream transfer nip start point N15KU on the most upstream side (front side) of the most upstream transfer nip location N15K to the most downstream transfer nip end point N15CL on the most downstream side (rear side) of the most downstream transfer nip location N15C.

[0134] Furthermore, in the above-described embodiment, the range where the paper P contacts the transfer belt 18 in the transfer section 15 (FIG. 2) is considered to be the range from the tension roller upper end 17T to the drive roller upper end 16T, and the roller distance Lr is the distance corresponding to this range. However, the present invention is not limited to this. For example, as shown in FIG. 15(A), when the paper P contacts the transfer belt 18 in the downstream expansion range 16A up to the downstream expansion end 16E downstream (rear side) of the drive roller upper end 16T, the roller distance Lr may be the range from the tension roller upper end 17T (FIG. 2) to the downstream expansion end 16E. Alternatively, as shown in FIG. 15(B), when the paper P contacts the transfer belt 18 in the upstream expansion range 17A up to the upstream expansion end 17E upstream (front side) of the tension roller upper end 17T, the roller distance Lr may be the range from the upstream expansion end 17E to the drive roller upper end 16T (FIG. 2). Furthermore, by combining both, the roller distance Lr may be set to a distance corresponding to the range from the upstream expansion end 17E to the downstream expansion end 16E.

[0135] Furthermore, in the above embodiment, one slack limit paper length LD is associated with each paper thickness category TD in the paper condition table T1 (FIG. 11), and the paper spacing setting process procedure RT1 (FIG. 12) switches the paper spacing distance D(n) to two levels, a shortened distance Lex or an extended distance Lst, based on the comparison result between the set paper length and the slack limit paper length LD. However, the present invention is not limited to this, and for example, two or more slack limit paper lengths LD may be associated with each paper thickness category TD, and the paper spacing distance D(n) may be switched to three or more levels.

[0136] Further, in the above-mentioned embodiment, the paper condition table T1 (FIG. 11) corresponds to each paper thickness category TD and the slack limit paper length LD, and the paper interval setting process procedure RT1 (FIG. 12) compares the slack limit paper length LD, which is associated with the paper thickness category TD to which the set paper thickness belongs, with the set paper length. However, the present invention is not limited to this, and for example, a temperature sensor and a humidity sensor for measuring temperature and humidity may be provided in the image forming apparatus 1, and multiple paper setting tables corresponding to the temperature and humidity may be created in advance. In this case, the image forming apparatus 1 may select a paper setting table corresponding to the measured values ​​of temperature and humidity obtained by the temperature sensor and humidity sensor in the paper interval setting process, and then compare the slack limit paper length LD, which is associated with the paper thickness category TD to which the set paper thickness belongs, with the set paper length in the paper setting table. Alternatively, the value of the constant α (FIG. 2) may be changed according to the values ​​of temperature and humidity. With these, even if the frictional force between the paper P and the transfer belt 18 changes according to the temperature and humidity, the appropriate slack limit paper length LD can be used to compare with the set paper length. Further, the present invention is not limited to a configuration in which both temperature and humidity are used, and it is also possible to use only either temperature or humidity.

[0137] Furthermore, in the above embodiment, a paper condition table T1 (FIG. 11) that associates the paper thickness category TD with the slack limit paper length LD is stored in advance in the ROM 52 (FIG. 3), and the slack limit paper length LD is read out by referring to the paper condition table T1 in the paper interval setting procedure RT1 (FIG. 12). However, the present invention is not limited to this, and the slack limit paper length LD may be obtained based on the paper thickness category TD by various other methods. Specifically, for example, a function that represents the relationship between the paper thickness category TD and the slack limit paper length LD may be created in advance, and the slack limit paper length LD may be calculated using the function in the paper interval setting procedure RT1 (FIG. 12). In this case, the set paper thickness value may be used instead of the paper thickness category TD.

[0138] Furthermore, in the above embodiment, the paper interval setting process procedure RT1 (FIG. 12) has been described as a case in which the set paper thickness and set paper length set by the user are read out and used. However, the present invention is not limited to this, and for example, a paper thickness sensor that measures the thickness of paper P may be provided, and the value of the paper thickness measured by the paper thickness sensor may be used. Also, the paper length may be calculated based on the time when paper P is detected by a sensor such as the hopping sensor 9, the transport speed of the paper P, etc., and the obtained value may be used.

[0139] Furthermore, in the above-described embodiment, the case has been described where the distance D(n) between sheets is adjusted by appropriately controlling the rotation or stop of the pair of transport rollers 12 in the paper feed transport section 6 (FIG. 1) based on the control of the control unit 3. However, the present invention is not limited to this, and the distance D(n) between sheets may be adjusted, for example, by appropriately controlling the rotation speed of the pair of transport rollers 12 in the paper feed transport section 6. The distance D(n) between sheets may also be adjusted by controlling the timing of paper feed by the hopping roller 8, the transport speed of the pair of transport rollers 10, etc.

[0140] Furthermore, in the above-described embodiment, the image forming section 22 (FIGS. 1 and 2) is described as having four image forming units 23 each forming a toner image of one color. However, the present invention is not limited to this, and for example, three or less or five or more image forming units 23 may be provided in the image forming section 22. Alternatively, for example, the image forming section 22 may be provided with one image forming unit that forms toner images of four colors.

[0141] Furthermore, in the above-described embodiment, the various programs executed by the main control unit 41 are stored in advance in the ROM 52 (FIG. 3). However, the present invention is not limited to this, and for example, the main control unit 41 may be executed after connecting to various server devices (not shown) via the interface unit 55 and receiving various programs from the server devices.

[0142] Furthermore, in the above-described embodiment, the present invention has been described as being applied to the image forming apparatus 1 which is a single-function printer. However, the present invention is not limited to this, and may be applied to image forming apparatuses having various other functions, such as an MFP (Multi Function Peripheral) having the functions of a copier or facsimile machine.

[0143] Furthermore, the present invention is not limited to the above-mentioned embodiment and other embodiments. That is, the scope of application of the present invention extends to embodiments in which the above-mentioned embodiment and the other embodiments are combined in part or in whole in any manner, or embodiments in which a part is extracted.

[0144] Furthermore, in the above-described embodiment, the image forming apparatus is configured with the image forming unit 23 as the image forming section, the transfer section 15 as the transfer section, and the control section 3 as the control section. However, the present invention is not limited to this, and the image forming apparatus may be configured with an image forming section, a transfer section, and a control section having various other configurations. [Industrial Applicability]

[0145] The present invention can be used, for example, when a toner image formed in an image forming unit by electrophotography is transferred and fixed onto a sheet of paper being transported by a transfer belt. Available at. [Explanation of symbols]

[0146] 1: image forming apparatus, 3: control section, 6: paper feed conveying section, 9: hopping sensor, 11: paper sensor, 13: writing sensor, 15: transfer section, 16: drive roller, 16T: upper end of drive roller, 17: tension roller, 17T: upper end of tension roller, 18: transfer belt, 19: transfer roller, 22: image forming section, 23: image forming unit, 24: photoconductor drum, 25: transfer section, 26: curved conveying guide, 27: fixing section, 41: main control section, 51: CPU, 52: ROM, 53: RAM, 54: timer counter, D: paper distance, Lr: roller Distance, Lkc...distance between transfer nips, Lex...shortened distance, Lst...extended distance, L1...shortest conveying distance, L2...conveying distance when slack, dL...differential conveying distance, N15...transfer nip location, N15C...most downstream transfer nip location, N15K...most upstream transfer nip location, N27...fixing nip location, P...paper, T1...paper condition table, TD...paper thickness classification, LD...limit paper length, V15...transfer conveying speed, V27...fixing conveying speed, VP...conveying speed, dV...differential speed, W...conveying path, W2...transfer conveying path, W3...transfer conveying path, W3C...curved transfer conveying path, W3L...straight transfer conveying path.

Claims

1. A plurality of image forming units arranged in parallel along a conveying direction of a recording medium to form an image on the recording medium; a transfer section including a belt member that conveys the recording medium and a transfer member that forms a transfer nip location for each of the plurality of image forming units; a control unit that controls conveyance of the recording medium so that, when the length of the recording medium along the conveying direction is equal to or longer than a predetermined limit paper length, a media inter-distance from an end in the conveying direction of a preceding recording medium that is the recording medium conveyed first to a start in the conveying direction of a succeeding recording medium that is the recording medium conveyed next is equal to or longer than a transfer nip inter-distance that is a distance from an upstream transfer nip location that is the transfer nip location located on the most upstream side in the media conveying direction in the multiple image forming units to a downstream transfer nip location that is the transfer nip location located on the most downstream side; An image forming apparatus comprising:

2. The control unit controls the subsequent recording medium so that the trailing end in the transport direction of the preceding recording medium leaves the most downstream transfer nip location and then the subsequent recording medium reaches the most upstream transfer nip location.

2. The image forming apparatus according to claim 1,

3. a conveying section disposed upstream of the belt body and configured to convey the recording medium; a sensor that detects the recording medium at a detection point set between the belt body and the conveying unit; Further comprising: the transport unit requires a predetermined transport time to transport the recording medium by the inter-medium distance, When the length of the recording medium is equal to or longer than the limit paper length and the sensor detects the passage of the preceding recording medium at the detection location and the subsequent recording medium reaches the detection location before the conveying time has elapsed, the control unit controls the conveying unit to suspend the conveying and to resume the conveying of the subsequent recording medium by the conveying unit after the conveying time has elapsed.

3. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction.

4. a conveying section disposed upstream of the belt body and configured to convey the recording medium; Further comprising: The control unit controls the transport unit to make the inter-medium distance shorter than the inter-transfer nip distance when the length of the recording medium is less than the critical paper length.

3. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction.

5. a fixing section disposed downstream of the belt body, which conveys the recording medium and fixes the image on the recording medium; a slack forming unit disposed between the belt body and the fixing unit, the slack forming unit forming a slack in the recording medium; Further comprising: The conveying speed of the recording medium in the fixing section is faster than the conveying speed of the recording medium in the transfer section.

5. The image forming apparatus according to claim 1, wherein the image forming apparatus further comprises: a first fixing unit;

6. A difference between a slack conveying distance, which is a conveying distance when the recording medium is slackened by the slack forming unit, and a shortest conveying distance when the recording medium is conveyed without slack is defined as a differential conveying distance dL, A conveyance distance from the most downstream transfer nip location to the fixing unit when the recording medium sags is defined as L, The conveying speed of the belt body is V, When the difference between the conveying speeds of the fixing unit and the transfer unit is dV, The limit paper length is set to be equal to or greater than (dL÷dV×V+L).

6. The image forming apparatus according to claim 5,

7. The control unit uses a first limit paper length as the limit paper length when the recording medium has a predetermined first thickness, and uses a second limit paper length smaller than the first limit paper length as the limit paper length when the recording medium has a second thickness larger than the first thickness.

7. The image forming apparatus according to claim 1, wherein the image forming apparatus further comprises:

8. A plurality of image forming units arranged in parallel along a conveying direction of a recording medium to form an image on the recording medium; a transfer section including a belt body that transports the recording medium, and tensioners that are respectively disposed on the upstream side and downstream side of the image forming unit in the transport direction, and that stretch the belt body and transport the recording medium; a control unit that controls the conveyance of the recording medium so that, when the length of the recording medium along the conveyance direction is equal to or greater than a predetermined limit paper length, a media-to-media distance from an end in the conveyance direction of a preceding recording medium that is the recording medium conveyed first to a start in the conveyance direction of a succeeding recording medium that is the recording medium conveyed next is equal to or greater than a distance from the tension body on the upstream side to the tension body on the downstream side; An image forming apparatus comprising:

Images