JP6160238B2 - Image forming apparatus, roller control method, and computer program - Google Patents

Description

  The present invention relates to an image forming apparatus having a roller pair for heating a sheet and control thereof.

  An image forming apparatus that prints an image on a sheet (printing sheet) by electrophotography includes a fixing device that heats the sheet onto which the toner image is transferred. The toner, which is a color material, is melted by heating and fixed on the paper. A typical fixing device includes a heating roller (also referred to as a fixing roller) having a heat source that raises the temperature of a peripheral surface, and a pressure roller that contacts the heating roller to form a nip portion. The sheet is conveyed to the nip portion and heated while passing through the nip portion while being pressurized.

  Fixing conditions related to print quality are heating temperature, pressure, and conveyance speed. In order to sufficiently heat the toner image passing through the nip portion, it is necessary to set the fixing conditions in consideration of the heat of fusion accompanying the melting of the toner. Patent Document 1 proposes an image forming apparatus that minimizes energy consumed for heating per sheet by controlling fixing conditions according to the maximum amount of toner adhered to the sheet.

  Regarding the pressurization at the nip portion of the fixing device, there is a prior art in which the pressure is changed while the paper is passing. In the image forming apparatus disclosed in Patent Document 2, in order to prevent a fixing jam in which a sheet that has passed through the nip is wound around a roller, a pressure load applied when the leading end of the sheet passes through the nip is applied to a portion other than the leading end. Make it lighter than the pressurized load when the part passes. Thereby, sticking to the roller of a front-end | tip part is reduced.

  In addition, regarding control according to the printing pattern, there is a technique for weakening the pressure applied to the roller of the pressing member that cleans the fixing roller when a sheet printed with a high density pattern or a high printing rate pattern passes through the nip portion. (Patent Document 3). There is a description that toner is prevented from collecting on the upstream side of the contact portion of the fixing roller with the pressing member by weakening the pressure.

JP 2004-286806 A JP 2000-089604 A JP 2009-063906 A

  In order to reduce power consumption of the image forming apparatus, it is necessary to minimize power consumption in fixing.

  In view of such circumstances, an object of the present invention is to reduce power consumption in heating a printing sheet.

An image forming apparatus that achieves the above-described object is an image forming apparatus having a pair of rollers for heating a sheet for printing, and a transport mechanism that transports the sheet to a nip portion sandwiched between the pair of rollers. A pressure switching mechanism that changes the relative position between one and the other of the pair of rollers in a direction in which the pressure applied to the sheet changes, and at least part of the width direction perpendicular to the transport direction of the printing surface of the sheet. A first pressure is applied to the sheet over a period in which a colored region to which a coloring material is attached passes through the nip portion, and the non-colored region that is not the colored region on the printing surface passes through the nip portion. as low gastric pressure force than the first pressure is applied to the sheet, and a controller for controlling the pressurized pressure changeover sorting mechanism, the controller, Printing that indicates the position of the colored region in the transport direction on a transported body that is the single sheet transported by the transport mechanism or a sheet row composed of the plurality of sheets when printing on one or a plurality of sheets. An acquisition unit that acquires position information, a detection unit that detects an intermediate blank portion that is a portion between the two colored regions arranged in the transport direction in the transported body, based on the print position information, and the intermediate A determination unit that determines whether the length of the blank portion in the transport direction is equal to or greater than a set length, and when the determination unit determines that the length is equal to or greater than a set length, After the front end of the intermediate blank portion passes through the nip portion, the applied pressure is switched from the first pressure to a second pressure lower than the first pressure, and the rear end of the intermediate blank portion is in the nip portion. When the pressurization switching mechanism is controlled so that the applied pressure is switched from the second pressure to the first pressure before wearing, and the determination unit determines that the length is not equal to or greater than a set length. After the front end of the intermediate blank portion has passed through the nip portion, the applied pressure is switched from the first pressure to a third pressure that is a pressure between the first pressure and the second pressure, The pressure switching mechanism is controlled so that the applied pressure is switched from the third pressure to the first pressure before the rear end of the intermediate blank portion reaches the nip portion.

  By switching the applied pressure from the first pressure to the second pressure, the nip width that is the dimension in the conveyance direction of the nip portion is narrowed. Thereby, the time for the non-colored region to contact the roller is shortened, and the thermal diffusion from the roller to the non-colored region that does not require heating is reduced.

  On the other hand, in the past, a constant pressurizing force whose value was previously set according to the thickness of the paper and the content of printing was applied to the paper over a period during which the paper passed through the nip portion in the fixing device. Even when the technique of Patent Document 2 is applied, the applied pressure is kept constant during a period in which a portion other than the leading edge of the sheet passes through the nip portion.

  The constant pressure means that the nip width is constant and any position on the paper is in contact with the heating roller for a certain time. That is, it means that the blank portion where the toner is not attached on the paper is heated in the same manner as the colored portion where the toner is attached.

  According to the present invention, it is possible to reduce power consumption by suppressing wasteful heat diffusion from a roller for heating a printing sheet to the sheet.

1 is a diagram illustrating a configuration of an image forming apparatus. It is a figure which shows an example of the pressurization switching mechanism assembled | attached to a fixing device. It is a figure which shows the structure of the engine controller which controls a pressurization switching mechanism. It is a figure which shows the timing of pressurization control. It is a figure which shows the coloring area | region and intermediate | middle blank part which concern on pressurization control. It is a figure which shows how to determine the position of the conveyance direction in a to-be-conveyed body. It is a figure which shows the timing of switching of the applied pressure in an intermediate | middle blank part. It is a figure which shows the relationship between the length of the conveyance direction of an intermediate | middle blank part, and the presence or absence of switching of a pressurizing force. It is a figure which shows the several position which concerns on the pressurization control in a conveyance path. It is a figure which shows the specific example of time and distance which concerns on pressurization control in a table | surface form. FIG. 6 is a diagram illustrating a state where a fixing loop is formed on a conveyance path. It is a timing chart which shows the relationship between a loop sensor signal and a conveyance speed. 6 is a graph showing a change in a moving distance of a sheet when a fixing loop is formed. It is a flowchart of the 1st example of pressurization control. It is a flowchart of the 2nd example of pressurization control. FIG. 10 is a diagram illustrating an example of an encoder signal acquisition method for detecting the progress of paper conveyance on the upstream side of a nip portion. It is a figure which shows the specific example of the control parameter which concerns on the acquisition method of FIG. It is a figure which shows the example of the acquisition method of the encoder signal for detecting the progress condition of the paper conveyance in a nip part. It is a figure which shows the specific example of the control parameter which concerns on the acquisition method of FIG. It is a figure which shows the other example of pressurization control.

  An image forming apparatus 1 illustrated in FIG. 1 is a tandem color printer that forms a color image or a monochrome image by electrophotography. Since four color toner images of yellow (Y), magenta (M), cyan (C), and black (K) are formed in parallel when forming a color image, the image forming apparatus 1 includes an intermediate transfer belt 20 and an intermediate transfer belt. There are four process units UY, UM, UC, UK arranged along the belt 20. The process unit UK for forming a black (K) image includes a photosensitive drum 11, a charging device 12, a print head 13, a developing device 14, a primary transfer device 15, and a cleaning device 16, and other process units. UY, UM, and UC also have similar components except for the difference in toner colors.

  A lamp heating type fixing device 30 is mounted on the image forming apparatus 1. The fixing device 30 includes a heating roller (fixing roller) 31, a pressure roller 32, and a pressure switching mechanism 36. The material of the peripheral surface portion of the heating roller 31 is sponge, and the heat source for raising the temperature of the peripheral surface is a halogen lamp. The material of the peripheral surface portion of the pressure roller 32 is solid rubber.

  When a print job is given to the image forming apparatus 1 from an external device (not shown) (for example, a personal computer), a series of printing operations controlled by the engine controller 200 starts. For example, in the case of a color print job, the charging device 12 uniformly charges the surface of the photosensitive drum 11 in the process units UY, UM, UC, UK. The light beam emitted from the print head 13 is controlled on and off in accordance with image data from the external device, and an electrostatic latent image is formed on the surface of the photosensitive drum 11. The electrostatic latent image is developed into a toner image by the developing device 14 and is primarily transferred to the intermediate transfer belt 20 by the primary transfer roller 15. The toner images of the four colors are superimposed on the intermediate transfer belt 20 by primary transfer synchronized with the movement of the intermediate transfer belt 20.

  In parallel with the formation of the four-color toner images, the paper Q is sent out from the paper storage unit 24 to the conveyance path 50 indicated by a two-dot chain line in the drawing. Thereafter, the paper Q is conveyed by the registration roller 26 and proceeds to a secondary transfer position where the intermediate transfer belt 20 and the secondary transfer roller 27 face each other. The paper Q onto which the toner image has been secondarily transferred from the intermediate transfer belt 20 passes through the fixing device 30 and is then discharged to the paper discharge tray 29 by the paper discharge roller 28. When the paper Q passes through the fixing device 30, it is sandwiched between a pair of a heating roller 31 and a pressure roller 32 (hereinafter, this may be referred to as a fixing roller pair). The toner is melted by heating and pressing at the nip portion of the fixing roller pair, and the image is fixed on the paper Q.

  The pressure switching mechanism 36 illustrated in FIG. 2 includes a motor 65, a gear 66, and a cam 67. The pressure switching mechanism 36 changes the relative position of the pressure roller 32 with respect to the heating roller 31 in the direction in which the pressure applied to the paper changes. That is, the cam 67 in contact with the rotating shaft 321 of the pressure roller 32 rotates, so that the pressure roller 32 moves in a direction toward and away from the heating roller 31. The heating roller 31 is variably supported by the distance between the axes of the fixing roller pair, and is urged away from the heating roller 31 by, for example, a coil spring (not shown).

  The solid line in FIG. 2 shows a state where the pressure roller 32 is in strong contact with the heating roller 31 and a nip portion having a large nip width is formed. A chain line in the figure indicates a state where the pressure roller 32 is weakly in contact with the heating roller 31 and a nip portion having a small nip width is formed. When the motor 65 is rotated so that the gear 66 rotates in the direction indicated by the white arrow in the figure, the pressure roller 32 is moved by the urging force in the direction in which the distance between the shaft centers is increased, and the nip portion is narrowed. . When the motor 65 is rotated in the reverse direction, the pressure roller 32 is moved by being pushed by the cam 67, the distance between the shaft centers of the fixing roller pair is reduced, and the nip portion is expanded.

  In the image forming apparatus 1, the nip width is changed while the paper Q passes through the nip portion. That is, the distance between the axes of the fixing roller pair is increased or decreased so that the pressure applied to the paper Q changes within a predetermined time determined by the conveyance speed of the paper Q. For this reason, as the motor 65, an inner rotor type DC brushless motor having excellent responsiveness to start and stop instructions among various motors is used.

  As shown in FIG. 3, the engine controller 200 includes a CPU (Central Processing Unit) 201 as a computer that executes a control program. The CPU 201 loads a program and data necessary for control from a ROM (Read Only Memory) 202 or a nonvolatile memory 204 to a RAM (Random Access Memory) 203 in a timely manner. The engine controller 200 controls the electrophotographic process in response to an instruction from the main controller 100 that is responsible for overall control of the image forming apparatus 1.

  The CPU 201 controls the rotation of the fixing roller pair by transmitting an enable signal (ENABLE) and a clock signal (CLK) to the drive circuit 206 of the fixing motor 61. Further, the CPU 201 transmits a PWM Duty signal, a brake signal, and a rotation direction signal to a drive circuit 650 built in the motor 65 of the pressure switching mechanism 36 as necessary, so that the nip of the nip portion in the fixing device 30 is obtained. Switch the width. Detection signals are input to the CPU 21 from the paper sensor 41 and the loop sensor 42.

  Other control objects 45 controlled by the CPU 201 include process units UY, UM, UC, UK, a paper conveyance motor and clutch, and a heat source of the heating roller 31.

  4A to 4F show the timing of pressure control for switching the nip width by the pressure switching mechanism 36. FIG. As described below, the pressure is switched (the nip width is changed) during the period in which the sheet Q1 passes through the nip portion.

  In FIG. 4A, the front end of the colored region 85 in the paper Q1 moving in the transport direction M1 is located upstream of the pressure increase start position p4. The pressure increase start position p4 is a position set in consideration of the time required for switching the pressurizing force, and is set on the upstream side with respect to the nip position p6 that is the center in the transport direction in the nip portion. The coloring area 85 is an area to which the toner specified by the image data indicating the printing content is to be attached. Usually, since the toner image is formed almost faithfully to the image data, it can be said that the colored region 85 is a region where the toner is attached. In the state of FIG. 4A, the pressure applied to the fixing roller pair is a reduced pressure P2 lower than the fixing pressure P1 suitable for toner fixing, and the nip width is small.

  In FIG. 4B, the front end of the colored region 85 has just arrived at the pressure increase start position p4. When the state shown in FIG. 4B is reached, rotation in a direction to increase the pressure of the motor 65 of the pressure switching mechanism 36 is started. The nip width is small at the start of rotation, but thereafter the nip width increases as the cam 67 rotates (see FIG. 2).

  In FIG. 4C, the colored region 85 has arrived immediately before the nip position p6. At the stage of FIG. 4C, the applied pressure has already been switched to the fixing pressure P1, and the nip width is large. The motor 65 stops rotating.

  In FIG. 4D, the colored region 85 is passing through the nip position p6. The applied pressure is maintained at the fixing pressure P1. The stage of FIG. 4E is a stage where the colored region 85 has finished passing through the nip position p6. At this stage, rotation in a direction to reduce the pressure applied by the motor 65 of the pressure switching mechanism 36 is started. The nip width is large at the start of rotation, but thereafter the nip width decreases as the cam 67 rotates.

  In FIG. 4F, the rear end of the colored region 85 has reached the pressure reduction completion position p8. The pressure reduction completion position p8 is a position on the downstream side of the nip position p6, and is separated from the nip position p6 by a conveyance distance corresponding to the time required to switch the applied pressure from the fixing pressure P1 to the reduced pressure P2. At the stage of FIG. 4 (F), the nip width is large, and the motor 65 stops rotating.

  FIG. 5 shows a colored region and an intermediate blank portion related to pressure control. FIG. 5A shows an intermediate blank portion 81 in a print job that uses only one sheet Q1. FIG. 5B shows an intermediate blank portion 81 in a print job using a plurality of sheets Q1 and Q2.

  In FIG. 5A, the printing surface of one sheet Q1 as a transported body that is transported to the nip portion for fixing is divided into a colored region 85 and a non-colored region that is other than that. The colored region 85 is a region where at least a part of the width direction M2 orthogonal to the transport direction M1 is a portion to which toner is attached. That is, the colored region 85 is a pixel row including colored pixels in the main scanning direction (width direction M2) represented by the image data, and a region in which such a pixel row is continuous in the sub-scanning direction (conveying direction M1). is there. The colored region 85 extends over the entire length in the width direction M2.

  The non-colored region is divided into a blank portion 80 that is both ends of the paper Q1 in the transport direction M1 and an intermediate blank portion 81 that is a portion between two colored regions 85 arranged in the transport direction M1. When the blank portion 80 passes through the fixing nip portion, the pressing force is set to the reduction pressure P2 in order to reduce the nip width and reduce the thermal diffusion to the paper Q1. Although it is desirable that the pressure applied to the intermediate blank portion 81 is the reduced pressure P2, in the image forming apparatus 1, the colored area 85 on the rear end side of the intermediate blank portion 81 is returned to the fixing pressure P1 before reaching the nip portion. Can be applied only to the intermediate blank portion 81 that is long in the transport direction M1.

  In FIG. 5B, the transported body 3 is composed of two sheets Q1 and Q2 that are continuously transported at a predetermined interval. As in the case of FIG. 5A, the printing surface of each sheet Q1, Q2 is divided into a colored area 85 and a non-colored area other than that. Unlike FIG. 5A, in FIG. 5B, the gap between the colored areas 85 in each of the sheets Q1 and Q2, together with the gap between the sheets and the blank portions 80 on both sides thereof are combined. The portion is treated as a series of intermediate blank portions 81. When one or both of the blank portion 80 on the rear end side of the paper Q1 and the blank portion 80 on the front end side of the paper Q2 are wide, and the length of the intermediate blank portion 81 including these is longer than a predetermined value, The intermediate blank portion 81 is a target of pressure control.

  As shown in FIG. 6, the position in the transport direction M1 of an arbitrary part on the transported body is specified by the distance L from the front end including the sheet interval. In the transported member 4 shown in the figure, which is a sheet row of four sheets Q1, Q2, Q3, and Q4, for example, the distance L (Y) from the front end of the center position Y of the third sheet Q3 is each sheet. If the length of Y is y and the sheet interval is d, then 2.5y + 2d.

  FIG. 7 shows the switching timing of the pressing force in the intermediate blank portion. The intermediate blank portion 81k, which is a non-colored region between the colored region 85j and the colored region 85k, is longer in the transport direction M1 than the sum of the two distances L1 and L2. Here, the distance L1 is the moving distance of the intermediate blank portion 81k in the required time (T1) for switching from the fixing pressure P1 to the reduced pressure P2, and the distance L2 is the required time for switching from the reduced pressure P2 to the fixing pressure P1 ( This is the moving distance of the intermediate blank portion 81k in T2). That is, the intermediate blank portion 81k can be returned by reducing the applied pressure within a period from when the colored region 85j on the front end side passes through the nip portion to when the colored region 85k on the rear end side reaches the nip portion. Length.

  Actually, the pressing force is switched at a timing when a predetermined margin Lm is provided as shown in the figure. That is, when the intermediate blank portion 81k is longer than the sum of the distance L1m obtained by adding the margin Lm to the distance L1 and the distance L2m obtained by adding the margin Lm to the distance L2, the intermediate blank portion 81k passes the nip portion during the period. Pressure can be switched. Specifically, switching from the fixing pressure P1 to the reduced pressure P2 is performed when the conveyance time corresponding to the margin Lm has elapsed from the time when the rear end of the colored region 85j has finished passing through the nip portion. Then, the switching from the reduced pressure P2 to the fixing pressure P1 is started so that the switching is completed at a time earlier than the time when the front end of the coloring region 85 arrives at the nip portion by the conveyance time corresponding to the margin Lm.

  FIG. 8 shows the relationship between the length of the intermediate blank portion in the conveyance direction and whether or not the pressing force is switched.

  Switching from the reduced pressure P2 to the fixing pressure P1 is performed during the period when the blank portion 80 on the front end side of the first sheet Q1 passes through the nip portion. Since the length L81a of the first intermediate blank portion 81a in the transport direction M1 is shorter than the sum of the distances L1 and L2, when the intermediate blank portion 81a passes through the nip portion, the applied pressure remains unchanged at the fixing pressure P1.

  The length L81b in the transport direction M1 of the intermediate blank portion 81b including the gap between the sheets Q1 and Q2 is sufficiently long. Accordingly, switching is performed in which the fixing pressure P1 is once lowered to the reduced pressure P2 during the period when the intermediate blank portion 81b passes through the nip portion, and is returned to the fixing pressure P1 after a predetermined time has elapsed. For the intermediate blank portion 81c having a sufficient length L81c, the pressure is switched similarly to the intermediate blank portion 81b.

  The length L81d of the intermediate blank portion 81d in the transport direction M1 is exactly equal to the distance obtained by adding two margins Lm to the distance L3 that is the sum of the distances L1 and L2. In the intermediate blank portion 81d, the pressure is switched in the same manner as the intermediate blank portions 81b and 81c.

  FIG. 9 shows a plurality of positions related to the pressure control in the transport path.

  A paper sensor 41 that detects the presence or absence of paper is disposed in the vicinity of the downstream side of the registration position p1 that is the nip position of the registration roller 26 in the conveyance path 50. The arrangement position of the paper sensor 41 is set as a reference position p2 of the pressure control timing. That is, the CPU 201 detects that the front end of the transported body passes through the reference position p2, and pays attention to an area (intermediate blank) on the transported body that is moving from the distance from the reference position p2 and the transport speeds V1 and V2. The current position of the edge of the part or blank part) is calculated. Then, the time point at which the site of interest arrives at the pressure increase start position p4 separated from the reference position p2 by the distance L24 or further the pressure reduction completion position p8 by the distance Lx is calculated, and the applied pressure is switched when the calculated time point arrives. .

  A conveyance speed V2 in a section from the registration position p1 through the secondary transfer position p3 to the nip position p6 is equal to the moving speed (system speed) of the intermediate transfer belt 20. The conveyance speed V1 downstream from the nip position p6 is determined by the rotation speed of the fixing roller pair or the rotation speed of the paper discharge roller 28 (see FIG. 1). Basically, the transport speed V1 is the same as the transport speed V2. The distance L1 described above is a product of the conveyance speed V1 and the required time T1, and the distance L2 is a product of the conveyance speed V2 and the required time T2.

  When the switching timing is set in consideration of the nip width, the time point when the target portion on the transported body passes through the upstream end position p5 and the downstream end position p7 of the nip portion is calculated. Is done.

  FIG. 10 shows a specific example of time and distance relating to pressurization control in a tabular form. In the example, the required times T1 and T2 for switching the pressing force are both 51.7 ms. The margin Lm is the sum of the error (1 mm) of the image position on the paper and the estimated positional deviation (1 mm) for paper conveyance.

  FIG. 11 shows a state in which a fixing loop is formed on the conveyance path. The rotational peripheral speed of the fixing roller pair varies depending on component variations, changes with time, thermal expansion, paper type, and the like. When the rotational peripheral speed changes, a conveyance speed difference occurs between the secondary transfer position p3 and the nip position p6, causing the following problems. When the speed of the nip position p6 becomes larger and the paper Q is stuck, transfer deviation occurs. When the speed at the nip position p6 becomes smaller and the fixing loop in which the paper Q bends before the nip position p6 becomes excessive, the paper comes into contact with the conveyance guide and unfixed toner adheres to the conveyance guide. This causes paper smearing.

  In order to prevent the occurrence of these problems, the CPU 201 performs loop control for adjusting the rotation speed of the fixing roller pair so that the fixing loop falls within a predetermined range. In the loop control, the output of the loop sensor 42 disposed before the nip position p6 is monitored.

  As shown in FIG. 12, when the loop sensor signal S42 is on, the CPU 201 gives an instruction to the motor 61 to make the rotation speed of the pair of fixing rollers larger than the normal speed Vs. When the loop sensor signal S42 changes from the on state to the off state, the CPU 201 gives an instruction to the motor 61 to make the rotation speed smaller than the normal speed Vs.

As indicated by the solid line in the graph of FIG. 13, by changing the rotation speed of the fixing roller pair, the moving speed of the sheet changes in the section from the secondary transfer position p3 to the pressure increase start position p4. On the other hand, when the loop control is not performed, the sheet moves at a constant speed from the reference position p2 to the pressure increase start position p4 as indicated by a chain line in FIG. For this reason, there is a time difference dT in the arrival of the paper at the pressure increase start position p4 between when the loop control is performed and when it is not performed. When performing the loop control, the switching timing of the pressing force is set in consideration of the time difference dT.
FIG. 14 is a flowchart of a first example of pressurization control.

  For example, the CPU 201 acquires print position information indicating the position of the colored region 85 on the print surface of the paper based on raster image data used for on / off control of the print head 13 when forming the electrostatic latent image (S01). The CPU 201 detects which part of the transported body is passing the pressure increase start position p4 (S02), and when the first colored region 85 of the transported body reaches the pressure increase start position p4 (YES in S03). ), The applied pressure is switched from the reduced pressure P2 to the fixing pressure P1 (S04).

  Next, the CPU 201 extracts the position information of the intermediate blank portion 81 on the transported body from the print position information and stores it in the queue in the order close to the front end of the transported body (S05). In the queue, the position information of the uninterested intermediate blank portion 81 is stored. The first position information in the queue is noticed, and the noticed position information is sequentially deleted from the queue after the check in the next step S06.

  When the length of the noticed intermediate blank 81 in the transport direction M1 is equal to or longer than the distance Lx (YES in S06), it is detected which part of the transported body is passing the nip position p6 (S07). As described above, the distance Lx is a distance obtained by adding the margin Lm to the sum L3 of the distances L1 and L2 corresponding to the required times T1 and T2. When the front end of the target intermediate blank 81 arrives at the nip (YES in S08), the CPU 201 switches the applied pressure from the fixing pressure P1 to the reduced pressure P2 (S09).

  Subsequently, the CPU 201 detects which part of the transported body is passing through the pressure increase start position p4 (S10), and the rear end of the focused intermediate blank 81 arrives at the pressure increase start position p4. Then (YES in S11), the applied pressure is returned from the reduced pressure P2 to the fixing pressure P1 (S12). Thereafter, the flow returns to step S06.

  In step S06, when the length of the noticed intermediate blank portion 81 in the transport direction M1 is less than the distance Lx (NO in S06), it is checked whether or not the last colored region 85 on the transported body has passed through the nip portion. (S13). When the last colored region 85 passes through the nip portion (YES in S13), the CPU 201 switches the applied pressure from the fixing pressure P1 to the reduced pressure P2 (S14). The processes from step S06 to step S13 are repeated until all of the intermediate blank part 81 in which the position information is stored in the queue has been noticed.

  FIG. 15 is a flowchart of a second example of pressurization control. In the second example, the pressure control timing is set in consideration of the nip width of the nip portion. The basic flow is the same as in the first example of FIG. Here, processing different from the first example will be described.

  In step S06b following step S05, it is checked whether the length of the noticed intermediate blank portion 81 in the transport direction M1 is equal to or longer than the distance Lxb. The distance Lxb is a distance obtained by adding the nip width of the fixing roller pair to the distance Lx. If the check result in step S06b is YES, the CPU 201 detects which part of the transported body is passing the position p7 at the downstream end of the nip portion (S07b). When the front end of the target intermediate blank 81 arrives at the position p7 at the downstream end of the nip (YES in S08b), the CPU 201 switches the applied pressure from the fixing pressure P1 to the reduced pressure P2 (S09).

  On the other hand, when the check result in step S06b is NO, the CPU 201 checks whether or not the last colored region 85 in the transported body has passed the downstream end of the nip portion (S13b). When the last colored region 85 passes the downstream end of the nip portion (YES in S13b), the CPU 201 switches the applied pressure from the fixing pressure P1 to the reduced pressure P2 (S14).

  FIG. 16 shows an example of an encoder signal acquisition method for detecting the progress of paper conveyance on the upstream side of the nip portion.

  As shown in FIG. 16A, the encoder signal S20 for detecting the rotation amount of the roller can be obtained from the motor 71 that rotates the roller around which the intermediate transfer belt 20 is wound. When using a motor 71b that does not have an encoder instead of the motor 71, the encoder 75 is attached to the shaft of the roller around which the intermediate transfer belt 20 is wound, as shown in FIG. Thus, the encoder signal S20 can be obtained.

  By detecting the pulse period of the encoder signal S20, the rotation speed of the roller is obtained to calculate the transport speed V2 at the secondary transfer position p3, and the current position of the target portion in the transported body moving toward the nip position p6. Can be detected. A specific example of the control parameter according to FIG. 16 is listed in FIG.

  FIG. 18 shows an example of an encoder signal acquisition method for detecting the progress of paper conveyance in the nip portion.

  As shown in FIG. 18A), an encoder signal S31 for detecting the amount of rotation of the heating roller 31 can be obtained from the motor 61 that rotates the heating roller 31. When using a motor 61b that does not have an encoder instead of the motor 61, an encoder signal S31 is obtained by attaching an encoder 64 to the shaft of the heating roller 31 as shown in FIG. 18B. be able to.

  By detecting the pulse period of the encoder signal S31, the rotation speed of the heating roller 31 is obtained to calculate the conveyance speed V1 at the nip position p6, and the region of interest in the conveyed object moving downstream from the nip position p6 is calculated. The current position can be detected. Specific examples of control parameters according to FIG. 18 are listed in FIG.

  In the above embodiment, in step S01 of FIGS. 14 and 15, the CPU 201 operates as an acquisition unit that acquires print position information. In step S05, the CPU 201 operates as a detection unit that detects the intermediate blank portion. In step S06 or step S06b, the CPU 201 operates as a determination unit that determines whether the length of the intermediate blank portion in the conveyance direction is equal to or greater than the set length.

  In the above-described embodiment, the pressure control for switching the pressure applied to the fixing roller pair between the fixing pressure P1 and the reduced pressure P2 is performed. Even if the applied pressure is reduced to the reduced pressure P2 during the conveyance of the paper Q, the applied pressure is switched only when the applied pressure can be returned before the colored region 85 reaches the nip portion. However, the effect of reducing thermal diffusion can be obtained without necessarily reducing the applied pressure to the reduced pressure P2. That is, as shown in FIG. 20, the pressure control may be performed on the intermediate blank portion 81 regardless of the length of the intermediate blank portion 81x in the transport direction M1.

  The fixing pressure P1 and the reduction pressure P2 can be changed according to the thickness of the paper, the material of the paper, the environmental temperature, the system speed, and the like.

  In FIG. 20, the length L81x of the intermediate blank portion 81x in the transport direction M1 on the paper Qx is shorter than the sum of the distances L1 and L2. However, in the section of the intermediate blank portion 81x excluding the margins Lm at both ends, pressurization control is performed to lower the applied pressure to the pressure P2b and return the fixing pressure P1. The value of the pressure P2b is a value between the fixing pressure P1 and the reduced pressure P2.

  In the above embodiment, the configuration of the image forming apparatus 1 is within the scope of the present invention, including the image forming method, the configuration of the pressure switching mechanism 36, the timing of the pressure control, the material and dimensions of the component parts, and the like. Can be changed as appropriate. For example, the reference position p2 is not limited to the downstream side of the registration position p1, but may be upstream from the nip portion of the fixing roller pair. When the CPU 201 can control the front end position of the paper at a certain point with a predetermined accuracy, such as a case where a conveyance sequence in which the conveyance is temporarily stopped at the registration position p1 and the paper is fed by the registration roller 26 in synchronization with the primary transfer is adopted. In this case, detection by the paper sensor 41 can be omitted.

  Using either the conveyance speed or the estimated value of the sheet conveyance speed, the fixing nip diameter and the nip width, the timing of switching the pressing force can be adjusted in time for the unfixed toner to reach the nip. When the applied pressure is lowered, the nip diameter is reduced, and the speed of paper conveyance by the fixing roller pair is reduced. At the same time, the timing is adjusted in consideration of the increase in the nip width.

1 Image forming apparatus Q, Q1, Q2, Q3, Q4, Qx Paper (sheet for printing)
3, 4 Object to be transported 31 Heating roller 32 Pressure roller 20 Intermediate transfer belt (conveying mechanism)
26 Registration roller (conveyance mechanism)
36 Pressure switching mechanism 81, 81a, 81b, 81c, 81d Intermediate blank portion 85 Colored region M1 Transport direction M2 Width direction 200 Engine controller (controller)
P1 Fixing pressure (first pressure)
P2 Low pressure reduction (second pressure)
P2b pressure (second pressure)
L1 distance L2 distance L3 distance p2 reference position p6 nip position

Claims (8)

An image forming apparatus having a pair of rollers for heating a sheet for printing,
A transport mechanism for transporting the sheet to a nip portion sandwiched between the pair of rollers;
A pressure switching mechanism that changes the relative position between one and the other of the pair of rollers in a direction in which the pressure applied to the sheet changes;
A first pressure is applied to the sheet over a period in which a coloring region that attaches a coloring material to at least a part of a width direction orthogonal to the conveyance direction of the printing surface of the sheet passes through the nip portion, wherein as low gastric pressure force than the non-colored region is not a colored region is said first pressure during passing through the nip portion is applied to the sheet, and a controller for controlling the pressurized pressure changeover replacement mechanism,
The controller is
Indicates the position of the colored region in the transport direction in a transported body that is the single sheet transported by the transport mechanism or a sheet row composed of the plurality of sheets when printing on a single sheet or a plurality of sheets. An acquisition unit for acquiring print position information;
Based on the printing position information, a detection unit that detects an intermediate blank portion that is a portion between the two colored regions arranged in the transport direction in the transported body;
A determination unit that determines whether or not the length of the intermediate blank portion in the transport direction is a set length or more,
When the determination unit determines that the length is equal to or longer than a set length, the applied pressure is changed from the first pressure to the first pressure after the front end of the intermediate blank portion has passed through the nip portion. The pressure switching mechanism is controlled so that the applied pressure is switched from the second pressure to the first pressure before the rear end of the intermediate blank portion reaches the nip portion. And
When the determination unit determines that the length is not equal to or longer than a set length, the applied pressure is changed from the first pressure to the first pressure after the front end of the intermediate blank portion has passed through the nip portion. The pressure is switched to the third pressure, which is a pressure between the second pressure, and the applied pressure is switched from the third pressure to the first pressure before the rear end of the intermediate blank portion reaches the nip portion. And an image forming apparatus that controls the pressure switching mechanism . The set length is the sum L3 of the transport distance L1 in the time required for switching from the first pressure to the second pressure and the transport distance L2 in the time required for switching from the second pressure to the first pressure. The image forming apparatus according to claim 1 , wherein the image forming apparatus has the length described above. The controller determines a point in time when the front end of the transported body has passed a reference position upstream from the nip portion of the transport path in which the transported body is transported in order to determine a start time of switching of the pressurizing force. reference as the intermediate blank portion image forming apparatus front end according to claim 1 or 2, wherein the rear end of the time and the intermediate blank portion passing completely through the nip calculates when to arrive at the nip portion of the. A sensor for detecting the presence or absence of a sheet at the reference position;
The image forming apparatus according to claim 3 , wherein the controller detects a time point when a front end of the transported body has passed the reference position based on an output of the sensor. The controller detects the amount of rotation of the pair of rollers and calculates the time for the intermediate blank portion to pass through the nip portion when calculating the time point when the front end of the intermediate blank portion finishes passing through the nip portion. The image forming apparatus according to claim 3 or 4 . The controller detects the amount of rotation of a motor that is a drive source of the transport mechanism when calculating the time point when the rear end of the intermediate blank portion reaches the nip portion, and the intermediate blank portion is moved from the reference position to the reference position. the image forming apparatus according to any one of claims 3 to 5 to calculate the time to move to the nip portion. An image forming apparatus having a pair of rollers for heating a sheet for printing and a transport mechanism for transporting the sheet to a nip portion sandwiched by the pair of rollers. A roller control method for changing a relative position in a direction in which the pressure applied to the sheet changes ,
A transport direction of a printing surface of the sheet in a transported body that is the single sheet transported by the transport mechanism or a sheet row composed of the plurality of sheets when printing on a single sheet or a plurality of sheets; Obtaining print position information indicating a position in the transport direction of a colored region to which a color material is attached to at least a portion of the orthogonal width direction ;
Based on the print position information, an intermediate blank portion that is a portion between the two colored regions arranged in the transport direction in the transported body is detected,
Determine whether the length of the intermediate blank portion in the transport direction is greater than or equal to a set length;
The colored region controls the relative position so that the first pressure is Kuwawa to the seat over a period of time to pass through the nip portion,
When it is determined that the length is equal to or longer than the set length, after the front end of the intermediate blank portion passes through the nip portion, a non-colored region that is not the colored region on the printing surface is the nip. The pressure is switched to a second pressure that is lower than the first pressure during a period of passing through the section, and the pressure is changed from the second pressure before the rear end of the intermediate blank portion reaches the nip. Changing the relative position to switch to the first pressure ;
When it is determined that the length is not equal to or longer than the set length, the applied pressure is applied during a period in which the non-colored region passes through the nip portion after the front end of the intermediate blank portion passes through the nip portion. Is switched from the first pressure to a third pressure which is a pressure between the first pressure and the second pressure, and before the rear end of the intermediate blank portion reaches the nip portion, the applied pressure is The roller control method , wherein the relative position is changed so as to switch from a third pressure to the first pressure . The relative positions of a pair of rollers for heating the sheet for printing, a transport mechanism for transporting the sheet to a nip portion sandwiched by the pair of rollers, and one of the pair of rollers and the other are added to the sheet. A computer program executed in an image forming apparatus having a pressure switching mechanism that changes pressure in a changing direction,
In the computer that the image forming apparatus has,
A transport direction of a printing surface of the sheet in a transported body that is the single sheet transported by the transport mechanism or a sheet row composed of the plurality of sheets when printing on a single sheet or a plurality of sheets; Executing a process of acquiring print position information indicating a position in the transport direction of a colored region to which a color material is attached to at least a part of the orthogonal width direction ;
Based on the print position information, to execute a process of detecting an intermediate blank portion that is a portion between the two colored regions arranged in the transport direction in the transported body,
Executing a process of determining whether the length of the intermediate blank portion in the transport direction is equal to or longer than a set length;
The colored region so that the implementation of the process for controlling the pressurized pressure changeover sorting mechanism so that the first pressure is Kuwawa to the seat over a period of time to pass through the nip portion,
When it is determined that the length is equal to or longer than the set length, a non-colored area that is not the colored area on the printing surface after the front end of the intermediate blank part passes through the nip part. The pressure is switched from the first pressure to a second pressure lower than the first pressure during the period of passing through the section, and before the rear end of the intermediate blank portion reaches the nip portion, the pressure is Executing a process of controlling the pressurization switching mechanism so as to switch from the second pressure to the first pressure ;
When it is determined that the length is not longer than a set length, the applied pressure is applied during a period in which the non-colored region passes through the nip portion after the front end of the intermediate blank portion passes through the nip portion. The pressure is switched from the first pressure to a third pressure that is a pressure between the first pressure and the second pressure, and before the rear end of the intermediate blank portion reaches the nip portion, the applied pressure is changed to the first pressure. A computer program for executing processing for controlling the pressurization switching mechanism so as to switch from three pressures to the first pressure .

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