JP2011158809A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To facilitate paper conveyance control in the upstream of a nip, while uniforming the fixing quality of an image in paper. <P>SOLUTION: A fixing device includes a fixing roller, a pressure roller, a control circuit, a temperature sensor for detecting the surface temperature of the pressure roller, and a paper sensor that is arranged at the upstream position to the nip in paper conveyance and detects presence of paper at the upstream position. The control circuit detects, using the paper sensor, that the rear end of paper during passing through the nip finishes passing through the upstream position. Then, when the temperature detected by the temperature sensor is lower than an allowable temperature range, the control circuit reduces the rotation speed of the pressure roller and returns the rotation speed after the paper finishes passing through the nip. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a fixing device for forming an image on a recording sheet, an image forming device having the fixing device, and a roller rotation speed control method. The image forming apparatus includes a printer, a copier, a facsimile machine, and a multifunction peripheral (also referred to as MFP: Multifunction Peripherals).

  An image forming apparatus that forms an image on a sheet by electrophotography includes a fixing device that heats the sheet on 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 fixing roller (also referred to as a heating roller) having a heat source that raises the temperature of the surface, and a pressure roller that contacts the fixing roller to form a nip portion. The sheet is conveyed to the nip portion and heated and pressurized while passing through the nip portion. A halogen lamp is widely used as a heat source for the fixing roller. There is also a fixing device including a belt-like magnetic conductor that generates heat by induction heating (IH) as a heat source.

  When the paper passes through the nip portion, the heat of fusion accompanying the melting of the toner and the heat dissipation to the paper occur in the nip portion. When the sheet is conveyed at a constant speed in the fixing device, the temperature of the nip portion gradually decreases from the leading end to the trailing end of the sheet. Normally, in consideration of such a temperature drop, the temperature of the nip portion at the start of fixing is set high so that the temperature at the rear end of the sheet does not fall below the temperature range where proper fixing is possible.

  There is a prior art that changes the sheet conveyance speed during fixing. One of them is to detect the surface temperature of the fixing roller and decrease the rotation speed of the fixing roller in response to the decrease in the surface temperature (Patent Document 1). According to this, since the nip time is extended due to the decrease in speed, the fixing limit (lower limit temperature) is lowered and the number of sheets that can be continuously formed is increased. The other is to reduce the rotation speed of the fixing roller during a period when one sheet passes through the nip (Patent Documents 2 and 3). According to this, it is possible to equalize the fixing temperature condition from the leading edge to the trailing edge of the sheet, and to uniform the fixing quality including gloss and strength.

JP-A-57-79975 No. 4-1564 JP-A-8-95422

  Conventionally, it is necessary to control the conveyance speed and the delivery timing upstream of the nip portion in accordance with the conveyance deceleration in fixing. Further, since the conveyance is always decelerated when the sheet passes through the nip portion, the productivity of image formation is reduced.

  The present invention has been made in view of such circumstances, and an object thereof is to simplify paper conveyance control upstream of the nip portion while achieving uniform image fixing quality in the paper.

  The fixing device that achieves the above object includes a fixing roller having a heat source that raises the temperature of the surface, a pressure roller that forms a nip portion that contacts the fixing roller and heats the paper, the fixing roller, and a heating roller. A fixing device having a control circuit for controlling a rotation speed of the pressure roller, the temperature sensor detecting a surface temperature of the pressure roller, and disposed at an upstream position with respect to the nip portion in paper conveyance; A paper sensor for detecting the presence or absence of paper. The control circuit detects that the trailing edge of the paper passing through the nip portion has passed the upstream position, and then detects the temperature detected by the temperature sensor is lower than the allowable temperature range. In this case, the rotational speed is decreased, and the rotational speed is restored after the paper has passed through the nip portion.

  According to the present invention, it is possible to simplify paper conveyance control upstream of the nip portion while achieving uniform image fixing quality in the paper. Since the conveyance speed is reduced only when it is necessary, a decrease in productivity can be minimized.

6 is a timing chart showing an outline of a driving control method for a fixing roller. It is a figure which shows an example of the temperature fall amount table. It is a figure which shows an example of a control table. 1 is a diagram illustrating an outline of a configuration of an image forming apparatus. It is a figure which shows the structure of a fixing device. It is a figure which shows the structure of a fixing device provided with a fixing device. 6 is a timing chart illustrating an example of rotation driving control of a roller of a fixing device. 6 is a timing chart illustrating another example of rotation driving control of a roller of a fixing device.

  In the electrophotographic image forming apparatus, as shown in FIG. 1, the roller rotation speed is switched according to the change in the surface temperature of the pressure roller. In a state where the surface temperature of the pressure roller is maintained at the normal temperature T0 that is a standby temperature, the sheet is carried into the nip portion from upstream. At that time, the pressure roller rotates at the normal speed S0 and presses the sheet against the fixing roller while feeding the sheet downstream at a peripheral speed corresponding to the rotation speed. During the period from time t0 when the leading edge of the paper reaches the nip portion to time t5, t5 ′, t5 ″ when the trailing edge of the paper finishes passing through the nip portion (this is referred to as a paper passage period) TP as necessary as follows The roller rotation speed is changed stepwise.

  The temperature change A is a change in which the temperature decrease amount in the paper passage period TP is smaller than the difference between the normal temperature T0 and the first threshold temperature T1 (T1 <T0). When the surface temperature change of the pressure roller is the temperature change A, the surface temperature is sufficiently higher than the lower limit Tmin of the proper fixing range, so it is not necessary to reduce the roller rotation speed. In this case, the roller rotation speed is maintained at the normal speed S0. Since the transport speed does not decrease, the paper passage period TP is the shortest.

  The temperature change B is a change in which the temperature decrease amount reaches the difference between the normal temperature T0 and the first threshold temperature T1 at the time point t3. When the surface temperature change of the pressure roller is the temperature change B, it is necessary to reduce the roller rotation speed in order to extend the nip time. In this case, the roller rotation speed is switched to the first correction speed (deceleration speed) S1 that is smaller than the normal speed S0 at or near the time t3. That is, the conveyance speed is reduced by one step. The end of the paper passage period TP is a time point t5 'after the time point t5. After the time t5 ', the roller rotation speed is returned from the first correction speed S1 to the normal speed S0.

  In the temperature change C, the temperature decrease amount reaches the difference between the normal temperature T0 and the first threshold temperature T1 at the time point t2 before the time point t3, and further, the temperature decrease amount is changed from the normal temperature T0 to the second threshold temperature at the subsequent time point t4. The change reaches a difference from T2 (T2 <T1). Even when the surface temperature change of the pressure roller is the temperature change C, it is necessary to reduce the roller rotation speed in order to extend the nip time. In this case, the roller rotation speed is switched from the normal speed S0 to the first correction speed S1 at a time point near the time point t2 or later, and the roller rotation speed is changed to the first correction speed S1 at a time point near the time point t4 or later. To a smaller second correction speed S2. That is, the conveyance speed is decelerated by two steps one by one. The end of the paper passage period TP is a time point t5 ″ after the time point t5 ′. After the time point t5 ″, the roller rotation speed is returned from the second correction speed S2 to the normal speed S0.

  The first threshold temperature T1 and the second threshold temperature T2 are set based on a required heating time for obtaining an appropriate fixing quality. Since the size of the nip portion is constant, the amount of heat received by the sheet in the nip portion depends on the time required to move the distance corresponding to the size of the nip portion (nip time). The lower the surface temperature, the more heat is required, and the required heating time, which is the nip time for obtaining the required heat, is long. However, if the nip time is controlled for each temperature range of a predetermined width, a practically suitable fixing quality can be obtained. The first threshold temperature T1 and the second threshold temperature T2 are set values that determine the lower limit of a temperature range (a range in which application of a certain required heating time is allowed) related to nip time control.

The first threshold temperature T1 and the second threshold temperature T2 are not fixed values but relative values to the normal temperature T0. Thereby, even when the setting of the normal temperature T0 is changed according to the operating environment and the type of paper, the nip time can be optimized as in the case of the standard setting. However,
The first threshold temperature T1 and the second threshold temperature T2 may be fixed values.

  When it is necessary to switch the roller rotation speed, it is desirable to switch at a timing that does not affect the sheet conveyance upstream of the nip portion. That is, the roller rotation speed may be reduced after the sheet is separated from the upstream conveying roller. The device configuration for that purpose will be described later.

  A first method for determining whether or not to switch the roller rotation speed is to detect the surface temperature and compare it with the first threshold temperature T1 and the second threshold temperature T2. In the second method, the elapsed time from the start time t1 of the paper passage period TP specified by the paper conveyance timing information is counted, and the surface temperature of the pressure roller is determined based on the temperature drop characteristic obtained in advance through experiments or calculations. And the estimated value is regarded as the actual surface temperature and compared with the first threshold temperature T1 and the second threshold temperature T2.

  The temperature drop characteristic is related to the heat capacity of the paper and the length of the paper passage period TP at the normal speed S0. Therefore, as shown in FIG. 2, for each type of paper classified by heat capacity, a temperature reduction amount table TA1 that associates the temperature reduction amount for each paper size is stored in an appropriate memory so that the control means can refer to it. Then, for example, according to the designation of the paper type and paper size in the operation setting operation, the surface temperature of the pressure roller can be estimated based on the corresponding temperature decrease characteristics.

  In the illustration of FIG. 2, A3, A4, and A5 are given as representative examples of the paper size, but the temperature reduction amount is also supported for various sizes such as B5, B4, postcard, letter, and legal. It can be attached. Further, for each size, the temperature decrease amount can be associated with the case where the long side is along the conveyance direction and the case where the short side is along.

  The first correction speed S1 and the second correction speed S2 related to the roller rotation control are set based on the required heating time calculated for each predetermined temperature section. In the control table TA2 illustrated in FIG. 3, the required heating time, the roller peripheral speed, and the roller rotation speed are associated with each temperature drop amount category. The quotient (nip width / required heating time) obtained by dividing the conveyance direction dimension (nip width) of the nip portion by the required heating time is the roller peripheral speed. In the example, the nip width is calculated as 4.5 mm. The roller rotation speed is determined from the outer diameter of the heating roller and the roller peripheral speed.

  Hereinafter, the configuration of the image forming apparatus having a function of switching the roller rotation speed will be described.

  An image forming apparatus 1 illustrated in FIG. 4 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 fixing roller (heating roller) 31 and a pressure roller 32. As shown in FIG. 5, the fixing roller 31 has a halogen lamp 310 as a heat source for raising the temperature of the surface thereof. The pressure roller 32 is in contact with the fixing roller 31 to form a nip portion 35 for heating the paper P. A temperature sensor (for example, an infrared sensor) 314 that detects the surface temperature in a non-contact manner is disposed in the vicinity of the peripheral surface of the fixing roller 31, and a contact type that detects the surface temperature in the vicinity of the peripheral surface of the pressure roller 32. A temperature sensor 315 is disposed.

  The fixing roller 31 and the pressure roller 32 are rotatably supported by a bearing (not shown). The pressure roller 32 is pressed against the fixing roller 31 by a biasing mechanism (not shown) using a spring or an elastic member. The fixing roller 31 is driven to rotate at a predetermined peripheral speed in a direction (counterclockwise in the drawing) in which the paper P is fed out by the driving mechanism. The pressure roller 32 is rotated by the rotation of the fixing roller 31 by the frictional force with the fixing roller 31 at the nip portion 35. However, on the contrary, a mechanism configuration in which the pressure roller 32 is driven to rotate and the fixing roller 31 is driven to rotate, or a mechanism configuration in which both the fixing roller 31 and the pressure roller 32 are driven to rotate may be employed.

  The outline of the image forming operation is as follows. For example, when a color print job is given from an external device (not shown) (for example, a personal computer), the charging device 12 uniformly charges the surface of the photosensitive drum 11 in the process units UY, UM, UC, UK. A light beam emitted from the print head 13 is controlled on and off in accordance with image data from an external device, and a latent image is formed on the surface of the photosensitive drum 11. The 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 device 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 P of the type and size specified by the job is taken out from the paper storage unit 24, and the intermediate transfer belt 20 and the secondary transfer roller 27 are opposed to each other by the conveying roller 26. It is conveyed to the next transfer position. The sheet P on which the toner image is secondarily transferred from the intermediate transfer belt 20 passes through the fixing device 30 and is then discharged to the discharge tray 29 by the discharge roller pair 28. When the paper P passes through the fixing device 30, the toner is melted and the image is fixed on the recording paper.

  The transport roller closest to the fixing device 30 on the upstream side of the fixing device 30 is a secondary transfer roller 27. The distance between the nip portion 35 of the fixing device 30 and the secondary transfer position is shorter than the minimum value of the conveyance direction dimension of the compatible size paper that can be used. In other words, in the image forming apparatus 1, there is a period in which the sheet spans both the nip portion 35 of the fixing device 30 and the nip portion of the secondary transfer. In order to switch the conveyance speed in the fixing device 30 at a timing that does not hinder upstream conveyance, the paper sensor 60 is disposed downstream of the secondary transfer roller 27 and close to the secondary transfer roller 27. . The sheet sensor 60 is disposed at an upstream position with respect to the nip portion 35 of the fixing device 30.

  The operation of the image forming apparatus 1 is controlled by the control circuit 100. As illustrated in FIG. 6, the control unit 100 includes a CPU that executes a control program, a storage unit that stores programs and various data, and a communication unit that communicates with an external device. Detection signals are input to the control unit 100 from the temperature sensors 314 and 315 and the paper sensor 60. The heater driver 117 that drives the halogen lamp 310 of the fixing roller 31 is controlled based on the output of the temperature sensor 314, and the rotation drive mechanism 120 that rotates the fixing roller 31 is controlled based on the output of the temperature sensor 315. In the temperature control of the fixing roller 31, setting adjustment is performed such that the normal temperature is raised or returned to the standard according to the type and size of the sheet designated by the operation unit 110. Such a control circuit 100 and the fixing device 30 constitute the fixing device 3.

  7 and 8 show examples of the rotational speed control of the pressure roller 32. FIG. The example here is an example of control when images are formed on two sheets. In the example of FIG. 7, the paper size is A4 (297 mm × 210 mm) and the short direction is the transport direction. In the example of FIG. 8, the paper size is A5 (210 mm × 148 mm), and the short direction is the transport direction.

  The rotation speed control of the pressure roller 32 is performed in order to suppress a decrease in the surface temperature of the pressure roller 32 within an allowable range in terms of fixing quality. Since the fixing roller 31 in contact with the pressure roller 32 has a heat source and is appropriately controlled in temperature, the surface temperature of the fixing roller 31 is basically constant although there is some variation due to the responsiveness of the heat source. Kept. On the other hand, the surface temperature of the pressure roller 32 gradually decreases when the sheet passes through the nip portion 35.

  In FIG. 7, the sheet conveyed from the upstream at time t <b> 11 reaches the nip portion 35, and fixing on the sheet starts. Time t11 is the leading edge of the paper passage period TP. At time t10 before time t11, the output of the paper sensor 60 arranged at the upstream position viewed from the nip portion 35 changes from “absent” to “present”. This indicates that the leading edge of the paper has reached the upstream position.

  The surface temperature of the pressure roller 32 gradually decreases during the paper passage period TP. At time t12, since the surface temperature has decreased to the threshold temperature on the lower limit side of the normal speed application range, it is necessary to reduce the conveying speed so that the nip time becomes the required heating time corresponding to the current temperature. However, at time t12, there is a sheet in the upstream position. That is, the sheet is not released from the secondary transfer roller 27. Accordingly, the vehicle does not decelerate at this time t12. If the speed is decelerated, the secondary transfer roller 27 sends out the paper faster than the conveyance speed of the nip portion 35, so that the paper is curved in the conveyance path between the nip portion 35 and the secondary transfer roller 27 and comes into contact with the conveyance guide. The unfixed toner image is disturbed.

  The rotation speed of the pressure roller 32 is reduced after the trailing edge of the sheet passes through the upstream position where the sheet sensor 60 is disposed and the sheet is released from the secondary transfer roller 27. That is, it is performed in response to the control circuit 100 detecting that the paper has changed from “present” to “no” at the upstream position based on the output of the paper sensor 60. In the figure, at the time t13, the rotation speed of the pressure roller 32 is switched from the normal speed (for example, 150 mm / s) to the correction speed (for example, 100 mm / s) lower by one step. As a result, the nip time is extended, so that the surface temperature decreases more slowly than before.

  Thereafter, when the paper passage period TP ends at time t14, heat absorption by the paper is lost, and the surface temperature of the pressure roller 32 returns to the normal temperature due to heat conduction from the fixing roller 31. The reduced rotational speed is returned to the normal speed within the inter-paper period from the time t14 until the paper passage period TP of the next paper starts. The rotation speed control is performed on the second sheet as well as the first sheet.

  In FIG. 8, the leading edge of the paper reaches the upstream position at time t20, and the leading edge of the paper reaches the nip portion 35 at time t21 thereafter. In the paper passage period TP from time t21 to time t22, the surface temperature of the pressure roller 32 gradually decreases, but does not decrease to the threshold temperature. Accordingly, the rotation speed of the pressure roller 32 is set to a normal speed over the entire period including the sheet passage period TP of the first sheet, the sheet passage period TP of the second sheet, and the inter-paper period therebetween. Kept.

  In the above embodiment, the fixing device 30 is not limited to the lamp heating type, but may be an induction heating type. The image forming apparatus 1 may be any apparatus that heats and presses paper, and the present invention can be applied to various image forming apparatuses such as a monochrome printer, a copying machine, a facsimile machine, and a multifunction machine.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 3 Fixing apparatus 31 Fixing roller 310 Halogen lamp (heat source)
32 Pressure roller 35 Nip part 100 Control circuit 315 Temperature sensor 60 Paper sensor TA2 Control table

Claims (6)

A fixing roller having a heat source for raising the temperature of the surface, a pressure roller that forms a nip for contacting the fixing roller to heat the paper, and a control for controlling the rotation speed of the fixing roller and the pressure roller A fixing device having a circuit,
A temperature sensor for detecting a surface temperature of the pressure roller;
A sheet sensor that is disposed at an upstream position with respect to the nip portion in the sheet conveyance and detects the presence or absence of the sheet at the upstream position;
The control circuit detects that the trailing edge of the paper passing through the nip portion has passed the upstream position, and then detects the temperature detected by the temperature sensor is lower than the allowable temperature range. In this case, the fixing device reduces the rotational speed and returns the rotational speed to the original after the paper has passed through the nip portion. The control circuit includes a required heating time required for heating the conveyance time corresponding to the length of the nip portion in accordance with the amount of decrease in the surface temperature of the pressure roller during a period in which the sheet passes through the nip portion. The fixing device according to claim 1, wherein the rotation speed is decreased stepwise so that A control table for associating a temperature and a rotational speed respectively inside and outside the allowable temperature range;
The fixing device according to claim 1, wherein the control circuit determines the rotation speed with reference to a control table. A fixing device according to any one of claims 1 to 3,
An image forming apparatus, wherein a toner image transferred onto a sheet is fixed by the fixing device. A method of controlling the roller rotation speed of a fixing device having a fixing roller having a heat source for raising the temperature of the surface and a pressure roller that forms a nip for heating the paper in contact with the fixing roller,
A paper sensor for detecting the presence or absence of paper at the upstream position is disposed at an upstream position with respect to the nip portion in paper conveyance,
When the surface temperature of the pressure roller is lower than the allowable temperature range after the paper sensor detects that the trailing edge of the paper passing through the nip portion has passed the upstream position, the fixing roller And a rotation speed control method for a fixing device, wherein the rotation speed of the pressure roller is reduced and the rotation speed is restored after the paper has passed through the nip portion. The type of paper to be used is determined by using a temperature reduction amount table prepared in advance that associates the type of paper with the estimated value of the amount of reduction in the surface temperature of the pressure roller when the paper passes through the nip portion. 6. The surface temperature of the pressure roller is estimated from the designation information shown and the sheet conveyance timing information, and the estimated value is regarded as the surface temperature of the pressure roller and compared with the lower limit value of the allowable temperature range. The roller rotation speed control method of the fixing device as described.

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