JP2017198976A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that can suppress extension/contraction of an image occurring due to a change in external force imposed on a recording material and output an image without density unevenness.SOLUTION: An image forming apparatus comprises: a photoreceptor; an exposure part that exposes the photoreceptor according to image information and forms a latent image on the photoreceptor; a developing part that develops the latent image with toner; and a transfer part that transfers a toner image from the photoreceptor to a recording material. The apparatus changes a surface moving speed of the photoreceptor at a timing prior by a predetermined time to a predetermined timing at which the moving speed of the recording material at the transfer part changes.SELECTED DRAWING: Figure 5

Description

  The present invention relates to an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer.

  An example of an electrophotographic image forming apparatus will be described with reference to FIG. FIG. 13 is a cross-sectional view showing a schematic configuration of an example of an electrophotographic image forming apparatus (monochrome printer).

  In the image forming apparatus 100, the outer peripheral surface (front surface) of the photosensitive drum 1 that is rotating is uniformly charged to a predetermined potential and polarity by the charging roller 2.

  Reference numeral 3 denotes a laser scanner type exposure unit 3. The exposure unit 3 irradiates the rotating polygon mirror with the laser beam L output from the laser oscillator. The polygon mirror scans (irradiates) pixels for one horizontal line (= 1 scan) on one mirror surface in the main scanning direction (bus line direction) of the photosensitive drum 1. By this scanning exposure, an electrostatic latent image corresponding to target image information is formed on the charged surface of the photosensitive drum 1 that is rotating.

  The developing unit 4 develops a toner image by attaching toner to the latent image on the surface of the photosensitive drum 1.

  The paper feeding unit 25 supplies the uppermost recording material P on the tray 11 to the transfer nip Pb by the roller 12 having the separation pad 26 pressed against it. The transfer nip Pb is formed by the photosensitive drum 1 and the transfer roller 5. The recording material P is nipped and conveyed by the transfer nip Pb, and the toner image is transferred from the surface of the photosensitive drum 1 onto the recording material by the transfer roller 5 in the conveyance process.

  The recording material P carrying the unfixed toner image is conveyed to the fixing nip Pd of the fixing unit 6. The fixing nip Pd rotates while being in contact with a heater (not shown), a cylindrical film 24 and a pressure roller 23. Is formed by. The recording material P is heated while being nipped and conveyed by the fixing nip Pd, whereby the toner image is heated and fixed on the recording material.

  The recording material exiting the fixing nip Pd is discharged out of the apparatus by the roller 14.

  The image forming apparatus 100 is required to output target image information input from an image scanner (not shown) or an external device such as a computer as imaged. For this purpose, it is necessary to match the peripheral speed at the transfer nip Pb on the surface of the photosensitive drum 1 with the moving speed at the transfer nip of the recording material P to which the toner image is to be transferred.

  The scanning interval in the sub-scanning direction (circumferential direction) by scanning with the polygon mirror is always kept constant. Therefore, if the photosensitive drum 1 is always rotating at a constant speed Vdrum, the magnification in the sub-scanning direction of the toner image formed on the photosensitive drum 1 is always constant.

  However, if the moving speed Vpaper of the recording material P in the transfer nip Pb becomes slow, the toner image on the surface of the photosensitive drum 1 is transferred onto the recording material one after another as the photosensitive drum rotates. It will be in a contracted state.

  For example, assume that toner images are developed on the surface of the photosensitive drum 1 at intervals of 10 mm in the sub-scanning direction. On the other hand, if the recording material P moves 0.2% later in the transfer nip Pb, the recording material moves only 9.8 mm while the photosensitive drum 1 moves 10 mm. Therefore, the interval between the toner images that are actually transferred onto the recording material is 9.8 mm in the sub-scanning direction, and an image that is 0.2% smaller in the conveyance direction of the recording material P than the target image information is obtained. .

  Conversely, when the recording material P moves 0.2% faster, the recording material moves 10.2 mm while the surface of the photosensitive drum 1 advances 10 mm in the transfer nip Pb. Therefore, the interval between the toner images that are actually transferred onto the recording material is 10.2 mm in the sub-scanning direction, and an image that is 0.2% larger than the target image information is obtained.

  The moving speed Vpaper of the recording material P in the actual transfer nip Pb may not be constant. This is because various external forces act on the recording material when nipped and conveyed by the transfer nip Pb.

  The external force acting on the recording material P includes a frictional resistance received from the paper feeding unit 25, a sliding resistance received from the entrance guide 28, and a tensile force received from the fixing unit 6.

  In the paper feeding unit 25, the separation pad gives frictional resistance to the recording material. In addition, in the conveyance direction of the recording material P, the inlet guide 28 provided on the upstream side of the transfer nip Pb gives sliding resistance to the recording material. In the conveyance direction of the recording material P, an inlet guide 27 provided on the upstream side of the fixing nip Pd gives sliding resistance to the recording material. These frictional resistance and sliding resistance are both external forces that act in the direction of inhibiting the conveyance of the recording material P.

  In the fixing unit 6, the pressure roller 23 may thermally expand, and the surface movement speed of the pressure roller may be relatively faster than the movement speed of the recording material P. At this time, at the moment when the leading edge of the recording material P enters the fixing nip Pd, the recording material receives a tensile force due to the conveying force of the pressure roller 23.

  When a force acts in a direction that hinders conveyance of the recording material P, such as frictional resistance or sliding resistance, the moving speed Vpaper of the recording material in the transfer nip Pb gradually decreases, and the toner transferred to the recording material The image shrinks. Conversely, when a pulling force acts on the recording material P and the moving speed Vpaper of the recording material in the transfer nip Pb increases, the toner image transferred to the recording material expands.

  In this way, whenever the external force applied to the recording material changes, the toner image transferred onto the recording material expands or contracts.

  In response to such a problem, a change in the conveyance speed of the recording material in the transfer nip is detected, and based on the detection result, the driving speed of the motor for driving the transfer roller is changed, and the moving speed of the recording material is changed. It is possible to keep it constant. By keeping the moving speed of the recording material in this way, it is possible to eliminate the deviation between the surface moving speed of the photosensitive drum that always rotates at a constant speed and the conveying speed of the recording material. According to this method, the toner image transferred to the recording material does not expand or contract.

  However, in order to realize the above method, at least a motor for driving the photosensitive drum and a motor for driving the transfer roller are required. When the exposure unit 3 is a laser scanner system, a motor (scanner motor) that rotates the polygon mirror is also required. In an image forming apparatus, it is required to reduce the number of motors in order to meet the demand for reduction in size and weight and to achieve cost reduction.

  Patent Document 1 discloses an image forming apparatus that can roughly match the peripheral speed of the surface of the photosensitive drum and the moving speed of the recording material to which the toner image is to be transferred. This image forming apparatus has a main motor and a scanner motor, and is an apparatus premised on the following phenomenon. That is, the moving speed of the recording material increases at the timing when the leading edge of the recording material enters the fixing portion, and the recording material is transferred onto the recording material after the leading edge of the recording material enters the fixing nip to the trailing edge of the recording material. It is assumed that the toner image is gradually extended in the sub-scanning direction.

JP 2012-98590 A

  In recent years, in image forming apparatuses, as the image forming apparatus further increases in speed, it is possible to suppress scattering of a toner image transferred from the surface of the photosensitive drum 1 onto the recording material in the transfer nip and to improve dot reproducibility. It has been demanded. If there is a difference between the peripheral speed of the photosensitive drum surface and the moving speed of the recording material, the image not only expands and contracts, but also the density of the image changes. If the density of the image changes, image unevenness may occur.

  FIGS. 14A to 14C show how the density of an image changes as the image expands or contracts. The degree of influence of image unevenness due to image expansion / contraction varies depending on the resolution. FIG. 14A shows the case where the resolution is low, and FIGS. 14B and 14C show the case where the resolution is high. A numerical value (%) represents the density of the image.

  First, (a) and (b) of FIG. 14 are compared. Even if an attempt is made to output a 50% halftone image from an external device such as an image scanner or a computer, as shown in FIG. In the case of an image having a low resolution as shown in FIG. 14A, the size of the grid is larger than that in the case of a high resolution shown in FIG. For this reason, even if the peripheral speed of the photosensitive drum surface and the moving speed of the recording material are slightly deviated from each other, a white portion remains and the difference in image density is not noticeable.

  However, in response to the recent demand for outputting fine lines and small characters, when trying to output a high-resolution image as shown in FIG. It becomes smaller than the case of low resolution. As shown in FIGS. 15A to 15D, the scattering degree (width) of the toner image does not change regardless of the image resolution and the image magnification. For this reason, when the image is shrunk at the time of transfer in high resolution printing, the white portion is filled with toner, and the density looks dark.

  The density error between the image transferred from the external device and the image actually output on the recording material due to toner scattering can be eliminated to some extent by correction. Specifically, the actual output image density (commonly called γ curve) with respect to the transfer image density from the external device is measured in advance. Then, the image information transferred from the external device is corrected.

  Therefore, when the same image information as in FIG. 14A is transferred from an external device, the image information is actually corrected as shown in FIG. 14C. Specifically, when low resolution image information is transferred at an image density of 50% as shown in FIG. 14A, the image density is changed to 37 while converting the resolution to a high resolution as shown in FIG. Reduce to 5%. By such correction, the density of the actually output image can be adjusted to 60% as in the case of FIG. However, since the image in FIG. 14C has a high resolution, if the image expands or contracts, the effect on the image density is large.

  In the case of FIG. 14A, the density of the image without expansion / contraction is 60%, the density of the contracted image is 65%, and the density of the expanded image is 55%. On the other hand, in the case of FIG. 14C, the density of the image without expansion / contraction is 60%, the density of the contracted image is 75%, and the density of the expanded image is 45%. 14 (a) and 14 (c), both of the images without expansion / contraction have a density of 60%, but when expansion / contraction occurs, the image of FIG. 14 (c) is more than the image of FIG. 14 (a). It can be seen that the concentration fluctuates greatly.

  In the method of Patent Document 1, the expansion and contraction of the toner image formed on the surface of the photosensitive drum cannot be exactly matched to the fine speed change when the recording material is conveyed. For this reason, when an image with higher resolution than before is formed, density unevenness may occur.

  An object of the present invention is to provide an image forming apparatus capable of suppressing the expansion and contraction of an image caused by a change in external force applied to a recording material and capable of outputting an image without density unevenness.

In order to achieve the above object, an image forming apparatus according to the present invention includes:
A photoreceptor,
An exposure unit that exposes the photoconductor according to image information and forms a latent image on the photoconductor;
A developing unit for developing the latent image with toner;
A transfer portion for transferring a toner image from the photoreceptor to a recording material;
In an image forming apparatus having
The apparatus is characterized in that the surface moving speed of the photoconductor is changed at a timing a predetermined time before a predetermined timing at which the moving speed of the recording material in the transfer portion changes.

An image forming apparatus according to the present invention is
A photoreceptor,
An exposure unit that exposes the photoconductor according to image information and forms a latent image on the photoconductor;
A developing unit for developing the latent image with toner;
A transfer portion for transferring a toner image from the photoreceptor to a recording material;
In an image forming apparatus having
The apparatus is characterized in that the exposure interval in the sub-scanning direction by the exposure unit is changed at a timing before a predetermined time with respect to a predetermined timing at which the moving speed of the recording material in the transfer unit changes.

  According to the present invention, it is possible to provide an image forming apparatus that can suppress the expansion and contraction of an image caused by a change in external force applied to a recording material and can output an image without density unevenness.

1 is a diagram for explaining a configuration of an image forming apparatus in Embodiment 1. FIG. Sectional view showing the configuration of the paper feeder Sectional view showing the relationship between the photosensitive drum and the transfer roller and the configuration of the fixing unit The figure for demonstrating the timing when changing the surface moving speed of a photosensitive drum Timing chart showing the timing of changing the surface movement speed of the photosensitive drum in the image forming apparatuses of Example 1 and Comparative Example The figure which shows the comparison result of the image unevenness of an output image about the image forming apparatus of Example 1 and a comparative example FIG. 6 is a diagram for explaining a configuration of an image forming apparatus according to a second embodiment. Timing chart showing the timing of changing the surface movement speed of the photosensitive drum in the image forming apparatus according to the second embodiment. FIG. 6 is a diagram for explaining a configuration of an image forming apparatus according to a third embodiment. Timing chart showing change timing of surface movement speed of photosensitive drum in image forming apparatus of embodiment 3 FIG. 6 is a diagram for explaining a configuration of an image forming apparatus according to a fourth embodiment. FIG. 6 is a diagram for explaining a configuration of an image forming apparatus according to a fifth embodiment. The figure for demonstrating the structure of the conventional image forming apparatus. Diagram showing the effect of image expansion / contraction on density unevenness due to differences in resolution Diagram for explaining unevenness in density when low resolution, high resolution, and image expansion / contraction

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Although the preferred embodiment of the present invention is an example of the best embodiment of the present invention, the present invention is not limited by the following examples, and various other configurations are possible within the scope of the idea of the present invention. It is possible to replace with the configuration of

[Example 1]
(Image forming device)
FIG. 1A is a sectional view showing a schematic configuration of an example of an image forming apparatus (monochrome printer in this embodiment) 50 using an electrophotographic recording technique. FIG. 2B is a plan view showing a schematic configuration of the exposure unit 3. FIG. 2C is a block diagram illustrating a drive system for the main motor 22 and the scanner motor 20.

The configuration of the image forming apparatus shown in this embodiment will be described with reference to FIG. In (a), a broken line is a conveyance path (recording material P path) of the recording material P.
The photosensitive drum (photosensitive member) 1 is rotated by driving a main motor 22 (see FIG. 1C) by a control device (CPU) 21 that controls driving of the image forming apparatus 50. The outer diameter of the photosensitive drum 1 is Φ20 mm. At the normal driving speed of the main motor 22, the moving speed (process speed) Vdrum of the outer peripheral surface (surface) of the photosensitive drum 1 is 200 mm / sec.

  The surface of the photosensitive drum 1 is uniformly charged to a predetermined polarity and potential by a charging roller (charging unit) 2 during a rotating operation.

  Reference numeral 3 denotes a laser scanner type exposure unit. The exposure unit 3 outputs laser light L that is modulated on / off in accordance with target image information input from an external device such as an image scanner (not shown) or a computer. Laser light L emitted from the laser oscillator 52 shown in FIG. 1B is reflected by the polygon mirror 51, refracted by the fθ lens 53, and reaches the photosensitive drum 1 through the reflection mirror 54. The polygon mirror 51 is driven by the scanner motor 20 (see FIG. 1C). The laser beam L reflected by the polygon mirror 51 scans (irradiates) the photosensitive drum 1 in the main scanning direction (the generatrix direction of the photosensitive drum 1).

  The number of rotations of the scanner motor 20 and the output timing of the laser light L are controlled by the control device 21 so as to obtain a predetermined resolution. In this embodiment, the resolutions in the main scanning direction and the sub-scanning direction (the circumferential direction of the photosensitive drum 1) are both controlled to be 1200 dpi.

  An electrostatic latent image corresponding to target image information is formed on the surface of the photosensitive drum 1 by scanning exposure by the exposure unit 3. The exposure position Pe on the surface of the photosensitive drum 1 is located upstream of the transfer nip (transfer position) Pb described later in the rotational direction of the photosensitive drum. In the circumferential direction of the surface of the photosensitive drum 1, the distance Dbe from the center Pb1 of the transfer nip Pb to the exposure position Pe is 31.4 mm (see FIG. 1A).

  The developing unit 4 develops the latent image into a toner image by attaching the toner (developer) stored in the toner container 4a to the latent image on the surface of the photosensitive drum 1 using the developing sleeve 4b.

  The photosensitive drum 1, the charging roller 2, and the developing unit 4 are held by a frame (not shown) and integrated as a cartridge (hereinafter referred to as CRG). The CRG is detachably attached to the image forming apparatus main body 50a, and can be exchanged for a new one as necessary.

  The roller 12 of the pad type paper feeding unit 25 rotates based on the start signal. The roller 12 cooperates with a separation pad (separation member) 26 to separate the recording materials one by one from the bundle of recording materials P loaded on the tray 11 and supply the recording materials into the apparatus. The recording material P is introduced into the transfer nip Pb by the entrance guide 28. The entrance guide 28 is a guide for assisting the recording material P to enter the transfer nip Pb. The transfer nip Pb is formed by the photosensitive drum 1 and a transfer roller (transfer portion) 5 brought into pressure contact with the photosensitive drum.

  In the paper supply unit 25, a position Pa indicates a pressure contact portion formed by the roller 12 and the separation pad 26 pressed on the surface of the roller 12 by the spring S. The distance from the outlet of the press contact portion Pa to the center Pb1 of the transfer nip Pb is preferably as short as possible in order to cope with the higher speed. In this embodiment, the distance (linear distance) Dab between the outlet of the press contact portion Pa and the center Pb1 of the transfer nip Pb is set to 40 mm (see FIG. 1A).

  When the leading edge of the recording material P enters the transfer nip Pb, the toner image on the surface of the photosensitive drum 1 also reaches the transfer nip. The recording material P that has entered the transfer nip Pb is conveyed at a moving speed Vpaper by a photosensitive drum (surface moving speed (peripheral speed) is Vdrum) while being nipped by the transfer nip. During the conveyance process, a predetermined transfer voltage (transfer bias) is applied to the transfer roller 5 from a transfer bias application power source (not shown). By applying a transfer bias to the transfer roller 5, toner images are electrostatically and sequentially transferred from the surface of the photosensitive drum 1 to the recording material P at the transfer nip Pb.

In this embodiment, as the transfer roller 5, a roller in which a nickel-plated steel rod having a diameter of 5 mm is covered with a foam sponge of NBR (nitrile butadiene rubber) having a resistance value adjusted to 5 × 10 ⁷ Ω is used. It was. The resistance value can be adjusted by mixing a conductive material such as hydrin hydrin or carbon into NBR. The outer diameter of the foamed sponge is Φ13 mm. The width in the direction (longitudinal direction) perpendicular to the conveying direction of the recording material P in the foamed sponge is 216 mm assuming a letter size as the maximum size recording material used in the apparatus.

  The higher the pressure contact force of the transfer roller 5 to the photosensitive drum 1, the more easily the moving speed Vpaper of the recording material P in the transfer nip Pb follows the surface moving speed Vdrum of the photosensitive drum 1. However, when exchanging the CRG, the user must bring the photosensitive drum 1 into contact with the transfer roller 5, and if the pressure contact force is too high, it becomes difficult to attach the CRG to the apparatus main body 50a. Therefore, the pressure contact force is preferably (4.9 to 12.74 N) (500 to 1300 gf). In this example, the pressure contact force was 9.8 N (1000 gf). In the conveyance direction of the recording material P, the width of the transfer nip Pb is about 1 mm.

  The recording material P that has received the transfer of the toner image at the transfer nip Pb is separated from the surface of the photosensitive drum 1 and conveyed toward the fixing unit 6 for fixing the unfixed toner image on the recording material. The recording material P carrying the unfixed toner image T (see FIG. 3) is introduced into the fixing nip Pd by the entrance guide 27. The entrance guide 27 is a guide for assisting the recording material P to enter the fixing nip Pd.

  The fixing nip Pd is formed by a cylindrical film 24 that encloses a heater 241 (see FIG. 3) as a heating body, and a pressure roller 23 that pressurizes the heater through the film. The film 24 is heated by the heater 241. The recording material P is nipped and conveyed while being pressed and heated at the fixing nip Pd, whereby the toner image is heated and fixed on the recording material.

  The distance Dbd from the transfer nip Pb to the fixing nip Pd is preferably shortened for speeding up. However, if the distance from the transfer nip Pb to the fixing nip Pd is too short, the temperature of the CRG is raised by the heat of the fixing unit 6 and the durability of the constituent members of the CRG is reduced, and the electrical characteristics of the constituent members are changed. Therefore, there is a possibility that the above-described charging, exposure, development, and transfer processes may be hindered.

  In this embodiment, the distance Dbd between the transfer nip Pb and the fixing nip Pd is a total distance (= 50 mm) of the straight line La and the straight line Lb. Here, the straight line La is a line connecting a tangent line at the center Pb 1 of the transfer nip Pb of the photosensitive drum 1 and an intersection point Pc intersecting the fixing entrance guide 27. The straight line Lb is a line connecting the intersection Pc and the center of the fixing nip Pd in the transport direction.

  The interval between the preceding recording material P and the succeeding recording material P supplied next to the preceding recording material is 45 mm so that 35 letter-size sheets are output per minute at a process speed of 200 mm / sec. (0.9 sec in time). Therefore, while the trailing edge of the preceding recording material P passes through the transfer nip Pb, the photosensitive drum 1 immediately prepares for each process of charging, exposure, development, and transfer for the succeeding recording material P.

  The recording material P that has passed through the fixing unit 6 is discharged out of the apparatus by the roller 14.

  The print processing operation of the image forming apparatus 50 has been described above.

  Next, an external force that affects the moving speed Vpaper of the recording material P in the transfer nip Pb will be described. Here, as external forces, there are a frictional resistance due to the separation pad (separation member) 26 of the paper feeding unit 25 and a pulling force due to a conveying force during thermal expansion of the pressure roller 23 of the fixing unit 6.

(Friction resistance by separation pad 26)
First, the configuration of the paper feeding unit 25 will be described with reference to FIG.

  In the paper feeding unit 25, the separation pad 26 is disposed to face the surface of the roller 12. The separation pad 26 is pressed by a spring S and pressed against the surface of the roller 12.

  The separation pad 26 has a width of 10 mm in the conveyance direction of the recording material P and a width in the longitudinal direction perpendicular to the conveyance direction of the recording material P of 40 mm. The outer diameter of the roller 12 is Φ20 mm. A material having friction such as rubber is used for the surfaces of the roller 12 and the separation pad 26. Therefore, when the recording material P is supplied by the roller 12, even if a plurality of recording materials P are simultaneously supplied by the roller, the second and subsequent recording materials are prevented from being conveyed by the frictional resistance with the separation pad 26 at the press contact portion Pa. Is done. For this reason, only the topmost recording material P can be supplied by the roller 12.

  The contact pressure of the separation pad 26 with respect to the roller 12 is 0.98 in consideration of the balance between the state in which the recording material P cannot be supplied and the state of double feeding in which the recording materials are fed out in a trade-off relationship. -4.9N (100-500 gf) is preferred. In this example, the contact pressure was 3.43 N (350 gf).

  The separation pad 26 is pressed against the roller 12 by a spring S in order to prevent the second and subsequent recording materials P from being fed into the apparatus by the first recording material. For this reason, the first recording material P supplied into the apparatus is directly given a frictional resistance by the separation pad 26. Alternatively, a frictional resistance is indirectly applied to the first recording material P by the separation pad 26 via the second recording material.

  The moving speed Vpaper of the recording material P in the transfer nip Pb should match the surface moving speed Vdrum of the photosensitive drum 1 because the recording material is nipped and conveyed by the transfer nip Pb. However, the frictional resistance when the transfer roller 5 and the photosensitive drum 1 convey the recording material P is smaller than the frictional resistance that the separation pad 26 gives to the recording material. Therefore, the frictional resistance applied to the recording material P by the separation pad 26 is affected, and the moving speed Vpaper of the recording material in the transfer nip Pb is slower than the surface moving speed Vdrum of the photosensitive drum 1.

  Due to this influence, the toner image transferred onto the recording material is contracted.

(Tensile force due to the conveying force at the time of thermal expansion of the pressure roller 23)
Next, a description will be given of the tensile force generated at the moment when the recording material P enters the fixing nip Pd due to the thermal expansion of the pressure roller 23 of the fixing unit 6.

  First, the configuration of the fixing unit 6 will be described with reference to FIG.

  The heater 241 has a rectangular parallelepiped alumina substrate 241a having a width in the conveyance direction of the recording material P of 6 mm, a width in the longitudinal direction perpendicular to the conveyance direction of the recording material of 270 mm, and a thickness of 1 mm. A resistance heating element 241b such as Ag / Pd (silver palladium) that generates heat when energized was applied to the surface of the substrate 241a on the film 24 side by screen printing along the longitudinal direction of the substrate. The thickness of the resistance heating element 241b is 10 μm. The resistance heating element 241b is covered with glass as the protective layer 241c. The thickness of the protective layer 241c is 50 μm.

  Reference numeral 240 denotes a heater holder as a holding member. The heater holder 240 made of a heat-resistant resin liquid crystal polymer (LCP) preferably has a low heat capacity so as not to take heat of the heater 241. The heater 241 is held by being fitted in a groove provided in the heater holder 240.

  The film 24 has an outer diameter of Φ18 mm in a cylindrical state where the film is not deformed. The film 24 has a multilayer structure, and has a base layer (not shown) for maintaining the strength of the film and a release layer (not shown) provided on the surface of the base layer. The material of the base layer includes a polyimide resin having both heat resistance and sliding resistance, and a carbon-based filler added to the polyimide resin in order to improve thermal conductivity and strength. The material of the release layer is a perfluoroalkoxy resin (PFA) that is excellent in releasability and heat resistance among fluororesins.

  The pressure roller 23 is disposed to face the heater 241 with the film 24 interposed therebetween. In the heater holder 240, both ends in the longitudinal direction orthogonal to the conveyance direction of the recording material P are pressed in a direction orthogonal to the generatrix direction of the film by a pressurizing mechanism (not shown). ) Is in pressure contact with the protective layer 241c of the heater.

  The pressure roller 23 has an iron cored bar 230 having a diameter of 11 mm and an elastic layer 232 having a thickness of 3.5 mm made of silicone rubber provided on the surface of the cored bar. A smaller outer diameter of the pressure roller 23 can suppress the heat capacity. However, if the outer diameter of the pressure roller 23 is too small, the width of the fixing nip Pd in the conveyance direction of the recording material P becomes narrow, so that an appropriate diameter is required. In this embodiment, the outer diameter of the pressure roller 23 is Φ18 mm. A release layer 231 made of PFA is provided on the surface of the elastic layer 232.

  As the surface hardness of the pressure roller 23 is lower, the width of the fixing nip Pd can be obtained with a smaller applied pressure. However, if the surface hardness is too low, the durability of the pressure roller decreases. In the embodiment, the surface hardness of the pressure roller 23 is 40 ° in Asker-C hardness (4.9 N (500 g load)).

  The stronger the pressing force of the heater holder 240 against the pressure roller 23, the larger the width of the fixing nip Pb and the better the fixability. However, the amount of deformation of the elastic layer 232 of the pressure roller increases and the durability of the elastic layer 232 increases. There is a possibility of shortening. Therefore, the applied pressure is preferably about 49N to 294N (5 to 30 kgf). In this example, the applied pressure was 147 N (15 kgf). At this time, the width of the fixing nip Pd in the conveyance direction of the recording material P is 7 mm.

  The pressure roller 23 is rotated in the direction of the arrow by the main motor 22. The film 24 follows the rotation of the pressure roller 23 and rotates in the direction of the arrow while sliding the inner surface of the film on the protective layer 241c of the heater 241.

  The heat of the heater 241 is transmitted to the elastic layer 232 of the pressure roller 23 through the film 24, whereby the elastic layer is thermally expanded. When the elastic layer 232 is thermally expanded, the outer diameter of the pressure roller 23 is increased (see 23 'in FIG. 3). Therefore, the conveyance speed of the recording material P by the pressure roller 23 in which the elastic layer 232 is thermally expanded is faster than the conveyance speed of the recording material by the pressure roller before the elastic layer is thermally expanded.

  If printing is continued, the amount of heat is given to the elastic layer 232 of the pressure roller 23, so that the amount of thermal expansion of the elastic layer 232 further increases. Depending on the amount of thermal expansion of the elastic layer 232, the surface movement speed of the pressure roller is also increased. The width of the fixing nip Pdw formed by the pressure roller 23 and the film 24 whose surface moving speed is increased by thermal expansion is the width of the fixing nip Pd formed by the pressure roller and the film before thermal expansion. Wider than. At the moment when the recording material P enters the wide fixing nip Pdw, a tensile force due to the conveying force of the pressure roller 23 is applied to the recording material.

  Further, since the width of the fixing nip Pdw when the pressure roller 23 is thermally expanded is wider than the transfer nip Pb before the thermal expansion, the pressure applied to the recording material P at the wide fixing nip is stronger than that of the transfer nip. . Therefore, the force with which the pressure roller 23 grips the recording material P at the fixing nip Pdw is stronger than that of the transfer nip Pb.

  Therefore, the moving speed Vpaper of the recording material P in the transfer nip Pb is controlled by the surface moving speed (circumferential speed) of the pressure roller 23 after the leading edge of the recording material enters the fixing nip Pdw. That is, from the moment when the recording material P enters the fixing nip Pdw, the moving speed Vpaper of the recording material becomes faster than the surface moving speed Vdrum of the photosensitive drum 1. In this case, the moving speed Vpaper of the recording material P becomes faster than the peripheral speed of the surface of the photosensitive drum 1 in the transfer nip Pb, and the toner image transferred onto the recording material becomes stretched.

(Rotation drive control)
As shown in FIG. 1C, the image forming apparatus 50 shown in this embodiment has two motors, ie, the scanner motor 20 and the main motor 22. Both the motors 20 and 22 are rotationally driven by a control device (CPU) 21.

  The scanner motor 20 that rotates the polygon mirror is controlled by the control device 21 so as to maintain a constant speed.

  On the other hand, the drive speed of the main motor 22 can be changed by the control device 21. The rotational driving force of the main motor 22 is transmitted to the roller 12, the photosensitive drum 1, the pressure roller 23, and the roller 14 through a power transmission mechanism (not shown). That is, when the driving speed of the main motor 22 is reduced in order to reduce the surface moving speed Vdrum of the photosensitive drum 1, the roller 12, the pressure roller 23, and the roller 14 are reduced at the same ratio as the main motor 22.

  The transfer roller 5 rotates following the rotation of the photosensitive drum 1 when there is no recording material P in the transfer nip Pb. Therefore, it is possible to prevent the transfer roller 5 from changing the moving speed Vpaper of the recording material P in the transfer nip Pb. However, when the recording material P exists in the transfer nip Pb, the transfer roller 5 rotates following the moving speed Vpaper of the recording material conveyed by the rotation of the photosensitive drum 1.

(Rotation speed change of main motor 22 and its timing)
As described above, the moving speed Vpaper of the recording material P in the transfer nip Pb changes due to the external force applied to the recording material P, and expands and contracts in the toner image transferred onto the recording material, thereby causing density unevenness. In order to suppress the expansion and contraction of the toner image and the density unevenness caused by the external force applied to the recording material P, it is necessary to match the peripheral speed of the photosensitive drum with the moving speed of the recording material.

  This method will be described by taking as an example a case where horizontal lines at intervals of 10 mm are drawn on the recording material in the conveyance direction of the recording material P.

  For example, the external force applied to the recording material P is a force in a direction that hinders the conveyance of the recording material, and the moving speed Vpaper of the recording material in the transfer nip Pb is always 2% slower than the normal speed (process speed 200 mm / sec). Suppose there was. In this case, since the recording material P moves only 9.8 mm while the surface of the photosensitive drum 1 moves by 10 mm, the toner image transferred onto the recording material becomes 9.8 mm, which is smaller than the original image.

  For this phenomenon, the surface moving speed Vdrum of the photosensitive drum 1 at the transfer nip Pb is increased by 2%. Then, since the scanning exposure interval of the polygon mirror 51 of the exposure unit 3 is constant, the interval between the latent images formed on the surface of the photosensitive drum 1 in the sub-scanning direction is 10.2 mm (10 mm at the normal speed). Since this latent image is developed into a toner image by the developing unit 4, the interval between the toner images on the surface of the photosensitive drum 1 is extended to 10.2 mm. Even if the surface moving speed Vdrum of the photosensitive drum 1 is increased by 2%, the relative speed of the recording material P with respect to the surface moving speed Vdrum of the photosensitive drum is 2% slower.

  If the rotational driving speed of the main motor 22 is increased by 2%, the speeds of the roller 12, the pressure roller 23, and the roller 14 are also increased by the same ratio, so the moving speed Vpaper of the recording material P in the transfer nip Pb is also 2%. Get faster. That is, the correlation between the surface moving speed Vdrum of the photosensitive drum 1 in the transfer nip Pb and the moving speed Vpaper of the recording material P does not change.

  If the moving speed Vpaper of the recording material P in the transfer nip Pb changes, the magnitude of the external force applied to the recording material may change. However, the change in the external force is considered small and can be ignored. Therefore, by increasing the interval between the toner images formed on the surface of the photosensitive drum 1 to 10.2 mm in advance, the recording material P moves approximately 10 mm while the surface of the photosensitive drum moves 10.2 mm in the transfer nip Pb. Therefore, it is possible to draw the intended horizontal lines at 10 mm intervals on the recording material P.

  It should be noted that the timing for changing the surface moving speed Vdrum of the photosensitive drum 1 at the transfer nip Pb is not the same as the timing when the external force applied to the recording material P is changed. In order to change the image interval of the toner image in the sub-scanning direction on the surface of the photosensitive drum 1, it is necessary to change the formation interval of the latent image when the latent image is formed on the surface of the photosensitive drum.

  FIG. 4 shows an image of timing for changing the surface moving speed Vdrum of the photosensitive drum 1.

  For example, consider a case in which the timing at which the moving speed Vpaper of the recording material P in the transfer nip Pb changes due to an external force is when the recording material position PKa that is a distance Ka away from the leading edge of the recording material moves through the transfer nip Pb (see FIG. (See (a) of FIG. 4).

  In this case, it is necessary to change the surface moving speed Vdrum of the photosensitive drum in advance at a timing before the position of the photosensitive drum surface overlapping the recording material position PKa passes through the exposure position Pe (see FIG. 4 (b)). Here, the change in the surface moving speed Vdrum of the photosensitive drum is based on the exposure start time of the surface of the photosensitive drum 1 in the exposure unit 3. That is, the timing at which the surface moving speed Vdrum of the photosensitive drum 1 is changed is the timing before the timing at which the external force applied to the recording material P actually changes by the distance from the center Pb1 of the transfer nip Pb to the exposure position Pe. (See (c) of FIG. 4).

(Change of the surface moving speed Vdrum of the photosensitive drum 1)
FIG. 5 is a timing chart showing changes in the surface moving speed Vdrum of the photosensitive drum 1. FIG. 5 shows a case where two Xerox Multi-purpose White Papers (LTR size, 75 g) P1 and P2 are continuously printed.
As shown in FIG. 5, the relative movement speed of the recording material P1 with respect to the surface movement speed of the photosensitive drum 1 gradually increases as v1 ′ → v2 ′ → v3 ′ in the figure.

  When the printer is in a print ready state (RDY = ON) and a print signal is input (PRINT = ON), the recording material is fed from the paper feed unit 25. As described above, the frictional resistance due to the separation pad 26 acts on the recording material after feeding. Therefore, even if the recording material P1 reaches the transfer nip Pb, the relative movement speed of the recording material P1 with respect to the surface movement speed of the photosensitive drum 1 is negative (v1 '). This state continues until the leading edge of the recording material P1 reaches the fixing nip Pd.

  The distance between the outlet of the pressure nip Pa and the inlet of the transfer nip Pb is distance Dab (40 mm) −half length of the width 1 mm of the transfer nip Pb (0.5 mm) = 39.5 mm. The distance between the entrance of the transfer nip Pb and the entrance of the fixing nip Pd is a distance Dbd (50 mm) + half length of 1 mm width of the transfer nip Pb (0.5 mm) −half length of 7 mm width of the fixing nip Pd ( 3.5 mm) = 47 mm.

  At timing t1 ', the leading edge of the recording material P1 enters the fixing nip Pd. At this time, the recording material P1 is also sandwiched between the press contact portion Pa and the transfer nip Pb. Further, it is assumed that the pressure roller 23 is in a state where the peripheral speed is increased by the expansion of the elastic layer 232.

  At the timing t1 ′ when the leading edge of the recording material P1 enters the fixing nip Pd, the pressure roller is instantaneously caused by the relative speed difference between the surface moving speed of the pressure roller 23 in the fixing nip and the moving speed of the recording material P1 in the transfer nip Pb. A tensile force is applied to the recording material. As a result, the relative movement speed of the recording material P1 with respect to the surface movement speed of the photosensitive drum 1 in the transfer nip Pb gradually increases from the timing t1 ′ to the timing t2 ′ where the steady state is reached, and changes from the speed v1 ′ to the speed v2 ′. .

  The time from the start of acceleration at timing t1 ′ to the steady state at timing t2 ′ is not only based on the friction coefficient and recording material clamping force of the member that sandwiches the recording material P, but also on the basis weight and friction coefficient of the recording material. Also depends. Here, the members that sandwich the recording material P refer to the separation pad 26 and the roller 12, the photosensitive drum 1 and the transfer roller 5, the pressure roller 23, and the film 24. In this example, the above time was 0.3 sec (60 mm in distance) when Xerox Multi-purpose White Papers (LTR size, 75 g) was used.

  Next, at the timing t3 ′ at which the trailing edge of the recording material P passes through the paper supply unit 25 and the frictional resistance of the separation pad 26 disappears, the force in the direction that impedes the conveyance of the recording material suddenly disappears. It seems that force is applied in the transport direction. That is, the tension in the pulling direction is applied to the recording material P here as well. The relative movement speed of the recording material P relative to the surface movement speed of the photosensitive drum 1 in the transfer nip Pb gradually increases from t3 ′ to timing t4 ′ (timing immediately before the trailing edge of the recording material passes through the transfer nip Pb). It changes from v2 ′ to speed v3 ′.

  The time from the start of acceleration at timing t3 'to the steady state at timing t4' was 0.1 sec (distance 20 mm). Here, timing t4 'is timing immediately before the trailing edge of the recording material P passes through the transfer nip Pb.

  As described above, the relative movement speed of the recording material with respect to the surface movement speed of the photosensitive drum 1 is changed by the change of the external force applied to the recording material P.

  In the comparative image forming apparatus, the driving speed of the main motor 22 is changed at the timing tk1 corresponding to the timing t1 '. Similarly, the driving speed of the main motor 22 is changed at timings tk2, tk3, and tk4 corresponding to the timings t2 ', t3', and t4 ', respectively.

  In the image forming apparatus of the comparative example, the toner formed on the surface of the photosensitive drum by continuously reducing the surface moving speed of the photosensitive drum from the time when the leading edge of the recording material enters the fixing nip to the trailing edge of the recording material. The image is gradually shrunk in the sub-scanning direction. By contracting the toner image formed on the surface of the photosensitive drum in the sub-scanning direction with respect to the image on the recording material originally tending to extend in the sub-scanning direction, the image expansion phenomenon can be suppressed.

  The characteristics of the image forming apparatus 50 of the present embodiment are as follows. That is, photosensitivity occurs at a timing t1 that is a distance of 31.4 mm from the exposure position Pe on the surface of the photosensitive drum 1 in the exposure unit 3 to the center in the width direction of the transfer nip Pb before the timing t1 ′ at which the external force applied to the recording material P changes. Change the surface movement speed of the drum.

  For this purpose, the driving speed of the main motor 22 is changed in advance from the timing t1 that is a distance of 31.4 mm before the timing t1 '. In this way, the image interval (image size) of the toner image transferred onto the recording material at the timing t1 ′ at which the recording material P starts to accelerate is already on the surface of the photosensitive drum. Is appropriately shortened at the timing of exposure at which is formed.

  Similarly, for the timings t2 ′, t3 ′, and t4 ′ at which the moving speed of the recording material P changes, the surface moving speed of the photosensitive drum 1 is previously obtained at the timings t2, t3, and t4, respectively, which are 31.4 mm away. Change. In other words, the timing t1 ′ to t4 ′ at which the external force applied to the recording material changes based on the timing reference when the leading edge of the recording material P enters the transfer nip Pb is the timing t1 to t4 considered based on the timing reference at the start of exposure. What is necessary is just to change the drive speed of 22 each.

  Timing t1 'is the timing when the leading edge of the recording material P enters the fixing nip Pd, and is the timing when the leading edge of the recording material advances 47 mm after passing through the entrance of the transfer nip Pb. The timing t1 at which the drive speed of the main motor 22 starts to change is a timing 31.4 mm before the timing t1 'and 0.235 sec after the start of exposure.

  The timing t2 'at which the recording material moving speed reaches the steady state is the timing 0.3 sec (60 mm in distance) from the timing t1' as described above. The timing t2 at which the driving speed of the main motor 22 starts to be in a steady state is a timing 31.4 mm before the timing t2 ', and is 0.535 seconds after the start of exposure.

  Timing t3 'is a timing at which the trailing edge of the recording material P passes through the paper feeding unit 25, and is a timing at which the distance until the trailing edge of the recording material reaches the exit of the transfer nip Pb is 39.5 mm. When LTR size paper having a length of 279 mm in the conveyance direction of the recording material is used as the recording material P, it is a timing advanced 239.5 mm after the leading end of the LTR size paper passes through the entrance of the transfer nip Pb. . The timing t3 is a timing 31.4 mm before the timing t3 'and is 1.193 sec after the start of exposure.

  The timing t4 'at which the recording material moving speed reaches the steady state is the timing 0.1 sec (20 mm in distance) from the timing t3' as described above. The timing t4 at which the drive speed of the main motor 22 starts to be in a steady state is a timing 31.4 mm before the timing t4 'and is 1.243 sec after the start of exposure.

  Incidentally, in preparation for the second print of continuous printing, the timing at which the exposure process for the first sheet is completed (the distance before the distance between the transfer nip and the exposure position at the timing when the rear end of the recording material P1 passes through the transfer nip Pb). At the timing, the surface moving speed of the photosensitive drum may be returned to the normal speed (200 mm / sec).

  The interval from the end of latent image formation for the trailing edge of the preceding recording material P1 by the exposure unit 3 to the start of latent image formation for the leading edge of the subsequent recording material P2 is 45 mm. By returning the surface moving speed of the photosensitive drum 1 to the normal speed during the interval of 45 mm, the charging, exposure, development, and transfer processes can be repeated again from the timing indicated by C in the figure. Become.

(Effect of this embodiment)
In the image forming apparatus of the comparative example, the surface moving speed of the photosensitive drum in the transfer nip Pb is changed at the timing t1 ′ at which the external force applied to the recording material changes.

  As in the comparative example, it is assumed that the surface moving speed of the photosensitive drum 1 is decelerated at the timing t1 'when the external force applied to the recording material changes and the moving speed of the recording material in the transfer nip Pb changes. In this case, it is impossible to change the magnification of the toner image already formed in the region of the distance of 31.4 mm between the exposure position on the photosensitive drum surface and the transfer nip.

  Therefore, in the control method of the comparative example, there is a period in which the moving speed of the photosensitive drum surface does not match the moving speed of the recording material. Then, the density of the image on the recording material changes in an area corresponding to the period.

  A method for controlling the surface movement speed of the photosensitive drum in the image forming apparatuses of the present embodiment and the comparative example will be described below. The reference for comparison is the difference between the image density and the image density unevenness when a 50% halftone image is output at 600 dpi and 1200 dpi on the entire surface of the letter size paper. Table 1 shows a comparison result of image density. The case where there is almost no change in shading is marked as ◯, and the case where the change in shading is noticeable is marked as x.

  FIG. 6 shows a comparison result of unevenness in image density. With a resolution of 600 dpi, image density unevenness that was not noticeable also has a large density difference at 1200 dpi. For this reason, unevenness in image density becomes conspicuous in a region where the peripheral speed of the photosensitive drum and the moving speed of the recording material do not match.

  According to the method of the present embodiment, it is possible to suppress fluctuations in image magnification throughout the recording material P. For this reason, even in a high-resolution image, unevenness in image density can be almost eliminated.

  The image forming apparatus 50 of this embodiment changes the surface moving speed of the photosensitive drum prior to the change of the external force applied to the recording material P. The timing for changing the surface moving speed of the photosensitive drum 1 is the timing t1 to t4 before the distance between the exposure position and the transfer nip than the timing t1 'to t4' when the external force changes.

  That is, the surface moving speed of the photosensitive drum is changed from t1 to t4 before t1 'to t4' where the external force applied to the recording material changes and the moving speed of the recording material in the transfer nip starts to change.

  Thereby, image expansion and contraction caused by a change in external force applied to the recording material P can be suppressed, and an image without density unevenness can be output.

  The timing for changing the surface moving speed of the photosensitive drum 1 is not limited to the timing before the distance between the exposure position and the transfer nip. For example, the recording material P may be curved in the apparatus main body depending on the type of the recording material and the configuration of the image forming apparatus. As described in Japanese Patent Application Laid-Open No. 2000-181262, in order to suppress the occurrence of wrinkling of the recording material P and image disturbance by the fixing unit 6, the entrance amount of the entrance guide 27 into the conveyance path of the recording material P is increased. There is a configuration in which the conveyance path is distorted toward the film 24 side of the fixing unit 6. In such a configuration, the recording material is easily bent due to the stiffness of the recording material P.

  As described above, it is assumed that the recording material is curved in the conveyance path from the transfer nip Pb to the fixing unit 6 due to the large amount of the entrance guide 27 entering the conveyance path of the recording material P.

  In this case, even if the leading edge of the recording material enters the fixing nip Pd, the force pulling the recording material of the pressure roller 23 does not immediately change the moving speed of the recording material in the transfer nip. Only after the curvature is eliminated, the moving speed of the recording material P in the transfer nip Pb is accelerated. That is, the moving speed of the recording material in the transfer nip Pb changes at a timing slightly later than the timing at which the leading edge of the recording material P enters the fixing nip Pd (t1 'in FIG. 5).

  In such a case, the surface movement speed of the photosensitive drum 1 may be changed at a timing slightly later than the timing t1.

  As described above, the timing for changing the surface moving speed of the photosensitive drum 1 may be appropriately changed according to the configuration of the image forming apparatus and the type of the recording material. The timing may be a timing in the vicinity of the timing t1 and a timing before the timing t1 'at which the external force on the recording material P actually changes. As a result, it is possible to suppress image expansion and contraction due to a deviation between the surface moving speed of the photosensitive drum 1 and the moving speed of the recording material P in the transfer nip Pb.

[Example 2]
Another example of the image forming apparatus 50 will be described. The features of the image forming apparatus 50 shown in this embodiment are that a recording material can be printed on both sides using a reversing mechanism (reversing unit) 17 that reverses the front and back of the recording material P, and the surface moving speed of the photosensitive drum 1. Is that the first surface and the second surface of the recording material are different.

Hereinafter, the configuration of the image forming apparatus 50 of the present embodiment will be described with reference to FIG. FIG. 7A is a cross-sectional view showing a schematic configuration of the image forming apparatus 50 of the present embodiment. FIG. 2B is a block diagram illustrating a drive system for the main motor 22 and the scanner motor 20.
In the present embodiment, the same members as those in the first embodiment are denoted by the same reference numerals, and description of the members is omitted.

  The reversing mechanism 17 includes an introduction guide 18 provided between the fixing unit 6 and the roller 14, a conveyance guide 16 that forms a conveyance path U for inverting the recording material P from the introduction guide, and the conveyance guide. And a roller 19 for nipping and conveying the recording material in 16 conveyance paths.

  When double-sided printing is performed on the recording material P, after the printing on the first surface is completed and the trailing end of the recording material P has passed the leading end Q of the introduction guide 18, the roller 14 reverses and the recording material P is conveyed to the conveyance guide 16. The roller 19 is guided along the conveyance path U.

  The roller 19 is rotated by driving the main motor 22. The surface moving speed of the roller 19 is set to be the same as the surface moving speed of the photosensitive drum 1. The recording material P sandwiched and conveyed by the roller 19 is substantially U-shaped provided in the conveyance path U of the conveyance guide 16 in order to transfer the toner image to the second surface (the first non-printing surface side) of the recording material. Through the inversion path U1. Then, the recording material P is again introduced into the transfer nip Pb by the entrance guide 28 with the second side facing the photosensitive drum 1 side.

  The recording material P is nipped and conveyed by the transfer nip Pb, and a toner image on the surface of the photosensitive drum 1 is transferred to the second surface by applying a predetermined transfer voltage to the transfer roller 5 in the conveyance process.

  The recording material P carrying the unfixed toner image T on the second surface is introduced into the fixing nip Pd of the fixing unit 6 by the entrance guide 27. The recording material P is nipped and conveyed by the fixing nip Pd, whereby the toner image on the second surface of the recording material is heated and fixed on the recording material.

  The recording material P that has exited the fixing unit 6 is discharged to the outside by the roller 14.

  As described above, when double-sided printing is performed on the recording material P, the conveyance path is different between the first side printing and the second side printing of the recording material. The external force that affects the moving speed of the recording material P in the transfer nip Pb is also different between the first side printing and the second side printing.

  When the recording material P is printed on the second side, since the recording material does not pass through the paper feeding unit 25, the recording material is not subjected to frictional resistance by the separation pad 26. Therefore, at the time of printing the first surface, the moving speed of the recording material P in the transfer nip Pb is slower than the moving speed of the surface of the photosensitive drum 1 until the leading edge of the recording material enters the fixing nip Pd. However, at the time of printing the second surface, the surface moving speed of the photosensitive drum 1 and the moving speed of the recording material P coincide with each other until the leading edge of the recording material enters the fixing nip Pd.

  Further, during the printing of the first side, even after the leading edge of the recording material P entered the pressure roller 23, the force from the paper feeding unit 25 was applied to the recording material for a while. However, the force does not act on the recording material during the second side printing. Furthermore, when the second side is printed, there is no frictional resistance due to the separation pad 26 applied to the recording material when the first side is printed. Therefore, during the second side printing, the moving speed of the recording material P in the transfer nip Pb when the leading edge of the recording material enters the fixing nip is faster than during the first side printing.

  Therefore, it is necessary to change the surface moving speed of the photosensitive drum 1 during the first-side printing and the second-side printing.

  FIG. 8 is a timing chart showing changes in the surface moving speed Vdrum of the photosensitive drum 1 in the image forming apparatus 50 of the present embodiment. FIG. 8 shows a case where Xerox Multi-purpose White Papers (LTR size, 75 g) are printed on both sides.

  The relative moving speed of the recording material P with respect to the surface moving speed of the photosensitive drum 1 is as shown in FIG. 8, and differs depending on the difference in external force applied to the recording material during the first side printing and the second side printing. In the transfer nip Pb, the change in the relative movement speed of the recording material P and the change in the surface movement speed of the photosensitive drum 1 during the first-side printing are the same as those in the first embodiment. Regarding the second side printing, the relative movement speed of the recording material P changes at timing t5 'and at timing t6' at which the steady speed is reached. In this embodiment, the timing t6 'is 0.2 sec after the timing t5' (distance is 40 mm).

  The interval between the recording material being printed on the first side through the transfer nip Pb, the recording material being turned upside down and entering the transfer nip again (first side-second side interval) is 100 mm.

  The timing at which the surface movement speed of the photosensitive drum should be changed is t5, t6, which is 31.4 mm before the distance 31.4 mm between the exposure position on the surface of the photosensitive drum and the transfer nip with respect to t5 ′ and t6 ′ where the relative movement speed of the recording material changes. It is. In this way, the timing for changing the surface moving speed of the photosensitive drum 1 is changed during the first-side printing and the second-side printing.

  Further, as described above, the recording material does not pass through the paper feeding unit 25 during the second side printing, and thus is not subjected to frictional resistance by the separation pad 26. Therefore, the surface moving speed of the photosensitive drum 1 at the exposure start timing is different between the first side printing and the second side printing.

  At the time of printing the first surface, the moving speed of the recording material P in the transfer nip Pb was slower than the moving speed of the surface of the photosensitive drum 1 until the recording material entered the pressure roller 23. However, at the time of printing the second side, the surface moving speed of the photosensitive drum and the moving speed of the recording material coincide with each other until the recording material enters the pressure roller 23. The apparatus of this example can cope with such a change in the moving speed at the time of printing the first side and the second side of the recording material P.

  Further, at the time of printing the first side, even after the recording material enters the pressure roller 23, the force by the paper feeding unit 25 is applied to the recording material for a while. However, the force does not act on the recording material during the second side printing. Depending on the presence or absence of this external force, the slope of the relative speed change between timing t1 ′ and timing t2 ′ and the slope of the relative speed change between timing t5 ′ and timing t6 ′ are different. That is, the change in the relative movement speed of the recording material with respect to the surface movement speed of the photosensitive drum is different between the first side printing and the second side printing.

  Therefore, the acceleration from timing t1 to t2 and the acceleration from timing t5 to t6 are changed.

  Even when double-sided printing is performed as in the present embodiment, if the surface movement speed of the photosensitive drum is appropriately changed according to the magnitude of the external force at a timing before the actual external force change timing, the embodiment 1 can be obtained.

[Example 3]
Another example of the image forming apparatus 50 will be described. The external force applied to the recording material P in the apparatus main body 50a is not limited to the frictional resistance by the separation guide 26 of the paper feeding unit 25 and the pulling force of the pressure roller 23 shown in the first and second embodiments. In the image forming apparatus 50 according to the first embodiment, after the leading edge of the recording material P enters the fixing nip Pd, the recording material P is applied with force in the direction of pulling the recording material P as an external force. For this reason, an apparatus for gradually reducing the surface moving speed of the photosensitive drum 1 is shown. In this embodiment, an example in which the surface moving speed of the photosensitive drum 1 is accelerated will be described.

  The apparatus shown in FIG. 9A has a configuration in which an entrance guide 281 to the transfer nip Pb is inserted into the photosensitive drum 1 side. With this configuration, the disturbance of the toner image transferred to the recording material can be suppressed. In the case of this configuration, the friction between the recording material P and the entrance guide 281 increases. That is, the entrance guide 281 for assisting the entry of the recording material P into the transfer nip Pb gives sliding resistance to the recording material while the recording material is in contact with the entrance guide.

  The apparatus further includes a frame 282 that restricts the recording material conveyance direction so that the recording material separated from the surface of the photosensitive drum 1 is directed to the fixing unit 6. This frame 282 also gives sliding resistance to the recording material.

  These rubbing resistances become back tension, and the moving speed Vpaper of the recording material P in the transfer nip Pb decreases.

  The configuration of the image forming apparatus 50 of the present embodiment will be described with reference to FIG. FIG. 9A is a cross-sectional view showing a schematic configuration of the image forming apparatus 50 of the present embodiment. FIG. 2B is a block diagram illustrating a drive system for the main motor 22 and the scanner motor 20.

  Also in the present embodiment, the same members as those in the first embodiment are denoted by the same reference numerals, and description of the members is omitted.

  The entrance guide 281 greatly enters the photosensitive drum 1 side with respect to a straight line B connecting the pressure contact portion Pa of the paper feeding unit 25 and the upstream end (see FIG. 3) of the transfer nip Pb.

  The larger the penetration amount b of the entrance guide 281 is, the better the toner image disturbance due to transfer is. However, if the penetration amount b is too large, the opening c between the entrance guide 281 and the surface of the photosensitive drum 1 becomes narrow, and the recording material P is difficult to be stably conveyed to the transfer nip Pb. In this example, the penetration amount b was 1.6 mm, and the frontage c was 2 mm.

  Further, the entrance guide 281 is provided so that the length on the straight line C connecting the apex of the entrance guide 281 on the surface side of the photosensitive drum 1 and the center Pb1 of the transfer nip is 5 mm.

  A frame 282 is provided between the transfer nip Pb and the entrance guide 27 because the recording material conveyance path needs to be regulated so that the recording material P faces the fixing unit 6. The recording material P that has passed through the transfer nip Pb travels straight along the straight line C, hits the frame 282 at the intersection Pf between the straight line and the frame 282, and thereafter heads toward the fixing unit 6 while sliding on the frame.

  In the conveyance direction of the recording material P, the distance from the center Pb1 of the transfer nip Pb to the intersection Pf is 10 mm. The total of the distance from the intersection Pf to the vertex Pg of the entrance guide 27 and the distance from the vertex to the center Pd1 of the fixing nip Pd is 40 mm.

  FIG. 10 shows a timing chart for changing the surface moving speed Vdrum of the photosensitive drum 1 in the image forming apparatus 50 of the present embodiment. FIG. 10 shows a case where two Xerox Multi-purpose White Papers (LTR size, 75 g) are continuously printed.

  The relative moving speed of the recording material P with respect to the surface moving speed Vdrum of the photosensitive drum 1 is slower than the surface moving speed of the photosensitive drum because the recording material receives frictional resistance from the separation pad 26 and the entrance guide 281. Therefore, the recording material P enters the transfer nip Pb at a speed slower than the surface moving speed of the photosensitive drum 1.

  Thereafter, the recording material P moves from the downstream end (see FIG. 3) in the recording material conveyance direction of the transfer nip Pb to the intersection Pf on the frame 282 at a linear distance of 9.5 mm, and the leading edge of the recording material is on the frame. The frictional resistance of the frame 282 starts to be received from the timing t7 ′ at which it hits the intersection Pf. Therefore, after the timing t7 ', the moving speed of the recording material P starts to gradually decrease.

  The distance from the transfer nip Pb to the fixing nip Pd is 47 mm. This distance includes a straight line connecting the downstream end of the transfer nip Pb in the conveyance direction of the recording material P and the intersection Pf, a straight line connecting the intersection and the vertex Pg of the inlet guide 27, and conveyance of the recording material of the vertex and the fixing nip Pd. This is the total distance of the straight line connecting the upstream ends in the direction (see FIG. 3). Since the distance between the transfer nip Pb and the fixing nip Pd is as short as 47 mm, the leading edge of the recording material P enters the fixing nip Pd (timing t8 ') before starting to decelerate at the timing t7' and before the steady state is reached. It is conveyed by the pressure roller 23.

  After the recording material P is conveyed by the pressure roller 23 whose surface moving speed is faster than the surface moving speed of the photosensitive drum 1, the recording material P that has been curved along the frame 282 is gradually curved. It is canceled and it leaves the frame. The relative movement speed of the recording material P is accelerated by the pulling force due to the conveying force of the pressure roller 23 after the bending is eliminated. In this embodiment, the timing t8 ″ at which the bending is eliminated is the timing 0.075 sec after the timing t8 ′ at which the leading edge of the recording material P actually enters the pressure roller 23 (15 mm distance). It was.

  After the timing t8 ″, the timing t9 ′ at which the recording material P is conveyed in a steady state where the surface movement speed of the pressure roller 23 is dominant is 0.2 seconds later (40 mm in distance). )Met.

  Thereafter, the relative movement speed of the recording material P is gradually accelerated at a timing t10 ′ when the sliding resistance of the entrance guide 282 disappears and at a timing t11 ′ when the friction resistance of the separation pad 26 disappears, and reaches a steady state at the timing t12 ′. became.

  As described above, the relative movement speed of the recording material changes as the external force applied to the recording material P changes at the timings t7 'to t12'. In this embodiment, the surface movement speed of the photosensitive drum 1 is changed prior to the change in the relative movement speed of the recording material P.

  Therefore, the surface moving speed of the photosensitive drum 1 is changed in advance at a timing 31.4 mm before the distance between the exposure position on the surface of the photosensitive drum 1 and the transfer nip before the timing at which the relative moving speed of the recording material P changes. Keep it. Here, the timing at which the relative movement speed of the recording material P changes is t7 ′, t8 ″, t9 ′, t10 ′, t11 ′, t12 ′, and the distance 31 between the exposure position on the surface of the photosensitive drum 1 and the transfer nip 31. Timing before 4 mm is t7, t8, t9, t10, t11, t12.

  The external force applied to the recording material P actually changes at timing t8 ′. However, since the loop is canceled and the relative movement speed of the recording material changes at timing t8 ″, timing t8 is from timing t8 ″. It should be changed 31.4mm before. With respect to the timing t8 ', it is the timing before (t8'-t8) = 0.082 sec (16.4 mm in distance).

  In this way, if the surface moving speed of the photosensitive drum 1 is appropriately changed according to the magnitude of the external force at a timing 31.4 mm before the timing at which the external force actually applied to the recording material P changes, the first embodiment will be described. The same effect can be obtained.

[Example 4]
Another example of the image forming apparatus 50 will be described. The image forming apparatus 50 of this embodiment uses an LED print head type apparatus as the exposure unit 3.

  The exposure unit 3 is not limited to the laser scanner system. As disclosed in JP 2012-101497 A, an LED print head type apparatus in which an LED (light emitting diode) light source is disposed as the exposure unit 3 may be used. In this case, it is possible to obtain the same effect as in the first embodiment by changing the light emission timing of the LED light source with respect to the photosensitive drum 1 rotating at a constant speed. The exposure unit 3 of the LED print head type can eliminate the mechanism of a scanner motor for rotating the polygon mirror and a polygon mirror for scanning the laser beam as compared with the laser scanner type. Can be reduced in size.

  The image forming apparatus 50 of the present embodiment is characterized in that the surface movement speed of the photosensitive drum 1 is set to a constant speed, and the light emission interval of the exposure unit 3 that is an LED print head system is changed at a timing earlier than before. . In the exposure unit 3, the timing at which the light emission interval is changed is the distance (= 31.4 mm) from the exposure position Pe on the surface of the photosensitive drum 1 to the center Pb1 of the transfer nip Pb, rather than the timing at which the external force applied to the recording material P changes. ) It is necessary that the timing is just before.

  The configuration of the image forming apparatus 50 of the present embodiment will be described with reference to FIG. FIG. 11A is a cross-sectional view showing a schematic configuration of the image forming apparatus 50 of the present embodiment. FIG. 4B is a front view showing a light emission source 72 of the exposure unit 3. (C) is a block diagram showing a signal generation circuit 74 and a drive system of the main motor 22.

  Also in the present embodiment, the same members as those in the first embodiment are denoted by the same reference numerals, and description of the members is omitted.

  The exposure unit 3 includes a light emission source 72 including a light emitting element (LED) 71 and a print head 73 including a plurality of lenses (optical means) that form an image of light La emitted from the light emission source on the surface of the photosensitive drum 1. . The exposure unit 3 is controlled by a signal generation circuit 74. The signal generation circuit 74 performs processing such as rearrangement of image data and correction of light quantity based on target image information input from an image scanner (not shown) or an external device such as a computer. The resolution is controlled to be 1200 dpi in both the main scanning direction and the sub-scanning direction. The signal generation circuit 74 is controlled by the control device 21.

  When the exposure unit 3 is of the LED print head type as in the image forming apparatus 50 of the present embodiment, the signal generating circuit 74 changes the light emission interval of the light emission source 72 to change the surface of the photosensitive drum 1 in the sub-scanning direction. The latent image interval can be changed. For example, if the light emission interval is lengthened while the surface moving speed Vdrum of the photosensitive drum 1 is kept constant, the latent image formed in the sub-scanning direction on the surface of the photosensitive drum is in a state extended more than usual.

  An example will be described in which an external force applied to a recording material is a force in a direction that impedes the conveyance of the recording material when horizontal lines at intervals of 10 mm in the conveying direction of the recording material P are drawn on the recording material.

  When the moving speed Vpaper of the recording material P in the transfer nip Pb is 2% slower than the normal speed (process speed 200 mm / sec), the recording material moves only 9.8 mm while the surface of the photosensitive drum 1 moves 10 mm. Therefore, the toner image transferred onto the recording material P is 9.8 mm, which is smaller than the original image.

  For this phenomenon, the light emission interval of the light source 72 is increased by 2% with respect to the surface moving speed Vdrum (= process speed 200 mm / sec) of the photosensitive drum 1 which is a constant speed. As a result, the latent image interval in the sub-scanning direction on the surface of the photosensitive drum 1 is 10.2 mm, compared with 10 mm for the normal light emission interval. Since the latent image on the surface of the photosensitive drum 1 is developed into a toner image by the developing unit 4, the image interval between the toner images on the surface of the photosensitive drum is extended to 10.2 mm.

  Since the moving speed Vpaper of the recording material P is 2% slower than the surface moving speed Vdrum of the photosensitive drum 1, the image interval (image size) of the toner image on the surface of the photosensitive drum 1 is extended to 10.2 mm in advance. Thus, the recording material P can move about 10 mm while the surface of the photosensitive drum moves 10.2 mm at the transfer nip Pb. Accordingly, it is possible to draw the intended horizontal lines at 10 mm intervals on the recording material P. The timing of changing the light emission interval of the light source 72 is a timing that is retroactive from the timing at which the external force applied to the recording material P changes by a distance of 31.4 mm between the exposure position Pe and the transfer nip Pb.

  As described above, if the light emission interval of the light emission source 72 in the exposure unit 3 is changed in a state where the surface moving speed Vdrum of the photosensitive drum 1 is made constant according to the change in the external force applied to the recording material P, the same as in the first embodiment. The effect of can be obtained.

[Example 5]
Another example of the image forming apparatus will be described. The image forming apparatus is not limited to the monochrome printer of the first to fourth embodiments, and may be an apparatus using an intermediate transfer belt (image carrier) as disclosed in Japanese Patent Application Laid-Open No. 2008-309906.

  The configuration of the image forming apparatus 60 of the present embodiment will be described with reference to FIG. FIG. 12A is a cross-sectional view showing a schematic configuration of an example of an image forming apparatus (full color printer in this embodiment) 60 using electrophotographic technology. FIG. 2B is a block diagram illustrating a drive system for the main motor 22 and the scanner motor 20.

  Also in the present embodiment, the same members as those in the first embodiment are denoted by the same reference numerals, and description of the members is omitted.

  The image forming apparatus 60 shown in the present exemplary embodiment has an outer peripheral surface (front surface) of an intermediate transfer belt (intermediate transfer unit) 105 that forms a toner image of each color formed according to image information separated into yellow, magenta, cyan, and black color components. To form a full-color toner image. The full-color toner image is secondarily transferred to the recording material P. The process speed is 100 mm / sec, and the specification is such that four letter sizes are output per minute as the maximum size recording material used in the apparatus.

  In the image forming apparatus 60, the photosensitive drum 1 has an outer diameter of Φ20 mm. At the normal driving speed, the surface moving speed of the photosensitive drum 1 is a process speed of 100 mm / sec. The exposure unit 3 is of the same laser scanner type as the exposure unit shown in the first embodiment. In the exposure unit 3, the rotation speed of the scanner motor 20 and the output timing of the laser light L are controlled by the control device 21 so as to obtain a predetermined resolution. In this embodiment, the resolution of the photosensitive drum 1 in the main scanning direction (bus line direction) and the sub-scanning direction (circumferential direction) are both controlled to be 1200 dpi.

  Reference numeral 104 denotes a rotary developing unit. The developing unit 104 includes a cartridge housing unit that is rotatably provided in the apparatus main body 60a. A yellow developing cartridge 104a, a magenta developing cartridge 104b, a cyan developing cartridge 104c, and a black developing cartridge 104d are detachably mounted in the cartridge housing portion.

  The cartridge housing portion is rotated in the direction of the arrow by the main motor 22. By the rotation of the cartridge housing portion, the developing cartridges 104a to 104d for each color are sequentially switched and conveyed to a developing position Ph for developing a latent image formed on the surface of the photosensitive drum 1 into a toner image.

  An endless intermediate transfer belt (intermediate transfer portion) 105 for transferring a toner image formed on the surface of the photosensitive drum is disposed downstream of the development position Ph in the rotation direction of the photosensitive drum 1. The intermediate transfer belt 105 is an endless belt-like film stretched around a driving roller 116 for rotating the intermediate transfer belt and a tension roller 117 for applying tension to the intermediate transfer belt. The intermediate transfer belt 105 is moved (rotated) in the direction of the arrow at the same surface moving speed as that of the photosensitive drum 1 when the driving roller 116 is driven by the main motor 22.

In this embodiment, the intermediate transfer belt 105 is formed of an endless belt having a thickness of 100 μm and a volume resistivity of 10 ¹⁰ Ωcm formed of PVDF (polyvinylidene fluoride), a resin having good releasability and durability. A film was used. Further, as the driving roller 116, an aluminum cored bar coated with EPDM (ethylene / propylene / diene rubber) having a resistance value of 10 ⁴ Ω and a wall thickness of 0.5 mm, in which carbon is dispersed as a conductive agent, has a diameter of 25 mm. A roller was used. As the tension roller 117, an aluminum metal rod having a diameter of 25 mm was used.

  The tension roller 117 applies tension to the intermediate transfer belt 105 by urging both ends in the longitudinal direction perpendicular to the conveyance direction of the recording material P in a direction away from the driving roller 116. In this embodiment, the tension applied to the intermediate transfer belt 105 is 39.2 N (4 kgf).

  A transfer brush (first transfer portion) 108 is disposed at a position facing the photosensitive drum 1 with the intermediate transfer belt 105 interposed therebetween, and the surface of the photosensitive drum 1 and the outer peripheral surface (surface) of the intermediate transfer belt 105 are arranged. Thus, a primary transfer nip (first transfer position) Pi is formed. As the photosensitive drum 1 and the intermediate transfer belt 105 rotate, a primary transfer voltage (primary transfer bias) is applied to the transfer brush 108 from a transfer bias power source (not shown). As a result, the toner image is primarily transferred from the surface of the photosensitive drum 1 to the surface of the intermediate transfer belt 105 at the primary transfer nip Pi.

  By performing the above transfer process for the developing cartridges 104a (yellow color), 104b (magenta color), 104c (cyan color), and 104d (black color), toner images of a plurality of colors are formed on the surface of the intermediate transfer belt 105. Is done. For example, in the case of a full-color image, four color toner images of yellow, magenta, cyan, and black are formed in an overlapping manner.

  A secondary transfer roller (second transfer unit) 109 is disposed at a position facing the driving roller 116 with the intermediate transfer belt 105 interposed therebetween, and the surface of the intermediate transfer belt 105 and the outer peripheral surface (surface of the secondary transfer roller 109). ) To form a secondary transfer nip (second transfer position) Pj. The recording material P is supplied to the secondary transfer nip Pj into the apparatus at a predetermined timing by the roller 12 of the paper feeding unit 25.

  The recording material P is introduced into the secondary transfer nip Pj by the entrance guide 28. Simultaneously with the introduction of the recording material P into the secondary transfer nip Pj, a secondary transfer voltage (secondary transfer bias) is applied to the secondary transfer roller 109 from a secondary transfer bias power source (not shown). As a result, the toner image is secondarily transferred onto the recording material P from the surface of the intermediate transfer belt 105 at the secondary transfer nip Pj.

In this embodiment, as the secondary transfer roller 109, a nickel plated steel rod having a diameter of 6 mm and an NBR foam sponge having a resistance value adjusted to 5 × 10 ⁷ Ω are used. The outer diameter of the foamed sponge is Φ13 mm. The width in the longitudinal direction perpendicular to the conveying direction of the recording material P of foamed sponge is 216 mm assuming a letter size as the maximum size recording material used in the apparatus. The secondary transfer roller 109 is in contact with the intermediate transfer belt 105 with a contact pressure of 58.8 N (6 kgf), and rotates following the rotation of the intermediate transfer belt.

  The recording material P that has received the transfer of the toner image at the secondary transfer nip Pj is separated from the surface of the intermediate transfer belt 105 and conveyed toward the fixing unit 6. The recording material P carrying an unfixed toner image is introduced into the fixing nip Pd by the entrance guide 27. The entrance guide 27 is a guide for assisting the recording material P to enter the fixing nip Pd. The recording material P is nipped and conveyed by the fixing nip Pd, whereby the toner image is heated and fixed on the recording material.

  The recording material P that has passed through the fixing unit 6 is discharged out of the apparatus by the roller 14.

  In the image forming apparatus 60 of this embodiment, the number of motors is two, that is, the main motor 22 and the scanner motor 20. Also in this embodiment, the exposure interval in the sub-scanning direction of the polygon mirror 51 of the exposure unit 3 is constant. On the other hand, the drive speed of the main motor 22 can be changed by a control device (CPU) 21.

  The rotational driving force of the main motor 22 is applied to the photosensitive drum 1, the roller 12 of the paper feeding unit 25, the cartridge storage unit of the developing unit 104, the driving roller 116 of the intermediate transfer belt 105, the pressure roller 22 of the fixing unit 6, and the roller 14. Each is transmitted via a power transmission mechanism (not shown). That is, the driving speed of the main motor 22 is reduced in order to reduce the surface moving speed Vdrum of the photosensitive drum 1. Then, the roller 12 of the paper feeding unit 25, the cartridge housing unit of the developing unit 104, the driving roller 116 of the intermediate transfer belt 105, the pressure roller 23, and the roller 14 are decelerated at the same reduction ratio.

  For example, when the surface moving speed of the photosensitive drum 1 is reduced by 1%, the photosensitive drum 1, the roller 12 of the paper feeding unit 25, the cartridge housing unit of the developing unit 104, the driving roller 116 of the intermediate transfer belt 105, the pressure roller 23, the roller 14 operates at 99 mm / sec. The secondary transfer roller 105 rotates following the rotation of the intermediate transfer belt in order to suppress a change in the conveyance speed of the recording material P in the secondary transfer nip Pj due to the outer peripheral length of the secondary transfer roller.

  Also in the image forming apparatus 60 of the present embodiment, the moving speed Vpaper of the recording material P in the secondary transfer nip Pj may change due to an external force applied to the recording material, as in the first embodiment. In this case, the timing at which the toner image transferred to the surface of the intermediate transfer belt 105 reaches the secondary transfer nip Pj and the timing at which the image position on the recording material P to which the toner image is to be transferred reaches the secondary transfer nip. The image is expanded and contracted.

  In order to prevent image expansion and contraction, it is necessary to match the moving speed Vpaper of the recording material in the secondary transfer nip Pj, which is changed by an external force applied to the recording material P, with the moving speed of the surface of the intermediate transfer belt 105.

  For this purpose, in the exposure process for each color, the surface moving speed Vdrum of the photosensitive drum 1 may be changed with respect to a constant exposure interval in the sub-scanning direction of the polygon mirror 51 in the exposure unit 3. As the surface moving speed of the photosensitive drum 1 is changed, the latent image interval on the surface of the photosensitive drum in the sub-scanning direction is changed, whereby the magnification of the toner image transferred onto the surface of the intermediate transfer belt 105 can be changed. As a result, the size of the toner image that reaches the secondary transfer nip Pj can be changed.

  That is, the magnification in the sub-scanning direction of the toner image formed on the photosensitive drum is changed by changing the rotation speed of the main motor 22. Accordingly, the magnification of the toner image transferred to the intermediate transfer belt 105 is changed.

  The timing for changing the surface moving speed Vdrum of the photosensitive drum 1 is a timing before the change timing of the moving speed Vpaper of the recording material P in the secondary transfer nip Pj. In other words, the surface moving speed of the intermediate transfer belt is changed from the timing before the timing at which the moving speed of the recording material in the secondary transfer nip starts to change due to the change in external force applied to the recording material.

  Here, the near timing is a timing before the following distance from the timing when the external force applied to the recording material P changes. The distance is the rotational movement distance that the surface of the photosensitive drum rotates from the exposure position Pe of the photosensitive drum 1 to the primary transfer nip Pi, and the surface of the intermediate transfer belt 105 rotates from the primary transfer nip to the secondary transfer nip Pj. This is the distance obtained by adding the rotational movement distance.

  As described above, if the magnification of the toner image formed on the intermediate transfer belt is changed prior to the timing at which the moving speed of the recording material in the secondary transfer nip fluctuates due to external force, the same effect as in the first embodiment can be obtained. Can do.

  The image forming apparatus 60 according to the present exemplary embodiment may be configured to drive only the driving roller 116 by a motor (not shown) different from the main motor 22. In this case, the surface moving speed Vdrum of the photosensitive drum 1 is made constant, and in this state, the driving of another motor is controlled by the control device 21 to change the moving (rotating) speed of the intermediate transfer belt 105. The exposure interval in the sub scanning direction of the polygon mirror 51 in the exposure unit 3 is constant, and the magnification of the toner image in the sub scanning direction on the surface of the photosensitive drum 1 is also constant.

  Therefore, for example, when the moving speed of the intermediate transfer belt 105 is slowed down, the surface moving speed of the intermediate transfer belt 105 at the primary transfer nip Pi is slow relative to the moving speed of the surface of the photosensitive drum 1. As a result, the image interval of the toner image transferred to the surface of the intermediate transfer belt 105 can be reduced (the image magnification can be reduced).

  Each time the toner image for one color formed on the surface of the photosensitive drum 1 is transferred from the photosensitive drum to the surface of the intermediate transfer belt 105, the moving speed of the intermediate transfer belt is changed in advance to change the toner image on the surface of the intermediate transfer belt. You just have to stretch and contract.

  In this way, according to the change in the external force applied to the recording material, the surface movement speed of the intermediate transfer belt and the surface of the photosensitive drum in the primary transfer nip at an appropriate timing before the timing at which the external force applied to the recording material changes. Change the relative speed of the moving speed. As a result, the expansion and contraction of the image caused by the change in the external force applied to the recording material can be suppressed, and an image without density unevenness can be output.

[Other embodiments]
In the image forming apparatus 50 described above, the pressure roller 23 of the fixing unit 6 may be driven by a motor (not shown) different from the main motor 22. In this case, the other motor and the main motor 22 are driven by the controller 20 so that the conveyance speed of the recording material P by the pressure roller 23 and the conveyance speed of the recording material by the photosensitive drum 1 are substantially constant. In that case, even if the pressure roller 23 is thermally expanded, the recording material P is not pulled.

  For this reason, the moving speed of the recording material P is kept slow due to the frictional resistance of the separation pad 26 of the paper feeding unit 25 even if the leading edge of the recording material enters the fixing nip Pd. Then, at the moment when the trailing edge of the recording material passes through the paper feeding unit and the frictional resistance disappears, the moving speed of the recording material P is accelerated by momentarily acting on the recording material in the pulling direction. Thereafter, the moving speed of the recording material P matches the surface moving speed of the photosensitive drum 1.

  When the moving speed of the recording material P changes as described above, the first embodiment can be achieved by changing the exposure interval in the sub-scanning direction and the relative speed of the surface moving speed of the photosensitive drum so as to return to the normal state. The same effect can be obtained.

  Further, in the image forming apparatus 50 described above, in order to eliminate the phenomenon that the moving speed of the recording material becomes slow due to the frictional resistance by the separation pad of the paper feeding unit, the recording material is arranged downstream of the paper feeding unit in the recording material transport direction. It is good also as a structure which provides the roller for conveying. The surface moving speed of the roller is set faster than the process speed.

  In this case, when the trailing edge of the recording material P passes through the paper feeding unit 25 and the frictional resistance disappears, the recording material is accelerated by the conveying force of the roller. Since the moving speed of the recording material P at this time depends on the surface moving speed of the roller, the moving speed of the recording material in the transfer nip Pb is faster than the normal moving speed (= process speed). Then, as soon as the trailing edge of the recording material P passes through the roller, the assist of the roller that has sent the recording material faster than the normal moving speed is lost, and thus the force in the direction of decelerating the recording material works momentarily. The recording material has a normal moving speed.

  Even in such a case, the relative speed between the exposure interval in the sub-scanning direction and the surface movement speed of the photosensitive drum is continuously changed at an appropriate timing before the timing at which the force applied to the recording material P changes. Thus, the magnification of the toner image on the surface of the photosensitive drum 1 can be changed according to the moving speed of the recording material that changes in accordance with the change in external force applied to the recording material P. Therefore, the same effect as in the first embodiment can be obtained.

  The process speeds shown in the image forming apparatuses 50 and 60 of each embodiment, the size, material, and shape of each member, and the configuration of the fixing unit 6 and the paper feeding unit 25 are merely examples, and are not limited thereto.

1 photosensitive drum, 3 exposure section, 4 development section, 5 transfer roller (transfer section),
22 Main motor

Claims (3)

A photoreceptor,
An exposure unit that exposes the photoconductor according to image information and forms a latent image on the photoconductor;
A developing unit for developing the latent image with toner;
A transfer portion for transferring a toner image from the photoreceptor to a recording material;
In an image forming apparatus having
The image forming apparatus, wherein the apparatus changes the surface moving speed of the photoconductor at a timing before a predetermined time with respect to a predetermined timing at which the moving speed of the recording material in the transfer unit changes. The apparatus has a motor for driving the photoconductor,
By changing the rotational speed of the motor at a timing before a predetermined time with respect to a predetermined timing at which the moving speed of the recording material in the transfer unit changes, the surface moving speed of the photoconductor is changed, and the photoconductor The image forming apparatus according to claim 1, wherein a magnification in a sub-scanning direction of a toner image formed on the image is changed. A photoreceptor,
An exposure unit that exposes the photoconductor according to image information and forms a latent image on the photoconductor;
A developing unit for developing the latent image with toner;
A transfer portion for transferring a toner image from the photoreceptor to a recording material;
In an image forming apparatus having
The image forming apparatus is characterized in that the exposure interval in the sub-scanning direction by the exposure unit is changed at a timing before a predetermined time with respect to a predetermined timing at which the moving speed of the recording material in the transfer unit changes.