Image forming device

Abstract

To provide an image forming apparatus that operates in a plurality of operation modes different in process speed, and to prevent a change in image quality depending on the operation mode.SOLUTION: An image forming apparatus comprises: exposure means that reflects a laser beam emitted from a light source on a polygon mirror having a plurality of reflection surfaces to form a latent image on a photoreceptor; and control means that, in response to an instruction input thereto, controls the operation of the image forming apparatus in a plurality of operation modes including a standard mode and a slow mode having a process speed slower than that of the standard mode. When receiving input of an instruction to designate the standard mode, or when not receiving input of designation of the operation mode, the image forming apparatus reflects the laser beam by using all the plurality of reflection surfaces of the polygon mirror, and when receiving input of an instruction to designate the slow mode, increases the process speed to be faster than 0.5 times that of the standard mode, increases the rotation speed of the polygon mirror to be faster than that of the standard mode, and reflects the laser beam on some reflection surfaces of the plurality of reflection surfaces of the polygon mirror.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] When forming images on recording materials such as rough paper or thick paper, which are more difficult for toner to fuse to than plain paper, laser beam printers use a low-speed mode to ensure good fixability. Slowing the process speed increases the time the recording material takes to pass through the fuser, allowing sufficient heat to be transferred to the toner and recording material to fuse the toner image, improving fixability.

[0003] One method for achieving the low-speed mode is to slow down the rotation speed of the photosensitive member without changing the rotation speed of the polygon mirror of the exposure device that exposes the photosensitive member from that of the standard mode. With this method, the resolution in the sub-scanning direction is changed by image processing so that the resolution in the sub-scanning direction does not differ between the standard mode and the low-speed mode. Another method for achieving the low-speed mode is to use the reflective surfaces of the polygon mirror in a face-skipping manner (see Patent Documents 1 and 2). For example, by using the reflective surfaces of the polygon mirror in a face-skipping manner and setting the rotation speed of the photosensitive member to half that of the standard mode, the resolution of the latent image in the sub-scanning direction can be maintained unchanged between the standard mode and the low-speed mode. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5896620 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-298793 Summary of the Invention [Problem to be solved by the invention]

[0005] When the process speed changes significantly between low-speed mode and standard mode, image quality such as halftone density, the appearance of outline text, and the size of small dots changes. In particular, in image forming devices that develop toner images using a jumping development method, a change in process speed changes the number of times the toner travels back and forth in the development nip. As a result, the image quality changes significantly between low-speed mode and standard mode, and there are limits to suppressing the change in image quality through image processing such as tone curve correction. The present invention aims to suppress the change in image quality due to the operating mode in an image forming device that operates in multiple operating modes with different process speeds. [Means for solving the problem]

[0006] The present invention comprises a photoreceptor and a polygon mirror that is rotationally driven by a driving means and has a plurality of reflecting surfaces; an exposure unit that forms a latent image on the photosensitive member by reflecting a laser beam emitted from a light source on the polygon mirror and scanning the photosensitive member; a developing means for depositing toner onto the latent image formed on the photosensitive member to form a toner image on the photosensitive member; a transfer means for transferring the toner image formed on the photosensitive member to a recording material directly or via an intermediate transfer member; a fixing unit that fixes the toner image by heating the recording material onto which the toner image has been transferred; a control means for controlling an image forming mode for forming an image on a recording material so as to be executable; The image forming mode is Factory default The default operating mode is a first image forming mode in which A process speed that defines the speed of the recording material passing through the fixing means. is in front a second image forming mode having a process speed slower than that of the first image forming mode, The control means When an instruction to designate the first image forming mode is input and when a specific image forming mode is input, When no instruction to designate a mode is input, control is performed so that the laser light is reflected using all of the plurality of reflective surfaces of the polygon mirror of the exposure means, when an instruction to designate the second image formation mode is input, the process speed is made faster than 0.5 times the process speed of the first image formation mode, the rotation speed of the polygon mirror is made faster than the rotation speed of the polygon mirror in the first image formation mode, and control is performed so that laser light is reflected by only some of the plurality of reflection surfaces of the polygon mirror of the exposure means; The image forming apparatus is characterized in that in the second image forming mode, the output of the laser light emitted from the light source is controlled to be greater than in the first image forming mode. [Effects of the Invention]

[0007] According to the present invention, in an image forming apparatus that operates in a plurality of operation modes with different process speeds, it is possible to suppress changes in image quality due to the operation mode. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to an embodiment. [Figure 2] FIG. 1 is a top view showing a schematic configuration of an exposure apparatus according to an embodiment. [Figure 3] FIG. 2 is a conceptual diagram of a developing unit according to the embodiment. [Figure 4] 10 is a diagram showing the relationship between the number of times toner travels back and forth in the development gap and the amount of developable toner. FIG. [Figure 5] FIG. 4 is a diagram showing the relationship between process speed and image density in an embodiment. [Figure 6] FIG. 2 is a conceptual diagram of a fixing unit according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Preferred embodiments of the present invention will be described in detail below with reference to the drawings. Note that the following embodiments are merely examples, and the present invention is not limited to the contents of the embodiments. Furthermore, in the following drawings, components that are not necessary for explaining the embodiments are omitted.

[0010] FIG. 1 is a schematic diagram of an image forming apparatus 9 according to this embodiment. The image forming apparatus 9 is an A4 monochrome laser beam printer. A laser driver 300 of an exposure device 400 emits a laser beam 208 based on image data output from an image signal generator 100. The laser beam 208 scans and exposes a photoconductor 4, which has been charged by a charging unit 2, such as a conductive rubber roller, to form a latent image on the surface of the photoconductor 4. A developing unit 3 develops the latent image on the surface of the photoconductor 4 with toner to form a toner image. A recording material supplied from a paper feed unit 8 is transported by a roller 5 to a nip region between the photoconductor 4 and a transfer roller 41. The transfer roller 41 is a transfer means that transfers the toner image formed on the photoconductor 4 to the recording material transported from the roller 5. While this embodiment illustrates a configuration in which a toner image is directly transferred from the photoconductor 4 to the recording material, a configuration in which a toner image is indirectly transferred to the recording material via an intermediate transfer member may also be used. Residual toner remaining on the surface of the photoreceptor 4 without being transferred to the recording material is cleaned by a cleaning unit (not shown), and the surface of the photoreceptor 4 is prepared for the next image formation. Meanwhile, the recording material with the transferred toner image is transported to the fixing unit 6. The fixing unit 6 is a fixing unit that heats and presses the recording material to fix the toner image to the recording material. The recording material with the fixed toner image is discharged to the outside of the image forming apparatus 9 by a paper discharge roller 7. The image forming apparatus 9 has a control unit 500 that controls the operation of the image forming apparatus 9 and an input unit 510 through which a user inputs instructions to the control unit 500. The input unit 510 is, for example, an input unit such as a button provided on the main body of the image forming apparatus 9, a touch panel display, a keyboard, or a computer connected directly or indirectly via a network. The control unit 500 is a control unit that controls the operation of the image forming apparatus 9 in multiple operating modes, including a standard mode and a low-speed mode (to be described later), based on instructions input from the input unit 510. The control unit 500 also controls the operation of the image forming apparatus 9 in accordance with instructions input from the input unit 510. In accordance with the specified operation mode, the control unit 500 controls the operations of the exposure device 400, photoreceptor 4, roller 5, fixing unit 6, etc., and controls the process speed of the image forming device 9. The process speed defines the speed at which the recording material passes through the fixing unit 6. If an instruction to specify a specific operation mode is not input from the input unit 510, the control unit 500 controls the operation of the image forming device 9 in the standard mode as the default operation mode.

[0011] <Exposure equipment> FIG. 2 shows a cross-sectional view in the main scanning direction of an exposure device 400, which is an exposure means according to this embodiment. Laser light 208 emitted by a light source 401 is shaped into an elliptical shape by an aperture stop 402 and enters a coupling lens 403. After passing through the coupling lens 403, the laser light 208 is converted into substantially parallel light and enters an anamorphic lens 404. Note that the term "substantially parallel light" includes weakly convergent light and weakly divergent light. The anamorphic lens 404 has positive refractive power in the main scanning cross section and converts the incident light beam into convergent light in the main scanning cross section. The anamorphic lens 404 also focuses the light beam near a reflecting surface 405a of a polygon mirror 405 in the sub-scanning cross section, forming a long line image in the main scanning direction.

[0012] The light beam passing through the anamorphic lens 404 is reflected by a reflecting surface 405a of a polygon mirror 405. The polygon mirror 405 has multiple reflecting surfaces 405a. In this embodiment, the polygon mirror 405 has four reflecting surfaces 405a. The laser light 208 reflected by the reflecting surface 405a passes through an imaging lens 406 and is focused on the surface of the photosensitive member 4, forming a predetermined spot-shaped image (hereinafter referred to as a spot). The polygon mirror 405 is driven to rotate at a constant angular velocity by a driving motor 408, which serves as a driving means. As a result, the spot moves in the main scanning direction on the scanned surface 407 of the photosensitive member 4, forming an electrostatic latent image on the scanned surface 407. The main scanning direction is a direction parallel to the surface of the photosensitive member 4 and perpendicular to the direction of movement of the surface of the photosensitive member 4.

[0013] The beam detect (hereinafter referred to as BD) sensor 409 is a synchronization optical system that determines the timing of writing an electrostatic latent image on the scanned surface 407. The laser beam 208 is incident on and detected by the BD sensor 409, which includes a photodiode. The writing timing is controlled based on the timing at which the BD sensor 409 detects the laser beam 208. The BD sensor 409 has a range of laser output power within which it can detect timing with high accuracy. If the laser output power falls below this range, the detection accuracy decreases and the timing of writing the electrostatic latent image may be shifted. To improve detection accuracy, a condenser lens can be installed in front of the BD sensor 409 to increase detection accuracy at low laser output power, but this increases the space and cost required for installation. The light source 401 may be composed of a single light-emitting element, or may be composed of multiple light-emitting elements whose light emission can be controlled independently.

[0014] <Standard mode and slow mode> The control unit 500 controls the operation of the image forming device 9 in multiple operating modes, including operating modes with different process speeds that define the speed of the recording material passing through the fixing unit 6. In this embodiment, the process speed is also the rotational speed of the photoconductor 4. Specifically, the control unit 500 controls the operation of the image forming device 9 in multiple operating modes, including a standard mode, which is the default operating mode used for standard image formation, and a low-speed mode, which has a process speed slower than that of the standard mode. In the standard mode, the image forming device 9 prints at a transport speed of 200 mm / sec for the photoconductor 4 and recording material. The standard mode is the most commonly used mode for A4 monochrome laser beam printers and is the factory default mode. In other words, the standard mode is the mode that is compatible with the widest range of recording materials. A4 and letter-size so-called plain paper is printed at a resolution of 600 dpi in the standard mode. The low-speed mode is an operating mode suitable for forming images on recording materials with low toner fixability, such as cardboard or rough paper. The user can input an instruction specifying a desired operating mode via the input unit 510. The control unit 500 controls the operation of the image forming apparatus 9 according to the operating mode specified by the instruction input from the input unit 510. The input unit 510 may be configured to allow the user to directly specify the operating mode (standard mode or low-speed mode), or may be configured to allow the user to specify the type of recording material, with the control unit 500 setting the optimal operating mode according to the specified type of recording material. For example, if the user specifies plain paper as the recording material, the control unit 500 may set the operating mode to standard mode, and if the user specifies cardboard or rough paper, the control unit 500 may set the operating mode to low-speed mode. The input unit 510 may also be configured to receive image data and instructions related to image formation, including a specification of the operating mode, from a computer connected to the image forming apparatus 9 directly or via a network. In this case, the control unit 500 controls the operation of the image forming apparatus 9 in the operating mode specified in the instruction input from the computer. Furthermore, if an instruction explicitly specifying a specific operating mode is not input from the input unit 510, the control unit 500 controls the operation of the image forming apparatus 9 in standard mode as the default operating mode.

[0015] <Relationship between polygon mirror rotation speed and resolution> When the process speed in the low-speed mode is slowed down to α times (α<1) that in the standard mode, there are two possible methods for forming an image in the low-speed mode with the same resolution as in the standard mode.

[0016] The first method is to use the entire reflective surface 405a of polygon mirror 405 in the low-speed mode, just as in the standard mode, and to slow the rotation speed of polygon mirror 405 by α times that of the standard mode. In this case, the time it takes for the surface of photoconductor 4 to advance by one dot in the low-speed mode is the same as the time it takes for polygon mirror 405 to rotate 90°, just like in the standard mode. Therefore, by slowing down the rotation speed of polygon mirror 405, it is possible to form images in the low-speed mode with the same resolution as in the standard mode.

[0017] The second method is to expose the photoconductor 4 using every other reflective surface 405a of the polygon mirror 405 in low-speed mode, and to increase the rotation speed of the polygon mirror 405 by 2α times compared to standard mode. In this case, because every other reflective surface 405a of the polygon mirror 405 is used, the time it takes for the surface of the photoconductor 4 to advance by one dot is the time it takes for the polygon mirror 405 to rotate 180°. Therefore, by increasing the rotation speed of the polygon mirror 405, it is possible to form images with the same resolution in low-speed mode as in standard mode. When α>0.5, the rotation speed of the polygon mirror 405 in low-speed mode is increased compared to standard mode.

[0018] <Relationship between polygon mirror rotation speed and laser output> When the rotation speed of polygon mirror 405 changes, the amount of light per dot on the surface of photoconductor 4 changes, assuming that the laser output of light source 401 is constant. When the rotation speed of polygon mirror 405 becomes β times that of the standard mode, the amount of light per dot on the surface of photoconductor 4 becomes 1 / β times that of the standard mode. In other words, when the laser output is constant, as the rotation speed of polygon mirror 405 slows (β<1), the amount of light per dot on the surface of photoconductor 4 increases. As the rotation speed of polygon mirror 405 increases (β>1), the amount of light per dot on the surface of photoconductor 4 decreases.

[0019] Changes in the amount of light per dot on the surface of the photoreceptor 4 affect the density of the image formed on the recording material. Therefore, in order to prevent changes in the density of the image formed in the standard mode and the low-speed mode, it is necessary to keep the amount of light per dot on the surface of the photoreceptor 4 constant in the standard mode and the low-speed mode.

[0020] When the rotation speed of the polygon mirror 405 in the low-speed mode is β times that in the standard mode, the light source 401 By increasing the laser output from the polygon mirror 405 by β times that in the standard mode, the amount of light per dot on the surface of the photosensitive member 4 can be made the same as that in the standard mode. That is, when the rotation speed of the polygon mirror 405 is made faster than that in the standard mode, the laser output is made larger than that in the standard mode. On the other hand, when the rotation speed of the polygon mirror 405 is made slower than that in the standard mode, the laser output is made smaller than that in the standard mode. This makes it possible to prevent the density of the formed image from changing significantly from that in the standard mode, even when the rotation speed of the polygon mirror 405 is changed in the low-speed mode from that in the standard mode.

[0021] <Laser output adjustment range limit> However, there are limitations to the adjustable range of the laser output. For example, the maximum value of the laser output is determined by the characteristics of the laser element, so the adjustable range of the laser output has an upper limit based on the characteristics of the laser element. Furthermore, when the laser output is low, the BD sensor 409 cannot perform detection with high accuracy. Therefore, in order to appropriately control based on the detection results of the BD sensor 409, the laser output cannot be made lower than the lower limit that the BD sensor 409 can detect.

[0022] For example, suppose the upper limit of the adjustable range of the laser output is γ times (γ>1) the laser output in standard mode, and the lower limit is δ times (δ<1) the laser output in standard mode. When the rotation speed of the polygon mirror 405 is increased to β times (β>1) that of the standard mode in low-speed mode, if β>γ, the laser output cannot be increased to β times that of the standard mode. As a result, the amount of light on the photoconductor 4 is insufficient compared to the standard mode, and the density of the image formed in low-speed mode is lighter than that in standard mode. On the other hand, when the rotation speed of the polygon mirror 405 is decreased to β times (β<1) that of the standard mode in low-speed mode, if β<δ, the laser output cannot be decreased to β times that of the standard mode. As a result, the amount of light on the photoconductor 4 is excessive compared to that in standard mode, and the density of the image formed in low-speed mode is darker than that in standard mode.

[0023] To prevent the laser output adjustment amount from becoming too large, if the process speed in the low-speed mode is α times that in the standard mode (α<1), the control of the polygon mirror 405 in the low-speed mode can be varied depending on the value of α. For example, when α is close to 1 (e.g., 0.75≦α<1), the entire reflective surface 405a of the polygon mirror 405 is used, the rotation speed of the polygon mirror 405 is slowed to α times that in the standard mode, and the laser output is reduced to α times, thereby suppressing an increase in image density. On the other hand, when α is a value far from 1 (e.g., 0.5≦α<0.75), every other reflective surface 405a of the polygon mirror 405 is used, and the rotation speed of the polygon mirror 405 is increased to 2α times that in the standard mode. Furthermore, increasing the laser output to 2α times suppresses a decrease in image density. If the adjustment amount of the laser output is small, restrictions on the design of the sensitivity of the BD sensor 409, the output characteristics of the laser element, the sensitivity of the photosensitive member 4, and the like are reduced, which is preferable.

[0024] <Developing section> FIG. 3 shows a conceptual diagram of the developing unit 3. The developing unit 3 is a developing means that attaches toner to a latent image formed on the photoreceptor 4 to form a toner image on the photoreceptor 4. The developing unit 3 develops the latent image on the photoreceptor 4 into a toner image using a magnetic one-component jumping development method. The developing unit 3 has a developing sleeve 301 as a rotatable toner carrier facing the photoreceptor 4. As the developing sleeve 301 rotates, magnetic toner regulated by a metal blade coats the developing sleeve 301. The magnetic toner is held on the developing sleeve 301 by a magnet inside the developing sleeve 301. In this embodiment, magnetic toner with an average particle size of 8 μm is used.

[0025] The developing sleeve 301 rotates at a predetermined speed ratio to the rotation speed of the photosensitive member 4. This speed ratio is determined by the gear ratio that rotates the photosensitive member 4 and the developing sleeve 301, and is therefore constant regardless of the rotation speed of the photosensitive member 4. In this embodiment, the developing sleeve 301 rotates at a predetermined speed ratio to the rotation speed of the photosensitive member 4. It rotates at 110% of the normal speed.

[0026] A development gap DG of 350 μm is provided between the surface of the photoconductor 4 and the development sleeve 301. A development bias in which a rectangular AC bias (frequency 3000 Hz, Vpp: 1.7 kV) is superimposed on a DC bias is applied to the development sleeve 301. The toner on the development sleeve 301 flies in a cloud-like manner to the surface of the photoconductor 4, and a toner image corresponding to the latent image on the surface of the photoconductor 4 is developed. In this embodiment, the development potential, expressed as the average bias value over one cycle of the AC bias, is −300 V. The contrast between the potential of the portion of the surface of the photoconductor 4 exposed to the laser light 208 and the development potential is 200 V. Therefore, the potential of the exposed portion of the photoconductor 4 is −100 V.

[0027] <Relationship between photoreceptor rotation speed and developability> In Figure 3, area DL indicates the area where toner can fly due to the electric field of the developing bias. In this area DL, toner on the developing sleeve 301 receives the force of the electric field of the developing bias and moves back and forth between the photosensitive member 4 and the developing sleeve 301. The size of area DL is constant and does not depend on the rotation speed of the photosensitive member 4 (the conveyance speed of the recording material). Therefore, if the rotation speed of the photosensitive member 4 is fast, the number of times the toner moves back and forth is small, and if the rotation speed of the photosensitive member 4 is slow, the number of times the toner moves back and forth is large. In this embodiment, the size of area DL is approximately 2 to 3 mm.

[0028] Figure 4 shows the relationship between the number of times the toner travels back and forth in the toner flight area DL (horizontal axis) and the amount of toner that flies through the development gap DG and is available for development (vertical axis). The toner that flies through the development gap DG and travels back and forth between the photoconductor 4 and the development sleeve 301 collides with toner that is magnetically or electrostatically attracted to the development sleeve 301, knocking it out into the development gap DG and generating new flying toner. Therefore, the more toner that travels back and forth through the development gap DG, the more toner that flies through the development gap DG and is available for development. Even if the electrostatic latent image on the surface of the photoconductor 4 remains the same, the more toner is available for development, the darker the resulting image. The change in image density due to the change in the amount of toner available for development is particularly noticeable in halftones, small dots, thin lines, and outline characters.

[0029] Figure 5 shows the relationship between the rotation speed of the photoconductor 4 and halftone density. The horizontal axis shows the halftone image data value, and the vertical axis shows the density of the halftone actually formed on the recording material. Figure 5 shows the relationship between the data value and density of the halftone image when the rotation speed of the photoconductor 4 is 200 mm / sec, 120 mm / sec, and 100 mm / sec. From this relationship, it can be seen that even if the image data value is the same, the density of the image formed on the recording material increases as the rotation speed of the photoconductor 4 decreases.

[0030] <Fixing section> The fixing unit 6 will be described with reference to FIG. 6. The fixing unit 6 of this embodiment is configured as a film heating type heat fixing device. The fixing unit 6 has a film unit 10 as a heating device and a pressure roller 20. The film unit 10 has a heat-resistant fixing film 13 which is a heating rotatable body as a heat transfer member, a heater 11 which is a heating member, and a holder 12 which is a heater holding member. The heater 11 is provided inside the fixing film 13. The pressure roller 20 which is a pressure rotatable body is provided opposite the film unit 10. A recording material P on which a toner image t has been formed is sandwiched in a fixing nip formed between the fixing film 13 and the pressure roller 20 and is conveyed. The toner image t is fixed to the recording material P as it moves together with the fixing film 13.

[0031] A thermistor 14 as a temperature detection member is disposed in contact with the surface of the heater 11 opposite to the surface that slides against the fixing film 13. Based on the temperature detected by the thermistor 14, the control unit 500 controls the current of the heater 11 so that the temperature of the heater 11 reaches a predetermined temperature. In this embodiment, the temperature detected by the thermistor 14 when the recording material passes through the fixing nip portion in the standard mode is controlled to be 180°C.

[0032] <Low speed mode setting> When using thick paper or rough paper, the heat required to melt the toner is not easily transferred from the heater 11 to the toner as it passes through the fixing nip, making it difficult for the toner to fuse to the paper. When forming an image on such a recording material, fixing can be ensured by performing image formation in low-speed mode. This is because the process speed is slow in low-speed mode, and the time the recording material takes to pass through the fixing nip is longer, allowing sufficient heat to be transferred from the heater 11 to the toner.

[0033] On the other hand, as mentioned above, slowing the process speed increases the number of times the toner travels back and forth within the toner flight area DL, resulting in a higher density of the formed image. Therefore, significantly slowing the process speed to ensure fixability is not desirable. To achieve the same resolution in low-speed mode and standard mode, either the entire reflective surface 405a of the polygon mirror 405 is used to slow down the rotation speed, or every other reflective surface 405a of the polygon mirror 405 is used to speed up the rotation speed. Slowing the rotation speed of the polygon mirror 405 results in a higher density of the formed image, whereas speeding up the rotation speed of the polygon mirror 405 results in a lower density of the formed image.

[0034] Considering the balance between image quality and fixability, in this embodiment, the control unit 500 controls the process speed in the low-speed mode to be faster than 0.5 times and slower than 0.75 times that in the standard mode. Furthermore, in the low-speed mode, the control unit 500 reflects the laser light using every other reflective surface 405a of the polygon mirror 405, controlling the rotation speed of the polygon mirror 405 to be faster than that in the standard mode. If the process speed in the low-speed mode is α times that in the standard mode (0.5<α<0.75), the control unit 500 controls the rotation speed of the polygon mirror 405 to be 2α times that in the standard mode (1<2α<1.5). This allows images to be formed in the low-speed mode with the same resolution as in the standard mode. In the standard mode, the control unit 500 controls the process speed so that all reflective surfaces 405a of the polygon mirror 405 are used to reflect the laser light. The number of reflecting surfaces 405a of polygon mirror 405 is not limited to four, and in the low-speed mode, control may be performed so that laser light 208 is reflected by only some of the reflecting surfaces 405a of polygon mirror 405. In this case, the rotation speed of polygon mirror 405 in the low-speed mode may be set appropriately based on the ratio between the total number of reflecting surfaces 405a and the number of surfaces that are used.

[0035] Because the process speed in low-speed mode is more than half that of standard mode, excessively slow process speeds can be prevented from resulting in excessively high image density. Therefore, even in low-speed mode, images whose image quality, such as halftone density, outline character visibility, and small dot size, is sensitive to density changes, can be produced satisfactorily. Furthermore, low-speed mode ensures sufficient heating time for the toner to melt in the fixing nip, enabling high-quality image formation even on recording materials with low toner fixability, such as rough paper. In low-speed mode, the effect of increasing image density due to a slower process speed and the effect of decreasing image density due to an increased rotation speed of the polygon mirror 405 simultaneously occur, thereby suppressing image density fluctuations. If necessary, the laser output can be increased to suppress the effect of density reduction due to an increased rotation speed of the polygon mirror 405.

[0036] <Evaluation of image quality changes between standard mode and slow mode> The image forming apparatus according to the embodiment was evaluated for changes in image quality when forming images in standard mode and low-speed mode for several examples in which the conditions of the low-speed mode were varied. Halftones from solid white (0% density) to solid black (100% density) were printed in 10% increments. Using these images as evaluation images, images were formed in both standard and slow modes, and the image densities were compared. The lightness L* of each of the 11 halftones in the formed image was determined, and the maximum difference in lightness between standard and slow modes was taken as the representative value ΔL*. The subjective impression of the ΔL* value was as follows, with ΔL*≦5 being considered to be within the acceptable range.

[0037] ΔL*≦3: When comparing two images side by side, there is almost no difference. 3<ΔL*≦5: When comparing two images side by side, there may be a slight difference, but it is within the acceptable range. 5<ΔL*...A clear difference is visible when comparing the two images side by side.

[0038] As with halftone images, density differences can occur in images formed in standard mode and low-speed mode for outline characters and small dots, but the results showed a similar tendency to the subjective evaluation of density differences observed in halftone images.

[0039] The fixation of rough paper in low speed mode was also evaluated. Neenah Bond Classic Laid Text paper was used as the rough paper. An image consisting of 12 pt text was used as the image for evaluation. The images formed on the rough paper were visually inspected for the presence or absence of missing characters due to insufficient toner melting. In the evaluation results below, "◯" indicates that no missing characters occurred, and "×" indicates that missing characters occurred. Table 1 shows the conditions for each example and comparative example, and the evaluation results for the change in image quality in standard mode and low-speed mode. [Table 1]

[0040] Example 1 The process speed in low-speed mode was 0.57 (4 / 7) times that of standard mode, the reflective surfaces 405a of the polygon mirror 405 were used alternately, and the rotation speed of the polygon mirror 405 was 1.14 (8 / 7) times that of standard mode. The process direction resolution of the latent image formed on the surface of the photoreceptor 4 was 600 dpi in both standard and low-speed modes. The target temperature control value for the thermistor 14 detected when the recording material passed through the nip of the fixing unit 6 was 180°C in both standard and low-speed modes. The maximum brightness difference ΔL* between the standard and low-speed modes for the 11 halftone images was 3.5, and the density difference between the halftone images in standard and low-speed modes was within the acceptable range. This is likely due to the small difference in process speed between low-speed and standard modes. No missing characters were observed in the fixing results for rough paper in low-speed mode. This is likely due to sufficient heating time in the fixing nip.

[0041] Example 2 The process speed in the low-speed mode was set to 0.67 (2 / 3) times that of the standard mode, the reflecting surfaces 405a of the polygon mirror 405 were used every other surface, and the rotation speed of the polygon mirror 405 was set to 1.33 (4 / 3) times that of the standard mode. Other conditions were the same as in Example 1. The maximum value ΔL* of the brightness difference between the standard mode and the low-speed mode was 2.8, and the difference in brightness between the standard mode and the low-speed mode was There was almost no difference in tone density. This is thought to be because the difference in process speed between the low-speed mode and the standard mode is small. No missing characters were observed in the fixing results for rough paper in the low-speed mode. This is thought to be because sufficient heating time was ensured in the fixing nip.

[0042] Example 3 The process speed in low-speed mode was set to 0.55 (5 / 9) times that of standard mode, the reflective surfaces 405a of the polygon mirror 405 were used every other surface, and the rotation speed of the polygon mirror 405 was set to 1.11 (10 / 9) times that of standard mode. Other conditions were the same as in Example 1. The maximum value ΔL* of the brightness difference between standard mode and low-speed mode was 4.1, and the difference in halftone density between standard mode and low-speed mode was within the allowable range. This is thought to be because the difference in process speed between low-speed mode and standard mode is not large. No missing characters were observed in the fixing results for rough paper in low-speed mode. This is thought to be because sufficient heating time was ensured in the fixing nip.

[0043] (Comparative Example 1) The process speed in the low-speed mode was set to 0.5 (1 / 2) times that of the standard mode, the reflective surfaces 405a of the polygon mirror 405 were used every other surface, and the rotation speed of the polygon mirror 405 was set to the same as that in the standard mode. Other conditions were the same as in Example 1. The maximum value ΔL* of the brightness difference between the standard mode and the low-speed mode was 5.2, and a clear difference in density was observed. This is thought to be due to the large difference in process speed between the low-speed mode and the standard mode. No missing characters were observed in the fixing results for rough paper in the low-speed mode.

[0044] (Comparative Example 2) The process speed in low-speed mode was set to 0.86 (6 / 7) times that of standard mode, the entire reflective surface 405a of the polygon mirror 405 was used, and the rotation speed of the polygon mirror 405 was set to 0.86 (6 / 7) times that of standard mode. Other conditions were the same as in Example 1. The maximum value ΔL* of the brightness difference between standard mode and low-speed mode was 2.3, and there was almost no difference in halftone density between standard mode and low-speed mode. This is thought to be because the difference in process speed between low-speed mode and standard mode was small. Character chipping was observed in the fixing results for rough paper in low-speed mode. This is thought to be because the process speed in low-speed mode was too fast to sufficiently heat and melt the toner.

[0045] (Comparative Example 3) The process speed in the low-speed mode was 0.57 (4 / 7) times that in the standard mode, the entire reflective surface 405a of the polygon mirror 405 was used, and the rotation speed of the polygon mirror 405 was 0.57 (4 / 7) times that in the standard mode. Other conditions were the same as in Example 1. The maximum brightness difference ΔL* between the standard mode and the low-speed mode was 7.0, and a clear density difference was observed. The lower limit of the laser output detectable by the BD sensor 409 used in Comparative Example 3 was 0.7 times the laser output in the standard mode. Therefore, the light intensity per dot on the surface of the photoreceptor 4 in the low-speed mode was 0.7 × (1 / 0.57) = 1.23 times that in the standard mode, and could not be achieved at the same level as in the standard mode. As a result, the image formed in the low-speed mode had a density 10% or more higher than that in the standard mode. This is thought to be why a clear density difference was observed. No missing characters were observed in the fixing results on rough paper in the low-speed mode.

[0046] As described above, in the image forming apparatuses according to Examples 1 to 3 to which the present invention is applied, it is possible to suppress the change in image quality between the standard mode and the low-speed mode, and it is possible to form images with stable image quality regardless of the process speed. Therefore, when forming an image on a recording material with low toner fixability, it is possible to sufficiently fix the toner to the recording material by using the low-speed mode, and In this case, it was possible to form an image with the same image quality as when the standard mode was used.

[0047] In the embodiment, the present invention is applied to an image forming apparatus using a magnetic single-component jumping development system, but the development system is not limited to this example. For example, even in non-magnetic contact single-component development systems and two-component development systems, image density changes when the process speed changes, as with the jumping development system, to varying degrees. Therefore, applying the present invention can suppress changes in image quality in low-speed mode. Also, in the embodiment, an image forming apparatus using a fixing film heating system as a fixing device is illustrated, but the fixing device is not limited to this. For example, in fixing systems such as roller heating and flash fixing, the tendency for fixation to improve by lowering the process speed is similar to that in the fixing film heating system. Therefore, applying the present invention can improve fixation in low-speed mode. [Explanation of symbols]

[0048] 3: developing unit, 4: photosensitive member, 6: fixing unit, 41: transfer roller, 400: exposure device, 405: polygon mirror, 500: control unit, 510: input unit

Claims

1. A photoreceptor; a polygon mirror that is rotationally driven by a driving means and has a plurality of reflecting surfaces; an exposure unit that forms a latent image on the photosensitive member by reflecting a laser beam emitted from a light source on the polygon mirror and scanning the photosensitive member; a developing means for depositing toner onto the latent image formed on the photosensitive member to form a toner image on the photosensitive member; a transfer means for transferring the toner image formed on the photosensitive member to a recording material directly or via an intermediate transfer member; a fixing unit that fixes the toner image by heating the recording material onto which the toner image has been transferred; a control means for controlling an image forming mode for forming an image on a recording material so as to be executable; the image forming modes are comprised of a first image forming mode which is a default operation mode at the time of shipment from the factory, and a second image forming mode in which a process speed which defines the speed of the recording material passing through the fixing means is slower than the process speed of the first image forming mode, The control means when an instruction to designate the first image formation mode is input and when an instruction to designate a specific image formation mode is not input, control is performed so that the laser light is reflected using all of the plurality of reflecting surfaces of the polygon mirror of the exposure means, when an instruction to designate the second image formation mode is input, the process speed is made faster than 0.5 times the process speed of the first image formation mode, the rotation speed of the polygon mirror is made faster than the rotation speed of the polygon mirror in the first image formation mode, and control is performed so that laser light is reflected by only some of the plurality of reflection surfaces of the polygon mirror of the exposure means, an output of the laser beam emitted from the light source in the second image forming mode being controlled to be greater than that in the first image forming mode;

2. The control means controls the process speed in the second image forming mode to a speed that is faster than 0.5 times and slower than 0.75 times the process speed in the first image forming mode.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is controlled by the control unit.

3. 3. The image forming apparatus according to claim 1, wherein the control means controls the exposure means so that, in the second image forming mode, the plurality of reflective surfaces of the polygon mirror of the exposure means are used alternately to reflect the laser light.

4. 4. The image forming apparatus according to claim 3, wherein the control means controls the rotation speed of the polygon mirror in the second image forming mode to 2α times the rotation speed of the polygon mirror in the first image forming mode when the process speed in the second image forming mode is α times the process speed in the first image forming mode.

5. 5. The image forming apparatus according to claim 1, wherein the developing means performs development using a magnetic one-component jumping development method.

6. an input unit for receiving an input of a user's instruction specifying the type of recording material; An image forming apparatus according to any one of claims 1 to 5, wherein the control means controls the operation of the image forming apparatus in the image forming mode corresponding to the type of recording material specified in the instructions input by the input means.

7. an input unit for receiving an input of a user's instruction for specifying the image forming mode; 7. The image forming apparatus according to claim 1, wherein the control means controls the operation of the image forming apparatus in the image forming mode designated in the instruction input by the input means.

8. 8. The image forming apparatus according to claim 6, wherein when no instruction is input by said input means, said control means controls the operation of said image forming apparatus in said first image forming mode.

Images