JP6012480B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer equipped with a fixing device that fixes an unfixed toner image formed on a recording material to the recording material using electrophotographic recording technology or the like.

  A method for visualizing image information through an electrostatic latent image such as electrophotography is currently used in various fields such as a copying machine and a printer with the development of technology and the expansion of market demand.

  Particularly in recent years, demands for environmental friendliness and cost reduction have increased, and technology for reducing toner consumption has become very important. This technology for reducing toner consumption is also important from the viewpoint of reducing the energy generated in the process of permanently fixing the toner to the recording material, especially in an image forming apparatus using an office type electrophotographic system. It has come to play an important role from the demand for the development.

  Patent Documents 1 to 3 describe that a toner with high coloring power is used and the amount of toner transferred onto a recording material is reduced so that a toner image after fixing has a required image density. .

JP 2004-295144 A JP-A-2005-195670 JP-A-2005-195664

  However, the above-described conventional techniques still have the following problems that cannot be solved. In other words, it is possible to reduce the amount of toner consumed by increasing the amount of pigment in the toner and reducing the total amount of toner applied to that amount. However, if the amount of toner applied is reduced, the amount of toner in a solid color will decrease. As a result, the toner cannot be in close contact with each other, and the surface of the recording material with unevenness cannot be hidden with the toner. At this time, image defects such as blurring or loss of characters / line drawings occur.

  In such a state, in the secondary color (two toner layers of different colors are stacked), the area where the toners of different colors overlap each other decreases, so that the saturation of the secondary color is significantly reduced. As a result, there arises a problem that the color reproduction range becomes narrow.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve such problems and to provide an image forming apparatus capable of suppressing a decrease in saturation even when a small amount of toner is used and at the same time suppressing a decrease in image quality after fixing. To do.

  In order to solve the above-described problems, the present invention provides an image forming unit capable of forming a toner image in which a plurality of color toners are laminated on a recording material, and a toner image while nipping and conveying the recording material carrying the toner image at the nip portion. In the image forming apparatus having the fixing unit that fixes the toner image on the recording material, the image forming unit records the toner image so that the maximum width per pixel of the toner image is shorter than the width of one pixel in a predetermined direction on the recording material. An image is formed on the material, and the fixing portion is structured to apply pressure so as to stretch the toner image on the recording material in the predetermined direction at the nip portion.

  As described above, according to the present invention, it is possible to provide an image forming apparatus that can suppress a decrease in saturation even when a small amount of toner is used, and can suppress a decrease in image quality after fixing.

1 is a configuration model diagram of an example of an image forming apparatus. FIG. 3 is a schematic cross-sectional view of a fixing unit according to the first exemplary embodiment. FIG. 3 is a front sectional view of a fixing device that slides a fixing roller in a longitudinal direction. FIG. 3 is a schematic cross-sectional view showing a state of the fixing device after completion of fixing one sheet. FIG. 4 is a schematic cross-sectional view showing a series of operations of a fixing roller slide. FIG. 6 is a schematic cross-sectional view showing a series of operations of a fixing roller slide when a second and subsequent sheets are passed. The graph which showed the relationship between the sliding amount of a fixing roller, and the elongation amount of a line edge part. FIG. 6 is a schematic diagram illustrating toner deformation at a fixing unit. FIG. 3 is a schematic diagram illustrating a state of a dot image before and after fixing. FIG. 3 is a schematic diagram for explaining an image forming unit according to the first exemplary embodiment. FIG. 3 is a diagram illustrating an image after fixing when image processing of Example 1 is not performed. FIG. 3 is a diagram illustrating an image after fixing when image processing of Example 1 is performed. The figure which showed the other example of the image processing. 6A and 6B are diagrams for explaining a difference in a toner layer on a recording material when image processing according to Embodiment 1 is not performed and when image processing is performed. FIG. 3 is a diagram for explaining a developing bias potential of the apparatus according to the first exemplary embodiment. FIG. 6 is a diagram for explaining an image forming unit according to a second embodiment. FIG. 6 is a schematic cross-sectional view of a fixing unit according to a second embodiment. FIG. 6 is a diagram illustrating an image after fixing when image processing of Example 2 is not performed. FIG. 6 is a diagram illustrating an image after fixing when image processing of Example 2 is performed. Explains the difference in the toner image after fixing between the fixing device that presses perpendicularly to the surface of the recording material and the fixing device that pressurizes the component in the direction parallel to the surface of the recording material. Photo to do. FIG. 4 is a diagram illustrating a toner amount and “a toner layer forming state of a single color and a secondary color”. (A) is a model diagram showing a close-packed arrangement of toner, and (b) is a model diagram showing an arrangement of toner with a gap t.

  Hereinafter, the present invention will be described more specifically with reference to examples. Although these examples are examples of the best mode of the present invention, the present invention is not limited to these examples.

  First, a phenomenon that occurs when the amount of toner on the recording material is small will be described. FIG. 21 is a schematic diagram showing an observation result of the toner amount of the unfixed toner image formed on the recording material and the “monolayer and secondary color toner layer formation state”. In addition to the toner 401 (cyan in the description) for the single color, the second color toner 403 (yellow in the description) is shown. In the figure, the state of forming a single color toner layer when the amount of toner is small is (a), the state of forming a secondary color toner layer is (b), and further when the amount of toner is large (when aligned without gaps). The monochromatic toner layer formation state of (c) and the secondary color toner layer formation state are shown in (d).

  When the toner amount is small, it can be seen that there are many gaps in the lower cyan toner 401 as shown in (a), and the upper yellow toner 403, which is the second color, as shown in (b). It can be seen that the toner 401 is placed in the gap formed. Needless to say, when particles such as toner form a layer, the particles placed thereon fall between the underlying particles. As described above, the upper yellow toner 403 is placed on the lower cyan toner 401 having a gap. Therefore, as shown in (transmission state) of (b), when viewed through the upper layer toner, a portion 404 where only the upper layer yellow toner 403 exists, a portion 405 where only the lower layer cyan toner 401 exists, and an upper layer toner It can be seen that the yellow toner 403 and the lower cyan toner 401 overlap to form an overlapping portion 406 that forms green.

  On the other hand, when the toner amount is large (when they are arranged without gaps), the cyan toner 401 in the lower layer is in contact with the adjacent toner as shown in FIG. Recognize. Further, as shown in (d), the upper yellow toner 403 for the second color is placed in the gap formed by the cyan toner 401 as in (b), and is further placed on the yellow toner 403. It can be seen that the toner 403 is also placed in the gap formed by the yellow toner itself. In the monochromatic state of (c), the recording material is already well concealed, and the yellow toner 403 itself located in the upper layer is also in a state of concealing the lower layer with the yellow toners. Therefore, as can be seen from the transmission state of (d), unlike the transmission state of (b) when the toner amount is small, many portions where the yellow toner 403 is present are overlapped portions 406 that form green. It turns out that it becomes.

  As described above, when the toner amount is large, many portions are overlapped portions 406 that favorably form the secondary color, whereas when the toner amount is small, the lower the toner amount, The portions (404, 405) that are only monochromatic increase in the gaps between the lower layers. Therefore, when the toner amount on the recording material is reduced, the color development (saturation) of the secondary color is deteriorated, and at the same time, the concealment of the recording material is also deteriorated in the single color formation portion, thereby causing an extremely wide gamut reproduction range. To drop.

  From the above observation results, it was found that the amount of the gap generated between the single color toners affects the color gamut reproduction range. The gap generated between the single color toners increases as the toner amount decreases. As can be seen from the observation, when there is a sufficient amount of toner to form a single color and multiple layers, the upper layer toner fills the gap between the lower layer toners. As the toner amount decreases, multilayer formation cannot be performed, and the gap gradually increases. Therefore, a boundary condition as to whether or not the surface of the recording material is covered with a single toner layer (a layer having a thickness corresponding to one toner particle) will be considered.

When the shape of the toner is a true sphere, the amount of toner required for the true sphere toner to form a single layer in an ideal close-packed arrangement was calculated. The close-packed array is an array in which adjacent toner particles of the same color are in contact with each other as shown in FIG. FIG. 22 shows a state in which the toner is placed on the recording material in a close-packed arrangement (FIG. 22A) and a state in which the toner is placed on the recording material at intervals of the gap t (FIG. 22B). Is shown. The parameters used for the calculation are toner particle size (weight average particle size) L [μm] and toner density (specific gravity) ρ [g / cm ³ ].

The volume of toner V _○ [μm ^3], _○ projected area of the planar toner S [μm ^2], (rhombic portion of FIG. 22 (a)) unit area in the toner one minute is S _■ [[mu] m ² ], which are as follows.

  From these, the toner loading amount H [μm] of the single layer (one color) when the toners are closely packed (arrangement in FIG. 22A) (the volume of the toner per unit area = the average height of the toner) Is calculated as follows.

The toner loading amount A [mg / cm ² ] (weight per unit area) is

(Where 1/10 is for unit matching).

That is, A = ρπL / (30√3) [mg / cm ² ] is the applied toner amount in the close-packing arrangement. If the amount of toner on the recording material is smaller than this amount of toner, the gap between the toner particles becomes too large, and even when a toner image in which a plurality of color toners are laminated is fixed on the recording material, the saturation of the toner image is lowered. It is easy to do. Therefore, in each color, in the image forming apparatus in which the maximum applied amount A [mg / cm ² ] of the unfixed toner image on the recording material is set to A <ρπL / (30√3), the toner consumption is suppressed. The gap between the toner particles on the material is large, and the saturation tends to decrease.

Therefore, in the image forming apparatus of the present embodiment, the maximum applied amount A [mg / cm ² ] of the unfixed toner image on the recording material is set to A <ρπL / (30√3) for each color. Nevertheless, the reduction in saturation can be suppressed.

(Image forming part)
In the image forming apparatus shown in FIG. 1, first, second, third, and fourth image forming units Pa, Pb, Pc, and Pd are provided side by side. It is formed through a transfer process.

  Each of the image forming portions Pa, Pb, Pc, and Pd includes a dedicated image carrier, in this example, the electrophotographic photosensitive drums 3a, 3b, 3c, and 3d, and each color is provided on each of the photosensitive drums 3a, 3b, 3c, and 3d. The toner image is formed. An intermediate transfer member 30 is installed adjacent to each of the photosensitive drums 3a, 3b, 3c, and 3d, and the toner images of the respective colors formed on the photosensitive drums 3a, 3b, 3c, and 3d are primary on the intermediate transfer member 30. Transferred and transferred onto the recording material P at the secondary transfer portion. Further, the toner image formed on the recording material is heated and pressed by the fixing unit 9 and fixed on the recording material, and is then discharged out of the apparatus as a recorded image.

  Drum chargers 2a, 2b, 2c, and 2d, developing devices 1a, 1b, 1c, and 1d, primary transfer chargers 24a, 24b, 24c, and 24d, and a cleaner are disposed on the outer periphery of the photosensitive drums 3a, 3b, 3c, and 3d, respectively. 4a, 4b, 4c, 4d are provided. A laser scanner for forming an electrostatic latent image on the photosensitive drum in accordance with image information is installed above these portions.

  The developing devices 1a, 1b, 1c, and 1d contain cyan, magenta, yellow, and black toners. The developing devices 1a, 1b, 1c, and 1d develop the latent images on the photosensitive drums 3a, 3b, 3c, and 3d, respectively, and visualize them as cyan toner images, magenta toner images, yellow toner images, and black toner images.

  The intermediate transfer member 30 is driven to rotate at the same peripheral speed as the photosensitive drum 3 in the direction of the arrow. The yellow toner image of the first color formed on the photosensitive drum 3 a passes through the nip portion between the photosensitive drum 3 and the intermediate transfer body 30, and the effect of the primary transfer bias applied to the intermediate transfer body 30. Thus, the image is transferred to the outer peripheral surface of the intermediate transfer body 30. Similarly, a magenta toner image of the second color, a cyan toner image of the third color, and a black toner image of the fourth color are sequentially superimposed and transferred onto the intermediate transfer body 30, and a composite color toner image corresponding to the target color image is intermediate. It is formed on the transfer body.

  A secondary transfer roller 11 is disposed in contact with the intermediate transfer member 30. A desired secondary transfer bias is applied to the secondary transfer roller 11 by a secondary transfer bias source. The composite color toner image superimposed and transferred on the intermediate transfer member 30 is transferred from the paper feed cassette 10 to the recording material P that is conveyed to the contact nip between the intermediate transfer member 30 and the secondary transfer roller 11 via the registration roller 12. Is done. In this way, an unfixed toner image in which a plurality of color toners are superimposed is formed on the recording material. Thereafter, the recording material is conveyed to the fixing unit 9. The unfixed toner image formed on the recording material is heated and pressed at the fixing nip portion of the fixing unit 9 and fixed on the recording material.

  The photosensitive drums 3a, 3b, 3c and 3d after the primary transfer are cleaned by the respective cleaners 4a, 4b, 4c and 4d. Further, the intermediate transfer member 30 is also cleaned by the cleaner 19.

(Fixing device)
The fixing device (fixing unit) 9 of this example is configured to apply a shearing force that is perpendicular to the toner stacking direction and is in a certain direction to the toner image during the fixing process of one recording material at the fixing nip. It will continue to be granted.

  Examples of the fixing device will be described below. In this embodiment, the fixing roller is rotated and simultaneously moved (slid) in the longitudinal direction of the fixing roller to stretch the toner while melting the unfixed toner. Even when the amount of unfixed toner is small (the toner layer is small), the color developability (saturation) of the secondary color can be improved. This will be described in detail below.

  FIG. 2 is a schematic cross-sectional view of the fixing device in this embodiment. A fixing roller (first rotating body that contacts an unfixed toner image) 100 has an outer diameter of φ40 mm, and an elastic layer 105 made of silicone rubber is formed on the outer side of an aluminum cored bar 104 of φ36 mm. On the elastic layer 105, a release layer made of PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer) is formed as a toner release layer with a thickness of 30 μm. In this example, a PFA tube having excellent durability was used as the release layer. As a material for the release layer, a fluororesin such as polytetrafluoroethylene (PTFE) or tetrafluoroethylene-hexafluoropropylene resin (FEP) may be used in addition to PFA.

  In this embodiment, the pressure roller (second rotating body that forms a fixing nip portion together with the first rotating body) 101 has the same configuration as the fixing roller 100. That is, the outer diameter is φ40 mm, the elastic layer 105 made of silicone rubber is formed outside the φ36 mm aluminum cored bar 104, and the release layer made of PFA is provided on the outermost layer. The pressure roller 101 is pressed with a force of 400 [N] in the direction of arrow A1 in the drawing by a pressure spring 103 and contacts the fixing roller, thereby forming a fixing nip N having a width of 9 mm in the recording material conveyance direction. . Further, the pressure roller 101 is rotated by a drive motor 1109 (see FIG. 3) in the direction of arrow R1 in the drawing at a surface speed of 117 mm / sec. Following the rotation of the pressure roller 101, the fixing roller 100 also rotates at a surface speed of 117 mm / sec (arrow R2 in the figure). The fixing roller 100 and the pressure roller 101 form a pair of rotating bodies that form a fixing nip portion.

  A halogen heater 102 is provided in each of the fixing roller 100 and the pressure roller 101. By supplying power to the halogen heater 102, the halogen heater 102 generates heat, and the heat is transmitted to the cored bar 104 by radiant heat transfer or heat transfer via air, and then the elastic layer 105 and the release layer are warmed. A temperature detection element (not shown) is arranged in contact with the surface of the fixing roller 100, and the surface temperature of the fixing roller 100 is adjusted by controlling the power supplied to the halogen heater according to the signal of the temperature detection element. doing.

  When the recording material P to which the unfixed toner image T has been transferred is conveyed to the fixing nip portion N by a conveying means (not shown), the heat of the fixing roller 100 is heated while the recording material is nipped and conveyed by the fixing nip portion. The toner image T is transmitted to the image T and the recording material P, and the toner image T is fixed on the surface of the recording material P.

  Next, a mechanism for stretching the toner (mechanism for applying a shearing force) while melting the unfixed toner image T will be described below. FIG. 3 is a front cross-sectional view of a fixing device of this embodiment that slides the fixing roller in the longitudinal direction. The pressure roller 101 is rotated in the direction of arrow R1 by the drive motor 1109, and the fixing roller 100 is driven to rotate in the direction of arrow R2. Both the fixing roller 100 and the pressure roller 101 are smoothly rotated by bearings 111 at both ends. The pressure roller 101 is fixed in the longitudinal direction, but the fixing roller 100 can move (slide) in the longitudinal direction.

  A mechanism for sliding the fixing roller 100 in the longitudinal direction will be described. Side metal plates 106 are provided at both ends of the fixing roller 100, and the side metal plates 106 are further fixed to the movable support metal plate 107. A shaft 108 passes through the movable support metal plate 107, and a motor 109 for rotating the shaft 108 is disposed at one end of the shaft 108. When the motor 109 rotates in the direction of arrow R3, the shaft 108 also rotates in the direction of arrow R3. As the shaft 108 rotates, the movable support metal plate 107 moves smoothly along the slide rail 110 in the direction of arrow A2. Accordingly, the fixing roller 100 fixed to the movable support metal plate 107 also slides in the direction of the arrow A2. When the motor 109 rotates in the reverse direction (in the direction of arrow R4), the fixing roller 100 slides in the direction of the arrow A3 by the same mechanism as described above.

  In this way, the recording material P is passed through the fixing nip portion N while rotating the fixing roller 100 while sliding in the longitudinal direction, and the unfixed toner on the recording material P is fixed. At this time, it is necessary to prevent the surface of the fixing roller 100 from coming into contact with the recording material P by sliding the fixing roller 100 while the recording material P passes through the fixing nip portion. Therefore, the length of the fixing roller 100 in the longitudinal direction needs to be longer than that of the pressure roller 101 according to the amount to be slid. As shown in FIG. 3, in this embodiment, the length of the fixing roller 100 is longer than the pressure roller 101 by 2D (= D + D). Here, the length D represents the length from the end of the pressure roller 101 to the end of the fixing roller 100 when the centers of the fixing roller 100 and the pressure roller 101 are aligned in the longitudinal direction. The setting of the length D will be described later.

  As described above, when the fixing roller 100 slides in the arrow A2 direction or the arrow A3 direction, the pressure roller 101 is fixed in the longitudinal direction and does not slide, so that the toner on the recording material P is transferred to the toner on the recording material P in the fixing nip N. A shear force parallel to the moving direction acts. In the configuration in which the fixing roller 100 is not slid in the longitudinal direction, only the pressing force perpendicular to the recording material acts on the toner on the recording material. Therefore, when the amount of toner is small, the secondary color is developed by the above-described mechanism. Remarkably deteriorates. On the other hand, when the pressure roller 101 is fixed in the longitudinal direction and the fixing roller 100 is slid in the longitudinal direction as in this embodiment, a shearing force (toner parallel to the recording material other than the pressing force perpendicular to the recording material) Force to stretch the toner) acts on the toner. Accordingly, since the toner can be stretched in the longitudinal direction while being melted, the above-described mechanism can improve the color developability of the secondary color even when the amount of toner is small.

  Regardless of whether the recording material P is coated paper or plain paper, the color developability increases as the fixing roller slide amount increases. However, as the slide amount is increased, the saturation tends to be saturated at a certain value or more, so that a sufficient effect can be obtained by applying a slide amount at which the saturation starts to show a saturation tendency. In the apparatus of this example, the width of the fixing nip portion N is 6.5 mm, but it has been found that the saturation is saturated at a sliding amount (about 200 μm) of about 3% of the width of the fixing nip portion. That is, when the recording material P passes through the fixing nip portion, if the fixing roller 100 is slid by 200 μm in the longitudinal direction (an amount of about 3% of the fixing nip width), a sufficient saturation enhancement effect can be obtained.

  It should be noted that if the sliding direction of the fixing roller 100 is changed while the recording material P passes through the fixing nip N, the fixing roller is moved in the longitudinal direction for a short time to change the direction of the sliding direction. It is not moving. As a result, in the fixed image, the color developability of the portion where the direction of the slide is changed is lowered. Therefore, while one recording material P passes through the fixing nip N, the sliding direction of the fixing roller 100 needs to be fixed in one direction (A2 direction or A3 direction). That is, it is preferable to continue to apply a shearing force that is perpendicular to the toner stacking direction and in a certain direction to the toner image during the fixing process of one recording material at the fixing nip portion.

  Here, as a specific example, a case where an A4 size recording material P is passed through the fixing nip in the horizontal direction will be described. If the required slide amount is 3% of the fixing nip width for the above-described reason, the fixing roller 100 is moved in the direction of the arrow A2 (arrow) from the state of FIG. 3 until one A4 size recording material passes the fixing nip in the horizontal direction. It may slide 6.3 mm (= 210 mm × 3%) in the A3 direction). At this time, since the speed at which the fixing roller 100 is slid is 3% of the process speed, in this embodiment, it is 3.5 mm / sec (= 117 mm / sec × 3%). FIG. 4 shows the state of the fixing device after fixing one sheet. When the second sheet is continuously fixed, the state shown in FIG. 3 is restored by sliding the sheet 6.3 mm in the A3 direction (A2 direction when the first sheet is moved in the A3 direction). Further, when fixing the third sheet continuously, it may be slid in the A2 direction in the same manner as the first sheet. However, when only the same part in the longitudinal direction of the fixing roller 100 comes into contact with the recording material, there is a problem that the part is quickly deteriorated. Therefore, it is preferable to slide the fixing roller 100 in the direction of the arrow A3 when passing the third sheet. FIG. 5 shows a series of operations of the fixing roller 100 described above. However, the manner in which the recording material P passes through the fixing nip N is not shown.

  As shown in FIG. 4, if the end of the fixing roller 100 and the end of the pressure roller 101 are aligned before the sheet is passed, a maximum 2D slide amount can be secured in the A2 direction. The length D may be set according to product specifications. In the case of this embodiment, since the recording material having the maximum width among the recording materials that can be used in the image forming apparatus is 19 inches, 14.5 mm (19 × 25.4 mm × 3%) is a value of 2D, and D is It is about 7.2 mm. The fixing roller 100 may be made longer than the pressure roller 101 by a value of 2D. The size of the recording material that can be fixed in the state in which the longitudinal central portions of the fixing roller 100 and the pressure roller 101 are aligned, that is, the series of operations shown in FIG. For other recording material sizes up to 19 inches, the first sheet is slid in the direction of arrow A3 from the state shown in FIG. A series of operations when the second and subsequent sheets are continuously fed are shown in FIG. Here, however, the state in which the recording material P passes through the fixing nip N is not shown. In the case of fixing by the above procedure, the positional relationship between the fixing roller 100 and the pressure roller 101 is changed to (1) in FIG. 5 before passing the first sheet according to the size of the recording material to be fixed, or Control must be made in (2) of FIG.

  In addition to the above operation, for example, when the length D is 14.5 mm, fixing can be continuously performed by the operation shown in FIG. 5 for any recording material size up to 19 inches. At this time, the fixing roller 100 and the pressure roller 101 may be aligned at the longitudinal center after fixing. However, the length of the fixing roller 100 in the longitudinal direction is restricted by the space in which the fixing device is disposed, and if it is too long, the energy saving performance is impaired due to heat radiation from the end of the fixing roller. Therefore, it is necessary to determine the sliding means according to the specifications of the product on which the fixing device is mounted. In this embodiment, the slide amount is set to 3% of the fixing nip width. However, it may be set to 3% or less depending on the product specifications, or may be set to 3% or more in consideration of fluctuations in the effect.

  Although the example in which the fixing roller 100 is slid in the longitudinal direction has been described so far, a configuration in which the fixing roller 100 is fixed in the longitudinal direction and the pressure roller 101 is slid in the longitudinal direction may be used. In that case, the fixing roller 100 is driven (rotated) in the circumferential direction, and the pressure roller 101 is driven by the fixing roller 100. Further, in order to slide the pressure roller 101, the length of the pressure roller 101 must be longer than the fixing roller 100. Since the configuration is upside down in FIG. 3 and the effect is the same, detailed description is omitted.

  Up to this point, the configuration has been described in which either the fixing roller 100 or the pressure roller 101 is fixed in the longitudinal direction and the unfixed side is slid in the longitudinal direction. In order to apply a shearing force, both the fixing roller 100 and the pressure roller 101 may be slid. In other words, the toner image may be stretched by sliding at least one of the pair of rotating bodies in a direction perpendicular to the recording material conveyance direction while rotating the pair of rotating bodies. However, when the fixing roller 100 and the pressure roller 101 slide in the same direction and in synchronization, naturally no shearing force is generated, so that no effect is obtained. If the fixing roller 100 and the pressure roller 101 are slid in the opposite direction or asynchronously in the same direction, a shearing force is generated and the same effect can be obtained. When either the fixing roller 100 or the pressure roller 101 is slid, the recording material meanders slightly when passing through the fixing nip N, but the fixing roller 100 and the pressure roller 101 are slid by the same amount in the opposite directions. In this case, there is an advantage that the meandering of the recording material is suppressed.

  When the fixing device as described above is used, the toner is greatly deformed (stretched), so that the color development (saturation) is improved even if the amount of toner placed on the recording material is small. FIGS. 9A and 9B are schematic diagrams illustrating an example of the state of the dot image before and after performing the fixing process in the fixing device of this example. A black circle indicates a dot image formed using toner before the fixing process, and a gray part indicates a state after the fixing process and melted and spread by fixing. As shown in FIGS. 9A and 9B, when the fixing device of this example is used, a shearing force in a direction parallel to the surface of the recording material perpendicular to the toner stacking direction is applied to the toner. The dot image extends in the direction of the shearing force that works parallel to the surface.

  However, since the toner is stretched, the toner image extends in the direction of stretching the toner as shown in the lower diagram of FIG. The conventional fixing shown in the upper diagram of FIG. 20 is a case where the fixing process is performed with a fixing unit having a structure in which pressure is applied perpendicularly to the surface of the recording material. Is shown. FIG. 8 is a diagram schematically showing the deformation of the toner when it is fixed using the fixing device of this example. As shown in FIG. 8, all the toners in the image are deformed approximately in the same direction and in the same direction. However, even if the toner inside the image (not the edge of the image) is deformed, the deformation of the toner is not noticeable because there is toner at the deformed tip. Therefore, it seems that only the edge of the image is extended.

  Since the toner at the edge of the image is slightly stretched, it is a level that is hardly noticeable in an image having many solid portions such as a photograph. However, if there is an image that has a combination of vertical and horizontal lines, such as characters or a table, the image recognizes that the line width has been increased by extending the characters and lines in the direction of toner deformation. It becomes easy to be done. Further, in an image such as a barcode where the line width is important, it is difficult to ignore the increase in the line width due to the above-described toner elongation.

(Elongation of image edge)
When the above-described fixing device is used, a shearing force is generated in the fixing nip N in the longitudinal direction (axial direction of the roller), and the color development property of the secondary color and the concealment rate of the recording material are improved. In this embodiment, the longitudinal direction of the fixing roller to which the shearing force acts is the main scanning direction (predetermined direction) when the photosensitive member is irradiated with laser light. Therefore, the image end portion in the main scanning direction is extended by the shearing force. The predetermined direction in this example, that is, the main scanning direction is also a direction orthogonal to the recording material conveyance direction.

  FIG. 7 shows the amount of increase (increase) in the line width when a 3 dot line toner image perpendicular to the shearing direction (sliding direction) is fixed using the fixing device of this example (that is, while sliding the roller). Amount). The difference between the line width of the toner image after fixing when the toner image is fixed without sliding the fixing roller and the line width of the toner image after fixing by the fixing device of this example is the line width (line end portion). ) On the vertical axis. From this figure, it can be seen that the line width increases as the slide amount increases. This is because, as described above, when the amount of deformation of the toner increases, the amount of elongation at the image edge in the direction of deformation of the toner also increases. As shown in FIG. 8, all the toner in the toner image is actually deformed. Although the end portion of the line has been described here, it goes without saying that the end portion in the direction in which the shearing force acts is extended in any image. For example, in order to make Green's saturation 80, the slide amount must be about 390 μm. At this time, the extension amount of the line end (image) is about 15 μm.

(Details of image formation)
Next, the image forming apparatus of this embodiment will be described in detail with reference to FIG. The image forming unit of this example includes a rotating photosensitive member and a light source that emits light according to image information, and scans the photosensitive member with light emitted from the light source to form an electrostatic latent image on the photosensitive member. A developing unit that supplies toner to the electrostatic latent image and develops it, and a transfer unit that transfers the toner image formed on the photosensitive member to a recording material. Further, by shortening the electrostatic latent image formed on the photoconductor in a predetermined direction, the maximum width per pixel of the toner image on the recording material is made shorter than the width of one pixel. FIG. 10 shows one image forming station among the four image forming stations Pa to Pd shown in FIG. The other three image forming stations have the same configuration.

  In FIG. 10, an image of a color document to be copied is projected onto an image sensor 90 such as a CCD by a lens. The image sensor 90 decomposes an image for each color of a color original into 600 dpi pixels and generates a photoelectric conversion signal corresponding to the density of each pixel for each color. The photoelectric conversion signal (analog image signal) for each color output from the image sensor 90 is sent to the image signal processing circuit 91, where a pixel signal (digital signal) having an output level corresponding to the density of the pixel for each pixel. ) And sent to a pulse width modulation (PWM) circuit 92.

  For each input pixel image signal, the pulse width modulation circuit 92 forms and outputs a laser driving pulse having a width (time length) corresponding to the level. The laser drive pulse output from the pulse width modulation circuit 92 is supplied to the semiconductor laser 80 and causes the semiconductor laser 80 to emit light for a time corresponding to the pulse width. Therefore, the semiconductor laser 80 is driven for a longer time per pixel for high density pixels and for a shorter time for low density pixels. Therefore, the photosensitive drum 3 is exposed to a long range in the main scanning direction for high density pixels and a short range in the main scanning direction for low density pixels by the following optical system. .

  Laser light emitted from the semiconductor laser 80 is deflected by a rotating polygon mirror 81, passes through a lens 82 such as an f / θ lens, and is spotted on the photosensitive drum 3 via a fixed mirror 83 that is directed toward the photosensitive drum. Imaged.

  The photosensitive drum 3 is an electrophotographic photosensitive drum having amorphous silicon, selenium, OPC or the like on its surface and rotating in the direction of the arrow. After the charge is uniformly removed by the exposure device 34, the photosensitive drum 3 is uniformly distributed by the primary charging device 2. Charged. Thereafter, exposure scanning is performed with a laser beam modulated in accordance with the above-described image information, whereby an electrostatic latent image corresponding to the image information is formed. This electrostatic latent image is reversely developed by a developing device 1 using a two-component developer in which toner particles and carrier particles are mixed to form a visible image (toner image).

  Here, the reversal development is a development method in which a toner charged with the same polarity as the latent image is attached to a region exposed to light of a photosensitive member to visualize the toner. The toner image developed on the photoreceptor is primarily transferred to the intermediate transfer material 30 by the action of the primary transfer device 24.

  Thereafter, the untransferred residual toner adhering to the surface of the photosensitive drum 3 after the toner image is primarily transferred is removed by the cleaning device 4. The collected residual toner and the like are discharged by a conveying screw and stored in a waste toner box.

(Image processing)
Next, image processing for forming a pre-fixed toner image short in advance in the direction in which the toner is deformed in the fixing process will be described.

  As an example of image processing, a line composed of four pixels of a single color (4 dot line) will be described. FIG. 11A shows an image signal before image processing. The minimum square in the figure is one pixel (here, one pixel of 600 dpi), and its size is 42 μm × 42 μm. In this one pixel, the semiconductor laser 80 can emit light in the time of PWM 0% to 100%, but cannot be turned on / off twice or more. For example, after the PWM is turned on for 30%, the PWM is turned off for 50%, and the remaining PWM 20% cannot be turned on again in the pixel. One pixel is a minimum area (here, one pixel of 600 dpi) in which the laser can be turned on only once.

  The shaded area in FIG. 11A is a region in which the laser is turned on (corresponding to the laser turn-on time), and one pixel constituting the line image is turned on with PWM 100%. The latent image portion in the shaded area is developed with toner to form a 4 dot line.

  When the toner image obtained from FIG. 11A is fixed at a slide amount of 390 μm, the line image is deformed and the end of the line extends about 15 μm in the shearing (slide) direction, and the line width of the 4 dot line is It becomes as large as 15 μm (see FIG. 11B).

  In order to prevent such deterioration in image quality due to the extension of the image end, an image processing circuit (not shown) in the image signal processing circuit 91 is used to reduce the length of each pixel of 4 dot lines by 20 μm in the main scanning direction. The laser lighting time in the pixel is set (FIG. 12 (a)). That is, the maximum light emission time is changed from PWM 100% to PWM 52%. As a result, the maximum image width (laser lighting width) in the main scanning direction within one pixel is reduced from 42 μm to 22 μm. The reason why the shortening amount of the image is slightly increased to 20 μm instead of 15 μm will be described later. Thus, by shortening the electrostatic latent image formed on the photosensitive member in a predetermined direction, the maximum width per pixel of the toner image on the recording material is made shorter than the width of one pixel.

When the toner image obtained from FIG. 12A is fixed with a slide amount of 390 μm as described above, a desired (maximum width of one pixel) is obtained after fixing, as shown in FIG. 12B. ) 4dot line width can be obtained. That is, it is possible to prevent an increase in the image width due to the extension of the line end. The laser spot diameter (main scanning Gaussian distribution spot 1 / e ² ) used here is 20 μm.

  In FIG. 12A, the laser lighting time is shortened in the direction opposite to the direction in which the shearing force acts (slide direction), but the laser lighting time may be shortened in the same direction as the direction in which the shearing force acts. Further, as shown in FIG. 13, the laser may be turned on within one pixel. This is because any means can obtain a desired line width after fixing.

  Here, another feature of this example will be described. When the original image signal shown in FIG. 11A is developed as it is without performing the above image processing (the height of the unfixed toner image formed on the recording material is approximately the height of one toner particle), Let T be the toner amount (mounting amount) per unit area in an image region (region where toner is placed) in one pixel. In this example, the toner amount per pixel remains T, and the image width in one pixel is shortened in the direction in which the toner is deformed. Therefore, the applied amount (toner layer height) of the image area in one pixel increases.

  FIGS. 14A and 14B schematically show a cross section in a state where development is performed on one pixel. FIG. 14A shows the case of the original image signal, and FIG. 14B shows the case where the above image processing is performed. As described above, since the image width is shortened while keeping the amount of toner in one pixel as T, the toner layer of one pixel becomes high as shown in FIG.

  In the state of FIG. 14A, the toner layer is one layer or less (a layer having a thickness corresponding to one toner particle) and the gap between the toners is large. On the other hand, in the state of FIG. 14B, the toner layer has a two-layer structure, and there is almost no gap between the toners.

  As described above, the image forming unit (image forming station) of this example forms an image on the recording material so that the maximum width per pixel of the toner image on the recording material is shorter than the width of one pixel in a predetermined direction. . In addition, the fixing unit has a structure in which pressure is applied so that the toner image on the recording material is stretched in the same direction as the shortening direction of the toner image at the nip portion. Therefore, it is possible to provide an image forming apparatus that can suppress a decrease in saturation even when a small amount of toner is used, and can suppress a decrease in image quality after fixing.

  Also, if the toner amount per pixel remains T and the image area of one pixel is shortened in the direction in which the toner extends, the amount of image area in one pixel increases and the toner layer increases. When the toner layer is increased, when the toner is stretched while being melted, the toner can be stretched in a state of sticking to each other, so that the monochromatic hiding ratio is improved. Even when the slide amount is the same, the higher the toner layer, the slightly longer the image extends. Therefore, the amount of shortening the image was set to 20 μm instead of 15 μm.

  In this embodiment, the development contrast (Vcon) is increased in order to keep the toner amount in one pixel as T (increase the amount of applied toner M / S (mass per square) in the image area in one pixel) ( FIG. 15). The dotted line shown here schematically represents the high electrostatic latent image potential formed by the digital latent image. A solid line represents the potential of the developing bias. In the figure, Vl is the latent image potential of the image area, Vdc is the DC component of the developing bias, Vd is the potential of the non-image area, and Vcon = Vdc−Vl. In this embodiment, the latent image potential setting without image processing is Vl = −140 [V], Vdc = −350 [V], Vd = −500 [V], and Vl = − when image reduction is performed. Vcon was increased with 140 [V], Vdc = −390 [V], and Vd = −540 [V].

  The change in the development contrast is performed by inputting an output signal corresponding to the amount of shortening of the image from a main image processing circuit (not shown) to a development bias control circuit (not shown) provided in the developing device 1. It is.

  By performing the image processing as described above, when the fixing roller is slid, it is possible to prevent the line width from being increased and to obtain a desired 4-dot line after fixing (FIG. 12B). Furthermore, since the amount of toner in one pixel remains T, the gap between toners in the image area in one pixel is reduced. As a result, there is an advantage that the toners are more likely to stick to each other at the time of fixing, and the concealment rate on the surface of the recording material is improved.

  In the case of secondary colors such as RGB, it is possible to prevent the line width from being increased by the above processing, and at the same time, there is an advantage of improving the color development property because the overlapping portion of the toner in the image region within one pixel increases.

  As described above, lines are used as examples of image reduction. However, it is possible to perform image processing on characters and solid images with the same operation.

(Example 2)
Example 2 will be described below. The image forming unit of this example also shortens the electrostatic latent image formed on the photosensitive member in a predetermined direction so that the maximum width per pixel of the toner image on the recording material is shorter than the width of one pixel. . In this embodiment, the image forming apparatus for forming an unfixed toner image is substantially the same as that in the first embodiment, but as shown in FIG. 16, spot diameter changing means 84 is provided. Since other elements are the same as those in the first embodiment, description thereof is omitted. Next, the image forming apparatus of this embodiment will be described in detail with reference to FIG. In this embodiment, image processing in the case where a shearing force acts in the fixing roller rotation direction will be described.

(Fixer)
Examples of the fixing device in which a shearing force acts in the fixing roller rotation direction include an oblique pressurization method and a peripheral speed difference method. In this embodiment, the oblique pressurization method will be described. FIG. 17 shows a schematic cross-sectional view of an oblique pressure type fixing device used in this embodiment. This is a belt heating type heat fixing device using electromagnetic induction heating. In the figure, 330 is a heating unit as a heating rotator including heating means. Reference numeral 331 denotes a cylindrical fixing film as an electromagnetic induction heat generating rotating body having an electromagnetic induction heat generating layer (conductor layer, magnetic layer, resistor layer). Reference numeral 332 denotes a film guide member, and the cylindrical fixing film 331 is loosely fitted outside the film guide member 332. The magnetic field generating means includes an exciting coil 333 disposed inside the film guide member 332 and an E-type magnetic core (core material) 334. Reference numeral 320 denotes an elastic pressure roller, which forms a fixing nip portion N having a predetermined width with a predetermined pressure contact force with a sliding member 336 disposed on the lower surface of the film guide member 332 with the fixing film 331 interposed therebetween. I'm allowed. Reference numeral 335 denotes a pressurizing rigid stay. The magnetic core 334 of the magnetic field generating means is disposed so as to correspond to the fixing nip portion N.

  The pressure roller 320 is rotationally driven by the driving means M in the clockwise direction indicated by the arrow. A rotational force acts on the fixing film 331 by a frictional force between the pressure roller 320 and the outer surface of the fixing film 330 by the rotational driving of the pressure roller 320. The fixing film 331 rotates in the counterclockwise direction indicated by the arrow while the inner surface of the fixing film 331 slides in close contact with the sliding member 336 disposed on the lower surface of the film guide member 332 in the fixing nip N. The outer periphery of the film guide member 332 is rotated at a peripheral speed substantially corresponding to the peripheral speed (pressure roller drive system). In this state, the fixing film 331 rotates with a certain amount of resistance due to the friction of the sliding member 336 in close contact with the inner surface. Since the fixing film 331 on the side receiving the rotational force rotates with resistance, it is suitable for effectively applying a shearing force to the toner image on the recording material P with the pressure roller 320 on the driving side. Yes.

  The film guide member 332 serves to pressurize the fixing nip N, support the exciting coil 333 and the magnetic core 334 as magnetic field generating means, support the fixing film 331, and transport stability when the fixing film 331 rotates. do. The film guide member 332 is an insulating member that does not hinder the passage of magnetic flux, and a material that can withstand a high load is used.

  The exciting coil 333 generates an alternating magnetic flux by an alternating current supplied from an exciting circuit (not shown). The alternating magnetic flux is intensively distributed in the fixing nip portion N by the E-type magnetic core 334 corresponding to the position of the fixing nip portion N, and the alternating magnetic flux is generated by electromagnetic induction of the fixing film 331 in the fixing nip portion N. Generate eddy currents in the layer. This eddy current generates Joule heat in the electromagnetic induction heating layer due to the specific resistance of the electromagnetic induction heating layer. The electromagnetic induction heat generation of the fixing film 331 is concentrated in the fixing nip portion N where the alternating magnetic flux is concentrated, and the fixing nip portion N is heated with high efficiency. The temperature of the fixing nip N is controlled so that a predetermined temperature is maintained by controlling the current supply to the exciting coil 333 by a temperature control system including a temperature detection unit (not shown).

The pressing direction is set to a direction L ₂ having an angle θ with respect to the normal direction (substantially the toner overlapping direction) L ₁ of the sliding surface of the sliding member 336. The pressurizing method is not particularly limited, and a spring or the like can be used. For example, after setting the angle of the heating unit 330 to θ so that the normal direction of the contact surface of the fixing member 331 of the sliding member 336 is L ₁ , a spring (not shown) is attached to the heating unit 330 with L _2. annexed direction, guided by a member (not shown), by the heating unit 330 to be pressed against the L ₂ direction, it is possible to the pressurizing direction to the L _2. With the above configuration, the applied pressure is set to 600N.

Arrow near the fixing nip portion N in FIG. 17 is a direction of the force acting in the fixing nip N, which shows the component of force and L ₂ direction of the force. By applying pressure obliquely with respect to the toner overlapping direction, the in-plane component force (shearing force) applied to the toner is increased. As a result, the toner spreads in the in-plane direction. In particular, in the secondary color, a region where different color toners overlap increases, and the saturation and color gamut increase. Since the shearing force applied to the toner increases as the angle θ increases, the effect increases. However, if the angle θ is excessively large, the force for crushing in the toner overlapping direction is insufficient, so that the fixability is deteriorated. In addition, it is difficult to stably maintain the high-pressure direction in the apparatus configuration. In view of the above, as an example of the fixing operation condition in this embodiment, when the angle θ = 60 ° formed by the inter-axis direction L ₁ and the pressing direction L ₂ is set, compared to θ = 0 °. The saturation of the secondary color green increased by about 8 (when glossy paper was used as the recording material).

(Image processing)
At this time, the line width increases by about 10 μm in the fixing roller rotation direction. Therefore, in this embodiment, an image within one pixel must be performed in the sub-scanning direction. A means for shortening the image in the sub-scanning direction will be described below.

  Similar to the first embodiment, as an example of image processing, 4 dot lines having a resolution of 600 dpi in the main scanning direction and the sub scanning direction are used (FIG. 18A). Unlike Example 1, since a shearing force acts in the sub-scanning direction, when image processing is not performed, if fixing is performed using the oblique pressure fixing method of this example, the line width increases by 10 μm in the sub-scanning direction (FIG. 18). (B)).

  In order to prevent the extension of the line in the sub-scanning direction due to the shearing force, each pixel of the 4 dot line is shortened by 15 μm in the sub-scanning direction as shown in FIG. The reason why the amount to be shortened is increased by 5 μm is the same as in Example 1. If the fixing of the present embodiment is performed in a state where such image processing is performed, a desired 4-dot line width can be obtained as an image after fixing as shown in FIG. 19B.

  In this embodiment, the means for shortening the image in the sub-scanning direction reduces the laser spot diameter in the sub-scanning direction. In this example, the laser spot diameter in the sub-scanning direction was switched from 42 μm to 27 μm. The laser spot diameter is switched by a slit provided in front of the mirror 83 when a spot diameter switching signal is inputted to the spot diameter changing means 84 from a circuit (not shown) for processing an image in the image signal processing circuit 91 shortly. This can be done by switching the diameter of 85.

As in the first embodiment, at the same time as the above image processing is performed, the toner amount in the image area in one pixel remains T, so that the applied toner amount M / S in the image area in one pixel is 0.30 [ mg / cm ² ] to 0.46 [mg / cm ² ]. Accordingly, the toner is almost densely arranged from the state where there is a gap between the toners. Therefore, since the toner is stretched in a state where the toners are adhered to each other at the time of fixing, the concealment rate of a single color is also improved. The method for increasing the height of the toner layer was the same as in the example, and was realized by changing the development contrast.

Claims (5)

An image forming unit capable of forming a toner image in which a plurality of color toners are laminated on a recording material;
A fixing unit for fixing the toner image to the recording material while nipping and conveying the recording material carrying the toner image at the nip portion;
In an image forming apparatus having
The image forming unit forms an image on the recording material such that the maximum width per pixel of the toner image on the recording material is shorter than the width of one pixel in a predetermined direction, and the fixing unit is configured to record the recording material at the nip portion. An image forming apparatus having a structure in which pressure is applied so as to stretch the upper toner image in the predetermined direction.   The image forming unit includes a rotating photosensitive member and a light source that emits light according to image information, and scans the photosensitive member with light emitted from the light source to form an electrostatic latent image on the photosensitive member. A scanning unit; a developing unit that supplies toner to the electrostatic latent image to develop; and a transfer unit that transfers a toner image formed on the photoconductor to a recording material. 2. The image forming apparatus according to claim 1, wherein the latent image is shortened in the predetermined direction so that the maximum width per pixel of the toner image on the recording material is shorter than the width of one pixel.   The image forming apparatus according to claim 1, wherein the predetermined direction is a direction orthogonal to the recording material conveyance direction.   The fixing unit includes a pair of rotating members that form the nip portion, and the toner is obtained by sliding at least one of the pair of rotating members in a direction perpendicular to the recording material conveyance direction while rotating the pair of rotating members. The image forming apparatus according to claim 3, wherein the image is stretched. In the case of forming an image using a plurality of color toners, the image forming unit is configured to record a recording material for each color, assuming that the specific gravity of the toner is ρ (g / cm ³ ) and the weight average particle diameter of the toner is L (μm). The maximum applied amount A (mg / cm ² ) of the upper unfixed toner image is set to A <ρπL / (30√3), according to any one of claims 1 to 4. Image forming apparatus.