JP7206656B2 - image forming device - Google Patents

Description

The present invention relates to an image forming apparatus.

Patent Documents 1 and 2 disclose a configuration in which a transfer electric field having an AC component is applied between an intermediate transfer member and a transfer roll to transfer a toner image on the intermediate transfer member to a recording medium.

JP 2016-177252 A JP 2016-188991 A

Here, in a configuration in which an image is transferred by applying a transfer electric field having an AC component, toner scattering occurs periodically. It may be the cause.

According to the present invention, in a color image having the largest difference between the L* of a single-color solid image after fixing and the L* of a recording medium, the screen and the toner scattering that occurs periodically due to the application of the transfer electric field are eliminated. An object of the present invention is to make it possible to suppress poor image quality as compared with a configuration that interferes.

A first aspect of the invention comprises holding means for holding a toner image, transfer means for transferring the image onto a recording medium, and application for applying a transfer electric field having an AC component between the holding means and the transfer means. and fixing means for fixing the image transferred to the recording medium to the recording medium, and the color image having the largest difference between the L* of the single-color solid image after fixing and the L* of the recording medium. In the above, interference between the screen and toner scattering that occurs periodically due to the application of the transfer electric field is prevented.

In the second aspect of the invention, in the color image, the angle of the dot screen with respect to a predetermined reference line is an angle different from 0°, 45°, 90° and 135°.

In the third aspect of the invention, the transfer means has a contact area that contacts the holding means, and the angle of the dot screen with respect to a line along the longitudinal direction of the contact area as the reference line in the color image are angles different from 0°, 45°, 90°, and 135°.

A fourth aspect of the invention comprises holding means for holding an image, transfer means for transferring the image onto a recording medium, and applying means for applying a transfer electric field having an AC component between the holding means and the transfer means. , wherein the angles of the dot screen with respect to a predetermined reference line are different from 0°, 45°, 90° and 135° in a black color image.

In the fifth aspect of the invention, the applying means selectively applies a first transfer electric field having no AC component and a second transfer electric field having an AC component between the holding means and the transfer means. and when the applying means applies the first transfer electric field, the angle of the dot screen in the image of the color is set to any one of 0°, 45°, 90° and 135°, and the applying means The angles of the dot screens in the color images are different from 0°, 45°, 90° and 135° when a second transfer field is applied.

In the sixth aspect of the invention, when the applying means applies the first transfer electric field, the angle of the dot screen in the image of a different color different from the color is different from the angle of the dot screen in the image of the color. When the application means applies the second transfer electric field, the angle of the dot screen in the different color image is set to the angle of the dot screen in the different color image when the first transfer electric field is applied. Make a different angle.

In the seventh aspect of the invention, the angles different from 0°, 45°, 90° and 135° are angles different from 0°, 45°, 90° and 135° by 3° or more.

The eighth aspect of the invention is that, in the color image, the angle of the line screen with respect to a predetermined reference line is different from 0°.

In the ninth aspect of the invention, the applying means selectively applies a first transfer electric field having no AC component and a second transfer electric field having an AC component between the holding means and the transfer means. and when the applying means applies the first transfer electric field, the angle of the line screen is set to 0°, and when the applying means applies the second transfer electric field, the angle of the line screen is set to Make an angle different from 0°.

The tenth aspect of the invention uses an error diffusion screen in the color image.

In the eleventh mode of the invention, the applying means selectively applies a first transfer electric field having no AC component and a second transfer electric field having an AC component between the holding means and the transfer means. A screen other than the error diffusion screen is used when the applying means applies the first transfer electric field, and the error diffusion screen is used when the applying means applies the second transfer electric field.

According to the configuration of the first aspect of the present invention, in the color image having the largest difference between the L* of the single-color solid image after fixing and the L* of the recording medium, the difference is generated periodically by the application of the screen and the transfer electric field. Image quality defects can be suppressed as compared with a configuration in which toner particles interfere with each other.

According to the configuration of any one of the second to fourth aspects of the present invention, poor image quality is reduced compared to the configuration in which the angle of the dot screen is 0°, 45°, 90°, or 135°. can be suppressed.

According to the configuration of the fifth aspect of the present invention, regardless of the electric field applied by the application means, the angle of the dot screen is different from 0°, 45°, 90°, and 135°. When applying the first transfer electric field, graininess or character reproduction in the image of the color (the color with the largest difference between the L* of the single-color solid image after fixing and the L* of the recording medium, or the black color) It is possible to suppress the decline in sexuality.

According to the configuration of the sixth aspect of the present invention, regardless of the electric field applied by the applying means, compared to the case where the angle of the dot screen in the different color image is maintained, the color (L* of the single color solid image after fixing) is reduced. and L* of the recording medium (or black)) and an image of a different color.

According to the configuration of the seventh aspect of the present invention, it is possible to suppress poor image quality compared to a configuration in which the angles of the dot screen differ within a range from 0°, 45°, 90°, 135° to less than 3°.

According to the configuration of the eighth aspect of the present invention, poor image quality can be suppressed as compared with the configuration in which the angle of the line screen is set to 0°.

According to the configuration of the ninth aspect of the present invention, regardless of the electric field applied by the applying unit, compared to the configuration in which the angle of the line screen is different from 0°, when the applying unit applies the first transfer electric field, , it is possible to suppress deterioration of graininess or character reproducibility in an image.

According to the configuration of the tenth aspect of the present invention, image quality defects can be suppressed compared to the configuration using the dot screens arranged periodically.

According to the configuration of the eleventh aspect of the present invention, regardless of the electric field applied by the applying means, when the applying means applies the first transfer electric field, the graininess or A decrease in character reproducibility can be suppressed.

1 is a schematic diagram showing the configuration of an image forming apparatus according to an embodiment; FIG. 3A and 3B are diagrams showing a dot screen and a screen angle used in the toner image forming section according to the embodiment; FIG. FIG. 5 is a schematic diagram showing toner scattering occurring in a dot screen in a comparative example; FIG. 5 is a schematic diagram showing toner scattering that occurs on the dot screen according to the present embodiment. 7 is a flow showing an example of a procedure for setting a secondary transfer electric field and a screen angle; and FIG. 11 is a schematic diagram showing the configuration of an image forming apparatus according to a modification. 7 is a table showing the screen angle and the number of lines of a dot screen used for each color image in a comparative example. 4 is a table showing screen angles and line numbers of dot screens used for images of respective colors in Example 1. FIG. 10 is a table showing screen angles and line numbers of dot screens used for images of respective colors in Example 2. FIG. 10 is a table showing screen angles and line numbers of dot screens used for images of respective colors in Example 3. FIG. 10 is a table showing screen angles and line numbers of dot screens used for images of respective colors in Example 4. FIG. 10 is a table showing screen angles and line numbers of dot screens used for images of respective colors in Example 5. FIG.

An example of an embodiment according to the present invention will be described below with reference to the drawings.
<Image forming apparatus 10>
The configuration of the image forming apparatus 10 according to this embodiment will be described. FIG. 1 is a schematic diagram showing the configuration of an image forming apparatus 10 according to this embodiment.

An image forming apparatus 10 shown in FIG. 1 is an example of an image forming apparatus that forms an image on a recording medium. Specifically, the image forming apparatus 10 is an electrophotographic image forming apparatus that forms a toner image (an example of an image) on the recording medium P. As shown in FIG. More specifically, the image forming apparatus 10 includes an accommodation section 12 , a conveying section 15 , an image forming section 14 , a fixing device 16 , an operation section 90 and a control section 20 . Hereinafter, each section (storage section 12, conveying section 15, image forming section 14, fixing device 16, operation section 90, and control section 20) of image forming apparatus 10 will be described.

(Accommodating portion 12 and transporting portion 15)
The accommodation unit 12 has a function of accommodating a recording medium P such as paper. The transport unit 15 has a function of transporting the recording medium P. As shown in FIG. Specifically, the transport unit 15 transports the recording medium P from the storage unit 12 to the image forming unit 14 (specifically, the secondary transfer position T2 described later), and transports the recording medium P from the image forming unit 14 to the fixing device 16. have a function.

More specifically, the transport unit 15 includes a delivery roll 46 that delivers the recording medium P accommodated in the storage unit 12, and a plurality of transport rolls that transport the recording medium P delivered by the delivery roll 46 to the image forming unit 14. 50 and . Further, the conveying section 15 has a conveying belt 59 for conveying from the image forming section 14 to the fixing device 16 . The conveying belt 59 is a conveying member that conveys the recording medium P by rotating while holding the recording medium P on the outer peripheral surface of the belt by suction or the like.

(Image forming unit 14)
The image forming section 14 has a function of forming a toner image (an example of an image) on the recording medium P. As shown in FIG. Specifically, the image forming section 14 has a toner image forming section 22 and a transfer device 17 . Note that the image forming unit 14 is not limited to the configuration described below, and may have another configuration as long as it forms an image on the recording medium P. FIG.

[Toner image forming unit 22]
A plurality of toner image forming units 22 shown in FIG. 1 are provided so as to form a toner image for each color. In the present embodiment, toner image forming units 22 for a total of four colors of yellow (Y), magenta (M), cyan (C), and black (K) are provided. (Y), (M), (C), and (K) shown in FIG. 1 indicate components corresponding to the respective colors.

In the image forming apparatus 10, when it is necessary to distinguish between yellow (Y), magenta (M), cyan (C), and black (K), (Y), (M), (C) and (K) are attached, and (Y), (M), (C) and (K) may be omitted when there is no need to distinguish between colors. Further, since the toner image forming units 22 of each color are configured in the same manner except for the toner used, each portion of the toner image forming unit 22 (Y) in FIG. is attached.

Specifically, the toner image forming section 22 for each color has a photoreceptor drum 32 (photoreceptor) that rotates in one direction (for example, the clockwise direction in FIG. 1). Further, each color toner image forming section 22 has a charger 23 , an exposure device 36 , a developing device 38 , and a removing device 40 .

In the toner image forming unit 22 of each color, the charger 23 charges the photosensitive drum 32 . Further, the exposure device 36 exposes the photosensitive drum 32 charged by the charger 23 based on the image signal sent from the controller 20 to form an electrostatic latent image on the photosensitive drum 32 . Further, the developing device 38 develops the electrostatic latent image formed on the photosensitive drum 32 by the exposure device 36 to form a toner image. The removing device 40 removes toner remaining on the photosensitive drum 32 after the toner image is transferred to the transfer belt 24, which will be described later.

The toner image forming section 22 for each color is configured to form a toner image for each color by means of a dot screen DS (halftone dots) shown in FIG. The screen angle θ (see FIG. 2) can be changed in each of the magenta, cyan, and black toner image forming units 22 . Specifically, in the magenta toner image forming section 22 (M), the screen angle θ can be changed between 75° and 72°. In the cyan toner image forming section 22(C), the screen angle θ can be changed between 15° and 12°. In the black toner image forming portion 22(K), the screen angle θ can be changed between 45° and 42°.

The screen angles of 75°, 15°, and 45° in the toner image forming portions 22 of magenta, cyan, and black are used when a DC electric field is applied as the secondary transfer electric field, as will be described later. Further, the screen angles of 72°, 12°, and 42° in the toner image forming portions 22 of magenta, cyan, and black are used when a superimposed electric field is applied as a secondary transfer electric field, as will be described later.

The yellow toner image forming portion 22 (Y) is configured such that the screen angle θ is not changed. In the yellow toner image forming portion 22(Y), the screen angle θ is 90°.

Here, the screen angle θ (see FIG. 2) is an example of the angle of the dot screen with respect to a predetermined reference line. Specifically, it is the angle of the arrangement direction (arrangement direction) of the dot screen with respect to a predetermined reference line. Specifically, the predetermined reference line is, for example, the reference line S1 along the longitudinal direction of the contact area 28A (nip area) where the secondary transfer roll 28 (to be described later) contacts the transfer belt 24 . Note that FIG. 2 shows the contact area 28A (hatched portion) when viewed from above. Further, the predetermined reference line can also be said to be a reference line along an intersecting direction (specifically, an orthogonal direction) that intersects the conveying direction of the recording medium P. In addition, it can also be said to be a reference line along the axial direction of the secondary transfer roll 28, which will be described later, or the axial direction of the opposing roll 42B, which will be described later.

[Transfer device 17]
The transfer device 17 shown in FIG. 1 is a device that transfers the toner image formed by the toner image forming section 22 onto the recording medium P. As shown in FIG. Specifically, the transfer device 17 superimposes and primarily transfers the toner images of the photosensitive drums 32 of each color onto the transfer belt 24 as an intermediate transfer member, and transfers the superimposed toner images to the secondary transfer position T2 (described later). is secondarily transferred to the recording medium P at the contact area 28A). Specifically, the transfer device 17 includes a transfer belt 24, a primary transfer roll 26, a secondary transfer roll 28, and an application section 29, as shown in FIG.

The primary transfer roll 26 is a roll that transfers the toner image of each color photoreceptor drum 32 onto the transfer belt 24 at a primary transfer position T1 between the photoreceptor drum 32 and the primary transfer roll 26 . In this embodiment, a primary transfer electric field is applied between the primary transfer roll 26 and the photoreceptor drum 32, so that the toner image formed on the photoreceptor drum 32 is transferred to the transfer belt 24 at the primary transfer position T1. be transcribed.

The transfer belt 24 is an example of a holding unit that holds a toner image. Specifically, the transfer belt 24 has a function of holding the toner images transferred from the photosensitive drums 32 of each color. More specifically, the transfer belt 24 is configured as follows. As shown in FIG. 1, the transfer belt 24 has an endless shape and is wrapped around a plurality of rolls 42 to determine its posture. The transfer belt 24 is rotated in the arrow A direction by, for example, the roll 42D of the plurality of rolls 42 being rotationally driven by a drive unit (not shown). Among the plurality of rolls 42, the roll 42B shown in FIG. 1 functions as the opposing roll 42B of the secondary transfer roll 28.

The secondary transfer roll 28 is an example of transfer means for transferring an image onto a recording medium. Specifically, the secondary transfer roll 28 has a function of transferring the toner image held on the transfer belt 24 onto the recording medium P. As shown in FIG. More specifically, the secondary transfer roll 28 is configured as follows. The secondary transfer roll 28 has a contact area 28A (nip area) that contacts the transfer belt 24 . This contact area 28A is the secondary transfer position T2. Further, the secondary transfer roll 28 conveys the recording medium P while sandwiching it between itself and the transfer belt 24 in the contact area 28A.

The application unit 29 is an example of an application unit that applies a transfer electric field having an AC component between the holding unit and the transfer unit. Specifically, the application section 29 has a function of applying a secondary transfer electric field having an AC component between the secondary transfer roll 28 and the opposing roll 42B (transfer belt 24).

More specifically, the application unit 29 selectively applies, as the secondary transfer electric field, a DC electric field having a DC component and no AC component, and a superimposed electric field in which an AC component is superimposed on a DC component. It is configured. A DC electric field is an example of a first transfer electric field. A superimposed electric field is an example of a second transfer electric field.

In the transfer device 17, the application unit 29 applies a secondary transfer electric field (DC electric field or superimposed electric field) between the secondary transfer roll 28 and the opposing roll 42B (transfer belt 24), thereby superimposing the image on the transfer belt 24. The toner image thus formed is transferred onto the recording medium P at the secondary transfer position T2.

(Fixing device 16)
A fixing device 16 shown in FIG. 1 is an example of fixing means for fixing an image transferred onto a recording medium. Specifically, the fixing device 16 is a device that fixes the toner image transferred onto the recording medium P by the secondary transfer roll 28 onto the recording medium P. As shown in FIG. More specifically, as shown in FIG. 1, the fixing device 16 has a heating roll 82 as a heating member and a pressure roll 84 as a pressure member.

In the fixing device 16 , the toner image formed on the recording medium P is fixed on the recording medium P by heating and pressing the recording medium P with the heating roll 82 and the pressure roll 84 .

(Operation unit 90)
An operation unit 90 (operation means) shown in FIG. 1 allows an operator (user) to select the type of recording medium P to be used. As the types of the recording medium P to be selected, there is a predetermined first type suitable for application of a DC electric field as a secondary transfer electric field (hereinafter referred to as a DC compatible type), and an application of a superimposed electric field is suitable for application of a secondary transfer electric field. There is a predetermined second type (hereinafter referred to as a superimposition compatible type).

The direct current compatible type includes paper having irregularities smaller than a predetermined size on the surface, specifically plain paper. Examples of types suitable for superimposition include paper having irregularities of a predetermined size or more on the surface, specifically embossed paper, rough paper, and the like.

It should be noted that the operation section can also be said to be an input section (input means) for inputting the type of recording medium P to be used from the outside.

(control unit 20)
The control section 20 shown in FIG. 1 has a function of controlling the operations of each section of the image forming apparatus 10 including the conveying section 15 , the image forming section 14 and the fixing device 16 . Each section of the image forming apparatus 10 including the conveying section 15 , the image forming section 14 and the fixing device 16 is connected to the control section 20 .

Further, the control section 20 has a secondary transfer electric field setting section 202 and a screen angle setting section 204 . The secondary transfer electric field setting unit 202 has a function of setting either a superimposed electric field or a DC electric field as the secondary transfer electric field according to the type of recording medium P selected through the operation unit 90 . Specifically, the secondary transfer electric field setting unit 202 sets the DC electric field as the secondary transfer electric field when the type of recording medium P selected through the operation unit 90 is the DC compatible type. Further, the secondary transfer electric field setting unit 202 sets the superimposed electric field as the secondary transfer electric field when the type of recording medium P selected through the operation unit 90 is the superimposition suitable type.

Then, the control unit 20 causes the application unit 29 to apply the secondary transfer electric field (superimposed electric field or DC electric field) set by the secondary transfer electric field setting unit 202 .

In the secondary transfer electric field setting unit 202, the DC electric field is initially set, and when the type of recording medium P selected through the operation unit 90 is the superimposition compatible type, the setting is changed to the superimposition electric field. may be

The screen angle setting unit 204 sets the screen angle θ (see FIG. 2) used by the toner image forming units 22 of magenta, cyan, and black according to the type of recording medium P selected through the operation unit 90 .

Specifically, when the type of recording medium P selected through the operation unit 90 is the DC compatible type, the screen angle setting unit 204 sets the screen angles used by the toner image forming units 22 of magenta, cyan, and black. 75°, 15°, and 45° are set as θ.

Further, when the type of recording medium P selected through the operation unit 90 is the type suitable for superimposition, the screen angle setting unit 204 sets the screen angle θ used in the toner image forming units 22 of magenta, cyan, and black to: Set 72°, 12°, 42°. This setting does not change the relative angles between the colors magenta, cyan and black.

Then, the control unit 20 uses the screen angle θ set by the screen angle setting unit 204 to cause the toner image forming units 22 of magenta, cyan, and black to form toner images. In the magenta, cyan, and black toner image forming units 22 , based on image signals image-processed using the screen angle θ set by the screen angle setting unit 204 , the photosensitive drums 32 are exposed and electrostatically charged. A toner image is formed by forming a latent image.

As a result, in the present embodiment, when a DC electric field is applied as the secondary transfer electric field, the screen angle θ is 45°. Further, when a DC electric field is applied as the secondary transfer electric field, the screen angle θ is set to 75° in the magenta image formed by the magenta toner image forming section 22(M). Further, when a DC electric field is applied as the secondary transfer electric field, the screen angle θ is set to 15° in the cyan image formed by the cyan toner image forming section 22(S).

A "black image" is an example of "a color image having the largest difference between the L* of the single-color solid image after fixing and the L* of the recording medium P". The “single-color solid image after fixing” is a solid image formed with toner of one color, transferred to the recording medium P, and further fixed on the recording medium P by the fixing device 16 . "L* (Elster)" is a lightness index defined in JIS Z 8781-4. L* (Elster) can be measured using, for example, X-Rite 938 (manufactured by X-Rite Co., Ltd.). When the recording medium P is blank, the color with the largest difference is black.

"Magenta image and cyan image" is an example of "a different color image different from the color with the largest difference between the L* of the single-color solid image after fixing and the L* of the recording medium P".

“75°” used for the screen angle θ in the magenta image and “15°” used for the screen angle θ in the cyan image are examples of “an angle different from the angle of the dot screen in the color image”, i.e. This is an example of "an angle different from the screen angle θ (45°) used in a black image".

Furthermore, in this embodiment, when a superimposed electric field is applied as the secondary transfer electric field, the screen angle θ is set to 42° in the black image formed by the black toner image forming section 22(K). Further, when a superimposed electric field is applied as the secondary transfer electric field, the screen angle θ is set to 72° in the magenta image formed by the magenta toner image forming unit 22(M). Further, when a superimposed electric field is applied as the secondary transfer electric field, the screen angle θ is set to 12° in the cyan image formed by the cyan toner image forming section 22(S).

“42°” used for the screen angle θ in a black image is an example of “an angle different from 0°, 45°, 90°, and 135°”. The "42°" is also an example of "an angle different from 0°, 45°, 90° and 135° by 3° or more". Furthermore, "42°" is also an example of "an angle that does not deviate from any one of 0°, 45°, 90° and 135° by 22.5° or more". The "42°" is an angle that deviates from any one of 0°, 45°, 90°, and 135° (45° in this embodiment) within a range of less than 22.5°. In other words, the "42°" is the intermediate value of two adjacent angles (eg, 0°, 45°) out of 0°, 45°, 90°, and 135°, that is, 22.5° , 67.5° and 112.5°”.

"72°" used for the screen angle θ in the magenta image and "12°" used for the screen angle θ in the cyan image are "dot screens in different color images when the first transfer electric field is applied. is an example of "an angle different from the angle of That is, the screen angle θ used in the magenta image and the cyan image differs depending on whether the DC electric field is applied as the secondary transfer electric field or the superimposed electric field.

Here, when a superimposed electric field is applied as the secondary transfer electric field, as shown in FIG. direction) and return direction (direction from the secondary transfer roll 28 to the opposing roll 42B). Note that FIG. 3 shows an example in which a rectangular wave in which the ratio of the transfer direction to the return direction in one cycle is reduced is used as the superimposed electric field. In FIG. 3, the square wave is indicated by a thick line. Also, in FIG. 3, a TH portion indicates a portion where the direction of the electric field is the transfer direction.

When the recording medium P and the toner image on the recording medium P are led out from the contact area 28A (nip area) between the secondary transfer roll 28 and the transfer belt 24, the direction of the secondary transfer electric field is the transfer direction. In some cases, toner scattering occurs in the toner image. Toner scattering is a phenomenon in which toner dots spread and become larger. This phenomenon occurs when the direction of the secondary transfer electric field changes from the return direction to the transfer direction, and each dot of the toner image collides with the recording medium P when led out from the contact area 28A. guessed.

As described above, when the toner image on the recording medium P is led out from the contact area 28A, if the direction of the secondary transfer electric field is in the transfer direction, the toner image will be scattered. Variation occurs with a period corresponding to the period of the AC component of the superimposed electric field. Specifically, the toner scattering occurs in a belt-like (line-like) range having a length along the longitudinal direction of the contact area 28A at intervals in the short side direction of the contact area 28A in the period. (See Figure 3).

In FIG. 3, a dot screen (halftone dot) in which toner scattering occurs is indicated by a symbol DT. Further, in FIG. 3, the portions where the toner has diffused are indicated by shading. Further, in FIG. 3, the arrow X indicates the direction corresponding to the longitudinal direction of the contact area 28A, and the arrow Y indicates the direction corresponding to the lateral direction of the contact area 28A.

If the dot screen DS (halftone dots) is arranged along the longitudinal direction (X direction) of the contact area 28A on the recording medium P in the cycle of occurrence of toner scattering, the density becomes higher than that of the surrounding area. streaks (banding) are generated. Note that the streak-like portion may be visually recognized as roughness of the image.

The state in which the dot screen DS is arranged along the longitudinal direction (X direction) of the contact area 28A on the recording medium P in the generation period of the toner scattering (see TH portion in FIG. 3) is the screen and the toner scattering. This is an example of a state of interference with the

On the other hand, in this embodiment, as described above, when the superimposed electric field is applied as the secondary transfer electric field, the screen angle θ is set to 42° in the black image. , the dot screen DS (halftone dots) is arranged on the recording medium P so as to be inclined with respect to the longitudinal direction (X direction) of the contact area 28A. be done. That is, the dot screen DS (halftone dots) is arranged on the recording medium P in a direction along the longitudinal direction (X direction) of the contact area 28A in the generation cycle of toner scattering (see TH portion in FIG. 4). Absent. Therefore, it is designed not to interfere with the screen and toner scattering. In other words, in this embodiment, when a superimposed electric field is applied as a secondary transfer electric field, the dot screen DS is positioned on the recording medium P with respect to the longitudinal direction (X direction) of the contact area 28A in a black image. It can also be said to be a configuration in which they are arranged obliquely.

In the screen angle setting unit 204, 75°, 15°, and 45° are initially set as the screen angle θ used in the toner image forming units 22 of magenta, cyan, and black. When the type of the medium P is the superimposition compatible type, the screen angle θ used in the toner image forming units 22 of magenta, cyan, and black may be changed to 72°, 12°, and 42°. . This setting change does not change the relative angles between the colors magenta, cyan and black.

<Action of this embodiment>
The operation of the image forming apparatus 10 of this embodiment will be described using an example of the flow shown in FIG.

In the image forming apparatus 10 , first, in step 300 , the operator (user) selects the type of recording medium P to be used through the operation section 90 .

In step 302, it is determined whether the type of recording medium P selected in step 300 is a predetermined superimposition suitable type (eg, embossed paper, rough paper) suitable for application of a DC electric field as a secondary transfer electric field. judgment is made.

As a result, when it is determined that the type of the selected recording medium P is the superimposed type, the secondary transfer electric field setting unit 202 sets the superimposed electric field as the secondary transfer electric field in step 304 .

Further, at step 306, the screen angle setting unit 204 sets 72°, 12°, and 42° as the screen angle θ used in the toner image forming units 22 of magenta, cyan, and black.

Then, in step 308, an image forming operation is executed. As a result, the image forming operation is performed using the superimposed electric field as the secondary transfer electric field and using the screen angle θ of 72°, 12°, and 42° for the toner image forming portions 22 of magenta, cyan, and black. . That is, in the present embodiment, when a superimposed electric field is applied as the secondary transfer electric field, a screen angle θ of 42° is used for a black image formed by the black toner image forming section 22(K).

If it is not determined in step 302 that the type of recording medium P selected is the superimposition compatible type, that is, if the selected type of recording medium P is the DC compatible type, At step 310, the secondary transfer electric field setting unit 202 sets a DC electric field as the secondary transfer electric field.

Further, in step 312, the screen angle setting unit 204 sets screen angles θ75°15°45° as the screen angles θ used in the toner image forming units 22 of magenta, cyan, and black.

Then, in step 308, an image forming operation is executed. As a result, an image forming operation is performed using a DC electric field as the secondary transfer electric field and using a screen angle θ of 75°, 15°, and 45° for the toner image forming portions 22 of magenta, cyan, and black. .

As described above, in this embodiment, when the superimposed electric field is applied as the secondary transfer electric field, the screen angle θ of 42° is set for the black image formed by the black toner image forming unit 22 (K). Used.

Thereby, as shown in FIG. 4, the dot screen DS (halftone dots) is arranged on the recording medium P so as to be inclined with respect to the longitudinal direction (reference line) of the contact area 28A. For this reason, it is designed not to interfere with the scattering of toner on the screen.

As a result, when a superimposed electric field is applied as a secondary transfer electric field, in a black image, compared to the configuration (first configuration) using screen angles θ of 0°, 45°, 90°, and 135°, the dot screen It is suppressed that the DS continues along the longitudinal direction of the contact area 28A and the toner scattering occurs. In this embodiment, the dot screen on which toner scattering occurs has a length of about several dots (for example, 1 mm) in the longitudinal direction of the contact area 28A. As a result, compared to the first configuration, streaky portions with higher density than the surrounding area are less likely to occur, and poor image quality is suppressed.

Note that the transfer belt 24 may be inclined with respect to the longitudinal direction of the contact area 28A due to the oblique movement, but this inclination is about 0.1°. With this inclination, the effect of generating a streak-like portion with a higher density than the surroundings is small.

In addition, 42° used as the screen angle θ in a black image when a superimposed electric field is applied as a secondary transfer electric field is an angle different from 0°, 45°, 90°, and 135° by 3° or more. , the screen angle θ varies from 0°, 45°, 90°, 135° to less than 3°, the dot screen is arranged continuously along the length of the contact area 28A. Hateful. Therefore, poor image quality is suppressed.

The 42° is also an angle that does not deviate from any of 0°, 45°, 90°, and 135° by 22.5° or more. Therefore, out of the four prohibited angles (0°, 45°, 90°, 135°), different deviation amounts are ensured from adjacent two prohibited angles (eg, 0°, 45°).

Further, in this embodiment, when a superimposed electric field is applied as the secondary transfer electric field, the screen angles θ of 72° and 12° are used for each of the magenta and cyan images.

Here, if the screen angle θ of each of the magenta and cyan images has an angular difference of 30° with respect to the screen angle θ of the black image, moire is less likely to occur between the images. In this embodiment, the screen angles θ of 72° and 12° are used for the magenta and cyan images so that the angle difference from the screen angle θ for the black image is maintained. Therefore, compared to the case where the screen angles θ of the magenta and cyan images are maintained regardless of the applied secondary transfer electric field, moiré between the magenta and cyan images and the black image is reduced. Suppressed.

Further, in this embodiment, when a DC electric field is applied as the secondary transfer electric field, a screen angle θ of 45° is used for a black image formed by the black toner image forming section 22(K).

As a result, the dot screen (halftone dots) is arranged on the recording medium P along the longitudinal direction of the contact area 28A. , the deterioration of graininess and character reproducibility is suppressed in a black image when the application unit 29 applies a DC electric field.

<Modification of this embodiment>
In this embodiment, when a DC electric field is applied as the secondary transfer electric field, the screen angle θ is set to 45° for a black image, but it is not limited to this. For example, when a DC electric field is applied as the secondary transfer electric field, the screen angle θ may be 0°, 90°, or 135° in a black image.

Further, in the present embodiment, when a superimposed electric field is applied as a secondary transfer electric field, the screen angle θ is set to 42° for a black image, but the screen angle θ is not limited to this. For example, when a superimposed electric field is applied as the secondary transfer electric field, the screen angle θ in a black image may be an angle in the range of 42° or more and 87° or less (excluding 42°). Any angle other than °, 45°, 90°, and 135° may be used.

Further, in the present embodiment, either the DC electric field or the superimposed electric field is set as the secondary transfer electric field according to the type of the recording medium P selected by the operator (user), but the present invention is not limited to this. . For example, an operator (user) may directly select either the DC electric field or the superimposed electric field as the secondary transfer electric field. In this case, for example, the screen angle θ is set according to the secondary transfer electric field selected by the operator (user). That is, when the secondary transfer electric field selected by the operator (user) is a DC electric field, the screen angles θ used in the toner image forming units 22 of magenta, cyan, and black are 75°, 15°, and 45°. °. Further, when the secondary transfer electric field selected by the operator (user) is the superimposed electric field, the screen angles θ used in the toner image forming units 22 of magenta, cyan, and black are 72°, 12°, and 42°. °.

Further, in the present embodiment, the DC electric field and the superimposed electric field are selectively used as the secondary transfer electric field, but the present invention is not limited to this. For example, the configuration may be such that only the superimposed electric field is used as the secondary transfer electric field. In this case, for example, the screen angle θ of 72°, 12°, and 42° is used for each of the magenta, cyan, and black images.

Further, in the present embodiment, an intermediate transfer type image forming apparatus using the transfer belt 24 as an intermediate transfer member is used as an example of the holding means, but the present invention is not limited to this. For example, a direct transfer type image forming apparatus that directly transfers an image held by a photoreceptor, which is an example of a holding means, onto a recording medium P may be used.

Further, in the present embodiment, the four-color toner image forming units 22 of yellow (Y), magenta (M), cyan (C), and black (K) are provided, but the present invention is not limited to this. For example, the image forming apparatus may be provided with only the black (K) toner image forming section 22 . That is, it may be a monochrome image forming apparatus.

<Modified Example of Image Forming Apparatus>
As an image forming apparatus, as shown in FIG. 6, a toner image forming section 22 for a total of five colors of yellow (Y), magenta (M), cyan (C), black (K), and special color (W) is provided. It may be the image forming apparatus 200 provided. The special colors are colors different from the normal colors (yellow (Y), magenta (M), cyan (C), and black (K)). For example, white is used as the special color.

The special color (W) toner image forming section 22 is arranged downstream of the normal color toner image forming section 22 and upstream of the secondary transfer position T2 in the rotational direction of the transfer belt 24, for example. there is The special color (W) toner image forming section 22 is arranged upstream of the normal color toner image forming section 22 and downstream of the secondary transfer position T2 in the rotational direction of the transfer belt 24. may (Y), (M), (C), (K), and (W) shown in FIG. 6 indicate components corresponding to the respective colors.

In the image forming apparatus 200, when it is necessary to distinguish between yellow (Y), magenta (M), cyan (C), black (K), and special color (W), the code of each member is followed by (Y), (M), (C), (K), and (W) are added, and (Y), (M), (C), (K), and (W) are omitted when there is no need to distinguish between colors. sometimes.

Further, since the toner image forming units 22 of each color are configured in the same manner except for the toner used, each portion of the toner image forming unit 22 (W) in FIG. is attached.

In the image forming apparatus 200, when a DC electric field is applied as the secondary transfer electric field, 45° is used as the screen angle θ in the white image formed by the special color toner image forming unit 22(W). .

A “white image” is an example of “a color image having the largest difference between the L* of the single-color solid image after fixing and the L* of the recording medium P”. Specifically, when the recording medium P is black paper, the color with the largest difference is white.

Further, in the image forming apparatus 200, when a superimposed electric field is applied as the secondary transfer electric field, the screen angle θ of 48° is used for a white image. “48°” used for the screen angle θ in a white image is an example of “an angle different from 0°, 45°, 90° and 135°”. The "48°" is also an example of "an angle different from 0°, 45°, 90° and 135° by 3° or more". Furthermore, the "48°" is also an example of "an angle that does not deviate from 0°, 45°, 90°, 135° by 22.5° or more".

It should be noted that the screen angle θ used for each color image of yellow, magenta, cyan, and black is the same as that of the image forming apparatus 10 described above.

In this modification, when a superimposed electric field is applied as the secondary transfer electric field, the screen angle θ of 48° is used for a white image. 28A is arranged obliquely with respect to the longitudinal direction (see FIG. 4). For this reason, it is designed not to interfere with the scattering of toner on the screen.

As a result, when a superimposed electric field is applied as a secondary transfer electric field, in a white image, compared to the configuration (second configuration) using screen angles θ of 0°, 45°, 90°, and 135°, the dot screen However, continuous toner scattering along the longitudinal direction of the contact area 28A is suppressed. As a result, compared to the second configuration, streaky portions with a higher density than the surrounding area are less likely to occur, and image quality defects are suppressed.

Thus, this modification also has the same effects as the image forming apparatus 10 described above. Silver, gold, or the like may be used as the special color. In this case, when the paper is black, the colors with the largest difference are silver and gold, and the configuration of this modification can be applied.

<First modification of screen>
In the above-described embodiment, when a superimposed electric field is applied as the secondary transfer electric field, the screen angle θ of the dot screen is set to 42° in a black image, but the present invention is not limited to this. For example, an error diffusion screen may be used in a black image when a superimposed electric field is applied as the secondary transfer electric field. An error diffusion screen is a screen in which dot screens (halftone dots) are arranged aperiodically. For this reason, the dot screen is not arranged along the longitudinal direction of the contact area 28A, so as not to interfere with the screen and toner scattering.

As a result, when a superimposed electric field is applied as the secondary transfer electric field, the dot screen is arranged in the longitudinal direction of the contact area 28A, compared to the configuration using the dot screens arranged periodically (third configuration). , the occurrence of toner scattering is suppressed. Therefore, as compared with the third configuration, streaky portions having a higher density than the surrounding area are less likely to occur, and image quality defects are suppressed.

Note that in this modification, when a superimposed electric field is applied as the secondary transfer electric field, an error diffusion screen may also be used for magenta and cyan images.

Further, in this modified example, when a DC electric field is applied as the secondary transfer electric field, a dot screen having a screen angle θ of, for example, 45° is used for a black image as in the above-described embodiment. .

As a result, the dot screen (halftone dots) is arranged on the recording medium P along the longitudinal direction of the contact area 28A. When 29 applies a DC electric field, deterioration of graininess and character reproducibility is suppressed in a black image.

When a DC electric field is applied as the secondary transfer electric field, any screen other than the error diffusion screen may be used.

<Second modification of screen>
In this modification, a linear screen is used instead of the dot screen. Specifically, in this modified example, when a superimposed electric field is applied as the secondary transfer electric field, a line screen with a screen angle of, for example, 90° is used for a black image. Note that "90°" is an example of "an angle different from 0°".

Thereby, the line screen is arranged on the recording medium P at an angle with respect to the longitudinal direction of the contact area 28A. For this reason, it is designed not to interfere with the scattering of toner on the screen.

As a result, when a superimposed electric field is applied as the secondary transfer electric field, the line screen is arranged in the longitudinal direction of the contact area 28A compared to the configuration (fourth configuration) in which the angle of the line screen is set to 0° in a black image. Continuously along the line, the occurrence of toner scattering is suppressed. Therefore, as compared with the fourth configuration, streaky portions having a higher density than the surrounding area are less likely to occur, and image quality defects are suppressed.

Further, when a DC electric field is applied as the secondary transfer electric field, a line screen with a screen angle of 0° is used for black images. As a result, since the line screen is arranged along the longitudinal direction of the contact area 28A on the recording medium P, regardless of the applied secondary transfer electric field, compared to the configuration using the line screen angle of 90°, the When the application unit 29 applies a DC electric field, graininess and lowering of character reproducibility are suppressed in a black image.

The present invention is not limited to the above-described embodiments, and various modifications, changes, and improvements are possible without departing from the scope of the invention. For example, the modifications shown above may be configured by appropriately combining a plurality of them.
<Evaluation>
In Examples and Comparative Examples, the presence or absence of streak-like portions (banding) where the density is higher than the surroundings was evaluated.
In this evaluation, in Examples 1, 2, 4, and 5 and the comparative example, printers configured in the same manner as the image forming apparatus 10 described above were used. In Example 3, a printer configured in the same manner as the aforementioned image forming apparatus 200 having a special color toner image forming section 22 (W) was used.
In Examples 1 to 3 and Comparative Example, a superimposed electric field was applied to form four-color images of yellow (Y), magenta (M), cyan (C), and black (K). The process speed was 444 mm/s and the frequency of the superimposed electric field was 500 Hz.
(Comparative example)
As shown in the table of FIG. 7, the screen angles of the dot screen were 90°, 75°, 15°, and 45° for each of the yellow, magenta, cyan, and black images. Also, in the yellow, magenta, cyan, and black images, the line number of the dot screen was set to 200 lines/inch.
(Example 1)
As shown in the table of FIG. 8, the screen angles of the dot screen were set to 90°, 75°, 15°, and 42° for each of the yellow, magenta, cyan, and black images. Also, in the yellow, magenta, cyan, and black images, the line number of the dot screen was set to 200 lines/inch.
(Example 2)
As shown in the table of FIG. 9, the screen angles of the dot screen were 90°, 72°, 12°, and 42° for each of the yellow, magenta, cyan, and black images. Also, in the yellow, magenta, cyan, and black images, the line number of the dot screen was set to 200 lines/inch.
(Example 3)
As shown in the table of FIG. 10, the screen angles of the dot screen were 90°, 72°, 12°, 42°, and 48° for each of the yellow, magenta, cyan, black, and white images. Also, in the yellow, magenta, cyan, black, and white images, the line number of the dot screen was set to 200 lines/inch.
(Example 4)
As shown in the table of FIG. 11, the screen angles of the dot screen were set to 90°, 75°, and 15° for each of the yellow, magenta, and cyan images. Also, in the yellow, magenta, and cyan images, the line number of the dot screen was set to 200 lines/inch. An error diffusion screen was used in the black image.
(Example 5)
In Example 5, a line screen was used to form the image. As shown in the table of FIG. 12, the screen angles of the line screen were 90°, 75°, 15°, and 90° for each of the yellow, magenta, cyan, and black images. Also, in the yellow, magenta, cyan, and black images, the line number of the line screen was set to 200 lines/inch.
(Evaluation results)
Banding occurred in the comparative example, whereas no banding occurred in Examples 1-5. Further, in Example 2, no moiré occurred between colors.

10 image forming device 16 fixing device (an example of fixing means)
24 transfer belt (an example of holding means)
28 secondary transfer roll (an example of transfer means)
28A contact area 29 application unit (an example of application means)
200 image forming apparatus θ screen angle S1 reference line

Claims (6)

holding means for holding a toner image;
a transfer means for transferring the image onto a recording medium;
applying means for applying a transfer electric field having an AC component between the holding means and the transfer means;
fixing means for fixing the image transferred to the recording medium onto the recording medium;
with
the transfer means has a contact area that contacts the holding means;
The applying means selectively applies a first transfer electric field having no AC component and a second transfer electric field having an AC component between the holding means and the transfer means,
When the applying means applies the first transfer electric field, the difference between the L* of the monochromatic solid image after fixing and the L* of the recording medium is the largest in the color image along the longitudinal direction of the contact area. The angle of the dot screen with respect to the reference line is 0°, 45°, 90°, or 135°,
When the applying means applies the second transfer electric field, the angle of the dot screen with respect to the reference line in the image of the color is set to an angle different from 0°, 45°, 90°, and 135°. Device. a holding means for holding an image;
a transfer means for transferring the image onto a recording medium;
applying means for applying a transfer electric field having an AC component between the holding means and the transfer means;
with
the transfer means has a contact area that contacts the holding means;
The applying means selectively applies a first transfer electric field having no AC component and a second transfer electric field having an AC component between the holding means and the transfer means,
When the applying means applies the first transfer electric field, the angle of the dot screen with respect to the reference line along the longitudinal direction of the contact area in the black image is 0°, 45°, 90°, 135°. ° either
When the applying means applies the second transfer electric field, the angle of the dot screen with respect to the reference line in the image of the color is different from 0°, 45°, 90°, 135°.
Image forming device. when the applying means applies the first transfer electric field, the angle of the dot screen in the image of a different color different from the color is set to an angle different from the angle of the dot screen in the image of the color;
When the applying means applies the second transfer electric field, the angle of the dot screen in the different color image is different from the angle of the dot screen in the different color image when the first transfer electric field is applied. 3. The image forming apparatus according to claim 1 , wherein the image forming apparatus is angled. The angle different from 0°, 45°, 90°, 135° is an angle different from 0°, 45°, 90°, 135° by 3° or more, according to any one of claims 1 to 3 . Image forming device. holding means for holding a toner image;
a transfer means for transferring the image onto a recording medium;
applying means for applying a transfer electric field having an AC component between the holding means and the transfer means;
fixing means for fixing the image transferred to the recording medium onto the recording medium;
with
the transfer means has a contact area that contacts the holding means;
The applying means selectively applies a first transfer electric field having no AC component and a second transfer electric field having an AC component between the holding means and the transfer means,
When the applying means applies the first transfer electric field, in the color image having the largest difference between the L* of the single-color solid image after fixing and the L* of the recording medium, along the longitudinal direction of the contact area. The angle of the line screen with respect to the reference line drawn is set to 0°,
When the applying means applies the second transfer electric field, the angle of the line screen with respect to the reference line is an angle different from 0°.
Image forming device. holding means for holding a toner image;
a transfer means for transferring the image onto a recording medium;
applying means for selectively applying a first transfer electric field having no AC component and a second transfer electric field having an AC component between the holding means and the transfer means;
fixing means for fixing the image transferred to the recording medium onto the recording medium;
with
In the color image having the largest difference between the L* of the single-color solid image after fixing and the L* of the recording medium,
using a screen other than an error diffusion screen when the applying means applies the first transfer electric field,
An image forming apparatus using the error diffusion screen when the applying means applies the second transfer electric field.

Images