Image forming apparatus

Abstract

To improve image quality when embossed paper is used as a recording material with a simple configuration.SOLUTION: An image forming apparatus 100 has an image carrier 1K, toner image forming means 4K, an intermediate transfer body 7, a toner electrifier 17, a toner electrification bias power supply 132, a control unit 120, and an acquisition unit 121 that acquires information on a recording material S to which a toner image is transferred. The control unit 120 can perform control to execute a first image forming mode when a type of the recording material S indicated by the information acquired by the acquisition unit 121 is a first type, and execute a second image forming mode when the type of the recording material S indicated by the information acquired by the acquisition unit 121 is a second type having larger irregularities on a surface than the recording material of the first type. The control unit controls to increase an absolute value of a toner electrification bias in a predetermined environment in the second image forming mode compared with the first image forming mode.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile apparatus using an electrophotographic method or an electrostatic recording method.

Background Art

[0002] Conventionally, as an image forming apparatus using, for example, an electrophotographic method, there is an intermediate transfer type image forming apparatus having an intermediate transfer body as a second image carrier that conveys a toner image primarily transferred from a photosensitive member as a first image carrier to a recording material for secondary transfer. Further, as such an image forming apparatus, there is a tandem type image forming apparatus in which toner images formed on the photosensitive members of a plurality of image forming units are primarily transferred to an intermediate transfer body at the primary transfer unit of each image forming unit and then secondarily transferred to a recording material at the secondary transfer unit. As the intermediate transfer body, an intermediate transfer belt formed of an endless belt is often used. Further, primary transfer and secondary transfer are often performed by applying a transfer bias to a transfer member such as a transfer roller that abuts on an image carrier to form a transfer unit. Hereinafter, a tandem type image forming apparatus having an intermediate transfer belt employing an intermediate transfer method will be described as an example. Further, since paper is often used as the recording material, in the following description, the recording material may be referred to as "paper" in some cases, but the recording material is not limited to paper, and may be, for example, synthetic paper formed using a synthetic resin.

[0003] In the production printing market and the like where such image forming apparatuses are required to support various recording materials, there is a demand to transfer toner images well even to recording materials having a larger surface irregularity than plain paper, such as embossed paper, which is a type of irregular paper having an irregular shape on its surface, like embossed paper and rough paper (hereinafter simply referred to as "embossed paper"). However, when embossed paper is used, it is known that toner hardly reaches the concave portions on its surface during secondary transfer, and the secondary transfer property deteriorates compared to normal papers such as plain paper.

[0004] Patent Document 1 discloses an invention that controls the pressing force of a primary transfer roller against a photoreceptor based on the surface shape information of the recording material. Specifically, when embossed paper is selected by the user, the pressing force of the primary transfer roller is reduced by a variable mechanism, thereby mitigating the toner cohesive force on the intermediate transfer belt and improving the secondary transfer performance to the recesses of the embossed paper.

[0005] Patent Document 2 discloses an invention in which the pressure received by an intermediate transfer belt having an elastic layer from a secondary transfer outer roller is controlled by a variable mechanism. By making the elastic layer of the intermediate transfer belt follow the recesses of the embossed paper, the gap between the recesses and the belt is reduced, improving the secondary transfer performance to the recesses. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-126192 [Patent Document 2] Japanese Patent Publication No. 2018-60032 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, it has been found that in tandem-type image forming apparatuses, it can be difficult to set an appropriate secondary transfer bias when using embossed paper due to the amount of charge on the toner on the intermediate transfer belt. In relation to the image forming section and primary transfer section of a tandem-type image forming apparatus, "upstream" and "downstream" refer to the direction of movement of the surface of the intermediate transfer belt.

[0008] The toner image transferred to the intermediate transfer belt in the upstream primary transfer section undergoes a discharge as it passes through the downstream primary transfer section, causing the charge on the toner on the intermediate transfer belt to increase. On the other hand, the toner image transferred in the downstream primary transfer section does not have the opportunity to undergo a similar discharge, so the charge on the toner on the intermediate transfer belt tends to be lower compared to the toner image transferred upstream.

[0009] Thus, the appropriate range of secondary transfer bias differs for toner images with different charge levels. For example, multi-color toner images, especially those with a large amount of toner, that are primary transferred upstream require a strong secondary transfer bias because the amount of charge supplied for secondary transfer increases. However, halftone toner images, especially those with a small amount of toner, that are primary transferred downstream only require a weak secondary transfer bias. Therefore, if the secondary transfer bias for toner images primary transferred downstream is matched to that of toner images primary transferred upstream, strong skipping is likely to occur. This strong skipping is a phenomenon where the polarity of the toner reverses due to an overly strong secondary transfer bias, causing it to remain on the intermediate transfer belt without being secondary transferred to the recording material. In particular, on embossed paper with large irregularities, discharge is more likely to occur in the recesses, making low-charge toners more susceptible to strong skipping.

[0010] In other words, when embossed paper is used as the recording material S, it becomes difficult to ensure a transfer latitude (a range of conditions where there is neither weak nor strong ink bleeding) that provides appropriate transferability for each single-color toner. Moreover, as mentioned above, it becomes difficult to ensure a transfer latitude that provides appropriate transferability for toners of different colors or toners with different amounts, each with a different appropriate secondary transfer bias (secondary transfer current).

[0011] Patent documents 1 and 2 do not mention the amount of charge on the toner on the intermediate transfer belt, and instead improve the secondary transfer performance to embossed paper through a variable mechanism in the hardware configuration of the primary and secondary transfer sections. Such a configuration leads to a larger device and increased costs.

[0012] Therefore, the objective of the present invention is to improve image quality when embossed paper is used as the recording material with a simple configuration. [Means for solving the problem]

[0013] The above objective is achieved by the image forming apparatus according to the present invention. In summary, the present invention is Multiple image forming units, A rotatable image carrier that holds a toner image, Developing Department in Toner is supplied to the image carrier. A toner image is formed on the surface of the image carrier. Developing device and, Each of the multiple image forming units is provided with a primary transfer unit that contacts the image carrier of each of the multiple image forming units and forms a primary transfer unit, and each of the multiple image forming units is The toner image transferred from the image carrier in the primary transfer section is transported to the recording material in the secondary transfer section for transfer. 、 A rotatable intermediate transfer body, The plurality of image forming units include a first image forming unit, and the image carrier of the first image forming unit is located at the downstream end of the image carriers of each of the plurality of image forming units with respect to the direction of movement of the surface of the intermediate transfer body, in an image forming apparatus, the first image forming unit is the first image forming unit Regarding the rotational direction of the image carrier , the first image forming unit The aforementioned Developing Department Downstream and The first image forming unit Upstream of the primary transfer section , the first image forming unit Toner charger for charging the toner on the surface of the image carrier Equipped with The system includes a toner charging bias power supply that supplies a toner charging bias to the toner charger, a control unit capable of controlling the toner charging bias power supply, and an acquisition unit that acquires information about the recording material on which the toner image is transferred. The control unit executes a first image forming mode when the type of recording material indicated by the information acquired by the acquisition unit is a first type, and executes a second image forming mode when the type of recording material indicated by the information acquired by the acquisition unit is a second type having greater surface irregularities than the first type of recording material. death The absolute value of the toner charging bias in a given environment is larger in the second image forming mode than in the first image forming mode. The toner charging bias power supply control 、 This is an image forming apparatus characterized by the following features. [Effects of the Invention]

[0014] According to the present invention, it is possible to improve image quality when embossed paper is used as the recording material with a simple configuration. [Brief explanation of the drawing]

[0015] [Figure 1] It is a schematic cross-sectional view of an image forming apparatus. [Figure 2] It is a schematic cross-sectional view of a black image forming unit. [Figure 3] It is a graph showing the charge amount of toner on the intermediate transfer belt when the charge amount of toner is not adjusted. [Figure 4] It is a schematic block diagram showing the control mode of the main part of an image forming apparatus. [Figure 5] It is a flowchart of the control of Example 1. [Figure 6] It is a graph showing the charge amount of toner on the intermediate transfer belt for explaining the effect of Example 1. [Figure 7] It is a graph showing the image quality characteristics for embossed paper for explaining the effect of Example 1. [Figure 8] It is a flowchart of the control of Example 2. [Figure 9] It is a graph showing the charge amount of toner on the intermediate transfer belt for explaining the effect of Example 2. [Figure 10] It is a graph showing the image quality characteristics for embossed paper for explaining the effect of Example 2. [Figure 11] It is a schematic block diagram showing the control mode of the main part of the image forming apparatus of Example 3. [Figure 12] It is a schematic diagram of the selection screen of the image forming mode in Example 3. [Figure 13] It is a flowchart of the control of Example 3. [Embodiments for Carrying Out the Invention]

[0016] Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

[0017] [Example 1] 1. Explanation of the Configuration and Operation of the Image Forming Apparatus Figure 1 is a schematic cross-sectional view of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is a tandem-type multifunction device (having the functions of a copier, printer, and facsimile machine) employing an intermediate transfer method. The image forming apparatus 100 can form a full-color image on a sheet-like recording material S, such as paper, using an electrophotographic method, based on an image signal transmitted from an external device, such as a personal computer connected to the image forming apparatus 100. The image forming apparatus 100 can also form a full-color image on the recording material S based on an image signal generated when an original image is read by an image reading unit 80 provided in the image forming apparatus 100.

[0018] The image forming apparatus 100 has four image forming units 11Y, 11M, 11C, and 11K, each forming images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. These image forming units 11Y, 11M, 11C, and 11K are arranged in a line along the direction of movement of the image transfer surface, which is positioned approximately horizontally on the intermediate transfer belt 7, which will be described later. Elements in each image forming unit 11Y, 11M, 11C, and 11K that have the same or corresponding function or configuration may be described collectively by omitting the Y, M, C, and K at the end of the symbols indicating that they are elements for one of the colors. In this embodiment, the image forming unit 11 is composed of a photosensitive drum 1 (1Y, 1M, 1C, 1K), a charger 2 (2Y, 2M, 2C, 2K), an exposure device 3 (3Y, 3M, 3C, 3K), a developer 4 (4Y, 4M, 4C, 4K), a primary transfer roller 5 (5Y, 5M, 5C, 5K), a cleaning device 6 (6Y, 6M, 6C, 6K), and the like, as will be described later. As will be described in more detail later, the black image forming unit 11K is provided with a toner charger 17 as a toner charge amount control means for charging (applying charge to or controlling the amount of charge) the toner of the toner image on the surface of the photosensitive drum 1K.

[0019] The photosensitive drum 1, a rotatable drum-shaped (cylindrical) photoreceptor (electrophotographic photoreceptor) serving as the first image carrier for holding the toner image, is driven to rotate in the direction of arrow R1 (counterclockwise) in Figure 1 by a drum drive motor (not shown) acting as a drive source. The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential with a predetermined polarity (negative polarity in this embodiment) by a charger 2 acting as a charging means. During the charging process, a predetermined charging bias (charging voltage) is applied to the charger 2 by a charging bias power supply (not shown). The charged surface of the photosensitive drum 1 is scanned and exposed according to an image signal by an exposure device 3 acting as an exposure means (electrostatic image forming means), and an electrostatic image (electrostatic latent image) is formed on the photosensitive drum 1. In this embodiment, the exposure device 3 is composed of a laser scanner device that irradiates the photosensitive drum 1 with laser light modulated according to an image signal (image information).

[0020] The electrostatic image formed on the photosensitive drum 1 is developed (visualized) by a developer unit 4, which is a developing means (toner image forming means), when toner is supplied as a developer, and a toner image (toner image, developer image) is formed on the photosensitive drum 1. In this embodiment, toner charged with the same polarity as the charging polarity of the photosensitive drum 1 (negative polarity in this embodiment) adheres to the exposed area (image area) on the photosensitive drum 1, where the absolute value of the potential has decreased after uniform charging treatment and exposure (reverse development method). In this embodiment, the normal charging polarity of the toner, which is the main charging polarity of the toner during development, is negative polarity. The developer unit 4 has a developing roller, which is a rotatable developer carrier (developing member) that carries and transports the developer. The developing roller supplies toner to the photosensitive drum 1 at a developing section (developing position, toner image forming section) G (see Figure 2), which is a position on the photosensitive drum 1 opposite the developing roller in the rotational direction of the photosensitive drum 1, and forms a toner image on the photosensitive drum 1. The developing roller is rotated by a driving force transmitted, for example, from the drive system of the photosensitive drum 1. During development, a predetermined developing bias (developing voltage) is applied to the developing roller by a developing bias power supply (not shown).

[0021] Opposite the four photosensitive drums 1Y, 1M, 1C, and 1K is an intermediate transfer belt 7, a rotatable intermediate transfer body composed of an endless belt, which serves as a second image carrier for holding the toner image. The intermediate transfer belt 7 is stretched under a predetermined tension by being wrapped around a plurality of tension rollers (support rollers): a drive roller 22, an upstream auxiliary roller 23a, a downstream auxiliary roller 23b, a tension roller 25, a pre-secondary transfer roller 24, and a secondary transfer inner roller 21. The drive roller 22 transmits driving force to the intermediate transfer belt 7. The tension roller 25 applies a predetermined tension to the intermediate transfer belt 7 and controls the tension of the intermediate transfer belt 7 to be constant. The pre-secondary transfer roller 24 forms the surface of the intermediate transfer belt 7 near the upstream of the secondary transfer section N2 (described later) with respect to the rotational direction (direction of surface movement, running direction) of the intermediate transfer belt 7. The secondary transfer inner roller (secondary transfer opposing roller, inner member) 21 functions as an opposing member (opposing electrode) of the secondary transfer outer roller 9 (described later). The upstream auxiliary roller 23a and the downstream auxiliary roller 23b form an image transfer surface that is arranged substantially horizontally. The drive roller 22 is rotationally driven by a driving force transmitted from a belt drive motor (not shown) as a drive source. As a result, the intermediate transfer belt 7 receives drive input from the drive roller 22 and rotates (moves in a circular motion) in the direction of arrow R2 (clockwise direction) in Figure 1. In this embodiment, the intermediate transfer belt 7 is rotationally driven to a peripheral speed of 150 to 470 mm / sec. On the inner circumferential surface (back surface) side of the intermediate transfer belt 7, primary transfer rollers 5Y, 5M, 5C, and 5K, which are roller-shaped primary transfer members as primary transfer means, are arranged corresponding to each photosensitive drum 1Y, 1M, 1C, and 1K. The primary transfer roller 5 presses the intermediate transfer belt 7 toward the photosensitive drum 1, forming a primary transfer section (primary transfer nip, primary transfer position) N1, which is the contact area between the photosensitive drum 1 and the intermediate transfer belt 7. The tension rollers other than the drive roller 22 among the multiple tension rollers, and each primary transfer roller 5, rotate in accordance with the rotation of the intermediate transfer belt 7.

[0022] As described above, the toner image formed on the photosensitive drum 1 is transferred (primary transfer) in the primary transfer section N1 to the rotating intermediate transfer belt 7, which is the transfer target, by the action of the primary transfer roller 5. During primary transfer, the primary transfer roller 5 is subjected to a primary transfer bias (primary transfer voltage), which is a DC voltage with the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, by the primary transfer bias power supply 133 (Figure 2), which serves as a primary transfer bias application means. For example, when forming a full-color image, the toner images of yellow, magenta, cyan, and black formed on each photosensitive drum 1 are sequentially primary transferred so that they are superimposed on the same image forming region on the intermediate transfer belt 7. In this embodiment, the primary transfer section N1 is the image forming position where the toner image is formed on the intermediate transfer belt 7. The intermediate transfer belt 7 is an example of a rotatable endless belt that transports the toner image carried at the image forming position. The adjustment of the toner charge amount in the black image forming section 11K will be described later.

[0023] On the outer circumferential surface (surface) side of the intermediate transfer belt 7, a secondary transfer outer roller (secondary transfer roller, outer member) 9, which is a roller-shaped secondary transfer member serving as a secondary transfer means, is positioned opposite the secondary transfer inner roller 21. The secondary transfer outer roller 9 is pressed toward the secondary transfer inner roller 21 and contacts the secondary transfer inner roller 21 via the intermediate transfer belt 7, forming a secondary transfer section (secondary transfer nip, secondary transfer position) N2, which is the contact point between the intermediate transfer belt 7 and the secondary transfer outer roller 9. The secondary transfer outer roller 9 may rotate in conjunction with the rotation of the intermediate transfer belt 7, or it may be rotationally driven independently of the intermediate transfer belt 7. The toner image formed on the intermediate transfer belt 7 is transferred (secondary transfer) in the secondary transfer section N2 to the recording material S, which is being transported sandwiched between the intermediate transfer belt 7 and the secondary transfer outer roller 9, by the action of the secondary transfer outer roller 9. During secondary transfer, a secondary transfer bias (secondary transfer voltage), which is a DC voltage with the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the secondary transfer outer roller 9 by the secondary transfer bias power supply 134, which serves as a secondary transfer bias application means. In this embodiment, the secondary transfer inner roller 21 is electrically grounded. Alternatively, a configuration may be provided in which a secondary transfer bias with the same polarity as the normal charging polarity of the toner is applied to the inner member corresponding to the secondary transfer inner roller 21 in this embodiment, and the outer member corresponding to the secondary transfer outer roller 9 in this embodiment is electrically grounded.

[0024] The recording material (transfer material, sheet material, recording medium, media) S is fed from the feeding unit 60 and supplied to the secondary transfer unit N2. The feeding unit 60 is composed of a cassette 61, a feeding roller 62, a transport roller 63, etc. The recording material S is stored in the cassette 61, which is the storage unit. The recording material S is separated and fed one sheet at a time from the cassette 61 by the feeding roller 62, which is the feeding member. The recording material S sent out from the cassette 62 is transported to the register roller 10 by the transport roller 63, which is the transport member. This recording material S is then transported to the secondary transfer unit N2 by the register roller 10, with the timing synchronized with the toner image on the intermediate transfer belt 7.

[0025] The recording material S on which the toner image has been secondarily transferred is transported to a fixing device 40, which is a fixing means, by a transport belt 41, which is a transport member. The fixing device 40 heats and pressurizes the recording material S carrying the unfixed toner image to fix (melt and solidify) the toner image onto the recording material S. The recording material S on which the toner image has been fixed is discharged (output) to an output tray 42 provided outside the main body 101 of the image forming apparatus 100.

[0026] Toner remaining on the photosensitive drum 1 after the primary transfer (primary transfer residue toner) is removed and recovered from the photosensitive drum 1 by a cleaning device 6, which serves as a cleaning means. In this embodiment, the cleaning device 6 cleans the photosensitive drum 1 by scraping off and recovering the primary transfer residue toner from the rotating photosensitive drum 1 using a cleaning blade positioned in contact with the surface of the photosensitive drum 1. In addition, a belt cleaning device 12, which serves as an intermediate transfer body cleaning means, is positioned on the outer circumferential surface of the intermediate transfer belt 7, opposite the drive roller 22. Toner remaining on the intermediate transfer belt 7 after the secondary transfer (secondary transfer residue toner) and other adhering materials such as paper dust from the recording material S are removed and recovered from the intermediate transfer belt 7 by the belt cleaning device 12. In this embodiment, the belt cleaning device 12 cleans the intermediate transfer belt 7 by electrostatically recovering adhering materials such as secondary transfer residue toner from the intermediate transfer belt 7.

[0027] In this embodiment, the intermediate transfer belt unit 20, which serves as a belt conveying device, comprises an intermediate transfer belt 7 stretched over multiple tension rollers, each primary transfer roller 5, a belt cleaning device 12, and a frame that supports these components. The intermediate transfer belt unit 20 is detachable from the main device body 100 for maintenance or replacement.

[0028] Here, the intermediate transfer belt 7 can be made of a single-layer or multi-layer resin-based material, or a multi-layer structure with an elastic layer made of an elastic material.

[0029] In this embodiment, the primary transfer roller 5 is constructed by providing an elastic layer made of ion-conductive foamed rubber on the outer circumference of a metal core (core material). In this embodiment, the primary transfer roller 5 has an outer diameter of 15 to 20 mm, and its electrical resistance is 1 × 10 when measured with a voltage of 1 kV applied in an environment of 23°C and 50% RH. 5 ~1 × 10 8 It is Omega.

[0030] In this embodiment, the secondary transfer outer roller 9 is constructed by providing an elastic layer of ion-conductive foamed rubber on the outer circumference of a metal core (core material). In this embodiment, the secondary transfer outer roller 9 has an outer diameter of 20-25 mm, and its electrical resistance is 1 × 10⁻¹⁰ when measured with a voltage of 1 kV applied in an environment of 23°C and 50% RH. 5 ~1 × 10 8 It is Ω. Also, the secondary transfer outer roller 9 contacts the secondary transfer inner roller 21 with a predetermined pressure, with the intermediate transfer belt 7 in between, to form the secondary transfer section (secondary transfer nip) N2.

[0031] In this embodiment, the secondary transfer roller 21 is constructed by providing an elastic layer of electronically conductive rubber on the outer circumference of a metal core (core material). In this embodiment, the secondary transfer roller 21 has an outer diameter of 20-22 mm, and its electrical resistance is 1 × 10 when measured with a voltage of 50 V applied in an environment of 23°C and 50% RH. 5 ~1 × 10 8 It is Omega.

[0032] Furthermore, in this embodiment, the rotational axis directions of the tension rollers of the intermediate transfer belt 7, including the secondary transfer inner roller 21, and the secondary transfer outer roller 9 are substantially parallel to each other.

[0033] Figure 2 is a schematic cross-sectional view showing the configuration of the black image forming unit 11K. In this embodiment, among the multiple image forming units 11Y, 11M, 11C, and 11K, only the black image forming unit 11K is provided with a toner charger 17 as a toner charge amount control means. In this embodiment, the toner charger 17 is composed of a corona charger. The toner charger 17 is positioned downstream of the developing unit by the developer unit 4K and upstream of the primary transfer unit N1 with respect to the rotation direction R1 of the photosensitive drum 1, and irradiates the toner image before transfer with charged particles. The toner charger 17 has a wire electrode (discharge electrode) 18 and a shield electrode 19. The shield electrode 19 surrounds the wire electrode 18 in three directions with the wire electrode 18 at its center, and has an opening that is diagonally upward on the photosensitive drum 1K side. In this embodiment, the shield electrode 19 has an opening length of 14 mm and an opening depth of 15 mm. The toner charger 17 has a cleaning mechanism (not shown) that cleans foreign matter by rubbing a pad along the wire electrode 18. In this embodiment, the external dimensions of the toner charger 17, including the cleaning mechanism, are 30 mm × 15 mm × 350 mm. The wire electrode 18 is subjected to a toner charging bias by a toner charging bias power supply 132, which is a DC voltage with the same polarity (negative polarity in this embodiment) as the normal charging polarity of the toner. In this embodiment, the toner charging bias is controlled by a constant current, and a predetermined DC current is supplied to the wire electrode 18 when the toner charging bias is applied.

[0034] 2. Explanation of the assignment Here, "upstream" and "downstream" in relation to the image forming unit 11 and the primary transfer unit N1 refer to "upstream" and "downstream" with respect to the movement direction R2 of the surface of the intermediate transfer belt 7. The toner image primary transferred to the intermediate transfer belt 7 in the upstream primary transfer unit N1 receives a discharge as it passes through the downstream primary transfer unit N1, and the amount of charge on the toner on the intermediate transfer belt 7 tends to increase. On the other hand, the toner image primary transferred in the downstream primary transfer unit N1 does not have the opportunity to receive a similar discharge, so the amount of charge on the toner on the intermediate transfer belt 7 tends to be lower than that of the toner image primary transferred upstream.

[0035] Figure 3(a) is a graph showing the charge levels of yellow, magenta, cyan, and black toners on the intermediate transfer belt 7 before reaching the secondary transfer section N2 (high temperature and high humidity environment). Figure 3(b) is a graph showing the charge level distribution of yellow and black toners on the intermediate transfer belt 7 before reaching the secondary transfer section N2 (high temperature and high humidity environment). As shown in Figure 3(a), the charge level of the toners on the intermediate transfer belt 7 before reaching the secondary transfer section N2 tends to be higher for toners that have been primary transferred upstream (especially yellow and magenta) and lower for toners that have been primary transferred at the very downstream end (black). Similarly, as shown in Figure 3(b), the charge level distribution of the toners also tends to show a lower charge level distribution for black toner compared to yellow toner. Thus, a discrepancy in charge levels can occur between toners of colors other than black (also referred to here as "color toners") and black toner. The reason why the charge level of black toner is lower than that of color toners is due to the difference in charge levels between color toners and black toners due to the tandem arrangement described above, as well as the following reasons. In other words, the reason why the charge of black toner is lower than that of color toner is that black toner contains carbon and has lower electrical resistance compared to color toner, allowing the charge to escape more easily.

[0036] Thus, the appropriate range of secondary transfer bias differs for toner images with different charge levels. For example, multi-color toner images with a particularly large amount of toner applied, transferred upstream, require a strong secondary transfer bias because the amount of charge supplied for secondary transfer increases. However, halftone toner images with a particularly small amount of toner applied, transferred downstream, only require a weak secondary transfer bias. Therefore, if the secondary transfer bias for toner images transferred downstream is matched to that of toner images transferred upstream, strong skipping is likely to occur. This strong skipping is a phenomenon where the polarity of the toner reverses due to an excessively strong secondary transfer bias, causing it to remain on the intermediate transfer belt 7 without being transferred to the recording material S.

[0037] To address these issues, a toner charger 17 is installed only in the black image forming section 11K. Furthermore, especially in high-temperature and high-humidity environments, negatively charged particles are irradiated onto the black toner image supported on the black photosensitive drum 1K to reinforce the charge level of the toner in the black toner image to be on par with that of the color toner image. On the other hand, in environments other than high-temperature and high-humidity environments, such as normal temperature and humidity environments, or low-temperature and low-humidity environments, the increased charge level of the black toner increases the primary transfer bias required for primary transfer, which may make re-transfer of the color toner image more likely in the black primary transfer section N1K. This re-transfer is a phenomenon in which, for example, in the black primary transfer section N1, the polarity of the toner on the intermediate transfer belt 7, which was primary transferred upstream of the black primary transfer section N1, reverses and moves to the black photosensitive drum 1K. As a result, color changes in the color image may become more apparent, especially on plain paper or coated paper with small surface irregularities (surface roughness). Therefore, considering the balance between the strong leakage in the secondary transfer section N2 and the re-transfer of the primary transfer section N1, it is conceivable to apply a toner bias to the toner charger 17 in the black image forming section 11K only in high-temperature, high-humidity environments where the toner charge is significantly reduced.

[0038] However, especially with embossed paper, which has a large surface irregularity (surface roughness), discharge is more likely to occur in the voids of the depressions. As a result, black toner with a relatively low charge is more prone to strong gaps, and there is a high possibility of white gaps occurring. Furthermore, discharge in voids is more likely to occur in normal or low humidity environments than in high temperature and high humidity environments. Therefore, in order to improve the secondary transferability to the depressions of embossed paper, it has been found that the charge of the toner in the black toner image needs to be reinforced to the same level as the toner in the color toner image, even in environments below normal humidity (normal temperature and humidity environments other than high temperature and high humidity environments, such as low temperature and low humidity environments).

[0039] 3. Description of the control in this embodiment Next, the adjustment of the toner charge amount in this embodiment will be described. Figure 4 is a schematic block diagram showing the control configuration of the main parts of the image forming apparatus 100 in this embodiment. Figure 5 is a flowchart diagram showing the schematic procedure of a job (a series of operations that start with a single start command to form and output an image on one or more recording materials S) including the adjustment of the toner charge amount in this embodiment.

[0040] Using Figure 4, the configuration of the control system of the image forming apparatus 100 in this embodiment will be explained. In this embodiment, the image forming apparatus 100 is equipped with a control unit 120 that can comprehensively control each part of the image forming apparatus 100. The image forming apparatus 100 is also equipped with an environmental sensor 13 that detects the temperature and humidity of the environment in which the image forming apparatus 100 is installed. The environmental sensor 13 sends the detected information to the control unit 120. The image forming apparatus 100 is also equipped with an operation unit 110 that allows an operator, such as a user (here simply referred to as "user"), to arbitrarily set the type of recording material S used for image formation (paper type, media type) and image formation conditions. The type of recording material S includes any classification that can distinguish the recording material S based on any information that can distinguish the recording material S, such as attributes based on general characteristics such as plain paper, cardboard, thin paper, and embossed paper (so-called paper type category), numerical values ​​or numerical ranges such as basis weight and thickness, or brand name (including manufacturer, product name, product number, etc.). Furthermore, the type of recording material S is not limited to being directly specified. The type of recording material S may be specified by selecting a cassette 61 containing the relevant recording material S from among a plurality of cassettes 61, or by selecting an operating setting of the image forming apparatus 100, such as "plain paper mode" or "embossed paper mode". The operation unit 110 sends the input information to the control unit 120. The environmental sensor 13 is an example of an environmental detection means that detects environmental information, which is at least one of the temperature or humidity inside or outside the image forming apparatus 100. In this embodiment, it detects temperature and humidity information (temperature and humidity information) inside the image forming apparatus 100. In this embodiment, the environmental sensor 31 can determine absolute humidity (absolute moisture content) as temperature and humidity information. The operation unit 110 also has a display unit such as a liquid crystal display that displays information under the control of the control unit 120, and an input unit such as keys for inputting various setting information and operation start instructions (signals) to the control unit 120 by user operation. The operation unit 110 may be configured to have a touch panel that has the functions of both a display unit and an input unit.

[0041] The control unit 120 includes a recording material information acquisition unit 121, a toner charging bias control unit 122, and a primary transfer bias control unit 123. The recording material information acquisition unit 121 acquires information about the recording material S set by the user (information about the type of recording material S, such as surface properties and basis weight, and information about its size). The toner charging bias control unit 122 determines a corresponding output value from a plurality of preset output values ​​based on the recording material S information obtained by the recording material information acquisition unit 121 and the temperature and humidity information obtained by the environmental sensor 13, and controls the toner charging bias power supply 132 to output the toner charging bias. The primary transfer bias control unit 123 also determines a corresponding output value from a plurality of preset output values ​​based on the recording material S information obtained by the recording material information acquisition unit 121 and the temperature and humidity information obtained by the environmental sensor 13, and controls the primary transfer bias power supply 133 to output the primary transfer bias. Although not shown in Figure 4, in this embodiment, the primary transfer bias power supply 133 is provided individually for each of the four image forming units 11Y, 11M, 11C, and 11K. Specifically, the control unit 120 is configured to include a CPU as an arithmetic processing means, which is the central element that performs arithmetic processing; a memory (storage element) such as ROM or RAM as a storage means; and an input / output unit that controls the exchange of signals between the control unit 120 and the elements connected to it. Sensor detection results, calculation results, etc. are stored in the RAM, and control programs, pre-determined data tables, etc. are stored in the ROM (including rewritable ones). In this embodiment, the recording material information acquisition unit 121, toner charging bias control unit 122, and primary transfer bias control unit 123, which are the control blocks described above, are realized by the CPU executing a predetermined program stored in memory.

[0042] Next, the procedure for the operation of the job in this embodiment will be explained using Figure 5. When the job starts (S101), the control unit 120 determines whether the type of recording material S indicated by the information of the recording material S acquired by the recording material information acquisition unit 121 is a predetermined embossed paper (S102). If the control unit 120 determines in S102 that the type of recording material S is not embossed paper (it is something other than embossed paper) ("No"), it decides to perform image formation in normal operation (here referred to as "image formation mode 1") (S103). The control unit 120 also decides to output a normal bias P1 [-μA] based on the temperature and humidity information of the environmental sensor 13 as the toner charging bias, using constant current control from the toner charging bias power supply 132 (S104). Then, the control unit 120 controls the toner charging bias power supply 132 with the determined settings using the toner charging bias control unit 122 to perform image formation (S107) and finish the job (S108).

[0043] Furthermore, if the control unit 120 determines in S102 that the type of recording material S is embossed paper ("Yes"), it decides to perform image formation using the operation corresponding to embossed paper (referred to here as "image formation mode 2"), which will be described in detail later (S105). The control unit 120 also decides to output a bias P2 (>P1) [-μA] with an absolute value greater than the output value P1 in image formation mode 1 as the toner charging bias from the toner charging bias power supply 132 using constant current control (S106). Then, the control unit 120 controls the toner charging bias power supply 132 with the determined settings by the toner charging bias control unit 122 to perform image formation (S107), and then finishes the job (S108).

[0044] Next, we will describe in more detail the image formation mode 1 and image formation mode 2 in this embodiment. Image formation mode 1 is selected when the type of recording material S indicated by the information of the recording material S acquired by the recording material information acquisition unit 121 is not embossed paper. In image formation mode 1, the toner charging bias power supply 132 is controlled by the toner bias control unit 122 to output a toner charging bias of a preset value based on the detection information of the environmental sensor 13. In image formation mode 1, the absolute value of the output of the toner charging bias is increased, especially in high temperature and high humidity environments where the amount of charge of the toner decreases significantly. In this embodiment, if the absolute humidity detected by the environmental sensor 13 is less than 11 [g / kg Air (hereinafter referred to as "g / kg")] (low temperature and low humidity environment), the output of the toner charging bias is turned OFF. Also, if the absolute humidity is 11 [g / kg] or more and less than 21 [g / kg] (normal temperature and normal humidity environment), constant current control of the output of the toner charging bias is performed to supply a current of -150 [μA] (target current). Furthermore, when the absolute humidity is 21 g / kg or higher (high temperature and high humidity environment), constant current control of the toner charging bias output is performed to supply a current of -350 μA.

[0045] Image forming mode 2 is selected when the type of recording material S indicated by the information of the recording material S acquired by the recording material information acquisition unit 121 is embossed paper. In image forming mode 2, the toner charging bias power supply 132 is controlled by the toner charging bias control unit 122 to output a toner charging bias with a larger absolute value than the toner charging bias in image forming mode 1. For example, in this embodiment, constant current control of the toner charging bias output is performed to supply a uniform current of -350 [μA] regardless of the detection information of the environmental sensor 13. However, the absolute value of the toner charging bias (supplied current) to be increased in image forming mode 2 compared to image forming mode 1 is not particularly limited. For example, it is also possible to add a fixed value to the output in image forming mode 1 based on the detection information of the environmental sensor 13.

[0046] The effects of this embodiment will be explained using Figure 6. Figure 6(a) is a graph showing the charge amounts of yellow, magenta, cyan, and black toners on the intermediate transfer belt 7 before reaching the secondary transfer section N2 (at normal temperature and humidity). Figure 6(b) is a graph showing the charge amount distribution of yellow and black toners on the intermediate transfer belt 7 before reaching the secondary transfer section N2 (at normal temperature and humidity). In Figures 6(a) and (b), the charge amount of the black toner is shown for both the case where the charge amount is not adjusted and the case where the charge amount is adjusted according to this embodiment. By executing image forming mode 2 according to this embodiment, the charge amount of the black toner on the intermediate transfer belt 7 increases (Figure 6(a)), and the toner charge amount distribution becomes similar for the yellow toner and the black toner (Figure 6(b)). In this way, the discrepancy in charge amounts between the color toners and the black toner is reduced.

[0047] As a result, the image quality can be improved when embossed paper is used as the recording material S. Next, we will explain the results of confirming this point. As the embossed paper, we used Lezack 66 (registered trademark) with a basis weight of 250 g / m². 2 The following was used. The secondary transferability to embossed paper is indicated by the standard deviation σ of the brightness (density) of each uneven part on the surface of the embossed paper. The standard deviation σ of brightness can be calculated, for example, as follows: In the image reading unit 80 (Figure 1) located at the top of the image forming apparatus 100, a sheet (recording material S on which the toner image is fixed) placed on the platen glass 82 is illuminated by a light source (not shown). Then, in the image reading unit 80, the image on the sheet is read by an image reading element (not shown) at a predetermined dot density. Here, for example, the toner image on the embossed paper is read at about 300 dpi, and the standard deviation σ is calculated from the brightness value derived from 0 to 255. The lower the secondary transferability to the recesses of the embossed paper, and the more white areas there are, the greater the difference in brightness between the uneven areas, and the larger the value of the standard deviation σ of brightness.

[0048] Figure 7 is a graph showing the improvement in secondary transferability to embossed paper according to this embodiment (under normal temperature and humidity conditions). In Figure 7, the horizontal axis represents the secondary transfer current [μA], and the vertical axis represents the standard deviation σ of brightness, which is an indicator of the secondary transferability of the black toner image to the embossed paper. Figure 7 shows the results when image formation mode 2 is performed according to this embodiment and the results when image formation mode 1 is performed as comparative example 1. When the standard deviation σ exceeds 10, white areas in the recesses become visible to the naked eye. From Figure 7, it can be seen that in comparative example 1, it is difficult to set the secondary transfer current so that the standard deviation σ is 10 or less. In contrast, in this embodiment, the standard deviation σ of brightness is small, and it can be seen that the secondary transfer current can be set so that the standard deviation σ is 10 or less.

[0049] In this embodiment, when image formation mode 2 is executed, the charge level of the black toner increases, which relatively reduces the primary transferability of the black toner compared to the charge level in normal operation (image formation mode 1). This leads to a decrease in the amount (density) of black toner on the intermediate transfer belt 7. This is because the amount of charge required to transfer the black toner from the black photosensitive drum 1K onto the intermediate transfer belt 7 increases. If the primary transfer bias supplied to the black primary transfer roller 5K is increased to match the highly charged black toner, there is a possibility that the re-transfer of the color toner that has been primary transferred upstream will occur more easily. In other words, in the black primary transfer section N1, there is a possibility that the polarity of the color toner on the intermediate transfer belt 7 that has been primary transferred upstream will reverse and move to the black photosensitive drum 1K. For this reason, in this embodiment, the primary transfer bias supplied to the black primary transfer roller 5K is not increased. Therefore, in order to increase the amount (density) of black toner on the intermediate transfer belt 7 (to make it approximately constant between image formation mode 1 and image formation mode 2), density correction can be performed as follows. For example, a gradation pattern with varying toner densities is created immediately before image formation or between sheets of paper, and the density of each pattern is read by a reading unit (not shown) consisting of an optical sensor or the like, which is provided to read the photosensitive drum 1K or the intermediate transfer belt 7. Based on the reading results, the image formation conditions are corrected to make the amount of toner approximately constant as described above. As image formation conditions, for example, the intensity of the laser light irradiated from the exposure device (laser scanner device) 3 can be corrected. In addition, the charging bias applied to the charger 2 and the developing bias applied to the developer 4 may also be corrected. At least one (or more) of these exposure amounts (amount of light irradiated per unit area per unit time), charging bias, and developing bias from the exposure device 3 can be corrected. The density correction using gradation patterns itself can be performed using any available method, such as a known method.

[0050] Furthermore, it is not desirable to always set the toner charging bias to a high bias regardless of whether the recording material S used for image formation is embossed paper or not. This is because, as mentioned above, the toner charging bias increases the amount of charge of the black toner, which increases the primary transfer bias required for primary transfer, potentially making re-transfer of the color toner image more likely in the primary transfer section N1K of the black toner. As a result, color changes in the color image may become more apparent on plain paper with small surface irregularities or coated paper with high smoothness. Therefore, considering the balance between strong gaps in the secondary transfer section N2 and re-transfer of the primary transfer section N1, the control described in this embodiment (image formation mode 2) is executed only on embossed paper, in particular, where strong gaps in the secondary transfer section are more likely to occur.

[0051] Furthermore, the amount of charge on the toner on the intermediate transfer belt 7 can be determined by the blow-off measurement method. The blow-off measurement method can be performed using a known method.

[0052] Furthermore, the charge distribution of the toner can be evaluated using a charge distribution measuring device (Hosokawa Micron Corporation; model EST-3), and the standard deviation can be used as an indicator of the extent of the charge distribution from the obtained q / d distribution.

[0053] Thus, in this embodiment, the image forming apparatus 100 includes a rotatable image carrier 1K that carries a toner image, a toner image forming means 4K that forms a toner image on the surface of the image carrier 1K in the toner image forming unit G, a rotatable intermediate transfer body 7 that transports the toner image transferred from the image carrier 1K in the primary transfer unit N1K for transfer to the recording material S in the secondary transfer unit N2, a toner charger 17 that charges the toner on the surface of the image carrier 1K downstream of the toner image forming unit G and upstream of the primary transfer unit N1K in the rotation direction of the image carrier 1K, a toner charge bias power supply 132 that supplies a toner charge bias to the toner charger 17, and a control of the toner charge bias power supply 132 The system includes a control unit 120 and an acquisition unit 121 that acquires information about the recording material S onto which the toner image is transferred. The control unit 120 can be controlled to execute a first image forming mode ("image forming mode 1") when the type of recording material S indicated by the information acquired by the acquisition unit 121 is a first type, and to execute a second image forming mode ("image forming mode 2") when the type of recording material S indicated by the information acquired by the acquisition unit 121 is a second type with greater surface irregularities than the first type of recording material. The system is controlled so that the absolute value of the toner charging bias in a predetermined environment is greater in the second image forming mode than in the first image forming mode. In other words, the control unit 120 can control the system to execute a first image forming mode when the type of recording material S indicated by the information acquired by the acquisition unit 121 is a first type with a higher surface smoothness than a second type, and to execute a second image forming mode when the type of recording material S indicated by the information acquired by the acquisition unit 121 is a second type, and to control the system so that the absolute value of the toner charging bias in a predetermined environment is smaller in the first image forming mode than in the second image forming mode. Note that the absolute value of the toner charging bias in the predetermined environment in the first image forming mode may be 0 (toner charging bias output OFF).In this embodiment, the image forming apparatus 100 includes an image forming unit 11K equipped with an image carrier 1K, a toner image forming means 4K, and a toner charger 17, and another image forming unit 11Y equipped with another rotatable image carrier 1Y positioned downstream of the secondary transfer unit N2 and upstream of the image carrier 1K with respect to the rotational direction of the intermediate transfer unit 7 and carrying a toner image, and another toner image forming means 4Y that forms a toner image on the surface of the other image carrier 1Y. The intermediate transfer unit 7 is transported by another primary transfer unit N1Y corresponding to the other image carrier 1Y and a primary transfer unit N1K corresponding to the image carrier 1K, for transfer of the toner images transferred from the other image carrier 1Y and the image carrier 1K to the recording material S in the secondary transfer unit N2. In this embodiment, the toner image forming means 4K forms a black toner image on the surface of the image carrier 1K, and the other toner image forming means 4Y forms a toner image of a color other than black on the surface of the other image carrier 1Y. For example, the first type mentioned above is plain paper, and the second type mentioned above is embossed paper. In this embodiment, the toner charger 17 is a corona charger that irradiates the toner on the surface of the image carrier 1K with charged particles.

[0054] As explained above, in this embodiment, when embossed paper is selected by the user, the output value of the toner charging bias is increased, thereby raising the amount of charge on the black toner on the intermediate transfer belt 7 to the same level as the color toner. This makes it possible to improve the image quality when using embossed paper. In other words, according to this embodiment, it is possible to improve the image quality when using embossed paper with large surface irregularities as the recording material S with a simpler configuration compared to configurations that include a variable mechanism in the hardware configuration.

[0055] [Example 2] Next, other embodiments of the present invention will be described. The basic configuration and operation of the image forming apparatus in this embodiment are the same as those of the image forming apparatus in Embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus in Embodiment 1 are denoted by the same reference numerals as in Embodiment 1, and detailed descriptions are omitted.

[0056] In Example 1, we described a control method that, when embossed paper is selected by the user, increases the output value of the toner charging bias to raise the charge level of the black toner on the intermediate transfer belt 7 to the same level as that of the color toner.

[0057] In this embodiment, in addition to increasing the charge amount of the black toner to reinforce it to the same level as the color toner as described above, we will explain a control method that reduces the charge amount of the color toner on the intermediate transfer belt 7 to reduce the discrepancy in charge amounts between the color toner and the black toner.

[0058] In other words, the toner image that is first transferred to the intermediate transfer belt 7 in the upstream primary transfer section N1 is discharged as it passes through the downstream primary transfer section N1, and the charge amount of the toner on the intermediate transfer belt 7 tends to increase. In particular, the yellow toner that is first transferred at the very upstream end increases in charge amount each time it passes through the downstream magenta, cyan, and black primary transfer sections N1, and the discrepancy with the charge amount of the black toner at the very downstream end tends to become large.

[0059] Therefore, in this embodiment, in addition to increasing the charge amount of the black toner to reinforce it to the same level as the color toner, the charge amount of the color toner on the intermediate transfer belt 7 is reduced to minimize the difference in charge amounts between the color toner and the black toner. In this embodiment, the charge amounts of the yellow, magenta, and cyan toners are controlled to be reduced. However, it is sufficient to reduce the charge amount of the toner that is primarily transferred in at least one of the image forming units 11 other than the downstream image forming unit 11.

[0060] Next, the procedure for the job operation in this embodiment will be explained using Figure 8 (and Figure 4). Figure 8 is a flowchart illustrating the general procedure for the job, including the adjustment of the toner charge amount, in this embodiment.

[0061] When a job is started (S201), the control unit 120 determines whether or not the type of the recording material S indicated by the information of the recording material S acquired by the recording material information acquisition unit 121 is a predetermined embossed paper (S202). When the control unit 120 determines in S202 that the type of the recording material S is not embossed paper (other than embossed paper) ("No"), it decides to perform image formation in the normal operation ("image formation mode 1") (S203). Also, the control unit 120 decides to output a normal bias P1 [-μA] based on the temperature and humidity information of the environment sensor 13 as the toner charging bias from the toner charging bias power supply 132 by constant current control (S204). Next, the control unit 120 decides to output a normal bias T0 [V] based on the temperature and humidity information of the environment sensor 13 as the color primary transfer bias from the primary transfer bias power supply 133 by constant voltage control (S205). After that, the control unit 120 controls the toner charging bias power supply 132 and the primary transfer bias power supply 133 with the settings determined respectively by the toner charging bias control unit 122 and the primary transfer bias control unit 123 to perform image formation (S209), and ends the job (S210).

[0062] Also, when the control unit 120 determines in S202 that the type of the recording material S is embossed paper ("Yes"), it decides to perform image formation in the operation corresponding to the embossed paper (to be described in detail later) ("image formation mode 2") (S206). Also, the control unit 120 decides to output a bias P2 (>P1) [-μA] having an absolute value larger than the output value P1 in the image formation mode 1 as the toner charging bias from the toner charging bias power supply 132 by constant current control (S207). Next, the control unit 120 decides to output a bias T1 (0<T1<T0) [V] having an absolute value smaller than the output value T0 in the image formation mode 1 and larger than 0 as the color primary transfer bias from the primary transfer bias power supply 133 by constant voltage control (S208). After that, the control unit 120 controls the toner charging bias power supply 132 and the primary transfer bias power supply 133 with the settings determined respectively by the toner charging bias control unit 122 and the primary transfer bias control unit 123 to perform image formation (S209), and ends the job (S210).

[0063] Next, the image formation mode 2 in this embodiment will be described in more detail. Image formation mode 2 is selected when the type of recording material S indicated by the information of the recording material S acquired by the recording material information acquisition unit 121 is embossed paper. The control of the toner charging bias in image formation mode 2 is the same as in embodiment 1. For the primary transfer bias of color in image formation mode 2, the primary transfer bias power supply 133 is controlled by the primary transfer bias control unit 123 so as to output a bias (target voltage) whose absolute value is smaller than the output T0 [V] in image formation mode 1 and whose absolute value is greater than 0 [V]. For example, in this embodiment, constant voltage control of the primary transfer bias output is performed so as the primary transfer bias of color (yellow, magenta, cyan), a voltage T1 (T1 = T0 - 300) [V] is applied, which is uniformly 300 [V] lower than T0 [V]. However, the absolute value of the primary transfer bias (applied voltage) to be reduced in image formation mode 2 compared to image formation mode 1 is not particularly limited. For example, it is also possible to output a fixed value without relying on the output in image formation mode 1 based on detection information from the environmental sensor 13.

[0064] The effects of this embodiment will be explained using Figure 9. Figure 9(a) is a graph showing the charge levels of yellow and black toners on the intermediate transfer belt 7 before reaching the secondary transfer section N2 (under normal temperature and humidity conditions). Figure 9(a) shows the charge levels of the toners when the charge levels are not adjusted and when the charge levels are adjusted according to this embodiment. Figure 9(b) is a graph showing the charge level distribution of yellow and black toners on the intermediate transfer belt 7 before reaching the secondary transfer section N2 (under normal temperature and humidity conditions). Figure 9(b) shows the charge level of the toners when the charge levels are adjusted according to this embodiment. By executing image forming mode 2 according to this embodiment, the charge level of the yellow toner on the intermediate transfer belt 7 decreases, and the charge level of the black toner increases (Figure 9(a)). In addition, the charge level distribution of the yellow toner and black toner on the intermediate transfer belt 7 becomes similar (Figure 9(b)). In other words, compared to the distribution shown in Figure 3(b) when the toner charge amount is not adjusted, the difference in charge amount between the color toner and the black toner becomes smaller.

[0065] Figure 10 is a graph showing the improvement in secondary transferability to embossed paper according to this embodiment (under normal temperature and humidity conditions). In Figure 10, the horizontal axis represents the secondary transfer current [μA], and the vertical axis represents the standard deviation σ of brightness, which is an indicator of the secondary transferability of the toner image to the embossed paper. Figure 10 shows the results for black toner and color toner, with the color toner results specifically showing the results for secondary colors (e.g., blue) which are particularly difficult to transfer to recessed areas. Figure 10 also shows the results when image formation mode 2 is executed according to this embodiment and the results when image formation mode 1 is executed as comparative example 2. When the standard deviation σ exceeds 10, white areas in recessed areas become visible. From Figure 10, it can be seen that in comparative example 2, it is difficult to set the secondary transfer current so that the standard deviation σ is 10 or less. In contrast, in this embodiment, the standard deviation σ of brightness of the secondary color image and the black image is small, and it can be seen that the secondary transfer current can be set so that the standard deviation σ is 10 or less.

[0066] In this embodiment, when image formation mode 2 is executed, the primary transferability of the color toner decreases by lowering the primary transfer bias of the color toner. This leads to a decrease in the amount (density) of toner on the intermediate transfer belt 7 of the color toner. Therefore, in order to increase the amount (density) of toner on the intermediate transfer belt 7 of the color toner (to make it approximately constant between image formation mode 1 and image formation mode 2), density correction can be performed as follows. For example, a gradation pattern with varying toner densities is created just before image formation or between sheets of paper, and the density of each pattern is read by a reading unit (not shown) consisting of an optical sensor or the like, which is provided to read on the color photosensitive drums 1Y, 1M, 1C or on the intermediate transfer belt 7. Based on the reading result, the image formation conditions are corrected to make the amount of toner approximately constant as described above. As image formation conditions, for example, the intensity of the laser light irradiated from the exposure device (laser scanner device) 3 can be corrected. Alternatively, the charging bias applied to the charger 2 or the developing bias applied to the developer 4 may also be corrected. The exposure amount (amount of light irradiated per unit area per unit time), charging bias, and development bias of these exposure devices 3 can be corrected for at least one (or more). The density correction using the gradation pattern itself can be performed using any available method, such as a known method.

[0067] Furthermore, it is not desirable to always set the primary color transfer bias to a low bias, regardless of whether the recording material S used for image formation is embossed paper or not. This is because, as mentioned above, lowering the primary color transfer bias reduces primary transferability. In contrast, even if the density is corrected to compensate for the decrease in toner amount (density) on the intermediate transfer belt 7 as mentioned above, the color tones of multi-order colors are particularly susceptible to the effects of reduced primary transferability. Therefore, plain paper with small surface irregularities and coated paper with high smoothness may make color changes in color images more apparent. For this reason, considering the balance between strong color dropouts in the secondary transfer section N2 and the gradation of multi-order colors, the control described in this embodiment (image formation mode 2) is executed only on embossed paper, where strong color dropouts in the secondary transfer section are particularly likely to occur.

[0068] On the other hand, in this embodiment, the absolute value of the primary transfer bias for color is reduced, but the absolute value of the primary transfer bias for black is not reduced. As mentioned above, if the primary transfer bias for black toner is reduced for black toner whose charge level has increased due to the toner charging bias, the amount of charge supplied required for primary transfer of toner to the intermediate transfer belt 7 will be further reduced. This is because the primary transferability of the black toner will be relatively lower than when the black toner is in a normal charged state, and the amount of black toner on the intermediate transfer belt 7 will be greatly reduced. Although this is not done in this embodiment because it increases the consumption of black toner, it is also possible to correct the density from the reading result of the gradation pattern, similar to the color toner described above.

[0069] Thus, in this embodiment, the image forming apparatus 100 has an image forming unit 11K equipped with an image carrier 1K, a toner image forming means 4K, and a toner charger 17, and another image forming unit 11Y equipped with another image carrier 1Y and another toner image forming means 4Y. The intermediate transfer body 7 is transported by another primary transfer unit N1Y and primary transfer unit N1K to transfer the toner images transferred from the other image carrier 1Y and image carrier 1K to the recording material S in the secondary transfer unit N2. In this embodiment, the control unit 120 can control the power supply 133 that supplies a primary transfer bias to another primary transfer unit N1Y in order to transfer the toner image from the other image carrier 1Y to the intermediate transfer body 7, and controls the second image forming mode to have a smaller absolute value of the primary transfer bias supplied to the other primary transfer unit N1Y in a predetermined environment than the first image forming mode. Furthermore, in this embodiment, the control unit 120 can control the power supply 133 that supplies the primary transfer bias to the primary transfer unit N1K in order to transfer the toner image from the image carrier 1K to the intermediate transfer unit 7, and controls it so that the primary transfer bias supplied to the primary transfer unit N1K in the above predetermined environment is substantially the same in the first image formation mode and the second image formation mode.

[0070] As explained above, in this embodiment, when embossed paper is selected by the user, the output value of the toner charging bias is increased, thereby increasing the amount of charge on the black toner on the intermediate transfer belt 7. At the same time, the absolute value of the primary color transfer bias is lowered, thereby decreasing the amount of charge on the color toner on the intermediate transfer belt 7. This reduces the difference in toner charge between the color toner and the black toner, making it possible to improve image quality when using embossed paper.

[0071] [Example 3] Next, other embodiments of the present invention will be described. The basic configuration and operation of the image forming apparatus in this embodiment are the same as those of the image forming apparatuses in Embodiments 1 and 2. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatuses in Embodiments 1 and 2 are denoted by the same reference numerals as in Embodiments 1 and 2, and detailed descriptions are omitted.

[0072] In Examples 1 and 2, the toner charging bias control unit 122 and the primary transfer bias control unit 123 were controlled based on information about the recording material S, which was arbitrarily selected by the user and acquired by the recording material information acquisition unit 121. For example, when density correction is performed to suppress the decrease in toner density on the intermediate transfer belt 7 due to the control of the toner charging bias and primary transfer bias as described above, the amount of toner consumed will increase.

[0073] On the other hand, embossed paper available on the market varies in terms of the size and periodicity of the surface irregularities. Therefore, in some cases, sufficient image quality can be obtained using the normal image formation mode 1, even without using image formation mode 2 as described in Examples 1 and 2.

[0074] Therefore, in this embodiment, for example, from the viewpoint of toner consumption as described above, the image formation mode 2 is not executed when the recording material information acquisition unit 121 acquires information on the embossed paper, and the user is allowed to arbitrarily select the image formation mode when they wish to improve the image quality.

[0075] The adjustment of the toner charge amount in this embodiment will now be described. Figure 11 is a schematic block diagram showing the control configuration of the main parts of the image forming apparatus 100 in this embodiment. Figure 12 is a schematic diagram of the image forming mode selection screen in this embodiment. Figure 13 is a flowchart diagram showing the general procedure of a job including the adjustment of the toner charge amount in this embodiment.

[0076] The configuration of the control system of the image forming apparatus 100 in this embodiment will be explained using Figure 11. In this embodiment, the control unit 120 has an image forming mode control unit 124 in addition to the configuration of Embodiment 1 explained using Figure 4. The image forming mode control unit 124 controls the execution of the selected image forming mode when the user selects an arbitrary image forming mode using the operation unit 110. In this embodiment, the image forming mode control unit 124 as a control block is realized by the CPU executing a predetermined program stored in memory.

[0077] Next, the image formation mode selection screen in the operation unit 110 will be explained using Figure 12. The control unit 110 controls the operation unit 110 to display a mode selection screen 111 as shown in Figure 12, in response to user operations on the operation unit 110 (such as calling up a user mode to set arbitrary operation settings). On the mode selection screen 111 displayed on the operation unit 110, the user can arbitrarily select between "0", which indicates the default setting, normal mode (image formation mode 1), and "1", which indicates the emboss transferability improvement mode (image formation mode 2). When the user sets the selection button 112 to "1" and then presses the OK button 113 or the Apply button 115, information indicating that image formation mode 2 is enabled is stored in the memory of the control unit 120. When image formation mode 2 is enabled, the control unit 120 controls the image formation mode control unit 124 to execute image formation mode 2, as described in Examples 1 and 2, when performing image formation on a predetermined embossed paper for the next job. Note that the method of selecting the mode is not limited to directly specifying whether or not to enable image formation mode 2. For example, the depth of the embossing, which indicates the degree of unevenness of the embossed paper to be used, may be specified, and if the default setting of "shallow" is selected, image formation mode 1 may be executed, and if "deep" is selected, image formation mode 2 may be executed.

[0078] Next, the procedure for the operation of the job in this embodiment will be explained using Figure 13. When the job starts (S301), the control unit 120 determines whether the type of recording material S indicated by the information of the recording material S acquired by the recording material information acquisition unit 121 is a predetermined embossed paper (S302). If the control unit 120 determines in S302 that the type of recording material S is not embossed paper (it is something other than embossed paper) ("No"), it decides to perform image formation in normal operation ("image formation mode 1") (S304). Also, if the control unit 120 determines in S302 that the type of recording material S is embossed paper ("Yes"), it determines whether the user has selected image formation mode 2 in the operation unit 110 (whether image formation mode 2 is enabled) (S303). If the control unit 120 determines in S303 that image formation mode 2 is selected ("Yes"), it controls the image formation mode control unit 124 to perform image formation in image formation mode 2 (S307). On the other hand, if the control unit 120 determines in S303 that image formation mode 2 is not selected ("No"), it controls the image formation mode control unit 124 to perform image formation in image formation mode 1 (S304). In this embodiment, the operation of image formation mode 1 by S304, S305, S306, S310, and S311 is the same as the operation of image formation mode 1 by S203, S204, S205, S209, and S210 in Figure 8 of Embodiment 2. Furthermore, in this embodiment, the operation of image formation mode 2 by S307, S308, S309, S310, and S311 is the same as the operation of image formation mode 2 by S206, S207, S208, S209, and S210 in Figure 8 of Embodiment 2. Although the operation is the same as in Embodiment 2 here, the operation can also be the same as in Embodiment 1.

[0079] Thus, in this embodiment, there is an input unit (operation unit in this embodiment) 110 that inputs information to the control unit 120 in response to an operation by the operator. The control unit 120 controls the system to execute the second image forming mode when the type of recording material S indicated by the information acquired by the acquisition unit 121 is the second type, and information instructing the execution of the second image forming mode is input from the input unit 110.

[0080] As explained above, in this embodiment, if embossed paper is selected by the user and the user arbitrarily selects image formation mode 2 because they wish to improve image quality, it is possible to execute image formation mode 2 in the same manner as in embodiments 1 and 2. This means that, for example, if sufficient image quality can be obtained with the normal operation of image formation mode 1 depending on the embossed paper used by the user, image formation mode 1 can be executed.

[0081] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the embodiments described above.

[0082] In the embodiments described above, the toner charging bias was explained as being controlled by a constant current, but it may also be controlled by a constant voltage. When using constant current control, in order to increase the absolute value of the toner charging bias in image formation mode 2 compared to the absolute value of the toner charging bias in image formation mode 1 in a given environment, the absolute value of the target current of the toner charging bias power supply should be increased. However, in this case as well, typically the absolute value of the output voltage of the toner charging bias power supply will be greater in image formation mode 2 than in image formation mode 1. When using constant voltage control, in order to increase the absolute value of the toner charging bias in image formation mode 2 compared to the absolute value of the toner charging bias in image formation mode 1 in a given environment, the absolute value of the target voltage of the toner charging bias power supply should be increased. However, in this case as well, typically the absolute value of the output current of the toner charging bias power supply will be greater in image formation mode 2 than in image formation mode 1.

[0083] Furthermore, although the primary transfer bias was described as being controlled by a constant voltage in the above embodiment, it may also be controlled by a constant current. When using constant voltage control, in order to make the absolute value of the primary transfer bias in image formation mode 2 smaller than the absolute value of the primary transfer bias in image formation mode 1 in a given environment, the absolute value of the target voltage of the primary transfer bias power supply should be reduced. However, in this case as well, typically the absolute value of the output current of the primary transfer bias power supply will be smaller in image formation mode 2 than in image formation mode 1. Similarly, when using constant current control, in order to make the absolute value of the primary transfer bias in image formation mode 2 smaller than the absolute value of the primary transfer bias in image formation mode 1 in a given environment, the absolute value of the target current of the primary transfer bias power supply should be reduced. However, in this case as well, typically the absolute value of the output voltage of the primary transfer bias power supply will be smaller in image formation mode 2 than in image formation mode 1. The same applies to the secondary transfer bias.

[0084] Furthermore, the information that is input from the operation unit of the image forming apparatus in the above embodiment may be input from an external device such as a personal computer connected to the image forming apparatus. For example, the recording material information acquisition unit may acquire recording material information input from the operation unit, or it may acquire recording material information input from an external device. When information is input from an external device to the control unit, the input unit is configured with an input / output unit that controls the sending and receiving of signals to and from the control unit 120.

[0085] Furthermore, in the above-described embodiment, the recording material information acquisition unit acquires information about the recording material input by the user. However, the recording material information may also be acquired based on detection results from a sensor provided in the image forming apparatus that is capable of detecting information about the recording material, including its surface properties. As such a sensor, for example, a known media sensor combining an ultrasonic sensor or an optical sensor can be used.

[0086] Furthermore, in the above-described embodiment, a toner charger was provided only in the downstream image forming unit among the multiple image forming units, particularly in the black image forming unit. However, a toner charger may be provided in at least one of the image forming units other than the downstream image forming unit. The toner charging bias applied to the toner charger of the image forming unit other than the downstream image forming unit can be kept substantially constant between image forming mode 1 and image forming mode 2.

[0087] Furthermore, in the above-described embodiment, an example was explained in which density correction using a gradation pattern is performed to suppress the decrease in density caused by the control of toner charging bias and primary transfer bias in image formation mode 2. However, it is not always necessary to perform density correction using a gradation pattern to correct the image formation conditions in order to suppress the decrease in density. For example, image formation conditions that have been adjusted in advance to compensate for the decrease in density can be determined and the information of these image formation conditions can be stored in the memory unit. Then, when executing image formation mode 2, the information of the image formation conditions stored in the memory unit can be used.

[0088] The present invention can also be implemented in other embodiments in which some or all of the configurations of the above-described embodiments are replaced with alternative configurations. The present invention can be implemented without distinction between tandem type / single-drum type, charging method, electrostatic image formation method, developing method, transfer method, and fixing method. In the above-described embodiments, the main parts related to toner image formation / transfer were mainly explained, but the present invention can be implemented in various applications such as printers, various printing machines, copiers, fax machines, and multifunction devices by adding the necessary equipment, equipment, and housing structure. Furthermore, in the above-described embodiments, examples of constant current control or constant voltage control were shown in various bias controls, but this is not limited to these, and the invention can be implemented without distinction in other high-voltage controls as well. [Explanation of Symbols]

[0089] 5. Primary transfer roller 7. Intermediate transfer belt 13 Environmental Sensors 17 Toner Charger 18 Wire electrodes 19 Shielding electrode 21 Secondary transfer inner roller 51 Primary Transfer Bias Power Supply 52 Secondary Transfer Bias Power Supply 100 Image forming apparatus 110 Operation section 121 Recording Material Information Acquisition Unit 122 Toner charging bias control unit 123 Primary Transfer Bias Power Supply 124 Image Forming Mode Selection Section 132 Toner Charging Bias Power Supply 133 Primary Transfer Bias Power Supply

Claims

1. A plurality of image forming units, each comprising: a rotatable image carrier that carries a toner image; and a developing unit that supplies toner to the image carrier to form a toner image on the surface of the image carrier; A rotatable intermediate transfer body that contacts the image carrier of each of the plurality of image forming units to form a primary transfer unit, and transports the toner image transferred from the image carrier in the primary transfer unit of each of the plurality of image forming units for transfer to a recording material in the secondary transfer unit, It has, The plurality of image forming units include a first image forming unit, The image carrier of the first image forming unit is located in an image forming apparatus that is the furthest downstream of the image carriers of each of the plurality of image forming units with respect to the direction of movement of the surface of the intermediate transfer body, The first image forming unit includes a toner charger that charges the toner on the surface of the image carrier of the first image forming unit, downstream of the developing unit and upstream of the primary transfer unit of the first image forming unit, with respect to the rotation direction of the image carrier of the first image forming unit. A toner charging bias power supply that supplies a toner charging bias to the toner charger, A control unit capable of controlling the toner charging bias power supply, An acquisition unit that acquires information about the recording material on which the toner image is transferred, It has, The control unit executes a first image forming mode when the type of recording material indicated by the information acquired by the acquisition unit is a first type, executes a second image forming mode when the type of recording material indicated by the information acquired by the acquisition unit is a second type having greater surface irregularities than the first type of recording material, and controls the toner charging bias power supply such that the absolute value of the toner charging bias in a predetermined environment is larger in the second image forming mode than in the first image forming mode. An image forming apparatus characterized by the following features.

2. The plurality of image forming units include a second image forming unit, The second image forming unit has a second primary transfer bias power supply that supplies a primary transfer bias to the primary transfer unit of the second image forming unit in order to transfer a toner image from the image carrier of the second image forming unit to the intermediate transfer unit, The control unit controls the second primary transfer bias power supply such that the absolute value of the primary transfer bias supplied to the primary transfer unit of the second image forming unit in the predetermined environment is smaller in the second image forming mode than in the first image forming mode. The image forming apparatus according to feature 1.

3. The first image forming unit has a first primary transfer bias power supply that supplies a primary transfer bias to the primary transfer unit of the first image forming unit in order to transfer a toner image from the image carrier to the intermediate transfer unit, The control unit controls the first primary transfer bias power supply so that the primary transfer bias supplied to the primary transfer unit of the first image forming unit in the predetermined environment is substantially the same in the first image forming mode and the second image forming mode. The image forming apparatus according to claim 1 or 2.

4. The image forming apparatus according to any one of claims 1 to 3, characterized in that the developing device of the first image forming unit forms a black toner image, and the developing device of the image forming units other than the first image forming unit forms a toner image of a color other than black.

5. It has an input unit that inputs information to the control unit in response to operations performed by the operator. The control unit controls the system to execute the second image forming mode when the type of recording material indicated by the information acquired by the acquisition unit is the second type, and information instructing the execution of the second image forming mode is input from the input unit. The image forming apparatus according to any one of claims 1 to 4.

6. The image forming apparatus according to any one of claims 1 to 5, characterized in that the first type is plain paper and the second type is embossed paper.

7. The image forming apparatus according to any one of claims 1 to 6, characterized in that the toner charger is a corona charger that irradiates the toner on the surface of the image carrier of the first image forming unit with charged particles.

Images