JP2012042809A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable transfer-superimposition of a plurality of images on a sheet of recording material, and to improve productivity when a single image is transferred on the sheet of recording material.SOLUTION: An image forming apparatus includes: a photoreceptor drum 11 which is rotatably arranged to hold a toner image on the outer peripheral surface; a transfer drum 21 which is rotatably arranged to transfer the toner image held on the outer peripheral surface of the photoreceptor drum 11 to a sheet S held between the transfer drum 21 and the photoreception drum 11; a front end gripper 22 and a rear end gripper 23 which hold the sheet S supplied toward the transfer drum 21 on the outer peripheral surface of the transfer drum 21; and a transfer roll 31 which is rotatably arranged to transfer the toner image held on the outer peripheral surface of the photoreceptor drum 11 to the sheet S held between the transfer roll 31 and the photoreceptor drum 11.

Description

  The present invention relates to an image forming apparatus.

  As a conventional technique described in the publication, an image carrier that holds and rotates a toner image, a transfer material carrier that carries and rotates a transfer material, and a visible image formed on the image carrier on the transfer material. In the image forming apparatus having the transfer means for transferring to the transfer material, the transfer material carrier is made large enough to carry a plurality of small-size transfer materials and the transfer material carrier carries the transfer material There are those provided with a plurality of transfer material gripping means and adsorption means (see Patent Document 1).

Japanese Patent Laid-Open No. 6-230687

  SUMMARY OF THE INVENTION An object of the present invention is to make it possible to transfer a plurality of images on a single recording material, and to improve productivity when transferring a single image to a single recording material.

  According to the first aspect of the present invention, there is provided an image forming unit that forms a single color image or sequentially forms a plurality of color images for each color, and is rotatably provided to hold the image formed by the image forming unit. An image carrier and a function of gripping the recording material on the outer peripheral surface and rotatably provided. The image held on the image carrier is transferred to a recording material sandwiched between the image carrier and the image carrier. 1 transfer means, which has no function of gripping the recording material on the outer peripheral surface and is rotatably provided, and transfers the image held on the image holding body to the recording material sandwiched between the image holding body A second transfer unit, a multicolor mode in which the image forming unit sequentially forms a plurality of color images for each color, and the image forming unit forms a single color image and image formation is performed per unit time. More recording material than the number of recording materials In the first monochrome mode in which priority is given to the positional accuracy of the image to be transferred, the transfer operation using the first transfer unit is executed, the image forming unit forms a monochrome image, and the unit is higher than the positional accuracy of the image with respect to the recording material. In the monochrome second mode in which priority is given to the number of recording materials on which image formation is performed per time, the image forming apparatus includes a control unit that executes a transfer operation using the second transfer unit.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the first transfer unit grips a leading end and a trailing end in the transport direction of the recording material to be transported.
The invention according to claim 3 further includes a supply means for supplying a recording material for transferring the image held on the image holding body, wherein the control means is configured such that the supply interval of the recording material is the first mode of the single color. 3. The image forming apparatus according to claim 1, wherein the supply unit is controlled so as to be narrower in the monochrome second mode.
According to a fourth aspect of the present invention, in the multi-color mode and the single-color first mode, the control unit rotates the first transfer unit more than the facing portion between the image carrier and the first transfer unit. The recording material is supplied toward the upstream side, and the supplied recording material is gripped by the first transfer unit. In the single color second mode, the recording material is provided at a facing portion between the image holding member and the second transfer unit. The image forming apparatus according to claim 3, wherein the recording material is supplied toward the image forming apparatus.

  According to a fifth aspect of the present invention, there is provided an image holding body that is rotatably arranged and holds an image on an outer peripheral surface thereof, and a recording material that is rotatably arranged and sandwiched between the image holding body. A first transfer member that transfers an image held on the outer peripheral surface of the first transfer member; a gripping member that grips the recording material supplied toward the first transfer member on the outer peripheral surface of the first transfer member; and a rotatable member An image forming apparatus including a second transfer member that is disposed and transfers an image held on the outer peripheral surface of the image holding body to a recording material sandwiched between the image holding body and the recording material.

The invention according to claim 6 is characterized in that the gripping member grips the front end in the transport direction of the recording material transported toward the first transfer member on the outer peripheral surface of the first transfer member. Item 6. The image forming apparatus according to Item 5.
According to a seventh aspect of the present invention, an elastic layer is provided on the outer peripheral surface of the first transfer member, except for one region along the axial direction of the first transfer member. The second transfer member is attached to a portion of the outer peripheral surface of the first transfer member where the elastic layer is not provided so as to be rotatable with respect to the first transfer member. Item 7. The image forming apparatus according to Item 5 or 6.
According to an eighth aspect of the present invention, the position where the recording material is sandwiched between the image carrier and the first transfer member and the position where the recording material is sandwiched between the image carrier and the second transfer member are a common transfer. The image forming apparatus according to claim 5, wherein the image forming apparatus is set as a position.

According to the first aspect of the present invention, a plurality of images can be transferred onto a single recording material, and productivity can be improved when a single image is transferred onto a single recording material. be able to.
According to the second aspect of the present invention, the positioning accuracy of the recording material with respect to the image on the image carrier can be further improved as compared with the case where the present configuration is not provided.
According to the third aspect of the present invention, productivity in the case where a single image is transferred to a single recording material can be further improved as compared with the case where this configuration is not provided.
According to the fourth aspect of the present invention, in the multi-color mode and the single-color first mode, the image transfer with the recording material held can be performed more easily than in the case where this configuration is not provided. In the monochrome second mode, it is possible to shorten the time required for transporting the recording material.
According to the fifth aspect of the present invention, it is possible to transfer a plurality of images on a single recording material, and to improve productivity when transferring a single image to a single recording material. be able to.
According to the sixth aspect of the present invention, the positioning accuracy of the recording material with respect to the image on the image holding member can be further improved as compared with the case without this configuration.
According to the seventh aspect of the present invention, it is possible to reduce the size of the image forming apparatus as compared with the case where this configuration is not provided.
According to the eighth aspect of the present invention, the recording material conveyance path can be made more common as compared with the case where the present configuration is not provided.

1 is a diagram illustrating an example of a configuration of an image forming apparatus according to Embodiment 1. FIG. FIG. 3 is a diagram for explaining a transfer unit and a photosensitive drum constituting the image forming apparatus, and a size of a sheet used in the image forming apparatus. 6 is a timing chart for explaining an example of an image forming operation in a multi-color mode. 6 is a timing chart for explaining an example of an image forming operation in a single color image quality priority mode. 6 is a timing chart for explaining an example of an image forming operation in a monochromatic productivity priority mode. FIG. 6 is a diagram for explaining a relationship between a component of a transfer unit that passes a transfer position and a sheet in each mode. 6 is a diagram illustrating an example of the configuration of an image forming apparatus according to Embodiment 2 (during an image forming operation in a multicolor mode and a monochromatic image quality priority mode). 6 is a diagram illustrating an example of a configuration of an image forming apparatus according to Embodiment 2 (during an image forming operation in a monochromatic productivity priority mode). FIG.

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.
<Embodiment 1>
FIG. 1 is a diagram illustrating an example of the configuration of the image forming apparatus according to the first embodiment.
The image forming apparatus includes an image forming unit 10 that forms a toner image, a transfer unit that transfers a toner image formed by the image forming unit 10 to a sheet S as an example of a recording material, and a sheet S that is formed by the transfer unit. A fixing unit 40 that fixes the transferred toner image, and a paper supply unit 50 that supplies a paper S for transferring the toner image to the transfer unit are provided. Here, the transfer unit includes a drum-type transfer device 20 and a roll-type transfer device 30, and either one of them is used in the image forming operation. The image forming apparatus is also controlled as an example of a control unit that controls operations of the image forming unit 10, the transfer unit (the drum type transfer device 20 and the roll type transfer device 30), the fixing unit 40, the paper supply unit 50, and the like. A part 100 is further provided.

  Among these, the image forming unit 10 includes a photosensitive drum 11 as an example of an image carrier, a charging device 12 that charges the photosensitive drum 11, and an exposure device 13 that exposes the charged photosensitive drum 11. The rotary developing device 14 that develops the electrostatic latent image formed on the photosensitive drum 11 by charging and exposure with toner, and the toner remaining on the photosensitive drum 11 after the developed toner image is transferred is cleaned. And a cleaning device 15.

  The photosensitive drum 11 includes a photosensitive layer (not shown) having a negative charging polarity on the surface thereof, and is attached so as to rotate in the direction of arrow A about the rotation shaft 11a. The charging device 12, the exposure device 13, the rotary developing device 14, and the cleaning device 15 are arranged around the photosensitive drum 11 in this order along the arrow A direction.

The charging device 12 is a contact roller type discharging device in the present embodiment, and charges the photosensitive drum 11 while rotating together with the photosensitive drum 11.
The exposure device 13 forms an electrostatic latent image by selectively irradiating the surface of the charged photosensitive drum 11 with light. The exposure apparatus 13 of the present embodiment has a configuration in which a plurality of light emitting elements (for example, LEDs) are arranged along the axial direction of the photosensitive drum 11.

The rotary developing device 14 includes a rotating shaft 14a extending along the axial direction of the rotating shaft 11a of the photosensitive drum 11, and yellow (Y), magenta (M), cyan (C), and the like arranged around the rotating shaft 14a. Each of the black (K) developing devices 14Y, 14M, 14C, and 14K is provided. Further, the rotary developing device 14 is configured to rotate in the direction of arrow D about the rotation shaft 14a, and at any position facing the photosensitive drum 11 (hereinafter referred to as a developing position), any one of the developing devices. The vessel stops. The rotary developing device 14 is configured to develop the electrostatic latent image formed on the photosensitive drum 11 formed by the exposure device 13 using toner of a developing device stopped at the developing position.
Each color developing device 14Y, 14M, 14C, 14K contains a two-component developer containing the corresponding color component toner and carrier. Although a two-component developer is used in this example, a one-component developer that does not include a carrier may be used.

The cleaning device 15 removes toner remaining on the surface of the photosensitive drum 11 and adhering matters other than toner. The cleaning device 15 of the present embodiment is configured by a blade type cleaner.
In the present embodiment, the charging device 12, the exposure device 13, and the rotary developing device 14 have a function as image forming means.

Next, the drum type transfer device 20 and the roll type transfer device 30 constituting the transfer unit will be described.
First, the drum-type transfer device 20 includes a transfer drum 21 that is disposed so as to be rotatable about a rotating shaft 21a that faces the photosensitive drum 11 and extends along the axial direction of the rotating shaft 11a of the photosensitive drum 11. A front end gripper 22 that grips the front end side in the transport direction of the paper S on the outer peripheral surface of the drum 21, a rear end gripper 23 that similarly grips the rear end in the transport direction of the paper S on the outer peripheral surface of the transfer drum 21, and a rotating transfer. And a phase sensor 25 that detects the phase of the drum 21. Here, the transfer drum 21 is configured to rotate in the direction of arrow B, which is the same direction as the direction of rotation of the photosensitive drum 11 (direction of arrow A), at a position facing the photosensitive drum 11.

Among these, the transfer drum 21 as an example of the first transfer member includes a base 21A having a cylindrical shape and to which the rotary shaft 21a is attached, and an elastic layer 21B provided on the outer peripheral surface of the base 21A. Yes. Here, the elastic layer 21B covers a portion of the outer peripheral surface of the base portion 21A excluding a partial region (one region) along the axial direction of the base portion 21A. Therefore, a portion of the outer peripheral surface of the base 21A in the transfer drum 21 that is not covered by the elastic layer 21B is an exposed portion 21C where the base 21A is exposed. Therefore, the exposed portion 21C is closer to the surface portion of the elastic layer 21B when viewed from the rotating shaft 21a (the portion facing the photosensitive drum 11 is closer than the surface portion of the elastic layer 21B when viewed from the photosensitive drum 11). It is located on the far side.
The base portion 21A in the present embodiment is composed of a conductive hollow tube, and for example, metal is used. On the other hand, the elastic layer 21B is made of an elastic member having semiconductivity, and for example, a resin such as polyurethane is used. Further, a transfer bias having a polarity opposite to that of the toner is applied to the base portion 21A by a high voltage power source (not shown). On the other hand, the photosensitive drum 11 is grounded.

  Further, the tip gripper 22 as an example of a gripping member is on the outer peripheral surface of the transfer drum 21 in the rotational direction tip side of the exposed portion 21C and the elastic layer 21B (the elastic layer tip 21L side described later: FIG. 2A described later). ) See)) is provided along the rotation axis 21 a of the transfer drum 21. The front end gripper 22 is attached to the transfer drum 21, and the front end gripper 22 rotates with the rotation of the transfer drum 21. The leading edge gripper 22 grips an end portion of the sheet S in the traveling direction (referred to as a sheet leading edge SL in the following description: see FIG. 2C described later), and grips the leading edge gripper 22. Is configured to release.

  On the other hand, a rear end gripper 23 as an example of a gripping member is provided on the outer peripheral surface of the transfer drum 21 along the axial direction of the rotation shaft 21 a of the transfer drum 21. The rear end gripper 23 is rotatably attached to the rotation shaft 21 a of the transfer drum 21, and can rotate independently of the transfer drum 21. Thereby, in this embodiment, the positional relationship (distance) between the front end gripper 22 and the rear end gripper 23 on the outer peripheral surface of the transfer drum 21 can be changed. The rear end gripper 23 holds an end portion on the rear end side in the traveling direction of the paper S (referred to as a paper rear end ST in the following description: see FIG. 2C described later) between the rear end gripper 23 and the transfer drum 21. Moreover, it is comprised so that the holding | grip may be cancelled | released.

  Further, the phase sensor 25 is disposed to face the outer peripheral surface of the transfer drum 21, and detects a mark (not shown) provided on the outer peripheral surface of the transfer drum 21, thereby detecting the rotation of the transfer drum 21 that rotates. The phase is measured.

  Next, the roll type transfer device 30 is disposed on the base portion 21A of the exposed portion 21C of the transfer drum 21 so as to be rotatable about a rotation shaft 31a attached along the axial direction of the rotation shaft 21a of the transfer drum 21. A transfer roll 31 is provided. For this reason, in the present embodiment, as the transfer drum 21 rotates in the arrow B direction around the rotation shaft 21a, the transfer roll 31 also rotates in the arrow B direction around the rotation shaft 21a. Yes. Further, when the transfer roll 31 is disposed so as to face the photosensitive drum 11, the transfer roll 31 is configured to rotate alone in the direction of arrow C, which is the same direction as the rotation direction of the photosensitive drum 11 (arrow A direction). ing.

As the transfer roll 31 as an example of the second transfer member, for example, a surface of a foamed urethane rubber roll in which carbon is dispersed is coated with fluorine, and the volume resistivity is 10 ³ to 10 ¹⁰ Ω · It is set within the range of cm. Here, unlike the transfer drum 21, the transfer roll 31 has a circular cross section by covering the entire outer peripheral surface.
In the present embodiment, the transfer drum 21, the front end gripper 22, and the rear end gripper 23 function as a first transfer unit, and the transfer roll 31 functions as a second transfer unit.

In the present embodiment, the rotating shaft 11a of the photosensitive drum 11, the rotating shaft 14a of the rotary developing device 14, and the rotating shaft 21a of the transfer drum 21 are respectively provided on a main body frame (not shown) of the image forming apparatus. It is fixed. Therefore, in the image forming apparatus, the positional relationship between the rotating shaft 11a of the photosensitive drum 11 and the rotating shaft 14a of the rotary developing device 14, and the positional relationship between the rotating shaft 11a of the photosensitive drum 11 and the rotating shaft 21a of the transfer drum 21. The positional relationship between the rotary shaft 14a of the rotary developing device 14 and the rotary shaft 21a of the transfer drum 21 is kept constant. On the other hand, when the rotation shaft 31a of the transfer roll 31 is attached to the transfer drum 21, the rotation shaft 31a of the transfer roll 31, the rotation shaft 11a of the photosensitive drum 11, the rotation shaft 14a of the rotary developing device 14, and the transfer drum. The positional relationship between the transfer drum 21 and the rotation shaft 21a changes according to the rotation state of the transfer drum 21 (the position of the exposed portion 21C). In this embodiment, the rotational drive of the photosensitive drum 11, the rotational drive of the rotary developing device 14, the rotational drive of the transfer drum 21, and the rotational drive of the transfer roll 31 are executed independently. ing.
In the following description, a position where the photosensitive drum 11 and the transfer drum 21 or the transfer roll 31 on the transfer drum 21 face each other is referred to as a transfer position Tr.

The fixing unit 40 includes a heating roll 41 that has a heating source (not shown) and is rotatably installed, and a pressure roll 42 that is pressed against the heating roll 41.
Further, a paper supply unit 50 as an example of a supply unit is taken out by a paper storage unit 51 that stores the paper S inside the transfer drum 21, a take-out roll 52 that takes out the paper S from the paper storage unit 51, and a take-out roll 52. A registration roll 53 that conveys the sheet S in a timely manner and a gate 54 that switches the conveyance direction of the sheet S fed from the resist roll 53 are provided.

  Here, in the following description, the sheet S shown on the left side in the drawing is directed to the transfer drum 21 from the registration path 53 to the transfer drum 21 through the conveyance path of the sheet S from the sheet storage unit 51 to the registration roll 53. Is referred to as a first paper feed path 61, and a paper S transport path shown on the right side in the drawing from the registration roll 53 to the transfer position Tr via the gate 54 is referred to as a second paper feed path 62. Further, the conveyance path of the sheet S on the outer peripheral surface of the transfer drum 21 is referred to as a rotation path 63, and the conveyance path of the sheet S from the transfer position Tr to the fixing unit 40 is referred to as a discharge path 64. The sheet feeding position on the transfer drum 21 via the first sheet feeding path 61 is referred to as a first sheet feeding position P1, and the sheet feeding position to the transfer roll 31 via the second sheet feeding path 62 is the second sheet feeding position. This is referred to as a paper feeding position P2. In the present embodiment, the transfer position Tr and the second paper feed position P2 are common.

  FIG. 2 illustrates the size of the transfer unit (the drum-type transfer device 20 and the roll-type transfer device 30) and the photosensitive drum 11 that constitute the image forming apparatus shown in FIG. 1, and the size of the paper S used in the image forming apparatus. It is a figure for doing. Here, FIG. 2A shows the sizes of the transfer drum 21 and the transfer roll 31 constituting the transfer unit, FIG. 2B shows the size of the photosensitive drum 11, and FIG. It is a figure for demonstrating the magnitude | size of each.

  First, the sizes of the transfer drum 21 and the transfer roll 31 constituting the transfer unit will be described with reference to FIG. Of the elastic layer 21 </ b> B constituting the transfer drum 21, the end located on the upstream side in the rotation direction of the transfer drum 21 is called an elastic layer tip 21 </ b> L, and the end located on the downstream side in the rotation direction of the transfer drum 21 is elastic. It is called the layer rear end 21T.

  First, let the diameter of the transfer drum 21 be the transfer drum diameter Dd. Here, as shown in FIG. 2A, the transfer drum diameter Dd is a diameter of a portion of the transfer drum 21 where the elastic layer 21B exists on the outer peripheral surface. At this time, the peripheral length of the transfer drum 21 is set as a transfer drum peripheral length Ld. The transfer drum circumferential length Ld is obtained by multiplying the transfer drum diameter Dd by the circumference ratio π. When the entire area of the transfer drum 21 is the transfer drum diameter Dd (existence of the exposed portion 21C). Is a hypothetical value.

  Next, in the transfer drum 21, the circumferential length of the elastic layer 21B extending from the elastic layer front end 21L to the elastic layer rear end 21T along the rotation direction of the transfer drum 21 is defined as an elastic layer length Le. In the transfer drum 21, the circumferential length of the exposed portion 21C from the elastic layer rear end 21T to the elastic layer front end 21L along the rotation direction of the transfer drum 21 is defined as a gap length Lg. In this embodiment, the sum of the elastic layer length Le and the gap length Lg is the transfer drum circumferential length Ld.

  Further, the diameter of the transfer roll 31 is set as the transfer roll diameter Dr, and the circumference of the transfer roll 31 is set as the transfer roll circumference Lr. The transfer roll circumferential length Lr is obtained by multiplying the transfer roll diameter Dr by the circumferential ratio π.

Next, the size of the photosensitive drum 11 will be described with reference to FIG.
Here, the diameter of the photosensitive drum 11 is defined as a photosensitive drum diameter Dp, and the circumferential length of the photosensitive drum 11 is defined as a photosensitive drum circumferential length Lp. The photosensitive drum circumferential length Lp is obtained by multiplying the photosensitive drum diameter Dp by the circumferential ratio π.

Finally, the size of the paper S will be described with reference to FIG.
Of the paper S, the part that is the leading edge in the transport direction is the paper leading edge SL, and the part that is the trailing edge in the transport direction is the paper trailing edge ST. At this time, the distance from the front end SL of the paper to the rear end ST of the paper, that is, the length in the transport direction of the paper S is called a paper length Ls.

  In this embodiment, the photosensitive drum diameter Dp is smaller than the transfer drum diameter Dd and larger than the transfer roll diameter Dr. Therefore, the transfer drum diameter Dd is larger than the transfer roll diameter Dr. The transfer drum diameter Dd in the present embodiment is an integral multiple of the photosensitive drum diameter Dp (four times in this example).

  Further, the elastic layer length Le in the transfer drum 21 is larger than the transfer roll circumferential length Lr. On the other hand, the gap length Lg in the transfer drum 21 is larger than the transfer roll diameter Dr.

In this embodiment, the transfer drum diameter Dd is set to 120 mm, and the transfer drum circumferential length Ld is about 370 mm. The elastic layer length Le in the transfer drum 21 is set so as to be longer than the longitudinal length (355.6 mm) of the legal size paper S.
Furthermore, in this embodiment, the transfer roll diameter Dr is set to 10 mm, and the transfer roll circumferential length Lr is about 31.4 mm.
In this embodiment, the photosensitive drum diameter Dp is set to 30 mm, and the photosensitive drum circumferential length Lp is about 94.2 mm.

  Furthermore, the distance from the rotation shaft 21a of the transfer drum 21 to the outer peripheral surface of the elastic layer 21B and the distance from the rotation shaft 21a of the transfer drum 21 to the outer peripheral surface of the transfer roll 31 via the rotation shaft 31a of the transfer roll 31 are as follows. , Are set to be equal. For this reason, in the transfer roll 31 attached to the transfer drum 21, the outer end of the outer peripheral surface of the transfer roll 31 overlaps the virtual outer peripheral surface of the elastic layer 21 </ b> B of the transfer drum 21.

Next, an image forming operation in the image forming apparatus shown in FIG. 1 will be described.
In the image forming apparatus according to the present embodiment, a multicolor mode in which an image is formed on one sheet S using toner images of two to four colors of yellow, magenta, cyan, and black, and yellow, magenta, and cyan. In addition, a monochrome mode in which an image is formed on one sheet S using a toner image of one color of black is selectively executed. In the monochromatic mode, the image quality priority mode gives priority to the image quality of the image formed on the paper S over the number of sheets S on which image formation is performed per unit time, or the image quality of the image formed on the paper S. Also, the productivity priority mode that prioritizes the number of sheets S on which image formation is performed per unit time is selectively executed. Therefore, the image forming apparatus according to the present embodiment operates in any one of “multicolor mode”, “monochrome image quality priority mode”, and “monochrome productivity priority mode”. Here, the “monochromatic image quality priority mode” corresponds to the first monochrome mode, and the “monochromatic productivity priority mode” corresponds to the second monochrome mode. Among these, the “monochromatic image quality priority mode” can be selected when image formation is performed on a sheet S in which an image recording area is predetermined, such as a slip sheet on which a postcard or form is recorded in advance. Hereinafter, an image forming operation using the image forming apparatus shown in FIG. 1 will be described for each mode described above.

(Multicolor mode)
FIG. 3 shows a multicolor mode in which the image forming apparatus shown in FIG. 1 is used to form a multicolor image (full color image in this example) including four colors of yellow, magenta, cyan, and black on one sheet S. An example of a timing chart is shown. Here, FIG. 3 shows the passage of time, the driving of the photosensitive drum 11 (ON / OFF), the developing device disposed at the developing position, the position of the gate 54 (first / second), and the transfer drum. 21 (ON / OFF), application of transfer bias to the transfer drum 21 (ON / OFF), the state of the front end gripper 22 (open / closed), the state of the rear end gripper 23 (open / closed), Driving the transfer roll 31 (ON / OFF), applying a transfer bias to the transfer roll 31 (ON / OFF), an image on the photosensitive drum 11 passing through the transfer position Tr (transfer position passing image), and a transfer position The relationship with the sheet S (transfer position passing sheet) passing through Tr is shown. Here, the first position of the gate 54 is a position where the first paper feed path 61 is opened and the second paper feed path 62 is closed as the gate 54 rotates counterclockwise in FIG. The second position of the gate 54 is a position where the first paper feed path 61 is closed and the second paper feed path 62 is opened as the gate 54 rotates in the clockwise direction in FIG.

  In FIG. 3, “Y”, “M”, “C”, and “K” correspond to yellow, magenta, cyan, and black, respectively, and “S1” is the first sheet supplied first. The sheet S and “S2” indicate the second sheet S supplied after the first sheet S, respectively. “1st”, “2nd”, “3rd”, and “4th” indicate the number of times that the same sheet S passes the transfer position Tr. Therefore, for example, “M (S1)” means a state in which the magenta image formed on the photosensitive drum 11 passes through the transfer position Tr in order to transfer it to the first sheet S. Further, for example, “S1 (2nd)” means a state in which the first sheet S that has once passed the transfer position Tr has passed the transfer position Tr again (second time).

  In the initial state before the image forming operation in the multicolor mode is started, the driving of the photosensitive drum 11, the transfer drum 21, and the transfer roll 31 is all OFF (stopped state). In the initial state, it is assumed that the bias applied to the transfer drum 21 and the bias applied to the transfer roll 31 are both set to OFF. Further, in the initial state, as shown in FIG. 1, the black developing device 14K is disposed at the development position, and the gate 54 is a position where both the first paper feed path 61 and the second paper feed path 62 are opened ( It is assumed that it is arranged at the standby position). Furthermore, in the initial state, both the front end gripper 22 and the rear end gripper 23 are in an open state. In the initial state, as shown in FIG. 1, the transfer drum 21 is stopped so that the transfer roll 31 is positioned at the transfer position Tr.

  Note that these preconditions are the same not only in the “multicolor mode” described below, but also in the “monochrome image quality priority mode” (see FIG. 4) and the “monochrome productivity priority mode” (see FIG. 5) described later. It is. Therefore, in the “monochrome image quality priority mode” and “monochrome productivity priority mode” to be described later, description of these preconditions is omitted.

  With the start of the image forming operation in the multicolor mode, the control unit 100 switches the driving of the photosensitive drum 11 and the transfer drum 21 from OFF to ON, thereby rotating each of them at a predetermined peripheral speed. At this time, the photosensitive drum 11 and the transfer drum 21 rotate so as to be in the same direction at the transfer position Tr while being in contact with each other. Here, the peripheral speeds of the photosensitive drum 11 and the transfer drum 21 are set so that the peripheral speeds are constant or within a range of 1%. At this time, the control unit 100 keeps the drive of the transfer roll 31 OFF. The control unit 100 moves the gate 54 from the standby position to the first position with the start of the image forming operation in the multicolor mode. As a result, the supply path 60 and the first paper feed path 61 are coupled, while the supply path 60 and the second paper feed path 62 are separated.

  Next, the control unit 100 rotates the rotary developing device 14 to place the yellow developing device 14Y at the developing position. Then, the control unit 100 drives the charging device 12, the exposure device 13, and the developing device at the developing position (here, the yellow developing device 14Y). Accordingly, the photosensitive layer of the rotating photosensitive drum 11 is charged by the charging device 12 and then exposed by the exposure device 13, whereby an electrostatic latent image is formed on the photosensitive drum 11. Subsequently, the electrostatic latent image formed on the photosensitive drum 11 is developed by the yellow developing unit 14Y, and a yellow toner image corresponding to the electrostatic latent image is formed on the photosensitive drum 11. Thereafter, the yellow toner image formed on the photosensitive drum 11 moves toward the transfer position Tr as the photosensitive drum 11 further rotates.

  On the other hand, in response to the start of the image forming operation in the multi-color mode, the control unit 100 causes the paper supply unit 50 to supply the first sheet S1. In other words, the control unit 100 causes the first sheet S1 taken out from the paper storage unit 51 using the take-out roll 52 to enter the supply path 60, and blocks the paper leading end SL side by the resist roll 53 that has stopped rotating. To temporarily stop. When the leading edge gripper 22 attached to the rotating transfer drum 21 reaches the first paper feeding position P1, the control unit 100 moves the paper leading edge SL of the first sheet S1 to the first paper feeding position P1. The rotation of the resist roll 53 is started so as to arrive. Accordingly, the first sheet S1 reaches the first sheet feeding position P1 from the supply path 60 via the first sheet feeding path 61. Then, the control unit 100 shifts the leading edge gripper 22 from the open state to the closed state as the leading edge SL of the first sheet S1 reaches the first sheet feeding position P1. As a result, the leading edge SL of the first sheet S1 is mechanically held by the transfer drum 21. At this time, the sheet leading end SL side of the first sheet S1 is conveyed along the rotation path 63 while being wound around the elastic layer 21B of the transfer drum 21, and the sheet trailing end ST side of the first sheet S1 is conveyed. Are transported along the first paper feed path 61 and the supply path 60.

  Next, the leading end SL of the first sheet S1 held by the transfer drum 21 reaches the transfer position Tr (first time). At this time, the controller 100 forms the yellow toner image formation area on the photosensitive drum 11 as the sheet leading end SL of the first sheet S1 held by the transfer drum 21 reaches the transfer position Tr. The exposure apparatus 13 is controlled based on the phase signal from the phase sensor 25 so that the leading edge in the moving direction reaches the transfer position Tr. Then, the control unit 100 switches the transfer bias applied to the transfer drum 21 from OFF to ON as the leading edge SL of the first sheet S1 reaches the transfer position Tr. As a result, the transfer of the yellow toner image to the first sheet S1 is started at the transfer position Tr.

  In this example, after the leading edge SL of the first sheet S1 reaches the transfer position Tr, the trailing edge ST of the first sheet S1 reaches the first feeding position P1. Then, the control unit 100 shifts the trailing edge gripper 23 from the open state to the closed state as the sheet trailing edge ST of the first sheet S1 reaches the first sheet feeding position P1. As a result, the sheet trailing edge ST of the first sheet S1 is mechanically held by the transfer drum 21. Accordingly, the first sheet S1 is gripped by the leading edge gripper 22 and the trailing edge ST of the sheet S1 by the trailing edge gripper 23. As a result, the entire first sheet S1 is conveyed along the rotation path 63 while being wound around the elastic layer 21B of the transfer drum 21.

  Furthermore, in this example, after the trailing edge ST of the first sheet S1 reaches the first sheet feeding position P1, the yellow toner image on the photosensitive drum 11 corresponding to the first sheet S1 is changed. Development ends. Subsequently, the sheet trailing edge ST of the first sheet S1 held by the transfer drum 21 passes the transfer position Tr. Then, the control unit 100 switches the transfer bias applied to the transfer drum 21 from ON to OFF in accordance with the trailing edge ST of the first sheet S1 reaching the transfer position Tr. As a result, the transfer of the yellow toner image to the first sheet S1 is completed at the transfer position Tr. In this example, as the transfer of the yellow toner image to the first sheet S1 is completed, the rotary developing device 14 is driven to switch the developing device disposed at the developing position (yellow developing device 14Y). To magenta developing device 14M). Then, the control unit 100 starts forming a magenta toner image on the photosensitive drum 11.

  Here, during the period when the transfer bias is applied to the transfer drum 21, the yellow image (Y (S 1)) for the first sheet S 1 formed on the photosensitive drum 11 passes through the transfer position Tr. In addition, the first sheet S1 passes the first time (S1 (1st)).

  As the transfer drum 21 rotates, the exposed portion 21C of the transfer drum 21 passes through the transfer position Tr, and is gripped by the transfer drum 21, whereby the leading edge SL of the first sheet S1 that moves in the rotation path 63. Reaches the transfer position Tr (second time). At this time, the controller 100 forms a magenta toner image formation area on the photosensitive drum 11 as the leading edge SL of the first sheet S1 held by the transfer drum 21 reaches the transfer position Tr. The exposure device 13 and the rotary developing device 14 are controlled on the basis of the phase signal from the phase sensor 25 so that the leading edge in the moving direction reaches the transfer position Tr. Then, the control unit 100 switches the transfer bias applied to the transfer drum 21 from OFF to ON as the leading edge SL of the first sheet S1 reaches the transfer position Tr. As a result, at the transfer position Tr, transfer of the magenta toner image to the first sheet S1 is started.

  In this example, after the sheet trailing edge ST of the first sheet S1 reaches the first sheet feeding position P1, the magenta toner image on the photosensitive drum 11 corresponding to the first sheet S1 is displayed. Development ends. Subsequently, the sheet trailing edge ST of the first sheet S1 held by the transfer drum 21 passes the transfer position Tr. Then, the control unit 100 switches the transfer bias applied to the transfer drum 21 from ON to OFF in accordance with the trailing edge ST of the first sheet S1 reaching the transfer position Tr. Thereby, at the transfer position Tr, the transfer of the magenta toner image to the first sheet S1 is completed. In this example, as the transfer of the magenta toner image to the first sheet S1 is completed, the rotary developing device 14 is driven to switch the developing device disposed at the developing position (magenta developing device 14M). To Cyan developing device 14C). Then, the control unit 100 starts forming a cyan toner image on the photosensitive drum 11.

  Here, during the period in which the transfer bias is applied to the transfer drum 21, the magenta image (M (S1)) for the first sheet S1 formed on the photosensitive drum 11 passes through the transfer position Tr. In addition, the first sheet S1 passes the second time (S1 (2nd)).

  As the transfer drum 21 rotates, the exposed portion 21C of the transfer drum 21 passes through the transfer position Tr and is gripped by the transfer drum 21, so that the leading edge SL of the first sheet S1 that moves in the rotation path 63 is moved. The transfer position Tr is reached (third time). At this time, the control unit 100 forms a cyan toner image formation region on the photosensitive drum 11 as the leading edge SL of the first sheet S1 held by the transfer drum 21 reaches the transfer position Tr. The exposure device 13 and the rotary developing device 14 are controlled on the basis of the phase signal from the phase sensor 25 so that the leading edge in the moving direction reaches the transfer position Tr. Then, the control unit 100 switches the transfer bias applied to the transfer drum 21 from OFF to ON as the leading edge SL of the first sheet S1 reaches the transfer position Tr. As a result, the transfer of the cyan toner image to the first sheet S1 is started at the transfer position Tr.

  In this example, after the sheet trailing edge ST of the first sheet S1 reaches the first sheet feeding position P1, the cyan toner image on the photosensitive drum 11 corresponding to the first sheet S1 is displayed. Development ends. Subsequently, the sheet trailing edge ST of the first sheet S1 held by the transfer drum 21 passes the transfer position Tr. Then, the control unit 100 switches the transfer bias applied to the transfer drum 21 from ON to OFF in accordance with the trailing edge ST of the first sheet S1 reaching the transfer position Tr. Thus, the transfer of the cyan toner image to the first sheet S1 is completed at the transfer position Tr. In this example, as the transfer of the cyan toner image to the first sheet S1 is completed, the rotary developing device 14 is driven to switch the developing device disposed at the developing position (cyan developing device 14C). To a black developing device 14K). Then, the control unit 100 starts forming a black toner image on the photosensitive drum 11.

  Here, during the period when the transfer bias is applied to the transfer drum 21, the cyan image (C (S 1)) for the first sheet S 1 formed on the photosensitive drum 11 passes through the transfer position Tr. In addition, the first sheet S1 passes the third time (S1 (3rd)).

  As the transfer drum 21 rotates, the exposed portion 21C of the transfer drum 21 passes through the transfer position Tr, and is gripped by the transfer drum 21, whereby the leading edge SL of the first sheet S1 that moves in the rotation path 63. Reaches the transfer position Tr (fourth time). At this time, the control unit 100 forms a black toner image formation region on the photosensitive drum 11 as the sheet leading end SL of the first sheet S1 held by the transfer drum 21 reaches the transfer position Tr. The exposure device 13 and the rotary developing device 14 are controlled on the basis of the phase signal from the phase sensor 25 so that the leading edge in the moving direction reaches the transfer position Tr. Then, the control unit 100 switches the transfer bias applied to the transfer drum 21 from OFF to ON as the leading edge SL of the first sheet S1 reaches the transfer position Tr. Thereby, at the transfer position Tr, transfer of the black toner image to the first sheet S1 is started.

  In this example, the control unit 100 shifts the leading edge gripper 22 from the closed state to the opened state as the leading edge SL of the first sheet S reaches the transfer position Tr. As a result, the sheet leading end SL of the first sheet S1 passes through the transfer position Tr, then moves away from the rotation path 63 and moves toward the fixing unit 40 via the sheet discharge path 64. When the first sheet S enters the fixing unit 40, the full-color toner image superimposed and transferred onto the first sheet S1 is fixed on the first sheet S1. At this time, the first sheet S1 is conveyed in a state of being sandwiched between the photosensitive drum 11 and the elastic layer 21B of the transfer drum 21 at the transfer position Tr.

  In this example, after the trailing edge ST of the first sheet S1 reaches the first sheet feeding position P1, the black toner image on the photosensitive drum 11 corresponding to the first sheet S1 is displayed. Development ends. Subsequently, the transfer bias applied to the transfer drum 21 is switched from ON to OFF in accordance with the arrival of the sheet trailing edge ST of the first sheet S1 at the transfer position Tr. As a result, the transfer of the black toner image to the first sheet S1 is completed at the transfer position Tr. In this example, as the transfer of the black toner image to the first sheet S1 is completed, the rotary developing device 14 is driven to switch the developing device disposed at the developing position (black developing device 14K). To yellow developing device 14Y). Then, the control unit 100 starts to form the next yellow toner image on the photosensitive drum 11.

  Here, during the period in which the transfer bias is applied to the transfer drum 21, the black image (K (S1)) for the first sheet S1 formed on the photosensitive drum 11 passes through the transfer position Tr. In addition, the first sheet S1 passes the fourth time (S1 (4th)).

  On the other hand, in this example, the control unit 100 shifts the trailing edge gripper 23 from the closed state to the opened state in accordance with the timing at which the sheet trailing edge ST of the first sheet S1 reaches the transfer position Tr. As a result, the sheet rear end ST of the first sheet S1 passes through the transfer position Tr, then leaves the rotation path 63 and is conveyed to the fixing unit 40 via the sheet discharge path 64. When the first sheet S1 passes the transfer position Tr, the first sheet S1 does not receive the conveying force from the transfer drum 21 side. At this time, the first sheet S1 Since the leading edge SL has already passed through the fixing unit 40, the first sheet S1 is conveyed by the conveying force received from the fixing unit 40.

  In this example, while the first sheet S1 is passing the transfer position Tr in order to transfer the black toner image, the control unit 100 performs the second sheet S2 from the sheet supply unit 50. Start supplying. In other words, the control unit 100 causes the second sheet S2 taken out from the sheet storage unit 51 using the take-out roll 52 to enter the supply path 60, and stops the paper leading end SL side by the resist roll 53 that has stopped rotating. To temporarily stop. When the leading edge gripper 22 attached to the rotating transfer drum 21 reaches the first paper feeding position P1, the control unit 100 moves the paper leading edge SL of the second sheet S2 to the first paper feeding position P1. The rotation of the resist roll 53 is started so as to arrive. Accordingly, the second sheet S2 reaches the first sheet feeding position P1 from the supply path 60 via the first sheet feeding path 61. Then, the control unit 100 shifts the leading edge gripper 22 from the open state to the closed state as the leading edge SL of the second sheet S2 reaches the first sheet feeding position P1. As a result, the leading edge SL of the second sheet S2 is mechanically gripped by the transfer drum 21. At this time, the sheet leading end SL side of the second sheet S2 is conveyed along the rotation path 63 while being wound around the elastic layer 21B of the transfer drum 21, and the sheet trailing end ST side of the second sheet S2 is conveyed. Are transported along the first paper feed path 61 and the supply path 60.

  The sheet trailing edge ST of the first sheet S1 passes through the transfer position Tr. Subsequently, the exposed portion 21C of the transfer drum 21 passes through the transfer position Tr, and then the second sheet gripped by the transfer drum 21. The leading edge SL of the sheet S2 reaches the transfer position Tr (first time). At this time, the control unit 100 forms the yellow toner image formation area on the photosensitive drum 11 as the sheet leading end SL of the second sheet S2 held by the transfer drum 21 reaches the transfer position Tr. The exposure apparatus 13 is controlled based on the phase signal from the phase sensor 25 so that the leading edge in the moving direction reaches the transfer position Tr. Then, the control unit 100 switches the transfer bias applied to the transfer drum 21 from OFF to ON as the leading edge SL of the second sheet S2 reaches the transfer position Tr. Thereby, at the transfer position Tr, transfer of the yellow toner image onto the second sheet S2 is started.

  Further, in this example, after the leading edge SL of the second sheet S2 reaches the transfer position Tr, the trailing edge ST of the second sheet S2 reaches the first sheet feeding position P1. Then, the control unit 100 shifts the trailing edge gripper 23 from the open state to the closed state in accordance with the trailing edge ST of the second sheet S2 reaching the first sheet feeding position P1. As a result, the trailing edge ST of the second sheet S2 is mechanically held by the transfer drum 21. Accordingly, the second sheet S2 is gripped by the leading edge gripper 22 and the trailing edge ST of the sheet S2 by the trailing edge gripper 23. As a result, the entire second sheet S2 is conveyed along the rotation path 63 while being wound around the elastic layer 21B of the transfer drum 21.

  Further, in this example, after the trailing edge ST of the second sheet S2 reaches the first sheet feeding position P1, the yellow toner image on the photosensitive drum 11 corresponding to the second sheet S2 is detected. Development ends. Following this, the sheet trailing edge ST of the second sheet S2 held by the transfer drum 21 passes the transfer position Tr. Then, the control unit 100 switches the transfer bias applied to the transfer drum 21 from ON to OFF in accordance with the trailing edge ST of the second sheet S2 reaching the transfer position Tr. As a result, the transfer of the yellow toner image to the second sheet S2 is completed at the transfer position Tr. In this example, as the transfer of the yellow toner image to the second sheet S2 is completed, the rotary developing device 14 is driven to switch the developing device disposed at the developing position (yellow developing device 14Y). To magenta developing device 14M). Then, the control unit 100 starts forming a magenta toner image on the photosensitive drum 11.

  Here, during the period when the transfer bias is applied to the transfer drum 21, the yellow image (Y (S2)) for the second sheet S2 formed on the photosensitive drum 11 passes through the transfer position Tr. In addition, the second sheet S2 passes the first time (S2 (1st)).

Thereafter, in the same procedure as that for the first sheet S1, the transfer operation of the magenta, cyan, and black toner images to the second sheet S2, and the full-color toner image are overlaid and transferred. The fixing operation for the second sheet S2 is sequentially executed.
In the multicolor mode, the transfer roll 31 is not driven, and no transfer bias is applied to the transfer roll 31.

(Monocolor priority mode)
FIG. 4 shows an example of a timing chart in the monochromatic image quality priority mode in which the image forming apparatus shown in FIG. 1 is used to form a monochromatic image (monochrome image in this example) consisting of one black color on one sheet S. ing.

  Along with the start of the image forming operation in the monochromatic image quality priority mode, the control unit 100 switches the driving of the photosensitive drum 11 and the transfer drum 21 from OFF to ON, thereby rotating each at a determined peripheral speed. At this time, the photosensitive drum 11 and the transfer drum 21 rotate so as to be in the same direction at the transfer position Tr while being in contact with each other. The relationship between the peripheral speeds at this time is set to the same conditions as in the above-described multicolor mode. At this time, the control unit 100 keeps the driving of the transfer roll 31 OFF as in the above-described multicolor mode. In addition, the control unit 100 moves the gate 54 from the standby position to the first position with the start of the image forming operation in the monochrome image quality priority mode. As a result, as in the multi-color mode, the supply path 60 and the first paper feed path 61 are coupled, while the supply path 60 and the second paper feed path 62 are separated.

  Further, when a black monochrome image is formed in the monochrome image quality priority mode, the control unit 100 does not rotate the rotary developing device 14 in which the black developing device 14K is stopped at the developing position, and the charging device 12 and the exposure device. 13 and the developing device in the developing position (here, black developing device 14K) are driven. Accordingly, the photosensitive layer of the rotating photosensitive drum 11 is charged by the charging device 12 and then exposed by the exposure device 13, whereby an electrostatic latent image is formed on the photosensitive drum 11. Subsequently, the electrostatic latent image formed on the photosensitive drum 11 is developed by the black developing device 14 </ b> K, and a black toner image corresponding to the electrostatic latent image is formed on the photosensitive drum 11. Thereafter, the black toner image formed on the photosensitive drum 11 moves toward the transfer position Tr as the photosensitive drum 11 further rotates.

  On the other hand, in response to the start of the image forming operation in the monochrome image quality priority mode, the control unit 100 causes the paper supply unit 50 to supply the first sheet S1. In other words, the control unit 100 causes the first sheet S1 taken out from the paper storage unit 51 using the take-out roll 52 to enter the supply path 60, and blocks the paper leading end SL side by the resist roll 53 that has stopped rotating. To temporarily stop. When the leading edge gripper 22 attached to the rotating transfer drum 21 reaches the first paper feeding position P1, the control unit 100 moves the paper leading edge SL of the first sheet S1 to the first paper feeding position P1. The rotation of the resist roll 53 is started so as to arrive. Accordingly, the first sheet S1 reaches the first sheet feeding position P1 from the supply path 60 via the first sheet feeding path 61. Then, the control unit 100 shifts the leading edge gripper 22 from the open state to the closed state as the leading edge SL of the first sheet S1 reaches the first sheet feeding position P1. As a result, the leading edge SL of the first sheet S1 is mechanically held by the transfer drum 21. At this time, the sheet leading end SL side of the first sheet S1 is conveyed along the rotation path 63 while being wound around the elastic layer 21B of the transfer drum 21, and the sheet trailing end ST side of the first sheet S1 is conveyed. Are transported along the first paper feed path 61 and the supply path 60.

  Next, the leading end SL of the first sheet S1 held by the transfer drum 21 reaches the transfer position Tr (first time). At this time, the control unit 100 forms a black toner image formation region on the photosensitive drum 11 as the sheet leading end SL of the first sheet S1 held by the transfer drum 21 reaches the transfer position Tr. The exposure apparatus 13 is controlled based on the phase signal from the phase sensor 25 so that the leading edge in the moving direction reaches the transfer position Tr. Then, the control unit 100 switches the transfer bias applied to the transfer drum 21 from OFF to ON as the leading edge SL of the first sheet S1 reaches the transfer position Tr. Thereby, at the transfer position Tr, transfer of the black toner image to the first sheet S1 is started.

  In this example, the control unit 100 shifts the leading edge gripper 22 from the closed state to the opened state as the leading edge SL of the first sheet S1 reaches the transfer position Tr. As a result, the sheet leading end SL of the first sheet S1 passes through the transfer position Tr, then moves away from the rotation path 63 and moves toward the fixing unit 40 via the sheet discharge path 64. When the first sheet S1 enters the fixing unit 40, the monochrome toner image transferred to the first sheet S1 is fixed on the first sheet S1. At this time, the first sheet S1 is conveyed in a state of being sandwiched between the photosensitive drum 11 and the elastic layer 21B of the transfer drum 21 at the transfer position Tr.

  Further, in this example, after the leading edge SL of the first sheet S1 reaches the transfer position Tr, the trailing edge ST of the first sheet S1 reaches the first feeding position P1. Then, the control unit 100 shifts the trailing edge gripper 23 from the open state to the closed state as the sheet trailing edge ST of the first sheet S1 reaches the first sheet feeding position P1. As a result, the sheet trailing edge ST of the first sheet S1 is mechanically held by the transfer drum 21. Accordingly, the first sheet S1 is sandwiched between the photosensitive drum 11 and the transfer drum 21 at an intermediate portion between the sheet leading edge SL and the sheet trailing edge ST, and the sheet trailing edge ST is the trailing edge. It is gripped by the gripper 23. As a result, the first sheet S1 is conveyed along the sheet discharge path 64 on the sheet leading end SL side, and along the rotation path 63 on the sheet trailing end ST side.

  In this example, after the trailing edge ST of the first sheet S1 reaches the first sheet feeding position P1, the black toner image on the photosensitive drum 11 corresponding to the first sheet S1 is displayed. Development ends. Following this, the control unit 100 switches the transfer bias applied to the transfer drum 21 from ON to OFF as the sheet trailing edge ST of the first sheet S1 reaches the transfer position Tr. As a result, the transfer of the black toner image to the first sheet S1 is completed at the transfer position Tr. In the single color image quality priority mode, since the toner image of the same color (here, black) is repeatedly formed, the control unit 100 keeps the black developing device 14K stopped at the development position. Then, the control unit 100 starts to form the next black toner image on the photosensitive drum 11.

  On the other hand, in this example, the control unit 100 shifts the trailing edge gripper 23 from the closed state to the opened state in accordance with the timing at which the sheet trailing edge ST of the first sheet S1 reaches the transfer position Tr. As a result, the sheet rear end ST of the first sheet S1 passes through the transfer position Tr, then leaves the rotation path 63 and is conveyed to the fixing unit 40 via the sheet discharge path 64. When the first sheet S1 passes the transfer position Tr, the first sheet S1 does not receive the conveying force from the transfer drum 21 side. At this time, the first sheet S1 Since the leading edge SL has already passed through the fixing unit 40, the first sheet S1 is conveyed by the conveying force received from the fixing unit 40.

  Here, during the period in which the transfer bias is applied to the transfer drum 21, the black image (K (S1)) for the first sheet S1 formed on the photosensitive drum 11 passes through the transfer position Tr. In addition, the first sheet S1 passes the first time (S1 (1st)).

  In this example, while the first sheet S1 is passing the transfer position Tr in order to transfer the black toner image, the control unit 100 performs the second sheet S2 from the sheet supply unit 50. Start supplying. In other words, the control unit 100 causes the second sheet S2 taken out from the sheet storage unit 51 using the take-out roll 52 to enter the supply path 60, and stops the paper leading end SL side by the resist roll 53 that has stopped rotating. To temporarily stop. When the leading edge gripper 22 attached to the rotating transfer drum 21 reaches the first paper feeding position P1, the control unit 100 moves the paper leading edge SL of the second sheet S2 to the first paper feeding position P1. The rotation of the resist roll 53 is started so as to arrive. Accordingly, the second sheet S2 reaches the first sheet feeding position P1 from the supply path 60 via the first sheet feeding path 61. Then, the control unit 100 shifts the leading edge gripper 22 from the open state to the closed state as the leading edge SL of the second sheet S2 reaches the first sheet feeding position P1. As a result, the leading edge SL of the second sheet S2 is mechanically gripped by the transfer drum 21. At this time, the sheet leading end SL side of the second sheet S2 is conveyed along the rotation path 63 while being wound around the elastic layer 21B of the transfer drum 21, and the sheet trailing end ST side of the second sheet S2 is conveyed. Are transported along the first paper feed path 61 and the supply path 60.

  The sheet trailing edge ST of the first sheet S1 passes through the transfer position Tr. Subsequently, the exposed portion 21C of the transfer drum 21 passes through the transfer position Tr, and then the second sheet gripped by the transfer drum 21. The leading edge SL of the sheet S2 reaches the transfer position Tr (first time). At this time, the control unit 100 forms a black toner image formation region on the photosensitive drum 11 as the sheet leading end SL of the second sheet S2 held by the transfer drum 21 reaches the transfer position Tr. The exposure apparatus 13 is controlled based on the phase signal from the phase sensor 25 so that the leading edge in the moving direction reaches the transfer position Tr. Then, the control unit 100 switches the transfer bias applied to the transfer drum 21 from OFF to ON as the leading edge SL of the second sheet S2 reaches the transfer position Tr. Thereby, transfer of the black toner image to the second sheet S2 is started at the transfer position Tr.

  In this example, the control unit 100 shifts the front end gripper 22 from the closed state to the open state as the front end SL of the second sheet S2 reaches the transfer position Tr. As a result, the sheet leading edge SL of the second sheet S2 passes through the transfer position Tr, then moves away from the rotation path 63 and moves toward the fixing unit 40 via the sheet discharge path 64. When the second sheet S2 enters the fixing unit 40, the monochrome toner image transferred to the second sheet S2 is fixed on the second sheet S2. At this time, the second sheet S2 is conveyed in a state of being sandwiched between the photosensitive drum 11 and the elastic layer 21B of the transfer drum 21 at the transfer position Tr.

  Further, in this example, after the leading edge SL of the second sheet S2 reaches the transfer position Tr, the trailing edge ST of the second sheet S2 reaches the first sheet feeding position P1. Then, the control unit 100 shifts the trailing edge gripper 23 from the open state to the closed state in accordance with the trailing edge ST of the second sheet S2 reaching the first sheet feeding position P1. As a result, the trailing edge ST of the second sheet S2 is mechanically held by the transfer drum 21. Accordingly, the second sheet S2 has the intermediate portion between the sheet leading edge SL and the sheet trailing edge ST sandwiched between the photosensitive drum 11 and the transfer drum 21, and the sheet trailing edge ST is the trailing edge. It is gripped by the gripper 23. As a result, the sheet leading edge SL side of the second sheet S2 is conveyed along the sheet discharge path 64, and the sheet trailing end ST side is conveyed along the rotation path 63.

  In this example, after the sheet trailing edge ST of the second sheet S2 reaches the first sheet feeding position P1, the black toner image on the photosensitive drum 11 corresponding to the second sheet S2 is formed. Development ends. Following this, the control unit 100 switches the transfer bias to be applied to the transfer drum 21 from ON to OFF as the trailing edge ST of the second sheet S2 reaches the transfer position Tr. Thereby, at the transfer position Tr, the transfer of the black toner image to the second sheet S2 is finished. In the single color image quality priority mode, since the toner image of the same color (here, black) is repeatedly formed, the control unit 100 keeps the black developing device 14K stopped at the development position. Then, the control unit 100 starts to form the next black toner image on the photosensitive drum 11.

  On the other hand, in this example, the control unit 100 shifts the rear end gripper 23 from the closed state to the open state in accordance with the timing when the rear end ST of the second sheet S2 reaches the transfer position Tr. As a result, the sheet trailing edge ST of the second sheet S2 passes through the transfer position Tr, then leaves the rotation path 63, and is conveyed to the fixing unit 40 via the sheet discharge path 64. When the second sheet S2 passes through the transfer position Tr, the second sheet S2 does not receive the conveying force from the transfer drum 21 side, but at this time, the second sheet S2 Since the leading edge SL has already passed through the fixing unit 40, the second sheet S2 is conveyed by the conveying force received from the fixing unit 40.

  Here, during the period when the transfer bias is applied to the transfer drum 21, the transfer position Tr is set to be the black image (K (S 2)) for the second sheet S 2 formed on the photosensitive drum 11. The second sheet S2 passes the Tr (S2 (1st)).

Thereafter, the black toner for the third sheet S3, the fourth sheet S4, and the fifth sheet S5 is processed in the same procedure as the first sheet S1 and the second sheet S2. The image transfer operation and the fixing operation for the third sheet S3, the fourth sheet S4, and the fifth sheet S5 onto which the black toner image has been transferred are sequentially executed.
In the monochromatic image quality priority mode, similarly to the above-described multicolor mode, the transfer roll 31 is not driven, and no transfer bias is applied to the transfer roll 31.

(Monochromatic productivity priority mode)
FIG. 5 shows an example of a timing chart in the monochromatic productivity priority mode in which the image forming apparatus shown in FIG. 1 is used to form a single color image (monochrome image in this example) consisting of one black color on one sheet S. Show.

  Along with the start of the image forming operation in the monochromatic productivity priority mode, the control unit 100 switches the driving of the photosensitive drum 11 and the transfer roll 31 from OFF to ON, thereby rotating each at a determined peripheral speed. At this time, in the initial state, since the photosensitive drum 11 and the transfer roll 31 are disposed at positions facing each other (see FIG. 1), the photosensitive drum 11 and the transfer roll 31 are in contact with each other and are transferred to the transfer position Tr. To rotate in the same direction. Here, the peripheral speeds of the photosensitive drum 11 and the transfer roll 31 are set so that the peripheral speeds are constant or within a range of 1%. At this time, the control unit 100 keeps driving of the transfer drum 21 OFF. Further, the control unit 100 moves the gate 54 from the standby position to the second position with the start of the image forming operation in the monochrome productivity priority mode. As a result, unlike the above-described multicolor mode and single color image quality priority mode, the supply path 60 and the second paper feed path 62 are combined, while the supply path 60 and the first paper feed path 61 are separated.

  In addition, when forming a black monochrome image in the monochrome productivity priority mode, the control unit 100 sets the rotary developing device 14 in which the black developing device 14K is stopped at the development position, as in the above-described monochrome image quality priority mode. Without rotating, the charging device 12, the exposure device 13, and the developing device at the developing position (here, the black developing device 14K) are driven. Accordingly, the photosensitive layer of the rotating photosensitive drum 11 is charged by the charging device 12 and then exposed by the exposure device 13, whereby an electrostatic latent image is formed on the photosensitive drum 11. Subsequently, the electrostatic latent image formed on the photosensitive drum 11 is developed by the black developing device 14 </ b> K, and a black toner image corresponding to the electrostatic latent image is formed on the photosensitive drum 11. Thereafter, the black toner image formed on the photosensitive drum 11 moves toward the transfer position Tr as the photosensitive drum 11 further rotates.

  On the other hand, in response to the start of the image forming operation in the monochromatic productivity priority mode, the control unit 100 causes the paper supply unit 50 to supply the first paper S1. In other words, the control unit 100 causes the first sheet S1 taken out from the paper storage unit 51 using the take-out roll 52 to enter the supply path 60, and blocks the paper leading end SL side by the resist roll 53 that has stopped rotating. To temporarily stop. Then, the control unit 100 adjusts the leading end SL of the first sheet S1 to the second position as the leading end in the moving direction of the black toner image formation region on the photosensitive drum 11 reaches the transfer position Tr. The rotation of the registration roll 53 is started so as to reach the paper feed position P2, that is, the transfer position Tr. Accordingly, the first sheet S1 is conveyed from the supply path 60 to the second sheet feed position P2, that is, the transfer position Tr via the second sheet feed path 62.

  Next, the leading edge SL of the first sheet S1 supplied via the second sheet feeding path 62 reaches the transfer position Tr (first time). Then, the control unit 100 switches the transfer bias applied to the transfer roll 31 from OFF to ON as the leading edge SL of the first sheet S1 reaches the transfer position Tr. Thereby, at the transfer position Tr, transfer of the black toner image to the first sheet S1 is started.

  Further, the sheet leading edge SL of the first sheet S1 moves toward the fixing unit 40 via the sheet discharge path 64 after passing through the transfer position Tr. When the first sheet S1 enters the fixing unit 40, the monochrome toner image transferred to the first sheet S1 is fixed on the first sheet S1. At this time, the first sheet S1 is transported while being sandwiched between the photosensitive drum 11 and the transfer roll 31 at the transfer position Tr.

  Further, in this example, after the leading edge SL of the first sheet S1 reaches the transfer position Tr, the first sheet S1 before the trailing edge ST of the first sheet S1 reaches the transfer position Tr. The development of the black toner image on the photosensitive drum 11 corresponding to the sheet S1 is completed. Following this, the control unit 100 switches the transfer bias applied to the transfer roll 31 from ON to OFF as the trailing edge ST of the first sheet S1 reaches the transfer position Tr. Thereby, at the transfer position Tr, the transfer of the black toner image onto the first sheet S1 is completed. In the single color productivity priority mode, the toner image of the same color (here, black) is repeatedly formed, so the control unit 100 keeps the black developing device 14K stopped at the development position. . Then, the control unit 100 starts forming the next black toner image on the photosensitive drum 11 at an interval shorter than the above-described single color image quality priority mode.

  On the other hand, the sheet rear end ST of the first sheet S1 that has passed the transfer position Tr is transported to the fixing unit 40 via the sheet discharge path 64 after passing the transfer position Tr. When the first sheet S1 passes the transfer position Tr, the first sheet S1 does not receive the conveying force from the transfer roll 31 side, but at this time, the first sheet S1 Since the leading edge SL has already passed through the fixing unit 40, the first sheet S1 is conveyed by the conveying force received from the fixing unit 40.

  Here, during the period when the transfer bias is applied to the transfer roll 31, the black image (K (S1)) for the first sheet S1 formed on the photosensitive drum 11 passes through the transfer position Tr. In addition, the first sheet S1 passes the first time (S1 (1st)).

  Further, in this example, after the first sheet S1 is supplied to the transfer position Tr in order to transfer the black toner image, the control unit 100 performs an interval shorter than the above-described single color image quality priority mode. The supply of the second sheet S2 from the sheet supply unit 50 is started. In other words, the control unit 100 causes the second sheet S2 taken out from the sheet storage unit 51 using the take-out roll 52 to enter the supply path 60, and stops the paper leading end SL side by the resist roll 53 that has stopped rotating. To temporarily stop. Then, the control unit 100 adjusts the leading edge SL of the second sheet S2 to the second position as the leading end in the moving direction of the black toner image formation region on the photosensitive drum 11 reaches the transfer position Tr. The rotation of the registration roll 53 is started so as to reach the paper feed position P2, that is, the transfer position Tr. Accordingly, the second sheet S2 is conveyed from the supply path 60 to the second sheet feed position P2, that is, the transfer position Tr via the second sheet feed path 62.

  Next, the leading edge SL of the second sheet S2 supplied via the second sheet feeding path 62 reaches the transfer position Tr (first time). Then, the control unit 100 switches the transfer bias to be applied to the transfer roll 31 from OFF to ON as the leading edge SL of the second sheet S2 reaches the transfer position Tr. Thereby, transfer of the black toner image to the second sheet S2 is started at the transfer position Tr.

  Further, the sheet leading end SL of the second sheet S2 moves toward the fixing unit 40 via the sheet discharge path 64 after passing through the transfer position Tr. When the first sheet S1 enters the fixing unit 40, the monochrome toner image transferred to the second sheet S2 is fixed on the second sheet S2. At this time, the second sheet S2 is conveyed while being sandwiched between the photosensitive drum 11 and the transfer roll 31 at the transfer position Tr.

  Further, in this example, after the leading edge SL of the second sheet S2 reaches the transfer position Tr, the second sheet S2 before the trailing edge ST of the second sheet S2 reaches the transfer position Tr. The development of the black toner image on the photosensitive drum 11 corresponding to the sheet S2 is completed. Following this, the control unit 100 switches the transfer bias applied to the transfer roll 31 from ON to OFF as the trailing edge ST of the second sheet S2 reaches the transfer position Tr. Thereby, at the transfer position Tr, the transfer of the black toner image to the second sheet S2 is finished. In the single color productivity priority mode, the toner image of the same color (here, black) is repeatedly formed, so the control unit 100 keeps the black developing device 14K stopped at the development position. . Then, the control unit 100 starts to form the next black toner image on the photosensitive drum 11.

  On the other hand, the sheet trailing edge ST of the second sheet S2 that has passed the transfer position Tr is transported to the fixing unit 40 via the sheet discharge path 64 after passing the transfer position Tr. When the second sheet S2 passes through the transfer position Tr, the second sheet S2 is not subjected to the conveying force from the transfer roll 31 side. The sheet leading end SL has already passed through the fixing unit 40, and the second sheet S2 is conveyed by the conveying force received from the fixing unit 40.

  Here, during the period in which the transfer bias is applied to the transfer roll 31, the black image (K (S2)) for the second sheet S2 formed on the photosensitive drum 11 passes through the transfer position Tr. In addition, the second sheet S2 passes the first time (S2 (1st)).

Thereafter, the third sheet S3, the fourth sheet S4, the fifth sheet S5, and the sixth sheet are processed in the same procedure as the first sheet S1 and the second sheet S2. The fixing operation for the sheet S6 and the seventh sheet S7 is sequentially executed.
In the monochromatic productivity priority mode, unlike the above-described multicolor mode and single color image quality priority mode, the transfer drum 21 is not driven and no transfer bias is applied to the transfer drum 21. In the monochromatic productivity priority mode, the front end gripper 22 and the rear end gripper 23 are maintained in the open state, and the transition to the closed state is not performed.

  6 passes through the transfer position Tr in each of the “multicolor mode” shown in FIG. 3, the “monochrome image quality priority mode” shown in FIG. 4, and the “monochrome productivity priority mode” shown in FIG. FIG. 5 is a diagram for explaining a relationship between components of a transfer unit and paper S. 6A shows the relationship in the multicolor mode, FIG. 6B shows the relationship in the monochrome image quality priority mode, and FIG. 6C shows the relationship in the monochrome productivity priority mode. 6A and 6B show the transfer drum 21 as a component of the transfer unit, and FIG. 6C shows the transfer roll 31 as a component of the transfer unit. FIGS. 6A to 6C illustrate the case where the same size paper S is used, and the paper length Ls is, for example, the length in the vertical direction (Ls = 297 mm) of JISA4 size. .

  First, in the multicolor mode shown in FIG. 6A, the transfer drum 21 is used, the leading edge gripper 22 (see FIG. 1) is used to grip the sheet leading edge SL of the sheet S, and the trailing edge gripper 23 (see FIG. 6). 1) is used to grip the trailing edge ST of the sheet S. For this reason, in the multicolor mode, the paper front end SL of the paper S is positioned at the elastic layer front end 21L of the elastic layer 21B provided on the transfer drum 21. At this time, the sheet trailing edge ST of the same sheet S is positioned on the elastic layer 21B, and the position of the trailing edge gripper 23 on the elastic layer 21B is changed according to the sheet length Ls of the sheet S. ing. As a result, in the multicolor mode, the sheet S is positioned on the elastic layer 21B excluding the exposed portion 21C on the outer peripheral surface of the transfer drum 21.

  Next, in the monochromatic image quality priority mode shown in FIG. 6B, similarly to the multi-color mode, the transfer drum 21 is used and the leading edge gripper 22 (see FIG. 1) is used to grip the leading edge SL of the sheet S. At the same time, the trailing edge ST of the sheet S is gripped using the trailing edge gripper 23 (see FIG. 1). For this reason, even in the monochromatic image quality priority mode, the paper tip SL of the paper S is positioned at the elastic layer tip 21L of the elastic layer 21B provided on the transfer drum 21. At this time, the sheet trailing edge ST of the same sheet S is positioned on the elastic layer 21B, and the position of the trailing edge gripper 23 on the elastic layer 21B is changed according to the sheet length Ls of the sheet S. ing. As a result, even in the monochromatic image quality priority mode, the sheet S is located on the elastic layer 21B excluding the exposed portion 21C on the outer peripheral surface of the transfer drum 21.

  Further, in the monochromatic productivity priority mode shown in FIG. 6C, unlike the above-described multicolor mode or single color image quality priority mode, the transfer roll 31 is used and the paper using the leading edge gripper 22 and the trailing edge gripper 23 is used. Do not hold S. For this reason, in the monochromatic productivity priority mode, it is not necessary to position the leading edge SL of the sheet S with respect to a specific portion of the transfer roll 31. Further, since the transfer roll 31 does not have a portion corresponding to the exposed portion 21 </ b> C of the transfer drum 21, the sheet leading end SL and the sheet trailing end ST of the same sheet S are always on the outer peripheral surface of the transfer roll 31. Will be located. As described above, in the single color productivity priority mode, unlike the above-described multicolor mode and single color image quality priority mode, the positions of the paper leading edge SL and the paper trailing edge ST of the paper S are limited due to the components of the transfer unit. I do not receive it.

  In the multicolor mode and the monochromatic image quality priority mode, the transfer drum 21 having the elastic layer 21B and the exposed portion 21C is used, and the exposed portion 21C of these cannot be used for transfer. The maximum value of the sheet length Ls of the sheet S that can be used for the forming operation is limited to be equal to or less than the elastic layer length Le of the elastic layer 21B. On the other hand, in the monochromatic productivity priority mode, since the transfer roll 31 having a circular cross section is used, the maximum value of the sheet length Ls of the sheet S that can be used for the image forming operation is subjected to such a restriction. Absent.

  Here, in the multi-color mode, since the sheet S is gripped using the leading edge gripper 22 and the trailing edge gripper 23, the photosensitive drum 11 (see FIG. 1) passing through the transfer position Tr (see FIG. 1). The positioning accuracy of the paper S with respect to the upper toner image is improved. As a result, in the multicolor mode, the accuracy of superimposing the toner images when a plurality of toner images are sequentially transferred onto one sheet S is improved.

  Also in the monochrome image quality priority mode, since the paper S is gripped by using the leading edge gripper 22 and the trailing edge gripper 23, the positioning accuracy of the paper S with respect to the toner image on the photosensitive drum 11 passing through the transfer position Tr. Will improve. As a result, in the single color image quality priority mode, the positioning accuracy of the toner image with respect to one sheet S is improved as compared with the single color productivity priority mode described later.

  On the other hand, in the monochromatic productivity priority mode, since the sheet S is not gripped using the leading edge gripper 22 and the trailing edge gripper 23, the photoconductor passes through the transfer position Tr as compared with the above-described monochrome image quality priority mode. There is a concern that the positioning accuracy of the paper S with respect to the toner image on the drum 11 is lowered. However, in the monochromatic productivity priority mode, the paper interval Li, which is the distance between the sheets S that continuously pass through the transfer position Tr, is shortened compared to the monochromatic image quality priority mode, thereby improving the productivity compared to the monochromatic image quality priority mode. It is possible to improve. Hereinafter, this reason will be described.

  In the present embodiment, the sheet interval Li is defined as the distance between the sheets S that continuously pass through the transfer position Tr as described above. Here, in the multi-color mode, a case where the sheet interval Li is defined for the same sheet S (for example, the first sheet S1) and a different adjacent sheet S (for example, the first sheet S1 and the second sheet S1) are used. There is a case where the sheet interval Li is defined in the sheet S2). On the other hand, in the monochromatic image quality priority mode and the monochromatic productivity priority mode, there is only a case where the sheet interval Li is defined for different adjacent sheets S (for example, the first sheet S1 and the second sheet S2).

  In the multicolor mode shown in FIG. 6A and the monochromatic image quality priority mode shown in FIG. 6B, the sheet interval Li is the difference between the transfer drum circumferential length Ld and the sheet length Ls in the transfer drum 21 (Li = Ld). -Ls). Therefore, the size of the transfer drum circumferential length Ld affects the sheet interval Li in the multicolor mode and the monochromatic image quality priority mode. Further, as described above, the transfer drum circumferential length Ld is represented by the sum (Ld = Le + Lg) of the elastic layer length Le of the elastic layer 21B and the gap length Lg of the exposed portion 21C. Accordingly, the sheet interval Li in the multicolor mode and the monochromatic image quality priority mode is represented by the difference between the sum of the elastic layer length Le and the gap length Lg and the sheet length Ls (Li = Le + Lg−Ls). Here, the gap length Lg takes a constant value regardless of the paper length Ls, and the paper length Ls is limited to the elastic layer length Le at the maximum, so the paper interval in the multicolor mode and the monochromatic image quality priority mode. Li is at least the gap length Lg or more.

In the multi-color mode, it is necessary to position the sheet leading end SL of the sheet S on the elastic layer leading end 21L of the elastic layer 21B provided on the transfer drum 21, and the four color toner images are printed on one sheet S. It is necessary to transfer sequentially. For this reason, in the multi-color mode, one sheet S is supplied every time the transfer drum 21 rotates four times.
In the monochromatic image quality priority mode, it is necessary to position the paper tip SL of the paper S on the elastic layer tip 21L of the elastic layer 21B provided on the transfer drum 21. For this reason, in the monochrome image quality priority mode, one sheet S is supplied every time the transfer drum 21 rotates once.

  On the other hand, in the monochromatic productivity priority mode shown in FIG. 6C, the sheet interval Li is not limited by the transfer roll circumferential length Lr in the transfer roll 31 and, of course, the gap length Lg in the transfer drum 21. There are no restrictions on the above. Therefore, in the monochromatic productivity priority mode, the paper leading edge SL and the paper trailing edge ST of the paper S can be positioned regardless of the position of the outer peripheral surface of the rotating transfer roll 31. For this reason, in the present embodiment, the sheet interval Li in the monochromatic productivity priority mode can be made shorter than the sheet interval Li in the multicolor mode and the monochromatic image quality priority mode. As a result, the number of prints per unit time in the monochrome productivity priority mode can be increased more than the number of prints per unit time in the monochrome image quality priority mode, that is, productivity can be improved.

  In the monochromatic image quality priority mode, even when the paper S having a paper length Ls shorter than the example shown in FIG. 6B is used, every time the transfer drum 21 rotates, that is, every transfer drum circumferential length Ld. One sheet S is supplied to the printer. Therefore, in the monochromatic image quality priority mode, the paper interval Li is increased to cope with it, and the productivity does not change even if the paper length Ls of the paper S is shortened.

  On the other hand, in the monochromatic productivity priority mode, when the paper S having a shorter paper length Ls than the example shown in FIG. 6C is used, the paper S can be supplied without changing the paper interval Li. become. Accordingly, in the monochromatic productivity priority mode, when the paper length Ls of the paper S is shortened, the productivity is further improved.

<Embodiment 2>
7 and 8 are diagrams illustrating an example of the configuration of the image forming apparatus according to the second embodiment. Here, FIG. 7 shows the positional relationship of each component when executing the image forming operation in the multi-color mode and the monochromatic image quality priority mode, and FIG. 8 shows the image forming operation in the monochrome productivity priority mode. The positional relationship of each component is shown. In the present embodiment, the same components as those described in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  In the first embodiment, the transfer roll 31 is attached to the transfer drum 21. In contrast, in this embodiment, the transfer drum 21 and the transfer roll 31 are provided independently.

  The transfer drum 21 is provided so as to be able to advance and retreat with respect to the photosensitive drum 11. In the following description, a position where the transfer drum 21 approaches the photosensitive drum 11 and forms the transfer position Tr together with the photosensitive drum 11 (position shown in FIG. 7) is referred to as a transfer drum use position. A position (a position shown in FIG. 8) where the transfer position Tr is not formed away from the photosensitive drum 11 is referred to as a transfer drum non-use position.

  On the other hand, the transfer roll 31 is provided so as to be movable along the outer peripheral surface of the photosensitive drum 11. In the following description, the position (position shown in FIG. 8) where the transfer roll 31 moves in the counterclockwise direction in the drawing to form the transfer position together with the photosensitive drum 11 is referred to as a transfer roll use position. A position (position shown in FIG. 7) where 31 moves clockwise in the figure and no longer forms the transfer position Tr is referred to as a transfer roll non-use position.

  For this reason, in the present embodiment, the position of the rotation shaft 11a of the photosensitive drum 11 and the rotation shaft 14a of the rotary developing device 14 are fixed to the main body frame (not shown) of the image forming apparatus. The rotating shaft 21a of the drum 21 and the rotating shaft 31a of the transfer roll 31 are movably attached to the main body frame of the image forming apparatus.

  In the multi-color mode and the monochromatic image quality priority mode, as shown in FIG. 7, the transfer drum 21 is disposed at the transfer drum use position, and the transfer roll 31 is disposed at the transfer roll non-use position. In the monochromatic productivity priority mode, as shown in FIG. 8, the transfer drum 21 is disposed at the transfer drum non-use position, and the transfer roll 31 is disposed at the transfer roll use position. That is, in the image forming apparatus according to the present embodiment, one of the transfer drum 21 and the transfer roll 31 constitutes the transfer position Tr together with the photosensitive drum 11.

In the multicolor mode, after the transfer drum 21 and the transfer roll 31 are arranged in the positional relationship shown in FIG. 7 in the initial state, the image forming operation can be basically performed along the timing chart shown in FIG.
In the monochromatic image quality priority mode, after the transfer drum 21 and the transfer roll 31 are arranged in the positional relationship shown in FIG. 7 in the initial state, the image forming operation can be basically performed along the timing chart shown in FIG. .
Further, in the monochromatic productivity priority mode, after the transfer drum 21 and the transfer roll 31 are arranged in the positional relationship shown in FIG. 8 in the initial state, the image forming operation is basically performed along the timing chart shown in FIG. it can.

  In the first and second embodiments, the transfer drum 21 is driven while the transfer roll 31 is not driven in the image forming operation in the multicolor mode and the single color image quality priority mode. However, the transfer roll 31 may be driven together with the transfer drum 21. In the image forming operation in the monochromatic productivity priority mode according to the first embodiment, when the transfer roll 31 is driven, the transfer drum 21 is stopped without being driven. In the image forming operation in the monochromatic productivity priority mode, the transfer drum 21 may be driven when the transfer roll 31 is driven.

  Furthermore, in the first and second embodiments, when a single color image is formed on the paper S, the monochrome image quality priority mode in which the image is transferred to the paper S using the transfer drum 21 or the paper S using the transfer roll 31. One of the monochromatic productivity priority modes for transferring an image is selectively executed, but the present invention is not limited to this. For example, the transfer drum 21 is used in the multicolor mode, and the image is transferred to the paper S gripped by the transfer drum 21 by the leading edge gripper 22 and the trailing edge gripper 23, and the transfer roll 31 is used in the single color mode. An image may be transferred to S.

DESCRIPTION OF SYMBOLS 10 ... Image forming part, 11 ... Photosensitive drum, 12 ... Charging device, 13 ... Exposure device, 14 ... Rotary developing device, 15 ... Cleaning device, 20 ... Drum type transfer device, 21 ... Transfer drum, 21A ... Base, 21B DESCRIPTION OF SYMBOLS Elastic layer, 21C ... Exposed part, 21L ... Elastic layer front end, 21T ... Elastic layer rear end, 22 ... Front end gripper, 23 ... Rear end gripper, 30 ... Roll type transfer device, 31 ... Transfer roll, 40 ... Fixing part, DESCRIPTION OF SYMBOLS 50 ... Paper supply part, 60 ... Paper feed path, 61 ... 1st paper feed path, 62 ... 2nd paper feed path, 63 ... Rotation path, 64 ... Paper discharge path, 100 ... Control part, S ... Paper, SL ... Paper leading edge, ST ... paper trailing edge

Claims (8)

An image forming means for forming a single color image or sequentially forming a plurality of color images for each color;
An image holding member that is rotatably provided and holds an image formed by the image forming unit;
A first transfer means that has a function of gripping a recording material on an outer peripheral surface and is rotatably provided, and transfers an image held by the image holding body to a recording material sandwiched between the image holding body;
A second transfer means that does not have a function of gripping the recording material on the outer peripheral surface, is rotatably provided, and transfers an image held on the image holding body to a recording material sandwiched between the image holding body and ,
The multi-color mode in which the image forming unit sequentially forms a plurality of color images for each color, and the number of recording materials on which the image forming unit forms a single color image and image formation is performed per unit time. In the monochrome first mode in which priority is given to the positional accuracy of the image with respect to the recording material, a transfer operation using the first transfer unit is executed, the image forming unit forms a monochrome image, and the positional accuracy of the image with respect to the recording material An image forming apparatus including a control unit that executes a transfer operation using the second transfer unit in the single-color second mode in which priority is given to the number of recording materials on which image formation is performed per unit time.   The image forming apparatus according to claim 1, wherein the first transfer unit grips a leading end in a transport direction and a rear end in a transport direction of a recording material being transported. A supply unit for supplying a recording material for transferring the image held on the image holding member;
3. The image forming apparatus according to claim 1, wherein the control unit controls the supply unit so that a recording material supply interval is narrower in the monochrome second mode than in the monochrome first mode. apparatus. The control means includes
In the multicolor mode and the monochrome first mode, the recording material is supplied to the upstream side in the rotation direction of the first transfer unit from the facing portion of the image carrier and the first transfer unit, and the supply is performed. The first transfer means grips the recorded recording material,
4. The image forming apparatus according to claim 3, wherein, in the single color second mode, a recording material is supplied toward a facing portion between the image carrier and the second transfer unit. An image carrier that is rotatably arranged and holds an image on an outer peripheral surface;
A first transfer member that is rotatably arranged and transfers an image held on the outer peripheral surface of the image carrier to a recording material sandwiched between the image carrier and the image carrier;
A gripping member for gripping the recording material supplied toward the first transfer member on the outer peripheral surface of the first transfer member;
An image forming apparatus comprising: a second transfer member that is rotatably arranged and transfers an image held on an outer peripheral surface of the image holding body to a recording material sandwiched between the image holding body and the recording material.   The image forming apparatus according to claim 5, wherein the gripping member grips a leading end in a transport direction of a recording material transported toward the first transfer member on an outer peripheral surface of the first transfer member. On the outer peripheral surface of the first transfer member, there is provided an elastic layer for wrapping the recording material having elasticity except for one region along the axial direction of the first transfer member,
The said 2nd transfer member is rotatably attached with respect to the said 1st transfer member in the site | part in which the said elastic layer is not provided in the outer peripheral surface of the said 1st transfer member. Image forming apparatus.   The position where the recording material is sandwiched between the image carrier and the first transfer member and the position where the recording material is sandwiched between the image carrier and the second transfer member are set as a common transfer position. The image forming apparatus according to claim 5, wherein the image forming apparatus is an image forming apparatus.

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