JPH1165309A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To excellently transfer a toner image on an image carrier to a sheet of paper, regardless of the kind of paper such as a plain paper, a cardboard and an OHP sheet by providing a nip width by paper kind storage means for storing the respective nip widths of the cardboard, the plain paper and the OHP sheet. SOLUTION: The nip width by paper kind storage means for storing the nip widths set according to the kind of paper P and the density of a recorded image, stores the nip width Wa set according to the case that the kind of paper P is the cardboard, the nip width Wf set according to the case that the kind of paper P is the plain paper and the nip width Wo set according to the case that the kind of paper P is the OHP sheet, when the density of an image recorded on the paper P is a prescribed value or above. Then, these widths are set to be Wa<Wf<Wo. Therefore, when the density of the recorded image is the prescribed value or above and the kind of paper P is the cardboard, the plain paper or the OHP sheet, the nip widths of transferring region Qk, Qy, Qm and Qc can be properly adjusted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for transferring a toner image formed on a surface of an image carrier to a sheet in a transfer area,
The present invention relates to an electrophotographic image forming apparatus that heats and fixes a transferred sheet, and more particularly, to an image forming apparatus that adsorbs the sheet to a sheet-shaped sheet transporter and transports the sheet to the transfer area.
An image forming apparatus that adsorbs the sheet onto a sheet-shaped sheet conveyance body and conveys the sheet to the transfer area may sequentially convey the sheet to each transfer area that transfers a toner image of a plurality of image carriers arranged in tandem. Accordingly, it is possible to form a composite toner image in which a plurality of toner images overlap on a sheet in a short time. Such an image forming apparatus is used in a copying machine, a printer, or the like.

[0002]

2. Description of the Related Art In an image forming apparatus of the type described above, in order to reliably transfer a toner image on an image carrier to the surface of a sheet in a transfer area, the back of a sheet-like sheet transporter, that is, a sheet for adsorbing a sheet. The surface opposite to the suction surface is elastically pressed against the image carrier by a pressing member. The pressing member is a member that presses the back surface of the sheet via a sheet-shaped sheet conveying member such that the surface of the sheet adsorbed on the sheet adsorbing surface follows the curved surface of the image carrier. In the present specification, “paper” means a member to which a toner image is transferred, such as general paper, thick paper, and OHP paper.

[0003]

When thick paper is used as the paper, the nip width, which is the contact length in the moving direction of the image carrier on which the toner image is formed and the sheet-like paper transporter, is reduced by the toner image. If the high-density portion of the toner image is adjusted so as to be satisfactorily transferred to the paper (thick paper), there is a problem that the high-density portion of the toner image cannot be satisfactorily transferred to the paper when OHP paper or plain paper is used. Further, when the thick paper is used, if the nip width is set so that the high density portion of the toner image can be satisfactorily transferred to the paper, there is a problem that the medium and low density portions are not transferred well.

In view of the above-mentioned circumstances, the present invention relates to an image forming apparatus configured to transfer a toner image on an image carrier to a sheet adsorbed on a sheet-like sheet transporter, as described below (O0).
The contents described in 1) shall be the subject. (O01) To be able to satisfactorily transfer a toner image on an image carrier onto a sheet irrespective of the type of sheet, such as plain paper, thick paper, and OHP paper.

[0005]

Means for Solving the Problems Next, the present invention which has solved the above-mentioned problems will be described. Elements of the present invention include elements of the embodiments to facilitate correspondence with the elements of the embodiments described later. The sign of parentheses is added in parentheses. The reason why the present invention is described in correspondence with the reference numerals of the embodiments described below is to facilitate understanding of the present invention and not to limit the scope of the present invention to the embodiments.

According to the present invention, there is provided an image forming apparatus according to the present invention, which has the following requirements. (A01) A toner is provided on a rotatable endless surface. A plurality of image carriers (16k, 16y,
16m, 16c), (A02) a sheet suction position (Q1) for adsorbing the sheet (P) on the sheet (P) onto which the toner image is transferred, and adsorbing the sheet (P) on the sheet adsorbing surface; The paper suction surface is used for the plurality of image carriers (16k, 16y,
16m, 16c), and a plurality of transfer areas (Qk, Qy, Qy, 16y, 16y, 16m, 16c) that move in the same direction at the same speed while contacting the paper suction surface and the surfaces of the image carriers (16k, 16y, 16m, 16c). Qm, Qc), and a sheet-like sheet transporter (45) that sequentially rotates through a sheet peeling position for peeling the sheet (P) from the sheet suction surface,
(A03) A paper supply device (51) for adsorbing the paper (P) on the paper adsorption surface at the paper adsorption position (Q1)
To 54), (A04) each of the transcription regions (Qk, Qy, Qm,
In Qc), the toner image on the surface of the image carrier (16k, 16y, 16m, 16c) is arranged on the opposite side of the sheet-adsorbing surface of the sheet-shaped sheet transporter (45), and is applied to the sheet-adsorbing surface Transfer device (46k, 46y, 46m, 46c) for transferring to the paper (P) adsorbed, (A05)
A nip width adjusting device (a nip width adjusting device for adjusting a nip width which is a contact length of the transfer area (Qk, Qy, Qm, Qc) between the surface of the image carrier (16k, 16y, 16m, 16c) and the paper suction surface in the moving direction. Tk, Ty, Tm, Tc: Ts), (A0
6) The type of paper (P) to which the toner image is transferred is any of thick paper, plain paper, and OHP.
(A4) a recording image density discriminating means (C2) for detecting whether or not a recording image density on a sheet (P) is equal to or greater than a predetermined value; P) nip width storage means (C) for storing a nip width determined according to the type of recording image and the recording image density.
1), (A09) The type of paper (P) and the recorded image density determining means (C
The nip width adjusting devices (Tk, Ty, Tn) are adjusted so that the nip width corresponds to the recording image density detected by 2).
m, Tc: Ts) a nip width adjustment signal output means (C3) for outputting a nip width adjustment signal.

(Operation of the Present Invention) In the image forming apparatus of the present invention having the above-described configuration, a toner image is formed on the surface of a plurality of endless image carriers (16k, 16y, 16m, 16c) that rotate. You. A sheet-like sheet transporter (4) having a sheet suction surface for sucking a sheet (P) onto which the toner image is transferred.
5) is a paper suction position (Q1), a plurality of transfer areas (Qk,
Qy, Qm, Qc), and the sheet peeling position, and sequentially rotates. The paper supply device (51-54) is configured to control the paper (P) on the paper suction surface at the paper suction position.
Is adsorbed. The recording image density determining means (C2) detects whether the recording image density on the sheet (P) is equal to or higher than a predetermined value. The paper determination means (C4) determines whether the type of paper (P) to which the toner image is transferred is thick paper, plain paper, or OHP. The paper-specific nip width storage means (C1) stores a nip width determined according to the type of the paper (P) and the recording image density. The nip width adjustment signal output means (C3) includes a nip width corresponding to the type of the paper (P) determined by the paper determination means (C4) and the recording image density detected by the recording image density determination means (C2). Thus, a nip width adjustment signal for controlling the nip width adjustment device (Tk, Ty, Tm, Tc: Ts) is output. Nip width adjustment device (Tk,
Ty, Tm, Tc: Ts) are the transcription regions (Qk, Qy, Q
m, Qc) of the image carrier (16k, 16y, 16
m, 16c) Adjust the nip width, which is the length of contact between the surface and the paper suction surface in the moving direction. The paper suction surface is pressed against the surface of each image carrier (16k, 16y, 16m, 16c) in each transfer area (Qk, Qy, Qm, Qc), and the paper suction surface and each image carrier (16k, 16y, 16
m, 16c) The surfaces move in the same direction at the same speed while making contact. In each of the transfer areas (Qk, Qy, Qm, Qc), a transfer device (46k, 46y, 46y, 46y,
46m, 46c) are the image carriers (16k, 16y, 16c).
m, 16c) The toner image on the surface is transferred onto the sheet (P) adsorbed on the sheet adsorbing surface. The transfer region (Qk, Qy,
Qm, Qc) on which the toner image is transferred (P)
Is peeled off from the sheet suction surface at the sheet peeling position. As described above, in the present invention, each transfer region (Qk, Qk)
Since the nip width in (y, Qm, Qc) is adjusted to an appropriate value according to the type of paper (P) and the recording image density, good transfer can be performed regardless of the type of paper (P). .

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1) The image forming apparatus of Embodiment 1 of the present invention is characterized in that the image forming apparatus of the present invention has the following requirements: (A010) The paper (P) The nip width Wa determined for the case where the recording image density is equal to or more than the predetermined value and the type of the paper (P) is thick paper, and the nip width Wf determined for the case of plain paper , And the nip widths Wo determined corresponding to the OHP, and the nip widths Wa, Wf, and Wo whose sizes are set to Wa <Wf <Wo. Means (C1).

(Operation of First Embodiment of the Invention) In the image forming apparatus of the first embodiment of the present invention, the nip width determined according to the type of the paper (P) and the recording image density is stored. The paper-specific nip width storage means (C1) stores a nip determined when the recording image density on the paper (P) is equal to or more than a predetermined value and the type of the paper (P) is thick paper. The width Wa, the nip width Wf determined for plain paper, and the nip width Wo determined for OHP are stored. And their sizes are set to Wa <Wf <Wo. Therefore, when the recording image density is equal to or higher than a predetermined value and the type of the paper (P) is thick paper, plain paper, or OHP paper, the transfer area (Q
Since the nip width of k, Qy, Qm, Qc) can be appropriately adjusted, good transfer can be performed.

(Embodiment 2) An image forming apparatus according to a second embodiment of the present invention is characterized in that the image forming apparatus according to the present invention or the first embodiment of the present invention has the following requirements. (A011) The transfer region (Qk, Q
y, Qm, Qc), the image carrier (16k, 16y,
16m, 16c) a pressing member (61) for pressing a surface of the sheet-shaped paper transport body (45) opposite to the paper suction surface so as to press the paper suction surface against the surface, and the pressing member (61). To adjust the position of the image carrier (16k,
16y, 16m, 16c) the nip width adjusting device (Tk, Ty, Tm, Tc) for adjusting the nip width which is the contact length of the surface and the paper suction surface in the moving direction, (A012) the paper discriminating means (C4) Width adjustment for controlling the position of the pressing member (61) so that the nip width corresponds to the type of the paper (P) determined by the above and the recording image density detected by the recording image density determining means (C2). The nip width adjustment signal output means (C3) for outputting a signal to the nip width adjustment device (Tk, Ty, Tm, Tc).

(Operation of Second Embodiment of the Invention) In the image forming apparatus according to the second embodiment of the present invention, the nip width adjusting device (Tk, Ty, Tm, Tc: Ts) includes the transfer region (Qk, Tk).
Qy, Qm, Qc), the image bearing members (16k, 16k)
(y, 16m, 16c) a pressing member (61) for pressing the opposite surface of the sheet-shaped paper transport body (45) from the paper suction surface to press the paper suction surface against the surface. The nip width adjustment signal output means (C3) includes a nip width corresponding to the type of the paper (P) determined by the paper determination means (C4) and the recording image density detected by the recording image density determination means (C2). So that the pressing member (6
The nip width adjustment signal for controlling the position 1) is output to the nip width adjustment device (Tk, Ty, Tm, Tc). The nip width adjusting device (Tk, Ty, Tm, Tc) adjusts the position of the pressing member (61) based on the nip width adjustment signal, and adjusts the position of the image carrier (16k, 16y, 16m, 16c). The nip width, which is the length of contact between the front surface and the paper suction surface in the moving direction, is adjusted.

(Embodiment 3) An image forming apparatus according to a third embodiment of the present invention is characterized in that the image forming apparatus according to the present invention or the first embodiment of the present invention has the following requirements. (A013) The transfer region (Qk, Q
y, Qm, Qc), the image carrier (16k, 16y,
16m, 16c) The transfer area (Q) of the sheet-shaped sheet transporter (45) for pressing the sheet suction surface against the surface.
k, Qy, Qm, Qc) are separated from and approached to the image carrier (16k, 16y, 16m, 16c) by a peeling roll which is disposed downstream of the sheet-like sheet transporter (45) and is supported by the image carrier (16k, 16y, 16m, 16c). A peeling roll elevating device (R) for adjusting the position of the peeling roll (42) by the peeling roll elevating device (R) to thereby control the image carrier (16k, 16y, 16).
m, 16c) The nip width adjusting device (T) for adjusting the nip width which is the length of contact between the surface and the paper suction surface in the moving direction.
s), (A014) The type of paper (P) and the recorded image density determining means (C) determined by the paper determining means (C4).
Outputting the nip width adjustment signal for controlling the position of the peeling roll (42) to the nip width adjustment device (Ts) so that the nip width corresponds to the recording image density detected by 2). Signal output means (C3).

(Operation of Third Embodiment of the Invention) In the image forming apparatus according to the third embodiment of the present invention, the nip width adjusting device (Ts) operates in the transfer area (Qk, Qy, Qm, Qc). The plurality of transfer areas (Qk, Qy, Qm, Qm, and Qm) of the sheet-like sheet transporter (45) for pressing the sheet suction surface against the surfaces of the image carriers (16k, 16y, 16m, and 16c).
It has a peeling roll elevating device (R) arranged downstream of Qc). Nip width adjustment signal output means (C3)
Corresponds to the type of the paper (P) determined by the paper determination means (C4) and the nip width corresponding to the recording image density detected by the recording image density determination means (C2). ) Is output to the nip width adjusting device (Ts). The nip width adjusting device (Ts) moves the peeling roll (42) supporting the sheet-shaped paper transport body (45) by the peeling roll elevating device (R) to the image carrier (16k, 1).
6y, 16m, 16c), the position of the peeling roll (42) is adjusted by moving away from and approaching the same, and the contact between the surface of the image carrier (16k, 16y, 16m, 16c) and the moving direction of the paper suction surface is adjusted. Adjust the nip width, which is the length.

[0014]

Next, specific examples (embodiments) of the embodiment of the image forming apparatus of the present invention will be described with reference to the drawings, but the present invention is not limited to the following embodiments. In addition,
In order to facilitate understanding of the following description, an X axis, a Y axis, and a Z axis, which are orthogonal to each other, are defined in the drawings. The X direction is a forward direction, the -X direction is a rear direction, and the Y direction is a left direction. To the right, the Z direction to the upward direction, and the -Z direction to the downward direction. Furthermore, in the figure, those with "•" in "○" mean arrows pointing from the back of the paper to the front, and those with "x" in "○" are from the table on the paper. It shall mean an arrow pointing to the back. (Embodiment 1) FIG. 1 is an overall explanatory view of an image forming apparatus (tandem digital color copying machine) according to Embodiment 1 of the present invention. A tandem digital color copying machine F as an image forming apparatus has a copy start key UI1 (see FIG. 3), a numeric keypad UI2, a copy set number input key UI3,
UI having paper type input key UI4, display unit UI5, etc.
(User interface) and a transparent platen glass 2 on which the original 1 is placed. The paper type input key UI4 includes a thick paper designation key UI4a and a plain paper designation key UI4.
b, composed of an OHP sheet designation key UI4c.

On the lower side of the platen glass 2, the original 1
A document illumination unit 3 that scans while illuminating the document is arranged. The document illumination unit 3 has a document illumination light source 4 and a first mirror 5. In addition, platen glass 2
Below the moving speed of the document illumination unit 3
A mirror unit 6 that moves at a speed of 2 is provided. The mirror unit 6 has a second mirror 7 and a third mirror 8 which are emitted from the illumination light source 4 and reflected by the original 1 and reflect original image light reflected by the first mirror 5. The document image light reflected by the third mirror 8 passes through the imaging lens 9 and is read as R, G, and B analog signals by a CCD (color image reading sensor).

The R (red), G (green), and B (blue) image signals read by the CCD are input to an IPS (image processing system). The IPS is an image processing device having a CPU for image processing, and its operation is controlled by the controller C (see FIGS. 1 and 3).
reference). The IPS is R, obtained by the CCD,
Image read data output means 11 for converting analog electric signals of G and B read images into digital signals and outputting the digital signals, and converting the RGB image data into K (black), Y (yellow), M
(Magenta) and C (cyan) image data, and has image data output means 12 for performing data processing such as density correction and enlargement / reduction correction, and outputting as write image data (laser drive data). ing. The image data output means 12 has an image memory 13 for temporarily storing the KYMC image data.

The IPS write image data output means 12
Output the KYMC four-color image writing data (laser drive data) output by the laser drive signal output devices 14k, 14y, 14m, and 14c of the respective colors K, Y, M, and C. Is input to The laser drive signal output devices 14k, 14y, 14m, and 14c for each color are
At a predetermined timing, a laser drive signal corresponding to the input image data is applied to the exposure optical systems Uk, Uk,
It has the function of outputting to Uy, Um and Uc. The exposure optical systems Uk, Uy, Um, Uc are respectively K (black), Y
(Yellow), M (magenta), and C (cyan) toner images of the respective colors are transferred to the image carriers 16k, 16y, 1
6m, 16c writing device. Around the image carrier 16k on which a black image is formed, a charger 17k and a developing device 1
8k, a cleaner 19k and the like are arranged. Then, the same chargers 17y and 17 around the other image carriers 16y, 16m and 16c as those around the image carrier 16k.
m, 17c, developing devices 18y, 18m, 18c, cleaner 1
9y, 19m, 19c, etc. are arranged. The developing devices 18k, 18y, 18m and 18c are provided with image carriers 16k and 16k.
This device develops the electrostatic latent images on y, 16m, and 16c into toner images of K (black), Y (yellow), M (magenta), and C (cyan).

Each of the exposure optical systems Uk, Uy, Um, Uc is
A laser diode 21 and a light source optical system for causing a laser beam emitted from the laser diode 21 to enter a rotary polygon mirror 22 having a vertical rotation axis, and a laser beam reflected from the rotary polygon mirror 22 to the image carriers 16k, 16y, and
It has a scanning optical system for converging and scanning on 16m and 16c. A scanning optical system for each exposure optical system Uk, Uy, Um, Uc (reflected beams Lk, Ly, Lm,
An optical system for converging Lc on the image carriers 16k, 16y, 16m, and 16c) includes a rotating polygon mirror 22 and the rotating polygon mirror 22.
Has a cylindrical reflecting mirror 23 for correcting surface tilt. The cylindrical reflecting mirror 23 has a function as a return mirror.

The exposure optical system U for each color of the exposure optical system U
k, Uy, Um, Uc are the laser drive signal output devices 14
k, Y, input from k, 14y, 14m, 14c
According to the laser drive signals of the respective colors M and C, the charger 1
An electrostatic latent image is written on the image carriers 16k, 16y, 16m, and 16c uniformly charged by 7k, 17y, 17m, and 17c. The electrostatic latent images on the image carriers 16k, 16y, 16m, and 16c are converted to K by developing devices 18k, 18y, 18m, and 18c.
(Black), Y (yellow), M (magenta), and C (cyan) are developed into toner images of respective colors.

The image carriers 16k, 16y, 16m, 16c
A paper transport device H is disposed below the printer. The sheet transport device H has the belt module B shown in FIG. The belt module B includes a belt supporting drive roll 41 rotatably supported by a front plate and a rear plate (not shown) provided at both ends in the front-rear direction (the direction perpendicular to the paper surface in FIG. 1, the X-axis direction). Roll 4
2. Tension roll 43 and idler roll 44
have. The rolls 41 to 44 support a belt for transporting the paper (that is, a sheet-shaped paper transport body) 45. Upper surface of the belt 45 (image carrier 16
The surfaces that come into contact with k, 16y, 16m, and 16c and that adsorb the paper on which the toner image is transferred on the image carrier) are disposed horizontally. A driven gear (not shown) is mounted on the rear end of the driving roll 41, and is configured to transmit a rotational driving force.

Each of the image carriers 16k, 16y, 16m, 1
6c and the transfer areas Qk, Qy, Qm,
Qc is a corotron transfer device 46k, 46y, 46m, 4
Nip width adjusting devices Tk, Ty, Tm and Tc are arranged to press the belt 6c and the belt 45 against the respective image carriers and adjust the nip width which is the length of the contact surface in the moving direction. A peeling corotron 47 is disposed on the upstream side of the peeling roll 42, and a stripper (peeling claw) 4 is located on the downstream side.
8 are arranged. A fixing device 49 is disposed on the left side of the belt 45, and a belt cleaner 50 for collecting toner adhered to the surface of the belt 45 is disposed between the tension roll 43 and the driving roll 41. . In FIG. 1, paper (paper) P is stored in a paper feed cassette 51 disposed below the paper transport device H. The paper P is taken out by the paper take-out roll 52 and transported to the registration roll 53. Cash register roll 53
Transports the transported paper P to the paper suction position Q1 between the belt 45 and the suction roll 54 at a predetermined timing. The suction roller 54 is a member for pressing the sheet P against the belt module B to suck the sheet P. A sheet supply device (51 to 5) for adsorbing the sheet (P) on the sheet adsorbing surface of the sheet-shaped sheet conveying body 45 at the sheet adsorbing position Q1 by the elements indicated by the reference numerals 51 to 54.
4) is configured.

The sheet P adsorbed on the belt 45 at the sheet adsorbing position Q1 is conveyed by the belt 45. At this time, the image forming start position on the sheet P conveyed by the belt 45 and the K located at the most upstream side in the sheet conveying direction.
The transport timing of the sheet P and the image writing timing so that the leading end of the K (black) image on the (black) image carrier 16k coincides with the transfer area Qk between the transfer device 46k and the image carrier 16k. Is determined. The toner image on the image carrier 16k is transferred onto the sheet P having reached the transfer area Qk by the transfer unit 46k. The paper P to which the K (black) toner image has been transferred is sequentially transferred to the transfer areas Qy, Qm, Qc between the image carriers 16y, 16m, 16c and the transfer units 46y, 46m, 46c.
However, the image writing timing of the image carriers 16y, 16m, and 16c is such that the leading ends of the Y, M, and C toner images coincide with the leading ends of the K (black) toner images transferred to the sheet P. It is decided as follows.

A sheet P on which the toner images of the respective colors have been transferred.
Is separated at the separation point on the outer periphery of the separation roll 42 by the separation corotron 47 and the stripper 48 and the like, and is conveyed to the fixing device 49. The paper P on which the color toner image is fixed by the fixing device 49 is discharged from the discharge roll 55 to the discharge tray TR. The surfaces of the image carriers 16k, 16y, 16m, and 16c after the transfer of the toner images are cleaned by cleaners 19k, 19y, 19m, and 19c.

In the multi-transfer type image forming apparatus (tandem type digital color copying machine) F for sequentially transferring a plurality of toner images onto a sheet, the main scanning direction and the sub-scanning direction of the toner image of each color on each sheet P are described. If the writing start position is shifted in the scanning direction, a color shift occurs and the image quality deteriorates. Therefore, at a position on the downstream side of the optical writing device Uc of the belt 45,
Light sources 56, 56 (only one is shown in FIG. 1) for detecting a positional deviation of a writing start position separated in the belt width direction and image position sensors 57, 57 are arranged. An image position signal 57a output from the image position sensor 57 is input to a controller C, and the laser drive signal output device 14
The writing start timings of k, 14y, 14m, and 14c are adjusted.

FIG. 2 is an explanatory view of a nip width adjusting device Tk for adjusting the nip width by pressing the belt 45 against the surface of the image carrier 16k on which a black toner image is formed. In addition,
The nip width adjusting devices Ty, Tm, and Tc for transferring the toner images of the respective colors from the image carriers 16y, 16m, and 16c on which the toner images of the other colors Y, M, and C are formed on the paper P are the nip widths. Since the configuration is the same as that of the adjusting device Tk, the description thereof is omitted. The nip width adjusting device Tk is an elastic material disposed adjacent to a corotron-type transfer device 46k that transfers a toner image onto a sheet P passing through the transfer area Qk while being attracted to the sheet attracting surface of the sheet-shaped sheet transport body 45. Pressing member 61 made of
have. The pressing member 61 is supported by a support member 62. The support member 62 is supported so as to be rotatable around a shaft 63, and is constantly receiving a clockwise rotational force by a tension spring 64. Therefore, the leading end of the pressing member 61 always lifts the belt 45 to keep the image carrier 16
Pressing on the k surface.

The shaft 63 supporting the support member 62 is supported by a slide arm 67 slidably supported by a slide mechanism 66. The slide arm 67
Is configured to be moved by a driving device (not shown) using a rack and a pinion. Incidentally, as the slide moving mechanism of the slide arm 67, various conventionally known slide moving mechanisms can be adopted. The pressing member 61 also moves in the left-right direction according to the movement of the slide arm 67 in the left-right direction. The nip width, which is the contact width of the image carrier 16k and the belt 45 in the moving direction, changes depending on the position of the pressing member 61 in the left-right direction.

In the first embodiment, the position of the leading end of the pressing member 61 (the position of the portion pressing the belt 45) is set as follows according to the type of paper and the density of recorded images on the paper. In addition, the following “recorded image density on paper” is 5
Whether it is 0% or more can be detected from the image data stored in the image memory 13 of the IPS (image processing system). That is, when forming a recorded image of one page, the laser drive signal output device 14 scans the surface of the image carrier 16k while turning on and off the laser light based on the image data stored in the image memory 13, It can be detected based on whether the ratio of the ON state of the laser beam at that time is 50% or more. (1) In the case where the recording image density to be recorded on the paper is 50% or more, (1-1) In the case of thick paper 0.5 mm from the position immediately below the image carrier 16k to the upstream in the paper transport direction (1-2) In the case of plain paper, a position 1.0 mm upstream from the position directly below the image carrier 16 k in the paper transport direction (1-3) In the case of OHP paper 1. From the position directly below the image carrier 16 k to the upstream in the paper transport direction 1. 3mm position

(2) The density of a recorded image to be recorded on a sheet is 50
%, (2-1) In the case of thick paper, a position 1.0 mm upstream from the position directly below the image carrier 16k in the paper transport direction (2-2) In the case of plain paper, the position immediately below the image carrier 16k From the position just below the image carrier 16k and 1.0 mm upstream from the position directly below the image carrier 16k.

Since the nip width is determined by setting the tip position of the pressing member 61 as described above, the set value of the tip position of the pressing member 61 of the first embodiment can be used for controlling the nip width. Parameter. In the first embodiment, instead of controlling the value of the nip width itself, control is performed such that the nip width is adjusted by adjusting the tip position of the pressing member 61. The set value of the nip width in the control is a set value of the tip position of the pressing member 61. Therefore, the nip width determined for the case where the type of paper is thick paper is W
a, the nip width determined for plain paper is Wf,
And the nip width determined for OHP
If o is set, the nip width (set value of the nip width) set in the first embodiment is as follows.

(1) The density of an image recorded on a sheet is 50
% Or more; (1-1) For thick paper: Wa = 0.5 mm from the position directly below the image carrier 16k to the upstream side in the paper transport direction (1-2) For plain paper: Wf = Image carrying A position of 1.0 mm upstream from the position directly below the body 16 k in the paper transport direction (1-3) For OHP paper Wo = a position 1.3 mm upstream from the position directly below the image carrier 16 k in the paper transport direction

(2) The density of a recorded image to be recorded on a sheet is 50
%: (2-1) In the case of thick paper Wa = 1.0 mm upstream from the position directly below the image carrier 16k in the paper transport direction (2-2) In the case of plain paper Wf = Image carrier 16k (2-3) In the case of OHP paper, Wo = 1.0 mm upstream from the position directly below the image carrier 16k in the paper transport direction.

Therefore, when the density of the image recorded on the paper is equal to or higher than a predetermined value (50%), the nip width Wa is determined for the case where the type of paper is thick paper, and when the type of paper is plain paper. Nip width Wf and OHP
The nip width Wo determined corresponding to the case is set so that their sizes satisfy the following expression. Wa <Wf <Wo

FIG. 3 is a block diagram of a control portion of the tandem digital color copying machine (image forming apparatus) F of the first embodiment. In FIG. 3, a controller C receives a setting signal of the UI (user interface). The controller C is constituted by a microcomputer and outputs operation signals of the belt motor drive circuit D1 and the fixing device motor drive circuit D3. Further, the controller C outputs operation control signals of the nip width adjusting motor drive circuits D2k to D2c provided corresponding to the transfer areas Qk, Qy, Qm, Qc in response to the input signal from the UI. ing. The paper transport belt drive motor M1 is rotationally driven by the belt motor drive circuit D1, and the belt (sheet-like paper transport body) 45 is rotated accordingly. The nip adjustment drive motors M2k to M2c are rotationally driven by the nip width adjustment motor drive circuits D2k to D2c, and accordingly, the slide arms 67 slide and adjust the position of the pressing member 61. The fixing device driving motor M3 is driven by the fixing device motor driving circuit D3.
Is driven to rotate, and a pair of fixing rolls of the fixing device 49 rotates accordingly.

The controller C is constituted by a computer and has functions such as a paper-specific nip width storage means C1, a recording image density determination means C2, a nip width adjustment signal output means C3, and a paper determination means C4. C1: Nip width storage means for each sheet The nip width storage means for each sheet C1 stores the sheet conveyance distance between the secondary transfer area Q4 and the fixing area Q5 when the sheet conveyance belt 43 is located at the normal position P0. doing. C2: Recording Image Density Determining Unit The recording image density determining unit C2 has a function of detecting whether or not the recording image density input from the IPS is equal to or higher than a predetermined value (50%). C3: Nip width adjustment signal output / output means The nip width adjustment signal output / output means C3 controls the type of paper (thick paper, plain paper, or paper) depending on whether the recording image density is equal to or more than a predetermined value (50%). OHP paper), the nip width determined according to the paper type nip width storage means C
It has a function of reading out from 1 and outputting it to each of the nip width adjusting motor drive circuits D2k to D2c. C4: Sheet Discriminating Unit The sheet discriminating unit C4 has a function of discriminating whether the sheet on which the image is to be recorded is thick paper, plain paper, or OHP paper based on the input of the UI sheet type input keys UI4a to UI4c. Have.

(Operation of Embodiment 1) FIG. 4 is a diagram showing the relationship between the tip position of the pressing member 61 and the transferability of thick paper, plain paper, and OHP paper, and FIG.
FIG. 4B is a diagram showing the low density portion transferability. As can be seen from the graph shown in FIG. 4, the transferability of the high density portion is good when the thickness is 0.5 mm for thick paper, 1.0 mm for plain paper, and 1.3 mm for OHP paper. However, low density area transferability is 1.0m for thick paper, plain paper and OHP paper.
m is optimal. Next, the reason why such a phenomenon occurs will be described.

FIG. 5 is an explanatory view of the transfer operation depending on the position of the leading end of the pressing member 61. FIG. 5A is an explanatory view of the transfer operation for thick paper, and FIG. 5B is an explanatory view of the transfer operation for OHP paper. is there. In the case of the thick paper P1 shown in FIG. 5A, the transfer electric field is less likely to effectively act on the surface of the image carrier 16k than in the case of the plain paper due to the thickness of the thick paper P1. In this case, the distal end position of the pressing member 61 is moved to a position immediately below the image carrier 16k, and the distance D1 between the distal end position of the pressing member 61 and the immediately lower position (lower end position) of the image carrier 16k is shortened. Since the discharge region of the transfer unit 46 is narrowed by the pressing member 61 acting as a discharge regulating member and the electric charge is concentrated, it is considered that the high density portion transferability to the thick paper P1 is improved. In the case of the OHP sheet shown in FIG.
1 is moved from immediately below (the lower end) of the image carrier 16k to the upstream side, and the tip position of the pressing member 61 and the image carrier 1 are moved.
By increasing the distance D2 from the position immediately below 6k, the discharge area of the transfer unit 46 is widened and the electric charges are dispersed, so that the output of the transfer unit 46 is the same as that of the thick paper P1 having a large thickness. OH transfer electric field suitable for thin OHP paper P2
It is considered that the toner can act on the toner on the surface of the image carrier 16k via the P paper.

FIG. 6 is a diagram showing the relationship between the transfer electric field and the transferability for a low density image and a high density image. In FIG. 6, when the transfer electric field is large, the high density portion transferability is improved, but the low density portion transferability is deteriorated. FIG. 7 is an explanatory diagram of a nip width adjustment flow in the first embodiment of the image forming apparatus of the present invention. This flow starts when the power of the image forming apparatus is turned on, and is processed in parallel with other image recording flows by multitasking. When the nip width adjustment flow starts, ST1
In (Step 1), it is determined whether the copy start key is turned on. If no (N), ST1 is repeatedly executed, and if yes (Y), the process moves to ST2. In ST2, it is determined whether or not the density of the image recorded on the sheet is 50% or more. If yes (Y), the process moves to ST3. In ST3, it is determined whether the paper on which the image is to be recorded is plain paper. This determination is based on the thick paper designation key UI4a, the plain paper designation key UI4b, the OHP paper designation key U4 of the paper type input key UI4 of the UI.
This depends on which of I4c is specified. ST
If the answer is 2 (No), the process proceeds to ST4 if the answer is OHPST3 and the answer is Yes (Y).

In ST4, the leading end of the pressing member 61 is moved 1 m from the lower end (immediately below) of the image carrier to the upstream side in the sheet transport direction.
The slide arm 67 is moved to the position of m. If it is already at the position of 1 mm, it is left as it is. If no (N) in ST3, ST5
Move on to In ST5, it is determined whether the sheet is a thick sheet. In the case of Yes (Y), in ST6, the leading end position of the pressing member 61 is 0.5 m upstream from the lower end of the image carrier in the sheet transport direction.
The slide arm 67 is moved to the position of m. In the case of NO (N) in ST5, in ST7, the slide arm 67 is moved so that the leading end position of the pressing member 61 is 1.3 mm upstream from the lower end of the image carrier in the sheet transport direction.

Next, in ST7, it is determined whether or not the transfer to the sheet has been completed. If no (N), ST8 is repeatedly executed. If yes (Y), the process moves to ST9. S
At T9, it is determined whether or not the job (all copying work, or copying work of the specified number of all originals when the automatic document feeder is used) is completed. If no (N), the process proceeds to ST2, and if yes (Y), ST10.
Move on to In ST10, the slide arm 67 is moved so that the tip end of the pressing member 61 is located at a position 1 mm upstream from the lower end of the image carrier in the sheet transport direction. Then ST1
Return to

(Embodiment 2) FIG. 8 is an overall explanatory view of an image forming apparatus (tandem digital color copying machine) according to Embodiment 2 of the present invention. FIG. 9 is an explanatory diagram of a nip width adjusting device used in the image forming apparatus according to the second embodiment. FIG. 10 shows the second embodiment.
Tandem digital color copier (image forming apparatus) F
FIG. 3 is a block diagram of a control part of FIG. FIG. 11 is a graph showing the relationship between the transferability and the distance between the lower end of the image carrier 16k and the upper surface of the paper transport belt (sheet-like paper transporter) 45 with respect to the type of paper, and FIG. FIG. 11B is a graph showing low density transferability.
FIG. 12 is an explanatory diagram of a nip width adjustment flow of the image forming apparatus according to the second embodiment of the present invention. In the description of the second embodiment, components corresponding to the components of the first embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted. In the first embodiment, the nip width is adjusted by adjusting the position of the pressing member 61, whereas in the second embodiment, the pressing member 61 is fixedly arranged and the nip width is adjusted by adjusting the position of the peeling roll 42. Is configured. Therefore, in the second embodiment, the pressing member 61 is
At the fixed position (image carriers 16k, 16y, 16m, 1)
(The position 0.5 mm upstream from the lower end of 6c in the paper transport direction)
Is pressed upward. In this embodiment, the device for adjusting the position by moving the pressing member 61 such as the slide mechanism 66 and the slide arm 67 of the first embodiment is omitted.

As can be seen from FIG.
By adjusting the vertical position of the belt 2, the belt (sheet-shaped paper transport body) 45 and the image carriers 16 k, 16 y, 16 m, 16 c
The distance from the lower end can be adjusted. In the second embodiment, the reference position of the peeling roll 42 is set so that the distance is 0 mm. That is, the peeling roll 42
Is held at the reference position, the pressing member 6
Assuming that there is no image carrier 1, each of the image carriers 16k, 16k
The reference position is set such that the lowermost ends of y, 16m, and 16c are in line contact with the belt 45. By the way, since the amount of toner on a sheet passing through the nip on the downstream side in the sheet conveying direction is larger, the necessity of adjusting the nip width is higher on the downstream side than on the upstream side in the sheet conveying direction.

As can be seen from FIG.
2 moves upward, the image carriers 16k, 1
The nip width between 6y, 16m, 16c and belt 45 increases. Since the position of the driving roll 41 is fixed, an increase in the nip width between the image carrier 16k, 16y, 16m, 16c and the belt 45 when the peeling roll 42 moves upward is caused by the peeling roll 42. The image carrier 16c on the near side (downstream in the sheet transport direction) is the largest.

That is, in the second embodiment, the nip width on the most downstream side in the sheet conveying direction, in which the nip width needs to be adjusted, is preferentially adjusted. Since the recording image density differs for each color image, the recording image density used for controlling the nip width is the recording image density of the image carrier 16c on which a C (cyan) toner image is formed. .
In the second embodiment, the vertical position of the peeling roll 42 is set as follows in accordance with the type of paper and the density of an image recorded on the paper. (1) In the case where the recording image density of C (cyan) is 50% or more, (1-1) In the case of thick paper The peeling roll reference position (that is, +0.0 from the reference position)
(1-2 mm) (1-2) In the case of plain paper Release roll reference position + 0.1 mm position (1-3) In the case of OHP paper Release roll reference position + 0.2 mm position

(2) C (cyan) recording image density is 50
%, (2-1) In the case of thick paper, the release roll reference position + 0.1 mm position (2-2) In the case of plain paper Release roll reference position + 0.1 mm position (2-3) In the case of OHP paper Release Roll reference position + 0.1 mm position

Since the nip width is determined by setting the tip position of the peeling roll 42 as described above, the set value of the vertical position of the peeling roll 42 in the second embodiment can be used for controlling the nip width. Parameter. Example 2
In the second embodiment, instead of controlling the value of the nip width itself, control is performed to adjust the nip width by adjusting the vertical position of the peeling roll 42.
The set value of the nip width in the nip width control is a set value of the vertical position of the peeling roll. Therefore, the nip width determined for the case of thick paper is Wa, the nip width determined for plain paper is Wf, and the nip width determined for OHP. Is Wo, the nip width (set value of the nip width) set in the second embodiment is as follows.

(1) When the density of the image recorded on the paper is 50% or more, (1-1) For thick paper: Wa = peeling roll reference position (that is, position +0.0 mm from the reference position) (1-2) ) In the case of plain paper: Wf = peeling roll reference position +
0.1 mm position (1-3) For OHP paper Wo = peeling roll reference position + 0.2 mm position

(2) When the density of the image recorded on the paper is less than 50%, (2-1) For thick paper Wa = reference position of peeling roll + 0.1 mm (2-2) For plain paper Wf = peeling Roll reference position + 0.1 mm position (2-3) For OHP paper Wo = peeling roll reference position + 0.1 mm position

Therefore, when the density of the image recorded on the paper is equal to or greater than a predetermined value (50%), the nip width Wa is determined for the case where the type of paper is thick paper, and when the type of paper is plain paper. Nip width Wf and OHP
The nip width Wo determined corresponding to the case is set so that their sizes satisfy the following expression. Wa <Wf <Wo

Next, a nip width adjusting device Ts (see FIG. 10) for adjusting the nip width will be described with reference to FIG. 9 and 10, the nip width adjusting device Ts includes a nip width adjusting motor drive circuit D2 (see FIGS. 9 and 10) controlled by the controller C, a nip width adjusting motor M2, a peeling roll elevating device R, And peeling roller 4
2 and the like. In FIG. 9, the peeling roll 42 that moves between a rising position (two-dot chain line) and a lower position has a rotating shaft 42a. The rotary shaft 42a is rotatably supported by a bearing 71, and the bearing 71 is supported by a cylindrical roll support member 72 extending in the vertical direction. The roll support member 72 has a cam contact surface 72 a at an end on the side that does not support the peeling roll 42, and a guided member extending in the vertical direction is provided on an outer peripheral surface of the roll support member 72. 72
b is fixed.

On the outer peripheral side of the roll support member 72, a support member guide 73 supports the roll support member 72 so as to be vertically movable. A guide groove 73a is formed in the inner peripheral surface of the support member guide 73, and slidably engages with the guided member 72b of the roll support member 72 to guide the roll support member 72 in the vertical direction. It has become. An eccentric cam 74 is disposed so as to contact the cam contact surface 72a of the roll support member 72. The eccentric cam 74 is supported by a cam rotation shaft 75, and the cam rotation shaft 75 is rotatably supported by bearings 76, 76. A pulley 77 is supported at one end of the cam rotation shaft 75, and the pulley 77 has a rotational force of a pulley Pm attached to an output shaft of the nip width adjusting drive motor M2 (see FIGS. 9 and 10). Is transmitted via the belt Bm. The nip width adjusting drive motor M2 is driven by a nip width adjusting motor drive circuit D2 controlled by a controller C (see FIGS. 9 and 10). Therefore, when the pulley 77 (see FIG. 9) is rotated by the nip width adjusting drive motor M2, the eccentric cam 74 is also rotated, and the roll supporting member 72 abutting on the eccentric cam 74 moves up and down. When the peeling roll 42 supported by the roll supporting member 72 moves from the lower position to the raised position, the peeling roll 42
Is lifted from the back surface, and the image carriers 16k, 16y, 16
The nip width is increased by pressing against the surfaces of the m and 16c.

(Operation of Embodiment 2) FIG. 11 is a graph showing the relationship between the distance between the lower end of the image carrier 16k and the upper surface of the paper transport belt (sheet-like paper transporter) 45 and the transferability with respect to the type of paper. 11A is a graph showing high density transferability, and FIG. 11B is a graph showing low density transferability. In FIG. 11, the image carrier 16k
The optimal value of the distance between the belt and the belt 45 is 0.0 m for thick paper, 0.1 mm for plain paper up from directly below the image carrier 16 k, and 0.2 mm for OHP paper up from just below the image carrier 16 k. Value. In the case of low-density transfer, thick paper, plain paper,
For both OHP sheets, the optimum value is 0.1 mm upward from directly below (lower end) the image carrier 16k.

Next, the nip width adjustment flow of the image forming apparatus according to the second embodiment of the present invention will be described with reference to FIG. The nip width adjustment flow of the second embodiment is the same as that of the first embodiment shown in FIG.
ST4 ', ST4 instead of T4, ST6, ST7, ST10
6 ', ST7 and ST10' are performed. Example 2
Other STs (steps) are the same as those in the first embodiment. That is, it is determined whether or not the paper is plain paper in ST3.
If yes (Y), the peeling roll 42 in ST4 '
Is set to W = reference position + 0.1 mm. Also, S
At T5, it is determined whether the paper is thick paper. If yes (Y), the position W of the peeling roll 42 is set to W = reference position (that is, reference position + 0.0 mm) in ST6 ', and if no (N), the position W of the peeling roll 42 is set to W in ST7'. = Reference position + 0.2 mm. ST10 '
, The position W of the peeling roll 42 is defined as W = reference position +
0.1 mm. That is, as can be seen from the process of ST10 ', the position of the peeling roll 42 is always the reference position +
It is held at a position of 0.1 mm.

(Modifications) Although the embodiments of the present invention have been described in detail, the present invention is not limited to the above-described embodiments, but falls within the scope of the present invention described in the appended claims. Thus, various changes can be made. Modified embodiments of the present invention will be exemplified below. (H01) It is possible to dispose the thick paper designation key UI4a for designating that the paper is thick paper, and to dispose a thick paper detection sensor on the paper transport path.

[0054]

The image forming apparatus of the present invention using the plurality of optical scanning devices in which the rotation axes of the above-mentioned rotary polygon mirror are vertically arranged and the plurality of image carriers horizontally arranged is as follows. Can play. (E01) The toner image on the image carrier can be satisfactorily transferred to the paper irrespective of the type of paper such as plain paper, thick paper, and OHP paper.

[Brief description of the drawings]

FIG. 1 is an overall explanatory diagram of an image forming apparatus (tandem-type digital color copying machine) according to a first embodiment of the present invention.

FIG. 2 shows an image carrier 1 on which a black toner image is formed.
FIG. 8 is an explanatory view of a nip width adjusting device Tk for pressing the belt 45 against a 6k surface.

FIG. 3 is a block diagram of a control portion of a tandem type digital color copying machine (image forming apparatus) F of the first embodiment.

FIGS. 4A and 4B are diagrams showing the relationship between the tip position of the pressing member 61 and the transferability of thick paper, plain paper, and OHP paper. FIG. 4A is a diagram showing a high density portion transferability, and FIG. 4B is a low density portion. It is a figure which shows a transfer property.

5 is an explanatory diagram of a transfer operation based on a difference in the position of the leading end of the pressing member 61. FIG. 5A is an explanatory diagram of a transfer operation for thick paper, and FIG. 5B is an explanatory diagram of a transfer operation for OHP paper. It is.

FIG. 6 is a diagram illustrating a relationship between a transfer electric field and a transfer property for a low-density image and a high-density image.

FIG. 7 is an explanatory diagram of a nip width adjustment flow in Embodiment 1 of the image forming apparatus of the present invention.

FIG. 8 is an overall explanatory diagram of an image forming apparatus (tandem digital color copying machine) according to a second embodiment of the present invention.

FIG. 9 is an explanatory diagram of a nip width adjusting device used in the image forming apparatus according to the second embodiment.

FIG. 10 is a block diagram of a control portion of a tandem digital color copying machine (image forming apparatus) F according to a second embodiment.

FIG. 11 shows an image carrier 16 corresponding to the type of paper.
11A is a graph showing the relationship between the distance between the lower end of k and the upper surface of the paper transport belt (sheet-like paper transport body) 45 and the transferability. FIG. 11A is a graph showing high-density transferability, and FIG. FIG.

FIG. 12 is a second embodiment of the image forming apparatus of the present invention.
FIG. 4 is an explanatory diagram of a nip width adjustment flow of FIG.

[Explanation of symbols]

C1: nip width storage means for each paper, C2: recording image density determination means, C3: nip width adjustment signal output means, C4: paper determination means, P: paper, Q1: paper suction position, Qk, Qy, Qm,
Qc: transfer area, 16k, 16y, 16m, 16c: image carrier, 45: sheet-like sheet carrier, (51 to 54): sheet supply device, (Tk, Ty, Tm, Tc: Ts): nip width adjustment apparatus,

Claims (4)

[Claims] 1. An image forming apparatus having the following requirements: (A01) a plurality of image carriers on which a toner image is formed on a rotatable endless surface; and (A02) the toner image is A sheet suction surface for sucking a sheet to be transferred, a sheet suction position for sucking the sheet on the sheet suction surface, the sheet suction surface being pressed against the surface of each of the plurality of image carriers, and the sheet suction surface; A plurality of transfer areas in which the surfaces of the image carriers move in the same direction at the same speed while contacting each other; and a sheet-shaped paper transport body that rotates and sequentially passes through a sheet peeling position where the paper is peeled from the paper suction surface. ,
(A03) a sheet supply device for adsorbing the sheet on the sheet adsorbing surface at the sheet adsorbing position; (A04) a sheet feeding unit disposed on the opposite side of the sheet adsorbing surface of the sheet-shaped sheet transporter in each of the transfer areas; A transfer device for transferring the toner image on the surface of the image carrier onto the sheet adsorbed on the sheet suction surface, and (A05) a transfer length of the transfer surface between the image carrier surface and the sheet suction surface in each transfer area. A nip width adjusting device for adjusting a certain nip width, (A06) a sheet discriminating unit for discriminating whether the type of the sheet to which the toner image is transferred is thick paper, plain paper, or OHP; And (A08) storing a nip width determined according to the type of the paper and the recording image density. Paper by the nip width memory means, (A
09) A nip width adjustment signal for controlling the nip width adjustment device is output so that the nip width corresponds to the type of paper determined by the paper determination unit and the recording image density detected by the recording image density determination unit. Nip width adjustment signal output means. 2. The image forming apparatus according to claim 1, wherein the following conditions are satisfied: (A010) When the density of an image recorded on the sheet is equal to or higher than a predetermined value, and the type of the sheet is thick paper. And the nip width Wf determined for plain paper, and OH
The nip width Wo corresponding to the case of P,
The sheet-specific nip width storage means for storing the nip widths Wa, Wf, and Wo whose sizes are set to Wa <Wf <Wo. 3. The image forming apparatus according to claim 1, wherein the sheet is provided so as to press the sheet suction surface against the surface of the image carrier in the transfer area. A pressing member that presses the opposite side of the sheet-conveying body from the sheet-adsorbing surface, and adjusting the position of the pressing member to determine a contact length of the surface of the image carrier and the sheet-adsorbing surface in the moving direction. (A012) the nip width adjusting device for adjusting the nip width, and (A012) the pressing so that the nip width corresponds to the type of paper determined by the paper determining means and the recording image density detected by the recording image density determining means. Nip width adjustment signal output means for outputting a nip width adjustment signal for controlling the position of the member to the nip width adjustment device. 4. The image forming apparatus according to claim 1, wherein the sheet is provided so that the sheet suction surface is pressed against the surface of the image carrier in the transfer area. A peeling roll lifting / lowering device that is arranged downstream of the transfer area of the sheet-like sheet transporter and that separates and approaches a peeling roll that supports the sheet-like sheet transporter with respect to the image carrier. A nip width adjusting device that adjusts a position of a peeling roll by an elevating device to adjust a nip width which is a contact length of the surface of the image carrier and a sheet suction surface in a moving direction, and (A014) discrimination by the paper discriminating means. A nip width adjustment signal for controlling the position of the peeling roll so as to have a nip width corresponding to the type of paper and the recording image density detected by the recording image density determining means. Nip width adjustment signal output means for outputting to the nip width adjustment device.