JP6064499B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that electrostatically transfers a toner image formed on an image bearing rotating body onto a conveyed sheet and then separates the transferred sheet from the image bearing rotating body. It relates to technology to improve.

An image forming apparatus such as a printer transfers a toner image formed on an image bearing rotating body such as a photosensitive drum or an intermediate transfer belt to a recording sheet to be conveyed between the image bearing rotating body and a transfer member. Generally, the sheet is transferred electrostatically at a transfer position, and the sheet after the toner image is transferred is conveyed to a fixing unit, and the toner image on the sheet is fixed.
In this electrostatic transfer configuration, excess charge remains on the sheet during electrostatic transfer, and the sheet after passing through the transfer position does not move away from the image bearing rotator and is attached to the image bearing rotator. It is easy for the separation failure to remain. If the sheet is wound around the image bearing rotating body due to poor separation, the sheet cannot be conveyed to the fixing unit, and jamming (paper jam) occurs.

For poor sheet separation, Patent Document 1 provides a transfer charger for transferring the toner on the photosensitive drum to the sheet, and a separation charger for discharging to remove residual charges. A configuration is disclosed in which the transfer bias supplied to the transfer charger and the separation bias supplied to the separation charger are variably controlled in accordance with the moisture content.
Specifically, as the moisture content of the paper increases, the rigidity (waist) of the paper becomes weaker due to moisture absorption of the paper and the separation performance decreases. Therefore, the transfer bias is reduced and the separation bias is increased to improve the separation performance. Control is performed.

JP 2005-249889 A

However, as the moisture content of the paper increases, the paper resistance decreases and the electrical resistance value of the paper also decreases. If the electrical resistance value of the paper decreases, the transfer current easily flows through the paper, so that the residual charge of the paper is reduced and the separability is improved, and the separability is lowered due to the weakness of the paper. Are in conflicting relations.
This means that if the moisture content of the paper increases, it cannot be said that the separability is necessarily lowered. In the control of Patent Document 1, an unnecessarily large separation bias is used even though the separability is not degraded. Can occur.

The toner image on the paper immediately after the transfer has toner particles adhering to the paper by electrostatic force, but the discharge of the separation charger acts in a direction that weakens the electrostatic force between the toner particles and the paper, so the separation bias As the discharge amount from the separation charger increases, the toner particles are more likely to be scattered, and the image quality is likely to deteriorate due to so-called static elimination noise.
On the other hand, when the moisture content of the paper is small, the rigidity of the paper is increased and the electrical resistance value of the paper is increased. When the rigidity of the paper increases, the separability improves. However, when the electrical resistance value increases, the residual charge of the paper increases and the separability decreases. Therefore, as in the case where the moisture content of the paper is large, the improvement and reduction of the separation property are in a contradictory relationship.

In other words, the moisture content of the paper cannot be said to represent the separation characteristics of the paper without contradiction, and is not suitable for controlling the separation.
SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus that can further improve the separation of a sheet from an image bearing rotating body.

In order to achieve the above object, an image forming apparatus according to the present invention transfers a toner image formed on an image carrying rotator between the image carrying rotator and a transfer member onto a conveyed sheet. An image forming apparatus that electrostatically transfers at a position and separates a sheet onto which a toner image has been transferred from the image carrying rotator after passing through the transfer position, and contacts the image carrying rotator of a sheet to be conveyed A first acquisition unit that acquires sheet information indicating the smoothness of the side surface; a second acquisition unit that acquires a value representing the bending rigidity and an electrical resistance value of the sheet in units of sheets; and the image carrier. A value representing the smoothness of the surface of the rotator is acquired once every fixed number of sheets or every fixed period for each of a plurality of different locations along the direction orthogonal to the rotation direction of the image bearing rotator . Acquisition means and single sheet Therefore, the separation parameter that affects the separability when the sheet is separated from the image bearing rotating body is set so that the separability increases as the smoothness of the sheet obtained from the sheet information increases. And a control means for controlling, wherein the control means divides the value representing the bending rigidity of the sheet by the electrical resistance value of the sheet, to the reciprocal of the value representing the smoothness of the sheet obtained from the sheet information. The separability index G obtained by multiplying the values is obtained, and the separability index G obtained for the sheet in units of sheets is added to the plurality of locations on the surface of the image bearing rotating body acquired last time. Is corrected to a new value G1 obtained by multiplying the reciprocal of the highest smoothness value among the values representing the smoothness of each of the first and second sheets having the same smoothness and different separability indices G1, Second seat If the separation index G1 than the first sheet is small, so that towards the second sheet separability is higher than the first sheet, and controls the separation parameter.

Further, the first acquisition unit may be a detection unit that detects the smoothness of the sheet to be conveyed.
Here, the detected smoothness may be Beck smoothness or an index value of smoothness having a correlation therewith.
Further, the detecting means receives the reflected light from the light emitting part toward the surface of the sheet that contacts the image bearing rotating body, and receives the reflected light from the light receiving part. An optical sensor that outputs a signal indicating the magnitude of the quantity may be used, and the smoothness index value may be the output signal.

A sheet feeding cassette that accommodates the sheet; and a conveyance unit that conveys the sheet accommodated in the sheet feeding cassette one by one to the conveyance path and conveys the fed sheet to the transfer position. The target sheet may be a sheet stored in the sheet feeding cassette or a sheet being conveyed through the conveyance path.
Further, the sheet information may be information at a front end portion in a sheet conveying direction of a surface of the sheet to be conveyed that is in contact with the image carrying rotator.

Further, the first acquisition unit may be a reception unit that receives an input of the sheet information from a user.
The sheet information may be a type name or name of a sheet to be conveyed.
Further, the control unit may execute the control until the front end in the sheet conveyance direction of the conveyed sheet passes the transfer position and is conveyed by a predetermined distance, and thereafter stops the control.

Further, the sheet having passed through the transfer position is provided with a charge removal member that performs discharge for separating the sheet from the image bearing rotator, and the separation parameter is a separation bias supplied to the charge removal member. The control means may perform control to increase the separation bias with an absolute value as the smoothness increases.
Further, the separation parameter is a voltage or current as a transfer bias supplied to the transfer member, and the control unit performs control to lower the transfer bias by an absolute value as the smoothness increases. Also good.

In addition, a driving unit for rotating the image bearing rotator is provided, the separation parameter is a rotation speed of the image bearing rotator, and the control unit is configured to increase the smoothness of the image bearing rotator. The driving means may be controlled so that the rotation speed of the rotating body is slow.
Further, the image carrying rotator is a photoconductor, and prior to the transfer step for performing the transfer, a charging step for charging the surface of the photoconductor, and exposing the charged photoconductor surface to form a latent image. And a developing step of forming a toner image by developing the surface of the photoreceptor on which the latent image is formed, and the separation parameter is a surface potential of the photoreceptor, and the control means The control may be performed such that the surface potential of the photoconductor is decreased in absolute value as the smoothness increases.

A fixing step of thermally fixing the toner image on the surface of the sheet separated from the image bearing rotating body to the sheet; a step of feeding back the fixed sheet to the transfer position; and a back surface of the fed back sheet. A step of electrostatically transferring a toner image different from the toner formed on the image carrying rotator at the transfer position, and a step of separating the sheet that has passed the transfer position from the image carrying rotator. The separation parameter is a fixing temperature when the toner image on the surface of the sheet is thermally fixed in the fixing step, and the first acquisition unit is configured to perform the two-sided printing function. Sheet information indicating the smoothness of the back surface of the sheet is acquired, and the control unit performs control to lower the fixing temperature as the smoothness of the back surface of the sheet increases. And it may be.

  Furthermore, an image forming unit for forming a transparent, white or yellow toner image different from the toner image on the image carrier is provided, and the separation parameter is formation of the toner image by the image forming unit. The control unit forms an image of the toner on the sheet having the smoothness equal to or greater than the threshold by the image forming unit, and transfers the formed toner image to the leading end region in the sheet conveyance direction. The image formation control may not be performed on a sheet having smoothness lower than a threshold value.

  Further, the conveyed sheet includes plain paper and coated paper having a higher surface smoothness than this, and the plain paper and the coated paper are selectively conveyed to the transfer position. May be.

  As described above, as the smoothness of the sheet increases in units of sheets as described above, control is performed to change the separation parameter that affects the separation of the sheet from the image bearing rotating body so as to increase the separation. As a result, the separability of the sheet from the image bearing rotating body can be further improved.

1 is a diagram illustrating an overall configuration of a printer according to Embodiment 1. FIG. FIG. 6A is a schematic diagram illustrating a state in which a sheet is wound around the surface of an intermediate transfer belt in a configuration example in which a static elimination needle is not provided, and FIG. 5B is a configuration example in which a static elimination needle according to the embodiment is provided. FIG. 6 is a schematic diagram illustrating a state where the sheet is separated from the surface of the intermediate transfer belt. It is a block diagram which shows the structure of a control part. (A) is the figure which showed and showed the relationship between the electrical resistance value of a paper, and separation property from the relationship between environment (temperature / humidity) and a paper type, (b) is the relationship between separation property and rigidity of a paper. Is derived from the relationship between environment (temperature and humidity), paper type, and rigidity. It is a figure which shows the experimental result of the separation property quality of a paper from an intermediate transfer belt. 6 is a diagram illustrating an example of a list of paper types. FIG. It is a figure which shows the relationship between the smoothness and separation property of a paper. It is a figure which shows the relationship between a paper kind and Beck smoothness. It is a figure which shows the relationship between Beck smoothness and isolation | separation voltage. It is a figure which shows the result of having confirmed by experiment whether the separation voltage was constant irrespective of Beck smoothness (comparative example) and the case where it changed based on Beck smoothness (Example 1). It is a table formed by associating the detection value of the paper smoothness detection sensor with the Beck smoothness. It is a flowchart which shows the content of the separation voltage determination process. 10 is a plan view showing a configuration of an operation unit according to Embodiment 2. FIG. 10 is a diagram illustrating a configuration example of a separation voltage table according to Embodiment 2. FIG. FIG. 9 is a diagram illustrating a configuration for detecting Beck smoothness, bending rigidity, and electrical resistance value of a sheet according to a third embodiment. It is a figure which shows the structure of a conveyance path | route detection sensor. It is a figure which shows the structural example of the separation voltage table which shows the correspondence of a separability index | exponent and a separation voltage. 10 is a flowchart showing the content of a separation voltage determination process according to the third embodiment. It is a figure which shows the result of having confirmed by experiment the quality of the separability at the time of determining a separable voltage from a separability index. FIG. 10 is a diagram illustrating a configuration for detecting Beck smoothness, bending rigidity, electrical resistance value, and smoothness change rate of the surface of an intermediate transfer belt according to a fourth embodiment. It is a figure which shows the result of having confirmed by experiment the quality of the separability at the time of determining a separable voltage from a separability index. It is a schematic diagram for demonstrating the structure which detects the smoothness of the several location of a belt surface. FIG. 10 is a diagram illustrating a configuration example of a direct transfer type image forming apparatus according to a modification.

Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described by taking a tandem color printer (hereinafter simply referred to as “printer”) as an example.
[Embodiment 1]
(1) Overall Configuration of Printer 1 FIG. 1 is a diagram illustrating an overall configuration of the printer 1.

  As shown in the figure, the printer 1 forms an image by a well-known electrophotographic system, and includes an image process unit 10, an intermediate transfer unit 20, a feeding unit 30, a fixing unit 40, and a control unit. 50, an operation unit 60, and a separation unit 70, which are connected to a network (for example, a LAN) and receive a print (print) job execution instruction from an external terminal device (not shown), based on the instruction Thus, color image formation of yellow (Y), magenta (M), cyan (C), and black (K) is executed.

  The image processing unit 10 includes image forming units 10Y, 10M, 10C, and 10K corresponding to Y to K colors. The image forming unit 10Y includes a photosensitive drum 11Y that rotates in a direction indicated by an arrow, and a cleaner for cleaning the surfaces of the charging unit 12Y, the exposure unit 13Y, the developing unit 14Y, and the photosensitive drum 11Y disposed around the photosensitive drum 11Y. 15Y and the like, and a Y-color toner image is formed on the photosensitive drum 11Y through charging, exposure, and development processes. This configuration is the same for the other image forming units 10M to 10K. The image forming units 10M, 10C, and 10K are charged units 12M, 12C, and 12K, exposure units 13M, 13C, and 13K, and developing units 14M, 14C, and 14K. Further, cleaners 15M, 15C, and 15K for cleaning the surface of the photosensitive drum 11Y are provided, and toner images of corresponding colors are formed on the photosensitive drums 11M, 11C, and 11K.

The intermediate transfer unit 20 includes an intermediate transfer belt 21, a driving roller 22, a driven roller 23, primary transfer rollers 24Y, 24M, 24C, and 24K, a secondary transfer roller 25, and the like. The intermediate transfer belt 21 is an endless belt.
The drive roller 22 is located on one end side in the direction in which the image forming units 10Y to 10K are arranged (the left-right direction in the figure), and the driven roller 23 is located on the other end side. The primary transfer rollers 24Y to 24K are between the driving roller 22 and the driven roller 23, and are disposed to face the corresponding photosensitive drums 11Y to 11K with the intermediate transfer belt 21 interposed therebetween.

The intermediate transfer belt 21 is stretched by these rollers along the direction in which the image forming units 10Y to 10K are arranged, and is driven to rotate in the direction indicated by the arrow in FIG.
When performing color printing, the surface of the photosensitive drums 11Y to 11K is charged by the charging units 12Y to 12K (charging process) for each of the image forming units 10Y to 10K (charging process), and the charged photosensitive drums 11Y to 11K are charged. Are exposed and scanned by the exposure units 13Y to 13K to form a latent image (exposure process), and the latent images on the photosensitive drums 11Y to 11K are developed by the developing units 14Y to 14K, so that the colors Y to K are developed. The toner image is formed (development process).

Then, each color toner image formed on the photosensitive drums 11Y to 11K is generated between the photosensitive drums 11Y to 11K and the primary transfer rollers 24Y to 24K by the primary transfer voltage applied to the primary transfer rollers 24Y to 24K. It is electrostatically transferred onto the intermediate transfer belt 21 by the action of an electric field (primary transfer step).
In this image forming operation for each of the colors Y to K, the timing is shifted from the upstream side toward the downstream side in the circumferential direction so that the toner images of the respective colors are primary-transferred superimposed on the same position of the circulating intermediate transfer belt 21. Executed.

The secondary transfer roller 25 is pressed against the driving roller 22 via the intermediate transfer belt 21, and forms a transfer nip N between the secondary transfer roller 25 and the intermediate transfer belt 21.
The feeding unit 30 includes a paper feed cassette 31, a feeding roller 32, a transport roller pair 33, and a timing roller pair 34.
The paper feed cassette 31 stores a plurality of sheets, here, paper S, and the feed roller 32 feeds the paper S stored in the paper feed cassette 31 one by one onto the transport path 35. In the paper feed cassette 31, as the paper S to be used, different sizes, for example, B5 to A3 size paper, different types, for example, plain paper, reprint paper, coated paper including coated paper and art paper, etc. Can be accommodated selectively. Here, the coated paper is a paper whose aesthetics and smoothness are enhanced by applying paint or pigment on the paper surface. The type of coated paper will be described later. Plain paper other than coated paper or reprinted paper is called non-coated paper.

The transport roller pair 33 sends the paper S fed from the paper feed cassette 31 to the timing roller pair 34. The timing roller pair 34 sends the sheet S conveyed from the conveyance roller pair 33 to the secondary transfer roller 25 in accordance with the timing of the image forming operations of Y to K colors.
When the sheet S sent to the secondary transfer roller 25 (transfer member) passes through the transfer nip N where the secondary transfer roller 25 and the intermediate transfer belt 21 are in pressure contact with each other, Each of the color toner images that have been multiplex-transferred on the sheet S is sandwiched between the intermediate transfer belt 21 by the secondary transfer voltage applied to the secondary transfer roller 25, and the sheet S is generated by the action of an electric field generated between the secondary transfer roller 25 and the drive roller 22. It is electrostatically transferred onto the top (secondary transfer process).

In this embodiment, since the polarity of the toner is negative, the secondary transfer voltage is a positive polarity voltage opposite to this. The transfer nip N is a position between the intermediate transfer belt 21 and the secondary transfer roller 25 and can be said to be a transfer position at which the toner image on the intermediate transfer belt 21 (image carrier) is transferred to the paper S.
The separation unit 70 includes a static elimination needle 71 and a separation voltage output unit 72 that applies a separation bias to the static elimination needle 71, and removes the residual charge of the sheet S that has passed through the transfer nip N by discharge from the static elimination needle 71. By separating the sheet S from the surface of the intermediate transfer belt 21, the paper S is prevented from being wound around the surface of the intermediate transfer belt 21.

FIG. 2A is a schematic diagram illustrating a state in which the sheet S is wound around the surface of the intermediate transfer belt 21 in a configuration example in which the neutralizing needle is not provided, and FIG. 2B is a neutralization according to the embodiment. 4 is a schematic diagram illustrating a state where a sheet S is separated from the surface of an intermediate transfer belt 21 in a configuration example in which a needle is provided. FIG.
In the configuration shown in FIG. 2A, the rotational axis of the driving roller 22 is grounded, and a secondary transfer voltage having a positive polarity is applied to the rotational axis of the secondary transfer roller 25 from the secondary transfer voltage output unit 250. Has been. The sheet S that has passed through the transfer nip N retains a lot of positive charges during secondary transfer, and this residual charge electrostatically attracts the leading edge Sa in the transport direction of the sheet S to the surface of the intermediate transfer belt 21. Attracting power to try.

On the other hand, the sheet S is subjected to a restoring force to return to the original straight posture due to its waist, but is conveyed by the fact that the attracting force on the surface of the intermediate transfer belt 21 due to the residual charge becomes larger. The leading edge Sa of the sheet S cannot be separated from the surface of the intermediate transfer belt 21 and is wound around the surface of the intermediate transfer belt 21 that circulates.
When the sheet S that has passed through the transfer nip N is wound around the surface of the intermediate transfer belt 21, it is not conveyed to the fixing unit 40, and printing is interrupted due to the occurrence of a jam.

On the other hand, if the configuration in which the static elimination needle 71 is provided as shown in FIG. 2B, a negative polarity voltage is applied to the static elimination needle 71 from the separation voltage output unit 72 as a separation bias, and the tip of the static elimination needle 71 is applied. By discharging the negative polarity charge from 71a, the positive polarity charge remaining on the paper S can be removed (cancelled).
FIG. 2B shows that the charge of the positive polarity remaining on the sheet S is smaller than that of FIG. 2A due to the charge eliminating action of the charge eliminating needle 71, and thus the residual charge on the surface of the intermediate transfer belt 21 of the sheet S. The restoring force of the sheet S becomes larger than the attracting force by the sheet S, and when the leading end Sa of the sheet S passes through the transfer nip N, the sheet S is separated from the surface of the intermediate transfer belt 21 and is not jammed. Is conveyed toward the fixing unit 40.

Here, the static elimination needle 71 is made of a thin plate made of metal, for example, about 500 [μm], and is long along the width direction of the paper S (direction perpendicular to the transport direction: equivalent to the direction perpendicular to the paper surface of the figure). The side opposite to the transfer nip N is formed in a saw shape. The apex of the saw-tooth is equivalent to the tip 71 a of the static elimination needle 71.
The separation voltage output unit 72 has a function of variably outputting a negative separation voltage having a polarity opposite to that of the secondary transfer voltage as a separation bias, and applies the output separation voltage to the static elimination needle 71. The value of the separation voltage is varied according to an instruction from the control unit 50. The controller 50 determines the value of the separation voltage based on the surface smoothness of the paper S as will be described later, and instructs the separation voltage output unit 72 of the determined voltage value.

Returning to FIG. 1, the sheet S after passing through the transfer nip N is conveyed to the fixing unit 40.
The fixing unit 40 includes a fixing roller 41 that is heated by a heating unit (not shown) and a pressure roller 42 that is pressed against the fixing roller 41. The toner image is fixed on the paper S (fixing step). The sheet S after fixing is discharged onto the discharge tray 39 via the discharge roller pair 38.

  The operation unit 60 is arranged on the front side of the main body of the printer 1 at a position where the user can easily operate. The operation unit 60 is a touch panel type that displays a numeric keypad for the user to input the number of prints and a key for setting and inputting the size (A4, B4, etc.) of the paper S stored in the paper feed cassette 31. It has a liquid crystal display. Information such as the accepted paper size is sent to the control unit 50.

Near the transport path 35 and at a position upstream of the timing roller pair 34 in the paper transport direction, a paper smoothness detection sensor for detecting the smoothness of the surface of the transported paper S, here the surface smoothness. 80 is arranged.
Here, the surface smoothness corresponds to the surface roughness per unit area. For example, the coated paper has a characteristic that the smoothness is higher (the surface roughness is smaller) than the plain paper or the additional paper. Yes.

The paper smoothness detection sensor 80 is an optical sensor including a light emitting unit 81 and a light receiving unit 82, and the light emitted from the light emitting unit 81 is brought into contact with the surface of the paper S conveyed through the conveyance path 35 (contacted with the intermediate transfer belt 21). The light is diffused and reflected from the surface of the paper S by the light receiving unit 82 and converted into an electric signal indicating the magnitude of the amount of light received.
The diffuse reflection light from the surface of the paper S has a characteristic that the reflection angle thereof changes depending on the surface smoothness, and here the diffuse reflection light decreases as the surface smoothness increases.

  Accordingly, if the amount of diffuse reflected light received and the surface smoothness are associated in advance, the sheet S is detected from the detection signal indicating the amount of diffuse reflected light received from the sheet smoothness detection sensor 80 for the sheet S to be used. Can be obtained. In this sense, the detection value of the paper smoothness detection sensor 80 can be said to be sheet information that indicates the smoothness of the surface of the paper S to be transported that contacts the intermediate transfer belt 21. A detection signal of the paper smoothness detection sensor 80 is sent to the control unit 50.

(2) Configuration of Control Unit 50 FIG. 3 is a block diagram showing the configuration of the control unit 50.
As shown in the figure, the control unit 50 includes a communication interface (I / F) unit 51, a CPU 52, a ROM 53, a RAM 54, a paper surface smoothness detection unit 55, a separation voltage control unit 56, and a smoothness table. 57 and a separation voltage table 58, and each unit can exchange signals and data with each other.

The communication interface unit 51 is an interface such as a LAN card or a LAN board for connecting to a network, here, a LAN, and receives print job data sent from an external terminal via the LAN to receive an image (not shown). Store in memory.
The CPU 52 reads a necessary program from the ROM 53, and based on the print job data stored in the image memory and information input by the user from the operation unit 60, the image processing unit 10, the intermediate transfer unit 20, and the feeding unit 30. Then, the image forming operation (printing) is smoothly executed by controlling the fixing unit 40 and the like.

The RAM 54 serves as a work area for the CPU 52.
The paper surface smoothness detection unit 55 detects Beck smoothness (described later) representing the paper surface smoothness for each sheet of paper S conveyed from the detection signal of the paper smoothness detection sensor 80.
The separation voltage control unit 56 determines a separation voltage based on the Beck smoothness detected by the paper surface smoothness detection unit 55 for each sheet, and the determined separation voltage is applied to the static elimination needle 71. The separation voltage output unit 72 is controlled.

The reason why such control is performed will be described in order below, comparing (a) the relationship between the electrical resistance value of the sheet S and the separation property and (b) the relationship between the smoothness and the separation property of the sheet S.
(A) Relationship between electrical resistance value and separation of paper S FIG. 4A shows a relationship between electrical resistance value of paper S and separation from the relationship between environment (temperature and humidity) and paper type. FIG.

Here, HH in the figure represents a high-temperature and high-humidity environment, NN represents a normal temperature and normal humidity environment, and LL represents a low-temperature and low-humidity environment. Hereinafter, they are referred to as HH environment, NN environment, and LL environment.
Looking at the relationship between the electrical resistance of the paper S and the environment, the electrical resistance of both the A-type paper and the B-type paper decreases with the shift from the LL environment to the HH environment. This is because the moisture absorption of the paper S increases in the order of the LL environment, the NN environment, and the HH environment, and it can be seen that the electrical resistance of the paper S is dependent on the environment regardless of the paper type.

On the other hand, looking at the relationship between the electrical resistance of the paper S and the separability, it is an inversely proportional relationship where the separability decreases as the electrical resistance increases. This is because as the electric resistance of the sheet S increases, the transfer current becomes less likely to flow, and the amount of positive charge (FIG. 2) remaining on the sheet S after the secondary transfer increases, resulting in a decrease in separability.
FIG. 4B is a diagram showing the relationship between the separation property and the rigidity of the paper S derived from the relationship between the environment (temperature and humidity), the paper type, and the rigidity.

Looking at the relationship between the rigidity of the paper S and the environment, as for the paper type A and B paper, as the paper moves from the LL environment to the HH environment, the moisture absorption of the paper S increases and the paper becomes weak. For this reason, the rigidity decreases as the LL environment moves to the HH environment. From this, it can be seen that the rigidity of the paper S is dependent on the environment regardless of the paper type.
On the other hand, looking at the relationship between the rigidity of the paper S and the separability, it is in a proportional relationship in which the separability decreases as the rigidity decreases. This is because the lower the rigidity of the paper S, the lower the stiffness of the paper S, and the higher the adsorption force of the paper S against the surface of the intermediate transfer belt 21 such as electrostatic force and van der Waals force, the lower the separation. Because.

As described above, the separability is inversely proportional to the electrical resistance of the paper S with respect to the environmental change, but is proportional to the rigidity of the paper S. From this, it is considered that the separability satisfies the following relationship (Equation 1).
Separability = (Rigidity of paper S) / (Electrical resistance value of paper S) (Equation 1)
Therefore, whether or not the separation property in calculation obtained by (Equation 1) represents the separation property of the sheet S that is actually conveyed is determined by experiment using various types of sheets S. Confirmed by

FIG. 5 is a diagram showing experimental results of whether the paper S is separable from the intermediate transfer belt 21. The paper type, paper name, basis weight, environment, bending stiffness and resistance as paper characteristics, separability index, The experimental results of the separation voltage and the separability are associated with each other and arranged in order from the highest separability index to the lowest.
There are two types of paper, coated paper and non-coated paper. The coated paper includes various types of paper such as coated paper and art paper as shown in FIG. As the paper, “OK Kanto + (T eye)” (manufactured by Oji Paper) shown in FIG. 5 is used. The paper name “J” means “J paper” (manufactured by Konica Minolta), and the recycled quality means “recycled quality” (made by Nippon Paper Industries).

  Here, the T-th line indicates the fiber direction of the paper, and is the paper on which the paper fiber flows parallel to the long side of the paper. On the other hand, a paper in which paper fibers flow parallel to the short side of the paper is called a Yth eye. The paper used in the experiment was A4 size, and this was carried out in the horizontal direction (the direction of the paper being transported in a posture in which the short side of the paper is parallel to the paper transport direction). The Y eye is stronger than the T eye.

The bending stiffness corresponds to the stiffness of the paper S, and is measured by a stiffness measuring module (manufactured by Kumagai Riki Kogyo Co., Ltd.) for a fully automatic paper property measuring device. The unit is [mN · m]. Yes.
The resistance corresponds to the electrical resistance of the paper S, and is measured by Hiresta manufactured by Mitsubishi Chemical Corporation. The unit is [(log) MΩ].
The separability index corresponds to the separability of (Formula 1), and the greater the separability index, the higher the separability. The separation voltage was fixed at -2 [kV] in the experiment. The experimental machine used was a bizhub C652 made by Konica Minolta modified for experimentation.

  As the experimental results of the separability, “good” indicates that the separability is good, and “poor” indicates that the separability is poor. In order to judge whether the separation is good or bad, the separation claw 99 indicated by the broken line in FIG. This was done by judging that the paper that was photographed and that did not hit the separation claw 99 from the photographed image was good, and that the paper that hit was bad.

  Looking at the experimental results of separability, it can be seen that those having a high separability index are good (◯), and those having a low separability index are poor (×). However, some of the coated papers are poor (×) even when the separability index is 1.56, and non-coated papers with a separability index of 1.41 that is lower than this and are good (◯). In consideration of the fact, the separability index is not necessarily suitable for expressing the separability of the paper S.

  In recent years, in offices such as companies, image forming devices have entered the field of in-house printing, and in order to create leaflets and catalogs, the ability to pass thinner coated paper has become especially important. Therefore, there is a demand for higher-quality image output that is more luxurious than plain paper because of its glossy paper characteristics. When a separation failure occurs, the coated paper is wasted due to the occurrence of a jam. If the separation bias is increased too much to avoid the separation failure, toner particles may scatter or the separation nail may be applied to the paper S when the separation nail is used. The generation of these image noises, such as separation claw traces that leave traces and rubbing by the separation claw on the image surface, makes it impossible to achieve high image quality.

As a result of intensive studies to solve the above problems, the present inventors have found that the smoothness of the paper S affects the separability.
(B) Relationship between Smoothness and Separability of Paper S FIG. 7 is a diagram showing the relationship between smoothness and separation of the paper S, and also shows the relationship between the smoothness of the paper S and the environment.

As shown in the figure, the smoothness of the paper S does not depend on the environment, and it can be seen that the smoothness and the separation of the paper S have an inverse relationship in which the higher the smoothness, the lower the separation.
The reason why the separability becomes lower as the smoothness of the paper S becomes higher is considered as follows.
That is, when the smoothness of the paper S increases, the effective contact area per unit area of the surface of the paper S microscopically with the surface of the intermediate transfer belt 21 becomes wider and more easily adheres. Therefore, the effect of physical adsorption including van der Vaals force increases. This is because when the attracting force is increased, the sheet S in close contact with the surface of the intermediate transfer belt 21 at the time of secondary transfer is hardly separated from the surface of the intermediate transfer belt 21 after the secondary transfer.

It was confirmed by an experiment whether the relationship between the smoothness and the separation property of the paper S represents the smoothness and the separation property of the paper S actually conveyed.
FIG. 8 is a diagram showing the result of confirming the smoothness and separation of the sheet S actually transported by experiment. Only three of the sheet types shown in FIG. It shows the result of the experiment under the same conditions. Since the smoothness of the paper S does not depend on the environment, the experiment is performed only in the NN environment.

  The Beck smoothness shown in FIG. 8 is generally used as a value representing the surface smoothness and corresponds to the smoothness of the paper S. In the experiment, for each paper S used, Kumagaya Riki Kogyo Co., Ltd. The result measured using a Beck smoothness meter manufactured by the company is shown, and the unit is [(log) s] (seconds). As the Beck smoothness increases, the surface smoothness of the paper S increases (roughness decreases), and the separability decreases.

  As shown in FIG. 8, when the experiment was performed under the same conditions for the environment and the separation voltage, among the three types of paper S, the non-coated paper with the smallest Beck smoothness showed a good separation result (◯). Yes, both the non-coated paper and the coated paper having a higher Beck smoothness than the above showed poor (×) separation results. From this result, it can be seen that the Beck smoothness increases, that is, the smoother the paper, the lower the separability, and it was proved that the smoothness and the separability of the paper have the correlation shown in FIG. It will be.

Therefore, in the present embodiment, for the paper S having low separation due to the Beck smoothness, the separation voltage is increased as an absolute value so that the separation does not deteriorate for any paper type. The intermediate transfer belt 21 is prevented from being wound around.
FIG. 9 is a diagram showing the relationship between the Beck smoothness X and the separation voltage V according to the present embodiment, and the Beck smoothness X and the separation voltage V are divided into three stages of large, medium, and small Beck smoothness X. When divided, the separation voltage V becomes smaller in absolute value in this order.

For example, for a paper S with a Beck smoothness X of 1, the separation voltage V is set to −2 [kV], and for a paper S with a Beck smoothness X of 3.5, the separation voltage V is −4 [ kV].
FIG. 10 shows a case where the separation voltage is constant regardless of the Beck smoothness (comparative example) and a variable based on the Beck smoothness on the same paper as the type of paper shown in FIG. It is a figure which shows the result which confirmed the quality of the separability of Example 1) by experiment. Here, in FIG. 5, each type of paper is arranged in order from the smallest Beck smoothness, which is partly different from the order shown in FIG. 5.

As shown in FIG. 10, in the comparative example, for non-coated paper and coated paper having Beck smoothness X of 2.4 or more, the separation performance is 2−kV. It is defective (x), and separability cannot be ensured.
On the other hand, when Example 1 is seen, the separation voltage V is varied according to the Beck smoothness X, and the separability is good (◯) for all types of non-coated paper and coated paper. It can be seen that separability is ensured.

If the separation voltage is varied from the separation index shown in FIG. 5, among the paper types, the third coated paper “OK Kanto + (Y eyes)” from the top is −2 [ Since the separation voltage is poor (x) at a separation voltage of kV], it is conceivable to increase the separation voltage (absolute value).
However, the fourth non-coated paper “J” from the top has a separation voltage of −2 [kV] and has a good separation (◯). Therefore, if the separation voltage (absolute value) is increased, An excessive separation voltage acts on the non-coated paper “J”, which is not preferable.

To avoid this, the separation voltage (absolute value) of the third coated paper “OK Kanto + (Y eyes)” remains for the fourth non-coated paper “J” with the separation voltage being −2 kV. However, this is not possible because of the magnitude of the separability index, and in the configuration using the separability index, the separability cannot be improved.
By performing control to vary the separation voltage V using the Beck smoothness X having characteristics that do not depend on the environment as described above, the separability can be improved.

FIG. 11 is a diagram illustrating a configuration example of the smoothness table 57 illustrated in FIG. 3, and is a table in which the detection value α of the paper smoothness detection sensor 80 and the Beck smoothness X are associated with each other. In advance, the value of the detected value α of the paper smoothness detection sensor 80 and the value of the Beck smoothness X are obtained from experiments, and the obtained result is written in the smoothness table 57. .
Returning to FIG. 3, the paper surface smoothness detection unit 55 is transported by reading the Beck smoothness X corresponding to the detection value α of the paper smoothness detection sensor 80 with reference to the smoothness table 57. The Beck smoothness X of the surface of the paper S is detected.

  The separation voltage table 58 is a table in which the Beck smoothness X and the separation voltage V are associated with each other, and the same information as the correspondence between the Beck smoothness X and the separation voltage V shown in FIG. The correspondence between the Beck smoothness X and the separation voltage V is obtained in advance from experiments or the like, but it is sufficient that the separation voltage V within a range in which the separability is good is set, and the values shown in FIG. It is not limited to. In addition, the configuration example in which the Beck smoothness X is divided into three stages has been described. However, the present invention is not limited to this, and the separation voltage V suitable for the stage may be set in advance in two or four or more stages.

(3) Contents of Separation Voltage Determination Process FIG. 12 is a flowchart showing the contents of the separation voltage determination process. This separation voltage determination process is started by the control unit 50 along with job execution.
As shown in the figure, it is determined whether or not the first (first) sheet S has been fed out from the sheet feeding cassette 31 toward the transport path 35 by the start of the job (step S1). This determination is made by determining whether or not the operation of the feeding roller 32 of the feeding unit 30 has been started.

When it is determined that the first sheet S has been fed out by starting the operation of the feeding roller 32 (“YES” in step S1), the sheet smoothness detection sensor 80 for the sheet S being conveyed through the conveyance path 35. The sensor detection value α from is acquired (step S2).
Acquisition of the sensor detection value α (sheet information) is performed on the leading end portion of the sheet S, for example, a sheet portion in a range of 3 mm from the leading end in the transport direction toward the trailing end. Once the leading edge of the sheet S is separated from the intermediate transfer belt 21, the separated state is almost always continued only by the stiffness of the sheet S, that is, the rigidity. It is because it is desirable to acquire.

In addition, the acquisition range of the sensor detection value α of the sheet S is not limited to this. For example, instead of 3 [mm], a range from the leading end of the sheet S to 10 [mm], or from the leading end in the transport direction to the center in the transport direction. It is good also as the range to.
Then, the Beck smoothness X of the sheet S with respect to the acquired sensor detection value α is acquired (step S3). This acquisition is performed by referring to the smoothness table 57.

A separation voltage V for the paper S is determined from the acquired Beck smoothness X (step S4). This determination is made by referring to the separation voltage table 58. For example, when the Beck smoothness X is 1, the separation voltage V is determined to be −2 [kV] from FIG.
The output value of the separation voltage output unit 72 is controlled so that the application of the determined separation voltage V to the static elimination needle 71 is started at the timing when the leading edge Sa of the sheet S reaches the transfer nip N. .

The application of the separation voltage V to the static elimination needle 71 is started when the leading end Sa of the sheet S reaches the transfer nip N, and is conveyed by a predetermined distance, for example, 10 mm after the leading end Sa passes through the transfer nip N. Then it stops.
Thus, the output control of the separation voltage V is executed until the leading end Sa of the paper S passes through the transfer nip N until it is conveyed by a predetermined distance, and is stopped thereafter for the following reason.

That is, if the leading edge Sa of the sheet S is separated from the surface of the intermediate transfer belt 21, the separated state is almost always continued by the rigidity of the sheet S (strength of waist). This is because it is not necessary to continue the separating action by the charge eliminating needle 71 after the toner is separated from the surface of the intermediate transfer belt 21.
Depending on the configuration of the apparatus, not only the leading end of the sheet S but also the entire sheet S, that is, until the trailing end of the sheet S passes through the transfer nip N, is separated into the charge eliminating needle 71. The application of the voltage V may be continued. In this case, the predetermined distance is the length in the transport direction of the paper S.

Further, the separation voltage V determined by the Beck smoothness X is used only for the front end portion of the paper S, and a portion other than the front end portion of the paper S (the center portion, the rear end portion, etc.) The separation voltage V may be restored.
For example, if the reference separation voltage V is −2 [kV] and the separation voltage V determined by the Beck smoothness X is −4 [kV], −4 [kV] is applied to the leading edge of the paper S. After the application and the leading edge of the sheet S has passed through the transfer nip N, control to return to the reference −2 [kV] may be performed.

Further, the application start time of the separation voltage V to the static elimination needle 71 is not limited to the time when the leading edge Sa of the paper S reaches the transfer nip N. Depending on the apparatus configuration, for example, the leading edge Sa of the paper S reaches the transfer nip N. It is also possible to set the time immediately before the start or the time when the leading edge Sa of the paper S passes through the transfer nip N.
The timing at which the leading edge Sa of the sheet S reaches the transfer nip N, the timing at which the leading edge Sa passes through the transfer nip N, the timing at which the trailing edge of the sheet S passes through the transferring nip N, and the like detect the leading edge and trailing edge of the conveyed sheet S. A sensor (not shown) for this purpose can be separately arranged along the transport path 35 and detected based on the detection timing of the sensor. This sensor may be shared by the paper smoothness detection sensor 80.

  In step S5, it is determined whether there is a sheet S to be fed out next. If it is determined that it exists (“YES” in step S5), the process returns to step S1 to determine whether or not the second sheet S has been fed out. If it is determined that the second sheet S has been fed out ("YES" in step S1), the processes subsequent to step S2 are executed for the second sheet S.

Until it is determined in step S5 that the next sheet S does not exist, the processes of steps S1 to S5 are repeatedly performed on the sheet S for each sheet S to be fed.
When it is determined that the next sheet S does not exist, that is, after the final sheet has been fed out (“NO” in step S5), the separation voltage determination process ends.
As described above, in the present embodiment, for each sheet S to be conveyed, the separation voltage V is absolute as the Beck smoothness X of the sheet S increases (smoothness increases). The value is determined to be large (highly separable), and control is performed so that the determined separation voltage V is applied to the static elimination needle 71 in accordance with the timing when the sheet S passes through the transfer nip N.

Therefore, even if the surface smoothness of the type of paper S to be conveyed, such as coated paper or non-coated paper, is different, the separation voltage V suitable for the paper S can be determined for each paper, and the apparatus is installed. The separation of the paper S, particularly coated paper, from the intermediate transfer belt 21 can be improved without being affected by changes in the electrical resistance value and rigidity of the paper S due to the environment (HH or the like).
Further, since it is not necessary to extremely increase the separation voltage V as an absolute value regardless of the surface smoothness of the paper S in order to improve the separation property, the influence on the secondary transfer due to the increase in the discharge amount from the static elimination needle 71 is eliminated. Can be avoided.

  Furthermore, when the discharge amount from the static elimination needle 71 increases, the number of discharge products also increases. When the increased discharge product adheres to and accumulates on the static elimination needle 71, the electrical resistance value of the static elimination needle 71 increases. The separation current is difficult to flow, and the charge removal performance may be reduced. On the other hand, in the present embodiment, the separation voltage V can be set to an appropriate value according to the surface smoothness of the paper S, so that the neutralization performance is reduced due to the increase in the discharge amount from the static elimination needle 71. It can be avoided.

In the above description, an example in which the separation bias supplied to the static elimination needle 71 is the separation voltage V has been described. However, the present invention is not limited to the voltage, and for example, a configuration in which the separation current can be varied according to the magnitude of the Beck smoothness X. It may be taken.
Further, although the paper smoothness detection sensor 80 is disposed along the transport path 35 and the smoothness of the surface of the paper S being transported is detected by the paper smoothness detection sensor 80, the present invention is not limited to this.

It is only necessary that the surface smoothness can be detected before the leading edge Sa of the paper S passes through the transfer nip N. For example, a paper smoothness detection sensor 80 is disposed immediately above the paper feed cassette 31 and is accommodated in the paper feed cassette 31. In such a state, the surface smoothness of the uppermost sheet S can be detected.
[Embodiment 2]
In the first embodiment, the configuration example in which the Beck smoothness X is acquired from the sensor detection value α of the paper smoothness detection sensor 80 and the separation voltage V is determined from the acquired Beck smoothness X has been described.

  On the other hand, in the second embodiment, if the user inputs paper information such as paper type and basis weight from the operation unit 60 without using the paper smoothness detection sensor 80, the input paper information is used. The separation voltage V corresponding to the Beck smoothness is determined, which is different from the first embodiment. Hereinafter, in order to avoid duplication of description, the description of the same contents as those of Embodiment 1 is omitted, and the same components are denoted by the same reference numerals.

FIG. 13 is a plan view showing the configuration of the operation unit 60.
As shown in the figure, the operation unit 60 includes a key group 61 and a display unit 62.
The key group 61 includes a numeric keypad for inputting the number of prints, a start key for starting a job, a stop key for stopping the job halfway, and the like.
The display unit 62 includes a touch panel type liquid crystal display unit, and a screen for selecting the size and type of the paper S accommodated in the paper feed cassette 31 and other various settings is selectively selected by a user operation. It is displayed.

This figure shows an example in which a paper type selection screen is displayed. In the paper type selection screen, a column 211 for selecting non-coated paper and coated paper, and non-coated paper are displayed. Among them, a column 212 for selecting plain paper and high-quality paper and a column 213 for selecting the paper basis weight are provided.
The user can complete the selection by touching the OK button 214 after touching the button displayed in each column 211 to 213 to select the paper type. The selected paper information is sent to the control unit 50.

In the second embodiment, the control unit 50 is not provided with the paper surface smoothness detection unit 55 (FIG. 3) and the smoothness table 57 in the first embodiment. The separation voltage table 221 shown in FIG.
As shown in FIG. 14, the separation voltage table 221 includes a paper type 1 field, a paper type 2 field, a basis weight field, a Beck smoothness field, and a separation voltage field, and the paper type, Beck smoothness, and separation voltage correspond to each other. It is attached.

For example, when the user selects “non-coated paper, high-quality paper, basis weight (60-90 [g / m ² ]”) on the operation unit 60, the control unit 50 performs the Beck smoothness of the conveyed paper S. Is 2.4 and the corresponding separation voltage V is −3 [kV], and the separation voltage V can be set to −3 [kV] for the paper S. It can be said that the sheet type input by the user is sheet information indicating the smoothness of the sheet S to be conveyed.

As described above, in the second embodiment, since the separation voltage V is determined from the sheet information such as the paper type by the user in the operation unit 60, it is not necessary to use the paper smoothness detection sensor 80, and the apparatus is more than the case where this is used. The configuration can be simplified, and the cost can be reduced because the sensor is not required.
In FIG. 14, the paper type, the basis weight, the Beck smoothness, and the separation voltage are associated with each other. However, if the Beck smoothness is uniquely determined from the paper type and the basis weight, the paper type and the basis weight are set to the paper S. Therefore, it is possible to adopt a configuration in which only the paper type, the basis weight, and the separation voltage are associated with each other. A separation voltage suitable for the Beck smoothness determined from the type and basis weight of each sheet S is determined in advance by experiments or the like.

If the Beck smoothness is determined by the paper type regardless of the basis weight, the basis weight may be excluded from the selection. In this case, the user only needs to perform an operation of selecting the paper types 1 and 2, and the operability for the user is improved.
If the separation voltage V can be made different only from the difference between the non-coated paper and the coated paper, the separation property can be improved as compared with the configuration in which the separation voltage is not changed. You can remove it. In this case, the user only needs to perform an operation of selecting only coated paper and non-coated paper, which leads to further improvement in operability. Further, if the paper name and the separation voltage are associated with each other instead of the paper type name, input or selection of the paper name may be accepted.

Note that the types, basis weights, names, and the like of the non-coated paper and coated paper shown in FIG. 14 are not limited to those described above, and may be further subdivided.
Further, the configuration example in which the operation unit 60 is a reception unit that receives sheet information has been described. However, the configuration is not limited thereto as long as the sheet information can be acquired. For example, if the user can transmit sheet information such as the paper type to the printer 1 from a terminal device connected to the network, the sheet information is received from the terminal device via the network instead of the operation unit 60. The receiving interface may be the receiving means.

[Embodiment 3]
In the first embodiment, the configuration example in which the separation voltage is determined from the Beck smoothness of the paper S has been described. In the third embodiment, in addition to the Beck smoothness, the bending rigidity and the electrical resistance of the paper S are considered. This is different from the first embodiment in that the separation voltage V is determined.
FIG. 15 is a diagram illustrating a configuration for detecting the Beck smoothness, the bending rigidity, and the electric resistance value of the paper S, and also illustrates a partial block diagram of the control unit 50.

As shown in the figure, a resistance detection unit 301, a paper smoothness detection sensor 80, and a conveyance path detection sensor 320 are arranged along the conveyance path 35, and the control unit 50 includes a paper surface smoothness detection unit 55. In addition, a sheet bending stiffness detection unit 302 and a separation voltage control unit 303 are provided.
The resistance detection unit 301 detects an electrical resistance value of the conveyed paper S, and includes conveyance rollers 311 and 312, a constant voltage power source 313, and an ammeter 314.

  When a constant voltage is applied from the constant voltage power source 313 between the transport rollers 311 and 312 while the paper S is nipped and transported by the transport rollers 311 and 312, it flows between the transport rollers 311 and 312 via the paper S. The current value is measured by an ammeter 314, and the electrical resistance value of the sheet S currently being conveyed is detected from the measurement result. In this sense, the measured current value can be said to be information indicating the electrical resistance of the paper S.

The detected electric resistance value is converted into a value measured by the Hiresta, and the unit is [(log) MΩ]. For this conversion, an equation (not shown) for converting the detected electrical resistance value into a value measured by Hiresta is used. A value (electric resistance value R) representing the electric resistance of the converted paper S is sent to the separation voltage control unit 303.
The paper smoothness detection sensor 80 and the paper surface smoothness detection unit 55 are the same as those in the first embodiment, and information indicating the detected Beck smoothness is sent to the separation voltage control unit 303.

The conveyance path detection sensor 320 is a CCD laser displacement sensor (ranging sensor) including a light emitting unit 321 and a light receiving unit 322 as shown in the enlarged view of FIG. 16, and is used to detect the rigidity of the paper S.
As shown in the figure, a guide 391 and an auxiliary transport roller 393 disposed opposite to the guide 391 are disposed upstream of the transport path detection sensor 320 in the paper transport direction. A guide 392 is disposed.

The sheet S being conveyed is conveyed on the guide 391 while being guided on the guide 391 while being guided on the guide 391 while being guided on the guide 391. Head for.
When the paper S is transported from the guide 391 to the guide 392, if the rigidity (waistness) of the paper S is strong, the paper S that has passed through the guide 391 proceeds obliquely upward along the guide surface of the guide 391, and is shown by a one-dot chain line. If the paper S has a low rigidity, the posture is as if the waist is bent immediately after passing through the guide 391, and the straight line cannot be moved upward as it is, and the broken line 381 is slightly tilted. It becomes easy to take the route shown.

This means that there is a difference in the transport path of the paper S due to the rigidity of the paper S. If the transport path to be taken by the rigidity of the paper S is known in advance, the actual transport of the paper S is in progress. Further, by detecting the transport path of the paper S, the rigidity of the paper S can be known.
Therefore, in the present embodiment, the conveyance path that is displaced due to the difference in rigidity of the paper S is detected by the distance from the light emitting unit 321 of the conveyance path detection sensor 320 using the conveyance path detection sensor 320 that is a distance measuring sensor. Based on the information indicating the detected distance, the paper bending stiffness detecting unit 302 shown in FIG. 15 detects the bending stiffness of the paper S.

  Specifically, a table (not shown) in which the detected distance and the bending stiffness of the paper S are associated with each other is stored in advance, and the paper bending stiffness detection unit 302 refers to the detected distance by referring to the table. The bending stiffness of the paper S corresponding to is obtained. In this sense, the detected distance can be said to be information indicating the bending stiffness of the paper S. The unit of bending stiffness is [mN · m]. The obtained value (bending stiffness B) indicating the bending stiffness of the sheet S is sent to the separation voltage control unit 303.

  The separation voltage control unit 303 is an alternative to the separation voltage control unit 56 according to the first embodiment, and is detected by the electrical resistance value R of the paper S detected by the resistance detection unit 301 and the paper surface smoothness detection unit 55. The separation index G is obtained based on the Beck smoothness X of the sheet S and the bending stiffness B of the sheet S detected by the sheet bending stiffness detector 302, and the separation voltage V is determined from the obtained separation index G. The separability index G is obtained from the following (Equation 2).

Separability index G = (1 / Beck smoothness X [(log) s]) × (bending stiffness B [mN · m] / electric resistance R [(log) MΩ]) (Equation 2)
The term “bending stiffness B / electric resistance value R” in (Equation 2) corresponds to the separability obtained in (Equation 1), and a value obtained by multiplying this by the reciprocal of Beck smoothness X is used as the separability index G. Yes.
If the Beck smoothness X increases, the separability index G decreases in inverse proportion to this, indicating that separation becomes difficult. By multiplying by “bending stiffness B / electric resistance value R”, it is possible to Since the changing separability component is taken into consideration, the separation voltage V can be determined to a value more suitable for the environment than the configuration in which the separation voltage V is determined only by the Beck smoothness X.

FIG. 17 is a diagram illustrating a configuration example of the separation voltage table 304 indicating the correspondence between the separation index G and the separation voltage V, and the separation voltage table 304 is provided in the control unit 50.
As shown in the figure, there is a relationship in which the separation voltage V becomes smaller in absolute value as the separation index G becomes larger. Information on the separation voltage table 304 is obtained in advance by experiments or the like and stored in the separation voltage table 304.

The separation voltage control unit 303 refers to the separation voltage table 304 and reads the value of the separation voltage V corresponding to the separation index G obtained by (Equation 2) from the separation voltage table 304, thereby obtaining the separation voltage V. Can be determined.
FIG. 18 is a flowchart showing the contents of the separation voltage determination process according to the third embodiment.
As shown in the figure, when it is determined that the first sheet S has been fed out (“YES” in step S31), the electric resistance value R [(log) MΩ] of the sheet S is obtained from the resistance detector 301. Obtain (step S32).

Next, the Beck smoothness X [(log) s] of the paper S is acquired from the paper surface smoothness detection unit 55 (step S33). This acquisition uses the same method as the method according to the first embodiment. Subsequently, the bending stiffness B [mN · m] of the paper S is acquired from the paper bending stiffness detection unit 302 (step S34).
A separability index G for the paper S is obtained from the obtained electrical resistance value R, Beck smoothness X, and bending stiffness B of the paper S by (Equation 2) (step S35).

The separation voltage V for the paper S is determined from the obtained separation index G (step S36). This determination is made by referring to the separation voltage table 304.
If there is a sheet S to be fed next (“YES” in step S37), the process returns to step SS31 to determine the separation voltage V for the sheet S (steps S31 to S36). If there is no sheet S to be fed out next (“NO” in step S37), the separation voltage determination process ends.

FIG. 19 is a diagram showing the result of confirming the quality of the separation when the separation voltage V is determined from the separation index G on the same paper as the type of paper shown in FIG. is there. In this figure, each type of paper is arranged in descending order of the separability index G and is the same as the order shown in FIG.
As shown in FIG. 19, since the paper names “OK Kanto + (T eye)” and “OK Kanto + (Y eye)” have the Beck smoothness X of 3.16, the separation voltage is determined only by the Beck smoothness X. When V is determined, the separation voltage V is determined to be the same value regardless of whether the environment is NN or LL.

On the other hand, if the separability index G is applied, even if the Beck smoothness X is 3.16 and the same value, the separability index G is 0.49, 0.41, and 0.30 depending on the environment. Thus, the separation index can be subdivided as compared with the case where only the Beck smoothness X is used.
In the present embodiment, assuming that the separation voltage V can be switched in three stages according to the separation index G as shown in FIG. 17, in FIG. However, an example in which the same separation voltage V of −4 [kV] is applied is shown. However, if the separation voltage V is configured to be switchable in more stages, the separability index G is 0.49, 0. Different separation voltages V can be set in each of the cases .41 and 0.3.

  Specifically, for example, from the experimental results, the separation voltage V = −5 [kV] when 0.3 ≦ separation index G <0.4, and the separation voltage when 0.4 ≦ separation index G <0.45. If V = −4.5 [kV], 0.45 ≦ separability index G <0.5, if it is found in advance that the separation voltage V = −4 [kV] is optimal, the separability index shown in FIG. For “OK Kanto + (T eye)” with G = 0.3, the separation voltage V is “−5” [kV], and for “OK Kanto + (T eye)” with the separation index G of 0.41. The separation voltage V is set to “−4.5” [kV], and the separation voltage V is set to “−4” [kV] for “OK Kanto + (Y-th)” with the separation index G of 0.49. be able to.

That is, for the paper S having the same Beck smoothness but different bending stiffness and electrical resistance depending on the environment, the separation voltage V is increased in absolute value as the separation index G decreases (separability is increased). To be higher).
In this way, by using the separability index G obtained from the Beck smoothness X, the bending stiffness B, and the electrical resistance value R of the paper S, a more suitable separation voltage V is applied to the paper S even if the environment changes. By applying, it becomes possible to further improve the separability.

In the above description, the bending stiffness B is obtained from the detection result of the displacement amount in the conveyance path of the paper S. However, the present invention is not limited to this, and other detection methods may be used. Further, the unit is not limited to the bending stiffness, and any other unit may be used as long as the value represents the bending stiffness of the paper S.
Furthermore, although the electrical resistance value R is obtained from the detection result of the current value flowing between the transport rollers 311 and 312 that sandwich and transport the paper S, the present invention is not limited to this. In another method, for example, a constant current circuit, a voltage value that varies depending on the resistance value of the paper S can be detected, and the electrical resistance value of the paper S can be obtained from the detection result.

Further, the configuration is not limited to the configuration in which the bending stiffness B and the electrical resistance value R are detected. For example, a configuration in which the user performs setting input from the operation unit 60 in advance as in the second embodiment may be adopted. A configuration in which any one or two of the Beck smoothness X, the bending stiffness B, and the electric resistance value R are detected and the rest is set by the user may be employed.
[Embodiment 4]
In the third embodiment, the separability index G is obtained using the Beck smoothness X, the bending stiffness B, and the electric resistance value R of the paper S. However, in the fourth embodiment, the separability index G is obtained. Furthermore, the smoothness change rate E of the surface of the intermediate transfer belt 21 is added, and this point is different from the third embodiment.

FIG. 20 shows a configuration for detecting the smoothness change rate of the surface of the intermediate transfer belt 21 in addition to the configuration for detecting the Beck smoothness, the bending stiffness, and the electric resistance value of the paper S. It is a figure shown combining with a figure.
As shown in the figure, a resistance detection unit 301, a paper smoothness detection sensor 80, a conveyance path detection sensor 320, a paper surface smoothness detection unit 55, a paper bending stiffness detection unit 302, a belt surface smoothness change rate detection unit 401, A separation voltage control unit 402 and a belt smoothness detection sensor 410 are provided.

The resistance detection unit 301 to the sheet bending stiffness detection unit 302 are the same as those in the third embodiment.
The belt smoothness detection sensor 410 emits light toward the surface of the intermediate transfer belt 21 and the surface of the intermediate transfer belt 21 of light emitted from the light emitting unit 411 (hereinafter referred to as “belt surface”). ) Is an optical sensor having a light receiving unit 412 that receives reflected light from the light source and outputs an electrical signal corresponding to the received reflected light amount.

The intermediate transfer belt 21 is made of, for example, polyimide, and is in direct contact with the sheet S at the transfer nip N during job execution. Due to the occurrence of friction with the surface of the sheet S at that time, the job execution time elapses. Accordingly, depending on the type of paper S used, the belt surface is often roughened or conversely polished, and the smoothness of the belt surface often changes.
The higher the smoothness of the belt surface, the larger the area per unit area of the belt surface that comes into contact with the paper S after transfer, so that the surface of the belt and the paper S are the same as when the smoothness of the surface of the paper S is increased. The adsorptive power generated by the electrostatic force and van der Waals force generated at the contact portion increases with the result that the separability decreases. The change in the separability due to the change in the smoothness of the belt surface means that the change in the smoothness of the belt surface also affects the separability index G.

Therefore, in the present embodiment, the separation index G1 is obtained from the following (Equation 3) in consideration of the belt surface smoothness change rate (belt surface change rate) E.
Separability index G1 = (1 / Beck smoothness X) × (1 / belt surface change rate E) × (bending stiffness B / electric resistance R) (Equation 3)
The separability index G1 is obtained by multiplying the separability index G by (1 / belt surface change rate E), and becomes a new corrected index obtained by correcting the separability index G based on the belt surface change rate E.

The belt surface change rate E indicates a change rate of the amount of light received by the light receiving unit 412 of the belt smoothness detection sensor 410 with respect to a reference value. That is, the belt surface change rate E is expressed as a ratio (= Ea / Es) with the light reception amount Ea detected after the new transfer time when the light reception amount when the intermediate transfer belt 21 is new is the reference value Es.
For example, assuming that the smoothness of the belt becomes higher (mirrored) as the belt surface is polished as the usage time elapses, the light emitting portion of the belt smoothness detection sensor 410 becomes more mirror-finished. Since the amount of light reflected from the belt surface of the light emitted from 411 increases, the amount of light received by the light receiving unit 412 also increases. In this case, the relationship of the reference value Es <light reception amount Ea is established, and the belt surface change rate E> 1.

On the contrary, when the belt surface is roughened, the amount of light reflected from the belt surface decreases, so that the relationship of reference value Es> light reception amount Ea is established, and the belt surface change rate E <1. In this sense, the belt surface change rate E is a value representing the smoothness of the belt surface, and can be said to be a value that increases as the smoothness increases.
Since the belt surface change rate E and the separability have a relation that the separability decreases as the belt surface change rate E increases, the reciprocal of the belt surface change rate E should be applied as shown in (Equation 3). Thus, the separability index G1 in consideration of the belt surface change rate E can be obtained.

FIG. 21 shows an experiment to confirm whether the separation voltage V is determined from the separation index G1 obtained by the above (Equation 3) for the sheet S having the sheet name “recycle quality” shown in FIG. It is a figure which shows the result. The experimental conditions were basically the same as described above, but the experiment was performed by separately mounting three intermediate transfer belts 21 having different belt surface smoothness.
As shown in the figure, the separability index G1 differs depending on the belt surface change rate E even in the same paper and the same environment (even in the same separability index G).

When the experiment was performed with the separation voltage V set according to the different separation index G1, the separation results were all good (◯) as shown in Example 4 and the separation performance was further improved. I understand.
In FIG. 21, the fact that the belt surface change rate E is 1 is the same condition as in the third embodiment not taking the belt surface change rate E into account (separability index G1 = 0.57), and the separation voltage V Becomes -3 [kV], and the separation result is the same as the result shown in FIG.

On the other hand, for example, when the belt surface change rate E is 1.25, which is greater than 1, since the intermediate transfer belt 21 has a higher belt surface smoothness, the separability index G1 is 0.46. For S, the separation voltage V is appropriately -4 [kV].
For the intermediate transfer belt 21 having such characteristics, if the separation voltage V is set to -3 [kV] lower in absolute value than -4 [kV], intermediate transfer due to higher belt surface smoothness. As the suction force between the belt 21 and the paper S increases, the separation of the paper S decreases.

Further, when the belt surface change rate E is 0.57 which is smaller than 1, the belt surface has a somewhat low smoothness to the intermediate transfer belt 21, the separability index G1 becomes 1, and the separability index G1 becomes 1. For the sheet S to be, the separation voltage V is appropriately −2 [kV].
The low smoothness of the belt surface means that the separability is high to some extent. Therefore, if the separation voltage V is set to -3 [kV], which is higher in absolute value than -2 [kV], the separation bias. Is unfavorable because it may cause excessive influence on the transferability and a burden on the static elimination needle 71.

In the present embodiment, the separation voltage V is determined according to the smoothness of the belt surface by taking into account the belt surface change rate E, and therefore the belt surface smoothness has a characteristic that changes with the passage of job execution time. When the intermediate transfer belt 21 is used, the separation property of the paper S can be maintained at a certain level or more.
In FIG. 21, the example of obtaining the separability index G1 for the paper S having the paper name “recycled quality” has been described, but the separability index G is similarly determined by the belt surface change rate E for other types and other names of paper. Is changed to G1, the separation voltage V with respect to the separation index G1 can be obtained, so that the separation can be improved as compared with the configuration not considering the belt surface change rate E.

In the above description, one belt smoothness detection sensor 410 is provided and the smoothness is detected at one place on the belt surface of the intermediate transfer belt 21, but the present invention is not limited to this. For example, it is possible to adopt a configuration in which smoothness at a plurality of different locations is detected and the belt surface change rate E is obtained from the detection result.
FIG. 22 is a schematic diagram for explaining a configuration for detecting the smoothness of a plurality of locations on the belt surface, and shows a view when the intermediate transfer belt 21 is viewed from the direction indicated by the arrow A in FIG.

As shown in FIG. 22, the intermediate transfer belt 21 has three belt smoothness detection sensors 410a, 410b, 410c spaced apart from each other along a direction orthogonal to the belt conveyance direction (corresponding to the main scanning direction). It is arranged opposite to the surface.
A broken line D1 indicates a detection area by the belt smoothness detection sensor 410a, a broken line D2 indicates a detection area by the belt smoothness detection sensor 410b, and a broken line D3 indicates a detection area by the belt smoothness detection sensor 410c.

By using the three belt smoothness detection sensors 410a to 410c, the smoothness at three different positions along the main scanning direction can be detected on the belt surface. For example, the belt surface change rate at each location is obtained, and the highest value among them can be applied to (Equation 3) as the belt surface change rate E.
If the separation voltage V1 corresponding to the lowest belt surface change rate is obtained when there is some variation in smoothness depending on the location on the belt surface, the separation voltage V1 is the belt surface change of the belt surface. The portion with the lowest rate is suitable, but the portion with the highest belt surface change rate is too low in absolute value.

  If the belt surface change rate is the lowest in the detection area D1 and the highest in the detection areas D2 and D3, the sheet width (length in the main scanning direction) W of one sheet S to be used is changed from the detection areas D1 to D3. If the distance L is longer than the distance L, the separation voltage V1 is in contact with the detection regions D2 and D3 even if the separation of the sheet portion of the sheet S that is in contact with the detection region D1 on the belt surface can be secured. Separation of the paper portion that falls is likely to be difficult to separate from the belt surface.

On the other hand, if the separation voltage V2 corresponding to the highest belt surface change rate is obtained, the separation can be ensured even for the belt portion having the lowest separation among the variations in the smoothness of the belt surface. This is because, even if there is a variation in smoothness on the belt surface, the separability can be maintained above a certain level.
The deterioration of the smoothness of the belt surface over time is likely to occur in a region Z corresponding to the feeding position of the paper S by the feeding roller 32 in the entire main scanning direction of the belt surface. This is due to the following reason.

That is, when the paper S in the paper feed cassette 31 is fed to the conveyance path 35 by the feed roller 32, the surface portion of the paper surface that is in contact with the feed roller 32 is minute due to friction with the surface of the feed roller 32. However, it may be roughened.
The fed sheet S passes through the transfer nip N in the transfer nip N while the roughened surface portion is in contact with the belt surface of the intermediate transfer belt 21. If the cumulative number of sheets S that have passed through the transfer nip N is small, the belt surface will not become rough just because it contacts the roughened surface portion of the sheet S.

  However, as the number of job executions increases, the number of times of contact with the roughened paper S increases, and when in contact with the roughened paper surface portion, out of the entire main scanning direction of the belt surface, The area Z in contact with the roughened paper surface portion gradually becomes rough, and when this continues for a long period of time, the roughening of the area Z proceeds, and the area other than the area Z in the main scanning direction This is because the smoothness of the region Z is significantly reduced.

As described above, the smoothness of the belt surface may vary depending on the location, and therefore, it can be said that it is more desirable to individually obtain the belt surface change rates at a plurality of different locations.
In addition, when the belt surface change rate at different locations is obtained, it is not limited to the configuration that applies the highest belt surface change rate, and if the variation in smoothness of the belt surface falls within a certain range, for example, an average method May be used.

  Further, since the belt surface change rate E often changes gradually over a long period of time, it is detected only once, for example, every fixed number of prints or every fixed period, and is the same as the detected belt surface change rate E. The value may be applied to (Equation 3) until the next number or period arrives. Compared to the configuration in which the belt surface change rate E is detected for each sheet, the processing load on the CPU 52 and the like required for the detection can be reduced.

Further, although the smoothness change rate E of the surface of the intermediate transfer belt 21 is obtained, it may be a value representing the smoothness of the belt surface, and the smoothness at the time of detection may be obtained instead of the change rate. In this case, since the separability index G1 obtained by (Equation 3) is a value corresponding to the smoothness, a new separation voltage V corresponding to this is obtained in advance.
The present invention is not limited to the image forming apparatus, and may be a method of separating the transferred sheet from an image carrier such as the intermediate transfer belt 21. The method may be a program executed by a computer. The program according to the present invention includes, for example, a magnetic disk such as a magnetic tape and a flexible disk, an optical recording medium such as a DVD-ROM, DVD-RAM, CD-ROM, CD-R, MO, and PD, and a flash memory recording medium. It can be recorded on various computer-readable recording media, and may be produced, transferred, etc. in the form of the recording medium, wired and wireless various networks including the Internet in the form of programs, In some cases, the data is transmitted and supplied via broadcasting, telecommunication lines, satellite communications, or the like.

(Modification)
As described above, the present invention has been described based on the embodiment. However, the present invention is not limited to the above-described embodiment, and the following modifications may be considered.
(1) In the above-described embodiment, the configuration example using the static elimination needle 71 as the static elimination member for separating the paper S after the secondary transfer from the intermediate transfer belt 21 has been described. However, the configuration is limited to a needle-like member. There is no. For example, it is possible to use a separation charger that performs discharge for separation from an ultrafine wire to which a separation voltage is applied. Further, although the separation voltage is a direct current voltage having the same polarity as that of the toner, the separation voltage is not limited thereto, and may be an alternating current, for example.

(2) In the above embodiment, the sheet S is used for each sheet to be used, with the discharging needle 71 for discharging being used as the discharging member and the separation voltage V applied to the discharging needle 71 as the separation parameter that affects the separation. Although the configuration has been described in which the separation parameter is variably controlled so that the separability becomes higher as the smoothness becomes higher, the present invention is not limited to the discharge, and other means may be used.
For example, the separation parameter can be a secondary transfer voltage as a transfer bias instead of the separation bias.

If the secondary transfer voltage is lowered in absolute value, the amount of residual charge remaining on the sheet S after the secondary transfer is reduced, so that the space between the surface of the sheet S after the secondary transfer and the surface of the intermediate transfer belt 21 is reduced. The adsorptive power of is weakened and the separability is improved.
Therefore, if the control for lowering the secondary transfer voltage is executed for each sheet as the smoothness of the sheet surface increases, the effect of improving the separation similar to the embodiment can be obtained. The secondary transfer voltage can be varied by configuring the output of the secondary transfer voltage output unit 250 to be variable.

Further, since the secondary transfer voltage is varied according to the paper S to be used, the secondary transfer voltage is not excessively increased or decreased more than necessary, and the secondary transfer voltage is secured while maintaining a certain level of separation. It is possible to reduce a load on the secondary transfer voltage output unit 250 and the secondary transfer roller 25 due to an excessive increase in the output value of the transfer voltage output unit 250.
Since the change in the secondary transfer voltage affects the transfer efficiency at the time of secondary transfer, the smoothness and the secondary transfer voltage of the sheet S are improved so that the transfer efficiency is within an appropriate range while improving the separability. Relationship is determined in advance through experiments. Further, instead of the secondary transfer voltage, the secondary transfer current may be varied as the transfer bias.

Further, both the separation bias and the transfer bias may be variably controlled according to the smoothness of the paper surface. The variable control of both is the same for the other separation parameters described below.
(3) Further, for example, the separation parameter can be the rotational speed (peripheral speed) of the intermediate transfer belt 21.

That is, if the rotation speed of the intermediate transfer belt 21 is decreased, the conveyance speed of the sheet S passing through the transfer nip N is also decreased accordingly. The lower the transport speed of the paper S, the more the front end of the paper S is transported by a unit distance when the front end of the paper S passes through the transfer nip N and is transported while being stuck to the surface of the intermediate transfer belt 21. It takes longer time to complete.
The longer the transport time of the paper S, the longer the time for the restoring force to return to the original straight posture due to the rigidity of the paper S to act on the leading edge of the paper S, and the longer the working time of the restoring force. As a result, the leading edge of the sheet S is easily separated from the surface of the intermediate transfer belt 21.

Therefore, if the control for slowing the rotation speed of the intermediate transfer belt 21 is executed as the smoothness of the surface of the sheet increases for each sheet of paper, the smoothness of the surface of the sheet decreases due to the increase in smoothness. The separability can be supplemented by the improvement in separability due to the decrease in the conveyance speed, and the separability can be maintained at a certain level or more.
In addition, since the rotational speed of the intermediate transfer belt 21 is reduced only when necessary according to the paper S to be used, while maintaining a certain level of separation without reducing print productivity more than necessary, Productivity can be improved compared to a configuration that is always slowed down.

Further, in addition to slowing down the rotation speed of the intermediate transfer belt 21, if the transfer current is reduced, the separation can be further improved.
That is, if the rotation speed of the intermediate transfer belt 21 is decreased, the conveyance speed of the sheet S is also decreased, and it takes a long time for the sheet S to pass through the transfer nip N. This means that when a certain unit area on the paper S is viewed, the time during which the unit area of the paper S stays in the transfer nip N becomes longer, and the transfer time becomes longer.

When the transfer time is long, the amount of current supplied to the secondary transfer roller 25 per unit time is smaller than when the transfer time is short, and the amount of residual charge remaining on the sheet S after transfer is correspondingly reduced. The reason why the residual charge remaining on the sheet S after transfer is reduced is that the separability is improved as described above.
Further, instead of the above, on the premise of supplying a separation bias, it is possible to adopt a configuration in which the rotational speed of the intermediate transfer belt 21 is decreased as the smoothness of the paper surface increases.

That is, if the rotation speed of the intermediate transfer belt 21 is decreased, the conveyance speed of the sheet S is also decreased, and it takes a long time for the sheet S to pass through the transfer nip N. Therefore, the smoothness of the sheet surface is increased. The separability that is reduced by the above can be compensated by the improvement in the separability due to the longer application time of the separation bias to the paper S, and a certain level of separability can be secured.
In this case, the separation bias may be a constant voltage, or the separation bias voltage (absolute value) may be controlled to be increased to some extent while the rotational speed of the intermediate transfer belt 21 is slowed down.

  Note that changing the rotation speed of the intermediate transfer belt 21 is equivalent to changing the so-called system speed, so that each rotation member such as the photosensitive drums 11Y to 11K, the pair of conveyance rollers 33 and 34, and the fixing roller 41 is also subjected to intermediate transfer. The rotation is controlled so as to be the same speed as the belt 21. Naturally, the exposure scanning speed by the exposure units 13Y to 13K is also changed to a speed that matches the system speed.

(4) Further, for example, in an apparatus having a double-sided printing function for forming images on the front and back surfaces of a single sheet S, the separation parameter can be the fixing temperature.
Here, the double-sided printing function is the following function.
That is, when the transported paper S passes through the transfer nip N, the toner image on the intermediate transfer belt 21 is secondarily transferred to the surface thereof, and then separated, transported to the fixing unit 40, and the fixing unit 40 The toner image on the surface is heated and fixed.

The fixed sheet S is conveyed to a feedback conveyance path and returned to the transfer nip N by feedback. When the sheet S passes the transfer nip N again, the sheet S is placed on the back surface of the sheet S on the intermediate transfer belt 21. This is a function of secondarily transferring a toner image different from the above, then separating it, transporting it to the fixing unit 40, and fixing the toner image on the back surface thereof.
One sheet S passes through the transfer nip N twice. The fixing temperature when the toner image transferred onto the surface of the sheet S is fixed by the fixing unit 40 at the first time is 2 In order to improve the separability at the second time (at the time of printing on the back side), the control is performed to lower the reference temperature.

  Lowering the fixing temperature means that when the toner image on the surface is thermally fixed, the moisture contained in the paper S is less likely to be taken away by evaporation or the like, and the moisture is less likely to be taken away. This means that the moisture content remaining on the sheet S after being processed is large. The fact that the moisture content remaining on the paper S is large means that the electrical resistance value of the paper S can be lowered, in other words, not too high.

The low electrical resistance value of the paper S after passing through the fixing unit 40 means that when the paper S passes through the transfer nip N again by feedback, the toner image is transferred to the back surface by the amount of low electrical resistance. Later separability can be improved.
Therefore, in the double-sided printing function for forming an image in the order of the front surface and the back surface of the sheet S in units of one sheet, the fixing temperature at the time of thermal fixing of the toner image onto the front surface of the sheet S If the control of decreasing the smoothness of the sheet S is performed, the separability after transfer to the back surface of the paper S can be improved as in the embodiment.

Further, since the fixing temperature is varied according to the paper S to be used, the fixing temperature is not excessively increased or decreased more than necessary, and the fixing temperature is excessively increased while ensuring a certain degree of separation. Therefore, the load on the fixing unit 40 can be reduced.
The smoothness of the back surface of the paper S may be obtained by separately providing a sensor for detecting the smoothness of the back surface of the paper S or may be obtained by input from the operation unit 60.

This acquisition is executed before the sheet S having the toner image transferred on the front surface reaches the fixing unit 40, and the sheet S reaches the fixing unit 40 even when the fixing temperature varies according to the acquired smoothness of the back surface. Executed before.
Note that since the change in the fixing temperature affects the fixing property, the relationship between the smoothness of the paper S and the fixing temperature is determined in advance by experiments or the like so that the fixing property is within an appropriate range while improving the separation property. It is done. The fixing temperature can be varied by providing a heating unit such as a heater (not shown) that heats the fixing roller 41 and a detection unit such as a sensor that detects the temperature of the fixing roller 41, and the temperature of the fixing roller 41 is detected by the detection unit. Obtained by controlling the power supplied to the heating unit so that the temperature of the fixing roller 41 becomes the target fixing temperature.

(5) In addition to the image forming units 10Y to 10K, another image forming unit for forming a transparent toner image is provided in the image processing unit 10, and the formed transparent toner image and Y to K color toners are provided. In a configuration that allows multiple transfer of an image onto the intermediate transfer belt 21, the separation parameter can be the image formation of a transparent toner image.
An area (leading end area) of about several millimeters from the leading end Sa of the sheet S toward the trailing end is often a non-image forming area. In this case, Y to K color toner images are not transferred. If a transparent toner image is formed in this non-image forming area, the positive charge residual charge in the front end area of the paper S is attracted to the negative polarity transparent toner particles and is neutralized to some extent. When passing, the amount of charge in the leading end region of the paper S is likely to decrease.

When the charge amount in the leading end region of the sheet S decreases, the attracting force that the leading end region of the sheet S tries to electrostatically attract to the surface of the intermediate transfer belt 21 is weakened, and the separability is improved. Further, since transparent toner particles are interposed between the surface of the paper S and the surface of the intermediate transfer belt 21, the adhesion between the surface of the paper S and the surface of the intermediate transfer belt 21 is lowered, and in this respect also the separation property is improved. To do.
Therefore, when a sheet S having high smoothness, for example, a Beck smoothness equal to or greater than a threshold value, is used for each sheet S, a transparent toner image is formed and transferred to the leading end region in the conveyance direction of the sheet S. If the image formation control of the transparent toner image to be performed is performed, the image is not reflected on the human eye and the separability can be improved.

On the other hand, when the paper S whose smoothness is lower than the threshold value is used, it is possible not to perform image formation control of the transparent toner image.
Since the formation of the transparent toner image is controlled in accordance with the smoothness of the paper S, the transparent toner image is formed more than a certain amount without increasing the consumption of the transparent toner more than necessary as compared with the configuration in which the transparent toner image is formed regardless of the smoothness. Separability can be ensured.

In the above description, the transparent toner image formation region is only the front end region in the transport direction of the paper S. However, the present invention is not limited to this. For example, the entire area of one paper can be used.
Further, the transparent toner may be formed in the front end region in the transport direction of the paper S and in the entire width direction, but may be formed only in the both ends in the width direction, for example. If the separability at both ends in the width direction of the paper S is improved, the separation starts from both ends in the width direction of the paper S being transported, and the center from the both ends is restored by the restoring force due to the waist strength of the paper S along with the transport. This is because it may be gradually separated over the part.

Also, it is possible to perform control to increase the density of the transparent toner image as the smoothness increases, for example, instead of whether or not the transparent toner image is formed. Increasing the density of the transparent toner image reduces the residual charge on the paper S and increases the amount of transparent toner interposed between the paper S and the intermediate transfer belt 21, and therefore works in a direction of further improving the separability. Because it becomes.
In the above, the example using the transparent toner has been described, but the present invention is not limited to this. It is also possible to use a toner that is assumed not to be recognized by the user as a drop in image quality, such as white or Y toner.

(6) Further, in a configuration of a so-called direct transfer system in which, for example, a toner image on the photoconductor is directly transferred to the paper S, the separation parameter can be the surface potential of the photoconductor.
FIG. 23 is a diagram illustrating a configuration example of a direct transfer type image forming apparatus.
As shown in the figure, the image forming apparatus is an apparatus that forms a monochrome image including a scanner unit 501 and a print unit 502.

The scanner unit 501 reads an original image and obtains image data.
The printing unit 502 forms an image on the paper S based on the image data obtained by reading, and includes an image processing unit 510, a feeding unit 520, a conveying unit 530, and a fixing unit 540.
The image processing unit 510 includes a photosensitive drum 511 that rotates in a direction indicated by an arrow B in the drawing, a charging unit 512, an exposure unit 513, a developing unit 514, a transfer unit 515, a separation unit 516, and a cleaner 517. Is provided.

The charging unit 512 charges the surface of the photosensitive drum 511 (charging process). Here, a charging charger is used in which a minus polarity charging voltage is applied to an extremely fine wire. This charging voltage is output from the charging voltage output unit 518.
The exposure unit 513 drives and modulates the semiconductor laser based on the image data, emits a laser beam Lb from the semiconductor laser, exposes and scans the charged photosensitive drum 511, and forms an electrostatic latent image on the photosensitive drum 511. An image is formed (exposure process).

The developing unit 514 develops the electrostatic latent image on the photosensitive drum 511 with, for example, negative polarity K toner, and forms a K toner image on the photosensitive drum 511 (developing process).
The transfer unit 515 transfers (directly transfers) the toner image on the photosensitive drum 511 onto the sheet S conveyed from the feeding unit 520 along the direction indicated by the arrow C in the drawing at the transfer position. Here, a transfer charger in which a plus polarity transfer voltage is applied to an extremely fine wire is used as a transfer member.

The separation unit 516 separates the transferred paper S from the surface of the photosensitive drum 511, and here, a separation charger is used in which a minus polarity or alternating voltage separation voltage is applied to a very thin wire. The transfer voltage and the separation voltage are output from an output unit (not shown).
The cleaner 517 cleans the photosensitive drum 511 by removing the residual toner after transfer on the photosensitive drum 511.

The feeding unit 520 feeds the paper S stored in the paper feed cassette 521 to the transport path 526 by the feed roller 522, transports the fed paper S by the transport rollers 523, 524, and 525, and transfers the transfer unit 515. Send to.
In the vicinity of the conveyance path 526, the same paper smoothness detection sensor 80 as that in the first embodiment is arranged, and the surface of the paper S to be conveyed (the photosensitive drum 511 (image carrier) and the sheet smoothness detection sensor 80). The smoothness of the contact side surface) is detected.

The transport unit 530 transports the separated sheet S to the fixing unit 540 by the belt 531 stretched between the driving roller 532 and the driven roller 533.
The fixing unit 540 fixes the toner image on the conveyed paper S to the paper S. The sheet S that has passed through the fixing unit 540 is discharged out of the apparatus by the discharge roller 541 and is stored in the tray 550.

In such a configuration, for each sheet, control is performed to lower the surface potential (drum surface potential) of the photosensitive drum 511 by an absolute value as the smoothness of the sheet S increases.
This is because, when the drum surface potential is lowered, the adsorption force that is electrostatically attracted to the photosensitive drum 511 at the transfer position where the sheet S first contacts the photosensitive drum 511 is reduced, so that the separability is improved. Here, the drum surface potential is changed by changing the charging voltage to the charging unit 512 output from the charging voltage output unit 518.

If the drum surface potential is varied as described above, the separation property of the paper S having different smoothness can be improved for each paper without changing the transfer or separation voltage.
Further, since the drum surface potential is varied according to the smoothness of the paper S, the drum surface potential is not increased or decreased more than necessary, and the drum surface potential is set while ensuring a certain level of separation. The load on the photosensitive drum 511 due to excessive increase can be reduced.

Note that the change in the surface potential of the photosensitive drum 511 affects the density of the reproduced image. Therefore, the smoothness and the charging voltage of the sheet S are improved so that the density of the reproduced image is within an appropriate range while improving the separability. Relationship is determined in advance through experiments.
(7) In the above embodiment, an example in which the image forming apparatus according to the present invention is applied to a tandem type color printer has been described. However, the present invention is not limited to this. Any image forming apparatus that electrostatically transfers a toner image on an image carrier such as a photosensitive drum or an intermediate transfer belt onto a recording sheet such as paper S and separates the transferred sheet from the image carrier. For example, the present invention can be applied to a copying machine, a facsimile machine, an MFP (Multiple Function Peripheral), and the like.

  Moreover, although the detection value by the paper smoothness detection sensor 80 is converted into Beck smoothness, the present invention is not limited to this. Any value that indicates the smoothness having a correlation with the Beck smoothness can be converted, and a detection value of another sensor or the like may be used. Further, if a detection unit such as a sensor that can directly detect the Beck smoothness from the paper S is used, the detected Beck smoothness becomes sheet information as it is. Can be reduced. Any configuration that detects a value indicating the smoothness of the sheet S may be used.

Further, although the reflective optical sensor is used as the paper smoothness detection sensor 80, the present invention is not limited to this, and other optical sensors such as a CCD laser displacement sensor, a contact type roughness measuring sensor, or the like may be used. .
Further, the photoconductor as the image carrier is not limited to a drum shape, and for example, a belt shape can be used. Similarly, the intermediate transfer member as the image carrier is not limited to a belt shape, and for example, a drum shape can be used.

Furthermore, in the above embodiment, the case of using a negative polarity toner has been described, but instead of this, for example, when a positive polarity toner is used, the polarity of the transfer bias, the separation bias, etc. is opposite to the above. Become.
Further, the contents of the above embodiment and the above modifications may be combined as much as possible.

  The present invention can be applied to an image forming apparatus configured to transfer a toner image on an image carrier to a sheet and then separate the sheet from the image carrier.

DESCRIPTION OF SYMBOLS 1 Printer 11Y, 11M, 11C, 11K, 511 Photosensitive drum 12Y, 12M, 12C, 12K, 512 Charging part 21 Intermediate transfer belt 25 Secondary transfer roller 40 Fixing part 50 Control part 55 Paper surface smoothness detection part 56,303 , 402 Separation voltage control unit 57 Smoothness table 58, 221 and 304 Separation voltage table 60 Operation unit 71 Static elimination needle 72 Separation voltage output unit 80 Paper smoothness detection sensor 211 Paper type selection column 250 Secondary transfer voltage output unit 301 Resistance detection Section 302 Paper bending stiffness detection section 401 Belt surface smoothness change rate detection section 410, 410a, 410b, 410c Belt smoothness detection sensor 516 Separation charger 518 Charging voltage output section N Transfer nip S Paper

Claims (16)

The toner image formed on the image carrying rotator is electrostatically transferred onto the conveyed sheet at a transfer position between the image carrying rotator and the transfer member, and the sheet on which the toner image is transferred is transferred to the transfer position. An image forming apparatus for separating from the image bearing rotating body after passing,
First acquisition means for acquiring sheet information indicating the smoothness of the surface of the sheet to be conveyed on the side in contact with the image carrying rotator;
Second acquisition means for acquiring a value representing the bending stiffness of the sheet and an electrical resistance value in sheet units;
A value representing the smoothness of the surface of the image carrying rotator is obtained once every fixed number of sheets or every fixed period for each of a plurality of different locations along the direction orthogonal to the rotation direction of the image carrying rotator. A third acquisition means for:
Separation parameters that affect the separability of the sheet from the image bearing rotator on a sheet basis increase as the smoothness of the sheet obtained from the sheet information increases. Control means for controlling
With
The control means includes
Obtain the separability index G obtained by multiplying the reciprocal of the value representing the smoothness of the sheet obtained from the sheet information by the value obtained by dividing the value representing the bending rigidity of the sheet by the electrical resistance value of the sheet,
The separability index G obtained for the sheet in units of sheets, and the most smoothness among the values representing the smoothness of the plurality of locations on the surface of the image bearing rotating body obtained last time . To a new value G1 multiplied by the inverse of the higher value of
When the second sheet has a smaller separability index G1 than the first sheet for the first and second sheets having the same smoothness and different separability index G1, the second sheet The image forming apparatus controls the separation parameter so that the separation property is higher than that of the first sheet. The first acquisition means includes
The image forming apparatus according to claim 1, wherein the image forming apparatus is a detection unit that detects the smoothness of a sheet to be conveyed. The detected smoothness is:
The image forming apparatus according to claim 2, wherein the image forming apparatus is an index value of Beck smoothness or smoothness having a correlation therewith. The detection means includes
A signal indicating the magnitude of the amount of light received by the light receiving unit receiving reflected light from the light emitted from the light emitting unit toward the surface of the sheet that is in contact with the image bearing rotator. An optical sensor that outputs
The smoothness index value is:
The image forming apparatus according to claim 3, wherein the output signal is the output signal. A paper cassette for storing sheets;
Conveying means for feeding the sheets stored in the sheet feeding cassette one by one to the conveying path, and conveying the fed sheets to the transfer position;
The sheet to be conveyed is
5. The image forming apparatus according to claim 2, wherein the image forming apparatus is a sheet accommodated in the sheet feeding cassette or a sheet conveyed through the conveyance path. The sheet information is
6. The image forming apparatus according to claim 2, wherein the image forming apparatus is information at a leading end portion in a sheet conveying direction of a surface of the sheet to be conveyed, which is in contact with the image carrying rotating body. The first acquisition means includes
The image forming apparatus according to claim 1, wherein the image forming apparatus is a receiving unit that receives an input of the sheet information from a user. The sheet information is
The image forming apparatus according to claim 7, wherein the image forming apparatus is a type name or a name of a sheet to be conveyed. The control means includes
9. The control according to claim 1, wherein the control is executed until a front end in a sheet conveying direction of the conveyed sheet passes the transfer position and is conveyed by a predetermined distance, and thereafter, the control is stopped. The image forming apparatus according to claim 1. A neutralizing member that performs discharge for separating the sheet from the image bearing rotating body with respect to the sheet that has passed the transfer position,
The separation parameter is:
A voltage or current as a separation bias supplied to the static elimination member,
The control means includes
The image forming apparatus according to claim 1, wherein control is performed to increase the separation bias with an absolute value as the smoothness increases. The separation parameter is:
A voltage or current as a transfer bias supplied to the transfer member;
The control means includes
The image forming apparatus according to claim 1, wherein control is performed to lower the transfer bias with an absolute value as the smoothness increases. Drive means for rotating the image bearing rotator,
The separation parameter is a rotation speed of the image bearing rotator,
The control means includes
The image forming apparatus according to claim 1, wherein the driving unit is controlled such that the rotation speed of the image bearing rotating body decreases as the smoothness increases. The image bearing rotator is a photoconductor,
Prior to the transfer step for performing the transfer,
A charging step for charging the surface of the photoconductor, an exposure step for exposing the charged surface of the photoconductor to form a latent image, and developing the surface of the photoconductor on which the latent image is formed to form a toner image. And a development process.
The separation parameter is a surface potential of the photoreceptor,
The control means includes
The image forming apparatus according to claim 1, wherein control is performed to lower the surface potential of the photoconductor in absolute value as the smoothness increases. A fixing step of thermally fixing the toner image on the surface of the sheet separated from the image bearing rotating body to the sheet;
Feeding back the fixed sheet to the transfer position;
Electrostatically transferring, at the transfer position, a toner image different from the toner, formed on the image bearing rotator, on the back surface of the fed back sheet;
A step of separating the sheet that has passed through the transfer position from the image bearing rotator, and performing a double-sided printing function,
The separation parameter is a fixing temperature when the toner image on the surface of the sheet is thermally fixed in the fixing step,
The first acquisition means includes
Obtaining sheet information indicating the smoothness of the back surface of the sheet;
The control means includes
The image forming apparatus according to claim 1, wherein control is performed to lower the fixing temperature as the smoothness of the back surface of the sheet increases. An image forming unit for forming a transparent, white or yellow toner image different from the toner image on the image carrier;
The separation parameter is formation of a toner image by the image forming unit,
The control means includes
For a sheet having a smoothness equal to or greater than a threshold value, image formation control is performed such that a toner image is formed by the image forming unit, and the formed toner image is transferred to a front end region in the conveyance direction of the sheet. The image forming apparatus according to claim 1, wherein the image forming control is not performed on a sheet having a lower than threshold value. In the conveyed sheet,
Includes plain paper and coated paper with higher surface smoothness than this,
The image forming apparatus according to claim 1, wherein plain paper and coated paper are selectively conveyed to the transfer position.

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