JP6931477B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus.

Conventionally, an image forming apparatus having the following configuration is known. That is, a power supply that outputs a superposed voltage for transferring the toner image to the recording sheet sandwiched between the transfer nip due to the contact between the image carrier supporting the toner image and the contact body, and the test transferred to the contact body. The configuration includes an optical characteristic detecting means for detecting the optical characteristics of the toner image.

For example, the image forming apparatus described in Patent Document 1 is a recording sandwiched between an endless intermediate transfer belt, which is an image carrier, and a secondary transfer nip, which is a contact between an endless secondary transfer belt 41, which is an abutting body. The toner image on the intermediate transfer belt is secondarily transferred to the sheet. For this secondary transfer, a secondary transfer bias consisting of DC and AC superimposed voltages output from the secondary transfer power supply is output, and the intermediate transfer belt is directed toward the secondary transfer belt inside the loop of the intermediate transfer belt. It is applied to the pressing secondary transfer bias roller. The test toner image formed on the intermediate transfer belt is secondarily transferred to the secondary transfer belt instead of the recording sheet, and then the image density is detected by the toner adhesion amount sensor which is an optical characteristic detecting means. By adjusting the image formation conditions based on this detection result, it is said that an image can be formed with a stable image density over a long period of time.

However, in this image forming apparatus, in order to increase the printing speed, a concavo-convex sheet such as Japanese paper may cause remarkable uneven image density. Furthermore, if an attempt is made to suppress image density unevenness on a concavo-convex sheet while increasing the printing speed, transfer defects may occur on a smooth sheet such as surface coated paper, or transfer dust may cause optical characteristics of the test toner image. There was a risk of reducing the detection accuracy.

In order to solve the above-mentioned problems, the present invention superimposes DC and AC for transferring a toner image to a recording sheet sandwiched between transfer nip due to contact between an image carrier carrying a toner image and an abutting body. In an image forming apparatus including a power source that outputs a superposed voltage due to the above and an optical characteristic detecting means for detecting the optical characteristics of the test toner image transferred to the abutting body, the image bearing body is used as a base material. In the test transfer mode in which an elastic layer made of a material having excellent elasticity is laminated and the test toner image is transferred to the abutting body, the transfer nip is contained within one cycle of the superimposed voltage. Of the transfer peak value at which the electrostatic force in the transfer direction from the image carrier side toward the contact body side acts most strongly on the toner and the reverse peak value which is the peak value on the opposite side, the reverse peak value is A process of outputting a superposed voltage, which is a value that weakens the electrostatic force in the direction opposite to the transfer direction, from the power supply as compared with the superposed voltage in the sheet transfer mode in which the toner image on the image carrier is transferred to the recording sheet. It is characterized in that a control means for carrying out is provided.

According to the present invention, there is an excellent effect that the printing speed can be increased while suppressing a decrease in detection accuracy of the optical characteristics of the test toner image.

The schematic block diagram which shows the printer which concerns on embodiment. An enlarged configuration diagram showing an enlarged electrophotographic unit for K in the printer. The block diagram which shows the main part of the electric circuit of the secondary transfer power supply in the printer together with the secondary transfer backside roller and the secondary transfer nip lining roller. The block diagram which shows the main part of the electric circuit of the printer. A flowchart showing a processing flow in process control. The schematic diagram explaining the patch pattern image on the secondary transfer belt of the printer. A characteristic diagram showing the relationship between the development potential and the amount of toner adhered. An enlarged cross-sectional view partially showing a cross section of an intermediate transfer belt of the printer. An enlarged plan view showing the intermediate transfer belt partially enlarged. The graph which shows the 1st example of the waveform (duty = 50%) of the secondary transfer bias which consists of a superposed voltage. The graph which shows the 2nd example of the waveform (duty = 50%) of the secondary transfer bias which consists of a superposed voltage. The graph for demonstrating the duty in the secondary transfer bias shown in FIG. The graph for demonstrating the duty in the secondary transfer bias shown in FIG. The graph which shows an example of the waveform of the secondary transfer bias which made the duty 35 [%]. An enlarged configuration diagram showing a secondary transfer nip and its surroundings in a configuration using a single-layer structure as the intermediate transfer belt, which is different from that of the printer. An enlarged cross-sectional view showing a secondary transfer nip and its surrounding configuration in a configuration in which a multi-layered elastic belt is used as the intermediate transfer belt as in the printer. The graph which shows an example of the waveform in the high-duty secondary transfer bias which the polarity is reversed in one cycle, and the duty is 80 [%]. The block diagram which shows the electric circuit of the operation display part of the printer which concerns on embodiment. The graph which shows the relationship between the secondary transfer rate of a toner image to a secondary transfer belt, the secondary transfer current, and the characteristic of a secondary transfer bias in a printer testing machine having a basic configuration. The schematic diagram which shows the misalignment detection pattern together with a belt. The block diagram which shows the paper feed path of the printer which concerns on 1st Embodiment. The schematic diagram for demonstrating the switching of the secondary transfer bias when the type of a recording sheet is changed from a concavo-convex sheet to a smooth sheet during a continuous print job. The flowchart which shows the processing flow in the continuous print job executed by the power supply control unit 200 of the printer which concerns on 1st Embodiment. The block diagram which shows the electric circuit of the operation display part of the printer which concerns on 2nd Example.

Hereinafter, embodiments of an electrophotographic color printer (hereinafter, simply referred to as a printer) as an image forming apparatus to which the present invention is applied will be described. The field of application of the present invention is not limited to printers, and the present invention can also be applied to copiers, facsimiles, multifunction devices having a copying function and a fax function, and the like.

First, the basic configuration of the printer according to the embodiment will be described. FIG. 1 is a schematic configuration diagram showing a printer according to an embodiment. In the figure, the printer according to the embodiment is an electrophotographic unit 1Y that constitutes a part of an image forming means for forming a toner image of yellow (Y), magenta (M), cyan (C), and black (K). , 1M, 1C, 1K. It also includes a transfer unit 30, an optical writing unit 80, a fixing device 90, a feeding cassette 100, a resist roller pair 101, and the like.

The electrophotographic units 1Y, 1M, 1C, and 1K for Y, M, C, and K use Y, M, C, and K toners of different colors, but other than that, they have the same configuration and reach the end of their service life. Sometimes exchanged. Taking an electrophotographic unit 1K for forming a K toner image as an example, as shown in FIG. 2, this is a drum-shaped photoconductor 2K as a latent image carrier, a drum cleaning device 3K, a charging device 6K, and a developing device. It has a device 8K and the like. It also has a static elimination device and the like. By integrally attaching and detaching these devices to the printer body while holding them in a common holder, the devices can be replaced at the same time.

The photoconductor 2K has an organic photosensitive layer formed on the surface of the drum substrate, and is rotationally driven in the clockwise direction in the drawing by a driving means. The charging device 6K makes the surface of the photoconductor 2K one by generating an electric discharge between the charging roller 7K and the photoconductor 2K while contacting or bringing the charging roller 7K to which the charging bias is applied into contact with or close to the photoconductor 2K. Charge like.

In this printer, the surface of the photoconductor 2K is uniformly charged to the same negative polarity as the normal charging polarity of the toner. As the charging bias, the one in which the AC voltage is superimposed on the DC voltage is adopted. The charging roller 7K is formed by coating the surface of a metal core metal with a conductive elastic layer made of a conductive elastic material. Instead of the method of bringing a charging member such as a charging roller into contact with or close to the photoconductor 2K, a method using a charging charger may be adopted.

The surface of the uniformly charged photoconductor 2K is light-scanned by a laser beam emitted from an optical writing unit 80, which will be described later, to carry an electrostatic latent image for K. The electrostatic latent image for K is developed by a developing device 8K using K toner to become a K toner image. Then, the primary transfer is performed on the intermediate transfer belt 31, which will be described later.

The drum cleaning device 3K removes the transfer residual toner adhering to the surface of the photoconductor 2K after undergoing the primary transfer step (primary transfer nip described later). It has a cleaning brush roller 4K that is driven to rotate, a cleaning blade 5K that brings the free end into contact with the photoconductor 2K while being cantilevered, and the like. The rotating cleaning brush roller 4K scrapes the transfer residual toner from the photoconductor 2K surface, and the cleaning blade scrapes the transfer residual toner from the photoconductor 2K surface.

The static eliminator removes the residual charge of the photoconductor 2K after being cleaned by the drum cleaning device 3K. By this static elimination, the surface of the photoconductor 2K is initialized to prepare for the next image formation.

The developing apparatus 8K has a developing unit 12K that houses a developing roll 9K that is a developer carrier, and a developing agent conveying unit 13K that agitates and conveys the K developer. The developer transport unit 13K has a first transport chamber for accommodating the first screw member 10K and a second transport chamber for accommodating the second screw member 11K. Each of the screw members includes a rotary shaft member whose both ends in the axial direction are rotatably supported by bearings, and a spiral blade spirally projecting from the peripheral surface thereof.

The first transport chamber accommodating the first screw member 10K and the second transport chamber accommodating the second screw member 11K are separated by a partition wall, but both ends of the partition wall in the screw axis direction. Each section is formed with a communication port for communicating both transport chambers. The first screw member 10K conveys the K developer held in the spiral blade from the back side to the front side in the direction orthogonal to the paper surface in the drawing while stirring in the rotation direction with the rotation drive. do. Since the first screw member 10K and the developing roll 9K, which will be described later, are arranged in parallel in a posture facing each other, the transport direction of the K developer at this time is also the direction along the rotation axis direction of the developing roll 9K. .. Then, the first screw member 10K supplies the K developer to the surface of the developing roll 9K along the axial direction thereof.

The K developer transported to the vicinity of the front end in the drawing of the first screw member 10K enters the second transport chamber through the communication opening provided in the vicinity of the front end in the drawing of the partition wall. , It is held in the spiral blade of the second screw member 11K. Then, as the second screw member 11K is rotationally driven, it is conveyed from the front side to the back side in the drawing while being agitated in the rotational direction.

In the second transport chamber, a toner concentration sensor is provided on the lower wall of the casing to detect the K toner concentration of the K developer in the second transport chamber. As the K toner concentration sensor, a sensor made of a magnetic permeability sensor is used. Since the magnetic permeability of the K developer containing the K toner and the magnetic carrier correlates with the K toner concentration, the magnetic permeability sensor detects the K toner concentration.

This printer is provided with Y, M, C, K toner replenishment means for individually replenishing Y, M, C, K toner in the second storage chamber of the developing apparatus for Y, M, C, K. There is. Then, the control unit of the printer stores the Vtref for Y, M, C, and K, which is the target value of the output voltage value from the Y, M, C, and K toner concentration detection sensors, in the RAM. If the difference between the output voltage value from the Y, M, C, K toner concentration detection sensor and the Vtref for Y, M, C, K exceeds a predetermined value, Y, for the time corresponding to the difference. Drives M, C, K toner replenishment means. As a result, the Y, M, C, and K toners are replenished in the second transport chamber of the developing apparatus for Y, M, C, and K, and the K toner concentration of the K developer is maintained within a predetermined range.

The developing roll 9K housed in the developing unit 12K faces the first screw member 10K and also faces the photoconductor 2K through the opening provided in the casing. Further, the developing roll 9K includes a tubular developing sleeve made of a non-magnetic pipe that is driven to rotate, and a magnet roller fixed inside the sleeve so as not to rotate with the sleeve. Then, while the K developer supplied from the first screw member 10K is supported on the sleeve surface by the magnetic force generated by the magnet roller, it is conveyed to the developing region facing the photoconductor 2K as the sleeve rotates.

A development bias having the same polarity as the toner, having an absolute value larger than the potential of the electrostatic latent image of the photoconductor 2K, and having an absolute value smaller than the uniform charging potential of the photoconductor 2K is applied to the developing sleeve. ing. As a result, a development potential that electrostatically moves the K toner on the developing sleeve toward the electrostatic latent image acts between the developing sleeve and the electrostatic latent image of the photoconductor 2K. Further, a background potential that moves the K toner on the developing sleeve toward the sleeve surface acts between the developing sleeve and the background portion of the photoconductor 2K. Due to the action of the development potential and the background potential, the K toner on the developing sleeve is selectively transferred to the electrostatic latent image of the photoconductor 2K, and the electrostatic latent image is developed into a toner image.

In FIG. 1, in the electrophotographic units 1Y, M, C for Y, M, C as well as the electrophotographic unit 1K for K, Y, M, C toner images are displayed on the photoconductors 2Y, 2M, 2C. Is formed. Above the electrophotographic units 1Y, 1M, 1C, and 1K, an optical writing unit 80, which is a latent image writing means, is arranged. The optical writing unit 80, which forms a part of the image forming means, uses laser light emitted from a laser diode based on image information sent from an external device such as a personal computer to generate photoconductors 2Y, 2M, 2C, and 2K. Is lightly scanned. By this optical scanning, electrostatic latent images for Y, M, C, and K are formed on the photoconductors 2Y, 2M, 2C, and 2K.

The optical writing unit 80 irradiates the photoconductor through a plurality of optical lenses and mirrors while polarized the laser beam L emitted from the light source in the main scanning direction by a polygon mirror rotationally driven by a polygon motor. Is. An LED array may be used in which light writing is performed by LED light emitted from a plurality of LEDs in the LED array.

Below the electrophotographic units 1Y, 1M, 1C, and 1K, a transfer unit 30 is arranged so that the endless intermediate transfer belt 31 is stretched and moved endlessly in the counterclockwise direction in the drawing. In addition to the intermediate transfer belt 31 which is an image carrier, the transfer unit 30 includes a drive roller 32, a secondary transfer back surface roller 33, a cleaning backup roller 34, and primary transfer rollers 35Y, 35M, 35C for Y, M, C, and K. , 35K, etc. It also has a belt cleaning device 37, a secondary transfer power supply 45, a nip forming unit 38, and the like.

The intermediate transfer belt 31 is stretched by a drive roller 32, a secondary transfer back surface roller 33, a cleaning backup roller 34, and four primary transfer rollers 35Y, 35M, 35C, and 35K arranged inside the loop. Then, it is endlessly moved in the same direction by the rotational force of the drive roller 32 which is rotationally driven in the counterclockwise direction in the drawing by the drive means.

The primary transfer rollers 35Y, 35M, 35C, 35K for Y, M, C, and K sandwich an intermediate transfer belt 31 that can be moved endlessly between the photoconductors 2Y, 2M, 2C, and 2K. As a result, a primary transfer nip for Y, M, C, K is formed in which the front surface of the intermediate transfer belt 31 and the photoconductors 2Y, 2M, 2C, 2K for Y, M, C, K come into contact with each other. Has been done. A primary transfer bias that is individually output from the primary transfer power supply is applied to the primary transfer rollers 35Y, 35M, 35C, and 35K. As a result, a primary transfer electric field is formed between the Y, M, C, K toner images on the photoconductors 2Y, 2M, 2C, 2K and the primary transfer rollers 35Y, 35M, 35C, 35K. The Y toner formed on the surface of the photoconductor 2Y for Y enters the primary transfer nip for Y as the photoconductor 2Y rotates. Then, by the action of the primary transfer electric field and the nip pressure, the primary transfer is performed from the photoconductor 2Y onto the intermediate transfer belt 31. The intermediate transfer belt 31 on which the Y toner image is primarily transferred in this way then sequentially passes through the primary transfer nips for M, C, and K. Then, the M, C, and K toner images on the photoconductors 2M, 2C, and 2K are sequentially superposed on the Y toner image and primarily transferred. By this superposition primary transfer, a four-color superposition toner image is formed on the intermediate transfer belt 31. Instead of the primary transfer rollers 35Y, 35M, 35C, 35K, a transfer charger, a transfer brush, or the like may be adopted.

In the transfer unit 30, a nip forming unit is arranged below the intermediate transfer belt 31. The nip forming unit 38 includes a secondary transfer nip lining roller 36, a separation roller 44, a tension roller 43, a cleaning backup roller 39, a secondary transfer belt 41 as a nip forming body, a belt cleaner 42, an optical sensor unit 40, and the like. ing.

The endless secondary transfer belt 41 is tension-tensioned by the secondary transfer nip lining roller 36, the separation roller 44, the tension roller 43, and the cleaning backup roller 39 arranged inside the loop. It moves endlessly in the clockwise direction. Then, in the entire area in the circumferential direction on the front surface (loop outer peripheral surface) of the intermediate transfer belt 31, the secondary transfer nip is formed by abutting on the hooking portion with respect to the secondary transfer back surface roller 33.

The secondary transfer nip backing roller 36 arranged in the loop of the secondary transfer belt 41 is grounded, whereas the secondary transfer back surface roller 33 arranged in the loop of the intermediate transfer belt 31 is attached to the secondary transfer back surface roller 33. The secondary transfer bias output from the secondary transfer power supply 45 is applied. As a result, negatively polar toner is electrostatically moved between the secondary transfer back surface roller 33 and the secondary transfer nip lining roller 36 from the secondary transfer back surface roller 33 side toward the secondary transfer nip lining roller 36 side. A secondary transfer electric field is formed.

Below the transfer unit 30, a feeding cassette 100 that houses a plurality of recording sheets P in the form of a stack of sheets is arranged. In the feed cassette 100, the paper feed roller 100a is brought into contact with the recording sheet P at the top of the sheet bundle, and the recording sheet P is brought into the feed path by rotationally driving the paper feed roller 100a at a predetermined timing. Send out towards.

A resist roller pair 101 is arranged near the end of the supply path. The tip of the recording sheet P conveyed in the feed path is detected by a resist sensor (507 described later) arranged slightly upstream of the resist nip due to the contact between the two rollers in the resist roller pair 101. NS. The transportation of the recording sheet P is temporarily stopped after a predetermined time has elapsed from the timing of this detection. As a result, the recording sheet P is slightly bent with its tip abutting against the resist nip, thereby correcting the skew. After that, the resist roller pair 101 is rotationally driven at a timing capable of synchronizing the recording sheet P with the four-color superimposed toner image on the intermediate transfer belt 31 in the secondary transfer nip, and directs the recording sheet P toward the secondary transfer nip. And send it out.

The four-color superimposed toner image on the intermediate transfer belt 31 that is brought into close contact with the recording sheet P by the secondary transfer nip is collectively secondary transferred onto the recording sheet P by the action of the secondary transfer electric field and nip pressure, and is a full-color toner. It becomes a statue. When the recording sheet P on which the full-color toner image is formed on the surface in this way passes through the secondary transfer nip, the curvature is separated from the intermediate transfer belt 31. Further, the curvature is separated from the secondary transfer belt 41 by the curvature of the separation roller 44 around which the secondary transfer belt 41 is hung.

The transfer residual toner that has not been transferred to the recording sheet P is attached to the intermediate transfer belt 31 after passing through the secondary transfer nip. This is cleaned from the belt surface by the belt cleaning device 37 that is in contact with the front surface of the intermediate transfer belt 31. The cleaning backup roller 34 arranged inside the loop of the intermediate transfer belt 31 backs up the cleaning of the belt by the belt cleaning device 37 from the inside of the loop.

A fixing device 90 is arranged on the downstream side in the sheet transport direction with respect to the secondary transfer nip. The fixing device 90 forms a fixing nip by a fixing roller 91 including a heat generating source such as a halogen lamp and a pressure roller 92 that rotates while contacting the fixing roller 91 with a predetermined pressure. The recording sheet P that has passed through the secondary transfer nip and is fed into the fixing device 90 is sandwiched between the fixing nips in a posture in which the surface supporting the unfixed toner image is brought into close contact with the fixing roller 91. Then, the toner in the toner image is softened by the influence of heating and pressurization, and the full-color image is fixed. The recording sheet P discharged from the fixing device 90 is discharged to the outside of the machine after passing through a transport path after fixing.

When forming a monochrome image, this printer changes the posture of the support plate supporting the primary transfer rollers 35 (Y, M, C) for Y, M, C in the transfer unit 30 by driving a solenoid or the like. Let me. As a result, the primary transfer rollers 35 (Y, M, C) for Y, M, and C are moved away from the photoconductor 2 (Y, M, C), and the front surface of the intermediate transfer belt 31 is placed on the photoconductor 2 Separate from (Y, M, C). In this way, in the state where the intermediate transfer belt 31 is in contact with only the black photoconductor 2K, among the electrophotographic units 1 (Y, M, C, K) for Y, M, C, K, Only the black electrophotographic unit 1K is driven to form a K toner image on the black photoconductor 2K.

FIG. 3 is a block diagram showing a main part of the electric circuit of the secondary transfer power supply 45 and the power supply control unit 200 together with the secondary transfer back surface roller 33, the secondary transfer nip lining roller 36, and the like. The secondary transfer power supply 45 includes a DC power supply 110, a detachable AC power supply 140, and the like.

The DC power supply 110 is a power supply for outputting a DC voltage for applying an electrostatic force from the belt side to the recording sheet side in the secondary transfer nip to the toner on the surface of the intermediate transfer belt 31. It also includes a DC output control unit 111, a DC drive unit 112, a DC voltage transformer 113, a DC output detection unit 114, an output abnormality detection unit 115, an electrical connection unit 221 and the like.

The AC power supply 140 is a power supply that outputs an AC voltage for forming an alternating electric field in the secondary transfer nip. It also includes an AC output control unit 141, an AC drive unit 142, an AC voltage transformer 143, an AC output detection unit 144, a removal unit 145, an output abnormality detection unit 146, an electrical connection unit 242, and an electrical connection unit 243. There is.

The power supply control unit 200, which is a control means, controls the output from the secondary transfer power supply, and is from a control device having a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like. Become. A DC_PWM signal for controlling the magnitude of the DC voltage output is input from the power supply control unit 200 to the DC output control unit 111. Further, the output value of the DC voltage transformer 113 detected by the DC output detection unit 114 is also input. Then, the DC output control unit 111 performs the following control based on the duty ratio of the input DC_PWM signal and the output value of the DC voltage transformer 113. That is, the drive of the DC voltage transformer 113 is controlled via the DC drive unit 112 so that the output value of the DC voltage transformer 113 becomes the output value indicated by the DC_PWM signal.

The DC drive unit 112 drives the DC voltage transformer 113 according to the control from the DC output control unit 111. Further, the DC voltage transformer 113 is driven by the DC drive unit 112 to output a negative DC high voltage. When the AC power supply 140 is not connected, the electrical connection portion 221 and the secondary transfer back surface roller 33 are electrically connected by the harness 301, so that the DC voltage transformer 113 is connected via the harness 301. A DC voltage is output (applied) to the secondary transfer back surface roller 33. On the other hand, when the AC power supply 140 is connected, the electrical connection unit 221 and the electrical connection unit 242 are electrically connected by the harness 302, so that the DC voltage transformer 113 is connected to the AC power supply 140 via the harness 302. Outputs DC voltage.

The DC output detection unit 114 detects the output value of the DC high voltage from the DC voltage transformer 113 and outputs it to the DC output control unit 111. Further, the DC output detection unit 114 outputs the detected output value to the power supply control unit 200 as an FB_DC signal (feedback signal). This is because the power supply control unit 200 controls the duty of the DC_PWM signal so that the transferability does not deteriorate due to the environment and the load. In this printer, since the AC power supply 140 can be attached to and detached from the main body of the secondary transfer power supply 45, the impedance of the output path of the high voltage output depends on whether the AC power supply 140 is connected or not. Changes. Therefore, when the DC power supply 110 performs constant voltage control and outputs a DC voltage, the voltage division ratio changes because the impedance in the output path changes depending on the presence or absence of the AC power supply 140. Further, since the high voltage applied to the secondary transfer back surface roller 33 changes, the transferability changes depending on the presence or absence of the AC power supply 140.

Therefore, in this printer, the DC power supply 110 performs constant current control to output a DC voltage, and the output voltage is changed according to the presence or absence of the AC power supply 140. As a result, even if the impedance in the output path changes, the high voltage applied to the secondary transfer back surface roller 33 can be kept constant, and the transferability can be kept constant regardless of the presence or absence of the AC power supply 140. can. Further, the AC power supply 140 can be attached and detached without changing the value of the DC_PWM signal. As described above, in this printer, the DC power supply 110 is controlled by a constant current, but the following configuration may be adopted. That is, if the high voltage applied to the secondary transfer back surface roller 33 can be kept constant by changing the value of the DC_PWM signal when the AC power supply 140 is attached or detached, a configuration is adopted in which the DC power supply 110 is controlled at a constant voltage. You may.

The output abnormality detection unit 115 is arranged on the output line of the DC power supply 110, and when an output abnormality occurs due to a ground fault of an electric wire or the like, a signal indicating an output abnormality such as a leak is output to the power supply control unit 200. do. This makes it possible for the power supply control unit 200 to perform control for stopping the high-voltage output from the DC power supply 110.

The AC_PWM signal for controlling the magnitude of the AC voltage output and the output value of the AC voltage transformer 143 detected by the AC output detection unit 144 are input to the AC output control unit 141 from the power supply control unit 200. Then, the AC output control unit 141 performs the following control based on the duty ratio of the input AC_PWM signal and the output value of the AC voltage transformer 143. That is, the drive of the AC voltage transformer 143 is controlled via the AC drive unit 142 so that the output value of the AC voltage transformer 143 becomes the output value indicated by the AC_PWM signal.

An AC_CLK signal that controls the output frequency of the AC voltage is input to the AC drive unit 142. Then, the AC drive unit 142 drives the AC voltage transformer 143 based on the control from the AC output control unit 141 and the AC_CLK signal. By driving the AC voltage transformer 143 based on the AC_CLK signal, the AC drive unit 142 can control the output waveform generated by the AC voltage transformer 143 to an arbitrary frequency indicated by the AC_CLK signal. ..

The AC voltage transformer 143 is driven by the AC drive unit 142 to generate an AC voltage, and superimposes the generated AC voltage and the high DC voltage output from the DC voltage transformer 113 to generate a superposed voltage. When the AC power supply 140 is connected, that is, when the electrical connection portion 243 and the secondary transfer back surface roller 33 are electrically connected by the harness 301, the AC voltage transformer 143 transfers the generated superimposed voltage. It is applied to the secondary transfer back surface roller 33 via the harness 301. When the AC voltage transformer 143 does not generate an AC voltage, the AC voltage transformer 143 outputs (applies) a high DC voltage output from the DC voltage transformer 113 to the secondary transfer back surface roller 33 via the harness 301. .. The voltage (superimposed voltage or DC voltage) output to the secondary transfer back surface roller 33 is then returned to the DC power supply 110 via the secondary transfer nip backing roller 36.

The AC output detection unit 144 detects the output value of the AC voltage of the AC voltage transformer 143 and outputs it to the AC output control unit 141. Further, the detected output value is output to the power supply control unit 200 as an FB_AC signal (feedback signal). This is because the power supply control unit 200 controls the duty of the AC_PWM signal so that the transferability is not lowered by the environment and the load. Although the AC power supply 140 performs constant voltage control, a power supply 140 that performs constant current control may be used. The AC voltage waveform generated by the AC voltage transformer 143 (AC power supply 140) may be either a sine wave or a square wave, but this printer uses a short pulse square wave. .. This is because the image quality can be further improved by making the waveform of the AC voltage a short pulse-shaped square wave.

The secondary transfer power supply 45 outputs a DC voltage by a constant current control method that adjusts the output voltage value so that the output current value matches a predetermined target current value. Further, the AC voltage is output by a constant voltage control method in which the amplitude is adjusted so that the peak-to-peak value Vpp matches a predetermined target value.

As described in Patent Document 1, if an alternating electric field is formed in the secondary transfer nip by using a secondary transfer bias consisting of a superposed voltage, the toner can be satisfactorily applied to the recesses on the surface of the recording sheet having abundant surface irregularities. It is already known that it can be secondarily transcribed. It is also known that the principle is as follows. That is, when a secondary transfer bias consisting of only DC voltage is used, only a very small number of toner particles constituting the toner image on the intermediate transfer belt in the secondary transfer nip are from the belt surface to the sheet surface. Cannot be transferred into the recess. Even if the secondary transfer bias consisting of the superimposed voltage is used, the same applies until the first cycle of the AC component of the secondary transfer bias elapses after the toner image enters the secondary transfer nip. Only a very small amount of toner particles can be transferred into the recesses on the surface of the sheet. However, when the next cycle elapses, the amount of toner particles transferred from the belt surface into the sheet surface recesses increases slightly. Specifically, first, in the first half of the next cycle, when the toner particles in the recesses on the sheet surface return to the belt surface, they collide with the toner particles that had been attached to the belt surface until then, and the toner particles. Weakens the adhesion between the toner particles and other toner particles and the belt surface. Then, in the latter half, the toner particles whose adhesive force is weakened as described above are transferred from the belt surface into the sheet surface recess together with the toner particles returning to the belt surface. Further, in the next cycle, the number of toner particles transferred into the recesses on the sheet surface further increases due to the same phenomenon. In the process of the toner particles reciprocating between the belt surface and the sheet surface recesses many times in the secondary transfer nip, the number of toner particles transferred into the sheet surface recesses gradually increases. Then, by the time the toner image finally passes through the secondary transfer nip, a sufficient amount of toner particles have been transferred into the recesses on the surface of the sheet.

In FIG. 1, the optical sensor unit 40 of the nip forming unit 38 includes four reflective optical sensors, and the reflective optical is provided through a predetermined gap with respect to the front surface of the secondary transfer belt 41. The sensors are facing each other. These reflective optical sensors detect the optical characteristics of the secondary transfer belt 41 and the test toner image secondarily transferred from the intermediate transfer belt 31 to the secondary transfer belt 41 by the secondary transfer nip. As a result, the position of the test toner image secondarily transferred on the secondary transfer belt 41 is detected, and the image density (toner adhesion amount per unit area) of the test toner image is detected.

For the reason described later, in this printer, by using an intermediate transfer belt 31 made of an elastic belt having a dark color tone, it is possible to satisfactorily detect the optical characteristics of the toner image on the intermediate transfer belt 31. Can not. Therefore, the test toner image is secondarily transferred from the dark color intermediate transfer belt 31 to the light color secondary transfer belt 41, and the optical characteristics of the test toner image on the secondary transfer belt 41 are detected by the optical sensor unit 40. It has become.

FIG. 4 is a block diagram showing a main part of the electric circuit of this printer. In the figure, the main control unit 300 including the CPU 300a, the RAM 300b, the ROM 300c, the flash memory 300d, and the like controls the drive of each device in the printer based on the program stored in the ROM 300c, and performs various arithmetic processes. To do. The main control unit 300 includes a resist sensor 506, a resist motor 507, electrophotographic units 1Y, 1M, 1C, 1K for Y, M, C, and K, a transfer unit 30, a write control unit 505, and a power supply control unit 200. Etc. are connected. Further, the optical writing unit 80 is connected via the writing control unit 505. Further, the environment sensor 50, the operation display unit 501, the charged power supply unit 508, the developing power supply unit 503, the primary transfer power supply unit 504, and the secondary transfer power supply 45 are connected via the power supply control unit 200.

The resist motor 507 is a motor that is a drive source for the resist roller pair 101. Further, the writing control unit 505 controls the drive of the optical writing unit 80 based on the image data sent from an external personal computer or a scanner, and sends the image data to the main control unit 300.

The environment sensor 50 detects the temperature and humidity inside the machine and sends the detection result to the power supply control unit 200. The power supply control unit 200 calculates the absolute humidity based on the temperature detection result and the humidity detection result sent from the environment sensor 50.

The operation display unit 501 includes a touch panel and a plurality of operation keys, and accepts a touch operation on the touch panel by a user, accepts a key operation on the operation key, and displays an image on the touch panel.

The charging power supply unit 508 outputs the charging bias applied to the charging roller of the charging device for Y, M, C, and K individually for each color. The output values of these charging biases are individually controlled by the power supply control unit 200.

The developing power supply unit 503 outputs the development bias applied to the developing sleeve of the developing apparatus for Y, M, C, and K individually for each color. The output values of these development biases are individually controlled by the power supply control unit 200.

The primary transfer power supply unit 504 individually controls the primary transfer bias applied to the primary transfer rollers 35Y, 35M, 35C, 35K for Y, M, C, and K for each color. The output values of these primary transfer biases are individually controlled by the power supply control unit 200.

The main control unit 300 executes the process control process at a predetermined timing, such as when the main power is turned on, when the standby time is after a predetermined time has elapsed, or when the main control unit 300 is on standby after outputting a predetermined number of prints or more. FIG. 5 is a flowchart showing a processing flow in the process control.

When the above-mentioned predetermined timing arrives, the main control unit 300 first acquires environmental information such as the number of sheets to be printed, the printing rate, the temperature, and the humidity (step 1: hereinafter, the step is referred to as S). Next, the development characteristics of the electrophotographic units 1Y, 1M, 1C, and 1K are grasped. Specifically, the development gamma γ and the development start voltage are calculated for each color (S2).

The calculation process is as follows. That is, first, while rotating the photoconductors 2Y, 2M, 2C, and 2K, each of them is uniformly charged. Regarding this charging, the absolute value of the charging bias Vc is gradually increased, unlike a uniform value (for example, −700V) at the time of normal printing. By scanning the laser beam with the optical writing unit 80, a patch-shaped Y test toner image, M test toner image, C test toner image, and electrostatic latent image for the K test toner image are formed on the photoconductors 2Y, 2M, 2C, and 2K. To form. By developing them with a developing device for Y, M, C, and K, the photoconductors 2Y, 2M, 2C, and 2K have a Y patch pattern image YPP, an M patch pattern image MPP, and a C patch pattern image CPP, K patch pattern. Form the image KPP. At the time of development, the main control unit 300 gradually increases the absolute value of the development bias Vb applied to the development sleeves for Y, M, C, and K. Both the development bias Vb and the charging bias Vc are negative DC voltages.

The patch pattern images (YPP, MPP, CPP, KPP) for Y, M, C, and K are secondarily transferred from the photoconductors 2Y, 2M, 2C, and 2K to the secondary transfer belt 41 via the intermediate transfer belt 31. NS. As shown in FIG. 6, the secondary transfer is performed so as to line up in the belt width direction without overlapping each other. Specifically, the Y patch pattern image YPP is transferred to one end of the secondary transfer belt 41 in the width direction. Further, the M patch pattern image MPP is transferred to a position slightly shifted to the center side of the Y patch pattern image YPP in the belt width direction. Further, the K patch pattern image KPP is secondarily transferred to the other end of the secondary transfer belt 41 in the width direction. Further, the C patch pattern image CPP is secondarily transferred to a position slightly shifted to the center side of the K patch pattern image KPP in the belt width direction.

The optical sensor unit 40 includes a Y-reflective optical sensor 40Y, an M-reflective optical sensor 40M, a C-reflective optical sensor 40C, and a K-reflective optical sensor 40K that detect the light reflection characteristics of the belt at different positions in the belt width direction. Have. Of these four reflective optical sensors, the K reflective optical sensor 40K employs one that detects only specularly reflected light so as to detect changes in the light reflection characteristics of the belt surface due to the adhesion of black toner. ing. On the other hand, other reflective optical sensors detect both specular and diffuse reflected light so as to detect changes in the light reflection characteristics of the belt surface due to adhesion of Y, M or C toner. It's a type.

The Y reflection type optical sensor 40Y detects the amount of toner adhered to a plurality of Y test toner images of the Y patch pattern image YPP formed at one end in the width direction of the secondary transfer belt 41. Further, the M reflection type optical sensor 40M detects the amount of toner adhered to a plurality of M test toner images of the M patch pattern image MPP formed next to the Y patch pattern image YPP on the secondary transfer belt 41. Further, the K reflection type optical sensor 40K detects the amount of toner adhered to a plurality of K test toner images of the K patch pattern image KPP formed at the other end in the width direction of the secondary transfer belt 41. Further, the C reflection type optical sensor 40C detects the amount of toner adhered to a plurality of K test toner images of the K patch pattern image KPP formed next to the K patch pattern image KPP on the secondary transfer belt 41.

The main control unit 300 calculates the light reflectance of the test toner image of each color based on the output signals sequentially sent from the four reflective optical sensors of the optical sensor unit 40, and the amount of toner adhered based on the calculation result. Is stored in the RAM 300b. The test toner image of each color that has passed through the position facing the optical sensor unit 40 as the secondary transfer belt 41 travels is removed from the secondary transfer belt 41 by the belt cleaner 42.

Next, the main control unit 300 uses the image density data (toner adhesion amount) stored in the RAM 300b and the exposure unit potential (latent image potential) data separately stored in the RAM 300b to form a linear approximation formula (figure) shown in FIG. Y = a × Vb + b) is calculated. In the two-dimensional coordinates of the figure, the x-axis shows a value obtained by subtracting the development bias Vb applied at that time from the exposure potential Vl, that is, the development potential (Vl−Vb). The Y-axis indicates the amount of toner adhered (y) per unit area. In the figure, the number of data corresponding to the number of test toner images is plotted on the XY plane. Based on the plotted data, the interval on the XY plane to be linearly approximated is determined. Then, within that section, a linear approximation equation (y = a × Vb + b) is obtained by the least squares method. At this time, the development gamma γ and the development start voltage Vk are calculated based on the linear approximation formula. The development gamma γ is calculated as the slope of the linear approximation formula (γ = a), and the development start voltage Vk is calculated as the intersection of the linear approximation formula and the X axis (Vk = −b / a). In this way, the development characteristics of the electrophotographic units 1Y, 1C, 1M, and 1K of Y, M, C, and K are calculated (S2).

Next, based on the obtained development characteristics, the target value of the charging potential (ground potential) Vd (target charging potential), the target value of the exposure potential Vl (target exposure potential), and the development bias Vb are determined. Required (S3). Specifically, the target charging potential and the target exposed portion potential are obtained based on a data table in which the relationship between the developing gamma γ and the charging potential Vd and the exposed portion potential Vl is predetermined. Thereby, the target charging potential and the target exposed portion potential suitable for the developed gamma γ can be selected. The development bias Vb is obtained as follows. That is, the development potential for obtaining the maximum toner adhesion amount is obtained by the combination of the development gamma γ and the development start voltage Vk, and the development bias Vb capable of obtaining the development potential is obtained. Then, the target charging potential is obtained based on the development bias Vb and the background potential. Since the surface of the developing sleeve of the developing roller has almost the same value as the developing bias Vb, the surface of the photoconductor is charged to the target charging potential, and if it is properly exposed, the target developing potential and background potential can be obtained. be able to.

The main control unit 300 then determines the charge bias Vc. Specifically, the charging bias Vc at which the target charging potential is obtained changes depending on the amount of wear of the surface layer of the photoconductor, the electric resistance of the charging roller affected by the environment, and the like. Therefore, the main control unit 300 stores an algorithm for obtaining the charging bias Vc capable of obtaining the target charging potential from the combination of the absolute humidity and the mileage of the photoconductor. This algorithm was constructed based on prior experiments. Then, the charging bias Vc capable of obtaining the target charging potential is obtained by using an algorithm based on the combination of the temperature / humidity detection result by the environment sensor 50 and the mileage of the photoconductor stored in the RAM.

By periodically performing such a process control process, it is possible to form an image with a stable image density for a long period of time regardless of environmental changes.

In a continuous print job in which an image is continuously formed on a plurality of recording sheets P, the toner charge amount (Q / M) in the developing agent of the developing device becomes the average output image in the continuous print job over a long period of time. It changes according to the area ratio. Specifically, when the average output image area ratio becomes relatively high, the amount of toner replenished per unit time becomes relatively large, and the average residence time of toner from replenishment to contributing to development in the developing apparatus is relatively large. It gets shorter. Then, the toner charge amount (Q / M) becomes relatively low and the adhesive force of the toner to the magnetic carrier becomes relatively weak, so that the developing ability of the developing apparatus becomes relatively high and the image density is increased.

On the other hand, when the average output image area ratio is relatively low, the amount of toner replenished per unit time is relatively small, and the average residence time of toner from replenishment to contributing to development in the developing apparatus becomes relatively long. Then, the toner charge amount (Q / M) becomes relatively high and the adhesive force of the toner to the magnetic carrier becomes relatively strong, so that the developing ability of the developing apparatus becomes relatively low and the image density is lowered.

In order to suppress fluctuations in image density due to such differences in average output image area ratio, the main control unit 300 sets Vtref (target of developer) for each color of Y, M, C, and K during a continuous print job. A Vtref adjustment process for adjusting the toner concentration) is performed. In this Vtref adjustment process, a Y test toner image, an M test toner image, a C test toner image, and a K test toner image are formed one by one every time a predetermined number of prints are performed. Specifically, it is a region between a region in contact with the preceding recording sheet P at the secondary transfer nip and a region in contact with the subsequent recording sheet P in the entire area in the circumferential direction of the secondary transfer belt 41. These test toner images are formed in the inter-sheet correspondence area. Then, for each of Y, M, C, and K, Vtref is corrected so that a predetermined image density can be obtained based on the result of detecting the toner adhesion amount of the test toner image by the optical sensor unit 40.

Next, a characteristic configuration of the printer according to the embodiment will be described.
In the old days, it was common to use a belt substrate made of a hard material such as a polyimide belt as an intermediate transfer belt. However, it was found that such a configuration causes the following problems. That is, for example, while moving the intermediate transfer belt endlessly at an extremely high linear velocity of about 630 [mm / s], a toner image is formed on an uneven sheet such as a surface uneven sheet (trade name: Rezac 66) manufactured by Tokai Paper Co., Ltd. Suppose that the secondary transcription is performed. Then, despite the fact that a superposed voltage is used as the secondary transfer bias, the toner cannot be satisfactorily transferred to the surface recesses of the uneven sheet, resulting in uneven image density that follows the surface irregularities. Will cause. In other words, with an intermediate transfer belt consisting of only a hard belt substrate, if the printing speed is increased to an ultra-high speed for business users, unevenness will occur even if an alternating electric field is formed in the secondary transfer nip by the secondary transfer bias consisting of the superimposed voltage. Causes poor toner transfer in the recesses on the surface of the sheet.

On the other hand, the present inventors prepared a printer testing machine equipped with an elastic belt as an intermediate transfer belt, and made an image on a concavo-convex sheet (Rezac) at an ultra-high speed (process linear speed = 630 mm / s) for business users. An experiment of secondary transcription was carried out in 66). Then, the toner was satisfactorily secondarily transferred to the surface recesses of the uneven sheet, and the occurrence of image density unevenness following the sheet surface irregularities could be effectively suppressed. It is considered that this is because the distance between the surface of the intermediate transfer belt and the surface recess of the uneven sheet is reduced by flexibly changing the elastic layer of the intermediate transfer belt composed of the elastic belt in the secondary transfer nip. In view of the experimental results, in the printer according to the embodiment, the intermediate transfer belt 31 is made of an elastic belt.

FIG. 8 is an enlarged cross-sectional view partially showing a cross section of an intermediate transfer belt 31 made of an elastic belt mounted on the printer according to the embodiment. The intermediate transfer belt 31 has an endless belt-shaped base layer (belt substrate made of a hard material) 31a made of a material having a certain degree of flexibility and high rigidity, and is excellent in flexibility laminated on the front surface thereof. It is provided with an elastic layer 31b made of an elastic material. Particles 31c are dispersed in the elastic layer 31b, and the particles 31c are densely packed in the belt surface direction as shown in FIG. 9 in a state where a part of the particles 31c protrudes from the surface of the elastic layer 31b. And lined up. A plurality of protrusions are formed on the belt surface by the plurality of particles 31c.

As the material of the base layer 31a, a resin in which an electric resistance adjusting material composed of a filler or an additive for adjusting the electric resistance is dispersed can be exemplified. From the viewpoint of flame retardancy, the resin is preferably a fluorine-based resin such as PVDF (polyvinylidene fluoride) or ETFE (ethylene / ethylene tetrafluoride copolymer), a polyimide resin, a polyamide-imide resin, or the like. .. Further, from the viewpoint of mechanical strength (high elasticity) and heat resistance, a polyimide resin or a polyamide-imide resin is particularly preferable.

Examples of the electric resistance adjusting material dispersed in the resin include metal oxides, carbon blacks, ionic conductive agents, and conductive polymer materials. Examples of the metal oxide include zinc oxide, tin oxide, titanium oxide, zirconium oxide, aluminum oxide, silicon oxide and the like. In order to improve the dispersibility, the metal oxide which has been surface-treated in advance may be used.

The coating liquid (a liquid resin before curing in which an electric resistance adjusting material is dispersed), which is a precursor of the base layer 31a, contains a dispersion aid, a reinforcing material, a lubricating material, and a heat conductive material, if necessary. , Antioxidants and the like may be added. The amount of the electric resistance adjusting material added to the base layer 31a of the seamless belt preferably equipped as the intermediate transfer belt 31 is preferably 1 × 10 ⁸ to 1 × 10 ¹³ [Ω / □] in terms of surface resistance, and volume resistance. The amount is 1 × 10 ⁶ to 1 × 10 ¹² [Ω · cm].

The thickness of the base layer 31a is not particularly limited and may be appropriately selected depending on the situation, but is preferably 30 μm to 150 μm, more preferably 40 μm to 120 μm, and particularly preferably 50 μm to 80 μm. If the thickness of the base layer 31a is less than 30 μm, the belt is likely to be torn due to cracking, and if it exceeds 150 μm, the belt may be torn due to bending. On the other hand, if the thickness of the base layer 31a is in the above-mentioned particularly preferable range, it is advantageous in terms of durability.

In order to improve the belt running stability, it is preferable to reduce the layer thickness unevenness of the base layer 31a as much as possible. The method for adjusting the thickness of the base layer 31a is not particularly limited, and can be appropriately selected depending on the situation. For example, a method of measuring with a contact type or eddy current type film thickness meter and a method of measuring a cross section of a film with a scanning electron microscope (SEM) can be mentioned.

As described above, the elastic layer 31b of the intermediate transfer belt 31 has a plurality of convex shapes formed by the plurality of dispersed particles 31c on the surface.

Among the elastic materials constituting the elastic layer 31b, acrylic rubber is most preferable from the viewpoints of ozone resistance, flexibility, adhesion to particles, flame retardancy, environmental stability and the like.

The appropriate range of the amount of the cross-linking agent to be blended is preferably 0.05 to 20 parts by weight, more preferably 0.1 to 5 parts by weight, based on 100 parts by weight of the acrylic rubber. If the amount of the cross-linking agent is too small, the cross-linking is not sufficiently performed, and it becomes difficult to maintain the shape of the cross-linked product. On the other hand, if the content is too large, the crosslinked product becomes too hard and the elasticity of the crosslinked rubber is impaired.

The acrylic rubber used for the elastic layer 31b may be blended with a cross-linking accelerator for the purpose of promoting the cross-linking reaction of the above-mentioned cross-linking agent.

Regarding the amount of the electric resistance adjusting material added, the resistance value of the elastic layer 31b is 1 × 10 ⁸ to 1 × 10 ^{13 [Ω / □] for the surface resistance and 1 × 10 6} to 1 × 10 ¹² [Ω / □] for the volume resistance. It is preferable to adjust the range so that it is in the range of [cm].

The layer thickness of the elastic layer 31b is preferably 200 μm to 2 mm, more preferably 400 μm to 1000 μm. If the layer thickness is smaller than 200 μm, the ability of the recording sheet to follow the surface irregularities and the effect of reducing the transfer pressure are lowered, which is not preferable. Further, if the layer thickness is larger than 2 mm, the elastic layer 31b tends to bend due to its own weight, which makes the running performance unstable, and the belt tends to crack when it is hung on the roller on which the belt is stretched. It is not preferable because it may occur. As a method for measuring the layer thickness, a method for measuring the cross section by observing the cross section with a scanning microscope (SEM) can be exemplified.

The particles 31c dispersed in the elastic material of the elastic layer 31b have an average particle diameter of 100 μm or less, a spherical shape, are insoluble in an organic solvent, and have a 3% thermal decomposition temperature of 200 ° C. or higher. Use certain resin particles. The resin material of the particles 31c is not particularly limited, and examples thereof include acrylic resin, melamine resin, polyamide resin, polyester resin, silicone resin, fluororesin, and rubber. The surface of the base of the particles made of these resin materials may be surface-treated with a different material. A hard resin may be coated on the surface of spherical base particles made of rubber. Further, as the parent particles, hollow ones or porous ones may be used.

Among the resin materials exemplified so far, silicone resin particles are most preferable from the viewpoint of excellent slipperiness, releasability to toner, abrasion resistance, and the like. The particles are preferably particles obtained by finishing the resin material into a spherical shape by a polymerization method or the like, and those closer to a true sphere are preferable. Further, as the particles 31c, it is desirable to use particles having a volume average particle diameter of 1.0 μm to 5.0 μm and being monodisperse particles. The monodisperse particles are not particles having a single particle size, but particles having an extremely sharp particle size distribution. Specifically, the particles have a distribution width of ± (average particle size × 0.5 μm) or less. If the particle size of the particles 31c is less than 1.0 μm, the effect of promoting the transfer performance by the particles 31c cannot be sufficiently obtained. On the other hand, if the particle size is larger than 5.0 μm, the gap between the particles becomes large and the belt surface roughness becomes large, so that the toner cannot be transferred well or the intermediate transfer belt 31 is cleaned. It is easy to cause defects. Furthermore, since the particles 31c made of a resin material generally have high insulating properties, if the particle size is too large, the electric charge of the particles 31c tends to cause image distortion due to the accumulation of the electric charge during continuous printing.

As the particles 31c, specially synthesized particles may be used, or commercially available products may be used. The particles 31c can be easily and uniformly aligned by applying the particles 31c directly to the elastic layer 31b and smoothing them. By doing so, it is possible to substantially eliminate the overlap of the particles 31c in the belt thickness direction. The diameter of the cross section of the elastic layer 31b of the plurality of particles 31c in the surface direction is preferably as uniform as possible, and specifically, the distribution width is preferably ± (average particle size × 0.5 μm) or less. Therefore, it is preferable to use a powder having a small particle size distribution as the powder of the particles 31c, but if a method for selectively applying only the particles 31c having a specific particle size to the surface of the elastic layer 31b is adopted. It is also possible to use a powder having a relatively large particle size distribution. The timing of applying the particles 31c to the surface of the elastic layer 31b is not particularly limited, and may be either before or after the cross-linking of the elastic material of the elastic layer 31b.

Regarding the projected area ratio between the portion where the particles 31c are present and the portion where the surface of the elastic layer 31b is exposed in the surface direction of the elastic layer 31b in which the particles 31c are dispersed, the particles 31c are present. It is desirable that the projected area ratio of the existing part is 60% or more. If it is less than 60%, the chance of direct contact between the toner and the solid surface of the elastic layer 31b is increased, so that good toner transferability cannot be obtained, or the toner cleaning property from the belt surface is lowered. , It reduces the filming resistance of the belt surface. As the intermediate transfer belt 31, it is also possible to use a belt in which the particles 31c are not dispersed in the elastic layer 31b.

As shown in FIG. 9, on the surface of the intermediate transfer belt 31, almost no overlap of the particles 31c is observed. The diameter of the cross section of the particles 31c on the surface of the elastic layer 31b is preferably as uniform as possible, and specifically, the distribution width is preferably ± (average particle size × 0.5) μm or less. In order to realize such a distribution width, it is preferable to use a particle powder having a narrow particle size distribution, but an elastic layer 31b is adopted by adopting a method of selectively localizing particles 31c having a specific particle size on the surface. If the above is formed, a particle powder having a wide particle size distribution may be used.

As the recording sheet P, in order to satisfactorily secondarily transfer the toner to each of the plurality of recesses on the surface of the uneven sheet and suppress the occurrence of image density unevenness due to the surface unevenness, the elastic layer 31b has some flexibility ( It is necessary to use one with excellent elasticity). If such an elastic layer 31b is adopted, the elastic layer 31b alone cannot withstand actual use because it stretches immediately when it is stretched. Therefore, it is an essential condition that the base layer 31a, which is more rigid than the elastic layer 31b, is provided, and the elongation of the entire belt is suppressed for a long period of time by the rigidity of the base layer 31a.

As described above, in the printer according to the embodiment, the intermediate transfer belt 31 made of an elastic belt in which the elastic layer 31b is laminated on the base layer 31a is used. As a result, even if an image is formed at an ultra-high speed of belt linear velocity (process linear velocity) = 630 [mm / s] for business users, a sufficient amount of toner is transferred into the concave surface of the concave-convex sheet, resulting in unevenness. It was possible to suppress the occurrence of image density unevenness that was similar to the surface unevenness of the sheet.

Since the intermediate transfer belt 31 having the elastic layer 31b as described above has a low reflectance of light on its surface and a dark color tone, the detection accuracy may be poor when the optical characteristics of the test toner image are detected on the surface of the intermediate transfer belt 31. The optical characteristics cannot be detected. On the other hand, the secondary transfer belt 41 is a single-layer belt made of a polyimide resin, and since the light reflectance of the surface thereof is high and the color tone is bright, the optical characteristics of the test toner image on the surface can be detected satisfactorily. can.

Therefore, in the test transfer mode, the test toner image is secondarily transferred from the dark color intermediate transfer belt 31 to the light color secondary transfer belt 41, and the optical characteristics of the test toner image on the secondary transfer belt 41 are measured by an optical sensor. Detected by the unit 40. As a result, the optical characteristics of the test toner image can be detected satisfactorily. As the secondary transfer belt 41, a fluororesin such as PVDF (polyvinylidene fluoride), ETFE (ethylene / ethylene tetrafluoride copolymer), or a polyamide-imide resin may be used.

FIG. 10 is a graph showing a first example of a waveform of a secondary transfer bias composed of a superimposed voltage output from the secondary transfer power supply 45. The waveform of the secondary transfer bias shown in the figure is a sine wave. The offset voltage Voff is the value of the DC component (DC voltage) of the secondary transfer bias composed of the superimposed voltage. The polarity of the offset voltage Voff in the figure is negative. When the waveform of the secondary transfer bias is a sine wave as shown in the figure, the offset voltage Voff and the average potential Wave per cycle (T) of the secondary transfer bias have the same value. Therefore, in the figure, the average potential Wave is also negatively polar.

In the configuration in which the secondary transfer bias is applied to the core metal of the secondary transfer back surface roller (33 in FIG. 1) as in the printer of the embodiment, the polarity of the secondary transfer bias becomes the same as the normal charging polarity of the toner. Occasionally, the toner in the secondary transfer nip is electrostatically moved in the transfer direction. The transfer direction is a direction from the surface side of the intermediate transfer belt 31 toward the surface side of the secondary transfer belt 41 in the secondary transfer nip. On the other hand, when the polarity of the secondary transfer bias becomes opposite to the normal charge polarity of the toner, the toner in the secondary transfer nip electrostatically moves in the direction opposite to the transfer direction. As shown in the figure, by setting the average potential Wave to a negative polarity that is the same polarity as the normal charging polarity of the toner, the toner is reciprocated between the belt surface side and the sheet surface side in the secondary transfer nip while moving back and forth. It is relatively moved from the belt surface side to the sheet surface side. As a result, the toner image on the surface of the intermediate transfer belt 31 can be secondarily transferred onto the surface of the recording sheet P.

In the figure, the transfer peak value Vt is a peak value that electrostatically moves the toner in the secondary transfer nip toward the transfer direction with the strongest electrostatic force within one cycle (cycle T) of the secondary transfer bias. .. Further, the inverse peak value Vr is a peak value that is not the transcription peak value Vt. In the secondary transfer bias shown in the figure, the reverse peak value Vr has the opposite polarity (positive polarity) to the transfer peak value Vt.

The waveform of the secondary transfer bias is not limited to the sine wave as shown in the figure. A secondary transfer bias of a triangular wave or a square wave may be adopted. FIG. 11 is a graph showing a second example of the waveform of the secondary transfer bias composed of the superimposed voltage. The waveform of the secondary transfer bias shown in the figure is a rectangular wave. Both the sine wave of the secondary transfer bias shown in FIG. 10 and the square wave of the secondary transfer bias shown in FIG. 11 have a duty of 50 [%], which will be described later. In any of the secondary transfer biases composed of waveforms having such characteristics, the average potential Wave per cycle (cycle T) becomes the same value as the offset voltage Voff. That is, the value of the DC component becomes the same as the average potential Wave.

FIG. 12 is a graph for explaining the duty in the secondary transfer bias shown in FIG. In the figure, the central potential VC is the central potential at the peak toe peak value Vpp of the AC component (AC voltage) of the secondary transfer bias. Further, the reverse peak side time tr sets the reverse peak value Vr from the moment when the value of the secondary transfer bias starts to rise from the central potential VC toward the reverse peak value Vr within one cycle (cycle T) of the AC component. It is the time until it returns to the central potential VC. Further, the transfer peak side time tf sets the transfer peak value Vt from the moment when the value of the secondary transfer bias starts to rise from the central potential VC toward the transfer peak value Vt within one cycle (period T) of the AC component. It is the time until it returns to the central potential VC. The duty is the ratio of the reverse peak side time tr in the period T, and is 50 [%] in the case of the illustrated waveform. That is, the duty in the illustrated waveform is 50 [%].

FIG. 13 is a graph for explaining the duty in the secondary transfer bias shown in FIG. Even in the illustrated square wave, the duty as a ratio of the reverse peak side time tr in the period T is 50 [%]. Hereinafter, the characteristic that the duty is less than 50 [%] is referred to as low duty. On the other hand, the characteristic that the duty exceeds 50 [%] is called high duty.

In order to electrostatically move the toner in the transfer direction in the secondary transfer nip using the secondary transfer bias with duty = 50 [%], the absolute value of the transfer peak value Vt is changed to the absolute value of the inverse peak value Vr. Need to be larger than. If the absolute value of the transfer peak value Vt becomes too large, a discharge will be generated between the surface of the intermediate transfer belt 31 and the uneven sheet in the secondary transfer nip. This discharge causes the toner particles to be backcharged and significantly inhibits the secondary transfer of the toner particles, so that many white spots are generated in the image and the image quality is significantly impaired. Therefore, it is necessary to keep the absolute value of the transcription peak value Vt at a certain value.

On the other hand, if the absolute value of the reverse peak value Vr is made too small, a sufficient amount of toner cannot be transferred to the surface recesses of the uneven sheet. Specifically, if the absolute value of the reverse peak value Vr is made too small, the toner particles once transferred from the belt surface to the surface recesses of the uneven sheet in the secondary transfer nip cannot be returned to the belt surface. .. Then, it becomes impossible to weaken the adhesive force by hitting the toner particles returned from the inside of the surface recess against the other toner particles or the toner particles adhering to the belt surface on the belt surface. By simply vibrating the toner particles, it is not possible to increase the number of toner particles that are transferred into the surface recesses of the concavo-convex sheet due to the vibration, so that the amount of toner transferred into the surface recesses is insufficient. It is.

One of the methods for increasing the inverse peak value Vr is to increase the peak toe peak value Vpp of the AC component. However, if the peak toe peak value Vpp is increased in order to eliminate the shortage of the reverse peak value Vr, the transfer peak value Vt is increased at the same time, so that white spots due to discharge are likely to be caused.

Another method of increasing the inverse peak value Vr is to decrease the offset voltage Voff. However, when the offset voltage Voff is reduced, the average potential Wave is also reduced accordingly, so that the toner cannot be satisfactorily electrostatically moved from the belt surface side to the sheet surface side in the secondary transfer nip. , May cause secondary transfer failure.

Therefore, in order to transfer a sufficient amount of toner to the surface recesses of the concavo-convex sheet, it is desirable to set the duty to 50 [%] or less. More preferably, it has a low duty of less than 50%. This is because when the duty is set to a high duty larger than 50 [%], white spots due to discharge and secondary transfer defects are likely to occur. Specifically, when the duty is set to high duty, the average potential Wave is shifted to the opposite peak side and the absolute value is reduced by that amount, so that secondary transfer defects are likely to occur. Then, in order to avoid the occurrence of the secondary transfer defect, if the peak toe peak value Vpp is increased to increase the average potential Wave, the transfer peak value Vt is increased accordingly, which is caused by the discharge. White spots are likely to occur. Therefore, it is desirable to set the duty to a low duty of less than 50 [%].

That is, when a concavo-convex sheet having a low surface smoothness is used as the recording sheet, it is desirable to adopt a secondary transfer bias having a low-duty superposed voltage. In other words, low duty is a characteristic according to low smoothness. In addition, in this specification, low smoothness means that it is inferior to the surface smoothness of plain paper, such as the surface smoothness of Japanese paper. Further, hereinafter, what is superior to the surface smoothness of plain paper, such as the surface smoothness of surface coated paper, is referred to as high smoothness. Further, what is equivalent to the surface smoothness of plain paper is called medium smoothness.

FIG. 14 is a graph showing an example of a waveform of the secondary transfer bias in which the duty is set to 35 [%], which is smaller than 50 [%]. The reverse peak value Vr of the secondary transfer bias shown in FIG. 11 is the same as Vr among the reverse peaks of the secondary transfer bias shown in FIG. Further, the transfer peak value Vt of the secondary transfer bias shown in FIG. 14 is the same as the transfer peak value Vt of the secondary transfer bias shown in FIG. The only difference between the secondary transfer bias shown in FIG. 14 and the secondary transfer bias shown in FIG. 11 is the reverse peak side time tr and the transfer peak side time tf. In the secondary transfer bias shown in FIG. 11, the reverse peak side time tr and the transfer peak side time tf are the same, whereas in the secondary transfer bias shown in FIG. 14, the reverse peak side time tr is the transfer peak. It is shorter than the side time tf. More specifically, the reverse peak side time tr of the secondary transfer bias shown in FIG. 11 is 50 [%] of the period T, whereas the reverse peak side time of the secondary transfer bias shown in FIG. 14 is tr is 35 [%] of the period T. That is, the duty of the secondary transfer bias shown in FIG. 14 is 35 [%].

In the secondary transfer bias with duty = 50 [%] shown in FIG. 11, as described above, the offset voltage Voff and the average potential Wave have the same value. On the other hand, in the secondary transfer bias with duty = 35 [%] shown in FIG. 14, the average potential Wave is larger than the offset voltage Voff. The peak-to-peak value Vpp is the same for the secondary transfer bias shown in FIG. 7 and the secondary transfer bias shown in FIG. That is, by setting the duty to less than 50 [%], the average potential Vave does not change the peak toe peak value Vpp, the transfer peak value Vt, and the inverse peak value Vr as compared with the case of setting the duty to 50 [%]. Can be increased. Therefore, it is possible to suppress the occurrence of secondary transfer defects and white spots as compared with the case where the duty is set to 50 [%] or more.

Therefore, when the toner image is secondarily transferred to the concavo-convex sheet, the power supply control unit 200 reverses the polarity within one cycle as a secondary transfer bias, and starts with a low-duty superimposed voltage of less than 50 [%]. Is to be output from the secondary transfer power supply 45. In such a configuration, it is possible to suppress the occurrence of secondary transfer defects and white spots due to discharge, as compared with a configuration using a secondary transfer bias composed of a high-duty superimposed voltage of 50 [%] or more.

By the way, the present inventors secondary transfer a halftone image to a smoothing sheet (surface smoothness is high smoothness) made of surface-coated paper having excellent surface smoothness under the condition of using a low-duty secondary transfer bias. An experiment was conducted. Then, a remarkable lack of image density due to a secondary transfer defect was caused in the halftone image.

As a result of diligent research on the causes of such secondary transcription defects, the following was found. That is, in the halftone image, the entire image portion is not covered with the toner, and the toner adhesion portion forming a relatively small number of dot groups and the blank portion to which the toner is not adhered are mixed in the image portion. doing. When the intermediate transfer belt 31 provided with the elastic layer 31b is used and a smoothing sheet having excellent surface smoothness is used as the recording sheet P, the elastic layer 31b constitutes a small number of dots in the halftone image in the secondary transfer nip. It is flexibly deformed according to the shape of the small number of dot toner lumps. Then, due to this deformation, not only the surface of the minority dot toner mass in the halftone image but also the side surface of the minority dot toner mass is wrapped with the elastic layer 31b. Then, a charge having a polarity opposite to the normal charging polarity is injected from the elastic layer 31b into each toner particle of the small number of dot toner lumps to reduce the charge amount (Q / M) of the toner or reverse charge the toner. .. As a result, it was found that the secondary transfer failure of the toner image was caused. When an uneven sheet is used as the recording sheet P, the elastic layer 31b is deformed into an irregular shape according to the unevenness of the sheet surface, so that the elastic layer 31b wraps the side surface of the small number of dot toner lumps. Almost disappears. Therefore, even on the surface convex portion of the concavo-convex sheet, secondary transfer failure does not occur.

FIG. 15 is an enlarged configuration diagram showing a secondary transfer nip and its surroundings in a configuration using a single-layer structure as the intermediate transfer belt 31 unlike that of the printer according to the embodiment. When a single-layer structure as shown in the figure is used as the intermediate transfer belt 31, a secondary transfer current flows between the secondary transfer back surface roller 33 and the secondary transfer nip lining roller 36 as follows. That is, as shown by the arrows in the figure, the secondary transfer current is concentrated in the nip center position (center position in the belt movement direction) and flows in a straight line. Does not flow so much. Due to the flow of the secondary transfer current in this way, the time during which the secondary transfer current is applied to the toner in the secondary transfer nip becomes relatively short. Therefore, it is almost impossible for the secondary transfer current to excessively inject a charge having a polarity opposite to that of the normal polarity into the toner.

FIG. 16 is an enlarged cross-sectional view showing a secondary transfer nip and its peripheral configuration in a configuration in which the intermediate transfer belt 31 is composed of an elastic belt having a multi-layer structure, similar to the printer according to the embodiment. In the configuration using the intermediate transfer belt 31 composed of the elastic belt having a multi-layer structure, the secondary transfer current flows between the secondary transfer back surface roller 33 and the secondary transfer nip lining roller 36 as follows. That is, at the interface between layers, the secondary transfer current flows in the belt thickness direction while spreading in the belt circumferential direction. As a result, the secondary transfer current wraps around not only at the center position of the nip but also near the inlet and exit of the nip. Therefore, in the secondary transfer nip, the time for applying the secondary transfer current to the toner is relatively long. It will be a long time. Then, the secondary transfer current tends to excessively inject a charge having a polarity opposite to that of the normal polarity into the toner. This also greatly reduces the amount of charge of the normal polarity of the toner and causes the toner to be backcharged, thereby impairing the secondary transferability. Such inhibition and the soft intermediate transfer belt 31 flexibly deforming and wrapping the convex shape of the toner image on the surface of the intermediate transfer belt 31 tend to cause secondary transfer defects of the halftone image. Conceivable.

Next, the present inventors use a high-duty (duty = 80%) secondary transfer bias shown in FIG. 17 instead of a low-duty one or one consisting only of a DC voltage. An experiment was conducted in which the image was secondarily transferred to a smooth sheet. As the smoothing sheet, an OK top coat (128 gsm) manufactured by Oji Paper Co., Ltd. was used (so-called coated paper). When a black halftone image (2by2) was secondarily transferred to a smoothing sheet under the condition of process linear velocity = 630 [mm / s] under an environment of 27 ° C./80%, smoothing was performed without causing secondary transfer failure. The black halftone image could be successfully secondary transferred to the sheet.

It is considered that the reason why the secondary transferability of the halftone image with respect to the smooth sheet could be improved by using the high-duty secondary transfer bias is as described below. That is, when the intermediate transfer belt 31 that moves endlessly enters the secondary transfer nip, the secondary transfer bias starts charging the belt portion that has entered the secondary transfer nip. Then, when the charge amount exceeds a certain threshold value, injection of reverse charge into the small number of dot toner lumps in the halftone image starts. Since charging of the belt portion that has entered the secondary transfer nip occurs mainly within the transfer peak side time tf, the longer the transfer peak side time tf, the greater the amount of reverse charge injected into the minority dot toner mass. Since the high-duty secondary transfer bias has a shorter transfer peak side time tf than the low-duty secondary transfer bias, the amount of reverse charge injected into a small number of dot toner lumps is reduced, resulting in secondary transfer failure. It is thought that it will be possible to suppress the occurrence.

From the results of this experiment, it was found that high duty is a characteristic of the surface smoothness of the recording sheet according to the high smoothness. Therefore, the power supply control unit 200 outputs a high-duty secondary transfer bias from the secondary transfer power supply 45 when the toner image is secondarily transferred to the smoothing sheet.

In addition, another experiment conducted by the present inventors also revealed the following. That is, as shown in FIG. 17, it is better to use a high-duty secondary transfer bias that does not reverse the polarity than to use a high-duty secondary transfer bias that reverses the polarity within one cycle. Can be improved.

In the secondary transfer bias shown in FIG. 17, the polarity of the voltage is changed to the transfer peak side time so as to reverse the direction of the electric field in the direction of returning the toner from the sheet surface side to the belt surface side within the reverse peak side time tr. The polarity is opposite to the polarity at tf. Such a secondary transfer bias and a secondary transfer bias that does not reverse the polarity have the same duty and the same intensity integral value (Vave) of the secondary transfer electric field in one cycle to make the same secondary. Trying to obtain transferability. Then, it is necessary to make the transfer peak value Vt in the secondary transfer bias of FIG. 17 larger than the transfer peak value Vt in the secondary transfer bias that does not reverse the polarity. As a result, despite the same duty and average potential Wave, the secondary transfer bias in FIG. 17 has a reverse charge for a small number of dot toner masses in the halftone image than the secondary transfer bias that does not reverse the polarity. Increase the injection volume. In other words, the secondary transfer bias that does not reverse the polarity can reduce the transfer peak value Vt and reduce the amount of reverse charge injected into a small number of dot toner lumps as compared with the secondary transfer bias of FIG. The secondary transferability can be further improved.

FIG. 18 is a block diagram showing an electric circuit of the operation display unit 501 of the printer according to the embodiment. As shown in the figure, the operation display unit 501 includes a smooth paper button 501a, an uneven paper button 501b, and a plain paper button 501h. In this printer, an explanation for having the user perform the following operations is described in the instruction manual. That is, when a smoothing sheet having excellent surface smoothness is set as the recording sheet P on the feeding cassette (100 in FIG. 1), the smoothing paper button 501a is pressed. On the other hand, when a concavo-convex sheet such as Japanese paper is set as the recording sheet P on the feeding cassette, the concavo-convex paper button 501b is pressed. When plain paper is set as the recording sheet P in the feeding cassette, the plain paper button 501h is pressed. That is, the operation display unit 501 functions as an information acquisition means capable of acquiring the following information. That is, it is possible to grasp at least whether the recording sheet P to be the secondary transfer target of the toner image is a smoothing sheet having excellent surface smoothness or an uneven sheet having a surface smoothness inferior to that of smoothing sheet or plain paper. Information that can be done.

Based on the acquisition result of the information by the operation display unit 501, the power supply control unit 200 switches the transfer mode when the toner image is secondarily transferred to the recording sheet between the uneven mode, the smoothing mode, and the normal mode. Specifically, when the concavo-convex paper button 501b is pressed, a concavo-convex mode using a secondary transfer bias composed of a low-duty superposed voltage is performed as a sheet transfer mode for transferring the toner image to the recording sheet P. When the smoothing paper button 501a is pressed, a smoothing mode using a secondary transfer bias composed of a high-duty superimposed voltage is performed as the sheet transfer mode. When the plain paper button 501h is pressed, the normal mode is executed as the sheet transfer mode. In this normal mode, as the secondary transfer bias, the one consisting only of the DC voltage and the one same as the smoothing mode are switched according to the environment.

In such a configuration, it is possible to suppress the occurrence of transfer failure of a halftone image due to injecting a charge having a polarity opposite to the normal charge polarity into the toner in the secondary transfer nip.

In the smoothing mode in which the toner image is transferred to the smoothing sheet, it is not necessary to reciprocate the toner between the surface of the intermediate transfer belt 31 and the surface of the sheet in the secondary transfer nip. Therefore, it is desirable to adopt any of the following values for the reverse peak value Vr of the secondary transfer bias consisting of the superimposed voltage.
(A) A reverse peak value Vr that is opposite to the normal charging polarity of the toner and whose absolute value is small enough not to cause the toner to return from the sheet surface side to the belt surface side.
(B) Reverse peak value Vr having the same polarity as the normal charging polarity of the toner.
Regardless of which reverse peak value Vr is adopted, the toner is not reciprocated between the sheet surface and the surface of the intermediate transfer belt 31 in the secondary transfer nip.

As already mentioned, in the smoothing mode, it is possible to better suppress the secondary transfer failure of the halftone image by using the one that does not invert the polarity than the one that inverts the polarity in one cycle of the AC voltage. .. Therefore, in this printer, in the smoothing mode, the superimposed voltage of the reverse peak value Vr of the above (b) is output from the secondary transfer power supply 45. The same applies when a secondary transfer bias consisting of a high-duty superimposed voltage is used in the normal mode.

For reference, Table 1 below shows examples of various numerical values of the secondary transfer bias used in the uneven mode.

Table 2 below shows examples of various numerical values of the secondary transfer bias used in the smoothing mode.

In this printer, as a combination of the secondary transfer bias in the uneven mode and the secondary transfer bias in the smooth mode, the latter combination of making the peak-to-peak value Vpp smaller is adopted. This is due to the reasons explained below. That is, in the smoothing mode, since it is not necessary to reciprocate the toner in the secondary transfer nip, no electrostatic force is generated in the direction opposite to the transfer direction (the polarity is not reversed), or a very weak force is used. Even if there is, it does not cause secondary transfer failure. The reason for using the AC voltage is that the voltage of the polarity (minus in this example) that generates the electrostatic force in the transfer direction is temporarily greatly lowered to suppress the injection of the reverse charge into the toner. For the peak-to-peak value Vpp of the AC voltage used for this reason (same as the Vpp of the superimposed voltage), it is sufficient that the electrostatic force in the transfer direction is a value required for transfer, and the uneven mode It doesn't have to be as big as. Nevertheless, if the peak-to-peak value Vpp is set to be as large as the uneven mode, unnecessary alternating electric shocks are applied to the intermediate transfer belt 31 and the secondary transfer belt 41 to accelerate their deterioration. In other words, by setting the peak toe peak value Vpp in the smoothing mode to a lower value, deterioration due to alternating electric shock can be suppressed.

Further, in this printer, as a combination of the secondary transfer bias in the uneven mode and the secondary transfer bias in the smoothing mode, a combination in which the frequency of the latter is higher than that in the former is adopted. This is due to the reasons explained below. That is, in the concavo-convex mode in which the toner image is secondarily transferred to the concavo-convex sheet, the higher the frequency, the shorter the time for forming the electric field in the direction opposite to the transfer direction. If it is set too high, the time cannot be secured sufficiently, and the toner cannot be returned from the recess on the sheet surface to the surface of the intermediate transfer belt 31, and the low-duty secondary transfer cannot be performed. Even if a bias is used, it causes poor transfer to the recess. On the other hand, in the smoothing mode in which the toner image is secondarily transferred to the smoothing sheet, the lower the frequency, the longer the time during which a relatively high voltage is continuously applied on the transfer peak value Vt side, so that the toner is transferred to the toner. It becomes easy to cause the injection of the reverse charge of. By making the frequency of the high duty secondary transfer bias higher than that of the low duty, the following is possible. That is, the secondary transfer of the halftone image caused by the injection of the reverse charge into the toner by making the frequency too low in the smoothing mode while suppressing the transfer failure to the concave part of the sheet surface due to the frequency being made too high in the uneven mode. The occurrence of defects can be suppressed.

In the concavo-convex mode in which the toner image is transferred to the concavo-convex sheet, the toner is reciprocated between the sheet surface and the intermediate transfer belt 31 multiple times in the secondary transfer nip, so that the toner is moved around the image portion. It is easy to generate transfer dust that scatters particles. However, the deterioration of image quality due to poor transfer to the recesses on the sheet surface due to not moving the toner back and forth is more noticeable than the deterioration of image quality due to the transfer dust. Therefore, in order to preferentially suppress the deterioration of image quality of the latter, the toner A secondary transfer bias with the characteristic of reciprocating the toner is adopted.

In the process control process described above and the Vtref adjustment process during the continuous print job, a test transfer mode in which the test toner image is secondarily transferred from the intermediate transfer belt 31 to the secondary transfer belt 41 is executed. Since the surface of the secondary transfer belt 41 has almost no irregularities, a secondary transfer bias having a low duty and a relatively high reverse peak value Vr as in the case of secondary transfer of a toner image to an uneven sheet is used. No need. Nevertheless, for example, suppose that the following processing is performed during a continuous print job. That is, immediately after the toner image is secondarily transferred to the concavo-convex sheet using the low-duty secondary transfer bias in the concavo-convex mode, a test toner image for Vtref adjustment processing is applied to the inter-sheet corresponding region of the secondary transfer belt 41. It is assumed that the secondary transfer is performed with the same low-duty secondary transfer bias. Then, the image quality of the test toner image is deteriorated due to the transfer dust, and the detection accuracy of the toner adhesion amount (image density) is lowered.

Therefore, in the test transfer mode in which the power supply control unit 200 transfers the test toner image to the secondary transfer belt 41, any of the following is listed regardless of the surface smoothness information of the recording sheet acquired by the operation display unit 501. The secondary transfer bias of is output from the secondary transfer power supply 45.
(Α) Secondary transfer bias consisting only of DC voltage.
(Β) Secondary transfer bias same as smoothing mode.
(Γ) Different from the secondary transfer bias in smooth mode, and the reverse peak value Vr is a value that makes the electrostatic force in the direction opposite to the transfer direction weaker than the reverse peak value Vr of the secondary transfer bias in uneven mode. Secondary transfer bias consisting of high-duty superimposed voltage.

In all of the secondary transfer biases (α) to (γ), the reciprocating movement of the toner is suppressed or the reciprocating movement is eliminated, as compared with the case where the same secondary transfer bias as in the uneven mode is used. Occurrence can be suppressed.

FIG. 19 is a graph showing the relationship between the secondary transfer rate of the test toner image with respect to the secondary transfer belt 41 in the printer testing machine having the above-mentioned basic configuration, the secondary transfer current, and the characteristics of the secondary transfer bias. Is. This graph was created based on the results of experiments conducted in a laboratory set in a weakly humid environment of 27 [° C.] 80 [%] (absolute humidity 20.6 [g / m ^3]). It is an experiment. The secondary transfer rate indicates the ratio of the toner secondarily transferred onto the secondary transfer belt 41 among the toners of the test toner image consisting of the solid image primary transferred onto the intermediate transfer belt 31. In the figure, "(DC)" indicates that a secondary transfer bias consisting of only a DC voltage was used. Further, "(DC + high DutyAC)" indicates that a high-duty superimposed voltage was used as the secondary transfer bias.

In a weak and high humidity environment, the amount of charge (Q / M) of the toner is relatively small, so when a secondary transfer bias consisting of only DC voltage is used, a relatively large amount of secondary transfer current is generated. If it is flowed, secondary transfer defects are likely to occur. In particular, in K toner having a lower charge amount than Y toner, M toner, and C toner (hereinafter, these toners are collectively referred to as color toner), such secondary transfer defects are remarkably likely to occur. It can be seen that the drop in the graph on the high current side is steeper in the K toner than in the color toner. As described above, even if the image is not a halftone image but a solid image, the image density may decrease due to the secondary transfer failure depending on the environment and the amount of charge of the toner. Therefore, when the test toner image is secondarily transferred to the secondary transfer belt 41 by using the secondary transfer bias consisting of only the DC voltage, transfer failure of the test toner image is likely to occur.

On the other hand, when a high-duty superimposed voltage is used as the secondary transfer bias, overcharging of the toner in the secondary transfer nip is suppressed, so even K toner (solid image) with a small amount of charge can be used. , Sufficient secondary transcription rate has been obtained.

Therefore, in the test transfer mode, the secondary transfer bias consisting only of the DC voltage is not adopted as in the above (α), but is composed of the high-duty superimposed voltage as in the above (β) and the above (γ). It is easy to think that it is advantageous to adopt a secondary transfer bias. However, the test toner image is not an image for being provided to the customer for viewing, but an image for investigating how much toner is attached to form the toner image. Even if a transfer defect occurs, if the secondary transfer rate reflecting the transfer defect is obtained by an experiment in advance, it is possible to accurately determine the toner adhesion amount of the test toner image before the secondary transfer.

When the power control unit 200 executes the test transfer mode immediately after the toner image is secondarily transferred to the smooth sheet by the smoothing mode, the power control unit 200 performs the following control in the test transfer mode. That is, the same secondary transfer bias as in the smoothing mode is output from the secondary transfer power supply 45. Since the test toner image secondaryly transferred to the highly smooth secondary transfer belt 41 under these conditions causes almost no transfer failure due to the injection of the reverse charge into the toner, the secondary is investigated by prior experiments. It is not always necessary to determine the amount of toner adhered in consideration of the transfer rate. In addition, the generation of transfer dust can be suppressed as compared with the case of using a secondary transfer bias having a low-duty superimposed voltage. Since the same secondary transfer bias as the immediately preceding sheet transfer mode is used, it is possible to avoid complicated control due to unnecessary switching of the characteristics of the output voltage from the secondary transfer power supply 45.

Table 3 below shows the relationship between the transfer mode used in this printer, the type of recording sheet P, and the characteristics of the secondary transfer bias. In Table 3, the values of duty [%], peak toe peak value Vpp [kV], and frequency [Hz] are 23 [° C.], 50 [%] = absolute humidity 10.3 [g / m ³ ]. This is an example in a medium-humidity environment, and if it is not in a medium-humidity environment, the values may differ. The relationship between the environment and those numerical values will be described later.

As shown in Table 3, in the normal mode in which the toner image is secondarily transferred to plain paper, the secondary transfer bias consists of only a DC voltage and a high-duty superposed voltage depending on the environment. Switch with. Hereinafter, an environment in which the absolute humidity is 15.5 [g / m ³ ] or less is referred to as an environment in which the absolute humidity is medium humidity or less. An environment in which the absolute humidity is more than 15.5 [g / m ³ ] to 23.8 [g / m ³ ] or less is called a weak and high humidity environment. An environment in which the absolute humidity exceeds 23.8 [g / m ³ ] is called a strong and high humidity environment.

When the normal mode is executed in an environment of medium humidity or lower, even if a secondary transfer bias consisting of only a DC voltage is adopted, transfer failure due to injection of a reverse charge into the toner is unlikely to occur. Therefore, when the normal mode is executed in an environment of medium humidity or lower, the power supply control unit 200 outputs a secondary transfer bias consisting of only a DC voltage from the secondary transfer power supply 45. As a result, it is possible to reduce the electrical load on the secondary transfer belt 41 and the intermediate transfer belt 31 by using a secondary transfer bias containing an AC component. Therefore, deterioration of the belt due to an electric load can be suppressed.

On the other hand, in a weak and high humidity environment or a strong and high humidity environment, the charge amount (Q / M) of the toner is relatively low, so even if it is plain paper, it is charged by injecting a reverse charge into the toner. Insufficient amount is likely to occur. Therefore, in the normal mode, if a secondary transfer bias consisting of only a DC voltage is used, transfer defects due to injection of a reverse charge are likely to occur. Therefore, when the normal mode is executed in a weak and high humidity environment or a strong and high humidity environment, the power supply control unit 200 outputs the same as the smoothing mode as the secondary transfer bias from the secondary transfer power supply 45. ..

As already described, this printer uses a secondary transfer bias consisting of a low-duty superimposed voltage in the concave-convex mode in which the toner image is secondarily transferred to the concave-convex sheet. Further, in the smoothing mode in which the toner image is secondarily transferred to the smoothing sheet, a secondary transfer bias composed of a high-duty superposed voltage is used.

In the test transfer mode executed immediately after the sheet transfer mode (normal mode) in which the toner image is transferred to plain paper, the power supply control unit 200 outputs the same secondary transfer bias as the immediately preceding normal mode from the secondary transfer power supply 45. .. At this time, depending on the environment, a secondary transfer bias consisting of only a DC voltage is output, and injection of a reverse charge into the toner may cause transfer failure of the test toner image on the secondary transfer belt 41. However, for the reason described below, the main control unit 300 can accurately determine the amount of toner adhered to the test toner image. That is, the main control unit 300 preliminarily determines the secondary transfer rate (value investigated by a prior experiment) when the test toner image is secondarily transferred to the secondary transfer belt 41 by the secondary transfer bias consisting of only the DC voltage. I remember. Then, when the test transfer mode is executed with the secondary transfer bias consisting only of the DC voltage, the test is performed based on the secondary transfer rate and the detection result of the optical characteristics of the test toner image by the optical sensor unit 40. Obtain the amount of toner adhered to the toner image.

Since the test transfer mode is executed using the same secondary transfer bias as the immediately preceding normal mode, it is possible to suppress the complexity of the control content due to switching the characteristics of the secondary transfer bias according to the mode switching. Furthermore, it is possible to suppress a decrease in the detection accuracy of the amount of toner adhering due to transfer dust as compared with the case of using a secondary transfer bias composed of a low-duty superimposed voltage.

Further, in the test transfer mode executed immediately after the sheet transfer mode (smoothing mode) in which the toner image is transferred to the smoothing sheet, the power supply control unit 200 applies the same secondary transfer bias as the immediately preceding smoothing mode from the secondary transfer power supply 45. Output. Since the secondary transfer bias is the same as in the smoothing mode, even if the test toner image is transferred to the highly smooth secondary transfer belt 41, transfer failure of the test toner due to injection of the reverse charge into the toner does not occur. Since the test transfer mode is executed using the same secondary transfer bias as the immediately preceding smoothing mode, it is possible to suppress the complexity of the control content due to switching the characteristics of the secondary transfer bias according to the mode switching. Furthermore, it is possible to suppress a decrease in the detection accuracy of the amount of toner adhering due to transfer dust as compared with the case of using a secondary transfer bias composed of a low-duty superimposed voltage.

In the test transfer mode executed immediately after the sheet transfer mode (concavo-convex mode) in which the toner image is transferred to the concavo-convex sheet, the power supply control unit 200 performs the same secondary transfer bias as the smoothing mode in the test transfer mode, which is different from the immediately preceding concavo-convex mode. It is output from the transfer power supply 45. Although the characteristics of the secondary transfer bias are changed when the mode is switched, the accuracy of detecting the amount of toner adhering due to transfer dust is lower than when the secondary transfer bias consisting of the same low-duty superimposed voltage as in the smoothing mode is used. Can be suppressed.

In the test transfer mode, instead of outputting the secondary transfer bias ((β)) having the same superimposed voltage as in the smoothing mode, the secondary transfer bias of (γ) may be output.

When a secondary transfer bias consisting of a high-duty superposition bias is used in the smoothing mode or the test transfer mode, the secondary transfer rate differs depending on the environment. Specifically, the higher the humidity, the smaller the amount of charge [Q / M] of the toner, and the more likely it is that secondary transfer defects will occur.

Table 4 shown below was prepared based on the results of experiments conducted by the present inventors. The K color secondary transfer rate represents the secondary transfer rate of the K test toner image (solid image) secondarily transferred on the secondary transfer belt 41 in the experiment.

Focusing on Experiment No. 2 and Experiment No. 3 in Table 4, the only condition different between the two is the environment, and the absolute humidity is higher because the temperature is higher than that of Experiment No. 3. Focusing on the K-color secondary transfer rates of both, Experiment No. 3 is about 10 [%] lower. This is because Experiment No. 3 had a higher humidity and a lower charge amount of the K toner, so that the decrease in the secondary transfer rate due to the injection of the reverse charge became more remarkable.

Focusing on Experiment No. 3 and Experiment No. 4, the only difference between them is the duty, and Experiment No. 4 has a higher duty by 5 [%]. Focusing on the K-color secondary transfer rates of both, Experiment No. 4 is about 7 [%] higher.

From these facts, it can be seen that the higher the detail, the more the decrease in the secondary transcription rate due to the injection of the reverse charge can be suppressed. However, under the same absolute humidity condition, the higher the detail, the weaker the electrostatic force in the transfer direction at the secondary transfer nip, so that secondary transfer defects due to insufficient electrostatic force are likely to occur. In order to suppress the occurrence of such secondary transfer defects, it is necessary to increase the peak-to-peak value Vpp by the amount of the increased duty to prevent the weakening of the electrostatic force. However, under the same absolute humidity condition, the larger the peak toe peak value Vpp, the easier it is to generate a discharge in the secondary transfer nip, and the white spots (small white spots in the image) caused by the discharge become more likely to occur. It becomes easy to generate.

As described above, in order to improve the secondary transfer rate of the test toner image as much as possible, there is a combination of the optimum duty according to the absolute humidity and the peak-to-peak value Vpp. The higher the absolute humidity, the less likely it is that an electric discharge will occur in the secondary transfer nip. Therefore, even if the peak toe peak value Vpp is increased, the deterioration of the white spot does not occur. Therefore, by increasing the peak-to-peak value Vpp and increasing the detail, it is possible to suppress the secondary transfer failure of the test toner image due to the injection of the reverse charge into the toner.

Therefore, the power supply control unit 200 describes the environment as having a medium humidity or less, a weak high humidity, and a strong high humidity based on the absolute humidity calculated based on the detection result of the temperature and humidity sent from the environment sensor 50. Determine which one it corresponds to. The environment below medium humidity is an environment with moderate humidity or lower humidity based on 23 [° C] 50 [%] (absolute humidity 10.3 [g / m ³ ]). Is an environment in which the absolute humidity is 15.5 [g / m ³ ] or less. The weak and high humidity environment is based on 27 [° C.] 80 [%] (absolute humidity 20.6 [g / m ³ ]), and specifically, the absolute humidity is 15.5 [g / m 3]. The environment is from more than g / m ³ ] to 23.8 [g / m ³ ] or less. The strong and high humidity environment is an environment based on 32 [° C.] 80 [%] (absolute humidity 27.0 [g / m ³ ]), and specifically, the absolute humidity is 23.8 [g]. / M ³ ] is exceeded.

When the power supply control unit 200 that determines the environment uses a secondary transfer bias composed of a high-duty superimposed voltage in the smoothing mode or the test transfer mode, the characteristics of the superimposed voltage are changed as follows. That is, in an environment of medium humidity or lower, the secondary transfer bias having the same characteristics as that of Experiment No. 1 in Table 4 is output from the secondary transfer power supply 45. Further, in a weak and high humidity environment, a secondary transfer bias having a superimposed voltage having the same characteristics as that of Experiment No. 2 in Table 4 is output from the secondary transfer power supply 45. Further, in a strong and high humidity environment, a secondary transfer bias having a superimposed voltage having the same characteristics as that of Experiment No. 4 in Table 4 is output from the secondary transfer power supply 45. That is, the higher the absolute humidity, the higher the detail and the peak toe peak value Vpp, respectively.

In such a configuration, the toner image can be secondarily transferred to the smooth sheet or the test toner image can be secondarily transferred to the secondary transfer belt 41 with a good secondary transfer rate regardless of the change in humidity.

As a process for forming a test toner image, a misalignment correction process is known in addition to the process control process described above and the Vtref adjustment process during a continuous print job. The misalignment correction process is a process for correcting the relative misalignment of the toner images of Y, M, C, and K. In the misalignment correction process, the misalignment detection pattern shown in FIG. 20 is formed at one end, the center, and the other end of the belt (secondary transfer belt 41 in the embodiment) in the width direction. Each position deviation detection pattern includes a plurality of sets (three sets in the figure) of chevron patch patterns arranged in the belt moving direction. One Chevron patch pattern comprises four orthogonal patches (K orthogonal patch SPk, M orthogonal patch SPm, C orthogonal batch SPc, Y orthogonal patch SPy) extending in the belt width direction. Further, four inclined patches (K inclined patch TPk, M inclined patch TPm, C inclined patch TPc, Y inclined patch TPy) extending in a posture inclined by 45 [°] from the belt width direction are also provided.

Each patch, which is a test toner image of the pattern for detecting misalignment, is detected by an optical sensor (optical sensor unit 40 in the embodiment). If the formation timing of each patch is appropriate, the detection time intervals of the patches arranged in the belt moving direction are equal, but if they are inappropriate, the detection time intervals become non-uniform. If the optical system for optical writing is not skewed, patches of the same color are detected at the same timing among the three positional deviation detection patterns, but if skew is generated, the detection timing is different. It will be different. The control unit (for example, the main control unit 300) measures the detection time interval and the deviation of the detection timing of each patch in the main scanning direction (same as the belt width direction) and the sub scanning direction (same as the belt moving direction). Then, based on the measurement result, the inclination of the optical mirror as an image forming condition is adjusted, and the optical writing timing as an image forming condition is corrected. By such a positional deviation reduction process, it is possible to suppress superposition deviation and image skew of each color.

When such a misalignment correction process is periodically performed, the secondary transfer bias in the test transfer mode (mode in which each patch is transferred to the secondary transfer belt 41) is similar to the process control process and the Vtref adjustment process. It is desirable to control. In this case, it is possible to suppress the detection error of the position of each patch due to the transfer dust generated in each patch. Therefore, it is possible to suppress the occurrence of residual position shift (residual color shift) due to the detection error.

Next, the printer according to each embodiment in which a more characteristic configuration is added to the printer according to the embodiment will be described. Unless otherwise specified below, the configuration of the printer according to each embodiment is the same as that of the embodiment.
[First Example]
The printer according to the first modification does not have a smooth paper button, an uneven paper button, or a plain paper button on the operation display unit 501. Therefore, the specifications are not such that the information on the surface smoothness of the recording sheet P is input by the user. Instead, it is equipped with a smoothness detection sensor that detects the surface smoothness of the recording sheet.

FIG. 21 is a configuration diagram showing a paper feed path of the printer according to the first embodiment. The paper feed path guides the recording sheet P sandwiched between the first guide plate 509 and the second guide plate 510 to the resist nip of the resist roller pair 101. The first guide plate 509 is provided with a through-hole, and the smoothness detection sensor 502 is fitted in the through-hole. The smoothness detection sensor 502 composed of a reflective optical sensor irradiates the light emitted from the light emitting element toward the recording sheet P in the paper feed path, and the specularly reflected light reflected by the surface of the recording sheet P is reflected by the light receiving element. Receive light. The amount of specularly reflected light obtained on the surface of a smooth sheet such as coated paper is larger than the amount of specularly reflected light obtained on the surface of a concavo-convex sheet such as Japanese paper.

The smoothness detection sensor 502 is electrically connected to the power supply control unit 200. The power control unit 200 calibrates the smoothness detection sensor 502 when the device is started immediately after the main power of the printer is turned on. Specifically, in a state where the light emitting element is turned on and the light from the light emitting element is reflected on the surface of the white second guide plate 510, the light emitting amount (supply voltage) of the light emitting element is obtained so that a predetermined specular reflected light amount can be obtained. ) Is adjusted. The supply voltage value at this time is stored in the storage circuit, and thereafter, when the smoothness detection sensor 502 detects the amount of specularly reflected light on the surface of the recording sheet P, it is the same value as the supply voltage value stored in the storage circuit. The voltage of is supplied to the light emitting element.

When the print job is started, the recording sheet P sent out from the feed cassette 100 at a predetermined timing is abutted against the resist nip of the non-driven resist roller pair 101 for skew correction, and the transfer is temporarily performed. It will be stopped. At this time, it faces the smoothness detection sensor 502 in the paper feed path. In this state, the power supply control unit 200 detects the amount of specularly reflected light obtained on the sheet surface by the smoothness detection sensor 502. Then, based on the detection result, it is determined whether the recording sheet P is an uneven sheet, plain paper, or a smooth sheet.

In such a configuration, it is possible to automatically determine whether the recording sheet P is a concavo-convex sheet, plain paper, or a smooth sheet without the user's operation, and improve the user's operability. Further, even when a user sets different types of recording sheets on the feed cassette 100, it is possible to automatically determine the difference in type and execute the sheet transfer mode according to the determination result.

FIG. 22 is a schematic diagram for explaining switching of the secondary transfer bias when the type of the recording sheet P is changed from the uneven sheet to the smooth sheet during the continuous print job. In the figure, the rectangular frame shown by the dotted line indicates a sheet-corresponding area in the entire area of the secondary transfer belt 41 that contacts the recording sheet P at the secondary transfer nip. When the concavo-convex sheet is used, the concavo-convex mode is executed at the timing when the concavo-convex sheet has entered the secondary transfer nip. Further, at the timing when the inter-sheet correspondence region of the secondary transfer belt 41 enters the secondary transfer nip, the secondary transfer bias output from the secondary transfer power supply 45 differs depending on the state of the inter-sheet correspondence region. Specifically, when the inter-sheet correspondence region in which the test toner images (YT, MT, CT, KT) of each color for Vtref adjustment control are formed enters the secondary transfer nip, it is the same as the immediately preceding uneven mode. The next transfer voltage is output. On the other hand, when the inter-sheet correspondence region where the test toner image of each color is formed enters the secondary transfer nip, the same secondary transfer bias as in the smoothing mode is output (examples in weak high humidity and high humidity). ).

When the type of the recording sheet P changes from the uneven sheet to the smooth sheet, the toner image is secondarily transferred to the smooth sheet in the smooth mode corresponding to the smooth sheet. After the type of sheet is changed, before the secondary transfer to the changed recording sheet P (smooth sheet in the illustrated example) is completed, the next recording sheet P is abutted against the resist nip to determine the type. NS. At this time, if the type is also the same as the type of the previous recording sheet P, the formation of the image is temporarily suspended, the process control process is performed as shown in the figure, and the patch pattern image of each color ( YPP, MPP, CPP, KPP) is formed.

As described above, in the printer according to the first embodiment, when the secondary transfer of the toner image to the recording sheet P of the same type is continued twice after the type of the recording sheet P is changed, the second secondary transfer is performed. A process control process is performed prior to transcription. The reason why the process control process is not performed prior to the first time is that the toner image corresponding to the first secondary transfer is already formed when the type change is detected, so that the image formation cannot be interrupted. Because.

The process control process is carried out under the condition of outputting the secondary transfer bias according to the changed sheet type. In the illustrated example, since the sheet type after the change is a smooth sheet, it is performed under the condition of outputting the same secondary transfer bias as the smoothing mode corresponding to the smooth sheet.

When the process control process is completed, image formation is resumed. Then, at the timing when the smoothing sheet enters the secondary transfer nip, the smoothing mode is executed. That is, the same secondary transfer bias as the immediately preceding process control process is continuously output. After that, at the timing when the inter-sheet correspondence region of the secondary transfer belt 41 enters the secondary transfer nip, the same secondary transfer bias as in the smoothing mode is continuously output regardless of whether or not a test toner image of each color is formed. Will be done.

FIG. 23 is a flowchart showing a processing flow during a continuous print job executed by the power supply control unit 200 of the printer according to the first embodiment. In this processing flow, the power supply control unit 200 first waits until the feeding of the recording sheet P to the resist nip is completed (N in step 1; hereinafter, the step is referred to as S). Then, when the feeding is completed (Y in S1), the type of the recording sheet P is determined based on the detection result by the smoothness detection sensor 502 (S2), and then the secondary transfer bias according to the type and the environment is applied. Output from the secondary transfer power supply 45 (S3).

After that, it waits until the timing between seats arrives (N in S4). The sheet-to-sheet timing is the sheet between the region in close contact with the preceding recording sheet P and the region in close contact with the subsequent recording sheet P at the secondary transfer nip in the entire area in the circumferential direction of the secondary transfer belt 41. This is the timing when the corresponding region enters the secondary transfer nip. When the inter-sheet timing arrives (Y in S4), the necessity of secondary transfer of the test toner image within the inter-sheet timing is determined (S5). In other words, it is determined whether or not the test toner image is formed in the above-mentioned inter-sheet correspondence region.

When the secondary transfer of the test toner image is unnecessary (N in S5), the output of the secondary transfer bias having the same characteristics as in the sheet transfer mode is continued, and the presence or absence of the next print is determined (S10). , If there is the next print (Y in S10), the processing flow is looped to S1. If there is no next print (N in S10), the output of the secondary transfer bias is stopped (S11), and then the series of processing flows is terminated.

On the other hand, in S5 above, when the secondary transfer of the test toner image is required (Y in S5), it is determined whether or not the immediately preceding sheet transfer mode is a concavo-convex mode using a low-duty secondary transfer bias. (S6). Then, in the case of the uneven mode (Y in S6), the transfer dust is deteriorated by the secondary transfer bias as it is, so the output of the secondary transfer bias is switched to the one with the same high duty as the smoothing mode (Y). S7). On the other hand, in the case where the concavo-convex mode is not used (N in S6), the secondary transfer bias is maintained as it is because the transfer dust is not deteriorated even if the secondary transfer bias is maintained as it is (S8).

After the steps of S7 and S8, when the secondary transfer of the test toner image to the secondary transfer belt 41 is completed (Y in S9), the processing flow proceeds to S10 described above.

[Second Example]
FIG. 24 is a block diagram showing an electric circuit of the operation display unit 501 of the printer according to the second embodiment. Unlike the embodiment, the operation display unit 501 does not have a smooth paper button, an uneven paper button, and a plain paper button. Instead, it has a menu key 501c, an up key 501d, a down key 501e, a decision key 501f, a display 501g, and the like.

When the menu key 501c is pressed by the user, the main control unit 300 causes the display 501g to display the menu screen. The user selects the menu by pressing the enter key 501f with the cursor on the desired menu among the plurality of menus displayed on the menu screen by operating the up key 501d or the down key 501e. be able to. When the "sheet type input" menu is selected by the user's key operation, the main control unit 300 causes the display 501g to display the sheet brand list. By operating the up key 501d and the down key 501e, the user can select the same brand as the recording sheet set in the feed cassette 100 from among the plurality of brands displayed in the brand list. Since the brand and the surface smoothness of the brand on the recording sheet have a one-to-one relationship, the brand can function as information indicating the surface smoothness.

The power supply control unit 200 stores in the data storage circuit a data table in which the brand is associated with the type of the recording sheet P (three types of smooth sheet, uneven sheet, and plain paper). From this data table, the type corresponding to the brand input by the user is specified, and the specific result is grasped as the type of the recording sheet P.

The above description is an example, and the effect peculiar to each of the following aspects is exhibited.
[Aspect A]
Aspect A is a recording sheet sandwiched between a transfer nip (for example, a secondary transfer nip) due to contact between an image carrier (for example, an intermediate transfer belt 31) carrying a toner image and an abutting body (for example, a secondary transfer belt 41). Optical characteristics of a power supply (for example, a secondary transfer power supply 45) that outputs a superposed voltage by superimposing direct current and alternating current for transferring a toner image to (for example, recording sheet P) and a test toner image transferred to the abutting body. In an image forming apparatus (for example, a printer) provided with an optical characteristic detecting means (for example, an optical sensor unit 40) for detecting an image, an elastic layer made of a material having higher elasticity is laminated on a base material as the image carrier. In the test transfer mode in which the test toner image is transferred to the abutting body, only the DC voltage out of the DC voltage and the AC voltage is output from the power supply (for example). It is characterized in that the power supply control unit 200) is provided.

In the aspect A, by using an image carrier provided with an elastic layer that can be elastically deformed, a condition in which the transport speed of the recording sheet is made higher than in the case of using an image carrier without an elastic layer. This enables good transfer of the toner image to the uneven sheet. As a result, the printing speed can be increased. Further, by using a voltage consisting only of a DC voltage as a voltage for transferring the test toner image to the contact body in the test transfer mode, the toner is transferred between the surface of the image carrier and the surface of the contact body in the transfer nip. Do not move it back and forth. As a result, it is possible to suppress a decrease in the detection accuracy of the optical characteristics of the test toner image by avoiding the generation of transfer dust caused by the reciprocating movement of the toner.

[Aspect B]
Aspect B is a power supply that outputs a superposed voltage by superimposing DC and AC for transferring the toner image to the recording sheet sandwiched between the transfer nip due to the contact between the image carrier carrying the toner image and the abutting body. In an image forming apparatus including an optical characteristic detecting means for detecting the optical characteristics of a test toner image transferred to the abutting body, the image carrier is made of a base material and a material having higher elasticity. In the test transfer mode in which the layers are laminated and the test toner image is transferred to the abutting body, the toner in the transfer nip is hit by the toner from the image carrier side within one cycle of the superimposed voltage. Of the transfer peak value (for example, transfer peak value Vt) that exerts the strongest electrostatic force in the transfer direction toward the contact side and the reverse peak value (for example, reverse peak value Vr) that is the peak value on the opposite side, the above. The power supply uses a superposed voltage at which the inverse peak value is a value that weakens the electrostatic force in the direction opposite to the transfer direction as compared with the superposed voltage in the sheet transfer mode in which the toner image on the image carrier is transferred to the recording sheet. It is characterized in that a control means for carrying out a process of outputting from is provided.

In the aspect B, by using an image carrier provided with an elastic layer that can be elastically deformed, a condition in which the transport speed of the recording sheet is made higher than in the case of using an image carrier without an elastic layer. This enables good transfer of the toner image to the uneven sheet. As a result, the printing speed can be increased. Further, as a voltage for transferring the test toner image to the contact body in the test transfer mode, a superposition of a value whose reverse peak value makes the electrostatic force in the reverse direction weaker than the reverse peak value of the superposed voltage in the sheet transfer mode. Use voltage. When such a superposed voltage is used, the momentum of the reciprocating movement of the toner is reduced or the reciprocating movement of the toner is almost eliminated as compared with the sheet transfer mode. Since the abutting body does not have a surface recess unlike the uneven sheet, it is not necessary to reciprocate the toner for the purpose of suppressing poor transfer of toner to the surface recess. Therefore, there is no problem even if the momentum of the reciprocating movement is reduced or the reciprocating movement is almost eliminated. Rather, it has the advantage of suppressing transfer dust generated by reciprocating the toner. Then, by suppressing the transfer dust, it is possible to suppress a decrease in the detection accuracy of the optical characteristics of the test toner image.

[Aspect C]
Aspect C is the toner in the transfer nip in one cycle of the superimposed voltage as compared with a predetermined reference value in the sheet transfer mode in which the toner image on the image carrier is transferred to the recording sheet in the aspect A. Of the transfer peak value that exerts the strongest electrostatic force in the transfer direction from the image carrier side to the contact body side, and the reverse peak value that is the peak value on the opposite side, the reverse peak value side The control means so as to perform a process of switching the duty, which is a ratio within one cycle of the valued time, between a mode in which the duty is higher than 50 [%] and a mode in which the duty is lower than 50 [%]. Is characterized by the fact that. In such a configuration, when the toner image is transferred to the concavo-convex sheet, a low-duty superimposed voltage is output from the power source, so that poor transfer of the toner to the recesses on the sheet surface can be suppressed. In addition, by outputting a high-duty superimposed voltage from the power supply when transferring the toner image to the smooth sheet, it is possible to suppress transfer defects of the halftone image due to injection of reverse charge into the toner in the transfer nip. ..

[Aspect D]
In the test transfer mode performed immediately after the sheet transfer mode in which the duty is made lower than 50 [%] in the aspect D, only the DC voltage is output from the power supply, while the duty is 50. In the test transfer mode carried out immediately after the sheet transfer mode set to be higher than [%], the control means is used so as to perform a process of outputting the same superimposed voltage as the immediately preceding sheet transfer mode from the power supply. It is characterized by being configured. In such a configuration, in the test transfer mode immediately after the sheet transfer mode in which the duty is made higher than 50 [%], it is possible to avoid complicated control by switching the output voltage from the power supply unnecessarily.

[Aspect E]
In the aspect E, in the sheet transfer mode, the duty, which is a ratio within one cycle of the time when the value is on the side of the reverse peak value with respect to the predetermined reference value, is 50 [%]. The control means is configured so as to perform a process of switching between a mode in which the value is higher than 50 and a mode in which the value is lower than 50 [%]. In such a configuration, when the toner image is transferred to the concavo-convex sheet, a low-duty superimposed voltage is output from the power source, so that poor transfer of the toner to the recesses on the sheet surface can be suppressed. In addition, by outputting a high-duty superimposed voltage from the power supply when transferring the toner image to the smooth sheet, it is possible to suppress transfer defects of the halftone image due to injection of reverse charge into the toner in the transfer nip. ..

[Aspect F]
In the test transfer mode performed immediately after the sheet transfer mode in which the duty is lowered to less than 50 [%] in the aspect F, the reverse peak value is higher than the superimposed voltage of the sheet transfer mode. In the test transfer mode performed immediately after the sheet transfer mode in which the superimposed voltage, which is a value that weakens the electrostatic force in the reverse direction, is output from the power source while the duty is made higher than 50 [%], the test transfer mode is performed. It is characterized in that the control means is configured so as to carry out a process of outputting the superimposed voltage from the power source, which is the same as the sheet transfer mode immediately before. In such a configuration, in the test transfer mode in which the high-duty superimposed voltage is output from the power supply, it is possible to avoid complicated control by switching the output voltage from the power supply unnecessarily.

[Aspect G]
In the G aspect, in any of the C to F aspects, the superimposed voltage of the sheet transfer mode in which the environment detecting means (for example, the environment sensor 50) for detecting the environment is provided and the duty is made higher than 50 [%]. The control means is configured so as to perform a process of changing at least one of the duty and the peak-to-peak value according to the detection result by the environment detection means. be. In such a configuration, by changing at least one of the duty and peak toe peak values according to the environment, it is possible to effectively suppress the transfer failure of the halftone image even in a high humidity environment. Further, when the environment is not highly humid, the toner image can be transferred satisfactorily by using a voltage under conditions suitable for an environment without restrictions on duty and peak toe peak value.

[Aspect H]
In the aspect H, the environment detecting means for detecting the environment is provided in the aspect D or F, and at least one of the duty and the peak-to-peak value is set to the environment for the superimposed voltage in the test transfer mode. It is characterized in that the control means is configured so as to carry out a process of changing according to a detection result by the detection means. Even in such a configuration, by changing at least one of the duty and peak toe peak values according to the environment, it is possible to effectively suppress transfer defects of the test toner image even in a high humidity environment. Further, when the environment is not highly humid, the test toner image can be transferred satisfactorily by using a voltage under conditions suitable for an environment without restrictions on duty and peak toe peak value.

[Aspect I]
In aspect I, in any of aspects C to H, the peak-to-peak value of the superimposed voltage in the sheet transfer mode, which makes the duty higher than 50 [%], is made lower than 50 [%]. It is characterized in that the control means is configured so as to carry out a process of making the superimposed voltage smaller than the peak-to-peak value of the sheet transfer mode. In such a configuration, when a concavo-convex sheet is used as the recording sheet, the surface of the concavo-convex sheet is elastically deformed by conforming the surface shape of the concavo-convex sheet with a transfer nip, so that the surface recesses of the concavo-convex sheet and the image carrier are in close contact with each other. Increase sex. As a result, it is possible to suppress poor transfer of toner to the surface recesses. As described above, even in a high-speed machine for business use, toner can be satisfactorily transferred to the surface recesses of the concave-convex sheet.

[Aspect J]
In aspect J, in any of aspects C to I, the sheet transfer is such that the frequency of the superimposed voltage in the sheet transfer mode that makes the duty higher than 50 [%] is lower than the duty 50 [%]. It is characterized in that the control means is configured so as to carry out a process of making the frequency of the superimposed voltage of the mode higher than the frequency. In such a configuration, by setting the peak toe peak value in the high duty mode to a lower value, it is possible to suppress deterioration of the image carrier and the abutting body due to the alternating electric shock in the high duty mode. Further, in the low duty mode, it is possible to suppress transfer failure of the uneven sheet to the surface concave portion due to the value of the reverse peak value being too small.

[Aspect K]
In aspect K, in any of aspects C to J, information acquisition means (for example, operation display unit 501 or smoothness detection sensor 502) for acquiring information on the surface smoothness of the recording sheet used is provided, and the information is compared. If the surface smoothness is relatively low, the sheet transfer mode is executed in which the duty is lower than 50 [%], while the information is higher than the relatively low surface smoothness. When it exhibits smoothness, the control means is configured to execute the process of executing the sheet transfer mode in which the duty is made higher than 50 [%]. be. In such a configuration, the type of recording sheet can be grasped based on the information acquired by the information acquisition means.

[Aspect L]
The aspect L is characterized in that, in the aspect K, the information acquisition means is configured so as to acquire the information of the brand of the recording sheet as the information of the surface smoothness. In such a configuration, the type of recording sheet can be grasped only by having the user input the brand information without having the user determine the type of recording sheet.

[Aspect M]
Aspect M is characterized in that, in Aspect K, a smoothness detecting means for detecting the surface smoothness of the recording sheet used is used as the information acquisition means. In such a configuration, it is possible to acquire the type information while saving the user's trouble of inputting the type information of the recording sheet.

31: Intermediate transfer belt (image carrier)
40: Optical sensor unit (optical characteristic detection means)
41: Secondary transfer belt (contact body)
45: Secondary transfer power supply (power supply)
50: Environmental sensor (environmental detection means)
200: Power supply control unit (control means)
501: Operation display unit (information acquisition means)
502: Smoothness detection sensor (information acquisition means, type detection means)
Vt: Transfer peak value Vr: Reverse peak value YT, MT, CT, KT: Test toner image YPP, MPP, CPP, KPP: Patch pattern including test toner image

Japanese Unexamined Patent Publication No. 2016-156927

Claims (10)

A power supply that outputs a superposed voltage by superimposing direct current and alternating current for transferring a toner image to a recording sheet sandwiched between transfer nip by contact between an image carrier supporting a toner image and an abutting body, and the abutting body. In an image forming apparatus including an optical characteristic detecting means for detecting the optical characteristics of the test toner image transferred to the test toner image.
As the image carrier, a base material on which an elastic layer made of a material having better elasticity is laminated is used.
Moreover, in the test transfer mode in which the test toner image is transferred to the contact body, static electricity in the transfer direction from the image carrier side to the contact body side with respect to the toner in the transfer nip within one cycle of the superimposed voltage. Of the transfer peak value at which the force acts most strongly and the reverse peak value which is the peak value on the opposite side, the reverse peak value is a sheet transfer mode in which the toner image on the image carrier is transferred to the recording sheet. An image forming apparatus provided with a control means for performing a process of outputting a superposed voltage, which is a value that weakens the electrostatic force in the direction opposite to the transfer direction, from the power source. In the image forming apparatus of claim 1,
In the sheet transfer mode, a mode in which the duty, which is a ratio within one cycle of the time when the value is on the side of the reverse peak value with respect to the predetermined reference value, is made higher than 50 [%], and 50. An image forming apparatus comprising the control means so as to perform a process of switching between modes lower than [%]. In the image forming apparatus of claim 2,
In the test transfer mode performed immediately after the sheet transfer mode in which the duty is made lower than 50 [%], the reverse peak value makes the electrostatic force in the reverse direction weaker than the superimposed voltage in the sheet transfer mode. The test transfer mode, which is carried out immediately after the sheet transfer mode in which the duty is set to be higher than 50 [%] while the superimposed voltage is output from the power source, is the same as the sheet transfer mode immediately before. An image forming apparatus comprising the control means so as to perform a process of outputting the superposed voltage from the power source. In the image forming apparatus of claim 2 or 3,
Provided an environment detection means to detect the environment
Regarding the superimposed voltage in the sheet transfer mode in which the duty is made higher than 50 [%], at least one of the duty and the peak-to-peak value is changed according to the detection result by the environment detection means. An image forming apparatus characterized in that the control means is configured so as to carry out a process of causing the image to be generated. In the image forming apparatus of claim 3,
Provided an environment detection means to detect the environment
The control means so as to perform a process of changing at least one of the duty and the peak-to-peak value of the superimposed voltage in the test transfer mode according to the detection result by the environment detection means. An image forming apparatus characterized in that the above is configured. In the image forming apparatus according to any one of claims 2 to 5.
The peak toe peak value of the superimposed voltage in the sheet transfer mode that makes the duty higher than 50 [%], and the peak toe peak value of the superimposed voltage in the sheet transfer mode that makes the duty lower than 50 [%]. An image forming apparatus characterized in that the control means is configured so as to carry out a process of making the size smaller than the above. In the image forming apparatus according to any one of claims 2 to 6.
The process of increasing the frequency of the superimposed voltage in the sheet transfer mode to be higher than 50 [%] and making the duty higher than the frequency of the superimposed voltage in the sheet transfer mode to be lower than 50 [%]. An image forming apparatus characterized in that the control means is configured to be carried out. In the image forming apparatus according to any one of claims 2 to 7.
An information acquisition means for acquiring information on the surface smoothness of the recording sheet used is provided.
If the information exhibits relatively low surface smoothness, the sheet transfer mode is performed with the duty lower than 50%, while the information exhibits relatively low surface smoothness. When the surface smoothness is higher than 50 [%], the control means is configured to execute the process of executing the sheet transfer mode in which the duty is higher than 50 [%]. Image forming apparatus. In the image forming apparatus of claim 8,
An image forming apparatus comprising the information acquisition means so as to acquire information on a brand of a recording sheet as information on surface smoothness. In the image forming apparatus of claim 8,
An image forming apparatus characterized in that a smoothness detecting means for detecting the surface smoothness of a recording sheet used is used as the information acquisition means.

Images