JP7467873B2 - Image forming apparatus and image forming method - Google Patents

Description

This invention relates to an image forming apparatus and an image forming method, and in particular to an image forming apparatus that forms an image using toner, and an image forming method that is executed by the image forming apparatus.

Image forming devices, such as multi-function peripherals (MFPs), are equipped with a fixing device for fixing toner to a recording medium such as paper. A recording medium onto which a toner image made of toner has been transferred is supplied to this fixing device, which applies heat and pressure to the recording medium, melting the toner and fixing it to the recording medium. However, in this fixing device, heat is also applied to the recording medium at the same time to melt the toner, which uses unnecessary thermal energy and results in a large energy loss.

A fixing device known as a simultaneous transfer and fixation method that reduces this energy loss is known. For example, Japanese Patent Application Laid-Open No. 2007-65348 describes a fixing device having a first nip section in which, as a primary pressure step, a powdered particulate image forming material formed on an image carrier is pressed while applying thermal energy on the image carrier, and a second nip section in which, as a secondary pressure step, the image forming material is transferred and fixed to a recording medium downstream of the first nip. This fixing device applies heat to the toner image before transferring the toner image to the recording medium, and then transfers the toner image to the recording medium, so the area and time of contact with the recording medium can be shortened compared to a method of applying heat and pressure to the recording medium.

On the other hand, if the area and time of contact with the recording medium are shortened, even if the toner reaches its softening temperature, it may not be able to deform to the unevenness of the paper due to its viscoelasticity. This causes problems such as a decrease in image quality. To address this problem, in JP 2007-65348 A, in order to apply heat energy to the image carrier while pressing it, the first nip is formed by pressing the primary pressure roller against the backup roller via the transfer and fixing belt using a primary pressure spring.

However, in the fixing device described in JP 2007-65348 A, the primary pressure roller and the backup roller come into contact with the transfer and fixing belt, so heat is conducted to the primary pressure roller and the backup roller. This results in unnecessary heat energy being used, resulting in a large energy loss.

JP 2007-65348 A

One of the objectives of this invention is to provide an image forming device that can reduce the loss of thermal energy while maintaining image quality.

Another object of the invention is to provide an image forming method that can reduce the loss of thermal energy while maintaining image quality.

According to one aspect of the invention, an image forming apparatus includes an image creating means for forming a toner image on an image carrier, a fixing means for heating the toner image formed by the image creating means and then fixing the toner image to a recording medium, and a pressure applying means for non-contactly increasing the contact area between a plurality of toner particles that constitute the toner image while the toner image is heated, wherein the plurality of toner particles that constitute the toner image are charged, the pressure applying means generates an electric field in the path along which the toner image moves, the direction of the electric field generated by the pressure applying means is such that the toner is subjected to a force in the direction in which the toner is directed toward a transfer fixing member that carries the toner image, and the pressure applying means increases the strength of the electric field the greater the amount of toner that constitutes the toner image.

Preferably, the toner image further includes a charge amount determination means for determining the charge amount of the toner that constitutes the toner image, and the pressure means changes the strength of the electric field according to the charge amount.

In accordance with this aspect, it is possible to apply a force equal to or greater than a predetermined force to each of the multiple toner particles, even if the charge amount of the toner changes.

According to another aspect of the present invention, an image forming apparatus includes an image creating means for forming a toner image on an image carrier, a fixing means for heating the toner image formed by the image creating means and then fixing the toner image to a recording medium, and a pressure applying means for non-contactly increasing the contact area between a plurality of toner particles constituting the toner image while the toner image is heated, wherein the plurality of toner particles constituting the toner image are charged, the pressure applying means generates an electric field in a path along which the toner image moves, and the direction of the electric field generated by the pressure applying means is a direction in which the toner is subjected to a force in a direction in which the toner moves toward a transfer fixing member carrying the toner image, and the image forming apparatus further includes a charge amount determining means for determining the charge amount of the toner constituting the toner image, and the pressure applying means increases the strength of the electric field as the charge amount of the plurality of toner particles constituting the toner image decreases.

Preferably, the device further includes a humidity detection means for detecting humidity, and the charge amount determination means determines the charge amount of the toner based on the humidity.

In this aspect, the charge amount of the toner is determined based on the humidity, so the charge amount of the toner can be easily determined.

Preferably, the charge amount determination means determines the charge amount of the toner based on the operation history of the imaging means.

In this aspect, the charge amount of the toner is determined based on the movement history, so the charge amount of the toner can be easily determined.

Preferably, the pressure applying means changes the strength of the electric field depending on the number of types of toner constituting the toner image.

In this aspect, the strength of the electric field is changed according to the number of types of toner, so even if the number of types of toner that make up the toner image changes, a force equal to or greater than a predetermined force can be applied to each of the multiple toners.

According to another aspect of the present invention, an image forming apparatus includes an image creating means for forming a toner image on an image carrier, a primary intermediate transfer member onto which the toner image is transferred from the image carrier, a secondary intermediate transfer member onto which the toner image is transferred from the primary intermediate transfer member, a fixing means for heating the toner image formed by the image creating means onto which the toner image is transferred from the secondary intermediate transfer member and then fixing the toner image onto a recording medium, and a pressure means provided in the fixing means for non-contactly increasing the contact area between multiple toner particles constituting the toner image while the toner image is heated, wherein the multiple toner particles constituting the toner image are magnetic, and the pressure means generates a magnetic field in a path along which the toner image moves.

In this aspect, a magnetic field is generated in the path along which the toner image moves, so a predetermined force can be applied to the multiple toner particles that make up the toner image without contact.

According to yet another aspect of the present invention, an image forming apparatus includes an image creating means for forming a toner image on an image carrier, a fixing means for heating the toner image formed by the image creating means and then fixing the toner image to a recording medium, the toner image being composed of a plurality of magnetic toners, and a pressure applying means for generating a magnetic field in the path along which the toner image moves while the toner image is heated .

According to this aspect, a predetermined force is applied in a non-contact manner to multiple magnetic toner particles passing through a magnetic field. This increases the contact area between the multiple toner particles in a non-contact manner. As a result, it is possible to provide an image forming device that can reduce the loss of thermal energy while maintaining image quality.

Preferably, the pressure means changes the magnetic force of the magnetic field depending on the amount of toner that makes up the toner image.

In this aspect, the magnetic force is changed according to the amount of toner, so that even if the amount of toner changes, a force equal to or greater than a predetermined force can be applied to each toner.

Preferably, the pressure means increases the absolute value of the magnetic force as the amount of toner that constitutes the toner image increases.

Preferably, the pressure applying means changes the strength of the magnetic field depending on the number of types of toner constituting the toner image.

In this aspect, the strength of the magnetic field is changed according to the number of types of toner, so even if the number of types of toner that make up the toner image changes, a force equal to or greater than a predetermined force can be applied to each of the multiple toners.

According to another aspect of the present invention, an image forming method is an image forming method executed by an image forming apparatus, the image forming apparatus comprising: an imaging means for forming a toner image on an image carrier; a primary intermediate transfer member onto which the toner image is transferred from the image carrier; a secondary intermediate transfer member onto which the toner image is transferred from the primary intermediate transfer member; a fixing means for heating the toner image formed by the imaging means onto a recording medium, onto which the toner image is transferred from the secondary intermediate transfer member, and then fixing the toner image onto a recording medium; and a pressure means provided in the fixing means, the toner image being composed of a plurality of charged toners, and generating an electric field in a path along which the toner image moves while the toner image is heated, the direction of the electric field generated by the pressure means being a direction in which the toner receives a force in a direction in which the toner is directed toward the transfer and fixing member carrying the toner image, and the image forming apparatus includes a step of increasing the strength of the electric field generated by the pressure means as the amount of toner constituting the toner image increases .

According to this aspect, it is possible to provide an image forming method capable of reducing the loss of thermal energy while maintaining image quality.
Preferably, the method further includes a switching step of switching a state between a contact state in which the secondary intermediate transfer member is abutted against the primary intermediate transfer member at the first nip portion and against the fixing means at the second nip portion, and a separation state in which the secondary intermediate transfer member is separated from the primary intermediate transfer member and the fixing means.

According to yet another aspect of the present invention, an image forming method is an image forming method executed by an image forming apparatus, the image forming apparatus comprising an image creating means for forming a toner image on an image carrier, a fixing means for heating the toner image formed by the image creating means and then fixing the toner image to a recording medium, the toner image being composed of a plurality of magnetic toners, and a pressure applying means for generating a magnetic field in a path along which the toner image moves while the toner image is heated , and including a step of controlling the strength of the magnetic field generated by the pressure applying means.

In accordance with this aspect, it is possible to provide an image forming method that can reduce the loss of thermal energy while maintaining image quality.

1 is a first perspective view showing an external appearance of an MFP according to an embodiment of the present invention; FIG. 2 is a block diagram showing an example of a hardware configuration of an MFP. FIG. 2 is a cross-sectional view illustrating an example of an internal configuration of an MFP. FIG. 2 is a diagram illustrating a detailed configuration of a transfer and fixing device. FIG. 4 is a diagram showing a schematic diagram of a pressure applying unit. FIG. 2 is a block diagram showing an example of functions of a CPU included in the MFP. FIG. 4 is a diagram showing an example of a first voltage table. 5 is a flowchart showing an example of the flow of an image forming process according to the first embodiment. FIG. 11 is a diagram illustrating a schematic view of a pressure applying unit according to a first modified example. FIG. 11 is a block diagram showing an example of functions of a CPU included in an MFP according to a second modified example. FIG. 11 is a diagram showing an example of a second voltage table. FIG. 13 is a block diagram showing an example of functions of a CPU included in an MFP according to a third modified example. FIG. 13 is a diagram illustrating an example of a third voltage table. FIG. 13 is a block diagram showing an example of functions of a CPU included in an MFP according to a fourth modified example. FIG. 13 is a diagram showing an example of a fourth voltage table. 13 is a diagram illustrating a schematic diagram of a pressure unit included in an MFP according to a second embodiment. FIG. 13 is a block diagram showing an example of functions of a CPU included in an MFP according to a second embodiment. FIG. FIG. 11 is a diagram illustrating an example of a first grid voltage table. FIG. 13 is a diagram illustrating a schematic view of a pressure unit included in an MFP according to a fifth modified example. FIG. 13 is a block diagram showing an example of functions of a CPU included in an MFP according to a sixth modified example. FIG. 11 is a diagram illustrating an example of a second grid voltage table. FIG. 13 is a block diagram showing an example of functions of a CPU included in an MFP according to a seventh modified example. FIG. 13 is a diagram illustrating an example of a third grid voltage table. FIG. 13 is a block diagram showing an example of functions of a CPU included in an MFP according to an eighth modified example. FIG. 13 is a diagram illustrating an example of a fourth grid voltage table. 13 is a diagram illustrating a schematic diagram of a pressure unit included in an MFP according to a third embodiment. FIG. FIG. 13 is a block diagram showing an example of functions of a CPU included in an MFP according to a third embodiment. FIG. 4 is a diagram showing an example of a first magnetic flux density table. FIG. 13 is a block diagram showing an example of functions of a CPU included in an MFP according to a ninth modified example. FIG. 11 is a diagram showing an example of a second magnetic flux density table.

Below, an embodiment of the present invention will be described with reference to the drawings. In the following description, identical parts are given the same reference numerals. Their names and functions are also the same. Therefore, detailed descriptions thereof will not be repeated.

First Embodiment
Fig. 1 is a first perspective view showing the appearance of an MFP according to an embodiment of the present invention. Fig. 2 is a block diagram showing an example of a hardware configuration of the MFP. With reference to Figs. 1 and 2, an MFP (Multi Function Peripheral) 100 is an example of an image forming apparatus, and includes a main circuit 110, an image forming unit 140 for forming an image on paper or the like based on image data, a paper feed unit 150 for supplying paper to the image forming unit 140, and an operation panel 160 as a user interface.

The main circuit 110 includes a CPU (Central Processing Unit) 111 that controls the entire MFP 100, a communication interface (I/F) unit 112, a ROM (Read Only Memory) 113, a RAM (Random Access Memory) 114, a hard disk drive (HDD) 115 as a large-capacity storage device, a facsimile unit 116, and an external storage device 117. The CPU 111 is connected to the image forming unit 140, the paper feed unit 150, and the operation panel 160, and controls the entire MFP 100.

The paper feed unit 150 transports the paper stored in the paper feed tray to the image forming unit 140. The image forming unit 140 is controlled by the CPU 111 and forms images using a well-known electrophotographic method. Based on image data input from the CPU 111, the image is formed on the paper transported by the paper feed unit 150, and the paper on which the image is formed is discharged to the discharge tray 39. The image data that the CPU 111 outputs to the image forming unit 140 includes image data such as print data received from an external personal computer, etc.

ROM 113 stores the programs executed by CPU 111 or data required to execute the programs. RAM 114 is used as a working area when CPU 111 executes the programs.

Operation panel 160 is provided on the top surface of MFP 100. Operation panel 160 includes display unit 161 and operation unit 163. Display unit 161 is, for example, a liquid crystal display (LCD) that displays an instruction menu for the user and information related to acquired image data. Note that instead of an LCD, for example, an organic electroluminescence (EL) display can be used as long as it is a device that displays images.

The operation unit 163 includes a touch panel 165 and a hard key unit 167. The hard key unit 167 includes a plurality of hard keys. The hard keys are, for example, contact switches. The touch panel 165 detects a position on the display surface of the display unit 161 that is designated by the user.

The communication I/F unit 112 is an interface for connecting the MFP 100 to a network. The communication I/F unit 112 communicates with other computers connected to the network using a communication protocol such as TCP (Transmission Control Protocol) or FTP (File Transfer Protocol). The network to which the communication I/F unit 112 is connected is a local area network (LAN), and the connection form may be either wired or wireless. The network is not limited to a LAN, and may be a wide area network (WAN), a public switched telephone network (PSTN), the Internet, or the like.

Facsimile unit 116 is connected to the public switched telephone network (PSTN) and transmits facsimile data to the PSTN or receives facsimile data from the PSTN. Facsimile unit 116 stores the received facsimile data in HDD 115, converts it to print data that can be printed by image forming unit 140, and outputs it to image forming unit 140. In this way, image forming unit 140 forms an image of the facsimile data received by facsimile unit 116 on paper. Facsimile unit 116 also converts the data stored in HDD 115 into facsimile data and transmits it to a facsimile device connected to the PSTN.

The external storage device 117 is controlled by the CPU 111, and is equipped with a CD-ROM (Compact Disk Read Only Memory) 118 or a semiconductor memory. In this embodiment, an example is described in which the CPU 111 executes a program stored in the ROM 113, but the CPU 111 may also control the external storage device 117 to read a program from the CD-ROM 118 for the CPU 111 to execute, store the read program in the RAM 114, and execute it.

The recording medium for storing the program executed by CPU 111 is not limited to CD-ROM 118, but may be a flexible disk, cassette tape, optical disk (MO (Magnetic Optical Disc)/MD (Mini Disc)/DVD (Digital Versatile Disc)), IC card, optical card, mask ROM, EPROM (Erasable Programmable ROM), EEPROM (Electrically EPROM), or other semiconductor memory medium. Furthermore, CPU 111 may download the program from a computer connected to the network and store it in HDD 115, or a computer connected to the network may write the program to HDD 115, and the program stored in HDD 115 may be loaded into RAM 114 and executed by CPU 111. The programs referred to here include not only programs that can be executed directly by the CPU 111, but also source programs, compressed programs, encrypted programs, etc.

Figure 3 is a cross-sectional view showing a schematic example of the internal configuration of an MFP. For the sake of explanation, the left-right direction in Figure 3 is referred to as the left-right direction, and the front-to-back direction is referred to as the depth direction. The direction from left to right in the left-to-right direction is referred to as the right side direction, and the direction from right to left is referred to as the left side direction. The direction from front to back in the depth direction is referred to as the front direction, and the direction from back to front is referred to as the back direction.

The image forming section 140 includes image forming units 20Y, 20M, 20C, and 20K for yellow, magenta, cyan, and black, respectively. Here, "Y", "M", "C", and "K" represent yellow, magenta, cyan, and black, respectively. An image is formed by driving at least one of the image forming units 20Y, 20M, 20C, and 20K. When all of the image forming units 20Y, 20M, 20C, and 20K are driven, a full-color image is formed. Print data for yellow, magenta, cyan, and black is input to the image forming units 20Y, 20M, 20C, and 20K, respectively. The image forming units 20Y, 20M, 20C, and 20K differ only in the color of the toner they handle, so here, the image forming unit 20Y for forming a yellow image will be described.

The image forming unit 20Y includes an exposure device 21Y to which yellow printing data is input, a photoconductor drum 23Y as an image carrier, a charging roller 22Y for uniformly charging the surface of the photoconductor drum 23Y, a developing device 24Y, a primary transfer roller 25Y for transferring the toner image formed on the photoconductor drum 23Y onto the primary intermediate transfer belt 30 as an image carrier by the action of an electric field force, a drum cleaning blade 27Y for removing residual toner from the photoconductor drum 23Y, a toner bottle 41Y, and a toner hopper 42Y.

Toner bottle 41Y contains yellow toner. Toner bottle 41Y rotates using a toner bottle motor as a drive source and discharges toner to the outside. The toner discharged from toner bottle 41Y is supplied to toner hopper 42Y. Toner hopper 42Y supplies toner to developing device 24Y when the remaining amount of toner contained in developing device 24Y falls below a predetermined lower limit.

A charging roller 22Y, an exposure device 21Y, a developing device 24Y, a primary transfer roller 25Y, and a drum cleaning blade 27Y are arranged around the photoconductor drum 23Y in the direction of rotation of the photoconductor drum 23Y.

After the photoconductor drum 23Y is charged by the charging roller 22Y, the laser light emitted by the exposure device 21Y is irradiated and scans the photoconductor drum 23Y. The exposure device 21Y exposes the image corresponding portion of the surface of the photoconductor drum 23Y to form an electrostatic latent image. As a result, an electrostatic latent image is formed on the photoconductor drum 23Y. Next, the developer 24Y develops the electrostatic latent image formed on the photoconductor drum 23Y with charged toner. The toner is placed on the electrostatic latent image formed on the photoconductor drum 23Y by the action of the electric field force. Specifically, a development bias voltage is applied to the developer 24Y. The development bias voltage is set between the charging potential of the toner and the exposure potential of the photoconductor drum 23Y. As a result, a toner image is formed on the photoconductor drum 23Y due to the potential difference between the development bias voltage and the exposure potential. In this way, the developer 24Y creates a toner image on the photoconductor drum 23Y.

The toner image formed on the photoreceptor drum 23Y is transferred by the action of electric field force by the primary transfer roller 25Y onto the primary intermediate transfer belt 30, which is an image carrier. Specifically, a positive primary transfer bias voltage is applied to the primary transfer roller 25Y. The toner image on the photoreceptor drum 23Y is transferred to the primary intermediate transfer belt 30 by the electric field generated between the primary transfer roller 25Y and the photoreceptor drum 23Y. The toner remaining on the photoreceptor drum 23Y that has not been transferred is removed from the photoreceptor drum 23Y by the drum cleaning blade 27Y.

Meanwhile, the primary intermediate transfer belt 30 is suspended by a drive roller 33 and a driven roller 34 so as not to slacken. When the drive roller 33 rotates counterclockwise in FIG. 3, the primary intermediate transfer belt 30 rotates counterclockwise in the figure at a predetermined speed. As the primary intermediate transfer belt 30 rotates, the driven roller 34 rotates counterclockwise.

As a result, image forming units 20Y, 20M, 20C, and 20K sequentially transfer toner images onto primary intermediate transfer belt 30. The timing at which each of image forming units 20Y, 20M, 20C, and 20K transfers a toner image onto primary intermediate transfer belt 30 is adjusted by detecting a reference mark on primary intermediate transfer belt 30. As a result, yellow, magenta, cyan, and black toner images are superimposed on primary intermediate transfer belt 30.

When forming a full-color image, the MFP 100 drives all of the image forming units 20Y, 20M, 20C, and 20K, but when forming a monochrome image, it drives one of the image forming units 20Y, 20M, 20C, and 20K. It is also possible to form an image by combining two or more of the image forming units 20Y, 20M, 20C, and 20K. Note that, here, an example is described in which the MFP 100 employs a tandem system with image forming units 20Y, 20M, 20C, and 20K that form four color toners on paper, but a four-cycle system in which the four color toners are transferred to paper in sequence using one photosensitive drum may also be employed.

The toner image carried by the primary intermediate transfer belt 30 is transferred to the transfer fixing device 50. A belt cleaning blade 28 is provided upstream of the image forming unit 20Y of the primary intermediate transfer belt 30. The belt cleaning blade 28 removes toner remaining on the primary intermediate transfer belt 30 that has not been transferred to the transfer fixing device 50.

A number of sheets of paper are set in the paper feed cassette 35. The sheets stored in the paper feed cassette 35 are fed one by one to the transport path by the take-out rollers 36 attached to the paper feed cassette 35, and are sent to the timing rollers 31 by the paper feed rollers 37. In addition, when one or more sheets of paper are set in the manual feed cassette 35A, the one or more sheets of paper set in the manual feed cassette 35A are fed one by one to the transport path by the take-out rollers 36A attached to the manual feed cassette 35A, and are sent to the timing rollers 31 by the paper feed rollers 37.

The timing rollers 31 transport paper transported from the paper feed cassette 35 or the manual feed cassette 35A to the transfer and fixing device 50. The transfer and fixing device 50 transfers and fixes a toner image onto the paper transported by the timing rollers 31. This melts the toner and fixes it onto the paper. The timing rollers 31 start transporting the paper in time with the toner image transferred to the transfer and fixing device 50 reaching the position where the transfer and fixing device 50 transfers the toner image onto the paper. This adjusts the position where the toner image is transferred onto the paper. The transfer and fixing device 50 discharges the paper onto which the toner image has been transferred and fixed onto the paper output tray 39.

Figure 4 is a diagram showing a schematic diagram of the detailed configuration of the transfer and fixing device. Referring to Figure 4, the transfer and fixing device 50 includes an intermediate transfer unit 51, a simultaneous transfer and fixing unit 61, and a pressure roller 71. The intermediate transfer unit 51 includes a secondary transfer roller 53, which is an image carrier, and a separation mechanism 59. The secondary transfer roller 53 is a roller having a hollow cylindrical shape.

The rotation axis of the secondary transfer roller 53 is a rotationally symmetrical axis. The secondary transfer roller 53 is supported by the intermediate transfer frame 10. The detachment mechanism 59 moves the rotation axis of the secondary transfer roller 53 to either the drive position or the detachment position by sliding the intermediate transfer frame 10 relative to the main frame. The detachment mechanism 59 biases the rotation axis of the secondary transfer roller 53 in the direction toward the drive roller 33 and the direction toward the heating roller 63 when the secondary transfer roller 53 is located at the drive position. The detachment mechanism 59 has an elastic body such as a spring to bias the rotation axis of the secondary transfer roller 53.

When the secondary transfer roller 53 is located at the drive position, a first nip portion N1 is formed between the secondary transfer roller 53 and the drive roller 33. A second transfer bias voltage is applied to the secondary transfer roller 53, and the toner image carried by the primary intermediate transfer belt 30 is transferred to the secondary transfer roller 53 at the first nip portion N1. The secondary transfer bias voltage has a larger absolute value than the primary transfer bias voltage. When the secondary transfer roller 53 is located at the drive position, a second nip portion N2 is formed between the secondary transfer roller 53 and a heating roller 63 described later. A third transfer bias voltage is applied to the heating roller 63, and the toner image carried by the secondary transfer roller 53 is transferred to the transfer fixing belt 67 at the second nip portion N2. The third transfer bias voltage has a larger absolute value than the secondary transfer bias voltage. The first nip portion N1 is a portion where the secondary transfer roller 53 contacts the primary intermediate transfer belt 30 between the secondary transfer roller 53 and the drive roller 33. The second nip portion N2 is the portion between the secondary transfer roller 53 and the heating roller 63 where the secondary transfer roller 53 comes into contact with the transfer and fixing belt 67.

When the secondary transfer roller 53 is in the detached position, the secondary transfer roller 53 is separated from the drive roller 33 and from the heating roller 63.

The simultaneous transfer and fixation unit 61 includes a heating roller 63, a driven roller 65, an endless transfer and fixation belt 67 which is an image carrier, and a pressure unit 69. The heating roller 63 is a hollow cylindrical member and has a heat source 66 built in. The inner diameter of the heating roller 63 is set to a size that does not allow the heat source 66 to come into contact with it. The heating roller 63 is made of stainless steel. Since the heating roller 63 is made of stainless steel, it is strong and easy to process. The heat source 66 is, for example, a halogen heater. In this embodiment, two halogen heaters with different light emission lengths are used as the heat source 66. The heat source 66 is not limited to a halogen heater, and a resistance heating element or IH (Induction Heating) may be used.

When the heat source 66 generates heat, the heating roller 63 is heated, and the temperature of the heating roller 63 rises. By thinning the heating roller 63, the thermal capacity of the heating roller 63 is reduced. This increases the rate at which the temperature of the heating roller 63 rises, and the warm-up time until the heating roller 63 reaches a predetermined temperature can be shortened. In addition, the power consumption of the heat source 66 can be reduced.

An endless transfer fixing belt 67 is stretched around the outer circumference of the heating roller 63 and the outer circumference of the driven roller 65. The heating roller 63 is rotated at a predetermined rotation speed by a motor. The transfer fixing belt 67 and the driven roller 65 rotate in accordance with the rotation of the heating roller 63. The transfer fixing belt 67 is heated while in contact with the heating roller 63.

A thermistor is disposed outside the heating roller 63, but is not in contact with the heating roller 63. This thermistor detects the surface temperature of the transfer fixing belt 67 to which heat is transferred by the heating roller 63, and controls the amount of heat generated by the heat source 66. Specifically, the amount of heat generated by the heat source 66 is controlled so that the surface temperature of the transfer fixing belt 67 becomes a predetermined temperature.

The heat source 66 of the heating roller 63 may be provided inside the driven roller 65 instead of inside the heating roller 63.

The pressure roller 71 is disposed at a position facing the driven roller 65. The pressure roller 71 is a cylindrical member. The rotation shaft of the pressure roller 71 is slidably supported on the main body frame. The rotation shaft of the pressure roller 71 is biased toward the heating roller 63 by an elastic body such as a spring, and a third nip portion N3 is formed between the pressure roller 71 and the driven roller 65. The third nip portion N3 is the portion where the pressure roller 71 contacts the transfer fixing belt 67. The biasing force applied to the pressure roller 71 is adjusted by changing the length of the spring. The length of the third nip portion N3 is preferably 0 mm or more to 1 mm or less. In addition, the pressure roller 71 is provided with a movement mechanism that changes its state between a biased state in which it is biased in a direction toward the driven roller 65 and a standby state in which it is separated from the driven roller 65 and does not contact the transfer fixing belt 67.

In the energized state, the pressure roller 71 contacts the transfer fixing belt 67, so the heat generated by the heat source 66 is conducted, but in the standby state, the pressure roller 71 does not contact the transfer fixing belt 67, so the heat is not conducted. Therefore, in the standby state, the heat generated by the heat source 66 is not conducted to the pressure roller 71, so the time until the temperature of the transfer fixing belt 67 reaches a predetermined value can be shortened. This makes it possible to prevent unnecessary consumption of thermal energy.

When the secondary transfer roller 53 is in the detached position, the secondary transfer roller 53 is separated from the drive roller 33 and the heating roller 63. Therefore, when the secondary transfer roller 53 is in the detached position, the heat generated by the heat source 66 is not conducted to the primary intermediate transfer belt 30. Therefore, the temperature rise of the primary intermediate transfer belt 30 and the image forming units 20Y, 20M, 20C, and 20K can be suppressed. This makes it possible to prevent the toner contained in the image forming units 20Y, 20M, 20C, and 20K from solidifying, and to prevent the image forming units 20Y, 20M, 20C, and 20K from being destroyed. In addition, the toner is prevented from adhering to the primary intermediate transfer belt 30, and the belt cleaning blade 28 is prevented from being damaged.

The pressure applying section 69 increases the contact area between the multiple toner particles that make up the toner image while the toner image transferred to the transfer fixing belt 67 at the second nip N2 is being heated. The toner image transferred to the transfer fixing belt 67 is carried by the transfer fixing belt 67. The toner image carried by the transfer fixing belt 67 is heated by the transfer fixing belt 67 while moving from the second nip N2 to the third nip N3.

FIG. 5 is a schematic diagram of the pressure applying unit. Referring to FIG. 5, the pressure applying unit 69 generates an electric field between the second nip portion N2 and the third nip portion N3 of the transfer fixing belt 67. Specifically, the pressure applying unit 69 includes a first electrode plate 81 and a second electrode plate 83 arranged to sandwich the transfer fixing belt 67 between the second nip portion N2 and the third nip portion N3, and a voltage applying unit 85 that generates a potential difference between the first electrode plate 81 and the second electrode plate 83. The first electrode plate 81 is arranged on the inside opposite to the side on which the toner image of the transfer fixing belt 67 is carried, and the second electrode plate 83 is arranged on the outside of the transfer fixing belt 67. The voltage applying unit 85 applies a voltage to the first electrode plate 81 and the second electrode plate 83 so that the potential of the first electrode plate 81 is greater than the potential of the second electrode plate 83. Therefore, the direction of the electric field is from the first electrode plate 81 to the second electrode plate 83.

The toner constituting the toner image carried by the transfer-fixing belt 67 is negatively charged. Therefore, as the toner image passes through the electric field generated by the pressure unit 69 while moving from the second nip portion N2 to the third nip portion N3, the toner constituting the toner image is pressed against the transfer-fixing belt 67.

The toner carried by the transfer fixing belt 67 is attracted to the transfer fixing belt 67 by surface tension. However, surface tension alone is insufficient to deform the toner. The force applied to the toner by the pressure unit 69 is greater than the surface tension. This reduces the space between the toner particles that make up the toner image, and increases the contact area between the toner particles. In this way, the pressure unit 69 increases the contact area between the multiple toner particles that make up the toner image without contacting the toner particles when the toner image carried by the transfer fixing belt 67 is heated. When the toner that makes up the toner image carried by the transfer fixing belt 67 is positively charged, the voltage application unit 85 applies a voltage such that the potential of the first electrode plate 81 is greater than the potential of the second electrode plate 83.

In addition, it is preferable that the material of the first electrode plate 81 and the second electrode plate 83 is aluminum. In this case, the second electrode plate 83 reflects radiant heat, so the toner can be heated, and the first electrode plate 81 reflects radiant heat, so the temperature drop of the transfer fixing belt 67 can be reduced.

The pressure unit 69 is controlled by the CPU 111. The CPU 111 controls the voltage application unit 85 of the pressure unit 69, and the greater the amount of toner carried by the transfer fixing belt 67, the greater the electric field. The greater the amount of toner carried by the transfer fixing belt 67, the greater the voltage with which the voltage application unit 85 applies a voltage with a greater potential difference. This allows the force with which the toner is pressed against the transfer fixing belt 67 to be greater than or equal to a predetermined value even if the amount of toner fluctuates. In the MFP 100 of this embodiment, the greater the amount of toner, the greater the absolute value of the voltage applied by the voltage application unit 85 to the first electrode plate 81 and the second electrode plate 83, so that the force with which the multiple toners constituting the toner image are pressed against the transfer fixing belt 67 can be greater than or equal to a predetermined value even if the amount of toner fluctuates.

FIG. 6 is a block diagram showing an example of functions of a CPU included in an MFP. The functions of CPU 111 included in MFP 100 are functions that are realized by CPU 111 as CPU 111 executes an image formation control program. Referring to FIG. 6, CPU 111 includes a toner amount determination unit 201, a voltage determination unit 203, and a voltage control unit 205.

The toner amount determination unit 201 determines the amount of toner adhesion based on image data that is the subject of image formation in the image forming unit 140. The adhesion amount is the amount of toner carried by the transfer fixing belt 67. Specifically, the toner amount determination unit 201 calculates the adhesion amount based on the pixel values of multiple pixels included in the image data. The voltage determination unit 203 refers to the first voltage table and determines a voltage determined by the first voltage table for the adhesion amount determined by the toner amount determination unit 201. Note that, although an example using the first voltage table is described here, an arithmetic formula for determining a voltage value from the toner amount may also be used. The voltage control unit 205 controls the voltage application unit 85 to apply the voltage determined by the voltage determination unit 203 to the voltage application unit 85.

Figure 7 shows an example of the first voltage table. The first voltage table determines the voltage relative to the amount of adhesion. The first voltage table is obtained through experimentation. The first voltage table determines a voltage whose absolute value is larger as the amount of adhesion increases.

Figure 8 is a flow chart showing an example of the flow of image formation processing in the first embodiment. The image formation processing is processing that is executed by CPU 111 included in MFP 100 as CPU 111 executes an image formation control program. With reference to Figure 8, CPU 111 separates pressure roller 71 (step S01). It controls the movement mechanism to change the state of pressure roller 71 to a standby state.

In the next step S02, the CPU 111 separates the secondary transfer roller 53. Specifically, the CPU 111 controls the separation mechanism 59 to move the secondary transfer roller 53 to the separation position.

In step S03, the preparatory operation of the simultaneous transfer and fixing unit 61 is started. Specifically, the CPU 111 causes the heat source 66 to generate heat and rotates the heating roller 63. As a result, the transfer and fixing belt 67 is heated by the heating roller 63 while rotating around the heating roller 63 and the driven roller 65.

In step S04, it is determined whether the temperature of the transfer fixing belt 67 is a predetermined temperature. The process waits until the surface temperature of the transfer fixing belt 67 detected by the thermistor reaches the predetermined temperature (NO in step S04), and if it is the predetermined temperature, the process proceeds to step S05.

In step S05, it is determined whether print data has been received. For example, the device waits until print data is received from an external computer (NO in step S05), and if print data has been received (YES in step S05), the process proceeds to step S06.

In step S06, the amount of toner is determined. The amount of toner to be used in forming the image is determined based on the image data to be used for image formation, which is included in the print data. For example, the amount of toner is determined for each line of the image data. One line includes multiple pixels lined up in the main scanning direction. The main scanning direction is the direction in which the exposure device 21Y scans the photoconductor drum 23Y, and is parallel to the depth direction. The amount of toner to be used for each of the multiple pixels included in one line is predicted, and the sum of these is set as the amount of toner for that line. The amount of toner is determined for each of the multiple lines included in the image data.

In step S07, a voltage pattern is determined. Using the first voltage table, a voltage determined by the first voltage table is determined for each of the multiple toner amounts determined for each of the multiple lines. Then, a voltage pattern is determined in which voltages are determined in a time series in accordance with the image forming speed at which image forming units 20Y, 20M, 20C, and 20K form images.

In step S08, image formation is started. Image forming units 20Y, 20M, 20C, and 20K are controlled to form a toner image on primary intermediate transfer belt 30. In the next step S09, secondary transfer roller 53 is energized. CPU 111 controls separation mechanism 59 to move secondary transfer roller 53 to the drive position. As a result, the toner image carried by primary intermediate transfer belt 30 is transferred to secondary transfer roller 53 at first nip portion N1, and the toner image carried by secondary transfer roller 53 is transferred to transfer fixing belt 67 at second nip portion N2.

In step S10, an electric field is generated. The CPU 111 controls the voltage application unit 85 to apply a voltage determined by the voltage pattern determined in step S07 to the first electrode plate 81 and the second electrode plate 83. As a result, since the multiple toners constituting the toner image carried by the transfer fixing belt 67 are negatively charged, the multiple toners constituting the toner image receive a force in a direction toward the transfer fixing belt 67. Therefore, the contact surface area between the multiple toners constituting the toner image increases. Since the voltage pattern determines a voltage corresponding to the amount of toner in the toner image carried by the transfer fixing belt 67, even if the amount of toner changes, the force of the multiple toners constituting the toner image carried by the transfer fixing belt 67 in the direction toward the transfer fixing belt 67 can be made to be equal to or greater than a predetermined value.

In step S11, it is determined whether it is time for paper to be fed to the third nip portion N3. The timing at which the paper is transported to the third nip portion N3 is adjusted by the timing roller 31, so the timing at which the paper is fed to the third nip portion N3 is determined based on when the timing roller 31 starts feeding the paper. The process waits until it is time for paper to be fed to the third nip portion N3 (NO in step S11), and if it is time for paper to be fed to the third nip portion N3, the process proceeds to step S12.

In step S12, the pressure roller 71 is energized, and the process proceeds to step S13. The CPU 111 controls the movement mechanism to change the state of the pressure roller 71 to the energized state. As a result, the pressure roller 71 is energized in the direction toward the driven roller 65, and the toner image carried by the transfer and fixing belt 67 is transferred and fixed to the paper transported to the third nip portion N3.

In step S13, it is determined whether image formation by the image forming unit 140 has been completed. The process remains on standby until image formation is completed (NO in step S13), and if image formation is completed (YES in step S13), the process proceeds to step S14.

In step S14, the operation of generating an electric field is terminated, and the process proceeds to step S15. The CPU 111 controls the voltage application unit 85 so that no voltage is applied to the first electrode plate 81 and the second electrode plate 83.

In step S15, the CPU 111 separates the pressure roller 71 in the same manner as in step S01. Next, in step S16, the CPU 111 separates the secondary transfer roller 53 in the same manner as in step S02, and ends the process.

In the above example, when the rotation axis of the secondary transfer roller 53 is located in the drive position, the secondary transfer roller 53 contacts the primary intermediate transfer belt 30 at the first nip portion N1, and the secondary transfer roller 53 contacts the transfer fixing belt 67 at the second nip portion N2. However, if a toner image can be transferred by the action of an electric field force at the first nip portion N1, the secondary transfer roller 53 may be spaced a predetermined distance from the primary intermediate transfer belt 30, and the secondary transfer roller 53 may be spaced a predetermined distance from the transfer fixing belt 67 at the second nip portion N2.

<First Modification>
9 is a schematic diagram of a pressure unit in the first modified example. Referring to FIG. 9, the pressure unit 69 is different from the pressure unit 69 shown in FIG. 5 in that the first electrode plate 81 is omitted. The other configurations are the same as those shown in FIG. 9, and therefore the description will not be repeated here. The second transfer bias voltage is applied to the heating roller 63. Therefore, the potential of the transfer fixing belt 67 is determined by the second transfer bias voltage. The voltage application unit 85 applies a voltage to the second electrode plate 83 so that the potential of the second electrode plate 83 is smaller than the potential of the transfer fixing belt 67. Therefore, the direction of the electric field is from the transfer fixing belt 67 to the second electrode plate 83.

The toner constituting the toner image carried by the transfer-fixing belt 67 is negatively charged. Therefore, as the toner image passes through the electric field generated by the pressure unit 69 while moving from the second nip portion N2 to the third nip portion N3, the toner constituting the toner image is pressed against the transfer-fixing belt 67.

<Second Modification>
As described above, the toner image is formed by layering the toner in the order of yellow toner, magenta toner, cyan toner, and black toner by the image forming units 20Y, 20M, 20C, and 20K. Therefore, the more types of toner, the more times the toner is layered, and the greater the height of the toner image from the transfer and fixing belt 67. The MFP 100 in the second modified example determines the voltage to be applied to the voltage application unit 85 depending on the number of types of toner.

Figure 10 is a block diagram showing an example of the functions of a CPU included in an MFP in the second modified example. Referring to Figure 10, the functions differ from those shown in Figure 6 in that the toner amount determination unit 201 has been changed to a toner type number determination unit 201A, and the voltage determination unit 203 has been changed to a voltage determination unit 203A. The other functions are the same as those shown in Figure 6, so the description will not be repeated here.

The toner type number determination unit 201A determines the number of toner types carried on the transfer and fixing belt 67. Specifically, the toner type number determination unit 201A calculates the number of toner types based on the pixel values of multiple pixels included in the image data. The image data includes four bitmap data corresponding to the four colors of cyan, magenta, yellow, and black. The number of toner types indicates the number of colors applied to one pixel. The number of colors used in multiple pixels included in several lines can be set as the number of toner types.

The voltage determination unit 203A refers to the second voltage table and determines a voltage determined by the second voltage table for the number of toner types determined by the toner type number determination unit 201A. Note that, although an example using the second voltage table is described here, an arithmetic formula for determining a voltage value from the number of toner types may also be used.

Figure 11 shows an example of the second voltage table. The second voltage table determines the voltage value for the number of toner types. The second voltage table is obtained through experiments. The second voltage table determines a voltage whose absolute value is larger as the number of toner types increases.

In the second modified example, the MFP 100 increases the absolute value of the voltage applied to the voltage application unit 85 as the number of toner types increases, so that the force with which the multiple toners constituting the toner image are pressed against the transfer and fixing belt 67 can be made to be equal to or greater than a predetermined value even if the number of toner types varies.

In the second modified example, an image forming process is performed in which steps S06 and S07 of the image forming process shown in FIG. 8 are modified. In step S06, the number of types of toner to be used for each of the multiple lines is determined. In step S07, the second voltage table is used to determine a voltage determined by the second voltage table for the number of types of toner for each of the multiple lines. Then, a voltage pattern is determined in which voltages are determined in a time series in accordance with the image formation speed. Note that the voltage may be determined by combining the amount of toner and the number of types of toner.

<Third Modification>
Fig. 12 is a block diagram showing an example of functions of a CPU included in an MFP in the third modified example. Referring to Fig. 12, the functions differ from those shown in Fig. 6 in that toner amount determination unit 201 is changed to charge amount detection unit 201B, and voltage determination unit 203 is changed to voltage determination unit 203B. The other functions are the same as those shown in Fig. 6, and therefore description thereof will not be repeated here.

The charge amount detection unit 201B detects the charge amount of the toner carried on the transfer fixing belt 67 based on the output value of a potential sensor arranged near the transfer fixing belt 67. The voltage determination unit 203 refers to the third voltage table and determines a voltage determined by the third voltage table for the charge amount of the toner detected by the charge amount detection unit 201B. Note that, although an example using the third voltage table is described here, an arithmetic formula for determining a voltage value from the charge amount of the toner may also be used.

Figure 13 shows an example of the third voltage table. The third voltage table determines the voltage relative to the charge amount of the toner. The third voltage table is obtained through experimentation. The third voltage table determines a voltage whose absolute value is larger as the charge amount of the toner is smaller.

In the third modified example, the MFP 100 increases the absolute value of the voltage applied to the voltage application unit 85 as the charge amount of the toner decreases, so that the force with which the multiple toner particles that make up the toner image are pressed against the transfer and fixing belt 67 can be made to be equal to or greater than a predetermined value even if the charge amount of the toner fluctuates.

In the third modified example, steps S06 and S07 of the image forming process shown in FIG. 8 are also deleted, and an image forming process is executed in which step S10 is changed. In step S10, the charge amount of the toner carried on the transfer fixing belt 67 is detected based on the output value of the potential sensor, and a voltage determined by the third voltage table for the detected charge amount is determined, and the voltage application unit 85 is controlled so that the determined voltage is applied to the first electrode plate 81 and the second electrode plate 83.

<Fourth Modification>
It has been found through experiments that the charge amount of the toner varies depending on the absolute humidity. It has also been found through experiments that the charge amount of the toner varies depending on the number of sheets of paper on which images are formed by the image forming unit 140. When the total number of sheets of paper on which images are formed by the image forming units 20Y, 20M, 20C, and 20K increases, the carriers in the developers 24Y, 24M, 24C, and 24K provided in the image forming units 20Y, 20M, 20C, and 20K, respectively, wear out. When the carriers wear out, the toner becomes less likely to be electrostatically charged. The total number of sheets of paper printed by the image forming unit 20Y is the number of sheets of paper on which images have been formed since the MFP 100 was manufactured.

In the fourth variant, the MFP 100 determines the charge amount of the toner based on the absolute humidity and the total number of pages printed by the image forming unit 140.

Figure 14 is a block diagram showing an example of functions of a CPU included in an MFP in the fourth modified example. Referring to Figure 14, the differences from the functions shown in Figure 12 are that charge amount detection unit 201B has been removed, voltage determination unit 203 has been changed to voltage determination unit 203C, and humidity detection unit 207, temperature detection unit 209, and operation history acquisition unit 211 have been added. The other functions are the same as those shown in Figure 12, so the description will not be repeated here.

The humidity detection unit 207 controls the hygrometers arranged around the image forming units 20Y, 20M, 20C, and 20K to detect the humidity. The temperature detection unit 209 controls the thermometers arranged around the image forming units 20Y, 20M, 20C, and 20K to detect the temperature.

The operation history acquisition unit 213 acquires the operation history of each of the image forming units 20Y, 20M, 20C, and 20K. Here, the operation history is the total number of printed sheets, which is the number of sheets of paper on which an image is formed by the image forming unit 140.

The voltage determination unit 203C determines the absolute temperature from the temperature detected by the temperature detection unit 209 and the humidity detected by the humidity detection unit 207. The voltage determination unit 203C then determines the voltage determined in the fourth voltage table for the absolute temperature and the total number of printed sheets. The voltage control unit 205 causes the voltage application unit 85 to apply the determined voltage.

Figure 15 is a diagram showing an example of the fourth voltage table. The fourth voltage table determines the voltage for each combination of absolute humidity, total number of printed pages, and toner. The unit kp indicates 1000 pages. The fourth voltage table is determined by experiment. The fourth voltage table determines a voltage with a larger absolute value as the absolute humidity is higher, and also determines a voltage with a larger absolute value as the total number of printed pages is larger.

In the fourth modified example, the MFP 100 increases the absolute value of the voltage as the absolute humidity increases, and increases the absolute value of the voltage as the total number of printed pages increases, so that the force with which the multiple toner particles that make up the toner image are pressed against the transfer and fixing belt 67 can be made to be equal to or greater than a predetermined value even if the charge amount of the toner fluctuates.

In the fourth modified example, an image formation process is performed in which steps S06 and S07 of the image formation process shown in FIG. 8 are modified. In step S06, the absolute temperature is detected and the total number of printed sheets is obtained. In step S07, the fourth voltage table is used to determine a voltage determined by the fourth voltage table for a pair of the absolute temperature and the number of printed sheets.

In addition, instead of the total number of printed sheets, the total number of rotations of the developers 24Y, 24M, 24C, and 24K provided in the image forming units 20Y, 20M, 20C, and 20K, respectively, may be used.

In addition, instead of the fourth voltage table, the voltage determination unit 203C may determine the toner charge amount using a table that defines the toner charge amount for a combination of absolute temperature and total number of printed pages, and may further determine the voltage to be applied by the voltage application unit 85 from the toner charge amount using the third voltage table.

In addition, the voltage determination unit 203C may determine the voltage to be applied by the voltage application unit 85 from the absolute temperature using a table that defines the voltage for the absolute temperature instead of the fourth voltage table.In addition, the voltage determination unit 203C may determine the voltage to be applied by the voltage application unit 85 from the total number of printed sheets using a table that defines the voltage for the total number of printed sheets instead of the fourth voltage table.

Second Embodiment
The MFP 100 in the second embodiment is a modification of the pressurizing unit 69 of the MFP 100 in the first embodiment.

16 is a schematic diagram of a pressure unit included in the MFP in the second embodiment. Referring to FIG. 16, the pressure unit 69A included in the MFP 100 in the second embodiment charges the transfer fixing belt 67 with a charge of a different sign from the charge of the toner image carried by the transfer fixing belt 67. Specifically, the pressure unit 69 includes a charger 82 that discharges to the transfer fixing belt 67, and a charge control unit 84 that controls the output of the charger 82. The charger 82 is disposed upstream of the second nip portion N2 and in the vicinity of the outer side of the transfer fixing belt 67 on which the toner image is carried. The charge control unit 84 is controlled by the CPU 111 and causes the charger 82 to discharge a positive charge so that the transfer fixing belt 67 is positively charged. As a result, the transfer fixing belt 67 is positively charged. The charge control unit 84 adjusts the grid voltage Vg to change the discharge amount of the charger 82. In addition, when the potential of the transfer fixing belt 67 is higher than the potential of the secondary transfer roller 53, the second transfer bias voltage is not applied to the heating roller 63.

The toner that constitutes the toner image carried by the transfer fixing belt 67 is negatively charged. The transfer fixing belt 67 is positively charged upstream of the second nip portion N2, so that the toner that constitutes the toner image is pressed against the transfer fixing belt 67 as the toner image moves from the second nip portion N2 to the third nip portion N3.

The toner carried by the transfer fixing belt 67 is attracted to the transfer fixing belt 67 by surface tension. However, surface tension alone is insufficient to deform the toner. The potential of the transfer fixing belt 67 is determined so that the force with which the positively charged transfer fixing belt 67 attracts the toner is greater than or equal to the surface tension. This reduces the space between the toner particles that make up the toner image, and increases the contact area between the toner particles. When the toner that makes up the toner image carried by the transfer fixing belt 67 is positively charged, the charge control unit 84 causes the charger 82 to discharge a negative charge.

The pressure unit 69A is controlled by the CPU 111. The CPU 111 controls the charging control unit 84 of the pressure unit 69, and the greater the amount of toner carried by the transfer fixing belt 67, the greater the absolute value of the grid voltage Vg. This makes it possible to ensure that the force with which the multiple toners constituting the toner image are pressed against the transfer fixing belt 67 is equal to or greater than a predetermined value, even if the amount of toner fluctuates.

FIG. 17 is a block diagram showing an example of functions of a CPU included in an MFP in the second embodiment. The functions of CPU 111 included in MFP 100 in the second embodiment are functions that are realized by CPU 111 as CPU 111 executes an image formation control program. Referring to FIG. 17, CPU 111 includes a toner amount determination unit 201, a grid voltage determination unit 221, and an output control unit 223.

The toner amount determination unit 201 determines the amount of toner adhesion based on image data that is the subject of image formation by the image forming unit 140. The grid voltage determination unit 221 refers to the first grid voltage table and determines the voltage determined by the first grid voltage table for the adhesion amount determined by the toner amount determination unit 201. Note that, although an example using the first grid voltage table is described here, an arithmetic formula for determining the grid voltage Vg from the toner amount may also be used. The output control unit 223 controls the charging control unit 84 to discharge the charger 82 at the grid voltage Vg determined by the grid voltage determination unit 221.

Figure 18 is a diagram showing an example of the first grid voltage table. The first grid voltage table determines the grid voltage Vg with respect to the amount of adhesion. The first grid voltage table is obtained by experiment. The first grid voltage table determines the grid voltage Vg with a larger absolute value as the amount of adhesion increases.

<Fifth Modification>
Fig. 19 is a schematic diagram of a pressure unit included in an MFP in the fifth modified example. Referring to Fig. 19, the difference from the pressure unit 69A shown in Fig. 16 is that the charger 82 is disposed downstream of the second nip N2 and in the vicinity of the inner side opposite to the side on which the toner image is carried of the transfer-fixing belt 67. When the charge control unit 84 causes the charger 82 to discharge a positive charge, the transfer-fixing belt 67 is positively charged.

<Sixth Modification>
As described above, the more types of toner there are, the more times the toner is laminated, and therefore the greater the height of the toner image from the transfer-fixing belt 67. The MFP 100 in the sixth modified example determines the grid voltage Vg in accordance with the number of types of toner.

Figure 20 is a block diagram showing an example of the functions of a CPU included in an MFP in the sixth modified example. Referring to Figure 20, the functions differ from those shown in Figure 17 in that the toner amount determination unit 201 has been changed to a toner type number determination unit 201A, and the grid voltage determination unit 221 has been changed to a grid voltage determination unit 221A. The other functions are the same as those shown in Figure 17, so the description will not be repeated here.

The toner type number determination unit 201A determines the number of toner types carried on the transfer fixing belt 67. Specifically, the toner type number determination unit 201A calculates the number of toner types based on the pixel values of multiple pixels included in the image data. The grid voltage determination unit 221A refers to the second grid voltage table and determines the grid voltage Vg determined by the second grid voltage table for the number of toner types determined by the toner type number determination unit 201A. Note that, although an example using the first grid voltage table is described here, an arithmetic formula for determining a voltage value from the number of toner types may also be used.

Figure 21 shows an example of the second grid voltage table. The second grid voltage table determines the grid voltage Vg for the number of toner types. The second grid voltage table is obtained through experiments. The second grid voltage table determines a grid voltage Vg whose absolute value is larger as the number of toner types increases.

In the sixth modified example, the MFP 100 increases the absolute value of the grid voltage Vg of the charging control unit 84 as the number of toner types increases, so that the force with which the toner is pressed against the transfer fixing belt 67 can be made equal to or greater than a predetermined value even if the number of toner types varies. The grid voltage Vg may be determined by combining the amount of toner and the number of toner types.

<Seventh Modification>
Fig. 22 is a block diagram showing an example of functions of a CPU included in an MFP in the seventh modified example. Referring to Fig. 22, the functions differ from those shown in Fig. 17 in that the toner amount determination unit 201 is changed to a charge amount detection unit 201B, and that the grid voltage determination unit 221 is changed to a grid voltage determination unit 221B. The other functions are the same as those shown in Fig. 17, and therefore description thereof will not be repeated here.

The charge amount detection unit 201B detects the charge amount of the toner carried on the transfer fixing belt 67 based on the output value of a potential sensor arranged near the transfer fixing belt 67. The grid voltage determination unit 221B refers to the third grid voltage table and determines the grid voltage Vg determined by the third grid voltage table for the charge amount of the toner detected by the charge amount detection unit 201B. Note that, although an example using the third grid voltage table is described here, an arithmetic formula for determining the grid voltage Vg from the charge amount of the toner may also be used.

Figure 23 shows an example of the third grid voltage table. The third grid voltage table determines the grid voltage Vg relative to the charge amount of the toner. The third grid voltage table is obtained through experiments. The third grid voltage table determines a grid voltage Vg whose absolute value is larger as the charge amount of the toner is smaller.

In the seventh modified example, the MFP 100 increases the absolute value of the grid voltage Vg of the charge control unit 84 as the charge amount of the multiple toners that make up the toner image decreases, so that the force with which the multiple toners that make up the toner image are pressed against the transfer and fixing belt 67 can be made equal to or greater than a predetermined value even if the charge amount of the multiple toners that make up the toner image fluctuates.

<Eighth Modification>
It has been found through experiments that the charge amount of the multiple toners constituting a toner image varies depending on the absolute humidity. It has also been found through experiments that the charge amount of the toner varies depending on the number of sheets of paper on which images are formed by the image forming unit 140. When the total number of sheets of paper on which images are formed by the image forming units 20Y, 20M, 20C, and 20K increases, the carriers in the developers 24Y, 24M, 24C, and 24K provided in the image forming units 20Y, 20M, 20C, and 20K, respectively, wear out. When the carriers wear out, the toner becomes less likely to be electrostatically charged.

In the eighth variant, the MFP 100 determines the charge amount of the toner based on the absolute humidity and the total number of pages printed by the image forming unit 140.

Figure 24 is a block diagram showing an example of functions of a CPU included in an MFP in the eighth modified example. Referring to Figure 24, the differences from the functions shown in Figure 22 are that charge amount detection unit 201B has been removed, grid voltage determination unit 221B has been changed to grid voltage determination unit 221C, and humidity detection unit 207, temperature detection unit 209, and operation history acquisition unit 211 have been added. The other functions are the same as those shown in Figure 22, so the description will not be repeated here.

The humidity detection unit 207 controls the hygrometers arranged around the image forming units 20Y, 20M, 20C, and 20K to detect the humidity. The temperature detection unit 209 controls the thermometers arranged around the image forming units 20Y, 20M, 20C, and 20K to detect the temperature.

The operation history acquisition unit 211 acquires the operation history of each of the image forming units 20Y, 20M, 20C, and 20K. Here, the operation history is the number of sheets of paper on which images are formed by the image forming unit 140.

The grid voltage determination unit 221C determines the absolute temperature from the temperature detected by the temperature detection unit 209 and the humidity detected by the humidity detection unit 207. Then, the grid voltage determination unit 221C determines the grid voltage Vg defined in the fourth grid voltage table for the absolute temperature and the total number of printed sheets. The output control unit 223 causes the voltage application unit 85 to apply the determined grid voltage Vg.

Figure 25 shows an example of the fourth grid voltage table. The fourth grid voltage table determines the grid voltage Vg for a combination of absolute humidity, total number of printed pages, and toner. The fourth grid voltage table is obtained by experiment. The fourth grid voltage table determines a grid voltage Vg with a larger absolute value as the absolute humidity is higher, and determines a grid voltage Vg with a larger absolute value as the total number of printed pages is larger.

In the eighth modified example, the MFP 100 increases the absolute value of the grid voltage Vg as the absolute humidity increases, and increases the absolute value of the grid voltage Vg as the total number of printed pages increases, so that the force with which the multiple toner particles that make up the toner image are pressed against the transfer-fixing belt 67 can be made to be equal to or greater than a predetermined value even if the charge amount of the toner fluctuates.

In addition, instead of the total number of printed sheets, the total number of rotations of the developers 24Y, 24M, 24C, and 24K provided in the image forming units 20Y, 20M, 20C, and 20K, respectively, may be used.

In addition, the grid voltage determination unit 221C may determine the toner charge amount using a table that defines the toner charge amount for a combination of absolute temperature and total number of printed pages, instead of the fourth grid voltage table, and may further determine the grid voltage Vg to be applied to the charge control unit 84 from the toner charge amount using the third grid voltage table.

In addition, the grid voltage determination unit 221C may determine the grid voltage Vg to be applied to the charging control unit 84 from the absolute temperature using a table that defines the grid voltage Vg for the absolute temperature instead of the fourth grid voltage table.In addition, the grid voltage determination unit 221C may determine the grid voltage Vg to be applied to the charging control unit 84 from the total number of printed sheets using a table that defines the voltage for the total number of printed sheets instead of the fourth grid voltage table.

Third Embodiment
The MFP 100 in the third embodiment is a modified version of the pressure applying unit 69 of the MFP 100 in the first embodiment. The toner used in the MFP 100 in the third embodiment is a magnetic toner having magnetic properties. Therefore, the toner contains a known substance having magnetic properties. The substance having magnetic properties may be any known magnetic material.

26 is a schematic diagram of a pressure unit included in the MFP in the third embodiment. Referring to FIG. 26, the pressure unit 69B included in the MFP 100 in the third embodiment generates a magnetic field between the second nip portion N2 and the third nip portion N3 of the transfer fixing belt 67. Specifically, the pressure unit 69B includes a coil 82B that generates a magnetic field, and a magnetic field control unit 84B that controls the magnetic flux density of the magnetic field generated by the coil 82B. The coil 82B is disposed near the inner side opposite to the side on which the toner image is carried of the transfer fixing belt 67 between the second nip portion N2 and the third nip portion N3. The magnetic field control unit 84B is controlled by the CPU 111, and generates a magnetic field between the second nip portion N2 and the third nip portion N3 of the transfer fixing belt 67. The direction of the magnetic field may be any direction. The magnetic field control unit 84B changes the magnetic flux density of the magnetic field generated by the coil 82B by adjusting the voltage applied to the coil 82B.

The toner that constitutes the toner image carried by the transfer-fixing belt 67 is magnetic, so as the toner image moves from the second nip portion N2 to the third nip portion N3, the toner that constitutes the toner image is pressed against the transfer-fixing belt 67 by the magnetic force generated by the coil 82B.

The toner carried by the transfer fixing belt 67 is attracted to the transfer fixing belt 67 by surface tension. However, surface tension alone is insufficient to deform the toner. The magnetic force with which the coil 82B attracts the toner is determined by a magnetic flux density that is greater than or equal to the surface tension. The magnetic flux density is determined by the voltage applied to the coil 82B. This reduces the space between the toner particles that make up the toner image, and increases the contact area between the toner particles.

The pressure unit 69B is controlled by the CPU 111. The CPU 111 controls the magnetic field control unit 84B of the pressure unit 69B, and the greater the amount of toner carried by the transfer fixing belt 67, the greater the magnetic flux density. This makes it possible to ensure that the force with which the multiple toners constituting the toner image are pressed against the transfer fixing belt 67 is equal to or greater than a predetermined value, even if the amount of toner fluctuates.

FIG. 27 is a block diagram showing an example of functions of a CPU included in an MFP in the third embodiment. The functions of CPU 111 included in MFP 100 in the third embodiment are functions that are realized by CPU 111 as CPU 111 executes an image formation control program. Referring to FIG. 27, CPU 111 includes a toner amount determination unit 201, a magnetic flux density determination unit 225, and a magnetic flux density control unit 227.

The toner amount determination unit 201 determines the amount of toner adhesion based on image data that is the subject of image formation by the image forming unit 140. The magnetic flux density determination unit 225 refers to the first magnetic flux density table and determines the magnetic flux density determined by the first magnetic flux density table for the adhesion amount determined by the toner amount determination unit 201. Note that, although an example using the first magnetic flux density table is described here, an arithmetic formula for determining the magnetic flux density from the toner amount may also be used. The magnetic flux density control unit 227 controls the magnetic field control unit 84B to apply a voltage corresponding to the magnetic flux density determined by the magnetic flux density determination unit 225 to the coil 82B so that the magnetic flux density is generated from the coil 82B.

Figure 28 is a diagram showing an example of a first magnetic flux density table. The first magnetic flux density table determines the magnetic flux density with respect to the amount of adhesion. The first magnetic flux density table is obtained through experiments. The first magnetic flux density table determines a larger magnetic flux density as the amount of adhesion increases.

In the third embodiment, the MFP 100 increases the magnetic flux density of the magnetic field generated by the coil 82B as the amount of toner adhesion increases, so that the force with which the multiple toner particles that make up the toner image are pressed against the transfer and fixing belt 67 can be made to be equal to or greater than a predetermined value even if the amount of toner adhesion fluctuates.

<Ninth Modification>
As described above, the more types of toner there are, the more times the toner is laminated, and therefore the greater the height of the toner image from the transfer-fixing belt 67. The MFP 100 in the ninth modified example determines the magnetic flux density in accordance with the number of types of toner.

Figure 29 is a block diagram showing an example of the functions of a CPU included in an MFP in the ninth modified example. Referring to Figure 29, the functions differ from those shown in Figure 27 in that the toner amount determination unit 201 has been changed to a toner type number determination unit 201A, and that the magnetic flux density determination unit 225 has been changed to a magnetic flux density determination unit 225A. The other functions are the same as those shown in Figure 27, so the description will not be repeated here.

The toner type number determination unit 201A determines the number of toner types carried on the transfer fixing belt 67. Specifically, the toner type number determination unit 201A calculates the number of toner types based on the pixel values of multiple pixels included in the image data. The magnetic flux density determination unit 225A refers to the second magnetic flux density table and determines the magnetic flux density determined by the second magnetic flux density table for the number of toner types determined by the toner type number determination unit 201A. Note that, although an example using the second magnetic flux density table is described here, an arithmetic formula for calculating the magnetic flux density from the number of toner types may also be used.

Figure 30 shows an example of the second magnetic flux density table. The second magnetic flux density table defines the magnetic flux density for the number of toner types. The second magnetic flux density table is obtained through experiments. The second magnetic flux density table defines a larger magnetic flux density as the number of toner types increases.

In the ninth modified example, the MFP 100 increases the magnetic flux density of the magnetic field generated by the coil 82B as the number of toner types increases, so that the force with which the multiple toners constituting the toner image are pressed against the transfer fixing belt 67 can be made to be equal to or greater than a predetermined value even if the number of toner types varies.

In this embodiment, the MFP 100 has been described as an example of an image forming device, but it may also be a copying machine, a laser beam printer, a facsimile machine, etc.

The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is indicated by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

100 MFP, 20Y, 20M, 20C, 20K image forming unit, 21Y, 21M, 21C, 21K exposure device, 22Y, 22M, 22C, 22K charging roller, 23Y, 23M, 23C, 23K photoconductor drum, 24Y, 24M, 24C, 24K developing device, 25Y, 25M, 25C, 25K primary transfer roller, 27Y, 27M, 27C, 27K drum cleaning blade, 28 belt cleaning blade, 30 primary intermediate transfer belt, 31 timing roller, 33 driving roller, 34 driven roller, 35 paper feed cassette, 39 paper discharge tray, 41Y, 41M, 41C, 41K toner bottle, 42Y, 42M, 42C, 42K toner hopper, 50 transfer fixing device, 51 Intermediate transfer unit, 53 Secondary transfer roller, 59 Separation mechanism, 61 Transfer and simultaneous fixing unit, 63 Heat roller, 65 Driven roller, 66 Heat source, 67 Transfer and fixing belt, 69, 69A, 69B Pressure unit, 71 Pressure roller, 82 Charger, 82B Coil, 84 Charge control unit, 84B Magnetic field control unit, 85 Voltage application unit, 201 Toner amount determination unit, 201A Toner type number determination unit, 201B Charge amount detection unit, 203, 203A, 203B, 203C Voltage determination unit, 205 Voltage control unit, 207 Humidity detection unit, 209 Temperature detection unit, 211 Operation history acquisition unit, 213 Operation history acquisition unit, 221, 221A, 221B, 221C Grid voltage determination unit, 223 Output control unit, 225, 225A Magnetic flux density determination unit, 227 Magnetic flux density control unit.

Claims (14)

an imaging means for forming a toner image on an image carrier;
a fixing unit for heating the toner image formed by the imaging unit and then fixing the toner image on a recording medium;
a pressurizing unit that increases a contact area between a plurality of toner particles that constitute the toner image in a non-contact manner while the toner image is heated,
The toner particles constituting the toner image are charged,
The pressure applying means generates an electric field in a path along which the toner image moves,
the direction of the electric field generated by the pressure applying means is a direction in which the toner is subjected to a force in a direction in which the toner moves toward a transfer fixing member carrying the toner image,
The pressure applying means increases the strength of the electric field as the amount of toner constituting the toner image increases. a charge amount determining unit for determining a charge amount of the toner constituting the toner image,
2. The image forming apparatus according to claim 1 , wherein said pressure applying means changes the strength of said electric field in accordance with the amount of charge. an imaging means for forming a toner image on an image carrier;
a fixing unit for heating the toner image formed by the imaging unit and then fixing the toner image on a recording medium;
a pressurizing unit that increases a contact area between a plurality of toner particles that constitute the toner image in a non-contact manner while the toner image is heated,
The toner particles constituting the toner image are charged,
The pressure applying means generates an electric field in a path along which the toner image moves,
the direction of the electric field generated by the pressure applying means is a direction in which the toner is subjected to a force in a direction in which the toner moves toward a transfer fixing member carrying the toner image,
a charge amount determining unit for determining a charge amount of the toner constituting the toner image,
The pressure applying means increases the strength of the electric field as the charge amounts of the plurality of toner particles constituting the toner image decrease. Further comprising a humidity detection means for detecting humidity,
4. The image forming apparatus according to claim 2, wherein said charge amount determining means determines the charge amount of said toner based on humidity. 5. The image forming apparatus according to claim 2, wherein said charge amount determining means determines the charge amount of said toner based on an operation history of said image forming means. 6. The image forming apparatus according to claim 1, wherein said pressure applying means changes the strength of said electric field in accordance with the number of types of toner constituting said toner image. an imaging means for forming a toner image on an image carrier;
a primary intermediate transfer member onto which a toner image is transferred from the image carrier;
a secondary intermediate transfer member onto which a toner image is transferred from the primary intermediate transfer member;
a fixing unit for fixing a toner image transferred from the secondary intermediate transfer member and formed by the image forming unit onto a recording medium after heating the toner image;
a pressure applying unit provided in the fixing unit and configured to increase a contact area between a plurality of toner particles constituting the toner image in a non-contact manner while the toner image is heated,
The toner particles constituting the toner image have magnetic properties,
The pressure applying means generates a magnetic field in a path along which the toner image moves. an imaging means for forming a toner image on an image carrier;
a fixing unit for heating the toner image formed by the imaging unit and then fixing the toner image on a recording medium;
The toner image is composed of a plurality of toner particles having magnetic properties,
a pressure applying unit that applies a magnetic field to a path along which the toner image moves while the toner image is heated. 9. The image forming apparatus according to claim 7, wherein said pressure applying means changes the magnetic force of said magnetic field in accordance with an amount of toner constituting said toner image. The image forming apparatus according to claim 9, wherein the pressure means increases the magnetic force as the amount of toner constituting the toner image increases. 11. The image forming apparatus according to claim 7, wherein the pressure applying means changes the strength of the magnetic field in accordance with the number of types of toner constituting the toner image. An image forming method performed in an image forming apparatus, comprising:
The image forming apparatus includes an image forming unit for forming a toner image on an image carrier;
a primary intermediate transfer member onto which a toner image is transferred from the image carrier;
a secondary intermediate transfer member onto which a toner image is transferred from the primary intermediate transfer member;
a fixing unit for fixing a toner image transferred from the secondary intermediate transfer member and formed by the image forming unit onto a recording medium after heating the toner image;
The toner image is composed of a plurality of charged toner particles,
a pressure applying unit provided in the fixing unit and configured to generate an electric field in a path along which the toner image moves while the toner image is heated;
the direction of the electric field generated by the pressure applying means is a direction in which the toner is subjected to a force in a direction in which the toner moves toward a transfer fixing member carrying the toner image,
The image forming method includes a step of increasing the strength of the electric field generated by the pressure applying means as the amount of toner constituting the toner image increases . 13. The image forming method according to claim 12, further comprising a switching step of switching a state between a contact state in which the secondary intermediate transfer member is contacted with the primary intermediate transfer member at a first nip portion and with the fixing means at a second nip portion, and a separation state in which the secondary intermediate transfer member is separated from the primary intermediate transfer member and the fixing means. An image forming method performed in an image forming apparatus, comprising:
The image forming apparatus includes an image forming unit for forming a toner image on an image carrier;
a fixing unit for heating the toner image formed by the imaging unit and then fixing the toner image on a recording medium;
The toner image is composed of a plurality of toner particles having magnetic properties,
a pressure applying unit that applies a magnetic field to a path along which the toner image moves while the toner image is heated;
An image forming method comprising the step of controlling the intensity of the magnetic field generated by the pressure means.

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