JP2009211019A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus, such as a copying machine, a facsimile apparatus or a printer, which has a plurality of image bearers and performs transfer smoothly, while using a two-component developer including a relatively soft toner and a carrier, and to provide an image forming method using the image forming apparatus. <P>SOLUTION: The image forming apparatus uses a plurality of pressers pressing receptors 11, which are arranged corresponding to the respective image bearers 20Y, 20M, 20C, 20BK and onto which toner images on the image bearers 20Y, 20M, 20C, 20BK are transferred, to the corresponding image bearers 20Y, 20M, 20C, 20BK at a predetermined pressure, and a two-component developer in which toner comprises a binder resin and an additive, wherein the additive in the toner has a burial rate of ≥40%, and a pressure of the presser corresponding the image bearer 20BK bearing a black toner image among the image bearers 20Y, 20M, 20C, 20BK is made lower than those of the other pressers. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, a printer, and the like, which has a plurality of image carriers, and forms an image using a two-component developer containing toner and a carrier, and the image forming apparatus The present invention relates to an image forming method using.

  2. Description of the Related Art An image forming apparatus such as a copying machine, a facsimile machine, or a printer provided with an image carrier such as a photoconductor is known which has a plurality of image carriers for the purpose of color image formation. In this type of image forming apparatus, a toner image formed on a plurality of image carriers is directly transferred onto a sheet such as paper, and is transferred onto a sheet such as paper, and fixed by applying heat energy or the like. There is an intermediate transfer method in which the image is transferred, and then transferred to a sheet and fixed. The intermediate transfer method is sometimes called an indirect transfer method.

  For example, an intermediate transfer type color image forming apparatus includes a primary transfer unit that transfers a toner image on an image carrier to an intermediate transfer member, a secondary transfer unit that transfers a toner image on an intermediate transfer member onto a sheet, Primary cleaning means for cleaning the toner remaining on the image carrier after the toner image is transferred to the intermediate transfer member, and cleaning the toner remaining on the intermediate transfer member after the toner image is transferred to the sheet Secondary cleaning means and the like are provided. These cleaning means generally include a cleaning blade for scraping off toner remaining on the image carrier and the intermediate transfer member to be cleaned.

  As a developer used for image formation, a two-component developer containing a toner and a carrier is well known. In an image forming apparatus using a two-component developer, the toner in the developer is carried on an image carrier to form a toner image, the toner image is transferred to a sheet such as paper, and heat energy is applied to form the toner image. It is fixed on the sheet.

  The toner used for developing an electrostatic image is generally colored particles in which a binder, a charge control agent, and other additives are contained in a binder resin. There are chemical toners.

  In order to obtain high-quality and high-quality images, improvements have been made by reducing the particle diameter of the toner, but the pulverized toner produced by the usual kneading and pulverization method has an irregular particle shape. Therefore, it lacks the graininess and sharpness of the image. In addition, the pulverized toner has poor fluidity as a powder because of its shape, and requires a large amount of external additive as a fluidizing agent, and a low filling rate in the toner bottle reduces the size of the device. Has become an obstruction factor. In addition, the pulverization method has a limit on the particle size and cannot cope with further reduction in the particle size.

  Furthermore, in an image forming apparatus that creates a full-color image, a transfer process such as transfer of a toner image from an image carrier to an intermediate transfer member and transfer of a toner image from an intermediate transfer member to a sheet has become complicated. Due to the poor transferability due to the irregular shape such as pulverized toner, there has been a problem that the transferred image is missing and the toner consumption is large to compensate for it.

In order to compensate for the problem of the irregular toner, various methods for producing a chemical toner have been devised in order to produce a spherical toner.
Chemical toner production methods include a suspension polymerization method and a toner production method using an emulsion polymerization aggregation method. [Patent Document 1] discloses a method called a polymer dissolution suspension method. This method is a method in which a toner material is dispersed and dissolved in a volatile solvent such as a low-boiling organic solvent, and this is emulsified and formed into droplets in an aqueous medium containing a dispersant, and then the volatile solvent is removed.

  In [Patent Document 2] and [Patent Document 15], the resin used in the polymer dissolution suspension method has a low molecular weight, and the viscosity of the dispersed phase is lowered, thereby improving the dispersibility of the toner material and facilitating emulsification. A polymerization reaction is carried out in the interior to improve the fixing property.

  Unlike the suspension polymerization method and emulsion polymerization aggregation method, these methods are versatile resins that can be used, especially excellent in low-temperature fixability, and full color that requires transparency and smoothness of the image area after fixing. It is excellent in that a polyester resin useful for the process can be used. This production method is particularly called an ester extension polymerization method.

  However, the toner having a small particle diameter and a spherical shape has remarkably poor cleaning performance by the cleaning blade. In addition, although spherical toner has excellent transfer efficiency, there is also a problem that the transfer efficiency decreases as the adhesion with the photosensitive member increases as the particle size decreases.

  Therefore, in order to improve cleaning properties and transfer properties, it is disclosed in [Patent Document 3] that medium-sized inorganic fine particles having an average particle size of 20 to 40 nm are used in combination with external additives. In addition, by using inorganic fine particles having an average particle size of 100 nm or more as an external additive, cleaning properties are ensured, and further, small spacer particles caused by stress in the developing machine due to the spacer effect of the large particle inorganic particles. It is described in [Patent Document 4] to [Patent Document 6] to suppress the burying of the external additive.

However, these techniques are limited to suppressing the burying, and good cleaning properties and transferability can be obtained in the initial stage, but the external additive is also buried over time.
In recent years, demands for resource saving and power saving are strong, and reduction of fixing energy is indispensable. To reduce the fixing energy, the softening point of toner made of resin powder has been lowered. In order to lower the softening point of the toner, for example, it is preferable to select a relatively soft resin such as a polyester resin as the binder resin constituting the toner.

  Under such circumstances, even when the techniques described in [Patent Document 3] to [Patent Document 6] are used, when the polyester resin is used as the binder resin, the mechanical strength of the polyester resin is otherwise Therefore, it is difficult to maintain good cleaning properties and transferability over time. In view of this, it is possible to obtain a high-quality and high-quality image over a long period of time even if the toner uses a polyester resin having excellent low-temperature fixability and the external additive is easily embedded. An imaging system that can be used has been desired.

  On the other hand, in transferring the toner image from the image carrier to the intermediate transfer member and transferring the toner image from the intermediate transfer member to the sheet, it is required to transfer the toner image faithfully and stably to the transfer target.

  Therefore, in the color image forming apparatus of the intermediate transfer system as described above, a pressing unit that presses the intermediate transfer member against the surface of the image carrier may be provided at each transfer position. By this pressing means, pressure is applied between the image carrier and the intermediate transfer member at the time of primary transfer to increase transfer efficiency and prevent occurrence of transfer defects such as white spots. By using the pressing means, it is possible to suppress the unevenness of the transfer by suppressing the undulation of the intermediate transfer member at each transfer position and bringing the intermediate transfer member into uniform contact with the surface of the image carrier.

  However, when pressure is applied between the image carrier and the intermediate transfer member, stress concentrates on a part of the toner image on the intermediate transfer member, so that the center of the image on the intermediate transfer member, particularly the line portion or There is a case where a phenomenon in which the image at the center of the character portion is lost is transferred. As a countermeasure against such a transfer omission, as described in [Patent Document 7] to [Patent Document 9], the contact pressure of the primary transfer unit with respect to the image carrier is set within a certain range, and the predetermined pressure or more. There is known a technique for preventing the transfer dropout phenomenon from being applied to the toner image.

  However, the optimum contact pressure differs depending on the amount of toner adhesion such as a single color image and a multicolor overlaid image. If the amount is less than the optimum value or too much, a sufficient effect may not be obtained. , It turned out to be worse. In order to solve this problem, in [Patent Document 10] and [Patent Document 11], the contact pressure of the downstream transfer unit is made weaker than that of the upstream, or the contact pressure of the black transfer unit and the most upstream transfer unit is set differently. Proposes different technologies. As for the configuration in which the intermediate transfer member is brought into contact with the image carrier, in [Patent Document 12] and [Patent Document 13], the transfer unit including the intermediate transfer member is brought into contact with the image carrier by its own weight, thereby being stable. Techniques have been proposed for ensuring the contact state. [Patent Document 14] proposes a technique for ensuring a stable contact state by bringing a transfer paper into contact with an image carrier by its own weight.

JP-A-7-152202 JP-A-11-149179 Japanese Patent Laid-Open No. 3-100161 Japanese Patent No. 3328013 JP-A-9-319134 Japanese Patent No. 3056122 JP2003-098770 JP2000-162899 JP 2000-155476 A JP2002-014515 JP-A-2005-024936 JP 2006-301673 A JP 2004-264559 A Japanese Patent No. 2978367 Japanese Patent No. 3762078

  However, it has been found that adjusting the pressure on the image carrier of the primary transfer means affects not only the transfer dropout but also the secondary transfer performance. For example, when the sheet is paper with low smoothness, the transferability varies depending on the unevenness of the surface of the paper, so that the image becomes rough or blurred. In particular, in an image forming apparatus that forms an image using a two-component developer containing a soft toner, since the adhesion force of the toner is high, the pressure on the primary transfer unit is considered after considering the influence on the performance of the secondary transfer. Careful consideration is required.

  The present invention is an image forming apparatus such as a copying machine, a facsimile machine, a printer, and the like, and has a plurality of image carriers, and an image that is favorably transferred while using a two-component developer containing a relatively soft toner and a carrier. An object is to provide a forming apparatus and an image forming method using the same.

In order to achieve the above object, the invention described in claim 1 includes a plurality of image carriers, a transfer object that faces the plurality of image carriers, and onto which a toner image on the image carrier is transferred, and a plurality of image carriers. And a plurality of pressing means for pressing the transferred object against the corresponding image carrier with a predetermined pressing force, and includes toner and a carrier. In an image forming apparatus using a two-component developer, wherein the toner includes a binder resin and an additive, the following additive burying ratio X of the toner is 40% or more, and a plurality of the images The pressing force of the pressing unit corresponding to the image carrier that carries a black toner image among the supporting members is smaller than the pressing force of the other pressing units.
Additive burial rate X = {(A−B) / A} × 100
A: BET specific surface area of the toner (cm ² / g)
B: BET specific surface area (cm ² / g) after the toner was subjected to additive burying treatment

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the BET specific surface area of the toner base is 3 (m ² / g) or less.

  According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the pressing unit includes a pressing member that comes into contact with the transferred body, and the pressing member causes the vertical direction to be from the upper side to the lower side. The image receiving member is pressed against the corresponding image bearing member.

  According to a fourth aspect of the present invention, in the image forming apparatus according to the third aspect, the pressing unit presses the transferred body against the corresponding image carrier by the weight of the pressing member. .

  According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the pressing member has an Asker C hardness of 50 or less.

  According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, the transfer target is an intermediate transfer in which a toner image transferred from the image carrier is transferred to a sheet. A first transfer unit that transfers the toner image on the image carrier to the intermediate transfer member, and a second transfer unit that transfers the toner image on the intermediate transfer member to a sheet. Features.

  According to a seventh aspect of the present invention, in the image forming apparatus according to the sixth aspect, among the plurality of toner images transferred from the image carrier to the intermediate transfer body, the toner image transferred last to the intermediate transfer body. Is a black toner image.

  According to an eighth aspect of the present invention, in the image forming apparatus according to the sixth or seventh aspect, out of a plurality of toner images transferred from the image carrier to the intermediate transfer body, the toner image is first transferred to the intermediate transfer body. The toner image is a yellow toner image.

  According to a ninth aspect of the present invention, in the image forming apparatus according to the eighth aspect, out of a plurality of toner images transferred from the image carrier to the intermediate transfer body, the second and third transfer to the intermediate transfer body. The toner images are magenta and cyan toner images, respectively.

  According to a tenth aspect of the present invention, in the image forming apparatus according to any one of the first to ninth aspects, the binder resin is a polyester resin.

  According to an eleventh aspect of the present invention, there is provided an image forming method for forming an image using the image forming apparatus according to any one of the first to tenth aspects.

The present invention corresponds to each of a plurality of image carriers, a transfer member that faces the plurality of image carriers, and onto which a toner image is transferred, and the plurality of image carriers. A two-component developer comprising a plurality of pressing means for pressing the transfer target body against the corresponding image carrier with a predetermined pressing force, the toner including a toner and a carrier. In an image forming apparatus using a two-component developer containing a resin and an additive, the following additive burying ratio X of the toner is 40% or more, and a black toner image is carried among the plurality of image carriers. The pressing force of the pressing means corresponding to the image carrier is made smaller than the pressing force of the other pressing means, and the additive burying rate X = {(A−B) / A} × 100, and A Is the BET specific surface area (cm ² / g) of the toner, and B is the toner Is the BET specific surface area (cm ² / g) after the additive burying treatment, so that the transfer can be performed satisfactorily while using a two-component developer containing a relatively soft toner and carrier, and a good image It is possible to provide an image forming apparatus that can form images and can maintain fixability even when the fixing temperature is lowered, and can reduce energy consumption.

If the BET specific surface area of the base of the toner is 3 (m ² / g) or less, the additive coverage is increased when the additive is externally added to the toner, and the toner comes into contact with each other or the toner When the toner contacts with the transfer target, the probability that the additive is interposed between the toners or between the toners or between the toner and the transfer target decreases, and the transferability is improved. Transfer can be performed satisfactorily while using a two-component developer containing a carrier and a carrier, and good image formation can be performed, and fixability can be maintained even when the fixing temperature is lowered. An image forming apparatus capable of reducing energy consumption can be provided.

  The pressing means has a pressing member that contacts the transferred body, and presses the transferred body against the corresponding image carrier from the upper side to the lower side in the vertical direction by the pressing member. As a result, it is possible to omit the structure in which the pressing by the pressing member is performed against the gravity, and the pressing force of the transfer target with respect to the image carrier can be made uniform, and transfer defects can be prevented or suppressed. The pressure can be set low, transfer can be performed satisfactorily using a two-component developer containing a relatively soft toner and carrier, good image formation can be performed, and the fixing temperature Therefore, it is possible to provide an image forming apparatus that can maintain the fixing property even when the image quality is lowered and can reduce energy consumption.

  If the pressing means presses the transferred body against the corresponding image carrier by the weight of the pressing member, a configuration in which pressing by the pressing member is performed against gravity is omitted. The non-uniformity of the pressing force of the transfer target with respect to the image bearing member caused by the use can be prevented or suppressed to prevent or suppress the transfer failure, and the pressing force can be set low and the toner is relatively soft Transfer can be performed satisfactorily while using a two-component developer containing a carrier and a carrier, and good image formation can be performed, and fixability can be maintained even when the fixing temperature is lowered. An image forming apparatus capable of reducing energy consumption can be provided.

  If the pressing member has an Asker C hardness of 50 or less, the pressing member is liable to be crushed, the contact area with the transferred body is increased, and the pressing force of the transferred body against the image carrier is made uniform. Transfer failure can be prevented or suppressed, the pressing force can be set low, and transfer can be performed well while using a two-component developer containing a relatively soft toner and carrier. Therefore, it is possible to provide an image forming apparatus that can perform stable image formation, can maintain fixing performance even when the fixing temperature is lowered, and can reduce energy consumption.

  The transfer target is an intermediate transfer member on which a toner image transferred from the image carrier is transferred to a sheet, and a first transfer unit that transfers the toner image on the image carrier to the intermediate transfer member; And a second transfer means for transferring the toner image on the intermediate transfer member to the sheet, the primary transfer and the secondary transfer using a two-component developer containing a relatively soft toner and a carrier. An image forming apparatus capable of achieving good image formation, performing good image formation, maintaining the fixability even when the fixing temperature is lowered, and reducing energy consumption is provided. can do.

  Of the toner images transferred from the plurality of image carriers to the intermediate transfer member, the toner image transferred last to the intermediate transfer member is a black toner image. Using a two-component developer containing a carrier, a black toner image can be transferred at the end, making it possible to perform primary transfer and secondary transfer well, and it is easy to stand out and does not overlap with other colors. By doing so, it is possible to provide an image forming apparatus that can perform particularly good image formation, can maintain the fixing property even when the fixing temperature is lowered, and can reduce energy consumption. .

  Of the toner images transferred from the plurality of image carriers to the intermediate transfer member, the first toner image transferred to the intermediate transfer member is a yellow toner image. While using a two-component developer containing a carrier, primary transfer and secondary transfer can be performed satisfactorily, and good image formation can be performed by first transferring an inconspicuous yellow toner image, In addition, it is possible to provide an image forming apparatus capable of maintaining the fixability even when the fixing temperature is lowered and reducing the energy consumption.

  Of the toner images transferred from the plurality of image carriers to the intermediate transfer member, the second and third toner images transferred to the intermediate transfer member are magenta and cyan toner images, respectively. Then, while using a two-component developer containing a relatively soft toner and carrier, primary transfer and secondary transfer can be performed satisfactorily, and excellent image formation is achieved by performing transfer in a color order that is not noticeable. In addition, it is possible to provide an image forming apparatus that can maintain the fixing property even when the fixing temperature is lowered and can reduce energy consumption.

  If the binder resin is a polyester resin, transfer can be carried out satisfactorily while using a two-component developer containing a relatively soft toner containing a polyester resin and a carrier, and good image formation can be achieved. In addition, it is possible to provide an image forming apparatus that can be performed and that can easily maintain the fixing property even when the fixing temperature is lowered and can reduce the energy consumption.

  The present invention resides in an image forming method for forming an image using such an image forming apparatus. Therefore, transfer can be performed satisfactorily while using a two-component developer containing a relatively soft toner and a carrier. It is possible to provide an image forming method capable of forming images, maintaining the fixing property even when the fixing temperature is lowered, and reducing energy consumption.

(First embodiment)
FIG. 1 shows an outline of an image forming apparatus according to a first embodiment to which the present invention is applied. The image forming apparatus 100 is a multifunction peripheral of a copying machine, a printer, and a facsimile machine, and can perform full color image formation. When used as a printer, the image forming apparatus 100 performs an image forming process based on an image signal corresponding to image information received from the outside. This is the same when the image forming apparatus 100 is used as a facsimile.

  The image forming apparatus 100 can form an image on a sheet-like recording medium using not only plain paper generally used for copying, but also OHP sheets, cardboard, cardboard, cardboard, and envelopes. It is. The image forming apparatus 100 is also a double-sided image forming apparatus capable of forming images on both sides of a sheet as a recording medium.

  The image forming apparatus 100 includes a main body 101 that occupies a center position in the vertical direction, a reading device 21 that is positioned above the main body 101 and that reads a document, and a document that is loaded and loaded with a document toward the reading device 21. An automatic document feeder 22 called ADF, and a sheet feeding table on which sheets conveyed below the photosensitive drums 20Y, 20M, 20C, 20BK and the intermediate transfer belt 11 are positioned below the main body 101. As a sheet feeding device 23.

  The image forming apparatus 100 includes a plurality of photosensitive drums 20Y, 20M, 20C, and 20BK that can form images as images corresponding to colors separated into yellow, magenta, cyan, and black, respectively. 1 is a tandem type image forming apparatus that employs a tandem structure in which the two are arranged side by side. The photosensitive drums 20Y, 20M, 20C, and 20BK have the same diameter, and an intermediate transfer as an intermediate transfer body that is an endless belt that is an almost endless belt disposed in the inside of the main body 101 of the image forming apparatus 100. They are arranged at equal intervals on the outer peripheral surface side of the belt 11, that is, on the image forming surface side.

  The intermediate transfer belt 11 is movable in the arrow A1 direction, which is the clockwise direction in the figure, while facing the respective photosensitive drums 20Y, 20M, 20C, and 20BK. The visible image, that is, the toner image formed on each of the photosensitive drums 20Y, 20M, 20C, and 20BK is superimposed and transferred to the intermediate transfer belt 11 that moves in the direction of the arrow A1, and is then collectively transferred to the sheet. It has become. As described above, the image forming apparatus 100 employs an intermediate transfer method, in other words, an indirect transfer method.

  In the superimposing transfer to the intermediate transfer belt 11, the toner images formed on the photosensitive drums 20Y, 20M, 20C, and 20BK are superimposed on the same position on the intermediate transfer belt 11 in the process in which the intermediate transfer belt 11 moves in the A1 direction. A primary transfer device serving as a first transfer unit serving as a transfer charger disposed at a position facing each of the photosensitive drums 20Y, 20M, 20C, and 20BK with the intermediate transfer belt 11 interposed therebetween. By applying a voltage from a certain primary transfer roller 12Y, 12M, 12C, or 12BK, the timing is shifted from the upstream side toward the downstream side in the A1 direction, and the position immediately below each photosensitive drum 20Y, 20M, 20C, or 20BK, that is, the transfer position. Is done.

Although the manufacturing method and material of the intermediate transfer belt 11 are not particularly limited, polyimide resin is most suitable as the material.
When the intermediate transfer belt 11 is made of polyimide, the manufacturing method is preferably as follows.

  That is, carbon black is dispersed in a polyamic acid solution, the dispersion is poured into a metal drum and dried, and then the film peeled off from the metal drum is stretched at a high temperature to form a polyimide film. An endless belt made of polyimide resin is produced by cutting into a large size.

  Film forming is a common method. A polymer solution in which carbon black is dispersed is poured into a cylindrical mold, and the cylindrical mold is rotated while being heated to 100 to 200 ° C. to form a film by centrifugal molding.

The film thus obtained is demolded in a semi-cured state and covered with an iron core, and the polyimide transfer reaction is allowed to proceed at 300 to 450 ° C. to be cured to obtain the intermediate transfer belt 11.
The characteristics of the intermediate transfer belt 11 can be adjusted by changing the carbon amount, the firing temperature, the curing speed, and the like. By this method, the volume resistivity and the surface resistivity can also be adjusted.

  For measurement of volume resistivity and surface resistivity, which will be described later, a Hiresta UP (MCP-HT450) high resistance meter manufactured by Mitsubishi Chemical is used, and a URS probe (MCP-HTP14) of the same company is used as a probe.

  The photosensitive drums 20Y, 20M, 20C, and 20BK are arranged in this order from the upstream side in the A1 direction. The photosensitive drums 20Y, 20M, 20C, and 20BK are provided in image stations 60Y, 60M, 60C, and 60BK, respectively, for forming yellow, magenta, cyan, and black images.

  The image forming apparatus 100 is arranged opposite to the image forming unit 60 as an image forming unit constituted by four image stations 60Y, 60M, 60C, and 60BK, and below the respective photosensitive drums 20Y, 20M, 20C, and 20BK. A transfer belt unit 10 as an intermediate transfer unit provided with the intermediate transfer belt 11, and disposed so as to be opposed to the intermediate transfer belt 11 and in contact with the intermediate transfer belt 11. A secondary transfer roller 17 that is a secondary transfer device as a second transfer unit that rotates in the same direction as the intermediate transfer belt 11 and transfers a toner image on the intermediate transfer belt 11 to a sheet, and a secondary transfer roller 17 A conveyance device 76 that conveys a sheet on which the toner image on the intermediate transfer belt 11 has been transferred, and a toner image that is disposed to face the intermediate transfer belt 11. And an intermediate transfer belt cleaning device 14 for cleaning the upper intermediate transfer belt 11 after being transferred to the sheet.

  The image forming apparatus 100 also includes an optical scanning device 8 that is an exposure unit that is a writing unit as an optical writing device that is a writing unit disposed facing the image stations 60Y, 60M, 60C, and 60BK. The sheet conveyed from the feeding device 23 is transferred between the intermediate transfer belt 11 and the secondary transfer roller 17 at a predetermined timing in accordance with the toner image formation timing by the image stations 60Y, 60M, 60C, and 60BK. And a sensor (not shown) for detecting that the leading edge of the sheet has reached the registration roller pair 13.

  The image forming apparatus 100 also includes a fixing device 6 as a belt-fixing type fixing unit for fixing a toner image on the sheet when the sheet conveyed by the conveying device 76 enters, and a fixed sheet outside the main body 101. The sheet discharge unit 79 includes a sheet discharge path for discharging the sheet and a reversing path for conveying the sheet toward the registration roller pair 13 again. The sheet discharge unit 79 conveys the sheet to any one of the paths, and the sheet discharge unit 79 forms an image on one surface. And a duplex unit 96 that is a sheet reversing device as a sheet refeeding unit that switches the sheet back and reverses the sheet when the sheet is conveyed to the reversing path and conveys the sheet again toward the registration roller pair 13. ing.

  The image forming apparatus 100 also includes a paper discharge tray 75 that is disposed outside the main body 101 and stacks sheets on which images have been formed, a manual sheet feeding device 33 that is disposed on the right side surface of the main body 101 in FIG. An operation panel (not shown) that operates the apparatus 100 and a control unit (not shown) that controls the operation of the entire image forming apparatus 100 are provided.

  In addition to the intermediate transfer belt 11, the transfer belt unit 10 includes primary transfer rollers 12Y, 12M, 12C, and 12BK, a driving roller 72, a transfer inlet roller 73, and a tension roller 74 around which the intermediate transfer belt 11 is wound. And driving means (not shown).

  The transfer belt unit 10 is swung counterclockwise around the drive roller 72 in FIG. 1 by the driving means, so that the photosensitive drum 20BK is kept in contact with the intermediate transfer belt 11 and the photosensitive drum 20BK is maintained. 20Y, 20M, and 20C can be separated from the intermediate transfer belt 11.

  The state shown in FIG. 1 is a case where full-color image formation is performed, and the photosensitive drums 20Y, 20M, and 20C are separated from the intermediate transfer belt 11 when black single-color image formation is performed. The operation of the driving means is performed by the control means.

  Since the image forming apparatus 100 includes the intermediate transfer belt cleaning device 14, in principle, black residual toner is also transferred from the intermediate transfer belt 11 to the photosensitive drums 20 </ b> Y and 20 </ b> M even when a black monochrome image is formed. Therefore, the photosensitive drums 20Y, 20M, and 20C need not be separated from the intermediate transfer belt 11, but the intermediate transfer belt is cleaned for some reason. In consideration of the case where the residual toner cannot be completely removed even with the apparatus 14, the photosensitive drums 20Y, 20M, and 20C are separated from the intermediate transfer belt 11 when a black monochrome image is formed. This is because the black toner is easily noticeable and has a great influence on the image when it adheres to the photosensitive drums 20Y, 20M, and 20C.

  For the same reason, the photosensitive drum 20BK for forming a black image is disposed downstream of the other photosensitive drums 20Y, 20M, and 20C in the A1 direction. When full-color image formation is performed, the black toner is passed through the intermediate transfer belt 11 so that the intermediate transfer belt 11 and the photosensitive drums 20Y, 20M, 20C, and 20BK are kept in contact with each other. In order to prevent or suppress adhesion to 20M and 20C, it is most preferable that the photosensitive drum 20BK is disposed at the most downstream position in the A1 direction and the transfer to the intermediate transfer belt 11 is performed last. is there.

  The registration roller pair 13 is grounded. Generally, in the intermediate transfer system such as the image forming apparatus 100, the paper powder is difficult to move to the image carrier such as the photosensitive drums 20Y, 20M, 20C, and 20BK. This is because there is little need to consider this, and there is no problem even if it is grounded.

However, a bias may be applied to the registration roller pair 13 in order to remove paper dust from the sheet. For example, when a bias is applied using a conductive rubber roller having a diameter of 18 mm and a surface covered with a conductive NBR rubber having a thickness of 1 mm, the volume resistance of the rubber material is about 10 ⁹ Ωcm, and the toner transfer side (surface Side) and a voltage of about +200 V is applied to the back side of the sheet.

  When applying a voltage in this way, a DC bias is generally applied as the applied voltage, but an AC voltage having a DC offset component can also be applied to charge the sheet more uniformly.

  Since the sheet surface after passing through the registration roller pair 13 to which a bias is applied is slightly charged to the negative side, no voltage is applied to the registration roller pair 13 in the transfer from the intermediate transfer belt 11 to the sheet. In some cases, the transfer conditions change and the transfer conditions change.

The conveying device 76 includes an endless conveying belt 5 that conveys a sheet, and a driving roller 15 and a driven roller 16 around which the conveying belt 5 is wound.
The secondary transfer roller 17 faces the transfer entrance roller 73 and is in pressure contact with the intermediate transfer belt 11 with the transfer entrance roller 73. The secondary transfer roller 17 may be configured to employ a non-contact charger, or is used as a member common to the driven roller 16 of the conveying device 76 and conveys the sheet toward the fixing device 6. A transfer conveyance unit as indicated by 17 may be configured.

  The optical scanning device 8 scans and exposes the surfaces to be scanned formed by the surfaces of the photosensitive drums 20Y, 20M, 20C, and 20BK, and forms a laser beam based on an image signal for forming an electrostatic latent image. The laser beam is scanned by a polygon mirror (not shown) that emits a laser beam (not shown), a polygon mirror (not shown) that scans the laser beam emitted by the light source, and a polygon motor (not shown) that rotates the polygon mirror. The laser beam is imaged on the photosensitive drums 20Y, 20M, 20C, and 20B, and a large number of optical elements (not shown) for scanning are provided. The main scanning direction in which the laser beam is scanned on the photosensitive drums 20Y, 20M, 20C, and 20B in the optical scanning device 8 is a direction perpendicular to the paper surface in FIG.

  The fixing device 6 includes a heating roller 62 having a heat source therein, a fixing belt 64 wound around the heating roller 62, a fixing roller 65 wound around the fixing belt 64 together with the heating roller 62, and the fixing roller 65. And a pressure roller 63 that presses the fixing belt 64. By passing a sheet carrying a toner image through a fixing portion that is a pressure contact portion between the fixing belt 64 and the pressure roller 63, heat and pressure can be obtained. By this action, the carried toner image is fixed on the surface of the sheet.

  The paper discharge unit 79 conveys the fixed sheet conveyed from the fixing device 6 toward the duplex unit 96, the paper discharge roller 98 that discharges the sheet to the outside of the main body 101, and the fixed sheet. There is provided a switching claw 94 for switching between discharging to the outside of the main body 101 through the paper discharge path with the conveying roller 97 or guiding it into the reverse path with the paper discharge roller 98 to enter the duplex unit 96.

  The duplex unit 96 includes a tray 92 on which a sheet having an image formed on one surface, which has been conveyed from the sheet discharge unit 79, is temporarily stacked, a reverse roller 93 that switches back the sheet on the tray 92, and a reverse roller 93. A sheet feed roller 95 for feeding the switched back sheet toward the registration roller 13 is provided.

The sheet feeding device 23 includes a paper bank 26 having a paper feeding cassette 25 on which a large number of sheets are stacked, and a paper feeding roller that contacts the upper surface of the topmost sheet among the sheets stacked on the paper feeding cassette 25. A conveyance roller 24, a separation roller 27 that separates the sheets fed by the feeding roller 24 one by one, and a conveyance that conveys the sheet fed by the sheet feeding roller 24 and the separation roller 27 toward the registration roller pair 13. A roller 28 and a paper feed path 29 through which a sheet conveyed by the conveying roller 28 passes are provided.
The sheet feeding path 29 is provided so as to be continuous from the sheet feeding device 23 into the main body 101, and a conveyance roller 28 is also disposed in the sheet feeding path 29 in the main body 101.

  In the sheet feeding device 23, the feeding roller 24 is rotationally driven in the counterclockwise direction in the drawing, and the separation roller 27 acts to guide the uppermost sheet into the sheet feeding path 29 and rotate the conveying roller 28. Thus, the sheet is fed toward the registration roller pair 13 and the conveyed sheet is abutted against the registration roller pair 13 and stopped.

  The manual sheet feeding device 33 includes a manual feed tray 34 on which sheets are stacked, a feed roller 35 as a feed roller that contacts the upper surface of the uppermost sheet among the sheets stacked on the manual feed tray 34, and a feed roller 35. , A separation roller 36 that separates the sheets fed out one by one, and a paper sensor that detects that the sheet is placed on the manual feed tray 34.

  In the manual sheet feeding device 33, the feeding roller 35 is driven to rotate in the clockwise direction in the drawing, and the separation roller 36 acts to guide the uppermost sheet into the sheet feeding path 29 on the main body 101 side and the registration roller. The sheet fed toward the pair 13 is stopped by abutting against the registration roller pair 13.

  The reading device 21 includes a contact glass 21a on which a document is placed, a light source (not shown) that irradiates light on the document placed on the contact glass 21a, and a first light source (not shown) that reflects light reflected from the light source. A first traveling body 21b that includes a reflector and travels in the left-right direction in FIG. 1; a second traveling body 21c that includes a second reflector (not shown) that reflects light reflected by the reflector of the first traveling body 21b; An image forming lens 21d for forming an image of light from the second traveling body 21c, a reading sensor 21e for receiving the light passing through the image forming lens 21d and reading the contents of the document are provided.

  The automatic document feeder 22 has a document table 22a on which a document is placed. The automatic document feeder 22 is rotatable with respect to the reading device 21, and exposes the contact glass 21a when rotated upward. Yes. When copying using the image forming apparatus 100, the document is set on the document table 22a of the automatic document feeder 22, or the automatic document feeder 22 is turned upward to manually move the document onto the contact glass 21a. After the document is placed on the automatic document feeder 22, the automatic document feeder 22 is closed and the document is pressed against the contact glass 21a.

The operation panel has a start button for starting copying, a numeric keypad for inputting the number of copies, a mode selection key for selecting an image forming mode such as whether to perform full color image formation or black single color image formation, etc. It has.
The control means includes a CPU, a memory as a storage means, and the like.

  Regarding the image stations 60Y, 60C, 60M, and 60BK, the configuration will be described as a representative of the configuration of the image station 60Y including the photosensitive drum 20Y. Since the configuration of other image stations is substantially the same, in the following description, for the sake of convenience, reference numerals corresponding to the reference numerals assigned to the configuration of the image station including the photosensitive drum 20Y are used for other image stations. A detailed description will be omitted as appropriate for the station configuration, and those with Y, C, M, and K at the end of the reference numerals are used to form yellow, cyan, magenta, and black images, respectively. It is assumed that this is the configuration.

  As shown in FIG. 2, the image station 60Y provided with the photosensitive drum 20Y is in contact with the intermediate transfer belt 11 around the photosensitive drum 20Y along its rotational direction B1, which is counterclockwise in the drawing. A secondary transfer roller 12Y, a primary transfer roller 12Y including a primary transfer roller 12Y, the primary transfer roller 12Y pressing the intermediate transfer belt 11 against the photosensitive drum 20Y with a predetermined pressing force, a cleaning device 40Y as a cleaning unit, It has a charging device 30Y as a charging unit as a charging means, a developing device 50Y as a developing unit as a developing means, and a static eliminating device as a static eliminating means (not shown).

  The photosensitive drum 20Y, the cleaning device 40Y, the charging device 30Y, the developing device 50Y, and the charge removal device are integrated to form a process cartridge 95Y. The process cartridge 95Y can be pulled out of the main body 101 along a guide rail (not shown) fixed to the main body 101, can be pushed into the main body 101, and is detachably installed on the main body 101. .

  When the process cartridge 95Y is pushed into the main body 101, it is loaded and positioned at a predetermined position suitable for image formation. Making a process cartridge in this way is very preferable because it can be handled as a replacement part, so that the maintainability is remarkably improved.

  The process cartridge 95Y is configured by integrating at least the photosensitive drum 20Y and the developing device 50Y among the photosensitive drum 20Y, the cleaning device 40Y, the charging device 30Y, the developing device 50Y, and the charge eliminating device. The unit is detachably installed on the main body 101.

  The charging device 30Y includes a charging roller 31Y that rotates in contact with the surface of the photosensitive drum 20Y, and a cleaning roller 32Y that rotates in contact with the charging roller 31Y. The charging roller 31Y is connected to a voltage applying means (not shown) for applying a bias of an alternating current component to a direct current, and in the charging region facing the photosensitive drum 20Y, the surface of the photosensitive drum 20Y is neutralized at the same time as the charging roller 31Y. The polarity is charged.

The cleaning roller 32Y is driven to rotate by the charging roller 31Y to clean the charging roller 31Y.
As described above, in this embodiment, a charging system using a contact roller is used. However, the charging system may use a proximity roller or a non-contact type such as a scorotron system. It may be.

  The primary transfer roller 12Y is a member that functions as a pressing member that contacts the intermediate transfer belt 11 and presses the intermediate transfer belt 11 against the photosensitive drum 20Y. The primary transfer roller 12Y has a shaft 37Y that is rotatably supported by the main body 101 and serves as a rotation center of the primary transfer roller 12Y. Although not shown, the primary transfer roller 12Y has a metal cored bar and an elastic layer formed so as to cover the outer peripheral surface of the cored bar, and the cored bar has a shaft 37Y. is doing. The primary transfer roller 12Y has an Asker C hardness of 50 or less due to the elastic layer. A predetermined voltage suitable for primary transfer is applied to the primary transfer roller 12Y by a bias application unit and a bias control unit having a power source (not shown). The shaft 37Y extends in a direction perpendicular to the paper surface in FIG. 1, that is, the main scanning direction, and this direction is referred to as an axial direction of the primary transfer roller 12Y.

  In addition to the primary transfer roller 12Y, the pressing means 18Y is positioned between the shaft 37Y and the main body 101, and a pressing spring 19Y as a biasing member that biases the shaft 37Y toward the intermediate transfer belt 11, and a shaft And supporting means (not shown) for supporting the 37Y in a vertically displaceable manner. The pressing unit 18Y presses the intermediate transfer belt 11 against the photosensitive drum 20Y from the lower side to the upper side in the vertical direction by the primary transfer roller 12Y by the urging force of the pressing spring 19Y. The magnitude of the pressing force of the pressing spring 19Y against the intermediate transfer belt 11 against the photosensitive drum 20Y will be described later as the primary transfer pressure shown in FIGS.

  As shown in FIG. 2, the optical scanning device 8 shown in FIG. 1 irradiates a region between the charged region and the developing region on the photosensitive drum 20Y with a laser beam L that is light-modulated according to image information. The surface of the photosensitive drum 20Y after being charged by the charging roller 31Y is exposed to form an electrostatic latent image that is visualized as a yellow toner image by the developing device 50Y.

  The cleaning device 40Y has a cleaning case 43Y having an opening at a portion facing the photoconductor drum 20Y, and scrapes off unnecessary materials such as residual toner, carrier, and paper dust on the photoconductor drum 20Y in contact with the photoconductor drum 20Y. The brush roller 45Y as a rotating brush to be cleaned and the rotating direction B1 of the photosensitive drum 20Y contact the photosensitive drum 20Y at a position downstream of the brush roller 45Y and scrape off unnecessary matter on the photosensitive drum 20Y. And a cleaning blade 41Y as a blade for cleaning.

  The cleaning device 40Y is also rotatably supported by the cleaning case 43Y, and scrapes the brush roller 45Y and the cleaning blade 41Y, and removes unnecessary materials such as waste toner generated by removal to a waste toner tank (not shown). And a discharge screw 42Y constituting a part of a waste toner path (not shown) for transporting the toner.

  The developing device 50Y includes a developing case 55Y having an opening at a portion facing the photoreceptor drum 20Y, and a developer carrying member disposed in close proximity to the photoreceptor drum 20Y so as to face the photoreceptor drum 20Y from the opening. A developing roller 51Y as a body, and a developing blade 52Y as a doctor as a regulating member that regulates the developer on the developing roller 51Y to a certain height.

  The developing device 50Y is also disposed below the developing case 55Y so as to face each other, and is driven to rotate in opposite directions to stir the developer and to supply the developer to the developing roller 51Y. The first conveying screw 53Y and the second agitating screw 54Y as supply members, the partition wall 57Y provided between the first conveying screw 53Y and the second conveying screw 54Y, and the first conveying screw partitioned by the partition wall 57Y. 53Y, and a second storage chamber 59Y that constitutes a developer storage section as a developer storage device that stores the second transport screw 54Y.

  The developing device 50Y also includes a toner hopper 80Y that stores yellow toner, and a T sensor as a toner concentration measuring sensor that is provided at the bottom of the second storage chamber 59Y and serves as a toner concentration detecting unit that measures the toner concentration in the developer. A toner concentration detection sensor 56Y as a toner concentration detection device and a double-sided tape 86Y as an attachment member to which the toner concentration detection sensor 56Y is attached to the second storage chamber 59Y are provided.

  The developing device 50Y also includes a bias applying unit (not shown) that applies a developing bias of a DC component, a developing driving unit (not shown) that drives the developing roller 51Y, and the first transport screw 53Y and the second transport screw 54Y in opposite directions. (Not shown) for supplying toner to the second storage chamber 59Y from the toner hopper 80Y.

  The developing roller 51Y includes a magnet roller 81Y as a magnetic field generating unit, and a nonmagnetic developing sleeve 82Y that includes the magnet roller 81Y and is driven in the C1 direction, which is the clockwise direction in the drawing, by the developing driving unit.

  Although not shown, the magnet roller 81Y has a plastic roller fixed to the developing case 55Y and a magnet block that is a plurality of magnets forming a plurality of magnetic poles embedded in the plastic roller 84Y.

  The developing sleeve 82Y is rotatably supported by the developing case 55Y and the magnet roller 81Y. A developing bias of an appropriate size is applied between the developing sleeve 82Y and the photosensitive drum 20Y by a bias applying unit. The gap between the developing sleeve 82Y and the photosensitive drum 20Y in the developing region, that is, the developing gap is set to be 0.3 ± 0.05 mm.

  The developing blade 52Y is formed of a SUS material. A gap between the developing sleeve 82Y and the developing blade 52Y, that is, a doctor gap is set to be 0.5 ± 0.04 mm.

The developer is a two-component developer containing toner and carrier.
The carrier is a magnetic carrier, and has a core material and a resin coating layer formed on the surface of the core material. The resin coating layer contains conductive particles in which a conductive coating layer comprising a tin dioxide layer and an indium oxide layer containing tin dioxide provided on the tin dioxide layer is provided on the surface of the substrate particles. .
Details of the toner will be described later.

  The toner concentration in the developer is controlled to be within a predetermined range of about 4 to 11% by weight based on detection by the toner concentration detection sensor 56Y, and will be described later by the control means. Is always kept at an appropriate value, and contributes to obtaining a high-quality image. The toner density decreases with the consumption of toner by development, and when the toner density detection sensor 56Y detects that the toner density falls below the lower limit value of the predetermined range described above, the toner replenishing means from the toner hopper 80Y. Toner is supplied to the second storage chamber 59Y.

  The first transport screw 53Y and the second transport screw 54Y are disposed so as to extend in the width direction of the developing roller 51Y, in other words, in the direction perpendicular to the paper surface in FIG. 2, which is the longitudinal direction of the developing roller 51Y.

  The first transport screw 53Y is rotationally driven by the transport driving means, and supplies the developer in the first storage chamber 58Y to the developing roller 51Y while transporting the developer from the back side to the front side in FIG. The developer transported to the vicinity of the end in the first storage chamber 58Y by the first transport screw 53Y enters the second storage chamber 59Y through an opening (not shown) formed in the partition wall 57Y.

  In the second storage chamber 59Y, the second transport screw 54Y is rotationally driven by the transport drive means to transport the developer sent from the first storage chamber 58Y in the opposite direction to the first transport screw 53Y. . At this time, when the toner is replenished from the toner hopper 80Y, the replenished toner is conveyed while being agitated and mixed in the developer. The developer transported to the vicinity of the end of the second storage chamber 59Y by the second transport screw 54Y returns to the first storage chamber 58Y through another opening (not shown) provided in the partition wall 57Y.

  The toner thus supplied is agitated and mixed by the first conveying screw 53Y and the second conveying screw 54Y while being agitated and conveyed with the developer, frictionally charged, and supplied and carried on the developing roller 51Y.

  The developing roller 51Y whose developer thickness is regulated by the developing blade 52Y and whose layer thickness is regulated is developed in the developing area between the developing roller 51Y and the photosensitive drum 20Y by the rotation and the developing bias by the bias applying means. An appropriate amount of developer is carried by the developing blade 52Y, and the yellow toner in the developer is electrostatically transferred to the electrostatic latent image formed on the surface of the photosensitive drum 20Y, and the electrostatic latent image is formed. A visible image is formed as a yellow toner image.

The developer that has consumed the yellow toner by the development is returned to the developing device 50Y as the developing roller 51Y rotates.
In this embodiment, the DC component developing bias is applied by the bias applying means. However, the developing bias may be an AC component, or an AC component superimposed on the DC component.

  Thus, in the developing device 50Y, the developer stirred and transported by the first transport screw 53Y and the second transport screw 54Y is pumped up by the magnetic force of the magnet roller 81Y and carried on the developing sleeve 82Y, and the photosensitive drum 20Y. The toner is supplied to the latent image on the photosensitive drum 20Y and developed. The developer that has consumed the toner after development is released from the surface of the developing sleeve 82Y into the first storage chamber 58Y, and is stored in the first storage chamber 58Y and the second storage chamber 59Y by the first transport screw 53Y and the second transport screw 54Y. The cycle in which the developer is stirred and pumped up to the surface of the developing sleeve 82Y is repeated. The magnet block is arranged to repeat such a cycle.

  In such a cycle, since the toner in the developer is consumed, the toner density is lowered. The decrease in toner density is detected by the toner density detection sensor 56Y.

  The toner concentration detection sensor 56Y measures the toner concentration based on the magnetic permeability of the developer, and its output is taken out as a voltage Vout and input to the control means, and the toner concentration is discriminated based on the magnitude of the voltage Vout. The The toner concentration is in units of% by weight.

  In a developer containing toner and a magnetic carrier, if the toner concentration is low, the carrier ratio increases and the magnetic permeability is increased.If the toner concentration is high, the carrier ratio is decreased and the magnetic permeability is decreased. The decrease in toner density TC and the increase in voltage Vout are in a substantially direct relationship.

  Therefore, when the control unit recognizes a decrease in toner density based on the output voltage Vout from the toner concentration detection sensor 56Y, the control unit drives the toner supply unit until the output voltage Vout recovers to a predetermined level. Toner is supplied from the toner hopper 80Y to the second storage chamber 59Y.

  In the image forming apparatus 100 having such a configuration, when copying, the operation panel is started with the document set on the automatic document feeder 22 or the document placed on the contact glass 21a as described above. Press the button. When the image forming apparatus 100 is used as a printer, an image forming start operation is performed after image data to be formed is selected and input by an external input device such as a PC connected to the image forming apparatus 100.

  When copying is performed and the original is set on the automatic document feeder 22, the read original is read by the reading device 21 after the set original is fed onto the contact glass 21a. Is placed on the contact glass 21a, the document is read by the reading device 21 when the start button is pressed, and image data is generated.

  When reading a document, the first traveling body 21b and the second traveling body 21c travel, the light from the light source is emitted toward the document, and the reflected light from the document surface is reflected by the first reflector to the second traveling body. Reflected in the direction of 21c, the direction is changed by 180 degrees by the second reflector, enters the reading sensor 21e through the imaging lens 21d, and the contents of the original are read by the reading sensor 21e.

  Based on the generated image data or the input image data, the image stations 60Y, 60M, 60C, and 60BK having the above-described configuration operate.

  In the image station 60Y, the surface of the photosensitive drum 20Y is uniformly charged by the charging roller 31Y as it rotates in the B1 direction, and corresponds to the yellow color by exposure scanning of the laser light L from the optical scanning device 8. An electrostatic latent image is formed, and the electrostatic latent image is satisfactorily developed with yellow toner by the developing device 50Y, and the yellow toner image obtained by the development is moved in the A1 direction by the primary transfer roller 12Y. Unnecessary materials including toner that has been primarily transferred to the intermediate transfer belt 11 and remained after the transfer are removed by the cleaning device 40Y, and are used for the next static elimination and charging by the static elimination device and the charging roller 31Y.

  Similarly, the toner images of the respective colors are formed on the other photoconductive drums 20C, 20M, and 20BK, and the formed toner images of the respective colors are transferred intermediately by the primary transfer rollers 12C, 12M, and 12BK. Primary transfer is sequentially performed at the same position on the belt 11 to form a full-color composite color image. The toner image superimposed on the intermediate transfer belt 11 is moved to the secondary transfer nip which is a position facing the secondary transfer roller 17 as the intermediate transfer belt 11 rotates in the A1 direction, and is secondarily transferred to the sheet. The

  A sheet conveyed between the intermediate transfer belt 11 and the secondary transfer roller 17 is fed from the corresponding sheet feeding cassette 25 by the selection of one feeding roller 24 of the sheet feeding device 23, and is fed. Is fed from the manual feed tray 34 by the rotation of the feed roller 35 of the manual paper feed device 33, or fed from the duplex unit 96 by the paper feed roller 95. The timing at which the front end portion of the toner image on the intermediate transfer belt 11 faces the secondary transfer roller 17 by the registration roller pair 13 based on the detection signal from the sensor. It was sent out by.

  When the toner images of all colors are transferred and carried on the sheet, the sheet is transported to the transport device 76 and enters the fixing device 6, and is heated when passing through the fixing portion between the fixing belt 64 and the pressure roller 63. By the action of pressure and pressure, the carried toner image is fixed and a color image is formed on the sheet. The fixed sheet that has passed through the fixing device 6 is stacked on the discharge tray 75 via the discharge roller 98 or enters the duplex unit 96 via the conveyance roller 97 according to the position of the switching claw 94. In preparation for double-sided image formation. On the other hand, the intermediate transfer belt 11 that has finished the secondary transfer is cleaned by removing residual toner remaining on the intermediate transfer belt cleaning device 14 to prepare for the next image formation.

The toner contained in the two-component developer used in the image forming apparatus 100 will be described.
The toner is not particularly limited as long as it satisfies the conditions of the invention according to the present application, and a toner obtained by a conventionally known production method can be used. As the binder resin and the colorant used for the toner, conventionally known ones can be used as long as these conditions are satisfied.

  Examples of the binder resin include polyester resins, styrene resins, acrylic resins, styrene-acrylic resins, polyol resins, and epoxy resins. In particular, the binder resin used from the viewpoint of low-temperature fixability is preferably a polyester resin. The glass transition point (Tg) of the binder resin is 40 to 75 ° C., preferably 45 to 65 ° C. If Tg is too low, the heat-resistant storage stability of the toner is deteriorated. Conversely, if Tg is too high, the low-temperature fixability becomes insufficient. Tg can be measured by a differential scanning calorimeter (DSC). Tg was obtained from a DSC curve obtained using DSC-60A (manufactured by Shimadzu Corporation) under a temperature increase rate of 10 ° C./min.

  All known dyes and pigments can be used as the colorant. For example, carbon black, nigrosine dye, naphthol yellow, hansa yellow, polyazo yellow, oil yellow, pigment yellow, permanent yellow, brilliant carmine, permanent red, oil red, quinacridone red, pyrazolone red, polyazo red, phthalocyanine blue, anthraquinone blue, Anthraquinone violet, naphthol green, phthalocyanine green and the like can be mentioned. The amount of the colorant used is usually 0.5 to 15% by weight, preferably 3 to 10% by weight, as the content in the toner base. The colorant used in the present invention can also be used as a master batch combined with a resin. As the resin used for manufacturing the masterbatch, a resin similar to the binder resin for toner is usually used, but is not particularly limited. The toner base means a structure that does not contain an additive before the additive is externally added.

  The toner may contain a release agent together with the binder resin and the colorant. A well-known thing can be used as a mold release agent. Examples thereof include polyethylene wax, polypropylene wax, paraffin wax, sazol wax, carnauba wax, and montan wax. The amount of the release agent used is usually an amount of 0 to 40% by weight, preferably 5 to 20% by weight, as the content in the toner base.

  Moreover, you may contain a charge control agent as needed. Known charge control agents can be used. Examples include nigrosine dyes, triphenylmethane dyes, chromium-containing metal complex dyes, molybdate chelate pigments, quaternary ammonium salts, fluorine-modified quaternary ammonium salts, salicylic acid metal salts, and metal salts of salicylic acid derivatives. The amount of the charge control agent used is determined by the toner production method including the type of binder resin, the presence or absence of additives used as necessary, and the dispersion method, but is not uniquely limited. The content in the toner base is usually 0.1 to 10% by weight, preferably 0.2 to 5% by weight. The charge control agent may be used by being dispersed in the toner, or may be externally added or fixed to the toner surface.

  On the other hand, inorganic fine particles contained as an external additive as an additive in toner particles are used for the purpose of improving flow characteristics, development characteristics, charging characteristics, and the like. Usually, the primary particle diameter of the inorganic fine particles is preferably 5 nm to 2 μm. Although the proportion of the inorganic fine particles used depends on the type, it is often used in the range of 0.01 to 5% by weight based on the toner particles. Specific examples of the inorganic fine particles include, for example, silica, alumina, titanium oxide, barium titanate, magnesium titanate, calcium titanate, strontium titanate, zinc oxide, tin oxide, silica sand, clay, mica, wollastonite, Examples thereof include diatomaceous earth, chromium oxide, cerium oxide, pengala, antimony trioxide, magnesium oxide, zirconium oxide, barium sulfate, barium carbonate, calcium carbonate, silicon carbide, and silicon nitride. These can be used singly or in combination of two or more.

-Low-temperature fixing polyester toner The most preferable toner used in the image forming apparatus 100 is a polyester resin obtained by an ester extension polymerization method and having excellent low-temperature fixing property as a binder resin.

  As described above, in recent years, there has been a strong demand for resource saving and power saving, and it is essential to reduce fixing energy. To lower the fixing energy, the lower softening point of toner made of resin powder has progressed. This is because.

  In the ester elongation polymerization method, an organic solvent phase containing a polyester prepolymer is dispersed in an aqueous medium phase together with an active hydrogen-containing compound, and an extension reaction and / or a crosslinking reaction is performed in the aqueous medium phase to remove the organic solvent. In this method, toner particles are formed by washing and drying. This production method is also excellent in granulation properties, and the particle size, particle size distribution, and shape can be easily controlled. Below, the said manufacturing method and the material used are demonstrated.

The polyester prepolymer is a component that forms a higher molecular weight toner binder by an extension reaction and / or a crosslinking reaction with an active hydrogen-containing compound in an aqueous medium.
Examples of the polyester prepolymer include a polyester prepolymer having a functional group that reacts with active hydrogen such as an isocyanate group.

  The polyester prepolymer preferably used is a polyester prepolymer having this isocyanate group. This polyester prepolymer is produced by reacting a polyisocyanate (PIC) with a polyester having an active hydrogen group, which is a polycondensate of a polyol (PO) and a polycarboxylic acid (PC). Examples of polycondensates of polyols (PO) having active hydrogen groups and polycarboxylic acids (PC) include, for example, bisphenol A alkylene oxide adducts and dicarboxylic acids (succinic acid, adipic acid, maleic acid, fumaric acid, phthalic acid) And polycondensates of trivalent or higher polycarboxylic acids (trimellitic acid, pyromellitic acid, etc.).

  As polyisocyanate (PIC), aliphatic polyisocyanate (tetramethylene diisocyanate, hexamethylene diisocyanate, 2,6-diisocyanatomethyl caproate, etc.), alicyclic polyisocyanate (isophorone diisocyanate, cyclohexylmethane diisocyanate, etc.), aromatic Diisocyanates (tolylene diisocyanate, diphenylmethane diisocyanate, etc.), araliphatic diisocyanates (α, α, α ′, α′-tetramethylxylylene diisocyanate, etc.), isocyanurates, the polyisocyanates as phenol derivatives, oximes, caprolactam, etc. And a combination of two or more of these.

  The number of isocyanate groups contained per molecule in the polyester prepolymer having isocyanate groups is usually 1 or more, preferably 1.5 to 3 on average, more preferably 1.8 to 2.5 on average. is there. When the number is less than 1 per molecule, the molecular weight of the polyester after the elongation reaction becomes low, and the hot offset resistance deteriorates. Further, as described above, the polyester prepolymer is used by being dissolved in the organic solvent phase. The amount used and the amount of the polyester prepolymer is 10 to 55% by weight, preferably 10 to 40% as the content in the toner base. % By weight, more preferably 15 to 30% by weight.

Moreover, a non-reactive polyester can be dissolved in an organic solvent phase and used together with the polyester prepolymer.
By using this non-reactive polyester in combination, the low-temperature fixability of the toner and the gloss when used in a full-color device are improved, which is preferable to the case where the polyester prepolymer is used alone.

  Examples of the non-reactive polyester include a polycondensate of a polyol and a polycarboxylic acid similar to the polyester used for the reaction with the polyisocyanate (PIC), and preferable ones are the same as described above.

  When the non-reactive polyester is contained in the organic solvent phase, the blending amount is 10/90 to 55/45, preferably 10/90 to 40/60, as a weight ratio of the polyester prepolymer to the non-reactive polyester. More preferably, it is 15 / 85-30 / 70. If the weight ratio of the polyester prepolymer is too low, the hot offset resistance deteriorates and it is difficult to achieve both heat-resistant storage stability and low-temperature fixability. A resin other than non-reactive polyester may be used. For example, a conventionally known toner binder resin such as styrene resin, acrylic resin, epoxy resin, styrene-acrylic acid ester copolymer may be further blended. There is no problem.

  As the active hydrogen compound, amines are preferably used, and a urea-modified polyester resin can be obtained by reaction with the isocyanate group of the polyester prepolymer. Examples of the amines include diamines, trivalent or higher polyamines, amino alcohols, amino mercaptans, amino acids, and those obtained by blocking the amino groups of these amines. Preferred examples include 4,4'-diaminodiphenylmethane, isophoronediamine, hexamethylenediamine, ethanolamine, aminoethyl mercaptan, aminopropionic acid, and ketimine compounds obtained by blocking these amino groups with ketones such as methyl ethyl ketone.

  Most preferably, the colorant or colorant masterbatch is previously dissolved or dispersed in the organic solvent phase together with the polyester prepolymer and the non-reactive polyester. Further, if necessary, a release agent or a charge control agent may be dissolved or dispersed in the organic solvent phase.

  As an aqueous medium for forming the aqueous medium phase, water alone may be used, but an organic solvent may be used in combination. In particular, in order to reduce the viscosity when the resin component contained in the organic solvent phase is dispersed in an aqueous medium, it is preferable to use an organic solvent in which the resin component is soluble. In addition, the organic solvent is preferably volatile with a boiling point of less than 100 ° C. from the viewpoint of easy distillation thereof. For example, toluene, xylene, benzene, carbon tetrachloride, methylene chloride, 1,2-dichloroethane, 1,1,2-trichloroethane, trichloroethylene, chloroform, monochlorobenzene, dichloroethylidene, methyl acetate, ethyl acetate, methyl ethyl ketone, methyl isobutyl ketone These may be used alone or in combination of two or more.

  Further, it is preferable to use resin fine particles dispersed in an aqueous medium. The resin fine particles are used for the purpose of controlling the toner shape (circularity, particle size distribution, etc.) and are mainly distributed on the surface of the toner particles formed. The resin fine particles may be any resin as long as it can form a dispersion in an aqueous medium, and may be a thermoplastic resin or a thermosetting resin. For example, vinyl resins, polyurethane resins, epoxy resins, polyester resins , Polyamide resin, polyimide resin, silicon resin, phenol resin, melamine resin, urea resin, aniline resin, ionomer resin, polycarbonate resin and the like. These can be used singly or in combination of two or more. Among these, vinyl resins, polyurethane resins, epoxy resins, polyester resins, or combinations thereof are preferable because an aqueous dispersion of fine spherical resin particles is easily obtained.

  Vinyl resins are polymers obtained by homopolymerization or copolymerization of vinyl monomers, such as styrene- (meth) acrylic acid ester copolymers, styrene-butadiene copolymers, (meth) acrylic acid-acrylic acid ester copolymers. Examples thereof include a polymer, a styrene-acrylonitrile copolymer, a styrene-maleic anhydride copolymer, and a styrene- (meth) acrylic acid copolymer.

  The dispersion / blending amount of the resin fine particles in the aqueous medium is preferably 0.5 to 10 wt% with respect to the organic solvent phase. If it is not within this range, emulsification failure occurs and granulation cannot be performed. More preferably, it is 1 to 3 wt%. The average particle diameter of the resin fine particles is 5 to 200 nm, preferably 20 to 300 nm, from the viewpoint of granulation. Further, from the viewpoint of low-temperature fixability and toner storage stability, the glass transition point (Tg) is preferably 40 to 90 ° C, and more preferably in the range of 50 to 70 ° C.

  The following methods are employed for measuring the toner particle diameter and the toner circularity, respectively.

-Toner particle size measurement method The weight average particle size and number average particle size are measured using Coulter Multisizer II (Coulter). The measurement count is 50,000 counts.

Specifically, first, 0.1 to 5 ml of a surfactant (preferably alkyl benzene sulfonate) is added as a dispersant in 100 to 150 ml of an electrolytic aqueous solution. Here, the electrolytic solution is prepared by preparing a 1% NaCl aqueous solution using primary sodium chloride, and for example, ISOTON-ll (manufactured by Coulter) can be used. Here, 2 to 20 mg of a measurement sample is further added. The electrolytic solution in which the sample is suspended is subjected to a dispersion treatment with an ultrasonic disperser for about 1 to 3 minutes, and the measurement device is used to measure the volume and number of toner particles or toner using a 100 μm aperture as the aperture. Volume distribution and number distribution are calculated. From the obtained distribution, the weight average particle diameter (D ₄ ) and number average particle diameter of the toner can be obtained.

-Toner circularity measurement method The average circularity is measured using a flow type particle image analyzer FPIA-2100 (manufactured by Sysmex Corporation) for measurement of ultra fine toner, and analysis software (FPIA-2100 Data Processing Program for FPIA). Analysis is performed using version 00-10).

  Specifically, 0.1 to 0.5 ml of 10 wt% surfactant (alkylbenzene sulfonate Neogen SC-A, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) was added to a glass 100 ml beaker, and each toner 0.1 to 0.00. 5 g was added and stirred with a microspatel, and then 80 ml of ion-exchanged water was added. The obtained dispersion was subjected to a dispersion treatment for 3 minutes with an ultrasonic disperser (manufactured by Honda Electronics Co., Ltd.). The shape and distribution of the toner were measured with the FPIA-2100 until the concentration of 5000 to 15000 / μl was obtained. In this measurement method, from the viewpoint of measurement reproducibility of the average circularity, it is important that the dispersion concentration is 5000 to 15000 / μl. In order to obtain the dispersion concentration, it is necessary to change the conditions of the dispersion, that is, the amount of surfactant to be added and the amount of toner. The amount of the surfactant varies depending on the hydrophobicity of the toner as in the measurement of the toner particle diameter described above. When a large amount is added, noise due to bubbles is generated, and when the amount is small, the toner cannot be sufficiently wetted and dispersion is not achieved. It will be enough. Further, the amount of toner added varies depending on the particle size, and is small for small particle sizes and large for large particle sizes. When the toner particle size is 3 to 7 μm, the toner amount is 0.1 to 0. By adding 0.5 g, the dispersion concentration can be adjusted to 5000 to 15000 pieces / μl.

-Additive burying rate As described above, when a relatively soft polyester resin or the like is used as the binder resin for the toner, the burying of the additive may affect the transferability. The additive burying rate is known as an index representing the ease of burying the additive. Hereinafter, the additive burying rate will be described.

  The additive burying rate is an index indicating the degree to which the additive externally added to the toner surface is buried in the toner due to the agitation stress over time in the developing device. A toner whose surface is weak in mechanical strength and in which the additive is easily embedded has a higher additive burying rate, and conversely, a toner in which the additive is difficult to bury has a lower additive burying rate.

The additive burying rate is X (%), the BET specific surface area of the toner used in the image forming apparatus 100 is A (cm ² / g), and the BET specific surface area of the toner after the additive burying treatment of the toner is B (cm ^2). / G), the additive burial ratio X is
Additive burial rate X (%) = {(A−B) / A} × 100
It is represented by

The procedure of the additive burying process is as follows.
A polyethylene ointment bottle having an internal volume of 300 to 500 mL is charged with 10 g of toner and 100 g of a resin-coated ferrite carrier, and mixed at 100 rpm for 30 minutes using a tumbler mixer.

As a resin-coated ferrite carrier, conventionally known ones can be used, but in this embodiment, a ferrite carrier EF963-60B (particle size 35 to 85 μm, manufactured by Powdertech Co., Ltd.) coated with a silicone resin is used. did.
As the tumbler mixer, a tumbler mixer T2F type (manufactured by Willy et Bacofen (WAB)) was used.

  After mixing for 30 minutes, 300 mL of water was added to the ointment bottle, lightly stirred with a stirring rod to separate the toner and the carrier in water, and the toner dispersion as a supernatant was filtered. The toner obtained by filtration was dried under reduced pressure in a room temperature environment to obtain a toner after the additive embedding treatment.

  The BET specific surface area of the toner and the toner after the additive burying treatment was measured using an automatic specific surface area / pore distribution measuring device TriStar 3000 (manufactured by Shimadzu Corporation). Specifically, 1 g of toner was put in a dedicated cell, and the degassing process in the dedicated cell was performed using TriStar degassing dedicated unit Bacuprep 061 (manufactured by Shimadzu Corporation). The deaeration treatment was performed at room temperature, and was performed for 20 hours under a reduced pressure condition of at least 100 mtorr. The dedicated cell that has been deaerated can automatically obtain the BET specific surface area using TriStar 3000. The adsorption gas was nitrogen gas.

-Manufacturing method of toner with adjusted additive burying rate The additive burying rate of the toner used in this embodiment is 42%.
The method for producing such toner is as follows.

  950 parts of water, 20 parts of a vinyl resin (styrene salt copolymer of styrene-methacrylic acid-butyl acrylate-methacrylic acid ethylene oxide adduct sulfate) (manufactured by Sanyo Chemical Industries), dodecyl diphenyl ether disulfonic acid 16 parts of a 48.5% aqueous solution of sodium (Eleminol MON-7, manufactured by Sanyo Chemical Industries), 12 parts of a 3.0% aqueous solution of polymer protective colloid carboxymethylcellulose (Serogen BSH, manufactured by Sanyo Chemical Industries), and 130 ethyl acetate The parts were mixed and stirred to obtain a milky white liquid. This is the aqueous phase.

  Add 1200 parts of water, 50 parts of carbon black (Regal 400R, manufactured by Cabot), 50 parts of polyester resin (RS801, manufactured by Sanyo Chemical Industries) and 30 parts of water, and mix with a Henschel mixer (manufactured by Mitsui Mining). The mixture was kneaded at 150 ° C. for 30 minutes using two rolls, rolled and cooled, and pulverized with a pulverizer to obtain a carbon black masterbatch.

  A container equipped with a stir bar and a thermometer was charged with 500 parts of a polyester resin (RS801, manufactured by Sanyo Chemical Industries, weight average molecular weight 19,000, Tg 64 ° C.), 30 parts of carnauba wax, and 850 parts of ethyl acetate. The mixture was kept at 80 ° C. for 5 hours, cooled to 30 ° C. over 1 hour, and using a bead mill (Ultra Visco Mill, manufactured by IMEX), liquid feeding speed: 1.2 Kg / hr The wax was dispersed under the conditions of disk peripheral speed: 8 m / sec, 0.5 mm zirconia bead filling amount: 80% by volume, and number of passes: 3 times. Next, 110 parts of the carbon black masterbatch and 500 parts of ethyl acetate were charged in a container and mixed for 1 hour to obtain a dissolved product. Thereafter, 240 parts of ethyl acetate was further added, and using the above-mentioned bead mill, liquid feeding speed: 1.2 Kg / hr, disk peripheral speed: 8 m / sec, 0.5 mm zirconia bead filling amount: 80 vol%, number of passes: A dispersion was obtained under three conditions. This is the oil phase.

  1780 parts of the above oil phase, 100 parts of a 50% ethyl acetate solution of polyester prepolymer (manufactured by Sanyo Chemical Co., Ltd., number average molecular weight 3800, weight average molecular weight 15000, Tg 60 ° C.), 15 parts isobutyl alcohol, and 7.5 parts isophoronediamine And mix for 1 minute at 6,000 rpm using a TK homomixer (manufactured by Tokushu Kika), add 1200 parts of the aqueous phase to the container, and mix for 20 minutes at a rotational speed of 7,500 rpm with a TK homomixer. An aqueous medium dispersion was obtained.

  The aqueous medium dispersion is charged into a container equipped with a stirrer and a thermometer, desolvated at 30 ° C. for 12 hours, and then aged at 45 ° C. for 8 hours to obtain a dispersion from which the organic solvent has been distilled off. It was. After filtering 100 parts of this dispersion under reduced pressure, 500 parts of ion-exchanged water was added to the cake after filtration, followed by mixing with a TK homomixer (10 minutes at a rotation speed of 12000 rpm), followed by filtration under reduced pressure again. The filter cake was dried at 45 ° C. for 48 hours with a circulating dryer and sieved with a mesh having a mesh size of 75 μm to obtain a base of toner particles.

  100 parts by weight of the toner particle base obtained above, 1.2 parts by weight of hydrophobic silica (manufactured by Clariant Japan) having an average primary particle diameter of about 12 nm as an external additive, and hydrophobic titanium oxide having an average primary particle diameter of about 12 nm 0.5 parts by weight (manufactured by Teika) and 0.8 parts by weight of hydrophobic silica (manufactured by Shin-Etsu Chemical) having an average primary particle size of about 120 nm are mixed by a Henschel mixer and passed through a sieve having an aperture of 38 μm to remove aggregates. Thus, a toner was obtained.

The obtained toner had a weight average particle diameter (D ₄ ) of 5.8 μm, a number average particle diameter (Dn) of 5.1 μ, an average circularity of 0.97, and an additive burying rate of 42%. These measurement methods are as described above.

The additive burying rate can be adjusted by adjusting the molecular weight of the resin. For example, when a polyester resin (RS801, manufactured by Sanyo Chemical Co., Ltd., weight average molecular weight 19,000, Tg 64 ° C.) is changed to a polyester resin (manufactured by Sanyo Chemical Co., Ltd., weight average molecular weight 12,000, Tg 56 ° C.), the weight average particle diameter (D ₄ ) was 5.7 μm, the number average particle diameter (Dn) was 5.1 μ, the average circularity was 0.98, and the additive burying rate was 56%. When styrene-acrylic resin was used as the resin, a toner with an additive burying rate of 30% was obtained.

  As described above, there is a demand for an image forming system that can obtain a high-quality and high-quality image over a long period of time even with a toner that is relatively soft and easily embedded in an external additive. In the transfer of a toner image from a carrier to an intermediate transfer member and the transfer of a toner image from an intermediate transfer member to a sheet, it is required to transfer the toner image faithfully and stably to the transfer target. It has been found that adjusting the pressing force that presses the transfer member against the surface of the image carrier affects the performance of the secondary transfer.

  Therefore, in the image forming apparatus 100 that forms an image using a two-component developer containing a soft toner, a good image can be obtained by considering the effect on the performance of the secondary transfer in consideration of the effect of the primary transfer. Whether or not the formation was performed was determined by various experiments.

  The primary transfer pressure, that is, the primary transfer pressure is determined by the pressing springs of the pressing means provided corresponding to the photosensitive drums 20Y, 20M, 20C, and 20BK, respectively, to press the intermediate transfer belt 11 against the photosensitive drum 20Y. It is adjusted by changing the pressure. For example, the pressure spring 19Y described above is provided corresponding to the photosensitive drum 20Y.

  The primary transfer pressure is a pressure at which the primary transfer rollers 12Y, 12M, 12C, and 12BK actually press the intermediate transfer belt 11. In this embodiment, the primary transfer rollers 12Y, 12M, 12C, Since 12BK is in contact with the intermediate transfer belt 11 from the lower side to the upper side in the vertical direction, the primary transfer pressure is determined by the weight of the primary transfer rollers 12Y, 12M, 12C, and 12BK from the pressing force of the pressing spring. The pressure is obtained by subtracting.

  Further, since the primary transfer rollers 12Y, 12M, 12C, and 12BK have an Asker C hardness of 50 or less due to the elastic layer, the primary transfer rollers 12Y, 12M, 12C, and 12BK are easily crushed by the primary transfer pressure, and the contact area with the intermediate transfer belt 11 is increased. For this reason, the balance of the primary transfer pressure is reduced in the axial direction of the primary transfer rollers 12Y, 12M, 12C, and 12BK due to assembly tolerances or the like, or the primary transfer pressure deviates from the target pressing force. Even if this is not the case, deterioration of the secondary transfer performance is prevented or suppressed, and image formation is performed satisfactorily. Therefore, the primary transfer pressure can be set to a low value.

  The experimental results are shown in FIGS. In each figure, what is described as “Example” has conditions under which image formation is good or relatively good, and can be applied to the image forming apparatus 100 according to the present invention. It is a condition. In addition, what is described as “comparative example” is a condition where image formation is not good.

  In each of the drawings, “missing” means a defective image in which an image at the center of the formed image and at the center of the character portion is missing. In FIG. 3 and FIG. 5, “bottle” means a defective image whose formed image is not smooth. In the same figure, the evaluation results are shown by the five-level evaluation results of ranks 1 to 5, rank 1 is the worst and rank 5 is the best. The allowable range is rank 4 or higher.

  FIG. 3 shows the evaluation results of the blur and void of the formed image when the primary transfer pressure is changed. In the drawing, the hollow rank “1C” indicates that one color image is formed, and “2C” indicates that two color superimposed images are formed.

In the figure, Comparative Examples 1 to 3 are cases where the primary transfer pressures of the respective colors are equal, Comparative Example 2 is lower in pressure than Comparative Example 1, and Comparative Example 3 is lower in pressure than Comparative Example 2. ing.
In Comparative Example 1, there is no problem with hollowing out, but the magenta and cyan blurs are in an unacceptable rank. Since the soft toner is generally easily deformed, when the primary transfer pressure is high, when the toner is deformed and adheres to the intermediate transfer belt 11, the contact area with the intermediate transfer belt 11 is increased and the adhesion force is further increased. As a result, blurring and voids are likely to occur.

  In Comparative Examples 2 and 3, the roughness improved compared to Comparative Example 1, but the rank of 2C was not acceptable. This is because by lowering the pressing force of the primary transfer, the adhesion between the toner and the intermediate transfer belt 11 is reduced, the secondary transfer property is improved and the blur is improved. This indicates that the pressing force is insufficient for the transfer, and the transfer is defective as a dropout.

  In contrast to these comparative examples, in Examples 1 and 2 in which only the primary transfer pressure of black, which is the final transfer color, was lowered, both the blur and the void were improved within an allowable range. This is because, as described above, by lowering the pressing force of the primary transfer, the adhesion force between the toner and the intermediate transfer belt 11 is lowered, the secondary transfer property is improved, the blur is improved, and the pressing force is lowered at the same time. It is shown that if only the black is, there is no effect on the drop in the two-color overlap.

  In general, black hardly occupies the same position as the image of other colors on the intermediate transfer belt 11 or the sheet, and therefore, even when two colors are overlapped, there is actually little overlap of colors. It is considered that the result that the omission of the two-color superposition does not have an effect is obtained.

In addition, it can be seen from the comparison between Example 1 and Comparative Example 2 and the comparison between Example 2 and Comparative Example 3 that the effect is slightly smaller than when the pressing force of all colors is lowered.
The additive burying rate of the toner used for this evaluation was 42%. The intermediate transfer belt 11 was made of polyimide resin, and had a volume resistivity of 1 × 10 ⁹ Ω · cm and a surface resistivity of 1 × 10 ¹¹ Ω / □.

FIG. 4 shows the evaluation results of the blur and the void as in FIG. 3, but the difference from the experiment shown in FIG. 3 is that the additive burying rate of the used toner is also adjusted. The fixability at low temperature and low humidity was also examined. The primary transfer pressure of the comparative example was unified at 100 g / cm ² for each color as in comparative example 1.

  Comparative Examples 4 to 6 are cases where toners with an additive burying rate of less than 40% were used. In this case, the hollowing out was no problem as in Comparative Example 1, but the fixability at low temperature and low humidity (10 ° C., 15%) was insufficient.

  On the other hand, Comparative Examples 7 to 9 using toners with an additive burying rate of 40% or more were good because there was no problem of voids and the fixability at low temperature and low humidity was improved. However, compared with Comparative Examples 4-6, the bossiness was getting worse, and all were in an unacceptable range.

  Thus, as shown in Examples 3 to 5, both toner and voids were within the allowable range by using toner having an additive burying rate of 40% or more and reducing only the primary transfer pressure of black. Fixability at low temperature and low humidity was also good.

The use of a polyester resin as the toner resin has an advantage that it is easy to produce a toner having a characteristic that the mechanical strength is low and the additive burying rate is high, but the toner is easily fixed at a low temperature.
In this evaluation, the intermediate transfer belt 11 was made of polyimide resin, the volume resistivity was 1 × 10 ⁹ Ω · cm, and the surface resistivity was 1 × 10 ¹¹ Ω / □.

  FIG. 5 shows the degree of blur of a two-color superimposed image when the image forming order is changed. “◯” indicates that the degree of blur is acceptable, and “×” indicates that it is not acceptable. The evaluated image is a solid portion of a two-color superimposed image such as red (yellow + magenta), green (yellow + cyan), and blue (magenta + cyan) in which the image forming order affects the blur. Further, for example, when the notation of the image forming order in the drawing is YMCK, the image forming and primary transfer are performed in the order of yellow, magenta, cyan, and black from the upstream in the A1 direction that is the moving direction of the intermediate transfer belt. Shows the case.

  From this figure, it can be seen that only when the image forming order is yellow, magenta, cyan, and black, all the blurs of the two-color superimposed image (red, green, blue) are acceptable.

This is the case of this image sequence.
・ When yellow and magenta are overlaid ... Even if yellow is removed with red, the background is not noticeable with magenta. ・ When yellow and cyan are overlaid ... Even when yellow is missing with green, the background is not easily noticeable with cyan. When cyan and cyan are overlaid, it is considered that even if magenta is removed with blue, the background becomes less noticeable with cyan, so that the blur is less noticeable and does not become a problem. In other image forming orders, the reverse phenomenon occurs, for example, the blue color becomes cyan and the background is magenta.

  As described above, it has been found that it is preferable to perform image formation with colors that are less noticeable in order from the upstream side in the A1 direction. In addition to this, the disturbance of the image of each color due to the transfer bias is more likely to occur as the color is located on the upstream side of the A1 direction, but even if the disturbance occurs, this is less noticeable, and the toner of the color on the upstream side of the A1 direction. Is not noticeable even if it adheres to the color image carrier on the downstream side in the same direction, so that there is an advantage that image defects do not easily occur, which is extremely advantageous for good image formation.

The additive burying rate of the toner used for this evaluation was 42%. The intermediate transfer belt 11 was made of polyimide resin, and had a volume resistivity of 1 × 10 ⁹ Ω · cm and a surface resistivity of 1 × 10 ¹¹ Ω / □.

  FIG. 6 is a graph in which the magenta bump rank is examined by adjusting the BET specific surface area of the toner base without changing the conditions regarding the additive burying rate, toner resin, and primary transfer pressure in Example 3 shown in FIG. It is. From FIG. 6, it was found that the boss rank is also affected by the BET specific surface area of the toner base.

Specifically, the BET specific surface area of the toner base is larger than 3 (m ² / g) even if the primary transfer pressure is adjusted to improve the blur as in Example 3 shown in FIG. It was found that the soot rank was below acceptable level 4.

This is because, when the BET specific surface area of the toner base is larger than 3 (m ² / g), the additive coverage is low when the additive is externally added to the toner and the toner comes into contact with each other or the toner is covered. When the contact with the intermediate transfer belt 11 serving as a transfer body is low, the adhesion between the toners or between the toner and the intermediate transfer belt 11 is lowered due to the low probability that the additive is interposed therebetween, resulting in insufficient transferability. This is probably because of this.

Therefore, by using a toner having an additive burying rate of 40 or more and making the primary transfer pressure of black smaller than that of multiple colors, the toner base has a BET specific surface area of 3 (m ² / g) or less. , The blurring is more highly suppressed and more reliably prevented.

  There are other indices indicating the toner shape, such as circularity, SF-1, and SF-2. Since these are calculated on a two-dimensional plane onto which the toner is projected, only a two-dimensional uneven structure can be shown. I can't. On the other hand, since the BET specific surface area is a value representing the surface area of the toner, it shows a three-dimensional uneven structure. Therefore, the BET specific surface area has a high correlation with the coverage of the additive, and on the other hand, the coverage of the additive has a high correlation with the transferability that influences the blurring. There is a good correlation. For these reasons, the BET specific surface area is suitable as an index indicating the toner shape, and the circularity and SF-1 and SF-2 are not suitable.

From the above experimental results, it can be seen that the conditions for performing a good image are as follows.
The toner additive burying ratio X is 40% or more, and it is essential to make the primary transfer pressure of the black toner image smaller than the primary transfer pressure of the other color toner images.

In addition, it is desirable that the toner base be 3 (m ² / g) or less in the BET specific surface area ratio having a high correlation with the coverage of the additive as a countermeasure against the blur.

  Further, as described above, considering that black toner is easily noticeable and has a great influence on an image when it adheres to the photosensitive drums 20Y, 20M, and 20C, the toner that is finally transferred to the intermediate transfer belt 11 It is desirable that the image be a black toner image. This also has the advantage that the blur and void are improved by adding the primary transfer pressure of the black toner image, which is smaller than the primary transfer pressure of the toner image of a color other than black, at the end.

  Also, it is desirable to transfer the yellow toner image to the intermediate transfer belt 11 first. This is because yellow is the most inconspicuous color and the blur is improved.

  Further, it is desirable to transfer the magenta and cyan toner images to the intermediate transfer belt 11 second and third, respectively. This is because the colors are less noticeable in this order, and the blur is improved.

  It is desirable that the binder resin constituting the toner is a polyester resin. This is because it is easy to obtain an additive burying ratio X of 40% or more, the blur is improved even if the primary transfer pressure is low, and the fixability at low temperature and low humidity is good.

  In order to obtain a good image, it is most desirable to satisfy all of these conditions. Therefore, the image forming apparatus 100 satisfies all these conditions.

(Second Embodiment)
FIG. 6 shows an outline of an image forming apparatus according to the second embodiment to which the present invention is applied.
In the present embodiment, parts different from those in the first embodiment will be mainly described. Regarding the remaining parts, the components shown in FIG. 6 are denoted by the reference numerals corresponding to the components in the first embodiment. The description will be omitted as appropriate.

  In the image forming apparatus 100 of this embodiment, the transfer belt unit 10 is positioned vertically above the image stations 60Y, 60M, 60C, and 60BK, and the intermediate transfer belt 11 is moved to the primary transfer rollers 12Y, 12M, 12C, and 12BK. The pressing means 18Y, 18M, 18C, and 18BK that press the photosensitive drums 20Y, 20M, 20C, and 20BK respectively move the primary transfer rollers 12Y, 12M, 12C, and 12BK from the upper side to the lower side in the vertical direction. The intermediate transfer belt 11 is brought into contact with the intermediate transfer belt 11 and is pressed against the photosensitive drums 20Y, 20M, 20C, and 20BK.

  Specifically, the pressing means 18Y, 18M, 18C, and 18BK are configured to support the primary transfer rollers 12Y, 12M, 12C, and 12BK and the primary transfer rollers 12Y, 12M, 12C, and 12BK so that they can be displaced in the vertical direction. The supporting means (not shown) is provided. The pressing means 18Y, 18M, 18C, and 18BK include a member that urges the primary transfer rollers 12Y, 12M, 12C, and 12BK toward the intermediate transfer belt 11, such as the pressing spring 19Y in the first embodiment. The primary transfer rollers 12Y, 12M, 12C, and 12BK are in contact with the intermediate transfer belt 11 only by their own weights. Therefore, the pressing means 18Y, 18M, 18C, and 18BK cause the intermediate transfer belt 11 to move from the upper side to the lower side in the vertical direction by the weight of the primary transfer rollers 12Y, 12M, 12C, and 12BK. Press to 20M, 20C, 20BK.

The configurations of the primary transfer rollers 12Y, 12M, 12C, and 12BK are the same as those in the first embodiment, and are a metal core and an elastic layer formed so as to cover the outer peripheral surface of the core. And has an Asker C hardness of 50 or less due to the elastic layer.
The primary transfer pressure is set and adjusted by the mass of the cored bar.

  In the image forming apparatus 100 according to the second embodiment, the pressing units 18Y, 18M, 18C, and 18BK are arranged such that the intermediate transfer belt 11 is moved from the upper side to the lower side in the vertical direction by the primary transfer rollers 12Y, 12M, 12C, and 12BK. The reason why the photosensitive drums 20Y, 20M, 20C, and 20BK are configured to be pressed is that the primary transfer roller 12Y such as the pressing spring 19Y in the image forming apparatus 100 according to the first embodiment, Depending on tolerances generated by the biasing force of a member that biases 12M, 12C, and 12BK toward the intermediate transfer belt 11, density unevenness occurs in the main scanning direction in the black image, or the blur occurs on one side in the main scanning direction. This may be caused by

  When the primary transfer rollers 12Y, 12M, 12C, and 12BK are positioned vertically below the photosensitive drums 20Y, 20M, 20C, and 20BK, respectively, the primary transfer rollers 12Y, 12M, 12C, and 12BK are set to their own weights. In order to press against the photosensitive drums 20Y, 20M, 20C, and 20BK, the primary transfer rollers 12Y, 12M, 12C, and 12BK are biased toward the photosensitive drums 20Y, 20M, 20C, and 20BK. The biasing force of such a member must be increased. However, if the biasing force is increased, the balance of the pressing force in the main scanning direction is likely to be lost due to tolerances and the like, and this causes density unevenness and blurring. .

Specifically, as in Example 1 described above, when the primary transfer pressure of black is set to 70 g / cm ² , the primary transfer pressure at one of the end portions in the main scanning direction is larger than this value. It was confirmed that it could become larger and the blur could not be improved. Further, as in Example 2 described above, when the primary transfer pressure of black is set to 50 g / cm ² , the primary transfer pressure of one of the end portions in the main scanning direction becomes smaller than this value. It has been confirmed that transfer efficiency may decrease and transfer defects may occur.

  On the other hand, when the primary transfer rollers 12Y, 12M, 12C, and 12BK are positioned vertically above the photosensitive drums 20Y, 20M, 20C, and 20BK, respectively, as in the present embodiment, the primary transfer rollers 12Y, It is not necessary to press the 12M, 12C, and 12BK against the photosensitive drums 20Y, 20M, 20C, and 20BK against their own weights, and the balance of the primary transfer pressure in the main scanning direction is improved. Is prevented or suppressed as compared with the case where the configuration in the first embodiment is adopted.

  In the present embodiment, the primary transfer pressure is generated only by the weight of the primary transfer rollers 12Y, 12M, 12C, and 12BK. However, a member such as a pressure spring 19Y is used for forming the primary transfer pressure. Also good. Even in such a configuration, the urging force by such a member does not have to be large enough to oppose the weight of the primary transfer rollers 12Y, 12M, 12C, and 12BK, and therefore the urging force assists to generate the primary transfer pressure. Therefore, the primary transfer pressure is well balanced in the main scanning direction. However, it is preferable to generate the primary transfer pressure only by the weight of the primary transfer rollers 12Y, 12M, 12C, and 12BK from the viewpoint of simplification of structure, size reduction, and cost reduction.

  Generation of the primary transfer pressure with such a configuration is compatible with the primary transfer rollers 12Y, 12M, 12C, and 12BK having an Asker C hardness of 50 or less. This is because of this hardness, the primary transfer pressure is liable to be crushed and the contact area with the intermediate transfer belt 11 becomes large. Therefore, the balance of the primary transfer pressure is dependent on the formation and assembly tolerances in the main scanning direction. In this case, the deterioration of the secondary transfer performance is prevented or suppressed to a higher degree than in the first embodiment, and the image formation is performed satisfactorily, and the primary transfer pressure is thereby set to a low value. This is also possible.

  The image forming apparatus 100 according to the second embodiment is also configured as a combined printer / facsimile machine, and the secondary transfer device also has a function of conveying the sheet toward the fixing device 6. The image forming apparatus 100 differs from the image forming apparatus 100 according to the first embodiment in that it is configured as a transfer conveyance unit 17 and a double-sided image forming function is omitted.

  The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the specific embodiments, and the present invention described in the claims is not specifically limited by the above description. Various modifications and changes are possible within the scope of the above.

  For example, a direct transfer system that directly transfers toner images of each color onto a sheet or the like without using an intermediate transfer member may be employed. That is, the transfer target may be a sheet. In this case, the toner images on the plurality of image carriers are directly transferred to the sheet.

The intermediate transfer member is not limited to a belt shape but may be a drum shape.
The image forming apparatus may not be a copier, a printer, and a facsimile machine, but may be a single unit thereof, or may be a multi-function machine of another combination such as a copier and printer.

  The effects described in the embodiments of the present invention are only the most preferable effects resulting from the present invention, and the effects of the present invention are limited to those described in the embodiments of the present invention. is not.

1 is a schematic front view of an image forming apparatus according to a first embodiment to which the present invention is applied. FIG. 2 is a schematic front view showing a configuration around one image carrier among a plurality of image carriers provided in the image forming apparatus shown in FIG. 1. It is the evaluation result of the blur and the void of the formed image when the primary transfer pressure is changed. FIG. 6 is a result of evaluating the fixability of a formed image when it is changed in primary transfer pressure and toner additive burying rate, and in a low-temperature and low-humidity state. It is an evaluation result of the degree of blur of a two-color superimposed image when the image forming order is changed. FIG. 4 is a correlation diagram between a BET specific surface area of a toner base and a bump rank. It is a schematic front view of the image forming apparatus which concerns on 2nd Embodiment to which this invention is applied.

Explanation of symbols

11 Transfer object, intermediate transfer member 12Y, 12M, 12C, 12BK First transfer unit, pressing member 17 Second transfer unit 18Y, 18M, 18C, 18BK Pressing unit 20BK Image carrier 20Y carrying a black toner image , 20M, 20C, 20BK Image carrier 100 Image forming apparatus

Claims (11)

A plurality of image carriers;
A transfer object which is opposed to the plurality of image carriers and onto which a toner image on the image carrier is transferred;
A plurality of pressing means provided corresponding to each of the plurality of image carriers, and pressing the transferred body against the corresponding image carrier with a predetermined pressing force;
In an image forming apparatus using a two-component developer including a toner and a carrier, wherein the toner includes a binder resin and an additive,
The following additive burying ratio X of the toner is 40% or more,
Image forming characterized in that the pressing force of the pressing means corresponding to the image bearing member carrying a black toner image among the plurality of image bearing members is made smaller than the pressing force of the other pressing means. apparatus.
Additive burial rate X = {(A−B) / A} × 100
A: BET specific surface area of the toner (cm ² / g)
B: BET specific surface area (cm ² / g) after the toner was subjected to additive burying treatment The image forming apparatus according to claim 1.
The image forming apparatus, wherein the toner base has a BET specific surface area of 3 (m ² / g) or less. The image forming apparatus according to claim 1 or 2,
The pressing means has a pressing member that contacts the transferred body, and presses the transferred body against the corresponding image carrier from the upper side to the lower side in the vertical direction by the pressing member. An image forming apparatus. The image forming apparatus according to claim 3.
The image forming apparatus, wherein the pressing unit presses the transferred body against the corresponding image carrier by the weight of the pressing member. The image forming apparatus according to any one of claims 1 to 4,
The image forming apparatus according to claim 1, wherein the pressing member has an Asker C hardness of 50 or less. The image forming apparatus according to any one of claims 1 to 5,
The transfer target is an intermediate transfer member on which a toner image transferred from the image carrier is transferred to a sheet,
First transfer means for transferring a toner image on the image carrier to the intermediate transfer member;
An image forming apparatus comprising: a second transfer unit that transfers the toner image on the intermediate transfer member to a sheet.   7. The image forming apparatus according to claim 6, wherein a toner image transferred last to the intermediate transfer member among a plurality of toner images transferred from the image carrier to the intermediate transfer member is a black toner image. An image forming apparatus.   8. The image forming apparatus according to claim 6, wherein a toner image transferred to the intermediate transfer member among a plurality of toner images transferred from the image carrier to the intermediate transfer member is a yellow toner image. An image forming apparatus.   9. The image forming apparatus according to claim 8, wherein the second and third toner images transferred from the plurality of image carriers to the intermediate transfer member are respectively magenta. An image forming apparatus, which is a color and cyan toner image.   The image forming apparatus according to claim 1, wherein the binder resin is a polyester resin.   An image forming method for forming an image using the image forming apparatus according to claim 1.

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