JP2005077842A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus of an intermediate transfer system, in which developer images formed with developers in a plurality of colors on an intermediate transfer body can be transferred secondarily together in a proper state and image failures, such as low density in the image, can be prevented. <P>SOLUTION: The image forming apparatus 100 is equipped with an image forming means 10 which can form developer images in a plurality of colors; an intermediate transfer body 40 where the developer images in a plurality of colors are transferred from the image forming means 10; and a transfer means 42 which transfers in batch the developer images in a plurality of colors formed on the intermediate transfer body 40 to a transfer material P. The image forming means 10 has at least one image carrying body 21 and a plurality of development devices 23, each being equipped with a developer carrying body 23a which rotates and supplies a developer of the respective color different from others to the image carrying body 21. The peripheral speed ratio of the developer carrying body 23a to the image carrying body 21 is varied in the plurality of developing devices. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus using an electrophotographic system such as a copying machine, a facsimile machine, or a printer. More specifically, a developer image formed on an image carrier is subjected to a primary transfer process with an intermediate transfer member interposed therebetween. The present invention relates to an image forming apparatus using an intermediate transfer system that transfers to a transfer material through a secondary transfer process.

  Conventionally, as an image bearing member, for example, an electrostatic image formed on a cylindrical electrophotographic photosensitive member (photosensitive drum) is converted into a visible image (developer image) using a developer by a developing unit. There is an image forming apparatus using an electrophotographic method in which an image is transferred to a transfer material and fixed to obtain a recorded image.

  In recent years, for example, an image forming apparatus using an electrophotographic method has been improved in speed, function, and color, and various types of image forming apparatuses have been proposed. Further, from the viewpoint of speeding up, research and development of an in-line apparatus for forming an image by arranging a plurality of image forming units that form images of different colors in series and driving them simultaneously is progressing. .

  Since an in-line apparatus can form a high-speed color image, it is considered to be extremely useful in, for example, business use where high-speed printing is highly demanded.

  In this in-line type image forming apparatus, a plurality of color toner images are temporarily superimposed on an intermediate transfer member and subjected to primary transfer, and then collectively transferred onto a transfer material such as a recording sheet or an OHP sheet. A method using an intermediate transfer method in which a final image is formed by subsequent transfer is well known.

  On the other hand, conventionally, a plurality of developing means are provided for one image carrier, and electrostatic images formed on the image carrier are successively developed using a predetermined developer containing a desired developer. The process of primary transfer of the agent image to the intermediate transfer member is repeated to form a superimposed image composed of a plurality of types of developer on the intermediate transfer member, and then the developer image is collectively transferred to the transfer material. Such image forming apparatuses are also widely used.

  With the intermediate transfer method, compared to the method in which the transfer material is transferred to the transfer belt and the toner image is transferred directly from the image carrier to the transfer material, the transfer bias can provide a good full-color toner image regardless of the transfer material. Therefore, complicated control can be omitted and high image quality can be obtained.

  In the developing means for forming a toner image on the image carrier, the one-component developing method using no carrier is more suitable than the two-component developing method using a two-component developer mainly including toner and carrier. There is an advantage that the configuration becomes simple and the cost is low. In the case of the one-component system, the developing device, the image carrier, the cleaning device, and the charging device can be easily integrated to form a process cartridge, so that maintenance is very good. In the case of the one-component developing method, it is easy to make the developing device a cartridge (developing cartridge) that can be detachably attached to the apparatus main body.

  Further, as the developing means, a non-magnetic toner can be used by using an elastic roller having an elastic layer on the developer carrying member and bringing the developer into contact with the image carrying member. Even in the apparatus using the toner, there is an advantage that the amount of toner scattered in the apparatus is small and the image scattering is good.

  In the conventional image forming apparatus, the developer is charged by controlling the charge amount by externally adding a material for controlling the charge amount to the powder containing the pigment. Further, the developer coating amount is controlled on the developer carrying member by a developer regulating blade, but in order to make a thin layer effective for imparting a high charge amount (tribo) and uniform chargeability to the developer. Management of the developer regulating blade mounting position, contact pressure, surface roughness, and the like are performed.

  In general, a color image forming apparatus forms a full-color image using three primary color toners of yellow, magenta, and cyan as a developer and black (black) toner. Further, it has been proposed to use many kinds of toners, and the present invention can be applied to such an image forming apparatus. Hereinafter, a black toner as a general developer and a color other than black will be described. A case where three primary color toners of yellow, magenta, and cyan (hereinafter simply referred to as “color toner”) will be described.

  As described above, the intermediate transfer method has excellent advantages such as not selecting a transfer material. However, in particular, black toner generally uses carbon black as a pigment as a colorant. Since the resistance of this carbon black is relatively low, the charging property of black toner is different from that of other color toners. It tends to be particularly low compared. For this reason, in the secondary transfer process, the black toner is deviated in transferability with respect to the transfer bias with respect to the color toner.

  That is, since the color toner has a high charge amount, the optimum secondary transfer bias is higher than that of the black toner. Therefore, for example, when a secondary transfer bias adapted to color toner is used, the bias is stronger than the optimum value for black toner. As a result, the secondary transfer bias does not act on the toner image on the intermediate transfer member, and a current leaks to the intermediate transfer member, resulting in a so-called pass-through image, and the density may be low.

  Conversely, when a secondary transfer bias matched to black toner is used, the bias becomes weaker than the optimum value for the color toner, and the density after the secondary transfer tends to become thin. At the same time, since the coating is performed with a thin layer, the density of the black toner image tends to be thin.

  As described above, conventionally, it has been difficult to select an optimal secondary transfer bias for all color toners.

Here, in Patent Document 1, each station is provided in an image forming apparatus configured to transfer a toner image formed on each photosensitive drum provided in a plurality of stations onto a transfer sheet conveyed by a transfer sheet conveyance sheet. It is disclosed that the peripheral speed of the developing sleeve of the developing device using the two-component developing system is made faster at the black station than at the other stations. However, such conventional technology does not relate to an intermediate transfer type image forming apparatus, and does not mention anything about adjusting the charge amount of toner on the intermediate transfer member.
Japanese Patent Laid-Open No. 9-22156

  The present invention has been made in view of the above problems.

  In other words, an object of the present invention is to allow a secondary transfer of a developer image on an intermediate transfer member formed of a plurality of colors of developer in an intermediate transfer type image forming apparatus. An object of the present invention is to provide an image forming apparatus capable of preventing image defects such as low density.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image forming unit capable of forming a developer image of a plurality of colors, an intermediate transfer member to which a developer image of a plurality of colors is transferred from the image forming unit, and formed on the intermediate transfer member. Transfer means for collectively transferring the developer images of a plurality of colors onto a transfer material, and the image forming means is different from at least one image carrier and rotated relative to the image carrier. And a plurality of developing units each provided with a developer carrying body for supplying a color developer, and a peripheral speed ratio of the developer carrying body to the image carrying body between the plurality of developing units. An image forming apparatus characterized by changing.

  According to an embodiment of the present invention, the peripheral speed ratio is changed for each of the plurality of developers according to the electric resistance of each developer used by the plurality of developers. In one embodiment, the peripheral speed ratio in a developing device using a developer having a relatively low electrical resistance among the plurality of developing devices is set as the peripheral speed ratio in the developing device using a developer having a relatively large electrical resistance. Make it larger than the speed ratio. In one embodiment, at least one of the plurality of developing devices is a black developing device using a black developer, and the peripheral speed ratio in the black developing device is set to the peripheral speed ratio in another developing device. Larger than. The black developer may contain carbon black. In one embodiment, the peripheral speed of the developer carrier of the black developer is 105% to 130% of the peripheral speed of the developer carrier of another developer.

  According to one embodiment of the present invention, the charge per unit area of the developer of each color of the developer images of a plurality of colors formed on the intermediate transfer body before being transferred to the transfer material by the transfer unit. The peripheral speed ratio in each of the plurality of developing devices is set so that the amounts are all within a predetermined range for all colors.

  In the present invention, the image forming means has a plurality of image carriers, and the plurality of developing devices are provided so as to supply developers of different colors to the image carriers, respectively. The developer image formed in (1) may be sequentially superimposed and transferred onto the intermediate transfer member. Alternatively, the image forming means is provided with the plurality of developing units so as to sequentially supply different color developers from the plurality of developing units to one of the at least one image carrier. The developer image formed on the at least one image carrier may be sequentially superimposed and transferred onto the intermediate transfer member. In the present invention, the developer may be a non-magnetic one-component developer.

  According to the present invention, in an intermediate transfer type image forming apparatus, a developer image on an intermediate transfer member formed of a plurality of colors of developer can be secondary transferred in a good batch, and the image density can be reduced. It is possible to prevent image defects such as. In particular, since the secondary transfer characteristics of black toner colored with carbon black can be aligned with other colors, the density of the image is reduced or transferred due to the so-called sticking-out phenomenon due to the transfer bias being too large. It is possible to satisfactorily prevent density thinning due to the bias being too small.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Example 1
(Overall configuration of image forming apparatus)
FIG. 1 shows a schematic cross section of an image forming apparatus according to the present invention. The image forming apparatus 100 according to the present embodiment corresponds to an image information signal transmitted from an external device such as a personal computer that is communicably connected to the apparatus main body 100A or a document reading apparatus connected to the apparatus main body 100A. This is an intermediate transfer type full color laser beam printer capable of forming and outputting a recorded image on a transfer material, for example, a recording sheet, an OHP sheet, a cloth or the like, using an electrophotographic system.

  First, a schematic overall configuration and operation of the image forming apparatus 100 will be described. The image forming apparatus is an intermediate transfer body (intermediate transfer) that is driven in the direction of arrow A in FIG. Belt) 40. As image forming means for forming developer images of a plurality of colors on the intermediate transfer belt 40 around the intermediate transfer belt 40, yellow (Y) and magenta (M ), Cyan (C), and black (K), a plurality of image forming units 10Y, 10M, 10C, and 10K are provided. For example, when a four-color full-color image is formed, the toner images formed in the respective image forming units are transferred to the respective images by the primary transfer rollers 41Y, 41M, 41C, and 41K that are primary transfer members via the intermediate transfer belt 40. In the primary transfer portion T1 facing the carriers 21Y, 21M, 21C, and 21K, the primary transfer rollers 41Y, 41M, 41C, and 41K to which the primary transfer bias is applied are electrostatically applied on the intermediate transfer belt 40. Thus, a primary transfer is performed, and a full-color toner image superimposed on the intermediate transfer belt 40 is formed.

  The toner image formed on the intermediate transfer belt 40 moves as the surface of the driven intermediate transfer belt 40 moves, and the secondary transfer roller 42 as the secondary transfer member and the intermediate transfer belt 40 come into contact with each other. At the nip portion (secondary transfer portion) T2, the secondary transfer roller 42 to which a secondary transfer bias is applied is electrostatically subjected to secondary transfer collectively onto the transfer material P. The transfer material P is supplied by a transfer material supply roller 51 or the like from a cassette 51 serving as a transfer material storage unit of the transfer material supply unit 50 in synchronization with the timing at which the toner image on the intermediate transfer belt 40 reaches the secondary transfer unit T2. Paper is conveyed.

  Thereafter, the transfer material P is separated from the intermediate transfer belt 40 and conveyed to the fixing unit 60, where the transfer material P is heated and pressed by a fixing roller 61 and a pressure roller 62 provided with a heating unit. The image is fixed. The transfer material P on which the toner image is fixed is discharged to a tray 73 which is a transfer material discharge portion through a discharge path 72 by a discharge roller 71 of the transfer material discharge portion 70. Thus, a recorded image is formed on the transfer material P, and image formation is completed.

  Next, each of the image forming units 10Y, 10M, 10C, and 10K will be described in more detail. In the image forming apparatus 100 of this embodiment, the basic configuration and basic operation of each of the image forming units 10Y, 10M, 10C, and 10K are common except that the color of the toner image to be formed is different. When there is no particular distinction, the subscripts Y, M, C, and K attached to the reference numerals in the drawing are omitted to indicate that the element belongs to one of the image forming units, and a general description will be given. .

  FIG. 2 shows an enlarged view of the vicinity of the process cartridge 20 and the primary transfer unit T1 included in the image forming unit 10.

  In this embodiment, each image forming unit 10 is rotated in the direction of an arrow R1 in the figure as an image carrier, that is, in the direction of an arrow R4 in the figure as a photosensitive drum 21 and a charging means. A charging roller 22 that can be developed, a developing device 23 that is a developing means, and a cleaning member 24 that is a cleaning means for the image carrier, and a primary transfer bias is applied to the back surface of the intermediate transfer belt 40 at the primary transfer portion T1. A primary transfer roller 41 which is a primary transfer member in contact therewith is provided. The charging roller 22, the cleaning member 24, and the photosensitive drum 21 are supported by a cleaner container 25. The developing device 23 includes a developing roller 23a as a developer carrying member, a developing blade 23b as a developer regulating member, a supply roller 23c as a developer supplying member, a developer stirring member 23d, and the like in a developing container 23e.

  In this embodiment, the photosensitive drum 21, the charging roller 22, the developing device 23, and the cleaning member 24 are integrated into a cartridge by integrally connecting the developing container 23 e and the cleaner container 25, thereby configuring the process cartridge 20. The main body 100A is detachable. The process cartridge 20 is detachably mounted on the apparatus main body 100A via a mounting means 80, such as a guide member and a positioning member, provided in the apparatus main body 100A. When the process cartridge 20 is properly attached to the apparatus main body 100A, a charging roller is supplied from a voltage applying means (not shown) provided in the apparatus main body 100A via contacts (not shown) on the apparatus main body side and the process cartridge side. 22, a predetermined bias can be applied to each of the developing roller 23a and the like. Similarly, when the process cartridge 20 is properly attached to the apparatus main body 100A, drive transmission members (not shown) on the apparatus main body side and the process cartridge side are connected, and a drive source (see FIG. The drive from (not shown) can be transmitted to the process cartridge 20. In this embodiment, the drive from the drive source provided in the apparatus main body 100A is transmitted to the photosensitive drum 21 via the drive transmission member, and this drive force is supplied to the developing roller 23a of the developing unit 23 via the gear train and the like. It is transmitted to the roller 23c, the developer stirring member 23d, and the like.

  The aspect of the process cartridge is not limited to that of the present embodiment, and at least the developing means is integrated into a cartridge as an image carrier and process means acting on the image carrier, or further At least one of the charging unit and the cleaning unit may be integrally formed as a cartridge so as to be detachable from the image forming apparatus main body. Further, the developing means may be a cartridge (developing cartridge) that can be detachably attached to the apparatus main body 100A.

  In the developing container 23e, a nonmagnetic one-component developer (toner) that is substantially composed only of nonmagnetic resin toner particles (may have an external additive as usual) is stored as a developer. In the cleaner container 25, toner remaining on the photosensitive drum 21 after the primary transfer process, toner transferred from the intermediate transfer belt 40 to the photosensitive drum 21 in the primary transfer portion T1 due to a retransfer phenomenon, and other foreign matters are contained. It is peeled off from the photosensitive drum 21 by the cleaning member 24 and stored.

  The developing roller 23a, which is a developer carrying member, has an elastic layer having an electrically medium resistance, and is the same as the photosensitive drum 21 that is driven to rotate in the direction of arrow R2 in the drawing, that is, the direction of arrow R1 in the drawing. It is rotationally driven in the direction. The developing roller 23 a rotates in contact with the photosensitive drum 21, and a predetermined bias is applied to supply the toner to the photosensitive drum 21. In this embodiment, the developing roller 23a has a diameter of 18 mm, and is a multilayer in which an elastic layer made of urethane rubber as a base layer and carbon as a surface layer is provided around a stainless steel core. The configuration. However, the present invention is not limited to this, and a metal cylindrical body such as aluminum, aluminum alloy, or stainless steel is used as the core metal, and the base layer of the elastic layer is made of a rubber material such as NBR, EPDM, silicone rubber, urethane rubber, or the like. The surface layer may be composed of ether urethane or nylon.

  On the other hand, the supply roller 23c, which is a developer supply member, has a foamed elastic layer and comes into contact with the development roller 23a that is driven to rotate in the direction of arrow R1 in the drawing, that is, the direction of arrow R2 in the drawing. Driven in the reverse direction. When a non-magnetic one-component developer is used as the developer, the developer cannot be supplied to the developer carrying member by magnetic force. Therefore, the supply roller 23c rotates in contact with the development roller 23a, thereby developing roller. The toner in the developing container 23e is adhered to the developing roller 23a while peeling off the toner coated on the 23a. In the present embodiment, the supply roller 23c is a sponge roller having a diameter of 18 mm in which urethane foam is provided as an elastic layer around a metal core such as aluminum, aluminum alloy, or stainless steel.

  The developing blade 23b, which is a developer regulating member, is provided with a resin coating on a metal elastic plate. By adjusting the contact pressure and the tip position of the developing blade 23b, a high-charge amount and a thin toner coat ( Toner layer) can be obtained. In this embodiment, the developing blade 23b is formed by coating a nylon bronze plate on a contact surface with the developing roller 23a on a phosphor bronze plate having a straight spring elasticity. However, this is not restrictive, and a stainless steel plate may be used instead of the phosphor bronze plate, and a rubber plate or the like is bonded or coated on the metal plate on the contact surface between the developing blade 23b and the developing roller 23a instead of nylon. May be.

FIG. 3 shows the contact nip N between the developing roller 23a and the developing blade 23b in a further enlarged manner. From the tip of the developing blade 23b to the end of the nip portion N is called a nip inlet NE, and its length is called an NE length. FIG. 4 shows the relationship between the toner coat amount (M / S) (mg / cm ² ) on the developing roller 23a and the NE length. From the figure, it is understood that when the NE length is large, the toner is taken into the nip portion N and the toner coat amount (M / S) is increased. However, even if the toner coat amount (M / S) is large, it is well known that the toner that is actually charged is the toner below the toner coat of the developing roller 23a (on the developing roller 23a side). That is, the toner on the upper layer of the toner coat (developing blade 23b side) on the developing roller 23a has a small charge amount, and development and transfer cannot be controlled by an electric field, which causes fogging and transfer failure. Therefore, it is preferable to reduce the toner coat amount (M / S).

  Next, FIG. 5 shows the relationship between the toner coat amount (M / S) and the charge amount (Q / M) (μC / mg) of the toner on the developing roller 23a. From the figure, it can be seen that the toner charge amount (Q / M) increases as the toner coat amount (M / S) decreases. This indicates that the upper layer portion of the toner coat on the developing roller 23a, which is weakly charged, is reduced. That is, it can be said that the toner on the developing roller 23a is uniformly charged by reducing the toner coat amount (M / S).

In this embodiment, the developing blade 23b was set to have a NE length of 1.0 mm and a contact pressure of about 30 gf / cm ² .

  The toner coat on the developing roller 23a is preferably a thin layer, but in order to obtain a necessary image density, the peripheral speed of the photosensitive drum 21 (100 mm / second in this embodiment) is supplied to supply an appropriate amount of toner. On the other hand, it is driven to rotate with a peripheral speed ratio of about 150%. The peripheral speed ratio between the developing roller 23a and the photosensitive drum 21 will be described in detail later.

  Here, the developer used in the image forming apparatus 100 of this embodiment will be described.

  In this embodiment, a negatively chargeable non-magnetic one-component developer is used as the developer used in each image forming unit 10. As the toner, a publicly known and arbitrary toner obtained by coloring a binder resin with a coloring component (pigment, dye) can be used. Preferably, those having a volume average particle diameter of 5 μm to 8 μm are suitably used for high definition of images. In this embodiment, the black toner contains carbon black as a pigment.

  Here, the volume average particle diameter of the toner was measured as follows. A Coulter Counter TA-II type (manufactured by Coulter Inc.) was used as a measuring device, and an interface (manufactured by Nikka) and a CX-1 personal computer (manufactured by Canon) for outputting the number average distribution and volume average distribution were connected to the electrolyte solution. Prepare 1% NaCl aqueous solution using primary sodium chloride.

  As a measuring method, 0.1 to 5 ml of a surfactant, preferably alkylbenzene sulfonate, is added as a dispersant to 100 to 150 ml of the above electrolytic aqueous solution, and 0.5 to 50 mg of a measurement sample is further added. The electrolyte solution in which the sample is suspended is subjected to dispersion treatment with an ultrasonic disperser for about 1 to 3 minutes, and a particle size distribution of 2 to 40 μm particles is measured using a 100 μm aperture as an aperture with a Coulter counter TA-II type. Obtain volume average distribution and number average distribution. The volume average particle diameter is obtained from the volume distribution and number distribution thus obtained.

  With reference to FIG. 6, the charging characteristics of black toner will be described in comparison with the charging characteristics of magenta toner. FIG. 6 is a graph obtained by collecting the toner coated on the developing roller 23a and measuring the charge amount and the number of particles. The horizontal axis represents the charge amount of the toner particles, and the vertical axis represents the charge amount. This is the number of particles with that charge amount. That is, it represents the toner charge amount distribution. A curve 101 in the figure represents the charge amount distribution of the magenta toner, and a curve 102 represents the charge amount distribution of the black toner.

  The black toner uses carbon black as a representative pigment for producing a black color. It is known that the amount of charge is small because the volume resistivity of toner using carbon black is lower than that of other color toners. Therefore, comparing the curve 101 (magenta toner) and the curve 102 (black toner) in FIG. 6, the peak of the black toner charge amount is smaller than the peak of the magenta toner charge amount. . This indicates that the charge amount of the black toner is small. In this embodiment, the charging tendency of cyan toner and yellow toner, which are color toners other than magenta not using carbon black, is almost the same as that of magenta. Therefore, illustration and description of charging characteristics of color toners other than magenta are omitted.

(Image forming operation)
The image forming operation in each image forming unit 10 will be further described. At the time of image formation, the members of the photosensitive drum 21, the developing roller 23a, the supply roller 23c, and the charging roller 22 are shown by arrows R1, R2, It is rotationally driven in the direction of R3 and R4. The charging roller 22 is applied with a charging bias, is pressed against the photosensitive drum 21 and is driven to rotate, and charges the photosensitive drum 21 to a predetermined charging potential. Thereafter, the charged surface of the photosensitive drum 21 is subjected to an exposure L corresponding to the print image in the exposure unit that passes through, and an electrostatic image is formed on the surface of the photosensitive drum 21. In this embodiment, the laser beam scanner 30 provided in each image forming unit 10 serving as an exposure unit (image writing unit) emits laser light in accordance with an image information signal, and the developing container 23e and the cleaner container 25 of the process cartridge 20 are used. The surface of the rotating photosensitive drum 21 is scanned and exposed through a slit S formed therebetween.

  Next, the electrostatic image formed on the photosensitive drum 21 reaches a portion (developing region) facing the developing roller 23a. In the developing area, a predetermined DC bias is applied as a developing bias in this embodiment, and the toner on the developing roller 23 that is rotationally driven in contact with the photosensitive drum 21 is formed on the photosensitive drum 21 by an electrostatic image. The toner image is transferred to the photosensitive drum 21 and a toner image is formed on the photosensitive drum 21. In this embodiment, development is performed by reversal development in which toner charged to the same polarity as the charging polarity (negative) of the photosensitive drum 21 is attached to a portion of the photosensitive drum 21 where the potential is attenuated by exposure.

  Next, the toner image formed on the photosensitive drum 21 is transferred onto the intermediate transfer belt 40 by an electric field formed by the photosensitive drum 21 and the transfer roller 41 to which the primary transfer bias is applied in the primary transfer portion T1. Primary transfer.

  In each image forming unit 10, image formation is performed in synchronization so that the toner image transferred from each photosensitive drum 21 becomes a full-color image on the intermediate transfer belt 40, and the intermediate transfer belt is transferred from each photosensitive drum 21. The toner images are sequentially superposed onto the toner 40 and transferred to the primary position. Thus, a full color toner image is formed on the intermediate transfer belt 40.

  By the way, as the primary transfer bias applied to the primary transfer roller 41 in the primary transfer process, an optimum bias can be applied to each primary transfer portion T1 of each color if the power source is made independent. The primary transfer bias is generally controlled according to the resistance value of the primary transfer roller 41, but it is well known that the optimum value is slightly affected by the chargeability of the toner. For example, in the primary transfer portion T1 of black toner, a larger bias than that of the primary transfer portion T1 of a color other than black is applied to perform optimal primary transfer.

  Next, in the secondary transfer portion T2, a secondary transfer bias is applied to the secondary transfer roller 42 as a secondary transfer member, and the full-color toner image primarily transferred onto the intermediate transfer belt 40 is transferred onto the transfer material P. Secondary transfer at once.

  As described above, a black toner colored with carbon black as a pigment tends to have a smaller charge amount than a color toner. However, in the secondary transfer process, the transfer bias cannot be controlled in accordance with the charge amounts of all color toners as in the primary transfer.

  That is, since the color toner has a large charge amount, the optimum secondary transfer bias is higher than that of the black toner. Therefore, for example, when a secondary transfer bias adapted to color toner is used, the bias is stronger than the optimum value for black toner. As a result, a so-called click-through image may be obtained and the density may be low.

  Conversely, when a secondary transfer bias matched to black toner is used, the bias becomes weaker than the optimum value for the color toner, and the density after the secondary transfer tends to become thin. At the same time, since the coating is performed with a thin layer, the density of the black toner image tends to be thin.

  Therefore, in this embodiment, the peripheral speed ratio of the developing roller 23a to the photosensitive drum 21 is changed between a plurality of developing units to adjust the charge amount of the developer on the intermediate transfer belt 40. Preferably, the peripheral speed ratio in the developing unit 23 using a developer having a relatively small electrical resistance among the plurality of developing units 23 is equal to the peripheral speed ratio in the developing unit 23 using a developer having a relatively large electrical resistance. Larger than. In particular, in this embodiment, the peripheral speed ratio of the developing roller 23aK to the photosensitive drum 21K of the black developing device 23K using black toner colored with carbon black as a developer is set to other developing devices, that is, yellow, magenta. The peripheral speed ratio of the developing rollers 23aY, 23aM, 23aC of the cyan color developing devices 23Y, 23M, 23C to the photosensitive drums 21Y, 21M, 21C is set larger.

  The peripheral speed ratio of the developing roller 23a to the photosensitive drum 21 is not limited to this, but when the developing roller 23a is driven from the photosensitive drum 21, the configuration of the drive transmission means such as a gear ratio is changed. Can be achieved. In addition, when the developing roller 23a is driven by driving from an independent driving source such as a stepping motor, the peripheral speed of the developing roller 23a can be easily changed by controlling the driver of the driving source. it can. Any of them can be achieved by using well-known and conventional techniques, so further detailed description of the means for changing the peripheral speed will be omitted.

  Hereinafter, it will be described in more detail with reference to specific experimental examples.

(Experimental example 1)
Table 1 shows the amount of charge (Q / S) per unit area (Q / S) of toner image transferred to the intermediate transfer belt 40 by changing the peripheral speed ratio of the developing roller 23aK of the black developing device 23K to the photosensitive drum 21K (μC / cm ² ), fog on the transfer material P, density, and degree of deterioration of the developing device 23K for black over a long period of use of the developing roller 23aK.

  Here, the peripheral speed ratio of the developing rollers 23aY, 23aM, and 23aC of the color developing units 23Y, 23M, and 23C to the photosensitive drums 21Y, 21M, and 21C is fixed, and the black developing unit 23K includes the toner to be used and the developing roller. Except for changing the peripheral speed ratio of the 23aK to the photosensitive drum 21K, the same configuration as that of the color developing devices 23Y, 23M, and 23C was maintained.

  The charge amount (Q / S) per unit area of the toner image on the intermediate transfer belt 40 is a value measured by the following apparatus and method under the following image conditions.

  The image forming apparatus 100 is operated so as to form a solid image of 10 mm × 10 mm on the intermediate transfer belt 40, and the image forming apparatus 100 is turned off before the secondary transfer process. A 10 mm × 10 mm toner image is formed thereon. After that, using a measurement container having a cylindrical filter paper, a suction port made of metal is attached, and the suction pressure is adjusted so that the toner forming the toner image on the surface of the intermediate transfer belt 40 can be sucked uniformly without excess or deficiency. Aspirate the toner. The charge Q of the developer sucked at this time is measured with a 616 digital electrometer (manufactured by KEITHLEY). A value obtained by dividing the charge Q by the area of the toner image is defined as a charge amount (Q / S) per unit area.

The evaluation criteria for fog (after fixing) on the transfer material P are as follows.
○: Almost △: Covered but practical use ×: Not practical

The evaluation criteria for the image density (after fixing) on the transfer material P are as follows.
○: Uniform solid density can be obtained. △: Slight solid density unevenness is possible but practical. ×: Not practical.

Further, the evaluation criteria for the deterioration of the developing roller 23aK of the black developing device 23K are the results of examining the developing roller 23aK after the 10,000 pages are operated by intermittent printing, and the evaluation criteria are as follows.
○: No problem Δ: Slight toner fusing, but unrecognizable level in image ×: Toner fusing is severe and can be confirmed in image In addition, peeling of the surface layer may occur.

  As for the secondary transfer bias, an image was evaluated by applying an optimum bias (in this embodiment, the same for yellow, magenta, and cyan) to toner other than black. Here, a transfer bias of 900 V was applied at the secondary transfer portion T2.

When the peripheral speed ratio of the developing roller 23aK of the black developing unit 23K to the photosensitive drum 21K increases, the charge amount (Q / S) per unit area of the black toner image on the intermediate transfer belt 40 increases. For the developing rollers 23aY, 23aM, and 23aC of the color developing units 23Y, 23M, and 23C, the charge amount per unit area of the color toner image is set by setting the peripheral speed ratio with respect to the photosensitive drums 21Y, 21M, and 21C to 150%. Q / S) was in the range of ^{20 to} 22 μC / cm ² in which these toners could be satisfactorily transferred to the secondary transfer bias under the above conditions. In the configuration of this embodiment, in the color developing units 23Y, 23M, and 23C, if the peripheral speed ratio exceeds 150%, the absolute amount of toner that is not highly charged increases, and fogging may occur.

  On the other hand, when the black image transferred to the transfer material P is observed, in order to obtain a sufficient density, the peripheral speed ratio of the developing roller 23aK of the black developing device 23K to the photosensitive drum 21K is larger than 155%, more preferably. Has been confirmed to be 160% or more. By doing so, the charge amount (Q / S) per unit area of the black toner on the intermediate transfer belt 40 can be brought close to that of the color toner, that is, aligned. Then, the optimum secondary transfer bias range can be brought close to, that is, aligned.

  On the other hand, when the peripheral speed ratio of the developing roller 23aK of the black developing unit 23K to the photosensitive drum 21K is set to 200% or more, the absolute amount of toner that is not a high charge amount increases, and thus deterioration of fog can be confirmed. Further, when the peripheral speed is set to 200% or more, the developing roller 23aK of the black developing device 23K is rapidly deteriorated due to sliding friction with the photosensitive drum 21K and the supply roller 23c.

  Therefore, the peripheral speed ratio of the developing roller 23aK of the black developing unit 23K to the photosensitive drum 21K is set to be larger than 155% and smaller than 200%. More preferably, the peripheral speed ratio is 160% or more and 195% or less. That is, the peripheral speed of the developing roller 23aK of the black developing unit 23K is set to 105% or more and 130% or less with respect to the peripheral speed of the developing rollers 23aY, 23aM, and 23aC of the color developing units 23Y, 23M, and 23C.

  By setting the peripheral speed ratio in such a range, it was confirmed that even if the observation result of the full-color image in the secondary transfer was observed, the density reduction due to black penetration was suppressed and a good image was obtained.

In this embodiment, the peripheral speed ratio of the developing rollers 23aY, 23aM, 23aC of the color developing units 23Y, 23M, 23C to the photosensitive drums 21Y, 21M, 21C is 150%, and the photosensitive drum 21K of the developing roller 23aK of the black developing unit 23K. By setting the peripheral speed ratio to 170%, the charge amount (Q / S) per unit area on the intermediate transfer belt 40 before secondary transfer of all toners is in the range of 20 μC / cm ^{2 to} 22 μC / cm ² . Almost evenly aligned. As a result, it was possible to obtain a very good result with better image density and no fogging or slipping out.

  As described above, according to this embodiment, it is possible to improve the secondary transfer property and form a high-quality color image. That is, as described above, the configuration in which the peripheral speed ratio of the developing roller 23a to the photosensitive drum 21 is changed between the developing devices, and the toner of the toner image on the intermediate transfer belt 40 before the secondary transfer is preferably changed. The amount of charge per unit area can be set within a predetermined range for the black toner and the color toner, and the optimum value of the secondary transfer bias of each color toner can be set, so that the toner is formed of a plurality of color toners. The toner images on the intermediate transfer belt 40 can be secondarily transferred in a good batch. In particular, when using a toner whose electrical resistance is significantly different from that of other colors, such as black toner colored using carbon black, and whose secondary transfer characteristics are significantly different from those of other colors, the secondary transfer characteristics of the toner are reduced. It is extremely useful because it can be aligned with other colors.

  In this embodiment, the yellow, magenta, and cyan color toners have the same charging characteristics, and the developing rollers 23aY, 23aM, and 23aC of the photosensitive drums 21Y and 21M of the developing units 23Y, 23M, and 23C for these colors are used. The peripheral speed ratio for 21C was the same. However, the present invention is not limited to this, and the peripheral speed ratio with respect to the developer carrying member 23a of the photosensitive drum 21 can be changed for each developing device 23 for each color according to the charging characteristics of each toner. Accordingly, it is preferable that the charge amount per unit area of the toner of each color (primary color) toner image before the secondary transfer step can be set within a predetermined range for all the colors. The optimum value of the next transfer bias can be made uniform.

  In the present embodiment, the intermediate transfer member is described as a belt member. However, the present invention is not limited to this, and any known transfer member can be used. For example, it may be a drum type in which a sheet is stretched on a frame forming an end portion, or a hollow or solid cylindrical drum type.

Example 2
Next, another embodiment of the present invention will be described.

  In the first exemplary embodiment, the image forming apparatus 100 includes a plurality of image carriers arranged side by side, and a toner image that is sequentially primary-transferred by superimposing each image carrier on the intermediate transfer member, and then transferring the toner images onto the transfer material in a batch. Although the case where the present invention is applied to a so-called in-line image forming apparatus that performs next transfer has been described, the present invention is not limited to such an embodiment.

  FIG. 7 shows a schematic cross-sectional configuration of another example of an image forming apparatus to which the present invention can be applied. Elements having substantially the same or corresponding functions and configurations as those of the image forming apparatus 100 according to the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  The illustrated image forming apparatus 200 includes a photosensitive drum 21 that can rotate in the direction of an arrow in the drawing. Around the photosensitive drum 21, a charging roller 22, a laser scanner 30, a rotary developing device (rotary developing unit) 90, a cleaning member 24, and the like are arranged. Further, an intermediate transfer belt 40 is disposed so as to face the photosensitive drum 21 so as to be capable of rotating in the direction of the arrow in the drawing, and a primary transfer roller 41 is provided so as to face the photosensitive drum 21 via the intermediate transfer belt 40. It has been. In addition, a secondary transfer roller 42 that is detachable from the intermediate transfer belt 40 is provided. The rotary developing device 90 includes a developing unit 23Y for each color containing toner of each color of yellow, magenta, cyan, and black as a developer. 23M, 23C, 23K. Then, by rotating the rotating body 91, the developing unit 23 for a desired color can be arranged at a position (developing position) facing the photosensitive drum 21. The photosensitive drum 21, the charging roller 22, the laser scanner 30, the rotary developing device 90, and the primary transfer roller 41 constitute an image forming unit that forms a plurality of colors of developer on the intermediate transfer belt 40.

  In such a configuration, similarly to the image forming process in each image forming unit described in the first embodiment, the charging and exposure steps are performed, and the electrostatic image corresponding to, for example, yellow image information as the first color. Are formed on the photosensitive drum 21. Then, this electrostatic image is developed by the yellow developing device 23Y disposed at the developing position to obtain a yellow toner image. This toner image is then primarily transferred to the intermediate transfer belt 40 that rotates in the primary transfer portion T1.

  In the same manner as described above, charging, exposure, development, and primary transfer are performed for the second to fourth colors, and a toner image in which each color is superimposed is formed on the intermediate transfer belt 40. At this timing, the secondary transfer roller 42 comes into contact with the intermediate transfer belt 40, and then the toner image on the intermediate transfer belt 40 is collectively collected on the transfer material P conveyed from a recording material supply unit (not shown). Transcript.

  In such an image forming apparatus 200, as in the image forming apparatus 100 of the first embodiment, for example, as a developer, a non-magnetic one-component developer black toner and yellow, magenta, and cyan color toners are used. The black toner and the color toner have different charging characteristics. Therefore, the transferability at the secondary transfer portion T2 is greatly different between the black toner and the color toner.

  Therefore, as in the first embodiment, the present invention is applied. Here, the peripheral speed ratio of the developing roller 23aK of the black developing unit 23K to the photosensitive drum 21 is set to the developing rollers 23aY, 23aM of the color developing units 23Y, 23M, and 23C. It is faster than the peripheral speed ratio of 23aC to the photosensitive drum 21.

  Accordingly, as described in the first embodiment, it is preferable that the charge amount per unit area of the toner image on the intermediate transfer belt 40 before the secondary transfer is set to a predetermined value between the black toner and the color toner. Since the optimum values of the secondary transfer bias of each color toner can be made uniform, good secondary transfer can be performed for a plurality of color developers.

  In addition, a plurality of sets each provided with a plurality of developing means are provided for one image carrier, and a superimposed image composed of a plurality of types of developer is formed on each intermediate transfer member from each image carrier. However, the present invention can also be applied in the same manner as described above even if this is transferred to a transfer material. By changing the peripheral speed ratio of the developer carrier to the image carrier between a plurality of developing means according to the charging characteristics of the toner to be used, for example, the electrical resistance, a toner having a small electrical resistance is typically used. The same effect can be obtained by increasing the peripheral speed ratio of the developer carrying member of the developing means to the image carrier.

  As described above, even with the configuration of this example, the same effects as those of Example 1 can be obtained, and the secondary transfer property can be improved and a high-quality color image can be formed.

  In the above-described embodiments, the developer is assumed to be a non-magnetic one-component developer, but the present invention is not limited to this. For example, even in a configuration in which one of a plurality of developing units uses a non-magnetic one-component developer and one of them uses a magnetic one-component developer, the developer of the developing unit depends on the transfer characteristics and charging characteristics of the toner. By changing the peripheral speed ratio of the carrier to the image carrier between the developing devices, good secondary transfer can be performed on the developer images of all colors as in the above-described embodiment. The present invention can also be applied to a well-known two-component developer mainly including non-magnetic resin toner particles and magnetic carrier particles. In this case, the toner charging characteristics, typical According to the electrical resistance (volume resistivity) of the toner and / or carrier, the peripheral speed ratio of the developer carrier of the developer to the image carrier is changed between the developers (toner and / or carrier). The peripheral speed ratio is increased for a developing device having a relatively low electrical resistance), and good secondary transfer can be performed on developer images of all colors as in the above-described embodiment.

1 is a schematic longitudinal sectional view of an embodiment of an image forming apparatus according to the present invention. FIG. 2 is an enlarged view of an image forming unit of the image forming apparatus in FIG. 1. FIG. 2 is an enlarged view of a nip portion between a developing roller and a developing blade in the image forming apparatus of FIG. 1. FIG. 6 is a graph for explaining the relationship between the length (NE length) of the nip inlet of the developing blade and the toner coat amount (M / S). FIG. 6 is a graph for explaining a relationship between a toner coat amount (M / S) and a toner charge amount (Q / M). FIG. 6 is a graph for explaining toner charging characteristics of a black toner and a magenta toner on a developing roller. FIG. 6 is a schematic cross-sectional configuration diagram of another embodiment of an image forming apparatus according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Image formation part 20 Process cartridge 21 Photosensitive drum (image carrier)
22 Charging roller (charging means)
23 Developer (Developing means)
23a Development roller (developer carrier)
23b Development blade (developer regulating member)
23c Supply roller (developer supply member)
23d Developer stirring member 23e Developer container 24 Cleaning member (cleaning means)
25 Cleaner container 40 Intermediate transfer belt (intermediate transfer member)
41 Primary transfer roller (primary transfer member)
42 Secondary transfer roller (secondary transfer member)
60 Fixing device P Transfer material

Claims (10)

Image forming means capable of forming developer images of a plurality of colors, an intermediate transfer body to which developer images of a plurality of colors are transferred from the image forming means, and developer images of a plurality of colors formed on the intermediate transfer body A transfer unit that collectively transfers the image to a transfer material, wherein the image forming unit rotates at least one image carrier and rotates to supply developers of different colors to the image carrier. In an image forming apparatus having a plurality of developing units each including an agent carrier,
An image forming apparatus, wherein a peripheral speed ratio of the developer carrier to the image carrier is changed between the plurality of developing devices.   2. The image forming apparatus according to claim 1, wherein the peripheral speed ratio is changed for each of the plurality of developing devices in accordance with an electric resistance of each developer used by the plurality of developing devices.   The peripheral speed ratio in a developing device using a developer having a relatively small electrical resistance among the plurality of developing devices is larger than the peripheral speed ratio in a developing device using a developer having a relatively large electrical resistance. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   At least one of the plurality of developing devices is a black developing device using a black developer, and the peripheral speed ratio in the black developing device is set larger than the peripheral speed ratio in the other developing devices. The image forming apparatus according to claim 1, 2, or 3.   The image forming apparatus according to claim 4, wherein the black developer contains carbon black.   6. The image formation according to claim 4, wherein a peripheral speed of the developer carrier of the black developer is 105% to 130% of a peripheral speed of the developer carrier of another developer. apparatus.   The charge amount per unit area of the developer of each color of the developer images of a plurality of colors formed on the intermediate transfer body before being transferred to the transfer material by the transfer unit is set within a predetermined range for all colors. The image forming apparatus according to claim 1, wherein the peripheral speed ratio in each of the plurality of developing devices is set.   The image forming means has a plurality of image carriers, and the plurality of developing devices are provided so as to supply developers of different colors to the image carriers, and the development formed on each image carrier The image forming apparatus according to claim 1, wherein the agent images are sequentially superimposed and transferred onto the intermediate transfer member.   The image forming means is provided with the plurality of developing units so as to sequentially supply developers of different colors from a plurality of developing units to one of the at least one image carrier. The image forming apparatus according to claim 1, wherein a developer image formed on one image carrier is sequentially superimposed and transferred onto the intermediate transfer member.   The image forming apparatus according to claim 1, wherein the developer is a non-magnetic one-component developer.

Images