JP2006323301A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of suppressing distortion of an unfixed toner image formed on the carrying member side surface of a recording body. <P>SOLUTION: The toner image height (pile height) of a solid image on the carrying belt side surface of recording paper is made smaller than the pile height of a solid image on a surface of the recording paper different from the carrying belt side surface. Distortion of the unfixed image on the carrying belt side surface of the recording paper due to rubbing with a carrying belt 51 can be suppressed as compared with the case of the same pile height of solid images on both surfaces of recording paper. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, and a printer, and more particularly to an image forming apparatus that forms an image on both sides of a recording medium such as transfer paper by a one-pass method.

  Conventionally, as a method for forming images on both sides of a recording medium, a switchback method, a one-pass method, and the like are known. In the switchback method, the recording medium is passed through a transfer unit and a fixing unit, an image is recorded on one surface of the recording unit, and then the recording unit is reversed by a reverse conveyance path. Then, the image is recorded on the other side by switching back to the transfer unit and the fixing unit. On the other hand, the one-pass method is a method in which a recording body on which images are transferred on both sides by a double-side transfer means is passed through a fixing means, and an image can be recorded on both sides without switching back the recording medium. The one-pass method is superior to the switchback method in the following points. In other words, cost increases by providing a complicated mechanism for switchback, longer image formation time by switchback, and jamming by switching back the curled recording medium by heating by the fixing means are all avoided. It is a point to get.

  However, such a one-pass method has a problem that the image is easily disturbed when the recording medium after the double-side transfer is sent from the double-side transfer means to the fixing means. When the recording medium separated from the double-sided transfer unit is transferred to the fixing unit, the sliding surface of the recording surface is slid along with the guide member disposed between the double-sided transfer unit and the inside of the fixing unit. The unfixed toner image is disturbed. In the case of an apparatus that transfers a toner image to only one side of a transfer paper, the design is such that the non-transfer surface of the toner image and the guide member are rubbed by means such as a guide member and layout, etc. It is possible to avoid disturbance of the toner image. However, in the case of double-sided transfer, the transfer surface of the toner image inevitably becomes a sliding surface with the guide member, so that the image is easily disturbed.

  Therefore, in Patent Document 1, a driven rotatable spur having a plurality of protrusions on the peripheral surface is provided between the double-sided transfer unit and the fixing unit, thereby guiding the recording body from the double-sided transfer unit to the fixing unit. An image forming apparatus has been proposed. According to Patent Document 1, by rotating the spur that supports the recording body while piercing the protrusion from the lower side with the movement of the recording body, without disturbing the image of the recording body surface on the side supported by the spur, The recording medium can be guided from the double-side transfer means to the fixing means. However, even if the spur protrusion is sharp, if it is pierced, the unfixed image will be disturbed.

JP-A-10-142869

  On the other hand, the present applicant is developing the following new image forming apparatus. That is, the conveying means for receiving the recording body fed from the double-side transfer means by the conveying belt which is endlessly moved while being stretched by a plurality of stretching rollers and conveying it toward the fixing means, and directly delivering the leading edge to the fixing means. Is an image forming apparatus. According to this image forming apparatus, the conveyance belt supports the recording body sent out from the double-side transfer means while moving at the same speed, thereby guiding the recording body toward the fixing means while suppressing the friction with the recording body. To do. As a result, disturbance of the unfixed toner image due to rubbing can be suppressed.

However, even in this image forming apparatus, an unfixed image on the recording medium may be disturbed for the following reason.
A spur, a conveyor belt, or the like provided for conveyance and delivery between the double-side transfer unit and the fixing unit rubs the unfixed toner image on the recording medium and disturbs the unfixed image.
Further, when the recording medium is transferred from the double-side transfer means to the conveying belt of the conveying means, the recording medium is slightly rubbed against the belt by a reaction caused by contact with the conveying belt, and the unfixed toner image is slightly disturbed.
Further, the conveyance is performed to the vicinity of the fixing unit by the conveyance belt. When the recording medium is transferred from the conveyance belt to the fixing unit, the recording medium is moved by a reaction caused by contact between the conveyance member in the fixing unit and the recording medium. The unfixed toner image is slightly disturbed by slightly sliding on the conveying member in the fixing device.

  In view of this, the present inventor conducted intensive research on the cause of disturbing the unfixed toner image on the surface of the recording medium on the conveying belt side. As a result, the unfixed toner image having a higher toner image height (hereinafter referred to as pile height) It was found that the unfixed toner image was disturbed by rubbing.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of suppressing disturbance of an unfixed toner image formed on a side surface of a conveyance member of a recording medium. It is.

In order to achieve the above object, according to the first aspect of the present invention, the first toner image formed by the image forming means is transferred to the first surface of the recording medium, and the second toner image formed by the image forming means is recorded. A double-sided transfer means for transferring to the second surface of the body and a conveying member for carrying the recording body, and recording from the double-sided transfer means to a fixing means for fixing the toner image on the recording body onto the recording body. In an image forming apparatus comprising a recording medium conveying means for conveying a recording medium, the surface of the recording medium on the conveying member side of the unfixed solid image has a surface different from the surface of the recording medium on the conveying member side. The height of the toner image is lower than that of the unfixed solid image.
According to a second aspect of the present invention, in the image forming apparatus of the first aspect, the image forming means includes an image carrier that carries a latent image, an exposure device that forms a latent image on the image carrier, and the image And a developing device that develops the latent image on the carrier, and the toner of the unfixed solid image on the surface of the recording member on the conveying member side according to the development potential when developing the latent image on the image carrier. The image height is lower than the toner image height of an unfixed solid image on a surface different from the surface on the conveying member side of the recording medium.
According to a third aspect of the present invention, in the image forming apparatus according to the second aspect, the image forming apparatus further comprises a detecting means for detecting a toner image height of an unfixed solid image transferred to the surface of the recording member on the conveying member side. The developing potential is controlled based on the detection result of the means.
According to a fourth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the toner image height of the single-color unfixed solid image transferred to the surface of the recording member on the conveying member side is set to a toner particle size. On the other hand, it is characterized by being 1.25 times or less.
According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, a first image forming unit that forms the first toner image and a second image forming unit that forms the second toner image. It is characterized by comprising.
The invention according to claim 6 is the image forming apparatus according to any one of claims 1 to 5, wherein the toner used for forming the toner image has a weight average particle diameter of 3 to 7 [μm] and a weight average particle diameter. The value obtained by dividing the number by the number average particle diameter is 1.05-1.30.
According to a seventh aspect of the present invention, in the image forming apparatus according to any one of the first to sixth aspects, as the toner used for forming the toner image, the shape factor SF-1 is 100 to 180, and the shape factor SF-2 is What is 100-180 is used.
According to an eighth aspect of the present invention, in the image forming apparatus according to the first aspect, or the image forming apparatus according to any one of the second to seventh aspects, comprising the developing device, the developing device carries a developer and the image carrying device. And a developer regulating member for regulating the amount of the developer carried on the developer carrying body, and the developer regulating member is used as the toner for forming the toner image. Among the toners on the developer carrying member that has passed through the member, a toner in which the number of toners of −0.1 [femto C / μm] or more is less than 10 [%] is used.
According to a ninth aspect of the present invention, in the image forming apparatus according to any one of the first to eighth aspects, the toner used for forming the toner image is formed by adding an additive to the base particles composed of at least a binder resin and a colorant. And an additive having an average primary particle size of 50 to 500 [nm] and a bulk density of 0.3 [g / cm ³ ] or more is used as the additive. To do.
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 weight average particle diameter of the toner used for forming a toner image transferred to the surface of the recording member on the conveying member side is It is characterized in that it is smaller than the weight average particle diameter of the toner used for the toner image transferred to a surface different from the surface on the conveying member side of the recording medium.
According to an eleventh aspect of the present invention, in the image forming apparatus according to any one of the first to tenth aspects, the toner shape factors SF-1 and SF-2 used for forming a toner image on the surface of the recording member on the conveying member side are determined. The shape factor SF-1 and the shape factor SF-2 of the toner used for forming a toner image on a surface different from the surface on the conveying member side of the recording body are characterized by being smaller.
According to a twelfth aspect of the present invention, in the image forming apparatus according to any one of the first to eleventh aspects, the length of a part of the conveying member from the double-sided transfer unit to the fixing unit is set to the maximum size capable of image formation. It is characterized in that it is larger than the length of.
According to a thirteenth aspect of the present invention, in the image forming apparatus according to any one of the first to twelfth aspects, the speed of the conveying member that conveys the recording body to the fixing means after the recording body has passed through the double-side transfer means. Is controlled based on the type of the recording medium.
According to a fourteenth aspect of the present invention, in the image forming apparatus according to any one of the first to thirteenth aspects, a charge imparting unit that imparts a charge to the surface of the conveying member before receiving the recording material from the double-sided transfer unit is provided. It is characterized by.

  According to the first to fourteenth aspects of the present invention, the solid toner image height on the conveyance member side of the recording medium is set lower than the solid toner image height on the surface different from the surface on the conveyance member side of the recording medium. Thereby, it is possible to suppress the unfixed image on the surface of the recording member on the conveying member side from being disturbed by the sliding of the conveying member as compared with the case where the solid toner image heights on both surfaces of the recording member are the same.

Hereinafter, embodiments of the image forming apparatus of the present invention will be described. FIG. 1 illustrates a full-color printer capable of duplex printing by an electrophotographic method as the image forming apparatus 100 according to the first embodiment.
In the image forming apparatus main body 100 shown in FIG. 1, the first image forming unit 20 is disposed at the upper part and the second image forming unit 30 is disposed at the lower part with the recording paper conveyance path 43A as a boundary. . The first image forming unit 20 includes a first intermediate transfer belt 21 that is a first toner image carrier that moves endlessly in the direction of the arrow, and the second image forming unit 30 is a second toner image carrier that is endlessly moved in the direction of the arrow. Two intermediate transfer belts 31 are provided. Four first image forming units 80 </ b> Y, 80 </ b> C, 80 </ b> M, and 80 </ b> K, which are first image forming units, are arranged on the upper tension surface of the first intermediate transfer belt 21. On the other hand, four second image forming units 81Y, 81C, 81M, and 81K as second image forming units are disposed on the upper stretched surface of the second intermediate transfer belt 31. Y, C, M, and K along the numbers of the first and second image forming units correspond to the colors of the toner to be handled. Y is yellow, C is cyan, M is magenta, and K is black. Means. Y, C, M, and K are similarly applied to the photosensitive member 1 that is provided in the first and second image forming units and rotates together with the first intermediate transfer belt 21 and the second intermediate transfer belt 31. It is along. The photoreceptors 1Y to 1K are arranged at the same interval in each image forming unit, and at least at the time of image formation, are in contact with a part of the stretched portion between the intermediate transfer belts 21 and 31, respectively. These contacting surfaces are referred to as a first image receiving surface F1 and a second image receiving surface F2, respectively, and their positional relationships are shown in FIG.

As shown in FIG. 2, when the angle of the angle including the first and second image forming units among the angles formed by the first image receiving surface F1 and the second image receiving surface F2 is α, the value of α is 180. It should be larger than 270 ° and smaller than 270 °.
Further, a configuration is employed in which at least a part of the horizontal projection surface of the first intermediate transfer belt 21 in the upper portion and the horizontal projection surface of the second intermediate transfer belt 31 in the lower portion overlap each other. That is, the second intermediate transfer belt 31 at the lower part is shaped so as to be recessed below the first intermediate transfer belt 21 at the upper part. By adopting such a configuration, a more compact layout is possible in the lateral direction.

Although the toner colors to be handled are different, the configurations of the first image forming units 80Y, 80C, 80M, and 80K are the same. Therefore, the configuration of the first image forming unit 80 will be described with reference to FIG.
In FIG. 3, when the image forming apparatus 100 is in operation, a photosensitive member 1 that is a cylindrical image carrier that is rotatably supported by a drive source (not shown) so as to rotate in the direction of the arrow is disposed. Then, an image forming member such as a scorotron charger 3, an exposure device 4, a developing device 5, a cleaning device 2, and a photostatic device Q, which are charging means according to an electrophotographic process, an electric potential sensor S1, and an image around the photoreceptor 1. A sensor S2 is provided.
The photoreceptor 1 is obtained by forming an organic photosensitive layer (OPC), which is a photoconductive substance, on an aluminum cylinder surface having a diameter of about 30 to 120 mm, for example. A photoconductor on which an amorphous silicon (a-Si) layer is formed can also be used. A belt-like photoreceptor can also be employed. The cleaning device 2 includes a cleaning brush 2a, a cleaning blade 2b, and a recovery member 2c, and removes and recovers foreign matters such as toner remaining on the surface of the photoreceptor.

  The exposure device 4 scans the light corresponding to the image data for each color on the surface of each photoreceptor 1 that has been uniformly charged by the charging means, and forms an electrostatic latent image. The exposure apparatus 4 in the illustrated example is an exposure apparatus that includes an LED (light emitting diode) array and an imaging element as light emitting elements, but uses a laser light source, a polygon mirror, etc., and beam light modulated according to image data to be formed. It is also possible to adopt a laser scanning type exposure apparatus. In addition to the charger 3, a type that makes contact with the surface of the photoreceptor 1, such as a charging roller, can be used as the charging means.

The development at this time is a development system that employs a two-component developer composed of toner and carrier. The electrostatic latent image for each color formed on the surface of each photoconductor 1 by the exposure device 4 with respect to the negatively charged photoconductor 1 is a toner of a predetermined color having the same polarity (negative polarity) as the charged polarity of the photoconductor. Developed to become a visible image. So-called reversal development is performed.
The yellow (Y), cyan (C), magenta (M), and black (K) toners are detected by the magnetically permeable toner detecting means 5e when consumed by the developing device that handles each color. Then, the toner of each color is supplied to each developing device 5 from a toner cartridge 86 or 87 provided in the toner cartridge housing portion 85 inside the image forming apparatus 100 by a supply unit (not shown).
As this supply means, a system using a known MONO pump can be adopted. According to this method, there are few restrictions on the installation location of the toner cartridge, which is advantageous for space allocation in the image forming apparatus. Further, since the toner can be replenished in a timely manner, it is not necessary to provide a large toner storage space in the developing device, and the developing device can be downsized.

  The developing device 5 is provided with screws 5c and 5d for stirring and conveying the toner and carrier. When the developing device 5 is mounted on the image forming apparatus 100, one end of the toner replenishing unit is connected to a part of the screw 5d. The toner is supplied by the screw 5c to the developing roller 5a that is a developer carrying member that rotates in the direction of the arrow, but the thickness of the toner layer on the surface of the developing roller 5a becomes a predetermined thickness by the blade 5b that is a developer regulating member. It is regulated as follows. The developing roller 5a is a cylinder made of stainless steel or aluminum, and is supported by the frame of the developing device 5 so as to be rotatable and a distance from the photoreceptor is properly secured, and a predetermined magnetic force line is formed inside. Has a magnet.

Since the second image forming units 81Y, 81M, 81C, and 81K used in the second image unit 30 have the same configuration, the second image forming unit 81 will be described with reference to FIG.
The second image forming unit 81 shown in FIG. 4 has the same constituent members as the first image forming unit 80, but the rotational direction of the photosensitive member 1 is different from that of FIG. However, they are mutually shaped with respect to the y-axis passing through the rotation axis 1a of the photosensitive member 1 indicated by an arrow in the drawing. This shape is an important matter although it is related to the arrangement of image forming members provided around the photoreceptor 1. In other words, consideration is given to a method of connecting a connecting portion with the image forming apparatus 100, for example, a connecting portion with a driving unit, an electrical connecting portion, a toner supplying portion, and a toner discharging portion. Thereby, the first image forming units 80Y, 80M, 80C, and 80K can be compatible with the second image forming units 81Y, 81M, 81C, 81K. Therefore, it is not necessary to manufacture the developing device, the cleaning device, and the parts individually for the first image forming unit and the second image forming unit, and the efficiency in manufacturing parts and managing parts is high, and the overall cost can be reduced. It is peeled off.
There are the following two conditions that can be used in the first image unit 20 and the second image unit 30 in the form of an axial object with the first and second image forming units as the center of the y-axis. As a first condition, a peripheral surface where no member for image formation is provided around the photosensitive member 1 is secured, that is, a wide range in which the intermediate transfer belt can contact is secured. The second condition is that the arrangement angle between the image receiving surface F1 of the first intermediate transfer belt 21 and the image receiving surface F2 of the second intermediate transfer belt 31 is appropriate.

The first condition and the second condition will be described with reference to FIGS. 3 or 4, the intermediate transfer belt 21 or 31 can be installed between the arrows A1 and A2. That is, the A1 side is the developing device 5, and the A2 side is that the intermediate transfer belt can be installed in a range where the belt does not contact the cleaning device 2. The inclusion of the second image bearing belt 31 in FIG. 3 shows an intermediate transfer belt on which the target image forming unit can be arranged with respect to the y axis including the rotation axis 1 a of the photoreceptor 1.
If the image forming unit 80 is turned upside down with respect to an axis orthogonal to the y axis and used as the image forming unit 81, the toner is stirred at least in the developing device 5 due to the influence of the earth's gravity and the toner on the developing roller 5a. Conditions such as replenishment are different. As a result, the members of the image forming unit 80 must be optimized, and parts cannot be shared.

  As shown in FIG. 2, as the positional relationship between the image receiving surface of the first intermediate transfer belt 21 and the image receiving surface of the second intermediate transfer belt 31, the angle α is greater than 180 degrees and 270 degrees or less. When the angle is preferably 210 ° to 255 °, the first intermediate transfer belt shown in FIG. 3 and the second intermediate transfer belt shown in FIG. 4 can be used as the image forming unit. That is, it is possible to avoid an image forming unit that makes it impossible to share parts as described above.

As shown in FIG. 2, since the recording paper conveyance path 43 can be secured almost horizontally and linearly above the second image forming unit 81, the recording paper conveyance and reliability are excellent, and the entire image forming apparatus The unity becomes good.
In particular, the first intermediate transfer belt is long and flat in the horizontal direction, and the second intermediate transfer belt 31 is long and inclined in the vertical direction. 2 The space in the height direction of the intermediate transfer belt 31 is increased. However, as shown in FIGS. 3 and 4, the image forming unit has a vertical dimension with respect to the horizontal dimension by ensuring a wide range in which the image forming unit can contact the intermediate transfer belt. Is a short configuration. For this reason, in the second image unit, the image forming unit is disposed obliquely in the vertical direction with respect to the first image unit in which the image forming unit is disposed substantially horizontally. In addition, the interval between the image forming units can be set short, and the space in the height direction of the second image unit can be saved. As a result, the degree of freedom in layout in the height direction is increased, so that the recording paper conveyance path can be set to an ideal height considering operability, a paper feeding device, and a post-processing device (not shown). .

In addition, since the paper feeders having the same height as that occupied by the second intermediate transfer belt 31 can be installed side by side, a paper feeder capable of storing a large amount of recording paper can be installed. In addition, the sheet feeding surface on the upper surface of the sheet feeding device and the recording sheet conveyance path can be made substantially the same height, and the recording sheet feeding / conveying reliability can be ensured.
As described above, the image forming units 80 and 81 have many common points due to the arrangement of the image forming apparatus around the photosensitive member and the arrangement of the intermediate transfer belt, that is, the realization of the second condition. But it is very advantageous.

  Next, the intermediate transfer belt will be described. The first intermediate transfer belt 21 as a first toner image carrier is supported by a plurality of rollers 23, 24, 25, 26 (two), 27, 28, 29 and travels in the direction of the arrow. The first image forming units 80Y to 80K are provided below the photoreceptors 1Y, 1C, 1M, and 1K. The first intermediate transfer belt 21 is endless, and is stretched and arranged so that a part of each photoconductor after the developing process comes into contact. In addition, a primary transfer roller 22 serving as an intermediate transfer unit is provided on the inner peripheral portion of the first intermediate transfer belt 21 so as to face the photoreceptors 1Y, 1C, 1M, and 1K.

A cleaning device 20 </ b> A is provided on the outer periphery of the first intermediate transfer belt 21 at a position facing the roller 23. The cleaning device 20 </ b> A wipes away unnecessary toner remaining on the surface of the first intermediate transfer belt 21 and foreign matters such as paper dust.
Members related to the first intermediate transfer belt 21 are integrally configured as the first image forming unit 20 and can be attached to and detached from the image forming apparatus 100.

On the other hand, the second intermediate transfer belt 31 is supported by a plurality of rollers 33, 34, 35, 36 (two), 37, 38 and travels in the direction of the arrow. The second image forming units 81Y to 81K are provided in contact with the photoreceptors 1Y, 1C, 1M, and 1K. The second intermediate transfer belt 31 is endless, and is stretched and arranged so that a part of each photoconductor after the developing process comes into contact. A primary transfer roller 32 serving as an intermediate transfer unit is provided on the inner peripheral portion of the second intermediate transfer belt 31 so as to face the photoreceptors 1Y, 1C, 1M, and 1K. Further, the roller 34 facing the conveyor belt 51 is connected to the ground.
A cleaning device 30 </ b> A is provided on the outer periphery of the second intermediate transfer belt 31 at a position facing the roller 33. The cleaning device 30 </ b> A wipes off unnecessary toner remaining on the surface of the intermediate transfer belt 31 and foreign matters such as paper dust.
Members related to the second intermediate transfer belt 31 are integrally configured as the second image forming unit 30 and can be attached to and detached from the image forming apparatus 100.

The intermediate transfer belts 21 and 31 are belts having, for example, a resin film or rubber having a base thickness of 50 to 600 μm as a base. The toner image carried by each photoconductor 1 has a resistance value that enables electrostatic transfer onto the belt surface by a bias applied to the primary transfer rollers 22 and 32. As an example of such a belt, carbon is dispersed in polyamide, and the volume resistance value thereof is adjusted to a resistance of about 10 ⁶ to 10 ¹² Ωcm. Belt detent ribs for stabilizing the running of the belt are provided on one side or both ends of the belt.

As the primary transfer rollers 22 and 32 serving as intermediate transfer means, a metal roller serving as a core metal is coated with a conductive rubber material, and a bias is applied to the core metal part from a power source (not shown). Raised. Here, as for the conductive rubber material, carbon is dispersed in urethane rubber, and the resistance is adjusted to a volume resistance of about 10 ⁵ Ωcm.

  The image forming apparatus 100 can perform monochrome recording using only black toner. In such a case, there are photosensitive members that are not used. Therefore, not only the unused photoreceptors 1Y, 1C, 1M or the developing device 5 are operated, but also a mechanism for keeping these unused photoreceptors and the image carrying belt 21 or 31 in a non-contact state is provided. An internal frame (not shown) that supports the roller 26 and the primary transfer roller 22 is provided, and is supported so as to be rotatable about a certain point. Then, by rotating in a direction away from the photosensitive member, only the photosensitive member 1K comes into contact with the first intermediate transfer belt 21 or the second intermediate transfer belt 31, and an image forming process is executed, whereby monochrome with black toner is performed. Create an image. This is advantageous in terms of improving the life of the photoreceptor.

Further, on the outer periphery of the first intermediate transfer belt 21, in the vicinity of the support roller 28, a first secondary transfer roller 46 serving as a pre-stage transfer unit is provided. The secondary transfer roller 46 is provided on the inner peripheral portion of the second intermediate transfer belt 31 and is in contact with the inner peripheral surface of the second intermediate transfer belt. The secondary transfer roller 46 is made of a metal roller serving as a core metal and is coated with conductive rubber, and a bias is applied to the core metal part from a power source (not shown). Carbon is dispersed in the conductive rubber, and the volume resistance is adjusted to about 10 ⁷ Ωcm. By applying a bias to the secondary transfer roller 46 while passing the recording paper P between the first intermediate transfer belt 21 and the secondary transfer roller 46, an image by the toner carried by the first image carrying belt 21 is recorded on the recording paper. Transferred to P.

On the outer periphery of the second intermediate transfer belt 31, in the vicinity of the support roller 34, a transfer charger 47, which is a subsequent transfer means, is provided. The transfer charger 47 is a known type, and a thin wire of tungsten or gold is used as a discharge electrode, held by a casing, and a transfer current is supplied to the discharge electrode from a power source (not shown). By supplying a transfer current while passing the recording paper P between the second intermediate transfer belt 31 and the transfer charger 47, an image by the toner carried by the second image carrying belt 31 is transferred to the recording paper P. The polarity of the transfer current supplied to the transfer roller 46 and the transfer charger 47 is a positive polarity opposite to the polarity of the toner.
As described above, the toner image is formed on both sides of the recording paper P by the double-sided transfer device as the double-sided transfer means including the first intermediate transfer belt 21, the secondary transfer roller 46, the second intermediate transfer belt 31, and the transfer charger 47. Is transcribed.

  On the right side of the image forming apparatus 100, a paper feeding device 40 that accommodates the recording paper P is provided. Paper feed device (tray) 40a storing a large amount of recording paper in a plurality of stages, for example, the upper stage, and three stages of paper feed cassettes 40b, 40c, and 40d below each are pulled out to the front (operation surface side) perpendicular to the paper surface. It is arranged to be possible. Different types of recording paper P are stored in the paper feed tray 40a and the paper feed cassettes 40b, 40c, and 40d, respectively. Among these, the uppermost recording paper is selectively fed and separated by the corresponding paper feeding / separating means 41a to 41d, and only one sheet is reliably fed by the recording paper transport paths 43B and 43A by the plurality of transport roller pairs 42B. Sent to.

  A pair of registration rollers 45 are provided in the recording paper conveyance path 43A in order to take a feeding timing for sending the recording paper P to the secondary transfer position which is the first and second transfer positions. Further, a lateral registration correction mechanism 44 for setting the position perpendicular to the conveyance direction of the recording paper to a normal position is provided in the recording paper conveyance path 43A. The lateral registration correction mechanism 44 includes the following. The recording paper P is slid and conveyed so that a horizontal reference guide (not shown) and a skew roller pair are formed, and the horizontal end of the recording paper is pressed against the reference guide. Then, the recording paper is aligned with a predetermined position. The reference guide is moved and arranged at a predetermined position depending on the size of the recording paper. The lateral registration correction mechanism 44 is composed of a regulating member that presses both sides of the recording paper for a short time and a plurality of times from both lateral directions of the recording paper with respect to the recording paper conveyance direction to align the recording paper at a predetermined position. The jogger method may be used.

The recording paper P is conveyed from the registration roller pair 45 toward the preceding transfer area formed by the first intermediate transfer belt 21 and the second intermediate transfer belt 31 contacting each other. Thereafter, the sheet is conveyed toward a subsequent transfer area constituted by the second intermediate transfer belt 31 and the transfer charger 47.
Note that recording paper can be supplied to the recording paper conveyance path 43C having the conveyance roller pair 42C from another paper feeding device 300 that can be installed upstream in the conveyance direction. The upper paper feed surface of the paper feed tray 40a is arranged so that the uppermost recording paper of the paper feed tray 40a is fed and then conveyed substantially horizontally without being bent. Therefore, even thick recording paper and highly rigid paperboard can be reliably fed. In addition, it is convenient to adopt an air paper feed composed of a vacuum mechanism so that the paper feed tray 40a can reliably feed even when recording papers having various characteristics are stored. Although not shown, a sensor for detecting a recording sheet is provided at a key point of the recording sheet conveyance path, and serves as a trigger for various signals based on the presence of the recording sheet.

  In order to convey the recording sheet that has passed through the latter transfer area on the extension of the recording sheet conveying path 43A to the fixing nip in the fixing device 60 as fixing means provided downstream in the conveying direction of the recording sheet while maintaining a flat state. A recording paper transport device 50 is provided.

  FIG. 5 is a schematic configuration diagram of the recording paper transport device 50. The recording paper transport device 50 includes rollers 52, 53, 54, 55, and 56 that support a transport belt 51 that is a transport member that moves endlessly in the direction of the arrow. The conveyance belt 51 is a sheet conveyance belt 51 where the recording paper P discharged from the second transfer unit of the second image unit 30 is wound around a belt by a receiving roller 52 which is one of a plurality of stretching rollers. Receive on. At a timing earlier than this reception, a negative polarity charge is applied to the front surface of the conveyor belt 51 by the electrostatic chuck charger 57 as charge applying means. The recording paper P is charged with a positive polarity by the transfer charger 47 or the like, and the recording paper P is applied to the front surface of the transport belt 51 by applying a negative polarity charge to the front surface of the transport belt 51. It can be electrostatically adsorbed.

The conveyance belt 51 having the recording paper P electrostatically attracted to the front surface conveys the recording paper P from the right side to the left side in the drawing along with its endless movement. Then, the recording paper P is delivered to the fixing device 60 as fixing means disposed on the left side of the paper transport unit 50 in the drawing. At a timing earlier than this delivery, a positive polarity charge is applied to the transport belt 51 by the charge removal charger 58. By applying this electric charge, the transport belt 51 that has been charged with a negative polarity is discharged. By neutralizing the transport belt 51 by the charge removal charger 58, the recording paper P that has been electrostatically attracted to the front surface of the transport belt 51 until then is easily separated from the belt. Of the plurality of stretching rollers, a belt that suddenly changes the moving direction in accordance with the curvature of the separation roller 54 at a belt winding position by the separation roller 54 disposed closest to the fixing device 60. The recording paper P is separated from the paper and transferred to the fixing device 60.
Further, a cleaning device 50 </ b> A is provided outside the transport belt 51 so as to contact the cleaning blade 50 </ b> B so as to face the roller 55.

  As the conveyance belt 51, a base material made of polyimide or polyamide is used. This base material is the belt itself when the transport belt 51 has a single-layer structure. Further, when the conveyor belt 51 has a multilayer structure, it is the thickest layer. Since polyimide or polyamide is a material having extremely high heat resistance, the use of them as the base material of the transport belt 51 allows the transport belt 51 to be disposed in the vicinity of the fixing device 60 for paper delivery. Exhibits excellent heat resistance. Further, the expansion and contraction of the conveyance belt 51 and the life reduction due to the influence of heat from the fixing device 60 can be suppressed.

  Further, the distance L2 from the receiving roller 52 to the separation roller 54 is set to be longer than the length of the maximum size recording paper P that can be stored in the recording paper storage means such as the paper feed cassette 40. Thereby, the paper conveyance between the double-sided transfer device and the fixing device 60 can be made independent of the double-sided transfer device. Specifically, it is possible to create a state in which the recording paper P that has passed through the latter transfer area is completely separated from the second intermediate transfer belt 31 and does not enter the fixing nip. In such a state, the recording paper P can be transported satisfactorily even if the process linear speed, which is the linear speed of each intermediate transfer belt or each photoconductor, is different from the linear speed of the paper transport belt 51. With such a configuration, the paper conveyance speed by the fixing device 60 can be changed according to the required amount of fixing heat, and the amount of fixing heat per unit time for the recording paper can be changed. Then, the amount of heat for fixing the recording paper P can be changed without changing the heating temperature by the fixing device 60. For this reason, even when the required fixing heat amount becomes considerably large, the fixing process can be performed on the recording paper with a sufficient heating amount without considerably increasing the heating temperature by the fixing device 60. Even if the required fixing heat amount is considerably small, the fixing process can be performed with an appropriate heating amount without excessively heating the recording paper. Therefore, it is assumed that the required fixing heat amount becomes considerably large, and avoiding an increase in cost due to setting the heat resistance of the fixing device 60, and fixing failure and hot offset due to a change in the required fixing heat amount for each printout. Can be suppressed.

  For the detection of the recording paper, for example, one of the paper feed trays 40a to 40d is set exclusively for the thick paper, and when the recording paper is fed from the paper feeding tray dedicated to the thick paper, the speed of the conveying belt is set to the thick paper. Decelerate to the corresponding speed. Further, the surface roughness or thickness of the recording paper P may be measured by an optical sensor while passing through the roller 42A, and the speed of the conveying belt may be changed based on the measurement result.

  Whether the trailing edge of the recording paper has passed through the transfer charger 47 is determined as follows. A sensor for detecting the transfer paper immediately after passing through the transfer charger 47 is provided, and when this sensor detects the leading edge of the transfer paper, time measurement is started. The storage unit stores a time corresponding to each paper size in a table form. Information on the size of the transferred transfer paper is acquired from the image data input to the exposure apparatus or the paper feed tray 40. A corresponding time is determined from the acquired transfer sheet size information. If the measured time exceeds the determined time, it is determined that the transfer paper has passed the transfer charger 47, and the transport speed of the transport belt 51 is reduced.

  A fixing device 60 having a heating unit is provided on the downstream side of the recording paper transport device 50 in the recording paper transport direction. A type having a heater inside the roller, a belt fixing device that runs a heated belt, a fixing device that employs induction heating as a heating method, and the like can be employed. In order to make the color and glossiness of the images on both sides of the recording paper the same, the material, hardness, surface property, etc. of the fixing roller and the fixing belt are made up and down the same. Further, the fixing condition is controlled by a full color and monochrome image, or one side or both sides, or controlled by a control unit (not shown) so as to obtain an optimum fixing condition according to the type of recording paper. A cooling roller pair 70 having a cooling function is provided in the conveyance path after fixing in order to cool the recording sheet after fixing and stabilize the unstable toner state at an early stage. As the cooling roller pair 70, a heat pipe structure roller having a heat radiating portion can be adopted. The cooled recording sheet is discharged and stacked on a discharge stack unit 75 provided on the left side of the printer main body 100 by a discharge roller pair 71. In order to stack a large amount of recording paper, the paper discharge stack unit employs a mechanism in which a receiving member moves up and down according to a stack level by an elevator mechanism (not shown). Note that the recording paper can also be transported to another post-processing apparatus through the paper discharge stack unit 75. As another post-processing apparatus, an apparatus for bookbinding such as drilling, cutting, folding, and binding can be connected.

  In the space 85, the first toner cartridges 86 </ b> Y to 86 </ b> K for the respective colors in which unused toner corresponding to the developing devices of the first image forming units for the respective colors are accommodated are detachably accommodated by a toner conveyance unit (not illustrated). ing. In addition, in the space 85, the second toner cartridges 87Y to 87K for each color in which unused toner corresponding to the developing device of the second image forming unit for each color is accommodated by a toner conveying unit (not shown) can also be attached and detached. It is stored. In the configuration shown in FIG. 1, the toner cartridges are separated for the image forming units arranged above and below, but they can be shared. The toner cartridges 86K and 87K for black toner, which is highly consumed, can have a particularly large capacity. This storage space 85 is on the back side when viewed from the operation direction on the upper surface of the image forming apparatus, and a plane portion is secured on the front side of the upper surface of the image forming apparatus, so that it can be used as a work table.

The operation and display unit 90 provided on the upper surface of the image forming apparatus 100 includes a keyboard and the like. Accordingly, conditions for image formation can be input, the state of the apparatus is displayed on the display unit, and information exchange between the operator and the image forming apparatus 100 is facilitated. The waste toner storage unit 88 provided in the image forming apparatus 100 is connected to the image forming unit cleaning device 2, the intermediate transfer belt cleaning devices 20 </ b> A and 30 </ b> A, and the conveyance belt 51 cleaning device 50 </ b> A. Then, foreign substances such as waste toner and paper dust sent from these are collected and stored in a lump. Since these cleaning devices (2, 20A, 20B, 50A) are not provided with a large-capacity waste toner storage section, the cleaning device can be reduced in size, and the waste toner disposal operability is also good. A full sensor (not shown) is used to issue a warning about toner disposal in the waste toner storage unit 88 or container replacement.
Further, in the electrical / control device 95 provided inside the image forming apparatus 100, various power supplies, control boards, and the like are protected and housed in a sheet metal frame.
The inside of the image forming apparatus becomes hot due to heat from the fixing device 60 or heat generated from the electrical device. However, as a countermeasure against this, a fan 96 is provided to prevent deterioration of the function due to heat of the internal members. The fan 96 is coupled to the heat radiating portion of the cooling roller pair 70 to ensure the cooling effect of the cooling roller pair 70.
In FIG. 1, reference numeral 200 denotes a document reading device, and 300 denotes a paper feeding device that can be additionally installed.

Next, an operation at the time of single-sided recording in which the image forming apparatus 100 forms a full-color image on one side of the recording paper P will be described.
There are basically two types of single-sided recording methods that can be selected. One of the two types is a method in which an image carried on the first intermediate transfer belt 21 is directly transferred to one side of the recording paper, and the other method is that an image carried on the second intermediate transfer belt 31 is transferred. This is a method of transferring directly to one side of a recording paper. Here, due to the configuration of the image forming apparatus 100, when the image carried on the first intermediate transfer belt 21 is directly transferred to one side of the recording paper, the image is formed on the upper surface of the recording paper. On the other hand, when the image carried on the second intermediate transfer belt 31 is directly transferred to one side of the recording paper, the image is formed on the lower surface of the recording paper. In the case where the data to be recorded is a plurality of pages, it is convenient to control the image forming order so that the pages are aligned on the paper discharge stack unit 75. An explanation will be given of a method in which an image is carried on the first intermediate transfer belt 21 and then transferred onto a recording sheet so that the pages are arranged in order from the image data of the last page.

When the image forming apparatus 100 is operated, the photoreceptors 1Y, 1C, 1M, and 1K in the first intermediate transfer belt 21 and the first image forming units 80Y to 80K rotate. At the same time, the second intermediate transfer belt 31 rotates, but the photoreceptors 1Y, 1C, 1M, and 1K in the second image forming units 81Y to 81K are separated from the second intermediate transfer belt 31 and are not rotated. First, image formation by the image forming unit 80Y is started. By the operation of the exposure device 4 composed of an LED (light emitting diode) array and an imaging element, light corresponding to image data for yellow emitted from the LED is irradiated onto the surface of the photoreceptor 1Y uniformly charged by the charging device 3. Thus, an electrostatic latent image is formed.
This electrostatic latent image is developed with yellow toner by the developing roller 5a, becomes a visible image, and is electrostatically transferred onto the first intermediate transfer belt 21 that moves in synchronization with the photoreceptor 1Y by the transfer action of the primary transfer roller 22. Primary transfer. Such latent image formation, development, and primary transfer operations are sequentially performed in the same manner on the photosensitive members 1C, 1M, and 1K.
Thereafter, yellow, cyan, magenta, and black toner images are carried on the first intermediate transfer belt 21 as sequentially overlapping full-color toner images and are moved in the direction of the arrow together with the first intermediate transfer belt 21. .

  At the same time, the recording paper P used for recording is fed out from the paper feed tray 40a or the paper feed cassettes 40b to 40d in the paper feed device 40 by one of the paper feed / separation means 41a to 41d for its supply. Then, the paper is transported to the recording paper transport path 43A by the transport roller pairs 42B and 42C. Further, the surface roughness and thickness maintenance of the recording paper P are measured by the optical sensor unit 140 while passing through the roller 42A. Before the leading edge of the recording paper is not picked up by the registration roller pair 45, the lateral registration correction mechanism 44 operates so as to push both sides of the recording paper from both lateral directions with respect to the conveyance direction of the recording paper. Can be aligned. The registration roller pair 45 is stationary, and the leading edge of the recording paper is stationary while entering the nip of the registration roller pair 45. Then, the registration roller pair 45 is rotated at a timing so that the position of the image on the first intermediate transfer belt 21 is normal, and the recording paper is conveyed to the transfer area.

  The full color toner image on the first intermediate transfer belt 21 is transferred to the upper surface of the recording paper P conveyed in synchronization with the first intermediate transfer belt 21 by receiving a transfer action by the secondary transfer roller 46. The bias applied to the secondary transfer roller 46 has a positive polarity opposite to the charging polarity of the toner. After the transfer, the surface of the first intermediate transfer belt 21 is cleaned by the belt cleaning device 20A. Further, foreign matters such as toner remaining on the surfaces of the photoreceptors 1Y, 1C, 1M, and 1K in the first image forming units 80Y to 80K that have finished the primary transfer are removed by the cleaning brush 2a and the cleaning blade 2b of the cleaning device 2. Made. The surface of each photoconductor is neutralized by a residual potential by the static eliminator Q to prepare for the next image formation / transfer process. The removed foreign matter such as toner is sent to the collection unit 88 by the collection unit 2c. Sensors S1 and S2 detect whether the surface potential after exposure on the surface of the photoconductor and the concentration of toner attached to the surface of the photoconductor after the development process are appropriate, and appropriately set image forming conditions. Information is sent to a control means (not shown) for control.

The recording paper P onto which the toner image that has been superposed on the first intermediate transfer belt 21 is transferred is transported toward the fixing device 60 by the transport belt 51 of the recording paper transport device 50.
The toners of the respective colors superimposed on the recording paper P are subjected to the fixing action by the heat of the fixing device 60, and are melted and mixed to form a completely color image. Since toner is contained only on one side (upper surface) of the recording paper, less heat energy is required for fixing compared to double-sided recording having toner on both sides. A control means (not shown) optimally controls the power used by the fixing device according to the image. Until the fixed toner is completely fixed on the recording paper, it is rubbed against a guide member or the like in the conveyance path, and an image is lost or distorted. In order to prevent this problem, the cooling roller pair 70 which is a cooling means is operated to cool the toner and the recording paper. Thereafter, the paper is discharged by the paper discharge roller 71 onto the paper discharge stack 75 with the image surface facing upward. In the paper discharge stack section 75, the image forming order is programmed so that the recorded matter of the young pages is stacked so as to be sequentially stacked on top, so the page order is aligned. Since the paper discharge stack unit 75 descends as the number of discharged recording sheets increases, the recording sheets can be stacked in an orderly and reliable manner, and the page order is not disturbed. Instead of directly stacking the recorded recording paper on the paper discharge stack section 75, a punching process can be performed, or the recording paper can be conveyed to a post-processing device such as a sorter, a collator, a binding device, or a folding device.

Next, the operation during double-sided recording in which images are formed on both sides of the recording paper P will be described. When a start signal is input to the printer main body, images for each color sequentially formed by the first image forming units 80Y, 80C, 80M, and 80K described in the single-sided recording operation are sequentially applied to the first intermediate transfer belt 21 in order. Transfer. Substantially in parallel with the step of supporting the first image, the images for the respective colors sequentially formed by the second image forming units 81Y, 81C, 81M, 81K are sequentially primary-transferred to the second intermediate transfer belt 31, and the second The process of carrying as an image is performed. As shown in FIG. 1, the interval between the second image forming units in the belt conveyance direction is set closer than the interval between the first image forming units. Thereby, in order for the first image and the second image to coincide with each other at the leading end of the recording paper in the conveyance direction, the formation of the second image is started after the start of the formation of the first image. . Since the recording paper is stopped and retransmitted by the registration roller pair 45, the recording paper is fed in consideration of the time, and is aligned by the lateral registration correction mechanism 44. The registration roller pair 45 transports the recording paper to the preceding transfer area constituted by the transfer roller 46 serving as the preceding transfer means and the first intermediate transfer belt 21 at a timing. A transfer current having a positive polarity is supplied to the transfer roller 46, and the image is transferred from the first intermediate transfer belt to one side (upper surface in the drawing) of the recording paper P.
In this way, the recording paper P having an image on one side is continuously fed to the subsequent transfer area where the transfer charger 47 as the subsequent transfer means is provided by the conveying action of the second intermediate transfer belt 31. Then, by supplying a positive polarity transfer current to the charger, the full-color second image previously carried on the second intermediate transfer belt 31 is collectively transferred onto the lower surface of the recording paper P.

The recording paper P having the full color toner image transferred on both sides in this way is transported to the fixing device 60 by the transport belt 51. Then, the toner image on both sides of the recording paper is melted and mixed under the fixing process by heat of the fixing device 60. The recording sheet subsequently passes through the cooling roller pair and is discharged onto the discharge stack unit 75 by the discharge roller 71.
When double-sided recording is performed on a plurality of pages of recording paper, the image forming order is controlled so that an image of a young page becomes the lower surface and is stacked on the paper discharge stack unit 75. When the image formation is completed, it is taken out from the stack, and when the top and bottom surfaces are reversed, the recorded matter is one page in order from the top, the second page on the back, the third page on the second sheet, and the fourth page on the back. Such control of image forming sequence and control such as increasing the power input to the fixing device compared to the one-side recording are executed by a control means (not shown).

  As for the single-sided recording and the double-sided recording operation, an example in which full-color recording is executed has been described, but monochrome recording using only black toner is also possible. When the need for maintenance or replacement of parts arises, maintenance is performed by opening an unillustrated exterior cover or the like. At this time, it is convenient to unitize the members constituting the image forming units 80 and 81 shown in FIGS. 3 and 4 and replace them as process cartridges. The configuration of the process cartridge is such that the image forming members excluding the intermediate transfer belts 21 and 31 and the primary transfer roller 22 and the exposure device 4 from the configuration shown in FIGS. It is housed and configured. Further, it can be realized by appropriately providing a guide part and a handle for attaching and detaching to the printer main body 100.

  Next, features of the present embodiment will be described. FIG. 6 is a graph showing the relationship between the height of the second toner image formed on the lower surface of the recording paper (hereinafter referred to as pile height) and the granularity of the image formed on the lower surface of the recording paper. For pile height, the solid image was transferred onto the coated paper with a double-sided transfer device while changing the adhesion amount, and the solid image transferred onto the coated paper was measured. The pile height was measured using a laser microscope (VK-9500) manufactured by Keyence Corporation. The granularity of the image is based on the solid image that is transferred onto the lower surface of the coated paper with the amount of adhesion corresponding to each pile height, conveyed to the fixing device by the conveyor belt 51, and fixed on the lower surface of the fixed coated paper. evaluated. A toner having a particle diameter of 6.5 [μm] was used.

  As shown in FIG. 6, as the pile height increases, the granularity of the image deteriorates and the image has a rough feeling. This is presumably because the unfixed image formed on the surface of the recording paper on the conveyance belt side was disturbed by the conveyance belt 51 because the pile height was high.

This will be specifically described below. FIG. 7A is a diagram illustrating a state in which an unfixed image having a high pile height is disturbed by the conveyance belt 51. FIG. 7B shows a state in which an unfixed image having a low pile height is disturbed by the conveyance belt 51.
On the recording paper P, toner particles T are adhered in layers, and as shown in FIGS. 7A and 7B, an unfixed image having a high pile height is compared with an unfixed image having a low pile height. Many toner particles T are stacked. Here, if there is a difference in speed between the second intermediate transfer belt 31 and the paper conveyance belt 51, the conveyance speed of the recording paper P when the recording paper P is transferred to the conveyance belt 51 and the conveyance belt 51. There is a speed difference between the transfer speed. Further, when the speed of the transport belt 51 is rapidly changed while the transport belt 51 is transporting the recording paper P, the recording paper P tends to move at a constant speed due to inertia. A speed difference occurs between the two. When such a speed difference occurs (in the drawing, the speed of the transport belt 51 is lower than the transport speed of the recording paper P), the transport belt 51 rubs against the unfixed image, and the stacked toner particles. T is shifted. At this time, in the case of an unfixed toner image formed by stacking the three toner particles T in FIG. 7A, the toner particles T in the second layer and the third layer are shifted by the conveying belt 51. On the other hand, in the case of the unfixed toner image formed by stacking the two toner particles T in FIG. 7B, only the toner image in the second layer is merely shifted by the conveying belt 51, so that FIG. The amount of displacement D ′ is smaller than the amount of displacement D of the unfixed image of the three layers having a high pile height. As described above, when the same speed difference is generated between the conveyance belt 51 and the recording paper P, it can be seen that the lower pile height can suppress the image disturbance than the higher pile height. In the above description, for the sake of simplicity, the description is given schematically. However, in practice, if the height of the pile height of the unfixed image exceeds 2.7 times the toner particle size, the image is distorted. Became prominent.

  Therefore, in this embodiment, the pile height of the unfixed image formed on the surface (lower surface) of the recording paper on the conveyance belt side is set lower than the pile height of the unfixed image formed on the upper surface of the recording paper. The unfixed image formed on the surface (lower surface) on the side of the conveying belt is prevented from being disturbed by the conveying belt 51.

  In order to suppress the pile height of the unfixed image to 2.7 times or less in the full-color image, the pile height of the monochromatic solid image is 10 [μm] or less when the toner particle size is 6.5 [μm]. Experiments have confirmed that the toner particle size needs to be 1.25 times or less. If the toner particle size is reduced, the gap between the toner particles is improved, so that the amount of deviation by the conveying belt 51 is reduced, and there is some margin in the height of the pile height with respect to the toner particle size. .

  In the image forming apparatus of Embodiment 1, the development potential of the second image forming unit 81 is controlled so that the pile height of the single-color solid image is 1.25 times or less of the toner particle size. The relationship between the pile height and the development potential when a monochromatic solid image is formed is as shown in FIG. 8, and based on this relationship, the pile height of the monochromatic solid image is 1.25 times the toner particle size. The second image forming unit 81 is controlled so as to have the following development potential. The development potential may be controlled by controlling the development bias applied to the development roller 5a, or by controlling the exposure energy in the exposure apparatus 4.

Further, the development potential may be controlled based on the detection result of the pile height detecting means provided with a pile height detecting means for detecting the pile height of the single color solid image formed on the second intermediate transfer belt. The pile height is detected at a predetermined timing such as when the power is turned on or after a predetermined number of sheets are passed.
FIG. 9 is an explanatory diagram for explaining detection of pile height. As shown in FIG. 9, the pile height detection means 100 includes a first optical sensor 101 and a second optical sensor 102. As shown in FIG. 10A, the first optical sensor 101 is a regular reflection type optical sensor in which a light receiving unit 101b is disposed at a position where light from a regularly reflected light emitting unit 101a is detected. As shown in FIG. 10B, the second optical sensor 102 is an irregular reflection type optical sensor in which a light receiving portion 102b is disposed at a position where light from the irregularly reflected light emitting portion 102a is detected. The first optical sensor 101 detects the pile height of a solid image of K color. In the case of K color toner, it is not necessary to consider the reflected light component of the toner. Therefore, by detecting the light regularly reflected from the second intermediate transfer belt 31, it is possible to know how much light is blocked by the K color toner on the second intermediate transfer belt 31. FIG. 11 is a diagram illustrating the relationship between the output value of the first optical sensor and the pile height of the solid image. As can be seen from the figure, the higher the pile height, the higher the probability that the toner shields the light. Therefore, the amount of light detected by the light receiving unit 101b decreases and the output value decreases. Based on the relationship shown in FIG. 11, the pile height of the K-color solid image is detected.
The second optical sensor 102 detects the pile height of each solid image of C, M, and K colors. In the case of color toner, since the toner irregularly reflects light, the pile height of each solid image of C, M, and K colors is detected by detecting the component of the irregularly reflected toner by the light receiving unit 102b. FIG. 12 is a diagram illustrating the relationship between the second optical sensor output value and the pile height of the solid image. When the pile height of the solid image is high, the amount of components in which the toner diffuses and reflects light increases, so that the amount of light detected by the light receiving unit 102b increases and the output value increases. Then, the pile height of each color solid image is detected based on the relationship shown in FIG.

  As shown in FIG. 9, when the pile height is detected at a predetermined timing such as after a predetermined number of sheets have passed, first, a solid color of K color or a halftone test pattern between images on the second intermediate transfer belt 31, and M Create a solid color or halftone test pattern. Next, the pile height of the K color test pattern is detected by the first optical sensor 101, and the pile height of the M color test pattern is detected by the second optical sensor 102. Then, based on the detected pile height, the development potentials of the K-color second image forming unit 81K and the M-color second image forming unit 81M are changed. Between the next images on the second intermediate transfer belt 31, a C-color test pattern is created, and the pile height of the C-color test pattern is detected by the second optical sensor 102. Then, based on the detected pile height, the developing potential of the C-color second image forming unit 81C is changed. A Y-color test pattern is created between the next images on the second intermediate transfer belt 31, and the pile height of the Y-color test pattern is detected by the second optical sensor 102. Then, based on the detected pile height, the developing potential of the Y-color second image forming unit 81Y is changed. In this way, the development potential is changed so that the pile height of the solid images of K, M, C, and Y colors becomes a predetermined height.

  The detection results of Y, C, M, and K may be used for toner replenishment control and image formation condition setting. In the above description, a test pattern is formed on the second intermediate transfer belt 31 to detect the pile height. However, the present invention is not limited to this. As shown in FIG. 4, it is detected whether the density of the toner adhering to the surface of the photoconductor after the development process is appropriate, but the pile height of each color may be detected using the image sensor S2.

Next, a toner suitably used in the present embodiment will be described.
The image forming apparatus according to the present embodiment allows the user to use Y, M, C, and K toners that satisfy the following conditions (a) to (f) as toner images. To be specified.
(A) The weight average particle diameter D4 is 3 to 7 [μm].
(B) A value (D4 / D1) obtained by dividing the weight average particle diameter D4 by the number average particle diameter D1 is 1.05 to 1.30.
(C) The shape factor SF-1 is 100 to 180.
(D) The shape factor SF-2 is 100 to 180.
(E) Fine particles having an average particle diameter of 50 to 300 [nm] and a bulk density of 0.3 [g / cm ³ ] or more are held on the surface (externally added).
(F) Among the toner on the developing roller 5a after passing through the blade 5b of the developing device, the number of toners of −0.1 [femto C / μm] or more is less than 10 [%].

  By using a toner having any of the above conditions (a) to (f), the transferability of the toner in each transfer region can be improved. In addition, an image having excellent dot reproducibility can be obtained, and an image on the upper surface of the recording paper can be made a high quality image. In particular, the following merits can be obtained by using the toner having any of the above conditions (a) to (f) in the second image forming unit for forming an unfixed image on the lower surface of the recording paper. That is, a predetermined image density can be obtained even if the pile height is kept low. Therefore, the pile height of the unfixed image formed on the conveyance belt side surface of the recording paper can be lowered, and the unfixed image formed on the conveyance belt side surface of the recording paper is disturbed by the conveyance belt 51. Can be suppressed. In addition, since a high-quality unfixed image having excellent dot reproducibility is formed, even if the unfixed image is somewhat disturbed by the conveyor belt 51, a grainy image with a large loss of granularity can be obtained. There is no. In other words, the margin for undisturbed images is improved.

  As a method for instructing the user to use such toner, for example, a toner having all of the above conditions (a) to (f) can be packaged and shipped together with the image forming apparatus. Further, for example, the product number or product name of the toner may be specified in the image forming apparatus main body or the instruction manual. Further, for example, it may be performed by notifying the user of the product number, product name, etc. in writing or electronic data. Further, for example, it can be performed by shipping a toner bottle, which is a toner storage unit that stores such toner, in a state of being set in the main body of the image forming apparatus. The image forming apparatus employs all these methods, but it is sufficient to employ at least one of the methods.

  The reason why the toner satisfying the condition (a) is designated is as follows. In other words, when the toner has a weight average particle size smaller than 3 [μm], when used as a two-component developer, the toner fuses to the surface of the carrier during long-term agitation in the developing device, thereby reducing the charging ability of the carrier Because. When used as a one-component developer, toner filming on the developing roller and toner fusion to a member such as a blade for thinning the toner are likely to occur.

In the case of reproducing minute dots having a resolution of 600 dpi or more, the characteristic of weight average particle diameter of 3 to 7 [μm] is particularly effective. In this range, since the toner particles have a sufficiently small particle size with respect to the minute latent image dots, the dot reproducibility is excellent. Further, since the micro gap between the toner adhering on the recording paper is reduced and the toner density in the toner image is increased (so-called solid filling is improved), a sufficient image density can be obtained with a smaller amount of adhesion. FIG. 13 is a diagram showing the relationship between the minimum pile height for obtaining a predetermined density and the toner particle size. As can be seen from FIG. 13, as the toner particle size decreases, the height of the pile height for obtaining a predetermined density decreases. That is, by using a toner having a small particle diameter, it is possible to obtain a predetermined image density while keeping the pile height low. Therefore, by using a toner having a weight average particle diameter of 3 to 7 [μm], a good image density can be obtained even if the pile height is lowered.
If the weight average particle size is less than 3 [μm], phenomena such as a decrease in transfer efficiency and a decrease in blade cleaning properties tend to occur. Moreover, when the weight average particle diameter (D4) exceeds 7 [μm], it is difficult to suppress scattering of characters and lines.

  The reason why the toner satisfying the condition (b) is specified is as follows. The closer the value obtained by dividing the weight average particle size by the number average particle size is closer to 1.00, the sharper the particle size distribution. That is, a toner in which the value obtained by dividing the weight average particle diameter by the number average particle diameter is in the range of 1.05 to 1.30 has various merits because the particle diameter distribution is narrow. For example, a phenomenon in which toner particles having a particle size suitable for an electrostatic latent image pattern contributes to development preferentially over other toners, so that various patterns of images can be stably formed. There is a merit that it becomes possible to do. In addition, when the apparatus adopts a configuration in which toner remaining on an image carrier such as a photoreceptor is collected and recycled, a large amount of small-size toner particles that are difficult to be transferred are recycled. In such recycling, when the above-mentioned one having a relatively large value is used, the development performance is adversely affected due to a large change in the toner particle size from the new toner supply to the next toner supply. Further, the toner charge distribution is uniform, and a high-quality image with little background fogging can be obtained. In the electrostatic transfer method, the transfer rate can be increased.

  The weight average particle diameter and number average particle diameter of the toner can be measured by a measuring device using a Coulter counter method, for example, Coulter Counter TA-II or Coulter Multisizer II (both manufactured by Coulter). Specifically, first, 0.1 to 5 ml of a surfactant (preferably alkylbenzene sulfonate) is added as a dispersant to 100 to 150 ml of the electrolytic aqueous solution. As the electrolytic aqueous solution, approximately 1% NaCl aqueous solution is prepared using primary sodium chloride, and for example, ISOTON-II (manufactured by Coulter) can be used. Further, 2 to 20 mg of a measurement sample is added to the obtained solution. Then, the solution is dispersed for about 1 to 3 minutes with an ultrasonic disperser, and the weight and number of toners are measured by the above-described measuring apparatus using a 100 μm aperture as an aperture to calculate the weight distribution and the number distribution. To do. From the obtained distribution, the weight average particle diameter and the number average particle diameter of the toner can be obtained. In addition, as a channel, it is less than 2.00-2.52 micrometer; 2.52-3.17 micrometer; 3.17-4.00 micrometer; 4.00-5.04 micrometer; 5.04-6.35 micrometer 6.35 to less than 8.00 μm; 8.00 to less than 10.08 μm; 10.08 to less than 12.70 μm; 12.70 to less than 16.00 μm; 16.00 to less than 20.20 μm; Intended for toner particles having a particle size of 2.00 μm to less than 40.30 μm using 13 channels of less than 25.40 μm; 25.40 to less than 32.00 μm; 32.00 to less than 40.30 μm.

The reason why the toner having the above conditions (c) to (d) is specified is as follows. That is, the shape factor SF-1 and the shape factor SF-2 are parameters representing the shape of the toner, and are familiar parameters in the field of powder engineering. The shape factor SF-1 referred to here is a value indicating the degree of roundness in a spherical substance such as toner particles. As shown in FIG. 14, the square of the maximum diameter portion length MXLNG in an elliptical figure obtained by projecting a spherical substance on a two-dimensional plane is divided by the area AREA, and further multiplied by 100π / 4. . That is, it can be expressed by an equation “shape factor SF-1 = {(MXLNG) ² / AREA} × (100π / 4)”. A spherical substance having a shape factor SF-1 of 100 is a true sphere, and the larger the value of SF-1, the more irregular the shape of the spherical substance.

The shape factor SF-2 is a numerical value indicating the degree of unevenness on the surface of the spherical substance. As shown in FIG. 15, the value obtained by dividing the square of the perimeter PERI of the figure obtained by projecting the spherical substance on the two-dimensional plane by the area AREA, and further multiplying by 100 / 4π. In other words, the shape factor SF-2 can be expressed by a mathematical expression of “shape factor SF-2 = {(PERI) ² / AREA} × (100 / 4π)”. Note that the spherical material having a shape factor SF-2 of 100 has no irregularities on the surface. As the value of the shape factor SF-2 increases, the irregularities on the surface of the spherical substance become more prominent.

  It has been clarified by the present inventors that the transfer efficiency increases as the toner shape approaches a true sphere (both SF-1 and SF-2 approach 100). This is because the closer to the true sphere, the smaller the contact area between the toner particles and the thing (toner particles, image carrier, etc.) in contact with the toner particles, and the toner fluidity is increased. This is considered to be because the (mirror power) is weakened and is easily affected by the transfer electric field. Further, since the suction force between the conveyance belt 51 and the unfixed image on the recording paper is also reduced, it is possible to suppress the unfixed image from being disturbed by the conveyance belt 51. According to the inventor's research, it has been clarified that when the shape factor SF-1 exceeds 180 or the shape factor SF-2 exceeds 180, the transfer efficiency starts to deteriorate rapidly. Further, the suction force between the conveyance belt 51 and the unfixed image is increased, and the image on the second surface of the recording paper is greatly disturbed. By setting each to 180 or less, it is possible to maintain a good transfer rate and stably form dots even on unevenness on the paper surface with relatively low smoothness, so that small dots can be stabilized. It can be formed in a shape. Further, the closer to the true sphere, the smaller the micro gap between the toner adhering on the recording paper and the toner density in the toner image increases (so-called solid filling is improved), so that a sufficient image density can be obtained with a smaller amount of adhesion. be able to. As a result, a predetermined image density can be obtained even if the pile height is kept low. In the toner having the above shape, since the progress of the burying of the external additive is fast, it is necessary to appropriately control the stirring conditions in the development.

  The reason why the toner satisfying the condition (e) is specified is as follows. Due to the presence of fine particles having appropriate characteristics on the surface of the toner, an appropriate gap is formed between the toner particles and the thing (toner particles, image carrier, etc.) in contact therewith. Further, the fine particles have a very small contact area with toner particles, a photoconductor, a conveyance belt, and the like, and are in uniform contact. For this reason, the attracting force (mirroring force) to the object is weakened, and the toner of the unfixed image formed on the surface of the recording paper on the conveying belt side is difficult to be attracted to the conveying belt 51, thereby suppressing the image disturbance. Further, development / transfer efficiency is improved, and dot reproducibility is improved. For this reason, an unfixed image formed on the surface of the recording paper on the conveyance belt side has a high margin against image disturbance caused by the conveyance belt 51. In addition, since the fine particles play a role of roller, they are buried in the toner particles even during cleaning under high stress (high load, high speed, etc.) between the cleaning blade and the photoconductor without wearing or damaging the photoconductor. Therefore, the toner can be detached and restored even if it is buried a little, so that stable toner characteristics can be maintained over a long period of time. Further, the fine particles are appropriately detached from the surface of the toner and accumulated at the tip of the cleaning blade, and the so-called dam effect has an effect of preventing the toner from passing through the blade. Since these characteristics have an effect of reducing the share received by the toner particles, the filming effect of the toner itself due to the low rheological component contained in the toner is exhibited for high-speed fixing (low energy fixing). In addition, when fine particles having an average primary particle size in the range of 50 to 500 [nm] are used, the excellent cleaning performance can be fully utilized and the particle size of the toner is extremely small. Does not reduce fluidity. Further, although details are not clear, even when fine particles contaminate the carrier, the degree of developer deterioration is small. Therefore, since there is little change in toner fluidity and chargeability over time, stable image quality is always formed, and image disturbances formed on the surface of the recording paper conveyance belt proceed according to the number of output sheets. There is nothing.

The average primary particle size (hereinafter referred to as the average particle size) of the fine particles is 50 to 500 [nm], and particularly preferably 100 to 400 [nm]. If it is less than 50 [nm], the fine particles may be buried in the concave and convex portions of the unevenness of the toner surface, reducing the role of the rollers. On the other hand, when the particle size is larger than 500 [nm], when the fine particles are positioned between the blade and the surface of the photoreceptor, the toner particles are in the same order as the contact area of the toner itself, and the toner particles to be cleaned are allowed to pass through. It becomes easy to generate. In addition, when the bulk density is less than 0.3 [mg / cm ³ ], although there is a contribution to the improvement of fluidity, the scattering property and adhesion of the toner and fine particles are increased, so the effect as a toner and a roller, A function such as a so-called dam effect that accumulates in the cleaning unit and prevents toner cleaning failure is reduced.

As the fine particles, inorganic compounds and organic compounds can be used. Examples of inorganic compounds include SiO ₂ , TiO ₂ , Al ₂ O ₃ , MgO, CuO, ZnO, SnO ₂ , CeO ₂ , Fe ₂ O ₃ , BaO, CaO, K ₂ O, Na ₂ O, ZrO ₂ , CaO. Examples thereof include SiO ₂ , K ₂ O (TiO ₂ ) n, Al ₂ O ₃ .2SiO ₂ , CaCO ₃ , MgCO ₃ , BaSO ₄ , MgSO ₄ , SrTiO _{3 and the} like, preferably SiO ₂ , TiO _2. Al ₂ O ₃ . Particularly, preferred is SiO _2. These inorganic compounds may be hydrophobized with various coupling agents, hexamethyldisilazane, dimethyldichlorosilane, octyltrimethoxysilane, and the like.
The organic compound fine particles may be a thermoplastic resin or a thermosetting resin. For example, vinyl resin, polyurethane resin, epoxy resin, polyester resin, polyamide resin, polyimide resin, silicon resin, phenol resin, melamine resin, Examples include urea resin, aniline resin, ionomer resin, and polycarbonate resin. As the resin fine particles, two or more of the above resins may be used in combination. Of these, vinyl resins, polyurethane resins, epoxy resins, polyester resins, and combinations thereof are preferred because an aqueous dispersion of fine spherical resin particles is easily obtained. Specific examples of vinyl resins include polymers obtained by homopolymerization or copolymerization of vinyl monomers, such as styrene- (meth) acrylic acid ester copolymers, styrene-butadiene copolymers, (meth) acrylic acid. -Acrylic ester copolymer, styrene-acrylonitrile copolymer, styrene-maleic anhydride copolymer, styrene- (meth) acrylic acid copolymer, and the like.

The bulk density of the fine particles can be expressed by a mathematical formula: “bulk density (g / cm ³ ) = fine particle amount (g / 100 ml) ÷ 100”, and the fine particle amount was measured by the following method. Using a 100 ml graduated cylinder, fine particles were gradually added to make 100 ml. At that time, no vibration was applied. The amount of fine particles was measured by the difference in weight before and after the fine particles in the graduated cylinder were placed.

  The fine particles are externally added and adhered to the toner surface by a method in which the toner base particles and the fine particles are mechanically mixed and adhered using various known mixing devices, or the toner base particles in the liquid phase. There is a method in which fine particles are uniformly dispersed with a surfactant or the like, and are dried after an adhesion treatment.

  The reason why the toner satisfying the condition (f) is designated is as follows. FIGS. 16A and 16B show the measurement of the toner charge amount distribution on the developing sleeve after passing through the development regulating blade. The solid lines in FIGS. 16A and 16B indicate the charge amount distribution of the toner suitably used in the present embodiment, and the dotted line in the drawing indicates the charge amount distribution of the conventional toner. In addition, the charge amount distribution was measured using an E-spart analyzer manufactured by Hosokawa Micron. The measurement principle of this E spurt analyzer will be briefly described. The toner particles are vibrated by dropping the toner particles between two electrodes having opposite acoustic polarities, and the toner particles are moved to the electrodes by the electric field action of the electrodes. By simultaneously measuring the frequency and moving distance of the toner at this time by the laser Doppler method, the particle diameter and the charge amount of each particle are calculated. According to this apparatus, it is possible to measure the charge amount distribution of particle groups in a specific particle diameter range and to easily measure the total charge amount of these particle groups. In this embodiment, the total number of measurement toners is 3000.

  As shown in FIGS. 16A and 16B, in the toner suitably used in this embodiment, the number ratio of weakly charged and reversely charged toner of −0.1 [femto C / μm] or more is 10%. It can be seen that it is normally charged. However, in the conventional toner, as shown in FIG. 16A, the charge amount distribution is shifted to a lower charge side than the toner suitably used in the present embodiment, or as shown in FIG. In addition, there is a two-peak distribution in which small peaks are formed on the low charging side. As a result, in the conventional toner, the number ratio of weakly charged and reversely charged toner of −0.1 [femto C / μm] or more is 10% or more. As described above, when the number ratio of the reversely charged toner is 10% or more, the toner image is easily scattered at the developing portion, or the so-called background stain that adheres the toner to the non-image portion is generated. Also, in transfer, dust is generated at the dots and line portions and the image quality is deteriorated. On the other hand, in the toner suitably used in the present embodiment, the number ratio of the weakly charged toner and the reversely charged toner is 10% or less. Stabilize. As a result, pile height is easy to control and image quality stability is high. Therefore, it is possible to increase the margin for image disturbance of the image of the surface of the recording paper that is in contact with the conveyance belt.

  As described above, if the pile height on the surface of the recording paper on the conveyor belt side is lower than the pile height on the upper surface of the recording paper, the amount of toner attached to the lower surface of the recording paper is reduced compared to the upper surface of the recording paper. Compared with the upper surface, the image density on the lower surface of the recording paper becomes lighter. In order to eliminate such a density difference, the toner used in the first image forming unit 80 and the toner used in the second image forming unit 81 are made different. Specifically, the weight average particle diameter of the toner used in the second image forming unit 81 is made smaller than that of the toner used in the first image forming unit 80. Thus, the toner used in the second image forming unit 81 is more solid than the toner used in the first image forming unit 80, and a predetermined image density can be obtained with a small amount of adhesion. . As a result, the pile height of the unfixed image on the lower surface of the recording paper is made lower than the pile height of the unfixed image on the upper surface of the recording paper without the image density on the lower surface of the recording paper becoming lower than the image density on the upper surface of the recording paper. be able to.

  Further, the shape factor SF-1 and the shape factor SF-2 of the toner used in the second image forming unit 81 are made smaller than the toner used in the first image forming unit 80. Thus, the toner used in the second image forming unit 81 is more solid than the toner used in the first image forming unit 80, and a predetermined image density can be obtained with a small amount of adhesion. . As a result, the pile height of the unfixed image on the lower surface of the recording paper is made lower than the pile height of the unfixed image on the upper surface of the recording paper without the image density on the lower surface of the recording paper becoming lower than the image density on the upper surface of the recording paper. be able to.

[Embodiment 2]
Next, an image forming apparatus according to the second embodiment will be described. Note that the configuration of the image forming apparatus according to the second embodiment is the same as that of the image forming apparatus according to the first embodiment unless otherwise specified.

  FIG. 17 is a schematic configuration diagram illustrating a main configuration of the image forming apparatus according to the second embodiment. This image forming apparatus is different from the image forming apparatus according to the first embodiment in the following points. That is, in the image forming apparatus according to the first embodiment, the first image forming units 80Y, 80M, 80C, 80K, and 80K for exclusively forming only the first toner image out of the first toner image and the second toner image. And a second image forming unit (81Y, M, C, K) for exclusively forming the second toner image. On the other hand, the image forming apparatus according to the second embodiment has only four first image forming units 80Y, 80M, 80C, 80K as image forming units. These first image forming units 80Y, 80M, 80C, 80K, and 80K form a first toner image and a second toner image on the first intermediate transfer belt 21, which is a first toner image carrier. In the image forming apparatus according to the second embodiment, the first intermediate transfer belt 21 and the second intermediate transfer belt 31 are in contact with the preceding transfer area, and the second toner image on the first intermediate transfer belt is transferred to the second intermediate transfer belt. It is configured to be used as an intermediate transfer area for transferring to the medium. In the figure, the illustration of the image reading device (scanner) is omitted for convenience.

  In the image forming apparatus according to the second embodiment, first, each first image forming unit 80Y, 80M, 80M, 80C, 80K, the second single-color toner image to be transferred to the second surface of the recording paper. It is formed before the first single color toner image to be transferred onto the surface. Then, each of them is transferred onto the first intermediate transfer belt 21 in a superimposed manner. Thus, a second multicolor toner image is first formed on the first intermediate transfer belt 21. The second multi-color toner image is secondarily transferred onto the second intermediate transfer belt 31 in a front transfer area where the first intermediate transfer belt 21 and the second intermediate transfer belt 31 are in contact with each other.

  Each of the first image forming units 80Y, 80M, 80C, 80K forms a second single color toner image and after a while, forms a first single color toner image and superimposes it on the first intermediate transfer belt 21. And transcribe. Thereafter, the recording paper P is fed into the preceding transfer area. In the preceding transfer area, the first multicolor toner image on the first intermediate transfer belt 21 is in close contact with the first surface of the recording paper P and is collectively secondary transferred. Next, when the recording paper P passes through the preceding transfer area and is conveyed to a position facing the transfer charger 47, the second multicolor toner image on the second intermediate transfer belt 31 is collectively on the second surface of the recording paper P. Secondary transferred.

  In the image forming apparatus having such a configuration, the first image forming unit 20 including the first image forming unit dedicated for forming the first toner image and the second image dedicated for forming the second toner image. A toner image can be formed on each side of the recording paper by the one-pass method without providing both the second image forming unit 30 including a forming unit. However, since it is necessary to form the first toner image and the second toner image at different times, compared to the image forming apparatus according to the first embodiment that can form both the toner images in parallel. The image forming speed is reduced. In contrast, in the image forming apparatus according to the first embodiment, providing both the first image forming unit 20 and the second image forming unit 30 can increase the image forming speed instead of causing an increase in cost. it can.

Also in the image forming apparatus according to the second embodiment, when the pile height of an unfixed image formed on the surface (lower surface) of the recording paper on the conveyance belt side is high, the recording paper is transferred from the intermediate transfer belt 31 to the conveyance belt 51. In addition, the unfixed image is disturbed. Therefore, also in the image forming apparatus of the second embodiment, the pile height of the unfixed image formed on the surface (lower surface) of the recording paper on the conveying belt side is set to be higher than the pile height of the unfixed image formed on the upper surface of the recording paper. I try to lower it.
Specifically, the development potential when forming the first toner image is different from the development potential when forming the second toner image. The second toner image is formed at a development potential at which the pile height of the single color solid image obtained based on the relationship shown in FIG. 8 is 1.25 times or less of the toner particle size. Accordingly, the pile height of the color toner image formed by overlapping the Y, M, C, and K toner images formed on the lower surface of the recording paper can be made 2.7 times or less of the toner particle diameter. This suppresses the unfixed image formed on the lower surface of the recording paper from being disturbed by the conveyance belt 51.

  Further, a pile height detection unit 100 is provided on the first intermediate transfer belt or on each photoconductor, and the development potential when the second toner image is formed is controlled based on the detection result of the pile height detection unit 100. Also good. Further, by using the toner satisfying the conditions (a) to (f) described above, it is possible to suppress a decrease in density of the unfixed image formed on the lower surface of the recording paper.

  Further, as shown in FIG. 18, each image forming unit may include a first developing device 500 that develops the first toner image and a second developing device 501 that develops the second toner image. In this case, the toner stored in the second developing device 501 that develops the second toner image has a weight average particle size or shape factor (SF-1, SF-2) as compared with the toner stored in the second developing device 501. Keep it small. As a result, even if the pile height of the unfixed image (second toner image) formed on the lower surface of the recording paper is low, a predetermined image density can be obtained, and the image density formed on the upper surface of the recording paper and the recording paper can be obtained. It is possible to suppress the difference in image density formed on the lower surface. Further, since the dot reproducibility of the unfixed image formed on the lower surface of the recording paper is improved, even if the unfixed image is somewhat disturbed by the transport belt 51, the image quality is not affected.

  In the above embodiment, the paper transport unit 50 is provided between the double-side transfer means and the fixing device 60, but the present invention is not limited to this. As illustrated in FIG. 19, a conveyance belt that conveys a recording medium from the double-side transfer device to the fixing device 60 by the second intermediate transfer belt 31 without providing the paper conveyance unit 50 between the double-side transfer unit and the fixing device 60. You may make it serve as. With this configuration, it is not necessary to transfer the recording paper between the double-sided transfer device and the nip portion that is the fixing position. Therefore, since the recording medium can be transferred from the double-side transfer means to the fixing device without rubbing the unfixed toner image on the recording medium, the disturbance due to the rubbing of the unfixed toner image can be suppressed. In addition, since the recording medium is not transferred from the double-sided transfer device to the conveying belt, it is possible to avoid the jamming of the recording medium that occurs between them.

  In the case of the image forming apparatus of FIG. 19, since the second intermediate transfer belt 31 passes near the high-temperature fixing device, it is preferable to use a belt having high heat resistance as the second intermediate transfer belt 31. Examples of the belt having high heat resistance include a belt whose base is made of polyimide or polyamide. The belt base means the belt itself when the second intermediate transfer belt 31 has a single layer structure. Moreover, when it is set as a multilayer structure, it means the layer with the largest thickness among each layer. By constituting at least the belt substrate with polyimide or polyamide, it is possible to suppress the expansion and contraction of the entire belt due to heating and to suppress the deterioration thereof.

  In the image forming apparatus shown in FIG. 19 as well, if the pile height of an unfixed image formed on the surface (lower surface) of the recording paper on the second intermediate transfer belt side is high, the recording paper passes through the transfer charger 47 and then passes through the recording paper. When the speed of the second intermediate transfer belt 31 is increased or decreased so that the recording paper passes through the fixing device according to the type, the unfixed image formed on the lower surface is disturbed. For this reason, also in the image forming apparatus shown in FIG. 19, the development potential of the image forming unit 81 for forming the second toner image is controlled so that the pile height of the unfixed image formed on the lower surface of the recording paper is placed on the upper surface of the recording paper. Lower than the pile height of the formed unfixed image. Thereby, it is possible to suppress the unfixed image formed on the lower surface of the recording paper from being disturbed by the second intermediate transfer belt 31.

(1)
As described above, according to the image forming apparatus of this embodiment, the toner image height (pile height) of the solid image on the conveyance belt side of the recording paper is set higher than the pile height of the solid image on the surface different from the surface on the conveyance belt side of the recording paper. It is low. As a result, it is possible to suppress the unfixed image on the surface of the recording paper on the conveyance belt side from being disturbed by the friction of the conveyance belt, as compared with the case where the pile heights of the solid images on both sides of the recording paper are the same.
(2)
Further, according to the image forming apparatus of the present embodiment, the pile height of the solid image transferred to the surface on the conveyance belt side of the recording paper by the development potential when developing the latent image on the photoconductor as the image carrier, It is lower than the pile height of a solid image transferred to a surface different from the surface on the conveyance belt side of the recording paper. In this way, by controlling the pile height of a solid image transferred to a surface different from the surface on the conveyance belt side of the recording paper by the development potential, a solid image formed on the surface of the recording paper on the conveyance belt side can be easily controlled. The pile height of the image can be controlled.
(3)
Further, the pile height of the unfixed image transferred to the surface of the recording paper on the conveyance belt side may not reach a predetermined height with the development potential set at the initial stage due to environmental fluctuations or deterioration of the photoreceptor. However, in the present embodiment, by controlling the development potential based on the detection result of the pile height detection means, even if the environmental change or the photoconductor deteriorates, it is transferred to the surface of the recording paper on the conveying belt side. The development potential can be set so that the pile height of the unfixed image becomes a predetermined height. Therefore, it is possible to suppress the unfixed image on the conveyance belt side of the recording paper from being disturbed by the conveyance belt over a long period of time.
(4)
Further, according to the image forming apparatus of the present embodiment, the pile height of the single-color solid image transferred to the surface of the recording paper on the conveying belt side is set to 1.25 times or less with respect to the toner particle diameter, thereby The pile height of a color image formed by overlapping toner colors can be made 2.7 times or less with respect to the toner particle diameter. Thereby, even if a color image is formed on the surface of the recording paper on the conveying belt side, it is possible to suppress the image from being largely disturbed by the conveying belt.
(5)
In addition, according to the image forming apparatus of the present embodiment, the first image forming unit that is the first image forming unit that forms the first toner image and the second image forming unit that is the second image forming unit that forms the second toner image. By providing the unit, the image forming speed can be increased as compared with the case where the first toner image and the second toner image are formed by one image forming unit.
(6)
Further, according to the image forming apparatus of the present embodiment, the solid filling is improved by using a weight average particle diameter of 3 to 7 [μm] as the toner used for forming the toner image. Image density can be obtained. Further, since the dot reproducibility is excellent, even if an unfixed image formed on the surface of the recording paper on the conveying belt side is somewhat disturbed by the conveying belt, the image quality is not greatly affected. In other words, the margin of the unfixed image formed on the conveyance belt side surface of the recording paper is improved against image disturbance caused by the conveyance belt. In addition, since a value obtained by dividing the weight average particle diameter by the number average particle diameter is 1.05 to 1.30, a high-quality image developed with stable development performance can be obtained.
(7)
In the image forming apparatus according to the present exemplary embodiment, toner having a shape factor SF-1 of 100 to 180 and a shape factor SF-2 of 100 to 180 is used as the toner used for forming the toner image. As a result, an appropriate gap is formed between the toner particles and the thing (toner particles, image carrier, etc.) in contact therewith, and the adsorption force (mirror power) to the thing is weakened. Therefore, the releasability of toner from a transfer source such as a transfer belt is improved, and excellent transfer properties can be exhibited. Therefore, the dot reproducibility of the image is improved, and even if the unfixed image formed on the surface of the recording paper on the conveyance belt side is somewhat disturbed by the conveyance belt, the image quality is not greatly affected. In other words, the margin of the unfixed image formed on the conveyance belt side surface of the recording paper is improved against image disturbance caused by the conveyance belt. In addition, since the adsorption force with the conveyance belt is weakened, it is possible to suppress the unfixed image formed on the conveyance belt side surface of the recording paper from being disturbed by the conveyance belt.
In addition, since the micro gap between the toner adhering on the recording paper is reduced and the toner density in the toner image is increased, a sufficient image density can be obtained with a smaller adhering amount. As a result, it is possible to suppress a decrease in image density when the pile height is lowered.
(8)
In the image forming apparatus according to the present embodiment, as the toner used for forming the toner image, −0.1 [femtoC / of toner on the developing roller that is a developer carrying member after passing through the blade that is a developer regulating member. [mu] m] or more is used in which the number of toners is less than 10%. As a result, the toner moves faithfully with respect to the development electric field and the transfer electric field, and the adhesion amount of the toner is stabilized. As a result, the pile height of the unfixed image formed on the surface of the recording paper on the conveying belt side can be easily controlled, and the image quality stability is high. Therefore, the disturbance of the unfixed image formed on the surface of the recording paper on the conveyance belt side can be stably suppressed.
(9)
In the image forming apparatus of the present embodiment, as an additive for the toner used for forming the toner image, the average primary particle size is 50 to 500 [nm] and the bulk density is 0.3 [g / cm. ³ ] or more is used. Due to this additive, an appropriate gap is formed between the toner particles and the thing (toner particles, image carrier, etc.) in contact therewith, and the adsorbing power (mirror power) to the thing is weakened. Therefore, the releasability of toner from a transfer source such as a transfer belt is improved, and excellent transfer properties can be exhibited. Therefore, the dot reproducibility of the image is improved, and even if the unfixed image formed on the surface of the recording paper on the conveyance belt side is somewhat disturbed by the conveyance belt, the image quality is not greatly affected. In other words, the margin of the unfixed image formed on the conveyance belt side surface of the recording paper is improved against image disturbance caused by the conveyance belt. In addition, since the adsorption force with the conveyance belt is weakened, it is possible to suppress the unfixed image formed on the conveyance belt side surface of the recording paper from being disturbed by the conveyance belt. In addition, deterioration of the developer can be suppressed, and stable image quality can be obtained over a long period of time.
(10)
Further, according to the image forming apparatus of the present embodiment, the weight average particle diameter of the toner used for forming the toner image transferred onto the surface of the recording paper on the conveyance belt side is different from the surface of the recording paper on the conveyance member side. It is smaller than the weight average particle diameter of the toner used for the toner image transferred to the surface. As a result, the toner used in the second image forming unit is more solid than the toner used in the first image forming unit, and a predetermined image density can be obtained with a small amount of adhesion. As a result, the pile height of the unfixed image on the lower surface of the recording paper is made lower than the pile height of the unfixed image on the upper surface of the recording paper without the image density on the lower surface of the recording paper becoming lower than the image density on the upper surface of the recording paper. be able to.
(11)
Further, according to the image forming apparatus of the present embodiment, the toner shape factors SF-1 and SF-2 used for forming the toner image transferred onto the surface of the recording paper on the conveying member side are transferred to the recording medium. It is made smaller than the shape factors SF-1 and SF-2 of the toner used for the toner image transferred to a surface different from the member side surface. As a result, the toner used in the second image forming unit is more solid than the toner used in the first image forming unit, and a predetermined image density can be obtained with a small amount of adhesion. As a result, the pile height of the unfixed image on the lower surface of the recording paper is made lower than the pile height of the unfixed image on the upper surface of the recording paper without the image density on the lower surface of the recording paper becoming lower than the image density on the upper surface of the recording paper. be able to.
(12)
Further, in the image forming apparatus of the present embodiment, the partial length of the conveying belt from the double-sided transfer device to the fixing device is made larger than the length of the maximum size recording body capable of image formation. With such a configuration, the trailing edge of the recording paper can be removed from the secondary transfer nip before the leading edge of the recording paper is sandwiched in the fixing nip of the fixing device 60. This avoids a situation in which the leading edge of the recording paper is caught in the fixing nip while the trailing edge of the recording paper is caught in the secondary transfer nip. Further, it is possible to avoid the rubbing between the recording paper and the second intermediate transfer belt 31 due to the tension applied to the recording paper between both nips, and to more reliably suppress the disturbance due to the rubbing of the unfixed toner image. . Further, unlike the illustrated example, even if the secondary transfer nip for the second surface of the recording paper is formed in the post-transfer area, the leading edge of the recording paper separated from the second intermediate transfer belt 31 is connected to the fixing device 60. Prior to being inserted into the fixing nip, the trailing edge of the recording sheet can be removed from the secondary transfer nip in the subsequent transfer area. Therefore, it is possible to avoid the disturbance of the unfixed toner image due to the tension applied to the recording paper between these nips.
(13)
Further, in the image forming apparatus according to the present embodiment, after the recording paper passes through the double-side transfer unit, the speed of the conveying belt is controlled based on the type of the recording paper, so The amount of fixing heat can be changed. In other words, in the case of recording paper having a large amount of heat required for fixing, such as thick paper, it is possible to lengthen the passage time of the recording paper through the fixing device by reducing the speed of the conveying belt. Then, the amount of heat for fixing the recording paper P can be increased without changing the heating temperature by the fixing device 60. For this reason, even when the required fixing heat amount becomes considerably large, the fixing process can be performed on the recording paper with a sufficient heating amount without considerably increasing the heating temperature by the fixing device 60. Therefore, it is assumed that the required fixing heat amount becomes considerably large, and avoiding an increase in cost due to setting the heat resistance of the fixing device 60, and fixing failure and hot offset due to a change in the required fixing heat amount for each printout. Can be suppressed. Also, in the case of recording paper such as thin paper that requires a relatively small amount of heat for fixing, the time for which the recording paper passes through the fixing device is shortened by increasing the speed of the conveying belt. As a result, the fixing process can be performed with an appropriate heating amount without excessively heating the recording paper.
(14)
Further, in the image forming apparatus of the present embodiment, an electrostatic adsorption charger is provided as a charge applying unit that applies a charge to the surface of the transport belt before receiving the recording paper from the double-side transfer unit. As a result, a negative polarity charge is applied to the front surface of the conveyor belt by the electrostatic chuck charger. The recording paper is charged with a positive polarity by a transfer charger or the like, and the recording paper is electrostatically attracted to the front surface of the transport belt by applying a negative polarity charge to the front surface of the transport belt. Can do. As a result, the recording paper can be stably conveyed to the fixing device, and even when the conveyance belt is accelerated / decelerated, the recording paper can be prevented from being deviated from the conveyance belt. Therefore, it is possible to suppress the unfixed image from being disturbed by the rubbing of the conveyance belt. Further, a negative polarity charge is applied by the electrostatic adsorption charger, so that the charge polarity on the paper transport belt is the same as that of the toner. Therefore, the adsorption force between the conveyance belt 51 and the unfixed image on the recording paper is reduced, and the unfixed image can be prevented from being disturbed by the sliding of the conveyance belt.

1 is a schematic configuration diagram of an image forming apparatus according to Embodiment 1. FIG. Explanatory drawing of angle (alpha) which two image receiving surfaces form. FIG. 3 is an enlarged configuration diagram illustrating one of four first process units in a printer unit of the image forming apparatus. FIG. 3 is an enlarged configuration diagram illustrating one of four second process units in the printer unit of the image forming apparatus. FIG. 3 is a diagram illustrating a recording paper conveyance device of the image forming apparatus. The figure which shows the relationship between a pile height and the granularity of an image. FIG. 6A is a diagram illustrating a state in which an unfixed image having a high pile height is disturbed by the conveyance belt 51. FIG. 6B is a diagram illustrating a state in which an unfixed image having a low pile height is disturbed by the conveyance belt 51. The figure which shows the relationship between pile height and development potential. The figure explaining the detection of pile height. (A) is a figure which shows a 1st optical sensor. (B) is a figure which shows a 2nd optical sensor. The figure which showed the relationship between the output value of a 1st optical sensor, and pile height. The figure which showed the relationship between the output value of a 2nd optical sensor, and pile height. The figure which showed the relationship between a toner particle size and pile height. FIG. 3 is an explanatory diagram schematically showing a toner shape for explaining a shape factor SF-1. FIG. 4 is an explanatory diagram schematically showing a toner shape for explaining a shape factor SF-2. (A), (b) is a diagram showing toner charge distribution. FIG. 3 is a schematic configuration diagram of an image forming apparatus according to a second embodiment. FIG. 3 is an enlarged configuration diagram illustrating one of four process units in the printer unit of the image forming apparatus. FIG. 3 is a schematic configuration diagram illustrating another image forming apparatus.

Explanation of symbols

1Y, M, C, K photoconductor 5 developing device 20 first transfer unit 21 first intermediate transfer belt 30 second transfer unit 31 second intermediate transfer belt 46 secondary transfer roller 47 transfer charger 50 recording paper transport device 51 transport belt 60 fixing device 80Y, M, C, K first image forming unit 81Y, M, C, K second image forming unit 100 pile height detecting means 101 first optical sensor 102 second optical sensor

Claims (14)

A double-side transfer means for transferring the first toner image formed by the image forming means to the first surface of the recording medium, and transferring the second toner image formed by the image forming means to the second surface of the recording medium; An image forming apparatus comprising: a conveying member that supports the recording medium; and a recording medium conveying unit that conveys the recording medium from the double-sided transfer unit to a fixing unit that fixes the toner image on the recording medium onto the recording medium. In
The toner image height of the unfixed solid image on the surface of the recording member on the conveying member side is set lower than the toner image height of the unfixed solid image on the surface different from the surface on the conveying member side of the recording member. Image forming apparatus. The image forming apparatus according to claim 1.
The image forming means includes an image carrier that carries a latent image, an exposure device that forms a latent image on the image carrier, and a developing device that develops the latent image on the image carrier. Due to the development potential when developing the latent image on the image carrier, the toner image height of the unfixed solid image on the surface of the recording member on the conveying member side is different from the surface on the conveying member side of the recording member. An image forming apparatus having a toner image height lower than that of an unfixed solid image. The image forming apparatus according to claim 2.
The image forming apparatus includes a detecting unit that detects a height of a toner image of an unfixed solid image transferred to the surface of the recording member on the conveying member side, and controls the development potential based on a detection result of the detecting unit. Image forming apparatus. The image forming apparatus according to any one of claims 1 to 3,
An image forming apparatus characterized in that the height of a toner image of a single-color unfixed solid image transferred to the surface of the recording member on the conveying member side is 1.25 times or less with respect to the toner particle diameter.   5. The image forming apparatus according to claim 1, further comprising: a first image forming unit that forms the first toner image; and a second image forming unit that forms the second toner image. Image forming apparatus. The image forming apparatus according to claim 1,
As the toner used for forming the toner image, a toner having a weight average particle diameter of 3 to 7 [μm] and a value obtained by dividing the weight average particle diameter by the number average particle diameter is 1.05 to 1.30. An image forming apparatus characterized by comprising: The image forming apparatus according to claim 1.
An image forming apparatus using a toner having a shape factor SF-1 of 100 to 180 and a shape factor SF-2 of 100 to 180 as toner used for forming the toner image. The image forming apparatus according to claim 1, or the image forming apparatus according to claim 2, comprising a developing device.
The developing device includes a developer carrier that carries a developer and conveys the developer to the image carrier, and a developer regulating member that regulates the amount of the developer carried on the developer carrier. Among the toners on the developer carrier after passing through the developer regulating member, the number of toners of −0.1 [femto C / μm] or more is less than 10 [%] as the toner used for forming the toner image. An image forming apparatus using the toner. The image forming apparatus according to claim 1,
The toner used for forming the toner image is obtained by externally adding an additive to base particles composed of at least a binder resin and a colorant, and the average primary particle size of the additive is 50 to 500 [nm. ] And having a bulk density of 0.3 [g / cm ³ ] or more. The image forming apparatus according to claim 1,
The weight average particle diameter of the toner used for forming the toner image transferred to the surface of the recording member on the conveying member side is set to be the same as that of the toner used for the toner image transferred to a surface different from the surface of the recording member on the conveying member side. An image forming apparatus characterized by being smaller than the weight average particle diameter. The image forming apparatus according to claim 1,
Toner used for forming a toner image on a surface different from the surface on the conveying member side of the recording medium, using the toner shape factors SF-1 and SF-2 used for forming the toner image on the conveying member side surface of the recording medium. An image forming apparatus characterized in that it is smaller than the shape factor SF-1 and the shape factor SF-2. 12. The image forming apparatus according to claim 1, wherein
An image forming apparatus, wherein a partial length of the conveying member from a double-sided transfer unit to a fixing unit is made larger than a maximum length of a recording medium capable of forming an image. The image forming apparatus according to claim 1.
An image forming apparatus, comprising: controlling a speed of a conveying member that conveys the recording body to a fixing unit after the recording body has passed through the double-side transfer means, based on a type of the recording body. The image forming apparatus according to claim 1,
An image forming apparatus, comprising: a charge applying unit configured to apply a charge to the surface of the conveying member before receiving the recording body from the double-sided transfer unit.

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