JP2010066515A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus that can suppress faulty transfer of a second toner image. <P>SOLUTION: A polarity of toner of the second toner image on a transfer belt 10, which has been transferred from a photoreceptor 1 that is a first image carrier to the transfer belt 10 that is a second image carrier, is changed to a polarity opposite to the polarity of the toner on the photoreceptor 1 by a separation charger 21. The second toner image on the transfer belt 10, the polarity of the toner of which is changed, is conveyed to a transfer nip again. The second toner image on the transfer belt 10 is electrostatically transferred to a back surface of transfer paper when a first toner image on the photoreceptor 1 is electrostatically transferred to a front surface of the transfer paper at the transfer nip. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile. More specifically, the present invention relates to a one-pass image forming apparatus that transfers a toner image onto both sides of a recording medium such as transfer paper and fixes the toner images on both sides simultaneously.

  Conventionally, a so-called switchback method and a one-pass method are known as methods for transferring an image onto both surfaces of a recording medium such as transfer paper. In the switchback method, first, a toner image is transferred onto one surface of the transfer member that transfers a toner image on an image carrier such as a photosensitive member to the recording member through the recording member. Then, after the recording body is reversed and switched back to the transfer means, the toner image is transferred to the other surface. On the other hand, in the one-pass method, as a transfer unit, a toner image is transferred from both the front and back sides of the recording medium, thereby forming images on both sides without switching back the recording medium. The one-pass method is superior to the switchback method in the following points. That is, it is possible to avoid an increase in cost by providing a complicated mechanism for switchback and an increase in image formation time due to switchback. As an image forming apparatus that realizes such a one-pass method, for example, the one described in Patent Document 1 is known.

  The image forming apparatus disclosed in Patent Document 1 forms a second toner image to be transferred to the back surface of a recording body on a photoconductor that is a first image carrier. The second toner image formed on the photoconductor is conveyed from the back surface of the transfer belt, which is the second image carrier, to the transfer nip where the photoconductor formed by pressing the transfer roller toward the photoconductor and the transfer belt are in contact with each other. Is done. The second toner image on the photoconductor conveyed to the transfer nip is electrostatically transferred to the transfer belt by the action of a transfer bias having a polarity opposite to the polarity of the toner forming the second toner image. Next, a first toner image to be transferred to the front surface of the recording member is formed on the photosensitive member, and the first toner image on the photosensitive member is electrostatically transferred to the front surface of the recording member at the transfer nip. . After the first toner image is transferred to the front surface of the recording medium, the transfer belt is transferred by a transfer charger disposed downstream of the transfer nip in the recording medium conveyance direction and facing the belt with a predetermined gap. The upper second toner image is electrostatically transferred to the back surface of the recording medium. The recording medium having the toner images transferred on both sides is separated from the transfer belt and conveyed to the fixing device.

JP-A-1-209470

  As described above, the second toner image is recorded by the transfer charger disposed so as to face the belt with a predetermined gap so as not to disturb the first toner image transferred to the front surface of the recording medium. It is transferred to the back of the body. For this reason, when the second toner image is transferred to the back surface of the recording medium, the recording medium may float from the transfer belt. When the recording body floats from the transfer belt, the transfer electric field between the back surface of the recording body and the transfer belt is weakened, and the second toner image cannot be sufficiently electrostatically transferred to the back surface of the recording body. As a result, there is a problem that a transfer failure occurs in the second toner image.

  The present invention has been made in view of the above problems, and an object thereof is to provide an image forming apparatus capable of suppressing transfer failure of a second toner image.

To achieve the above object, the invention of claim 1 includes a first image carrier, a second image carrier, and a toner image forming means for forming a toner image on the first image carrier, After the second toner image is transferred from the first image carrier to the second image carrier, the second toner image on the second image carrier transferred to the second image carrier is transferred. A double-sided transfer unit that transfers the first toner image on the first toner image carrier onto the front surface of the recording body, and transfers the first toner image on the first toner image carrier to the front surface of the recording body. Polarity conversion means for converting the polarity of the toner of the second toner image before transfer of the recording medium on the second image carrier to a polarity opposite to the polarity of the toner on the first image carrier is provided, Forming a transfer nip where the recording medium on the second image carrier and the recording medium on the second image carrier are in contact with each other, and the second image carrier By applying a transfer bias having a polarity opposite to the toner polarity of the first toner image from the back surface, the first toner image on the first image carrier is electrostatically transferred to the front surface of the recording body, The double-side transfer means is configured to electrostatically transfer the second toner image on the second image carrier to the back surface of the recording medium.
According to a second aspect of the present invention, in the image forming apparatus according to the first aspect of the present invention, the image forming apparatus further comprises a neutralizing unit that neutralizes the recording body on the second image carrier having a toner image formed on both sides thereof. It is characterized by using a static eliminating means.
According to a third aspect of the present invention, in the image forming apparatus of the second aspect, the neutralization of the recording body and the polarity conversion of the toner of the second toner image are performed in a non-contact manner.
According to a fourth aspect of the present invention, there is provided the image forming apparatus according to the second or third aspect, wherein the charge eliminating unit includes a discharge member that discharges toward the second image carrier, and a toner image is formed on both sides. When neutralizing the recording body on the two-image carrier, an AC bias is applied to the discharge member to convert the polarity of the toner of the second toner image before transferring the recording body on the second image carrier. Is characterized in that the charge eliminating means is configured to apply a DC bias to the discharge member.
According to a fifth aspect of the present invention, in the image forming apparatus of the fourth aspect, the direct current bias applied to the discharge member is obtained by rectifying alternating current.
According to a sixth aspect of the present invention, in the image forming apparatus according to the fourth or fifth aspect, a discharge needle is used as the discharge member.
According to a seventh aspect of the present invention, in the image forming apparatus according to the fourth or fifth aspect, a conductive thin plate member is used as the discharge member.
According to an eighth aspect of the present invention, in the image forming apparatus according to any of the first to eighth aspects, the second image carrier is stretched endlessly by a driving roller and at least one stretching roller. This is an endless belt.
According to a ninth aspect of the present invention, in the image forming apparatus according to the eighth aspect, the driving roller is provided in a separation portion that separates the recording body from the second image carrier, and the diameter of the driving roller is 16 [ mm] or more.
According to a tenth aspect of the present invention, in the image forming apparatus according to the eighth or ninth aspect, as the second image carrier, a belt having a surface resistance of 10 ⁵ [Ω / □] or more and 10 ¹² [Ω / □] or less. It is characterized by using a member.
According to an eleventh aspect of the present invention, in the image forming apparatus according to any one of the eighth to tenth aspects, the second image carrier and a plurality of stretching rollers that stretch at least the first image carrier are unitized. It is characterized by this.
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 image forming apparatus further comprises a mark detection unit that detects a mark provided on the surface of the second image carrier, and the detection result of the mark detection unit Based on the above, the image forming operation is controlled.
According to a thirteenth aspect of the present invention, in the image forming apparatus according to the twelfth aspect of the present invention, the image forming apparatus further includes a dedicated driving unit for driving the second image carrier.
According to a fourteenth aspect of the present invention, in the image forming apparatus according to any one of the first to thirteenth aspects, the second image carrier is configured to be able to contact and separate from the first image carrier, and at the time of image formation. Other than the above, the second image carrier is separated from the first image carrier.
The invention according to claim 15 is the image forming apparatus according to any one of claims 1 to 14, further comprising a cleaning member that contacts the surface of the second image carrier and cleans the surface of the second image carrier. The second image carrier is configured to be able to contact and separate.
According to a sixteenth aspect of the present invention, in the image forming apparatus according to any one of the first to fifteenth aspects, a toner having an average circularity of 0.90 or more and 0.99 or less is used as the toner constituting the toner image. It is a feature.
According to a seventeenth aspect of the present invention, in the image forming apparatus according to any one of the first to sixteenth aspects, as the toner constituting the toner image, the shape factor SF-1 is 120 or more and 180 or less, and the shape factor SF-2 is It is characterized by using a toner of 120 or more and 190 or less.
According to an eighteenth aspect of the present invention, in the image forming apparatus according to any one of the first to seventeenth aspects, as the toner constituting the toner image, a value obtained by dividing the weight average particle diameter by the number average particle diameter is 1.05 to 1. 30 toner is used.

According to the present invention, the polarity conversion means converts the polarity of the toner of the second toner image on the second image carrier to a polarity opposite to the polarity of the toner on the first image carrier. The toner polarity of the first toner image on the first image carrier and the toner polarity of the second toner image on the second image carrier can be made different. When a transfer bias having a polarity opposite to the toner polarity of the first toner image is applied from the back surface of the second image carrier, the first toner image on the first image carrier is electrostatically attracted toward the recording body, The first toner image is electrostatically transferred to the front surface. On the other hand, the second toner image on the second image carrier is electrostatically transferred from the back surface of the second image carrier to the surface of the second image carrier by a transfer bias having a polarity opposite to the toner polarity of the first toner image. While repelling, it is electrostatically attracted to the toner on the first image carrier. Thereby, the second toner image on the second image carrier is transferred to the back surface of the recording medium.
In this way, the polarity conversion means converts the polarity of the toner of the second toner image on the second image carrier to a polarity opposite to the polarity of the toner on the first image carrier, so that the first in the transfer nip. The toner image and the second toner image can be simultaneously transferred to the recording medium. As a result, the second toner image is electrostatically transferred to the recording body while the second image carrier and the recording body are in close contact with each other. Therefore, as compared with the conventional case where the toner image on the second image carrier is transferred to the back surface of the recording body by a transfer means that is not in contact with the recording body, transfer defects of the second toner image are suppressed. Can do.

Hereinafter, an electrophotographic printer (hereinafter simply referred to as a printer) will be described as an embodiment of an image forming apparatus to which the present invention is applied.
First, the basic configuration of the printer will be described. FIG. 1 is a schematic configuration diagram of the printer. In the figure, the printer includes a photosensitive member 1 as a first image carrier at a substantially central position in the apparatus. Around this photosensitive member, a drum cleaning device 2, a charge eliminating device L, a charging device 3, a developing device 5, and the like are arranged. Alternatively, the photosensitive member 1 and the drum cleaning device 2, the charge removal device L, the charging device 3, the developing device 5, etc. may be replaced at the end of their lifetime as a process unit.

  The charging device 3 uniformly charges, for example, negatively on the surface of the photoreceptor 1 rotated clockwise in the drawing by a driving unit (not shown). The surface of the photoreceptor 1 thus uniformly charged is exposed by an exposure apparatus described later and carries an electrostatic latent image.

  The developing device 5 develops the electrostatic latent image on the photosensitive member 1 into a visible toner image with the toner carried on the developing roller exposed from the opening of the casing. The developed toner image is transferred to a transfer belt 10 as a second image carrier or transfer paper as a recording body by a transfer unit described later. Further, the drum cleaning device 2 cleans the photoreceptor 1 by removing the toner as the image forming material remaining on the photoreceptor 1 after the toner image transfer. The static eliminator L neutralizes residual charges on the photoreceptor 1 after cleaning. By this charge removal, the surface of the photoreceptor 1 is initialized and prepared for the next image formation.

  An exposure device 4 is disposed above the apparatus. The exposure device 4 deflects and scans a laser beam 4A emitted from a light source (not shown) based on image information with a polygon mirror as a deflector, and passes through a plurality of mirrors and an fθ lens 4C. The photosensitive member 1 is irradiated from a writing position between the developing devices 5.

  A paper feed cassette 26 that is a recording body accommodating portion is disposed below the apparatus. A plurality of transfer papers P as recording members are stored in the paper feed cassette 26, and a paper feed roller 27 is in contact with the uppermost transfer paper P. When the paper feeding roller 27 is rotated counterclockwise in the drawing by a driving means (not shown), the uppermost transfer paper P is drawn out, guided by a guide 29 and conveyed to the registration roller pair 28. The registration roller pair 28 feeds the transfer paper P toward a transfer nip described later at an appropriate timing.

  A manual feed tray 35 that can be opened and closed is provided on the right side of the apparatus. A manual paper feed roller 36 is in contact with the uppermost transfer paper P set on the bottom plate 37 of the manual feed tray 35. When the manual paper feed roller 36 is rotated counterclockwise in the drawing by a driving means (not shown), the uppermost transfer paper P is drawn out and conveyed to the registration roller pair 28.

  In this printer, a toner image forming unit is realized by the charging device 3, the exposure device 4, and the developing device 5.

  On the other hand, between the photosensitive member 1 and the paper feed cassette 26, a transfer belt 10, which is a second image carrier, a driving roller 11, stretching rollers 12, 13, backing rollers 14, 15, 16a, 16b, A transfer unit 50 having a transfer roller 20 as a double-side transfer means, a belt cleaning unit 25, and the like is disposed.

FIG. 2 is a schematic perspective view of the transfer unit 50.
As shown in FIG. 2, the transfer unit 50 is configured to support the transfer belt 10 inside a box-shaped frame 51. Stretch rollers 11, 12, and 13 are pivotally supported on the frame 51, and the transfer belt 10 is stretched around these three rollers. Both upper sides of the frame 51 are connected by a reinforcing member 51b. Note that illustrations of the transfer roller 20 and the like that are not necessary for the description here are omitted.

  In order to transfer the transfer paper on the transfer belt 10 to the fixing device, a coupling (not shown) is provided on one shaft end of the stretching roller 11 provided in the separation unit that separates the transfer paper from the transfer belt 10. And is connected to a rotating shaft extending from the driven pulley 52. A driving belt 54 is stretched between the driven pulley 52 and a driving pulley fixed to an output shaft of a stepping motor 53 as a driving means. When the stepping motor 53 is driven, the transfer belt 10 travels. The stepping motor 53 is provided separately from the motor for driving the photoreceptor 1 as the first image carrier, and is a dedicated drive unit for driving the transfer belt 10. That is, in this embodiment, the tension roller 11 is used as a drive roller. The driving roller 11 has a diameter of 16 [mm] or more. By setting the diameter of the drive roller 11 to 16 [mm] or more, the winding area with the transfer belt increases, the slip between the transfer belt and the drive roller can be suppressed, and the speed fluctuation of the transfer belt can be suppressed. In addition, by using the tension roller 11 provided in the separation unit for separating the transfer paper on the transfer belt as a driving roller, the transfer paper carrying area of the transfer belt is pulled by the driving roller, and the transfer belt transfer is performed. The bending of the transfer belt in the paper carrying area can be suppressed. As a result, a shift or the like hardly occurs between the transfer paper and the transfer belt, and the disturbance of the toner image on the transfer belt can be suppressed.

  The shaft of the driving roller 11 is rotatably supported by a support body (not shown) provided so as to be slidable with respect to the apparatus main body, and the transfer unit 50 can swing around the shaft of the driving roller 11. . The frame 51 of the transfer unit 50 is pushed upward (in the direction of the photoreceptor 1) by a spring 56 from below the end on the tension roller 13 side. Due to the pressure of the spring 56, the transfer belt 10 is brought into contact with the photosensitive member 1 with a predetermined pressure to form a transfer nip. A member (not shown) provided on the frame 51 is in contact with the support member of the photoconductor 1 so that the positional relationship between the transfer belt 10 and the photoconductor 1 is appropriately ensured.

  Further, bosses 55 are provided so as to protrude from the side surfaces on both sides of the end portion of the frame 51 on the tension roller 13 side. The boss 55 is fitted with a U-shaped boss receiving portion 58 provided at the tip of a U-shaped yoke member 57. The yoke member 57 is rotatably supported on a support body (not shown) by a shaft 59 that passes through the vicinity of the center of the U-shaped opposed surface. A protruding shaft 60 is provided on the U-shaped side surface portion of the yoke member 57. Above the protruding shaft 60, a solenoid 61 is attached to a support (not shown), and the plunger 62 and the protruding shaft 60 are connected by a spring 63.

  When the solenoid 61 is turned on, the plunger 62 is drawn, whereby the yoke member 57 rotates counterclockwise as shown by the arrow M in the drawing. Then, in the frame 51 of the belt unit, the boss 55 is pushed downward against the pressure of the spring 56, and the transfer unit 50 rotates clockwise as shown by an arrow N in the drawing. That is, the transfer belt 10 is separated from the photoreceptor 1. When the solenoid 61 is turned off, the pull-in of the plunger 62 is released, and the transfer unit 50 rotates in the direction opposite to the arrow N by the pressure of the spring 56 and returns to the initial position. That is, the transfer belt 10 is brought into contact with the photoreceptor 1. At this time, it goes without saying that the yoke member 57 also rotates in the direction opposite to the arrow M.

  In this embodiment, the transfer belt 10 is separated from the photoconductor 1 during non-image formation, and the solenoid 61 is turned on so that the transfer belt 10 contacts the photoconductor 1 when image formation is started. Control off / off.

  Further, as shown in FIG. 1, a mark (not shown) is formed in the non-image forming area at the end of the transfer belt 10, and a mark detection sensor 71 serving as a mark detection means for detecting this mark is provided on the back side. It is arranged so as to face the front surface of the transfer belt between the contact roller 16a and the back roller 16b. In the present embodiment, an optical sensor is used as the mark detection sensor 71. The transfer belt 10 is composed of a member that absorbs light such as black, and a mark (not shown) composed of a member that reflects light is attached to the end of the transfer belt 10. When the transfer belt 10 faces the detection location of the mark detection sensor 71, the light emitted from the mark detection sensor 71 is absorbed without being reflected, and the mark detection sensor 71 does not detect the light. On the other hand, when the mark faces the detection location of the mark detection sensor 71, the light emitted from the mark detection sensor 71 is reflected and the mark detection sensor 71 detects the reflected light. Thereby, the mark detection sensor 71 can detect the mark on the transfer belt. In this embodiment, although details will be described later, an image forming process for forming images on both sides of the transfer paper P is performed based on the detection result of the mark detection means. The mark may be formed on the back surface of the transfer belt 10 and the mark detection sensor 71 may be disposed so as to face the back surface of the transfer belt 10.

Further, the transfer belt 10 is an endless belt, and has an electric resistance condition that can realize electrostatic toner transfer. Specifically, the surface resistivity is adjusted to 10 ⁵ to 10 ¹² [Ω / □].
Depending on the surface resistance of the transfer belt 10, the ease of current flow in the plane direction changes. For this reason, the voltage at the transfer nip has a different voltage drop slope depending on the surface resistivity of the transfer belt 10. That is, when the surface resistivity of the transfer belt 10 is a low surface resistivity of less than 10 ⁵ (Ω / □), the difference between the voltage applied to the transfer roller 20 and the voltage at the transfer nip position is small, and the applied voltage is lowered. be able to. However, when the surface resistivity is a low value of less than 10 ⁵ (Ω / □), the potential at the transfer nip inlet portion upstream of the transfer nip also increases. As a result, the gap electric field between the transfer belt 10 at the entrance of the transfer nip and the photosensitive member is increased, and a part of the toner image formed on the surface of the photosensitive member is transferred through the gap, and is particularly noticeable in characters. Will result in a Chile image.

On the other hand, when the surface resistivity of the intermediate transfer belt 10 exceeds 10 ¹² [Ω / □], the applied voltage applied to the transfer roller 20 increases to ensure good transferability, resulting in insufficient power capacity. To do. In addition, the potential of the intermediate transfer belt at the transfer nip outlet becomes high, and abnormal discharge is likely to occur in the gap portion of the intermediate transfer nip outlet between the transfer belt 10 and the photosensitive member.

In the present embodiment, since the surface resistivity of the transfer belt 10 is set to 10 ⁵ to 10 ¹² [Ω / □], a good image without image deterioration such as a dust image can be obtained. Further, the transfer bias can be kept low, and the power source capacity of the power source can be kept low.

  The transfer roller 20 serving as a double-sided transfer means is disposed so that the transfer belt 10 is sandwiched between the transfer roller 10 and the photosensitive member 1 at the transfer nip. The transfer bias is applied. Then, a transfer electric field is formed between the transfer bias and the photoreceptor 1.

  The transfer paper is charged by the influence of the transfer bias and is electrostatically attracted to the transfer belt 10. The transfer sheet electrostatically attracted to the transfer belt 10 changes its moving direction abruptly in accordance with the curvature of the drive roller 11 at the belt wrapping position by the drive roller 11 disposed closest to the heat fixing device 30. The transfer paper is separated from the transfer belt 10 and transferred to the heat fixing device 30. However, in this printer, as described above, in order to stably move the transfer belt 10 endlessly, the diameter of the drive roller 11 is 16 mm or more. The curvature of the drive roller 11 at the location is large. As a result, the transfer paper cannot be separated in curvature, and paper jam may occur.

  Therefore, in the present printer, as shown in FIG. 1, as a charge eliminating unit that discharges the transfer paper by facing the front surface of the transfer belt 10 at the separation portion where the transfer belt 10 and the transfer paper are separated. The separation charger 21 is provided. By neutralizing the transfer paper with the separation charger 21, the electrostatic attraction between the transfer paper and the transfer belt 10 can be weakened. As a result, even when the diameter of the driving roller 11 is 16 [mm] or more and the curvature of the driving roller 11 at the belt-wrapped portion by the driving roller 11 is large, the transfer paper can be satisfactorily separated from the transfer belt 10.

Further, the separation charger 21 functions as a polarity conversion unit that converts the polarity of the toner of the toner image carried on the transfer belt 10.
FIG. 3 is a diagram for explaining the polarity conversion of the toner of the toner image carried on the transfer belt 10 by the separation charger 21.
As shown in FIG. 3, an AC power source 201 for applying an AC bias to the separation charger and a rectifier 202 that rectifies AC to become DC are provided. When converting the polarity of the toner of the toner image carried on the transfer belt 10, as shown in FIG. 3, an alternating current from an alternating current power supply 201 is passed through the rectifier 202 and rectified by the rectifier 202 to generate a direct current. This direct current is applied to the separation charger 21. As a result, a DC bias is applied to the separation charger. When the transfer paper on the transfer belt is discharged, the switch 203 is switched so that an AC bias is applied to the discharge bias. Further, an AC power source and a DC power source may be provided, and when the toner polarity of the toner image carried on the transfer belt 10 is converted, the AC power source may be switched to the DC power source. Note that the polarity of the DC bias applied to the separation charger 21 when converting the polarity of the toner of the toner image carried on the transfer belt 10 is opposite to the polarity of the toner on the photoconductor.

  When converting the polarity of the toner of the toner image carried on the transfer belt 10, the toner carried on the transfer belt 10 is applied to the separation charger 21 by applying a DC bias having a polarity opposite to the polarity of the toner on the photoreceptor. Charges of polarity (negative polarity) and reverse polarity (positive polarity) of the toner of the image are applied to the toner on the transfer belt. As a result, the polarity of the toner of the toner image carried on the transfer belt 10 is converted to a polarity (positive polarity) opposite to the toner polarity on the photoreceptor. As described above, the toner polarity of the toner image on the transfer belt is converted to a polarity opposite to the toner polarity on the photoconductor, so that the toner image on the photoconductor 1 is transferred at the transfer nip as will be described later. At the same time as being transferred to the front surface of the paper, the toner image on the transfer belt 10 can be transferred to the back surface of the transfer paper.

  When neutralizing the transfer paper on the transfer belt, by applying an AC bias, the transfer paper can be given a charge having the opposite polarity to the charge polarity of the transfer paper, and the transfer belt has the opposite polarity to the charge polarity of the transfer belt. Charge can be applied to the transfer belt. Thereby, not only the transfer paper but also the transfer belt can be neutralized, and the transfer paper can be satisfactorily separated by the separation unit.

  A heat fixing device 30 is disposed on the left side of the transfer unit 50 in the drawing. The transfer paper P that has been neutralized by the separation charger 21 is delivered to the heating and fixing device 30 from the separation portion of the transfer belt.

  The heat fixing device 30 includes a heating roller 30A having a heater (not shown), a pressure roller 30B, and the like, and sandwiches the transfer paper P fed from the transfer belt 10 between both rollers. Then, the toner image is fixed on the transfer paper P by an action such as heat generated by the heating roller 30 </ b> A as a heating conveyance member or pressure between the rollers, and then the transfer paper P is sent out. As described above, in the heat fixing device 30 that fixes the toner image to the transfer paper P while heating it, the fixing property of the toner image onto the transfer paper P can be improved as compared with the fixing device that does not heat the transfer paper P. . The transfer paper P delivered from the heat fixing device is a first paper discharge tray provided above the device in accordance with the swing stop position of the switching guide 42 configured to be swingable about the swing shaft 43. 40 or the second discharge tray 44 that is provided on the left side of the apparatus and can be opened and closed. When the paper is fed toward the first paper discharge tray 40, it is reversed and conveyed by the first paper discharge roller pair 34A and the second paper discharge roller pair while being guided by the guide members 31A and 31B. Stacked on the tray 40. When the paper is sent toward the second paper discharge tray 44, it is transported by the paper discharge roller pair 32 and stacked on the second paper discharge tray 44. The pressure roller 30B may also be a heating roller having a heat source.

  A control device E2 disposed below the electrical component E1 and the switching guide 42 disposed between the paper feed cassette 26 and the transfer unit 50 performs electrical control on each device. The fan F1 disposed in the printer main body forcibly discharges the air in the apparatus to the outside and suppresses an excessive increase in the internal temperature.

  In this embodiment, the transfer belt 10 is separated from the photoreceptor 1 during non-image formation. Therefore, when the front door D is opened and the transfer unit 50 is pulled out together with the support or when the process unit is attached or detached, the transfer belt 10 and the photosensitive member 1 are not rubbed. Therefore, it is possible to prevent the photosensitive member 1 and the transfer belt 10 from being damaged.

The printer 100 is configured to form a toner image not only on one side of the transfer paper P but also on both sides by a process as described below. That is, the printer 100 is configured to form an image based on an image information signal sent from a personal computer 200 (not shown). When this image information signal is received by the printer 100, the transfer belt 10 is driven to rotate, the mark on the transfer belt 10 is detected by the mark detection sensor 71, and the transfer belt 10 is initialized to a fixed position. In the present embodiment, since the driving means for driving the transfer belt 10 is the stepping motor 53, the transfer belt 10 can be accurately stopped at a fixed position. Further, since the stepping motor 53 is a dedicated driving means for driving the transfer belt 10, the start position can be controlled by driving only the transfer belt 10.
Further, when the start position of the transfer belt 10 is controlled, the transfer belt 10 is separated from the photoconductor 1 to prevent the transfer belt 10 and the photoconductor 1 from being damaged by rubbing.

  Thus, when the transfer belt 10 is initialized to a fixed position, the solenoid 61 is turned off to bring the transfer belt 10 into contact with the photoreceptor 1. The photosensitive member 1 and the transfer belt 10 are simultaneously driven at a predetermined timing, and the exposure device 4 is driven and controlled based on the image information signal. Laser light (4A) emitted from the exposure apparatus 4 by this drive control is scanned by a polygon mirror 4B that is rotationally driven by a motor (not shown). Then, a second electrostatic latent image corresponding to a second image (second page image) formed on the back surface of the transfer sheet P is formed by irradiating the photoreceptor 1 through a mirror, an fθ lens 4C, and the like.

  The second electrostatic latent image is developed by the developing device 5 using toner which is an image forming substance, and becomes a second toner image on the photoreceptor 1. The second toner image moves to the transfer nip as the photosensitive member 1 rotates. At this time, the transfer paper P is not yet fed into the transfer nip, and the second toner image is transferred onto the transfer belt 10 instead of the transfer paper P. Since the transfer belt 10 is started to be driven after being initialized to a fixed position by start position control, the second toner image is transferred to a predetermined position on the transfer belt 10. Then, after the transfer, when the transfer belt 10 moves endlessly by about one round, it returns to the transfer nip again.

  Thus, while the transfer belt 10 moves endlessly by about one round, the mark is detected again by the mark detection sensor 71, and the next exposure process and development process are started based on the detection result of the mark detection sensor 71, A first toner image corresponding to the first image (first page image) formed on the front surface of the transfer paper P is formed on the photoreceptor 1. Further, based on the detection result of the mark detection sensor 71, the transfer paper P is sent from the paper feed cassette 26 to the registration roller pair 28. Based on the detection result of the mark detection sensor 71, the registration roller pair 28 has a timing at which the back surface (the lower surface in the figure) of the transfer paper P can be superimposed on the second toner image that returns to the transfer nip again. Send to transfer nip.

  As described above, each process is controlled based on the detection result of the mark detection sensor 71, so that the first toner image is transferred to the front surface of the transfer paper P sent to the transfer nip (upper surface in the drawing). ) On the photosensitive member 1 at a timing that exactly overlaps the Therefore, in the transfer nip where the second toner image has returned again, the second toner image on the transfer belt 10, the transfer paper P, and the first toner image on the photoreceptor 1 are sandwiched.

  Then, as shown in FIG. 4, the first is affected by the transfer electric field formed by the transfer roller 20 to which a transfer bias having a reverse polarity (positive polarity) to the first toner image on the photoconductor is applied from the power source 20a. The toner image is transferred to the front surface of the transfer paper P. As described above, the toner that forms the second toner image on the transfer belt is converted to the polarity (positive polarity) opposite to the polarity of the toner on the photosensitive member by the separation charger 21. For this reason, when the first toner image is transferred to the front surface of the transfer paper P, it is located between the transfer belt 10 and the back surface of the transfer paper P due to the influence of the transfer electric field formed by the transfer roller 20. A repulsive force acts on the second toner image, and the second toner image is electrostatically transferred to the back surface of the transfer paper P by the repulsive force.

  As described above, in the present printer, the toner that forms the second toner image on the transfer belt is converted into a polarity (positive polarity) opposite to the polarity of the toner on the photosensitive member by the separation charger 21, so The second toner image can be transferred to the back surface of the transfer paper. Accordingly, the second toner image can be electrostatically transferred to the back surface of the transfer paper P while the transfer paper and the transfer belt are in close contact with each other, and transfer defects of the second toner image can be suppressed.

  The transfer paper P that has exited the transfer nip carries the first toner image transferred at the transfer nip on the front surface, carries the second toner image on the back surface, and moves together with the transfer belt 10. When passing through the position facing the separation charger 21, a high-voltage AC bias is applied, and the transfer paper on the transfer belt 10 is neutralized. At this time, a predetermined gap is maintained between the front surface of the transfer paper P and the separation charger 21, and the first toner image is not disturbed by contact with the separation charger 21.

  The second toner image is formed on the photoreceptor 1 so as to be a normal image instead of a mirror image. This is for the reason explained below. That is, in the case of direct transfer from the photosensitive member 1 to the transfer paper P, a mirror image is formed on the photosensitive member 1, and when this is directly transferred to the transfer paper P, a normal image is obtained. However, in indirect transfer of photoconductor → transfer belt → transfer paper P, if a mirror image is formed on the photoconductor 1, it becomes a mirror image on the back surface of the transfer paper P. Needless to say, the first toner image directly transferred to the transfer paper P is formed as a mirror image on the photoreceptor 1 as usual.

  The transfer paper P with the toner images transferred on both sides is sent to the heat fixing device 30, where the first toner image and the second toner image on the transfer paper are fixed at a time and conveyed to the switching guide 42. When the position of the switching guide 42 is the position shown in FIG. 1, the first image (first page image) formed on the front surface of the transfer paper P becomes the lower surface, and is placed on the first paper discharge tray 40. The paper is ejected. On the other hand, when the switching guide 42 is swung in the direction of the arrow J, the first image (first page image) formed on the front surface of the transfer paper P becomes the lower surface, and the first paper discharge is performed. The paper is discharged to the tray 44.

In order to align the recorded transfer paper on the first paper discharge tray 40 during continuous printing, the second image of the second page is created first, and the toner corresponding to the second image is formed on the transfer belt 10. The image is held, and a first image corresponding to the first page is created later, and is controlled to be directly transferred from the surface of the photosensitive member to the transfer paper.
On the other hand, in order to align the pages of the recorded transfer paper on the second paper discharge tray 44 during continuous printing, the order of image formation is reversed. That is, the first image of the first page is created first, the toner image corresponding to the first image is held on the transfer belt 10, and the second image corresponding to the second page is created later, from the surface of the photoreceptor. Control is performed so that the image is directly transferred onto the transfer paper.

  When forming an image on thick paper, it is preferable to set the thick paper on the manual feed tray 35, feed the thick paper from the manual feed tray 35, and discharge the paper to the second paper discharge tray 44. Since the transport path is almost linear, thick and rigid cardboard can be transported without resistance, and jamming can be suppressed.

  When the image forming operation is completed and the transfer belt 10 and the photoconductor 1 are stopped, the transfer belt 10 is separated from the photoconductor 1.

  The above is the image forming process performed when the toner image is formed on both sides of the transfer paper P. When the toner image is formed on one side of the transfer paper P, the following image forming process is performed. That is, first, the transfer belt 10 is brought into contact with the photosensitive member, and an image forming operation is started. A mirror image of the toner image is formed on the photoreceptor 1 and sent to the transfer nip. On the other hand, the transfer paper P is sent from the paper feed cassette 26 toward the registration roller pair 28 and is sent out to the transfer nip at a timing such that the registration roller pair 28 overlaps the toner image. The transfer paper P that passes through the transfer nip and carries a toner image on its front surface is charged from the separation charger 21 and discharged, and then transferred to the fixing device 30.

  This printer employs a contact transfer system in which the toner image on the photosensitive member 1 is transferred to the belt side while the transfer belt 10 is positively brought into contact with the photosensitive member 1 in the transfer nip. In such a contact transfer method, unlike a non-contact transfer method in which a toner image is projected toward a transfer body, transfer is performed without causing the toner image to fly, thereby avoiding a transfer shift caused by a deviation in the flight path of the toner image. be able to. Further, the toner image on the photoconductor is transferred to the front surface of the transfer paper by the contact transfer method, and the toner image on the transfer belt is transferred to the back surface of the transfer paper. Therefore, it is possible to avoid the transfer deviation of both the toner image transferred to the front surface of the transfer paper and the toner image transferred to the back surface.

  After the toner on the transfer belt is transferred to the transfer paper, some toner inevitably remains on the transfer belt 10. When the residual toner comes into contact with the transfer paper P sent out from the registration roller pair 28 to the transfer nip, the back surface of the transfer paper P is soiled.

  Therefore, the printer includes a belt cleaning unit 25 as shown in FIG. The belt cleaning unit 25 includes a cleaning roller 25A, which is a cleaning member, a scraping blade 25B, a transport screw 25C, a contact / separation mechanism (not shown), and the like.

  The cleaning roller 25A is disposed downstream of the driving roller 11 of the transfer unit 50 in the belt moving direction. Then, the transfer belt 10 is rotated while being sandwiched between the tension roller 13 and the tension roller 13 that contacts the back surface of the transfer belt 10. A cleaning bias is applied to the cleaning roller 25A by a power source (not shown). Thus, a cleaning electric field is formed between the cleaning roller 25A and the stretching roller 13 so as to electrostatically move the toner from the stretching roller 13 side to the cleaning roller 25A side. Residual toner on the front surface of the transfer belt 10 is transferred to the cleaning roller 25 </ b> A from the transfer belt 10 by the influence of the cleaning electric field and removed. The scraping blade 25B scrapes off the residual toner transferred to the surface of the cleaning roller 25A and drops it onto a conveying screw 25C disposed below the scraping blade 25B. Further, the conveying screw 25C rotates while receiving the residual toner falling from the scraping blade 25B, thereby conveying the residual toner in the axial direction and sending it to a collecting unit (not shown).

  The belt cleaning unit 25 is also provided with a contact / separation mechanism (not shown) for bringing the cleaning roller 25 </ b> A into contact with and away from the transfer belt 10. This contact / separation mechanism swings the belt cleaning unit 25 around the transport screw 25C as shown by an arrow in FIG. 1 by turning on / off a solenoid (not shown), for example. By this swinging, the cleaning roller 25 </ b> A comes into contact with and separates from the transfer belt 10. According to such a contact / separation mechanism, a situation where the second toner image is cleaned can be avoided as follows. When not the residual toner but the second toner image moves to the cleaning position, the cleaning roller 25A is separated from the transfer belt 10. In addition, the load on the rotation driving unit of the cleaning roller 25A and the transfer belt 10 can be reduced without losing the cleaning performance as follows. That is, the cleaning roller 25A is brought into contact with the transfer belt 10 only when cleaning is necessary, and is separated when cleaning is not required. For example, the cleaning roller 25 </ b> A is brought into contact with the transfer belt 10 at a predetermined timing after the AC bias is applied to the separation charger 21. Then, the separation charger 21 is turned off, and the cleaning roller 25A is separated from the transfer belt 10 at a predetermined timing.

  Further, in the above, a separation charger is used as a charge eliminating unit, and a bias is applied to a wire extending in the transfer belt width direction as a discharge member. For example, as shown in FIG. 5, a conductive thin plate member 21a is used as a discharge member. It may be used. Moreover, as shown in FIG. 6, you may use as a discharge member what processed so that the one long side of the electroconductive thin plate member might become sawtooth shape. In the configuration shown in FIG. 6, the sawtooth portion constitutes the discharge needle 21b. Then, by discharging from these discharge needles 21b, the polarity of the toner on the transfer belt is converted, or the transfer paper on the transfer belt is discharged.

  In the above description, the neutralization charger as the neutralization unit is used as the polarity conversion unit for converting the polarity of the toner on the transfer belt. However, the separation charger and the polarity conversion unit may be provided separately. The polarity conversion unit may be disposed anywhere as long as the toner image on the transfer belt is in a position where charge can be imparted to the toner on the transfer belt before reaching the transfer nip. In addition, it is preferable that the polarity conversion unit does not contact the transfer belt 10 so as not to disturb the toner image on the transfer belt 10.

Next, toner that can be suitably used in the image forming apparatuses according to the first and second embodiments will be described.
In this printer, the user is designated to use toner that satisfies all of the following conditions (a) to (d) as toner used for forming a toner image.
(A) The average circularity is 0.90 to 0.99.
(B) The shape factor SF-1 is 120 to 180.
(C) The shape factor SF-2 is 120 to 190.
(D) The particle size distribution (volume average particle diameter Dv / number average particle diameter Dn) is 1.05 to 1.30.

  As a method for designating the user to use such toner, for example, a toner having all of the above conditions (a) to (d) can be packaged and shipped together with a printer. Further, for example, the product number or product name of the toner may be specified in the printer 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, the toner bottles (BY, BM, BC, BK), which are toner storing means for storing such toner, can be shipped in a state of being set in the printer main body. This printer 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. That is, a toner having an average circularity of less than 0.90, that is, a toner having an irregular shape rather than a spherical shape, deteriorates transferability rapidly and causes transfer dust during electrostatic transfer. It is because it becomes easy. Further, if it is less than 0.90, it becomes difficult to form a high-definition image having a reproducibility with an appropriate density. Furthermore, if the average circularity exceeds 0.99, in a device that employs blade cleaning, a cleaning defect such as a photosensitive member or an intermediate transfer belt may occur and the image may be easily stained. It is also from. When outputting an image having a relatively low image area ratio, there is little transfer residual toner, and poor cleaning is unlikely to be a problem. However, when outputting an image with a high image area ratio, such as a color photographic image, or when an untransferred image remains on the photoconductor due to a paper feed failure or the like, cleaning defects are particularly likely to occur. . A more preferable range of the average circularity is 0.93 to 0.97, and it is more preferable that the toner particles having a circularity of less than 0.94 are limited to 10% or less.

  The average circularity of the toner can be measured as follows. That is, first, a suspension containing toner particles of the toner to be tested is passed through an imaging unit detection zone on a flat plate, and the particle image is optically captured by a CCD camera. Then, for each particle image, an average value of the values obtained by dividing the perimeter of an equivalent circle having the same projected area by the perimeter of the actual particle is calculated. This average value is the average circularity. In order to measure the average circularity, for example, a flow type particle image analyzer FPIA-2100 (manufactured by Toa Medical Electronics Co., Ltd.) may be used. In the case of using this apparatus, 0.1 to 0.5 ml of a surfactant, preferably alkylbenzene sulfonate is added as a dispersant in 100 to 150 [ml] of water from which impure solids have been removed in advance. Further, about 0.1 to 0.5 [g] of the test toner is added. Then, this suspension was subjected to dispersion treatment with an ultrasonic disperser for about 1 to 3 minutes, and the dispersion concentration was adjusted to 3000 to 1 [10,000 / μl], and the shape and distribution of the toner were measured using the above apparatus. To do.

The reason why the toner satisfying the conditions (b) and (c) 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. 7, it is a value obtained by dividing the square of the length MXLNG of the maximum diameter portion in an elliptical figure obtained by projecting a spherical substance on a two-dimensional plane by the area AREA and further multiplying by 100π / 4. . That is, it can be expressed by the following equation. 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.
(Formula 1)
Shape factor SF-1 = {(MXLNG) ² / AREA} × (100π / 4)

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. 8, it is a value obtained by dividing the square of the perimeter PERI of the figure obtained by projecting a spherical substance on a two-dimensional plane by the area AREA and further multiplying by 100 / 4π. That is, the shape factor SF-2 can be expressed by the following equation. 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.
(Formula 2)
Shape factor SF-2 = {(PERI) ² / AREA} × (100 / 4π)

  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. According to the inventor's research, it has been clarified that when the shape factor SF-1 exceeds 180 and the shape factor SF-2 exceeds 190, the transfer efficiency starts to deteriorate rapidly.

  However, the closer the toner shape is to a true sphere, the more disadvantageous it is for mechanical cleaning (blade cleaning, etc.). This is presumably because the toner fluidity increases or the toner easily passes through a slight gap between the cleaning member and the member to be cleaned. According to the study by the present inventors, it became clear that when both the shape factor SF-1 and the shape factor SF-2 are less than 120, the cleaning property starts to deteriorate rapidly.

  The shape factor SF-1 and the shape factor SF-2 can be obtained as follows. That is, using a FE-SEM (S-800) manufactured by Hitachi, 100 images of toner particles are selected at random, and the images are sequentially taken, and the image information is introduced into an image analyzer (LUSEX 3) manufactured by Nireco. Obtain MXLING, AREA, and PERI. And it calculates as an average value per 100 shape factors obtained by the above-mentioned formula.

  The reason why the toner satisfying the condition (d) is specified is as follows. That is, the particle size distribution (volume average particle size Dv / number average particle size Dn) is one of the parameters representing the particle size distribution of the toner. A dry toner having a volume average particle diameter Dv / number average particle diameter Dn of 1.05 to 1.30, preferably 1.10 to 1.25 has various advantages because the particle size distribution of the toner becomes narrow. .

  For example, a powder toner having a volume average particle diameter Dv of 4 to 8 [μm] and a volume average particle diameter Dv / number average particle diameter Dn of 1.05 to 1.30 has the following merits. There is. That is, among them, a phenomenon that toner particles having a particle size suitable for the pattern of the electrostatic latent image tends to contribute to development preferentially over other toners, so that images of various patterns are stably formed. be able to. 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. When such a recycle having a relatively large particle size distribution is used, the variation in the particle size from the new toner supply to the next toner supply becomes large, which adversely affects the development performance. Further, in the case of a toner having a volume average particle diameter Dv smaller than the above range, when used as a two-component developer, the toner is fused to the surface of the carrier during a long period of stirring in the developing device, thereby reducing the charging ability of the carrier. . 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. On the contrary, if the volume average particle system Dv is larger than the above range, it becomes difficult to obtain a high-resolution and high-quality image, and the toner particle diameter when the balance of the toner in the developer is performed. In many cases, the fluctuations of

  Further, when the above characteristics are satisfied, the solid filling of the toner is improved. Therefore, a good image density can be obtained even if the toner adhesion amount is reduced. In general, the toner adhesion amount in a portion with a high image density increases, and the toner adhesion amount in a portion with a low image density decreases. If the difference between the part with a large amount of toner adhesion and the part with a small amount of toner is large, the amount of toner adhering will be significantly different, and the pressure applied to the part with a small amount of toner adhesion will be weaker than the part with a large amount of toner adhesion Become. As a result, there is a possibility that the portion where the toner adhesion amount is small does not sufficiently lower the viscosity due to the pressure and the fixability is deteriorated. However, by satisfying the above characteristics, a good image density can be obtained even if the toner adhesion amount is reduced. For this reason, it is possible to suppress the toner adhesion amount in the high image density portion, and to reduce the difference in the toner adhesion amount between the high image density portion and the low image density portion. As a result, the toner volume is not significantly different between the high image density portion and the low image density portion, and the viscosity of the low image density portion is sufficiently reduced by the pressure and can be fixed on the transfer paper.

  The particle size distribution of the toner can be measured by a measuring device using a Coulter counter method, for example, a 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 volume and number of toner particles or toner are measured with the above-described measuring apparatus using a 100 μm aperture as the aperture. Calculate the distribution. From the obtained distribution, the volume average particle diameter Dv and the number average particle diameter Dn 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.

  As described above, according to the image forming apparatus of the present embodiment, the polarity of the toner of the second toner image on the transfer belt as the second image carrier by the polarity conversion unit is changed to the polarity of the toner on the photosensitive member as the first image carrier. The polarity is reversed. As a result, the first toner image and the second toner image can be simultaneously transferred onto the transfer paper at the transfer nip. As a result, the second toner image can be electrostatically transferred in a state in which the second image carrier and the transfer paper are in close contact with each other. Therefore, as compared with the conventional case where the toner image on the second image carrier is transferred to the back surface of the transfer paper by a non-contact transfer means to the transfer paper, the transfer failure of the second toner image is suppressed. Can do.

  In addition, by using a separation charger as a neutralization unit as the polarity conversion unit, the apparatus can be made cheaper than the case where the polarity conversion unit and the separation charger are individually provided.

  Further, by performing non-contacting of the transfer paper charge removal and toner polarity conversion of the second toner image, the transfer sheet charge removal may disturb the first toner image transferred to the front surface of the transfer paper sheet. Absent. Further, the second toner image on the transfer belt is not disturbed during polarity conversion.

When the transfer paper on the transfer belt is neutralized, by applying an AC bias, both the transfer belt and the transfer paper charged to the opposite polarity to the transfer belt can be neutralized. Can be separated well.
In addition, when converting the polarity of the toner of the second toner image on the transfer belt, only a charge having a polarity opposite to the toner polarity of the second toner image can be applied by applying a DC bias. The polarity of the toner of the two toner image can be converted well.

  In addition, the DC bias can be obtained by rectifying the alternating current, so that the alternating current bias and the direct current bias can be applied to the separation charger with only the alternating current power supply.

  Further, the discharge needle may be used to discharge the toner image on the transfer belt and the transfer paper. Even in such a configuration, the polarity of the toner of the toner image can be converted in a non-contact manner, and the transfer paper on the transfer belt can be discharged without contact.

  Further, the toner image on the transfer belt may be discharged using a conductive thin plate member. Even in such a configuration, the polarity of the toner of the toner image can be converted in a non-contact manner, and the transfer paper on the transfer belt can be discharged without contact.

  Further, by making the transfer belt an endless belt, the toner image on the transfer belt can be transported to the transfer nip again only by moving the belt surface in one direction, and the apparatus configuration can be simplified. An image forming apparatus.

The drive roller is provided at a separation portion where the transfer paper is separated from the transfer belt, and the diameter of the drive roller is 16 [mm] or more. By providing the driving roller in the separation portion where the transfer paper is separated from the transfer belt, the region of the transfer belt carrying the transfer paper is pulled by the driving force of the drive roller. And the second toner image transferred between the transfer belt and the back surface of the transfer paper is rubbed against the transfer belt and transferred to the back surface of the transfer paper. Disturbing the image can be suppressed.
Further, by setting the diameter of the drive roller to 16 [mm] or more, the amount of wrapping around the transfer belt increases, and slipping or the like hardly occurs between the transfer belt and the drive roller. As a result, the speed fluctuation of the transfer belt can be suppressed.

Further, by using a belt member having a surface resistance of 10 ⁵ [Ω / □] or more and 10 ¹² [Ω / □] or less as the transfer belt 10, it is possible to obtain a good image without image deterioration such as a dust image. . Further, the transfer bias can be kept low, and the power source capacity of the power source can be kept low.

  By unitizing the transfer belt 10 and at least a plurality of stretching members that stretch the transfer belt, the transfer belt 10 can be easily detached from the apparatus main body, and exchange workability can be improved.

  In addition, a mark detection sensor serving as a mark detection unit that detects a mark provided on the surface of the transfer belt is provided, and an image forming operation is controlled based on a detection result of the mark detection sensor. That is, the toner image can be transferred to a predetermined position on the transfer belt by starting the image forming operation after the mark is detected by the mark detection sensor. As a result, the positional relationship between the mark on the transfer belt and the toner image on the transfer belt becomes a predetermined relationship, and the back surface of the transfer paper P is controlled by controlling the registration roller pair based on the detection result of the mark detection sensor. (The lower surface in the figure) can be sent to the transfer nip at a timing at which it can be superimposed on the second toner image returning to the transfer nip again. Further, by controlling the formation timing of the toner image to be transferred to the front surface of the transfer paper based on the detection result of the mark detection sensor, the toner image on the photosensitive member is transferred to the front surface of the transfer paper P (see FIG. It can be sent out to the transfer nip at a timing that perfectly overlaps the upper middle surface. Thus, double-sided transfer can be performed at a predetermined position on the transfer paper without the image forming position being shifted in the sub-scanning direction.

  In addition, by having a dedicated drive unit for driving the transfer belt, it is possible to perform mark detection by the mark detection sensor before the image forming operation by rotating only the transfer belt.

  Further, the transfer belt is configured to be able to contact and separate from the photosensitive member, and the transfer belt is separated from the photosensitive member except during image formation. As a result, when the photoconductor is taken out from the apparatus main body or when the transfer belt is exposed from the apparatus main body, the photoconductor and the transfer belt are not rubbed and the surface of the photoconductor and the transfer belt can be prevented from being damaged. .

  In addition, a cleaning roller that is a cleaning member that contacts the transfer belt surface and cleans the transfer belt surface is configured to be able to contact and separate from the transfer belt. As a result, when the toner image on the transfer belt is conveyed again to the transfer nip, a situation in which the toner image on the transfer belt is cleaned by the cleaning roller can be avoided.

  Further, according to the image forming apparatus of the present embodiment, a toner having an average circularity of 0.90 to 0.99 is used as a toner used for forming a toner image. This makes it possible to form a high-quality image with reduced transfer dust while exhibiting a stable electrostatic transfer rate and suppressing transfer shortage.

  Further, according to the image forming apparatus of the present embodiment, the toner used for forming the toner image is designated with the shape factor SF-1 of 120 to 180, so that the shortage of transfer and the transfer dust are further suppressed. A high-quality image can be formed. Further, since the toner used for forming the toner image has a shape factor SF-2 of 120 to 190, it is possible to form a high-quality image in which transfer shortage and transfer dust are further suppressed.

Further, according to the image forming apparatus of the present embodiment, a toner having a value obtained by dividing the volume average particle diameter Dv by the number average particle diameter Dn is 1.05 to 1.30 as the toner used for forming the toner image. Therefore, a high-quality image developed with stable development performance can be obtained.
Further, when the above characteristics are satisfied, the solid filling of the toner is improved. Therefore, a good image density can be obtained even if the toner adhesion amount is reduced. In general, the toner adhesion amount in a portion with a high image density increases, and the toner adhesion amount in a portion with a low image density decreases. If the difference between the part with a large amount of toner adhesion and the part with a small amount of toner is large, the volume of toner adhering will be greatly different, and the pressure applied to the part with a small amount of toner adhesion will be higher than the pressure applied to the part with a large amount of toner adhesion Become weaker. As a result, there is a possibility that the portion where the toner adhesion amount is small does not sufficiently lower the viscosity due to the pressure and the fixability is deteriorated. However, by satisfying the above characteristics, a good image density can be obtained even if the toner adhesion amount is reduced. For this reason, it is possible to suppress the toner adhesion amount in the high image density portion, and to reduce the difference in the toner adhesion amount between the high image density portion and the low image density portion. As a result, the toner volume is not significantly different between the high image density portion and the low image density portion, and the viscosity of the low image density portion is sufficiently reduced by the pressure and can be fixed on the transfer paper.

1 is a schematic configuration diagram of a printer according to an embodiment. FIG. 3 is a schematic perspective view of a transfer unit. FIG. 4 is a diagram for explaining toner polarity conversion of a toner image carried on a transfer belt by a separation charger. The figure explaining simultaneous transfer to the both sides of transfer paper in a transfer nip. The schematic block diagram which shows the example which used the electroconductive thin plate member as a discharge member. The schematic block diagram which shows the example using what provided the needle electrode in the electroconductive thin plate member as a discharge member. The schematic diagram for demonstrating shape factor SF-1. The schematic diagram for demonstrating shape factor SF-2.

Explanation of symbols

1: Photoconductor 2: Drum cleaning device 4: Exposure device 5: Development device 10: Transfer belt 20: Transfer roller 21: Separation charger 25: Belt cleaning unit 26: Paper feed cassette 28: Registration roller pair 30: Heat fixing device 32 : Switching guide 35: manual feed tray 40: first paper discharge tray 44: second paper discharge tray 50: transfer unit 53: stepping motor 71: mark detection sensor 100: printer 300: scanner

Claims (18)

A first image carrier;
A second image carrier;
Toner image forming means for forming a toner image on the first image carrier;
After the second toner image is transferred from the first image carrier to the second image carrier, the second toner image on the second image carrier transferred to the second image carrier is transferred. A double-sided transfer unit that transfers the first toner image on the first toner image carrier onto the front surface of the recording medium, and transfers the first toner image on the first toner image carrier to the front surface of the recording medium;
A polarity conversion means for converting the polarity of the toner of the second toner image before transfer of the recording material on the second image carrier to a polarity opposite to the polarity of the toner on the first image carrier;
A transfer nip is formed in which the recording body on the first image carrier and the recording body on the second image carrier abut, and a polarity opposite to the toner polarity of the first toner image is formed from the back surface of the second image carrier. By applying the transfer bias, the first toner image on the first image carrier is electrostatically transferred to the front surface of the recording body, and at the same time, the second toner image on the second image carrier is transferred. An image forming apparatus characterized in that a double-side transfer means is configured to electrostatically transfer the image onto the back surface of the recording medium. The image forming apparatus according to claim 1.
A neutralizing means for neutralizing the recording body on the second image carrier having the toner images formed on both sides;
An image forming apparatus using the charge eliminating unit as the polarity converting unit. The image forming apparatus according to claim 2.
An image forming apparatus, wherein neutralization of the recording material and toner polarity conversion of the second toner image are performed in a non-contact manner. The image forming apparatus according to claim 2 or 3,
The static elimination means includes a discharge member that discharges toward the second image carrier,
When neutralizing the recording medium on the second image carrier having the toner images formed on both sides, an AC bias is applied to the discharge member, and the second toner before the recording medium is transferred onto the second image carrier. An image forming apparatus comprising: a discharging unit configured to apply a DC bias to the discharge member when converting the polarity of toner of an image. The image forming apparatus according to claim 4.
The image forming apparatus according to claim 1, wherein the direct current bias applied to the discharge member is obtained by rectifying alternating current. The image forming apparatus according to claim 4 or 5,
An image forming apparatus using a discharge needle as the discharge member. The image forming apparatus according to claim 4 or 5,
An image forming apparatus using a conductive thin plate member as the discharge member. The image forming apparatus according to claim 1,
An image forming apparatus, wherein the second image carrier is an endless belt that is stretched by a driving roller and at least one stretching roller and moves endlessly. The image forming apparatus according to claim 8.
The drive roller is provided in a separation unit that separates the recording body from the second image carrier,
An image forming apparatus, wherein a diameter of the driving roller is 16 [mm] or more. The image forming apparatus according to claim 8 or 9,
An image forming apparatus using a belt member having a surface resistance of 10 ⁵ [Ω / □] or more and 10 ¹² [Ω / □] or less as the second image carrier. The image forming apparatus according to claim 8,
An image forming apparatus, wherein the second image carrier and at least a plurality of stretching rollers for stretching the first image carrier are unitized. 12. The image forming apparatus according to claim 1, wherein
A mark detection means for detecting a mark provided on the surface of the second image carrier,
An image forming apparatus that controls an image forming operation based on a detection result of the mark detection means. The image forming apparatus according to claim 12.
An image forming apparatus comprising a dedicated driving unit for driving the second image carrier. The image forming apparatus according to claim 1,
The second image carrier is configured to be able to contact and separate from the first image carrier,
An image forming apparatus, wherein the second image carrier is separated from the first image carrier except during image formation. The image forming apparatus according to claim 1.
An image characterized in that a cleaning member that comes into contact with the surface of the second image carrier and cleans the surface of the second image carrier is configured to be able to contact and separate from the second image carrier. Forming equipment. The image forming apparatus according to claim 1,
An image forming apparatus using toner having an average circularity of 0.90 or more and 0.99 or less as toner constituting the toner image. The image forming apparatus according to claim 1,
An image forming apparatus using a toner having a shape factor SF-1 of 120 or more and 180 or less and a shape factor SF-2 of 120 or more and 190 or less as toner constituting the toner image. The image forming apparatus according to claim 1,
An image forming apparatus comprising: a toner having a weight average particle diameter divided by a number average particle diameter of 1.05 to 1.30 as a toner constituting the toner image.

Images