JP2007114500A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid toner scattering on a tension roller of a toner image on an intermediate transfer belt.
The distance in the rotational drive direction along the intermediate transfer body from the center of the primary transfer member to the center of the stretch member disposed immediately downstream of the primary transfer portion of the stretch member is L [ m], where τT [sec] is the characteristic time of the intermediate transfer member carrying toner, and V [m / sec] is the speed of the intermediate transfer member when subjected to rotational driving, L ≧ V It is configured to satisfy × 2 × τT.
[Selection] Figure 1

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an intermediate transfer type image forming apparatus in which a toner image formed by an image forming unit is temporarily transferred onto an intermediate transfer member and then transferred onto a transfer material. Can be embodied in facsimiles, printers, copiers, etc.

  As a technique for transferring a toner image using an intermediate transfer member in an image forming apparatus, a technique as shown in FIG. 2 is known.

  The surface of the photosensitive drum is uniformly charged by a charging roller, and an electrostatic latent image is formed on the surface by receiving laser irradiation corresponding to image information by a laser driver. The electrostatic latent image is visualized by electrostatically adhering charged toner by the developing means.

  The toner image on the photosensitive drum is electrostatically primarily transferred onto the intermediate transfer member by the primary transfer roller. Further, the secondary transfer roller is electrostatically secondary-transferred onto a transfer material conveyed at a predetermined timing. The transfer material onto which the toner image has been transferred is conveyed to a fixing unit, where it is heated and fixed to become a permanent image.

  In the figure, a one-roller downstream roller is a roller disposed immediately downstream of the primary transfer portion among rollers that stretch an intermediate transfer belt as an intermediate transfer member. It is a member that is indispensable for increasing the pattern reading accuracy, and is opposed to the reference pattern detecting means such as a reference density pattern and a position information pattern (not shown).

For example, Patent Document 1 can be cited as a conventional example.
JP 2000-298408 A

  In the one-roll downstream roller as described above, when the toner image carried on the intermediate transfer belt touches the roller, the toner scatters and the toner image is disturbed. This is because, in the primary transfer portion, the toner carrying charge supplied from the primary transfer roller to the intermediate transfer belt causes rearrangement in the first transfer downstream roller portion.

  As shown in FIG. 3, in the primary transfer portion, the toner carrying charge is injected from the primary transfer roller to the intermediate transfer belt and moved to the toner carrying surface, so that the toner image is transferred onto the intermediate transfer belt. It is carried on. Here, the + mark in the figure schematically represents the toner carrying charge, and the polarity is opposite to that of the toner. In the figure, negative toner charged to a negative polarity is used, and the polarity of the toner carrying charge is positive. Further, the total amount of charge for toner carrying is larger than the total amount of charged charge of the toner image. Therefore, immediately after the primary transfer, excess charge exists on the toner carrying surface of the belt.

  At this time, as shown in FIG. 4, the toner carrying charge is different from where the toner is present and where it is not. This is because, as shown in FIG. 5, when primary transfer is performed, the current value differs depending on whether the toner is present or not. In the absence of toner, since the surface layer of the photosensitive drum is an insulating layer, the electric charge can be transferred to the belt only by electric discharge, and therefore the electric discharge is caused by the gap between the toner carrying surface of the intermediate transfer belt and the photosensitive drum. Until a sufficient electric field can be applied to start, the charge does not move in the primary transfer section, and therefore no current flows. On the other hand, when toner is present, the charged toner can be moved from the photosensitive drum to the intermediate transfer belt only by applying an electric field. Therefore, even with a voltage at which no discharge occurs, a charge corresponding to the amount of toner is applied to the intermediate transfer belt. Can move. For the above reasons, when a voltage necessary for primary transfer of toner to the intermediate transfer belt is applied, a current flows in the presence of toner, but only a small amount of current flows in the absence of toner. Therefore, since the amount of charge for carrying the toner moved to the belt is proportional to the current value, there is a large amount of charge where there is toner and only a small amount of charge where there is no toner. Occurs.

  However, in some places where the toner is present, the charge amount equal to the total charge amount of the toner in the toner carrying charge cancels out with the toner, so that the macro has only a surplus charge. become.

  Further, it is known that the toner carrying charge distributed inside the intermediate transfer belt is attenuated during driving. There are two types of attenuation: attenuation due to natural extinction and attenuation due to escape to a grounded roller.

  As shown in FIG. 5, the intermediate transfer belt, which is a dielectric that retains electric charge, is equivalent to an open-ended RC parallel circuit. At this time, in a state where a constant charge is held in the capacitor, a potential difference is generated in the RC parallel circuit, so that a phenomenon occurs in which current flows through the R component and the capacitor charge is discharged. The spontaneous discharge phenomenon of the RC parallel circuit is a damping behavior that occurs in a float state in which the intermediate transfer belt passes through the primary transfer portion and is not in contact with any member such as a grounded member such as a stretch roller. Further, this attenuation behavior is a phenomenon that depends on the conduction characteristics and dielectric characteristics of the dielectric itself, and is not affected by the presence of toner.

  On the other hand, when the intermediate transfer belt touches the installed stretching roller, the attenuation behavior in which the charge is lost to the roller is greatly influenced by the presence of the toner. As shown in FIG. 6, when the back surface touches the grounded stretching roller, one end of the RC parallel circuit equivalent to the belt is grounded. On the other hand, since the toner carried on the belt is insulated, it is regarded as a capacitor holding a certain electric charge, connected in series with the RC parallel circuit, and the other end is in an electrically floating state. The charge flow at this time is only the flow of the charge of the RC parallel circuit to the ground plane, but the attenuation behavior is not only the RC parallel circuit but also the capacitance of the capacitor equivalent to the toner layer connected in series. Under the influence of the components, there is a property that the characteristic time of relaxation becomes longer. That is, when the roller grounded on the back surface touches, the attenuation caused by the escape of electric charge to the roller is characterized in that the characteristic time is longer when the toner is present than when the toner is absent. .

  In summary, the behavior of the toner carrying charge injected to the intermediate transfer belt in the primary transfer portion and the behavior of the electric field inside the belt caused by the distribution of the toner carrying charge are as follows. Become.

  In the primary transfer portion, charged charges are injected where there is toner and where there is toner, and the amount of charge increases when there is toner, but the amount of charge equal to the total charge amount of toner is macroscopic. Therefore, only the surplus charge amount larger than the total toner charge amount is present. Therefore, the difference in effective charge amount is small where there is toner and where it is not. For this reason, the electric field in the intermediate transfer belt from where the toner is present to where there is no toner is small.

  When both the toner carrying surface and the back surface of the belt are floated after passing through the primary transfer portion, natural attenuation of charge occurs inside the belt regardless of the presence of toner, and the attenuation characteristic time is the same. . As a result, in the primary transfer portion, the difference in the amount of charge inside the belt caused by the presence or absence of toner is maintained. As a result, the electric field from the location where the toner is present to the location where no toner is present in the intermediate transfer belt is the same as that of the primary transfer portion.

  When the intermediate transfer belt is conveyed and touches the grounded stretching roller, attenuation due to escape of toner-carrying charges to the roller occurs. At this time, the characteristic time of attenuation changes depending on the presence / absence of toner, and the characteristic time becomes longer when the toner is present, so that the charge is less likely to attenuate. As a result, the electric field in the intermediate transfer belt from the place where there is toner to the place where there is no toner becomes the largest when touching the ground roller.

  In this way, when the grounding roller is touched, the electric field in the belt from where the toner is present to where there is no toner increases, so as shown in FIG. A portion of the toner carrying charge that has been transferred moves to a place where there is essentially no toner. The toner that can no longer be electrostatically held on the belt due to the rearrangement of the toner carrying charge is repelled by the repulsion of the polarity between the toners, attracted to the toner carrying charge after the rearrangement, and moved. A toner scattering phenomenon occurs.

  The above is the explanation of the cause of toner scattering in the ground roller.

  There is a known countermeasure against the toner scattering phenomenon by, for example, grounding the roller through a resistor instead of directly grounding the roller (see, for example, Patent Document 1). By grounding the roller via a resistor, the characteristic time of attenuation when the roller is touched can be extended, and the amount of attenuation of the toner carrying charge is reduced, so that charge relocation is unlikely to occur. Therefore, it is a fact that the toner scattering phenomenon is alleviated. However, since the configuration is such that the roller is not directly grounded, the configuration becomes complicated, which may lead to an increase in cost.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an intermediate transfer that can avoid toner scattering on a grounded tension roller section without taking a complicated configuration. Another object is to provide an image forming apparatus of the type.

  A typical configuration of the present invention for achieving the above object is as follows.

An intermediate transfer type image forming apparatus that primarily transfers a toner image on an image carrier onto an intermediate transfer body and performs secondary transfer collectively on a transfer material.
The intermediate transfer member is composed of an endless belt, and is stretched by at least a plurality of stretching members to receive rotational driving,
The distance in the rotational drive direction along the intermediate transfer body from the center of the primary transfer member to the center of the stretch member disposed immediately downstream of the primary transfer portion of the stretch member is L [m], Assuming that the characteristic time of the intermediate transfer member in the state where the toner is carried is τT [sec], and the speed of the intermediate transfer member when rotating is V [m / sec], L ≧ V × 2 × τT It is embodied by an image forming apparatus characterized by satisfying the above.

  As described above, according to the image forming apparatus of the present invention, it is possible to effectively avoid the scattering of the toner image by effectively reducing the surplus charge until it contacts the stretching member.

  Therefore, by using such an image forming apparatus, it is possible to provide an image forming apparatus that can stably maintain high image quality without causing scattering.

  Exemplary embodiments of the present invention will be described in detail below with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified. Absent. Further, the materials, shapes, etc. of the members once described in the following description are the same as those in the first description unless otherwise described.

  A schematic configuration of an electrophotographic full-color laser printer as an example of an image forming apparatus according to an embodiment of the present invention will be described with reference to FIG.

  This embodiment is an image forming apparatus that forms a full-color image by superimposing toner images of a total of four colors, yellow, magenta, cyan, and black.

  In the figure, reference numeral 1 denotes a photosensitive drum as a carrier for yellow image information. The photosensitive drum is configured by applying an organic photosensitive member (OPC) to an aluminum cylinder having an outer diameter of 84 mm. After the surface is uniformly charged by the charging roller 2, the photosensitive drum is irradiated with the laser 3. In response, an electrostatic latent image corresponding to the yellow component is formed on the surface.

  The yellow toner image is developed with respect to the formed electrostatic latent image by the yellow toner developing device 4y containing yellow toner, and becomes a visible image. The yellow toner image formed on the photosensitive drum is primarily transferred onto the intermediate transfer belt 11 by the primary transfer roller 5. The surface of the photosensitive drum 1 having the yellow toner image transferred to the intermediate transfer belt is cleaned by the photosensitive drum cleaner 12 and is used for the next and subsequent image forming operations.

  Toner images of magenta, cyan, and black color components are formed by the same process as described above, and are superimposed on the intermediate transfer belt.

  The toner image formed on the intermediate transfer belt is transported to a secondary transfer portion composed of a secondary transfer inner roller 71 and a secondary transfer outer roller 72, and is collectively transferred onto a transfer material transported at a predetermined timing. Then, the image is subjected to heat and pressure fixing by the fixing device 9 and becomes a permanent image.

  After the toner image is transferred to the transfer material, the intermediate transfer belt is cleaned by the intermediate transfer belt cleaner 10 and used for image formation from the next time.

  Hereinafter, the configuration of each member and the image forming conditions in the image forming unit that forms a visible image of yellow toner will be described.

  The yellow toner developing device 4y conveys yellow toner to the developing sleeve by a yellow toner conveying mechanism in a developing device (not shown), and applies a thin layer of yellow toner to the outer periphery of the developing sleeve by a regulating blade pressed against the outer periphery of the developing sleeve. Then, after the electric charge is applied to the yellow toner, the developing latent image formed on the photosensitive drum 1 is developed by applying a developing bias in which an AC bias is superimposed on a DC bias to the developing sleeve. The developing sleeve is disposed at a position facing the photosensitive drum 1 with a minute interval (300 μm).

  In this embodiment, the potential of the photosensitive drum, the potential of the developing sleeve, and the potential applied to the primary transfer roller are set as described below.

  Under an environment of 23 ° C. and 50% Rh, the surface potential of the photosensitive drum is controlled to be −450 V by applying an AC bias obtained by superimposing an AC bias of 900 Vp-p on a DC bias of −450 V to the charging roller. It is carried out.

  On the other hand, an AC bias in which a 1.2 kVp-p AC component is superimposed on a −300 V DC component is applied to the developing sleeve. The waveform of the AC component at this time is a blank pulse waveform, and a waveform combining a 9 kHz AC waveform and a 4.5 kHz blank is applied as a developing bias.

  When subjected to laser exposure, the photosensitive drum has a bright portion potential of −200 V at a portion where an electrostatic latent image that is a maximum density image is formed.

  At this time, a potential difference (primary transfer contrast) between the primary transfer roller and the bright portion of the photosensitive drum becomes 600 V by applying a potential of 400 V as the primary transfer bias to the primary transfer roller. By this primary transfer contrast, the negatively charged toner is primarily transferred onto the intermediate transfer belt.

The intermediate transfer belt uses a polyimide resin film having a thickness of 85 μm as a base material, disperses carbon black, and has a surface resistivity of 1 × 10 ¹² Ω / □ and a volume resistivity of 1 × 10 ^9.5 Ω · cm. The resistance was adjusted as follows. The peripheral length of the intermediate transfer belt was 527.5 mm, and the driving speed (process speed) was 130 mm / sec.

The secondary transfer outer roller is a sponge roller in which a foamed rubber layer based on foamed NBR (nitrile butadiene rubber) is provided on a steel cored bar having an outer diameter of 12 mm, and includes an NBR rubber layer. The diameter was 24 mm. The resistance was adjusted by dispersing an ion conductive resistance adjusting agent so that the resistance value of the roller was 10 ^7.5 Ω (when 2 kV was applied) in an environment of 23 ° C. and 50% Rh.

  The primary transfer downstream roller 6 is a SUS roller having an outer diameter of 16 mm, and is grounded. In the primary transfer portion, the intermediate transfer belt charged with the primary transfer roller 5 is carried from the back surface, so there is a risk of charging up if not grounded, leaking to surrounding members, and the image forming apparatus main body It is desirable to ground it because it may cause electrical stress on the cable.

  In this embodiment, the tension member of the intermediate transfer belt that contacts the toner image carried on the intermediate transfer belt first after passing through the primary transfer is the primary transfer downstream roller 6. By setting the distance from the center position of the primary transfer roller 5 to the center position of the primary transfer downstream roller 6 to 50 mm in this embodiment, the image forming apparatus capable of maintaining high image quality without causing toner image scattering. We were able to.

  The scattering of the toner image is due to the difference between the excess charge amount where the toner is present and the absence of the toner, excluding the charge amount corresponding to the total charge amount of the toner among the toner carrying charges existing in the intermediate transfer belt 11. to cause. In other words, regardless of the presence or absence of toner, if there is no difference in the toner carrying charge, scattering does not occur.

  As shown in FIG. 8, by measuring the surface potential of the intermediate transfer belt when the intermediate transfer belt 11 contacts the primary transfer downstream roller 6, the surplus charge amount of the toner carrying charge amount is measured. be able to. In some places where the toner is present, the charge corresponding to the total charge amount of the toner out of the total charge amount of the toner and the toner carrying charge has the same absolute value and different polarity. Even if it is measured, it cancels in appearance, so only the excess charge can be measured. For this reason, if the surface potential where the toner is present is larger than that where there is no toner, it indicates that the toner is more likely to scatter due to more excess charge.

  FIG. 9 shows the measurement result of the surface potential. In the figure, the horizontal axis indicates the distance L from the center of the primary transfer roller to the primary transfer downstream roller, and the vertical axis indicates the relative value of the measurement result of the surface potential. The solid line represents the measured value of the surface potential where the toner is present and the broken line is the position where the toner is absent. As is apparent from the figure, the measured value of the surface potential is higher when the toner is present up to L = 50 [mm], but when L ≧ 50 [mm], the surface potential is present where there is no toner. There is no difference in the measured values. That is, the distance between the central position of the primary transfer downstream roller and the primary transfer roller, which is a condition of L ≧ 50 [mm], is separated by 50 [mm] or more, regardless of the presence or absence of toner. The surplus charge in the intermediate transfer belt becomes uniform and no scattering occurs.

  The measurement result of the relative amount of the surplus charge amount will be considered. Scattering occurs due to the difference in the characteristic time of the decay behavior of surplus charge after touching the primary transfer downstream roller. On the other hand, if all surplus charges are alleviated by the time they come into contact with the primary transfer downstream roller, it is considered that scattering does not occur. Therefore, in order to examine the attenuation amount of surplus charges at a distance of L = 50 [mm], the relaxation time of natural discharge that occurs when the belt is in the float is measured. FIG. 10 shows a schematic diagram of a measuring instrument for measuring the natural discharge relaxation time of the intermediate transfer belt. In the drawing, the potential regulating plate is a grounded metal plate, and is arranged 4 mm from the back surface of the intermediate transfer belt 11 and parallel to the belt. By disposing a potential regulating plate at a position facing the surface potential meter via the intermediate transfer belt, it has a role of improving the accuracy of the surface potential measurement result. As a measurement method, when the toner carrying portion that has been subjected to normal image formation and has undergone primary transfer reaches the above measurement system, the operation of the image forming apparatus is forcibly stopped, and then the belt surface potential measurement value is attenuated. Measure behavior. Since the surface potential of the belt can relatively express the surplus charge existing inside the belt, the attenuation behavior of the surplus charge can be accurately measured.

FIG. 11 shows the measurement result of the relaxation behavior of the excess charge in the toner. As shown in the figure, at the moment when the image forming apparatus is forcibly stopped, there is some surface potential because of the surplus charge inside the belt, but the surplus charge is attenuated by the natural decay of the charge inside the belt. In addition, the surface potential of the belt is also attenuated. At this time, by fitting with an exponential function V = V ₀ × exp (−t / τ), the characteristic time τT = 192 [msec] of the natural attenuation of the intermediate transfer belt in the presence of toner can be obtained.

  In this embodiment, since the driving speed of the intermediate transfer belt is 130 [mm / sec], when the characteristic time of natural attenuation is simply applied, it is 25 [mm]. Thus, the intermediate transfer belt in this embodiment shows that the surplus charge attenuates to 1 / e, that is, approximately 36.8%, while receiving the 25 mm drive.

When the primary transfer downstream roller is disposed 25 mm downstream from the primary transfer portion, the surplus charge is naturally attenuated to 36.8% of the surplus charge amount of the primary transfer portion. From the measurement result of the surface potential, it can be seen that it is not the condition that the excess charge is sufficiently attenuated without scattering. This shows that even if the surplus charge is reduced to about 36.8%, the attenuation is insufficient to suppress scattering. As shown in FIG. 9, since the surplus charge does not disappear sufficiently unless it is arranged downstream of 50 [mm], it takes twice the relaxation characteristic time, and the charge attenuation at this time is (1 / e) It can be seen that ² = 13%.

  As described above, in order to effectively avoid the scattering at the primary transfer downstream roller portion, it is necessary to dispose a distance that is twice the natural relaxation characteristic time of the belt away from the primary transfer roller. In this embodiment, since the distance between the primary transfer downstream roller and the primary transfer roller is set to 50 mm, scattering of the toner image can be effectively avoided.

1 is a schematic diagram of one embodiment of the present invention. It is a figure for demonstrating the prior art. FIG. 5 is a schematic diagram for explaining toner-carrying charges inside a belt. It is a schematic diagram for explaining the IV characteristics of the primary transfer. It is a figure for demonstrating the natural attenuation | damping of the electric charge inside a belt. It is a figure for demonstrating attenuation | damping by dissipation of the electric charge from a baltic. It is a schematic diagram for demonstrating rearrangement of an excess charge. It is a schematic diagram for demonstrating the measurement system of the surface potential of a belt. It is a figure showing the measurement result of the surface potential of a belt. It is a schematic diagram for demonstrating attenuation | damping of the surface potential of a belt. It is a figure showing the measurement result of attenuation | damping of the surface potential of a belt.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charging roller 3 Exposure means 4 Rotating developing device 4y Yellow toner developing device 4m Magenta toner developing device 4c Cyan toner developing device 4k Black toner developing device 5 Primary transfer roller 6 Primary transfer downstream roller 71 In secondary transfer Roller 72 Secondary transfer outer roller 81 Tension roller 82 Tension roller 9 Fixing device 10 Intermediate transfer belt cleaner 11 Intermediate transfer belt 12 Photoconductor drum cleaner a1 Driving direction of developer a2 Driving direction of photoconductor drum a3 Driving of intermediate transfer belt direction

Claims (3)

An intermediate transfer type image forming apparatus that primarily transfers a toner image on an image carrier onto an intermediate transfer body and performs secondary transfer collectively on a transfer material.
The intermediate transfer member is composed of an endless belt, and is stretched by at least a plurality of stretching members to receive rotational driving,
The distance in the rotational drive direction along the intermediate transfer body from the center of the primary transfer member to the center of the stretch member disposed immediately downstream of the primary transfer portion of the stretch member is L [m], Assuming that the characteristic time of the intermediate transfer member in the state where the toner is carried is τT [sec], and the speed of the intermediate transfer member when rotating is V [m / sec], L ≧ V × 2 × τT An image forming apparatus characterized by satisfying the above.   The image forming apparatus according to claim 1, wherein the tension member disposed immediately downstream of the primary transfer portion among the tension members is configured to be grounded.   The image forming apparatus according to claim 1, wherein the intermediate transfer body is made semiconductive by using a resin as a base material and a resistance adjusting agent.

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