JP2006267674A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high quality image forming apparatus capable of optimally controlling a transfer bias and maintaining transfer efficiency and concentration constant even when there is circumferential unevenness of resistance values generated in a manufacturing process of a transfer roller. <P>SOLUTION: The image forming apparatus has an image carrier on the surface of which a development image is formed and the transfer roller 14 which forms a pressure welding nip part with the image carrier and transfers the development image from the image carrier to an intermediate transfer body or a transfer material as a transfer medium at the pressure contacting nip part by application of the transfer bias, and the image forming apparatus performs a pre-processing process for determining the transfer bias to be applied to the transfer roller 14 in a transfer process. In the image forming apparatus, it has a position detection means for detecting a specific position on the circumference of the transfer roller, the transfer bias is determined based on relation between voltage applied to the transfer roller and current flowing in the pressure contacting nip part except the specific position in the pre-processing process. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as an electrophotographic or electrostatic recording type copying machine, printer, or the like when transferring a developed image formed on an image carrier to an intermediate transfer member or transfer material as a transfer medium. The present invention relates to a method for determining an optimum transfer bias.

  2. Description of the Related Art Conventionally, for example, in an electrophotographic image forming apparatus, an electrostatic latent image is formed on one or a plurality of photosensitive drums as an image carrier, and the latent image is formed in each of yellow, magenta, cyan, and black colors. After sequentially developing with toner to form a developed image (toner image) of each color, these toner images are once transferred onto a drum-shaped or belt-shaped intermediate transfer member as a transfer medium (primary transfer), Subsequently, a full-color image forming apparatus of an intermediate transfer type that obtains a recorded image by batch transfer (secondary transfer) to a transfer material is known. Alternatively, there is known a monochrome image forming apparatus that forms only a black toner image on a photosensitive drum and transfers the toner image directly onto a transfer material.

  In these apparatuses, a conductive transfer roller or the like is used as a transfer unit in a transfer process in which primary transfer or secondary transfer is performed. The transfer roller is used in contact with the photosensitive drum or the intermediate transfer member, and has an advantage that the amount of ozone generation is small because the charge necessary for transferring the toner image is applied by charge injection regardless of discharge.

  By the way, it is known that the resistance of the transfer roller is likely to fluctuate due to temperature and humidity in the apparatus and energization. In particular, when an ion conductive transfer roller in which an ion conductive agent or a surfactant is dispersed as a material is used, the above resistance variation is likely to occur.

  On the other hand, when an electronic conductive transfer roller in which conductive fillers such as carbon and metal oxide are dispersed as a conductive agent is used, resistance variation due to temperature and humidity and current flow is reduced. However, if the surface of the transfer roller is contaminated with toner due to long-term durability, or if the thickness of the surface of the photoreceptor facing the transfer roller decreases due to wear, the overall resistance including the transfer roller and the photoreceptor drum changes.

  In any case, since these resistances fluctuate, there is a problem that even when a constant voltage is applied as a transfer bias to the transfer roller, the current flowing through the transfer roller fluctuates and the toner image cannot be transferred optimally. .

  Therefore, in order to prevent the occurrence of transfer failure due to resistance fluctuation of the transfer roller, as a pre-processing step, for example, as described in Patent Document 1, the relationship between the voltage applied to the transfer roller and the current flowing through the transfer portion is expressed as follows. A method is employed in which the transfer bias applied to the transfer roller is optimally controlled in accordance with the measurement result.

  In this control method, a constant voltage under constant voltage control is applied to the transfer roller during the pre-rotation before image formation (image formation), the current value at that time is detected, and the voltage applied to the transfer roller and the flow to the transfer site. The optimum voltage V0 necessary for obtaining the optimum current I0 is calculated from the current relationship, and the voltage V0 is applied as a transfer bias during transfer during image formation. By doing so, it is possible to always flow an optimum current to the transfer portion even if the resistance of the transfer roller varies.

  In this specification, in the image forming process, after an external print signal is transmitted to the image forming apparatus, the first transfer material arrives at a position (transfer portion) where the developer image is transferred. The time period in which each image forming unit is operating in the period up to is referred to as pre-rotation.

JP 2001-125338 A

  However, there are the following problems in performing the transfer bias control.

  That is, as shown in FIG. 9, in the manufacturing process of the transfer roller, the core metal 14a is inserted into the roller-shaped die 14b, and the kneaded rubber as the roller material is injected from the direction C perpendicular to the core metal. . The rubber material collides with the core metal, branches in two directions, merges again, and the mold is filled with the rubber material. At this time, since the rubber material is compressed and the rubber density is increased in the merging portion L, the density of the conductive material is also increased and the resistance value is lower than that of the surrounding area.

  FIG. 10 shows the change in resistance in the circumferential direction of the transfer roller manufactured as described above. The change in current flowing through the transfer portion when a constant voltage is applied to the core metal while rotating the transfer roller at a constant speed is shown in FIG. It is shown. The current fluctuates with the rotation period of the roller, and a large amount of current flows locally at the junction L. That is, it can be seen that the resistance value is low at the junction L. For this reason, when a constant voltage is applied to the roller, a current change of Δi occurs in the circumferential direction.

  When transfer bias control is performed using the above-described transfer roller, when a constant voltage is applied to the transfer roller and current is detected, if a current is detected at the junction, the target value is obtained because the resistance value is low. As a result, there is a problem that the transfer current is insufficient at a portion other than the joining portion, and the toner image is not sufficiently transferred. In order to avoid this, a method has been adopted in which the current is detected at a plurality of points in the circumferential direction, and the transfer bias is determined from the average value of the detection results to reduce the influence of circumferential unevenness of the resistance value. Since the detection points are determined at random, there is a problem in that the transfer bias value varies depending on whether the detection point includes or does not include the joining portion, the transfer efficiency varies, and the image density varies.

  Accordingly, an object of the present invention is to provide a high-quality image forming apparatus capable of optimally controlling the transfer bias and maintaining the transfer efficiency and density constant even when there is a circumferential unevenness of the resistance value generated in the transfer roller manufacturing process. That is.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image forming apparatus having the following configuration.

(1) An image carrier on which a developed image is formed on the surface, an intermediate transfer body as a transfer medium at the pressure nip portion by forming a pressure nip portion with the image carrier and applying a transfer bias In an image forming apparatus having a transfer roller for transferring the developed image from the image carrier to a transfer material, and performing a preprocessing step for determining a transfer bias to be applied to the transfer roller in a transfer step,
A position detecting means for detecting a specific position on the circumference of the transfer roller, and based on a relationship between a voltage applied to the transfer roller and a current flowing in the pressure nip portion excluding the specific position in the preprocessing step; An image forming apparatus for determining a transfer bias.

  (2) A position detecting means for detecting a plurality of specific positions on the circumference of the transfer roller is provided, and in the pre-processing step, a voltage applied to the transfer roller and a current flowing in the pressure nip portion excluding the specific position at that time The image forming apparatus according to (1), wherein the transfer bias is determined based on the relationship.

  According to the present invention, in an image forming apparatus that performs transfer bias control for determining a transfer bias in an image pre-processing step, the transfer bias can be determined so as not to be affected by the low resistance portion that occurs in the transfer roller manufacturing process. Therefore, it is possible to prevent a decrease in density associated with a decrease in transfer efficiency.

  Hereinafter, an image forming apparatus according to the present invention will be described in detail with reference to the drawings.

<Embodiment 1>
FIG. 1 shows a schematic configuration of an example of an image forming apparatus to which the present invention is applied.

  In the present embodiment, a development image (toner image) of a plurality of colors is formed on a photosensitive drum 1 as an image carrier, and is primarily transferred onto an intermediate transfer belt 6 as an intermediate transfer body as a transfer medium. An application example of the present invention in a color image forming apparatus adopting an intermediate transfer system in an electrophotographic system in which the composite toner images of a plurality of colors are collectively transferred onto a transfer material P in a secondary transfer unit will be described.

  First, the overall configuration of the image forming apparatus will be described.

  In FIG. 1, a photosensitive drum 1 is an image carrier, and rotates in the direction of arrow A. The charging device 2 provided around the photosensitive drum 1 and the exposure device 3 that performs exposure based on the image information change the image information. A corresponding electrostatic latent image is formed. A developing unit 8 including developing units corresponding to yellow (Y), magenta (M), cyan (C) and black (K) colors is disposed around the photosensitive drum 1. The electrostatic latent image formed on the photosensitive drum 1 is developed with any of the above developing devices to form a toner image.

  The intermediate transfer belt 6 disposed so as to be in contact with the surface of the photosensitive drum 1 is stretched around a plurality of stretching rollers 9 to 13 and is rotated in the direction of arrow B. In the present embodiment, the stretching rollers 10 and 11 are disposed in the vicinity of the primary transfer position, and the flat surface of the intermediate transfer belt 9 is brought into contact with the photosensitive drum 1 to form a nip for primary transfer.

  The tension roller 12 is a tension roller for keeping the tension of the intermediate transfer belt 9 constant, and is biased by a pressure spring (not shown). The stretching roller 13 is a driving roller 13 that rotates the intermediate transfer belt 6, and the stretching roller 9 is in pressure contact with the secondary transfer roller 14 and forms a nip portion serving as a secondary transfer portion. A roller (back-up roller) 9.

As the intermediate transfer belt 6, an appropriate amount of carbon black is contained in a resin such as polyimide, polycarbonate, polyethylene terephthalate, polyvinylidene fluoride, the volume resistivity is 1 × 10 ⁸ to 1 × 10 ¹³ Ω · cm, and the thickness is 70. Those having a thickness of ˜100 μm are used.

  Further, at the primary transfer position of the intermediate transfer belt 6 facing the photosensitive drum 1, a primary transfer roller 7 serving as transfer means is disposed on the inner surface side of the intermediate transfer belt 6. The toner image on the photosensitive drum 1 is primarily transferred onto the intermediate transfer belt 6 by applying a transfer bias having a polarity opposite to the charging polarity of the toner from a high voltage power source (transfer bias applying means) 17.

  Further, a drum cleaner 5 for removing the toner remaining on the photosensitive drum 1 after the primary transfer is provided opposite to the photosensitive drum 1. After the cleaning by the drum cleaner 5 is completed, the residual charge of the photosensitive drum 1 is attenuated by the charge eliminating lamp 30 and is prepared for the next image formation.

  Further, the secondary transfer roller 14 that is disposed in pressure contact with the toner image carrying surface side of the intermediate transfer belt 6 and the intermediate transfer belt 6 are positioned at the secondary transfer position of the intermediate transfer belt 6 facing the conveyance path of the transfer material P. A grounded backup roller 9 disposed on the inner surface side and serving as a counter electrode of the secondary transfer roller 14 is provided. A secondary transfer bias having a polarity opposite to the charging polarity of the toner is applied to the secondary transfer roller 14 by a high voltage power source 18.

  Further, a belt cleaner 16 for removing toner remaining on the intermediate transfer belt 6 after the secondary transfer is provided on the downstream side of the secondary transfer position.

  The secondary transfer roller 14 and the belt cleaner 16 are arranged so as to be able to contact and separate from the intermediate transfer belt 6. When a color image of a plurality of colors is formed, the toner image before the final color is second. The secondary transfer roller 14 and the belt cleaner 16 are separated from the intermediate transfer belt 9 until they pass through the secondary transfer roller 14 and the belt cleaner 16, and the secondary transfer roller 14 and the belt cleaner 16 reach the secondary transfer position before the final color toner image reaches the secondary transfer position. 6 is contacted.

  The transfer material P is sandwiched between the registration rollers 15 and once positioned and stopped, and then sent to the secondary transfer position at a predetermined timing. Further, the transfer material P after the secondary transfer is a conveying member (not shown). ) To a fixing device (not shown) so that the toner image is fused and fixed to the transfer material P.

  Next, an image forming process of this apparatus will be described.

  First, an electrostatic latent image is written on the photosensitive drum 1 and developed by a developing device corresponding to the electrostatic latent image. For example, if the electrostatic latent image written on the photosensitive drum 1 corresponds to yellow image information, the electrostatic latent image is developed by the developing unit Y containing yellow toner, A yellow toner image is formed on the photosensitive drum 1.

  The toner image formed on the photosensitive drum 1 is transferred from the photosensitive drum 1 to the surface of the intermediate transfer belt 9 at a primary transfer position where the photosensitive drum 1 and the intermediate transfer belt 6 are in contact with each other.

  At this time, when a single color image is formed, the toner image primary-transferred to the intermediate transfer belt 6 is immediately secondary-transferred to the transfer material P, but a color image in which a plurality of color toner images are superimposed is formed. In this case, the toner image formation on the photosensitive drum 1 and the primary transfer process of the toner image are repeated for the number of colors. For example, when a full color image is formed by superimposing four color toner images, yellow, magenta, cyan and black toner images are formed on the photosensitive drum 1 for each rotation. Are successively transferred onto the intermediate transfer belt 6 in order.

  On the other hand, the intermediate transfer belt 6 rotates with the same period as the photosensitive drum 1 while carrying the toner image first transferred first, and yellow, magenta, cyan and black are transferred on the intermediate transfer belt 6 for each rotation. The toner images are transferred in a superimposed manner.

  The toner image primarily transferred to the intermediate transfer belt 6 in this way is conveyed to the secondary transfer position as the intermediate transfer belt 6 rotates. On the other hand, the transfer material P is supplied to the secondary transfer position at a predetermined timing by the registration roller 15, and is sandwiched between the backup roller 9 and the secondary transfer roller 14. At this time, a secondary transfer bias is applied to the secondary transfer roller 14, and the toner image carried on the intermediate transfer belt 6 is transferred by the action of a transfer electric field formed between the secondary transfer roller 14 and the backup roller 9. It is electrostatically transferred to the material P.

As the primary transfer roller 7 or the secondary transfer roller 14, a roller whose resistance value is adjusted to a value of about 1 × 10 ⁶ to 1 × 10 ¹⁰ (Ω) is used. These transfer rollers are provided with a conductive elastic layer on the outer peripheral surface of a metal core, and there are mainly the following two methods for imparting conductivity. The electroconductive transfer roller is made by dispersing a conductive filler such as carbon or metal oxide in a sponge such as EPDM or urethane, and the resistance value is adjusted by the amount of the conductive filler added. The ion-conducting transfer roller contains an ion-conducting material, such as NBR, urethane, epichlorohydrin, or other rubber or sponge material itself, or a surfactant dispersed in them. Use

  Next, a method for controlling the transfer bias of the secondary transfer roller 14 in the pretreatment process will be described with reference to FIG.

  As a pre-processing step, the secondary transfer roller 14 is rotated in a state where the photosensitive drum 1 and the intermediate transfer belt 6 are rotated during the pre-rotation immediately before the image forming step, and the secondary transfer roller 14 is pressed against the intermediate transfer belt 6. Voltages 2500V, 3000V, and 3500V are applied at a constant voltage, and the current value at that time is detected. An appropriate transfer bias is determined by calculating a transfer bias V0 (= 2700 V) necessary for obtaining the target current I0 (= 45 μA) (= 35 μA) by linear interpolation from the relationship between the detected current and voltage.

  During transfer in the image forming process, a transfer bias is applied with constant voltage control. The reason for this is that when a transfer current is applied under constant current control, current does not flow easily due to the resistance of the paper, so the current escapes out of the paper passing area, and the current in the paper passing portion, that is, the image portion is insufficient. This is because the ink cannot be sufficiently transferred. If constant voltage control is performed, a necessary electric field is formed also in the sheet passing portion, and current can be secured.

  When determining the transfer bias, it is not determined by applying three constant voltages and linearly interpolating the current and voltage as described above, but by determining the voltage by applying only one point of the target current I0. You may do it.

  Next, the characteristic configuration of the present invention and the operation of the pretreatment process will be described with reference to FIGS.

  FIG. 2 shows the configuration of one end of the transfer roller 14. As described above, the joining portion L is a portion having a low resistance value, and the mark 42 is pasted at the same position as the joining portion L on the circumference of the cored bar 14a. The mark 14a is an optical reflective sticker, and the position of the mark 42 can be detected by a photosensor 41 incorporating light emitting and light receiving elements. Further, the photo sensor 41 is arranged on a straight line connecting the shafts of the transfer roller 14 and the backup roller 9, whereby the joining portion L comes to the secondary transfer nip portion with respect to the rotating transfer roller 14. Sometimes the mark 42 can be detected.

  3 and 4 show the position and timing at which current is detected in the preprocessing step.

  As shown in FIG. 3, the transfer roller 14 is divided at equal intervals in the circumferential direction, and the current is detected at a total of six points P1 to P6 excluding the joining portion L.

  FIG. 4 shows the timing of sampling the current in synchronization with the mark detection signal. Sampling is performed after time t starting from the time when the mark detection signal is detected, that is, the time when the junction L passes through the nip. Start and sample a total of 6 times. Time t is the rotation time from the junction L to P1. As a result, current detection in a region excluding the junction L can be performed. When determining the transfer bias, the average value of these six points of current is taken to determine the current-voltage relationship. The reason why the current is detected at a plurality of points is to set an optimum transfer bias on the average for the entire region excluding the merge portion L.

  FIG. 5 shows the effect of the present invention.

  FIG. 5 shows a change in transfer efficiency with respect to the transfer current. When the transfer efficiency is 90% or more, transfer is considered good. According to FIG. 5, the transfer efficiency is 90% or more when the current is set to the region a (target current center value = 45 μA). Δi indicates a current change due to resistance unevenness in the circumferential direction of the transfer roller 14. Conventionally, the current is randomly detected in the circumferential direction of the transfer roller 14 to determine the transfer bias, and therefore sampling may be performed including the junction L. In this case, the resistance of the junction L is low. In addition, the transfer bias may be set low. For this reason, for example, the region b may be set, the current becomes insufficient in the circumferential direction, the transfer efficiency falls below 90%, and the toner is not sufficiently transferred, resulting in a decrease in density. On the other hand, in the present invention, since sampling is performed except for the merging portion L, the region a is set, and good transfer can be performed in the entire circumferential direction.

<Embodiment 2>
6 and 7 show a method of controlling the transfer roller having two joining portions L1 and L2 so as to exclude the joining portion.

<Embodiment 3>
Next, Embodiment 3 will be described.

  The control method described above can be applied not only to a color image forming apparatus but also to a monochrome image forming apparatus. FIG. 8 shows an example of an electrophotographic monochrome image forming apparatus.

  8, parts having the same functions as those of the color image forming apparatus shown in FIG. 1 are denoted by the same reference numerals as those in FIG. The photosensitive drum 1 is rotationally driven in the direction of arrow A at a predetermined peripheral speed. The peripheral surface of the photosensitive drum 1 is charged to a predetermined polarity and potential by the charging device 2. The exposure device 3 outputs laser light whose emission is controlled in accordance with image information to be recorded, and an electrostatic latent image corresponding to the target image information is formed on the photosensitive drum 1.

  The developing device 8 visualizes the electrostatic latent image on the photosensitive drum 1 with black toner and forms a toner image. The toner used was a magnetic one-component developer.

  A pre-transfer charger 4 is disposed downstream of the developing device 8 so as to face the photosensitive drum 1 and imparts a charge to the toner image.

  Here, a transfer material P serving as a transfer medium is accommodated in a paper feed cassette (not shown), and a paper feed roller (not shown) is driven based on a paper feed start signal so that the transfer material P in the paper feed cassette is moved. Each sheet is fed one by one, and the transfer material P is conveyed in the direction of arrow B by the registration roller 15 and introduced at a predetermined timing into a transfer portion which is a contact nip portion between the photosensitive drum 1 and the transfer roller 19. That is, the transfer of the transfer material P is controlled by the registration roller 15 so as to synchronize with the timing at which the leading end of the toner image on the photosensitive drum 1 reaches the transfer site.

  The transfer material P introduced into the transfer site is held and conveyed by the transfer site. At that time, a transfer bias controlled to a predetermined level from a high voltage power source 20 is applied to the transfer roller 19 at a constant voltage. This transfer bias control will be described later. By applying a transfer bias having a polarity opposite to that of the toner to the transfer roller 19, the toner image on the photosensitive drum 1 is electrostatically transferred to the transfer material P. Thereafter, the transfer material P is separated and conveyed from the photosensitive drum 1, conveyed to a fixing device (not shown), and subjected to a heat and pressure fixing process of the toner image.

  The present invention can be similarly applied to the transfer roller 19 as described above.

  The present invention can be similarly applied to the primary transfer roller of FIG.

  The dimensions, materials, shapes, relative positions, and the like of the component parts of the image forming apparatus described above are not intended to limit the scope of the present invention only to those unless otherwise specified. Even in the electrostatic recording type, the present invention can be applied by changing the method of changing the potential of the surface of the image carrier.

1 is a schematic configuration diagram of an image forming apparatus according to the present invention. It is a figure which shows the detection means which detects the confluence | merging part L of a transfer roller. It is a figure which shows the electric current detection position of a transfer roller. It is a figure which shows the electric current detection timing of a transfer roller. It is a figure which shows the relationship between the applied voltage and detection current in transfer bias control. It is a figure which shows the electric current detection position of the transfer roller which has a some low resistance part. It is a figure which shows the electric current detection timing of the transfer roller which has a some low resistance part. It is a schematic block diagram of the image forming apparatus which concerns on Embodiment 3 of this invention. It is a figure which shows the manufacturing process of a transfer roller. It is a figure which shows the result of having measured the resistance change of the circumferential direction of a transfer roller.

Explanation of symbols

1 Photosensitive drum (image carrier)
6 Intermediate transfer belt (intermediate transfer member, transfer medium)
7 Primary transfer roller (transfer means)
14 Secondary transfer roller (transfer means)
17, 18, 20 High voltage power supply (transfer bias applying means)
19 Transfer roller (transfer means)
P transfer material 42 mark 41 mark detection means

Claims (2)

An image carrier on which a developed image is formed, and a pressure nip portion formed with the image carrier, and a transfer bias is applied to the intermediate transfer member or transfer material as a transfer medium at the pressure nip portion. An image forming apparatus having a transfer roller for transferring the developed image from the image carrier and performing a pretreatment step for determining a transfer bias applied to the transfer roller in a transfer step;
A position detecting means for detecting a specific position on the circumference of the transfer roller, and based on a relationship between a voltage applied to the transfer roller and a current flowing in the pressure nip portion excluding the specific position in the preprocessing step; An image forming apparatus for determining a transfer bias.   Position detecting means for detecting a plurality of specific positions on the circumference of the transfer roller, and in the pre-processing step, a relationship between a voltage applied to the transfer roller and a current flowing through the pressure nip portion excluding the specific position at that time 2. The image forming apparatus according to claim 1, wherein the transfer bias is determined based on the transfer bias.

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