JPH10198186A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of preventing transfer defect under the low humidity environment and a drum memory under the high humidity environment, and possible of forming the color image of an excellent quality being free from the image defect derived from the transfer defect and the drum memory. SOLUTION: As for the image forming device, in transferring the toner image on a photosensitive drum to a transfer material by electrostatically attracting a recording material by means of attract-charge on a surface of a transfer sheet to potential in reverse polarity corresponding to a photosensitive drum (potential rise of transfer sheet reverse side Vq), and transferring by means of the transferring electric charge in reverse polarity (potential rise Vt × four colors portion), the initialization of transfer sheet before the attract-charge (potential-V0) is performed, so as to control the transfer sheet rear side potential corresponding to the environment under which the image forming device is mounted. The rear side potential at the last color transferring time stands for, V1=-V0+Vq+4×Vt, and the initial potential -V0 is preferred to be large under the low humidity environment while -V0 is preferred to be small under the high humidity environment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an image forming apparatus for electrostatically forming an image on a recording material such as paper, such as a laser beam printer.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as a copying machine or a laser beam printer, an image carrier on which a toner image is formed and a recording material carrier for carrying a recording material onto which the toner image is transferred are used. What is provided is known.

A four-color full-color copying machine will be described as an example of such an image forming apparatus. As shown in FIG. 8, this copying machine has a photosensitive drum 1 which is an image carrier, and a periphery thereof, A primary charger 2 for charging a photosensitive drum 1 to a predetermined potential, and a traveling optical device 3 as a latent image forming means
Developing units 4Y, 4M, 4C, and 4K for yellow, magenta, cyan, and black, and a transfer drum 5 that carries a recording material and conveys it to a transfer unit facing the photosensitive drum 1;
5b for transferring the toner image on the photosensitive drum 1 to the conveyed recording material, and a cleaner 6 for cleaning the photosensitive drum 1
Is formed.

A process until a color image is formed on a recording material will be described. First, the surface of the photosensitive drum 1 is uniformly charged by the primary charger 2 and the reader unit 30 is charged on the surface of the photosensitive drum 1. The first color of the original sent by
For example, image exposure based on yellow image information is performed by the optical device 3 to form a first-color electrostatic latent image on the surface of the photosensitive drum 1. The electrostatic latent image on the photosensitive drum 1 is developed by the developing device 4Y for the first color and visualized as a yellow toner image, and this toner image is carried on the transfer drum 5 and transported by the paper feeding cassette 7. The image is transferred onto the recording material from (7a, 7b or 7c) by a transfer charger (transfer charging brush) 5b at a transfer section.

The transfer drum 5 has a thickness of 150 μm
A transfer sheet 5f made of polycarbonate is stretched, and the recording material is conveyed while being adsorbed to the transfer sheet 5f. This polycarbonate is a material that is superior in durability to PVDF (polyvinylidene fluoride) and the like, and that can be disposed of after use very easily.

Around the transfer drum 5, inside or outside, a suction charging brush 5c, a suction roller 5g, the above-described transfer charging brush 5b, a separation roller 8b, a separation charger 5h, a separation claw 8a, an inner discharger 5d, and an outer A static eliminator 5e and transfer cleaners 16 and 17 are provided.

[0007] After the transfer of the first color toner image is completed, the photosensitive drum 1 is subjected to the next image formation after removing the residual toner on the surface by the cleaner 6. And
The above steps are performed for the remaining three colors of magenta, cyan, and black, and a four-color toner image of yellow, magenta, cyan, and black is superimposed on the recording material to form a full-color image that is multiplex-transferred.

The recording material on which the transfer of the toner images of the four colors has been completed is separated from the transfer drum 5 by a charging device 5h, a separating claw 8a, and a separating roller 8b of a separating means, and is fixed by a fixing device 9 to a tray. Reaches 10. The transfer drum 5 from which the recording material has been separated removes unnecessary toner adhered to the surface by the transfer cleaners 16 and 17 and prepares to carry the next recording material.

The fixing device 9 includes a fixing roller 9b containing a heater 9a and a pressing roller 9c pressed against the fixing roller 9b from below.
Heating and pressurizing while nipping and transporting the recording material at the nip portion between both rollers, the toner image on the recording material is melted and fixed to the recording material and fixed.

In the above-described full-color copying machine using the multiple transfer system, the potential on the back surface of the transfer sheet 5f of the transfer drum 5 varies as shown in FIG.

[0011] First, the surface of the transfer sheet 5f is initialized by the inner static eliminator 5d and the outer static eliminator 5e (the potential of the front and back surfaces of the transfer sheet is 0 V), and thereafter, the recording material is absorbed and carried. Therefore, a suction current is applied from the suction charging brush 5c to the back surface of the transfer sheet 5f, whereby the potential on the back surface of the transfer sheet 5f increases by Vq. Similarly, in order to transfer the toner image on the photosensitive drum 1 to the recording material, a transfer current is applied from the transfer charging brush 5b to the back surface of the transfer sheet 5f, whereby the potential of the back surface of the transfer sheet 5f becomes Vt (t = M , C, Y, K). In a full-color copying machine, this transfer step is performed four times, and assuming that the transfer current is constant in all four transfers, the potential on the back surface of the transfer sheet 5f finally becomes V1 (V) =-V0 + V
q + 4 × Vt = Vq + 4 Vt (−V0: initial back potential of the transfer sheet (abbreviated as initial sheet back potential)
Is 0).

In this embodiment, the transfer sheet 5f has a thickness of 150 μm.
However, since the resin has a dielectric constant of about 3 which is considerably lower than that of polyvinylidene fluoride of 10, the potential rise Vq of the back surface of the transfer sheet 5f due to the above-mentioned adsorption of the recording material, and the toner The potential rise Vt due to image transfer charging is a large value. However,
There is a limit to the potential that can be applied to the transfer sheet 5f. If the limit potential is Vlimit, the transfer current becomes more difficult to flow as V1 approaches Vlimit. In a low-humidity environment, the charge amount of the toner on the photosensitive drum increases, and the current value necessary for transfer increases, and therefore, the potential rise Vt increases. For this reason, in a low-humidity environment, V1 is close to Vlimit, and the transfer current of the final color becomes small, which often causes problems such as insufficient transfer of the toner image of the final color.

Therefore, as shown by the solid line in FIG. 10, at the stage of initializing the transfer sheet 5f before carrying the recording material, the potential of the back surface of the transfer sheet enters the claim having the same polarity as the charged polarity of the photosensitive drum. This word is necessary. Therefore, by making the polarity opposite to the transfer current (the surface potential of the transfer sheet is the same polarity as the transfer current), that is, V1 = -V0 + Vq +
At 4 Vt, by setting -V0 to have the opposite polarity to Vt, the value of the final sheet back potential V1 can be set small, and the transfer of the final color can be performed well.

Thus, the inner and outer static eliminators 5d, 5e
A method of effectively realizing subsequent multiple transfer by charging the back surface potential of the transfer sheet to a potential having a polarity opposite to that of the transfer current by initializing the transfer sheet 5f having a low dielectric constant due to the following has been proposed. I have.

[0015]

However, in a full-color image forming apparatus including the above-described copying machine, a photosensitive drum using an organic photosensitive member (OPC) as the photosensitive drum 1 is often used. It is general to employ a method (reversal development method) in which the photosensitive drum 1 is charged to a potential having the same polarity as the toner to form a latent image and used for development.

According to this, when the toner has a negative charge, the photosensitive drum 1 also has a negative potential, and in this state, the transfer sheet 5 charged positively
When f (the surface potential of the transfer sheet (the potential on the side supporting the recording material) is indicated by a dotted line in FIG. 10) directly contacts the photosensitive drum 1, charging unevenness is likely to occur on the photosensitive drum 1, and an image is formed during subsequent image formation. It turned out that there was a problem that caused a defect (drum memory). For example, since the surface of the transfer sheet 5f before carrying the recording material is positively charged, when the transfer sheet 5f and the photosensitive drum 1 come into contact with each other and stop due to a jam or a malfunction, a memory phenomenon or the like occurs on the photosensitive drum. It has been confirmed that a defect on the image appears.

The above-mentioned phenomenon of the drum memory is particularly remarkable in a high-humidity environment. This is because the resistance values of the transfer sheet 5f, the photosensitive drum 1, and the recording material decrease, and the charge of the transfer sheet affects the photosensitive drum. It is because it becomes easy to give.

When a full-color image is formed, there is no problem since the surface potential of the transfer sheet becomes the same polarity as that of the photosensitive drum after the transfer of the cyan toner image. In this case, as shown in FIG. 11, the surface potential of the transfer sheet is kept in contact with the photosensitive drum for a long time with the opposite polarity as shown in FIG. As a result, it has been confirmed that the memory phenomenon on the photosensitive drum gradually accumulates and, for example, a problem such as a change in color on an image.

An object of the present invention is to provide an image forming apparatus capable of preventing a transfer failure in a low-humidity environment and a drum memory in a high-humidity environment and forming a high-quality color image free from transfer failure and image defects due to the drum memory. To provide.

[0020]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention relates to an image carrier on which an image is formed, and a recording material that is electrostatically attracted and is transported to a transfer unit facing the image carrier. And a charging unit that supplies an electric charge to the recording material carrier to transfer an image formed on the image carrier onto a recording material carried on the recording material carrier. Before the recording material is carried on the recording material carrier, the back surface of the recording material carrier opposite to the side carrying the recording material is charged to the same polarity as the surface potential of the image carrier, In an image forming apparatus in which the recording material supporting side of the recording material supporting member is charged with a polarity opposite to the surface potential of the image supporting member, the back surface potential of the recording material supporting member before supporting the recording material is determined by an image forming apparatus. Change according to at least one or more differences in the installation environment and operating conditions of the main unit The image forming apparatus according to claim.

According to the present invention, the difference in the installation environment of the image forming apparatus main body can be a difference in the absolute water content of the installation environment of the image forming apparatus main body. The absolute value of the back surface charge amount of the recording material carrier before supporting the recording material in an installation environment of the image forming apparatus main body is lower than the absolute moisture content of the installation environment. The back surface charge amount of the recording material carrier before carrying the recording material can be controlled so as to be smaller than the absolute value of the back surface charge amount. The difference in the operating conditions of the image forming apparatus main body may be a difference in the properties of the recording material. The difference in the properties of the recording material may be a difference in at least one or more of the thickness, the dielectric constant, the electric resistance, and the rigidity of the recording material. Further, the difference in the operating conditions of the image forming apparatus main body may be a difference in the number of times of the transfer step or the recording material adsorption step in one image formation.

[0022]

Embodiments of the present invention will be described below in detail with reference to the drawings.

Embodiment 1 FIG. 1 is a schematic diagram showing the area around a transfer drum in an embodiment of the image forming apparatus of the present invention. The configuration around the transfer drum of the image forming apparatus of the present invention is basically the same as that of the conventional image forming apparatus shown in FIG. 8, and in FIG. 1, the same members as those in FIG. According to the present invention, before the recording material P is carried on the transfer sheet 5f of the transfer drum 5, the back surface of the transfer sheet 5f opposite to the recording material carrying side is exposed to the photosensitive drum 1.
When the transfer sheet is electrically initialized, the surface potential on the recording material carrying side is charged to the opposite polarity to the charge polarity of the photosensitive drum by electrically charging the transfer sheet. Is characterized by being changed in accordance with at least one or more differences in the installation environment and operating conditions of the image forming apparatus main body.

In the first embodiment of the present invention, the transfer sheet is charged by changing the potential on the back surface according to the environment.

In FIG. 1, on the surface of the transfer drum 5,
A transfer sheet 5f, which is a recording material carrying member that electrostatically attracts the recording material P and conveys the recording material P in the direction of arrow C, is wound thereon. The rear end of the transfer sheet 5f is connected to a connecting member (not shown) of the transfer drum 5. ). In this example, a polycarbonate (PC) sheet having a thickness of 150 μm and a deposition resistivity of 10 ¹⁵ Ωcm or more is used as the transfer sheet 5f.

Around the transfer drum 5, inside or outside, a suction charging brush 5c, a suction roller 5g, the above-described transfer charging brush 5b, a separation roller 8b, a separation charger 5h, a separation claw 8a, an inner static eliminator 5d, and an outer A static eliminator 5e and transfer cleaners 16 and 17 are provided.

To carry the recording material P on the transfer drum 5,
First, the inner static eliminator 5d and the outer static eliminator 5e, which are opposed to each other across the transfer sheet 5f, are operated to electrically initialize the transfer sheet 5f. The recording material P is fed to the transfer sheet 3f from the direction of arrow B by a sheet feeding means (not shown). The attraction roller 5g and the attraction / charging brush 5c which are opposed to each other across the transfer sheet 5f are operated, and the attraction roller 5g moves the transfer sheet 5f in accordance with the supply of the recording material P.
, And positively attracted charges are injected from the attraction charging brush 5c to the back surface (the side opposite to the recording material carrying side) of the transfer sheet 5f. Suction roller 5g
Is electrically grounded, an electric charge is induced in the recording material P in a direction opposite to the electric charge injected from the suction charging brush 5c. As a result, the recording material P and the transfer sheet 5f are polarized and attracted to each other. The recording material P is electrostatically attracted to the surface of the transfer sheet 5f. When the suction operation of the recording material onto the transfer drum is completed, the suction roller 5g is quickly separated from the transfer drum.

Next, at the transfer section facing the photosensitive drum 1, positive transfer charges are similarly injected from a high-voltage power supply (not shown) into the transfer sheet 5f via the transfer charging brush 5b, and formed on the photosensitive drum 1. The negatively charged toner image is transferred onto the recording material P. In this example, a photosensitive drum having an organic OPC photosensitive member on the surface layer is used as the photosensitive drum 1. To form an image, the photosensitive drum 1 is charged to the same polarity as the toner, that is, a negative potential, and a laser is used. Development is performed by a so-called reversal development method in which toner is attached to a portion where the amount of charge is reduced by exposure.

The above-described transfer operation is repeated a desired number of times, so that the toner images of each color are multiplex-transferred onto the recording material P. Thereafter, the recording material P is separated from the transfer drum 5 by a separating roller 8h, a claw 8a, and a charger 5h. I do. Inner static eliminator 5d, outer static eliminator 5
e operates before and after the above-described series of adsorption, transfer, and separation operations, and electrically initializes the transfer sheet 5f.

In the above-described full-color image forming apparatus,
The potential state of the surface of the transfer sheet 5f of the transfer drum 5 is shown in FIG.
It fluctuates like To explain, first, the transfer sheet 5f
Is initialized by the inner static eliminator 5d and the outer static eliminator 5e,
An electric charge is uniformly applied to the back side of the transfer sheet 5f so as to be -V0 (V) (V0: initial back potential of the transfer sheet (initial back potential of the sheet)). Thereafter, when the recording material P is adsorbed and carried by the adsorbing operation, the adsorbing and charging brush 5 is used.
When a charge of Vq (V) is applied from c to the back side of the transfer sheet, the charge amount (potential) on the back side of the transfer sheet becomes -V0 + Vq +
Vt. In the full-color copying machine of this embodiment, the recording material is separated after this transfer step is performed four times. As a result, the back surface of the transfer sheet 5f finally reaches V1 (V).
= −V0 + Vq + 4 × Vt.

As described above, actually, the transfer sheet 5
There is a limit to the potential that can be applied to f. If the limit potential is Vlimit, as V1 approaches Vlimit, the transfer current is less likely to flow, and the charge does not get on the transfer sheet. In a low-humidity environment, the amount of charge of the toner on the photosensitive drum increases, the current value Vt required for transfer increases, and V1 approaches Vlimit. In particular, the transfer current of the final color decreases. In many cases, adverse effects such as the inability to transfer the image to the printer have occurred.

Therefore, also in this embodiment, as shown in FIG. 2, the initial back potential V0 of the transfer sheet 5f is increased so that the transfer process for the fourth color can be performed effectively.

On the other hand, in a high-humidity environment, the value of V1 does not increase so much because Vt may be small because the charge amount of the toner is small. However, since the resistance values of the photosensitive drum 1, the transfer sheet 5f, the recording material P, and the like become low, the charge on the surface of the transfer sheet easily moves toward the photosensitive drum. In the present embodiment, as described above, since the OPC-based photosensitive drum is employed, if a positive charge is generated on the photosensitive drum, it is difficult to cancel this, and the drum appears on the image as a drum memory. Therefore, when the value of the initial back surface charge -V0 of the transfer sheet is a negatively large value,
Positive charges on the surface of the transfer sheet move in large quantities to the photosensitive drum, and the problem of generating a drum memory is likely to occur.

Therefore, in this embodiment, as shown in FIG. 3, in the high humidity environment, the initial charge amount of the transfer sheet, that is, the initial back potential (initial surface potential) is set smaller than that in the low humidity environment.

As described above, in this embodiment, the initial back potential of the transfer sheet 5f is changed in accordance with the environment in which the image forming apparatus is placed. And a high quality color image free from transfer failure and image defects due to the drum memory can be obtained.

An example of the current setting for the inner static eliminator 5d and the outer static eliminator 5e in this embodiment is shown in a low humidity environment (relative humidity 5%).
%) For the inner static eliminator, and +50 for the outer static eliminator.
0 μA DC discharge was performed, and a high humidity environment (relative humidity 80
%), The inner static eliminator was subjected to DC discharge of -250 μA, and the outer static eliminator was subjected to DC discharge of +250 μA.

The settings of the inner static eliminator 5d and the outer static eliminator 5e can be made as follows. That is, an AC discharge is performed on the outer static eliminator, and a discharge in which a DC component is superimposed on AC is performed on the inner static eliminator.
Change the value of the component. For example, in a high-humidity environment, both the inner and outer static eliminators perform static elimination of only AC and set the initial surface potential of the transfer sheet to 0, thereby preventing the memory of the photosensitive drum. On the other hand, in a low-humidity environment, by applying a negative DC to the inner static eliminator, the initial back potential of the transfer sheet can be maintained on the negative side, and particularly, transfer failure of the fourth color can be effectively prevented. Compared to a case where both the inner and outer static eliminators are controlled only by DC, the device configuration of these high-voltage power supplies is slightly more expensive, but the initial sheet charge amount can be accurately controlled.

Further, control may be performed as follows. That is, irrespective of the environment, −500 μm is applied to the inner static eliminator 5d.
A: The outer static eliminator 5e is caused to perform +500 μA DC discharge. In a high-humidity environment, the conductor is grounded through an appropriate resistor between the neutralizers 5d and 5e and the transfer section (transfer charging brush 5b). Alternatively, the initial potential of the transfer sheet may be lowered. By configuring the part of the conductor ground to be detachable and separated so that the conductor ground is not performed in a low humidity environment etc., the initial back potential of the transfer sheet can be changed depending on the installation environment of the apparatus, It is possible to effectively prevent transfer failure in a low humidity environment and drum memory in a high humidity environment. Although the device configuration is slightly complicated, there is an advantage that environmental control of the high-voltage power supply of the static eliminator is not required.

Embodiment 2 FIGS. 4 and 5 are explanatory diagrams showing a second embodiment of the present invention. 4 shows a case where the thickness of the recording material is large, and FIG. 5 shows a case where the thickness of the recording material is small.

As described above, when the surface potential of the transfer sheet 5f is opposite in polarity to the surface potential of the photosensitive drum 1, a memory phenomenon may occur on the photosensitive drum when they come into direct contact with each other.

Therefore, in the present embodiment, as shown in FIGS. 4 and 5, the initial back potential of the transfer sheet 5f is set to a potential that is reduced by the same degree as the potential rise Vq of the transfer sheet due to the charging of the recording material by suction. (The initial surface potential of the transfer sheet is positive). With this configuration, even when the potential of the transfer sheet surface is initialized to the opposite polarity to the photosensitive drum, the potential of the photosensitive drum surface is the same polarity as the photosensitive drum by the time the transfer sheet comes into contact with the photosensitive drum. Therefore, the rate of occurrence of the memory phenomenon on the photosensitive drum is significantly reduced.

When the recording material is electrostatically attracted to the transfer sheet, the required electrostatic attraction force varies depending on the rigidity and thickness of the recording material.

When the rigidity and thickness of the recording material are large, the so-called stiffness of the recording material is increased, and the recording material is easily peeled off from the transfer sheet. In order to maintain a stronger suction force, the suction current is increased, or the transfer drum is rotated by one or more turns, and the suction step is divided into two or more turns. It is necessary to supply a current, and the potential rise Vq of the transfer sheet due to the adsorption process becomes large. Particularly, when a transfer sheet having a low dielectric constant is used, adverse effects such as a difficulty in flowing the transfer current of the final color tend to occur.

In this embodiment, in order to enable the final color transfer process to be performed effectively, as shown in FIG. 4, the initial surface potential of the transfer sheet is opposite to that of the photosensitive drum and the transfer is performed. The sheet was charged so that the initial backside potential of the sheet was substantially the same as the transfer sheet potential increase Vq due to the attracting current, that is, about -Vq (V). The transfer sheet that has been initialized and charged becomes almost zero potential or more when it comes into contact with the photosensitive drum by applying an attraction current, and does not cause a memory on the photosensitive drum. Of course, the potential of the back surface of the sheet at the time of transferring the final color can be lowered, so that a sufficient transfer current can be given to transfer the final color satisfactorily.

On the other hand, in the case of a recording material having extremely small stiffness and thickness, applying an excessively large adsorption current tends to cause an adverse effect when the recording material is separated from the transfer sheet after the completion of image formation. As shown in FIG. 5, the potential rise Vq due to the attracting current in the case of a recording material having an extremely small thickness is smaller than that of a recording material having a large thickness, as shown in FIG.

If the initial back potential of the transfer sheet is set to be as low as that of a thick recording material for a thin recording material, the potential of the front surface of the transfer sheet after passing through the recording material suction portion is changed to the photosensitive drum. When the device is emergency stopped due to a jam or abnormal operation, a memory phenomenon occurs in the photosensitive drum due to the contact between the transfer sheet and the photosensitive drum. Such a problem does not occur for a thin recording material that is weak enough, because the initial back potential of the transfer sheet is set to a potential closer to 0 than in the case of a thick recording material.

In this embodiment, the initial potential of the transfer sheet is controlled by the difference in the thickness of the recording material. However, the initial potential of the transfer sheet may be controlled by the difference in the rigidity of the recording material. In addition to this, the present invention is also applicable to a case where there is a difference in properties of a recording material such as a difference in volume resistivity or a surface resistance that causes a difference in required suction force between the recording material and the transfer sheet. Can be applied. That is, the initial back surface potential of the transfer sheet may be charged to a polarity opposite to that of the photosensitive drum by an amount corresponding to an increase in the potential of the transfer sheet due to a necessary attracting current.

Third Embodiment In the second embodiment, an example was described in which the initial back potential of the transfer sheet was changed according to the thickness of the recording material. Is the case. The present embodiment is characterized in that the initial back potential of the transfer sheet is optimally controlled in accordance with the environment in which the image forming apparatus is placed and the properties of the recording material, such as the thickness.

In general, the stiffness of a recording material also changes depending on the environment in which the apparatus in which the recording material is to be used is placed. In the case of a recording material such as paper, the stiffness decreases in a high-humidity environment. Even with cardboard, a lower adsorption current is required.
On the other hand, as described in the first embodiment, the memory phenomenon of the photosensitive drum becomes more severe in a high humidity environment. As a result, even for thick paper, the attraction current, and thus the initial potential of the transfer sheet, can be changed according to the installation environment of the apparatus, and it is preferable to change it.

As described above, in this embodiment, by combining Embodiments 1 and 2, the installation environment of the apparatus, the thickness of the recording material,
Because the initial back potential of the transfer sheet is optimally controlled according to the properties of the material, etc., the final color transfer process can be performed more completely and well, and the memory phenomenon of the photosensitive drum can be effectively prevented. Became possible.

Embodiment 4 FIGS. 6 and 7 are explanatory diagrams showing a fourth embodiment of the present invention.

Even if the photosensitive drum 1 is electrostatically damaged by the surface of the transfer sheet 5f having a charge of the opposite polarity to the photosensitive drum surface during image formation, the photosensitive drum is immediately charged and exposed to damage. Can be recovered. For this reason, in full-color image formation, the back surface potential of the transfer sheet 5a has a polarity opposite to that of the photosensitive drum.
As shown in FIG. 6, when there is a change up to the first color, the transfer sheet causes electrostatic damage to the photosensitive drum until the transfer of the first color (magenta (M) in the figure). The electrostatic damage on the photosensitive drum surface is sufficiently recovered by the time the final color is transferred (black (K)).

However, when a large number of continuous copies are made in a single-color mode (for example, in a single black color), as shown in FIG. 11 of the conventional example, the surface potential of the transfer sheet has a polarity opposite to that of the photosensitive drum for a long time. You will keep making contact. As a result, a memory phenomenon on the photosensitive drum gradually accumulates, and a problem such as a change in color on an image occurs and has been confirmed.

Therefore, in this embodiment, the number of transfer steps for one recording material is recognized, and the charging state of the initial back surface potential of the transfer sheet is changed according to the number of transfer steps.
For example, in the monochromatic mode, as shown in FIG. 7, the initial surface potential of the transfer sheet is set to almost zero. Then, the variation width of the back surface potential of the transfer sheet is only Vq + Vt at the time when the single-color mode image formation on one recording material is completed. Therefore, even if the initial back surface potential of the transfer sheet is 0, the transfer width is changed. There is no problem because it is much smaller than the limit value Vlimit of the potential that can be applied to the sheet and does not reach Vlimit at all. Also, unlike the case of FIG. 11, since the surface potential of the transfer sheet is always the same polarity as that of the photosensitive drum, even when a large number of continuous copies are performed in the single-color mode, almost no electrostatic damage is accumulated on the photosensitive drum. Absent.

As described above, in this embodiment, the initial back potential of the transfer sheet is changed according to the number of transfer steps during the formation of one image, so that a large number of continuous copies can be made in a specific image forming mode. Due to
It has become possible to suppress adverse effects such as a change in the tint of an image.

In this embodiment, the number of transfer steps is considered as described above, but the same can be said for the suction step. That is, depending on the thickness of the recording material, the installation environment of the apparatus, and the shock at the time of suction, the suction step may be performed in a plurality of times. Of the rear surface potential changes.

For example, even in the same monochrome mode, the adsorption process 1
When the transfer is performed only once, the color change on the image occurs due to the accumulation of the memory phenomenon on the photosensitive drum as described above. Since it is expected that the harm caused by the charge-up of the transfer sheet is more likely to occur, the initial charge state of the transfer sheet should be set to the same potential as in the full-color mode, thereby effectively performing the transfer process. Can be.

Although the present invention has been described with reference to a full-color copying machine having a transfer drum as an example, the present invention relates to all types of multi-transfer copying machines having a recording material carrier such as a transfer drum and a transfer belt. It is needless to say that the present invention can be applied to an image forming apparatus such as an image forming apparatus and an LBP.

[0059]

As described above, according to the present invention,
The surface of the transfer sheet is attracted and charged to a potential of a polarity opposite to that of the photosensitive drum to electrostatically attract the recording material, and when the toner image on the photosensitive drum is transferred to the recording material by the transfer charge of the polarity opposite to that of the photosensitive drum, Since the electrical initialization of the transfer sheet before the adsorption charging was performed so as to control the potential of the back surface of the transfer sheet according to the environment in which the image forming apparatus is installed and the difference in rigidity, thickness, etc. of the recording material. Even when a transfer sheet with a low dielectric constant is used, especially, the transferability of the final color in a low-humidity environment is ensured, the drum memory is effectively prevented, and a high-quality image free from transfer defects and defects due to the drum memory. Can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a periphery of a transfer drum in an embodiment of an image forming apparatus according to the present invention.

FIG. 2 is a diagram showing a change in the back surface potential of the transfer sheet during charging in a low humidity environment according to the first embodiment of the present invention.

FIG. 3 is a diagram illustrating a change in the back surface potential of the transfer sheet during charging in a high humidity environment according to the first embodiment.

FIG. 4 is a diagram illustrating a change in a back surface potential of a transfer sheet due to charging when a thick recording material is used according to a second embodiment of the present invention.

FIG. 5 is a diagram illustrating a change in the back surface potential of a transfer sheet due to charging when a thin recording material is used according to the second embodiment.

FIG. 6 is a diagram illustrating a change in the back surface potential of a transfer sheet during charging according to Embodiment 4 of the present invention.

FIG. 7 is a diagram illustrating a change in the rear surface potential of a transfer sheet during charging during continuous copying in a single-color mode according to a fourth embodiment.

FIG. 8 is a schematic configuration diagram illustrating a conventional image forming apparatus.

9 is a diagram illustrating a change in the back surface potential of the transfer sheet due to charging in the image forming apparatus of FIG. 8;

FIG. 10 is a diagram illustrating a change in the back surface potential of the transfer sheet when the back surface potential of the transfer sheet is made to have the same polarity as the front surface potential of the photosensitive drum at the stage of initialization by charging in the conventional image forming apparatus.

FIG. 11 shows the fluctuation of the back surface potential of the transfer sheet during continuous copying in the monochromatic mode when the back surface potential of the transfer sheet is set to the same polarity as the front surface potential of the photosensitive drum at the stage of initialization by charging in the conventional image forming apparatus. FIG.

[Explanation of symbols]

 Reference Signs List 1 photosensitive drum 5 transfer drum 5b transfer charging brush 5c adsorption charging brush 5d inner static eliminator 5e outer static eliminator 5f transfer sheet 5g suction roller P recording material

Claims (6)

[Claims] An image carrier on which an image is formed, and a recording material that carries the electrostatically attracted recording material and conveys it to a transfer section facing the image carrier, and travels while engaging with the image carrier. A carrier, and a charging unit that supplies an electric charge to the recording material carrier to transfer an image formed on the image carrier onto a recording material carried on the recording material carrier; Before the recording material is carried on the recording material carrier, the back surface of the recording material carrier opposite to the side on which the recording material is carried is charged to the same polarity as the surface potential of the image carrier, so that the recording material is carried. In an image forming apparatus in which the recording material carrying side of the image bearing member is charged to a polarity opposite to the surface potential of the image carrying member, the back potential of the recording material carrying member before carrying the recording material is determined by installing the image forming apparatus main body. Characterized by being changed according to at least one or more differences in environment and operating conditions. Image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the difference in the installation environment of the image forming apparatus main body is a difference in the absolute moisture content of the installation environment of the image forming apparatus main body. 3. An image forming apparatus according to claim 1, wherein the absolute value of the back surface charge amount of the recording material carrier before carrying the recording material in an installation environment in a certain installation environment is lower than the absolute moisture content of the installation environment. 3. The image forming apparatus according to claim 2, wherein the back surface charge amount of the recording material carrier before carrying the recording material is controlled so as to be smaller than the absolute value of the back surface charge amount of the recording material carrier. apparatus. 4. The image forming apparatus according to claim 1, wherein the difference in operating conditions of the image forming apparatus main body is a difference in properties of a recording material. 5. The recording material according to claim 1, wherein at least one of a thickness, a dielectric constant, an electric resistance, and a rigidity of the recording material is selected.
5. The image forming apparatus according to claim 4, wherein the difference is at least one of the properties. 6. The image forming apparatus according to claim 1, wherein the difference in operation conditions of the image forming apparatus main body is a difference in the number of times of a transfer step or a recording material suction step in one image formation.