JPH08334990A - Image forming device - Google Patents

Abstract

PURPOSE: To improve image quality by stably performing the attraction and the transfer of a transfer sheet even when the kind of the transfer sheet is changed. CONSTITUTION: This image forming device is provided with a photoreceptor drum 15 on the surface of which a toner image is to be formed, and a transfer drum 11 transferring the toner image formed on the drum 15 on the transfer sheet P by making the sheet P abut on the drum 15. The drum 11 is obtained by laminating a dielectric layer 28, a semiconductor layer 27 and a conductor layer 26 in order from the sheet P side, and provided with a power source part 32 impressing a specified voltage on the conductor layer 26 and a conductive roller 12 grounded and coming into contact through the sheet P on the surface on an upstream side from a transfer point X on the surface of the dielectric layer 28. Then, nip time when the optional position of the sheet P passes through nip width between the drum 11 and the roller 12 is changed according to the kind of the sheet P.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus used in laser printers, copying machines, laser fax machines and the like.

[0002]

2. Description of the Related Art Conventionally, toner is attached to an electrostatic latent image formed on a photosensitive drum to develop it, and this toner image is
There is an image forming apparatus that transfers onto a transfer sheet wound around a transfer drum.

An example of such an image forming apparatus is shown in FIG.
As shown in FIG. 1, a corona charger 102 for adsorbing the transfer sheet P is provided inside a cylinder 101 having a dielectric layer 101a,
A corona charger 104 for transferring the toner image formed on the surface of the photoconductor drum 103 to the transfer sheet P is separately arranged, and the charging and transferring of the transfer sheet P are separately performed by the chargers 102 and 104. I am supposed to do it.

Further, as shown in FIG. 22, a cylinder 2 having a two-layer structure of an outer semiconductive layer 201a and an inner base material 201b.
01 and a grip mechanism 202 for holding the conveyed transfer sheet P along the cylinder 201. In this image forming apparatus, after the transferred transfer sheet P is gripped by the grip mechanism 202 so as to be along the surface of the cylinder 201, a voltage is applied to the outer semiconductive layer 201a of the cylinder 201. Alternatively, the surface of the cylinder 201 is charged by discharging by a charger provided inside the cylinder 201,
The toner image No. 03 is transferred to the transfer sheet P.

However, in the image forming apparatus shown in FIG. 21, since the cylinder 101, which is the transfer roller, has a single-layer structure including only the dielectric layer 101a, the corona chargers 102 and 104 described above are disposed inside thereof. There is a need to. Therefore, the size of the cylinder 101 is limited, and there is a problem that the device cannot be downsized.

Further, in the image forming apparatus shown in FIG. 22, the cylinder 201, which is the transfer roller, has a two-layer structure, so that the cylinder 201 for transferring the toner image onto the transfer sheet P is charged. Therefore, the number of chargers is small. However, since the grip mechanism 202 is provided, the configuration of the entire image forming apparatus becomes complicated, which causes a problem that the number of parts of the entire apparatus increases and the cost for manufacturing the apparatus increases. There is.

In order to solve the above problems, for example, Japanese Patent Laid-Open No. 2-74975 discloses a transfer sheet for a transfer drum in which a conductive rubber and a dielectric film are laminated on a grounded metal roll. An image forming apparatus is disclosed in which a corona charger driven by a unipolar power source is provided near the peeling position.

In the above-mentioned image forming apparatus, the corona charger induces electric charges in the dielectric film to attract the transfer sheet to the transfer drum. Further, when the transfer sheet is adsorbed, charges are further induced and transfer is performed.

Therefore, according to the above-mentioned image forming apparatus, since the surface of the transfer drum is charged by one charging device to adsorb and transfer the transfer sheet, only one charging device is required. The drum can be miniaturized. Further, a mechanism such as the above-mentioned gripping mechanism 202 for holding the transfer sheet is not required, and the transfer sheet can be adsorbed with a simple structure.

Further, Japanese Patent Laid-Open No. 173435/1993 includes a transfer drum having at least an elastic layer made of foam and a dielectric layer covering the elastic layer, and of each color sequentially formed on the photosensitive drum. An image forming apparatus is disclosed in which toner images are sequentially superimposed and transferred onto a transfer sheet adsorbed on the transfer drum to form a color image on the transfer sheet.

In the above-mentioned image forming apparatus, as a method of supporting the transfer sheet on the transfer drum, the transfer sheet is electrostatically adsorbed on the transfer drum by using an adsorption roller as a charge applying means. Further, in the image forming apparatus, a void portion is provided between the elastic layer and the dielectric layer in order to improve the suction ability, that is, the suction ability of the transfer sheet.

[0012]

SUMMARY OF THE INVENTION However, Japanese Patent Laid-Open No.
In the image forming apparatus disclosed in Japanese Patent No. 74975, the surface of the transfer drum is charged by air discharge of a corona charger. Therefore, in the case of color printing, that is, in the case where the transfer process is performed a plurality of times, the charge is replenished by the corona charger after each transfer. Therefore, a charging unit including a unipolar power source for controlling the drive of the corona charger is required, and the number of constituent parts of the apparatus increases. As a result, there is a problem that the cost for manufacturing the device increases.

Further, if the surface of the transfer drum is flawed, the electric field region becomes small due to charging by air discharge, the electric field balance is disturbed at the flawed portion, and transfer defects such as white spots occur at that portion. Cause a decrease in

Further, since the surface of the transfer drum is charged by the air discharge, the voltage applied to the charging becomes large and the driving energy of the image forming apparatus increases. Further, since the air discharge is easily affected by the environment such as the temperature and humidity of the air, the surface potential of the transfer drum is likely to vary, and there is a problem in that the transfer sheet is apt to be improperly attracted and the print spots are likely to occur. .

Further, in the image forming apparatus described in JP-A-5-173435, the hardness of the elastic layer (foam layer) and the
The contact pressure between the suction roller and the transfer drum is not specified. Further, regarding the width of the close contact portion (that is, the nip width) formed between the suction roller and the transfer drum and the time for any position of the transfer sheet to pass through the above nip width (that is, the nip time), Not specified.
From this, it is considered that the nip time is constant in any type of transfer sheet.

However, it is generally known that different types of transfer sheets cause different amounts of charge on the transfer sheet within a certain nip time. From this, it is considered that the electrostatic attraction force when the transfer sheet is electrostatically attracted onto the transfer drum varies considerably depending on the type of the transfer sheet. In other words, when the nip time is constant for any type of transfer sheet, the charge amount of the transfer sheet charged within the fixed time varies depending on the type of transfer sheet. The sheet may not be stably electrostatically adsorbed on the transfer drum. In this case, during color printing, the electrostatic attraction force of the transfer sheet on the transfer drum is reduced, and the transfer sheet is transferred before all the toner images of each color formed on the transfer drum are transferred to the transfer sheet. There is a problem in that it peels off from the drum and good transfer cannot be performed.

Further, in the above image forming apparatus, a suction roller power source for attracting the transfer sheet to the transfer drum, and a power source for applying a voltage having a polarity opposite to that of the toner to the transfer sheet when the toner transfer is performed, Requires at least two power supplies. As a result, there is a problem that the cost for manufacturing the device increases.

The present invention has been made in view of the above problems, and an object thereof is to stably electrostatically adsorb a transfer sheet onto the surface of a transfer means such as a transfer drum.
An object of the present invention is to provide an image forming apparatus which eliminates defective transfer of a toner image onto a transfer sheet, improves an image formed on the transfer sheet, and has a low-cost configuration.

[0019]

An image forming apparatus according to claim 1, wherein an image carrier (for example, a photoconductor drum) on which a toner image is formed and a transfer sheet are brought into contact with the image carrier. Thus, in the image forming apparatus having a transfer unit (for example, a transfer drum) that transfers the toner image formed on the image carrier, the transfer unit is provided with, for example, polyvinylidene fluoride from the contact surface side of the transfer sheet. A dielectric layer made of, for example, a semiconductive layer made of urethane foam, and a conductive layer made of aluminum, for example, are laminated in order, and a voltage applying means (for example, a power source section) for applying a predetermined voltage to the conductive layer. And a grounded electrode member (for example, a conductive roller) contacting the transfer sheet on the upstream side from the transfer position on the surface of the dielectric layer. Of Type (for example, paper, synthetic resin sheet for OHP) by, is characterized by changing the nip time any position closely partial transfer sheet passes between the transfer means and the electrode member.

An image forming apparatus according to a second aspect is the image forming apparatus according to the first aspect.
In addition to the above configuration, the nip time is changed on the basis of the relationship between the nip time previously obtained according to the type of the transfer sheet and the charge amount of the transfer sheet.

An image forming apparatus according to a third aspect is the image forming apparatus according to the first aspect.
Alternatively, in addition to the constitution of 2, the nip time is changed by adjusting the hardness of the semiconductive layer and / or the contact pressure between the transfer means and the electrode member.

[0022]

According to the structure of the present invention, since the electrode member is in contact with the surface of the dielectric layer surface on the upstream side from the transfer position via the transfer sheet and is grounded, the electrode member is not grounded. A charge having the same polarity as that of the applied voltage is accumulated in the semiconductive layer, and further, a charge having the same polarity is induced on the surface of the dielectric layer and the transfer sheet pressed against the surface of the dielectric layer. . That is, on the back surface of the transfer sheet that is in contact with the dielectric layer, charges having a polarity opposite to the polarity of the voltage applied to the conductor layer are induced. As a result, the transfer sheet can be electrostatically adsorbed to the surface of the dielectric layer, that is, the surface of the transfer means by simply applying a voltage to the conductor layer, and the potential of the charge on the surface of the transfer sheet and the surface of the image carrier are The toner image can be transferred to the transfer sheet by providing a predetermined difference between the toner image and the electric potential of the toner image.

As a result, the transfer sheet is attracted and transferred by inducing electric charges, instead of the attraction and transfer of the transfer sheet by injecting charges by air discharge as in the prior art, so that the voltage can be low and the voltage can be reduced. Easy to control. In addition, it is possible to eliminate variations in voltage due to external pressure. Further, it is necessary to separately provide a power source for electrostatically adsorbing the transfer sheet to the surface of the dielectric layer, that is, the surface of the transfer unit, and a power source for transferring the toner image formed on the image carrier to the transfer sheet. Since there is no such a problem, an inexpensive structure can be obtained.

Further, if the type of transfer sheet is different, the amount of charge charged on the transfer sheet within the nip time is also different. Therefore, the charge amount of the transfer sheet can be adjusted by changing the nip time depending on the type of the transfer sheet. Thereby, no matter what kind of transfer sheet is used, the transfer sheet can be stably electrostatically adsorbed on the dielectric layer of the transfer means. From this, the transfer sheet does not separate from the transfer means before all the toner images of the respective colors formed on the image carrier are transferred to the transfer sheet, and good toner transfer from the image carrier to the transfer sheet is achieved. It can be carried out. Therefore, it is possible to always supply a stable image.

According to the structure of claim 2, claim 1
In addition to the action of, if the relationship between the nip time and the charge amount of the transfer sheet is obtained in advance according to the type of transfer sheet, the nip time can be calculated according to the type of transfer sheet used.
It is possible to change to a suitable nip time that can efficiently provide a charge amount for stably adsorbing the transfer sheet to the transfer unit. Furthermore, depending on the type of transfer sheet used,
It is possible to easily determine how to change the nip time in order to stably attract the transfer sheet onto the dielectric layer of the transfer unit.

The snail and the transfer sheet have different physical properties such as resistance values depending on the type of the transfer sheet.
Further, the charge amount of the transfer sheet differs not only according to the physical properties of the transfer sheet, but also depending on the physical properties (resistance) of the semiconductive layer, the physical properties (resistance) of the dielectric layer, or the applied voltage. However, the relationship between the nip time and the charge amount of the transfer sheet is, for example, the resistance of the semiconductive layer, the resistance of the dielectric layer,
Even if conditions such as applied voltage and type of transfer sheet change, 3
It is roughly divided into patterns. Therefore, if the relationship between the charge amount of the transfer sheet and the nip time in the case of using an arbitrary semiconductive layer or dielectric layer for each type of transfer sheet is calculated in advance, for example, the semiconductive layer Even if the resistance, the resistance of the dielectric layer, or the type of the transfer sheet is changed, the transfer sheet can be efficiently charged simply by determining the type of the transfer sheet to be used, depending on which pattern the transfer sheet to be used belongs to. The nip time can be easily determined.

For example, when the charge amount of the transfer sheet with the change of the nip time has the maximum value (pattern 1),
By setting and changing the nip time so that the charge amount of the transfer sheet does not become lower than the initial charge amount, the transfer sheet can be stably electrostatically adsorbed on the dielectric layer. Further, by setting the nip time to the maximum value described above, it is possible to efficiently inject charges and efficiently charge the transfer sheet.

When the charge amount of the transfer sheet increases as the nip time becomes longer (pattern 2), the nip time is within the range of 0 V ± 1000 V between the potential before charge injection and the potential after charge injection. So that
By changing it, the transfer sheet can be stably electrostatically adsorbed on the dielectric layer. From the experimental results, it was found that the electrostatic attraction force of the transfer sheet P was weakened by providing a potential difference of over 1000 V before and after the charge injection.

Further, when the charge amount of the transfer sheet decreases from the initial charge amount as the nip time becomes longer (pattern 3), the charge amount of the transfer sheet becomes
By setting and changing the nip time so as to be 50% or more of the initial charge amount, the transfer sheet can be stably electrostatically adsorbed on the dielectric layer.

As described above, if the relationship between the nip time and the charge amount of the transfer sheet is obtained in advance according to the type of the transfer sheet, the charge amount of the transfer sheet to be used can be determined only by determining the type of the transfer sheet. And the nip time, the nip time for efficiently charging the transfer sheet can be easily determined. Furthermore, like this,
By changing the nip time based on the relationship between the charge amount of the transfer sheet used and the nip time, it is possible to efficiently provide the charge amount for adsorbing the transfer sheet on the dielectric layer of the transfer means. As a result, the transfer sheet can be stably electrostatically adsorbed on the dielectric layer.

According to the third aspect of the invention, the nip time can be easily changed by adjusting the hardness of the semiconductive layer. Further, the nip time can be easily changed by adjusting the contact pressure between the transfer means and the electrode member.

As described above, the nip time is changed by adjusting not the rotation speed of the transfer means but the hardness of the semiconductive layer and / or the contact pressure between the transfer means and the electrode member. There is no fear of lowering the transfer efficiency as in the case of changing the nip time depending on the rotation speed. When changing the nip time according to the rotation speed of the transfer means, it is necessary to slow down the rotation speed of the transfer means in order to increase the nip time. When the rotation speed of the transfer means is slowed in this way, the transfer efficiency per minute is lowered. Therefore, it is preferable to change the nip time depending on the hardness of the semiconductive layer and / or the contact pressure between the transfer means and the electrode member.

[0033]

Embodiment 1 Embodiment 1 of the present invention will be described with reference to FIGS.
The description will be made based on 0 as follows.

As shown in FIG. 2, the image forming apparatus according to the present embodiment stocks and feeds a transfer sheet P (see FIG. 1) as a recording sheet on which an image is formed by toner, A transfer unit 2 for transferring the toner image onto the transfer sheet P,
It is composed of a developing unit 3 for forming a toner image and a fixing unit 4 for fusing and fixing the toner image transferred onto the transfer sheet P.

The sheet feeding section 1 is detachably arranged at the bottom of the main body, stocks the transfer sheet P, and transfers the sheet to the transfer section 2.
To the sheet feeding cassette 5, and a manual sheet feeding section 6 provided on the front side of the main body for manually feeding the transfer sheets P one by one from the front side. A pickup roller 7 that sends the transfer sheets P one by one, a PF roller 8 that conveys the transfer sheets P sent by the pickup roller 7, a manual feed roller 9 that conveys the transfer sheets P supplied from the manual feed supply unit 6, and the above-mentioned. A pre-curl roller 10 that curls the transfer sheet P conveyed by the PF roller 8 and the manual feed roller 9 is provided.

The sheet feeding cassette 5 is provided with a sending member 5a biased upward by a spring or the like, and the transfer sheet P is laminated on the sending member 5a. As a result, the transfer sheet P of the paper feed cassette 5 is
Has its uppermost portion brought into contact with the pickup roller 7 and the PF is rotated one by one by the rotation of the pickup roller 7 in the arrow direction.
It is sent out to the roller 8 and conveyed to the pre-curl roller 10.

On the other hand, the transfer sheet P supplied from the manual feed section 6 is also fed by the manual feed roller 9 to the pre-curl roller 1.
Transported to 0.

The pre-curl roller 10 curls the transfer sheet P conveyed as described above. This is because the transfer sheet P is adsorbed on the surface of the cylindrical transfer drum 11 provided in the transfer section 2. This is because it is easy to be done.

The transfer section 2 is provided with a transfer drum 11 as the above-mentioned transfer means. Around the transfer drum 11, there are a conductive roller 12 as an earthed electrode member and a transfer drum 11. A guide member 13 for guiding the transfer sheet so as not to fall, a peeling claw 14 for forcibly peeling off the transfer sheet P adsorbed on the transfer drum 11, and the like are provided. Further, the transfer drum 11 is adapted to adsorb the transfer sheet P on its surface by static electricity. Therefore, further, around the transfer drum 11,
A static eliminator 11 that acts on the transfer drum 11 after the transfer sheet P is peeled off from the transfer drum 11, and removes the residual charge applied to the transfer drum 11 when the transfer sheet P is peeled off.
a is provided. The static eliminator 11a is provided on the upstream side of the conductive roller 12. As a result, there is no residual charge on the transfer drum 11, and the next transfer sheet P is stably adsorbed. In addition, after the transfer sheet P is peeled from the transfer drum 11 around the transfer drum 11 on the upstream side of the static eliminator 11a, the transfer sheet P acts on the transfer drum 11 to remove the residual toner attached to the transfer drum 11. A cleaning device 11b is provided. As a result, the transfer drum 11 moves to the next transfer sheet P.
Is cleaned by the time when is absorbed, and the next transfer sheet P is stably absorbed. The above-mentioned peeling claw 14 is provided on the surface of the transfer drum 11 so as to be freely contactable to and detachable from it. The details of the structure of the transfer drum 11 will be described later.

Further, in the developing section 3, the transfer drum 11 is
There is provided a photosensitive drum 15 as an image bearing member that comes into pressure contact with the photosensitive drum 15. The photosensitive drum 15 is composed of a grounded conductive aluminum base tube 15a, and the surface thereof has an OPC.
The film 15b (see FIGS. 8 and 9) is applied.

Further, around the photosensitive drum 15,
The developing devices 16, 17, 18, and 19 containing the yellow, magenta, cyan, and black toners are radially arranged, and the charging device 2 that charges the surface of the photosensitive drum 15 is provided.
A cleaning blade 21 that scrapes off and removes the residual toner on the surface of the photosensitive drum 15 is provided, and a toner image is formed on the photosensitive drum 15 for each toner. That is, according to the photoconductor drum 15, charging, exposure, development and transfer are repeated for each color.
Therefore, in the case of color transfer, with respect to the transfer sheet P electrostatically attracted to the transfer drum 11, each time the transfer drum 11 makes one rotation, a toner image of one color is transferred to the transfer sheet P, and a maximum of four rotations occur. It is designed to obtain one color image.

The photosensitive drum 15 and the transfer drum 11 are pressed against each other so that a pressure of 8 kg is applied at the transfer site in view of transfer efficiency and image quality.

In the fixing unit 4, a fixing roller 23 that fuses a toner image at a predetermined temperature and pressure to fix the toner image on the transfer sheet P, and after the toner image is transferred, is peeled from the transfer drum 11 by a peeling claw 14. A fixing guide 22 for guiding the transfer sheet P to the fixing roller 23 is provided.

A discharge roller 24 is provided on the downstream side of the transfer sheet conveyance of the fixing section 4 so that the transfer sheet P after fixing is discharged from the apparatus main body onto a discharge tray 25.

Now, the structure of the transfer drum 11 will be described. As shown in FIG. 3, the transfer drum 11 uses a cylindrical conductor layer 26 made of aluminum as a base material, and a semi-conductor layer 27 made of elastic urethane foam on the upper surface of the conductor layer 26. Is provided.

Furthermore, on the upper surface of the semiconductive layer 27,
A dielectric layer 28 made of polyvinylidene fluoride or PET (polyethylene terephthalate) is provided.

Further, a power source unit 32 (see FIG. 1) as a voltage applying means is connected to the conductor layer 26,
A stable voltage is maintained over the entire circumference of the conductor layer 26.

Further, the above-mentioned layers are not adhered by an adhesive agent or the like, but are provided at both ends of the sheet-like semiconductive layer 27 and dielectric layer 28 as shown in FIG. 3, for example. A plurality of through holes 29 ...
The bosses 30a provided on the sheet pressing plate 30 are fitted to each other, and the bosses 30a are fitted to the openings 26a provided on the upper surface of the conductor layer 26 so that the semiconducting layer 27 and the dielectric layer 28 are fitted. Are fixed to the conductor layer 26.

In the fixing method described above, the semi-conductive layer 27 and the dielectric layer 28 are adapted to apply tension to the inside of the conductive layer 26 by the sheet pressing plate 30, whereby the floating of each layer is prevented. It is designed to prevent loosening.

In this case, each of the above layers is the sheet pressing plate 3
Since it is only fixed by 0, each layer can be easily replaced.

Further, in addition to the above fixing method, as shown in FIG. 4, for example, a sheet pressing member 31 having bosses 31a at both ends and a fixing member 31b at the center is used.
There is a method of fixing a sheet composed of the semiconductive layer 27 and the dielectric layer 28 to the conductive layer 26.

In the fixing method described above, the bosses 31a of the sheet pressing member 31 are fitted into the fitting holes 26b provided at both ends of the opening 26a of the conductor layer 26, and the opening 26a is formed. The fixing member 3 of the sheet pressing member 31
1b is inserted and the sheet made of the semiconductive layer 27 and the dielectric layer 28 is fixed to the conductive layer 26. Even if each layer is fixed by such a method, each layer can be easily replaced as described above.

As shown in FIG. 7, the width of the dielectric layer 28 of the transfer drum 11 is larger than the width of the photoconductor tube (aluminum tube 15a) forming the photoconductor drum 15.
In addition, this photoconductor tube width is larger than the effective transfer width,
Further, this effective transfer width is equal to the effective image width (OPC film 15
It is larger than the coating width b).

The width of each layer of the transfer drum 11 is as shown in FIG.
As shown in FIG. 3, when the conductive layer 26> semiconductor layer 27> dielectric layer 28 is formed, the semiconductive layer 2 is formed.
7 is a grounded aluminum tube 1 of the photosensitive drum 15.
There is a risk of contact with 5a.

That is, the conductor layer 26 is formed by the power source section 32.
When a + voltage is applied to the conductive layer 26, a + charge is induced in the conductor layer 26, and the + charge moves to the surface of the semiconductive layer 27. At this time, if the grounded aluminum tube 15a of the photoconductor drum 15 and the semiconductive layer 27 come into contact with each other, all the charges charged in the semiconductive layer 27 are transferred to the aluminum tube 15a, and the dielectric It becomes impossible to induce a positive charge on the surface of the body layer 28. Therefore, the transfer drum 11 cannot adsorb the negatively-charged toner adsorbed on the OPC film 15b, and a transfer failure occurs.

Therefore, in each layer of the transfer drum 11, as shown in FIG. 9, the widths of the conductor layer 26 and the dielectric layer 28 are the same, and the width of the semi-conductor layer 27 is smaller than the above widths. By making it small, it is possible to prevent the contact between the semiconductive layer 27 and the grounded aluminum base tube 15a, and to prevent the leakage of charges.

As a result, the transfer drum 11 can adsorb the negatively charged toner adsorbed on the OPC film 15b, and the transfer failure can be eliminated.

Further, the diameter of the transfer drum 11 is such that one transfer sheet P is wound without overlapping, that is, the maximum width of the transfer sheet P that can be used in the present image forming apparatus,
Alternatively, it is formed in a size corresponding to the length.

As a result, the transfer sheet P can be stably wound around the transfer drum 11, and as a result, the transfer efficiency can be improved and the image quality can be improved.

Here, the suction / transfer operation of the transfer sheet P by the transfer drum 11 will be described below with reference to FIGS. 5, 6 and 10. The transfer drum 1
It is assumed that a positive voltage is applied from the power supply section 32 to the first conductor layer 26.

First, the suction process of the transfer sheet P will be described. The charging of the dielectric layer 28 using the conductive roller 12 is
Mainly consists of Paschen discharge and charge injection. That is, as shown in FIG. 5, the transfer sheet P conveyed to the transfer drum 11 is pressed against the surface of the dielectric layer 28 by the conductive roller 12, and the charge accumulated in the semiconductive layer 27 is applied to the dielectric layer 28. Move to. As a result, the dielectric layer 28
Positive charges are induced on the surface, and as shown in FIG. 10, an electric field is generated from the transfer drum 11 side toward the conductive roller 12 side. The surface of the transfer drum 11 is uniformly charged by the rotation of the conductive roller 12 and the transfer drum 11. Then, as the distance between the conductive roller 12 and the dielectric layer 28 of the transfer drum 11 becomes shorter and the electric field strength applied to the close contact portion between the dielectric layer 28 and the conductive roller 12, that is, the nip, becomes stronger, the airborne Dielectric breakdown occurs, and in the area (I), the conductive roller 1 is transferred from the transfer drum 11 side.
Discharge to the 2 side, that is, Paschen discharge occurs.

Further, after the discharge is completed, the conductive roller 1 is
2, the nip between the transfer drum 11 and the transfer drum 11, that is, the area (II)
In the above, charge injection occurs from the conductive roller 12 side to the transfer drum 11 side, and negative charges are accumulated on the surface of the transfer drum 11. That is, by the Paschen discharge and the charge injection accompanying the Paschen discharge, −charges are accumulated inside the transfer sheet P, that is, on the contact surface side with the dielectric layer 28.
As a result, the transfer sheet P is electrostatically attracted to the transfer drum 11. The attracting force does not vary as long as the applied voltage is stable, and the transfer sheet P can be stably attracted to the transfer drum 11.

As described above, since the charging is performed by contact instead of charging by air discharge, the conductive layer 26 is used.
The voltage applied to is low. In addition, from various experimental results,
The applied voltage is suitably +3 kV or less, and more preferably +2 kV, good charging can be performed.

The transfer sheet P adsorbed to the transfer drum 11 is charged to the outside with +,
The toner image transfer point X
Be transported to.

Next, the transfer process of the transfer sheet P will be described. As shown in FIG. 6, the photosensitive drum 15 has a negative surface.
The charged toner is adsorbed. Therefore, the transfer sheet P whose surface is positively charged becomes the transfer point X.
When the sheet is conveyed to, the toner is adsorbed and transferred onto the surface of the transfer sheet P due to the potential difference between the positive charge on the surface of the transfer sheet P and the negative charge on the toner.

By the way, in general, since the types of the transfer sheet P are different, the time for any position of the transfer sheet P to pass through the nip width formed between the conductive roller 12 and the transfer drum 11, that is, the nip time. It is known that the charge amount of the transfer sheet P in the inside is different.

Here, a method for adjusting the nip time will be described. As shown in FIG. 1, the image forming apparatus includes a transfer sheet detection sensor 33 for detecting the type of the transfer sheet P. The transfer sheet detection sensor 33
Is connected to a control unit (not shown), and under the control of this control unit, the transfer sheet P conveyed to the transfer drum 11 before the transfer sheet P is electrostatically attracted to the transfer drum 11.
The type of the transfer sheet P is detected by measuring the physical properties of the sheet. That is, the transfer sheet detection sensor 33 detects whether it is paper or a synthetic resin sheet for OHP by measuring the transmittance, or detects the thickness of the transfer sheet P, for example. For example, it detects whether the paper is thick paper or thin paper. Then, the type of the transfer sheet P detected here (for example, paper, a synthetic resin sheet for OHP, or
The nip time is adjusted according to the thickness of the transfer sheet P).

The nip time is determined by (nip width formed between the transfer drum 11 and the conductive roller 12) / (rotational speed of the transfer drum 11). The nip width can be adjusted by changing the hardness of the semiconductor layer 27, for example. The semiconductive layer 27
The hardness of is specified by Asker C. The above-mentioned Asker C is a standard in the Japan Rubber Association, and a hardness measuring needle with a spherical tip is pressed against the surface of the sample by the force of the spring, and the balance between the resistance of the sample and the force of the spring is The hardness is represented by the depth (pushing depth) at which the needle pushes the sample when it is taken. According to the Asker C standard, the hardness of the sample is 0 degree so that the pushing depth of the needle when a load of 55 g is applied to the spring is equal to the maximum displacement of the needle, and a load of 855 g is applied to the spring. The hardness of the sample is set to 100 degrees so that the needle pressing depth becomes 0. Table 1 shows the relationship between the Asker C and the suction effect of the transfer sheet P.

[0069]

[Table 1]

In Table 1, (◯) indicates that the adsorption effect is large, and the transfer sheet P is stably attached to the transfer drum 11 during four rotations of the transfer drum 11 (transferring toner of four colors). The state of being electrostatically adsorbed. Also, (△)
Indicates that the adsorption effect is small, and the transfer sheet P is electrostatically adsorbed to the transfer drum 11 during four rotations of the transfer drum 11, but the front end or the rear end of the transfer sheet P is floating. Say. Furthermore, (x) means that there is no adsorption effect, and while the transfer drum 11 rotates four times,
A state in which the transfer sheet P is peeled off from the transfer drum 11.

From the results shown in Table 1, it can be understood that the hardness of the semiconductive layer 27 is set within the range of 20 to 80 in Asker C to have the effect of adsorbing the transfer sheet P. That is, the hardness of the semiconductive layer 27 is 20 to 80 in Asker C.
The range is preferable because the transfer sheet P can be electrostatically adsorbed onto the transfer drum 11 during four rotations of the transfer drum 11. Further, the semiconductive layer 27
A hardness of 25 to 50 in Asker C is most preferable because the transfer sheet P can be more stably electrostatically adsorbed onto the transfer drum 11 during four rotations of the transfer drum 11.

The hardness of the semiconductive layer 27 is 20 in Asker C.
If it is smaller, the hardness is too low and the transfer sheet P is curled in the opposite direction (not along the transfer drum 11). As a result, the transfer sheet P cannot be stably electrostatically adsorbed on the transfer drum 11, which is not preferable.

On the other hand, the hardness of the semiconductive layer 27 is Asker C.
If it is larger than 80, the hardness is too high, and the nip width between the transfer drum 11 and the conductive roller 12 becomes small, and as a result, the transfer sheet P is stably transferred.
It is not preferable because it cannot be electrostatically adsorbed on the top. Further, the hardness of the semi-conductor layer 27 is 80 in Asker C.
If it is larger, the hardness is too high, so that the photosensitive drum 1
5, an excessive contact pressure is applied between the transfer drum 11 and the transfer drum 11, and the durability of the photoconductor deteriorates.

The nip width can be adjusted by changing the contact pressure between the transfer drum 11 and the conductive roller 12. For the contact pressure between the transfer drum 11 and the conductive roller 12, for example, an eccentric cam 34 for pressing the conductive roller 12 shown in FIG. This can be changed by adjusting the force with which the conductive roller 12 presses the conductive roller 12. The eccentric cam 34 includes a shaft (core) 34a and a shaft 34a.
It is composed of pressing members 34b and 34b, which are formed of the same elliptical flat plate provided at both ends of a. This eccentric cam 34
Means that the pressing members 34b and 34b are connected to the conductive roller 12
Is provided so as to come into contact with the rotary shaft 12a of the conductive roller 12 extending in the longitudinal direction from the center of both side surfaces in the longitudinal direction. The shaft 34a supports the pressing member 34b at a position deviated from the center of the pressing member 34b, and is provided so as to be parallel to the conductive roller 12.

The contact pressure between the transfer drum 11 and the conductive roller 12 is the same as that of the transfer drum 11, the conductive roller 12, and the eccentric cam 34, as shown in FIG. Is maximized when the distance between the shaft 34a and the rotary shaft 12a is the closest (in the figure, the distance from the shaft 34a to the peripheral edge of the pressing member 34b is H), and the distance between the shaft 34a and the rotary shaft 12a is the closest (Fig. The distance from the shaft 34a to the peripheral portion of the pressing member 34b is G). As a result, by rotating the eccentric cam 34, the force with which the eccentric cam 34 presses the conductive roller 12 is adjusted, and thus the contact pressure between the transfer drum 11 and the conductive roller 12 is adjusted. The pressing member 3
4b is not particularly limited as long as it has a contact portion with the rotating shaft 12a, that is, a peripheral portion formed by a curved line, and may be a circular plate or a sphere. here,
Table 2 shows the relationship between the nip width and the suction effect of the transfer sheet P.

[0076]

[Table 2]

In Table 2, (◯) indicates that the adsorption effect is large, and the transfer sheet P is stably attached to the transfer drum 11 during the four rotations of the transfer drum 11 (transferring toner of four colors). The state of being electrostatically adsorbed. Also, (△)
Indicates that the adsorption effect is small, and the transfer sheet P is electrostatically adsorbed to the transfer drum 11 during four rotations of the transfer drum 11, but the front end or the rear end of the transfer sheet P is floating. Say. Furthermore, (x) means that there is no adsorption effect, and while the transfer drum 11 rotates four times,
A state in which the transfer sheet P is peeled off from the transfer drum 11.

From the results shown in Table 2, the nip width was 0.5.
By setting the transfer sheet P within the range of 5.0 mm to 5.0 mm, the transfer sheet P is transferred during the four rotations of the transfer drum 11.
It can be seen that it can be electrostatically adsorbed on 1. That is,
The nip width within the range of 0.5 mm to 5.0 mm is suitable for mechanical strength (mechanical strength), and 1.0 mm
Within the range of up to 4.0 mm, the mechanical strength (mechanical strength) is most suitable. When the nip width is smaller than 0.5 mm, the conductive roller 12 does not follow the transfer drum 11, and the transfer sheet P cannot be stably adsorbed and conveyed while the transfer drum 11 rotates four times. Not preferable. On the other hand, if the nip width is larger than 5.0 mm,
The nip pressure is too strong, and the transfer sheet P is curled in the opposite direction (not along the transfer drum 11). As a result, the transfer sheet P cannot be stably electrostatically adsorbed on the transfer drum 11, which is not preferable.

As described above, for the nip time, if the rotation speed of the transfer drum 11 is constant, the hardness of the semiconductive layer 27 and / or the contact pressure between the transfer drum 11 and the conductive roller 12 should be changed. Can be easily changed. On the other hand, the nip time can be adjusted by keeping the nip width constant and changing the rotation speed of the transfer drum 11. However, when changing the nip time according to the rotation speed of the transfer drum 11, it is necessary to slow down the rotation speed of the transfer drum 11 in order to increase the nip time. When the rotation speed of the transfer drum 11 is slowed in this way, the transfer efficiency per minute decreases. From the above, it is more preferable to change the nip time by adjusting the hardness of the semiconductive layer 27 and / or the contact pressure between the transfer drum 11 and the conductive roller 12.

Here, the relationship between the type of the transfer sheet P and the charge amount of the transfer sheet P during the nip time will be described below with reference to FIGS. 13 to 16.

FIG. 13 shows the charge injection mechanism after the Paschen discharge, and the charge injection corresponds to the charge being accumulated in the capacitor by the current flowing through the circuit. That is, E is from the power supply section 32 to the conductor layer 26.
R1 represents the resistance of the semiconductive layer 27, r2 represents the resistance of the dielectric layer 28, r3 represents the resistance of the transfer sheet P, and r4 represents the transfer between the conductive roller 12 and the conductive roller 12. It represents the resistance of the nip with the drum 11. Also, C
1 represents the capacitance of the dielectric layer 28, C2 represents the capacitance of the transfer sheet P, and C3 represents the capacitance of the nip between the conductive roller 12 and the transfer drum 11.

Here, the amount of charge (potential) accumulated in C2
In order to obtain the above, the above-mentioned equivalent circuit was solved for the potential difference applied to C2 with the amount of electric charge (electric potential) charged by Paschen discharge as the initial potential, and the charging potential considering both Paschen discharge and charge injection was obtained. The analytical expression of the final charging potential (V2) of the transfer paper P thus obtained is

[0083]

[Equation 1]

It is represented by Note that α, β, γ, B, and C represent constants depending on the circuit.

Here, the resistance value (volume resistivity) of the semiconductive layer 27 is 10 ⁷ Ω · cm, the resistance value (volume resistivity) of the dielectric layer 28 is 10 ⁹ Ω · cm, and the applied voltage is 3.0 KV. The transfer sheet P was used as a paper, and the relationship between the nip time and the charge potential (charge amount) of the transfer sheet P, which was obtained based on the above-described analytical expression, was calculated.
As a result, as shown in FIG. 14, it was found that the charge amount of the transfer sheet P has a maximum value as the nip time changes.

Here, the rotational speed of the transfer drum 11 is set to 85
mm / sec, assuming that the nip width formed between the transfer drum 11 and the conductive roller 12 is 4 mm, the nip time is
It will be 0.047 seconds. From the result of FIG. 14, the charge amount of the transfer sheet P at the nip time of 0.047 seconds (-1
740 V) is smaller than the initial charge amount (-1800 V), the electrostatic attraction force of the transfer sheet P is
It turns out to be weak.

In this case, in order to set the charged charge amount after the charge injection is not lower than at least the initial charged charge amount, the nip width is reduced (for example, 3 mm) or the transfer drum 11 is rotated. It is conceivable to adjust the nip time by increasing the speed (for example, 95 mm / sec). Further, in order to perform the charge injection efficiently, the nip width is set to 0.85 mm so that the charge injection is performed when the charge amount of the transfer sheet P reaches the maximum value (nip time 0.01 seconds). Set or
It is conceivable to set the rotation speed of the transfer drum 11 to 300 mm / sec. Thus, the electrostatically optimum nip width is calculated from the relationship between the nip time and the charge injection amount during the nip time, and the electrostatically optimum nip width and the optimum nip width in the mechanical strength are calculated. By setting the nip width taking both factors into consideration as the optimum nip width, it is possible to set the nip time that allows efficient charge injection.

From the above, when the charge amount of the transfer sheet P has a maximum value due to the change of the nip time, the nip time is set so that the charge amount of the transfer sheet P does not become lower than the initial charge amount. By doing so, the transfer sheet P can be stably electrostatically adsorbed onto the dielectric layer 28 of the transfer drum 11. Further, by setting the maximum value as the nip passage time, it is possible to more efficiently inject charges and to charge the transfer sheet P more efficiently. As a result, the transfer sheet P can be more stably electrostatically adsorbed on the dielectric layer 28. As a result, before all the toner images of the respective colors formed on the photoconductor drum 15 are transferred to the transfer sheet P, the transfer sheet P does not peel off from the transfer drum 11 and the photoconductor drum 1
Good toner transfer from No. 5 to the transfer sheet P can be performed. Therefore, a stable image can always be supplied.

Further, except that the transfer sheet P was changed from paper to a synthetic resin sheet for OHP, the same conditions (resistance value (volume resistivity) of the semiconductive layer 27, 10 ⁷ Ω · cm, dielectric layer 28) were used. The resistance value (volume resistivity) of 10 ⁹ Ω · cm, applied voltage of 3.0 KV), and the relationship between the nip time and the charge potential obtained by the amount of charge injection at the nip time based on the above-mentioned analytical formula were graphed. However, the results shown in FIG. 15 were obtained.

As a result, it can be seen that when the synthetic resin sheet for OHP is used as the transfer sheet P, the charge amount of the transfer sheet P tends to increase as the nip time increases. That is, the set nip time is set to, for example, the mechanical nip condition shown in Table 1 or Table 2 (the hardness of the semiconductor layer 27 is set to 20 to 80 in Asker C, or
Alternatively, if the nip width between the transfer drum 11 and the conductive roller 12 is set to 0.5 mm to 5.0 mm), charge injection is always performed. here,
Table 3 shows the relationship between the charge potential difference of the transfer sheet P after the charge injection with respect to that before the charge injection, the adsorption effect of the transfer sheet P, and the printing efficiency.

[0091]

[Table 3]

In Table 3, (◯) indicates that the adsorption effect is large and the printing efficiency is good. here,
The large adsorption effect means a state in which the transfer sheet P is stably electrostatically adsorbed to the transfer drum 11 while the transfer drum 11 rotates four times (transfers toner of four colors). Also,
(X) means that there is no adsorption effect or the printing efficiency is poor. Here, having no suction effect means a state in which the transfer sheet P is peeled off from the transfer drum 11 while the transfer drum 11 rotates four times.

From the results shown in Table 3, 1 before and after the charge injection.
It is understood that when a potential difference exceeding 000 V is provided, the attraction force of the transfer sheet P is weakened and the transfer sheet P is peeled off from the transfer drum 11 while the transfer drum 11 rotates four times. The reason for this is that in order to increase the charge application amount,
For example, by increasing the nip width and increasing the nip time, the nip pressure between the transfer drum 11 and the conductive roller 12 is increased, and the transfer sheet P is moved in the opposite direction to the transfer drum 11 (along the transfer drum 11). There is a mechanical reason such as curling. On the other hand, the nip time can be increased by slowing the process speed, that is, slowing down the rotation speed of the transfer drum 11 while keeping the nip width unchanged in order to increase the amount of charge application. In this case, since the process speed that can give the amount of charge application enough to generate a potential difference exceeding 1000 V is too slow, the printing efficiency per minute becomes poor. From the above, the potential difference before and after the charge injection is 0V ± 1000V.
The range of is most suitable.

Therefore, as described above, when the charge amount of the transfer sheet P increases as the nip time increases, the nip time is set to the charge potential of the transfer sheet P before the charge injection and the charge potential after the charge injection. The transfer sheet P can be stably electrostatically adsorbed on the dielectric layer 28 by setting the difference between the transfer sheet P and the transfer layer P within the range of 0 V ± 1000 V.
This allows the transfer sheet P to be transferred from the photosensitive drum 15 to the transfer sheet P without being peeled off from the transfer drum 11 before all the toner images of the respective colors formed on the photosensitive drum 15 are transferred to the transfer sheet P. Good toner transfer can be performed. Therefore, a stable image can always be supplied.

Furthermore, the resistance value (volume resistivity) of the semiconductive layer 27 and the resistance value (volume resistivity) of the dielectric layer 28 are increased (resistance value of the semiconductive layer 27 (volume resistivity) 10 ⁹ Ω.・
cm, resistance value of the dielectric layer 28 (volume resistivity) 10 ¹⁰ Ω
16), the applied voltage was set to 3.0 KV, the transfer sheet P was used as paper, and the relationship between the nip time and the charging potential obtained by the charge injection amount at the nip time based on the above-described analytical expression was plotted. The results shown in are obtained.

As a result, when the transfer sheet P is paper, if the resistance values of the semiconductive layer 27 and the conductive layer 28 are increased, no charge injection is performed after passing through the nip width, and the transfer sheet P is charged. It can be seen that the amount tends to decrease from the initial charge amount as the nip time increases. Here, Table 4 shows the relationship between the charge potential ratio after charge injection with respect to before charge injection and the adsorption effect of the transfer sheet P.

[0097]

[Table 4]

In Table 4, (◯) indicates that the adsorption effect is large, and the transfer sheet P is stably attached to the transfer drum 11 during the four rotations of the transfer drum 11 (transferring toner of four colors). The state of being electrostatically adsorbed. Also, (×)
Means that there is no adsorption effect, and the transfer drum 11 has 4
The state in which the transfer sheet P is peeled off from the transfer drum 11 during rotation.

From the results shown in Table 4, if the charging potential (charged charge amount) ratio after charge injection to that before charge injection is 50% or more of the initial potential (initial charged charge amount), the transfer drum 11 rotates four times. During transfer, the transfer sheet P is transferred to the transfer drum 11
It can be seen that it can be stably electrostatically adsorbed on top.

From the above, when the charge amount of the transfer sheet P tends to decrease from the initial charge amount as the nip time becomes longer, the nip time becomes less than that of the machine shown in Table 1 or Table 2, for example. Nip condition (semiconductor layer 2
The hardness of No. 7 is set to 20 to 80 by Asker C, or the nip width between the transfer drum 11 and the conductive roller 12 is set to 0.5 mm to 5.0 mm),
The charge amount of the transfer sheet P is 50% of the initial charge amount.
If the nip time is set as described above, the transfer sheet P can be stably electrostatically adsorbed onto the transfer drum 11. In this case, the above mechanical nip conditions are satisfied,
The charge amount of the transfer sheet P is 50% of the initial charge amount.
In order to achieve the above, for example, the nip width is set to 0.85 mm, or the rotation speed of the transfer drum 11 is set to 300 mm / sec to set the nip time to 0.
It may be set to 01 seconds.

Therefore, as described above, when the charge amount of the transfer sheet P decreases from the initial charge amount as the nip time becomes longer, the charge amount of the transfer sheet P becomes smaller than the initial charge amount. By setting the nip time to be 50% or more, the transfer sheet P can be stably electrostatically adsorbed on the dielectric layer 28. This allows
Before all the toner images of the respective colors formed on the photoconductor drum 15 are transferred to the transfer sheet P, the transfer sheet P is not peeled off from the transfer drum 11 and good transfer from the photoconductor drum 15 to the transfer sheet P is performed. Toner transfer can be performed. Therefore, a stable image can always be supplied.

The type of the transfer sheet P, the semiconductive layer 2
No. 7, the physical property value (volume resistivity), the physical property value (volume resistivity) of the dielectric layer 28, and the applied voltage were changed, and experiments were conducted. The relationship between the nip time and the charging potential of the transfer sheet P was found. It can be confirmed that the tendency of the graph shown in (1) corresponds to any of the graphs shown in FIGS.

As described above, the relationship between the nip time and the charge amount of the transfer sheet P is as follows: the physical properties (resistance) of the semiconductive layer 27, the physical properties (resistance) of the dielectric layer 28, the applied voltage, or the transfer sheet P. 3 types of patterns (a pattern in which the charge amount of the transfer sheet P has a maximum value as the nip time changes, and a pattern in which the charge amount of the transfer sheet P increases as the nip time increases). , And a pattern in which the charge amount of the transfer sheet P decreases as the nip time becomes longer).

Therefore, in advance, for each type of transfer sheet P,
If the relationship between the charge amount of the transfer sheet P and the nip time when the optional semiconductive layer 27, the dielectric layer 28, or the like is used, the physical properties (resistance) of the semiconductive layer 27, the dielectric Even if the physical properties (resistance) of the layer 28, the applied voltage, or the type of the transfer sheet P is changed, it is necessary to stably electrostatically attract the transfer sheet P onto the dielectric layer 28 depending on the type of the transfer sheet P used. It is possible to easily determine how to change the nip time.

Further, if the relationship between the nip time and the charge amount of the transfer sheet P is obtained in advance depending on the type of the transfer sheet P, the nip time can be changed according to the type of the transfer sheet P used. Can be changed to a suitable nip time that can efficiently provide the amount of electric charge for stably electrostatically adsorbing the electric charges on the dielectric layer 28. Furthermore, like this,
By changing the nip time based on the relationship between the charge amount of the transfer sheet P used and the nip time, the transfer sheet P
Can be stably electrostatically adsorbed on the dielectric layer 28.

As described above, by changing the nip time according to the type of the transfer sheet P detected by the transfer sheet detection sensor 33, the charge injection can be efficiently performed. Thereby, the transfer sheet P can be stably electrostatically adsorbed onto the transfer drum 11.

The means for detecting the type of the transfer sheet P is not particularly limited, and what is used to determine the type of the transfer sheet P is not particularly limited. The type of the transfer sheet P can be visually judged by the user, and the user can perform the process of changing the nip means based on the result. However, the type of the transfer sheet P can be detected (for example, a transfer sheet). The detection sensor 33) is used to detect the type of the transfer sheet P, and based on the relationship between the nip time stored in the storage means in advance and the charge amount of the transfer sheet P, for example, the control of the eccentric cam 34 in the previous period. Thus, by changing the contact pressure between the transfer drum 11 and the conductive roller 12, the nip time can be automatically changed so that the transfer sheet P can be stably electrostatically attracted to the transfer drum 11. .

Here, the image forming process in the image forming apparatus having the above configuration will be described with reference to FIGS.
Will be described below with reference to.

First, as shown in FIG. 2, in the case of automatic sheet feeding, the transfer sheet P (see FIG. 5) is picked up by the pickup roller 7 in order from the top by the sheet feeding cassette 5 provided at the bottom of the main body. The sheets are sent to the PF roller 8 one by one. Next, the transfer sheet P passing through the PF roller 8 is curled by the pre-curl roller 10 along the shape of the transfer drum 11.

On the other hand, in the case of manual paper feeding, when the transfer sheets P are sent out one by one from the manual feed section 6 provided on the front surface of the main body, the transfer sheets P are conveyed to the pre-curl roller 10 by the manual feed roller 9. Then, the transfer sheet P is
The pre-curl roller 10 curls along the shape of the transfer drum 11.

Next, the transfer sheet P curled by the pre-curl roller 10 is conveyed between the transfer drum 11 and the conductive roller 12, as shown in FIG. Then
The charge accumulated in the semiconductive layer 27 of the transfer drum 11 is
Electric charges are induced on the surface of the transfer sheet P by moving through the surface of the semiconductive layer 27 and the inner surface of the transfer sheet P.
As a result, the transfer sheet P is electrostatically attracted to the surface of the transfer drum 11.

After that, the transfer sheet P attracted to the transfer drum 11 is conveyed to the transfer point X, which is a pressure contact portion between the transfer drum 11 and the photosensitive drum 15, as shown in FIG. Transfer sheet P due to the potential difference between the charge of the toner formed on the transfer sheet and the charge on the surface of transfer sheet P.
The above toner image is transferred to.

At this time, the photosensitive drum 15 is charged, exposed, developed and transferred for each color. Therefore, the transfer sheet P rotates on the transfer drum 11 while being attracted to the transfer drum 11, and one color transfer is performed every one rotation, and one full-color image is obtained by a maximum of four rotations. There is. However, when a monochrome image or a monocolor image is obtained, the transfer drum 11 may be rotated once.

Further, when all the toner images are transferred onto the transfer sheet P, the transfer sheet P is forced from the surface of the transfer drum 11 by the peeling claws 14 provided on the circumference of the transfer drum 11 so as to be separable from and in contact with the transfer drum 11. Are peeled off and guided to the fixing guide 22.

After that, the toner image on the transfer sheet P is guided to the fixing roller 23 by the fixing guide 22, and is fused and fixed on the transfer sheet P by the temperature and pressure of the fixing roller 23.

Then, the fixed transfer sheet P is discharged onto the discharge tray 25 by the discharge roller 24.

As described above, the transfer drum 11 is
The conductive layer 26 made of aluminum, the semiconductive layer 27 made of urethane foam, and the dielectric layer 28 made of PVDF are formed from the inside. Thereby, the conductor layer 2
By applying a voltage to 6, the electric charge is sequentially induced from the conductor layer 26, and the electric charge is accumulated in the semi-conductor layer 27. Then, when the transfer sheet P is conveyed between the transfer drum 11 and the conductive roller 12, the charges accumulated in the semiconductive layer 27 move to the transfer sheet P, and the transfer sheet P is transferred. 11 is electrostatically adsorbed.

Therefore, since the transfer sheet P is attracted and transferred by inducing electric charges instead of the attraction and transfer of the transfer sheet P by injecting charges by air discharge as in the conventional case, the voltage can be low and the voltage can be reduced. Easy to control. In addition, it is possible to eliminate variations in voltage due to external pressure.

As a result, the voltage applied to the transfer drum 11 can be kept constant without being affected by the environment such as humidity and temperature, thus improving the transfer efficiency,
The image quality can be improved.

Further, the surface of the transfer drum 11 can be charged more stably than in the conventional case where the surface of the transfer drum 11 is charged by inducing an electric charge by electric discharge, so that the transfer sheet P is attracted and absorbed. The transfer can be performed stably.

Moreover, by simply applying a voltage to the conductor layer 26, charges can be induced in the order of the semi-conductor layer 27 and the dielectric layer 28 to charge the surface of the transfer drum 11, so that it is possible as in the conventional case. Compared with the case where the surface of the transfer drum 11 is charged by air discharge, a lower voltage is required, so that voltage control becomes easier and less driving energy is required.

Further, since it is only necessary to apply a voltage to one place, it is not necessary to apply a voltage to each charger as in the conventional case, the apparatus can be simplified and the manufacturing cost can be reduced. It can be anything.

Further, since the transfer drum 11 is charged by contact charging, the electric field region does not change even if the surface of the transfer drum 11 is flawed, so that the electric field balance is disturbed at the flawed portion of the surface of the transfer drum 11. There is no. Thereby, the transfer efficiency can be improved.

Further, unlike air discharge, it is less affected by the environment such as temperature and humidity of the air, so that the surface potential of the transfer drum 11 does not vary, and the adsorption failure of the transfer sheet P and print bleeding can be eliminated. it can. Also by this, the transfer efficiency can be improved and the image quality can be improved.

Further, since the amount of electric charge that the transfer sheet P is charged within a fixed time varies depending on the type of the transfer sheet P, the nip time is changed depending on the type of the transfer sheet P, so that the transfer sheet P is transferred. 11 can be stably electrostatically adsorbed on the surface of the substrate 11.

At this time, the relationship between the charge amount of the transfer sheet P and the nip time when an arbitrary semiconductive layer 27, dielectric layer 28 or the like is used can be obtained in advance for each type of the transfer sheet P. For example, the physical properties (resistance) of the semiconductive layer 27,
Even if the physical properties (resistance) of the dielectric layer 28, the applied voltage, or the type of the transfer sheet P changes, the transfer sheet P is stably electrostatically adsorbed on the dielectric layer 28 depending on the type of the transfer sheet P used. It is possible to easily judge how to change the nip time.

Further, if the relationship between the nip time and the charge amount of the transfer sheet P is obtained in advance according to the type of the transfer sheet P, the nip time can be changed according to the type of the transfer sheet P used. Can be changed to a suitable nip time that can efficiently provide the amount of electric charge for stably electrostatically adsorbing the electric charges on the dielectric layer 28. Furthermore, like this,
By changing the nip time based on the relationship between the charge amount of the transfer sheet P used and the nip time, the transfer sheet P
Can be stably electrostatically adsorbed on the dielectric layer 28.

As a result, before the toner images of the respective colors formed on the photosensitive drum 15 are all transferred to the transfer sheet P, the transfer sheet P is transferred from the photosensitive drum 15 without being peeled off from the transfer drum 11. Good toner transfer to the sheet P can be performed. Therefore, a stable image can always be supplied.

The nip time is the same as the transfer drum 11
It can be easily adjusted by changing the nip width formed between the conductive roller 12 and the conductive roller 12, or the rotation speed of the transfer drum 11.

The nip width can be easily changed by changing the hardness of the semiconductive layer 27. That is, the nip time can be easily adjusted by changing the hardness of the semiconductor layer 27. At this time, the transfer sheet P can be stably electrostatically adsorbed on the dielectric layer 28 by setting the hardness of the semiconductive layer 27 within the range of 20 to 80 by Asker C.

The nip width can be easily changed by adjusting the contact pressure between the transfer drum 11 and the conductive roller 12. That is, the nip time can be easily changed by adjusting the contact pressure between the transfer drum 11 and the conductive roller 12. Transfer drum 11 and conductive roller 1
The contact pressure with 2 can be easily adjusted by using an eccentric cam 34 as shown in FIGS. 11 and 12, for example.

At this time, the nip time is preferably adjusted so that the nip width is within the range of 0.5 mm to 5.0 mm. The nip width is 0.5 mm
The transfer sheet P can be stably electrostatically adsorbed on the dielectric layer 28 by adjusting the transfer sheet P to be within the range of 5.0 mm.

As described above, the nip time is adjusted not for the rotation speed of the transfer drum 11, but for the hardness of the semiconductive layer 27 and / or the contact pressure between the transfer drum 11 and the conductive roller 12. Therefore, the nip time can be changed without lowering the transfer efficiency.

Incidentally, instead of the transfer drum 11 shown in FIG.
As shown in FIG. 7, another transfer drum 41 having the semiconductive layer 27 and the dielectric layer 28 may be used. In the transfer drum 41, instead of the conductor layer 26 of the transfer drum 11,
Cylindrical base material (base layer) made of resin having a conductive thin film layer 43 such as copper foil or aluminum foil provided on the surface
42, and a semiconductive layer 27 and a dielectric layer 28 are sequentially provided on the upper surface of the thin film layer 43.

By connecting the power source section 32 to the thin film layer 43 and applying a voltage, it is possible to stably induce an electric charge on the surface of the dielectric layer 28, similarly to the transfer drum 11. Adsorption of the transfer sheet P to the transfer drum 41 and transfer of the toner image can be stably performed.

As described above, the base material 42 at the center of the transfer drum 41 is formed of resin, and a conductor such as a thin copper foil is provided on the surface of the base material 42, so that the conductor layer 26 of the transfer drum 11 serves as a conductor layer. The manufacturing cost can be reduced as compared with the case of using.

Further, the semiconductive layer 27 and the dielectric layer 2 are formed.
As another transfer drum having 8, the transfer drum 51 shown in FIG. 18 may be used. The transfer drum 51 uses the base material 42 of the transfer drum 41 as a central base material, a semiconductive elastic layer 52 is provided on the surface of the base material 42, and the upper surface of the elastic layer 52 is further provided. As shown in FIGS. 19 and 20, a non-continuous electrode layer (conductor layer) 53 in which a plurality of conductor plates (conductive members) 53a ... Has been formed.

Further, on the upper surface of the electrode layer 53, a semiconductive layer 27 and a dielectric layer 28 are sequentially provided.

Further, by connecting the power source section 32 to the electrode layer 53 and applying a voltage thereto, like the transfer drum 11 and the transfer drum 41, a stable charge is induced on the surface of the dielectric layer 28. Thus, the transfer sheet P can be attracted to the transfer drum 41 and the toner image can be transferred stably.

In this case, the same effect can be obtained by connecting the power source section 32 to the semiconductive layer 27 and applying a voltage.

In the transfer drum 51 having the above structure, the electrode layer 5
3 is a conductor plate 5 discontinuously arranged on the elastic layer 52.
3a ..., the voltage drop in the electrode layer 53 occurs only when the vicinity of the grounded conductive roller 12 is approached, and at other times, the conductor plates 53a do not contact each other. Since it is continuous, there is no movement of charges between the conductor plates 53a, and as a result, no voltage drop occurs.

Therefore, since the voltage drop at the transfer point X can be eliminated, the transfer failure can be eliminated, the transfer efficiency can be improved, and the image quality can be improved.

Further, as described above, since the electrode layer 53 as a conductor layer is merely provided on the surface of the base material 42 with the conductor plates 53a at regular intervals as described above, the transfer drum 51 of The cost of manufacturing can be reduced,
This can reduce the manufacturing cost of the entire device.

[0144]

The image forming apparatus according to the invention of claim 1 is
As described above, the image carrier has a toner image formed on the surface thereof, and the transfer unit transfers the toner image formed on the image carrier by bringing the transfer sheet into contact with the image carrier. In the image forming apparatus, the transfer means is
A dielectric layer, a semiconductive layer, and a conductive layer are laminated in this order from the contact surface side of the transfer sheet, and voltage applying means for applying a predetermined voltage to the conductive layer, and a transfer position on the surface of the dielectric layer. The surface of the transfer sheet has a grounded electrode member that is in contact with the transfer sheet via the transfer sheet, and depending on the type of the transfer sheet, the contact portion between the transfer unit and the electrode member may be located at any position. This is a configuration for changing the passing nip time.

Therefore, since the transfer sheet is attracted and transferred by inducing the electric charge instead of adsorbing and transferring the transfer sheet by injecting charge by air discharge as in the conventional case, the voltage can be low and the voltage Easy to control. In addition, it is possible to eliminate variations in voltage due to external factors. From this, the voltage applied to the transfer means is
Since the voltage can be kept constant without being affected by the environment such as temperature, it is possible to improve the transfer efficiency and the image quality.

Further, if the type of the transfer sheet is different, the amount of charge charged on the transfer sheet within the nip time is different. Therefore, the charge amount of the transfer sheet can be adjusted by changing the nip time depending on the type of the transfer sheet. Thereby, no matter what kind of transfer sheet is used, the transfer sheet can be stably electrostatically adsorbed on the dielectric layer of the transfer means. From this, the transfer sheet does not separate from the transfer means before all the toner images of the respective colors formed on the image carrier are transferred to the transfer sheet, and good toner transfer from the image carrier to the transfer sheet is achieved. It can be carried out. Therefore, there is an effect that the image can always be stably supplied.

As described above, in the image forming apparatus according to the second aspect of the present invention, in addition to the configuration of the first aspect, the nip time is the nip time previously obtained according to the type of the transfer sheet and the charging of the transfer sheet. The configuration is changed based on the relationship with the charge amount.

Therefore, in addition to the effect of the first aspect, it is preferable that the nip time can efficiently provide the charge amount for stably adsorbing the transfer sheet to the transfer means depending on the type of the transfer sheet used. You can change the nip time. Further, depending on the type of transfer sheet used, it is possible to easily determine how to change the nip time in order to stably attract the transfer sheet onto the dielectric layer of the transfer means.

The snails and the transfer sheet have different physical values such as resistance values depending on the type of the transfer sheet.
Further, the charge amount of the transfer sheet differs not only according to the physical properties of the transfer sheet, but also depending on the physical properties (resistance) of the semiconductive layer, the physical properties (resistance) of the dielectric layer, or the applied voltage. However, the relationship between the nip time and the charge amount of the transfer sheet is, for example, the resistance of the semiconductive layer, the resistance of the dielectric layer,
Even if conditions such as applied voltage and type of transfer sheet change, 3
It is roughly divided into patterns. Therefore, if the relationship between the charge amount of the transfer sheet and the nip time in the case of using an arbitrary semiconductive layer or dielectric layer for each type of transfer sheet is calculated in advance, for example, the semiconductive layer Even if the resistance, the resistance of the dielectric layer, or the type of the transfer sheet is changed, the transfer sheet can be efficiently charged simply by determining the type of the transfer sheet to be used, depending on which pattern the transfer sheet to be used belongs to. The nip time can be easily determined. Further, by changing the nip time based on the relationship between the charge amount of the transfer sheet used and the nip time as described above, the charge amount for adsorbing the transfer sheet on the dielectric layer of the transfer unit can be efficiently obtained. In addition, the effect that the transfer sheet can be stably electrostatically adsorbed on the dielectric layer is also obtained.

As described above, in the image forming apparatus according to the invention of claim 3, in addition to the structure of claims 1 and 2, the nip time is set to the hardness of the semiconductive layer and / or the transfer means. The configuration is changed by adjusting the contact pressure between the electrode members.

Therefore, in addition to the effect of claim 1 or 2, by adjusting the hardness of the semiconductive layer and / or the contact pressure between the transfer means and the electrode member, the transfer efficiency is not lowered. In addition, the nip time can be easily changed.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram in the vicinity of a transfer drum included in an image forming apparatus according to an exemplary embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of an image forming apparatus including the transfer drum shown in FIG.

FIG. 3 is a conductor layer that constitutes the transfer drum shown in FIG.
It is explanatory drawing which shows the connection state with a sheet-shaped semiconductive layer and a dielectric layer.

FIG. 4 is a conductor layer forming the transfer drum shown in FIG.
It is another explanatory view showing a connection state with a semiconductive layer and a dielectric layer.

5 is an explanatory diagram showing a charged state of the transfer drum shown in FIG. 1, and is an explanatory diagram showing an initial state in which a transfer sheet is conveyed to the transfer drum.

6 is an explanatory diagram showing a charged state of the transfer drum shown in FIG. 1, and is an explanatory diagram showing a state in which a transfer sheet is conveyed to a transfer position of the transfer drum.

FIG. 7 is an explanatory diagram showing a comparison between the charging width of the transfer drum shown in FIG. 1 and the effective image width.

8 is an explanatory view showing charge transfer between the transfer drum and the photosensitive drum shown in FIG. 1, and showing charge transfer when the width of each layer of the transfer drum is such that dielectric layer <semiconductor layer <conductor layer. Is.

9 is an explanatory view showing charge transfer between the transfer drum and the photosensitive drum shown in FIG. 1, showing charge transfer when the width of each layer of the transfer drum is such that semiconductive layer <dielectric layer = conductive layer. Is.

10 is an explanatory diagram showing Paschen discharge in a close contact portion between the transfer drum and the conductive roller shown in FIG.

FIG. 11 is an explanatory diagram showing a configuration made to change the contact pressure between the transfer drum and the conductive roller shown in FIG.

FIG. 12 is an explanatory view showing a configuration for changing a contact pressure between the transfer drum and the conductive roller shown in FIG. 1 from a side surface of the conductive roller.

13 is a circuit diagram showing an equivalent circuit of a charge injection mechanism between the transfer drum and the conductive roller shown in FIG.

FIG. 14 is a graph showing a relationship between a charge amount of a transfer sheet and a nip time.

FIG. 15 is a graph showing the relationship between the amount of charge on the transfer sheet and the nip time when the conditions are different from those in FIG.

FIG. 16 is a graph showing the relationship between the charge amount of the transfer sheet and the nip time when the condition is further different from that of FIG.

FIG. 17 is a schematic configuration diagram of the vicinity of another transfer drum provided in the image forming apparatus according to the present invention.

FIG. 18 is a schematic configuration diagram in the vicinity of still another transfer drum provided in the image forming apparatus according to the present invention.

FIG. 19 is an explanatory view schematically showing the structure of the electrode layer of the transfer drum shown in FIG.

20 is a perspective view showing a structure of an electrode layer of the transfer drum shown in FIG.

FIG. 21 is a schematic configuration diagram of a conventional image forming apparatus.

FIG. 22 is a schematic configuration diagram of another conventional image forming apparatus.

[Explanation of symbols]

 11 Transfer Drum (Transfer Means) 12 Conductive Roller (Electrode Member) 15 Photoreceptor Drum (Image Carrier) 26 Conductor Layer 27 Semiconductive Layer 28 Dielectric Layer 32 Power Supply Section (Voltage Applying Means) 33 Transfer Sheet Detection Sensor P Transfer sheet X Transfer point (transfer position)

Claims (3)

[Claims] 1. An image carrier on which a toner image is formed,
In an image forming apparatus having a transfer unit that transfers a toner image formed on the image carrier by bringing the transfer sheet into contact with the image carrier, the transfer unit is provided from the contact surface side of the transfer sheet. , A dielectric layer, a semiconductive layer, and a conductive layer are sequentially stacked, and a voltage applying means for applying a predetermined voltage to the conductive layer, and a transfer position on the upstream side from the transfer position of the dielectric layer surface. And a grounded electrode member contacting through the sheet, and changing the nip time for any position of the transfer sheet to pass through the close contact portion between the transfer means and the electrode member depending on the type of the transfer sheet. An image forming apparatus characterized by. 2. The image forming apparatus according to claim 1, wherein the nip time is changed based on the relationship between the nip time previously obtained by the type of the transfer sheet and the charge amount of the transfer sheet. 3. The nip time is changed by adjusting the hardness of the semiconductive layer and / or the contact pressure between the transfer means and the electrode member. Image forming apparatus.