JPH06130839A - Image forming device - Google Patents

Abstract

PURPOSE:To surely prevent a middle omission phenomenon from occurring even to an OHP sheet and to prevent ozone from being produced by setting a toner image carrier and a roller type transfer device in a non-contact state. CONSTITUTION:An electrostatic latent image formed on the surface of a photosensitive drum 1 after the surface of the drum 1 is uniformly electrostatically charged by an electrostatic charging roller connected to an AC power source biased to a negative polarity is developed with negatively electrostatically charged toner by a developing device using magnetic one-component toner, and then a toner image is transferred on a transfer material 6 by a transfer roller 5. The transfer roller 5 is constituted by coating the periphery of a metallic core bar 13 with an elastic layer 14 consisting of semiconducting foaming urethane rubber. Both ends of the core bar 13 of the transfer roller 5 are pivotally supported by a tracking roll 15, and a gap between the roller 5 and the photosensitive drum 1 is held to 0.1-0.3mm by butting the tracking rolls 15 and 15 against the drum 1.

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 such as an electrophotographic copying machine or a laser beam printer, and more particularly to an apparatus for transferring a toner image from a toner image carrier to a transfer material using a roller type transfer device. And at least OHP
The present invention relates to an image forming apparatus capable of transferring a toner image in a good state onto a transfer material including a sheet (sheet for an overhead projector).

[0002]

2. Description of the Related Art Conventionally, as an image forming apparatus such as the above-mentioned electrophotographic copying apparatus, an elastic roller which rotates via a transfer material is pressed against a toner image carrier, and the elastic roller is connected to the toner roller to determine the polarity of the toner. It is known that a bias voltage of reverse polarity is applied to transfer a toner image from a toner image carrier onto a transfer material.

Since this type of image forming apparatus does not use a corona discharger such as a corotron as a transfer means, it produces less ozone and conveys the transfer material in a state of being sandwiched between a toner image carrier and an elastic roller. At the same time, since the toner image is transferred onto the transfer material, there is an advantage that it is possible to prevent an image shift due to the waviness of the transfer material or the discrepancy between the transfer material conveying speed of the transfer material conveying roller and the peripheral speed of the toner image carrier. is there. Therefore, in the above-described image forming apparatus, by combining a biased elastic roller that also generates a small amount of ozone as a charging unit that primarily charges the toner image bearing member, it becomes possible to eliminate or greatly reduce ozone countermeasures. Therefore, in recent years, it has been used as a small printer.

However, in the case of the above-mentioned image forming apparatus, the transfer material is pressed against the toner image carrier by the elastic roller,
Since the toner image is transferred from the toner image carrier onto the transfer material, there is a problem that a phenomenon called "middle void" occurs in which a transfer failure occurs inside a character or line image. This defective transfer phenomenon called "outer void" is caused by the contact pressure of the transfer material on the toner image carrier concentrated in the central portion of the character or line image, and the toner particles and toner located in the central portion of the character or line image. It is considered that since the adhesive force with the surface of the image bearing member increases, the toner particles located in the central portion of the character or line image are hard to separate from the surface of the toner image bearing member and are generated. In particular, a transfer material having a smooth surface, which is represented by an OHP sheet, has a smooth surface when the transfer material presses the toner image onto the surface of the toner image bearing member. Since the adhesive force is less likely to be dispersed and the adhesive force between the toner particles located in the central portion of the character or line image and the surface of the toner image bearing member is concentrated, the occurrence of transfer failure called "middle void" is remarkable.

Therefore, in order to prevent the occurrence of transfer failure in the image forming apparatus using the roller type transfer device,
Various techniques as shown below have already been proposed.

(1) As disclosed in JP-A-58-142362, JP-A-3-1214621, etc., a method of using a transfer auxiliary agent for weakening the adhesive force between the toner image carrier and the toner. .

(2) A method of providing a speed difference between a toner image carrier and a transfer material, as disclosed in JP-A-2-262175.

(3) A method of stabilizing the contact pressure of the transfer roller weakly, as disclosed in Japanese Patent Laid-Open No. 1-316774.

[0009]

However, the above-mentioned prior art has the following problems. That is, in the case of the device according to the above (1), it is necessary to provide a device for supplying a transfer aid that weakens the adhesive force between the toner image carrier and the toner, or to mix the toner with the transfer aid. Therefore, there is a problem that the size of the device is increased and the running cost is increased.

Further, in the case of the device according to the above-mentioned proposals (2) and (3), there is a problem that the results of the experiments by the present inventors show no effect on the OHP sheet.

That is, the inventors of the present invention prototyped the device according to the above-mentioned proposals (2) and (3), and conducted an experiment for investigating the occurrence of the image dropout phenomenon.

At this time, the contact angle between the photosensitive drum and water becomes large as shown in FIG. 6 by repeatedly charging by using a contact type charging roll as a charging means for primary charging the photosensitive drum. While the phenomenon was confirmed, when the scorotron charger was used as the primary charging means, there was no change in the contact angle.

As a result of examining the adhesive force between the photosensitive drum and the toner, the adhesive force between the photosensitive drum and the toner is shown in FIG.
As shown in, it became larger by repeating charging by the charging roll. From this fact, when the contact type charging roll is used as the primary charging means, the phenomenon of hollow characters and the like is remarkable, especially in the OHP sheet, and the device according to the above (2) and (3) In case of OHP
It was found that the hollow defect cannot be prevented for the sheet.

Therefore, the present invention has been made to solve the above-mentioned problems of the prior art, and its object is to apply an OHP sheet to an OHP sheet even when a contact type charging roll is used as the primary charging means. On the other hand, it is an object of the present invention to provide an image forming apparatus capable of reliably preventing the occurrence of a hollow defect and also preventing ozone from being generated.

[0015]

The above-mentioned technical problems are as follows.
In the image forming apparatus according to the present invention, after the surface of the toner image carrier is primarily charged by the charging roller, an electrostatic latent image is formed and the electrostatic latent image is developed into a toner image. Image formation in which a toner image formed on a body is electrostatically transferred onto a transfer material by using a roller type transfer device which is arranged to face the toner image carrier via a transfer material and to which a predetermined bias voltage is applied. In the apparatus, it is achieved by setting the toner image carrier and the roller type transfer device in a non-contact state. The gap between the toner image carrier and the roller type transfer device is, for example, 0.1 mm or more and 0.3 mm.
It is set below.

At this time, when the transfer material is plain paper, the toner image carrier and the roller type transfer device are brought into contact with each other, and only when the transfer material is an OHP sheet, the toner image carrier and the roller type transfer device are contacted. The gap of the transfer unit may be set to 0.1 mm or more and 0.3 mm or less. Further, only when the transfer material is plain paper, the moving speed of the toner image carrier and the moving speed of the transfer material may be different from each other. Further, the roller type transfer device may be configured to rotate at the same speed as the toner image carrier. As the bias power source, a power source that performs constant current control at least during the transfer operation may be used.

[0017]

The inventors of the present invention carry out primary charging of the surface of the toner image bearing member by the charging roller, form an electrostatic latent image, and develop this electrostatic latent image to form a toner image. Image formation in which a toner image formed on a body is electrostatically transferred onto a transfer material by using a roller type transfer device which is arranged to face the toner image carrier via a transfer material and to which a predetermined bias voltage is applied. In the device, a prototype of the device in which the gap between the toner image carrier and the roller type transfer device is variously changed,
As a result of an experiment for forming an image, the gap between the toner image carrier and the roller type transfer device is 0.1 mm or more and 0.3 mm.
It has been clarified that the transfer failure such as the image dropout can be prevented by setting the following.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to illustrated embodiments.

FIG. 2 shows a laser beam printer as an embodiment of the image forming apparatus according to the present invention.

In the figure, reference numeral 1 denotes a photosensitive drum as a toner image bearing member. As the photosensitive drum 1, for example, negative charging having a diameter of 30 mm rotated in a direction of an arrow by a driving means (not shown) is used. A system consisting of an OPC photoconductor is used. The surface of the photoconductor drum 1 is uniformly charged to −500 V by a charging roller 2 connected to an AC power source biased to a negative polarity, and then is turned on and off based on an image signal from a semiconductor laser. The image is exposed by the laser beam 3 which is emitted and scanned along the axial direction of the photosensitive drum 1 by the polygon mirror which rotates at high speed, and an electrostatic latent image is formed. This image exposure is performed by, for example, image portion exposure in which the laser beam 3 is exposed to the image portion. The electrostatic latent image formed on the surface of the photoconductor drum 1 is developed with a negatively chargeable toner by a developing device 4 using a magnetic one-component toner, and then this toner image is transferred by a transfer roller 5. It is transferred onto the material 6.

The transfer material 6 is supplied from a cassette 7 containing the transfer material 6 by a feeding roller 8 and the photosensitive material drum 1 by transfer material conveying rollers 9 and 10.
Is conveyed to the surface of the photosensitive drum 1 in synchronism with the rotation. Then, the toner image formed on the surface of the photosensitive drum 1 is transferred onto the transfer material 6 by the transfer roller 5, and the transfer material 6 onto which the toner image is transferred is fixed to the fixing device 1.
1, the toner image is fixed on the transfer material 6,
It is discharged to the outside of the machine.

The photosensitive drum 1 after the transfer process is completed.
The surface of the sheet is cleaned of residual toner by the blade cleaner 12 and is ready for the next image forming cycle.

By the way, as shown in FIG. 3, the transfer roller 5 has a semi-conductive (10 ⁵
The transfer roller 5 is formed by covering it with an elastic layer 14 made of urethane foam rubber of 10 ⁹ Ω · cm), and the outer diameter of the transfer roller 5 is set to 16 mm, for example. Further, both ends of the core bar 13 of the transfer roller 5 are axially supported by tracking rolls 15 and 15, and by abutting these tracking rolls 15 and 15 on the photosensitive drum 1, the transfer roller 5 and the photosensitive drum 1 are exposed. The gap between the body drums 1 is maintained at 0.1 mm. Further, the transfer roller 5 is provided on the photosensitive drum 1 by a driving unit (not shown).
It is designed to rotate at a speed equal to the peripheral speed of.

On the other hand, the core metal 13 of the transfer roller 5 has
The transfer material 6 is transferred to the photosensitive drum 1 by the transport rollers 9 and 10.
When the leading end of the transfer material 6 enters the gap at a substantially constant speed, a predetermined bias voltage (for example, +1000 V) is applied by the transfer roller bias power source 16.

With the above arrangement, the laser beam printer according to this embodiment is adapted to transfer a toner image as follows. That is, the surface of the photoconductor drum 1 is uniformly charged to −500 V by the charging roller 2 connected to the AC power source biased to the negative polarity, and then the image is exposed by the laser beam 3 to electrostatic latent image. An image is formed. The electrostatic latent image formed on the surface of the photosensitive drum 1 is developed by the developing device 4 with a negatively chargeable toner, and then the toner image is transferred to the transfer roller 5.
Is transferred onto the transfer material 6.

At this time, on the core metal 13 of the transfer roller 5, the transfer material 6 is introduced into the gap between the photosensitive drum 1 and the transfer roller 5 at substantially the same speed as the photosensitive drum 1 by the conveying rollers 9 and 10. When the tip of the transfer material 6 enters the gap, a predetermined bias voltage (for example, + 1000V) is applied by the transfer roller bias power supply 16. Then, a toner image is transferred from the surface of the photosensitive drum 1 to the transfer material 6 by a transfer electric field formed between the photosensitive drum 1 and the transfer roller 5.
Transcribed on.

In this case, the transfer roller 5 and the photosensitive drum 1
Is set to 0.1 mm, the transfer material 6
In the case of the transfer material 6 having a thickness of 100 μm or less, which is normally used when the toner passes through the gap between the transfer roller 5 and the photosensitive drum 1, the contact pressure does not act on the transfer material 6. There is.

Therefore, the inventors of the present invention prototyped a laser beam printer constructed as described above, conducted an experiment to actually form an image and evaluated the image quality, and clarified the conditions for preventing the hollow defect. .

In the experiment, 20 sheets of A4 size transfer material were printed, and the image of the 20th transfer material 6 was evaluated from the following three viewpoints. Table 1 described below shows the results of this experiment and the following examples and comparative examples.

(1) Missing Characters Missing characters were evaluated by print samples of English characters on an OHP sheet (thickness 100 μm) and two types of plain paper (thickness 84 μm, thickness 108 μm).

Here, ◯ is good, when the character omission does not occur, Δ is practically acceptable, and the character omission occurs slightly, but there is no problem in the image quality in practical use. , ×
Indicates a case where the character is defective, a blank character occurs, and is not suitable for practical use.

(2) Image unevenness Image unevenness was evaluated by printing a gradation pattern on the entire surface of three types of transfer materials similar to the case of hollow characters.

Here, ◯ is good, when the image unevenness does not occur, Δ is practical, and the image unevenness occurs slightly, but there is no problem in the image quality in practical use, and x is defective. In each case, image unevenness occurs, which is not suitable for practical use.

(3) Image deviation The image deviation is evaluated with respect to the printed image in (2) (a phenomenon in which the image being transferred is displaced when the trailing edge of the transfer material leaves the transport roll). did.

Here, ◯ is good, when no image shift occurs, Δ is practical, and some image shift occurs, but there is no problem in image quality in practical use, x is bad. In each case, image misregistration occurs and is inappropriate for practical use.

(Example 2) Tracking rolls 15 and 1
The same as Example 1 except that the diameter of the photosensitive drum 1 was changed and the gap between the photosensitive drum 1 and the transfer roller 5 was changed to 0.2 mm.

(Example 3) Tracking rolls 15 and 1
The same as Example 1 except that the diameter of 5 is changed and the gap between the photosensitive drum 1 and the transfer roller 5 is set to 0.3 mm.

(Embodiment 4) As shown in FIG. 5, the OHP mode in which the gap between the photosensitive drum 1 and the transfer roller 5 is set to 0.2 mm only when the toner image is transferred onto the OHP sheet is shown in FIG. Added to the beam printer.

That is, when the transfer material 6 is plain paper,
As shown in FIG. 5A, the transfer roller 5 is brought into contact with the photosensitive drum 1 by the solenoid 20 via the roller support 21 and the transfer material 6 having a thickness of 100 μm with a force of 48 gf / cm to transfer the transfer bias. Transfer is performed at a voltage of + 500V. On the other hand, when the OHP sheet is selected as the transfer material 6, for example, the OHP print button on the operation panel causes the solenoid 20 to operate in a direction to move the transfer roll 5 away from the photoconductor drum 1, and the photoconductor drum 1 and the transfer roll 5 are separated from each other. The gap is 0.2 mm. Further, the OHP print button is used to transfer the toner image onto the OHP sheet by the transfer bias voltage increased to + 2000V. Of course, for OHP sheet detection,
Providing detection means instead of manual operation of the print button,
The above-mentioned changes may be automatically made.

Table 1 shows the result of the evaluation described in the first embodiment in the laser beam printer according to the second embodiment to which the above OHP mode is added.

Example 5 The same as Example 4 except that the bias power source for the transfer roll 5 was a constant current (for example, 2 μA) type bias power source.

(Embodiment 6) The speeds of the transfer material conveying rolls 9 and 10 and the transfer roller 5 of the laser beam printer to which the OHP mode shown in Embodiment 4 is added are increased by 1.6% only when transferring plain paper. . That is, by increasing the speed of the drive motor (not shown) that rotationally drives the transport rollers 9 and 10 and the transfer roller 5 by 1.6%, the transport speed of the transfer material 6 that was almost the same as that of the photosensitive drum 1 is increased. Then, a speed difference was established between the photosensitive drum 1 and the transfer material 6. Other than that, it is the same as that of the fifth embodiment.

(Comparative Example 1) Tracking rolls 15 and 1
5, the transfer roller 5 is set to 0.1 on the photosensitive drum 1.
Same as Example 1 except that it was bited in by 5 mm.

(Comparative Example 2) Tracking rolls 15 and 1
Same as Example 1 except that the diameter of 5 is changed so that the transfer roller 5 just contacts the photosensitive drum 1.

(Comparative Example 3) Tracking rolls 15 and 1
Same as Example 1 except that the diameter of 5 is changed and the gap between the photosensitive drum 1 and the transfer roller 5 is 0.4 mm.

Comparative Example 4 The same as Example 3 except that the transfer roll 5 was not rotated.

(Comparative Example 5) Same as Comparative Example 1 except that the charging roller 2 is replaced by a scorotron charger.

(Comparative Example 6) In Example 3, the drive motor was controlled so as to increase the speed of the transfer roll 5 and the transport rolls 9 and 10 including the OHP sheet by 1.6%.

Table 1 shows the experimental results of the above Examples and Comparative Examples.

[0050]

[Table 1]

As is clear from Table 1, when the toner image is transferred onto the OHP sheet, the void phenomenon can be prevented by setting the gap between the photosensitive drum 1 and the transfer roll 5 to 0.1 mm or more and 0.3 mm or less. Recognize. On the other hand, as shown in Comparative Example 3, if the gap between the photosensitive drum 1 and the transfer roll 5 is 0.4 mm or more, other image defects occur, which is not practical. Further, when a gap is formed on the plain paper, the gap range that satisfies both the hollow image and other image defects satisfactorily with the OHP sheet becomes narrow, and the gap is formed only when the image is transferred to the OHP sheet. It turns out that the method is the best. At this time, by making a difference between the photosensitive member speed and the transfer material conveying speed only when transferring to plain paper, it is possible to further improve the hollowness of plain paper which is in the practical range, and at the time of transferring to OHP sheet. Also makes it difficult to cause image defects.

Further, as a power source for the transfer device, by performing constant current control at least in the transfer area, it is possible to perform transfer with the same current value regardless of the presence or absence of a gap. Therefore, it is not necessary to switch the power supply output depending on the presence or absence of a gap, and the configuration can be simplified.

Further, according to the present invention, as is clear from the experimental results of Comparative Example 5, when the scorotron charger is used as the primary charger, a hollow defect or the like occurs in the OHP sheet even under the same conditions as the conventional one. However, it can be seen that the effect is more exerted by the combination with the contact type charging roll connected to the biased AC power source as the charger of the photosensitive drum.

The bias voltage applied to the transfer roll 5 may be changed as shown in Table 2 below depending on the type of the transfer material 6 and the gap between the photosensitive drum 1 and the transfer roll 5.

[0055]

[Table 2]

[0056]

EFFECT OF THE INVENTION The present invention has the above-mentioned constitution and operation, and reliably prevents the occurrence of the hollow defect even in the OHP sheet even when the contact type charging roll is used as the primary charging means. It is possible to provide an image forming apparatus capable of preventing the generation of ozone.

[Brief description of drawings]

FIG. 1 is a main part configuration diagram showing an embodiment of an image forming apparatus according to the present invention.

FIG. 2 is a block diagram showing an embodiment of an image forming apparatus according to the present invention.

FIG. 3 is a front view showing a transfer roller used in the embodiment.

FIG. 4 is a side view of the same.

5 (a) and 5 (b) are explanatory views each showing a transfer state.

FIG. 6 is a graph showing the relationship between the contact angle and the number of prints.

FIG. 7 is a graph showing the adhesion of the photosensitive drum and toner with respect to the number of printed sheets.

[Explanation of symbols]

1 photoconductor drum, 2 charging roller, 3 laser beam, 4 developing device, 5 transfer roller, 6 transfer material.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshinari Ueno 1600 Takematsu, Minamiashigara City, Kanagawa Prefecture Fuji Xerox Co., Ltd.

Claims (6)

[Claims] 1. A toner image is formed on a surface of a toner image bearing member by primary charging the surface of the toner image bearing member with a charging roller, forming an electrostatic latent image and developing the electrostatic latent image. In the image forming apparatus, in which the formed toner image is electrostatically transferred onto the transfer material by using a roller-type transfer device which is arranged to face the toner image carrier via the transfer material and to which a predetermined bias voltage is applied, An image forming apparatus characterized in that a toner image carrier and a roller type transfer device are set in a non-contact state. 2. The image forming apparatus according to claim 1, wherein the gap between the toner image carrier and the roller type transfer device is set to 0.1 mm or more and 0.3 mm or less. 3. When the transfer material is plain paper, the toner image carrier and the roller type transfer device are brought into contact with each other, and the transfer material is
The image forming apparatus according to claim 1, wherein the gap between the toner image carrier and the roller type transfer device is set to 0.1 mm or more and 0.3 mm or less only when the sheet is an HP sheet. 4. The image forming apparatus according to claim 3, wherein the moving speed of the toner image carrier is different from the moving speed of the transfer material only when the transfer material is plain paper. . 5. The image forming apparatus according to claim 1, wherein the roller type transfer device is rotated at a constant speed with respect to the toner image carrier. 6. The image forming apparatus according to claim 1, wherein the bias power source is a power source that performs constant current control at least during a transfer operation.