JP2000098769A - Transfer device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transfer device for making the nip amount of a transfer roller uniform in an axial direction, and also, for removing the material sticking to the surface of a transfer roller. SOLUTION: A backup roller 13 is installed opposite to a photoreceptor drum 11 with reference to a transfer roller 12 which is rotated in contact with the photoreceptor drum 11. The backup roller 13 is constituted of a metallic shaft, and the backup roller 13 is pressed toward the transfer roller 12 by springs 17a and 17b through holders 18a and 18b. The attaching position of the springs 17a and 17b is set so as to satisfy the separation of about 2:6:2 in the axial direction of the transfer roller 12. By attaching the springs 17a and 17b at the aforesaid positions, the distortion of the backup roller 13 with reference to the transfer roller 12 is made uniform to the maximum. The backup roller 13 is rotated at a circumferential speed slightly different from that of the transfer roller 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transfer device for transferring toner on a printing medium in an electrophotographic printer.

[0002]

2. Description of the Related Art Conventionally, as a transfer device incorporated in an electrophotographic printer, a transfer roller is pressed against a photosensitive drum,
2. Description of the Related Art A method of transferring a toner on a photosensitive drum to a print medium by applying a voltage to a transfer roller has been used. The transfer roller is formed of a soft semiconductive rubber made of a material, and is pressed by a photosensitive drum to deform a portion of the rubber to form a so-called nip.

When a toner is transferred to a printing medium, a voltage having a polarity opposite to that of the toner is applied to the transfer roller. However, if the nip, that is, the deformation amount of the transfer roller is not uniform in the axial direction of the transfer roller, the transfer roller and the photosensitive drum And the charge density per unit area is different between the two, and the toner transfer varies. Therefore, keeping the nip constant is an important point in maintaining print quality.

[0004]

However, the transfer roller usually has a structure in which only both end portions are supported, and when the transfer roller is pressed against the photosensitive drum in this mode, the central portion of the transfer roller bends. The contact state between the photosensitive drum and the transfer roller at both end portions is different from the contact state between both at the center portion.

A specific description will be given with reference to the drawings. FIG. 9 is an explanatory view showing a conventional problem. In FIG. 9, a transfer roller 1 is an A4 size transfer roller having a rubber hardness of about 30, a diameter of 16 mm, and a shaft diameter of 6 mm. The transfer roller 1 is in pressure contact with the photosensitive drum 2, and the nip amount when the transfer roller 1 is deformed by this pressure contact is set to 0.4 mm. However, in actuality, as shown in FIG. 9, a nip amount close to 0.4 mm is obtained at both ends of the transfer roller 1, but only a nip amount of 0.3 mm or less is obtained at the center portion due to the bending of the shaft 3 of the transfer roller 1. Absent. The width of contact of the transfer roller 1 with the photosensitive drum 2 is also denoted by reference numeral 4 in FIG.
As shown by, there is a large difference between both ends and the center.
From the above, there has been a problem that the electric charge applied to the transfer roller 1 is different between the both ends and the central part, and the transfer of the toner to the print medium is not uniform.

Further, a small amount of toner and a component contained in the toner (for example, silica as a fluid material) always adhere to the photosensitive drum in addition to the portion where the toner image is formed.
These deposits are generated when the photosensitive drum and the transfer roller rotate before and after printing (when the print medium does not exist between the photosensitive drum and the transfer roller, and both are in direct contact and rotate).
Then, the photosensitive drum moves from the photosensitive drum onto the transfer roller. These deposits are insulators, and when the transfer roller surface is thinly covered, the apparent resistance value of the transfer roller increases to about 1.5 to 2 times. When the print medium is conveyed between the photosensitive drum and the transfer roller in this state, the deposit on the transfer roller moves to the print medium this time. The apparent resistance value of the transfer roller also decreases with the movement of the attached matter to the print medium. As a result, the resistance value of the transfer roller immediately after the start of printing and immediately before the end of printing are different.

The voltage applied to the transfer roller is determined by measuring the resistance value of the transfer roller in advance before printing, and is determined according to the measurement result. However, the voltage applied to the transfer roller is included in the toner and the toner. Since the measurement result of the resistance value differs depending on whether or not the component is attached, or how much the component is attached, there is also a problem that a correct transfer voltage cannot be determined.

[0008]

In order to solve the above-mentioned problems, the present invention conveys a printing medium between a photosensitive drum and a transfer roller rotating in contact with the photosensitive drum, and applies a voltage to the transfer roller. A transfer device that transfers the toner on the photosensitive drum to the print medium by using a backup roller that presses the transfer roller against the photosensitive drum over the entire axial direction from the opposite side of the transfer roller to the transfer roller. I do.

According to the present invention having the above structure, the transfer roller is pressed against the photosensitive drum over the entirety in the axial direction by the backup roller, so that the nip amount becomes uniform.
The transfer of the toner is performed uniformly. Further, since the backup roller is in contact with the transfer roller, it is possible to move the deposits on the transfer roller onto the backup roller.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. Elements common to the drawings are assigned the same reference numerals. FIG. 1 is a front view showing the transfer device according to the first embodiment, FIG. 2 is an end view taken along line BB of FIG. 1, and FIG. 3 is a side view showing the transfer device according to the first embodiment.

In FIGS. 1, 2 and 3, the photosensitive drum 1
1 is pressed against the transfer roller 12, and the transfer roller 12
The backup roller 13 is pressed against the opposite side, that is, the lower portion of the photosensitive drum 1. A gear 14 is attached to one end of the photosensitive drum 11, and this gear 14
The second shaft 15 meshes with a gear 16 attached to one end of the second shaft 15. The backup roller 13 is formed of a metal shaft, and is provided with a holder 18 by springs 17a and 17b.
The transfer roller 12 is pressed to the transfer roller 12 side via a and 18b. As a result, the backup roller 13 becomes the transfer roller 1
2 is pressed over the entire axial direction.

The mounting positions of the springs 17a and 17b with respect to the backup roller 13 are as shown in FIG.
Assuming that the length of the transfer roller 12 is l, about 2:
6: 2. Spring 1
By attaching 7a and 17b to this position, the deflection of the backup roller 13 with respect to the transfer roller 12 becomes the most uniform. FIG. 2 shows the contact area of the transfer roller 12 with the photosensitive drum 11. As can be seen from the figure, the contact area 12a of the transfer roller 12 is uniform in the axial direction,
Therefore, the nip amount becomes uniform.

A gear 1 is provided at one end of the backup roller 13.
9 is attached, and the gear 19 is the gear 16 of the transfer roller 12.
And are engaged. The gear ratio of the gears 16 and 19 is set such that the peripheral speed of the transfer roller 12 and the peripheral speed of the backup roller 13 are slightly different. Further, the shaft 15 of the transfer roller 12 and the backup roller 13
Electrodes 20 and 21 are connected to the other end of the transfer roller 12, respectively, and a voltage is applied to the transfer roller 12 and the backup roller 13 through these electrodes 20 and 21. The transfer roller 1
2. The backup roller 13 is rotatably supported by a guide (not shown).

Next, the operation of the first embodiment will be described.
When a print command is sent from the host device to the electrophotographic printer, the photosensitive drum 11 is rotated in the direction of the arrow shown in FIG. Thus, the transfer roller 12 in contact with the photosensitive drum 11 and the backup roller 13 in contact with the transfer roller 12 also rotate. The peripheral speed of the transfer roller 12 and the peripheral speed of the backup roller 13 are slightly different, and slippage occurs between the transfer roller 12 and the backup roller 13. At this time, the photosensitive drum 11 is negatively charged, a positive voltage is applied to the transfer roller 12 via the electrode 20, and a positive voltage having the same potential as the voltage applied to the transfer roller 12 is applied to the backup roller 13. Has been applied. Also, the photosensitive drum 1
1 is negatively charged.

Thereafter, the print medium 22 is conveyed and enters between the photosensitive drum 11 and the transfer roller 12. Photosensitive drum 1
The toner image on 1 is attracted toward the transfer roller 12 by the potential difference between the photosensitive drum 11 and the transfer roller 12 and moves onto the print medium 22. As a result, the transfer of the toner image is performed, and the print medium 22 includes the photosensitive drum 11 and the transfer roller 12.
Then, the sheet is conveyed to the fixing unit.

As described above, in the first embodiment, since the nip amount is uniform in the axial direction of the transfer roller 12, the potential difference applied to the transfer roller 12 is equal to the both ends of the transfer roller 12 and the center. It occurs uniformly without unevenness in the part. Therefore, there is no variation in the transfer of the toner image, and the toner is transferred uniformly. Further, while the voltage is applied to the transfer roller 12, the same voltage is applied to the backup roller 13, so that the flow of current from the transfer roller 12 to the backup roller 13 is prevented. Therefore, the transfer efficiency is also maintained well.

Since the nip amount of the transfer roller 12 is uniform in the axial direction, the transport speed of the print medium 22 is not different between the both ends and the central portion of the transfer roller 12, and the transport unevenness does not occur.

Further, the peripheral speed Vt of the transfer roller 12 and the peripheral speed Vb of the backup roller 13 are made slightly different from each other so that a slight slippage occurs between the two rollers. It is possible to scrape off the attached matter such as the toner not used for image formation and components contained in the toner from the transfer roller 12.
The scraped-off matter is received by a tray (not shown) provided below the backup roller 13.
By scraping off the deposits, the transfer roller 12 can always maintain a constant resistance value.

Transfer roller 12 and backup roller 13
Providing a difference in peripheral speed between the above also leads to the following effects. That is, the surface of the transfer roller 12 gradually deteriorates with long-term use. This is because the surface is oxidized by the discharge generated during the transfer, and the oxide insulating layer finally covers the entire surface of the transfer roller 12. Therefore, the resistance value of the transfer roller 12 increases, and the applied voltage must be increased in order to obtain a necessary potential difference. As the number of insulating layers increases, the resistance increases, and the applied voltage also increases. If the applied voltage exceeds the allowable range, transfer failure occurs, which eventually leads to the life of the transfer roller 12, but it goes without saying that it is desirable to extend the life of the transfer roller 12. By providing a peripheral speed difference between the transfer roller 12 and the backup roller 13, an oxide insulating layer formed on the surface of the transfer roller 12 for a long time can be removed. Thus, the life of the transfer roller 12 can be extended.

In the above-described embodiment, the adherence or oxide on the surface of the transfer roller 12 is removed by providing a peripheral speed difference between the transfer roller 12 and the backup roller 13. The removal effect can be provided by making the surface irregularities by surface roughening or the like. Furthermore, the removal effect can be further improved by using both the provision of the peripheral speed difference and the provision of the unevenness on the surface of the backup roller 13.

In the above embodiment, the backup roller 13 is constituted by one roller. However, the backup roller 13 divided as shown in FIG. 4 may be used. FIG. 4 is a front view showing a modification of the first embodiment. In FIG. 4, the backup roller 13 is divided into three parts 13a, 13b and 13c, and the holders 18a and 18 are separated by the springs 17a and 17b at the boundary of the division.
b. Spring 17a, 17b
Is set at a position divided about 2: 6: 2 in the axial direction in the same manner as in the first embodiment. In this way, the backup roller can be discontinuously pressed against the transfer roller 12 over the entire axial direction.

When the conductive material of the transfer roller 12 uses an ionic substance, the transfer voltage is always positive. If the transfer roller 12 is used for a long period of time, ions in the roller rubber will be in the vicinity of the shaft (positive electrode) of the transfer roller. And the conductive material is biased. As a result, the number of ions is different between the vicinity of the center of the transfer roller and the vicinity of the surface, and the resistance value is also biased in the transfer roller. If the resistance value is biased, the transfer voltage cannot be set accurately, as described above. The second embodiment of the present invention also solves such a problem.

Next, a second embodiment of the present invention will be described. FIG. 5 is a block diagram showing a control system according to the second embodiment. In FIG. 5, a control unit 31 controls the operation of the transfer device, and a power supply unit 32 is connected. The power supply unit 32 applies a voltage to the transfer roller 12 and the backup roller 13. The time for which a voltage is applied to the transfer roller 12 and the time for which a voltage is applied to the backup roller 13 are controlled by the control unit 31. The mechanical configuration of the second embodiment is the same as that of the first embodiment. The following operation will be described with reference to FIG.

Next, the operation of the second embodiment will be described with reference to FIGS. 1, 6, 7 and 8. FIG. 6 is a time chart showing the operation of the second embodiment, and FIGS. 7 and 8 are side views showing the operation of the second embodiment.

The transfer operation of the toner image is the same as in the first embodiment. FIG. 6 shows the time T1 for applying the voltage to the transfer roller 12 in the upper part, and the time for applying the voltage of the backup roller 13 in the lower part. Et voltage is applied to the transfer roller 12, and E is also applied to the backup roller 13.
A voltage of Eb having the same potential as t is applied. Transfer roller 12
When the print medium 22 passes between the photosensitive drum 11 and the transfer roller 12, the application of the voltage to the transfer roller 12 is stopped. However, backup roller 13
Is continued for a time T2 thereafter. During this time, the rotation of the transfer roller 12 and the backup roller 13 is continued.

FIG. 7 shows the direction of the current in the transfer roller 12 at the time T1, and FIG. 8 shows the direction of the current in the transfer roller 12 at the time T2. In both figures, when a voltage is applied to both the transfer roller 12 and the backup roller 13, a current flows from the vicinity of the shaft 15 toward the photosensitive drum 11, and ions gather near the shaft 15. However, when a voltage is applied only to the backup roller 13, a current flows from the backup roller 13 toward the shaft 15, and the ions are pulled toward the backup roller 13. Since the transfer roller 12 is rotating, the ions move in the circumferential direction of the transfer roller 12. This can prevent the ions from being biased to the vicinity of the shaft 15 and keep the resistance value constant.

As described above, in the second embodiment, a positive voltage is applied to the backup roller 13 at a time other than the time of transfer, and a potential difference is provided between the backup roller 13 and the transfer roller 12, so that the vicinity of the shaft 15 is provided. The collected ions are uniformly dispersed in the roller, and the resistance value of the transfer roller 12 itself can be kept constant.

[0028]

As described above in detail, according to the present invention, the transfer roller is pressed against the photosensitive drum over the entire axial direction by the backup roller. Performed uniformly. Further, since the backup roller is in contact with the transfer roller, it is possible to move the deposits on the transfer roller onto the backup roller.

[Brief description of the drawings]

FIG. 1 is a front view illustrating a transfer device according to a first embodiment.

FIG. 2 is an end view taken on line BB of FIG. 1;

FIG. 3 is a side view illustrating the transfer device according to the first embodiment.

FIG. 4 is a front view showing a modification of the first embodiment.

FIG. 5 is a block diagram illustrating a control system according to a second embodiment.

FIG. 6 is a time chart showing the operation of the second embodiment.

FIG. 7 is a side view showing the operation of the second embodiment.

FIG. 8 is a side view showing the operation of the second embodiment.

FIG. 9 is an explanatory diagram showing a conventional problem.

[Explanation of symbols]

 Reference Signs List 11 photosensitive drum 12 transfer roller 13 backup roller 16 gear 17a, 17b spring 19 gear

Claims (5)

[Claims] 1. A transfer device for transferring a print medium between a photosensitive drum and a transfer roller rotating in contact with the photosensitive drum, and transferring a toner on the photosensitive drum to the print medium by applying a voltage to the transfer roller. A transfer roller provided with a backup roller for pressing the transfer roller against the photosensitive drum over the entire axial direction from a side opposite to the photosensitive drum with respect to the transfer roller. 2. The transfer apparatus according to claim 1, wherein the backup roller is pressed from a side opposite to the transfer roller at a point where the backup roller is divided into 2: 6: 2 in the axial direction. 3. The backup roller rotates at a peripheral speed slightly deviated from the peripheral speed of the transfer roller.
The transfer device as described in the above. 4. The backup roller is made of a conductive metal, is disposed continuously in the axial direction of the transfer roller, and a voltage having the same potential as the voltage applied to the transfer roller during transfer is applied to the backup roller. The transfer device according to claim 1. 5. The transfer device according to claim 4, wherein a voltage having the same potential as a voltage applied to the transfer roller during transfer is applied to the backup roller, and a potential difference is provided between the transfer roller and the backup roller during non-transfer.