JPH06118814A - Image forming device - Google Patents

Abstract

PURPOSE:To secure the excellent transfer of a toner image and simultaneously, to prevent the generation of the nonuniformity of the density of an image, even if the environmental conditions of the temperature, humidity, etc., of an image forming device are changed. CONSTITUTION:A transfer roller 43 comes into contact with a photosensitive drum 39 forming the toner image at the time of transferring. The transfer roller 43 can be advanced or retreated to or from the photosensitive drum 39 by the control of the movement of transfer roller supporting matter 58 by the drive of a stepping motor 71. In a stage before the toner image is transferred, a current flowing into the transfer roller 43 via a constant voltage circuit 75 is detected by a current detecting circuit 74 and the position of the transfer roller supporting matter 58 is controlled so as to keep the detected current in a prescribed range, to realize the excellent transfer of the image.

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 electrostatic copying machine or a laser printer, which forms an image on a photosensitive member by using an electrostatic process, and particularly, to press a sheet on the photosensitive member. The present invention relates to an image forming apparatus that improves transfer characteristics when a toner image is transferred.

[0002]

2. Description of the Related Art A technique of forming a latent electrostatic image on a photoconductor such as a photoconductor drum or a photoconductor belt and transferring the developed toner image onto a sheet to obtain a recorded image is an electrostatic latent image. It is used in various image forming apparatuses such as copying machines. Most of such devices include a corotron called a transfer corotron, and employ a transfer system in which a toner image on a photoconductor is electrostatically attracted to a sheet.

Apart from this, for example, Japanese Patent Laid-Open No. 2-2642
As disclosed in Japanese Patent Laid-Open No. 78, there is also a system in which a toner image on a photoconductor is brought into direct contact with a sheet and transferred. In this method, when the paper is passed between the photoconductor and the transfer member, the paper is pressed against the photoconductor by the transfer member. At this time, a bias voltage is applied to the transfer member to transfer the toner image to the paper side.

A blade-shaped or cylindrical elastic member is usually used as the transfer member. In the case of a cylindrical elastic member, it rotates so as to be in contact with the photoconductor, so it will be referred to as a transfer roller hereinafter.

FIG. 7 shows the basic configuration of a conventional image forming apparatus using a transfer roller. A charging roller 12 is rollingly contacted with an upper portion of the photoconductor drum 11, and a power source 13 uniformly charges the drum surface. In this state, the photosensitive drum 11 is indicated by the arrow 1
It rotates in four directions at a constant speed, and the image information writing means 15 writes the image information to the charged portion. Writing of image information may be performed, for example, by laser light modulated by the image information, or may be performed by slit exposure of an image of an original document (not shown) on the surface of a drum using an ordinary optical system. is there. By writing the image information, the charges are selectively lost, and an electrostatic latent image corresponding to the image information is formed on the surface of the photosensitive drum 11.

The electrostatic latent image is developed by the developing device 16 and a toner image is formed by selectively attracting the toner image. This toner image moves to the transfer position where the drum surface and the transfer roller 17 are in rolling contact with the rotation of the photosensitive drum 11. A power source 18 for adsorbing the developed toner particles is connected to the transfer roller 17.

When the writing of image information by the image information writing means 15 is started, the feed roller 22 that is in contact with the sheets 21 stacked on the sheet feed tray 19 is driven at a predetermined timing. The paper 21 sent out by this is conveyed on the conveyance path 23 shown by the broken line, and passes between the photosensitive drum 11 and the transfer roller 17. At this time, a charge having a polarity opposite to that of the toner particles is applied to the back surface of the reading means 22 through the transfer roller 17, and the toner image is transferred onto the paper 22. The paper 22 onto which the toner image is transferred is a fixing device 25 including a heat roll or the like.
Through which the toner image is fixed. And
The sheet is discharged to a discharge tray (not shown), an intermediate tray for temporary storage during double-sided recording, or the like.

On the other hand, the toner not transferred onto the paper 22 by the transfer roller 17 reaches the cleaning device 27 by the rotation of the photosensitive drum 11, and is collected from the drum surface here. After that, the photosensitive drum 11 is recharged by the charging roller 12 to prepare for the next recording.

An image forming apparatus which employs such a toner image contact transfer system does not require a high voltage power source as compared with an image forming apparatus which uses a transfer corotron to adsorb toner particles by corona discharge. For this reason,
The cost of the device can be reduced. Also, since the use of wire electrodes for corona discharge can be avoided,
No trouble such as dirt inside the device due to discharge occurs.
Furthermore, neither ozone nor nitride is generated due to the high-voltage discharge, and there is no problem such as deterioration of the photoreceptor or deterioration of image quality due to abnormal discharge.

[0010]

However, the image forming apparatus adopting the toner image contact transfer system has a problem that the transfer characteristics vary depending on the environment and it is difficult to perform stable transfer. For example, it is known that in a low-temperature and low-humidity environment, the resistance value of the transfer roller 17 made of an elastic material such as foamed urethane rises by several orders of magnitude higher than that at room temperature and normal humidity. On the other hand, in a high temperature and high humidity environment, the resistance value of such a transfer roller 17 becomes 1 to 2 digits lower than that in the normal temperature and normal humidity. When such a large variation in the resistance value occurs, the value of the current flowing when a bias voltage is applied to the transfer roller 17 changes greatly, and the transfer characteristic changes.

That is, when the resistance value of the transfer roller 17 becomes high and the transfer current becomes too small, the force of attracting the toner particles from the photosensitive drum 11 to the paper 21 becomes weak and a transfer failure occurs. On the contrary, the transfer roller 17
When the resistance value of 1 becomes low and the transfer current becomes too large, a developing bias is applied to the photosensitive drum 11 which is initially negatively (negatively) charged by the charging roller 12, and development is performed with toner particles negatively charged. In such a case, a large amount of positive (plus) current for transferring the toner particles from the transfer roller 17 flows, and the drum surface is charged to the opposite plus side. In such a case, the photoconductor drum 11 can be obtained by using a charge eliminating means (not shown).
It becomes difficult to sufficiently eliminate the charge on the positive side of the, and unevenness in the removal of charge occurs. If unevenness occurs in the charge removal, uniform negative charging by the charging roller 12 becomes impossible, which causes uneven density such as "fog" in the image.

It is therefore an object of the present invention to provide an image forming apparatus capable of ensuring good transfer of a toner image and preventing uneven density even when such environmental conditions change.

[0013]

According to a first aspect of the present invention, a photoconductor such as a photoconductor drum or a photoconductor belt for forming an electrostatic latent image and an electrostatic latent image on the photoconductor are developed. Developing means for forming a toner image, an elastic transfer material for bringing a paper into contact with a photoconductor to transfer the toner image, and a variable contact pressure for changing the contact pressure when the transfer material is brought into contact with the photoconductor Means, a current value detecting means for applying a voltage to the transfer material before the transfer of the toner image onto the sheet and detecting the current value flowing at that time, and a current value detected by the current value detecting means. The image forming apparatus is provided with a contact pressure adjusting unit that controls the contact pressure varying unit so that the value becomes a predetermined value.

That is, according to the first aspect of the invention, the contact pressure at which the transfer material comes into contact with the photosensitive member is changed according to the environment so that the current value flowing through the transfer material becomes a predetermined value. When the current value is too low, the transfer failure is prevented, and the current value is prevented from becoming excessive, so that the charging characteristic for the photoconductor is kept good.

[0015]

EXAMPLES The present invention will be described in detail below with reference to examples.

FIG. 1 shows the configuration of a laser printer as an embodiment of the image forming apparatus of the present invention. The laser printer 31 includes a laser scanning device 32. The laser scanning device 32 includes an image information processing device 33.
A semiconductor laser (not shown) that modulates and outputs the laser light according to the image signal processed in (1) is arranged. The laser beam emitted from the semiconductor laser is the polygon mirror 3
4 and is deflected according to this rotation. The deflected laser beam passes through the fθ lens 35 and then the mirror 3
The traveling direction is changed by 6, 37, and the laser beam is output from the laser scanning device 32.

A recording cartridge 38 is disposed on the left side of the laser scanning device 32 in the drawing. The laser beam output from the laser scanning device 32 repeatedly scans a predetermined position of the photosensitive drum 39 arranged in the recording cartridge 38 in the drum axis direction (main scanning direction). The photosensitive drum 39 is rotated clockwise at a constant speed by a main motor described later. In this embodiment, this causes the drum surface to move at a speed of 28 mm / sec. The charge roller 4 is provided around the photosensitive drum 39.
1, the developing roller 42, the transfer roller 43, and the cleaning device 44 are arranged in contact in the clockwise direction in this order. Then, in FIG. 7, the charge roller 41 uniformly charges the photosensitive drum 11 as described in the principle, and the electrostatic latent image is formed by the laser beam. This electrostatic latent image is developed by the developing roller 42.

The developing roller 42 is disposed in the developing device 46, and is supplied with the toner by the toner supply mechanism 47 so as to develop the electrostatic latent image by magnetically rising. A predetermined developing bias is applied to the developing roller 42. The toner image obtained by the development of the electrostatic latent image is electrostatically transferred onto the paper (plain paper) not shown by the transfer roller 43.

By the way, the paper on which the toner image is transferred is sent out from a paper feed tray (not shown) arranged at the bottom of the apparatus main body, and is carried on a carrying path 49 indicated by a dotted line. At this time, the paper passes between the photoconductor drum 39 and the transfer roller 43 at a predetermined timing, and the toner image is transferred. Then, the sheet is guided by the guide 51 and passes through the inside of the fixing device 52. The fixing device 52 is provided with a heat roller 53 having a heating source and a pressure roller 54 that rolls against the heat roller 53 at a predetermined pressure, and the toner image on the sheet is thermally fixed when passing between them. It
The fixed sheet is further advanced along the conveyance path 49 and discharged to the outside of the machine by the pair of discharge rollers 56.

On the other hand, the photosensitive drum 39 is not transferred to the paper.
The toner particles remaining on the surface are removed and collected from the surface of the drum by a blade-shaped cleaning device. The surface of the photosensitive drum 39, which has been cleaned in this manner, is uniformly charged again by the charge roller 41, and the cycle for the next recording operation is started.

By the way, the transfer roller 43 is a roller having a diameter of 18 mm and made of urethane foam as a raw material, and its resistance value is 4 × when a DC voltage of 500 V is applied by applying a load of 500 g to both ends of the shaft of the roller. 10
^It shows a value of ⁷ Ω. The hardness of the transfer roller 43 is 28 degrees. The transfer roller 43 is
It is arranged at one end of a rod-shaped transfer roller support tool 58.
Teeth are engraved on the side portion of the transfer roller support tool 58 at a predetermined pitch to form a rack. A pinion 59 attached to the rotating shaft of a stepping motor (not shown) meshes with this rack. Therefore, when the stepping motor rotates in the forward direction or the opposite direction, the transfer roller 43 moves forward in the direction in which it comes into pressure contact with the photosensitive drum 39, or reversely moves backward in the direction away from it. A predetermined voltage generated by the power supply device 61 is applied to the transfer roller 43. Also,
The overall control of the laser printer 31 having such a configuration is performed by the control device 62.

FIG. 2 specifically shows the transfer roller and its peripheral circuits. The transfer roller 43 comes into contact with the photosensitive drum 39 a predetermined time before the transfer of the paper, and is separated from the photosensitive drum 39 again when the transfer of the paper is completed. The advancing / retreating mechanism of the transfer roller support 58 for this purpose has been described with reference to FIG. The stepping motor 71 that rotates the pinion 59 is driven and controlled by a stepping motor drive circuit 73 that is controlled by the control circuit 72. The control circuit 72 also detects the value of the current flowing through the transfer roller 43 from the current detection circuit 74. This current flows through the photosensitive drum 39 which is grounded based on the voltage applied to the transfer roller 43 by the constant voltage circuit 75.
In this embodiment, the transfer characteristics are kept good by adjusting the contact pressure of the transfer roller 43 with respect to the photosensitive drum 39 so that this current falls within a predetermined range.

FIG. 3 shows a control circuit and its peripheral circuits for performing such control. Control circuit 7
Reference numeral 2 is composed of a CPU (central processing unit) 81, a ROM (read only memory) 82 and a RAM (random access memory) 83, and a power supply device 33 and a stepping motor drive through an input / output port (not shown). The circuit 73, the current detection circuit 74, the main motor drive circuit 85, and the operation panel 86 are connected. Here, the main motor drive circuit 85 is the photosensitive drum 39 (FIG. 1).
The drive of the main motor 87, which is a drive source for rotating the motor at a constant speed, is controlled. The operation panel 86 is a panel for an operator to input / output data to / from the laser printer 31, and various key switches, components such as a liquid crystal display, etc. are arranged therein.

FIG. 4 shows the contents of control of the transfer roller for transferring the toner image onto the paper. The CPU 81 shown in FIG. 3 determines whether or not the sheet being conveyed on the conveying path 49 (FIG. 1) reaches a position before the transfer position of the toner image by a predetermined distance, depending on the passage of time from the start of feeding the sheet. It is determined (step S101 in FIG. 4).
When this predetermined position is reached (Y), the constant voltage circuit 75 in the power supply device 33 is turned on to start voltage application to the transfer roller 43 (step S102). From this stage, the CPU 81 detects that the current detected by the current detection circuit 74 is 1.2.
It is checked whether or not μA or more (step S1)
03). When the transfer roller 43 is still kept in non-contact with the photosensitive drum 39, this current value is zero. In such a case, or when the transfer roller 43 is in contact with the photosensitive drum 39 but its pressure is low (N), the CPU 81 sends a control signal to the stepping motor drive circuit 73 to send the control signal to the stepping motor 7.
1 is normally rotated by one pulse (step S104).

In this embodiment, when the stepping motor 71 is supplied with a drive pulse for one pulse, the motor angle is rotated by 7.5 ° and the transfer roller supporting member 58 is moved by 0.2.
Only mm will move back and forth with respect to the surface of the photosensitive drum 39. In the example of step S104, the distance toward the photosensitive drum 39 is increased by 0.2 mm.

When the drive pulse for one pulse is supplied to the stepping motor 71 in this manner, the process returns to step S103 and the same control is performed. That is, the current value detected by the current detection circuit 74 is 1.2 μ.
The stepping motor 71 is continuously rotated forward until it becomes A or more.

FIG. 5 shows the relationship between the pressure difference and the resistance value when the transfer roller is pressed against the photosensitive drum. As shown in this figure, the resistance value decreases as the pressure (load) increases. That is, as the stepping motor 71 in FIG. 2 is rotated in the normal direction and the transfer roller 43 is strongly pressed against the photosensitive drum 39, the resistance value decreases and the current increases.

Returning to FIG. 4, the description will be continued. If the current detected as a result of the forward rotation of the stepping motor 71 (FIG. 3) is 1.2 μA or more (step S1
03; Y), it is determined whether this current value is 1.8 μA or less (step S105). If the following is true (Y), the resistance value of the transfer roller 43 is within an appropriate range, so the driving of the stepping motor 71 is stopped at that position. On the other hand, when the current value detected by the current detection circuit 74 exceeds 1.8 μA (step S105; N), the transfer roller 43 comes into contact with the photosensitive drum 39 with an unnecessarily strong pressure. ing. Therefore, in this case, the stepping motor 71 is reversed by one drive pulse (step S106), the process returns to step S103, and the same control is repeated until an appropriate current value is detected.

FIG. 6 shows the timing of each part during the recording operation. Of these, FIG. 9A shows the timing of voltage application to the transfer roller 43. As described in step S102 of FIG. 4, voltage application by the constant voltage circuit 75 (FIG. 2) is first started, and the stepping motor is driven at substantially the same time (see FIG. 6).
(B)) is started. In the laser printer of the present embodiment, when 585 mS has passed from these, transfer to the sheet is started as shown in FIG. 6C. The stepping motor 71 is controlled to turn on / off the drive pulse every 15 mS. Therefore, step S1
The circuit is designed so that the transfer roller 43 is adjusted to an appropriate contact pressure within 585 mS after the constant voltage circuit 75 is turned on in 02.

In this manner, the toner image is transferred onto the paper while the contact pressure of the transfer roller 43 with respect to the photosensitive drum 39 is maintained in an appropriate state. CPU
In this state, 81 monitors whether or not the transfer to the paper is completed (step S107 in FIG. 4), and when the transfer is completed, the voltage application to the transfer roller 43 by the constant voltage circuit 75 is stopped (step S108). Then, thereafter, the stepping motor 71 is rotated in the reverse direction (step S110) until the current value detected by the current detection circuit 74 becomes zero (step S109). This is because the transfer roller 43 is pulled away from the photosensitive drum 39 except when the paper is being transferred, so that the transfer roller 43 is prevented from being worn and the transfer roller 43 is prevented from being contaminated on the drum surface. Therefore, if such measures are unnecessary, step S109
The following operations may be omitted and all operations may be completed.

Although the transfer roller 43 is used as the elastic transfer material in the embodiment described above, it is also possible to use a blade-shaped elastic body. Further, in the embodiment, the transfer roller 43 is moved back and forth by the mechanism of the rack and pinion, but the transfer roller 43 or another transfer material may be moved back and forth by using a known mechanism other than this.

In the embodiment, the contact pressure of the elastic member with respect to the photosensitive drum 39 is changed, but it goes without saying that the present invention can be applied to the photosensitive belt. Further, although the present invention is applied to the laser printer in the embodiment, it is needless to say that the present invention can be applied to other image forming apparatuses such as a copying machine.

[0033]

As described above, according to the present invention, a voltage is applied to an elastic transfer material, the current flowing at this time is detected, and the contact pressure of the transfer material with respect to the photosensitive member is set to an appropriate value. Therefore, not only the transfer of the toner image can be stabilized at all times even when the environment changes, but also it is possible to easily control the contact / separation of the transfer material with respect to the photoconductor by detecting the current change. There is also.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a laser printer as an embodiment of an image forming apparatus of the present invention.

FIG. 2 is an explanatory diagram specifically showing a transfer roller and its peripheral circuits in the present embodiment.

FIG. 3 is a block diagram showing a control circuit and its peripheral circuits in the present embodiment.

FIG. 4 is a flowchart showing the content of control of a transfer roller for transferring a toner image onto a sheet in this embodiment.

FIG. 5 is a characteristic diagram showing a relationship between a pressure difference and a resistance value when a transfer roller is pressed against a photosensitive drum.

FIG. 6 is a timing chart showing the timing of each unit during a recording operation in this embodiment.

FIG. 7 is a principle diagram showing a principle configuration of a conventional image forming apparatus using a transfer roller.

[Explanation of symbols]

31 ... Laser printer (image forming apparatus), 33 ... Power supply device, 39 ... Photosensitive drum, 43 ... Transfer roller, 58 ... Transfer roller support tool, 59 ... Pinion, 71 ... Stepping motor, 72 ... Control circuit, 73 ... Stepping Motor drive circuit, 74 ... Current detection circuit, 75 ... Constant voltage circuit, 81
… CPU, 82… ROM

Claims (1)

[Claims] 1. A photosensitive member for forming an electrostatic latent image, a developing unit for developing the electrostatic latent image on the photosensitive member to form a toner image, and a sheet contacting the photosensitive member to form a toner image. An elastic transfer material for transferring, a contact pressure varying means for changing the contact pressure when the transfer material is brought into contact with the photoconductor, and the transfer material before the transfer of the toner image onto the paper. Current value detecting means for applying a voltage to the circuit to detect the current value flowing at that time, and contact pressure adjusting means for controlling the contact pressure varying means so that the current value detected by the current value detecting means becomes a predetermined value. An image forming apparatus comprising: