JPH08160826A - Image forming device - Google Patents

Abstract

PURPOSE: To shorten a preprotating process for stabilizing the potential by electrification of an image carrier by applying a bias having the same polarity as that of the electrification to a transfer means, until an electrification starting position on the image carrier reaches a transfer position. CONSTITUTION: An AC high-voltage on which a DC high-voltage is superimposed is applied to an electrifying roller 7, to start the electrification of the surface of a photoreceptor drum 8 to a prescribed potential, at the same time when the photoreceptor drum 8 starts to rotate. Simultaneously with this, the DC high-voltage having the same polarity as that of the potential of the electrification on the photoreceptor drum 8 is applied to a transfer roller 13. The application of the bias to the transfer roller 13 is continued until the contact position of the drum 8 with the electrifying roller 7 reaches the transfer position. Thus, a deficiency in the electrostatic charge quantity of the first rotation for electrifying of the drum 8 can be supplied. When the part of the drum 8 after passing through both of the transfer roller 13 and the electrifying roller 7 reaches an exposing position, a laser diode 3 starts to emit light in accordance with an image signal.

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 a copying machine, a laser printer, and a facsimile which applies electrophotography.

[0002]

2. Description of the Related Art A conventional image forming apparatus using electrophotography is constructed as shown in FIG. 1 is an interface controller, 2 is a laser drive circuit, 3 is a laser diode, 4 is a polygon mirror, 5 is an f-θ lens,
6 is a reflection mirror, 7 is a charging roller, 8 is a photosensitive drum, 9
Is a developing device, 9'is a developing sleeve, 10 is a feed roller, 1
1 is a paper cassette, 12 is a register roller, 13 is a transfer roller, 14 is a fixing device, 15 is a cleaner, and 16 is a collimator lens. The image information sent from the host computer (not shown) is input to the interface controller 1 and signal-processed, and the output causes the laser diode 3 to appropriately emit light in the laser drive circuit 2. The laser light is condensed by the collimator lens 16 and is projected onto the photosensitive drum 8 via the rotating polygon mirror 4, the f · θ lens 5 and the reflection mirror 6. The photosensitive drum 8 is charged in a negative manner by the charging roller 7, and then is irradiated with the laser light described above, and an electrostatic latent image is formed according to ON / OFF of the laser light. Then, with the rotation of the photosensitive drum in the direction of the arrow, the developing device 9 applies a developer according to the electrostatic latent image, and a visible image is obtained on the drum. On the other hand, the transfer papers taken out one by one from the paper cassette 11 by the paper feed roller 10 are brought into contact with the drum 8 at the same timing by the register roller 12 and then transferred to the transfer roller 1.
By 3, the visible image is transferred onto the transfer paper. The visible image on the transfer paper is fixed on the transfer paper by the fixing device 14. After the transfer process, the cleaner 15 is applied to the photosensitive drum 8,
The residual developer is removed and the next image forming process is started.

[0003]

In the above-mentioned conventional example, the sixth example is used.
As shown in the figure, the potential on the first lap of the drum from the start of charging on the drum surface is lower by ΔVD than on the second lap and thereafter, so the potential when laser exposure is applied to this part from the second lap and later. Is also reduced by ΔVL.

This is because in the contact charging method such as the charging roller, the drum potential does not converge to the voltage applied to the charging roller only once when the surface potential of the photosensitive drum is 0 V or close to it, so that this rotation is completed. It is known that the difference in the drum potential between the first and second rounds appears as density unevenness on the image, and in general, a portion having a lower potential has a higher density in reversal development and a lower density in regular development.

To solve this problem, in the conventional apparatus, the image drawing (that is, laser exposure) is started after the portion where the drum potential is stable reaches the exposure position in the second round of charging from the start of charging the drum. This is dealt with by forming a sequence in which the pre-rotation is performed once or more before printing the first sheet.

However, according to this method, the time for at least one rotation of the drum + the distance between the charging and the exposure is required until the image can be drawn. Therefore, the first print time is that much regardless of the paper path of the apparatus. It will be long.

Particularly in recent laser printers, the time required for the polygon motor to rise to a predetermined number of revolutions or to reach the fixing temperature of the fixing device, which has been a main factor in determining the first print time, tends to be shortened. For this reason, the wait time for stabilizing the charging potential has been further highlighted.

[0008]

According to the present invention, image exposing means for converting image information into light to expose the image carrier to form a latent image, and charging means for uniformly charging the image carrier. When,
An image forming apparatus including: a developing unit that develops a latent image on an image carrier with a toner developer; a transfer unit that transfers the developed toner image onto a transfer material; an image exposing unit; and a control unit that controls the transferring unit. In at least from the start of charging the image carrier to the transfer start position on the image carrier until the charging start position reaches the transfer position, a bias having the same polarity as that of the charging is applied to the transfer unit, and the image carrier by the image exposure unit is applied. By drawing the image information on the body on the image carrier that has passed through the transfer position and the charging position, the pre-rotation process for stabilizing the charging potential of the image carrier can be shortened and the charging can be performed. It is possible to reduce the density unevenness of the image due to the potential difference.

[0009]

【Example】

(Embodiment 1) FIGS. 1 to 3 show an embodiment of the present invention. FIG. 1 is a sequence chart showing the operation timing of an image forming apparatus, FIG. 2A, FIG. FIG. 2C shows the main members around the photosensitive drum and changes in the state of the photosensitive drum surface when the drum rotates.

Since the mechanical structure of the image forming apparatus of this embodiment is the same as that of the conventional one shown in FIG. 5, the same reference numerals are given to the same members and the description thereof will be omitted.

When the print start signal is input, the photosensitive drum 8 starts rotating, and at the same time, the charging roller 7 is rotated.
AC high voltage superimposed with DC high voltage VDC is applied to
The surface of the photosensitive drum 8 starts to be charged to a predetermined potential. At this time, simultaneously with the rotation of the photosensitive drum 8, a DC high voltage VT having the same polarity as the charging potential of the photosensitive drum 8 is applied to the transfer roller 13 by constant voltage control. For example, when the photosensitive drum is negatively charged, a negative bias is applied. By applying the bias to the transfer roller 13, the photosensitive drum 8 is charged though it is slightly smaller than that by the charging roller 7.

Here, in FIG. 2A, when the contact positions of the charging roller 7 and the transfer roller 13 of the photosensitive drum 8 at rest are C and T, respectively, C and T are shown as the photosensitive drum 8 rotates. 2 will move in the direction of arrow A, and will eventually come to the position shown in (b) of FIG. Up to this point, the portion of the surface of the photosensitive drum that has passed only the charging roller position has the potential V
It is charged to D '. Generally, DC high voltage V to charging roller
In contrast to DC, the drum surface potential VD converges to VDC and is charged so that VD = VDC.
As described above, the potential is lowered by VD.

Therefore, VD '= VD-ΔVD.
In addition, the potential passing through only the transfer roller position is the potential VT '.
The portion that has been electrically charged to and has passed both the transfer roller and the charging roller is electrically charged to the potential VDT. The magnitude relationship of this potential is VT '<VD'<VDT≤VD. That is, by applying a bias having the same polarity as the charging potential to the transfer roller, it is possible to compensate for the shortage of the charge amount of the first rotation of the photosensitive drum.

As shown in FIG. 2B, when the portion passing through both the transfer roller and the charging roller reaches the exposure position, the laser diode 3 starts to emit light according to the image signal. As a result, an electrostatic latent image is formed, and the developer is applied by the developing device 9 to form a visible image, as in the conventional example. The photosensitive drum 8 is further rotated from the state of FIG. 2B, and a bias having the same polarity as the charging potential is applied to the transfer roller 13 at least until C reaches the transfer position as shown in FIG. Continue to apply. As shown in (b) of FIG. 2, the purpose is to apply the bias without fail when the transfer roller faces the portion where the charging potential of the photosensitive drum 8 is 0V so that the portion VD ′ is not formed. . In this embodiment, when C reaches the transfer position, the transfer roller 1
Constant current control is performed at a predetermined current value to detect the optimum transfer voltage value, and then the optimum transfer voltage VTR determined by the constant current control is applied to the transfer roller 13 according to the transfer material entering the transfer nip. It is applied and the transfer of the visible image is executed. V
TR has the opposite polarity to charging in the case of reversal development and the same polarity in regular development.

With the above-described structure, the charging potential VD ′ is obtained on the first rotation of the photosensitive drum in the conventional image forming apparatus.
In this embodiment, a latent image is formed with respect to VDT that is closer to the original charging potential VD, as compared with the case where an image is written in the area, so that the density unevenness due to the charging potential difference is very slight or not noticeable at all. be able to. Therefore, it is not necessary to wait until the photosensitive drum is charged for one round as in the conventional case, and then the image formation is not required, and the first print time can be shortened accordingly.

(Embodiment 2) In Embodiment 1, the bias of the same polarity applied to the transfer roller for assisting the charging of the photosensitive drum is controlled by the constant voltage control, but in this embodiment, it is controlled by the constant current control. . In general, the resistance of the transfer roller varies by more than one digit due to manufacturing variations and environmental changes.Therefore, when the bias applied to the transfer roller is a constant voltage, the current value flowing through the transfer roller is large. Varies greatly depending on the roller or the usage environment. On the other hand, the amount of charge applied to the photosensitive drum by the transfer roller greatly depends on the current value.Therefore, when constant voltage control is used, the transfer roller with a high resistance has a smaller charge than the transfer roller with a low resistance. The amount will be significantly reduced. That is, depending on the resistance of the transfer roller, uneven density may be noticeable.

In this embodiment, since a constant current is supplied to the transfer roller, it is possible to always apply a constant charge amount to the photosensitive drum irrespective of the roller and the environment, and it is possible to stably prevent uneven density.

(Embodiment 3) A sequence chart of the image forming apparatus of this embodiment is shown in FIG. In the present embodiment, the drawing of the image information is started one drum after the charging start position of the photosensitive drum 8 reaches the exposure portion, and the forced emission of the laser diode 3 which is different from the writing of the image information until then is started. Is carried out for one round of the photosensitive drum. The purpose of this is to eliminate the uneven charge accumulated in the photosensitive layer by pre-exposing the entire surface of the photosensitive drum with a laser before drawing an image, and it is effective in improving image defects such as so-called "ghosts". There is. In this way, the potential of the surface of the photosensitive drum that is forcibly turned on by the laser drops to the potential of the bright area.Therefore, the amount of charge is not stable just by charging once and the potential difference with other parts occurs. After all, this will appear as uneven density. Therefore, in this embodiment, the area in which the laser is forcibly emitted (one rotation from the photosensitive drum charging start position)
On the other hand, a bias having the same polarity as that of the charge is applied by the transfer roller to assist the recharge of the region where the potential is forcibly emitted and the potential is lowered to the bright portion potential.

The operation of each main process of this embodiment will be described below with reference to FIG.
(A), FIG. 4 (b), FIG. 4 (c), FIG. 4 (d)
Will be explained.

The photosensitive drum 8 starts to rotate in response to the input of the print start signal, and at the same time, AC high voltage in which DC high voltage VDC is superimposed is applied to the charging roller 7, and the image on the photosensitive drum 8 is charged to a predetermined potential. Begin to. At this time, in this embodiment, the voltage applied to the transfer roller is set to 0 V, but there is no problem even if a bias having the same charging polarity is applied at the same time when the drum starts rotating as in the first and second embodiments. (B) in Figure 4
When the charging start position C reaches the exposure section as described above, the forced light emission of the laser starts, and this is because the drum rotates and C returns to the exposure position again ((d) in FIG. 4). One lap is done.

From the state of (b) of FIG. 4 to the state shown in (c) of FIG. 4 by the rotation of the drum, that is, from the time when the first charging start position C reaches the transfer position, the transfer roller 13 is reached. Is applied with a DC high voltage VT having the same polarity as the charging potential of the photosensitive drum 8. As a result, the potential of the portion exposed to laser on the photosensitive drum and lowered to the bright portion potential VL ′ is VL.
Become T. When the drum further rotates and reaches the state of (4) in FIG. 4, the forced light emission of the laser is stopped, and this time the laser emission for drawing the image information is started.

In the state of FIG. 4D, the potential of the surface of the photosensitive drum will be described. The potential of the bright portion after laser exposure is V
L ', the part that passes only the transfer position from there is VLT,
The portion further passing the charging position from that point becomes VLTD, and the magnitude relationship of each potential is VL ′ <VLT <VD ′ <V.
LTD ≦ VD (normal charging potential).

As in the present embodiment, the photosensitive drum is exposed before the image information is drawn so that the electric charge staying in the photosensitive layer is made uniform to prevent the occurrence of "ghost", and the transfer roller is also charged. By applying a bias of polarity, the charge amount at the time of recharging the laser-exposed photoconductor is adjusted to the normal potential V.
Similar to D, a good image without density unevenness can be obtained.

In this embodiment, in order to obtain the maximum effect, forcible light emission of the laser is performed once for the photosensitive drum. However, this does not necessarily have to be one revolution, and when the laser lighting is less than one revolution. The timing of applying the bias to the transfer may be adjusted accordingly.

[0025]

As described above, at least from the start of charging the photosensitive drum until the charging start position on the photosensitive drum reaches the transfer position by the rotation of the drum, a bias having the same polarity as the charging is applied to the transfer means. By applying the image information and drawing the image information on the photosensitive drum by the image exposing means on the photosensitive drum which has passed the transfer position and the charging position, the image information is drawn from the first charging cycle. However, the uneven density of the image due to the difference in charging potential does not occur. Therefore, the pre-rotation step for stabilizing the charging potential on the surface of the photosensitive drum can be shortened and the first printing can be accelerated.

Further, by applying a laser pre-exposure for one round of the photosensitive drum before drawing the image information, by applying a bias of the same polarity as the charging to the transfer means to the area pre-exposed by the laser, It is possible to reduce uneven density due to pre-laser exposure.

[Brief description of drawings]

FIG. 1 is an operation timing chart of main members of a first embodiment of the present invention.

FIG. 2 is a state change diagram of the surface of the photosensitive drum according to the first embodiment of the present invention.

FIG. 3 is an operation timing chart of main members of the second embodiment of the present invention.

FIG. 4 is a state change diagram of a photosensitive drum surface according to a second embodiment of the present invention.

FIG. 5 is a diagram showing a conventional image forming apparatus.

FIG. 6 is a diagram showing a drum potential of a conventional image forming apparatus.

[Explanation of symbols]

1 ... Interface controller, 2 ... Laser drive circuit, 3 ... Laser diode, 4 ...
Polygon mirror, 5 ... f.theta. Lens, 7 ... charging roller, 8 ... photosensitive drum, 9 ... developing device, 13 ...
..Transfer rollers, 15 ... Cleaners,

Claims (2)

[Claims] 1. An image exposure means for converting image information into light to expose the image carrier to form a latent image, a charging means for uniformly charging the image carrier, and a latent image on the image carrier. An image forming apparatus comprising: a developing unit that develops with a toner developer; a transfer unit that transfers the developed toner image onto a transfer material; an image exposing unit; and a control unit that controls the transferring unit. From the start of charging until the charging start position on the image carrier reaches the transfer position, a bias having the same polarity as that of the charging unit is applied to the transfer unit, and image information is drawn on the image carrier by the image exposure unit. Including
An image forming apparatus, which is performed on an image carrier that has passed both a transfer position and a charging position. 2. The drawing of image information on the image carrier by the image exposing means is started at the same time when the upper portion of the image carrier at the transfer position at the start of charging reaches the exposure position. Image forming apparatus.