JPS61185758A - Image forming device - Google Patents

Abstract

PURPOSE:To prevent the defect of separation of a sheet from a drum and the occurrence of the retransfer phenomenon by making the transfer electrostatic charge output at the rear part in the carrying direction of the sheet larger than that of the part other than the rear end of the sheet. CONSTITUTION:The carried sheet is subjected to transfer electrostatic charge and an image on a photosensitive drum 1 is transferred onto the sheet. The sheet is flowed in the direction of an arrow B when the image is copied to the one surface, and the sheet is flowed in the direction of an arrow C when the images are copied to two surfaces. For the purpose of preventing transfer defects and separation defects, a transfer current is set to 350muA for one surface and is set to 200muA for two surface. Further, the rear end of the sheet carried from an intermediate tray 26 is detected by a form sensor 35 when the images are copied to the two surfaces, and the transfer current is raised from 200muA to 350muA for one surface in accordance with detection of the rear end of the sheet. Thus, the defect of separation of the sheet from the drum and the occurrence of the retransfer phenomenon are prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION Scope of Industrial Application The present invention relates to an image forming apparatus having a transfer function of transferring an image on an image bearing member to a sheet, such as a copying machine or a recording machine, and particularly relates to an image forming apparatus having a double-sided recording function. This is effective for image forming apparatuses that have

BACKGROUND OF THE INVENTION For example, in an electrophotographic copying machine having an automatic double-sided copying function, a sheet on which the first side has been copied is temporarily stored in an intermediate tray in the main body, and then the second side is copied from the intermediate tray. There is a method in which this is performed on a fed sheet and the sheet is ejected from the main body.

However, during the first side copying process, it passes through a fixing device (heat roller fixing method in most cases), or the transport section from the intermediate tray to the intermediate tray for copying the second side, or from the intermediate tray to the transfer section for second side copying. It was inevitable that the sheet would be curled and deformed due to passing through the conveyance section.

In addition, when the sheet passes through a heat-generating fixing device, water contained in the sheet oozes out, which causes a phenomenon in which the electric resistance value of the sheet differs between one-sided copying and two-sided copying. Therefore, if the transfer is performed with the same transfer charging output on both the first and second sides,
Since the electric resistance value of the sheet is higher when two sides are used than when one side is used, the sheet receives a stronger charge than when one side is used, which has the disadvantage that the sheet is more strongly charged and becomes difficult to separate from the drum. Was. This not only causes sheet jams due to poor separation, but also makes the separation of the sheet from the drum unstable, resulting in a thinner image due to the toner image that was previously transferred to the sheet being transferred to the drum again, so-called re-transfer. It was the cause.

In order to prevent the above drawbacks, a method has been proposed in which the transfer charging output for two sides is made smaller than the transfer charging output for one side.

However, when the sheet is curled or deformed during one-sided copying as described above, reducing the transfer charging output during two-sided copying causes a problem in that transfer skipping and transfer misalignment tend to occur. This transfer omission and transfer misalignment occur most notably at the trailing end of the sheet in the conveyance direction. The reasons why the trailing edge is noticeable are: ■ The trailing edge of the sheet is particularly prone to curling. ■ The trailing edge in the conveying direction is in close contact with the drum because other parts have already been separated from the drum. ShiK
It is thought that it will be difficult.

Problems to be Solved by the Invention The present invention aims to solve the problem of trailing edge transfer defects such as transfer skipping and transfer misalignment at the trailing edge of a sheet, and to eliminate trailing edge transfer defects.

OBJECTS OF THE INVENTION It is an object of the present invention to provide a new image formatting device which improves the above-mentioned disadvantages of the conventional image formatting apparatus.

Another object of the present invention is to provide an image forming apparatus that does not cause image omission, image shift, etc. at the rear end of a sheet conveyed through a transfer section.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention makes the transfer charging output at the trailing edge of the sheet in the transport direction larger than the transfer charging output at areas other than the trailing edge. .

Furthermore, specifically, the strength of the transfer charge in the transfer process,
In other words, the transfer current is changed when transferring the substantially trailing edge of the sheet, and the transfer current for a portion other than the substantially trailing edge of the sheet, that is, the portion before the substantially trailing edge is also increased. It's there. Note that the above-mentioned approximate trailing edge usually refers to a portion approximately 2 mycorrhizae from the trailing edge of the sheet (normally, when a sheet is used in a copying machine and a trailing edge transfer failure occurs, the area from the trailing edge to the maximum (often about 2α).

EXAMPLES Hereinafter, specific examples of the present invention will be described in detail with reference to the drawings. FIG. 2 is a sectional view showing an example of a copying apparatus to which the present invention is applicable.

In the figure S2, I is a photosensitive drum, 2 is a charging exposure section,
3 is a developing section, 4 is a transfer/separation charger, and 5 is a cleaning section. 6 is a paper feed cassette, 7 is a paper feed roller, 8, 9
10 is a paper guide, 10 is a paper sensor, and 11 and 12 are registration rollers for aligning the leading edge of the paper with the leading edge of the toner image on the drum 1. Reference numeral 13 denotes a conveyor belt for guiding the transfer paper that has undergone the transfer separation process to the fixing device 14. The destination of the sheet of paper on which the toner image has been fixed by the fixing device 14 is determined by a flapper 15. In other words, in the case of normal one-sided copying and in the case of second-sided copying in double-sided copying, the transport guide 16. The paper passes through the paper discharge rollers 17 and 1B and is discharged onto the paper discharge tray 19. The position of the flumper 15 in FIG. 1 shows the state at this time. The flamper 15 is controlled by an unillustrated lens or the like so that it can be displaced between the positions indicated by broken lines.

During double-sided copying, when the first side is being copied, the flapper 15 rotates about 300 times counterclockwise around the rotating shaft 20, as indicated by the broken line 15' in the figure, and the paper is transported as described below. . 21 and 22 are conveyance guides, 25 and 24 are conveyance rollers, 25 is a conveyance belt, and 26 is an intermediate tray. Due to these, the paper is temporarily stored in the intermediate tray 26. At this time, the copy side of the one-sided copy is facing upward.

Now, when the second side copying starts in double-sided copying, the copy paper that has been copied on the first side and is housed in the intermediate tray 26 is conveyed as follows.

2'7.28 is the intermediate tray paper feed roller, 29°3
0 is 4E! Feeding guide, 31.32 and 33.34
are a pair of transport rollers, and the paper fed by the paper feed rollers 27 and 2B is transferred to the registration roller 1 by transport guides 29, 50 and transport rollers 31, 32, 33, and 34.
Reach 1,12. 35 is a paper sensor similar to the paper sensor 10.

The flow of paper explained above can be easily illustrated using arrows.
It will look like Figure 4.

The paper flow indicated by arrow A in FIG. 3 is for normal one-sided copying, and that shown in FIG. 4 is for double-sided copying. The flow indicated by arrow B in Figure 4 is for one-sided copying, and the flow indicated by arrow C
This flow is for two-sided copying.

This copying device uses an electrostatic separation method, but when making two-sided copies, the paper has already passed through the fixing device 14, so the moisture content is reduced compared to when copying one-sided paper, and the paper is thinner. For reasons such as the fact that the drum 1 is separated from the drum 1, For this reason, defective transfer and separation may occur.
: The transfer current is lower than that for one side. Specifically, the current is 350 μA when one surface is used, and 200 μA when two surfaces are used. However, since the reverse side transfer vL flow was made low, the adhesion to the drum 1 tended to be weakened due to the curling deformation of the paper caused by one side copying. This tendency occurred mostly at the trailing edge of the paper. As for 111ii, the rear end,
This causes defects such as transfer omission and transfer misalignment in a portion approximately two sub-edges from the trailing edge.Therefore, when the trailing edge of the paper passes through the transfer section during two-sided printing, the transfer current is increased. This is to eliminate transfer omissions, transfer deviations, etc. at the trailing edge of the paper. Specifically, the paper sensor 35 detects the trailing edge of the paper fed and conveyed from the intermediate tray, and the transfer current is adjusted. 200μA
The amplifier was set to 550 μA, the same as when the first page was used. Immediate transfer 1 by detecting the trailing edge: When the flow is changed, the position at which the transfer current is switched will change depending on the distance between the position of the paper sensor 35 and the transfer position. , a certain amount of time lag may be provided depending on the machine to which it is applied.

In this embodiment, the present invention is applied to the second side of double-sided copying, but considering the effect, it can also be applied to single-sided copying or the first side of double-sided copying.

Although the transfer current was switched by detecting the trailing edge of the paper using the paper sensor, it goes without saying that the transfer current may be determined based on the size of the cassette in which the paper is fed during one-sided copying.

In addition, although the electrostatic separation method was used in the embodiment, it goes without saying that it can also be applied to a so-called separation belt method.

In order to be able to apply the above invention even if the paper size is different, a paper sensor is provided in the paper conveyance path before the transfer process, and after the trailing edge of the paper passes through this paper sensor, a constant A method may be adopted in which the transfer output is switched after a certain period of time, or if only standard sizes are used, the timing for switching may be determined by determining which size paper was used from the paper feed cassette used.

Further, although the present invention is effective even when applied to a normal one-sided copying machine, its effectiveness is highest when applied to a two-sided copying machine such as a two-sided copying machine. Furthermore, in this case, if the trailing edge transfer output for two pages is set to be the same as the transfer output for one page, only two transfer current switching levels are required, and the apparatus and control can be simplified.

Next, specific transfer high voltage switching control of the present invention will be explained with reference to FIG. Corona current control for transfer begins with H
By the VTS signal, when IKTTS = 1, the switch circuit is turned off, the differential amplifier circuit is turned off, and the high voltage transformer is turned off.

When Hv'rs=o, the switch circuit is turned on, the differential amplifier circuit is turned on, the high voltage transformer is also turned on, and a transfer high voltage is applied. Note that the timing of the HVTS signal is controlled according to the size of the transfer paper and the copying sequence. ON - OFF of transfer high pressure l-j Although controlled as described above, the transfer paper flow value when the transfer high pressure is ON is controlled as follows.

The transfer current value is DC controller) when the signal HVTL from o-ra is 1
Controlled to two values depending on whether it is 1″ or 0°.HV
The transfer when TL="1#" is the flow value T1 and HVTL=
The transfer current value T2 at "O" is determined by the transfer current setting circuit. In the example, 'r, = 550μ
A, T2 = -200 μA.

・Bago! (VTL [, varies depending on the copy sequence (single-sided copy or double-sided copy) and the state of the trailing edge detection sensor 35.

The status is shown in the table below.

In this embodiment, transfer 1! of the trailing edge of the paper to the second side in the double-sided copying sequence! The flow is strengthened to prevent transfer defects. In the above table, HVTL=@1 '' during the second side of double-sided copying, ie, "from the change from 0 to 02 to after a clock" is HVTL=@1, and the transfer current becomes stronger.

The a clock is the time during which the transfer current is T1, and as shown in FIG. 5, it may be within the range of Y≦a≦X.

In the example, a=Y. Note that sl and S2 indicate the transfer paper, and P indicates the location of the paper at the transfer position when the trailing edge detection sensor output changes from "1°" to "0".

Therefore, the length of the trailing edge of the paper transferred by the transfer current T1 is approximately 5 cnt from the trailing edge detection sensor 35 to the transfer section.
).

In this embodiment, the trailing edge of the paper is approximately 5
Although the transfer current for the crR portion is switched, it may be changed depending on the paper size. This is because the amount of paper curl generally differs depending on the gap in the paper and the direction in which it enters the fixing device. This is because the amount of curl differs depending on the force and the degree of defective trailing edge transfer differs. However, switching according to the paper size may be complicated in terms of software, and it is considered best to determine the trailing edge amount of paper to perform T-flow switching for the worst case. (For your reference, in order to switch according to the paper size, ■ The paper path length between the transfer unit and the paper trailing edge detection sensor must be larger than the maximum switching length. ■ The transfer current control signal HVTL must be set to "1". It is necessary to delay the timing at which `` occurs (by providing a delay circuit) according to the paper size.) As described in detail, the present invention amplifies the transfer current at the trailing edge of the sheet. There was no occurrence of defective separation from the drum 1 or retransfer phenomenon, and therefore it was possible to achieve very good transfer performance over the entire surface.

This is because the separation of the sheet from the drum is determined almost entirely at the leading edge of the sheet, and retransfer is also more likely to occur near the front edge of the sheet, so even if the transfer is stronger at the trailing edge, This is because there is no adverse effect on separability or occurrence of re-transfer.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the main parts of the present invention, FIG. 2 is a cross-sectional view showing a copying device to which the present invention can be applied, and FIGS. 3 and 4 are cross-sectional views showing the sheet conveyance direction. FIG. 5 is an explanatory diagram showing the relationship between the sheet and the transfer section. In the figure, 4 is a transfer/separation charger, and 10.35 is a paper sensor.

Claims (1)

[Claims] In an image forming apparatus that applies transfer charging to transfer an image on an image bearing member to a conveyed sheet, the transfer charging output at the rear end of the sheet in the conveying direction is made larger than the transfer charging output at other parts of the sheet. An image forming apparatus characterized by: