JPH10254258A - Electrostatic charge image forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method simultaneously preventing a void due to two causes of the under- and over-transfers of the top end part, by switching a current applied at the time of performing a transfer to the top end of a recording medium in plural stages (over several times). SOLUTION: In the electrostatic charge image forming method, a corona discharger 2 is arranged opposedly to an image carrier 1 and at the time of performing the transfer, the recording medium is fed to between the corona discharger 2 and the image carrier 1, while being guided by the entrance guide of the corona discharger 2, to attain a corona discharge by the corona discharger 2 from the rear side of the recording medium, so that the recording medium is electrified to a polarity opposite to that of the charge of a toner image formed on the image carrier 1, to transfer the toner image formed on the image carrier 1 onto the recording medium. In such a case, at the time of performing the transfer to the top end of the recording medium, the current applied to the corona discharger 2 is made higher than that applied to the central part of the recording medium and further, the current value of a transfer current applied to the top end of the recording medium is switched in the plural stages.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electrophotographic copying machine,
In an electrostatic image forming method used in an image forming apparatus such as a laser printer and a facsimile, a method for applying a current to a corona discharger used when a toner image is transferred to a recording medium is improved, so that a tip portion of the recording medium is improved. In which insufficient transfer and excessive transfer are prevented.

[0002]

2. Description of the Related Art In an electrostatic image forming apparatus, a corona discharger is arranged so as to face an image carrier, and when transferring a toner image on the image carrier onto a recording medium, an entrance guide of the corona discharger is used. The recording medium is fed between the corona discharger and the image carrier while guiding the recording medium with the recording medium, and the corona discharge is performed by the corona discharger from the back of the recording medium, so that the recording medium is formed on the image carrier. A step of transferring the toner image formed on the image carrier to a recording medium by charging the image with a polarity opposite to that of the image is performed. There are two types of configurations of this electrostatic image forming apparatus: (1) a type in which the image carrier is configured by a drum-shaped photoconductor, and (2) a type in which the image carrier is configured by an intermediate transfer belt. . In the electrostatic image forming apparatus of the type (1), an electrophotographic process is applied to a drum-shaped photoreceptor as an image carrier, and charging, exposure,
A toner image is formed through a development process, and the toner image is transferred to a recording sheet as a recording medium by a corona discharger. In the electrostatic image forming apparatus of the type (2), charging means, exposure means, developing means,
In addition to the primary transfer means, a medium-resistance intermediate transfer belt as an image carrier is provided between the photoreceptor and the primary transfer means, and a corona discharge device as a secondary transfer means is provided around the intermediate transfer belt elsewhere. And a recording medium is passed between the secondary transfer unit and the intermediate transfer belt. In this type of electrostatic image forming apparatus, when forming an image, a charging step of charging a photoreceptor by a charging unit, an exposing step of exposing the charged photoreceptor according to image information by an exposing unit, and exposing the exposed portion by a developing unit A developing step of visualizing the toner image, a primary transfer step of applying a voltage having a polarity opposite to that of the toner image to the primary transfer means to transfer the toner image onto the intermediate transfer medium, and a toner image on the intermediate transfer medium. A secondary transfer step of applying a voltage of opposite polarity to a corona discharger as secondary transfer means to transfer a toner image on an intermediate transfer belt as an image carrier to recording paper. Among these electrostatic image forming apparatuses, in the case of the type (1), when forming a full-color image, a color superimposed transfer image is formed on an image carrier, and the superimposed transferred image is collectively formed. And transfer it to recording paper. In the electrostatic image forming apparatus of the type (2), when forming a full-color image, the primary transfer is repeated to form a full-color toner image on the intermediate transfer belt by a superimposed toner image. Are collectively secondarily transferred onto recording paper.

For an intermediate transfer belt, the volume resistivity is 1
× 10 ⁸ to 10 ¹² Ω · cm, surface resistivity 1 × 10 ⁸ to
A medium resistance belt of 10 ¹¹ Ω (JIS K6911) is used. The reason for using a medium resistor in this way is that if a high resistor is used, the primary transfer is repeated when creating a full-color image, and the charged potential gradually increases due to the bias applied during the primary transfer. This causes inconvenience in the primary transfer and the secondary transfer. On the other hand, if an intermediate transfer belt made of a medium resistor is used as an image carrier and discharge is performed from the intermediate transfer belt via a support member, even if primary transfer is repeated in the process of forming a full-color toner image. The charge amount of the intermediate transfer belt can be maintained substantially constant.
In such an electrostatic image forming apparatus, the toner image carried on the intermediate transfer belt having a medium resistance has a characteristic that the toner has a weak toner adhering force because the charged charges are discharged through the support member of the intermediate transfer belt. There is. On the other hand, as a transfer means, there are a method using a bias roller and a method using a corona discharger. In the case where the latter transfer bias roller is used, in the process of forming a full-color toner image, the bias roller uses the primary transfer image. In order to avoid this because the image is disturbed when it contacts the transfer bias roller, it is necessary to keep the transfer bias roller away from the intermediate transfer medium as the image carrier until the primary transfer image passes through the bias roller.
The necessity of a separating mechanism for this purpose causes a problem that the mechanism becomes complicated and large, and the cost increases. For this reason, a corona transfer device capable of maintaining a non-contact state with a toner image without using a bias roller as a transfer unit is often employed.

In a configuration using a corona transfer unit, the recording paper is fed between the corona discharger and the image carrier while being guided from the leading end thereof by an inlet / outlet guide of the corona discharger, and the leading end of the recording paper is transferred to the image carrier. And the recording paper is sent out, so that the paper is bent by the electrostatic force generated by the transfer current applied to the corona discharger, and the rear part from the leading end of the recording paper also comes into close contact with the image carrier. However, since there is a gap between the leading end of the entrance guide and the image carrier, the recording paper is extremely slid until the leading end of the recording paper comes out of the entrance guide and contacts the image carrier. This is an unstable state, causing a problem of poor adhesion at the leading end, resulting in poor transfer such as white spots. For this reason, conventionally, from the time when the leading edge of the recording paper enters the transfer electric field area to the time when it contacts the image carrier, the transfer current when transferring the central portion of the recording paper in the conveying direction to the corona discharger is smaller than the transfer current. A method has been employed in which a large transfer current is applied to prevent poor adhesion at the leading end.

[0005]

However, as described above, if a current larger than the current applied to the central portion of the image is switched and applied only once to the leading end of the recording medium, the transfer failure due to the poor adhesion of the recording paper may occur. Even if it can be prevented, excessive transfer occurs, so that it is not possible to satisfy the two demands which are mutually exclusive. In particular, at the time of backside copying of double-sided copying, the problem of white spots at the leading end due to back curl after passing through the fixing device appears remarkably. In view of the above, according to the present invention, white spots due to two causes, that is, insufficient transfer at the leading end portion and excessive transfer, are prevented at the same time by switching the applied current at the time of transferring to the leading end of the recording medium in a plurality of stages (over multiple times). The aim is to propose a way to do it.

[0006]

In order to achieve the above object,
The invention according to claim 1 is arranged such that a corona discharger is disposed so as to face the image carrier, and a transfer medium is guided between the corona discharger and the image carrier while guiding a recording medium by an entrance guide of the corona discharger during transfer. The recording medium is sent to the recording medium, and the corona discharge is performed by the corona discharger from the back surface of the recording medium, so that the recording medium is charged to a polarity opposite to the charging polarity of the toner image formed on the image carrier. An electrostatic image forming method for transferring a toner image formed on the image carrier to a recording medium, wherein a current applied to the corona discharger when the toner image is transferred to a leading end of the recording medium is recorded. And According to a second aspect of the present invention, in the electrostatic image forming method according to the first aspect, the transfer current applied to the front end portion, which is switched in a plurality of stages, is set such that the current value increases as the front end portion increases. According to a third aspect of the present invention, in the electrostatic image forming method according to the second aspect, the application end portion of the foremost stage of the transfer current applied to the leading end of the recording medium by switching in a plurality of stages,
It is characterized by being within the range of the width of the leading edge image of the recording medium. According to a fourth aspect of the present invention, in the electrostatic image forming method according to the first, second, or third aspect, the image carrier is an intermediate transfer member of a medium resistance material.

[0007]

According to the first aspect of the present invention, since the applied current when transferring the image to the leading end of the recording medium is controlled in a plurality of steps by a current larger than the central part of the recording medium, the electrostatic image forming method is controlled. By selecting the current value appropriately
Provided is an electrostatic image forming method in which adhesion to an image carrier is improved and white spots due to two causes, that is, insufficient transfer at a leading end portion and excessive transfer are prevented at the same time. According to a second aspect of the present invention, in addition to the electrostatic image forming method as described in the first aspect, the current is set to be larger at the front portion of the front end portion, so that the front end portion is in close contact with the image carrier. When the transferability is improved, the current is controlled with a reduced current value so as not to cause an excessive transfer, so that white transfer due to two causes, that is, insufficient transfer due to poor recording paper adhesion at the leading end portion and excessive transfer due to excessive transfer current, is performed. Provided is an electrostatic image forming method capable of simultaneously preventing detachment. According to a third aspect of the present invention, in addition to the electrostatic image forming method of the second aspect, since the application end portion of the first stage falls within the range of the leading end image chipping width, excessive transfer may occur. Even if a discharge occurs at the top stage where
It does not appear in the image within the range of the image chipping width. Claim 4
According to the present invention, in addition to the invention of the electrostatic image forming methods according to the first, second, and third aspects, even when the image carrier is an intermediate transfer body of a medium-resistance material, the current at the leading end is Can be set to be large, and the adhesion to the image carrier can be improved at the foremost portion, so that an electrostatic image forming method free from poor transfer at the leading end can be provided.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described in detail with reference to the accompanying drawings. First, an electrostatic image forming apparatus suitable for carrying out the present invention is classified into a type using an intermediate transfer belt and a type not using an intermediate transfer belt. A. FIG. 1 shows an overall configuration of a color electrostatic image forming apparatus using an intermediate transfer belt as an image carrier, and FIG. 2 shows a main configuration of the same. In these figures, a writing optical unit 21 as an exposure unit converts a color image data from a color scanner 20 into a color image data which is an electrical signal, and reads out the color image data of the original image. Then, optical writing corresponding to the original image is performed, and an electrostatic latent image is formed on the photoconductor 3 as an image carrier. The writing optical unit 21 includes a laser light emitting means (laser diode) 30 and its light emission drive control unit (not shown), a polygon mirror 31 and its rotation motor 32, an f / Θ lens 33, a reflection mirror 34, and the like. I have. Photoconductor 3
Rotates counterclockwise as indicated by the arrow. Around the photosensitive member cleaning unit 35 and the static elimination lamp 3
6, a developing device selected from among the potential sensor 38 and the rotary developing device 22 as a developing unit (the developing device 4 in the example of FIG. 1).
0), a development density pattern detector 39, an intermediate transfer belt 1 as an intermediate transfer medium, and the like. The volume resistivity of the intermediate transfer belt 1 is 1 × 10 ⁸ to 10 ¹² Ω ·
cm, surface resistivity of 1 × 10 ⁸ to 10 ¹¹ Ω (JISK6
911), a medium resistance belt, ethylene tetrafluoroethylene (ETFE), epichlorohydrin rubber or the like is used. Alternatively, it is used not as a belt but as a drum.

The rotary developing device 22 includes a black developing device 41, a cyan developing device 42, a magenta developing device 43, a yellow developing device 44, and a rotation drive unit (not shown) for rotating each developing device. I have. Each developing device includes a developing sleeve that rotates by contacting the ears of the developer with the surface of the photoconductor 3 in order to visualize the electrostatic latent image, and a developing sleeve that rotates to pump up and agitate the developer. It is composed of paddles and the like. In the standby state, the rotary developing device 22 is set at the position of black development, and when a copying operation is started, reading of black image data is started at a predetermined timing by the color scanner 20, and this image data is read. (Hereinafter, an electrostatic latent image based on black image data is referred to as a black latent image, and the same applies to cyan, magenta, and yellow). In order to develop from the leading end of the black latent image, before the leading end of the latent image reaches the developing position of the black developing device, the rotation of the developing sleeve is started to develop the black latent image with black toner. Thereafter, the developing operation of the black latent image area is continued, but when the rear end of the latent image passes the black developing position, the rotary developing device 22 is quickly moved from the black developing position to the next color developing position.
Rotates. The above operation is completed at least before the leading end of the latent image based on the next image data arrives.

When the image forming cycle is started, first, the photosensitive member 3 is rotated in a counterclockwise direction as indicated by an arrow, and the intermediate transfer belt 1 is rotated in a clockwise direction by a drive motor (not shown). . As the intermediate transfer belt 1 rotates, black toner image formation, cyan toner image formation, magenta toner image formation, and yellow toner image formation are performed, and finally black (Bk), cyan (C), and magenta (M) , Yellow (Y), and the intermediate transfer belt 1
A toner image is created on top of it.

Here, in order to reduce the time required for aligning the leading edge of the image when forming the superimposed toner image as described above, the support roller R of the intermediate transfer belt 1 is moved, as shown in FIG. As shown by the broken line, the intermediate transfer belt 1 is separated from the photoconductor 3, and the intermediate transfer belt 1 is rapidly fed under this separated state. First, black image formation is performed as follows. The charger 37 as charging means uniformly charges the photoconductor 3 with a negative charge to about -700 V by corona discharge (charging step). Subsequently, the laser diode 30 performs raster exposure based on the black signal (exposure step). When the raster exposure is performed in this manner, the exposed portion of the photoconductor 3 that is initially uniformly charged loses the charge proportional to the amount of exposure light, and an electrostatic latent image is formed.

The toner in the developing device is charged to a negative polarity by stirring with the ferrite carrier, and the black developing sleeve of the present developing device is negatively charged by a power source (not shown) with respect to the metal base layer of the photoreceptor 3. A bias of a potential in which a DC potential and an AC are superimposed is applied. As a result, the toner does not adhere to the portion of the photoconductor 3 where the charge remains, and the black toner is adsorbed to the portion having no charge, that is, the exposed portion, and a black visible image similar to the latent image is formed. It will be formed (development step).

The intermediate transfer belt 1 is stretched around a driving roller 4, transfer opposing rollers 5, 6, a cleaning opposing roller 7, and a group of driven rollers, and is driven and controlled by a driving motor (not shown). The black toner image formed on the photoconductor 3 is transferred to the surface of the intermediate transfer belt 1 that is driven at a constant speed in a state of contact with the photoconductor 3 by a belt corona discharger 8 as a primary transfer unit (primary transfer). Process). Hereinafter, the transfer of the toner image from the photoconductor 3 to the intermediate transfer belt 1 is referred to as primary transfer. The discharge efficiency (distribution ratio) of the belt corona discharger 8 is about 20 to 40%.

A small amount of untransferred residual toner present on the photoconductor 3 is cleaned by the photoconductor cleaning unit 35 in preparation for reuse of the next color of the photoconductor 3. The collected toner is stored in a waste toner tank (not shown) via a collection pipe. The black, cyan, magenta, and yellow toner images sequentially formed on the photoreceptor 3 are accurately and sequentially aligned on the intermediate transfer belt 1 to form a four-color superimposed belt transfer image. Using the transfer corona discharger 2 as a secondary transfer means, batch transfer is performed on recording paper as a recording medium (secondary transfer step). On the photoconductor 3 side, the process proceeds to the cyan process after the black process. At a predetermined timing, the reading of cyan image data by the color scanner 20 starts, and the formation of a cyan latent image is performed by writing laser light based on the image data. Do it. The cyan developing device 42 performs the rotation operation of the rotary developing device 22 with respect to the developing position after the previous rear end of the black development has passed and before the leading end of the cyan latent image has arrived. The latent image is visualized with cyan toner. Thereafter, the development of the cyan development area is continued, but when the rear end of the latent image passes, the rotation operation of the cyan development unit is performed in the same manner as in the case of the black developing device. This is also completed before the leading end of the magenta latent image arrives.
In the magenta and yellow processes, the respective operations of reading image data, forming a latent image, and developing are the same as those in the above-described black and cyan processes, and thus description thereof is omitted.

Transfer corona discharger 2 as secondary transfer means
Transfers a multicolor superimposed toner image on the intermediate transfer belt 1 onto a recording medium by applying a DC or AC + DC component by a corona discharge method. The distribution ratio is about 20 to 40%, which is the same as that of the belt corona discharger 8. The recording paper cassettes 51, 52, and 53 in the paper supply bank 23 contain recording papers of various sizes. Of these, the recording paper cassettes of the designated (selected) size are removed from the paper supply rollers 56. , 5
The paper is fed and conveyed in the direction of the registration roller pair 9 by 7, 58. Reference numeral 54 denotes a double-sided tray. The single-sided recording paper once stored on the double-sided tray is re-fed to the transfer position by the paper feed roller 59. Reference numeral 55 denotes a manual paper feed tray for OHP paper, thick paper, and the like.

At the time when the image formation is started, the recording paper is fed from the paper feed port of any of the above-mentioned cassettes, and waits at the nip portion of the registration roller pair 9. Then, when the leading end of the toner image on the intermediate transfer belt 1 approaches the transfer corona discharger 2, the registration roller pair 9 is driven so that the leading end of the recording paper coincides with the leading end of the image, and the registration of the paper and the image is performed. Done. In this way, the recording paper is superposed on the intermediate transfer belt 1 and passes over the transfer corona discharger 2 connected to the positive potential. At this time, corona discharge is discharged,
The discharge current charges the recording paper with a positive charge, and a substantial part of the toner image is transferred onto the recording paper. Subsequently, when the recording paper passes a portion of an unillustrated neutralization brush arranged on the left side of the transfer corona discharger 2 in the drawing, the recording paper is neutralized, peels off from the intermediate transfer belt 1 and moves to the paper transport belt 45.

The recording paper on which the four-color superimposed toner image is collectively transferred from the intermediate transfer belt 1 is conveyed to the fixing device 24 by a paper conveying belt 45, and is formed by a fixing roller 46 and a pressure roller 47 controlled at a predetermined temperature. The toner image is melted and fixed in the nip portion, sent out of the apparatus by a discharge roller pair 48, and stacked face up on a copy tray (not shown) to obtain a full color copy. The surface of the photoconductor 3 after the image transfer to the intermediate transfer belt 1 is cleaned by a photoconductor cleaning unit 35 including a brush roller and a rubber blade, and is uniformly discharged by a discharge lamp 36. The surface of the intermediate transfer belt 1 after transferring the toner image onto the recording paper is cleaned by pressing the blade again by the blade contacting / separating mechanism in the cleaning device 49. In the case of the repeat copy, the operation of the color scanner and the image formation on the photosensitive member 3 are performed after the first-color image process of the first sheet, and the process proceeds to the first-color image process of the second sheet at a predetermined timing. . In the intermediate transfer belt 1, the second black toner image is primarily transferred to the surface area cleaned by the cleaning device 49, following the batch transfer process of the first four-color superimposed image onto the recording paper. You. Thereafter, the same operation as the first sheet is performed as described above.

The above is a description of the copy mode for obtaining four full-colors of A4 size horizontal feed. In the case of the three-color copy mode or the two-color copy mode, the same as above with respect to the designated color and the number of times. Is performed.
In the case of the single-color copy mode, only the developing device of the predetermined color of the rotary developing device 22 is in the developing operation state, that is, the cleaning device 49 is set in the predetermined color developing position until the predetermined number of sheets is completed. The copying operation is continuously performed while the blade is pressed against the belt.

(B) Type without Intermediate Transfer Belt The image forming apparatus shown in FIGS. 1 and 2 is of a type that uses an intermediate transfer belt. A toner image is formed on a drum-shaped photoreceptor as a carrier by a well-known electrostatographic method, and the toner image is subjected to a corona discharge from a back surface of the recording paper by a corona discharger to place the recording paper on the photoreceptor. The formed toner image is charged with a polarity opposite to that of the charged toner and transferred, and the recording paper after the transfer is sent to a fixing device, and a copy is obtained through a fixing process. As described above, even in the electrostatic image forming apparatus of the type that does not use the intermediate transfer belt, when forming a full-color image, the photoconductor cleaning unit is separated from the photoconductor each time the toner image passes so that the photoconductor cleaning unit is separated from the photoconductor. It is possible to successively superimpose the color images to create a superimposed transfer image. Then, after the full-color toner image is collectively transferred to the recording paper, the photoconductor cleaning unit is brought into contact with the photoconductor to clean the photoconductor. As described above, for the intermediate transfer belt of the medium resistance used in the electrostatic image forming apparatus of the type (1), the adhesion force to the recording paper is weak because the intermediate transfer belt easily discharges electric charges. This is particularly significant because the transfer failure at the leading end of the recording paper, which is a problem of the present invention, tends to occur.

FIG. 3 is an enlarged view of the transfer corona discharger 2 provided as the secondary transfer means in FIG. 2 together with the leading end 10F of the recording paper 10. In FIG. 3, the intermediate transfer belt 1 is opposed to a casing portion of the transfer corona discharger 2 with a gap allowing paper to pass, and this opposed portion is supported by two transfer facing rollers 5 and 6. Thus, a flat portion is formed. In the drawing, the right side of the transfer corona discharger 2 is the entrance side of the recording paper, and the left side is the exit side of the recording paper. An entrance guide is formed by plate-like guide members 11 to 14 at the entrance side. These guide members 1
Reference numerals 1 to 14 are vertically opposed to each other, and guide the recording paper 10 on the entrance side with a guide member 13 and a guide member 14 made of a synthetic resin thin plate. 13 is fixed to the guide member 11 and located on the upper side, and the guide member 14 is fixed to the guide member 12 which is a part of the casing of the transfer corona discharger 2 and is located on the lower side. Reference numerals 14 are opposed to each other, and are arranged so as to gradually narrow the facing interval in the feeding direction of the recording paper 10 indicated by a broken line 15 so that the feeding direction of the recording paper 10 is closer to the transfer facing roller 5 side of the flat portion. To be specified.

The guide member 14 of the guide members 13 and 14 is slightly longer than the guide member 13 in the direction in which the recording paper 10 advances, and receives the recording paper 10 that hangs downward. I have to. On the other hand, on the exit side, the guide member 16 is for guiding the leading end 10F of the recording paper 10 so as not to enter the transfer corona discharger 2, and the guide member 16 further transports the recording paper to the transport belt 45 (see FIG. 1). The discharge wire 17 of the transfer corona discharger 2 extends in a direction penetrating the paper surface in FIG. 3 corresponding to the width of the intermediate transfer belt 1, and forms an image formed on the intermediate transfer belt 1 at the time of secondary transfer. A transfer current having a polarity opposite to the polarity of the charged toner is applied, and corona discharge is performed. The corona discharge covers the flat portion formed by the intermediate transfer belt between the transfer opposing rollers 5 and 6 so that the end of the guide member 14 on the entrance side and the guide member 16 on the exit side.
A transfer electric field region 19 having a depth in a direction penetrating the paper of FIG.

In the drawing, the leading end 10F of the recording paper 10 which advances in the feed direction indicated by a broken line 15 advances from the leading end of the guide member 14 into the transfer position, and is wound around the transfer opposing roller 5. , At the position 18, and further advanced, and the leading end 10 </ b> F is slightly separated from the intermediate transfer belt 1. In this embodiment, the recording paper 10
In order to prevent the leading edge 10F from separating, a transfer current whose size is reduced stepwise is applied to the leading edge of the recording paper as shown in FIG. In the present embodiment, 600
By setting the transfer current to μА, 400 μА in the next stage, and 200 μА in the central part of the image, which is the majority, sufficient effects could be obtained without causing insufficient transfer and excessive transfer. 600 which is the transfer current applied to the foremost part
μА is a current value suitable for bringing the recording paper 10 into close contact with the belt, but excessive transfer occurs for an image, and discharge occurs to cause an abnormal image. In addition, the most advanced part is 200μ
In the case of А, poor transferability occurs when the recording paper 10 lacks in adhesion to the belt and has a slightly curled tip such as double-sided copy. Therefore, in this embodiment, after applying 600 μA to the predetermined width portion at the forefront and 400 μA sequentially to the next predetermined width portion, 200 μA is applied to the remaining portion, so that the belt at the forefront portion is applied. While increasing the adhesive force to increase the adhesive stability of the succeeding part, it also reduces the excessive transfer while slightly reducing the adhesive strength for the next predetermined width, and improves the transferability of the remaining part exclusively. A small current that is optimal for the operation.

FIG. 5 shows the current value 6 at the initial stage when the transfer is excessive.
FIG. 4 is a diagram illustrating a state in which an application end portion of 00 μА is accommodated within an image missing area of the recording paper 10. In this way, even if a corona discharge occurs at the initial stage of the current value of 600 μА, the range affected by the discharge is outside the recording paper 10 or over the image lacking range. It is not output as an image. The image missing width is a portion that is controlled in advance so that the image does not appear because the recording paper easily winds around the fixing roller when an image exists in this portion. The current value of 400 μА at the next stage is a high current value in a range that does not cause an abnormal image, and is a value that can prevent the recording paper 10 that has once adhered to the belt from being separated again. After that,
An optimal current value of 200 μА is applied for image transfer.
These stepwise changes in the current value are performed by a control unit (not shown) to control the discharge wire 1 of the transfer corona discharger 2.
7 to the recording paper 10.

FIG. 6 is a control block diagram relating to the supply of voltage to the transfer corona discharger 2. As shown in FIG. When the full color mode is designated by a predetermined key of the operation unit 60, the CP
In U61, the data is output to the I / O board 64, which is the control interface of the transfer corona discharger 2, based on the image forming data stored in the ROM 62 and the RAM 63. The primary transfer power pack (not shown) is activated by the data given to the I / O board 64,
A high voltage for development and primary transfer is supplied. After the timing is adjusted, pulse width modulation (PW
M) The controlled signal is converted into an analog voltage value to be supplied by the D / A converter 65 and applied to the secondary transfer power pack 66. In the secondary transfer power pack 66, the stepwise current shown in FIGS.
Is supplied to the secondary transfer corona discharger 67. In the above embodiment, the transfer current is controlled in two stages as a stage before the main transfer current of 200 μA is applied. However, this is an example, and three or more stages may be used. In addition, the transfer current value, the application time, the timing, and the like of each current shown in the drawings are merely examples, and are not limited thereto.

[0025]

As described above, according to the first aspect of the present invention,
The applied current at the time of transferring to the leading end of the recording medium is made larger than the applied current to the central part of the recording medium, and the former applied current is switched to a plurality of stages (decreased in stages). Since the adhesiveness of the leading end portion of the recording paper to the image carrier is improved, it is possible to provide an electrostatic image forming apparatus that prevents white spots due to two causes, that is, insufficient transfer at the leading end portion and excessive transfer. . According to the second aspect of the present invention, in addition to the first aspect of the present invention, the current for switching to a plurality of stages is set to be larger in the front part (the foremost part) of the front end part, so that insufficient transfer and excessive transfer of the front end part are reduced. It has become possible to provide an electrostatic image forming apparatus in which prevention is assured. According to the third aspect of the present invention, in addition to the second aspect of the present invention, the application end portion of the first stage is configured to be within the range of the leading image chipping width.
Even if a discharge occurs at the top stage where there is a high possibility of excessive transfer, it is no longer output as an abnormal image because it is outside the recording paper or within the image missing range. According to the fourth aspect of the present invention, in addition to the first, second, and third aspects of the present invention, it is possible to improve the adhesion to the image carrier at the most distal end portion. An intermediate transfer member made of a medium-resistance material, which is liable to cause white spots, can be used as an image carrier without any problem.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a main part of an electrostatic image forming apparatus suitable for carrying out the present invention.

FIG. 2 is a diagram illustrating a main configuration of an electrostatic image forming apparatus suitable for carrying out the present invention.

FIG. 3 is an explanatory diagram illustrating a state when a leading end portion of a recording sheet reaches an entrance of a corona discharger.

FIG. 4 is an explanatory diagram illustrating a stepwise supply current supplied to the leading end of a recording sheet via a corona discharger of the present invention.

FIG. 5 is an explanatory diagram illustrating a relationship between a stepwise supply current supplied to a leading end portion of a recording sheet and an image missing area of the recording sheet according to the present invention.

FIG. 6 is a block diagram related to a voltage supply to a transfer corona discharger.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Image carrier or intermediate transfer belt 2 Corona discharger or transfer corona discharger 3 Photoconductor 5 Transfer opposing roller 6 Transfer opposing roller 10 Recording paper 10F Tip 17 Discharge wire

Claims (4)

[Claims] 1. A corona discharger is disposed facing an image carrier, and a recording medium is guided between the corona discharger and the image carrier while transferring a recording medium by an entrance guide of the corona discharger during transfer. The recording medium is charged with a polarity opposite to that of a toner image formed on the image carrier by performing corona discharge from the back surface of the recording medium with the corona discharger. An electrostatic image forming method for transferring a toner image formed on a holding body onto a recording medium, comprising applying a current applied to the corona discharger to a central portion of the recording medium when transferring the toner image to a leading end of the recording medium. The method of forming an electrostatic image, wherein the current value of the transfer current applied to the leading end of the recording medium is switched in a plurality of stages. 2. The electrostatic image forming method according to claim 1, wherein the transfer current of the front end portion, which is switched and applied in a plurality of stages, is set such that the current value is set to be larger toward the front. Method. 3. The electrostatic image forming method according to claim 2, wherein the application end portion of the foremost stage of the transfer current applied to the leading end of the recording medium by switching in a plurality of stages is missing the leading end image of the recording medium. An electrostatic image forming method characterized by being within a range of a width. 4. The electrostatic image forming method according to claim 1, wherein the image carrier is an intermediate transfer member made of a medium-resistance material. Forming method.