JP2009175359A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of preventing the occurrence of image deterioration and color mixture even when a Bk small-area image is consecutively output while toner recycling is performed in such a state that a Bk imaging unit is arranged at the most downstream position. <P>SOLUTION: In the image forming apparatus, out of a plurality of imaging units 2 to 5, the Bk imaging unit 5 is arranged independently, and a formed Bk image is directly transferred to transfer paper P conveyed by a transfer conveying belt 36. The Bk imaging unit 5 includes a toner collection mechanism 40 restoring toner removed by a cleaning part 23 after transfer by using two-component developer to a developer stirring part 27a of a developing means 27, and a control means 41 setting a threshold of output image area per sheet and performing control to develop predetermined amount of toner after forming an image when the image area is smaller than the threshold. When a toner image passes through a transfer nip between the transfer conveying belt 36 and an image carrier 9, bias having a polarity to transfer the toner image to the transfer conveying belt 36 is applied. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a color PPC and a color printer.

A full-color electrophotographic apparatus is provided with a plurality of color developing devices around one photoconductor, and a toner is attached to these developing devices to form a composite toner image on the photoconductor, and then the toner image is transferred. A so-called 1-drum type that records color images on transfer paper, and a plurality of photoconductors arranged side by side are each provided with a developing device, and a single color toner image is formed on each photoconductor, and these single color toner images are sequentially There is a so-called tandem type in which a composite color image is recorded on a transfer paper by transfer (see, for example, Patent Document 1).
Patent Document 1 is merely listed to show a conventional example of a tandem type electrophotographic apparatus. A direct transfer method in which images on each photoconductor are sequentially transferred onto a transfer paper conveyed by a conveyance belt by a transfer device, and images on each photoconductor are sequentially transferred to an intermediate transfer member once in a primary transfer unit, There is an indirect transfer method in which images on the intermediate transfer body are collectively transferred onto a transfer sheet at a secondary transfer portion.

FIG. 5 is a schematic diagram showing a configuration of a conventional example of a tandem type image forming apparatus. FIG. 6 is an enlarged schematic view showing the periphery of the image forming unit of the monochrome image forming apparatus. Here, an intermediate transfer type image forming apparatus will be described as a tandem type image forming apparatus. Since the black and white image forming apparatus in FIG. 6 does not employ the intermediate transfer system, the constituent members around the image forming unit are described with the same reference numerals as those in FIG.
As shown in FIG. 5, in the image forming apparatus 1 that employs the tandem type intermediate transfer system, yellow (along the lower traveling side of the intermediate transfer belt 10 that is an intermediate transfer body that is rotationally driven in the direction of arrow A in the drawing. Four image forming units 2 to 5 corresponding to the respective color toners of Y), cyan (C), magenta (M), and black (K) are arranged.
An intermediate transfer member 10 made of rubber or resin having a single layer or a multilayer structure is stretched by a secondary transfer counter bias applying roller 11 and support rollers 12, 13, and 14, and in the illustrated example, the intermediate transfer member A is counterclockwise. It is rotationally driven in the direction. Further, a secondary transfer roller 15 is disposed opposite to the secondary transfer counter bias applying roller 11 across the intermediate transfer body 10.

The secondary transfer counter bias applying roller 11 can form an electric field having the same polarity as that of the toner by a secondary transfer electric field forming unit (not shown). The toner can be secondarily transferred to the transfer paper.
On the left side of the support roller 14, an intermediate transfer member cleaning unit 34 that removes residual toner remaining on the intermediate transfer member 10 after image transfer is provided.
Inside the intermediate transfer member 10 between the support roller 12 and the support roller 13, primary transfer bias rollers 16, 17, 18, and 19 that generate an electric field during primary transfer can contact the intermediate transfer member 10. Is arranged in.
Further, on the opposite side of the primary transfer roller (primary transfer bias roller) 16, 17, 18, 19 across the intermediate transfer body 10, four photosensitive materials of yellow, cyan, magenta, and black are arranged along the conveyance direction. A tandem type image forming apparatus is configured by arranging image forming units 2, 3, 4, 5 around the bodies 6, 7, 8, 9.

A toner image secondarily transferred by a secondary transfer roller 15 onto a transfer sheet conveyed from a paper feeding unit (not shown) is then fixed to a fixing roller 28 constituting a fixing unit in order to fix the toner image on the transfer sheet. In the nip portion with the pressure roller 29, fixing is performed on the transfer paper by heat and pressure.
Around the respective photosensitive members 6 to 9 of the image forming apparatus 1, a photosensitive member charging unit 20a, a photosensitive member cleaning unit 20 to 23, and a developing unit 24 to 27 are arranged. In the figure, toner cartridges 30, 31, 32 and 33 shown above the intermediate transfer member 10 are connected to developing units 24 to 27 of respective colors, although not described in detail.
In contrast to the direct transfer method, the intermediate transfer method requires one transfer step, but it is highly compatible with paper types and has a short transport path from the paper feed unit to the paper discharge unit. The first print time of Bk can be shortened by arranging in B.
In this way, the intermediate transfer system has many advantages, but the toner transferred to the intermediate transfer member by the image forming unit arranged on the upstream side in the rotation direction of the intermediate transfer member passes through the image forming unit on the downstream side. In this case, there is a problem that a part of toner is reversely transferred from the intermediate transfer member to the photosensitive member.

Next, toner recycling will be described with reference to FIG. Here, since the same applies to the other image forming units 2, 3, 4, 5 (FIG. 4), the image forming unit 2 will be described.
The cleaned toner on the photoreceptor (hereinafter referred to as the photoreceptor drum) 6 is conveyed from the cleaning unit 20 to a waste toner container (not shown) and stored in the waste toner container together with the intermediate transfer cleaned toner. . However, if the cleaned toner is discarded as waste toner, a certain percentage of toner is wasted.
Therefore, in the black and white machine, the toner collected by the cleaning unit 20 is reused with the configuration shown in FIG. The photosensitive drum 6 rotates in the direction of the arrow in the figure, and the developer carrier 24a (hereinafter referred to as a developing roller) is close to the photosensitive drum 6 with a very narrow gap Gp (hereinafter referred to as a developing gap). Yes.
In the developer stirring section 24b, a two-component developer composed of toner and a magnetic carrier is stirred while being circulated by two screws arranged in parallel with the developing roller 24a, and the toner is charged and adhered to the carrier. It is in the state.

The developing roller 24a has a magnet fixed inside, and when the outer cylindrical developing sleeve rotates in the direction shown in the figure, the developer in the developer supply unit (hereinafter referred to as the single shaft side) is pumped up by magnetic force. Thus, a magnetic brush is formed at the facing portion between the photosensitive drum 6 and the developing sleeve 24a with a predetermined pumping amount by the developer regulating member 24c.
The surface speed of the developing sleeve 24a (developing sleeve linear velocity) is set faster than the surface velocity of the photosensitive drum 6 (photosensitive linear velocity), and toner is supplied by the tip of the magnetic brush rubbing the photosensitive drum 6. The Due to the potential difference between the bias applied to the developing sleeve 24a and the latent image potential of the photosensitive drum 6, negatively charged toner adheres to the latent image.
The developed toner is transferred to the transfer paper P at the transfer nip portion between the photosensitive drum 6 and the roller 16 as a transfer body, but all the toner is not transferred and remains on the photosensitive drum 6 without being transferred. It remains as toner.
The toner is cleaned by the cleaning blade 20 b and conveyed to the rear side of the cleaning unit 20 by the conveying member 35. On the rear side of the cleaning unit 20, the toner is transported in the direction of arrow B (outlined) in the diagram along a transport path indicated by a dotted line in the diagram, and the toner is dropped to the developer stirring unit 24 b and reused.

With such a configuration, it is possible to form an image without wasting toner, but when a low-area image is continuously output, the developing unit for a long time until the toner in the developing unit 24 is developed. Since it circulates in the interior 24, a lot of stress is applied until it is developed. For this reason, there is a problem in that the toner deteriorates compared to when the image is not a low area image, the transferability is deteriorated, and the image deteriorates.
In order to prevent this problem, the average image output area in the image forming unit 2 is counted, and when the area is low, a predetermined toner is developed on the photosensitive drum 6 after image formation (hereinafter referred to as toner discharge). As a result, it is possible to reduce the time required for the toner to circulate in the developing means 24 until it is developed.
JP 2001-5243 A

However, in the case of using the above-mentioned toner recycling type image forming unit, even if the toner is discharged, it returns to the developing means, so that the toner is deteriorated before being developed on the image portion, and the image The effect on deterioration will be lost.
Further, when toner recycling is performed in the intermediate transfer type image forming apparatus, the toner image formed by the upstream image forming unit is reversely transferred to the downstream image forming unit. It is cleaned and mixed into the developing means. Therefore, there arises a problem that the image forming colors at the downstream position are mixed.
In recent years, there has been a strong demand to reduce CPP (the cost of replacement parts, toner, etc. divided by the number of sheets before replacement, the cost per copy). There is a strong demand for a value of. Therefore, a configuration in which only Bk toner is recycled in a color machine can be considered.
Therefore, in view of the above situation, the object of the present invention is to arrange a Bk image forming unit at the most downstream position and perform toner recycling, even when a Bk low area image is continuously output. An object of the present invention is to provide an image forming apparatus capable of preventing image degradation and color mixing problems.

  In order to solve the above problems, the invention according to claim 1 forms a plurality of color images on the intermediate transfer member by arranging a plurality of image forming units for forming toner images on the image carrier in parallel. In the image forming apparatus for forming a color image by superimposing the plurality of color images on a transfer sheet, the Bk image forming unit among the plurality of image forming units is independently arranged and the Bk image forming unit The Bk image formed in step 1 is directly transferred to the transfer paper transported by the transfer transport belt, and the Bk image forming unit uses the two-component development to transfer the toner cleaned by the cleaning unit after the transfer to the developing means. A toner recovery mechanism that returns the developer to the developer agitation unit; and a control unit that sets a threshold value for the output image area per sheet and controls the development of a predetermined amount of toner after image formation when the image area is lower than the threshold value. Wherein the toner image, characterized in an image forming apparatus for applying a bias polarity to be transferred to the transfer conveyor belt as it passes through the transfer nip between the image bearing member and the transfer conveyance belt.

According to a second aspect of the present invention, the intermediate transfer member includes the cleaning unit, and a portion on the transfer conveyance belt to which the Bk toner developed after image formation is attached forms a nip portion with the intermediate transfer member. The image forming apparatus according to claim 1, wherein a bias for moving toner is applied to the intermediate transfer member at a timing of passing.
According to a third aspect of the present invention, the transfer / conveying belt includes a transfer / conveying belt cleaning unit, and when the Bk image is formed, the transfer / conveying belt and the intermediate transfer belt are separated from each other, and the toner developed after the image is formed The image forming apparatus according to claim 1, wherein the image forming apparatus is cleaned by the cleaning unit of the transfer conveyance belt.

  According to the present invention, the Bk image forming unit is independently arranged and directly transferred onto the transfer paper conveyed by the transfer conveyance belt, and the polarity bias transferred to the transfer conveyance belt when the discharged toner passes through the transfer nip. Therefore, even when the Bk toner is recycled, color mixing does not occur, and the discharge effect can be further improved.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of an example of an image forming apparatus according to the present invention. FIG. 2 is a block diagram for explaining a main part of the present invention based on FIG.
The image forming apparatus 1 adopting the tandem type intermediate transfer system in FIG. 1 is the same as the image forming apparatus described as the conventional example in terms of the basic structure. The same reference numerals are used and description thereof is omitted.
In the drawing, yellow (Y), cyan (C), magenta (M), and black (K) toners are respectively transferred along the lower traveling side of the intermediate transfer belt 10 that is an intermediate transfer member that is rotationally driven in the direction of arrow A. Four corresponding image forming units 2 to 5 are arranged. The intermediate transfer belt 10 is stretched by a secondary transfer counter bias applying roller 11 that also serves as a support roller and a support roller 14, and is driven to rotate in the A direction counterclockwise in the illustrated example.
Further, a secondary transfer roller 15 is disposed opposite to the secondary transfer counter bias applying roller 11 across the intermediate transfer belt 10. A transfer conveyance belt 36 runs between the secondary transfer counter bias applying roller 11 and the secondary transfer roller 15, and this transfer conveyance belt 36 is stretched on support rollers 37 and 38, one of which is a drive roller. Has been.

The secondary transfer counter bias applying roller 11 can form an electric field having the same polarity as that of the toner by a secondary transfer electric field forming unit (not shown). The toner can be secondarily transferred to a transfer sheet that is a recording medium conveyed from a non-feeding unit. An intermediate transfer belt cleaning unit 34 for removing residual toner remaining on the intermediate transfer belt 10 after image transfer is provided on the left of the support roller 14.
Inside the intermediate transfer belt 10 between the support roller 12 and the support roller 13, primary transfer bias rollers 17, 18, and 19 that generate an electric field during primary transfer are arranged in contact with the intermediate transfer belt 10. Has been.
In this image forming apparatus 1, the Bk image forming unit 5 is configured to be independently transferred and directly transferred onto the transfer paper P conveyed by the transfer conveyance belt 36. In this case, the primary transfer bias to the Bk image forming unit 5 is applied from the back side of the photosensitive drum 9 with the transfer conveyance belt 36 interposed therebetween.

Further, on the opposite side of the primary transfer rollers (primary transfer bias rollers) 17, 18, 19 across the intermediate transfer belt 10, three photosensitive drums 6 of yellow, cyan, magenta, A tandem type image forming apparatus is configured by arranging image forming units 2, 3, 4 centered on 7 and 8 side by side.
The toner image secondarily transferred by the secondary transfer roller 15 onto the transfer paper P transported from a paper feeding unit (not shown) is then fixed in the fixing unit in order to fix the toner image on the transfer paper P. In the nip portion between the roller 28 and the pressure roller 29, the toner image is fixed on the transfer paper P by heat and pressure.

Around each of the photosensitive drums 6 to 9 of the image forming apparatus 1, a photosensitive member charging unit (not shown), a photosensitive member cleaning unit 20 to 23, and a developing unit 24 to 27 are arranged. In the figure, toner cartridges 30, 31, 32 and 33 shown above the intermediate transfer member 10 are connected to developing units 24 to 27 of respective colors, although not described in detail.
Furthermore, a description will be given with reference to FIG. In this image forming apparatus 1, the three color image forming units 2 to 4 are arranged in parallel to primarily transfer the toner image onto the intermediate transfer belt 10, and Bk is a sheet feeding unit (not shown) in the transfer conveyance belt 36. Is directly transferred onto the transfer paper P conveyed.
The transfer paper P onto which Bk has been transferred is transported by the transfer transport belt 36 and superimposed on the three color toner images on the intermediate transfer belt 10 by the secondary transfer roller 15 of the secondary transfer section to form a fixing section. Fixing is performed by the fixing roller 28 and the pressure roller 29. The Bk image forming unit 5 performs toner recycling, and returns the toner cleaned by the image forming unit cleaning unit 23 to the developer of the developing unit 27 for reuse.

Referring to FIG. 2, in the image forming apparatus 1, a plurality of image forming units 2 to 5 are arranged in parallel to form toner images on the photosensitive drums 6 to 9 as image carriers, and the intermediate transfer belt 10. A plurality of color images are formed thereon, and the plurality of color images are superimposed on the transfer paper P to form a color image.
Among the plurality of image forming units 2 to 5, the Bk image forming unit 5 is arranged independently, and the formed Bk image is transferred to the transfer paper P conveyed to the transfer conveyance belt 36 from a paper supply unit (not shown). Directly transcribed.
The Bk image forming unit 5 includes a toner collecting mechanism 40, and after the transfer using the two-component developer, the toner cleaned by the cleaning unit 23 is returned to the developer stirring unit 27a of the developing unit 27 by the toner collecting mechanism 40.
The Bk image forming unit 5 also includes a control means 41 for setting a threshold value for the output image area per sheet and for developing a predetermined amount of toner after image formation when the image area is lower than the threshold value. It is out.

When the toner image formed by the Bk image forming unit 5 passes through the transfer nip between the transfer conveyance belt 36 and the photosensitive drum 9, a bias having a polarity to be transferred to the transfer conveyance belt 36 is applied. This bias is applied from the back side of the photosensitive drum 9 with the transfer conveyance belt 36 interposed therebetween.
With such a configuration, the toner imaged on the upstream side is not reversely transferred to the Bk photoconductor drum 9, so that the toner of other colors is developed by the developing means 27 even if the toner recycling is performed in the Bk image forming unit 5. No color mixing occurs. Further, by arranging the Bk image forming unit 5 at the most downstream position, the first print time at the time of Bk single-color image forming can be shortened to the same level as a monochrome machine.
In the image forming apparatus 1 in FIGS. 1 and 2, the image forming units 2 to 5 are arranged in the order of yellow (Y), cyan (C), magenta (M), and black (Bk). A form is demonstrated.

FIG. 3 is a timing chart showing the timing of LD writing and primary transfer bias when toner is discharged after image formation when the image area ratio of Bk is lower than the threshold value.
Referring to FIGS. 1 to 3, in a normal full-color image forming sequence, Y, C, M, and Bk LD writing is performed after each photosensitive drum 6-9 is charged to form a latent image, and developing means The latent image is developed at 24-27 to form a toner image.
In the Y, C, and M image forming units 2 to 3, intermediate transfer is performed by applying a primary transfer bias by the primary transfer bias rollers 17, 18, and 19 when the formed toner image passes through the primary transfer portion. A toner image is transferred onto the belt 10.
In the Bk image forming unit 5, the formed toner image is directly transferred onto the transfer paper P by the transfer conveyance belt 36 as a transfer portion. The control means 40 counts the average image output area in the image forming unit 2 and, when the Bk image area ratio is lower than the threshold value, discharges Bk toner in the non-image portion after the full color image is formed. Do.

When the Bk toner image on the photosensitive drum 9 passes through the nip portion with the transfer conveyance belt 36, the Bk toner is discharged to transfer the toner image from the photosensitive drum 9 to the transfer conveyance belt 36. The biasing unit 42 applies the Bk transfer bias as described above from the back side of the photosensitive drum 9 with the transfer conveyance belt 36 interposed therebetween.
In this way, the discharged toner does not enter the cleaning means 23 of the Bk photoconductor drum 9 so that the toner discharge effect can be obtained even when the toner is recycled.
The above-described toner discharge is performed only when a low-area image is continuously output. Under normal conditions of use, the transfer residual toner and the scumming toner are returned to the developing means 27 and reused. The usage rate can be increased to reduce the toner consumption.
In this way, since the polarity bias transferred to the transfer conveyance belt 36 when the discharged toner passes through the transfer nip is applied, no color mixing occurs even when the Bk toner is recycled.

Next, although there are several possible cleaning methods for the transfer conveyance belt 36, a bias is applied to move the toner to the intermediate transfer belt 10 at the nip portion with the intermediate transfer belt 10 so that the toner adheres to the intermediate transfer belt 10. There is a method of cleaning by the transfer belt cleaning means 34 of the intermediate transfer belt 10.
FIG. 4 is a timing chart showing the timing of LD writing and primary transfer bias in the case where a bias is applied at the nip portion between the intermediate transfer belt and the transfer conveyance belt and the discharged toner adheres to the intermediate transfer belt.
1, 2, and 4, the bias (secondary transfer in FIG. 1) is performed at the nip portion between the intermediate transfer belt 10 and the transfer conveyance belt 36 (Bk toner is not transferred but is referred to as a secondary transfer portion here). (Transfer bias) is applied, and the discharged toner adheres to the intermediate transfer belt 10.

In the image forming apparatus 1 described above, the Bk discharge toner adheres to the transfer / conveyance belt 36. Therefore, in the timing chart shown in FIG. 4, the nip portion between the intermediate transfer belt 10 and the transfer / conveyance belt 36 (Bk toner is not transferred). Here, a bias (secondary transfer bias in FIG. 4) is applied by a secondary transfer portion), and the discharged toner is adhered onto the intermediate transfer belt 10.
The toner is cleaned by the transfer belt cleaning means 34 of the intermediate transfer belt 10. This method eliminates the need for a cleaning unit for the transfer / conveying belt 36, and is advantageous in that the apparatus can be reduced in size and cost.
Bias for moving the toner to the intermediate transfer belt 10 at the timing when the portion on the transfer conveyance belt 10 to which the Bk toner developed after image formation is attached passes through the nip portion between the transfer conveyance belt 36 and the intermediate transfer belt 10. Therefore, it is not necessary to arrange a cleaning unit on the transfer / conveying belt 3, and the image forming apparatus can be reduced in size and cost.

The above-described configuration in which the discharged toner adheres to the intermediate transfer belt 10 eliminates the need for the cleaning unit of the transfer conveyance belt 36, and has an advantage that the image forming apparatus can be reduced in size and cost. However, in order to allow the discharged toner to adhere to the intermediate transfer belt 10 during Bk single-color image formation, the intermediate transfer belt 10 must be driven even during Bk single-color image formation.
On the other hand, if a transfer conveyance belt cleaning means (shown as a dotted line 39 in FIG. 1) is arranged on the transfer conveyance belt 36, the transfer conveyance belt 36 and the intermediate transfer belt 10 are connected when forming a Bk image. Toner that is separated and developed after image formation (discharge toner) can be cleaned by the cleaning means 39 of the transfer conveyance belt 36.
In general color machines, the ratio of Bk single color is high, so if the intermediate transfer belt 10 is not driven during Bk single color image formation, the life of the intermediate transfer belt 10 and the transfer belt cleaning unit 34 of the intermediate transfer belt 10 is considerably extended. be able to. This configuration requires the cleaning means 39 for the transfer / conveying belt 36, which requires space and is disadvantageous for miniaturization and cost reduction. However, there is a merit for a high-speed machine that requires a large number of sheets to be used and has a long service life. large.

1 is a schematic diagram illustrating a configuration of an example of an image forming apparatus according to the present invention. It is a block diagram explaining the principal part of this invention based on FIG. 6 is a timing chart showing the timing of LD writing and primary transfer bias when toner is discharged after image formation when the image area ratio of Bk is lower than a threshold value. 6 is a timing chart showing the timing of LD writing and primary transfer bias when a bias is applied at the nip portion between the intermediate transfer belt and the transfer conveyance belt and the discharged toner adheres to the intermediate transfer belt. It is the schematic which shows the structure of the prior art example of a tandem type image forming apparatus. FIG. 3 is a schematic diagram showing an enlargement of the periphery of an image forming unit of a black and white image forming apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus, 2 Image forming unit, 5 Image forming unit, 9 Image carrier (photosensitive drum), 10 Intermediate transfer body (Intermediate transfer belt), 23 Cleaning part (Image forming unit cleaning means), 27 Developing means, 27a Developer stirrer, 34 Intermediate transfer belt cleaning means (cleaning part), 39 Transfer conveyance belt cleaning means, 40 Toner recovery mechanism, 41 Control means, 42 Transfer bias applying means, P transfer paper

Claims (3)

A plurality of image forming units that form toner images on the image carrier are arranged in parallel to form a multi-color image on the intermediate transfer body, and the multi-color image is superimposed on the transfer paper to form a color image. In the image forming apparatus to
Among the plurality of image forming units, the Bk image forming unit is independently arranged, and the Bk image formed by the Bk image forming unit is directly transferred to the transfer paper conveyed by a transfer conveyance belt, The Bk image forming unit uses a two-component development, a toner recovery mechanism for returning the toner cleaned in the cleaning section after transfer to the developer stirring section of the developing means,
Control means for setting a threshold value for the output image area per sheet and for developing a predetermined amount of toner after image formation when the image area is lower than the threshold value;
An image forming apparatus, wherein a bias having a polarity to be transferred to the transfer conveyance belt is applied when the toner image passes through a transfer nip between the transfer conveyance belt and the image carrier.   The intermediate transfer member includes a cleaning unit, and toner is applied to the intermediate transfer member at a timing at which a portion on the transfer conveyance belt to which Bk toner developed after image formation has adhered passes through a nip portion with the intermediate transfer member. The image forming apparatus according to claim 1, wherein a bias to be moved is applied.   The transfer / conveying belt includes a transfer / conveying belt cleaning unit, and when forming a Bk image, the transfer / conveying belt and the intermediate transfer belt are separated from each other, and the toner developed after the image formation is cleaned by the transfer / conveying belt cleaning unit. The image forming apparatus according to claim 1.

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