JP2000019863A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of preventing toner preshed out of transfer paper from the adherence on transferring means. SOLUTION: As for this device, in the case of impossible to preliminarily recognize the length of transfer paper at the time of feeding paper by hand or the like, when the image length is longer than the transfer paper length, the toner on an intermediate transfer belt 10 swollen out from the transfer paper 28 is adhered on the secondary transfer bias roller 14 by which the transfer paper soil on the subsequent print and transfer defect are caused, therefore by determining the transfer paper length based on the time when the leasing end and the trailing end of the transfer paper 28 is respectively allowed to pass through the detecting position by the paper feed sensor 15, in comparing the obtained transfer paper length and the image length, when the image length is longer than the transfer paper length, the secondary transfer bias roller 14 is separated from the intermediate transfer belt 10 by an attach/detaching mechanism 16 before the trailing end of the transfer paper 28 is allowed to pass through the secondary transfer region. Then, the toner is prevented from adhering on the secondary transfer bias roller 14, and the transfer paper soiling and the transfer defect for the subsequent print, therefore the excellent color image can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile, etc., and more particularly to a method for carrying a toner image transferred from an image carrier to the image carrier. The present invention relates to an image forming apparatus having an intermediate carrier that conveys a toner image until a transfer process.

[0002]

2. Description of the Related Art In a conventional image forming apparatus, a non-contact type corona charger has been used as a transfer means for transferring a toner image on an image carrier onto transfer paper. However, in order to generate corona discharge in the corona charger, a high voltage must be applied, and ozone is generated. For this reason, in recent years, the use of a contact transfer method in which a bias voltage is directly applied to the back surface of a transfer sheet has been increasing in many cases. This is because the contact transfer method enables transfer at a lower voltage than a corona charger and generates less ozone.

[0003] In such an image forming apparatus, a transfer paper suitable for the image size is usually supplied, so that there is no particular problem. If the size of the transfer paper is smaller than the size, the toner on the image carrier in a portion protruding from the transfer paper is directly transferred to the transfer roller. Changing the resistance value of the roller may cause a partial transfer failure. In particular, a full-color printer often has a plurality of toner layers, and the effect of toner attached to the transfer roller is great.

[0004] Such a problem is caused when the toner adhesion width on the transfer roller coincides with the transfer paper width direction of the next print.
That is, it is remarkable when the image length is longer than the transfer paper length. If the image data width is larger than the transfer paper width,
Since the toner adhering on the transfer roller and the transfer paper width of the next print do not match, the above-mentioned problem is unlikely to occur. For this reason,
If the image size is larger than the transfer paper size by comparing the image size with the transfer paper size, a bias having a polarity opposite to that of the transfer bias is applied to the transfer roller between the first and second images, so that the image on the transfer roller is reduced. A method has been proposed in which the adhered toner is transferred to an image carrier and the transfer roller is cleaned (see JP-A-6-161308). Also, if the image size is larger than the transfer paper size by comparing the image size with the transfer paper size, a method of increasing the cleaning bias value applied to the transfer roller or increasing the application time according to the amount of protrusion is also proposed. (See Japanese Patent Application Laid-Open No. 7-287476).

[0005]

However, in the former method, it is necessary to clean the transfer roller at least one or more times between images, so that the distance between images during continuous printing must be relatively long. In the image forming apparatus using the intermediate carrier, the circumferential length of the intermediate carrier is determined by a value obtained by adding the distance between images to the maximum transfer paper length. Must be lengthened, which causes a problem that the device becomes large. In addition, the length of the intermediate carrier in the circumferential direction is increased, so that the time required for printing is inevitably increased. Also, if the outer diameter of the transfer roller is reduced in order to reduce the distance between images, the strength of the roller is insufficient. Therefore, there is a problem that a transfer failure due to the deflection of the roller is caused.

In the latter method as well, in a full-color printer using a plurality of color toners, the absolute amount of the adhered toner is large, and even if the cleaning bias is increased, there is a problem that back stains are generated. Prolonging the cleaning bias application time causes problems such as a longer print time.

SUMMARY OF THE INVENTION In view of the above-mentioned conventional problems, it is an object of the present invention to provide an image forming apparatus capable of preventing toner that has run off a transfer sheet from adhering to a transfer unit. Another object of the present invention is to provide an image forming apparatus capable of preventing unnecessary trailing edge omission of an image when a problem does not originally occur, that is, when the length of the transfer sheet is longer than the image length.
Still another object of the present invention is to provide an image forming apparatus capable of preventing adhesion of toner on an intermediate carrier to a transfer unit and preventing transfer paper conveyance failure from a secondary transfer portion to a fixing unit. And

[0008]

According to a first aspect of the present invention, there is provided an image forming apparatus for carrying a toner image obtained by visualizing an electrostatic latent image to achieve the above object. A primary transfer means for transferring a toner image on the image carrier onto the intermediate carrier; and a primary transfer means which can be brought into contact with and separated from the intermediate carrier, and which contacts the back surface of the transfer paper to impart a charge to the intermediate carrier. Secondary transfer means for transferring the toner image on the body to the transfer paper; paper feed means for feeding the transfer paper to a secondary transfer site; and a leading end and a rear end of the transfer paper in the paper feed means Paper length detecting means for detecting the paper length by detecting the paper length, and image length specifying means for specifying the image length in advance, wherein the paper length detected by the paper length detecting means and the image length If the result of comparison with the image length specified by the The transfer means and changes the timing for separating from the intermediate carrier.

According to a second aspect of the present invention, when the comparison result is different, the timing for separating the secondary transfer means from the intermediate carrier only when the image length is longer than the transfer paper length. Is changed.

According to a third aspect of the present invention, when the comparison result is different, the operation of separating the secondary transfer means from the intermediate carrier is performed before the rear end of the transfer paper passes through the secondary transfer portion. Is performed at the timing described above.

According to a fourth aspect of the present invention, in the case where the image length is longer than the transfer paper length and the toner image does not exist in an image area which is longer than the transfer paper length, the secondary transfer means is provided. Is not changed at the timing of separating from the intermediate support.

[0012] pertaining to the fifth aspect, when the distance of the secondary transfer unit and nip upstream end which is formed when pressed against the intermediate carrier and the transfer sheet trailing edge as l _1, the secondary The timing at which the transfer means is separated from the intermediate support is zero.
≦ l ₁ ≦ 100 mm.

The distance between the downstream end of the nip formed when the secondary transfer means is pressed against the intermediate carrier and the upstream end of the fixing nip formed by a pair of fixing rollers is defined as l _2. When the length of the transfer paper is l ₃ , l ₃ −l ₁ > l ₂
The distance l ₁ is set so as to satisfy the following relationship.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an enlarged view around a photoconductor and an intermediate transfer belt in a color image forming apparatus showing an embodiment of the present invention in which an intermediate carrier is an intermediate transfer belt. In the figure, reference numeral 1 denotes a photoreceptor rotating in the direction of the arrow shown in the figure, and a photoreceptor cleaning unit 2, a charger 4,
Exposure means 5, black developing device 6, cyan developing device 7, magenta developing device 8, yellow developing device 9, intermediate transfer belt 10
Etc. are arranged. Each of the developing devices 6 to 9 includes a developing sleeve that rotates so that the developer faces the photoconductor 1 in order to develop the electrostatic latent image, a developing paddle that rotates to pump up and agitates the developer, and a toner of the developer. It is composed of a density detection sensor and the like. Reference numeral 3 in the drawing denotes a rubber blade of the photoconductor cleaning unit 2.

The intermediate transfer belt 10 includes a driving roller 13,
It is stretched over a group of rollers such as a belt transfer bias roller 11 and a driven roller 12, and is driven by a drive motor (not shown). In the figure, reference numeral 19 denotes a belt cleaning unit, and a rubber blade 18 and the belt 1
It is composed of a contact / separation mechanism 20 for contacting / separating from 0,
After the first color black image is transferred to the belt, the second and fourth colors are separated from the belt 10 by the contact / separation mechanism 20 during the belt transfer.

In FIG. 1, reference numeral 21 denotes a paper transfer unit, which comprises a secondary transfer bias roller 14 and a contact / separation mechanism 16 for bringing the secondary transfer bias roller 14 into and out of contact with the belt 10. Although the secondary transfer bias roller 14 is normally separated from the surface of the intermediate transfer belt 10,
When the four-color superimposed images formed on the surface are collectively transferred to the transfer paper 28, they are pressed by the contact / separation mechanism 16 in a timely manner,
The transfer to the transfer paper 28 is performed by applying a predetermined bias voltage (for example, +1.5 kV).

The transfer paper 28 is fed by the feed roller 22 and the registration roller 23 at the timing when the leading end of the four-color superimposed image on the surface of the intermediate transfer belt 10 reaches the paper transfer position. In the vicinity of the registration roller 23, a paper feed sensor 15 for detecting the leading and trailing ends of the transfer paper 28 is arranged. The transfer paper 28 is transported by the transport belt 29 to the fixing device 17.
After the transferred four-color superimposed image is fixed by the fixing unit 17, the sheet is discharged. At the time of continuous printing of a plurality of colors, the above-described bias voltage is turned off at a timing when the rear end of the transfer paper 28 sufficiently passes through the secondary transfer bias roller 14, and thereafter, the toner is not attached to the intermediate transfer belt 10 of the next page. Next, the secondary transfer bias roller 14 is separated from the intermediate transfer belt 10 by the contact / separation mechanism 16.

As shown in the timing chart of FIG. 2, at the end of the last page during single-sheet printing or continuous printing, the secondary transfer bias roller 14 is not separated from the surface of the intermediate transfer belt 10 and has a polarity opposite to that of the transfer bias. Cleaning bias (for example, -1 kV) is applied for a certain period of time, and the toner adhered to the surface of the secondary transfer bias roller 14 is transferred to the intermediate transfer belt 10 to clean the surface of the secondary transfer bias roller 14. The secondary transfer bias roller 14 is separated from the intermediate transfer belt 10 by the contact / separation mechanism 16.
The toner transferred to the intermediate transfer belt 10 is collected from the belt surface by a belt cleaning unit 19.

In the above configuration and operation, the paper feed tray 2
In the case where the transfer paper 28 is stored in the printer 7 and the paper feed tray 27 is provided with a detecting means for detecting the size of the transfer paper 28 or a size designating means by a user, the transfer paper length and the image length before the printing operation are performed. Can be compared, it is possible to determine whether or not to execute printing. However, in manual sheet feeding or the like, the transfer paper length may not be recognized in advance. In this case, there is no particular problem if the image length matches the transfer paper length or the image length is smaller than the transfer paper length. On the contrary, if the image length is larger than the transfer paper length, the transfer paper 28 Protruding intermediate transfer belt 1
0 adheres to the secondary transfer bias roller 14,
Transfer paper stains and poor transfer of the next print occur.

For this reason, as shown in FIG.
The length of the transfer paper is determined from the time when the leading edge and the trailing edge of the transfer paper 28 pass through the position detected by Step 5 (Steps 1 and 2), and the obtained transfer paper length is compared with the image length (Step 3). Only when the length is larger than the transfer paper length, the transfer paper 28
The secondary transfer bias roller 14 is separated from the intermediate transfer belt 10 by the contact / separation mechanism 16 before the rear end of the secondary transfer portion passes through the secondary transfer portion (steps 4 and 5). As a result, the toner does not adhere to the secondary transfer bias roller 14, the transfer paper of the next print is not stained and the transfer is not defective, and good color is obtained.
An image is obtained.

However, even if the image length is larger than the length of the transfer paper in the determination in step 3, if the toner image does not exist in the area beyond the length of the transfer paper (step 4), the secondary The timing for separating the transfer bias roller 14 is not changed. As a result, all the toner images in the transfer paper area can be transferred, and no toner adheres to the secondary transfer bias roller 14, so that a good color image with no trailing edge transfer omission can be obtained.
After the above steps, if the image is of the last page (step 6), the secondary transfer bias roller 1
The cleaning process 4 is performed (step 7).

In this embodiment, as shown in FIG. 4, the distance between the upstream end of the nip formed when the secondary transfer bias roller 14 is pressed against the intermediate transfer belt 10 and the rear end of the transfer paper 28 is set to l ₁ . When the timing at which the secondary transfer bias roller 14 is separated from the intermediate transfer belt 10 is 0 ≦ l ₁ ≦
Set within a range of 100 mm. However, in consideration of the trailing edge image omission range and the prevention of toner adhesion to the secondary transfer bias roller 14 due to transfer paper conveyance variations, 5 mm ≦ l ₁ ≦ 2
It is desirable to set the range to 0 mm.

In recent years, there has been an increasing demand for printing on postcard-size paper with a full-color printer. However, due to cost restrictions, it is often not possible to provide the transport belt 29 for transporting the transfer paper to the fixing nip portion as in the above-described embodiment. In such a printer, when the present invention is applied to a small-size transfer sheet, if the secondary transfer bias roller 14 is separated before the leading end of the transfer sheet is carried into the fixing nip portion, a jam due to a conveyance failure occurs. Resulting in. Therefore, the secondary transfer bias roller 14
When the distance between the downstream end of the nip formed when the sheet is pressed against the intermediate transfer belt 10 and the upstream end of the fixing nip formed by the fixing roller pair is l ₂ (see FIG. 5), and the minimum transfer paper length is l ₃ , L ₃ −l ₁ > l ₂ , the jam can be prevented by setting the value of l ₁ so as to satisfy the relationship.

When the above operation is performed, unlike the normal printing, as shown in the timing chart of FIG. 6, when the printing of the last page is completed, the secondary transfer bias roller 14 once separated is moved to the intermediate transfer belt. At the timing when the rear end of the image toner on 10 has passed the secondary transfer portion, it is brought into contact again, and a cleaning bias having a polarity opposite to that of the transfer bias is applied for a predetermined time,
The surface of the secondary transfer bias roller 14 is cleaned by transferring the toner attached to the surface of the secondary transfer bias roller 14 to the intermediate transfer belt 10.

FIG. 7 shows an example in which the transfer brush is different from the transfer means of the apparatus shown in FIG. In the case of this example, similarly to the above-described embodiment, only when the image length is larger than the transfer paper length, the transfer by the contact / separation mechanism 16 before the rear end of the transfer paper 28 passes through the secondary transfer portion. Brush 2
By separating 4, toner can be prevented from adhering to the transfer brush 24, and a good color image can be obtained without transfer paper contamination and transfer failure of the next print.

FIG. 8 also shows an example in which a transfer belt 25 is used and the transfer bias applying means 26 is arranged in contact with the back surface of the transfer belt 25 in a different manner from the transfer means of the apparatus of FIG. In the case of this example, similarly to the above-described embodiment, only when the image length is larger than the length of the transfer sheet, the contact / separation mechanism 1 before the rear end of the transfer sheet 28 passes through the secondary transfer portion.
By separating the transfer belt 25 according to 6, the toner can be prevented from adhering to the transfer belt 25, and a good color image can be obtained without transfer paper contamination and transfer failure of the next print.

Of course, the present invention can be applied to any other contact transfer method than the one described above, as long as it can be contacted and separated.

[0028]

According to the first aspect of the present invention, as described above, when the comparison result between the sheet length detected by the sheet length detecting means and the image length specified by the image length specifying means is different. In addition, the timing at which the secondary transfer means that can contact and separate from the intermediate carrier to transfer the toner image on the intermediate carrier to the transfer paper by applying a charge by contacting the back surface of the transfer paper is separated from the intermediate carrier is changed. As a result, it is possible to prevent the toner that has protruded from the transfer paper from adhering to the transfer means. There is an effect that a color image can be obtained.

In the image forming apparatus according to the second aspect, when the paper length is different from the designated image length, especially when the image length is longer than the transfer paper length, the secondary transfer means is moved from the intermediate carrier. Since the separation timing is changed, in addition to the above-mentioned common effects, the normal printing operation is performed when the length of the transfer paper, which originally does not cause a problem, is longer than the image length, so that unnecessary trailing edge omission of the image is prevented. The effect is that it can be prevented.

In the image forming apparatus according to the third aspect, when the paper length is different from the designated image length, the operation of separating the secondary transfer means from the intermediate support is performed by moving the rear end of the transfer paper past the secondary transfer portion. Since it is performed at a timing before the transfer, there is an effect that, in addition to the above-described common effects, the toner on the intermediate carrier can be prevented from adhering to the secondary transfer unit.

In the image forming apparatus according to the present invention, when the image length is longer than the length of the transfer paper and the toner image does not exist in the image range protruding beyond the length of the transfer paper, the secondary transfer means is provided. Since the timing of separation from the intermediate carrier is not changed, there is an effect that in addition to the above-described common effects, unnecessary trailing edge omission of the image can be prevented.

In the image forming apparatus according to the fifth aspect, the timing for separating the secondary transfer means from the intermediate support is determined by adjusting the timing at which the secondary transfer means is pressed against the intermediate support by the nip upstream end and the transfer paper rear end. With distance l ₁ , 0 ≦ l ₁
Since it is set to be within a range of ≦ 100 mm, in addition to the above-mentioned common effect, there is an effect that the adhesion of the toner on the intermediate carrier to the transfer unit can be reliably prevented.

In the image forming apparatus of the present invention, the distance l ₁ between the upstream end of the nip formed when the secondary transfer means is pressed against the intermediate support and the rear end of the transfer paper, and the secondary transfer means is connected to the intermediate support. distance l ₂ between the fixing sites nip upstream end formed by the fixing roller pair nip downstream end which is formed when pressed against the, transfer paper length _{l 3, l 3 -l 1>} l 2 relationship to become so Since the value of the distance l ₁ is set to the above, in addition to the above-mentioned common effect, even when the timing of separating the secondary transfer means from the intermediate support is changed, the distance between the secondary transfer portion and the fixing means is changed. The transfer paper transfer failure can be prevented.

[Brief description of the drawings]

FIG. 1 is an enlarged view around a photosensitive member and an intermediate transfer belt in a color laser printer which is an embodiment of an image forming apparatus according to the present invention.

FIG. 2 is a transfer timing chart during normal printing in the embodiment of FIG. 1;

FIG. 3 is a flowchart showing an operation sequence of the embodiment of FIG. 1;

4 is an enlarged view showing a distance between an upstream end of a nip and a rear end of a transfer sheet when a secondary transfer bias roller used for setting a separation operation timing of a secondary transfer unit of the embodiment of FIG. 1 is pressed against an intermediate transfer belt; FIG.

FIG. 5 illustrates a nip downstream end when a secondary transfer bias roller used for setting a separation operation timing of a secondary transfer unit of the embodiment of FIG. 1 is pressed against an intermediate transfer belt and a fixing nip upstream formed by a fixing roller pair. It is an enlarged view which shows the distance with an edge.

FIG. 6 is a transfer timing chart of the embodiment of FIG.

FIG. 7 is an enlarged view of another transfer unit different from the embodiment of FIG.

FIG. 8 is an enlarged view of still another transfer unit that is different from the embodiment of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoconductor 2 Photoconductor cleaning unit 3 Rubber blade 4 Charger 5 Exposure means 6 Black developing device 7 Cyan developing device 8 Magenta developing device 9 Yellow developing device 10 Intermediate transfer belt 11 Belt transfer bias roller 12 Follower roller 13 Drive roller 14 Two Next transfer bias roller 15 Paper feed sensor 16 Contact / separation mechanism 17 Fixing means 18 Rubber blade 19 Belt cleaning unit 20 Contact / separation mechanism 21 Paper transfer unit 22 Feed roller 23 Registration roller 24 Transfer brush 25 Transfer belt 26 Transfer bias applying means 27 Feeding Paper tray 28 Transfer paper 29 Conveyor belt

Claims (6)

[Claims] An image carrier that carries a toner image obtained by visualizing an electrostatic latent image; a primary transfer unit that transfers a toner image on the image carrier onto an intermediate carrier; A secondary transfer means that is detachable, transfers the toner image on the intermediate carrier to the transfer paper by applying a charge by contacting the back surface of the transfer paper, and supplies the transfer paper to a secondary transfer portion. Paper feeding means for feeding paper, paper length detecting means for detecting a paper length by detecting a leading end and a trailing end of the transfer paper in the paper feeding means, and an image length for designating an image length in advance When the comparison result between the paper length detected by the paper length detection means and the image length specified by the image length specification means is different, the secondary transfer means is separated from the intermediate carrier. An image forming apparatus, wherein a timing of the image forming is changed. 2. When the comparison result is different, the timing for separating the secondary transfer unit from the intermediate support is changed only when the image length is longer than the transfer paper length. The image forming apparatus according to claim 1, wherein: 3. When the comparison result is different, the operation of separating the secondary transfer unit from the intermediate carrier is performed at a timing before the rear end of the transfer paper passes through the secondary transfer portion. 3. The image forming apparatus according to claim 1, wherein: 4. When the image length is longer than the transfer paper length, and the toner image does not exist in an image range protruding beyond the transfer paper length, the secondary transfer means is separated from the intermediate support. 4. The image forming apparatus according to claim 1, wherein a timing of the image forming is not changed. 5. When the distance of the secondary transfer unit and nip upstream end which is formed when pressed against the intermediate carrier and the transfer sheet rear end was l _1, the intermediate bearing member the secondary transfer unit claims 1, wherein the timing for separating is in the range of 0 ≦ l ₁ ≦ 100 mm from one of the image forming apparatus 4. 6. The distance between the downstream end of the nip formed when the secondary transfer means is pressed against the intermediate carrier and the upstream end of the fixing nip formed by a pair of fixing rollers is l ₂ , and the transfer paper length is l. ₃ and with time was, l ₃ -l _1> an image forming apparatus according to claim 5, characterized in that setting the distance l ₁ so as to be in the relationship of l _2.