JP2012063740A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem by setting between sheets, the problem in which a bias applied to a second transfer member for transferring toner on a transfer belt to a sheet leaves a strong charge on the transfer belt when the trail edge of the sheet passes through a secondary transfer part, and the charge manifests itself as an image failure at a primary transfer in the subsequent imaging process.SOLUTION: No problem occurs if there is no belt potential memory in an intermediate transfer belt 15. If, however, there remains the belt potential memory in the intermediate transfer belt 15, toner 204 jumps from a surface of a photoreceptor 1 to a region with a high potential of the intermediate transfer belt 15 to be adhered thereto (adhered toner: 205), with the phenomenon intensifying with magnitude of the potential memory. Consequently, when a potential history that remains on the intermediate transfer belt 15 approaches the primary transfer part when the trail edge of the sheet passes through the secondary transfer part, the toner on a photoreceptor drum 1 is pre-transferred, and thereby space between dots are filled, and a black streak results.

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multi-function machine that performs secondary transfer, and more particularly, to an electrophotographic image forming apparatus using an intermediate transfer belt. The present invention relates to an image forming apparatus in consideration of the influence on primary transfer in the next image forming process due to charge accumulation on the belt.

  In an image forming apparatus such as a copying machine that performs secondary transfer, it is necessary to prevent influence on primary transfer in the next image forming process due to charge accumulation on the belt in the secondary transfer unit. In other words, when the paper leading and trailing edges pass through the transfer roller nip during the secondary transfer, the problem is that they are generated at the position after one round of the belt due to the electric charge moved (discharged) in the step gap at the paper leading and trailing edges.

  In the image forming apparatus as described above, when the potential history remaining on the belt after the secondary transfer (the portion having a high + potential when passing through the leading edge of the paper) approaches the primary transfer portion, the toner on the photoconductor As a result of pre-transferring, the above-mentioned problems occur. That is, if primary transfer is performed at a regular position (near the primary transfer nip), the interval between dots is constant, but only a portion of toner is pre-transferred (a high-potential potential history remains on the belt). The area between the dots and the dots is clogged, and when viewed macroscopically, white stripes or dots that are too clogged cause black stripes.

  Conventionally, for example, in the invention disclosed in Patent Document 1, a non-contact type static elimination unit using corona discharge or a contact type static elimination unit by rubbing against a brush roller made of a conductive fiber is provided. An image forming apparatus is proposed in which the intermediate transfer member is neutralized after secondary transfer. However, the non-contact type static elimination means using corona discharge has a problem that ozone is generated by the discharge of negative charge, which is not preferable from the viewpoint of environmental protection. Further, not only the contact method by rubbing with the brush roller but also a contact-type neutralizing means for bringing a certain contact member into contact with the primary transfer body cleans the toner adhering to the primary transfer body before neutralization. Even if the cleaning means is provided, the toner that cannot be completely removed may come into contact with the contact member. Then, after transferring the toner from the primary transfer body to the contact member, it may transfer again to the transfer target area of the primary transfer body to form a smudge image, and additional members may be added. This increases the cost, and there remains a problem that it is necessary to secure a space for mounting the member.

  Further, for example, the invention disclosed in Patent Document 2 includes contact-type static elimination means that neutralizes the transfer surface while bringing the contact member into contact with the transfer surface of the primary transfer body after the secondary transfer at the static elimination position, An electric field that electrostatically moves the visible substance from the contact member toward the primary transfer body is formed while the primary transfer body is transported by the primary transfer body transport means during the forming operation. Is. In this image forming apparatus, even if the contact member of the contact-type static eliminator is soiled with a visible material during an image forming operation, the visible material is re-transferred to the primary transfer member by forming the electric field during the non-image forming operation. By this re-transfer, the contact member contaminated with the visible substance during the image forming operation can be cleaned. Also in the invention according to Patent Document 2, there is a problem that the cost is increased from the point of adding a member and it is necessary to secure a space for mounting the member.

  Furthermore, in the invention disclosed in, for example, Patent Document 3, an image carrier, an intermediate transfer member that transfers a toner image from the image carrier, a primary transfer unit thereof, a static elimination member that neutralizes the intermediate transfer member, Means for applying a bias to the neutralizing member, a contactable / separable secondary transfer roller for transferring a visible image from the intermediate transfer member to the paper, and a secondary transfer opposite member that stretches the intermediate transfer member facing the secondary transfer roller An image forming apparatus having a roller, a means for applying a bias to a secondary transfer counter roller, and a heat fixing means for fixing an unfixed image on a sheet, has a single high-voltage power supply, And a control means for applying a bias to the secondary transfer member. However, the invention according to Patent Document 3 also increases the cost from the point of adding a member, and also reduces the space for mounting the member. A problem that it is necessary to guarantee remains.

  Accordingly, in the present invention, in view of the above-described conventional problems, the position of the belt when the trailing edge of the secondary transfer is lost or the position of the belt when the leading edge of the paper enters the secondary transfer nip portion is determined. It is an object of the present invention to provide an image forming apparatus capable of preventing an image defect at the time of primary transfer caused by a belt memory by using a gap between sheets.

  The image apparatus according to the first aspect of the present invention includes a plurality of image carriers, and a transfer belt unit that rotates an endless belt by at least a driven roller and a driving roller. By transferring toner onto the transfer belt, the primary transfer unit that forms a color image on the transfer belt and the drive roller are disposed opposite to each other, and the color images on the transfer belt are collectively transferred to a sheet. In an image forming apparatus having a secondary transfer portion having a secondary transfer member for changing the value, an interval between the sheets is changed according to the number of sheets simultaneously related to the transfer belt. It is characterized by making it.

The invention according to claim 2 includes a plurality of image carriers and a transfer belt unit that rotates an endless belt by at least a driven roller and a drive roller, and transfers toner from the image carriers onto the transfer belt. By performing the above, a primary transfer portion that forms a color image on the transfer belt and a secondary transfer that is disposed to face the drive roller and collectively transfer the color images on the transfer belt onto a sheet. In an image forming apparatus having a secondary transfer portion having a member,
When the length of the paper simultaneously related to the transfer belt is A [mm], the length of the belt is B [mm], and the distance between the papers is x [mm], these are the expressions (B−nA). ) / (N + 1) <x <(B-nA) / n (n: integer)
The paper interval x is defined and changed so as to satisfy the above relationship.

The invention according to claim 3 includes a plurality of image carriers and a transfer belt unit that rotates an endless belt by at least a driven roller and a driving roller, and transfers toner from the image carriers onto the transfer belt. By performing the above, a primary transfer portion that forms a color image on the transfer belt and a secondary transfer that is disposed to face the drive roller and collectively transfer the color images on the transfer belt onto a sheet. In an image forming apparatus having a secondary transfer portion having a member,
When the paper length is A [mm], the belt length is B [mm], the paper gap is x [mm], and the leading edge margin of the paper is D [mm], these are expressed by the equation 0 <{B− (N-1) A} / n <x <{B- (n-1) A + D} / n (n: integer)
The paper interval x is defined and changed so as to satisfy the above relationship.

According to the fourth aspect of the present invention, there is provided a plurality of image carriers and a transfer belt unit that rotates an endless belt by at least a driven roller and a driving roller, and toner transfer from each of the image carriers onto the transfer belt. By performing the above, a primary transfer portion that forms a color image on the transfer belt and a secondary transfer that is disposed to face the drive roller and collectively transfer the color images on the transfer belt onto a sheet. In an image forming apparatus having a secondary transfer portion having a member,
When the paper length is A [mm], the belt length is B [mm], the paper interval is x [mm], and the paper trailing edge margin is E [mm], 0 <(B−nA−E ) / N <x <(B-nA) / n (n: integer)
The paper interval x is defined and changed so as to satisfy the above relationship.

The invention according to claim 5 includes a plurality of image carriers and a transfer belt unit that rotates an endless belt by at least a driven roller and a drive roller, and transfers toner from the image carriers onto the transfer belt. By performing the above, a primary transfer portion that forms a color image on the transfer belt and a secondary transfer that is disposed to face the drive roller and collectively transfer the color images on the transfer belt onto a sheet. In an image forming apparatus having a secondary transfer portion having a member,
When the paper length is A [mm], the belt length is B [mm], the paper gap is x [mm], and the paper trailing edge margin is E [mm] (B−nA) / n < x <(B-nA) / n-1 (n: integer)
The paper interval x is defined and changed so as to satisfy the above relationship.

  According to the sixth aspect of the present invention, in the image forming apparatus according to any one of the second to fifth aspects, the image is transferred from a developing device including an uppermost image carrier in a conveying direction of the sheet on the transfer belt. The change in the sheet interval x is applied when the toner color is other than yellow.

  According to the seventh aspect of the present invention, in the image forming apparatus according to any one of the second to fifth aspects, the change in the sheet interval x is applied only when the travel distance of the process cartridge located at the uppermost stream is equal to or less than a threshold value. It is characterized by making it.

  According to an eighth aspect of the present invention, in the image forming apparatus of the seventh aspect, as a means for calculating a travel distance of the process cartridge, a value read by an ID chip attached to the process cartridge is used.

  According to a ninth aspect of the present invention, in the image forming apparatus of the seventh aspect, the means for calculating the travel distance of the process cartridge is calculated from the number of rotations of a motor that drives the process cartridge.

  According to the tenth aspect of the present invention, in the image forming apparatus according to any one of the second to fifth aspects, the change in the sheet interval x is applied only when the dot configuration of an image to be printed is less than 100%. It is characterized by.

  According to the eleventh aspect of the present invention, in the image forming apparatus according to any one of the second to fifth aspects, among the process cartridges in which the photoconductor, the charger, and the developer are integrated, a sheet of paper on the transfer belt is integrated. The change in the sheet interval x is applied only when the travel distance of the process cartridge positioned at the uppermost stream in the transport direction is equal to or less than a threshold value.

  According to the twelfth aspect of the present invention, in the image forming apparatus according to any one of the second to fifth aspects, only when the temperature and humidity are detected by an environmental sensor and a predetermined threshold value is exceeded, the sheet interval x It is characterized by applying a change of.

  The image forming apparatus according to any one of claims 1 to 12, wherein the transfer belt has a common logarithm value of surface resistivity of 9 logΩ / □ or more and 14 logΩ / □ or less, and a volume resistance. The common logarithm of the rate is 9 logΩ · cm or more and 14 logΩ · cm or less.

  According to the present invention, the position where the trailing edge of the sheet is removed during the secondary transfer is set so as to be positioned between the sheets in the next image forming process, or when the leading edge of the sheet enters the secondary transfer nip portion. The conventional problems described above can be avoided by setting the position of the belt so as to be positioned between the sheets of the next image.

Conceptual sectional view (A) of image forming apparatus and conceptual diagram of essential part (B) The figure which shows the image malfunction occurrence mechanism in the primary transfer due to the belt memory The figure which is Example 1 of this invention, Comprising: The example in case a paper gap is 180 mm The figure which shows the example (problem example) when the same paper gap is 60mm Another figure showing the mechanism of image defect occurrence in primary transfer due to belt memory The figure which is Example 2 of this invention, Comprising: The example in case a paper gap is 50 mm The figure which shows the example (problem example) when the same paper gap is 65mm The figure which shows Example 2 of this invention The figure which shows Example 3 of this invention The figure which shows the correlation with the horizontal black streak level which generate | occur | produces in the 3rd sheet at the time of printing 3 sheets of black halftone images continuously with the paper spacing setting of 60 mm, and the common logarithm value of the volume resistivity of the intermediate transfer belt The figure which shows the travel distance correlation of the horizontal black streak level and the process cartridge Conceptual diagram for explaining in detail the operation of the seventh embodiment. FIG. 10 is a diagram showing a horizontal black streak level generated on the third sheet when a black halftone image is printed continuously with a black halftone image set at 60 mm and a running distance correlation of the process cartridge. Conceptual diagram for explaining in detail the operation of the eighth embodiment.

  In the present invention, a bias is applied to the secondary transfer member in order to transfer the toner on the transfer belt to the paper, but when the trailing edge of the paper passes through the secondary transfer portion, a strong charge is left on the belt side, The problem that the electric charge becomes apparent as an image defect at the time of the primary transfer in the next image forming process is solved by setting between papers.

  That is, the paper length is changed so that the length of the paper, the length of the belt, and the distance between the papers satisfy the predetermined relationship, and the portion where the rear edge of the paper is removed during the secondary transfer is the space between the papers in the next image forming process. To avoid problems.

  Further, the present invention applies the above-described gap between papers when the developing device having the most upstream image carrier is other than yellow. In other words, by performing in colors other than yellow, in which image defects due to the belt memory are likely to be manifested, it is possible to prevent a decrease in productivity at the time of yellow where image defects due to the belt memory do not occur.

  Further, the present invention is applied only when the dot configuration of an image to be printed is less than 100%. That is, a condition that the belt memory is likely to be manifested at the time of the primary transfer is that the belt memory is remarkably generated in an image (halftone) in which dots are spaced from each other. Therefore, a solid image having a dot configuration of 100% or more in the printed image is less affected by the memory, so that it is possible to prevent a decrease in productivity when printing an image having a dot configuration of 100% or more that does not cause an image defect due to the belt memory. .

  In the present invention, the common logarithm of the surface resistivity of the transfer belt is 9 logΩ / □ or more and 14 logΩ / □ or less, and the common logarithm of the volume resistivity is 9 logΩ · cm or more and 14 logΩ · cm or less. When the transfer belt has a common logarithmic value of the surface resistivity of less than 9 log Ω / cm and a common logarithm of the volume resistivity of less than 9 log Ω · cm, it is difficult to generate a belt memory in the secondary transfer portion. The productivity can be improved by applying the above-mentioned gap between sheets.

  Further, the present invention applies the above-described paper gap only when the travel distance of the process cartridge located at the uppermost stream is below a certain threshold value. As the travel distance of the process cartridge increases, the toner deteriorates with time. When there is little deterioration of the toner over time, the chargeability of the toner is high and the physical adhesion is weak, so that it is highly sensitive to electrostatic disturbances such as memory during primary transfer and secondary transfer. ing. However, as the toner deteriorates with time, the influence of disturbance during primary transfer and secondary transfer is reduced, and image defects due to the belt memory are also reduced. Therefore, it is possible to increase productivity by applying the above-described paper gap only when toner deterioration is small.

  Further, the present invention makes it possible to easily obtain the travel distance data of the process cartridge by calculating the travel distance of the process cartridge by reading it with an ID chip attached to the process cartridge. The means for calculating the travel distance of the process cartridge includes the number of rotations of a motor that drives the process cartridge, and the travel distance data of the process cartridge can be obtained more easily using a known technique.

  Further, the present invention applies the above-described paper gap only when the temperature and humidity are detected by an environmental sensor and the detected value exceeds a predetermined threshold value. That is, by detecting whether the resistance member is installed by detecting the environment, the above-described paper gap is set only in an environment in which belt memory is likely to occur, and productivity in an environment in which no belt memory is generated is improved.

  In the present specification, the term “paper” is not limited to paper, but includes other known and well-known transfer materials used in image forming apparatuses.

  FIG. 1 schematically shows an image forming apparatus according to an embodiment of the present invention. In the drawing, an image forming unit having four photosensitive drums as an image carrier is shown. In the figure, reference numeral 1 denotes a cylindrical photosensitive drum having a diameter of 24 mm (hereinafter simply referred to as a photosensitive member), which rotates at a peripheral speed of 120 mm / s. A roller-shaped charger 2 serving as charging means is pressed against the surface of the photosensitive member 1, and is driven to rotate by the rotation of the photosensitive member 1, and a bias in which AC is superimposed on DC or DC by a high voltage power source (not shown). Is applied, the photoreceptor 1 is uniformly charged to a surface potential of −500V.

  Subsequently, the photosensitive member 1 is exposed to image information by an exposure unit 3 which is a latent image forming unit, and an electrostatic latent image is formed. In this exposure process, the laser light modulated by the laser diode (LD) element 3a forms an image on the surface of the photosensitive member. By scanning this laser beam, a latent image is written on the photoconductor 1 corresponding to a desired image, and an electrostatic latent image is formed on the photoconductor 1.

  In the figure, reference numeral 4 denotes a one-component contact developing device as developing means, which visualizes the electrostatic latent image of the photoreceptor 1 as a toner image by a predetermined developing bias supplied from a high voltage power source (not shown). In the figure, reference numeral 10 denotes a process cartridge in which a photoconductor, a charger, and a developer are integrated.

  Four process cartridges 10 are arranged in parallel, and when a full-color image is formed, visible images are formed in the order of yellow, magenta, cyan, and black, and the visible images of the respective colors are applied to the intermediate transfer belt 15 that is in contact with each other. A full-color image is formed by successively transferring images.

  The intermediate transfer belt 15 is a secondary transfer counter roller 21 that also serves as a drive roller (hereinafter referred to as a drive roller 21 and a secondary transfer counter roller 21 according to the contents of the description, but the same roller), and is made of metal. The cleaning counter roller 16, the primary transfer roller 5 (described later), and the tension roller 20 (described later) are stretched, and are driven to rotate via a driving roller 21 by a driving motor (not shown). Although not shown, the belt tension is applied by springs on both sides of the tension roller 20. The tension roller 20 is, for example, in the shape of an aluminum pipe.

  As the primary transfer roller 5 that is a primary transfer member, a conductive blade, a conductive sponge roller, a metal roller, or the like can be used. In this embodiment, a metal roller having a diameter of φ8 mm is used. A transfer electric field is formed through the intermediate transfer belt 15 by applying a predetermined transfer bias in common to the next transfer roller 5 by a single high voltage power source (not shown), and the toner image on the photoreceptor 1 is transferred to the intermediate transfer belt 15. Let

  In the figure, reference numeral 17 denotes a toner mark sensor (TM sensor), which measures the toner image density and each color position on the intermediate transfer belt 15 by a regular reflection type or diffusion type sensor and adjusts the image density and color matching. In the figure, reference numeral 32 denotes an intermediate transfer belt cleaning unit, which performs cleaning by scraping off the transfer residual toner on the intermediate transfer belt 15 by the cleaning blade 31. As the cleaning blade material, for example, urethane rubber having a thickness of 1.5 to 3 mm and a rubber hardness of 65 to 80 ° is used, and is brought into counter contact with the intermediate transfer belt 15.

  The transfer residual toner thus scraped off is stored in the intermediate transfer member waste toner storage section 33 through a toner conveyance path (not shown). At least one of the portion corresponding to the cleaning nip portion of the intermediate transfer belt 15 or the edge portion of the cleaning blade 31 is coated with a coating agent such as a lubricant, toner, or zinc stearate when assembled. In addition to preventing the blade from rolling up, a dam layer is formed in the cleaning nip to improve the cleaning performance. Each roller that stretches the intermediate transfer belt 15 is supported from both sides of the intermediate transfer belt 15 by an intermediate transfer belt unit side plate (not shown).

  Materials used for the intermediate transfer belt 15 are PVDF (vinylidene fluoride), ETFE (ethylene-tetrafluoroethylene copolymer), PI (polyimide), PC (polycarbonate), TPE (thermoplastic elastomer), PAA (polyamide). A resin film-like endless belt is used by dispersing a conductive material such as carbon black in a polymer material such as alloy). In this embodiment, a common logarithm of surface resistivity of 9 to 10 logΩ, volume resistivity is used. A belt having a common logarithm value of 9 to 10 log Ω · cm, a tensile modulus of 1000 to 2500 MPa, a thickness of 90 to 160 μm, and a circumferential length of 650 mm can be used.

  Further, in the figure, reference numeral 25 denotes a secondary transfer roller. The secondary transfer roller 25 is configured by coating a metal core such as SUS having a diameter of, for example, φ6 mm with an elastic body such as urethane adjusted to a resistance value of 106 to 1010 Ω with a conductive material. As the material, an ion conductive roller (urethane + carbon dispersion, NBR, hydrin), an electronic conductive type roller (EPDM) or the like is used. In this embodiment, a urethane roller having Φ20 and Asker C hardness of 35 to 50 °. Was used. The paper P is fed by a paper feed / conveying roller 23 and a registration roller pair 24 at the timing when the leading edge of the toner image on the surface of the intermediate transfer belt 15 reaches the secondary transfer position, and a predetermined transfer bias is supplied from a high voltage power source (not shown). Is applied, the toner image on the intermediate transfer belt 15 is transferred to the paper P. In this configuration, the intermediate transfer belt 15 rotates in the direction of the arrow in FIG. 1B, the paper feed takes a vertical path, and the formed color image is a predetermined image with respect to the secondary transfer roller 5. By applying a voltage, the image is transferred to a sheet P such as a sheet. The sheet P is separated from the intermediate transfer belt 15 by the curvature of the secondary transfer counter roller 21, and the toner image transferred to the sheet P is fixed by the fixing unit 40. After that, the paper is discharged.

  Although illustration is omitted, a fan is disposed in the vicinity of the fixing unit 40 and a temperature sensor is also disposed in the vicinity of the secondary transfer unit to monitor and control the temperature control inside the apparatus main body. . Further, the toner that cannot be transferred by the secondary transfer roller 21 and remains on the intermediate transfer belt 15 is collected by a cleaning blade 31 into a cleaner unit (not shown). In this embodiment, the configuration of the intermediate transfer unit is a biaxial stretch, and the secondary transfer counter roller has various functions. However, the present invention is not limited to this configuration, and any transfer system having secondary transfer may be used. For example, the present invention can have the features described below.

<Example 1>
A first embodiment of the present invention will be described. FIG. 2 simply shows the mechanism of occurrence of image defects in the primary transfer caused by the belt memory. In the figure, 201 is the toner developed on the surface of the photosensitive drum 1 which is an example of the image carrier, and the part A is a nip portion between the photosensitive member 1 and the intermediate transfer belt 15, that is, a position immediately before the primary transfer portion. The state of FIG. 2A is an image Ia when the primary transfer is normally performed, and the toner image that has arrived on the photosensitive drum 1 as the toner 201 is not clogged with dots at the position indicated by the arrow A in the figure. In the case of FIG. 2B, the potential history remaining on the intermediate transfer belt 15 when the trailing edge of the sheet passes through the secondary transfer portion (passing the leading and trailing edges of the sheet) is transferred to the sheet normally. When the potential potential is higher, the higher the potential memory, the higher the potential memory, the more toner 204 flows from the surface of the photosensitive member 1 to the portion with the higher sheet potential, and the toner 201 on the photosensitive drum 1 is transferred to B. The pre-transfer is performed at the position, and the dots are clogged and black stripes are generated (see image Ib). In other words, a bias is applied to the secondary transfer unit in order to transfer the toner on the intermediate transfer belt 15 to a sheet such as a sheet, but when the rear end of the sheet passes through the secondary transfer unit, it is strongly applied to the intermediate transfer belt 15 side. The charge remains, and the charge becomes apparent as a memory image (horizontal stripe) during the primary transfer in the next image forming process.

  In the image forming apparatus used in this embodiment, the sheet interval x is set at three levels of 25 mm, 60 mm, and 180 mm, and the printing dot rate of 25 sheets of A4 size (image length 294 mm in the intermediate transfer belt traveling direction). % Images were continuously passed through to check for image defects. When the paper gap x is 25 mm and 180 mm, the portion 306 where the trailing edge of the paper P is removed at the primary transfer portion is the space between the papers in the next image forming process, that is, the toner image p2 to be secondarily transferred is secondary transferred. Since it is in a state before reaching the position, the primary transfer is not affected and no image defect occurs.

  FIG. 3 shows the case where the paper gap is 180 mm. This figure shows that the missing portion 306 at the rear end of the sheet P1 is between the sheets of the next process. That is, when the trailing edge of the first sheet P1 passes through between the secondary transfer counter roller 21 and the secondary transfer roller 25, which are secondary transfer portions, a potential history is left on the surface of the intermediate transfer belt 15. When the potential history remaining (belt memory) portion 306 comes to the image forming position 301 of the most upstream photoconductor 1, since it is before the next image forming process by the image p2, that is, between the sheets, the belt moving direction Thus, there is no influence on the primary transfer at the image forming position 301 of the most upstream photoreceptor 1. The same applies to the photoconductor downstream of the most upstream photoconductor 1.

  On the other hand, when the gap between the sheets is 60 mm, the belt memory when the trailing edge of the paper P1 is pulled out at the secondary transfer portion has an influence on the primary transfer portion in the subsequent process, resulting in an image defect called horizontal black streak.

FIG. 4 shows the primary transfer portion and the secondary transfer portion when the sheet interval is 60 mm. When the surface of the intermediate transfer belt 15 reaches the most upstream image forming position when the trailing edge of the first sheet P1 passes through the secondary transfer portion, the memory portion 306 transfers an image to the third sheet. It can be seen that the primary transfer image is disturbed by overlapping with the image formation of p3. Accordingly, when the paper length is A [mm], the belt length is B [mm], and the paper gap is x [mm], the equation (B−nA) / (n + 1) <x <(B− nA) / n
By setting the sheet interval x so as to satisfy the above relationship, it is possible to prevent image defects caused by the belt memory.

In the above formula, when n = 1, (BA) / 2 <x <(BA) / 1, so half of the value obtained by subtracting the paper length from the belt length is the length of the paper gap x. It can be seen that the length of the paper interval x is smaller than the belt length and is greater than the belt length minus the paper length.
If n = 2, then (B-2A) / 3 <x <(B-2A) / 2.
In other words, as the number of sheets simultaneously existing on the intermediate transfer belt 15 increases, the free length on the intermediate transfer belt 15 that is allowed as the length of the sheet interval x becomes shorter, and the restriction becomes stricter. .

<Example 2>
FIG. 5 is a diagram depicting the mechanism of occurrence of an image defect in primary transfer caused by the belt memory in a different manner from FIG. 5A shows the case where the intermediate transfer belt 15 does not have a belt potential memory, and FIG. 5B shows the case where the intermediate transfer belt 15 has a potential memory remaining on the belt. Similar to FIG. 2, no problem occurs in the case of FIG. 5A, but if the belt potential memory remains on the intermediate transfer belt 15, as shown in FIG. The higher the potential memory, the easier the toner 204 at the corresponding position B from the toner 203 adhering to the surface of the photoreceptor 1 to the higher portion, and adhering to the intermediate transfer belt 15 (the toner adhering to the intermediate transfer belt 15). Is indicated by reference numeral 205). As a result, the potential history remaining on the intermediate transfer belt 15 when the trailing edge of the sheet has passed through the secondary transfer portion A (the portion with a high + potential when passing through the leading edge of the sheet) approaches the primary transfer portion A. At this time, the toner on the photoconductor 1 is pre-transferred, the dots are clogged, and black streaks are generated.

  In the image forming apparatus used in the present embodiment, the sheet spacing is set at two levels of 50 mm and 65 mm, and two A4 size images (image length 294 mm in the belt traveling direction) with a print dot rate of 25% are continuously two sheets. The paper was passed through and image defects were confirmed. When the paper interval is 50 mm, as shown in FIG. 6, the portion where the leading edge of the paper P1 enters the secondary transfer nip portion is between the papers in the next image forming process, so that there is no effect at the time of the primary transfer and the image defect Does not occur. FIG. 6 shows a schematic diagram in which the primary transfer portion and the secondary transfer portion in the case where the sheet interval is 50 mm are written, and the portion where the leading end of the sheet enters the secondary transfer nip portion is the sheet interval of the next process.

  When the trailing edge of the first sheet P1 that has received the transfer of the toner image p1 passes through the secondary transfer portion, a potential history remains on the belt surface 306 (FIG. 6A), but the potential history remains. When the belt memory location 306 reaches the image forming position 301 of the most upstream photoconductor 1, the image p3 transferred to the third sheet passes through the image forming position 301, that is, between the sheets (see FIG. 6 (B)), the primary transfer at the image forming position 301 of the most upstream photoconductor 1 is not affected at all. The same applies to the photoconductor downstream of the most upstream photoconductor 1.

  On the other hand, as shown in FIG. 7, when the sheet interval is 65 mm, the belt memory portion 306 affects the primary transfer portion in the next process, and an image defect called horizontal black streak occurs. FIG. 7 shows the primary transfer portion and the secondary transfer portion when the sheet interval is 65 mm. When the belt memory location 306 generated when the leading edge of the first sheet P1 enters the nip portion of the secondary transfer portion reaches the image forming position 301 of the most upstream photoconductor 1, it becomes the third sheet. Overlaying the transferred image p3, the primary transfer image at the image forming position 301 of the most upstream photoreceptor 1 is disturbed. In the figure, P2 indicates the second sheet.

Therefore, when the paper length is A [mm], the belt length is B [mm], and the paper gap is x [mm], the formula (B−nA) / n <x <(B−nA) / N-1
By setting the paper interval x so as to satisfy the above relationship, it is possible to prevent image defects caused by the belt memory.

<Example 3>
A third embodiment of the present invention is shown in FIG. In this embodiment, the sheet interval x is set to 60 mm, and three images with an image length of 294 mm in the belt traveling direction and a print dot rate of 25% are printed continuously. At that time, when the uppermost development unit is changed to black, yellow, magenta, cyan, and the image is printed, the image defect caused by the belt memory when the rear end of the first sheet passes through the secondary transfer portion. As a result of confirmation, in the colors other than yellow, horizontal streaks, which are image defects, were seen, but in yellow, they were not seen at all. Although it is considered that an image defect actually occurred in the primary transfer portion, horizontal black streaks could not be confirmed on the yellow image. Also, when black is placed in the most upstream developing device and arranged downstream in the order of magenta, cyan, and yellow, and the continuous black printing dot rate of 25% is printed on 3 sheets, the trailing edge of the third sheet Although horizontal black streaks occur at a position of 60 mm, in this configuration, black streaks did not occur when three consecutive printing dot ratios of 25% for cyan or magenta single color were printed. This is because when the belt memory on the intermediate transfer belt passes between the uppermost photosensitive member and the intermediate transfer belt, the surface of the intermediate transfer belt is neutralized due to the negative charge of the photosensitive member. Therefore, the belt memory has an effect only during the most upstream printing.

<Example 4>
A third embodiment of the present invention is shown in FIG. In this embodiment, the level of the printing dot rate is changed, the paper interval x is set to 60 mm, and three images with an image length of 294 mm in the belt traveling direction and a printing dot rate of 25% are printed continuously. The graph of FIG. 8 shows the correlation between the ratio of the printed dots and the level of the horizontal black stripe at this time.

  The level of the horizontal stripes is divided into five levels, and each level is set to “5: not visible at all”, “4: hardly visible”, “3: visible slightly”, “1: clearly understood”. If the composition of the printed dots is 100% or more, the horizontal black stripes are not noticeable. If the printed dot composition is less than 100%, the dot composition ratio decreases, and as the dot composition ratio decreases, the horizontal black stripes become noticeable. Here, 100% is defined as a solid image. Therefore, when the dot composition ratio is less than 100%, it is possible to prevent the occurrence of horizontal black streaks by setting the sheet interval x as described in the previous embodiment.

<Example 5>
A third embodiment of the present invention is shown in FIG. FIG. 9 shows the correlation between the horizontal black streak level generated on the third sheet and the common logarithm of the volume resistivity of the intermediate transfer belt when three continuous black black halftone images are printed with the paper spacing x set to 60 mm. Indicates. When the resistance of the intermediate transfer belt is low, charges are unlikely to be stored in the belt, so that image defects due to the belt memory are unlikely to occur. If the resistance is too high, charges are accumulated in the belt and are not easily attenuated. Therefore, an abnormal image other than the belt memory at the time of the trailing end of the paper, which is to be improved by the present invention, is generated. From FIG. 9, when the common logarithmic value of the volume resistivity of the intermediate transfer belt is less than 8 log Ω · cm, no horizontal black streak occurs, and from 9 log Ω · cm to 14 log Ω · cm, the horizontal black streak occurs. Further, when the common logarithmic value of the volume resistivity of the intermediate transfer belt is 15 log Ω · cm, transfer unevenness other than the horizontal black streaks occurs. Therefore, the intermediate transfer belt has a common logarithmic value of surface resistivity of 9 logΩ / □ or more and 14 logΩ · cm or less, and a common logarithm of volume resistivity of 9 logΩ · cm or more and 14 logΩ · cm or less as described above. By setting the paper interval, it is possible to prevent the occurrence of horizontal black streaks.

<Example 6>
In Example 6 of the present invention, the sensitivity to electrostatic disturbance during primary transfer and secondary transfer becomes dull as the toner deteriorates over time. That is, it becomes dull with respect to disturbances such as the belt memory of the intermediate transfer belt. The toner deterioration referred to here means that the external charge of the toner surface layer is peeled off and the chargeability is lowered, or the toner adhesion is increased by being buried in the toner. The travel distance of the process cartridge is used as an index of toner deterioration. That is, as the travel distance of the process cartridge increases, the deterioration of the toner with time progresses. When there is little deterioration of the toner over time, the chargeability of the toner is high and the physical adhesion is weak, so that it is highly sensitive to electrostatic disturbances such as memory during primary transfer and secondary transfer. ing. However, as the toner deteriorates with time, the influence of disturbance during primary transfer and secondary transfer is reduced, and image defects due to the belt memory are also reduced. Therefore, productivity can be improved by applying the paper gap according to claim 1 only when toner deterioration is small. As a means for reading the travel distance of the process cartridge, for example, a means for reading the travel distance of the process cartridge from an ID chip provided in the process cartridge (calculated from the number of rotations of a motor driving the process cartridge) can be cited. Only when the travel distance is equal to or less than a predetermined threshold, the sheet interval control is performed. Other known and well-known techniques may be used.

  FIG. 10 shows the horizontal black streak level generated on the third sheet and the travel distance of the process cartridge when three black black halftone images are continuously printed with the paper spacing x set to 60 mm. It was found that the horizontal black streaks became noticeable when the running distance was less than 2500 m. Therefore, it is possible to prevent the occurrence of horizontal black streaks by setting the sheet distance according to claim 1 when the travel distance is less than 2500 m.

<Example 7>
In the seventh embodiment of the present invention, although not shown, a temperature / humidity sensor is provided on the side plate of the main body. When the reading value of the sensor is 5, 10, 20, 30 g / m3, the black halftone image is generated on the third sheet when three continuous prints are made with the paper spacing x set to 60 mm. When the level of horizontal black stripes was confirmed, the horizontal black stripes were almost invisible at 20 g / m3 and 30 g / m3. This is because when the absolute water content increases, the member resistance of the secondary transfer portion and the primary transfer portion decreases, so that the charge accumulated in the intermediate transfer belt is likely to leak. In this configuration, it is possible to prevent the occurrence of horizontal black streaks by performing the above-mentioned paper gap setting only when it is below 20 g / m3.

<Example 8>
When the length of the sheet is A [mm], the length of the intermediate transfer belt 15 is B [mm], the interval between the sheets is x [mm], and the leading edge margin of the sheet is D [mm], these are the formulas. 0 <{B− (n−1) A} / n <x <{B− (n−1) A + D} / n (n: integer)
By defining and changing the paper interval x so as to satisfy this relationship, the belt position from which the trailing edge of the sheet is removed during the secondary transfer is located in the margin at the leading edge of the sheet after the next image forming step. To achieve. In the case of the present embodiment, when the peripheral length of the intermediate transfer belt 15 is 650 mm, the paper (paper) size is A4 (294 mm), n = 2, and the margin at the front end of the paper is 4 mm, the continuous paper passing so as to satisfy the above formula is performed. If the paper gap x is set so as to satisfy 178 mm <B <180 mm, the portion where the trailing edge of the sheet is removed at the secondary transfer portion corresponds to the margin at the leading edge of the sheet after the next image forming process, and this has no influence on the primary transfer. There was no image defect.

  FIG. 11 is a conceptual diagram for explaining the operation of this embodiment in detail. When the trailing edge of the first sheet P1 passes through the secondary transfer portion, a charge + (1) is left on the intermediate belt 15. When the portion where the electric charge at the trailing edge of the sheet is left, that is, the portion where the potential history remains (belt memory) comes to the next secondary transfer position (2), the sheet P is not at the secondary transfer position. There is no problem (as in the second sheet P2 in the figure), and if the position (2) is located at the leading edge margin as in the third sheet P3, the position of the + potential is within the leading margin range. Since it is located inside, it does not become a white image defect (streaks). However, for example, as described on the rear end side of the third sheet P3, if the position (2) arrives in the image (it is possible that the sheet interval x = 60 mm or the like), as described above An image defect occurs due to a streak caused by the phenomenon. Therefore, when the belt memory location when the trailing edge of the first sheet P1 passes through the secondary transfer portion reaches the most upstream image forming position (position B in FIG. 2B), the sheet interval x is set. If 178 mm <B <180 mm, the position of the positive potential is the margin at the leading edge of the sheet, and therefore, there is no effect on the image form at the time of primary transfer.

  The operation of the configuration of this embodiment will be described. When the sheet interval x of the sheet P is set to 60 mm, the image length in the advancing direction of the intermediate transfer belt 15 is 294 mm, and three images with a printing dot rate of 25% are printed in succession, When the developer is changed from the black developer to the yellow developer, magenta developer, and cyan developer to change the toner color and print an image, the trailing edge of the first sheet passes through the secondary transfer section. When an image defect due to the belt memory was confirmed, horizontal black streaks that were an image defect were seen when the toner color was other than yellow, but was not seen at all. Although it is considered that an image defect actually occurred in the primary transfer portion, horizontal black streaks could not be confirmed on the yellow image. In addition, a black developing device is arranged at the most upstream developing device in the rotation direction of the intermediate transfer belt 15 and arranged downstream in the order of magenta, cyan, and yellow, and a continuous black single color printing dot rate of 25% is printed on three sheets. In this case, horizontal black streaks occur at the position of the rear edge of the third sheet of 60 mm. In this configuration, when black dots of 25% in the cyan single color or magenta single color configuration are continuously printed, the horizontal black stripes are generated. Did not occur. This is because when the memory location on the intermediate transfer belt 15 passes between the uppermost photosensitive drum 1 and the intermediate transfer belt 15, the surface of the intermediate transfer belt 15 is neutralized by the negative charge of the photosensitive drum 1. . Therefore, it can be seen that the belt memory has an effect only during the most upstream printing.

  Although it has already been described that the travel distance of the process cartridge is used as an index of toner deterioration, the third sheet when a black halftone image is printed in succession in FIG. 3 shows the correlation between the level of horizontal black stripes and the travel distance of the process cartridge. It can be seen that the horizontal black stripes are noticeable when the travel distance is less than 2500 m. Therefore, if the travel distance is less than 2500 m and the sheet interval according to the present invention is set, the occurrence of horizontal streaks can be effectively prevented. As described above, the means for reading the travel distance of the process cartridge can also be realized by providing the process cartridge with an ID chip. In FIG. 12, “AIO” means an All-In-One type cartridge.

<Example 9>
In this embodiment, the length of the sheet is A [mm], the length of the intermediate transfer belt 15 is B [mm], the interval between the sheets is x [mm], and the trailing edge margin of the sheet is E [mm]. These are the formulas 0 <(BnA-E) / n <x <(BnA) / n (n: integer)
By defining and changing the paper interval x so as to satisfy this relationship, the belt position from which the trailing edge of the sheet is removed during the secondary transfer is located in the margin at the leading edge of the sheet after the next image forming step. To achieve. In the case of this embodiment, when the peripheral length of the intermediate transfer belt 15 is 650 mm, the paper (paper) size is A4 (294 mm), n = 2, and the paper trailing edge margin is 4 mm, the continuous paper passing so as to satisfy the above equation is performed. If the paper gap x is set to be 29 mm <x <31 mm, the portion where the trailing edge of the sheet is removed in the secondary transfer portion corresponds to the margin of the trailing edge of the sheet after the next image forming process, and therefore, there is no influence on the primary transfer. There was no image defect.

  FIG. 13 is a conceptual diagram for explaining the operation of this embodiment in detail. When the trailing edge of the sheet P1 passes through the secondary transfer portion, a charge + (1) is left on the intermediate belt 15. There is no problem if the sheet P is not in the secondary transfer position when it reaches the next secondary transfer position (2) (as in the second sheet P2 in the figure), and the third sheet If the position (2) is located at the rear end margin as in P3, the position of the + potential is located within the range of the rear end margin, so that it does not become a white image defect (streak). However, for example, if the position (2) arrives in the image as described on the leading end side of the third sheet P3 (it is possible that the paper interval x = 60 mm or the like), the phenomenon as described above As a result, the image defect occurs. However, as described above, the belt memory location on the surface of the intermediate transfer belt 15 when the trailing edge of the first sheet P1 passes through the secondary transfer portion is the most upstream image forming position (FIG. 2B). In other words, if the sheet spacing x is 29 mm <x <31 mm, the position of the + potential is the trailing edge margin of the sheet, so that there is nothing in the image form during the primary transfer. There is no impact. The operation of the configuration of the present embodiment is the same as that of the previous embodiment, and thus a duplicate description is omitted.

1: Photosensitive drum (photosensitive member)
2: Charger 3: Exposure means 3a: Laser diode (LD) element 5: Primary transfer roller 10: Process cartridge 15: Intermediate transfer belt 16: Cleaning counter roller 20: Tension roller 21: Driving and secondary transfer counter roller 23 : Feeding roller 24: Registration roller pair 25: Secondary transfer roller 31: Cleaning blade 32: Intermediate transfer belt cleaning unit 33: Waste toner storage unit for intermediate transfer member 41: Component contact developer (developer)
201, 204, 205: Toner 301: Image forming position on the most upstream photoconductor 306: Belt memory location A: Nip portion between photoconductor and intermediate transfer belt P, P1, P2, P3: Paper
p1, p2, p3: Images to be transferred

JP-A-6-161298 JP 2001-188418 A Japanese Patent Laying-Open No. 2005-10491

Claims (13)

A plurality of image carriers, and a transfer belt unit that rotates an endless belt by at least a driven roller and a drive roller, and performs toner transfer from each of the image carriers onto the transfer belt. A primary transfer portion that forms a color image, and a secondary transfer portion that is disposed opposite to the drive roller and has a secondary transfer member that collectively transfers the color image on the transfer belt onto a sheet. In an image forming apparatus having
An image forming apparatus, wherein a value between papers, which is an interval between the papers, is changed according to the number of papers simultaneously related to the transfer belt. A plurality of image carriers, and a transfer belt unit that rotates an endless belt by at least a driven roller and a drive roller, and performs toner transfer from each of the image carriers onto the transfer belt. A primary transfer portion that forms a color image, and a secondary transfer portion that is disposed opposite to the drive roller and has a secondary transfer member that collectively transfers the color image on the transfer belt onto a sheet. In an image forming apparatus having
When the length of the paper simultaneously related to the transfer belt is A [mm], the length of the belt is B [mm], and the distance between the papers is x [mm], these are the expressions (B−nA). ) / (N + 1) <x <(B-nA) / n (n: integer)
An image forming apparatus characterized in that the sheet interval x is defined and changed so as to satisfy the above relationship. A plurality of image carriers, and a transfer belt unit that rotates an endless belt by at least a driven roller and a drive roller, and performs toner transfer from each of the image carriers onto the transfer belt. A primary transfer portion that forms a color image, and a secondary transfer portion that is disposed opposite to the drive roller and has a secondary transfer member that collectively transfers the color image on the transfer belt onto a sheet. In an image forming apparatus having
When the paper length is A [mm], the belt length is B [mm], the paper gap is x [mm], and the leading edge margin of the paper is D [mm], these are expressed by the equation 0 <{B− (N-1) A} / n <x <{B- (n-1) A + D} / n (n: integer)
An image forming apparatus characterized in that the sheet interval x is defined and changed so as to satisfy the above relationship. A plurality of image carriers, and a transfer belt unit that rotates an endless belt by at least a driven roller and a drive roller, and performs toner transfer from each of the image carriers onto the transfer belt. A primary transfer portion that forms a color image, and a secondary transfer portion that is disposed opposite to the drive roller and has a secondary transfer member that collectively transfers the color image on the transfer belt onto a sheet. In an image forming apparatus having
When the paper length is A [mm], the belt length is B [mm], the paper interval is x [mm], and the paper trailing edge margin is E [mm], 0 <(B−nA−E ) / N <x <(B-nA) / n (n: integer)
An image forming apparatus characterized in that the sheet interval x is defined and changed so as to satisfy the above relationship. A plurality of image carriers, and a transfer belt unit that rotates an endless belt by at least a driven roller and a drive roller, and performs toner transfer from each of the image carriers onto the transfer belt. A primary transfer portion that forms a color image, and a secondary transfer portion that is disposed opposite to the drive roller and has a secondary transfer member that collectively transfers the color image on the transfer belt onto a sheet. In an image forming apparatus having
When the paper length is A [mm], the belt length is B [mm], the paper gap is x [mm], and the paper trailing edge margin is E [mm] (B−nA) / n < x <(B-nA) / n-1 (n: integer)
An image forming apparatus characterized in that the sheet interval x is defined and changed so as to satisfy the above relationship. The image forming apparatus according to any one of claims 2 to 5,
An image forming apparatus that applies a change in the paper interval x when a toner color transferred from a developing device including an image carrier on the most upstream side in a conveying direction of the paper on the transfer belt is other than yellow. . The image forming apparatus according to any one of claims 2 to 5,
The image forming apparatus, wherein the change in the sheet interval x is applied only when the travel distance of the process cartridge positioned at the uppermost stream is equal to or less than a threshold value. The image forming apparatus according to claim 7.
An image forming apparatus using a value read by an ID chip attached to a process cartridge as means for calculating a travel distance of the process cartridge. The image forming apparatus according to claim 7.
An image forming apparatus characterized in that, as means for calculating a travel distance of a process cartridge, calculation is performed from the number of rotations of a motor that drives the process cartridge. The image forming apparatus according to any one of claims 2 to 5,
An image forming apparatus that applies the change in the sheet interval x only when the dot configuration of an image to be printed is less than 100%. The image forming apparatus according to any one of claims 2 to 5,
Of the process cartridges in which the photosensitive member, the charger, and the developing unit are integrated, only when the travel distance of the process cartridge located in the uppermost stream in the transfer direction of the paper on the transfer belt is equal to or less than a certain threshold, An image forming apparatus characterized by applying a change. The image forming apparatus according to any one of claims 2 to 5,
An image forming apparatus, wherein temperature and humidity are detected by an environmental sensor, and the change in the sheet interval x is applied only when a predetermined threshold value is exceeded. The image forming apparatus according to claim 1,
The transfer belt has a common logarithm of surface resistivity of 9 logΩ / □ or more and 14 logΩ / □ or less, and a common logarithm of volume resistivity of 9 logΩ · cm or more and 14 logΩ · cm or less. .