JP2024057158A - Image forming device - Google Patents

Abstract

To obtain a larger transfer rate, while achieving both an improvement in the transfer rate and prevention of image disturbance caused by discharge at the entrance of a transfer nip.SOLUTION: An image forming apparatus 100 has: an image carrier belt 31; nip forming means 30, 41 that sandwich the image carrier belt with an inside contact member 33 in contact with the inside of the image carrier belt and an outside contact member 404 in contact with the outside of the image carrier belt, and form a transfer nip N between an outside surface of the image carrier belt and the outside contact member; and transfer bias application means 39 that applies an AC transfer bias between the inside contact member and the outside contact member. The length in a recording sheet conveyance direction of a pre-transfer nip that is a portion not in contact with the inside contact member, of a portion of the image carrier belt in contact with the outside contact member, becomes the shortest length within a length range in which the peak-to-peak voltage of the AC transfer bias can be set so as to prevent the occurrence of discharge on an entrance side of the transfer nip.SELECTED DRAWING: Figure 1

Description

The present invention relates to an image forming device.

Conventionally, there is known an image forming device in which an image carrier belt is sandwiched between an inner contact member that contacts the inside of the image carrier belt and an outer contact member that contacts the outside of the image carrier belt to form a transfer nip, and an AC transfer bias is applied between the inner contact member and the outer contact member.

For example, Patent Document 1 discloses an image forming device in which a portion of an intermediate transfer belt (image carrier belt) wrapped around a secondary transfer roller (outer abutment member) is brought into contact with a portion of the secondary transfer belt wrapped around the secondary transfer roller (inner abutment member) to form a secondary transfer nip. In this image forming device, a superimposed bias (AC transfer bias) in which an AC voltage and a DC voltage are superimposed is applied between the secondary transfer back roller and the secondary transfer roller.

In this image forming apparatus, the position of the secondary transfer roller is offset upstream in the recording material conveying direction with respect to the secondary transfer back roller. This forms a pre-transfer nip, which is a portion of the intermediate transfer belt that is in contact with the secondary transfer belt portion wrapped around the secondary transfer roller but is not in contact with the secondary transfer back roller. The formation of the pre-transfer nip prevents discharge from occurring in the gap between the outer surface of the intermediate transfer belt and the recording material when the recording material enters the secondary transfer nip, suppressing image distortion caused by discharge. On the other hand, the formation of the pre-transfer nip causes the toner to be overcharged in the secondary transfer nip, lowering the transfer rate. Therefore, in order to improve this transfer rate, the image forming apparatus uses a superimposed bias as the secondary transfer bias, with the application time of the peak voltage Vt in the transfer direction that moves the toner image from the intermediate transfer belt side to the recording material side being less than 50%.

In conventional image forming devices, while it was necessary to suppress image distortion caused by discharge at the entrance of the transfer nip and improve the transfer rate at the same time, the improvement in the transfer rate was insufficient.

In order to solve the above-mentioned problems, the present invention provides an image forming apparatus having an image carrier belt, a nip forming means for sandwiching the image carrier belt between an inner contact member that contacts the inside of the image carrier belt and an outer contact member that contacts the outside of the image carrier belt, forming a transfer nip between the outer surface of the image carrier belt and the outer contact member, and a transfer bias applying means for applying an AC transfer bias between the inner contact member and the outer contact member, characterized in that the length in the recording sheet conveyance direction of the pre-transfer nip portion, which is the portion of the image carrier belt that contacts the outer contact member but does not contact the inner contact member, is configured to be the shortest length within the length range in which the peak-to-peak voltage of the AC transfer bias can be set so that no discharge occurs on the entrance side of the transfer nip.

According to the present invention, it is possible to obtain a higher transfer rate while simultaneously suppressing image disturbance caused by discharge at the entrance of the transfer nip and improving the transfer rate.

FIG. 1 is a schematic configuration diagram showing a printer according to an embodiment. FIG. 2 is an enlarged schematic configuration diagram of an image forming unit in the printer. 3A is a partially enlarged cross-sectional view of an intermediate transfer belt made of an elastic belt having a coating layer, FIG. 3B is a partially enlarged cross-sectional view of an intermediate transfer belt made of an elastic belt having particles, and FIG. 3C is a partially enlarged plan view of FIG. 1A is an explanatory diagram showing a configuration in which the secondary transfer roller is not offset to the upstream side in the recording sheet transport direction, and FIG. 1B is an explanatory diagram showing a configuration in which the secondary transfer roller is offset to the upstream side in the recording sheet transport direction. 1A is an explanatory diagram of a secondary transfer nip in a configuration in which the secondary transfer roller 36 is not offset, and FIG. 1B is an explanatory diagram of a secondary transfer nip in a configuration in which the secondary transfer roller 36 is offset to the upstream side in the recording sheet transport direction indicated by an arrow Z. FIG. 1 is a schematic diagram showing the configuration of a roller transfer type printer that uses a secondary transfer roller as a transfer member. 5A to 5C are explanatory diagrams showing the occurrence of entrance discharge due to differences in the usage environment.

Hereinafter, an embodiment in which the present invention is applied to an electrophotographic color printer (hereinafter, simply referred to as the "printer") as an image forming apparatus will be described.
In this embodiment, the symbols Y, M, C, and K are suffixes given to components corresponding to yellow, magenta, cyan, and black, and may be omitted as appropriate.

The basic configuration of the printer 100 according to this embodiment will be described.
FIG. 1 is a schematic diagram showing the configuration of a printer 100 according to the present embodiment.
This printer 100 includes four image forming units 1Y, 1M, 1C, and 1K for forming toner images of yellow (Y), magenta (M), cyan (C), and black (K). The printer 100 also includes a transfer unit 30 as a transfer device, a cassette 60 for storing recording sheets P, a fixing device 90, and a control unit 300.

The four image forming units 1Y, 1M, 1C, and 1K use different color toners, Y, M, C, and K, as developers, but are otherwise configured the same and are replaced when they reach the end of their life. In other words, the four image forming units 1Y, 1M, 1C, and 1K are detachably mounted on the printer body 100A and are replaceable.

FIG. 2 is an enlarged schematic diagram of one of the four image forming units 1Y, 1M, 1C, and 1K.
The four image forming units 1Y, 1M, 1C, and 1K have the same configuration except for the colors of toner they use, and for this reason, the suffixes Y, M, C, and K indicating the colors of toner they use are omitted in FIG.

The image forming unit 1 includes a drum-shaped photoconductor 2 that is a latent image carrier, a drum cleaning device 3, a static eliminator, a charging device 6, a developing device 8, etc. In the image forming unit 1, these multiple devices are held by a common holder to form a process cartridge that is detachable as a unit from the printer main body 100A, and can be replaced as a unit.

The photoreceptor 2 is drum-shaped with an organic photosensitive layer formed on the surface of the drum base, and is rotated counterclockwise in the figure by a driving means such as a motor. The charging device 6 uniformly charges the surface of the photoreceptor 2 by generating a discharge between the charging roller 7 and the photoreceptor 2 while bringing the charging roller 7, which is a charging member to which a charging bias is applied, into contact with or close to the photoreceptor 2. Instead of a method of bringing a charging member such as a charging roller into contact with or close to the photoreceptor 2, a method using a charging charger may be adopted.

The surface of the photoconductor 2, which is uniformly charged by the charging roller 7, is optically scanned by exposure light such as laser light emitted from the optical writing unit 101 to carry an electrostatic latent image for each color. This electrostatic latent image is developed by the developing device 8 using toner of each color to become a toner image for each color. The toner image on the photoconductor 2 is primarily transferred onto the outer peripheral surface (outer surface) 31a of the intermediate transfer belt 31, which serves as an image carrier belt.

The drum cleaning device 3 removes residual toner adhering to the surface of the photoconductor 2 after passing through the primary transfer nip. The drum cleaning device 3 has a cleaning brush roller 4 that is rotated, a cleaning blade 5 that contacts the photoconductor 2, and the like. The drum cleaning device 3 cleans the surface of the photoconductor 2 by scraping the residual toner with the cleaning brush roller 4 and by scraping the residual toner with the cleaning blade 5 from the surface of the photoconductor 2. The static eliminator is a well-known device that eliminates residual charge on the photoconductor 2 after it has been cleaned by the drum cleaning device 3. The surface of the photoconductor 2 is initialized by this static elimination in preparation for the next image formation.

The developing device 8 has a developing section 12 that contains a developing roller 9 that serves as a developer carrier, and a developer transport section 13 that agitates and transports the developer. The developer transport section 13 has a first transport chamber that houses a first screw member 10, and a second transport chamber that houses a second screw member 11. The first screw member 10 and the second screw member 11 are rotatably supported on the case of the developing device 8, and when driven to rotate, they circulate and transport the developer, supplying it to the developing roller 9.

As shown in FIG. 1, an optical writing unit 101 serving as a latent image writing means is disposed above the image forming units 1Y, 1M, 1C, and 1K. This optical writing unit 101 optically scans the photoconductors 2Y, 2M, 2C, and 2K with laser light emitted from a laser diode based on image data sent from an external device such as a personal computer. This optical scanning forms electrostatic latent images for Y, M, C, and K on the photoconductors 2Y, 2M, 2C, and 2K, respectively.

Transfer unit 30 is disposed below image forming units 1Y, 1M, 1C, and 1K as a transfer device that moves intermediate transfer belt 31 endlessly in the clockwise direction in the figure while tensioning it. The rotational movement direction of intermediate transfer belt 31 is defined as belt movement direction a. In addition to intermediate transfer belt 31, transfer unit 30 is equipped with a drive roller 32, a secondary transfer back roller 33 as an inner abutment member, a cleaning backup roller 34, four primary transfer rollers 35Y, 35M, 35C, and 35K, and a pre-transfer roller 37 as a pressing member. Transfer unit 30 is detachable (replaceable) as a unit from printer main body 100A.

The intermediate transfer belt 31 is supported and tensioned by being wrapped around a drive roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, four primary transfer rollers 35Y, 35M, 35C, 35K, and a pre-transfer roller 37, which are arranged on the inner peripheral surface (inner side) side of the intermediate transfer belt 31. The rotational force of the drive roller 32, which is driven to rotate in the clockwise direction in the figure by a drive means such as a drive motor, causes the intermediate transfer belt 31 to move endlessly in the same direction and be transported.

In this embodiment, the intermediate transfer belt 31 is an intermediate transfer body having elasticity, in which at least a base layer 310, an elastic layer 311, and a surface coat layer 312 are laminated, as shown in FIG. 3(a). Alternatively, the intermediate transfer belt 31 may be a body in which the base layer 310 and the elastic layer 311 are laminated, and a large number of particles 313 are dispersed in the elastic layer 311, as shown in FIG. 3(b). The particles 313 are densely arranged in the belt surface direction, as shown in FIG. 3(c), with a part of them protruding from the surface of the elastic layer 311. The particles 313 form a plurality of irregularities on the outer peripheral surface 31a, which is the belt surface. The base layer 310 is an endless belt-like member made of a material having a certain degree of flexibility and high rigidity. The elastic layer 311 is made of an elastic material with excellent flexibility laminated on the outer peripheral surface of the base layer 310.

The four primary transfer rollers 35Y, 35M, 35C, and 35K sandwich the endlessly moving intermediate transfer belt 31 between the photoconductors 2Y, 2M, 2C, and 2K, forming primary transfer nips for Y, M, C, and K where the outer circumferential surface 31a of the intermediate transfer belt 31 contacts the photoconductors 2Y, 2M, 2C, and 2K. A primary transfer bias is applied to the primary transfer rollers 35Y, 35M, 35C, and 35K from a well-known transfer bias power source. This forms a transfer electric field between the Y, M, C, and K toner images on the photoconductors 2Y, 2M, 2C, and 2K and the primary transfer rollers 35Y, 35M, 35C, and 35K.

The Y toner image formed on the surface of the yellow photoconductor 2Y enters the yellow primary transfer nip as the yellow photoconductor 2Y rotates. Then, due to the action of the transfer electric field and nip pressure, it is primarily transferred from the yellow photoconductor 2Y to the intermediate transfer belt 31. The intermediate transfer belt 31 onto which the Y toner image has been primarily transferred in this way then passes through the primary transfer nips for M, C, and K in sequence. Then, the M, C, and K toner images on the photoconductors 2M, 2C, and 2K are sequentially superimposed and primarily transferred onto the Y toner image. This superimposed primary transfer forms a four-color superimposed toner image on the intermediate transfer belt 31. As the primary transfer member, a transfer charger or a transfer brush may be used instead of the primary transfer rollers 35Y, 35M, 35C, and 35K.

The image forming process described so far has been described on the assumption that a four-color full-color image is formed. However, it is also possible to form a single-color toner image of any of yellow, magenta, cyan, and black, or a toner image using toner of at least two of the above colors, and transfer it to the intermediate transfer belt 31.

A belt-type secondary transfer unit 41 equipped with a secondary transfer belt 404 as a transfer member is arranged around the outer peripheral surface side of the intermediate transfer belt 31. The secondary transfer belt 404 is stretched around three rotating bodies, namely rollers 401, 402, and 403 and secondary transfer roller 36. The secondary transfer unit 41 sandwiches the intermediate transfer belt 31 between the secondary transfer back roller 33, which is an inner contact member, and the secondary transfer belt 404, which is an outer contact member, to form a secondary transfer nip N as a transfer nip where the outer peripheral surface 31a of the intermediate transfer belt 31 and the outer peripheral surface of the secondary transfer belt 404 come into contact with each other.

In this embodiment, a power source 39 is connected to the secondary transfer back roller 33 as a transfer bias application means, and a secondary transfer bias is applied between the secondary transfer back roller 33 and the secondary transfer belt 404 (the portion wrapped around the secondary transfer roller 36). As a result, a secondary transfer electric field is formed between the secondary transfer back roller 33 and the secondary transfer belt 404, which moves negative polarity toner from the secondary transfer back roller 33 side toward the secondary transfer belt 404 side. In this embodiment, the secondary transfer bias output from the power source 39 uses constant current control. In addition, the secondary transfer bias output from the power source 39 may also use constant voltage control.

The secondary transfer roller 36, together with the secondary transfer belt 404, performs a secondary transfer of the toner image on the outer peripheral surface 31a of the intermediate transfer belt 31 onto the recording sheet P. The secondary transfer roller 36 is in contact with the inner peripheral surface of the secondary transfer belt 404 and is disposed opposite the secondary transfer back roller 33. The secondary transfer roller 36 sandwiches the intermediate transfer belt 31 and the secondary transfer belt 404 between itself and the secondary transfer back roller 33. The secondary transfer roller 36 is biased against the secondary transfer belt 404, forming a secondary transfer nip N between the intermediate transfer belt 31 and the secondary transfer belt 404.

The secondary transfer belt 404 can be made of a belt material selected from resin materials such as polyimide (PI), polyamide-imide (PAI), and polyvinylidene fluoride (PVDF). The secondary transfer belt 404 can also be made of a belt material made of an elastic material instead of these materials. In this embodiment, a polyimide resin belt (PI belt) with a thickness of 80 μm is used.

The roller 401 separates the recording sheet P, which is stuck to the secondary transfer belt 404 by electrostatic attraction, from the secondary transfer belt 404 by the curvature separation of the roller 401. The roller 403 urges the secondary transfer belt 404 from the inside to the outside by a tension spring 406 as a urging means, forming a tension roller. A cleaning blade 405, which serves as a cleaning member, is in contact with the outside of the secondary transfer belt 404 opposite the roller 403, and scrapes off paper dust and toner remaining on the secondary transfer belt 404. In other words, the roller 403 also functions as a cleaning blade opposing roller.

In the secondary transfer unit 41, a pattern detection sensor 407 as a density detection means is disposed outside the secondary transfer belt 404 facing the roller 402. The pattern detection sensor 407 is used to adjust the image density.

In this embodiment, the power source 39 is connected to the secondary transfer back roller 33, but the power source 39 may be connected to the secondary transfer roller 36. When the power source is connected to the secondary transfer roller 36, a secondary transfer voltage of the opposite polarity to the toner is applied to the secondary transfer roller 36, and when the power source is applied to the secondary transfer back roller 33, a secondary transfer voltage of the same polarity as the toner is applied to the secondary transfer back roller 33.

The secondary transfer bias applied by the power source 39 is an AC bias in which an AC current, which is an AC component, is superimposed on a DC current, which is a DC component. Specifically, the secondary transfer bias applied by the power source 39 alternates between a transfer direction bias that moves the toner image from the intermediate transfer belt 31 side to the recording sheet P side, and a return direction bias of the opposite polarity to the transfer direction bias, at least when the toner image on the intermediate transfer belt 31 is transferred to the recording sheet P.

Below the secondary transfer unit 41, a cassette 60 is provided that contains a stack of recording sheets P, such as various types of paper and resin sheets. The cassette 60 has a roller 60a in contact with the topmost recording sheet P of the stack, and by rotating the roller 60a at a predetermined timing, the recording sheet P is sent from the cassette 60 to a conveying path 65 formed between the cassette 60 and the secondary transfer nip N. A conveying roller pair, a registration roller pair 61, and a lower guide member 62 are provided on the conveying path 65. The registration roller pair 61 rotates at a timing that allows the recording sheet P sent from the cassette 60 to be synchronized with the four-color superimposed toner image on the outer circumferential surface 31a of the intermediate transfer belt 31 in the secondary transfer nip N, and sends the recording sheet P toward the secondary transfer nip N.

The four-color superimposed toner image on the outer peripheral surface 31a of the intermediate transfer belt 31 is secondarily transferred onto the recording sheet P by the action of the secondary transfer electric field and nip pressure in the secondary transfer nip N, and becomes a full-color toner image in combination with the white color of the recording sheet P. After passing through the secondary transfer nip N, residual toner that was not transferred to the recording sheet P adheres to the intermediate transfer belt 31. The residual toner is cleaned from the belt surface by a belt cleaning device 38 that is in contact with the outer peripheral surface 31a of the intermediate transfer belt 31. A cleaning backup roller 34 arranged inside the loop of the intermediate transfer belt 31 backs up the belt cleaning by the belt cleaning device 38 from inside the loop.

A fixing device 90 is disposed downstream of the secondary transfer nip N in the recording sheet transport direction b. A recording sheet P onto which a toner image has been secondarily transferred is fed into the fixing device 90. The fed recording sheet P is sandwiched in the fixing nip where a fixing roller 91, which has a heat source inside, comes into contact with a pressure roller 92, and the toner in the full-color toner image is softened and fixed by the application of heat and pressure. After fixing, the recording sheet P is discharged from the fixing device 90 and ejected outside the machine.

In this configuration, at the entrance side of the secondary transfer nip N, discharge (entrance discharge) occurs in the gap between the outer circumferential surface 31a of the intermediate transfer belt 31, which is wound around the secondary transfer back roller 33 to which a power source 39 is connected, and the recording sheet P. For example, as shown in FIG. 4A, if the secondary transfer roller 36 is not offset upstream (to the right in the figure) in the recording sheet transport direction b from the position facing the secondary transfer back roller 33, an entrance gap S is formed between the intermediate transfer belt 31 and the recording sheet P at the entrance side of the secondary transfer nip, and discharge can occur in this gap S.

On the other hand, as shown in FIG. 4B, when the secondary transfer roller 36 is offset upstream in the recording sheet conveying direction b, the secondary transfer nip N has a main nip portion, which is a portion of the intermediate transfer belt 31 that is sandwiched between the secondary transfer back roller 33 and the secondary transfer belt 404, and a pre-transfer nip portion, which is a portion of the intermediate transfer belt that is in contact with the secondary transfer belt 404 but is not in contact with the secondary transfer back roller 33. By forming the pre-transfer nip portion, the recording sheet P can come into contact with the outer circumferential surface 31a of the intermediate transfer belt 31 before entering the main nip portion. As a result, the gap S generated between the outer circumferential surface 31a of the intermediate transfer belt 31 and the recording sheet P is farther from the main nip portion to which a large secondary transfer bias is applied than in the configuration of FIG. 4A. The bias (voltage) applied to the gap S becomes smaller as it moves away from the main nip portion, so that the occurrence of entrance discharge at the gap S is suppressed.

The likelihood of inlet discharge occurring varies depending on the voltage applied to the secondary transfer back roller 33, the electrical resistance values and materials of the intermediate transfer belt 31, secondary transfer back roller 33, secondary transfer roller 36, and secondary transfer belt 404, the type of recording sheet P, and the state of conveyance. In particular, when the intermediate transfer belt 31 is an elastic belt having an elastic layer as in this embodiment, inlet discharge is likely to occur. For this reason, this embodiment employs a configuration in which the secondary transfer roller 36 is offset upstream in the recording sheet conveyance direction b, as shown in FIG. 4(b).

FIG. 5A is an explanatory diagram showing a configuration in which the secondary transfer roller 36 is not offset.
FIG. 5B is an explanatory diagram showing a configuration in which the secondary transfer roller 36 is offset to the upstream side in the recording sheet transport direction indicated by the arrow Z.
5A and 5B, the center distance between the secondary transfer back roller 33 and the secondary transfer roller 36 is the same, and assuming that the rollers 33, 36 and the secondary transfer belt 404 are crushed in the same manner, the secondary transfer nip N is determined geometrically. That is, in both of the configurations of FIG. 5A and 5B, the length L1 of the main nip n1 area sandwiched between the two rollers, the secondary transfer back roller 33 and the secondary transfer roller 36, is the same. In the configuration in which the secondary transfer roller 36 is offset, as shown in FIG. 5B, a pre-transfer nip n2 of length L2 is present in which the intermediate transfer belt 31 is wound around the outer circumferential surface 36a of the secondary transfer roller 36 via the secondary transfer belt 404 on the inlet side (upstream side in the recording sheet conveying direction) of the main nip n1.

Note that here, the secondary transfer belt 404 is used as the transfer member, but in the case of a roller transfer method in which the secondary transfer roller 36 is used alone as the transfer member as shown in FIG. 6, the length L2 of the intermediate transfer belt 31 directly wrapped around only the outer peripheral surface 36a of the secondary transfer roller 36 becomes the pre-transfer nip portion n2.

In the configuration in which the pre-transfer nip portion n2 is formed as in this embodiment, as shown in FIG. 5B, not only when the recording sheet P is in the main nip portion n1, but also when it is in the pre-transfer nip portion n2, the back surface of the recording sheet P contacts the secondary transfer belt 404 and the front surface of the recording sheet P contacts the intermediate transfer belt 31. Therefore, in the configuration in which the recording sheet P is not offset as shown in FIG. 5A, the length L of the secondary transfer nip N is the length L1 of the main nip portion n1, while in the configuration in which the recording sheet P is offset as shown in FIG. 5B, the length L of the secondary transfer nip N is the length L1 of the main nip portion n1 + the length L2 of the pre-transfer nip portion n2. As a result, in the offset configuration, the time that the toner on the intermediate transfer belt 31 passes through the secondary transfer nip N is longer than in the non-offset configuration, and the amount of transfer current flowing into the toner is increased. As a result, the amount of weakly charged toner and reversely charged toner increases due to overcharging of the toner, and the transfer rate is likely to decrease.

Here, when an AC bias is used as the secondary transfer bias as in this embodiment, the transfer rate can be improved by increasing the peak-to-peak voltage (peak-to-peak voltage) Vpp value of the secondary transfer bias (AC transfer bias). However, the higher the peak-to-peak voltage Vpp value, the more likely it is that an entrance discharge will occur at the secondary transfer nip N. Therefore, by adjusting the combination of the length L2 of the pre-transfer nip portion n2 and the peak-to-peak voltage Vpp of the secondary transfer bias, it is possible to both suppress image disturbance caused by entrance discharge at the secondary transfer nip N and improve the transfer rate.

Table 1 shows the experimental results of the relationship between the combination of the length of the pre-transfer nip portion and the peak-to-peak voltage Vpp of the secondary transfer bias and the transfer rate when thin paper is used as the recording sheet P.
Table 2 shows the experimental results of the relationship between the combination of the length of the pre-transfer nip portion and the peak-to-peak voltage Vpp of the secondary transfer bias and the transfer rate when thick paper is used as the recording sheet P.

The above experiment confirmed the transfer rate and the occurrence of entrance discharge at the secondary transfer nip N when a halftone image was formed using only black toner at a process linear speed of 630 mm/s in an environment with a temperature of 27°C and a humidity of 80% for each of thin paper and thick paper by changing the pre-transfer nip and the peak-to-peak voltage Vpp of the secondary transfer bias. In Tables 1 and 2, the places where a transfer rate value is not entered are places where the occurrence of entrance discharge at the secondary transfer nip N was confirmed. Therefore, the places in Tables 1 and 2 where a transfer rate value is not entered indicate that the entered transfer rate was obtained without the occurrence of entrance discharge at the secondary transfer nip N.

In Tables 1 and 2, the experimental results in which the peak-to-peak voltage Vpp is 0.0 kV were obtained by using a secondary transfer bias without an AC component (i.e., a DC transfer bias). In addition, due to the difference between thin paper and thick paper, the secondary transfer bias in this experiment used a constant current control of -110 μA for thin paper and a constant current control of -120 μA for thick paper.

The results of this experiment show that thick paper has a higher margin for generating an entrance discharge relative to the peak-to-peak voltage Vpp than thin paper. In addition, it can be seen that the length L2 of the pre-transfer nip n2, which allows the use of an AC transfer bias instead of a DC transfer bias as the secondary transfer bias without generating an entrance discharge, is 2 mm or more for thin paper and 1 mm or more for thick paper.

In addition, in the results of this experiment, when compared at the same peak-to-peak voltage Vpp, the longer the length L2 of the pre-transfer nip n2, the lower the transfer rate. This indicates that as the pre-transfer nip n2 becomes longer, the amount of transfer current flowing into the toner increases, and the amount of weakly charged toner and reversely charged toner increases due to toner overcharging, resulting in a lower transfer rate.

On the other hand, in the results of this experiment, when comparing the length of the same pre-transfer nip portion n2, the transfer rate increases as the peak-to-peak voltage Vpp value increases. This indicates that the transfer rate increases as the peak-to-peak voltage Vpp value increases, and the transfer rate improves due to the so-called AC transfer bias.

According to the results of this experiment, when trying to obtain a high transfer rate without generating an entrance discharge to the secondary transfer nip N for both thin paper and thick paper, the length L2 of the pre-transfer nip n2 is set to 2 [mm]. In this case, the secondary transfer bias for thin paper has a peak-to-peak voltage Vpp value of 1.0 [kV], and for thick paper, the secondary transfer bias for thick paper has a peak-to-peak voltage Vpp value of 4.0 [kV].

In order to achieve both suppression of image disturbance caused by entrance discharge of the secondary transfer nip N and improvement of the transfer rate by adjusting the combination of the length L2 of the pre-transfer nip n2 and the peak-to-peak voltage Vpp, it is important to increase the transfer rate as much as possible without causing entrance discharge of the secondary transfer nip N. Methods for increasing the transfer rate without causing entrance discharge of the secondary transfer nip N include increasing the value of the peak-to-peak voltage Vpp as much as possible without causing entrance discharge of the secondary transfer nip N, and shortening the length L2 of the pre-transfer nip n2 as much as possible without causing entrance discharge of the secondary transfer nip N.

Based on the above-mentioned experimental results, the inventors have found that a greater transfer rate can be obtained by shortening the length L2 of the pre-transfer nip n2 than by increasing the value of the peak-to-peak voltage Vpp. In other words, when increasing the transfer rate within a range in which no inlet discharge occurs at the secondary transfer nip N, the inventors have found that the transfer rate is improved more effectively by shortening the length L2 of the pre-transfer nip n2 than by increasing the value of the peak-to-peak voltage Vpp.

According to this embodiment, the length L2 of the pre-transfer nip n2 is configured to be the shortest length (2.0 mm according to the experimental results described above) within the length range (a range of 2.0 mm to 5 mm according to the experimental results described above) in which the peak-to-peak voltage Vpp of the AC transfer bias can be set so that no discharge occurs at the entrance of the secondary transfer nip N. This makes it possible to obtain a higher transfer rate while simultaneously suppressing image disturbance caused by discharge at the entrance of the transfer nip and improving the transfer rate.

The above experimental results are for the configuration of this embodiment, and the specific length of the pre-transfer nip, the peak-to-peak voltage Vpp of the secondary transfer bias, and the transfer rate will vary depending on factors such as the film thickness and electrical resistance of various belts, the diameter of various rollers, the electrical resistance and hardness, and the process linear speed. Therefore, the optimal length of the pre-transfer nip will vary depending on the configuration of the image forming apparatus. However, as described above, if the length L2 of the pre-transfer nip n2 is configured to be the shortest length within the length range in which the peak-to-peak voltage Vpp of the AC transfer bias can be set so that no entrance discharge occurs at the secondary transfer nip N, a larger transfer rate can be obtained while simultaneously suppressing image disturbance due to entrance discharge at the transfer nip and improving the transfer rate.

For example, Table 3 below shows experimental results for different device configurations.
In this Table 3, too, if the length L2 of the pre-transfer nip portion n2 is configured to be the shortest length (6 mm, as seen in Table 3) within the length range (a range of 6 mm or more and 10 mm, as seen in Table 3) in which the peak-to-peak voltage Vpp of the AC transfer bias can be set so that no entrance discharge occurs at the secondary transfer nip N, then the highest transfer rate (90% as seen in Table 3) can be obtained.

Even when the length of the pre-transfer nip is set to the optimum length, as can be seen from the experimental results shown in Tables 1 and 2, the optimum value of the peak-to-peak voltage Vpp of the secondary transfer bias varies depending on the type of recording sheet P used. That is, according to the experimental results described above, in a configuration in which the length L2 of the pre-transfer nip n2 is set to the optimum 2 [mm], the optimum value of the peak-to-peak voltage Vpp of the secondary transfer bias for thin paper is 1.0 [kV], and the optimum value of the peak-to-peak voltage Vpp of the secondary transfer bias for thick paper is 4.0 [kV]. Therefore, by changing the peak-to-peak voltage Vpp of the secondary transfer bias to an appropriate value according to the type of recording sheet P, a higher transfer rate can be obtained for each type of recording sheet P within the range in which no discharge occurs at the entrance of the secondary transfer nip N.

The discrimination means for discriminating the type of recording sheet P is not particularly limited, and any known means can be used. For example, it may be a means for discriminating the type of recording sheet P based on input information input by a user's instruction operation. Also, for example, a paper discrimination sensor installed in the cassette 60 may be used as the discrimination means.

Even when the length of the pre-transfer nip is set to the optimum, the optimum value of the peak-to-peak voltage Vpp of the secondary transfer bias changes because the electrical resistance value of each part varies depending on the usage environment (temperature and humidity) of the image forming apparatus. FIG. 7 is a table showing the occurrence of inlet discharge due to differences in usage environment. Note that FIG. 7 shows the occurrence of inlet discharge of the secondary transfer nip N confirmed in each of the first usage environment with a temperature of 27° C. and a humidity of 80% and the second usage environment with a temperature of 19° C. and a humidity of 40%. In FIG. 7, the case where the occurrence of inlet discharge was confirmed in the first usage environment is indicated by the upper right diagonal line, and the case where the occurrence of inlet discharge was confirmed in the second usage environment is indicated by the upper left diagonal line. When the occurrence of inlet discharge was confirmed in both usage environments, the upper right diagonal line and the upper left diagonal line are overlapped.

According to FIG. 7, in a configuration in which the length L2 of the pre-transfer nip n2 is optimally set to 2 [mm], the optimal value of the peak-to-peak voltage Vpp of the secondary transfer bias in the first usage environment is 4.0 [kV], which is the largest value in the range of the settable peak-to-peak voltage Vpp (range of 1.0 [kV] or more and 4.0 [kV] or less). However, in the second usage environment, if the peak-to-peak voltage Vpp of the secondary transfer bias is 4.0 [kV], an entrance discharge of the secondary transfer nip N occurs, so the optimal value of the peak-to-peak voltage Vpp of the secondary transfer bias in the first usage environment is 1.0 [kV]. Therefore, by changing the peak-to-peak voltage Vpp of the secondary transfer bias to an appropriate value according to the usage environment, a higher transfer rate can be obtained in each usage environment within the range in which an entrance discharge of the secondary transfer nip N does not occur.

As a means for detecting the usage environment, a detection means for detecting at least one of temperature and humidity can be used. The set value of the peak-to-peak voltage Vpp for each usage environment may be determined according to a calculation formula that calculates the set value of the peak-to-peak voltage Vpp from the detected information of temperature and humidity, or it may be determined by providing categories of temperature and humidity and selecting from the set values of the peak-to-peak voltage Vpp assigned to each category.

In this embodiment, an apparatus that transfers an image from the intermediate transfer belt 31 to the recording sheet P (a so-called intermediate transfer type image forming apparatus) has been described as an example, but the present invention is not limited to this. For example, the present invention can also be applied to an apparatus that directly transfers an image from an image carrier belt such as a photosensitive belt to the recording sheet P (a so-called direct transfer type image forming apparatus). In addition, the transfer unit may use a transfer device that does not form a transfer nip (a charger type transfer charger).

In this embodiment, an image forming device has been described in which the recording sheet P is transported horizontally in the transfer section (secondary transfer nip N), but the present invention can also be applied to image forming devices configured to transport the recording sheet P upward, downward, diagonally upward, or diagonally downward in the transfer section.

Although the preferred embodiment of the present invention has been described above, the present invention is not limited to such a specific embodiment, and various modifications and changes are possible within the scope of the spirit of the present invention described in the claims, unless otherwise specifically limited in the above description.
For example, the image forming apparatus may not be a printer, but may be a single copy machine or facsimile, or a multifunction machine having at least two of the functions of a copy machine, printer, facsimile, and scanner.
The effects described in the embodiments of the present invention are merely a list of the most preferable effects resulting from the present invention, and the effects of the present invention are not limited to those described in the embodiments of the present invention.

The above description is merely an example, and each of the following aspects provides unique effects.
[First aspect]
The first aspect is a nip forming means (e.g., transfer unit 30 and secondary transfer unit 41) that sandwiches the image carrier belt between an inner contact member (e.g., secondary transfer back roller 33) that contacts the inner side of the image carrier belt and an outer contact member (e.g., secondary transfer belt 404) that contacts the outer side of the image carrier belt, and forms a transfer nip (e.g., secondary transfer nip N) between the outer surface (e.g., outer peripheral surface 31a) of the image carrier belt and the outer contact member, and an AC transfer belt (e.g., AC transfer belt 403) that is disposed between the inner contact member and the outer contact member. and a transfer bias application means (e.g., power source 39) for applying a bias (e.g., a secondary transfer bias), wherein the length L2 in the recording sheet transport direction of a pre-transfer nip portion n2, which is a portion of the image carrier belt that is in contact with the outer abutment member but not in contact with the inner abutment member, is configured to be the shortest length within a length range in which the peak-to-peak voltage Vpp of the AC transfer bias can be set so that discharge does not occur on the entrance side of the transfer nip.
In a conventional image forming apparatus, in order to suppress image disturbance caused by entrance discharge occurring at the entrance side of the transfer nip, some of the image carrier belt portions that are in contact with the outer contact member but are not in contact with the inner contact member, i.e., a pre-transfer nip portion, are formed. The longer the length of the pre-transfer nip portion in the recording sheet conveying direction (hereinafter simply referred to as "length"), the longer the distance between the transfer nip center (the closest position between the inner contact member and the outer contact member) and the transfer nip entrance end. Therefore, the gap generated between the image carrier belt and the recording sheet at the entrance side of the transfer nip is moved away from the transfer nip center. Since the highest voltage is applied to the transfer nip center within the sheet conveying direction range of the transfer nip, and the voltage applied decreases as it moves away from the transfer nip center, if the length of the pre-transfer nip portion is increased, the voltage applied to the gap is reduced, and the occurrence of discharge (entrance discharge) occurring in the gap is suppressed.
However, the longer the length of the pre-transfer nip portion, the longer the time that the toner on the image carrier belt takes to pass through the pre-transfer nip portion, and therefore the amount of transfer current flowing into the toner increases, causing the toner to become overcharged, increasing the amount of weakly charged toner and reversely charged toner, and reducing the transfer rate.
On the other hand, the transfer rate can be improved by increasing the peak-to-peak voltage Vpp of the AC transfer bias applied between the inner contact member and the outer contact member, but increasing the peak-to-peak voltage Vpp makes it easier for discharge to occur on the entrance side of the transfer nip (entrance discharge).
Therefore, by adjusting the combination of the length of the pre-transfer nip and the value of the peak-to-peak voltage Vpp, it is possible to both suppress image disturbance caused by discharge at the entrance of the transfer nip and improve the transfer rate.
In order to achieve both, it is important to increase the transfer rate within a range where no inlet discharge occurs at the transfer nip. Methods for increasing the transfer rate within a range where no inlet discharge occurs at the transfer nip include a method of increasing the value of the peak-to-peak voltage Vpp as much as possible while adjusting the length of the pre-transfer nip portion within a range where no inlet discharge occurs at the transfer nip, and a method of shortening the length of the pre-transfer nip portion as much as possible while adjusting the value of the peak-to-peak voltage Vpp within a range where no inlet discharge occurs at the transfer nip. As a result of research by the present inventors, it was found that the latter method can increase the maximum transfer rate obtained within a range where no inlet discharge occurs at the transfer nip compared to the former method.
Therefore, in this embodiment, the length of the pre-transfer nip is set to the shortest length within the range of lengths in which the peak-to-peak voltage of the AC transfer bias can be set so that no discharge occurs at the entrance of the transfer nip, thereby making it possible to obtain a higher transfer rate while simultaneously suppressing image disturbance caused by discharge at the entrance of the transfer nip and improving the transfer rate.

[Second aspect]
In a second aspect, in the first aspect, the transfer bias application means applies an AC transfer bias having a maximum peak-to-peak voltage within a range that does not cause discharge on the entrance side of the transfer nip.
The higher the peak-to-peak voltage of the AC transfer bias, the greater the effect of improving the transfer rate due to the AC transfer bias. Therefore, according to this embodiment, a larger transfer rate can be obtained while simultaneously suppressing image disturbance due to discharge at the entrance of the transfer nip and improving the transfer rate.

[Third aspect]
A third aspect is characterized in that, in the first or second aspect, it has a discrimination means for discriminating the type of the recording sheet, and the transfer bias application means changes the peak-to-peak voltage value of the AC transfer bias to be applied depending on the discrimination result of the discrimination means.
According to this, a larger transfer rate can be obtained for each type of recording sheet within a range in which no discharge occurs at the entrance of the transfer nip.

[Fourth aspect]
A fourth aspect is characterized in that, in any of the first to third aspects, the transfer bias applying means has a detection means for detecting at least one of temperature and humidity, and changes the peak-to-peak voltage value of the AC transfer bias applied in accordance with the detection result of the detection means.
According to this, even if the temperature and humidity fluctuate, a larger transfer rate can be obtained within a range in which no discharge occurs at the entrance of the transfer nip.

[Fifth aspect]
A fifth aspect is the image carrier belt according to any one of the first to fourth aspects, characterized in that the image carrier belt has a multi-layer structure in which an elastic layer is laminated on a base layer.
Since such a multi-layer image carrier belt is prone to generating discharge at the entrance of the transfer nip, it is beneficial to obtain a higher transfer rate within a range where no discharge at the entrance of the transfer nip occurs.

1: image forming unit 2: photoconductor 30: transfer unit 31: intermediate transfer belt 33: secondary transfer back roller 35: primary transfer roller 36: secondary transfer roller 39: power supply 41: secondary transfer unit 60: cassette 90: fixing device 100: printer 101: optical writing unit 300: control unit 404: secondary transfer belt N: secondary transfer nip n1: main nip n2: pre-transfer nip P: recording sheet S: gap a: belt movement direction b: recording sheet transport direction

JP 2016-218152 A

Claims (5)

An image carrier belt;
a nip forming means for sandwiching the image carrier belt between an inner contact member that contacts the inner side of the image carrier belt and an outer contact member that contacts the outer side of the image carrier belt, and forming a transfer nip between the outer surface of the image carrier belt and the outer contact member;
a transfer bias applying unit that applies an AC transfer bias between the inner contact member and the outer contact member,
An image forming apparatus characterized in that the length in the recording sheet transport direction of a pre-transfer nip portion, which is a portion of the image carrier belt that is in contact with the outer abutment member but not in contact with the inner abutment member, is configured to be the shortest length within a range of lengths in which the peak-to-peak voltage of the AC transfer bias can be set so that discharge does not occur on the entrance side of the transfer nip. 2. The image forming apparatus according to claim 1,
The image forming apparatus according to the present invention, wherein the transfer bias applying means applies an AC transfer bias having a maximum peak-to-peak voltage within a range that does not cause discharge on the entrance side of the transfer nip. 3. The image forming apparatus according to claim 1,
A discrimination means for discriminating the type of the recording sheet,
The image forming apparatus according to claim 1, wherein the transfer bias application means changes a peak-to-peak voltage value of the applied AC transfer bias in response to the determination result of the determination means. 3. The image forming apparatus according to claim 1,
A detection means for detecting at least one of temperature and humidity,
The image forming apparatus according to claim 1, wherein the transfer bias application means changes a peak-to-peak voltage value of the applied AC transfer bias in response to a detection result of the detection means. 3. The image forming apparatus according to claim 1,
The image forming apparatus is characterized in that the image carrier belt has a multi-layer structure in which an elastic layer is laminated on a base layer.

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