JP2016109753A - Transfer device and image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress deterioration in image quality due to image voids formed at an image front end part in a low-temperature, low-humidity environment.SOLUTION: A transfer device which comprises an endless intermediate transfer belt 7 stretched rotatably by a plurality of stretch members, a nip formation member such as a secondary transfer roller 15 which abuts on a surface of the intermediate transfer belt to form a nip, and contacting/separating means such as a contact/separating mechanism 20 which performs switching between a separation position where the intermediate transfer belt and the nip formation member separate from each other and a contacting position where the intermediate transfer belt and the nip formation member come into contact with each other to form the nip, and transfers a toner image carried on a surface of the intermediate transfer belt to a sheet held in the nip has: temperature/humidity measurement means such as a temperature/humidity sensor 208 which measures the temperature and humidity in an image formation device comprising the transfer device; and separation quantity varying means 218 of varying a separation quantity at the separation position where the front end of the sheet enters the nip according to measurement results of the temperature/humidity measurement means.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a transfer device and an image forming apparatus including the transfer device.

  Conventionally, when the leading edge of the paper enters the secondary transfer nip, the intermediate transfer belt and the secondary transfer roller, which is a nip forming member, are separated from each other than during the transfer process, thereby reducing the shock jitter when the leading edge of the paper enters. The device is known.

  Patent Document 1 discloses a transfer device that forms a minute gap between an intermediate transfer belt and a secondary transfer roller before the leading edge of a sheet enters the secondary transfer nip by rotation driving of a rotary cam. Yes.

  In the transfer device disclosed in Patent Document 1, the rotation cam position is a contact position where the secondary transfer roller and the intermediate transfer belt are in contact, and a separation position where a minute gap is formed between the secondary transfer roller and the intermediate transfer belt. It has been switched to. Before the leading edge of the sheet enters the secondary transfer nip, the rotational cam position is switched from the contact position to the separation position by the rotational drive of the rotary cam, and a minute gap is formed between the secondary transfer roller and the intermediate transfer belt. Immediately after the leading edge of the sheet enters the secondary transfer nip, switching of the rotational cam position from the separation position to the contact position is started. As a result, it is possible to suppress the occurrence of impact when the paper leading edge enters the secondary transfer nip, and an event that causes horizontal stripe-like density unevenness in the printed image due to the speed difference between the intermediate transfer belt and the photoreceptor caused by the impact. It can be reduced.

In the transfer device disclosed in Patent Document 1, when the rotational cam position is switched from the separation position to the contact position, depending on the image front end position with respect to the paper front end, the image front end moves to the contact position before entering the secondary transfer nip. May not be completed. In this case, at the leading edge of the image that passes through the secondary transfer nip until the switching of the rotating cam position to the contact position is completed, the space between the sheet and the intermediate transfer belt or the secondary transfer roller is within the secondary transfer nip. The sheet cannot be completely adhered, and a minute gap is generated between the sheet and the intermediate transfer belt or the secondary transfer roller.
In a low-temperature and low-humidity environment, the intermediate transfer belt, the secondary transfer roller, or the paper has a relatively high electrical resistance, so that the contact between the paper and the intermediate transfer belt or the secondary transfer roller in the secondary transfer nip. The transfer current is difficult to flow through the portion. For this reason, if the minute gap is locally generated in the secondary transfer nip, a voltage higher than the discharge start voltage is likely to be applied to the minute gap, and discharge is likely to occur in the minute gap. Due to the discharge, the toner at the portion where the discharge has occurred at the leading edge of the image or the toner near the discharge portion is skipped by the impact of the discharge, and there is a problem that local white spots may occur. It was.

  In order to solve the above-described problems, the invention of claim 1 is characterized in that an endless intermediate transfer belt that is rotatably stretched by a plurality of stretching members, and a nip formed by contacting the surface of the intermediate transfer belt. And a contact / separation means for switching between a separation position where the intermediate transfer belt and the nip formation member are separated and a contact position where the intermediate transfer belt and the nip formation member are in contact with each other to form a nip. A transfer device for transferring a toner image carried on the surface of the intermediate transfer belt to a paper sandwiched in a nip, a temperature / humidity measuring means for measuring the temperature and humidity inside the image forming apparatus provided with the transfer device, According to the measurement result of the temperature / humidity measuring means, the apparatus has a separation amount changing means for changing the separation amount at the separation position when the leading edge of the sheet enters the nip.

  According to the present invention, it is possible to obtain an excellent effect that it is possible to suppress deterioration in image quality due to white spots in an image leading edge portion in a low temperature and low humidity environment.

1 is a schematic configuration diagram of a printer according to an embodiment. The figure which represented the shape of the toner typically in order to demonstrate shape factor SF-1. The figure which represented the shape of the toner typically in order to demonstrate shape factor SF-2. Explanatory drawing of the control system of a printer. The figure explaining the apparatus which spaces apart a secondary transfer roller and a secondary transfer opposing roller. The figure which showed the mode of the process linear velocity fluctuation | variation of an intermediate transfer belt. The figure explaining a mode that the eccentric cam is pushing down the secondary transfer roller in the stop position C. FIG. 6 is a diagram illustrating a state in which an eccentric cam stops at a stop position B without pushing down a secondary transfer roller. The figure explaining a mode that the eccentric cam is pushing down the secondary transfer unit in the stop position A. The graph which shows the result of having measured the external shape change amount of the secondary transfer roller in a LL environment, MM environment, and HH environment, and a secondary transfer opposing roller.

Hereinafter, as an image forming apparatus to which the present invention is applied, an embodiment of an electrophotographic printer (hereinafter simply referred to as a printer) will be described. First, a basic configuration of the printer according to the present embodiment will be described.
FIG. 1 is a schematic configuration diagram of the printer. An outline of a four-color full-color image forming apparatus as a printer will be described. The printer 100 is a quadruple tandem type color image forming apparatus in which four image forming units 1Y, 1M, 1C, and 1K are arranged along the traveling direction of the intermediate transfer belt 5. In the image forming unit 1Y, a photoconductor cleaning blade 6Y, a charger 3Y, an exposure unit 4Y, a developing device 5Y, and the like are arranged around a photoconductor 2Y as an image carrier. The image forming units 1C to 1K are also configured similarly to 1Y. In the following description, the subscripts Y, C, M, and K indicate members for yellow, cyan, magenta, and black, respectively. When forming a full-color image, a visible image is formed in the order of the image forming unit 1Y, the image forming unit 1M, the image forming unit 1C, and the image forming unit 1K, and the visible images of the respective colors are sequentially transferred onto the intermediate transfer belt 7. As a result, a full-color image is formed.

  The intermediate transfer belt 7 is stretched by a driving roller 8 and a tension roller 9 and is driven by a driving motor. The process speed is adjusted to 350 [mm / sec]. The intermediate transfer belt 7 includes primary transfer bias rollers 10 </ b> Y to 10 </ b> K that are driven rollers on the inner side, and a belt cleaning counter roller. Each roller is supported by the intermediate transfer belt unit side plate from both sides of the intermediate transfer belt via bearings and arms.

The primary transfer bias roller 10 is disposed at a contact portion between the photoreceptor 2 and the intermediate transfer belt 7, and a predetermined transfer bias is applied to the primary transfer bias roller 10. In this embodiment, +1800 [V] is set to be applied. The intermediate transfer belt 7 is composed of PVDF (vinylidene fluoride), ETFE (ethylene-tetrafluoroethylene copolymer), PI (polyimide), PC (polycarbonate) or the like in a single layer or a plurality of layers, such as carbon black. The conductive material is dispersed, and the volume resistivity is adjusted to be in the range of 10 ⁸ to 10 ¹² [Ω · cm] and the surface resistivity is adjusted to be in the range of 10 ⁹ to 10 ¹³ [Ω · cm]. If necessary, a release layer may be coated on the surface of the intermediate transfer belt 5. Materials used for the coating include ETFE (ethylene-tetrafluoroethylene copolymer), PTFE (polytetrafluoroethylene), PVDF (vinylidene fluoride), PEA (perfluoroalkoxy fluorocarbon resin), FEP (four-fluorocarbon). Fluorine resin such as ethylene fluoride-propylene hexafluoride copolymer) or PVF (vinyl fluoride) can be used, but is not limited thereto.

  The method of manufacturing the intermediate transfer belt 7 includes a casting method and a centrifugal molding method, and the surface thereof may be polished as necessary. If the volume resistivity of the intermediate transfer belt 7 exceeds the above-described range, the bias required for transfer increases, which increases the power supply cost, which is not preferable. Further, since the charging potential of the intermediate transfer belt 5 becomes high and the self-discharge becomes difficult in the transfer process, the transfer paper peeling process, etc., it is necessary to provide a static eliminating means. Furthermore, if the volume resistivity and surface resistivity are below the above ranges, the charge potential decays faster, which is advantageous for static elimination by self-discharge, but toner scatter occurs because the current during transfer flows in the surface direction. End up. Therefore, the volume resistivity and the surface resistivity of the intermediate transfer belt 7 in this embodiment must be within the above ranges.

  The volume resistivity and the surface resistivity were measured by connecting an HRS probe (inner electrode diameter 5.9 [mm], ring electrode inner diameter 11 [mm]) to a high resistivity meter (Mitsubishi Chemical Corporation: Hiresta). Then, a voltage of 100 [V] was applied to the front and back of the intermediate transfer belt 7, and the measured value after 10 seconds was used. A cleaning blade 11 made of urethane rubber is pressed against the intermediate transfer belt 7 to block the toner and clean it.

  In order to facilitate cleaning, the lubricant 12 is applied by the lubricant application member 13. As the lubricant 12, a fatty acid metal salt having a linear hydrocarbon structure is used. The fatty acid metal salt contains at least one fatty acid selected from stearic acid, palmitic acid, myristic acid and oleic acid, and at least one metal selected from zinc, aluminum, calcium, magnesium and lithium The fatty acid metal salt containing is mentioned. Among them, zinc stearate is the most preferable material in terms of cost, quality stability, and reliability because it is produced on an industrial scale and has been used in many fields. However, the higher fatty acid metal salts that are generally used industrially are not the simple substance composition of the name, but include other fatty acid metal salts, metal oxides, and free fatty acids that are more or less similar. Fatty acid metal salts in form are no exception.

  A small amount of these lubricants are supplied in the form of powder. As a specific method, the lubricant applied on the block is scraped off and applied by a lubricant application means 13 such as a brush. And a method of supplying the toner externally. However, when the lubricant is supplied externally to the toner, the supply amount depends on the output image area and cannot always be supplied to the entire belt surface. For this reason, when it is intended to supply the lubricant stably to the entire belt surface with a simple apparatus configuration, a method of scraping and applying the solid lubricant with a brush as in this embodiment is preferable.

  In order to scrape the solid lubricant 12 with the brush of the lubricant application member 13, the force of the solid lubricant 12 is 1 [N] to 4 [N] by the lubricant pressurizing means which is an elastic body such as a spring. Press to the brush. Since the width of the solid lubricant 12 needs to be set wider than the image width, it is set to 304 [mm] or more. The width of the brush needs to be larger than the width of the solid lubricant 12 in order to evenly scrape the solid lubricant 12.

  Further, the intermediate transfer belt 7 is pressurized by the pressure roller 14 in order to suppress vibration of the brush on the intermediate transfer belt 7. In this embodiment, the pressure roller 14 is provided on the outer side of the intermediate transfer belt 7, but the same effect can be obtained on the inner side. Of the primary transfer bias rollers 10K, 10C, 10M, and 10Y, the primary transfer bias rollers 10C, 10M, and 10Y for color can be brought into contact with and separated from the intermediate transfer belt 7 by a contact / separation mechanism as a contact / separation unit similar to the conventional one. ing. Accordingly, the primary transfer bias rollers 10C, 10M, and 10Y can contact the intermediate transfer belt 7 from the state in the monochrome mode in the full color mode as shown in FIG. The intermediate transfer belt 7 can be brought into contact with and wound around the body 2. The intermediate transfer belt 7 is wound around the monochrome primary transfer bias roller 10K regardless of the contact / separation mechanism. The secondary transfer roller 15 and the secondary transfer counter roller 16 as nip forming members are arranged with the intermediate transfer belt 7 interposed therebetween.

The toner used in this embodiment is a polymerized toner generated by a polymerization method. Further, the shape factor SF-1 of the toner used in this embodiment is preferably in the range of 100 to 180, and the shape factor SF-2 is preferably in the range of 100 to 180. 2 and 3 are diagrams schematically showing the shape of the toner in order to explain the shape factor SF-1 and the shape factor SF-2. The shape factor SF-1 indicates the ratio of the roundness of the toner shape and is represented by the following formula (1). This is a value obtained by dividing the square of the maximum length MXLNG of the shape formed by projecting the toner on a two-dimensional plane by the figure area AREA and multiplying by 100π / 4.
SF-1 = {(MXLNG) ² / AREA} × (100π / 4) (1)
When the value of SF-1 is 100, the shape of the toner becomes a true sphere, and becomes larger as the value of SF-1 increases.

The shape factor SF-2 indicates the ratio of the unevenness of the toner shape, and is represented by the following formula (2). A value obtained by dividing the square of the perimeter PERI of the figure formed by projecting the toner on the two-dimensional plane by the figure area AREA and multiplying by 100 / 4π.
SF-2 = {(PERI) ² / AREA} × (100 / 4π) (2)
When the value of SF-2 is 100, there is no unevenness on the toner surface, and as the value of SF-2 increases, the unevenness of the toner surface becomes more prominent.

  Specifically, the shape factor is measured by taking a photograph of the toner with a scanning electron microscope (S-800: manufactured by Hitachi, Ltd.), introducing it into an image analyzer (LUSEX 3: manufactured by Nireco) and analyzing it. Calculated. When the shape of the toner is close to a sphere, the contact state between the toner and the toner or the toner and the photoconductor becomes a point contact, so that the adsorbing force between the toners becomes weak, and thus the fluidity increases, and the toner and the photoconductor The attractive force is also weakened and the transfer rate is increased. If either of the shape factors SF-1 and SF-2 exceeds 180, the transfer rate is lowered and the cleaning property when attached to the transfer means is also lowered, which is not preferable. The toner particle size is preferably in the range of 4 to 10 [μm] in terms of volume average particle size. When the particle size is smaller than this, it becomes a cause of background stains at the time of development, fluidity is deteriorated, and further, aggregation tends to occur, so that voids are likely to occur. On the other hand, when the particle size is larger than this, it is impossible to obtain a high-definition image due to toner scattering or resolution deterioration. In this embodiment, a toner having a volume average particle diameter of 6.5 [μm] is used.

  The main body control unit 200 controls the operation of each unit that needs to be operated and provided in the main body of the printer 100 and the devices included in each unit. The main body control unit 200 will be described with reference to FIG.

  FIG. 4 is an explanatory diagram of the control system of the printer 100. As shown in FIG. 4, the main body control unit 200 includes a central processing unit (CPU) 201, a memory including a ROM 202 and a RAM 203, input / output I / O ports 204 and 205, and the like. The I / O port 204 is connected to the operation unit 206. The I / O port 205 includes a sheet position detection unit 207, a temperature / humidity sensor 208, a photoconductor drive motor 209, a belt drive motor 210, an intermediate transfer contact / separation clutch 211, a primary transfer high-voltage power supply 212, a secondary transfer high-voltage power supply 213, and charging. It is connected to a high voltage power source 214, a development high voltage power source 215, an LED array 216, an image position detector 217, a separation amount changing means 218, and the like.

  The paper position detection unit 207 calculates the position of the paper P from the timing when the registration roller pair starts to rotate. The temperature / humidity sensor 208 measures the temperature and humidity inside the printer 100. Further, the intermediate transfer contact / separation clutch 211 moves the path of the intermediate transfer belt 7 so that the photosensitive drums 2 and the intermediate transfer belt 7 included in the image forming units 1 of other colors are separated during monochrome image formation. Switch.

  Although the speed of the intermediate transfer belt 7 in FIG. 1 is required to be constant, torque is applied to the intermediate transfer belt 7 due to an impact when the paper P as a transfer material enters or leaves the secondary transfer nip. A load may be generated, and a speed difference may occur between the photoreceptor 2 and the intermediate transfer belt 7. This is caused by a difference in speed between the intermediate transfer belt and the photosensitive member caused by an impact when the sheet P enters and leaves the secondary transfer nip. As a result, it is known that horizontal streak-like density unevenness occurs in the printed image and causes printing defects. Such an abnormal image is called a shock jitter.

  In order to reduce this shock jitter, when the paper P enters or exits the secondary transfer nip, the secondary transfer roller and the secondary transfer counter roller of the secondary transfer nip are separated in advance, and the paper The impact when P enters and leaves is suppressed. For example, by using a contact / separation mechanism having a stepping motor or an eccentric cam, which will be described later, when the paper enters or leaves, the eccentric cam is rotated to a predetermined stop position by the stepping motor and the secondary transfer roller is pushed down. As a result, it is separated from the secondary transfer counter roller. As a result, the transfer pressure between the secondary transfer roller and the secondary transfer counter roller is reduced, and the impact when the sheet enters between the secondary transfer nips is reduced. Thereafter, in order to apply a predetermined transfer pressure necessary for transferring the toner image on the intermediate transfer belt onto the sheet, the eccentric cam is rotated to a predetermined stop position by a stepping motor. The toner image is transferred to the sheet by bringing the secondary transfer roller and the secondary transfer counter roller into contact with each other after the leading edge margin of the sheet enters the nip. Thereafter, in order to reduce an impact when the sheet passes through the secondary transfer nip, the secondary transfer roller and the secondary transfer counter roller are separated again while the trailing edge margin portion of the sheet passes through the secondary transfer nip.

Next, a contact / separation mechanism for separating the secondary transfer roller and the secondary transfer counter roller will be described with reference to the drawings.
FIG. 5 is a diagram illustrating a contact / separation mechanism that separates the secondary transfer roller and the secondary transfer counter roller. As shown in FIG. 5, the secondary transfer roller 15 and the secondary transfer counter roller 16 are arranged with the intermediate transfer belt 7 interposed therebetween. A biasing force is applied to the shaft support member 17 of the secondary transfer roller 15 toward the secondary transfer counter roller 16 by a biasing means 18 such as a compression spring or a tension spring. A predetermined nip pressure can be applied to the sheet and the intermediate transfer belt 7 by the urging unit 18. The secondary transfer roller 15 and the secondary transfer counter roller 16 have a structure that can be freely separated within a certain range by a contact / separation mechanism 20 as contact / separation means.

  As shown in FIG. 5, in the contact / separation mechanism 20, eccentric cams 21 are installed on both ends of the secondary transfer counter roller 16 coaxially with the roller. The eccentric cam 21 is configured to abut against ball bearings 22 attached to both ends of the secondary transfer roller 15 so as not to prevent the rotation of the secondary transfer roller 15. When the shaft to which the eccentric cam 21 is attached rotates, grooves that engage with each other are attached so that the eccentric cam 21 also rotates at the same timing and at the same angle. The shape of the eccentric cam 21 is such that the portion B where the distance between the cam shaft 21a and the outer shape portion 21b of the eccentric cam 21 is the shortest is shorter than the diameter of the secondary transfer opposing roller 16, and the rotational center and outer shape of the eccentric cam 19 are. The part with the longest distance connecting the parts is shaped to be longer than the diameter of the secondary transfer counter roller 16. The shaft to which the eccentric cam 19 is attached is configured such that the rotation can be freely controlled by a stepping motor. In FIG. 5, the rotation of the stepping motor 26 is transmitted to the shaft attached to the eccentric cam 21 through the gears 23 and 24 and the timing belt 25. The stepping motor 26 can control rotation at a step angle of 1.8 [°].

  The eccentric cam 21 is rotated by the stepping motor 26 before the sheet P enters the nip portion of the secondary transfer roller 15. The eccentric cam 21 is abutted against the ball bearing 22. By rotating the eccentric cam 21, (the distance from the rotation center of the eccentric cam 21 to the contact portion of the ball bearing 22 of the eccentric cam 21 + the radius of the ball bearing)> (radius of the secondary transfer counter roller 16 + secondary transfer) The secondary transfer roller 15 urged by the urging means 18 is pushed down in a direction away from the secondary transfer counter roller 16. When the leading edge of the paper P starts to enter the gap between the secondary transfer roller 15 and the secondary transfer counter roller 16, the stepping motor 26 starts the rotation of the eccentric cam 21 again.

  Thereafter, (the distance from the rotation center of the eccentric cam 21 to the contact portion of the ball bearing 22 of the eccentric cam 21 + the radius of the ball bearing) <(the radius of the secondary transfer counter roller 16 + the radius of the secondary transfer roller 15). Then, the secondary transfer roller 15 and the secondary transfer counter roller 16 come into contact with each other and start to apply a predetermined transfer pressure to the paper P. By adopting such a configuration, the secondary transfer roller 15 and the secondary transfer counter roller 16 are separated from each other when the paper P enters, thereby suppressing the impact when the paper P enters and the process linear velocity fluctuation of the intermediate transfer belt. Can do. Similarly, when the paper P is detached, the eccentric cam 21 is rotated by the stepping motor 26, and the secondary transfer roller 15 and the secondary transfer counter roller are rotated before the trailing edge margin of the paper P passes through the secondary transfer nip. By separating 16, the impact when the paper P is detached can be reduced.

  FIG. 6 is a diagram showing the process linear velocity fluctuation of the intermediate transfer belt. The intermediate transfer belt is usually driven at the same process linear velocity within a certain range with the set process linear velocity as the center. However, when the sheet enters or the secondary transfer roller and the secondary transfer counter roller come into contact with each other, the process linear velocity is reduced as indicated by the valley of the graph of FIG. 6 due to the impact and torque load. After that, when the transfer material is detached, the torque load factor due to the paper is removed, so that acceleration starts as shown in the peak portion of the graph of FIG. If the impact due to the entry of the sheet into the secondary transfer nip and the contact between the secondary transfer roller and the secondary transfer counter roller is large, the degree of deceleration of the intermediate transfer belt increases accordingly. For this reason, the drop of the valley part of the graph of FIG. 6 becomes large. Then, it appears as a horizontal streak abnormal image in the printed image.

FIG. 7 is a diagram illustrating how the eccentric cam pushes down the secondary transfer roller at the stop position C, and FIG. 8 is a diagram illustrating how the eccentric cam stops at the stop position B without pushing down the secondary transfer roller. FIG. 9 and FIG. 9 are diagrams for explaining how the eccentric cam pushes down the secondary transfer unit at the stop position A. As shown in FIGS. 7 to 9, the eccentric cam 21 has a length R from the cam shaft 21 a to the outer shape portion 21 b as R _A , R _B , and R _C at the stop positions A, B, and C, respectively. a eccentric cam having such a relation such that _{a> R} C> _{R B.}

  The stop position A is the stop position where the distance from the cam shaft 21a to the outer portion 21b is the longest in the eccentric cam 21, and the gap amount between the intermediate transfer belt 7 and the secondary transfer roller 15 is separated by 0.51 [mm]. be able to. 0.51 [mm] is an example, and is a gap amount determined by the maximum paper thickness of the image forming apparatus in which this contact / separation mechanism is mounted, that is, a gap amount larger than the maximum paper thickness.

  The stop position C is a stop position that is a distance from the second cam shaft 21a to the outer portion 21b in the eccentric cam 21, and the gap amount between the intermediate transfer belt 7 and the secondary transfer roller 15 is set to 0 to 0.1 [. mm] is formed. This stop position can be managed with a minute gap that does not cause discharge by separating the intermediate transfer belt 7 and the secondary transfer roller 15 by a minute gap. As a result, it is possible to prevent discharge in the transfer nip while reducing the impact when the paper P enters the transfer nip.

  The stop position B is a stop position where the distance from the cam shaft 21a to the outer portion 21b is the shortest stop position in the eccentric cam 21, and the intermediate transfer belt 7 and the secondary transfer roller 15 are completely in contact with each other. Do not push down at all. This stop position is a stop position where the radius of the cam is the shortest and the secondary transfer unit is not pushed down at all. By setting this stop position B, a predetermined transfer pressure can be reliably applied, and a pressure sufficient for transfer can be applied during the secondary transfer.

Next, an embodiment of a transfer device which is a characteristic part of the present invention will be described.
Depending on the position of the leading edge of the image relative to the leading edge of the paper, the movement to the contact position may not be completed before the leading edge of the image enters the secondary transfer nip. In this case, the gap between the intermediate transfer belt and the paper, or the gap between the paper and the secondary transfer roller is present at the leading edge of the image that passes through the secondary transfer nip before the movement to the contact position is completed. The transfer process is performed in a state where it occurs or a state where the transfer nip pressure is insufficient. At the leading edge of the image, there is a minute gap in the secondary transfer nip or the transfer nip pressure is insufficient. Therefore, the sheet and the intermediate transfer belt or the secondary transfer roller cannot be completely brought into close contact within the secondary transfer nip. As a result, a minute gap is generated between the secondary transfer belt and the paper or between the paper and the secondary transfer roller.

  In a normal temperature and humidity environment, the electrical resistance value of the intermediate transfer belt, the secondary transfer roller, or the paper is relatively low. Therefore, in the secondary transfer nip, the transfer current tends to flow through the contact portion of the intermediate transfer belt, the secondary transfer roller, and the paper. As a result, even if a minute gap is locally generated in the secondary transfer nip, a voltage higher than the discharge start voltage is not easily applied to the gap, and discharge in the secondary transfer nip is less likely to occur.

  On the other hand, in the low temperature and low humidity environment, the electrical resistance value of the intermediate transfer belt, the secondary transfer roller, or the paper is relatively high. Therefore, in the secondary transfer nip, the transfer current hardly flows through the contact portion of the intermediate transfer belt, the secondary transfer roller, and the paper. As a result, if a minute gap is locally generated in the secondary transfer nip, a voltage higher than the discharge start voltage is likely to be applied to the gap, and discharge is likely to occur in the gap. When a discharge occurs in the gap, the toner at the location where the discharge has occurred or the toner near the discharge location is blown away by the impact of the discharge, and an image white spot occurs at that location.

  On the other hand, it is known that the transfer efficiency is lowered due to a small transfer nip pressure applied to the image front end portion. In order to suppress the transfer efficiency, for example, the image front end portion is transferred more than the remaining image portions. There is a transfer bias technique that increases current. When such a technique is applied, discharge is likely to occur in a low-temperature and low-humidity environment, particularly because a larger transfer bias acts on the leading edge of the image.

  Therefore, the transfer apparatus according to the present embodiment performs the following control. During normal printing, before the paper enters the secondary transfer nip, the paper P is placed in the secondary transfer nip by waiting at a stop position A in FIG. 9 with a gap larger than the paper thickness. The impact at the time of entry is sufficiently mitigated. Thereafter, the eccentric cam 21 is moved to the stop position B in FIG. 8 to release the pressing of the secondary transfer roller 15 and image printing is performed in a state where the transfer pressure is sufficiently applied. The movement of the eccentric cam 21 starts from the time when the sheet P enters the secondary transfer nip to the image forming unit, but the movement of the eccentric cam 21 is not in time at the leading edge of the image, and the movement is very small. In some cases, it is necessary to start image formation with a gap or insufficient transfer pressure. This lack of transfer pressure is compensated for by increasing the transfer current only at the leading edge of the paper. In a low-temperature and low-humidity environment (eg, 10 [° C.] 15 [%]) where the electrical resistance value of the secondary transfer roller or the paper is high, the electrical resistance value of the paper P is high, or solids of two or more colors overlap. Printing is performed under conditions such as an image pattern with a large amount of toner adhesion (the electric resistance value of the toner is large). Then, for the reasons described above, a discharge occurs at the front end of the sheet in the secondary transfer nip, and the unfixed toner image near the occurrence location or in the vicinity thereof may be blown off, resulting in a locally blank image.

  Since the toner is blown off in a circular shape around the discharge portion in such a portion where the discharge has occurred, white spots with a diameter of about 0 to 2 [mm] may occur in a large amount at the front end of the paper. From time to time, there was a complaint as a blank image. As a transfer device having a conventional configuration, by setting the gap amount so that discharge does not occur even in a low-temperature and low-humidity environment, the gap amount (so that a blank image is not generated no matter what environment the user prints) In general, the distance is set to a distance. The eccentric cam 21 is moved to the stop position C in FIG. 7, and the gap amount between the intermediate transfer belt 7 and the secondary transfer roller 15 is set to a minute gap of about 0 to 0.1 [mm]. Then, at the moment when the paper P enters the secondary transfer nip, the intermediate transfer belt 7, the paper P, and the secondary transfer roller 15 are completely in contact with each other depending on the thickness of the paper P. Become. Since the gap is secured between the intermediate transfer belt 7 and the secondary transfer roller 15 by a minute gap before the sheet P enters the secondary transfer nip, the pressure is reduced more than usual. . As a result, the impact at the moment when the paper P enters the secondary transfer nip can be reduced more than usual.

  However, if the gap amount is set such that no discharge occurs in a low-temperature and low-humidity environment, the impact at the time of paper entry, which is a trade-off, cannot be sufficiently mitigated. For this reason, there is a problem that the shock jitter cannot be sufficiently mitigated in an environment of normal temperature and normal humidity (23 [° C.] 50 [%]) or higher where no discharge occurs. Therefore, in the present invention, as shown in FIG. 4, by providing a temperature / humidity sensor 208 in the image forming apparatus, the temperature / humidity inside the apparatus is constantly monitored and the temperature / humidity inside the apparatus falls below a preset threshold value. In this case, it is determined that the environment is low temperature and low humidity, and the cam position of the eccentric cam is switched from the stop position C to the stop position A.

  Specifically, as described above, even if a sufficient gap is secured in a room temperature and humidity environment, the electrical resistance values of the secondary transfer roller, paper, etc. are low. Therefore, before the sheet P enters the secondary transfer nip, the gap between the intermediate transfer belt 7 and the secondary transfer roller 15 is opened by +0.51 [mm] at the stop position A in FIG. Thereafter, at the moment when the sheet P enters the secondary transfer nip, the cam position of the eccentric cam 21 is moved to the stop position C in FIG. 7, and the gap between the intermediate transfer belt 7 and the secondary transfer roller 15 is +0.1. The gap amount is changed to [mm]. As a result, even if a sufficient gap is ensured by largely pressing down the secondary transfer roller 15, no discharge occurs in the nip. For this reason, the gap amount is made larger than the sheet thickness so that the impact when the sheet enters the secondary transfer nip is minimized.

  On the other hand, in a low-temperature and low-humidity environment where the electrical resistance value of the secondary transfer roller, paper, etc. is high, as described above, if a large gap is secured between the intermediate transfer belt and the secondary transfer roller, a white-out image due to discharge occurs. Resulting in. Therefore, before the sheet P enters the secondary transfer nip, the gap between the intermediate transfer belt 7 and the secondary transfer roller 15 is opened by +0.11 [mm] at the stop position B in FIG. Thereafter, the eccentric cam 21 is moved to the stop position B in FIG. 8 at the moment when the paper P enters the secondary transfer nip, and the gap between the intermediate transfer belt 7 and the secondary transfer roller 15 is −0.18 [mm]. ] Change the gap amount to Thereby, in a low-temperature and low-humidity environment, white spot images are more likely to be generated due to discharge in the nip than in a normal-temperature and normal-humidity environment. Therefore, the gap at the moment when the paper P enters the secondary transfer nip is reduced by about −0.28 [mm] so that no white spot image is generated. As described above, in the case of a low temperature and low humidity ring, by reducing the gap distance, discharge can be reliably prevented even in a low temperature and low humidity environment.

  Depending on the environmental conditions, the diameter of the secondary transfer roller or the secondary transfer counter roller may change. In that case, the gap between the intermediate transfer belt 7 and the secondary transfer roller 15 is adjusted in consideration of the amount of change in the roller diameter according to environmental conditions. Specifically, FIG. 10 shows a low temperature and low humidity environment (LL environment: 10 [° C.] 15 [%]), a normal temperature and normal humidity environment (MM environment: 23 [° C.] 50 [%]), and a high temperature and high humidity environment (HH). (Environment: 28 [° C.] 85 [%]) shows the result of measuring the external change amount of the secondary transfer roller and the secondary transfer counter roller. As shown in FIG. 10, the −0.11 [mm] diameter is smaller in the LL environmental condition than in the MM environmental condition. When converted to the gap amount between the intermediate transfer belt and the secondary transfer roller, −0.055 [mm] is not obtained. Therefore, in the LL environmental condition, in addition to the possible gap amount that increases the electric resistance value and prevents discharge in the nip, the amount of −0.055 [mm] is converted and the gap amount is adjusted. More desirable results.

  An abnormal image due to shock jitter is an abnormal image that appears as a faint horizontal black streak when the dot pitch is disturbed when a halftone image is printed. For this reason, solid images or characters or photographic images may not appear as abnormal images. On the other hand, the abnormal image due to the discharge generates a local white-out image by blowing off the toner in the discharge portion. Therefore, if there is a print image in the discharge corresponding portion, the abnormal image is surely an abnormal image. Therefore, it is necessary to avoid an abnormal image due to discharge, and in a situation where discharge occurs, priority should be given to reduction of shock jitter.

  In a low temperature and low humidity environment, the diameter of the secondary transfer roller is also small, so the diameter of the secondary transfer roller is smaller than usual. For this reason, the gap amount between the intermediate transfer belt 7 and the secondary transfer roller 15 tends to be larger than that in the normal temperature and normal humidity environment. Since the shock jitter tends to be improved as compared with the normal temperature and humidity environment, the deterioration of the shock jitter tends to be small even if the amount of pressing down of the secondary transfer roller is smaller than usual in order to prevent discharge.

  As described above, in an image forming apparatus having a mechanism capable of providing a gap between the intermediate transfer belt and the secondary transfer roller for the purpose of reducing the impact when the paper enters the secondary transfer nip, While preventing discharge, the impact at the moment when the paper enters the secondary transfer nip can be sufficiently reduced even in an environment other than low temperature and low humidity, and abnormal images called shock jitter can be prevented.

What has been described above is merely an example, and the present invention has a specific effect for each of the following modes.
(Aspect A)
An endless intermediate transfer belt 7 that is rotatably supported by a plurality of tension members, a nip forming member such as a secondary transfer roller 15 that abuts against the surface of the intermediate transfer belt to form a nip, and an intermediate transfer belt And a contact / separation means such as a contact / separation mechanism 20 for switching between a separation position where the nip forming member is separated and a contact position where the intermediate transfer belt and the nip forming member are in contact with each other to form a nip. And a temperature / humidity measuring means such as a temperature / humidity sensor 208 for measuring the temperature and humidity inside the image forming apparatus provided with the transfer device in the transfer device for transferring the toner image carried on the sheet to the paper sandwiched in the nip. And a separation amount changing means 218 for changing the separation amount at the separation position when the leading edge of the paper enters the nip according to the measurement result.
According to this aspect, in the low-temperature and low-humidity environment where the electrical resistance value of the intermediate transfer belt, the nip forming member, or the paper is increased, the separation amount at the separation position when the paper leading edge enters the nip can be made relatively small. It becomes. As described above, the distance from the separation position to the contact position can be narrowed by making the separation amount at the separation position relatively small. Thereby, in the low temperature and low humidity environment, the operation to the contact position can be completed at an earlier time after the leading edge of the sheet enters the transfer nip. As a result, it is possible to prevent the situation where the image leading edge enters the transfer nip before the operation up to the contact position is completed, or to reduce the number of image leading edge portions that enter the transfer nip before the operation up to the contact position is completed. it can. As a result, it is possible to suppress deterioration in image quality due to white spots in the image that has conventionally occurred at the leading edge of the image in a low temperature and low humidity environment.
In a low-temperature and low-humidity environment, since the separation amount is relatively small, the effect of suppressing shock jitter is weakened. However, the occurrence of image blanking has a greater influence on image quality than shock jitter.

(Aspect B)
In (Aspect A), the separation amount changing unit determines that the inside of the image forming apparatus is in a low temperature and low humidity environment when the measurement result of the temperature / humidity measurement unit is equal to or lower than a predetermined threshold, and the separation amount changing unit. To reduce the distance.
According to this, as described in the above embodiment, by reducing the gap separation amount in the case of a low-temperature and low-humidity ring, it is possible to suppress the occurrence of discharge that is likely to occur in a low-temperature and low-humidity environment.

(Aspect C)
In (Aspect A) or (Aspect B), the separation amount changing unit changes the separation amount set in advance for the low-temperature and low-humidity environment to a separation amount that is smaller than the thickness of the thinnest paper among the sheets used during image formation. According to this, as described in the above embodiment, by reducing the separation amount compared to the paper thickness of the thinnest paper among the papers used at the time of image formation, the paper thickness used at the time of image formation at the low temperature and low humidity ring. When printing on any of these sheets, the intermediate transfer belt, the sheet, and the nip forming member can be brought into contact with each other. Thereby, generation | occurrence | production of the discharge which is easy to occur in a low-temperature low-humidity environment can be suppressed.

(Aspect D)
In an image forming apparatus including a toner image forming unit that forms a toner image on an image carrier and a transfer unit that transfers a toner image on the image carrier to a sheet, the transfer unit includes (Aspects A) to (A) Any transfer device of embodiment C) is used.
According to this, as described in the above embodiment, it is possible to sufficiently reduce the impact at the moment when the paper enters the secondary transfer nip in an environment other than the low temperature and low humidity while preventing the discharge in the low temperature and low humidity environment. The occurrence of abnormal images called shock jitter can be prevented. Therefore, an image can be formed satisfactorily.

DESCRIPTION OF SYMBOLS 1 Image forming unit 2 Photoconductor 3 Charger 4 Exposure means 5 Developer 6 Photoconductor cleaning blade 7 Intermediate transfer belt 8 Driving roller 9 Tension roller 10 Primary transfer bias roller 11 Cleaning blade 12 Lubricant 13 Lubricant application member 14 Pressurization Roller 15 Secondary transfer roller 16 Secondary transfer counter roller 17 Shaft support member 18 Biasing means 20 Contact / separation mechanism 21 Eccentric cam 21a Cam shaft 21b Outer portion 22 Ball bearing 23 Gear 24 Gear 25 Timing belt 26 Stepping motor 100 Printer 208 Temperature Humidity sensor 218 Separation amount changing means

Japanese Patent No. 5509939

Claims (4)

An endless intermediate transfer belt that is rotatably supported by a plurality of tension members, a nip forming member that abuts against the surface of the intermediate transfer belt to form a nip, the intermediate transfer belt, and the nip forming member A separation position where the intermediate transfer belt and the nip forming member come into contact with each other to switch to a contact position where a nip is formed, and a toner image carried on the surface of the intermediate transfer belt is transferred into the nip. In the transfer device that transfers to the paper sandwiched between
Temperature and humidity measuring means for measuring the temperature and humidity inside the image forming apparatus provided with the transfer device;
And a separation amount changing unit configured to change a separation amount at the separation position when the leading end of the sheet enters the nip according to a measurement result of the temperature and humidity measurement unit. The transfer apparatus according to claim 1, wherein
The separation amount changing unit determines that the inside of the image forming apparatus is in a low-temperature and low-humidity environment when the measurement result of the temperature / humidity measurement unit is equal to or lower than a predetermined threshold, and the separation amount changing unit determines that the separation amount changing unit A transfer device characterized in that the amount is reduced. In the transfer device according to claim 1 or 2,
The separation amount changing unit changes the separation amount set in advance for a low-temperature and low-humidity environment to the separation amount smaller than the thickness of the thinnest paper among the papers used during image formation. An image forming apparatus comprising: a toner image forming unit that forms a toner image on an image carrier; and a transfer unit that transfers the toner image on the image carrier to a sheet.
An image forming apparatus using the transfer device according to claim 1 as the transfer unit.