DE69813584T2 - Intermediate transfer unit - Google Patents

Description

The present invention relates to an intermediate transfer unit used in an image forming apparatus employing an electrophotographic process such as a copying machine, a printer and a facsimile machine. The present invention also relates to a recording medium conveying system applied to the image forming apparatus.

For a copying machine, a printer, a facsimile machine and other image forming apparatuses each employing electrophotography, particularly an image forming apparatus using a laser beam writing apparatus, a function of transferring and fixing a toner image while conveying a recording medium at high speed is required in order to make good use of the function of the writing apparatus, and also an operability enabling easy action against paper jam and the like caused by the provision of such a function is necessary.

In general, an image forming apparatus, such as known from US-A-5,351,114, US-A-5,040,028, US-A-5,075,731, using an electrophotographic technology is provided with: a photoconductive drum formed with a photosensitive layer as the peripheral surface, charging means for uniformly charging the peripheral surface of the photoconductive drum, exposing means for selectively exposing the peripheral surface uniformly charged by the charging means to form an electrostatic latent image, developing means for applying toner as a developer for the electrostatic latent image formed by the exposure means to form a visible image (a toner image), and transfer means for transferring the toner image developed by the developing means to a transfer medium such as paper.

As a transfer medium for transferring a toner image developed on a photoconductive drum to a transfer medium such as paper, transfer means are known which are formed with an intermediate transfer belt onto which a toner image formed on a photoconductive drum is transferred (primary transfer) and which further transfer the toner image to a recording medium (secondary transfer), and are formed with a drive roller for rotationally driving the intermediate transfer belt.

With respect to the conventional transmission means, there is a problem that since the distance between a primary transmission position and the drive roller is large, the shrinkage amount of the intermediate transmission belt between them is increased, the drive speed of the intermediate transmission belt in the primary transmission position is unstable, as a result of which it is difficult to achieve a satisfactory primary transmission.

According to the above conventional transfer means, there is still a problem that a transfer roller is directly connected to the seam of the intermediate transfer belt, that a secondary transfer roller is soiled by toner which accumulates in a stage of the seam of the intermediate transfer belt, and that toner adheres to the back surface of a recording medium at the next secondary transfer.

According to the above conventional transfer means, there is still a problem that when an image having a thin line is transferred to a recording medium having a smooth surface, failure of transfer of the toner (dropout) occurs.

According to the above conventional transfer means, there is further a problem that even if transfer to a recording medium having a smooth surface is satisfactory, transfer to a recording medium having a rough surface is insufficient and, particularly, when multiple toner layers are transferred as a multi-color image, failure of transfer from toner of a layer far from the surface of a recording medium occurs.

According to the above conventional transfer means, there is still a problem that deterioration of transfer efficiency and omission (dropout) of a part of a toner image in transfer occur in primary or secondary transfer. In addition, in secondary transfer, there is a problem that it is difficult to transfer to a recording medium having an extremely irregular surface such as recycled paper and banknote paper without omission of a part of an image. There is a problem such that, particularly, when toner having high fluidity is used, the toner is easily smeared when transferred, particularly, when primary or secondary transfer means serving as a transfer electrode for applying a transfer voltage to a transfer position is arranged at a position remote from the transfer position, an electric field for transfer in the transfer position cannot be concentrated at the transfer position, whereby a toner image is smeared due to electrostatic force, and when, for example, the intermediate transfer belt is attached to the photoconductive drum without means for firmly pressing the intermediate transfer belt to the photoconductive drum is wound, or a recording medium in a transfer position area in which the photoconductive drum and the intermediate transfer belt are in contact in a transfer position is large, and the swirling of a toner image due to mechanical force caused by slight differences in speed between both and others easily occurs.

Furthermore, according to the above conventional transfer means, a single or multi-layer belt in which a conductive, a semi-conductive or an insulating resin layer is generally formed at least as the surface layer is used for the intermediate transfer belt. Therefore, there is a problem such that since the surface is made of resin as described above, friction and scratching are easily generated. In particular, a large amount of metal oxide particles generally adhere to the surface of a toner particle as an additive, and there is a problem such that since the above additive is much harder than synthetic resin forming the surface of the intermediate transfer belt, it is easily embedded in the intermediate transfer belt, and further, a phenomenon (so-called filming) occurs in which toner adheres to the intermediate transfer belt at the above embedded locations, whereby deterioration of an image, for example, deterioration of transfer efficiency in primary or secondary transfer and absence of a part of a toner image in transfer (dropout) occurs. In addition, in secondary transfer, there is a problem such that it is difficult to transfer to a recording medium having an extremely irregular surface, such as recycled paper and banknote paper, without causing a defect in the image.

According to the above conventional transmission means, there is still a problem such that a phenomenon occurs, namely, a part of a toner image transferred onto the intermediate transfer belt in primary transfer, particularly the center, is missing, i.e., a so-called dropout. In addition, there is a problem in secondary transfer in that it is difficult to transfer even onto a recording medium having an extremely irregular surface, such as recycled paper and banknote paper, without causing a defective image in addition to the above problem of dropout. Furthermore, in an image forming apparatus for forming a full-color image by overlapping multiple colors, for example, secondary transfer means are prevented from being soiled by controlling the drive of the secondary transfer means to perform secondary transfer so that the means are not in contact with the intermediate transfer belt while images of each color are being formed and so that the means contact the intermediate transfer belt after the final image is formed, and that when secondary transfer is started before primary transfer is completed, an image on the intermediate transfer belt is prevented from being smeared. However, there is a problem such that the intermediate transfer belt is vibrated, the speed is varied, and blurring of an image occurs when the state of the secondary transfer means is switched from a contact to a non-contact state with the intermediate transfer belt.

Furthermore, according to the above transfer means, the transferability is insufficient in a primary transfer part. Specifically, there are problems in the toner amount (the thickness of the layer), resistance distribution and resistance change between each member, the change in transfer efficiency due to the change in resistance, a dropout phenomenon, and the stability of density due to aging.

According to the above conventional transfer means, furthermore, transferability is insufficient in a secondary transfer part. Specifically, there are problems in the amount of toner (the thickness of the layer), the type of a recording medium such as plain paper, a postcard, and overhead transparencies, resistance distribution and resistance change between each member, the change in transfer efficiency due to the change in resistance, a dropout phenomenon, and the stability of density due to aging.

In the above conventional transmission means, furthermore, in view of the resistance which is the important characteristic of a primary transmission element and a secondary transmission element, elements having approximately the same change in resistance due to the environment are used for both the primary and secondary transmission elements.

Therefore, if a member having a small change in resistance due to the environment is used for both, a current may be lost in a part not related to the transfer, and the failure of the transfer may occur in a case where a recording medium such as a postcard and an envelope, which are smaller than the width of the secondary transfer member, are printed in an environment of low temperature and humidity, where the resistance of the recording medium is higher than that of the secondary transfer member in a secondary transfer part. To avoid the above situation, it is conceivable to increase the resistance of the secondary transfer member and thus reduce a leakage current. However, since a member having a small change in resistance due to the environment generally has a large resistance distribution, there is a problem that the non-uniformity of the transfer partially occurs.

If a member with large resistance change due to the environment is used for both, failure due to leakage does not occur in the secondary transmission because the resistance of the secondary transmission member changes approximately as the resistance of a recording medium changes due to the environment. However, voltage required in a primary transmission member in the low temperature and low humidity environment causes an increase in cost.

Furthermore, in conventional transfer means as disclosed in Japanese Patent Application No. Hei. 7-322667, a defective image is prevented from occurring by simultaneously matching the primary transfer and the secondary transfer by providing a conductive layer for the intermediate transfer belt and setting a ratio between a resistance RT of a part of a power source applying a primary transfer bias to the conductive layer and an apparent resistance R1 in a primary transfer part to be RT < R1.

According to the above conventional transfer means, there is a case that it is insufficient to prevent a defective image depending on the environment and the type of paper while matching primary and secondary transfer. Specifically, the phenomenon is noticeable when a current flowing in secondary transfer is larger than a current flowing in primary transfer.

The present invention has been made to solve the above problems, and an object thereof is to provide an intermediate transfer unit capable of easily dealing with various difficulties caused by high-speed transport of the recording paper.

Another object of the invention is to provide an intermediate transfer unit by which the speed of an intermediate transfer belt can be stabilized in a primary transfer position.

Another object of the invention is to provide an intermediate transfer unit by which the back surface of a recording medium is not soiled by using an intermediate transfer belt with a seam.

Another object of the invention is to provide an intermediate transfer unit for enabling satisfactory transfer to a recording medium the surface of which is smooth as in overhead projector (OHP) transparencies. The object is also to provide an intermediate transfer unit which enables satisfactory transfer to a recording medium the surface of which is smooth in a general area in the direction of the shaft of a transfer roller. The object is further to provide an intermediate transfer unit which enables satisfactory transfer to a recording medium the surface of which is smooth and maintains high quality of the image for a long period of time and also enables satisfactory transfer to a recording medium the surface of which is rough. The object is further to provide an intermediate transfer unit for enabling formation of an image which is uniform in color in each density area on a recording medium the surface of which is smooth.

Still another object of the invention is to provide an intermediate transfer unit for enabling satisfactory transfer to a recording medium having a rough surface such as banknote paper. The object is also to provide a compact and to provide an inexpensive intermediate transfer unit for enabling satisfactory transfer to a recording medium having a rough surface and at the same time enabling reduction of a torque for driving a transfer roller. The object is further to provide an intermediate transfer unit for enabling satisfactory transfer to a recording medium having a rough surface and at the same time maintaining high image quality for a long period of time. The object is further to provide an intermediate transfer unit for enabling formation of an image approximately uniform in color in each density area on a recording medium having a rough surface.

Still another object of the invention is to provide an intermediate transfer unit for forming a satisfactory image without missing a part of an image such as a dropout in transfer.

Yet another object of the invention is to provide an intermediate transfer unit to enable the stabilization of the transferability (transfer efficiency) in a primary transfer part.

According to a further object of the invention, it is to provide an intermediate transfer unit to enable the stabilization of the transferability (transfer efficiency) in the secondary transfer part.

Yet another object of the invention is to provide an intermediate transmission unit to enable the stabilization of the transmittability (transmission efficiency) in the secondary transmission part and the reduction of the capacity of the high voltage power source.

Still another object of the invention is to provide an intermediate transfer unit which can prevent the deterioration of an image during simultaneous transmission of the primary transfer and secondary transfer.

To achieve these objects, the present invention provides an intermediate transfer unit as defined in claim 1. Preferred embodiments of the invention are described in the dependent claims.

According to a first aspect, in a recording medium carrying system, a paper feeding mechanism for carrying a recording medium to a transfer part, a mechanism for transferring a toner image to a recording medium, a mechanism for fixing the transferred toner image to the recording medium, and a mechanism for ejecting the recording medium from a fixing part are each formed as an independent unit.

According to a second aspect of the invention, there is provided an intermediate transfer unit having an intermediate transfer belt onto which a toner image formed on a photoconductive drum is first transferred, and which then further transfers this toner image onto a recording medium, and having a drive roller for rotationally driving the intermediate transfer belt, characterized in that the above primary transfer position is located near the drive roller.

According to the intermediate transfer unit of the second aspect, since its primary transfer position is located near the drive roller, the shrinkage of the intermediate transfer belt between the primary transfer position and the drive roller is reduced, the speed of the intermediate transfer belt in the primary transfer position is stable, and as a result, the primary Transmission in a satisfactory condition is easily achieved.

According to a third aspect of the invention, there is provided an intermediate transfer unit having an intermediate transfer belt with a seam, onto which intermediate transfer belt a toner image formed on a photoconductive drum is first transferred by a primary transfer member and which further then transfers the toner image to a recording medium using a secondary transfer roller and having a drive roller for rotationally driving the intermediate transfer belt, and is characterized in that an electric field is formed in a direction in which the above toner is returned from the secondary transfer roller to the intermediate transfer belt while the secondary transfer roller is pressed onto the intermediate transfer belt when no image is formed, and the secondary transfer roller is released when the seam of the intermediate transfer belt is opposed to the secondary transfer roller.

According to the intermediate transfer unit of the third aspect, it is possible to avoid the phenomenon that toner adheres to the secondary transfer roller by direct contact with the joint of the intermediate transfer medium, so that the back side of a recording medium can be prevented from being soiled and the intermediate transfer unit can easily achieve a satisfactory transfer.

According to a fourth aspect of the invention, there is provided an intermediate transfer unit having an intermediate transfer belt on which a toner image is formed on a photoconductive drum and is primarily transferred by a primary transfer member and which then further transfers this toner image to a recording medium using a secondary transfer roller and a drive roller for rotationally driving the intermediate transfer belt, and characterized in that the above intermediate transfer belt comprises dispersed fluorine particles at least in the surface layer and the above secondary transfer roller is pressed onto the intermediate transfer belt under the linear pressure of 27 gf/mm or less.

In addition, in the above intermediate transfer unit, the hardness of the above secondary transfer roller is set to 70º or less in case the hardness is measured with an Asker-C hardness meter.

Furthermore, in the above intermediate transfer unit, plural kinds of additives which are different in particle diameter are added to the above toner, and the surface coverage of them is two or more.

Furthermore, in the above intermediate transfer unit, the above toner image transferred to the above intermediate transfer belt is 1.5 mg/cm² or less per unit area in each density area.

According to the intermediate transfer unit of the fourth aspect of the invention, since the intermediate transfer belt has an excellent print-releasing property, toner is easily peeled off in secondary transfer, and when an image having a thin line is transferred to a recording medium whose surface is smooth, satisfactory transfer is enabled even if the pressure applied to the toner is not fixed. Furthermore, since the hardness of the secondary transfer roller is set to 70° or less in the case where the hardness is measured with an Asker-C hardness meter, the concentration of the transfer pressure of a linear image on the intermediate transfer belt is avoided and the occurrence of dropout can be reduced.

In addition, according to the intermediate transfer unit, since pressure applied to toner is uniform, when an image having a thin line is transferred to a recording medium having a smooth surface, satisfactory transfer is enabled.

In addition, according to the intermediate transfer unit, since an additive having a relatively large particle diameter is added, the additive is not embedded in a mother particle for a long period of time, but fluidity is maintained and image quality is stable, and since an additive having a relatively small particle diameter is added, the surface coverage is large compared with the added weight, and even if pressure applied to the toner is not fixed, when an image having a thin line is transferred to a recording medium having a smooth surface, satisfactory transfer is enabled.

Furthermore, according to the intermediate transfer unit, since the height of a toner layer is limited and pressure applied to the toner is made uniform when an image having a thin line is transferred to a recording medium having a smooth surface, satisfactory transfer is enabled by forming a toner layer in each density area under the condition that the amount of toner to be transferred is secondarily 1.5 mg/cm².

According to a fifth aspect of the invention, there is provided an intermediate transfer unit comprising an intermediate transfer belt onto which a toner image formed on a photoconductive drum is first transferred by a primary transfer member and which further secondarily transfers the toner image onto the recording medium using a secondary transfer roller and comprising a drive roller for circulating of the intermediate transfer belt, and characterized in that the above toner is coated with an additive at the surface coverage of 2 or more and the above secondary drive roller is pressed onto the intermediate transfer belt with the linear pressure of 15 gf/mm or more.

In addition, in the above intermediate transfer unit, the hardness of the above secondary transfer roller is set to 50º or more in case the hardness is measured with an Asker-C hardness meter.

Furthermore, in the above intermediate transfer unit, several kinds of additives different in a particle diameter are added to the above toner.

In addition, in the above intermediate transfer unit, the toner image transferred to the intermediate transfer belt is 1.5 mg/cm² or less per unit area in each density area.

According to the intermediate transfer unit of the fifth aspect of the invention, since toner is coated with a sufficient amount of additive, the force on the toner adhering to the intermediate transfer belt can be reduced, toner can be transferred even in a concave portion of a recording medium having a rough surface, and secondary transfer in a satisfactory state can be easily achieved. Furthermore, since a recording medium having a rough surface is pressed onto the intermediate transfer belt under sufficient linear pressure, the concave portion of the recording medium can be brought into close contact with a toner image on the intermediate transfer belt, and secondary transfer in a satisfactory state can be easily achieved.

In addition, according to the above intermediate transfer belt, since the increase in driving torque by excessively widening a secondary transfer nip formed by the secondary transfer roller and the intermediate transfer belt can be avoided, and a driving motor and an intermediate transfer unit can be miniaturized, which does not take up much space, high cost can be easily achieved.

In addition, according to the above intermediate transfer unit, since an additive having a relatively large particle diameter is added, the additive is not embedded in a mother particle for a long period of time, but the fluidity is maintained and the image quality is stable. In addition, since an additive having a relatively small particle diameter is added, the surface coverage is large compared to the added weight, and sufficient transfer to a recording medium having a rough surface is enabled.

Furthermore, according to the above intermediate transfer unit, the occurrence of irregular color due to the transfer failure of toner of a layer farthest from a recording medium is prevented by forming a toner layer in each density area under the condition that the amount of toner to be transferred is secondarily 1.5 mg/cm2 or less.

According to a sixth aspect of the invention, there is provided an intermediate transfer unit comprising an intermediate transfer belt to which a toner image formed on a photoconductive drum is first transferred in a primary transfer position and which further secondarily transfers the toner image to a recording medium in a secondary transfer position; primary transfer means disposed within the intermediate transfer belt, the intermediate transfer belt between the photoconductive drum and the primary transfer means in the primary transfer position; and auxiliary means disposed in the intermediate transfer belt and secondary transfer means disposed outside the intermediate transfer belt, the intermediate transfer belt being supported between the auxiliary means and the secondary transfer means in the secondary transfer position, and characterized in that the loose apparent density of the toner is set to 0.35 g/cc or more, the shape factor SF-1 of the toner is set to 150 or less, and SF-2 is set to 140 or less.

According to the intermediate transfer unit of the sixth aspect, dropout is prevented from occurring in the transfer by pressing the primary transfer means and the secondary transfer means onto the intermediate transfer belt at the respective transfer positions, and sufficient transfer onto a recording medium having a highly irregular surface is also enabled, such as recycled paper and banknote paper.

According to a seventh aspect of the invention, there is provided an intermediate transfer unit having an intermediate transfer belt to which a toner image formed on a photoconductive drum is first transferred at a primary transfer position and which further secondarily transfers the toner image to a recording medium at a secondary transfer position, wherein primary transfer means are arranged in the intermediate transfer belt and secondary transfer means are arranged outside the intermediate transfer belt, characterized in that the load of the secondary transfer position is larger than a load in the primary transfer position.

In the intermediate transfer unit of the seventh aspect, the ratio of the load in the secondary transfer position to the load in the primary transfer position is 1.5 or more.

According to the intermediate transfer unit of the seventh aspect, dropout is prevented in transfer by pressing the primary transfer means onto the intermediate transfer belt by a relatively small load, sufficient transfer is also enabled even on a recording medium having an extremely irregular surface such as recycled paper and banknote paper by pressing the secondary transfer means onto the intermediate transfer belt by a relatively large load, and further, the durability of the intermediate transfer belt can be increased.

According to an eighth aspect of the invention, there is provided an intermediate transfer unit having an intermediate transfer belt for primarily transferring a toner image formed on a photoconductive drum and further secondarily transferring the toner image to a recording medium, wherein primary transfer means are disposed inside the intermediate transfer belt and secondary transfer means are disposed outside the intermediate transfer belt, characterized in that the hardness of the secondary transfer means is higher than that of the primary transfer means.

In the intermediate transfer unit of the eighth aspect, the hardness of the secondary transfer means is higher than that of the primary transfer means by 10º or more, when measured with an Asker-C hardness meter.

According to the intermediate transfer unit of the eighth aspect of the invention, since the hardness of the primary transfer means is relatively low, dropout in transfer is avoided. Since the hardness of the secondary transfer means is relatively high, sufficient transfer is also enables a recording medium having an extremely irregular surface and further the blurring of an image caused by switching the position of the secondary transfer means between positions in contact and not in contact with the intermediate transfer belt can be avoided.

According to a ninth aspect of the invention, an intermediate transfer unit is characterized in that a toner image formed on the photoconductive drum is primarily transferred to an intermediate transfer belt by applying a bias voltage from a high-voltage power source to a primary transfer member disposed on the back side of the intermediate transfer belt, the resistance of the primary transfer member is set to 106 to 108 Ω, the surface resistance of the intermediate transfer belt is set to 109 to 1012 Ω/, the volume resistance is set to 108 to 1012 Ω cm, the high-voltage power source has a constant current control when the impedance in the primary transfer part is large, and has a constant voltage control when the impedance is small.

According to the intermediate transfer unit of the ninth aspect of the invention, the control of the high voltage power source is optimized. Therefore, since control is carried out at a fixed current in the case of a print pattern in which two to four toner layers overlap, that is, when the impedance is large, a required transfer electric field is ensured for each toner layer. Now, since control is carried out at a fixed voltage in the case of a pattern in which the ratio of printing is small, that is, when the impedance is small, a required and minimum electric field strength for transferring toner is ensured. In addition, since the resistance of the primary transfer member and the intermediate transfer belt is optimized, Transmission at required and minimum voltage and current is possible, and faulty image can be avoided due to abnormal discharge and others.

In addition, since the hardness of the primary transfer member and a load on the photoconductive drum by the primary transfer member are optimized, the shift of an image in primary transfer is prevented and a dropout can be avoided.

In addition, dropout can be avoided by optimizing the amount of an additive having a small particle diameter of two kinds of additives which are different in particle diameter added to the toner and ensuring the fluidity of the toner, and the deterioration of density due to aging can also be suppressed by a function that the surface state of the toner hardly varies due to aging by optimizing the amount of additive having a large particle diameter.

According to a tenth aspect of the invention, an intermediate transfer unit is characterized in that a toner image formed on a photoconductive drum is primarily transferred to an intermediate transfer belt, the toner image is secondarily transferred to a recording medium by applying a bias from a high voltage power source to a secondary transfer member which is pressed onto the auxiliary roller, the resistance of the secondary transfer member is set to 106 to 109 Ω, the surface resistivity of the intermediate transfer belt is set to 109 to 1012 Ω/ , the volume resistivity is set to 109 to 1012 Ω cm, the high voltage power source has constant current control when impedance in the secondary transmission element is large and constant voltage control when the impedance is small.

According to the intermediate transfer unit of the tenth aspect of the invention, the control of the high-voltage power source is optimized. Therefore, when impedance is large, as in transferring to a recording medium in an environment in which temperature and humidity are low and on an overhead film, a transfer electric field required for constant current control is ensured and high transfer efficiency is maintained. Since constant voltage control is carried out when impedance is small, as in transferring to a recording medium in an environment in which temperature and humidity are high and on a recording medium whose width is narrower than that of the secondary transfer member, a required and minimum electric field strength for transferring toner is ensured. In addition, since the resistance of the secondary transfer member and the intermediate transfer belt are optimized, transfer is enabled at required and minimum voltage and current, and an erroneous image can be avoided due to abnormal discharge and others.

In addition, since the hardness of the secondary transfer member and a load on the auxiliary roller by the secondary transfer member are optimized, the misalignment of an image in secondary transfer is avoided and satisfactory transfer is enabled even on a recording medium having a rough surface such as banknote paper for example.

In addition, dropout can be prevented by optimizing the amount of a small particle diameter additive of two kinds of additives different in particle diameter added to the toner and ensuring the fluidity of the toner. Furthermore, the deterioration of density due to aging can also be suppressed by the function that the surface state of the toner is hardly changed. is due to aging by optimizing the amount of additives with large particle diameter.

According to an eleventh aspect of the invention, there is provided an intermediate transfer unit for primarily transferring a toner image formed on a photoconductive drum to an intermediate transfer belt by applying a bias voltage from a high voltage power source to a primary transfer member disposed on the back side of the intermediate transfer belt, and for secondarily transferring the toner image to a recording medium by applying a bias voltage from a bias power source to a secondary transfer member pressed onto an auxiliary roller, characterized in that the primary transfer member and the secondary transfer member are constituted by an elastic body and are set to be larger than that of the primary transfer member by changing the resistance of the secondary transfer member due to the environment.

According to the intermediate transfer unit of the eleventh aspect of the invention, the change in resistance of the primary transfer member and the secondary transfer member due to the environment is optimized. Since the primary transfer member is made of a member with a small change in resistance due to the environment, the capacity of a primary transfer power source can be reduced. Since the secondary transfer member is made of a member with a large change in resistance due to the environment, no failure of transfer occurs in both the low temperature and low humidity environment and the high temperature and high humidity environment because the resistance changes approximately as that of a recording medium such as paper.

According to a twelfth aspect of the invention, an intermediate transfer unit primarily transfers a toner image formed on a photoconductive drum to an intermediate transfer belt by applying a bias voltage from a high voltage power source to a primary transfer member disposed at a position different from a primary transfer member on the surface of the intermediate transfer member, and secondarily transfers the toner image to a recording medium by applying a bias voltage to a secondary transfer member, and is characterized in that an auxiliary member in the primary transfer part is an elastic body, the resistance of the primary transfer member is set to 1 MΩ or less, and a high voltage power source for applying a bias voltage to the primary transfer member has a current-absorbable constant voltage controller.

According to a thirteenth aspect of the invention, an intermediate transfer unit primarily transfers a toner image formed on a photoconductive drum to an intermediate transfer belt by applying a bias from a high voltage power source to a primary transfer member arranged at a position different from a primary transfer part on the surface of the intermediate transfer belt, and secondarily transfers the toner image to a recording medium by applying a bias to a secondary transfer member, and characterized in that an auxiliary member in the primary transfer part is an elastic body, the resistance of the primary transfer member is set to 1 MΩ or less, and a resistor is connected in parallel to a high voltage power source which applies a bias to the primary transfer member.

Further details and advantages of the invention will become apparent from the following description taken in conjunction with the drawings, of which:

Fig. 1 is a block diagram of an apparatus showing an embodiment of the present invention; and

Fig. 2 is a timing diagram showing the operation of the above-mentioned device.

Fig. 3 is a schematic drawing showing an example of an image forming apparatus using an embodiment of an intermediate transfer unit according to the present invention.

Fig. 4 is a side view omitting a part and mainly showing the intermediate transfer unit.

Fig. 5 shows the main part of a gear train.

Figs. 6(a) to 6(c) show an example of a particle size distribution of the toner in the present invention.

Fig. 7 is a side view omitting a part, while mainly showing an intermediate transfer unit of an embodiment.

Fig. 8 explains the function of an embodiment of the present invention.

Fig. 9 explains the function of an embodiment of the present invention.

Fig. 10 explains the function of an embodiment of the present invention.

Preferred embodiments of the present invention will now be described.

Fig. 1 shows the outline of a color image forming apparatus provided with a recording medium carrier system of an embodiment of the present invention.

First, the entire system of the apparatus will be described. Around a photoconductive drum 2 in Fig. 1, there are provided in order from the upstream side in the rotation direction a charging roller 3, a laser beam scanning type latent image forming unit 4, developing units for yellow, magenta, cyan and black 5, 6, 7 and 8, and a cleaning unit 10 facing a first transfer part 9. The above-mentioned apparatus is constructed so that a toner image according to the recording information is formed on the photoconductive drum 2 by repeating each image forming process in yellow, magenta, cyan and black at each rotation of the photoconductive drum 2.

Further, an endless intermediate transfer belt 11, which is in contact with or separated from the photoconductive drum 2 in the transfer part 9, is constructed so that a color toner image formed on the peripheral surface of the photoconductive drum 2 is transferred to the intermediate transfer belt by a primary transfer roller 12, and then transferred to a recording medium 13 by a backup roller 13. The recording paper S stacked on a paper feed cassette 20 reaches a secondary transfer part via a take-up roller 24 and pairs of paper conveying rollers 31 and 33, and in the secondary transfer part, a color toner image is transferred to the recording paper. After the transferred color toner image is fixed by a fixing unit 50, the recording paper is then transferred to a Paper output tray 66 via a pair of paper ejection rollers 62 and 64.

In the following, a recording paper carrying mechanism, which does not form part of the invention, will be described in detail. The paper feed cassette 20 is designed so that it can be installed in the lower part at the front of the frame 1 of the apparatus, that is, in the lower part in Fig. 1, and the fixing unit 50 can be rotated forward so that the recording paper S can be fed immediately and measures can be taken for a paper jam.

A paper push-up plate 21 provided above the paper feed cassette 20 is connected to a drive motor via a stepping clutch, not shown, and is stopped at 120° and 240° so that the paper push-up plate is driven by a single drive motor, not shown, to drive a cam 45 to bring into contact or disengage a secondary transfer roller 41 and all pairs of paper separation rollers 26 and the pairs of paper conveying rollers 31 and 33 between the take-up roller 24 and the pair of gate rollers 35, and can be moved vertically. The paper push-up plate is designed to be raised when the entire apparatus starts operation and lowered after completion of the printing operation. Further, a press roller 22 made of resin for pressing an envelope or the like to the paper feed cassette 20 is provided at the back of the take-up roller 24 so that a slant in paper feed can be prevented which occurs when the edge of the topmost envelope of a stack is lifted and brought into contact with the take-up roller 24 at an angle.

Furthermore, the take-up roller 24 for feeding recording paper S which is pushed up by the paper push-up plate 21 is designed as a roller with a length of about 40 mm, made of rubber having a hardness of 25 to 40º, and is formed so that the take-up roller comes into contact with the center of the paper width and is driven via a first clutch, not shown, so that the take-up roller is locked to the pair of separating rollers 26.

The pair of paper separation rollers 26 arranged near the downstream side in a direction in which paper is fed from the take-up roller 24 consists of an upper separation roller 27 rotating in the paper conveying direction and a lower separation roller 28 rotating normally and reversely via a torque limiter, and both are each formed as a roller having a length of about 40 mm so that each roller comes into contact with the center of the width of the recording paper S and feeding of multiple sheets is prevented.

Further, a paper conveying path between the pair of paper separating rollers 26 and the secondary transfer part serves as a paper reversing path 30 for reversing the recording paper. In this section, a first and second pair of conveying rollers 31 and 33 and a pair of gate rollers 35 are arranged at a pitch at which a postcard can be fed lengthwise or are arranged at a pitch at which an envelope can be fed sideways as appropriate, and are arranged so that the driving force is transmitted via a second clutch.

The first pair of conveyance rollers 31 are arranged near the downstream side of the pair of paper separation rollers 26 and are formed as rollers having a length equal to the width of the recording paper S in order to compensate for the unstable feeding of the pair of paper separation rollers 26 which hold and support only the center of the paper width.

The pair of gate rollers 35 are supported by a sliding bearing, while these first and second pairs of conveyance rollers 31 and 33 are supported by a ball bearing. The above-mentioned rollers are designed so that the free torque of these pairs of conveyance rollers is smaller than that of the pair of gate rollers 35, and even if recording paper S fed at high speed collides with the pair of gate rollers 35, the pair of gate rollers 35 are prevented from moving by the force of collision.

Further, in the paper reversal path 30, a tractive force is prevented from being applied to the recording paper S in the conveying process by setting the peripheral speed of each pair of rollers 26, 31 and 33 between the take-up roller 24 and the pair of gate rollers 35 to be increasingly slower, and further, slippage of the recording paper S in the secondary transfer part is prevented by setting the peripheral speed of the pair of gate rollers 35 to be higher than that of the transfer belt 11.

The peripheral speed of each pair of rollers 26, 31 and 33 needs only to be adjusted to an extent that the peripheral speed of the roller on the upstream side, when the tolerance of the diameter of the rollers on the downstream side is maximum, is equal to or slower than the peripheral speed of the roller on the upstream side, when the tolerance of the diameter of the rollers on the upstream side is maximum. Likewise, the peripheral speed of the pair of gate rollers 35 needs only to be adjusted to an extent that the peripheral speed of the gate roller, when the tolerance of the diameter of the gate roller is minimum, is equal to or faster than the speed of the transmission belt 11.

In the paper reversal path 30, first and second paper sensors 32 and 34 are arranged near the downstream side of the first pair of conveying rollers 31 and near the upstream side of the pair of gate rollers 35. If the recording paper does not reach the first paper sensor 32 after the elapse of a certain time after the take-up roller 24 starts feeding the recording paper, the second paper sensor 34 is deemed to detect an abnormality regardless of whether the second paper sensor 34 detects the recording paper or not, and a signal is output to the control means. Therefore, the amount of information sent to the control means is reduced by the quantity of the signal.

The take-up roller 24, the pair of paper separation rollers 26, the first and second pairs of carrier rollers 31 and 33, and the pair of gate rollers 35 as described above are assembled as a paper feed unit 37 as shown by a dashed line in Fig. 1. The paper feed unit is attached to the body 1 of the apparatus so that it can be detached from the body, and is designed so that it can also be connected to a high-capacity paper feed cassette.

Further, reference numeral 40 denotes a secondary transfer roller unit disposed on the downstream side of the pair of gate rollers 35 via a paper guide member 38. The unit 40 is composed of a swing lever 41 which can swing around a support point 43, the swing lever being pressed by the spring 43 so that the secondary transfer roller 42 supported by the swing lever is always in contact with the transfer belt 11, and the cam 45 for swinging the swing lever 41 so that the secondary transfer roller 42 is disengaged from the transfer belt 11 by a cam follower 44.

The cam 45 for engaging and disengaging is coupled to the drive motor via the step clutch 4, not shown, so that the cam is stopped at several positions in one rotation, in this embodiment at 120° and at 240°, and the front of the cam is formed with an extremely small sine curve so that the secondary transfer roller is disengaged from the transfer belt 11 in a range of, for example, about 1 mm, in which atmospheric discharge can take place by applying tension to the transfer belt 11.

The impact when the secondary transfer roller 42 comes into contact with the transfer belt 11 is reduced and a reduction in the quality of an image due to the impact is prevented by the arrangement described above. The application of a voltage to the secondary transfer roller 42 is controlled so that after the secondary transfer roller 42 comes into contact with the transfer belt 11, the application of current is started and before the secondary transfer roller 42 is released, the application of current ends to prevent an actual atmospheric discharge from taking place.

Reference numeral 50 denotes a fixing unit for fixing a transferred toner image on recording paper S. The fixing unit 50 is mounted so as to be able to turn outward with a supporting part 51 provided at the inner lower end as a supporting point, and is designed so that a paper jam occurring on the paper discharge path can be easily handled and each developing unit 5 to 9 can be easily replaced.

The fixing unit 50 is constituted by a heating roller 52, first and second pressing rollers 54 and 56 pressed against the heating roller 52, and a heat insulating member 55, arranged among them. Toner can be better fixed at a higher speed if a large nip length and a large contact pressure are provided to the first pressing roller 54 to provide a function of fusing the toner, while at the same time a large curvature is provided to the second pressing roller 56 to provide a function of fixing the toner, and further a function of guiding the recording paper and a function of controlling the heat radiation from the heat roller 52 to the heat insulating member 56.

A group of pairs of paper ejection rollers following the fixing unit 50, that is, two pairs of paper ejection rollers 62 and 64 in this embodiment, are mounted to the front of the device 1 as a paper ejection roller unit.

These pairs of paper ejection rollers 62 and 64 are designed so that the recording paper can be ejected onto the paper ejection tray 66 with the recording paper S taut by setting the paper conveying speed of these pairs of paper ejection rollers 62 and 64 to be higher than that of the fixing unit 50 and setting the paper conveying speed of the pair of paper ejection rollers 64 on the downstream side in the paper conveying direction to be higher than that of the pair of paper ejection rollers 62 on the upstream side.

The peripheral speed of each pair of paper discharge rollers 62 and 64 need only be adjusted to an extent that the peripheral speed of the roller on the downstream side when the diameter of the roller is maximum is equal to or not slower than that of the roller on the upstream side when the diameter of the roller is minimum, with the same consideration as in the case of the above-mentioned rollers on the paper feed path. Reference numerals 61 and 63 denote each paper detection sensor arranged on the paper ejection path.

Next, the recording paper conveying operation of the apparatus constructed as described above will be described with reference to Fig. 2.

When the operation of the entire apparatus is started at time "a" after an initial phase for feeding paper, the paper push-up plate 21 pushes up loaded recording paper S and brings the center of the width of the uppermost paper into contact with the take-up roller 24.

When a paper feed/separation roller clutch is connected at time "b" with respect to an image forming process and the pickup roller 24 being rotated feeds the first recording paper S, the pair of paper separation rollers 26 disposed near the downstream side of the pickup roller prevents multiple sheets from being fed by counter-rotating the lower separation roller 28. A paper conveyance roller clutch connected in common with the paper feed/separation roller clutch transmits rotation to each first and second pair of conveyance rollers 31 and 33 for a time corresponding to the length of a paper path between the paper feed tray 20 and the pair of gate rollers 35, that is, until time c, and is brought into contact with the full width of the recording paper S by the pair of paper separation rollers 26 to stably convey it to the pair of gate rollers 35.

At time "d" after a certain time has elapsed after the initial conveyance, a gate roller transmits the driving force to the pair of gate rollers 35 over a period of time corresponding to the path length between the pair of gate rollers 35 and the secondary transfer roller 42, that is, until time e, and simultaneously conveys the recording paper S to a transfer part in cooperation with the first and second pairs of conveying rollers 31 and 33 to which the driving force is transmitted by means of the paper conveying roller clutch, and then carries out the required transfer operation thereon.

Although different depending on the length in the conveying direction of the recording paper S, the paper feed/separation roller clutch for conveying the second recording paper S is connected at the time "f" before or after the operation of the gate roller clutch at the following time "g", the paper carrier roller clutch transmits the driving force to the first and second pairs of conveying rollers 31 and 33 over the period corresponding to the length of a path between the first pair of conveying rollers 31 and the pair of gate rollers 35, that is, until the time "h", and conveys the second recording paper S to the pair of gate rollers 35.

Furthermore, in such an apparatus in which recording paper is continuously conveyed, high durability and advanced paper conveyance control means are provided. However, abrasion and wear of parts and occurrence of paper jam and the like cannot be avoided. When such a situation occurs, a target unit of units each independently mounted as the paper feed unit 37, a transfer unit 40, the fixing unit 50 and a paper discharge unit 60 is detached or exchanged from the apparatus body 1 by the user.

As described above, according to the present invention, a paper feed mechanism, a transfer mechanism, a fixing mechanism and a paper ejection mechanism constituting a recording medium carrying system are provided as independent unit, a user can cope with such a situation that a paper jam or abrasion and wear of parts occurs in this type of image forming apparatus which constantly conveys a recording medium at high speed by detaching or exchanging a unit. Thus, the maintenance cost can be reduced and the operating speed of the apparatus can be significantly improved.

Fig. 3 is a schematic drawing showing an example of an image forming apparatus using an embodiment of an intermediate transfer unit according to the present invention.

First, an outline of the image forming apparatus will be described, and then mainly the intermediate transfer unit will be described in detail.

A full-color image can be formed using developing machines for four colors of toner yellow, cyan, magenta, and black by the above image forming apparatus.

In Fig. 3, reference numeral 150 denotes a casing of the body of the apparatus, and in this casing 150, there are provided an exposure unit 160, a paper feeding unit 70, a photoconductor unit 100, a developing unit 200, an intermediate transfer unit 300, a fixing unit 400, a control unit 80 for controlling the entire apparatus, and others.

The photoconductor unit 100 is formed with a photoconductive drum 110, a charging roller 120 as charging means which comes into contact with the peripheral surface of the conductive drum 110 and uniformly charges the peripheral surface, and cleaning means 130.

The developing unit 200 is formed with a developing area 210Y for yellow, a developing area 210C for cyan, a developing area 210M for magenta and a developing area 210K for black as developing means. These developing areas 210X, 210C, 210M and 210K contain yellow, cyan, magenta and black toner, respectively. The above developing areas are formed with developing rollers 211Y, 211C, 211M and 211K, and are set so that only one of the above developing areas can come into contact with the photoconductive drum 110.

The intermediate transfer unit 300 is formed with a drive roller 310, a primary transfer roller 320, a smoothing roller 330, a tension roller 340, an auxiliary roller 350, an endless intermediate transfer belt 360 extending around each roller, and cleaners 370 which may or may not contact the intermediate transfer belt 360.

A secondary transfer roller 380 is arranged opposite to the auxiliary roller 350. The secondary transfer roller 380 is supported such that the secondary transfer roller can be swung by an arm 382, which is supported by a support shaft 381, so that the arm can be swung. The secondary transfer roller is contacted or not contacted by the intermediate transfer belt 360 when the arm is swung by the operation.

A gear 311 shown in Fig. 5 is attached to the end of the drive roller 310, and is rotated at approximately the same rotational speed as the photoconductive drum 110 because the gear 311 is in engagement with a gear 144 (see Fig. 5) of the photoconductive unit 100. Therefore, the intermediate transfer belt 360 circulates at approximately the same rotational speed as the photoconductive drum 110.

In an operation in which the intermediate transfer belt 360 circulates, a toner image on the photoconductive drum 110 is transferred to the intermediate transfer belt 360 between the primary transfer roller 320 of the photoconductive drum 110, and the toner image transferred to the intermediate transfer belt 360 is transferred to a recording medium S such as paper, which is arranged between the intermediate transfer belt and the secondary transfer roller 380. The recording medium S is supported by the paper feed unit 70.

The paper feed unit 70 is formed with a tray 71 on which a plurality of sheets of a recording medium S are stacked, a pickup roller 72, a spring plate 73 for pressing the recording media S stacked on the tray 71 toward the pickup roller 72, and a pair of separation rollers 74 for securely separating the recording media fed from the pickup roller 72.

A recording medium S fed from the paper feed unit 70 is fed to a secondary transfer part, that is, between the intermediate transfer belt 360 and the secondary transfer roller 380 by a pair of first conveyance rollers 91, a first paper sensor 91S, a pair of second conveyance rollers 92, a second paper sensor 92S, and a pair of input rollers 93, and thereafter ejected onto the casing 50 through the fixing unit 400, a pair of first ejection rollers 94, and a pair of second ejection rollers 95.

The fixing unit 400 is formed with a fixing roller 410, which is provided with a heating source, and a pressure roller 420, which is pressed onto the fixing roller.

The operation of the above whole image forming apparatus is as follows:

(i) When a print command signal (an image forming signal) is supplied from a host computer, not shown, such as a PC, to the control unit 80, the photoconductive drum 110, the developing roller and the like of the developing unit and the intermediate transfer belt 360 are rotated.

(ii) The peripheral surface of the photoconductive drum 110 is uniformly charged by the charging roller 120.

(iii) Selective exposure L corresponding to the image formation of a first color (for example, yellow) is applied to the peripheral surface of the uniformly charged photoconductive drum 110 by the exposure unit 60 so that an electrostatic latent image for yellow is formed.

(i,v) Only the developing roller 211Y of the developing section for the first color (for example, yellow) contacts the photoconductive drum 110, thereby the above electrostatic latent image is developed and the toner image of the first color (for example, yellow) is formed on the photoconductive drum 110.

(v) The toner image formed on the photoconductive drum 110 is transferred to the intermediate transfer belt 360 in a primary transfer part, that is, between the photoconductive drum 110 and the primary transfer roller 320. Now, the cleaning means 370 and the secondary transfer roller 380 are removed from the intermediate transfer belt 360.

(vi) After toners remaining on the photoconductive drum 110 are removed by the cleaning means 130, the photoconductive drum 110 is deelectrified by deelectrification light L' from the deelectrification means.

(vii) The operation shown in items (ii) to (vi) above is repeated if necessary. That is, the operation for second, third and fourth colors is repeated according to the contents of the above print command signal, and a toner image according to the contents of the print command signal is formed superimposed on the intermediate transfer belt 360 and on the intermediate transfer belt 360.

(viii) A recording medium S is fed from the paper feed unit 70 at a predetermined timing. Immediately before or after the end of the recording medium S reaches the secondary transfer part (in short, at a timing at which a toner image on the intermediate transfer belt 360 is transferred to a desired position on the recording medium S), the secondary transfer roller 380 is pressed onto the intermediate transfer belt 360, and the toner image (substantially a full-color image) on the intermediate transfer belt 360 is transferred to the recording medium S. The cleaning means 370 come into contact with the intermediate transfer belt 360, and after secondary transfer, toner left on the intermediate transfer belt 370 is removed.

(ix) When the recording medium S passes through the fixing unit 400, a toner image is fixed on the recording medium S and thereafter the recording medium S is ejected into the casing 150, namely by means of a pair of paper ejection rollers 94 and 95.

The plan view of the image forming apparatus is described above. Next, the details of the intermediate transfer unit 300 will be mainly described.

Fig. 4 is a side view with a part omitted, essentially showing the intermediate transfer unit 300.

As described above, the intermediate transfer unit 300 is formed with the drive roller 310, the primary transfer roller 320, the smoothing roller 330, the tension roller 340, the auxiliary roller 350, the endless intermediate transfer belt 360 extending around each of the above rollers, and the cleaning means 370 which can be applied or not applied to the intermediate transfer belt 360. Each of the above members and others are fixed to a frame 301 as shown in Fig. 3.

The frame 301 is composed of a pair of side plates (in Fig. 4, the side plate on this side is omitted), and each of the above members and others are fixed between both side plates. In other words, the frame is constructed such that a pair of side plates are coupled by the shaft of each of the above members. In Fig. 2, each member on this side of a pair of members described later is omitted.

The drive roller 310 is supported on the frame 301 by its shaft 312 so that the drive roller can be rotated and the above gear 311 shown in Fig. 5 is attached to the end thereof. The drive roller is designed to be rotated at approximately the same peripheral speed as the photoconductor unit 100 because the gear 311 is connected to the gear 144 of the Photoconductor unit 100. As shown in Fig. 5, reference numeral 500 denotes a drive motor. The photoconductive drum 110 is rotated because a gear 510 fixed to its output shaft 501 is engaged with the gear 144 provided at the end of the photoconductive drum 110 through a reduction gear 520. The gear 511 is engaged with the drive gear 133b of a toner conveying screw 133 in the photoconductor unit 100 shown in Fig. 3 through an intermediate gear 520 and a reduction gear 521, here by which the toner conveying screw 133 is rotated.

As shown in Fig. 4, the shaft 321 of the primary transfer roller 320 is supported by the frame 301 through a pair of bearing members 322 so that the primary transfer roller can be rotated. An electrode plate 323 for applying a voltage to the primary transfer roller 320 is supported by fixing its elongated hole 323a to a threaded hole with which the frame 301 is formed. The bearing member 322 is supported by a concave portion 303 with which the frame 301 is formed so that the bearing member is slidable (can be moved vertically in Fig. 1), and a compression spring 324 as a pressing means is provided between the bearing member 322 and the frame 301.

Therefore, the primary transfer roller 320 is pressed onto the photoconductive drum 110 by means of the intermediate transfer belt 360 since the two ends of the shaft 321 are pressed by the pair of compression springs 324, respectively.

The smoothing roller 330 is supported on the frame 301 by its shaft 331 so that the smoothing roller can be rotated.

The pressure roller 340 is supported in such a way that its shaft 341 can be rotated and can slide in a slot 304, which is provided on the frame 301. One end 324a of an arm 324 forming a pair at both ends is in contact with the shaft 341. The arm 324 is supported on the frame 301 by its shaft 343 so that the arm can be pivoted, and a tension spring 344 is provided between the other end 342b and the frame 301.

Therefore, the pressure roller 340 is pressed by the arm 342 by the tension spring 344 in a direction in which the intermediate transfer belt 360 is always tensioned.

The auxiliary roller 350 is supported on the frame 301 by its shaft 351 so that the auxiliary roller can be rotated.

The intermediate transfer belt 360 extends over each of the above rollers 310, 320, 330, 340 and 350 and is circulated by drive roller 310 in a direction (clockwise) shown by arrows in Fig. 4.

The cleaning means 370 are formed with a fur brush 371 for brushing off the toner remaining on and adhering to the peripheral surface of the intermediate transfer belt 360, a cleaning blade 372 for further scraping off the toner still remaining and adhering to the peripheral surface of the intermediate transfer belt 360, and a toner conveying screw 373 as conveying means for conveying the toner brushed off or scraped away by the above fur brush 371 or cleaning blade 372, and each of the above members is installed in a housing 374.

A toner accommodating chamber 375 is formed in the lower part of the casing 374, and the above fur brush 371, cleaning blade 372 and toner conveying screw 373 are arranged in the toner accommodating chamber 374.

The fur brush 371 is mounted on its shaft 371a, which penetrates the side plate of the housing 374 and which in the direction indicated by the arrows in Fig. 4, by the shaft 371a which is driven by the drive means not shown.

The cleaning blade 372 is fixed to the housing 374 by means of a fixing plate 372a and is constructed such that the end (the lower end) comes into contact with the peripheral surface of the intermediate transfer belt 360 and scrapes off the toner.

The toner conveying screw 373 is rotated in the direction shown by the arrows in Fig. 4 by means of its shaft 373a piercing the side plate of the casing 374, driven by driving means not shown, and conveys toner collected in the toner receiving chamber 375 to a waste toner casing not shown as waste toner.

Its cylindrical part 374a, which is formed on both sides of the case 374, is supported on the frame 301 by means of a bearing member 376 so that the cylindrical part can be rotated.

A hook 377 is attached to both sides of the lower end of the case 374, and a tension spring 378 is provided between the hook 377 and the frame 301.

Therefore, the housing 374 is always pressed against the tension spring 387 in a direction (clockwise) in which the fur brush 371 and the cleaning blade 372 press the intermediate transfer belt 360. However, the rotation of the housing 374 is regulated because a cam 55 is provided on the intermediate transfer unit 300 as shown in Fig. 3 and is in contact with the lower end of the housing 374.

The cam 55 is driven by a drive means not shown. When the cam is located in a position shown in Fig. 4, it rotates the housing 374 counterclockwise as shown by an alternating long and short dashed line and removes the fur brush 371 and the cleaning blade 372 from the intermediate transfer belt 360.

In Fig. 4, reference numeral 156 denotes a position detecting sensor (see Fig. 3) with which the case of the image forming apparatus is formed so that the position detecting sensor is opposite to the drive roller 310. The position detecting sensor is formed so as to detect the position of the intermediate transfer belt 360.

The above intermediate transfer unit 300 is designed so that it can be attached or detached from the body of the image forming apparatus.

Furthermore, since various devices are made or can be made in this embodiment, they will now be described.

< With reference to the drive roller 310>

(1) The outer diameter of the drive roller 310 is designed such that the peripheral speed of the intermediate transfer belt 360 is slightly faster (within a tolerance range) than that of the photoconductive drum 110.

It is desirable that the peripheral speed of the photoconductive drum 110 be exactly equal to that of the intermediate transfer belt 360 on which a toner image is transferred from the photoconductive drum 110.

However, since there is a tolerance between the outer diameter of the photoconductive drum 110 and the drive roller 310, it is impossible to make the above peripheral speeds completely equal. In such a state, if the peripheral speed of the intermediate transfer belt 360 in a part where the intermediate transfer belt is wound on the drive roller 310 is slightly lower than that of the photoconductive drum 110, a force attempting to loosen the intermediate transfer belt 360 is applied to the intermediate transfer belt 360 between a position (a primary transfer position T1) in which the photoconductive drum 110 and the primary transfer roller 320 are in contact and the drive roller 310 is very small by the force. Therefore, a state of the intermediate transfer belt 360 in the primary transfer position T1 is unstable.

In this embodiment, the outer diameter of the drive roller 310 is set such that the peripheral speed of the intermediate transfer belt 360 is slightly faster (within a tolerance range) than that of the photoconductive drum 110.

When the above structure is made, since the intermediate transfer belt 360 is always tensioned between the position (the primary transfer position T1) in which the photoconductive drum 110 and the primary transfer roller 320 are in contact and the drive roller 310, although the amount of tension is small, the state of the intermediate transfer belt 360 is stabilized in the primary transfer position T1.

The bending amount of the peripheral surface of the drive roller 310 is set to ± 0.05 mm or less.

(2) The intermediate transfer belt 360 is designed such that the period is equivalent to an integer multiple of the period of the drive roller 310.

The bending of the shaft or peripheral surface of the drive roller 310 caused by the amount of shift between toner images of each color overlapped on the intermediate transfer belt 360 can be reduced by a construction as described above.

In this specific case, the above ratio is set at 5 to 1.

(3) The intermediate transfer belt 360 is designed such that the period is equivalent to an integer multiple of the period of the photoconductive drum 110.

The amount of displacement of the shaft or peripheral surface of the photoconductive drum 110 caused by the bending between toner images of each color overlapping on the intermediate transfer belt 360 can be reduced by a construction as described above.

In this specific case, the above ratio is set to 2 to 1.

(4) The angle at the contact point of the intermediate transfer belt 360 with the drive roller 310 is set to 90º or more so that the angle of contact is larger than the angle of contact with the other roller.

The intermediate transfer belt 360 can be stably driven by a construction as described above even when a friction coefficient between the drive roller 310 and the intermediate transfer belt 360 is small or the friction coefficient is reduced due to long-term use.

In the specific case, the above angle at the contact point is set to approximately 151º.

In order to increase the above friction coefficient, a urethane coating is applied to the peripheral surface of the drive roller 310.

< With reference to the auxiliary roller 350>

For a method of separating the intermediate transfer belt 360 and a recording medium S at a location where the auxiliary roller 350 and the secondary transfer roller 380 are in contact, that is, a secondary transfer part T2 shown in Fig. 4, a curvature separation method is used. The diameter of the auxiliary roller 350 is set to 35 mm or less, and the contact angle of the intermediate transfer belt 360 with the auxiliary roller 350 is set to 90° or more.

A recording medium S is securely separated from the intermediate transfer belt 360 by a construction as described above.

It is desirable that the diameter of the auxiliary roller 350 is set to 30 mm or less and the contact angle of the intermediate transfer belt 360 with the auxiliary roller 350 is set to 105º or more. In the concrete case, the above diameter is set to 30 mm diameter and the above contact angle is set to 109º.

It is desirable that the surface resistance of the intermediate transfer belt 360 is set to 10¹² Ω or less.

< With reference to cleaning agents 370>

(1) The tension roller 340 is set closer to the side of the cleaner 370 in a horizontal direction as compared with the auxiliary roller 350, and a part of the toner receiving chamber 375 is open below a part in which the fur brush 371 and the intermediate transfer belt 360 are in contact.

According to the above construction, toner brushed off by the fur brush 371 is easily collected in the toner receiving chamber 375.

It is desirable that an angle between the intermediate transfer belt 360 and a vertical line V between the tension pulley 350 and the auxiliary pulley 350, that is, an angle θ between a common tangent of the tension pulley 350 and the auxiliary pulley 350 and a vertical line V is set to 10° or more, and it is more desirable that the above angle is set to 15° or more.

According to the above construction, toner brushed off by the fur brush 371 is more securely and easily collected in the toner receiving chamber 375, and toner falling off when the cleaners 370 are detached from the intermediate transfer belt 360 is also more easily collected in the toner receiving chamber 375.

(2) The tension roller 340 also serves as a means for receiving the printer’s cleaning agents 370 onto the intermediate transfer belt 360.

The manufacturing cost can be reduced by a construction as described above. Since another tension pulley is not required and the number of pulleys can be reduced, the contact angle of the intermediate transmission belt with each pulley is increased.

< With reference to the smoothing roller 330>

The smoothing roller 330 is arranged on the upstream side near the primary transfer position T1 in a direction in which the intermediate transfer belt 360 is driven, and the contact angle of the intermediate transfer belt 360 with the smoothing roller 330 is set to 10° or more.

A fold formed on the intermediate transfer belt 360 between the tension roller 340 and the smoothing roller 330 (a wavy state when viewed from the smoothing roller 330 to the tension roller 340) is removed by the smoothing roller 330, and the intermediate transfer belt 360 in the primary transfer position T1 can be smoothed by a construction as described above.

It is desirable that the contact angle of the intermediate transfer belt 360 with the smoothing roller 330 is set to 15º or more. In the concrete case, the above angle is set to 17.6º.

Means for changing the running direction of the intermediate transfer belt 360 by 10° or more, such as a guide plate, may be provided instead of the smoothing roller 330.

< With reference to the primary transmission position T1>

(1) The drive roller 310, the primary transfer roller 320 and the smoothing roller 330 are arranged such that the intermediate transfer belt 360 is stretched straight in a direction tangent to the photoconductive drum 110 at the primary transfer position T1.

A transfer point can be stabilized without depending on the belt tension by a construction as described above. If the intermediate transfer belt 360 is placed on the primary transfer roller 320 and the primary transfer position T1 is formed at the wound position, the change in tension of the intermediate transfer belt 360 has a large effect on the primary transfer position T1. However, the above effect can be reduced by a construction such that the intermediate transfer belt 360 is tensioned in a direction of a tangent of the photoconductive drum 110 without winding the intermediate transfer belt 360 on the primary transfer roller 320.

(2) The primary transfer position T1 is located near the drive roller 310.

If the distance between the primary transfer position T1 and the drive roller 310 is large, the shrinkage of the intermediate transfer belt 360 between them is increased and the speed of the intermediate transfer belt 360 in the primary transfer position T1 becomes unstable.

In this embodiment, the speed of the intermediate transfer belt 360 at the primary transfer position T1 is stabilized by arranging the primary transfer position T1 near the drive roller 310.

It is desirable that the distance L1 shown in Fig. 4 between the primary transfer position T1 and the drive roller 310 is set to 40 mm or less, and it is more desirable that the above distance is set to 35 mm or less. In the concrete case, the distance is set to about 30.5 mm.

(3) For the length of the straight part of the intermediate transfer belt 360 from the smoothing roller 330 toward the drive roller 310, the ratio is set to 0.25 or less. It is more preferable that it is set to 0.15 or less.

This follows from the fact that the above effect can be prevented more effectively by a fold.

In the specific case, the length of the straight part above is set to approximately 55.5 mm.

<Regarding position detection>

As described above, the position detecting sensor 56 is arranged opposite to the drive roller 310 to detect the position of the intermediate transfer belt 360 on the drive roller 310.

This allows the cycle of the intermediate transfer belt 360 to be accurately recorded.

The position detecting sensor 56 comprises a reflector type optical sensor, and a mark to be detected by the position detecting sensor 56 is provided on the intermediate transfer belt 360 by printing.

When the position detection sensor is constituted by a through light sensor and a hole to be detected by the sensor is made on the intermediate transfer belt 360, mechanical stress is concentrated in the hole and the hole is deformed so that accurate detection is impossible. However, in this embodiment, since the position detection sensor 56 is constituted by a reflector-type optical sensor and a mark to be detected by the sensor is provided on the intermediate transfer belt 360 by printing, the cycle of the intermediate transfer belt 360 can be accurately detected.

<Regarding the structure by which the intermediate transmission belt 360 is tensioned and stretched>

For a construction in which the intermediate transfer belt 360 is tensioned, the length of the intermediate transfer belt 360 from the primary transfer position T1 to the secondary transfer position T2 is set to the length in the transverse direction of an A4 paper or longer, and the length of the intermediate transfer belt 360 from the secondary transfer position T2 to the primary transfer position T1 is also set to the length in the transverse direction of an A4 paper or longer. That is, the intermediate transfer belt 360 is tensioned and stretched to realize the length described above.

According to the above construction, when printing on A4 paper is continuously carried out, at a time when the secondary transfer roller 380 is brought into contact with the intermediate transfer belt 360, it can be set in holding of the paper, that is, the secondary transfer roller 380 does not contact the intermediate transfer belt 360 during the primary transfer.

When the secondary transfer roller 380 is brought into contact with the intermediate transfer belt 360 during the primary transfer, an image through the primary transfer may be deformed by the collision. However, such a situation can be avoided by a construction as described above.

< With reference to cleaning agents 370>

(1) The cleaning blade 372 is made of urethane rubber, the free length is about 8 mm, the thickness is about 3 mm, the elastic modulus is about 7 to 9 MPa, the holding angle (an angle between the blade in a state of no load and the tangent of the roller in the contact position) is about 20º, and the contact pressure on the intermediate transfer belt 360 is about 45 gf/cm.

According to the above construction, failure of cleaning caused by the passage of toner through the sheet, vibration and lifting of the sheet can be avoided.

(2) The waste toner box is provided separately from the casing 374.

Because of the above construction, it can be avoided that a large amount of waste toner is accumulated in the casing 374, the change in load when the casing 374 is swung and force acting on the casing 374 after the casing is swung can be reduced. As a result, the contact pressure of the cleaning blade 372 on the intermediate transfer belt 360 can be stabilized.

(3) The shaft 373a (see Fig. 4) of the toner conveying screw 373 is arranged at the fulcrum of the housing.

According to the above construction, a relative positional relationship between the housing and the other fixed part, for example, between the waste toner carriage opening of the housing 374 and the toner receiving opening of the waste toner housing, is easily secured.

(4) The cam 155 is formed by a SIN cam.

A shock applied to the intermediate transmission belt 360 can be reduced by a construction as described above.

< With reference to material recording>

Material detection, that is, detection of a toner amount during trial printing, is carried out on the intermediate transfer belt 360 on the drive roller 310.

The above material detection can be carried out at a place where the contact angle is large and velocity is stable by a construction as described above.

< With reference to bead>

A bead is a protrusion provided on the back of the intermediate transmission belt 360 along the rotational direction, wherein the position (in the direction of the axis of each pulley) of the belt is regulated by fitting the beads in a concave groove (a regulating groove) formed on each pulley around which the belt is wound.

The above beads are not necessarily provided and in the form shown in Fig. 4 they are not provided. If they are provided, they are constructed as follows:

(1) Silicone rubber is used for the bead, with the thickness (the height of the bump) set to about 1.5 mm and the width to about 4 mm.

(2) The friction coefficient between the bead and the regulating bead is set to be smaller than that between the base material of the intermediate transmission belt 360 and each roller.

The occurrence of a tension tendency in the axial direction of the belt due to frictional force between the bead and the regulating bead can be reduced by the design described above.

The friction coefficient between the base material of the intermediate transfer belt 360 and each roller is about 0.4.

(3) The elastic strength of the bead is set to about 2.0 to 8.0 MPa. Namely, if the bead is too soft, mechanical stress against the drive in a regulating part is applied only at one place, that is, a small area in which the bead is connected.

In contrast, if the bead is too hard, the effect of the bead on the bent part of the bead is too large. It is desirable that if t1 is the thickness of the belt, t2 is the thickness of the bead, and E1 is the elastic modulus (up to 4.0·10³ MPa) of the belt, the elastic strength of the bead is set to {1.0 to (t1/t2)²}·E1 [MPa].

(4) The bead regulating bead is provided for each roller which is not adjacent to the primary transfer position T1.

According to the above construction, a shift between the toner images of each color overlapping on the intermediate transfer belt 360 can be reduced by the random change by contact between the bead and the regulating bead of the intermediate transfer belt 360.

For example, the bead control bead is reconstructed by adding a stepped flange to the end of the auxiliary roller 350.

(5) The regulating bead is formed such that the width is slightly larger than that of the bead and the regulating bead has an edge for the uniform adhesion of the bead.

For example, if the width of the bead is about 4 mm, that of the regulating bead is set to about 4.2 mm.

<Regarding replacement and handling of the intermediate transfer unit 300>

(1) The intermediate transfer unit 300 is designed such that the intermediate transfer belt 360 does not come into contact with the surface of a desk or others when the intermediate transfer unit 300 is placed on the desk or others. That is, the intermediate transfer belt 360 is not damaged or foreign matter does not adhere to the intermediate transfer belt.

(2) The intermediate transfer unit 300 is designed such that a drive transmission part such as the gear 311 does not come into contact with the surface of a table or others when the intermediate transfer unit 300 is placed on the table or others. Therefore, the deformation and damage of the drive transmission part is prevented.

(3) The electrode part of the intermediate transfer unit 300 is provided on the back of the drive transfer part so that an electrode is not contaminated and contact failure is avoided,

(4) The intermediate transfer unit 300 is designed such that the photoconductor unit 100 cannot be installed unless the intermediate transfer unit 300 is installed. Therefore, erroneous attachment is avoided.

(5) The intermediate transfer unit 300 is designed such that the capacity of the waste toner housing is matched to the service life of the intermediate transfer belt 360 and the waste toner case is also replaced when the intermediate transfer unit 300 is replaced. Therefore, the handling is improved.

<With reference to the sequence>

(1) When the position of the intermediate transfer belt 360 is detected as the basis of the exposure writing timing, a bias voltage for primary transfer is applied, that is, a bias voltage for primary transfer is applied before detecting the position.

The load of each of the four colors on the intermediate transfer belt 360 in the primary transfer position T1 from the detection position to the primary transfer is approximately equal, and displacement (called misregistration) of toner images of each color overlapping on the intermediate transfer belt 360 can be avoided by the setting described above.

(2) The position of the mark for detecting the position when the intermediate transfer belt 360 is stopped is set so as to be located on the upstream side of the primary transfer position T1. For example, the above position on the upstream side is a position represented by M in Fig. 4.

Since the position is detected when the tension of the intermediate transfer belt 360 is stable due to the application of the pretension of the initial circulation of the intermediate transfer belt 360, misalignment caused by the shift of the period can be avoided by setting as described above.

<Referring to the frame 301 of the intermediate transfer unit 300>

The side plate of the frame 301 is formed by an insulating member such that insulation to a roller shaft for applying a preload to a roller (and or a bearing member) is not required.

The thermal expansion coefficient of the frame 301 is approximately equal to that of the intermediate transfer belt 360 by using acrylonitrile butadiene styrene resin (ABS resin) as the above insulating member, and relative misalignment due to temperature change can be avoided.

[Embodiments]

Further specific embodiments are described below.

The following description mainly concerns a transfer procedure:

< To stabilize the efficiency of primary transmission>

(1) A high-voltage power source which has constant current control when the impedance of the primary transmission is large (about 30 MΩ or more) and has constant voltage control when the impedance is small (about 30 MΩ or less) is used.

Therefore, even if dispersion occurs in the amount (film thickness) of toner, environment, and resistance of a member, the transfer is satisfactorily carried out.

(2) The surface resistance of the intermediate transfer belt 360 is set to 10⁸ to 10¹² Ω/ and the volume resistance is set to 10⁸ to 10¹² Ω cm.

The primary transfer roller 320 is made of urethane in which carbon is dispersed, the resistance thereof is set to 10⁶ to 10⁸ Ω (preferably about 10⁷ Ω), and the hardness is set to 45±5°, and the load on the photoconductive drum 110 by the primary transfer roller is set to 1.0 to 3.5 kg (preferably about 2.5 kg).

Transmission is enabled at 1200 V or less by setting the resistance value in the above range.

The occurrence of a so-called misfire can be avoided by setting the hardness and the load in the above range.

(3) Regarding an additive amount added to the toner, the amount of an additive having a large particle diameter is set to 0.5 to 4.0 wt% (preferably about 0.7 wt%), and the amount of an additive having a small particle diameter is set to 1.5 to 4.0 wt% (preferably about 2.0 wt%).

The additive having a large particle diameter is mainly required to increase the stability of the toner preservation, and therefore, it is better if the amount of the above additive is large. However, if the amount of the above additive exceeds 4.0 wt%, the fluidity of the toner deteriorates and dropout and the like may occur. Therefore, too large an amount of the above additive is not desirable.

Now, the additive having a small particle diameter is mainly required to increase the transferability on rough paper, and therefore, it is better if the amount of the above additive is higher. However, if the amount of the above additive exceeds 4.0 wt%, the photoconductive drum 110 and the intermediate transfer belt 360 are easily coated with a flowing silicate layer. Therefore, too much amount is not desirable.

The deterioration of an image due to interference in simultaneous primary and secondary transmission can be avoided and the capacity of the high-voltage power source can be reduced to a minimum under the conditions as described in (1) to (3) above.

< To stabilize the secondary transmission efficiency>

(1) A high-voltage power source which has constant current control when the impedance of the secondary transmission is large (about 20 MΩ or more) and constant voltage control when the impedance is small (about 20 MΩ or less) is used.

Even if dispersion occurs here due to the type of paper, environment and resistance of a member, the transfer is carried out satisfactorily.

(2) The surface resistance of the intermediate transfer belt 360 is set to 10⁹9 to 10¹² Ω/, and the volume resistance is set to 10⁹8 to 10¹² Ω cm.

The secondary transfer roller 380 is an ionic roller, the resistance thereof is set to 10⁶ to 10⁹ Ω, the hardness is set to 60±5°, and the load on the auxiliary roller 350 by the secondary transfer roller is set to 5.0 to 9.0 kg (preferably about 7.0 kg).

Transmission is enabled at 4000 V or less and 200 uA or less by setting the resistance in the above range.

The auxiliary roller 350 is earthed.

(3) Regarding the amount of an additive added to the toner, the amount of an additive having a large particle diameter is set to 0.5 to 4.0 wt% (preferably about 0.7 wt%)

and the amount of an additive having a small particle diameter is set to 1.5 to 4.0 wt% (preferably about 2.0 wt%).

The reason is as described above.

< To prevent the back of the recording medium S such as paper from becoming dirty>

When transfer to paper or transfer of color is not performed while the secondary transfer roller 380 is in contact with the intermediate transfer belt 360, a voltage of about 0 to -600 V is applied in a direction in which toner is returned to the intermediate transfer belt 360.

Toner adhering to the secondary transfer roller 380 is reduced and a stain on the back of a recording medium S is reduced by a construction as described above.

<For satisfactory transfer to rough (bound or banknote) paper>

(1) 'The hardness of the secondary transfer roller 380 is set to 60±5º and the load on the auxiliary roller 350 by the secondary transfer roller is set to 5.0 to 9.0 kg (preferably about 7.0 kg).

(2) Regarding the amount of an additive added to the toner, the amount of an additive having a large particle diameter is set to 0.5 to 4.0 wt% (preferably about 0.7 wt%), and the amount of an additive having a small particle diameter is set to 1.5 to 4.0 wt% (preferably about 2.0 wt%).

Toner with high pigment density is used as the toner, with the particle diameter being about 7 µm.

(3) The toner amount from the secondary transfer, that is, the toner amount on the intermediate transfer belt 360 is set to 1.5 mg/cm² or less.

A satisfactory transfer condition can also be achieved on rough paper such as banknote paper by setting the parameters as described above under (1) to (3).

That is, the paper surface can be brought into close contact with the toner by setting the hardness of the secondary transfer roller 380 to a high value as described above and setting a load on the secondary transfer roller to a high value. Therefore, even if a high electric field is formed, the transfer error due to discharge is reduced. A state in which paper is conveyed is also stabilized by applying the high load.

Furthermore, the transfer efficiency of the toner can be increased by reducing the amount of toner before secondary transfer as described above.

< To avoid the occurrence of a dropout >

(1) The intermediate transfer belt 360 is made of ethylene-tetra-fluoro-ethylene (ETFE) in which carbon black and others are dissolved as a conductor, polyethylene terephthalate (PET) which is produced by depositing aluminum and further coating with urethane paint excluding fluorine particles, or polyamide in which carbon black and others are dissolved as a conductor.

The photoconductive drum 110 is made of polycarbonate.

(2) The hardness of the primary transfer roller 320 is set to 45 ± 5º and the load on the photoconductive drum 110 by the primary transfer roller is set to 1.0 to 3.5 kg.

(3) The hardness of the secondary transfer roller 380 is set to 60 ± º5 and the load on the auxiliary roller by the secondary transfer roller is set to 5.0 to 9.0 kg.

(4) As the amount of an additive used for the toner, the amount of an additive having a large particle diameter is set to 0.5 to 4.0 wt% (preferably about 0.7 wt%), and the amount of an additive having a small particle diameter is set to 1.5 to 4.0 wt% (preferably about 2.0 wt%).

The fluidity of the toner is set to about 0.35 g/cc.

The following function and effect can be achieved by setting the above parameters:

That is, for the transfer state from the photoconductive drum 110 to the intermediate transfer belt 360 in the primary transfer part, the low hardness, the low load and the high fluidity of the toner are utilized, so that the occurrence of dropout is avoided.

For the transfer state of the intermediate transfer belt 360 in the secondary transfer part, the high hardness and the high load of the toner are used. However, since the intermediate transfer belt 360 is made of fluorine and the toner is very fluid, the occurrence of dropout is avoided.

< To reduce toner scattering >

(1) The smoothing roller 330 is provided close to the upstream side of the primary transfer position T1.

(2) The amount of an additive having a large particle diameter is set to 0.5 to 4.0 wt% (preferably about 0.7 wt%), and the amount of an additive having a small particle diameter is set to 1.5 to 4.0 wt% (preferably about 2.0 wt%).

The fluidity of the toner is set to 0.35 g/cc and the amount of electrostatic charge is set to -10 uC/g or more.

(3) The surface roughness of the intermediate transfer belt 360 is set to Rmax 1 µm or less (preferably 0.7 µm).

The surface resistance of the intermediate transfer belt 360 is set to 10⁹9; to 10¹² Ω/, and the volume resistance is set to 10⁹8; to 10¹² Ωcm.

The following function and effect can be achieved by setting the above parameters:

That is, in the primary transfer part, wrinkles of the intermediate transfer belt 360 are reduced by the smoothing roller 330 and scattering of toner.

In the secondary transfer part, toner is stably conveyed onto the intermediate transfer belt 360 and scattering is reduced.

< To reduce costs>

(1) The endless intermediate transfer belt 360 is formed by coating a sheet-shaped PET material on which aluminum is deposited with urethane paint in which PEFT particles and SnO as a conductor are dissolved, and joining both ends by ultrasonic welding.

A height difference by connecting both ends is set to be 50 µm or less, and preferably 30 µm or less. The elastic modulus of the paint is set to be about 1.5·104 kgf/cm2. The surface resistance of the paint is set to be about 10⁸ to 10¹² Ω/, and the surface roughness is set to be Rmax 1 µm (preferably 0.7 µm) or less. Regarding the construction of an electrode, a conductive layer is printed on the surface of the aluminum at one end, and a bias voltage is applied by a roller electrode (1 MΩ or less).

(2) The high-voltage power source has a current absorption type constant voltage control in the primary transmission part and applies a primary transmission voltage until the secondary transmission is completed.

The transfer efficiency and the cleaning property can be increased by setting the parameters as described under (1) and (2).

The primary transfer roller serves only as the auxiliary roller and it is not required for it to fulfill the function as an electrode.

Furthermore, the deterioration of an image due to interference during simultaneous primary and secondary transmission can be avoided by constructing the electrode and the power source as described above.

As described above, according to the intermediate transfer unit, the shrinkage of the intermediate transfer belt between the primary transfer position and the drive roller is reduced, so that the speed of the intermediate transfer belt is stable in the primary transfer position and as a result, the primary transfer in a satisfactory condition can easily be achieved.

Although the embodiments or examples of the present invention have been described above, the present invention is not limited to the above embodiments or examples and can be appropriately varied within the scope of the present invention.

For example, the following changes are possible.

< For satisfactory transfer to rough paper (bound paper)>

(1) The outer diameter of the elastic body of the secondary transfer roller 380 is set to 25 mm, the outer diameter of the shaft is set to 15 mm, the length of the elastic body in the direction of the shaft is set to 323 mm, the hardness of the secondary transfer roller is set to 60 ± 10º (preferably about 60±5º), and the load on the auxiliary roller 350 by the secondary transfer roller is set to 5.0 to 9.0 kg (or 15 gf/mm to 27 gf/mm), and preferably about 7.0 kg (or 21 gf/mm).

2) The additive amount of large particle diameter is set to 0.5 to 4.0 wt% (preferably about 0.7 wt%), and the additive amount of small particle diameter is set to 1.5 to 4.0 wt% (preferably about 2.0 wt%). The surface coverage can be calculated according to the following expression 1, and the surface coverage for toner having a mother particle diameter of 7 µm to which silica having a particle diameter of 40 nm is added at 0.7 wt% and silica having a particle diameter of 9 nm at 2.0 wt% is 2.8. [Expression 1]

R: Outer diameter of the toner mother particle

ri: outer diameter of the i-th additive

&rho;: Density of the toner mother particle

&rho;i: density of the i-th additive

Amount (wt%) of the 1st additive added to the toner mother particle

i: i-th additive

n: number of types of additives

(3) The toner amount before secondary transfer, that is, the toner amount on the intermediate transfer belt 360 is set to 1.5 mg/cm² or less.

A satisfactory transfer state can also be achieved on rough paper such as banknote paper having a rough surface as a recording medium by setting the parameters as described above in (1) to (3).

That is, if the linear pressure of the secondary transfer roller 380 is set to 20 gf/mm or more, a sufficient electric field can be formed in a toner layer by setting a concave portion of rough (bonded) paper to a toner image on the intermediate transfer belt 360 and bringing the concave portion close to the toner image, whereby failure of transfer due to discharge in a high electric field is reduced. Furthermore, if the hardness of the secondary transfer roller 380 is set to 50° or more, in the case where the hardness is measured by an Asker-C hardness meter, no increase in torque occurs due to excessive roller width, and the state in which paper is conveyed is also stabilized by a stable nip.

Furthermore, since the fluidity of the toner is secured and the adhesion strength to the intermediate transfer belts can be reduced by adding a small particle diameter additive so that the surface coating of the additive for the toner is 2.0 or more, the efficiency of transfer to rough paper can be increased. Furthermore, an additive is hardly embedded in a toner mother particle or hardly detached during long-term use by adding the large particle diameter additive as described above, whereby the increase in durability and transferability to rough paper are compatible.

Furthermore, the transfer efficiency of the toner can be increased by reducing the amount of toner before secondary transfer as described above. That is, if a primary transfer image consisting of two overlapped layers of complete images on the photoconductive drum is transferred to rough paper, a potential difference to be applied between the surface of the intermediate transfer medium and the surface of a recording medium can be reduced and failure of transfer due to discharge can be avoided by setting the total amount of toner in the primary transfer image to 1.5 mg/cm² or less.

< To avoid the occurrence of a dropout>

(1) The intermediate transfer belt 360 is made of ethylene tetrafluoroethylene (ETFE) in which carbon black and others are dissolved as a conductor, polyethylene terephthalate (PET) is produced by depositing aluminum and further coating with urethane paint including fluorine particles, or polyamide in which carbon black or others are dissolved as a conductor.

The photoconductive drum 110 is made of polycarbonate.

(2) The outer diameter of the elastic body of the primary transfer roller 320 is set to 22 mm, the outer diameter of the shaft is set to 12 mm, the length of the elastic body in the direction of the shaft is set to 358 mm, the hardness of the primary transfer roller 320 is set to 45 ± 5º, and the load on the photoconductive drum 110 by the primary transfer roller is set to 1.0 to 3.5 kg.

The outer diameter of the elastic body of the secondary transfer roller 380 is set to 25 mm, the outer diameter of the shaft is set to 15 mm, the length of the elastic body in the direction of the shaft is set to 332 mm, the hardness of the secondary transfer roller 380 is set to 60 ± 10º (preferably about 60 ± 5º), and the load on the auxiliary roller 350 by the secondary transfer roller is set to 5.0 to 9.0 kg (or 15 gf/mm to 27 gf/mm), and preferably about 7.0 kg or about 21 gf/mm.

(4) The additive amount with a large particle diameter is set to 0.5 to 4.0 wt% (preferably about 0.7 wt%), and the additive amount with a small particle diameter is set to 1.5 to 4.0 wt% (preferably about 2.0 wt%). The surface coverage can be calculated according to Expression 1, and the surface coverage of the additive for toner with a mother particle diameter of 7 µm to which silica with a particle diameter of 40 nm is added at 0.7 wt% and silica with a particle diameter of 9 nm at 2.0 wt% is 2.8.

The fluidity of the toner is set to about 0.35 g/cc.

With parameters such as (1) to (3), a satisfactory transfer state to a recording medium such as OHP can also be achieved, whereby the surface is smooth. That is, as for the state of transfer from the photoconductive drum 110 to the intermediate transfer belt 360 in the primary intermediate transfer part, the low hardness, the low load and the high fluidity of the toner are utilized, so that the occurrence of dropout is avoided.

For the transfer state of the intermediate transfer belt 360 in the secondary transfer part, the high hardness and the high load of the toner are used. However, since the intermediate transfer belt 360 is made of fluorine and can be easily released from a mold, the occurrence of dropout is avoided.

Furthermore, since the concentration of the transfer pressure on a linear image on the intermediate transfer belt 360 is avoided, since the hardness of the secondary transfer roller is set to 70º or less in the case where the hardness is measured with an Asker-C hardness meter, the occurrence of dropout is avoided.

Furthermore, since the toner fluidity is ensured and the adhesion force to the intermediate transfer belt can be reduced by adding an additive with a small particle diameter so that the surface coverage of the additive to the toner is 2.0 or more, the occurrence of dropout is prevented. Furthermore, an additive is hardly embedded in a toner mother particle or hardly detached in long-term use by adding the additive with a large particle diameter as described above, the increase in durability and transferability to rough paper are compatible.

Furthermore, since the thickness of a toner layer is limited by reducing the amount of toner before secondary transfer as described above, the pressure on the toner is uniform and the occurrence of dropout is avoided. <

To prevent the back of the recording medium S, such as paper, from becoming dirty>

When the secondary transfer roller 380 is in direct contact with the intermediate transfer belt 360, an electric field in a direction in which toner is returned from the secondary transfer roller 380 to the intermediate transfer belt 360 (for example, the voltage of about 0 to -600 V) is applied to the secondary transfer roller 380, and when the seam of the intermediate transfer belt 360 is located at the secondary transfer position T2, the secondary transfer roller 380 is released.

Toner adhering to the secondary transfer roller 380 is reduced, and a stain adhering to the back of a recording medium S is reduced by setting the parameters as described above.

That is, although toner which cannot be removed by the cleaners 370 is left in a step region of the seam of the intermediate transfer belt 360, since the secondary transfer roller 380 is not in direct contact with the region, the secondary transfer roller 380 can be cleaned at another location by a bias voltage, and a stain caused by toner on the secondary transfer roller 380 can be reduced, whereby a stain on the back side of a recording medium can be reduced.

Furthermore, according to the intermediate transfer unit of the invention, it is possible to prevent a phenomenon in which toner adheres to the secondary transfer roller by directly contacting the secondary transfer roller with the joint of the intermediate transfer medium, and thereby it can be prevented that the back side of a recording medium is soiled, wherein the intermediate transfer unit for satisfactory transmission can be easily achieved.

Furthermore, according to the intermediate transfer unit of the invention, since the intermediate transfer belt has excellent cast-peeling properties, toner is easily peeled off in secondary transfer. Furthermore, since the hardness of the secondary transfer roller is set to 70° or less in the case where the hardness is measured with an Asker-C hardness meter, the concentration of the transfer pressure on a linear image on the intermediate transfer belt 360 can be avoided and as a result, when a thin line image is transferred to a recording medium having a smooth surface, the occurrence of so-called dropout can be reduced.

Furthermore, according to the intermediate transfer unit of the invention, since toner is covered with a sufficient amount of additive, the force of the toner adhering to the intermediate transfer belt can be reduced. Furthermore, since a recording medium having a rough surface is printed on the intermediate transfer belt under sufficient linear pressure, a concave portion of the recording medium can be brought close to a toner image on the intermediate transfer belt, and as a result, a satisfactory transfer state can be achieved for rough paper such as banknote paper which is a recording medium having a rough surface.

The present invention may also be further modified as follows.

< To stabilize the efficiency of the primary transfer roller>

(1) A high voltage power source which has a constant current control when the impedance of the primary transmission is large (about 30 MΩ or more) and a Constant voltage control when the impedance is small (about 30 MΩ or less) is used.

As a result, even if there is a variation in the toner amount (film thickness), environment and resistance of a member, the transfer is carried out satisfactorily.

(2) The surface resistance of the intermediate transfer belt 360 is set to 10⁸ to 10¹² Ω/, and the volume resistance is set to 10⁹⁹ to 10¹² Ω cm.

The primary transfer roller 320 is a roller with the diameter of 22 mm in which an elastic layer of urethane resin in which carbon is dissolved is formed on the peripheral surface of a metallic shaft with the diameter of 12 mm. The resistance of the roller is set to 10⁵ to 10⁸ Ω (preferably about 10⁷ Ω), the hardness is set to 45±5°, and the load on the photoconductive drum 110 by the primary transfer roller is set to 1.0 to 3.5 kg (preferably about 2.5 kg).

Transmission is enabled at 1200 V or less by setting the resistance values in the above range.

The occurrence of a so-called misfire can be avoided by setting the hardness and the load in the above range.

Hardness is measured by an Asker-C hardness meter, which is known to those skilled in the art. Such a hardness meter is called an indentation hardness meter and it should be noted that the thickness of an elastic layer has an influence on the value of hardness measured by such a hardness meter. Hardness in the sense of the present invention does not refer to the result of measuring the hardness of an elastic body itself which forms an elastic layer, but refers to the result of measurement in a state in which an elastic layer is formed on a roll.

(3) The amount of an additive having a large particle diameter is set to 0.5 to 4.0 wt% (preferably about 0.7 wt%), and the amount of an additive having a small particle size is set to 1.0 to 4.0 wt% (preferably about 2.0 wt%).

The large particle diameter additive is mainly required to increase the stability of the durability of the toner, and in view of the above, the more of the above additive, the better. However, if the above additive amount exceeds 4.0 wt%, the fluidity of the toner deteriorates. That is, too much of the above additive causes the occurrence of dropout and others, which is undesirable.

Now, the additive having a small particle diameter is mainly required to increase the transferability to rough paper, and in view of this, a large amount of the above additive is better. However, if the amount of the above additive exceeds 4.0 wt%, a film layer containing flowing silica is easily formed on the photoconductive drum 110 and the intermediate transfer belt 360, which is undesirable.

The deterioration of an image due to interference during simultaneous primary and secondary transmission can be avoided and the capacity of the high-voltage power source can be reduced to the minimum under the conditions as described above in (1) to (3).

(4) The particle diameter of the toner is set to 9 µm or less.

The reason is that the resolution is deteriorated if the particle diameter is 9 µm or more.

Fig. 6(a) to 6(c) show the particle size distribution of the toner used this time. The particle size distribution of the above toner is measured with a coal tar counter model TA-II. The aperture is 100 µm and ISOTON-II is used as an electrolyte. In a table shown in Fig. 6(a), the number is shown in a right panel, the volume in a left panel, the result of measurement in the lower column, and a calculated value based on the result of measurement is shown in the upper column. However, the above volume means a volume in the case where a measured toner particle is considered as a sphere.

In the curves shown in Fig. 6(b) and 6(c), a bar graph shows numerical data and a connected line shows cumulative data.

In the table shown in Fig. 6(a), the meaning of each point showing the result of measurement in the lower column is as follows:

DIF N: most basic data and represents numeric data , data representing number of toners input through an input/output device for each channel.

DIF %: shows above numerical data (DIF N) for each channel in %.

CUM N: shows data obtained by accumulating the above numerical data (DIF N).

CUM %: shows data obtained by accumulating the above DIF %.

The meaning of each point representing a calculated value in the above column is as follows:

25.4 um: indicates cumulative percentage value of 25.4 um or more.

6.35 um: indicates cumulative percentage of 6.35 um or less.

KURTOSITY: shows kurtosis of the distribution. An image which is satisfactory in transferability and whose resolution never deteriorates can be obtained by setting the particle size distribution in volume to 0.8 or more and setting the particle size distribution in number to 0.3 or more.

DISTORTION: indicates distortion of distribution. An image that is satisfactory in transferability and never deteriorates in resolution can be obtained by setting distortion to 0.6 or less in an absolute value in the particle size distribution by volume, and setting distortion to 0.1 or less in an absolute value in the particle size distribution by number.

Mean: shows arithmetic mean for particle size.

25%: indicates particle size value when cumulative percentage reaches 25%. (See the curves in Fig. 6(b) and 6(c).)

50%: indicates particle size value when cumulative percentage reaches 50%. (See curves in Fig. 6(b) and 6 (c).)

75%: indicates particle size value when cumulative percentage reaches 75%. (See curves in Fig. 6(b) and 6(c).)

CV%: Coefficient (%) of change. An image which is satisfactory in transferability and whose resolution never deteriorates can be obtained by setting both the particle size distribution in volume and the particle size distribution in number to 28% or less.

SDu: Standard deviation: (um).

(S) Shape of the toner

With respect to the shape factor of the toner, 100 pieces of toner images are sampled which are magnified up to 500 times at random using an FE-SEM (S-800) manufactured by Hitachi, Ltd., for example, the image information is analyzed using an image analyzer Luzex III manufactured by Nireco, Ltd., for example, and values calculated according to the following expressions are defined as a shape factor.

Form factor (SF - 1) = (MXLNG)²/AREA·π/4·100

Form factor (SF - 2) = (PERI)²/AREA·1/4 ?·100

In the above expressions, MXLNG means the absolute maximum length of the toner, PERI means the peripheral length of the toner, and AREA means the projected area of the toner.

The shape factor SF-1 shows the degree of roundness of the toner, and that of SF-2 shows the degree of irregularity of the toner. It is preferable that the shape factor SF-1 of the toner is 100 to 150, and it is even more preferable that SF-1 is 100 to 130. It is preferable that the shape factor SF-2 of the toner is 100 to 140, and it is even more preferable that SF-2 is 100 to 124. Transfer efficiency in primary and secondary transfers is increased by setting the form factors SF-1 and SF-2 as described above.

In the embodiment of the present invention, an electric field for transfer in each transfer position can be concentrated on the transfer position since primary or secondary transfer means serving as a transfer electrode for applying a transfer voltage to a transfer position is in contact with each transfer position even when toner having the high fluidity of A.D. 0.35 g/cc or more is used. Furthermore, transfer means in each transfer position are pressed and toner having a shape which is approximately spherical and the surface thereof is smooth is used. Therefore, a toner image can be easily compressed in the direction of height in a transfer position so that cohesion in the toner is increased. As a result, the transfer efficiency is increased and at the same time, the occurrence of dropout can be better prevented. Blurring of a toner image due to mechanical force caused by slight difference in speed between the photoconductive drum or a recording medium and the intermediate transfer belt at a transfer position and others can also be better prevented.

There is also the effect that the melting of each toner is accelerated because a toner image can be easily compressed in the direction of height without causing the turbulence of an image, and an image satisfactory in color and transparency can be formed when a toner image is fixed on a recording medium.

< To stabilize the secondary transmission efficiency>

(1) A high voltage power source having a constant current control when the impedance of the secondary transmission is large (about 20 MΩ or more) and a Constant voltage control when the impedance is small (about 20 MΩ or less) is used.

As a result, transfer is carried out satisfactorily even if there is dispersion in the type of paper, environment and resistance of a member.

(2) The surface resistance of the intermediate transfer belt 360 is set to 10⁹9 to 10¹² Ω/, and the volume resistance is set to 10⁹9 to 10¹² Ωcm.

The secondary transfer roller 380 is a roller with a diameter of 25 mm on which an elastic layer is formed by applying or melting an ion-conductive material such as lithium perchlorate in urethane resin, and is formed on the peripheral surface of the metal shaft with a diameter of 15 mm. The resistance of the roller is set to 10⁶ to 10⁸ Ω, the hardness to 60 ± 5°, and the load on the auxiliary roller 350 by the secondary transfer roller is set to 5.0 to 9.0 kg (preferably about 7.0 kg).

Transmission is enabled at 4000 V or less and 200 uA or less by setting the resistance to the above ranges.

Hardness is measured by an Asker-C hardness meter, which is known to those skilled in the art, and as described above, hardness described in the present invention does not mean the result of measuring an elastic body which itself forms an elastic layer, but means the result of measurement in a state in which an elastic layer is formed on a roll.

The auxiliary roller 350 is earthed.

(3) The amount of additive used in the toner is set to the amount of an additive with a large particle diameter to 0.5 to 4.0 wt% (preferably about 0.7 wt%) and the amount of small particle diameter additive is set to 1.5 to 4.0 wt% (preferably about 2.0 wt%).

The reason for this is as described above.

< To avoid the occurrence of a dropout>

The durability of the intermediate transfer belt can be increased by setting the load of the secondary transfer means to be larger than that of the primary transfer means. This is based on the inventor's knowledge that the filming of the toner on the intermediate transfer belt is caused by the additive of the toner left on the intermediate transfer belt and embedded in the intermediate transfer belt by the cleaning means such as the cleaning sheet for cleaning the surface of the intermediate transfer belt; the isolation of an additive often occurs when colors overlap in order in primary transfer; Since an additive which is isolated from the toner and adheres to the intermediate transfer belt again adheres to a relatively soft toner and a relatively soft paper phase in comparison with the intermediate transfer belt, when the above additive is removed by a load which exceeds a fixed load among toner or paper, the additive can be removed from the intermediate transfer belt.

In general, the primary transfer roller 320 is always pressed onto the intermediate transfer belt 360 and the secondary transfer roller 380 is pressed onto the intermediate transfer belt 360 when a full-color image in which the overlapping of colors is finished is transferred. However, the secondary transfer roller is removed from the intermediate transfer belt 360 while images of each color are formed in order. However, since a phenomenon occurs (so-called reverse transfer) in which a part of an image of the n-th color is returned from the intermediate transfer belt to the photoconductive drum when an image of the ("n" + 1)-th color is superimposed on the image of the n-th color already formed on the intermediate transfer belt, if the load of the primary transfer roller 320 is set to a load exceeding a load by which an isolated additive on the intermediate transfer belt can be removed by toner according to the above constitution, it is desirable that the load on the secondary transfer roller 380 is set to a fixed load or more and the load of the primary transfer roller 320 is set to a fixed load or less. A load (a load required to remove an additive from the intermediate transfer belt under toner) obtained in an experiment according to the embodiment of the present invention is 150 g/cm or more, and it is desirable that the above load is 200 g/cm or more.

In order to avoid back transfer in primary transfer, a load obtained in an experiment according to the embodiment of the present invention is 100 g/cm or less, and it is preferable that the above load is 70 g/cm or less.

Therefore, the ratio of the respective loads on the primary transmission means and secondary transmission means is 1.5 or more, and it is desirable that the above ratio is 2 or more.

In order to prevent the primary and secondary transfer rollers from being bent due to a load, it is necessary that the shaft of each roller be formed with a strength according to the load, and therefore it is desirable that the outer diameter of the shaft of the secondary transfer roller be larger than that of the primary transfer roller.

According to the intermediate transfer unit of the present invention, occurrence of dropout in transfer is prevented, satisfactory transfer to rough paper can be realized, and further durability of the intermediate transfer belt can be increased.

The following change is also possible.

< To avoid the occurrence of a dropout >

Since resonance between the primary transfer means and the secondary transfer means can be avoided by distinguishing the frequency of vibration caused by impact when the secondary transfer means comes into contact with the intermediate transfer belt from the frequency of the primary transfer means by setting the hardness of the secondary transfer roller 380 to be higher than the hardness of the primary transfer means 320, the vibration of the intermediate transfer belt and the change in speed respectively caused by the contact and non-contact of the secondary transfer means with the intermediate transfer belt can be avoided. In particular, to reduce time required between a paper and a subsequent paper and speed up the output of an image by switching the state of the secondary transfer means from non-contact with the intermediate transfer belt to contact before primary transfer is completed and starting secondary transfer, the above is very effective. It is more effective to distinguish the hardness of all the rollers arranged such that each roller contacts the intermediate transfer belt. However, in the intermediate transfer unit, the quality of a toner image on the intermediate transfer belt or the quality of a toner image on a recording medium is mainly determined by a contact state between the primary or secondary transfer means and the intermediate transfer belt in the primary or secondary transfer position. Therefore, at least by constructing as in the embodiment of the present invention, a satisfactory effect can be obtained by avoiding vibration in the above transfer position.

Furthermore, the vibration of the intermediate transfer belt can be further satisfactorily prevented by setting the hardness of the secondary transfer roller 380 to be higher than the hardness of the primary transfer roller 320 by 10° or more.

Even if a belt having a joint is used for the intermediate transfer belt, a vibration caused when the primary (or the secondary) transfer means transfers to the joint in the primary (or the secondary) transfer position can be resonated by the secondary (or the primary) transfer means by setting the hardness of the secondary transfer roller 380 so that it is similarly higher than the hardness of the primary transfer roller 320.

The following change is also possible.

< To stabilize the efficiency of primary transmission>

(1) A high voltage power source which has constant current control when the impedance of primary transmission (the ratio of the output voltage and the output current of a power source for primary transmission is not shown) is large (about 30 MΩ or more) and has constant voltage control when the impedance is small (about 30 MΩ or less) is used. The above constant current is set to 15 uA and the above constant voltage is set to 450 V.

Here, even if there is dispersion in the amount (film thickness) of toner, environment, and resistance of a member, satisfactory transfer as shown in Table 1 is carried out.

For comparison, Table 2 shows the result when simple constant current control (set to 15 uA) is performed and Table 3 shows the result when simple constant voltage control (set to 450 V) is performed.

(2) The surface resistance of the intermediate transfer belt 360 is set to 10⁹9 to 10¹² Ω/ and the volume resistance is set to 109 to 10¹² Ωcm.

The primary transfer roller 320 is a roller with the outer diameter of 22 mm and the width of 358 mm on a shaft of 12 mm in diameter. It is made of urethane in which carbon is dispersed, the resistance is set to 10⁶ to 10⁹9 Ω (preferably about 10⁷ Ω), the hardness is set to 45±5°, and a load on the photoconductive drum 110 by the primary transfer roller is set to 1.0 to 3.5 kg (preferably about 3.5 kg). That is, the above load is set to 28 to 98 g/cm (preferably about 70 g/cm).

Transmission is enabled at the relatively low voltage of 1200 V or less by setting the resistance value in the above range.

The occurrence of a so-called misfire can be avoided by setting the hardness and load to the above range. [Table 1] [Table 2] [Table 3]

(3) For the amount of additive used in the toner, the amount of an additive having a large particle diameter (the primary particle diameter of 40 µm) is set to 0.5 to 4.0 wt% (preferably about 0.7 wt%), and the amount of an additive having a small particle diameter (the primary particle diameter of 14 µm) is set to 1.5 to 4.0 wt% (preferably about 2.0 wt%).

The large particle diameter additive is mainly required to increase the durable stability (density stability) of the toner, and in view of the above, the higher the amount of the above additive, the better. However, if the amount of the above additive exceeds 4.0% by weight, the fluidity of the toner is deteriorated. Therefore, too much amount of the above additive causes the occurrence of dropout and others and is therefore undesirable.

Now, the additive having a small particle diameter is mainly required to increase the transferability to rough paper, and in view of the above, the higher the amount of the above additive, the better. However, if the amount of the above additive exceeds 4.0 wt%, a silicate layer is easily formed on the photoconductive drum 110 and the photoconductive drum 360, which is undesirable.

A high voltage power source which has constant current control when the impedance of secondary transmission (the ratio of the output voltage and the output current of a power source for secondary transmission not shown) is large (about 20 MΩ or more) and constant voltage control when the impedance is small (about 20 MΩ or less) is used. The constant current is set to 30 uA and the constant voltage is set to 600 V.

Here, even if there is dispersion in the type of paper, environment and resistance of a link, transfer is carried out satisfactorily.

(2) The surface resistance of the intermediate transfer belt 360 is set to 10⁸ to 10¹² Ω/, and the volume resistance is set to 10⁹9 to 10¹² Ωcm.

The secondary transfer roller 380 is a roller with the outer diameter of 25 mm and the width of 332 mm on a shaft of 15 mm in diameter. Ion-conductive material such as lithium perchlorate is applied to the transfer roller, the resistance is set to 10⁶ to 10⁸ Ωcm, and the hardness is set to 60 ± 5°, and a load on the auxiliary roller 360 by the secondary transfer roller is set to 5.0 to 9.9 kg (preferably about 7.0 kg.) That is, the above load is set to 150 to 270 g/cm (preferably about 210 g/cm).

Transmission is enabled at 4000 V or less and 200 uA or less by setting the resistance in the above range.

The auxiliary roller 350 is earthed.

(3) For the amount of additive used in the toner, the amount of an additive having a large particle diameter is set to 0.5 to 4.0 wt% (preferably about 0.7 wt%), and the amount of an additive having a small particle diameter is set to 1.5 to 4.0 wt% (preferably about 2.0 wt%).

The reason for this is as described above.

According to the above conditions, the deterioration of an image due to interference during simultaneous primary and secondary transmission can be avoided and the Capacity of the high voltage power source can be reduced to a minimum.

As described above, according to the intermediate transfer unit of the present invention, satisfactory transferability can be ensured without depending on a print pattern because the control of the high-voltage power source is optimized.

In addition, transfer of required and minimum voltage and current and defective image can be avoided due to abnormal discharge and others since the resistance of the primary transfer member and the intermediate transfer belt is optimized.

In addition, the shift of images in primary transfer can be avoided and a dropout phenomenon can be avoided because the hardness of the primary transfer member and a load on the photoconductive drum by the primary transfer member are optimized.

In addition, the dropout phenomenon can be avoided since the amount of an additive with a small particle diameter of the additives added to the toner is optimized, and the deterioration of density due to aging can be avoided since the amount of an additive with a large particle diameter is optimized.

The following change is also possible.

< To stabilize the secondary transmission efficiency>

(1) A high-voltage power source which has constant current control when the impedance of secondary transmission (the ratio of the output voltage and the output current of a power source for secondary transmission not shown) is large (about 20 MΩ or more) and has constant voltage control when the Impedance is small (about 20 MΩ or less) is used. The constant current is set to 30 uA and the constant voltage is set to 600 V.

As a result, as shown in Table 4, even if there is dispersion in the type of paper, environment and resistance of a member, transfer is carried out satisfactorily.

For comparison, Table 5 shows the result for simple constant current control (current is set to 30 uA) and Table 6 shows the result for simple constant voltage control (voltage is set to 600 V). [Table 4] [Table 5] [Table 6]

According to the intermediate transfer unit of the present invention, satisfactory transferability can be ensured without being influenced by the type of recording medium and the environment because the control of the high-voltage power source is optimized.

In addition, transfer of required and minimum voltage and current is enabled, and defective image can be avoided due to abnormal discharge and others because the resistance of the primary transfer member and the intermediate transfer belt is optimized.

In addition, the shift of images in secondary transfer can be avoided and satisfactory transfer to a recording medium with a rough surface such as bound paper is also enabled because the hardness of the secondary transfer member and a load on the auxiliary drum by the secondary transfer member are optimized.

In addition, the dropout phenomenon can be avoided since the amount of an additive with a small particle diameter of the additives added to the toner is optimized, and the deterioration of density due to aging can be avoided since the amount of an additive with a large particle diameter is optimized.

The following change is also possible.

< To stabilize the efficiency of primary transmission>

(1) A high-voltage power source which has constant current control when the impedance of primary transmission (the ratio of the output voltage and the output current of a power source for primary transmission is not shown) is large (about 20 MΩ or more) and has constant voltage control when the impedance is small (about 20 MΩ or less) is used. The above constant current is set to 30 uA and the above constant voltage is set to 600 V.

Here, even if there is dispersion in the amount (film thickness) of toner, environment, and resistance of a member, satisfactory transfer as shown in Table 1 is carried out.

(2) The surface resistance of the intermediate transfer belt 360 is set to 10⁹9 to 10¹² Ω/ and the volume resistance is set to 10⁹9 to 10¹² Ωcm.

The secondary transfer roller 380 is a roller with the outer diameter of 25 mm and the width of 332 mm on a shaft of 15 mm in diameter. Ion-conductive material such as lithium perchlorate is applied to the secondary transfer roller, the resistance is set to 3 x 10⁷ to 1 x 10⁸ Ω in the low temperature and low humidity environment, and to 1 x 10⁷ to 1 x 10⁸ Ω in the high temperature and high humidity environment, the hardness is set to 60 ± 5°, and a load on the auxiliary drum 350 by the secondary transfer roller is set to 5.0 to 9.0 kg (preferably about 7.0 kg). That is, the above load is set at 150 to 270 g/cm (preferably about 210 g/cm).

Transmission is enabled at the voltage of 4000 V or less and 200 uA or less by setting the resistance value in the above range.

The auxiliary roller 350 is earthed.

(3) For the amount of additive used in the toner, the amount of an additive having a large particle diameter is set to 0.5 to 4.0 wt% (preferably about 0.7 wt%), and the amount of an additive having a small particle diameter is set to 1.5 to 4.0 wt% (preferably about 2.0 wt%).

The reason is as described above.

Table 7 shows an example of an experiment of the above primary transmission part and the secondary transmission part. [Table 7]

(*) Fail of paper transfer at small size occurs in the environment of 10ºC, 15%, RH.

As shown in Experiment No. 1, satisfactory secondary transmittability and reduction of the capacity of the primary transmission power source can be realized by using a member with small variation in resistance due to the environment for the primary transmission roller and using a member with large variation in resistance due to the environment for the secondary transmission roller.

According to the intermediate transmission unit of the invention, since the change in resistance of the primary transmission member and the secondary transmission member due to the environment is optimized, the capacity of the primary transmission power source can be reduced and no failure of transmission in the secondary transmission part occurs in both the environment of low temperature and low humidity and the environment of high temperature and high humidity.

Fig. 7 is a side view showing a modification of the intermediate transfer unit 300.

In this variation, the intermediate transfer unit 300 is formed with a roller electrode 600 which is an example of the primary transfer member. Other portions in this intermediate transfer unit are the same as those in Fig. 4.

The roller electrode 600 is a conductive elastic member about 10 mm in diameter and 5 mm in width, is disposed at the end of the intermediate transmission belt 360, and is in light contact with the belt. Voltage is applied to the roller electrode 600 from a high-voltage power source for primary transmission (not shown).

Fig. 8 shows an equivalent circuit for primary transmission. "V1" denotes the voltage of a primary Transfer power source, "R1" denotes apparent resistance generated when a charged photoconductive drum, an intermediate transfer belt equipped with a resistance layer are rotated or moved, "RT" denotes the resistance of a primary transfer member and contact resistance, and "I1" denotes current for enabling primary transfer (current required for primary transfer.)

Fig. 9 shows an equivalent circuit in the case where primary transmission and secondary transmission are carried out simultaneously. "V2" denotes the voltage of a secondary transmission power source, "R2" denotes apparent resistance generated by a secondary transmission member and a recording medium, and "I2" denotes current for enabling secondary transmission (current required for secondary transmission). It is the electric potential at a point A that is important in Fig. 9. If this electric potential varies greatly, the point A is outside of a suitable transmission electric field and primary transmission fails. To avoid the above failure, "I2" is set to flow on the side of the primary transmission power source by setting it such that RT < R1. Concretely, this means that the resistance of the primary transmission member is set to 1 MΩ. or less is set.

If the relationship of "I1 > I2" is satisfied under the above conditions, the transmission failure during simultaneous primary and secondary transmission is avoided.

However, depending on environmental conditions and the type of a recording medium, 11 is smaller than 12. In this case, since current cannot be supplied from the primary transmission power source, an electric potential at the point A is increased and transmission failure occurs.

"IT" denotes the current of the primary transmission power source and under the above condition, which can be shown by an expression, IT = I1 - I2. Therefore, under the condition "I1 < I2", the current IT of the primary transmission power source requires a function (a current absorbing function) to output a negative current while outputting a positive voltage.

Fig. 10 illustrates a case where a resistor Rx is connected in parallel to the high voltage power source. The current ITO of the primary transmission power source can be expressed by an expression "ITO = Ix + (I1 - I2)" using a current Ix flowing in the resistor Rx and the above currents I1 and I2. Therefore, since ITO is positive, even if "I1 - I2 < 0", the electric potential at the point A can be maintained.

The following change is also possible.

The following essentially concerns a transfer process.

(1) The endless intermediate transfer belt 360 is formed by coating a sheet-shaped PET material on which aluminum is deposited with urethane paint in which PEFT particles and SnO as a conductor are dissolved, and joining both ends by ultrasonic welding.

A height difference by connecting both ends is set to 50 µm or less, and preferably 30 µm or less. The elastic modulus of the paint is set to about 1.5· 10⁴ kgf/cm². The surface resistance of the paint is set to about 10⁸ to 10¹² Ω/, and the surface roughness is set to Rmax 1 µm (preferably 0.7 µm) or less. Regarding the construction of an electrode, a conductive layer is coated on the surface of the aluminum printed at one end and a bias voltage is applied through a roller electrode 600 (1 MΩ or less). The primary transfer member may also be a brush, a blade and the like other than the roller electrode in this embodiment. It is important that the resistance of the primary transfer member is 1 MΩ or less.

The transfer efficiency and cleaning property can be increased as described above.

(2) The high-voltage power source has a current absorption type constant voltage control in the primary transmission part and applies a primary transmission voltage until the secondary transmission is completed.

The primary transfer roller (the primary transfer auxiliary roller) serves only as an auxiliary roller.

Even if the secondary transmission current is larger than the primary transmission current, the deterioration of an image due to interference during simultaneous primary and secondary transmission can be avoided by designing the electrode and the power source as described above.

Table 8 presents the result of the above experiment. [Table 8]

O: No deterioration in image quality

Δ: Change visible, but within permissible limits

x: Significant deterioration in image quality

The difference between the comparative example and this embodiment is only the difference caused by the high voltage power source.

When a secondary transfer current value is larger than a primary transfer current value by 10 uA or more, significant deterioration of image quality occurs. However, according to the present invention, a high-quality image can be obtained regardless of the environment and the kind of paper.

< To stabilize the efficiency of primary transmission>

(1) The primary transmission high-voltage power source is set to 500 V. Current flowing through during primary transmission is about 20 to 50 uA.

Since the primary transfer roller (primary transfer auxiliary roller) 320 and the toner additive used are the same as those in the above-described embodiment, the description thereof is omitted.

The following change is also possible.

The following essentially concerns a transfer process.

(1) The endless intermediate transfer belt 360 is formed by coating a sheet-shaped PET material on which aluminum is deposited with urethane paint in which PEFT particles and lead oxide as a conductor are dissolved, and joining both ends by ultrasonic welding.

A height difference by joining both ends is set to 50 µm or less, and preferably 30 µm or less. The elastic modulus of the paint is set to about 1.5· 10⁴ kgf/cm². The surface resistance of the paint is set to about 10⁸ to 10¹² Ω/, and the surface roughness is set to Rmax 1 µm (preferably 0.7 µm) or less. Regarding the construction of an electrode, a conductive layer is printed on the surface of the aluminum at one end, and a bias voltage is applied through a roller electrode 600 (1 MΩ or less). The primary transfer member may also be a brush, a blade, and the like other than the roller electrode in this embodiment. It is important that the resistance of the primary transfer member is 1 MΩ or less.

The transfer efficiency and cleaning property can be increased as described above.

(2) A resistance of 5 MΩ is connected in parallel to the primary transmission high voltage power source. The primary transmission high voltage power source applies a primary transmission voltage until the secondary transmission is completed.

The primary transfer roller (the primary transfer auxiliary roller) serves only as an auxiliary roller.

Even if the secondary transmission current is larger than the primary transmission current, the deterioration of an image due to interference during simultaneous primary and secondary transmission can be avoided by designing the electrode and the power source as described above.

Table 9 presents the result of the above experiment. [Table 9]

O: No deterioration in image quality

Δ: Change visible, but within permissible limits

x: Significant deterioration in image quality

Differences between the comparative example and this embodiment depend only on whether a resistor is connected in parallel with the high voltage power source or not.

The symbols I1 and I2 in the table are the same as described previously.

When a secondary transfer current value is larger than a primary transfer current value by 10 uA or more, significant deterioration of image quality occurs. However, according to the present invention, a high-quality image can be obtained regardless of the environment and the kind of paper.

According to the intermediate transfer unit of the invention, since the control of the high voltage power source and the resistance of the primary transfer member are optimized, the deterioration of the image quality in simultaneous primary and secondary transfer can be avoided regardless of the environment and the type of paper.

A recording medium carrying system used for an image forming apparatus of a type in which a toner image formed according to an electrophotographic method is transferred and fixed on a recording medium S comprises: a paper feed mechanism 24, 31, 33 for conveying a recording medium S to a transfer part, a transfer mechanism 12, 13 for transferring a toner image to a recording medium, a fixing mechanism 50 for fixing the transferred toner image to the recording medium, and an ejection mechanism 60 for ejecting the recording medium from a fixing part, wherein the paper feed mechanism 24, 31, 33, the transfer mechanism 12, 13, the fixing mechanism 50 and the ejection mechanism 60 are each constructed as an independent unit.

The paper feed unit 24, 31, 33 includes roller pairs 31, 33, and a conveying speed of each roller pair 31, 33 is set such that the closer the roller pairs are to a pair of gate rollers 35, the lower the conveying speed of the roller pair 31, 33.

The paper feed unit comprises roller pairs 31,33 and a free rotation torque of each roller pair 31, 33 is smaller than that of a pair of gate rollers 35.

A conveyance speed of the paper ejection unit 60 is faster than that of the fixing unit 50, and a conveyance speed of a pair of paper ejection rollers 62, 64 on a downstream side in a conveyance direction in the paper ejection unit 60 is faster than that of a pair of paper ejection rollers on an upstream side.

The rapier feeding unit 24, 31, 33 includes roller pairs 31, 33, and a conveyance capacity of each roller pair 31, 33 is set such that the closer a conveyance roller pair 31, 33 is to a downstream side in a conveyance direction, the larger the conveyance capacity of the roller pairs 31, 33 is.

Claims (30)

1. Intermediate transfer unit comprising: an intermediate transfer belt (360) to which a toner image formed on a photoconductive drum (110) is primarily transferred and which secondarily transfers the toner image to a recording medium (S); and a drive roller (310) for rotating the intermediate transfer belt (360), characterized in that a primary transfer position (T1) where the intermediate transfer belt (360) is brought into contact with the photoconductive drum (110) to primarily transfer the toner image is arranged in the vicinity of the drive roller (310) upstream thereof with respect to a rotational direction of the intermediate transfer belt (360). 2. The intermediate transfer unit of claim 1, further comprising: a primary transfer member (320) for primarily transferring the toner image formed on the photoconductive drum (110); a secondary transfer roller (380) for secondary transfer of the toner image to the recording medium (S), wherein the intermediate transfer belt (360) has a seam ; wherein, when no image is formed, an electric field is formed in a direction in which toner is returned from the secondary transfer roller (380) to the intermediate transfer belt (360) while the secondary transfer roller (380) presses the intermediate transfer belt (360), and wherein when the seam of the intermediate transfer belt (360) is disposed opposite the secondary transfer roller (380), the secondary transfer roller (380) is detached from the intermediate transfer belt (360). 3. An intermediate transfer unit according to claim 1 or 2, wherein the intermediate transfer belt (360) comprises dispersed fluorine particles at least in its surface layer, and wherein the secondary transfer roller (380) is pressed onto the intermediate transfer belt (360) under a linear pressure of 0.265 N/mm (27 gf/mm) or less. 4. The intermediate transfer unit according to claim 3, wherein a hardness of the secondary transfer roller (380) is 70° or less in case the hardness is measured with an Asker-C hardness meter. 5. The intermediate transfer unit according to claim 3 or 4, wherein different types of additives different in particle diameter are added to the toner, and wherein a surface coverage of the additives to the toner is 2 or more. 6. Intermediate transfer unit according to one of claims 3 to 5, wherein the toner image which is applied to the intermediate transfer belt (360) is 1.5 mg/cm² or less. 7. An intermediate transfer unit according to any one of claims 1 to 6, wherein toner is coated with an additive at a surface coverage of 2 or more, and wherein the secondary transfer roller (380) is pressed onto the intermediate transfer belt (360) under a linear pressure of 0.147 N/mm (15 gf/mm) or more. 8. The intermediate transfer unit according to claim 7, wherein a hardness of the secondary transfer roller is 50° or more in the case that the hardness is measured with an Asker-C hardness meter. 9. The intermediate transfer unit according to claim 7 or 8, wherein different types of additives which differ in particle diameter are added to the toner. 10. The intermediate transfer unit according to any one of claims 7 to 9, wherein the toner image transferred onto the intermediate transfer belt (360) is 1.5 mg/cm² or less. 11. Intermediate transfer unit according to one of claims 1 to 10, further comprising: the primary transfer means (320) disposed within the intermediate transfer belt (360), the intermediate transfer belt (360) being held and supported between the photoconductive drum (110) and the primary transfer means (320) at a primary transfer position; Pressure means (350) disposed within the intermediate transfer belt (360); and the secondary transfer means (380) disposed outside the intermediate transfer belt (360), the intermediate transfer belt (360) being held and supported between the pressure means (350) and the secondary transfer means (380) at a secondary transfer position, wherein a loose bulk density of the toner is 0.35 g/cm³ or more, a shape factor SF-1 of the toner is 150 or less, and a shape factor SF-2 is 140 or less. 12. Intermediate transfer unit according to one of claims 1 to 11, further comprising: the primary transfer means (320) disposed within the intermediate transfer belt (360) at a primary transfer position where the toner image on the photoconductive drum (110) is primarily transferred; and the secondary transfer means (320) which is arranged outside the intermediate transfer belt (119) at a secondary transfer position, where the toner image is secondarily transferred; wherein a load of the secondary transmission means (380) is greater than a load of the primary transmission means (320). 13. An intermediate transfer unit according to claim 12, wherein the primary and secondary transfer means are pressed against the intermediate transfer belt (360) such that a ratio of the load of the secondary transfer means (380) to the load of the primary transfer means (320) is 1.5 or more. 14. Intermediate transfer unit according to one of claims 1 to 13, wherein the hardness of the secondary transfer means (380) is higher than that of the primary transfer means (320). 15. An intermediate transfer unit according to claim 14, wherein the hardness of the secondary transfer means (380) is higher than that of the primary transfer means (320) by 10º or more when measured with an Asker-C hardness meter. 16. Intermediate transfer unit according to one of claims 1 to 15, further comprising: the primary transfer member (320) which is arranged on a back side of the intermediate transfer belt (360); and a high voltage power supply for applying a bias voltage to the primary transfer member (320) so that the toner image formed on the photoconductive drum (110) is primarily transferred to the intermediate transfer belt (360), wherein the primary transfer member (320) has a resistivity of 10⁶ to 10⁸ Ω, wherein the intermediate transfer belt (360) has a surface resistivity of 10⁸ to 10¹² Ω/ , and a volume resistivity of 10⁸ to 10¹² Ωcm, and wherein the high-voltage power supply performs constant current control when an impedance in a primary transmission part is high, and performs constant voltage control when the impedance is small. 17. An intermediate transfer unit according to claim 16, wherein the primary transfer member (320) is an elastic roller made of an electrical conductor made of carbon. 18. An intermediate transfer unit according to claim 16 or 17, wherein the hardness of the primary transfer member (320) is 40° to 50° when measured with an Asker-C hardness meter; and wherein a load on the photoconductive drum (110) by the primary transfer member (320) is 28 to 98 g/cm. 19. The intermediate transfer unit according to any one of claims 16 to 18, wherein two or more types of additives different in particle diameter are added to the toner. 20. The intermediate transfer unit according to claim 19, wherein an added amount of an additive having a large particle diameter among the additives added to the toner is 0.5 to 4.0 wt%; and an added amount of an additive having a small particle diameter is 1.5 to 4.0 wt%. 21. Intermediate transfer unit according to one of claims 1 to 20, further comprising: a pressure roller (350) disposed within the intermediate transfer belt (360); the secondary transmission member (380) which is pressed onto the pressure roller (350); and a high-voltage power supply for applying a bias voltage to the secondary transfer member (380) so that the primarily transferred toner image is secondarily transferred to the recording medium (S), wherein the secondary transmission element (380) has a resistance of 10⁶ to 10⁶ Ω, wherein the intermediate transfer belt (360) has a surface resistivity of 10⁸ to 10¹² Ω/ , and a volume resistivity of 10⁸ to 10¹² Ωcm, and wherein the high voltage power supply performs constant current control when impedance in a secondary transmission part is large, and performs constant voltage control when the impedance is small. 22. An intermediate transfer unit according to claim 21, wherein the secondary transfer member (380) is an elastic roller made of an electrical conductor of ion-conductive material; 23. An intermediate transfer unit according to claim 21 or 22, wherein hardness of the secondary transfer member (380) is 55º to 56º degrees when measured with an Asker-C hardness meter; and wherein a load on the pressure roller (350) by the secondary transfer member (380) is 150 to 270 g/cm. 24. The intermediate transfer unit according to any one of claims 21 to 23, wherein two or more types of additives different in particle diameter are added to the toner. 25. The intermediate transfer unit according to claim 24, wherein an added amount of an additive having a large particle diameter among the additives added to the toner is 0.5 to 4.0 wt%, and an added amount of an additive having a small particle diameter is set to 1.5 to 4.0 wt%. 26. Intermediate transfer unit according to one of claims 1 to 25, further comprising: the primary transfer member (320) disposed on a back side of the intermediate transfer belt (360); a high voltage power supply for applying a bias voltage to the primary transfer member (320) so that the toner image formed on the photoconductive drum (110) is primarily transferred to the intermediate transfer belt (360); a pressure roller (350) disposed within the intermediate transfer belt (360); and the secondary transmission member (380) which is pressed onto the pressure roller (350); a high-voltage power supply for applying a bias voltage to the secondary transfer member (380) so that the primarily transferred toner image is secondarily transferred to the recording medium (S), wherein the primary transmission member (320) and the secondary transmission member (380) are formed by an elastic body; and wherein the deviation of the resistance of the secondary transmission element (380) due to the environment is greater than that of the primary transmission element (320). 27. An intermediate transfer unit according to claim 26, wherein the primary transfer member (320) is an elastic roller made of an electric conductor by carbon black. 28. An intermediate transfer unit according to claim 26 or 27, wherein the secondary transfer member (380) is an elastic roller made of an electrical conductor of ion-conductive material. 29. Intermediate transfer unit according to one of claims 1 to 28, further comprising: the primary transfer member (320) disposed at a position different from a primary transfer part on a surface of the intermediate transfer belt (360); a high voltage power supply for applying a bias voltage to the primary transfer member (320) so that the toner image formed on the photoconductive drum (110) is primarily transferred to the intermediate transfer belt (360); and the secondary transfer member (380), wherein the toner image is secondarily transferred to the recording medium (S) by applying a bias voltage to the secondary transfer member (380), wherein a pressure member (350) in the primary transmission part is an elastic body, wherein a resistance value of the primary transmission element (320) is 1 MΩ or less, and wherein the high voltage power supply performs a current absorbable constant voltage control for applying a bias voltage to the primary transmission element (320). 30. Intermediate transfer unit according to one of claims 1 to 29, further comprising: the primary transfer member (32U) arranged at a position different from a primary transfer part on a surface of the intermediate transfer belt (360); a high voltage power supply for applying a bias voltage to the primary transfer member (320) so that the toner image formed on the photoconductive drum (110) is primarily transferred to the intermediate transfer belt (360); the secondary transfer member (380), wherein the toner image is secondarily transferred to the recording medium (S) by applying a bias voltage to the secondary transfer member (380), wherein a pressure member (350) in the primary transmission part is an elastic body, wherein a resistance value of the primary transmission element (320) is 1 MΩ or less, and wherein a resistor is connected in parallel to the high voltage power supply for applying a bias voltage to the primary transmission element (320).