Patent Application
20240103410
The present disclosure relates to an image forming apparatus using an electrophotographic process, such as a laser beam printer, copying machine, and facsimile.
Typical image forming apparatuses, such as copying machines and laser beam printers, are known to use an intermediate transfer belt as an intermediate transfer member.
In a primary transfer process, an image forming apparatus transfers a toner image formed on the surface of a photosensitive drum serving as an image carrier onto an intermediate transfer belt by applying a voltage to a primary transfer roller serving as a primary transfer member disposed at a portion facing the photosensitive drum. The image forming apparatus then repetitively performs this primary transfer process for a plurality of color toner images to form a multi-color toner image on the front surface of the intermediate transfer belt. Subsequently, as a secondary transfer process, the apparatus collectively transfers the multi-color toner image formed on the front surface of the intermediate transfer belt onto the surface of a recording material, such as paper, by applying a voltage to a secondary transfer member. The toner image transferred onto the surface of the recording material is then fixed thereto by a fixing unit, and a color image is formed on the recording material.
The specification of U.S. Pat. No. 10,684,577 discloses a configuration for performing a primary transfer process in which a primary transfer roller is grounded to a metal frame of an image forming apparatus and a photosensitive drum is applied with a voltage from a power source.
According to an aspect of the present disclosure, an image forming apparatus includes an image carrier configured to carry a toner image, an intermediate transfer member onto which the toner image carried on the image carrier is transferred, wherein the intermediate transfer member is rotatable and endless, a primary transfer member configured to perform primary transfer of the toner image carried on the image carrier onto the intermediate transfer member, wherein the primary transfer member is disposed in a state where the primary transfer member is electrically grounded at a position corresponding to the image carrier across the intermediate transfer member, a voltage application member configured to apply a voltage to the image carrier, and a secondary transfer member in contact with an outer circumferential surface of the intermediate transfer member, wherein the secondary transfer member is configured to perform secondary transfer of the toner image carried on the intermediate transfer member onto a transfer material, wherein the toner image carried on the image carrier is primarily transferred onto the intermediate transfer member in a state where the voltage application member applies the voltage to the image carrier, wherein the intermediate transfer member has a volume resistivity from 5×107 Ω·cm to 2×1011 Ω·cm inclusive, and wherein a relation ρs1/ρs2≥1.5 is satisfied, where ρs1 denotes a surface resistivity which is measured from the outer circumferential surface of the intermediate transfer member and ρs2 denotes a surface resistivity which is measured from an inner circumferential surface of the intermediate transfer member.
Further features of the present disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Preferred exemplary embodiments of the present disclosure will be exemplarily described in detail below with reference to the accompanying drawings. However, sizes, materials, shapes, and relative arrangements of components according to the following exemplary embodiments are to be modified as appropriate depending on the configuration of an apparatus to which the present disclosure is applied and other various conditions. Therefore, unless otherwise specifically described, the scope of the present disclosure is not limited to the following exemplary embodiments.
The first image forming unit Sa includes a photosensitive drum 1a as an image carrier, a charge roller 2a as a charge member, a developing unit 4a, and a drum cleaning unit 5a.
The photosensitive drum 1a is rotatably driven at a predetermined process speed (200 mm/second according to the present exemplary embodiment) in the direction of the arrow R1 in
When the image forming operation is started in response to a controller 274 (illustrates it in
An intermediate transfer belt 10 which is a seamlessly (endlessly) movable intermediate transfer member is disposed so as to come into contact with the photosensitive drums 1a to 1d of the image forming units Sa to Sd, respectively, and stretched by two different shafts, a stretching roller 11 and a secondary transfer counter roller 13 serving as a drive roller. The intermediate transfer belt 10 is stretched with a tension of a total pressure of 58.8 N by the stretching roller 11, and moves in the direction of the arrow R2 in
When the toner image formed on the photosensitive drum 1a passes through a primary transfer portion N1a at which the photosensitive drum 1a and the intermediate transfer belt 10 are in contact with each other, the toner image is primarily transferred onto the intermediate transfer belt 10 by a negative voltage applied to the photosensitive drum 1a from a primary transfer power source 23 (voltage application member). Then, residual toner on the photosensitive drum 1a, in other words, toner not having been primarily transferred onto the intermediate transfer belt 10, is collected by the drum cleaning unit 5a.
Similarly, a magenta toner image of the second color, a cyan toner image of the third color, and a black toner image of the fourth color are formed on the corresponding image forming units and then sequentially primarily transferred onto the intermediate transfer belt 10 in an overlapped way. Thus, a four-color toner image corresponding to the target color image is formed on the intermediate transfer belt 10. Subsequently, when the four-color toner image carried on the intermediate transfer belt 10 passes through the secondary transfer portion N2 at which the secondary transfer roller 20 and the intermediate transfer belt 10 are in contact with each other, the four-color image is secondarily transferred at one time onto the surface of the transfer material (recording material) P, such as paper, supplied by a sheet feeding unit 50.
The secondary transfer roller 20 having an 18 mm outer diameter is formed of a nickel-plated steel bar with an 8 mm outer diameter coated by a foamed sponge material consisting primarily of nitrile butadiene rubber (NBR) and epichlorohydrin rubber, adjusted to a volume resistivity of 108 Ω·cm and a thickness of 5 mm. The rubber hardness of the foamed sponge material is measured by using the Asker Rubber Hardness Meter Type C (from KOBUNSHI KEIKI CO., LTD.) conforming to Japanese Industrial Standards (JIS) K 7312, and is 30 degrees with a load of 4.9 N. The secondary transfer roller 20 in contact with the outer circumferential surface of the intermediate transfer belt 10 is pressed onto the secondary transfer counter roller 13 disposed at a position facing the secondary transfer roller 20 via the intermediate transfer belt 10, with a pressure of 49.0 N to form the secondary transfer portion N2.
When the secondary transfer roller 20 is driven and rotated according to the rotation of the intermediate transfer belt 10, and is applied with a voltage from the secondary transfer power source 21, a current flows from the secondary transfer roller 20 to the secondary transfer counter roller 13. Thus, the toner image carried on the intermediate transfer belt 10 is secondarily transferred onto the transfer material P at the secondary transfer portion N2. When the toner image is secondarily transferred onto the transfer material P, the voltage to be applied to the secondary transfer roller 20 from the secondary transfer power source 21 is controlled so that a constant current flows from the secondary transfer roller 20 to the secondary transfer counter roller 13 via the intermediate transfer belt 10. The magnitude of the current for performing the secondary transfer is predetermined in accordance with the ambient environment where the image forming apparatus 100 is installed and the type of the transfer material P. The secondary transfer power source 21 is connected to the secondary transfer roller 20 and applies the transfer voltage to the secondary transfer roller 20.
The transfer material P with the four-color toner image transferred thereon via the secondary transfer is then pressurized and heated by a fixing unit 30. The four-color toner melts and mixes, and the four-color toner image is fixed to the transfer material P. Meanwhile, the toner remaining on the intermediate transfer belt 10 after the secondary transfer is cleaned and removed by a belt cleaning unit 16 disposed downstream of the secondary transfer portion N2 in the moving direction of the intermediate transfer belt 10. The belt cleaning unit 16 includes a cleaning blade 16a as a contact member in contact with the outer circumferential surface of the intermediate transfer belt 10 at a position facing the secondary transfer counter roller 13, and a waste toner container 16b for storing the toner collected by the cleaning blade 16a.
With the above-described operation, a full-color image is formed by the image forming apparatus 100.
Control according to the present exemplary embodiment will be described below with reference to
As illustrated in
The formatter 273 transfers the exposure data resulting from the conversion to an exposure control unit 277 which is an exposure control apparatus in the controller 274 (control unit). The exposure control unit 277 controls the exposure unit 3 based on an instruction from a central processing unit (CPU) 276. In the image forming apparatus 100, the image halftone is controlled with the area gradation based on the ON/OFF state of the exposure data. When the CPU 276 receives a printing instruction from the formatter 273, the CPU 276 starts an image forming sequence. The controller 274 includes the CPU 276 and a memory 275 and performs programmed operations. The CPU 276 controls a charge power source 281, a developing power source 280, and the primary transfer power source 23 to control the formation of an electrostatic latent image and the transfer of a developed toner image, thus performing the image forming process.
In performing the correction control for correcting the position and density of the image to be formed, the CPU 276 also performs a process of receiving a signal from an optical sensor 60 which is a detection unit. In the image correction control, the optical sensor 60 measures the amount of reflected light from a test patch (detection toner image) formed at a position on the outer circumferential surface of the intermediate transfer belt 10 at a position facing the optical sensor 60. The signal detected by the optical sensor 60 is subjected to the analog-to-digital (A/D) conversion via the CPU 276 and then stored in the memory 275. The controller 274 performs calculations by using the detection result by the optical sensor 60 to correct the image.
According to the present exemplary embodiment, the primary transfer roller 6a serving as a primary transfer member is a metal roller that is made of metal and is not coated by an elastic material, such as rubber. As illustrated in
More specifically, when viewed from the rotational axis direction of the photosensitive drum 1a, the rotation center Rtr of the primary transfer roller 6a is located downstream of the rotation center Rdc of the photosensitive drum 1a in the moving direction of the intermediate transfer belt 10 at the primary transfer portion N1a. The distance from the rotation center Rdc of the photosensitive drum 1a to the rotation center Rtr of the primary transfer roller 6a along the moving direction of the intermediate transfer belt 10 is a distance Dd. More specifically, the primary transfer roller 6a is disposed at a position corresponding to the photosensitive drum 1a across the intermediate transfer belt 10 in a state where the rotation center Rtr is offset relative to the rotation center Rdc or the primary transfer portion N1a serving as a contact portion between the photosensitive drum 1a and the intermediate transfer belt 10.
In the configuration of the present exemplary embodiment, a distance Lc which is the length of the straight line connecting the rotation centers Rdc and Rtr satisfies the following inequality (1):
Lc>(Da/2)+(Db/2)+Tc (1),
where Da denotes the diameter of the photosensitive drum 1a, Db denotes the diameter of the primary transfer roller 6a, and Tc denotes the thickness of the intermediate transfer belt 10.
If the relation of the inequality (1) is satisfied over a predetermined region in the longitudinal direction of the photosensitive drum 1a orthogonal to the moving direction of the intermediate transfer belt 10, it is possible for the primary transfer roller 6a to stably come into contact with the back surface (inner circumferential surface) of the intermediate transfer belt 10.
According to the present exemplary embodiment, the primary transfer roller 6a is a roller member (transfer roller) formed of a straight round bar made of a nickel-plated stainless-steel material having an outer diameter of 6 mm. The primary transfer roller 6a is driven and rotated according to the rotation of the intermediate transfer belt 10. The photosensitive drum 1a has an outer diameter of 24 mm and has an aluminum cylinder, serving as a base material, which is coated by thin films having different functions, such as a conducting layer, an electric charge generation layer, and an electric charge transport layer. In the configuration of the present exemplary embodiment, the distance Dd between the photosensitive drum 1a and the primary transfer roller 6a is 3.0 mm. The photosensitive drum 1a is applied with a negative voltage from the inner side of the aluminum cylinder by the primary transfer power source 23. Thus, a current flows from the primary transfer roller 6a toward the photosensitive drum 1a via the intermediate transfer belt 10.
The intermediate transfer belt 10 having a configuration which is peculiar to the present exemplary embodiment will be described below.
The intermediate transfer belt 10 has a circumference of 700 mm and a thickness of 90 μm and includes a base layer 10a as a first layer and a surface layer 10b as a second layer. The base layer 10a includes endless polyethylene naphthalate (PEN) containing an ion conducting agent as a conducting agent. The surface layer 10b includes acrylic resin containing a metallic oxide as a conducting agent. As illustrated in
While the present exemplary embodiment uses PEN as the material of the base layer 10a of the intermediate transfer belt 10, other materials are also applicable. For example, materials, such as polyester, acrylonitrile-butadiene-styrene copolymer (ABS), polybutylene naphthalate (PBN), and their mixed resins are also useable. While the present exemplary embodiment uses acrylic resin as the material of the surface layer 10b of the intermediate transfer belt 10, other materials, such as polyester, are also useable.
For the intermediate transfer belt 10, the volume resistivity measured from the surface layer 10b, the surface resistivity on the front surface measured from the surface layer 10b, and the surface resistivity on the back surface measured from the base layer 10a are prescribed as suitable resistance values.
The volume resistivity is measured by using the Hiresta-UP (MCP-HT450) together with the Ring Probe Type UR (Type MCP-HTP12) from Mitsubishi Chemical Corporation. The metal surface of the Register Table UFL is used as the probe opposing electrode. The surface resistivity is measured by using the same measuring instrument as that used in the measurement of the volume resistivity with the Teflon (registered trademark) surface of the Register Table UFL as the probe opposing electrode. Various resistivities were measured in an environment with an indoor temperature of 23° C. and an indoor humidity of 50% after the intermediate transfer belt 10 was let stand for at least one day in the relevant environment.
The volume resistivity of the intermediate transfer belt 10 was measured with an applied voltage of 250 V and a measuring time of 10 seconds in a state where the probe was placed on the front surface (outer circumferential surface) of the intermediate transfer belt 10 with an applied pressure of 9.8 N and the probe opposing electrode was disposed on the back surface (inner circumferential surface) of the intermediate transfer belt 10. The volume resistivity is the resistance value in the thickness direction of the intermediate transfer belt 10.
The surface resistivity of the intermediate transfer belt 10 was measured for each of the back surface (inner circumferential surface) where the base layer 10a was disposed and the front surface (outer circumferential surface) where the surface layer 10b was disposed. The surface resistivity of the back surface (inner circumferential surface) where the base layer 10a was disposed was measured in a state where the probe was placed on the back surface (inner circumferential surface) of the intermediate transfer belt 10 and the probe opposing electrode was disposed on the front surface (outer circumferential surface) thereof. The surface resistivity of the front surface (outer circumferential surface) where the surface layer 10b was disposed was measured in a state where the probe was placed on the front surface (outer circumferential surface) of the intermediate transfer belt 10 and the probe opposing electrode was disposed on the back surface (inner circumferential surface) thereof. Each measurement was conducted with an applied voltage of 250 V and a measuring time of 10 seconds in a state where the probe was placed on the relevant surface of the intermediate transfer belt 10 with an applied pressure of 9.8 N.
As a result of the measurement under the above-described conditions, a volume resistivity ρv of the intermediate transfer belt 10 according to the present exemplary embodiment was 2.50×1010 (Ω·cm). A surface resistivity ρs1 measured from the front surface was 2.20×1011 (Ω/sq.), and a surface resistivity ρs2 measured from the back surface was 5.00×109 (Ω/sq.). The ratio of the surface resistivity of the front surface of the intermediate transfer belt 10 to that of the back surface thereof, ρs1/ρs2, is about 44. This means that the surface resistivity measured from the front surface is higher than that measured from the back surface.
A reason the surface resistivity ρs1 measured from the surface layer 10b is set high will be described below with reference to
The secondary transfer roller 20 is applied with a secondary transfer voltage Vt2 from the secondary transfer power source 21. At this time, a voltage Vt2′ illustrated in
Referring to
Vt1 denotes the primary transfer voltage applied from the primary transfer power source 23. Id denotes the current flowing from the secondary transfer portion N2 toward the primary transfer portion N1a near the secondary transfer portion N2, and It1 denotes the primary transfer current.
A current interference that occurs between the secondary and primary transfer portions in the configuration of the comparative example will be described below with reference to
As illustrated in the equivalent circuit in the configuration of the comparative example in
If the transfer current flowing at the primary transfer portion N1a increases with respect to the target value of the transfer current due to the influence of the interference current, the transfer current may be excessive at the primary transfer portion N1a. The excessive transfer current then reverses the polarity of the toner at the primary transfer portion N1a, so that the toner may be reversely transferred (re-transferred) from the intermediate transfer belt 10 toward the photosensitive drum 1a. In a case where the transfer current becomes further excessive, electric discharge may occur between the intermediate transfer belt 10 and the photosensitive drum 1a at a portion upstream of the primary transfer portion N1a in the moving direction of the intermediate transfer belt 10. In this case, the image obtained after the image forming process may have image defects due to toner scattering or electric discharge patterns.
More specifically, in a case where the interference current from the secondary transfer roller 20 flows into the primary transfer portion N1a, the transfer current deviates from the appropriate primary transfer current value, so that the transfer efficiency may be degraded or image defects may occur. Further, in the secondary transfer control, the interference current changes with change in the applied voltage at the secondary transfer portion N2. Thus, the primary transfer may be affected by the secondary transfer control and become unstable.
In contrast, in the configuration of the present exemplary embodiment in
Here, the interference current Id_t1 can be represented by the following equation (2) by solving the equivalent circuit in
I
d_t1=(Vt2′×Roffset/(Rsa+Roffset)−Vt1)/RVb (2)
As indicated in the equation (2), the current Id_t1 changes by the resistance value of the intermediate transfer belt 10, the primary transfer voltage Vt1, and the distance Dd. In particular, the resistance RVb of the surface layer 10b largely affects the current Id_t1. More specifically, increasing the electrical resistance of the surface layer 10b of the intermediate transfer belt 10 enables effectively reducing the current Id_t1. To increase the electric resistance of the surface layer 10b of the intermediate transfer belt 10, it is effective to reduce the content of the conducting agent contained in the surface layer 10b. Reducing the content of the conducting agent to be applied to the surface layer 10b of the intermediate transfer belt 10 increases the surface resistivity ρs1 of the front surface (outer circumferential surface) of the intermediate transfer belt 10.
The electrical resistance of the surface layer 10b has a high correlation with the surface resistivity ρs1 of the front surface of the intermediate transfer belt 10. Thus, it is desirable to use the surface resistivity ρs1 as a management parameter.
Increasing the surface resistivity ρs2 of the back surface (inner circumferential surface) of the intermediate transfer belt 10 reduces the current component Id_GND passing through the current path AA and flowing in the primary transfer roller 6a. At this time, the component Id_t1 flowing in the photosensitive drum 1a which serves as an interference current increases, possibly resulting in the excessive transfer current. More specifically, to prevent image defects caused by the above-described current interference, it is effective to set the resistance value of the surface layer 10b of the intermediate transfer belt 10 to a high value relative to that of the base layer 10a. In other words, increasing the ratio of the surface resistivity of the surface layer 10b of the intermediate transfer belt 10 to that of the base layer 10a thereof, ρs1/ρs2, as the management parameter, enables preventing image defects possibly caused by the current interference.
In contrast, the primary transfer efficiency is determined by the amount of current flowing in the photosensitive drum 1a via the primary transfer portion N1a. If the volume resistivity and/or surface resistivity of the intermediate transfer belt 10 are(is) increased to achieve an appropriate primary transfer process, and accordingly a high primary transfer voltage is to be set. The current Id_t1 can be decreased by simply increasing the volume resistivity of the intermediate transfer belt 10. However, setting the primary transfer voltage to a high value may possibly increase the current Id_t1. Thus, in the present exemplary embodiment, it is desirable to set the volume resistivity ρv to an intermediate resistance region to be used as an intermediate transfer member, in other words, a range from 5×107 (Ω·cm) to 2×1011 (Ω·cm) (inclusive).
As described above, in the configuration of the present exemplary embodiment, the ratio of the surface resistivity, ρs1/ρs2, is set within a predetermined range to prevent the interference current from flowing from the secondary transfer portion N2 toward the photosensitive drum 1a via the primary transfer portion N1a near the secondary transfer portion N2.
<Setting ρs1/ρs2>
A preferred relation between the surface resistivities ρs1 and ρs2 will be described in detail below. To achieve the optimum primary transfer according to the present exemplary embodiment, the optimum transfer current to be applied to the primary transfer portion N1a is to be determined, initially. The desired transfer current is settable by applying the primary transfer current in the primary transfer portion N1a in a state where no voltage is applied from the secondary transfer power source 21, and checking the efficiency of the toner image transfer from the photosensitive drum 1a to the intermediate transfer belt 10. The setting of the primary transfer current will be described below with reference to
An ammeter (not illustrated) which is a current detection unit is disposed between the primary transfer power source 23 and the photosensitive drum 1a illustrated in
As illustrated in Table 1, the residual toner decreased with increasing value of the primary transfer current, and a tendency of favorable results for the visual evaluation was observed. This is because, if the primary transfer current is too small, the current for transferring the toner image carried on the photosensitive drum 1a to the intermediate transfer belt 10 is insufficient, which increases the amount of residual toner. Meanwhile, for the primary transfer current of 13.0 μA, almost no residual toner was visually recognized. However, increasing the primary transfer current increased the amount of the residual to degrade the transfer efficiency. This is because, if the primary transfer current is too large, electric discharge occurs in the primary transfer portion N1a to reverse the toner polarity, resulting in residual toner on the photosensitive drum 1a. More specifically, in the configuration of the image forming apparatus 100 according to the present exemplary embodiment, we found in this examination that the suitable primarily transfer current to be applied in the primary transfer portion N1 was 13.0 μA to perform the optimum primary transfer.
Subsequently, as targets of the comparison with the intermediate transfer belt 10 according to the present exemplary embodiment, we prepared seven intermediate transfer belts for the first and the second comparative examples and the first to the fifth modifications of the present exemplary embodiment, performed the image forming operation, and performed various evaluations. The intermediate transfer belt for the first comparative example is a single-layer intermediate transfer belt. We prepared the belt by using polyimide as the main raw material and, to obtain a desired electrical resistance, distributed carbon black as a conducting agent. While intermediate transfer belts for the second comparative example and the first to the fifth modifications are made of the same materials as the intermediate transfer belt 10 according to the present exemplary embodiment, we prepared the intermediate transfer belts having different resistance values by changing the content of the conducting agent.
For the single-layer intermediate transfer belt used in the first comparative example, the surface resistivity measured from the outer circumferential surface is the same as that measured from the inner circumferential surface. For the two-layer intermediate transfer belts used in the second comparative example and the modifications, the surface resistivity measured from the outer circumferential surface is larger than that measured from the inner circumferential surface, as illustrated in Table 2 (described below).
We performed the image forming process by using the intermediate transfer belts in the present exemplary embodiment, the first and the second comparative examples, and the first to the fifth modifications, and evaluated the primary transfer efficiency, the density of the test image secondarily transferred to the transfer material, and the occurrence of a secondary transfer memory. Table 2 illustrates results of the evaluations. In the image forming operation for the following evaluations, the primary transfer voltage was determined by performing a predetermined control sequence so that the target current of 13.0 μA flowed as the primary transfer current. In the primary transfer process during the image forming process, constant-voltage control was performed by using the primary transfer voltage determined in the above-described control sequence. In the secondary transfer process, constant-current control was performed so that an appropriate current flowed in the secondary transfer portion N2.
As in the evaluation corresponding to the results in Table 1, we evaluated the primary transfer efficiency by forcibly stopping the image forming operation in the primarily transfer, collecting the residual toner on the photosensitive drum 1a by using an adhesive tape, and visually checking the amount of residual toner. We also evaluated the residual toner based on the same evaluation criteria (three different levels A, B, and C) in Table 1. The primary transfer portions N1 subjected to the evaluation include the primary transfer portion N1a near the secondary transfer portion N2 and a primary transfer portion N1b hardly affected by the current interference from the secondary transfer portion N2. In the evaluation environment, the temperature was set to 23° C. and the humidity to 50%.
In the evaluation for the density of the test image secondarily transferred to the transfer material, plain paper CS-680 from Canon was used as the transfer material, and a 5×5 mm yellow square patch image formed by using the image forming unit Sa was used as the test image. We performed the evaluation by forming the test image on the transfer material and measuring the optical reflection density of the test image after the secondary transfer. EXACT BASIC from X-Rite, Inc. was used to measure the optical reflection density.
The secondary transfer memory which is an evaluation item in Table 2 refers to a residual electrostatic latent image corresponding to the image pattern secondarily transferred to the transfer material at the secondary transfer portion N2, remaining on the intermediate transfer belt 10 as a trace. If a secondary transfer memory occurs, image defects may possibly occur depending on image forming conditions, for example, when a subsequent image is formed at the position where a secondary transfer memory has occurred in a continuous image forming process. More specifically, an image defect becomes revealed as a phenomenon where, when the intermediate transfer belt 10 with a secondary transfer memory formed thereon rotates one round and reaches the secondary transfer portion N2 again, the density of subsequent images decreases. In particular, an image defect is likely to occur with the high surface resistivity of the outer circumferential surface of the intermediate transfer belt 10.
Initially, density evaluation results for the test image will be described below. In the examination of the present exemplary embodiment, we visually recognized that the threshold value of the optical reflection density for determining the density reduction was 1.40 and that the density lower than the threshold value was determined to be a density reduction. In the above-described examination, the density is determined to be suitably controlled in the secondary transfer process, and the density variation in the evaluation results is caused by the primary transfer process. In the first and the second comparative examples, a density reduction of the test image became reveled; however, the density reduction of the test image did not occur in the first to the fifth modifications. Further, the image forming unit Sb for magenta that is hardly affected by the interference current from the secondary transfer portion N2 provided a favorable primary transfer efficiency in all of the above-described configurations.
As for the primary transfer efficiency, while the first and the second comparative examples resulted in a large amount of residual toner and the evaluation level C, the first to the fifth modifications resulted in an amount of residual toner within the permissible range. In particular, the third to the fifth modifications provided a favorable primary transfer efficiency.
In view of the above-described results, it is desirable that ρs1/ρs2 satisfies the following inequality (3) to prevent the occurrence of image defects in primarily transferring the toner image from the image carrier to the intermediate transfer member.
ρs1/ρs2≥1.5 (3)
If ρs1/ρs2 satisfies the following inequality (4), further favorable primary transfer efficiency can be achieved. For example, if a sufficient capacity of the drum cleaning unit 5a cannot be allocated because of product dimensions, the amount of residual toner can be further reduced by satisfying the inequality (4).
ρs1/ρs2≥2.0 (4)
The fifth modification provides a large value of ρs1/ρs2, in other words, a large difference between the surface resistivity ρs1 of the outer circumferential surface of the intermediate transfer belt 10 and the surface resistivity ρs2 of the inner circumferential surface thereof. Thus, the occurrence of a secondary transfer memory was observed with this modification. It is desirable that the following inequality (5) is satisfied to prevent the occurrence of a secondary transfer memory which is a cause of image defects in the secondary transfer process.
ρs1/ρs2≤100.0 (5)
As described above, if ρs1/ρs2 satisfies the inequality (3), the configuration in the present exemplary embodiment enables preventing the occurrence of image defects in primarily transferring the toner image from the photosensitive drum 1a to the intermediate transfer belt 10. A description will be provided of the appropriate distance Dd from the rotation center Rdc of the photosensitive drum 1a to the rotation center Rtr of the primary transfer roller 6.
The resistance Roffset changes when the distance Dd changes. That the current Id_t1 changes when the resistance Roffset changes is seen from the equation (2). If the distance Dd increases, the resistance Roffset also increases, resulting in increase in the component Id_t of the interference current. If the distance Dd increases, the voltage Vt1 (negative polarity) is to be increased to apply an appropriate primary transfer current according to the value of the distance Dd. The component Id_t of the interference current then further increases.
We evaluated the primary transfer efficiency with the value of the distance Dd changed, using the intermediate transfer belts according to the present exemplary embodiment, the second comparative example, and the first to the fifth modifications. We compared the distance Dd with six different values 1 mm, 3 mm, 5 mm, 7 mm, 10 mm, and 13 mm, which include 3.0 mm according to the configuration of the present exemplary embodiment. D=1.0 mm is the shortest distance Dd for stably forming the primary transfer nip portion in consideration of the clearance of mechanical dimensions in the configuration of the present exemplary embodiment. The target current for the primary transfer is 13.0 μA. We used the same evaluation method and the same evaluation environment as those in the evaluation of the primary transfer efficiency in Table 2. Evaluation results are illustrated in Table 3.
As illustrated in Table 3, there was a tendency that the primary transfer efficiency decreases with increasing distance Dd.
In a case where the intermediate transfer belt 10 according to the present exemplary embodiment was used, the primary transfer efficiency was favorable with the distance Dd of up to 7 mm and was within the permissible range, without influence on the image with the distance Dd of 10 mm. As described above, it is desirable to set the distance Dd to 10.0 mm or less. In the configuration of the present exemplary embodiment, it was found that setting the distance Dd to 7 mm or less enabled obtaining a higher primary transfer efficiency.
The output value of the negative voltage Vt1 to be applied to the photosensitive drum 1a in the primary transfer process may be changed according to environmental variations, such as temperature and humidity variations, and the operating life of the cartridge. In such a case, according to this change, the potential (dark portion potential Vd′) formed on the photosensitive drum 1a by the charge roller 2 or the developing potential Vdc′ to be applied to the developing roller 42 may be changed. For example, in a case where the relation represented by the following equation (6) is satisfied, where Vt1′ denotes the primary transfer voltage after the change and 66 Vt1 denotes the variation of the primary transfer voltage after the change, it is desirable to change the values of the dark portion potential Vd′ and the developing potential Vdc′ of the photosensitive drum 1a as expressed by equations (7) and (8). The photosensitive drum 1a is uniformly charged. Referring to the equation (7), the dark portion potential Vd denotes the dark portion potential of the photosensitive drum 1a corresponding to the voltage Vt1 before being changed to the voltage Vt1′. Referring to the equation (8), the developing potential Vdc is the developing potential corresponding to the voltage Vt1 before being changed to the voltage Vt1′.
ΔVt1=Vt1′−Vt1 (6)
Vd′=Vd+ΔV
t1 (7)
Vdc′=Vdc+ΔV
t1 (8)
The foregoing is a description of changing the primary transfer voltage according to environmental variations, such as temperature and humidity variations, and the operating life of the cartridge, and accordingly changing the developing potential and the dark portion potential of the photosensitive drum 1a. Alternatively, change targets are not limited to the developing potential and the dark portion potential but may include the light portion potential which is changed by changing the amount of laser beam.
While the present exemplary embodiment has been described above using a two-layer intermediate transfer belt as an example, an intermediate transfer belt formed of three or more layers is also applicable as long as the ratio of the surface resistivity from the outer circumferential surface to the surface resistivity from the inner circumferential surface satisfies the relation represented by the equation (2). A single-layer intermediate transfer belt made of the same base material and the same conducting agent material is also applicable. In this case, the surface resistivity is differentiated between the front and back surfaces by differentiating the conducting agent, for example, by using different density or distribution state of carbon black.
Although the present exemplary embodiment has been described using a metal roller as the primary transfer roller as an example, a general rubber roller formed of a metal shaft coated by an elastic material (elastic layer), such as rubber, is also applicable. When using a rubber roller as the primary transfer roller, it may be disposed right under the photosensitive drum 1a with the intermediate transfer belt 10 therebetween. In this case, the relation represented by the inequality (1) does not need to be satisfied.
A second exemplary embodiment of the present disclosure will be described below. An intermediate transfer belt 110 according to the second exemplary embodiment differs from the intermediate transfer belt 10 according to the first exemplary embodiment in that the contact resistance between the photosensitive drum 1a and the intermediate transfer belt 110 is changed by controlling the surface roughness of the intermediate transfer belt 110 by applying predetermined groove profiles to the outer circumferential surface. In the following descriptions, only configurations different from those according to the first exemplary embodiment will be described below, and descriptions of configurations common to those according to the first exemplary embodiment will be omitted.
The surface profile of the intermediate transfer belt 110 is able to be managed, for example, based on a surface roughness Rzjis. The surface roughness is measured by using the Surface Roughness/Contour Profile Measuring Instrument SURFCOM 1500SD (from TOKYO SEIMITSU CO., LTD.) in conformance with JIS B0601:2001 under conditions of a cutoff wavelength of 0.25 mm, a measurement reference length of 0.25 mm, and a measurement length of 1.25 mm. The surface roughness Rzjis on the outer circumferential surface of the intermediate transfer belt 110 is measured by scanning the surface with the stylus of the measuring instrument in the direction substantially orthogonal to the moving direction of the intermediate transfer belt 110. The average of values measured at least five different positions is used as a management value. In consideration of the cleaning property of the belt cleaning unit 16 for the intermediate transfer belt 110, it is preferable that the roughness Rzjis of the intermediate transfer belt 110 is within a range from 0.26 μm to 0.67 μm, inclusive.
The contact area between the photosensitive drum 1a and the front surface of the intermediate transfer belt 110 with the grooves 84 provided in the front surface of the intermediate transfer belt 110 is smaller than the contact area without the grooves 84. When the contact area decreases, the contact resistance increases to increase the apparent electrical resistance on the outer circumferential surface of the intermediate transfer belt 110. Thus, providing the grooves 84 in the front surface of the intermediate transfer belt 110 enables controlling the contact resistance between the intermediate transfer belt 110 and the photosensitive drum 1a. More specifically, the ratio of the surface resistivity measured from the outer circumferential surface of the intermediate transfer belt 110 to the surface resistivity measured from the inner circumferential surface thereof is settable with the surface profiles and surface roughness of the intermediate transfer belt 110.
As described above, even with a single-layer intermediate transfer belt, controlling the surface profile and surface roughness of the intermediate transfer belt 110 enables setting ρs1/ρs2 described in the first exemplary embodiment to a desired range. This enables providing a favorable primary transfer efficiency and a favorable image quality, as in the first exemplary embodiment.
In the present exemplary embodiment, the mold transfer (imprint processing) is exemplified as a means for providing groove profiles to the front surface of the intermediate transfer belt 110. Alternatively, the intermediate transfer belt 110 may be subjected to roughening process by using sandpaper or wrapping paper. In molding the intermediate transfer belt 110, a method for providing predetermined profiles to the intermediate transfer belt 110 is also applicable. With this method, uneven shapes are provided in advance on the mold to be in contact with the front surface of the intermediate transfer belt 110.
While the present exemplary embodiment has been described about a configuration for providing groove profiles to a single-layer intermediate transfer belt, the present disclosure is not limited thereto. A specified surface resistivity ratio may be implemented in conformance with the electrical characteristics of the material by providing predetermined profiles to the front surface of the intermediate transfer belt 110.
A third exemplary embodiment will be described below with reference to
In the primary transfer process, applying the primary transfer voltage to the photosensitive drum 1a from the primary transfer power source 23 in as short time as possible is advantageous to the operating lives of the photosensitive drum 1a, the intermediate transfer belt 10, and other components. More specifically, in the image forming operation, it is desirable to apply the primary transfer voltage as late as possible immediately before starting the primary transfer of the toner image from the photosensitive drum 1a to the intermediate transfer belt 10. This prevents the degradation of components due to electrical conduction and is desirable from the viewpoint of durability. The present exemplary embodiment is characterized in controlling the timing of applying the primary transfer voltage in synchronization with the timing when the toner image carried on the photosensitive drum 1a enters the primary transfer portion N1.
As illustrated in
At time t1, the leading end of the toner image carried on the photosensitive drum 1a (in the rotational direction of the photosensitive drum 1a) enters the primary transfer portion N1. At the time t1, the photosensitive drum 1a is applied with the primary transfer voltage Vt1 by the primary transfer power source 23.
In the control according to the present exemplary embodiment, the primary transfer voltage Vt1 is applied from the primary transfer power source 23 to the photosensitive drum 1a when the toner image developed on the photosensitive drum 1a reaches the primary transfer portion N1. With the application of the primary transfer voltage Vt1, the CPU 276 controls the charge power source 281 and the developing power source 280 to changes the dark portion potential Vd and the developing potential Vdc, respectively. Thus, the CPU 276 maintains various potential differences before and after the application of the primary transfer voltage Vt1 to prevent the occurrence of failures in the charging and the developing processes.
While the primary transfer voltage Vt1 is applied to the photosensitive drum 1a at the time t1 when the leading end of the toner image carried on the photosensitive drum 1a enters the primary transfer portion N1 in the exemplary embodiment, the application timing of the primary transfer voltage Vt1 is not limited thereto. For example, the primary transfer voltage Vt1 may be applied before the time t1. In such a case, an appropriate primary transfer current is applicable in the primary transfer portion N1 before the toner image reaches the primary transfer portion N1. The configuration for applying the primary transfer voltage Vt1 at least after the time t0 enables preventing the degradation of components due to electrical conduction to a further extent than that in the configuration for applying the primary transfer voltage Vt1 at the time t0.
As described above, combining the configuration of the present exemplary embodiment with the configurations of the first and the second exemplary embodiments enables not only producing the effects obtained by the configurations of the first and the second exemplary embodiments but also preventing the degradation of components due to electrical conduction.
While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2022-154906, filed Sep. 28, 2022, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-154906 | Sep 2022 | JP | national |
