Image forming apparatus

Abstract: An image forming apparatus capable of surely transferring an image to produce an attractive image and, in addition, surely separating a transfer medium from a photoconductive element with no regard to the environment. A transfer belt (3) is passed over a drive roller (4) and driven rollers (5-7). The rollers 6 and 7 are connected to ground. The roller 5 plays the role of a bias roller or bias electrode while the roller 4 remains in an electrically floating state. The belt (3) contacts the photoconductive drum (1) over a predetermined nip width. Optionally the belt (3) acts as an intermediate transfer medium.

Description: BACKGROUND OF THE INVENTION The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus of the type transferring a toner image from a photoconductive element to a transfer medium. An image forming apparatus of the type described includes a transfer and separation device for transferring a toner image from a photoconductive element to a sheet or similar transfer medium and then separating the sheet from the element. This kind of device has customarily been implemented by a corona transfer and separation system, i.e., a first and a second corona discharger assigned to image transfer and sheet separation, respectively. The first corona discharger effects corona discharge at the rear of the sheet to transfer the toner image from the photoconductive element to the front of the sheet. The second corona discharger applies corona discharge to the rear of the sheet carrying the toner image to separate it from the photoconductive element. A contact type transfer and separation system is another conventional system available for the image forming apparatus. In this type of system, while a transfer belt is in rotation, a bias potential is applied to the belt from a power source to transfer the toner image from the photoconductive element to a sheet or similar medium carried on the belt. The sheet with the toner image is separated from the photoconductive element by being electrostatically adhered to the belt. The transfer belt is sometimes replaced with a transfer roller. The contact type transfer and separation system has been proposed in various forms, as disclosed in Japanese Patent Laid-Open Publication No. 2 123385, 2 123386, and 2187380 by way of example. Another conventional image forming apparatus includes a carrier for carrying a toner image transferred thereto at a transfer position and transporting it while being rotated. In this type of apparatus, a toner image formed on a photoconductive element is transferred to a belt at a first transfer position. As the belt is rotated to transport the toner image to a second transfer position, the toner image is transferred from the belt to a sheet. At a position upstream of the first transfer position, a transfer potential is applied to the belt to transfer the toner image from the photoconductive element to the belt. The contact type image transfer and sheet separation system is advantageous over the corona type system in that it reduces ozone and requires only a low power source voltage. However, the problem with the transfer belt is that the adequate bias voltage to be applied from the power source to the belt changes due to various causes including irregularities in the resistance of the belt, varying ambient conditions, kind of sheets, and area of a toner image. This prevents the toner image from being surely transferred from the belt to the sheet. Specifically, the amount of charge deposited on the belt by the bias potential from the power source deviates from one required to effect desirable image transfer due to irregularities in the resistance of the belt ascribable to the production line, changes in the resistance ascribable to the varying ambient conditions, changes in the material and thickness of sheets, etc. More specifically, when the amount of charge required to effect desirable image transfer is deposited on a transfer belt, discharge does not occur in a pretransfer region upstream of the nip portion between the photoconductive element and the belt. In this condition, a toner charged to positive polarity, for example, i s transferred to the sheet carried on the belt in a transfer region. In this case, a bias potential is applied from a power source to the belt. When the actual amount of charge on the belt is deviated from the expected one due to the above-stated reasons, discharge occurs in the pretransfer region. This causes a negative charge to deposit on the toner and thereby charges the front and the rear of the belt to positive polarity and negative polarity, respectively. As a result, despite that the bias potential from the power source is adequate, the toner is prevented from being transferred from the photoconductive element to the sheet, resulting in the local omission of an image on the sheet. Further, the transfer belt not only transfer the toner image from the photoconductive element to a sheet or similar transfer medium, but also separates the sheet from the element by electrostatically retaining it thereon. However, the problem is that the separation of the sheet from the photoconductive element depends on the ambient conditions. Particularly, when the water content of the sheet increases in a hot and humid environment, it is likely that the sheet is adhered to the photoconductive element and not to the belt and cannot be separated from the element. Should the sheet be forcibly separated from the photoconductive element by a pawl or similar implementation, it would be scratched or creased to degrade the image quality. In the electrophotographic image forming apparatus, a transfer potential is applied to the belt at a position upstream of the first transfer position so as to transfer the toner image from the photoconductive element to the belt. This brings about a problem that the toner flies toward the belt at a position upstream of the first transfer position, thickening lines, blurring characters, reducing sharpness or otherwise degrading images. SUMMARY OF THE INVNTION The problem to be solved is to avoid charge being transferred from the belt to the sheet. This problem is solved by the independent claims 1, 2, 3, 6, 7, 8, 10, and 11. Dependent claims pertain to embodiments of advantage. Advantageously an image forming apparatus is provided which is capable of surely transferring an image to produce an attractive image and, in addition, insuring desirable separation of a transfer medium from a photoconductive element with no regard to the environment. Advantageously an image forming apparatus comprises a photoconductive element for forming a toner image thereon, a transfer medium movable in contact with the photoconductive element over a predetermined nip width to allow the toner image to be transferred from the photoconductive element to the transfer medium, a transfer bias applying device for applying a predetermined transfer bias to the transfer medium, and a potential gradient generating device for providing the transfer bias with a potential gradient such that an amount of transfer of the toner image from the photoconductive element to the transfer medium increases in a region upstream of a transfer region with respect to the photoconductive element and terminating at a point where the photoconductive element and transfer medium begin to contact each other. Also advantageously an image forming apparatus comprises a photoconductive element for forming a toner image thereon, a transfer belt movable in contact with the photoconductive element over a predetermined nip width for transporting a sheet to allow the toner image to be transferred from the photoconductive element to the sheet, a transfer potential bias applying device for applying a predetermined transfer bias to the transfer belt, a potential gradient generating device for providing the transfer bias with a potential gradient such that an amount of transfer of the toner image from the photoconductive element to the sheet increases in a region upstream of a nip portion with respect to the photoconductive element and terminating at a point where the photoconductive element and transfer belt begin to contact each other, and a bias applying device for applying a bias to the potential gradient generating device after the sheet has moved a predetermined distance away from the nip portion. Further advantageously, an image forming apparatus comprises a photoconductive element for forming a toner image thereon, a transfer belt movable in contact with the photoconductive element over a predetermined nip width for transporting a sheet to allow the toner image to be transferred from the photoconductive element to the sheet, a transfer bias applying device contacting the transfer belt at a position downstream of the photoconductive element for applying a predetermined bias to the transfer belt, and a potential gradient generating device having a dielectric layer on a surface thereof and contacting the transfer belt at a position upstream of the photoconductive element for providing the transfer bias with a potential gradient such that an amount of transfer of the toner image from the photoconductive element to the sheet increases in a region upstream of the nip portion with respect to the photoconductive element and terminating at a point where the photoconductive element and transfer belt contact each other. Moreover advantageously, an image forming apparatus comprises a photoconductive element for forming a toner image thereon, a transfer belt movable in contact with the photoconductive element over a predetermined nip width for transporting a sheet to allow the toner image to be transferred from the photoconductive element to the sheet, a transfer bias applying device contacting said transfer belt at a position downstream of the photoconductive element for applying a predetermined bias to the transfer belt, and a potential gradient generating device having an elastic dielectric layer on a surface thereof and contacting a rear of the transfer belt in a position upstream of the photoconductive element for providing the transfer bias with a potential gradient such that an amount of transfer of the toner image from the photoconductive element to the sheet increases in a region upstream of the nip portion with respect to said transfer belt and terminating at a point where the photoconductive element and transfer belt contact each other. In addition advantageously an image forming apparatus comprises a photoconductive element for forming a toner image thereon, a transfer medium contacting the photoconductive element over a predetermined nip width in a transfer region and undergoing a step o f transferring the toner image formed on the photoconductive element a plurality of times, a first electrode contacting the transfer medium at a position downstream of the transfer region, a second electrode contacting the transfer medium a t a position upstream of the transfer region, and a potential gradient generating device for providing the transfer bias with a potential gradient by applying a transfer bias to the first electrode or both of the first and said second electrodes, and applying, when a toner is absent on the transfer medium, a bias of the same polarity as the toner to the second electrode or applying, when the toner is present on the transfer medium, a bias of opposite polarity to the toner to the second electrode, such that an amount of transfer of the toner image from the photoconductive element to the transfer medium increases in a region upstream of a transfer region with respect to the photoconductive element and terminating at a point where the photoconductive element and transfer medium contact each other. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: FIGS. 1 and 2 are respectively a fragmentary front view and a section showing an image forming apparatus embodying the present invention; FIG. 3 is a graph indicative of the results o f experiments; FIGS. 4-8 are front views each showing an alternative embodiment of the present invention; FIG. 9 is a timing chart demonstrating a specific operation of the embodiment shown in FIGS. I and 2; FIG. 10 is a timing chart representative of a specific operation of the embodiment shown in FIG. 5; FIG. 11 is a timing chart representative of a specific operation of the embodiment shown in FIG. 6; FIG. 12 is a timing chart demonstrating a specific operation of the embodiment shown in FIG. 7; FIGS. 13 and 14 are block diagrams each schematically showing another alternative embodiment of the present invention; FIGS. 15 and 16 are front views each showing another alternative embodiment of the present invention; FIGS. 17 and 18 are views showing problems particular to a conventional image forming apparatus; FIGS. 19, 20 and 21 are graphs indicative of, respectively, potential distributions particular to the embodiments shown in FIGS. 1 and 2, FIG. 6, and FIG. 7; FIG. 22 is a side elevation showing another alternative embodiment of the present invention; FIG. 23 shows a potential gradient of an intermediate transfer belt included in the embodiment of FIG. 22; FIG. 24 is a graph showing a relation between the potential between a bias roller and a contact point particular to the embodiment of FIG. 22. DESCRIPTION OF THE PREFERRED EMBODIMENTS To better understand the present invention, a brief reference will be made to a conventional image forming apparatus, particularly a contact type image transfer and sheet separation system available therewith, shown in FIGS. 17 and 18. The contact type image transfer and sheet separation system is advantageous over a corona type system in that it reduces ozone and requires only a low power source voltage, as discussed earlier. However, the problem with a transfer belt is that the adequate bias voltage to be applied from a power source to the belt changes due to various causes including irregularities in the resistance of the belt, varying ambient conditions, kind of sheets, and area of a toner image. This prevents a toner image from being desirably transferred from the belt to a sheet. Specifically, the amount of charge deposited on the belt by the bias potential from the power source deviates from one required to effect desirable image transfer due to irregularities in the resistance of the belt ascribable to the production line, changes in the resistance ascribable to the varying ambient conditions, changes in the material and thickness of sheets, etc. More specifically, as shown in FIG. 17, when the amount of charge required to effect desirable image transfer is deposited on a transfer belt, discharge does not occur in a pretransfer region upstream of the nip portion between a photoconductive element 37 and the belt. In this condition, a toner 39 charged to positive polarity, for example, is transferred to a sheet 38 carried on the belt in a transfer region. In this case, a bias potential is applied from a power source, not shown, to the belt 38. As shown in FIG. 18, when the actual amount of charge on the belt is deviated from the expected one due to the above-stated reasons, discharge occurs in the pretransfer region. This causes a negative charge to deposit on the toner 39 and thereby charges the front and the rear of the belt to positive polarity and negative polarity, respectively. As a result, despite that the bias potential from the power source is adequate, the toner 39 is prevented from being transferred from the photoconductive element 37 to the sheet 38, resulting in the local omission of an image on the sheet. Referring to FIG. 2, part of an image forming apparatus embodying the present invention is shown and implemented as an electrophotographic copier. As shown, the apparatus includes an image carrier in the form of a photoconductivc drum 1. The drum 1 is uniformly charged by a main charger while being rotated by a drive mechanism, not shown. A writing device writes image data on the charged surface of the drum 1 to form an electrostatic latent image. A developing unit develops the latent image to produce a corresponding toner image. A recording medium in the form of a sheet is fed from a sheet feed device to a register roller 2, brought to a stop for a moment, and then driven toward a transfer belt 3 in synchronism with the toner image formed on the drum 1. At least the front of the transfer belt 3 is made of a dielectric material. The transfer belt 3 is passed over a drive roller 4 and driven rollers 5-7. The rollers 6 and 7 are connected to ground. The roller 5 plays the role of a bias roller or bias electrode while the roller 4 remains in an electrically floating state. As soon as the leading edge of the sheet approaches a portion where the drum 1 and transfer belt 3 are to contact, a solenoid 9 is energized to urge a lever 10 upward. The lever 10 in turn raises one side of the belt assembly, i.e., the belt 3 and rollers 4-7 until the belt 3 contacts the drum 1. The drive roller 4 is driven by a motor to in turn rotate the transfer belt 3. The belt 3 contacts the drive roller 4 at a position upstream of the portion where it is capable of contacting the drum 1, and contacts the bias roller 5 at a position downstream of the contact portion. The belt 3 contacts the drum 1 over a predetermined nip width. As shown in FIGS. 1 and 9, when the belt 3 contacts the drum 1, a power source 8 applies to the bias roller 5 a predetermined bias voltage whose polarity is opposite to the polarity of the toner deposited on the drum 1, thereby depositing a corresponding charge on the belt 3. The belt 3 is made of a material having a specific volume resistivity (10 6 Ωcm to 1 0 12 Ωcm). Hence, a current flows toward the rollers 6 and 7 due to the bias voltage from the bias roller 5, resulting in the fall of voltage. While the sheet is transported between the belt 3 and the drum 1, the toner image is transferred from the drum 1 to the sheet due to the above-mentioned bias voltage applied from the power source 8 to the belt 3. The sheet is polarized by the charge applied from the power source 8 to the belt 3. The polarizing voltage of the sheet and the true charge of the belt 3 generate an electrostatic force. As a result, the sheet is conveyed by the belt 3 while being electrostatically adhered to the belt 3. While the sheet is conveyed by the belt 3, the charge thereof is reduced by being released to ground via the belt 3 and rollers 6 and 7. The rate at which the charge of the sheet decreases greatly depends on the resistance R and capacitance C of the sheet and is determined in terms of a time constant τ = R·C. The sheet is transported toward a fixing station by the belt 3. As the sheet approaches the inlet of the fixing station, the charge thereof is reduced to in turn reduce the electrostatic force acting between the sheet and the belt 3. Consequently, the sheet is separated from the belt 3 by the roller 7 connected to ground and having a small diameter and the elasticity of the sheet. Then, the toner image carried on the sheet is fixed at the fixing station. Preferably, the roller 7 has a diameter ranging from 14 mm to 16 mm. As soon as the trailing edge of the sheet moves away from the nip portion between the drum 1 and the belt 3, the solenoid 9 is deenergized to retract the lever 10 and, therefore, the belt assembly including the belt 3 and rollers 4-7. As a result, the belt 3 is brought out of contact with the drum 1. This is to protect the drum I from deterioration while the transfer of a toner image is not performed. While the belt 3 is in rotation, the toner scattered around from the drum 1 without being transferred to the sheet is directly deposited on the belt 3. This part of the toner has the charge thereof reduced by the rollers 6 and 7 connected to ground and then scraped off by a cleaning blade 11 into a collecting bottle 12. In the illustrative embodiment, only the bias roller 5 deposits a charge on the belt 3. When the bias voltage from the power source is applied not only to the bias roller 5 but also to the drive roller 4, the potential distribution on the belt 3 has a linear gradient in a portion T between the rollers 4 and 5. Experiments showed that the belt 3 fails to electrostatically retain a sheet thereon as the water content of the sheet is as great as 8 % to 11 % due to a hot and humid environment. By contrast, when the bias voltage is applied only to the bias roller 5 as shown in FIG. 9, the embodiment insures the separation of the sheet from the drum 1 as shown in FIG. 3. This is presumably because when the charge is applied by the drive roller 4 contacting the belt 3 at a position upstream of the drum I, the charge penetrates into the leading edge of the sheet as the water content of the sheet increases, preventing the sheet from electrostatically adhering to the belt 3. When the drive roller 4 does not apply a charge to the belt 3 as in the embodiment, a charge does not penetrate into the leading edge of a sheet being transported by the belt 3. Hence, a repulsive force is not generated between the belt 3 and the sheet which would prevent the sheet from being fully separated from the drum 1. FIG. 4 shows a second embodiment of the present invention. As shown, the drive roller 4 plays the role of a bias roller or bias electrode at the same time. The drive roller 4 is connected to ground via a varistor, Zener diode or similar constant voltage element 13. While the bias voltage to be applied to the bias roller is open to choice, it should preferably be close to the bias voltage to be applied to the downstream bias roller 5. Specifically, in this embodiment, the bias voltage from the power source 8 is applied to the downstream bias roller 5. The upstream bias roller 4 is maintained at substantially the same potential as the downstream bias roller 5. This is successful in maintaining the potential at the position where the drum 1 and belt 3 contact stable and, therefore, insuring desirable toner image transfer with no regard to, for example, irregularities in the resistance of the belt 3. In addition, the charge injection into the sheet in a humid environment is reduced to promote sure separation of the sheet from the drum 1. Particularly, since this embodiment is even more stable than the first embodiment regarding the transfer of the toner image, the bias voltage to be applied from the power source 8 to the bias roller 5 can be low. Referring to FIG. 5, a third embodiment of the present invention will be described which is similar to the first embodiment except for the following. The drive roller 4 plays the role of a bias roller or bias electrode. As shown in FIG. 10, a power source 14 starts applying a bias voltage to the drive roller 4 at the time when the sheet enters the nip portion between the drum 1 and the belt 3 and contacts the drum 1. Again, as shown in FIG. 19, the potential distribution of the belt 3 has a linear gradient in the portion T. The third embodiment, like the first embodiment, enhances the separation of the sheet from the drum 1 and transfers the toner image to the sheet stably with no regard to irregularities in the resistance of the belt 3. Consequently, the belt 3 can be produced and selected at a high yield. FIG. 6 shows a fourth embodiment of the present invention which is similar to the third embodiment except for a variable power source 15 substituted for the power source 14. As shown in FIG. 11, the variable power source 15 applies a bias voltage lower than the bias voltage to the bias roller 5 to the bias roller 4 and at the same time as the voltage to the bias roller 5. As soon as the leading edge of the sheet enters the nip portion between the drum 1 and the belt 3, the power source 15 applies the same bias voltage as applied to the bias roller 5 to the bias roller 4. The resulting potential distribution of the belt 3 is shown in FIG. 20. As stated above, the fourth embodiment maintains the bias voltage to the bias roller 4 lower than the bias voltage to the bias roller 5 until the leading edge of the sheet enters the nip portion between the drum 1 and the belt 3. This also enhances the separation of the sheet from the drum 1. FIG. 7 shows a fifth embodiment of the present invention which is similar to the first embodiment except that a power source 16 applies a bias voltage to the bias rollers 4 and 5 at a timing shown in FIG. 12. Specifically, the power source 16 starts applying a bias voltage to the bias rollers 4 and 5 at the time when the leading edge of the sheet has moved a predetermined distance shorter than 8 8 mm away from the nip between the drum L and the belt 3. As a result, the toner image is not transferred to the sheet over the predetermined distance as measured from the leading edge thereof. Specifically, the sheet is simply left blank over several millimeters as measured from the leading edge thereof. However, the sheet is surely separated from the belt 3 at the inlet of the fixing station, causing the toner image to be fixed there. In this case, the belt 3 has a potential gradient shown in FIG. 21. FIG. 8 is representative of a sixth to an eighth embodiment corresponding to the third to fifth embodiments, respectively. As shown, in the sixth to eighth embodiments, the distance LB between the nip portion between the drum 1 and the belt 3 and the bias roller 5 is selected to be shorter than the distance LA between the nip portion and the bias roller 4. The power source 8, 15 or 16 starts applying the bias voltage to the bias roller 4 or 5 when the leading edge of the sheet has moved to a point A which is downstream of the roller 4 by the distance LB. Such alternative embodiments are also successful in surely separating the sheet from the drum 1. A ninth embodiment of the present invention is similar to the third embodiment of FIG. 5 except that the time for applying the bias voltage to the bias roller 4 is adjustable at the outside of the apparatus. While the third embodiment starts applying the bias voltage to the bias roller 4 from the power source 14 after the sheet has contacted the drum 1, it is likely that the sheet transport control differs from one machine to another or changes within the same machine due to the wear of a sheet transport system. When the time for applying the bias voltage to the upstream bias roller 4 is too early, a charge is apt to deposit on the belt 3 before the sheet contacts the drum 1, making the separation of the sheet from the drum 1 unstable in a humid environment. Conversely, when the above-mentioned time is too late, the bias voltage from the power source 14 is applied to the bias roller 4 after the leading edge of the sheet has moved away from the nip portion between the drum 1 and the belt 3. Then, it is likely that the toner image cannot be surely transferred to the leading edge portion of the sheet when the belt 3 has a resistance component. In the light of this, the ninth embodiment allows the time for applying the bias to the bias roller 4 to be changed at the outside of the apparatus. FIG. 13 shows circuitry for implementing the ninth embodiment. As shown, a main control section 17 has a CPU (Central Processing Unit 18, a ROM (Read Only Memory) 19, a RAM (Random Access Memory) 20, an input circuit 21, a load driver 22, and a system control interface (I/F) 23. A system controller 24, a high tension power source 25 and an operating section (SP mode) 26 are connected to the main control section 17. A serviceman, for example, manipulates the operating section 26 to condition the apparatus for a serviceman program (SP) mode and again manipulates it to enter an adjusting value associated with the application of the bias voltage to the bias roller 4. The ROM 19 stores a program according to which the CPU 18 operates. As a signal indicative of the SP mode is entered on the operating section 26 and applied to the CPU 18 via the input circuit 21, the CPU 18 sets up the SP mode. When the adjusting value associated with the application of the bias voltage is entered on the operating section 26, it is written to the RAM 20 which is backed up by a battery. The CPU 18 adjusts the time for applying the bias voltage to the bias roller 4 from the power source 14 via the load driver 22 on the basis of the adjusting value stored in the RAM 20. If desired, in the fourth to eighth embodiments, the time for applying the the bias to the upstream bias roller 4 may also be adjusted at the outside of the apparatus to insure both of sure image transfer and sure sheet separation. Referring to FIG. 14, circuitry representative of a tenth embodiment of the present invention is shown. As shown, a main control section 26 has a CPU 27, a ROM 28, a RAM 29, an input section 30, a load driver 31, and a system control I/F 32. A system controller 33, a high tension power source 34 and a humidity sensor 35 are connected to the main control section 26. Located in close proximity to the sheet feed device, the humidity sensor senses humidity around sheets stacked in the sheet feed device. During an ordinary mode operation (e.g. during copying), as the humidity sensor 35 sends a humidity signal to the CPU 27 via the input circuit 30, the CPU 27 determines whether or not the sensed humidity is higher than a predetermined humidity, e.g., 70 % in terms of relative humidity. If the actual humidity is higher than the predetermined one, the CPU 27 retards the time for applying the bias voltage to the bias roller 4 from the power source 14 via the load driver 31, i.e., causes the power source 14 to start applying the bias voltage to the roller 4 after the sheet has contacted the drum 1. When the sensed humidity is lower than 70 %, the CPU 27 causes the power source 14 to start applying the bias voltage to the bias roller 4 at the same time as the power source 8 applies the bias voltage to the bias roller 5 via the load driver 31. In this configuration, even when the water content of the sheet is high due to high humidity, the charge injection from the upstream bias roller 4 into the sheet does not occur, so that the sheet is stably separated from the drum 1. After the leading edge of the sheet has been separated from the drum 1, the bias rollers 4 and 5 inject charges into the sheet. Hence, the toner image is stably transferred to the sheet at the rear of the leading end portion of the belt 3. So long as the humidity is lower than 70 %, the separation of the sheet from the drum I is satisfactory. Since the bias rollers 4 and 5 sequentially deposit a charge on the sheet, the toner image is stably transferred from the leading edge toward the trailing edge of the sheet at all times. The above-stated advantages of the tenth embodiment are also achievable with an absolute humidity sensor in place of the relative humidity sensor. If desired, the humidity sensor may be used in combination with a temperature sensor to control the time for applying the bias voltage from the power source 14 to the bias roller 4. Further, the humidity sensor scheme of the tenth embodiment is also applicable to the fourth to eighth embodiments, if desired. FIG. 15 shows an eleventh embodiment which is similar to the fifth embodiment except that the bias roller 4 is provided with a surface layer 4a made of a dielectric material. The surface layer 4a has a specific volume resistivity of, for example, 10 6 Ωcm to 10 12 Ωcm and a thickness of 0.2 mm to 3 mm. The rollers 5 and 7 contacting the belt 3 at positions downstream of the drum 1 are made of metal. As the power source 16 applies a voltage to the bias rollers 4 and 5, the rollers 4 and 5 sequentially deposit a voltage on the belt 3 to maintain the potential at the portion where the belt 3 contacts the drum 1 stable, thereby insuring desirable transfer of the toner image. Since the bias roller 5 deposits a charge on the belt 3 more efficiently than the bias roller 4, the charge injection from the upstream bias roller 4 into the sheet sparingly occurs in a humid environment. This promotes sure separation of the sheet from the drum 1 in such a humid environment. Again, as shown in FIG. 19, the potential distribution of the belt 3 has a linear gradient in the portion T between the portions where the rollers 4 and 5 contact the belt 3. FIG. 16 shows a twelfth embodiment of the present invention. As shown, this embodiment is similar to the eleventh embodiment except that a bias roller 36 is held in contact with the rear of the belt 3 at the position where the belt 3 contacts the drum 1, and in that the power source 16 applies a voltage to a bias roller 36 as well as to the roller 5. The bias roller 36 has a surface layer made of an elastic dielectric material. For example, the surface layer of the roller 36 is made of a material having a specific volume resistivity of 10 6 Ωcm to 10 12 Ωcm and a hardness less than 50° in terms of modulus hardness.and provided with a thickness of greater than I mm. As shown in FIG. 19, the potential of the belt 3 linearly changes in the portion T between the portions where the rollers 4 and 5 contact the belt 3. As the power source 16 applies a voltage to the bias rollers 36 and 5, the rollers 36 and 5 each efficiently deposits a charge on the belt 3 even when the transfer voltage in the position where the belt 3 contacts the drum 1 is low. High transfer voltages are apt to cause an image to be locally omitted when transferred to a sheet. Since the surface layer of the bias roller 36 has a medium resistance, there can be eliminated the damage to the bias roller 36 and belt 3 due to the leakage of the charge and, therefore, the resulting defective image transfer. Moreover, since the charge deposition on the belt 3 by the bias rollers 36 and 5 does not occur at the upstream side with respect to the transfer station, no charges are injected into the sheet in a humid environment before the image transfer, insuring positive sheet separation. In the embodiments shown and described, the transfer belt 3 may be replaced with a transfer roller, if desired. The transfer roller is as effective as the belt 3 regarding the stable sheet separation and, in addition, frees the sheets from the trace of a separator, creases and jams. In summary, it will be seen that the present invention provides an image forming apparatus having various unprecedented advantages, as enumerated below. (1) An image can be desirably transferred without any degradation, and a transfer medium can be surely separated. (2) The separation of the transfer medium is not effected by the environment. (3) Desirable image transfer is achievable with no regard to irregularities in the resistance of a transfer belt. (4) Although sheet transport control changes from one machine to another or within the same machine due to aging, an image can be surely transferred. (5) There can be eliminated the damage to electrodes and transfer belt due to charge leakage and, therefore, defective image transfer ascribable to such damage. Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.

Claims: An image forming apparatus comprising: a photoconductive element (1) for forming a toner image thereon; a transfer belt (3) movable in contact with said photoconductive element (1) over a predetermined nip portion for transporting a sheet to allow the toner image to be transferred from said photoconductive element to said sheet; a first transfer potential bias applying means (5, 8) for applying a first predetermined transfer bias to said transfer belt downstream from the nip portion by contacting the transfer belt; a second transfer potential means (4, 13; 4, 14; 4, 15; 4, 16) being upstream from the nip portion and contacting the transfer belt; and characterized in that said second transfer applying means remains in an electrically floating state and in that the first predetermined transfer bias is started to be applied when the leading edge of the sheet approaches the nip portion. An image forming apparatus comprising: a photoconductive element (1) for forming a toner image thereon; a transfer belt (3) movable in contact with said photoconductive element (1) over a predetermined nip portion for transporting a sheet to allow the toner image to be transferred from said photoconductive element to said sheet; a first transfer potential bias applying means (5, 8) for applying a first predetermined transfer bias to said transfer belt downstream from the nip portion by contacting the transfer belt; a second transfer potential bias applying means (4, 13; 4, 14; 4, 15; 4, 16) for providing a second transfer bias to said transfer belt upstream from the nip portion by contacting the transfer belt; and characterized in that the second applying means is connected to ground via a varistor or Zener diode or similar constant voltage element (13) which results in a potential close to the first predetermined transfer bias. An image forming apparatus comprising: a photoconductive element (1) for forming a toner image thereon; a transfer belt (3) movable in contact with said photoconductive element (1) over a predetermined nip portion for transporting a sheet to allow the toner image to be transferred from said photoconductive element to said sheet; a first transfer potential bias applying means (5, 8) for applying a first predetermined transfer bias to said transfer belt downstream from the nip portion by contacting the transfer belt; a second transfer potential bias applying means (4, 13; 4, 14; 4, 15; 4, 16) for providing a second transfer bias to said transfer belt upstream from the nip portion by contacting the transfer belt; and characterized in that the second transfer bias is started to be applied when the sheet enters the nip portion such that the potential distribution of the belt (3) has a linear gradient in the nip portion increasing in upstream direction. An apparatus as claimed in claim 3, wherein a time for applying the second transfer bias is variable. An apparatus as claimed in claim 3, further comprising: humidity sensing means for sensing humidity; and control means responsive to an output of said humidity sensing means for adjusting the time for applying the second transfer bias when humidity is higher than a predetermined value or not adjusting said time when humidity is lower than said predetermined value. An image forming apparatus comprising: a photoconductive element (1) for forming a toner image thereon; a transfer belt (3) movable in contact with said photoconductive element (1) over a predetermined nip portion for transporting a sheet to allow the toner image to be transferred from said photoconductive element to said sheet; a first transfer potential bias applying means (5, 8) for providing a transfer bias to said transfer belt downstream from the nip portion by contacting the transfer belt; a second transfer potential bias applying means (4, 13; 4, 14; 4, 15; 4, 16) for providing a transfer bias to said transfer belt upstream from the nip portion by contacting the transfer belt; and characterized in that the same transfer bias is started to be applied to the first and second transfer potential bias applying means when the leading edge of the sheet enters the nip portion or at the time when the leading edge of the sheet has moved a predetermined distance shorter than 8 mm away from the nip. An image forming apparatus comprising: a photoconductive element (1) for forming a toner image thereon; a transfer belt (3) movable in contact with said photoconductive element (1) over a predetermined nip portion for transporting a sheet to allow the toner image to be transferred from said photoconductive element to said sheet; a first transfer potential bias applying means (5, 8) for applying a first predetermined transfer bias to said transfer belt downstream from the nip portion by contacting the transfer belt; a second transfer potential bias applying means (4, 13; 4, 14; 4, 15; 4, 16) for providing a second transfer bias to said transfer belt upstream from the nip portion by contacting the transfer belt; and characterized in that the second transfer bias is started to be applied to the second bias applying means after the leading edge of the sheet has been transported from the second transfer potential bias applying means toward the nip over a distance exceeding the distance between said first bias applying means and said photoconductive element, said distance being shorter than the distance between the second transfer potential bias applying means and the nip. An image forming apparatus comprising: a photoconductive element (1) for forming a toner image thereon; a transfer belt (3) movable in contact with said photoconductive element (1) over a predetermined nip portion for transporting a sheet to allow the toner image to be transferred from said photoconductive element to said sheet; a first transfer potential bias applying means (5, 8) for applying a transfer bias to said transfer belt downstream from the nip portion by contacting the transfer belt; a second transfer potential bias applying means (4, 13; 4, 14; 4, 15; 4, 16) for providing a transfer bias to said transfer belt upstream from the nip portion by contacting the transfer belt; and characterized in that the same transfer bias is started to be applied to the bias applying means after the leading edge of the sheet has been transported from the second transfer potential bias applying means toward the nip over a distance exceeding the distance between said first bias applying means and said photoconductive element, said distance being shorter than the distance between the second transfer potential bias applying means and the nip. An apparatus as claimed in claim 7 or 8, further comprising: humidity sensing means for sensing humidity; and control means responsive to an output of said humidity sensing means for adjusting the time for applying the second transfer bias when humidity is higher than a predetermined value or not adjusting said time when humidity is lower than said predetermined value. An image forming apparatus comprising: a photoconductive element (1) for forming a toner image thereon; a transfer belt (3) movable in contact with said photoconductive element (1) over a predetermined nip portion for transporting a sheet to allow the toner image to be transferred from said photoconductive element to said sheet; a first transfer potential bias applying means (5, 8) for applying a first predetermined transfer bias to said transfer belt downstream from the nip portion by contacting the transfer belt; a second transfer potential bias applying means (4, 13; 4, 14; 4, 15; 4, 16) for providing a second DC-transfer bias to said transfer belt upstream from the nip portion by contacting the transfer belt; and characterized in that the second transfer potential bias applying means is provided with a surface layer (4a) made of a dielectric material of a specific volume resistivity of 10 6 -10 12 Ω cm. An image forming apparatus comprising: a photoconductive element (1) for forming a toner image thereon; a transfer belt (3) movable in contact with said photoconductive element (1) over a predetermined nip portion for transporting a sheet to allow the toner image to be transferred from said photoconductive element to said sheet; a first transfer potential bias applying means (5, 8) for applying a first predetermined transfer bias to said transfer belt downstream from the nip portion by contacting the transfer belt; a second transfer potential bias applying means (36) for providing a second transfer bias to said transfer belt at the nip portion by contacting the transfer belt; and characterized in that the second transfer potential bias applying means (36) has a surface made of an elastic dielectric material of a specific volume resistivity of 10 6 -10 12 Ω cm.

Inventors: BISAIJI TAKASHI, HARASAWA YUKO, KAMIYAMA HIDEKI, KAWAISHI YASUNORI, MATSUDA ITARU, MOTOHASHI TOSHIAKI, TAKAHASHI MITSURU, TAKANO SATOSHI, YU HIDEO, BISAIJI, TAKASHI, HARASAWA, YUKO, KAMIYAMA, HIDEKI, KAWAISHI, YASUNORI, MATSUDA, ITARU, MOTOHASHI, TOSHIAKI, TAKAHASHI, MITSURU, TAKANO, SATOSHI, YU, HIDEO
Applicants: RICOH KK, RICOH COMPANY, LTD
Application year: 1993
Main classification: G03G15
IPC codes: H04N1, G03G15, G03G21