DE69330579T2 - Image forming apparatus - Google Patents

Description

BACKGROUND OF THE INVENTION

The present invention relates to a copier, a printer, a facsimile machine or a similar image forming apparatus of the type in which a toner image is transferred from a photoconductive member 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 member to a sheet or similar transfer medium and then separating the sheet from the member. This type of device has been conventionally implemented by a corona transfer and separation system, i.e., first and second corona dischargers dedicated to image transfer and sheet separation, respectively. The first corona discharger applies a corona discharge to the trailing end of the sheet to transfer the toner image from the photoconductive member to the leading end of the sheet. The second corona discharger applies the corona discharge to the trailing end of the sheet carrying the toner image to separate it from the photoconductive member.

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 rotating, a bias potential is applied to the belt from a power source to transfer the toner image from the photoconductive member to a sheet or similar medium carried on the belt. The sheet with the toner image is separated from the photoconductive member by electrostatically adhering it to the belt. The transfer belt is sometimes replaced by a transfer roller. The contact type transfer and separation system has been proposed in various forms, as disclosed in Japanese Laid-Open Patent Publication Nos. 2123385, 2123386 and 2187380, for example.

Another conventional image forming apparatus includes a carrier for carrying a toner image transferred thereon at a transfer position and transporting it while being rotated. In this type of apparatus, a toner image formed on a photoconductive member is transferred to a belt at a first transfer position. When 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 transferred to the belt to transfer the toner image from the photoconductive member 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 an appropriate 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, variations in environmental conditions, types of sheets, and area of a toner image. This prevents the toner image from being transferred securely from the belt to the sheet. More specifically, the amount of charge deposited on the belt by the bias potential from the power source differs from that required to effect a desired image transfer due to irregularities in the resistance of the belt attributable to manufacturing, changes in resistance attributable to varying environmental conditions, changes in the material and thickness of sheets, etc. When the amount of charge required to effect a desirable image transfer is deposited on a transfer belt, more specifically, discharge does not occur in a pre-transfer region downstream of the nip between the photoconductive member and the belt. Under this condition, a toner charged to a positive polarity, for example, is transferred to the sheet carried on the belt in a transfer region. In this case, a bias potential is applied to the belt from a power source. If the actual amount of charge on the belt differs from that expected based on the above reasons, a discharge occurs in the pre-transfer area. This causes a negative charge to be deposited on the toner, thereby charging the front and rear ends of the belt to a positive polarity and a negative polarity, respectively. As a result, even though the bias potential from the power source is appropriate, toner is prevented from being transferred from the photoconductive element to the sheet, resulting in a local image missing on the sheet.

Further, the transfer belt not only transfers the toner image from the photoconductive member to a sheet or similar transfer medium, but also separates the sheet from the member by electrostatically holding it thereon. However, the problem is that the separation of the sheet from the photoconductive member depends on the environmental conditions. In particular, when the water content of the sheet increases in a hot and humid environment, the sheet is likely to stick to the photoconductive member and cannot be separated from the belt and the member. Should the sheet be forcibly separated from the photoconductive member by a claw or similar implementation, it would be scratched or curled, thereby deteriorating 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 member to the belt. This involves a problem that the toner does not fly onto the belt at a position upstream of the first transfer position, thereby thickening lines, blurring characters, reducing sharpness, or otherwise deteriorating images.

SUMMARY OF THE INVENTION

The problem to be solved is to avoid charge being transferred from the belt to the sheet. This problem is solved by independent claims 1, 2, 3, 6, 7, 8, 10 and 11. Dependent claims relate to advantageous embodiments. Advantageously, an image forming apparatus is provided which is adapted to do so in the Capable of securely transferring an image to produce an attractive image and additionally ensuring a desired separation of a transfer medium from a photoconductive element, regardless of the environment.

Advantageously, an image forming apparatus comprises: a photoconductive member for forming a toner image thereon, a transfer medium movable in contact with the photoconductive member over a predetermined nip width to allow the toner image to be transferred from the photoconductive member 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 applying the transfer bias with a potential gradient such that the amount of transfer of the toner image from the photoconductive member to the transfer medium increases in an area upstream of a transfer area with respect to the photoconductive member and ends at a point where the photoconductive member and the transfer medium start to contact each other.

Also advantageously, an image forming apparatus comprises: a photoconductive member for forming a toner image thereon, a transfer belt movable in contact with the photoconductive member over a predetermined nip width for transporting a sheet to enable the toner image to be transferred from the photoconductive member 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 member to the sheet increases in a region upstream of a nip portion with respect to the photoconductive member and ends at a point where the photoconductive member and the transfer belt start to contact each other, and a bias applying device for applying a bias to the potential gradient generating device after the sheet has been moved a predetermined distance away from the clamping section.

Further advantageously, an image forming apparatus comprises: a photoconductive member for forming a toner image thereon, a transfer belt movable in contact with the photoconductive member over a predetermined nip width for transporting a sheet to enable the toner image to be transferred from the photoconductive member to the sheet, a transfer bias applying device which contacts the transfer belt at a position downstream of the photoconductive member to apply a predetermined bias to the transfer belt, and a potential gradient generating device which has a dielectric layer on a surface thereof and which contacts the transfer belt at a position upstream of the photoconductive member to provide the transfer bias with a potential gradient such that an amount of transfer of the toner image from the photoconductive member to the sheet increases in an area upstream of the nip portion with respect to the photoconductive member and at a point where the photoconductive element and the transfer belt touch each other.

Furthermore, an image forming apparatus advantageously comprises: a photoconductive member for forming a toner image thereon, a transfer belt movable in contact with the photoconductive member over a predetermined nip width to transport a sheet to allow the toner image to be transferred from the photoconductive member to the sheet, a transfer bias applying device which contacts the transfer belt at a position downstream of the photoconductive member to apply a predetermined bias to the transfer belt, and a potential gradient generating device which has an elastic dielectric layer on a surface thereof and which contacts a rear end of the transfer belt at a position upstream of the photoconductive member to provide the transfer bias with a potential gradient such that an amount of transfer of the toner image from the photoconductive member increases in an area upstream of the nip portion with respect to the transfer belt and at a Point where the photoconductive element and the transfer belt touch each other ends.

In addition, advantageously, an image forming apparatus comprises: a photoconductive member for forming a toner image thereon, a transfer medium that contacts the photoconductive member over a predetermined nip width in a transfer region and undergoes a step of transferring the toner image formed on the photoconductive member many times, a first electrode that contacts the transfer medium at a position downstream of the transfer region, a second electrode that contacts the transfer medium at 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 to both the first electrode and the second electrode, and, when a toner is not present on the transfer medium, applying a bias of the same polarity as the toner to the second electrode or, when the toner is present on the transfer medium, applying a Applying a bias voltage having a polarity opposite to the polarity of the toner of the second electrode such that the amount of transfer of the toner image from the photoconductive element to the transfer medium increases in an area upstream of a transfer area with respect to the photoconductive element and ends at a point where the photoconductive element and the 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 in conjunction with the accompanying drawings, in which:

Figs. 1 and 2 are a partial front view and a section, respectively, showing an image forming apparatus embodying the present invention;

Fig. 3 is a graph showing the results of the experiments;

Figures 4 to 8 are front views each showing an alternative embodiment of the invention;

Fig. 9 is a timing chart showing a specific operation of the embodiment shown in Figs. 1 and 2;

Fig. 10 is a timing chart illustrating a specific operation of the embodiment shown in Fig. 5;

Fig. 11 is a timing chart showing a specific operation of the embodiment shown in Fig. 6;

Fig. 12 is a timing chart showing a specific operation of the embodiment shown in Fig. 7;

Figs. 13 and 14 are block diagrams each 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 each showing problems specific to a conventional image forming apparatus;

Figs. 19, 20 and 21 are graphs respectively showing potential distributions specific to the embodiments shown in Figs. 1 and 2, Figs. 6 and 7;

Fig. 22 is a side elevation showing an alternative embodiment of the present invention;

Fig. 23 shows a potential gradient of an intermediate transfer belt included in the embodiment of Fig. 22; and

Fig. 24 is a graph showing a relationship between the potential between a bias roller and a contact point specific to the embodiment of Fig. 22.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to better understand the present invention, reference will be made briefly to a conventional image forming apparatus, particularly a contact type image transfer and sheet separation system available therewith, as 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 requires low power source voltage as previously discussed. However, the problem with a transfer belt is that the appropriate bias voltage to be applied from a power source to the belt varies due to various causes including irregularities in the resistance of the belt, variation in environmental conditions, type of sheets and area of the toner image. This prevents a toner image from being desirably transferred from the belt to a sheet. In particular, the amount of charge deposited on the belt by the bias potential from the power source differs from that required to effect desirable image transfer due to irregularities in the resistance of the belt attributable to manufacturing, changes in resistance attributable to changing environmental conditions, changes in the material and thickness of the 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 pre-transfer region downstream of the nip portion between a photoconductive member 37 and the belt. In this state, a toner 39 charged to positive polarity is transferred, for example, to a sheet 38 carried on the belt in a transfer region. In this case, a bias potential from a power source, not shown, is applied to the belt 38. As shown in Fig. 18, when the actual amount of charge on the belt deviates from that expected due to the reasons set forth above, a discharge occurs in the pre-transfer area. This causes a negative charge to be deposited on the toner 39 and thereby the leading end and trailing end of the belt are charged to a positive polarity and a negative polarity, respectively. As a result, even though the bias potential from the power source is appropriate, the toner 39 is prevented from being transferred from the photoconductive member 37 to the sheet 38, resulting in local omission of an image on the sheet.

Referring to Fig. 2, a portion 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 photoconductive 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 form a corresponding toner image. A recording medium in the form of a sheet is fed from a sheet feeding device to an aligner roller 2, stopped for a moment, and then driven toward a transfer belt in synchronization with the toner image formed on the drum 1. At least the front end of the transfer belt 3 is made of a dielectric material. The transfer belt 3 is passed over a drive roller 4 and drive rollers 5 to 7. The rollers 6 and 7 are connected to the ground. The roller 5 plays the role of a bias roller or a bias electrode, while the roller 4 remains in an electrically floating state. As soon as the leading edge of the sheet reaches a portion where the drum 1 and the transfer belt 3 are to contact, a solenoid 9 is energized to urge upwards a lever 10. The lever 10 in turn lifts one side of the belt assembly, ie the belt 3 and the rollers 4 to 7, until the belt 3 contacts the drum 1.

The drive roller 4 is driven by a motor to rotate the transfer belt 3 in turn. The belt 3 contacts the drive roller 4 at a position upstream of the portion where it is able to contact 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 a predetermined voltage to the bias roller 5 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 volume resistivity (106 Ω cm to 1012 Ω cm). Consequently, a current flows toward the rollers 6 and 7 due to the bias voltage from the bias roller 5, resulting in a drop in the 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 to the belt 3 from the power source 8. The sheet is polarized by the charge applied to the belt 3 from the power source 8. The polarizing voltage of the sheet and the true charge of the belt 3 generates an electrostatic force. As a result, the sheet is transported by the belt 3 while electrostatically adhering to the belt 3.

As the sheet is conveyed by the belt 3, its charge is reduced by being discharged to earth via the belt 3 and the rollers 6 and 7. The rate at which the charge of the sheet decreases depends essentially on the resistance R and the capacitance C of the sheet and is expressed as a time constant T = R C. The sheet is conveyed toward a fixing station by the belt 3. As the sheet approaches the inlet of the fixing station, its charge 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 which is connected to earth and has 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 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 de-energized to retract the lever 10 and therefore the belt assembly including the belt 3 and the rollers 4 to 7. As a result, the belt 3 is brought out of contact with the drum 1. This is to protect the drum 1 from deterioration while the transfer of a toner image is not being carried out.

While the belt 3 is rotating, the toner scattered by the drum 1 without being transferred to the sheet is deposited directly on the belt 3. The charge of this part of the toner is reduced by the rollers 6 and 7, which are connected to the earth and is then scraped by a cleaning blade 11 into a collection bottle 12.

In the embodiment shown, 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 of the belt 3 has a linear gradient in a section T between the rollers 4 and 5. Experiments showed that the belt 3 is unable to electrostatically hold a sheet thereon when the water content of the sheet is greater than 8% to 11% due to a hot and humid environment. In contrast, when the bias voltage is applied only to the bias roller 5 as shown in Fig. 9, the embodiment ensures that the sheet is separated from the drum 1 as shown in Fig. 3. This is presumably because when the charge is applied to the drive roller 4 contacting the belt 3 at a position upstream of the drum 1, the charge penetrates into a leading edge of the sheet because the water content of the sheet increases, preventing the sheet from electrostatically adhering to the belt 3.

Unless the drive roller 4 applies a charge to the belt 3 as in the embodiment, a charge does not penetrate the leading edge of a sheet being transported by the belt 3. Consequently, a repulsive force is not generated between the belt 3 and the sheet which would prevent the sheet from being completely 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 simultaneously. The drive roller 4 is connected to ground through a varistor, Zener diode or similar constant voltage element 13. While the bias voltage to be applied to the bias roller can be selected, it should preferably be close to the bias voltage applied to the downstream bias roller 5.

Particularly, in this embodiment, the bias voltage from the power source 8 is applied to the downstream bias roller S. The upstream bias roller 4 is maintained at substantially the same potential as the downstream bias roller S. This is successful in maintaining the potential at the position where the drum 1 and the belt 3 stably contact and therefore ensures desired toner image transfer regardless of, for example, irregularities in the resistance of the belt 3. In addition, charge injection into the sheet is reduced in a humid environment to ensure sure separation of the sheet from the drum 1. Particularly, since this embodiment is even more stable than the first embodiment in terms of transfer of the toner image, the bias voltage 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. 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 section 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 section T.

The third embodiment, similarly to the first embodiment, improves the separation of the sheet from the drum 1 and transfers the toner image to the sheet stably regardless of the irregularities of the resistance of the sheet 3. Consequently, the belt 3 can be produced and selected with 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 which replaces the power source 14. As shown in Fig. 11, the variable power source 15 applies a bias voltage lower than the bias voltage on the bias roller 5 to the bias roller 4 and at the same time as to the bias roller 5. As soon as the leading edge of the sheet enters the nip section between the drum 1 and the When the belt 3 enters the belt 3, the power source 15 applies the same bias voltage as is 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 mentioned above, the fourth embodiment keeps the bias on the bias roller 4 lower than the bias on the bias roller 5 until the leading edge of the sheet enters the nip section between the drum 1 and the belt 3. This also improves 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 mm from the nip portion between the drum 1 and the belt 3. As a result, the toner image is not transferred from the sheet to the predetermined distance when measured from the leading edge thereof. Specifically, the sheet is simply left white for several millimeters when measured from the leading edge thereof. However, the sheet is surely separated from the belt 3 at the inlet of the fixing station, thereby causing the toner image to be fixed there. In this case, band 3 has a potential gradient shown in Fig. 21.

Fig. 8 is representative of a sixth and an eighth embodiment, which correspond 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 to the bias rollers 4 and 5 when the leading edge of the sheet moves to a point A, which is downstream of the roller 4, by the distance LB. Such alternative embodiments are equally successful in safely 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 timing for applying the bias voltage to the bias roller 4 is adjustable on 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, the sheet transport control is likely to differ from one machine to another or differ within the same machine due to wear of a sheet transport system. If the timing for applying the bias voltage to the upstream bias roller 4 is too early, a charge tends 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. In contrast, if the above-mentioned timing is too late, the bias voltage is applied from the power source 14 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 securely transferred to the leading edge portion of the sheet if the belt 3 has a resistance component. In view of this, the ninth embodiment allows the timing for applying the bias voltage to the bias roller 4 to be changed on the outside of the apparatus.

Fig. 13 shows a circuit 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 voltage power source 25 and an operating section (SP mode) 26 are connected to the main control section 17. For example, a maintenance man manipulates the operating section 26 to condition the apparatus for a maintenance man program (SP) mode and manipulates it in turn to enter a setting value related to the application of preload to the preload roller 4.

The ROM 19 stores a program according to which the CPU 18 operates. When a signal indicating the SP mode is input to the operating section 26 and applied to the CPU 18 via the input circuit 21, the CPU 18 sets the SP mode. When the setting value related to the application of the bias voltage is input to the operating section 26, it is written into the RAM 20 backed up by a battery. The CPU 18 sets the time for applying the bias voltage to the bias roller 4 from the power source 14 via the load driver 22 based on the setting value stored in the RAM 20. If desired, in the fourth to eighth embodiments, the time for applying the bias voltage to the upstream bias roller 4 can also be set on the outside of the apparatus to ensure both secure image transfer and secure sheet separation.

Referring to Fig. 14, there is shown a circuit representing a tenth embodiment of the present invention. 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-voltage power source 34, and a humidity sensor 35 are connected to the main section 26. Near the sheet feeder is the humidity sensor which senses the humidity around the sheets stacked in the sheet feeder. During operation in the ordinary mode (for example, during copying), 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, for example, 70%, in terms of relative humidity. If the actual humidity is higher than the predetermined one, the CPU 27 delays the time for applying the bias voltage to the bias roller 4 from the power source 14 via the load driver 31, that is, it causes the power source 14 to start applying to begin the pre-tensioning of the roller 4 after the sheet has touched 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 charge driver 31. With this configuration, even if the water content of the sheet is high due to the 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 is separated from the drum 1, the bias rollers 4 and 5 inject charges into the sheet. Consequently, the toner image is stably transferred to the sheet at the trailing end of the leading end portion of the belt 3. As long as the humidity is lower than 70%, the separation of the sheet from the drum 1 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-described advantages of the tenth embodiment are also achievable with an absolute humidity sensor instead of the relative humidity sensor. If desired, the humidity sensor can be used in combination with a temperature sensor to control the timing of applying the bias voltage from the power source 14 to the bias roller 4. Furthermore, the humidity sensor scheme of the tenth embodiment is also applicable to the fourth through 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 volume resistivity of, for example, 10⁶ Ω cm to 10¹² Ω cm and a thickness of 0.2 mm to 3 mm. The rollers 5 and 7 which contact the belt 3 at positions downstream of the drum 1 are made of metal. When 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 keep the potential stable at the portion where the belt 3 contacts the drum 1, thereby ensuring a desired 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 hardly occurs in a humid environment. This promotes the safe 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 end of the belt 3 at the position where the belt 3 contacts the drum 1 and in that the power source 16 applies a bias 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 volume resistivity of 10⁶ Ω cm to 10¹² Ω cm and having a hardness of less than 50° in terms of modulus hardness and is provided with a thickness of more than 1 mm. As shown in Fig. 19, the potential of the belt 3 changes linearly in the section T between the sections where the rollers 4 and 5 touch the belt 3.

When the power source 16 applies a voltage to the bias roller 36 and 5, the rollers 5 and 36 each efficiently deposit a charge on the belt 3 even if the transfer voltage at the position where the belt 3 contacts the drum 1 is low. High transfer voltages tend to cause an image to be locally omitted when it is transferred to a sheet. Since the surface layer of the bias roller 36 has a medium resistance, the damage to the bias roller 36 and the belt 3 due to the leakage of the charge and must therefore be eliminated due to the resulting defective image transfer. Moreover, since the charge deposition on the belt 3 by the bias rollers 36 and 5 does not occur on the downstream side with respect to the transfer station, no charges are injected into the sheet in a humid environment prior to image transfer, thereby ensuring positive sheet separation.

In the embodiments shown and described, the transfer belt 3 can be replaced by a transfer roller if desired. The transfer roller is as effective as the belt 3 in providing stable sheet separation and in addition frees the sheets from the trail of a separator, curls and jams.

In summary, it will be seen that the present invention provides an image forming apparatus having several unforeseen advantages, which are enumerated below.

(1) An image can be transmitted as desired without any deterioration, and a transmission medium can be safely separated.

(2) The separation of the transmission medium is not affected by the environment.

(3) Desired image transfer is achievable regardless of irregularities in the resistance of a transfer belt.

(4) Although a sheet transport control changes from one machine or device to another or within the same device or machine due to aging, an image can be securely transferred.

(5) The damage to electrodes and transfer belt due to charge leakage and therefore faulty image transfer attributable to such damage can be avoided.

Various modifications will be possible for one skilled in the art after receiving the teachings of the present invention without departing from the scope thereof.

Claims (11)

1. An image forming apparatus comprising: a photoconductive member (1) for forming a toner image thereon; a transfer belt (3) movable in contact with the photoconductive member (1) over a predetermined nip portion for transporting a sheet to allow the toner image to be transferred from the photoconductive member to the sheet; a first transfer potential bias applying means (5, 8) for applying a first predetermined transfer bias to the transfer belt downstream of the nip section by contacting the transfer belt; a second transfer potential device (4, 13; 4, 14; 4, 15; 4, 16) which is upstream of the nip section and contacts the transfer belt; and characterized in that the second transfer applying means remains in an electrically floating state and by starting the application of the first predetermined transfer bias voltage when the leading edge of the sheet approaches the nip section. 2. Image forming apparatus comprising: a photoconductive member (1) for forming a toner image thereon; a transfer belt (3) movable in contact with the photoconductive member (1) over a predetermined nip portion to transport a sheet to allow the toner image to be transferred from the photoconductive member to the sheet; a first transfer potential bias applying means (5, 8) for applying a first predetermined transfer bias to the transfer belt downstream of the clamping portion by contacting the transfer belt; a second transfer potential bias applying means (4, 13, 4, 14; 4, 15; 4, 16) for applying a second transfer bias to the transfer belt upstream of the clamping portion by contacting the transfer belt; and characterized in that the second applying means is connected to ground via a varistor or a Zener diode or a similar constant voltage element (13) resulting in a potential that is close to the first predetermined transfer bias voltage. 3. Image forming apparatus comprising: a photoconductive member (1) for forming a toner image thereon; a transfer belt (3) movable in contact with the photoconductive member (1) over a predetermined nip portion to transport a sheet to allow the toner image to be transferred from the photoconductive member to the sheet; a first transfer potential bias applying means (5, 8) for applying a first predetermined transfer bias to the transfer belt downstream of the clamping portion by contacting the transfer belt; a second transfer potential bias applying means (4, 13, 4, 14; 4, 15; 4, 16) for applying a second transfer bias to the transfer belt upstream of the nip section by contacting the transfer belt; and characterized in that the second transfer bias voltage is started to be applied when the sheet enters the nip section or gap section, such that the potential distribution of the belt (3) has a linear gradient in the nip section which increases in the upstream direction. 4. An apparatus according to claim 3, wherein a time for applying the second transfer bias voltage is variable. 5. 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 timing for applying said second transfer bias voltage when humidity is higher than a predetermined value or not adjusting the timing when humidity is lower than the predetermined value. 6. Image forming apparatus comprising: a photoconductive member (1) for forming a toner image thereon; a transfer belt (3) movable in contact with the photoconductive member (1) via a predetermined nip portion for transporting a sheet to allow the toner image to be transferred from the photoconductive member to the sheet; a first transfer potential bias applying means (5, 8) for applying a transfer bias to the transfer belt downstream of the nip portion by contacting the transfer belt; a second transfer potential bias applying means (4, 13, 4, 14; 4, 15; 4, 16) for applying a transfer bias to the transfer belt upstream of the clamping portion by contacting the transfer belt ; and characterized in that starting to apply the same transfer bias 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 which is less than 8 mm from the nip portion or gap. 7. Image forming apparatus comprising: a photoconductive member (1) for forming a toner image thereon; a transfer belt (3) movable in contact with the photoconductive member (1) via a predetermined nip portion to transport a sheet to allow the toner image to be transported from the photoconductive member to the sheet; a first transfer potential bias applying means (5, 8) for applying a first predetermined transfer bias to the transfer belt downstream of the nip portion by contacting the transfer belt; a second transfer potential bias applying means (4, 13, 4, 14; 4, 15; 4, 16) for applying a second transfer bias to the transfer belt upstream of the clamping portion by contacting the transfer belt ; and characterized in that starting to apply the second transfer bias to the second bias applying means; after the leading edge of the sheet has been transported by the second transfer potential bias applying means toward the nip portion over a distance exceeding the distance between the first bias applying means and the photoconductive member, the distance being shorter than the distance between the second transfer potential bias applying means and the nip portion. 8. Image forming apparatus comprising: a photoconductive member (1) for forming a toner image thereon; a transfer belt (3) movable in contact with the photoconductive member (1) via a predetermined nip portion for transporting a sheet to allow the toner image to be transferred from the photoconductive member to the sheet; a first transfer potential bias applying means (5, 8) for applying a transfer bias to the transfer belt downstream of the clamping portion by contacting the transfer belt; a second transfer potential bias applying means (4, 13, 4, 14; 4, 15; 4, 16) for applying a first transfer bias to the transfer belt upstream of the clamping portion by contacting the transfer belt ; and characterized in that starting to apply the same transfer bias 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 the first bias applying means and the photoconductive member, the distance being shorter than the distance between the second transfer potential bias applying means and the nip. 9. Apparatus as claimed in claim 7 or 8, further comprising: a humidity sensing device for sensing a humidity; and a control device responsive to an output signal of the humidity sensing device for setting the time for applying the second transfer bias voltage when the humidity is higher than a predetermined value or not setting the time when a humidity is lower than the predetermined value. 10. Image forming apparatus comprising: a photoconductive member (1) for forming a toner image thereon; a transfer belt (3) movable in contact with the photoconductive member (1) via a predetermined nip portion for transporting a sheet to allow the toner image to be transferred from the photoconductive member to the sheet; a first transfer potential bias applying means (5, 8) for applying a first predetermined transfer bias to the transfer belt downstream of the clamping portion by contacting the transfer belt; a second transfer potential bias applying means (4, 13, 4, 14; 4, 15; 4, 16) for applying a second DC transfer bias to the transfer belt upstream of the clamping 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 having a volume resistivity of 10⁶-10¹² Ωcm. 11. Image forming apparatus comprising: a photoconductive member (1) for forming a toner image thereon; a transfer belt (3) movable in contact with the photoconductive member (1) via a predetermined nip portion for transporting a sheet to allow the toner image to be transferred from the photoconductive member to the sheet; a first transfer potential bias applying means (5, 8) for applying a first predetermined transfer bias to the transfer belt downstream of the clamping portion by contacting the transfer belt; a second transfer potential bias applying means (36) for applying a second transfer bias to the transfer belt at the clamping 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 having a volume resistivity of 106-1012 Ωcm.