JP5321076B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that forms an image by transferring a toner image onto a transfer material such as paper.

2. Description of the Related Art Conventionally, an electrophotographic printer that forms characters and images on paper by transferring and fixing a toner image formed on a photoreceptor onto a transfer material such as paper is known.
In such an electrophotographic printer, a toner image is directly transferred from a photosensitive member to a sheet, and a toner image is temporarily transferred (primary transfer) from the photosensitive member to an intermediate transfer member, and further transferred from the intermediate transfer member to the sheet. Some transfer (secondary transfer).

  In recent years, the secondary transfer method is becoming mainstream from the standpoints of ease of maintenance work for repairing when a paper jam occurs and miniaturization of the entire apparatus. In such a secondary transfer system, a belt is used as an intermediate transfer member, a transfer counter roller that stretches the belt from the inside, and a transfer pressure roller that rotates by pressing the transfer counter roller through the belt. Generally, the secondary transfer portion is configured.

  In this secondary transfer unit, a transfer current applied to the paper for secondary transfer of the toner image to the paper is transferred from a transfer pressure roller (hereinafter simply referred to as a transfer roller) to a transfer counter roller (hereinafter simply referred to as a counter roller). ) And a method of flowing from the opposite roller to the transfer roller.

  By the way, in order to press the transfer roller against the counter roller, there are many systems in which both ends of the shaft of the transfer roller are pressed against the counter roller side by an appropriate urging member. However, not only the transfer roller but also printer rollers have a relatively small roller diameter with respect to the length of the roller corresponding to the paper width in order to reduce the size of the printer body. That is, the roller shaft has a small diameter and is easily bent.

  Therefore, the pressure applied to both ends of the shaft of the transfer roller in the opposite roller direction does not reach the roller circumferential surface uniformly, and a strong pressure is applied to both ends of the shaft, and a weak pressure is applied to the center of the shaft.

  When the diameters of the opposing roller and the transfer roller are respectively uniform straight in the axial direction, the contact area of the paper gripping portion formed between the opposing roller and the transfer roller by the pressing of the transfer roller is larger than both ends as described above. However, since the load applied to the central portion is weak, the contact area width at the central portion of the opposing roller is narrow and the contact area width at both end portions is wide.

  For this reason, if the opposing roller has, for example, an ion conduction type resistance distribution with high uniformity, a large amount of current flows through both ends of the secondary transfer portion where the contact area width of the paper gripping portion is wide. There is a problem that transfer current hardly flows in a narrow central portion, resulting in transfer failure.

  In order to solve this problem, in an image forming apparatus that transfers a toner image formed on a belt to a sheet by applying a current or voltage from a counter roller, the counter roller is coated on the crown-shaped rubber roller part and the roller. A configuration of a counter roller that is formed from a high-resistance coating material and has a straight appearance is known as a conventional technique. (For example, refer to Patent Document 1.)

JP 2008-033191 A

  However, the prior art disclosed in Patent Document 1 requires a high level of technology in order to form a crown-shaped rubber roller portion, particularly when the opposing roller is formed of a crown-shaped rubber roller portion and a high-resistance coating material applied thereon. Furthermore, in order to apply a high resistance coating material thereon to make the entire appearance straight, there is a problem that it requires more advanced processing techniques and labor.

In order to solve the above problems, the image forming apparatus of the present invention primarily transfers a toner image formed on an image carrier to an intermediate transfer member, and the intermediate transfer member transfers the toner image that has been primarily transferred to the intermediate transfer member. In the image forming apparatus that performs secondary transfer from the member to the image forming medium being conveyed, the transfer counter roller that supports and rotates the intermediate transfer member in the secondary transfer unit, and the intermediate transfer member is moved with respect to the transfer counter roller. and a transfer press roller which rotates in pressing through, the transfer press roller has a circumferential surface of uniform diameter formed of an electrically conductive elastic layer having a characteristic that is a resistor receiving the pressure rises, the the pressing force for pressing is set high at the both ends than at the central portion, the electric resistance of the end portions at the time of the pressing is higher than the electrical resistance of the central portion, and the image type of a conductive type of the electronic conductivity type Configured to apply a transfer voltage to the medium, the transfer opposite roller, the peripheral surface of the roller body will be covered by a tube-like resistance member, the resistance value of the opposite in response to the resistance value of the transfer pressing roller Thus, it is comprised so that the electrical resistance of the center part of the said tubular resistance member may become higher than the electrical resistance of both ends .

The elastic layer is, for example, ethylene - propylene - formed Ru diene rubber (EPDM).

  The present invention does not form a crown-shaped rubber roller portion, and therefore does not use an advanced technique for applying a high-resistance coating material to make the entire appearance straight. However, there is an effect that it is possible to easily form the roller of the secondary transfer portion by using a variety of types of elastic resin without trouble.

FIG. 2 is a schematic side cross-sectional view schematically showing the internal configuration of the printer according to the first embodiment of the invention. FIG. 3 is a diagram schematically illustrating the secondary transfer unit in order to explain the function of the secondary transfer unit in the configuration of the printer according to the first embodiment of the invention. In order to examine the degree of change (distribution degree) of the electrical resistance value due to the pressure applied to both ends of the roller shaft of the transfer roller of the printer according to the first embodiment of the present invention, it is arranged so as to be in direct contact with the transfer roller experimentally. It is a figure which shows the structure of the made driven roller. 6 is a chart showing current detected through each electrode foil on the driven roller side and electric resistance value calculated from the current when a voltage of 1000 V is applied to the transfer roller of the printer according to Example 1 of the invention. . (a) is a graph showing the distribution of electrical resistance values obtained by plotting the electrical resistance values calculated in FIG. 4, and (b) is image formation when, for example, black toner is solid-printed on paper in this state. It is a figure which shows the state of. It is a figure which shows typically the structure of the driven roller which gave the resistance which differs in an axial direction so that height may be opposite to the resistance value of the transfer roller of the printer which concerns on Example 1 of this invention. 7 is a chart showing a state of a combined resistance value with respect to a transfer current obtained by an experiment similar to the case of FIG. 3 using the driven roller having the configuration of FIG. (a) is a graph showing the distribution state of the combined resistance value of FIG. 7 and the resistance value of the secondary transfer roller alone before combining, and (b) is a state of a formed image when, for example, black toner is solid-printed on a sheet in this state. It is a figure which shows a state.

  Hereinafter, embodiments of the present invention will be described in detail. In the following description, the image carrier described in the claims includes, for example, the photosensitive drum 7, the intermediate transfer member includes, for example, the conveyance belt 4, and the image forming medium includes, for example, the paper 3, The opposing roller is made up of, for example, a driven roller 6 and the transfer pressing roller is made up of, for example, a secondary transfer roller 18 or the like.

FIG. 1 is a schematic cross-sectional side view schematically showing the internal configuration of the image forming apparatus according to the first embodiment. The figure shows a tandem color image forming apparatus as an example.
A color image forming apparatus 1 shown in FIG. 1 is a small color image forming apparatus having a size that can be placed on, for example, a desktop personal computer rack. A paper cassette 2 is detachably attached to the bottom of a main body base. I have. A large number of sheets 3 are placed and stored in the sheet cassette 2.

  An intermediate transfer belt 4 that is a main member of a belt unit that conveys a sheet is arranged in a flat loop shape in the front and rear directions at substantially the center inside the color image forming apparatus 1. The intermediate transfer belt 4 is looped around the driving roller 5 and the driven roller 6 and circulates in the counterclockwise direction indicated by arrows a and b in the figure.

  In the upper circulation portion of the intermediate transfer belt 4, four photosensitive drums 7 (7m, 7c, 7y, 7k) are arranged side by side in a multistage manner from the upstream side to the downstream side in the paper transport direction (from the right to the left in the figure). Has been. Then, each of the photosensitive drums 7 is surrounded (hereinafter, only the peripheral devices around the photosensitive drum 7k are representatively shown by reference numerals), and the cleaner 8, the charging roller 9, the optical writing head 11, and the development are developed. A developing roller 13 and a primary transfer roller 14 of the container unit 12 are disposed.

  From the developing unit 12 corresponding to the photosensitive drum 7m to the developing unit 12 corresponding to the photosensitive drum 7k, magenta (m), cyan, which are the three primary colors of subtractive color mixing, are contained in the toner container 15 of each developing unit 12. Each toner of (c) and yellow (y) and black (k) toner dedicated to printing of black portions of characters and images are accommodated.

  The four optical writing heads 11 are supported by the image forming apparatus main body frame via a support member (not shown). The primary transfer roller 14 is disposed as a member belonging to the belt unit.

  In FIG. 1, the intermediate transfer belt 4 is always urged outward by a tension roller 16 that is in pressure contact with the back surface of the downstream end portion and a tension roller 17 that is in pressure contact with the back surface of the upstream end portion, thereby maintaining an appropriate tension. The intermediate transfer belt 4 transfers the toner image directly to the belt surface (primary transfer), and conveys the toner image to a transfer position to the paper for further transfer (secondary transfer) to the paper.

  The driven roller 6 disposed at the upstream end of the intermediate transfer belt 4 serves as a tension holding roller of the intermediate transfer belt 4 and a counter roller of the secondary transfer unit, and is driven as the counter roller. A secondary transfer roller 18 that is pressed and rotated via the intermediate transfer belt 4 is disposed opposite to the roller 6. The secondary transfer portion c is driven by the driven roller 6, the intermediate transfer belt 4, and the secondary transfer roller 18. Is forming.

  Further, a paper take-out roller 19, a feeding roller 21 and a separating roller 22 are disposed in the vicinity of the sheet feeding opening (right side in the drawing) of the sheet cassette 2, and the secondary transfer portion c is connected to the feeding roller 21 and the separating roller 22. A standby roller pair 23 is arranged in the middle of the sheet conveyance path up to the point.

  A belt-type fixing device 24 is disposed downstream (upward in the drawing) of the secondary transfer portion c. On the further downstream side of the belt-type fixing device 24, although not shown in particular, a pair of carry-out rollers for carrying out the fixed paper from the belt-type fixing device 24, and a paper discharge formed on the upper surface of the paper. A pair of paper discharge rollers for discharging the paper to the tray is provided.

  As shown in FIG. 1, the image forming apparatus 1 does not directly transfer a toner image onto a conventional sheet, but performs intermediate transfer onto a sheet that is conveyed vertically to a secondary transfer unit c by a standby conveyance roller pair 23. The toner image is transferred via the belt 4.

Therefore, it is possible to cope with a maintenance process for recovering from a problem such as a paper jam occurring in the paper transport path by simply opening the right side of FIG.
In addition, since troubles such as paper jam do not occur in the arrangement section of the kits, the operation for attaching and detaching consumables such as kits concentrated on the left side of FIG. It is configured as follows.

  Thereby, the dimension between kits is reduced as much as possible, and size reduction of the whole apparatus main body is achieved. Further, the optical writing head itself is also miniaturized and is configured closer to the photosensitive drum.

  As shown in FIG. 1, the intermediate transfer belt 4 disposed with the upper circulation surface sandwiched between the photosensitive drum 7 and the primary transfer roller 14 is also not shown under the control of a control unit (not shown). A primary transfer voltage is applied from the primary transfer voltage generation circuit via the primary transfer roller 14, whereby the toner image is transferred onto the intermediate transfer belt 4.

  The toner image transferred to the intermediate transfer belt 4 is moved to the secondary transfer portion c where the secondary transfer roller 18 and the driven roller 6 are pressed against each other via the intermediate transfer belt 4 due to the circulating movement of the intermediate transfer belt 4. It is conveyed.

  On the other hand, the paper 3 as the image forming medium taken out from the paper cassette 2 by the paper take-out roller 19 shown in FIG. 1 is sandwiched between the feeding roller 21 and the separating roller 22 and is moved along the conveyance path indicated by the arrow d in FIG. Then, the sheet is held by the standby roller pair 23 and temporarily stopped, and then sent to the secondary transfer portion c in accordance with the transfer timing.

  The control unit notifies the secondary transfer voltage generation circuit (not shown) of the secondary transfer voltage to be applied to the sheet 3. The secondary transfer voltage generation circuit generates a secondary transfer voltage notified from the control unit, and outputs the generated secondary transfer voltage to the secondary transfer roller 18. This secondary transfer voltage is applied to the sheet 3 passing through the secondary transfer portion c via the secondary transfer roller 18.

Here, the transfer roller 18 will be described.
FIG. 2 is a diagram schematically illustrating the secondary transfer unit c in order to explain the function of the secondary transfer roller 18 of the secondary transfer unit c in the configuration of the printer according to the first embodiment. In FIG. 2, the same components as in FIG. 1 are given the same reference numerals as in FIG.

  First, the secondary transfer roller 18 of this example has a circumferential surface of a uniform diameter made of a conductive elastic layer having a characteristic that resistance increases when subjected to pressure, and is applied to a paper by an electronic conductive type conductive method. A transfer voltage is applied.

  The elastic layer is generally composed of ethylene-propylene-diene rubber (EPDM). Ethylene-propylene-diene rubber (EPDM) is an ethylene-propylene rubber (EPM), which is a copolymer of ethylene and propylene, with a small amount of a third component introduced to give a double bond in the main chain. is there.

  Various synthetic rubbers are commercially available depending on the type and amount of the third component. In any case, the electronic conductive type elastic body (foam) obtained by this method has a variation in electrical resistance value under environmental conditions. It has excellent properties such as low stability, abundant types of foam, and high durability.

  The elastic layer is not limited to ethylene-propylene-diene rubber (EPDM), and an elastic body having the same properties can be obtained even if urethane, NBR rubber, or the like is used as the base resin body. As the conductive agent, carbon black, metal oxide powder, or the like can be used.

  These conductive agents are evenly dispersed in the base resin body, and the resin body is foam-molded to form a final sponge-like roller shape. In this embodiment, a carbon powder dispersed in EPDM is foam-molded, and the hardness is set to about 40 ° with Asker C.

  Further, the secondary transfer roller 18 is formed so that the diameter of the roller is relatively small with respect to the length of the roller corresponding to the width of the paper 3 in order to reduce the size of the main body of the printer 1. That is, the diameter of the roller shaft 25 is thin and easy to bend.

  By the way, when the toner image on the intermediate transfer belt 4 is transferred to a sheet, a stable contact area of the curved surface of the intermediate transfer belt 4 with respect to the sheet 3, that is, the peripheral surface of the driven roller 6 and the peripheral surface of the secondary transfer roller 18. In order to obtain the width, it is necessary to apply a suitable pressure to the secondary transfer roller 18.

  Therefore, as shown in FIG. 2, a large urging force is applied to both ends of the roller shaft 25 of the secondary transfer roller 18 in the direction of the driven roller 6 as indicated by arrows e1 and e2. However, as described above, since the diameter of the roller shaft 25 is thin and easily bent, the applied pressure does not reach the roller peripheral surface of the secondary transfer roller 18 uniformly, and the two ends of the shaft are indicated by arrows f1 and f2. There is a tendency that a strong pressure is applied to the shaft and a weak pressure is applied at the center of the shaft as indicated by an arrow g.

  EPDM, which is the material of the secondary transfer roller 18, is an industrially inexpensive and extremely general synthetic resin, and if it is used, there is a great cost advantage. However, as one drawback, as described above, the resistance increases when subjected to pressure.

  As described above, since the secondary transfer roller 18 has a characteristic that the resistance increases when it receives pressure, the electrical resistance greatly increases at both ends where a strong pressure is applied, and the electrical resistance is so large at the center where a weak pressure is applied. Does not rise.

  Accordingly, since there is a difference in the electric resistance value between the both end portions and the central portion, the high voltage constant voltage is applied to the secondary transfer roller 18 in order to transfer the toner image on the intermediate transfer belt 4 to the sheet 3. Alternatively, when a constant current is applied, transfer unevenness occurs.

  FIG. 3 shows a direct contact with the secondary transfer roller 18 experimentally in order to examine the degree of change (distribution degree) of the electrical resistance value due to the pressure applied to both ends of the roller shaft 25 of the secondary transfer roller 18. It is a figure which shows the structure of the driven roller 6 arrange | positioned so as to oppose.

  FIG. 3 shows an electrode foil on the peripheral surface of the driven roller 6 for extracting the constant voltage (or constant current) applied to the secondary transfer roller 18 to the driven roller 6 side at both ends and the center. The state where it was affixed on the peripheral surface of the roller 6 is shown.

  The electrode foils 26 and 28 at both ends are arranged with a width h of 5 mm and an interval i of 5 mm. The three electrode foils 27 in the central part are also 5 mm wide, but the distance between the two electrode foils 27 on both sides of the central part is approximately 100 mm from the end of the roller, one in the center of the central part. The electrode foil 27 is disposed at the center of the roller.

  The exact arrangement position of the central portion of the eleven electrode foils 26, 27, and 28 is measured from the left end of the roller, 10, 20, 30, 40, 95, 159, 218, 278, 288. 11 positions of 298 and 308 mm.

  FIG. 4 shows the current detected through the electrode foils 26, 27, and 28 on the driven roller 6 side when the voltage of 1000 V is applied to the secondary transfer roller 18, and the current detection calculated from the current. It is a graph which shows the electrical resistance value (henceforth only resistance value) of a position.

  In the left column, the arrangement positions 10, 20, 30, 40, 95, 159, 218, 278, 288, as viewed from the left end of the roller at the center of the eleven electrode foils 26, 27, and 28, Eleven positions at 298 and 308 mm are shown, and the two columns on the right side show the current detected at that position and the resistance value calculated from the current.

  FIG. 5A is a graph showing a distribution state of resistance values obtained by plotting the resistance values calculated as described above. FIG. 5B is a graph showing, for example, black toner on the paper 3 in this state. FIG. 6 is a diagram illustrating a state of a formed image when solid printing is performed.

  As shown in FIG. 6A, the distribution of the resistance value of the secondary transfer roller 18 indicated by the current value detected by the driven roller 6 is concentrated between the resistance values of 1.00E + 08Ω and 1.00E + 09Ω at both ends. It is extremely high, and in the central part, it is concentrated in the vicinity of approximately 1.00E + 08Ω.

  When solid printing is performed on the paper 3 using black toner in this state, the transfer current flows well in the central portion where the resistance value is low as shown in FIG. Is formed. On the other hand, the flow of the transfer current is poor at both ends having a high resistance value, and an image having a low density is formed.

  Therefore, the inventor follows the transfer roller 18 as a transfer counter roller corresponding to the different resistance values in the axial direction of the transfer roller 18 in order to absorb the imbalance in the axial direction of the current flowing during the transfer. The roller 6 has different resistance values in the axial direction in which the resistance value of the transfer roller 18 is opposite.

  FIG. 6 is a diagram schematically illustrating the configuration of the driven roller 6 having different resistances in the axial direction so that the level of the resistance value of the transfer roller 18 is opposite. As shown in the figure, the peripheral surface of the driven roller 6 is covered with a tubular resistance member 29 (29a, 29b) or 31 having a different resistance.

  Low resistance tube-shaped resistance members 29 are coated on the peripheral surfaces of both ends of the driven roller 6 corresponding to both ends of the secondary transfer roller 18 exhibiting a high resistance value during secondary transfer to form a low resistance layer. doing.

  In contrast, the peripheral surface of the central portion of the driven roller 6 corresponding to the central portion of the secondary transfer roller 18 exhibiting a low resistance value during secondary transfer is covered with a high resistance tubular resistance member 31. A high resistance layer is formed.

  FIG. 7 is a chart showing a state of a combined resistance value with respect to a transfer current obtained by an experiment similar to the case of FIG. 3 using the driven roller 6 having the configuration of FIG. In the following description, the driven roller 6 is referred to as a transfer counter roller or simply as a counter roller.

  In the chart shown in FIG. 7, as shown in the counter roller column 32 at the left end, the low resistance layers 29a and 29b in the position column 33 have resistance values (corresponding to the resistance value column 34 of the tubular resistance member 29). As shown by resistance value), a tubular resistance member having a relatively low resistance value of 5.00E + 08Ω is used.

  On the other hand, the high resistance layer 31 in the position column 33 of the counter roller column 32 at the left end of the same table also has a resistance value (resistance value of the tubular resistance member 31) shown in the corresponding resistance value column 34. As shown, a tubular resistance member having a relatively high resistance value of 1.00E + 09Ω is used.

  As shown in the position column 36 of the secondary transfer roller column 35 in the figure, the low resistance layer 29a corresponds to a position 10 to 40 mm from the left end of the secondary transfer roller 6, and the high resistance layer 31 The low resistance layer 29 b corresponds to a position 278 to 308 mm from the left end of the secondary transfer roller 6, and corresponds to a position of 95 to 218 mm from the left end of the secondary transfer roller 6.

  Note that the numerical values shown in the position column 36 and the resistance value column 37 of the secondary transfer roller column 35 in the table are the values shown in the position column and the resistance value column of the table shown in FIG. In the combined resistance value column 38 of the chart of FIG. 7, a combined value obtained by adding the resistance value shown in the resistance value column 34 of the counter roller column 32 and the resistance value shown in the resistance value column 37 of the secondary transfer roller column 35 is added. Resistance values are shown.

  FIG. 8 (a) shows the combined resistance value obtained as described above added to the graph shown in FIG. 5 (a) and plotted to obtain the combined resistance value (plot of ◆) and the pre-synthesis value. FIG. 6B is a graph showing a distribution state of resistance values (plots of ■) of the secondary transfer roller alone, and FIG. 5B shows a state of a formed image when, for example, black toner is solid-printed on the paper 3 in this state. FIG.

  As shown in FIG. 8 (a), the combined resistance value indicated by the ♦ plot is the secondary resistance against the non-uniform distribution state of the resistance value of the secondary transfer roller alone indicated by the ■ plot for each axial position. A substantially uniform distribution is shown in the axial direction of the transfer roller.

With this configuration, it has been found that, for example, when black toner is solid-printed on the paper 3, an image having a uniform density with no unevenness can be obtained on the entire surface, as shown in FIG.
Thus, according to the present embodiment, a resistance layer is provided on the peripheral surface of the opposing roller corresponding to the uneven distribution state for each axial position of the resistance value of the secondary transfer roller during transfer, As a result, the axial resistance is stabilized by the sum of the non-uniform resistance value in the axial direction of the secondary transfer roller and the resistance value on the opposite roller side, and non-uniformity at the end and center of the secondary transfer roller. Therefore, it is possible to obtain an image having a stable density in the axial direction by uniformizing the flow of the transfer current.

  In addition, the roller is made using the most inexpensive and widely available resin body without using a technique that requires a high manufacturing cost, such as making the core of the roller into a crown shape, and without causing instability of the shape. It is economical because it can be created easily.

  In the above-described embodiment, the driven roller 6 is provided with different resistances in the axial direction by covering the peripheral surfaces of both end portions and the central portion of the roller with tubular resistance members having different resistances. However, it is not limited to this.

  For example, the driven roller may be formed by combining cylindrical hard high-hardness solid rubbers having different resistances at both ends and the center of the roller. Alternatively, the driven roller may be formed of integrally molded rubber in which different resistances are distributed in gradation at both ends and the center.

  The present invention can be used in an image forming apparatus that forms a uniform secondary transfer image of a toner image.

DESCRIPTION OF SYMBOLS 1 Color image forming apparatus 2 Paper cassette 3 Paper 4 Intermediate transfer belt 5 Drive roller 6 Driven roller (transfer opposing roller)
7 (7m, 7c, 7y, 7k) Photosensitive drum 8 Cleaner 9 Charging roller 11 Optical writing head 12 Developing unit 13 Developing roller 14 Primary transfer roller 15 Toner container 16, 17 Tension roller 18 Secondary transfer roller (transfer pressing roller) )
DESCRIPTION OF SYMBOLS 19 Paper extraction roller 21 Feeding roller 22 Rolling roller 23 Standby roller pair 24 Belt type fixing device 25 Roller shaft 26, 27, 28 Electrode foil 29 (29a, 29b) Low resistance layer 31 High resistance layer 32 Opposing roller column 33 Position column 34 Resistance value column 35 Secondary transfer roller column 36 Position column 37 Resistance value column 38 Composite resistance value column

Claims (2)

An image forming apparatus that primarily transfers a toner image formed on an image carrier to an intermediate transfer member, and secondarily transfers the primary transferred toner image from the intermediate transfer member to an image forming medium being conveyed in a secondary transfer unit. In
A transfer counter roller that rotates while supporting the intermediate transfer member in the secondary transfer unit;
A transfer pressing roller that rotates while pressing against the transfer counter roller via the intermediate transfer member;
With
The transfer pressing roller has a circumferential surface of a uniform diameter made of a conductive elastic layer having a characteristic that resistance increases when subjected to pressure, and the pressing pressure to be pressed is set higher at both ends than the central portion. The electrical resistance of the both end portions at the time of pressing is higher than the electrical resistance of the central portion, and is configured to apply a transfer voltage to the image forming medium by an electronic conductive type conductive method,
The transfer counter roller is formed by covering the peripheral surface of the roller body with a tube-shaped resistance member, and at the center of the tube-shaped resistance member so as to have an opposite resistance value corresponding to the resistance value of the transfer pressure roller. The electrical resistance is configured to be higher than the electrical resistance at both ends .
An image forming apparatus.   The image forming apparatus according to claim 1, wherein the elastic layer is made of ethylene-propylene-diene rubber (EPDM).