JP5407400B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic copying machine and printer in which a toner image developed on a photosensitive member is temporarily transferred onto an intermediate transfer member and then secondarily transferred onto paper or another transfer material to obtain a final image. And the like.

In electrophotographic copying machines, printers, etc., a toner image developed on a photosensitive member is first transferred onto an intermediate transfer member and then secondarily transferred onto paper or other transfer material to obtain a final image. Therefore, an intermediate transfer type image forming apparatus that can reduce the occurrence of color image misregistration and color misregistration is widely known (see, for example, Patent Documents 1 to 4).
For example, in Patent Document 1 and others, many intermediate transfer belt type color image forming apparatuses having a plurality of photoconductors are commercially available, and the belt material has a high elastic modulus from the viewpoint of reducing misalignment during high-speed driving. It is described that many polyimide resins, polyamideimide resins and the like are employed.
However, such a material exhibiting a high elastic modulus has a problem that it is difficult to obtain a uniform transfer image on paper with large irregularities. The cause is that, in the concave portion of the paper, a gap is generated between the toner and the toner on the intermediate transfer belt. If the transfer electric field becomes weak even a little, the background of the paper can be seen, and it is difficult to obtain a uniform transfer image.

Therefore, if the secondary transfer current and the secondary transfer voltage are easily increased in order to increase the transfer electric field, discharge occurs in the gaps in the recesses, and the polarity of the toner is reversed, and the transfer efficiency is reduced. The result is a more noticeable background.
In particular, in an image forming apparatus in which the secondary transfer current is constant-current controlled with the same current value, the toner flows (transfers) from the intermediate transfer belt to the paper depending on the toner printing rate, charge amount, and paper resistance. Since the balance of the amount of current, the amount of discharge, and the amount of current that flows directly to the non-image area of the paper changes, a uniform image, that is, a constant transfer efficiency and image density can be obtained for images of various patterns (printing rates). It was difficult to realize.
On the other hand, in order to increase the adhesion between the concave portion and the toner on the intermediate transfer belt, a secondary transfer bias roller (hereinafter referred to as “secondary transfer roller”) as shown in Patent Document 1, and a secondary If the pressure between the transfer opposing roller (hereinafter referred to as “opposite roller”) is increased, stress is concentrated on the toner in contact with the convex portion, and the non-between the toner particles and between the toner and the intermediate transfer belt. There is a problem in that electrostatic adhesion increases, and toner is not transferred particularly in character images and fine line images.

In order to solve such a problem, for example, Patent Document 2 and others have proposed many intermediate transfer belts in which a high elastic modulus material is used as a base layer and an elastic layer is laminated thereon. It was necessary to bond each layer with an agent or the like, which was difficult in terms of resistance unevenness, durability, cost, and the like.
By the way, as described above, in order to achieve a uniform image regardless of the image pattern and printing rate on paper with large unevenness, the maximum electric field is stabilized within a range where no discharge occurs in the concave portion of the paper. It is important to form.
That is, it is necessary to create a situation in which most of the current supplied from the opposing roller or the secondary transfer roller is used to move the toner from the intermediate transfer belt to the paper without causing unnecessary discharge.
However, as described above, when the secondary transfer current is controlled at a constant current, the printing rate, that is, the total charge amount of the transferred toner is changed in the main scanning direction in the transfer nip, and the toner is transferred from the intermediate transfer belt. Since the balance of the amount of current that flows by moving to paper, the amount of discharge, and the amount of current that flows directly to the non-image area of the paper changes, supply as much current as necessary to transfer toner to various images It is impossible in principle.
In order to solve such a problem, for example, Patent Document 3 or Patent Document 4 proposes a method of changing the bias applied to the transfer roller based on image data.

However, in the case of the secondary transfer process, both the opposing roller and the secondary transfer roller are conductive, and the influence of the flow in the non-image portion of the paper is large. Even if the secondary transfer current or the secondary transfer voltage is controlled based on the image data, a uniform and good image cannot be obtained without depending on the image pattern.
Therefore, the object of the present invention is to provide a uniform final image in which the background of the paper is not conspicuous regardless of the image pattern and the printing rate, even if there are large irregularities on the paper surface, in consideration of the above-described situation. An object is to provide an image forming apparatus.

In order to solve the above problems, the invention described in claim 1 is characterized in that a transfer material is sandwiched between an intermediate transfer body to which a toner image formed on an image carrier is transferred and the intermediate transfer body. In the image forming apparatus, comprising: a conductive member that is disposed at a position facing the conductive member via the intermediate transfer member and that forms an electric field between the conductive member and the conductive member. Transfer material resistance increasing means for increasing the resistance of the transfer material, and temperature and humidity detected by an environmental sensor before the transfer of the toner image on the intermediate transfer body to the transfer material at the contact portion A resistance prediction unit that predicts the resistance of the transfer material based on at least one of them, a resistance detection unit that detects the resistance of the transfer material, and a prediction result by the resistance prediction unit or a detection by the resistance detection unit The result is predetermined The transfer material resistance increasing means controls to increase the resistance of the material to be transferred before transferring the toner image on the intermediate transfer body to the material to be transferred at the contact portion. The secondary current value that flows through the conductive member or the opposing member when the toner image on the intermediate transfer member at the contact portion is transferred to the transfer material is transferred to the transfer material. An image forming apparatus comprising: a current control unit configured to control the toner image so as to increase according to a printing rate or the number of pixels of the toner image.
The invention according to claim 2 further includes a conveyance path for conveying the transfer material that has passed through the transfer material resistance increasing means toward a registration roller that matches a secondary transfer timing, and the transfer material The resistance increasing means is a fixing device used to fix the transferred toner image transferred to the transfer material to the transfer material by heat, and the contact between the conductive member and the intermediate transfer member The image forming apparatus according to claim 1, wherein the transfer material is heated by the fixing device before the toner image transferred onto the intermediate transfer member is transferred to the transfer material.

The invention according to claim 3, wherein said current control means, a to electric current values flow in the conductive member or the opposing member, so as to increase in accordance with the charge amount of the toner on the intermediate transfer member is increased the image forming apparatus according to claim 1 or 2, wherein the control shall be the features.

According to a fourth aspect of the present invention , the resistance control means is transferred onto the intermediate transfer member at a contact portion between the conductive member and the intermediate transfer member based on the type of the transfer material. before the toner image is transferred to the transfer material, and wherein the image forming apparatus of any one of claims 1 to 3 increase the resistance of the transfer material by said transfer material resistance increase means.
In the invention according to claim 5 , the user selects the type of the material to be transferred on the operation panel when copying the document on the print screen when printing the document from the personal computer or the image forming apparatus main body. Thus, the resistance control means is configured to prevent the transfer material resistance before the toner image transferred onto the intermediate transfer member at the contact portion between the conductive member and the intermediate transfer member is transferred to the transfer material. wherein the image forming apparatus of any one of claims 1 to 4 raise the resistance of the transfer material by the lifting means.
According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, the intermediate transfer member is a belt-shaped member, and the tensile elastic modulus of the belt-shaped member is 2 GPa or more. It is characterized by.

According to the present invention, before the toner is transferred to the paper, a process for increasing the resistance of the paper is performed, and the transfer current is controlled based on the number of pixels of the toner image transferred to the paper, the printing rate, and the charge amount of the toner. . That is, the value of the secondary transfer current is determined based on the toner printing rate in the main scanning direction in the nip (for each pixel in the sub scanning direction).
As a result, the influence of the current flowing through the non-image portion of the paper can be eliminated, so that only the amount of current necessary for transferring the toner can be accurately applied, and a uniform final image is output even on paper with a large surface unevenness. be able to.

1 is a schematic view showing one embodiment of an image forming apparatus of the present invention. 2 is a flowchart showing a control flow for carrying out the present invention in the printer of FIG. 1. It is a schematic diagram which shows an example of a solid pattern. It is a schematic diagram which shows the other example of a solid pattern. It is a figure which shows the relationship between the secondary transfer current and applied voltage in a prior art example with a graph. It is a figure which shows the relationship between the secondary transfer current and applied voltage in this invention with a graph. It is a figure which shows the relationship between a secondary transfer rate, a secondary transfer electric current, the volume resistivity of a recording sheet, and a printing rate with a graph. It is a figure which shows the subjective evaluation of the image density of the convex part of the surface of the solid pattern formed on the 2nd surface of rough surface paper with the printing rate of 5 [%], and the image density of a recessed part by a table | surface.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic view showing one embodiment of the image forming apparatus of the present invention. An embodiment in which the present invention is applied to a color printer (hereinafter simply referred to as a printer) A that forms a color image by a tandem type image forming unit as an image forming apparatus will be described. First, a basic configuration of the printer according to the embodiment will be described.
The printer A of this embodiment includes an optical writing unit (not shown), a tandem image forming unit 10, a transfer unit 20, a fixing device 40, a retransmission device 50, and the like. The tandem image forming unit 10 includes four image forming units 1Y, M, C, and K for forming toner images of colors of Y (yellow), M (magenta), C (cyan), and K (black). ing.
The transfer unit 20 includes an endless intermediate transfer belt 6, a driving roller 22, a driven roller 23, a secondary transfer counter roller 24, four primary transfer rollers 5Y, 5M, 5C, 5K, a secondary transfer roller 26, and the like. doing. The endless intermediate transfer belt 6 as an image carrier is wound around the driving roller 22, the driven roller 23, and the secondary transfer counter roller 24 in a posture in which the side view is an inverted triangular shape. .

The intermediate transfer belt 6 is moved endlessly in the clockwise direction in the figure by the rotational drive of the drive roller 22. In addition to the driving roller 22, the driven roller 23, and the secondary transfer counter roller 24, four primary transfer rollers 5 Y, M, C, and K are also disposed inside the loop of the intermediate transfer belt 6. The roles of the primary transfer rollers 5Y, M, C, K and the secondary transfer roller 26 will be described later.
The tandem image forming unit 10 is disposed above the transfer unit 20 in a posture in which the four image forming units 1Y, 1M, 1C, and 1K are arranged in a horizontal direction along the overlaid surface of the intermediate transfer belt 6. . The image forming units 1Y, 1M, 1C, and 1K are drum-shaped photoreceptors 2Y, M, C, and K that are driven to rotate counterclockwise in the drawing, and developing units 4Y, M, C, and K, and charging units Means 3Y, M, C, and K are included.
It also has drum cleaning devices for Y, M, C, and K (not shown). The photoconductors 2Y, 2M, 2C, and 2K are respectively in contact with the overlying surface of the intermediate transfer belt 6 to form primary transfer nips for Y, M, C, and K, and by driving means (not shown). It is rotated in the counterclockwise direction in the figure.
The developing units 4Y, M, C, and K are for developing the electrostatic latent images formed on the photoreceptors 2Y, M, C, and K with Y, M, C, and K toners. The charging units 3Y, 3M, 3C, and 3K uniformly charge the surfaces of the photoreceptors 2Y, 2M, 2C, and 2K to the same polarity as the toner charging polarity.

Below the primary transfer nips for Y, M, C, and K, in the loop of the intermediate transfer belt 6, the primary transfer rollers 5Y, M, C, and K attach the intermediate transfer belt 6 to the photoreceptors 2Y, M, and C. , Pressing toward K. A primary transfer bias is applied to these primary transfer rollers 5Y, 5M, 5C, and 5K by primary transfer power supplies 11Y, 11M, 11C, and 11K.
An optical writing unit (not shown) is disposed above the tandem image forming unit 10. This optical writing unit is uniformly charged by the charging means 3Y, 3M, 3C, and 3K, and performs an optical writing process with the scanning light L on the surface of the photoreceptors 2Y, 2M, 2C, and 2K to form an electrostatic latent image. To form.
The electrostatic latent images formed on the photoconductors 2Y, 2M, 2C, and 2K are developed by the developing units 4Y, 4M, 4C, and 4K to become Y, M, C, and K toner images. These Y, M, C, and K toner images are primarily transferred while being superimposed on the front surface of the intermediate transfer belt 6 in the above-described primary transfer nips for Y, M, C, and K. As a result, a four-color superimposed toner image is formed on the front surface of the intermediate transfer belt 6.
In the printer A, non-contact charging rollers 3Y, 3M, 3C, and 3K, which are charging members to which a charging bias is applied by the charging bias power sources 12Y, 12M, 12C, and 12K, are used as charging means. The non-contact charging rollers 3Y, 3M, 3C, and 3K are in close proximity to the photoconductors 2Y, M, C, and K, and the non-contact charging rollers 3Y, M, C, and K and the photoconductors 2Y, M, C, and K A discharge is generated between the photosensitive member 2Y and the photosensitive member 2Y, M, C, K. Instead of such charging means (non-contact charging roller) 3Y, M, C, K, a scorotron charger or the like may be employed.

The transfer unit 20 has a secondary transfer roller 26 below the intermediate transfer belt 6. The secondary transfer roller 26 as a nip forming member is in contact with the secondary transfer counter roller 24 of the intermediate transfer belt 6 in contact with the secondary transfer opposite roller 24 from the front surface side of the belt to form a secondary transfer nip. doing.
On the other hand, the secondary transfer counter roller 24 that wraps around the intermediate transfer belt 6 above the secondary transfer nip is subjected to secondary transfer having the same polarity as the toner charging polarity by the secondary transfer bias power source 14. A bias is applied.
Thus, in the secondary transfer nip between the secondary transfer counter roller 24 and the secondary transfer roller 26, the secondary transfer that electrostatically moves the toner from the secondary transfer counter roller 24 side to the secondary transfer roller 26 side. An electric field is formed.
A recording sheet (not shown) is fed into the secondary transfer nip at a predetermined timing. The four-color superimposed toner image on the intermediate transfer belt 6 is secondarily transferred to the recording sheet collectively by the action of the nip pressure and the secondary transfer electric field.
The recording sheet onto which the four-color superimposed toner image has been secondarily transferred at the secondary transfer nip is fixed via the sheet conveying belt 8 that is moved endlessly in the counterclockwise direction in the drawing after exiting the secondary transfer nip. Into the device 40.
In the fixing device 40, the heat fixing roller 41 containing a heat source such as a halogen lamp and the pressure roller 42 pressed against the heat fixing roller 41 are sandwiched between the fixing nips to perform pressure or heat treatment. A toner image fixing process is performed.

As described above, the fixing device 40 fixes the toner image on the recording sheet, and as described later, before the toner image is secondarily transferred to the recording sheet, the fixing device 40 is previously set. It plays the role of drying the recording sheet.
When an image is formed on the opposite side of the recording sheet having an image fixed on one side by the fixing device 40, that is, when a double-sided printing moving image is performed, the retransmission device 50 sends the recording sheet sent from the fixing device 40. Is introduced into the conveyance path 17 by the conveyance guide 12, and is switched back and up by a switchback mechanism (transfer material reversing device) 51 for reversing the recording sheet, thereby reversing the top and bottom. Thereafter, the recording sheet is retransmitted to the secondary transfer nip.
On the other hand, when a recording sheet having no toner image is dried by the fixing device 40, the retransmission device 50 introduces the recording sheet sent from the fixing device 40 into the conveyance path 16 by the conveyance guide 12, and The image is retransmitted to the secondary transfer nip without reversing the upper and lower sides.
The registration roller 13 corrects the skew of the recording sheet by abutting the leading end of the recording sheet in a state where the rotation of the two rollers is stopped. Thereafter, the two rollers are rotated to hold the leading edge of the recording sheet into the registration nip, but immediately after that, the rotation of the rollers is stopped. Then, the rotation of the roller is resumed at a timing at which the recording sheet can be synchronized with the toner image on the intermediate transfer belt 6 at the secondary transfer nip.

FIG. 2 is a flowchart showing a control flow for carrying out the present invention in the printer of FIG. The image forming operation will be described with reference to FIGS. In this printer A, when color image data is generated by a scanner or the like, first, a recording sheet that is a transfer material such as paper or an OHP (overhead projector) sheet is fed as a transfer material feeding device (not shown). It is conveyed from the cassette through the resistance measuring roller pair 31 and the registration roller 13 (S1).
At this time, if the recording sheet is paper, it is determined whether the resistance of the paper is high (S2). For example, if the humidity of the paper is higher than 50%, it is determined that the resistance is low (for example, the volume resistivity is 1E9 [Ω · cm] or less) depending on the paper type.
If it is determined that the resistance of the paper is low, the intermediate transfer belt 6, the counter roller 24, the secondary transfer roller 26, the conveyance belt 8, the fixing device 40, and the like shown in FIG. 1 are driven (S 3). The paper is dried with heat through the paper to increase the resistance of the paper (S4). The paper that has passed through the fixing device 40 is guided to the conveyance path 16 by the conveyance guide 12 (S5), and is sent again to the registration roller 13 (S6).
In this case, by using the fixing device 40 as a transfer material resistance increasing means for increasing the resistance of the paper in advance before the secondary transfer, it is possible to provide reliable without providing an extra mechanism or device for increasing the resistance of the paper. Increases the resistance of paper.

On the other hand, in parallel with the drying of the paper by the fixing device 40, in the image forming unit 1Y of the primary transfer image forming unit 10, first, the non-contact charging roller 3Y that is negatively biased by the power source is the photosensitive drum 2Y. Is uniformly charged negatively, and an exposure device (not shown) forms an electrostatic latent image on the surface of the photosensitive drum 2Y (S7).
Subsequently, the developing unit 4Y reversely develops toner having a negative charge, thereby forming a toner image on the photosensitive drum 2Y (S8). A positive bias having a polarity opposite to the polarity of the toner is applied to the primary transfer roller 5Y, and the toner image on the photosensitive drum 2Y is intermediated by a transfer electric field formed between the primary transfer roller 5Y and the primary transfer roller 5Y. The image is transferred onto the transfer belt 6 (S9), and a primary transfer image is formed.
Similarly, in the primary transfer image forming units 1M, 1C, and 1K, image formation is performed at each timing, and a primary transfer image composed of four colors of toner is formed on the intermediate transfer belt 6. .
Subsequently, in accordance with the timing at which the primary transfer image on the intermediate transfer belt 6 reaches the secondary transfer nip (S10), the recording sheet whose resistance is increased in step S4 as described above is transferred from the registration roller 13 to the intermediate transfer. It is conveyed to a secondary transfer nip formed by the belt 6 and the secondary transfer roller 26 (S11).

A current having the same polarity as the toner (hereinafter referred to as “secondary transfer current”) is applied to the opposing roller 24 (S12). The value of the secondary transfer current is a value calculated from the following formula (1) based on the print rate of the toner image in the main scanning direction at the secondary transfer nip exit and the estimated value of the toner charge amount. Control is performed for each pixel in the scanning direction.
I = A × Σ (ηi × Qi) + B (1)
Where I: secondary transfer current value [μA]
A, B: Constant ηi: Print rate of toner of each color Qi: Charge amount of toner of each color (μC / g)
The secondary current value that flows through the secondary transfer roller 26 that is a conductive member or the counter roller 24 that is a counter member is controlled to increase as the charge amount of toner on the intermediate transfer belt 6 that is an intermediate transfer member increases. . Stable and uniform transfer can be realized by accurately determining and controlling the optimum secondary transfer current value in consideration of the printing rate of the transferred toner and the charge amount of the toner.
In the embodiment of FIG. 1, the fixing device 40 is used as a transfer material resistance increasing means for increasing the resistance of the paper to be transferred before the secondary transfer, and heating by this is used. The transfer material resistance increasing means is not limited to this.
For example, a mechanism for heating the paper in contact / non-contact with the paper between the registration roller 13 and the secondary transfer nip may be provided, or the paper in the paper feed tray is constantly heated, a dehumidifier, etc. It is also possible to carry out dehumidification by the above.

Next, experiments conducted by the present inventors will be described. The inventors prepared a printer testing machine having the same configuration as the printer A shown in FIG. Various conditions in this printer testing machine are listed below.
(1) Intermediate transfer belt 6
A carbon-dispersed polyimide belt having a thickness of 60 [μm] and a volume resistivity of 1E9 [Ω · cm] (measured under a voltage application condition of 100 V with a Hirester UP MCP HT450 manufactured by Mitsubishi Chemical).
(2) Toner Manufactured by a pulverization method, and the base material resin is a polyester resin.
(3) Secondary transfer roller 26
The roller portion is coated with an elastic layer made of a conductive rubber material, and the volume resistivity of the roller portion is adjusted to 1E9 [Ω · cm].
(4) Secondary transfer counter roller 24
The roller portion is coated with an elastic layer made of a conductive rubber material, and the volume resistivity of the roller portion is adjusted to 1E9 [Ω · cm].
(5) Fixing device 40
The fixing temperature (the surface temperature of the heat fixing roller 41) was set to 165 [° C.].
(6) One roller of the registration roller 13 A stainless roller.
(7) Process linear speed In the experiment, the process that is the linear speed of the intermediate transfer belt 6 or the like was set to 280 [mm / s].
(8) Length W of secondary transfer nip in belt movement direction
It was set to 3 [mm].
Using such a printer testing machine, a test test of a test image was performed. As the size of the recording sheet, an A3 size of 420 [mm] × 297 [mm] was adopted. As the toner, a toner having a charge amount of about −20 [μC / g] at normal temperature and humidity is used.

FIG. 3 is a schematic diagram showing an example of a solid pattern. FIG. 4 is a schematic diagram showing another example of a solid pattern. The test image in FIG. 3 employs a belt-like solid pattern extending in the sub-scanning direction (sheet conveyance direction), and the solid pattern is formed on an A3 size recording sheet conveyed along the long side direction.
Regarding the solid pattern printing rate, two patterns of 0.05 (5 [%]) and 1.00 (100 [%]) were employed. The printing rate is a numerical value indicating the ratio of the sum of the color image sizes to the recording sheet size in the main scanning direction (direction orthogonal to the transport direction).
For example, the solid pattern shown in FIG. 3 has only one strip pattern made of black (K) toner, and its size in the main scanning direction is 29.7 [mm]. Since this corresponds to 0.10 (10 [%]) of the recording sheet size (297 mm) in the main scanning direction, the illustrated solid pattern is formed with a printing rate of 0.10 (10 [%]). It will be.

For example, the solid pattern shown in FIG. 4 includes a belt-like pattern B1 made of black toner and a belt-like pattern B2 made of magenta (M) toner, and the size in the main scanning direction is 29.7 [mm], respectively. It has become. Since the total corresponds to 0.20 (20 [%]) of the recording sheet size (297 mm) in the main scanning direction, the solid pattern shown is formed with a printing rate of 0.20 (20 [%]). Will be.
In the example of FIG. 4, the belt-like pattern B1 made of black toner and the belt-like pattern B2 made of M toner are formed so as to be shifted from each other. However, even when both patterns are superimposed, the printing rate is 0.20. (20 [%]). This is because the printing rate is a ratio of the total size of each color image.
In this print test, plain paper (My Paper) manufactured by NBS Ricoh Company was used as an A3 size recording sheet. For the secondary transfer bias output from the secondary transfer bias power supply 14 (FIG. 1), constant voltage control was performed instead of constant current control.

FIG. 5 is a graph showing the relationship between the secondary transfer current and the applied voltage in the conventional example. FIG. 6 is a graph showing the relationship between the secondary transfer current and the applied voltage in the present invention.
FIG. 5 shows the secondary in the case of the conventional technique in which plain paper (manufactured by NBS Ricoh, My Paper) is not passed through the fixing device as a recording sheet before secondary transfer in an environment of a temperature of 26 ° C. and a relative humidity of 60%. It shows the printing rate dependency in the relationship between the transfer current and the applied bias.
FIG. 6 shows the secondary transfer current in the case of this embodiment in which plain paper (manufactured by NBS Ricoh, My Paper) is passed through the fixing device as a recording sheet before the secondary transfer in the same environment as in FIG. It shows the printing rate dependency in relation to the applied bias. In the present embodiment, black toner (charge amount-20 μC / g) is used.
As shown in FIG. 5 showing the conventional example, when the resistance of the recording sheet is low without passing through the fixing device, the secondary transfer current-voltage characteristics are hardly different depending on the toner printing rate. It can also be seen that the current that flows when the toner is transferred is buried in the discharge current and the current that flows through the background of the recording sheet (paper).

For this reason, it is easily estimated that the effect of controlling the secondary transfer current for each pixel in the sub-scanning direction is small according to the printing rate and the number of pixels. However, since the balance between the current derived from the toner, the discharge current, and the current flowing through the background of the recording sheet varies greatly depending on the humidity, constant current control is performed at a constant value without considering the printing rate and the number of pixels. However, the density of the final image obtained varies.
On the other hand, as shown in FIG. 6 showing the present embodiment, once the resistance of the recording sheet is increased through the fixing device, it can be seen that the influence of the transferred toner charge appears remarkably. In the region where the applied voltage is low, the higher the printing rate, the larger the current flows at the same voltage. Incidentally, the reason why the difference between the curves becomes small in the region where the applied voltage is high is that the influence of the discharge current becomes strong.
Here, the higher the printing rate, the larger the current flows at the same voltage. The current flows when the toner itself moves (transfers), so the larger the toner adhesion amount (printing rate) and charge amount, the lower the applied voltage. (Transfer) A lot of current flows by voltage. For example, the transfer efficiency when the transfer voltage is 0 V is roughly estimated to be 50%, but the amount of current flowing at that time depends on the printing rate and the charge amount.

As described above, before the toner image on the intermediate transfer belt 6 is transferred to the recording sheet (paper) as the transfer material at the contact portion between the secondary transfer roller 26 and the intermediate transfer belt 6, measurement or prediction is performed. When the resistance of the recording sheet is lower than a predetermined value, the resistance of the recording sheet is increased by the transfer material resistance increasing means.
The operation for increasing the resistance of the recording sheet is performed only when the resistance of the recording sheet is expected to be low by actually measuring the temperature and humidity and the resistance of the recording sheet. When the resistance of the recording sheet is high, since the fixing device is not passed, a reduction in productivity can be reduced.
The value of the current that flows through the secondary transfer roller 26 that is a conductive member or the counter roller 24 that is a counter member is controlled to increase as the charge amount of toner on the intermediate transfer belt 6 that is an intermediate transfer member increases.

FIG. 7 is a graph showing the relationship between the secondary transfer efficiency, the secondary transfer current, the volume resistivity of the recording sheet, and the printing rate. FIG. 8 is a table showing subjective evaluations of the image density of the convex portions on the surface of the solid pattern formed on the second surface of the rough paper at a printing rate of 0.05 (5 [%]) and the image density of the concave portions. FIG.
FIG. 7 shows the secondary transfer efficiency (= the mass of toner secondarily transferred to the recording sheet / primary transfer onto the intermediate transfer belt in advance when the resistance of the recording sheet, which is paper, is increased once through the fixing device. The printing rate dependency of the toner mass) and the secondary transfer current is shown. It can be seen that the current value that maximizes the secondary transfer efficiency increases as the printing rate increases.
On the other hand, FIG. 8 shows the image density of the convex portion and the concave portion of the recording sheet (NBS Ricoh FC Japanese paper type “ripple wave”) having a large surface irregularity under the condition of a printing rate of 0.05 (5 [%]). The results of subjective evaluation of the image density (the appearance of the background of the recesses) are shown. From this result, it can be understood that when the secondary transfer current value is increased, the background of the concave portion of the recording sheet is conspicuous and the image quality is deteriorated.

As described above, FC Japanese paper type rough surface paper (trade name “Sazanami” manufactured by NBS Ricoh Co., Ltd.) with large surface irregularities was used as the recording sheet. The recording sheet (rough paper) is passed through the transfer nip and the fixing device 40 under the condition of a printing rate of 0, and the volume resistivity of the recording sheet is increased. A solid pattern having a printing rate of 0.05 (5 [%]) was formed on the surface.
And on the 2nd surface, the image density of the convex part of the surface and the image density of a recessed part were subjectively evaluated. The result is shown in FIG. In FIG. 8, ◯, Δ, and X are good (sufficient image density is obtained), acceptable (image density within an allowable range is obtained), and bad (only significant low image density is obtained). The result of the evaluation is “Unable to be white = blank”.
As shown in FIG. 8, when a recording sheet having a high volume resistivity is used, a good image density is obtained at the convex portion as the secondary transfer current is increased. On the contrary, the image density is lowered at the concave portion. You can see that. This is because as the secondary transfer current is increased, the amount of minute discharge in the recess is increased and the amount of reversely charged toner is increased.

From the above, it has been found that when the volume resistivity of paper as a recording sheet is high, it is necessary to appropriately correct the control target value of the transfer current according to the volume resistivity and the printing rate. Simply, the control target value is determined so as to ensure a constant secondary transfer efficiency at an assumed printing rate (for example, 0.20 (20 [%]) to 0.50 (50 [%])). Depending on the volume resistivity, there is a possibility that toner reverse charging due to minute discharge may be remarkably generated in the concave portion on the surface of the recording sheet.
As described above, this is because discharge occurs when the secondary transfer current increases, and it is concluded that the toner charge on the intermediate transfer belt is reversed by the discharge in the concave portion of the recording sheet and is not transferred to the recording sheet. The
In the present invention, by setting a secondary transfer current at which the secondary transfer efficiency is 0.9 for each printing rate and toner charge amount, a good final image in which the background is not noticeable even on a recording sheet having large unevenness. Successfully realized.

Specifically, the secondary transfer current is controlled in accordance with the printing rate and the toner charge amount using the mathematical formula shown in the following formula (2). As a result, a good final image is realized. That is, the secondary transfer current value is increased as the printing rate is higher. Further, the secondary transfer current value is increased as the toner charge amount (absolute value) is increased.
I = 0.50 × Σηi × Qi + 12.42 Formula (2)
I: Secondary transfer current value [μA]
0.50 and 12.42: Constant ηi: Print rate of toner of each color Qi: Charge amount of toner of each color (μC / g)
The intermediate transfer belt mounted on the printer, which is an image forming apparatus, is a polyimide belt having a tensile elastic modulus of 2.6 GPa. By using a material having a high elastic modulus as the intermediate transfer belt, positional deviation and color deviation are suppressed.

Since it has a higher elastic modulus than rubber (approximately 1 to 10 MPa), its adhesion to a recording sheet with large irregularities on the surface is small, but it is uniform by controlling the secondary transfer current according to the printing rate and toner charge amount. Secondary transfer can be realized.
Therefore, it has been possible to realize uniform transfer on a recording sheet with large unevenness, suppression of color shift and positional shift at the primary transfer portion, and improvement of durability, which have been problems in the past. In order to suppress color shift and position shift, it is desirable to use a material having an elastic modulus of at least 2 GPa as the material of the intermediate transfer belt.
As described above, in the present invention, a material having a high elastic modulus is used as the intermediate transfer belt, thereby suppressing a positional shift and a color shift. With such a belt, it has been a problem that it cannot be uniformly transferred on a recording sheet having a large unevenness on the surface, but it is possible to achieve both suppression of misalignment and uniform transfer by combining with transfer current control depending on the printing rate.

By the way, in this embodiment, only when the recording sheet as the transfer material is paper and the relative humidity exceeds 50%, drying by the fixing device 40 is performed before the secondary transfer. This is to suppress a decrease in productivity due to passing the fixing device 40 once before the secondary transfer every time. However, the higher the resistance of the recording sheet, the higher the effect of current control based on the printing rate of the secondary transfer current and the toner charge amount. Therefore, the above criteria are appropriately determined in consideration of the balance between productivity and image quality. .
Incidentally, in the case of a recording sheet that is a transfer material having high resistance from the beginning, there is no point in drying it before the secondary transfer. For this reason, for example, an OHP sheet is detected by an optical sensor so that drying before secondary transfer is not performed.
In the configuration of FIG. 1, which is one embodiment of the present invention, this intermediate transfer is performed at the contact portion (FIG. 1) between the secondary transfer roller 26 and the intermediate transfer belt 6 according to the type of recording sheet (paper). Before the toner image transferred onto the belt 6 is transferred to the recording sheet, the resistance of the recording sheet is increased by the fixing device 40 which is a transfer material resistance increasing unit.

As described above, the present invention is most effective with paper (recording sheet) having a large surface unevenness. Therefore, a user who uses the printer prints a document when printing a document from a personal computer, or the image forming apparatus. For example, a specific recording sheet such as “NBS RICOH FC Japanese paper type ripple” can be selected on the operation panel when copying a document with the main body. Further, even if the name of the recording sheet is not known, when the user selects the “uneven paper mode”, the paper is passed through the fixing device 40 before the secondary transfer.
As a result, before the toner image transferred onto the intermediate transfer belt 6 at the contact portion between the secondary transfer roller 26 and the intermediate transfer belt 6 is transferred to the recording sheet, the fixing material serving as a transfer material resistance increasing unit is fixed. The device 40 increases the resistance of the recording sheet. When the user designates a recording sheet having a large surface irregularity, a uniform secondary transfer is realized by always including an operation of increasing the resistance of the recording sheet.
In this printer, optimum transfer is realized by controlling the secondary transfer current based on the charge amount of the toner of each color to be transferred, as shown by the equations (1) and (2). However, if there is not much difference in the charge amount of each color toner, or if the toner charge amount in the development unit is stable against the environment and stress, the secondary transfer current is controlled only by the printing rate during transfer. Even if a big effect is acquired.

The function used for the control is not limited to the formulas (1) and (2), but a simpler function, and conversely, other physical quantities such as the temperature and humidity and the amount of toner deposited on the intermediate transfer belt are considered. It is possible to use more complex functions. Further, the control may be performed based on a predetermined table according to the charge amount and the printing rate of each toner without using a function.
It should be noted that the printing rate that serves as a reference for the control is not limited to the exit portion of the secondary transfer nip, and the average value of the printing rates existing in the center of the nip or in the nip can also be used. Further, the current control is not performed for each pixel in the sub-scanning direction, but may be performed more roughly (for example, every second transfer nip width / process linear velocity (seconds)).
In the present invention, the value of the secondary transfer current is determined based on the toner printing rate in the main scanning direction in the secondary transfer nip (for each pixel in the sub-scanning direction). As a result, the influence of the current flowing through the non-image portion of the recording sheet (paper) can be eliminated, so that only the amount of current necessary for transferring the toner can be accurately applied.
At that time, if the resistance of the recording sheet is low, the current flowing through the non-image portion of the recording sheet increases, and the amount of current necessary to transfer the toner is buried in the error. Since a stable image (stable transfer efficiency) cannot be obtained, the resistance of the recording sheet is increased in advance before the secondary transfer. The secondary transfer efficiency is a numerical value indicating the ratio of the amount of toner transferred to the recording sheet to the amount of toner on the belt surface.

A combination of a resistance measuring roller pair 31 including rollers 31a and 31b, an ammeter 34, a high voltage power source 35, a constant current control unit, and the like functions as a resistance detection unit that detects the resistance of the recording sheet. In this manner, not only the temperature and humidity environment but also the actual paper resistance can be measured and judged.
In the present invention, the value of the secondary transfer current is determined based on the toner printing rate in the main scanning direction in the secondary transfer nip (for each pixel in the sub-scanning direction). As a result, the influence of the current flowing through the non-image portion of the recording sheet (paper) can be eliminated, so that only the amount of current necessary for transferring the toner can be accurately applied.
At that time, if the resistance of the recording sheet is low, the current flowing through the non-image portion of the recording sheet increases, and the amount of current necessary to transfer the toner is buried in the error. Since a stable image (stable transfer efficiency) cannot be obtained, the resistance of the recording sheet is increased in advance before the secondary transfer.

  A Image forming apparatus (printer), 1Y, M, C, K Image forming unit, 2Y, M, C, K Image carrier (photosensitive drum), 4Y, M, C, K Developing unit, 5Y, M, C , K primary transfer roller, 6 intermediate transfer member (intermediate transfer belt), 8 transfer material transfer belt, 10 tandem image forming unit, 12 transfer guide, 13 registration roller, 14 secondary transfer bias power supply, 16 transfer path, 20 Transfer unit, 24 Opposing member (opposing roller, nip forming member), 26 Conducting member (secondary transfer roller, nip forming member), 31 Resistance measuring roller pair (part of resistance detecting means), 34 Ammeter (resistance detecting means) 35) High voltage power supply (Part of resistance detection means), 36 Environmental sensor, 40 Fixing device, 41 Fixing roller, 42 Pressure roller, 50 Retransmission device

JP2007-121619A Japanese Patent Laid-Open No. 2001-100545 Japanese Patent Laid-Open No. 06-289682 Japanese Patent Application Laid-Open No. 07-175340

Claims (6)

An intermediate transfer member to which a toner image formed on the image carrier is transferred, a conductive member that sandwiches a transfer material between the intermediate transfer member, and the conductive member via the intermediate transfer member An image forming apparatus having an opposing member disposed at an opposing position and forming an electric field between the conductive member,
A transfer material resistance increasing means for increasing the resistance of the transfer material;
Before the transfer of the toner image on the intermediate transfer body to the transfer material at the contact portion, the resistance of the transfer material is predicted based on at least one of temperature and humidity detected by an environmental sensor. A resistance predicting means for detecting or a resistance detecting means for detecting the resistance of the material to be transferred;
When the prediction result by the resistance prediction unit or the detection result by the resistance detection unit is smaller than a predetermined value, the toner image on the intermediate transfer member at the contact portion is transferred to the transfer material before the transfer. Resistance control means for performing control to increase the resistance of the transfer material by means of the transfer material resistance increasing means;
When the toner image on the intermediate transfer body at the contact portion is transferred to the transfer material, a secondary current value that is passed through the conductive member or the opposing member is printed on the toner image transferred to the transfer material. An image forming apparatus comprising: a current control unit configured to control the rate so as to increase according to the rate or the number of pixels.   A transfer path for transferring the transfer material that has passed through the transfer material resistance increasing means toward a registration roller that matches a secondary transfer timing; and the transfer material resistance increasing means transfers the transfer material to the transfer material A fixing device used to fix the transferred toner image to the transfer material by heat, and is transferred onto the intermediate transfer member at a contact portion between the conductive member and the intermediate transfer member. The image forming apparatus according to claim 1, wherein the transfer material is heated by the fixing device before the toner image is transferred to the transfer material. The current control means controls a secondary current value passed through the conductive member or the opposing member so as to increase as the charge amount of the toner on the intermediate transfer member increases. 2. The image forming apparatus according to 2. The resistance control unit is configured to transfer a toner image transferred onto the intermediate transfer member at the contact portion between the conductive member and the intermediate transfer member based on the type of the transfer target material. 4. The image forming apparatus according to claim 1, wherein the resistance of the transfer material is increased by the transfer material resistance increasing unit. 5.   When the document is printed from a personal computer or when the document is copied on the image forming apparatus main body, the resistance control unit is configured to select the type of the material to be transferred on the operation panel. Before the toner image transferred onto the intermediate transfer body at the contact portion between the intermediate transfer body and the intermediate transfer body is transferred to the transfer material, the transfer material resistance increasing means increases the resistance of the transfer material. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   The image forming apparatus according to claim 1, wherein the intermediate transfer member is a belt-like member, and the tensile elastic modulus of the belt-like member is 2 GPa or more.

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