Patent Application
20080181636
A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by any one of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever.
1. Field of the Invention
The present invention relates to a control technique of a transfer bias voltage in an image forming apparatus for transferring a toner image onto a sheet by applying the transfer bias voltage to the sheet through a transfer member.
2. Description of the Related Art
Hitherto, the determination of a processing condition of a transfer processing of a toner image onto a sheet has been performed such that a user selects the basis weight of a sheet to be used, a voltage value of a transfer bias voltage is determined based on the information, and transfer onto the sheet is performed. Besides, in order to save the user the trouble of selecting the sheet, after the thickness of the sheet is detected by a thickness detecting sensor or the like, the transfer bias voltage is determined and the transfer processing onto the sheet has been performed.
In general, in sheets of the same material, even if the basis weight or electric resistance is changed, a necessary transfer current is constant. Accordingly, when a transfer bias voltage by which a specified transfer current flows is applied according to the basis weight or electric resistance, an excellent image can be obtained. However, in sheets of different materials, even if the basis weights or electric resistances are almost the same, optimum transfer currents are not always coincident with each other, and poor transfer (poor image) can often occur.
The invention has been made to solve the foregoing problem, and has an object to provide a technique of preventing poor transfer from occurring by suitably controlling a transfer bias voltage according to a processing condition in an image forming apparatus in which a toner image is transferred onto a sheet by applying the transfer bias voltage to the sheet through a transfer member.
Hereinafter, embodiments of the invention will be described with reference to the drawings.
Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus, methods and programs of the present invention.
As shown in the drawing, the image forming apparatus M is provided with process units 1a, 1b, 1c and 1d as image forming means. The respective process units include photoconductive drums 3a, 3b, 3c and 3d as image bearing bodies, and developer images are formed on these photoconductors.
The process unit la will be described. In
The following are provided around the photoconductive drum 3a along the rotation direction. First, a charging charger 5a is provided to be opposite to the surface of the photoconductive drum 1a. The charging charger 5a uniformly negatively (−) charges the photoconductive drum 3a. An exposure device 7a to expose the charged photoconductive drum 3a to form an electrostatic latent image is provided at the downstream side (right in
A cleaner 19a is provided at the downstream side of the contact position between the photoconductive drum 3a and the belt 11. After transfer, the cleaner 19a removes the surface charge of the photoconductive drum 3a by uniform light irradiation, and removes and receives residual toner on the photoconductor. By this, one cycle of image formation is completed, and at a next image forming process, the charging charger 5a again uniformly charges the non-charged photoconductive drum 3a.
The process unit 1a includes the photoconductive drum 3a, the charging charger 5a, the exposure device 7a, the developing unit 9a, and the cleaner 19a.
The belt 11 has a length (width) almost equal to the length of the photoconductive drum 1a in a direction (depth direction of the drawing) perpendicular to a transport direction (direction of an arrow e shown in the drawing). This belt 11 has a shape of an endless (seamless) belt, and is supported on a drive roller 15 to rotate the belt at a specified speed and some driven rollers.
The belt 11 is made of polyimide uniformly dispersed with carbon and having a thickness of 100 μm. This belt 11 has an electric resistance of 109 Ωcm and exhibits semiconductivity.
As a material of the belt 11, any material may be used as long as it has a volume resistance value of 108 to 1011 Ωcm and exhibits semiconductivity. For example, a material obtained by dispersing a conductive particle of carbon or the like into polyethylene terephthalate, polycarbonate, polytetrafluoroethylene, polyvinylidene fluoride or the like may be used in addition to the polyimide dispersed with carbon. The conductive particle is not used, but a high molecular film whose electric resistance is adjusted by composition adjustment may be used. Further, a material obtained by mixing an ion conductive material into such a high molecular film, or a rubber material, such as silicone rubber or urethane rubber, having relatively low electric resistance may be used.
The process units 1b, 1c and 1d, in addition to the process unit 1a, are arranged on the belt 11 between the drive roller 15 and a secondary transfer opposite roller 13 along the transport direction of the belt 11.
Each of the processing units 1b, 1c and 1d has the same structure as the process unit 1a. That is, the photoconductive drums 1b, 1c and 1d are provided almost at the centers of the respective process units. Charging chargers 5b, 5c and 5d are provided around the respective photoconductive drums. Exposure devices 7b, 7c and 7d are provided at downstream sides of the charging chargers. A structure in which developing units 9b, 9c and 9d and cleaners 19b, 19c and 19d are provided at downstream sides of the exposure devices is also similar to the process unit 1a. A difference is a developer contained in the developing unit. The developing unit 19b contains a magenta developer, the developing unit 19c contains a cyan developer, and the developing unit 19d contains a black developer.
The belt 11 sequentially comes in contact with the respective photoconductive drums. In the vicinities of the contact positions between this belt 11 and the respective photoconductive drums, transfer devices 23a, 23b, 23c and 23d as transfer means are provided to correspond to the respective photoconductive drums. That is, the transfer device 23 is provided below the corresponding photoconductive drum to be in back contact with the belt 11 and is opposite to the process unit through the belt 11.
The transfer member 23a is connected to a not-shown positive (+) DC power source 25a as voltage application means. Similarly, the transfer members 23b, 23c and 23d are respectively connected to not-shown DC power sources 25b, 25c and 25d.
On the other hand, in
Besides, a fixing unit 33 to fix the developer onto the sheet P and an in-barrel paper discharge unit 34 to which the sheet P fixed by this fixing unit is discharged are provided at the front right of the belt 11.
Next, a color image forming operation of the image forming apparatus M constructed as described above will be described.
When the start of an image forming processing is instructed, the photoconductive drum 3a receives a drive force from a not-shown drive mechanism and starts to rotate. The charging charger 5a uniformly charges the photoconductive drum 3a to about −600 V. The exposure device 7a irradiates light corresponding to an image to be recorded to this photoconductive drum 3a uniformly charged by the charging charger 5a and forms an electric static latent image. The developing unit 9a contains the developer (yellow (Y) toner+ferrite carrier; two-component developer), a developing bias voltage value of −380 V is given to a not-shown developing sleeve by a not-shown developing bias power source, and a developing electric field is formed between it and the photoconductive drum 3a. Reversal development is performed in which the negatively charged Y toner is attached to an area of an image part potential (high potential part; signs are considered) of the electrostatic latent image of the photoconductor 3a. Next, by a method different from that in which the developing unit 9a forms the Y toner image on the photoconductive drum 3a, the developing unit 9b develops the electrostatic latent image with the magenta developer and forms a magenta toner (M toner) image on the photoconductive drum 3b. At this time, similarly to the Y toner, the M toner has an average particle diameter of about 7 microns and is negatively charged by friction charging with a ferrite magnetic carrier particle (not shown) with an average particle diameter of 60 microns. Similarly to the developing unit 3a, the developing bias voltage value is about −380 V and is applied to the developing sleeve (the structure of the developing unit is the same as the developing unit 9a) by a not-shown bias power source. The direction of a developing electric field is directed from the surface of the photoconductive drum 3b toward the developing sleeve, and the negatively charged M toner is attached to a high potential part of the latent image.
In a transfer area Ta including the photoconductive drum 3a, the belt 11 and the transfer member 23a, a bias voltage of about +1000 V is applied to the transfer member 23a. A transfer electric field is formed between the transfer member 23a and the photoconductive drum 3a, and the yellow toner image on the photoconductive drum 3a is transferred onto the belt 11 in accordance with this transfer electric field.
Next, a portion relating to the transfer device will be described in more detail.
The transfer device 23a is a conductive urethane foam roller which is made conductive by dispersing carbon. A roller 41 with an outer diameter of φ18 mm is formed on a cored bar 40 of φ10 mm. An electric resistance between the cored bar and the roller surface is about 106 Ω. A constant voltage DC power source 25a is connected to the cored bar.
A feeder device in the transfer device may be a conductive brush, a conductive rubber blade, a conductive sheet or the like in addition to the roller. The conductive sheet is a rubber material dispersed with carbon or a resin film, and may be a rubber material such as silicone rubber, urethane rubber or EPDM, or a resin material such as polycarbonate. It is desirable that a volume resistance value is 105 to 107 Ωcm.
A spring 47 and a spring 49 as urging means are provided at both ends of a roller shaft, and by the springs 47 and 49, the transfer roller 23a is urged to come in elastic contact with the transport belt 11 in the vertical direction. The magnitude of the urging force by the spring 47 and the spring 49 provided to each of the transfer rollers is 600 gf. Here, the urging force means the sum of an urging force of 300 gf by the spring 47 and an urging force of 300 gf by the spring 49.
The structures of the transfer devices 23b, 23c and 23d are similar to the transfer device 23a, and the structures of elastic contact with the transport belt 11 are also similar to each other with respect to the respective transfer members, and therefore, the explanation of the structures of the transfer devices 23b, 23c and 23d will be omitted.
An image on the belt 11 on which the Y (yellow) toner image is transferred in the transfer area Ta is transported to a transfer area Tb. In the transfer area Tb, a bias voltage of about +1200 V is applied to the transfer member 23b from a DC power source, so that the magenta toner image is transferred to overlap with the Y toner image. A bias voltage of about +1400 V is applied to the transfer member 23c in a transfer area Tc, and further, a voltage of about +1600 V is applied to the transfer member 23d in a transfer area Td, so that the cyan developer image and the black developer image are sequentially multiplex-transferred to overlap with the already transferred developer image. On the other hand, the pickup roller 27 takes out the sheet P from the paper feed cassette 26, and the register roller pair 29 supplies this sheet P to the secondary transfer unit.
In the secondary transfer unit, a specified transfer bias voltage is applied to the secondary transfer opposite roller, a transfer electric field is formed between it and the secondary transfer roller through the belt, and the multiplex color toner images on the belt 11 are transferred onto the sheet P at the same time. The secondary transfer opposite roller 15, the belt 11 and the secondary transfer roller 24 here are equivalent to the transfer member.
As stated above, the developer images of the respective colors transferred at the same time are fixed on the sheet P by the fixing unit 33, and a color image is formed. The fixed sheet P is discharged onto the in-barrel paper discharge unit 34.
The roughness detecting unit 101 serves to detect the surface roughness of the sheet.
The roughness information acquiring unit 102 serves to acquire, as information relating to the surface roughness of the sheet, information relating to a surface roughness value operation-inputted to the operation input unit 112 or a surface roughness value detected by the roughness detecting unit 101.
The sheet information acquiring unit 103 serves to acquire information relating to an electric resistance of the sheet. Specifically, the sheet information acquiring unit 103 acquires, as information relating to the electric resistance of the sheet, at least one of the model number, thickness, basis weight and material of the sheet based on the operation input to the operation input unit 112.
The environment detecting unit 104 serves to detect at least “humidity” as the installation environment of the image forming apparatus.
The resistance value calculating unit 105 serves to calculate the electric resistance value of the transfer member based on a voltage value at the time when a specified current is made to flow to the transfer member.
The resistance value estimating unit 106 serves to estimate the electric resistance value of the transfer member based on the humidity detected by the environment detecting unit 104.
The resistance value information acquiring unit 107 serves to acquire the electric resistance value calculated by the resistance value calculating unit 105 or the electric resistance value estimated by the resistance value estimating unit 106 as information relating to the electric resistance of the transfer member.
The first transfer voltage calculating unit 108 serves to calculate a first transfer voltage as a voltage for the transfer member in the transfer bias voltage based on the information acquired by the resistance value information acquiring unit 107 and a specified current value.
The second transfer voltage estimating unit 109 serves to estimate a second transfer voltage as a voltage for the sheet in the transfer bias voltage based on the humidity detected by the environment detecting unit 104 and the information acquired by the sheet information acquiring unit 103.
The voltage correcting unit 110 serves to correct the voltage value of the second transfer voltage estimated by the second transfer voltage estimating unit 109 based on the information acquired by the roughness information acquiring unit 102, so that the transfer current flowing to the sheet at the time when the toner image is transferred onto the sheet becomes a specified optimum current value. Incidentally, it is preferable that the voltage correcting unit 110 makes a correction based on the information relating to the surface roughness of the sheet acquired by the roughness information acquiring unit 102, so that as the value of the surface roughness of the sheet becomes large, the post-correction value of the voltage value of the second transfer voltage estimated by the second transfer voltage estimating unit 109 becomes large.
The voltage control unit 111 serves to apply, as the transfer bias voltage, the sum of the first transfer voltage calculated by the first transfer voltage calculating unit 108 and the second transfer voltage of the voltage value corrected by the voltage correcting unit 110.
The operation input unit 112 includes a keyboard, a mouse or the like, and serves as an interface to receive the operation input of the user. Besides, the display unit 113 includes, for example, a liquid crystal display, and serves to screen-display the processing content in the image forming apparatus M. Of course, it is also possible to realize the functions of the operation input unit 112 and the display unit 113 by a touch panel display or the like.
The CPU 801 serves to perform various processings in the image forming apparatus, and also serves to realize various functions by executing programs stored in the MEMORY 802. The MEMORY 802 includes, for example, a ROM or a RAM, and serves to store various information and programs used in the image forming apparatus.
In general, since the first transfer voltage depending on the material of the transfer member and the surface state does not change greatly, the accuracy of the correction can be improved by correcting, as in the structure of the embodiment, only the second transfer voltage for the sheet, which is much changed in accordance with the environment.
Incidentally, in the foregoing structure, although the transfer bias voltage is divided into the “first transfer voltage” for the transfer member and the “second transfer voltage” for the sheet, for example, in the case where the voltage value of the first transfer voltage can also be estimated based on an environment factor such as humidity, the “first transfer voltage” and the “second transfer voltage” are integrated into one transfer bias voltage, and a structure as described below may be adopted.
Specifically, in an image forming apparatus for transferring a toner image onto a sheet by applying a transfer bias voltage to the sheet through a transfer member, there can be provided the image forming apparatus including a roughness information acquiring unit configured to acquire information relating to a surface roughness of the sheet, a sheet information acquiring unit configured to acquire information relating to an electric resistance of the sheet, an environment detecting unit configured to detect humidity as an installation environment of the image forming apparatus, a voltage calculating unit configured to calculate a transfer bias voltage based on the information acquired by the sheet information acquiring unit and the humidity detected by the environment detecting unit, a voltage correcting unit configured to correct a voltage value of the transfer bias voltage calculated by the voltage calculating unit based on the information acquired by the roughness information acquiring unit so that a transfer current flowing to the sheet at a time when the toner image is transferred onto the sheet becomes an optimum current value, and a voltage control unit configured to apply the transfer bias voltage of the voltage value corrected by the voltage correcting unit.
Incidentally, the humidity detected by the environment detecting unit is, for example, relative humidity. Besides, although the environment detecting unit detects at least the humidity as an environment factor which becomes a primary factor to change the electric resistance of the sheet in the thickness direction, no limitation is made to this, and for example, in the case where temperature changes the electric resistance of the sheet according to the material of the sheet, the temperature or the like may also be detected.
As a result of diligent investigation as to the cause of this difference, it has been found that this difference is caused by the surface roughness of the sheet.
When the surface roughnesses of two kinds of sheets shown in
Hereinafter, the flow of a transfer bias voltage control in this embodiment will be described in detail.
In the transfer bias voltage control in this embodiment, there are two flows as shown in
In the other processing flow, a relative humidity sheet correction voltage (second transfer voltage) applied to the selected sheet is calculated from the sheet kind selected by the user and the relative humidity information detected by the environment detecting unit 104. These two correction voltages are combined to form the transfer bias voltage.
Next, a control method of a secondary transfer transformer TR2 will be described by use of
An intermediate transfer belt is driven, and when it is confirmed that the secondary transfer roller is in contact with the intermediate transfer belt, an electric resistance detecting control becomes possible, and the control is started. First, the secondary transfer transformer is turned ON, switching to the constant current output is performed by the control switching signal, and when the control voltage is set so that a specified current is obtained and is inputted to the transformer, the specified constant current output is applied from the secondary transfer transformer to the secondary transfer unit. Further, a voltage generated at that time is outputted as a monitor voltage from the secondary transfer transformer. After a specified time has passed since the secondary transfer current was applied, that is, after the applied current becomes stable, this monitor voltage is detected. Although depending on the characteristic of the transformer, the time from the application of the secondary transfer current to the detection of the voltage is about 50 msec. Besides, although the time in which the voltage is detected is suitable to be one or more rounds of the secondary transfer roller, the detection may be performed in one round or less according to circumstances.
For example, when the diameter of the secondary transfer roller is 28 mm, the process speed is 150 mm/sec, and the sampling period is 24 msec, the number of times of sampling is about 24, and averaging is performed for this and a measurement voltage is obtained. A relation between the measurement voltage and the secondary transfer roller correction voltage is stored in the MEMORY 802 as a table of six points, and the secondary transfer roller correction voltage is calculated by linear interpolation between the respective two points. When the detection current and the transfer current are identical to each other, the measured voltage substantially becomes the secondary transfer roller correction voltage as it is. Since the detection current is fixed to 30 μA, in the case where the process speed varies, a desired transfer current varies, and therefore, the measurement voltage and the secondary transfer roller correction voltage are different from each other. Next, the relative humidity sheet correction voltage will be described. This corresponds to a divided voltage of the transfer voltage applied to the electric resistance of the sheet and the toner layer. Besides, with respect to relative humidities of 6 points, values of relative humidity sheet correction voltage are stored in the MEMORY 802 as a table, and the relative humidity sheet correction voltage is calculated by the linear interpolation between the respective two points. These tables are prepared for respective kinds of sheets, for example, normal paper, thick paper, thin paper, recycled paper and the like and for the respective sheets which the user can set by a control panel or a printer driver.
When the second side in two-sided printing is printed, the sheet once passes through the fixing unit at the time of the print processing to the first side, so that the sheet becomes rid of moisture and the electric resistance becomes high, and therefore, even if the other condition is the same, the same table as that of the printing of the first side can not be used. Accordingly, it is preferable to prepare a back side correction voltage table for each sheet. However, with respect to a sheet, such as an OHP or special paper, which is known not to be subjected to the two-sided printing, it is not necessary to prepare the table for the second side.
When the control is started, based on the kind of the sheet specified by the user and the relative humidity value detected by the environment detecting unit 104, the relative humidity sheet correction voltage is calculated by the linear interpolation between two points from the table of the relative humidity sheet correction voltage with respect to the relative humidity. However, in the case where the value of the surface roughness of the sheet is different from a specified surface roughness value, the relative humidity sheet correction voltage is corrected according to the difference of the value. For example, in the case where the value of the surface roughness is larger than the specified value, the relative humidity sheet correction voltage is set to be larger than a normal value. On the other hand, in the case where the value of the surface roughness is smaller than the specified value, the relative humidity sheet correction voltage is set to be smaller than the normal value. The calculated secondary transfer roller correction voltage (first transfer voltage) and the relative humidity sheet correction voltage (second transfer voltage) are summed to obtain the transfer bias voltage.
First, the roughness detecting unit 101 detects the surface roughness of the sheet (roughness detecting step) (S101). Specifically, as the detecting method of the surface roughness of the sheet, for example, a method of two-dimensionally grasping the sheet surface, such as a method of calculating the surface roughness by using a CCD sensor to take a picture of the sheet surface and by an image processing, or a method of using a CMOS area sensor, is effective.
The roughness information acquiring unit 102 acquires, as information relating to the surface roughness of the sheet, “information relating to the surface roughness value of the sheet” operation-inputted to the operation input unit 112 or the surface roughness value detected at the roughness detecting step (roughness information acquiring step) (S102).
The sheet information acquiring unit 103 acquires the “information relating to the electric resistance of the sheet” operation-inputted to the operation input unit 112 (sheet information acquiring step) (S103). Specifically, at the sheet information acquiring step, at least one of the model number, thickness, basis weight and material of the sheet is acquired as the information relating to the electric resistance of the sheet.
Incidentally, in this embodiment, although the structure is such that the acquisition of the information in the roughness information acquiring unit 102 and the sheet information acquiring unit 103 is executed each time the secondary transfer processing to the sheet is performed, no limitation is made to this, and in the case where the kind of the sheet to be used is the same as one used in the past, the information relating to the sheet is held in the MEMORY 802, and the information may be used.
The environment detecting unit 104 includes a temperature humidity sensor or the like, and detects at least the “humidity” as the installation environment of the image forming apparatus (environment detecting step) (S104).
As shown in
The resistance value estimating unit 106 estimates the electric resistance value of the transfer member based on the humidity detected at the environment detecting step (resistance value estimating step) (S106).
The resistance value information acquiring unit 107 acquires the electric resistance value calculated at the resistance value calculating step or the electric resistance value estimated at the resistance value estimating step as the information relating to the electric resistance of the transfer member (resistance value information acquiring step) (S107).
The first transfer voltage calculating unit 108 calculates the first transfer voltage as the voltage for the transfer member in the transfer bias voltage based on the information acquired at the resistance value information acquiring step and a specified current value (first transfer voltage calculating step) (S108).
The second transfer voltage estimating unit 109 estimates the second transfer voltage as the voltage for the sheet in the transfer bias voltage based on the humidity detected at the environment detecting step and the information acquired at the sheet information acquiring step (second transfer voltage estimating step) (S109) (see
The voltage correcting unit 110 corrects the voltage value of the second transfer voltage estimated at the second transfer voltage estimating step based on the information acquired at the roughness information acquiring step, so that the transfer current flowing to the sheet at the time when the toner image is transferred onto the sheet becomes a specified optimum current value (voltage correcting step) (S110). Incidentally, at the voltage correcting step, it is preferable that based on the information relating to the surface roughness of the sheet acquired at the roughness information acquiring step, the correction is performed so that as the value of the surface roughness of the sheet becomes large, the post-correction value of the voltage value of the second transfer voltage estimated at the second transfer voltage estimating step becomes large.
The voltage control unit 111 applies, as the transfer bias voltage, the sum of the first transfer voltage calculated at the first transfer voltage calculating step and the second transfer voltage of the voltage value corrected at the voltage correcting step (voltage control step) (S111). As stated above, in this embodiment, the secondary transfer roller correction voltage (first transfer voltage) Va and the relative humidity sheet correction voltage (second transfer voltage) Vc are calculated, and both are added to obtain the transfer bias voltage.
As described above, according to this embodiment, in an image forming method for transferring a toner image onto a sheet by applying a transfer bias voltage to the sheet through a transfer member, there can also be provided the image forming method including a roughness information acquiring step of acquiring information relating to a surface roughness of the sheet, a sheet information acquiring step of acquiring information relating to an electric resistance of the sheet, an environment detecting step of detecting a humidity as an installation environment of the image forming apparatus, a voltage calculating step of calculating a transfer bias voltage based on the information acquired at the sheet information acquiring step and the humidity detected at the environment detecting step, a voltage correcting step of correcting a voltage value of the transfer bias voltage calculated at the voltage calculating step based on the information acquired at the roughness information acquiring step so that a transfer current flowing to the sheet at a time when the toner image is transferred onto the sheet becomes an optimum current value, and a voltage control step of applying the transfer bias voltage of the voltage value corrected at the voltage correcting step. Besides, in the image forming method of the structure as stated above, at the voltage correcting step, it is desirable that the correction is performed based on the information relating to the surface roughness of the sheet acquired at the roughness information acquiring step, so that as the value of the surface roughness of the sheet becomes large, the post-correction value of the voltage value of the transfer bias voltage calculated at the voltage calculating step becomes large. Besides, in the image forming method of the structure as stated above, there is provided an operation input step of receiving an operation input of a user, and at the roughness information acquiring step, the information relating to the surface roughness of the sheet may be acquired based on the operation input at the operation input step. Besides, in the image forming method of the structure as stated above, there is provided a roughness detecting step of detecting the surface roughness of the sheet, and at the roughness information acquiring step, a value of the surface roughness detected at the roughness detecting step can also be acquired. Besides, in the image forming method of the structure as stated above, at the sheet information acquiring step, it is preferable that at least one of the model number, thickness, basis weight and material of the sheet is acquired as the information relating to the electric resistance of the sheet.
Next, a second embodiment of the invention will be described. This embodiment is a modified example of the first embodiment, and the structure of a secondary transfer unit in an image forming apparatus of this embodiment is similar to that of the first embodiment as shown in
In this embodiment, a relative humidity sheet correction voltage table is not used, and as shown in
For example, in the case where the value of the surface roughness is larger than the specified value, the sheet correction voltage is set to be larger than a normal value. On the other hand, in the case where the value of the surface roughness is smaller than the specified value, the sheet correction voltage is set to be smaller than the normal value. The calculated secondary transfer roller correction voltage and the relative humidity sheet correction voltage are added to obtain a transfer bias voltage.
The operation input unit 608 serves to receive an operation input of a user.
The roughness detecting unit 601 serves to detect a surface roughness of a sheet.
The roughness information acquiring unit 602 serves to acquire information relating to the surface roughness of the sheet based on a value of the surface roughness detected by the roughness detecting unit 601 or the operation input to the operation input unit 608.
The environment detecting unit 603 serves to detect temperature and humidity as an installation environment of the image forming apparatus.
The resistance value calculating unit 604 serves to calculate electric resistance values of the transfer member and the sheet at a timing when the sheet is nipped in the secondary transfer unit and based on a voltage value at a time when a specified current is made to flow to the sheet through the transfer member.
The transfer voltage calculating unit 605 serves to calculate a transfer bias voltage in the secondary transfer unit based on the electric resistance value calculated by the resistance value calculating unit 604 and the specified current value.
The voltage correcting unit 606 serves to correct the voltage value of the transfer bias voltage calculated by the transfer voltage calculating unit 605 based on the information acquired by the roughness information acquiring unit 602 so that a transfer current flowing to the sheet at the time when the toner image is transferred onto the sheet becomes a specified optimum current value.
The voltage correcting unit 606 performs a correction based on the information relating to the surface roughness of the sheet acquired by the roughness information acquiring unit 602, so that as the value of the surface roughness of the sheet becomes large, a post-correction value of the voltage value of the transfer bias voltage calculated by the transfer voltage calculating unit 605 becomes large.
In the case where the temperature and humidity detected by the environment detecting unit 603 is a specified high temperature and high humidity environment, the voltage control unit 607 applies, as the transfer bias voltage, the sum of a specified correction voltage value according to the temperature and humidity and the voltage value corrected by the voltage correcting unit 606.
Next, an image forming apparatus according to a third embodiment of the invention will be described.
In this embodiment, by using the surface roughness of the sheet, the transfer bias voltage can be determined more precisely than the foregoing embodiment.
First, the surface roughness of the sheet is detected by the roughness detecting unit. It is preferable that the detection of the surface roughness of the sheet is performed in the vicinity of a position where the sheet is nipped by register rollers. The detection of the surface roughness is performed such that the sheet surface is two-dimensionally grasped as a grayscale picture by a CMOS sensor, and is converted into roughness information by an image processing. The optimum transfer current is determined by referring to a previously defined relation between a sheet surface roughness and a suitable transfer current. For example, in an “A” sheet with a surface roughness of about 1.4 μm, the suitable transfer current becomes 40 μA (see
Next, in order to calculate a secondary transfer roller correction voltage, similarly to the first embodiment, a constant current of 30 μA is applied in a state where the sheet is not nipped in the secondary transfer unit, and a voltage Va applied to the secondary transfer unit is measured. Here, since a suitable transfer current and a resistance detection current are different between the case of the “A” sheet and the case of the “B” sheet, the voltage measured at 30 μA is converted into that of the case of 40 μA and 60 μA by using a conversion table. In this case, a converted secondary transfer roller correction voltage Va′ is 1000V in the case of the “A” sheet, and 1400V in the case of the “B” sheet.
Next, in order to calculate the sheet correction voltage, similarly to the second embodiment, a constant current of 30 μA is applied in the state where the sheet is nipped in the secondary transfer unit, and a voltage Vb applied to the secondary transfer unit is measured. Here, although Vb−Va is a voltage applied to the sheet, that is, the sheet correction voltage Vc, also in this case, since the suitable transfer current and the resistance detection current are different, the voltage Vc measured at 30 μA is converted into that of the case of 40 μA and 60 μA by using a conversion table. In this case, the converted sheet correction voltage Vc′ is 600 V in the case of the “A” sheet, and 900 V in the case of the “B” sheet. Finally, the secondary transfer roller correction voltage Va′ and the sheet correction voltage Vc′ are added to obtain the secondary transfer bias. In the case of this embodiment, the secondary transfer biases of the “A” sheet and the “B” sheet become 1600 V and 2300 V, respectively. In most cases, since the resistance of the sheet is higher than the resistance of the toner, the secondary transfer bias may be Va′+Vc′, however, under a high temperature and high humidity environment, since the resistance of the sheet becomes low, about 50 V to 100 V may be added as a voltage Vt for the resistance of the toner. In this case, the secondary transfer bias becomes Va′+Vc′+Vt.
As stated above, in this embodiment, the specified constant current is applied to the transfer member or the opposite member at the time when the sheet is not nipped at the secondary transfer position, the voltage applied to the secondary transfer unit at that time is detected, and the transfer member correction voltage is calculated based on the voltage, and further, the transfer material correction voltage is calculated by detecting the electric resistance of the transfer material and the surface roughness, and the transfer bias voltage is determined by adding the transfer member correction voltage, the transfer material correction voltage, and the specified toner correction voltage.
Next, a fourth embodiment of the invention will be described. In the foregoing respective embodiments, the transfer bias voltage is obtained based on the kind of the sheet, the humidity, the resistance value of the transfer member and the like, and the whole or part of the transfer bias voltage is corrected based on the surface roughness of the sheet, whereas in this embodiment, the surface roughness of the sheet is detected to determine an optimum transfer current, and further, the electric resistance of the sheet is detected, and an optimum transfer bias voltage is calculated from the optimum transfer current and the electric resistance of the sheet to control the secondary transfer bias.
The roughness detecting unit 701 serves to detect a surface roughness of a sheet.
The roughness information acquiring unit 702 serves to acquire information relating to the surface roughness of the sheet based on a value of the surface roughness detected by the roughness detecting unit 701 or an operation input to the operation input unit 707.
The sheet information acquiring unit 703 serves to acquire information relating to an electric resistance of the sheet based on the operation input to the operation input unit 707.
The optimum current value setting unit 704 serves to set an optimum value of a transfer current at the time when a toner image is transferred onto the sheet based on the information acquired by the roughness information acquiring unit 702 and the sheet information acquiring unit 703. Specifically, for example, table data in which the surface roughness of the sheet, the kind of the sheet and the like are made to correspond to the optimum transfer current value is stored in the MEMORY 802, and the optimum current value setting unit 704 refers to the table data and determines the optimum value of the transfer current.
The voltage detecting unit 705 serves to detect (measure) a voltage value at the time when the transfer current of the current value set by the optimum current value setting unit 704 is made to flow to the sheet through the transfer member.
The voltage control unit 706 serves to apply (voltage constant control) the transfer bias voltage of the voltage value detected by the voltage detecting unit 705.
The roughness information acquiring unit 702 acquires the information relating to the surface roughness of the sheet (roughness information acquiring step) (S701).
The sheet information acquiring unit 703 acquires the information relating to the electric resistance of the sheet (sheet information acquiring step) (S702).
The optimum current value setting unit 704 sets the optimum value of the transfer current at the time when the toner image is transferred onto the sheet based on the information acquired at the roughness information acquiring step and the sheet information acquiring step (optimum current value setting step) (S703).
The voltage detecting unit 705 detects a voltage value at the time when the transfer current of the current value set at the optimum current value setting step is made to flow to the sheet through the transfer member (voltage detecting step) (S704).
The voltage control unit 706 applies the transfer bias voltage of the voltage value detected at the voltage detecting step (voltage control step) (S705).
The respective steps of the processing of the image forming apparatus in the foregoing respective embodiments are realized by causing the CPU 801 to execute an image forming program stored in the MEMORY 802.
In the foregoing respective embodiments, although the example has been mentioned in which the sheet as the object of the image forming processing is the copy paper or thick paper, no limitation is made to these, and for example, it is needless to say that an OHP film and the like may be used.
In the embodiment, although the description has been given to the case where the function to carry out the invention is previously recorded in the inside of the apparatus, no limitation is made to this, and the same function may be downloaded from a network, or the same function is stored on a recording medium and may be installed in the apparatus. As the recording medium, any form may be used as long as the recording medium, such as a CD-ROM, can store the program and can be read by the apparatus. Besides, the function obtained previously by installation or download at stated above may be realized by the cooperation with an OS (Operating System) or the like in the inside of the apparatus.
Although the invention has been described with reference to the specific modes, it would be apparent for one of ordinary skill in the art that various modifications and improvements can be made without departing from the sprit and scope of the invention.
As described above in detail, according to the invention, in the image forming apparatus for transferring the toner image onto the sheet by applying the transfer bias voltage to the sheet through the transfer member, there can be provided the technique to prevent the occurrence of poor transfer by suitably controlling the transfer bias voltage according to the processing condition.
