JP4898260B2 - Image forming apparatus - Google Patents

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a digital copying machine, printer, or facsimile that forms an image using an electrophotographic method, and more particularly to image formation capable of detecting the image density of a recording medium being conveyed and adjusting the image density. Relates to the device.

  As is well known, in an image forming apparatus using an electrophotographic system, a visible image carried on a photosensitive member or a transfer member is transferred to a recording medium such as plain paper to obtain a recorded image. It has become. For this reason, the recording medium on which the visible image is transferred from the photosensitive member or the transfer member is transported to the fixing device and discharged after being fixed to the visible image.

  By the way, in recent years, as the demand for higher image quality and higher stability is increasing, for example, a technique for controlling the density of an image as shown below is used in order to always maintain the density of an image formed by an image forming apparatus in an appropriate state. Many have been proposed. That is, after forming a reference image having a predetermined density, the density of the reference image is measured, the obtained density measurement value is compared with the density target value, and a conversion table is created, and the created conversion table Is used to convert density characteristics of image data.

  As a measurement of the density of the reference image necessary for performing such density control, before transferring the toner image formed as the reference image to the recording medium, that is, on the drum or the intermediate transfer member, Many measure the density of the reference image. However, in such a density measurement method, due to fluctuations in the transfer amount of toner to the recording medium, the degree of fixing, and the like, a difference occurs between the density of the actually obtained image and the measured image, and high-precision density control is performed. I couldn't.

In view of this, conventionally, an output image is read by its own document reading device, and density control is performed based on the reading result, or a density sensor is provided in a recording medium conveyance path after transfer or fixing, Some of them perform density adjustment by an image on a recording medium. (For example, refer to Patent Document 1.)
Incidentally, it is well known that an optical sensor is used as a density sensor provided after the fixing.
JP 2000-1332013 A JP 2001-209217 A

  However, when the recording medium fluctuates in the vertical direction as a characteristic of the optical sensor, it is impossible to measure the density with high accuracy. This is because a normal optical sensor is vulnerable to vertical fluctuations, and when the measurement object fluctuates up and down, its output value changes greatly. If there is such an output fluctuation, it is difficult to detect a minute difference between the density patches on the paper to be detected.

  As a countermeasure, it is conceivable to widen the allowable range of vertical fluctuation of the object to be measured by using a lens for the optical sensor. This can provide a greater effect by increasing the lens diameter or by stacking a plurality of lenses. In addition, in order to correct the variation in the center position of the allowable variation range and match the center position of the allowable variation range, it is possible to take measures by providing a mechanism for adjusting the distance between each transmitting / receiving unit and the recording medium in the optical sensor itself.

  However, since these measures lead to an increase in the size of the sensor and an increase in parts cost, the number of parts, the number of assembly steps, etc., the sensor itself becomes a big problem in terms of size and cost. This also becomes a major obstacle in reducing the size and cost of the entire image forming apparatus.

  As another countermeasure, a proposal has been made to provide a position regulating member and a backup roller having a light transmission member for regulating the distance between the density sensor and the recording medium (see, for example, Patent Document 2).

  However, since this measure regulates the position of the light transmissive member in physical contact with the surface of the recording medium, the surface of the light transmissive member is damaged by minute foreign matter on the recording medium. Alternatively, when the toner is not satisfactorily fixed on the recording medium due to a defect in the image forming apparatus, the surface of the light transmission member is stained with unfixed toner, and the measurement accuracy is deteriorated.

  By the way, since various toners handled in the image forming apparatus have an extremely minute shape, it is very difficult to completely eliminate contamination due to scattering in the product. In order to maintain the measurement accuracy over a long period of time, it is necessary to make the structure resistant to dirt. However, in this measure, no special measures are taken against dirt. For example, when the backup roller is durable and dirty, the density on the back of the paper is low when the density patch on the recording medium is thin or the thickness of the recording medium itself is thin. It will change and will reduce the measurement accuracy.

  The present invention reduces the amount of vertical fluctuation of the recording medium without regulating the surface of the recording medium positioned at the position read by the image density detecting means with a regulating means or the like, and performs density adjustment with high accuracy and measurement with durability. The purpose is to improve accuracy stability.

In order to achieve the above object, the present invention provides:
An image forming unit for forming an image on a recording medium;
By the image forming unit, and the density concentration readable image density detecting means of the patch of the recording medium being conveyed which density patches are formed,
With
Using the output value of the image density detecting unit, an image forming apparatus which performs density adjustment of the image formed on the recording medium by the image forming section,
A guide having an opening for guiding the recording medium and reading the density of the density patch of the recording medium being conveyed on which the density patch is formed by the image density detection unit;
A backup roller for pressing the recording medium against the guide from the side opposite to the image density detecting means;
With
The backup roller is pressed against the guide opening so as to sandwich the recording medium being conveyed with the guide,
The maximum width of the dimension of the opening of the guide in the recording medium conveyance direction is A,
The maximum width of the dimension in the direction perpendicular to the recording medium conveyance direction of the opening of the guide is B,
The width of the backup roller is C,
The diameter of the backup roller is D,
If the relative relationship in A, B, C, D is
0.3 ≦ A / D ≦ 0.8
B / C ≦ 0.8
And
The backup roller has a hardness of 10 ° or more and 30 ° or less in the ASKER C measurement.

  According to the present invention, the surface of the recording medium positioned at the position read by the image density detecting means is not regulated by the regulating means or the like, and the vertical fluctuation amount of the recording medium is reduced, the density is adjusted with high accuracy, and the durability is high. It is possible to improve the stability of measurement accuracy.

  Hereinafter, exemplary embodiments of an image forming apparatus according to the present invention will be described in detail with reference to the drawings.

  In the following embodiments, an intermediate transfer member is used as an image forming apparatus, and toner images of each color are sequentially transferred onto the intermediate transfer member, and the toner image carried on the intermediate transfer member is used as a recording medium. Although an image forming apparatus that performs batch transfer is illustrated, the present invention is not limited to this. For example, an image forming apparatus that uses a recording medium carrier and sequentially superimposes and transfers the toner images of the respective colors onto the recording medium carried on the recording medium carrier. In addition, the printer is exemplified as one aspect of the image forming apparatus, but the present invention is not limited to this. For example, the image forming apparatus is another image forming apparatus such as a copying machine or a facsimile machine, or a multi-function machine combining these functions. Other image forming apparatuses may be used.

  In addition, the shapes of the component parts described in the following embodiments, the relative arrangement thereof, and the like should be appropriately changed according to the configuration of the apparatus to which the present invention is applied and various conditions, and the scope of the present invention. Is not intended to be limited only to those exemplified below.

[Schematic configuration of image forming apparatus]
FIG. 1 is a schematic sectional view showing an image forming apparatus 100 using an electrophotographic system according to an embodiment of the present invention.

  The image forming apparatus 100 includes an image forming unit having an electrophotographic photosensitive drum (hereinafter referred to as a “photosensitive drum”) 1 as an image carrier that rotates at a constant speed, and conveys a recording medium to the image forming unit. The image forming unit includes a conveying unit that conveys a recording medium on which an image is formed.

  As shown in FIG. 1, the image forming unit includes a pre-exposure lamp 6, a charger 7, a laser exposure optical system 2, a rotary developer 5, an intermediate transfer member 3, and a cleaning device disposed around the photosensitive drum 1. It is comprised by the container 4 grade | etc.,. Here, the pre-exposure lamp 6 is for discharging the photosensitive drum 1. The charger 7 charges the surface of the photosensitive drum 1 uniformly. The laser exposure optical system 2 forms a latent image on the photosensitive drum 1. The rotary developer 5 is a toner image that is developed by attaching toner to the latent image on the photosensitive drum 1. The intermediate transfer member 3 is for transferring and carrying the toner images formed on the photosensitive drum 1 sequentially. Further, the cleaning device 4 removes transfer residual toner remaining on the surface of the photosensitive drum 1 after transfer.

  The rotatable rotary developer 5 has six colors: black developer 5K, yellow developer 5Y, magenta developer 5M, cyan developer 5C, light magenta developer 5LM, and light cyan developer 5LC. Development unit. The rotary developer 5 rotates around a cylindrical rotation shaft provided at the center of the rotary developer 5 in the direction of arrow a in the figure, and a developer of a desired color is connected to the photosensitive drum 1 when necessary. It is possible to move to the opposite development position.

When forming an image, the photosensitive drum 1 is rotated, and the photosensitive drum 1 after being neutralized by the pre-exposure lamp 6 is uniformly charged by the charger 7, and the optical image E of the first color is obtained by the laser exposure optical system 2. To form a latent image on the photosensitive drum 1. Next, the latent image on the photosensitive drum 1 is developed by a first color developing device, and a toner image using a resin and a pigment as a base is formed on the photosensitive drum 1. Thereafter, the toner image formed on the photosensitive drum 1 is primarily transferred to the intermediate transfer member 3.

  When the development of the first color is completed, the rotary developer 5 is rotated by 60 ° in the direction of arrow a in the figure, and the developer of the second color is moved to a development position facing the photosensitive drum 1. The photosensitive drum 1 that has been subjected to the primary transfer of the first color and has been cleaned by the cleaning device 4 repeats the latent image, development, and primary transfer for the second, third, fourth, fifth, and sixth colors as in the first color. Then, the toner images of the respective colors are sequentially superimposed on the intermediate transfer member 3.

  On the other hand, the transport means for transporting the recording medium to the image forming unit and transporting the recording medium on which the image is formed by the image forming unit first stores the recording media stored in the storage units 61, 62, 63, 64. Each feed roller 71, 72, 73, 74 selectively feeds one by one. Then, the skew is corrected by the registration roller 75 and conveyed to the secondary transfer unit 76 at a desired timing.

  On the recording medium conveyed to the secondary transfer unit 76, the toner images superimposed and transferred onto the intermediate transfer body 3 are transferred collectively (secondary transfer). The recording medium on which the toner image is transferred by the secondary transfer unit 76 passes through the conveyance unit 77, the toner image is fixed by the fixing device 8, and is discharged onto the discharge tray 65 by the discharge roller 79.

  When images are formed on both sides of the recording medium, the recording medium passes through the fixing device 8 and is once guided to the reverse path 66 by the first switching guide 80. The recording medium guided to the reversing path 66 is conveyed in a direction opposite to the direction fed from the rear end of the recording medium when fed by the reverse rotation of the reversing roller 78 capable of forward / reverse rotation, and the second switching is performed. It is sent to the duplex conveyance path 67 through the guide 81. Thereafter, the paper passes through the double-sided conveyance path 67 and is conveyed to the registration roller 75, and is again sent to the image forming unit described above to transfer the image to the other surface.

  In FIG. 1, reference numeral 90 denotes a control unit that controls the operation of each of the above parts constituting the image forming apparatus 100.

  Next, image density adjustment in the image forming apparatus will be described in detail.

  The image forming apparatus 100 includes a density sensor 9 (9a, 9b) as an image density detecting unit that detects the density of an image formed on a recording medium.

  FIG. 2 is a schematic cross-sectional view of the vicinity of the density sensor 9 applied to the image forming apparatus 100 as image density detection means and a recording medium.

  The recording medium is guided and conveyed by a double-sided conveyance upper guide 95 and a double-sided conveyance lower guide 96.

  The density sensor 9 includes a white LED (irradiation unit) 91 and a charge storage type sensor (light receiving unit) 92 with an RGB on-chip filter. The white LED 91 is incident on the recording medium 93 on which the patch 97 after fixing is formed at an angle of 45 degrees, and the intensity of irregular reflection light in the 0 degree direction is detected by the charge storage sensor 92 with an RGB on-chip filter. As shown in FIG. 3, the light receiving portion of the charge storage sensor 92 with the RGB on-chip filter is an independent pixel for RGB.

  The charge accumulation amount is adjusted to obtain a desired dynamic range by arbitrarily changing the LED output or the integration time.

  Then, the toner density on the recording medium is measured with high accuracy from the amount of reflected light by making the above adjustment using a reference index in advance.

  The charge storage sensor of the charge storage sensor 92 with the RGB on-chip filter may be a photodiode. Several sets of three RGB pixels may be arranged. Further, a configuration in which the incident angle is 0 degree and the reflection angle is 45 degrees may be employed. Furthermore, you may comprise by LED which emits RGB three colors, and a sensor without a filter.

  CMYK can be identified and the density can be detected from three different outputs obtained by this, for example, RGB output. It is also possible to detect the chromaticity by subjecting the RGB output to a mathematical process such as linear conversion or by converting the RGB output using a lookup table (LUT) (see, for example, Japanese Patent Application Laid-Open No. 2003-060922). .

  FIG. 4 is a schematic diagram showing the relationship between the double-sided transport upper guide 95 and the backup roller 94 as viewed from the density sensors 9a and 9b. In the present embodiment, two density sensors are arranged side by side in a direction crossing the recording medium conveyance direction, and a double-sided conveyance upper guide opening 98 is formed at a corresponding position. The arrangement of the density sensors 9a and 9b makes it possible to read gradation density patches for two colors at the same time, thereby realizing a reduction in density control time. In addition, a configuration in which density is measured in a shorter time by arranging a plurality of density sensors in the same column, or a configuration in which density measurement is performed at a low cost by arranging a single density sensor 9, etc. Of course, the image forming apparatus may be optimized.

  The image forming unit forms a test pattern image as shown in FIG. 5 on the recording medium when adjusting the image density. That is, at the time of image density adjustment, the latent image, development, and primary transfer are repeated similarly to the above-described normal double-sided image formation, and the test pattern image (toner image) is transferred to the recording medium conveyed to the secondary transfer unit. To do. The transferred test pattern image is fixed on the recording medium by the fixing device 8. The recording medium on which the test pattern image is formed in this manner is guided to the reversing path 66 by the first switching guide 80, reversed by the reversing roller 78, and the density sensor 9 is arranged as shown in FIG. It is sent to the duplex conveyance path 67.

  When the recording medium sent to the duplex conveyance path 67 passes the density sensor 9, the test pattern image density is detected.

  The density sensor 9 irradiates the patch on the recording medium conveyed at a predetermined timing with reference light from the irradiating means 91 in the density sensor 9 and receives the reflected light from the recording medium with the light receiving means 92. A signal corresponding to the amount of received light is output. Then, the control unit 90 compares the density measurement value obtained from the output value of the density sensor 9 with a preset density target value to create a conversion table, and uses the created conversion table to create an image. The density characteristics of the data are converted and the density of the image is adjusted.

  As shown in FIG. 5, the test pattern images are arranged in two rows in the recording medium conveyance direction. Each row is composed of one color. Each column is divided into two, each consisting of 22 patches, 11 patches starting with the trigger patch. The trigger patch indicates a reference position for density sensor reading. These 22 patches are arranged so as to gradually become lighter starting from the dark side so that the color changes uniformly.

  The size of one patch is 12 mm × 32 mm, and all the patches including the trigger patch are arranged so as to be within the range of 105 mm × 310 mm. As a result, image density adjustment can be performed on A3 and LGL size recording media.

  It is also possible to adjust the image density with a smaller A4 size or LTR size recording medium by arranging more density sensors or reducing the patch size by lowering the conveyance speed of the recording medium.

  For one dither pattern, the reading of a total of six colors is completed for the first M, C, the second LM, LC, the third Bk, Y, and the third. Since the present embodiment has four image processing dither patterns, all reading is completed with a total of 12 sheets.

  In order to shorten the adjustment time, it is also possible to perform density adjustment for only one dither pattern as a simple mode.

  In order to measure the density that changes minutely over 22 patches with high accuracy, it is necessary to reduce the variation (fluttering) in the distance between the density sensor 9 and the recording medium over almost the entire area in the conveyance direction of the recording medium. .

  Factors that increase the fluttering include a rush shock to the rollers on the conveyance path of the recording medium and a decrease in stiffness of the recording medium due to a difference in the leaving condition.

  In the present embodiment, as shown in FIG. 2, as a means for making it difficult to affect the patch reading of the density sensor 9 even if there is a rush shock or a decrease in the stiffness of the recording medium, a counter guide of the density sensor 9 is used. A backup roller is provided at the opening and directly below the guide opening.

  When reading the patch density, the backup roller is pressed against the guide opening so as to sandwich the recording medium. The guide opening is drawn to prevent the image on the recording medium from being damaged and the leading edge of the recording medium from being caught and causing a conveyance failure.

  As shown in FIG. 6, the opening dimensions A and B described in the present embodiment are defined at the place where the upper surface portion of the double-sided transport upper guide sheet metal and the inside of the opening portion of the drawing bent portion intersect.

Example 1
As a result of various studies, the present inventor has found that there is an optimum condition for the relative shape of the guide opening and the backup roller that is optimum for suppressing the fluttering of the recording medium.

  In this embodiment, the upper limit of the density measurement accuracy is set to the color difference ΔE ≦ 3. The reason for this is that if ΔE <3.2, it is generally a region where it is difficult for a person to distinguish the difference in color.

ΔE is the distance between the two chromaticity coordinates (L1, a1, b1) and (L2, a2, b2) ((L1−L2) ² + (a1−a2) ² + (b1−b2) ² ) ^1/2
Defined by

  By the way, in the toner of this embodiment, it is known that the relationship between the ratio | ΔD | / D of the true image density D and the density difference ΔD obtained from the measurement and the color difference ΔE is as shown in FIG. . (The concentration is measured with X-rite 520 (manufactured by X-rite), and the chromaticity is measured with Spectrolino (manufactured by Gretag Macbeth))

  Therefore, ΔE is used as an index by converting the deviation ΔD between the density D detected by the density sensor 9 and the true density into ΔE.

FIG. 7 shows the relationship between the ratio of the maximum width A of the guide opening in the recording medium conveyance direction dimension to the backup roller diameter D and the density measurement accuracy.

  The range bar in the figure shows the measurement error range when the same density patch on the recording medium is repeatedly measured.

  From this result, the range of 0.3 ≦ A / D ≦ 0.8 is the optimum condition.

  The minimum value in the above range is that the nip between the backup roller and the double-sided conveyance upper guide is too large, and the density patch on the recording medium is pressed against the double-sided conveyance upper guide, causing defects such as scratches on the density patch. It is determined. The maximum value is determined from the fact that the recording medium flutters due to the protrusion of the backup roller from the opening.

  In this embodiment, the relationship between A and D is set to A / D = 0.6, which is the optimum condition.

  Further, the hardness of the backup roller used in the study is 15 ° in the ASKER C measurement.

  FIG. 8 shows the relationship between the ratio of the maximum value B of the direction dimension intersecting the recording medium conveyance direction of the guide opening and the backup roller length C and the density measurement accuracy. From this result, the range of B / C ≦ 0.8 is the optimum condition. The maximum value of the use range is determined from a decrease in measurement accuracy due to a decrease in the distance between the density sensor 9 and the recording medium due to the protrusion of the backup roller from the opening.

  In the present embodiment, the relationship between B and C is set to B / C = 0.55 which is an optimum condition.

  The hardness of the backup roller used in this experiment is also 15 ° in the ASKER C measurement.

  When more detection accuracy is required, the numerical range may be narrowed, as is clear from the examination results of FIGS.

FIG. 9 shows the relationship between the backup roller hardness and the density measurement accuracy in ASKER C measurement. The range bar in the figure shows the measurement error range when the basis weight of the recording medium is prepared in the range of 60 to 250 g / m ^{2 and} the density is measured while changing the leaving condition of each recording medium.

From this result, can maintain the concentration measurement accuracy with respect to the recording medium basis weight in the range of 60~250g / ^{m 2} is 30 ° or less 10 ° or more in ASKER C measurement.

  The lower limit value of the hardness is determined from the fact that, when thick paper is used, the roller loses the stiffness of the recording medium and the flutter cannot be suppressed. Further, the upper limit value is determined because the behavior of the recording medium at the guide opening portion becomes unstable, especially when the nip width becomes too narrow, especially in thin paper left in a high humidity environment.

In this embodiment, the color of the backup roller is L * a * b * color system,
L * = 23.5, a * = 0.55, b * = 1.85
Are using things. However, in this case, the color of the backup roller does not affect the flickering of the recording medium, and therefore any color may be used.

(Example 2)
In the configuration of Example 1, when density detection was performed after forming hundreds of thousands of images, it was found by the inventors that the detection accuracy changes depending on the color of the backup roller.

Specifically, in the L * a * b * color system (CIE 1976),
L * = 76, a * = 11, b * = − 9
With the so-called pink roller, the density measurement accuracy after endurance of 100,000 sheets has dropped to ΔE = 4.5.

  FIG. 10 summarizes the results of the study of the effect of the present inventors on the density measurement accuracy of dirt due to the backup roller color and durability. From this result, it can be seen that when the backup roller is close to white, the measurement accuracy is low due to durability, whereas when gray or black, the rate of change is small.

  This is due to durability and contamination of the backup roller with toner or the like, and when the density patch on the recording medium is low, the density on the back of the paper changes.

From this result, the backup roller color is L * a * b * color system (CIE1976).
L * ≦ 40, −5 ≦ a * ≦ 5, −8 ≦ b * ≦ 8,
It is preferable to satisfy so-called gray or black.

In the present embodiment, the color of the backup roller is L * a * b * color system,
L * = 23.5, a * = 0.55, b * = 1.85
Are using things.

  As described above, according to the present embodiment, the amount of vertical fluctuation of the recording medium is reduced and the density is adjusted with high accuracy without restricting the surface of the recording medium located in the density sensor reading unit with a regulating means or the like. It is possible to provide an image forming apparatus that improves the stability of measurement accuracy in durability.

1 is a main cross-sectional view of an image forming apparatus in which a density sensor that is an example of an embodiment of the present invention is disposed in a double-sided conveyance path. FIG. 2 is a cross-sectional view of the vicinity of a density sensor and a recording medium in the image forming apparatus of FIG. 1. Pixel model of charge storage type sensor light receiving part. Schematic diagram of double-sided transport upper guide and backup roller as seen from the density sensor side. Test pattern image used for density measurement. Definition of guide opening dimensions on both sides transport. The graph which shows the relationship between the ratio of a recording opening direction of a recording medium of a guide opening part and a backup roller diameter, and density | concentration measurement precision. The graph which shows the relationship between the direction dimension which cross | intersects the recording medium conveyance direction of a guide opening part, the ratio of backup roller length, and density | concentration measurement accuracy. The graph which shows the relationship between backup roller hardness and density measurement accuracy. The graph which shows the influence which it has on the density | concentration measurement accuracy of dirt by the backup roller color and durability. 10 is a graph showing a relationship between a deviation ΔD between a true density D and an actual measurement value | ΔD | / D and ΔE.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Laser exposure optical system 3 Intermediate transfer body 4 Cleaning device 5 Rotating type developing body 6 Pre-exposure lamp 7 Charger 8 Fixing device 9a, 9b Density sensor 61, 62, 63 Storage part 65 Discharge tray 66 Reverse path 67 Double-sided conveyance path 71, 72, 73, 74 Feed roller 75 Registration roller 76 Secondary transfer part 78 Reverse roller 79 Discharge roller 80 First switching guide 90 Control means 91 White LED
92 Charge Accumulation Sensor with RGB On-Chip Filter 93 Recording Medium 94 Backup Roller 95 Double-sided Transport Upper Guide 96 Double-sided Transport Lower Guide 97 Density Patch 98 Double-sided Transport Upper Guide Opening

Claims (2)

An image forming unit for forming an image on a recording medium;
By the image forming unit, and the density concentration readable image density detecting means of the patch of the recording medium being conveyed which density patches are formed,
With
Using the output value of the image density detecting unit, an image forming apparatus which performs density adjustment of the image formed on the recording medium by the image forming section,
A guide having an opening for guiding the recording medium and reading the density of the density patch of the recording medium being conveyed on which the density patch is formed by the image density detection unit;
A backup roller for pressing the recording medium against the guide from the side opposite to the image density detecting means;
With
The backup roller is pressed against the guide opening so as to sandwich the recording medium being conveyed with the guide,
The maximum width of the dimension of the opening of the guide in the recording medium conveyance direction is A,
The maximum width of the dimension in the direction perpendicular to the recording medium conveyance direction of the opening of the guide is B,
The width of the backup roller is C,
The diameter of the backup roller is D,
If the relative relationship in A, B, C, D is
0.3 ≦ A / D ≦ 0.8
B / C ≦ 0.8
And
The image forming apparatus, wherein the backup roller has a hardness of 10 ° to 30 ° in the ASKER C measurement. The color of the backup roller is L * a * b * color system,
L * ≦ 40, −5 ≦ a * ≦ 5, −8 ≦ b * ≦ 8,
The image forming apparatus according to claim 1 , wherein the image forming apparatus satisfies gray color or black color.

Images