JP4845425B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine that forms an image using an electrophotographic method, and particularly detects the density of an image formed on a recording material after the image is heated. The present invention relates to an image forming apparatus that performs adjustment.

  As is well known, in an image forming apparatus using an electrophotographic method, 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 is like that. For this reason, the recording medium on which the visible image is transferred from the photosensitive member or the transfer member is conveyed to the fixing device and discharged after being fixed to the visible image.

  By the way, in recent years, in a situation where demands for higher image quality and higher stability are intensifying, in order to always maintain the density of an image formed by the image forming apparatus in an appropriate state, the reference image is formed after a reference image having a predetermined density is formed. Image density control, comparing the obtained density measurement value with the density target value, creating a conversion table, and converting the density characteristics of the image data using the created conversion table, thereby controlling the image density Many techniques have been proposed.

  As a method for measuring the density of the reference image necessary for such density control, there are a method that is performed before the toner image formed as the reference image is transferred to the recording medium and a method that is performed after the transfer and before the fixing. In the density measurement method, due to fluctuations in the amount of toner transferred to the recording medium, the degree of fixing, etc., the density of the actually obtained image and the measured image will differ, and high-precision density control is performed. I could not.

  For this reason, a method has been proposed in which an output image is read by its own image reading device and density control is performed based on the reading result. However, according to this method, the output image must be set in the image reading device. Therefore, the operation is troublesome, and the density adjustment cannot be performed in the case of an apparatus that does not include an image reading device such as a printer.

  Therefore, in a conventional image forming apparatus, as shown in FIG. 6, a density sensor 109 is provided in the recording medium conveyance path immediately after the fixing device 108, and the density is adjusted based on the density of the image after fixing (for example, Patent Document 1). In FIG. 6, 101 is a photosensitive drum, 102 is a laser exposure optical system, 104 is a cleaning device, 105 is a developing device, 111 is a primary charger, and 1112 is a transfer charger.

  Also, as shown in FIG. 7, there is a type in which density adjustment is performed by providing a density sensor 209 in the reverse path portion R3 after the fixing device 208 (see, for example, Patent Document 2). In FIG. 7, 201k, 201y, 201m, and 201c are photosensitive drums, and 203 is an intermediate transfer member.

JP 2000-1332013 A JP-A-10-268589

  However, in the conventional image forming apparatus, the toner does not include wax for improving the releasability from the fixing unit, but it is assumed that the toner of the conventional image forming apparatus includes wax. In this case, the following problems occur.

  That is, since the density sensor is arranged immediately after the fixing device, in the fixed image detected by the density sensor, the wax is in a liquid state by heating at the time of fixing. It will be different. For this reason, when the density is measured using a light reflection type density sensor, the amount of reflected light varies depending on the progress of solidification of the wax, and the measurement result also changes. As a result, even if the image density in this state is detected, there is a possibility that the measured value varies for each detection, and the density adjustment cannot be performed with high accuracy.

  Accordingly, a plurality of patches (images) having different densities are used as reference images for image density adjustment, and a conversion table is created by comparing the density measurement values obtained from the plurality of patches with the density target values, When image density control is performed by converting the density characteristics of image data using the created conversion table, the time for the wax temperature to fall to the solidification temperature (melting temperature) if the image density is high Therefore, the degree of progress of solidification of the wax differs depending on the patch, and the difference between the density measurement value and the density target value varies depending on the density of the patch, which causes a further deterioration in density adjustment accuracy.

  The present invention has been made in view of the above problems, and an object of the present invention is to perform density adjustment with high accuracy and stability in an image forming apparatus that performs density adjustment by detecting the density of an image on a recording material after image heating. And

In order to achieve the above object, an image forming apparatus according to the present invention includes an image forming unit that forms an image on a recording material with a toner containing wax, and an image on the recording material formed by the image forming unit. Heating means, a conveyance path for conveying the recording material carrying the toner image formed by the image forming means to the heating means, and a reversing unit for reversing the conveyance direction of the recording material heated by the heating means A linear conveyance path for linearly conveying the recording material conveyed via the reversing unit toward the image forming unit, and a density of the image on the recording material heated by the heating unit. Density detecting means for detecting, and adjusting means for adjusting the density of the image formed on the recording material based on the detection result detected by the density detecting means, the density detecting means comprising the heating hand The position where the maximum temperature of the recording material when heated by the density detecting means after being heated by the temperature is equal to or lower than the coagulation temperature of the wax is above the linear conveyance path in the vertical direction of the image forming apparatus. And the detection surface of the density detection means is arranged downward in the vertical direction .

  According to the present invention, in an image forming apparatus that detects an image density of a recording material after fixing and adjusts the image density, the image density is highly accurate and low-cost after the image density on the recording material after fixing is stabilized. Therefore, image density adjustment can be performed with high accuracy and stability without increasing the size of the main body.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a schematic cross-sectional view of an image forming apparatus according to the present invention. In FIG. 1, reference numeral 1 denotes a photosensitive drum which is an image carrier, and the photosensitive drum 1 is rotatably supported. A pre-exposure lamp, a corona charger, a laser exposure optical system 2, a potential sensor, an intermediate transfer member 3, a cleaning device 4 and a rotary developing device 5 are disposed as forming means.

  The rotatable rotary developing device 5 has four color developing devices: a black developing device 5K, a yellow developing device 5Y, a magenta developing device 5M, and a cyan developing device 5C. The rotary developing device 5 rotates around a cylindrical rotation shaft provided at the center thereof in the direction of arrow a (counterclockwise) in the drawing, and a developing device of a desired color is provided on the photosensitive drum 1 when necessary. It is possible to move it to a development position opposite to.

  Further, in the laser exposure optical system 2 and the image processing unit 37 as an adjusting unit for adjusting the image density, the image signal from the reader unit is subjected to image processing so as to satisfy a desired image forming condition (desired image density). The laser light converted into an optical signal by the laser output unit is reflected by the polygon mirror and projected onto the surface of the photosensitive drum 1 through the lens and each reflecting mirror.

  FIG. 5 shows a block diagram of the image processing unit 37.

The image processing unit 37 includes an A / D conversion unit 41, a shading correction unit 42, a LOG conversion unit 43, a masking UCR unit 44, an LUT (look-up table) unit 45, and a pulse width modulation unit 46. A pattern generator 47 is connected to the pulse width modulation unit 46, a CPU 48 having a ROM 50 and a RAM 51 is connected to the LUT unit 45, and an external input I / F 52 is connected to the LOG conversion unit 43. The image signal processed by the image processing unit 37 is output to the laser output unit.
The image forming apparatus according to the present embodiment uses a look-up table based on image density information on a recording material after heat-fixing in order to maintain good image quality of a full-color image, particularly color and gradation. Create (update the image formation conditions) and perform image formation based on it. The information of the look-up table updated last time is stored in the RAM 51 as storage means (storage means), and the CPU 48 causes the image forming means to execute image formation using the updated look-up table stored.

  In the present embodiment, the image forming apparatus can select a mode for performing density adjustment. When performing density adjustment, the user selects a density adjustment mode selection button (not shown). Then, a predetermined recommended paper is set in a predetermined paper feeding unit, and an image for density adjustment is formed on the recommended paper, and a lookup table is created based on the image density information on the recording material after heat fixing ( Update the image forming conditions).

  Thus, at the time of image formation, the photosensitive drum 1 is rotated, and the photosensitive drum 1 after being neutralized by the pre-exposure lamp is uniformly charged by the corona charger to irradiate the first color, for example, the yellow light image E, A latent image is formed on the photosensitive drum 1. Next, the latent image on the photosensitive drum 1 is developed by the yellow developing device 5Y, and an image of yellow toner using a resin and a pigment as a base is formed on the photosensitive drum 1.

  Thereafter, the yellow toner image formed on the photosensitive drum 1 is primarily transferred to the intermediate transfer member 3.

  When the development of the first color yellow is completed, the rotary developing device 5 is rotated by 90 ° in the direction of the arrow a in the figure, and the second color magenta developing device 5M is moved to the development position facing the photosensitive drum 1 to perform the primary transfer. The photosensitive drum 1 cleaned by the end cleaning device 4 repeats the latent image, development, and primary transfer for the second color magenta, the third color cyan, and the fourth color black similarly to the first color yellow, and the intermediate transfer member 3. The toner images of each color are sequentially stacked on top.

  Here, in order to keep the toner ratio (or toner amount) in the developing devices 5K, 5Y, 5M, and 5C constant, the toner is transferred from the toner storage unit to the developing devices 5K, 5Y, 5M, and 5C at a desired timing. Are replenished as needed.

  On the other hand, a recording medium as a recording material is conveyed one by one from any of the storage units 61, 62, 63, 64 by any of the paper feeding means 71, 72, 73, 74 and skewed by the registration roller 75. After being corrected, the toner image is conveyed to the secondary transfer unit 76 at a desired timing, and the secondary transfer unit 76 receives the transfer of the toner image on the intermediate transfer body 3.

  The recording medium on which the toner image is transferred by the secondary transfer unit 76 is conveyed to the heat roller fixing device 8 as a heating unit through the conveying unit 77, and the toner image is fixed (image) by the heat roller fixing device 8. After being heated, the paper is discharged to a paper discharge tray 65 or a paper post-processing device (not shown).

  When forming images on both sides of the recording medium, the conveyance path switching guide is driven immediately after the recording medium passes through the heat roller fixing device 8, and the recording medium is once guided to the reversing path 66 and then reversed. Due to the reverse rotation of the roller 78, the recording medium is fed to the front, with the trailing edge as the leading edge, and then withdrawn in the direction opposite to the fed direction, and sent to the duplex conveyance path 67. Thereafter, the recording medium that has passed the double-sided conveyance path 67 is corrected for skew by a double-sided conveyance roller 79 as a conveyance member, temporarily stopped for timing, and conveyed to the registration roller 75 at a desired timing. The image is transferred to the other surface again by the above-described image forming process.

  Next, the configuration of image density adjustment will be described with reference to FIGS.

  FIG. 2 is a partial cross-sectional view showing the density sensor arrangement in the recording medium conveyance path, FIG. 3 is a cross-sectional view of the density sensor and the recording medium, and FIG.

  When adjusting the image density, similarly to the above-described normal double-sided image formation, the latent image, development, and primary transfer steps are repeated, and the toner image is secondary transferred to the recording medium conveyed to the secondary transfer unit 76. To do. The secondary transferred recording medium is fixed with the toner image by the heat roller fixing device 8 and when the conveyance path switching guide is driven, the recording medium is reversed by the reversing roller 78 via the reversing path 66. It is sent to the duplex conveyance path 67.

  The image formed here is a standard image having a predetermined density. As shown in FIG. 3, the standard image on the conveyed recording medium is irradiated with reference light from the irradiation means 91 in the density sensor 9, and The reflected light from the reference light is received by the light receiving means 92 and a signal corresponding to the received light amount is output. The image processing unit 37 as an adjusting unit compares the density measurement value obtained from the output value of the density sensor 9 with the density target value, creates a conversion table, and uses the created conversion table to create the density of the image data. The density is adjusted by converting the characteristics.

  In the present embodiment, the density sensor that detects the image density on the recording material includes a near-infrared light LED as a light emitting element and a photodiode as a light receiving element, and is an image heated and fixed on the recording material. The reflected light from is detected and the image density is detected. Control is performed by changing image forming conditions based on the detection result of the density sensor 9.

  Next, the toner (and wax) will be described.

  The toner used in the present embodiment contains paraffin wax as a wax for improving the releasability. Here, the toner components and the production method will be briefly described. Of the toners used in this embodiment, cyan toner will be described as an example.

・ Hybrid resin with vinyl polymer unit and polyester resin unit
100 parts by mass-Paraffin wax (melting temperature (melting point): 72 ° C) 9 parts by mass-C.I. I. Pigment Blue 15: 3 5 parts by mass -Di-tert-butylsalicylic acid aluminum complex 6 parts by mass was sufficiently premixed with a Henschel mixer, melt-kneaded with a twin-screw extruder, cooled and then granulated with a hammer mill. Coarsely pulverized to a diameter of about 1-2 mm. Subsequently, it was finely pulverized with an air jet type fine pulverizer. Further, the obtained finely pulverized product was classified by a multi-division classifier to obtain cyan toner particles having a weight average particle size of 7.6 μm.

Hydrophobic silica fine powder surface-treated with 7 parts of hexamethyldisilazane on 100 parts by mass of silica fine powder having a primary particle size of about 20 nm (BET specific surface area = 85 m ²
/ G) was added to 1.1 parts by mass with respect to 100 parts by mass of the cyan toner particles to obtain a cyan toner. Further, cyan toner and magnetic carrier particles (average particle diameter 50 μm) surface-coated with a silicone resin were mixed so that the toner concentration was 6% by mass to obtain a two-component developer.

  In this embodiment, the toner uses a pulverization method. However, the present invention is not limited to this. Needless to say, the toner may be produced by a known method as long as the toner and the wax are mixed. .

  Incidentally, the density sensor 9 is provided in the conveyance path so as to detect the density of the recording material in the recording material conveyance path, and the temperature of the wax formed on the recording material and heated by the heat fixing roller device. Is disposed at a position lower than the solidification temperature of the wax (which is also the melting temperature (melting point) of the wax) (hereinafter, the solidification temperature of the wax is referred to as “wax melting point”). The melting point of the wax is 72 ° C.).

  In the present invention, the solidification temperature of the wax is a temperature at which the wax changes from a liquid state to a solid state (or vice versa). If the temperature is lower than this temperature, the wax is in a solidified state. Therefore, if density detection is performed at a temperature lower than this temperature, it is possible to prevent variations in detection results of the density sensor due to different surface properties due to differences in the toner phase state (liquid phase, solid phase, etc.). .

  Here, a method for measuring the melting point of the wax will be described.

  The melting point of the toner wax is measured by differential thermal analysis (DSC) shown below after separating and collecting the wax by dissolving the toner in THF and hot toluene.

<Differential thermal analysis measurement>
Measurement is performed according to ASTM D3418-82 using a differential scanning calorimeter (DSC measuring device), DSC-7 (manufactured by Perkin Elmer) or DSC2920 (manufactured by TA Instruments Japan). The measurement sample is precisely weighed in an amount of 2 to 10 mg, preferably 5 mg. This is put in an aluminum pan, and an empty aluminum pan is used as a reference, and measurement is performed at a temperature rising rate of 10 ° C./min within a measurement temperature range of 30 to 200 ° C.

  In the measurement, the temperature is raised and then the temperature is lowered once, and then the temperature is raised again, and the endothermic peak of the DSC curve in the temperature range of 30 to 200 ° C. is measured. At this time, the peak showing the maximum value among one or a plurality of endothermic peaks is defined as the maximum endothermic peak, which is defined as the melting point in the present invention.

  In the present embodiment, the wax is a wax having one or a plurality of endothermic peaks in the temperature range of 30 to 200 ° C., and the peak temperature of the maximum endothermic peak in the endothermic peak is 72 ° C. .

  Here, the change in the image temperature with time is shown in FIG. 4. As shown in the figure, the image temperature decreases hyperbolically with time, and at time T, the image temperature decreases to the melting point (72 ° C.) of the toner-containing wax. . Accordingly, the density sensor 9 is provided at a position where a time T or more elapses until the recording medium that has passed through the heat roller fixing device 8 follows the conveyance path and reaches the density sensor 9.

  The temperature of the wax on the recording material can be measured as follows.

  In other words, if there is a recommended paper for density adjustment as the recording material, the recommended paper is used (if there is no recommended paper, the recording material is a color laser copier paper A4 size (product code 5548A002) manufactured by Canon Inc.). I will decide).

  The temperature measuring means for measuring the temperature of the wax may be either a contact type or non-contact type temperature sensor, and can be measured by a commercially available infrared sensor or thermistor. The measurement environment is an environment with a temperature of 23 ° C. and a humidity of 50%. The image (toner patch) for density adjustment is used as a recording material from the state in which the image forming apparatus is turned on and allowed to form an image for 10 minutes. Let it form. Then, after the formed toner patch is heated by the heating unit, the temperature measuring unit measures the temperature of the recording material (including the image forming unit) when the toner patch is conveyed to the position (detection position) facing the density sensor 9. . At this time, the measurement area by the temperature measuring means is arranged so as to include an area (density detection area of the density sensor 9) where density adjustment toner is formed on the recording material.

  Here, it is only necessary that the highest measured value measured by the temperature measuring means is lower than the melting point (solidification temperature) of the wax. According to the experiment, the variation in the detection result of the concentration sensor could be prevented if the temperature of the wax was below the melting point. That is, even when the temperature of the wax was just the melting point, it was possible to prevent occurrence of variations in detection results.

  For this reason, when the density of the image is detected by the density sensor 9, most of the toner-containing wax in the image is already in a solidified state, thereby preventing variations in detection results due to variations in the solidification progress state of the wax (image). Can do. Therefore, the density of the reference image is stable, and highly accurate density adjustment based on this reference density can be performed. Further, since the density sensor 9 is provided in the double-sided conveyance path 93, it is not necessary to increase the size of the main body, and the main body does not increase in size.

  Thereafter, the recording medium on which the reference image is formed is discharged to the discharge tray 65 (see FIG. 1) through the double-sided conveyance roller 79, the registration roller 75, the secondary transfer unit 76, and the heat roller fixing device 8.

  As in the present embodiment, the density sensor 9 is more preferably disposed in the horizontal double-sided conveyance path. Reasons include the following points.

  That is, when the density sensor is arranged in the reverse path, the recording medium passes through the sensor facing portion twice, and the density is detected only when the recording medium having a stable density passes the sensor facing portion for the second time. However, it is only necessary to detect the density when the recording medium passes once in the double-sided conveyance path, and it is sufficient to form an image serving as a trigger on the recording medium. Can do.

  In addition, the reversing path cannot secure a sufficient straight path (straight conveyance path) for density measurement, and the tip of the recording medium hits the bending path (bending conveyance path) during density measurement. Since the recording medium fluctuates in the direction perpendicular to the density sensor due to the rush shock, density measurement cannot be performed with high accuracy. This is because, as is well known, a normal optical sensor is vulnerable to vertical fluctuation, and its output value changes greatly when the measurement object fluctuates up and down.

  Furthermore, when a density sensor is provided in the reversing path, in many cases, the sensor light-receiving unit faces substantially in the horizontal direction, and the sensor light-receiving unit may be affected by scattered toner or paper dust. As a countermeasure against this, it is possible to provide a cover, a shutter, etc., but this leads to an increase in cost.

  In addition, in an image forming apparatus that has been speeded up in recent years, there is a possibility that a sufficient cooling distance cannot be secured in the reversing path portion. No longer. This naturally increases the cost, and the recording medium fluctuates up and down due to the effect of air discharged by the cooling fan, and the density adjustment cannot be performed with high accuracy due to the up and down fluctuation of the recording medium. Because.

  Here, as shown in FIG. 2, the density sensor 9 is a double-sided horizontal conveyance path 93 (straight conveyance path) that can convey the recording medium in a substantially horizontal direction through a reversing path 66, a feeding portion near the reversing roller 78, and the like. Since the light receiving surface of the density sensor 9 is arranged vertically downward, the density sensor 9 is not easily affected by scattered toner or paper dust.

  If the density sensor 9 is arranged at a position away from the bent conveyance path 94 on the downstream side of the double-sided horizontal conveyance path 93 from the maximum size of the recording medium used for image adjustment, the leading end of the recording medium is measured during density measurement. Since it does not hit the bent conveyance path 94, the recording medium does not fluctuate up and down due to the rush shock, and stable density adjustment is performed while minimizing the influence of the vertical fluctuation of the object to be measured, which is a problem of the scanning sensor. be able to.

  If the density measuring sensor 9 cannot be disposed at a position away from the bent conveyance path 94 on the downstream side of the double-sided horizontal conveyance path 93 from the maximum size of the recording medium used for image adjustment, the density image adjustment is performed. If the density measurement is performed by the density sensor 9 only for the time until the leading edge of the recording medium is conveyed to the bent conveyance path 94, stable density adjustment can be performed while minimizing the influence of the vertical fluctuation of the recording medium. it can.

  Further, the roller interval between the upstream roller 95 provided immediately upstream of the density sensor 9 and the downstream roller 96 provided immediately downstream is made shorter than the minimum size of the recording medium used for image density adjustment, and the downstream roller 96. By setting the recording medium conveyance speed at 1 to the same or faster than the recording medium conveyance speed of the upstream roller 95, the vertical fluctuation of the recording medium can be further reduced, and more stable density adjustment can be achieved.

  The reference light of the density sensor 9 may be white light such as an LED, a halogen lamp, or a xenon lamp, or red, blue, green light or the like. The photoelectric conversion element in the light receiving means 92 may be a CCD, A photodiode, photomultiplier, CMOS sensor, or the like may be used.

  As described above, according to the present embodiment, in the image forming apparatus that detects the image density of the recording medium after image heating and adjusts the image density, the image density on the recording medium after fixing becomes stable. Since the image density is detected, the image density adjustment can be performed with high accuracy and stability.

  In this embodiment, development is performed using a rotary developing device. However, the present invention is not limited to this, and any form can be applied as long as the effects of the present invention can be obtained.

  The present invention is particularly useful for an image forming apparatus that adjusts image density by detecting image density on a recording material after fixing.

1 is a main cross-sectional view of an image forming apparatus according to the present invention. FIG. 4 is a cross-sectional view of a density sensor arrangement portion in a recording medium conveyance path of the image forming apparatus according to the present invention. 1 is a cross-sectional view of a density sensor and a recording medium of an image forming apparatus according to the present invention. FIG. 3 is a diagram showing a relationship between image temperature and time in the image forming apparatus according to the present invention. 2 is a block diagram illustrating a configuration of an image processing unit of the image forming apparatus according to the present invention. FIG. It is sectional drawing of the conventional image forming apparatus. It is sectional drawing of the conventional image forming apparatus.

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 device 8 Heat roller fixing device 9 Density sensor 37 Image processing part 41 A / D conversion part 42 Shading correction part 43 LOG conversion part 44 Masking UCR part 45 LUT unit 46 Pulse width modulation unit 47 Pattern generator 48 CPU
50 ROM
51 RAM
52 External input I / F
66 Reverse path 67 Double-sided conveyance path 75 Registration roller 76 Secondary transfer part 77 Conveying part 78 Reverse roller 79 Double-sided conveyance roller 91 Irradiation means 92 Light-receiving means 93 Double-sided horizontal conveyance path 94 Bending conveyance path 95 Upstream roller 96 Downstream roller

Claims (5)

An image forming means for forming an image on a recording material with toner containing wax, a heating means for heating an image on the recording material formed by the image forming means, and a toner image formed by the image forming means A conveying path for conveying the recording material to be carried to the heating unit, a reversing unit for reversing the conveying direction of the recording material heated by the heating unit, and the recording material conveyed via the reversing unit A linear conveyance path for linearly conveying the recording material toward the image forming means, a density detecting means for detecting the density of the image on the recording material heated by the heating means, and a detection detected by the density detecting means Adjusting means for adjusting the density of the image formed on the recording material based on the result, and the density detecting means is heated by the heating means and then conveyed to the density detecting means Kino maximum temperature of the recording material a position such that below the solidification temperature of the wax, is arranged below the conveying path in the vertical direction A above the linear transport path of the image forming apparatus, the density detection The image forming apparatus is characterized in that the detection surface of the means is disposed downward in the vertical direction .   The interval between the conveying members arranged immediately upstream and immediately downstream of the density detecting means is set to be shorter than the minimum size of the recording material for image density adjustment, and the recording material is held by the conveying member. The image forming apparatus according to claim 1, wherein the image density is detected.   The recording material conveyance speed of the conveyance member arranged immediately downstream of the density detection means is equal to or faster than the recording medium conveyance speed of the conveyance member arranged immediately upstream of the density detection means. The image forming apparatus according to claim 2.   The image forming apparatus according to claim 1, wherein the density detecting unit includes a reflective density sensor that detects reflected light obtained by irradiating light on an image on a recording material.   The image forming apparatus according to claim 1, wherein the solidification temperature is a maximum endothermic peak temperature of a differential thermal analysis curve of the wax.

Images