Abstract:
According to an embodiment of the invention, an image forming apparatus comprises a fixing unit including: a pair of rollers rotating with a recording sheet holding a toner image interposed therebetween; a driving motor rotating at least one of the pair of rollers; a heater being built in at least one of the pair of rollers to heat the roller; a timer measuring a period of time after starting operation; and a rotation speed control unit controlling the driving motor so that one of the rollers rotates in a speed higher than a rotation speed under normal conditions until a predetermined time measured by the timer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The application is based upon and claims the benefit of priority from U.S. provisional application No. 61/012,764, filed on Dec. 10, 2007, the content of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to an image forming apparatus, and more particularly to an image forming apparatus having a fixing unit which controls a rotation speed of a driving roller. 
       BACKGROUND 
       [0003]    An image forming apparatus using various schemes, such as an electro-photographic scheme, an ink jet scheme, thermal transfer scheme or the like, is widely used, which uses plain papers that are not specially processed in sheet material. 
         [0004]    The image forming apparatus, first of all, generates an electrostatic latent image, develops the electrostatic latent image by the use of toners, transfers the developed image to a recording sheet such as the plain paper directly or to another transfer medium firstly and then to the recording sheet. In order to permanently fix the image, it is necessary to perform a fixing of the transferred toner image to the recording sheet. 
         [0005]    In order to perform the fixing, the recording sheet holding the transferred toner image may be inserted between a pressurizing roller and a fixing belt or a fixing roller in many cases. The fixing belt or the fixing roller is heated at a predetermined temperature, and comes in contact with a toner holding area (surface) of the recording sheet. On the other hand, the pressurizing roller comes in contact with a rear surface of the recording sheet and applies pressure to the recording sheet so as to perform the fixing. 
         [0006]    The fixing of the toner image on the recording sheet is related with a temperature of the recording sheet and a processing speed. When the toner image is fixed at a predetermined speed in a predetermined temperature, a predetermined fixing is optimally made. The fixing speed depends on a rotation speed of a motor which drives the fixing roller wound with the fixing belt or the pressurizing roller. 
         [0007]    For keeping the temperature of the recording sheet constant, the temperature of a heater in the roller (heating roller) wound with the fixing belt is controlled in order to keep the temperature of the fixing belt constant by detecting the surface temperature of the fixing belt. Further, temperature of a built-in heater is controlled in order to keep the surface temperature of the pressurizing roller constant by detecting the surface temperature thereof. 
         [0008]    However, when the image forming apparatus is turned on, the temperature of the heating roller and the pressurizing roller is still at a low temperature similarly to the outside, and it takes much time for the built-in heater to heat these rollers. Moreover, since the pressurizing roller and the fixing roller which are driven at a constant rotation speed are expanded as the temperature thereof rises, diameters of the rollers become large, so that a peripheral surface speed of the driving roller increases. For this reason, the time for the toner-held recording sheet to pass through the fixing unit becomes short and the fixing speed increases, so that fixing conditions are changed. Further, a speed difference between the transfer roller before the fixing unit and a paper discharge roller after the fixing unit occurs to cause sheet bending, so that the sheet is rubbed against a sheet guide. Therefore, image defects occur due to excessively pulling the sheet. 
         [0009]    In order to solve the above problems, for example, Japanese Patent Application Laid-Open No. 2008-3297 discloses a fixing device for detecting the number of rotations of a fixing roller and a temperature of a fixing belt wound around the roller, obtaining a diameter of the fixing roller corresponding to the detected temperature, and controlling the rotation of the fixing roller according to the diameter. 
         [0010]    However, in the fixing device, since the temperature of the fixing belt is also detected and it takes much time until a rise in temperature is detected, it is hard to change the speed of the fixing belt at a proper speed to perform the fixing at a predetermined speed if the temperature is rapidly changed similarly to the case of turning on the power. 
       SUMMARY 
       [0011]    The invention is made in consideration of the above problems in the fixing unit of the image forming apparatus, and is to provide an image forming apparatus which is able to perform a fixing at a proper speed even though the temperature of the fixing unit rapidly rises similarly to the case of turning on the power. 
         [0012]    If the rotation speed of the driving roller in the fixing unit is constant, the diameter of the driving roller is changed when the temperature is changed, so that the peripheral surface speed of the driving roller is varied. The fixing speed corresponds to the peripheral surface speed of the driving roller. If the diameter of the driving roller is small though the rotation speed thereof is constant, the peripheral surface speed of the driving roller decreases, so that the fixing speed also decreases. 
         [0013]    Although the driving roller is heated by a built-in heater, it takes much time for the surface temperature of the driving roller to rise up to a predetermined temperature after starting operation and the diameter of the driving roller cannot be large accordingly. In the invention, the peripheral surface speed of the driving roller, that is, the fixing speed is raised by raising the rotation speed of the driving roller during a predetermined period of time after starting operation (start of heating). Therefore, the driving roller comes to be in a constant temperature and the diameter thereof becomes large, so that the fixing speed can be increased similarly to the case when the peripheral surface speed of the driving roller increases. 
         [0014]    The change in the rotation speed of the driving roller may be performed in 2 steps or 3 steps or more. When the change in the rotation speed of the driving roller is performed in 2 steps, first, the driving roller is rotated at a first rotation speed V 1  until a first time T 1  after starting operation. The first rotation speed V 1  is higher than a rotation speed V 0  of the driving roller which is in a steady temperature. Then, after the first time T 1 , the driving roller is rotated at a rotation speed until a second time T 2  (T 2 &gt;T 1 ) after starting operation, which is lower than the first rotation speed but higher than the rotation speed V 0  in the steady temperature. 
         [0015]    As described above, the rotation speed of the driving roller is made to be increased right after starting operation and to be decreased until the temperature is in the steady state. Therefore, it is allowed to correct that the diameter of the driving roller increases by thermal expansion of the roller and the fixing speed increases. 
         [0016]    According to an aspect of the invention, there is provided an image forming apparatus comprising a fixing unit including: a pair of rollers being rotated with a recording sheet holding a toner image interposed therebetween; a driving motor rotating at least one of the pair of rollers; a heater being built in at least one of the pair of rollers to heat the roller; a timer measuring a period of time after starting operation; and a rotation speed control unit controlling the driving motor so that one of the rollers rotates at a speed higher than a rotation speed under normal conditions until a predetermined time measured by the timer. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a diagram schematically illustrating a configuration of an image forming apparatus according to an embodiment of the invention. 
           [0018]      FIG. 2  is a diagram schematically illustrating a structure of a fixing unit of the image forming apparatus shown in  FIG. 1 . 
           [0019]      FIG. 3  is a diagram illustrating a configuration including an electric circuit of the fixing unit of the image forming apparatus shown in  FIG. 1 . 
           [0020]      FIG. 4  is a diagram illustrating result data obtained by experimenting on a peripheral surface speed of a transfer belt after starting operation in the image forming apparatus according to the embodiment of the invention. 
           [0021]      FIG. 5  is a graph diagram illustrating the data shown in  FIG. 4 . 
           [0022]      FIG. 6  is an explanatory diagram illustrating an operation according to another embodiment of the invention in which a rotation speed of a driving roller is changed in 2 steps. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Hereinafter, an image forming apparatus according to an embodiment of the invention will be described with reference to the drawings. 
         [0024]      FIG. 1  shows a diagram schematically illustrating a configuration of an image forming apparatus (MFP, Multi-Functional Peripheral) to which the invention is applicable. 
         [0025]    The image forming apparatus  10  shown in  FIG. 1  includes an image forming apparatus body  11 , a sheet feeding unit  12 , and an image acquisition unit  13 . The image forming apparatus body  11  outputs image information, for example, as an output image called as hard-copy or printout. The sheet feeding unit  12  can supply a recording sheet of an arbitrary size used in the image output with respect to the image forming apparatus body  11 . The image acquisition unit  13  acquires the image information, which is subjected to image formation in the image forming apparatus body  11 , as image data from an object (document) holding the image information. 
         [0026]    The image acquisition unit  13  includes a document table (document glass)  13   a  supporting the object and an image sensor, for example, a CCD sensor, converting the image information into the image data. An illumination device (not shown) radiates illumination light to the document set on the document table  13   a , and the image acquisition unit  13  converts the light reflected from the document into an image signal by the CCD sensor. 
         [0027]    An instruction input unit, that is, a controller panel (operation unit) is provided at a predetermined position (not shown in  FIG. 1 ) in the image forming apparatus  10 , which instructs the image forming apparatus body  11  on a start of image formation or the image acquisition unit  13  on a start of reading the image information of the document. 
         [0028]    The image forming apparatus body  11  includes: a first to a fourth photoconductive drums  14   a  to  14   d  holding a latent image; developing units  15   a  to  15   d  developing the latent image by supplying developers, that is, toners to the latent image held on the photoconductive drums  14   a  to  14   d ; a transfer belt  16  sequentially holding toner-images held on the photoconductive drums  14   a  to  14   d ; cleaners  17   a  to  17   d  removing toners remaining on the photoconductive drums  14   a  to  14   d  from the photoconductive drums  14   a  to  14   d , respectively; a transfer unit  18  transferring the toner image held on the transfer belt  16  to a sheet-shaped recording medium, such as a plain paper or an OHP sheet formed of a transparent resin sheet; a fixing unit  19  fixing the toner image transferred to the recording medium by the transfer unit  18  to the recording medium; and an exposure unit  20  forming the latent image on the photoconductive drums  14   a  to  14   d.    
         [0029]    The image signals obtained from the image acquisition unit  13  are sent to the exposure unit  20 . The photoconductive drums  14   a  to  14   d  rotate in arrow directions, respectively, and the surfaces thereof are uniformly charged in a charging unit (not shown) before being exposed by the exposure unit  20 . Light images corresponding to Y (yellow), M (magenta), C (cyan) and Bk (black) are radiated to the photoconductive drums  14   a  to  14   d , so that the latent images corresponding to the Y, M, C and Bk are formed. 
         [0030]    The first to fourth developing units  15   a  to  15   d  contain an arbitrary color of toners of Y (yellow), M (magenta), C (cyan) and Bk (black) used for obtaining a color image and develop the latent images held on each of the photoconductive drums  14   a  to  14   d  by the use of a toner with one of colors in Y, M, C and Bk, that is, the latent images is made to be visible. The order of colors is determined by a predetermined order according to an image forming process or a toner property. The toner images formed on the photoconductive drums  14   a  to  14   d  are transferred primarily to the transfer belt  16  by fitting phases. 
         [0031]    The transfer belt  16  holds the toner images of colors formed by the developing units  15   a  to  15   d  in the order of the formed toner images. Therefore, a color toner image is formed on the transfer belt  16 . 
         [0032]    The transfer unit  18  includes a secondary transfer roller  18   a  and a secondary transfer backup roller  18   b , and the color toner image is supplied between both of these two rollers by the rotation of the transfer belt  16 . 
         [0033]    On the other hand, the sheet feeding unit (sheet cassette)  12  supplies a recording medium P to the transfer unit  18 . The recording medium P supplied to the transfer unit  18  is also supplied between the secondary transfer roller  18   a  and the secondary transfer backup roller  18   b  and the color toner image on the transfer belt  16  is secondly transferred to the recording medium P. 
         [0034]    In  FIG. 1 , although the sheet feeding unit  12  is illustrated just one, in general, there are plural sheet cassettes, in which various sizes and types of the recording mediums are contained in the plural sheet cassettes. The recording medium is withdrawn from a sheet cassette selected among the plural sheet cassettes. Alternatively, the recording medium inserted from a manual mechanism  22  is supplied to the transfer unit  18  through a medium path  21 . The size of the recording medium corresponds to the size of the color toner image which is formed by the image forming apparatus body  11 . 
         [0035]    In the transfer unit  18 , the color toner image on the transfer belt  16  is secondly transferred to the recording medium P which is sent through the medium path  21 , and the recording medium P is sent to the fixing unit  19  through a medium path  23 . The fixing unit  19  will be described in detail later. The recording medium P on which the toner image is fixed by the fixing unit  19  is loaded on a stock unit  25  located at a space between the image acquisition unit  13  and the image forming apparatus body  11  through a medium path  24 . 
         [0036]    The fixing unit  19  has a function of fixing the color toner image on the recording medium P. The perspective view illustrating the structure of the fixing unit  19  is shown in  FIG. 2 . The fixing unit  19  includes a heating roller  31  heated, a fixing roller  32  for fixing the toner image, an endless fixing belt  33  wound around the fixing roller  32  and the heating roller  31 , and a pressurizing driving roller  34  press-contacted with the fixing belt  33  by a spring (not shown). The recording medium P of which a surface having the toner image is faced to the fixing belt  33  is made to be passed between the pressurizing driving roller  34  and the fixing belt  33  from the bottom to the top. 
         [0037]      FIG. 3  shows a structure of each roller of the fixing unit  19  and a connection relationship between these rollers and electric circuits thereof. The heating roller  31  has a metal layer on the surface thereof. The fixing roller  32  has a sponge layer with a predetermined thickness on the surface thereof, and a metal core is located therein. The pressurizing driving roller  34  has a rubber layer with a predetermined thickness on the surface thereof, and a metal core is located therein. 
         [0038]    The heating roller  31  is made of a metal pipe with a 1 mm thickness, the surface thereof is coated in order to prevent rust and improve a lubricating property. The heating roller  31  has a temperature detector  31   s  on the outer periphery thereof, and a temperature measured by the temperature detector  31   s  is input to an external heating circuit  31   h . The heating circuit  31   h  controls heaters  31   a  and  31   b  located in the heating roller  31 . The pressurizing driving roller  34  also has a temperature detector  34   s  on the outer peripheral thereof, and a temperature measured by the temperature detector  34   s  is input to an external heating circuit  34   h . The heating circuit  34   h  controls a heater  34   a  located in the pressurizing driving roller  34 . The heaters  31   a ,  31   b , and  34   a  are configured with, for example, halogen lamps. 
         [0039]    The heating roller  31  being heated by the heaters  31   a  and  31   b  heats the fixing belt  33  and comes to heat the surface holding the color toner image on the recording medium P, where the surface of the recording medium P is press-contacted with the fixing belt  33  when passing between the fixing belt  33  and the pressurizing driving roller  34 . At this time, the pressurizing driving roller  34  heats the surface of the pressurizing driving roller  34  and comes to heat the recording medium P from the rear surface thereof. The surface temperature of the pressurizing driving roller is kept, for example, at 120° C. 
         [0040]    The pressurizing driving roller  34  is connected to a driving motor  35  via a gear, and is rotatably driven by the driving motor  35 . A rotation-speed control circuit  36  controls a rotation speed of the driving motor  35 . The driving motor  35  is a kind of pulse motor whose rotation is controlled by pulses input thereto. 
         [0041]    The rotation-speed control circuit  36  is connected to a timer  37 . The timer  37  measures a period of time lapsed after starting operation, that is, a warming-up (WU) time when the image forming apparatus  10  starts, and inputs the lapsed time to the rotation-speed control circuit  36 . 
         [0042]    The rotation-speed control circuit  36  has a table as a memory (not shown), through which the rotation speed of the pressurizing driving roller  34  is changed according to a period of time after starting operation. Specifically, the rotation-speed control circuit  36  includes a speed control memory which has a correspondence relationship of the number of pulses per every minute to be supplied to the driving motor with respect to the time lapsed after the WU time. In a normal fixing in a steady temperature, the pressurizing driving motor is rotated at a rotation speed when 128 pulses per one minute are input to the driving motor  35 . 
         [0043]    Based on the number of pulses (in this case, 128 pulses) per one minute in the general case, the number of adding pulses according to a period of time after the WU time may be stored in the speed control memory as a table. 
         [0044]    Results experimented on the image forming apparatus having the above structure will be described. The peripheral surface speed of the fixing belt  33  after starting operation (WU time) is measured with respect to an image forming apparatus capable of outputting 35 sheets per one minute and an image forming apparatus capable of outputting 45 sheets per one minute, respectively. 
         [0045]    The heating roller  31  has a metal layer with a 1 mm thickness on the surface thereof. The fixing roller  32  has a sponge layer with an 8 mm thickness and hardness 350 in ASKER-C on the surface thereof, and a metal core is located therein. The pressurizing driving roller  34  has an outer diameter of 38 mm, a rubber layer (hardness 720 in ASKER-C) with a predetermined thickness from the surface thereof, and a metal core therein. 
         [0046]    The rubber thickness on the surface of the pressurizing driving roller  34  is 2 mm in the case of the 35-sheet machine, and 4 mm in the case of the 45-sheet machine. 
         [0047]    In  FIG. 4 , the measurement results of the speed in these two kinds of image forming apparatus are shown every one minute after starting operation. In the 45-sheet machine, a pulse signal of 128 bits/sec corresponding to a normal speed is input to the driving motor  35  from the rotation-speed control circuit  36 . In the 35-sheet machine, a signal of 131 bits which is larger than 128 bits by 3 bits is supplied to the driving motor  35  for one minute. 
         [0048]    When the number of pulses is increased by 10 bits, the rotation speed of the pressurizing driving roller rises 0.6%. Since 131 bits−128 bits=3 bits, the fixing speed in the 35-sheet machine rises about 0.18% higher than that in the 45-sheet machine. 
         [0049]      FIG. 5  shows a graph diagram illustrating the speed data of the 35-sheet machine and the speed data of the 45-sheet machine. The horizontal axis indicates a time lapsed after starting operation, and the vertical axis indicates a peripheral surface speed of the transfer belt, this is, a moving speed (mm/sec) of the recording sheet in fixing. 
         [0050]    In  FIG. 5 , (♦) indicates data of the fixing speed of the 35-sheet machine, and the solid line indicates an approximated exponential curve of these data. In  FIG. 5 , (▴) indicates data of the fixing speed of the 45-sheet machine, and the dotted line indicates an approximated exponential curve of these data. 
         [0051]    In the 35-sheet machine, the speed when starting operation is 151 mm/sec and changed to about 151.5 mm/sec, and then to about 151.6 mm/sec. It is understood that, in the 45-sheet machine, the speed when starting operation is 150.4 mm/sec and changed to about 152 mm/sec. 
         [0052]    From this measurement results, it is possible to understand that, as coming near to the operation start time, the peripheral surface speed of the pressurizing driving roller becomes considerably low. 
         [0053]    In addition, the thickness of the rubber provided on the surface of the pressurizing driving roller  34  is 2 mm in the case of the 35-sheet machine, and 4 mm in the case of the 45-sheet machine. The reason why the thickness of the rubber of the 45-sheet machine, which is a high-speed machine, is larger than that of the 35-sheet machine is to increase the contact area (nip width) between the rollers. In the 35-sheet machine, the nip width is 8 mm, and is about 9.5 mm in the 45-sheet machine. In this way, when the nip width of the pressurizing driving roller  34  is made to be increased for speed-up (for example, from 2 mm to 4 mm), the speed variation becomes large due to the thermal expansion. Therefore, it is necessary to correct the speed. 
         [0054]    In normal room temperature (about 20° C.), the pressurizing driving roller  34  has a roller diameter of 38 mm in both the 35-sheet machine and the 45-sheet machine, and the peripheral surface speed thereof is 150 mm/sec in both in the normal room temperature. 
         [0055]    From  FIG. 5 , it can be seen that the peripheral surface speed of the fixing belt when starting operation is decreased as the thickness of the rubber on the surface of the pressurizing driving roller  34  increases. That is, it can be understood that the large expansion rate and the high temperature cause the diameter of the pressurizing driving roller  34  to vary largely. Further, it can be understood that, if the thickness of the rubber is large, it is difficult to transfer the heat, and it takes much time for the heater  34   a  located in the pressurizing driving roller  34  to heat the surface of the roller. 
         [0056]    From the experimental results, for example, in the 35-sheet machine, it can be understood that the rotation speed is preferably raised about 0.3% for 10 minutes after starting operation. 
         [0057]    Next, in the fixing unit  19  shown in  FIG. 3 , another embodiment in which the rotation speed of the pressurizing driving roller  34  is changed in 2 steps will be described with reference to  FIG. 6 . 
         [0058]    In this case, it is assumed that the rotation speed of the pressurizing driving roller  34  is constant. When the temperature is in a desired constant steady state passing enough time after starting operation, the actual diameter of the pressurizing driving roller  34  comes to be in a predetermined size. At this time, the peripheral surface speed of the pressurizing driving roller  34  is set to Vc, and the rotation speed thereof is set to V 0 . 
         [0059]    When the pressurizing driving roller is made to be rotated at the rotation speed V 0  even though the temperature when starting operation is low, the actual diameter of the roller is small because the temperature is low. Therefore, the peripheral surface speed of the pressurizing driving roller  34  is lower than the peripheral surface speed Vc. In this case, the rotation speed of the pressurizing driving roller rises up to, for example, V 1  (V 1 &gt;V 0 ) by a first predetermined time T 1  after starting operation even though the temperature is low, in order that the peripheral surface speed of the pressurizing driving roller  34  becomes Vc. 
         [0060]    Next, the temperature of the pressurizing driving roller  34  increases, but if the temperature does not yet reach to the steady temperature, the peripheral surface speed does not reach to Vc. At this time, during a period of time from starting operation to the secondary time T 2  after being lapsed from the first time T 1 , the rotation speed V 2  of the pressurizing driving roller  34  is controlled to be lower than the rotation speed V 1  but higher than the rotation speed V 0 , that is, V 1 &gt;V 2 &gt;V 0 , so that the peripheral surface speed almost equals to Vc. 
         [0061]    Specifically, the first predetermined time T 1  is 2 minutes, and the second predetermined time T 2  is 15 minutes. The peripheral surface speed V 1  is a speed more increased than normal by 0.8%, that is, corresponds to 128+13=141 bits. The peripheral surface speed V 2  is a speed more increased than normal by 0.4%, that is, corresponds to 128+7=135 bits. 
         [0062]    Such being this case, if the rotation speed of the pressurizing driving roller is controlled in 2 steps, the peripheral surface speed of the pressurizing driving roller due to the small roller diameter is corrected and kept almost constant. Moreover, it is possible to keep the fixing speed almost constant. 
         [0063]    As the embodiment, if the rotation speed of the pressurizing driving roller is controlled by being separated in 2 steps, the change in the fixing speed can be suppressed more than in the case of controlling in 1 step of the above embodiment. 
         [0064]    In the above-mentioned embodiment, the rubber is provided on the surface of the pressurizing driving roller. However, in the invention, it is possible to use a sponge instead of the rubber on the surface of the pressurizing driving roller. Further, the rubber or the sponge can be also provided on the surface of the fixing roller. Moreover, in the invention, mini-cell rollers can be adopted as these rollers. 
         [0065]    In the above-mentioned embodiment, it is described about the fixing after the toner image is transferred from the transfer belt to the recording sheet. However, the invention is applicable to the fixing after the toner image is transferred from the transfer roller to the recording sheet. 
         [0066]    In the above-mentioned embodiment, it is described about the fixing of the color toner image. However, the invention is applicable to the fixing of a monochrome toner image as well as the color toner image. 
         [0067]    In the above-mentioned embodiment, it is described about that the pressurizing driving roller comes in contact with the rear surface of the recording sheet, that is, that the rotation is driven by the pressurizing roller. However, the invention is also applicable to that the fixing roller which rotatably moves the fixing belt is rotatably driven. 
         [0068]    In the above-mentioned embodiment, it is described about the fixing using the fixing belt. However, the invention is not limited thereto, and is also applicable to that the fixing roller is directly provided opposite to the pressurizing roller and the recording sheet holding the toner image is moved by being interposed between these rollers without using the fixing roller. In this case, the roller which is driven rotatably may be the pressurizing roller or the fixing roller. 
         [0069]    In the above-mentioned embodiment, it is described about that the driving motor is a pulse motor and the rotation thereof is changed by varying the number of pulses input to the pulse motor, so that the rotation of the roller driven by the driving motor is changed. However, in the invention, the change in the rotation of the roller may be made without using the pulse motor. 
         [0070]    Obviously, many, modifications and variations of this invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, this invention may be practiced otherwise than as specification.