Abstract:
An image forming apparatus of the present invention is capable of protecting a sheet or recording medium from defective fixation and creases and enhancing image quality. These advantages are achievable even when the operator of the apparatus switches the kind of sheets to use or selects, e.g., a special sheet needing a particular fixing condition.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus.  
           [0002]    Generally, an image forming apparatus includes a photoconductive element, an exposing unit, a developing unit, an image transferring unit, a fixing unit, a cleaning unit, a sheet feeder, a registration roller pair, an outlet roller, and an outlet sensor. The apparatus additionally includes a heat discharge fan for discharging heat generated in the apparatus, e.g., heat generated by the fixing unit.  
           [0003]    Assume that the image forming apparatus forms an image with high resolution based on a low linear velocity, and that the heat discharge fan continuously rotates from the time when a sheet starts being driven by the registration roller pair to the time when it moves away from the outlet sensor. Then, when the leading edge of the sheet passes the outlet sensor, a stream of air generated by the heat discharge fan is apt to cause the leading edge to shake and therefore crease, degrading image quality. A thin sheet, in particular, cannot smoothly enter the fixing unit due to the influence of the stream of air and therefore creases.  
           [0004]    Japanese Patent Laid-Open Publication No.7-319370, for example, teaches countermeasures against the creasing of a sheet ascribable to the heat discharge fan. In accordance with this document, when use is made of a thin sheet, a heat discharge fan is rotated at a lower speed than usual. At the same time, a control member adjoining the heat discharge fan for controlling airflow is replaced with one having a smaller air passage in order to reduce airflow. Further, a motor drives the control member in order to vary its angular position, thereby reducing the airflow of the heat discharge fan.  
           [0005]    There is an increasing demand for an image forming apparatus capable of printing images with a plurality of different resolutions and thereby enhancing image quality. An image forming apparatus with this capability may be constructed to feed, for resolution as low as 600 dpi (dots per inch), a sheet at a usual linear velocity or feed, for resolution as high as 1,200 dpi, a sheet by reducing the usual linear velocity to one half.  
           [0006]    Laid-Open Publication No. 7-319370 mentioned earlier does not control the rotation speed of the heat discharge fan in accordance with resolution. As a result, when a sheet is fed at the lower linear velocity for the resolution of 1,200 dpi, the stream of air generated by the fan is apt to cause the leading edge of the sheet to shake and crease. We have proposed an implementation for protecting a sheet fed at the lower linear velocity or a thin sheet from creases ascribable to the heat discharge fan. The implementation consists in stopping the rotation of the heat discharge fan from the time when a registration roller pair starts driving the sheet to the time when the leading edge of the sheet moves away from an outlet sensor.  
           [0007]    Some problems arise when images with different resolutions are continuously printed on consecutive sheets. For example, assume that a first sheet is conveyed at the higher linear velocity for the lower resolution or is implemented by an ordinary sheet, and then a second sheet is conveyed at the lower linear velocity for the higher resolution or is implemented by a thin sheet. Then, high fixing temperature is assigned to the first sheet. Despite that lower target fixing temperature is assigned to the second sheet that immediately follows the first sheet, the temperature cannot be lowered to the target temperature and causes the second sheet to crease.  
           [0008]    Also, assume that a first sheet is conveyed at the lower linear velocity or is implemented by a thin sheet, and then a second sheet is conveyed at the higher linear velocity or is implemented by a usual sheet. Despite that the higher target fixing temperature is assigned to the second sheet that immediately follows the first sheet, the temperature cannot be raised to the target temperature and fails to fully fix toner on the second sheet. In this manner, when different resolutions and different kinds of sheets are dealt with together, sheet feed occurs before the fixing temperature reaches the target temperature. This makes it impossible to fully protect sheets from creases and other defects.  
           [0009]    Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-332330 and 2000-259045.  
         SUMMARY OF THE INVENTION  
         [0010]    It is therefore an object of the present invention to provide an image forming apparatus capable of protecting a sheet from defective fixation and creases and enhancing image quality even when the kind of sheets to be used is varied or when a special sheet, for example, needing a particular fixing condition is selected. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:  
         [0012]    [0012]FIG. 1 is a view showing an image forming apparatus embodying the present invention;  
         [0013]    [0013]FIG. 2 is a schematic block diagram showing a control system included in the illustrative embodiment;  
         [0014]    [0014]FIG. 3 is a flowchart demonstrating a main routine to be executed by a controller included in the control system of FIG. 2;  
         [0015]    [0015]FIGS. 4 through 6 are flowcharts each showing a particular subroutine included in the main routine in detail;  
         [0016]    [0016]FIG. 7 is a table listing a relation between conditions selected on a control panel included in the illustrative embodiment and the operation of a heat discharge fan also included in the illustrative embodiment;  
         [0017]    [0017]FIGS. 8A and 8B are tables listing experimental data relating to the temporary stop of rotation of the heat discharge fan; and  
         [0018]    [0018]FIG. 9 is a flowchart demonstrating resolution-by-resolution fan control practicable with the illustrative embodiment.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    Referring to FIG. 1 of the drawings, an image forming apparatus embodying the present invention is shown and generally designated by the reference numeral  1 . As shown, the image forming apparatus  1  includes a photoconductive element implemented as a drum  2 . A developing unit  3  and an image transfer roller  7  adjoin the drum  2 . A fixing unit  6  includes a heat roller  4  and a press roller  5 . While the drum  2  is in rotation, a charger, not shown, uniformly charges the surface of the drum  2 . A latent image is electrostatically formed on the charged surface of the drum  2  in accordance with image data representative of a document image. The developing unit  3  develops the latent image with a developer or toner to thereby produce a corresponding toner image. A cassette or sheet feeder  8  is removably positioned in the lower portion of the apparatus  1  and loaded with a stack of sheets. A pickup roller  9  pays out the top sheet from the cassette  8  toward a registration roller pair  10 . The registration roller pair  10  drives the sheet at such timing that the leading edge of the sheet meets the leading edge of the toner image. The image transfer roller  7  transfers the toner image from the drum  2  to the sheet.  
         [0020]    More specifically, the registration roller pair  10  once stops the movement of the sheet and again drives it toward the image transfer roller  7  at the above-mentioned timing. A controller  27  (see FIG. 2) causes the registration roller pair  10  to so operate by sending a drive start signal thereto. A registration sensor  13  senses the sheet arrived at the registration roller pair  10 .  
         [0021]    The fixing unit  6  fixes the toner image carried on the sheet with heat and pressure. An outlet roller  14  delivers the sheet with the fixed image, i.e., a copy to the outside of the apparatus  1 . At this instant, an outlet sensor  12  senses the sheet and causes its output to go high.  
         [0022]    A manual sheet feed section  15  allows the operator of the apparatus  1  to feed an OHP (OverHead Projector) sheet, thick sheet or similar special sheet by hand. A pickup roller  16  delivers such a special sheet from the manual sheet feed section  15  to the registration roller pair  10 . The registration roller pair  10  drives the special sheet toward the image transfer roller  7  in the same manner as it drives the usual sheet paid out from the cassette  8 . A heat discharge fan  11  discharges heat generated inside the apparatus  1 , e.g., heat generated by the fixing unit  6  to the outside of the apparatus  1 .  
         [0023]    The illustrative embodiment is characterized in that when the kind of sheets to be used or resolution is varied, the controller  11  controls the rotation of the heat discharge fan  11  in order to protect the sheets from creases, which are ascribable to an air stream generated by the fan  11 .  
         [0024]    Specifically, as shown in FIG. 2, the controller or engine  27  includes a CPU (Central Processing Unit)  18 , an I/O (Input/Output) controller  19 , a ROM (Read Only Memory)  20 , a RAM (Random Access Memory)  21 , and an EEPROM (Electrically Erasable Programmable ROM)  22 . The ROM  20  stores a program, i.e., commands meant for the CPU  18 . The RAM  21  serves as a volatile memory when the control program is being executed. In the illustrative embodiment, the controller  27  temporarily stops the operation of the heat discharge fan  11  or switches it in accordance with the kind of sheets to be used or resolution. Further, the controller  27  selectively raises or lowers fixing temperature in a sheet feed mode or a standby mode in accordance with a fixation control program installed in the controller  27 , as will be described specifically later.  
         [0025]    The EEPROM  22  is a nonvolatile memory for storing data that should be preserved even when a power switch, not shown, is turned off, e.g., the contents of counters for maintenance. Just after the turn-on of the power switch, the CPU  18  reads the control program out of the ROM  22  and delivers commands to the RAM  21  and EEPROM  22 . The registration roller pair  10 , outlet sensor  12 , heat discharge fan  11  and other mechanical sections and the CPU  18  interchange information via the I/O controller  19 . A heater  30  and a thermistor  29  are included in the fixing unit  6 . The thermistor  29  is responsive to the surface temperature of the heat roller  4 . The CPU  18  controls the heat roller  4  to preselected temperature in accordance with temperature data received from the thermistor  29  via the I/O controller  19 . A communication interface  23  is connected to the CPU  18  via the I/O controller  19  in order to transfer a print command or print signal received from a personal computer to the CPU  18 . A control panel  26  is connected to the CPU  18  and allows the operator to select a desired kind of sheets or desired resolution.  
         [0026]    [0026]FIG. 3 shows a specific main routine to be executed by the controller  27 . The main routine to be described is assumed to be executed in a period of the order of as short as 10 milliseconds. During each period, there are executed all of control for error detection, standby control, and sheet feed control. Specifically, the controller  27  starts executing the main routine substantially at the same time as the turn-on of the power switch (step S 1 ). The controller  27  then initializes various parameters for driving the apparatus  1  and controls the warm-up of the apparatus  1  (step S 2 ). Subsequently, the controller  27  executes control for detecting sheet jams around the cassette  8  and on the conveyance path and control for detecting a fixing temperature error, a high-tension voltage control error and so forth (step S 3 ). When the engine enters a standby mode after the warm-up control, the controller  27  executes stand-by control (step or subroutine S 4 ). After the step S 4 , as soon as the engine enters a sheet feed mode, the controller  27  executes sheet feed control (step or subroutine S 5 ). Thereafter, the controller  27  determines whether or not one period assigned to the main routine has elapsed (step S 6 ). If the answer of the step S 6  is positive (YES), the controller  27  again executes the steps S 3  through S 5 . If the answer of the step S 6  is negative (NO), the controller  27  repeats the step S 6 . For example, one period elapses in a preselected period of time (YES, step S 6 ) since the execution of the step S 3 . For the step S 6 , use is made of a timer included in the CPU  18 .  
         [0027]    The subroutine S 4  will be described specifically with reference to FIG. 4. As shown, the controller  27  starts executing the subroutine S 4  (step S 1 ) and then determines whether or not a transition-to-standby request has appeared or whether or not the standby mode is under way (step S 2 ). If the transition-to-standby request has appeared or if the standby mode is under way (YES, step S 2 ), then the controller  27  sets up fixing temperature matching with the kind of a sheet and resolution in the standby mode (step S 3 ). At this instant, the controller  27  sets up the standby mode before the step S 3  if the current mode is not the standby mode. The transition-to-standby request appears after the warm-up operation and after the feed of sheets. If the answer of the step S 2  is NO, then the controller  27  ends the subroutine S 4 , FIG. 3.  
         [0028]    [0028]FIG. 5 shows the subroutine S 5 , FIG. 3, in detail. As shown, the controller  27  starts executing the subroutine S 5  (step S 1 ) and then determines whether or not the sheet feed mode has been setup (step S 2 ). If the answer of the step S 2  is YES, then the controller  27  determines whether or not to execute resolution-by-resolution fan control (step S 3 ). If the answer of the step S 3  is YES, then the controller  27  executes resolution-by-resolution fan control (step S 4 ). On the other hand, if the answer of the step S 2  is NO, then the controller  27  determines whether or not a transition-to-feed request has appeared (step S 5 ). If the answer of the step S 5  is YES, then the controller  27  executes sheet feed start control (step S 6 ). More specifically, in the step S 6 , the controller  27  determines whether or not to cause sheet feed to start. If the answer of the step S 5  is NO, then the controller  27  ends the subroutine S 5 , FIG. 3.  
         [0029]    After the step S 6 , the controller  27  determines whether or not a feed start flag is set (step S 7 ). If the answer of the step S 7  is YES, then the controller  27  sets up the sheet feed mode (step S 8 ) and then executes the steps S 3  and S 4 . If the answer of the step S 7  is NO, then the controller  27  inhibits sheet feed until the engine reaches target temperature. The resolution-by-resolution fan control (step S 4 ) will be described later in detail. The step S 4  is followed by a step S 9  for controlling the fixing temperature to target temperature matching with the kind of a sheet to be used or resolution. The controller  27  then ends the subroutine step S 5 , FIG. 3.  
         [0030]    The sheet feed start control (step S 6 , FIG. 5) will be described hereinafter in detail. Assume that a thin sheet is to be fed after an ordinary sheet fed last time. Then, in the illustrative embodiment, the controller  27  causes the heat discharge fan  11  to rotate at a higher speed than during usual sheet feed for a preselected period of time, thereby rapidly lowering the fixing temperature to preselected one. On the other hand, assume that an ordinary sheet is to be fed after a thin sheet fed last time. Then, the controller  27  stops the rotation of the heat discharge fan  11  in order to raise the fixing temperature as rapidly as possible. Further, assume that high resolution is to be set up by a low linear velocity after low resolution set up by a high linear velocity last time. Then, the controller  27  causes the heat discharge fan  11  to rotate at a higher speed than during usual sheet feed for a preselected period of time, thereby rapidly lowering the fixing temperature. On the other hand, assume that low resolution is to be set up by a high linear velocity after high resolution set up by a low linear velocity last time. Then, the controller  27  stops the rotation of the heat discharge fan  11  in order to raise the fixing temperature as rapidly as possible.  
         [0031]    Reference will be made to FIGS. 6 and 7 for describing the sheet feed start control (step S 6 , FIG. 5) in detail. As shown in FIG. 6, after the start of the sheet feed start control (step S 1 ), the controller  27  determines whether or not fan/sheet feed switch flag is set (step S 2 ). If the answer of the step S 2  is YES, then the controller  27  executes a step S 3  that will be described later specifically. If the answer of the step S 2  is NO, then the controller  27  unconditionally sets the previously mentioned sheet feed start flag, so that a sheet is fed without regard to resolution or the kind of the sheet (step S 4 ).  
         [0032]    In the step S 3 , the controller  27  compares resolution and the kind of a sheet selected last time and resolution and the kind of a sheet selected this time. As shown in step S 5 , assume that a low linear velocity, i.e., high resolution is selected in place of a high linear velocity, i.e., low resolution selected last time (condition (4), FIG. 7) or that a thin sheet is selected in place of an ordinary sheet selected last time (condition (1), FIG. 7). Then, the controller  27  causes the heat discharge fan  11  to rotate at a high speed (step S 6 ). It is to be noted that the linear velocity refers to the rotation speed of the drum  2 , FIG. 1.  
         [0033]    More specifically, by causing the heat discharge fan  11  to rotate at a high speed (step S 6 ), the controller  27  lowers high temperature set last time in matching relation to low resolution or an ordinary sheet. The controller  27  then determines whether or not the fixing temperature has lowered below target temperature (step S 7 ). If the answer of the step S 7  is YES, then the controller  27  sets the sheet feed start flag; if otherwise, it resets the sheet feed start flag. This allows sheet feed to start as soon as the fixing temperature lowers below the target temperature. It follows that offset or similar defect is obviated even when the kind of a sheet or resolution is switched.  
         [0034]    Assume that resolution selected this time and resolution selected last time both are high or that thin sheets are selected this time and last time (step S 8 ). Then, the controller  27  unconditionally sets the sheet feed start flag in order to start sheet feed immediately. On the other hand, assume that high resolution is selected in place of low resolution (condition (3), FIG. 7) or that an ordinary sheet is selected in place of a thin sheet (condition (2), FIG. 7) (step S 9 ). Then, the controller  27  stops the rotation of the heat discharge fan  11  (step S 10 ) in order to raise the low fixing temperature set for high resolution or the thin sheet last time. In this manner, when the kind of a sheet or resolution is varied, the illustrative embodiment selectively lowers or raises the fixing temperature by causing the heat discharge fan  11  to rotate at a high speed or causing it to stop rotating.  
         [0035]    After the step S 10 , the controller  27  determines whether or not the fixing temperature has risen above target temperature (step S 11 ). If the answer of the step S 11  is YES, then the controller  27  sets the sheet feed start flag; if otherwise, the controller  27  resets the sheet feed start flag. In this manner, the controller  27  can start sheet feed as soon as the fixing temperature rises above the target temperature, obviating defective fixation or similar trouble. Further, assume that resolution selected this time and resolution selected last time both are high or that ordinary sheets are selected this time and last time (step S 12 ). Then, the controller  27  unconditionally sets the sheet feed start flag in order to start sheet feed immediately. The sheet feed start control ends after the sheet feed start flag has been set, as stated above.  
         [0036]    In the case of a sheet thicker than an ordinary sheet, for example, fixing temperature higher than one assigned to the ordinary sheet is set. Therefore, when such a thick sheet is selected in place of a thin sheet, the controller  27  stops the rotation of the heat discharge fan  11 . Further, when a thick sheet is selected in place of an ordinary sheet, the controller  27  maintains the ordinary rotation speed of the heat discharge fan  11 .  
         [0037]    As shown in FIG. 7, after the feed of a sheet, the controller  27  stops the rotation of the heat discharge fan  11  in the conditions (1) and (4) set on the control panel  26 , FIG. 2, but does not stop it in the conditions (2) and (3). More specifically, in the conditions (1) and (4), the controller  27  stops the rotation of the heat discharge fan  11  just after the drive of the registration roller  10  and then resumes it just after the leading edge of a sheet has moved away from the outlet sensor  12 . In the conditions (2) and (3), the controller  27  does not stop the rotation of the heat discharge fan  11  after the start of sheet feed, but continues the usual rotation of the fan  11 .  
         [0038]    Further, the illustrative embodiment executes unique processing when the operation mode is switched from the sheet feed mode to the standby mode, as will be described herein after. Assume that the heat discharge fan  11  is rotating at the usual speed when the sheet feed mode is replaced with the standby mode, that the fixing temperature in the standby mode is T in the event of replacement of the standby mode with the sheet feed mode, and that the fixing temperature in the sheet feed mode is t. Further, assume that when the temperature T is higher than the temperature t, i.e., when the fixing temperature is lowered at the time of transition from the standby mode to the sheet feed mode, the allowable temperature difference is t down . In addition, assume that that when the temperature T is lower than the temperature t, i.e., when the fixing temperature is to be raised at the time of transition from the standby mode to the sheet feed mode, an allowable temperature difference is t up . Then, when the difference T−t down  is greater than t, the controller  27  causes the heat discharge fan  11  to rotate at a higher speed than during usual sheet feed. When the difference T−t down  is equal to or smaller than t, which is in turn equal to or smaller than a sum T+t up  the controller  27  causes the heat discharge fan  11  to rotate at the usual speed. Further, when the sum T+t up  is smaller than t, the controller  27  causes the heat discharge fan  11  to stop rotating for a moment. Such unique processing will be described more specifically with reference to FIGS. 7, 8A and  8 B.  
         [0039]    [0039]FIGS. 8A and 8B show experimental results showing a relation between the kind of sheets and resolution and the fixing temperature in the sheet feed mode and standby mode, respectively. It should be noted that temperatures (° C.) shown in FIGS. 8A and 8B are only illustrative, and that t down  and t up  are selected to be 10° C. and 30° C., respectively.  
         [0040]    In conditions (5) and (7) shown in FIG. 7, the controller  27  causes the heat discharge fan  11  to stop rotating up to the start of sheet feed on the basis of the temperature data shown in FIGS. 8A and 8B. In a condition (6), the controller  27  causes the heat discharge fan  11  to rotate at a high speed for a preselected period of time up to the start of sheet feed. In a condition (8), the controller  27  causes the heat discharge fan  11  to rotate at the usual speed up to the start of sheet feed. Further, in the conditions (5), (6) and (8), the controller  27  causes the heat discharge fan  11  to stop rotating after the start of sheet feed. In addition, in the condition (7), the controller  27  does not cause the heat discharge fan  11  to stop rotating after the start of sheet feed.  
         [0041]    The resolution-by-resolution fan control will be described with reference to FIG. 9. In the illustrative embodiment, when a low linear velocity, i.e., high resolution or a thin sheet is selected, the controller  27  can cause the heat discharge fan  11  to stop rotating just after the start of drive of the registration roller pair  10  during sheet feed. Also, the controller  27  can cause the heat discharge fan  11  to again start rotating at the usual speed just after the leading edge of a sheet being conveyed has moved away from the outlet sensor  12 .  
         [0042]    Specifically, as shown in FIG. 9, the controller  27  starts executing the resolution-by-resolution fan control (step S 1 ) and then determines whether or not resolution as high as, e.g., 1,200 dpi is selected or whether or not a thin sheet is selected (step S 2 ). If the high resolution is selected or if a thin sheet is selected (YES, step  2 ), then the controller  27  executes a step S 3  for determining whether or not the registration roller pair  10  has started being driven. If the answer of the step S 3  is YES, then the controller  27  determines whether or not the outlet sensor  12  has turned on (step S 4 ). If the answer of the step S 3  is NO, then the controller  27  once ends the resolution-by-resolution fan control (RETURN). More specifically, the controller  27  again executes the above control in the next routine and, because the answer of the step S 2  is YES in the previous routine, again executes the step S 3 . The controller  27  repeats such a sequence of steps in the period of the routine until the registration roller pair  10  starts being driven.  
         [0043]    If the answer of the step S 4  is NO, then the controller  27  causes the heat discharge fan  11  to stop rotating (step S 5 ). If the answer of the step S 4  is YES, then the controller  27  causes the heat discharge fan  11  to rotate at the usual speed (step S 6 ). In this manner, the heat discharge fan  11  rotates at the usual speed until the registration roller pair  10  starts rotating, stops rotating from the time when the registration roller pair  10  starts rotating to the time when the leading edge of the sheet moves away from the outlet sensor  12 , and again starts rotating at the usual speed after the leading edge of the sheet has moved away from the sensor  12 . After the step S 5  or S 6 , the controller  27  ends the control.  
         [0044]    In summary, it will be seen that the present invention provides an image forming apparatus capable of obviating the creasing of a sheet ascribable to excessive heat during fixation, reducing a waiting time up to sheet refeed, and obviating defective fixation ascribable to defective heating and thereby enhancing image quality. This is also true even when the kind of a sheet to be used or resolution is varied. In addition, the apparatus of the present invention is capable of executing particular control for, e.g., each special sheet that needs a particular fixing condition.  
         [0045]    Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.