Abstract:
It is one objective of the present invention to provide a sheet feeding apparatus that can steadily separate and feed individual sheets without causing an image detect, such as a transfer failure. The sheet feeding apparatus, for employing a sheet feeding portion to feed sheets stacked on elevatable sheet stacking means, includes an air blowing portion for blowing air against an end face of a sheet stack supported by the sheet stacking means, a sheet position detector for detecting that a top face of the sheet stack has reached a position whereat a sheet feeding operation by the sheet feeding portion is enabled, wherein, when the sheet position detector has detected that the top face of the sheet stack has reached the position for sheet feeding, or when the sheet feeding portion starts the sheet feeding operation, and when a sheet is not actually fed after a predetermined waiting time has elapsed, the air blowing portion starts an air blowing operation during a predetermined air blowing period.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a sheet feeding apparatus and an image forming apparatus equipped with this sheet feeding apparatus, and particularly to a configuration for separating sheets that tend to stick to each other and feeding individual sheets. 
   2. Related Background Art 
   A conventional image forming apparatus, such as a copier or a printer, includes a sheet feeding apparatus wherein sheets stacked on sheet stacking means are sequentially fed, beginning with the uppermost sheet, by sheet feeding means, which is a pickup roller, and are separated by a sheet separating portion and individually supplied to an image forming portion. 
   Cut sheets, generally of high-quality paper or of a standard paper designated by a copier maker, are employed for the sequential feeding performed by a thus arranged sheet feeding apparatus. And to steadily separate and feed cut sheets individually, various sheet separating systems have been employed, such as a sheet separating pad system that, to prevent the double feeding of sheets, brings a friction member into contact with a feed roller. 
   As another separating system, there is a retard separating system wherein a separating portion is constituted by a feed roller, which is rotated in a sheet conveying direction, and a separating roller, which is driven at a predetermined torque in a direction opposite to the sheet conveying direction and which contacts the feed roller under a predetermined pressure, and wherein the separating portion passes only the uppermost sheet of a stack of sheets that is fed by a pickup roller, and returns, toward the sheet mounting means, other sheets that accompany the uppermost sheet, so that double feeding is prevented. 
   When one of the sheet separating systems, such as a retard separating system, is employed to steadily separate and individually feed sheets, a return torque and a pressurization force for a separation roller are optimized while taking the friction force of a sheet into account. 
   Recently, as the variety of types of sheets (recording media) has increased, the demand has likewise increased for the forming of images not only on very thick paper, OHP sheets and art films, but also on coated sheets, for which a surface coating process has been performed to obtain white and glossy colors that satisfy market demands for color. 
   However, when very thick paper is to be fed, it can not be picked up because the weight of the paper resists its conveyance, and a paper jam occurs. Further, when resin sheets, such as OHP sheets and art films, that tend to acquire a charge are to be fed in a low relative humidity environment, the surfaces of the sheets are gradually charged by rubbing against other sheets, and a Coulomb force causes then to attract to each other. As a result, either a sheet cannot be picked up, or the double feeding of sheets occurs. 
   Furthermore, a property of coated sheets the surfaces of which are covered with a coating material, is that when stacked they attract to each other, especially in a high relative humidity environment. Therefore, the coated sheets cannot be picked up individually, and the double feeding of sheets occurs. 
   The friction force exerted between the special sheets described above is equal to or smaller than the friction force for standard paper. However, in a low relative humidity environment, the attraction of resin sheets to each other is induced by an attractive force considerably stronger than the force generated by friction, and in a high relative humidity environment, the attraction of coated sheets to each other is induced by another attractive force that is considerably higher than the friction force. Therefore, the conventional separation system cannot perform individual sheet separation. 
   That is, since for the conventional sheet separation system only the friction force exerted between sheets is considered, this system cannot steadily separate individual sheets when an attractive force other than the friction force acts on sheets. 
   In order to eliminate the very high attractive force exerted between the sheets, conventionally, the printing industry and some copier manufacturers have adopted a sheet separation and feeding system as disclosed in Japanese Patent Application Laid-Open No. H11-005643. According to this system, individual sheets are raveled out in advance by blowing air against the side of a stack of sheets to remove attractions between sheets. In this state, the individual sheets are picked up, start with the uppermost, and are separated by a sheet separating portion located downstream. In the sheet separation and feeding system that comprises means (hereinafter referred to auxiliary raveling-out means) for blowing air against the side of a stack of sheets, the sheets (recording media) that tend to attract to each other are raveled out before the sheet feeding, and the attractions removed. Therefore, the efficiency of the sheet separation function is increased compared with the previously described system that relies only on the friction force.  FIG. 18  is a diagram showing the configuration of a sheet feeding apparatus that includes such auxiliary raveling-out means. A sheet feeding apparatus  155  comprises: a sheet supply tray  59  on which sheets S are stacked; sheet feeding means (not shown), for feeding the sheets S from the sheet supply tray  59 ; air blowing means  71 , for blowing air against the side of the stacked sheets S; and flow path moving means  157 , for vertically moving the air blowing means  71  along the side of the stack of sheets S. 
   The flow path moving means  157  includes a guide rail (not shown), used to support the air blowing means  71  so it is movable vertically; an electric motor  121 ; and a cam plate  123 , which contacts the lower face of the air blowing means  71  and moves the air blowing means  71  vertically. In the flow path moving means  157 , when the electric motor  121  is rotated, the air blowing means  71  is moved vertically by the cam plate  123 , and accordingly, an air channel is moved vertically. Since the opening (air blowing port) of the air blowing means  71  has a constant predetermined opening dimension, the side of the sheet S is exposed at the opening as the air blowing means  71  is lowered. Then, the dimension of the opening is reduced, and the direction in which air is blown from the opening is narrowed. As a result, the sheets P are floated beginning with the uppermost sheet S, and the attraction between all the sheets S is removed. Another example sheet separating and feeding system for blowing air against the side of a stack of sheets is disclosed in Japanese Patent Application Laid-Open No. 2001-048366. According to this system, blown air is heated by a heater to remove humidity from the sheets P in order to reduce the attractive force between the sheets (coated sheets), especially in a high relative humidity environment. 
   However, for a sheet feeding apparatus that employs the sheet separation and feeding system for blowing air against the side of a sheet stack, when air is blown, especially in a low relative humidity environment, only part of the stacked sheets close to the air blowing port is dried. 
   When the sheets are only partially dried, the surface resistance on the sheet plane is uneven, and as a result, when a sheet is fed to the image forming portion of the image forming apparatus, this dry portion causes a transfer failure, and an image defect occurs. Especially for an electrophotographic system wherein the image forming portion employs an electrostatic charge to transfer a toner image to a sheet, since the transfer function is greatly affected by the surface resistance of the sheet, the uneven surface resistance causes an uneven image transfer, so that considerable image deterioration occurs and the obtained image is very unsatisfactory. 
   SUMMARY OF THE INVENTION 
   While taking these shortcomings into account, it is one objective of the present invention to provide a sheet feeding apparatus that can steadily separate and feed individual sheets without causing an image detect, such as a transfer failure, and an image forming apparatus employing this sheet feeding apparatus. 
   According to one aspect of the present invention, a sheet feeding apparatus for feeding sheets, comprises: 
   sheet feeding means for feeding sheets stacked on elevatable sheet stacking means, 
   air blowing means for blowing air against an end face of a sheet stack supported by the sheet stacking means; 
   sheet position detection means for detecting that a top face of the sheet stack has reached a sheet feeding position whereat a sheet feeding operation by the sheet feeding means is enabled, 
   wherein, after a predetermined waiting time that a sheet is not actually fed by said sheet feeding means has elapsed since the sheet position detection means has detected that the top face of the sheet stack has reached the sheet feeding position, or since the sheet feeding means has finished the sheet feeding operation, the air blowing means starts an air blowing operation during a predetermined air blowing period. 
   According to another aspect of the invention, a sheet feeding apparatus for feeding sheets comprises: 
   a elevatable lifter support on which a stack of sheets is mounted; 
   a pickup roller for conveying the sheets from the lifter support; 
   an air blow opening located opposite an end face of the sheet stack mounted on the lifter support; 
   a fan for blowing air from the air blow opening; and 
   a paper position sensor for detecting that a top face of an uppermost sheet of the sheet stack on said lifter support has reached a sheet feeding position whereat a sheet feeding operation is enabled, 
   wherein, after a predetermined waiting time that a sheet is not fed by said pickup roller has elapsed since the paper position sensor has detected that the top face of the uppermost sheet on the sheet stack has reached the sheet feeding position, or since the pickup roller has finished the sheet feeding operation, the fan blows air during a predetermined air blowing period. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a printer, an example image forming apparatus that includes a sheet feeding apparatus according to one embodiment of the present invention; 
       FIG. 2  is a plan view of the configuration of the sheet feeding apparatus; 
       FIG. 3  is a side cross-sectional view of the sheet feeding apparatus; 
       FIG. 4  is a block diagram showing the printer; 
       FIG. 5  is a graph showing-relationship between attractive force and relative humidity; 
       FIGS. 6A ,  6 B,  6 C and  6 D are diagrams for explaining an attractive mechanism for a coated sheet; 
       FIG. 7  is a plan view of the state wherein small sheets are stored in the sheet feeding apparatus; 
       FIG. 8  is a graph showing the temporal change of an attractive force immediately after a package of coated sheets is opened; 
       FIG. 9  is a graph showing the temporal change of the attractive force after the coated sheets are raveled out; 
       FIG. 10  is a control table for controlling the initial swing time for the sheet feeding apparatus; 
       FIG. 11  is a control table for controlling a pre-job swing time for the sheet feeding apparatus; 
       FIG. 12  is a control table controlling the temperature of the heater of the sheet feeding apparatus; 
       FIG. 13  is a time control table for controlling a swing operation in the waiting state of the sheet feeding apparatus; 
       FIG. 14  is a flowchart showing the initial swing operation of the sheet feeding apparatus; 
       FIG. 15  is a flowchart showing the pre-job swing operation of the sheet feeding apparatus; 
       FIG. 16  is a flowchart showing the swing control, in the waiting state, for the sheet feeding apparatus; 
       FIG. 17  is a flowchart showing the pre-job swing operation for the sheet feeding apparatus performed after the swing operation in the waiting state has been performed; and 
       FIG. 18  is a diagram for explaining the configuration of a conventional sheet feeding apparatus. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiment of the present invention will now be described in detail while referring to the accompanying drawings. 
     FIG. 1  is a cross-sectional view of a printer, an example image forming apparatus that includes a sheet feeding apparatus according to the embodiment of the invention. 
   In  FIG. 1 , a printer  1000  comprises a printer main body  1001  and a scanner  2000  arranged on the top face of the printer main body  1001 . 
   The scanner  2000  for reading a document includes an optical scanning system light source  201 , an original glass  2020 , a document pressing plate  203  that is opened and closed, a lens  203 , a light-receiving (photo-electric) device  205 , an image processing portion  206 , and a memory portion  208  used to store image processing signals obtained by the image processing portion  206 . 
   To read a document, the optical scanning system light source  201  emits light to illuminate a document (not shown) placed on the original glass  202 . The obtained document image is processed by the image processing portion  206 , and is converted into an electric signal  207 , which is an electrical code, and the electric signal  207  is transmitted to a laser scanner  111   a  that serves as image production means. The image data that are encoded by the image processing portion  206  may be temporarily stored in the memory portion  208 , and in accordance with a signal transmitted by a controller  120 , which will be described later, the image data may be transmitted to the laser scanner  111   a , as needed. 
   The printer main body  1001  includes: a sheet feeding apparatus  1002 , for feeding a sheet S; a sheet conveying apparatus  1004 , for conveying, to an image forming portion  1003 , the sheet S received from the sheet feeding apparatus  1002 ; and a controller  120 , which serves as control means for the printer  1000 . 
   The sheet feeding apparatus  1002  includes cassettes  100 , pickup rollers  101 , and separating portions constituted by feed rollers  102  and retard rollers  103 . The sheets S in the cassettes  100  are separated individually and fed by the pickup rollers  101 , which are elevated/rotated at a predetermined timing, and the separating portions. Further, sheet feeding sensors  104  are located downstream, in the sheet conveying direction, in the vicinities of the feed rollers  102  and the retard rollers  103 . The sheet feeding sensors  104  detect the passage of sheets S. 
   Cassette storage portions  1005 , wherein the cassettes  100  are stored, are provided in the lower portion of the printer main body  1001 . Partitions  106  and  107  delimit the cassette storage portions  1005 , and are closed at a predetermined tightness. Temperature and humidity sensors  108 , which are provided for the individual cassettes  1005 , are means for detecting the temperature and humidity in the vicinities of the cassettes  100  stored in the cassette storage portions  1005 . These sensors  108  can independently detect the temperatures and humidities in the cassette storage portions  1005 . 
   A large capacity paper deck  1010  is detachably mounted as an option. A sheet feeding apparatus  1002  and a lifter support (not shown) are provided for the paper deck  1010 , as well as for the printer main body  1001 . The paper deck  1010  is closed at a predetermined tightness, and a temperature and humidity sensor  108  is provided to detect the temperature and the humidity in the paper deck  1010 . 
   The sheet conveying apparatus  1004  includes conveying roller pairs  105  and a registration roller portion constituted by a pre-registration roller pair  130  and a registration roller pair  110 . A sheet S, fed by the sheet feeding apparatus  1002 , is conveyed by the conveying roller pair  105  along a sheet conveying path  1008  formed by a guide plate, and is introduced to the registration roller pair  110 . Thereafter, the sheet S is conveyed by the registration roller pair  110  to the image forming portion  1003 . 
   The image forming portion  1003  includes a photosensitive drum  112 , the laser scanner  111   a , a developing device  114 , a transfer charging device  115 , and a separating charging device  116 . For image forming, a laser beam emitted by the laser scanner  111   a  is reflected by a mirror  113  and projected onto an exposure position  112   a  on the photosensitive drum  112 , which is rotated clockwise. As a result, a latent image is formed on the photosensitive drum  112  and is thereafter visualized as a toner image by the developing device  114 . 
   The toner image on the photosensitive drum  112  is transferred to the sheet S at the transfer portion  112   b  by the transfer charging device  115 . The sheet S bearing the toner image is then electrostatically separated from the photosensitive drum  112 , by the separating charging device  116 , and is conveyed along a conveying belt  117  to a fixing apparatus  118  to fix the toner image. Thereafter, the resultant sheet P is discharged by discharging rollers  119 . A sheet discharging sensor  119   a , which detects the passage of the sheet P that is to be discharged, is located along the conveying path extending between the fixing apparatus  118  and the sheet discharging roller  119 . 
   In this embodiment, the printer main body  1001  and the scanner  2000  are separate members; however, they may be integrally formed. Regardless of whether the printer main body  1001  is separately or integrally formed with the scanner  2000 , the printer main body  1001  can either function as a copier when the laser scanner  111   a  receives a signal from the scanner  2000  or as a facsimile machine when a facsimile signal is received, or can also function as a printer when a signal is received from a personal computer. 
   Further, when a signal obtained by the image processing portion  206  of the scanner  2000  is to be transmitted to another facsimile machine, the printer main body  1001  can also function as a facsimile machine. In addition, an automatic document feeding apparatus  250 , indicated by a dashed double-dotted line, may be mounted instead of the pressing plate  203 , so that a document can be automatically read. 
     FIG. 2  is a plan view of the configuration of the sheet feeding apparatus  1002 , and  FIG. 3  is a side cross-sectional view of the sheet feeding apparatus  1002 . In this embodiment, the cassettes  100  are inserted into, or removed from the cassette storage portions  1005  in a widthwise direction, perpendicular to the sheet conveying direction. 
   In  FIG. 2 , side regulation plates  1  and  2  control the widthwise position of the sheets S stored in each of the cassettes  100 , and can be displaced in the widthwise direction in accordance with the width of the sheets S. A rear end regulation plate  3  controls the position of the sheets S at the rear, in the sheet conveying direction, and can be displaced in the sheet conveying direction in accordance with the length of the sheets S. 
   The cassette  100  can be pulled along rails  19  and  20  in  FIG. 3 . When a user sets up the cassette  100 , he or she need only pull the cassette  100  out from the front of the printer main body  1001 . As is shown in  FIG. 2 , a protrusion  100   a  is formed for each cassette  100 . When the cassette  100  is stored in the cassette storage portion  1005 , the protrusion  100   a  is detected by a cassette attachment and detachment detection sensor  17  that is provided for the cassette storage portion  1005 . 
   A detection signal obtained by the cassette attachment and detachment detection sensor  17  is transmitted to the controller  120 , which employs the received detection signal to determine whether the cassette  100  is attached to the cassette storage portion  1005  or has been pulled out. 
   In each of the cassettes  100 , as is shown in  FIG. 3 , a lifter support  16  is provided as elevatable sheet stacking means used to mount the sheets S. As the cassette  100  is inserted or removed, the lifter support  16  is elevated or lowered by a lifter motor  18  in  FIG. 4 . 
   For example, when a user stores a cassette  100  in which sheets S are mounted, and when the controller  120  detects this based on a signal received from the cassette attachment and detachment detection sensor  17 , the controller  120  drives the lifter motor  18  to elevate the lifter support  16 . Then, when the user pulls out the cassette  100  to set sheets S and the controller  120  detects this, based on a signal received from the cassette attachment and detachment detection sensor  17 , and the controller  120  drives the lifter motor  18  to lower the lifter support  16  to a lower limit position. 
   At the upper portion of each of the cassette storage portions  1005 , a sheet surface position detection sensor  15  is provided to determine whether the face of the uppermost sheet S 1  mounted on the lifter support  16  is appropriately positioned for sheet feeding, i.e., to determine whether the face of the uppermost sheet has reached the sheet feeding position. 
   When the lifter support  16  is to be elevated, the rotation of the lifter motor  18  is continued until the sheet surface position detection sensor  15  detects the position of the face of the uppermost sheet S 1 . When the sheet surface position detection sensor  15  detects the uppermost sheet S 1 , the controller  120 , based on a detection signal received from the sheet surface position detection sensor  15 , halts the lifter motor  18 . Through this processing, an appropriate height can be maintained for the sheet S 1 . 
   As the sheet feeding operation is initiated, the sheets S are sequentially fed, from the uppermost location, and as the height of the sheets is gradually reduced and the sheet surface position detection sensor  15  is turned off, the controller  120  drives the lifter motor  18  again to elevate the lifter support  16  Through-this processing, the height of the face of the uppermost sheet can be constantly controlled, within a predetermined range. 
   As is described above, for coated sheets, an attraction phenomenon occurs in high humidity. The clarification of an attraction mechanism, obtained by the present inventor, will now be explained. 
     FIG. 5  is a graph showing the results of an attractive force measurement experiment conducted in advance in order to clarify the attraction mechanism. For the attractive force measurement experiment, the attractive forces for two types of coated sheets (coated sheets A and coated sheets B) and standard sheets were measured in different environments. In  FIG. 5 , the horizontal axis represents the relative humidity during the experiment and the vertical axis represents the attractive force, at a fixed temperature of 30° C. 
   As is apparent from in  FIG. 5 , the results obtained for the coated sheets A and B were extremely different from those for the standard sheets, and the attractive force readings for the coated sheets A and B depended very much on the humidity. In an environment wherein the relative humidity was 40% or lower, for all sheets, including the standard, almost no attractive forces occurred, while when the relative humidity exceeded 40%, the attractive forces increased linearly. The same measurements were conducted at temperatures of 20° C. and 40° C., and the same results were obtained. Based on the results, it was found that the attractive force for the coated sheets depended more on the relative humidity than on the absolute amount of water contained in the air. 
   Through various experiments conducted by the present inventor, the attraction mechanism for the coated sheets can be explicated as follows. 
   As is shown in  FIG. 6A , when a sheet stack SA of coated sheets is exposed in a high relative humidity environment, moisture absorption occurs only on the obverse surface of the uppermost sheet S 1  of the sheet stack SA and on the side edge portions. When moisture is absorbed, as is shown in  FIG. 6B , the obverse surface of the uppermost sheet S 1  is elongated and the side edge portions of the sheet stack SA swell. 
   Since the reverse surface of the uppermost sheet S 1  is less elongated than the obverse, as is shown in  FIG. 6C , a convex deformation phenomenon of the uppermost sheet S 1  occurs. While since the coated sheets are very smooth and do not transmit much air, substantially no air flows between the sheets. Therefore, when the convex deformation phenomenon of the uppermost sheet S 1  occurs, a defined volume, between the uppermost sheet S 1  and the second sheet S 2 , is increased, a negative pressure is generated, and the second sheet S 2  is attracted to the uppermost sheet S 1 . This phenomenon is hereinafter called attraction to the attraction of the uppermost sheet through the absorption of moisture. 
   When the moisture absorption occurs on the side edge portions of the sheet stack SA for sheets other than the uppermost sheet S 1 , the center of the sheet stack SA does not swell while the side edge portions do. Thus, the volume is increased in the direction of the thickness of the sheets, and a negative pressure is generated between the sheets that causes the sheets to attract to each other. This phenomenon is hereinafter called attraction through moisture absorption by the side edge portions. 
   Furthermore, as is shown in  FIG. 6D , when the convex deformation occurs on the second coated sheet S 2  because of the convex deformation of the uppermost sheet S 1 , a negative pressure is generated between the second coated sheet S 2  and the third coated sheet S 3 , and the sheets S 2  and S 3  attract to each other. This phenomenon is called attraction through chain deformation. The attraction through chain deformation may occur for several tens of sheets, from the third sheet down. 
   As is described above, for the coated sheet attraction mechanism in high relative humidity are three types of attraction phenomena, the attraction through moisture absorption by the uppermost sheet, the attraction through moisture absorption by the side edge portions, and the attraction through chain deformation. Since these three attraction phenomena are caused by the swelling or the elongation of the coated sheets through moisture absorption, and the generation of a negative pressure, the attraction phenomena can be prevented and negative pressure removed by the flow of air between the coated sheets. Further, since the temperature of the air flow is increased, the coated sheets can be dehumidified and dried, and protected from swelling, and the phenomenon, where coated sheets again attract to each other, can be prevented. 
   Therefore, according to the embodiment, as is shown in  FIGS. 2 and 3  as previously explained, in the side regulation plate  2  that is located to the rear in the widthwise direction, a plurality (two in this embodiment) of air duct ports  2   a  and  2   b  are formed at a predetermined interval in the sheet conveying direction, and at a height that, at the least, corresponds to the side edge of the sheet S that is located at the position for sheet feeding. Ducts  9  and  12  are provided wherein fans  4  and  5 , which are air flow means, are mounted, upstream of the air duct ports  2   a  and  2   b . The fans  4  and  5  blow air onto the sheets S through the air duct ports  2   a  and  2   b.    
   Between the fans  4  and  5  and the air duct ports  2   a  and  2   b , shutters  10  and  11  are provided that are elevatable by a swing motor  13  and an elevating mechanism (not shown). As air is blown onto the sheets S, the shutters  10  and  11  are gradually swung vertically to direct the air so that it sequentially flows between the sheets S, and the effect produced by raveling-out the sheets is increased. 
   The fans  4  and  5  and the swing motor  13  are independently driven in accordance with signals transmitted, by the controller  120 , via fan driver circuits  4   a  and  5   a  and a swing motor driver circuit  13   a  shown in  FIG. 4 . 
   Furthermore, as is shown in  FIG. 2 , air heating means  8 , which includes a heater  6  and a heat sink  7 , is provided near an air inlet  9   a  for the duct  9  that leads to the air duct port  2   a  on the pickup roller side. The air heating means, which is located upstream in the direction in which the fan  5  blows air, heats air supplied through the air inlet  9   a  in the direction indicated by arrows, and expels warm air through the air duct port  2   a.    
   A thermistor  7   a  is attached to the heat sink  7  to detect the temperature of the surface of the heat sink  7 , and a detection signal is transmitted by the thermistor  7   a  to the controller  120 , as is shown in  FIG. 4 . In accordance with the detection signal received from the thermistor  7   a , the controller  120 , via the driver circuit  6   a , turns the heater  6 , of the air heating means  8 , on or off, so as to adjust the temperature of the warm air supplied through the air duct port  2   a.    
   As is shown in  FIG. 2 , the fans  4  and  5 , the ducts  9  and  12 , the air heating means  8  and the shutters  10  and  11  are integrally attached to the side regulation plate  2  located to the rear in the widthwise direction. With this arrangement, when sheets S having the size shown in  FIG. 2  are exchanged for smaller sheets S 2  shown in  FIG. 7 , the fan  5  and the other components are moved, together with the side regulation plate  2  located to the rear in the widthwise direction, so that the positional relationship, relative to the ends of the sheets S 2 , can be continuously maintained. 
   In this case, when the rear ends of sheets S, such as the small sheets S 2  shown in  FIG. 7 , do not reach the air duct port  2   b  located downstream in the sheet conveying direction, and when the fan  4  is driven, the air supplied by the fan  4  is wasted. 
   Therefore, a sheet size detection sensor  14 , as shown in  FIG. 14 , is provided for the cassette  100  to detect the sheet size in accordance, for example, with the locations of the side regulation plates  1  and  2  and the rear end regulation plate  3 . And when, in accordance with a sheet size data signal received from the sheet size detection sensor  14 , the controller  120  determines the sheets S stored in the cassette  100  are small, it independently halts the fan  4 . 
   Through this process, a negative pressure is eliminated by flowing air between the sheets S, and the temperature of the air is increased to dehumidify and dry the wet, coated sheets, to prevent them from swelling and to prevent the occurrence of attraction. 
   The present inventor found through an experiment that, as the characteristic of the coated sheets, the attractive force reached its highest level immediately after a package of coated sheets was opened. 
     FIG. 8  is a graph showing data obtained by measuring the temporal change in the attractive force of the coated sheets immediately after a package of the coated sheets was opened. In  FIG. 8 , the vertical axis represents attractive force, and the horizontal axis represents time. The environment for the measurement was a temperature of 30° C. and a relative humidity of 80%. 
   As is apparent from the measurement results shown in  FIG. 8 , the attractive force of the coated sheets is highest immediately after the package is opened, and gradually reduces as time elapses. That is, the attractive force of coated sheets is highest immediately after a cassette  100 , in which coated sheets have been stored by a user, has been loaded into the cassette storage portion  1005 . The attraction phenomenon is hereafter called attraction immediately after a package of coated sheets is opened. 
   Next, the present inventor supplied air at a high temperature to coated sheets that were attracted to each other, raveling-out the sheets, and measured the temporal change in the attractive force.  FIG. 9  is a graph showing data obtained by measuring the temporal change in the attractive force in a state wherein the attractive force was released. In  FIG. 9 , the vertical axis represents the attractive force and the horizontal axis represents time. 
   As is apparent from the measurement results shown in  FIG. 9 , the attractive force is eliminated immediately after the sheets are raveled out; however, the re-attraction of sheets is begun as time elapses, and a considerably high attractive force is generated, although it is not as high as the attractive force immediately after a package is opened. Hereinafter, this attraction phenomenon is called re-attraction occurring as time elapses. The present inventor found that re-attraction occurring-as time elapses and attraction immediately after a package was opened also cause double feeding and erroneous feeding. 
   Furthermore, in order to examine the affect on an image (transfer performance) when a coated sheet is partially dried using air at a high temperature, the present inventor blew warm air at 45° C. onto a coated sheet for one minute at a temperature of 30° C. and a relative humidity of 80%, and at a temperature of 5° C. and a relative humidity of 10%, and measured the water content in part of the coated sheet. As a result, in the environment at a temperature of 30° C. and a humidity of 80%, uneven water content was almost not observed, while in the environment at a temperature of 5° C. and a humidity of 10%, a considerably uneven water content was observed. 
   Further, when the image forming portion  1003  transferred an image to the coated sheet used in the environment at a temperature of 30° C. and a humidity of 80%, no problems occurred. However, when the image was transferred to the coated sheet used in the environment at a temperature of 5° C. and a humidity of 10%, the transfer performance was deteriorated at the portion having a small water content, and a satisfactory density could not be obtained. 
   That is, through the experiment performed by the present inventor, in an environment at a high temperature and a high humidity, the coated sheets absorb considerably moisture, and image forming is not affected by raveling-out these sheets using air at a high temperature. However, since the coated sheets do not attract to each other in an environment at a low temperature and a low humidity, the sheets need not be raveled out by air blown at a high temperature, and if air is blown onto the sheets, an image defect occurs. 
   The present inventor also found that an image detect due to a transfer failure in the low humid environment was correlated not only with the temperature of the air, but also with the period air was blown and the air flow rate. 
   Based on these obtained results, in this embodiment, the following arrangement is employed. 
   Since the coated sheets tend to attract to each other immediately after a package is opened, when a cassette  100  is loaded into the cassette storage portion  1005 , and when the surface of the uppermost coated sheet is detected by the sheet surface detection sensor  14 , i.e., when the stack of coated sheets reaches a position for sheet feeding, air blown to fan the sheets for a predetermined time T 1  is enough. This operation is hereinafter called the initial swing operation. 
   Furthermore, before the sheet feeding is initiated, air is blown for a predetermined time T 2  to sufficiently fan the sheets, This operation is hereinafter called a pre-job swing operation. 
   In addition, as is described above, since the coated sheets strongly attract to each other in a high relative humidity environment, and do not attract in a low relative humidity environment, the temperature of the heater  6  must be designated in accordance with the environment. 
   When a predetermined period, i.e., a period beginning after a cassette  100  is loaded into the cassette storage portion  1005  and is raised to the position for sheet feeding, and continuing until the sheet feeding operation is started, or a period beginning after that, since the sheet feeding operation initiated and continued until the next sheet feeding operation is started is long, the sheets may not be appropriately raveled out by the pre-job swing operation. 
   In this embodiment, therefore, a waiting operation interval T 3 , which is an operating interval time (waiting time) for the swing operation in the waiting state, is determined in accordance with the detection results obtained by the temperature and humidity sensor  108 . When the waiting operation interval T 3  has elapsed, air is blown during a waiting swing operation time T 4 . This operation is hereinafter called a swing operation on waiting. When the swing operation on waiting is repeated until the sheet feeding operation is started, the re-attraction as time elapses, which occurs while the sheet feeding apparatus  1002  is in the waiting state, can be eliminated. 
     FIGS. 10 to 13  are control tables for optimal air blowing periods (the initial swing time T 1  and the pre-job swing time T 2 ), the temperature of air (temperature adjusted by the heater  6 ), the waiting operation interval T 3 , the waiting swing operation time T 4 , and a halt time T 5  for the swing operation, during a job that will be described later, all of which the prevent inventor defined while taking into account the affect of the transfer performance in each environment wherein the sheet feeding apparatus  1002  was employed. 
   The air blowing period control table for the initial swing operation and the pre-job swing operation, the heating temperature control table and the swing operation control table, which is a time control table shown in  FIGS. 10 to 13 , and a fan air flow rate control table (not shown) are stored in storage means  30  in  FIG. 4 . 
   When a sheet type input portion  21  included in an operating portion in  FIG. 4  is employed, for example, to enter coated sheets to be stored in the cassette  100 , and when the cassette  100  is loaded into the cassette storage portion  1005 , the initial swing operation is performed for the predetermined time T 1  in accordance with the environmental condition of the cassette storage portion  1005  or the cassette  100 . 
   For resin sheets, such as OHP or art films, since attraction immediately after a package is opened or re-attraction as time elapses does not occur in a high relative humidity environment, the initial swing operation, the pre-job swing operation and the swing operation in the waiting state need not be performed. Further, since the attraction mechanism for these sheets is attraction due to charging, the air need not be heated by the heater  6 . Therefore, a period required for temperature adjustment to be completed by the heater  6  can be removed. 
   Further, since standard sheets do not originally attract to each other, raveling-out of these sheets using air is not required during the sheet feeding operation. As is described above, since the initial swing operation, the pre-job swing operation, the swing operation in the waiting state, and the temperature control operation by the heater  6  are not performed if not necessary, the FCOT is quickly ready, and for a user, the usability of a printer can be increased. 
   The initial swing operation will now be described while referring to a flowchart in  FIG. 14 . 
   When the cassette  100  is loaded into the cassette storage portion  1005 , and when the cassette attachment and detachment detection sensor  17  detects this and is in the ON state (Y at step  1 ), the controller  120  rotates the lift motor  18  to raise the lifter support  16  (step  2 ). Then, the level of the sheet stack is gradually raised, together with the lifter support  16 , until the sheet surface position detection sensor  15  detects the surface of the uppermost sheet and is set in the ON state (Y at step  3 ) Thereafter, the lift motor  18  is halted (step  4 ) 
   Next, the temperature and humidity sensor  108  detects the temperature and the humidity in the cassette storage portion  1005  (or the cassette  100 ) (step  5 ), and based on the temperature and humidity reading thus obtained, data for the temperature of the heater  6  and the initial swing time T 1  are read from the control tables shown in  FIGS. 10 and 12  (step  6 ). Then, to adjust the temperature of the heater  6 , the heater  6  is rendered conductive via the heater driver circuit  6   a  (see  FIG. 4 ). 
   When the temperature control operation performed by the heater  6  is completed (Y step  7 ), the fans  4  and  5  and the swing motor  13  are turned on (step  8 ). Following which, when the initial swing time T 1  obtained from the control table has elapsed (Y at step  9 ), the fans  4  and  5  and the swing motor  6  are turned off (halted) (step  10 ). 
   Through this processing, immediately after a package of coated sheets is opened, warm air can be blown onto the coated sheets to remove attractions, so that the coated sheets can be appropriately raveled out. As a result, a reliable sheet feeding apparatus can be provided that prevents the occurrence of a paper jam or double feeding. In addition, since the initial swing time T 1  and the adjusted temperature are designated based on the optimal tables that have been determined, through experiment, to establish both the coated sheet raveling-out capability and the image quality, image deterioration, such as a transfer failure, does not occur. 
   When there is a possibility that the coated sheets are not appropriately raveled out during the initial swing operation, the job start instruction may be rejected until the initial swing operation has been completed, or a job may be started after the job start instruction has been accepted and the initial swing operation has been completed. 
   While referring to a flowchart in  FIG. 15 , an explanation will now be given for the pre-job swing operation performed before the sheet feeding operation is started in order to eliminate the occurrence of re-attraction as time elapses. 
   When a user depresses a job start button, first, the temperature and humidity sensor  108  detects the temperature and the humidity in the cassette storage portion  1005  (the cassette  100 ) (step  21 ), and based on the obtained temperature and humidity, data for the pre-job swing time T 2  and the adjusted temperature for the heater  6  are read from the control tables in  FIGS. 11 and 12  (step  22 ). 
   Thereafter, the controller  120  renders the heater  6  conductive to adjust the temperature of the heater  6 , and when the temperature control operation for the heater  6  has been completed (Y at step  23 ), the fans  4  and  5  and the swing motor  13  are turned on to perform the pre-job swing operation (step  24 ). When the pre-job swing time T 2  obtained from the control table has elapsed (Y at step  25 ), the sheet feeding operation is initiated (step  26 ). And when a predetermined job is terminated, i.e., when the final sheet for the job has been fed (Y at step  27 ), the fans  4  and  5  and the swing motor  13  are turned off (halted) (step  28 ). 
   Through this processing, before the sheet feeding operation is initiated for coated sheets that have been exposed from a package for a while; warm air can be blown onto the coated sheets to eliminate re-attraction, and the coated sheets can be appropriately raveled out. 
   Next, while referring to the flowchart in  FIG. 16 , an explanation will be given for the swing operation in the waiting state performed when there has been a long sheet feeding waiting time. The swing operation in the waiting state in  FIG. 16  is performed for a long waiting time when the period since the cassette  100 , loaded in the cassette storage portion  1005 , reached a sheet feeding enabled position until the sheet feeding operation began is extended. 
   When the above described initial swing operation in  FIG. 14  has been completed, based on the temperature and humidity obtained by the temperature and humidity sensor  108 , the controller  120  reads, from the time control table in  FIG. 13 , the predetermined waiting operation interval T 3 , following which the swing operation in the waiting state is started in accordance with the temperature and humidity, and the waiting swing operation time T 4 , which is a predetermined time during which the swing operation in the waiting state is performed (step  31 ). Then, the controller  120  renders the heater  6  conductive via the heater driver circuit  6   a  to adjust the temperature of the heater  6 . 
   Following this, a time  109  (see  FIG. 4 ) is activated (step  32 ), and the start of the sheet feeding operation is waited for (step  33 ). Specifically, the controller  120  waits until the user depresses the job start button. When the sheet feeding operation is not begun (N at step  33 ), and when the waiting operation interval T 3  has elapsed (Y at step  34 ), the fans  4  and  5  and the swing motor  13  are turned on (step  35 ). Thereafter, when the waiting swing operation time T 4  previously obtained from the time control table has elapsed (Y at step  36 ), the fans  4  and  5  and the swing motor  13  are turned off (halted) (step  37 ). 
   For a case wherein there is a long waiting time since a sheet feeding operation was performed before the next sheet feeding operation is started, at step  27  in  FIG. 15 , the final sheet for the job is fed, at step  28 , the fans  4  and  5  and the swing motor  6  are turned off, and the controller  120  performs the same process as in  FIG. 16 . The swing operation in the waiting state is repeated until the sheet feeding operation is initiated. 
   Furthermore, the pre-job swing operation and the sheet feeding operation shown in the flowchart in  FIG. 17  may be performed. The processes at steps  41  to  47  in  FIG. 17  are the same as those at steps  21  to  27  in  FIG. 15 . 
   In the flowchart in  FIG. 17 , after a predetermined job has been terminated, i.e., after the final sheet for the job has been fed, and when, at step  48 , the job swing operation halt time T 5 , which is obtained from the time control table, has elapsed (Y at step  48 ), the fans  4  and  5  and the swing motor  13  are turned off (step  49 ). 
   Since the fans  4  and  5  and the swing motor  13  are halted after the job swing operation halt time T 5  has elapsed, the coated sheets can be appropriately and smoothly raveled out in the next pre-job swing operation. 
   As is described above, when a predetermined waiting time that a sheet is not fed has elapsed since it was detected that the sheets had reached the sheet feeding enabled position, or since the sheet feeding operation was finished, the air blowing operation need only be performed during a predetermined period to eliminate the attraction between the sheets that occurs while waiting for the sheet feeding operation. Thus, an image defect, such as an image failure, does not occur, and various types of sheets, such as coated sheets, OHP sheets, art films and very thick paper sheets, can be individually separated and fed. Further, the next sheet feeding operation can be performed smoothly. For each swing operation, only the air blowing operation may be performed while the vertical movements of the shutters  10  and  13  are halted. 
   In addition, since the temperature for the heater  6  is set in accordance with a signal received from the temperature and humidity sensor  108  located near the cassette  100 , a satisfactory sheet feeding function and a high-quality image, without a defect such as an image failure, can be provided. 
   In this embodiment, the operation for the coated sheets has been explained in detail. However, the present invention is not limited to the coated sheets, and control tables may be prepared for OHP films, art films, very thick paper and other standard paper, in addition to the coated sheets for which the characteristic differs depending on the environment. 
   For example, as is described above, for an OHP file or an art film, since attraction in a low relative humidity environment occurs as a result of charging, air must be blown at a high flow rate in a low relative humidity environment, while since in a high relative humidity environment attraction by charging almost does not occur, air can be blown at a low flow rate. Further, since resin sheets do not absorb water, warm air is not required, and therefore, the heater can be turned off. In addition, since re-attraction as time elapses does not occur for these types of sheets, the swing operation in the waiting state need not be performed. 
   For very thick paper, the conveying resistance is increased by its own weight, and a pickup failure occurs. Thus, environmental dependency is not present, and the blowing of air is required in all environments. Further, since the attraction by moisture absorption does not occur for very thick paper, as well as the OHP, warm air is not required, and the heater can be turned off. Furthermore, since re-attraction as time elapses does not occur, the swing operation in the waiting state is not required. 
   As is described above, the optimal control tables for the heater temperature, the air flow rate and the air blowing period may be prepared for each type of sheet material, the sheet type input portion  21  shown in  FIG. 4  may be provided as sheet type input means, and the controller  120  may select and employ one of the time control tables in accordance with the sheet type data obtained from the sheet type input portion  21 . Further, since the attractive characteristic and the transfer characteristic differ depending on the type and brand of coated sheet, optimal control tables may be provided for each type and brand. Thus, a more reliable sheet feeding apparatus can be provided. 
   Furthermore, a data input portion  22  in  FIG. 4  may be provided to rewrite data in the time control table or the temperature control table, or to add a new table, and a user or a maintenance person may employ the data input portion  22  to freely create and store each of the above described control tables in accordance with the purpose. 
   In this embodiment, the fans  4  and  5  and the air duct ports  2   a  and  2   b  are located on the side (at one end in the direction of the width of a sheet) of a sheet stack that is mounted on the lifter support  16 , and the air is blown onto the side end of the sheet stack. However, the present invention is not limited to this arrangement, and can be applied for a configuration wherein air duct ports are provided to the front, in the direction in which the mounted sheets are fed, and air is blown onto the front end of the sheet stack. 
   Furthermore, since the initial swing operation, the pre-job swing operation and the swing operation in the waiting state are performed for the sheet deck, a image defect such as an image failure does not occur, and the individual sheets can be appropriately separated and fed. 
   Further, although in this embodiment the retard system has been employed as sheet separating means, a Duplo system or an air feeding system may be employed. 
   This application claims priority from Japanese Patent Application No. 2003-301028 filed on Aug. 26, 2003, which is hereby incorporated by reference herein.