Patent Publication Number: US-6341861-B1

Title: Sheet conveying apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a conveying apparatus disposed between first and second processing portions having independent conveying systems, and more particularly, it relates to a sheet conveying apparatus disposed between an image forming means and a fixing means. 
     2. Related Background Art 
     As disclosed in Japanese Patent Application Laid-open No. 4-73152, there is an image forming apparatus having an image forming means as a first processing portion and a fixing means as a second processing portion. 
     In such an image forming apparatus, a sheet in which cationic polymer (PVA) as an ink holding layer is coated on a RET (polyethylene terephthalate) film as a substrate and a white ink transporting layer is coated on the ink holding layer is used as a recording material. The ink transporting layer is obtained by mixing activator and polyethylene particles by using ionomer as binder. 
     An image is recorded by an ink jet recording head (image forming means) of line type covering the entire width of the recording material, and the ink transporting layer is melted by a pair of pressure/heat rollers (fixing means) disposed adjacent to and at a downstream side of the recording head to make the transporting layer smooth and transparent, thereby forming a transparent smooth layer coating the imaged surface of the recording material. 
     In the above-mentioned image forming apparatus, since image accuracy is greatly influenced by conveying accuracy of the image forming means, it is desirable that the image forming means is not subjected to a force from a conveying system of the fixing means as less as possible. 
     When a distance between the processing portions is selected to become greater than a length of a maximum sheet to be treated, since the sheet is conveyed to the second processing portion after it leaves the first processing portion, the interference between the processing portions can be prevented. 
     However, if a sheet to be treated has a considerably great length, the entire apparatus including the sheet conveying apparatus becomes bulky accordingly. 
     SUMMARY OF THE INVENTION 
     The present invention aims to eliminate the above-mentioned conventional drawback, and an object of the present invention is to provide a compact conveying apparatus which is disposed between the processing portions and which can cope with recording materials having various sizes without interference between conveying systems of the two processing portions. 
     To achieve the above object, according to the present invention, there is provided a conveying apparatus disposed between first and second processing portions having independent conveying systems and including a convey path for feeding a processed medium conveyed from the first processing portion to the second processing portion, in which the convey path is divided into first and second convey path portions, and an escape portion for temporarily escaping the processed medium conveyed in the first convey path portion and for conveying the processed medium to the second convey path portion with a tail end of the processed medium facing forwardly is provided between the first and second convey path portions, and there is provided a conveying means which does not apply any load to the processed medium when the processed medium is being conveyed by the conveying system of the first processing portion and starts to apply a conveying force to the processed medium after the conveyance of the processed medium effected by the conveying system of the first processing portion is finished. 
     In the specification, “processing” means the fact that the processed medium (sheet) is subjected to various phenomena for changing quality of the sheet. The “sheet” means all of sheet-shaped processed media, as well as a cut sheet and an OHP sheet. 
     With the arrangement as mentioned above, when the conveyance in the first processing portion is finished, the conveying means starts to convey the processed medium in a direction along which the tip end of the processed medium is escaped. After the processed medium is conveyed until the tail end of the processed medium reaches the vicinity of the escape portion, the conveying means is rotated reversely so that the processed medium starts to be conveyed toward the second convey path portion with the tail end of the processed medium facing forwardly. 
     The conveying means may be provided at the escape portion. 
     It is preferable that the convey path is bent around the escape portion. 
     It is preferable that the conveying means comprises a roller shiftable in an up-and-down direction. 
     The conveying means may comprise a D-cut roller. 
     The D-cut roller is a roller in which a part of periphery of a roller is cut away. 
     There may be provided a shutter for directing the tail end of the processed medium conveyed from the first convey path portion to the second convey path portion. 
     The convey means may comprise a vertically shiftable roller provided in the first convey path portion, a D-cut roller provided at the escape portion, first and second sheet (processed medium) detecting means provided in the first convey path portion, and convey rollers provided in the second convey path portion and may be designed so that the above elements are disposed to satisfy a relation “L3&lt;L1&lt;L2&lt;L” when a length of an arc of the D-cut roller is L, a distance from a nip point of the vertically shiftable roller to the D-cut roller is L1, a distance from the first sheet detecting means to the D-cut roller is L2 and a distance from the D-cut roller to an most upstream convey roller (among the convey rollers) is L3, and the second sheet detecting means is disposed immediately in front of the escape portion. 
     It is preferable that, in a sheet conveying apparatus, the processed medium treated by the above-mentioned conveying apparatus is a sheet and the first processing portion is an image forming means. 
     As mentioned above, the “sheet” means all of sheet-shaped processed media, as well as a cut sheet and an OHP sheet. 
     The second processing portion may be a fixing means. 
     At least one roller having a driving force may be disposed in the second convey path portion between the escape portion and the fixing means, and such a roller may be a one-way roller driven by a conveying force of the fixing means not to generate slip between the sheet and the roller. 
     A plurality of rollers may be provided in the second convey path portion, and, by a conveying force of a most downstream roller among the rollers contacted with the sheet, the other rollers may be rotatingly driven. 
     A temperature of a surface of the sheet may be increased before the fixing by fixing heat generated by the fixing means. 
     The present invention further provides an image forming apparatus comprising the above-mentioned sheet conveying apparatus, an image forming means as the first processing portion, and a fixing means as the second processing portion. 
     The present invention also provides an image forming apparatus for forming an image on a processed medium obtained by coating ink permeable polymer film on a substrate, wherein, after the image is formed on the processed medium by the image forming means in an ink jet manner, the processed medium is thermally treated by the fixing means to make the polymer film finer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1,  2 ,  3 ,  4  and  5  are enlarged views for explaining an operation of a sheet conveying apparatus according to a first embodiment of the present invention; 
     FIG. 6 is a sectional view of an image forming apparatus to which the sheet conveying apparatus according to the first embodiment is applied; 
     FIG. 7 is a perspective view of the image forming apparatus of FIG. 6; 
     FIG. 8 is a sectional view of a sheet used in the sheet conveying apparatus according to the first embodiment; 
     FIG. 9 is a flow chart showing an operation sequence of the sheet conveying apparatus according to the first embodiment; and 
     FIG. 10 is a sectional view of an image forming apparatus to which the sheet conveying apparatus according to a second embodiment of the present invention is applied. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be explained in connection with embodiments thereof with reference to the accompanying drawings. However, dimensions, materials, configurations and relative positions of constructural elements described in the embodiments do not limit the present invention, except for special mention. 
     First Embodiment 
     An arrangement in which a sheet conveying apparatus according to a first embodiment of the present invention is applied to an image forming apparatus is shown in FIGS. 1 to  9 . 
     Among these Figures, FIGS. 1 to  5  are enlarged views for explaining an operation of the sheet conveying apparatus, FIG. 6 is a sectional view of the image forming apparatus, FIG. 7 is a perspective view of the image forming apparatus, FIG. 8 is a sectional view of a sheet used in the sheet conveying apparatus, and FIG. 9 is a flow chart showing an operation sequence of the sheet conveying apparatus. 
     Construction of Sheet 
     Since the image forming apparatus according to the first embodiment mainly treats special media and has various features corresponding to features of the media (sheets), a construction of the sheet will firstly be described. 
     FIG. 8 shows the sheet pinched between fixing rollers. As shown in FIG. 8, the sheet is constituted by four layers, i.e., a latex layer (ink permeable polymer film)  41   a , a coloring layer  40  mainly absorbing ink, a substrate  39  formed from a RET (polyethylene terephthalate) sheet or a baryta sheet, and a back coating  38 . 
     As shown, the latex layer  41   a  having the noticeable feature is melted by heat (110-180° C.) of a fixing roller  14   a  and is changed to a transparent laminated layer  41   b  after cooling. As a result, gloss, water-resistance and anti-weather of an image are greatly improved. 
     Entire Construction of Apparatus 
     In FIGS. 6 and 7 showing the entire apparatus, an image forming apparatus  1  comprises a sheet supplying system, an image forming system as a first processing portion, a conveying system (switch-back conveying system) as a convey path or a convey means including an escape portion, and a fixing system as a second processing portion. The image forming system and the fixing system are disposed side by side in a vertical direction so that sheet convey path portions are disposed side by side in a vertical direction. 
     The sheet supplying system includes an upper lid  21  which can be opened and closed around a hinge  21   a  so that a cartridge  3  containing a roll-shaped sheet  2  can be incorporated into the sheet supplying system, a pair of pick-up rollers  20 ,  20   a  for drawing the sheet from the cartridge  3 , and a pair of pulling rollers  22 ,  22   a  for sending the sheet to the image forming system. A loop having a substantially constant amount is formed in the sheet between the pair of pick-up rollers  20 ,  20   a  and the pair of pulling rollers  22 ,  22   a . A nip between the paired pulling rollers  22  and  22   a  can be released by a cam (not shown). 
     The image forming system as the first processing portion mainly includes an image forming portion  5  of ink jet type, a line feed roller (LF roller)  6  which is a conveying system as a second conveying means for effecting conveyance of the sheet during image formation, an LF driven roller  6   a , a spurred roller  7   a , and a downstream LF roller  7  so that the sheet conveyed from the sheet supplying system can be conveyed intermittently by a predetermined length Li (about 10 mm). The image forming system is constituted by the image forming portion  5 , LF roller  6 , LF driven roller  6   a , downstream LF roller  7  and spurred roller  7   a . Further, a sheet insertion opening  25  and an open/close switch (not shown) for the sheet insertion opening are provided to permit a sheet manual insertion. 
     A cutter  8  for cutting the sheet to a desired length is disposed between the image forming system and the switch-back conveying system. 
     The switch-back conveying system comprises a sheet conveying apparatus for conveying the sheet between the image forming system and the fixing system which are not synchronous with each other mechanically and are not interfered with each other. Further, there are provided a shutter  9 , a D-cut roller  10  as a conveying means, and a plurality of pairs of convey rollers  12  disposed along a conveying direction. The convey path is divided into a first convey path portion  13   a  and a second convey path portion  13   b  before and after the D-cut roller  10 . An escape portion  11   b  is provided so that the sheet conveyed through the first convey path portion is temporarily escaped from the convey path to a discharge tray  11  as a guide without fail, and the entire convey path is bent at the escape portion  11   b.    
     Lower rollers of the plural pairs of rollers  12  are rotatably mounted on an open/close door (not shown) so that a jammed sheet can easily be removed. 
     The fixing system includes a heating means or fixing means  14  comprised of a pair of fixing rollers including a heat roller  14   a  and a pressure roller  14   b.    
     Operation of Apparatus 
     When a new cartridge  3  is loaded, in a condition that the upper lid  21  is opened, the sheet is drawn from the cartridge  3  and a tip end of the sheet is inserted into a nip of the pair of pulling rollers  22 ,  22   a . A sensor  23  detects the tip end of the sheet to rotate the pulling roller  22 . Thereafter, when the upper lid  21  is closed, the pair of pick-up rollers  20 ,  20   a  start to rotate, thereby forming the loop  24  in the sheet between the pair of pick-up rollers  20 ,  20   a  and the pair of pulling rollers  22 ,  22   a . When the tip end of the sheet supplied from the pair of pulling rollers  22 ,  22   a  reaches the image forming means and is pinched by a nip between the LF roller  6  and the LF driven roller  6   a  and a nip between the downstream LF roller  7  and the spurred roller  7   a , the nip of the pair of pulling rollers  22 ,  22   a  is released by the cam (not shown). 
     Since image forming accuracy is greatly influenced upon conveying accuracy of the LF roller  6  and the downstream LF roller  7 , by forming the loop and by releasing the nip in this way, while the sheet is being conveyed in the image forming system, the sheet is not subjected to any load from other conveying members. 
     Accordingly, the loop  24  must be maintained during the image formation. To this end, there is provided a look sensor (not shown) associated with the pair of pick-up rollers  20 ,  20   a.    
     The image forming portion  5  serves to convey the sheet pinched between a platen (not shown) and a sheet hold-down member (not shown) intermittently in the conveying direction by the predetermined length (about 10 mm). The image forming portion  5  utilizes cyan color ink, light cyan color ink, magenta color ink, light magenta color ink, yellow color ink and black color ink to achieve high quality image formation. 
     That is to say, the image forming portion  5  includes a recording head  5   a  having six kinds of nozzles for discharging the cyan color ink, light cyan color ink, magenta color ink, light magenta color ink, yellow color ink and black color ink. A carriage  5   b  on which the recording head  5   a  is mounted is shifted along a carriage guide  5   c  in a direction perpendicular to the sheet conveying direction (scanning) and ink is discharged from the recording head  5   a  integrally shifted together with the carriage  5   b  in response to image data, thereby forming an image. The carriage  5   b  is shifted by a drive means (not shown). The carriage  5   b  and the drive means constitute a shift means. 
     Whenever one scan is effected, an image is formed on an area having a predetermined length Li in the conveying direction (recording width) through the entire width of the sheet. Whenever the image formation by one scan is finished, the sheet is conveyed by the LF roller  6  and the downstream LF roller  7  by a distance same as the predetermined length Li and then is stopped. After the sheet is stopped, next scan is effected to form an image on a next similar area. 
     By repeating the image formation on the recording width Li effected by the recording head  5   a  and the conveyance of the sheet by the recording width Li effected by the LF roller  6  and the downstream LF roller  7  alternately, the entire image continuous in the conveying direction is formed on the sheet. 
     A nip roller  16  provided in the first convey path portion  13   a  is supported on a free end of an arm  16   c  rockable around a shaft  16   b.    
     During the image formation, the nip roller  16  provided in the first convey path portion  13   a  is spaced apart from a driving roller  16   a  so that the sheet is not pinched between these rollers. Further, the D-cut roller is normally positioned so that a cut-out portion  10   c  (of the D-cut roller) having a smaller radius is opposed to a driven roller  10   a  not to pinch the sheet therebetween. The reason is that any conveying load is not applied to the sheet during the image formation to improve the conveying accuracy of the LF roller  6  and the downstream LF roller  7 . 
     As the image formation is continued, when the image having a predetermined length or more is formed, the tip end of the sheet is directed onto the discharge tray  11 . 
     When the image formation is finished, the sheet is conveyed by the conveying system of the image forming system until a portion of the sheet to be a tail end after cutting reaches the position of the cutter  8  to obtain a cut sheet having a desired length. After the sheet is pinched by the switch-back conveying systems so that the sheet is not dropped out of the apparatus, the sheet is cut by the cutter  8 . 
     After the sheet having the desired length is transferred to the switch-back conveying system in this way, the remaining sheet portion is returned to a position where the tip end of the sheet is positioned at the image formation start position, thereby preparing for next image formation. 
     Regarding the sheet transferred to the switch-back conveying system, after the sheet is escaped until the tail end thereof reaches a position immediately below the D-cut roller  10 , the shutter (guide means)  9  is switched and the D-cut roller  10  is rotated reversely. Although the tail end of the sheet is conveyed toward the image forming portion, due to the presence of the switched shutter  9 , the sheet is directed to the second convey path portion  13   b . In the second convey path portion, the plurality of pair of convey rollers  12  are disposed side by side and are rotated at the same speeds so that the sheet is conveyed with the tail end thereof facing forwardly. Each of these paired convey rollers is constituted by a one-way roller so that, if the pairs of convey rollers  12  are rotated at different speeds for some reason, the rotational speed of the convey rollers is changed to become the same as that of a most downstream pair of convey rollers among the pairs of convey rollers contacted with the sheet and the other convey rollers are rotatingly driven in synchronous with the most downstream pair of convey rollers. Accordingly, any load is not applied to the sheet and no slip is generated between the sheet and the rollers. Lower rollers of the plural pairs of convey rollers  12  are biased by springs (not shown) toward the corresponding lower rollers to adjust nips between the upper rollers and the lower rollers, thereby preventing the latex layer of the sheet from being damaged. 
     The sheet passed through the second convey path portion is transferred to the fixing system. At the same time, the driving of the pairs of convey rollers  12  is stopped so that these rollers are rotatingly driven by the conveying force of the fixing rollers  14   a  ,  14   b . The latex layer formed on the surface of the sheet is melted by the heat (110-180° C.) of the fixing roller. After cooled, the latex layer becomes a transparent laminated layer (FIG.  8 ). Further, the heat generated in the fixing system is transmitted to the sheet existed in the second convey path portion  13   b  by means of a fan  26 , thereby achieving the pre-heat effect. 
     A curl formed in the sheet left the fixing system is corrected by an uncurling means or curl correcting means  27  and then is discharged to a discharge portion  15 . 
     Construction and Operation of Switch-back Conveying System 
     Next, the switch-back conveying system which is the most important characteristic of the present invention will be explained with reference to FIGS. 1 to  5  and FIG.  9 . 
     The switch-back conveying system is a sheet conveying apparatus in which the sheet conveyed from the image forming means is switched back to convey it to the fixing means and which comprises the shutter  9 , D-cut roller  10 , discharge tray  11 , convey rollers  12 , vertically shiftable nip roller  16 , first sheet detecting means  30  and second sheet detecting means  31 . The reference numeral  12 a denotes a most upstream convey roller (nearest to the D-cut roller) among the convey rollers  12 . 
     Now, the operation of this sheet conveying apparatus will be described with reference to an operation sequence shown in FIG.  9 . 
     A control means (not shown) controls the cutter  8 , a drive means such as a motor and a clutch for shifting the nip roller  16  in the vertical direction, a drive means such as a motor and a clutch for rotating the driving roller  16   a , a drive means such as a motor and a clutch for rotating the D-cut roller  10 , and a solenoid for shifting the shutter  9 . The control means also controls motors for driving the LF roller  6  and the downstream LF roller  7 , a drive means for shifting the carriage  5   b  and a drive means for discharging the ink from the recording head  5   a.    
     During the image formation, the sheet conveying apparatus is firstly positioned in a condition shown in FIG.  1 . That is to say, the nip roller  16  is waiting at the upper position to be spaced apart from the driving roller  16   a , with the result that the sheet is not pinched between the rollers  16  and  16   a . Further, the cut-out portion  10   c  of the D-cut roller  10  having the small radius is opposed to the driven roller  10   a  so that the sheet is not pinched between the rollers  10  and  10   a . The portion of the sheet on which the image was formed is conveyed toward the discharge tray  11  by the LF roller  6  and the downstream LF roller  7  without applying any load from the rollers  16 ,  16   a ,  10 ,  10   a  to the sheet portion. In this case, the guide  32  disposed below the D-cut roller  10  is lowered together with the driven roller  10   a  by the cam (not shown) to be brought to a position shown by the broken line in FIG. 1, thereby providing, below the D-cut roller  10 , an adequate space through which the sheet is discharged onto the discharge tray  11 . 
     When the image formation is finished (flow F 1 ), the sheet is conveyed by the LF roller  6  and the downstream LF roller  7  to the position where the portion of the sheet to be cut is positioned at the cutting position of the cutter  8  so that the sheet can be cut to the desired length (flow F 2 ). 
     When the sheet is conveyed to the cutting position, the arm  16   c  is rocked downwardly, so that the sheet is pinched between the nip roller  16  and the driving roller  16   a  (flow F 3 ), and, the sheet is cut by the cutter  8  (flow F 4 ). 
     When the size of the sheet is large, at this point, the tip end of the sheet is suspended from the discharge tray  11 . The reason why the sheet is pinched between the nip roller  16  and the driven roller  16   a  before the sheet is cut by the cutter  8  is that the sheet is prevented from dropping from the discharge tray by the weight of the sheet itself upon cutting the sheet. Although the sheet can be pinched between the D-cut roller  10  and the driven roller  10   a  for preventing the sheet from being dropped, due to the design limitation, since a distance between the cutter  8  and the D-cut roller  10  should be longer than a smallest length (for a post card) of the sheet, in the illustrated embodiment, the nip roller  16  is provided to convey such a sheet to the D-cut roller  10 . 
     Incidentally, a roller  11   a  or member having an arch-like curvature is provided at a tip end of the discharge tray to improve the sliding effect of the sheet on the discharge tray. 
     When the sheet is cut by the cutter  8 , the driving roller  16   a  starts to be rotate to convey the sheet in a first direction (flow F 5 ). Thereafter, when the tail end of the sheet is detected by the first sheet detecting means  30  (flow F 6 ), the D-cut roller starts to rotate normally (in a clockwise direction in FIG.  2 ). At the same time when the rotation of the D-cut roller  10  is started, the guide  32  is shifted upwardly to the position shown by the solid line to form the nip between the D-cut roller  10  and the driven roller  10   a  of the guide  32 . 
     When the tail end of the sheet is detected by the second sheet detecting means  31  (FIG. 3) (flow F 7 ), the shutter  9  is switched by rotating it in a clockwise direction around the shaft  9   a  (flow F 7 ), and, at the same time, the convey rollers  12  start to rotate (flow F 8 ) to rotate the D-cut roller  10  reversely while pinching the sheet (FIG. 4) (flow F 9 ). 
     The tail end of the sheet is directed, by the shutter  9 , to the most upstream convey roller  12   a  among the convey rollers  12 , with the result that the sheet is conveyed toward the fixing means by the convey rollers. 
     In order to effect such conveyance of the sheet, the arrangement of various elements or members is predetermined. 
     More specifically, when it is assumed that a length of an arc of the D-cut roller  10  is L, a distance from a nip point of the nip roller  16  to the D-cut roller  10  is L1, a distance from the first sheet detecting means  30  to the D-cut roller  10  is L2 and a distance from the D-cut roller  10  to the convey rollers  12   a  is L3, the elements must be arranged to satisfy a relation “L 3 &lt;L1&lt;L2&lt;L”. 
     As mentioned above, since the sheet is once escaped, in dependence upon the size of the discharge tray, the image formation can be effected regarding the sheet having greater length. 
     Second Embodiment 
     FIG. 10 shows a case where a sheet conveying apparatus according to a second embodiment of the present invention is applied to an image forming apparatus. 
     While an example that the second convey path portion  13   b  is disposed below the first convey path portion  13   a  was explained in the first embodiment, in the second embodiment, the second convey path portion is disposed above the first convey path portion  13   a . Since the other functions and effects are the same as those in the first embodiment, the same elements are designated by the same reference numerals and explanation thereof will be omitted. 
     Other Embodiments 
     In the above-mentioned embodiment, while an example that the sheet conveying apparatus according to the present invention is applied to the image forming apparatus was explained, the sheet conveying apparatus can be applied to any apparatuses so long as the sheet conveying apparatus is disposed between two processing portions having independent conveying systems. 
     Further, while an example that the D-cut roller is used as the conveying means of the escape portion was explained, in place of the D-cut roller, a vertically shiftable nip roller may be used. 
     As the first and second sheet detecting means, sensors of light permeable type may be used or mechanical sensors may be used. When the mechanical sensor is used as the second sheet detecting means, it is desirable that the operation of the sensor does not interfere with the shutter, and, it is preferable that such a mechanical sensor is provided integrally with the shutter  9 . 
     After the image formation, it is important that an adequate waiting time during which the image formed on the sheet is absorbed to the sheet is provided before the sheet conveyed by the nip roller, so that the image (ink) can be prevented from being transferred onto the nip roller. In this case, it is preferable that the waiting time is controlled on the basis of material of the sheet and/or an amount of ink used in the image formation. By doing this, the adequate waiting time during which the image formed on the sheet is absorbed to the sheet can be set, thereby improving the image quality and shortening the image forming time. 
     The D-cut roller may be disposed nearer to the discharge roller, and two or more nip rollers  16  may be provided. 
     The present invention provides the sheet conveying apparatus disposed between the first and second processing portions having the independent convey systems and including the convey path for feeding the processed medium conveyed from the first processing portion to the second processing portion, and wherein the convey path includes the first convey path portion, second convey path portion, and escape portion for temporarily escaping the processed medium from the first convey path portion and for conveying the processed medium to the second convey path portion with the tail end of the processed medium facing forwardly, and further wherein the convey path is provided with the convey means in which any load is not applied to the processed medium during the conveyance of the processed medium in the first processing portion and thereafter the conveying force is applied to the processed medium. With this arrangement, the processed media having various sizes can be conveyed without making the apparatus bulky and preventing the interference between the two processing portions. 
     In the above-mentioned embodiment, while an example that the present invention is applied to the image forming apparatus was explained, the present invention is not limited to such an example, but, may be applied to any sheet processing apparatus in which treatment is effected in first and second processing portions. 
     In this case, when sheet conveying speed, sheet conveying sequence and/or sheet conveying method of the first processing portion differ from those of the second processing portion, the advantages of the present invention can be achieved more effectively. 
     Further, the ink permeable layer may be formed from a porous polymer layer (particularly, a resin porous layer). By effecting heat treatment regarding such a layer, a transparent polymer (resin) layer is formed on the imaged surface of the sheet.