Patent Publication Number: US-9885983-B2

Title: Method for heat treatment of mixed media sheets

Description:
The invention relates to a method for heat treatment of mixed media sheets in an image reproduction apparatus having an image forming station, a heat treatment station, and a conveying path for conveying the sheets one by one through the image forming station and the heat treatment station, the apparatus further having a duplex loop for looping sheets back from the heat treatment station to the image forming station, the method comprising a step of pre-heating a sheet before an image is formed thereon in the image forming station. 
     US 2006216091 describes an image reproduction apparatus having a mechanism capable of performing printing on both sides of a sheet of paper by using a liquid toner as the developer. 
     JP2009163064 describes a double-sided printing machine and a double-sided printing method for a liquid-developing electrophotographic system by which a highly precise double-sided printing is performed. 
     In an image reproduction apparatus, a heat treatment of the image-receiving media sheets may be necessary for example in order to fuse the images that have been formed in the image forming station. The necessary duration of the heat treatment depends upon the heat capacity of the media sheets. Sheets with a higher heat capacity must be conveyed through the heat treatment station at a lower speed in order to raise the temperature of the sheets to a sufficient level. 
     A print job may require printing on mixed media sheets which have different heat capacities. For example, the print job may consist of printing a plurality of sets of copies on relatively thin media sheets, but each set may have a cover sheet which has a significantly larger thickness and, consequently, a higher heat capacity. In such cases, the printing speed and hence the productivity will be determined by the sheets with the highest heat capacity. 
     A higher productivity may be obtained when the image reproduction apparatus has an extra pre-heating station where the thicker media sheets may be pre-heated, so that they will reach the fuse station already with an elevated temperature. 
     U.S. Pat. Nos. 7,324,779 B2 and 7,336,920 B2 disclose image reproduction apparatus which have a plurality of fusing stations, so that the printed sheets may be subjected to a plurality of fusing steps for improving the permanence or appearance of the printed image. 
     However, an extra pre-heating station or an additional fuse station adds to the space requirements for the image reproduction apparatus and to the complexity and costs of the apparatus. Further operating the pre-heating station and the main heat treatment station simultaneously will temporarily increase the power consumption, which may be problematic when the power capacity of the grid is limited. 
     It is an object of the invention to provide a method tor heat treatment that permits to increase the productivity of an image reproduction apparatus without requiring additional equipment. 
     In order to achieve this object, the method according to the invention is characterized in that the step of pre-heating comprises the sub-steps of:
         passing the sheet through the image forming station while the image forming station is idle,   passing the sheet through the heat treatment station for pre-heating, and   looping the sheet back to the image forming station.       

     Thus, according to the invention, one and the same heat treatment station may be used for pre-heating the sheets and for the proper heat treatment. The sheets will be pre-heated when they are passed through the heat treatment station for a first time, and the proper heat treatment will be performed when the sheets are passed through the heat treatment station once again after an image has been formed. Consequently, a high conveying speed may be used even for the sheets with the higher heat capacity, so that a high productivity can be achieved. 
     In a typical scenario, only a relatively small fraction of the mixed media sheets to be processed will need pre-heating, so that extra time for passing sheets through the heat treatment station before an image is formed is required only for a small number of sheets, whereas the majority of the sheets which do not require pre-heating need to be passed through the heat treatment station only after an image has been formed. Consequently, the extra time for looping the sheets back will be outweighed by the increased conveying speed. 
     More specific optional features of the invention are indicated in the dependent claims. 
     In case of duplex printing, the normal sheets which do not need pre-heating will be passed through the image forming station and the heat treatment station twice, whereas the few thicker sheets will be passed through the image forming station and the heat treatment station at least three times. 
     A gap scheduling routine may be employed for controlling the feed of blank sheets to the image forming station at timings that lead to a highest possible productivity while assuring that the pre-heated sheets that are looped back from the heat treatment station will be appropriately inserted into gaps in the stream of blank sheets, with the desired output sequence of the sheets being preserved. 
    
    
     
       An embodiment example will now be described in conjunction with the drawings, wherein: 
         FIG. 1  is a view of an image reproduction apparatus to which the invention is applicable; 
         FIGS. 2 to 6  show the image reproduction apparatus in different stages in the execution of the method according to the invention; 
         FIG. 7  is a diagram illustrating a conventional gap scheduling routine for duplex printing; 
         FIG. 8  is a diagram of a gap scheduling routine as modified in accordance with the invention; and 
         FIGS. 9 and 10  diagrams of gap scheduling routines according to other embodiments. 
     
    
    
     As is shown in  FIG. 1 , an image reproduction apparatus  10 , e.g. an electrostatic printer, comprises an input section  12 , a main body  14 , and a finisher  16 . 
     In the simple example shown here, the input section  12  has two input trays  18  accommodating stacks of media sheets  20 ,  22  of two different types. The sheets  20  in the upper tray have a relatively large thickness and are intended to form cover sheets for copy sets to be printed, whereas the sheets  22  are thinner and are intended to constitute all the other sheets of the sets of copies. 
     The input section  12  is arranged to withdraw the sheets  20 ,  22  from the trays  18  upon demand and to feed them one by one into a sheet conveying path  24  that extends from an exit  26  of the input section  12  through the main body  14  and to the finisher  16 . 
     The main body  14  includes an image forming station  28  and a heat treatment station  30  which are arranged in that order along the sheet conveying path  24 . The main body  14  further includes a duplex loop  32  that leads from the downstream side of the heat treatment station  30  back to the input side of the image forming station  28  and includes a sheet reversing mechanism  34  for reversing the orientation in which the sheets are fed back to the image forming station  28 . A switch  36  is provided at the output side of the heat treatment station  30  for directing the sheets that leave the heat treatment station  30  either into the duplex loop  32  or into the finisher  16  where the sheets are stacked on an output tray  38  and optionally subjected to finishing operations such as stapling, punching or the like. 
     An electronic controller  40  is provided for controlling the operation of the image reproduction apparatus and communicates with a user interface  42 . 
     The controller  40  analyses job specifications of a print job that has been submitted via the user interface  42 , the job specifications determining among others for each of the printed copies, which of the two types of media sheets  20  and  22  is to be used. Based on this information, a gap scheduling routine that is implemented in the controller  40  determines a sequence in which the sheets  20 ,  22  are withdrawn from the trays  18  and fed through the apparatus. 
     In the condition shown in  FIG. 1 , one of the thicker sheets  20  has been fed into the image forming station  28 . In this stage, however, the image forming station  28  is idle, so that no image is formed on the sheet  20 . 
     As is shown in  FIG. 2 , the sheet  20  is then passed-on from the image forming station  28  to the heat treatment station  30  where a heat treatment is applied for pre-heating the sheet  20 . 
     The pre-heated sheet  20  is then directed into the duplex loop  32 , as has been shown in  FIG. 3 . There, the sheet reversing mechanism  34  will reverse the orientation of the sheet, and the sheet will be fed once again to the image forming section  28 . 
       FIG. 4  shows a condition where an image  44  has been formed on the side of the sheet  20  which is now the top side, and the sheet has been conveyed once again to the heat treatment station  30  where, now, a heat treatment is applied for fusing the image  44  on the sheet. Meanwhile, one of the thinner sheets  22  has been fed into the image forming station  28  where an image will be formed on the top side of that sheet. 
       FIG. 5  shows the thinner sheet  22  with an image formed thereon in the heat treatment station  30  where the image is fused. The thicker sheet  20  has again been directed into the duplex loop  22  with the image  44  now facing downwards. The sheet reversing mechanism  34  will assure that the image  44  will still face downwards when the sheet  20  is once again returned to the image forming station  28 . 
       FIG. 6  shows a condition where another image  46  has been formed on the top side of the sheet  20  and the sheet passes through the heat treatment station  30  for a third time for fusing the image  46 . Thereafter, the switch  36  is operated to direct the sheet  20 , which now bears images  44 ,  46  on both sides, into the finisher  16 . Meanwhile, another one of the thinner sheets  22  has been fed into the image forming station  28 . This new sheet  22  will then receive an image on its top side and will be fused in the heat treatment section  30  and then directed into the duplex loop  32 , whereas the other thin sheet  22  that is ready in the duplex loop will be returned to the image forming station  28  for forming an image on the second side. 
     In the process illustrated in  FIGS. 1 to 6 , a pre-heating treatment is applied only to the thick sheets  20  when they pass through the heat treatment station  30  for the first time, and these sheets will then pass through the heat treatment station  30  a second and a third time for fusing the images  44 ,  46 . In contrast, the thinner sheets  22  which do not need pre-heating will pass through the heat treatment station  30  only twice, a first time when a first image has been formed and the second time when the second image has been formed on the second side of the sheet. 
     Of course, in case of simplex printing, the thick sheets will pass through the heat treatment station  30  twice whereas the thinner sheets pass through the heat treatment station  30  only once. 
     The supply of sheets into the sheet conveying path  24  is scheduled such that, whenever a sheet  20  or  22  returns from the duplex loop  32 , it will be inserted in a gap in the stream of sheets that are supplied from the input section  12 . Preferably, the supply of sheets should also be scheduled such that the printed sheets are output to the finisher  16  in the desired order, even though the numbers of times which these sheets pass through the heat treatment station  30  may differ from sheet to sheet. 
     An examples of a suitable gap scheduling routines will now be explained in conjunction with  FIGS. 7 to 9 . 
       FIG. 7  illustrates a straightforward scheduling routine that may be applied when all the sheets have the same thickness and, consequently, none of the sheets requires pre-heating. It shall be assumed in this example that the print job is composed of sets of duplex sheets S 1 -S 7 , and each set comprises seven sheets. The left column “feed” in  FIG. 7  illustrates the sequence in which the sheets S 1 -S 7  for a first set and then the sheets S 1 -S 7  for a second set are fed into the sheet conveying path  24  at the exit  26  of the input section  12 . Rectangles shown in dashed lines in this column represent gaps  48  in the stream of sheets, each gap having a size of one or more sheets. 
     It shall further be assumed that the duplex loop  32  has a capacity of three sheets, so that it is possible to feed three sheets in immediate succession through the image forming station  28  and the heat treatment station  30  and then into the duplex loop  32  before the first of these sheets will arrive again at the image forming station  28 . Consequently, a gap  48  with a size of three sheets has to be provided after each set of three sheets. 
     The second column “image forming” in  FIG. 7  illustrates the sequence in which the sheets are processed in the image forming station  28 . When the first image  44  has been formed and fused on each of the first three sheets S 1 -S 3 , these sheets are inserted into the gap  48  for forming the images  46  on the second side. The same applies to also subsequent batches of three sheets each. 
     The right column in  FIG. 7  illustrates the sequence in which the printed duplex sheets are output to the finisher. 
     Now, another print job shall be considered wherein the first sheet S 1  of each set of seven sheets shall be a cover sheet for which the media type should be that of the thicker sheets  20 , whereas all the other sheets are sheets  22  of the thinner media type. In that case, the scheduling routine may be modified as shown in  FIG. 8 . 
     The stream of sheets and gaps in the conveying path  24  has been divided into feed cycles C 1 -C 9  each of which comprises three time slots for feeding either one of the sheets or a gap with the size of one sheet. 
     In the first cycle C 1 , only the (thick) first sheet S 1  is supplied in the first time slot, and the other two time slots are left empty. In the column “image forming”, the rectangle that symbolizes the sheets S 1  has been shown in dashed lines in order to indicate that the image forming station is idle and no image is formed on the sheet S 1 . The sheet will then be pre-heated in the heat treatment station  30 . The first image on the sheet S 1  is formed only when this sheet has returned from the duplex loop  32  for the first time. 
     Immediately thereafter, in the second cycle C 2 , the (thin) second sheet S 2  is supplied, so that an image will be formed on the first side of that sheet. Although the duplex loop  32  could accommodate three sheets, the next time slot is left empty, so that only the two sheets S 1  and S 2  will be in the duplex loop. 
     The feed cycle C 3  consists of a gap with a size of three sheets, and the sheets S 1  and S 2  are inserted into this gap for receiving the image  46  on the second side. Thereafter, these two sheets S 1  and S 2  will be output to the finisher. 
     In the next feed cycle C 4 , the next two (thin) sheets S 3  and S 4  will be supplied. The third time slot of that cycle is again left empty. 
     The next cycle C 5  is again a three sheet gap into which the sheets S 3  and S 4  are inserted for receiving an image on the second side. 
     In the next cycle S 6 , the (thin) sheets S 5  and S 6  are fed for receiving an image  44  on the first side. In this case, however, a gap is left between the two sheets S 5  and S 6 . The reason for this will become clear from the description that follows. 
     In the cycle C 7 , the sheets S 5  and S 6  are recirculated for receiving the image  46  on the second side, and the gap between them is filled by feeding the thick sheet S 1  which will be the cover sheet of the next set. As in the first cycle C 1 , this sheet is passed through the idle image forming station  28 . 
     Then, in the next cycle C 8 , the last (thin) sheet S 7  of the present set is fed for receiving the first image  44  in the image forming station. Immediately thereafter, the sheet S 1  returns from the duplex loop and also receives a first image  44  on the first side. The last time slot in this cycle is again left empty. 
     Then, in the cycle C 9 , the sheets S 7  and S 1  receive an image  46  on the second side and are output to the finisher. 
     As can be seen in the last column in  FIG. 8 , the sheets are output in the ordered sequence from S 1  to S 7 , followed by the first sheet S 1  of the next set. This has been achieved by supplying the sheet S 1  for receiving the first image in the cycle C 8  in the time slot immediately behind S 7 , i.e. in the second time slot of the cycle C 8 . A condition for this was that, in the preceding cycle C 7 , the sheet S 1  has been passed through the idle image forming station  28  also in the second time slot, so that it will be returned to the image forming station in the cycle C 8  just in time. 
     In turn, in order to be able to supply the sheet S 1  in the cycle C 7  in the second time slot, a gap had to be present in this time slot. This gap has been created in the cycle C 6  by feeding the sheets S 5  and S 6  in the first and third time slot, thereby leaving a gap therebetween. 
     This principle may be applied repeatedly for printing the further sheets of the subsequent sets without disrupting the ordered sequence of sheets. 
     The necessary freedom for providing the gaps at the image forming station  28  at the right timings is obtained here by not fully exploiting the capacity of the duplex loop  32 , i.e. by using only two of the three available time slots. 
     There may however be feed cycles in which it is possible to use the full capacity of the duplex loop and thereby to further enhance productivity. As an example,  FIG. 9  shows a modified scheduling scheme which differs from that shown in  FIG. 8  in that two sheets S 2  and S 3  are fed in the second cycle C 2 , so that, together with the sheet S 1 , the duplex loop is filled completely. In order to provide the gap for the sheet S 1  in the correct position in the cycle C 5 , it is sufficient to leave a corresponding gap in the cycle C 4 . 
     On the other hand, there may be print jobs in which it is necessary to leave two or more slots empty in the duplex loop. This may for example be the case when the number of sheets per set (which was seven in  FIGS. 8 and 9 ) is smaller than the capacity of the duplex loop  32  (which was three in  FIGS. 8 and 9 ). 
     When comparing the first column “feed” in  FIG. 9  to the first column in  FIG. 7 , it can be seen that even in the improved scheme according to  FIG. 9 , the number of empty slots or gaps is larger than in  FIG. 7 , which implies a certain loss in productivity. However, most jobs with mixed media sheets, this loss in production is more than compensated by the fact that pre-heating can be applied selectively to the thicker sheets, so that the conveying speed can be increased. 
     Moreover, in a realistic embodiment, the capacity of the duplex loop  32  will be significantly larger than three. It may for example be as large as eight or twenty or even more. In that case, the number of empty slots in relation to the number of filled slots will decrease significantly, so that the loss in productivity becomes smaller, especially when the job consists of a relatively large number of sets. Further, the productivity will of course be higher when the ratio of thin sheets to thick sheets in each set becomes larger. 
     When the heat capacity of the thick sheets is very large and/or the heating power of the heat treatment station  30  is small, the principle of the invention may also be generalized to the case that two or more pre-heating steps are performed for each of the thicker sheets before a first image is printed thereon. 
     When the number of pre-heating steps per sheet is odd, e.g. 1 or 3 , it will be observed that the pre-heated sheets pass through the sheet reverse mechanism  34  one or three times more than the sheets that are not pre-heated. Consequently when the thin sheets  22  receive the first image  44  on the top side, the thick sheets  20  will receive the first image  44  on the bottom side. This effect may be undesirable when the surface properties of the two sides of the sheets are not identical, e.g. when one side is coated and the other is not. In such a case, however, the effect may be compensated for by placing the sheets  20  in the bin  18  in reverse orientation, so that the sides receiving the first image  44  will face downwards for the sheets  20  whereas they face upwards for the sheets  22 . 
       FIG. 10  shows yet another gap scheduling scheme which leads to a more regular output of the printed sheets. This is achieved by interleaving sheets that arrive at the image forming station for the first time with sheets that return from the duplex loop. 
     In the example shown in  FIG. 10 , it is assumed that each set consists of three duplex sheets. The sheets of the second set are marked with a star “*”, the sheets of the third set with two stars “**”, and so on. It can be seen that printing the first side of the third sheet S 3 , S 3 *, etc of each set is interleaved with printing the second side of the first and second sheets which return from the duplex loop.