Abstract:
A hardcopy apparatus including a main roller and a loading mechanism to load a medium into the hardcopy apparatus, said loading mechanism including a vacuum holddown input unit to hold media down onto a surface of said holddown unit. In addition, a method of loading a medium into a hardcopy apparatus including a vacuum holddown unit, a driving roller and a secondary roller, comprises the steps of: manually positioning the medium onto a surface of the holddown unit; by rotating the secondary roller, advancing the medium towards the main roller; and engaging the medium to the main roller.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to hardcopy apparatus, such as copiers, printers, scanners, facsimiles, and more particularly to improved media holddown devices for such apparatus. 
     BACKGROUND OF THE INVENTION 
     Hardcopy apparatus and particularly in apparatus handling media of big size, such as large format printers, are supplied by moving the end of the printing media manually towards the input portion of the apparatus, the printing media usually being a roll of printing media or sheet of printing media or the like, the printing media being pushed manually until its front edge is gripped between the feed roller and the drive roller which advance the sheet or the roll. 
     A problem which frequently arises in apparatus of this type is that, given the manual guidance of the printing media has to be carried out at a considerable distance from the front edge of the printing media, it is difficult to achieve good alignment of the printing media and , in many cases, this delays the operation of the printer when this operation has to be repeated, since the printer cannot tolerate appreciable deviation of the printing media from the theoretical position. 
     As is clear, the need from a repetition of the said loading operation results in loss of time. 
     Furthermore, a feeding operation of a printing media not properly positioned may result in a paper jam, with a consequent loss of paper, which may be very expansive depending on its size and quality, as well as in a further loss of time for removing the paper jam. 
     Alternatively, an improved method of loading media into a printer is disclosed in the European patent application no. 97 500153.8. In this application the method of loading the printing media into the printer provides for the operation of the rollers for advancing the printing media to be activated manually by the operator himself at the moment when the said printed media is introduced into the printer and at the moment when the operator estimates that the sheet of printing media is correctly positioned, without deviation. 
     However, also in this case manual loading and positioning of media appears to be cumbersome, in particular when cut sheet of media are employed. In fact it is still difficult to achieve good alignment of the printing media since few references are provided to the operator in order to estimate if the media is correctly positioned or not. 
     Finally, both the solutions cause a sharp change of status of the media, i.e. from completely free to stably locked. This change may cause an unintentional modification of the position of the media, which may result in an incorrect feeding of the sheet into the printer. 
     Conventionally, sheet holddown devices such as electrostatic or suction devices are employed only to reduce the effects of paper curl and cockle on dot placement during printing. In vacuum holddown devices, sheet flatness is maintained by providing suction between a support plate and the back surface of a sheet to be handled. 
     Cockle effect is the reluctance of the paper to bend smoothly. Instead it bends locally in a sharp fashion, creating permanent wrinkles. 
     Although conventional vacuum holddown devices are fairly effective in maintaining sheet flatness during printing, they have drawbacks. One drawback is the complexity of maintaining the same holddown force along the entire width of the medium while printing, i.e. in the direction of the printheads motion. This is due to the losses of air that the conventional devices allow, causing the medium to be subject to different forces, i.e. forcing the medium to rotate while it is advanced in the direction of the media motion. 
     Another drawback is that on one hand the maximum holddown force on a sheet is limited because of the necessity to maintain low frictional loads on transport devices which index the sheets. In conventional inkjet printers, such limitations can cause pen-to-sheet spacing distances to vary from swath to swath. Consequently, the holddown pressure at a localised area being printed may be insufficient to flatten cockles and other paper irregularities. On the other hand the vacuum required to eliminate cockle wrinkles in a printout would be so high that is normally unfeasible; in fact, high vacuum may suck the ink right through the paper and at the same time generate a lot of noise. 
     Applicant has then experimented that the employment of a vacuum holddown input unit may help media to be properly manually positioned by the operator. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide an improved hardcopy apparatus and method of loading a printing medium in the hardcopy apparatus. 
     According to an aspect of the present invention, there is provided a hardcopy apparatus comprising a main roller and a loading mechanism to load a medium into the hardcopy apparatus, said loading mechanism comprises a vacuum holddown input unit to hold media down onto a surface of said holddown unit. 
     This reduces the risk of any unintentional modifications of its positioning. In fact the medium always stays in a sort of partially bounded condition, i.e. it is stably held, unless a significant force is applied in order to have it moved, since the vacuum is not switched off during the positioning operation. It should be noted that, direct contact of the holddown device with the printing surface is avoided to minimize signs and other adverse affects on print appearance. 
     Preferably, said holddown unit comprises a vacuum source, connected to atmosphere through a plurality of first apertures formed into the surface, and a vacuum channel to generate a negative pressure capable of holding down media onto the surface, while loading a medium. 
     In a preferred embodiment, said surface further comprises a plurality of recesses, at least one of said plurality of first apertures is located in one of said plurality of recesses. 
     However, the air flow between the surface and the back of the medium may generate noise in correspondence of the first apertures. 
     Accordingly, in a preferred arrangement, at least one of said plurality of recesses comprises a first portion capable of uniformly distributing the vacuum on an area substantially bigger than the aperture itself, and a second portion so shaped to reduce air flow noise interference with media. 
     This result is achieved since the shape of the recesses is designed for providing the air flow with a smooth transition, reducing the resulting noise. 
     Typically, said holddown unit further comprises a plurality of second apertures formed into the surface to create an additional negative pressure capable to increase the stability of media onto the surface. Advantageously, said holddown unit further comprises means to extend the additional negative pressure to a position closer to the drive roller. 
     This feature allows the media to be loaded in a position closer to the drive roller, reducing the distance to be covered during the feeding operation to engage an edge of the medium with the drive roller. 
     Preferably, said means for extending the negative pressure comprise a plurality of grooves extending towards the drive roller and a number of said plurality of second apertures are located within said grooves and each aperture ( 330 ) of said plurality of second apertures being located within a groove. 
     In a further preferred embodiment, said holddown unit further comprises feeding means, being the generated negative pressure capable to engage the back of the medium with said feeding means and transfer said medium to engage said drive roller. 
     Accordingly, the apparatus may be kept free from any additional elements which may reduce the operator&#39;s access to the input surface, reducing the complexity of his manual activity. 
     Additionally, said feeding means comprises one or more overdrive wheels disposed to form on the surface an alternating sequence of recesses and overdrive wheels in the direction perpendicular to the direction of motion of media. 
     In a further preferred arrangement, the holddown unit further comprises a first reference for placing the medium in a correct position on the surface, said first reference crossing the surface in the direction perpendicular to the direction of motion of media and a second reference for placing the medium in the right position on the surface, said second reference crossing the surface in the direction of motion of media. 
     Thanks to the input surface mainly free of obstacles the operator can be provided with a full visual control of a plurality of references, to help him in positioning the medium. 
     Preferably, said first reference is tangent to the portion of the grooves closer to the main roller. In this way, one edge of medium can be positioned at the very limit of the area capable of applying vacuum to the back of the medium. 
     More preferably, said medium is a cut sheet of media. 
     Viewing another aspect of the present invention, there is also provided, a method of loading a medium into a hardcopy apparatus including a holddown unit provided with a vacuum source, a main driving roller and a secondary roller, which comprises the steps of: manually positioning the medium onto a surface of the holddown unit; by rotating the secondary roller, advancing the medium towards the main roller; and engaging the medium to the main roller. 
     Preferably, the step of keeping the vacuum source activated during all the previous steps, in order to achieve the engagement of the secondary roller with the back of the medium. 
     In a preferred embodiment, the hardcopy apparatus further comprises one or more wheels, while the step of advancing the medium comprises the step of moving the one or more wheels clockwise or counter-clockwise, depending on the advancing direction of the medium. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described further, by way of example only, with reference to an embodiment thereof as illustrated in the accompanying drawings in which: 
     FIG. 1 is a perspective view of an inkjet printer incorporating the features of the present invention; 
     FIG. 2 is a more detailed diagram of a holddown system within the printer of FIG. 1; 
     FIG. 3 depicts a portion of the holddown system of FIG. 2; 
     FIG. 4 is a section of the main hardware components of the holddown system within the printer of FIG. 1; 
     FIG. 5 depicts a test curve of nominal values of the pressure applied to a medium vs. air flow provided by a vacuum device, employed in the holddown system of the preceding figures, in the rated voltage of 24 V. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a printer  110  includes a housing  112  mounted on a stand  114 . The housing has left and right drive mechanism enclosures  116  and  118 . A control panel  120  is mounted on the right enclosure  118 . A carriage assembly  100  illustrated in phantom under a cover  122 , is adapted for reciprocal motion along a carriage bar  124 , also shown in phantom. The carriage assembly  100  comprises four inkjet printheads  102 ,  104 ,  106 ,  108  that store ink of different colours, e.g. black, magenta, cyan and yellow ink respectively, and an optical sensor  105 . As the carriage assembly  100  translates relative to the medium  130  along the X and Y axis, selected nozzles of the printheads  102 ,  104 ,  106 ,  108  are activated and ink is applied to the medium  130 . The colours from the three colour printheads are mixed to obtain any other particular colour. The position of the carriage assembly  100  in a horizontal or carriage scan axis (Y) is determined by a carriage positioning mechanism with respect to an encoder strip. (not shown). A print medium  130  such as paper is positioned along a vertical or media axis by a media axis mechanism (not shown). As used herein, the media axis is called the X axis denoted as  101 , and the scan axis is called the Y axis denoted as  103 . 
     Referring now to FIG. 2, an holddown system is globally referenced as  200 . Such a holddown system  200  is located between the left and right drive mechanism enclosures  116  and  118 . The width of the holddown system along the Y axis is at least equal to the maximum allowable width of the media. In this example it should allow the employment of medium having width up to 36″, i.e. 914 mm. A more detailed description of the various components of the holddown system  200  will be made further with reference to FIG.  3 . The inkjet printheads  102 ,  104 ,  106 ,  108 , are held rigidly in the movable carriage  100  so that the printhead nozzles are above the surface of a portion of the medium  130  which lays substantially flat on a flat stationary support platen  400  of said holddown system  200 . 
     With reference to FIG. 3, the flat platen  400  is shown in more details, and is located in a front position of the printer  110  and co-operate with a main driving roller  300 , in the following identified also as the main roller, located in a rear position, and a plurality of pinch wheels  310 , in this example 12 pinch wheels  310  are employed, which are controlled to periodically index or convey the medium across the surface of the platen  400 . The force between each pinch wheels  310  and the main roller  300  is comprised between 3.33 N and 5 N, preferably 4.15 N. 
     This pinch wheel distribution and force helps to drive the medium  130  straight with irrelevant lateral slippage, to share the medium  130  expansion on all its width. In fact has been observed that printers with low forces, e.g. about 1 N, allow media expansion accumulates in a particular place and this may cause a wrinkle to get so big to create a crash of the printhead. 
     The main roller  300  is provided with a conventional surface having a plurality of circumferencial recesses  305  housing a corresponding plurality of protrusions  405  of the platen  400  extending towards the rear of the printer  110 . This combination of features allows the medium  130  to reliably move from the main roller  300  to the platen  400  and vice versa. In fact the gap between the roller  300  and the platen  400  may allow an edge of the medium to engage the back of the platen itself causing a paper jam. 
     The printer  110  comprises, a vacuum source, in this case a fan not shown in the drawings, connected to the atmosphere through a plurality of holes, or apertures,  330 ,  350  and a vacuum channel  380 ; such vacuum source generates an air flow by sucking air from the atmosphere. 
     Due to the pressure differential between atmosphere pressure on the surface of the medium  130  and the vacuum applied through the vacuum channel  380  and the holes  330 ,  350  to the back of the medium, the portion of the medium  130  close to the holes  330 ,  350  is suckingly adhered to the platen  400 . 
     In order to reduce the losses of air from the vacuum channel  380 , the holes  330 ,  350  are distributed at a certain distance from the main roller. According to this embodiment a plurality of first holes  330  lays in a line at a distance comprised between 10 mm and 30 mm, preferably 19 mm and a plurality of secondary holes  350 , distributed preferably in line. 
     Furthermore, the platen  400  is provided, according to this preferred example, with a plurality of substantially linear grooves having one end closer to and the opposed end further from the main roller  300 . Such grooves are linked together to form a continues slot  320 , which crosses substantially the whole width of the platen  400 , where such a continuous slot  320  is arranged to have a waved shape. 
     The plurality of first holes, or slot holes  330 , having a diameter comprises between 1.5 mm and 3.5 mm, preferably about 2.5 mm, are then distributed inside the waved slot  320 , and in this embodiment are preferably located in the further part of the slot  320  with respect to the main roller  300 . 
     It is important to note that since the main roller  300  is not included within the vacuum channel  380 , the vacuum can be only directly generated at a certain distance from the main roller  300  itself. However, if the slot  320  is included in the unit, when the vacuum source is activated and in presence of a medium on the platen  400 , the vacuum can be expanded along all the slot extending the vacuum closer to the main roller  300 . 
     In this application extending the vacuum means that the vacuum generated at one aperture, which is normally supplied to an area of the back of medium, is now supplied to an area of the back of the medium which is at least 10% bigger, preferably bigger than 500%. 
     This helps in more uniformly apply the vacuum to the back of the medium, reducing the risk of having peak of vacuum that may crease the medium. Furthermore, thanks to the slot  320  there is no need to conventionally include the main roller  300  into the vacuum channel  380  and this means that: a) the air losses are minimized, since in conventional systems, having the main roller included in the vacuum channel, most of the air is lost around the main roller itself; b) the air flow is forwarded towards the main roller  300 , meaning that a print zone  450  can be defined closer to the main roller  300 ; and c) the dimensions of the vacuum channel can be better controlled, giving more design freedom for designing the holddown system. 
     Size of the vacuum channel is a further parameter relevant to apply the proper vacuum to the back of the medium. Experiments run by the Applicant have shown that the surface of squared section of the vacuum channel  380 , as depicted in FIG. 3, is preferably bigger than the sum of the surface of all the apertures  330 ,  350  distributed within the platen  400 . More preferably the surface of the squared section is as big as twice, or more, the sum of the surface of all the apertures  330 ,  340 . 
     According to the above, it is possible to print closer to the edges of a cut medium. In fact the medium can still be indexed by the main roller  300  and the pinch rollers  310  even when we are printing close to the very end of the medium itself. 
     Applicant&#39;s extended tests have revealed that a width too wide of the slot can reduce the capability of maintaining the medium substantially flat while printing, so affecting the printing quality. On the contrary, a width too narrow and/or an insufficient depth may affect the air flow direction, i.e. the vacuum force is not extended close enough to the main roller  300 . 
     Furthermore, high vacuum may crease the paper especially if the grooves of the slot  320  are wide and run parallel to the paper advance direction. Therefore is advisable to run the grooves at about  450  respect to the media axis X and optimise the slot width to minimize creases in the paper and to evenly distribute the vacuum. In addition, if the groove is parallel to the advance direction, it may make the ink to migrate and create localised dark areas. 
     This means that it is not necessary that the plurality of grooves are linked together in order to form a continuous slot for achieving the above advantage. 
     Accordingly, the slot  320  has a depth deeper than 0.5 mm, preferably 1 mm, and a width comprises between 3 mm and 8 mm, preferably 5 mm. 
     However, the continuous shape of the waved slot  320  helps the holddown system  200  to evenly distribute the vacuum along the print zone  450 . In fact, an interrupted sequence of grooves may create areas, having a reduced vacuum, which cross the complete print zone  450 , in the media axis direction X. This may force the ink applied in those areas to migrate and create localised dark or clear portions in the printout. 
     Further from the waved slot  320 , along the media axis (X), the platen  400  is provided with a plurality of secondary recesses  360 , distributed in one line along the scan axis (Y). In this example each recess  360  is composed by two parts, a first one substantially squared and a second one substantially triangular, where the triangular part lays on a plane which deeper than the plane on which the squared part lays. 
     Furthermore, each squared part is provided with a secondary hole  350 , having a diameter comprises between 1.5 and 2.5 mm, preferably 2.0. Such sequence of secondary recesses  360  is combined with a sequence of overdrive wheels  340 , forming a secondary roller  345 , such that a group of 3 consecutive secondary recesses  360  is disposed between two consecutive wheels  340 . Such a secondary roller is housed in the vacuum channel  380 . 
     Thus, this holddown system  200  comprises 12 overdrive wheels  340  equally separated along the scan axis (Y) to supply equal traction to each part of the medium. 
     In this description an overdrive wheel may mean a single wheel as well as a plurality of wheels in strict contact one to another, in order to build a wheel having a larger width. 
     A secondary recess  360  is distanced by each adjacent element, both a further secondary recess  360  or a wheel  340 , by a rib  370 . The ribs are employed to reduce the risk of generating cockle wrinkles which may extend towards the print zone  450 . 
     Accordingly, two consecutive ribs  370 , having a preferably height of 1 mm, are distanced one to another by a distance comprised between 15 mm and 25 mm, preferably about 20 mm if the two ribs  370  are separated by a secondary recess  360 . 
     The plurality of secondary holes  350  provides the vacuum channel  380  with further apertures for the air flow generated by the vacuum source. 
     Since the air flow between the top of the platen  400  and the back of the medium  130  may generate noise in correspondence of the secondary holes  350 , the particular shape of the recesses  360  helps to provide the air flow with a smooth transition, reducing the resulting noise. 
     As for the slot holes  330 , the vacuum generated in correspondence of the secondary holes  350  is extended, in order to apply a negative pressure to most of the medium  130  laying on the platen  400 . The vacuum is extended particularly due to the presence of the overdrive wheels  340 , and the ribs  370 , which create a larger empty space between the medium  130  and the platen  400 . 
     Furthermore, the design of this part of the holddown system helps the printer to reduce the cockle effect on the printout. 
     Tensioning the paper in the feeding direction intuitively does not help, because cockle wrinkles mainly extend in the feeding direction as well. Anyway, overdrive forces can reduce the height reached by the cockle wrinkles by as much as a half. In addition, it was noted how the paper works in compression, some very thin papers may even buckle and create loops between the main roller  300  and the print zone. 
     This means that the presence of a secondary roller  345 , having the function of tensioning the paper during the printing operation, may help in controlling the occurrence of the cockle wrinkles in the printout. 
     However, it should be kept in mind that such a secondary roller  345  provide the printer  110  with more capabilities, which will be described further. 
     In this portion of the platen  400 , vacuum is furnished through the plurality of holes  350  and the gap between each overdrive wheel  340  and its surrounding portion of the platen  400 . 
     Vacuum is used to provide the force between medium and overdrive wheels  340 ; the design has been done in such a way that it can provide the required force to the overdrive wheel  340 , preferably comprised between 0.6 N and 1 N, in this example 0.8 N per each wheel  340 , without employing starwheels. Elimination of starwheels is an important issue since it helps to avoid a) the risk of damaging the printout with starwheel marks, b)the need to employ a mechanism or a structure to hold the starwheels themselves. 
     In addition, according to this example, in order to transmit the proper traction force to the medium, the overdrive interference, i.e. the distance between the surface of the platen  400  and the top of the a overdrive roller  340 , is preferably maintained between 0.3 mm and 0.6 mm. Below 0.25 mm the traction falls quickly, towards zero traction at zero interference; if the interference is bigger than 0.65 mm, wrinkles created by the overdrive roller  340  can extend to the print zone  450 . 
     In FIGS. 2 and 3 it is also shown a first reference sign  390 , according to this example, in the form of a phantom line, but any kind of suitable reference can be employed, e.g. a continuous or dotted line. This first reference  390  is traversing all the platen  400  from the right to the left side in the scan axis (Y) direction. Preferably the first reference  390  is tangent to the slot  320 , on the side closer to the main roller  300 , and it could be in colour and/or in under-relief. This feature is used preferably in combination with a second reference  392 , placed at one side end of the platen  400 . The second reference is traversing the platen  400  in the media axis (X) direction, preferably starting from the first reference  390  to the end of the platen  400  further from the main roller  300 . 
     Accordingly, the user is provided with two references for placing correctly the edges of a cut media sheet, or a media roll, onto the platen  400  in order to load and feed the sheet into the printer  110 . Particularly, the first reference  390  is providing the user with a reference which can fully match an edge of the sheet, so simplifying the loading operation. 
     In this embodiment a second reference is placed at one end of the platen  400 , which is conventionally located at the right end of the printer, respect to the user placing the sheet. 
     This combination of references enhances the easiness of the loading operation by the user, reducing the occurrence of inaccurate positioning of the medium, which may cause a paper jam, during the feeding or the printing phases. 
     Referring now to FIG. 4, it is shown the main roller  300  and one of the pinch wheels  310  co-operating with one protrusion  405  of the platen  400  holding the medium  130 . One of the overdrive wheels  340 , tensioning the medium  130  in the print zone  450 , is also shown. From FIG. 4 it is better depicted that the vacuum channel  380  does not extend underneath the complete print zone  450 , particularly the vacuum channel  380  is partially overlapped by a portion of the print zone  450  which is less than 90% of the complete print zone  450 , preferably less than 50%, and more preferably about 30-35%. 
     Referring now to FIG. 5, a diagram showing nominal values supplied by the vacuum source, a fan, employed in this example. Those values have been measured running the fan at its full power of 24 V. The pressure unit on the Y axis is Pascal and air flow unit on the X axis is m 3 /min. 
     Vacuum required to eliminate cockle wrinkles in a printer would be so high that is normally unfeasible; in fact, high vacuum may suck the ink right through the paper and at the same time generate a lot of noise. 
     The vacuum level has been preferably set between 380 Pa and 440 Pa, which can be achieved by a small fan, producing acceptable level of noise, i.e. about 65 dBA. 
     Several test run by the Applicant have verified that this level is enough for rigid roll paper, like high glossy photo roll, in order to flatten the curling during printing. In addition, it has been verified with many print modes that this level of vacuum is unlikely to suck the ink through the paper. 
     Five operational levels of vacuum have been defined for the following activities: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Normal CAD printing 
                 21 V 
               
               
                   
                 Thick paper and high density prints 
                 24 V 
               
               
                   
                 Loading and cutting media 
                 22 V 
               
               
                   
                 Hoiddown during cut sheet ioading 
                 16 V 
               
               
                   
                 Managing thin Japanese rice paper, always 
                 14 V 
               
               
                   
                   
               
             
          
         
       
     
     According to FIG.  5  and to the tests run by the Applicant, one characteristic of the fan considered particularly valuable has been the capability of providing a pressure of 300 Pa, when the air flow is at about 0.5 m 3 /min. 
     Now reference is made to FIGS. 1,  2 ,  3  and  4  in order to describe how a medium can be loaded into, printed with and outputted from the printer  110 . 
     LOADING OPERATION 
     A loading operation can be activated in a plurality of different ways, e.g. by a user selection of the operation from the front panel  120  of the printer  110 , or more easily, as in this embodiment, by opening the cover  122 . 
     Once that the loading operation is activated the vacuum source is powered on, at 16 V, in order to help the loading operation. 
     In the following an example on how to load a cut sheet of media will be described. However a skilled in the art may appreciate that, similarly, a roll of media may also be load. 
     In order to load a cut sheet of media into the printer, a user should place the top edge of the medium  130  in correspondence of the first reference  390 , and the top portion of the right edge of the same medium  130  in correspondence of the second reference. During all this phase the vacuum on is helping the user in holding the medium  130  adherent to the platen  400 , so that small adjustments in the position of the medium  130  can be done using only one hand. Accordingly, the risk of inadvertently damaging the medium  130  (e.g. due to fingerprints or to the fall of the medium  130  on the ground) are minimized. 
     Once that the loading step has been completed, the medium  130  is fed into the printer for the printing phase. The feeding step may be activated in several ways. For instance, it is automatically activated after that sensors have sensed the proper positioning of the medium  130 , or by user selection of the feeding operation from the front panel  118 , or, as in this embodiment, by closing the cover  122 . 
     Once that feeding step is activated, the overdrive wheels  340  start to move clockwise in order to advance the medium  130  towards the main roller  300 , until the medium  130  itself is engaged between the main roller and the pinch wheels  310 . The vacuum is maintained on to transmit the traction force from the overdrive wheels  340  to the medium  130 . 
     As soon as main roller is fed with the medium  130 , conventional steps are carried on in order to remove the medium  130  from the platen  400  and to convey the medium  130 , into a feeding guide for a subsequent printing phase. Finally, the vacuum source is switched off. 
     PRINTING OPERATION 
     When a printing operation is activated, the main roller  300  in co-operation with the pinch rollers  310  and other conventional elements of the printer  110 , starts to convey the medium, from the feeding guide, across the print zone defined onto the platen  400 . Contemporarily, the vacuum source is switched on, at a power according to the kind of media employed and/or to the kind of plot which will be printed. Thus, the vacuum is keeping the medium  130  substantially flat onto the print zone  450  defined on the platen  400  to allow a quality printing. Preferably, before starting printing, the main roller is advancing the medium towards the overdrive wheels  340 , to have the medium engaged by them. In fact, as already explained, the medium should be tensioned in the media direction X to keep the cockle wrinkles under control. Alternatively, the printing may start even if the overdrive wheels  340  are not engaged yet with the medium. 
     Once that the medium  130  is also engaged by the overdrive wheels the advance of the medium in the print zone along the media axis direction X is performed by a pushing force provided by the main roller  300 , moving counter-clockwise, and the pinch wheels  310 , moving clockwise, and by a pulling force provided by the overdrive wheels  340 , moving counter-clockwise too. 
     Conventional printing steps allow the carriage assembly  100  to move the printheads  102 ,  104 ,  106 , and  108 , relative to the medium  130  along the scan axis Y, in order to apply ink to the medium  130 , in one or more passes, and so reproducing the desired image. 
     OUTPUTTING OPERATION 
     An outputting operation may be activated for instance a) automatically when a printing operation has been completed or aborted, or b) manually by a user request. 
     When the operation is activated the printer verifies if the medium  130  to be outputted is a cut sheet or a roll. If the medium  130  is a roll a cutting step is performed. This means that the medium  130  is advanced in the cutting position and the vacuum source is powered on, at 22 V, to hold the medium substantially flat and minimize the movement of the same while a blade, not show, is traversing the medium  130  along the scan axis Y to cut the medium. 
     If the medium  130  is a cut sheet or after that the roll has been cut, the medium is advanced in the media axis direction X towards the front of the printer  110 , i.e. further from the main roller  300 . 
     The advancement of the medium is performed by the counter-clockwise movement of the overdrive wheels  340 , frictionally engaging a portion of the back of the medium  130 , due to the negative pressure generated by the vacuum source applied to the medium  130 . If a cut sheet of media  130  is still engaged with the main roller  300  and the pinch wheels  310 , those elements are also co-operating to advance the medium. 
     In case that the printout printed onto the medium  130  requires an additional dry time, the overdrive wheels movement is stopped when most of the printout is advanced out of the printer, e.g. as shown in FIG.  1 . The vacuum source is kept on for the required time to dry the medium, so holding only an end region of the medium  130 , preferably having length equal to the width of the medium  130  and about 50 mm in the media axis direction X. 
     Finally, the vacuum is switched off to drop the medium  130 , e.g. into a conventional collecting bin, not shown. 
     The skilled in the art may appreciate that, in accordance to this preferred embodiment, the same holddown system, e.g. having one platen and one vacuum source, may be capable of being employed to perform a plurality different operations, such as loading and feeding operation, printing operation and outputting operation. However, each of this operations may be performed also using independent holddown systems, i.e. independent holddown surfaces and/or independent vacuum source. Furthermore, the skilled in the art is now aware that only some of those operations may be performed by means of a vacuum holddown system while the remaining ones may be performed employing conventional systems.