The present invention relates to a sheet handling device including a sheet transport path, a transport mechanism adapted to advance a sheet along the transport path with a non-uniform speed, and a discharge mechanism arranged at the transport path for taking over the sheet from the transport mechanism and discharging it into a tray.
Ink jet printers, for example, often work on a scanning principle. That is, a carriage which includes a number of ink jet printheads reciprocates across a sheet. In each pass of the carriage, a number of pixel lines are printed on the sheet by means of the printheads which eject droplets of ink onto the sheet in accordance with image information supplied to the printheads. Between the printing passes, a transport mechanism advances the sheet by a suitable sheet advance step. In large format printers, the sheet advance movement is simultaneously performed in different parts of the printer. For example, while a leading part of a sheet has already been discharged onto the tray by the discharge mechanism, a rear part of the sheet may still be printed and advanced by the transport mechanism.
In order to achieve a high printing speed, the sheet advance movements have to be performed as quickly as possible. However, the printing quality depends on the accuracy of the sheet advance movements. When the sheet is simultaneously advanced by different transport means, one transport means may influence the accuracy of the movement of another transport means. For example, a discharge mechanism that discharges the printed sheet onto a tray may exert a force onto the sheet portion which is still being printed. This problem is even more pronounced in large format printers where the load that has to be exerted to move the sheet is higher. Moreover, the faster the sheet advance step is to be performed, the more power is required of the transport means. This leads to a higher load on the sheet and an increased noise generation.
From JP 62211263 A, a transport mechanism for discharging a sheet is known that transports the sheet from a pair of fixing rollers towards a discharge tray. A first driving roll is driven at a peripheral speed which is higher than that of the fixing rolls. Thereby, a waving of the sheet is reduced. However, the driving roll exerts a pulling force on the sheet during the fixing process.
JP 08268615 A also shows a transport mechanism for discharging a sheet that transports the sheet from a pair of fixing rollers towards a discharge tray. A sheet is held between the fixing rollers and first carrying rollers while a previous sheet is discharged by further carrying rollers at a speed which is higher than the paper carrying speed of the fixing rollers. The first carrying rollers are driven by means of a torque limiter, but nevertheless exert a pulling force on the sheet while it is held between the fixing rollers and the first carrying rollers.
From JP 2004196483 A, a transport mechanism is known which is adapted to appropriately maintain the amount of deflection of a sheet being conveyed along a curved path between two pairs of conveyance rollers. The amount of deflection of the sheet is measured by a pivotable contact arm, and the peripheral speed ratio of the two pairs of conveyance rollers is controlled to appropriately maintain the amount of deflection. However, a constant amount of deflection can only be maintained when the two pairs of conveyance rollers are continuously driven and their speed variations are small.
From WO 2004/041542 A1, a device for delivering a printed coupon is known wherein a web is advanced into a reserve chamber by a transport roller which participates in printing the web. The web is further advanced by a pair of discharge rollers at an discharge opening of the chamber. The transport rollers and the discharge rollers are operated to hold the web under tension against a resiliently pivotable arm during the printing process and to pull the web against a cutting edge after the printing is completed. Thus, a pulling force is exerted on the web during printing and also for cutting the web after printing.