Digital printing machines can take on a variety of configurations. One common process is that of electrostatographic printing, which is carried out by exposing a light image of an original document to a uniformly charged photoreceptive member to discharge selected areas. A charged developing material is deposited to develop a visible image. The developing material is transferred to a medium sheet (paper) and heat fixed.
Another common process is that of direct to paper ink jet printing systems. In ink jet printing, tiny droplets of ink are sprayed onto the paper in a controlled manner to form the image. Other processes are well known to those skilled in the art. The primary output product for a typical digital printing system is a printed copy substrate such as a sheet of paper bearing printed information in a specified format.
The output sheet can be printed on one side only, known as simplex, or on both sides of the sheet, known as duplex printing. In order to duplex print, the sheet is fed through a marking engine to print on the first side, then the sheet is inverted and fed through the marking engine a second time to print on the reverse side. The apparatus that turns the sheet over is called an inverter.
FIG. 1 (prior art) shows a state-of-the-art digital printing machine 20. Printer 20 includes a marking engine 22, a media sheet feeder 24, and a finisher 26. Printer 20 has an inverter 28 to turn the sheet over for duplex printing. A second inverter 30 turns the sheet over again, so that the first side is facing up. Notice that the second inverter 30 has an optional bypass 32.
A commonly employed inverter 28 is shown in FIG. 2. Sheets following a prior art inversion path 49 enter at a sheet entry 34, travel downward through leg 35, nips 36 and 38 into a holding portion 40. The nip 38 then reverses, sending the sheet through nip 42 to the opposite leg 44, and out the sheet exit 46 onto the process path 48. The sheet is inverted, and the trail edge becomes the lead edge. Inverter 28 takes a considerable volume of space within the printer, because the legs 35, 44 must be of a minimum radius to avoid marking or creasing the sheets. Because the legs are sheet material such as metal, the paper slides across the surface of the metal, creating marks. This construction also tends to limit the speed at which sheets can be conveyed through inverter 28.
Accordingly, there is a need to provide a sheet inverting system that will occupy a minimum of volume within a printer.
There is a further need to provide a sheet inverting system of the type described and that will match the high production rate of a digital printing machine.
There is a still further need to provide a sheet inverting system of the type described and that is capable of selective simplex and duplex operation.
There is a yet further need to provide a sheet inverting system of the type described and that is mechanically simple and robust, thereby minimizing cost and avoiding the problems associated with the prior art.