The present invention relates to a sheet feeder and electrostatographic printing machines employing such sheet feeder which enables the improved feeding of copy substrates which have been passed through the printing machine one time and have a heat fused toner image thereon through a second pass into the printing machine to provide a duplex image or a second color image on the same side the first image is printed.
In an electrostatographic reproducing apparatus commonly in use today, a photoconductive insulating member is typically charged to a uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the usual document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with developing powder referred to in the art as toner. Most development systems employ a developer material which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development the toner particles are attracted from the carrier particles by the charge pattern of the image areas in the photoconductive insulating area to form a powder image on the photoconductive area. This image may subsequently be transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure. Following transfer of the toner image to a support surface, the photoconductive insulating member is cleaned of any residual toner that may remain thereon in preparation for the next imaging cycle.
Commercial applications of this apparatus have become increasing complex offering the users a variety of printing and copying options. One of the options of particular interest to a growing variety of customer applications is the capability for such machines to produce duplex prints and copies. By duplex copying or printing it is intended to define copies on which both sides of a single sheet are provided with fused toner images. Another capability being provided in copying and printing machines with increasing frequency is that of being capable of providing two color or highlight color simplex, toner images on one side only of the copy or print. Both of these capabilities present significant problems in the handling of the copy substrates in an automatic printing or copying machine since the copy substrate on which the duplex image is formed or the second color or highlight color image is formed has already passed through the printing machine once to have the first toner image formed. In completing the first toner image the copy substrate is passed through a fuser which typically is at a temperature of about 400.degree. F. to thereby raise the temperature of the thermoplastic toner material to a level at which it will coalesce and penetrate into the substrate typically the paper fibers. As a result of this heating which is also typically performed in a heated roll fuser using both heat and pressure between the fuser roll and a pressure roll, the copy substrates are subjected to sufficient stress that they have a slight amount of curl or other nonplanar deformity resulting in poor stacking between adjacent sheets. As a result, the first produced copy substrates having fused toner images thereon when collected in a supply tray within the printing machine for subsequent duplex or second color reproduction do not stack the way virgin copy substrates stack but rather stack with nonuniform gaps or air pockets between adjacent copy substrates. This tendency of such copy substrates to fluff creates feeding difficulties in sheet feeders used to feed the copy substrates from the collection tray for the second pass through the printing or copying apparatus. The difficulties are particularly pronounced in sheet feeders wherein a feed roll is reciprocally moved between a sheet feeding position where it is in feeding engagement with the top sheet in a stack of sheets in the supply tray and a retracted position wherein it is retracted from feeding engagement with the top sheet. The difficulty is encountered in that typically the feed roll is rotated at times other than when it is actually feeding a sheet while in the feeding position and a fluffed sheet may come in contact with the feed roll producing undesired sheet feeding including random feeds and multisheet feeds. In some applications this difficulty may be exacerbated by the generation of static electricity in the feeding system resulting in the top sheet in the stack being electrostatically attracted to the feed roll even when it is not rotating so that upon subsequent rotation it can also result in undesired sheet feeding. This occurs because the movement of the feed roll as well as the reciprocal movement of the feed head can generate static electricity even in virgin paper. This is particularly true in those systems wherein the feed roll and the feed head are moving at times when they are is not feeding sheets. Thus, even if a feed roll is not rotating it may pick up a sheet because of the static electricity so that when the feed roll is subsequently rotated an undesired feed may be realized.
While the presence of a static eliminator devices may be effective in removing some of the effects of static electricity they have no effect on the undesired feeding of the top sheet in a stack created by the curl in the paper. The difficulties described above may be more fully appreciated with reference to schematics illustrated in FIGS. 1A and 1B. In FIG. 1A, which is a stack of fresh or virgin paper, paper that has not been passed through the printing or copying machine fuser system at least once, is provided on the sheet support platform 10. The articulating or reciprocating movable feed roll 12 is illustrated in solid line as being in the retracted standby position and in dashed line as being in sheet feeding engagement with the top sheet in the stack and the retard pad. In FIG. 1B, however, the sheets on the sheet support platform have been passed through the printer or copy or fuser system at least once resulting in the creation of curl in the copy sheets so that they do not lie flat during stacking but rather air pockets are created between adjacent sheets such that even when the sheet feed roll is in the retracted standby position and rotating it is possible for the top sheet in the stack of sheets on the sheet support platform to contact the rotating feed roll and be fed forward at a time when such feeding is not desired.