Generally, in a commercial electrostatographic reproduction apparatus (such as copier/duplicators, printers or the like), a latent image charge pattern is formed on a uniformly charged photoconductive or dielectric member. Pigmented marking particles (toner) are attracted to the latent image charge pattern to develop such image on the dielectric member. A receiver member, such as paper, is then brought into contact with the dielectric member and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric member and the image is fixed or fused to the receiver member by heat and/or pressure to form a permanent reproduction thereon. In a typical fusing process where the toner is fused to the paper or receiving member, two rolls are used through which the paper travels during the toner fusing. One roll, usually the harder roll, is a fuser roll, the second roll is the pressure roll or the softer roll.
Typical pressure rolls (“Softer Roll”) that are used in a fusing system have an elastomeric coating like silicone rubber which may or may not have a thin layer of another material over the surface of the roll. A functional nip is formed when the softer roll is pressed into the fuser roll (“Harder Roll”). The fuser roll generally comprises a metal core with a hard Teflon(™ of DuPont) coating or thin elastomer.
The pressure rolls or softer rolls are typically constructed of a cylindrical steel core or rod having positioned over it an elastomer or rubber material cylindrical roll. At the ends of the elastomer roll, generally, are positioned end plates or steel washers. In any system when a hard roll (fuser roll) is pressed against and contacts a softer roll nips are formed throughout the length of the pressure roll in contact with the fuser roll. These pressure zones ultimately cause the softer material to contact the support plates and create wear, shortening roll life and causing debris in the system. Also, once excessive wear takes place and an uneven nip is formed, improper fusing of the toner can result causing imperfect copies on the paper or receiving member. In addition, because of this wear problem, frequent changes requiring new softer rolls are required. Generally, the elastomeric rolls have typically been manufactured from a single elastomeric material, such as silicon rubber, of a uniform hardness as determined by a durometer. From both a cost standpoint and performance standpoint, any improvement in the softer roll construction that would extend roll life and improve performance at the fuser station would be very desirable. Thus, eliminating an uneven nip and material deterioration of the pressure roll would extend pressure roll life and improve fusing performance.
Nip forming rollers in fusers are frequently not long enough to avoid the effect the lack of constrain on the end face of the rubber has on the nip width uniformity at the edges of the widest paper path. That is, the nip width decreases as the proximity to the roller end decreases due to rubber deforming axially at the ends versus only circumferentially in the middle of the roller when subjected to radial interference with the indenting roll. In the past, a rigid washer has been mounted over the core of the roller and biased against the rubber with a spring (wavy washer) and constrained with a retainer ring. Or a fairly rigid washer has been fastened to the core end via several small screws. The screwed-on prior art method uses less space along the axis of the roll than the wavy washer method but requires very tight control on the rubber to core dimension since there is very little compliance. Also, thermal expansion of the rubber produces large increases in the washer to rubber load.
One reason for rapid degradation of the elastomer end face at the washer contact area causing formation of an uneven or non-uniform nip is the presently used rigid washers that are located at each end of the core of the pressure rolls. These washers are biased against the rubber coating on the soft pressure roll. These washers are fastened to the core ends via several small screws. Rubber trimming tolerance and thermal expansion was to large variation in contact force between the washer and rubber. The use of these washers invariably causes rubber wear allowing the nips to get narrower as it gets closer to the roll end as the roll ages. As above noted, a non-uniform nip causes uneven contact with the paper, uneven fusing of the toner, paper wrinkles and excessive pressure roll wear eventually requiring replacement.