Electrophotography – Image formation – Transfer
Reexamination Certificate
2001-02-16
2003-04-15
Beatty, Robert (Department: 2852)
Electrophotography
Image formation
Transfer
C399S313000, C399S328000
Reexamination Certificate
active
06549745
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to apparatus and methods for using frictional drives including conformable rollers in electrostatography, and more particularly to the use of frictional drives for transferring toner images in electrophotography.
BACKGROUND OF THE INVENTION
During the production of color images in an electrostatographic engine in general and in an electrophotographic engine in particular, latent images on photoconductive surfaces are developed by electrostatic attraction of triboelectrically charged colored marking toners. A latent image is created in a color electrophotographic engine by exposing a charged photoconductor (PC) using, for example, a laser beam or LED writer. Individual writing of each latent image must be properly timed so that the various toner images developed from the latent images can be transferred in registry. Each of these toner images corresponds to one of several color separations that will make up a final color image. The toned image separations must then be transferred, in register, to either a receiver or to an intermediate transfer member (ITM). The toner images can be transferred, either sequentially from a plurality of photoconductive elements to a common receiver in proper register, or transferred, sequentially, in proper register, to one or more ITMs from which all images are then transferred to a receiver. Alternately, each photoconductive surface may be associated with its own ITM, which transfers its toned image, in proper register with those of the other ITMs, to a receiver, for the purpose of enhancing the transfer efficiencies as described more fully in T. Tombs et al., U.S. Pat. No. 6,075,965. A toner image on the receiver is thermally fused in a fusing station, typically by passing the receiver through a pressure nip which includes a fuser roller and a pressure roller.
A key feature is that transfers must be performed in proper registry. The degree of misregistration that can be tolerated in an acceptable print depends on the image quality specifications. For high image quality color applications, allowable misregistration is typically less than 0.004 inch (0.1 mm) and preferably less than 0.001 inch (0.025 mm). Misregistration is often examined using 10× to 20× loupes to determine relative positions of interpenetrating fiducial line or rosette patterns. In systems involving elastomeric rollers and in particular in machines including compliant incompressible elastomeric rollers as intermediate transfer members as described by D. Rimai et al., U.S. Pat. No. 5,084,735, the rollers are known to deform as they roll under pressure against a photoconductive surface which may include a web or a drum. These intermediate transfer members also undergo deformations as they roll against receiver materials either as continuous webs or as cut sheets that can be supported by a web or by a backup roller assembly, or by combinations of these. Other prior art disclosing ITMs include U.S. Pat. Nos. 5,110,702; 5,187,526; 5,666,193 and 5,689,787.
Deformations of conformable members produce a phenomenon known as overdrive. Overdrive refers to the fact that in a nip including an elastomeric roller and a relatively rigid roller that roll without slipping, the surface speed of the rigid roller exceeds the surface speed of that portion of the elastomeric roller that is far from the nip. Far away from the nip means at a location where any distortions caused by the nip are negligible. The difference in peripheral speeds far from the nip is a result of the strains occurring in the elastomeric roller surface as it approaches and enters the nip.
The concept of overdrive may be better understood by referring to the sketches in
FIGS. 1 and 2
.
In
FIG. 1
a
, a rigid cylindrical wheel or roller is driven without overdrive. In such an example, each point on the periphery has a velocity v
0
given by the product of the angular velocity &ohgr; and the radius r of the roller, i.e., v
0
=&ohgr;r.
In
FIG. 1
b
, a deformable externally driven roller is illustrated. The deformation illustration is exaggerated to facilitate explanation of the concept that when a substantially incompressible compliant member is in a transfer nip, for example, a deformation will occur that causes the radius to be smaller in the nip area but to bulge out at pre-nip and post-nip areas. The dotted line shows the original circular rigid case of
FIG. 1
a
for comparison. The relationship of v
0
=&ohgr;r still holds true for points on the roller far from the nip area where there is no deformation. However, this relationship is not true for the points in the pre-nip, nip and post-nip areas. For the roller illustrated in
FIG. 1
b
the speed of a point in the nip area has a higher magnitude than that far from the nip. The speed ratio of the roller surface in the nip divided by the speed at a point far from the nip area characterizes overdrive.
More particularly consider, for example, a conformable roller having an externally driven axle, frictionally driving with negligible drag a movable planar element having a nondeformable surface. If the external radius of the roller far from the nip is r and the peripheral speed of the roller far from the nip is v
0
, then the surface velocity V
nip
of the distorted portion of the roller in nonslip contact with the planar surface is given by
v
nip
=&lgr;&ohgr;r
where &lgr; is a speed ratio defined by
&lgr;=(
v
nip
/v
0
).
As defined here, overdrive (or underdrive) is numerically equal to the absolute value of the speed ratio minus one. The value of &lgr; is determined principally by an effective Poisson's ratio of the roller materials, such as produced by a roller including one or more layers of different materials, and secondarily, by the deformation geometry of the nip produced by the engagement. The Poisson ratios of high polymers, including elastomeric polymers which for practical purposes are almost incompressible, approach 0.5. The Poisson ratios for highly compressible soft polymeric foams approach zero. It has been shown by K. D. Stack, “Nonlinear Finite Element Model of Axial Variation in Nip Mechanics with Application to Conical Rollers” (Ph.D. Thesis, University of Rochester, Rochester, N.Y. (1995),
FIGS. 5-6
and
5
-
7
, pages 81 and 83) that the value of Poisson's ratio for &lgr;=1 is about 0.3 for a roller driving a rigid planar element. For values of Poisson's ratio larger than about 0.3, the circumference of the roller distorted by the nip is greater than 2&pgr;r, producing overdrive of the planar element with respect to the roller, i.e., the surface speed v
nip
of the distorted portion of the elastomeric roller within the nip and hence that of the planar element is greater than v
0
(i.e., &lgr;>1). For values of Poisson's ratio smaller than about 0.3, the circumference of the elastomeric roller distorted by the nip is less than 2&pgr;r, producing underdrive of the planar element with respect to the roller, i.e., the surface speed v
nip
within the nip is smaller than v
0
(i.e., &lgr;<1). Conversely, if a nondeformable planar element frictionally drives, with negligible drag, a roller having a Poisson ratio less than about 0.3 and causes it to rotate, one may speak of overdrive of the roller with respect to the planar element because the surface speed of the driven roller far from the nip is faster than the speed of the planar element.
With reference to
FIG. 2
b
, when a roller transfer member formed of an elastomer that has a Poisson ratio of about 0.45 to about 0.5 is driving a rigid planar element that is moving through a nip and there is no slippage between the roller and the rigid element, the rigid element will be overdriven relative to the speed of the roller far from the nip. Where the roller is formed of a compressible material (i.e., experiences relatively large volume reduction upon compression), such as a foam, the distortion of the roller may be such (see
FIG. 2
a
) that the surface of the roller is contracted rather
Dickhoff Andreas
May John W.
Quesnel David J.
Rimai Donald S.
Tombs Thomas N.
Beatty Robert
NexPress Solutions LLC
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