Electrophotography – Image formation – Transfer
Reexamination Certificate
2000-10-04
2002-09-24
Braun, Fred L (Department: 2852)
Electrophotography
Image formation
Transfer
Reexamination Certificate
active
06456816
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electrostatography and, more particularly, to a reproduction method and apparatus that employs transfers of toner images to and from intermediate transfer members.
BACKGROUND OF THE INVENTION
In an electrophotographic process a photoconductive element is initially electrically charged. An electrostatic latent image is first formed by image-wise exposing the photoconductive element using an exposure source such as a laser scanner or an LED array. The latent image is developed into a visible image by bringing the electrostatic latent image into close proximity to a developer such as contained in a magnetic brush or other known type of development station. The developer can be what is typically referred to as a single component developer containing toner particles. Also, the developer can have two or more components with nonmarking magnetic carrier particles and marking non-magnetic toner particles. To produce color images, electrostatic latent images corresponding to the appropriate color are separately formed and developed. The resulting toner images are transferred to a receiver, such as a paper or a plastic sheet for example, preferably by using an electrostatic field to urge the toner in the direction of the receiver. The electrostatic field is commonly applied in one of several manners. For example, charge can be sprayed on to the back of a receiver using a corona device. However, it is frequently preferable to use an electrically biased transfer roller to apply the field, especially in instances where color images are to be produced.
It is often advantageous to transfer a toner image from an imaging member to an intermediate transfer member (ITM) in a first transfer nip, and from ITM to a receiver, e.g., paper, in a second transfer nip, rather than transferring directly from imaging member to a receiver. The second nip can be formed in a variety of ways, such as utilizing a biased transfer drum wrapped in a tensioned, electrically biasable transport web to which a receiver sheet is attached, or by using a dielectric transport web and an electrically biasable backup transfer roller. Color images are produced by developing and transferring toner images corresponding to the appropriate color separations. For example, to produce a full-color image, color separations having cyan, magenta, yellow and black toners typically are used. Color separation toner images are commonly transferred sequentially, in a registered manner, to an ITM and, subsequently, the complete color toner image is transferred from the ITM to a receiver in one step. Alternatively, individual color toner separation images can be transferred to separate ITMs, or to different sections of a single ITM, and then transferred in register to a receiver in multiple steps. Following a final toner transfer, a toner image on a receiver is permanently fixed using known processes such as thermal or radiant fusing.
In an electrographic process, a latent electrostatic image is created on a dielectric imaging member, e.g., by ionography or by electrified stylus or by other known means, and then toned. The resulting toner image, which can be a color separation image, can then be transferred to an ITM, and subsequently transferred to a receiver and fused as described above. An electrographic press can include a sequential series of modules, each module having an electrographic dielectric imaging member and an ITM for generating and transferring a color separation toner image to a receiver.
The nip is an engagement area of a roller under pressure with another member. This engagement area will result in some deformation. Pressure nips formed by rollers coated with elastomers are known to exhibit overdrive. Overdrive is the phenomena where the tangential speed of a roller within the nip engagement area is actually different than the tangential speed of the roller in an area not near the nip. Overdrive can be understood from a hypothetical consideration of a roller having an externally driven axle, frictionally driving a movable planar element having a nondeformable surface. If the external radius of the roller in an area not near the nip is R and the tangential speed of the roller in this area not near the nip is v
0
, then the surface velocity v of the distorted portion of the roller which is in nonslip contact with the planar surface is given by Equation 1.
v=&lgr;&ohgr;R
Equation 1
where &ohgr; is the nominal angular rotational rate of the roller around its axis (radians per unit time) and where &lgr; is a peripheral speed ratio defined by
&lgr;=(
v/v
0
).
The value of &lgr; is determined principally by the roller materials effective Poisson's ratio and moduli, by the engagement and drag torque forces. The Poisson ratio, &ngr;, of high polymers (those having a high molecular weight) approaches 0.5, and approaches zero for very soft polymeric foams. It has been shown in theoretical model computations 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 for a special case of a rigid cylindrical roller coated by a layer of deformable material frictionally driving with no drag, a nondeformable moving planar element, the deformable material should have a value of Poisson's ratio of about 0.3 in order to have negligible overdrive, i.e., &lgr;≈1. 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 within the nip of the coated roller (and hence that of the planar element) is greater than v
0
. For values of Poisson's ratio smaller than about 0.3, the circumference of the roller is less than 2&pgr;R, producing underdrive of the planar element with respect to the roller, i.e., the surface speed is smaller than v
0
within the nip. Conversely, if a nondeformable planar element frictionally drives, with negligible drag, a roller having &ngr; less than about 0.3 and causes it to rotate, one can 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.
A foam or sponge can include a “felted” material, as is well known in the art. Felted foams can be made, for example, by compressing under heat, typically uniaxially, an elastomeric, previously made foam, followed by cooling it under compression and then removing the compressive load. Felted foams have anisotropic mechanical properties. For example, both the Young's modulus and Poisson's ratio of a felted foam material made by uniaxial compression will be different along the direction of compression that lies in a plane at right angles to the direction of compression. Moreover, Poisson's ratio, which tends to be small for soft foams, can even take on negative values in felted foams or sponges.
Overdrive associated with two materials in contact in a pressure nip can result in squirming and undesirable stick-slip behavior. Such behavior can adversely affect image quality when more than one intermediate transfer member is used to make a color print in a color reproduction machine, e.g., by degrading the mutual registration of color separation images if the amount of overdrive produced by each ITM roller varies from roller to roller, or, by causing toner smear. Moreover, variations in overdrive from a given roller, sometimes referred to as “differential overdrive”, can occur axially or radially along the length of a transfer nip, such variations being produced, for example, by local changes in engagement, such as caused by runout, or by a lack of parallelism, or by variations of dimensions of the members forming a transfer nip.
An electrophotographic printing press can include a number of modules, one module for e
Chowdry Arun
Cormier Steven
May John W.
Mills Borden H.
Patterson Bonnie
Braun Fred L
Leimbach James D.
NexPress Solutions LLC
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