Electrophotography – Image formation – Fixing
Reexamination Certificate
2002-09-16
2004-09-21
Chen, Sophia S. (Department: 2852)
Electrophotography
Image formation
Fixing
C219S216000
Reexamination Certificate
active
06795677
ABSTRACT:
BACKGROUND
This invention relates generally to electrostatographic imaging, and more particularly, it relates to a high-speed heat and pressure belt fusing apparatus for fixing images to a final substrate.
In a typical electrophotographic copying or printing process, a charge retentive surface such as a photoconductive member is charged to a substantially uniform potential so as to sensitive the surface thereof. The charged portion of the photoconductive member is selectively exposed to light to dissipate the charges thereon in areas subjected to the light. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing one or more developer materials into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules either to a donor roll or to a latent electrostatic image on the photoconductive member. When attracted to a donor roll the toner particles are subsequently deposited on the latent electrostatic images. The toner powder image is then transferred from the photoconductive member to a final substrate. The toner particles forming the toner powder images are then subjected to a combination of heat and/or pressure to permanently affix the powder images to the copy substrate.
In order to fix permanently or fuse the toner material onto a substrate or support member such as plain paper by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers and/or into the pores of the support member or otherwise upon the surface thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the final substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the substrate to which the toner images are electrostatically adhered is moved through a nip formed between the pressure engaged rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. In a Nip Forming Fuser Roll (NFFR), the heated fuser roll is provided with a layer or layers that are deformable (i.e. conformable) by a harder pressure roll when the two rolls are pressure engaged. The length of the nip determines the dwell time or time that the toner particles remain in contact with the surface of the heated roll, the dwell time being also determinative of the fuser's speed.
The layer or layers usually comprise an abhesive (low surface energy) material for preventing toner offset to the fuser member. Three materials, which are commonly used for such purposes, are fluoropolymers, fluoroelastomers and silicone rubber.
Roll fusers work well for fusing color images at lower speeds since the required process conditions such as temperature, pressure and dwell can be achieved. When process speeds approach faster speeds, for example 100 pages per minute (ppm), roll fusing performance is no longer acceptable. As fusing speed increases dwell time must be maintained above a minimum value which means an increase in nip length. Increasing the nip length can be accomplished either by increasing the fuser roll rubber thickness, and/or reducing the modulus and/or increasing the outside diameter of the roll. However, each of these solutions reach their maximum effectiveness at about 100 ppm. Specifically, for an internally heated fuser roll, the fuser roll deformable layer thickness is limited by the maximum temperature the material forming the layers can withstand, and the thermal gradient across the layer. The roll size also becomes a critical issue for reasons of space, weight, cost and substrate stripping therefrom.
In order to obtain much higher fusing speeds than heretofore possible for color, very large or long fusing nips are necessary. One way to achieve longer fusing nips for this purpose is to use a thick deformable belt instead of a fuser roll with a thick deformable layer or layers. Due to poor thermal conductivity, however, it is necessary to heat the outer surface of a thick elastomer belt over an extended contact zone using a source of thermal energy. To create a long nip for extending fusing dwell time, it is desired that the belt be as thick as possible. However, belt flexibility can be compromised with relatively large belt thicknesses. Additional nip length can also be obtained using an elastomeric layer or layers on a pressure roll that contact the internal surface of the thick belt. The thicknesses of the elastomers on the pressure roll and the fuser belt along with other characteristics of the elastomers such as Shore A hardness contribute to the desired characteristics of the fusing nip. The thickness and the durometer of both elastomers can be varied to obtain the desired dwell times in the fusing nip.
Heat transfer to the belt from an external source such as an external heater roll, especially for belts with macro non-uniformities and/or a rough surface can be improved and the belt life extended. External heating of such a belt can be accomplished though the wrapping of a portion of the belt around the external heater roll to create a large wrap therearound and, therefore a large area of contact therebetween. Such contact between the belt and the external heater roll results in a low-pressure contact, in the order of 1 to 3 psi. Because of the poor transfer of heat through the relatively thick belt, no heat is provided from inside of the belt. Low-pressure contact between the belt and pressure roll results in inefficient heat transfer therebetween, particularly, where the belt has macro non-uniformities and/or a rough surface. Poor contact and non-uniform contact result in a cold belt or a belt with cold spots and an unacceptable image gloss and fix level. Additionally, toner fusing and belt contamination are an ongoing problem where the thermal contact is poor and non-uniform which in the worst case results in cold offset. Cold offset is a fusing condition where the toner attaches to the fuser belt surface, which results in both a fuser belt cleaning problem and an image deletion on the paper.
Following is a discussion of references that may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description of the present invention to follow, may provide a better understanding of the invention. The references that are discussed herein are hereby incorporated by reference in their entirety.
U.S. patent application Ser. No. 10/217,683 filed on Aug. 12, 2002 and assigned to the same assignee as the present invention discloses a high speed heat and pressure belt fuser apparatus or structure for fixing toner images including an endless belt and a pair of pressure members between which the endless belt is sandwiched for forming a fusing nip through which substrates carrying toner images pass with the toner images contacting an outer surface of the endless belt. Thus, one of the pressure rolls is supported internally of the endless belt while the other pressure roll is supported externally of the belt. The belt has at least one conformable or deformable layer which cooperates with a deformable or conformable layer on at least one of the pressure members to provide a large nip that yields high gloss images, long belt life, minimal edge wear and reliable stripping at high speeds.
Effective substrate stripping is accomplished by wrapping a portion of the belt about the external roll in a post-nip area.
U.S. patent a
Berkes John S.
Bott Donald M.
Condello Anthony S.
Chen Sophia S.
Sklar Benjamin
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