Coating processes – Immersion or partial immersion
Reexamination Certificate
2001-08-15
2003-05-06
Barr, Michael (Department: 1762)
Coating processes
Immersion or partial immersion
C427S385500, C427S407100, C427S412100
Reexamination Certificate
active
06558751
ABSTRACT:
TECHNICAL FIELD
The present invention relates to electrophotographic processes. In particular, it relates to a process for making fuser belts used to fix toner in such processes.
BACKGROUND OF THE INVENTION
In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing an area of the material's surface to light. A difference in electrostatic density is created between the areas on the surface exposed and those unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners which contain pigment components and thermoplastic components. The toners, which may be liquids or powders, are selectively attracted to the surface of the photoconductor, either exposed or unexposed to light, depending upon the relative electrostatic charges on the photoconductor surface, the development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to the photoconductor surface, pulling the toner from that surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor surface. A set of fuser rolls or belts, under heat, melts and fixes the toner in the paper subsequent to transfer, producing the printed image.
The electrostatic printing process, therefore, comprises an intricate and on-going series of steps in which the surface of the photoconductor is charged and discharged as the printing takes place. In addition, during the process, various charges are formed on the photoconductor surface, the toner and the paper surface to enable the printing process to take place. Having the appropriate charges in the appropriate places at the appropriate times is critical to making the process work.
After the image is transferred to the paper or other recording medium, it goes to the fuser where the paper is moved through a nip where it is heated and pressed. This melts the thermoplastic portion of the toner, causing it to bond with the fibers of the paper, thereby fixing the image onto the paper or recording medium. In the past, the majority of fuser assemblies used a fuser roll. These fuser rolls are typically aluminum cylinders with a heating lamp inside and a release coating on the outside. In this system, paper or transparency film with a toner image on it is passed through the fusing nip formed between the fuser roll and a backing roll. As the toner image passes through the fuser nip, the heat and pressure fuse the toner image to the paper or transparency film. This system has been used for many years because of its simplicity and functionality in high speed systems. The problem with this system, however, is that it requires the fuser roll to preheat before it may operate. This is true even with the machine in standby mode between printing or copying jobs. The need for preheating is the result of the large heat capacity of fuser roll. This preheating means a delay for the user before each printing or copying job can be started.
In recent years, a new on-demand fusing system has been developed and is being used in the industry to minimize this delay problem. This system is mainly composed of a ceramic heater and a thin film belt through which the heat is transferred to the toner-laden image. The ceramic heater is thin and has a small heat capacity compared to the fuser roll. The fuser belt is designed to conduct the heat from the heater to the toner image with minimal resistance. The fuser belt is not a gear or roller driven belt. The contact of the belt with the paper going through the fuser nip is the driving force for the belt to turn around the ceramic heater. The nip for the fuser belt system is formed by the fuser belt and a backing roll. The net result is a system which essentially eliminates any significant time delay caused by initial heating of the fuser system.
The fuser belt described above is typically composed of three layers. The first layer is a polymer film. This polymer film is the main substrate which gives the fuser belt structural integrity. The film must have specific properties regarding flexibility, as well as physical tolerances to high temperatures and repeated heating and cooling cycles. The polymer film must also be a good conductor of heat. The polymer chosen for this film is typically a polyimide with a material, such as boron nitride, dispersed within it for improved heat conduction. The current industry standard for producing this polymer film is a vertical dip coating technique carried out on a specially coated metal mandrel. After the polymer has been cured, it is slipped off the coating mandrel and is coated with one, two or more layers to form the finished belt.
The second layer of the fuser belt typically is a conductive primer coating. This layer provides a path within the finished belt for static charge dissipation. At one end of the fuser belt, there is a strip of exposed conductive primer, providing a place to ground the belt and to remove the static charge generated during belt operation. This is important because there can exist an electrostatic offset phenomenon whereby the toner is electrostatically transferred from the paper or transparency film to the fuser belt. This is undesirable since, when it happens, the toner transferred previously as a result of the offset is then fused to the print copy. This phenomenon produces a ghost-like image that deteriorates the overall image quality. The grounding clip, which is in contact with the strip of the exposed conductive primer coating, provides a path for charge dissipation and thus reduces the electrostatic offset phenomenon.
The third layer of the fixing belt is composed of a release coating. A primary purpose of this coating is to provide a surface to which the toner will not adhere during fusing. A second purpose of this coating layer is to provide a strong wear layer for the belt. As stated above, the preferred method for manufacturing fuser belts utilizes a vertical dip coating method. A number of problems result when attempts are made to apply a typical topcoat material, such as a fluoropolymer, to the fuser belt using this vertical dip coating technique. First, the coating solution does not wet the polymer tube in a uniform sheet. Instead, the coating runs and drips off the belt during coating. Second, the topcoat tends to crack when it is exposed to high temperatures (for example, greater than 350° C.), such as during the sintering operation which is necessary for drying and curing the polymer and other coatings. Finally, the topcoat coating solution is frequently not stable for long enough to remain uniform throughout the coating process. When the solution sits for a few minutes, it frequently tends to separate into a non-homogenous solution. It therefore would be useful to develop a process which allows topcoat materials, such as fluoropolymers, to be applied to fuser belts, using the vertical dip coat technique in an efficient and effective manner. The present invention addresses that objective.
U.S. Pat. No. 5,853,892, Chen, et al., issued Dec. 29, 1998, describes the use of an amorphous fluoropolymer in the outer layer of a belt used for fusing a thermoplastic resin toner image to a substrate. The fuser belt coatings are said to require a lower sintering temperature than the conventional semi-crystalline fluoropolymers.
U.S. Pat. No. 5,778,295, Chen, et al., issued Jul. 7, 1998, describes the preparation and use of a fuser belt comprising a seamless polyimide substrate belt, a cross-linked silicone resin intermediate layer and a surface layer containing a silsesquioxane polymer.
U.S. Pat. No. 5,709,973, Chen, et al., issued Jan. 20, 1998, discloses a metal fuser belt comprising an unmatted powder-coated polytetrafluoroethylene-co-perfluoropropyl vinyl ether copolymer (PFA), an unm
Bellino Mark Thomas
Broce Scott Moreland
Neely Jennifer Kaye
Nguyen Dat Quoc
Srinivasan Kasturi Rangan
Barr Michael
Brady John A.
Lexmark International Inc.
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