Electrophotography – Control of electrophotography process – Control of fixing
Reexamination Certificate
1997-10-02
2002-08-06
Beatty, Robert (Department: 2852)
Electrophotography
Control of electrophotography process
Control of fixing
C399S328000, C399S384000
Reexamination Certificate
active
06430381
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is directed to a thermal fixing system for fixing toner images on the front side of a web-shaped recording medium in an electrographic printer or copier device, whereby the back side of the recording medium can already have a fixed toner image.
Thermal fixing devices that comprise a pre-heating saddle with a following fixing zone composed of a heated fixing drum and a pressure roller are employed in printer or copier devices for heat transfer fixing of toner images on a recording medium that is usually composed of paper.
Such thermal fixing devices are disclosed, for example, by U.S. Pat. No. 4,147,922 or Japan Abstract Vol. 13, No. 120, Mar. 24, 1989 (Japan-A-63-292177).
It is beneficial in electrographic printer devices that work in the highest speed range with, for example, a printing speed of more than 0.5 m/s, and that employ a heat transfer fixing station for fixing, to heat the paper web or the paper sheet to temperatures of approximately 100° C. or more before the actual heat transfer fixing process in order to thus obtain a good joining of the toner image to the paper surface.
When a paper web or a single sheet of paper that is already printed and fixed on one side, for example on the back side, is to be printed and fixed on the other side, then the first side which is already fixed must be conducted over the hot surface of the pre-heating saddle for heating the paper for the second fixing process. The following problems thereby arise in this second fixing process:
a) Continuous printer operation:
The print image that is already fixed and that runs over the hot surface of the pre-heating saddle is heated to such an extent that it assumes a condition ranging from tacky through fluid, and is partly smeared on the saddle surface. The more toner is transferred from the toner image onto the saddle surface the more toner collects on the saddle surface, until a visible destruction of the toner image on the paper occurs.
b) Waiting or Standby Operation:
While the printer is in the waiting or standby mode, the paper web having the already fixed print image lies on the hot saddle. The print image is heated to such an extent in the region of the surface of the pre-heating saddle that it assumes a tacky through fluid condition and sticks to the hot surface of the pre-heating saddle. When the paper web is started, the toner image is then torn from the surface of the paper web and remains sticking on the hot surface of the saddle.
In the case of the known fixing devices, there is another problem. It has previously been assumed that it is necessary to preheat the paper very rapidly over a relatively short path, via the preheating saddle, and then to fix the toner image on the paper via the rollers. For this purpose, the heating elements are arranged in the preheating saddle in such a way that the greatest quantity of heat is emitted to the recording substrate in the region of the paper inlet of the preheating saddle and that the emitted quantity of heat is then reduced over the heating elements in the direction of the paper exit. Thus, the relatively hottest region of the saddle is the paper inlet.
However, it has appeared that a rapid heating up of the paper over a short path leads to a high loading of the paper. This loading is expressed as a deformation, an embrittlement or an ageing of the paper and as a non-uniform loss of water from the paper during passage through the fixing station. Hence, post-processing of the paper by cutting or sorting is made more difficult or there occurs a non-uniform fixing of the toner images and thus an impairment of the quality of the print.
In addition, a rapid heating up requires a high specific heating power using high-power heating elements and a complicated control system. Because of the high heating power it is therefore necessary to lift the recording substrate immediately from the saddle in the event of a printer stop, in order to prevent buning of the paper. His makes comprehensive control devices necessary, which impairs the paper handling as a whole.
In modem electrophotograhic printing devices, furthermore, recording substrates of the most different widths are processed in the same machine. If the same amount of energy is fed to the saddle over the entire width, the saddle heats up severely in that region where there is no paper running, since in this region no energy is dissipated, apart from losses due to convection.
A temperature distribution of this type has considerable disadvantages. The paper is heated up non-uniformly, which leads to fluctuations in the fixing quality and can also cause paper running problems. The maximum heating saddle temperature must be reduced, since there exists the risk of overheating of the heating elements and the lifetime of the heating elements is thereby shortened. The energy losses are relatively large and the inner region of the machine is heated up unnecessarily.
In the case of thermofixing devices with a preheating saddle, the recording substrate is guided over a heated gliding surface of the saddle. Direct contact between paper and saddle is essential for a good thermal transfer between paper and saddle surface. In the case of high printing speeds and in the use of pre-folded papers or papers of non-uniform thickness, fluttering movements of the paper can occur in the region of the saddle. In consequence, the paper lifts partially off from the saddle, which impairs the thermal transfer. Also, paper contains a relatively high proportion of water, which is released during warming. The released steam can be deposited in the machine and can lead there to disturbances or to corrosion.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a thermal fixing means having a pre-heating saddle for fixing toner images on the front side of a recording medium in an electrographic printer or copier device, whereby the back side of the recording medium can already have a fixed toner image.
It is another object of the invention to provide a thermofixing device and a process for fixing, in which the recording substrate is exposed to as small a thermal loading as possible during passage through the fixing device.
It is a further object of the invention to provide a thermofixing device which makes it possible, without fluctuations of the fixing quality, to fix recording substrates of the most different widths and in which warping and deformations of the fixed recording substrate are avoided.
According to the invention, a thermal fixing system is provided for fixing toner images on a front side of a recording medium in an electrographic printer or copier device wherein a back side of the recording medium already has a fixed toner image. A heat transfer fixing station is provided for fixing the toner images on the recording medium. A pre-heating saddle precedes the heat transfer fixing station and a running direction of the recording medium has a sliding surface allocated thereto for accepting the recording medium over its back side. The sliding surface comprises a toner-repellant material at least in a contact region of the recording medium.
The specification of front side and back side of a recording medium is a purely relative matter for describing the two sides of a recording medium.
When the recording medium, which can be composed of single sheets or of continuous form paper, is conducted over a pre-heating saddle having a sliding surface that exhibits a repellant property for the tacky through fluid toner and has high abrasion resistance with respect to the paper web sliding thereon, then the thermal fixing means can be employed in printer or copier devices that work both in a simplex as well as in a duplex mode.
Materials that are manufactured of fluorine compounds such as, for example, PTFE or, respectively, PFA compounds, have proven beneficial. The material can be vapor-deposited, sprayed, or glued on an appropriate acceptance surface of a pre-heating saddle. PTFE or, respectively, PFA compounds exhibit extremely good repellency with respect to the t
Beatty Robert
Océ Printing Systems AG
Schiff & Hardin & Waite
LandOfFree
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