Electrophotography – Image formation – Transfer
Reexamination Certificate
2001-12-06
2003-03-18
Braun, Fred L. (Department: 2852)
Electrophotography
Image formation
Transfer
C399S336000, C399S411000, C430S124300
Reexamination Certificate
active
06535711
ABSTRACT:
FIELD OF THE INVENTION
The invention involves a process for the double-sided printing and/or coating of a substrate, in particular, of paper or cardboard, using at least one toner.
BACKGROUND OF THE INVENTION
A known process is electrostatic printing, in which a latent electrostatic image is developed by charged toner particles. These particles are transferred onto an image-receiving substrate, called substrate for short in the following. Afterwards, the developed image that has been transferred onto the substrate is fixed by the toner particles being heated up and fused. To fuse the toner particles, contacting processes are often used in which the toner particles are brought into contact with suitable devices, for example hot rollers or cylinders. It is disadvantageous that it is usually necessary to use silicone oil as a separating agent that should prevent an adhesion of the fused toner onto the heating device. In addition, the design, the maintenance and the operating costs of these heating devices that operate by contact are expensive and thus cost-intensive. Furthermore, the defect rate caused by the contacting heating devices is relatively high. In order to fix the toner that is transferred onto the paper, for example, heating devices and processes are also known that operate in a contactless manner, in which for example, the toner particles are fused using heat radiation and microwave radiation or with hot air.
In the contacting and non-contacting fusing processes, toner is used, for example, that has a glass transformation temperature (T
G
) in a range from 45° C. to 75° C. The glass transformation temperature, at which the toner—starting from the solid state—begins to soften, can be influenced by the choice of raw materials and by the addition of certain additives to the toner. The lower value of the temperature range, in which the glass transformation point lies, is limited at the bottom by the storage conditions of the toner and the heat generated in the printer, in particular within the development station, and it is limited at the top by the fusing and fixing conditions. In a fusing device for the toner, both the toner as well as the substrate itself is heated up. In order to be able to ensure a good fixing of the toner onto the substrate, the surface temperature of the substrate must be in the range of the glass transformation temperature of the toner above it. The toner easily reaches and/or exceeds the glass transformation temperature (T
G
) in the area of the fusing device.
Processes and devices are known in which the substrate is printed or coated double-sided, whereby for the printing of the upper side and the lower side, one and the same toner transfer device and fusing device is often used. After a first side of the substrate is printed, the substrate is automatically reversed, supplied back to the beginning of the processing line, and supplied again to the transfer device and fusing device, where the other side of the substrate is printed. While the toner located on the second side of the substrate is fused, the substrate, the image that has already been fixed on the rear side of the substrate, and the image to be fixed are heated. The second heating affects the print quality in an undesirable way, in particular the gloss of the image that has already been fixed and is located on the first side. By the repeated heating of the substrate, the gloss can change at individual locations or over the entire side of the substrate. The gloss value of the second side of the substrate is larger than on the first side of the substrate. Furthermore, the toner already fixed on the first side of the substrate tends to smudge when the second side of the substrate is heated up to a temperature that is above the glass transformation point of the toner. The renewed fusing of the toner that has already been fixed and is located on the first side of the substrate leads to errors in the printed image and to the smudged toner dirtying a transport device that conducts the substrate along the processing line. In the worst case, the substrate can adhere to the transport device. The same problems also occur in a device in which two complete print units each have a toner transfer device and a fusing device. In these known devices, a first image is transferred and fixed by a first print unit to a first side of the substrate, while subsequently a second image is transferred and fixed onto the rear side of the substrate using the second print unit.
SUMMARY OF THE INVENTION
The purpose of the invention is to produce a process in which a double-sided printing and/or coating of a substrate is possible with a simultaneously high quality of the images and/or coatings applied onto the front side and the rear side of the substrate.
In order to achieve this purpose, a process is proposed that provides for the double-sided printing and/or coating of a substrate, for example, a paper sheet or a paper web, while using at least one liquid or dry toner that has at least one polymer, at first at least one toner layer, or a first image that has at least one toner layer, is transferred onto a first side of the substrate. Then, this toner is heated up to its glass transformation temperature (T
G
) or a temperature above it. In the process, the toner and/or the toner layers are preferably fused until a certain gloss becomes set. This state of the toner is then transformed by the fixing of the toner onto the substrate using ultraviolet radiation, for example. The toner present in the form of individual molecules has the property that its original glass transformation temperature shifts to a higher temperature level as a result of the cross-linking of its polymer chains, and the viscosity of the toner increases. In other words, after the toner has been heated up for the first time to its glass transformation point, or beyond, and cross-linked, and cooled off again, its glass transformation temperature increases so that this toner first softens at a higher temperature when it is re-heated—starting from the solid state. The cross-linking process increases the glass transformation temperature and the viscosity of the toner, so that the toner no longer becomes liquid above its new glass transformation temperature when it is re-heated, but instead it obtains a thermoplastic, rubber-like structure. After the toner has then been fixed to the first side of the substrate, at least one toner layer or a second image that has at least one toner layer is transferred to the other, second side of the substrate in the next step. The toner located on the second side of the substrate is then warmed or heated up to a temperature that is equal to or greater than its own glass transformation temperature. Next, a cross-linking of the molecules of the second toner also occurs here, which leads to the changes in the properties of the toner as described above. Since the toner already fixed onto the first side of the substrate can no longer become liquid (as mentioned), but stays highly viscous when heated above its new glass point, it can be ensured that the toner applied and fixed onto the first side of the substrate does not smudge on its support, for example, a conveyor belt or a roller, or experience a change in its gloss, by the fixing of the toner on the other, second side of the substrate.
It is especially advantageous in the process according to the invention that the temperature of the first side of the substrate and the toner fixed on it, which becomes set during the heating of the second side of the substrate in order to fix the second toner image, can also be above the new glass transformation point of the first toner image, provided the first toner image is not harmed by this. By the toner located on the first side of the substrate no longer becoming liquid when it is re-heated, it is thus possible to prevent a smudging and thus a dirtying of the printing and/or coating machine and/or copier, in which the process according to the invention is applied, by the toner applied and fixed on the first side of the substrate
Rohde Domingo
Schulze-Hagenest Detlef
Braun Fred L.
Kessler Lawrence A.
NexPress Solutions LLC
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