Electrophotography – Image formation – Fixing
Reexamination Certificate
2001-12-18
2003-08-19
Pendegrass, Joan (Department: 2852)
Electrophotography
Image formation
Fixing
C399S069000, C430S124300
Reexamination Certificate
active
06608987
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns a method for printing and/or coating of a substrate, especially paper or cardboard using at least one meltable, curing toner and a machine for application and fixation of curable toner onto a substrate.
BACKGROUND OF THE INVENTION
A known printing and/or coating method is electrostatic printing in which a latent electrostatic image is developed by charged toner particles. These are transferred to an image-receiving substrate, hereafter substrate for short. The developed image transferred to the substrate is then fixed, the toner particles being heated and melted and the substrate optionally heated. Contact methods are frequently used to melt the toner particles, in which the toner particles are brought into contact with the corresponding devices, for example, hot rolls or rollers. The shortcoming here is that the design, maintenance and operating costs of these contact-heating devices are demanding and therefore cost-intensive. Consequently, the use of silicone oil as parting agent is necessary, which is supposed to prevent adhesion of the melted toner to the heating device. The error rate caused by the contact heating devices is also relatively high.
For fixation of toner transferred to paper, for example, contactless heating devices and methods are also known in which the toner particles are melted, for example, with heat/microwave radiation or hot air so that they adhere to the paper.
In the printing and copying methods in which the image applied to the substrate or the coating is developed by toner, the luster of the toner does not follow the luster of the paper, as in offset printing. This is true both for noncontact and contact toner fixation methods. The reason for this is the greater layer thickness of the toner relative to the color layers produced in offset printing methods. The relatively thick toner layers fill up the porosity of the paper so that a smooth surface structure and a certain luster of the toner layer are produced. After the toner is fixed on the paper, the luster of the toner layer can subsequently be increased by calender having hot rolls.
A shortcoming in the known method is that the luster of the toner layers after fixation on the paper differs as a function of the thickness of the toner layers so that after actual fixation of the toner subsequent treatment of the printed or coated paper, for example, with calenders is required in order to obtain an equally high luster even with different toner layer thicknesses.
SUMMARY OF THE INVENTION
The task of the invention is to offer a process in which control of the luster of the toner or coating applied to the substrate is possible largely independent of the luster of the substrate consisting of paper or cardboard. Another task consists of the fact that the method is to be used in particular for contactless toner melting methods. Another objective is improvement of the printing or copying quality, especially the quality of toner melting during fixation, especially cohesion of the toner layer. Finally, an objective of the invention is to keep heating of the substrate as low as possible in order to minimize moisture elimination from the substrate. Consequently, a machine for execution of the method is offered.
To solve the task, a method is proposed to print or coat the substrate, for example, a paper sheet or paper web in which at least one curable toner is used. The liquid or dry toner has at least one polymer and colored pigments as well as additional ingredients which will be taken up further below. In conjunction with the present invention, “curing” of the toner is understood to mean that this toner is heated to or above its glass transition point so that the toner particles melt. Exposure of the melted toner to UV radiation causes the polymer chains to crosslink and therefore become longer so that the glass transition point and viscosity of the toner become greater. These changes in properties mean that the toner no longer flows on the substrate from a certain chain length but instead exhibits rubber-like properties. This crosslinking of the toner materials is called curing. Melting and curing of the toner or toner layer(s), as described above, is referred to for short as fixation of the toner.
The method according to the invention is characterized by the fact that the degree of melting of at least one toner layer being fixed is controlled as a function of the desired luster of a fixed toner image. The toner is therefore only melted until the toner layer(s) situated on the substrate has a specific surface roughness or porosity at which a specific luster of the toner is produced. In order to almost freeze in the state of the heated toner melted and made flowable to the desired degree at which a desired luster is produced, an appropriate method is used as a function of the properties of the corresponding toner. To achieve high luster of the fixed toner layer, it is fully melted so that it can flow, through which it acquires a very even surface structure and therefore high luster. If a dull toner layer with low luster is to be produced, the toner according to the invention is not fully melted so that toner particles having a certain geometric shape are still situated in the melt so that the surface of the toner layer as corresponding unevenness or roughness and therefore a limited luster. This state of the melt is then frozen in with an appropriate method. The method according to the invention therefore presents, by appropriate control of the process parameters during fixation, among other things, duration and/or intensity of heating of the solvent, a rapid and reliable adjustment of the toner image luster in advantageous fashion without the requirement for this purpose, as in the known prior art methods, processing or final processing of the toner fixed on the substrate in a technical device expressly prescribed for this purpose, for example, a calender.
In a particularly preferred variant, a toner having at least one polymer is used which cures by exposure to ultraviolet rays, hereafter referred to as UV radiation for short, which means that its polymer chains begin to crosslink on exposure to UV radiation. Because of this property, a variant of the method according to the invention is obtained in which the toner layer being fixed is heated to its glass transition point or above it and exposed to UV radiation. Exposure to ultraviolet light, as mentioned, leads to crosslinking of the polymer chain and to an increase in toner viscosity so that the melted toner after a certain degree of crosslinking can no longer flow. The surface structure of the toner image and therefore its luster can then no longer change. Exposure of the toner layer with UV radiation for the purpose of crosslinking can occur already during the melting process or after conclusion of the melting process.
The highest degree of melting of the toner is preferably 100%, i.e., the toner layer(s) is heated to a high enough temperature and held sufficiently long at this temperature so that the toner layer(s) consisting of toner particles is melted and made flowable so that an even layer is formed on the surface of the substrate. The highest possible luster is achieved because of this. At a low degree of melting, i.e., at a degree of melting of 60%, in which the toner is already partially melted and made flowable, toner particles are still present in the melt, which have still not fully lost their shape in favor of a melt continuum and the toner layer therefore still exhibits a certain unevenness in roughness so that the luster that is produced is correspondingly lower than in a fully melted and flowing toner layer. By influencing the degree of melting in combination with corresponding initiation of the crosslinking/curing process, any arbitrary luster value (within certain tolerances) can be produced under practical conditions between dull and highly lustrous.
In the variants of the method according to the invention in which UV-curing toner is used, melting of the toner and crosslinking of the ton
Bartscher Gerhard
Rohde Domingo
Schulze-Hagenest Detlef
Kessler Lawrence P.
NexPress Solutions LLC
Pendegrass Joan
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