Printing – Processes – Specific ink
Reexamination Certificate
1998-10-02
2001-09-18
Hilten, John S. (Department: 2854)
Printing
Processes
Specific ink
C101S119000, C101S129000, C106S031130, C523S160000
Reexamination Certificate
active
06289812
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a novel printing process utilising a digital or a stencil duplicator, novel duplicating apparatus, and novel inks for use therein.
BACKGROUND OF THE INVENTION
Traditionally, the inks used on digital and stencil duplicators are of the water-in-oil emulsion type. The continued use of such inks in the duplicating field derives from their possessing a number of important properties.
In particular, water-in-oil emulsion inks can be formulated to have rheological characteristics suitable for use in rotary printing machines, making it possible to control the amount of ink used in the printing process and the degree of ink penetration into the printing substrate, and to prevent “tail flooding”, the effect whereby ink exudes from under the trailing edge of the stencil master on a single drum duplicator. Also, and of crucial importance, emulsion inks can be formulated to be non film-forming, which allows their retention within the printing drum of a duplicator during periods between use without film formation, skinning, or crystallisation, which would result in irreversible blocking of the mesh screen on the drum.
However, there are major disadvantages associated with the use of emulsion inks which limit the ability of current duplicating processes to compete with other imaging technologies. In particular, the degree of image-fixing that can be achieved in modern duplicating processes, which are relatively high speed printing processes typically printing up to 150 pages per minute, using an emulsion ink is limited. It can be appreciated that since emulsion inks do not solidify after printing, their continued liquid state on the printed sheet leads to the problems of set off and smudging. These problems become more serious when the substrate being printed is non-absorbent, rendering emulsion inks generally unsuitable for the printing of non-absorbent substrates.
Another problem with the use of emulsion inks in duplicating processes is that they achieve lower image density than other digital imaging processes, such as laser printing. This inferior image density is believed to result from the fact that in order to achieve image-fixing of an emulsion ink a certain proportion of that ink must be absorbed into the print medium, thereby detracting from image density.
Yet another problem with the use of emulsion inks in duplicating processes is that if left in the drum of a duplicator between periods of use the water content of the ink tends to be reduced by evaporation. As a consequence, print quality on start-up is less than optimum until the drum is purged by fresh ink from the supply container, or cartridge. Typically, therefore, prints on start-up are characterised by excessive ink usage accompanied by abnormally high levels of set off and smudging.
Yet another problem relating to the use of emulsion inks is the phenomenon of paper cockling which results from the absorption of water from the Ink into the fibres of the printed paper. This is particularly evident when a large quantity of ink is used, either to print a large surface area or to achieve a high print density. Paper cockle is not only unaesthetic, but also the high points of the distorted paper surface exacerbate set-off.
Since the introduction of the digital duplicating process, attempts have been made to address the above-identified shortcomings. However, most or all of the proposed improvements have retained the use of an emulsion ink. For instance, JP-A-06128516 discloses the use of a water-in-oil emulsion ink containing a hot-melt component, so that image-fixing can be enhanced after printing by the application of heat. However, the other disadvantages associated with emulsion inks described above still remain unresolved.
Further, the necessity to remove water during the drying/fixing stage of the process disclosed in JP-A-06128516 may impose limitations on the process, for instance in terms of print speed. In an Example given in that document prints are fixed by passing over a heated roll at a speed of 3 m/min, which is a significantly lower printing speed than the 18 -50 m/min range typical of modern digital duplicators.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, a printing process comprises applying a composition to a substrate using a digital or a stencil duplicator, the composition comprising a dispersion of particles of a thermoplastic or thermosetting polymer dispersed in an involatile organic liquid, wherein the size of the particles in the composition is less than 35 &mgr;m. Preferably, the printing process also includes a heat-fixing stage, to fix the composition to the substrate.
The printing process of the present invention possesses a number of significant advantages over conventional digital or stencil duplicating processes. In particular, it achieves a high print density comparable to that achieved in other digital printing methods. In addition, it achieves good image-fixing, particularly if a heat-fixing stage is included, and reduces considerably, if not eliminating altogether, set off and smudging. The process is therefore, readily applicable to non-absorbent media. Further, as the ink used in the process is not a water-in-oil emulsion ink, no water loss is experienced on standing of the printing machine, thereby overcoming the usual problems on starting-up the process after a period of non-use, and achieving greater process consistency. Additionally, the absence of water means that paper cockle is not a problem encountered when printing with this ink.
According to a second aspect of the present invention, an ink suitable for use on a digital or stencil duplicator comprises particles of a thermoplastic or thermosetting polymer dispersed in an involatile organic liquid, wherein the polymer particles have a diameter of less than 35 &mgr;m, and a colouring agent either dispersed in the liquid or incorporated into the polymer particles.
DESCRIPTION OF THE INVENTION
The compositions, or inks, for use in the process of the present invention are liquid at room temperature, i.e. 20 -25° C., with the dispersed polymeric particles being insoluble and solid in the liquid component of the composition. However, at higher temperatures the dispersed particles are capable of dissolving in or absorbing the liquid. On cooling, however, the composition fuses, or solidifies, to form a cohesive film that is substantially free of liquid character, although this does not preclude the presence of some free liquid on or around the film. The temperature to which the composition needs to be heated in order to render it fusible depends upon its components. Typically, however, temperatures of at least 60° C., and more typically at least 100° C., will be required, although the temperature used should not be so high as to damage any of the components of the composition, substrate or printing machine. While it is not essential that a heating stage be employed in the process of the present invention this is preferred, as improved fixing of the composition to the substrate to which it has been applied can be achieved in this manner.
The maximum particle size of the polymer, or resin, particles dispersed in the compositions, or inks, for use in the process of the present invention is critical in order to avoid blocking of the screen or stencil on the digital or stencil duplicator. It is essential that the particles have a size, or diameter, of less than 35&mgr;m. Preferably, however the particles have a diameter of less than 20&mgr;m, and more preferably less than 10&mgr;m, eg. less than 8&mgr;m or even less than 5&mgr;m. It may, in some instances, be desirable to use particle sizes of greater than 1&mgr;m, if for instance the polymer is relatively difficult to process to smaller particle size. In the context of the present Application particle sizes are measured by the Hegmann guage method carried out on the final composition so as to take into account any swelling of the particles in that composition.
As with all printing processes, it is essential
Adams John Christopher
Hamilton John
G. R. Advanced Materials Ltd.
Grohusky Leslie J.
Hilten John S.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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