Incremental printing of symbolic information – Thermal marking apparatus or processes – Density control
Patent
1997-09-25
1999-11-02
Le, N.
Incremental printing of symbolic information
Thermal marking apparatus or processes
Density control
347190, 4001201, B41J 2355
Patent
active
059780061
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of thermal printing by the deposition of dyes.
2. Description of the Related Art
The present invention relates more particularly to a method of continuous-tone dye diffusion printing of the type described in the articles by P. W. Webb and R. A. Hann, <<Measurement of thermal transients in a thermal print head used for dye diffusion color printing>> in IEEE Proceedings-A Vol. 138, No. 1, January 1991, and A. Kaneko, <<A Simple Simulation for Simultaneous Diffusion of Dye and Heat in Dye Diffusion Thermal Transfer Printing>>, in Journal of Imaging Science, volume 35, No. 4, July/August 1991.
A method of this kind, which can be used to achieve high-quality printing, can be applied in particular to the customization of plastic cards such as smart cards, magnetic cards, badges, etc.
FIG. 1 shows a printing device 1 according to this method, designed for the customizing of plastic cards of a known kind as already described in the French patent applications No. 90 14329 or No. 94 02116 filed on behalf of the present Applicant.
In very broad terms, the printing device 1 comprises two pairs 2, 3 of secondary rollers for the conveyance of a plastic card 4 to be printed, a main conveyance and printing roller 5, a print head 6 of which only the useful bar-shaped end is shown, an inking ribbon 7 with three sequences of dyes of primary colors, generally yellow, (J), magenta (M) and cyan blue (C). The card 4 is sandwiched between the print head 6 and the main roller 5 with the interposition of the inking ribbon 7. The card 4 moves step by step in a printing direction S identified in FIG. 1 and, to each shift of the card, there corresponds an equivalent shift of the inking ribbon 7 and the printing of a line. Thus, the printing of a pattern proceeds line by line for a first primary color sequence until the entire length of the card is crossed, then the card returns to the initial position for the printing of a second primary color sequence etc. After three printing sequences, a full range of colors is obtained by the combination of the three primary colors.
FIG. 2 shows the lower face of the print head 6 in contact with the ribbon 7, and FIG. 3 gives a schematic view of the electrical structure of the print head 6. Together, these two figures provide for a clearer understanding of the printing mechanism.
As can be seen FIG. 2, the print head 6 includes a row of n heating resistive points P.sub.i (P.sub.1, P2, . . . P.sub.n), i being a index ranging from 1 to n. For the printing of a line, each resistive point P.sub.i is activated by a train of voltage pulses of the same duration, and is thus brought up to a temperature of diffusion of the dye coating the ribbon 7, namely a temperature of about 200.degree. C. to 300.degree. C. Each resistive point P.sub.i thus ensures the printing of a pixel, the set of pixels constituting a line. Of course, when an pixel should not be printed, the corresponding resistive point P.sub.i is not activated.
In FIG. 3, it can be seen schematically that the constant-duration voltage pulses providing for the activation of the resistive points P.sub.i are applied by means of a plurality of switches I.sub.i (I.sub.1, I.sub.2, . . . I.sub.n) connected to a source 8 of voltage Va by means of an electrical cable 9. The switches I.sub.i are controlled by an electronic circuit 11 that opens and closes them alternately. Since the quantity of dye deposited on the card by diffusion (the term <<migration>> is also used) is a function of the temperature of the resistive points P.sub.i, the electronic circuit 11, depending on the image to be printed, determines the number of voltage Va pulses that should be applied to each resistive point P.sub.i. The quantity of primary color deposited for each pixel is thus modulated, making it possible to obtain a large variety of shades of colors after the combination of the three primary colors.
In view of the above, it can be understood th
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Gemplus
Le N.
Vo Anh T. N.
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