Electro-thermic Printing Device

Abstract

An electrothermal printer for non-impact printing on plain paper which makes use of a ribbon which is made up of a substrate having a thermal transferrable ink coated on the surface towards the plain paper and a coating of electrically resistive material on the other side. The ribbon is held in contact with the paper while a plurality of selectively energizable electrodes are held in contact with the resistive material coated side of the ribbon. The electrodes are selectively energized for causing a current to pass through an incremental portion of the resistive material to another electrode which is held in contact with said resistive material a short distance from said selectively energizable electrodes. The current in the incremental portion of the resistive material causes enough I.sup.2 R heating to soften the wax coated on the substrate directly opposite to the heated portion. The softened ink transfers to the plain paper sheet as a dot or a line.

Description

CROSS-REFERENCE TO RELATED APPLICATION

The convention priority application for this invention is Italian application No. 67046-A/70 filed on Jan. 9, 1970.

BACKGROUND OF THE INVENTION

The present invention refers generally to non-impact printing systems, and more specifically to an electro-thermic printing device, wherein a thermotransferable ink layer is printed on a receiving surface in areas where the ink is softened by a source of thermal energy.

Many thermal printing devices are known which print on plain paper whereon is superimposed a sheet coated with a pigmented and thermotransfereable layer, which layer is locally softened by heat transfer apparatus receiving exact registration, and which then adheres to the paper. One thermal printer uses as heat transfer apparatus a modulated electronic beam, which requires complex and bulky equipment not suitable, for example, for an office printing machine.

Another thermal printer uses a sheet having a resistive layer, a low resistance substrate and a pigmented wax layer, in contact with the receiving sheet. A voltage is applied between a point of the resistive layer and the substrate edge by means of an electrode, which voltage causes a current to flow between the point and the substrate edge through a portion of the substrate having a variable length. The I.sup.2 R heating in the segment having a relatively high resistance causes the wax to melt. This device has the disadvantage that, by changing the electrode position, it changes the circuit resistance, and therefore the current rate, which in some positions will be such as not to melt the wax while in others it will be such as to cuase excessive melting. Furthermore, if a plurality of electrodes operate simultaneously, the currents in the substrate are added and can produce enough I.sup.2 R heating to cause printing where it is undesired.

A further thermal printer uses a metal sheet of high conductivity superimposed on a thermotransferable ink layer which is in turn superimposed on plain paper sheet. An electrode of a different metal is pressed on the high conductivity sheet, which electrode is transversely displaceable on the sheet and connected to a generator of electric voltage. When the electrode is displaced while voltage is applied a current flows therethrough, which heats the ink lying below. This device, however, requires high rated currents; furthermore, since it is based upon the junction effect, it requires a system capable of holding the stylus at a constant pressure against the sheet of high conductivity material, it being required that the electric characteristics of the junction do not change with the stylus position.

SUMMARY OF THE INVENTION

The aforesaid disadvantages ore obviated by the device according to the present invention, according to which a layer of resistive material is superimposed on thermotransferable ink layer, and a plurality of electrodes which are selectively energized with a voltage are mounted in contact with the layer of resistive material, in such a manner that a corresponding limited portion of said resistive material is electrically connected in series with each of said electrodes, so that the current in each of said electrodes causes in a part of the corresponding limited portion of the resistive layer sufficient I.sup.2 R heating for locally softening the thermotransferable ink.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description represents a preferred embodiment of the device according to the invention, made by the aid of the accompanying drawings, wherein:

FIG. 1 represents a perspective view of the device according to the invention;

FIG. 2 represents a detail drawing of FIG. 1;

FIG. 3 represents a cross-section of FIG. 2 according to line III--III;

FIG. 4 represents an enlarged detail drawing of FIG. 3 not in scale;

FIG. 5 represents an enlarged detail drawing of FIG. 3, not in scale, according to a different embodiment of the device according to the invention.

DETAILED DESCRIPTION

In FIG. 1 there is schematically illustrated a typewriter which includes a printing device according to the invention. The machine comprises a usual alphanumeric keyboard 11 which controls, by means of a coding device of known type (not shown) a printing head 13. Head 13 is mounted on a carriage 14 which is step-by-step transversely displaceable with respect to the machine by means of a guiding system (not shown). The head 13 presses a special typewriter ribbon 16 on a platen 19. The ribbon 16 is unwound from a payout reel 17 and is wound on a take-up reel 18 by means of a system commonly used in the typewriters. The paper 21, which is to be printed upon, is fed between the platen 19 and the ribbon 16.

As shown in FIGS. 2 and 3, the head 13 includes a metal plate 22 whereon is provided a slot 23. Fitted into a projection 24 of the plate 22 are a plurality of L-shaped electrodes 25, for example, five, which are insulated from the plate 22. The shorter legs of the electrodes 25 are inserted into the slot 23 of the plate 22 so as not to contact it, and bear against the ribbon 16. Selectively applied between each of the electrodes 25 and the plate 22 is a voltage from the coding devices controlled by the keyboard 11. In FIG. 4 there is represented a section of head 13, of ribbon 16 and of paper 21. The thickness of ribbon 16 is exaggerated for sake of illustration.

The ribbon 16 comprises a flexible insulating substrate 26, for example, of paper, cotton, silk, polyethylene, etc., coated with a resistive layer 27 on the side facing the printing head 13, and a thermotransferable ink layer 28 on the side facing the paper 21. The resistive layer 27 may be a coat of varnish containing conductive pigments such as graphite or conductive carbon black and metal particles. In order to improve the characteristics of the layer, auxiliary agents as plasticizers, dispersants and stabilizers may be added to the varnish.

For satisfactory operation of the printer the resistivity of the layer 27 should be between approximately 50 ohms per square and 1,000 ohms per square. An example of a mixture for a conductive varnish which for a 10 .mu. thick layer has a resistivity of 800 ohms per square is the following:

Vinilyte (Union Carbide Int.) 60 grs Carbon Black (Columbia Carbon Co.) 70 grs Conductex DOP (Massimiliano Massa) 10 grs Raybo 6 (Raybo Chemical Co.) 3 grs MEC (RP. Erba) 100 grs

In the event of polymeric substrates 26 the conductive additives may be mixed in the same plastic which, when extruded, forms a conductive ribbon.

The thermotransferable ink 28 that is coated on the substrate 26 is made of a binding means colored with pigments or dyes, or both, suitable to melt at a well-determined melting point, to be transferred by pressure onto the paper 21, and to solidify as soon as heating is discontinued. The ink should be such as not to be transferable at room temperature by means of pressure alone. To this end there have been found to be useful vynil chloride acetate copolymers, butadiene-styrene polymers, acrylic-vynilic copolymers, terpene polymers, etc.

The ink should also include such auxiliary agents as plasticizers, dispersing means and stabilizers.

When the voltage is applied between an electrode 25 and the platen 22, the current paths 31 take up the pattern represented in FIG. 4. The current density is greater near the electrode 25, where the highest I.sup.2 R heating occurs. The heat is transmitted by conduction through the substrate 26 to the thermotransferable ink 28 which accordingly melts and is transferred to the paper 21. The adhesion of the ink 28 to the paper 21 is aided by the pressure the plate 22 exerts between the ribbon 16, the paper 21, and the platen 19 in the next character spacing. Since the ribbon is stationary during the character spacing of the head 13 across the line with respect to the paper, no quick cooling of the ink is required.

Upon striking a key on the keyboard 11, a voltage is selectively applied to the electrodes 25 of the head 13, whereupon dots corresponding to the energized electrodes are recorded on the paper 21. Subsequently the head 13 is advanced and another column of dots are selectively recorded on the paper 21, and so forth throughout five feeding steps. The selected character is in this way printed by means of a matrix of dots.

In FIG. 5 is represented a different embodiment of the device according to the invention, using a ribbon 16 comprising a resistive layer 27 of the type as previously described, a flexible highly conductive substrate 29 (for example, of alluminum or in any case of a material combining high flexibility, mechanical resistance and good electric conductivity features) and a thermotransferable ink layer 28. When a voltage is applied between an electrode 25 and the plate 22 the current paths 32 proceed through the resistive layer 27, along a line which is parallel to the electrodes 25, and in the highly conductive substrate 29 following a segment which is perpendicular to the first. Comparing FIG. 5 with FIG. 4, it is noted that in this second ribbon embodiment the area of the highest current path concentration, and therefore the highest heat dissipation, is located at the electrodes 25. In this way the recorded dots are better defined than in the foregoing ribbon. With a ribbon of this type it is also possible to print full characters, by the use of electrodes formed as characters, located, for example, along the periphery of a cylinder having its axis parallel to the ones of the platen 19, and by a character selection device of a known type.

Claims

We claim:

1. Electrothermal printing apparatus for printing with thermally transferable ink on a receiving surface comprising:

an ink layer of thermally transferable ink, said ink layer being immediately adjacent said receiving surface;

means for heating an isolated region of said thermally transferable ink layer so as to transfer ink to said surface; said heating means including:

a printing head having at least two electrodes of different electrical potential;

a conducting layer for conducting electrical current from one to the other of said electrodes attached to said ink layer, a resistive layer attached to said conducting layer, said resistive layer, said conducting layer and said ink layer forming an integral ribbon, said printing head, said ribbon and said receiving surface being movable with respect to each other;

said two electrodes being spaced a predetermined distance from each other and contacting said resistive layer, to allow current to flow through said conducting layer, the heat generated by said current heating said ink layer.

2. Apparatus according to claim 1 further comprising a second less resistive layer interposed between said ink layer and resistive layer, said electrical current passing from one of said electrodes through said resistive layer into said second resistive layer, through said second resistive layer parallel to said ink layer, and again through said resistive layer to the other said electrode.

3. Apparatus according to claim 1, wherein said printing head includes:

a row of selectively energizable points, and

two elongated electrodes parallel to said row and positioned on opposite sides of said row.

4. Electrothermic printing device for printing upon a receiving surface by means of a thermally transferable ink comprising, in combination:

a laminated ribbon comprising:

an ink layer carrying a thermally transferable ink,

an insulating layer adjacent to said ink layer, and

a resistive layer adjacent to said insulating layer for conducting electrical current,

first electrode means comprising an electrode member energizable to a first electrical potential, said electrode member having an elongated opening and a contact surface surrounding said elongated opening, said contact surface engaging said resistive layer,

second electrode means comprising a plurality of wire probes each selectively energizable to a second electrical potential, said probes being positioned in said elongated opening spaced from one another and from said electrode member, each said probe terminating in a contact point aligned with said contact surface engaging said resistive layer, and

means for moving said ribbon and said receiving surface with respect to said first and second electrode means.

5. Apparatus according to claim 4, wherein said resistive layer comprises a conductive varnish and a support material, said varnish being coated on said support material, said support material being attached to said ink layer and said varnish contacting said electrodes.