Thermoelectric Printing Apparatus

1. Field of the Invention

The present invention relates to a printing apparatus. More particularly, the invention relates to a printing apparatus employing an electrothermal transfer film which can be heated upon electric conduction therethrough. Specifically, the invention relates to a so-called electrothermal printing apparatus for printing characters, figures, or other patterns by using an electrothermal transfer film composed of a resistive layer and an ink layer, by applying a printing signal so as to pass a current higher than a predetermined level through a selected portion of the resistive layer in the film to fuse the thermally fusible ink by Joule heat, and by transferring the fused ink to a recording body such as a paper sheet, a film or the like.

2. Description of the Prior Art

One known electrothermal printing apparatus is disclosed in U.S. Pat. No. 4,350,449 wherein the disclosed printing device effects printing in response to a printing signal applied when a predetermined current is passed through a resistive layer in an ink film. With this device, if electrodes in the print head are not held sufficiently in contact with the resistive layer, then small dots cannot be printed, and dots of different diameters will be printed where larger dot diameters are desired.

Another known apparatus is disclosed in U.S. Pat. No. 4,236,834. The device of this patent uses head electrodes which are constructed as projecting needles, necessitating the use of large inter-electrode pitches which result in an inability to print dots at small pitches. Further, since current passes through each electrode, those electrodes which are better conductors of current wear more rapidly than other electrodes. Therefore, the lengths of the electrodes become irregular during use, with the consequence that the print head electrodes contact the recording body unevenly. The various problems referred to above have made it difficult to put the above-described thermal transfer printing apparatus to practical use.

A thermal transfer printing apparatus according to the present invention utilizes a print head in which the recording electrodes are formed on a substrate by thick film printing, plating, or the like. Printing is effected by pressing an electrothermal transfer film into contact with the end of the substrate and with the ends of the recording electrodes on the substrate and by moving the electrothermal transfer film and the recording electrodes relative to each other. The print head has a slanted end configuration so that the print head surface will cross the electrothermal transfer film at a prescribed angle when the recording electrodes are pressed there-against for printing.

When printing has been effected for a predetermined length of transfer film, the end of the print head which is held against the electrothermal transfer film is cleaned and reshaped to the desired contour.

Where the recording electrodes are substantially as hard as or softer than the substrate, the angle formed between the electrode-carrying end surface of the print head substrate and the electrothermal transfer sheet differs from the angle formed between the same end surface and the surface which does the cleaning and reshaping. When the recording electrodes are much harder than the substrate, the angles may be equal to each other.

It is an object of the present invention to provide a practical thermal printing apparatus which avoids irregular printing due to contact of print head electrodes with an electrothermal transfer film under uneven pressure.

Another object of the present invention is to provide a printing apparatus and a method for printing which, when used for gradation printing using the method of dot area modulation, will assume that the areas of the dots is varied as a function of the value of the applied current irrespective of the distance the electrothermal transfer film has travelled with respect to the print head, e.g. the wear of the print head, and of the position of print head.

The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.

For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings, in which:

FIGS. 1(a) through 1(c) are fragmentary cross-sectional views of electrothermal transfer films which can be used with the printing apparatus of the present invention;

FIGS. 2(a) through 2(c) show a print head and print electrodes in the printing apparatus of the present invention;

FIG. 3 is a side elevational view illustrating the manner of printing effected by the printing apparatus of the invention;

FIGS. 4(a) through 4(c) are diagrams showing the manner in which thermal printing is carried out by the apparatus of the invention;

FIG. 5 is a diagram of a switching arrangement for causing printing by the printing electrodes of the present invention;

FIG. 6 is a perspective view of the printing mechanism used in the printing apparatus of the present invention;

FIG. 7 is a perspective view of an electrode-carrying substrate used in a print head in the printing apparatus of the present invention; FIG. 8 is a fragmentary perspective view of the assembled print head;

FIG. 9(a) is a fragmentary plan view of the recording electrodes;

FIG. 9(b) is a view showing the manner in which printing is effected by the recording electrodes;

FIGS. 10(a), (b), (c-1), (c-2), (d-1), (d-2), (d-3) and (d-4) are views illustrating embodiments of the printing electrodes of the present invention under various conditions of use;

FIG. 11 is a block diagram of a circuit for a video printer which incorporates the printing apparatus of the present invention; and

FIGS. 12 and 13 are views of additional embodiments of printing devices according to the present invention.

A printing device according to the present invention will first be described briefly.

FIGS. 1(a) through 1(c) are sectional views of examples of electrothermal transfer films for use in electrothermal transfer recording which may be used with the printing device of the present invention. In each of FIGS. 1(a) through 1(c), reference numeral 1 denotes a fusible ink layer composed of wax and a pigment or a dye dispersed therein, the ink layer being thermally fusible at about 60.degree. C. In FIG. 1(a) the resin film 2 is made of PET, or the like, or of capacitor paper, and is a layer which supports ink layer 1, which in turn supports an electrically conductive layer 3 (hereinafter referred to as a resistive layer) composed of a resin and a fine powder of carbon dispersed therein. In FIGS. 1(b) and 1(c), an electrically conductive layer 4 similar to layer 3 serves to support the ink layer. A layer 5 for preventing color turbidity consists of a heat-resistant resin and is positioned between ink layer 1 and conductive layer 4. The ink layer can be fused by Joule heat generated by passing a current through the resistive layer of the film, and the fused ink layer can then be transferred to a transfer sheet for printing.

FIGS. 2(a) through 2(c) respectively show an end view, a fragmentary cross-sectional view through an electrode, and a fragmentary top view of a print head 22 for use in the electrothermal transfer recording which is used in the printing apparatus of the present invention. Electrodes 21 are integrally laid down on substrate 20 as more fully described below, to produce an end surface 23.

FIG. 3 illustrates the manner in which print head 22 contacts electrothermal transfer film 19 in the printing apparatus for the invention. Main roller or platen 30 is formed of a resilient material such as rubber with a transfer body, which may be a paper sheet 10 wound therearound and held thereto by a fixture 32. The electrothermal transfer film 19 is reeled off of film roll 33, delivered by feed rollers 35, 36 in the direction of the arrows, and wound around takeup roll 34. The print head 22 is urged by a spring 37 to turn about a pivot 38 in a direction which keeps the end 23 of the print head pressed against the main roller 30. By pressing the end 23 of the print head 22 having the construction of FIG. 2 against the electrothermal transfer film, there is assured a good electrical connection between the recording electrodes of the print head and the resistive layer in the film.

FIGS. 4(a) through 4(c) illustrate the manner in which the recording electrodes of the printing head of the present invention effect thermal printing. End portions 40, 41 of the recording electrodes are held in contact with the resistive layer of the film, causing currents 42 to flow in the resistive layer when a voltage is applied such that electrode 40 is at a negative potential and electrode 41 at a positive potential. By varying the length of time of application of the voltage, the ink layer attains the different temperature distributions indicated by curves 43 and 44. Curve 43 represents a shorter conduction time, and curve 44 a longer conduction time. The resulting different temperature distributions over the ink layer result in transfer of different ink configurations 45, 56 as shown in FIG. 4(c). Therefore, it is possible, in addition to mere printing, to produce gradations in area. FIGS. 4(a) through 4(c) show one example of a printing process used in the printing apparatus of the invention, in which voltages of different polarities are applied between adjacent recording electrodes. Where voltages are applied for printing in this manner across adjacent recording electrodes, a crosstalk-free multiplex driving mode can be obtained by simultaneously printing dots which are several dots apart and by subsequently applying voltages between the electrodes to print the intervening dots at different times, thereby effecting printing.

FIG. 5 shows a switching arrangement for effecting the process of FIG. 4 so as to record between all of the recording electrodes. Switches 50 are connected to the positive terminal of a power supply 8 and switches 51 are connected to the negative terminal of the power supply 8. The switches are connected alternately in pairs to recording electrodes H1 through Hm and L1 through Lm. The number of the recording electrodes is 2.times.m (m is a positive integer), and the number of recording pixels provided thereby is 2.times.m-1. This arrangement makes it possible to provide a high-speed, full-color printing apparatus which is capable of printing high-quality images.

The printing mechanism of FIG. 6 has been constructed for conducting various tests of the printing apparatus of the present invention. This illustrative printing mechanism will be described below.

A pring head HE is composed of a substrate BO and recording electrodes DD formed on the reverse side thereof. Between the print head HE and a roller or platen PT, there are disposed an electrothermal transfer film FL and a transfer recording body PA, such as paper, which are pressed against the platen PT by the spring-loaded recording head HE during printing. While the film FL and the recording body PA are pressed against the platen PT, an end of the print head HE having the recording electrodes DD on the reverse side thereof is held in abutment against the electrothermal transfer film FL and transfer recording body PA. The electrothermal transfer film FL is arranged so that the resistive layer faces the print head HE while the ink layer faces transfer recording body PA. In the printing mode, the electrothermal transfer film FL and the recording body PA are moved in the direction of the arrow and data items are successively printed on the transfer body PA. The platen PT is relatively soft as it has a hardness in the range of from 20.degree. to 50.degree..

The construction of print head HE in the printing mechanism of FIG. 6 is shown in FIG. 7, with the substrate BO fixed in position by head holder HP as illustrated in FIG. 8. The views of print head HE and holder HP shown in FIGS. 7 and 8 can be obtained by looking at the assembled apparatus in FIG. 6 from below.

In FIG. 7, the print head HE consists of: a recording electrode section WDD which is conveniently 1 to 5 mm in depth and carries recording electrodes DD having a thickness ranging from about 10 to 30 .mu.m and formed by thick film printing, plating, or the like; mounting section WIC for IC drivers 42 which transmit driving signals to the recording electrodes DD; and a common electrode signal wiring section WCM in which latch signal wires, power supply wires, and the like, are formed. All of the sections are mounted on the substrate BO, which has a thickness ranging from about 0.5 to 3 mm. The common electrode signal wiring section WCM has two layers of electrode wires. The ICs 42 in the driver IC mounting section WIC are arranged in a staggered fashion on the substrate BO and bonded face down. Each of the IC chips includes a register, a latch, and a driver.

As shown in FIG. 8, the substrate BO is fastened to heat radiation fins FI and held down by a springy head holder HP. The head holder HP is made of a metal such as aluminum or iron, or of a plastic, and has a curved section by which the substrate BO is held down. The head holder HP has slits HP-1 which provide individually acting portions which hold the substrate BO with an increased efficiency, and cooperates with an auxiliary holder plate HP-2 to uniformly hold the substrate BO.

The printing mechanism of FIG. 6 will now be further described. The printing mechanism employs head-raising cams CA-1 to press the print head HE against the ink film FL on platen PT and to release the print head HE off the platen PT. The printing mechanism also includes a head angle positioning cam CA-2 for changing the angle of the print head HE with respect to the platen PT when an abrasive sheet is used in place of the electrothermal ink transfer film for cleaning and reshaping the print head.

The print head HE is supported by pivots CA-51 on head side support members HE-3. Side support members HE-3 are, in turn, carried on side pivots HE-4 which are journalled in printing frame HE-5. A lever or cam follower CA-52 is supported on head HE and contacts the head angle changing cam CA-2. The head raising cams CA-1 serve to lift the front end of the print head off the platen PT to allow feeding of the transfer recording sheet PA and the electrothermal transfer film FL smoothly when printing is not to be effected. The head raising cams CA-1 are fixed to a cam drive shaft CA-30 which is operatively coupled to a cam drive motor CA-40. When cam drive motor CA-40 is energized, the head raising cams CA-1 are rotated about their own axes to permit springs S to raise the head support members HE-3.

The head angle changing cam CA-2 does not turn with shaft CA-30 but is secured to a head angle changing plate CA-20 by means of a coil spring (not shown) which is disposed around the cam drive shaft CA-30. When head angle changing plate CA-20 is engaged by the finger CA-21 on the iron core CA-22 of plunger CA-40, it cannot be turned about its axis. The coil spring around the cam drive shaft CA-30 is thus kept loose thereon, and no driving force is transmitted to the head angle changing cam CA-2 even when the cam drive shaft CA-30 is rotated. However, when the plunger CA-40 is energized so as to attract the iron core and head angle changing plate CA-20 is released by finger CA-21, the coil spring winds around cam drive shaft CA-30 and transmits driving force to head angle changing cam CA-2, whereupon the cam follower CA-52 is raised, rotating print head HE.

In the following, the angle formed between the plane of the substrate BO of print head HE and the end surface where electrothermal transfer film FL contacts the print head HE during printing is expressed by .THETA.1, and the angle between the plane of the print head HE and a cleaning material (such as abrasive paper, an abrasive platen, or the like) used for cleaning the front end of print head HE is expressed by .THETA.2. The relationship between angles .THETA.1 and .THETA.2 is illustrated in FIG. 9(b). FIGS. 9(a) and 9(b) are plan and side elevational views, respectively, of a portion of the print head HE. As seen in FIG. 9(a), the recording electrodes DD are formed on the substrate BO with an average electrode width of WDD and a pitch PDD. The angles .THETA.2 of FIG. 9(b) can easily be changed by means of the head angle-changing cam CA-2.

Where the electrothermal transfer film FL and the recording body PA are to be fed along in a so-called "paper feed" or "sheet feed" mode, the head raising cams CA-1 are first turned, using the foregoing mechanism, to press their longer radius portions against the head support members HE-3 so as to lift the print head HE off of the platen PT (the position shown in FIG. 6). The electrothermal transfer film FL and the recording body PA can then be fed along without being pressed against the print head HE for printing. For pressing the print head HE toward the platen PT, or truing the print head HE with a cleaning material such as an abrasive material, which may be coated on the surface of the platen PT, or carried on an abrasive sheet put in place of the electrothermal transfer film (FL), the head support members HE-3 are allowed to be raised by spring S by turning to the shorter radius portions of head raising cams CA-1. The head angle is selected to be .THETA.1 or .THETA.2 by means of the head angle changing cam CA-2.

The positions of the cams for the various modes of operation of the printing mechanism of FIG. 6 are shown in Table 1.

It is apparent that .THETA.1=.THETA.2 when the head angle changing cam CA-2 is not rotated.

Using the apparatus of FIGS. 6, 7 and 8, various tests were conducted to determine the effect of various hardnesses of the recording electrodes DD and of the substrate BO of print head HE. The pitch PDD of the recording electrodes DD was 320 .mu.m, the electrode width WDD was 180 .mu.m, the thickness of the recording electrodes DD was 20 .mu.m, the number of the recording electrodes DD was 640, and the width of the recording section was about 205 mm. .THETA.1=.THETA.2=45 degrees.

Measurements were made whenever any one of the 640 electrodes failed to print or when discontinuous printing resulted after one refreshing (cleaning) cycle was effected. Printing was carried out (without intermediate, cleaning) while feeding the electrothermal transfer film.

Table 2 shows the results of the above experiments. The substrates BO used were as follows. In the uppermost block of the left hand column, the substrate BO was formed of porous alumina which can be of a hardness ranging from 500 to 1000 HV (Vickers units), and the recording electrode DD was formed of tungsten which can have a hardness in the range of from 700 to 1000 HV. The uppermost block in the lefthand column indicates the experimental result when the print head substrate BO was made of porous alumina with a hardness of 900 HV and the recording electrodes DD were formed of tungsten having a hardness of 800 HV. The experimental result was that a print failure occurred when the distance travelled by an electrothermal transfer film past the print head was 5 m or less, this result being marked "x".

Where the substrate BO was formed of a material mainly composed of magnesium, silicon, aluminum, potassium, fluorine and oxygen, and fluorine having a hardness of 200 HV and the recording electrodes DD were formed of tungsten with a hardness of 800 HV, the experimental result as shown in the uppermost, right hand, data block, was that no print failure occurred when the distance of travel of an electrothermal transfer film past the print head was 100 m or more, the condition being marked " .circleincircle. ".

Those print heads marked with "x" in the respective data blocks actually suffered from a print failure when the distance of travel of transfer film FL was in the range of 2 to 3 m. Thus, where the hardness of the recording electrodes DD is significantly greater than that of the substrate BO, the cleaning interval may be 100 m or more, but where the recording electrodes DD and the substrate BO have about the same hardness, the print head must be cleaned frequently and is of low practical value for use in printing apparatus under actual working conditions.

A print head composed of a substrate formed of ceramics having a hardness of 750 HV and recording electrodes formed of tungsten molybdenum having the same hardness was marked with "x", and a print head having a substrate formed of ceramics having a hardness of 800 HV and recording electrodes formed of tungsten molybdenum having a hardness of 700 HV was also marked "x".

Where the substrate BO was made of magnesium and other compositions having a hardness of 300 HV and the recording electrodes DD were formed of molybdenum having 400 HV, the result was " .circleincircle. ".

The above experimental results were produced because the recording electrodes DD and the substrate BO of print head HE were pressed and slid simultaneously against the electrothermal transfer film FL and the ends of the recording electrodes DD were worn simultaneously (immediately) when the recording electrodes DD and the substrate BO head have the same hardness. The result is that the recording electrodes DD will not be held sufficiently against the electrothermal transfer film FL.

Distances of travel of the transfer film (wear) required to produce lower image quality in Table 2 above and Table 3 below are defined as follows:

.circleincircle. : 80-100 m or more

O: 50-60 m

.DELTA.: 20-25 m

x: 5 m or less.

Similar experiments were conducted under substantially the same conditions as above except that .THETA.1 was 50 degrees and .THETA.2 was 40 degrees. The results are shown in Table 3.

Here, there were no printing heads which were evaluated as "x", completely inappropriate for use.

A print head HE composed of a substrate BO of forsterite ceramic having a hardness of 750 HV and recording electrodes DD of tungsten molybdenum having the same hardness was evaluated "O", and a print head HE having a substrate BO of forsterite ceramic having a hardness of 800 HV and recording electrodes DD of tungsten molybdenum having a hardness of 700 HV was also evaluated "O".

A print head HE composed of a substrate BO of the composition of manganese oxide and others having a hardness of 300 HV and recording electrodes DD of molybdenum having a hardness of 400 HV was evaluated " .circleincircle. ".

It can be understood that those combinations of material evaluated ".DELTA." should be cleaned after use resulting from travel of the transfer film for distances ranging from 20 to 50 m past the print head. A printing apparatus which requires cleaning after transfer film travel past the print head for distances of 20 m or more is sufficiently practical for use.

Those combinations of material marked with "O" should be cleaned for each such travel distance ranging from 50 to 100 m, and those material combinations marked with " .circleincircle. " can be cleaned after travel distances of 100 m or more.

The results above were obtained for the reason that, with the differing angles .THETA.1 and .THETA.2, the limit of wear on the ends of recording electrodes DD is extended until the ends of the recording electrodes DD are worn down to the substrate BO, so that the cleaning from the end of the substrate to a new end can be increased into a practical range.

The relationship between changes in shape of the recording electrodes DD and cleaning thereof will now be described.

In general, FIG. 10 shows the manner in which the print head HE and the electrothermal transfer film FL are held in contact with each other during printing.

Designated at PA is the transfer recording body. The electrothermal transfer film FL is shown with the ink layer and the resistive layer omitted. The electrothermal transfer film FL is moved in the direction of the arrow with respect to the print head HE. However, the print head HE may be moved in an opposite direction, or both the print head HE and the electrothermal transfer film FL may be moved.

FIG. 10(a) shows conditions in which the hardness of recording electrodes DD is significantly larger than the hardness of substrate BO. In FIG. 10(a), the angles .THETA.1 and .THETA.2 are substantially the same as each other; they may be different from each other as shown in FIG. 10(b) and as described above. The end of print head HE changes in shape due to printing wear as indicated by the dashed lines, and at the time of cleaning has the shape indicted by the chain lines.

The distance from the end of the print head after cleaning to the point where the recording electrodes are worn to the largest extent is denoted WP. Since the hardness of recording electrodes DD in FIG. 10(a) is significantly larger than the hardness of substrate BO, the recording electrodes DD project beyond the substrate BO. Therefore, essentially no cleaning is required.

When recording electrodes DD are exposed and it is necessary to prevent short-circuiting or leakage between the recording electrodes DD which is due to the presence therebetween of conductive dust or the like, the recording electrodes DD should be cleaned. By covering the recording electrodes DD with a protective insulating layer, the above condition can be substantially improved.

FIG. 10(b) shows the condition in which the hardness of recording electrodes DD is equal to or less than the hardness of the substrate BO. In this case, the worn recording electrodes DD and substrate BO are shaped as indicated by the dotted line.

FIG. 10(c-1) illustrates a special case of the wear condition of FIG. 10(b). Here, current has been passed between adjacent recording electrodes DD as shown in FIG. 10(c-2) to fuse ink. The change in shape of recording electrodes DD shown by the dotted lines in FIG. 10(c-1) is due to an electric discharge which occurs between recording electrodes DD and the resistive layer in electrothermal transfer film FL, in addition to wear on the recording electrodes DD resulting from sliding movement between the print head HE and the electrothermal transfer film FL. As shown in FIG. 10(c-1), electrothermal transfer film FL confronts recording electrodes DD at points PV1, PV2. Since angles .THETA.1 and .THETA.2 are different and since the electrothermal transfer film FL and recording electrodes DD contact each other under different conditions in the vicinities of points PV1, PV2 (the portion PV1 is under pressure and the portion near point PV2 tends to come off, or the contact pressure on portion wear point PV1 is larger and the contact pressure on the portion of the electrode near point PV2 is smaller), portions of the electrode near to points PV1, PV2 are subjected to different potentials. Therefore, an electric discharge is produced between the electrothermal transfer film FL and recording electrode DD at the portion near point PV2, deforming the portions of the recording electrode DD which lie closer to the substrate BO.

The above condition will be described in greater detail with reference to FIG. 10(c-2). In FIG. 10(c-2), a recording current is first passed between recording electrodes DD1 and DD2 to effect thermal transfer; then a current is passed between recording electrodes DD2 and DD3 for thermal transfer, and finally a current is passed between recording electrodes DD3 and DD4 for thermal transfer. The recording electrodes DD have fixed polarities as shown. The recording electrodes DD of the positive polarity are deformed, as shown by the shadings, due to an electric discharge.

In the conditions shown in FIGS. 10(b), 10(c-1), and 10(c-2), printing failure occurs when the distance WP is about 0.5 .mu.m (in the range of from about 0.3 .mu.m to 0.7 .mu.m), since printing conditions change due to the changed shape of recording electrodes DD in contact with electrothermal transfer film FL. Thus, where printing is to be effected with a different printed area for a given dot, the area of the dot which is actually produced will be substantially different as a result of only a slight change in shape of the recording electrodes DD.

Therefore, the allowable wear distance WP should be in the range of from about 1 to 2 m.

Materials useful in the recording electrodes DD and the substrate BO will now be described in detail.

The recording electrodes DD in the print head HE can be made, for example, of compositions containing metal such as tungsten, molybdenum, manganese, or a mixture of these hard metal compositions, in a combination with an insulating material such as alumina, glass powder, glass fiber, or silicon oxide powder.

The substrate BO may also be made of a material such as a porous ceramic composed mainly of alumina, a ceramic composed mainly of magnesium and silicon, or a ceramic composed mainly of magnesium, silicon, aluminun, potassium, fluorine and oxygen.

The substrate BO may be composed solely of the materials indicated above, but may also contain a small amount of lead, zirconium, potassium, or the like mixed in a proportion in the range of from a few to ten and a few percent. Ceramics composed mainly of magnesium and silicon should be of 20 to 30 weight percent of magnesium and 70 to 80 weight percent of silicon. One example of such a ceramic is forsterite. Ceramics composed mainly of magnesium, silicon, aluminum, potassium, fluorine and oxygen may be of 10 to 20 weight percent of magnesium, 40 to 60 weight percent of silicon, 20 to 40 weight percent of aluminum, and 2 to 15 weight percent of potassium and fluorine, the proportion of aluminum, potassium and fluorine being about the same as that of aluminum and potassium while the proportion of fluorine is one-third (1/3) to five-sixths (5/6) of the above. One example of such a ceramic is mica.

Where an insulating material is to be mixed with a hard metal or a composition of hard metals, the proportion of the insulating material to be mixed may be in the range of from 5 to 20 percent by weight of the overall weight for making the metal electrodes DD harder.

An experiment was conducted to find optimum angles .THETA.1, .THETA.2.

To insure that the electrodes and the ink film are kept well pressed against each other, the angles .THETA.1, .THETA.2 should preferably be correlated to meet the requirement .THETA.2=.THETA.1/2. From the standpoint of the mechanism, however, the angles .THETA.1 and .THETA.2 should be as close alike as possible to permit design of the mechanism for changing the angle of the print head for ease of operation.

An experiment was carried out under the same conditions as those for the first example and in which recording electrodes DD were made of tungsten molybdenum having a hardness of 700 HV on a substrate BO which was made of forsterite ceramic having a hardness of 800 HV.

The results of the experiment are given in Table 4. The mark "x" indicates failure of proper printing occurred before the distance of transfer film travel had reached 5 m and that the print head is not therefore considered to be suited for practical use in a printing apparatus. The mark " .circleincircle. " shows that the experimental values shown in Table 3 were attained. The mark ".DELTA." indicates that the print head needed to be cleaned at values which were half the experimental values in Table 3. Although the head should preferably be used with the angle conditions marked " .circleincircle. ", it may be used in practice with the angle conditions marked ".DELTA.".

The same results were obtained with a print head having a substrate BO made of forsterite with a hardness of 750 HV and having recording electrodes DD made of tungsten-molybdenum with a hardness of 750 HV. Furthermore, the same results were reached with a print head having a substrate BO which was made of forsterite with a hardness of 800 HV and having recording electrodes DD made of tungsten molybdenum with a hardness of 700 HV.

Thus, the combinations evaluated with "O" and ".DELTA." in Table 3 produced the same results.

A print head HE was fabricated by a thick film printing process as follows: A material for substrate BO was prepared mainly from 25 to 30 weight percent of manganese oxide and 65 to 70 weight percent of silicon dioxide. The prepared material was crushed and mixed, and then atomized and dried to produce a green sheet. As the material for recording electrodes, tungsten and an organic binder of PET were mixed at a proportion of 9:1 weight percent, and the mixture was crushed and kneaded to produce a conductive paste. The conductive paste was printed on the green sheet, which was then heated in a reducing atmosphere to bake the substrate and the electrodes simultaneously, thus producing the print head. The electrodes had a thickness of 30 .mu.m.

While the ceramic substrate (forsterite) composed mainly of manganese oxide and silicon dioxide has a hardness ranging from 500 to 600 HV, tungsten has a hardness of about 700 HV.

The print head was refreshed for each travel distance of 100 m, using .THETA.1=50.degree., .THETA.2=45.degree.; the print head remained capable of clear printing after a distance of 2000 m had been travelled.

An experiment was carried out with the recording electrode pitch PDD being 254 .mu.m and the electrode width WDD ranging from 0.4-0.7 (100 .mu.m, 120 .mu.m, 140 .mu.m, 160 .mu.m, 170 .mu.m) of pitch PDD, and the same results as in Tables 2 and 3 were obtained.

However, insufficient printing quality was attained when the recording electrode pitch PDD was 0.3 or less. When the recording electrode pitch PDD was greater than 0.8, foreign matter (electrode dust or the like) was deposited between recording electrodes DD, tending to cause a short circuit therebetween.

When an insulating protective layer (e.g., an organic material such as PVA, PET, polysulfone, or polyimide, for example), which had a hardness lower than that of recording electrodes DD, was formed on recording electrodes DD, the insulating protective layer was capable of preventing short-circuits between the recording electrodes. With this arrangement, the protective layer was slightly backed off the electrode tips by cleaning, and good printing was achieved.

The protective layer covers the ends of the electrodes, as well as the spaces between the electrodes. When the ends of the electrodes are cleaned subsequently by the abrasive, as shown in FIGS. 10(d-1), 10(d-2), and 10(d-3), more of protective layer PROT is removed by the abrasive than of electrodes DD because of the relative hardnesses of the materials, so that the ends of the electrodes project out of the protective layer.

FIG. 10(d-1) shows the manner in which the contacting portions of electrodes DD on the end of the substrate BO and of protective layer PROT are cleaned.

The substrate BO and electrode DD are ground at the printing end by the grinding material GRI which is mounted on at least part of the surface of the platen PT. The ends of the angled surface of electrodes DD and substrate BO which contact electrothermal transfer film FL along with electrode DD project from protective layer PROT.

FIG. 10(d-2) shows an electrode DD in cross-section on substrate BO in which the acute angle end of electrode DD projects from protective layer PROT.

In addition, protective layer PROT' may be formed on electrode DD as shown in FIG. 10(d-3). The ends of electrodes DD are caused to project from protective layer PROT' by cleaning.

A print head HE was fabricated by a thick film printing process as with example A. A substrate BO was made of porous ceramic composed mainly of alumina, while recording electrodes DD were made of a material composed of tungsten and alumina mixed therein. The porous ceramic composed mainly of alumina was produced by baking alumina at a temperature which ranged from 300.degree. to 800.degree. C., lower than the full baking temperature of about 1600.degree. C. for alumina.

The hardness of the porous ceramics composed mainly of alumina ranged from about 200 to 600 HV depending upon the baking temperature and the purity of the alumina. The recording electrodes DD were made of a mixture of tungsten and alumina and had a hardness of 1000 HV.

The print head was refreshed for each distance of travel of transfer film of 300 m with .THETA.1=60.degree. and .THETA.2=50.degree., and no printing failure was experienced after the distance of travel of 2000 m had been cleared.

In making recording electrodes DD, a prescribed pattern was printed on substrate BO by thick film printing, and then a conducting layer was plated on the printed thick film pattern. The printed thick film should be of a thickness in the range of from 8 to 15 .mu.m, the depth of the plated layer should be in the range of from 5 to 10 .mu.m, and the overall thickness of the electrodes should range from about 10 to 30 .mu.m.

If recording electrodes DD were to be produced by thick film printing only, then the recording electrodes would be likely to collapse. However, where recording electrodes DD of the prescribed thickness are produced by plating on the printed thick film pattern, the recording electrodes DD are more resistant to changes in their shape.

The plating of the electrodes may be by means of electroplating or electroless plating.

A substrate BO was made of soda glass (of a hardness ranging from 300 to 600 HV). A thick film pattern having a thickness of 15 .mu.m of silver palladium was printed on the substrate by way of screen printing. Thereafter, the thick film pattern was electroplated with a nickel layer having a thickness of 5 .mu.m and a Vickers hardness of about 600 HV, thus producing recording electrodes DD. The other conditions were the same as those of example A.

Where the layer is to be plated on a thick film, the hardness of the plated layer should be selected as indicated in Table 2.

The thick film may be as described in Table 3, but may be formed of silver paste, gold paste, copper paste, or the like.

The material to be plated onto the thick film may be, in addition to nickel, rhodium or a mixture or composite of Ni and SiC.

After a thick film has been formed all over the substrate BO, a photoresist may be coated thereon and exposed through a film having a prescribed pattern to produce recording electrodes of the desired shape. With this arrangement, it is easy to shape the recording electrodes DD.

Alternatively, a thick film of uniform thickness may be coated on a substrate BO by printing, dipping, brushing, spraying or the like, and thereafter may be patterned to a prescribed shape by photolithography as described above.

The plated layer may be hardened by quenching.

A substrate BO was made of soft glass ceramic, and was plated with a nickel layer having a uniform thickness of 20 .mu.m. Substrate BO may be of a hardness ranging from 300 to 600 HV. Although the plated nickel layer originally had a hardness of 600 HV, it was quenched at about 500.degree. C. to a hardness in the range of from 800 to 1200 HV. Thereafter, a predetermined pattern of electrodes DD was formed by photolithography.

A print head having a dot density of 10 dots/mm or more was fabricated. Thereafter, a prescribed pattern of electrodes was formed by photolithography.

In the above example, the substrate BO was plated with a layer of uniform thickness, followed by shaping into the desired recording electrode configuration by photolithography. Thereafter, the plated layer was quench-hardened.

As an alternative, the substrate BO may be formed of glass, soft glass ceramics, or the like, and wires of a prescribed diameter may be placed on and fixed to the substrate by means of an inorganic adhesive.

Wires of diameters ranging from 100 to 170 .mu.m were placed at equal pitches of 254 .mu.m on a substrate BO of soft glass ceramic, and were then bonded thereto by an inorganic glass adhesive. The other conditions are the same as those in the preceding example. Tungsten wires may be employed to form recording electrodes DD of tungsten of high-purity, high-hardness bulk. The resulting recording electrodes were highly resistant to both electric-discharge and mechanical wear.

The recording electrodes on the substrate BO may be left exposed, but an insulating layer which is softer than the electrodes may be formed entirely thereover on substrate BO. The insulating layer may be made of epoxy resin, acrylic resin, urethane, silicone, PET, or other resins.

A length of 15 to 50 m of electrothermal transfer film FL is rolled. A grinding section which is 5 to 30 cm long is then provided on the leading end of the electrothermal transfer film FL by joining a suitably shaped portion of a commercially available abrasive grinding sheet thereto by means of an adhesive, or a double-sided adhesive tape, or by fixing particles of, for example, silicon carbide directly to the surface of the electrothermal transfer film FL using a suitable adhesive.

The above-described electrothermal transfer film FL was used for printing in combination with a print head HE of the character marked with " .circleincircle. " in Table 2, in which the hardness of the recording electrodes DD is greater than that of the substrate BO. With this combination, good printing resulted since the end of the print head HE was cleaned first each time that one roll of electrothermal transfer film FL was consumed for printing.

Printing was then carried out with print heads HE marked with "O" or ".DELTA." in Table 2, in which the hardness of the recording electrodes DD is the same as or less than that of the substrate BO. In this case, the angle .THETA.1 was not equal to angle .THETA.2, but they were related to each other as indicated by Table 4. More specifically, the end of the print head HE was ground at angle .THETA.1 when electrothermal transfer film FL was first wound back, during which time the cleaning section was pressed against the print head, and printing was effected at angle .THETA.2.

Likewise, the trailing end of a roll of electrothermal transfer film FL was provided with a grinding section, and the roll was used for a similar cleaning and printing operation. Such an embodiment is depicted in FIG. 10(d-4) where grinding material GRI is carried on a section of conductive layer 4 of transfer film FL.

By thus providing a cleaning section on the leading or the trailing section of a roll of electrothermal transfer film FL, the print head can be ground each time the roll is replaced. It is to be noted also that this arrangement can be used when the electrothermal transfer film is composed of an integral electrothermal transfer film and a recording body PA which will be separated after printing. Therefore, the grinding process can be carried out without fail after printing has been effected for a predetermined interval or distance. The user of the printing apparatus is not required to do any special maintenance work on the apparatus.

The length of a roll of electrothermal transfer film FL can also be made longer than the one interval after which the print head must be cleaned by providing grinding sections on the surface of the transfer film at the required intervals.

In still another manner of cleaning the print head, the electrothermal transfer film need not be provided with a cleaning or grinding section. Instead, the number of revolutions of the motor which feeds the electrothermal transfer film may be counted, an interval of printing on the electrothermal transfer film may be optically measured by slits defined in the film, or an interval of printing on the electrothermal transfer film may be magnetically measured by magnetic bodies mounted on the film, so that a light or buzzer signal can be generated when a predetermined printing interval has been completed. When such an alarm signal is generated, a separate cleaning material such as an abrasive sheet may be inserted between the platen PT and the print head HE to clean the end of the print head.

As shown in FIGS. 10(d-l), the platen PT may also be provided with a cleaning GRI section overall or over part of its surface. With this arrangement, the end of print head HE to be cleaned is, of necessity, held against platen PT and ground by the cleaning section prior to or subsequent to a printing process effected by a roll of electrothermal transfer film.

The above cleaning arrangements can also be realized using the transfer recording body PA instead of the electrothermal transfer film FL.

The electrothermal transfer film FL and the transfer recording body PA may be integrally combined prior to printing and may be separated after printing.

The abrasive or grinding cleaning material may be diamond, silicon carbide, aluminum oxide, or the like. Where such a material is used, it is employed in the form of particles which are bonded to a sheet of paper or plastic, a plastic film, a metal or rubber sheet, a platen, or the like, by means of a binder such as an adhesive, a tacking agent, a double-sided adhesive tape, or the like. Alternatively, the particles may be mixed in a plastic resin, and the mixture may be solidified.

The organic binder may be PET, ethylene butanol, acrylic resin, polystyrene, polycarbonate, polyethylene, general polyesters other than PET, or a mixture of these materials.

Where the width of each recording electrode in the print head HE is about 100 .mu.m, the cleaning material should be of a fineness of about #500 or more since, if the fineness were less, the ends of recording electrodes DD would be subject to breakage when ground. If the cleaning particles were too small, the cleaning efficiency would be lowered. Therefore, the fineness should be below about #2500, but preferably in the range of #600 to #2000.

The printing apparatus of the invention may employ electrothermal transfer films FL having two-color ink layers (red and black, blue and black, red and blue, or the like) arranged alternately in the direction of feed, so that characters may be printed in either one of the two colors.

An electrothermal transfer film FL may be coated successively with inks of yellow, magenta, and cyan for generating full color images.

FIG. 11 is a block diagram of a signal processing system for a full-color video printer incorporating the printing apparatus of the present invention. In the embodiment of FIG. 11, the video input signal and the sequence of operation of the mechanism are primarily controlled by an 8-bit central processing unit (CPU) 58. The video signal is fed to signal processing circuit 52 which separates the video signal into Y, M, and C chrominance signals, which are then selected by YMC select 5 for conversion by 6-bit A/D converter 56 into digital signals. The digital signals are fed to ROM table 60 which produces a serial signal matching the density characteristics of the printer of the invention and feeds the serial signal to print head control circuit 62. Print head control circuit 62 is composed of serial-parallel shift register 64, latch 66, data selector 68 for time-sharing drive, and driver 70. Shift register 64, latch 66, and data selector 68 are supplied with a shift clock signal, a latch signal, and an address signal, respectively, from control circuit 62 whose operation is regulated by CPU 58.

For forming a one-frame color image on a transfer recording sheet, the image is first printed using a yellow section of the ink film, then a magenta section, and finally a cyan section. Each of the three-color portions of the electrothermal transfer film FL has an ink layer extending over an area corresponding to one frame. Each time one color image is printed on a transfer recording sheet PA, the recording sheet is wound back for printing in another color.

The relationship between print head HE and ink film FL has mainly been described to show that the fusing of the ink layer of the electrothermal transfer film FL produces image transfer without error when the print head HE is operated under constant conditions.

Thus, the problems in making an effective printing apparatus are substantially solved as described above. However, the transfer recording body is improved, as described below, so as to be useful for the printing of superimposed colors or the clear printing of small dots.

The transfer recording body PA may be paper, a plastic film, or paper or plastic film which is coated with a resin. The resin may be organic or it may be inorganic, such as silicon dioxide. Although the recording body PA may be plain paper, small dots having diameters ranging from 40 to 70 .mu.m can be clearly printed by employing paper, a plastic film, or a combination thereof that has a smoothness of 1000 sec or more, expressed in Bekk units since the electrothermal transfer film FL and the recording body PA are more uniformly held during printing against each other by the print head HE and the platen PT.

Plain paper having a thickness ranging from 50 to 80 .mu.m may be coated with a smooth layer having a thickness ranging from 5 to 20 .mu.m. The coated layer may be of acrylic resin, urethane, polyester, PET, SBR (styrene-butadiene rubber), or the like.

A plastic film of a thickness ranging from 40 to 100 .mu.m may be coated as above with a smooth layer having a thickness in the range of 10 to 50 .mu.m. The plastic film may be of polyester, PET, polyethylene, polystylene, polypropylene, or the like. In the event that the smooth-coated layer is a good ink absorber, and where color printing is to be effected by superimposing colors, the first ink layer will not project upwardly, and hence the next ink layer can be printed with good smoothness and uniform pressure. In addition to improving smoothness, the coated resin layer is thus capable of good color printing.

However, the recording paper may be so constructed as to not absorb ink (or the ink may be such as does not permeate the recording paper) so that an applied ink layer will project onto the recording paper. With this modification, ink is in a fused state when the print head is in contact with the electrothermal transfer film, and thereafter is solidified, and hence there will be no problem of smoothness.

Also, the platen PT in the printing apparatus of the invention need not necessarily be cylindrical in shape, but may be flat.

The best results were achieved by the printing apparatus of the invention when the printing pressure ranged from 1.2 to 2.2 kg/cm.sup.2. When the printing pressure was about 0.8 kg/cm.sup.2, certain dots could not be printed if the heat generated in the dots was to be controlled and dots having diameters ranging from 40 .mu.m to 70 .mu.m were to be formed.

When the pressure rose to about 3 kg/cm.sup.2, the electrothermal transfer film FL and recording paper PA were wrinkled at the time they were fed along, and the paper was stained all over under the pressure of the print head HE.

The polarities of adjacent recording electrodes in print head HE may be switched around. More specifically, if one of two adjacent recording electrodes DD is at a positive potential while the other recording electrode is made negative at a predetermined time in a printing cycle, they may be switched to negative and positive potentials, respectively, for printing in every other cycle or every other few cycles.

The print head HE may be so constructed that a plurality of signal electrodes for fusing ink are formed on one surface of substrate BO while electrodes for giving polarities opposite to those of the signal electrodes are formed on the other surface of substrate BO. The relationship between the hardness of the recording electrodes DD and the hardness of the substrate BO, and the relationship between angles .THETA.1 and .THETA.2 hold true for this modification.

The printing apparatus of the present invention can effect gradation printing by controlling the current and voltage applied to the print head so as to control the heat (Joule heat) generated in electrothermal transfer film FL. Control of the generated heat is effective in controlling the amount of ink to be transferred to the recording body PA.

Rather than controlling the amount of ink to be transferred for each dot, the density of dots to be transferred may be changed for gradation printing provided that the amount of ink transferred for each dot remains the same.

The printing apparatus of the invention may be used for "lift-off" by fusing ink which has been transferred to the recording body PA and erasing the fused ink.

The printing apparatus of the invention can be incorporated in electrothermal transfer printing apparatus which use slightly different principles. The present invention can also be used with electrothermal transfer films and recording papers which are more or less different from the illustrated embodiments.

Examples of such arrangements will be given below.

FIG. 12 is illustrative of another embodiment of a printing apparatus to which the present invention is applicable. Here, electrothermal transfer film FL is composed of a resistive layer FL-1, a metal layer FL-2, a base layer FL-3, and an ink layer FL-4. Print head HE-a has a multiplicity of recording electrodes which are arranged substantially perpendicular to film FL and are held in contact with resistive layer FL-1. Another recording electrode HHa is held in direct contact with metal layer FL-2 by means of spikes HHa-1, for example. The other recording electrode HHa comprises pointed, elongate electrodes made of metal such as iron, stainless steel, aluminum, and copper. The tip of each electrode projects through resistive layer FL-1 of electrothermal transfer film FL and contacts metal layer FL-2 so as to prevent the generation of heat in the connecting portion of the other recording electrode HHa and the electrothermal transfer film FL. The required heat for melting the ink is thus generated at the area of connection of print head HE-a with the electrothermal transfer film FL. Print head HE-a and recording electrode HHa are connected to a power supply as shown so that a current flows through the film FL in the direction of the arrows. A portion of the resistive layer FL-1 which is contacted by print head HE-a is heated by the current to cause ink to be transferred to a recording body PA.

Metal layer FL-2 need not be of metal but may be of carbon or the like. Metal layer FL-2, however, has an electric resistance much lower than that of resistive layer FL-1. Recording electrodes DD (not shown) of print head HE-a may be kept at a negative potential rather than the indicated positive potential. Base layer FL-3 is optional, depending upon the tensile strength of the other materials.

FIG. 13 shows a modification of the printing apparatus of the present invention. Print head HE-b has a multiplicity of recording electrodes DD which are all kept at a positive or a negative potential by power source BA. Another electrode HHb which is maintained at the opposite potential is held in line-to-line or plane-to-plane contact with the resistive layer in electrothermal transfer film FL at a distance which is suitably removed from print head HE-b. In this arrangement, the area of contact of electrode HHb with the resistive layer in film FL is greater than the area of contact of print head HE-b with the resistive layer, with the result that the heat generated in the portion of the film which is in contact with the electrode HHb is so small that the ink layer will not be fused. As recording electrodes DD in print head HE-b are spaced sufficiently from electrode HHb, the portion of the print head HE-b which is held against film FL for forming dots can be kept in a constant position.

The electrothermal transfer film FL may include a conductive layer which has a resistance substantially smaller than that of the resistive layer.

Electrode HHb may be positioned upstream of the position in which electrothermal transfer film FL contacts print head HE-b, e.g. in the direction of feed of film FL.

The electrothermal transfer printing apparatus of the present invention is able to print dots at small intervals or pitches since the print head has finely spaced recording electrodes fabricated on a supportive substrate by means of thick film printing or the like.

Since the recording electrodes are pressed against the electrothermal transfer film at a predetermined angle, good printing quality can be maintained at all times.

Good printing quality can also be ensured by adjusting the relationship between the hardness of the recording electrodes and the hardness of the substrate.

Where the hardness of the recording electrodes is equal to or lower than that of the substrate, good printing can be maintained by cleaning the end of the print head at intervals. Better printing quality can be achieved by adjusting the angle at which the print head end is cleaned, selecting the surface roughness of the cleaning material used to a desired value, and by other refinements.

Colors can be printed in superimposed relation, or smaller dots can be printed, by employing a special transfer recording body.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above constructions without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.