Electric field assisted thermal recording apparatus

In a thermal recording apparatus, a thermal head heats an ink sheet to transfer ink to a recording medium such as paper. Even when recording on a low-smoothness plain paper, an image free of voids can be obtained. The apparatus is equipped with a mechanism for generating an electric field between the ink sheet and the recording medium. Due to this electric field, the ink particles liquified by the thermal head fly and are transferred to the recording medium.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a thermal recording apparatus using a thermal 
head, and more particularly to a thermal recording apparatus which makes 
it possible to record even on a rough-surfaced paper sheet or to perform 
half-tone recording. 
2. Description of the Related Art 
Since it is simple in structure, high in reliability and convenient to 
maintain, thermal recording and thermal transfer recording are currently 
the mainstream in facsimile and color printing, respectively. 
FIG. 13 of the accompanying drawings shows a typical conventional thermal 
transfer recording apparatus. This thermal transfer recording apparatus 
comprises a thermal head 1 having heat generating elements (not shown) for 
thermal transfer to a recording paper 6, a platen roller 21 against which 
the thermal head 1 is to be pressed, and an ink sheet 20 having an ink 
layer in which a plurality of sets of color regions of yellow (Y), magenta 
(M) and cyan (C), or black (K) if necessary, are arranged in a row. The 
ink sheet 20 also has a thin base film such as of polyethylene 
terephthalate (hereinafter abbreviated as "PET") formed over the ink 
layer. Generally the recording paper 6 is a high-quality paper or a 
dedicated paper having a high degree of smoothness. 
In color recording, firstly the yellow (Y) region of the ink sheet 20 is 
conveyed into the gap between the thermal head 1 and the platen roller 21 
with the recording paper 6. During recording, the heat generating elements 
of the thermal head 1 are energized for a predetermined time to generate 
joule heat and then the heat is transferred to the ink sheet 20. As a 
result, according to a record signal, solid ink on the ink sheet 20 is 
softened (liquefied) and is then transferred in part to the recording 
paper 6. The same operation is repeated in order for every color. In 
monochromatic recording, the same recording is made on the ink sheet 20 
whose ink layer consists of only a black (K) region. 
The common problem with the conventional thermal transfer recording 
apparatuses has been only a limited printing speed and the need for 
smooth-surfaced paper. For a better printing speed, on the one hand, it is 
now customary to improve the thermal head itself or to control the 
temperature of the heat generating elements precisely. 
On the other hand, studies are currently being made to enable recording on 
plain paper sheets (PPC paper), which are widely used in copying machines. 
However, in thermal transfer recording, the process to transfer ink to a 
paper depends on the roughness of the paper surface; for example, when a 
rough paper is used, mistransfer such as voids would occur to deteriorate 
the quality of recording. This results because ink could not adhere to 
recesses or dents on the paper surface but could be transferred to only 
other surface portions. 
In a serial printer, a fairly good printing can be obtained on a 
low-smoothness paper such as PPC paper by transferring ink in a bridge 
form; whereas in a line printer, an adequate result can not be obtained. 
For example, a publication "IMAGING Part 2" (Electronic Photographic 
Society's Hard Copy Series edited by the Electronic Photographic Society, 
pages 65-73) issued by Photographic Industries Publisher Co. Ltd. (a 
Japanese corporation) discloses: the concept of optimizing the break 
elongation rate and the viscoelasticity of ink at the ink peeling 
temperature (improvement to the ink sheet); an edge-type head with which 
the ink peeling timing is reduced in order to transfer ink while the ink 
viscosity is low at a high temperature and in which an adequately large 
angle of peeling ink can be realized (improvement to the ink peeling 
method); and the concept of bulging the graded layer right under the heat 
generating elements to cause an intimate contact with the recording paper 
(improvement to the head structure). In addition, studies are currently 
being made to apply an increased amount of power to cause ink to flow into 
recesses on the paper surface. 
When recorded on a low-smoothness paper such as bond paper, voids may occur 
in parts to varying degrees. When applying an increased amount of power to 
eliminate any voids, ink would bolt on the paper. 
The reasons why the edge peeling method could not eliminate the foregoing 
problems are that the solidifying time of ink was usually several ms and 
that solidification terminated before recording reached the peeling 
position in a line printer. Specifically, with the thermal head of 12 
dots/mm in which the heat generating elements are located in a position 1 
mm from the edge, recording of 1 line must be completed within 5/12 ms. 
Since the number of heat generating elements per line is about 2500, it 
requires a power supply of over several kW to meet the foregoing 
conditions, which is not practical. 
Another problem with thermal transfer recording is that half-tone recording 
could not be achieved; that is, only binary recording, i.e. recorded or 
not recorded, was possible. Generally thermal transfer recording done 
using energy immediately after the recording density has become saturated 
in the relationship between energy and recording density. This is because 
if transient energy before the recording density becomes saturated was 
used, the recording density would have varied widely to deteriorate the 
image quality considerably. This is partly because the probability of 
whether or not ink is transferred to the paper would be 50% and partly 
because components constituting as image noise would increase in this 
operating region. Therefore in half-tone recording, it was inevitable to 
use pseudo gradation such as area gradation so that resolution becomes 
impaired. 
SUMMARY OF THE INVENTION 
It is therefore an object of this invention to provide a thermal recording 
apparatus with which a high-quality image free of voids can be obtained 
even when recorded on a low-smoothness paper such as rough paper and with 
which half-tone gradation can be obtained and which consumes only a 
reduced amount of ink sheet. 
According to a first aspect of the invention, there is provided a thermal 
recording apparatus comprising: an ink sheet having an electrically 
conductive or chargeable ink layer; a thermal head for heating the ink 
sheet; and means for creating an electric field between the ink sheet and 
a recording medium. 
According to a second aspect of the invention, there is provided a thermal 
recording apparatus comprising: an ink sheet having a conductive or 
chargeable ink layer and a resistance layer for generating joule heat when 
energized; a pair of electrodes for applying a current to the resistance 
layer; and means for creating an electric field between the ink sheet and 
a recording medium. 
According to a third aspect of the invention, there is provided a thermal 
recording apparatus comprising: an ink sheet having a conductive or 
chargeable ink layer and a photo-thermal transducing layer for generating 
heat upon receipt of an electromagnetic wave; means for applying an 
electromagnetic wave to the photo-thermal transducing layer; and means for 
creating an electric field over the ink sheet and a recording medium. 
To obtain a better quality recording, electric field generating means may 
create an electric field obtained by superposing an a.c. voltage over a 
d.c. voltage. 
To reduce an amount of ink sheet to be consumed, the ink sheet feed speed 
may be smaller than the recording medium feed speed, and the heat 
generators of the thermal head may have a length, in the direction of 
conveying the recording medium. 
Means generating an electric field is provided according to a number of use 
of the sheet, by using the ink sheet repeatedly, and storing or recording 
the number of use onto the ink sheet, and reading on the data. 
An ink material permeated in the ink sheet may be a conductive material to 
form an ink layer serving as an electrode at one end of the electric field 
impressing means. 
To prevent any leak, the chargeable layer of the ink sheet may be formed of 
an ink holding material which is insulative at room temperature and 
conductive when heated. 
The thermal recording apparatus may further comprise means for retaining a 
gap between the ink sheet and the recording medium. 
The conductive layer of the ink sheet may have an area larger than that of 
the remaining layers and serves to make and contact with the electrode. 
In operation, an electric field is generated between the recording medium 
and the ink sheet, which has the conductive or chargeable ink layer, by 
the electric field generating means. Since due to this electric field the 
ink which has been softened by the heating means such as the thermal head 
is conveyed toward the recording medium so as to be transferred thereto, 
high-quality recording free of voids can be achieved. 
Particularly in the case where the electric field is provided by 
superposing an a.c. voltage over a d.c. voltage, since ink particles which 
are vibrating are made to reach the recording medium, the scattered 
recording densities are averaged based on the fact that ink particles 
reach the recording medium in different time periods depending on their 
size, thus resulting in high-quality recording and good half-tone graded 
recording. 
Further, since the thermal head presses against the ink sheet and the 
recording medium with a small amount of force, it is possible to make the 
ink sheet feed speed smaller than the recording paper feed speed so that 
the amount of ink sheet to be consumed can be reduced. Inksheet 
consumption can also be reduced by using the inksheets repeatedly. 
Since the heat generating elements of the thermal head can be reduced in 
size, the thermal head itself can also be reduced in size. 
By using, as an ingredient, an inorganic dye obtained from a dyed white 
conductive material, it is possible to realize color printing. 
Since the ink material permeated in the ink sheet is conductive, it is not 
necessary to add to the ink sheet a conductive layer such as a metal 
layer, thus making the ink sheet inexpensive. 
By using, as an ink holding material, a material which is insulative at 
room temperature and conductive when heated, it is possible to prevent any 
leakage from occurring. 
Since ink is made to transfer stably by the gap retaining means, it is 
possible to control the printing density precisely. 
Further, partly since the area of the conductive layer of the ink sheet is 
larger than that of the remaining layers, and partly since this expanded 
layer serves as a contact with the electrode through which the voltage for 
generating an electric field is to be impressed, it is possible to obtain 
a stable electric field.

DETAILED DESCRIPTION 
Various embodiments of this invention will now be described with reference 
to the accompanying drawings. FIGS. 1(A) and 1(B) are schematic 
cross-sectional views each showing the main portion of a thermal recording 
apparatus according to this embodiment; like reference numerals designate 
similar or corresponding parts or portions throughout FIGS. 1(A) and 1(B). 
The only difference between FIGS. 1(A) and 1(B) is that one uses only a 
d.c. bias voltage while the other uses both a.c. and d.c. bias voltages. 
In FIG. 1(A), reference numeral 1 designates a thermal head in which, for 
example, 2500 non-illustrated heat generating elements are arranged 
perpendicularly to the plane of this drawing sheet. 5 designates an ink 
sheet composed of a PET film (Mylar film), a metal layer 3 such as of 
aluminum, and an electrically conductive ink layer 4. 6 designates a 
recording paper such as low-smoothness paper, e.g. bond paper. Between the 
ink sheet 5 and the recording paper 6, there exists an air layer 10. Some 
air will also remain between the thermal head 1 and the ink sheets. 7 
designates a platen roller which is composed of a central metal portion 
and an outer conductive rubber and which is rotatable anticlockwise, while 
pressing the recording sheet 6 toward the ink sheet 5 between the thermal 
head 1 and the platen roller 7, the central metal portion being grounded. 
8 designates a d.c. voltage source for impressing a voltage between the 
metal portion of the platen roller 7 and the metal layer 3 of the ink 
sheet to create an electric field. The platen roller 7 and the D.C. 
voltage source 8 jointly constitute an electric field impressing means. 
9 in FIG. 1(B) designates a voltage source for impressing a bias voltage 
obtained by superposing a d.c. voltage and an a.c. voltage over each 
other. 
EMBODIMENT 1 
The operation of this thermal recording apparatus will now be described 
with reference to FIG. 1(A). A bias voltage is impressed between the ink 
sheet 5 and the platen roller 7 to create an electric field between the 
ink sheet 5 and the recording sheet 6. 
The conductive ink layer 4 may be liquefied in the same manner as the 
conventional thermal transfer recording method. Specifically, the thermal 
head 1 is driven by a signal from a non-illustrated thermal head control 
circuit to energize the heat generating elements of the thermal head 1 to 
generate joule heat is to be transmitted to the ink sheet 5. This joule 
heat is transmitted to the PET film 2, then the metal layer 3 and finally 
to the conductive ink layer 4 so that the conductive ink layer 4 will 
become liquefied. 
The conductive ink layer 4 is composed of carbon or metal such as silver, 
wax and dye, and preferably has a conductivity of about 10.sup.3 to 
10.sup.8 .OMEGA.cm as proved by experiments. Alternatively the conductive 
ink layer 4 may be composed of dye such as carbon, and a conductive 
processed material or a conductive resin binder. 
The conductive ink layer is selectively liquefied by transferred joule heat 
so that the ink particles of the conductive ink layer 4 will fly 
(downwardly in FIG. 1(A)) due to gravitation and the electric field caused 
by the bias voltage, so as to adhere to the recording paper 6. 
With the thermal recording apparatus, since the ink particles fly and 
adhere to the recording paper 6, recording can be done in a constant 
condition, irrespective of the kind of the recording paper 6. 
Generally, in thermal transfer recording, the rough-paper recording 
characteristic would be poor because the ink is liquefied by joule heat 
when adhering to the paper. In ink jet recording, although recording is 
possible on rough paper, ink would be bolted on the paper, and the ink 
nozzle would get clogged and hence there are difficulties with 
maintenance. This invention can eliminate these problems. 
EMBODIMENT 2 
The thermal recording apparatus of FIG. 1(B) is similar to that of FIG. 
1(A) except that an a.c. voltage is added as a bias voltage. If only a 
d.c. voltage is impressed like the case of FIG. 1(A), the same result as 
the previous embodiment can be obtained; if an a.c. voltage in addition to 
a d.c. voltage is impressed, higher-quality recording and excellent 
half-tone recording can be realized, compared to the embodiment of FIG. 
1(A). 
This embodiment will now be described from a view point of half-tone 
recording. In general, the liquefied ink particles in the conductive ink 
layer 4 are not constant in either size or weight. Therefore even when an 
electric field is created between the ink sheet 5 and the recording paper 
6, ink particles often do not fly constantly so that the recording density 
on the surface of the recording paper 6 tends to be not constant. This is 
due to the difference in charge between ink particles; in kinetic 
recording like recording using the thermal head 1, the difference in 
flying time period between ink particles of different sizes will cause the 
recording density to vary widely. Addition of an a.c. voltage will assist 
in eliminating this unevenness of the recording density. 
The result when an a.c. voltage is added will now be described. When an 
a.c. voltage is superimposed over a bias d.c. voltage, the ink particles 
adhere as they move up and down. Namely, when the a.c. voltage is 
impressed in the same direction as the d.c. voltage, the ink particles 
move toward the recording paper 6; when the a.c. voltage is impressed in 
the direction opposite to the d.c. voltage, the ink particles move toward 
the ink sheet 5. During that time, the smaller the ink particles, the more 
violently they move up and down. Since the amount of movement of ink 
particles from the conductive ink layer 4 is relatively the same, the ink 
particles arrive at the recording paper 6 substantially at the same time. 
Thus, half-tone recording can be realized by controlling both joule heat 
due to the thermal head 1, which causes the conductive ink layer 4 to 
melt, and the bias voltage. If with the bias voltage set to an optimum 
value, joule heat and thus the impressed voltage of the thermal head is 
varied, excellent half-tone recording can be achieved. The a.c. voltage 
should by no means be limited to a sine wave or a rectangular wave. 
Preferably the frequency should be within the range of 40 Hz to 200 kHz. 
If the frequency is less than 40 Hz, the recording speed would have to be 
made slow to obtain the required quality; if the frequency is over 200 
kHZ, the range of movement of ink particles is smaller so that it negates 
the effects gained by the addition of the a.c. voltage. 
EMBODIMENT 3 
In the thermal recording apparatus of FIG. 1(A), to the ink sheet 5 
composed of a conductive ink layer 4, whose resistance is 10.sup.6 
.OMEGA.cm and melting point is 70.degree. C., and a PET film 2 of 4.5 
.mu.m and a metal layer 3 (e.g., aluminum layer) of 1000 Angstrom, a d.c. 
bias voltage was impressed to energize the heat generating elements of the 
thermal head 1 and to thus melt the conductive ink layer 4. As the d.c. 
bias voltage was increased, the amount of ink adhered to the recording 
sheet 6 was increased and saturated at about 300 V. Upon the impression of 
a 300 V bias voltage, high-quality recording free of voids was achieved. 
EMBODIMENT 4 
In the thermal recording apparatus of FIG. 1(A), the platen roller 7 is 
grounded. Whereas in this case, a bias voltage of 300 V was impressed 
between the ink sheet 5, which was grounded, and the platen roller 7, and 
a high-quality image was obtained. 
EMBODIMENT 5 
In addition to the same conditions as Embodiment 3, an a.c. voltage of 100 
V was superimposed over the d.c. bias voltage. Also in this case, perfect 
recording with no voids was achieved as ink particles flew. Even in the 
case where bond paper, Japanese paper, an OHP film, thermal transfer 
recording paper (high-quality paper) and copying paper were used as the 
recording paper 6, good recording was achieved. By varying the heating 
time of the thermal head 1 from 300 ns to 1.0 ms, 32 level gradation 
recording was realized. 
EMBODIMENT 6 
As shown in FIG. 2, the ink sheet 5 was composed of a PET film 2 and a 
conductive ink layer 4. A bias voltage of 450 V was impressed between the 
conductive ink layer 4 and the platen roller 7. Also in this case, an 
image of the recording density 1.3 with no void was obtained. 
In the foregoing embodiments, the line-type thermal head was used. Also in 
the case of a serial-type thermal head, the same result can be obtained. 
In the ink sheet 5, when using either monochromatic (K) or color (Y, M, C), 
the same result was obtained. In color recording, colors are superimposed 
over one another usually in the order of Y, M and C. The recording of M 
and C may be made on the ink or on the recording paper 6; high-quality 
recording could not be achieved because ink is transferred with varying 
easiness. This is because the condition of the surface over which the next 
ink is superposed becomes uneven as inks are superimposed successively. In 
other words, the ink adheres to the paper surface or the previous ink 
surface, depending on the color of recording; or the ink is transferred to 
the valley-shape portions or flat portions where adjacent image elements 
are already recorded, depending on the pattern of recording. Practically, 
if the same energy is impressed, the same recording density cannot be 
obtained. The order to obtain better transfer efficiency is the flat ink 
surface, the flat paper surface, the valley-shape ink surface and the 
valley-shape paper surface; in the conventional apparatus, to realize good 
recording, control must be done in view of the recording pattern or the 
recording condition, which would have resulted in an expensive apparatus. 
Whereas in this invention, the foregoing problems can be overcome, and no 
external circuit is required and hence it is possible to reduce the cost 
of production. Further since ink flies, good recording can be achieved 
irrespective of the condition of the surface to which ink is to be 
transferred. 
In this invention, with the structure of FIGS. 1(A), 1(B) and 2, color 
recording can be realized using the ink sheet 5 composed of yellow (Y), 
magenta (M) and cyan (C) layers, plus a black (Bk) layer if necessary. 
Also in the case of a color ink sheet, the ink sheet 5 has a conductive 
ink layer 4. In particular color recording, the conductive material is 
preferably transparent. For example, as the ink material, a white 
conductive dye (such as tin oxide, titanium oxide or zinc oxide) dyed by 
an inorganic dye is used, or a resin material whose resistivity decreases 
when heated by the binder of the ink material. These materials are 
exemplified by a conductive material composed of such as polyamid resin, 
conductive polymer such as soluble polyaniline, fine powdery metal filler, 
a transparent conductive coating material such as antimony-containing tin 
oxide or tin-containing indium oxide, and ion conductive resin such as 
cationic polymer. 
EMBODIMENT 7 
In each of the foregoing embodiments, the means for heating the ink layer 
is the thermal head. The ink layer may be heated by an alternative means 
as shown in FIG. 3. 
FIG. 3 is similar to the structure of FIGS. 1(A), 1(B) and 2 except that a 
resistance layer 21 is added to the ink sheet 5 of the previous 
embodiments to constitute an ink sheet 20. The resistance layer 21 
generates joule heat due to a current flowing between a pair of electrodes 
22. Due to this joule heat the ink of the conductive ink layer 4 is 
liquefied, and the liquefied ink particles fly due to an electric field 
created by the bias voltage so as to adhere to the recording paper. This 
pair of electrodes 22, 22 corresponds to one of the heat generating 
elements of the thermal head 1. By providing a plurality of electrode 
pairs 22 corresponding to the respective heat generating elements of the 
thermal head 1, the same recording as in the previous embodiments can be 
achieved. 
EMBODIMENT 8 
FIG. 4 shows another embodiment in which the ink layer is heated by a 
heating means other than the thermal head. In this embodiment, parts or 
elements similar to those of the previous embodiments are designated by 
like reference numerals, and their description is omitted for clarity. The 
heating means of this embodiment includes a laser light source 32 and a 
photo-thermal transducing layer 31 added to an ink sheet 30. Upon receipt 
of laser light from the laser light source 32, the photo-thermal 
transducing layer 31 generates heat to heat the ink layer. Then the heated 
and liquefied ink adheres to the recording paper in the same manner as the 
previous embodiments. There may be provided a plurality of laser light 
sources 32 corresponding to the respective heat generating elements of the 
thermal head 1. Alternatively, laser light from a single laser light 
source may be refracted or reflected to radiate at a predetermined 
position on the ink sheet for heating. 
In this embodiment, laser light was used to heat the ink sheet. 
Alternatively heating may be done by electromagnetic waves such as 
microwaves. More preferably the photo-thermal transducing layer 31 is 
formed of a material whose transducing efficiency is good in the microwave 
band. 
Furthermore, if carbon is contained in the ink layer, the ink layer serves 
as both a photo-thermal layer and a metal layer. Thus it is possible to 
provide a simple ink sheet in a two-layer structure consisting of a PET 
film layer as the base and an ink layer containing carbon. 
EMBODIMENT 9 
An embodiment regarding the recording paper feed speed will now be 
described. 
In the thermal recording apparatus of this invention, the liquefied ink 
particles fly due to the electric field to adhere to the recording paper. 
Therefore, unlike the conventional apparatus, an air layer 10 may be 
located between the ink sheet 5 and the recording paper 6, as shown in 
FIGS. 1(A), 1(B), 2 and 3. It is therefore possible to reduce the amount 
of pressure with which the recording paper 6 is to be pressed toward the 
thermal head 1 by the platen roller 7. Since the ink particles are 
attracted between the ink sheet 5 and the recording paper 6 by the 
electric field between the ink sheet 5 and the platen roller 7, only a 
small amount of pressing force of the thermal head 1 is required. 
Therefore, even when any relative speed between the ink sheet 5 and the 
recording paper 6 happens to be created, neither the ink sheet 5 nor the 
recording paper 6 would become broken or creased. 
This embodiment utilizes the above-mentioned characteristic and will now be 
described with reference to FIGS. 5, 6(A) and 6(B). FIG. 5 shows the 
structure of this embodiment similar to that of FIGS. 1(A), 1(B). Parts or 
elements similar to those of FIGS. 1(A), 1(B) are designated by like 
reference numerals and their description is omitted for clarity. The only 
difference of this embodiment from the embodiment of FIGS. 1(A), 1(B) is 
that the ink sheet feed speed is different from the recording paper feed 
speed. In FIG. 5, v.sub.1 stands for the ink sheet feed speed and v.sub.2 
stands for the recording paper feed speed. The speed ratio is N (=v.sub.2 
/v.sub.1). 
Since there is a relative speed between the ink sheet 5 and the recording 
paper 6, the ink (designated by 41) heated by the thermal head 1 is 
transferred to the recording paper 6 at a portion designated by 42. 
Therefore, to record the feedwise length (l.sub.2) of 1 dot on the 
recording paper 6, the length on the ink sheet 5 may be l.sub.1 (=l.sub.2 
.times.1/N). This means that the feedwise length of the heat generating 
elements of the thermal head may be 1/N. This will now be described in 
connection with FIGS. 6(A) and 6(B). FIG. 6(A) shows the conventional 
thermal head whose heat generating elements are arranged in a square shape 
each side of which is substantially equal to the pitch P in a direction 
perpendicular to the paper feed direction. In this embodiment, the length 
in the paper feed direction may be P/N, as shown in FIG. 6(B). As a 
feature, the heat generating elements 43a of the thermal head 43 have a 
length P/N in the paper feed direction. Therefore downsizing of the 
thermal head can be realized. If the required thickness of the ink on the 
recording paper is d.sub.2 as shown in FIG. 5, the thickness d.sub.1 of 
the ink layer will be d.sub.1 =N.times.d.sub.2. 
EMBODIMENT 10 
FIGS. 7(A), 7(B), 7(C), 8(A) and 8(B) show a tenth embodiment. Parts or 
elements similar to those of FIG. 1 are designated by like reference 
numerals and their description is omitted for clarity. None of the 
foregoing embodiments mention anything about the number of times of use of 
the ink sheet. Also in almost all of the conventional apparatuses, the ink 
sheet is used once. Only a small number of apparatuses use the ink sheet 
repeatedly. As the number of times of use increases, printing difference 
and hence unevenness occurs between the faded portions where printing was 
previously made and the unfaded portions. This deteriorates the quality of 
recording. In this embodiment, for the first time use, as shown in FIG. 
7(A), a bias voltage V.sub.1 is impressed to cause the ink of the 
conductive ink layer 4 at a portion 4a nearest to the recording paper 6 to 
fly and adhere to the paper. When being used for the the second time, a 
bias voltage V.sub.2 is impressed as shown in FIG. 7(B). v.sub.2 is a 
product of increasing V.sub.1 by a predetermined voltage .DELTA.V. This 
increment causes the ink of the conductive ink layer 4 at a deeper portion 
4b to fly and adhere to the paper. Also, when being used for the third 
time, the bias voltage is further increased by .DELTA.V to cause the ink 
of the conductive ink layer 4 at an even more deeper portion 4c to fly and 
adhere to the paper. In this example, the ink sheet was used three times. 
Alternatively the ink sheet may be used more times. It is necessary to set 
an increment .DELTA. of the bias voltage for such additional use. 
In this embodiment, in order to ascertain the number of times the ink sheet 
has been used, the ink sheet 5 has a recording portion 5a where the number 
of times of use is to be recorded (FIGS. 8(A) and 8(B)). The recording 
portion 5a includes a magnetic recording medium, and the thermal recording 
apparatus is also equipped with a non-illustrated read head for reading 
the number of times of use recorded in the recording portion 5a and a 
non-illustrated write head for writing the number times of use in the 
recording portion 5a. FIG. 8(A) shows the case in which the ink sheet 5 
has at its leading end portion the recording portion 5a. In this case, the 
ink sheet 5 is wound up all the way to one side and is then wound back, 
and for reuse, the number of times it has been used recorded in the 
recording portion 5a is read out and the readout number of times of use 
plus one is stored in the recording portion 5a. The ink sheet 5 of FIG. 
8(B) has a plurality of recording portions 5a at regular distances. In 
this case, the ink sheet portion 5b between the first recording portion 
and the second recording portion is used a predetermined number of times 
(three times in this embodiment). Also during that time, each time it is 
used, the number of times use is read out from the recording portion 5a 
and then the readout number plus one is stored in the recording portion 
5a. When the number of times of use in the portion 5b between two adjacent 
recording portions reaches three, the ink sheet 5 is fed further and then 
recording is made in the portion 5c. 
In this embodiment, the recording portion 5a is constituted by a magnetic 
recording portion. This invention should by no means be limited to this 
specific example. For example, each time it is used, a small hole may be 
formed in the ink sheet at a predetermined position from which the number 
of uses is to be discriminated, or the number of times of use may be 
stored in a built-in recording portion of the apparatus rather than the 
recording portion of the ink sheet. This built-in recording portion can be 
previously stored in, for example, an E.sup.2 PROM. 
EMBODIMENT 11 
According to this invention, as described above, an electric field is 
created between the ink sheet 5 and the recording paper 6 to cause ink 
particles to fly toward the recording paper 6. According to the intensity 
of this electric field, the amount of ink caused to fly is controlled in 
order to control the recording density. However, if the distance between 
the recording paper 6 and the ink sheet 5, particularly the conductive ink 
layer 4, was not kept constant, the amount of ink caused to fly and adhere 
to the recording paper would vary so that a stable recording density could 
not be achieved. 
With this point in mind, according to this embodiment, there is provided a 
gap retaining means for retaining the distance between the conductive ink 
layer 4 and the recording paper 6. This embodiment will now be described 
with reference to FIG. 9. Parts or elements similar to those of FIGS. 
1(A), 1(B) are designated by like reference numerals and their description 
is omitted for clarity. 
In FIG. 9, a gap retainer 50 is inserted between the ink sheet 5 and the 
recording paper 6. The gap retainer 50 is fixed with respect to the 
thermal head 1 and has an opening 50a normally in confronting relation to 
the thermal head 1. The thickness of the gap retainer 50 is the distance 
between the conductive ink layer 4 and the recording paper 6. Through the 
opening 50a, the ink particles fly and adhere to the recording paper 6. 
With this gap retainer 50, it is possible to retain a constant distance. 
EMBODIMENT 12 
This is another embodiment regarding means for retaining the distance 
between the conductive ink layer 4 and the recording paper 6. FIG. 10 
shows an example in which the ink sheet has a mesh layer 52 at a side 
toward the recording paper 6. Thus the ink sheet 51 is a four-layer 
structure comprising the PET film 2, the metal layer 3, the conductive ink 
layer 4 and the mesh layer 52. With this mesh layer 52, the gap between 
the conductive ink layer 4 and the recording paper 6 is kept constant. The 
mesh size of the mesh layer 52 must be larger than the ink particle size. 
According to Embodiments 11 and 12, the distance between the conductive ink 
layer 4 and the recording paper 6 can be kept constant so that a stable 
recording density can be obtained. Particularly in half-tone recording, it 
is possible to realize a half-tone repeatability with high precision. 
EMBODIMENT 13 
This embodiment relates to how a potential is applied to the metal layer of 
the ink sheet. As shown in FIGS. 11(A), 11(B) and 11(C), the width of the 
metal layer 3 is larger than that of at least one of the PET film 2 and 
the conductive ink layer 4. Specifically, in FIG. 11(A), it is larger than 
the PET film 2 and is contactable at a contact portion 3a with the 
electrode. In FIG. 11(B), in which the one end of the metal layer 3 is 
lifted to show the contact portion 3a contactable with the electrode, it 
is larger than the conductive ink layer. In FIG. 11(C), in which a 
potential can be supplied from either the PET film 2 or the conductive ink 
layer 4, it is longer than both the PET film 2 and the conductive layer 4. 
FIGS. 12(A) and 12(B) each show an electrode for applying a potential to 
the ink sheet. As shown in FIG. 11(A), if the electrode needs to contact 
the electrode from the upper side, an electrode 60 is provided, through 
which a potential is applied. As shown in FIG. 11(B), if the electrode 
needs to contact the electrode from the lower side, the platen roller 
surface has a conductive portion 61a facing the contact portion 3a, 
through which a potential is applied. Since the platen roller must be 
grounded at a surface portion facing the ink sheet 5, the surface portion 
is insulated from the conductive portion 61a by an insulator 61b. 
The portion from which the potential is applied may be an ink sheet feed 
roller or a reel at the ink sheet other than the platen roller 7. In this 
case, the feedwise length of the metal layer 3 is longer than at least one 
of the PET film 2 and the conductive ink layer 4. Considering the possible 
danger to the operator or noise to the thermal head 1, the potential to be 
applied to the reel at the end of the ink sheet is preferably 0 V. 
Further, the metal layer 3 may be exposed between ink coated surfaces, and 
a voltage may be impressed by a roller between the thermal head 1 and an 
ink sheet supply or take-up roller (not shown). In either case, the area 
of the metal layer 3 (conductive layer) is preferably larger than that of 
the conductive ink layer 4 or the PET film 2. 
Also in the case where the ink layer is conductive, the ink layer is larger 
in width than the PET film so that a potential can be impressed in the 
same manner as this embodiment. 
In each of the foregoing embodiments, the conductive ink layer 4 is 
conductive. Alternatively the conductive ink layer 4 may be chargeable; 
that is, if there is a chargeability to a degree such that the melted ink 
particles can move by overcoming the coulomb force between the ink 
particles, the same result can be achieved. 
Further, by using, as the binder of the ink material, a resin material 
whose resistance value decreases when heated, it is possible to cause a 
current to flow only when heated by the thermal head so that any 
accidental leak and discharge are prevented. 
Further, in the illustrated embodiment, the ink sheet is pressed against 
the recording paper by the platen roller. However, this contact is not 
absolutely necessary, and it is possible to cause ink to adhere to the 
recording paper due to only gravity and the electric field. In this 
embodiment, the platen roller is composed of metal and rubber. 
Alternatively the platen roller may be formed of only metal or a 
conductive substance. In addition, in this embodiment, the ink sheet 5 
includes a metal layer 3. The metal layer 3 should by no means be limited 
to metal, and may be made of any other conductive substance. If the 
conductive ink layer is highly conductive, the metal layer 3 may be 
omitted. 
As described above, according to the thermal recording apparatus of this 
invention, since it comprises an ink sheet having a conductive or 
chargeable ink layer, a thermal head for heating the ink sheet, and an 
electric field impressing means for applying an electric field between the 
ink sheet and the recording medium, the ink which is softened by the heat 
of the thermal head flies and adheres to the recording medium under the 
influence electric field created by the electric impressing means, thus 
causing high-quality recording, free of voids. 
In the case where an electric field to be impressed is obtained by 
superimposing an a.c. voltage over a d.c. voltage, since due to the 
electric field of the a.c. voltage the vibrated ink particles fly to reach 
the recording medium, the unevenness of the recording density can be 
averaged, thus causing improved quality recording and good half-tone 
recording. 
By making the ink sheet feed speed slower than the recording paper feed 
speed, it is possible to reduce the consumption of the ink sheet and to 
reduce the size of the thermal head. 
By providing a gap retaining means for keeping the distance between the ink 
layer and the recording paper constant, it is possible to control the 
printing density with more stableness and more particularly it is possible 
to control the half-tone recording with high precision.