Incremental printing of symbolic information – Thermal marking apparatus or processes – Specific resistance recording element type
Reexamination Certificate
2001-06-18
2002-08-27
Tran, Huan (Department: 2861)
Incremental printing of symbolic information
Thermal marking apparatus or processes
Specific resistance recording element type
Reexamination Certificate
active
06441840
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thick-film thermal printhead which allows smooth transfer of a recording paper. The present invention also relates to a method of making such a thick-film thermal printhead.
2. Description of the Related Art
As is well known, thermal printheads are generally divided into the thick-film type and the thin-film type. A thick-film type thermal printhead includes a heating resistor which is larger in thickness than that of a thin-film thermal printhead.
FIGS. 15 and 16
of the accompanying drawings illustrate an example of prior art thick-film thermal printhead. The illustrated printhead includes an insulating head substrate
11
, a common electrode
12
, a plurality of individual electrodes
13
and a linear heating resistor
14
. The heating resistor
14
has a predetermined width and extends longitudinally of the head substrate
11
. As shown in
FIG. 15
, the common electrode
12
includes a plurality of comb-teeth
12
a
, and a main conductor
12
b
to which the comb-teeth
12
a
are commonly connected. The teeth
12
a
and the individual electrodes
13
are alternately arranged. The heating resistor
14
extends across the teeth
12
a
and the individual electrodes
13
. The distance between one edge
14
a
of the heating resistor
14
and the main conductor
12
b
is indicated by a reference sign S′.
As shown in
FIG. 16
, the main conductor
12
b
is provided with an auxiliary conductor
15
. The auxiliary conductor
15
, which is elongated in parallel with the heating resistor
14
, prevents a voltage drop at the common electrode
12
. As shown in this figure, there is provided a protective film
16
which covers the auxiliary conductor
15
as well as the common electrode
12
, the individual electrodes
13
and the heating resistor
14
. Due to the existence of the heating resistor
14
and the auxiliary conductor
15
, the protective film
16
includes a first projection
16
a
and a second projection
16
b
projecting toward a platen roller R. In operation, the platen roller R presses a recording paper P against the first projection
16
a
for conducting required printing.
For smoothly moving the recording paper P along a paper transfer path, it is required that the paper P does not come into contact with the second projection
16
b
during the printing. Therefore, in the prior art printhead, by making the distance S′ (between the heating resistor
14
and the main conductor
12
b
) larger (e.g. 200-300 &mgr;m) than the width of the heating resistor
14
, the auxiliary conductor
15
is sufficiently spaced from the heating resistor
14
for making the second projection
16
b
spaced from the paper transfer path as much as possible.
Although the prior art printhead takes the above-described measures, it still has the following drawbacks.
A first drawback is as follows. In printing, heat is generated at the heating resistor
14
. Part of the heat is transmitted directly to the head substrate
11
for dissipation, whereas other part of the heat is transmitted to the head substrate
11
via the common electrode
12
or the individual electrodes
13
for dissipation. However, when the distance S′ is increased as described above, the length of each tooth
12
a
of the common electrode
12
correspondingly increases. This causes difficulty in transmitting the heat generated at the heating resistor
14
to the main conductor
12
b
via the teeth
12
a
. As a result, the teeth
12
a
heated by the heating resistor
14
are caused to have a high temperature at portions corresponding to the distance S′. This high-temperature state is shown by a curve D′ in a graph of FIG.
14
. This graph generally illustrates how the temperature of the teeth
12
a
varies in accordance with a distance from the heating resistor
14
.
Due to the influence of the teeth
12
a
in a high-temperature state as described above, the recording paper P transferred by the platen roller R is exposed to the high temperature in moving the distance S′. As is well known, the paper P often generates dust when exposed to a high temperature. When such paper dust adheres to the protective film
16
of the printhead, it hinders the paper P from smoothly sliding over the protective film
16
, which may bar smooth paper transfer. Since the generation of paper dust increases in proportion to the printing speed of the printer, the printing speed need be limited to not more than 3 inch/sec in an apparatus using the prior-art printhead.
Another drawback is as follows. As described above, the recording paper P is transferred along the transfer path in press contact with the first projection
16
a
of the protective film
16
. At this time, unpleasant noise may be made due to strong rubbing of the recording paper P with the first projection
16
a
. Further, friction between the recording paper P and the first projection
16
a
may cause the recording paper P to thermally adhere to the protective film
16
(generation of sticking), which may prevent the smooth transfer of the paper P.
SUMMARY OF THE INVENTION
The present invention has been conceived under the circumstances described above. It is therefore an object of the present invention to provide a thermal printhead which is capable of smoothly transferring a recording paper. Another object of the present invention is to provide a method of making such a thermal printhead.
A thermal printhead provided in accordance with a first aspect of the present invention comprises an insulating substrate, a common electrode formed on the substrate, a linear heating resistor electrically connected to the common electrode, and a plurality of individual electrodes electrically connected to the heating resistor. The common electrode includes a plurality of comb-teeth and a main conductor connected to the comb-teeth. The heating resistor has a predetermined width. The distance between the main conductor and the heating resistor is smaller than the width of the heating resistor.
Preferably, the distance between the main conductor and the heating resistor may be no less than 0.25 times but no more than 0.75 times the width of the heating resistor.
Preferably, the thermal printhead according to the present invention may further comprise an auxiliary conductor provided on the main conductor.
Preferably, the distance between the auxiliary conductor and the heating resistor may be no less than twice the distance between the main conductor and the heating resistor.
Preferably, the thermal printhead according to the present invention may further comprise a protective coating for covering the heating resistor.
The protective coating may have a flat exposed surface.
Preferably, the thermal printhead according to the present invention may further comprise a glaze layer which supports the heating resistor and has a crescent cross section. The protective coating may include a bulging portion following the glaze layer.
Preferably, the glaze layer and the bulging portion may have respective curved profiles which are cross-sectionally parallel with each other.
Preferably, the protective coating includes a first protective film directly contacting the heating resistor and a second protective film formed on the first protective film.
Preferably, the first protective film may include a thin-wall portion located adjacent to the heating resistor.
Preferably, the first protective film and the second protective film may be formed of a same glass material. Alternatively, the first protective film may be made of glass, whereas the second protective film may be made of sialon.
A method of making a thermal printhead provided in accordance with a second aspect of the present invention comprises a step of preparing an insulating substrate, a step of forming on the substrate a conductor pattern and a heating resistor electrically connected to the conductor pattern, a step of forming a first protective film for covering the heating resistor, a step of removing a bulging portion formed in the fi
Sato Tadayoshi
Yamade Takumi
Bednarek Michael D.
Pittman LLP Shaw
Rohm & Co., Ltd.
Tran Huan
LandOfFree
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