Back to EveryPatent.com
| United States Patent |
6,236,424
|
|
Nakamura
|
May 22, 2001
|
Thermal head
Abstract
A thick film thermal head for making a stencil by thermally perforating a
stencil material includes an electrical insulating substrate and a glaze
layer superposed on a heat radiating plate in this order, a resistance
heater formed on the glaze layer to continuously extend in a main scanning
direction, a plurality of electrodes of at least two lines which extend in
a direction intersecting the main scanning direction in contact with the
resistance heater and are alternately arranged in the main scanning
direction, and a protective layer which covers exposed part of the
resistance heater and the electrodes. The resistance heater is not smaller
than 1 .mu.m and not larger than 10 .mu.m in thickness, the space between
each pair of adjacent electrodes in the main scanning direction is not
smaller than 20% and not larger than 60% of the center distance between
the adjacent electrodes, and the length in the sub-scanning direction of
the resistance heater at the portion between each pair of adjacent
electrodes is not smaller than 100% and not larger than 250% of the center
distance between the adjacent electrodes.
| Inventors:
|
Nakamura; Jun (Amimachi, JP)
|
| Assignee:
|
Riso Kajaku Corporation (Tokyo, JP)
|
| Appl. No.:
|
650818 |
| Filed:
|
August 30, 2000 |
Foreign Application Priority Data
| Aug 31, 1999[JP] | 11-245842 |
| Intern'l Class: |
B41J 002/335 |
| Field of Search: |
347/200,202,204,208
|
References Cited
U.S. Patent Documents
| 5216951 | Jun., 1993 | Yokoyama et al.
| |
| 5315319 | May., 1994 | Sato et al.
| |
| 5415090 | May., 1995 | Natori et al.
| |
| 5417156 | May., 1995 | Tateishi et al.
| |
| 5592209 | Jan., 1997 | Hasegawa et al.
| |
| Foreign Patent Documents |
| 63-191654 | Aug., 1988 | JP.
| |
| 2-67133 | Mar., 1990 | JP.
| |
| 4-71847 | Mar., 1992 | JP.
| |
| 4-314552 | Nov., 1992 | JP.
| |
| 5-185574 | Jul., 1993 | JP.
| |
| 5-345403 | Dec., 1993 | JP.
| |
| 5-345402 | Dec., 1993 | JP.
| |
| 5-345401 | Dec., 1993 | JP.
| |
| 6-115042 | Apr., 1994 | JP.
| |
| 7-171940 | Jul., 1995 | JP.
| |
| 8-132584 | May., 1996 | JP.
| |
| 8-142299 | Jun., 1996 | JP.
| |
Primary Examiner: Tran; Huan
Attorney, Agent or Firm: Nixon Peabody, LLP, Strudebaker; Donald R.
Claims
What is claimed is:
1. A thick film thermal head for making a stencil by thermally perforating
a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
electrodes of at least two lines which extend in a direction intersecting
the main scanning direction in contact with the resistance heater and are
alternately arranged in the main scanning direction, and a protective
layer which covers exposed part of the resistance heater and the
electrodes, wherein the improvement comprises that
the resistance heater is not smaller than 1 .mu.m and not larger than 10
.mu.m in thickness,
the space between each pair of adjacent electrodes in the main scanning
direction is not smaller than 20% and not larger than 60% of the center
distance between the adjacent electrodes, and
the length in the sub-scanning direction of the resistance heater at the
portion between each pair of adjacent electrodes is not smaller than 100%
and not larger than 250% of the center distance between the adjacent
electrodes.
2. A thick film thermal head for making a stencil by thermally perforating
a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
discrete electrodes and common electrodes which extend in a direction
intersecting the main scanning direction in contact with the resistance
heater and are alternately arranged in the main scanning direction, and a
protective layer which covers exposed part of the resistance heater and
the electrodes, the common electrodes comprising first and second groups
of common electrodes which are connected to each other by group and are
alternately arranged in the main scanning direction, wherein the
improvement comprises that
the resistance heater is not smaller than 1 .mu.m and not larger than 10
.mu.m in thickness, the space between each pair of adjacent electrodes in
the main scanning direction is not smaller than 20% and not larger than
60% of the center distance between the adjacent electrodes, and
the length in the sub-scanning direction of the resistance heater at the
portion between each pair of adjacent electrodes is not smaller than 100%
and not larger than 250% of the center distance between the adjacent
electrodes.
3. A thick film thermal head for making a stencil by thermally perforating
a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
discrete electrodes and common electrodes which extend in a direction
intersecting the main scanning direction in contact with the resistance
heater and are alternately arranged in the main scanning direction, and a
protective layer which covers exposed part of the resistance heater and
the electrodes, the common electrodes being connected to each other in one
line, wherein the improvement comprises that
the resistance heater is not smaller than 1 .mu.m and not larger than 10
.mu.m in thickness,
the sum of the space between each discrete electrode and the common
electrode on one side of the discrete electrode in the main scanning
direction and the space between the discrete electrode and the common
electrode on the other side of the discrete electrode in the main scanning
direction is not smaller than 20% and not larger than 60% of the center
distance between the common electrodes on the opposite sides of the
discrete electrode, and
the length in the sub-scanning direction of the resistance heater at the
portion between each discrete electrode and the common electrode on each
side of the discrete electrode is not smaller than 100% and not larger
than 250% of the center distance between the common electrodes on the
opposite sides of the discrete electrode.
4. A thick film thermal head for making a stencil by thermally perforating
a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
electrodes of at least two lines which extend in a direction intersecting
the main scanning direction in contact with the resistance heater and are
alternately arranged in the main scanning direction, and a protective
layer which covers exposed part of the resistance heater and the
electrodes, wherein the improvement comprises that
the following formula (1) is satisfied,
0.2.ltoreq.V/d.sup.2.ltoreq.10 (1)
wherein V (in .mu.m.sup.3) represents the volume of a part of the
resistance heater between each pair of adjacent electrodes, and d (in
.mu.m) represents the center distance between the adjacent electrodes.
5. A thick film thermal head for making a stencil by thermally perforating
a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
discrete electrodes and common electrodes which extend in a direction
intersecting the main scanning direction in contact with the resistance
heater and are alternately arranged in the main scanning direction, and a
protective layer which covers exposed part of the resistance heater and
the electrodes, the common electrodes comprising first and second groups
of common electrodes which are connected to each other by group and are
alternately arranged in the main scanning direction, wherein the
improvement comprises that
the following formula (1) is satisfied,
0.2.ltoreq.V/d.sup.2.ltoreq.10 (1)
wherein V (in .mu.m.sup.3) represents the volume of a part of the
resistance heater between each pair of adjacent electrodes, and d (in
.mu.m) represents the center distance between the adjacent electrodes.
6. A thick film thermal head for making a stencil by thermally perforating
a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
discrete electrodes and common electrodes which extend in a direction
intersecting the main scanning direction in contact with the resistance
heater and are alternately arranged in the main scanning direction, and a
protective layer which covers exposed part of the resistance heater and
the electrodes, the common electrodes being connected to each other in one
line, wherein the improvement comprises that
the following formula (2) is satisfied,
0.2.ltoreq.V/D.sup.2.ltoreq.10 (2)
wherein V (in .mu.m.sup.3) represents the sum of the volume of part of the
resistance heater between each discrete electrode and the common electrode
on one side of the discrete electrode in the main scanning direction and
the volume of a part of the resistance heater between the discrete
electrode and the common electrode on the other side of the discrete
electrode in the main scanning direction and D (in .mu.m) represents the
center distance between the common electrodes on the opposite sides of the
discrete electrode.
7. A thick film thermal head for making a stencil by thermally perforating
a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
electrodes of at least two lines which extend in a direction intersecting
the main scanning direction in contact with the resistance heater and are
alternately arranged in the main scanning direction, and a protective
layer which covers exposed part of the resistance heater and the
electrodes, wherein the improvement comprises that
the resistance heater is not smaller than 1 .mu.m and not larger than 10
.mu.m in thickness,
the space between each pair of adjacent electrodes in the main scanning
direction is not smaller than 20% and not larger than 60% of the center
distance between the adjacent electrodes,
the length in the sub-scanning direction of the resistance heater at the
portion between each pair of adjacent electrodes is not smaller than 100%
and not larger than 250% of the center distance between the adjacent
electrodes, and
the following formula (1) is satisfied,
0.2.ltoreq.V/d.sup.2.ltoreq.10 (1)
wherein V (in .mu.m.sup.3) represents the volume of a part of the
resistance heater between each pair of adjacent electrodes, and d (in
.mu.m) represents the center distance between the adjacent electrodes.
8. A thick film thermal head for making a stencil by thermally perforating
a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
discrete electrodes and common electrodes which extend in a direction
intersecting the main scanning direction in contact with the resistance
heater and are alternately arranged in the main scanning direction, and a
protective layer which covers exposed part of the resistance heater and
the electrodes, the common electrodes comprising first and second groups
of common electrodes which are connected to each other by group and are
alternately arranged in the main scanning direction, wherein the
improvement comprises that
the resistance heater is not smaller than 1 .mu.m and not larger than 10
.mu.m in thickness, the space between each pair of adjacent electrodes in
the main scanning direction is not smaller than 20% and not larger than
60% of the center distance between the adjacent electrodes,
the length in the sub-scanning direction of the resistance heater at the
portion between each pair of adjacent electrodes is not smaller than 100%
and not larger than 250% of the center distance between the adjacent
electrodes, and
the following formula (1) is satisfied,
0.2.ltoreq.V/d.sup.2.ltoreq.10 (1)
wherein V (in .mu.m.sup.3) represents the volume of a part of the
resistance heater between each pair of adjacent electrodes, and d (in
.mu.m) represents the center distance between the adjacent electrodes.
9. A thick film thermal head for making a stencil by thermally perforating
a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
discrete electrodes and common electrodes which extend in a direction
intersecting the main scanning direction in contact with the resistance
heater and are alternately arranged in the main scanning direction, and a
protective layer which covers exposed part of the resistance heater and
the electrodes, the common electrodes being connected to each other in one
line, wherein the improvement comprises that
the resistance heater is not smaller than 1 .mu.m and not larger than 10
.mu.m in thickness,
the sum of the space between each discrete electrode and the common
electrode on one side of the discrete electrode in the main scanning
direction and the space between the discrete electrode and the common
electrode on the other side of the discrete electrode in the main scanning
direction is not smaller than 20% and not larger than 60% of the center
distance between the common electrodes on the opposite sides of the
discrete electrode,
the length in the sub-scanning direction of the resistance heater at the
portion between each discrete electrode and the common electrode on each
side of the discrete electrode is not smaller than 100% and not larger
than 250% of the center distance between the common electrodes on the
opposite sides of the discrete electrode, and
the following formula (2) is satisfied,
0.2.ltoreq.V/D.sup.2.ltoreq.10 (2)
wherein V (in .mu.m.sup.3) represents the sum of the volume of a part of
the resistance heater between each discrete electrode and the common
electrode on one side of the discrete electrode in the main scanning
direction and the volume of a part of the resistance heater between the
discrete electrode and the common electrode on the other side of the
discrete electrode in the main scanning direction and D (in .mu.m)
represents the center distance between the common electrodes on the
opposite sides of the discrete electrode.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a thermal head for making a stencil by thermally
perforating a heat-sensitive stencil material, and more particularly to an
inexpensive thermal head formed by a thick film process.
2. Description of the Related Art
In stencil making apparatuses which have been put into practice, a
heat-sensitive stencil material is used, and there have been known two
stencil making systems. One of the stencil making systems is so-called a
flash system in which an original having a printing area containing
therein carbon is brought into a close contact with a heat-sensitive
stencil material is perforated by heat when a printing area of the
original is exposed through the stencil material to flashlight from a
flash bulb, a xenon flashtube or the like. The other stencil making system
is so-called a digital system in which a stencil material is thermally
perforated by selectively energizing heater elements of a thermal head
according to an image signal read out from an original through an image
sensor or the like, or an image signal representing a document and/or an
image created through a computer or the like. The digital system is now
prevailing over the flash system since the digital system permits the
document editing and the image processing. Though the thermal head was
once a device exclusively used in facsimiles, thermal recording printers
or the like, recently the thermal head has been modified so that it can be
used in thermal stencil making. Recently, the modified thermal head has
come to be used in a thermal stencil making apparatus of the digital
system. As the stencil material, there have been known one comprising
thermoplastic resin film laminated to a porous base sheet and one
comprising thermoplastic resin film with no base sheet.
Specific structures of thermal heads to be used in thermal stencil making
are disclosed, for instance, in the following patent publications.
In Japanese Unexamined Patent Publication Nos. 63(1988)-191654 and
6(1994)-191003, the thickness of the protective layer of the thermal head
is defined. In Japanese Unexamined Patent Publication Nos. 2(1990)-67133,
4(1992)-71847, 4(1992)-265759, 5(1993)-345401, 5(1993)-345402,
5(1993)-345403 and 6(1994)-115042, the length in the main scanning
direction of the heater element and/or the length in the sub-scanning
direction of the same is defined for the pitch of the heater elements in
each direction. In Japanese Unexamined Patent Publication Nos.
4(1992)-45936, 7(1995)-68807 and 7(1995)-171940, there is disclosed a
thermal head in which the heater element is not rectangular in shape. In
Japanese Unexamined Patent Publication Nos. 4(1992)-314552 and
8(1996)-142299, there is disclosed a thermal head in which a cooling
member is disposed between each pair of adjacent heater elements. In
Japanese Unexamined Patent Publication Nos. 4(1992)-369575 and
8(1996)-132584, the shape or thickness of the glaze layer is defined.
Further, in Japanese Unexamined Patent Publication No. 5(1993)-185574, the
ratio of the length in the main scanning direction to the length in the
sub-scanning direction of the heater element is defined.
Though not clearly described in the above patent publications, the thermal
heads disclosed in the above patent publications can be considered to be
of a thin film type except those disclosed in Japanese Unexamined Patent
Publication Nos. 5(1993)-345401, 5(1993)-345402 and 5(1993)-345403.
Actually, at present almost all the thermal stencil making apparatuses
using a thermal head use a thin film thermal head, and those using a thick
film type thermal head are limited to those for a postcard, those which
function also as a word processor printer and those which function also as
a heat transfer labeler. Only a very small fraction of the digital system
thermal stencil making apparatuses uses a thick film type thermal head.
As pointed out by many of the aforesaid patent publications, it is
preferred that the thermoplastic resin film of the stencil material be
perforated in such a manner that perforations are discrete and adjacent
perforations are not connected to each other. This is because of inherent
characteristics of stencil printing that ink is viscous fluid and spreads
wider than the area of the perforations when transferred to the printing
paper through perforations of the stencil, and when the perforations are
connected, the amount of ink transferred to the printing paper and the
thickness of printed ink layer on the printing paper are acceleratedly
increased and offset is caused. The thermal head for thermal stencil
making differs in this point from that for thermal recording in which that
recorded pixels overlaps each other is preferred.
In the digital system thermal stencil making, it is preferred that the
perforations be separated from each other, the proportion of open area
(the proportion of the area of the perforations per unit area of the
thermoplastic film of the stencil material) be in a predetermined range
(generally about 30 to 40% though depending upon the viscosity of the ink,
the kind of the printing paper, the pressure at which the stencil is
pressed against the printing paper, and the like) in order to ensure a
proper printing density, and the shapes and the areas of the perforations
be substantially uniform so that the unperforated portions between the
perforations are arranged in a regular pattern and the densities of large
printing areas such as solid parts are uniformed.
Typically, the thin film thermal head comprises a heat radiating plate of
metal, an electrical insulating substrate and a glaze layer formed on the
heat radiating plate in this order, a plurality of strip-like resistance
heaters which are formed on the glaze layer to extend in one direction
(the sub-scanning direction) and are arranged in a direction transverse to
said one direction (the main scanning direction), and a plurality of
electrodes each superposed on one of the strip-like resistance heaters
with a part of the resistance heater exposed through a gap formed in the
electrode. The exposed part of each strip-like resistance heater forms a
heater element. That is, a pair of electrodes are formed on each
resistance heater with their inner ends opposed to each other in the
sub-scanning direction with a gap between. One of the electrodes is
connected to a switching element for discretely energizing the heater
element and the other electrode are integrated with the corresponding
electrodes for the other heater elements to form a common electrode. When
producing such a thin film thermal head, an electrical insulating
substrate and a glaze layer are superposed on a heat radiating plate and a
solid resistance heater layer and a solid electrode layer are formed in
this order on the glaze layer. Then the electrode layer is removed along a
line extending in the main scanning direction, thereby exposing the
resistance heater layer in a line extending in the main scanning
direction, and the resistance heater layer and the electrode layer are
both removed in the sub-scanning direction at regular intervals in the
main scanning direction. Thus, a plurality of strip-like resistance heater
layers are formed each covered with a pair of electrode layers opposed to
each other in the sub-scanning direction with a gap between. One of the
electrode layer is connected to a switching element and forms a discrete
electrode for discretely energizing the part of the resistance heater
layer free from the electrode layer. The other electrode layer is
integrated with the corresponding electrode layer for the other strip-like
resistance heater layers to form a common electrode. A protective layer is
formed to cover the discrete electrodes, the exposed part of the
resistance heater layer and the common electrode. When an electric
potential different from the common electrode is applied to a discrete
electrode, the exposed part of each of the strip-like resistance heaters
between the discrete electrode and the common electrode is energized and
generates heat. That is, the exposed part of each of the strip-like
resistance heaters between the discrete electrode and the common electrode
forms a heater element.
Since the thin film thermal head is generally very small in heat capacity
as compared with the thick film thermal head and the heater elements are
separately independent of each other, the temperature distribution on the
thermal head during operation is clear and the temperature difference
between the high-temperature part and the low-temperature part (will be
referred to as "the temperature contrast", hereinbelow) is large, whereby
the thermoplastic resin film of the stencil material can be perforated in
relatively uniform shapes according to the clear pattern of the
temperature distribution. For this reason, in almost all of high-quality
stencil making apparatuses, a thin film thermal head has been employed.
In the thermal recording, thick film thermal heads have been much employed
as well as the thin film thermal heads. Typically, the thick film thermal
head comprises a heat radiating plate of metal, an electrical insulating
substrate and a glaze layer formed on the heat radiating plate in this
order, discrete electrodes and common electrodes which are formed on the
glaze layer alternately in the main scanning direction to extend in
opposite directions in the sub-scanning direction with their inner end
portions overlapping with each other in the main scanning direction, a
strip-like resistance heater formed over the discrete electrodes and the
common electrodes to extend in the main scanning direction across the
discrete electrodes and the common electrodes, and a protective layer
formed to cover the discrete electrodes, the common electrodes and the
strip-like resistance heater.
When an electric potential different from the common electrode is applied
to a discrete electrode, the parts of the strip-like resistance heater
between the discrete electrode and two common electrodes on opposite sides
of the discrete electrode are energized and generate heat. Each of the
parts between the discrete electrodes and the common electrodes forms one
heater element. However since on and off of the heater elements on
opposite sides of a discrete electrode cannot be controlled independently
of each other, that is, when one discrete electrode is applied with an
electric potential, both the heater elements generate heat, and when one
discrete electrode is not applied with an electric potential, none of the
heater elements generate heat, the two heater elements should be
considered to correspond to one pixel. The recording using such a thermal
head will be referred to as "twin-dot recording", hereinbelow. When first
common electrodes and second common electrodes of different lines are
alternately disposed in place of the common electrodes so that the first
and second common electrodes are electrically connected with one discrete
electrode at different timings, on and off of the heater elements on
opposite sides of a discrete electrode can be controlled independently of
each other. In this case, one heater element corresponds to one pixel. The
recording using such a thermal head will be referred to as "single-dot
recording", hereinbelow.
In Japanese Unexamined Patent Publication Nos. 5(1993)-345401,
5(1993)-345402 and 5(1993)-345403, there is disclosed a thick film thermal
head in which the lengths in the main and sub-scanning directions of each
heater element (corresponding to one pixel) are smaller than scanning
pitches in the main and sub-scanning directions, respectively, and the
ratio of the length of the heater element in the main scanning direction
to the main scanning pitch is substantially equal to the ratio of the
length of the heater element in the sub-scanning direction to the
sub-scanning pitch. The patent publications also say that the lengths in
the main and sub-scanning directions of each heater element are equal to
the diameters of a perforation in the main and sub-scanning directions,
respectively. However, a stencil making apparatus using such a thick film
thermal head has not been in wide use due to a problem in performance to
be described later.
As can be understood from the description above, presently, substantially
all the thermal stencil making apparatuses use the thin film thermal head.
The thick film thermal head is advantageous over the thin film thermal head
by the following reasons: First, the productive facilities for the thick
film thermal head is simpler and easier to manage than that for the thin
film thermal head and accordingly, the thick film thermal head can be
produced at lower cost. Second, unlike the thin film thermal head, the
thick film thermal head can be produced in an open atomosphere without
using, for instance, a sputter chamber in which the thermal head is to be
confined, and accordingly, the thick film thermal head can be easily
produced long. Accordingly, there has been demand for using the thick film
thermal head in thermally making a stencil.
However, when the conventional thick film thermal head is used in thermal
stencil making as it is, there arises a problem that printings made by the
use of a stencil made by the thick film thermal head become lower in image
quality. That is, as described above, the thick film thermal head is low
in the temperature contrast as compared with the thin film thermal head,
that is, the thick film thermal head is small in the temperature gradient
as compared with the thin film thermal head. Since the resistance heater
of the thick film thermal head is continuous in the main scanning
direction, heat generated by each heater element is easily propagated in
the main scanning direction. Accordingly, in the thick film thermal head,
the temperature contrast in the main scanning direction is lower than in
the thin film thermal head. Further, the thick film thermal head is larger
in size of each heater element than the thin film thermal head. Especially
in the thick film thermal head, the length in the sub-scanning direction
of each heater element is generally about three times the scanning pitch
in the sub-scanning direction, and accordingly, the temperature gradient
in the sub-scanning direction at a given time is small. The volume of each
heater element of the thick film thermal head is in the order of hundred
times that of the thin film thermal head so long as they are equivalent to
each other in resolution. Accordingly, the heater elements of the thick
film thermal head is larger in heat capacity than those of the thin film
thermal head, which results in slower temperature response to on and off
of the applied pulses. This also corresponds to a low temperature contrast
in the sub-scanning direction.
The shape of the perforations may be considered to basically correspond to
the shape of areas where the experienced temperature on the thermoplastic
film becomes not lower than a certain threshold value. However, actually,
the temperature fluctuates from heater element to heater element, and the
shape of the perforations are more apt to be affected by fluctuation in
the temperature of the heat elements as the temperature contrast on the
heater element becomes lower. Accordingly, the thick film thermal head is
larger than the thin film thermal head in fluctuation of the shape of the
perforations. Large fluctuation of the shape of the perforations results
in microscopic unevenness in printing density and deteriorates evaluation
of image quality. Further, fluctuation in the shape of the perforations is
apt to result in enlarged and/or connected perforations, which can result
in offset as described above.
Further, a state where the lengths in the main and sub-scanning directions
of each heater element are equal to the diameters of a perforation in the
main and sub-scanning directions as mentioned in Japanese Unexamined
Patent Publication Nos. 5(1993)-345401, 5(1993)-345402 and 5(1993)-345403
is a very special case. This is because, in the thick film thermal head,
the resistance heater is semi-cylindrical in cross-section and is the
thickest at the middle in the sub-scanning direction, and as the distance
from the middle of the resistance heater increases, the surface of the
resistance heater becomes remoter from the thermoplastic film of the
stencil material and the heat transfer efficiency deteriorates. The
resistance heater is about 3 to 20 .mu.m in thickness. Accordingly, the
distance between the surface of the resistance heater and the film of the
stencil material is about 3 to 20 .mu.m at the edges of the resistance
heater. In practical setting, at the time when the temperature of the
heater element is maximized, the temperature at the middle of the heater
element is, for instance, 350 to 400.degree. C., whereas the temperature
at edges of the heater element is only 200 to 250.degree. C., which is
substantially equal to the melting point of the film. Accordingly, when
the edges of the heater element is at a distance of, for instance, 10
.mu.m in the vertical direction (the direction substantially perpendicular
to the surface of the heater element), the perforation in the film can be
hardly enlarged to portions opposed to the edges of the heater element.
On the other hand, the resistance heater of the thick film thermal head is
substantially uniform in thickness in a cross-section in the main scanning
direction. Further since the resistance heater is continuous in the main
scanning direction, heat generated by each heater element is easily
propagated in the main scanning direction. When printing a solid printing
area, adjacent heater elements generate heat simultaneously, and
accordingly, the temperature of inter-element portions (portions between
the heater elements) is lower than the temperature of the heater elements
at the middle thereof (350 to 400.degree. C.) only by about 50.degree. C.
As described above, the temperature contrast of the thick film thermal head
highly depends upon the direction. Under such conditions, in order to make
the ratio of the length of the heater element in the main scanning
direction to the main scanning pitch smaller than 1 and substantially
equal to the ratio of the length of the heater element in the sub-scanning
direction to the sub-scanning pitch and to make the lengths of the heater
element in the main and sub-scanning directions equal to the diameters of
the perforation in the respective directions, it is necessary that the
heat shrinkage stress of the film is highly anisotropic, which is
practically impossible.
As can be understood from the description above, use of a thick film
thermal head in thermally making a stencil is practically difficult mainly
for reasons of quality of the perforations though proposed in Japanese
Unexamined Patent Publication Nos. 5(1993)-345401, 5(1993)-345402 and
5(1993)-345403.
SUMMARY OF THE INVENTION
In view of the foregoing observations and description, the primary object
of the present invention is to provide a thick film thermal head which can
make a stencil ensuring high quality printings and suppression of offset.
In accordance with a first aspect of the present invention, there is
provided a thick film thermal head for making a stencil by thermally
perforating a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
electrodes of at least two lines which extend in a direction intersecting
the main scanning direction in contact with the resistance heater and are
alternately arranged in the main scanning direction, and a protective
layer which covers exposed part of the resistance heater and the
electrodes, wherein the improvement comprises that
the resistance heater is not smaller than 1 .mu.m and not larger than 10
.mu.m in thickness,
the space between each pair of adjacent electrodes in the main scanning
direction is not smaller than 20% and not larger than 60% of the center
distance between the adjacent electrodes (the distance between the axes of
the adjacent electrodes extending in the sub-scanning direction), and
the length in the sub-scanning direction of the resistance heater at the
portion between each pair of adjacent electrodes is not smaller than 100%
and not larger than 250% of the center distance between the adjacent
electrodes.
In accordance with a second aspect of the present invention, there is
provided a thick film thermal head for making a stencil by thermally
perforating a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
discrete electrodes and common electrodes which extend in a direction
intersecting the main scanning direction in contact with the resistance
heater and are alternately arranged in the main scanning direction, and a
protective layer which covers exposed part of the resistance heater and
the electrodes, the common electrodes comprising first and second groups
of common electrodes which are connected to each other by group and are
alternately arranged in the main scanning direction, wherein the
improvement comprises that
the resistance heater is not smaller than 1 .mu.m and not larger than 10
.mu.m in thickness, the space between each pair of adjacent electrodes in
the main scanning direction is not smaller than 20% and not larger than
60% of the center distance between the adjacent electrodes, and
the length in the sub-scanning direction of the resistance heater at the
portion between each pair of adjacent electrodes is not smaller than 100%
and not larger than 250% of the center distance between the adjacent
electrodes.
In accordance with a third aspect of the present invention, there is
provided a thick film thermal head for making a stencil by thermally
perforating a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
discrete electrodes and common electrodes which extend in a direction
intersecting the main scanning direction in contact with the resistance
heater and are alternately arranged in the main scanning direction, and a
protective layer which covers exposed part of the resistance heater and
the electrodes, the common electrodes being connected to each other in one
line, wherein the improvement comprises that
the resistance heater is not smaller than 1 .mu.m and not larger than 10
.mu.m in thickness,
the sum of the space between each discrete electrode and the common
electrode on one side of the discrete electrode in the main scanning
direction and the space between the discrete electrode and the common
electrode on the other side of the discrete electrode in the main scanning
direction is not smaller than 20% and not larger than 60% of the center
distance between the common electrodes on the opposite sides of the
discrete electrode, and
the length in the sub-scanning direction of the resistance heater at the
portion between each discrete electrode and the common electrode on each
side of the discrete electrode is not smaller than 100% and not larger
than 250% of the center distance between the common electrodes on the
opposite sides of the discrete electrode.
In accordance with a fourth aspect of the present invention, there is
provided a thick film thermal head for making a stencil by thermally
perforating a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
electrodes of at least two lines which extend in a direction intersecting
the main scanning direction in contact with the resistance heater and are
alternately arranged in the main scanning direction, and a protective
layer which covers exposed part of the resistance heater and the
electrodes, wherein the improvement comprises that
the following formula (1) is satisfied,
0.2.ltoreq.V/d.sup.2.ltoreq.10 (1)
wherein V (in .mu.m.sup.3) represents the volume of a part of the
resistance heater between each pair of adjacent electrodes, and d (in
.mu.m) represents the center distance between the adjacent electrodes.
In accordance with a fifth aspect of the present invention, there is
provided a thick film thermal head for making a stencil by thermally
perforating a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
discrete electrodes and common electrodes which extend in a direction
intersecting the main scanning direction in contact with the resistance
heater and are alternately arranged in the main scanning direction, and a
protective layer which covers exposed part of the resistance heater and
the electrodes, the common electrodes comprising first and second groups
of common electrodes which are connected to each other by group and are
alternately arranged in the main scanning direction, wherein the
improvement comprises that
the following formula (1) is satisfied,
0.2.ltoreq.V/d.sup.2.ltoreq.10 (1)
wherein V (in .mu.m.sup.3) represents the volume of a part of the
resistance heater between each pair of adjacent electrodes, and d (in
.mu.m) represents the center distance between the adjacent electrodes.
In accordance with a sixth aspect of the present invention, there is
provided a thick film thermal head for making a stencil by thermally
perforating a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
discrete electrodes and common electrodes which extend in a direction
intersecting the main scanning direction in contact with the resistance
heater and are alternately arranged in the main scanning direction, and a
protective layer which covers exposed part of the resistance heater and
the electrodes, the common electrodes being connected to each other in one
line, wherein the improvement comprises that
the following formula (2) is satisfied,
0.2.ltoreq.V/D.sup.2.ltoreq.10 (2)
wherein V (in .mu.m.sup.3) represents the sum of the volume of a part of
the resistance heater between each discrete electrode and the common
electrode on one side of the discrete electrode in the main scanning
direction and the volume of a part of the resistance heater between the
discrete electrode and the common electrode on the other side of the
discrete electrode in the main scanning direction and D (in .mu.m)
represents the center distance between the common electrodes on the
opposite sides of the discrete electrode.
In accordance with a seventh aspect of the present invention, there is
provided a thick film thermal head for making a stencil by thermally
perforating a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
electrodes of at least two lines which extend in a direction intersecting
the main scanning direction in contact with the resistance heater and are
alternately arranged in the main scanning direction, and a protective
layer which covers exposed part of the resistance heater and the
electrodes, wherein the improvement comprises that
the resistance heater is not smaller than 1 .mu.m and not larger than 10
.mu.m in thickness,
the space between each pair of adjacent electrodes in the main scanning
direction is not smaller than 20% and not larger than 60% of the center
distance between the adjacent electrodes,
the length in the sub-scanning direction of the resistance heater at the
portion between each pair of adjacent electrodes is not smaller than 100%
and not larger than 250% of the center distance between the adjacent
electrodes, and
the following formula (1) is satisfied,
0.2.ltoreq.V/d.sup.2.ltoreq.10 (1)
wherein V (in .mu.m.sup.3) represents the volume of a part of the
resistance heater between each pair of adjacent electrodes, and d (in
.mu.m) represents the center distance between the adjacent electrodes.
In accordance with an eighth aspect of the present invention, there is
provided a thick film thermal head for making a stencil by thermally
perforating a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
discrete electrodes and common electrodes which extend in a direction
intersecting the main scanning direction in contact with the resistance
heater and are alternately arranged in the main scanning direction, and a
protective layer which covers exposed part of the resistance heater and
the electrodes, the common electrodes comprising first and second groups
of common electrodes which are connected to each other by group and are
alternately arranged in the main scanning direction, wherein th e
improvement comprises that
the resistance heater is not smaller than 1 .mu.m and not larger than 10
.mu.m in thickness, the space between each pair of adjacent electrodes in
the main scanning direction is not smaller than 20% and not larger than
60% of the center distance between the adjacent electrodes,
the length in the sub-scanning direction of the resistance heater at the
portion between each pair of adjacent electrodes is not smaller than 100%
and not larger than 250% of the center distance between the adjacent
electrodes, and
the following formula (1) is satisfied,
0.2.ltoreq.V/d.sup.2.ltoreq.10 (1)
wherein V (in .mu.m.sup.3) represents the volume of a part of the
resistance heater between each pair of adjacent electrodes, and d (in
.mu.m) represents the center distance between the adjacent electrodes.
In accordance with a ninth aspect of the present invention, there is
provided a thick film thermal head for making a stencil by thermally
perforating a stencil material comprising
an electrical insulating substrate and a glaze layer superposed on a heat
radiating plate in this order, a resistance heater formed on the glaze
layer to continuously extend in a main scanning direction, a plurality of
discrete electrodes and common electrodes which extend in a direction
intersecting the main scanning direction in contact with the resistance
heater and are alternately arranged in the main scanning direction, and a
protective layer which covers exposed part of the resistance heater and
the electrodes, the common electrodes being connected to each other in one
line, wherein the improvement comprises that
the resistance heater is not smaller than lm and not larger than 10 .mu.m
in thickness,
the sum of the space between each discrete electrode and the common
electrode on one side of the discrete electrode in the main scanning
direction and the space between the discrete electrode and the common
electrode on the other side of the discrete electrode in the main scanning
direction is not smaller than 20% and not larger than 60% of the center
distance between the common electrodes on the opposite sides of the
discrete electrode,
the length in the sub-scanning direction of the resistance heater at the
portion between each discrete electrode and the common electrode on each
side of the discrete electrode is not smaller than 100% and not larger
than 250% of the center distance between the common electrodes on the
opposite sides of the discrete electrode, and
the following formula (2) is satisfied,
0.2.ltoreq.V/D.sup.2.ltoreq.10 (2)
wherein V (in .mu.m.sup.3) represents the sum of the volume of a part of
the resistance heater between each discrete electrode and the common
electrode on one side of the discrete electrode in the main scanning
direction and the volume of a part of the resistance heater between the
discrete electrode and the common electrode on the other side of the
discrete electrode in the main scanning direction and D (in .mu.m)
represents the center distance between the common electrodes on the
opposite sides of the discrete electrode.
That is, the present invention is to compensate for disadvantage of the
thick film thermal head that it is low in temperature response and
temperature contrast in order to make a high quality stencil with a thick
film thermal head which is inexpensive. In accordance with the present
invention, temperature response and temperature contrast of the thick film
thermal head are improved by limiting the volume of each heater element
taking into account the conditions required in the thermal stencil making.
By limiting the thickness of the resistance heater (In this specification,
the thickness of the resistance heater or the heater element means a
maximum length of the resistance heater or the heater element as measured
in the vertical direction perpendicular to the surface plain of the under
layer, that is, a glaze layer.) to not larger than 10 .mu.m (preferably
not larger than 6 .mu.m), the heat capacity of each heater element is
reduced and response of the temperature of the heater element to on and
off of the applied pulses is increased, whereby the temperature contrast
in the sub-scanning direction is increased and fluctuation of the shapes
of the perforations in the sub-scanning direction can be suppressed. At
the same time, energy required to heat the heater element to a temperature
necessary to perforate the film of the stencil material is reduced and the
power consumption can be suppressed. Further, since the total amount of
heat to be generated by the heater element is reduced, accumulation of
heat is suppressed when a plurality of stencils are continuously made,
whereby fluctuation in printing density can be suppressed and offset can
be prevented. Further, when the thickness of the resistance heater is
smaller than 1 .mu.m, the shape of the resistance heater comes to largely
depend upon the position in the main scanning direction due to limitation
in precision of thick film printing process. In other words, uniformity of
the shape of the resistance heater in the main scanning direction largely
deteriorates, which results in fluctuation in shape, resistance and heat
generating properties of the heater elements and results in fluctuation of
the shape of the perforations obtained. Accordingly, the thickness of the
resistance heater should not be smaller than 1 .mu.m, and preferably
should not be smaller than 2 .mu.m.
By limiting the inter-electrode space in the main scanning direction as
described above, the following effect can be obtained. In "single-dot
recording" and in "twin-dot-recording" where two perforations
corresponding to one pixel are to be separated from each other (these
forms of perforation will be referred to as "single-dot independent
perforation", hereinbelow), when the space between each pair of adjacent
electrodes in the main scanning direction (the length in the main scanning
direction of each heater element) is not larger than 60% (preferably not
larger than 50%) of the center distance between the adjacent electrodes
(corresponding to the main scanning pitch), the temperature contrast of
the heater element is enhanced, whereby fluctuation in the shape of the
perforations can be suppressed in the main scanning direction and the
perforations can be prevented from connecting to each other in the main
scanning direction. Further, in twin-dot-recording" where two perforations
corresponding to one pixel are to be connected to each other though a pair
of perforations corresponding to one pixel are to be separated from
another pair of perforations corresponding to another pixel (this form of
perforation will be referred to as "twin-dot independent perforation",
hereinbelow), when the sum of the space between each discrete electrode
and the common electrode on one side of the discrete electrode in the main
scanning direction and the space between the discrete electrode and the
common electrode on the other side of the discrete electrode in the main
scanning direction is not larger than 60% (preferably not larger than 50%)
of the center distance between the common electrodes on the opposite sides
of the discrete electrode (corresponding to the main scanning pitch), the
temperature contrast of the heater element is enhanced, whereby
fluctuation in the shape of the perforations can be suppressed in the main
scanning direction and the perforations can be prevented from connecting
to each other in the main scanning direction. At the same time, energy
required to heat the heater element to a temperature necessary to
perforate the film of the stencil material is reduced and the power
consumption can be suppressed. Further, since the total amount of heat to
be generated by the heater element is reduced, accumulation of heat is
suppressed when a plurality of stencils are continuously made, whereby
fluctuation in density of printings can be suppressed and offset can be
prevented. On the other hand, when the space between each pair of adjacent
electrodes in the main scanning direction is smaller than 20% of the
center distance between the adjacent electrodes in the single-dot
independent perforation or when the sum of the space between each discrete
electrode and the common electrode on one side of the discrete electrode
in the main scanning direction and the space between the discrete
electrode and the common electrode on the other side of the discrete
electrode in the main scanning direction is smaller than 20% of the center
distance between the common electrodes on the opposite sides of the
discrete electrode in the twin-dot independent perforation, heat
generating areas become too small in the main scanning direction to form
perforations in a proper size (30 to 40% in terms of the proportion of
open area), which results in, for instance, a poor printing density.
Accordingly, the space between each pair of adjacent electrodes in the
main scanning direction should be not smaller than 20% (preferably not
smaller than 25%) of the center distance between the adjacent electrodes
in the single-dot independent perforation, and the sum of the space
between each discrete electrode and the common electrode on one side of
the discrete electrode in the main scanning direction and the space
between the discrete electrode and the common electrode on the other side
of the discrete electrode in the main scanning direction should be not
smaller than 20% (preferably not smaller than 25%) of the center distance
between the common electrodes on the opposite sides of the discrete
electrode in the twin-dot independent perforation.
By limiting the length of the resistance heater in the sub-scanning
direction as described above, the following effect can be obtained. When
the length in the sub-scanning direction of the resistance heater at the
portion between each pair of adjacent electrodes is not larger than 250%
(preferably not larger than 200%) of the center distance between the
adjacent electrodes in the single-dot independent perforation and when the
length in the sub-scanning direction of the resistance heater at the
portion between each discrete electrode and the common electrode on each
side of the discrete electrode is not larger than 250% (preferably not
larger than 200%) of the center distance between the common electrodes on
the opposite sides of the discrete electrode in the twin-dot independent
perforation, the length of the heater elements in the sub-scanning
direction can be not larger than 250% (preferably not larger than 200%) of
the sub-scanning pitch in the case where perforations are to be formed in
the sub-scanning direction in substantially the same pitch as in the main
scanning direction. When the length of the heater elements in the
sub-scanning direction is not larger than 250% (preferably not larger than
200%) of the sub-scanning pitch, the temperature contrast of the heater
element in the sub-scanning direction is enhanced as compared with the
conventional thick film thermal head where the length in the sub-scanning
direction of the resistance heater at the portion between each pair of
adjacent electrodes is about 300%, whereby fluctuation in the shape of the
perforations can be suppressed in the sub-scanning direction and the
perforations can be prevented from connecting to each other in the
sub-scanning direction. At the same time, energy required to heat the
heater element to a temperature necessary to perforate the film of the
stencil material is reduced and the power consumption can be suppressed.
Further, since the total amount of heat to be generated by the heater
element is reduced, accumulation of heat is suppressed when a plurality of
stencils are continuously made, whereby fluctuation in printing density
can be suppressed and offset can be prevented. On the other hand, when the
length in the sub-scanning direction of the resistance heater at the
portion between each pair of adjacent electrodes is smaller than 100% of
the center distance between the adjacent electrodes in the single-dot
independent perforation and when the length in the sub-scanning direction
of the resistance heater at the portion between each discrete electrode
and the common electrode on each side of the discrete electrode is smaller
than 100% of the center distance between the common electrodes on the
opposite sides of the discrete electrode in the twin-dot independent
perforation, the length of the heater elements in the sub-scanning
direction becomes smaller than 100% of the sub-scanning pitch in the case
where perforations are to be formed in the sub-scanning direction in
substantially the same pitch as in the main scanning direction. When the
length of the heater elements in the sub-scanning direction is smaller
than 100% of the sub-scanning pitch in the single-dot independent
perforation and the twin-dot independent perforation, heat generating
areas become too small in the sub-scanning direction to form perforations
in a proper size (30 to 40% in terms of the proportion of open area),
which results in, for instance, a poor printing density. Accordingly, the
length in the sub-scanning direction of the resistance heater at the
portion between each pair of adjacent electrodes should be not smaller
than 100% (preferably not smaller than 120%) of the center distance
between the adjacent electrodes in the single-dot independent perforation
and the length in the sub-scanning direction of the resistance heater at
the portion between each discrete electrode and the common electrode on
each side of the discrete electrode should be not smaller than 100%
(preferably not smaller than 120%) of the center distance between the
common electrodes on the opposite sides of the discrete electrode in the
twin-dot independent perforation.
By limiting the volume of the heater element as described above, the
following effect can be obtained. When formula (1) is satisfied in the
single-dot independent perforation and when formula (2) is satisfied in
the twin-dot independent perforation, the heater element can be optimal in
volume to any resolution, the heater element can be high in temperature
response and temperature contrast to any resolution, a high accuracy in
the shape of the heater element can be ensured and a heat generating area
necessary to perforation can be ensured in the case where perforations are
to be formed in the sub-scanning direction in substantially the same pitch
as in the main scanning direction. Specifically when V/d.sup.2 or
V/D.sup.2 is not larger than 10 .mu.m (preferably not larger than 5
.mu.m), the heater element can be high in temperature response and
temperature contrast to any resolution, and when V/d.sup.2 or V/D.sup.2 is
not smaller than 0.2 .mu.m (preferably not larger than 0.5 .mu.m), a high
accuracy in the shape of the heater element can be ensured and a heat
generating area necessary to perforation can be ensured.
Thus, in accordance with the present invention, a high quality stencil can
be thermally made by the use of a thick film thermal head which can be
produced at a lower cost than the thin film thermal head, whereby the
thermal stencil making apparatus can be manufactured at low cost.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a thermal stencil making apparatus provided
with a thermal head in accordance with an embodiment of the present
invention,
FIG. 2 is a fragmentary plan view of the thermal head employed in the
thermal stencil making apparatus,
FIG. 3 is a cross-sectional view taken along line A--A in FIG. 2,
FIG. 4 is a cross-sectional view taken along line B--B in FIG. 2,
FIG. 5 is a graph showing the change in the temperature of the surface of
the protective layer in response to on and off of the applied pulses for a
thermal head which is not larger than 10 .mu.m in thickness and a thermal
head which is larger than 10 .mu.m in thickness,
FIG. 6 is a graph showing the temperature contrast in the main scanning
direction on a thermal head of the present invention at the time the
temperature of the heater elements is maximized in comparison with that on
a thermal head of a comparative example,
FIG. 7A is a view showing the thermal head of the comparative example where
the space between each pair of adjacent electrodes in the main scanning
direction is larger than 60% of the center distance between the adjacent
electrodes,
FIG. 7B is a view showing the thermal head of the present invention where
the space between each pair of adjacent electrodes in the main scanning
direction is not larger than 60% of the center distance between the
adjacent electrodes, and
FIG. 8 is a view showing the temperature contrast on a thermal head of the
present invention in the sub-scanning direction passing through the center
of the heater element at the time the temperature of the heater element is
maximized in comparison with that on a thermal head of a comparative
example.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1, a thermal stencil making apparatus 10 comprises a thick film
thermal head 1 in accordance with an embodiment of the present invention,
and a stencil material 12 unrolled from a roll 11 is inserted between the
thermal head 1 and a platen roller 14 and is conveyed in response to
rotation of the platen roller 14.
As shown in FIGS. 2 to 4, the thermal head 1 comprises a strip-like
resistance heater 6 continuously extending in a main scanning direction X
(a direction of width of the stencil material 12) and a plurality of
discrete electrodes 5a and common electrodes 5b which extend in a
sub-scanning direction Y in contact with the resistance heater 6 and are
alternately arranged in the main scanning direction X. The parts 6a of the
resistance heater 6 between the discrete electrodes 5a and the common
electrodes 5b generate heat when energized through the discrete electrodes
5a and the common electrodes 5b as will be described in more detail later.
That is, each of the parts 6a of the resistance heater 6 between the
discrete electrodes 5a and the common electrodes 5b form a heater element,
and the thermal head 1 is provided with an array of heater elements 6a
extending in the main scanning direction X.
The stencil material 12 is conveyed in the sub-scanning direction Y with
its thermoplastic film kept in contact with the thermal head 1 while the
heater elements 6a of the thermal head 1 selectively energized through the
discrete electrodes 5a and the common electrodes 5b according to an image
signal representing the image of an original, whereby the thermoplastic
film of the stencil material is imagewise perforated. The stencil material
12 may be either one comprising thermoplastic resin film and a porous base
sheet or one comprising thermoplastic resin film with no base sheet.
A control section 15 controls power supply to the heater elements 6a and
controls the platen roller 14 by way of a platen roller drive motor (not
shown). That is, the control section 15 controls the electric voltages
applied to the respective heater elements 6a and/or applying times for
which the electric voltages are applied to the respective heater elements
6a, and the sub-scanning pitch at which the stencil material 12 is
conveyed in the sub-scanning direction.
The thermal head 1 is formed by thick film process. That is, as shown in
FIGS. 2 to 4, an electrical insulating substrate 3 and a glaze layer 4 is
superposed on a heat radiating plate 2 of metal, and a plurality of
discrete electrodes 5a and common electrodes 5b (in the form of thin
plates) are formed on the glaze layer 4 alternately in the main scanning
direction X to extend in the sub-scanning direction Y. The discrete
electrodes 5a and the common electrodes 5b extend in opposite directions
from a central portion of the glaze layer 4 with their central portions
overlapping with each other in the main scanning direction. A strip-like
resistance heater 6 is formed on the glaze layer 4 to extend in the main
scanning direction across the central portions of the discrete electrodes
5a and the common electrodes 5b. A protective layer 7 of, for instance,
glass is formed so as to cover the exposed part of the discrete electrodes
5a and the common electrodes 5b and the upper surface of the resistance
heater 6. The thermal head 1 is brought into contact with the stencil
material 12 at the surface of the protective layer 7.
The discrete electrodes 5a and the common electrodes 5b are respectively
connected to the corresponding lines through wire bonding or the like, and
electric voltages are applied across selected pairs of discrete electrode
5a and common electrode 5b adjacent to each other by a driver IC or the
like, whereby the heater elements 6a between the selected pairs of
discrete electrode 5a and common electrode 5b are energized and generate
heat.
The discrete electrodes 5a and the common electrodes 5b need not extend
accurately in the sub-scanning direction Y but may extend in any direction
which intersects the main scanning direction. Further, the discrete
electrodes 5a and the common electrodes 5b need not extend across the
resistance heater 6 but may extend midway between opposite edges of the
resistance heater 6. Further, the discrete electrodes 5a and the common
electrodes 5b may be provided either above or under the resistance heater
6 so long as they are in contact with the resistance heater. For example,
the discrete electrodes 5a may be provided above the resistance heater 6
with the common electrodes 5b provided below the same, and vice versa.
Anyway, the path along which an electric current flows when different
potentials are applied to the discrete electrode 5a and the common
electrode 5b functions as a heater element 6a and generates heat.
When the thermal head 1 is to be driven in the twin-dot recording and in
the single-dot independent perforation, the common electrodes 5b are
connected in one line, and the discrete electrodes 5a are selectively
applied with a pulse by a switching device according to on and off of
corresponding pixels of an image signal representing an original. When one
discrete electrode 5a is applied with a pulse, two heater elements 6a on
opposite sides of the discrete electrode 5a generate heat, and two
perforations are formed in the film of the stencil material 12 at two
portions opposed to the heater elements 6a through the protective layer 7.
In this case, two perforations correspond to one pixel. The center
distance d (FIG. 2) between the discrete electrode 5a and the common
electrode 5b adjacent to the discrete electrode 5a corresponds to the main
scanning pitch, and the distance d is set constant for all the heater
elements 6a. The sub-scanning pitch p (FIG. 2) is set to a constant value
equal to the center distance d. The reason why the sub-scanning pitch and
the main scanning pitch are made equal to each other is that it is the
most efficient to equalize the sampling frequencies in the main scanning
direction and the sub-scanning direction to each other from the viewpoint
of the amount of information in an image (quality of an image) so long as
the image is a typical image without any special anisotropy. The thickness
t of the resistance heater 6 (or each heater element 6a) is in a range of
1 .mu.m to 10 .mu.m (preferably 2 .mu.m to 6 .mu.m). The widths of the
electrodes 5a and 5b and the spaces therebetween are set so that the space
Lx between adjacent electrodes 5a and 5b (the length of the heater element
6a in the main scanning direction X) is 20% to 60% (preferably 25% to 50%)
of the center distance d between the electrodes 5a and 5b (the pitch of
the heater elements in the main scanning direction X). Further the length
Ly of the heater element 6a in the sub-scanning direction Y (the length in
the sub-scanning direction Y of the resistance heater 6 at the portion
between the adjacent electrodes 5a and 5b) is set to be 100% to 250%
(preferably 120% to 200%) of the center distance d between the adjacent
electrodes 5a and 5b. Further, the volume V (.mu.m3) of the heater element
6a (the volume of the portion of the resistance heater 6 between the
adjacent electrodes 5a and 5b) is set so that the value (V/d.sup.2)
obtained by dividing the volume V by the square of the center distance d
(.mu.m) is in the range of 0.2 to 10 (preferably 0.5 to 5).
When the thermal head 1 is to be driven in the single-dot recording and in
the single-dot independent perforation, the common electrodes 5b are
divided into first and second groups of common electrodes and are
connected in two lines by the group. The first and second groups of common
electrodes 5b are alternately disposed with a discrete electrode 5a
therebetween. The first and second groups of common electrodes 5b are
applied with a pulse at different timings by the group, while the discrete
electrodes 5a are selectively applied with a pulse by a switching device
according to on and off of corresponding pixels of an image signal
representing an original in time to the time sharing drive of the first
and second groups of the common electrodes 5b. When one discrete electrode
5a is applied with a pulse, a heater element 6a on one side of the
discrete electrode 5a generates heat, and one perforation is formed in the
film of the stencil material 12 at a portion opposed to the heater element
6a through the protective layer 7. In this case, one perforation
corresponds to one pixel. The center distance d between the discrete
electrode 5a and the common electrode 5b adjacent to the discrete
electrode 5a corresponds to the main scanning pitch, and the distance d is
set constant for all the heater elements 6a. The sub-scanning pitch p
(FIG. 2) is set to a constant value equal to the center distance d. The
reason why the sub-scanning pitch and the main scanning pitch are made
equal to each other is that it is the most efficient to equalize the
sampling frequencies in the main scanning direction and the sub-scanning
direction to each other from the viewpoint of the amount of information in
an image (quality of an image) so long as the image is a typical image
without any special anisotropy. The thickness t of the resistance heater 6
(or each heater element 6a) is in a range of 1 .mu.m to 10 .mu.m
(preferably 2 .mu.m to 6 .mu.m). The widths of the electrodes 5a and 5b
and the spaces therebetween are set so that the space Lx between adjacent
electrodes 5a and 5b (the length of the heater element 6a in the main
scanning direction X) is 20% to 60% (preferably 25% to 50%) of the center
distance d between the electrodes 5a and 5b (the pitch of the heater
elements in the main scanning direction X). Further the length Ly of the
heater element 6a in the sub-scanning direction Y (the length in the
sub-scanning direction Y of the resistance heater 6 at the portion between
the adjacent electrodes 5a and 5b) is set to be 100% to 250% (preferably
120% to 200%) of the center distance d between the adjacent electrodes 5a
and 5b. Further, the volume V (.mu.m3) of the heater element 6a (the
volume of the portion of the resistance heater 6 between the adjacent
electrodes 5a and 5b) is set so that the value (V/d.sup.2) obtained by
dividing the volume V by the square of the center distance d (.mu.m) is in
the range of 0.2 to 10 (preferably 0.5 to 5).
When the thermal head 1 is to be driven in the twin-dot recording and in
the twin-dot independent perforation, the common electrodes 5b are
connected in one line, and the discrete electrodes 5a are selectively
applied with a pulse by a switching device according to on and off of
corresponding pixels of an image signal representing an original. When one
discrete electrode 5a is applied with a pulse, two heater elements 6a on
opposite sides of the discrete electrode 5a generate heat, and two
perforations are formed in the film of the stencil material 12 at two
portions opposed to the heater elements 6a through the protective layer 7.
In this case, two perforations correspond to one pixel. Twice the center
distance d between the discrete electrode 5a and the common electrode 5b
adjacent to the discrete electrode 5a corresponds to the main scanning
pitch, and the distance d is set constant for all the heater elements 6a.
The sub-scanning pitch p (FIG. 2) is set to a constant value equal to the
center distance d. The reason why the sub-scanning pitch and the main
scanning pitch are made equal to each other is that it is the most
efficient to equalize the sampling frequencies in the main scanning
direction and the sub-scanning direction to each other from the viewpoint
of the amount of information in an image (quality of an image) so long as
the image is a typical image without any special anisotropy. The thickness
t of the resistance heater 6 (or each heater element 6a) is in a range of
1 .mu.m to 10 .mu.m (preferably 2 .mu.m to 6 .mu.m). The widths of the
electrodes 5a and 5b and the spaces therebetween are set so that the sum
of the space between the discrete electrode 5a and the common electrode 5b
on one side of the discrete electrode 5a in the main scanning direction X
(Lx) and the space between the discrete electrode 5a and the common
electrode 5b on the other side of the discrete electrode 5a in the main
scanning direction X (L'x), i.e., the sum of the lengths in the main
scanning direction X of the heater elements 6a on the opposite sides of
the discrete electrode 5a, is 20% to 60% (preferably 25% to 50%) of the
center distance D between the common electrodes 5b on the opposite sides
of the discrete electrode 5a (the main scanning pitch). Further the length
Ly of the heater element 6a in the sub-scanning direction Y (the length in
the sub-scanning direction Y of the resistance heater 6 at the portion
between the adjacent electrodes 5a and 5b) is set to be 100% to 250%
(preferably 120% to 200%) of the center distance D. Further, the sum V
(.mu.m3) of the volume of a part of the resistance heater 6 between the
discrete electrode 5a and the common electrode 5b on one side of the
discrete electrode 5a in the main scanning direction X and the volume of a
part of the resistance heater 6 between the discrete electrode 5a and the
common electrode 5b on the other side of the discrete electrode 5a in the
main scanning direction X, i.e., the sum of the volumes of the heater
elements 6a on the opposite sides of the discrete electrode 5a, is set so
that the value (V/D.sup.2) obtained by dividing the sum of the volumes V
by the square of the center distance D (.mu.m) between the common
electrodes 5b on the opposite sides of the discrete electrode 5a is in the
range of 0.2 to 10 (preferably 0.5 to 5).
The effect obtained by limiting the aforesaid factors in the thermal head 1
will be described with reference to FIGS. 5, 6, 7(7A and 7B) and 8,
hereinbelow.
In FIG. 5, the solid line shows the change, in response to on and off of
the applied pulses, in the temperature T1 of the surface of the protective
layer 7 at the center of the heater element 6a which is not larger than 10
.mu.m in thickness t (this invention) and the dashed line shows that of
the heater element which is larger than 10 .mu.m in thickness t
(comparative example).
As can be seen from FIG. 5, when the thickness t of the heater element 6a
is not larger than 10 .mu.m, the temperature T1 of the surface of the
protective layer 7 at the center of the heater element 6a quickly changes
in response to on and off of the applied pulses.
Further, as can be seen from FIG. 5, when application of a pulse is
repeated, the temperature T1 is gradually increased due to accumulation of
heat. However, in the case of the thermal head 1 which is not larger than
10 .mu.m in thickness, the degree of the temperature increase is less as
compared with the thermal head 1 which is larger than 10 .mu.m in
thickness.
When the thickness t of the resistance heater 6 (or the heater element 6a)
is limited to not larger than 10 .mu.m, the heat capacity of each heater
element 6a is reduced and response of the temperature of the heater
element 6a to on and off of the applied pulses is increased, whereby the
temperature contrast on the heater element 6a in the sub-scanning
direction Y is increased and fluctuation of the shapes of the perforations
in the sub-scanning direction Y can be suppressed. At the same time,
energy required to heat the heater element 6a to a temperature necessary
to perforate the film of the stencil material is reduced and the power
consumption can be suppressed. Further, when accumulation of heat is
large, perforations gradually become large in the sub-scanning direction
Y, which can result in fluctuation in printing density and offset.
Accordingly, by limiting the thickness t of the resistance heater 6 to not
larger than 10 .mu.m (preferably not larger than 6 .mu.m), fluctuation in
printing density can be suppressed and offset can be prevented.
Though the smaller the thickness t of the resistance heater 6 is, the
smaller the heat capacity of the heater element 6a is, when the thickness
t of the resistance heater 6 is smaller than 1 .mu.m, the shape of the
resistance heater 6 comes to largely depend upon the position in the main
scanning direction due to limitation in precision of thick film printing
process. In other words, uniformity of the shape of the resistance heater
6 in the main scanning direction largely deteriorates, which results in
fluctuation in shape, resistance and heat generating properties of the
heater elements 6a and results in fluctuation of the shape of the
perforations obtained. Accordingly, the thickness of the resistance heater
6 should not be smaller than 1 .mu.m, and preferably should not be smaller
than 2 .mu.m.
FIG. 6 shows the temperature contrast T2 in the main scanning direction X
on a thermal head of the present invention (solid line) at the time the
temperature of the heater elements 6a is maximized in single-dot
independent perforation in comparison with that on a thermal head of a
comparative example (dashed line). As shown in FIG. 7A, in the thermal
head of the comparative example, the space Lx between the adjacent
electrodes 5a and 5b in the main scanning direction X is larger than 60%
of the center distance d (constant irrespective of the position) between
the adjacent electrodes 5a and 5b, whereas in the thermal head of the
present invention, the space Lx between the adjacent electrodes 5a and 5b
in the main scanning direction X is not larger than 60% of the center
distance d between the adjacent electrodes 5a and 5b. As can be seen from
FIG. 6, in the thermal head of the present invention, the temperature
contrast is enhanced as compared with in the thermal head of the
comparative example.
That is, when the length in the main scanning direction of the heater
element 6a (Lx) is not larger than 60% of the main scanning pitch (d), the
temperature contrast of the heater element 6a in the main scanning
direction X is enhanced, whereby fluctuation in the shape of the
perforations can be suppressed in the main scanning direction X and the
perforations can be prevented from connecting to each other in the main
scanning direction X. At the same time, energy required to heat the heater
element 6a to a temperature necessary to perforate the film of the stencil
material is reduced and the power consumption can be suppressed. Further,
since the total amount of heat to be generated by the heater element 6a is
reduced, accumulation of heat is suppressed when a plurality of stencils
are continuously made, whereby the phenomenon that perforations gradually
become large in the sub-scanning direction Y, which can result in
fluctuation in printing density and offset, can be prevented. Accordingly,
the space Lx should be not larger than 60% (preferably not larger than
50%) of the center distance d.
On the other hand, though the smaller the space Lx is, the more the
temperature contrast of the thermal head is enhanced, when the space Lx is
smaller than 20% of the center distance d, heat generating areas become
too small in the main scanning direction X to form perforations in a
proper size (30 to 40% in terms of the proportion of open area) in the
main scanning direction X, which results in, for instance, a poor printing
density. Accordingly, the space Lx should be not smaller than 20%
(preferably not smaller than 25%) of the center distance d.
FIG. 6 also shows the temperature contrast T2 in the main scanning
direction X on a thermal head of the present invention (solid line) at the
time the temperature of the heater elements 6a is maximized in twin-dot
independent perforation in comparison with that on a thermal head of a
comparative example (dashed line). As shown in FIG. 7A, in the thermal
head of the comparative example, the sum (Lx+L'x) of the space between the
discrete electrode 5a and the common electrode 5b on one side of the
discrete electrode 5a in the main scanning direction X (Lx) and the space
between the discrete electrode 5a and the common electrode 5b on the other
side of the discrete electrode 5a in the main scanning direction X (L'x),
i.e., the sum of the lengths in the main scanning direction X of the
heater elements 6a on the opposite sides of the discrete electrode 5a is
larger than 60% of the center distance D between the common electrodes 5b
on the opposite sides of the discrete electrode 5a (constant irrespective
of the position), whereas in the thermal head of the present invention,
the sum (Lx+L'x) of the spaces is not larger than 60% of the center
distance D between the common electrodes 5b on the opposite sides of the
discrete electrode 5a. As can be seen from FIG. 6, in the thermal head of
the present invention, the temperature contrast is enhanced as compared
with in the thermal head of the comparative example.
That is, when the sum of the lengths in the main scanning direction X of
the heater elements 6a on the opposite sides of the discrete electrode 5a
(Lx+L'x) is not larger than 60% of the main scanning pitch (D), the
temperature contrast of the heater element 6a in the main scanning
direction X is enhanced, whereby fluctuation in the shape of the
perforations can be suppressed in the main scanning direction X and the
perforations can be prevented from connecting to each other in the main
scanning direction X. At the same time, energy required to heat the heater
element 6a to a temperature necessary to perforate the film of the stencil
material is reduced and the power consumption can be suppressed. Further,
since the total amount of heat to be generated by the heater element 6a is
reduced, accumulation of heat is suppressed when a plurality of stencils
are continuously made, whereby the phenomenon that perforations gradually
become large in the sub-scanning direction Y, which can result in
fluctuation in printing density and offset, can be prevented. Accordingly,
the sum (Lx+L'x) of the spaces should be not larger than 60% (preferably
not larger than 50%) of the center distance On the other hand, though the
smaller the sum (Lx+L'x) of the spaces is, the more the temperature
contrast of the thermal head is enhanced, when the sum (Lx+L'x) of the
spaces is smaller than 20% of the center distance D, heat generating areas
become too small in the main scanning direction X to form perforations in
a proper size (30 to 40% in terms of the proportion of open area) in the
main scanning direction X, which results in, for instance, a poor printing
density. Accordingly, the sum (Lx+L'x) of the spaces should be not smaller
than 20% (preferably not smaller than 25%) of the center distance D.
FIG. 8 shows the temperature contrast T3 on a thermal head of the present
invention (solid line) in the sub-scanning direction Y passing through the
center of the heater element 6a at the time the temperature of the heater
element 6a is maximized in comparison with that on a thermal head of a
comparative example (dashed line) in the case where the sub-scanning pitch
and the main scanning pitch are equal to each other. In the thermal head
of the comparative example, the length Ly in the sub-scanning direction Y
is larger than 250% of the sub-scanning pitch p or the main scanning pitch
(equal to the center distance d or D), whereas in the thermal head of the
present invention, the length Ly in the sub-scanning direction Y is not
larger than 250% of the sub-scanning pitch p or the main scanning pitch.
As can be seen from FIG. 8, in the thermal head of the present invention,
the temperature is more sharply lowered as the distance from the center of
the heater element 6a increases. Further as can be seen from FIG. 8, the
temperature at the part between the perforations in the sub-scanning
direction is lower in the thermal head of the present invention than in
the thermal head of the comparative example. In FIG. 8, the temperature
contrast T3 for (n-1)-th perforation is indicated at (n-1), the
temperature contrast T3 for n-th perforation is indicated at n, and the
temperature contrast T3 for (n+1)-th perforation is indicated at (n+1).
That is, when the length Ly in the sub-scanning direction Y of the heater
element 6a is not larger than 250% of the sub-scanning pitch p or the main
scanning pitch, the temperature contrast of the heater element 6a in the
sub-scanning direction Y is enhanced as compared with the thermal head of
the comparative example where the length Ly in the sub-scanning direction
Y of the heater element 6a is about 300% of the sub-scanning pitch p or
the main scanning pitch, whereby fluctuation in the shape of the
perforations can be suppressed in the sub-scanning direction Y and the
perforations can be prevented from connecting to each other in the
sub-scanning direction Y. At the same time, energy required to heat the
heater element 6a to a temperature necessary to perforate the film of the
stencil material is reduced and the power consumption can be suppressed.
Further, since the total amount of heat to be generated by the heater
element 6a is reduced, accumulation of heat is suppressed when a plurality
of stencils are continuously made, whereby the phenomenon that
perforations gradually become large in the sub-scanning direction Y, which
can result in fluctuation in printing density and offset, can be
prevented. Accordingly, the length Ly in the sub-scanning direction of the
heater element 6a should be not larger than 250% of the sub-scanning pitch
p or the main scanning pitch.
On the other hand, though the smaller the length Ly in the sub-scanning
direction Y of the heater element 6a is, the more the temperature contrast
of the heater element 6a in the sub-scanning direction is enhanced, when
the length Ly in the sub-scanning direction Y of the heater element 6a is
smaller than 100% of the sub-scanning pitch p or the main scanning pitch
(equal to the center distance d or D), heat generating areas become too
small in the sub-scanning direction Y to form perforations in a proper
size (30 to 40% in terms of the proportion of open area) in the
sub-scanning direction Y, which results in, for instance, a poor printing
density. Accordingly, the length Ly in the sub-scanning direction Y of the
heater element 6a should be not smaller than 100% (preferably not smaller
than 120%) of the sub-scanning pitch p or the main scanning pitch (equal
to the center distance d or D) and not larger than 250% (preferably not
larger than 200%) of the same.
When formula (1) is satisfied in the single-dot independent perforation and
when formula (2) is satisfied in the twin-dot independent perforation, the
heater element 6a can be optimal in size to any resolution in the case
where the sub-scanning pitch and the main scanning pitch are equal to each
other, the heater element 6a can be high in temperature response and
temperature contrast to any resolution, a high accuracy in the shape of
the heater element 6a can be ensured and a heat generating area necessary
to perforation can be ensured. V/d.sup.2 or V/D.sup.2 is set for the
purpose of making the horizontal projected area of the heater element 6a
proportional to a theoretical area d.sup.2 or D.sup.2 of a pixel and
making constant the thickness of the heater element 6a irrespective of the
value of d.sup.2 or D.sup.2. The former (to make the horizontal projected
area of the heater element 6.sup.a proportional to a theoretical area of a
pixel) is based on the fact that the two-dimensional shape of the
perforations is similar irrespective of resolution. The latter (to make
constant the thickness of the heater element 6a) is based on the fact that
heat propagates from the heater element 6a to the film of the stencil
material in a vertical direction (normal to the plane including both the
main scanning direction X and the sub-scanning direction Y) without
depending upon the horizontal shape in the plane including both the main
scanning direction X and the sub-scanning direction Y provided that
propagation of heat in the horizontal direction from the edge of the
heater element 6a is ignored, and the fact that the thickness of the film
is substantially constant irrespective of resolution in many of the
thermal stencil making apparatuses which have been put into practice. Data
obtained in the examples to be described later supports that formulae (1)
and (2) are reasonable. That is, when V/d.sup.2 or V/D.sup.2 is not larger
than 10 (preferably not larger than 5), the heater element 6a can be high
in temperature response and temperature contrast to any resolution, and
when V/d.sup.2 or V/D.sup.2 is not smaller than 0.2 (preferably not larger
than 0.5), a high accuracy in the shape of the heater element 6a can be
ensured and a heat generating area necessary to perforation can be
ensured.
Stencils were made by the use of the thermal heads in accordance with the
present invention (embodiments 1 to 6) and by the use of thermal heads not
in accordance with the present invention (comparative examples 1 to 10),
and the stencils obtained and printings made by the use of the stencils
were evaluated. The stencil making conditions and the result of the
evaluation were as shown in the following tables 1 and 2. In the tables,
"embodiment" is abbreviated as "em" (e.g., embodiment 1: em 1), and
"comparative example" is abbreviated as "cp" (e.g., comparative example 1:
cp 1). Further the main scanning direction is abbreviated as "m/d" and the
sub-scanning direction is abbreviated as "s/d". In comparative examples 1
and 2 and embodiment 1, resolution was 300 dpi in both the main scanning
direction and the sub-scanning direction, perforations were formed by
single-dot recording/single-dot independent perforation, and the target
proportion of open area were 40%. In comparative example 3 and embodiment
2, resolution was 300 dpi in the main scanning direction and 600 dpi in
the sub-scanning direction, perforations were formed by twin-dot
recording/single-dot independent perforation, and the target proportion of
open area were 30%. In this case, though the resolution in the main
scanning direction was 300 dpi, perforations were formed at the rate of
600/inch in both the main scanning direction and the sub-scanning
direction. In comparative examples 4 and 5 and embodiment 3, resolution
was 300 dpi in both the main scanning direction and the sub-scanning
direction, perforations were formed by twin-dot recording/twin-dot
independent perforation, and the target proportion of open area were 40%.
In this case, two perforations were formed by two heater elements for one
pixel and the two perforations formed were connected to each other in the
main scanning direction. In comparative examples 6 and 7 and embodiment 4,
resolution was 300 dpi in the main scanning direction and 400 dpi in the
sub-scanning direction, perforations were formed by single-dot
recording/single-dot independent perforation, and the target proportion of
open area were 37%. In comparative examples 8 and 9 and embodiment 5,
resolution was 400 dpi in both the main scanning direction and the
sub-scanning direction, perforations were formed by single-dot
recording/single-dot independent perforation, and the target proportion of
open area were 35%. In comparative example 10 and embodiment 6, resolution
was 600 dpi in both the main scanning direction and the sub-scanning
direction, perforations were formed by single-dot recording/single-dot
independent perforation, and the target proportion of open area were 30%.
In each of the comparative examples and the embodiments, the center
distance d or D between the electrodes and the sub-scanning pitch p were
set according to the resolution described above, and the length Lx or
Lx+L'x of the heater element in the main scanning direction (will be
referred to as "the length Lx(+L'x)", hereinbelow), the length Ly of the
heater element in the sub-scanning direction and the thickness t of the
heater element were set in different values.
In the tables 1 and 2, the length Lx(+L'x) of the heater element in the
main scanning direction, the length Ly of the heater element in the
sub-scanning direction, the thickness t of the heater element and the
recording system in the main scanning direction (single-dot recording or
twin-dot recording, and single-dot independent perforation or twin-dot
independent perforation: "1" denotes single-dot recording and single-dot
independent perforation, "2" denotes twin-dot recording and twin-dot
independent perforation) are shown. d or D denotes the center distance d
between the adjacent electrodes in the case of the single-dot independent
perforation and the center distance D between the common electrodes on the
opposite sides of the discrete electrode in the case of the twin-dot
independent perforation. Lx(+L'x) denotes the length Lx in the main
scanning direction of one heater element in the case of the single-dot
independent perforation and the sum Lx+L'x in the main scanning direction
of two heater elements corresponding to one pixel in the case of the
twin-dot independent perforation. Further, whether the conditions were
satisfied is shown in the tables 1 and 2. That is, (-) denotes that the
employed value was smaller than the lower limit, (+) denotes that the
employed value was larger than the upper limit, and (.largecircle.)
denotes that the employed value was between the upper and lower limits,
that is, satisfies the condition. Further evaluations of the stencils
obtained and printings made by the use of the stencils are shown in the
tables 1 and 2.
(1) Stencil Making Condition
The stencils were made by the use of pilot stencil making apparatuses which
satisfied the respective conditions shown in the tables 1 and 2. As the
heat-sensitive stencil material, RISOGRAPH GR MASTER 78W (RISO KAGAKU
CORPORATION, JAPAN) was used. The ambient temperature was 23.degree. C.
(2) Value of V/dp or V/Dp in Formula (1) or (2)
The value of V/dp or V/Dp employed is shown. In accordance with the present
invention, the value should be not smaller than 0.2 and not larger than
10.
(3) Evaluations of the Diameters of the Perforation, the S/N Ratio of the
Area of the Perforation and Influence of Heat Accumulation
As the evaluation of the shape of the perforation, the diameters of the
perforation in the main scanning direction and the sub-scanning direction,
the S/N ratio of the area of the perforation and influence of heat
accumulation were evaluated. The perforation is a separated opening
corresponding to one pixel. The diameters of the perforation in the main
scanning direction and the sub-scanning direction are defined as the
lengths of the orthogonal projections onto lines parallel to the
respective directions. The area of the perforation is defined as the area
of a projection of a penetration in the thermoplastic film of the stencil
material onto the film. The influence of heat accumulation was evaluated
in terms of the ratio (%) of the area of the perforation formed with heat
accumulation to that of the perforation formed without heat accumulation
in one frame.
Specifically, A3 size stencils were made at intervals of about five
minutes. Since there was a sufficient interval, the thermal head was
considered to accumulate no heat at the start of making each stencil. In
this state, A3 size stencils were made on the basis of an image including
a solid image area continuous in the longitudinal direction of the A3 size
stencil material (the sub-scanning direction) and images of an area which
was made immediately after the start of the stencil making (an area at a
distance of not smaller than 5 mm and not larger than 15 mm from the
starting line: will be referred at as "non-heat-accumulation area",
hereinbelow) and an area which was made a certain time after the start of
the stencil making (an area at a distance of not smaller than 300 mm and
not larger than 310 mm from the starting line: will be referred at as
"heat-accumulation area", hereinbelow) were taken by a CCD camera through
an optical microscope. Then by the use of an image analysis package
MacSCOPE (MITANI Commercial Company), 100 penetrations in the film was
taken out by binary-coding.
The average of the diameters of the perforations in the
non-heat-accumulation area was taken as the diameter of the perforation.
As the S/N ratio of the perforation, the S/N ratio of nominal-the-better
of the area of each perforation in the non-heat-accumulation area was
taken. Since the value of the S/N ratio of the perforation differs
according to the measuring condition, it is difficult to unitary evaluate
the S/N ratio of the perforation. However, it has been empirically known
that the S/N ratio should be not smaller 10 db in order to obtain uniform
transfer of ink through the perforation and preferably should be not
smaller than 13 db. If the S/N ratio is smaller than 10 db, a serious
problem arises.
The influence of heat accumulation was obtained by dividing the average of
the areas of the perforations in the heat-accumulation area by that in the
non-heat-accumulation area. In the case of comparative examples where the
perforations were connected in the sub-scanning direction, the value
obtained by the proportion of open area of an area of 10 pixel.times.10
pixel in the heat-accumulation area by that in the non-heat-accumulation
area was shown in parentheses in the tables 1 and 2. The influence of heat
accumulation is less as the value approaches 100 and is more as the value
increases beyond 100.
(4) Printing Conditions
In any of the comparative examples and the embodiments, the stencil
obtained was manually mounted on a printing drum of a stencil printer
RISOGRAPH GR 377 (RISO KAGAKU CORPORATION, JAPAN), and print was made by
the use of RISOGRAPH INK GR-HD under the standard conditions of the
stencil printer (setting for power-on). Wood free paper was used and the
ambient temperature was 23.degree. C.
(5) Printing Density
Optical reflection density at the solid area of the printing was measured
at 10 points on the printing by MACBETH reflection densitometer RD-918S
and the average was calculated.
(6) Uniformity of Solid Area
Microscopic fluctuation in density with position (at cycles of 1 mm or
less) in a solid area due to fluctuation in shape of the perforations was
subjectively evaluated and classified as follows.
.circleincircle.: No density fluctuation was observed.
.largecircle.: Slight density fluctuation was observed but at such a level
that problem would arise neither in solid reproduction of a letter
original nor in tone reproduction of a photographic original.
.DELTA.: Density fluctuation was observed at such a level that no problem
would arise in solid reproduction of a letter original but tone
reproduction of a photographic original would deteriorate.
.times.: Serious density fluctuation was observed at such a level that
solid reproduction of a letter original and tone reproduction of a
photographic original would both deteriorate.
(7) Blur of Thin Letters
The degree of blur (interruption in a pattern to be continuous) of thin
letters in the printing due to fluctuation in shape of the perforations
was subjectively evaluated and classified as follows.
.circleincircle.: No blur was observed.
.largecircle.: Slight blur was observed but at such a level that problem
would arise neither in reproduction of thin letters (black letters on a
white background) of a letter original nor in tone reproduction of
highlights of a photographic original.
.DELTA.: Blur was observed at such a level that no problem would arise in
reproduction of thin letters (black letters on a white background) of a
letter original but tone reproduction of highlights of a photographic
original would deteriorate.
.times.: Serious blur was observed at such a level that reproduction of
thin letters (black letters on a white background) of a letter original
and tone reproduction of highlights of a photographic original would both
deteriorate.
(8) Saturation of Thin Letters
The degree of saturation (loss of the white background between closely
opposed two patterns) in the area of thin letters in the printing due to
fluctuation in shape of the perforations was subjectively evaluated and
classified as follows.
.circleincircle.: No saturation was observed.
.largecircle.: Slight saturation was observed but at such a level that
problem would arise neither in reproduction of thin letters (black letters
on a white background) of a letter original nor in tone reproduction of
shadows of a photographic original.
.DELTA.: Saturation was observed at such a level that no problem would
arise in reproduction of thin letters (black letters on a white
background) of a letter original but tone reproduction of shadows of a
photographic original would deteriorate.
.times.: Serious saturation was observed at such a level that reproduction
of thin letters (black letters on a white background) of a letter original
and tone reproduction of shadows of a photographic original would both
deteriorate.
(9) Offset
The degree of stain on the backside of a printing with ink on the surface
of the immediately preceding printing in a stack of printings was
subjectively evaluated and classified as follows.
.circleincircle.: No offset was observed.
.largecircle.: Slight offset was observed but at such a level that no
problem would arise even if the amount of ink transfer was large and the
printings were acceptable as formal printings.
.DELTA.: Offset was observed at such a level that no problem would arise in
an area of thin letters (black letters on a white background) or a
highlight where the amount of ink transfer was relatively small but stain
was conspicuous in a large solid area where the amount of ink transfer was
relatively large. The printings were acceptable as informal printings
though not acceptable as formal printings.
.times.: Serious offset was observed at such a level that stain was
conspicuous in almost the whole area of the original and the printings
were not acceptable even as informal printings.
TABLE 1
cp 1 cp 2 em 1 cp 3 em 2
cp 4 cp 5 em 3
m/d
resolution (dpi) 300 300 300 300 300
300 300 300
recording 1 1 1 2 2
2 2 2
independent 1 1 1 1 1
2 2 2
d or D (.mu.m) 84.7 84.7 84.7 42.3 42.3
84.7 84.7 84.7
Lx(+ L'x) (.mu.m) 60 15 28 30 16 60
16 30
s/d
resolution (dpi) 300 300 300 600 600
300 300 300
pitch p (.mu.m) 84.7 84.7 84.7 42.3 42.3
84.7 84.7 84.7
length Ly (.mu.m) 250 75 130 150 70
250 75 130
thickness of element (.mu.m) 15 1.5 5 10 3.5
15 1.5 5
conditions
V/dp or V/Dp 24.652 0.185 1.994 19.721 1.718
24.652 0.197 2.136
20% .ltoreq. Lx(+ L'x) .ltoreq. 60% + - .smallcircle. + .smallcircle.
+ - .smallcircle.
Ly/p + - .smallcircle. + .smallcircle. +
- .smallcircle.
t + .smallcircle. .smallcircle.
.smallcircle. .smallcircle. + .smallcircle. .smallcircle.
formula (1) or (2) + - .smallcircle. + .smallcircle. +
- .smallcircle.
target value of proportion of open area (%) 40 40 40 30
30 40 40 40
master making conditions
energy applied (.mu.j) 192 70 93.75 74 41.25
206.4 77 103.125
power applied (mW) 400 200 250 185 125
430 220 275
applying time (.mu.s) 480 350 375 400 330
480 350 375
cycle (ms) 5 5 5 3 3
5 5 5
evaluation of perforations
diameter m/d (.mu.m) 42.3 37.5 60.8 23.9 25.8
44 40.2 62.3
diameter s/d (.mu.m) >84.7 39.3 59.8 >42.3 26.1 >84.7
38.8 58.6
open area (%) 41 17 40 34 30
42 18 39
perf. S/N ratio (db) -- 9.4 13.5 -- 12.7 -- 9.1
13.2
heat accu. (%) (151) 106 115 (131) 103
(155) 108 116
evaluation of printings
density 1.12 0.70 1.14 1.01 1.10
1.09 0.68 1.12
solid uniformity x x .circleincircle. x
.circleincircle. x x .circleincircle.
thin letter blur .DELTA. x .circleincircle. x
.largecircle. .DELTA. x .circleincircle.
thin letter sat. x .circleincircle. .smallcircle.
x .circleincircle. x .circleincircle. .smallcircle.
offset x .circleincircle.
.circleincircle. .smallcircle. .circleincircle. x
.circleincircle. .circleincircle.
TABLE 2
cp 6 cp 7 em 4 cp 8 cp 9
em 5 cp 10 em 6
m/d
resolution (dpi) 300 300 300 400 400
400 600 600
recording 1 1 1 1 1
1 1 1
independent 1 1 1 1 1
1 1 1
d or D (.mu.m) 84.7 84.7 84.7 63.5 63.5
63.5 42.3 42.3
Lx(+ L'x) (.mu.m) 60 15 28 41 11 22.5 30
16
s/d
resolution (dpi) 400 400 400 400 400
400 600 600
pitch p (.mu.m) 63.5 63.5 63.5 63.5 63.5
63.5 42.3 42.3
length Ly (.mu.m) 200 60 110 200 60
100 150 70
thickness of element (.mu.m) 15 1.5 5 10 0.9
5 10 3.5
conditions
V/dp or V/Dp 26.295 0.197 2.250 15.972 0.116
2.191 19.721 1.718
20% .ltoreq. Lx(+ L'x) .ltoreq. 60% + - .smallcircle. + -
.smallcircle. + .smallcircle.
Ly/p + - .smallcircle. + - .smallcircle.
+ .smallcircle.
t + .smallcircle. .smallcircle.
.smallcircle. - .smallcircle. .smallcircle. .smallcircle.
formula (1) or (2) + - .smallcircle. + - .smallcircle.
+ .smallcircle.
target value of proportion of open area (%) 37 37 37 35
35 35 30 30
master making conditions
energy applied (.mu.j) 139.5 59.2 80.5 103.5 33.6
57.8 74 41.25
power applied (mW) 310 185 230 230 120
170 185 125
applying time (.mu.s) 450 320 350 450 280
340 400 330
cycle (ms) 4 4 4 4 4
4 3 3
evaluation of perforations
diameter m/d (.mu.m) 41.5 35.1 57 31.6 25.6
42.5 24.1 25.5
diameter s/d (.mu.m) >63.5 29 46.6 >63.5 30.3 41.8
>42.3 26.2
open area (%) 37 16 37 36 15
35 33 30
perf. S/N ratio (db) -- 9.8 13.3 -- 8.8 13.1
-- 12.8
heat accu. (%) (146) 105 112 (139) 104
107 (133) 102
evaluation of printings
density 109 0.67 1.12 1.03 0.66
10.8 0.99 1.09
solid uniformity x x .circleincircle. x
x .circleincircle. x .circleincircle.
thin letter blur .DELTA. x .circleincircle.
.DELTA. x .circleincircle. x .smallcircle.
thin letter sat. x .circleincircle.
.circleincircle. x .circleincircle. .circleincircle. x
.circleincircle.
offset x .circleincircle.
.circleincircle. .DELTA. .circleincircle. .circleincircle.
.smallcircle. .circleincircle.
As can be seen from the tables 1 and 2, in the case of the embodiment 1,
parts where the pattern was slightly thicker than intended were observed
in evaluation of saturation of thin letters, but at such a level that
problem would arise neither in deciphering thin letters nor in tone
reproduction. The embodiment 1 was excellent in all the other items. In
the case of the embodiment 2, interruption in a pattern to be continuous
was slightly observed in evaluation of blur of thin letters, but at such a
level that problem would arise neither in deciphering thin letters nor in
tone reproduction. The embodiment 2 was excellent in all the other items.
In the case of the embodiment 3, parts where the pattern was slightly
thicker than intended were observed in evaluation of saturation of thin
letters, but at such a level that problem would arise neither in
deciphering thin letters nor in tone reproduction. The embodiment 3 was
excellent in all the other items. The embodiment 4 was excellent in all
the items. The embodiment 5 was excellent in all the items. In the case of
the embodiment 6, interruption in a pattern to be continuous was slightly
observed in evaluation of blur of thin letters, but at such a level that
problem would arise neither in deciphering thin letters nor in tone
reproduction. The embodiment 6 was excellent in all the other items.
In the case of the comparative example 1, the perforations were connected
in the sub-scanning direction. Accordingly, the diameters of the
perforations in the main scanning direction were made smaller to realize
the target proportion of open area, which resulted in perforations
extending in the sub-scanning direction like stripes in the solid area.
Further, though it was impossible to obtain the S/N ratio of the area of
the perforation since the perforations were not separated from each other,
molten resin grounds accumulated on parts of the film which were in a poor
contact with the base film or the heater element due to poor temperature
contrast and/or poor temperature response of the heater element, and local
fluctuation in the proportion of open area was very large. Further since
heat generation in one frame was large and influence of heat accumulation
was very large. Accordingly, reproduction of thin letters and/or fine
patterns largely depended upon the direction (the main scanning direction
or the sub-scanning direction), which resulted in poor pattern
reproduction. Further the large local fluctuation in the proportion of
open area resulted in fluctuation in printing density from position to
position in a solid area. Further, in an area where the proportion of
printing area was large, ink transfer became excessive due to connected
perforations, which resulted in significant offset. Further, due to large
influence of heat accumulation, printing density in a solid area in the
upper part of the printings largely differed from that in a solid area in
the lower part of the printings.
In the case of the comparative example 2, the size of the heater elements
was too small to obtain the target value of the proportion of open area,
and increase in the electric power (e.g., applied energy) resulted only in
promoted deterioration, for instance, in the resistance of the heater
element with the shape of the perforations kept substantially at the
values shown in table 1. Accordingly, the perforations were too small and
the proportion of open area was far smaller than the target value, whereby
the printing density was very poor.
Evaluation of the comparative example 3 was substantially equivalent to
that of the comparative example 1. That is, the perforations were
connected in the sub-scanning direction and accordingly, the diameters of
the perforations in the main scanning direction were made smaller to
realize the target proportion of open area, which resulted in perforations
extending in the sub-scanning direction like stripes in the solid area.
Further, though it was impossible to obtain the S/N ratio of the area of
the perforation since the perforations were not separated from each other,
local fluctuation in the proportion of open area was very large and
influence of heat accumulation was very large. Accordingly, reproduction
of thin letters and/or fine patterns was poor. Further the large local
fluctuation in the proportion of open area resulted in fluctuation in
printing density from position to position in a solid area. Further, due
to large influence of heat accumulation, printing density in a solid area
in the upper part of the printings differed from that in a solid area in
the lower part of the printings.
Evaluation of the comparative example 4 was substantially equivalent to
that of the comparative examples 1 and 3. That is, the perforations were
connected in the sub-scanning direction and accordingly, the diameters of
the perforations in the main scanning direction were made smaller to
realize the target proportion of open area, which resulted in perforations
extending in the sub-scanning direction like stripes in the solid area.
Further, though it was impossible to obtain the S/N ratio of the area of
the perforation since the perforations were not separated from each other,
local fluctuation in the proportion of open area was very large and
influence of heat accumulation was very large. Accordingly, reproduction
of thin letters and/or fine patterns was poor. Further, in an area where
the proportion of printing area was large, offset was severe. Further the
printing density fluctuated from position to position in a solid area.
Further, due to large influence of heat accumulation, printing density in
a solid area in the upper part of the printings largely differed from that
in a solid area in the lower part of the printings.
Evaluation of the comparative example 5 was substantially equivalent to
that of the comparative example 2. That is, the size of the heater
elements was too small to obtain the target value of the proportion of
open area, and increase in the electric power (e.g., applied energy)
resulted only in promoted deterioration of the heater element with the
shape of the perforations kept substantially at the values shown in table
1. Accordingly, the perforations were too small and the proportion of open
area was far smaller than the target value, whereby the printing density
was very poor.
Evaluation of the comparative example 6 was substantially equivalent to
that of the comparative examples 1, 3 and 4. That is, the perforations
were connected in the sub-scanning direction and accordingly, the
diameters of the perforations in the main scanning direction were made
smaller to realize the target proportion of open area, which resulted in
perforations extending in the sub-scanning direction like stripes in the
solid area. Further, though it was impossible to obtain the S/N ratio of
the area of the perforation since the perforations were not separated from
each other, local fluctuation in the proportion of open area was very
large and influence of heat accumulation was very large. Accordingly,
reproduction of thin letters and/or fine patterns was poor. Further, in an
area where the proportion of printing area was large, offset was severe.
Further the printing density fluctuated from position to position in a
solid area. Further, due to large influence of heat accumulation, printing
density in a solid area in the upper part of the printings largely
differed from that in a solid area in the lower part of the printings.
Evaluation of the comparative example 7 was substantially equivalent to
that of the comparative examples 2 and 5. That is, the size of the heater
elements was too small to obtain the target value of the proportion of
open area, and increase in the electric power resulted only in promoted
deterioration of the heater element with the shape of the perforations
kept substantially at the values shown in table 2. Accordingly, the
perforations were too small and the proportion of open area was far
smaller than the target value, whereby the printing density was very poor.
Evaluation of the comparative example 8 was substantially equivalent to
that of the comparative examples 1, 3, 4 and 6. That is, the perforations
were connected in the sub-scanning direction and accordingly, the
diameters of the perforations in the main scanning direction were made
smaller to realize the target proportion of open area, which resulted in
perforations extending in the sub-scanning direction like stripes in the
solid area. Further, though it was impossible to obtain the S/N ratio of
the area of the perforation since the perforations were not separated from
each other, local fluctuation in the proportion of open area was very
large and influence of heat accumulation was very large. Accordingly,
reproduction of thin letters and/or fine patterns was poor. Further, in an
area where the proportion of printing area was large, offset was severe.
Further the printing density fluctuated from position to position in a
solid area. Further, due to large influence of heat accumulation, printing
density in a solid area in the upper part of the printings largely
differed from that in a solid area in the lower part of the printings.
Evaluation of the comparative example 9 was substantially equivalent to
that of the comparative examples 2, 5 and 7. That is, the size of the
heater elements was too small to obtain the target value of the proportion
of open area, and increase in the electric power resulted only in promoted
deterioration of the heater element with the shape of the perforations
kept substantially at the values shown in table 2. Accordingly, the
perforations were too small and the proportion of open area was far
smaller than the target value, whereby the printing density was very poor.
Further the heater elements were small in thickness, 0.9 .mu.m,
fluctuation in shape of the heater elements were very large and the S/N
ratio of shape of the perforations was very poor.
Evaluation of the comparative example 10 was substantially equivalent to
that of the comparative examples 1, 3, 4, 6 and 8. That is, the
perforations were connected in the sub-scanning direction and accordingly,
the diameters of the perforations in the main scanning direction were made
smaller to realize the target proportion of open area, which resulted in
perforations extending in the sub-scanning direction like stripes in the
solid area. Further, though it was impossible to obtain the SIN ratio of
the area of the perforation since the perforations were not separated from
each other, local fluctuation in the proportion of open area was very
large and influence of heat accumulation was very large. Accordingly,
reproduction of thin letters and/or fine patterns was poor. Further, in an
area where the proportion of printing area was large, offset was severe.
Further the printing density fluctuated from position to position in a
solid area. Further, due to large influence of heat accumulation, printing
density in a solid area in the upper part of the printings largely
differed from that in a solid area in the lower part of the printings.
Top