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United States Patent |
6,239,826
|
Oike
|
May 29, 2001
|
Thermal head, method of manufacturing the same, and thermal stencil making
apparatus using the same
Abstract
A thermal head includes a heat radiating plate and an electrical insulating
substrate which is provided with a plurality of resistance heater elements
arranged in a direction over a predetermined length and a plurality of
electrodes for energizing the resistance heater and is integrated with the
heat radiating plate. The substrate is smaller than the heat radiating
plate in coefficient of thermal expansion and is fixed to the heat
radiating plate at a temperature higher than the normal working
temperature range of the thermal head. In the normal working temperature
range of the thermal head, the thermal head is convex toward the
resistance heater in a cross-section taken along a line parallel to the
direction in which the resistance heater elements are arranged due to the
difference in coefficient of thermal expansion between the heat radiating
plate and the substrate.
Inventors:
|
Oike; Hikaru (Amimachi, JP)
|
Assignee:
|
Riso Kagaku Corporation ()
|
Appl. No.:
|
650819 |
Filed:
|
August 30, 2000 |
Foreign Application Priority Data
| Aug 31, 1999[JP] | 11-245918 |
Intern'l Class: |
B41J 002/335 |
Field of Search: |
347/200,205
|
References Cited
Foreign Patent Documents |
61-24465 | Feb., 1986 | JP.
| |
6-115128 | Apr., 1994 | JP.
| |
9-290522 | Nov., 1997 | JP.
| |
Primary Examiner: Tran; Huan
Attorney, Agent or Firm: Nixon Peabody, LLP, Studebaker; Donald R.
Claims
What is claimed is:
1. A thermal head comprising a heat radiating plate and an electrical
insulating substrate which is provided with a plurality of resistance
heater elements arranged in a direction over a predetermined length and a
plurality of electrodes for energizing the resistance heater and is
integrated with the heat radiating plate, wherein the improvement
comprises that
the substrate is smaller than the heat radiating plate in coefficient of
thermal expansion and is fixed to the heat radiating plate at a
temperature higher than the normal working temperature range of the
thermal head so that the thermal head is convex toward the resistance
heater, in a cross-section taken along a line parallel to the direction in
which the resistance heater elements are arranged, in the normal working
temperature range of the thermal head due to the difference in coefficient
of thermal expansion between the heat radiating plate and the substrate.
2. A thermal head as defined in claim 1 in which the end portions of the
thermal head are lower than the middle portion of the same at least by
1/6000 of said predetermined length over which the resistance heater
elements are arranged.
3. A thermal head as defined in claim 1 in which the electrical insulating
substrate is provided with a glaze layer not larger than 60 .mu.m in
thickness and the resistance heater elements are provided on the surface
of the glaze layer.
4. A thermal stencil making apparatus comprising a thermal head and a
platen roller against which the thermal head is pressed with a stencil
material intervening therebetween, wherein the improvement comprises that
the thermal head comprises a heat radiating plate and an electrical
insulating substrate which is provided with a plurality of resistance
heater elements arranged in a direction over a predetermined length and a
plurality of electrodes for energizing the resistance heater and is
integrated with the heat radiating plate, and
the substrate is smaller than the heat radiating plate in coefficient of
thermal expansion and is fixed to the heat radiating plate at a
temperature higher than the normal working temperature range of the
thermal head so that the thermal head is convex toward the resistance
heater, in a cross-section taken along a line parallel to the direction in
which the resistance heater elements are arranged, in the normal working
temperature range of the thermal head due to the difference in coefficient
of thermal expansion between the heat radiating plate and the substrate.
5. A thermal stencil making apparatus as defined in claim 4 which is for
making a high resolution stencil not lower than 600 dpi.
6. A thermal stencil making apparatus as defined in claim 4 in which the
thermal head is pressed against the platen roller at a linear pressure not
lower than 150 g/cm.
7. A method of manufacturing a thermal head comprising a heat radiating
plate and an electrical insulating substrate which is provided with a
plurality of resistance heater elements arranged in a direction over a
predetermined length and a plurality of electrodes for energizing the
resistance heater and is integrated with the heat radiating plate, the
method comprising the steps of
heating a heat radiating plate of metal and an electrical insulating
substrate which is smaller than the heat radiating plate in coefficient of
thermal expansion to a temperature higher than the normal working
temperature range of the thermal head, and
fixing the heated substrate to the heated heat radiating plate and cooling
them so that the thermal head is convex toward the resistance heater, in a
cross-section taken along a line parallel to the direction in which the
resistance heater elements are arranged, in the normal working temperature
range of the thermal head due to the difference in coefficient of thermal
expansion between the heat radiating plate and the substrate.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a thermal head for thermally making a stencil for
use in a stencil printer, a method of manufacturing such a thermal head
and a thermal stencil making apparatus using such a thermal head.
2. Description of the Related Art
There has been known a stencil making apparatus having a stencil making
section such as shown in FIG. 5. The stencil making section comprises a
platen roller 3 having a metal support shaft 3a which is supported for
rotation on a side frame (not shown) at its opposite ends and a thermal
head 2 which is pressed against the platen roller 3 and is moved away from
the platen roller 3 by a head pressing mechanism (not shown).
The thermal head 2 comprises a heat radiating plate 21, a ceramic substrate
22 fixed to the heat radiating plate 21, and a glaze layer 23 which is
fixed to the surface of the ceramic substrate 22 and functions as a heat
accumulating layer. An array of resistance heater elements 24 is formed on
the surface of the glaze layer 23. The heater elements 24 are connected to
electrodes and a drive circuit (which are not shown) and are selectively
energized to thermally perforate a stencil material 4.
When making a stencil by imagewise perforating a stencil material 4, the
stencil material 4 is fed between the thermal head 2 and the platen roller
3, and then the thermal head 2 is pressed against the platen roller 3 with
the stencil material 4 intervening therebetween. With the thermal head 2
thus kept in a close contact with the stencil material 4, the resistance
heater elements 24 are selectively energized to thermally perforate the
stencil material 4. Thereafter, the platen roller 3 is rotated to bring
the thermal head 2 in contact with another part of the stencil material 4
and the resistance elements 24 are selectively energized again to
thermally perforate the stencil material 4. By repeating these steps, a
stencil master is made.
There has been a problem that, since the platen roller 3 is supported only
at opposite ends of the support shaft 3a, the platen roller 3 is deflected
at the middle thereof as shown in FIG. 5 in an exaggerated scale, whereas
the thermal head 2 is normally formed of highly rigid materials and is
hardly deflected. The thermal head 2 cannot be pressed against the platen
roller 3 under a sufficient pressure near the middle of the platen roller
3.
FIG. 6 shows the measured value of the pressure acting between the thermal
head 2 and the platen roller 3 per unit area when the thermal head 2 is
pushed toward the platen roller 3 under a predetermined force by the head
pressing mechanism. As can be seen from FIG. 6, the pressure acting
between the thermal head 2 and the platen roller 3 is low near the middle
of the platen roller 3 as compared with near the ends of the same, which
results in a higher probability of generating defective perforations near
the middle of the stencil.
When the pressure under which the thermal head 2 is pressed against the
platen roller 3 is reduced in order to suppress deflection of the platen
roller 3, the probability of generating defective perforations is
increased over the entire area of the stencil, which can result in
deterioration in printing density.
Recently, there is a tendency to make larger the stencil, and, as the size
of the stencil increases, the platen roller 3 must be larger in length,
which results in an increased probability of generating defective
perforations near the middle of the stencil.
There has been proposed a thermal stencil making apparatus in which a
thermal head convex near the middle is used in order to suppress reduction
in pressure between the platen roller 3 and the thermal head 2 due to
deflection of the platen roller 3.
Conventionally, since such a convex thermal head has been formed, for
instance, by pressing a convex heat radiating plate and fixing a ceramic
substrate provided with resistance heater elements to the convex heat
radiating plate, the degree of convexity of the thermal head obtained is
governed by the state in which the ceramic substrate is fixed to the heat
radiating plate, which makes it very difficult to obtain a desired degree
of convexity of the thermal head.
Further, there has been known a convex thermal head which is formed by
fixing a ceramic substrate to a flat heat radiating plate and then
applying a pressure to the assembly of the heat radiating plate and the
substrate to deform the assembly into a convex. However this method is
disadvantageous in that it is necessary to control the pressure to be
applied to the assembly according to the state in which the ceramic
substrate is fixed to the heat radiating plate and accordingly it is very
difficult to control the pressure to obtain a desired degree of convexity
of the thermal head.
Further, intention to quickly deform the assembly of the heat radiating
plate and the substrate into a convex is apt to result in breakage of the
ceramic substrate and/or the glaze layer on the substrate. When the
assembly is to be deformed by application of a pressure for a long time,
though fear of breakage of the ceramic substrate and/or the glaze layer on
the substrate is suppressed, productivity of the thermal head lowers and
accordingly the manufacturing cost of the thermal head increases.
SUMMARY OF THE INVENTION
In view of the foregoing observations and description, the primary object
of the present invention is to provide a thermal head which has a degree
of convexity proper to compensate for the aforesaid deflection of the
platen roller and can be manufactured at low cost.
Another object of the present invention is to provide a method of
manufacturing such a thermal head.
Still another object of the present invention is to provide a thermal
stencil making apparatus using such a thermal head.
In accordance with a first aspect of the present invention, there is
provided a thermal head comprising a heat radiating plate and an
electrical insulating substrate which is provided with a plurality of
resistance heater elements arranged in a direction over a predetermined
length and a plurality of electrodes for energizing the resistance heater
and is integrated with the heat radiating plate, wherein the improvement
comprises that
the substrate is smaller than the heat radiating plate in coefficient of
thermal expansion and is fixed to the heat radiating plate at a
temperature higher than the normal working temperature range of the
thermal head so that the thermal head is convex toward the resistance
heater, in a cross-section taken along a line parallel to the direction in
which the resistance heater elements are arranged, in the normal working
temperature range of the thermal head due to the difference in coefficient
of thermal expansion between the heat radiating plate and the substrate.
It is preferred that the end portions of the thermal head be lower than the
middle portion of the same at least by 1/6000 of said predetermined length
over which the resistance heater elements are arranged.
The electrical insulating substrate may be provided with a glaze layer not
larger than 60 .mu.m in thickness on the surface on which the resistance
heater elements are provided.
In accordance with a second aspect of the present invention, there is
provided a thermal stencil making apparatus comprising a thermal head and
a platen roller against which the thermal head is pressed against with a
stencil material intervening therebetween, wherein the improvement
comprises that
the thermal head comprises a heat radiating plate and an electrical
insulating substrate which is provided with a plurality of resistance
heater elements arranged in a direction over a predetermined length and a
plurality of electrodes for energizing the resistance heater and is
integrated with the heat radiating plate, and
the substrate is smaller than the heat radiating plate in coefficient of
thermal expansion and is fixed to the heat radiating plate at a
temperature higher than the normal working temperature range of the
thermal head so that the thermal head is convex toward the resistance
heater, in a cross-section taken along a line parallel to the direction in
which the resistance heater elements are arranged, in the normal working
temperature range of the thermal head due to the difference in coefficient
of thermal expansion between the heat radiating plate and the substrate.
The thermal stencil making apparatus of the present invention is especially
useful when making a high resolution stencil not lower than 600 dpi.
Further, it is preferred that the thermal head is pressed against the
platen roller at a linear pressure not lower than 150 g/cm.
In accordance with a third aspect of the present invention, there is
provided a method of manufacturing a thermal head comprising a heat
radiating plate and an electrical insulating substrate which is provided
with a plurality of resistance heater elements arranged in a direction
over a predetermined length and a plurality of electrodes for energizing
the resistance heater and is integrated with the heat radiating plate, the
method comprising the steps of
heating a heat radiating plate of metal and an electrical insulating
substrate which is smaller than the heat radiating plate in coefficient of
thermal expansion to a temperature higher than the normal working
temperature range of the thermal head, and
fixing the heated substrate to the heated heat radiating plate and cooling
them so that the thermal head is convex toward the resistance heater, in a
cross-section taken along a line parallel to the direction in which the
resistance heater elements are arranged, in the normal working temperature
range of the thermal head due to the difference in coefficient of thermal
expansion between the heat radiating plate and the substrate.
As the electrical insulating substrate, an electrical insulating plate such
as a ceramic plate may be used. The electrical insulating substrate may be
provided with a glaze layer on the surface on which the resistance heater
elements are provided. In this case, the glaze layer may be provided
either over the entire area of the surface of the substrate or only a part
of the same.
Thus, in accordance with the present invention, the thermal head is made
convex by a difference in coefficient of thermal expansion between the
heat radiating plate and the substrate. That is, when a substrate which is
smaller in coefficient of thermal expansion than a heat radiating plate is
fixed to the heat radiating plate at a temperature higher than the normal
working temperature range of the thermal head, and the assembly of the
heat radiating plate and the substrate is cooled to the normal working
temperature range, the assembly is deformed to a smooth convex which is
convex toward the surface of the substrate remote from the heat radiating
plate, that is, the surface on which the resistance heater elements are
provided, due to the difference in coefficient of thermal expansion, i.e.,
due to a so-called bimetal effect.
The degree of convexity of the thermal head can be easily controlled by
suitably selecting the temperature at which the substrate is fixed to the
heat radiating plate and accordingly a convex thermal head in a desired
convexity symmetrical in the direction of arrangement of the resistance
heater elements can be obtained. Further, since no external force is
applied to the substrate during production of the thermal head, the
substrate and/or the glaze layer cannot be broken and a convex thermal
head can be obtained without necessity of a long processing time, a
thermal head having a desired degree of convexity can be easily produced
at low cost. Further, the glaze layer may be not larger than 60 .mu.m in
thickness.
When the degree of convexity of the thermal head is such that the end
portions of the thermal head are lower than the middle portion of the same
at least by 1/6000 of the predetermined length over which the resistance
heater elements are arranged, even a large size stencil can be made
without deterioration of perforations near the middle thereof.
When a stencil of a resolution not lower than 600 dpi is made, the thermal
head is pressed against the platen roller under a high pressure (e.g., a
linear pressure of not lower than 150 g/cm) and the platen roller is
deflected as described above, which results in unsatisfactory perforations
near the middle of the stencil. In accordance with the present invention,
since the thermal head is convex at the middle thereof, the resistance
heater elements can be kept in close contact with the stencil material
even near the middle of the platen roller even if the platen roller is
pressed by the thermal head under a high pressure and is deflected to be
concave near the middle thereof, whereby generation of defective
perforations can be suppressed and a high quality stencil can be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic perspective view showing a stencil making apparatus
in accordance with an embodiment of the present invention,
FIG. 2 is a schematic perspective view showing the thermal head employed in
the stencil making apparatus,
FIGS. 3A to 3C are cross-sectional views for illustrating the manufacturing
process of the thermal head,
FIG. 4 is a graph showing the measured value of the pressure acting between
the thermal head employed in the stencil making apparatus of the
embodiment and the platen roller per unit area when the thermal head is
pressed against the platen roller under a predetermined force,
FIG. 5 is a fragmentary view for illustrating a problem in a conventional
thermal head, and
FIG. 6 is a graph showing the measured value of the pressure acting between
the conventional thermal head and the platen roller per unit area when the
thermal head is pressed against the platen roller under a predetermined
force.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1, a stencil making apparatus in accordance with an embodiment of
the present invention comprises a thermal head 1 and a platen roller 3.
The thermal head 1 is pressed against the platen roller 3 at a linear
pressure of 150 g/cm and is moved away from the platen roller 3 by a head
pressing mechanism (not shown). A stencil material 4 is fed between the
thermal head 1 and the platen roller 3 by a conveyor roller not shown.
The thermal head 1 comprises a heat radiating plate 11 of metal such
aluminum and a ceramic substrate 12 fixed to the heat radiating plate 11,
and the platen roller 3 comprises a cylindrical hard rubber roller
supported by a support shaft 3 extending through the hard rubber roller
along the longitudinal axis thereof. The platen roller 3 is rotated by a
drive mechanism (not shown) to convey the stencil material 4 in
synchronization with drive of the thermal head 1.
As shown in detail in FIG. 2, the ceramic substrate 12 is fixed to the heat
radiating plate 11 by a thermosetting adhesive layer 13. A glaze layer 14
60 .mu.m thick is formed on the ceramic substrate 12, and a plurality of
resistance heater elements 15 are formed on the glaze layer 14 arranged in
a row in the longitudinal direction of the substrate 12. The glaze layer
14 is of glass and functions as a heat accumulating layer. The resistance
heater elements 15 are arranged at a density corresponding to resolution
of 600 dpi. The length L over which the resistance heater elements 15 are
arranged is about 300 mm when an A3 size stencil is to be made.
Each of the resistance heater elements 15 is connected to a pair of
electrodes 16 extending in a direction substantially perpendicular to the
direction of arrangement of the resistance heater elements 15 and the
resistance heater elements 15 are selectively energized to generate heat
and thermally perforate the stencil material 4. Though not clear in FIG.
2, the thermal head 1 is convex toward the surface of the glaze layer 14
on which the heater elements 15 are formed in such an extent that the
middle portion of the surface of the glaze layer 14 is higher than the end
portions thereof by about 0.05 mm.
The manufacturing process of the thermal head 1 will be described with
reference to FIGS. 3A to 3C, hereinbelow. As shown in FIG. 3A, a
thermosetting adhesive layer 13 is formed on a heat radiating plate 11 and
a ceramic substrate 12 provided with a glaze layer 14, resistance heater
elements 15 and electrodes 16 is superposed on the adhesive layer 13. The
heat radiating plate 11 is slightly shorter than the ceramic plate 12.
Then the assembly of the heat radiating plate 11 and the ceramic substrate
12 is left in an oven at 100.degree. C. for two hours. At this time, the
thermosetting adhesive layer 13 is gradually set and the ceramic substrate
12 is bonded to the heat radiating plate 11. Since the coefficient of
thermal expansion of the ceramic substrate 12 is about 10.times.10.sup.-6
/.degree. C. and the coefficient of thermal expansion of the aluminum heat
radiating plate 11 is about 2300.times.10.sup.-6 /.degree. C., the ceramic
substrate 12 is bonded to the heat radiating plate 11 by the adhesive
layer 13 with the heat radiating plate 11 expanded to a length
substantially equal to the ceramic substrate 12 at a high temperature of
100.degree. C. as shown in FIG. 3B.
Then the assembly is taken out from the oven and is left stand at a room
temperature (23.degree. C.), whereby the heat radiating plate 11 and the
ceramic substrate 12 are cooled to the room temperature.
When cooled to the room temperature, the heat radiating plate 11, which is
larger in coefficient of thermal expansion is, contracts more than the
ceramic substrate 12, and accordingly, as the temperature of the assembly
lowers, the assembly (thermal head 1) curls toward the heat radiating
plate 11 and a convex thermal head 1 which is convex toward the surface of
the ceramic substrate 12 on which the resistance heater elements 15 are
provided is obtained as shown in FIG. 3C. In this particular embodiment,
the middle portion of the surface of the glaze layer 14 on the ceramic
substrate 12 is higher than the end portions thereof by about 0.05 mm
(indicated at h in FIG. 3C).
The degree of convexity h can be controlled by controlling the temperature
at which the thermosetting adhesive layer 13 is set, which should be set
higher than the upper limit of the normal working temperature range of the
thermal head 1 by at least 60.degree. C.
FIG. 4 is a graph showing the measured value of the pressure acting between
the thermal head 1 manufactured in the manner described above and the
platen roller 3 per unit area when the thermal head 1 is pressed against
the platen roller 3 under a predetermined force.
As can be understood from FIG. 4, the pressure is nor reduced even at the
middle portion of the thermal head 1 and accordingly, generation of
defective perforations near the middle of the stencil material 4 can be
suppressed. Further, since the thermal head 1 has a convexity symmetrical
about the middle thereof in the direction of arrangement of the resistance
heater elements 15, the pressure between the thermal head 1 and the platen
roller 3 can be substantially uniform over the entire length L over which
the resistance heater elements 15 are arranged, whereby the stencil
material 4 can be perforated in a desirable manner.
As can be understood from the description above, in accordance with the
present invention, a convex thermal head can be produced by only bonding a
substrate which is smaller in coefficient of thermal expansion than a heat
radiating plate to the heat radiating plate at a temperature higher than
the normal working temperature range of the thermal head, and cooling the
assembly of the heat radiating plate and the substrate to the normal
working temperature range. Further, the degree of convexity of the thermal
head can be easily controlled by suitably selecting the temperature at
which the substrate is bonded to the heat radiating plate and no external
force is applied to the substrate during production of the thermal head.
Accordingly, the substrate and/or the glaze layer cannot be broken and a
convex thermal head can be obtained without necessity of a long processing
time, whereby a thermal head having a desired degree of convexity can be
easily produced at low cost.
Further, since the assembly of the heat radiating plate and the ceramic
substrate gradually curls in the normal working temperature range of the
thermal head, a convexity which is smooth and substantially uniform in the
direction of arrangement of the resistance heater elements can be
obtained. Accordingly, the resistance heater elements can be kept in close
contact with the stencil material even near the middle of the platen
roller even if the platen roller is pressed by the thermal head under a
high pressure (e.g., 150 g/cm) and is deflected to be concave near the
middle thereof, whereby generation of defective perforations can be
suppressed and a high quality stencil can be obtained.
Needless to say, the present invention can be applied to both a thick film
thermal head and a thin film thermal head.
Further, though, in the embodiment described above, the ceramic substrate
and the heat radiating plate are bonded together by thermosetting
adhesive, they may be bonded in any manner so long as it can be bond them
at a high temperature.
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