Incremental printing of symbolic information – Thermal marking apparatus or processes – Specific resistance recording element type
Reexamination Certificate
2000-02-25
2001-04-10
Tran, Huan (Department: 2861)
Incremental printing of symbolic information
Thermal marking apparatus or processes
Specific resistance recording element type
C347S206000
Reexamination Certificate
active
06215510
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a thermal head for making a stencil by thermally perforating a stencil material, and more particularly to an improvement of a thick film type thermal head.
2. Description of the Related Art
A thermal head generally comprises a heater element array formed by a plurality of heater elements arranged in a row extending in one direction (this direction is generally referred to as “the main scanning direction”), and when making a stencil, the thermal head is moved along a stencil material in a direction intersecting the main scanning direction (this direction is generally referred to as “the sub-scanning direction”) while selectively energizing the heater elements, thereby thermally perforating the stencil material in an imagewise pattern. Such thermal heads are broadly divided by structure into a thin film type thermal head and a thick film type thermal head.
As shown in
FIGS. 8 and 9
, the thick film type thermal head conventionally comprises a ceramic substrate
85
, a heat insulating layer
82
formed on the ceramic substrate
85
, a plurality of comb-tooth electrodes
84
formed on the heat insulating layer
82
at predetermined intervals to extend in one direction in parallel to each other, and an electric heater strip
81
formed over the comb-tooth electrodes
84
to intersect the electrodes
84
in contact with the electrodes
84
. The direction in which the electric heater strip
81
extends is the aforesaid main scanning direction and each of the parts between adjacent two electrodes
84
forms a heater element, whereby the aforesaid heater element array is formed. The main scanning direction is indicated at X in FIG.
8
and the aforesaid sub-scanning direction is indicated at Y in FIG.
8
. The electric heater strip
81
is, for instance, of ruthenium oxide (RuO
2
), and is formed, for instance, by applying ruthenium oxide solution over the comb-tooth electrodes
84
by screen printing.
In order to improve recording density, the perforating pitch (that is, the distance by which the thermal head is moved in the sub-scanning direction at one time) should be as small as possible, and in order to reduce the perforating pitch, the width (the dimension as measured in the sub-scanning direction Y) of the heat generating area of the heater strip
81
(or each of the heater elements) should be as small as possible.
That is, if the width of the heat generating area of the heater strip
81
is larger than the perforating pitch, the perforations formed side by side in the sub-scanning direction Y will be merged with each other to form an elongated perforation as indicated at
102
in
FIG. 10
(reference numeral
101
in
FIG. 10
denotes a stencil material). When the perforations
102
are merged with each other into an elongated perforation, a large amount of ink flows out through the elongated perforation and an excessive amount of ink adheres to the printing paper, which can result in a phenomenon that the ink penetrates to the back side of the printing paper or the ink is seen from the back side of the printing paper. Accordingly, when the perforating pitch in the sub-scanning direction Y is to be reduced, it is necessary to reduce the width of the heat generating area of the electric heater strip
81
so that discrete perforations
102
can be formed in the sub-scanning direction as shown in FIG.
11
.
In the conventional thick film type thermal head, the electric heater strip
81
generates heat over its entire width W, that is, each heat generating area or each heater element
87
has a length equal to the distance between the adjacent comb-tooth electrodes
84
and a width equal to the width W of the heater strip
81
as shown in FIG.
12
. Accordingly, in order to reduce the width of the each heater element
87
, it is necessary to form a narrower heater strip
81
.
The heater strip
81
is generally formed by applying a paste-like mixture of, for instance, ruthenium oxide powder, glass powder and solvent by squeezing. In this case, the width of application of the paste-like mixture cannot be smaller than a mesh of the screen and the mesh of the screen cannot be smaller than the size of the particles in the paste-like mixture. As a result, it is difficult to form a narrower heater strip
81
. If the particles contained in the paste-like mixture can be smaller in size, the mesh of the screen can be smaller, whereby a narrower heater strip
81
can be formed. However, the particle size is in proportion to the electric resistance of the heater strip
81
and accordingly, reduction in the particle size is limited. Further since the paste-like mixture has a certain viscosity, the mixture is kept in a limited area just after application thereof. However as the time lapses, the mixture flows and spreads outward. This phenomenon also makes it difficult to form a narrower heater strip
81
.
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 type thermal head having a structure which is improved so that the width of each heat generating area or each heater element can be smaller though the heater strip is formed by use of a material and a screen which are the same as those employed in forming the conventional thick film type thermal head.
The thick film type thermal head in accordance with the present invention is characterized in that an electrical insulating layer is formed between the electrodes and the heater strip at least on one side of the heater strip so that the width of the contact area (the dimension as measured in the sub-scanning direction) between the electrodes and the heater strip becomes smaller than the width of the heater strip.
It is preferred that the electrical insulating layer be formed on both sides of the heater strip.
Further it is preferred that the electrical insulating layer be formed so that the width of the contact area between the electrodes and the heater strip becomes smaller than the perforating pitch in the sub-scanning direction.
In accordance with the present invention, the effective width of each heater element can be narrowed without reducing the width of the heater strip, and accordingly, the recording density can be increased without encountering the aforesaid difficulties in reducing the width of the heater strip and without fear that the perforations are merged into an elongate perforation.
REFERENCES:
patent: 4048470 (1977-09-01), Shindo et al.
patent: 5417156 (1995-05-01), Tateishi et al.
patent: 0 867 288 A2 (1998-09-01), None
patent: 063037962 (1988-02-01), None
patent: 06135031 (1994-05-01), None
Nixon & Peabody LLP
Riso Kagaku Corporation
Studebaker Donald R.
Tran Huan
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