Printing – Stenciling – Stencils
Reexamination Certificate
1999-11-17
2002-03-19
Funk, Stephan R. (Department: 2854)
Printing
Stenciling
Stencils
C347S186000, C347S192000, C400S120120
Reexamination Certificate
active
06357348
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a heat-sensitive stencil master making apparatus in which a stencil master is made by imagewise perforating a heat-sensitive stencil master material by a thermal head.
2. Description of the Related Art
There has been known a heat-sensitive stencil master making apparatus in which a thermal head having an array of heater elements is pressed against the thermoplastic film side of a heat-sensitive stencil master material while selectively energizing the heater elements, thereby perforating the thermoplastic film in a pattern representing image data.
FIG. 24
shows an example of such a stencil master making apparatus. In
FIG. 24
, the stencil master making apparatus
90
of this example comprises a thermal head
4
having an array of a plurality of heater elements
5
(only one is visible in FIG.
24
), and a platen roller
3
. A heat-sensitive stencil master material
1
is conveyed in the direction of arrow A when the platen roller
3
is driven by an electric motor (not shown) and passed between the platen roller
3
and the thermal head
4
with the side of a thermoplastic film
1
a
of the stencil master material
1
facing the thermal head
4
. Thus the heater elements
5
of the thermal head
4
are pressed against the thermoplastic film
1
a
of the stencil master material
1
and the thermoplastic film
1
a
is perforated in a pattern representing image data by selectively energizing the heater elements
5
by a head drive means (not shown).
Each of the perforations is formed in the following steps. When a heater element
5
starts to be energized and heated, the temperature of the part of the thermoplastic film
1
a
in contact with the heater element
5
is elevated. Since the temperature of the heater element
5
is the highest at the center thereof, the temperature of the thermoplastic film
1
a
is maximized at the part in contact with the center of the heater element
5
. When the temperature of this part reaches a perforation generation temperature to be described later, a small perforation is generated at this part. The small perforation is enlarged over an area circumscribed by an isothermal line at a shrinkage initiation temperature to be described later. After the heater element
5
is de-energized, the area circumscribed by the shrinkage initiation temperature line once enlarges and then narrows, and accordingly enlargement of the perforation stops.
When perforations are to be formed, each heater element
5
is generally applied with target power (more specifically, a voltage calculated on the basis of the mean resistance for all the heater elements
5
of the thermal head
4
and target power to be applied to the heater element
5
) continuously for a predetermined time as shown in FIG.
25
A. The power applied to each heater element
5
will be referred to as “the heater drive power” and the time for which the heater drive power is applied to the heater element
5
will be referred to as “the duration of heater drive power”, hereinbelow.
When the heater drive power is applied to the heater element
5
, the surface of the heater element
5
has a temperature distribution such that the temperature is the highest at the center of the heater element
5
and lowers as the distance from the center increases as shown in FIG.
26
. The temperature distribution changes depending on the shape and structure of the heater element
5
and the heater drive power and/or the time, and is an important factor which affects the shape of the perforation. In the following description, the temperature at the center of the surface of the heater element
5
is taken as a representative of the temperature of the heater element
5
, and “the temperature of the heater element
5
” as used hereinbelow means the temperature at the center of the surface of the heater element
5
unless otherwise noted.
So long as the shape and/or the structure are the same, the surface temperature distribution is similar, and accordingly, the temperature of the heater element
5
represents the state of heating of the heater element to some extent.
Since the heater drive power is of a square wave as shown in
FIG. 25A
, the temperature of the heater element
5
changes like an exponential function and asymptotically approaches a certain temperature with time as shown in
FIG. 25B
while the heater element
5
is energized. That is, the temperature of the heater element
5
is low at the beginning of application of the heater drive power, is monotonically increased and is maximized at the end of the application.
In order to improve quality of printed images, it is required that the perforations are as uniform as possible in shape. Nonuniformity in shape of the perforations is caused partly for systematic reasons and partly for random reasons. For example, the systematic reasons include the ambient temperature (when the ambient temperature is high, the perforations are enlarged, and vice versa), heat accumulation (the perforations are small at the beginning of stencil master making, and are gradually enlarged as the stencil master making process progresses due to accumulation of heat), common drop (when perforations are formed over a wide area, perforations are apt to become smaller in the end portions than in the middle portion in the main scanning direction, where the line resistance is higher), and the like. The random reasons include fluctuation in the temperature of the heater elements, dispersion of fibers in the support sheet of the stencil master material, nonuniformity in the state of contact between the thermoplastic film of the stencil master material and the heater elements due to surface roughness of the thermoplastic film, and the like. Unlike in other thermal recording such as those using heat-sensitive paper or thermal transfer, fluctuation in perforation size in the heat-sensitive stencil master is greatly affected by the random reasons. Accordingly, it is especially required in heat-sensitive stencil master making that nonuniformity in the perforation size is suppressed. Further improvement in printing durability and accuracy in the printing position, shortening the stencil master making time and the like are required. In order to meet these requirements, there have been made various studies on the material of the thermoplastic film.
For example, as disclosed in Japanese Patent Publication No. 2507612, there has been proposed use of thermoplastic film having two melting peaks in order to stabilize the shape of perforations. That is, by use of such thermoplastic film, generation of perforations is quickened by virtue of the resin component having the lower melting peak and the shape of the perforations is stabilized by virtue of the resin component having the higher melting peak. When a perforation is formed in heat shrinkable film, the perforation is generally enlarged over an area circumscribed by a shrinkage initiation temperature line (an isothermal line at a shrinkage initiation temperature) and fluctuation in size of the perforations is smaller as the temperature gradient near the shrinkage initiation temperature line is larger and as the degree to which the heat-shrinkage factor rises beyond the shrinkage initiation temperature increases. Actually, the heat-shrinkage temperature range of resin whose melting peak temperature is high is in a relatively high temperature range, and accordingly the temperature gradient near the shrinkage initiation temperature line is large. This stabilizes the shape of the perforations. However when the thermoplastic film is formed only of such high melting peak resin, sensitivity to perforation of the thermoplastic film, that is, the performance in forming perforations of a required size with low power, deteriorates. Accordingly in order to keep sufficient the sensitivity to perforation of the thermoplastic film, resin whose melting peak temperature is low is added. However, when the proportion of such low melting peak resin is increased giving precedence to the sensitivi
Irie Yukio
Kinoshita Hideyuki
Nakamura Jun
Nomura Kunio
Oike Hikaru
Funk Stephan R.
Nixon & Peabody LLP
Riso Kagaku Corporation
Studebaker Donald R.
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