Printing – Stenciling – Stencils
Reexamination Certificate
2001-10-02
2003-04-22
Evanisko, Leslie J. (Department: 2854)
Printing
Stenciling
Stencils
C101S128210, C400S120010, C400S120090, C347S171000, C347S188000
Reexamination Certificate
active
06550380
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for stencil plate making of a stencil sheet for stencil printing. More particularly, it relates to a method for stencil plate making of a stencil sheet for stencil printing using a micro porous plastic sheet as a stencil sheet for stencil printing and wherein the micro pores of the plastic sheet are closed by a thermal print head (referred to as TPH hereinafter).
2. Description of the Prior Art
Conventionally, as a sheet (stencil sheet) for stencil printing, a heat-sensitive sheet perforated by infrared radiation or TPH has been known. A sheet made of a thermoplastic film and a porous tissue paper stuck together by adhesive is used in general.
Further, rotary-type and simple press-type are known as a stencil plate printing machine using a heat-sensitive stencil sheet.
These printing machines perform printing by pushing the ink from the tissue paper side of the stencil sheet through pores perforated in the film corresponding to a line area of a print image thereby transferring ink onto print paper.
In the conventional stencil printing system, an improvement in ink drying has been sought because it takes a long time for ink to permeate print paper.
Namely, there are problems such that ink hardly permeates the surface of print paper, making fingers stained upon touching a printed sheet right after printing. Another problem is that when printing with a second color and subsequent colors in multicolor printing or printing on the back of the printed surface of the paper continuously, the ink is transferred to a rubber roller of the printing machine, thereby making the printed sheet stained owing to the ink transferred. The problem causes another disadvantage that a long time (10 to 20 minutes, for example) is needed to move to a further step.
To solve the above problem for obtaining instant dryness, it is effective to enhance ink permeability for print paper using low viscosity ink, to facilitate drying.
However, even if low viscosity ink is used in case that the amount of ink transferred is excessive, its drying worsens Therefore, in the conventional stencil plate printing system, when low viscosity ink is used, in order to suppress the amount of ink transferred, perforation diameter is required to be at least smaller than 20 &mgr;m.
However, when the perforation diameter is decreased as described above, heat element density (resolution) of TPH must be increased to make that a line are not fainted by increasing dot density to be perforated.
This would raise the cost of TPH as well as require level improvement in peripheral techniques such as securing of durability of TPH, yield improvement and increasing film sensitivity of a heatsensitive stencil sheet.
To solve the above problems, the present inventors have proposed a method for stencil plate making which comprising: preparing a stencil plate having a large number of continuous pores in the order of sub-micron (referred to as a micro porous sheet or a sheet hereinafter); and the pores corresponding to a non-line area are closed to obtain an ink impermeable area.
However, there is a problem that when making a stencil plate of a micro porous sheet in the aforementioned method, some pores without being closed (referred to as pinhole hereinafter) which are supposed to be closed by shrinking or melting a sheet, are produced in reality and that ink passes through the pinholes and transferred to printing paper.
A method for coating the surface of the sheet with a releasing agent is proposed to solve the problem. However, there is still a problem that some pinholes are produced even though the aforementioned method is used.
There is another problem that when the sheet is stencil plate made according to the above method, the sheet is shrunk with heat of TPH during stencil plate making, resulting in that the original dimension reproducibility is poor. Further, when the sheet is fed into the printing machine or attached to a printing drum, the sheet is wrinkled.
SUMMARY OF THE INVENTION
Namely, an object of the present invention is to provide a method for stencil plate making in which micro pores of a stencil sheet are closed by applying heat with TPH required for shrinking and melting the surface or up to the inside of the micro porous sheet, and wherein, no pinholes are produced; and excellent dimensional reproducibility of the sheet is obtainable.
To achieve the above object, the applicants have discovered the following conditions: driving conditions for TPH such as heating temperature, current-carrying time period and current-carrying cycle; a pressure condition (referred to as pressure condition for stencil plate making hereinafter) under which a sheet is pressed between TPH and its corresponding platen roller: and a thermal shrinkage condition of the sheet. These conditions allow producing no pinholes or very few pinholes on the sheet.
Further, the applicants have also discovered that when making a stencil plate, the rate of the thermal shrinkage of the micro porous sheet varies depending on TPH driving conditions, the stencil plate making pressure condition and the thermal shrinkage condition.
In other words, the method for stencil plate making of the present invention is characterized in that a stencil sheet is stencil plate made for a micro porous sheet by closing micro pores under the conditions which satisfies the following formulae at the same time.
<Driving Condition for TPH>
−30≦
Tp−Tm≦
300 (° C.) (1)
(wherein Tp represents a heating peak temperature of TPH, and Tm represents a melting temperature (melting point) of the sheet)
10≦
To×
100/
Ts≦
80 (%) (2)
(wherein To represents a current-carrying time period, Ts represents a current-carrying cycle, To×100/Ts represents the ratio of To to Ts)
<Pressure Condition for Stencil Plate Making>
0.1≦P≦1.0 (MPa) (3)
(wherein P represents a pressure for stencil plate making)
<Thermal Shrinkage Condition>
1≦S
Tm-30
≦20 (%) (4)
(wherein S
Tm-30
represents thermal shrinkage ratio at a temperature 30° C. lower than the melting point of the sheet Tm in TMA (thermal mechanical analysis)
Further, the applicants have discovered that a more preferable result can be obtained by controlling the distribution of heating temperature of TPH.
In other words, the present invention is characterized in that the ratio of the size and pitch of the heat element in the direction of main scanning and sub scanning satisfies the following formula (5) and (6). Provided that the direction of main scanning is the direction that heat element of TPH stand in line, and the direction of sub scanning is across the main scanning direction, that is, the direction stencil sheet is fed.
The present invention is characterized in that the ratio of the size of the heat element and the pitch of this heat element in the direction of the main scanning of the thermal print head is:
preferably
42≦
MR
S
/MR
P
≦88 (%) (5)
and more preferably satisfied with
54≦
MR
S
/MR
P
≦88 (%)
and the ratio of the size of the heat element and the pitch of this heat element in the direction of the sub-scanning of the thermal print head is:
preferably
42≦
SR
S
/SR
P
≦519 (%) (6)
and more preferably
54≦
SR
S
/SR
P
≦330 (%)
and furthermore preferably
65≦
SR
S
/SR
P
≦84 (%)
(wherein MR
S
is the size of the heat element In the direction of main scanning and MR
P
is the pitch (length) of the heat element in the direction of main scanning of the thermal print head, SR
S
is the size of the heat element in the direction of sub-scanning and SR
P
is the pitch (length) of the heat element in the direction of sub-scanning of the thermal print head.)
In the case of a thick film type thermal print head, “the size of the heat element in the direction of main scanning” is “the length between electrodes adjacent to each other”, and further, “the pitch (length) of the heat element” is “the pitch (lengt
Kinoshita Hideyuki
Yamamoto Yasuo
Berkowitz Marvin C.
Evanisko Leslie J.
Nath Gary M.
Novick Harold L.
Riso Kagaku Corporation
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