JPH11157041A - Printing block and printing block imaging method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To demonstrate simply imaging of a printing block by a thermal head. SOLUTION: The inkphilic nature or inkphobic nature of a printing block 3 is provided by removing a surface layer 14 or recesses and projections are formed by removing the surface layer 14, and the printing block 3 of the melting points or the thermal decomposition temperature or the ignition temperature of a second layer 13 or a layer 15 of bonding the surface layer 14 with the second layer 13 lower than the softening point or the thermal decomposition temperature of the surface layer 14 of the printing block 3 is used, and the second layer 13 or the bonding layer 15 is melted or decomposed by the temperature layer and then the softening point or the thermal decomposition temperature of the surface layer 14 by using a thermal head on which a plurality of heat elements are arranged on one line leaving the surface layer 14 as its is to carry out the imaging.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to a printing plate made using digitally controlled thermal output and a printing plate imaging method.

[0002]

2. Description of the Related Art Printing techniques using a printing plate include letterpress printing, gravure printing, and flat plate printing (offset printing). In these printing methods, a printing plate usually mounted on a cylinder is used to transfer ink to a desired pattern.

In letterpress printing, an image pattern is represented in the form of raised areas on a printing plate, which receive the ink and press it to transfer the ink onto a recording medium.

[0004] In gravure printing, in contrast to letterpress printing, an image pattern is formed by a series of wells, that is, recesses, which receive ink and express the density by the capacity of the recesses. Excess ink is removed by a doctor blade prior to contact between the plate and the recording medium.

[0005] In the case of offset printing, the image is printed on a printing plate by ink receptivity (lipophilicity, lipophilicity for oil-based ink) and repellency (ink-phobic, ink-repellent,
(If the ink is oily, hydrophilic). Thus, in a dry printing system (waterless printing system), the printing plate is simply inked and comes into contact with an ink-philic, rubber-like intermediate medium called a blanket to transfer the image to the blanket.
In a wet lithographic printing system (a printing system with water), the non-image area is hydrophilic, and water is applied to the printing plate prior to inking to develop ink repellency. Thereafter, the printing plate is inked, and thereafter the image is transferred onto a recording medium in the same manner as in a dry system.

On the other hand, multicolor printing requires a plurality of printing plates, and the above-described print transfer step is repeated a plurality of times to obtain multicolor printing. Each such printing plate is typically prepared photographically as described below.

In order to make a wet plate using a typical negative working subtractive method, first, an original is photographed to make a photographic negative film.
This negative film is placed on an aluminum plate with a moisture-receptive oxidized surface coated with a photopolymer and exposed to light (such as radiation) through the negative, exposing areas (dark areas of the original, print areas) Cures to a durable lipophilic state. The printing plate is then subjected to a development treatment to remove uncured areas of the coating (non-image areas, background areas of the original), exposing the hydrophilic surface of the aluminum plate.

[0008] The same applies to making a letterpress (letterpress).
The difference from the above is that the thickness of the cured region of the coating is several millimeters, and the non-image portion does not need to be ink-phobic because the unevenness is large when the uncured region is removed. .

In the case of gravure printing, in contrast to the case of the above-described letterpress, deep concave portions (30 to 40 μm) are provided with ink receptivity, and the convex portions (surfaces) are scraped cleanly by a doctor blade. A degree of ink sparseness or smoothness is provided. Further, there is a method in which a halftone dot is combined with a not-so-deep concave portion called a halftone gravure.

[0010] A similar photographic process is used to make dry printing plates, but the printing plates typically have an ink-repellent, such as silicone, coated on a photosensitive layer. It has a surface layer, and the photosensitive layer itself is coated on a substrate of suitable stable aluminum or the like. Upon exposure to actinic radiation, the photosensitive layer cures and breaks its bond to the surface layer. After exposure, treatment is performed to deactivate the light response of the photosensitive layer in the unexposed areas and increase the bonding of the surface layer to these areas. Immersion of the exposed plate in a developer solution results in dissolution of the surface layer at the location of the radiation-receiving plate surface, thereby exposing the cured ink-receiving photosensitive layer.

Traditionally, printing plates have been made in this manner, but the photographic plate making process is time consuming and requires appropriate equipment and equipment to perform the necessary chemical steps. In addition, a large amount of environmental pollutants such as a developing waste liquid is discharged as a waste liquid.

[0012] To avoid these drawbacks, a number of electronic alternatives have been developed for printing plate imaging. In these systems, a digitally controlled device changes the ink receptivity or shape of a blank with a pattern representing the image to be printed.
Such imaging devices have a pulsed source of electromagnetic radiation generated by one or more laser or non-laser sources, and the electromagnetic radiation energy causes a chemical change in the blank. This eliminates the need for a photographic negative film and solves the problems associated with the photographic process. Moreover, due to the recent increase in the sensitivity of the printing plate material, a high-power laser source is not required, and printing plate imaging with a light source of several W or less is possible.

[0013]

In the above-mentioned electronic method, the printing plate imaging takes a long time with a light source of several watts or less.
In the technique disclosed in Japanese Patent Application Laid-Open No. H10-209, a plurality of laser light sources are arranged to reduce the time. However, arranging a plurality of laser light sources has the following problems. Very high precision is required for the positional accuracy of each of the laser light sources. High-precision correction of variations in the output of each laser light source is required. The life of each laser light source varies. If the optical system is contaminated, the efficiency is extremely reduced, and operations such as replacement and cleaning are difficult. The influence of fluctuations in the temperature inside the apparatus is great, and it is necessary to correct each laser light source.

Due to such causes, a stripe pattern appears in a solid image. Further, the efficiency of converting the energy of the light source into heat energy is not 100%, the output of the light source is unnecessarily increased, and further, the cost cannot be reduced.

[0015]

According to the first aspect of the present invention,
A printing plate that develops ink-philicity or ink-phobicity by removing the surface layer or forms irregularities by removing the surface layer, which is higher than the softening point or thermal decomposition temperature of the surface layer of the printing plate. Using a printing plate having a lower melting point, thermal decomposition temperature, or ignition temperature of the second layer or the adhesion layer between the surface layer and the second layer, and using a thermal head in which a plurality of heating elements are arranged in a line, The second layer or the adhesive layer is melted or decomposed at a temperature lower than the softening point or the thermal decomposition temperature of the surface layer, and imaging is performed while leaving the surface layer. Therefore, since the heating element of the thermal head is used, a large number of heat sources can be easily arranged with high accuracy, and the speed can be increased.
It is easy to compensate for variations in each output, easy to replace, and easy to correct for the effects of fluctuations in the machine temperature and each temperature unevenness. As a result, there is no solid pattern in a solid image and high image quality Can be printed, and the cost can be reduced. Further, since the heat energy of the heating element is directly used, the efficiency is close to 100%, the output is not increased unnecessarily, and energy can be saved. Furthermore, even if the printing plate is made of a material from which dust such as sticky substances or ions is generated by heating, when the thermal head comes into contact with the surface layer and performs imaging, the surface layer does not peel off, thus damaging the thermal head. There is no dust, no dust adheres, and the image quality is maintained and the life is extended. Even if dust adheres to the thermal head, the user can perform maintenance such as wiping by hand without any special technique.

According to a second aspect of the present invention, the difference between the softening point or the thermal decomposition temperature of the surface layer and the melting point or the thermal decomposition temperature of the second layer or the adhesive layer between the surface layer and the second layer is 50 ° C. or more. The thickness of the surface layer is 1 to 12 μm, the specific heat × density is 0.8 cal / (cm ³ ℃) or less, and the thermal conductivity is 1 × 10 to ⁴ cal.
/ (Cms ° C.) or more and the thermal diffusivity is 1 × 10 to ⁴ cm ² / s or more. Therefore, the thermal efficiency of the surface layer is improved, and the thermal efficiency during printing plate imaging is improved.

According to a third aspect of the present invention, the thickness of the layer next to the layer melted or decomposed by the thermal head is 1 to 1
In 0 .mu.m, specific heat × density 0.2cal / (cm ³ ℃) above, the thermal conductivity of at ^{1 × 10 ~ 2 cal / (} cms ℃) or less, the thermal diffusivity is 5 ×
It was set to 10 to ² cm ² / s or less. Therefore, heat conduction to portions other than the heating section can be suppressed to the minimum necessary, and the thermal efficiency at the time of printing plate imaging is improved.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described with reference to the drawings. First, what is shown in FIG. 1 is a thermal plate imaging apparatus 1. This thermal plate imaging apparatus 1 includes at least one thermal head 2. The printing plate 3 is provided on the right side of the thermal head 2 in the form of a roll or a sheet laminated thereon, and has an appropriate speed (0) from the right side of the thermal head 2 by a transporting device such as a platen 4 or a belt transporting device. .5 to 10 inches / second) and transported. When the printing plate 3 is supplied, the thermal head 2 is driven at a predetermined image timing, and thereafter, is wound on a winding roll (not shown), stacked on a stacker, or supplied to a plate cylinder or the like of a printing apparatus. It is set automatically. Then, after being wound around the plate cylinder of the printing apparatus, the printing process starting from the known inking is executed as described above.

The thermal head 2 has the shape shown in FIG. 2, and is an end face head 6 in which the heating elements 5 are arranged at the corners. The end face head 6 may be arranged on the edge of the flat part.
The thermal head 2 having such a heating element 5 is driven by a drive circuit 7 shown in FIG. This drive circuit 7
Will be described below. The A block includes R ₁ , formed in an array on the substrate of the thermal head 2 as described above.
The heating element 5 is represented by R ₂ ... R ₆₄ . Since these heating elements 5 have a predetermined resistance temperature coefficient, it is possible to calculate the temperature of each heating element 5 by detecting each resistance value. The B blocks are switching elements 8 arranged in an array for controlling the energization of the heating elements 5. The C block is a control circuit that controls the switching element 8 according to a print signal, and is controlled by a clock pulse (CL), a latch pulse (XLT), and a strobe pulse (XSTB). 10 and a gate circuit 11. In addition, the D block, according to the print signal,
Immediately after printing is performed, control is performed by the changeover switch QM so that a current flows sequentially through the heating elements 5 to the detection resistor r. That is, a detection resistor r serving as a resistance element is arranged in series on the common electrode side of the heating element 5 of the thermal head 2, and the resistance value of the heating element 5 is determined by the heating element 5 and a voltage at which a current flowing through the detection resistor r is generated. Is detected.

Next, FIG. 4 shows a configuration of a typical printing plate 3. First, a base 12 and an absorbing layer 13 capable of absorbing infrared radiation
And a surface layer 14. An adhesive layer 15 having a predetermined thickness is interposed between the absorption layer 13 and the surface layer 14.
The base 12 is made of a strong, stable and flexible polymer film, paper, a metal sheet, or the like.
A polyester film having a thickness of about 0.18 mm is used. When metal is used, aluminum is preferred. In the case of paper, water resistance, dimensional stability, and strength are imparted by a polymer material or the like. Absorbing layer 13
Is generally composed of a polymer having absorption characteristics in the infrared region or a polymer in which a component that absorbs is dispersed or dissolved, but in the present embodiment, the thermal decomposition temperature or the melting temperature or the surface The peeling temperature or ignition point at the interface with the layer only needs to be lower than the thermal decomposition temperature or melting temperature of the surface layer, and the absorption performance is limited by the thermal properties.

The substrate 12 and the surface layer 14 have different affinities for ink or an ink-phobic liquid.
For example, the surface layer 14 is an ink-repellent silicone polymer that repels ink, and the substrate 12 is lipophilic polyester or aluminum. The result is a dry (waterless) printing plate. In the case of a wet (with water) printing plate, the surface layer 14 is a hydrophilic material such as polyvinyl alcohol, and the base 12 is lipophilic and hydrophobic.

The printing plate 3 having such a configuration is heated by the heating element 5 of the thermal head 2 using the thermal head 2 by the thermal plate imaging apparatus 1 shown in FIG. At a lower temperature, the absorption layer 13 or the adhesive layer 15 is melted or decomposed to perform plate imaging (plate printing). Residues of the surface layer 14 and the absorbing layer 13 or the adhesive layer 15 generated at this time may be peeled off by cleaning and peeling means after imaging, but as a feature of this embodiment, they can be used for printing as they are. . In this case, the absorbing layer 13 that melts or decomposes is recognized as the second layer.

The operation of the drive circuit 7 during the plate imaging will be described with reference to FIG. The print data SI is input to the shift register 9 under the control of the clock pulse CL. Next, the data is transferred to the latch circuit 10 under the control of the latch pulse XLT. The latched data is supplied to the gate circuit 1 by the strobe pulse XSTB.
1 is opened at the timing shown in FIG.
The power supply V is applied to the heating element 5 by the switching element 8 according to
The heat is applied by the HD. Immediately after the heat is generated, the switching element QM of the D block is turned off according to the mode signal in the timing chart, and when the detect pulse of the print data SI is input, the switch pulse is sequentially input to the shift register 9 and the latch circuit 10 by the clock pulse CL.
Switching element 8 sequentially by strobe pulse XLT
Is turned on, and a voltage is applied to the heating element 5. At that time, by detecting the voltage generated at both ends of the detection resistor r when a current flows through the detection resistor r, the temperature of the heating element 5 is calculated from the resistance value of each heating element 5 and the respective resistance temperature coefficient (TRC). Is calculated and input to a predetermined storage. Then, the temperature of each heating element 5 is fed back to the control circuit, and when the print data SI of the next line is input, each heating element 5 is controlled so as to generate heat to a predetermined temperature. Therefore, even if each heating element 5 has a variation such as a resistance value, the heating element 5 is controlled to generate heat at a constant temperature.

The printing plate 3 thus imaged has a portion where the surface layer 14 is fixed to the substrate 12 via the adhesive layer 15 and the absorbing layer 13, and a portion where the surface layer 14 has the absorbing layer 13 or the adhesive layer 13. Since 15 does not exist, there is a portion that is free from the base 12. Therefore, when used in a printing apparatus, there is a portion where the surface layer 14 is removed by receiving a pressure between the printing cylinder and the impression cylinder, and the printing plate 3 includes a portion where the surface layer 14 is left and a surface layer. 14 will have the removed portion. Thereby, the surface layer 14
The affinity of the ink with the substrate 12 or the absorption layer 13 differs from that of the ink, and is used as a printing plate 3 in offset printing. Further, in the case of letterpress printing or gravure printing using the unevenness of the printing plate 3, the portion where the surface layer 14 exists is convex, and the portion where the absorbing layer 13 or the adhesive layer 15 is removed is concave. Is done.

FIG. 5 shows a modification of the printing plate 3, in which a surface layer 14 is formed on a substrate 12 via an adhesive layer 15. That is, the absorption layer 13 in FIG. 4 does not exist. In this case, the surface layer 14 is heated by the heating element 5 of the thermal head 2.
Adhesive layer 1 at a temperature lower than the softening point or thermal decomposition temperature of
5 is melted or decomposed to perform plate imaging (printing plate). Therefore, the material is selected as a material in which the surface layer 14 and the base 12 have different affinity for ink, and is used as the printing plate 3 in offset printing. In the case of letterpress printing or gravure printing, a portion where the surface layer 14 exists is convex, and a portion where the adhesive layer 15 is removed is concave. In this case,
The substrate 12 is recognized as a second layer.

As a method of manufacturing such a printing plate 3, a case of a positive working dry printing plate 3 formed of a surface layer 14 made of silicone, a substrate 12 made of polyester, and an absorbing layer 13 made of nitrocellulose will be described. Nitrocellulose has a thermosetting crosslinking property, the preparation is 14 parts of nitrocellulose, 2 parts of hexamethoxymethylmelamine,
The coating main component is 236 parts of 2-butanone (methyl ethyl ketone). When infrared absorption is required, an absorbing component such as a metal oxide or carbon, or a mixture thereof is added and dispersed therein. In addition, nitrocellulose ignites at about 300 ° C, but the nitro type has a different ignition point from 100 ° C to several hundred ° C depending on the material,
By providing a difference of 50 ° C. or more as compared with the decomposition temperature of silicone of 400 ° C., separation between the interface and the substrate 12 can be realized without separating the surface layer 14. If the temperature difference is 50 ° C. or less, the surface layer 14 may be decomposed or melted, causing dust to adhere to the thermal head 2 or damaging the thermal head 2 by ions or the like, resulting in uneven image. . Furthermore, the surface layer 1
In order to suitably realize peeling between the interface and the substrate 12 without peeling the surface layer 4, the softening point or the thermal decomposition temperature of the surface layer 14 and the second layer (absorbing layer 13 or substrate 12) or the surface layer 14 The difference between the melting point or the thermal decomposition temperature of the adhesive layer 15 and the second layer is 50 ° C. or more, the thickness of the surface layer 14 is 1 to 12 μm, and the specific heat × density is 0.8 cal / (cm ³ ° C.) or less. The thermal conductivity is preferably 1 × 10 to ⁴ cal / (cms ° C.) or more, the thermal diffusivity is preferably 1 × 10 to ⁴ cm ² / s or more, and the thickness of the surface layer 14 and the condition of the thermal characteristics are correlated. For example, if the thickness is 1μ
m, the specific heat × density is 0.4 cal / (cm ³ ° C) or less,
The thermal conductivity may be 1 × 10 to ³ cal / (cms ° C.) or more, and the thermal diffusivity may be 1 × 10 to ³ cm ² / s or more. In this way, heat energy can be transmitted to the inside as efficiently as possible by the surface layer 14, and the energy efficiency required for plate imaging is improved.

Further, in order to preferably realize peeling between the interface and the substrate 12 without peeling the surface layer 14, the thickness of the layer next to the layer melted or decomposed by the thermal head 2 is 1 to 1. 10 μm, specific heat x density 0.2c
al / (cm ³ ℃) or more, thermal conductivity 1 × 10 ~ ² cal / (cms ℃)
Below, the thermal diffusivity is preferably 5 × 10 to ² cm ² / s or less. Although this also relates to the thickness, when the base 12 is made of a material having good thermal conductivity such as metal, the second layer (the absorption layer 13 or the base 1) is used.
2) Alternatively, heat radiation from the adhesive layer 15 between the surface layer 14 and the second layer is reduced as much as possible, and the energy efficiency of plate imaging is improved. Therefore, 20 μm
It is only necessary to provide a thermal insulating layer of a certain degree, but such a condition is necessary because it is desired to be several μm or less in consideration of the manufacturing cost. Further, the specific resistance of the substrate 12 is set to 1 × 10 ¹⁰ Ωcm
By doing so, the accumulation of charged charges on the printing plate 3 can be prevented, and damage to the thermal head 2 due to static electricity can be suppressed. Furthermore, by using a material composition in which the concentration of ions generated when heating the printing plate 3 is 500 ppm or less, damage to the thermal head 2 can be suppressed.

Here, when the energy required for plate imaging is compared with the thermal plate imaging using a laser light source, the applied energy is originally electrical energy for both the laser and the thermal head 2. Is 2 × 10 ⁵ W / inch ² or more, but with the thermal head 2, 7.4 × 10 ⁵ W / inch ²
Imaging became possible at W / inch ² and about 1/3.

By the way, the printing plate 3 has rigidity so that expansion and contraction of the printing plate 3 is reduced due to the requirement of its dimensional accuracy, and cannot be bent with a small radius like paper. Further, since the printing plate 3 is usually wound around a plate cylinder and used for printing, the curvature of the plate cylinder may be considered to be a minimum radius, so that the printing plate 3 has as much rigidity as permissible. This radius is
Even a very small one such as a business card printing machine is about 25 mm, usually 100 mm or more. Therefore, as the thermal head 2, an end face head type is optimal as shown in FIG. 2. With such a structure, imaging can be performed even when the printing plate 3 has a large radius or a flat surface. In addition, the required pressure can be obtained with a low load, and at the same time, since the area is small, it is easy to cope with the undulation. The undulation is related to the method of fixing to the plate cylinder and the rigidity of the plate itself. However, there are various factors such as the thickness accuracy of the plate, and it is desirable to use an apparatus as much as possible. In the case of an optical system, since the focal length must be automatically tracked, the cost is high, but the thermal head 2 can easily follow the focal length. Further, as described above, it has been confirmed that the end face head is effective also in the energy comparison. The above comparison is a comparison of average values in solid imaging.

[0030]

According to the first aspect of the present invention, there is provided a printing plate which exhibits ink-philicity or ink-phobicity by removing a surface layer or forms irregularities by removing the surface layer. Using a printing plate in which the melting point, thermal decomposition temperature, or ignition temperature of the second layer or the adhesive layer between the surface layer and the second layer is lower than the softening point or thermal decomposition temperature of the surface layer of the plate, and a plurality of heating elements are used. Using the thermal heads arranged in a line, the second layer is formed at a temperature lower than the softening point or thermal decomposition temperature of the surface layer while leaving the surface layer.
Since the imaging is performed by melting or decomposing the layer or the adhesive layer, a large number of heat sources can be easily arranged with high accuracy, and the speed can be increased because the heating elements of the thermal head are used. It is easy to compensate for variations in each output, easy to replace, and easy to correct for the effects of fluctuations in the machine temperature and each temperature unevenness. As a result, there is no solid pattern in a solid image and high image quality Can be printed, the cost can be reduced, and the thermal energy of the heating element is directly used. Therefore, the efficiency is close to 100%, the output is not increased unnecessarily, and the energy is saved. In addition, even if the printing plate is a material that generates dust such as sticky substances or ions by heating, when the thermal head contacts the surface layer and performs imaging, Since it does not peel, it does not damage the thermal head, does not cause dust to adhere to it, can maintain the image quality and extend the service life, and if the dust adheres to the thermal head, However, the present invention has an effect that the user can perform maintenance such as wiping by hand without any special technique.

The difference between the softening point or the thermal decomposition temperature of the surface layer and the melting point or the thermal decomposition temperature of the second layer or the adhesive layer between the surface layer and the second layer is 50 ° C. or more. The thickness of the surface layer is 1 to 12 μm, the specific heat × density is 0.8 cal / (cm ³ ℃) or less, and the thermal conductivity is 1 × 10 to ⁴ cal.
/ (Cms ° C.) or more and the thermal diffusivity is 1 × 10 to ⁴ cm ² / s or more, so that the thermal efficiency of the surface layer is improved and the thermal efficiency at the time of printing plate imaging is improved. .

According to a third aspect of the present invention, the thickness of the layer next to the layer melted or decomposed by the thermal head is 1 to 1
In 0 .mu.m, specific heat × density 0.2cal / (cm ³ ℃) above, the thermal conductivity of at ^{1 × 10 ~ 2 cal / (} cms ℃) or less, the thermal diffusivity is 5 ×
Since the flow rate is set to 10 to ² cm ² / s or less, heat conduction to portions other than the heating section can be suppressed to the minimum necessary, and there is an effect that thermal efficiency at the time of printing plate imaging is improved.

[Brief description of the drawings]

FIG. 1, showing an embodiment of the present invention, is a side view of a thermal plate imaging apparatus.

FIG. 2 is a perspective view of a thermal head.

FIG. 3 is a circuit diagram of a drive circuit.

FIG. 4 is a cross-sectional view illustrating an example of a printing plate.

FIG. 5 is a sectional view showing another example of the printing plate.

FIG. 6 is a timing chart of a driving circuit.

[Explanation of symbols]

 Reference Signs List 2 thermal head 3 printing plate 5 heating element 12 second layer 13 second layer 14 surface layer 15 adhesive layer

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Osamu Tsuchiya 6-78, Minamimachi, Mishima-shi, Shizuoka Prefecture

Claims (3)

[Claims] 1. A printing plate which exhibits ink-philicity or ink-phobicity by removing a surface layer, or forms irregularities by removing the surface layer, wherein the softening point of the surface layer of the printing plate or Use a printing plate that has a lower melting point or thermal decomposition temperature or ignition temperature of the second layer or the adhesive layer between the surface layer and the second layer than the thermal decomposition temperature, and use a thermal head in which a plurality of heating elements are arranged in a line. A printing plate characterized in that the second layer or the adhesive layer is melted or decomposed at a temperature lower than the softening point or thermal decomposition temperature of the surface layer while the surface layer remains, and imaging is performed. Imaging method. A difference between a softening point or a thermal decomposition temperature of the surface layer and a melting point or a thermal decomposition temperature of the second layer or an adhesive layer between the surface layer and the second layer is 50 ° C. or more; With a specific heat × density of 0.8 cal / (cm ³ ° C)
Hereinafter, a printing plate having a thermal conductivity of 1 × 10 to ⁴ cal / (cms ° C.) or more and a thermal diffusivity of 1 × 10 to ⁴ cm ² / s or more. 3. The layer next to the layer that is melted or decomposed by the thermal head has a thickness of 1 to 10 μm, a specific heat × density of 0.2 cal / (cm ³ ° C.) or more, and a thermal conductivity of 1 × 10 to 10 μm. ² cal
/ (Cms ° C) or less, and the thermal diffusivity is 5 × 10 to ² cm ² / s or less.