JP7599213B2 - Method for forming thin film portion using laser - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applied. The "method of forming a thin film portion by laser" of the present invention was described in the "Report on business results related to the Gifu Technology Innovation Center Management Council Joint Research Subsidy Program Grant for FY2019" and submitted to the Gifu Technology Innovation Center Management Council (1288 Oze, Seki City, Gifu Prefecture) (February 1, 2021). The "method of forming a thin film portion by laser" of the present invention was published on the website (https://www.mdpi.com/2073-4360/13/9/1448) (April 30, 2021).

This invention relates to a method for forming a thin film on a workpiece by irradiating the workpiece with a laser beam.

For example, food containers for storing processed foods sold at convenience stores and supermarkets are made from molded synthetic resin sheets such as polypropylene (PP) and polyethylene terephthalate (PET). In recent years, the issue of microplastics has been attracting attention, and efforts to reduce the use of synthetic resin materials and reuse them are being recommended in order to reduce the environmental impact.

For food containers made of synthetic resin, making them smaller (reducing the volume) is an effective way to improve the convenience of disposal and recovery after use. One known way to make synthetic resin containers smaller is to use a structure in which multiple bends are formed in the body of the container, making it easier to crush when the container is pushed in from above by deforming the bends (see Patent Document 1). In this way, to make synthetic resin molded products smaller, parts that can be easily deformed by applying force, such as bends, are provided. However, the bends make the molded product more complex in structure, so improvements were needed.

One way to make a molded product easily deformable while still having a simple structure is, for example, to form perforations or thin film sections in the deformed parts. Perforations can be easily formed by mechanical processing or laser processing, but if they are provided in food containers, they can easily cause leakage of contents or entry of foreign matter from the outside. In contrast, thin film sections are thought to enable appropriate deformation without the risk of leakage of contents or entry of foreign matter.

Laser processing is known to be an extremely effective synthetic resin processing technique that is fast and low cost. However, while synthetic resin sheets, for example, are thin and can be easily melted by irradiation with a laser beam, controlling the laser processing to form a thin film of an appropriate thickness without penetrating the material is not easy due to fluctuations in the irradiation conditions. Therefore, the inventors conducted extensive research and discovered a method that can stably form a thin film without penetrating the material when irradiating the material with a laser beam.

JP 2016-141468 A

The present invention has been made in consideration of these points, and provides a method for forming a thin film portion using a laser that can stably form a thin film portion on a workpiece.

That is, the invention of claim 1 relates to a method for forming a thin film portion by a laser, characterized in that when a laser beam is irradiated from the front side of a workpiece to perform laser processing, a temperature rise suppression means consisting of a base member arranged in close contact with the back side of the workpiece suppresses the temperature rise at the laser irradiation position of the workpiece, thereby preventing melting of the back side of the workpiece, thereby forming a thin film portion at the laser irradiation position of the workpiece.

The invention of claim 2 relates to the method of forming a thin film portion by using a laser according to claim 1, wherein the temperature rise suppression means is placed in close contact with the back surface side of the workpiece by vacuum suction .

The invention of claim 3 relates to the method of forming a thin film portion by using a laser according to claim 1 or 2, wherein the base member is a metal plate.

The invention of claim 4 relates to the method of forming a thin film portion by laser according to claim 3, in which the metal plate is made of copper, aluminum, brass, iron, or stainless steel.

The invention of claim 5 relates to a method for forming a thin film portion by a laser according to any one of claims 1 to 4, in which the workpiece is a sheet-like object containing a synthetic resin material.

According to the method for forming a thin film portion by a laser of the invention of claim 1, when a laser beam is irradiated from the front side of the workpiece to perform laser processing, a temperature rise suppression means consisting of a base member arranged in close contact with the back side of the workpiece suppresses the temperature rise at the laser irradiation position of the workpiece and prevents melting of the back side of the workpiece, thereby forming a thin film portion at the laser irradiation position of the workpiece, so that the temperature rise suppression effect can be efficiently applied to the back side of the workpiece, and even if the irradiation conditions for laser processing fluctuate, the back side of the workpiece will not be penetrated, and a thin film portion can be formed easily and stably.

According to the method of forming a thin film portion using a laser of the invention of claim 2, in the invention of claim 1, the temperature rise suppression means is positioned in close contact with the back side of the workpiece by vacuum suction, so that the base member can be closely attached to the back side of the workpiece.

According to the method for forming a thin film portion using a laser of the invention of claim 3, in the invention of claim 1 or 2, the base member is a metal plate, which has high thermal conductivity and high reflectivity and is suitable as a base member, and the base member is less likely to be damaged or deformed during laser processing.

According to the method for forming a thin film portion by a laser according to the invention of claim 4, in the invention of claim 3, the metal plate is made of copper, aluminum, brass, iron, or stainless steel, which is easily available, easy to handle, economical, and suitable for industrial use.

According to the method for forming a thin film portion by a laser according to the invention of claim 5, in the inventions of claims 1 to 4, the workpiece is a sheet-like object containing a synthetic resin material, so that laser processing can be easily performed.

1 is a schematic cross-sectional view of a method for forming a thin film portion by using a laser according to an embodiment of the present invention. FIG. 2 is an enlarged schematic cross-sectional view of a thin film portion of the workpiece. 13 is a graph showing the results of measuring the thickness of a thin film portion when laser processing is performed using a copper plate base member. 13 is a graph showing the results of measuring the thickness of a thin film portion when laser processing is performed without using a base member. 13 is a graph showing the results of measuring the thickness of a thin film portion when laser processing is performed using an aluminum plate as a base member. 13 is a graph showing the results of measuring the thickness of a thin film portion when laser processing is performed using a brass plate as a base member. 13 is a graph showing the results of measuring the thickness of a thin film portion when laser processing is performed using a steel plate as a base member.

Figure 1 is a schematic cross-sectional view showing a method for forming a thin film portion by a laser according to one embodiment of the present invention. In this embodiment, when laser processing is performed by irradiating a laser beam L from the front side of a sheet-shaped workpiece 10, melting of the back side of the workpiece 10 is suppressed by means 20 for suppressing the temperature rise at the laser irradiation position P of the workpiece 10, thereby forming a thin film portion 15 at the laser irradiation position P of the workpiece 10.

The laser processing may be a known processing method using an appropriate laser such as a _CO2 laser (carbon dioxide laser). The output of the laser is not particularly limited and may be a continuous wave (CW) laser or a pulsed laser, but a continuous wave laser capable of high-speed processing is suitable for industrial use such as mass production and is therefore preferred.

The workpiece 10 is a sheet-like object made of a synthetic resin material that is easy to laser process by irradiation with the laser beam L. Examples of the workpiece 10 include sheet materials made of known synthetic resin materials such as polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET). These resin materials are easy to obtain and have excellent formability, making them suitable for industrial use. From the perspective of reducing the environmental load, it is also possible to use sheet materials made of synthetic resin materials containing inorganic fillers such as talc (filler-containing polypropylene; PPF, etc.). The thickness of the workpiece 10 is appropriate, but is, for example, about 0.2 to 1 mm.

The thin film portion 15 is a portion formed by irradiating the workpiece 10 with a laser beam L from the front side of the workpiece 10, making it thinner than the thickness of the workpiece 10. As shown in FIG. 2, in this thin film portion 15, at the laser beam irradiation position P, the front side of the workpiece 10 is removed in a groove shape by the laser irradiation, and the back side is not melted and becomes a non-penetrating portion that maintains a specified thickness (T). The width and length of the thin film portion 15 can be appropriately designed.

The processed material 10, a sheet-like material containing synthetic resin, is suitable for use as a material for molded bodies such as containers for processed foods sold at convenience stores, etc. When the processed material 10 is molded and used as a food container, it is preferable that the thin film portion 15 is formed in the body of the container. In such a container, by pushing the container from above, the thin film portion 15 functions as a deforming portion without a complex structure, making it possible to make the container smaller (reduced in volume). In addition, since the thin film portion 15 is a non-penetrating portion, there is no risk of foreign matter being mixed in or the contents leaking when used as a container, and the strength of the container is not compromised.

The means 20 for suppressing the temperature rise prevents the back side of the workpiece 10, which is the opposite side to the front side irradiated with the laser beam, from reaching a temperature at which the back side would be melted by the laser irradiation, thereby preventing melting of the back side of the workpiece 10.

During laser processing, the workpiece 10 is melted or evaporated at the laser irradiation position P due to the heat of the laser irradiation, and is removed from the front side toward the back side. At this time, the temperature rise suppression means 20 acts on the back side corresponding to the laser irradiation position P of the workpiece 10, and it is thought that the temperature rise suppression effect exceeds the heating of the laser irradiation, and the part corresponding to the specified thickness (T) on the back side is kept at a temperature that does not melt. In this way, the back side of the workpiece 10 does not reach the melting temperature even during laser irradiation, so melting is suppressed. Therefore, the laser irradiation position P of the workpiece 10 becomes a thin film portion 15 where the front side has been removed and the back side is not penetrated.

The temperature rise suppression means 20 is not particularly limited as long as it is a means capable of controlling the back side of the workpiece 10 not to reach the melting temperature caused by laser irradiation during laser processing. Examples include a base member such as a metal plate as shown in the examples, air cooling means such as air blowing or assist gas, water cooling means, and other suitable temperature adjustment devices.

The temperature rise suppression means 20 is preferably a base member 20A that is placed in close contact with the back side of the workpiece 10. This base member 20A is a plate-like member with a flat surface on which the workpiece 10 can be placed, and it is believed that by placing the back side of the workpiece 10 in close contact with the flat surface, a state of thermal equilibrium is formed between the back side of the workpiece 10 and the base member 20A. Therefore, it is preferable that the base member 20A is made of a material that has a thermal conductivity that is significantly higher than the thermal conductivity of the workpiece and has a high reflectivity for laser beams (is less likely to absorb laser beams).

The high thermal conductivity base member has a higher thermal conductivity than the workpiece, and is therefore thought to function to maintain a state of thermal equilibrium with respect to the rear surface of the workpiece 10 rather than increasing in temperature due to laser irradiation. In addition, the high reflectivity (low absorptivity) base member is less likely to absorb the laser beam, so the increase in temperature due to absorption of the laser beam is suppressed, and damage, deformation, etc. can be suppressed, preventing deterioration in product quality, etc.

In practice, it is preferable to use a metal plate for the base member 20A. A metal plate has high thermal conductivity and high reflectivity, and also has a melting temperature higher than the sublimation temperature of the workpiece 10, which contains a synthetic resin material. Therefore, even if the laser irradiation position P of the workpiece 10 is heated to the sublimation temperature during laser processing, the base member 20A is unlikely to be damaged or deformed. The metal plate is preferably made of copper, aluminum, brass, iron, or stainless steel. These metal plates are easily available, can be installed with little modification to existing equipment, and are easy to handle, making them suitable for industrial use.

The workpiece 10 is placed in close contact with the base member 20A by an appropriate method such as vacuum suction. In this embodiment, suction holes 21 are formed in multiple locations on the base member 20A, and the workpiece 10 and base member 20A are vacuum-suctioned through the suction holes 21 to bring them into close contact. By bringing the workpiece 10 and base member 20A into close contact, the temperature rise suppression effect can be more efficiently applied to the back side of the workpiece 10.

In the thin film portion forming method of the present invention, by using the base member 20A, the thickness T of the thin film portion 15 (the remaining thickness on the back side of the workpiece 10) is formed to be almost constant even if the energy of the laser irradiation increases. Therefore, the thin film portion 15 can be formed easily and stably without finely controlling the conditions of the laser irradiation.

Although the base member 20A can be used in a room temperature environment, it may be heated or cooled as appropriate if it is possible to prevent the back side of the workpiece 10 from reaching the melting temperature during laser processing and thereby prevent melting. By controlling the temperature of the back side of the workpiece 10 that comes into contact with the base member 20A through heating or cooling, it is possible to adjust the thickness of the thin film portion 15 by adjusting the suppression of temperature rise on the back side.

[Formation of thin film portion]
A _CWCO2 laser (manufactured by Panasonic Corporation; LP-431U) with a spot size of 100 μm and a focal length of 111 mm was used, and a transparent PP sheet (workpiece) with a thickness of 300 μm was placed on a base member made of a copper plate with a thickness of 10 mm that can be vacuum-suctioned, and vacuum-suctioned at 21 kPa to bring the back side of the transparent PP sheet into close contact with the base member, and the laser was focused on the surface of the transparent PP sheet and irradiated with a laser beam so as to perform one scan of a length of 10 mm, thereby performing laser processing on the surface of the PP sheet. The conditions for laser irradiation were a wavelength of 10.6 μm, an output of 30 W, and a laser scan speed of 60 to 150 mm/s. The copper plate used as the base member had a thermal conductivity of 401 W/m·K, an absorptivity of 1% at a wavelength of 10.6 μm, and a melting temperature of 1084°C.

After laser processing, a laser interferometer (Keyence Corporation; SI-F1000V and SI-F80) was used to measure the thickness of the center of the thin film formed on the PP sheet. Table 1 shows the measurement results, with the energy linear density P/S (energy per unit length; J/m) and the thickness (μm) of the thin film for each laser scanning speed, and Figure 3 shows a graph with the energy linear density (P/S: unit is J/m) on the horizontal axis and the thin film thickness (μm) on the vertical axis. In the energy linear density, P is the laser output (W) and S is the laser scanning speed (mm/s).

As a comparative example, laser processing was performed on the surface of a PP sheet using a conventional laser processing method in which no base member was placed on the back side of the workpiece, and the back side of the workpiece was air. The laser scan speed was 115 to 600 mm/s. The measurement results are shown in Table 2 and the graph in Figure 4.

[Comparison of base materials]
Instead of the copper plate, an aluminum plate, a brass plate, and a steel plate (JIS SS400) were used as base members, and laser processing was performed on the surface of the PP sheet. The thickness of each base member and the conditions of vacuum suction were the same as those of the copper plate, and the laser scanning speed was 75 to 120 mm/s. The measurement results are shown in Table 3 and the graphs in Figures 5 to 7. The aluminum plate has a thermal conductivity of 236 W/m·K, an absorptivity of 1% at a wavelength of 10.6 μm, and a melting temperature of 660°C, the brass plate has a thermal conductivity of 111 W/m·K, an absorptivity of 2% at a wavelength of 10.6 μm, and a melting temperature of 930°C, and the steel plate has a thermal conductivity of 50 W/m·K, an absorptivity of 10% at a wavelength of 10.6 μm, and a melting temperature of 1500°C.

[Results and Discussion]
First, in the comparative example using the conventional laser processing method not using a base member shown in Table 2 and Figure 4, it was confirmed that the thickness of the thin film portion decreased (the processed portion of the PP sheet became deeper) as the energy linear density (P/S) increased, and when the energy linear density (P/S) was 210 to 250 J/m, there were some parts of the thin film portion that were partially penetrated. Also, when the energy linear density (P/S) was 260 J/m, the laser processed portion completely penetrated, and it was not possible to form a thin film portion.

In contrast, when a copper plate was placed in close contact with the back side of the PP sheet as a base member, as can be seen from Table 1 and Figure 3, the thickness of the thin film portion decreased with an increase in the energy linear density (P/S), and the thin film portion was stably formed at a thickness of 50 to 60 μm when the energy linear density (P/S) was 280 J/m or more. As can be seen from the comparison with the comparative example, in the conventional method, excessive laser irradiation would cause the laser-processed portion of the PP sheet to be penetrated, but by placing a copper plate (base member) in close contact with the back side of the PP sheet, it was found that even if excessive laser irradiation was performed, the laser-processed portion (thin film portion) would not be penetrated and an almost constant thickness was maintained.

In addition, when aluminum, brass, or steel plates were used as the base member instead of copper plates, as can be seen from Table 3 and Figures 5 to 7, a thin film portion with a nearly constant thickness that did not penetrate was formed even when excessive laser irradiation was performed, just like with copper plates. The thermal conductivity of copper, aluminum, brass, and steel plates is significantly higher than that of PP sheets (0.1 to 0.2 W/m·K), and it was found that they can be suitably used as base members.

As shown and explained above, in the method for forming a thin film portion by a laser of the present invention, when a laser beam is irradiated from the front side of a workpiece to perform laser processing, a means for suppressing a temperature rise at the laser irradiation position of the workpiece is used to suppress melting on the back side of the workpiece and form a thin film portion at the laser irradiation position. Therefore, even if the irradiation conditions for laser processing fluctuate and excessive laser irradiation is performed, the back side of the workpiece is not penetrated and a specified thickness can be maintained. Therefore, a thin film portion can be appropriately formed even if an error occurs in the output of the irradiated laser beam. In other words, a thin film portion of approximately constant thickness can be stably formed without finely controlling the laser irradiation. In addition, if a base member such as a metal plate is placed in close contact with the back side of the workpiece, a thin film portion can be formed more simply, efficiently and stably.

As described above, the method for forming a thin film portion using a laser according to the present invention can easily and stably form a thin film portion using laser processing without complex laser control. In addition to manufacturing commercially available food containers, it can be used to process a variety of other products.

10 Workpiece 15 Thin film portion 20 Temperature rise suppression means 20A Base member (metal plate)
21 Suction hole L Laser beam P Laser irradiation position T Thickness of thin film portion

Claims (5)

A method for forming a thin film portion by a laser, characterized in that, when laser processing is performed by irradiating a laser beam from the front side of a workpiece, a temperature rise suppression means consisting of a base member arranged in close contact with the back side of the workpiece suppresses the temperature rise at the laser irradiation position of the workpiece, thereby preventing melting of the back side of the workpiece, thereby forming a thin film portion at the laser irradiation position of the workpiece. 2. The method for forming a thin film portion by using a laser according to claim 1, wherein the temperature rise suppression means is placed in close contact with the back surface of the workpiece by vacuum suction. 3. The method for forming a thin film portion by using a laser according to claim 1, wherein the base member is a metal plate. The method for forming a thin film portion using a laser according to claim 3, wherein the metal plate is made of copper, aluminum, brass, iron, or stainless steel. The method for forming a thin film portion using a laser according to any one of claims 1 to 4, wherein the workpiece is a sheet-like object containing a synthetic resin material.

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