Resin molding and production method thereof

Abstract

To provide a resin molding and a production method thereof not only allowing visual and tactile wood texture to be acquired but also enabling a natural impression to be exhibited even when a plurality thereof are continuously arranged.SOLUTION: A resin molding 100 having a longitudinal direction and a transverse direction is configured to have a wood grain pattern 3 formed on its surface, the wood grain pattern 3 including a plurality of patterns among a cross grain pattern 33, a regular grain pattern 34, and a figured grain pattern 35, and being arranged in a width direction so that different patterns appear. This allows, when a plurality of the resin moldings are continuously arranged in the longitudinal and lateral directions, not only visual and tactile wood texture to be exhibited, but also enables an impressive pattern to be moderately softened by relatively milder patterns, thereby providing a moderate wood texture and a natural impression, and enabling a natural impression to be maintained even when a plurality of the resin moldings are continuously arranged.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a resin molded product having a wood grain surface, which is used for outdoor decks, interior materials for buildings, etc., and a method for producing the same. [Background technology]

[0002] Wood as a building material has been used for a long time in many countries around the world because it is strong despite its light weight, has a nice texture, and is easy to obtain. However, due to problems such as the expansion of deforestation, corrosion, and insect damage, the use of alternative materials is on the rise. Among these, the use of building materials made from resin molded products, which are lightweight, strong, and have good production efficiency, is also increasing.

[0003] However, the surface and texture of resin are different from that of wood, and it lacks the aesthetic appeal of being used in buildings and structures, so resin molded products with wood-like designs on the surface are being developed. For example, there is a method of printing a wood grain pattern or a method of forming a wood grain pattern by press molding using a mold.

[0004] Conventionally, a technique has been developed for expressing unevenness by using printing paint. For example, the technology described in Patent Document 1 involves printing a wood grain pattern on a base material, then using gravure printing to build up surface gloss in specified areas, while printing other areas with matte ink, thereby achieving a visually and tactilely three-dimensional wood texture.

[0005] In addition to printing, a technique has been developed for forming a wood grain pattern on the surface of a resin molded product by pressing with an embossing roll. For example, in the technology of Patent Document 2, a wood grain pattern of a predetermined depth is formed by an embossing roll. In this technology, in addition to embossing with the embossing roll, the non-embossed parts are also roughened to add a streaky pattern with a predetermined surface roughness, thereby improving the wood texture.

[0006] In the technology of Patent Document 1, a cross grain pattern such as a mountain shape, wave, or spiral pattern is arranged on the surface as shown in Fig. 7. This cross grain pattern is configured so that one large pattern is arranged for a given width. The same is true in Patent Document 2, and as shown in FIG. 8, a single large grain pattern is arranged on the surface. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 56-5788 [Patent Document 2] JP 2018-8512 A Summary of the Invention [Problem to be solved by the invention]

[0008] Wood has a cross grain, a straight grain, a figured grain, etc. depending on the processing direction when sawing, but a straight grain is a pattern of roughly parallel lines, and even if this is reproduced on the surface of a resin molded product, it will look like a monotonous line pattern and will have a poor wood texture. Therefore, it is common to reproduce the cross grain or figured grain, which are more characteristic patterns of wood, on the surface. On the other hand, in deck materials and interior and exterior materials for buildings, plate-like members of predetermined dimensions are used by arranging them continuously in the longitudinal and lateral directions.

[0009] In natural wood, planks with different sizes and patterns of knots are mixed, even if they have a flat or figured grain. In addition, decks and interior and exterior construction may use a mixture of straight-grained planks and planks with flat or figured grain.

[0010] However, in the case of wood grain pattern resin molded products, it is not economical to prepare a lineup of various cross grain patterns, straight grain patterns, etc., as in the case of natural wood. Therefore, resin molded products in which a wood grain pattern is formed using a press mold with a certain pattern are used by arranging them continuously in the longitudinal and lateral directions. As described above, conventional resin molded products have one large grain pattern to create a wood-like texture. In this case, when resin molded products having only one grain pattern are arranged in succession, the large grain pattern is arranged in succession on a plane, which creates a very unnatural appearance.

[0011] The present invention has been made in consideration of the above-mentioned problems, and an object of the present invention is to provide a resin molded product and a manufacturing method thereof that not only provides a visual and tactile wood-like texture, but also gives a natural impression even when multiple products are arranged in succession. [Means for solving the problem]

[0012] The means adopted by the present inventors to solve the above problems will be described below. The resin molded product of the present invention is a resin molded product having a longitudinal direction and a lateral direction, and has a wood grain pattern formed on the surface. The wood grain pattern may be a straight grain pattern, a cross grain pattern, a figured grain pattern, etc., and may include some or all of these patterns. The wood grain pattern includes a plurality of patterns selected from the cross grain pattern, the straight grain pattern, and the figured pattern, and the wood grain patterns are arranged in a plurality of patterns so that different patterns appear in the width direction.

[0013] In the present invention, a single resin molded product contains multiple patterns, such as straight grain, cross grain, and wood grain, in the width direction, and the multiple patterns are arranged so that different patterns appear in sequence. This allows the relatively soft patterns to moderately soften the impressive patterns, giving a moderate wood texture and natural impression, even when multiple resin molded products are arranged side by side.

[0014] In addition to the above means, the means adopted by the present invention to solve the above-mentioned problems can also be configured so that the short dimension is between 100 mm and 300 mm, and three or more rows of either the cross grain pattern or the straight grain pattern are arranged.

[0015] Among wood grain patterns, the cross grain pattern has a striking appearance as wood grain, while the straight grain pattern has a softer impression. On the other hand, deck materials and interior and exterior boards are often made up of widths of approximately 300 mm or less. In a width of 300 mm or less, by configuring the straight grain pattern and cross grain pattern to be arranged in three or more rows, when multiple resin molded products are arranged vertically and horizontally in succession, the striking cross grain pattern resembling wood grain and the soft straight grain pattern are arranged alternately, resulting in a natural appearance.

[0016] Another measure adopted by the present invention to solve the above-mentioned problems is to configure the short dimension to be between 100 mm and 300 mm, and the pitch between the cross grain patterns or the straight grain patterns to be between 20 mm and 100 mm. In the dimensions generally used for decking materials and interior and exterior boards, by arranging the pitch of the cross grain or straight grain pattern to be 20 mm or more and 100 mm or less, the striking cross grain pattern and the soft straight grain pattern will be spaced at appropriate intervals, resulting in a natural appearance.

[0017] In the straight grain pattern, the embossed recesses are continuously formed without being interrupted in the longitudinal direction. Therefore, in any of the above configurations, by using a wood grain pattern including a straight grain pattern, even if water accumulates in the resin molded product, it is easy to drain through the continuous embossed portions, improving drainage.

[0018] As another aspect of the present invention, the wood grain pattern can be formed by pressing to create unevenness, and the difference in the proportion of the area of ​​the embossed portion within each of the 10 mm strips along the short side can be within 50%. When a wood grain pattern is formed by the unevenness caused by press processing, the embossed portion is the portion formed by pressing with a die, and the non-embossed portion is the portion not pressed by the die. The portion pressed by the die is subjected to a force that deforms the resin molded body so that the resin molded body itself is bent, but depending on the structure of the resin molded body, there are some portions that are easy to deform and some that are difficult to deform.

[0019] For example, when the resin molded body is not simply plate-shaped but has a hollow shape with a certain internal space, ribs may be formed in the hollow portion. In this case, the position directly above the rib is unlikely to bend due to the pressing of the mold because the rib acts as a support, but the position between the ribs is likely to bend due to the pressing of the mold.

[0020] In such a case, for example, if only a certain position in the width direction is pressed intensively, the resin molding will bend at the pressed position, making it difficult to form a wood grain pattern of sufficient depth on the surface. On the other hand, at the position directly above the rib, the resin molding is less likely to bend when pressed by the mold, and the wood grain pattern can be firmly transferred.

[0021] In the present invention, the width direction is divided into 10 mm strips, and the wood grain pattern is constructed so that the difference in the proportion of the embossed area within each divided area is 50% or less, thereby making it possible to evenly distribute the pressure in the width direction of the molded product. This reduces the difference in pressure distribution across the width of the press die during press working of the molded product, making it possible to form a wood grain pattern of uniform depth overall.

[0022] In addition, when pressing force is concentrated locally in a press process, the mold in that part is easily worn. In the present invention, by setting the area ratio of the embossed portion as described above, the pressing force can be evenly distributed in the width direction of the molded product, and wear resistance is also improved.

[0023] In the present invention, a resin molded product having a wood grain pattern having a longitudinal direction and a transverse direction can also be manufactured by a method of forming a wood grain pattern on the surface by roll pressing so that a different pattern appears in the width direction. Here, the wood grain pattern includes a plurality of patterns selected from a cross grain pattern, a straight grain pattern, and a figured grain pattern.

[0024] In the above manufacturing method, it is also possible to form a wood grain pattern on the surface, with the short dimension being between 100 mm and 300 mm, and with three or more rows of either a cross grain pattern or a straight grain pattern. In addition, with the above dimensions, it is also possible to arrange the pitch between the cross grain patterns or the straight grain patterns to be 20 mm or more and 100 mm or less.

[0025] Furthermore, the surface can be formed with a wood grain pattern arranged so that the difference in the proportion of the area of ​​the embossed portions within each range when the short side direction is divided into 10 mm strips is within 50%. Effect of the Invention

[0026] As described above, the resin molded product of the present invention is a resin molded product having a longitudinal direction and a transverse direction, and multiple wood grain patterns formed on the surface are arranged alternately in the width direction. This has the effect that when the resin molded products are arranged continuously in the longitudinal direction and the transverse direction, not only can a visual and tactile wood texture be obtained, but also the impressive pattern is moderately softened by the relatively soft pattern, giving a moderate wood texture and natural impression, and even when multiple resin molded products are arranged continuously, a natural impression can be given. [Brief description of the drawings]

[0027] [Figure 1] 1A and 1B are a plan view and an enlarged cross-sectional view showing the surface of a resin molded product of the present invention. [Diagram 2] FIG. 2 is an explanatory diagram showing each range into which the wood grain pattern of the resin molded product of the present invention is divided. [Diagram 3] 1 is a graph showing the ratio of recesses in each range into which a resin molded product of the present invention is divided. [Figure 4] 1 is an explanatory diagram showing a method for producing a resin molded product according to the present invention. [Diagram 5] FIG. 2 is an explanatory diagram of an embossing roll used in the method for producing a resin molded product of the present invention. [Figure 6] FIG. 13 is a plan view showing a modified example of the present invention. [Figure 7] FIG. 1 is a plan view illustrating a conventional example of Patent Document 1. [Figure 8] FIG. 1 is a perspective view showing a conventional example of Patent Document 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. It should be noted that each drawing is drawn diagrammatically for the purpose of explanation, and some dimensions and shapes are shown exaggerated or simplified.

[0029] The resin molded product 100 of the present invention is used for decking materials for terraces and promenades, benches, wall materials, flooring materials, furniture, window louver materials, etc., and is a plate-shaped extrusion molded product having a specified cross-sectional shape as shown in FIG. 1. The resin molded product 100 has a two-layer structure in which a base member 1, which serves as a structural body, and a surface layer 2, which covers the outer periphery of the base member 1 except for both end faces, are integrated together.

[0030] Both the base member 1 and the surface layer 2 are made of synthetic resin, and for example, the base member 1 can be made of polyvinyl chloride (PVC) resin or acrylonitrile-butadiene-styrene (ABS) resin. The surface layer 2 can be made of acrylonitrile-styrene-acrylic ester (ASA) resin. Other resins that can be used include polyethylene (PE) resin, polypropylene (PP) resin, ethylene vinyl acetate (EVA) resin, polystyrene (PS) resin, etc. Additives such as pigments, foaming agents, ultraviolet absorbers, and flame retardants may be added to these resins as necessary.

[0031] In order to improve the woody texture, it is preferable that either of these materials contains wood flour. The higher the wood powder content relative to the synthetic resin, the higher the rigidity can be. On the other hand, molding defects such as deterioration of the appearance are more likely to occur during molding. For this reason, it is preferable that the base member 1, which is the non-appearance part, contains a larger amount of wood powder to provide rigidity that can withstand the load during embossing and use, and that the surface layer 2 contains less wood powder than the base member 1, with an emphasis on appearance. Specifically, the wood powder content of the base member 1 is preferably 10% or more higher than that of the surface layer 2, and more preferably 15% or more higher.

[0032] A wood grain pattern 3 is formed on the front surface 21 and back surface 22 of the surface layer 2 by embossing using a roll press, which will be described later, but the wood grain pattern 3 is not formed on the right side surface 23 and the left side surface 24. In addition, both front and rear end surfaces are cut surfaces that have been cut to a specified length, and the base member 1 is exposed. The cross-sectional shape is a horizontally long rectangle, and ribs 11 are formed in the hollow portion in the longitudinal direction at predetermined intervals to prevent bending.

[0033] The wood grain pattern 3 formed on the front surface 21 and the back surface 22 has an embossed portion 31 that is embossed so as to be recessed from the front surface, and a non-embossed portion 32 that is not embossed. The non-embossed portion 32 may be formed with a streak pattern by roughening, and the wood texture is improved by setting the arithmetic mean roughness of the non-embossed portion 32 to be 2 μm or more and 30 μm or less.

[0034] The thickness of the base member 1 and the ribs 11 is appropriately designed according to the required strength. From the standpoint of strength, it is preferable that the intervals between the ribs 11 are set to 10 mm or more and 30 mm or less, and in the embodiment shown in Fig. 1, nine ribs are arranged at intervals of 16 mm. The surface layer 2 has a substantially uniform thickness overall, and the thickness is preferably set to 0.5 mm or more and 5 mm or less according to the depth of the embossing. The width of the resin molded product 100 is preferably, for example, 100 mm to 300 mm, and may be set to about 160 mm for deck materials and about 100 mm for louver materials. The length in the longitudinal direction can be cut to any dimension depending on the application.

[0035] In the embodiment shown in FIG. 1, the wood grain pattern 3 formed on the surface 21 is a plurality of cross grain patterns 33 and straight grain patterns 34 arranged alternately in the width direction. It is preferable that the cross grain pattern 33 and the straight grain pattern 34 are arranged in three or more rows. In the embodiment shown in Fig. 1, the cross grain pattern 33 and the straight grain pattern 34 are arranged in four rows each alternately in the width direction on the front surface 21 of the resin molded article 2 having a width of 160 mm. The same is true on the back surface 22, and hereinafter, the front surface 21 will be described as a representative example.

[0036] It is rare that only one resin molded product 100 is used, and multiple resin molded products 100 are used by arranging them continuously in the width direction and the length direction depending on the application. Therefore, if the resin molded products 100 have only the board pattern 33 that stands out, the board pattern 33 will be visible everywhere when they are arranged continuously, which will cause discomfort. Therefore, by arranging the cross grain pattern 33 and the straight grain pattern 34 alternately in at least three rows, the cross grain pattern 33 gives a woody texture while the straight grain pattern 34 moderately softens the impression of the cross grain pattern 34, reducing the sense of incongruity when they are arranged continuously, and a more natural appearance can be achieved.

[0037] 1, the pitch between the cross grain patterns 33-33 and between the straight grain patterns 34-34 is 20 mm or more and 40 mm or less. The pitch between the cross grain patterns 33-33, the straight grain patterns 34-34, and other patterns, including the figured pattern not shown, is preferably 20 mm or more and 100 mm or less depending on the width of the resin molded product 100, to achieve a good balance between the wood texture and a natural impression, more preferably 20 mm or more and 80 mm or less, and even more preferably 20 mm or more and 65 mm or less.

[0038] The depth of the embossed portion 31 constituting the wood grain pattern 3 is preferably 100 μm to 2 mm, more preferably 150 μm to 1.5 mm, and even more preferably 200 μm to 1 mm. By setting the depth to such a level, the wood texture can be improved, and in the case of the straight grain pattern 34, even if water accumulates on the surface 21, the water can be absorbed in the embossed portion 31 and easily drained in the longitudinal direction.

[0039] The embossing is performed by pressing the protrusions of an embossing roll (described later) against the surface 21, causing plastic deformation. Therefore, a load acts on the pressing portion that forms the embossed portion 31 in a direction that deflects the surface 21 of the resin molded product 100. In this case, if the embossed portion 31 is directly above the rib 11 in the cross-sectional shape, the load from the convex portion of the embossing roll can be supported by the rib 11, so that the surface 21 does not bend and escape due to the load, and an embossment can be firmly formed on the surface 21 by the height of the convex portion of the embossing roll.

[0040] However, if the intermediate portions of the ribs 11 are embossed portions 31, the surface 21 will bend and escape under the load from the convex portions of the embossing roll since there is no rib 11 directly below the portion receiving the load. In this case, not only will the embossed portions 31 be shallower than the height of the protrusions of the embossing roll, but the ridge lines that are the boundaries between the embossed portions 31 and the non-embossed portions 32 will also be unclear, impairing the woody texture.

[0041] Therefore, in the present invention, as shown in FIG. 2, the wood grain pattern 3 is formed on the surface 21 of the resin molded product 100 so that the difference in the ratio of the area of ​​the embossed portion within each range when the short side direction is divided into strips of 10 mm each is within 50%.

[0042] The larger the proportion of the embossed portion in each range, the greater the load applied by the embossing roll in that range. In the present invention, the proportion of the area of ​​the embossed portion is calculated for each narrow range of 10 mm, and the wood grain pattern 3 is constructed so that the difference between the range with the maximum proportion and the range with the minimum proportion in the width direction is within 50%.

[0043] Figure 2 shows a binarized image of wood grain pattern 3 divided into 10 mm widths. To divide the 160 mm width into 10 mm widths, the image is divided into 16 ranges. Black pixels correspond to embossed areas 31, and white pixels correspond to non-embossed areas 32. Using this image, the number of black pixels in each range is counted relative to the total number of pixels, and the ratio is calculated. If a fraction occurs, the fraction is allocated to each end, and that range is not evaluated. If there is a range of 10 mm or less at either end, it may be excluded from the evaluation. This is because both ends are difficult to bend due to the right side 23 and left side 24.

[0044] Figure 3 is a graph in which the proportion of the area of ​​the embossed portion (black pixels) in each range is calculated and normalized. Section 14, which had the highest proportion of black, is set to 100%, and the graph shows how small the proportion in each section is compared to section 14. In other words, the graph shows the difference in proportion in each section compared to section 14, which had the largest proportion in the width direction. In this way, it can be seen that wood grain pattern 3 is such that when the difference between the range with the largest proportion of the area of ​​the embossed portion in the entire range and the range with the smallest proportion is calculated, the difference is within 50%.

[0045] By forming such a wood grain pattern 3, the difference in pressing force by the embossing roll is small whether directly above the rib 11 or between the ribs 11, and embossing can be performed with a uniform force across the entire width of the surface 21. Therefore, there is no variation in the depth of the embossed portions 31 or the clarity of the ridges across the entire surface 21, and a natural wood texture can be obtained. Furthermore, by applying a uniform pressing force in the width direction of the embossing roll, wear of the embossing roll, which is a press die, can be suppressed.

[0046] Such a resin molded product 100 can be manufactured by a manufacturing method using co-extrusion molding, as shown in Fig. 4. Specifically, the material of the base member 1 and the material of the surface layer 2 are melted and extruded into one mold M from separate extrusion heads, thereby obtaining a primary molded product P having a two-layer structure in which the base member 1 and the surface layer 2 are integrated.

[0047] In the embodiment shown in Fig. 4, the surface 21 and back surface 22 of the surface layer 2 of the primary molded product P obtained by extrusion molding are roughened. The roughening is performed by forming a stripe pattern using a belt sander B or the like. The wood texture is improved by adjusting the processing conditions so that the arithmetic mean roughness of the stripe pattern is in the range of 2 µm to 30 µm.

[0048] After the roughening, the resin molded product is embossed by an embossing roll R shown in Fig. 5. The embossing roll R is, for example, a roll having a width of 180 mm, and applies a predetermined pressing force to the resin molded product by sandwiching the resin molded product between two upper and lower rolls spaced apart according to the thickness of the primary molded product P. Each of the embossing rolls R·R is heated to a temperature equal to or higher than the thermal deformation temperature of the resin. The temperature is appropriately selected depending on the resin, but is generally about 90°C to 200°C.

[0049] This embossing roll R·R has a wood grain pattern 3 formed in an inverted concave-convex state such that when the roll is divided into the aforementioned 10 mm strips in the width direction, the difference in the area ratio of the embossed portion within each strip is within 50%. In this case, if the embossing roll R on the front surface 21 side and the embossing roll R on the back surface 22 side are formed so that the concaves and convexes of the wood grain pattern 3 correspond to each other, the balance of the upper and lower loads during the embossing process will be good, but the upper and lower embossed portions 31 may also have patterns with different arrangements.

[0050] The width of the embossing roll R is configured to be larger than the width of the primary molded product P, but by using the embossing roll R on which the wood grain pattern 3 of the above-mentioned proportions is formed, the resin molded product 100 can be processed with uniform pressing force regardless of the position in the width direction of the embossing roll R where the resin molded product 100 passes, and a wood grain pattern 3 of uniform depth can be obtained across the entire width direction of the resin molded product 100.

[0051] As described above, with the resin molded product 100 and its manufacturing method of the present invention, when multiple resin molded products 100 are arranged side by side, not only can a visual and tactile wood texture be obtained, but the impressive cross grain pattern 31 is moderately softened by the relatively soft impression of the straight grain pattern 32, giving a moderate wood texture and a natural impression, and a natural impression can also be given even when they are arranged continuously in the longitudinal and lateral directions.

[0052] 『Variation 1』 The present invention is not limited to the above-mentioned embodiment, and other embodiments may be adopted. For example, as shown in Fig. 6, the resin molded product 101 may be configured such that the wood grain pattern 3 is arranged between the cross grain pattern 33 and the straight grain pattern 34, and the figured grain pattern 35 is included. The grain is a complex pattern that appears when wood is cut at a location with localized twists or curves or at a knot, and is a distinctive pattern that gives the impression of natural wood.

[0053] Even in this modified example, the cross grain pattern 33 and the grain pattern 35 impart a woody texture, while the straight grain pattern 34 moderately softens the impression of the cross grain pattern 33 and the grain pattern 35, reducing the sense of incongruity when they are arranged next to each other, resulting in a more natural appearance. [Explanation of symbols]

[0054] 100,101 Resin molded products 1 Base material 11 Ribs 2 Surface layer 21 Surface 22 Back 23 Right side 24 Left side 3 Wood grain pattern 31 Embossed section 32 Non-embossed section 33 Woodgrain 34 Straight grain pattern 35 Grain pattern M mold B Belt sander R embossing roll P Primary molded product

Claims

1. A resin molded product having a longitudinal direction and a lateral direction, The wood grain pattern is formed on the surface, The wood grain pattern includes a plurality of patterns selected from the group consisting of a cross grain pattern, a straight grain pattern, and a wood grain pattern; A resin molded product characterized in that a plurality of molded parts are arranged so that different patterns appear in the width direction.

2. The short side dimension is between 100mm and 300mm.

2. The resin molded product according to claim 1, characterized in that either the cross grain pattern or the straight grain pattern is arranged in three or more rows.

3. The short side dimension is between 100mm and 300mm.

2. The resin molded product according to claim 1, wherein the pitch between the cross grain patterns or the straight grain patterns is 20 mm or more and 100 mm or less.

4. The wood grain pattern is formed by the unevenness caused by pressing, 4. The resin molded product according to claim 3, wherein the difference in the proportion of the area of ​​the embossed portion within each of the 10 mm rectangular sections in the short direction is within 50%.

5. A method for manufacturing a resin molded product having a wood grain pattern having a longitudinal direction and a lateral direction, the wood grain pattern including a cross grain pattern, a straight grain pattern, and a wood grain pattern, A method for producing a resin molded product, comprising forming a wood grain pattern on a surface by roll pressing so that a pattern that varies in the width direction appears.

6. The short side dimension is between 100mm and 300mm.

6. The method for producing a resin molded product according to claim 5, wherein a wood grain pattern having three or more rows of either a cross grain pattern or a straight grain pattern is formed on the surface.

7. The short side dimension is between 100mm and 300mm.

6. The method for producing a resin molded product according to claim 5, characterized in that a wood grain pattern is formed on the surface so that the pitch between the cross grain patterns or the straight grain patterns is 20 mm or more and 100 mm or less.

8. The resin molded product according to claim 7, characterized in that a wood grain pattern is formed on the surface so that the difference in the ratio of the area of ​​the embossed portion within each range when the short side direction is divided into 10 mm strips is within 50%.

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