Structure of living hinge and method for manufacturing living hinge

Abstract

A living hinge 10 includes: a first member 11A; a second member 11B; a hinge part 12 for connecting the first member 11A and the second member 11B; and a guide part 13 arranged in the first member 11A for limiting a curvature radius of the hinge part 12 within a predetermined range when the hinge part 12 is bent.

Description

Living hinge structure and manufacturing method thereof

[0001] The present disclosure relates to living hinge structures and methods of manufacturing living hinges.

[0002] A living hinge is a structure that connects two parts with a thin part, allowing the parts to be opened and closed. A living hinge is made by injecting molten resin into the gap in a mold to mold a container and a lid into an integrated shape, and is designed to be bendable. Living hinges generally use polymer materials with relatively high flexibility (large elongation at break), such as polypropylene or polyethylene. A properly designed living hinge is expected to withstand unlimited bending cycles (Non-Patent Document 1).

[0003] Living hinges are generally used indoors and are rarely exposed to harsh outdoor environments. However, there are also cases where living hinges are used outdoors, and in such cases, cracks and breakage have been reported (Non-Patent Document 2). In particular, living hinges used outdoors are thought to deteriorate due to the interaction of bending and ultraviolet degradation.

[0004] Generally, when stress is applied to polymeric materials such as polypropylene and polyethylene, they pass through an elastic deformation region, then crack in the region beyond the yield point (plastic deformation region), and if the load increases further, they will break (Non-Patent Document 3).

[0005] Hidetoshi Yokoi, "Improvement of Hinge Characteristics by In-Mold Press-In Hinge Molding," Production Research, Vol. 42, No. 6, 1990, pp. 97-100; Kaori Negishi, "Microscopic Analysis of Degraded Living Hinges of Polypropylene Aerial Cable Closure," Zairyo-to-Kankyo, Vol. 72, No. 3, 2023, pp. 76-84; Sakae Fujiki and Toshiya Hagiwara, "100 Case Studies: How to Observe the Fracture and Fracture Surfaces of Plastic Materials," Nikkan Kogyo Shimbun, 2015, pp. 18-21

[0006] When we look at the stresses acting on a living hinge, when it is bent, compressive stress is applied to the inside and tensile stress is applied to the outside. When a living hinge is repeatedly opened and closed, these stresses are thought to cause cracks and fractures to progress. The magnitude of stress concentration varies depending on the design of the living hinge. In other words, it is thought that the durability of a living hinge can be improved through structural design.

[0007] The present disclosure has been made in view of the above, and aims to improve the durability of living hinges.

[0008] The structure of a living hinge according to one aspect of the present disclosure comprises a first member, a second member, a hinge portion connecting the first member and the second member, and a guide portion disposed on the first member for limiting the radius of curvature of the hinge portion within a predetermined range when the hinge portion is bent.

[0009] According to the present disclosure, the durability of the living hinge can be improved.

[0010] FIG. 1 is a side view of a living hinge in a normal state. FIG. 2 is a side view of a living hinge when bent. FIG. 3 is a side view of a living hinge when bent. FIG. 4 is a side view of a living hinge in a normal state. FIG. 5 is a side view of a living hinge when bent. FIG. 6 is a side view showing an example of a living hinge exposed to sunlight. FIG. 7 is a diagram showing an example of a stress-strain curve of a polymer material. FIG. 8 is a diagram showing an example of physical property values ​​of a polymer material before and after degradation. FIG. 9 is an image of a hinge portion when bent. FIG. 10 is a diagram showing an example of a circle fitted to a hinge portion when bent. FIG. 11 is a diagram showing an example of an ellipse fitted to a hinge portion when bent. FIG. 12 is a diagram showing an example of the thickness of a hinge portion. FIG. 13 is a diagram showing an example of a hinge portion bent at 180°. FIG. 14 is a diagram showing an example of a hinge portion bent at 90°. Fig. 15 is a diagram showing an example of a mold for a living hinge in a state where the hinge portion is folded. Fig. 16 is a diagram showing an example of a hinge portion molded in a state where it is folded 90 degrees and an example of the state when the hinge portion is opened flat. Fig. 17 is a diagram showing an example of the size of the living hinge of Example 1 when folded. Fig. 18 is a diagram showing an example of experimental results for the living hinge of Example 1. Fig. 19 is a diagram showing an example of the size of the living hinge of Example 2 when it is folded. Fig. 20 is a diagram showing an example of experimental results for the living hinge of Example 2.

[0011] Hereinafter, an embodiment will be described with reference to the drawings.

[0012] [Structure of Living Hinge] An example of the structure of the living hinge of this embodiment will be described with reference to Figures 1 and 2. Figure 1 is a side view of the living hinge 10 in a normal state, and Figure 2 is a side view of the living hinge 10 when folded.

[0013] The living hinge 10 has a structure in which a first member 11A and a second member 11B are connected by a thin, plate-like hinge portion 12. The living hinge 10 can be bent along the hinge portion 12. The living hinge 10 includes a guide portion 13 that comes into contact with the hinge portion 12 when bent.

[0014] 1 and 2, the first member 11A and the second member 11B are plate-shaped members, but this is not limiting. For example, one of the first member 11A and the second member 11B may be a container, and the other may be a lid.

[0015] The hinge portion 12 is a thin plate-like member disposed between the first member 11A and the second member 11B, and connects the first member 11A and the second member 11B. The hinge portion 12 is bendable. For example, the hinge portion 12 has a thickness of 1 mm and a width of 1-3 cm, but is not limited to these. The hinge portion 12 may have any width that allows it to be bent within a desired range.

[0016] 2, the guide portion 13 is a protrusion that prevents the hinge portion 12 from bending excessively. When the hinge portion 12 is bent, the hinge portion 12 comes into contact with the guide portion 13 along the guide portion 13, thereby limiting the degree of bending (radius of curvature) of the hinge portion 12 within a predetermined range. This reduces the stress applied to the hinge portion 12.

[0017] When the living hinge 10 can be bent on both sides, the guide portions 13 may be provided on both sides of the first member 11A. The guide portions 13 may be provided on both the first member 11A and the second member 11B.

[0018] In Fig. 2, the cross-sectional shape of the guide portion 13 is an arc at the contact point with the hinge portion 12, with a radius of curvature that corresponds to the bending radius of the hinge portion 12. As shown in Fig. 3, the contact point with the hinge portion 12 may not be a curved surface, and the hinge portion 12 and the guide portion 13 may contact at several points. The shape of the guide portion 13 may be such that it limits the degree of bending of the hinge portion 12 within a predetermined range. The hinge portion 12 and the guide portion 13 may not contact each other, but the degree of bending of the hinge portion 12 may be limited within a predetermined range.

[0019] 4 and 5 show an example of a living hinge including a sub-guide portion 14. FIG. 4 is a side view of the living hinge 10 including the sub-guide portion 14 in a normal state, and FIG. 5 is a side view of the living hinge 10 including the sub-guide portion 14 when folded. As shown in FIG. 4, the sub-guide portion 14 is disposed on the second member 11B on the same side as the guide portion 13. Also, as shown in FIG. 5, the sub-guide portion 14 is designed so as not to interfere with the guide portion 13 when the living hinge 10 is folded. The height of the sub-guide portion 14 (height from the surface of the second member 11B) may be the same as or lower than the height of the guide portion 13 (height from the surface of the first member 11A).

[0020] Figure 6 shows an example of a living hinge that is irradiated with sunlight. The living hinge 10 in Figure 6(a) does not have a guide portion 13 or a sub-guide portion 14, so sunlight is irradiated onto the hinge portion 12 from an obliquely upward direction in the figure. The living hinge 10 in Figure 6(b) has a guide portion 13, so it blocks sunlight from an obliquely upward left direction in the figure. The living hinge 10 in Figure 6(c) has a guide portion 13 and a sub-guide portion 14, so it blocks sunlight from an obliquely upward left direction and an obliquely upward right direction in the figure.

[0021] As a result of continued sunlight irradiation, the exposed portions (exposed surfaces) of the hinge portion 12, guide portion 13, and sub-guide portion 14 gradually deteriorate from the surface due to ultraviolet light and become brittle. On the other hand, the hinge portion 12, which is less susceptible to exposure to sunlight due to the guide portion 13 and sub-guide portion 14, is less susceptible to ultraviolet light deterioration. Because stress is concentrated in the hinge portion 12, suppressing deterioration of the hinge portion 12 leads to improved durability of the living hinge 10.

[0022] The living hinge 10 is made by integrally molding a first member 11A, a second member 11B, a hinge portion 12, and a guide portion 13. The polymer material used for the living hinge 10 has flexibility enough to bend. For example, polyethylene or polypropylene can be used as the polymer material.

[0023] Figure 7 is an example of a stress-strain curve for a polymer material. When stress is applied to polymer materials such as polypropylene and polyethylene, they pass through the elastic deformation region, then crack in the region beyond the yield point (plastic deformation region), and if the load increases further, they will break. This type of curve can be drawn by conducting a tensile test using a dumbbell-shaped test piece (JIS K7161-1, ISO527-1). Note that stress-strain curves take various forms depending on the type of polymer material, so they may not necessarily resemble the shape of the graph in Figure 7.

[0024] An appropriate living hinge structure can be designed by predetermining the thickness of the hinge portion 12 and the radius of curvature when the hinge portion 12 is bent. The design of the living hinge structure will be described below.

[0025] [Structural design of living hinges focusing on the yield point of polymeric materials] In designs focusing on the yield point, for example, the yield point of the polymeric material used in the living hinge 10 is measured in advance. A radius of curvature is calculated so that the strain of the hinge portion 12 when bent is less than 100%, more preferably within 80%, of the strain at the yield point, and the guide portion 13 is designed so that the hinge portion 12 bends at a value greater than this radius of curvature. This makes it possible to suppress the occurrence of cracks in the living hinge.

[0026] The strain of the hinge portion 12 is the expansion of the length of the outside of the hinge portion 12 when folded, and the radius of curvature is the radius of curvature of the inside of the hinge portion 12 when folded. The outside and inside of the hinge portion 12 are the outside and inside as viewed from the side when the hinge portion 12 is folded. When the living hinge 10 is formed with the hinge portion 12 in a folded state, the strain of the hinge portion 12 is the expansion of the length of the inside of the hinge portion 12 in the open state, and the radius of curvature is the radius of curvature of the inside of the hinge portion 12 when formed.

[0027] [Structural Design of Living Hinge Focusing on the Breaking Elongation of Polymeric Materials] In Figure 7, the rate of change between the initial length and the length after break is referred to as the breaking elongation. In a design focusing on the breaking elongation, for example, the breaking elongation of the polymeric material used for the living hinge 10 is measured in advance. A radius of curvature is calculated so that the elongation of the outer length of the hinge portion 12 when bent is within 50%, more preferably within 20%, of the breaking elongation. The guide portion 13 is designed so that the hinge portion 12 bends at a value larger than this radius of curvature. This can suppress the occurrence of cracks in the living hinge. When forming the living hinge 10 with the hinge portion 12 bent, a radius of curvature is calculated so that the elongation of the inner length of the hinge portion 12 in the open state is within 50%, more preferably within 20%, of the breaking elongation. The living hinge 10 is formed with the hinge portion 12 bent at a value larger than this radius of curvature.

[0028] [Structural design of living hinges in polymer materials taking degradation into account] When polymer materials are degraded by ultraviolet rays or other factors, the physical properties of the stress-strain curve often change before and after degradation. Figure 8 shows an example of the physical properties of a polymer material before and after degradation. Figure 8 shows the compressive strength, tensile strength, range of elastic deformation, strain at yield point, elongation at break, and Young's modulus for polypropylene before and after degradation.

[0029] The physical properties of the polymer material are measured in advance to determine their values ​​when it deteriorates. The radius of curvature within a range in which the strain of the hinge portion 12 falls within a standard range is calculated based on the yield point or breaking elongation taking into account the deterioration, and the guide portion 13 is designed so that the hinge portion 12 bends at a value greater than that radius of curvature. This makes it possible to suppress deformation of the living hinge and prevent cracks from occurring even when the living hinge deteriorates.

[0030] If ultraviolet light hits the inside of the living hinge 10 when it is bent, the inside of the living hinge 10 will deteriorate. Therefore, it is necessary to design the guide portion 13 in accordance with the compressive stress.

[0031] The stress-strain curve in Figure 7 is an example of tensile stress. The compression side (the inner side when the living hinge 10 is bent) is not simply deformed by compression. As shown in Figure 9, when the living hinge 10 is bent, numerous wrinkles form on the inner side of the hinge portion 12. In other words, the compressed portion on the inner side of the hinge portion 12 also elongates, but it is difficult to accurately measure or calculate the extent of this. When a living hinge is bent, the inner side is compressed while the outer side is stretched. The degree of inner compression (wrinkle formation) varies depending on the radius of curvature. At the same time, the outer side is stretched, resulting in elongation. Because it is difficult to accurately measure the strength of inner compression, in this embodiment, the outer or inner elongation value is used as a surrogate indicator of the strength of inner or outer compression.

[0032] Figure 10 shows an image of the hinge portion 12 when bent. In the case of polypropylene with similar tensile and compressive strengths and Young's moduli in tension and compression, when the hinge portion 12 with a thickness of d is bent with a curvature radius of R, the length of the center in the thickness direction (the position halfway through the thickness) does not change due to the bending, but the inside of the hinge portion 12 is compressed by compressive stress and the outside of the hinge portion 12 is stretched by tensile stress. If the initial length of the innermost portion of the hinge portion 12 is 1, then the length is compressed to R / (R+d / 2). If the initial length of the outermost portion of the hinge portion 12 is 1, then the length is stretched to (R+d) / (R+d / 2).

[0033] Although a circle is fitted in the calculation of the radius of curvature R, depending on the shape of the hinge part, an ellipse may be fitted as shown in Figure 11. When fitting an ellipse, the minor and major axes are found and the compression and extension are calculated as described above.

[0034] [Hinge portion] As shown in Figure 12(a), it is theoretically desirable for the thickness d of the hinge portion 12 to be uniform, and it is advisable to devise a way to bend the hinge portion 12 in a planar manner with a large radius of curvature R. If the thickness of the hinge portion 12 is uniform, stress can be prevented from concentrating locally, making it less likely to break. However, since it may be difficult to make the thickness of the hinge portion 12 uniform due to molding considerations, it does not necessarily have to be uniform, for example, as shown in Figures 12(b) to 12(d). According to the results of FEM analysis, the thickness d of the thickest point of the hinge portion 12 is max is the thickness of the thinnest part, d min If the design is made to be within 150%, and more preferably within 120%, of the above, it is possible to prevent stress from concentrating locally.

[0035] Here, the definition of the hinge portion 12 will be explained. The corresponding part of the hinge portion 12 varies depending on the bending angle. For example, as shown in FIG. 13 , when the hinge portion 12 is bent 180° during use, a semicircle (or a semiellipse) with a central angle of 180° is fitted. The area (the shaded area in FIG. 13 ) outside the line obtained by extending the diameter of the semicircle (the side including the bent part) is defined as the hinge portion 12. FIG. 14 shows the hinge portion 12 when bent 90°. When the hinge portion 12 is bent 90°, a fan shape with a central angle of 90° is fitted. The area (the shaded area in FIG. 14 ) outside the line obtained by extending the radius of the fan shape is defined as the hinge portion 12. In this way, a fan shape with a central angle that matches the bending angle during use is fitted to the hinge portion 12, and the area outside the line obtained by extending the diameter or radius is defined as the hinge portion 12.

[0036] [Molding in a Bent State] In injection molding of a living hinge, as shown in Figure 15, molten resin (polymer material) may be injected into a mold with the hinge portion 12 bent, to mold a living hinge with the hinge portion 12 bent. In this case, when the hinge portion 12 is open and flat in its normal state (the state in Figure 1), tensile stress continues to be applied to the inside of the hinge portion 12 (the upper side in Figure 1). For this reason, when molding a living hinge using a mold with the hinge portion 12 bent, it is necessary to design the hinge portion 12 taking into account the tensile stress that is applied to the hinge portion 12 in its normal state.

[0037] Figure 16 shows the hinge section 12 during molding when bent 90 degrees and when opened flat. For polypropylene with comparable tensile and compressive strengths and Young's moduli in tension and compression, when a hinge section 12 molded in a 90-degree bent state with a thickness d and a curvature radius R is straightened, the length of the center of the thickness direction (d / 2, halfway down the thickness d) remains unchanged at π(R + d / 2) / 2. However, the inner side 12a of the hinge section 12 is stretched by tensile stress, while the outer side 12b of the hinge section 12 is compressed by compressive stress. If the initial length of the inner side 12a is 1, the inner side 12a is stretched to a length of (R + d / 2) / R. If the initial length of the outer side 12b is 1, the outer side 12b is compressed to a length of (R + d / 2) / (R + d).

[0038] Furthermore, when molding the living hinge 10 using a bent mold, if the sub-guide portion 14 cannot be molded because it would interfere with the mold, it is appropriate to use a structure in which only the guide portion 13 is attached.

[0039] In Example 1, living hinges with different radii of curvature R were fabricated, and the curvature radius R and durability of the hinge portion 12 after ultraviolet degradation were investigated, and an example of a criterion for designing a living hinge with a focus on breaking elongation was obtained. In Example 1, molten resin was injected into a mold with the hinge portion 12 in a flat open position, and a living hinge with the hinge portion open 180° was fabricated.

[0040] As Example 1, as shown in Figure 17, six living hinges A to F were prepared, each with a hinge portion 12 thickness d of 600 μm and a guide portion 13 of varying sizes. The guide portion 13 was designed so that when the living hinges A to F were bent, the radius of curvature R of the hinge portion 12 was 62, 130, 300, 600, 1200, and 1500 μm, respectively. When the living hinges were bent, the radius R+d / 2 to the thickness center of the living hinges A to F was 362, 430, 600, 900, 1500, and 1800 μm, respectively. When the living hinges were bent, the radius R+d to the outer edge of the living hinges A to F was 662, 730, 900, 1200, 1800, and 2100 μm, respectively. When the living hinges are bent, the outward elongation (R+d) / (R+d / 2) of living hinges A to F is 83, 70, 50, 33, 20, and 17%, respectively. Living hinges A to F were made using polypropylene, which has a breaking elongation of 600% before UV degradation and gradually decreases with UV degradation, eventually reaching a breaking elongation of approximately 100%.

[0041] Living hinges A to F were subjected to weather resistance tests and bending tests. The weather resistance test was conducted in accordance with JIS K 5600-7-7 and JIS K 7350-2, with the black panel temperature set at 63°C, the temperature inside the chamber set at 38°C, and the humidity set at 50% RH. The water spray time was 18 minutes out of 2 hours. The ultraviolet radiation intensity at wavelengths of 300-400 nm was 60 W / m 2 Under these conditions, accelerated deterioration of the living hinge was carried out, with the hinge being bent once every 20 hours. This cycle was repeated to determine how many hours it took for cracks to appear.

[0042] Figure 18 shows the test results. Living hinge A, which had the smallest R value of 62 μm, developed cracks after 100 hours. Living hinge B, which had an R value of 130 μm, developed cracks after 200 hours. Note that UV degradation varies widely, so even if the R values ​​are similar, there will be variation in the time it takes for cracks to develop. Living hinge C, with an R value of 300 μm, and living hinge D, with an R value of 600 μm, had relatively long times until cracks developed, at 400 and 600 hours, respectively, and thus longer lifespans than living hinges A and B. Living hinge E, with an R value of 1200 μm, and living hinge F, with an R value of 1500 μm, had even longer lifespans, at 800 and 1000 hours, respectively, until cracks developed.

[0043] From the above, it was found that living hinges C and D, which have guides that bend at R values ​​where the outer elongation is 50% and 33% of the breaking elongation after UV degradation, have a relatively long lifespan of 400-600 hours before cracks appear, and living hinges E and F, which have guides that bend at R values ​​where the outer elongation is 20% and 17% of the breaking elongation after UV degradation, have an even longer lifespan of 800-1000 hours before cracks appear. In other words, it was found that if the thickness of the hinge portion 12 is d, the bending radius of curvature of the hinge portion 12 is R, and the elongation at break of the polymer material is X%, the living hinge 10 can withstand long-term use by providing a guide portion 13 designed to cause the hinge portion 12 to bend with a bending radius of curvature R that satisfies X × 0.5 > 100 × (R + d) / (R + d / 2), or more preferably X × 0.2 > 100 × (R + d) / (R + d / 2).

[0044] In Example 2, living hinges were fabricated by molding the hinge portion 12 in a bent state with different radii of curvature R, and the curvature radius R and durability of the hinge portion 12 after ultraviolet degradation were investigated to determine an example of a criterion for designing a living hinge with a focus on breaking elongation. In Example 2, molten resin was injected into a mold with the hinge portion 12 in a bent state, and a living hinge with the hinge portion 12 in a bent state was fabricated.

[0045] As Example 2, six living hinges G to L were prepared by bending them with varying radii of curvature R, as shown in FIG. 19 . The radii of curvature R of the hinge portion 12 during molding for living hinges G to L were 62, 130, 300, 600, 1200, and 1500 μm, respectively. The thickness d of each hinge portion 12 was 600 μm. The radius R+d / 2 to the center of the thickness was 362, 430, 600, 900, 1500, and 1800 μm, respectively. The radius R+d to the outer edge of living hinges G to L was 662, 730, 900, 1200, 1800, and 2100 μm, respectively. When living hinges G to L were opened flat 180°, the inner elongation (R+d / 2) / R was 484, 231, 100, 50, 25, and 20%, respectively. Living hinges G to L were made using polypropylene, which has a breaking elongation of 600% before UV degradation, and whose breaking elongation gradually decreases with UV degradation, eventually reaching a breaking elongation of approximately 100%.

[0046] Living hinges G to L were subjected to weather resistance tests and bending tests. The weather resistance test was conducted in accordance with JIS K 5600-7-7 and JIS K 7350-2, with the black panel temperature set at 63°C, the temperature inside the chamber set at 38°C, and the humidity set at 50% RH. The water spray time was 18 minutes out of 2 hours. The ultraviolet radiation intensity at wavelengths of 300-400 nm was 60 W / m 2 Under these conditions, accelerated deterioration of the living hinge was carried out, with the hinge being bent once every 20 hours. This cycle was repeated to determine how many hours it took for cracks to appear.

[0047] Figure 20 shows the test results. Living hinge G, which had the smallest R=62 μm, cracked after 40 hours. Living hinge H, with R=130 μm, cracked after 60 hours, and living hinge I, with R=300 μm, cracked after 80 hours. Note that UV degradation varies widely, so even if the R values ​​are similar, there will be variation in the time it takes for cracks to appear. Living hinge J, with R=600 μm, took a relatively long time to crack at 400 hours, longer than living hinges G, H, and I. Living hinge K, with R=1200 μm, and living hinge L, with R=1500 μm, took 700 hours and 800 hours, respectively, before cracking, demonstrating even longer lifespans.

[0048] From the above, it was found that living hinge J, which was designed to bend uniformly at a value of R where the inner elongation is 50% of the breaking elongation after UV degradation, had a relatively long life of 400 hours before cracking, while living hinges K and L, which were designed to bend uniformly at values ​​of R where the inner elongation is 25% and 20% of the breaking elongation after UV degradation, respectively, had even longer lifespans of 700-800 hours before cracking. In other words, it was found that living hinge 10 can withstand long-term use by designing hinge portion 12 to bend at a bending curvature radius R that satisfies X × 0.5 > 100 × (R + d / 2) / R, more preferably X × 0.2 > 100 × (R + d / 2) / R, where d is the thickness of hinge portion 12, R is the bending radius of curvature of hinge portion 12, and X% is the breaking elongation of the polymer material.

[0049] In addition to elongation at break, similar design can also be performed using the elongation (strain) value at the yield point. Because the deformation before the yield point is elastic or plastic, it is clear that hinges with higher durability can be designed using elongation at break. For example, a living hinge with a guide that bends at a radius of curvature where the outer elongation is 100% of the strain at the yield point after UV degradation will have a cracking time of 800-1500 hours, while a living hinge with a guide that bends at a radius of curvature where the outer elongation is 80% of the strain at the yield point after UV degradation will have a cracking time of 1000-2000 hours, which is expected to be longer-lasting results.

[0050] In this case, if the thickness of hinge portion 12 is d, the bending radius of curvature of hinge portion 12 is R, and the elongation at the yield point of the polymer material is X%, then living hinge 10 can be determined to be durable for long-term use by providing guide portion 13 designed to allow hinge portion 12 to bend at a bending radius of curvature R that satisfies X > 100 × (R + d) / (R + d / 2), more preferably X × 0.8 > 100 × (R + d) / (R + d / 2). Alternatively, living hinge 10 can be determined to be durable for long-term use by molding living hinge 10 in a state in which hinge portion 12 is bent at a bending radius of curvature R that satisfies X > 100 × (R + d / 2) / R, more preferably X × 0.8 > 100 × (R + d / 2) / R.

[0051] Therefore, when designing a living hinge for an item that is expected to be used outdoors for a long period of time or be bent frequently, it is desirable to use the elongation value at the yield point. A hinge that bends with a curvature radius that is within the non-yielding range (less than 100% of the elongation at the yield point) is suitable. Near the yield point, the slope of the stress-strain curve approaches zero, as shown in Figure 7, but a hinge that bends with a curvature radius that results in an elongation within 80% of the elongation at the yield point is more suitable.

[0052] Furthermore, even in the case of no UV degradation, a similar design can be performed using the breaking elongation value before UV degradation and the elongation value at the yield point before UV degradation. It is clear that a living hinge with high durability against bending can be designed when used in an environment that is not exposed to UV rays.

[0053] As described above, the living hinge 10 of this embodiment includes a first member 11A, a second member 11B, a hinge portion 12 that connects the first member 11A and the second member 11B, and a guide portion 13 that is disposed on the first member 11A and that limits the radius of curvature of the hinge portion 12 to within a predetermined range when the hinge portion 12 is bent. This reduces the stress applied to the hinge portion 12, thereby improving the durability of the living hinge.

[0054] 10 Living hinge 11A First member 11B Second member 12 Hinge portion 13 Guide portion 14 Sub-guide portion

Claims

1. A living hinge structure comprising: a first member; a second member; a hinge portion connecting the first member and the second member; and a guide portion disposed on the first member for limiting the radius of curvature of the hinge portion within a predetermined range when the hinge portion is bent.

2. A living hinge structure as described in claim 1, wherein the minimum radius of curvature when the hinge portion is bent is determined based on the strain or breaking elongation at the yield point of the polymer material of the living hinge, and the guide portion is shaped to restrict the radius of curvature when the hinge portion is bent to be larger than the minimum radius of curvature.

3. A living hinge structure according to claim 2, wherein the minimum radius of curvature is determined so that the elongation of the hinge portion falls within a range of 100% to 80% of the strain at the yield point.

4. A living hinge structure according to claim 2, wherein the minimum radius of curvature is determined so that the elongation of the hinge portion is 50% or less of the breaking elongation.

5. A living hinge structure according to any one of claims 2 to 4, wherein the value of the strain or breaking elongation at the yield point after the polymer material is deteriorated by ultraviolet light is used as a criterion.

6. A living hinge structure as claimed in claim 1, wherein the thickness of the thickest point of the hinge portion is within 150% of the thickness of the thinnest point.

7. A living hinge structure as claimed in claim 1, further comprising a sub-guide portion disposed on the second member on the same side as the guide portion.

8. A method for manufacturing a living hinge comprising a first member, a second member, a hinge portion connecting the first member and the second member, and a guide portion for restricting the radius of curvature of the hinge portion within a predetermined range when the hinge portion is bent, comprising: injecting molten polymer material into a mold to mold a living hinge in which the hinge portion is bent with a radius of curvature larger than the minimum radius of curvature determined based on the strain or breaking elongation at the yield point of the polymer material.

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