Synthetic resin cap and manufacturing method thereof

Abstract

To provide a screw cap which can be molded by thermoforming from a synthetic resin sheet, is effectively prevented from being plastically deformed by forcible removal when released from a mold, has clear irregularities formed on an outer surface of a cap skirt part as well as on an inner surface of the skirt part, and is effectively prevented from being deformed when gripped with fingers.SOLUTION: A synthetic resin cap has a top surface and a skirt part, and has a screw part on an inner surface of the skirt part, a plurality of protrusion parts are arranged circumferentially on the inner surface of the skirt part, each protruding radially inward by less than the height of the screw threads, and the recesses that are recessed radially inward are formed on an outer surface of the skirt part corresponding to the positions of the protrusion parts, so that a plurality of uneven parts are arranged in parallel on the inner and outer surfaces of the skirt part in the circumferential direction, and the pitch of the protrusion parts on the inner surface of the skirt part is in the range of 1 to 10 mm.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a synthetic resin cap having a plurality of circumferentially irregularities formed on the inner and outer surfaces of the skirt portion, and a manufacturing method thereof. More specifically, the present invention relates to a synthetic resin cap having clearly formed irregularities on the outer surface of the skirt portion, similar to those on the inner surface of the skirt portion, and which is suppressed from deforming when gripped with fingers, and a manufacturing method thereof. [Background technology]

[0002] BACKGROUND ART Resin screw caps, which are formed by integral molding of plastic and can exhibit liquid-tightness without using liners or packing, are widely used as screw caps applied to containers such as polyester bottles. For example, Patent Document 1 listed below describes such a resin screw cap, which is made by integrally molding a top surface and a skirt hanging down from the periphery of the top surface from plastic, with threads on the inner peripheral surface of the skirt, and in which an inner ring and an outer ring hanging down from the inner surface of the top surface are in close contact with the inner and outer surfaces of the container mouth, ensuring liquid-tightness.

[0003] The screw cap described in Patent Document 1 has high sealing properties and ensures reliable liquid-tightness, but because of its complex structure, it is generally molded by injection molding or compression molding, and it has been difficult to easily mold a thin, lightweight screw cap. Synthetic resin caps have also been proposed that are formed by thermoforming a synthetic resin sheet, such as by compressed air molding. For example, Patent Document 2 listed below proposes a method for forming a screw cap by thermoforming a synthetic resin sheet. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 3872546 [Patent Document 2] Special Publication No. 5-57097 Summary of the Invention [Problem to be solved by the invention]

[0005] The above-mentioned Patent Document 2 describes that the cap is formed by thermoforming a synthetic resin sheet, which has the advantage of reducing the amount of material used compared to caps formed by injection molding, etc. Caps formed by thermoforming synthetic resin sheets are thinner than caps formed by injection molding, etc., and therefore tend to deform when held with the fingers, making them difficult to apply to the mouth of a container. There is also the problem that when the cap is turned, friction occurs due to deformation, making it difficult to rotate.

[0006] Furthermore, since caps equipped with a sealing portion or a screw portion have undercut portions that protrude radially inward, the core mold located inside the cap is forcibly removed when the shaped cap is released from the mold. However, caps made of thin synthetic resin sheets are easily stretched during forcible removal, and there is a risk of plastic deformation due to excessive stretching. If the sealing portion or screw portion of the cap is deformed, it becomes difficult to obtain the desired sealing performance. On the other hand, it is possible to suppress deformation of the undercut portion by rotating the core mold and releasing it, but this method reduces productivity and does not conform to the purpose of providing a cap that is easy to mold and economical.

[0007] To solve this problem, the inventor proposed a cap in which a plurality of protrusions, each protruding radially less than the height of the screw threads, are arranged circumferentially on the inner surface of the cap skirt (Patent Application No. 2022-177287). With this cap, even if the mold is forcibly removed after thermoforming, the cap side wall reversibly expands radially, preventing plastic deformation of the cap and enabling the molding of a cap with excellent sealing properties. In the above-mentioned cap, on the inner surface side of the cap that comes into direct contact with the mold, the convex portions formed on the inner surface of the skirt portion are shaped to accurately reproduce the shape of the mold, but it is difficult to form concave portions on the outer surface of the skirt portion that correspond to the convex portions on the inner surface of the skirt portion with sufficient depth. The concave and convex portions on the outer surface of the cap skirt portion are often used to rotate the cap by engaging with concave and convex portions formed on a jig when capping the mouth of a container or scoring to form a tamper-evident band. In the above-mentioned cap, if the concave and convex portions on the outer surface of the cap skirt portion are formed to be able to firmly engage with the concave and convex portions of a jig in existing equipment, it will be possible to perform capping, scoring, etc. using existing equipment.

[0008] Therefore, the object of the present invention is to provide a screw cap that can be formed by thermoforming from a synthetic resin sheet and that is effectively prevented from plastic deformation due to forceful removal when released from the mold, in which the outer surface of the cap skirt has clear irregularities similar to the inner surface of the skirt, and in which deformation when gripped with the fingers is effectively suppressed, and a method for manufacturing the same. [Means for solving the problem]

[0009] According to the present invention, there is provided a synthetic resin cap having a top surface and a skirt portion and a threaded portion on the inner surface of the skirt portion, characterized in that a plurality of convex portions are arranged circumferentially on the inner surface of the skirt portion, each protruding radially inward by less than the height of the screw threads, and a recess that is recessed radially inward is formed on the outer surface of the skirt portion corresponding to the position of the convex portions, so that a plurality of convex and concave portions are arranged in parallel circumferentially on the inner and outer surfaces of the skirt portion, and the pitch of the convex portions on the inner surface of the skirt portion is in the range of 1 to 10 mm.

[0010] In the synthetic resin cap of the present invention, (1) The protrusion extends in the axial direction. (2) The inner peripheral surface of the convex portion on the inner surface of the skirt portion is linear in horizontal cross section. (3) The container has a drop-lid shape, with an inner wall extending upward from the top surface and an annular portion extending outward from the upper end of the inner wall, which is connected to the skirt portion. (4) The top surface has a dome shape that is convex upward in the axial direction. (5) The synthetic resin is polyethylene terephthalate. is preferred.

[0011] The present invention also provides a method for manufacturing the above-mentioned synthetic resin cap, which is characterized in that a synthetic resin sheet is used to form a cap shape by thermoforming, and then the formed cap is released from the inside of the cap by forcibly removing it. [Effects of the Invention]

[0012] In the synthetic resin cap of the present invention, a plurality of convex portions are formed circumferentially on the inner surface of the cap skirt portion, each protruding radially inward by less than the height of the screw threads. This creates concave portions between adjacent convex portions, forming circumferential irregularities on the inner surface of the skirt portion. As a result, when a radially outward force is applied to the skirt portion, the skirt portion expands circumferentially to cancel out the irregularities. Therefore, even if the core mold positioned inside the cap after shaping is forcibly removed, the core mold easily passes through undercut portions such as the screw portion and the sealing portion. After the core mold passes through the undercut portion, the cap skirt portion can easily return radially inward. This effectively prevents the screw portion and the sealing portion from losing their intended shape and from being stretched due to plastic deformation, thereby preventing the desired sealing performance from being impaired.

[0013] Furthermore, in the synthetic resin cap of the present invention, the pitch of the convex portions formed on the inner surface of the skirt portion is formed to be within a specific range, making it possible to form clear concave and convex portions with sufficient depth on the outer surface of the skirt portion.When performing capping or scoring as described above, the concave and convex portions on the outer surface of the skirt portion can be interlocked using existing equipment for caps with knurled grooves on the skirt portion, making it possible to rotate the cap efficiently. Furthermore, by having the pitch of the convex portions on the inner surface of the skirt portion within the above range, even a thin-walled cap can be effectively prevented from being deformed due to bending when the outer surface of the skirt portion of the cap is grasped with the fingers, and the cap can be easily placed over the mouth of the container.

[0014] The effects of the screw cap of the present invention are also evident from the results of the examples described below: When the pitch of the convex portions formed on the inner surface of the skirt portion is less than 1 mm or more than 10 mm, the concave portions on the outer surface of the skirt portion are not sufficiently concave compared to the corresponding convex portions on the inner surface of the skirt portion, whereas when the pitch of the convex portions formed on the inner surface of the skirt portion is in the range of 1 to 10 mm, the concave amount of the concave portions on the outer surface of the skirt portion is approximately the same as the protrusion amount of the corresponding convex portion on the inner surface of the skirt portion, and it can be seen that concaves and convexities are reliably formed not only on the inner surface but also on the outer surface of the skirt portion. Similarly, when the pitch is less than 1 mm or more than 10 mm, the ease of deformation, as indicated by the magnitude of the reaction force when the same displacement is applied, is shown to be greater than when the pitch is within the above range.It is therefore clear that by keeping the pitch within the range of the present invention, deformation due to gripping can be suppressed even for thin-walled skirt portions. [Brief explanation of the drawings]

[0015] [Figure 1] 1A and 1B are a cross-sectional side view (right side) and a side view (left side) of an example of a screw cap of the present invention. [Figure 2] 2 is a partially enlarged side cross-sectional view showing the screw cap of FIG. 1 applied to the opening of a container. FIG. [Figure 3] 2 is a partial cross-sectional view showing only the left half of the screw cap taken along line XX in FIG. 1. [Figure 4] 4A and 4B are partial cross-sectional views for explaining the state in which pressure acts radially outward on the cap in the view shown in FIG. 3, where (a) shows the state before the diameter is expanded, and (b) shows the state after the diameter is expanded. [Figure 5] 10A and 10B are diagrams showing an example of the shape of the inner circumferential surface of a convex portion in a horizontal cross section. [Figure 6] 10 is a diagram showing another example of the shape of the inner circumferential surface of the convex portion in a horizontal cross section. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0016] The screw cap of the present invention will be described with reference to the accompanying drawings. As shown in Figures 1 and 2, the screw cap, generally designated by 1, is generally composed of a dome-shaped top surface 2 that covers the container mouth 30, an inner wall 3 that extends upward from the outer periphery of the top surface 2, and a skirt portion 6 that is connected to the inner wall via an annular portion 4 that extends outward from the upper end of the inner wall 3 and has a threaded portion 5. In the specific example shown in Figures 1 and 2, the inner wall 3 is formed of an upper inner wall 3a that is in a dogleg shape, i.e., an inversely tapered shape in which the outer diameter of the inner wall increases downward, and a tapered lower inner wall 3b that has an outer diameter that decreases downward, and the bent portion that is the boundary between the upper inner wall 3a and the lower inner wall 3b is formed as an annular protrusion 7, and the inner wall 3 can elastically deform radially inward with the annular protrusion 7 as its apex.

[0017] That is, as shown in Figure 2, the maximum outer diameter L1 of the annular protrusion 7 when not applied to a container is larger than the inner diameter L2 of the container mouth 30 to which it is applied.Therefore, when the screw cap 1 is applied to the container mouth 30 and the cap is screw-engaged to the bottom dead center in a sealed state, the annular protrusion 7 and the inner surface of the container mouth 30 are radially pressed against each other, thereby enabling excellent liquid-tightness to be achieved. 1 and 2, the top surface 2 is lower than the annular portion 4, forming a so-called drop lid, which can suppress deformation due to decompression when the container is cooled after hot filling. Furthermore, because the top surface 2 is dome-shaped upward, when applied to a container filled with a content that has autogenous pressure, such as a carbonated beverage, upward pressure on the top surface 2 causes the annular protrusion 7 to press more strongly against the inner surface of the container mouth, improving sealing performance. Note that the top surface 2 may also be dome-shaped downward.

[0018] 1 to 4, in the screw cap of the present invention, a plurality of protrusions 20, 20... that extend in the axial direction of the cap and protrude radially inward of the cap are formed at equal intervals in the circumferential direction on the inner surface of the skirt portion 6. The amount by which these protrusions 20, 20... protrude radially inward is less than the height of the screw portion 5, so they do not impair the turning of the screw. An important feature of the present invention is that the pitch of the convex portions 20 on the inner surface of the skirt portion 6 is in the range of 1 to 10 mm, particularly 2.5 to 8.0 mm, and furthermore 2.5 to 4 mm, which allows recesses 22 of sufficient depth to be formed on the outer surface of the skirt portion 6 at locations corresponding to the convex portions 20, and clear concave and convex portions are also formed on the outer surface of the skirt portion 6, allowing for secure engagement with existing jigs and enabling efficient capping, scoring, etc. In this specification, the pitch (P) of the convex portions on the inner surface of the skirt portion refers to the linear distance between the centers of the recesses located on both sides of the convex portion 20 (the centers of the convex portions on the outer surface of the skirt portion), as shown in Figure 3. Furthermore, it is preferable that the inner surface 20a in the horizontal cross section of the convex portion 20 formed on the inner surface of the skirt portion is linear (flat) or arc-shaped, and it is particularly preferable that it is linear (flat), which makes it possible to form the depth t2 of the recess 22 on the outer surface of the skirt portion to a depth approximately equal to the protrusion amount t1 of the convex portion 20 on the inner surface of the skirt portion.

[0019] In the present invention, as shown in Figure 3, the inner and outer surfaces of the cap are formed with corresponding concave and convex shapes, that is, the outer surface of the cap skirt portion 6 is formed with a spiral recess 21 recessed radially inward corresponding to the shape of the screw portion 5, and multiple axial recesses 22, 22... extending in the axial direction and recessed radially corresponding to the shape of the protrusions 20, 20... are formed at equal intervals around the circumferential direction, so that the skirt portion 6 has a roughly bellows shape in horizontal cross section. As a result, as shown in Figure 4(a), when radially outward pressure acts from the inner surface of the skirt portion on the protrusions 20, 20, 20..., the protrusions 20, 20, 20... are stretched in the circumferential direction, thereby expanding the diameter of the skirt portion 6 (Figure 4(b)). After that, when the pressure is removed, the skirt portion 6 can easily restore its original uneven shape. Therefore, as mentioned above, in the screw cap of the present invention, when the cap is released from the mold during molding, undercut portions such as the sealing portion and the screw portion can be easily removed, effectively preventing the sealing portion and the screw portion from being plastically deformed and losing their function.

[0020] In the cap of the present invention, uniform radial outward expansion and restoration are easily achieved as described above, while the pitch of the convex portions 20 on the inner surface of the skirt portion within the above range effectively suppresses deformation of the cap caused by gripping the cap skirt portion with fingers, i.e., deformation that causes the cap's horizontal cross section to become approximately elliptical, such as simultaneous radial inward contraction and radial outward expansion that intersects approximately perpendicularly with the contracted diameter. Therefore, even a thin-walled cap can be smoothly applied to a container mouth when gripped with fingers and fitted onto the container mouth. In addition, in the screw cap of the present invention, multiple convex portions that protrude radially less than the height of the threads are arranged at specific intervals on the inner surface of the skirt portion, so that even if force is applied to the outer surface of the cap skirt portion when the cap is fastened to the mouth of a container and placed in a bag, for example, the convex portions on the inner surface of the skirt portion abut against the threads of the mouth of the container, thereby suppressing deformation of the cap and preventing loosening of the cap. This effect is particularly pronounced in caps made by thermoforming using a synthetic resin sheet, because they are thinner than caps made by compression molding or injection molding, and the skirt portion is more susceptible to deformation by external forces.

[0021] As described above, in the cap of the present invention, various embodiments can be adopted as long as multiple convex portions, each protruding radially by less than the height of the screw portion, are arranged circumferentially on the inner surface of the skirt portion at a pitch in the range of 1 to 10 mm. For example, the horizontal cross section of the convex portion formed on the inner surface of the skirt portion is not limited to the trapezoidal shape shown in the figures, but may be circular, elliptical, triangular, or rectangular, or may have a linear (flat) inner circumferential surface 20a as shown in Fig. 4, or an arc-shaped inner circumferential surface 20a that is convex radially inward as shown in Fig. 5, or an arc-shaped inner circumferential surface 20a that is convex radially outward as shown in Fig. 6. As mentioned above, it is preferable that the horizontal cross section of the convex portion be trapezoidal or rectangular, since this allows for the formation of recesses with sufficient depth on the outer surface of the skirt portion. It is also possible to combine and arrange protrusions with different axial lengths, widths, or heights, arrange them so that the spacing between adjacent protrusions is different, or form a shape in which the height or shape of the protrusions changes between the top and bottom.

[0022] In the cap of the present invention, the height of the thread portion (the amount of protrusion from the inner surface of the skirt portion) can be changed as appropriate depending on the height of the thread portion of the mouth of the container to which the cap is applied and the synthetic resin sheet used, but for a cap with a 28 mm diameter that fits a typical bottle-shaped container, a height in the range of 0.4 to 1.5 mm is preferred. The height of the protrusions can be changed as appropriate depending on the thickness of the skirt portion, the width and length of the protrusions, or their arrangement (spacing and number), but is preferably 10 to 80% of the height of the thread portion, more preferably 30 to 50%. The width of the convex portions (distance on the inner peripheral surface) is in the range of 40 to 80% of the pitch of the convex portions, and from the viewpoint of uniform concavity and convexity, it is preferably 1 / 2 the pitch. From the viewpoint of being able to interlock the concavities and convexities on the outer surface of the skirt portion, if the pitch is less than 3 mm, the width of the convex portions is preferably 1 / 2 the pitch, but if the pitch is 3 mm or more, it is preferable to make the width of the convex portions larger, at 50 to 80%. This allows the jig to be fitted into the cap without catching, even if the concavities and convexities on the cap and jig do not match. It is also important that the convex portions are formed at an even pitch within the above range, regardless of the bottle opening diameter or the width of the inner peripheral surface of the convex portions, but in the case of a cap to be used with a bottle with a diameter of 28 mm, it is particularly preferable that the pitch of the convex portions be in the range of 1.0 to 10 mm, and expressed as a central angle in the range of 5 to 45°. The axial length of the convex portions can be changed as appropriate depending on the positions of the threaded portion and the sealed portion that form the undercut portion, but it is desirable that the length extend from a position above the top dead center of the threaded portion to a position below the bottom dead center, as in the specific example shown in Figures 1 and 2.

[0023] As described above, the cap of the present invention is particularly preferably a screw cap formed by thermoforming such as vacuum forming using a synthetic resin sheet. That is, a cap formed by thermoforming using a synthetic resin sheet is thinner than a cap formed by compression molding or injection molding, and therefore can be easily expanded in diameter, but is prone to plastic deformation. However, according to the screw cap of the present invention, a convex portion is formed on the inner surface of the skirt portion, and a concave portion corresponding to this convex portion is formed on the outer surface of the skirt portion, so that a substantially bellows-shaped skirt portion can be formed, as described above, and therefore deformation in the elastic range is possible, particularly in a cap having a thin skirt portion, and impairment of the function of the sealing portion and screw threads, which become undercut portions when released from the mold, can be effectively prevented. The thickness of the synthetic resin sheet that can be used to form the cap of the present invention varies depending on the synthetic resin sheet used, but if it is made of a polyester resin such as polyethylene terephthalate, it is preferably 0.5 to 1.2 mm thick, and if it is made of an olefin resin such as polypropylene, it is preferably 0.5 to 1.5 mm thick.

[0024] Furthermore, because the skirt portion of the cap of the present invention can expand and return to its original diameter due to elastic deformation when released from the mold, releasability can also be improved by molding methods such as compression molding and injection molding. That is, by forming convex portions on the inner surface of the skirt and concave portions corresponding to these convex portions on the outer surface of the skirt using a mold in compression molding or injection molding, a skirt portion having a generally bellows shape can be formed, just like the thermoformed cap using the synthetic resin sheet described above, which enables deformation in the elastic range and effectively prevents loss of function of the sealing portion and screw portion when released from the mold.

[0025] As described above, the screw cap of the present invention has excellent sealing and opening properties because the sealing portion and screw portion are not damaged even when forcibly removed. However, as shown in the specific example in Figures 1 and 2, the inner surface of the annular portion 4 is formed with a contact ring 9 that presses against the tip of the container mouth portion 30 when the cap reaches the bottom dead center due to screw engagement, thereby further improving the sealing properties of the cap. Furthermore, by forming the portion of the skirt portion 6 that connects to the annular portion 4 above the screw portion 5 (hereinafter referred to as the "upper skirt portion 10") in an inverted tapered shape in which the outer diameter of the skirt portion decreases as it goes downward, the tip of the container mouth portion 30 is fastened from the radially outside, forming a seal between the outer surface of the container mouth portion and the inner surface of the upper skirt portion 10, further improving the sealing performance of the cap. Furthermore, in the specific example shown in Figures 1 and 2, the formation of a countersink portion 8, combined with the fact that the countersink portion can also elastically deform in the radial direction of the cap when applied to a container, as described above, increases the radially outward pressing force on the inner surface of the container mouth due to the elastic deformation of the annular protrusion, making this a preferred embodiment because it can exhibit excellent sealing properties.However, depending on the thickness of the screw cap and the size of the top surface, it is possible to exhibit excellent sealing properties even if a countersink portion is not formed.

[0026] As described above, the screw cap of the present invention is preferably formed by using a synthetic resin sheet and thermoforming it using vacuum pressure forming, plug-assisted pressure forming, or other such methods. This results in a thin and lightweight cap, and, combined with the pitch of the convex portions on the inner surface of the skirt portion being within the above-described range, it is possible to form concave portions on the outer surface of the skirt portion that correspond to the convex portions on the inner surface of the cap skirt portion, thereby forming a skirt portion with a generally bellows shape. As a result, the cap is elastically deformable when released from the mold, easily expands and restores its diameter, and can be molded to the desired dimensions without plastic deformation of the sealing portion or thread portion, which would otherwise be undercut. Furthermore, by setting the pitch of the convex portions of the cap skirt portion within the above range, the concave portions on the outer surface of the skirt portion can be formed as concave portions of approximately the same depth as the protrusion amount of the convex portions on the inner surface of the skirt portion, and since clear concave and convex portions are formed on the outer surface of the skirt portion, they can reliably mesh with the concave and convex portions of the jig used for capping, scoring, etc., and capping, scoring, etc. can be performed using existing equipment.

[0027] In addition, by using a multilayer sheet as the synthetic resin sheet, which has a gas barrier intermediate layer made of a gas barrier resin such as an ethylene vinyl alcohol copolymer or aluminum foil, it is possible to provide a resin cap that is thin yet has excellent gas barrier properties. Furthermore, by using a multi-layer sheet with recycled material in the middle layer and virgin material in the inner and outer layers, it is possible to provide a cap that not only has no hygiene issues in the parts that come into contact with food and people, but also has a higher recycled material usage rate.

[0028] The screw cap of the present invention can be used for containers including all conventionally known containers made of metal, glass, resin, etc., and the diameter of the opening is not limited, but it is suitable for containers of 100 mm or less, particularly 15 to 50 mm, and is also suitable for containers with wider openings. That is, when forming a cap with a wider opening by compression molding or injection molding, compression molding requires a large mold and pressure, which inevitably leads to an increase in the size of the equipment, while injection molding requires a large projected area of ​​the molded product, which increases the required clamping force, which inevitably leads to an increase in the size of the equipment. However, in the case of a cap formed by thermoforming using a synthetic resin sheet, the wider the opening, the higher the sheet utilization rate and the more efficient it is. [Example]

[0029] Example 1 A commercially available 0.7 mm thick A-PET sheet was heated to approximately 110°C, and vacuum-pressure molding was performed using a resin mold in which the inner surface of the formed cap skirt had protrusions that were evenly arranged circumferentially and had a pitch of 2.53 mm (inner surface distance: 1 / 2 the pitch) extending in the axial direction and lower than the height of the screw threads, and the inner surface had linear axial extensions that were aligned in the circumferential direction.The cap was then released from the mold by force removal, and the part below the tamper-evident band was cut with scissors to obtain a sample. The molded sample was a 28mm diameter cap that fits a typical PET bottle, with a thread height of 0.65mm and a convex part height of 0.3mm.

[0030] (Examples 2 to 6, Comparative Examples 1 and 2) Samples were prepared in the same manner as in Example 1, except that the pitch of the convex portions was changed to the value shown in Table 1.

[0031] Example 7 A sample was produced in the same manner as in Example 1, except that the inner peripheral surface of the convex portion was formed in an arc shape as shown in FIG. 5, rather than in a straight line.

[0032] (Method for measuring the depth of the recess on the outer surface of the skirt) The top view was taken with an optical microscope to observe the unevenness of the outer peripheral surface of the skirt at the top of the cap and the unevenness of the inner peripheral surface of the skirt through the light, and the depth of the recesses formed on the outer peripheral surface of the skirt was measured. The results are also shown in Table 1.

[0033] (Evaluation of occurrence of free rotation between cap and capping jig) A commercially available PET bottle was fixed to the torque meter, and a sample of the cap formed in the above-mentioned Examples and Comparative Examples was placed on the bottle mouth. A chuck, a capping jig, was fitted into the cap and the cap was then manually capped. The chuck was shaped using a 3D printer to match the irregularities on the outer periphery of the skirt of the cap. Capping was performed to achieve a target torque of 1 N-m or 2 N-m, and it was confirmed whether or not freewheeling occurred between the cap and chuck. If freewheeling did not occur, it was marked with a ◯; if freewheeling did occur, it was marked with an ×. The results are also shown in Table 1.

[0034] (Analysis of deformation due to gripping) A model was created for a 0.5mm thick cap, and under the same conditions except for changing the pitch of the convex parts, the reaction force (1mm width) was calculated when a 1mm displacement was applied using 2D analysis to determine the ease of deformation. Solidworks Simulation was used as the analysis software. The results are also shown in Table 1.

[0035] (Evaluation of ease of opening and tightening the cap) The molded cap sample was placed on the mouth of a bottle, and the cap was opened and closed. The ease of opening and closing was evaluated. Good: Not easily deformed, and the uneven outer surface of the cap makes it easy to open and close. △: The ease of deformation and the unevenness of the outer surface of the cap make it slightly inferior, so it is slightly inferior in ease of opening and tightening, but there is no problem in use. ×: The cap is easily deformed and the uneven outer surface is difficult to feel catch on, so it is gripped tightly, making it even more difficult to open and close. The results are also shown in Table 1.

[0036] [Table 1]

[0037] (Consideration) A cap with a pitch of the convex portions on the inner skirt portion within the range of the present invention has a concave portion on the outer skirt portion that has a depth approximately equal to the protrusion of the convex portions. Therefore, when capped, free rotation between the cap and the chuck is unlikely to occur. Furthermore, because the reaction force when deformed is large, there is little displacement when gripped with fingers, and the small deformation. The large unevenness on the outer peripheral surface provides a good grip, making the cap easy to open and tighten. [Industrial Applicability]

[0038] The screw cap of the present invention can be formed by thermoforming from a resin sheet, and has resealability through screw engagement and can exhibit liquid-tightness, so it can be used as a cap for beverage containers such as polyester bottles. [Explanation of symbols]

[0039] 1 screw cap, 2 top surface, 3 inner wall, 4 annular portion, 5 screw portion, 6 skirt portion, 7 annular protrusion portion, 8 countersink portion, 9 contact ring, 10 upper skirt portion, 20 cap inner surface convex portion, 22 cap outer surface concave portion.

Claims

1. A synthetic resin cap having a top surface and a skirt portion, and a screw portion on the inner surface of the skirt portion, A plurality of convex portions are arranged in the circumferential direction on the inner surface of the skirt portion, each protruding radially inward by an amount smaller than the height of the screw threads, and a concave portion is formed in the outer surface of the skirt portion corresponding to the positions of the convex portions, the concave portion being recessed radially inward, so that a plurality of concave and convex portions are arranged in parallel on the inner and outer surfaces of the skirt portion in the circumferential direction, A synthetic resin cap characterized in that the pitch of the convex portions on the inner surface of the skirt portion is in the range of 1 to 10 mm.

2. 2. The synthetic resin cap according to claim 1, wherein the protrusion extends in the axial direction.

3. 3. The synthetic resin cap according to claim 1, wherein the inner peripheral surface of the convex portion on the inner surface of the skirt portion is linear in horizontal cross section.

4. 3. The synthetic resin cap according to claim 1, wherein the cap is in the form of a drop lid, and the cap is connected to the skirt portion via an inner wall extending upward from the top surface and an annular portion extending outward from the upper end of the inner wall.

5. 5. The synthetic resin cap according to claim 4, wherein the top surface is a dome-shaped surface that is convex upward in the axial direction.

6. 3. The synthetic resin cap according to claim 1, wherein the synthetic resin is polyethylene terephthalate.

7. 2. A method for manufacturing a synthetic resin cap according to claim 1, characterized in that a synthetic resin sheet is thermoformed into a cap shape, and then the shaped cap is released from the mold by forcibly removing it from the inside of the cap.

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