Plug and its manufacturing method

Abstract

To provide a method of manufacturing a mouthpiece that is light in weight due to foaming and that can adequately control liquid leakage.SOLUTION: A method of manufacturing a mouthpiece disclosed herein includes: a process (A) for preparing a molten resin composition containing a resin material and a supercritical fluid; a process (B) for injecting the molten resin composition into a cavity of a mold; a process (C) for foaming the molten resin composition in the cavity by decreasing the pressure; and a process (D) for recovering from the mold a mouthpiece with a plurality of voids originating from the foaming of the molten resin composition. When supercritical nitrogen is used in the process (A), 0.1 to 1.2 pts.mass of nitrogen is added to 100 pts.mass of the resin material. When supercritical carbon dioxide is used in the process (A), 0.8 to 4.2 pts.mass of carbon dioxide is added to 100 pts.mass of the resin material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a stopper and a method for making the same. [Background technology]

[0002] In recent years, plastic molded products have been used in a wide variety of everyday items and industrial products. As the quality of plastic molded products has improved and costs have decreased, their general use has progressed, increasing the demand for plastic molded products. Meanwhile, as seen in the problem of marine plastic waste, attention has been drawn to the impact of microplastics on the environment, leading to a growing trend toward eliminating plastic and reducing the use of plastic products.

[0003] When it comes to disposable plastic containers for food and daily necessities, there is a growing demand from users to reduce the amount of petroleum-derived plastic used. In response to these demands, efforts are being made to reduce the amount of plastic used, such as using plant-based resins as part of the raw material, utilizing recycled plastic materials, and devising dimensions and shapes.

[0004] Foam molding is known as a means of reducing the weight of plastic molded products. Foam molding can be broadly divided into chemical foam molding and physical foam molding. Chemical foam molding uses a blowing agent. On the other hand, physical foam molding uses a supercritical fluid, and this method is called supercritical fluid molding. Chemical foam molding has issues such as concerns about the negative environmental impact of the blowing agent and mold contamination. Supercritical fluid molding has traditionally been applied to relatively large industrial products such as automotive parts molding and office equipment. In recent years, with improvements in supercritical fluid generation technology and resin composition kneading technology, the application of supercritical fluid molding to high-cycle injection molding has been considered. Patent Documents 1 to 3 disclose food containers manufactured by supercritical fluid molding. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6085729 [Patent Document 2] Patent No. 6430684 [Patent Document 3] Japanese Patent Application Publication No. 2020-040690 Summary of the Invention [Problem to be solved by the invention]

[0006] In order to broaden the scope of application of supercritical fluid molding, the present inventors attempted to use supercritical fluid molding to manufacture stoppers for containers containing liquids such as drinks and jellies. As a result, they found that while the amount of resin material used can be reduced as the degree of foaming increases, liquid leakage tends to occur.

[0007] The present disclosure provides a plug that is lightweight due to foaming and can sufficiently suppress liquid leakage, and a method for manufacturing the same. [Means for solving the problem]

[0008] One aspect of the present disclosure relates to a method for producing a plug using a supercritical fluid. The production method according to the first aspect of the present disclosure uses nitrogen in a supercritical state. Specifically, this production method includes the steps of (A1) preparing a molten resin composition containing a resin material and nitrogen in a supercritical state, (B1) injecting the molten resin composition into a mold cavity, (C1) foaming the molten resin composition in the cavity by reducing the pressure, and (D1) recovering from the mold a plug having a plurality of voids resulting from the foaming, wherein the amount of nitrogen in a supercritical state is 0.1 to 1.2 parts by mass when the mass of the resin material in the molten resin composition is 100 parts by mass.

[0009] According to the above-mentioned manufacturing method, by setting the amount of nitrogen in the molten resin composition within the above-mentioned range, it is possible to sufficiently achieve both weight reduction and suppression of liquid leakage of the plug. According to the investigations of the present inventors, when the amount of nitrogen in the molten resin composition exceeds 1.2 parts by mass, patterns recognizable as traces of irregular flow of the resin material appear on the surface of the plug, and the tip of the spout in particular is likely to become uneven. It is presumed that this unevenness is one of the causes of liquid leakage.

[0010] Supercritical carbon dioxide may be used instead of supercritical nitrogen. That is, the production method according to the second aspect of the present disclosure includes the steps of (A2) preparing a molten resin composition containing a resin material and supercritical carbon dioxide, (B2) injecting the molten resin composition into a mold cavity, (C2) foaming the molten resin composition in the cavity by reducing the pressure, and (D2) recovering from the mold a plug having a plurality of voids resulting from the foaming, wherein the amount of supercritical carbon dioxide is 0.8 to 4.2 parts by mass when the mass of the resin material in the molten resin composition is 100 parts by mass.

[0011] According to the above-mentioned manufacturing method, by setting the amount of carbon dioxide in the molten resin composition within the above-mentioned range, it is possible to sufficiently achieve both weight reduction of the plug and suppression of liquid leakage. According to the investigations of the present inventors, when the amount of carbon dioxide in the molten resin composition exceeds 4.2 parts by mass, patterns recognizable as traces of irregular flow of the resin material appear on the surface of the plug, and the tip of the spout in particular is likely to become uneven. It is presumed that this unevenness is one of the causes of liquid leakage.

[0012] One aspect of the present disclosure relates to a stopper that, together with a container body, constitutes a container. The stopper includes a fusion portion having a surface to which the container body is heat-fused, a mouth portion having a threaded portion formed on the outer periphery, and a foam layer having voids derived from a supercritical fluid.

[0013] From the viewpoint of the mechanical strength of the plug, it is preferable that the plurality of voids in the foam layer are mutually independent. Furthermore, it is preferable that the plug has a foam layer in which the area ratio of voids determined by cross-sectional observation is 25% or less. By ensuring this area ratio of 25% or less, sufficient mechanical strength of the plug can be ensured and liquid leakage can be suppressed to an even greater extent. [Effects of the Invention]

[0014] According to the present disclosure, a plug that is lightweight due to foaming and can sufficiently suppress liquid leakage, and a method for manufacturing the same are provided. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a cross-sectional view showing a plug according to one embodiment of the present disclosure. [Figure 2] FIG. 2 is a front view schematically showing an example of a spout container equipped with the stopper shown in FIG. [Figure 3] FIG. 3 is a cross-sectional view showing a plug according to another embodiment of the present disclosure. [Figure 4] FIG. 4 is a cross-sectional view showing a plug according to another embodiment of the present disclosure. [Figure 5] FIG. 5 is a perspective view schematically illustrating an example of a container equipped with a plug according to another embodiment of the present disclosure. [Figure 6] FIG. 6 is a cross-sectional view schematically showing a plug provided in the container shown in FIG. [Figure 7] FIG. 7 is a CT image of the plug according to Comparative Example 1. [Figure 8] 8(a) to 8(c) are CT images of the plugs according to Examples 1 and 2 and Comparative Example 2. FIG. [Figure 9] 9(a) to 9(c) are CT images of the plugs according to Examples 3 and 4 and Comparative Example 3. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of the present disclosure will be described in detail, but the present invention is not limited to the following embodiments.

[0017] <Manufacturing method of stoppers> The method for manufacturing the plug according to this embodiment includes the following steps. (A) A step of preparing a molten resin composition containing a resin material and a supercritical fluid. (B) A step of injecting a molten resin composition into a cavity of a mold. (C) A step of foaming the molten resin composition in the cavity by reducing the pressure. (D) A step of recovering from the mold the plug having a plurality of voids resulting from the foaming of the molten resin composition. The series of steps (A) to (D) can be carried out using, for example, a MuCell injection molding machine ("MuCell" is a registered trademark of Trexel Co. Ltd.) (see Patent Documents 1 and 2).

[0018] [(A) Process] First, a molten resin composition containing a resin material and nitrogen or carbon dioxide in a supercritical state is prepared. Examples of resin materials include polypropylene resin and polyethylene resin. According to the inventors' studies, when nitrogen is used, the molten resin composition is prepared by adding 0.1 to 1.2 parts by mass of supercritical nitrogen per 100 parts by mass of the resin material. When the amount of nitrogen is 0.1 part by mass or more, weight reduction of the plug due to foaming can be achieved. On the other hand, when the amount of nitrogen is 1.2 parts by mass or less, liquid leakage can be sufficiently suppressed. From the viewpoint of weight reduction of the plug, the lower limit of the amount of nitrogen is preferably 0.15 parts by mass, more preferably 0.2 parts by mass. From the viewpoint of further suppressing liquid leakage and improving the mechanical strength of the plug, the upper limit of the amount of nitrogen is preferably 0.75 parts by mass, more preferably 0.55 parts by mass.

[0019] When carbon dioxide is used, 0.8 to 4.2 parts by mass of supercritical carbon dioxide is added to 100 parts by mass of the resin material to prepare a molten resin composition. When the amount of carbon dioxide is 0.8 parts by mass or more, weight reduction of the plug due to foaming can be achieved. On the other hand, when the amount of carbon dioxide is 4.2 parts by mass or less, liquid leakage can be sufficiently suppressed. From the viewpoint of weight reduction of the plug, the lower limit of the amount of carbon dioxide is preferably 1.0 parts by mass, and more preferably 1.2 parts by mass. From the viewpoint of further suppressing liquid leakage and improving the mechanical strength of the plug, the upper limit of the amount of carbon dioxide is preferably 2.8 parts by mass, and more preferably 1.7 parts by mass.

[0020] As described above, nitrogen or carbon dioxide can be used as the supercritical fluid. According to the studies of the present inventors, from the viewpoint of improving foaming properties, it is preferable to use nitrogen as the supercritical fluid rather than carbon dioxide.

[0021] When the resin material used is a polypropylene resin, the temperature of the molten resin composition (screw cylinder temperature) is preferably about 210 to 230° C. When the resin material used is a polyethylene resin, this temperature is preferably about 220 to 240° C. When this temperature is equal to or higher than the lower limit, the resin flows easily within the cavity, while when the temperature is equal to or lower than the upper limit, for example, burning of the resin tends to be suppressed.

[0022] The molten resin composition may contain components other than the resin material and the supercritical fluid, i.e., the molten resin composition may further contain, as necessary, for example, a filler, a colorant, a slip agent, an antistatic agent, etc.

[0023] [(B) process and (C) process] The molten resin composition prepared in step (A) is injected into the cavity through the gate of the mold. When the molten resin composition is introduced into the cavity, a decrease in pressure causes cell growth, resulting in the generation of independent cells within the resin composition. The injection speed is preferably 5 to 100 mm / sec, and more preferably 10 to 30 mm / sec. An injection speed of 5 mm / sec or higher tends to facilitate the resin reaching the end of the flow, thereby suppressing the occurrence of short shots. On the other hand, an injection speed of 100 mm / sec or lower tends to suppress the generation of sparsely populated areas within the molded product and the generation of unevenness on the surface of the molded product. According to the studies of the present inventors, when the resin material is a polyethylene resin and the supercritical fluid is nitrogen, the injection speed in step (B) is preferably 20 to 40 mm / sec, and more preferably 25 to 35 mm / sec. When the resin material is a polyethylene resin and the supercritical fluid is carbon dioxide, the injection speed in step (B) is preferably 20 to 40 mm / sec, and more preferably 25 to 35 mm / sec.

[0024] According to the inventors' investigations, when a plug to be manufactured has both thick and thin sections, foaming due to the supercritical fluid occurs primarily in the thick sections, while foaming is suppressed in the thin sections. Therefore, the weight of the plug as a whole can be reduced by foaming, while the thin sections can maintain sufficient strength. When the molten resin composition is introduced into the cavity through the gate, foaming occurs due to a decrease in pressure. While conventional injection molding involves a step of applying pressure to the cavity (holding pressure) after filling the resin, the above-described manufacturing method, for example, does not perform this pressure application step or applies a pressure lower than the peak filling pressure, thereby causing foaming in the thick sections. It is believed that the main reason foaming in the thin sections is suppressed is that the resin cools and solidifies before foaming occurs in the thin sections.

[0025] As described above, from the viewpoint of sufficiently promoting foaming in the thick-walled portion of the plug, it is not necessary to perform a step of applying pressure to the cavity (pressure holding) after filling the cavity with the molten resin composition. On the other hand, from the viewpoint of suppressing foaming in the thin-walled portion of the plug, it is preferable not to perform a step called "core-back" for reducing the pressure in the cavity. Core-back is a step of expanding the volume of the cavity by moving the movable part of the mold before the molten resin filled in the cavity has completely solidified (see Patent Document 1). In this embodiment, as described above, the pressure reduction caused by introducing the molten resin composition into the cavity can cause foaming in the region of the cavity corresponding to the thick-walled portion, while suppressing foaming in the region corresponding to the thin-walled portion.

[0026] [(D) Process] When the temperature of the plug drops to about 30 to 60°C, it is removed from the mold. The formation of voids inside the plug due to foaming reduces its weight and the amount of plastic material used. The formation of voids preferably reduces the weight by 5% or more by mass compared to a molded product without voids (a normal injection-molded product).

[0027] <Stop> The stopper 5 shown in FIG. 1 is manufactured through the above-described process. The stopper 5 comprises a fusion section 1, a mouth section 2, and a plurality of plates 3. The fusion section 1 has a surface 1a to which a container body 8 (see FIG. 2) is heat-fused. The mouth section 2 has a threaded section 2a formed on its outer periphery, and a cap 6 (see FIG. 2) is attached. The plurality of plates 3 are provided between the fusion section 1 and the mouth section 2, extending laterally. The plates 3 are used to transport and position the stopper 5 or the spout container 10 during the manufacturing process of the spout container 10 shown in FIG. 2 or during the process of filling the spout container 10 with a liquid.

[0028] The plug 5 has a foamed layer 5a having voids derived from nitrogen or carbon dioxide in a supercritical state. The surface of the plug 5 is preferably composed of a skin layer 5b. The skin layer 5b is preferably free of appearance defects such as minute irregularities (dimples) or foam marks (swirl marks). In the plug 5, a portion having a thickness of about 1.5 to 8 mm corresponds to the thick-walled portion. Specifically, the fused portion 1 corresponds to the thick-walled portion. On the other hand, in the plug 5, a portion having a thickness of about 0.8 to 0.9 mm corresponds to the thin-walled portion. Specifically, the tip of the mouth portion 2 and the plate 3 correspond to the thin-walled portion.

[0029] As shown in FIG. 1, in this embodiment, a foam layer 5a is formed in the fused portion 1. In the foam layer 5a, it is preferable that voids caused by foaming are independent of each other. In contrast, if multiple voids are not independent but integrated, the strength of the plug 5 tends to be insufficient. When nitrogen is used as the supercritical fluid, the diameter of the voids that exist independently of each other in the foam layer 5a is approximately 0.6 to 1.0 mm. On the other hand, when carbon dioxide is used as the supercritical fluid, the diameter of the voids that exist independently of each other in the foam layer 5a is approximately 1.2 to 1.8 mm. Note that voids are not completely formed in regions other than the thick portion of the plug 5; for example, fine voids with an average diameter of approximately 1 to 50 μm are formed.

[0030] The area ratio of voids in the foam layer 5a is preferably 25% or less. This area ratio of 25% or less ensures sufficient mechanical strength of the plug and can further suppress liquid leakage. On the other hand, this area ratio of 15% or more can more fully reduce the weight of the plug. This area ratio can be determined from cross-sectional images obtained by CT scanning.

[0031] <Spout container> The spout container 10 comprises a spout 5, a cap 6, and a container body 8. The cap 6 has a threaded portion formed on its inner surface so that it can be attached to the opening 2. The container body 8 is made of a flexible packaging material. The inner surface of the container body 8 is made of a resin with heat-sealing properties, and is heat-sealed to the surface 1a of the fusion portion 1.

[0032] Although the embodiments of the present disclosure have been described above, the present invention is not limited to the above embodiments. For example, in the above embodiments, the stopper 5 having the configuration shown in Fig. 1 is exemplified, but the stopper may have the configuration shown in Fig. 3. The stopper 15 shown in Fig. 3 further includes a straw portion 4 extending from the fused portion 1 in the direction opposite to the mouth portion 2.

[0033] The plug may also have the configuration shown in FIG. 4. The plug 25 shown in FIG. 4 includes two rods 21, 22 extending parallel to each other from the fused portion 1 toward the opposite side of the mouth 2, and first and second connecting portions 23a, 23b connecting the rods. The first connecting portion 23a connects the tips of the two rods 21, 22. The second connecting portion 23b is provided parallel to the first connecting portion 23a, near the fused portion 1. The distance between the first connecting portion 23a and the second connecting portion 23b may be approximately 8 to 20 mm. This configuration of the plug 25 is intended to form a gap within the spout container, allowing the liquid to easily flow to the mouth 2, even when there is little liquid remaining in the spout container. The second connecting portion 23b has a thickness of approximately 1.5 to 2 mm, which corresponds to a thick portion.

[0034] In the above embodiment, a stopper for a spout container is illustrated, but the stopper according to the present disclosure may be applied to other containers. FIG. 5 is a perspective view schematically showing an example of a container equipped with a stopper according to the present disclosure. The container 30 shown in this figure is intended for selling juice, soy sauce, or alcohol, for example. The container 30 comprises a container body 38, a stopper 35, and a cap 36. As shown in FIG. 6, the stopper 35 comprises a fused portion 31 having a surface 31a to which the container body 38 is heat-fused, and a mouth portion 32 having a threaded portion 32a formed on the outer periphery. [Example]

[0035] The present disclosure will be described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0036] (Comparative Example 1) The following materials were used to produce plugs (10 in total) having the configuration shown in Fig. 4 by ordinary injection molding. Fig. 7 is a cross-sectional image of the plug according to Comparative Example 1, obtained by CT scanning. [Resin material] High-density polyethylene (Prime Polymer Co., Ltd., 2100K (product name)) [Coloring agent (masterbatch)] PEMSSCMF0291 White (manufactured by Dainichi Seika Chemicals Co., Ltd.) [Plug configuration] Inner diameter of opening: 8.7mm Outer diameter of opening: 10.6mm Maximum thickness of fused part: 7.8mm Distance between the two rods 21 and 22 (see Figure 4): 14 mm Maximum thickness of the first connecting portion 23a (see FIG. 4): 1.5 mm Maximum thickness of the second connecting portion 23b (see FIG. 4): 2 mm ·Weight: 3.00g

[0037] Example 1 A molten resin composition was prepared by adding 0.5 parts by mass of supercritical nitrogen to 100 parts by mass of the resin material. This molten resin composition was used to produce stoppers (10 in total) of the same shape as in Comparative Example 1. The injection speed was 30 mm / sec. The stopper in Example 1 weighed 2.82 g, achieving a weight reduction of 6.0% compared to Comparative Example 1. Note that a MuCell injection molding machine ("MuCell" is a registered trademark of Trexel Co. Ltd.) was used in this example and the following examples and comparative examples.

[0038] FIG. 8(a) is a cross-sectional image of the plug according to Example 1 obtained by CT scanning. As shown in this cross-sectional image, visible voids were formed in the fused portion 1, and mutually independent voids (approximately 0.6 to 1.0 mm in diameter) were formed in the second connecting portion 23. From the cross-sectional image, the number of voids and the void area ratio in the second connecting portion 23b were determined. The results are shown in Table 1. The void area ratio is the ratio of the area of ​​the voids in the second connecting portion (the area of ​​the portion displayed in black in the cross-sectional image) to the cross-sectional area of ​​the second connecting portion.

[0039] Example 2 A molten resin composition was prepared by adding 1.0 part by mass of supercritical nitrogen to 100 parts by mass of the resin material. This molten resin composition was used to produce stoppers (10 in total) in the same manner as in Example 1. The weight of the stopper in Example 2 was 2.79 g, achieving a weight reduction rate of 7.0% compared to Comparative Example 1.

[0040] 8(b) is a cross-sectional image of the plug according to Example 2 obtained by CT scanning. As shown in this cross-sectional image, visible voids were formed in the fused portion 1 and the second connecting portion 23. From the cross-sectional image, the number of voids and the area ratio of the voids in the second connecting portion 23b were determined in the same manner as in Example 1. The results are shown in Table 1.

[0041] (Comparative Example 2) A molten resin composition was prepared by adding 2.0 parts by mass of supercritical nitrogen to 100 parts by mass of the resin material. This molten resin composition was used to produce stoppers (10 in total) in the same manner as in Example 1. The weight of the stopper in Comparative Example 2 was 2.73 g, achieving a weight reduction rate of 9.0% compared to Comparative Example 1.

[0042] 8(c) is a cross-sectional image of the plug according to Comparative Example 2 obtained by CT scanning. As shown in this cross-sectional image, visible voids were formed in the fused portion 1 and the second connecting portion 23. From the cross-sectional image, the number of voids and the area ratio of the voids in the second connecting portion 23b were determined in the same manner as in Example 1. The results are shown in Table 1.

[0043] Example 3 A molten resin composition was prepared by adding 1.5 parts by mass of supercritical carbon dioxide to 100 parts by mass of the above resin material. This molten resin composition was used to produce a stopper having the same shape as Comparative Example 1. The injection speed was 30 mm / sec. The weight of the stopper according to Example 3 was 2.83 g, achieving a weight reduction rate of 5.7% compared to Comparative Example 1.

[0044] 9(a) is a cross-sectional image of the plug of Example 3 obtained by CT scan. As shown in this cross-sectional image, visible voids were formed in the fused portion 1, and mutually independent voids (diameters of approximately 1.2 to 1.8 mm) were formed in the second connecting portion 23. From the cross-sectional image, the number of voids and the void area ratio in the second connecting portion 23b were determined in the same manner as in Example 1. The results are shown in Table 2.

[0045] Example 4 A molten resin composition was prepared by adding 3.0 parts by mass of supercritical carbon dioxide to 100 parts by mass of the above resin material. This molten resin composition was used to produce a stopper in the same manner as in Example 1. The weight of the stopper in Example 4 was 2.81 g, achieving a weight reduction rate of 6.3% compared to Comparative Example 1.

[0046] 9(b) is a cross-sectional image of the plug of Example 4 obtained by CT scan. As shown in this cross-sectional image, visible voids were formed in the fused portion 1 and the second connecting portion 23. From the cross-sectional image, the number of voids and the area ratio of the voids in the second connecting portion 23b were determined in the same manner as in Example 1. The results are shown in Table 2.

[0047] (Comparative Example 3) A molten resin composition was prepared by adding 6.0 parts by mass of supercritical carbon dioxide to 100 parts by mass of the above resin material. This molten resin composition was used to produce a stopper in the same manner as in Example 1. The weight of the stopper in Comparative Example 3 was 2.79 g, achieving a weight reduction rate of 7.0% compared to Comparative Example 1.

[0048] 9(c) is a cross-sectional image of the plug according to Comparative Example 3 obtained by CT scanning. As shown in this cross-sectional image, visible voids were formed in the fused portion 1 and the second connecting portion 23. From the cross-sectional image, the number of voids and the area ratio of the voids in the second connecting portion 23b were determined in the same manner as in Example 1. The results are shown in Table 2.

[0049] <Evaluation of liquid leakage> Spout containers were produced using the stoppers according to the Examples and Comparative Examples. The caps were produced by ordinary injection molding. The spout mouth was closed with a cap, and the penetrant liquid was poured into the mouth. The container was placed in an inverted position with the cap facing downwards, and after 24 hours, it was confirmed whether or not any liquid leakage was observed between the mouth and the cap. For each of the Examples and Comparative Examples, 10 spout containers were produced and evaluated. The number of spout containers that leaked (the number of occurrences of liquid leakage) out of the 10 spout containers is shown in Tables 1 and 2.

[0050] [Table 1]

[0051] [Table 2] [Explanation of symbols]

[0052] 1, 31... fused portion, 1a, 31a... surface, 2, 32... mouth portion, 2a, 32a... screw portion, 3... plate, 4... straw portion, 5, 15, 25, 35... stopper, 5a... foam layer, 5b... skin layer, 6, 36... cap, 8, 38... container body, 10... spout container, 21, 22... rod, 23a... first connecting portion, 23b... second connecting portion, 30... container

Claims

1. (A1) a step of preparing a molten resin composition containing at least one resin material selected from a polypropylene resin and a polyethylene resin and nitrogen in a supercritical state; (B1) injecting the molten resin composition into a mold cavity; (C1) foaming the molten resin composition in the cavity by reducing the pressure; (D1) recovering the plug having a plurality of voids resulting from the foaming from the mold; Including, the amount of nitrogen in a supercritical state is 0.1 to 1.2 parts by mass when the mass of the resin material in the molten resin composition is 100 parts by mass, A method for manufacturing a plug, wherein the plug has a foamed layer with an area ratio of voids of 25% or less as determined by cross-sectional observation (however, this does not include a method for manufacturing a spout that is manufactured by molding a mixture of low-density polyethylene resin and an elastomer resin compatible with it as a raw material).

2. (A2) a step of preparing a molten resin composition containing at least one resin material selected from a polypropylene resin and a polyethylene resin and carbon dioxide in a supercritical state; (B2) injecting the molten resin composition into a mold cavity; (C2) foaming the molten resin composition in the cavity by reducing the pressure; (D2) recovering the plug having a plurality of voids resulting from the foaming from the mold; Including, the amount of carbon dioxide in a supercritical state is 0.8 to 4.2 parts by mass when the mass of the resin material in the molten resin composition is 100 parts by mass, A method for manufacturing a plug, wherein the plug has a foamed layer with an area ratio of voids of 25% or less as determined by cross-sectional observation (however, this does not include a method for manufacturing a spout that is manufactured by molding a mixture of low-density polyethylene resin and an elastomer resin compatible with it as a raw material).

3. A stopper that constitutes a container together with a container body, a fusion portion having a surface to which the container body is heat-fused; a mouth portion having a threaded portion formed on its outer periphery; Equipped with A foam layer having voids derived from a supercritical fluid, A spout (excluding spouts manufactured by molding a mixture of low-density polyethylene resin and an elastomer resin compatible therewith) in which the foam layer has an area ratio of voids of 25% or less as determined by cross-sectional observation.

4. The plug according to claim 3, further comprising a straw portion extending from the fused portion in a direction opposite to the mouth portion.

5. two rods extending parallel to each other in a direction opposite to the mouth portion from the fused portion; At least one connecting portion connecting the two rods; The plug of claim 3 further comprising:

6. The foam layer is formed at the connecting portion, 6. The plug according to claim 5, wherein the voids visible upon cross-sectional observation of the connecting portion include voids that are independent of one another and have a diameter of 0.6 to 1.0 mm, and the number of the independent voids in the connecting portion is four or more.

7. The plug according to any one of claims 3 to 5, wherein a plurality of the voids in the foam layer are independent of one another.

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