Container and method for manufacturing the container

Abstract

To provide a container which suppresses the occurrence of sink marks and warpage with a simple configuration.SOLUTION: A container 10 is a container that is a molded body by supercritical fluid molding. The container 10 includes: a flange 3 and a fitting portion 4, which are opening ends provided in the opening of the container 10; and side wall parts 2a and 2b formed to connect to the flange 3 and the fitting portion 4. The opening end of the container 10 has: a flange 3 that is thicker than the side wall parts 2a and 2b; and a flange 3 and a fitting portion 4 that cause a difference in thickness within the opening ends. A plurality of closed cells is formed inside the flange 3 and the fitting portion 4 which are the open ends of the container 10.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a container manufactured by supercritical fluid molding and a method for manufacturing a container by supercritical fluid molding. [Background technology]

[0002] Patent Document 1 discloses an example of a cup container, in which a U-shaped cross-section cutout is provided below the outer periphery of the flange, thereby suppressing deformation of the adhesive surface of the flange and preventing poor adhesion of the heat seal. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-237477 Summary of the Invention [Problem to be solved by the invention]

[0004] Molded products manufactured by injection molding can suffer from sink marks and warpage at areas where the thickness changes, resulting in molding defects. This is because thicker areas shrink more during solidification than thinner areas. When a thinner area and a thicker area are adjacent to each other, the shrinkage forces become unbalanced at the boundary, causing the thinner area to be pulled toward the internal solidification of the thicker area. Patent Document 1 discloses an example of a configuration for suppressing the occurrence of such sink marks and warpage, but the structure is complex, and a simpler configuration for suppressing the occurrence of sink marks and warpage is desired.

[0005] An object of the present invention is to provide a container that can suppress the occurrence of sink marks and warping with a simple configuration, and a method for manufacturing the container. [Means for solving the problem]

[0006] One aspect of the present invention relates to a container formed by supercritical fluid molding. The container includes an opening edge provided at the opening of the container and a side wall portion formed so as to be continuous with the opening edge. The opening edge has at least one of a portion that is thicker than the side wall portion and a portion that creates a difference in thickness within the opening edge. A plurality of closed cells are formed inside the opening edge.

[0007] This container has an opening edge at the opening of the container that has at least one of a portion that is thicker than the side wall portion and a portion that creates a difference in thickness within the opening edge, but is configured so that multiple closed bubbles are formed within the opening edge. By forming voids within the opening edge to reduce the volume, the shrinkage rate of the thick portion of the container is reduced, and the drag phenomenon caused by solidification of the thick portion is reduced. This makes it possible to suppress the occurrence of sink marks and warpage. Note that the term "closed bubbles" as used herein refers to bubbles that exist independently and includes bubbles that are in contact with adjacent bubbles as long as they are not connected to each other.

[0008] In the container, each of the plurality of closed cells has a length of 10 μm or more and 3000 μm or less, where the length is the longest side passing through the center of gravity of the cell, and the number of the closed cells per unit volume is 0.3 cells / mm 3 More than 8.5 pieces / mm 3 It is preferable that the thickness is 0.1 or less. In this case, the drag phenomenon due to internal solidification of the thick portion can be more reliably prevented, and the occurrence of sink marks and warpage can be more reliably suppressed. Furthermore, if the thickness of the opening end where the closed bubbles are formed is sufficiently thin (for example, about 0.5 mm or less) to allow light to pass through, the number of bubbles per unit area can be specified. In this case, the number of bubbles per unit area of ​​a plurality of closed bubbles having a bubble length of 10 μm or more and 3000 μm or less is 0.1 bubbles / mm 2 More than 2.5 pieces / mm 2On the other hand, when the film is made of a material that does not transmit light, the region including the open end may be cut and the number of bubbles per unit area present on the cut surface may be counted. In this case, the number of bubbles per unit area of ​​the multiple closed bubbles having a bubble length of 10 μm or more and 3000 μm or less is 0.1 bubbles / mm, as in the above. 2 More than 2.5 pieces / mm 2 or less. The number of bubbles may be calculated by measurement using an optical microscope, or may be calculated directly from the obtained image, or may be calculated by subjecting the obtained image to predetermined image processing such as binarization. When observing using an optical microscope, it is preferable to obtain a cross-sectional image along the direction in which the opening end widens, but it is also possible to obtain a cross-sectional image perpendicular to the direction in which the opening end widens. When observing using an optical microscope, it is preferable to cut the container thin enough to allow observation.

[0009] In the above-mentioned container, it is preferable that the thickness of the side wall is 0.25 mm or more and 0.4 mm or less, and that no closed bubbles are formed inside the side wall. A container with such a thin side wall can reduce the amount of plastic used to form the container and lighten the weight of the container itself. Furthermore, even in a container with such a thin side wall, the formation of multiple closed bubbles inside the opening edge can suppress the occurrence of sink marks and warping due to the opening edge. Therefore, with such a container, it is possible to achieve both a reduction in the amount of plastic used, a lightweight container, and suppression of sink marks and warping. Note that "no closed bubbles formed" here means that there are no closed bubbles visible to the naked eye, or the number of bubbles per unit area visible under a microscope at a magnification of 35x is 0.01 bubbles / mm 2This means that the number of bubbles is equal to or less than 200 μm. Generally, the lower limit of the bubble length of bubbles that can be visually confirmed is 200 μm, so "there are no independent bubbles that can be visually confirmed" means that there are no bubbles with a bubble length exceeding 200 μm. Furthermore, in terms of determining whether or not independent bubbles are formed, the bubble length of bubbles that can be confirmed when the magnification of the microscope is set to 35 times may exceed 200 μm. In other words, even if bubbles exceeding 200 μm are confirmed when observed under a microscope, the number of bubbles per unit area may be less than 0.01 bubbles / mm. 2 If the value is less than this, it is determined that no closed cells have been formed.

[0010] In the above container, it is preferable that a skin layer is formed on the surface of the open end, and the closed cells are not exposed on the surface, thereby providing a container with a more aesthetically pleasing appearance.

[0011] In the above-described container, the open end may be configured so that another member that covers the opening of the container can be heat-sealed thereto or so that the open end can be fitted with another member.

[0012] In another aspect, the present invention relates to a method for manufacturing a container having an opening at one end by supercritical fluid molding. This manufacturing method includes the steps of (A) preparing a molten resin composition containing a resin material and a supercritical fluid, (B) injecting the molten resin composition into a cavity of a mold corresponding to the container, and (C) holding the pressure of the molten resin composition injected into the cavity. In the injection step, injection molding is performed so that the flow end of the molten resin composition injected into the cavity from the gate of the mold is aligned with the opening end provided at the opening of the container.

[0013] In this manufacturing method using supercritical fluid molding, injection molding is performed in the injection step so that the flow end of the molten resin composition injected into the cavity from the mold gate becomes the opening end that defines the opening of the container. According to this manufacturing method, the pressure of the molten resin composition injected at a predetermined pressure decreases at the flow end, causing cells in the molten resin composition to grow and foam, easily generating closed cells in the opening end corresponding to the flow end. This reduces the volume of the opening end, making it possible to easily manufacture containers while suppressing the occurrence of sink marks and warpage due to the opening end. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a container that has a simple configuration and that suppresses the occurrence of sink marks and warping, and a method for manufacturing the container. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a perspective view showing a container according to one embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view of the container shown in FIG. [Figure 3] FIG. 3 is a bottom view showing the bottom of the container shown in FIG. [Figure 4] FIG. 4 is a schematic diagram showing a method for producing the container shown in FIG. [Figure 5] FIG. 5 is a diagram showing a container according to a modified example of the present disclosure. [Figure 6A] FIG. 6A is a photograph showing a container according to a comparative example (normal molded product). [Figure 6B] FIG. 6B is a photograph showing a container according to an example (supercritical fluid molded product). [Figure 7A] FIG. 7A is a photograph of the flow end of a container manufactured by supercritical fluid molding, showing a high degree of foaming. [Figure 7B] FIG. 7B is a photograph of the flow end of a container manufactured by supercritical fluid molding, showing a medium degree of foaming. [Figure 7C]FIG. 7C is a photograph of the flow end of a container manufactured by supercritical fluid molding, showing a low degree of foaming. DETAILED DESCRIPTION OF THE INVENTION

[0016] A container manufactured by supercritical fluid molding according to one embodiment of the present invention will be described in detail below with reference to the drawings. In the description, the same elements or elements having the same functions may be designated by the same reference numerals, and redundant description will be omitted. Note that the present invention is not limited to the following embodiment.

[0017] FIG. 1 is a perspective view showing a container according to one embodiment of the present invention. The container 10 shown in FIG. 1 is a molded body formed by supercritical fluid molding. FIG. 2 is a cross-sectional view of the container 10. FIG. 3 is a bottom view of the container 10. In a plan view, the container 10 has a substantially rectangular shape with rounded corners. The container 10 can be used, for example, as a dessert cup, a beverage cup, or a thin-walled container for storing butter, margarine, cream cheese, or the like. The container 10 has excellent strength (e.g., buckling strength and drop resistance), so it may have a relatively large capacity. The internal volume of the container 10 is, for example, 280 cc or more, and may be 280 to 400 cc.

[0018] The container 10 comprises a bottom 1, a pair of side walls 2a, a pair of side walls 2b, a flange 3 (opening end), fitting portions 4 (opening end) provided at the four corners, and a support portion 5. In a plan view, the side walls 2a form the short sides of the container 10, while the side walls 2b form the long sides of the container 10. The fitting portions 4 serve to guide a lid (not shown) that fits into the container 10. The support portions 5 extend downward from the peripheral edge of the bottom 1 and serve to increase the strength of the container 10.

[0019] The bottom 1 has a generally rectangular shape with rounded corners in a plan view. The length of the short side of the bottom 1 (length La in FIG. 3) is, for example, 3 to 12 cm, and may be 5 to 10 cm, or 6 to 8.5 cm. The length of the long side of the bottom 1 (length Lb in FIG. 3) is, for example, 5 to 15 cm, and may be 7 to 12 cm, or 8 to 10.5 cm. The thickness of the bottom 1 is, for example, 0.3 to 0.6 mm, and may be 0.4 to 0.5 mm. When the thickness of the bottom 1 is 0.3 mm or more, rear bulging tends to be suppressed and drop resistance can be ensured. On the other hand, when the thickness of the bottom 1 is 0.6 mm or less, weight reduction is achieved.

[0020] Each of the side walls 2a and 2b extends obliquely upward from the periphery of the bottom 1 and is inclined so that the opening of the container 10 widens as it moves away from the bottom 1, as shown in Fig. 1. The side walls 2a and 2b may extend substantially vertically to the bottom 1.

[0021] The thickness of the side walls 2a, 2b is, for example, 0.25 to 0.4 mm, and may be 0.3 to 0.35 mm. Drop resistance can be ensured by having the thickness of the side walls 2a, 2b be 0.25 mm or more. On the other hand, weight reduction and a reduction in the amount of plastic used can be achieved by having the thickness of the side walls 2a, 2b be 0.4 mm or less. While the embodiment in which the entire side walls 2a, 2b have a thickness within the above range has been exemplified, a thin-walled portion having a thickness within the above range may also be used. From the viewpoint of reducing the amount of plastic used, the area ratio of the thin-walled portion in the side walls is preferably 50% or more, more preferably 70% or more, and even more preferably 90% or more. Unlike the flange 3 and fitting portion 4 described below, the side walls 2a, 2b are not internally foamed, i.e., do not have closed cells formed therein.

[0022] The flange 3 is an opening edge that defines the opening of the container 10 and has a generally rectangular frame shape with rounded corners in a plan view. The flange 3 is formed to be continuous with the sidewalls 2a and 2b and is configured to protrude outward beyond the sidewalls 2a and 2b. In other words, the flange 3 has a portion that protrudes from the inside toward the outside (lateral protrusion length), and is therefore formed to be thicker than the sidewalls 2a and 2b. The thickness of the flange 3 in the vertical direction may be the same as, thinner than, or thicker than the thickness of the sidewalls 2a and 2b (lateral thickness from the inside toward the outside). Meanwhile, the flange 3, together with the fitting portion 4, is an internally foamed portion, i.e., a portion in which closed cells are formed. Details will be described later. The fitting portion 4 is formed to protrude upward from the top surface of the flange 3. The inner upper surface of the flange 3 may be flat, allowing various sheets covering the contents to be heat-sealed thereto.

[0023] The fitting portions 4 are opening edges that, together with the flange 3, define the opening of the container 10, and are provided at the four rounded corners of the flange 3. The fitting portions 4 are wall portions that protrude upward from approximately the center in the width direction of the upper surface of the flange 3, and are portions that guide the lid (not shown) when the lid is fitted to the container 10 and engage with the lid. When viewed from above, the fitting portions 4 are formed to include a partial curve so that the corners are rounded. The fitting portions 4 are formed to be thinner than the flange 3, and are portions that create a difference in thickness at the opening edge including the flange 3. The four fitting portions 4 may be formed so that adjacent fitting portions 4 are continuous with each other. Note that a skin layer may be formed on the surface of the flange 3 and the fitting portions 4.

[0024] The support portion 5 is a portion that extends downward from the entire periphery of the bottom portion 1. The support portion 5 contributes to improving the strength of the container 10, and in particular improves the strength (buckling strength) of the container 10 against a force that crushes it. As shown in FIG. 3, the container 10 may further include a plurality of ribs 7 that reinforce the support portion 5.

[0025] In such a container 10, a plurality of closed cells (also referred to as a group of closed cells) are formed inside the opening end portion consisting of the flange 3 and the fitting portion 4 provided at the opening of the container 10 by a manufacturing method using supercritical fluid molding, which will be described later. On the other hand, no such closed cells are formed in the side wall portions 2a, 2b. Closed cells refer to cells that exist independently of each other, and include cells that are in contact with adjacent cells as long as they are not connected to each other. Each of the closed cells formed in the flange 3 or the fitting portion 4 has a cell length of 10 μm or more and 3000 μm or less, when the cell length is the longest side passing through the center of gravity of each cell. The number of closed cells per unit volume at the opening end portion of the flange 3 or the fitting portion 4 is, for example, 0.3 cells / mm 3 More than 8.5 pieces / mm 3 An example of such a closed bubble is shown in FIG. 7A. The image shown in FIG. 7A shows the flange 3 portion (bubble portion) at a magnification of 35 times using a microscope, measuring 6.5 mm long x 8.5 mm wide (area 55.25 mm). 2 ) range. Fig. 7A shows bubbles observed when observing a container that does not contain a colorant. Note that the bubbles in the flange 3 may be bubbles in the foaming degree shown in Fig. 7B or Fig. 7C depending on the relationship with the thickness of the side wall portions 2a, 2b, etc.

[0026] Furthermore, if the thickness of the opening end where the closed bubbles are formed is sufficiently thin (for example, about 0.5 mm or less) and light can be transmitted through it, it can also be specified by the number of bubbles per unit area. In this case, the number of bubbles per unit area of ​​the multiple closed bubbles at the opening end of the flange 3 or the fitting portion 4 is 0.1 bubbles / mm 2 More than 2.5 pieces / mm 2 On the other hand, when the flange 3 or the fitting portion 4 is made of a material that does not transmit light, the region including the opening edge may be cut and the number of bubbles per unit area present on the cut surface may be counted. In this case, the number of bubbles per unit area of ​​the multiple closed bubbles at the opening edge of the flange 3 or the fitting portion 4 may be 0.1 bubbles / mm or less, as in the above case. 2 More than 2.5 pieces / mm 2The number of bubbles may be measured using an optical microscope, calculated directly from the obtained image, or calculated by subjecting the obtained image to predetermined image processing such as binarization. When observing using an optical microscope, it is preferable to obtain a cross-sectional image along the direction in which the opening end widens, but a cross-sectional image perpendicular to the direction in which the opening end widens may also be obtained. When observing using an optical microscope, it is preferable to cut the container thin enough to allow observation.

[0027] The absence of independent bubbles in the sidewalls 2a and 2b means that there are no independent bubbles visible to the naked eye, or the number of bubbles visible per unit area when the magnification of the microscope is set to 35 times as described above is 0.01 bubbles / mm 2 This means that the number of bubbles is equal to or less than 200 μm. Generally, the lower limit of the bubble length of bubbles that can be visually confirmed is 200 μm, so "there are no independent bubbles that can be visually confirmed" means that there are no bubbles with a bubble length exceeding 200 μm. In addition, in terms of determining whether or not independent bubbles are formed, the bubble length of bubbles that can be confirmed when the magnification of the microscope is set to 35 times may exceed 200 μm. In other words, even if bubbles exceeding 200 μm are confirmed in the side wall portions 2a and 2b when observed with a microscope, the number of bubbles per unit area is not 0.01 bubbles / mm 2 If it is below this value, it is determined that no closed cells are formed in the side wall portions 2a and 2b.

[0028] In the container 10, the presence of such multiple closed cells in the open end reduces the volume of the flange 3, thereby reducing the volume difference with the sidewalls 2a and 2b where no closed cells are formed. This reduces the potential for sink marks, warpage, and other problems that could occur in the thin-walled sidewalls 2a and 2b during shrinkage after injection molding. Furthermore, the presence of the multiple closed cells reduces the volume difference with the fitting portion 4, thereby reducing the potential for sink marks, warpage, and other problems that could occur in the thin-walled fitting portion 4 during shrinkage after injection molding. Although closed cells are also formed in the fitting portion 4, the absolute value of the volume difference between the flange 3 and the fitting portion 4 is reduced by the formation of closed cells in both the flange 3 and the fitting portion 4, thereby reducing the potential for sink marks and warpage in this region as well. The method for forming closed cells primarily in the open end of the flange 3 and the fitting portion 4 as described above will be described in more detail in the manufacturing method below.

[0029] Next, a description will be given of a method for manufacturing the container 10. The container 10 is manufactured through 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) After the above step (B), a step of holding the cavity under pressure and cooling it. (D) Removing the container from the mold. 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, for example, Japanese Patent Nos. 6085729 and 6430684).

[0030] [(A) Process] First, a molten resin composition containing a resin material and a supercritical fluid is prepared. Examples of the resin material include thermoplastic resins, such as polypropylene resin and polyethylene resin. The melt flow rate of the thermoplastic resin is preferably 15 g / 10 min or more, more preferably 20 to 40 g / 10 min, and even more preferably 25 to 36 g / 10 min. A melt flow rate of 15 g / 10 min or more tends to suppress the occurrence of short shots, while a melt flow rate of 40 g / 10 min or less tends to produce containers with excellent drop resistance. The melt flow rate (MFR) is a value measured in accordance with the method described in JIS K7210-1:2014 under conditions of a temperature of 230°C and a load of 2.16 kg. Short shots refer to a phenomenon in which the resin material does not reach the flow end of the cavity.

[0031] To mold thin-walled containers using conventional injection molding, it was necessary to select a resin with high fluidity (a high MFR value) to prevent short shots. However, resin materials with high fluidity tend to have a relatively low molecular weight and low strength, making it difficult to manufacture thin-walled containers with excellent drop resistance. In contrast, in this embodiment, even if a resin material has a relatively low MFR value, the fluidity of the molten resin composition can be increased by using it in combination with a supercritical fluid. This makes it possible to achieve both short shot prevention and excellent strength (e.g., buckling strength and drop resistance).

[0032] When carbon dioxide is used as the supercritical fluid, 1 to 4 parts by mass, preferably 2 to 3 parts by mass, of supercritical carbon dioxide is added to 100 parts by mass of the resin material to prepare the molten resin composition. Using 2 parts by mass or more of carbon dioxide reduces the variation in filling pressure between molding shots, and the addition of carbon dioxide reduces the viscosity of the molten resin composition, thereby suppressing the occurrence of short shots. Additionally, foaming caused by supercritical carbon dioxide can be promoted, for example, at the flow end (e.g., flange 3 or fitting portion 4), to form voids within a portion of the molded body. On the other hand, using 3 parts by mass or less of carbon dioxide can impair foaming, resulting in an inability to form a uniform bubble layer, resulting in unevenness, or insufficient weight reduction.

[0033] When nitrogen is used as the supercritical fluid, 0.5 to 1.5 parts by mass of supercritical nitrogen is added to 100 parts by mass of resin material to prepare the molten resin composition. By adding 0.5 parts by mass or more of nitrogen, it is possible to reduce the variation in filling pressure between molding shots, and the addition of nitrogen reduces the viscosity of the molten resin composition, thereby suppressing the occurrence of short shots. In addition, the promotion of foaming due to supercritical nitrogen can form voids within the molded product. On the other hand, if the amount of nitrogen is 1.5 parts by mass or less, foaming properties are impaired, resulting in an inability to form a uniform bubble layer, resulting in unevenness, or insufficient weight reduction.

[0034] The temperature of the molten resin composition (screw cylinder temperature) may be set according to the melting point or MFR of the resin material. When polypropylene resin is used, this temperature is preferably about 210 to 250°C. When polyethylene resin is used, this temperature is preferably about 220 to 260°C. When this temperature is equal to or higher than the lower limit, the resin flows easily in the cavity, while when it is equal to or lower than the upper limit, burning of the resin tends to be suppressed.

[0035] 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.

[0036] [(B) Process] The molten resin composition prepared in step (A) is injected into the cavity through the gate portion of the mold corresponding to the container 10. At this time, as shown in FIG. 4, injection molding is performed with the side corresponding to the bottom 1 and support portion 5 as the gate portion and the side corresponding to the flange 3 and fitting portion 4 as the flow end portion. When the molten resin composition is introduced into the cavity through such a flow path, it is easy to maintain a predetermined pressure on the gate portion side. On the other hand, at the flange 3 and fitting portion 4, which are the flow end portions, a drop in pressure is likely to occur, as will be explained in step (C) below.

[0037] The injection speed of the molten resin composition in step (B) is preferably 100 to 400 mm / sec, more preferably 150 to 200 mm / sec. An injection speed of 100 mm / sec or higher tends to allow the resin to reach the end of the flow, thereby suppressing the occurrence of short shots. On the other hand, an injection speed of 400 mm / sec or lower tends to suppress the occurrence of flash defects in the molded article. In the container manufacturing method according to this embodiment, the injection speed may be set in multiple stages, with the initial injection speed being 250 to 350 mm / sec and the second-stage speed being reduced to 50 to 150 mm / sec. By reducing the injection speed in this manner, it is possible to improve the transferability of the mold and prevent solidification during molding.

[0038] [(C) Process] In step (C), after step (B), the cavity is cooled while being dwelled. The dwell pressure applied after injection may be 20 to 50 MPa, for example, 30 MPa. The dwell time is 0.5 to 1.5 seconds, for example, 1.0 second. This dwell pressure prevents supercritical fluids such as carbon dioxide and nitrogen from foaming in the bottom 1, support 5, and sidewalls 2a and 2b, which are close to the gate. Meanwhile, the dwell pressure tends to decrease in the flange 3 and fitting 4, which are located at the end of the flow path away from the gate, causing foaming of supercritical fluids such as carbon dioxide and nitrogen. While no bubbles are formed in the sidewalls 2a and 2b, multiple closed bubbles are formed inside the flange 3 and fitting 4, which are the open ends. The amount and size of bubbles generated can be adjusted by varying the dwell pressure and dwell time. For example, the foaming degree can be changed, as shown in Figures 7A to 7C.

[0039] [(D) Process] In step (D), after step (C) above, when the temperature of the molded body in the mold has dropped to about 30 to 60°C, the molded body (container 10) is recovered from the mold. This allows the container 10 to be obtained. In this embodiment, pressure is maintained in step (C), so large voids that are visible to the naked eye are not formed in the container 10. However, if bubbles formed in the flange 3 and fitting portion 4 of the container 10 deteriorate the appearance, this can be addressed by adding a colorant to the molten resin composition used for molding.

[0040] As described above, according to this embodiment, the flange 3 and fitting portion 4, which are the opening end portions of the container 10, have portions that are thicker than the sidewall portions 2a and 2b, and portions that cause thickness differences within the opening end portions. However, multiple closed bubbles are formed within the opening end portions. By forming voids within the opening end portions and reducing the volume, the shrinkage rate of the thick portions of this container is reduced, and the drag phenomenon caused by solidification of the thick portions is reduced. As a result, the container 10 can suppress the occurrence of sink marks and warpage.

[0041] Furthermore, in the container 10 according to this embodiment, each of the plurality of closed cells has a length of 10 μm or more and 3000 μm or less, where the length is the longest side passing through the center of gravity of the cell, and the number of the plurality of closed cells per unit volume is 0.3 cells / mm 3 More than 8.5 pieces / mm 3 or less. In this case, it is possible to more reliably prevent the drag phenomenon caused by internal solidification of the thick portion, and more reliably suppress the occurrence of sink marks and warpage. Furthermore, if the thickness of the opening end (flange 3, etc.) where the closed bubbles are formed is sufficiently thin (for example, about 0.5 mm) and light can be transmitted through it, it can also be specified by the number of bubbles per unit area. In this case, the number of bubbles per unit area of ​​a plurality of closed bubbles having a bubble length of 10 μm or more and 3000 μm or less is 0.1 bubbles / mm 2 More than 2.5 pieces / mm 2 On the other hand, when the film is made of a material that does not transmit light, the region including the opening end (flange 3, etc.) may be cut and the number of bubbles per unit area present on the cut surface may be counted. In this case, the number of bubbles per unit area of ​​a plurality of closed bubbles having a bubble length of 10 μm or more and 3000 μm or less is 0.1 bubbles / mm, as in the above. 2 More than 2.5 pieces / mm 2 It may be the following:

[0042] Furthermore, in the container 10 according to this embodiment, the thickness of the side walls 2a, 2b is 0.25 mm or more and 0.4 mm or less, and no closed bubbles are provided inside the side walls 2a, 2b. Having such thin side walls 2a, 2b makes it possible to reduce the amount of plastic used to form the container 10 and to lighten the weight of the container itself. Even in a container having such thin side walls 2a, 2b, providing multiple closed bubbles inside the open end can prevent sink marks and warping due to the open end (flange 3, etc.). Therefore, with the container 10, it is possible to reduce the amount of plastic used, reduce the weight of the container, and prevent sink marks and warping.

[0043] Furthermore, in the container 10 according to this embodiment, a skin layer is formed on the surface of the flange 3 and the fitting portion 4, so that the closed cells are not exposed on the surface. This makes it possible to provide a container with an excellent aesthetic appearance.

[0044] Furthermore, in the manufacturing method using supercritical fluid molding according to this embodiment, injection molding is performed in the injection step so that the flow end of the molten resin composition injected from the gate of the mold into the cavity becomes the opening end (flange 3 and fitting portion 4) that defines the opening of the container 10. According to this manufacturing method, the pressure of the molten resin composition injected at a predetermined pressure decreases at the flow end, causing cells in the molten resin composition to grow and foam, easily generating closed cells in the opening end corresponding to the flow end. This reduces the volume of the opening end, i.e., the flange 3 and fitting portion 4, and makes it possible to easily manufacture the container 10 while suppressing the occurrence of sink marks and warpage due to the flange 3, etc.

[0045] Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. For example, in the above embodiments, a container provided with a flange 3 and a fitting portion 4 (e.g., a container used for margarine, etc.) has been described as an example, but the scope of application of the present invention is not limited thereto. For example, the container 10 configured as shown in FIGS. 1 to 3 may be configured without the fitting portion 4. That is, the upper surface of the flange 3 may be a flat surface, and the entire upper surface of the flange 3 may be configured as a heat-sealable surface. In this case, too, multiple closed cells are formed inside the flange 3, thereby suppressing sink marks and warpage due to the flange 3 from occurring on the side wall portions 2a, 2b, etc. Furthermore, since cells are formed inside the flange 3, etc. of the container 10, when heat-sealing the flange 3, heat can be easily released during heat-sealing, thereby shortening the cooling time after heat-sealing.

[0046] Furthermore, for example, the present invention may be applied to a container 20 shown in Fig. 5. The container 20 includes a bottom 21, a sidewall 22, and a fitting portion 23. The fitting portion 23 of the container 20 is provided with a thread or undercut for fitting with a lid 24, so that a difference in wall thickness occurs at the fitting portion 23. This fitting portion 23 is molded as a flow end portion in the same manner as described above during injection molding using supercritical fluid molding. This results in multiple closed cells being formed inside the fitting portion 23.

[0047] In conventional injection molding, differences in wall thickness within the fitting portion 23 due to threads or the like formed in the fitting portion 23 would result in sink marks or the like. However, as in the above-described embodiment, in the container 20 according to this embodiment, the interior of the fitting portion 23 is foamed, thereby reducing the volumetric difference within the fitting portion 23. This reduces the volumetric difference between the fitting portion 23 and the side wall portion 22 adjacent to it. Therefore, in the container 20 according to the modified example, the occurrence of sink marks and warpage in the fitting portion 23 is suppressed. Moreover, because the container 20 is formed by supercritical fluid molding, the strength of the side wall portion 22 can be sufficiently ensured even if the side wall portion 22 has a thin shape. Note that, because the lid 24 is often formed primarily from resin or the like, it is possible to avoid the fitting portion 23 becoming weak when the lid 24 is fitted to the internally foamed fitting portion 23.

[0048] Furthermore, in the above-described embodiments, carbon dioxide or nitrogen is used as the supercritical fluid, but instead of these gases, argon or helium, for example, may be used. Furthermore, in the above-described embodiments, dairy products such as butter and margarine are given as examples of contents, but other contents may also be used. Examples of other contents include desserts such as parfait ice cream, pudding, shaved ice, and yogurt. When storing these contents, the shape of the container may be suitable for the contents. For example, the shape of the bottom of the container may be approximately square, circular, or elliptical. Furthermore, the sidewall portion does not have to be flat, but may be curved like a bowl. [Example]

[0049] The present invention will be described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0050] (Example) A container having the shape shown in FIG. 1 was produced by supercritical fluid molding using a MuCell injection molding machine ("MuCell" is a registered trademark of Trexel Co. Ltd.) as follows. First, the resin composition shown below was prepared. Then, 2.5 parts by mass of supercritical carbon dioxide was added to 100 parts by mass of the resin composition to prepare a molten resin composition.

[0051] [Composition of Resin Composition] Resin material: Polypropylene (Prime Polymer Co., Ltd., J667TG (model number), MFR: 36 g / 10 min) 100 parts by weight Coloring agent: Cream color 5 parts by weight

[0052] Next, a two-plate mold (hot runner) was prepared as the mold and installed in the injection molding machine described above. Then, as shown in Figure 4, injection molding was performed so that the injected molten resin composition flowed from the bottom of the container to the flange, etc., and then pressure was held. The design values ​​and molding conditions for the container were as follows: [Container design value] Side wall thickness (short side): 0.350 mm Side wall thickness (long side): 0.350 mm Flange thickness (vertical direction): 0.350mm Flange protrusion length (horizontal direction from inside to outside): 3.55 mm Thickness of mating part (horizontal direction from inside to outside): 0.350mm Length of support part on the outside of the container (short side): 4.55 mm Length of support part on the outside of the container (long side): 4.93 mm [Molding conditions] Screw cylinder temperature: 240℃ ·Injection speed: 250mm / sec - Holding pressure: 30MPa Pressure retention time: 1 second Maximum flow length in cavity: 99mm

[0053] (Comparative Example) A container having the same structure as in the example was produced by ordinary injection molding. More specifically, in the comparative example, molding was carried out using the same resin composition as in the example, under the same design values ​​and molding conditions as in the example, except that the supercritical fluid was not mixed into the resin composition.

[0054] FIGS. 6A and 6B show the conditions of sink marks that occurred in the flanges and fitting portions of containers manufactured by the above-described Examples and Comparative Examples. FIG. 6A is a photograph showing a container according to the Comparative Example (a conventional molded product). FIG. 6B is a photograph showing a container according to the Example (a supercritical fluid molded product). As shown in FIG. 6A, sink marks were observed in region S1 where the flanges and fitting portions of the container according to the Comparative Example are located. On the other hand, as shown in FIG. 6B, it was confirmed that sink marks did not occur in region S2 where the flanges and fitting portions of the container according to the Example are located. The bubbles present inside the flanges and fitting portions of the container according to the Example were bubbles like those shown in FIG. 7A. The photograph of the container shown in FIG. 7A was obtained by observing a container manufactured under the same conditions as the Examples but without adding a colorant. While strictly speaking, it differs from the Examples in that the colorant was omitted to make it easier to observe the closed bubbles. Thus, it was confirmed that by providing an internal void at the opening end of a flange or other portion with varying thickness, it is possible to reduce the occurrence of sink marks and warpage while thinning the sidewalls and other portions. [Explanation of symbols]

[0055] 2a, 2b, 22...side wall portion, 3...flange (opening end), 4, 23...fitting portion (opening end), 10, 20...container.

Claims

1. A container that is a molded body formed by supercritical fluid molding, an opening edge provided at the opening of the container; a side wall portion formed so as to be continuous with the opening end portion, the opening end has at least one of a portion that is thicker than the side wall portion and a portion that causes a thickness difference within the opening end, A plurality of closed cells are formed inside the open end, A container, wherein the thickness of the side wall portion is 0.25 mm or more and 0.4 mm or less, and no closed bubbles are formed inside the side wall portion.

2. each of the plurality of closed cells has a cell length of 10 μm or more and 3000 μm or less, when the cell length is the longest side passing through the center of gravity of the cell; The number of the plurality of closed cells per unit volume is 0.3 / mm ３ 8.5 pieces / mm or more ３ Below is the The container of claim 1.

3. a skin layer is formed on the surface layer of the opening end, and the plurality of closed cells are not exposed to the surface layer; 3. The container according to claim 1 or 2.

4. The opening end is formed so that another member covering the opening of the container can be heat-sealed to it or so that it can be fitted to the other member. A container according to any one of claims 1 to 3.

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