JP7790103B2 - Container and method for manufacturing the container - Google Patents

Description

The present invention relates to containers manufactured by supercritical fluid molding and to a method for manufacturing containers using supercritical fluid molding.

Patent Document 1 discloses an example of a cup container. In this cup container, a U-shaped cross-section cutout is provided below the outer periphery of the flange, which suppresses deformation of the adhesive surface of the flange and prevents poor adhesion during heat sealing.

Japanese Patent Application Laid-Open No. 2014-237477

Molded products produced by injection molding can develop sink marks and warpage in areas where the thickness changes, resulting in molding defects. This is because thicker areas shrink more during solidification than thinner areas. When a thin and thicker area adjoins a thicker area, the shrinkage forces become unbalanced in the boundary area, causing the thinner area to be pulled into the internal solidification of the thicker area. Patent Document 1 discloses an example of a configuration that prevents sink marks and warpage, but the structure is complex, and a simpler configuration is desired to prevent sink marks and warpage.

The present invention aims 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.

One aspect of the present invention relates to a container formed by supercritical fluid molding. This container has an opening edge provided at the opening of the container and a side wall portion formed 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.

This container has an opening edge that is thicker than the side wall and/or has a thickness difference 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 and reducing the volume, the shrinkage rate of the thicker portion of the container is reduced, reducing the drag phenomenon caused by solidification of the thicker portion. This reduces the occurrence of sink marks and warpage with this container. Note that "closed bubbles" here 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.

In the container described above, each of the plurality of closed bubbles preferably has a length of 10 μm to 3000 μm, where the length is the longest side passing through the center of gravity of each bubble, and the number of bubbles per unit volume of the plurality of closed bubbles is preferably ^0.3 to ^8.5 /mm. This more reliably prevents the drag phenomenon caused by internal solidification of the thick portion, thereby more reliably suppressing the occurrence of sink marks and warpage. Furthermore, if the thickness of the opening end where the closed bubbles are formed is sufficiently thin (e.g., 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 the plurality of closed bubbles having a length of 10 μm to 3000 μm may be ^0.1 to ^2.5 /mm. On the other hand, when the container is made of a material that does not transmit light, the region including the opening end may be cut and the number of bubbles present per unit area on the cut surface may be counted. In this case, the number of bubbles per unit area of multiple closed bubbles having a bubble length of 10 μm to 3000 μm may be 0.1 bubbles/ ^mm2 to 2.5 bubbles/ ^mm2 , as described above. The number of bubbles may be calculated 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.

In the above-described 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 the inside of the side wall is free of closed bubbles. A container with such a thin side wall can reduce the amount of plastic used to form the container and lighten the container itself. Furthermore, even with a container with such a thin side wall, the formation of multiple closed bubbles inside the open end can suppress the occurrence of sink marks and warping due to the open end. Therefore, with such a container, it is possible to achieve both a reduction in the amount of plastic used and a lightweight container, as well as the suppression of sink marks and warping. Here, "no closed bubbles formed" means that there are no closed bubbles visible to the naked eye, or that the number of bubbles per unit area visible at a magnification of 35x using a microscope is 0.01 bubbles/ ^mm² or less. Generally, the lower limit of the bubble length of bubbles visible to the naked eye is 200 µm, so "no closed bubbles visible to the naked eye" means that there are no bubbles with a length exceeding 200 µm. Furthermore, in terms of determining whether or not closed bubbles have been 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 with a microscope, it is determined that closed bubbles have not been formed as long as the number of bubbles per unit area is 0.01 bubbles/ ^mm2 or less.

In the above-mentioned container, it is preferable that a skin layer is formed on the surface of the open end, and that the multiple closed cells are not exposed on the surface. In this case, a container with a more aesthetically pleasing appearance can be provided.

In the above-mentioned container, the open end may be configured so that another member covering the opening of the container can be heat-sealed or can be fitted with another member.

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 comprises 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 becomes the opening end provided at the opening of the container.

In this supercritical fluid molding manufacturing method, injection molding is performed in the injection step so that the flow end of the molten resin composition injected from the mold gate into the cavity 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 within the opening end that corresponds 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 caused by the opening end.

The present invention provides a container with a simple structure that suppresses the occurrence of sink marks and warping, as well as a method for manufacturing the container.

FIG. 1 is a perspective view showing a container according to one embodiment of the present invention. FIG. 2 is a cross-sectional view of the container shown in FIG. FIG. 3 is a bottom view showing the bottom of the container shown in FIG. FIG. 4 is a schematic diagram showing a method for manufacturing the container shown in FIG. FIG. 5 shows a container according to a variation of the present disclosure. FIG. 6A is a photograph showing a container according to a comparative example (normal molded product). FIG. 6B is a photograph showing a container according to an example (supercritical fluid molded product). FIG. 7A is a photograph of the flow end of a container manufactured by supercritical fluid molding, showing a state in which the degree of foaming is high. FIG. 7B is a photograph of the flow end of a container manufactured by supercritical fluid molding, showing a medium degree of foaming. FIG. 7C is a photograph of the flow end of a container manufactured by supercritical fluid molding, showing a low degree of foaming.

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 with the same functions may be designated by the same reference numerals, and duplicate explanations will be omitted. Note that the present invention is not limited to the following embodiment.

Figure 1 is a perspective view showing a container according to one embodiment of the present invention. The container 10 shown in Figure 1 is a molded body formed by supercritical fluid molding. Figure 2 is a cross-sectional view of the container 10. Figure 3 is a bottom view of the container 10. In a plan view, the container 10 has a generally 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, etc. 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.

The container 10 comprises a bottom 1, a pair of side walls 2a, a pair of side walls 2b, a flange 3 (opening edge), fitting portions 4 (opening edge) 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 onto 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.

In a plan view, the bottom 1 has a generally rectangular shape with rounded corners. 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. Having a thickness of 0.3 mm or more tends to suppress rear bulging and ensures drop resistance. On the other hand, having a thickness of 0.6 mm or less contributes to weight reduction.

Each of the side walls 2a, 2b extends diagonally 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 Figure 1, for example. Note that the side walls 2a, 2b may also extend substantially vertically relative to the bottom 1.

The thickness of the side walls 2a and 2b is, for example, 0.25 to 0.4 mm, and may be 0.3 to 0.35 mm. Having a thickness of 0.25 mm or more for the side walls 2a and 2b ensures drop resistance. On the other hand, having a thickness of 0.4 mm or less for the side walls 2a and 2b reduces weight and the amount of plastic used. While the above-mentioned embodiment illustrates an embodiment in which the entire side walls 2a and 2b have a thickness within the above range, only a portion of the side walls may be thin-walled and have a thickness within the above range. From the perspective 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 and 2b are not internally foamed, i.e., do not contain closed cells.

The flange 3 is the opening edge that defines the opening of the container 10. In plan view, it has a generally rectangular frame shape with rounded corners. 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 vertical thickness of the flange 3 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 mating portion 4, is an internally foamed portion, i.e., a portion with closed cells formed inside. Details will be described later. The mating 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.

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

The support portion 5 extends downward from the entire periphery of the bottom portion 1. The support portion 5 contributes to improving the strength of the container 10, particularly improving the strength (buckling strength) of the container 10 against crushing forces. As shown in Figure 3, the container 10 may further include multiple ribs 7 that reinforce the support portion 5.

In this container 10, a plurality of closed cells (also referred to as a group of closed cells) are formed inside the opening end portion of the container 10, which is formed by the flange 3 and the fitting portion 4, using a manufacturing method using supercritical fluid molding, as described below. On the other hand, no such closed cells are formed in the sidewall portions 2a and 2b. Closed cells refer to bubbles that exist independently of each other, and include bubbles that are in contact with adjacent bubbles 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 length of 10 μm or more and 3000 μm or less, where the longest side passing through the center of gravity of each bubble is defined as the cell length. The number of closed cells per unit volume at the opening end portion of the flange 3 or the fitting portion 4 may be, for example, 0.3 cells/ ^mm3 or more and 8.5 cells/ ^mm3 or less. An example of such a closed cell is shown in FIG. 7A . The image shown in Fig. 7A shows the flange 3 portion (bubble portion) in an area of 6.5 mm length x 8.5 mm width (area 55.25 ^mm2 ) at a microscope magnification of 35x. Fig. 7A shows the bubbles observed when observing a container that does not contain a colorant. Note that the bubbles in the flange 3 may be the bubble portion with the foaming degree shown in Fig. 7B or 7C, depending on the relationship with the thickness of the side wall portions 2a, 2b, etc.

Furthermore, when 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 the multiple closed bubbles at the opening end of the flange 3 or the fitting portion 4 may be 0.1/ ^mm2 or more and 2.5/ ^mm2 or less. On the other hand, when the opening end is formed from a material that does not transmit light, the region including the opening end may be cut and the number of bubbles present per unit area 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 end of the flange 3 or the fitting portion 4 may be 0.1/ ^mm2 or more and 2.5/ ^mm2 or less, as described above. The 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 with an optical microscope, it is preferable to cut the container thin enough to allow observation.

The absence of closed bubbles in the sidewall portions 2a and 2b means that there are no closed bubbles visible to the naked eye, or that the number of bubbles visible per unit area at a magnification of 35x using a microscope is 0.01 bubbles/ ^mm² or less, as described above. Generally, the lower limit of the length of bubbles visible to the naked eye is 200 µm. Therefore, the absence of closed bubbles visible to the naked eye means that there are no bubbles with a length exceeding 200 µm. Furthermore, in determining whether closed bubbles are present, the length of bubbles visible at a magnification of 35x using a microscope may exceed 200 µm. In other words, even if bubbles exceeding 200 µm are observed in the sidewall portions 2a and 2b when observed using a microscope, it is determined that no closed bubbles are present in the sidewall portions 2a and 2b as long as the number of bubbles per unit area is 0.01 bubbles/ ^mm² or less.

In the container 10, the presence of multiple closed cells at the open end reduces the volume of the flange 3, reducing the difference in volume between the flange 3 and the sidewalls 2a and 2b where no closed cells are formed. This reduces the potential for sink marks and warping that could occur in the thin-walled sidewalls 2a and 2b during shrinkage after injection molding. Furthermore, the presence of multiple closed cells reduces the volume of the flange 3, reducing the potential for sink marks and warping 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 difference in volume 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 warping in this region as well. The method for forming closed cells primarily at 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.

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 the molten resin composition into the cavity of the 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. 6,085,729 and 6,430,684).

[(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) refers to 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 shot refers to a phenomenon in which the resin material does not reach the flow end of the cavity.

To mold thin-walled containers using conventional injection molding, it was necessary to select a resin with high fluidity (high MFR value) to prevent short shots. However, highly fluid resin materials 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 suppress short shots while also achieving excellent strength (for example, buckling strength and drop resistance).

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 per 100 parts by mass of 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. In addition, 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 properties, resulting in an inability to form a uniform bubble layer, resulting in unevenness, or insufficient weight reduction.

When nitrogen is used as the supercritical fluid, the molten resin composition is prepared by adding 0.5 to 1.5 parts by mass of supercritical nitrogen per 100 parts by mass of resin material. Using 0.5 parts by mass or more of nitrogen reduces 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, foaming caused by supercritical nitrogen can be promoted, forming voids inside the molded product. On the other hand, using an amount of nitrogen less than 1.5 parts by mass can impair foaming properties, resulting in an inability to form a uniform bubble layer, resulting in unevenness, or insufficient weight reduction.

The temperature of the molten resin composition (screw cylinder temperature) can be set according to the melting point or MFR of the resin material. When using polypropylene resin, this temperature is preferably approximately 210 to 250°C. When using polyethylene resin, this temperature is preferably approximately 220 to 260°C. When this temperature is above the lower limit, the resin flows more easily within the cavity; on the other hand, when it is below the upper limit, burning of the resin tends to be suppressed.

The molten resin composition may contain components other than the resin material and the supercritical fluid. That is, the molten resin composition may further contain, as needed, for example, fillers, colorants, slip agents, antistatic agents, etc.

[(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 Figure 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, a drop in pressure is likely to occur at the flange 3 and fitting portion 4, which form the flow end portion, as will be explained in step (C) below.

The injection speed of the molten resin composition in step (B) is preferably 100 to 400 mm/sec, and more preferably 150 to 200 mm/sec. An injection speed of 100 mm/sec or higher tends to make it easier for the resin to reach the end of the flow, and tends to suppress 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 product. Note that 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 speed in this manner, it is possible to improve the transferability of the mold and prevent solidification during molding.

[(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 varied, as shown in Figures 7A to 7C.

[(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.

As described above, according to this embodiment, the flange 3 and fitting portion 4, which form the opening edge of the container 10, have portions that are thicker than the side wall portions 2a and 2b, and portions that create a difference in thickness within the opening edge. However, multiple closed bubbles are formed inside the opening edge. In this way, by forming voids inside the opening edge 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 warping.

Furthermore, in the container 10 according to this embodiment, each of the plurality of closed bubbles may have a bubble length of 10 μm to 3000 μm, where the bubble length is the longest side passing through the center of gravity of each bubble. The number of bubbles per unit volume of the plurality of closed bubbles may be ^0.3 to 8.5/ ^mm² . This more reliably prevents the drag phenomenon caused by internal solidification of the thick portion, thereby more reliably suppressing the occurrence of sink marks and warpage. Furthermore, if the thickness of the opening end (e.g., flange 3) where the closed bubbles are formed is sufficiently thin (e.g., about 0.5 mm ⁾ to allow light to pass through, the number of bubbles per unit area may be specified. In this case, the number of bubbles per unit area of the plurality of closed bubbles having a bubble length of 10 μm to 3000 μm may be 0.1 to ^2.5 /mm². On the other hand, when the filter is made of a material that does not transmit light, the region including the opening end (such as the flange 3) may be cut and the number of bubbles present per unit area 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 may be 0.1 bubbles/ ^mm2 or more and 2.5 bubbles/ ^mm2 or less, as described above.

In addition, 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 present inside the side walls 2a, 2b. Having such thin side walls 2a, 2b makes it possible to reduce the amount of plastic used to make up the container 10 and to make the container itself lighter. Even in a container with 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.

In addition, 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, preventing the multiple closed cells from being exposed on the surface. This allows for the provision of a container with an even more aesthetically pleasing appearance.

In addition, in the supercritical fluid molding manufacturing method 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 mold gate 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., flange 3 and fitting portion 4, making it possible to easily manufacture the container 10 while suppressing the occurrence of sink marks and warpage caused by the flange 3, etc.

Although the above describes an embodiment of the present invention, the present invention is not limited to the above embodiment. For example, the above embodiment has been described using a container having a flange 3 and a fitting portion 4 (e.g., a container used for margarine, etc.) as an example, but the scope of application of the present invention is not limited to this. For example, the container 10 configured as shown in Figures 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-sealing surface. In this case, too, multiple closed bubbles are formed inside the flange 3, thereby suppressing sink marks and warpage caused by the flange 3 from occurring on the side wall portions 2a, 2b, etc. Furthermore, because bubbles are formed inside the flange 3, etc. of the container 10, when heat-sealing the flange 3, heat can be more easily released during heat-sealing, making it possible to shorten the cooling time after heat-sealing.

The present invention may also be applied to a container 20 shown in Figure 5, for example. The container 20 comprises 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 to the 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 above during injection molding using supercritical fluid molding. This results in multiple closed cells being formed inside the fitting portion 23.

With conventional injection molding, differences in 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 with the above-described embodiment, the container 20 according to this embodiment has foamed interior portions of the fitting portion 23, thereby reducing the volumetric difference within the fitting portion 23. This reduces the volumetric difference between the fitting portion 23 and the adjacent side wall portion 22. Therefore, the container 20 according to the modified example suppresses the occurrence of sink marks or warpage in the fitting portion 23. Moreover, because the container 20 is formed by supercritical fluid molding, the strength of the side wall portion 22 can be 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 a fitting portion 23 with foamed interior.

In addition, while the above embodiments illustrate the use of carbon dioxide or nitrogen as the supercritical fluid, argon or helium, for example, may be used instead of these gases. In addition, while the above embodiments illustrate dairy products such as butter and margarine as examples of contents, 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 appropriate for the contents. For example, the shape of the bottom of the container may be approximately square, circular, or oval. Furthermore, the sidewalls do not have to be flat, and may be curved like a bowl.

The present invention will be described below based on examples and comparative examples. Note that the present invention is not limited to the following examples.

(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 following resin composition 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.

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

Next, a two-plate mold (hot runner) was prepared as a 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 values]
- Thickness of the side wall (short side) (horizontal direction from inside to outside): 0.350 mm
- Thickness of the side wall (long side) (horizontal direction from inside to outside): 0.350 mm
Flange thickness (vertical direction): 0.350 mm
Flange protrusion length (horizontal direction from inside to outside): 3.55 mm
- Thickness of mating part (horizontal direction from inside to outside): 0.350 mm
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°C
Injection speed: 250 mm/sec. Holding pressure: 30 MPa
・Pressure holding time: 1 second ・Maximum flow length in cavity: 99 mm

(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.

Figures 6A and 6B show the state of sink marks that occurred in the flanges and fittings of containers manufactured according to the above-mentioned Examples and Comparative Examples. Figure 6A is a photograph showing a container according to the Comparative Example (normally molded product). Figure 6B is a photograph showing a container according to the Example (supercritical fluid molded product). As shown in Figure 6A, sink marks were confirmed in region S1, where the flange and fittings of the container according to the Comparative Example are located. On the other hand, as shown in Figure 6B, it was confirmed that sink marks did not occur in region S2, where the flange and fittings of the container according to the Example are located. Note that the air bubbles present inside the flange and fittings of the container according to the Example were similar to those shown in Figure 7A. Note, however, that the photograph of the container shown in Figure 7A was obtained by observing a container manufactured under the same conditions as the Examples, but without the addition of a colorant. While strictly speaking different from the Examples, the colorant was omitted to make it easier to observe the closed bubbles. In this way, it was confirmed that by providing an internal void at the opening end of a flange or other part with varying thickness, it is possible to reduce the occurrence of sink marks and warpage while also thinning the sidewalls.

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

Claims (4)

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 . 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 cells/ ^mm3 or more and 8.5 cells/ ^mm3 or less;
The container of claim 1. 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 . 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.
The container according to any one of claims 1 to 3 .