Food packaging containers

Abstract

This invention provides a microwave-safe food packaging container that suppresses the generation of resin powder by suppressing the occurrence of fluff. [Solution] A steam vent hole 30 is provided on the top surface of the lid, and a thickened portion 32 is provided on the opening edge of the steam vent hole 30. The cross-sectional shape of the thickened portion 32 is such that, when the length along the upper extension line 51 between the upper bulge start point 50 and the upper intersection point 52 is LU, the length along the lower extension line 54 between the lower bulge start point 53 and the lower intersection point 55 is LL, and the thickness of the sheet is T, then T ≤ LU and T ≤ LL.

Description

Technical Field

[0001] The present invention relates to a food packaging container for microwave oven heating, having steam vents provided on the top surface of the lid.

Background Art

[0002] For example, foods cooked at a supermarket or the like are stored in packaging containers. When reheating the stored food in a microwave oven, it is necessary to discharge the steam generated from the food to the outside. As one method of discharging the steam to the outside of the container, through holes for exhaust may be provided on the top surface of the lid of the container.

[0003] In Patent Document 1 below, as shown in FIG. 11, a lid having a plurality of fine holes 100 provided on the top surface of the lid by laser processing and an annular bulging portion 101 provided at the peripheral edge of the fine holes 100 is described. The cross section of the fine holes 100 is said to have a structure that expands from the bottom to the top.

[0004] However, since the lid is formed of a biaxially stretched sheet made of styrene resin, the lid is hard and brittle. Therefore, there is a risk that fluffing may occur on the wall surface of the fine holes 100 during the perforation operation. If fluffing occurs on the wall surface of the fine holes 100, there is a possibility that the fluffing may be removed and resin powder may be generated when the top surface of the lid is rubbed during packaging or transportation, etc., and it takes time to remove the resin powder.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present invention aims to provide a microwave-safe food packaging container that can suppress the generation of resin powder by suppressing the occurrence of fluffing in the steam vent holes. [Means for solving the problem]

[0007] The microwave-safe food packaging container according to the present invention comprises a container body and a lid thermoformed from a sheet of olefin resin that is attached to the container body, wherein the top surface of the lid is provided with a steam vent hole for releasing steam from inside the container to the outside during microwave heating, and the opening edge of the steam vent hole is provided with a thickened portion that bulges upward and downward around its entire circumference, and in a cross-sectional view when the sheet is cut vertically across the steam vent hole, the outer edge of the thickened portion has an upward curved portion that bulges upward from the top surface of the sheet and a downward curved portion that bulges downward from the bottom surface of the sheet. The container has an inwardly curved section that curves and bulges outwards toward the center of the steam vent hole, and in a cross-sectional view, the length along the upper extension line between the upper bulge initiation point where the upper curved section begins to bulge outwards from the upper surface of the sheet and the upper intersection point where the upper extension line extended from the upper surface of the sheet toward the center of the steam vent hole intersects with the outer shape of the thick section is denoted as LU, and the length along the lower extension line between the lower bulge initiation point where the lower curved section begins to bulge outwards from the lower surface of the sheet and the lower intersection point where the lower extension line extended from the lower surface of the sheet toward the center of the steam vent hole intersects with the outer edge of the thick section is denoted as LL, and when the thickness of the sheet is T, then T ≤ LU and T ≤ LL. The container body and lid form a storage space for storing food, and on the top surface of the lid, the storage space side is considered the lower side, and the side opposite the storage space is considered the upper side. On the top surface of the lid, the upper surface of the sheet is the outer surface of the container, and the lower surface of the sheet is the inner surface of the container.

[0008] In this configuration, the lid is made of an olefin resin sheet. Therefore, it is softer than a lid made of a styrene resin sheet. In addition, a thickened section is formed around the entire circumference of the steam vent opening edge, which improves the strength of the steam vent opening edge. In particular, an inwardly curved section is provided on the outer edge in the cross-sectional view of the thickened section, that is, an inwardly curved section is provided on the wall surface of the steam vent, and moreover, the relationship T≦LU and T≦LL is met. In other words, the cross-sectional shape of the thickened section is close to a circular shape. Therefore, the reinforcement effect of the thickened section on the steam vent opening edge is significant. Consequently, the steam vent is prevented from opening excessively during microwave heating, and the pressure inside the container can be maintained while releasing steam appropriately. Therefore, the food contained can be heated quickly. In other words, it is easy to achieve both appropriate steam release and maintenance of internal pressure. Furthermore, because the thick-walled section has a roughly circular cross-section, there is little resistance when steam passes through the steam vent holes, allowing steam to be discharged smoothly without having to make the opening area of ​​the steam vent holes excessively large.

[0009] Furthermore, because the lid is constructed from an olefin resin sheet and the thicker section has a roughly circular cross-section, fuzzing is less likely to occur on the walls of the steam vent holes. Consequently, the generation of resin powder due to fuzzing is suppressed, and the workload for removing the resin powder can be reduced.

[0010] In particular, in the above cross-sectional view, when the length of the line connecting the midpoint of the line connecting the upper bulge start point and the lower bulge start point and the innermost point in the inwardly curved portion that is closest to the center of the steam vent hole is denoted as A, and the distance in the thickness direction of the sheet between the uppermost point of the upper curved portion and the lowermost point of the lower curved portion is denoted as B, it is preferable that 0.8 ≤ B / A ≤ 1.1. With this configuration, the cross-sectional shape of the thickened portion becomes even closer to a circle. As a result, the reinforcing effect of the thickened portion on the opening edge of the steam vent hole is further improved, the resistance to steam discharge is further reduced, and the steam discharge efficiency is increased. Furthermore, the generation of resin powder due to fluffing is further reduced.

[0011] Furthermore, in the cross-sectional view described above, it is preferable that 1.8 ≤ B / T ≤ 2.2. With this configuration, the reinforcing effect of the thickened portion on the opening edge of the steam vent hole is further improved.

[0012] Furthermore, the shape of the steam vent hole in plan view is preferably an elongated hole shape with a width of 0.15 to 1 mm and a length of 2 to 30 mm along the steam vent hole. With this configuration, since the shape of the steam vent hole in plan view is an elongated hole shape with a predetermined width and length, the number of steam vent holes can be reduced, and the work process for forming the steam vent holes can be shortened. In addition, since the number of steam vent holes can be suppressed, the probability of resin powder generation due to fluffing can also be reduced. [Effects of the Invention]

[0013] As described above, by using an olefin resin for the lid sheet and making the cross-sectional shape of the thick section close to a circular shape, the occurrence of fuzzing on the wall surface of the steam vent can be suppressed, thereby suppressing the generation of resin powder and allowing steam to be smoothly discharged to the outside of the container. In addition, the strength of the opening edge of the steam vent is improved, and excessive opening of the steam vent can be suppressed during microwave heating, achieving both appropriate steam discharge and maintenance of internal pressure. [Brief explanation of the drawing]

[0014] [Figure 1] A perspective view showing the open state of a food packaging container according to one embodiment of the present invention. [Figure 2] Enlarged plan view of the lid of the food packaging container. [Figure 3] Enlarged plan view of the main part of the steam vent hole in the same lid. [Figure 4] Figure 3 shows a cross-sectional view of the PP (polypropylene) layer. [Figure 5] Enlarged view of section Q in Figure 4. [Figure 6] An enlarged cross-sectional view of the main part of the steam vent hole in the lid of a food packaging container in another embodiment of the present invention. [Figure 7] A photograph serving as a substitute for a diagram, showing a magnified cross-section of the steam vent. [Figure 8]Photograph substituting for drawing with enlarged cross-section of steam vent hole. [Figure 9] Partial enlarged plan view of lid of food packaging container in another embodiment of the present invention. [Figure 10] Partial enlarged plan view of main part of steam vent hole of the lid. [Figure 11] Cross-sectional view of conventional steam vent hole.

Mode for Carrying Out the Invention

[0015] Hereinafter, a food packaging container according to an embodiment of the present invention will be described with reference to FIGS. 1 to 5. The container in this embodiment is for accommodating various foods and is a microwave-compatible container that can be heated in a microwave with the lid closed without opening.

[0016] The container includes a container body 1 and a lid 2. Both the container body 1 and the lid 2 are formed from a synthetic resin sheet by various thermoforming (sheet forming) such as vacuum forming or pressure-air forming. The shape of the container in plan view is arbitrary and may be round, oval, substantially square, substantially polygonal, etc. In this embodiment, the shape of the container in plan view is circular.

[0017] <Container body 1> The container body 1 has a circular opening that opens upward. The lid 2 fits into the container body 1 to close the opening of the container body 1. The fitting between the container body 1 and the lid 2 preferably has an inner fitting structure in which the fitting portion 23 of the lid 2 is brought into contact with the inside of the fitting portion 13 of the opening of the container body 1 for fitting. By making the fitting structure an inner fitting structure, the sealing performance can be improved so that the contained soup etc. does not leak when the lid is closed. Incidentally, if necessary, a middle plate (not shown) may be accommodated between the container body 1 and the lid 2.

[0018] The container body 1 can hold various foods such as jelly-like soups, noodles, and dried ingredients, so that they can be heated directly in a microwave oven and eaten. The container body 1 can be obtained by thermoforming a sheet made of heat-resistant resin material. For use in a microwave oven, it is particularly preferable to use a foamed heat-resistant resin material for the container body 1, as it will not be hot to the touch when removed from the microwave after heating, thus preventing burns, and it will efficiently transfer the microwave's electromagnetic waves to the food inside, allowing for uniform and quick heating.

[0019] The container body 1 comprises a bottom portion 10, a side portion 11 extending upward from the outer edge of the bottom portion 10 while expanding upward, and a flange portion 12 extending outward from the upper end of the side portion 11. The bottom portion 10 may be provided with legs of various shapes (not shown). The bottom portion 10 may also be provided with a raised bottom portion (not shown). Preferably, a body fitting portion 13 is formed on the upper part of the side portion 11, into which the lid fitting portion 23 of the lid 2 (described later) fits inside. The shape of the body fitting portion 13 may vary, but one example is an inverse tapered shape that gradually decreases in diameter towards the top. When a body fitting portion 13 is provided, a body step portion 14 may extend inward (towards the center of the container) from the lower end of the body fitting portion 13. The fitting structure between the container body 1 and the lid 2 may be an internal fitting structure, an external fitting structure, or an internal and external fitting structure that has both internal and external fitting. The main flange portion 12 may be provided with a tab (not shown) to facilitate opening the lid. The main flange portion 12 may also be provided with a skirt portion (not shown) extending downwards.

[0020] <Lid 2> The lid 2 is preferably transparent, allowing the food contained in the container to be seen from the outside through the lid 2. The lid 2 comprises a top surface 20 which is entirely flat, a lid side surface 21 extending downward from the outer edge of the top surface 20, and a lid flange 22 extending outward from the lower end of the lid side surface 21. It is preferable that a lid fitting portion 23 is formed on the lid flange portion 22 which fits inside the main body fitting portion 13. The shape of the lid fitting portion 23 may vary, but one example is an inverse tapered shape that gradually decreases in diameter towards the top. The lid flange portion 22 may be provided with a tab (not shown) to facilitate opening the lid. The lid side surface 21 may be provided with a blocking prevention protrusion (not shown) to prevent the so-called blocking phenomenon, where the lids 2 become tightly interlocked and cannot be separated when many lids 2 are stacked together during collection. The blocking prevention protrusions are positioned at different locations for each lid 2.

[0021] Steam vents 30 are formed on the flat surface of the top portion 20 to release steam from inside the container to the outside during microwave heating. It is preferable that there are multiple steam vents 30. When multiple steam vents 30 are provided, it is preferable that they are arranged together in one place rather than dispersed. The area where multiple steam vents 30 are concentrated is called the steam vent area 31. Multiple steam vents 30 are concentrated in the steam vent area 31, forming a collection of holes consisting of multiple steam vents 30. It is preferable that the steam vents 30 are arranged in a predetermined arrangement in the steam vent area 31. The number and arrangement of the steam vents 30 are arbitrary, and the shape of the steam vents 30 is also arbitrary. This embodiment shows one example.

[0022] Figure 2 shows an enlarged plan view of the steam vent area 31 and the collection of holes, and Figure 3 shows the details of one steam vent hole 30. As shown in Figure 2, it is preferable that the multiple steam vent holes 30 are the same shape as each other. It is also preferable that the multiple steam vent holes 30 are arranged in a row along a predetermined direction. The direction in which the multiple steam vent holes 30 are arranged is defined as the horizontal direction, and the direction perpendicular to the horizontal direction is defined as the vertical direction. In Figures 2 and 3, the vertical direction is indicated by arrow Y, and the horizontal direction by arrow X. The steam vent area 31 is rectangular, preferably a square or rectangle, and in this embodiment, it is a rectangle that is long in the horizontal direction. The rows of steam vent holes 30 may be one row or two or more rows. The number of steam vent holes 30 is arbitrary, but for example, a range of 4 to 16 is preferred. For example, it can be 4 holes × 1 row = 4 holes in total, 8 holes × 1 row = 8 holes in total, 4 holes × 2 rows = 8 holes in total, or 3 holes × 3 rows = 9 holes in total.

[0023] The shape of the steam vent hole 30 may be circular in plan view, but it is preferable that it be elongated in plan view, as shown in Figure 3. The type of elongated hole is arbitrary, but in this embodiment, a curved one is exemplified, and a sinusoidal one is also exemplified. The sinusoidal steam vent hole 30 has a first arc-shaped portion 41 that bulges toward one side in the lateral direction and a second arc-shaped portion 42 that bulges toward the other side in the lateral direction, with the first arc-shaped portion 41 and the second arc-shaped portion 42 arranged alternately in the vertical direction. In this embodiment, one first arc-shaped portion 41 and one second arc-shaped portion 42 are provided, but the number is arbitrary. In Figure 3, the hole center 30c of the steam vent hole 30 is shown by a dashed line. In a plan view as shown in Figure 3, if the shape of the steam vent hole 30 is curved, the hole center 30c of the steam vent hole 30 will be a curve that follows the curve of the steam vent hole 30. Furthermore, the steam vent hole 30 may be straight, as shown in Figures 9 and 10, or it may be bent in the middle. In a plan view as shown in Figure 10, if the shape of the steam vent hole 30 is straight, the center 30c of the steam vent hole 30 will also be straight.

[0024] Next, the length along the hole of the steam vent 30 will be explained. The length along the hole of the steam vent 30 is the total length of the steam vent 30 if the steam vent 30 is straight. If the steam vent 30 is straight, the distance between the two ends 30a in the longitudinal direction of the steam vent 30 is the length along the hole of the steam vent 30. If the steam vent 30 is curved, the total length of the path of the steam vent 30 along the curve is the length along the hole. If the steam vent 30 is curved, it is the total length when the curve is extended into a straight line. Also, if the steam vent 30 is bent in the middle, it is the sum of the lengths of the multiple straight sections, and it is the total length when the multiple straight sections are extended into a straight line.

[0025] The length of the steam vent hole 30 along the opening is preferably 2 to 30 mm. The width W of the steam vent hole 30 is the dimension in the direction perpendicular to the direction along the opening (short side) when viewed from above. The width W of the steam vent hole 30 is preferably 0.15 to 10 mm. The steam vent hole 30 is formed by laser processing. If the width W of the steam vent hole 30 is 0.15 to 1 mm, it can be easily formed by laser.

[0026] The lateral spacing between adjacent steam vents 30 is preferably 1 mm to 8 mm. Here, spacing refers to the distance of the uncut portion between adjacent steam vents 30. If the lateral spacing between adjacent steam vents 30 is less than 1 mm, the adjacent steam vents 30 are more likely to connect during laser processing. If the spacing between steam vents 30 is wider than 8 mm, the steam vent area 31 becomes excessively wide.

[0027] <Thick part 32> A thickened portion 32 is formed at the opening edge of the steam vent hole 30. The thickened portion 32 is formed in a ridge-like manner around the entire circumference of the opening edge of the steam vent hole 30. That is, the thickened portion 32 is annular and surrounds the steam vent hole 30. Figure 4 shows the cross-sectional shape when the sheet is cut vertically so as to traverse the steam vent hole 30 in the width direction. Figure 5 shows the details of the thickened portion 32. Note that in Figure 5, hatching is indicated by numerous dots. The same applies to Figure 6, which will be described later. As shown in Figure 5, the thickened portion 32 is thickened by bulging outwards in a curved manner towards the upper side, the lower side, and toward the center 30c of the steam vent hole 30. That is, the thickened portion 32 is an annular projection that protrudes upward and downward. The thickened portion 32 is a part of the sheet where the thickness is locally increased at the opening edge of the steam vent hole 30, and is a thickened mass. In the cross-sectional view shown in Figure 5, the outer edge 32a of the thickened portion 32 has an upward curved portion 35 that curves upward and bulges from the upper surface 33 of the sheet, a downward curved portion 36 that curves downward and bulges from the lower surface 34 of the sheet, and an inward curved portion 37 that connects the upward curved portion 35 and the downward curved portion 36 and curves and bulges toward the center 30c of the steam vent hole 30. The thickened portion 32 is approximately circular in cross-sectional view.

[0028] In the cross-sectional view, each point is defined as follows: The point where the upper curved portion 35 begins to bulge from the sheet upper surface 33 is defined as the upper bulge start point 50. The upper bulge start point 50 is the point where the thickened portion 32 begins to bulge from the sheet upper surface 33. The straight line extended from the sheet upper surface 33 toward the center 30c of the steam vent hole 30 is defined as the upper extension line 51. The point where the upper extension line 51 intersects with the outer shape of the thickened portion 32 is defined as the upper intersection point 52. The point where the lower curved portion 36 begins to bulge from the sheet lower surface 34 is defined as the lower bulge start point 53. The lower bulge start point 53 is the point where the thickened portion 32 begins to bulge from the sheet lower surface 34. The straight line extended from the sheet lower surface 34 toward the center 30c of the steam vent hole 30 is defined as the lower extension line 54. The point where the lower extension line 54 intersects with the outer shape of the thickened portion 32 is defined as the lower intersection point 55. The uppermost point 56 is defined as the point located at the top of the upper curved section 35. The lowermost point 57 is defined as the point located at the bottom of the lower curved section 36. The innermost point 58 is defined as the point in the inward curved section 37 that is closest to the hole center 30c. As shown in Figure 4, the width W of the steam vent hole 30 is the dimension between two opposing innermost points 58. The hole center 30c of the steam vent hole 30 is located in the center of the width direction of the steam vent hole 30 in a cross-sectional view.

[0029] The upper curved section 35 is the portion between the upper bulge start point 50 and the upper intersection point 52. The upper curved section 35 rises from the upper bulge start point 50 to the highest point 56 and then descends from the highest point 56 to the upper intersection point 52. The upper curved section 35 connects with the sheet upper surface 33 at the upper bulge start point 50. The upper curved section 35 may be smoothly continuous with the sheet upper surface 33 at the upper bulge start point 50, as shown in Figure 6. The lower curved section 36 is the portion between the lower bulge start point 53 and the lower intersection point 55. The lower curved section 36 descends from the lower bulge start point 53 to the lowest point 57 and then rises from the lowest point 57 to the lower intersection point 55. The lower curved section 36 connects with the sheet lower surface 34 at the lower bulge start point 53. The downward curved portion 36 may be smoothly continuous with the sheet upper surface 33 at the downward bulging start point 53, as shown in Figure 6. The inward curved portion 37 is the portion between the upper intersection 52 and the lower intersection 55. The inward curved portion 37 curves toward the hole center 30c from the upper intersection 52 to the lower intersection 55. The inward curved portion 37 approaches the hole center 30c from the upper intersection 52 to the innermost point 58, and moves away from the hole center 30c from the innermost point 58 to the lower intersection 55. The inward curved portion 37 connects with the upper curved portion 35 at the upper intersection 52 and with the downward curved portion 36 at the lower intersection 55. The inward curved portion 37 is smoothly continuous with the upper curved portion 35 and smoothly continuous with the downward curved portion 36. The thickened portion 32 is preferably curved in an arc towards the hole center 30c from the uppermost point 56 through the innermost point 58 to the lowest point 57. The curvature may be constant from the uppermost point 56 to the lowest point 57, or it may vary.

[0030] The dimensions of each part in the cross-sectional view will be described. Let LU be the length along the upper extension line 51 between the upper bulging start point 50 and the upper intersection point 52, LL be the length along the lower extension line 54 between the lower bulging start point 53 and the lower intersection point 55, and T be the thickness of the sheet. Then, T ≤ LU and T ≤ LL. The straight line connecting the upper bulging start point 50 and the lower bulging start point 53 is referred to as the virtual outer line 60. The straight line connecting the midpoint 61 of the virtual outer line 60 and the innermost point 58 is referred to as the virtual center line 62. Let A be the length of the virtual center line 62, and B be the distance in the thickness direction of the sheet (the vertical dimension of the thick portion 32) between the uppermost point 56 of the upper curved portion 35 and the lowermost point 57 of the lower curved portion 36. Then, it is preferable that 0.8 ≤ B / A ≤ 1.1. That is, it is preferable that the dimension of the thick portion 32 in the direction along the sheet (the horizontal dimension) and the vertical dimension of the thick portion 32 are substantially equal.

[0031] Also, it is preferable that 1.8 ≤ B / T ≤ 2.2. That is, it is preferable that the vertical dimension of the thick portion 32 is approximately twice the thickness T of the sheet. Note that the thickness T of the sheet is from 0.12 to 0.8 mm, preferably from 0.15 to 0.6 mm, and more preferably from 0.18 to 0.4 mm. Here, the thickness T of the sheet is the reference thickness of the sheet and is the separation distance in the vertical direction (the thickness direction of the sheet) between the upper bulging start point 50 and the lower bulging start point 53.

[0032] Let BU be the dimension in the thickness direction of the sheet (the vertical dimension) from the upper surface 33 of the sheet to the uppermost point 56, and BL be the dimension in the thickness direction of the sheet (the vertical dimension) from the lower surface 34 of the sheet to the lowermost point 57. Then, BU < T and BL < T. BU is the amount of bulging upward from the upper surface 33 of the sheet, and BL is the amount of bulging downward from the lower surface 34 of the sheet. It is preferable that BU and BL are substantially equal. Also, it is preferable that LU is greater than twice BU, and it is preferable that LL is greater than twice BL. The straight line connecting the upper intersection point 52 and the lower intersection point 55 is referred to as the virtual inner line 63. The intersection point where the virtual inner line 63 and the virtual center line 62 intersect is referred to as the central intersection point 64. Let AC be the dimension on the virtual center line 62 from the central intersection point 64 to the innermost point 58. Then, it is preferable that AC < BU and it is preferable that AC < BL.

[0033] Furthermore, the upper and lower surfaces of the outer edges of the main body flange portion 12 and the lid flange portion 22 may be given a textured or embossed surface to create a textured shape. By creating this texture, the outer edges of the main body flange portion 12 and the lid flange portion 22 are reinforced, and fingers or other body parts will not be cut if they come into contact with the surface. When viewed magnified from the front, this texture may take the shape of waves, with the direction of the numerous peaks and valleys oriented in the direction of the inside and outside of the container in a plan view. If the container is rectangular in plan view, the texture will be formed in the radial direction in a plan view at the corners. In addition, this texture may be formed by knurling, such as flat knurling or diagonal knurling, and it is preferable to use diagonal knurling to make it less slippery.

[0034] The container body 1 is preferably formed from a synthetic resin sheet by various thermoforming (sheet molding) methods such as vacuum forming, pressure forming, or plug-assisted forming. Various synthetic resin sheets can be used for the container body 1. The synthetic resin sheet is composed of, for example, polyester resins such as polyethylene terephthalate, styrene resins such as polystyrene, and polyolefin resins such as polypropylene. Stretched sheets obtained by uniaxially or biaxially stretching these sheets are also preferred. The sheet constituting the container body 1 may be transparent, but it is preferable that it be a foamed material with heat insulating properties.

[0035] The lid 2 is formed from a sheet of synthetic resin by various thermoforming (sheet molding) methods such as vacuum forming or pressure forming. The lid 2 is formed from a sheet made of polyolefin resin. As described above, the thickness of the sheet is 0.12 to 0.8 mm, preferably 0.15 to 0.6 mm, and more preferably 0.18 to 0.4 mm.

[0036] The lid 2 is made of an olefin resin such as polypropylene or polyethylene, and polypropylene is particularly preferred from the viewpoint of heat resistance, oil resistance, and acid resistance. Furthermore, it is preferable that the lid has excellent transparency from the viewpoint of visibility of the contents contained in the container body 1.

[0037] It is preferable to use a polypropylene resin with excellent transparency. For example, propylene homopolymer (homoPP), block copolymer of α-olefins such as ethylene or 1-hexene with propylene (blockPP), and random copolymer of α-olefins with propylene (randomPP) can be used individually or in combination of several types.

[0038] In particular, for transparent polypropylene resin sheets, it is preferable to use a propylene homopolymer due to its excellent transparency. Furthermore, random copolymers of α-olefin and propylene, when polymerized using a metallocene catalyst, exhibit relatively good transparency and can be said to be preferable polypropylene resins to use.

[0039] Furthermore, a transparency nucleating agent may be used to improve the transparency of the polypropylene resin described above. As the transparency nucleating agent, those commonly used as nucleating agents to improve the transparency of crystalline polymers, including polypropylene resins, can be used. For example, aromatic carboxylic acid metal salts, aromatic phosphate metal salts, algitol derivatives, rosin metal salts, etc., can be used, and these can be used alone or in combination. In addition, in order to make the transparent polypropylene resin sheet have excellent transparency, the transparency nucleating agent is usually contained in an amount of 0.2 to 7 g per 1 kg of polypropylene resin, preferably 0.5 to 5 g, and particularly preferably 1.0 to 5 g. Among these, it is preferable to contain 0.5 to 5 g of an algitol derivative as the transparency nucleating agent per 1 kg of polypropylene resin, and particularly preferably 1.0 to 5 g.

[0040] Polypropylene resin sheets are obtained by melt-kneading an olefin resin composition in an extruder and extruding an unstretched sheet from a die (especially a T-die). At this time, the extruded sheet may be stretched sequentially or simultaneously in two axial directions: the longitudinal direction (sheet flow direction, MD; Machine Direction) and the transverse direction (direction perpendicular to the sheet flow direction, TD; Transverse Direction). The stretching ratio can be, for example, 1 to 3 times in both the MD and TD directions.

[0041] <Regarding the production of transparent polypropylene resin sheets> A polypropylene resin composition containing 2000 mg of a transparent nucleating agent (Milliken Corporation, bis(4-propylbenzylidene)propylsorbitol, trade name "Millad NX8000")) was prepared by heating and melt-kneading the mixture at approximately 250 °C in a 27 mm twin extruder, and extruding it at an extrusion rate of approximately 30 kg / h through a T-die set to approximately 220 °C to produce a transparent polypropylene resin sheet (Example 1) with a thickness of 0.23 mm. Similarly, a transparent polypropylene resin sheet (Example 2) with a thickness of 0.19 mm was prepared.

[0042] <About thermoformed products> The transparent polypropylene resin sheet with a thickness of 0.23 mm (Example 1) obtained above was supplied to a molding machine with the heater temperature in the heating zone set to 270 °C and thermoformed to produce a lid (Example 1) with a width of 130 mm (short side dimension) x a length of 160 mm (long side dimension) x a depth of 25 mm (vertical dimension). Similarly, a lid (Example 2) with a width of 130 mm (short side dimension) x a length of 160 mm (long side dimension) x a depth of 25 mm (vertical dimension) was produced from a transparent polypropylene resin sheet with a thickness of 0.19 mm (Example 2).

[0043] <Regarding the perforation process for thermoformed products> The following drilling operation was performed on the top surface of the lid obtained above, under the conditions described below. (Laser beam irradiation conditions) Laser beam irradiation device: Videojet VJ3640 Wavelength: 9.3μm Output: 60W Distance from the tip of the laser beam to the surface of the sheet: 254 mm

[0044] Figure 7 shows a photograph of the cross-section of the steam vent hole 30 in Example 1, which was formed in this manner. The sheet thickness T was 0.23 mm, LU = 0.48 mm, and LL = 0.51 mm. Also, A = 0.46 mm, B = 0.47 mm, BU = 0.11 mm, BL = 0.13 mm, and AC = 0.01 mm. B / A = 1.0 and B / T = 2.0. Note that the values ​​are the average values ​​of the left and right thickened sections 32.

[0045] Similarly, Figure 8 shows a photograph of the cross-section of the steam vent hole 30 in Example 2. The sheet thickness T was 0.19 mm, LU = 0.45 mm, and LL = 0.42 mm. Also, A = 0.45 mm, B = 0.39 mm, BU = 0.13 mm, BL = 0.07 mm, and AC = 0.03 mm. B / A = 0.9 and B / T = 2.1. Note that these values ​​are the average values ​​of the thickened sections 32 on the left and right sides.

[0046] As described above, in the microwave-safe food packaging container of this embodiment, the lid 2 is made of a sheet of olefin resin, and a thickened portion 32 with a roughly circular cross-section is formed around the entire circumference of the opening edge of the steam vent hole 30, thereby improving the strength of the opening edge of the steam vent hole 30. Furthermore, it prevents the steam vent hole 30 from opening excessively during microwave heating. As a result, the pressure inside the container is maintained at a high level while releasing steam appropriately, allowing the contained food to be heated quickly. Moreover, because the cross-sectional shape of the thickened portion 32 is roughly circular, there is little resistance when steam passes through the steam vent hole 30, and steam can be smoothly released to the outside of the container without making the opening area of ​​the steam vent hole 30 excessively large. Since it is not necessary to make the opening area of ​​the steam vent hole 30 excessively large, foreign matter contamination can be easily prevented.

[0047] Furthermore, since the lid is constructed from an olefin resin sheet and the thickened portion 32 is formed in a roughly circular shape in cross-section, fuzzing is less likely to occur on the wall surface of the steam vent hole 30 when it is formed. Consequently, the generation of resin powder due to fuzzing is suppressed, and the workload for removing the resin powder can be reduced.

[0048] Furthermore, a recess may be provided in the top surface 20, and a steam vent hole 30 may be provided in the bottom surface of the recess. Also, the container body 1 and the lid 2 may be connected by a hinge. [Explanation of symbols]

[0049] 1. Container body 2 lid 10 Bottom part 11 Side of the main unit 12 Main body flange section 13 Main body fitting part 14. Stepped section of the main body 20 Top section 21 Lid side part 22 Lid flange section 23 Lid fitting section 30 Steam vents 30a End in the longitudinal direction 30c hole center 31 Steam vent area 32 Thick wall part 32a Outer edge 33 Top surface of the seat 34 Underside of the seat 35 Upper curved section 36. Downward curved section 37 Inwardly curved section 41 First arc 42 Second arcuate part 50 Starting point of upward expansion 51 Upper extension line 52 Upper intersection 53 Starting point of downward expansion 54 Downward extension line 55 Lower intersection 56 Top score 57 Lowest point 58 Innermost point 60 Virtual External Lines 61 Midpoint 62 Virtual Central Line 63 Virtual Extension 64 Central intersection

Claims

1. A microwave-safe food packaging container comprising a container body and a lid thermoformed from a sheet of olefin resin that is attached to the container body, The top surface of the lid has a steam vent to release steam from inside the container to the outside during microwave heating. The opening edge of the steam vent is provided with thickened sections that bulge outwards on the upper and lower sides, extending around the entire circumference. In a cross-sectional view when the sheet is cut vertically across the steam vent, The outer edge of the thickened portion has an upward curved portion that curves upward and bulges out from the upper surface of the sheet, a downward curved portion that curves downward and bulges out from the lower surface of the sheet, and an inward curved portion that connects the upward curved portion and the downward curved portion and curves upward and bulges out towards the center of the steam vent hole. In the aforementioned cross-sectional view, Let LU be the length along the upper extension line between the upper bulge initiation point where the upward curved portion begins to bulge from the upper surface of the sheet, and the upper intersection point where the upper extension line, extended from the upper surface of the sheet toward the center of the steam vent hole, intersects with the outer shape of the thickened portion. Let LL be the length along the downward extension line between the downward curvature starting point where the downward curved portion begins to bulge from the underside of the sheet, and the downward intersection point where the downward extension line, extended from the underside of the sheet toward the center of the steam vent hole, intersects with the outer edge of the thickened portion. When the thickness of the sheet is T, A food packaging container where T ≤ LU and T ≤ LL.

2. In the aforementioned cross-sectional view, Let A be the length of the line connecting the midpoint of the line connecting the upper bulge initiation point and the lower bulge initiation point, and the innermost point in the inwardly curved section that is closest to the center of the steam vent hole. When B is the distance in the thickness direction of the sheet between the highest point of the upper curved section and the lowest point of the lower curved section, A food packaging container according to claim 1, wherein 0.8 ≤ B / A ≤ 1.

1.

3. In the aforementioned cross-sectional view, A food packaging container according to claim 2, wherein 1.8 ≤ B / T ≤ 2.

2.

4. A food packaging container according to any one of claims 1 to 3, wherein the shape of the steam vent in plan view is an elongated hole shape with a width of 0.15 to 1 mm and a length of 2 to 30 mm along the steam vent.

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