Lid and packaging container consisting of lid and container body

Abstract

To provide a lid body and a packaging container capable of discharging generated water vapor to the outside of the container by floating up a tip part of a flap with water vapor generated from a content during heating and forming a vent hole with a lid face part in between.SOLUTION: There is provided a lid body of a packaging container for storing a content to be heated by a microwave oven, the packaging container comprising a container body, and the lid body made from a synthetic resin for closing an opening of the container body. A lid face part of the lid body comprises one or more slits bored as through-holes passing through front and back faces of the lid body in a continuous U-shape by laser processing, a flap formed by the slits in an inner side part of the slits, and a swollen part formed around the slits. The swollen part has a volume equal to (thickness of the lid face part)×(width of the slits)×(overall length of the slits).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a lid made of synthetic resin that fits onto a container body to close the opening of the container body, and a packaging container consisting of the lid and the container body, for storing contents such as food to be heated in a microwave oven. [Background technology]

[0002] In recent years, convenience stores have increasingly sold instant foods, which are sold with food or other contents packaged in a container that consumers can simply heat in a microwave oven and enjoy immediately. In such food-packaging containers, it is necessary to allow steam generated from the contents to escape from the container during heating, thereby preventing the pressure inside the container from building up. To accomplish this, a method has been devised in which a U-shaped cutout, for example, is punched into the lid surface of the lid, and the U-shaped cutout functions as a valve element (referred to as a "flap" in this description, but not a check valve, commonly referred to as a "flap valve") that moves up and down in response to the pressure difference between the inside and outside of the container. Lids equipped with such flaps have been used for some time. Specifically, the flap is designed to lift up when the internal pressure rises during heating, using both ends of the U-shape, i.e., a straight line connecting the start and end points of the U-shape (referred to as the "base line") as a fulcrum, thereby forming a vent for steam between the lid surface and the U-shape. (In this description, the portion of the flap including the base line of the flap relative to the tip of the flap is referred to as the base end of the flap.) However, the flaps formed by incisions are formed by punching, and there is no gap between the side of the outer periphery of the flap (referred to as the flap outer periphery) and the side of the lid body that surrounds the flap (referred to as the flap outer shell). Therefore, when the flap is closed, i.e., when the flap and the lid body are on the same plane, the flap is in a state where it is firmly attached to the lid body, and therefore cannot rise up in response to an increase in internal pressure caused by steam generated by the contents of the packaging container, and therefore cannot function as a flap. In this case, there is a problem that the lid may come off the container body due to an increase in internal pressure, for example, when heated in a microwave oven.

[0003] To solve this problem, in conventional flaps, after forming the flap by making a U-shaped cut, the flap is forcibly pushed up toward the outside of the packaging container with a jig to release the adhesion between the outer periphery of the flap and the outer shell of the flap and to make the flap movable in advance. However, a flap that remains in the pushed-up position also leaves the air vent open at all times, and because the air vent is a gap that opens in a direction perpendicular to the lid surface (approximately perpendicular to the direction in which the flap moves), a new problem has arisen in that it increases the possibility of foreign objects, especially tiny insects (such as fruit flies), entering the packaging container through the gap.

[0004] Paragraph 0024 and Figure 5 of Patent Document 1 describe an invention of a lid in which a flap provided on the lid is raised in advance over the tip end. Furthermore, paragraph 0015 of the same document describes an invention in which the upper part of the flap is covered with an adhesive label to prevent the intrusion of foreign matter. However, this requires a secondary material called the adhesive label, and a separate process for attaching the adhesive label is also required, which reduces the economic and efficiency of product production. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2017-52518 Summary of the Invention [Problem to be solved by the invention]

[0006] In particular, in packaging containers intended to contain food to be heated in a microwave oven, it is preferable that the flap provided on the lid for venting water vapor should be able to provide as much ventilation as possible, i.e., have a wide opening area, so as to be able to cope with the water vapor that is generated due to a sudden rise in temperature, but at the same time, the flap needs to be designed with sufficient consideration given to preventing the intrusion of foreign matter, particularly tiny insects, etc. Conventionally disclosed flaps with a U-shaped cut in the lid surface of the lid make it relatively easy to ensure ventilation, but the tip of the flap is kept raised before microwave heating, so the risk of intrusion of foreign matter such as insects could not be completely eliminated.

[0007] The present invention was made in light of these circumstances. Specifically, the object of the present invention is to provide a lid for packaging food to be heated in a microwave oven, in which a flap formed by a U-shaped slit in the lid is flush with the lid surface before heating, minimizing the inclusion of foreign matter, and rises during heating in response to the increase in pressure caused by steam generated from the contents, forming a smooth vent between the flap and the lid surface around the flap, allowing the generated steam to be released outside the container; and to provide a packaging container comprising the lid and a container body. Note that the "U-shape" in the U-shaped slit does not necessarily mean a U-shape in the strict sense, but may also be a modified U-shape. In other words, the U-shape referred to here refers to an oval shape with one portion disconnected, and similar shapes in general. In this case, there are no particular limitations on the shape of the U-shape, except that the U-shaped slit and the base line of the U-shape do not intersect. For example, a partially disconnected circle, ellipse, oval, egg, or other shapes may be used. rectangle The outer peripheral shape is preferably a horseshoe shape or the like. The part corresponding to the inside of the U-shape functions as a flap for the lid of the present invention. Furthermore, there is no particular limitation on the number of slits in the U-shape. [Means for solving the problem]

[0008] The inventors of the present invention conducted extensive research into the overall structure of a flap that is provided on the lid surface of a lid that closes the opening of a container body, moves up and down in response to the pressure difference between the inside and outside of the packaging container, and automatically opens and closes. As a result, the inventors of the present invention have come up with a lid structure in which a U-shaped slit (referred to as a U-shaped slit) is formed on the lid surface of the lid, and a flap is provided on the inside of the U-shaped slit, with the base line connecting the start and end points of the U acting as a fulcrum. The line along the inside of the U around the slit is the flap outer periphery, and the line along the outside of the U is the flap outer shell. Furthermore, the U-shaped slit in the lid of the present invention is not a simple cut but a through-hole, and the flap outer periphery has a slit width that is long enough to prevent the flap outer shell from contacting the flap outer shell. Furthermore, in the lid of the present invention, the flap is positioned so that there is no gap between the flap outer periphery and the flap outer shell in a direction perpendicular to the lid surface. The lid body with the flap, under normal conditions, prevents minute foreign objects from getting mixed in. Furthermore, it was confirmed that the pressure of steam generated during heating in a microwave oven causes the tip of the flap to rise, forming a gap perpendicular to the lid surface between the outer periphery of the flap and the outer shell of the flap, which functions as a valve body for the vent that allows steam generated inside the container to be smoothly released outside the container, leading to the completion of the present invention.

[0009] That is, the present invention has the following configurations (1) to (6). (1) A lid for a packaging container for storing contents to be heated in a microwave oven, comprising a container body and a synthetic resin lid that closes the opening of the container body, The lid surface of the lid body has a slit having a U-shaped overall shape and a width of 0.1 mm or more and 1.0 mm or less, The inner side of the slit forms a flap with a straight line connecting both ends of the U-shape as a fulcrum, The flap is positioned so that there is no gap between the outer periphery of the flap and the outer shell of the flap in a direction perpendicular to the lid surface. (2) It is preferable that the lid body according to (1) has a bulge formed around the entire periphery of the U-shaped slit. (3) It is preferable that the lid body according to (1) or (2) is such that the U-shaped slit is formed in a recess provided in the lid surface portion. (4) It is preferable that the lid body described in either (1) or (2) has a protrusion formed around the U-shaped slit that protrudes upward from the lid surface. (5) It is preferable that the lid body according to any one of (1) to (4) above is such that the synthetic resin is a polystyrene-based resin. (6) A packaging container comprising the lid described in (1) to (5) above and a synthetic resin container body. [Effects of the Invention]

[0010] The lid of the packaging container of the present invention closes the opening of a packaging container that contains contents such as food to be heated in a microwave oven, and the flap formed by a U-shaped slit in the lid surface of the lid prevents the intrusion of minute foreign objects under normal conditions. Furthermore, when the pressure of steam generated during microwave heating rises, the tip of the flap rises, forming a gap between the outer periphery of the flap and the outer shell of the flap in a direction perpendicular to the lid surface, which functions as a vent valve that allows steam generated inside the container to escape. Therefore, with the lid of the present invention, there is no need to position the flap above the lid surface in advance, nor is there any need to cover the top of the flap with an adhesive label, for example. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic perspective view of the entire packaging container and the container body and lid body that constitute it. [Figure 2] 1A and 1B are plan views showing an example of a lid having a U-shaped slit and an enlarged view of a flap formed by the U-shaped slit. [Figure 3] 3 is a cross-sectional view of a flap formed by a U-shaped slit shown in FIG. 2 in a normal state, taken along line XX, and an enlarged view of the vicinity of the tip of the flap. [Figure 4]XX cross-sectional view of the flap formed by the U-shaped slit shown in FIG. 2 when the tip of the flap is lifted by the steam emitted when food contained in a packaging container is heated in a microwave oven. [Figure 5] FIG. 10 is a schematic perspective view of a cover showing another embodiment having a U-shaped slit in a recess provided in the cover surface portion. [Figure 6] FIG. 10 is a schematic perspective view of a lid showing another embodiment in which a convex portion that protrudes above the lid surface is formed around the U-shaped slit. [Figure 7] FIG. 10 is a plan view of a cover showing another embodiment of a U-shaped slit. DETAILED DESCRIPTION OF THE INVENTION

[0012] The following describes an embodiment of the present invention, but the present invention is not limited to the following embodiment.

[0013] <Lid and packaging container> Fig. 1 is a schematic perspective view showing the entire packaging container, separated into the container body and lid that constitute it. The lid 1 shown in Fig. 1, which is an embodiment of the present invention, has a slit 3 on the lid surface 11 of the lid 1 that is U-shaped overall and maintains the slit width specified in the present invention. The inside of the slit 3 is a flap 4, which functions as a valve body of a vent hole that releases water vapor. The lid 1 is fitted over an opening 21 at the top of the container body 2, and as the lid 1 that closes the opening 21 of the container body 2, it constitutes a part of a packaging container 5, which is another embodiment of the present invention.

[0014] Lid material The lid 1 according to the embodiment of the present invention is made of synthetic resin. Synthetic resins generally have excellent moldability and flexibility, allowing a portion of the lid surface 11 to function as a flap, i.e., a valve body for a vent that releases water vapor. It is preferable to use a transparent synthetic resin as the material for the lid 1 so that the contents inside the packaging container can be visually confirmed. Examples of transparent synthetic resins include polystyrene-based, polyethylene-based, polypropylene-based, and polyester-based synthetic resins.

[0015] From the perspective of mechanical strength, it is preferable that the lid 1 be obtained by molding a stretched sheet produced by stretching the synthetic resin to orient the synthetic resin molecules. Furthermore, biaxially stretched sheets are more preferable than uniaxially stretched sheets because they have a better balance of physical properties. Biaxially stretched sheets of polystyrene-based resins in particular offer a well-balanced performance in terms of rigidity, heat resistance, transparency, environmental friendliness, and processability, making them suitable as lid materials. Preferred examples of polystyrene-based resins include polystyrene resins, styrene-acrylic ester copolymer resins, and styrene-methacrylic ester copolymer resins. Furthermore, when using a sheet made of the polystyrene-based resin, the speed of perforation with a laser beam, as described below, is faster than with sheets made of other synthetic resins, thereby contributing to improved lid production efficiency. Therefore, the lid 1 used for the packaging container 5 of this embodiment was obtained by molding a commercially available biaxially stretched polystyrene sheet.

[0016] 《Container body material》 Additionally, while synthetic resins are generally preferred as the material for the container body 2, they are not particularly limited and may be paper, glass, ceramic, or other materials as long as they can be heated in a microwave oven. When the container body 2 is made of synthetic resin, the synthetic resin may be the same as or different from the synthetic resin used for the lid 1. Furthermore, in consideration of heat insulation so that the container body 2 can be held with bare hands even immediately after microwave heating, materials with relatively low thermal conductivity, such as expanded synthetic resins based on polystyrene, polyethylene, polypropylene, or polyester, are preferably used as the material for the container body 2. Therefore, the container body 2 used in the packaging container 5 of this embodiment is a container body obtained by molding an expanded polystyrene sheet.

[0017] <<Method for forming lid and container body>> There are no particular limitations on the method for molding the lid 1 and the container body 2. However, since a synthetic resin sheet is preferably used as the material, the lid 1 and the container body 2 are preferably molded by known heat molding methods such as vacuum molding, pressure molding, and vacuum pressure molding. Among heat molding methods, a method using a hot plate as a means for heating the sheet is sometimes called hot plate molding. Furthermore, the lid 1 and the container body 2 may be molded by injection molding or blow molding, without being limited to these molding methods.

[0018] "U-shaped slit and flap" The lid 1 of this embodiment has a U-shaped slit 3 formed in the lid surface 11. The lid surface here refers to the top surface of the lid excluding the peripheral portion involved in fitting with the container body. It is preferable that the portion of the lid surface where the U-shaped slit is formed is flat. FIG. 2 shows a plan view of the lid 1 having a U-shaped slit 3, which is an example of a slit, and an enlarged view of the U-shaped slit 3. As shown in FIG. 2, by making the overall shape of the slit 3 U-shaped, a flap 4 is formed inside the slit 3. The inside of the U-shape of the slit 3 is also the outer periphery of the flap 4, and similarly, the outside of the U-shape forms the outer contour of the flap 4. The outer periphery and outer contour of the flap 4 are connected at two points: the start point 31 and the end point 32 of the U-shape.

[0019] 3 is a cross-sectional view taken along line XX of the lid surface including the U-shaped slit 3 (flap 4) in FIG. 2 in a normal state, and an enlarged view of the vicinity of the tip 41 of the flap 4. The normal state here refers to a state in which a packaging container containing food or the like is not subjected to mechanical external forces or vibrations beyond those expected during distribution, until immediately before heating in a microwave oven.

[0020] As shown in FIG. 3 , the flap 4 formed by the U-shaped slit 3 is positioned flush with the lid surface 11 before the packaging container is heated. While the flap 4 and the lid surface 11 may not be perfectly flush due to warping or bending caused by distortion during slit 3 formation or bending due to the flap's own weight, the lid body of the present invention is a lid body in which the flap 4 is positioned so that there is no gap between the flap's outer periphery and its outer shell in a direction perpendicular to the lid surface. That is, as shown in the enlarged view of FIG. 3 , even if the flap 4 bends due to its own weight to the position indicated by the dashed line toward the inside of the packaging container, the gap is not formed, and the flap functions as a flap that can prevent the intrusion of foreign matter. Here, the slit 3 illustrated in FIG. 3 has a bulge 34 formed around its entire periphery, so the tolerance for warping or bending of the flap 4 before the gap is created is greater than that of a flap without a bulge. However, even if the slit does not have the bulging portion, in the present invention, at least the thickness of the sheet used as the lid material has a tolerance for warping and bending.

[0021] Fig. 4 is a cross-sectional view taken along line XX showing a state in which a tip 41 of a flap 4 is lifted by the released steam when a packaging container 5 containing food is heated, for example, in a microwave oven. The flap 4 is provided to efficiently release steam generated from the contents to the outside of the container when the packaging container 5 containing food is heated in a microwave oven, and the arrows in Fig. 4 show how the flap 4 is lifted from its normal position by the steam being released from inside the packaging container 5. Then, as shown in Fig. 4, the pressure of the steam generated from the food during heating causes the tip 41 to lift, forming a gap S between the flap outer peripheral portion 43 of the flap 4 and the flap outer shell portion 331 in a direction perpendicular to the lid surface. That is, during heating, the flap 4 is fixed at the base line 421 connecting the start point 31 and end point 32 of the U-shaped slit 3 due to the pressure of steam inside the container, and the tip end 41 thereof is raised, forming a gap S perpendicular to the lid surface between the flap outer peripheral portion 43 of the flap 4 and the flap outer shell 331. The size of the gap S is zero at the position of the base end 42 of the flap 4 and is maximum at the position of the tip end 41. Note that although the cross-sectional shape of the flap 4 in FIG. 4 is drawn linearly, it does not necessarily have to be raised while maintaining a linear cross-sectional shape, and it may have a curved shape.

[0022] 5 is a schematic perspective view of a lid body showing another embodiment in which a U-shaped slit 3 is formed in a recess 12 provided in the lid surface portion 11. By forming the slit 3 in the recess 12, the flap 4 is not pressed down by the band seal when the band seal is wrapped around the entire packaging container 5, and the tip 41 of the flap 4 can be smoothly lifted by the steam generated from the contents.

[0023] 6 is a schematic perspective view of a lid body showing another embodiment in which a protrusion 13 that protrudes upward from the lid surface 11, i.e., toward the outside of the packaging container, is formed around the U-shaped slit 3. By forming the protrusion 13 around the slit 3, the flap 4 is not pressed down by the seal when the seal is wrapped around the entire packaging container 5, and the tip 41 of the flap 4 can be smoothly lifted by the steam generated from the contents.

[0024] FIG. 7 is a plan view of a lid showing another example of a U-shaped slit 3. In FIG. 7(A), two U-shaped slits 3a, each smaller than the slit shown in FIG. 1, are provided on the lid surface 11, and flaps 4a are formed inside each of the U-shaped slits 3a. While FIG. 7(A) shows an embodiment in which the slits are arranged symmetrically in two locations, the size, number, and arrangement of the U-shaped slits can be selected as appropriate in the present invention. FIG. 7(B) shows an example in which the start and end points of the U-shaped slit are extended away from the flap 4, respectively, to form start point 311 and end point 321. The slit shape shown in FIG. 7(B) is also included in the U-shape defined in the present invention. By extending the start and end points of the U-shaped slit as in this embodiment, breakage of the flap at the base end can be more effectively prevented.

[0025] "Width of the U-shaped slit" The wider the U-shaped slit 3, the greater the risk of insects or other foreign objects entering the lid. To prevent this, the width of the U-shaped slit in the lid of the present invention, as shown by D in the enlarged view of Figure 2, is 1.0 mm or less, preferably 0.8 mm or less. Furthermore, when the U-shaped slit is formed by laser irradiation (described below), if the slit width is set too narrow, a thread-like connection formed by the molten resin between the flap outer periphery 43 of the flap 4 and the flap outer shell 331 will remain (this phenomenon is also referred to as "stringing"). In other words, the flap and the lid will be connected at points other than the base end, preventing them from functioning as a flap. Therefore, the slit width D is 0.1 mm or more, preferably 0.3 mm or more.

[0026] <Method for drilling U-shaped slits> The method for drilling the U-shaped slit 3 is not particularly limited as long as the flap outer periphery 43 of the flap 4 can bend about the base line 421 without interfering with the flap outer periphery 331, and a gap S perpendicular to the lid surface through which water vapor can be discharged can be formed between the flap outer periphery 43 of the flap 4 and the flap outer periphery 331. However, drilling by laser beam irradiation is preferred. Furthermore, when a synthetic resin sheet is irradiated with a laser beam to drill the slit while melting the synthetic resin, a bulge 34 (described later) is formed around the entire periphery 33 of the slit 3, i.e., at the edges of the start point 31 and end point 32 of the U-shaped slit 3 where the flap outer periphery 43 of the flap 4 and the flap outer periphery 331 are connected. When drilling the slit by laser beam irradiation, the slit width D can be controlled by appropriately adjusting the output power and travel speed of the laser beam.

[0027] There are various types of lasers, including carbon dioxide lasers, YAG lasers, semiconductor lasers, and argon lasers, but it is preferable to use a carbon dioxide laser with a wavelength range of 9 to 11 μm. In particular, it is preferable to irradiate with a carbon dioxide laser with an output of 10 to 100 W. If the output range of the carbon dioxide laser is lower than 10 W, workability is poor and the resin may not be penetrated, while if it exceeds 100 W, it may become overloaded and not be able to drill a hole of the desired width.

[0028] Furthermore, perforating U-shaped slits with a laser beam has relatively good dimensional accuracy and can be completed in a short time. In this case, smooth perforation can be achieved by irradiating the surface of the synthetic resin sheet with a laser beam at a transfer speed of typically 5 to 30,000 mm / s. A laser beam transfer speed slower than 5 mm / s is undesirable because it reduces workability and can result in excessive irradiation, which can cause the synthetic resin to overheat and reduce the dimensional accuracy of the slit. On the other hand, a laser beam transfer speed exceeding 30,000 mm / s may not be able to perforate a slit of the desired width.

[0029] 《Bulge》 When a laser beam is applied to the lid surface 11 of the synthetic resin lid to form a U-shaped slit 3, the molten synthetic resin accumulates around the entire periphery of the slit and then solidifies again, forming a bulge 34. The inner periphery of the U-shaped slit 3 becomes the flap outer periphery 43, so the bulge 34 formed on the inner periphery of the U-shaped slit 3 also serves as the bulge 34 formed on the flap outer periphery 43. The formation of the bulge 34 on the flap outer periphery 43 increases the bending strength of the flap 4, thereby suppressing deflection of the tip 41 due to the flap 4's own weight. The larger the bulge 34, the smaller the amount of deflection due to sagging of the tip 41. Furthermore, the bulge 34 formed on the outer periphery of the U-shaped slit 3 becomes the bulge 34 on the flap outer periphery formed on the lid surface 11. At this time, by forming a U-shaped slit 3 by irradiating a laser beam, the thickness of the flap outer shell 331 of the flap 4 can be made thicker than the thickness T of the cover surface. That is, when a slit is formed by irradiating a laser beam, the synthetic resin equivalent to the volume occupied by the slit becomes a bulge 34 around the slit periphery 33. Therefore, the wider the slit width D, the larger the bulge 34 becomes, and the larger the cover surface thickness (flap thickness) T, the larger the bulge 34 becomes. In other words, the total volume of the bulge 34 is approximately equal to the cover surface thickness T x the slit width D x the total length of the slit.

[0030] <Flap thickness> The lid 1 formed from a synthetic resin sheet has a lid surface thickness of 0.15 mm, which is the limit for the strength required for use as a lid; a thickness less than 0.15 mm will not function as a packaging container. Furthermore, a lid surface thickness of 0.7 mm or more not only makes it difficult to mold the sheet into a lid, but also requires a large amount of resin material, which increases the cost of the container and makes it uneconomical. Therefore, the lid surface thickness is preferably 0.15 mm or more, more preferably 0.2 mm or more, and preferably 0.7 mm or less, and more preferably 0.5 mm or less.

[0031] <Gap S perpendicular to the lid surface> As described above, a narrower width D of the U-shaped slit is preferable in terms of preventing the intrusion of foreign matter. However, even if the slit width D is narrowed, if the flap 4 is significantly warped or warped, a gap S may form between the flap outer periphery 43 of the flap 4 and the flap outer casing 331 in a direction perpendicular to the lid surface, increasing the risk of foreign matter intrusion. Therefore, in the lid of the present invention, as shown in FIG. 3 , before heating in a microwave oven, the amount of flap warping (bulge 34 indicated by the dashed-dotted line) does not exceed the thickness of the flap outer casing 331, including the position of the flap 4 reaching its tip 431. In other words, there is no gap S between the flap outer periphery 43 and the flap outer casing 331 in a direction perpendicular to the lid surface. Furthermore, as described above, the bulge 34 is simultaneously formed across the flap outer periphery 43 and the flap outer casing 331 by irradiating the lid surface 11 with a laser beam. In this case, the thickness of the outer shell 331 would be the same as the thickness T of the lid surface if there were no bulge 34 around the slit periphery 33; however, by providing the bulge 34, the thickness increases by the thickness of the bulge. Furthermore, since the bulge 34 is also formed in the flap outer periphery 43, it is less likely that a gap S will form between the tip 431 of the flap outer periphery and the flap outer periphery 331 of the flap 4 in a direction perpendicular to the lid surface due to the flexing of the flap 4. In other words, the presence of the bulge 34 is effective in preventing the intrusion of foreign matter. Furthermore, when the tip 41 of the flap 4 flexes to an amount greater than or equal to the thickness of the slit periphery 33 during microwave heating, a large amount of steam generated from the contents can be expelled to the outside of the container, preventing the lid from coming off due to an increase in pressure inside the container. [Example]

[0032] The following describes the embodiments of the present invention in more detail, but the present invention is not limited to these examples.

[0033] Example 1 The lid 1 in Example 1 was a disk-shaped lid with a lid surface diameter of 175 mm, formed by hot plate molding heat-resistant biaxially oriented polystyrene (Denka Thermosheet High Heat Resistant BOPS (R) manufactured by Denka Co., Ltd.). A U-shaped slit 3 was drilled near the center of the lid surface 11 of this lid 1 by irradiation with a laser beam. In the lid 1 shown in FIG. 2, the thickness of the lid surface 11 was 0.27 mm, the thickness of the slit periphery 33 including the bulge was 0.39 mm, the width D of the U-shaped slit was 0.7 mm, the width of the base end 42 of the flap 4 was 15 mm, and the length from the base line 421 of the flap 4 to the tip 431 of the flap periphery, i.e., the total length of the flap 4, was 18 mm.

[0034] A container body 2 made of expanded polystyrene into which the lid 1 of Example 1 was fitted was prepared. After pouring 400 ml of water into the container body 2, the lid 1 was fitted to the container body 2 to produce a packaging container 5 in its normal state. The flap position of the lid 1 before the heating test was visually confirmed, and the presence or absence of a gap perpendicular to the lid surface between the flap outer periphery 43 of the flap 4 and the flap outer shell 331 was visually confirmed. The fitting strength between the container body 2 and the lid 1 of the packaging container 5 was the maximum strength measured in a tensile test using a Tensilon (RTC-1210A, manufactured by Orientec Co., Ltd., tensile speed: 30 mm / min, temperature: 20-30°C). The lid was machined to have a hook attached to one edge (or to the tongue, if the edge of the lid has a so-called "fingers" that can be gripped with the fingers to facilitate opening) and was pulled upward at the hook position while the bottom of the container body was fixed. Next, a heating test was carried out using this packaging container in a 1500W microwave oven. The heating test involved heating the packaging container 5 for 2 minutes, and visually checking the open / closed state of the flap 4, the fitting state between the container body 2 and the lid 1, and the state of water vapor emission. After the heating test was completed, the fitting strength between the container body 2 and the lid 1 of a further used packaging container 5 was measured using the same method. The fitting strength between the lid 1 and the container body 2 in the packaging container 5 of Example 1 was 1500cN before microwave heating and 1400cN after microwave heating.

[0035] The results of the microwave heating test using the packaging container of Example 1 above were as follows. (1) The state of the flap before heating in the microwave Before heating in the microwave oven, the flap 4 was not bent and was almost flush with the lid surface, and the gap S shown in FIG. 4 was not observed. (2) Flap condition during microwave heating 90 seconds after the start of microwave heating, the tip 41 of the flap 4 began to lift, i.e., the gap S shown in Figure 4 became visible, and after 105 seconds, the tip 41 of the flap 4 rose about 5 mm above the lid surface and remained in that state. (3) The state of the lid fitting when heated in a microwave oven The lid 1 did not come off the container body 2 during microwave heating, the water vapor inside the packaging container 5 was smoothly discharged to the outside of the container, and the fitted state of the lid 1 and the container body 2 was maintained.

[0036] Example 2 A lid was created by scraping off the bulge 34 of the lid 1, which is the same as in Example 1, i.e., making the thickness of the flap outer periphery 43 and flap outer shell 331 of the flap 4 the same as the thickness of the lid surface.A microwave heating test was conducted under the same conditions as in Example 1, and the following results were obtained. (1) The state of the flap before heating in the microwave Before heating in the microwave oven, the tip 41 of the flap 4 was slightly hanging down from the lid surface, but it was visually observed that there was no gap S between the flap outer periphery 43 and the flap outer shell 331 in a direction perpendicular to the lid surface. (2) Flap condition during microwave heating 86 seconds after the start of microwave heating, the tip 41 of the flap 4 began to lift, and 99 seconds later, the tip 41 of the flap 4 remained in a state of being lifted about 6 mm above the lid surface. (3) The state of the lid fitting when heated in a microwave oven The lid 1 did not come off the container body 2 during microwave heating, the water vapor inside the packaging container 5 was smoothly discharged, and the fitted state between the lid 1 and the container body 2 was maintained.

[0037] From the above verification results of Examples 1 and 2, it was found that in a packaging container 5 using a lid body 1 having a U-shaped slit 3, there was no gap S perpendicular to the lid surface between the flap outer periphery 43 due to the deflection of the flap 4 and the flap outer shell 331 of the flap 4 before microwave heating, and furthermore, the effect of discharging water vapor from within the packaging container 5 during microwave heating was good.

[0038] Furthermore, when there is a bulge 34 around the slit 33, the flap hardly sags until a gap S is formed between the flap outer periphery 43 due to the deflection of the flap 4 before microwave heating and the flap outer shell 331 of the flap 4 in a direction perpendicular to the lid surface, and there is also a tolerance range, so the effect of expelling water vapor from the packaging container 5 during microwave heating is good. [Explanation of symbols]

[0039] 1 Lid 11 Lid surface part 12 recess 13 Convex part 2 Container body 21 Aperture 3,3a Slit 31 Starting Point 32 End Point 33 Slit perimeter 331 Flap outer shell 34 Bulge 4,4a Flap 41 Tip 42 Proximal end 421 Baseline 43 Flap outer periphery 431 Tip of outer periphery of flap 5 Packaging containers D Slit width T Thickness of the lid S Gap perpendicular to the lid surface

Claims

1. A method for manufacturing a lid for a packaging container for containing contents to be heated in a microwave oven, the lid comprising a container body and a lid made of a polystyrene-based resin that closes an opening of the container body, the method comprising: a step of forming a flap by forming one or more slits in a lid surface of the lid body by laser processing the front and back of the lid body so as to penetrate the front and back of the lid body in a continuous U-shape and to form a bulge around the slit, and forming the flap so that there is no gap in a direction perpendicular to the lid surface between the outer periphery of the flap and the outer shell of the flap at the inside part of the slit, A method for manufacturing a lid body in which the bulge portion has a volume equal to the thickness of the lid surface x the width of the slit x the total length of the slit, and the amount of deflection of the flap does not exceed the thickness of the outer shell of the flap.

2. The method for manufacturing a lid body according to claim 1, wherein the flap is formed so that the inner edge of the slit and the outer edge of the slit are connected only at the ends of the U-shape, with a straight line connecting the ends of the U-shape as a fulcrum.

3. The method for manufacturing a lid according to claim 1 or 2, wherein the U-shape is defined to include any of a circular, elliptical, oval, egg, rectangular, and horseshoe outer peripheral shapes, the outer peripheral shapes of which are not connected in part.

4. A method for manufacturing a packaging container, comprising the method for manufacturing a lid body according to any one of claims 1 to 3.

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