JP7789840B2 - Lid and packaging container consisting of lid and container body - Google Patents

Description

The present invention relates to a synthetic resin lid 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.

In recent years, convenience stores have increasingly sold instant foods, which are sold with food or other contents packaged inside. Consumers can simply heat the contents in a microwave oven and enjoy a quick meal. Such food packaging requires a way to release steam generated by the contents during heating, preventing internal pressure buildup. To release steam, a method has been devised in which a U-shaped cutout, for example, is punched into the lid surface of the lid, causing the U-shaped cutout to function as a valve (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 packaging. Lids equipped with such flaps have been in use for some time. Specifically, the flap's design uses 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. When heated, the flap's tip rises with the rise in internal pressure, forming a vent between the lid surface and the U-shape, forming a steam vent. (In this description, the portion of the flap including its base line, as opposed to its tip, is referred to as the base end of the flap.) However, flaps formed by notches are formed by punching, and there is no gap between the side of the flap's outer periphery (referred to as the flap outer periphery) and the side of the lid that surrounds the flap (referred to as the flap outer shell). As a result, when the flap is closed, i.e., when the flap and lid are on the same plane, the flap appears to be firmly attached to the lid, and is unable to rise in response to increased internal pressure caused by steam generated by the contents of the packaging container, preventing it from functioning as a flap. In this case, there is the problem that the lid may come off the container due to increased internal pressure, for example, when heated in a microwave oven.

To solve this problem, with conventional flaps, after a U-shaped cut is made to form the flap, the flap is then forcibly pushed up toward the outside of the packaging container with a jig, releasing the adhesion between the outer periphery of the flap and the outer shell of the flap and pre-conditioning the flap to be more flexible. However, a flap that remains in the pushed-up position also leaves the air vent open at all times, and because this air vent is a gap that opens perpendicular to the lid surface (roughly perpendicular to the direction in which the flap moves), a new problem arises: it increases the likelihood of foreign objects, particularly tiny insects (such as fruit flies), entering the packaging container through this gap.

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

JP 2017-52518 A

In particular, for packaging containers intended to hold food to be heated in a microwave oven, it is preferable for the flap on the lid to allow as much airflow as possible, i.e., have a wide opening area, so that it can handle the water vapor that is generated by a sudden rise in temperature. At the same time, however, the flap must be designed with sufficient consideration to prevent the intrusion of foreign matter, particularly tiny insects. Previously disclosed flaps with a U-shaped cut in the lid surface of the lid make it relatively easy to ensure sufficient airflow, but because the tip of the flap is held in a raised position before microwave heating, the risk of insects and other foreign matter entering the container cannot be completely eliminated.

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 to the outside of 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 point disconnected, and similar shapes in general. In this case, there are no particular restrictions on the shape of the U-shape, except that the U-shaped slit and the base line of the U-shape do not intersect, and for example, peripheral shapes such as a circle, ellipse, oval, egg, rectangle , or horseshoe shape with a partially disconnected portion are preferred shapes. The portion corresponding to the inside of the U-shape functions as a flap for the lid of the present invention. Furthermore, there is no particular restriction on the number of the U-shaped slits.

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. The flap moves up and down in response to the pressure difference between the inside and outside of the packaging container, automatically opening and closing. As a result, the inventors have determined that the lid of the present invention has a flap formed on the lid surface with a U-shaped slit (referred to as a U-shaped slit), the inner portion of which functions as a fulcrum for the base line connecting the start and end points of the U. The line along the inside of the U-shape around the slit is the outer periphery of the flap, and the line along the outside of the U is the outer shell of the flap. Furthermore, the U-shaped slit in the lid of the present invention is not a simple cut but a through-hole, and the outer periphery of the flap has a slit width that is long enough to prevent the flap outer shell from contacting the outer periphery of the flap. Furthermore, in the lid of the present invention, 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. Under normal conditions, the lid with the flap prevents minute foreign objects from getting mixed in. Furthermore, when heated in a microwave oven, the pressure of the steam generated 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. This function has been confirmed as a vent valve that allows steam generated inside the container to be smoothly released outside the container, leading to the completion of this invention.

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.

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. The flap, formed by a U-shaped slit on the lid surface of the lid, prevents the entry of minute foreign objects under normal conditions. Furthermore, the pressure of steam generated during microwave heating causes the tip of the flap to rise, forming a gap perpendicular to the lid surface between the outer periphery of the flap and its outer shell, 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 beforehand, nor is there any need to cover the top of the flap with an adhesive label.

1 is a schematic perspective view of the entire packaging container and the container body and lid body that constitute it. 1A and 1B are a plan view and an enlarged view of an example of a lid having a U-shaped slit, respectively, and a flap formed by the U-shaped slit. 3 is a cross-sectional view of the flap formed by the U-shaped slit shown in FIG. 2 in a normal state taken along the line XX, and an enlarged view of the vicinity of the tip of the flap. This is an X-X cross-sectional view of the flap formed by the U-shaped slit shown in Figure 2 when the tip of the flap is lifted up by the steam emitted when food contained in a packaging container is heated in a microwave oven. 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. FIG. 10 is a schematic perspective view of a lid body showing another embodiment in which a convex portion that protrudes above the lid surface is formed around the U-shaped slit. FIG. 10 is a plan view of a lid showing another embodiment of a U-shaped slit.

Embodiments of the present invention are described below. However, the present invention is not limited to the following embodiments.

<Lid and packaging container>
Fig. 1 is a schematic perspective view of 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.

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 for releasing 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.

From the perspective of mechanical strength, it is preferable that the lid 1 be formed from 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 offer 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 resin, styrene-acrylic ester copolymer resin, and styrene-methacrylic ester copolymer resin. 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, which contributes to improved lid production efficiency. Therefore, the lid 1 used for the packaging container 5 of this embodiment was formed from a commercially available biaxially stretched polystyrene sheet.

《Container body material》
Furthermore, 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 insulation properties so that the container 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.

<<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.

"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 also forms 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.

Figure 3 shows an X-X cross-sectional view of the lid surface including the U-shaped slit 3 (flap 4) in Figure 2 in its normal state, and an enlarged view of the vicinity of the tip 41 of the flap 4. Note that the "normal state" referred to 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, up until just before heating in a microwave oven.

As shown in Figure 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. Note that due to warping and bending caused by distortion when forming the slit 3, or bending due to the flap 4's own weight, the positional relationship between the flap 4 and the lid surface 11 may not be completely flush. However, in the lid body of the present invention, 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 Figure 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 to prevent the intrusion of foreign matter. Here, the slit 3 shown in Figure 3 has a bulge 34 formed around its entire periphery, so the tolerance for warping or bending of the flap 4 before the gap occurs is greater than that of a flap without a bulge. However, even if the slit does not have the bulge, in the present invention, there is a tolerance for warping or bending that is at least equivalent to the thickness of the sheet used as the lid material.

Figure 4 is an X-X cross-sectional view of a packaging container 5 containing food when it is heated, for example, in a microwave oven, causing the tip 41 of the flap 4 to rise due to the released steam. 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 Figure 4 represent the flap 4 being caused to rise from its normal position by the steam being released from inside the packaging container 5. Then, as shown in Figure 4, the pressure of the steam generated by the food during heating causes the tip 41 to rise, forming a gap S perpendicular to the lid surface between the flap outer periphery 43 of the flap 4 and the flap outer shell 331. That is, when heated, 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 the steam inside the container, and its tip 41 rises, forming a gap S perpendicular to the lid surface between the flap outer periphery 43 of the flap 4 and the flap outer shell 331. The size of this gap S is zero at the base end 42 of the flap 4 and is maximum at the tip 41. Note that although the cross-sectional shape of the flap 4 in Figure 4 is drawn linearly, it does not necessarily need to rise while maintaining a linear cross-sectional shape; a curved shape is also acceptable.

Figure 5 is a schematic perspective view of a lid showing another embodiment in which a U-shaped slit 3 is formed within a recess 12 provided in the lid surface 11. By forming the slit 3 within the recess 12, 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 smoothly rise due to the water vapor generated by the contents.

Figure 6 is a schematic perspective view of a lid showing another embodiment in which a protrusion 13 is formed around the U-shaped slit 3, protruding upward from the lid surface 11, i.e., toward the outside of the packaging container. 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 smoothly float up due to the water vapor generated from the contents.

Figure 7 is a plan view of a lid showing another example of a U-shaped slit 3. Figure 7(A) shows two U-shaped slits 3a, each smaller than the slit shown in Figure 1, formed on the lid surface 11, with a flap 4a formed inside each of the U-shaped slits 3a. Figure 7(A) shows an example in which the slits are arranged symmetrically in two locations, but the size, number, and arrangement of the U-shaped slits can be selected as appropriate in the present invention. Figure 7(B) shows an example in which the start and end points of the U-shaped slit are extended away from the flap 4, to form start point 311 and end point 321, respectively. The slit shape shown in Figure 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 example, it is possible to more effectively prevent the flap from breaking at the base end.

<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 indicated 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 and the flap outer shell 331 of the flap 4 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.

<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 shell 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 shell 331. However, drilling by laser beam irradiation is preferred. Furthermore, when drilling by irradiating a synthetic resin sheet with a laser beam to melt 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 shell 331 are connected. When drilling the slit by laser beam irradiation, the width D of the slit can be controlled by appropriately adjusting the output power and moving speed of the laser beam.

While there are various types of lasers, including carbon dioxide lasers, YAG lasers, semiconductor lasers, and argon lasers, it is preferable to use a carbon dioxide laser with a wavelength range of 9 to 11 μm. It is particularly preferable to use a carbon dioxide laser with an output of 10 to 100 W. If the carbon dioxide laser's output range is lower than 10 W, workability is poor and it may not be able to penetrate the resin, while if it exceeds 100 W, it may become overloaded and not be able to drill a hole of the desired width.

Furthermore, drilling U-shaped slits with a laser beam has relatively good dimensional accuracy and can be completed in a short time. In this case, smooth drilling can be achieved by irradiating the surface of the synthetic resin sheet with a laser beam at a travel speed of typically 5 to 30,000 mm/s. Laser beam travel speeds slower than 5 mm/s are undesirable, as they can lead to poor workability, excessive irradiation, and an overheating of the synthetic resin, which can reduce the dimensional accuracy of the slit. On the other hand, laser beam travel speeds exceeding 30,000 mm/s may not allow slits of the desired width to be drilled.

《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 lid 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 lid 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 lid surface thickness T x the slit width D x the total length of the slit.

<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 of 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.

<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 perpendicular to the lid surface will be formed between the flap outer periphery 43 of the flap 4 and the flap outer shell 331, 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 (the bulging portion 34 indicated by the dashed dotted line) does not exceed the thickness of the flap outer shell 331, including the position of the flap 4 reaching its tip 431. In other words, there is no gap S perpendicular to the lid surface between the flap outer periphery 43 and the flap outer shell 331. Furthermore, as described above, the bulging portion 34 is simultaneously formed across the flap outer periphery 43 and the flap outer shell 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, the provision of the bulge 34 increases the thickness by the amount 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 perpendicular to the lid surface between the tip 431 of the flap outer periphery and the flap outer shell 331 of the flap 4 due to deflection of the flap 4. In other words, the presence of the bulge 34 is effective in preventing the intrusion of foreign matter. Furthermore, when heated in a microwave oven, the deflection of the tip 41 of the flap 4 exceeds the thickness of the slit periphery 33, allowing a large amount of steam generated from the contents to be expelled to the outside of the container, preventing the lid from coming off due to an increase in pressure inside the container.

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

Example 1
The lid 1 in this 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-type) 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 end 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.

A container body 2 made of expanded polystyrene into which the lid 1 of Example 1 fits was prepared. After pouring 400 ml of water into the container body 2, the lid 1 was fitted to the container body 2 to create a packaging container 5 in its normal state. The flap position of the lid 1 was visually confirmed before the heating test, and the presence or absence of a gap perpendicular to the lid surface was visually confirmed between the flap outer periphery 43 of the flap 4 and the flap outer shell 331. 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). A hook was attached to one edge of the lid (or to the "flap" if the lid had a "flap" on the edge that can be gripped with the fingers to facilitate opening), and the bottom of the container body was fixed, and the lid was pulled upward at the hook. Next, a heating test was conducted 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 the packaging container 5 that had been used was measured using the same method. The fitting strength between the lid 1 and the container body 2 of the packaging container 5 of Example 1 was 1500cN before microwave heating and 1400cN after microwave heating.

The results of the microwave heating test using the packaging container of Example 1 above were as follows.
(1) State of the flap before microwave heating Before microwave heating, the flap 4 was not bent and was almost flush with the lid surface, and no gap S shown in Figure 4 was observed.
(2) State of flap 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. After 105 seconds, the tip 41 of the flap 4 rose about 5 mm above the lid surface and remained in that state.
(3) Fitting state of the lid body during microwave heating The lid body 1 did not come off the container body 2 during microwave heating, and the water vapor inside the packaging container 5 was smoothly discharged outside the container, maintaining the fitting state of the lid body 1 and the container body 2.

Example 2
A lid was created by scraping off the bulge 34 of the lid body 1, which is the same as in Example 1, i.e., by 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, and a microwave heating test was conducted under the same conditions as in Example 1, and the following results were obtained.
(1) Condition of the flap before microwave heating Before microwave heating, 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) State of flap during microwave heating 86 seconds after the start of microwave heating, the tip 41 of the flap 4 began to lift up, and after 99 seconds, the tip 41 of the flap 4 remained in a state of being lifted approximately 6 mm above the lid surface.
(3) Fitting state of the lid body during microwave heating The lid body 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 fitting state of the lid body 1 and the container body 2 was maintained.

From the above verification results of Examples 1 and 2, it was found that in a packaging container 5 using a lid 1 with a U-shaped slit 3, there was no gap S perpendicular to the lid surface between the flap outer periphery 43 and the flap outer shell 331 of the flap 4 due to the deflection of the flap 4 before microwave heating, and furthermore, the packaging container 5 was effectively able to release water vapor during microwave heating.

Furthermore, when the slit periphery 33 has a bulge 34, the flap 4 flexes before microwave heating, forming a gap S perpendicular to the lid surface between the flap outer periphery 43 and the flap outer shell 331 of the flap 4, and the flap sagging is barely noticeable. Furthermore, there is a certain tolerance, and the packaging container 5 is effectively able to expel water vapor during microwave heating.

REFERENCE SIGNS LIST 1 lid body 11 lid surface portion 12 concave portion 13 convex portion 2 container body 21 opening 3, 3a slit 31 starting point 32 ending point 33 slit periphery 331 flap outer shell portion 34 bulging portion 4, 4a flap 41 tip portion 42 base end portion 421 base line 43 flap outer periphery 431 tip of flap outer periphery 5 packaging container D slit width T lid surface thickness S gap in a direction perpendicular to the lid surface

Claims (4)

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. A method for manufacturing a lid as described in 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 both ends of the U-shape, with a straight line connecting the two ends of the U-shape as a fulcrum. 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. A method for manufacturing a packaging container comprising the method for manufacturing a lid according to any one of claims 1 to 3.