Replacement cartridge case

Abstract

To provide a replacement lead case capable of preventing an inward projection part from being deformed in releasing a die to secure shape stability and solving a problem of causing a rubbing scratch on a molded product, and a molding method for a replacement lead case part.SOLUTION: In the replacement lead case, right and left leg plates 13 are provided integrally at both sides of a principal plane plate 12, whereby a cross section perpendicular to a longitudinal direction is formed in a U-shape, and inward projection parts 14 are provided integrally so as to turn inward to the right and left leg plates to prepare a member which is used as one replacement lead case part 11; first step parts 16 having steps formed by the projections to an inner bottom face are formed on an inner bottom face 12a of the principal plane plate 12 interposed between the right and left leg plates of a replacement lead case part 11 along right and left leg plate sides excluding a center part in a longer direction of the inner bottom face.SELECTED DRAWING: Figure 12

Description

[Technical field]

[0001] The present invention relates to a replacement lead case for storing writing leads used in mechanical pencils and the like. S Regarding. [Background technology]

[0002] 2. Description of the Related Art Refill cases for accommodating writing leads used in mechanical pencils and the like have an elongated outer periphery that matches the linear shape of the writing lead. In this case, in order to remove the writing lead from the refill case, a configuration has been proposed in which an elongated inner case is slid relatively along the longitudinal direction within an elongated outer case, and this is disclosed in Patent Documents 1 and 2.

[0003] There has also been proposed a refill case that is assembled by pressing an elongated lower case into an elongated upper case. With this refill case, when the upper case is pressed into the lower case, a sliding cover that opens and closes the lead removal port is fitted between the upper and lower cases, making it easy to assemble the refill case. This is disclosed in Patent Document 3. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Utility Model Application Publication No. 5-44872 [Patent Document 2] Japanese Utility Model Application Publication No. 47-24230 [Patent Document 3] Japanese Utility Model Application Publication No. 57-98897 Summary of the Invention [Problem to be solved by the invention]

[0005] However, as mentioned above, this type of refill case is formed by combining two or more parts, so a certain degree of dimensional accuracy is required for each part, and in order to improve productivity, most of them are made using injection molding with resin as the raw material. Moreover, as mentioned above, each part that makes up the refill case is a combination of an elongated outer case and an inner case, or an elongated lower case and an upper case, etc. Therefore, in many cases, the cross-sectional shape of each of these parts perpendicular to its longitudinal direction is formed into, for example, a U-shape, and functional components are integrally molded as inward protrusions on each of the opposing inner surfaces of the U-shape so that they face inward toward each other.

[0006] When each refill lead case part with the above-mentioned inward protrusion is released from the injection molding die, the inward protrusion forms an undercut in the die. For this reason, a method known as "forced removal" is used, in which the molded resin product (refill lead case part) is released from the die while being deformed. Therefore, there is a risk that the inward protruding portion of the refill case part will sag and deform when released from the mold, and further, the inward protruding portion may adhere to the mold, making it difficult to release the resin product from the mold.

[0007] This invention was made with attention to the above-mentioned problems with mold molding of conventional refill case parts, and aims to provide a molding method for a refill case and refill case parts that can prevent deformation of the inward protruding part when released from the mold, ensure stability of the shape, and allow easy release from the mold. [Means for solving the problem]

[0008] The refill lead case of the present invention, which was made to solve the above-mentioned problems, is a refill lead case that uses a component as a single refill lead case part, in which left and right leg plates are integrally formed on both sides of a main panel, so that a cross section perpendicular to the longitudinal direction is U-shaped, and an inward protrusion is formed integrally with the leg plates so that it faces inward relative to the left and right leg plates, and is characterized in that on the inner bottom surface of the main panel sandwiched between the left and right leg plates of the refill lead case part, a first step portion having a step with respect to the inner bottom surface is formed along the left and right leg plate sides except for the longitudinal central portion of the inner bottom surface.

[0009] In this case, in one preferred embodiment, a second step portion having a step with respect to the inner surface is formed contiguous to the first step portion on the opposing inner surfaces of the left and right leg plates.

[0010] Furthermore, a third step having a different concave / convex direction from the first step is formed on the inner bottom surface of the main panel sandwiched between the left and right leg panels at a position offset from the first step. In this case, it is preferable that the first step is formed to protrude from the inner bottom surface, and the third step is formed to be recessed from the inner bottom surface.

[0011] On the other hand, the molding method of the present invention, which has been made to solve the above-mentioned problems, is a molding method of a refill lead case part in which left and right leg plates are integrally provided on both sides of a main panel, so that a cross section perpendicular to the longitudinal direction is formed into a U-shape, and an inward protrusion is formed integrally with the leg plates so as to face inward relative to the left and right leg plates, the molding method comprising: a first mold, a second mold, a core pin mounted on the second mold, a slide core which moves along the core pin in a direction inclined with respect to the mold opening direction of the first mold relative to the second mold, The process includes a resin injection process in which resin is injected into a cavity formed by the first mold, the second mold, the core pin, the slide core, and the ejection pin, a mold opening process in which the first mold is opened from the second mold, and a mold releasing process in which the slide core and the ejection pin are moved simultaneously in the mold opening direction of the first mold, thereby causing the ejection pin to release the replacement lead case part molded in the cavity from the slide core.

[0012] In this case, recesses for molding the inward protrusions are preferably formed on the opposing inner surfaces of the left and right leg plates, respectively, toward the outside near the tips of the pair of slide cores, and as the pair of slide cores move in the demolding process, each of the recesses is moved relatively toward the central sides of the left and right leg plates, thereby accompanying the operation of releasing the inward protrusions from the pair of slide cores. Effect of the Invention

[0013] According to the molding method of the present invention described above, a refill case part having a U-shaped cross section perpendicular to the longitudinal direction and having an inward protruding portion facing inward relative to the left and right leg plates, when released from the slide core, the end of the slide core moves relatively toward the center of the left and right leg plates of the refill case part. As a result, the inward protruding portion which is molded integrally with the refill case part can be easily released from the slide core and received by the ejection pin. Therefore, the refill case part having the inward protrusion integrally molded therein can be prevented from being deformed during removal from the mold without the forcible removal described above. This allows for the production of a refill case part with a stable shape of the inward protrusion.

[0014] On the other hand, during resin molding of the refill case part, due to the positional relationship between the core pin and the slide core, a first step portion having a step with respect to the inner bottom surface is formed on the inner bottom surface of the main panel of the refill case part along the left and right leg plate sides except for the longitudinal center part of the inner bottom surface. Therefore, when the lead refill case part is released from the slide core, the end of the slide core is moved toward the center of the left and right leg plates, but the step prevents the inner bottom surface of the lead refill case part from coming into contact with the slide core, which contributes to preventing product defects such as scratches on the inner bottom surface caused by the slide core. [Brief description of the drawings]

[0015] [Figure 1] 1 is a perspective view showing an example of a refill lead case using a refill lead case part obtained by the molding method according to the present invention. FIG. [Diagram 2] FIG. 11 is a perspective view showing the state in which a replacement lead is stored. [Diagram 3] FIG. 4 is a front view showing the individual configuration of the case body. [Figure 4] FIG. [Diagram 5] FIG. 4 is a perspective view showing the individual configuration of the inner lid. [Figure 6] This is also a front view. [Figure 7] 7 is a cross-sectional view taken along line aa in FIG. 6, as viewed in the direction of the arrow. [Figure 8] FIG. 4 is a perspective view showing a single component configuration of the cover member. [Figure 9] This is also a front view. [Figure 10] 10 is a cross-sectional view taken along line bb in FIG. 9, as viewed in the direction of the arrow. [Figure 11]2A to 2D are schematic diagrams showing the process for molding a refill case part according to the present invention in the order of steps (A) to (D). [Figure 12] 12 is a perspective view showing a first embodiment of a refill case part obtained by the molding method shown in FIG. [Figure 13] This is also a front view. [Figure 14] 14 is a cross-sectional view taken along line cc in FIG. 13, as viewed in the direction of the arrow. [Figure 15] 14 is a cross-sectional view taken along line dd in FIG. 13, as viewed in the direction of the arrow. [Figure 16] 14 is a cross-sectional view taken along line ee in FIG. 13, as viewed in the direction of the arrow. [Figure 17] 12 is a perspective view showing a second embodiment of a refill case part obtained by the molding method shown in FIG. [Figure 18] This is also a front view. [Figure 19] 19 is a cross-sectional view taken along line ff in FIG. 18, as viewed in the direction of the arrow. [Figure 20] 19 is a cross-sectional view taken along line gg in FIG. 18, as viewed in the direction of the arrow. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Figures 1 to 10 show examples of refill lead cases using refill lead case parts obtained by the molding method for refill lead case parts of the present invention. First, the examples of refill lead cases will be explained based on Figures 1 to 10, and then the molding method for refill lead case parts of the present invention will be explained based on Figures 11 to 20.

[0017] 1 and 2 show the external configuration of the entire assembled refill case, and FIG. 2 shows the case with a refill L housed therein. The cover member 1 and case body 2 that make up this refill lead case are molded from a resin material, and the inner lid 3 that is attached to the case body 2 and covered by the cover member 1 is also molded from a resin material. The case body 2 and the inner lid 3 are attached to the cover member 1 so as to be slidable along the longitudinal direction relative to the cover member 1, thereby assembling the refill lead case.

[0018] 3 to 10 show the individual components of the case body 2, inner lid 3 and cover member 1, and the configuration of each component will be described with reference to the drawings showing these individual components. Figures 3 and 4 show the case body 2, which is formed by a back panel 2a and left and right leg panels 2b, with a cross section perpendicular to the longitudinal direction being U-shaped, and a replacement core storage chamber 2c is formed in the inner space surrounded by the U-shape.

[0019] A bendable thin portion 2d is provided near the longitudinal end of the back panel 2a, and a lid portion 2e is formed on the end side, and a case portion 2f is formed on the rear side. A closing plate 2g is formed by connecting the left and right leg plates 2b to the cover body portion 2e to close the tip of the cover body portion 2e, and a rear closing plate 2h is formed by connecting the left and right leg plates 2b to the rear of the case portion 2f to close the rear of the case portion 2f.

[0020] A pair of guide bodies 2j are formed on the outer sides of the left and right legs 2b of the cover body 2e, protruding in the form of rails along the longitudinal direction. As shown in Fig. 4, the guide body 2j has a parallel region 2j1 parallel to the back plate 2a at the tip end side of the cover body 2e, and an inclined region 2j2 that is separated from the back plate 2a toward the thin portion 2d, which is continuous with the parallel region 2j1. In addition, the parallel region 2j1 and the inclined region 2j2 of this guide body 2j come into sliding contact with the sliding body 1g provided on the cover member 1 side as the lid body portion 2e of the case main body 2 slides out from the tip end of the cover member 1, thereby acting to impart a rotational motion to the lid body portion 2e to bend it towards the rear side via the thin-walled portion 2d.

[0021] The left and right leg plates 2b of the case portion 2f have fitting recesses 2k for attaching an inner lid 3 (described later) formed in approximately the center in the longitudinal direction and near the lid body portion 2e. Furthermore, a pair of slide protrusions 2m are formed on both outer sides of the left and right leg plates 2b of the case portion 2f along the longitudinal direction of the case body 2. The slide protrusions 2m engage with case support portions 1d (described later) provided on the cover member 1, and act to support the case body 2 so as to be slidable in the longitudinal direction.

[0022] In addition, a rib-like click protrusion 2n is formed near the rear of the left and right leg plates 2b of the case portion 2f, and when the cover member 1 slides relative to the case body 2, this click action occurs by climbing over the click body 1f formed on the case body 2 side. Therefore, this click action also occurs when the case body 2 is returned into the cover member 1. Furthermore, on the inner surface of the back panel 2a of the case portion 2f on the side of the refillable lead storage chamber 2c, multiple protrusions 2p to avoid surface contact with the stored refillable lead are formed perpendicular to the longitudinal direction of the case body 2. This serves to prevent the refillable lead L from being attracted by static electricity and sticking to the inner surface of the refillable lead storage chamber 2c of the case body 2 made of a resin material.

[0023] 5 to 7 show a single component structure of the inner lid 3 that is attached to the left and right leg plates 2b of the case body 2 and covers the replacement lead storage chamber 2c on the front side. The inner lid 3 is formed in a flat shape, and has fitting protrusions 3a formed in the longitudinal center and near the tip. The fitting protrusions 3a are fitted into the inner lid fitting recesses 2k formed in the case body 2, and are attached so as to cover the replacement lead storage chamber 2c of the case body 2 on the front side.

[0024] At this time, a pair of protective plates 3b formed at the tip of the inner lid 3 are arranged inside the left and right leg plates 2b of the case body 2 so as to be positioned from the thin portion 2d of the case body 2 to the lid portion 2e. This eliminates the problem of the replacement lead L being pinched between the leg plate 2b on the side of the lid body part 2e, which is bent toward the rear side, and the leg plate 2b on the side of the case part 2f, when the lid body part 2e, which has been extended by the sliding action, is returned into the cover member 1.

[0025] In addition, at the rear of the inner lid 3, a pair of spacers 3c formed protruding from the inner surface of the inner lid 3 act to maintain a constant distance between the rear of the inner lid 3 and the back plate 2a of the cover member 1. Furthermore, multiple protrusions 3d are formed on the inner surface of the inner lid 3 in a direction perpendicular to the longitudinal direction of the inner lid 3. This prevents the resin inner lid 3, which covers the front of the refill cartridge chamber 2c of the case portion 2f, from coming into surface contact with the refill cartridge L, and prevents the refill cartridge L from being attracted by static electricity and sticking to the inner lid 3.

[0026] 8 to 10 show the single-item configuration of the cover member 1, which is formed with a front plate 1a and left and right outer leg plates 1b, with a cross section perpendicular to the longitudinal direction being U-shaped, and a rear leg plate 1c that connects the left and right outer leg plates 1b is formed at the rear of the cover member 1. The front plate 1a covers the replacement lead storage chamber 2c of the cover member 1, and the left and right leg plates 2b of the case body 2 are positioned inside the left and right outer leg plates 1b to support the case body 2 so that it can slide in the longitudinal direction. The cover portion 2e of the case main body 2 is configured to be extended from the tip end side of the cover member 1 by a sliding movement.

[0027] Case support portions 1d are formed along the edges of the left and right outer leg plates 1b at two longitudinal locations, facing each other inward. These slidably support the left and right sliding protrusions 2m formed on the leg plates 2b of the case body 2, thereby slidably supporting the case body 2 and also functioning to prevent the case body 2 from coming off the cover member 1. A pair of slide protrusions 1e are formed facing each other on the inside near the rear of the left and right outer leg plates 1b. The rib-shaped click protrusions 2n formed on the case body 2 ride over these protrusions forward, generating a click action when the case body 2 is opened. Furthermore, a pair of click bodies 1f are formed facing each other on the inside of the rear side of the outer leg plate 1b, and when the case main body 2 is slid, these click bodies 1f are overcome rearward by the rib-shaped click protrusions 2n formed on the case main body 2, generating a clicking action.

[0028] Sliders 1g are provided facing each other in the outer leg plate 1b at the tip of the cover member 1. When the case body 2 slides and the lid body part 2e projects from the tip of the cover member 1, the pair of sliders 1g come into contact with the parallel region 2j1 and the inclined region 2j2 of the guide body 2j formed on the lid body part 2e, thereby bending the lid body part 2e toward the rear side.

[0029] Stoppers 1h are provided facing each other slightly toward the rear of the sliding body 1g at the tip of the cover member 1. When the case body 2 slides and the lid body part 2e is in a fully open state, the tips of the slide projections 2m formed on the leg plate 2b of the case part 2f come into contact with the pair of stoppers 1h, thereby stopping the sliding movement of the case body 2. In addition, during the sliding action of pulling the case body 2 back into the cover member 1, the pair of stoppers 1h also play a role in returning the cover body portion 2e, which is bent toward the rear side, to a state aligned with the longitudinal direction of the case body 2 by contacting the return guide portion 2j3 of the guide body 2j.

[0030] Furthermore, on the surface of the front plate 1a of the cover member 1, a circular recess 1k is formed in approximately the center of the cover member 1, as shown in FIGS. When the case body 2 is slid relative to the cover member 1, the depression 1k allows the sliding movement of the case body 2 to be performed smoothly by placing, for example, a thumb against the depression 1k.

[0031] 1 to 10 described above show one example of a refill lead case that uses refill lead case parts obtained by the molding method according to the present invention. In the following, the method of molding the refill case parts shown in Figs. 1 to 10 and the features of the resin molding of the refill case parts obtained by this method will be described with reference to Figs. 11 to 20.

[0032] FIG. 11 is a schematic diagram showing a molding method for a refill case part in the order of steps (A) to (D). The molding method for the refill lead case parts shown in Figure 11 is suitable for molding refill lead case parts that have a U-shaped cross section perpendicular to the longitudinal direction, such as the cover member 1 that constitutes part of the refill lead case already described, by having left and right outer leg plates 1b integrally formed on both sides of the front panel 1a.It is particularly suitable for molding refill lead case parts in which inward protruding parts, such as the case support part 1d, are integrally molded so that they face inward on the left and right outer leg plates 1b of the cover member 1.

[0033] Therefore, in the following explanation of the molding method for the refill case parts, the cover member 1 and case body 2 explained above will be referred to as refill case parts 11, and the front plate 1a of the cover member 1 and the back plate 2a of the case body 2 will be referred to as the main plate 12. Additionally, the left and right outer leg plates 1b of the cover member 1 and the left and right leg plates 2b of the case body 2 will be simply referred to as leg plates 13, and further the case support parts 1d, slide protrusions 1e, stoppers 1h, etc. molded inwardly on the outer leg plate 1b of the cover member 1 will be referred to as inward protrusions 14.

[0034] FIG. 11 is a schematic diagram of a resin molding apparatus showing a state in which the replacement lead case part 11, for example, which serves as the cover member 1 described above, is molded. This resin molding device includes, for example, a first mold 21 as a movable mold, a second mold 22 equipped with a core pin 23 that forms an isosceles trapezoid, a pair of slide cores 24 that move upward along the core pin in a direction inclined with respect to the mold opening direction of the first mold 21 relative to the second mold 22, and an ejection pin 25 that moves upward in the same direction as the mold opening direction of the first mold 21 relative to the second mold 22. In the example shown in FIG. 11, the ejector pin 25 is disposed in the center of the core pin 23, but when molding the case part 11 shown in FIGS. 12 to 20 described later, the ejector pin 25 will be disposed at a position overlapping a pair of slide cores 24 on the paper.

[0035] 11 shows the state after the first mold 21 is opened from the second mold 22. As is well known, the resin injection process before this process is performed by injecting resin into the cavity in a state in which the first mold 21 is closed on the second mold 22. In this case, the cavity is formed within a space surrounded by a first mold 21, a second mold 22, a core pin 23, a slide core 24, and an ejection pin 25 which constitute a resin molding apparatus shown in FIG.

[0036] FIG. 11B shows an initial state in which the pair of slide core 24 and ejector pin 25 simultaneously move in the mold opening direction of the first mold 21. 11(C), the pair of slide cores 24 and ejector pins 25 rise further from this initial state without changing their relative heights. As a result, the tips of the pair of slide cores 24 move toward the center of the case part 11 so as to approach each other. As a result, the inward protruding portions 14 of the case part 11 are released from the recesses 24 a formed on the outer side near the tip end of the slide core 24 , and the case part 11 is received by the tip end of the ejector pin 25 .

[0037] That is, in the state shown in Fig. 11(C), the inward protruding portion 14 as an undercut portion formed in the case part 11 can be vertically removed from the slide core 24. Therefore, as shown in Fig. 11(D), the resin-molded case part 11 can be easily removed by a robot arm (not shown) or the like.

[0038] According to the molding method of the refill case part described above, when the refill case part 11 is released from the mold, the slide core 24 is accompanied by an action of moving toward the center of the case part 11. As a result, the inward protrusion 14, which is molded integrally with the refill case part 11, is smoothly released from the slide core 24 and received by the ejection pin 25. Therefore, the refill lead case part 11 having the inward protruding portion 14 can be removed from the mold without relying on forcible removal, and a refill lead case part 11 can be obtained in which deformation of the inward protruding portion 14 caused by forcible removal is prevented.

[0039] 12 to 16 show a first embodiment of a case part 11 obtained by molding the cover member 1 described above according to the molding method shown in FIG. In this case part 11, the inner bottom surface 12a of the main panel 12 sandwiched between the left and right leg plates 13 has large and small rectangular first step portions 16 having steps formed by protruding molding relative to the inner bottom surface 12a, which are formed at six locations along the left and right leg plate 13 sides excluding the central portion in the longitudinal direction of the inner bottom surface 12a.

[0040] In addition, on the opposing inner surfaces of the left and right leg plates 13 of this case part 11, a second step portion 17 having a step with respect to this inner surface by protruding molding is formed continuously from the first step portion 16 described above. Furthermore, on the inner bottom surface 12a of the main panel 12 sandwiched between the left and right leg plates 13, third step portions 18 having a different concave / convex direction from the first step portions 16, i.e., the third step portions 18 having a semi-cylindrical shape recessed into the inner bottom surface 12a, are formed at four locations offset from the first step portions.

[0041] The inner bottom surface 12a of the case part 11 is formed by the top surface of the core pin 23 shown in Fig. 11. The first step portion 16 is formed by the upper end surfaces of a pair of slide cores 24. Furthermore, the second step 17 is formed by flat recesses (not shown) provided on both outer sides of the upper end of the core pin 23. And the third step 18 is formed by the upper end surface of the ejection pin 25.

[0042] 12 to 16, the first step 16 is formed to protrude from the inner bottom surface 12a, so that when the case part 11 is released from the slide core, the upper end of the slide core 24 slides along the surface of the first step 16. This makes it possible to prevent product defects such as scratches on the inner bottom surface 12a of the case part 11 caused by the movement of the slide core 24 accompanying release from the mold. Moreover, according to the first embodiment of the case part 11, the third step 18 is recessed relative to the inner bottom surface 12a. Therefore, the case part 11 released from the slide core 24 is received in a stable position by the ejection pin 25 in the recessed third step 18.

[0043] 17 to 20 show a second embodiment of a case part 11 obtained by molding the previously described case body 2 according to the molding method shown in FIG. In addition, in the respective parts of the case part 11 shown in the second embodiment, parts that perform the same functions as those in the first embodiment shown in Figs. 12 to 16 are indicated by the same reference numerals, and the description thereof will be omitted. In this case part 11, inward protrusions 14 are integrally formed with the leg plates 13 along the opening edges of the left and right leg plates 13. Then, on the inner bottom surface 12a of the main panel 12 sandwiched between the left and right leg plates 13, first step portions 16 having steps due to protrusion molding with respect to the inner bottom surface 12a are formed at four locations along the left and right leg plate 13 sides excluding the central portion in the longitudinal direction of the inner bottom surface 12a.

[0044] In addition, in this second embodiment, the inner bottom surface 12a of the main panel 12 has circular third step portions 18 recessed into the inner bottom surface 12a at six locations offset from the first step portions 16.

[0045] The inner bottom surface 12a of the case part 11 described above is formed by the top surface of the core pin 23 shown in Fig. 11, and the first step 16 is formed by the upper end surfaces of a pair of slide cores 24. Furthermore, the third step 18 is formed by the upper end surfaces of the ejection pin 25, and these relationships are the same as those in the first embodiment of the case part 11.

[0046] Therefore, also in the second embodiment of the case part 11 shown in Figures 17 to 20, the presence of the first step portion 16 makes it possible to prevent product defects such as scratches on the inner bottom surface 12a of the case part 11 due to the movement of the slide core 24 accompanying demolding. In addition, due to the presence of the third step 18, the case part 11 released from the slide core 24 can be received in a stable posture by the tip of the ejection pin 25 located within the third step 18, thereby obtaining the same advantageous effects as in the first embodiment. [Explanation of symbols]

[0047] 11 Replacement lead case parts 12 Main plate 12a Inner bottom surface 13 Leg board 14 Inward protrusion 16 First step 17 Second step 18 Third Section 21 First Mold 22 Second Mold 23 Core pin 24 Slide Core 24a Recess 25 Ejector pin

Claims

[Claim 1] A lead refill case is a lead refill case that uses a member having left and right leg plates integrally formed on both sides of a main panel, such that a cross section perpendicular to the longitudinal direction is formed in a U-shape, and an inward protrusion is formed integrally with the leg plates so as to face inward relative to the left and right leg plates, A refill lead case characterized in that a first step having a step with respect to the inner bottom surface is formed along the left and right leg plate sides except for the longitudinal center portion of the inner bottom surface on the inner bottom surface of the main panel sandwiched between the left and right leg plates, and a third step having a different concave-convex direction from the first step is formed on the inner bottom surface of the main panel sandwiched between the left and right leg plates at a position offset from the first step, the first step being molded to protrude with respect to the inner bottom surface, and the third step being molded to be recessed with respect to the inner bottom surface.

Images