A keytop sheet for a push-button switch

This invention relates to a keytop sheet for push-button switches capable of being reduced in size and thickness and suited to integration.

The reduction in the size and thickness of electronic devices that has been achieved in recent years has been accompanied by a demand to reduce the size and thickness also of the keytop sheet for the push-button switches used on the control panels of these devices.

In an effort to meet this demand, a keytop sheet is disclosed in Fig. 12 of European Patent Application (Publication No. 0 483 898 A2). Such a keytop includes a synthetic resin film deformed at a predetermined position thereof into so as to form an upwardly curved portion. A quantity moldable resin is filled into the recess defined by the upwardly curved portion. A portion of the moldable resin extends through a plurality small holes in the outer periphery of the curved portion of the film sheet, so as to form a plurality of film fixing portions and a plurality of film retaining portions on the upper and lower surfaces of the film sheet, respectively. The film sheet is clamped between the film fixing portion and the film retaining portions, so that the moldable resin is secured to the film sheet.

It is noted, however, that, with such a keytop construction, the film fixing portions and the film retaining portions should be protruded outwardly from the outer periphery of the curved portion of the resin film retaining portions should be protruded outwardly from the outer preriphery of the curved portion of the resin film. This disadvantageously increase the outer dimension of the keytop, as well as degrading appearance of the keytop when viewed from the above. If a nameplate or casing is placed on the resin film in order to prevent the protruded portions fom being viewed, the film fixing portions protruded from the resin film would prevent close contact between the film sheet and the nameplate or the like, so as to create gaps therebetween. Thus, reduction in thickness would not be achieved.

Accordingly, an object of the present invention is to provide a keytop sheet for a push-button switch which is reduced in outer dimension, which provides an increased appearance, and which provides reduction in total thickness of a film sheet and a nameplate or a casing placed thereon, by reason of a closely contact relationship therebetween.

The invention is mainly featured by a keytop sheet for a push-button switch which switch includes a keytop disposed above a switch contact and having a pusher portion in the lower surface thereof for depressing the switch contact for opening/closing operation thereof, the keytop being formed by means of a molding process at a predetermined position on a film sheet formed from a resin film, wherein: the film is formed from a resin film of a thermoplastic material, and the keytop is constructed by deforming the film sheet at a predetermined position thereof so as to form an upwardly curved portion, and filling a quantity of moldable resin into the inside of the upwardly curved portion so as to cause the opposed surfaces of the upwardly curved portion and the moldable resin to be directly and fusedly bonded together into a unitary construction.

The keytop is constructed by securing together the film sheet and the moldable resin solely through the fused bonding between the lower surface of the upwardly curved portion of the film sheet and the upper surface of the bulk of the moldable resin in directly contact with the lower surface of the film sheet. It is therefore unnecessary to use any additional members or elements for clamping together the film sheet and the moldable resin, as is used in prior art. Thus, the size of the keytop may be determined simply by the size of the upwardly curved portion of the film sheet. This contributes not only to reduction in outside dimension of the keytop, but also to elimination of a portion of the moldable resin which otherwise would be exposed outwardly or upwardly from the upper surface of the film sheet, whereby appearance of the keytop is improved When viewed from the above. It is further noted that, when a nameplate is provided on the keytop sheet, the opposed surfaces of the keytop sheet and the nameplate are closely adhered relative to one another, whereby the thickness thereof may be reduced.

The invention is also featured by a keytop sheet for a push-button switch which is characterized in that an adhesion layer is interposed between said film and the said quantity of moldable resin.

Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.

• Fig. 1 is a side sectional view illustrating a push-button switch one component of which is a keytop sheet according to a first embodiment of the present invention;
• Figs. 2(A) and 2(B) are diagrams illustrating the keytop sheet of Fig. 1, in which Fig. 2(A) is a plan view and Fig. 2(B) a side view;
• Figs. 3(A) and 3(B) are diagrams illustrating a film sheet in Figs. 2(A) and 2(B), in which Fig. 3(A) is a plan view and Fig. 3(B) a side view;
• Fig. 4 is a diagram illustrating a method of forming the keytop sheet of Fig. 1 by resin molding;
• Fig. 5 is a schematic side sectional view illustrating a push-button switch one component of which is a keytop sheet according to a second embodiment of the present invention;
• Figs. 6(A) and 6(B) are diagrams illustrating other structures of curved projections used in the present invention;
• Fig. 7 is a side sectional view illustrating a push-button switch one component of which is a keytop sheet according to a third embodiment of the present invention;
• Figs. 8(A) and 8(B) are diagrams illustrating the keytop sheet of Fig. 7, in which Fig. 8(A) is a plan view and Fig. 8(B) a side view;
• Figs. 9(A), 9(B), 9(C) are plan views illustrating a procedure for manufacturing a film sheet;
• Figs. 10(A), 10(B), 10(C) are plan views illustrating a procedure for manufacturing a nameplate;
• Fig. 11 is a diagram illustrating a method of forming the keytop sheet of Fig. 7 by resin molding; and
• Fig. 12 is an enlarged view of an encircled portion B in Fig. 7.

Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

Fig. 1 is a side sectional view illustrating a push-button switch one component of which is a keytop sheet according to a first embodiment of the present invention. As shown in Fig. 1, a push-button switch 1 includes a switch substrate 60 formed by superimposing an upper sheet 71, a spacer 63 and a lower sheet 65, and a keytop sheet 10 obtained by fixing a frame 40 to the outer periphery of the underside of a film sheet 20 provided with a keytop 50. A click spring 69 is attached to the switch substrate 60 over a switch contact 68, and the frame 40 of the keytop sheet 10 is affixed to the outer periphery of the switch substrate 60 via a sheet 71 having an adhesive on both sides thereof.

Figs. 2(A) and 2(B) are diagrams illustrating the keytop sheet 10, in which Fig. 2(A) is a plan view and Fig. 2(B) a side view. It should be noted that the keytop sheet 10 in Fig. 1 is a sectional view taken along line 1-1 in Fig. 2(A).

As shown in Figs. 1, 2(A) and 2(B), the keytop sheet 10 has three keytops 50 provided on the central portion of the film sheet 20, and the frame 40 is attached to the underside of the film sheet 20 along the outer periphery thereof. Further, the film sheet 20 surrounding the portions thereof on which the keytops 50 are molded is provided with curved projections 25, each of which protrudes in a downward direction from the surface of the film sheet 20, so as surround the respective keytops 50.

In order to manufacture the keytop sheet 10, first the film sheet 20 is prepared. As shown in Figs. 3(A) and 3(B), the film sheet 20 is manufactured by cutting a thermoplastic synthetic-resin film (e.g., polyethylene terephthalate, polyethylene naphthalate, etc.) into rectangular shape and printing a desired design (not shown) on the top side or underside of the sheet.

Next, as illustrated in Fig. 4, the film sheet 20 is clamped between an upper mold D and a lower mold E. The upper mold D is provided with a cavity D1 for forming the keytop 50, and the lower mold E is provided with a cavity E1 for forming the frame 40 and a cavity E2 for forming the pushing portion 51 of the keytop 50. Further, the cavities E1, E2 of the lower mold E are provided with pin gates E10, E20, respectively.

The portion of the upper mold D surrounding the cavity D1 is provided with a ring-shaped projection D2. The latter has a semicircular cross section.

The lower mold E is provided with a ring-shaped cavity E3 at a position opposing the projection D2. The cavity E3 has a semicircular cross section and is formed to have such dimensions that a gap equivalent to the thickness of the film sheet 20 will be delimited between the cavity E3 and the projection D2 when the upper mold D and lower mold E are closed.

When a molten resin under conditions of high temperature and pressure is forcibly introduced from the pin gates E10, E20, the frame 40 is formed on the outer peripheral portion on the underside of the film sheet 20, and the portion of the film sheet 20 situated at the cavity D1 is urged upward and deformed so as to adhere to the inner surface of the cavity D1, as indicated by the dashed line in Fig. 4. At the same time, the cavity D1 and the cavity E2 are filled with the molten resin. In other words, the film sheet 20 is formed to include a bulge 21 (see Fig. 1) having the shape of the inner surface of the cavity D1, and the cavity 21 is filled with charged resin 23 (se Fig. 23) that is allowed to harden to unify the film sheet 20 and the resin 23, thereby forming the keytop 50.

Since the molten resin introduced at this time is at a high temperature and pressure and the film sheet 20 consists of thermoplastic resin, the forcibly introduced resin and film sheet 20 are directly and strongly fused together and the film sheet 20 will not peel off.

The portion of the film sheet 20 clamped between the projection D2 and the cavity E3 shown in Fig. 4 is thermoplastically deformed into the clamped shape by the heat of the introduced molten resin and the heat from the upper and lower molds D, E themselves. As a result, the curved projection 25 is formed in the film sheet 20.

As an alternative to forming the curved projection 25 in the film sheet 20 by the upper and lower molds D, E, the film sheet may be furnished with the shape of the curved projection 25 in advance by a forming process, after which the film sheet may be clamped between the upper and lower molds D, E.

Separating the upper and lower molds D, E completes the keytop sheet 10 shown in Figs. 1, 2(A) and 2(B).

As shown in Fig. 1, the switch substrate 60 is an ordinary membrane switch and is produced by superimposing the upper and lower sheets 61, 65 with the spacer 63 sandwiched between them. The spacer 63 is provided with a hole 67 within which electrode patterns formed on respective ones of the upper and lower sheets 61, 65 are arranged in opposition to each other so as to construct the switch contact 68. The lower sheet 65 is provided with an air-venting hole 66.

The click spring 69, which is made of metal or resin, is attached over the switch contact 68.

Sheets 71, 73 both sides of each of which are provided with an adhesive are affixed to the upper and lower surfaces, respectively, of the switch substrate 60. The double-sided adhesive sheet 71 on the upper side is affixed so as to surround the entire outer periphery of the top side of the switch substrate 60, and the double-sided adhesive sheet 73 on the lower side covers the entire underside of the switch substrate 60 and is affixed in such a manner that the air-venting hole 75 is situated at a prescribed position of the switch substrate 60. It should be noted that a separator 77 is affixed to the underside of the lower double-sided adhesive sheet 73.

Bonding the upper double-sided adhesive sheet 71 to the underside of the frame 40 of the keytop sheet 10 completes the push-button switch 1.

When the push-button switch 1 is secured to another member, it will suffice to peel the separator 77 off and affix the underside of the doubled-sided adhesive sheet to the other member.

When the keytop 50 of the keytop sheet 10 is pressed, the pushing portion 51 presses the click spring 69, which snaps back to produce a clicking sensation, as well as the switch contact 68 to close the same.

Since the film sheet 20 is provided with the curved projection 25, the curved projection 25 is deformed when the keytop 50 is pressed to lower the keytop 50, as a result of which the pressing operation is facilitated. The reason for this is that when the keytop 50 is pressed, the stretching of the film by an amount equivalent to the stroke of the keytop 50 is compensated for (absorbed) by the shape deformation of the curved projection 25.

Further, since the film sheet 20 of the keytop sheet 10 is not provided with any holes, water cannot penetrate the push-button switch 1 to the underside of the film sheet 20 even if the switch is exposed to water from above.

In the embodiment described above, the keytop 50 can be brightly illuminated from its lower side if the resin 23 filling the keytop 50 consists of a transparent material and a light-emitting element is disposed at a prescribed position below the keytop 50.

Further, in the embodiment described above, a membrane switch comprising a flexible substrate is used as the switch substrate 60. However, it is permissible to use switch substrates of other types, such as a rigid substrate.

It is also noted that, in the keytop 50 according to the above embodiment, the film sheet 20 is deformed at its predetermined portion so as to form an upwardly curved portion by charging a quantity of molten moldable resin under a high-temperature and high-pressure condition into a space directly below the underside or lower surface of the film sheet 20. The lower surface of the upwardly curved portion of the film sheet 20 and the upper surface of the bulk of the moldable resin in directly contact with the lower surface of the film sheet 20 are directly bonded by means of the heat and the pressure included in charging the moldable resin. Thus, the film sheet 20 and the moldable resin are unitary fixed relative to one another solely by the respective fusedly bonded surfaces. It is therefore unnecessary to use any additional members or elements for clamping together the film sheet 20 and the moldable resin, as is used in prior art. Thus, the size of the keytop 50 may be determined simply by the size of the upwardly curved portion of the film sheet 20. This contributes not only to reduction in outside dimension of the keytop 50, but also to elimination of a portion of the moldable resin which otherwise would be exposed outwardly or upwardly from the upper surface of the film sheet 20, whereby appearance of the keytop 50 is improved when viewed from the above. It is further noted that, when a nameplate 30 is provided on the keytop sheet 10, the opposed surfaces of the keytop sheet 10 and the nameplate 30 are closely adhered relative to one another, whereby the thickness thereof may be reduced.

The film sheet is formed from a resin film free of pores or small apertures. Thus, the keytop sheet may be provided with a perfect water-resistant property, without using any other additional members.

In the above-mentioned keytop 50, the moldable resin is curved with the film sheet 20, whereby a smooth surface is obtained. Thus, soft touch and attractive appearance may be provided. It is also noted that a pattern or design printed on the film sheet 20 can be seen on the keytop 50, whereby the keytop 50 may be clearly and easily decorated. In this regard, it is noted that the film sheet 20, like papers, may be easily printed thereon with any desired patterns, using all kinds of printing methods including a multi-color printing process. Printing of a pattern directly on the surface of the moldable resin could not be easily performed, particularly when the surface of the moldable resin is curved.

Though the frame 40 is provided on the periphery of the underside of film sheet 20 so as to surround the keytop 50, it is not always necessary for the keytop sheet in this invention to have the frame 40. More specifically, a keytop sheet devoid of a frame may be arranged directly on a switch substrate, or a keytop sheet devoid of a frame may be retained by another member and this member may be arranged on the switch substrate.

Further, in the first embodiment described above, there are instances in which the adhesion between the film sheet 20 and keytop 50 is weak depending upon the quality and thickness of the film sheet 20 or the quality and melting temperature of the molding resin used to form the keytop 50 and the frame 40. In such case the union between the film sheet and keytop can be strengthened if an adhesive layer is interposed between them.

Fig. 5 is a schematic side sectional view illustrating a push-button switch 1-2 one component of which is a keytop sheet 10-2 according to a second embodiment of the present invention. Since a switch substrate 60-2 is identical with the switch substrate 60 of the first embodiment, the outline of the switch substrate 60-2 is indicated by the dashed line in Fig. 5.

This embodiment differs from the first embodiment in that a nameplate 30-2 is affixed to the upper side of a film sheet 20-2. The nameplate 30-2 is provided with a circular hole 31-2 through which a keytop 50-2 is passed. More specifically, the nameplate 30-2 is affixed to the film sheet 20-2 in such a manner that the keytop 50-2 passes through the hole 31-2.

If this arrangement is adopted, a curved projection 25-2 provided on the film sheet 20-2 will no longer be visible to the eye. In addition, the top side or underside of the nameplate 30-2 can be printed on in a variety of ways.

In the first and second embodiments described above, it is so arranged that the curved projection protrudes downward from the surface of the film sheet. However, as shown in Fig. 6(A), a curved projection 25-A may be formed so as to protrude upward from the surface of a film sheet 20-A, or, as shown in Fig. 6(B), curved projections 25-B may be formed so as to protrude upward and downward in the manner of a wave relative to the surface of a film sheet 20-B. In the latter case, the greater the length of the curved projections 25-B in comparison with the other embodiments, the easier it is to press the keytop.

Fig. 7 is a side sectional view illustrating a push-button switch 1-6 one component of which is a keytop sheet 10-6 according to a sixth embodiment of the present invention. As shown in Fig. 7, the push-button switch 1-6 includes a keytop sheet 10-6, in which a nameplate 30-6 and a frame 40-6 are secured to above and below a film sheet 20-6 provided with a keytop 50-6, and a switch substrate 60-6 (only the outline of which is shown) that same as the switch substrate 60 depicted in Fig. 1.

Figs. 8(A) and 8(B) are diagrams illustrating the keytop sheet 10-6 of Fig. 7, in which Fig. 8(A) is a plan view and Fig. 8(B) a side view. Figs. 9(A), 9(B), 9(C) are plan views illustrating a procedure for manufacturing the film sheet 20-6, and Figs. 10(A), 10(B), 10(C) are bottom views illustrating a procedure for manufacturing the nameplate 30-6.

In order to manufacture the film sheet 20-6, first a thermoplastic synthetic resin 21-6 (e.g., polyethylene terephthalate, polyethylene naphthalate, etc.) is prepared, as shown in Fig. 9(A), and a bonding agent 26-6 such as epoxy resin or urethane resin is printed on the top side of the film 21-6 to a thickness of 25 - 30 µm.

The bonding agent 26-6 is applied to the entirety of the film 21-6 with the exception of the interior of a circle a1-6, and to a ring-shaped portion 26A-6 located within the circle a1-6. The ring-shaped portion 26A-6 is a portion between two circles a2-6 and a3-6.

Next, as shown in Fig. 9(B), decorative printing 25-6 comprising a desired design or the like is applied to a prescribed position of the film 21-6. The location at which the decorative printing 25-6 is applied is the inner-side portion of a circle a4-6 between the circle a1-6 and the circle a2-6 illustrated in Fig. 9(A). It should be noted that the thickness of the decorative printing 25-6 is on the order of 5 - 7 µm.

Next, as shown in Fig. 9(C), the portion between the circles a1-6 and a2-6 of the film 21-6 is cut away to provide a C-shaped cut-out 22-6. As a result, a structure is obtained in which a circular keytop portion 23-6 is connected to the film 21-6 solely by a hinge portion 24-6 of small width.

In order to manufacture the nameplate 30-6, first a synthetic resin film 31-6 (e.g., polyethylene terephthalate or the like; a thermoplastic resin need not necessarily be used) is prepared and printing 33-6 such as a desired design is applied over the entire underside of the film 31-6, as illustrated in Fig. 10(A).

Next, as shown in Fig. 10(B), a bonding agent 35-6 such as epoxy resin or urethane resin is printed on the printing 33-6. It should be noted that the bonding agent 35-6 is not printed on the inner side of a circle b1-6. The diameter of the circle b1-6 is made the same as that of the circle a3-6 shown in Fig. 9(A).

The portion of the film 31-6 on which the bonding agent 35-6 has not been printed is cut away, as shown in Fig. 10(C), thereby providing a circular hole 32-6. The diameter of the hole 32-6 is the same as the diameter of the circle b1-6.

The nameplate 30-6 shown in Fig. 10(C) is placed upon the film sheet 20-6 of Fig. 9(C) in such a manner that the two layers of bonding agents 26-6, 35-6 contact each other. The nameplate 30-6 and film sheet 20-6 are bonded strongly together by thermocompression bonding.

Next, the keytop 50-6 and frame 40-6 are provided simultaneously, by resin molding, on the unitary body comprising the film sheet 20-6 and nameplate 30-6. This method will now be described with reference to Fig. 11.

As shown in Fig. 11, the film sheet 20-6 and nameplate 30-6 united in the manner set forth above are clamped between an upper mold D-6 and a lower mold E-6. The upper mold D-6 is provided with a cavity D1-6 for forming the keytop 50-6, and the lower mold E-6 is provided with a cavity E1-6 for forming the frame 40-6 and a cavity E2-6 for forming a pushing portion 51-6 of the keytop 50-6. The cavities E1-6, E2-6 of the lower mold E-6 are provided with pin gates E10-6, E20-6, respectively.

When a high-temperature, high-pressure molten resin is forcibly introduced from the pin gates E10-6, E20-6, a frame 40-6 is formed on the outer periphery of the underside of the film sheet 20-6, and the keytop 23-6 of the film sheet 20-6 is urged upwardly and deformed so as to adhere to the inner surface of the cavity D1-6, as indicated by the dashed line in Fig. 11. At the same time, the cavity D1-6 and the cavity E2-6 are filled with the molten resin.

Since the thermoplastic molten resin introduced at this time is at a high temperature and pressure and the film sheet 20-6 consists of thermoplastic resin, the forcibly introduced resin and film sheet 20-6 are directly and strongly fused together so that the film sheet 20 will not peel off. (It should be noted that an adhesive layer may be interposed between the resin and the film sheet, as described earlier in connection with the first embodiment.) In other words, the portion of the film sheet 20-6 having the keytop portion 23-6 is caused to protrude and deform upwardly, thereby forming a bulge having the shape of the inner surface of the cavity D1-6, and the synthetic resin is caused to fill the interior of the bulge and is allowed to harden so as to integrate the resin and the film sheet and form the keytop 50-6.

Separating the upper and lower molds D-6, E-6 completes the keytop sheet 10-6 shown in Figs. 7 and 8.

As shown in Figs. 7 and 8, the cut-out 22-6 [see Fig. 9(C)] provided in the film sheet 20-6 is concealed by being covered by the nameplate 30-6 and therefore cannot be seen from upper side. This makes it possible to maintain the attractive appearance of the keytop sheet 10-6.

Further, though the hinge portion 24-6 [see Fig. 9(C)] of the film sheet 20-6 develops a wrinkle when the keytop 50-6 is pressed, the hinge portion 24-6 also is covered and concealed by the nameplate 30-6 so that the attractive appearance of the keytop sheet is maintained in this sense as well.

Fig. 12 is an enlarged view of portion B in Fig. 7. As illustrated in Fig. 12, since the layer of bonding agent 26-6 of the film sheet 20-6 and the layer of bonding agent 35-6 of the nameplate 30-6 are in direct contact with each other, the two are bonded together by heat. However, since the decorative printing 25-6 has been applied to the ring-shaped portion 26A-6, the two are not bonded together at this portion even though heat is applied. In other words, the two members are merely in contact with each other at this portion.

This portion is provided with the ring-shaped portion 26A-6 for the following reason: If this portion were not provided with the ring-Shaped portion 26A-6 comprising a bonding agent of a prescribed thickness, a gas would be produced between the bonding agent 35-6 of the nameplate 30-6 and the decorative printing 25-6 of the film sheet 20-6 at this portion. If this were then clamped between the molds D-6, E-6 shown in Fig. 11, the film sheet 20-6 of the ring-shaped portion 26A-6 and the nameplate 30-6 could not be clamped together strongly and fixedly secured. If this portion cannot be fixedly secured, the molten resin will flow out from the gap to the side of the cut-out 22-6 of the film sheet 20-6 when the molten resin is charged. Such a molding operation is unsatisfactory.

A particular characterizing feature of this embodiment is that the thickness of the portion where the film sheet 20-6 of the ring-shaped portion 26A-6 and the nameplate 30-6 overlap is equal to the sum of the thickness of the film 21-6, the thickness of the ring-shaped portion 26A-6, the thickness of the decorative printing 25-6, the thickness of the bonding agent 35-6, the thickness of the decorative printing 33-6 and the thickness of the film 31-6. By contrast, the thickness of the portion where the film sheet 20-6 and nameplate 30-6 overlap is equal to the sum of the thickness of the film 21-6, the thickness of the bonding agent 26-6, the thickness of the bonding agent 35-6, the thickness of the decorative printing 33-6 and the thickness of the film 31-6. In other words, the thickness of the portion where the film sheet 20-6 of the ring-shaped 26A-6 and the nameplate 30-6 overlap is greater than that of other portions by the thickness of the decorative printing 25-6. As a consequence, the clamping of this portion by the molds D-6, E-6 is strengthened and made more certain.

In the above-described embodiment, the film sheet 20-6 and the nameplate 30-6 are bonded together by a thermocompression bonding step. However, this step is not always necessary. The reason is that if the above-mentioned members are clamped between the upper and lower molds D-6, E-6 shown in Fig. 11 in a state in which the film sheet 20-6 and nameplate 30-6 are merely superimposed (i.e., without being bonded together by thermocompression) and the molten resin is forcibly introduced, the two layers of bonding agents 25-5, 35-6 are melted and bonded together by the pressure and heat of the molten resin and the temperature of the high-temperature molds D-6, E-6.

If the keytop sheet 10-6 is attached to the switch substrate 60-6 and the keytop 50-6 of the keytop sheet 10-6 is pressed, the pushing portion 51-6 presses the click spring of the switch substrate 60-6 and the switch contact, thereby closing the switch contact.

When the keytop 50-6 is pressed, the hinge portions 24-6 flexes since the film sheet 20-6 is provided with the cut-out 22-6 and the keytop 50-6 is merely connected to the film sheet 20-6 by the hinge portion 24-6. As a result, the keytop 50-6 is very easy to depress.

In the embodiment described above, the keytop 50-6 can be brightly illuminated from its lower side if the charged resin consists of a transparent material and a light-emitting element is disposed on the switch-substrate 60-6.

In the third embodiment described above, the hinge portion 24-6 is provided at only one location. However, the invention is not limited to such an arrangement; the hinge portion may be provided at a plurality of locations.

As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.