JP2003001731A - Panel cores and door panels - Google Patents

Abstract

(57) [Problem] To provide a paper honeycomb which is excellent in design and excellent in strength and light in weight while maintaining the smoothness of the surface plate even when the surface plates made of thin metal plates are laminated. Panel core material and a door panel using the same. SOLUTION: A light weight, core material for a panel having excellent bending strength and torsion strength, comprising a paperboard having a basis weight of 400 to 500 g / m ² laminated on both sides of a paper honeycomb with an adhesive,
A surface plate and a back plate made of a metal plate having a thickness of 0.25 to 0.50 mm are laminated on both sides of the panel core material via an adhesive layer, and the front plate and the back plate are fitted at the peripheral edge. This is a door panel which is configured to be fixed together and has excellent surface smoothness, bending strength, and torsional strength and can be reduced in weight.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a paper which can be reduced in weight, has excellent surface plate adhesiveness, has excellent smoothness of the adhered surface plate and is excellent in design and strength. The present invention relates to a panel core material made of a honeycomb and a door panel using the panel core material.

[0002]

2. Description of the Related Art Conventionally, a door using a paper honeycomb as a core material and a thin plate on the surface, for example, a thin steel plate, has a groove shape made of metal as described in, for example, Japanese Patent Application Laid-Open No. 8-303129. The channels are arranged in a rectangular frame shape with the open surface facing inward, and the paper core is inserted and temporarily held in the space inside the frame fixed to the corner member. There is known a structure in which cores are sandwiched and arranged on both sides thereof, and these surface plates are adhered to the front and back surfaces of each channel material of a frame and the front and back surfaces of a paper core.

However, in the door having the above structure, a pair of surface plates made of thin steel plates are adhered to both sides of the frame and the paper core with the frame and the paper core sandwiched therebetween, and the surface plate made of a pair of thin steel plates is used for the frame and Since it is directly adhered to the paper core surface, unevenness of the joint between the frame and paper core and the paper core surface appears on the surface of the pair of adhered thin steel plates, impairing the smoothness of the surface plate. There is a drawback that the design is deteriorated.

[0004]

The object of the present invention is that the weight can be reduced, the adhesion of the surface plate is excellent, and the surface smoothness is maintained even when a thin metal plate is used as the surface plate. It is an object of the present invention to provide a core material for a panel made of paper honeycomb and a door panel using the core material for the panel, which is excellent in design and strength.

[0005]

[Means for Solving the Problems] By using a core material for a panel in which paperboard is laminated on both sides of a paper honeycomb with an adhesive, the core material is lightweight and excellent in bending strength and twisting strength. You can Further, since the paperboard is laminated on both sides of the paper honeycomb, even when a surface plate made of a thin metal plate is laminated on the surface of the paperboard, the paperboard functions as a cushioning material, so that the smoothness of the laminated surface plate is improved. can do.

A wood core material is arranged adjacent to a paper honeycomb, and a paper board is laminated on both sides of the paper honeycomb wall and the wood wood member with an adhesive to form a panel core material, thereby obtaining dimensional stability and strength in the thickness direction. In addition, the surface smoothness is excellent, and the accessory can be easily attached to the peripheral portion of the door panel manufactured by using the panel core material.

In the above-mentioned panel core material, by adjusting the basis weight of the paperboard to 400 to 500 g / m ² , even if there is some unevenness in the surface of the panel core material, the paper honeycomb and the wood material Even if the boundary becomes uneven, the thick paperboard acts as a cushioning material, so when a door panel is manufactured by laminating a front plate and a back plate made of thin metal plates on both sides of the panel core material. In addition, the surface plate has improved smoothness, and a door panel with excellent design can be obtained.

By providing a door panel having a structure in which a front plate and a back plate made of a thin metal plate are laminated on both sides of the above-mentioned panel core material with an adhesive layer interposed therebetween, surface strength, bending strength, and
It is possible to provide a lightweight door panel having excellent strength such as twisting strength.

In the above door panel, when the adhesive layer is made of a hot-melt type film adhesive, when laminating a front plate and a back plate made of thin metal plates on both sides of the panel core material, Laminating can be done simply by inserting a film adhesive between the panel core material and the front and back plates and hot pressing, so the workability in the laminating process is improved compared to the process of applying a liquid adhesive and laminating. You can do well. Moreover, since the thickness of the adhesive layer can be reduced to form a uniform layer, the surface smoothness of the laminated door panel can be made excellent, and an epoxy adhesive or the like can be used for lamination. It is possible to reduce the cost as compared with the case of doing it.

In the above door panel, the thin metal plate is
When the thickness is 0.25 to 0.50 mm, the weight of the panel can be reduced, and bending strength and twisting can be achieved even when the front plate and the back plate are metal plates having the above thickness. A door panel with excellent strength can be obtained.

In the above door panel, the peripheral edges of the front plate and the rear plate are respectively bent, and the peripheral edges of the front plate and the rear plate are fitted and fixed to each other.
A door panel can be easily manufactured by adhering the front surface plate and the back surface plate to the panel core material and fitting and fixing at the peripheral edge.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. 1 is a plan view showing the structure of a panel core material according to a first embodiment of the present invention, FIG. 2 is a sectional view taken along the line II of FIG. 1 in which the central portion is cut away, and FIG. 3 is a second embodiment of the present invention. The top view which shows the structure of the core material for panels, FIG. 4 is the II-II sectional view which notched the center part in FIG. 3, FIG. 5 is the figure which shows the door panel using the core material for panels of 1st Embodiment of this invention. 2 is a cross-sectional view corresponding to FIG. 2, FIG. 6 is a cross-sectional view corresponding to FIG. 2 showing another door panel using the panel core material of the first embodiment of the present invention, and FIG. 7 is a panel of the second embodiment of the present invention. 4 is a sectional view corresponding to FIG. 4, showing a door panel using a core material, FIG. 8 is a plan view of a paper honeycomb, 1 is a paper honeycomb, 2 and 5 are adhesive layers, 3 is paperboard, 4 and 4 ′.
Is a wood material, 6 is a front plate, 7 is a back plate, 8 and 8'are front plate edges, 9 and 9'are back plate edges, 11 is a flat portion, and 12 is a wavy portion.

The panel core material according to the first embodiment of the present invention is as shown in FIGS. As shown in FIG. 1, the planar shape is a panel core material used as a rectangular door panel. As shown in FIG. 2, the cross-sectional shape of the paper honeycomb 1 is formed on both sides of the paper honeycomb 1 by the adhesive layers 2 and the paperboard 3
Is bonded, and the basis weight of the paperboard 3 bonded to both sides of the paper honeycomb 1 is 400 to 500 g / m ² . With this configuration, the weight of the panel core material can be reduced, and the bending strength and the twisting strength can be increased. Basis weight is 400-50
By using 0 g / m ² of paperboard 3, the paperboard 3 acts as a cushioning material, and even if the surface of the paper honeycomb 1 has some unevenness, it does not appear on the surface of the laminated paperboard 3. Therefore, the core material for a panel having excellent surface smoothness can be obtained.

The panel core material according to the second embodiment of the present invention is as shown in FIGS. As shown in FIG. 3, the planar shape is a panel core material used as a rectangular door panel. As shown in FIG. 4, the cross-sectional shape is such that the wood material 4 is arranged on each of the two opposing short sides of the paper honeycomb 1, and the wood material 4 ′ is disposed on the center of each of the two opposing long sides. In this state, the entire surface of the paper honeycomb 1, the wood material 4 and the wood material 4'is covered with the adhesive layer 2 on both surfaces of the paper board 3 having a basis weight of 400 to 500 g / m ² .
Is a bonded structure. In the second embodiment, the wood members 4 are arranged on the opposite short sides, and the wood members 4'are arranged on the central portions of the opposite long sides, respectively. It suffices that the dimension in the thickness direction can be stabilized during pressing, and the wood material may be arranged on the entire peripheral edge of the paper honeycomb, that is, on four sides, or may be arranged only on the opposing short sides, respectively. . By arranging the wood materials 4 and 4'on the periphery of the paper honeycomb 1, it becomes easy to attach accessories to the periphery. In the second embodiment, the thickness of the paper honeycomb 1 is set to 0.1 rather than the thickness of the wood materials 4 and 4 '.
By making it thicker by about 0.2 mm and pressurizing it to the thickness of the wood materials 4 and 4 ', the surface of the paperboard 3 is uneven at the joint portion between the paper honeycomb 1 and the wood materials 4 and 4'. Since this can be prevented, excellent surface smoothness can be obtained. In terms of strength, the panel core material of the second embodiment also has the first
The same effect as the embodiment can be obtained.

The structure of the door panel using the panel core material of the first embodiment is as shown in FIG. The front plate 6 is laminated on one surface of the paperboard 3 of the panel core material of the first embodiment with the adhesive layer 5 interposed therebetween, and the back plate 7 is laminated on the other surface of the paperboard 3 with the adhesive layer 5 interposed therebetween. Then, the front plate edge 8 is formed into a shape having a groove formed by quadruple bending, the back plate edge 9 is bent at a right angle, and the back plate edge 9 is inserted into the groove of the front plate edge 8 and fixed. It has been configured. Surface board 6 on the entire surface of paperboard 3 on both sides of the panel core material
Since the back plate 7 is adhered by the adhesive layer 5,
The bending strength and twisting strength of the door panel are excellent. Further, when the front plate 6 and the back plate 7 are bonded to both sides of the paperboard 3 on both sides of the panel core material, the back plate end 9 is inserted into the groove of the front plate end 8 to insert the front plate 6 and the back plate 7. Since the peripheral portion of the door panel can be fixed, the manufacturing process of the door panel can be simplified.

Another construction of the door panel using the panel core material of the first embodiment is as shown in FIG. This door panel has two front plate edges 8'and two back plate edges 9 '.
It has a configuration in which the back plate edge 9'is fixed by inserting and fitting the back plate edge 9'inside the front plate edge 8'as a heavily folded shape. The door panel has the same structure as that of the door panel shown in FIG. 5, except that the fixing method is different. With the configuration shown in FIG. 6, when the front panel 6 and the back panel 7 are bonded to both sides of the panel core material by the adhesive layer 5, the back panel edge 9'is inside the front panel edge 8 '. The door panel can be manufactured more easily because it can be fixed simply by inserting and fitting. The door panel having this structure can also be reduced in weight and has excellent bending strength and twisting strength.

The structure of the door panel using the panel core material of the second embodiment is as shown in FIG. 7, except that the panel core material of the second embodiment is used.
It has the same structure as the door panel shown in FIG. This door panel has a configuration in which the wood members 4 and 4'are arranged at predetermined positions on the peripheral portion, so that it is easy to attach accessories to the peripheral portion of the door panel. In the door panel shown in FIG. 7, as the front plate edge 8'and the back plate edge 9'of the configuration shown in FIG. 6, the back plate edge 9'is inserted inside the front plate edge 8 '. Alternatively, the face plate 6 and the back plate 7 may be fitted together. This door panel can also be made lightweight and has excellent bending strength and twisting strength.

A plan view of the paper honeycomb 1 forming the core material for a panel of the first and second embodiments is as shown in FIG. The paper honeycomb 1 is composed of a flat portion 11 and a corrugated portion 12, and the top and bottom of the corrugated portion 12 are bonded and connected to the adjacent flat portions 11, respectively.
A single-sided corrugated board composed of 11 and the corrugated portion 12 is superposed in the same direction and bonded at a contact point. The shape of the paper honeycomb is not limited to the above-mentioned shape, and may be any honeycomb shape as long as the honeycomb holes are oriented perpendicularly to the paperboard surface. As the wood material 4, wood, LVL (single board laminated board), plywood, laminated wood or the like can be used.

As the paper forming the paper honeycomb,
A kraft liner of 150 to 350 g / m ² and a jute liner mainly made of waste paper can be used, but it is preferable to use a kraft liner because of its excellent strength. By forming a paper honeycomb with a craft liner, a jet liner, or the like having a weight of 150 to 350 g / m ^2, a paper honeycomb having a high compressive strength in the thickness direction can be obtained. As the paperboard 3, a Manila ball,
Paperboard such as white balls can be used. As the thickness of the paperboard grammage can be used those 210~600g / m ^2, the basis weight is preferably used paperboard 400 to 500 g / m ^2. Particularly, the basis weight is 400 to 500 g / m ^2.
By using paperboard such as Manila balls and white balls, it is possible to increase the bending strength and twisting strength of the panel core material, and since the paperboard acts as a cushioning material, unevenness such as unevenness on the paper honeycomb surface may occur. Even if there is unevenness or there is unevenness at the joint between the paper honeycomb and the wood material, the unevenness can be absorbed by the paperboard, so the surface smoothness of the panel core material in which paperboard is laminated on both sides of the paper honeycomb is excellent. Will be things. Therefore, the surface smoothness when a front plate and a back plate made of a thin metal plate are laminated on both surfaces of the core material for a panel to form a door panel can be made extremely excellent.

The paper such as the craft liner forming the paper honeycomb 1, the manila balls and the white balls forming the paperboard 3 may be resin-impregnated paper in which a water-resistant synthetic resin is mixed or impregnated. In this case, a panel core material having high water resistance and strength such as compression, bending, and twisting can be obtained. In addition, paper such as craft liner, Manila balls and white balls that become paperboard, mainly contain water-containing inorganic compounds such as self-extinguishing aluminum hydroxide, magnesium hydroxide, calcium hydroxide, clay, kaolin, talc, and dioxide. A flame-retardant paper prepared by mixing 10 to 80% by weight of titanium or the like as a filler can be used, and in that case, a flame-retardant panel core material can be used.

As the adhesive layer 2, a room temperature curing type epoxy adhesive, a moisture curing type urethane liquid adhesive, a moisture curing type urethane hot melt adhesive, an inorganic adhesive, an ethylene-vinyl acetate copolymer or the like is adhered. Agents can be used. As the adhesive layer 5, a room temperature curing type epoxy adhesive,
A moisture-curable urethane liquid adhesive, a moisture-curable urethane hot melt adhesive, etc. can be used. The adhesive layer 5 preferably has a large tensile elastic modulus. Adhesive layer 2,
The coating amount of No. 5 can be ²⁰ to 300 g / m ² , but is preferably 40 to 200 g / m ² . If the amount of adhesive applied is small, the strength of the produced panel core material cannot be obtained. The adhesive that forms the adhesive layer 2 is applied to both sides of the paper honeycomb 1 or both sides of the wood material 4 and adheres to the surface of the paperboard 3, and the adhesive that forms the adhesive layer 5 is a front plate or a back plate made of a thin metal plate. It is usually applied to the inner surface of and is adhered to the surface of the panel core material.

As the adhesive layer 5, a hot melt type film adhesive can be used. As the hot melt type film adhesive, ethylene-α-olefin copolymer, propylene-α-olefin copolymer, ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, ethylene-acrylic acid ester It is a film composed of a heat-adhesive resin such as a copolymer, an ethylene-methacrylic acid copolymer, an ethylene-methacrylic acid ester copolymer, a partially saponified ethylene-vinyl acetate copolymer, and a polyamide resin. The film adhesive is suitable as a single-layer film made of the above heat-adhesive resin, but a high melting point polyolefin resin having a higher melting point than the above heat-adhesive resin, nylon resin, polyester resin or the like as a core material A multi-layered structure in which the above thermoadhesive resin is laminated on both sides may be used. The thickness of the film adhesive is 30 to 200 μm, preferably 50 to 100.
μm. The melting point of the film adhesive is 70 to 1
Although the thing of 10 degreeC can be used, the thing of 90-100 degreeC is suitable. When a film adhesive is used, it can be laminated simply by sandwiching it between the panel core material and the front plate or back plate made of a thin metal plate, and hot pressing it. The step of applying the adhesive as in the case of using the adhesive can be omitted, and the laminating step can be made efficient. As the hot melt type film adhesive, for example, Cranbetter (Kurashiki Spinning Co., Ltd.), Admer (Mitsui Chemicals), Modic (Mitsubishi Chemical) and the like can be used.

The front plate and the rear plate forming the door panel include a galvanized steel plate, an aluminum plate, an aluminum-zinc alloy plated steel plate, a stainless steel plate, a titanium steel plate,
Alternatively, a metal plate colored with a general paint, a metal plate coated with a vinyl chloride resin, an acrylic film, a fluororesin, or the like is used. The thickness of the front plate or the back plate is preferably about 0.50 mm or less in order to reduce the weight. The front plate is about 0.40 to 0.45 mm, and the back plate is about 0.25 to 0.35 mm. It is preferable to use those of In the case of the door panel of the present invention, even if the thickness of the front plate or the back plate is reduced, the surface smoothness of the core material for the panel is excellent, and therefore the surface of the front plate and the back plate laminated on the surface of the core material for the panel. Since the smoothness is excellent, a door panel with a good design can be obtained.

The method for producing the core material for a panel of the first embodiment is as follows. A room temperature curing type epoxy adhesive or the like is applied on both sides of a paper honeycomb at about 180 g / m ² on one side and a paperboard is applied on both sides. 2 overlap each other with one corner
Align the paper honeycomb with the paperboard at the side,
Insert a partition plate consisting of medium-density fiberboard with a thickness of about 30 mm at every step and stack about 50 steps, 30 mm vertically
Applying a medium-density fiberboard with a thickness of 0.1 to 0.2 kg
The adhesive is hardened by leaving it at a normal temperature for about 24 hours while being pressurized with a pressure of / cm ² . In the case of manufacturing the panel core material of the second embodiment, the adhesive is applied to both sides of the paper honeycomb, and the adhesive is applied to both sides of the wood materials 4 and 4 ′ and arranged on the periphery of the paper honeycomb. It is manufactured by the same method as that of the panel core material of the first embodiment except that paperboard is laminated on both sides of the paper honeycomb and the wood materials 4 and 4 '.

A method for manufacturing a door panel using the core material for a panel of the first and second embodiments is as follows: the front plate 6 having the front plate edges 8 and 8 ′ made of a thin metal plate and the back plate edges 9 and 9. '
The back plate 7 having the above is prepared, the room temperature curing type epoxy adhesive or the like is applied to the inner surfaces of the front plate 6 and the back plate 7, and the panel core material produced above is sandwiched between the front plate 6 and the back plate 7. In this state, the front plate edges 8 and 8'and the back plate edges 9 and 9'are fitted and inserted between the metal partition plates that can be heated by circulating hot water. Stack several layers with metal spacers on the four corners of the plate,
1 at 50 ° C under pressure of 0.5 kg / cm ²
It is prepared by allowing the adhesive to harden by leaving it for about an hour. The thickness of the spacer is set to be about 0.2 mm smaller than the combined thickness of the front surface plate and the back surface plate on which the adhesive is applied with the panel core material sandwiched therebetween.

In the above method for producing a door panel, when a hot-melt type film adhesive is used instead of the room temperature curing type epoxy adhesive or the like, the surface plate edges 8 and 8'made of a thin metal plate are used. A front plate 6 having the above and a back plate 7 having the back plate edges 9 and 9 ′ are prepared, and a film adhesive having a thickness of 70 μm is sandwiched on one surface of the panel core material produced above to make the front plate. While overlapping 6
The back plate 7 is superposed on the other surface of the panel core material with a film adhesive having a thickness of 70 μm sandwiched between them, and the front plate edge 8 with the panel core material sandwiched between the front plate 6 and the back plate 7. , 8'and back plate edges 9, 9'fitted and inserted between metal partition plates that can be heated by a heater, and metal spacers are placed at the four corners of the metal partition plates. It can be produced by stacking several stages, heating at 90 to 120 ° C. for 1 to 5 minutes in a state of being pressurized with a pressure of 0.5 to 1.0 kg / cm ² , and then cooling for 1 to 5 minutes.

[0027]

EFFECT OF THE INVENTION By using a core material for a panel in which paperboard is laminated on both sides of a paper honeycomb with an adhesive, it is possible to make it lightweight and excellent in bending strength and twisting strength. Further, since the paperboard is laminated on both sides of the paper honeycomb, even when a surface plate made of a thin metal plate is laminated on the surface of the paperboard, the paperboard functions as a cushioning material, so that the smoothness of the laminated surface plate is improved. can do.

A wood core material is arranged adjacent to the paper honeycomb, and a paper core is laminated on both sides of the paper honeycomb wall and the wood wood member with an adhesive to form a panel core material, which has excellent strength and surface smoothness. It becomes easy to attach accessories to the peripheral portion of the door panel manufactured using this panel core material.

In the above panel core material, by setting the basis weight of the paperboard to 400 to 500 g / m ² , even when the surface of the panel core material is slightly uneven, the paper honeycomb and the wood material are Even if the boundary becomes uneven, the thick paperboard acts as a cushioning material, so when a door panel is manufactured by laminating a surface plate made of a thin metal plate on both sides of the panel core material, the surface plate The door panel has improved smoothness and excellent design.

By providing a door panel having a structure in which a front plate and a back plate made of a thin metal plate are laminated on both surfaces of the above-mentioned panel core material with an adhesive layer interposed therebetween, surface strength, bending strength, and
It is possible to provide a lightweight door panel having excellent strength such as twisting strength.

In the above door panel, when the adhesive layer is made of a hot-melt type film adhesive, when laminating a front plate and a back plate made of thin metal plates on both sides of the panel core material, Laminating can be done simply by inserting a film adhesive between the panel core material and the front and back plates and hot pressing, so the workability in the laminating process is improved compared to the process of applying a liquid adhesive and laminating. You can do well. Moreover, since the thickness of the adhesive layer can be reduced to form a uniform layer, the surface smoothness of the laminated door panel can be made excellent, and an epoxy adhesive or the like can be used for lamination. It is possible to reduce the cost as compared with the case of doing it.

In the above door panel, the thin metal plate is
When the thickness is 0.25 to 0.50 mm, the weight of the panel can be reduced, and bending strength and twisting can be achieved even when the front plate and the back plate are metal plates having the above thickness. A door panel with excellent strength can be obtained.

In the above door panel, the peripheral edges of the front plate and the rear plate are respectively bent, and the peripheral edges of the front plate and the rear plate are fitted and fixed.
A door panel can be easily manufactured by adhering the front surface plate and the back surface plate to the panel core material and fitting and fixing at the peripheral edge.

[Brief description of drawings]

FIG. 1 is a plan view showing a configuration of a panel core material according to a first embodiment of the present invention.

2 is a cross-sectional view taken along the line II of FIG. 1 in which the central portion is cut away.

FIG. 3 is a plan view showing the configuration of a panel core material according to a second embodiment of the present invention.

FIG. 4 is a sectional view taken along the line II-II of FIG.

FIG. 5 is a sectional view corresponding to FIG. 2, showing a door panel using the panel core material of the first embodiment of the present invention.

FIG. 6 is a cross-sectional view corresponding to FIG. 2, showing another door panel using the panel core material of the first embodiment of the present invention.

FIG. 7 is a sectional view corresponding to FIG. 2, showing a door panel using the panel core material according to the second embodiment of the present invention.

FIG. 8 is a plan view of a paper honeycomb.

[Explanation of symbols]

1 Paper honeycomb 2,5 adhesive layer 3 paperboard 4,4 'wood 6 surface plate 7 Back plate 8,8 'Front plate edge 9, 9'Back plate edge 11 Flat part 12 Wavy section

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2E016 HA09 HA10 KA05 LA01 LB01                       LB11 LB15 LB17 LC05 MA11                       MA25 NA02 NA09 QA13                 4F100 AP01 BA10A BA10C CC00                       DB17 DC02B DG10A DG10B                       DG10C GB07 JK01 JK04                       JK15 JL03 YY00A YY00C

Claims (7)

[Claims] 1. A core material for a panel comprising a paper honeycomb and a paperboard laminated on both sides of the paper honeycomb with an adhesive. 2. A core material for a panel, characterized in that a wood material is arranged adjacent to a paper honeycomb, and a board is laminated on both sides of the paper honeycomb and the wood material with an adhesive. 3. The paperboard has a basis weight of 400 to 500 g / m.
^The core material for a panel according to claim 1 or 2, wherein the core material is 2. 4. A door panel comprising a core plate for a panel according to claim 1 and a front plate and a back plate made of a thin metal plate laminated on both sides of the core layer with an adhesive layer interposed therebetween. . 5. The door panel according to claim 4, wherein the adhesive layer is made of a hot-melt type film adhesive. 6. The thin metal plate has a thickness of 0.25 to 0.
It is 50 mm, The door panel of Claim 4 or 5 characterized by the above-mentioned. 7. A structure in which the peripheral edges of the front plate and the back plate are respectively bent, and the peripheral edges of the front plate and the back plate are fitted and fixed to each other. Door panel according to any one of.