JPH07256674A - Foam and manufacture thereof - Google Patents

Abstract

PURPOSE:To easily manufacture foam large in the degree of freedom by forming a plastic preform so as to have a multilayered structure consisting of a layer containing a foaming agent and a magnetic material and a layer not containing them and applying a high frequency magnetic field thereto to activate a foamed layer. CONSTITUTION:A preform 1 consists of a resin layer la containing a foaming agent 2 and a magnetic material 3 and a resin layer 1b not containing them and is introduced into a high frequency magnetic field 4 to heat the magnetic material 3 in the resin layer 1a by magnetic field induction. The resin 1a' of the resin layer 1a is melted by this heating and the foaming agent 2 is foamed by thermal reaction to form a foamed layer. Since the resin layer 1b is not heated, the preformed shape is held by the resin layer 1b. Therefore, foam large in the degree of freedom, for example, a three-dimensional tubular member or a double pipe member can be easily and efficiently produced and foaming magnification is also easily controlled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plastic molded article having a foam layer and a method for producing the same. The foam article includes a foam layer containing a magnetic material and a non-foam layer for retaining the shape of a molded article. It consists of

Therefore, as a field of application, for example, in the case of a tubular body, it is effective for duct pipes that require noise reduction and duct pipes that require heat insulation and heat retention. Vibration absorption effect, including the body
Ducts that require sound wave absorption and electromagnetic wave absorption effects
Available for pipes. Moreover, even in the case of a flat plate-shaped molded body,
It can be applied as a member (for example, a wall material) that requires sound absorption, heat insulation, heat retention, and vibration absorption, and can also be used as a wall material that is required to have an effect as a radio wave absorber.

Particularly in the case of an automobile duct as an example, it is extremely effective for an intake duct required to eliminate intake noise generated during intake and a heater duct / cooler duct requiring heat insulation / heat retention. is there.

[0004]

2. Description of the Related Art Conventionally, as a method for molding a foam, as a foam-containing method, 1) a foaming agent decomposition method, 2) a gas mixing method, and 3).
There are solvent vaporization method, 4) chemical reaction method, etc., and various molding methods such as a) normal pressure foaming, b) extrusion foaming, c) injection foaming, d) press foaming, e) in-mold foaming. The above method has been put to practical use in combination with the bubble-containing method.

However, in general, many foamed molded products obtained by extrusion molding, calender molding, press molding and the like are in the form of sheets or flat plates, and have a simple shape. In the case of obtaining a complicated foam molded article, there is no choice but to use injection molding with a foaming agent or gas or in-mold foaming by a chemical reaction method.
Its shape was also limited. In particular, when obtaining a hollow tubular foam, extrusion molding or injection molding can be used, but it is difficult to control the molding in extrusion molding, and in injection molding, the shape must be simplified due to the restriction of the mold structure. There were problems such as not getting.

On the other hand, blow molding is a very effective molding method for obtaining a hollow tubular body,
In the case of obtaining a foamed hollow molded article, 1) the parison at the time of molding causes a decrease in melt tension due to foaming, the drawdown becomes large and molding is difficult, and 2) the hollow molding pressure at the time of hollow molding is high. , Foaming of the foam layer is suppressed,
It is not possible to obtain the desired foam that is sufficiently foamed. Further, when the hollow molding pressure is low, the transferability with the mold is deteriorated, the appearance is deteriorated, and the shape formation is insufficient. 3) There are problems such as difficulty in controlling the expansion ratio of the foam, and it has been difficult to obtain a desired foamed hollow molded article.

Further, the conventional foams have been manufactured only for the purpose of the effect as a foam, so that a foam having an effect of absorbing electromagnetic waves has not been provided.

[0008]

In the above-mentioned conventional method, a complicated shape cannot be obtained in extrusion molding, calender molding, press molding or the like for obtaining a desired foam,
There are problems such as molding control and shape restrictions in extrusion molding and injection molding that can obtain a foamed hollow body. Furthermore, in hollow molding that can obtain a hollow body with a complicated shape, a drawdown at the time of molding and a product with a desired expansion ratio There was a problem such as not getting. Therefore, the object of the present invention is to eliminate the above-mentioned conventional drawbacks, to efficiently and stably produce a foam having a desired shape and expansion ratio, and to muffle, heat-insulate and retain heat as a foam. It is another object of the present invention to provide a foam having an effect of absorbing electromagnetic waves and having a complicated shape.

[0009]

In order to achieve the above-mentioned object, the present invention forms, as a pre-molded article, a multi-layer molded article comprising a layer containing a foaming agent and a magnetic material and a layer not containing them. , The preform is placed in a high-frequency magnetic field to heat the magnetic material contained in the foaming agent containing layer, and the heat generated melts the resin in the foaming agent containing layer and decomposes the foaming agent to foam the foaming agent containing layer. By doing so, it is possible to provide a foam having a desired shape and expansion ratio, and having a foam effect and an added effect, and a method for producing the foam.

According to the present invention, since the foaming operation is not performed in the preforming step, the conventional drawbacks associated with the foam molding (eg, the decrease in melt tension and the control of the expansion ratio are difficult to perform in the preforming step). There is no). Therefore, the degree of freedom of molding in the preforming step is large, and various conventional molding methods can be used. For example, any molding method such as blow molding, injection molding, extrusion molding, press molding, and vacuum molding can be used. Furthermore, regarding the shape of the molded product, various molding methods can be used, so not only a simple shape (such as a flat plate shape or a straight cylindrical shape) but also a complicated shape (a bent hollow body, a long product or a structure). (Body etc.) can be easily molded, and there are no restrictions on the shape and structure of the molded product. Taking a hollow molded product as an example, the drawdown due to the decrease in melt tension during molding and the appearance defect due to the decrease in molding pressure during hollow molding are eliminated, and the desired shape and
A molded article having a structure can be obtained.

In the subsequent step of foaming the preform, the preform is placed in a high-frequency magnetic field, and only the foam layer of the preform containing the foaming agent and the magnetic material is melted by the heat generation of the magnetic material and foamed. As such, the non-foamed layer retains its shape during the preforming stage without being melted.
Therefore, the complicated shape formed in the preforming step is maintained until after the foaming step, and a molded article having a shape that cannot be obtained by the conventional foamed molded article can be easily obtained. Specifically, it functions as a duct or pipe in which the central axis of the tubular body is three-dimensionally bent in a hollow tubular molded body and a molded body in which additional parts such as brackets and pipes are added thereto, or a structure. A molded body having a complicated shape can be easily molded.

[0012] It is extremely easy to control the expansion ratio and the cell diameter by foaming in the foaming stage because foaming is carried out under normal pressure conditions, and it is also easy to obtain a high expansion ratio molded product that could not be obtained by conventional in-mold foaming. Can be obtained. By controlling the expansion ratio and bubble diameter, it is possible not only to control the effects of heat insulation, sound absorption, vibration absorption, etc., but also by controlling the content of the magnetic substance in the foam layer, the electromagnetic waves generated by the magnetic substance can be controlled. The effects such as absorption can also be controlled. Further, the effect of absorbing the transmitted sound by the high-mass magnetic material is also added, and the sound absorbing characteristic can be improved.

Further, since the foamed layer and the non-foamed layer are integrally formed in the preforming step, good adhesion is maintained even in the foaming step, and the foamed layer and the non-foamed layer are well adhered. A multilayer molded body is obtained. In addition, a multilayer body can also have a heat insulating effect and a sound absorbing effect.

[0014]

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIGS. 1 (a) to 1 (c) show a first embodiment of the present invention, and FIG. 1 (a) is a schematic view showing a cross section of a preform,
FIG. 1 (b) is a schematic view showing a state where a preform is put in a high frequency magnetic field, and FIG. 1 (c) is a schematic view showing a cross section of the foamed body foamed by the high frequency magnetic field.

In FIG. 1 (a), a preformed body 1 is a molded body composed of a resin layer 1a containing a foaming agent 2 and a magnetic material 3 and a resin layer 1b containing no foaming agent and a magnetic material. The foaming agent 2 and the magnetic material 3 are kneaded in the plasticizing step of the resin 1a 'in the preforming step to form the resin 1a' constituting the resin layer 1a.
It is contained by being dispersed therein. The containing layer resin 1a 'and the non-containing layer resin 1b' are multi-layered by a method such as coextrusion and shaped into a desired molded body shape in a molding die or an extrusion die. A preform 1 made of 1b is formed. Contained layer resin 1a 'and non-contained resin 1
The b's may be the same resin, or different resins as long as they have good compatibility and adhesion.

FIG. 1B is a schematic view showing a state in which the preform 1 is placed in the high frequency magnetic field 4. In FIG. 1 (b), the preform 1 placed in the high-frequency magnetic field 4 has a magnetic substance 3 contained in the resin layer 1 a of the preform 1 having a hysteresis loss due to induction of the high-frequency magnetic field 4 and an eddy current. The magnetic material 3 is heated by the contained resin layer 1a
The content layer resin 1a ′ therein is melted, and the content layer resin 1a ′ is further heated by heating the magnetic body 3 and melting the content layer resin 1a ′.
The foaming agent 2 therein foams by thermal decomposition or reaction, and the contained resin layer 1a forms a foamed layer. Since the non-containing resin layer 1b containing no magnetic material is not heated in the high frequency magnetic field 4, the preformed shape is maintained by the non-containing resin layer 1b.

FIG. 1 (c) is a schematic view showing a cross section of the foamed molded product after the foaming step in the high frequency magnetic field shown in FIG. 1 (b). The foamed molded body 5 is the non-foaming non-containing resin layer 1
Then, a multi-layer body including b and the contained resin layer 11a foamed in the high frequency magnetic field is formed. The foamed resin layer 11a is
It is composed of a foamed portion (bubble) 22 generated by foaming of the foaming agent, the magnetic body 3, and the containing layer resin 1a '. For the amount of foaming of the foaming part (bubbles) 22, the diameter of the bubbles, the expansion ratio of the foamed layer that has been foamed, etc., control the type and amount of the foaming agent, the type of resin, the heating conditions (magnetic substance content, etc.), etc. Controlled by.

2 and 3 (a) to 3 (d) are the second embodiment of the present invention, FIG. 2 is a schematic view of the hollow product (duct) of the second embodiment, and FIG. 3B is a partial cross-sectional schematic view of the non-foamed duct of FIG. 3, FIG. 3B is a partial cross-sectional schematic view of the foamed duct of FIG. 2, FIG. 3C is a schematic cross-sectional view of the bracket portion of the non-foamed duct of FIG. 2, and FIG. 3 is a schematic sectional view of a bracket portion of the foam duct of FIG. 2.

The hollow duct shown in FIG. 2 is an example formed by blow molding. In the hollow duct 6 shown in the schematic outline view of FIG. 2, a preformed body is molded by hollow molding to give a desired shape. The bracket 7 is integrally formed with the hollow duct 6 by insert molding during hollow molding of the preform. In the preformed body of the hollow duct 6, the tubular body portion has a resin layer (containing resin layer) 6a containing a foaming agent and a magnetic material and a resin layer (a resin layer containing no foaming agent and a magnetic material) as shown in the sectional view of FIG. The non-containing resin layer 6b is a molded body, and the bracket added portion is mechanically and thermally added to the non-containing resin layer 6b as shown in the sectional view of FIG. 3 (c). The preform molded by blow molding is then placed in a high frequency magnetic field to foam the contained resin layer 6a. 3 (b) and 3 (d) are schematic views showing a cross section of the duct after the foaming process in the high frequency magnetic field. In the foaming step in the high-frequency magnetic field, only the contained resin layer 6a containing the magnetic material is heated, so that the non-containing resin layer 6b maintains the shape of the preform molded by hollow molding. The resin layer (containing resin layer) 6a containing the foaming agent and the magnetic substance shown in FIGS. 3A and 3C is subjected to a foaming step in a high frequency magnetic field, and then the resin layer 6a in FIG.
The foam layer 66a shown in (b) and FIG. 3 (d) is formed.

As shown in the second example of the present embodiment, in the present invention, the additional parts such as the bracket can be integrally formed with the non-foamed resin layer, so that the bracket and other parts necessary for holding the duct can be provided. A functional part such as a holder for holding can be easily provided. Further, since the additional component is integrally formed with the non-foamed resin portion, it has mechanical strength and is highly reliable.

FIGS. 4A to 4D show a third embodiment of the present invention. FIG. 4A is a schematic external view of the resonator of the third embodiment, and FIG. 4B is a schematic sectional view thereof. 4 (c) is a schematic partial cross-sectional view of an unfoamed resonator, and FIG. 4 (d) is a schematic partial cross-sectional view of a foaming resonator.

In FIG. 4 (a), the resonator 8 is composed of a flow passage tube body 9 forming a gas flow passage and a resonance part 10, and has a double pipe structure as shown in the schematic sectional view of FIG. 4 (b). have. The flow path tube body 9 is a resin tube body that does not contain a foaming agent and a magnetic material and is formed by injection molding, hollow molding, or the like.
As shown in (b), it has a resonance port 9a. A resonator having a double tube structure shown in FIG. 4B is disclosed in Japanese Patent Laid-Open No. 4-82.
No. 13, etc., that is, the flow channel tube body 9
Is held at a predetermined position, a parison forming the resonance part 10 is extrapolated to the parison, and then the mold is clamped, and the parison is blown through the flow passage tube 9 and the resonance port 9a to define the resonance part. It is molded by a method of forming 10 shapes or the like.
In FIG. 4 (b), the unfoamed resonator, which is a preform, has a resonance part 1 as shown in the partial cross-sectional schematic view of FIG. 4 (c).
0 is a resin layer containing a foaming agent and a magnetic material (contained resin layer) 10
The flow path tube body 9 is a resin molded body that does not include a foaming agent and a magnetic substance, and is a multilayer molded body that includes a and a resin layer (non-containing resin layer) 10b that does not include a foaming agent and a magnetic substance. Figure 4 (d)
FIG. 4 is a schematic partial cross-sectional view of a foaming resonator after a pre-molded body, an unfoamed resonator, is placed in a high-frequency magnetic field and a foaming process is performed. The flow path tube body 9 and the non-containing resin amount 10b, which are the resin molded body portion not containing the foaming agent and the magnetic body, maintain the shape of the preformed body. The resin layer (containing resin layer) containing the foaming agent and the magnetic material of the resonance part 10 forms the foam layer 10a '.

In the third example of the present embodiment, the resonance part 10 is a multilayer molded body having a foam layer, but the flow path tube body 9 may be a multilayer foam molded body. That is, after preliminarily molding the flow channel tube body 9 as the preformed body of the present invention, in the hollow molding method, the tube body 9 is held at a predetermined position, the parison forming the resonance portion is externally inserted therein, and then the mold is clamped. And the flow channel tube 9
The parison is blown through the resonance port 9a to form the shape of the resonance part 10 defined by the mold, and the resonator having the double tube structure is obtained. By subjecting this to a foaming step in a high frequency magnetic field to foam the flow channel tube body 9 which is a preformed body, a resonator having a double tube structure in which the flow channel tube body portion is foamed can be obtained.

FIGS. 5 (a) and 5 (b) show a fourth embodiment of the present invention in which a foam layer is sandwiched between non-foam layers. FIG. 5A is a schematic sectional view of an unfoamed preform. FIG. 5B is a schematic sectional view of the foamed molded body.

In FIG. 5 (a), a preform 11 includes a resin layer (containing resin layer) 11a containing a foaming agent and a magnetic material and a resin layer (non-containing resin layer) 11 containing no foaming agent and a magnetic material.
It is a multi-layer molded body composed of b and 11c. The resin layer (non-containing resin layer) 11b containing no foaming agent and magnetic substance is a resin that does not thermally deform due to heat generation and melting of the resin layer (containing resin layer) 11a containing a foaming agent and a magnetic substance. It has a thickness and retains the product shape without being thermally deformed in the foaming process in a high frequency magnetic field. The other resin layer (non-containing resin layer) 11c containing no foaming agent and magnetic substance has a thickness or thermal property that can follow the foaming of the resin layer (containing resin layer) 11a containing a foaming agent and magnetic substance. Composed of resin. The pre-molded body 11 is a molded body molded by a molding method capable of multi-layer molding and has at least a three-layer structure.

FIG. 5 (b) shows the preform 1 of FIG. 5 (a).
1 is a schematic view showing a cross section of a foamed molded body 11 'after foaming 1 by subjecting 1 to a foaming step in a high frequency magnetic field. The foamed layer 11a 'formed by foaming the resin layer (containing resin layer) 11a containing the foaming agent and the magnetic substance is formed between the non-foamed layer 11b and the non-foamed layer 11c. The non-foamed layer 11b maintains the shape before the foaming step and retains the shape of the product. The non-foamed layer 11c keeps the non-foamed layer while following the change in the thickness of the foamed layer 11a 'due to foaming. Following the foaming of the non-foaming layer 11c can be achieved by reducing the layer thickness or forming the layer with a resin having a low melting temperature.

The fourth example of the present embodiment is effective in avoiding the unevenness of the foamed surface caused by foaming, and for example, in the case where the flow resistance is a problem in a duct, hose, pipe or the like forming a fluid flow path. It is valid. If the duct of the second embodiment of the present invention has the structure of the fourth embodiment of the present invention,
As a matter of course, a non-foamed layer is formed on the inner surface of the flow path, and a duct with less flow resistance can be obtained.

FIGS. 6 (a) and 6 (b) show a fifth embodiment of the present invention in which a foam layer is sandwiched between a non-foam layer and a magnetic material-containing layer. FIG. 6A is a schematic sectional view of an unfoamed preform. FIG. 6B is a schematic sectional view of the foamed molded body.

In FIG. 6 (a), the preform 12 is a resin layer containing a foaming agent and a magnetic material (foaming agent / magnetic material-containing resin layer) 12a and a resin layer containing no foaming agent and a magnetic material (non-containing resin). It is a multilayer molded body comprising a layer 12b and a resin layer (magnetic material-containing resin layer) 12c containing a magnetic material, and a resin layer (non-containing resin layer) 12b containing no foaming agent and magnetic material contains a foaming agent and a magnetic material. The resin layer (foaming agent / magnetic substance-containing resin layer) 12a and the resin layer (magnetic substance-containing resin layer) 12c containing a magnetic substance have a resin or a thickness that does not cause thermal deformation with respect to heat generation and melting. The preform 12 retains the shape of the product without being thermally deformed in the foaming process in a high-frequency magnetic field, and is formed by a molding method capable of multi-layer molding and has at least a three-layer structure.

FIG. 6 (b) shows the preform 1 of FIG. 6 (a).
2 is a schematic view showing a cross section of the foamed molded body 12 'after the foaming process is performed by placing No. 2 in a foaming process in a high frequency magnetic field and foaming. Resin layer containing foaming agent and magnetic substance (foaming agent / magnetic substance-containing resin layer) 12
The foam layer 12a 'formed by foaming a is formed between the non-foam layer 12b and the non-foam layer 12c. The non-foamed layer 12b containing no magnetic material maintains the shape before the foaming step and maintains the shape of the product. Non-foaming layer 12 containing magnetic material
c keeps the non-foamed layer while following the change in the thickness of the foamed layer 12a 'due to foaming. The non-foamed layer 12c containing the magnetic material is melted by heat generation of the magnetic material in the foaming step in the high frequency magnetic field, and facilitates foaming of the foamed layer 12a '. Further, since the non-foamed layer 12c containing a magnetic material forms a magnetic material layer having a high density, the electromagnetic wave absorbing effect is improved. Particularly, in this embodiment, since the sparse magnetic material layer is formed in the foamed layer 12a 'and the dense magnetic material layer is formed in the non-foamed layer 12c, the effect as the electromagnetic wave absorber is not only improved as compared with the single layer. Instead, the absorption wave band can be adjusted by changing the type of magnetic material added to the foamed layer 12a 'and the non-foamed layer 12c.

The fifth example of the present embodiment is effective for a foam which needs an electromagnetic wave absorbing effect, and can be particularly used for a wall surface which needs not only an electromagnetic wave absorbing effect but also a sound absorbing effect and a heat retaining effect.

The foaming agents usable in the above examples are:
Any foaming agent that thermally decomposes or reacts may be used. For example, taking an organic foaming agent as an example, ADCA, ABFA, D
PT, THT, OBSC, TSC, etc. are effective. Of course, it is possible to use a foaming aid, and by using the foaming aid together, desired foaming temperature, foaming ratio, foaming diameter (cell diameter), etc. can be easily controlled.

Further, taking a magnetic material as a heat source as an example, any magnetic material having a particle diameter that can be kneaded with a thermoplastic resin may be used, and for example, it can generate heat in a high frequency magnetic field such as magnetite, ferrite or iron. Any magnetic powder can be used.

The present invention, as shown in the above examples, is a layer containing a foaming agent and magnetic powder as a preform molded by a molding method such as a hollow molding method, an extrusion molding method or an injection molding method. And a multi-layered compact consisting of a layer not containing them, and then the preform is placed in a high frequency magnetic field to heat the magnetic powder contained in the foaming agent-containing layer, and the heat generation causes the foaming agent-containing layer to By melting the resin and decomposing the foaming agent to foam the foaming agent-containing layer,
The present invention provides a foam having a desired shape and expansion ratio and having a foam effect and an added effect, and a method for producing the foam. The above-mentioned examples are examples of embodiments of the present invention. It is needless to say that foams having various modes other than the above-mentioned embodiment are possible, and modes having various combinations of the foaming agent / magnetic material are also possible.

[0036]

The effects of the present invention are as follows.

In the plastic molding having a foam layer and the method for producing the same, (1) a foam having a large degree of freedom in shape (for example, a three-dimensional tubular body, a double tubular body or a functional structure) is easily and efficiently produced. can get.

(2) A foam having a vibration absorbing effect and an electromagnetic wave absorbing effect in addition to the sound deadening effect and the heat insulating effect as the functions of the foam can be obtained.

(3) A foam is obtained in which a function-added component such as a bracket or a pipe is added to the molded body.

(4) Not only various molding methods can be used for molding the foam, but also the foaming ratio and bubble diameter of the foam can be easily controlled.

[Brief description of drawings]

1A, 1B, and 1C are schematic sectional views and conceptual views showing each stage of the first embodiment.

FIG. 2 is a schematic external view of a hollow duct according to a second embodiment.

3 (a) and 3 (b) are schematic views showing a cross section of a tubular body portion of a second embodiment. (C), (d) is a schematic diagram showing a section of a bracket part of a 2nd example.

4 (a), (b), (c), and (d) are a schematic external view and a schematic sectional view showing a resonator of a third embodiment.

5 (a) and 5 (b) are schematic views showing a cross section of a three-layer molded body example (1) of the fourth embodiment.

6 (a) and 6 (b) are schematic views showing a cross section of a three-layer molded body example (1) of the fifth embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Preformed body 2 Foaming agent 3 Magnetic body 4 High frequency magnetic field 5 Foamed molded body 6 Hollow duct 7 Bracket 8 Resonator 9 Flow path pipe body 10 Resonance part 11 Preformed body 12 Preformed body

Claims (14)

[Claims] 1. A method for producing a plastic foam, wherein at least one layer of a plastic preform comprising at least two layers contains a foaming agent and a magnetic material, and at least another layer does not contain a foaming agent. A plastic preform consisting of layers is formed, the preform is allowed to exist in a high frequency magnetic field, and the magnetic substance is heated by hysteresis loss and eddy current loss of the magnetic substance generated by the high frequency magnetic field. A method for producing a plastic foam, which comprises melting a foaming agent-containing plastic portion and decomposing the foaming agent by heat generation of a magnetic material to form a foamed layer. 2. The method for producing a plastic foam according to claim 1, wherein the preform is formed by hollow molding, injection molding, extrusion molding, press molding, or vacuum molding. 3. The method for producing a plastic foam according to claim 1, wherein the foaming agent is a pyrolyzable foaming agent that generates gas by thermal decomposition. 4. The method for producing a plastic foam according to claim 1, wherein the magnetic material is magnetic powder of magnetite, ferrite, iron or the like. 5. The method for manufacturing a plastic foam according to claim 1, wherein the high-frequency magnetic field is applied by electromagnetic induction. 6. The method for producing a plastic foam according to claim 1, wherein the plastic containing the foaming agent and the magnetic material is a thermoplastic. 7. A plastic foam molded article formed of at least two layers, at least one layer forming a foam layer, at least another layer consisting of a non-foam layer, and at least the foam layer containing a magnetic material. A plastic foam molded article, which is formed by a body. 8. The plastic foam molding according to claim 7, wherein the plastic forming the foam layer is a thermoplastic. 9. The plastic foam according to claim 7, wherein the magnetic material is magnetic powder of magnetite, ferrite, iron or the like. 10. The plastic foam according to claim 7, wherein the foam layer is a foam layer foamed mainly by heat generated by a magnetic material heated by electromagnetic induction. 11. The plastic foam according to claim 7, wherein the plastic foam is a tubular molded body. 12. The plastic foam according to claim 11, wherein the plastic foam layer forms an inner layer of a tubular molded body. 13. The plastic foam according to claim 11, wherein the central axis of the tubular molded body is three-dimensionally bent. 14. The plastic foam according to claim 11, wherein an additional component such as a bracket or a pipe is added to the outer layer portion of the tubular molded body.