JP2005104998A - Manufacturing method of foam - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a foam satisfying both a density as low as 0.2 g/cm<SP>3</SP>or less and a shrinkage factor as low as 3% or less by incorporating a balloon with a thermoplastic resin which forms a framework of the foam and causing a supercritical fluid to be dissolved therein. <P>SOLUTION: In the method for manufacturing the foam from the necessary thermoplastic resin, the thermoplastic resin and the balloon are mixed at a volume percentage of 80:20-15:85 to give a mixture. The supercritical fluid is dissolved in the mixture under a specified pressure at a temperature not higher than the respective melting points of the thermoplastic resin and the balloon, and the mixture having the supercritical fluid dissolved therein is kept under a reduced pressure to give the foam having a density of at most 0.2 g/cm<SP>3</SP>and a shrinkage factor of at most 3% after the manufacture. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing a foam, and more specifically, in order to obtain a foam structure with a thermoplastic resin as a skeleton, a balloon made of resin or the like is mixed and a supercritical fluid is dissolved in the balloon mixture. Thus, the present invention relates to a method for producing a foam that can achieve low density and dimensional accuracy after production, that is, suppression of shrinkage.

  There are various manufacturing methods, such as (1) chemical foaming method, (2) mechanical stirring method, and (3) extraction method, when manufacturing foams with various thermoplastic resins as the skeleton. A suitable manufacturing method is appropriately selected from the above. In particular, as a method for producing a foam having a low density, (4) a production method in which various fine hollow materials, so-called balloons, are mixed with a thermoplastic resin forming a skeleton to form the balloon as a cell; (5) A method for producing a foam by dissolving a supercritical fluid having both characteristics as a liquid that exhibits high permeability to an individual and characteristics as a gas capable of forming a cell and reducing the pressure is known. It has been.

However, the following problems are pointed out when the foam is produced by the production method (4) or (5) described above. That is,
In the case of the method described in (4), the balloon is basically mixed and kneaded with a thermoplastic resin as a base material for forming a skeleton, and sufficiently dispersed to form a structure as a foam. Suitable dispersibility directly affects the cellular structure of the foam. Therefore, when a structure in which cells are uniformly dispersed is desired as a foam, it is necessary to sufficiently mix and knead the balloon. However, when the balloon is basically mixed with the thermoplastic resin, the balloon acts as a so-called thickening material such as a filler, so that the mixing ratio of the balloon is increased in order to obtain a low-density foam. The dispersibility will deteriorate. In order to avoid this, the driving force for mixing and kneading the thermoplastic resin and the balloon may be set large. However, in this case, the balloon having the hollow structure is crushed by an excessive driving force, and the low density that should be achieved by the balloon cannot be achieved by the presence of the balloon itself. That is, it is empirically known that when the density is reduced by increasing the mixing ratio of the balloon, it is difficult to achieve 0.3 g / cm ³ or less due to the thickening action of the balloon. In addition, the balloon is a method for easily achieving a low density, but its unit price is high, and when it is used in a large amount to reduce the density, there is a problem that the manufacturing cost increases.

  In the case of the method described in (5), the density can be easily set by appropriately setting conditions such as the dissolution amount of the supercritical fluid. In this method, after the supercritical fluid is dissolved in a thermoplastic resin as a base material that forms a skeleton under a pressure capable of maintaining the state, the gas obtained by vaporizing the supercritical fluid by being decompressed is a cell. To form a foam structure. In the case of such a production method, there is a problem that the obtained foam shrinks, and the cell shape is gradually deformed, or the cell becomes small, and a sufficient expansion ratio cannot be obtained.

In order to overcome the above-mentioned problems and achieve the intended object, a method for producing a foam according to the present invention is a method for producing a foam from a required thermoplastic resin.
The thermoplastic resin and balloon are mixed at a volume percentage of 80:20 to 15:85 to make a mixture,
Dissolving a supercritical fluid in the mixture under a predetermined pressure at a temperature below the melting point of each of the thermoplastic resin and the resin balloon;
Depressurizing the mixture in which the supercritical fluid is dissolved,
A foam having a density of 0.2 g / cm ³ or less and a shrinkage rate of 3% or less after manufacture is manufactured.

As described above, according to the foam manufacturing method of the present invention, 0.2 g / cm ³ can be obtained by manufacturing a foam using a hollow balloon and a supercritical fluid such as carbon dioxide. It is possible to produce a foam having both a low density of the following and a high dimensional accuracy after the production by achieving a low shrinkage of 3% or less. Further, by using various resins as the balloon forming material and extracting and removing the forming material from the foam, it is possible to easily produce a foam having enhanced air permeability, that is, a degree of communication, in addition to the above-described excellent physical properties.

Next, the method for producing a foam according to the present invention will be described below with reference to the accompanying drawings by way of preferred examples. The inventor of the present application, when producing a foam based on a thermoplastic resin, mixes a thermoplastic resin and a so-called balloon having a hollow structure at a predetermined mixing ratio, and a predetermined mixture is obtained with respect to the obtained mixture. By injecting and dissolving the supercritical fluid with pressure and then reducing the pressure, the density is as small as 0.2 g / cm ³ or less, and the shrinkage rate of the obtained foam is 3% or less. It has been found that a high foam can be obtained. Further, various resins may be employed as the balloon forming material, and the foam forming material may be communicated by extracting and removing the balloon forming material using a solvent.

  As shown in FIG. 1, the foam according to the present invention comprises a mixture of a thermoplastic resin and a balloon made of a resin as a raw material, mixed and kneaded using a predetermined device, and heated. Mixing step S1 and supercritical fluid dissolving step S2 for dissolving the supercritical fluid while putting the pressure in a molding die 30 having an inner contour shape that matches the outer contour shape of the foam and setting the pressure or the like to a predetermined value. Then, the mold 30 is manufactured under control by performing the decompression foaming step S3 in which the mold 30 is opened and decompressed to make the mixture into a foam, and the final step S5. In this embodiment, prior to the final step S5, an extraction step S4 is performed in which the foam is immersed in a predetermined solvent to extract and remove the balloon forming material. Here, as the balloon forming material, the extraction step S4 is performed by extracting and removing the resin by using a solvent. However, glass or the like is used as the balloon forming material and extraction is performed. You may make it not implement step S4.

  An example of the manufacturing apparatus which suitably implements the above manufacturing method is shown in FIG. The production apparatus 20 accommodates a molding die 30 therein, and mixes and kneads to produce a pressure vessel 22 that can be hermetically closed by an opening / closing lid 23 and a mixture 16 that is accommodated in the molding die 30 and foamed. It is basically composed of the mechanism 21. The pressure vessel 22 stores the mixture 16 including the required thermoplastic resin 12 and the balloon 14, and dissolves in the mold 30 that obtains the foam from the mixture 16 and the mixture 16 stored in the mold 30. The supercritical fluid is basically composed of a supercritical fluid supply source 24 that supplies a supercritical fluid via a pipe line 26 and a valve body 27. The mold 30 is configured as a box-like structure that can internally define a cavity 34 that regulates the outer shape of the foam and can withstand a required internal pressure. The mold 30 has a lid member 36, the upper opening of which can be opened and closed by the lid member 36, and an appropriate number of fluid introduction holes 38 and fluid escape holes 40 are formed. Note that the fluid introduction hole 38 and the fluid escape hole 40 are merely through holes that communicate the inside and outside of the mold 30, and the fluid introduction hole 38 is used for fluid escape or fluid escape. Relative in the sense that the holes 40 may be used for fluid introduction. The pressure vessel 22 includes a heating means (not shown) such as an electric heater so that the temperature in the vessel 22 can be controlled within a predetermined range.

(About mixing step S1)
The mixing ratio of the thermoplastic resin 12 and the balloon 14 performed in the mixing step S1 is set to 80:20 to 15:85 in volume percentage. When the mixing ratio of the thermoplastic resin 12 exceeds 80% by volume, that is, when the mixing ratio of the balloon 14 is less than 20% by volume, the density of the obtained foam exceeds 0.2 g / cm ³ Later shrinkage exceeds 3%. On the other hand, when the mixing ratio of the thermoplastic resin 12 is less than 15% by volume, that is, when the mixing ratio of the balloon 14 exceeds 85% by volume, the mixing / shearing force when the mixture 16 is produced increases. Will be crushed by the mixing / shearing force, and as a result, the density of the foam cannot reach 0.2 g / cm ³ or less regardless of the mixing ratio. In addition, about the mixing of the thermoplastic resin 12 and the balloon 14, when the viscosity is high and suitable mixing is difficult, you may perform premixing to such an extent that the balloon 14 is not crushed before mixing.

  Examples of the mixing and kneading mechanism 21 for mixing and kneading the thermoplastic resin 12 and the balloon 14 include a single-screw or twin-screw extruder, a pressure kneader, a kneader, a kneader, a Banbury mixer, a Henschel mixer, a rotor mixer, and the like. A kneading machine or the like that can uniformly disperse the balloon 14 in the thermoplastic resin 12 is preferably used. The temperature at the time of kneading is appropriately set depending on the heat melting point of the thermoplastic resin 12 to be used (both the thermoplastic resin 12 and the balloon 14 when the balloon 14 is made of various resins).

  The thermoplastic resin 12 forms a base material of the foam according to the present invention, that is, a skeleton portion. In addition to the olefin resin containing polyethylene (PE (LD-PE, HD-PE, LL-PE)), the TPE (Polyester, polyether, polyether polyester, styrene and polyamide, etc.), PP and TPO), TPU, polyamide, polyimide or polyacetal and any other resin that melts when heated Can also be used. Further, the balloon 14 is dispersed inside the thermoplastic resin 12 serving as a foam skeleton to reduce the density of the foam and increase the structural strength of the foam itself by its strong outer shell. In the reduced-pressure foaming step S3, the mixture 16 plays a role of suppressing shrinkage caused by a decrease in structural strength when the mixture 16 is foamed by vaporization of the supercritical fluid. For example, a commercially available resin balloon manufactured as a hollow structure using a resin such as an acrylic resin as a forming material, or a glass balloon manufactured from a glass material is used. When the forming material of the balloon 14 is made of a resin, the good solvent used for the extraction and the thermoplastic resin 12 so that the material can be suitably extracted and removed from the foam after molding. It is preferable to use a substance that is different from the good solvent. As described above, when the thermoplastic resin 12 is α-olefinated polyethylene and the forming material of the balloon 14 is an acrylic resin, suitable extraction and removal can be achieved by using n-dimethylacetamide (DMAC) as a solvent. Is done.

(About supercritical fluid dissolution process S2)
In the supercritical fluid dissolving step S2, the mixture 16 obtained from the thermoplastic resin 12 and the balloon 14 is charged into and stored in the mold 30 from the upper opening (see FIG. 3A). As a form of the mixture 16 at this time, it is preferable that the mixture 16 is preliminarily molded into a block shape having a volume smaller than the cavity 34 of the mold 30 by, for example, hot pressing. In this case, the molding shape of the block-like mixture 16 accommodated in the cavity 34 is similar to the inner shape of the cavity 34. For example, if the inner contour shape of the cavity 34 is a rectangular box shape, the mixture 16 is also compatible. In particular, it is more preferable if it is box-shaped and the mixture 16 is conical if the cavity 34 is conical.

  Then, the molding die 30 containing the mixture 16 is accommodated in the pressure vessel 22 and further closed by a lid member 36 to be in a sealed state, and the valve body 27 is opened, so that carbon dioxide is supplied from the supercritical fluid supply source 24. A supercritical fluid of such a material is introduced into the pressure vessel 22 (see FIG. 3B). At the initial stage when the supercritical fluid is introduced into the pressure vessel 22, the critical pressure of the supercritical fluid temporarily falls to a gaseous state, but the supercritical fluid is continuously supplied into the vessel 22. The critical pressure is reached and the fluid is filled with the supercritical fluid.

  The carbon dioxide as the supercritical fluid thus introduced into the pressure vessel 22 enters the mold 30 through the fluid introduction / escape holes 38 and 40 of the mold 30 housed in the pressure vessel 22, and enters the inside. In contact with the mixture 16 contained therein. As described above, the supercritical fluid has a density higher than that of gas and a viscosity lower than that of liquid, so that the supercritical fluid that is in contact with the mixture 16 easily penetrates into the mixture 16. . At this time, it is preferable for the high-efficiency dissolution / penetration of the supercritical fluid that the pressure vessel 22 is heated by the heating means described above through the mold 30 for high-efficiency dissolution / penetration of the supercritical fluid. It is necessary that the melting point of the thermoplastic resin 12 or the balloon 14 forming Preferably, the temperature ranges from the melting point of the thermoplastic resin 12 or the balloon 14 forming the mixture 16 to the melting point -20 ° C.

  As a substance used as a supercritical fluid, carbon dioxide having a critical temperature and a critical pressure close to normal temperature and normal pressure is preferable. Here is a brief explanation of supercriticality. When the temperature and pressure of a substance that generates a gas phase and a liquid phase are increased, the gas phase and the liquid phase are distinguished in a certain temperature range and pressure range. A supercritical state that cannot be obtained is generated. The temperature and pressure at this time are called the critical temperature and the critical pressure, respectively. In the supercritical state, the substance has both gas and liquid characteristics, and the fluid generated in this state is called the supercritical fluid. This supercritical fluid has characteristics that it has a higher density than a gas and a viscosity lower than that of a liquid, so that it can easily diffuse through a substance. Carbon dioxide becomes a supercritical state at a temperature of 31.2 ° C. or higher and a pressure of 7.47 MPa or higher, and exhibits a behavior as a fluid. Note that, at the initial stage when the supercritical fluid is introduced into the pressure vessel 22, the inside of the pressure vessel 22 is at a normal pressure, so that the critical pressure of the supercritical fluid temporarily drops and immediately before the supercritical state. The “subcritical state”, “critical state”, and “normal state” of FIG. 2 become gaseous carbon dioxide, but as described above, the supercritical fluid is rapidly supplied into the pressure vessel 22 as described above. The critical pressure is reached.

In the present invention, the pressure for dissolving the supercritical fluid is set to 20 MPa or less, preferably 15 MPa or less. When this pressure exceeds 20 MPa, the degree of crushing of the balloon 14 with a hollow structure increases, and the mixing ratio with the thermoplastic resin 12 is set within a predetermined range, and the crushing of the balloon 14 is suppressed by the force applied by mixing and kneading. Even so, the density of the finally obtained foam will not be 0.2 g / cm ³ or less. In the case of the balloon 14 made of resin, since the material is soft, the pressure is crushed from about 7.2 MPa, but if the pressure is 15 MPa or less, the density of 0.2 g / cm ³ or less described in the present invention is achieved. I know from experience that this is possible. In the present embodiment in which carbon dioxide is used as the supercritical fluid, the pressure is set to be at least the critical pressure of carbon dioxide of 7.4 MPa or more.

  The dissolution pressure of the supercritical fluid is the above-mentioned numerical value in relation to the balloon 14, but this pressure has a great influence on the size of bubbles formed from the supercritical fluid, that is, the cell diameter of the foam obtained. From this point, the numerical value of the pressure is controlled. Specifically, the dissolution pressure of the supercritical fluid is about 300 μm at 7 MPa and about 100 μm at 20 MPa. In addition, the dissolution pressure is also limited by the pressure reduction rate in the pressure reduction foaming step S3 described later ([0020]). That is, in the present invention, the depressurization speed is set to 0.6 MPa / second or less. Therefore, when the pressure is high, the depressurization time becomes long. In this case, since the production efficiency is lowered due to the prolonged decompression time, about 20 MPa or less, which completes decompression in about 1 minute, is preferable.

Further, the amount for dissolving the supercritical fluid is controlled by the time for dissolving the supercritical fluid. In the case of the present invention, carbon dioxide is used as a supercritical fluid as in this example, and the pressure is set to 7.4 MPa or more and set to 1 hour or more, whereby a density of 0.2 g / cm ³ or less is obtained. Can be achieved. The density further decreases as the dissolution time becomes longer, but the amount of supercritical fluid permeated from the foam during foaming under reduced pressure also increases. Therefore, the shrinkage of the foam can be suppressed even by the presence of the balloon 14. It has been confirmed that the limit is about 20 hours. Also, from the viewpoint of improving productivity, it is not preferable to dissolve the supercritical fluid for a long time.

(About reduced pressure foaming process S3)
The reduced pressure foaming step S3 is a step of gradually opening the pressure vessel 22 to the atmosphere by reducing the pressure at a predetermined speed to obtain a foam from the mixture 16 (see FIG. 3C). In this reduced pressure foaming step S3, the supercritical fluid in the mold 30 escapes from the fluid introduction / escape holes 38 and 40. When the pressure is thus released, carbon dioxide, which is a supercritical fluid dissolved in the mixture 16, undergoes a phase change (vaporization) into a normal gas (hereinafter referred to as a foaming gas). Along with the vaporization of the supercritical fluid, the mixture 16 expands and foams to generate extremely fine bubbles inside, and also expands in volume, and its outer shape is regulated by the cavity 34 of the mold 30.

  In addition, since the gas that changes phase from the supercritical fluid by decompression in this decompression foaming step S3 is basically generated randomly, the generation time is not limited. As a result, the vaporization time of the supercritical fluid, that is, vaporization, The size of the generated bubbles is not constant. However, in the case of the present invention, a large amount of the phase-changed gas from the supercritical fluid is generated at once using the balloon 14 present in the mixture 16 as a core material. The cell diameter is also characterized by being easily homogenized. The size of the bubbles generated from the gas, that is, the cell diameter of the foam is proportional to the size of the balloon 14 serving as a core material. It has been confirmed that the number of bubbles to be generated tends to decrease and the size increases. And about the speed concerning pressure reduction, the faster one is suitable. This is because the pressure reduction rate is high, that is, the more the pressure is reduced in the shortest possible time, the more gas is generated within the short time and the cell diameters are substantially the same.

(About the final process S5)
The final step S5 is a step in which the foam is removed from the mold 30 and cooled by water cooling or air cooling, and further predetermined post-processing and inspections are performed (see FIG. 3D).

(About extraction process S4)
The extraction step S4 is a step that is selectively performed prior to the execution of the final step S5. When a resin material is used as the formation material for the balloon 14, the formation material for the balloon 14 is further extracted and removed. This is a step of reducing the density of the foam and improving the degree of communication of the foam, that is, the air permeability. Specifically, as described above ([0012]), as the balloon 14, a resin balloon 14 made of a resin material that is soluble in a solvent that does not dissolve the thermoplastic resin 12 is adopted, and a foam is used as a solvent. What is necessary is just to immerse for time. The immersion time in the solvent is about 2 hours in the case where the foam has a sheet-like material having a thickness of about 500 μm, for example. In this step S4, since the decompression foaming step S3 has already been performed, no contraction occurs even if the balloon 14 is not present in the foamed body.

(Another example)
In the above-described embodiment, the foam is manufactured by putting the mixture 16 composed of the thermoplastic resin 12 and the balloon 14 into the mold 30 having the inner contour shape that matches the outer contour shape of the foam. However, the present invention is not limited to this. For example, an extruder may be used as an apparatus for mixing and kneading the thermoplastic resin 12 and the balloon 14, and the mixture 16 may be obtained by being extruded as a sheet-like material having a predetermined thickness. And the mixture 16 extruded and shape | molded in the sheet form is made into the foam which concerns on this invention by introduce | transducing a supercritical fluid, after accommodating in the pressure vessel 22. FIG.

  Moreover, about the pressure reduction speed | velocity, although the effect that it was quick was described in the Example, the upper limit of pressure reduction speed | velocity | rate is actually restrict | limited by the manufacturing equipment etc. The slower the pressure reduction rate, the more the generation time of the gas forming each cell fluctuates. As a result, the cell diameter varies greatly, so the lower limit is set to 0.1 MPa / second or more. It is preferable.

(Experimental example)
Hereinafter, when producing the foam according to the present invention, the mixing ratio of the thermoplastic resin and the balloon, the dissolution pressure and time of carbon dioxide as a supercritical fluid, and the thickness of the foam when foamed are changed. An example of an experiment in the case of being made is shown. The foam according to the present invention is not limited to this experimental example.

(Production of test piece)
The thermoplastic resin and the balloon were mixed at a required mixing ratio, and this was basically made into a test piece of 30.0 mm × 30.0 mm × 0.5 mm using the manufacturing apparatus 20 described above. The test piece was measured for density (g / cm ³ ) and shrinkage (%). The used thermoplastic resin, balloon, supercritical fluid, and test methods for each item are described below.
-Thermoplastic resin: Trade name Toughmer A4085; manufactured by Mitsui Chemicals (density: 0.89 g / cm ³ , melting point: about 80 ° C.)
Balloon: trade name Microsphere F-80ED; manufactured by Matsumoto Yushi Seiyaku (density: 0.267 g / cm ³ , melting point: about 140 ° C.)
-Supercritical fluid: carbon dioxide-Density measurement method: Measured by an underwater substitution method in accordance with JIS K 7112.
・ Measurement method of shrinkage ratio: The density (a) immediately after foaming and demolding from the mold and the density (b) after 24 hours were measured, and ((b)-(a) / (b )) X100 was calculated by applying the formula.

(Experiment 1: Supercritical fluid dissolution pressure and pressure reduction rate)
Test strips according to Examples 1-1 to 1-4 and Comparative Examples 1-1 to 1-4 are shown in Table 1, where the mixing ratio of the thermoplastic resin and the balloon, the supercritical fluid dissolution conditions, and the decompression conditions are shown in Table 1. Each was manufactured and measured for each item.

(Result of Experiment 1)
The results of Experiment 1 are also shown in Table 1 above. From this result, when the supercritical fluid is carbon dioxide, if the dissolution pressure of the supercritical fluid is in the range of carbon dioxide critical pressure of 20 MPa or less and the decompression speed during foaming is 0.1 MPa / second or more, It was confirmed that the foam according to the invention can be preferably produced. In particular, regarding the decompression speed, it was confirmed that the shrinkage rate could not be reduced by increasing the mixing amount of the balloon.

(Experiment 2: Mixing ratio of thermoplastic resin and balloon)
The test pieces according to Examples 2-1 to 2-3 and Comparative Examples 2-1 and 2-2 were prepared with the mixing ratio of the thermoplastic resin and the balloon, the dissolution conditions of the supercritical fluid, and the decompression conditions shown in Table 2. Each was manufactured and measured for each item. For reference, the contents related to Example 1-2 are also shown.

(Result of Experiment 2)
The results of Experiment 2 are also shown in Table 2 above. From this result, it was confirmed that the foam according to the present invention can be suitably produced when the mixing ratio of the thermoplastic resin and the balloon is 80:20 to 15:85 by volume percentage.

(Experiment 3: Dissolution temperature of supercritical fluid)
Test strips according to Example 3-1 and Comparative Examples 3-1 to 3-3 were produced, with the mixing ratio of the thermoplastic resin and balloon, the dissolution conditions of the supercritical fluid, and the decompression conditions shown in Table 3, respectively. Measurements were performed for each item. For reference, the contents related to Example 1-2 are also shown.

(Result of Experiment 3)
The results of Experiment 3 are also shown in Table 3 above. From this result, it was confirmed that the foam according to the present invention can be particularly suitably produced when the melting temperature of the supercritical fluid is lower than the melting points of both the thermoplastic resin and the balloon.

(Experiment 4: Dissolution time of supercritical fluid)
The test pieces according to Examples 4-1 to 4-3 and Comparative Examples 4-1 and 4-2 were prepared with the mixing ratio of the thermoplastic resin and the balloon, the dissolution conditions of the supercritical fluid, and the decompression conditions shown in Table 4. Each was manufactured and measured for each item. For reference, the contents related to Example 1-2 are also shown.

(Result of Experiment 4)
The results of Experiment 4 are also shown in Table 4 above. From this result, it was confirmed that the foam according to the present invention can be suitably produced unless the dissolution time of the supercritical fluid exceeds 20 hours. Further, the longer the main dissolution time, the lower the density can be achieved. However, considering the production efficiency and the reduction in density, about 1 hour is considered optimal.

In view of the problems inherent in the prior art, the present invention has been proposed to suitably solve this problem. A balloon is mixed with a thermoplastic resin forming a skeleton, and a supercritical fluid is dissolved. Thus, it is possible to provide a method capable of producing a foam having both a low density of 0.2 g / cm ³ or less and a low shrinkage of 3% or less.

It is a flowchart figure which shows the process of manufacturing the foam which concerns on the suitable Example of this invention. It is the schematic which shows an example of the manufacturing apparatus which manufactures the foam which concerns on an Example. It is a state diagram which shows each process which manufactures a foam using the manufacturing apparatus of FIG.

Explanation of symbols

12 Thermoplastic resin 14 Balloon 15 Mixture

Claims (4)

In the method for producing a foam from the required thermoplastic resin (12),
The thermoplastic resin (12) and balloon (14) are mixed at a volume percentage of 80:20 to 15:85 to form a mixture (16);
At a temperature below the melting point of each of the thermoplastic resin (12) and the balloon (14), a supercritical fluid is dissolved in the mixture (16) under a predetermined pressure,
Depressurizing the mixture (16) in which the supercritical fluid is dissolved,
A foam production method characterized by producing a foam having a density of 0.2 g / cm ³ or less and a shrinkage after production of 3% or less.   The balloon (14) is made of a resin, and the foam obtained by decompression is immersed in a substance that is a good solvent for the balloon (14) and a poor solvent for the thermoplastic resin (12). The method for producing a foam according to claim 1, wherein the balloon (14) dispersed in the product is extracted and removed.   The method for producing a foam according to claim 1 or 2, wherein a pressure at which the supercritical fluid is dissolved is set to 20 MPa or less. Carbon dioxide is used as said supercritical fluid, The manufacturing method of the foam in any one of Claims 1-3 melt | dissolved with the pressure of at least 7.4 MPa with respect to the said mixture (16).

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