JPH07116314B2 - Method for producing foamed polymer molded article - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing a foamed polymer molded article. More specifically, the pre-expanded particles are filled in a foaming mold having a hole through which water vapor passes, and heated with steam to fuse them to produce a foamed molded product useful as a cushioning material, packaging material, heat insulating material, etc. It is about how to do it.

[0002]

2. Description of the Related Art Conventionally, a method for obtaining a foam-molded article by in-molding thermoplastic resin foam particles mainly of polystyrene has been widely used. To obtain the expanded beads, in the case of a styrene resin, first, a volatile organic foaming agent such as butane or pentane is added to an aqueous dispersion medium having a dispersant under a vapor pressure of the foaming agent at a temperature equal to or higher than the softening point of the resin. The resin particles are impregnated, cooled, taken out, dehydrated and dried, surface-treated with a chemical or the like, and aged to produce expandable resin particles. Next, the expandable resin particles are expanded under atmospheric pressure of water vapor or under pressure up to 0.2 kg / cm ² G to obtain expanded resin particles usually called pre-expanded particles. The pre-expanded particles thus obtained are introduced into a predetermined molding die and used to provide various foam-molded articles by heat-foaming under steam.

On the other hand, in the case of a polyolefin resin such as polyethylene or polypropylene, a volatile organic blowing agent such as butane or dichlorofluoromethane is impregnated in a water dispersion system at a temperature higher than the softening point of the resin, and then the vapor of the blowing agent is used. Pre-expanded particles are obtained by opening one end below the water surface of the container while maintaining the pressure, and releasing into the atmosphere at a pressure lower than the internal pressure. As described above, in the conventional general method for producing pre-expanded particles, the volatile organic foaming agent is exclusively used as the foaming agent, and the impregnation thereof is mainly performed in the water dispersion system.

However, the above-mentioned volatile organic foaming agent has an affinity for the resin base material, is excellent in retention and is effective as a foaming agent, but on the other hand, it acts as a good solvent during impregnation and pre-foaming, and thus resin particles. It has a problem that it is easy to cause mutual adhesion. Others have the risk of toxicity and flammability. Some are expensive, and most of the time they are released to the atmosphere, which destroys the ozone layer,
This caused the problem of air pollution such as photochemical smog.

Therefore, it has recently been proposed to apply an inorganic gas such as carbon dioxide gas, nitrogen or air to the industrial production of pre-expanded particles in place of the volatile organic blowing agent. For example, in Japanese Patent Publication No. 62-61227, a polyolefin resin is dispersed in an aqueous medium in an airtight container, and then an inorganic gas is supplied to the resin while heating it to a softening temperature or higher while maintaining the vapor pressure of the inorganic gas to impregnate the resin. However, there is disclosed a method of producing pre-expanded particles by opening a discharge port under the surface of the water and discharging into a low-pressure atmosphere to foam. Further, Japanese Patent Publication No. 60-245648 discloses that a resin particle fusion preventive agent is used to overcome the drawback of requiring heat energy, since a large amount of water as a dispersion medium is also heated when the resin particles are heated to a softening temperature or higher. Disclosed is a method of releasing the inorganic gaseous medium from the high temperature and high pressure region in a state where they are separated from each other by using.

However, in the above method using an inorganic gas, since the resin is heated to a temperature higher than the softening temperature at the time of foaming, agitation or the like is required to keep the temperature of the entire impregnation system uniform, and there is a risk of coalescence of resin particles. In addition, there is a drawback that scale adheres to the vessel wall and stirring blades. And since all of them are release foaming which is instantly foamed, it is difficult to control the expansion ratio and the uniformity of the pre-expanded particles obtained,
There were problems such as poor reproducibility and the need for stabilization measures against fluctuations in the internal pressure of the container during the initial and late stages of release.

US Pat. No. 4,911,869 is impregnated with carbon dioxide gas, pre-expanded at a temperature not lower than the glass transition point of the resin, and re-impregnated with the pre-expanded particles with air, nitrogen or carbon dioxide gas to perform molding. It is disclosed. These inorganic gases have a low affinity for resin base materials and have a large dissipative property,
As a foaming agent, it is less effective than a volatile organic foaming agent, and there is a problem that a desired foaming multiple cannot be obtained by these methods particularly in a polyolefin resin.

Japanese Patent Publication No. 2-50945, Japanese Patent Laid-Open No. 2-756
No. 36, JP-A-62-18438, a foaming agent is impregnated to perform the first-stage foaming, and the expanded particles are further impregnated with the foaming agent to perform the second-stage foaming. A manufacturing method for obtaining pre-expanded particles is disclosed. However, in these methods, although the blowing agent used in the second step is an inorganic gas such as air, the blowing agent used in the first step is a volatile organic blowing agent, and in most cases, it is released. The foaming method had the same problem as above.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the prior art, and impregnation of polymer particles with an inorganic gas is carried out in a gas phase rather than in a conventional water dispersion system and in a specific manner. Performed under the conditions and pre-foaming without depending on the foaming released,
By foam-molding the obtained pre-expanded particles, it is possible to obtain high-quality pre-expanded particles having a uniform expansion ratio using an inorganic gas without causing troubles due to coalescence between polymer particles. The inventors have found that highly foamed and uniform foam-molded articles can be efficiently produced by foam-molding using pre-expanded particles, and have completed the present invention.

Thus, according to the present invention, the polyolefin polymer particles are vapor-phase impregnated with an inorganic gas at a temperature under which the particles do not coalesce and under a pressure so that the inorganic gas content is 0.05 mol / min. The primary expandable polymer particles of kg (polymer) or more are introduced into the foaming tank until the content of the inorganic gas is reduced to less than 0.05 mol / kg (polymer), and the above-mentioned polymer particles are melted. Peak temperature (T _P ) + 10 ° C to T _P
The primary pre-expanded particles are formed by heat-expanding by introducing steam in a temperature range of −10 ° C., and then the primary pre-expanded particles are vaporized with an inorganic gas at a moderate temperature at which the primary pre-expanded particles do not coalesce. Impregnated into secondary expandable foamed polymer particles having an inorganic gas content of 0.5 mol / kg (polymer) or more, and reducing the inorganic gas content to less than 0.5 mol / kg (polymer). It is introduced into the foaming tank until it decreases, and T _P +15
In the temperature range of ℃ to T _P -20 ℃, after heat-foaming by introducing steam to form secondary pre-expanded particles, inorganic gas is further added to the secondary pre-expanded particles at a moderate temperature at which the expanded particles do not coalesce. In addition, the secondary pre-expanded particles are subjected to internal pressure so that the inorganic gas content is 0.5 mol / kg (polymer) or more by impregnating in the gas phase under pressure, and the inorganic gas content is 0.5 It is characterized in that it is filled in a foam molding mold until it is reduced to less than mol / kg (polymer) and heat-foamed by introducing steam in a temperature range of T _P + 15 ° C to T _P -20 ° C to obtain a foam molded article. A method for producing a foamed polymer molded article is provided.

The melting peak temperature of the polymer particles in the present invention is measured according to the method for measuring the transition temperature of JIS K-7121 plastics and the differential scanning calorimetry (DSC) (a heating rate of about 3 to 4 mg of sample is 10 It is the peak temperature of the melting peak in the DSC curve obtained by increasing the temperature (° C / min). As the base resin of the polyolefin-based polymer particles in the present invention, polyethylene, polypropylene, ethylene propylene random copolymer, ethylene propylene block copolymer, ethylene propylene butene copolymer, polyethylene vinyl acetate copolymer, polybutene, styrene modified polyethylene cross-linked polymer, styrene A modified polypropylene crosslinked polymer or the like is used. In addition,
These may be a mixture of two or more kinds. Further, these polyolefin-based resins may be appropriately partially cross-linked in view of foam moldability and uniformity of foam cells.

The particle size of the polymer particles used in the present invention is generally about 0.2 to 3 mm. When impregnating such polymer particles with an inorganic gas, the surface of these particles may be coated with an appropriate surface treatment agent in advance. Examples of these surface treatment agents include prevention of coalescence during prefoaming, promotion of fusion during molding,
Examples include lubricity, an antistatic agent, and a coloring agent that improve mold filling property. These surface treatment agents include talc, calcium carbonate, silica-alumina colloids and slurries,
Examples thereof include ethylenebis stearamide, higher fatty acid, higher fatty acid alcohol, edible oil, higher fatty acid triglyceride, paraffin and polyethylene wax, and metal soap-modified polysiloxane such as zinc stearate.

The surface treatment is preferably performed on the polymer particles in advance with a ribbon blender, a tumbler, a Nauter mixer, a spur mixer, a Loedige mixer, etc. prior to the impregnation with the inorganic gas. However, as long as the inorganic gas impregnated container can be rotated or stirred, the surface treatment may be performed at the same time as the gas impregnation.

In the present invention, the polymer particles are first impregnated with an inorganic gas under pressure in the vapor phase. As the inorganic gas in the present invention, carbon dioxide gas, nitrogen, air and the like are used, and these may be used alone or in a mixture. The impregnation treatment is carried out by press-fitting an inorganic gas in a closed container. Such impregnation treatment is performed at a mild temperature at which the polymer particles do not coalesce. Here, the mild temperature is specifically a temperature range lower than the melting peak temperature of the polymer particles by 40 ° C. or more. It changes depending on the preferable temperature or the resin base material used, and also changes depending on the inorganic gas.

Usually, the impregnation temperature is desirably lower, preferably 40 ° C. or lower, because the substantial impregnation amount of the inorganic gas can be increased and the internal pressure of the pressure vessel can be set low. However, if the temperature is 0 ° C. or lower, the energy consumption is industrially increased due to normal cooling, which is not preferable. High temperature is desirable from the gas impregnation rate, but carbon dioxide is preferably 5 to 5 when industrially stable and high foaming property is obtained.
40 degreeC, and nitrogen and air are 0-30 degreeC. When the melting peak temperature of the polymer is −40 ° C. or higher, the polymer particles may adhere to each other during the impregnation, and gas escape occurs rapidly before defoaming and prefoaming after gas impregnation. The desired foamability cannot be obtained.

Under such conditions, there is no possibility of coalescence of the polymer particles with each other because the inorganic gas has no solvent property to the polymer and only mild heating is performed. Therefore, the impregnation may be performed while the polymer particles are allowed to stand. Also during impregnation,
The inorganic gas under pressure may be sucked from the upper part of the container with a pump or the like, passed through a greenhouse, and then returned to the lower part of the container for circulation.

A suitable pressure for impregnating the polymer particles with carbon dioxide is 15 kg / cm ² G or more, preferably 20 kg / cm ² G or more and 50 kg / cm ² G or less, and 30 kg / cm ^{2 in} the case of nitrogen or air.
G or more, preferably 40 kg / cm ² G or more and 90 kg / cm ² G or less. If the impregnation pressure is less than 15 kg / cm ² G, it is difficult to sufficiently impregnate the inorganic gas. On the other hand, the impregnation pressure is 90 kg
If it exceeds / cm ² G, the inorganic gas can be sufficiently impregnated at a pressure of 90 kg / cm ² G or less, so that the pressure container is required to have excessive pressure resistance, which is not preferable in industrial production.

By impregnating the inorganic gas, polymer particles uniformly impregnated with the inorganic gas can be obtained. This impregnation time varies depending on the type of the inorganic gas, the impregnation pressure, the type of the polymer, the particle size of the polymer, etc., but is carried out until at least 0.05 mol / kg (polymer) of the inorganic gas is impregnated, usually 1 to
Achieved in 10 hours. In particular, in the case of a polyolefin resin, the inorganic gas content is preferably 0.05 to 1.5 mol / kg (polymer) in order to obtain high quality pre-expanded particles.

The present invention uses an inorganic gas, and it is not necessary to use a volatile organic foaming agent also for that purpose. However, as long as the effect of the present invention is not impaired, it is also possible to use a small amount of a volatile organic foaming agent in combination, and propane, butane, dichlorofluoromethane, etc. are used in a small amount, for example, 0.2 mol or less per 1 mol of the inorganic gas. Even if it is, the same effect can be obtained.

The primary expandable polymer particles thus impregnated with the inorganic gas are then subjected to a pre-expanding step.
At this time, what is important is that the pre-foaming is performed under the condition that the content of the inorganic gas in the polymer particles is at least 0.05 mol / kg (polymer). That is, in the expandable polymer particles impregnated with the inorganic gas, the inorganic gas gradually dissipates when taken out from the impregnation pressure container to the atmosphere. When the gas content of the expandable polymer particles is reduced to less than 0.05 mol / kg (polymer), it becomes difficult to produce normal pre-expanded particles having a uniform expansion ratio.

Therefore, in industrial production, after the impregnation treatment, the primary expandable polymer particles are weighed for prefoaming while maintaining a pressurized state, and introduced into a prefoaming tank heated with steam almost simultaneously with depressurization. Alternatively, it is preferable to directly feed the pre-foaming tank under pressure from the pressurized state to heat and foam. When it is used for pre-foaming after being taken out into the air, it is necessary to heat-foam until the inorganic gas content falls below 0.05 mol / kg (polymer). These measures are appropriately selected depending on the situation, but all are within the scope of the present invention.

Pre-foaming is carried out by introducing steam into the foaming tank and heating it. As the steam in the present invention, it is suitable to use steam having a dew point of 60 to 100 ° C. or a steam-air mixed medium, and particularly the latter mixed medium having a dew point of 70 to 100 ° C. is preferable. A dew point of less than 60 ° C is not preferable because it is difficult to obtain a high foam.

The heating of the first-stage pre-foaming is performed by the above heated steam, and the melting peak temperature of the polymer particles (hereinafter referred to as T
_P referred) + 10 ℃ ~T _P -10 ℃ is controlled to be a temperature range of, preferably controlled to a temperature range of _{T P + 5 ℃ ~T P -5} ℃. When the control temperature is lower than T _P -10 ° C, the foaming becomes slow, the external escape of the inorganic gas becomes dominant, the foaming effect cannot be sufficiently obtained, and as a result, the high foaming property cannot be obtained. When the control temperature is higher than T _P + 10 ° C, the proper foaming range becomes extremely small, and industrially normal pre-expanded particles cannot be obtained.

It is preferable to preheat the inside of the foaming tank in the vicinity of the above heating control temperature during the pre-foaming, or to supply the expandable polymer particles under the steam in the foaming temperature range. It is preferable from the viewpoint of maintaining the sex, and it is appropriate to keep it under a pressure of about 0.5 to 2.0 kg / cm ² G. The foaming time is 10 to 120 seconds, preferably 10 to 60 seconds. At the time of this foaming, it is preferable to stir with a rotary blade to ensure uniform heating and prevention of coalescence of expanded particles.

Further, at the initial stage of supplying steam to the foaming tank, a part of the foaming tank is opened while maintaining the pressure under pressure, and the air and the inorganic gas foaming agent introduced together with the foamable polymer particles are supplied. It can also be discharged to improve the temperature rise and heat flow. The primary pre-expanded particles thus obtained are
Depending on the desired foaming multiple of the foamed article, the bulk multiple of 3 to 20
It is double-expanded, and the variation in expansion ratio between the pre-expanded particles is so small that it cannot be seen in other conventional production methods, and it is suitable as a raw material for the second-stage expansion. That is, in the industrial production, the dispersion of the expansion ratio among the particles of the primary pre-expanded particles tends to be further expanded by the second-stage impregnation expansion. This is a feature of the first-stage impregnation foaming in the method of the present invention for obtaining uniform primary pre-expanded particles for obtaining a homogeneous expanded molded article.

Next, in order to obtain more highly expanded pre-expanded particles, the first pre-expanded particles are impregnated with a second stage inorganic gas and pre-expanded to obtain secondary pre-expanded particles expanded to a bulk multiple of 20 times or more. . The second-stage inorganic gas impregnation of the primary pre-expanded particles is carried out under a mild temperature and under pressure at which the primary expanded particles do not coalesce in the same manner as in the case of the polymer particles described above. It is performed so that the content of the inorganic gas in the united particles is 0.5 mol / kg (polymer) or more. It is preferably 0.5 to 4.0 mol / kg (polymer). Impregnation pressure is 1 kg / cm ² G or more, preferably 10 to 50 kg / cm ²
G. If the impregnation pressure is less than 1 kg / cm ² G, the inorganic gas cannot be sufficiently impregnated. On the other hand, the pressure is 50 kg / cm ²
If it exceeds G, the primary pre-expanded particles are excessively shrunk in a state of being returned to the atmospheric pressure so as not to be restored to the original volume, which is not preferable.

The secondary expandable polymer particles are subjected to the second pre-expansion. What is important here is that the T content is at least 0.5 mol / kg (polymer) or more.
Pre-foaming is performed in a steam system in the temperature range of _P + 15 ° C to T _P -20 ° C. When the inorganic gas content is less than 0.5 / kg (polymer), it becomes difficult to obtain a high multiple normal secondary pre-expanded particles. Further, when the foaming temperature is lower than T _P by 20 ° C. or more, foaming is delayed, and the gas is diffused to the outside so that the foaming effect cannot be sufficiently obtained. On the other hand, when the temperature is higher than T _P by 15 ° C. or more, the proper foaming region becomes extremely small and stable and normal foamed particles cannot be obtained.

The pre-expansion of the second stage is carried out in substantially the same manner as the pre-expansion of the first stage, but the expansion time is 5 to 90 seconds, preferably 10 to 45 seconds. The secondary pre-expanded particles thus obtained are expanded to a bulk ratio of 20 times or more according to the desired expansion ratio of the foam-molded article. Then, the secondary pre-expanded particles are impregnated with an inorganic gas and internal pressure is applied to perform in-mold foam molding to obtain a highly foamed molded product.
The internal pressure applied to the secondary pre-expanded particles by the inorganic gas is at a moderate temperature, similar to the inorganic gas impregnation of the primary pre-expanded particles,
Moreover, it is impregnated so that the content thereof is 0.5 mol / kg (polymer) or more. In order to obtain good moldability, an inorganic gas content of 0.5 to 4.0 mol / kg (polymer) is preferable.

The secondary pre-expanded particles to which the internal pressure is applied have a water vapor system in the temperature range of T _P + 15 ° C to T _P -20 ° C under the condition that the inorganic gas content is at least 0.5 mol / kg (polymer). It is heated for 10 to 120 seconds, preferably 20 to 60 seconds, and subjected to so-called in-mold molding by a known method to obtain a foam molded article. In this case, when the gas content of the secondary pre-expanded particles applied with the internal pressure is less than 0.5 mol / kg (polymer), a normal foam-molded article cannot be obtained, and the molding temperature is 20% higher than T _P.
When the temperature is lower than 0 ° C, the foaming becomes slow and the external escape of the inorganic gas becomes dominant, so that the foaming effect cannot be sufficiently obtained. On the contrary, when the temperature is higher than T _P by 15 ° C. or more, the proper foaming region of the polymer becomes extremely small, and a normal foamed molded product cannot be obtained.

Impregnation of pre-expanded granules with an inorganic gas When expanded, the expanded granules are compressed and their volume is reduced. It may be filled in the molding die in the compressed state as it is, or may be filled in the molding die after depressurizing to restore the volume before compression to almost the same level. In the above description, the internal pressure is applied to the secondary pre-expanded particles to perform in-mold foam molding, but in order to obtain a molded article that is further highly foamed, for the secondary pre-expanded particles, Similar to the second-stage impregnation pre-expansion, after the third-stage impregnation pre-expansion treatment, internal pressure is applied to the obtained pre-expanded particles with an inorganic gas to perform in-mold expansion molding.

[0031]

EXAMPLES The present invention will be described below with reference to examples.
The present invention is not limited thereby. 300 m
Into a gas-impregnated container (1), 150 g of various polyolefin polymer particles surface-treated with 0.4 g of calcium carbonate, an anti-cohesion agent at the time of foaming, were sealed, and inorganic gas was pressure-inserted to maintain temperature for a certain period of time. After that, directly fill the foaming tank under heating steam pressure, or remove the internal pressure and put into the foaming tank within a short time (within about 30 minutes), and heat foaming in the steam system under pressure to perform the primary pre-foaming. The particles were obtained.

In a 1000 ml gas-impregnated container, 50 g of the obtained primary pre-expanded particles were put and sealed, and inorganic gas was press-fitted and held for a certain period of time while controlling the temperature, and then directly filled in a foaming tank under heating steam pressure. Alternatively, the internal pressure was removed, and the mixture was put into a foaming tank within a short time (within about 30 minutes) and heat-foamed in a steam system under pressure to obtain secondary pre-expanded particles.

In a 3000 ml gas-impregnated container, 50 g of the obtained secondary pre-expanded particles were placed and hermetically sealed, and an inorganic gas was pressure-inserted to maintain the temperature for a certain period of time, and then the internal pressure of the container was removed for a short time (about 30 Within minutes), it was filled in a mold tube (30 × 30 × 1.5 cm) of a preheated molding die, and heat-foam-molded with steam under pressure to obtain a foam-molded article. Tables 1 to 4 show the results of the conditions such as treatment pressure and treatment temperature and the properties and properties of the obtained pre-expanded particles and foam-molded products together with Comparative Examples.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4] The polymers used in Examples 1 and 5 and Comparative Examples 1 to 12 are Noblene S-131 ethylene-propylene random copolymers manufactured by Sumitomo Chemical Co., Ltd., and Example 2 is Nobrene W-produced by Sumitomo Chemical Co., Ltd. 501 polypropylene, and Example 3 is Nobrene FH- manufactured by Sumitomo Chemical Co., Ltd.
1015 polypropylene, Example 4 is PRO-FAX-814 special polypropylene manufactured by Highmont, and Example 6 is Yucaron HE-60 ethylene vinegar bicopolymer manufactured by Mitsubishi Petrochemical Co., Ltd.

The pre-expanded particles of Examples 1 to 6 did not cause coalescence or the like during the process, and the foam-molded products obtained were of high quality with uniform foaming and good fusion. On the other hand, in Comparative Example 1, the impregnation was too high to cause coalescence,
In Nos. 8, 11, and 12, the gas content was small and the foaming power was insufficient. In Comparative Examples 3, 5, 9 and 10, the heat was insufficient during foam molding, resulting in poor foaming. In Comparative Examples 4, 6, and 7, foaming failure accompanied by shrinkage was caused by excessive heat during foaming.

[0039]

According to the method for producing a foamed polymer molded article of the present invention, highly expanded pre-expanded particles having a small variation among the pre-expanded particles can be obtained by using the inorganic gas, and the pre-expanded particles are expanded. By molding, it is possible to efficiently and stably produce a high-foaming homogeneous foam-molded article. According to the present invention, the drawbacks found in the conventional use of inorganic gas are remarkably improved, and in addition to the use of a water dispersion system at the time of gas impregnation and there is no need to particularly heat, it is advantageous in the manufacturing process. In addition, it can be manufactured in a safe and favorable working environment, and industrial production can be performed without causing environmental pollution.

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Claims (4)

[Claims] 1. A polyolefin-based polymer particle is impregnated with an inorganic gas in a vapor phase at a moderate temperature at which the particle does not coalesce and under pressure, so that the inorganic gas content is 0.05 mol / kg (polymer ) The above primary expandable polymer particles are introduced into a foaming tank until the inorganic gas content is reduced to less than 0.05 mol / kg (polymer), and the melting peak temperature (T _P ) + 10 ° C. to T _P −10 ° C. to form primary pre-expanded particles by heat-expanding by introducing steam, and then primary pre-expansion at a moderate temperature at which the primary pre-expanded particles do not coalesce and under pressure. The particles are impregnated with an inorganic gas in a gas phase to obtain secondary expandable foamed polymer particles having an inorganic gas content of 0.5 mol / kg (polymer) or more, and the inorganic gas content is 0.5 mol / kg. Introduce into the foaming tank until it decreases to less than kg (polymer), _P + 15 ℃ ~T _P -20 after the formation of the temperature range in the secondary pre-expanded particles are heated foamed by the water vapor introduction of ° C., further expanded beads of an inorganic gas into the secondary pre-expanded particles do not coalesce mild temperatures The gas phase impregnation under the pressure of 0.5 mol / min.
Internal pressure is applied to the secondary pre-expanded particles so that the amount of the polymer is not less than kg (polymer), and this is filled in the foaming mold until the content of the inorganic gas is reduced to less than 0.5 mol / kg (polymer). , T _P +
Method for producing a foamed polymer molded article, characterized in that 15 ° C. through T heated foamed by vapor introduced at a _P -20 temperature range of ° C. to obtain a foamed molded article. 2. The mild temperature at which polymer particles do not coalesce during gas impregnation is 40 ° C. from the melting peak temperature of the polymer particles.
The manufacturing method according to claim 1, wherein the temperature is in the lower temperature range. 3. The introduction of steam has a dew point of 60 to 100 ° C. and the melting peak temperature ( _TP ) of the polymer particles + 15 ° C.
Through T _P -20 ° C. in a temperature range of steam or water vapor - The process of claim 1 performed using air mixed medium. 4. The secondary pre-expanded particles are further impregnated with an inorganic gas and pre-expanded to obtain pre-expanded particles of the third and subsequent pre-expanded particles, the internal pressure is applied to the pre-expanded particles, and the pre-expanded particles are heated in a molding die. The manufacturing method according to claim 1, wherein foaming is performed.