Manufacturing method for polypropylene-based resin foamed particle molded article

Abstract

In order to prevent the occurrence of a problem of losing steam in manufacturing of a large polypropylene-based resin foamed particle molded article, this manufacturing method includes: a filling step for filling a cavity (10) provided in split forming molds (70) with polypropylene-based resin foamed particles (50); a pre-pressurization step for adjusting the pressure in the cavity (10) to a pressure (P1) exceeding the atmospheric pressure, by supplying a pressurized gas other than steam to the cavity (10) filled with the polypropylene-based resin foamed particles (50); and a heating step for heating the polypropylene-based resin foamed particles (50) by supplying steam having a pressure (P2) higher than the pressure (P1) from an electric steam boiler (120) to the cavity having undergone the pre-pressurization adjustment. At least one mold of the split forming molds (70) is a recessed inner-type. The volume of the cavity (10) provided in the split forming molds (70) is adjusted to 0.1 m3 or more. The ratio (kg / m3) of the amount (kg) of the steam to be used when forming the molded article with respect to the volume (m3) of the cavity (10) is adjusted to be 100 or less.

Description

Method for producing expanded polypropylene resin beads

[0001] The present invention relates to a method for producing an expanded polypropylene resin bead molded article using expanded polypropylene resin beads.

[0002] It is known that, when producing foamed bead moldings by in-mold molding, steam is supplied to foamed beads filled in a cavity to heat them, causing the foamed beads to fuse together and thereby producing a foamed bead molding of a desired shape. More specifically, foamed bead moldings have typically been produced using a foamed bead molding production apparatus (200) equipped with multiple molding machines (210) and a large steam generator (220) connected to each of the multiple molding machines (210), as shown in FIG. 7 . Each molding machine (210) is equipped with a split mold (not shown). Hereinafter, the foamed bead molding production apparatus (200) may be simply referred to as the production apparatus (200). In such a system, the molding machines (210) are typically located inside a building (240) and the large steam generator (220) is typically located outdoors, as shown in FIG. 7 . In this manufacturing apparatus (200), one large steam generator (220) and a plurality of molding machines (210) are connected via piping (230), and steam is supplied to each molding machine (210) to heat the foamed beads and produce foamed bead molded articles. Known large steam generators (220) include those that utilize heat, such as oil combustion, gas combustion, high-frequency induction heating, turbine, and combustion furnace types.

[0003] However, when in-mold molding is performed using such a manufacturing apparatus (200), the distance from the large steam generator (220) to each molding machine (210) becomes long and the steam transport path becomes complex. This poses a problem in that the high-temperature, high-pressure steam transported through the piping (230) is prone to heat loss or pressure loss during transport. Hereinafter, this problem will be referred to as the steam loss problem or the energy loss problem. The first specific problem resulting from the steam loss problem is that the temperature of the steam discharged from the large steam generator (220) drops before it reaches each molding machine (210), resulting in energy loss and a long time required for the steam pressure to reach a level suitable for molding. The second problem is that drains are generated in the piping (230) and the molding machine (210), etc., and the drains absorb a large amount of heat from the subsequently supplied steam as heat of vaporization, resulting in heat loss.

[0004] Patent Document 1 attempts to propose a foam molding device that enables stable steam supply to the molding machine. Specifically, the proposed foam molding device is configured by pairing a molding machine main body used for in-mold molding with a steam generator that generates steam as a heating medium. It is stated that the heat source of the steam generator can be of any type, including oil combustion, gas combustion, electric heating, and high-frequency induction heating. While Patent Document 1 does not provide any detailed explanation of the foam resin beads used in the proposed device, it does provide an example of a molded product, such as a fish transport container called a "torobako." In other words, Patent Document 1 can be understood to suggest a polystyrene-based resin foam bead molded article commonly used as a fish transport container.

[0005] JP 2002-273752

[0006] It is generally known that the molding pressure during molding of expanded polypropylene resin beads in a mold is higher than the molding pressure during molding of expanded polyethylene resin beads or expanded polystyrene resin beads or other thermoplastic resin beads in a mold.

[0007] According to the investigations of the present inventors, it was presumed that the production of expanded polypropylene resin bead moldings, which requires high molding pressure, is more affected by the steam loss problem than the production of expanded polystyrene resin bead moldings, expanded polyethylene resin bead moldings, etc. In particular, a higher molding pressure is required when producing large expanded polypropylene resin bead moldings. Therefore, it was presumed that the steam loss problem is significantly affected when producing large expanded polypropylene resin bead moldings.

[0008] However, up until now, no substantial method has been proposed for producing large polypropylene resin expanded bead moldings that overcomes the steam loss problem and the various problems that arise from this problem.

[0009] Therefore, the present invention relates to the production of large-sized expanded polypropylene resin bead moldings, and proposes a novel production method that can prevent the occurrence of the problem of steam loss.

[0010] The method for producing an expanded polypropylene resin bead molded article of the present invention comprises an expanded bead molded article producing apparatus having a molding machine equipped with a split molding die consisting of a first die and a second die, and one or more steam generators provided for the molding machine, and the method comprises filling a cavity of the split molding die with expanded polypropylene resin beads and heating the expanded beads with steam to fuse the beads together, and at least one of the split molding die has a concave inner die, and the volume of the inner die defined by the opening surface of the concave inner die and the inner surface of the inner die is 0.1 m ３ The steam generator is an electric steam boiler, and the volume of the inner mold (m ３ ) to the ratio of the amount of steam (kg) used for molding (kg / m ３ ) is 100 or less.

[0011] According to the present invention, it is possible to produce large polypropylene resin foamed bead moldings by in-mold molding using an electric steam boiler instead of a large steam boiler such as a gas steam boiler having a large heat capacity. More specific configurations and effects of the present invention will be described in detail later.

[0012] FIG. 1 is a conceptual diagram showing an example of an expanded bead molding manufacturing apparatus group used in the manufacturing method of the present invention. FIG. 2 is a conceptual diagram showing an example of an expanded bead molding manufacturing apparatus used in the manufacturing method of the present invention. FIG. 3 is a schematic cross-sectional view showing an example of an expanded bead molding manufacturing apparatus used in the manufacturing method of the present invention. FIG. 4A is an explanatory diagram explaining the volume of the inner mold when one side of the split molding mold is a concave inner mold, and FIG. 4B is an explanatory diagram explaining the volume of the inner mold when both split molding molds are concave inner molds. FIG. 5 is a longitudinal cross-sectional view of an expanded bead molding manufacturing apparatus used in a first embodiment of the manufacturing method of the present invention. FIG. 6A to FIG. 6D are perspective views showing examples of expanded polypropylene resin beads used in a second embodiment of the manufacturing method of the present invention. FIG. 6 is a conceptual diagram of a conventional expanded bead molding manufacturing apparatus.

[0013] The method for producing an expanded bead molded article of the present invention (hereinafter, sometimes simply referred to as the production method of the present invention) will be described below. Drawings will be used as appropriate for the description. In all drawings, similar components will be designated by the same reference numerals, and duplicate explanations will be omitted where appropriate. In the following description, preferred numerical ranges of the present invention will be indicated as appropriate. In this case, preferred, more preferred, and particularly preferred ranges regarding the upper and lower limits of the numerical range can be determined from all combinations of the upper and lower limits. Furthermore, in the present invention, gauge pressure refers to a pressure determined based on atmospheric pressure, and is a pressure obtained by subtracting atmospheric pressure from absolute pressure. In this specification, the unit for gauge pressure is expressed as MPa (G).

[0014] First, an overview of the present invention will be described using Figures 1 to 4 as appropriate. Figure 1 is a conceptual diagram of an expanded bead molding manufacturing apparatus group (140) including five expanded bead molding manufacturing apparatuses (100) used in the manufacturing method of the present invention. Figure 2 is a conceptual diagram showing an example of an expanded bead molding manufacturing apparatus (100) used in the manufacturing method of the present invention. Figure 3 is a schematic cross-sectional view showing one embodiment of the manufacturing apparatus (100) used in the manufacturing method of the present invention. Figures 4A and 4B are explanatory diagrams illustrating the volume of the inner molds. Figure 4A shows an embodiment in which one inner mold is concave and the other is convex, and Figure 4B shows an embodiment in which both inner molds are concave. Note that, in this specification, the expanded bead molding manufacturing apparatus (100) may be simply referred to as the manufacturing apparatus (100).

[0015] As shown in Figure 3, the manufacturing apparatus (100) has a molding machine (110) equipped with a split molding die (70) consisting of a first die (34) and a second die (36), and one or more steam generators provided for each molding machine (110). Expanded polypropylene resin beads (50) are filled into a cavity (10) provided in the split molding die (70), and the expanded beads (50) are fused together by heating with steam to produce an expanded polypropylene resin bead molded article. In this specification, the expanded polypropylene resin beads (50) may be abbreviated to expanded beads (50). In the present invention, the volume of the inner die provided in the split molding die (70) is 0.1 m3. ３ The above-mentioned features are specified, and thereby a large-sized expanded polypropylene resin bead molding is provided. In producing such a large-sized expanded polypropylene resin bead molding, the production method of the present invention employs an electric steam boiler (120) as a steam generator. The electric steam boiler (120) is connected to the molding machine (110) through piping (130).

[0016] Although the electric steam boiler (120) is generally compact, the amount of steam generated per unit time is small, and it has been recognized that it is not suitable for producing large polypropylene resin foam bead moldings. However, the present invention is applicable to a boiler with a capacity of 0.1 m ３Even when using a large inner mold, the volume of the inner mold (m ３ ) to the ratio of the amount of steam (kg) used to form one molded body (kg / m ３ ) is adjusted to be 100 or less. This makes it possible to produce large polypropylene resin foam bead moldings even when using an electric steam boiler (120). Furthermore, as described above, the present invention uses one or more electric steam boilers (120) for one molding machine (110) rather than connecting one large boiler to multiple molding machines (110). This shortens the distance between the steam boiler and the molding machine (110) and prevents the piping from becoming complicated. This alleviates the energy loss problem, enables a stable supply of steam from the electric steam boiler (120) to the molding machine (110), and reduces steam loss in the transport path. As a result, fluctuations in the pressure of the steam supplied to the split mold (70) are suppressed, and the quality of the produced foam bead moldings is also suppressed.

[0017] Furthermore, the present invention utilizes a small steam boiler and can shorten the piping (130), thereby enabling a compact manufacturing system (100). This allows the manufacturing system (100) to be easily transported to the site of use. Alternatively, as shown in FIG. 1, a group of foamed bead molding manufacturing equipment (140) including an electric steam boiler (120) can be installed within a building (240). In the group of foamed bead molding manufacturing equipment (140), steam is supplied to multiple molding machines (110) from their respective steam boilers. Therefore, unlike the conventional manufacturing system (200) shown in FIG. 7, operating molding machines (110) are not affected by pressure fluctuations in the supplied steam due to the influence of other stopped molding machines (110). From the viewpoints of shortening the steam supply piping (130), enabling a compact manufacturing system (100), and minimizing the impact of pressure fluctuations in the supplied steam, it is preferable to use one electric steam boiler (120) per molding machine (110).

[0018] The manufacturing apparatus (100) used in the present invention may include not only the molding machine (110) and the electric steam boiler (120) but also other devices. For example, as shown in FIG. 2, the manufacturing apparatus (100) may include, in addition to the molding machine (110) and the electric steam boiler (120), a compressed gas generator (112) for supplying a compressed gas other than steam to the molding machine (110). The molding machine (110) and the compressed gas generator (112) may be connected by a pipe (131). The compressed gas other than steam may be, for example, compressed air. The manufacturing apparatus (100) may also be provided with a water source (114). Water supplied from the water source (114) may be connected by a pipe (132) so as to be supplied to the molding machine (110) and the electric steam boiler (120). Although not shown, the production apparatus (100) may further be equipped with a vacuum pump, a buffer tank pressure accumulator, and the like.

[0019] In the present invention, an electric steam boiler refers to a device that uses electricity as an energy source and can generate steam using the electricity and water. Unlike gas-fired steam boilers, which have traditionally been used as large steam boilers, electric steam boilers do not require fuel piping or exhaust ducts. Therefore, electric steam boilers can have a more compact overall configuration than gas-fired steam boilers. Therefore, as shown in FIG. 1 , a group of foamed bead molding production equipment (140) can be constructed within a building (240). Alternatively, as a mobile production equipment, the production equipment (100) can be moved and installed at any desired location where a power source is available, and foamed bead moldings can be produced. Furthermore, electric steam boilers have a relatively short start-up time, which allows for improved production capacity for foamed bead moldings.

[0020] The electric steam boiler (120) is connected to the molding machine (110) through a piping (130), and the electric steam boiler (120) and the molding machine (110) are preferably adjacent to each other. The distance of the piping (130) connecting the electric steam boiler (120) and the manufacturing apparatus (100) is not particularly limited. For example, from the viewpoint of preventing energy loss, compactly configuring the foamed bead molding manufacturing apparatus group (140), and facilitating the use of the manufacturing apparatus (100) as a mobile manufacturing apparatus, the length of the piping (130) is preferably 20 m or less, more preferably 10 m or less, even more preferably 5 m or less, and even more preferably 3 m or less. Furthermore, it is preferable that the piping (130) be short in length and have a moderately large diameter. Therefore, the diameter of the piping (130) is preferably 15 mm to 150 mm, and more preferably 25 mm to 100 mm. In particular, it is more preferable that the pipe (130) has a length within the above-mentioned preferred ranges and a diameter within the above-mentioned ranges.

[0021] Next, the volume of the inner mold will be described. In the manufacturing method of the present invention, at least one of the split molds has a concave inner mold, and the volume of the inner mold defined by the opening surface of the concave inner mold and the inner surface of the inner mold is 0.1 m ３ In order to obtain a larger molded product, the capacity of the inner mold is set to 0.2 m. ３ It is preferable that the length is 0.3 m or more. ３ On the other hand, the upper limit of the volume of the inner mold is not particularly limited, but it is generally 3 m ３ The volume of the inner mold is preferably 0.2 m or less. ３ More than 3m ３ Preferably, it is 0.3 m or less. ３ More than 3m ３More preferably, the split mold 70 is as follows. As shown in FIG. 3, the split mold 70 includes a first mold 34 and a second mold 36. The first mold 34 includes a first inner mold 30A, and the second mold 36 includes a second inner mold 30B. The pair of molds, consisting of the first inner mold 30A and the opposing second inner mold 30B, are clamped together to form a cavity 10, a space to be filled with the expanded beads 50. The inner molds are therefore molds for forming the cavity. The pair of molds are provided with a plurality of vent holes 64 that connect the cavity 10 and the chamber 20 in a ventilated manner. Each vent hole 64 may be provided with an opening / closing mechanism (not shown) for opening and closing the hole. Here, as shown in the upper part of Figure 4A, when the inner mold is composed of a concave first inner mold 30A and a convex second inner mold 30B, the volume of the inner mold refers to the volume of the first inner mold 30A. As shown in the lower part of Figure 4A, the volume of the first inner mold 30A refers to the volume of the internal space 80 measured when the opening 38 of the first inner mold 30A is closed. Also, as shown in the upper part of Figure 4B, when the first inner mold 30A and the second inner mold 30B constituting the inner mold are both concave, as shown in the lower part of Figure 4B, the volume of the inner mold is the sum of the volumes of the internal spaces 80A of the first inner mold 30A and 80B of the second inner mold 30B. In other words, in the present invention, the volume of the inner mold refers to the sum of the volumes of the concave inner molds among the inner molds provided in the split mold 70. 4A and 4B show the cavity 10 having a square cross section, the shape of the cavity 10 is not particularly limited, and any shape can be used. Examples of any shape include a concave inner mold having a bowl-like recess, a V-shaped recess, or a more complex recess. Another example of an inner mold in which both the first and second inner molds constituting the inner mold are concave is one in which the first and second inner molds are combined to form a spherical shape. The volume of the inner mold can be determined as follows.First, using a cross-sectional view of the inner mold as shown in Figures 4A and 4B, the area is determined from the internal space (80) formed by connecting the openings at both ends of the inner mold with a straight line so as to close the opening (38). Next, the vertical length of the inner mold is determined from a cross-sectional view perpendicular to the cross-sectional view. The volume of the inner mold is calculated from the product of the area determined from the internal space (80) and the vertical length. Furthermore, if the inner mold has a complex shape and it is difficult to determine the internal space (80) from the cross-sectional view, the volume of the inner mold when the opening (38) is closed can be calculated based on the CAD data of the inner mold.

[0022] The electric steam boiler used in the present invention is not limited in the upper limit of the amount of steam generated per hour. For example, from the viewpoint of compactness and ease of transport, the electric steam boiler is preferably one having an upper limit of steam generation per hour of about 100 kg to 200 kg. That is, the amount of steam generated per hour by the electric boiler is preferably 200 kg or less, 100 kg or less, or an intermediate amount between these. As the electric steam boiler, for example, the electric reflux boiler Ecofoot NK-150RR manufactured by Nippon Denetsu Co., Ltd. can be used. The above-mentioned compact electric steam boiler generates a very small amount of steam compared to gas steam boilers and the like, which have a maximum steam generation amount of 1000 kg or more per hour. Therefore, it is generally disadvantageous for producing large polypropylene resin foamed bead moldings. However, the production method of the present invention is advantageous in that it is possible to produce large polypropylene resin foamed bead moldings with a large inner mold volume (m ３ ) to the ratio of the amount of steam (kg) used to form one molded body (kg / m ３ ) is adjusted to be 100 or less. This enables the production method of the present invention to use an electric steam boiler and produce large polypropylene resin foamed bead moldings. ３) is preferably 70 or less, more preferably 50 or less, and even more preferably 30 or less. By adjusting the ratio within the above range, it is possible to produce a foamed bead molding using an electric steam boiler, even when using expanded polypropylene resin beads that require high molding pressure during in-mold molding. ３ The means for adjusting the temperature (Tc) to a desired range is not particularly limited, but first to third embodiments will be described below as examples of such means. The first to third embodiments described below enable the use of an electric steam boiler to satisfactorily produce an expanded polypropylene resin bead molding.

[0023] The expanded beads 50 may be filled into the cavity 10 in a state where the internal pressure of the expanded beads 50 has been increased by, for example, a pressurized gas in advance, thereby enhancing the secondary expansion property. The internal pressure of the expanded beads can be measured by the method described in JP-A-2003-201361. The internal pressure refers to the pressure inside the cells of the expanded beads.

[0024] The expanded beads 50 can be produced by appropriately following conventional methods for producing expanded polypropylene resin beads. For example, first, necessary materials are charged into an extruder and melt-kneaded to prepare a resin melt, which is then extruded from the extruder in the form of a strand and cut to an appropriate length to produce pellet-shaped resin beads. The resin beads are then charged into a pressure vessel together with a dispersion medium such as water, impregnated with a blowing agent, heated to the foaming temperature, and expanded by releasing the resin beads under a pressure lower than the pressure inside the pressure vessel, thereby producing the expanded beads 50.

[0025] The expanded beads (50) used in the present invention are composed of polypropylene-based resins at a ratio of 50% by mass or more of the resin components constituting the expanded beads to 100% by mass. Resin components other than polypropylene-based resins suitable for use in expanded bead moldings may also be blended as appropriate. Examples of resin components other than polypropylene-based resins include thermoplastic resins other than polypropylene-based resins. Examples of such thermoplastic resins include one or more selected from the group consisting of polyethylene-based resins, polystyrene-based resins, polycarbonate resins, polyvinyl chloride resins, polymethacrylic resins, acrylonitrile-based resins, polyester-based resins, polyamide-based resins, and blend polymers thereof. The polypropylene-based resin content of 100% by mass of the resin components is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 100% by mass. Here, the polypropylene-based resin refers to a propylene homopolymer and / or a propylene-based copolymer containing 50% by mass or more of propylene-derived structural units. Examples of the propylene homopolymer include propylene-based resins such as isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene. These resins exemplified as propylene homopolymers may be used alone or in combination of two or more. The propylene-based copolymer preferably has a propylene-derived structural unit content of 80% by mass or more, more preferably 90% by mass or more. The propylene-based copolymer preferably has a propylene-derived structural unit content of 99% by mass or less, more preferably 98% by mass or less. Examples of such propylene-based copolymers include copolymers of propylene and ethylene and / or an α-olefin having 4 to 20 carbon atoms. Examples of the α-olefin include 1-butene, 1-pentene, 1-hexene, 1-octene, and 4-methyl-1-butene. Other examples of propylene copolymers include ethylene-propylene random copolymers, propylene-butene random copolymers, and ethylene-propylene-butene random copolymers.These propylene-based copolymers may be, for example, random copolymers or block copolymers, but are preferably random copolymers. Examples of the propylene-based copolymers include impact-resistant polypropylenes (block polypropylenes) composed of two or more phases including a continuous phase of propylene polymer and a rubber phase present as a dispersed phase in the continuous phase. Examples of polymers constituting the rubber layer include ethylene-α-olefin copolymers. These resins exemplified as propylene-based copolymers may be used alone or in combination of two or more. Furthermore, the polypropylene-based resin may be a linear polypropylene-based resin, a branched polypropylene-based resin, or a combination thereof.

[0026] The melting point of the polypropylene-based resin is preferably 155°C or lower. In this case, a molded article having excellent appearance and rigidity can be molded at a lower molding temperature. From the viewpoint of improving this effect, the melting point of the polypropylene-based resin is more preferably 150°C or lower, and even more preferably 145°C or lower. On the other hand, from the viewpoint of further improving the heat resistance and mechanical strength of the molded article, the melting point of the polypropylene-based resin is preferably 135°C or higher, more preferably 138°C or higher, and even more preferably 140°C or higher. That is, the melting point of the polypropylene-based resin is preferably 155°C or lower but 135°C or higher, more preferably 150°C or lower but 138°C or higher, and even more preferably 145°C or lower but 140°C or higher.

[0027] Other Polymers: The expanded beads (50) may contain other polymers besides the thermoplastic resins described above, provided that the purpose and effects of the present invention are not impaired. Examples of such other polymers include thermoplastic elastomers such as olefin-based thermoplastic elastomers (TPO) and urethane-based thermoplastic elastomers (TPU). The other polymers may be one type or a combination of two or more types.

[0028] The content of the other polymer in the expanded beads 50 is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and particularly preferably 0% by mass. In other words, it is particularly preferable that the expanded beads 50 contain substantially only thermoplastic resin as the polymer.

[0029] Optional Additives: The expanded beads (50) may contain one or more optional additives as appropriate, provided that the additives do not impair the objects and effects of the present invention. For example, optional additives include various conventionally known additives such as conductive materials, antioxidants, flame retardants, flame retardant assistants, radiation suppressants, cell regulators, lubricants, crystal nucleating agents, light stabilizers such as ultraviolet inhibitors, antistatic agents, and colorants.

[0030] Bulk Density of Expanded Beads The bulk density of the expanded beads (50) is not particularly limited, but is preferably 10 kg / m ３ It is preferable that the viscosity is 15 kg / m or more. ３ More preferably, it is 20 kg / m or more. ３ More preferably, it is 200 kg / m or more. ３ More preferably, it is 100 kg / m or less. ３ More preferably, it is 50 kg / m or less. ３ That is, the bulk density of the expanded particles 50 is preferably 10 kg / m or less. ３ More than 200kg / m ３ More preferably, it is 15 kg / m or less. ３ More than 100kg / m ３ More preferably, it is 20 kg / m or less. ３ More than 50kg / m ３The bulk density of the expanded beads (50) is preferably within the above range, since it is possible to obtain an expanded bead molding that is lightweight and has excellent fusion properties. The method for measuring the bulk density of the expanded beads (50) is as follows. First, the expanded beads to be measured are left to stand for 24 hours or more in an environment of a temperature of 23°C, a relative humidity of 50%, and 1 atm. The expanded beads having the weight W (g) thus obtained are filled into a measuring cylinder, and the bottom of the measuring cylinder is lightly tapped on a horizontal surface several times to stabilize the filling height of the expanded beads in the measuring cylinder. The bulk volume V (L) of the expanded beads indicated on the measuring cylinder is read, and the weight W of the expanded beads is divided by the bulk volume V of the expanded beads (W / V). The value obtained in this manner is expressed as kg / m ３ The bulk density of the expanded beads (kg / m ３ ) can be obtained.

[0031] The expanded beads 50 may be single-layer expanded beads having only a particulate foam layer, or may be multi-layer expanded beads having a particulate foam layer as a core layer and a covering layer covering the foam layer. The covering layer may cover the entire surface of the foam layer, or may cover only a portion of the surface of the foam layer.

[0032] [First embodiment] In the first embodiment, in addition to the manufacturing process related to conventional in-mold molding, a pre-pressure process described below is performed. In this embodiment, pressure (P1) refers to the pressure inside the cavity adjusted by the pre-pressure process, pressure (P2) refers to the steam pressure used in the heating process, and pressure (P3) refers to the pressure inside the chamber during the heating process, which is the molding pressure when producing a molded body. In this specification, conventional in-mold molding that does not perform the pre-pressure process is sometimes referred to as normal molding, and in-mold molding that includes the pre-pressure process is sometimes referred to as the first embodiment.

[0033] In this embodiment, in-mold molding involves a filling step, a pre-pressure step, and a heating step, in which expanded polypropylene resin beads are filled into a cavity of a split mold consisting of a first mold and a second mold. The pre-pressure step is a step in which a pressurized gas other than steam is supplied into the cavity filled with the expanded beads to adjust the pressure in the cavity filled with the expanded beads to a pressure (P1) exceeding atmospheric pressure. The filling step may be performed before or overlapping with the pre-pressure step. The heating step is performed following or overlapping with the pre-pressure step. The heating step is a step in which the expanded beads are heated by supplying steam at a pressure (P2) higher than the pressure (P1) into the cavity adjusted to the pressure (P1) in the pre-pressure step. In this embodiment, a split mold having a cavity and a chamber that is ventilated may be used, and in the heating step, while supplying steam at the pressure (P2) into the cavity, the pressure in the chamber may be adjusted to a pressure (P3) that exceeds atmospheric pressure but is lower than the pressure (P2). A relationship of atmospheric pressure < pressure (P3) < pressure (P2) is preferred. Note that adjusting the pressure in the chamber to pressure (P3) makes it easier for steam to flow from the cavity to the chamber in the heating step.

[0034] According to the inventors' investigations, in conventional in-mold molding, much of the steam supplied to the split mold is used to increase the internal pressure of the entire split mold before the expanded beads are heated and fused, resulting in significant energy loss. This is because the steam is supplied into the split mold, which is at atmospheric pressure, and is consumed to heat the split mold, causing the pressure and temperature of the steam to drop immediately after supply. In other words, in conventional in-mold molding, it is estimated that the amount of steam supplied is several to several dozen times the amount actually required to heat-fuse the expanded beads filled in the cavity. In contrast, by performing the above-mentioned pre-pressure step and adjusting the pressure in the cavity to a pressure (P1) above atmospheric pressure, the amount of steam used in the heating step can be significantly reduced. In other words, in the manufacturing method of the first embodiment, by performing the heating step after the pre-pressure step, the supplied steam can be efficiently used to heat-fuse the expanded beads. According to this embodiment, in-mold molding including the pre-pressure step is performed, and the volume of the inner mold is 0.1 m ３ Even if the volume of the inner mold (m ３ ) to the ratio (kg / m) of the amount of steam (kg) used in molding one molding. ３ ) can be sufficiently suppressed to 100 or less. Furthermore, an expanded polypropylene resin bead molding requiring a high molding pressure can be molded using an electric steam boiler. The pressure inside the cavity in the pre-pressure step may be adjusted to a predetermined pressure as appropriate. Furthermore, the heating step can be performed by adjusting the supply of steam under conditions where the pressure inside the cavity reaches the predetermined pressure. This embodiment will be described in further detail below.

[0035] (Split Mold) First, a manufacturing apparatus 100 used in the manufacturing method of this embodiment will be described with reference to FIG. 5. FIG. 5 is a longitudinal cross-sectional view of the manufacturing apparatus 100 used in this embodiment. The manufacturing apparatus 100 shown in FIG. 5 is configured similarly to the manufacturing apparatus 100 shown in FIG. 3, except for the inclusion of a steam supply unit 60. In this embodiment, chambers 20 are provided on the rear side of the first inner mold 30A and the rear side of the second inner mold 30B. In FIG. 5, the chambers 20 refer to the space between the first inner mold 30A and the first frame 40A that covers it, and the space between the second inner mold 30B and the second frame 40B that covers it. The first frame 40A and the second frame 40B form a frame 40 that constitutes the exterior of the molding machine 110, and a pair of molds consisting of the first inner mold 30A and the second inner mold 30B are provided inside the frame 40. The frame 40 is provided with exhaust holes 66 that can exhaust gas from the chamber 20 toward the outside of the molding machine 110. The exhaust holes 66 allow ventilation between the chamber 20 and the outside of the molding machine 110. In this embodiment, the exhaust holes 66 are provided in the wall surfaces of the first frame 40A and the second frame 40B, and each exhaust hole 66 is provided with an opening / closing mechanism 67 that can open and close the opening. The opening / closing mechanism refers to a mechanism that switches the through-hole between an open state and a closed state, and includes, but is not limited to, a drain valve. Note that with regard to the above terms, the first inner mold 30A is also referred to as the mold cavity-side inner mold, and the second inner mold 30B is also referred to as the mold core-side inner mold.

[0036] The cavity (10) is a space into which expanded beads (50) for producing an expanded bead molding are filled. One end region of a filling feeder (52) that penetrates the frame (40) is inserted into the cavity (10). The other end region of the filling feeder (52) is located outside the manufacturing apparatus (100), and the expanded beads (50) are filled from that end. Each mold is provided with an air vent (64) to ensure ventilation between the cavity (10) and at least one of the chambers (20). Steam or a pressurized gas other than steam can ventilate the cavity (10) and the chambers (20) through the air vent (64). While the air vent (64) may be always open, in this embodiment, an opening / closing mechanism (65) is provided to open and close the opening to properly adjust the internal pressure of the cavity (10).

[0037] Next, the steam supply unit (60) will be described. The molding machine (110) is provided with a steam supply unit (60) for supplying steam to the interior thereof. In this embodiment, the steam supply unit (60) is a tubular body for supplying steam from an electric steam boiler (120) to the interior of the molding machine (110) through a pipe (130). With the electric steam boiler (120) side defined as the upstream side and the steam discharge side defined as the downstream side, the steam supply unit (60) is provided with one or more supply holes (68) for discharging steam at a downstream end region. More specifically, in this embodiment, the steam supply unit (60) is provided with a main flow path (60A) for introducing steam from the outside to the interior of the molding machine (110) and a plurality of sub-flow paths (60B) branching from the main flow path (60A). The sub-flow paths (60B) have supply holes (68) formed in their downstream end regions. In this embodiment, the supply holes 68 are located flush with the cavity surface, allowing steam to be introduced directly into the cavity 10. Here, the cavity surface refers to the same surface as the molding walls 32, which are the walls of a pair of molds forming the cavity 10 as an internal space. The term "flush" between the cavity surface and the supply holes 68 does not require strict flushness; rather, it is sufficient that the difference between the cavity surface and the opening surface of the supply holes 68 is approximately half the thickness of the molding walls 32. For example, if the thickness of the molding walls 32 is 10 mm, the difference between the cavity surface and the opening surface of the supply holes 68 may be 5 mm or less, and the difference between the outer surface of the molding walls 32 and the opening surface of the supply holes 68 may be 5 mm or less. Here, the difference between the cavity surface and the opening surface refers to the distance between the cavity surface and the opening surface opposite it. However, the supply hole 68 in this embodiment is not limited to this. Although not shown, this embodiment also encompasses an embodiment in which, for example, the downstream region of the sub-channel 60B penetrates the molding wall 32 and projects into the cavity 10, thereby providing the supply hole 68 inside the cavity 10. The supply hole 68 is open to the interior of the cavity 10 and can supply steam to the cavity 10 through this opening.In this embodiment, a steam supply section 60 is provided on the molding wall 32 of each of the first inner mold 30A and the second inner mold 30B, and steam is supplied to the cavity 10 from both opposing sides. The supply holes 68 are arranged alternately across the cavity 10, and the alternate arrangement of the supply holes 68 makes it easier for steam to reach the entire cavity 10.

[0038] The split mold described above is generally made of metal, but the heat of the steam supplied into the mold during the heating process described below may be absorbed by the metal. From this perspective, the split mold may be made of a material with a lower thermal conductivity than metal. The surface of the split mold made of metal may also be coated with a material with a lower thermal conductivity than metal. In particular, the molding wall (32) forming the cavity (10) may be made of a material with a lower thermal conductivity, or the inner and / or outer surfaces of the molding wall (32) may be coated with a material with a lower thermal conductivity. The coating method described above is not limited, and examples include applying a resin paint or the like to the entire split mold made of metal or to a partial area, such as the molding wall (32), to form a coating layer, or applying a resin tape or the like. Here, examples of materials with lower thermal conductivity than metal include resin materials, and more specific examples include polytetrafluoroethylene (PTFE), polyphenylene sulfide (PPS), polyether ether ketone (PEEK), and polyetherimide (PEI).

[0039] (Filling Process) The filling process in this embodiment will be described. The filling process is a process of filling the cavity 10 with the expanded beads 50. In this embodiment, the cavity 10 is filled with the expanded beads 50 from a filling feeder 52 provided in the molding machine 110. The method of filling the expanded beads 50 can be carried out by appropriately imitating the filling method used in conventional in-mold molding. In this embodiment, the cavity 10 provided inside a pair of clamped molds is filled with the expanded beads 50 under atmospheric pressure.

[0040] The manufacturing method of this embodiment involves a pre-pressurization step, which will be described later. Therefore, in order to make the expanded beads 50 less likely to be compressed by pre-pressurization, it is preferable to use expanded beads whose internal pressure has been increased by pre-pressurization with a pressurized gas. The internal pressure of the expanded beads 50 is not particularly limited, but is preferably 0.1 MPa (G) or more and 0.2 MPa (G) or less.

[0041] (Pre-pressurization Step) In this embodiment, the pre-pressurization step is performed either simultaneously with the filling step or after completion of the filling step. In this embodiment, a pressurized gas other than steam is supplied into each chamber 20 and the cavity 10 to adjust the pressure in the cavity 10 to a pressure exceeding atmospheric pressure. The pressurized gas is a gas for increasing the pressure in the cavity 10 to a pressure (P1) exceeding atmospheric pressure. The pressurized gas other than steam can be, for example, one or more gases selected from the group consisting of inorganic gases, organic gases, and mixtures thereof. From the standpoints of safety and economy, the pressurized gas is preferably one or a combination of two or more gases selected from inorganic gases such as air, nitrogen, and carbon dioxide, with air being particularly preferred.

[0042] The means for supplying pressurized gas during the pre-pressurization step is not particularly limited. For example, as shown in FIG. 5 , an air inlet (69) capable of supplying pressurized gas can be provided inside the molding machine (110), and the pressurized gas can be supplied through the air inlet (69). Alternatively, the air inlet (69) can be omitted, and one or more of the exhaust holes (66) can be used as the pressurized gas inlet. Alternatively, a flow path switching mechanism (not shown) can be provided in the steam supply unit (60) or in the upstream piping (130) to supply a pressurized gas other than steam from the steam supply unit (60) into the molding machine (110) during the pre-pressurization step, and then the flow path can be switched to supply steam into the molding machine (110) during the heating step described below. During the pre-pressurization step, the exhaust holes (66) not used to supply pressurized gas can be closed, and the vent holes (64) provided in the pair of molds can be left open. This allows the pressure inside the chamber (20) and cavity (10) to be adjusted to a pressure (P1) above atmospheric pressure.

[0043] The pressure (P1) inside the cavity 10 after the pre-pressure step has been performed need only exceed atmospheric pressure and is not limited to a predetermined value. From the viewpoint of preventing the filled foamed beads 50 from being compressed by the pressure of a pressurized gas other than steam, the pressure (P1) inside the cavity 10 in the pre-pressure step is preferably adjusted to 0.5 MPa (G) or less, more preferably adjusted to 0.45 MPa (G) or less, and even more preferably adjusted to 0.4 MPa (G) or less. When the pressure (P1) inside the cavity 10 adjusted by the pre-pressure step is equal to or less than the above pressure, the foamed beads 50 shrink, increasing the gap inside the cavity 10, and thus shrinkage and deterioration of internal fusion in the resulting foamed bead molding can be effectively prevented. Furthermore, from the viewpoint of enabling sufficient reduction in the amount of steam used in the heating step described below by carrying out the pre-pressure step, the pressure (P1) inside the cavity (10) is preferably adjusted to 0.1 MPa (G) or more, more preferably adjusted to 0.15 MPa (G) or more, and even more preferably adjusted to 0.2 MPa (G) or more. From the above viewpoints, the pressure (P1) inside the cavity (10) adjusted by the pre-pressure step is preferably 0.1 MPa (G) to 0.5 MPa (G), more preferably 0.15 MPa (G) to 0.45 MPa (G), and even more preferably 0.2 MPa (G) to 0.4 MPa (G).

[0044] In order to facilitate adjustment of the pressure (P1) in the cavity (10) to a desired pressure in the pre-pressurization step, it is preferable to supply pressurized gas as described above and to adjust the pressure by exhausting excess pressurized gas to the outside of the molding machine (110) through the exhaust hole (66).

[0045] (Heating Step) By carrying out the pre-pressure step, the pressure (P1) in the cavity (10) is already adjusted to a pressure exceeding atmospheric pressure at the start of the heating step. Under this condition, steam at a pressure (P2) higher than the pressure (P1) in the cavity (10) is supplied from the steam supply unit (60) provided in the molding machine (110) into the cavity (10) to heat the expanded beads (50). Furthermore, during the heating step, steam at the pressure (P2) may be supplied into the cavity while adjusting the pressure in the chamber to a pressure (P3) higher than atmospheric pressure but lower than the pressure (P2). During the heating step, the exhaust hole (66) may be closed, and the pressure (P3) may be adjusted by adjusting the relationship between the pressure (P1) and the pressure (P2). Alternatively, the pressure (P3) may be adjusted by exhausting air from the exhaust hole (66) to the outside of the molding machine (110). In order to quickly exhaust pressurized gas other than steam from the cavity in the pre-compression step, it is preferable to adjust the exhaust to the outside of the molding machine (110) through the exhaust hole (66).

[0046] Differential Pressure (P2-P1): From the viewpoint of fully demonstrating the effects of this embodiment, the differential pressure (P2-P1) obtained by subtracting the pressure (P1) in the cavity (10) to which pressurized gas is supplied by performing the pre-pressurization step from the pressure (P2) of the steam supplied from the steam supply section (60) into the cavity (10) during the heating step is preferably 0.1 MPa or more and 0.4 MPa or less, and more preferably 0.15 MPa or more and 0.35 MPa or less. That is, the lower limit of the differential pressure (P2-P1) is preferably 0.1 MPa, and more preferably 0.15 MPa. Furthermore, the upper limit of the differential pressure (P2-P1) is preferably 0.4 MPa, and more preferably 0.35 MPa. When the differential pressure (P2-P1) is equal to or greater than the lower limit of the preferred pressure range described above, the heating time during the heating step can be shortened. That is, the steam output time is shortened. As a result, the expanded beads 50 begin to expand during the heating process, making it difficult for steam to pass between the expanded beads 50, thereby suppressing the generation of a temperature difference between the periphery of the supply holes 68 and the end of the cavity 10. Therefore, by ensuring that the pressure difference (P2-P1) is equal to or greater than the lower limit of the preferred pressure range described above, a good expanded bead molding is obtained that is free of shrinkage in the area corresponding to the periphery of the supply holes 68, shrinkage of the molding as a whole, and uneven heating. On the other hand, by ensuring that the pressure difference (P2-P1) is equal to or less than the upper limit of the preferred pressure range described above, excessive compression of the expanded beads due to the pressure difference (P2-P1) is suppressed. Therefore, a high filling rate of the expanded beads 50 in the cavity 10 is maintained, and a uniformly expanded expanded bead molding with good fusion properties and free of uneven heating is easily obtained. The pressure (P2) of steam supplied from the steam supply unit 60 into the cavity 10 in the heating step means the average pressure of steam supplied from the steam supply unit 60 into the cavity 10. The pressure (P1) in the cavity 10 to which pressurized gas is supplied by carrying out the pre-pressurization step means the pressure in the cavity after carrying out the pre-pressurization step.

[0047] Differential pressure (P1-P3): From the viewpoint of fully demonstrating the effects of this embodiment, the differential pressure (P1-P3), obtained by subtracting the pressure (P3) in the heating step from the pressure (P1) in the pre-pressure step, is preferably −0.1 MPa or more and +0.1 MPa or less, more preferably −0.05 MPa or more and +0.05 MPa or less, and even more preferably, the pressure (P1) and the pressure (P3) are the same. That is, the lower limit of the differential pressure (P1-P3) is preferably −0.1 MPa, more preferably −0.05 MPa, and even more preferably 0 MPa. The upper limit of the differential pressure (P1-P3) is preferably +0.1 MPa, more preferably +0.05 MPa, and even more preferably 0 MPa. Note that the notation "+" used here indicates a positive numerical value. In other paragraphs, numerical values ​​without the notation "+" or "-" also indicate positive numerical values. When the differential pressure (P1-P3) satisfies the above range, compressed air other than steam in the cavity can be efficiently replaced with steam in the heating step, and molding can be completed with a short steam output time.

[0048] Steam Exhaust Adjustment: As described above, during the heating step, steam at a pressure (P2) higher than the pressure (P1) within the cavity (10) is supplied from the steam supply unit (60) into the cavity (10). This adjusts the pressure within the cavity (10) to a pressure suitable for thermal fusion of the expanded beads (50). To more fully implement this adjustment, steam exhaust adjustment may be performed as described below while supplying steam into the cavity (10) during the heating step. Adjusting the steam exhaust also facilitates adjustment of the pressure difference (P2-P1) described above. The steam exhaust adjustment involves venting a portion of the steam within the cavity (10) to each chamber (20) through vent holes (64) provided in the molding walls (32) that constitute a pair of molds. The vent hole (64) is provided in the molding wall (32) separating the cavity (10) from the chamber (20) and ensures ventilation between the chamber (20) and the cavity (10). In addition to the exhaust, steam exhaust control can be performed by adjusting the pressure in the cavity (10) by opening and closing an opening / closing mechanism (65) provided in the vent hole (64). In another embodiment, the steam exhaust control may be performed by exhausting steam from the cavity (10) to the chamber (20) through the vent hole (64) provided in the molding wall (32), and exhausting the steam from the chamber (20) to the outside of the molding machine (110) through an exhaust hole (66) that connects the chamber (20) to the outside of the molding machine (110). In addition to the exhaust, the pressure inside the cavity 10 can be adjusted by opening and closing the opening / closing mechanism 65 provided on the vent hole 64 and / or the opening / closing mechanism 67 provided on the exhaust hole 66. In the steam exhaust adjustment, the exhaust is preferably adjusted by the opening / closing mechanism 67 provided on the exhaust hole 66, as this facilitates adjustment of the pressure difference (P2-P1). By adjusting the steam exhaust as described above, it is possible to adjust the heating temperature of the expanded beads 50 filled in the cavity 10. This allows the expanded beads 50 to be more effectively fused together.

[0049] In addition to carrying out the steam exhaust adjustment described above, it is advisable to check the change in temperature inside the cavity 10 by monitoring a thermometer (not shown) installed inside the cavity 10. Here, the temperature inside the cavity 10 refers to one or more of the ambient temperature inside the cavity 10, the temperature of the expanded beads 50 filled in the cavity 10, and the temperature of the expanded bead molding molded in the cavity 10.

[0050] The foamed bead molding is produced by carrying out the above-described filling step, pre-pressure step, and heating step. The mold is generally opened after confirming that the surface pressure of the foamed bead molding formed in the cavity 10 has reached a level suitable for removal. In this embodiment, a cooling step or a combination of steps may be carried out as appropriate before the mold is opened.

[0051] Compared to conventional molding without a pre-pressure step, this embodiment, which includes a pre-pressure step, significantly shortens the heating time during the heating step. Therefore, the expanded beads (50) that have expanded during the heating step are sufficiently prevented from adhering to each other, and the supplied steam can be maintained in a state where it is easy to pass between the expanded beads. As a result, steam is supplied to the cavity (10) in a balanced manner, preventing localized increases in heating temperature and making it easier to obtain an excellent expanded bead molded product without dents.

[0052] [Second embodiment] Next, the volume of the inner mold (m ３ ) to the ratio of the amount of steam (kg) used to form one molded body (kg / m ３ As another means for suppressing the steam loss ratio (%) to 100 or less, a second embodiment using expanded beads having through holes will be described. In this embodiment, expanded beads having through holes are used. Desired effects can also be obtained by combining the first embodiment and / or the third embodiment described below with the second embodiment. In particular, combining the first embodiment and the second embodiment is preferable because it can more desirably solve the problem of steam loss described above.

[0053] A feature of this embodiment is that cylindrical polypropylene-based resin expanded beads (54) having through holes, as illustrated in Figures 6A to 6D, are used as the expanded beads. Note that hereinafter, the polypropylene-based resin expanded beads (54) may be simply referred to as expanded beads (54). Examples of cylindrical expanded beads (54) include, but are not limited to, a cylindrical body having a through hole (56) with a substantially circular cross section, as shown in Figure 6A, a triangular prism having a through hole (56) with a substantially triangular cross section, as shown in Figure 6B, a cylindrical body having a through hole (56) with a substantially square cross section, as shown in Figure 6C, and a barrel-shaped body with an outwardly bulging outer surface and having thin through holes (56) inside, as shown in Figure 6D.

[0054] Through research by the present inventors, it was discovered that the amount of steam used during in-mold molding can be reduced by using expanded beads (54) with internal through-holes. While the reasons for this are unclear, it is speculated that a first factor is that steam passes through the through-holes during the heating process, allowing the expanded beads (54) to be efficiently heated from both the inside and the outside. A second factor is speculated to be that the expansion of the expanded beads (54) undergoing secondary expansion during the heating process occurs not only on the outside but also on the through-hole side, making it more difficult for the internal pressure in the cavity to increase compared to when conventional expanded beads are used, shortening the cooling time until the surface pressure reaches a level suitable for removal. As a result, the temperature of the split mold can be maintained to a certain extent during the next molding operation, which is speculated to substantially reduce the amount of steam supplied to the split mold during continuous in-mold molding.

[0055] For details of the expanded beads (54) having through holes, reference can be made to, for example, JP-A-1996-108441 and WO2022-270425.

[0056] [Third embodiment] Next, the volume of the inner mold (m ３ ) to the ratio of the amount of steam (kg) used to form one molded body (kg / m ３A third embodiment will be described in which the volume of the chamber is reduced as another means for suppressing the ratio (%) of the volume of the chamber to 100 or less. In this embodiment, the volume of the chamber is reduced as described below. More specifically, the split molding mold used in the manufacturing method of the present invention has a first mold having a first inner mold and a second mold having a second inner mold, and a cavity is formed by the first inner mold and the second inner mold during in-mold molding, and the mold satisfies the following conditions (A) and / or (B). Here, the third embodiment in which the ratio of the sum of the volumes of the chambers provided in the first mold and the second mold to the volume of the cavity is adjusted to 1 or less is one of the preferred aspects of the present invention. (A) The first mold has a first frame covering the first inner mold, and the space between the first inner mold and the first frame forms a chamber. (B) The second mold has a second frame covering the second inner mold, and the space between the second inner mold and the second frame forms a chamber. In addition, in the calculation of the sum of the volumes of the chambers provided in the first mold and the second mold, the case where the volume of the chamber provided in the first mold is zero or the case where the volume of the chamber provided in the second mold is zero is also included. Furthermore, desirable effects can also be obtained by combining the first and / or second embodiments with the third embodiment. In particular, combining the first and third embodiments is preferable because it can more desirably solve the steam loss problem.

[0057] According to the inventors' investigations, in addition to the energy loss factors described in the first embodiment, it was presumed that in normal in-mold molding, a considerable proportion of the steam supplied to the split mold is absorbed by the split mold itself, which also contributes to energy loss. In this embodiment, to alleviate the energy loss problem described above, the volume of the chamber is reduced, thereby reducing the mass of the frame that constitutes the chamber, thereby suppressing the amount of steam absorbed by the frame.

[0058] Specifically, for example, in the split mold 70, the volume of the chamber 20 in either or both of the first mold 34 and the second mold 36 can be reduced. In the present invention, the volume of the chamber 20 refers to the total volume of the chambers 20 provided in the split mold 70.

[0059] From the viewpoint of reducing the amount of steam used in molding and making it easier to stabilize the steam pressure, in this embodiment, the ratio of the volume of the chamber (20) to the volume of the cavity (10) is preferably 1 or less, more preferably 0.9 or less, and even more preferably 0.8 or less.

[0060] [Reference Examples] Below, several Reference Examples are shown to explain the present invention in more detail. Note that the Reference Examples and various data shown below do not limit the present invention in any way. First, in order to confirm the effect of carrying out the pre-pressure step described in the first embodiment, Reference Example 1 in which the pre-pressure step was carried out and Reference Example 2, which is an example of normal molding, are shown. In Reference Examples 1 and 2, in-mold molding was carried out using a gas boiler and a conventional manufacturing apparatus shown in FIG. 7. Note that Reference Examples 1 and 2 were prepared using an ethylene-propylene random copolymer having a melting point of 141.5°C, an ethylene content of 3.1% by mass, a melt flow rate of 7 g / 10 min, and a bulk density of 29.0 kg / m ３ The melt flow rate was measured in accordance with JIS K7210-1:2014 under conditions of a load of 2.16 kg and 230°C.

[0061] Reference Example 1 (Molding with Pre-Pressure Step) First, polypropylene-based resin foam beads before pressurization were filled into a pressure filling tank. Then, pressurized air was supplied into the tank and the tank was left to stand for 24 hours, and the internal pressure of the polypropylene-based resin foam beads was set to 0.14 MPa (G). A split mold having the same configuration as in FIG. 5 was used to manufacture a foamed bead molding. Specifically, when the mold was clamped, the dimensions were 1640 mm long x 1400 mm wide x 140 mm high (inner mold volume: 0.31 m). ３A split mold having a cavity capable of molding a flat-plate-shaped molded article (size: 1000 mm x 1000 mm) was used. Next, the vertical length of the inner mold was determined from a cross-sectional view perpendicular to the cross-sectional view. The volume of the inner mold was calculated from the product of the area determined from the internal space and the vertical length. The split mold was adjusted so that the cracking distance was 6 mm, and in this state, a filling process was carried out by filling it with polypropylene resin foam particles to which internal pressure had been applied in advance. Note that under the above conditions, the cracking rate was 10%. Next, pressurized air was supplied through an air supply hole provided in the frame of the split mold to pressurize each chamber, and pressurized air was also supplied to the cavity through an air vent, thereby carrying out a pre-pressurization process until the pressure inside the cavity reached 0.25 MPa (G). Note that the pressure inside the cavity (P1) was determined from the value of a pressure gauge provided in the piping supplying the pressurized gas when the pressurized gas supplied into the cavity was stationary. Next, steam at an output pressure of 0.45 MPa (G) was directly supplied to the cavity for 6 seconds from a steam supply unit having a supply hole disposed in the cavity, and a heating step was carried out. The differential pressure (P2-P1) obtained by subtracting the pressure (P1) in the cavity adjusted in the pre-pressure step from the output pressure (P2) of the steam used in the heating step was 0.20 MPa. The steam output pressure (P2) was determined from the value of a pressure gauge installed in the piping supplying the pressurized gas. The pressure in the split mold was 0.25 MPa (G). The maximum temperature of the first inner mold in the heating step was 139°C. The amount of steam consumed in the heating step when molding one molded body was 5.0 kg. The volume (m ３ The amount of steam (kg) used to mold a single molded body is 16 kg / m ３The pressure inside the split mold refers to the pressure inside the chamber (P3). The maximum temperature of the first inner mold is the highest temperature measured during the heating process, measured by inserting a thermocouple into the cavity in advance to measure the temperature inside the molded body. The temperature inside the molded body here refers to the temperature measured at a position halfway between the vertical and horizontal directions of the molded body, and 20 mm from the cavity surface of the first inner mold toward the inside of the molded body. After the heating process was completed, a cooling process was carried out, the split mold was opened, and the expanded bead molded body was removed. The density of the obtained expanded bead molded body was 33.1 kg / m ３ The resulting expanded bead moldings were evaluated as follows: shrinkage (dentation) of the moldings was a, and fusion property was b. The evaluation criteria for each will be described later.

[0062] [Reference Example 2] (Normal Molding) A conventional split mold similar to the split mold shown in Figure 5 was prepared, except that it did not have a steam supply section. The expanded beads, with an internal pressure adjusted to 0.14 MPa, were filled into a chamber with a cracking distance of 6 mm, and then the mold was clamped. Under the above conditions, the cracking rate was 10%. Subsequently, without performing a pre-pressurization step, steam at an output pressure of 0.60 MPa (G) was supplied to the split mold for 5 seconds with the split mold's opening / closing mechanism open, replacing the air in the split mold with steam. Subsequently, as one-way heating, steam at an output pressure of 0.60 MPa (G) was supplied into the chamber from the first frame side for 5 seconds, raising the pressure to 0.26 MPa (G) to heat the expanded beads. Furthermore, as reverse one-way heating, steam at an output pressure of 0.60 MPa (G) was supplied into the chamber from the second frame side for 3 seconds, raising the pressure to 0.36 MPa (G) to heat the expanded beads. Next, double-sided heating was performed by applying steam at an output pressure of 0.60 MPa (G) for 7 seconds by opening and closing the supply holes on both sides of the split mold so that the pressure inside the split mold was maintained at 0.38 MPa (G), and the heating process was completed. Both sides of the split mold refer to the first frame side and the second frame side. During heating, the exhaust holes in the frame of the split mold were closed except for the part used to supply steam, and the vent holes on the pair of molds were left open. This allowed the steam supplied to the split mold to be introduced into the chamber. The pressure inside the split mold refers to the pressure inside the chamber. The total consumption of steam used in the heating process was 50.0 kg. The volume (m) of the inner mold was 1.0 m. ３ The amount of steam (kg) used to mold a single molded body is 160 kg / m ３ The maximum temperature of the first inner mold in the heating step was 146°C. After the above-mentioned heating step was carried out, the mold was cooled and the expanded bead molding was removed from the mold. The density of the expanded bead molding obtained was 32.8 kg / m ３ The resulting expanded bead moldings were evaluated as follows: shrinkage (dentation) of the moldings was a, and fusion property was b. The evaluation criteria for each will be described later.

[0063] The densities of the expanded bead moldings of Reference Examples 1 and 2 obtained as described above were measured as follows, and their shrinkage and fusion properties were evaluated as follows. (Density of Expanded Bead Molding) The density was calculated by dividing the mass of the expanded bead molding by the volume calculated based on the dimensions. (Shrinkage (Dentation)) First, the outer surface of the expanded bead molding removed from the split mold was visually observed. Next, the expanded bead molding was left to stand for 24 hours in an environment at a temperature of 23°C and a relative humidity of 50% for curing. The expanded bead molding after curing was visually observed and rated as follows: a (very good): Small shrinkage was observed before curing, and no shrinkage was observed after curing. b (good): Significant shrinkage was observed before curing, but the shrinkage recovered after curing. c (poor): Significant shrinkage was observed both before and after curing. d (bad): Significant shrinkage was observed both before and after curing, and some melted areas were observed. (Weldability) The obtained foamed bead molding was bent and broken, and the number of foamed beads present on the broken surface (C1) and the number of broken foamed beads (C2) were determined. The ratio of the number of broken foamed beads to the total number of foamed beads (C2 / C1 x 100) was calculated as the material failure rate. The above measurement was carried out five times using different test pieces, and the material failure rate was determined for each test piece. The arithmetic mean value was evaluated as follows: b (good): Material failure rate of 80% or more; d (bad): Material failure rate of less than 80%.

[0064] The steam consumption in Reference Example 1 was 5.0 kg, whereas the steam consumption in Reference Example 2 was 50.0 kg. That is, it was confirmed that the steam consumption in Reference Example 1, which was subjected to the pre-pressing step, was significantly smaller than that in Reference Example 2, which was a normal molding. In addition, the volume (m ３ The amount of steam (kg) used to mold a single molded body is 16 kg / m ３ The difference in steam consumption due to differences in manufacturing conditions was particularly large when the volume of the inner mold was 0.1 m or less. ３From the above, it is considered that this was significant because a large foamed bead molding of 0.1 m or more was produced. ３ It was confirmed that even when manufacturing large foamed bead moldings such as those described above, it was possible to mold them sufficiently using a single electric steam boiler. ３ ) was obtained by calculating the volume of the inner mold when the opening was closed based on the CAD data of the inner mold.

[0065] The present invention described above encompasses the following technical concepts: (1) A method for producing an expanded polypropylene resin bead molded article, comprising an expanded bead molded article producing apparatus having a molding machine equipped with a split molding die consisting of a first die and a second die, and one or more steam generators provided for the molding machine, in which a cavity of the split molding die is filled with expanded polypropylene resin beads and the expanded beads are fused together by heating with steam, at least one die of the split molding die having a concave inner die, and the volume of the inner die defined by the opening surface of the concave inner die and the inner surface of the inner die is 0.1 m ３ or more, the steam generator is an electric steam boiler, and the volume (m ３ ) to the ratio of the amount of steam (kg) used for molding (kg / m ３(2) A method for producing an expanded polypropylene resin bead molded article according to (1), characterized in that it comprises a pre-pressurizing step of supplying a pressurized gas other than steam into the cavity filled with the expanded beads to adjust the pressure in the cavity to a pressure (P1) higher than atmospheric pressure, and a heating step of heating the expanded beads by supplying steam at a pressure (P2) higher than the pressure (P1) into the cavity adjusted to the pressure (P1) in the pre-pressurizing step. (3) A method for producing an expanded polypropylene resin bead molded article according to (2), characterized in that in the pre-pressurizing step, the pressure (P1) is adjusted to be 0.1 MPa(G) or more and 0.5 MPa(G) or less. (4) The method for producing an expanded polypropylene resin bead molded article according to (2) or (3), wherein in the heating step, a differential pressure (P2-P1) obtained by subtracting the pressure (P1) from the pressure (P2) is 0.15 MPa or more and 0.35 MPa or less. (5) The method for producing an expanded polypropylene resin bead molded article according to any one of (1) to (4), wherein the expanded polypropylene resin beads are cylindrically shaped and have through holes. (6) A method for producing a polypropylene resin foamed bead molded article according to any one of (1) to (5), wherein the split molding mold has a first mold having a first inner mold and a second mold having a second inner mold, a cavity consisting of the first inner mold and the second inner mold is formed during in-mold molding, and satisfies the following conditions (A) and / or (B), and the ratio of the sum of the volume of the chamber provided in the first mold and the volume of the chamber provided in the second mold to the volume of the cavity is 1 or less: (A) the first mold has a first frame covering the first inner mold, and the chamber is formed by the space between the first inner mold and the first frame; (B) the second mold has a second frame covering the second inner mold, and the chamber is formed by the space between the second inner mold and the second frame.(7) A molding machine equipped with a split molding die consisting of a first die and a second die, and one or more steam generators provided for the molding machine, wherein at least one die of the split molding die has a concave inner die, and the volume of the inner die defined by the opening surface of the concave inner die and the inner surface of the inner die is 0.1 m. ３ (8) A group of apparatuses for producing expanded polypropylene resin bead molded products, comprising a plurality of the apparatuses for producing expanded polypropylene resin bead molded products according to (5) above, in a building, wherein the concave inner mold and another inner mold pair can form a cavity for filling expanded polypropylene resin beads, and the steam generator is an electric steam boiler.

[0066] DESCRIPTION OF SYMBOLS 10: Cavity 20: Chamber 30A: First inner mold 30B: Second inner mold 32: Molding wall 34: First mold 36: Second mold 38: Opening 40: Frame 40A: First frame 40B: Second frame 50: Expanded polypropylene resin beads 52: Filling feeder 54: Through-hole beads 56: Through-hole 60: Steam supply section 60A: Main flow path 60B: Sub-flow path 64: Vent 65: Opening / closing mechanism 66: Exhaust hole 67: Opening / closing mechanism 68: Supply hole 69: Air supply hole 70: Split molding mold 80: Internal space 80A: Internal space of first inner mold 80B: Internal space of second inner mold 100, 200: Expanded bead molding manufacturing apparatus 110, 210: Molding machine 112: Compressed gas generator 114: Water source 120: Electric steam boiler 130, 131, 132, 230: Piping 140: Expanded bead molding manufacturing equipment group 210: Molding machine 220: Large steam generator 240: Building

Claims

1. A method for producing expanded polypropylene resin bead molded articles, comprising a molding machine equipped with a split molding die consisting of a first die and a second die, and one or more steam generators provided for the molding machine, in which expanded polypropylene resin beads are filled into the cavity of the split molding die and heated with steam to fuse the expanded beads together, wherein at least one die of the split molding die has a concave inner die, and the volume of the inner die defined by the opening surface of the concave inner die and the inner surface of the inner die is 0.1 m ３ or more, the steam generator is an electric steam boiler, and the volume (m ３ ) to the ratio of the amount of steam (kg) used for molding (kg / m ３ ) is 100 or less.

2. A method for producing a polypropylene resin foamed bead molding as described in claim 1, characterized in that it includes a pre-pressurization process in which a pressurized gas other than steam is supplied into the cavity filled with the foamed beads to adjust the pressure in the cavity to a pressure (P1) higher than atmospheric pressure, and a heating process in which steam at a pressure (P2) higher than the pressure (P1) is supplied into the cavity adjusted to the pressure (P1) in the pre-pressurization process to heat the foamed beads.

3. The method for producing a polypropylene resin foamed bead molding according to claim 2, characterized in that in the pre-pressure step, the pressure (P1) is adjusted to 0.1 MPa (G) or more and 0.5 MPa (G) or less.

4. A method for producing a polypropylene resin foam bead molding according to claim 2 or 3, characterized in that in the heating step, the differential pressure (P2-P1) obtained by subtracting the pressure (P1) from the pressure (P2) is 0.15 MPa or more and 0.35 MPa or less.

5. A method for producing a polypropylene resin foamed bead molding according to any one of claims 1 to 4, wherein the polypropylene resin foamed beads are cylindrical foamed beads having through holes.

6. A method for producing a polypropylene resin foamed bead molded article according to any one of claims 1 to 5, wherein the split molding mold has a first mold having a first inner mold and a second mold having a second inner mold, a cavity consisting of the first inner mold and the second inner mold is formed during in-mold molding, and satisfies the following conditions (A) and / or (B), and the ratio of the sum of the volumes of the chambers provided in the first mold and the chambers provided in the second mold to the sum of the volumes of the cavities is 1 or less: (A) the first mold has a first frame that covers the first inner mold, and the chamber is formed by the space between the first inner mold and the first frame; (B) the second mold has a second frame that covers the second inner mold, and the chamber is formed by the space between the second inner mold and the second frame.

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