JP4773247B2 - Molding method of polypropylene resin foam sheet - Google Patents

Description

  The present invention relates to a method for molding a polypropylene resin foam sheet, and more specifically, using a polypropylene resin foam sheet, an electronic component such as an IC component, a liquid crystal display panel (LCD), a personal digital assistant (PDA), and a liquid crystal panel The present invention relates to a method for forming a buffer packaging container (including a tray) for storing electrical / electronic products (including parts) such as the above.

  A packaging container (tray) for electric / electronic products (including parts) molded using a polypropylene resin foam sheet is used as a buffer packaging material. Packaging containers (tray) that can be compactly packed to reduce transportation costs, storage costs, etc. are desired, especially the packaging container (tray) storage part that stores electrical and electronic products (including parts). In this case, the corners of the minute support portions that hold the electric / electronic products (including parts) in a fixed position are configured as a packaging container (tray) having a sharp shape with little rounding, and the cushioning property of the package is improved. There has been a demand for trays that maintain and improve storage.

Conventionally, as a method for forming a foamed polypropylene sheet, the foamed polypropylene sheet is heated by molding it in a sealed oven under molding conditions such as an atmospheric temperature of 155 to 200 ° C. and a sheet surface temperature of 160 ± 5 ° C. There is provided a molding method that shortens the time, prevents insufficient heating and overheating of the foamed PP sheet, prevents appearance defects of the molded product, and obtains a molded product with excellent quality (Patent Document 1).
JP-A-6-218805

Further, as a method for forming a foamed polypropylene sheet, after a foamed sheet made of polypropylene resin is heated in a heating furnace for 5 to 30 seconds, the difference between the upper and lower surface temperature of the polypropylene resin foamed sheet immediately before molding is 10 ° C. or less. Thus, there is provided a method for forming a polypropylene resin foam sheet having excellent appearance, rigidity, and heat resistance by guiding the heated polypropylene resin foam sheet to the molding position of the molding die and molding it. .
JP 2001-138349 A

Further, as another molding method of the foamed polypropylene sheet, in the molding method of the polypropylene resin foamed sheet in which a polypropylene resin foamed sheet is preheated and a desired molded product is obtained by a molding die, the temperature of the molding die is set as follows. It was set in advance to 40 ° C. to 100 ° C., and after setting the surface temperature of the polypropylene-based resin foam sheet to 160 ° C. to 200 ° C. by the preheating, it was molded by the mold, and had excellent heat insulation and buffering properties. A method for forming a polypropylene resin foam sheet is provided.
JP-A-11-254518

Since the method of Patent Document 1 is molded under molding conditions such as an atmospheric temperature of 155 to 200 ° C. and a sheet surface temperature of 160 ± 5 ° C., the expanded product of the polypropylene-based resin foam sheet is maximized, and a molded product is obtained. This is a good method for reducing the residual strain. Therefore, this method is suitable for a food container having a shape with few irregularities such as a dish, a shallow bowl, a lunch box, etc., although the storage surface is sharply formed or high heat resistance is required. .
However, in the method of Patent Document 1 and the method of Patent Document 3, the surface temperature of the sheet is too high, and there are many angular irregularities like the packaging tray for electrical / electronic products (including parts) described above. In the tray having a shape with a high degree of verticality of the side walls, if the storage surface is sharply formed, the side wall thickness of the molded product is likely to be partially thinned, so that appearance defects and insufficient strength tend to occur. Therefore, if the basis weight of the polypropylene resin foam sheet is increased in order to compensate for this, there is a problem that the weight of the container increases and the cost becomes high.

On the other hand, since Patent Document 2 is a method of forming a sheet surface temperature difference within 10 ° C. within a heating time of 5 to 30 seconds, the forming temperature is relatively low in order to form the storage surface sharply. When the temperature is increased, the side wall thickness of the molded product is likely to be partially reduced as in the above-described method, so that appearance defects and insufficient strength are likely to occur. If the molding temperature is lowered while setting the temperature difference between the upper and lower sheets to be within 10 ° C to prevent the side wall thickness of the molded product from becoming thinner, the partial thinning of the molded product will be reduced, but the molded surface will be sharply molded. There was a problem that could not.
Conventionally, when forming a sheet, it has been considered preferable to form the sheet with as little difference in sheet temperature as possible. However, in forming a container having a lot of angular irregularities and a shape with a high degree of verticality of the side wall, it is thinned. It was impossible to achieve both prevention and sharp shape.

  The problem to be solved by the present invention is a lightweight and strong container (tray) having a sharp cross-sectional shape and a storage surface in which partial thickness reduction is suppressed, particularly the electric / electronic product. The object of the present invention is to provide a method for forming a polypropylene resin foam sheet capable of forming a container (tray) for (including parts).

In order to solve the above problems, the present invention
In a method for molding a polypropylene resin foam sheet, in which a polypropylene resin foam sheet is heated in a heating furnace of a molding machine, and then a molded product having a product (including parts) storage portion is molded in the next molding zone.
Immediately before molding, the surface temperature of the foamed sheet forming the storage side surface of the product (including parts) is 150 to 155 ° C., the surface temperature of the opposite side is 135 to 145 ° C., the surface temperature of the foam sheet on the opposite side A method for molding a polypropylene-based resin foam sheet is employed, in which molding is performed at a temperature lower by 10 to 15 ° C. than the surface temperature of the storage side surface.

  As a result, the present invention can form a lightweight and strong container (tray) having a sharp cross-sectional shape and having a storage surface in which partial thickness reduction is suppressed.

Therefore, the present invention is not a food container that requires high heat resistance, but rather a container (tray) for an electric / electronic product (including parts) having a more angular uneven cross section than a food container. It is suitably used as a method. Specifically, the cross section is composed of a vertical side element and a horizontal plane element, and the product has a concavo-convex cross section which is angular on one or both side walls in at least one of the vertical and horizontal XY directions of the container (tray). The present invention can be preferably applied to a method for forming a polypropylene resin foam sheet for forming a buffer packaging container having a stepped portion for supporting (including parts). More specifically, the cross section is composed of a vertical side element and a horizontal plane element, and forms a concave-convex cross section that is angular on one or both side walls in at least one of the vertical and horizontal XY directions of the container (tray). It can be preferably applied to a molding method having a total number of convex portions of 3 or more / 150 mm. Still further, in the molding method of the container, the planar element that supports the product (including parts) from the bottom side is configured as a stepped portion, and the protrusion that supports the product (including parts) from both sides is formed on the side element. Is also applicable. The angle formed between the side element (side part) and the horizontal element (horizontal part) is preferably 90 to 100 degrees. When the angle exceeds 90 degrees, it is particularly preferable to further provide the protrusion (small protrusion).
The measurement location of 150 mm was measured by the plane length S1 or S2 shown in FIG.

  For this reason, the present invention can make a container made of a polypropylene resin foam sheet light and thin, can be compactly packed, and the step portion and the side wall portion or the inner side of the container on the container storage surface. A product (including parts) can be favorably supported by the projecting portion (small projecting portion) formed on the side wall, and a molded product having excellent cushioning and storage properties can be obtained.

The present invention relates to a polypropylene resin foam sheet that heats a polypropylene resin foam sheet in a heating furnace of a molding machine and then molds a molded product having a product (including parts) storage in a next molding zone. In the molding method,
Immediately before molding, the surface temperature of the foamed sheet forming the storage side surface of the product (including parts) is 150 to 155 ° C., the surface temperature of the opposite side is 135 to 145 ° C., the surface of the foam sheet on the opposite side A method for molding a polypropylene resin foam sheet, wherein the molding is performed such that the temperature is lower by 10 to 15 ° C. than the surface temperature on the storage side.

When the sheet surface temperature of the storage surface side of the foamed sheet is less than 150 ° C., it is difficult to form the storage surface side surface into a sharp cross-sectional shape. When the sheet surface temperature on the storage surface side exceeds 155 ° C., a thinning phenomenon tends to occur in the molded product. Further, when the surface temperature of the surface of the foam sheet opposite to the storage surface is less than 135 ° C., it is difficult to mold due to insufficient elongation. If the surface temperature of the surface of the foam sheet on the opposite side exceeds 145 ° C., a thinning phenomenon tends to occur in the molded product.
Furthermore, when the temperature difference between the surface temperature of the opposite foam sheet and the surface temperature of the storage side surface is less than 10 ° C., in order to pursue sharpness, when the molding temperature is set to a high temperature as described above, Thinning phenomenon is likely to occur. If the molding temperature is lowered while setting the temperature difference within 10 ° C., thinning can be suppressed, but sharp molding cannot be performed on the storage surface. As described above, when forming a sheet, it has been preferable to form the sheet with as little difference in sheet temperature as possible, but it is difficult to achieve both prevention of thinning and a sharp shape.
On the other hand, if the temperature difference exceeds 15 ° C., the sheet temperature difference is large, the difference in sheet elongation on both sides opposite to the storage side becomes too large, and it is difficult to obtain a good molded product.
The surface temperature can be measured using a non-contact thermometer (for example, a non-contact radiation thermometer IT-550L manufactured by HORIBA). In the temperature measurement, it is preferable to measure the temperature immediately before molding. Specifically, the measurement is performed after the foam sheet heated in the heating zone of the molding machine has moved to the molding zone and stopped at the molding position. If the surface temperature of the foamed sheet satisfies the above conditions, it is molded immediately. If the time to molding is long, the surface temperature of the foamed sheet may be excessively lowered, so that it is preferably within 3 seconds, and more preferably within 2 seconds.

  In molding, in order to make the shape of the storage surface sharper, it is preferable to vacuum and form at least the surface on the storage side of the foam sheet. Further, in the molding zone, a surface temperature difference of 10 to 15 ° C. can be stably applied to the foamed sheet. Therefore, it is preferable to mold the foamed sheet with the storage side surface facing downward.

(Polypropylene resin foam sheet)
The polypropylene resin foam sheet of the present invention has a melt tension of 20 to 40 g and a polypropylene resin (a) having a free-end long chain branch of 10 to 50% by weight, a melt tension of 0.01 g to 6 g, and Polypropylene resin foam sheet extruded and foamed using a polypropylene resin mixture comprising 90 to 50% by weight of a polypropylene resin (b) having a ratio of the weight average molecular weight to the number average molecular weight of 3 to 8 as a base resin. It is preferable that

  This is because the higher the proportion of (a), the easier it is to produce a polypropylene resin foam sheet having a low density and a high closed cell ratio, and excellent thermoformability. On the other hand, the resin (a) has a problem that the production method is complicated and expensive compared with a general-purpose resin, and the cost can be reduced by mixing (b) contained in an inexpensive general-purpose resin.

  The melt tension of the polypropylene resin (a) is 20 to 40 g, and more preferably 20 to 30 g. When the melt tension is lower than 20 g, good foamability cannot be obtained, and when it exceeds 40 g, the fluidity is extremely deteriorated or a gel is easily formed, and the extrusion processability is lowered. Moreover, 7-20 are preferable and, as for the value of the ratio (Mw / Mn) of the weight average molecular weight and number average molecular weight of polypropylene resin (a), 7-15 are more preferable. When this value is lower than 7, good foamability is difficult to obtain, and when it exceeds 20, gel is likely to be formed, and the appearance tends to be deteriorated.

  The resin (a) is produced by a method as disclosed in Japanese Patent Application Laid-Open No. 62-121704 and Japanese Patent Application Laid-Open No. 2-69533. It is sold as “PF-814”.

  The melt tension of the polypropylene resin (b) used in the present invention is 0.01 to 6 g. When the melt tension is lower than 0.01 g, since the tension is too low, a drooping phenomenon (drawdown) occurs during heating, and good thermoforming cannot be performed. On the other hand, when the melt tension exceeds 6 g, it becomes difficult to lower the resin temperature to near the melting point because the melt viscosity increases. As a result, the obtained foam tends to be open-celled and the thermoformability is lowered.

  The value of the ratio of the weight average molecular weight to the number average molecular weight of the polypropylene resin (b) is 3 to 8, preferably 4 to 6. If this value is less than 3, the elasticity is low and the thermoformability is poor, and if it exceeds 8, it is difficult to uniformly lower the resin temperature, and it becomes difficult to obtain a foam sheet having a low open cell ratio. Examples of such a polypropylene resin (b) include homopolypropylene and ethylene-propylene copolymers that satisfy the above conditions.

  The mixing ratio of the polypropylene resin (a) and the polypropylene resin (b) is (a) 10 to 50% by weight (b) 90 to 50% by weight, and (a) 20 to 50% by weight (b) 80. -50% by weight is more preferred, and (a) 30-50% by weight (b) 70-50% by weight is particularly preferred. When the mixing ratio of the polypropylene resin (a) is less than 10% by weight, it becomes difficult to improve the foamability of the polypropylene resin (b), and the thermoformed property of the obtained foamed sheet is inferior.

Examples of the foaming agent used in the production of the polypropylene resin foam sheet used in the present invention include various volatile foaming agents and decomposable foaming agents. Examples of the volatile blowing agent include hydrocarbons such as propane, butane, and pentane, and halogenated hydrocarbons such as tetrafluoroethane, chlorodifluoroethane, and difluoroethane. Examples of the decomposable foaming agent include organic foaming agents such as azodicarbonamide and dinitrosopentamethylenetetramine, organic acids such as sodium bicarbonate or citric acid, or a combination of a salt thereof and a bicarbonate. In some cases, carbon dioxide, nitrogen gas, water, or the like can be used as appropriate. These foaming agents may be used alone or in combination of two or more.
In addition to the above-mentioned foaming agent added to the base resin of the polypropylene resin foam sheet of the present invention, for example, talc, sodium bicarbonate-citric acid, etc., adjust the size of bubbles when foaming Various additives such as an air-conditioning agent, a pigment, a stabilizer, a filler, and an antistatic agent may be added. As the filler, for example, inorganic fillers such as talc, clay, calcium carbonate, silica, and magnesium hydroxide are preferable, and it is preferable to add 5 to 50 parts by weight with respect to 100 parts by weight of the base resin.

  In addition, in the polypropylene resin foam sheet of the present invention, a film made of non-foamable resin may be laminated on one side or both sides of the polypropylene resin foam sheet in order to improve the appearance and strength of the molded product.

  Although it does not specifically limit as resin which comprises a film, What has the propylene-type resin of the same type as a foam sheet as a main component is preferable.

  The film may be a stretched film or an unstretched film, and the thickness of the film layer is usually 0.02 to 1.0 mm, preferably 0.1 to 0.5 mm. This film layer may contain, for example, an inorganic filler such as talc, clay, calcium carbonate, silica, and magnesium hydroxide, an antistatic agent, and a conductive material such as carbon. Lamination of the film is performed by a conventional method such as a heat lamination method or a coextrusion method.

The polypropylene resin foam sheet of the present invention is a mold in which a base resin comprising a polypropylene resin (a) and a polypropylene resin (b) and a foaming agent are melt-kneaded with an extruder and then attached to the tip of the extruder. Produced by extrusion foaming through.
The polypropylene resin foam sheet produced in this way is molded to a thickness of 0.5 to 5.0 mm, preferably 0.5 to 3 mm, for example. If the thickness is less than 0.5 mm, the strength is weak and the buffering property is lowered, which is not preferable. On the other hand, if it exceeds 5.0 mm, the thermoformability is deteriorated.

The polypropylene resin foam sheet has a density of about 0.1 to 0.45 g / cm ³ , preferably about 0.18 to 0.45 g / cm ³ . When the density is lower than 0.1 g / cm ³ , the strength is unfavorable. Moreover, since buffer property will fall when it is higher than 0.45 g / cm < ³ >, it is not preferable.

If the basis weight of the polypropylene resin foam sheet is small, even if it is the molding method of the present invention, although it depends on the shape of the molded product, partial thinning may occur or the strength of the molded product may be insufficient. It is preferable to use one that is / m ² or more.

The melt tension of the polypropylene resin in the present invention is determined by the following method. Using Capillograph PMD-C manufactured by Toyo Seiki Seisakusho Co., Ltd., the polypropylene resin is heated to 230 ° C., and the melted polypropylene resin is nozzled with a piston extrusion plastometer (2.095 mm in diameter and 8 mm in length). The piston is extruded into a string while maintaining the piston descending speed at a constant speed of 10 mm / min, and then the string is passed through a tension detection pulley located 35 cm below the nozzle, and then is wound at about 66 m / At a rate of about ² minutes, the winding speed was gradually increased until the string-like material was cut, and the tension (g) at that time was taken as the melt tension of the resin. However, if the string did not cut even when the winding speed exceeded 60 m / min, the tension (g) at the winding speed of 60 m / min was used as the melt tension of the resin.

In the present invention, the ratio of the weight average molecular weight to the number average molecular weight of the polypropylene-based resin can be determined under the following conditions by molecular weight distribution measurement by gel permeation chromatography.
Measuring device: GPC 150-C manufactured by Waters
Column: 3 UT-806M (SHODEX)
Column temperature: 145 ° C
Injection temperature: 145 ° C
Pump temperature: 55 ° C
Solvent used: o-dichlorobenzene Flow rate: 1.0 ml / min

  The presence or absence of free end long chain branching in the polypropylene resins (a) and (b) used in the present invention was confirmed by observing the elongation characteristics of the resin according to the method described in JP-A No. 62-121704. .

  The electronic component storage tray shown in FIG. 1 was molded using the molding method of the present invention.

(Manufacture of foam sheet)
As polypropylene-based resin, trade name “PF-814” (MI value 3, melt tension 24.5 g) 40% by weight, manufactured by Sun Polymer Co., Ltd., trade name “BC6C” (MI value 2.5, melt tension) 2.3 g) Dry blend of 100 parts by weight of a mixed resin of 60% by weight with 0.3 parts by weight of a baking soda-citric acid master batch (trade name “Fine Cell Master PO410K”, manufactured by Dainichi Seika Co., Ltd.) as a foam regulator. Then, this mixture was fed through a hopper of a caliber φ65 mm extruder of a tandem extruder consisting of a first extruder and a second extruder having a caliber φ65-φ90 mm, heated and melted, and then butane (isobutane / normal butane as a blowing agent) = 35/65) was melted and mixed. Next, it is transferred to an extruder with a diameter of 90 mm and cooled uniformly, and then extruded and foamed from a cylindrical die with a diameter of 60 mm at a discharge rate of 40 kg / hour, and the obtained cylindrical foam is cooled with water having an inside of about 20 ° C. The outer surface is cooled and formed by blowing air with an air ring larger than the diameter of the mandrel having a diameter of φ170, cut at a point on the circumference with a cutter, and a basis weight of 700 g / m ^2. A polypropylene resin foam sheet having a thickness of 2.2 mm and a density of 0.318 g / cm ³ was obtained.

  The above-mentioned polypropylene resin foam sheet was used, and the molding machine used was a continuous molding machine having a heating furnace opened only at the inlet and outlet. The mold for molding was a match mold mold, and the above-mentioned foamed sheet was molded on the lower mold side with the storage side surface facing down, and the opposite side was molded on the upper mold side. Moreover, the said shaping | molding carried out vacuum suction (vacuum degree: 85 KPa) and shape | molded the surface by the side of the accommodation of the said foam sheet. The mold temperature was 40 ° C. or less.

  Table 1 shows the temperature conditions for forming the foamed sheet. The surface temperature of the foam sheet was measured by using a non-contact radiation thermometer HORIBA IT-550L, and the foam sheet moved from the heating furnace to the molding die surface, and the temperature immediately before molding was measured.

  1 is a plan view of an electronic component storage tray formed by the method of the embodiment of the present invention, FIG. 2 is a front view thereof, FIG. 3 is a right side view thereof, and FIG. 4 is a storage portion formed on the tray. FIG. 5 is a cross-sectional view taken along line AA in FIG.

  In the figure, reference numeral 1 denotes a tray for storing electronic components 3, and 2 denotes a concave storage portion for receiving and storing electronic components. The tray 1 has a vertical width of 550 mm, a horizontal width of 310 mm, and a height of 20 mm. As shown in the drawing, storage portions 2 for storing electronic components therein are formed in two horizontal rows in five vertical directions. As shown in FIGS. 1 and 5, the storage unit 2 has a vertical width S1 of 75 mm, a horizontal width S2 of 55 mm, and a depth S3 of 15 mm.

As shown in FIGS. 4 and 5, the main tray 1 has a depth direction from the side of the opening 2 a of the storage portion 2 in each side wall B, C inside the storage portion 2 in the vertical X direction of the container (tray). The horizontal plane element 22a, the cross-section is a vertical side element 21a, the cross-section is a horizontal plane element 22b, the cross-section is a vertical side element 21b, and the cross-section is a horizontal plane element 22c. It is configured. And each side wall B and C inside the storage part 2 inside each vertical X direction of the said container (tray) has the said convex part 23a, 23b in the depth direction of the storage part 2 by making the square uneven | corrugated cross section, respectively. is doing. Reference numeral 24a denotes a recess composed of the side element 21a and the planar element 22b, and reference numeral 24b denotes a recess composed of the side element 21b and the planar element 22c. The planar element 22 c constitutes the bottom surface portion 26 of the storage portion 2. The planar element 22b between the concave portion 24a and the convex portion 23b and the side surface element 21b between the convex portion 23b and the concave portion 24b are stepped portions 27 that support the electronic component 3 from the bottom side in the planar element 22b on each of the side walls B and C. Is configured. Further, on each of the side walls B and C, a minute protrusion 25a (small protrusion) that supports the electronic component 3 to be stored from both sides protrudes from a part of the side element 21a on the opening side of the storage 2 on the concave portion 24a side. Is formed.
In this example, there are two convex portions 23a and 23b on the side wall B and two on the side wall C, and a total of four convex portions per 75 mm in the length of the storage portion plane in the X direction. In this example, the angle formed between the side surface element 21a and the planar element 22b and the angle formed between the side surface element 21b and the planar element 22c on each side wall B and C are 97 degrees.

  Such a configuration is similarly formed in the cross section in the horizontal Y direction of the container (tray) shown in FIG.

  For comparison, a tray for storing electronic components shown in FIG. 1 was molded in the same manner as in the above example except for the conditions shown in Table 1.

Next, in order to evaluate the sharpness of the storage surface of each electronic component storage tray obtained by molding, the cross-sectional radius (R) of the convex portion 23 was measured. This cross-section R (R) was measured by cutting a cross-section and taking an enlarged photograph. N = 5 (number of samples) was extracted from the molded product, and the average value was evaluated according to the following criteria. Table 2 shows the results.
○ Convex part R is less than 1.3 mm Δ Convex part R is 1.3 mm or more and less than 2 mm × Convex part R is 2 mm or more

Next, moldability was evaluated according to the following criteria after forming 10 trays.
○ All the 10 trays have no portion or perforation with a thickness of 1/10 or less of the thickness of the foam sheet before molding due to thinning on the side wall portion which is a side element.
Δ In one or two trays, there are portions or perforations having a thickness of 1/10 or less of the thickness of the pre-molding foamed sheet due to thinning of the side wall portions which are side elements.
X Three or more trays have a portion or perforation having a thickness of 1/10 or less of the thickness of the foam sheet before molding by thinning on the side wall portion which is a side surface element.

Furthermore, comprehensive evaluation was evaluated according to the following criteria.
○ Both storage surface sharpness evaluation and moldability evaluation ○
× Storage surface sharpness evaluation and moldability evaluation are not ○

  By using the method of the present invention, a container having a lot of irregularities with a square cross section and a high degree of verticality of the side wall using a polypropylene resin foam sheet, for example, an electronic component such as an IC component, a liquid crystal display panel (LCD), a portable Buffer packaging containers (including trays) that store electrical and electronic products (including parts) such as information terminal devices (PDAs) and liquid crystal panels have a sharp cross-sectional shape and suppress partial thinning. It can shape | mold as a container (tray) provided with the accommodated storage surface.

It is a top view of the tray for electronic component accommodation shape | molded by the method of the Example of this invention. It is the same front view. It is the same right view. It is a principal part enlarged plan view of the accommodating part formed in the tray. It is the sectional view on the AA line in FIG.

Explanation of symbols

1 Tray 2 Storage 3 Electronic parts

Claims (5)

In a method for molding a polypropylene resin foam sheet, in which a polypropylene resin foam sheet is heated in a heating furnace of a molding machine, and then a molded product having a product (including parts) storage portion is molded in the next molding zone.
Immediately before molding, the surface temperature of the foamed sheet forming the storage side surface of the product (including parts) is 150 to 155 ° C., the surface temperature of the opposite side is 135 to 145 ° C., the surface of the foam sheet on the opposite side A method for molding a polypropylene resin foam sheet, wherein the molding is performed such that the temperature is lower by 10 to 15 ° C than the surface temperature on the storage side.   2. The method for molding a polypropylene resin foam sheet according to claim 1, wherein at least a surface on the storage side of the foam sheet is vacuum sucked and molded.   The polypropylene resin foam sheet has a melt tension of 20 to 40 g, a polypropylene resin (a) having a free-end long chain branch of 10 to 50% by weight, a melt tension of 0.01 to 6 g, and a weight average. A polypropylene resin foam sheet that is extruded and foamed using a polypropylene resin mixture comprising 90 to 50% by weight of a polypropylene resin (b) having a ratio of the molecular weight and the number average molecular weight of 3 to 8 as a base resin. The method for molding a polypropylene resin foam sheet according to claim 1 or 2. The cross section consists of vertical side elements and horizontal plane elements,
Forming a concavo-convex cross section at one or both side walls in at least one of the vertical and horizontal XY directions of the container (tray), and having a total of 3 or more / 150 mm of the convex portions, and a product (including parts) The method for molding a polypropylene resin foam sheet according to any one of claims 1 to 3, wherein a buffer packaging container having a stepped portion to be supported is molded. The method for molding a polypropylene resin foam sheet according to claim 4, wherein the product (including parts) is an electric / electronic product (including parts).

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