JP7517833B2 - Polystyrene resin molded product and its manufacturing method - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies. Sales date: December 28, 2019. Sales location: Aeon Group stores.

The present invention relates to a polystyrene resin molded product that has a small molding shrinkage rate, excellent impact resistance, and a constant sheet temperature, and a method for manufacturing the same.

Polystyrene resins are generally highly transparent and rigid, and have excellent processing properties such as sheet processing and vacuum forming, making them a material that can be used to easily mass-produce molded products. For this reason, molded products made from polystyrene resins are used for a variety of purposes, including packaging materials and food containers. Such molded products are usually manufactured by thermoforming polystyrene resin sheets.

Various attempts have been made to improve the properties of polystyrene resins and molded products. For example, Patent Document 1 discloses a polystyrene resin foam sheet that contains polystyrene resin, rubber-modified polystyrene resin, and hydrogenated polystyrene elastomer and can prevent damage to fruits and vegetables, and a fruit and vegetable packaging container made using the same. Patent Document 2 discloses a polystyrene resin composition and packaging container that contains polystyrene, impact-resistant polystyrene resin, non-hydrogenated styrene-butadiene block copolymer, and styrene-butadiene-butylene-styrene thermoplastic elastomer and can improve impact resistance and prevent resin burns. Patent Document 3 discloses a thermoplastic resin sheet that contains styrene resin, olefin resin, styrene thermoplastic elastomer, and talc, and has good moldability, water vapor barrier properties, and rigidity, and a container made using the same.

Patent No. 5523238 Patent No. 4987429 JP 2017-75210 A

When producing molded products by thermoforming, the molded product shrinks after thermoforming, and a discrepancy may occur between its dimensions and the dimensions of the mold. The numerical value indicating this discrepancy is the molding shrinkage ratio. If the molding shrinkage ratio is large, it becomes difficult to obtain a molded product with an accurate size, and the molded product will lack uniformity in dimensions. Therefore, a small molding shrinkage ratio is required. Furthermore, if the molded product is to be used as a container, it is also required to have excellent impact resistance.

Generally, crystalline resins (e.g., polyethylene-based resins) have a higher molding shrinkage rate than non-crystalline resins (e.g., polystyrene-based resins). Therefore, in a resin composition containing crystalline and non-crystalline resins, if the mixing ratio of the non-crystalline resin is increased, the molding shrinkage rate can be reduced. However, if the mixing ratio of the crystalline resin is reduced, there is a problem that the impact resistance of the molded product decreases.

To facilitate thermoforming, it is preferable to increase the amount of heat applied to the sheet. However, as the sheet temperature increases, drawdown (the phenomenon in which the resin sheet used for thermoforming becomes too soft and sags) is more likely to occur. Since drawdown can lead to a decrease in the dimensional uniformity of the molded product, it is preferable to prevent a sudden increase in the sheet temperature.

The present invention aims to provide a polystyrene resin molded product that has a small molding shrinkage rate, excellent impact resistance, and a constant sheet temperature, and a manufacturing method thereof.

The molded article of the present invention is a molded article obtained by thermoforming a resin sheet mainly composed of a polystyrene-based resin, and is characterized in that, when the entire resin sheet is taken as 100% by weight, the resin sheet contains 3 to 35% by mass of a polyethylene-based resin, and the mold shrinkage rate of the molded article, calculated by the following formula (I), is 0.65% or less.
Molding shrinkage rate (%) = 100 x (mold size - molded product size) / mold size (I)

The method for producing a molded product of the present invention is characterized in that a resin sheet mainly composed of polystyrene-based resin and containing 3 to 35 mass% polyethylene-based resin when the entire resin sheet is taken as 100 weight %, is thermoformed at a molding temperature that is the sheet softening temperature of the polystyrene-based resin and is equal to or higher than the melting point of the polyethylene-based resin.

The present invention has a small mold shrinkage rate and excellent impact resistance, and can suppress changes in the mold shrinkage rate even when the resin contains a polyethylene resin (crystalline resin) that has a large mold shrinkage rate. Furthermore, in thermoforming, drawdown caused by a sudden increase in sheet temperature can be suppressed, thereby achieving uniform dimensions of the molded product.

2 is a schematic cross-sectional view of a female mold 4 and a container molded with the mold 4. FIG. 2 is a schematic cross-sectional view of a male mold 5 and a container molded by the mold 5. FIG.

The following describes an embodiment of the present invention. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

(1) Molded Product The molded product according to this embodiment (hereinafter referred to as "this molded product") is a molded product obtained by thermoforming a resin sheet mainly composed of a polystyrene-based resin (PS-based resin), and when the entire resin sheet is taken as 100% by weight, the resin sheet contains 3 to 35% by mass of a polyethylene-based resin (PE-based resin), and the mold shrinkage rate of the molded product calculated by the following formula (I) is 0.65% or less.
Molding shrinkage rate (%) = 100 x (mold size - molded product size) / mold size (I)

The resin sheet is mainly composed of the PS resin, which is an amorphous resin. The PS resin may be used alone or in combination of two or more types. The resin sheet may contain, in addition to the PS resin, one or more of other polymers, resins, and rubbers as secondary components. Here, "main component" means that, when the entire resin sheet is taken as 100% by weight, the PS resin is contained in an amount of 50% by weight or more, preferably 60% by weight or more, more preferably 70% by weight or more, more preferably 80% by weight or more, and particularly preferably 90% by weight or more.

The PS resin may be a homopolymer of a styrene monomer, or a copolymer of two or more styrene monomers, or one or more styrene monomers and one or more other monomers. Specific examples of the styrene monomer include α-methylstyrene, vinyltoluene, ethylstyrene, i-propylstyrene, t-butylstyrene, dimethylstyrene, bromostyrene, and chlorostyrene. There is no particular limitation on the type of the other monomer, as long as it can be copolymerized with the styrene monomer. Examples of the other monomer include one or more vinyl monomers. Specific examples of the vinyl monomer include (meth)acrylic acid esters such as methyl (meth)acrylate, butyl (meth)acrylate, and cyclohexyl (meth)acrylate.

The PS resin may be rubber-modified as necessary. Specific examples of the rubber modification include rubber modification with the addition of a conjugated diene rubber-like polymer. Examples of the conjugated diene rubber-like polymer include polybutadiene, random or block copolymers of styrene-butadiene, polyisoprene, polychloroprene, random, block or graft copolymers of styrene-isoprene, ethylene-propylene rubber, ethylene-propylene-diene rubber, and the like. These may also be partially hydrogenated.

Specific examples of the PS resin include polystyrene (general-purpose polystyrene resin (GPPS) and the like), rubber-modified polystyrene (high-impact polystyrene resin (HIPS) and the like), ABS resin (acrylonitrile-butadiene-styrene copolymer), AS resin (acrylonitrile-styrene copolymer), MS resin (methyl methacrylate-styrene copolymer), AAS resin (acrylonitrile-acrylic rubber-styrene copolymer), and AES resin (acrylonitrile-ethylene propylene-styrene copolymer). The PS resin is at least one of HIPS and GPPS, and preferably both. Here, the HIPS contains a rubber component such as butadiene in addition to a styrene-based monomer, and examples of the HIPS include a copolymer in which the rubber component is copolymerized with a styrene-based monomer, or a blend resin of the copolymer with another homopolymer or copolymer. The general-purpose polystyrene resin (GPPS) is a resin component that is substantially composed of only styrene monomers polymerized repeatedly.

When the PS resin contains both HIPS and GPPS, there is no particular limit to the content of both. When the entire resin sheet is taken as 100% by weight, the content of the HIPS can be preferably 35 to 80% by weight. The lower limit of the HIPS content can be 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50% by weight. The upper limit of the HIPS content can be 79, 78, 77, 76, 75, 74, 73, 72, 71, 70, 69, 68, 67, 66, 65, 64, 63, 62, 61, 60, 59, 58, 57, 56, 55, 54, 53, 52, 51, or 50% by weight. The content of the HIPS can be within a range that is any combination of the lower limit and the upper limit. In addition, when the entire resin sheet is taken as 100% by weight, the content of the GPPS can be preferably 10 to 40% by weight. The lower limit of the content of the GPPS can be 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20% by weight. In addition, the upper limit of the content of the GPPS can be 39, 38, 37, 36, 35, 34, 33, 32, 31, or 30% by weight. The content of the GPPS can be within a range that is any combination of the lower limit and the upper limit.

When the PS resin contains both HIPS and GPPS, there is no particular limitation on the content ratio of the two. Preferably, the content ratio of HIPS to GPPS can be (1.3-3.5):1. The lower limit of the HIPS content in the content ratio can be 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, or 2.5. The upper limit of the HIPS content in the content ratio can be 3.5, 3.4, 3.3, 3.2, 3.1, 3.0, 2.9, 2.8, 2.7, 2.6, 2.5, 2.4, 2.3, 2.2, 2.1, or 2.0. The HIPS content in the content ratio can be in a range that is any combination of the lower limit and upper limit.

The PE resin is preferably an ethylene homopolymer, but may also be a copolymer of ethylene and one or more other monomers, such as α-olefins (1-butene, 4-methyl-1-pentene, 1-hexene). When the PE resin is a copolymer, the proportion of ethylene units can be 80% by weight or more, 90% by mass or more, or 95% by mass or more. Specific examples of the PE resin include low-density polyethylene (LDPE; density 0.910 to 0.930), linear low-density polyethylene (LLDPE; density 0.910 to 0.925), high-density polyethylene (HDPE; density 0.94 g/cm3 or more, preferably 0.942 to 0.970 g/cm3), ethylene-vinyl acetate copolymer, and ethylene-ethyl acrylate copolymer. The PE resin is preferably crystalline. The PE resin can be a crystalline resin, high-density polyethylene (HDPE).

The origin of the ethylene constituting the PE-based resin is not particularly limited. The ethylene may be derived from ordinary petroleum or may be derived from plants. Plant-derived ethylene can be obtained, for example, by distilling and separating alcohol components, particularly ethyl alcohol, from a fermented product of sugar or cellulose fermentation extracted from plant raw materials (such as sugarcane and corn), and subjecting the alcohol to a dehydration reaction. From such plant-derived ethylene, a PE-based resin can be obtained by a normal polymerization method. Such PE-based resins are called biopolyethylenes. The PE-based resin may be such a biopolyethylene. For example, HIPS containing biopolyethylene can be used as the HIPS.

When the entire resin sheet is taken as 100% by weight, the content of the PE resin is 3 to 35% by weight. If the content is less than 3% by weight, the impact resistance is poor, which is not preferable. On the other hand, if the content exceeds 35% by weight, the molding shrinkage rate is large, which is not preferable. The lower limit of the content can be 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15% by weight. The upper limit of the content can be 35, 34, 33, 32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, or 5% by weight. The content can be in a range that is any combination of the lower limit and the upper limit.

The combination of the PS resin and the PE resin is not particularly limited, and can be selected from various options as needed. In the present molded product, the combination of the PS resin and the PE resin can be such that the PS resin is at least one of HIPS and GPPS, and preferably both, and the PE resin is high-density polyethylene. The explanation of the PS resin and the PE resin in this paragraph also applies to the combination of the PS resin and the PE resin.

In this molded product, the mold shrinkage calculated by the above formula (I) is 0.65% or less. If the mold shrinkage is 0.65% or less, the dimensional deviation between the mold and the molded product is small, and it is preferable because molds used for molding conventional PS resins such as HIPS can be reused.

In formula (I), "molded product dimensions" refers to the dimensions between any two points on the molded product. Also, "mold dimensions" refers to the dimensions between two points on the mold that correspond to the two points at which the molded product dimensions are measured. Specifically, for example, the "molded product dimensions" can be the horizontal dimension between the two bending points of the recess at the top of the molded product or the horizontal dimension between the two bending points of the recess at the bottom of the molded product, whichever is thicker, and the "mold dimensions" can be the horizontal dimension between the two bending points of the convex part of the mold that corresponds to the recess at which the "molded product dimensions" are measured.

An example of "molded product dimensions" and "mold dimensions" will be described with reference to Figs. 1 and 2. Molded product 1 has molded product tops 2A and 2B (the top of the molded product) and molded product bottom 3 (the bottom of the molded product). Molded product tops 2A and 2B have recesses 21A and 21B, and molded product bottom 3 also has recess 31. Molded product tops 2A and 2B have recesses 21A and 21B on tops 211A and 211B on bends 212a and 212b that form top 211A, and bending points 212c and 212d that form top 211B. Similarly, recess 31 on bottom 3 has bottom 311 and bending points 312a and 312b.

Typically, the recesses 21A, 21B of the tops 2A, 2B of the molded product are formed by a mold 4 (female mold), and the recess 31 of the bottom 3 of the molded product is formed by a mold 5 (male mold). The mold 4 has protrusions 41A, 41B for forming the recesses 21A, 21B of the tops 2A, 2B of the molded product. Similarly, the mold 5 has a protrusion 51 for forming the recess 31 of the bottom 3 of the molded product. The protrusion 41A of the mold 4 has an upper surface 411A and bending points 412a, 412b corresponding to the upper surface 211A and bending points 212a, 212b of the top 2A of the molded product. Similarly, the protrusion 41B of the mold 4 has an upper surface 411B and bending points 412c, 412d corresponding to the upper surface 211B and bending points 212c, 212d of the top 2B of the molded product. In addition, the convex portion 51 of the mold 5 has a bottom surface portion 511 and bending points 512a, 512b that correspond to the bottom surface portion 311 and bending points 312a, 312b of the molded product bottom portion 3.

When thermoforming is performed using a female mold, the top surface portions 211A and 211B are the portions that are most vacuumed, and are also the portions that are the thickest when thermoforming is performed using a male mold, while the bottom surface portion 311 is the portion that is most vacuumed and is also the portion that is the thickest. In one embodiment, the "molded product dimensions" can be the horizontal dimension (X1) between the two bending points 212a and 212b of the recess 21A of the top portion 2A of the molded product, or the horizontal dimension (X2) between the two bending points 312a and 312b of the recess 31 of the bottom portion 3 of the molded product, whichever is thicker.

If the "molded product dimension" is X1 as shown in FIG. 1, then the "mold dimension" means the horizontal dimension (Y1) between the two bending points 412a and 412b of the protruding portion 41 of the mold 4 that corresponds to the recess 21A of the top portion 2A of the molded product (the recess from which the molded product dimension is measured). On the other hand, if the "molded product dimension" is X2 as shown in FIG. 1, then the "mold dimension" means the horizontal dimension (Y2) between the two bending points 512a and 512b of the protruding portion 51 of the mold 5 that corresponds to the recess 31 of the bottom portion 3 of the molded product (the recess from which the molded product dimension is measured).

In FIG. 1, the horizontal dimension between bending points 212a and 212b is indicated as "X1". When molding with a female mold, the "molded product dimensions" can be the horizontal dimension between any two bending points at the top of the molded product, such as between the two bending points 212a and 212d of the recess 21B of the top 2B of the molded product. Similarly, in FIG. 1, the horizontal dimension between bending points 412a and 412b is indicated as "Y1". When molding with a female mold, the "mold dimensions" can be the horizontal dimension between any two bending points at the top of the molded product, such as between bending points 412a and 412d of the protrusion 41 of the mold 4 corresponding to the recess for which the molded product dimensions were measured.

The resin sheet may further contain one or more types of compatibilizer. By containing the compatibilizer, the components in the resin sheet, particularly the PS-based resin and the PE-based resin, can be dispersed more uniformly, which is preferable. There is no particular limitation on the type of the compatibilizer. Specific examples of the compatibilizer include styrene-butadiene thermoplastic elastomers, styrene-based thermoplastic elastomers, and hydrogenated styrene-based thermoplastic elastomers. More specific examples include styrene-ethylene-butylene-styrene copolymers, styrene-butadiene-butylene-styrene copolymers, styrene-ethylene-propylene-styrene copolymers, and styrene-isoprene-styrene copolymers.

The content of the compatibilizer can be set to an appropriate range as necessary. When the entire resin sheet is taken as 100% by weight, the content of the compatibilizer is preferably 1 to 20% by weight. The lower limit of the content can be 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, or 3% by weight. The upper limit of the content can be 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9.5, 9, 8.5, 8, 7.5, or 7% by weight. The content can be set to a range that is any combination of the lower limit and upper limit.

The resin sheet is mainly composed of the PS resin, and there is no particular limitation on its specific structure, so long as it includes a layer containing 3 to 35 mass % of the PE resin when the entire resin sheet is taken as 100 weight % (hereinafter referred to as the "PE-containing PS resin layer"). The resin sheet may be a single-layer structure consisting of only a PE-containing PS resin layer, or a multi-layer sheet having two or more layers including the PE-containing PS resin layer and another layer. When the resin sheet is a multi-layer sheet, all of the layers may be PE-containing PS resin layers, or some of the layers may be PE-containing PS resin layers. For example, the resin sheet may include a skin layer and a core layer, and the core layer is the PE-containing PS resin layer.

There is no particular limitation on the specific shape of the resin sheet. The specific shape of the resin sheet can be appropriately determined as needed. Therefore, the term "sheet" also includes a film-like shape. The thickness of the sheet can be, for example, 0.1 to 5 mm, or 0.5 to 3 mm.

The resin sheet may contain other components as necessary, provided that the moldability is not significantly impaired. Specific examples of the other components include additives used in known resin sheets, such as flame retardants, ultraviolet absorbers, fluorescent brighteners, antistatic agents, antifogging agents, lubricants, antiblocking agents, flow improvers, plasticizers, dispersants, and antibacterial agents.

The molded product can be obtained by thermoforming the resin sheet. There is no particular limitation on the specific method of thermoforming, and known thermoforming methods, such as hot plate molding, vacuum molding, pressure molding, or vacuum pressure molding, can be used. There is also no particular limitation on the conditions of thermoforming. The molding conditions can be appropriately determined as necessary. The molded product can be preferably produced by thermoforming at a molding temperature that is the sheet softening temperature of the PS resin and is equal to or higher than the melting point of the PE resin (this point will be described in detail later).

There are no particular limitations on the shape, dimensions, or specific uses of the molded product. Uses of the molded product include containers such as packaging containers, for example, containers for packaging food and beverages. The term "container" includes not only the entire container, but also parts of the container. For example, the term "container" includes not only the main body of the container, but also the lid of the container.

(2) Manufacturing Method of Molded Article The manufacturing method of a molded article according to this embodiment (hereinafter referred to as "this method") includes thermoforming a resin sheet mainly composed of a PS-based resin, the resin sheet containing 3 to 35 mass % of a PE-based resin when the entire resin sheet is taken as 100 weight %, at a molding temperature which is the sheet softening temperature of the PS-based resin and is equal to or higher than the melting point of the PE-based resin.

In this method, by thermoforming at a molding temperature that is equal to the sheet softening temperature of the PS resin and equal to or higher than the melting point of the PE resin, the sheet temperature remains constant during thermoforming, and drawdown can be suppressed. This is thought to be because when molding at such a molding temperature, even if the amount of heat increases, the thermal energy is consumed in melting the PE resin, and as a result, the sheet temperature does not increase (this statement is merely an expression of the inventor's opinion, and is not intended to define or limit the present invention).

In this method, the above explanation applies to the PS resin, resin sheet, and PE resin.

The molded product obtained by this method usually has a mold shrinkage ratio of 0.65% or less, calculated by the above formula (I). If the mold shrinkage ratio is 0.65% or less, the dimensional deviation between the mold and the molded product is small, and it is preferable because molds used for molding conventional PS resins such as HIPS can be reused. The above explanation applies to the "molded product dimensions" and "mold dimensions" in formula (I).

The present invention will be specifically explained below with reference to examples. Note that the present invention is not limited to the forms shown in the examples. The embodiment of the present invention can be modified in various ways within the scope of the present invention depending on the purpose, application, etc.

1. Production of PS Sheet The following components were used as raw materials.
<Non-crystalline resin>
(A) HIPS: "E640N" manufactured by Toyo Styrene Co., Ltd.
(B) GPS: "G9305" manufactured by PS Japan Co., Ltd.
<Crystalline Resin>
(C) Bio-PE resin; "SHC7260" manufactured by Braskem
<Compatibilizer>
(D) SBS; Kraton "DX408"

The raw materials (A) to (D) were mixed in the ratios shown in Table 1, and then charged into an extruder and heated to melt. The molten resin was then extrusion molded to obtain the single-layer PS sheets (thickness: 0.50 mm) of Examples 1 to 4, Reference Example, and Comparative Examples 1 and 2. The raw materials (A) to (D) were mixed in the ratios shown in Table 2, and then a core layer sheet and a skin layer sheet were obtained in the same manner. These were then laminated to obtain a multilayer PS sheet (thickness: 0.50 mm) including a core layer and a skin layer.

2. Production of molded products The resin sheets of Examples 1 to 4, 6 , Reference Example , and Comparative Examples 1 to 3 were thermoformed using a mold at an indirect heater temperature of 430°C (upper) and 380°C (lower), for a heating time of 8.5 s, to produce rectangular containers (215 mm square, 38 mm deep) of Examples 1 to 6 and Comparative Examples 1 to 3.

3. Performance Evaluation (1) Mold shrinkage rate For the containers of Examples 1 to 4, 6 , Reference Example , and Comparative Examples 1 to 3, the horizontal dimension between the two bending points of the recess at the top of the molded product (molded product dimension) was measured. In addition, the horizontal dimension between the two bending points of the protrusion of the mold corresponding to this portion (mold dimension) was measured. Based on the above formula (I), the mold shrinkage rate (%) was calculated from the obtained measurements. The results are shown in Tables 1 and 2.

(2) Drawdown The drawdown of Examples 1 to 4, 6 , Reference Example, and Comparative Examples 1 to 3 was evaluated by visually checking whether or not wrinkles occurred in the molded products due to drawdown when the sheet was heated by an indirect heater before thermoforming. In Tables 1 and 2, "◯" means good (drawdown is small and the molded products are not wrinkled), and "×" means bad (drawdown is large and the molded products are wrinkled).

(3) Impact resistance The impact resistance of Examples 1 to 4, 6 , Reference Example, and Comparative Examples 1 to 3 was evaluated by a drop test in which a 500 g weight was placed inside the molded article and the molded article was dropped from a height of 1.5 m in an environment with an atmospheric temperature of 5° C. The results are shown in Tables 1 and 2. In Tables 1 and 2, "◯" means good (no cracks were observed in the molded article after the drop) and "×" means poor (cracks were observed in the molded article after the drop).

4. Results From Table 1, even though Examples 1 to 4 contain a crystalline resin (PE) having a large mold shrinkage rate, the mold shrinkage rate was about the same as that of Comparative Example 1, which does not contain the crystalline resin. In addition, all of Examples 1 to 4 were excellent in impact resistance. On the other hand, Comparative Example 1, which does not contain a crystalline resin (PE), had a mold shrinkage rate about the same as that of Examples 1 to 4 , but was inferior in impact resistance and had a large drawdown due to an increase in the sheet surface temperature during thermoforming. In addition, Comparative Example 2, which contains a large amount of crystalline resin, had excellent impact resistance, but the mold shrinkage rate was larger than that of Examples 1 to 4. From these results, it can be seen that the molded products of Examples 1 to 4 , which are embodiments of the present invention, have improved impact resistance by containing a crystalline resin, and even though they contain a crystalline resin (PE) having a large mold shrinkage rate, the change in the mold shrinkage rate is small and they have excellent moldability.

As can be seen from Table 2, even though Example 6 contains crystalline resin (PE) with a large mold shrinkage rate, it has a mold shrinkage rate similar to that of Comparative Example 3 which does not contain this resin. Furthermore, Example 6 had small drawdown and good impact resistance. On the other hand, Comparative Example 3 which does not contain crystalline resin (PE) had a mold shrinkage rate similar to that of Example 6, but had poor impact resistance and large drawdown due to the rise in sheet surface temperature during thermoforming. This result shows that the results are similar to those of a single layer sheet, even with a multilayer sheet.

1; molded product, 2A, 2B; top of molded product, 21A, 21B; recess of top of molded product, 211A, 211B; upper surface, 212a, 212b, 212c, 212d; bending point, 3; bottom of molded product, 31; recess of bottom of molded product, 311; bottom surface, 312a, 312b; bending point, 4; mold, 41A, 2B; protrusion of mold 4, 411A, 411B; upper surface, 412a, 412b, 412c, 412d: bending points, 5: mold, 51: convex portion of mold 5, 511: upper surface, 512a, 512b: bending points, X1: horizontal dimension between bending points 212a, 212b, X2: horizontal dimension between bending points 312a, 312b, Y1: horizontal dimension between bending points 412a, 412b, Y2: horizontal dimension between bending points 512a, 512b.

Claims (6)

A molded product obtained by thermoforming a resin sheet containing 70% by weight or more of a polystyrene-based resin,
The resin sheet contains a compatibilizer selected from styrene-butadiene thermoplastic elastomers ,
The resin sheet contains 3 to 29% by mass of a polyethylene-based resin when the entire resin sheet is taken as 100% by weight, and the mold shrinkage rate of the molded article calculated by the following formula (I) is 0.65% or less.
Molding shrinkage rate (%) = 100 x (mold size - molded product size) / mold size (I) The molded article according to claim 1, characterized in that the polystyrene-based resin includes at least one of high impact polystyrene (HIPS) and general purpose polystyrene (GPPS), and the polyethylene-based resin includes high density polyethylene (HDPE). The molded product according to claim 1 or 2, characterized in that the polyethylene resin contains biopolyethylene. A method for producing a molded product, comprising thermoforming a resin sheet, the resin sheet containing 3 to 29% by mass of a polyethylene-based resin, 70% by mass or more of a polystyrene-based resin, and containing a compatibilizer selected from styrene-butadiene-based thermoplastic elastomers , when the entire resin sheet is taken as 100% by mass, at a molding temperature that is the sheet softening temperature of the polystyrene-based resin and is equal to or higher than the melting point of the polyethylene-based resin. The method for producing a molded article according to claim 4, characterized in that the molded article has a mold shrinkage rate of 0.65% or less, as calculated by the following formula (I):
Molding shrinkage rate (%) = 100 x (mold size - molded product size) / mold size (I) The method for producing a molded product according to claim 4 or 5, characterized in that the polystyrene-based resin contains at least one of high impact polystyrene (HIPS) and general purpose polystyrene (GPPS), and the polyethylene-based resin contains high density polyethylene (HDPE).

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