JP2006035660A - Manufacturing method of foamed molded article of thermoplastic resin and molded article - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermoplastic resin foam which is thin, has a high expansion ratio, a smooth surface and a uniform fine cell structure, and to provide its manufacturing method. <P>SOLUTION: The desired, foamed molded article is obtained by a first mold opening step wherein a thermoplastic resin composition and a foaming agent are supplied to an injection molding machine, molten, injected and filled into a mold and immediately after completion of the filling, the mold is opened to a predetermined position not to reach the position of the final product shaping; a midway mold closing step wherein the mold is closed to a predetermined position exceeding the initial cavity; and a second mold opening step wherein the mold is opened to the final product shaping position. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a process for producing a thermoplastic resin injection foam molded article and a molded article comprising this process.

  In the injection molding of thermoplastic resins, so-called injection foam molding has been conventionally performed in which foaming is performed for the purpose of reducing the weight, reducing costs, and preventing warping and sink marks of the molded body. As a technique for making the thermoplastic resin highly foamable, a resin containing a foaming agent is injection-molded in a mold space that is held so that the mold can be opened. There is a method of foaming (for example, Patent Documents 1 and 2). However, it is difficult to stably obtain a high-magnification molded body because voids are easily generated inside by simply adopting this method. The void referred to in the present case is a coarse bubble generated when internal bubbles communicate with each other, and means a bubble whose diameter substantially exceeds 1.5 mm.

  As a method for solving this problem, the mold is opened to a position below the final cavity dimension after a predetermined time after the filling of the molten resin containing the foaming agent is completed, and then the mold opening is stopped to the final cavity dimension after the mold opening is stopped for a predetermined time. A manufacturing method has been proposed (Patent Document 3). Certainly, according to this method, a foamed molded article having a high foaming ratio can be obtained, but it is necessary to leave the resin temperature in the cavity for a predetermined time before injection until the mold is opened after injection. Cooling of the portion in contact with the mold is likely to proceed, and usually the non-foamed layer (skin layer) tends to be thick. When the skin layer is thick, it is necessary to foam the internal foam layer at a higher magnification when molding a molded product with a certain thickness, and this problem becomes more apparent especially when the initial cavity clearance during injection is small. It was difficult to obtain a molded product having a thin wall and a high expansion ratio. In injection foam molding, in order to achieve weight reduction, it is essential to increase the foaming ratio with a thin wall, and it has been difficult to meet the needs for weight reduction with these conventional technologies. As a general measure to suppress the formation of the skin layer lightly, there are methods such as setting the mold temperature high and providing a heat insulating layer on the mold, and it is possible to make the skin layer thin by using them together. However, each of these methods has problems such as longer cooling time and higher mold costs, and is not a good solution.

  In addition, when polypropylene resin is used as the material, the appearance of a silver streak (or swirl mark) is formed on the surface of the molded body because the polypropylene resin is crystalline and has low melt tension (melting tension) and easily breaks bubbles when foamed. Defects and voids are likely to occur inside, and it was particularly difficult to increase the expansion ratio.

  As a method for improving the foaming property of a resin, a method of increasing the melt tension of a polypropylene resin by adding a crosslinking agent or a silane graft thermoplastic resin has been proposed (for example, Patent Document 4 and Patent Document 5). However, in this method, although a foamed molded product having a high expansion ratio can be obtained, the viscosity at the time of melting is excessively increased, and injection molding becomes difficult, and the surface properties of the obtained molded product are also poor.

  By introducing long-chain branching by radiation irradiation, the melt tension is higher than that of ordinary linear polypropylene resin, and the viscosity increases as the stretch distortion of the melt increases. The resin is commercially available from Sun Allomer as HMS-PP (High Melt Strength Polypropylene) (Patent Document 6). It is known that a foam molded body can be obtained by using such HMS-PP as a base resin for injection foam molding (Patent Document 7). However, the HMS-PP used here has a melt flow rate of only about 4 g / 10 minutes, has a low fluidity at the time of melting, and has a thin part with a mold cavity clearance of about 1 to 2 mm. There was a problem with short shots. On the other hand, HMS-PP (30 g / 10 min) with a high melt flow rate is also known, but although it exhibits strain hardening, it has a melt tension of only about 0.3 cN and obtains a foamed molded article with a high expansion ratio. Was difficult. And since manufacture of these HMS-PP uses an expensive radiation equipment, manufactured HMS-PP also becomes expensive and it is difficult to provide the product obtained from it at low cost.

Also, a polypropylene resin (Patent Document 8) having a high melt flow rate and a high melt tension mixed with polyethylene having a specific intrinsic viscosity, and a high melt tension containing a component having a specific intrinsic viscosity by multistage polymerization. A method has also been proposed in which a mixture of a polypropylene resin and a high melt flow rate polypropylene resin (Patent Document 9) is used for injection foam molding. However, such a polypropylene-based resin does not exhibit remarkable strain-hardening properties like the HMS-PP having a long chain branch, and therefore, when the foaming ratio is higher than 2 times, bubbles are not generated. It was prone to breakage and internal voids were apt to occur, and could not meet the needs for high rigidity and light weight.
Japanese Examined Patent Publication No. 39-22213 Japanese Patent Publication No.51-8424 JP 2001-341154 A JP 61-152754 A Japanese Patent Laid-Open No. 7-109372 Japanese Patent Laid-Open No. 62-121704 JP 2001-26032 A JP 2003-128854 A JP 2003-268145 A

  An object of the present invention is to provide a production method that can easily obtain a foamed molded article that is thin, has a high foaming ratio, does not suffer from a decrease in rigidity due to voids, and is excellent in lightness.

  In the manufacturing method for injection foam molding of a thermoplastic resin, the inventors opened the mold a little immediately after the completion of injection, then closed the mold at one end, and then opened the mold to the final product shape. As a result, the present inventors have found that a foamed molded article having excellent properties can be easily obtained and completed the present invention.

  That is, the present invention relates to a method for producing a foamed molded article by injecting a molten mixture of a thermoplastic resin and a foaming agent into a mold and then opening the mold, and injecting the thermoplastic resin composition and the foaming agent. The first mold opening step of opening the mold to a predetermined position less than the final product shape position immediately after the filling is completed, and then to the predetermined position larger than the initial cavity. The present invention relates to a method for producing a thermoplastic resin foam molded article, comprising a mold closing process in the middle of closing a mold, and then a second mold opening process for opening the mold to a final product shape position.

  The cavity clearance after the mold opening in the first mold opening process is preferably 1.2 times or more and 3.0 times or less of the initial cavity clearance, and the cavity clearance after the mold closing in the middle mold closing process is the initial cavity clearance. It is desirable that it is 1.1 times or more and 0.95 times or less of the cavity clearance after the first mold opening step. The mold opening speed in the first mold opening process is 5 to 100 mm / sec, the mold closing speed in the intermediate mold closing process is 0.01 to 5 mm / sec, and the mold opening speed in the second mold opening process is 0. It is desirable that it is 05-50 mm / sec.

  Furthermore, the thermoplastic resin is preferably a polypropylene resin, and the polypropylene resin is (A) a linear polypropylene resin having a melt flow rate of 10 g / 10 min to 100 g / 10 min and a melt tension of 2 cN or less. 50 to 95 parts by weight, (B) 5 to 50 parts by weight of a modified polypropylene resin having a melt flow rate of 0.1 g / 10 min or more and less than 10 g / 10 min, a melt tension of 5 cN or more, and exhibiting strain hardening (However, the total of the linear polypropylene resin (A) and the modified polypropylene resin (B) is preferably 100 parts by weight). The modified polypropylene resin (B) is preferably a modified polypropylene resin obtained by melt-mixing a linear polypropylene resin, a radical polymerization initiator, and a conjugated diene compound.

  Furthermore, it is desirable that the initial cavity clearance when injecting the molten mixture is less than 3 mm.

  The second aspect of the present invention relates to a foamed molded article produced by the above-mentioned production method, preferably a foam layer having an average cell diameter of 500 μm or less and a maximum cell diameter of 3 times or less of the average cell diameter, and at least one side of the foam layer And a non-foamed layer having a thickness of 10 μm or more and 1000 μm or less formed on the surface of the foam molded article having a foaming ratio of 2 times or more and 10 times or less.

  According to the method for producing an injection-foamed molded article of the present invention, even when the initial cavity clearance is thin, a foamed molded article having a high foaming ratio and excellent in lightness and rigidity can be produced. Furthermore, the use of a predetermined polypropylene resin in the present invention dilutes a relatively expensive modified polypropylene resin with an inexpensive linear polypropylene resin, which has been particularly difficult in the past. A thin and highly foamed molded article can be provided easily and inexpensively.

  The major features of the method for producing a thermoplastic resin injection foam molding of the present invention are: 1) injection filling a foamable thermoplastic resin into a mold, and immediately after the filling is completed, the mold is opened to increase the cavity clearance; 2) Next, the mold is closed to decrease the cavity clearance, and 3) the mold is then opened again to increase the cavity clearance.

By opening the mold immediately after the completion of injection filling and increasing the cavity clearance, the formation of the surface skin layer can be minimized. In the present invention, “immediately after completion of filling” refers to shifting to the first mold opening step as quickly as possible. For the purpose of weight reduction, when molding with a small initial cavity clearance t ₀ , the thickness of the internal foam layer becomes extremely thin when the skin layer is thick. On the other hand, if the skin layer is thin, it has a thicker foam layer, so when trying to secure a certain molded product thickness, the internal foam layer has a lower foaming ratio than when the skin layer is thick. For this reason, it is considered that voids tend to be less likely to occur. In addition to this, the conventional method of thinning the skin layer is to increase the mold temperature, and the mold having a heat insulating layer on the mold surface allows the disadvantages of longer molding cycle and increased mold cost. It is also possible to use in combination as far as possible.

The effect of closing the mold after the first mold opening process is not clear, but is considered as follows. In the first mold opening process, there is a temperature distribution in the thickness direction of the molded product, the temperature that is in contact with the mold wall surface is low, and conversely, the temperature near the center is considered high. When the mold is opened and the cavity clearance is increased, foaming tends to proceed in the portion where the resin temperature is high compared to the portion where the resin temperature is low, and the bubbles tend to increase. If the cavity clearance is further increased from this point, the bubbles grow further, and the bubbles in the central part in the thickness direction, which are growing more than other parts, are easily destroyed. As a result, voids are generated with a slight increase in the cavity clearance. It becomes easy to do. Therefore, it is considered that the bubble diameter can be made uniform by closing the mold at one end from the cavity clearance t ₁ in the first mold opening process and reducing the coarsened bubble at the center in the thickness direction. By opening the mold, it is considered that the magnification can be increased while bubbles grow relatively uniformly in the thickness direction.

The cavity clearance t ₁ after opening the mold in the first mold opening process is preferably less than the final product shape position, and is large so that voids are not generated due to bubbles breaking inside and communicating. The reason for this is that if the cavity clearance t ₁ after the first step is large, the core layer foaming ratio at this time becomes high, and it itself serves as a heat insulating layer and is difficult to receive cooling from the mold surface. This is considered to be because it leads to securing a thicker foam layer in the opening process. Although the specific value of t ₁ varies depending on the resin used, it is generally preferably 1.2 to 3.0 times the initial cavity clearance, and more preferably 1.5 to 2 .5 times or less is preferable. By being in this range, it is possible to minimize the generation of voids and to minimize the cooling from the mold. The mold opening speed in the first mold opening process is preferably 5 to 100 mm / sec. By being in the above-mentioned range, the skin layer is thin and can generate a large number of bubbles inside, and the uneven pattern is hardly exhibited on the surface. Further, from the viewpoint that this tendency appears remarkably, the mold opening speed is more preferably 10 to 70 mm / sec.

In the present invention, the initial cavity clearance t ₀ when the molten mixture of the thermoplastic resin and the foaming agent is injected and filled into the mold is less than 3 mm. It is preferable from the viewpoint that the effect is suitably exhibited.

Further, the cavity clearance t _B to decrease by the middle mold closing step is smaller than the cavity clearance t ₁ during the initial cavity larger than the clearance t ₀ first mold opening process good, the t ₀ from the viewpoint of sustained heat insulating effect It is preferably 1.1 times or more, more preferably 1.3 times or more, and preferably 0.95 times or less, more preferably 0.9 times or less of t ₁ from the viewpoint of making the bubble diameter uniform.

The mold closing speed in the middle mold closing process tends to make the bubble diameter of the molded body more uniform. The reason is not clear, but when the first mold opening process is completed, there are bubbles with large diameter (mainly near the center in the thickness direction) and bubbles with small diameter (mainly near the skin layer), and the pressure is high. It is thought that there are parts (small bubbles) and low parts (large bubbles). When the mold is closed at a relatively high speed in such a situation, the entire resin in the cavity is compressed at once, and the part with high pressure is also compressed. On the other hand, the small bubbles are further reduced. On the other hand, when the mold is closed at a low speed, only the low pressure portion, that is, the large bubbles are efficiently compressed, and the bubble diameter is made uniform. It is considered that the bubbles tend to be uniformized even after the mold opening process. The specific value of the mold closing speed in the middle mold closing process is preferably 0.01 to 5 mm / sec, and more preferably 0.05 to 2.5 mm / sec. However, if the time required for mold closing is long, the internal cooling proceeds, and it may be difficult to foam to the final product shape in the second mold opening process, so it depends on the values of t ₁ and t _B. However, the time required for mold closing is preferably within 15 s, and it is preferable to adjust the mold closing speed so that the desired mold closing is completed within this range. If necessary, a time for stopping the mold opening may be provided for the purpose of equalizing the internal temperature after the intermediate mold closing process. This time is not particularly limited as long as it can be foamed to the final product shape in the next second mold opening step, but is preferably within approximately 10 seconds.

In the second mold opening process, the internal bubbles are grown by opening the mold to the cavity clearance t ₂ that is the final product shape. The mold opening speed at this time is preferably 0.05 to 50 mm / sec, and more preferably 0.1 to 25 mm / sec. By being in this range, a predetermined product thickness can be obtained, and surface roughness due to peeling between the mold and the skin layer can be prevented.

  As the thermoplastic resin used in the present invention, any known thermoplastic resin such as polystyrene, polyethylene, polypropylene, and ABS resin can be used without particular limitation as long as it is a main component. Therefore, it is preferable to be a polyolefin resin, particularly a polypropylene resin.

  Furthermore, in the present invention, it is most preferable to use a polypropylene resin comprising a linear polypropylene resin (A) and a modified polypropylene resin (B).

  The linear polypropylene resin (A) preferably has a melt flow rate of 10 g / 10 min or more and 100 g / 10 min or less, more preferably 15 g / 10 min or more and 50 g / 10 min or less, and a melt tension is preferable. Is 2 cN or less, more preferably 1 cN or less. When the melt flow rate is in the range of 10 g / 10 min or more and 100 g / 10 min or less, when producing an injection-foamed molded article, a short shot is required even in molding having a thin-walled portion with a mold cavity clearance of about 1 to 2 mm. It tends to be able to form continuously and stably. In addition, since the foaming ratio is high and the bubbles are not easily destroyed at the time of foaming, a foamed molded article having a beautiful surface appearance tends to be obtained. Moreover, if the melt tension is 2 cN or less, the transferability to the mold surface is good, and a foamed molded article having a beautiful surface appearance is easily obtained.

The melt flow rate is a value measured under a load of 2.16 kg at 230 ° C. in accordance with ASTM D-1238. Used, a strand lowered at a piston descending speed of 10 mm / min was taken out at 1 m / min from a die having a hole of φ1 mm and a length of 10 mm at 230 ° C., and the take-up speed was increased at 40 m / min ² after stabilization. Sometimes, it refers to the take-up load of the pulley with a load cell when it breaks.

The linear polypropylene resin (A) here is a polypropylene resin having a linear molecular structure, and is obtained from a usual polymerization method, for example, a transition metal compound supported on a carrier and an organometallic compound. Obtained in the presence of a catalyst system such as a Ziegler-Natta catalyst. Specific examples include propylene homopolymers, block copolymers, and random copolymers, which are crystalline polymers. As a copolymer of propylene, a copolymer containing propylene in an amount of 75% by weight or more is preferable in that the crystallinity, rigidity, chemical resistance, etc., which are characteristics of the polypropylene resin, are maintained. The copolymerizable α-olefin is ethylene, 1-butene, isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 3,4-dimethyl-1-butene. , 1-heptene, 3-methyl-1-hexene, 1-octene, 1-decene, and the like, α-olefin having 2 or 4 to 12 carbon atoms, cyclopentene, norbornene, tetracyclo [6,2,1 ^1,8 , 1 ^3,6 ] -4-dodecene and other cyclic olefins, 5-methylene-2-norbornene, 5-ethylidene-2-norbornene, 1,4-hexadiene, methyl-1,4-hexadiene, 7-methyl-1,6 -Diene such as octadiene, vinyl chloride, vinylidene chloride, acrylonitrile, vinyl acetate, acrylic acid, methacrylic acid, maleic acid, ethyl acrylate, acrylic Butyl Le, methyl methacrylate, maleic anhydride, styrene, methyl styrene, vinyl toluene, and vinyl monomers such as divinylbenzene. Among these, ethylene and 1-butene are preferable in terms of improving cold brittleness resistance and low cost.

  The modified polypropylene resin (B) used in the present invention preferably has a melt flow rate of 0.1 g / 10 min or more and less than 10 g / 10 min, more preferably 0.3 g / 10 min or more and 5 g / 10. It is preferable that the melt tension is 5 cN or more, more preferably 8 cN or more, and exhibits strain hardening properties. When the melt flow rate is 0.1 g / 10 min or more and less than 10 g / 10 min, the dispersibility in the linear polypropylene resin (A) is good, the foaming ratio is high, the bubbles are uniform, and the surface property is high. There exists a tendency for the good foaming molding of the present invention to be obtained. In addition, transferability to the mold surface is good, and a beautiful surface appearance is easily obtained. In addition, when the melt tension is 5 cN or more, it is easy to obtain a foamed molded product having uniform fine cells twice or more.

  Strain hardening here is defined as that the viscosity increases as the stretch strain of the melt increases, and is usually the method described in Japanese Patent Application Laid-Open No. 62-121704, that is, the elongation measured by a commercially available rheometer. This can be determined by plotting the relationship between viscosity and time. Further, for example, strain hardening can be determined from the breaking behavior of the molten strand at the time of melt tension measurement. That is, when the take-up speed is increased, the melt tension increases abruptly, and when cutting, strain hardening is exhibited. The modified polypropylene resin (B) exhibits strain-hardening properties, and when the melt tension is high, a foamed molded product having a foaming ratio exceeding 2 times can be obtained and broken at the molten resin flow front end during injection molding. Since it becomes difficult for silver streaks to occur due to easy foaming, a foam molded article having a beautiful surface appearance can be obtained.

  As such a modified polypropylene resin (B), for example, the linear polypropylene resin is irradiated with radiation, or the linear polypropylene resin, radical polymerization initiator, conjugated diene compound is melt mixed. Examples thereof include a modified polypropylene resin containing the resulting branched structure or high molecular weight component. Among these, in the present invention, it is inexpensive to use a modified polypropylene resin obtained by melt-mixing a linear polypropylene resin, a radical polymerization initiator, and a conjugated diene compound because it does not require expensive equipment. It is preferable from the point which can be manufactured. Examples of the raw material polypropylene resin used in the production of the modified polypropylene resin (B) include the same as the linear polypropylene resin (A).

  Examples of the conjugated diene compound include butadiene, isoprene, 1,3-heptadiene, 2,3-dimethylbutadiene, 2,5-dimethyl-2,4-hexadiene, and these may be used alone or in combination. Good. Among these, butadiene and isoprene are particularly preferable because they are inexpensive and easy to handle and the reaction easily proceeds uniformly.

  The addition amount of the conjugated diene compound is preferably 0.01 to 20 parts by weight and more preferably 0.05 to 5 parts by weight with respect to 100 parts by weight of the linear polypropylene resin. If the amount is less than 0.01 parts by weight, the effect of the modification may be difficult to obtain, and if the amount exceeds 20 parts by weight, the effect is saturated, which may not be economical.

  Monomers copolymerizable with the conjugated diene compounds, such as vinyl chloride, vinylidene chloride, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, vinyl acetate, acrylic acid, methacrylic acid, maleic acid, maleic anhydride, metal acrylate Salt, metal methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, and other acrylic esters, methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-methacrylic acid 2- Methacrylic acid esters such as ethylhexyl and stearyl methacrylate may be used in combination.

  The radical polymerization initiator generally includes peroxides, azo compounds, and the like, but those having a capability of extracting hydrogen from a polypropylene resin or the conjugated diene compound are preferable. Generally, ketone peroxides, peroxyketals, hydroperoxides are used. Organic peroxides such as oxides, dialkyl peroxides, diacyl peroxides, peroxydicarbonates, and peroxyesters are listed. Of these, those having particularly high hydrogen abstraction ability are preferred, such as 1,1-bis (t-butylperoxy) 3,3,5-trimethylcyclohexane, 1,1-bis (t-butylperoxy) cyclohexane, Peroxyketals such as n-butyl 4,4-bis (t-butylperoxy) valerate, 2,2-bis (t-butylperoxy) butane, dicumyl peroxide, 2,5-dimethyl-2,5 -Di (t-butylperoxy) hexane, α, α'-bis (t-butylperoxy-m-isopropyl) benzene, t-butylcumyl peroxide, di-t-butylperoxide, 2,5-dimethyl Diacyl peroxides such as dialkyl peroxides such as -2,5-di (t-butylperoxy) -3-hexyne and benzoyl peroxides Oxide, t-butyl peroxyoctate, t-butyl peroxyisobutyrate, t-butyl peroxylaurate, t-butyl peroxy 3,5,5-trimethylhexanoate, t-butyl peroxyisopropyl carbonate 1, peroxyesters such as 2,5-dimethyl-2,5-di (benzoylperoxy) hexane, t-butylperoxyacetate, t-butylperoxybenzoate, di-t-butylperoxyisophthalate A seed | species or 2 or more types is mentioned.

  The addition amount of the radical polymerization initiator is preferably 0.01 parts by weight or more and 10 parts by weight or less, more preferably 0.05 parts by weight or more and 2 parts by weight or less with respect to 100 parts by weight of the linear polypropylene resin. If the amount is less than 0.01 part by weight, the effect of reforming may be difficult to obtain, and if the amount exceeds 10 parts by weight, the effect of reforming may be saturated and not economical.

  The apparatus for reacting the linear polypropylene resin, the conjugated diene compound, and the radical polymerization initiator includes a roll, a kneader, a Banbury mixer, a Brabender, a single screw extruder, a kneader such as a twin screw extruder, a twin screw, etc. Examples of the surface renewal machine include a horizontal stirrer such as a biaxial multi-disk device, and a vertical stirrer such as a double helical ribbon stirrer. Among these, a kneader is preferably used, and an extruder is particularly preferable from the viewpoint of productivity.

  There is no particular limitation on the order and method of mixing and kneading (stirring) the linear polypropylene resin, the conjugated diene compound, and the radical polymerization initiator. The linear polypropylene-based resin, the conjugated diene compound, and the radical polymerization initiator may be mixed and then melt-kneaded (stirred). After the polypropylene-based resin is melt-kneaded (stirred), the conjugated diene compound or the radical initiator may be mixed. They may be mixed simultaneously or separately, collectively or divided. The temperature of the kneading (stirring) machine is preferably 130 to 300 ° C. in that the linear polypropylene resin melts and does not thermally decompose. The time is generally preferably 1 to 60 minutes.

  In this way, the modified polypropylene resin (B) used in the present invention can be produced.

  There is no restriction | limiting in the shape and magnitude | size of polypropylene resin (A) and (B), and a pellet form may be sufficient.

  Of the total 100 parts by weight of the linear polypropylene resin (A) and the modified polypropylene resin (B) used in the present invention, the linear polypropylene resin (A) may be 50 parts by weight or more and 95 parts by weight or less. Preferably, it is 60 parts by weight or more and 90 parts by weight or less. The modified polypropylene resin (B) is preferably 5 to 50 parts by weight, more preferably 10 to 40 parts by weight. If it is the said compounding quantity, it will be easy to obtain the foaming molding which has a uniform fine bubble and whose expansion ratio is 2 times or more. In addition, a short shot does not occur in the molding having a thin-walled portion, and continuous and stable production can be performed, and a foamed molded article having a beautiful surface appearance can be provided at a low cost.

  The polypropylene resin for injection foam molding used in the present invention is preferably a resin obtained by mixing a linear polypropylene resin (A) and a modified polypropylene resin (B). The mixing method is not particularly limited and can be performed by a known method. For example, dry blending of pellet-shaped resin using a blender, mixer, etc., melt mixing, melting and mixing in a solvent, etc. Can be mentioned. In the present invention, the method of dry blending and then subjecting to injection foam molding is preferable because it requires less heat history and decreases the melt tension.

  The foaming agent that can be used in the present invention is not particularly limited as long as it can be usually used for injection foam molding, such as a chemical foaming agent and a physical foaming agent. The chemical foaming agent is previously mixed with the resin composition and then supplied to the injection molding machine, and decomposes in the cylinder to generate a gas such as carbon dioxide. Examples of the chemical foaming agent include inorganic chemical foaming agents such as sodium bicarbonate and ammonium carbonate, and organic chemical foaming agents such as azodicarbonamide and N, N′-dinitrosopentatetramine. A physical foaming agent is injected into a molten resin in a cylinder of a molding machine as a gaseous or supercritical fluid, dispersed or dissolved, and functions as a foaming agent by being released from pressure after being injected into a mold. Is. Physical foaming agents include aliphatic hydrocarbons such as propane and butane, alicyclic hydrocarbons such as cyclobutane and cyclopentane, halogenated hydrocarbons such as chlorodifluoromethane and dichloromethane, nitrogen, carbon dioxide, air, etc. Inorganic gas. You may use these individually or in mixture of 2 or more types.

  Among these foaming agents, normal injection molding machines can be used safely, and uniform fine bubbles are easily obtained. Chemical foaming agents are inorganic chemical foaming agents, physical foaming agents are nitrogen and carbon dioxide. Inorganic gas such as air is preferable. These foaming agents include, for example, foaming aids such as organic acids such as citric acid and inorganic fine particles such as talc and lithium carbonate, in order to stably and uniformly make the foamed foam air bubbles. A nucleating agent such as may be added. Usually, the inorganic chemical foaming agent is preferably used as a masterbatch of a polyolefin resin having a concentration of 10 to 50% by weight from the viewpoints of handleability, storage stability, and dispersibility in a polypropylene resin.

  What is necessary is just to set the usage-amount of the said foaming agent suitably with the foaming magnification of the final product, the kind of foaming agent, and the resin temperature at the time of shaping | molding. For example, in the case of an inorganic chemical foaming agent, it is preferably 0.1 to 20 parts by weight, more preferably 0.5 to 10 parts by weight, with respect to 100 parts by weight of the polypropylene resin of the present invention. Used in the following ranges. By using in this range, it is easy to economically obtain a foamed molded article having a foaming ratio of 2 times or more and uniform fine cells.

  Further, if necessary, as long as the effects of the present invention are not impaired, antioxidants, metal deactivators, phosphorus processing stabilizers, UV absorbers, UV stabilizers, fluorescent brighteners, metal soaps, antacid adsorption An additive such as a stabilizer such as an agent, a crosslinking agent, a chain transfer agent, a nucleating agent, a lubricant, a plasticizer, a filler, a reinforcing material, a pigment, a dye, a flame retardant, and an antistatic agent may be used in combination. Of course, these additives used as necessary are used within the range not impairing the effects of the present invention, but generally 0 parts by weight with respect to 100 parts by weight of the polypropylene resin composition of the present invention. 0.01 parts by weight or more and 10 parts by weight or less is used.

  The foamed molded article of the present invention thus obtained has an average cell diameter of preferably 500 μm or less, more preferably 300 μm or less, and a maximum cell diameter of preferably 3 times or less, more preferably 2 And a non-foamed layer having a thickness of preferably 10 μm or more and 1000 μm or less, more preferably 100 μm or more and 500 μm or less, formed on at least one surface of the foamed layer. When the average cell diameter of the foamed layer exceeds 500 μm, excellent rigidity may not be obtained, and when the maximum cell diameter exceeds 3 times the average cell diameter, this tendency is particularly remarkable. If the thickness of the non-foamed layer is less than 10 μm, the surface is not beautiful and the rigidity tends to decrease. If it exceeds 1000 μm, the lightness may not be obtained. In the present invention, the “bubble diameter” refers to the diameter when the bubbles are spherical, and the average value of the long and short diameters when the bubbles are oval, and the average bubble diameter refers to the foam molded body in the thickness direction. In the micrograph of the cross section cut into two, the average value of 20 arbitrarily selected, and the maximum bubble diameter refers to the bubble diameter of the largest bubble in this micrograph.

  The expansion ratio of the foamed molded article of the present invention is preferably 2 to 10 times, more preferably 3 to 6 times. If the expansion ratio is less than 2 times, it may be difficult to obtain light weight, and if it exceeds 10 times, the rigidity tends to be significantly reduced. The expansion ratio is a value obtained from the ratio of the specific gravity of the foamed molded product and the non-foamed molded product that was injection molded under the same conditions except that the foaming agent was not added to the polypropylene resin composition for injection foam molding.

  Other molding conditions may be appropriately adjusted according to the MFR of each polypropylene resin, the type of foaming agent, the type of molding machine, or the shape of the mold. Usually, the resin temperature is 170 to 250 ° C., the mold temperature is 10 to 100 ° C., the molding cycle is 1 to 60 minutes, the injection speed is 10 to 300 mm / second, and the injection pressure is 10 to 200 MPa.

  The present invention will be described in more detail with reference to the following examples. However, the present invention is not limited to these examples.

In the examples and comparative examples, the test methods and criteria used in various evaluation methods are as follows.
(1) Melt flow rate: Measured in accordance with ASTM 1238 at a temperature of 230 ° C. and a load of 2.16 kg.
(2) Melt tension: A capilograph (manufactured by Toyo Seiki Seisakusho) with an attachment for measuring melt tension was used. When a strand lowered at a piston descending speed of 10 mm / min was drawn at 1 m / min from a die having a hole of φ1 mm and a length of 10 mm at 230 ° C., it broke when the take-up speed was increased at 40 m / min ² after stabilization. The take-up load of the pulley with the load cell at that time was taken as melt tension.
(3) Strain hardenability: When measuring the above-mentioned melt tension, when the take-up speed is increased, the take-up load increases abruptly, and when it reaches the fracture, it indicates “strain hardenability”; It does not show sex ".
(4) Injection foam moldability: The number of short shots (number of defects) when 40 shots were continuously molded was determined and evaluated in the following three stages.

The number of defects is 1 or less.
2 to 4 defectives ... △
The number of defects is 5 or more.
(5) Foaming ratio: A specimen including the non-foamed layer on the surface was cut out from the foamed molded product, and the ratio was determined from the specific gravity ratio with a non-foamed molded product having a thickness of 3 mm (Reference Example 1).
(6) Average cell diameter, non-foamed layer thickness: It was determined from a micrograph of a cross section of the foamed molded product cut in the thickness direction. The non-foamed layer thickness was an average value of the movable mold side and the fixed mold side. The average cell diameter was an average value of 20 arbitrarily selected, and evaluated as follows.

Average bubble diameter is 500μm or less.
Those whose average bubble diameter exceeds 500μm ・ ・ ・ ×
(7) Bubble uniformity: The cross section of the foamed product cut in the thickness direction is observed, and the largest bubble diameter in the foam layer is taken as the maximum bubble diameter, and evaluated as follows from the relationship with the average bubble diameter. did.

Maximum bubble diameter / average bubble diameter ≤ 2.5
2.5 <maximum bubble diameter / average bubble diameter ≦ 3...
3 <maximum bubble diameter / average bubble diameter ...
(8) Molded body thickness: For a cross section cut in the thickness direction, an average value of five points at the four corners and the central portion was used.
(9) Surface smoothness: Smoothness was evaluated by the difference between the maximum value and the minimum value among the five measurement results of the measurement of the molded product thickness.

The difference is within 0.2mm ... ○
The difference exceeds 0.2mm
Next, polypropylene resins and foaming agents used in Examples and Comparative Examples are shown below.
(A) Linear polypropylene resin PP-1: J708UG (Propylene / ethylene / block copolymer, melt flow rate 45 g / 10 min, melt tension 1 cN or less) manufactured by Grand Polymer Co., Ltd.
PP-2: J707G manufactured by Grand Polymer Co., Ltd. (propylene / ethylene / block copolymer, melt flow rate 30 g / 10 min, melt tension 1 cN or less)
(B) Modified polypropylene resin MP-1: 100 parts by weight of a polypropylene homopolymer having a melt flow rate of 3 g / 10 min as a linear polypropylene resin, and 0.3 wt. Of t-butyl peroxyisopropyl carbonate as a radical polymerization initiator Part of the mixture is fed from a hopper of a 44 mmφ twin screw extruder (L / D = 38) at 50 kg / hour, and isoprene monomer is fed at a rate of 0.25 kg / hour from the introduction part provided in the middle using a metering pump. A modified polypropylene resin obtained by supplying, cooling with water and chopping the strand (melt flow rate 0.5 g / 10 min, melt tension 12 cN, exhibiting strain hardening)
MP-2: PF814 manufactured by Sun Allomer (homopolymer, melt flow rate 3 g / 10 min, melt tension 10 cN, strain hardening)
(C) Foaming agent B-1: Chemical foaming agent master batch (Polyslen EE275 manufactured by Eiwa Kasei Co., Ltd., foaming agent concentration 27%, decomposition gas amount 40 ml / g)
B-2: Carbon dioxide having a purity of 99% or more (Example 1)
The injection molding machine uses “MD350S-IIIDP type” manufactured by Ube Machinery Co., Ltd., which has a shut-off nozzle mechanism at the tip of the cylinder. What has the flat plate-shaped cavity of variable thickness by position adjustment was used. As molding conditions, foam molding was performed at a resin temperature of 200 ° C., a mold temperature of 50 ° C., an injection speed of 100 mm / second, a back pressure of 5 MPa, and a cooling time of 90 seconds.

That is, 80 parts by weight of PP-1 as a linear polypropylene resin (A), 20 parts by weight of MP-2 as a modified polypropylene resin (B), and 5 parts by weight of B-1 as a foaming agent are dry blended. The polypropylene resin mixture for injection foam molding thus obtained was supplied to the injection molding machine. After plasticizing and kneading in the molding machine, a mold having an initial cavity clearance (t ₀ ) of 1.5 mm is injection-filled. Immediately after the filling is completed, a cavity clearance (t ₁ ) is obtained at a mold opening speed of 50 mm / sec by the first mold opening process. Was expanded to 2.5 mm. Next, the cavity clearance (t _B ) is reduced to 2.0 mm at a mold closing speed of 0.1 mm / sec by a mold closing process, and then the cavity clearance is enlarged at a mold opening speed of 5 mm / sec by a second mold opening process. A foamed molded product having a predetermined molded product thickness was obtained.

(Example 2)
Dry blend 70 parts by weight of PP-1 as the linear polypropylene resin (A), 30 parts by weight of MP-2 as the modified polypropylene resin (B), and 7.5 parts by weight of B-1 as the foaming agent. A foam molded article was obtained in the same manner as in Example 1 except that the polypropylene resin mixture for injection foam molding obtained was used and t ₁ was set to 3.0 mm.

Example 3
9 parts by weight of B-1 was mixed as a foaming agent, and a foamed molded article was obtained in the same manner as in Example 2 except that the mold closing speed was 0.05 mm / sec and t _B was 2.5 mm in the mold closing process. It was.

Example 4
9 parts by weight of B-1 as a foaming agent is mixed, t _{0 is set} to 2.0 mm, t _{1 is set} to 5.0 mm, t _{B is set} to 3.0 mm, and the cavity is opened at a mold opening speed of 10 mm / sec in the second mold opening process. A foamed molded article was obtained in the same manner as in Example 2 except that the clearance was enlarged.

(Example 5)
A foamed molded article was obtained in the same manner as in Example 2 except that MP-1 was used as the modified polypropylene resin (B) and t _B was 2.5 mm.

(Example 6)
A foamed molded article was obtained in the same manner as in Example 2 except that the mold closing speed was 10 mm / sec in the middle mold closing step.

（実施例７）
実施例１〜５で使用した射出成形機をベントタイプ仕様（シリンダ中央付近にベント口）に変えてベント部分を二酸化炭素で加圧できるようにした成形機を使用し、さらに旭エンジニアリング（株）製「炭酸ガス供給装置ＭＡＣ−１００」を用いて二酸化炭素を圧力一定で供給することで、溶融樹脂に対する二酸化炭素供給量を制御した。

(Example 7)
The injection molding machine used in Examples 1 to 5 was changed to a vent type specification (a vent port near the center of the cylinder), and a molding machine in which the vent part could be pressurized with carbon dioxide was used. Asahi Engineering Co., Ltd. The amount of carbon dioxide supplied to the molten resin was controlled by supplying carbon dioxide at a constant pressure using a “carbon dioxide supply device MAC-100” manufactured by the manufacturer.

  Injection foam molding obtained by dry blending linear polypropylene resin (A) and modified polypropylene resin (B) having a composition ratio shown in Table 3 with 0.5 part of B-1 as a nucleating agent. Foaming was performed in the same manner as in Example 3 except that the polypropylene resin mixture for use was supplied to the injection molding machine, and carbon dioxide (B-2) was supplied as a foaming agent as shown in Table 3. A molded body was obtained.

(Example 8)
A foamed molded article was obtained in the same manner as in Example 7, except that the supply pressure of B-2 was 3 MPa, the mold closing speed was 0.1 mm / sec and t _B was 2.0 mm in the middle mold closing step.

Tables 2 and 4 show the moldability in Examples 1 to 8, and the shapes and physical properties of the obtained foamed molded articles. As shown in Tables 2 and 4, the foamed molded products obtained in Examples 1 to 8 have good injection foaming moldability, and have a foaming ratio of 2.6 to 4.4 times. Is excellent. It can be seen that the surface has a non-foamed layer (skin layer) of 300 μm, the internal bubbles are fine and excellent in uniformity, and the surface smoothness is also good.
(Reference Example 1)
In the examples, a modified polypropylene resin and a foaming agent are not used, and only a linear polypropylene resin PP-2 is injected and filled into a mold having an initial cavity clearance (t ₀ ) of 3.0 mm, and after completion of injection filling, 90% is obtained. The non-foamed molded body was taken out after cooling for 2 seconds. The results are shown in Table 2.

(Comparative Example 1)
A foamed molded article was obtained in the same manner as in Example 2 except that the mold opening process was carried out in one step at a mold opening speed of 50 mm / sec after completion of filling. The results are shown in Table 2. The foam had a large and nonuniform cell structure near the center in the thickness direction.

(Comparative Example 2)
After the completion of filling, the state is maintained for 1 second, and then the cavity clearance is increased to 3.0 mm at a mold opening speed of 5 mm / sec, and the state is maintained for 5 seconds. Subsequently, a foamed molded article was obtained in the same manner as in Example 2 except that the cavity clearance was increased again at a mold opening speed of 10 mm / sec. The results are shown in Table 2. The non-foamed layer was a slightly thick foam, and the foam had a large and nonuniform cell structure near the center in the thickness direction. Further, the foam did not swell to the desired thickness, and was a foam having a partially uneven surface.

(Comparative Example 3)
Example 2 except that the first mold opening process is carried out after holding the state as it is for 1 second after completion of filling, and the second mold opening process is carried out after holding the state for 5 seconds after the first mold opening process. In the same manner, a foamed molded product was obtained. The results are shown in Table 2. The non-foamed layer was a slightly thick foam, and the foam had a large and nonuniform cell structure near the center in the thickness direction. Further, the foam did not swell to the desired thickness, and was a foam having a partially uneven surface.

(Comparative Example 4)
After maintaining the state for 5 seconds after the first mold opening process, a foamed molded article was obtained in the same manner as in Example 2 except that the second mold opening process was performed. The results are shown in Table 2. The foam had a large and nonuniform cell structure near the center in the thickness direction.

(Comparative Example 5)
A foamed molded article was obtained in the same manner as in Example 7 except that a mold opening process in one step was performed at a mold opening speed of 50 mm / sec after completion of filling. The results are shown in Table 4. The foam had a large and nonuniform cell structure near the center in the thickness direction.

(Comparative Example 6)
After the completion of filling, the state is maintained for 1 second, and then the cavity clearance is increased to 3.0 mm at a mold opening speed of 5 mm / sec, and the state is maintained for 5 seconds. Next, a foamed molded article was obtained in the same manner as in Example 7 except that the cavity clearance was increased again at a mold opening speed of 10 mm / sec. The results are shown in Table 4. The non-foamed layer was a slightly thick foam, and the foam had a large and nonuniform cell structure near the center in the thickness direction. Further, the foam did not swell to the desired thickness, and was a foam having a partially uneven surface.

  The thermoplastic resin injection foam molding method of the present invention is capable of high magnification, and the resulting foam molded article has a smooth surface and is excellent in light weight. Can be widely used for containers, home appliances, and building materials.

Claims (11)

  In a method for producing a foamed molded article by injecting a molten mixture of a thermoplastic resin and a foaming agent into a mold and then opening the mold, the thermoplastic resin composition and the foaming agent are supplied to an injection molding machine. 1st mold opening process which melts and injects and fills the mold, and opens the mold to a predetermined position less than the final product shape position immediately after the filling is completed, and then closes the mold to a predetermined position larger than the initial cavity A method for producing a thermoplastic resin foam molded article, comprising a mold closing process and a second mold opening process for opening the mold to a final product shape position.   The method for producing a thermoplastic resin foam molded article according to claim 1, wherein a cavity clearance after the mold opening in the first mold opening step is 1.2 times or more and 3.0 times or less of an initial cavity clearance.   The cavity clearance after mold closing in the middle mold closing process is 1.1 times or more of the initial cavity clearance and 0.95 times or less of the cavity clearance after the first mold opening process. 3. A method for producing a thermoplastic resin foam molded article according to 2.   The mold opening speed in the first mold opening step is 5 to 100 mm / sec. The method for producing a thermoplastic resin foam molded article according to any one of claims 1 to 3.   The method for producing a thermoplastic resin foam molded article according to any one of claims 1 to 4, wherein a mold closing speed in the intermediate mold closing step is 0.01 to 5 mm / sec.   The method for producing a thermoplastic resin foam molded article according to any one of claims 1 to 5, wherein a mold opening speed in the second mold opening step is 0.05 to 50 mm / sec.   The method for producing a thermoplastic resin foam molded article according to any one of claims 1 to 6, wherein the thermoplastic resin is a polypropylene resin.   The polypropylene resin is (A) 50 to 95 parts by weight of a linear polypropylene resin having a melt flow rate of 10 g / 10 min to 100 g / 10 min and a melt tension of 2 cN or less, and (B) the melt flow rate is 0. 5 to 50 parts by weight of a modified polypropylene resin having a melt tension of 5 cN or more and a strain hardening property (however, the linear polypropylene resin (A) and the modified polypropylene) The total amount of the resin (B) is 100 parts by weight). The method for producing a thermoplastic resin foam-molded article according to any one of claims 1 to 7.   The modified polypropylene resin (B) is a modified polypropylene resin obtained by melt-mixing a linear polypropylene resin, a radical polymerization initiator, and a conjugated diene compound. The manufacturing method of the thermoplastic resin foaming molding in any one of.   The method for producing a thermoplastic resin foam molded article according to any one of claims 1 to 9, wherein an initial cavity clearance when the molten mixture is injected is less than 3 mm.   Foam having an average bubble diameter of 500 μm or less and a maximum bubble diameter of 3 times or less of the average bubble diameter and a non-foamed layer having a thickness of 10 μm or more and 1000 μm or less formed on the surface of at least one side of the foam layer The expanded molded article obtained by the production method according to any one of claims 1 to 10, wherein the magnification is 2 to 10 times.