JP2007062365A - Manufacturing method for thermoplastic resin foamed molded body, and molded body - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a foamed molded body which is excellent in lightweight property by a high foaming ratio, and does not have the reduction of rigidity by voids or the like, and to provide a manufacturing method by which especially, a box-shaped molded body can easily be obtained. <P>SOLUTION: This manufacturing method for the foamed molded body manufactures the foamed molded body by injecting a molten mixture of a thermoplastic resin and a foaming agent into dies. In the manufacturing method, the molten mixture which is from 80% to 97% of a full filling amount is injection-filled into the dies being constituted of a fixed die and a movable die which can advance and retract, and of which the initial cavity clearance t<SB>0</SB>is from 0.8 mm to 3.0 mm. Then, the movable die is retracted, and foamed. <P>COPYRIGHT: (C)2007,JPO&INPIT

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 weight, reducing costs, and preventing warping and sink marks of a 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 so-called core back method (Moving Cavity method).

  When a molten resin containing a foaming agent is injected into a mold by the core back method, generally, as disclosed in Patent Document 1, after leaving without applying pressure after injection or immediately after completion of injection There is a method in which a mold is opened and foamed. According to this method, compared to the conventional non-foamed injection molding, since no holding pressure is applied, the amount of resin to be used can be reduced by that amount, leading to weight reduction. However, normally, even after the mold is filled with the molten resin, the molten resin is pushed into the mold at a high pressure, that is, an excessive amount of the molten resin is injected in excess of the amount corresponding to the mold volume. In order to obtain a molded article having a high foaming ratio, it is easy to obtain a molded article having no void inside by filling in the mold, and therefore an excessive molten resin is filled in the mold. As a result, it was unexpectedly difficult to reduce the weight. In particular, when the initial cavity clearance of the mold is reduced, it is necessary to set the injection pressure high to prevent short shots, and to inject the molten resin at a high pressure and fill the mold. In this case, the presence or absence of pressure retention hardly affected the amount of resin filled in the mold, and the effect of weight reduction due to the absence of pressure retention tended to be difficult to obtain.

  Further, in order to reduce the weight of the molded product by the core back method, it is necessary to reduce the thickness of the molded product at the stage before foaming. For this reason, it is necessary to perform injection filling with the mold clearance set small, and for this reason, it is necessary to fill the mold with molten resin at a high pressure. An expensive molding machine having a clamping device is required, and an expensive mold that can withstand the high pressure during injection is also required for the mold.

  Furthermore, when a polypropylene resin is used as a material, the polypropylene resin is crystalline and has a low melt tension (melting tension), and bubbles are easily destroyed during foaming. When it is set small, the tendency is strong, it is difficult to increase the expansion ratio, and it is difficult to reduce the weight of the molded product. 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.

  Further, when the shape of the molded body is a box-shaped object composed of a bottom surface and a standing wall, it is difficult to foam the standing wall portion as efficiently as the bottom surface portion by the conventional core back method. In order to reduce the weight of the box-shaped molded body, it is necessary to reduce the thickness of the bottom part before foaming, and it is necessary to reduce the overall weight. It is difficult to reduce the weight of a box-shaped object having an upright wall portion and a particularly high ratio of the upright wall portion to the bottom surface.

  Furthermore, the standing wall portion may be required to have a certain thickness from the viewpoint of maintaining rigidity. However, in the core back method in which the resin is foamed by opening the mold, it is almost parallel to the mold opening direction. It is difficult to increase the thickness of the standing wall portion, and it is necessary to give a certain thickness in advance. Therefore, under the present circumstances, the thickness of the molded body before foaming is set so that the standing wall portion is relatively thick and the bottom surface is thin, but when the molten resin is injected into the mold in such a situation, a part of the flow front stands. When reaching the wall part, the flow resistance of the corresponding part decreases compared to other parts, and the flow of the molten resin tends to concentrate on that part. As a result, in other parts, the flow velocity is reduced, the skin layer becomes thicker, and the internal cooling proceeds and solidifies easily, so that it is difficult to obtain a high expansion ratio, and the molded product tends to be indented. This tendency is conspicuous even when the shape of the bottom surface of the box is not uniform, such as the shape of a rectangle or ellipse, such as a rectangle or an ellipse, and the maximum flow length from the gate to the standing wall is particularly large. There is a tendency to become more prominent when the difference between the value and the minimum value is large, and there is a limit to weight reduction by thinning.

  As a method for solving such a problem, for example, Patent Document 2 discloses a method of foaming the standing wall portion at a high magnification by providing a moving nest on the standing wall portion and retracting the nest. If this method is used, it is possible to foam the standing wall portion at a high magnification, but there is a problem that the mold structure becomes complicated and the manufacturing cost becomes expensive.

As described above, it has heretofore been difficult to obtain a thermoplastic resin injection foam molded article having good injection foam moldability, high lightness, and high foaming ratio and excellent rigidity.
JP-A-2005-59224 JP 2001-105447 A

  An object of the present invention is to provide a production method capable of easily obtaining a foamed molded article having a high foaming ratio, excellent lightness, and no reduction in rigidity due to voids or the like.

  In the production method of injection foam molding, the inventors of the present invention have a high blending ratio and light weight by foaming after injection filling a resin amount in a specific range in a production method of injection foam molding of a molten mixture of a polypropylene resin and a foaming agent. It has been found that a foamed molded article can be easily obtained, and the present invention has been completed.

That is, the first aspect of the present invention is 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 comprises a fixed mold and a movable mold capable of moving forward and backward. A mold having an initial cavity clearance t ₀ of 0.8 mm to less than 3.0 mm is injected and filled with a molten mixture having a full filling amount of 80% to 97% by weight, and then the movable mold is moved backward to foam. The present invention relates to a method for producing a thermoplastic resin foam molded article.

As a preferred embodiment,
(1) The mold is filled with the molten mixture when injection filling is completed,
(2) Upon completion of injection filling, the mold is not filled with the molten mixture, and the foamed molten mixture is filled in the mold after the foaming step,
(3) The thermoplastic resin is a polypropylene resin,
(4) An injection step of injecting and filling the molten mixture into the mold cavity having the initial cavity clearance t ₀ , and then a cavity smaller than the cavity clearance t ₂ corresponding to the shape position of the final product and larger than t ₀ The first stage foaming step for retracting the movable mold to the clearance t ₁ , and then the second stage foaming for retracting the movable mold to the cavity clearance t ₂ corresponding to the shape position of the final product after holding the clearance of t ₁ for a predetermined time. Comprising the steps,
(5) The mold opening speed in the first stage foaming step is 7 mm / second or more and 100 mm / second or less,
(6) The mold opening speed in the second stage foaming step is 0.5 mm / second or more and 20 mm / second or less,
(7) The relationship between the t ₀ and t ₁ is expressed by the following equation:

（８）前記第二段発泡工程における金型キャビティクリアランスｔ₁を保持する所定時間が１秒以上２０秒以下であること、
を特徴とする前記記載の射出発泡成形体の製造方法に関する。

(8) The predetermined time for maintaining the mold cavity clearance t ₁ in the second stage foaming step is 1 second or more and 20 seconds or less,
A method for producing the injection foam molded article as described above.

  The second aspect of the present invention has a foaming layer having an average cell diameter of 500 μm or less, and a non-foaming layer having a thickness of 10 μm or more and 1000 μm or less formed on at least one surface of the foaming layer, and a foaming ratio of 2 to 10 The thermoplastic resin foam molded article produced by the production method according to any one of claims 1 to 9, characterized in that, as a preferred embodiment, the foam molded article comprises a bottom surface and a standing wall portion. The thermoplastic resin foam-molded article according to claim 10, wherein the thermoplastic resin foam-molded article has a box-like shape.

  According to the method for producing an injection-foamed molded article of the present invention, a foamed molded article having a high foaming ratio and excellent lightness can be easily produced without reducing the initial cavity clearance more than necessary.

The manufacturing method of the thermoplastic resin injection foam molded body of the present invention is characterized in that in the method of manufacturing a foam molded body by injecting a molten mixture of a thermoplastic resin and a foaming agent into a mold, the mold is moved forward and backward. Injecting and filling a molten mixture of 80 wt% or more and 97 wt% or less of the full filling amount into a mold having an initial cavity clearance t ₀ composed of a movable mold, and then moving the movable mold backward to foam. It is in.

  The full filling amount referred to in the present invention is a non-foamed injection molded product without warping or sinking obtained through a so-called pressure-holding process in which a resin is pushed into a cavity while pressure is applied even after filling is completed. The amount of resin (weight).

  When the amount of the molten mixture to be injected and filled in the mold is within the above range, it is possible to stably obtain a molded body that is easily reduced in weight without reducing the initial cavity clearance more than necessary. The amount of the molten mixture is further preferably 85% by weight or more and 95% by weight or less of the full filling amount.

  In the present invention, within the range of the above-mentioned filling amount, the movable mold is retracted from two states: a state where the mold is filled with the molten mixture when filling is completed, or a state where the mold is not filled with the molten mixture when filling is completed. It can be made to move to a foaming process. In the present invention, the “full state” means a state in which the molten mixture is completely filled in the mold, and the “unfilled state” means that the molten mixture fills a portion in the mold. This is a so-called short shot state that is not exhausted.

  By making the mold filled with the molten mixture when the former filling is completed, a molded article with good surface properties can be easily obtained. Moreover, it becomes easy to obtain the molded object reduced in weight by making it foam from the state which is not full of the latter. In this latter case, the portion of the mold that is not filled with the molten mixture fills the mold with the foaming power of the molten mixture itself after the foaming process is completed. The product satisfies the shape.

Here, the initial cavity clearance t ₀ needs to be 0.8 mm or more and less than 3 mm. Preferably they are 1.0 mm or more and 2 mm or less. When t ₀ is less than 0.8 mm, the mold is injected and filled with a molten mixture of 80% to 97% of the full filling amount, and the mold is filled with the foaming power of the molten mixture itself after the foaming process. It is difficult to achieve high foaming that is twice or more. Further, when t ₀ is 3 mm or more, it is difficult to obtain the effect of weight reduction.

  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. To polyolefin-based resins are preferable, and polypropylene-based resins are particularly preferable.

  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) has a melt flow rate of preferably 10 g / 10 min to 100 g / 10 min, more preferably 15 g / 10 min to 50 g / 10 min, and a melt tension of preferably 2 cN or less. More preferably, it is 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 foam molded article, the mold cavity clearance is relatively small even in molding having a thin part of about 1 to 2 mm. The molten resin can be filled in the mold at a low pressure, and there is a tendency that continuous and stable molding can be performed. In addition, since bubbles are not easily destroyed during foaming, a foamed molded article having a beautiful surface appearance tends to be obtained. 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 tends to be obtained.

The melt flow rate is a value measured in accordance with ASTM D-1238 and measured at 230 ° C. under a load of 2.16 kg. 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) that can be 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. The melt tension is preferably 5 cN or more, more preferably 8 cN or more, and exhibits strain hardening. 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 tends to be obtained. In addition, when the melt tension is 5 cN or more, there is a tendency that a foamed molded product having uniform and fine cells twice or more is obtained.

  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.

  In addition, the effect of the strain-hardening property of the modified polypropylene resin (B) is that the appearance of the surface is beautiful because the silver streaks due to foam breakage at the molten resin flow tip during injection molding are less likely to occur. It is easy to obtain a foamed molded article with a high magnification exceeding the expansion ratio of 2 times.

  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, a modified polypropylene resin obtained by melt-mixing a linear polypropylene resin, a radical polymerization initiator and a conjugated diene compound can be produced at low cost without requiring expensive radiation irradiation equipment. It is preferable from the point. 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. May be. 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 parts by weight or more and 20 parts by weight or less, and more preferably 0.05 parts by weight or more and 5 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 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 Peroxyesters such as 2,5-dimethyl-2,5-di (benzoylperoxy) hexane, t-butylperoxyacetate, t-butylperoxybenzoate, and di-t-butylperoxyisophthalate. Of these, one or more of them can be used.

  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.

  As an apparatus for melt-mixing the linear polypropylene resin, the conjugated diene compound, and the radical polymerization initiator, a kneading machine such as a roll, a kneader, a Banbury mixer, a Brabender, a single screw extruder, a twin screw extruder, Examples thereof include a shaft surface renewal machine, a horizontal stirrer such as a two-shaft multi-disc 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. from the viewpoint that the linear polypropylene resin melts and does not thermally decompose. The time is generally preferably 1 to 60 minutes.

  In this way, a modified polypropylene resin (B) that can be 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.

  The polypropylene resin for injection foam molding that can be suitably used in the present invention is a linear polypropylene resin (A) in a total of 100 parts by weight of the linear polypropylene resin (A) and the modified polypropylene resin (B). ) Is preferably 50 parts by weight or more and 95 parts by weight or less, and more preferably 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. There exists a tendency which can provide the foaming molding of 2 times or more which has a uniform fine cell as it is the said compounding quantity cheaply.

  The mixing method of the linear polypropylene resin (A) and the modified polypropylene resin (B) is not particularly limited, and can be performed by a known method. For example, the pellet-shaped resin is dried using a blender, a mixer, or the like. Examples of the method include blending, melt mixing, and dissolving and mixing in a solvent. 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′-dinitrosopentamethylenetetramine. Of these, an inorganic chemical foaming agent is usually preferred because it is difficult to be colored, there are few decomposition residues, and air bubbles are easily refined. These inorganic chemical foaming agents include foaming aids such as organic acids such as citric acid, talc, lithium carbonate, etc. A nucleating agent such as inorganic fine particles may be added. In addition, when using the said inorganic type chemical foaming agent, it is normally used as a masterbatch of the polyolefin resin of a 10 to 50 weight% density | concentration from the point of handleability, storage stability, and the dispersibility to a thermoplastic resin. Is preferred. The amount of these inorganic chemical foaming agents added varies depending on the type and concentration in the masterbatch, but is generally preferably 0.1 to 20 parts by weight, more preferably 100 parts by weight with respect to 100 parts by weight of the thermoplastic resin of the present invention. Is used in the range of 0.5 to 10 parts by weight. 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. Of these, inorganic gases, particularly nitrogen and carbon dioxide are preferred because they are inexpensive and easy to handle. These various foaming agents may be used alone or in combination of two or more. The amount of the physical foaming agent used varies depending on the type of foaming agent and the desired expansion ratio, but generally it is preferably 0.05% by weight or more and 10% by weight or less, and more preferably 0.1% by weight for the thermoplastic resin of the present invention. It is used in the range of not less than 5% by weight and not more than 5% by weight, particularly preferably not less than 0.2% by weight and not more than 3% by weight.

  In the case of a polypropylene resin that can be suitably used in the present invention, by using a chemical foaming agent and a physical foaming agent in the above ranges, the expansion ratio is economically twice or more and uniform fine bubbles. In addition to being easy to obtain a foamed molded article of this type, the generation of silver streaks can be minimized because an unnecessarily large amount of foaming agent is not used.

  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. These additives used as necessary are used within a range not impairing the effects of the present invention, but preferably 0.01 wt.% Relative to 100 parts by weight of the thermoplastic resin of the present invention. More than 10 parts by weight.

In the method for producing an injection-foamed molded article of the present invention described below, when the mold is box-shaped, each cavity clearance t ₀ , t ₁ , t ₂ indicates the cavity clearance of the bottom surface portion.

A resin composition for injection foam molding comprising a foaming agent and the thermoplastic resin is movable so that the mold can be moved forward and backward with a fixed mold after being melt-kneaded in an injection molding machine cylinder to form a molten mixture. A mold cavity having an initial cavity clearance t ₀ smaller than a mold cavity clearance t ₂ corresponding to the shape position of the final product is injection-filled (injection process).

Immediately after the injection process, the movable mold is retracted to a mold cavity clearance t ₁ smaller than t _{2 and} larger than t ₀ (first stage foaming process). Here, “immediately after the injection process” means that the movable mold is retracted, that is, the mold cavity is opened substantially simultaneously with the completion of filling. The set holding time from the completion of filling to the opening of the mold cavity is preferably less than 0.5 seconds, more preferably less than 0.3 seconds. If the time to open the mold cavity after filling is 0.5 seconds or more, the resin temperature in the mold cavity tends to fall below the foaming suitable temperature when injection filling into the thin cavity as in the present invention. Yes, it tends to be difficult to obtain a high expansion ratio of 2 times or more in a part or the whole of the molded body. Moreover, it is preferable that the mold cavity clearance t ₁ after the movable mold retreats in the first stage foaming step has a relationship of the following equation with t ₀ .

ｔ₁がこの範囲内にある場合は、金型内に射出充填する溶融混合物量がフル充填量の８０％以上９７％以下であっても、内部ボイドが少なく、表面平滑性の優れた高発泡成形体が得られやすい。

When t ₁ is within this range, even if the amount of the molten mixture to be injected and filled in the mold is 80% or more and 97% or less of the full filling amount, there is little internal void and high foaming with excellent surface smoothness. A molded body is easily obtained.

  The mold opening speed in the first stage foaming step is preferably 7 mm / second or more and 100 mm / second or less, and more preferably 10 mm / second or more and 70 mm / second or less. When the mold opening speed is less than 7 mm / second, the bubbles are not uniform, and voids are likely to be generated inside the molded body, so that the rigidity tends to decrease. When the mold opening speed exceeds 100 mm / second, the surface smoothness tends to be inferior because the base resin foam is difficult to follow the mold opening speed.

After the first stage foaming process, the movable mold is stopped, and the mold cavity clearance is smaller than the mold cavity clearance t ₂ corresponding to the shape position of the final product and larger than the initial cavity clearance t ₀ at the time of injection filling. After holding at t ₁ for a predetermined set time, the movable mold is retracted to the mold cavity clearance t ₂ corresponding to the shape position of the final product (second stage foaming step).

Here, the set time for maintaining the mold cavity clearance t ₁ is preferably 1 second to 20 seconds, and more preferably 2 seconds to 10 seconds. When the set time for maintaining t ₁ is less than 1 second, voids in the molded body are likely to be generated, and when it exceeds 20 seconds, it is difficult to achieve high foaming and the surface smoothness tends to deteriorate.

  Furthermore, the mold opening speed in the second stage foaming step is preferably 0.5 mm / second or more and 20 mm / second or less, more preferably 1 mm / second or more and 10 mm / second or less. When the mold opening speed is less than 1 mm / second, foaming may be poor and high foaming may be difficult. On the other hand, if it exceeds 20 mm / sec, foaming of the base resin may be difficult to follow the mold opening speed, and the surface smoothness may be inferior.

  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 120 minutes, the injection speed is 10 to 300 mm / second, and the injection pressure is 10 to 200 MPa.

  Furthermore, in the present invention, in order to easily form a molded article having a beautiful appearance, the thermoplastic resin melt-kneaded material mixed with the foaming agent in the molding machine has a pressure higher than the pressure at which foaming does not occur at the flow front of the melt-kneaded material. You may employ | adopt what is called a counter pressure method injected in the metal mold | die pressurized with the gas body. In this case, it is necessary to maintain the pressure inside the mold at the time of pressurization. In general, gas leakage from the mold is prevented by inserting an O-ring or the like on the mold dividing surface or sliding portion. It is desirable to have a structure. The gas body for pressurizing the inside of the mold is not particularly limited as long as it can suppress foaming at the flow front of the molten resin by pressurization, but it is an inorganic gas because it is inexpensive and easy to handle. In particular, nitrogen and carbon dioxide are preferable.

The foamed molded article of the present invention thus obtained has a foam layer with an average cell diameter of preferably 500 μm or less, more preferably 300 μm or less, from the viewpoint that it is excellent in lightness, rigidity and easily becomes a beautiful surface, The 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 the surface of at least one side of the foamed layer. Furthermore, the expansion ratio of the foamed molded article of the present invention is preferably 2 to 10 times, more preferably 3 to 6 times. When the magnification is in this range, a foamed molded article that is not significantly reduced in rigidity and excellent in lightness can be easily obtained. The expansion ratio can be obtained from the following equation using the mold cavity clearance t ₀ and the final product bottom surface thickness t _F when the resin is filled in the mold.

さらに本発明の成形体は表層部の非発泡層が薄くなる傾向にある。すなわち、内部の発泡層の厚みをより多く確保することが可能となり、効率よく高発泡倍率化が可能となると考えられる。

Furthermore, in the molded product of the present invention, the non-foamed layer in the surface layer portion tends to be thin. In other words, it is possible to secure a greater thickness of the internal foam layer, and to increase the expansion ratio efficiently.

  The method for producing an injection-foamed molded product in the present invention can produce various foamed molded products, and in particular, the molded product has a box-shaped molded product with a high foaming ratio having a standing wall portion and a bottom surface portion. In the case of manufacturing by the back method, the density of the standing wall portion can be reduced and the portion can be reduced in weight, the weight reduction of the entire molded body can be promoted, and the effects of the present invention are easily exhibited.

  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 following, “%” and “part” are based on weight unless otherwise specified.

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 breakage, it indicates “strain hardenability”; It does not show sex ".
(4) Injection foaming moldability: When 40 shots were continuously molded, the number of unfilled parts or dents (number of defects) 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: Obtained from the following formula using the mold cavity clearance t ₀ and the final product bottom surface thickness t _F when the resin is filled in the mold.

（６）最終製品厚みｔ_F：底面部を厚み方向に切断した断面について、両端部、中央部の３点の平均値とした。
（７）表面平滑性：成形体肉厚測定結果の３点の測定結果の内、最大値と最小値の差で平滑性を評価した。

(6) Final product thickness t _F : For the cross section obtained by cutting the bottom surface portion in the thickness direction, the average value of the three points of both end portions and the central portion was used.
(7) Surface smoothness: Smoothness was evaluated by the difference between the maximum value and the minimum value among the three measurement results of the molded body thickness measurement result.

The difference is within 0.2mm ...
The difference exceeds 0.2mm ... ×
(8) Internal Void: A cross section obtained by cutting the bottom surface of the foamed molded product in the thickness direction was observed, and the presence or absence of voids having a size of 1.5 mm or more in the foamed layer was examined.

No internal voids ...
Something ...
(9) filling factor: calculated by the following equation from the foamed molded the weight W _F and the weight W _S of non-foamed molded article were made based on the Reference Example in a mold clearance corresponding to t ₀ when the foamed molded molding Is done.

（１０）立壁密度：成形体立壁部の中心付近５０ｍｍ四方を切り出した切出片の密度のことを言う。
（１１）軽量化率：参考例に基づき初期のクリアランス３ｍｍの金型で成形した非発泡成形品重量Ｗ₀と各発泡成形体重量Ｗ_Fとから下記式により算出される。

(10) Standing wall density: Refers to the density of a cut piece obtained by cutting a 50 mm square near the center of the molded body standing wall.
(11) weight ratio: is calculated from the non-foamed molded article weight W ₀ was molded in a mold of the initial clearance 3mm based on Reference Examples and the foam molding the weight W _F by the following equation.

（実施例１〜３）
線状ポリプロピレン系樹脂Ａ（プロピレン・エチレン・ブロックコポリマー、メルトフローレート４５ｇ／１０分、メルトテンション１ｃＮ以下）７０％と改質ポリプロピレン系樹脂Ｂ（サンアロマー社製ＰＦ８１４（ホモポリマー、メルトフローレート３ｇ／１０分、メルトテンション１０ｃＮ、歪硬化性を示す））３０％を混合した基材樹脂に、造核材として無機系化学発泡剤マスターバッチ（永和化成社製ポリスレンＥＥ２７５、発泡剤濃度２７％、分解ガス量４０ｍｌ／ｇ）を０．５部加えてドライブレンドし、射出発泡成形用樹脂組成物を得た。また発泡剤としては純度９９％以上の二酸化炭素を使用した。

(Examples 1-3)
Linear polypropylene resin A (propylene / ethylene block copolymer, melt flow rate 45 g / 10 min, melt tension 1 cN or less) 70% and modified polypropylene resin B (PF814 (Homopolymer, melt flow rate 3 g / m) 10 minutes, melt tension 10cN, showing strain hardening))) 30% mixed with base resin, inorganic chemical foaming agent masterbatch (Polyslen EE275 manufactured by Eiwa Kasei Co., Ltd., foaming agent concentration 27%, decomposition) 0.5 parts of a gas amount of 40 ml / g) was added and dry blended to obtain a resin composition for injection foam molding. Carbon dioxide having a purity of 99% or more was used as the foaming agent.

  The injection molding machine has a shut-off nozzle mechanism at the end of the cylinder and is made of Ube Industries Co., Ltd. “MD350S-IIIDP” with a vent type specification (vent vent near the center of the cylinder) that can pressurize the vent with carbon dioxide. The carbon dioxide supply amount to the molten resin was controlled by supplying carbon dioxide at a constant pressure by using “type” and further using “carbon dioxide gas supply device MAC-100” manufactured by Asahi Engineering Co., Ltd.

The resin composition containing the nucleating material is supplied to the molding machine, carbon dioxide is supplied as a foaming agent at a pressure of 1.5 MPa at the molding machine vent, and melt-mixed at a molding machine cylinder temperature of 200 ° C. and a back pressure of 5 MPa. After that, a box-shaped cavity having a φ2.0 pin gate and composed of a fixed mold and a movable mold capable of moving forward and backward, having a vertical shape of 330 mm × width of 230 mm × height of 100 mm (standing wall portion: tilting 10 degrees, A mold having a clearance of 3 mm and a bottom surface portion: clearance t ₀ ), set at 40 ° C. and held at 2.0 MPa with carbon dioxide, was injected at an injection speed of 100 mm / sec. The gas inside was evacuated, and injection filling was performed so that the amount of molten resin shown in Table 1 was filled in the cavity (injection process). Immediately after the completion of injection filling (set holding time 0 second), the movable mold was retracted to the mold cavity clearance t ₁ at the bottom to foam the resin in the cavity (first stage foaming step).

Next, after holding for a predetermined time in the state of the clearance t _{1 on} the bottom surface of the mold, the movable mold is further retracted to the mold cavity clearance t ₂ on the bottom surface corresponding to the shape position of the final product, and foaming is performed again. (Second stage foaming step). After completion of foaming, the foamed molded product was taken out after cooling for 60 seconds.

  Table 1 shows the clearance of the bottom surface of each mold, the retraction speed of the movable mold, and the set holding time in each process. Table 2 shows the injection foaming moldability, the magnification of the obtained foamed molded product, the weight reduction rate, and the standing wall density.

The box-shaped foamed molding obtained by such a molding method has almost no surface irregularities and is excellent in surface smoothness, has a foaming ratio in the range of 3 to 4 times, and has a high foaming ratio. is there. The density of the standing wall portion was low, it had a non-foamed layer (skin layer) of 200 to 300 μm, and there was almost no void inside the molded body. As a result, despite the box-shaped foam molded article, a high weight reduction rate of 35.5 to 37.7% was achieved with respect to the non-foamed molded article having a molded product thickness of 3.0 mm.
Example 4
A foamed molded article was obtained in the same manner as in Example 3 except that the filling amount of the molten resin shown in Table 1 was injected and filled so as not to fill the cavity. The results are shown in Table 2.

The resulting box-shaped foamed molded article has a foaming ratio of 3 times, and there are locally inferior surface properties at a level where there is no problem in practical use. A high weight reduction rate of 40.1% was achieved.
(Reference example)
In the examples, a modified polypropylene resin and a foaming agent were not used, and after completion of injection filling, a 50 MPa pressure-holding step was performed for 10 seconds, and then cooled for 45 seconds to take out a non-foamed molded article. At this time, by changing the initial clearance of the bottom surface of the mold, molded bodies having different thicknesses of the bottom surface were obtained.
(Comparative Example 1)
A foamed molded product was obtained in the same manner as in Example 2 except that the filling rate was increased. The results are shown in Table 2. The obtained box-shaped foamed molded article had a weight reduction rate of 26.6%, even with a clearance t _{0 of} 1.5 mm, and the standing wall portion had a high density.
(Comparative Example 2)
A foamed molded article was obtained in the same manner as in Example 3 except that the filling rate was increased. The results are shown in Table 2. The obtained box-shaped foamed molded article had a weight reduction rate of 33.9%, even with a clearance t _{0 of} 1.2 mm, and the density of the standing wall portion was high.
(Comparative Example 3)
A foam molded article was obtained in the same manner as in Example 3 except that the clearance t ₀ was 0.7 mm and the holding times at t ₁ , t ₂ and t ₁ were as shown in Table 1. The results are shown in Table 2. The resulting box-shaped foam molded article achieved a high weight reduction rate of 46.9%, but 20 shots out of 40 shots were short shots, voids were present, and surface smoothness was poor.

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, an initial cavity clearance t ₀ composed of a fixed mold and a movable mold that can be moved forward and backward is set to 0. A thermoplastic resin characterized by injecting and filling a molten mixture having a full filling amount of 80% by weight to 97% by weight into a mold having a size of 8 mm or more and less than 3.0 mm, and then moving the movable mold backward to foam. A method for producing a foam molded article.   2. The method for producing a thermoplastic resin foam molded article according to claim 1, wherein the mold is filled with the molten mixture when injection filling is completed.   The thermoplastic resin foam according to claim 1, wherein the mold is not filled with the molten mixture at the completion of injection filling, and the foamed molten mixture is filled in the mold after the foaming step. Manufacturing method of a molded object.   The said thermoplastic resin is a polypropylene resin, The manufacturing method of the thermoplastic resin foam molding as described in any one of Claims 1-3 characterized by the above-mentioned. An injection step of injecting and filling the molten mixture into the mold cavity having the initial cavity clearance t ₀ , and then a cavity clearance t ₁ smaller than the cavity clearance t ₂ corresponding to the shape position of the final product and larger than t _0. A first-stage foaming step of retracting the movable mold to the next, and then a second-stage foaming process of retracting the movable mold to the cavity clearance t ₂ corresponding to the shape position of the final product after holding the clearance of t ₁ for a predetermined time. The method for producing a thermoplastic resin foam-molded article according to any one of claims 1 to 4, wherein:   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 first stage foaming step is 7 mm / second or more and 100 mm / second or less.   The method for producing a thermoplastic resin foam molded article according to any one of claims 1 to 6, wherein a mold opening speed in the second stage foaming step is 0.5 mm / second or more and 20 mm / second or less. . Method for producing an injection molded foam product according to any one of claims 1 to 7, the relationship of the t _0, t ₁ is equal to or represented by the following formula.

Production of an injection molded foam product according to any one of claims 1 to 8, wherein the predetermined time is less than 20 seconds 1 second to hold the mold cavity clearance t ₁ in the second step expansion process Method.   It has a foaming layer having an average cell diameter of 500 μm or less and a non-foamed layer having a thickness of 10 μm or more and 1000 μm or less formed on at least one surface of the foam layer, and a foaming ratio of 2 to 10 times A thermoplastic resin foam molded article produced by the production method according to any one of claims 1 to 9.   The thermoplastic resin foam molded article according to claim 10, wherein the foam molded article has a box shape including a bottom surface and a standing wall portion.