JP6429014B2 - Blow molding method - Google Patents

Description

  The present invention relates to a blow molding method, and more particularly, to wrinkle countermeasures when foam blow molding a hollow molded product having a relatively small thickness.

  For example, various air-conditioning ducts that are mounted in an instrument panel of an automobile are known as foam blow molded products. For these air conditioning ducts, foam ducts formed by molding a foamed resin material are widely used. The foam duct is lightweight and can be easily manufactured by, for example, adding a foaming agent to a resin material such as polyolefin resin, melt-kneading, and blow-molding the foam parison extruded from the die head (die slit) of the extruder. Can do.

  Polyolefin resins are widely used as resin materials used for foam blow molded products, and among them, polypropylene resins are common. In recent years, replacement with a polyethylene-based resin has been studied for the purpose of making the material structure cheaper.

  In blow molding, as described above, the molten resin is extruded from the die slit as a cylindrical parison and blow-molded in a mold. At this time, the thickness of the parison is adjusted by adjusting the distance (slit width) between the die slits so that the thickness of the hollow molded product to be molded becomes the design value.

  However, if the size of the hollow molded product is large or the wall thickness is large, the weight per shot becomes large, and there is a concern about drawdown. Drawdown is a phenomenon in which the parison is stretched by its own weight and the thickness of the upper portion decreases. Due to the drawdown, a difference in thickness occurs in the product (hollow molded product).

  In order to avoid the drawdown, for example, it is conceivable to change the composition of the resin to be used. However, this method is not preferable because the resin to be used is restricted. Therefore, a method of correcting the thickness of the parison extruded from the die slit has also been proposed (see, for example, Patent Document 1).

  Specifically, in Patent Document 1, an accumulator measuring stroke S is set based on a required resin weight W calculated from a parison target thickness t and a target length L, and the target thickness t is set to a swell ratio R. The die gap set value Dt obtained by dividing by the above is corrected for the thickness reduction amount caused by the draw-down phenomenon caused by the stroke length x, and the die gap final set value Df is set to form a desired parison. A blow molding method is disclosed.

JP 7-290563 A

  By the way, when foam-molding a relatively thin hollow molded product, the extrusion start portion (lower portion) of the parison may be undulated or wrinkled in a curtain shape. It is a factor that degrades quality. If the method described in Patent Document 1 described above is employed, the influence of drawdown can be minimized, but this problem cannot be solved.

  The present invention has been proposed in view of such a conventional situation, and even when blow molding a hollow molded product having a small thickness, generation of wrinkles of the parison can be suppressed, and high quality hollow It is an object of the present invention to provide a blow molding method capable of producing a molded product.

In order to achieve the above object, the blow molding method of the present invention sets the interval between the die slits in the die head according to the target thickness of the hollow molded product to be molded, and the molten resin in the accumulator is removed from the die slit. It is a blow molding method in which a parison is formed by extruding into a cylindrical shape, and the parison is molded in a mold, and the interval between the die slits is set larger than the value set according to the target thickness at the start of extrusion, After starting the extrusion of the parison P at such a die slit interval, the die slit interval is gradually reduced, and the die slit interval is set in accordance with a target thickness that is narrower than the die slit interval at the start of extrusion. It is characterized by having an interval that matches .

  When blow-molding a hollow molded product having a small thickness, it is necessary to narrow the interval between the die slits (slit width) and the thickness of the parison. However, if the die slit width is reduced in accordance with the required parison thickness from the beginning, there arises a problem that the parison undulates in a curtain shape or wrinkles. By making the interval between the die slits larger than the value set in accordance with the target wall thickness at the start of extrusion and increasing the thickness of the lowermost parison, the occurrence of waviness and wrinkles of the parison is suppressed.

  ADVANTAGE OF THE INVENTION According to this invention, it is possible to provide the blow molding method which can manufacture a hollow molded article with high quality, without a wrinkle generating in a parison.

It is a schematic perspective view which shows an example of a foam duct. It is a schematic sectional drawing which shows typically the aspect at the time of blow-molding a duct. It is a figure which shows the time-dependent change of the die-slit space | interval at the time of parison extrusion.

  Hereinafter, an embodiment of a blow molding method to which the present invention is applied will be described in detail with reference to the drawings, taking as an example blow molding of a foam duct.

  The foam duct 10 that is a foam blow-molded product is configured such that air-conditioning air supplied from an air conditioner unit (not shown) is circulated through an internal flow path and is ventilated to a desired portion. In addition, as a shape of the foam duct 10, it is not limited to what is shown in FIG. 1, It can be set as arbitrary shapes according to a use, an installation place, etc.

  The foam duct 10 of the present embodiment is obtained by blow molding by sandwiching a foam parison formed by extruding a foam resin from a die of an extruder with a mold. In addition, the duct immediately after blow molding is in a state where both ends are closed, and both ends are cut into an open shape by trimming after blow molding.

  The foam duct 10 of the present embodiment is formed of a hollow foamed resin molded product whose tube wall is constituted by a foam layer. By adopting a configuration in which the foamed layer has a closed cell structure, it is possible to obtain a duct that is lightweight and excellent in heat insulation. The closed cell structure is a structure having a plurality of independent cell cells, and means a cell having at least a closed cell ratio of 70% or more. With such a configuration, even when cooling air is circulated in the foam duct 10, the possibility of dew condensation can be almost eliminated.

  The foam duct 10 of this embodiment basically uses a polypropylene resin as the foam resin material. Polypropylene-based resins are characterized by being easily optimized in terms of physical properties and the like and having good foam moldability.

  Examples of the polypropylene resin to be used include a propylene homopolymer, a random or block copolymer of propylene and another α-olefin, and the like. Other α-olefins copolymerized with propylene include ethylene, butene, pentene, hexene, octene, methylpentene, and the like. The amount of α-olefin copolymerized with propylene is arbitrary, but is preferably about 0.1 to 20% by mass, for example, in order to maintain the excellent physical properties of polypropylene.

  The polypropylene resin preferably has a long chain branched structure. Polypropylene resin has the disadvantage that the melt tension at the time of melting is low and the molding processability is inferior in foam molding, but by introducing a long-chain branched structure, the melting characteristics can be improved. Can be eliminated.

  Furthermore, in the foam duct 10, in addition to the polypropylene resin, a polyethylene elastomer may be used in combination to improve the impact resistance and to achieve both foam moldability and impact resistance.

  Polyethylene elastomer is a fine dispersion of olefin rubber in a matrix of polyethylene resin. It has excellent compatibility with polypropylene resin, and gives rubber elasticity to resin material to improve impact resistance. It has the feature of being able to.

  The proportion of the polyethylene elastomer in the resin material is preferably 5% by mass or more for the purpose of improving impact resistance. If the ratio of the polyethylene-based elastomer is less than 5% by mass, the impact resistance of the foam duct 10 may be insufficient. The greater the proportion of polyethylene-based elastomer, the more advantageous for impact resistance improvement, but when the proportion of polyethylene-based elastomer is too large, the proportion of polypropylene-based resin is relatively reduced, such as foam moldability, It becomes difficult to maintain the excellent physical properties of the polypropylene resin. From such a viewpoint, the proportion of the polyethylene-based elastomer is preferably 35% by mass or less. That is, the proportion of the polyethylene elastomer is preferably 5 to 35% by mass.

  In order to manufacture the foam duct 10, an additive such as antioxidant is added to the above-described polypropylene-based resin or the like as necessary, and it is subjected to blow molding. In blow molding, foaming is performed using a foaming agent. As the foaming agent, inorganic foaming agents such as air, carbon dioxide gas, nitrogen gas and water, and organic foaming agents such as butane, pentane, hexane, dichloromethane and dichloroethane can be used. Among these, it is preferable to use air, carbon dioxide gas, or nitrogen gas as the foaming agent. By using these, mixing of organic substances can be prevented, and deterioration of durability and the like can be suppressed.

  Moreover, it is preferable to use a supercritical fluid as the foaming method. That is, it is preferable to make carbon dioxide gas or nitrogen gas into a supercritical state and foam the resin material. By using a supercritical fluid, foaming can be performed uniformly and reliably. The foamed duct 10 is formed by blow molding the foamed resin material by a known method. FIG. 2 is a view showing an aspect when the foam duct 10 is blow-molded.

  In blow molding, first, a resin material used for molding is kneaded in an extruder to produce a base resin. In the case of molding using only the virgin resin, a modifier is added to the virgin resin of the above-described resin material as necessary and kneaded to prepare a base resin. When the recovered resin material is used, a predetermined ratio of virgin resin is added to the pulverized recovered resin material and kneaded to prepare a base resin.

  After adding a foaming agent to such a base resin and mixing in an extruder, it is stored in an in-die accumulator (not shown), and then a ring-shaped piston (not shown) is stored after a predetermined amount of resin is stored. Press down in the direction perpendicular to the horizontal direction (vertical direction). 2 is extruded between the divided molds 31 and 32 constituting the mold clamping device 30 as a cylindrical parison P, for example, at an extrusion speed of 700 kg / hour or more from the die slit of the annular die 21 shown in FIG. Thereafter, the divided molds 31 and 32 are clamped to sandwich the parison P, and air is blown into the parison P in a pressure range of 0.05 to 0.15 MPa to form the foam duct 10.

  After molding, a portion other than the finished product in the resin material that has been cooled and solidified is pulverized into a recovered resin material, and the same blow molding is performed again using a mixed resin obtained by adding a predetermined amount of virgin resin to the recovered resin material. By repeating such a manufacturing cycle, the foam duct 10 can be mass-produced.

  In the blow molding described above, for example, when forming a hollow molded product (foaming duct 10) having a foaming ratio of 2.8 times and a thickness of 3 mm or less, drawdown, parison undulation, and generation of wrinkles become problems. Therefore, in the blow molding method of this embodiment, these phenomena are avoided by appropriately controlling the interval between the die slits.

  FIG. 3 is a diagram showing a change with time of the die slit interval when the parison P is extruded. In the blow molding method of this embodiment, the die slit interval T is gradually increased so that the thickness of the parison P gradually increases in consideration of drawdown. That is, in FIG. 3, as indicated by the line L1, the die slit interval T is gradually increased. As a result, the phenomenon that the upper parison P is pulled downward by its weight to become thin is canceled out.

  Accordingly, at the start of the extrusion of the parison P, the die slit interval is set to the interval represented by the point A that extends the line L1 to the extrusion start point. When the product (foaming duct 10) is molded, the parison is wavy and wrinkled. Therefore, in the present embodiment, the parison starts to be extruded at a die slit interval indicated by a point B wider than the point A.

  After the extrusion of the parison P is started at such a die slit interval, the die slit interval is reduced along the line L2, and after matching the previous line L1, the die slit is gradually enlarged along the line L1. Controlling the die slit interval T with such a trajectory suppresses the occurrence of undulations and wrinkles at the bottom of the parison.

  Note that, immediately before the end of the extrusion of the parison P, as indicated by a line L3 in FIG. Although the uppermost part of the parison P may be ruptured due to various factors, the rupture can be prevented by increasing the die slit interval and increasing the thickness.

  As mentioned above, although embodiment which applied this invention has been described, it cannot be overemphasized that this invention is not what is limited to the above-mentioned embodiment, In the range which does not deviate from the summary of this invention, a various change can be added. Is possible.

  Hereinafter, specific examples of the present invention will be described based on experimental results.

Using a polypropylene resin and a polyethylene elastomer as a raw material for producing the foam duct, the foam duct was blow-molded with the following composition. The raw materials used are as follows.
-Polypropylene resin A: manufactured by Boreales, trade name WB140 (polypropylene having a long chain branched structure)
-Polypropylene resin B: Product name Novatec PP / BC4BSW manufactured by Nippon Polypro Co., Ltd.
-Polyethylene elastomer: Product name DF605, manufactured by Mitsui Chemicals, Inc.
Formulation: Polypropylene resin A: Polypropylene resin B: Polyethylene elastomer = 70: 23: 7 (mass ratio)

  The types of gas were nitrogen gas (samples 1 to 5) and carbon dioxide gas (samples 6 to 8), and blow molding was performed under various conditions to produce a foam duct. Samples 5 and 8 are examples in which the present invention is applied and the parison extrusion is started at a die slit interval indicated by a point B in FIG. The other corresponds to the conventional method, and is an example in which the extrusion of the parison is started at the die slit interval indicated by the point A in FIG.

Evaluation In the production of the foam duct, it was evaluated whether the parison was wavy or wrinkled. The state of the parison was confirmed by visual inspection, and when the wavy or wrinkle was recognized in the parison, it was rated as “X”, when it was hardly recognized, and when it was not recognized at all.

  The results are shown in Table 1. As is apparent from Table 1, in Sample 5 and Sample 8 to which the present invention is applied, no wavy or wrinkle is generated in the parison, and a good foam duct is formed. On the other hand, the foamed ducts (samples 1 to 4, 6, and 7) produced by the conventional method were wrinkled in the parison and inferior in product quality.

10 Foam duct 21 Annular die 30 Clamping device 31, 32 Split mold P Parison

Claims (4)

The interval between the die slits in the die head is set according to the target thickness of the hollow molded product to be molded, and the molten resin in the accumulator is extruded from the die slit into a cylindrical shape to form the parison, and the parison is placed in the mold. A blow molding method for molding,
At the start of extrusion, the interval between the die slits is made larger than the value set according to the target wall thickness, and after starting the extrusion of the parison P at such a die slit interval, the die slit interval is gradually reduced, The blow molding method is characterized in that the gap is made to coincide with a value set in accordance with a target thickness that is narrower than the gap between the die slits at the start of extrusion .   The blow molding method according to claim 1, wherein a value set according to the target thickness is corrected in consideration of a thickness reduction due to drawdown.   3. The blow molding method according to claim 1, wherein the blow molding method is foam blow molding, and a thickness of a hollow molded product to be molded is 3 mm or less.   4. The blow molding method according to claim 1, wherein the resin is mainly composed of polypropylene.

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