JPH08174635A - Manufacture of hollow molded article - Google Patents

Abstract

PURPOSE: To provide a manufacturing method of a hollow molded article, by which the inner surface smoothness of a thermoplastic resin hollow molded article is improved and the molded article having a small residual stress is obtained. CONSTITUTION: In a manufacturing method, in which a resin is melted and kneaded with an extruder so as to be extruded from a mold being annexed to the extruder and having a mandrel and a land in order to manufacture a hollow molded article, a forming device, in which the mandrel 4 projects outward beyond the land 3 and the crosssectional area of the mandrel 4 at the portion projected beyond the land 3 gradually decreases towards the extrusion direction of the resin, is used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a pipe or a hollow molded article having a modified cross section using a thermoplastic resin.

[0002]

2. Description of the Related Art Conventionally, in order to extrude a hollow molded article using a thermoplastic resin, a mandrel is provided inside,
A mold having a land at the resin outlet is used. For example, in JP-A-5-131526, a sizing former is provided on the resin outlet side of a mold, and a mandrel of the same length is provided in the sizing former.
A method for manufacturing a resin pipe using a molding die device in which a mandrel is gradually reduced toward the tip is described. The manufacturing method described in the above publication improves the smoothness of the inner surface of the resin pipe.

[0003]

The sizing former described in the above publication is for regulating the outer shape of the molded product, and the stress of the molten resin itself released by the ballast effect is the cooling function of the sizing former. By this, it is frozen and becomes a residual stress, which causes deformation or cracking of the molded product. Also, it is difficult to deal with troubles that may occur during molding startup or during molding.

The present invention solves the above conventional problems, improves the inner surface smoothness of a hollow molded product made of a thermoplastic resin, and provides a method for producing a hollow molded product which can be obtained as a molded product with less residual stress. The purpose is to provide.

[0005]

Means for Solving the Problems The method for producing a hollow molded article according to the present invention comprises melt-kneading a resin in an extruder and extruding the molten resin from a die attached to the extruder and having a mandrel and a land. In a manufacturing method for manufacturing a molded product, a mandrel is projected outward from a land, and a molding device in which the cross-sectional area of the mandrel of the portion projected from the land is gradually reduced in the resin extrusion direction. It is characterized by being used.

The cause of the roughness of the inner surface of the extruded product is that when the molten resin is in contact with the inner surface of the mold, it flows at a low velocity of 0 to 0, and the molten resin exits the mold and is subjected to a tensile action. It is considered that when the speed suddenly increases, tensile strain occurs in the inner layer and the surface of the resin in the extrusion direction, resulting in uneven resin pressure.

When the mandrel is projected outward from the land, the pressure of the molten resin is released on the outer surface side. Therefore, the friction between the molten resin and the mandrel is reduced and the mandrel surface can be moved smoothly, so that the inner surface of the molded product is less likely to be roughened. Further, since the internal strain stress is released to the outer surface side in the molten state of the resin, cooling shaping can be performed without leaving the strain stress.

Further, since the cross-sectional area of the mandrel projecting outward from the land is gradually reduced in the resin extruding direction, the friction between the molded product and the mandrel is further reduced, and Inner surface smoothness is improved.

The length of the mandrel projecting outward from the land is preferably ¼ or more of the length of the shortest side forming the hollow portion. If the length is less than this, the internal pressure of the molten resin cannot be sufficiently released to the outer surface side by the time the molten resin reaches the tip of the mandrel, residual stress remains inside the molded product, and causes such as distortion. Becomes

In order to make the cross-sectional area of the mandrel projecting outward from the land gradually decrease in the resin extruding direction, the mandrel is provided with a taper. The angle of this taper is preferably 0.5 degrees or more and 5 degrees or less. If it is less than 0.5 degree, the friction between the molten resin and the mandrel due to the reduction of the cross-sectional area of the mandrel cannot be reduced.

If it exceeds 5 degrees, the friction between the molten resin and the mandrel becomes too small and the inner surface is not sufficiently sized, and the inner surface of the molten resin receives a tensile force, so that the hollow molded article obtained on the contrary is The surface condition and dimensional accuracy deteriorate.

The resin used as a molded article in the present invention is a thermoplastic resin which has been conventionally used for extrusion molding. For example, the crystalline resin is a polyolefin resin represented by polyethylene, polypropylene or polybutene, or polytetrafluoroethylene. , Fluorocarbon resins such as polyvinyl difluorite, and crystalline engineering plastics such as polyethylene terephthalate, nylon, polyphenylene sulfide, and polyether ether ketone. As the non-crystalline resin, acrylic resin such as polymethylmethacrylate, polyvinyl chloride, polyvinylidene, polycarbonate, polyvinyl alcohol,
Examples thereof include polyether sulfide, polyether imide, and polyimide.

Further, the resin may contain additives such as a filler, a flame retardant, an ultraviolet absorber, a coloring material and a plasticizer. Examples of such additives include the following. Filler: one that is effective in improving elastic modulus, lowering cost, and lowering specific gravity (for example, glass chop strands, calcium carbonate powder, balloon-shaped blast furnace ash, etc.) Flame retardant: various halogen-based, non-halogen-based, Inorganic flame retardants, etc. UV absorber: hindered amine type, benzophenone type, etc. Coloring materials: organic materials, organic color formers, inorganic pigments, etc. Plasticizer: phthalic acid ester, aliphatic dibasic acid ester type, glycol ester type, etc.

[0014]

Since the mandrel is projected outward from the land, the pressure of the molten resin is released on the outer surface side by the balance effect. Therefore, the friction between the molten resin and the mandrel is reduced and the mandrel surface can be moved smoothly, so that the inner surface of the molded product is less likely to be roughened. Further, since the internal stress strain is released to the outer surface side in the molten state of the resin, it is possible to perform cooling shaping without leaving the stress strain. As a result, it is possible to prevent the molded product from being distorted or cracked.

Further, since the cross-sectional area of the mandrel of the portion protruding from the land is gradually reduced in the resin extruding direction, the friction between the molded article and the mandrel is further reduced, and the inner surface smoothness of the molded article is reduced. Is improved.

[0016]

EXAMPLE An example of the method for producing a hollow molded article of the present invention will be described with reference to the drawings. (Examples 1 to 7) FIG. 1 is a sectional view of an apparatus for manufacturing a hollow molded article used in Examples. A die 2 is attached to the extruder 1, a land 3 is provided at a resin outlet of the die 2, a mandrel 4 supported by a bridge 21 is provided in the die 2, and an end portion 41 of the mandrel 4 is provided. Are projected to the outside of the land 3. Further, the mandrel 4 is provided with a taper angle θ by gradually reducing the cross-sectional area from the end of the land 3. Reference numeral 11 is a screw in the extruder 1. For cooling shaping, cooling was performed only from the outer surface by a vacuum spraying sizing device (not shown) used for ordinary pipe forming. Medium density polyethylene (Mitsui Petrochemical Co., Ltd., trade name: 2015M) is used as resin, and a resin pipe (outer diameter 114 mm, wall thickness 11 m) is manufactured by the manufacturing apparatus shown in FIG.
m) was molded under the conditions shown in Table 1.

(Comparative Example 1) As shown in FIG. 2, using a mold 2 provided with a land 3 similar to that of the embodiment and having a mandrel 40 whose end portion is aligned with the end portion of the land 3, this is carried out. Using the same resin as that used in the example, a pipe similar to that of the example was molded under the conditions shown in Table 1. The reference numerals used in FIG. 2 are the same as those in FIG.

Comparative Example 2 A mold 2 provided with a sizing former 5 shown in FIG. 3 described in JP-A-5-131526 and the same resin as that used in the example are used. A pipe similar to that of the example was molded under the conditions shown. still,
In this device, the end of the mandrel 4 is tapered toward the front end to form an angle θ. A cooling device 51 is provided inside the sizing former 5.

Performance evaluation (inner surface roughness) Using a surface roughness meter (manufactured by Tokyo Seimitsu Co., Ltd.),
Maximum height Rm of surface roughness specified in JIS B0601
The ax (μm) was measured. (Circumferential residual stress) A pipe was cut into a length of 1 inch and a part of the circumference was cut in a direction parallel to the axis, and the amount of deformation of the cut end was measured, and the residual stress in the circumferential direction σmax was calculated by the following formula. expressed.

[0020]

Embedded image Here, Ε (t) is the Young's modulus h after the time t has passed after the pipe is cut in the axial direction in a part of the circumferential direction, and the wall thickness D ₁ is the axial direction in a part of the circumferential direction of the pipe. The inner diameter D ₂ (t) before cutting represents the Poisson's ratio, and the inner diameter ν of the pipe after the time t has elapsed after cutting the pipe. Table 1 shows the above results.

[0021]

[Table 1]

As is clear from Table 1, in each of Examples 1 to 7, both the roughness of the inner surface and the residual stress in the circumferential direction are small, and the distortion of the molded product is small, but Comparative Example 1
It can be seen that the product of Comparative Example 2 has a large inner surface roughness, and the product of Comparative Example 2 has a large residual stress and a large distortion of the molded product.

[0023]

EFFECTS OF THE INVENTION The method for producing a hollow molded product of the present invention has the above-mentioned constitution, and improves the inner surface smoothness of the hollow molded product made of a thermoplastic resin and has a small residual stress and a small strain or crack. Can be obtained.

[0024]

[Brief description of drawings]

FIG. 1 is a sectional view of an apparatus showing an embodiment of a method for producing a hollow molded article of the present invention.

FIG. 2 is a sectional view of an apparatus used in a conventional manufacturing method.

FIG. 3 is a cross-sectional view of another device used in the conventional manufacturing method.

[Explanation of symbols]

 1: Extruder 2: Mold 3: Land 4: Mandrel 41: End of mandrel

Claims (1)

[Claims] 1. A manufacturing method for producing a hollow molded article by extruding a molten resin from a die having a mandrel and a land attached to the extruder by melting and kneading the resin with the extruder, wherein the mandrel is more than the land. Is projected outward, and
A method for manufacturing a hollow molded article, which comprises using a molding apparatus in which a cross-sectional area of a mandrel of a portion protruding from a land is gradually reduced in a resin extruding direction.