JPH0760778A - Injection molding method for molded product of hollow structural - Google Patents

Abstract

PURPOSE:To provide an injection-molded product wherein neithr molten resin flows preferentially within a thick part and nonuniform part nor the product possesses badness such as a weld line or an air trap or a flow mark and an external appearance of the surface is excellent. CONSTITUTION:In an injection molding method of a hollow body wherein only a thick part is made into hollow structure by casting insufficiently molten resin within a mold cavity of a molded product having partly a thick part and then a pressurized fluid is cast, a mold device wherein a movable mold part 2 forming the thick part is constituted freely contactable to or separatable from a cavity 1 forming a base and at the time of uncasting of the molten resin, the movable mold part 2 forming the thick part is arranged so as to join the movable mold 2 to the cavity 1 is used. Furthermore, in an injection molding method of a hollow molded product, synthetic resin is melted and cast into the mold device, then the movable mold part 2 is slid at the last stage of the casting or after the casting, the thick part is formed and then pressurized fluid is cast into the thick part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for molding a hollow mold by injecting a molten resin and a pressurized fluid such as nitrogen gas into a mold cavity. In particular, it has one or more reinforcing ribs or thickening portions or uneven thickness portions such as molded bodies used as automobile parts such as door handles, rear spoilers, radiator grilles, and home electric appliances parts such as housings of personal computers and televisions. It relates to an injection molded body.

[0002]

2. Description of the Related Art A method called gas injection molding or gas assist molding is known as one of methods for manufacturing hollow body injection molded products. These are a method of filling the mold cavity by injecting a molten resin in an insufficient amount to fill the mold cavity and then injecting a pressurized fluid such as nitrogen gas into the mold cavity. JP-A-48-41264, JP-A-50-71756, JP-A-60-24913, JP-A-63-268611, and JP-A-1-63122.
It is described in detail in Japanese Patent Publication No.

Further, as an application of the above hollow body injection-molded product, the strength of the structure is improved by not forming the entire molded product into a hollow structure but by forming only the thick wall portion of the uneven thickness structure product into a hollow structure. In addition, there is a method for producing an injection-molded product, which is characterized in that it is possible to avoid sink marks and warpage caused by heat shrinkage during cooling and solidification concentrating on the thick portion, and JP-A-3-207617.
It is known from the official gazette. In order to make only the thick wall portion in the uneven thickness structure a hollow structure, it is necessary to design the thick wall portion to be continuous. I'm calling. This method of manufacturing an injection-molded product is capable of adopting a bold thick wall and uneven thickness structure and at the same time satisfying the external appearance, and is a useful manufacturing method for product design. That is, this method takes a partially continuous thick structure in order to improve the strength of the molded product. As an example, in order to improve the flexural rigidity of the flat plate, only the outer peripheral portion has a thick structure as shown in FIG.
Consider the case of a frame-shaped molded product as shown in. Although FIG. 4 is a top view and FIG. 5 is a cross-sectional view, it can be seen from the cross-section that the hollow portion 7 is formed. The hollow portion 7 and the pressed portion 6 provide a reinforcing effect as a rib, and the hollow portion 7 reduces heat shrinkage to remove sink marks and warpage of the surface.

[0004]

However, when the hollow portion is formed, the gas body flows only through the thick wall portion and the uneven wall portion, so that the ideal hollow portion as designed can be obtained. Even when injecting, the molten resin easily flows in the thick wall portion and the uneven wall portion because the flow resistance is small. Therefore, when the molded product of FIG. 1 is molded by normal injection molding, the flow of the molten resin flows through the thick portion 6 on the outer periphery first, and the weld line 8 that is the association portion of the molten resin is formed. It will be formed as shown in the drawing of FIG. When forming a reinforcing structure by providing a gas channel (that is, a thick portion) also in other molded products, various shapes of weld lines are formed by flowing the molten resin first through the thick portion.

This weld line is an important defect in terms of appearance design, and a molding method that does not form such a weld line is desired. In addition, in FIG. 4, the portion where the weld line is formed has no escape path for the trapped air, and the phenomenon of an air trap occurs, resulting in poor appearance such as gas burning of the resin due to adiabatic compression. Further, since the resin flow is not uniform, there is a problem that flow marks are generated. At present, it has been difficult to set the gate position in order to reduce these problems.

[0006]

In view of such a situation, the inventors of the present invention arrived at the present invention as a result of intensive studies to improve the above problems in the injection molding method for hollow structures.

That is, the gist of the present invention is to inject a molten resin insufficiently into a mold cavity of a molded article having a thick portion in a part thereof, and then to inject a pressurized fluid so that only the thick portion is formed. In a hollow body injection molding method having a hollow structure, a movable mold portion 2 forming a thick portion is configured to come in contact with and separate from a cavity 1 forming a substrate, and when the molten resin is not injected, the thick portion is formed. Using a mold device in which the movable mold part 2 that forms the mold is arranged so as to be bonded to the cavity 1, the synthetic resin is melt-injected into the mold device, and the movable mold part 2 is injected at the end of injection or after the injection. Is slid to form a thick portion, and then a pressurized fluid is injected, which is an injection molding method for a hollow structure molded article.

[0008]

EXAMPLES Hereinafter, description will be given based on examples embodying the present invention. The injection molding method according to the present invention will be described with reference to FIGS. The product of FIG. 1 was manufactured by injection molding. The size of the molded product is 100 mm x 100 mm with a basic wall thickness of 3 m
m, the thick portion of the outer circumference has a width of 8 mm and a thickness of 8 mm. The molding material is ABS (Diapet ABS manufactured by Mitsubishi Rayon Co., Ltd.)
1001) was used. 2 shows the mold closed state before the injection of the molten resin, and there is no thick portion cavity corresponding to the thick portion 6 shown in FIG. The mold structure in the present invention is
As shown in FIG. 2, it comprises a fixed mold 3 having a sprue part 5 and a movable mold 4 which faces the fixed mold 3, and is characterized by having a movable mold part 2 for controlling a mold cavity of a thick part 6. . In FIG. 2, when the molten resin is injected in a state where the movable mold part 2 and the substrate cavity 1 are joined, the mold cavity does not have an uneven thickness structure, so that the problem described in the problem to be solved by the invention is The weld line 8 shown in FIG. 4 does not occur. When the movable mold part 2 is hydraulically slid at an arbitrary time when about 70% or more of the molten resin is injected with the mold kept in the state of FIG. 2, a cavity of a thick part appears as shown in FIG. . The mechanism for sliding may use drive by resin pressure using a spring. Further, by continuing the injection of the molten resin, the molten resin is also injected into the thick portion 6. By injecting the molten resin in an amount insufficient to form a molded product, and then injecting a gas body, the gas body is injected only into the thick portion 3 as shown in FIG. Can be formed.

[0009]

The method of injection molding a hollow structure molded article according to the present invention has the following effects. The molten resin does not flow preferentially through the thick and uneven parts, and defects such as weld lines, air traps and flow marks do not occur in the molded product, and an injection molded product with a hollow structure with an excellent surface appearance is provided. It can be molded.

[Brief description of drawings]

FIG. 1 is a perspective view of a molded product obtained in the present invention.

FIG. 2 is a cross-sectional view showing a state of an injection molding die before inflow of resin in the present invention.

FIG. 3 is a sectional view showing a resin filled state after the molten resin injection and the gas body injection in the present invention.

FIG. 4 is a top view of a hollow structure molded product according to a conventional method.

FIG. 5 is a cross-sectional view of a hollow structure molded product.

[Explanation of symbols]

 1 Substrate Cavity 2 Movable Mold Part 3 Fixed Side Mold 4 Movable Side Mold 5 Sprue 6 Thick Part 7 Hollow Part 8 Weld Line 9 Gate Part 10 Resin

Claims (1)

[Claims] 1. A hollow structure in which a molten resin is insufficiently injected into a mold cavity of a molded article having a thick portion in a part thereof, and then a pressurized fluid is injected to form a hollow structure only in the thick portion. In the body injection molding method, a movable mold part 2 forming a thick part is configured to be capable of coming into contact with and separating from a cavity 1 forming a substrate, and a movable mold forming a thick part when a molten resin is not injected. Using a mold device in which the mold part 2 is arranged so as to be bonded to the cavity 1, a synthetic resin is melt-injected into the mold device, and the movable mold part 2 is slid at the end of injection or after the injection. An injection molding method for a hollow structure molded article, which comprises forming a thick portion and then injecting a pressurized fluid.