METHOD OF MANUFACTURING PLASTIC MOLDINGS AND CORE USED IN SAME

Abstract: The present invention relates to a process for producing a plastic molded article (1) having at least partly a hollow portion, and molding is conducted using a core (2) composed of a water-soluble molded article which has been formed to have the shape of the hollow portion. A plastic material is injected to cover part of the surface of the core (2), and subsequently injected to cover the remainder of the surface of the core (2), followed by finally removing the core (2) with water.

Description: Field of the Invention The present invention relates to a process for producing a plastic molded article having at least partly a hollow portion according to the precharacterising part of claim 1. Background of the Invention When a plastic branch tube 1 as shown in Fig. 5 is integrally molded by conventional injection molding, slide molds 4 which are provided to an elevation mold 3 so that they can be moved back and forth are used. That is, the hollow portion of the branch tube 1 is formed by getting the tips of the three slide molds 4 butted against each other, and the branch tube 1 shown in Fig. 5 is produced by withdrawing the slide molds 4 after finishing injection. However, in the conventional process in which the slide molds are used, the slide molds must be withdrawn after finishing injection. Accordingly, a branch tube 1 which, for example, has a curved hollow portion as shown in Fig. 4 cannot be molded. Moreover, screw portions 1a cannot be integrally molded, and there is no other way but to form them by another step after molding. WO 90 08637 A discloses a method according to the preamble of claim 1, wherein a water-soluble core is formed by injection molding in a first mold which has at least three parts. Two parts of this mold are then replaced by corresponding parts of a second mold, so that a cavity is formed between said core and the second mold, while the core remains to be held by the first part of the first mold. Then, a plastic material is injected into the cavity to form at least a part of the molded article. If the molded article is to have a more complicated shape, it is possible to add further steps in which other core parts and other parts of the molded article are molded successively. Finally, the core is removed with water so that a hollow molded article is obtained. FR 948 461 A discloses a method comprising a step of molding an intermediate molded article which is to cover part of the surface of a water-soluble core. Then, said core is mounted on the intermediate molded article and plastic material is injected to cover the remainder of the surface of the core and thereby to complete the molded article. Finally, the core is removed with water. Disclosure of the Invention It is an object of the present invention to provide a process for producing a plastic molded article according to the preamble of claim 1, wherein the core can be held within the mold at surface portions which will later be covered by plastic material of the molded article. According to the invention, this object is achieved by the features indicated in claim 1. A particular embodiment of the invention is the subject of the dependent claim. In the process for producing the plastic molded article, a plastic material is injected to cover part of the surface of the core, and as a result a molded article is formed on part of the surface of the core. The remainder of the surface of the core is held by a mold. Next, while the molded article having been molded is held by a mold, the plastic material is injected to cover the remainder of the surface of the core. The procedure may be conducted by one step or a plurality of steps. Since the core can thus be held by the mold, a desired plastic molded article can be produced. In the process for producing a plastic molded article of the present invention, there is no specific limitation on the material of the water-soluble molded article. One of the examples of the material comprises polyvinyl alcohol as a principal component and an added water-soluble bulk filler. Although there is no specific restriction on the water-soluble bulk filler, examples of the bulk filler include starch, sodium chloride and corn starch. Furthermore, there is no specific limitation on the shape and structure of the molded article produced by the process for producing a plastic molded article according to the present invention, and the process can be applied to a plastic molded article so long as it has at least partly a hollow portion. Brief Description of the Drawings Fig. 1A-Fig. 1D are cross--sectional views showing one embodiment of the process for producing a plastic molded article of the present invention. Fig. 2 is a perspective view showing the embodiment in Fig. 1. Fig. 3A-Fig. 3C are cross-sectional views showing examples of molded articles which can be produced by the process for producing a plastic molded article of the present invention. Fig. 4 is a cross-sectional view showing a branch tube which can be produced by the process for producing a plastic molded article of the present invention. Fig. 5 is a cross-sectional view for illustrating a conventional process for producing a plastic molded article. Best Mode for Carrying out the Invention The embodiments of the present invention will be explained below with reference to drawings. First Embodiment Fig. 1A- 1D are cross-sectional views showing embodiment of the process for producing a plastic molded article of the present invention. Fig. 2 is a perspective view showing the embodiment. In the first embodiment, an explanation will be made on the case where the process is applied to the production of a branch tube 1 having a curved hollow portion as shown in Fig. 4. First, the branch tube 1 shown in Fig. 4 is formed with plastics such as polypropylene (hereinafter referred to as PP resin), has a screw portion 1a in the upper end opening, which is branched into two portions in the down stream. A screw portion 1a is formed in the lower end opening of each branched portion. Moreover, inner corners 1b of the branched portion and inner corners 1c of curved portions after being branched are each formed with a smooth curved surface. Use of such a plastic branch tube 1, for example, as a tube for transfusion can prevent destruction of leukocyte because the branch tube 1 has no sharp edge in the hollow portion. As explained above, since the conventional production process uses slide molds, the slide molds cannot be withdrawn after molding when the desired molded article 1 has curved hollow portions as shown in Fig. 4, and the molded article 1 cannot be produced by injection molding. In the production process in the present embodiment, a core 2 corresponding to the hollow portion of the branch tube 1 which is the desired plastic molded article is molded with a water-soluble material in advance. The core 2 is inserted into an injection mold 3, and injection molding is conducted. The core 2 remaining in the hollow portion is removed with water. Material containing highly water-soluble polyvinyl alcohol as a principal component and an added water-soluble bulk filler such as starch, sodium chloride or corn starch is preferably employed for the core 2 used in the present embodiment. There is no specific limitation on the method for molding the core 2 having the shape of the hollow portion. Any of the procedures such as machining and sticking in addition to injection molding, cast molding and extrusion molding can be applied. Fig. 2 shows the water-soluble core 2 thus molded to have the hollow shape of the branch tube 1. Screw shapes corresponding to the screw portions 1a are also formed. Fig. 1A is a cross-sectional view of the core along the line I-I in Fig. 2. Next, as shown in Fig. 1B and Fig. 2, the core 2 is held by a mold 31, and a cavity 32c is formed between another mold 32 and the core 2. The mold 31 is not a mold which is directly participated in molding the branch tube 1, but functions to hold the cure 2. Accordingly, there is no specific limitation on the shape and structure thereof so long as it can hold the core 2. Clamping is conducted as shown in Fig. 1B , and PP resin is injected into the cavity 32c formed between the surface of the core 2 and the inner surface of the mold 32, thereby forming a plastic 11 constituting the branch tube 1 on about half of the surface of the core 2. Next, the mold 31 in Fig. 1B is replaced with a mold 33 in Fig. 1C. As a result, a cavity 33c is formed between the surface of the core 2 not covered with the plastic 11 and the inner surface of the mold 33; PP resin is injected thereinto. The branch tube 1 which is composed of PP resin and the hollow portion of which is packed with the core 2 is obtained by the steps mentioned above, and is released from the mold 3. Water is injected into one of the top end openings or each top end opening to dissolve the core 2, thereby producing the PP branch tube 1 the top end openings of which each have the screw portion 1a as shown in Fig. 4. Moreover, examples of the procedure for dissolving the core 2 include a procedure wherein a jet is injected and a procedure wherein the branch tube 1 is immersed in water, and ultrasonic vibration is applied in addition to a procedure wherein the branch tube 1 is immersed in water. Furthermore, it is not necessary to dissolve the core 2 entirely, but it is satisfactory to dissolve the surface of the core 2 to some extent so as to remove the core from the hollow portion. Still furthermore, when the dissolved water-soluble resin is collected, it can be recycled to give the core 2. The embodiment explained above has been described not to restrict the present invention but to make the present invention easily understandable. Each element disclosed in the embodiment explained above, therefore, includes all design variations and equivalents belonging to the technical scope of the present invention. Fig. 3A-3C are cross-sectional views each showing one embodiment of a molded article which can be produced by the process for producing a plastic molded article of the present invention. The following articles can thus be easily produced: a branch tube 1 having smooth inner surfaces and curved inner surfaces as shown in Fig. 3A ; a tube 1 having various protrusions and ribs in the hollow portion as shown in Fig. 3B ; and an oval hollow molded article 1 having openings which may each have a screw portion as shown in Fig. 3C. As explained above, according to the process for producing a plastic molded article and a core used therefor of the present invention, the hollow portion of the plastic molded article can be formed to have an arbitrary shape. Moreover, screws and partial recesses and protrusions can be freely molded.

Claims: A process for producing a plastic molded article (1) having at least partly a hollow portion, by using a core (2) composed of a water-soluble molded article which has been formed to have a shape of the hollow portion and which is removed with water after the article (1) has been molded, comprising a step of holding said core (2) by a first mold (31) and forming a first cavity (32c) between a second mold (32) and said core, and a step of injecting a plastic material (11) into the first cavity (32c), whereby part of a surface of said core (2) is covered with said plastic material, characterized by a step of replacing said first mold (31) with a third mold (33) and forming a second cavity (33c) between said third mold and said core, a step of injecting a plastic material into the second cavity, whereby a remainder of the surface of said core is covered with plastic material and said molded article (1) is completed. A process for producing a plastic molded article as claimed in claim 1, wherein said core (2) comprises polyvinyl alcohol as a principal component and an added water-soluble bulk filler.

Inventors: NARUTOMI MASANORI, NARUTOMI, MASANORI, NARUTOMI, Masanori, Taisei Plas Co., Ltd.
Applicants: TAISEI PLAS CO LTD, TAISEI PLAS CO., LTD.
Application year: 1998
Main classification: B29C33
IPC codes: B29C45, B29C33, B29L22