The present invention relates to the core structure of a molding apparatus used for making hollow molded articles having an undercut or internal threading such as a container cap and the like made of plastic or other materials, by a molding process. 
One typical example of the conventional core structures used for an apparatus for making a molded article is disclosed in U.S. Pat.  No.  3,247,548. 
Such core structure, as shown in FIG.  18 to FIG.  20, substantially comprises a cylindrical body (1) and a core expanding rod (2) which is reciprocably movable in the cylindrical body (1) for radially outwardly expanding said cylindrical body. 
The cylindrical body (1) has the upper portion thereof longitudinally divided into a plurality of circumferentially disposed sections and such sections are made of a plurality of core members (3) with a radially outwardly expanding cross section and a plurality of core members (4) with a radially inwardly expanding cross section, wherein the core members (3) and (4) are circumferentially alternately disposed. 
The core members (3) and (4) are usually resiliently retracted in the radially inward direction as shown in FIG.  19, while the core members (4) are disposed more radially inwardly from the core members (3). 
On molding, the inserting of the core-expanding rod (2) into the inside of the main cylindrical body (1) allows the core members (3) and (4) to expand radially outwardly and to form a molding surface on the outermost surface of the core members (3) and (4) as shown in FIG.  20 and such molding surface, in cooperation with the metal mold (5), can provide a molding space for molding a molded article (P) such as a container cap. 
After molding, the axial withdrawal of the rod (2), which causes the resilient retraction of the cores (3) and (4) in the radially inward direction, allows the molded article (P) to separate in such a manner as shown in FIG.  9 so that the molded article (P) may be readily removed from the metal mold (5). 
Such conventional core structure for molding apparatus, however, has following drawbacks,
(a) Such core structure is made as an integral unit or as one piece.  In manufacturing the core, though the cylindrical body (1) has been cut into a plurality of divided core members (3) and (4) by metal-saw cutting, the cutting clearance (circumferential width of cut) is too narrow to make the cut easily and gives the machined surfaces (3a), (3b), (4a), and (4b) a rough finish, resulting in poor contact or abutting between the core members (3) and the core members (4).  PA1 (b) For improving the surface condition of the machined surfaces in general, a finishing by grinding or milling is utilized.  However, it is impossible to apply such finishing to the machined surfaces (3a), (3b), (4a), and (4b), which have an extremely narrow clearance or gap therebetween.  PA1 (1) The cylindrical core which is in the form of a plurality of main core members separated by a plurality of wide slits and a plurality of complementary core members which are inserted and fastened in the slits can be independently manufactured and thereafter assembled.  Accordingly, the main core members, without the complementary core plate members being attached, provide sufficiently wide machining clearance to allow speedy machining on the side faces of the main core members by a milling cutter in place of a metal saw.  Moreover, a subsequent grinding process can be also readily carried out.  This makes it possible to manufacture highly precise cylindrical cores at a lower cost.  PA1 (2) The complementary core members, as well, may be machined and ground quite readily and separately from the main core members.  This makes it possible, as well, to manufacture more precise complementary core members at a lower cost.  PA1 (3) The cylindrical core members and complementary core members can be produced in high precision as mentioned above so that the assembling of these members to form a cylindrical core structure also can be carried out precisely or accurately and such core structure can be installed in the molding apparatus precisely.  Accordingly, the manufacture of highly precise molded articles becomes possible. 
Accordingly, such core members (3) and (4) cannot be precisely assembled resulting in low manufacturing yield of the core and high manufacturing costs. 
(c) The molded articles which are produced by the above members also exhibit a poor precision. 
Accordingly, it is an object of the present invention is to provide a core structure of a molding apparatus for making a molded article which may solve the problems associated with an inherent in the prior art. 
In summary, this invention relates to the core structure of a molding apparatus for making a molded article comprising a) a cylindrical core of which a part is resiliently retractable in the radially inward direction, the core having the part longitudinally divided into a plurality of main core members which are circumferentially disposed at a predetermined distance while forming a plurality of longitudinal slits between each two main core members, the slit having a wide and arcuate cross section, b) a plurality of complementary core members corresponding to the shape of the slits, the complementary core members capable of being assembled in the slits for forming a cylinder together with the core members and c) a core-expanding rod, which is axially inserted into the cylindrical core to expand radially the main core members and the complementary core members causing the radially expanded outermost surfaces of the main core members and the complementary core members to form a cylindrical smooth surface whereon the article is molded. 
Due to such construction, the present invention has following advantages.