The present invention relates to a three-dimensional hollow molded article having a complicated form such as the form of a sphere, a box, a straight tube, a curved tube or a manifold, an over-molding method for producing the same by placing a core having a hollow portion inside a cavity of a mold and by injecting a molten resin into a space formed by the core and a cavity wall of the mold, and an optimizing method of an over-molding method for producing the above three-dimensional hollow molded article.  More specifically, it relates to a three-dimensional hollow molded article which permits the prevention of deformation or breaking of a core caused by a pressure of a molten resin at the time of over-molding, an over-molding method for producing the three-dimensional hollow molded article, and an optimizing method of an over-molding method. 
An over-molding method (also called "two-shells method" or "dual-molding method") is one method of producing a three-dimensional hollow molded article from a thermoplastic resin by an injection molding method.  In the over-molding method, a core having a hollow portion is placed or set in a cavity of a mold, and a molten resin is injected into a space formed by the core and a cavity wall of the mold through a resin injection portion (resin gate portion) provided in the mold.  In this manner, at least part of the outer surface of the core is covered with the resin to form a three-dimensional hollow molded article.  A covering member formed of a resin covering at least part of the outer surface of the core is called a shell. 
When the above over-molding method for a three-dimensional hollow molded article is used, there can be produced a three-dimensional hollow molded article having a complicated form and a hollow portion, which so far cannot be produced by a conventional injection molding method.  Further, the over-molding method is advantageous in that the number of parts for an end product can be decreased by integrating other parts, that the inner surface of the hollow portion is flat and smooth, that the molded article has excellent dimensional accuracy and that the production facilities are inexpensive.  The over-molding method is widely used as a molding method for producing three-dimensional hollow molded articles having complicated forms such as a sphere, a box, a straight tube, a curved tube or a manifold together with a hollow portion in the fields of automobile parts such as an air intake manifold and an air duct, tubings for liquid such as water and other various articles having a hollow portion. 
The over-molding method is disclosed, for example, in JP-A-5-305679, JP-A-62-218117, JP-A-63-277851, JP-A-59-198116 and JP-A-3-239516. 
In JP-A-5-305679, projections 4', 5', 14' and 15' are provided on the exterior of joining surfaces 4, 5 and 13.  In JP-A-62-218117, thickened flanges 13 and 23 are provided near a joining surface of pre-molded articles 1 and 2.  In JP-A-63-277851, projections 2a and 3a are provided in a joining portion of split articles 2 and 3.  In other words, these publications disclose a core member formed of a resin in which the joining portion is increased in thickness or a core member having a thickness-increased portion which corresponds to the vicinity of an opening portion provided in a hollow portion of a core of a resin. 
The biggest problem with the above over-molding method is that when a molten resin is injected into a space formed by a core and a cavity wall of a mold through a resin injection portion provided in the mold, the pressure caused by injecting the molten resin works on the core of resin to deform, break, damage or buckle the core (sometimes generically called "deforming" hereinafter), so that it is difficult to obtain the intended three-dimensional hollow molded article in some cases.  That is, at the time of over-molding, the core is deformed in, or in the vicinity of, that portion of the core which faces or opposes the resin injection portion. 
In a conventional over-molding method, the resin injection portion is formed of a one-point pin gate or a one-point direct gate.  The term "pin gate" refers to an orifice provided in that portion of a mold through which a molten resin flows from a sprue portion into the cavity. 
When a resin injection portion of the above one-point gate type is used, as schematically shown in FIG.  179A (cross section), a molten resin injected from the resin injection portion flows down a long distance in a space formed by a core and a cavity wall of a mold.  In some cases where the resin injection portion is not provided in a proper position in the mold, the flow pattern of molten resin is non-uniform, so that a high injection pressure is required.  However, when the injection pressure is increased in the over-molding, a high pressure derived from the flow of the molten resin is exerted on the core.  The pressure on portions of the core depends upon the pressure distribution of the injected molten resin.  When the resin injection portion is formed as a one-point gate type formed with one injection point as shown in FIG.  179A, the highest pressure works on that portion of the core which is near to the resin injection portion.  As a result, the core placed in the cavity of the mold is deformed, so that no good three-dimensional hollow molded article is obtained.  On the other hand, the deformation of the core caused by the pressure of the injected molten resin in the over-molding may be prevented by arranging the flow distance of the molten resin from the resin injection portion such that the distance is as small as possible, and by filling a molten resin uniformly in the space.  In FIGS.  179A, 179B and 179C, numerals 60 and 62 indicate mold members, numerals 60A and 62A indicate cavity walls of the mold members 60 and 62 forming a cavity, numeral 63 indicates a main sprue portion, numeral 44 indicates a space, numeral 42 indicates a support rod, numeral 20 indicates a core, and numeral 21 indicates a hollow portion in the core 20.  In FIG.  179A, the core has the form of a straight tube.  FIG.  179B shows a schematic cross section taken along a line B--B in FIG.  179A, and FIG.  179C shows a schematic cross section taken along a line C--C in FIG.  179A.
None of the above five publications describe or suggest any deformation which occurs in or near that part of a core which faces or opposes the resin injection portion, nor do they describe or suggest any means to solve such deformation. 
That is, in a conventional over-molding method, no core of a resin (or core member of a resin) has been designed or produced by taking into consideration structural strength or a distribution state of the pressure caused by the injected molten resin in the step of molding a resin on at least part of the outer surface of the core (sometimes to be referred to as "over-molding step" hereinafter).  The (wall) thickness of that portion of a core which faces or opposes the resin injection portion is generally the same as that of other portions of the core.  Further, no resin injection portion has been improved in view of the prevention of deformation of the core caused by the pressure of the injected molten resin.  In the over-molding step, therefore, that portion of a core inserted and placed in the cavity of a mold, which faces or opposes the resin injection portion, is deformed by the pressure caused by injecting the molten resin, so that it is difficult to produce good molded articles. 
Moreover, there is so far no means or method for quantitatively determining the occurrence of deformation of a core formed of a resin.  In a conventional over-molding method, the form of a three-dimensional hollow molded article, the form of the core of a resin, the design of a mold typified by the position(s) and number of resin injection portion(s) and molding conditions (all of these will be sometimes generally referred to as "over-molding conditions" hereinafter) are determined depending upon the experiences and intuition of skilled workers.  When the core of a resin is actually deformed, the over-molding conditions are optimized by trial and error.  It therefore requires considerable amounts of time and labor and is largely ineffective to optimize the over-molding conditions. 
For overcoming the deformation of a core of a resin, there is a method in which a non-compressed material is charged into the hollow portion of the core.  The method of charging a non-compressed material (low-melting-point metal, sand, glass beads or water) into the hollow portion of a core has a problem in that the working efficiency and productivity are poor, so that the production cost of the three-dimensional hollow molded article increases.