Patent Publication Number: US-6666941-B2

Title: Method of manufacturing ribbed structure by using biodegradable mold

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 09/190,388, filed on Nov. 12, 1998, now U.S. Pat. No. 6,350,337B1 and is based upon and claims the benefit of priority from prior Japanese Patent Application Nos. 9-310480, Nov. 12, 1997, 9-318248, filed Nov. 19, 1997, and 10-020943, filed Feb. 2, 1998, the entire contents of all which are incorporated by reference. 
    
    
     FIELD OF INVENTION 
     The present invention relates to a method of manufacturing a composite-material product, such as a container, a tubular product, a wing or another structure, reinforced by, for example, reinforcing fibers. More particularly, the present invention relates to a method of manufacturing a ribbed structure made of fiber-reinforced plastic or the like by using a mold, for example, a core, the removal of which from the structure has been difficult and which is made of a biodegradable material. 
     BACKGROUND 
     When a composite-material product reinforced by carbon-fiber-reinforced plastic (hereinafter called “CFRP”) or glass-fiber-reinforced plastic (hereinafter called “GFRP”), for example, a hollow structure having an undercut, is manufactured, a method structured as shown in FIG. 30 has been employed. 
     That is, a metal and split mandrel  401  composed of a shell  401   a  and a core  401   b  having shapes corresponding to a shape attempted to be molded is prepared. Then, CFRP or GFRP is laminated on the outer surface of the shell  401   a  of the split mandrel  401  so that a reinforcing-fiber-reinforced resin layer  402  is formed. The reinforcing-fiber-reinforced resin layer  402  is hardened with heat or at room temperatures. Then, the shell  401   a  and core  401   b  of the split mandrel  401  are mechanically decomposed (separated) so as to be removed from the inside portion of the reinforcing-fiber-reinforced resin layer  402 . As a result, a hollow structure  403  is molded. 
     If the shape of the hollow structure attempted to be molded by the metal and split mandrel is too complicated to easily be removed by mechanical decomposition after the molding process has been completed, the following method is employed. That is, the mandrel is made of a metal material having a low melting point. Moreover, the CFRP or GFRP is laminated on the outer surface of the mandrel as described above to form the fiber-reinforced resin layer. Then, the fiber-reinforced resin layer is hardened at room temperatures, and then the mandrel is heated at appropriate temperatures so as to be melted and removed. 
     Another method is known with which the mandrel is made of a material which can be melted with a chemical. Another method is known with which the mandrel is made of collapsible plaster which is crushed so as to be removed after the molding process has been completed. The above-mentioned manufacturing methods have been employed to mold a product, such as a duct  404  including a warped portion  404   a  and a branch portion  404   b,  as shown in FIG.  31 ( a ). Also the foregoing methods have been employed to mold, for example, a tubular member  405  having bent portions  405   a  at two ends thereof, as shown in FIG.  31 ( b ). 
     However, the split mandrel cannot easily be manufactured and thus the manufacturing cost is enlarged. If a complicated shape is attempted to be formed, the separation and removal which are performed after the molding process has been completed cannot easily be performed as well as the difficulty in manufacturing the same. In this case, excessively large force is added to the molded product and, therefore, the molded product is deformed or broken. 
     Any one of the above-mentioned method of removing the mandrel by heating and melting the same, the method of removing the mandrel by melting the same by using a chemical and the method of removing the mandrel by crushing the collapsible plaster require a large number of steps. Thus, all of the foregoing methods suffer from unsatisfactory productivity. When a molded product having a complicated shape is attempted to be manufactured, the mandrel cannot completely be removed. When the core is manufactured by aluminum, the solvent of the chemical is sodium hydroxide. However, a great cost is required to perform disposal of sodium hydroxide after the core has been dissolved. What is worse, environmental pollution is undesirably caused to take place. 
     In recent years, weight reduction and increase in the strength have been required. Therefore, prepreg has energetically been developed which contains thermosetting resin, such as epoxy resin or unsaturated polyester, serving as a matrix thereof and a reinforcing material, such as carbon fibers, aramide fibers or glass fibers, added thereto. Therefore, the needs for a variety of products containing the prepreg have considerably been grown. In addition, the needs for a composite-material product such that thermoplastic resin, such as nylon or polyether-ether ketone (PEEK), is used as the matrix have been grown. 
     Since the prepreg of the foregoing type is a material having excellent characteristics which enable light weight and strong structure to be manufactured, it can be considered that a composite material is an advantageous material to make various elements for use in an extreme condition in, for example, an aerospace industrial field. Since the foregoing structures usually have complicated shapes, complicated processes are required to manufacture the foregoing structures. 
     When the thermosetting resin or the thermoplastic resin is employed as the matrix of the core of the honeycomb for use in the composite-material structure and long carbon-fiber-reinforced plastic (hereinafter called “CFRP”) or the glass-fiber-reinforced plastic (hereinafter called “GFRP”) is employed as the reinforcing fiber, the prepreg must be laminated in a trapezoidal mold having asperities so as to be hardened by an autoclave or a pressing machine. 
     A fact is known that a structure that the long fiber CFRP or GFRP employed as the reinforcing fiber of the core material enables a strong and rigid honeycomb plate to be manufactured. However, there arises a problem in that long time and great effort are required to inject the material and to perform a laminating process when a waveform plate is molded to manufacture the core member. Further, since the honeycomb structure such as the honeycomb plate has normally a directional property, etc., it has been difficult to design and manufacture the three-dimensional honeycomb structure. However, the honeycomb plate suffers from unsatisfactory strength against a load added in a direction perpendicular to the longitudinal plate. 
     When an airplane or a wing structure such as wings or fan&#39;s blades are manufactured by using the known honeycomb structures, the main body of the wing  411  is constituted by honeycomb cores  412  having lower densities, that is, a large cell size (the length of one side of a hexagon is long), as shown in FIG.  32 . In this case, the weight of the wing  411  can be reduced. If the outer surface of the wing  411  is attempted to be smoothed or if the resistance against collision with an object is attempted to be somewhat enlarged, it is preferable that honeycomb cores  413  each having a high density, that is, a small cell size (the length of one side of a hexagon is short) is employed. 
     Therefore, a two-layer structure has been employed which is composed of the honeycomb cores  412  having the large cell size and the honeycomb cores  413  having the small cell size which are laminated through the prepreg  414 . However, the manufacturing process requires long time and great effort and a complicated three-dimensional curved surface cannot easily be manufactured. Therefore, the above-mentioned structure cannot practically be employed. Although the honeycomb can be preformed at high temperatures, a large heat-resisting mold is required to preform the honeycomb. Thus, the manufacturing cost is enlarged. 
     When a three-dimensional curved surface is manufactured by using the honeycomb, a core material  415  must be cut to form a rectangular block into the three-dimensional curved surface, as shown in FIG.  33 ( a ). As an alternative to this, a honeycomb core material  416  for forming a three-dimensional curved surface must be employed, as shown in FIG.  33 ( b ). In either case, the manufacturing cost cannot be reduced. Therefore, another requirement is imposed to manufacture a complicated structure of the foregoing type by using the composite material at a low cost. 
     SUMMARY 
     To achieve the above-mentioned objects, the present invention provides a method of manufacturing a ribbed structure with fiber-reinforced composite material. A mold for the structure is prepared having a mold surface. Non-hardened resin containing reinforcing fibers are placed on the mold surface of the structure mold. A mold for a rib is formed by using a material containing biodegradable polymers. Non-hardened resin containing reinforcing fibers are laminated on the rib mold, and then the rib mold is placed in a predetermined position on the non-hardened resin containing the reinforcing fibers tin the mold surface of the structure mold. The non-hardened resins are hardened and the rib mold is biochemically degraded wherein in the hardening of the resins, the resin hardened on the rib mold and the resin hardened on the mold surface of the structure mold are united with each other. 
     An alternative a method comprises preparing a mold for the ribbed structure, which has a mold surface and a rib formation groove formed in a predetermined position in the mold surface. A mold for a rib is formed by using a material containing biodegradable polymers. Non-hardened resin containing reinforcing fibers are laminated on the rib mold, and then the rib mold is placed in the rib formation groove formed in the mold surface of the structure mold. Non-hardened resin containing reinforcing fibers is placed on the mold surface of the structure mold to cover the non-hardened resin containing the reinforcing fibers on the rib mold placed in the rib formation groove. The non-hardened resins are hardened and the rib mold is biochemically degraded, wherein in the hardening of the resins, the resin hardened on the rib mold and the resin hardened on the mold surface of the structure mold are united with each other. 
     The present invention further provides a method of manufacturing a ribbed structure with fiber-reinforced composite material where the ribbed structure includes a T-shaped cross section having a wide head portion and a narrow body portion. A reference surface mold which has a reference surface and a wide head portion-formation groove for formation of the wide head portion, formed in a predetermined position in the reference surface is prepared. A mold for the ribbed structure, located in a predetermined position on the reference surface of the reference surface mold, providing a narrow body portion-formation groove for formation of the narrow body portion of the rib in association with the wide head portion-formation groove, and having a mold surface is prepared. A mold for the narrow body portion of the rib is formed by using a material containing biodegradable polymers. Non-hardened resin containing reinforcing fibers is placed in the wide head portion-formation groove formed in the reference surface of the reference surface mold. The structure mold is placed in a predetermined position on the reference surface of the reference surface mold. Non-hardened resin containing reinforcing fibers is laminated on the mold for the narrow body portion, and then the mold for the narrow body portion is placed in the narrow body portion-formation groove provided by the structure mold placed in the predetermined position on the reference surface of the reference mold. Non-hardened resin containing reinforcing fibers is placed on the mold surface of the structure mold to cover the non-hardened resin containing the reinforcing fibers on the narrow body portion mold in the narrow body portion-formation groove, All the resins are hardened and the narrow body portion mold is biochemically degraded, wherein in the hardening of the resins, the resin hardened in the wide head portion-formation groove, the resin hardened on the narrow body portion mold, and the resin hardened on the mold surface of the structure mold are united with each other. 
     The biodegradable material for use to make the above-mentioned mold is preferrably a polymer which can be degraded with microorganisms, enzymes or another biochemical means or a mixed material of the polymer and a biodegradable material. Each of the above-mentioned material is biochemically degraded into e.g., water and carbon dioxide after the structure has been molded. Therefore, the material can easily and completely be removed from the structure. Since the biodegradable material can be degraded into the harmless substances, the disposal cost can be reduced and a problem of environmental pollution does not arise. 
     The present invention has another characteristic for efficiently degrading the mold, such as the core, made of the biodegradable material. For example, a structure manufactured by using the above-mentioned mold is accommodated in a degrading tank. In the foregoing tank, a solution containing biochemically active substances, such as microorganisms, enzymes or the like, is circulated. The solution is added with substances for enhancing the action of the biochemically active substances, for example, nutrients for the microorganisms. The temperature, pH, components and so forth of the solution which is circulated in the degrading tank are adjusted. Moreover, substances, for example, metabolites of the microorganisms, for example, carbon dioxide, which deteriorate the action of the biochemically active substances are removed from the degrading tank. 
     The mold made of the above-mentioned biodegradable material has a structure which enhances the biochemical degradation. If the mold is employed as the core, the core is formed into a hollow shape to maintain a passage and surface of contact with the solution containing the biochemically active substances. The mold is made of open-cell foam composed of the biodegradable material to enhance passage of the solution containing the biochemically active substances. Moreover, the area of contact can be enlarged. 
     The above-mentioned mold is made of a composite material composed of biodegradable polymers, particles composed of the biodegradable material, porous particles or particles of a water-soluble material. The foregoing particles enhance penetration of the solution, enlarge the area of contact and provide a culture area for the microorganisms. Prior to or simultaneously with the biochemical degradation, the mold is irradiated with, for example, ultraviolet rays. Thus, the molecule chains of the biodegradable polymers are cut to collapse the polymers so as to enhance the biochemical degradation. Moreover, substances for enhancing the degradation are added to the biodegradable polymers. 
     The present invention is able to manufacture structures having a variety of shapes by using the characteristic of the mold made of the biodegradable material, that is, the characteristic with which the mold is degraded into liquid and gas. 
     If the mold made of the above-mentioned material is used as the core, the core can easily be degraded and removed. The hollow portions created by the core are required to have passages capable of removing the solution containing the biochemically active substances, liquid of the degraded substances and the gas. Therefore, a hollow structure having an arbitrary shape can easily be manufactured. 
     When the above-mentioned characteristics are used to surround, for example, a spherical core, with a prepreg made of the composite material so as to be filled into the mold, a strong hollow structure can be constituted. Since a hollow portion having an arbitrary shape can be formed, a structure having a multiplicity of hollow ribs or a structure in the form of an isogrid shape can easily be manufactured. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING(S) 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a diagram showing a process for manufacturing a hollow structure according to a first embodiment of the present invention; 
     FIG. 2 is a diagram showing a process for manufacturing a hollow structure according to a second embodiment of the present invention; 
     FIG. 3 is a schematic view showing an apparatus for use in a process for biochemically degrading a mold according to the present invention; 
     FIG. 4 is a vertical cross sectional view showing a first example of the structure of the mold according to the present invention; 
     FIG. 5 is a vertical cross sectional view showing a second example of the structure of the mold according to the present invention; 
     FIG. 6 is a vertical cross sectional view showing a third example of the structure of the mold according to the present invention; 
     FIG. 7 is a vertical cross sectional view showing a fourth example of the structure of the mold according to the present invention; 
     FIG. 8 is a vertical cross sectional view showing a fifth example of the structure of the mold according to the present invention; 
     FIG. 9 is a vertical cross sectional view showing the fifth example of the structure of the mold according to the present invention; 
     FIG. 10 is a vertical cross sectional view showing a sixth example of the structure of the mold according to the present invention; 
     FIG. 11 is a vertical cross sectional view showing a seventh example of the structure of the mold according to the present invention; 
     FIG. 12 is a vertical cross sectional view showing an eighth example of the structure of the mold according to the present invention; 
     FIG. 13 is a vertical cross sectional view showing a process which is performed prior to the process for degrading the mold according to the present invention; 
     FIG. 14 is a diagram showing a process for manufacturing a hollow structure according to a third embodiment of the present invention; 
     FIG. 15 is a diagram showing a process for manufacturing a hollow structure according to a fourth embodiment of the present invention; 
     FIG. 16 is a diagram showing a process for manufacturing a porous structure according to a fifth embodiment of the present invention; 
     FIG. 17 is a diagram showing a process for manufacturing a porous structure according to a sixth embodiment of the present invention; 
     FIG. 18 is a diagram showing a process for manufacturing a porous structure according to a seventh embodiment of the present invention; 
     FIG. 19 is a diagram showing a process for manufacturing a porous structure according to an eighth embodiment of the present invention; 
     FIG. 20 is a diagram showing a process for manufacturing a porous structure according to a ninth embodiment of the present invention; 
     FIG. 21 is a diagram showing a process for manufacturing a porous structure according to a tenth embodiment of the present invention; 
     FIG. 22 is a diagram showing a process for manufacturing a rib structure according to an eleventh embodiment of the present invention; 
     FIG. 23 is a perspective view showing the shape of a core member according to the eleventh embodiment of the present invention; 
     FIG. 24 is a diagram showing a process for laminating prepreg according to the eleventh embodiment of the present invention; 
     FIG. 25 is a perspective view showing a hollow structure which is manufactured in the eleventh embodiment of the present invention; 
     FIG. 26 is a diagram showing a process for manufacturing a rib structure according to a twelfth embodiment of the present invention; 
     FIG. 27 is a perspective view showing a hollow structure which is manufactured in the twelfth embodiment of the present invention; 
     FIG. 28 is a diagram showing a process for manufacturing a rib structure according to a thirteenth embodiment of the present invention; 
     FIG. 29 is a perspective view showing a reference surface mold or jig for manufacturing the rib structure according to the thirteenth embodiment of the present invention; 
     FIG. 30 is a diagram showing a method of manufacturing a conventional hollow structure; 
     FIG. 31 is a perspective view showing another conventional hollow structure; 
     FIG. 32 is a diagram showing a honeycomb core for use to manufacture a conventional honeycomb; and 
     FIG. 33 is a diagram showing a process for manufacturing a conventional hollow structure having three-dimensional curved surfaces. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 shows a first embodiment in which a container in the form of a hollow structure is manufactured. Reference numeral  11  shown in FIG.  1 ( a ) represents a core made of biodegradable polymers, for example, microorganism type Biopol (trade name of Monsalt) in the form of a copolymer of hydroxybutyrate and valirate or Bionol (trade name of Showa Highpolymer) in the form of fatty acid polyester or polyester of succinic acid and butanediol/ethylene glycol. The core  11  is made of polymers which are degraded by dint of the action of the biochemically active substance, such as bacteria or enzymes. The core  11  is molded by blow molding or injection molding and provided with a spherical core body  11   a  having an elongated opening  11   b.    
     The biodegradable polymer is not limited to the above-mentioned material and the same may be any one of a variety of materials, such as a microorganism type polymer, a chemical synthesis type polymer, a polymer using natural substances, a blend type polymer. 
     A CFRP or GFRP resin layer  12  is formed on the outer surface of the core  11 . As a means for forming the resin layer  12 , reinforcing fibers  13 , for example, carbon fibers or glass fibers, are substantially uniformly wound around the outer surface of the core  11 , as shown in FIG.  1 ( b ) (first step). Then, as shown in FIG.  1 ( c ), catalysts are added, and then non-hardened resin, such as unsaturated polyester, epoxy resin or phenol resin, or molten resin  14  are applied to the reinforcing fibers  13  (second step). 
     As a means for adding the non-hardened resin or the molten resin  14 , the non-hardened resin or the molten resin  14  accommodated in a tray  15  is allowed to adhere to a roller  16  so as to be applied to the surface of the reinforcing fibers  13 . The non-hardened resin or the molten resin  14  may be immersed in a molten resin tank (not shown) together with the core  11 . The non-hardened resin or the molten resin  14  is impregnated into the reinforcing fibers  13  so that the resin layer  12  having a predetermined thickness is formed. 
     In this embodiment, a catalyst-added non-hardened resin or the molten resin  14  is unsaturated polyester, epoxy resin or phenol resin. The non-hardened resin or the molten resin  14  is hardened when it is allowed to stand at room temperatures. When a formed product is to be so formed as to have a smooth surface, the resin layer  12  is surrounded by a film before the resin layer  12  is hardened, and then the inside portion of the film is vacuum-sucked. Thus, the film is brought into hermetic contact with the resin layer  12 . As a result, the outer surface of the resin layer  12  is smoothed. 
     After the resin layer  12  has been hardened as shown in FIG.  1 ( d ), muddy water containing biochemically active substances  17 , such as bacteria and enzymes, specifically, microorganisms, is injected into the core  11  through an opening  11   b  of the core  11 . Then, the core  11  is allowed to stand for several days to several weeks so that the core  11  is degraded (mainly into carbon dioxide and water) (third step). 
     After the core  11  has been degraded, the residues are discharged so that a container  18  made of the resin reinforced by the reinforcing fibers  13  is completed, as shown in FIG.  1 ( e ). Even if the inner surface of the container  18  has a complicated shape, the residues of the core  11  are not left. That is, removal of the core  11  can easily be performed. As a result, excessive external force is not added to the container  18 . 
     FIG. 2 shows a second embodiment which is different from the first embodiment in that another method of forming the resin layer is employed. That is, as shown in FIG.  2 ( a ), prepreg  19  is wound around the outer surface of the core  11  made of the biodegradable polymers (first step). Then, as shown in FIG.  2 ( b ), the core  11  around which the prepreg  19  has been wound is accommodated in an autoclave  20 . Then, the prepreg  19  is heated so as to harden the resin in the prepreg  19  so that the resin layer  12  reinforced with the reinforcing resin is formed on the outer surface of the core  11  (second step). After the resin layer  12  has been hardened, the biochemically active substances  17 , such as bacteria or enzymes, are injected into the core  11  through the opening  11   b  of the core  11 . Then, the core  11  is allowed to stand for several days to several weeks so that the core  11  is degraded (third step) similarly to the first embodiment. 
     The resin layer  12  is surrounded by a film before the resin layer  12  is hardened, and then the inside portion of the film is vacuum-sucked. Thus, the film is brought into hermetic contact with the resin layer  12 . As a result, the outer surface of the resin layer  12  is smoothed. 
     Although a variety of biodegradable polymers have been developed at present, all of the polymers have low degradation speeds. Therefore, long time is required to degrade the core  11 . Hence, the biochemical degradation of the core must be enhanced in the above-mentioned process. When the above-mentioned method is put into practical use, degradation of the core must reliably be controlled. 
     FIG. 3 schematically shows an apparatus for enhancing the degradation of the core and controlling degrading period of time. The apparatus includes a degrading tank  30 . In the degrading tank  30 , a solution S containing biochemically active substances, such as the microorganisms and enzymes, are accumulated. The solution S is, by a pump  31 , circulated through a solution-component control unit  32  and a solution-temperature control unit  33 . Since the container  18  has one opening, a solution is jetted from a nozzle  34  to the opening so that the solution S is circulated in the core. 
     The temperatures and components of the solution S in the degrading tank  30  and the solution which is circulated by the pump  31  are detected by temperature detectors  36  and  38  and component detectors  37  and  39 . The component detectors  37  and  39  are units for detecting the components, pH and other factors of the solution S. Signals transmitted from the detectors are supplied to a control unit  35 . The control unit  35  processes the supplied signals so as to transmit control signals to the solution-component control unit  32  and the solution-temperature control unit  33  so as to control the operations of the foregoing units and control the components and temperature of the solution S to satisfy predetermined ranges. 
     The biochemically active substances, for example, the microorganisms and enzymes, have an optimum temperature range for the operation thereof. The solution-temperature control unit  33  maintains the temperature of the solution to satisfy the optimum range so as to enhance the degradation of the core. Microorganisms have optimum pH for the operation thereof. In general, the operations of microorganisms deteriorate when the concentration of substances produced because of degradation of the core, that is, metabolites, such as carbon dioxide, has been raised. If the employed microorganisms are aerobic microorganisms, the microorganisms consume oxygen during the operation of the microorganisms. The solution-component control unit  32  maintains the components of the solution S to satisfy the optimum range for the operations. 
     When the above-mentioned apparatus is employed, the biodegradation of the core is enhanced to quickly complete the degradation. Moreover, the degradation condition can be controlled. Therefore, the period of time required to complete the degradation and whether or not the degradation has been completed can accurately be detected. 
     The mold, for example, the core, may have a structure which enhances the biochemical degradation. If a structure  40  has an elbow-like shape having two opened ends as shown in FIG. 4, a hollow core  41  is employed. Moreover, a cover  43  having solution communication opening  44  is joined to a flanges  42  at each of the two ends of the structure  40 . The solution is passed to the core  41  through the solution communication opening  44  so that the degradation of the core  41  is enhanced. In this embodiment, the structure  40  is not required to be accommodated in the degrading tank. A tube or the like is connected to the solution communication opening  44  to circulate the solution. 
     FIG. 5 shows a second example of the structure of the core. In this example, a structure  45  has only one opened end. In this example, a core  46  is formed into a hollow shape. Moreover, a solution communication nozzle  47  is joined to another end opposite to the opened end. Thus, the solution is passed into the core  46  through the solution communication opening  44  of the cover  43  and the solution communication nozzle  47 . In this case, the core  46  is degraded, and then the solution communication nozzle  47  is cut. The opened portion is closed with another composite material. 
     FIG. 6 shows a third example of the structure of the core. In this example, a cover  48  having solution communication openings  49  and  51  and an insertion nozzle  50  is joined to an opened end of a structure  45 . The solution is passed to the inside portion of a core  46  through the solution communication openings  49  and  51  and the insertion nozzle  50 . 
     FIG. 7 shows a fourth example of the core. Since a structure  52  according to this example has no opened end, a solution communication nozzle  54  is provided which penetrates the hollow core  53  and the wall of the structure  52 . The solution is passed to the inside portion of the core  53  through the solution communication nozzle  54 . After the biochemical degradation of the core  53  has been completed, the solution communication nozzle  54  is removed. Moreover, the opened end formed by the solution communication nozzle  54  is closed with another composite material, if necessary. 
     FIGS. 8 and 9 show a fifth embodiment of the core. In this example, one of the solution communication nozzles  54  is allowed to communicate with the inside portion of the hollow core  53 . Another solution communication nozzle  55  is allowed to communicate with a position between the outer surface of the hollow core  53  and the inner surface of a hollow structure  52 . Air in the inside portion of the hollow core  53  is exhausted to realize a negative pressure. Moreover, a positive pressure is acted on the outside of the core  53  through the other solution communication nozzle  55 . 
     The difference in the pressure between the outside portion and the inside portion is used so that the hollow core is collapsed as shown in FIG.  9 . Then, the solution is passed through the solution communication nozzles  54  and  55 . In this example, the core  53  is collapsed into fine pieces. Therefore, the degradation of the core  53  can furthermore be enhanced. 
     As described above, the degrading efficiency can be improved by devising the microscopic structure of a material for making the mold, such as the core, as well as devising the shape and the structure of the core or the like. FIG. 10 shows a sixth example of the improvement in the microscopic structure of the mold. 
     In this example, a biodegradable polymer material  56  for constituting the mold is a foam structure having a multiplicity of open cells  57 . Note that the foam having the open cells can be formed by a known technique. In this example, the solution is communicated or penetrated through the open cells  57 . A wall  58  of the foregoing material has a small thickness and a large area of contact with the solution. Therefore, the biochemical degradation can efficiently be performed. In this example, the passage for the solution can be formed by the above-mentioned structure. 
     FIG. 11 shows a seventh example of the microscopic contrivance of the mold. In this example, a multiplicity of particles  59  are mixed and dispersed in the material  56 , such as the biodegradable polymer. The particles  59  are made of, for example, a biodegradable material or water-soluble material. It is preferable that the particles  59  are made of a porous material. It is further preferable that the particles  59  have elongated shapes. 
     The material of the particles  59  is exemplified by ashes of burning dust, chips of wood and pulp. The foregoing materials are biodegradable material and also serve as culture area for microorganisms to enhance the degradation. The particles  59  are further exemplified by fly ashes, starch, chemical fertilizer and water-soluble inorganic substances. Since the above-mentioned particle are dissolved in the solution and form small cavities at the positions of the particles, the degradation of the polymer material  56  is enhanced. Note that the chemical fertilizer and so forth serve as nutrient for microorganisms. If the particles  59  have the elongated shapes, the solution can furthermore deeply penetrate the core, the degradation is furthermore enhanced. 
     FIG. 12 shows an eighth example of the microscopic structural contrivance. In this example, particles similar to those according to the seventh example are employed. The quantity of the particles  59  with respect to the quantity of the biodegradable polymers is enlarged. The polymer is used as a binder for the particles  59 . In this example, the solution is furthermore deeply and quickly penetrate the core. Moreover, the polymer portions have large thicknesses. Therefore, the degradation can furthermore be enhanced. 
     A molecular structural contrivance of the polymer enables the degradation to be enhance as well as the microscopic structural contrivance of the biodegradable polymer. 
     FIG. 13 shows an example of a process for enhancing the degradation of the polymers. In this example, an optical guide  62 , such as an optical fiber, or an optical system combined with an optical element such as a mirror, is inserted into a hollow core  61  through an end of an opening of a structure  60 . Light, for example, ultraviolet rays, is transmitted through the optical guide  62 . A radiant optical device  63  disposed at the leading end of the optical guide  62  radiates light so that the inner surface of the hollow core  61  is irradiated with light. 
     When the ultraviolet rays are applied as described above, main chains of molecules of the biodegradable polymers of the core  61  are cut. Thus, the polymers molecular structure is collapsed. As a result of the collapse, the polymers are made to be brittle. Moreover, fine irregularities and cracks are formed because of separation of the surface. Therefore, penetration of the solution is enhanced and the surface area is enlarged. Since the main chains are cut, the biochemical degradation of the polymers is furthermore enhanced. 
     To effectively cut the main chains of the polymers molecules by dint of light as described above, it is preferable that grafting copolymerization of light functional groups with the polymers is performed. Another effective means is to add an enhancer for light degradation or microorganism degradation to the polymers. The foregoing enhancers enhance, for example, the degradation by dint of microorganisms. Moreover, conditions are realized under which polymers are oxidized and degraded by dint of an automatic oxidization effect, metabolite, such as carbon dioxide, of microorganisms is degraded and degradation by dint of microorganisms is enhanced. As the foregoing degradation enhancer, an enhancer DEGRA NOVON which is trade name of NOVON JAPAN INC. is available. 
     The irradiation with light, such as ultraviolet rays, may be performed simultaneously with the biochemical degradation process. If an appropriate type of microorganisms for use to perform the degradation is selected, the degradation effect can furthermore be enhanced by dint of the irradiation with light. 
     The method of the degradation and collapse of the biodegradable polymer is not limited to the light irradiation. The degradation and collapse may be performed with, for example, heat generated when the composite material is hardened by heating the structure in an autoclave. 
     Although the description has been made about a structure having a relatively simple shape, a structure having a complicated shape can be manufactured by using the characteristic of the present invention. 
     FIG. 14 shows a third embodiment of the present invention which is different from the first and second embodiments in the shape of the core. That is, as shown in FIG.  14 ( a ), a core  121  made of biodegradable polymers is composed of a plurality of tubular or cylindrical core elements  121   a,  . . . The core elements  121   a,  . . . , are connected to one another by connectors  122  provided for the axial portion thereof. Therefore, the connected core elements  121   a,  . . . , are disposed apart from one another for predetermined distances in the axial direction. 
     The core  121  is employed such that reinforcing fibers  123  which are carbon fibers or glass fibers are substantially uniformly wound around the outer surface of the core  121  (first step). In this case, prepreg may be wound as is performed in the second embodiment. Then, as shown in FIG.  14 ( b ), and then the core  121  around which the reinforcing fibers  123  have been wound is introduced into a cylindrical cavity  127  of a mold  126  composed of an upper mold  124  and a lower mold  125 . 
     In the foregoing state, non-hardened resin, such as unsaturated polyester, epoxy resin or phenol resin or the molten resin  128  is, under pressure, injected through a resin injection port  126   a  of the mold  126 . Thus, the non-hardened resin or the molten resin  128  is filled into a portion between the cavity  127  and the core  121  and gaps in the core  121 . Thus, a resin layer  129  having the reinforcing fibers  123  embedded therein is formed (second step). 
     The resin layer  129  is hardened at room temperatures or with heat, and then the core  121  having the resin layer  129  is taken from the mold  126 . As described above, the core elements  121   a,  . . . , for constituting the core  121  are connected to one another by the connectors  122 . Therefore, injection of biochemically active substances  117 , such as bacteria or enzymes, into the core  121  at either end results in the biochemically active substances  117  being supplied to the core element  121   a,  the connectors  122  and the core element  121   a  in this sequential order. As a result, the core  121  is degraded (third step). 
     Therefore, a composite molded product  130  can be obtained which has independent cylindrical hollow portions  130   a  formed apart from one another for predetermined distances in the axial direction, as shown in FIG.  14 ( c ). The composite molded product  130  has partition wall  130   b  formed by the resin layer  129  charged between the core elements  121   a.  Therefore, the partition wall  130   b  serves as a bulkhead so that the composite molded product  130  has increased strength. 
     In the foregoing process, the various means for enhancing the degradation of the core may appropriately be employed. 
     FIG. 15 shows a fourth embodiment with which wings of an airplane in the form of a hollow structure are manufactured by a RTM (Resin Transfer Molding) method. As shown in FIG.  15 ( a ), grooves  132  are provided for the upper and lower surfaces of a core  131  in the longitudinal and lateral directions for forming ribs by machining, the core  131  being made of biodegradable polymers corresponding to the shape of a wing of an airplane. 
     The above-mentioned core  131  is employed in this embodiment. Reinforcing fibers  133  which are carbon fibers or glass fibers are wound around the outer surface of the core  131  to have portions of corresponding thickness (first step). In this case, prepreg may be wound as is performed in the second embodiment. Then, as shown in FIG.  15 ( b ), the core  131  around which the reinforcing fibers  133  have been wound is introduced into the wing-shape cavity  137  of a mold  136  composed of an upper mold  134  and a lower mold  135 . 
     In the above-mentioned state, non-hardened resin, such as unsaturated polyester, epoxy resin or phenol resin or the molten resin  138  is, under pressure, injected through a resin injection port  136   a  of the mold  136 . Thus, the non-hardened resin or the molten resin  138  is injected into a gap between the cavity  137  and the core  131  and into the groove  132 . As a result, a resin layer  139  having the reinforcing fibers  133  embedded therein is formed (second process). 
     The resin layer  139  is hardened at room temperatures or with heat, and then the core  131  having the resin layer  139  is taken from the mold  136 . Since the two lengthwise-directional ends of the core  131  are in contact with the end surface of the mold  136 , the two ends of the core  131  are exposed over the resin layer  139 . When biochemically active substances  117 , such as bacteria or enzymes, are injected into the core  131  (third step), the core  131  is degraded by the biochemically active substances. As a result, the resin layer  139  having the reinforcing fibers  133  embedded therein is left. 
     Therefore, as shown in FIG.  15 ( c ), a hollow composite molded product  140  having ribs  140   a  formed therein and formed into the wing shape can be obtained. In this embodiment, the two lengthwise-directional ends of the core  131  are brought into contact with the end surface of the mold  136  to cause the core  131  to expose over the resin layer  139 . When the two lengthwise-directional ends of the core  131  are made to be apart from the end surface of the mold  136 , a hollow resin layer  139  can be formed which has two closed ends. In this case, an opening is provided for a portion of the resin layer  139  so that biochemically active substances, such as bacteria or enzymes, are injected through the opening. 
     The manufacturing method according to the present invention is able to manufacture a structure having a further complicated shape. Although the wing in the form of the honeycomb structure has high strength and rigidity as described above, there arises a problem in that the honeycomb structure cannot easily be manufactured. FIG. 16 shows a method of manufacturing a hollow-structure wing having the honeycomb structure. 
     In FIG.  16 ( a ), reference numeral  211  represents a hollow spherical member made of biodegradable polymers, for example, microorganism type Biopol (trade name of Monsalt) in the form of a copolymer of hydroxybutyrate and valirate or Bionol (trade name of Showa Highpolymer) in the form of fatty acid polyester or polyester of succinic acid and butanediol/ethylene glycol. The foregoing biodegradable polymer is a polymer which is degraded by dint of the action of the biochemically active substances, such as oxygen. The spherical member  211  can be manufactured by blow molding or injection molding. An opening  211   b  is formed in a portion of the spherical-member body  211   a.  The diameter of the spherical-member body  211   a  is several millimeters to tens of millimeters. It is preferable that spherical members having a variety of diameters are employed in place of those having the same diameter. 
     A CFRP or GFRP resin layer is formed on the outer surface of the spherical member  211 . As a means for forming resin layer, reinforcing fibers  213  which are carbon fibers or glass fibers impregnated with non-hardened resin  212 , such as unsaturated polyester, epoxy resin or phenol resin, are substantially uniformly wound around the outer surface of the spherical member  211 , as shown in FIG.  16 ( b ). The non-hardened resin  212  are wound to have a thickness with which the fibers are slightly apart from each other (rough winding is required because the biochemically active substances, such as bacteria and enzymes cannot easily be introduced if the fibers are wound too closely). Thus, reinforcing-fiber spherical members  214  are formed (first step). 
     Then, as shown in FIG.  16 ( c ), a cavity  217  of a mold  216  composed of an upper mold  215   a  and a lower mold  215   b  is closely filled with the multiplicity of the reinforcing-fiber spherical members  214 . Then, the reinforcing-fiber spherical members  214  are heated or placed at room temperatures so that the non-hardened resin  212  is hardened. As a result, the reinforcing-fiber spherical members  214  are integrally combined with one another as the non-hardened resin  212  is hardened (second step). The reinforcing-fiber spherical members  214  having the same diameters may be closely filled. Reinforcing-fiber spherical members  214   a  having small diameters may be disposed in the outer peripheral portion of the cavity  217  and reinforcing-fiber spherical members  214   b  having large diameters may be disposed in the central portion of the cavity  217 . In this case, the reinforcing-fiber spherical members  214  are disposed at a high density in the outer layer, while the same are disposed at a low density in the inner layer. 
     After a spherical-member aggregate  218  composed of the multiplicity of the reinforcing-fiber spherical members  214  has been molded as described above, the spherical-member aggregate  218  is taken from the mold  216 . As shown in FIG.  16 ( d ), the spherical-member aggregate  218  is injected into a tank  220  accommodating biochemically active substances  219 , such as bacteria and enzymes, specifically muddy water containing microorganisms so that the spherical-member aggregate  218  is immersed in the biochemically active substances  219 . Then, the spherical-member aggregate  218  is allowed to stand for several days to several weeks so that the biochemically active substances  219  penetrate the reinforcing-fiber spherical members  214  to penetrate the spherical members  211  made of bio-degradable polymers disposed in the spherical-member aggregate  218 . As a result, the spherical members  211  are degraded (mainly into carbon dioxide and water) (third step). 
     After the spherical members  211  have been degraded, the residues of the spherical members  211  are discharged. Thus, a porous structure  221  composed of the reinforcing fibers  213  and the resin as shown in FIG.  16 ( e ) can be obtained. Then, the porous structure  221  is employed as the core, and then a surface plate  222  is joined so that, for example, a wing of an airplane made of the composite material is formed. 
     FIG. 17 shows a sixth embodiment which is different from the fifth embodiment in the method of forming the resin layer. That is, as shown in FIG.  17 ( a ), reinforcing fibers  213 , which are carbon fibers or glass fibers, are substantially uniformly wound around a spherical member  211  made of biodegradable polymers to have a thickness with which the fibers are slightly apart from each other (first step). Then, enzymes are added to the reinforcing fibers  213 . Then, as shown in FIG.  17 ( b ), the reinforcing fibers  213  are coated with non-hardened resin, such as unsaturated polyester resin, epoxy resin or phenol resin, or molten resin  223  (second step). As a means for adding the molten resin  223 , the non-hardened resin or the molten resin  223  accommodated in a tray  224  may be allowed to adhere to a roller  225  so as to be applied to the surface of the reinforcing fibers  213 . The molten resin  223  may be immersed in a molten-resin tank (not shown) together with the spherical members  211 . Note that the third step is similar to that according to the first embodiment. 
     FIG. 18 shows a seventh embodiment which is different from the fifth and sixth embodiments in the method of forming the resin layer. That is, as shown in FIG.  18 ( a ), prepreg  226  is wound around the outer surface of a spherical member  211  made of biodegradable polymers (first step). Then, as shown in FIG.  18 ( b ), the spherical members  211  around each of which the prepreg  226  has been wound, that is, a multiplicity of reinforcing-fiber spherical members  227  are closely filled. Then, the reinforcing-fiber spherical members  227  are heated to harden the resin in the prepreg  226 , causing the reinforcing-fiber spherical members  227  to integrally be combined with one another as the resin is hardened (second step). Note that the third step is similar to that according to the fifth embodiment. 
     FIG. 19 shows an eighth embodiment which is different from the fifth to seventh embodiments in the method of heating and hardening the reinforcing-fiber spherical members  227 . That is, as shown in FIG.  19 ( a ), prepreg  226  is wound around the outer surface of a spherical member  211  made of biodegradable polymers (first step). Then, as shown in FIG.  19 ( b ), the spherical members  211  around each of which the prepreg  226  has been wound, that is, a multiplicity of reinforcing-fiber spherical members  227  are closely filled into a cavity  217  of a mold  216  composed of an upper mold  215   a  and a lower mold  215   b.  Then, the reinforcing-fiber spherical members  227  are heated, causing the resin in the prepreg  226  to be expanded. As a result, adjacent reinforcing-fiber spherical members  227  press against one another so that gaps are plugged. Thus, the reinforcing-fiber spherical members  227  are formed into polygonal shapes each having a hexagonal or octagonal cross sectional shape. That is, irregular polyhedrons are formed and thus the reinforcing-fiber spherical members  227  are hardened. Moreover, the reinforcing-fiber spherical members  227  are integrally combined with one another as the resin is hardened (second step). Note that the third step is similar to that according to the fifth embodiment. 
     When the reinforcing-fiber spherical members  227  closely filled into the cavity  217  of the mold  216  are heated, air is sucked from the outside of the mold  216  to realize a vacuum state. Thus, effects can be obtained in that heat expansion of the resin can be enhanced and degree of adhesion among the reinforcing-fiber spherical members  227  to one another can be raised. When each of the spherical members  211  is formed into a hollow structure and air or volatile liquid, a foaming agent generating a gas by heating, or the like is previously filled, the fluid is expanded when the temperature is raised. As a result, expansion of the spherical members  211  is enhanced and the internal pressure is raised. Because of the foregoing effects, the degree of adhesion among the reinforcing-fiber spherical members  227  to one another can be raised. 
     FIG. 20 shows a ninth embodiment having a structure that another reinforcing member  228  is added to a portion which requires highest strength is added to the method of manufacturing the hollow structure according to the fifth embodiment. When the reinforcing-fiber spherical members  214  are filled in the cavity  217  of the mold  216 , the reinforcing member  228  is placed in the cavity  217 . Thus, a hollow structure having satisfactory strength can be manufactured. 
     FIG. 21 shows a tenth embodiment for manufacturing a hollow structure having an excellent heat insulation characteristic such that a multiplicity of, for example, spherical heat insulating members  229   a  are filled to constitute a heat insulating layer  229 . In the foregoing case, a reinforcing-fiber layer similar to that formed around the spherical members  211  is previously formed around the spherical heat insulating members  229   a.    
     When the reinforcing-fiber spherical members  214  are filled in the cavity  217  of the mold  216 , the multiplicity of the spherical heat insulating members  229   a  are filled in the cavity  217  to form layers. Then, heating is performed so that the reinforcing-fiber spherical members  214  and the spherical heat insulating members  229   a  are combined with one another. Thus, a hollow structure having an excellent heat insulating characteristic can be manufactured. Although the description has been made about the structure in which the heat insulating material is injected, a sound absorbing material is employed in place of the spherical heat insulating members  229   a  when the structure must have a sound absorbing characteristic or a sound insulating characteristic. Thus, a structure made of the porous structure having an excellent sound absorbing characteristic can be obtained by a similar method. 
     When a heat insulating layer is attempted to be formed by the conventional honeycomb sandwich plate, a partitioned heat insulating layer cannot be formed because the heat insulating material is injected into the overall body of the honeycomb core in the direction of the thickness of the same because the honeycomb core has no partition in the direction of the thickness. When the method according to the tenth embodiment is employed, a heat insulating layer having an arbitrary thickness meeting a purpose can be formed. 
     In each of the above-mentioned embodiments, the spherical members are manufactured by blow molding or injection molding. Moreover, complete spheres are employed. The spherical members are not required to be complete spheres. Cubes having rounded corners or members each having an elliptic cross sectional shape may be employed. 
     A method of manufacturing a rib structure having hollow ribs, such as the wings of an airplane, will now be described. FIGS. 22 to  25  show an eleventh embodiment for manufacturing a wing of an airplane which is a hollow rib structure. 
     To form the outer surface of the wing of an airplane, a jig  321  having a surface formed into a concave shape corresponding to the outer shape of the wing of the airplane must be used. The jig  321  is formed to correspond to the size of the wing of an airplane. A plurality of layers of prepreg  322  serving as a base layer are stacked. After the prepreg  322  has been placed, cores  323  are disposed on the prepreg  322 . The core  323  is made of, for example, microorganism type Biopol (trade name of Zeneka) in the form of a copolymer of hydroxybutyrate and valirate or Bionol (trade name of Showa Highpolymer) in the form of fatty acid polyester or polyester of succinic acid and butanediol/ethylene glycol. The cores  323  can be degraded by the actions of the biochemically active substances, such as bacteria and enzymes. Therefore, an advantage can be realized to protect the global atmosphere. 
     The core  323  has a cross sectional shape which is, for example, rectangular shape as shown in FIG. 23. A tape-shape prepreg  324  is wound around the core  323 . The shape of the core  323  is not limited to the rectangle. When an isogrid structure is manufactured, the core  323  may be formed into a triangular shape. 
     The cores  323  around each of which the prepreg  324  has been wound are placed on the prepreg  322  which is the base layer. In this case, the adjacent cores  323  are closely disposed. 
     As shown in FIG. 24, the cores  323  are disposed on the prepreg  322 , and then one or more layers of the prepreg  326  are disposed. Then, prepreg  327  made of CF cloth and serving as a final layer is laminated. The CF cloth prepreg  327  has predetermined strength because continuous fibers are mixed. Moreover, spaces in which the ribs  328  are not formed are previously formed to correspond to the cores  323 . As a result, the prepreg  327  is disposed at the position corresponding to the upper surface of the ribs  328 . The prepreg  326  may be omitted. In this case, CF cloth prepreg  327  is directly disposed on the cores  323 . 
     The overall body of a product molded by an autoclave is usually covered with a heat-resisting film before the heat hardening process. Then, inside air is sucked to realize a vacuum state so as to raise the degree of adhesion among the elements and the prepreg. In the foregoing state, the temperature is raised to a high level. 
     Then, the jig  321  is heated to a predetermined temperature so that the cores  323  and prepreg  322 ,  324  and  326  stacked on the jig  321  are brought into hermetically contact with one another and integrated with one another. Since the prepreg  326  is hardened, the ribs  328  are constituted. 
     As shown in FIG. 25, awing component  329  having lattice ribs  328  formed on the inside portion of the curved surface can be obtained. Two wing components  329  are manufactured, and then the two wing components  329  are stacked in such a manner that the curved surfaces are disposed opposite to each other. Then, the outer peripheries of the two wing components  329  are connected to each other by bonding or welding or with a connecting member. Thus, a hollow structure A having the ribs  328  and serving as a wing can be obtained. 
     The shape of the core  323  is not limited to the rectangular shape or the triangular shape. For example, the core may have a structure composed of elongated members and columnar members acting as bridges between the elongated members. 
     To degrade and remove the cores  323  made of the biodegradable polymers, holes are formed at arbitrary positions of prepreg  322 ,  324  and  326  which cover the cores  323 . A water solution containing biochemically active substances, such as bacteria and enzymes, for example, microorganisms, is injected into the core  323  through the holes. When the cores  323  are allowed to stand for several days to several weeks in the foregoing state, the cores are degraded (mainly into carbon dioxide and water). 
     After the cores  323  have been degraded, the residues of the cores  323  are discharged through the holes. Thus, the portions in which the cores  323  have existed are formed into hollow portions. As a result, wing components  329  having hollow ribs  328  can be completed. As a result, a rib structure  320 , the weight of which can be reduced and which has required strength, can be formed. 
     The hollow structure A formed by stacking the wing components  329  manufactured by the above-mentioned manufacturing method is formed such that the wing components are integrally formed with the ribs. Therefore, the strength can be raised as compared with the strength of the wing components  329  which are joined to each other. Since the cores  323  are made of the biodegradable polymers, degradation of the biodegradable polymers results in only the prepreg  324  which has covered the biodegradable polymers is left. As a result, the hollow rib can be formed. As a result, the weight of the rib structure can be reduced. 
     When sheet or tape shape prepreg  322 ,  324  and  326  are stacked or wound, a wing member having a required shape and thickness can be formed. 
     A twelfth embodiment for manufacturing a wing of an airplane in the form of a rib structure similar to that according to the foregoing embodiment will now be described with reference to FIGS. 26 and 27. 
     In this embodiment, a rib structure  330  is formed by using a mandrel  331  formed to correspond to the internal shape of the wing so that the outer shape of the wing is formed. The mandrel  331  has grooves  333  each having a depth corresponding to the shape of the rib  332 . The grooves  333  are formed into a lattices shape in the surface of the mandrel  331 . Then, a CF cloth prepreg  334  is disposed in the bottom portion of the grooves  333 . After the CF cloth prepreg  334  has been disposed, the prepreg  335  is disposed on the bottom surface and in the inside portion of the mandrel  331 . 
     Cores  336  are disposed to correspond to the shapes of the portions covered with the grooves  333 . Similarly to the first embodiment, the cores  336  are composed of the cores  336  and prepreg  337  wound around the cores  336 . The cores  336  are disposed in the grooves in such a manner that the adjacent cores  336  are disposed closely. Then, the prepreg  338  is laminated from an upper position of the mandrel  331 , and then the foregoing elements are heated and hardened. Thus, the outer shape of the wing member can be formed. 
     After the rib structure  330  has been formed by heating and hardening, holes are formed at arbitrary positions of the prepreg  338  which covers the rib structure  330  or at the lengthwise ends of the mandrel  331 . Then, biochemically active substances, such as bacteria and enzymes, are introduced through the holes. 
     In the wing member formed by the above-mentioned manufacturing method, the mandrel  331  disposed in the wing member and made of the biodegradable polymers is degraded and removed after the wing shape has been formed. Thus, an excellent hollow wing member can be manufactured. Since the mandrel  331  can be degraded and allowed to disappear, a hollow shape can easily be manufactured. Moreover, the weight of the wing member can satisfactorily be reduced. 
     When fine portions at the end of the wing, that is, portions in which the inside ribs  332  are formed, are first stacked, a shape corresponding to the fine portions can be manufactured. When also the cores  336  are made of the biodegradable polymers, reduction of the weights of the ribs  332  is permitted. As a result, a hollow shape, the weight of which can be reduced, can easily be formed. 
     A thirteenth embodiment for manufacturing a wing of an airplane which is a structure similar to that according to the foregoing embodiment will now be described with reference to FIGS. 28 and 29. 
     When a rib structure  340  having ribs  346  each having leading formed into an inverted-T-shape is formed, the prepreg cannot easily be laminated as is performed in the second embodiment. In this case, grooves  342  each having a thickness and a width corresponding to the leading ends  343  of the ribs  346  are formed in the surface of the reference surface mold or jig  341 . Then, CF cloth prepreg  343  serving as the leading end of the rib is disposed in the groove  342 . 
     After the leading ends of the ribs  346  have been disposed in the grooves  342 , a ribbed structure mold or plate-like mandrel  344  of biodegradable polymer of a predetermined thickness and a shape (a rectangular shape in this embodiment) corresponding to a space surrounded by the CF cloth prepreg  343  is disposed from an upper position. Since the ribbed structure mold or plate-like mandrel  344  is disposed, the ends of the CF cloth prepreg  343  are secured by the ribbed structure mold or plate-like mandrel  344 . 
     Then, the narrow body portion molds or cores  345  are disposed from an upper position of the CF cloth prepreg  343  such that the narrow body portion mold or core  345  are disposed adjacently. Thus, the inverted-T-shape ribs  346  are provided. Also provided are a mold for the wide headed portion  351  and resin  350 . Then, the prepreg  347  is disposed from an upper position of the rib  346  to cover the rib  346 . Thus, the prepreg  347  and the narrow body portion molds or cores  345  are brought into contact with one another through the ribbed structure mold or plate-like mandrel  344 . The narrow body portion molds or cores  345  are made of the biodegradable mandrel similarly to the first and twelfth embodiments. Moreover, a tape-shape prepreg  348  is wound around the narrow body portion mold or core  345 . 
     After the covering prepreg  347  has been disposed, a whole laminate structure including the reference surface mold or jig  341  is heated so that the prepreg is melted and hardened. As a result, the outer shape of the wing can be formed. The position of the ribbed structure mold or plate-like mandrel  344  is fixed by a locating pin  349  to locate the position with respect to the reference surface mold or jig  341 . 
     Since the method of manufacturing the rib structure  340  having the above-mentioned structure is formed as described above, the grooves  342  are previously formed in the reference surface mold or jig  341 . When the grooves  342  are used, the rib structure  340  integrally having the ribs  346  formed into the inverted-T-shape can easily be formed. Since the integral forming process is employed, the strength of the rib structure  340  can be raised. 
     After the formed rib structure  340  has been detached from the reference surface mold or jig  341 , as in the above-mentioned embodiment, the ribbed structure mold or mandrel  344 , core material, etc., are removed through a breakdown by a bioactive material such as bacteria and enzyme and it is possible to obtained a compact, highly rigid rib structure. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.