Patent Publication Number: US-2021187463-A1

Title: Autoclave molding apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to an autoclave molding apparatus for use in forming composite material molded articles used in aircraft, automobile and other general industries. 
     BACKGROUND ART 
     A molded article having a desired cross-sectional shape can be obtained by a known technique, in which sheets of prepreg, i.e., a composite material produced by impregnating a reinforcing material, such as carbon fiber, aramid fiber, or glass fiber, with a thermosetting resin matrix, such as epoxy resin or phenol resin, are heated and compression-molded. Carbon fiber and glass fiber have high elasticity. Therefore, these fibers are formed into thin fiber sheets, which are laminated such that the fibers in the adjacent sheets are oriented in different directions to obtain a composite material. Since lightweight and strong products can be obtained from such composite materials, these materials are widely used in aircraft, automobile and other general industries. A composite material having a thermosetting resin matrix is soft at room temperature, but when it is heated, it is cured at elevated temperatures above a predetermined temperature while generating heat of reaction. 
     An autoclave apparatus disclosed in Patent Literature 1 (Japanese Unexamined Patent Application Publication No. 2009-51074) below is used as an apparatus for molding these composite materials. As shown in  FIG. 7 , a composite material  101  is sealed together with a molding tool  102  in a heat and pressure resistant vacuum bag  103  and then the entire assembly is placed in a cylindrical vessel  105  through a door  104 . The air in the cylindrical vessel  105  is heated by a heater  106  to raise the temperature of the composite material  101  to a predetermined temperature. The pressure increases as the temperature increases. If higher pressure is needed, a compressed gas is introduced from outside (not shown) to generate a pressure difference between inside and outside of the vacuum bag  103 . Due to this pressure difference, the composite material  101  is pressed against the molding tool  102  and formed into the shape of the molding tool  102 . During this molding process, the heating gas is circulated by a fan  107  so that the composite material  101  can be heated uniformly. The composite material  101  is maintained at the predetermined temperature for a predetermined period of time until it is cured and the molding process is completed. Then, the composite material  101  is cooled. Cooling water is supplied from outside to cool the circulating gas through a cooler  108  and thereby cool the composite material  101 . 
     However, since heat is transferred through the gas in both the heating process and the cooling process, these processes take a lot of time, which results in a decrease in productivity. This is a problem to be solved. 
     In order to solve this problem, an autoclave apparatus disclosed in Patent Literature 2 (Japanese Unexamined Patent Application Publication No. 2012-153133) below is used. This apparatus is described with reference to  FIG. 8 . In this apparatus, steam is used as a heating medium instead of circulating air. A composite material  111  is sealed together with a molding tool  112  in a vacuum bag  113  and then the entire assembly is placed in a cylindrical vessel  115  through a door  114 . High-temperature and high-pressure steam is supplied into the cylindrical vessel  115  through a steam supply pipe  116 . The steam flows directly to the vacuum bag  113 , condenses on the top surface of the vacuum bag  113 , the bottom surface of the molding tool  112 , etc., releases a large amount of heat of condensation to the top surface of the vacuum bag  113  and the bottom surface of the molding tool  112 , and thereby raises the temperature of the composite material  111  by heat transfer. Concurrently, due to a pressure difference between the steam and the interior of the vacuum bag  113 , the composite material  111  is brought into close contact with the molding tool  112  and thus molded. When the steam condenses into water, its volume is reduced to about one hundredth or less. Therefore, fresh steam flows into a region where the condensation occurs, and the composite material  111  and the molding tool  112  are heated rapidly and at a uniform temperature. Therefore, a device for circulating the steam, such as a fan, is not needed. The composite material  111  is maintained at a predetermined temperature until it is cured and the molding process is completed. Then, the composite material  111  is cooled. Cooling is performed by spraying externally supplied cooling water through a cooling water supply pipe  117  to directly cool the vacuum bag  113 . Thereby, the time required for the heating process and the cooling process can be reduced significantly. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] Japanese Unexamined Patent Application Publication No. 2009-51074 
     [Patent Literature 2] Japanese Unexamined Patent Application Publication No. 2012-153133 
     SUMMARY OF INVENTION 
     Technical Problem 
     As described above, heating by steam allows rapid heating and is suitable for molded articles with complex shapes. However, in order to improve this autoclave molding apparatus, it would be necessary to solve the following problems. 
     Composite materials generate heat of curing reaction (hereinafter called “curing heat”) while curing. Thermosetting resins as matrices of composite materials have low heat conductivity, while fibers as reinforcing materials have higher heat conductivity. Therefore, the heat conductivity in the thickness direction as the lamination direction is lower than that in the planar direction as the fiber direction. Since heat of condensation of steam is transferred mainly in the thickness direction, a temperature difference occurs in the thickness direction of the composite material. At the start of heating, due to the heat of condensation of steam, the surface temperature of the composite material is higher than the internal temperature thereof. As curing proceeds, due to the curing heat generated in the interior, the internal temperature becomes higher. In the molding process, not only the surface but also the interior must be below a predetermined temperature or lower. If the temperature of the composite material is too high, sufficient strength cannot be obtained. 
     In view of the above, heat must be adequately dissipated from the surface of the vacuum bag and the molding tool in the curing process. In the steam environment, heating is a transfer of heat of condensation of steam and thus has high performance in temperature rise, while heat dissipation is another type of heat transfer, i.e., convection of heat of steam. Due to the absence of a fan for circulating the steam, the convection velocity of the steam is low. In addition, the heat conductivity of the steam is low. As a result, the heat transfer is significantly reduced. Therefore, the surface temperature of the composite material when it generates heat while curing is higher than that in a conventional autoclave in which the composite material is heated by a gas. As a result, it is difficult to maintain the temperature of the composite material at a predetermined temperature or lower. 
     Accordingly, it is a primary object of the present invention to provide an autoclave molding apparatus capable of effectively reducing the overshoot of the temperature of a composite material caused by poor dissipation of curing heat in a curing process in which steam is used as a heating and compressing medium, and thereby capable of producing a composite material molded article having uniform and sufficient strength. 
     Solution to Problem 
     In order to solve the above problems, the present invention provides an autoclave molding apparatus  10  configured as shown, for example, in  FIG. 1  to  FIG. 6 . 
     More specifically, in the autoclave molding apparatus  10  for use in molding a composite material  12  composed of a fibrous base material and a matrix and sealed together with a molding tool  14  in a vacuum bag  16 , using steam for heating and compressing the composite material  12 , a condensed water holder  18  configured to hold condensed water generated during the heating is mounted on at least a portion of the vacuum bag  16  or at least a portion of the molding tool  14 , or the condensed water holders  18  are mounted on at least a portion of the vacuum bag  16  and at least a portion of the molding tool  14 . 
     According to the present invention, a portion of a large amount of condensed water that is generated on the surface of the vacuum bag  16  and/or the exposed surface of the molding tool  14  when the composite material  12  and the molding tool  14  are heated is held in the condensed water holder  18 , and heat generated during the subsequent curing of a thermosetting resin is used as latent heat of evaporation of the condensed water, so as to allow dissipation of heat with a small temperature difference. 
     As a result, not only an increase in the surface temperature of the composite material  12  but also an increase in the internal temperature of the composite material  12  can be suppressed, and a composite material  12  molded article having uniform and sufficient strength can be obtained. 
     Preferably, in the present invention, the condensed water holder  18  is made of at least one selected from the group consisting of a porous material  18 A, a mesh material  18 B obtained by knitting fibers, wrinkles  18 C of the vacuum bag  16  that are formed when the composite material  12  is sealed together with the molding tool  14  in the vacuum bag  16 , and a recessed portion  18 D formed on a surface of the molding tool  14  opposite to a surface thereof on which the composite material  12  is placed. 
     Preferably, the present invention further includes particular components to be described in the following embodiments. 
     Advantageous Effects of the Invention 
     According to the present invention, it is possible to provide an autoclave molding apparatus capable of effectively reducing the overshoot of the temperature of a composite material caused by poor dissipation of curing heat in a curing process in which steam is used as a heating and compressing medium, and thereby capable of producing a composite material molded article having uniform and sufficient strength. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of an autoclave molding apparatus according to an embodiment of the present invention. 
         FIG. 2  is a model diagram showing temperature changes at different positions during operation of the autoclave molding apparatus according to the present invention. 
         FIG. 3  is a perspective view of an example of a condensed water holder according to the present invention. 
         FIG. 4  is a schematic view of another example of the condensed water holder according to the present invention. 
         FIG. 5  is a cross-sectional view of another example of the condensed water holder according to the present invention. 
         FIG. 6  is a cross-sectional view of another example of the condensed water holder according to the present invention. 
         FIG. 7  is a schematic diagram of a conventional autoclave molding apparatus. 
         FIG. 8  is a schematic diagram of a conventional steam heating type autoclave molding apparatus. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described with reference to drawings.  FIG. 1  is a schematic diagram of an autoclave molding apparatus  10  according to an embodiment of the present invention. As shown in this figure, when a composite material  12  made of a fibrous base material impregnated with a thermosetting resin matrix and placed on a molding tool  14  is sealed in a vacuum bag  16  and the entire assembly is placed in a molding chamber  20   a  in a cylindrical vessel  20 , the autoclave molding apparatus  10  of the present embodiment is configured to mold the composite material  12  by heating and compression. 
     In the autoclave molding apparatus  10  of the present embodiment, steam of a predetermined temperature suitable for molding the composite material  12  is supplied, as a heat source, from a steam supply unit  22  to the molding chamber  20   a.  The vacuum bag  16  is evacuated by a vacuum pump (not shown) placed outside the molding chamber  20   a . Due to a pressure difference between the interior of the molding chamber  20   a  and that of the vacuum bag  16 , the composite material  12  is pressed against the molding tool  14  and formed into the shape of the molding tool  14 . In  FIG. 1 , the reference numeral  21  refers to a door to open or close the molding chamber  20   a  when the assembly including the composite material  12  is placed thereinto or the resulting molded article of the composite material  12  is removed therefrom. 
     The condensed water holders  18  having heat resistance to a temperature of 100° C. or higher (more preferably, 120° C. or higher) and made of a water holding material are mounted on the top surface of the vacuum bag  16  and the bottom surface of the molding tool  14 . The condensed water holders  18  each have a shape with an opening large enough to supply steam, which is supplied from a steam supply unit  22  for heating, to the surface of the vacuum bag  16  and the surface of the molding tool  14  (the condensed water holder  18  will be described in detail later). The steam introduced into the molding chamber  20   a  condenses on the low-temperature surfaces of the vacuum bag  16  and the molding tool  14  and releases a large amount of heat of condensation to the vacuum bag  16  and the molding tool  14 . This heat is transferred to the composite material  12  by heat transfer, and thereby raises the temperature of the composite material  12 . When the steam condenses into water, its volume is reduced to about one hundredth or less, and fresh steam is further supplied. Therefore, compared to heating by circulation of the air or the like, the steam spreads more rapidly, and as a result, the vacuum bag assembly can be heated through all surfaces exposed to the steam. In addition, since an enormous amount of heat is transferred when the steam condenses, heating can be achieved at a uniform temperature and with a small temperature difference between the steam and the surface. 
     The steam condenses into water, and the condensed water is held in the condensed water holder  18 . The amount of water that can be held in the condensed water holder  18  is proportional to its volume. Therefore, when the amount of the condensed water exceeds the water holding capacity, the excess water accumulates on the vacuum bag  16 . If the thickness of the accumulated condensed water increases, it serves as a thermal resistance and reduces transfer of condensation heat. However, since the condensed water holder  18  for practical use is not very thick, it has little effect on the transfer of condensation heat. In order to prevent the thickness of the accumulated condensed water from increasing, it is also effective to tilt the composite material  12  and the molding tool  14  when they are placed in the apparatus. 
       FIG. 2  is a model diagram showing temperature changes at different positions of the composite material  12  during heating and molding. A dotted line indicates a change in the surface temperature of the composite material  12 , a dash-dot line indicates a change in the internal temperature of the composite material  12  placed in a conventional autoclave molding apparatus (conventional art), and a solid line indicates a change in the internal temperature of the composite material  12  placed in the autoclave molding apparatus of the present invention (the apparatus of the present invention). 
     When the composite material  12  is heated and compressed to be formed into a desired shape using the autoclave molding apparatus  10  of the present invention, the surface temperature of the composite material  12  increases as the ambient temperature in the molding chamber  20   a  increases, but the internal temperature thereof increases with a time lag after the ambient temperature increases (see  FIG. 2 ). This is because the thermal conductivity of the composite material  12  is lower than that of the molding tool  14 . Therefore, the thicker the composite material  12  is, the longer the time lag of the temperature rise is. 
     Next, when the temperature of the molding chamber  20   a  reaches a target temperature, the amount of the steam supplied from the steam supply unit  22  is controlled to maintain the temperature inside the molding chamber  20   a  constant. The surface temperature of the composite material  12  is approximately equal to the ambient temperature of the molding chamber  20   a.  On the other hand, the internal temperature of the composite material  12  increases with a considerable time lag. The composite material  12  begins to cure when it is heated to a curing reaction temperature and generates heat of curing reaction. This curing reaction heat accelerates an increase in the internal temperature, and as the curing reaction proceeds, the internal temperature exceeds the surface temperature. 
     Here, the molding chamber of the conventional steam heating type autoclave molding apparatus is almost filled with steam. Without a steam circulating means, the heat of the vacuum bag  113  and the molding tool  112  is dissipated by natural convection of the steam, and the internal temperature of the composite material  111  far exceeds and overshoots the ambient temperature of the molding chamber (which is almost equal to the surface temperature of the composite material  12 ), as indicated by a dash-dot line in  FIG. 2 . Therefore, the desired physical properties may not be obtained at the internal temperature in the conventional apparatus shown in  FIG. 2 . 
     In contrast, in the autoclave molding apparatus  10  of the present embodiment, the condensed water is held in the condensed water holder  18 , and therefore the curing heat of the composite material  12  heats the condensed water and causes it to evaporate. Since the curing heat of the composite material  12  is thus consumed as the latent heat of evaporation of the condensed water, the heat of the composite material  12  can be dissipated with a small temperature difference. As indicated by a solid line in  FIG. 2 , the internal temperature of the composite material  12  does not far overshoot the target temperature, and the resulting molded article obtained from the composite material  12  can have desired physical properties (such as strength). 
     It should be noted that when the composite material  12  containing another thermosetting resin is used, the same effect can be obtained even if the temperature and pressure of the steam are changed depending on the curing temperature of the resin. The amount of condensed water that the condensed water holder  18  can hold can be easily obtained from the amount of heat required for curing the composite material  12  and the latent heat of evaporation of the steam. 
     When the above-mentioned curing process is completed, cooling water falls down onto the composite material  12  from a cooling water supply unit  24  to cool it. It should be noted that even if another method is employed in the cooling process, the same effect can be obtained in the present invention. 
       FIG. 3  is a schematic view of the condensed water holders  18  mounted on the top surface of the vacuum bag  16  and the bottom surface of the molding tool  14  (i.e. an exposed surface of the molding tool  14  opposite to a surface on which the composite material  12  is placed) in the autoclave molding apparatus  10  of the present embodiment. 
     These condensed water holders  18  are made of a porous material  18 A, e.g., a metal such as aluminum, iron, or iron alloy, or a heat-resistant resin such as polyester or nylon. The condensed water can be held in the pores. Here, when a metal is used as the porous material  18 A, a metal material having a rough surface obtained by blasting can be suitably used (as the porous material  18 A). 
     The porous material  18 A of the condensed water holder  18  is a lattice of rods with wide openings  19 A, so that the steam is more likely to come into contact with the surface of the vacuum bag  16  and the exposed surface of the molding tool  14 . The condensed water holder  18  having such a shape has stiffness high enough to prevent accidental detachment from a surface facing downward in the gravity direction, like the bottom surface (exposed surface) of the molding tool  14 , when it is mounted on such a downwardly facing surface. If the condensed water holder  18  cannot be attached to the bottom surface of the molding tool  14  due to the lack of stiffness, it can be attached to the molding tool  14  with an attachment member or the like. 
     The condensed water holder  18  is not limited to that configured as described above as long as it can hold condensed water effectively. For example, the condensed water holder  18  may be mounted on either the top surface of the vacuum bag  16  or the bottom surface of the molding tool  14 . The condensed water holder(s)  18  do not necessarily have to be mounted on the entire top surface of the vacuum bag  16  and/or the entire bottom surface of the molding tool  14 , and may be mounted on only a predetermined portion of the top surface and/or a predetermined portion of the bottom surface. 
     Not only the mounting area of the condensed water holder  18  may be modified as described above, but also the form thereof may be modified in the following way. 
       FIG. 4  is a schematic view of the condensed water holder  18  according to another embodiment (a second embodiment) of the present invention. This condensed water holder  18  is made of a mesh material  18 B obtained by knitting fibers of a heat-resistant fibrous material capable of holding condensed water, for example, a synthetic fiber such as polyester or nylon, a hydrophilic natural fiber such as cotton or linen, or a glass fiber. Such a mesh material has openings  19 B through which steam can pass easily. The amount of supplied water can be easily controlled by changing the diameter of the fibers of the fibrous material, for example. 
       FIG. 5  is a cross-sectional view of the condensed water holder  18  according to another embodiment (a third embodiment) of the present invention. 
     The composite material  12  made of a fibrous base material impregnated with a thermosetting resin matrix is sealed together with a molding tool  14  in the vacuum bag  16 . The size of the vacuum bag  16  used is slightly larger than that of the assembly covered by the bag  16  so that the bag  16  can enclose the resulting molded article having a shape conforming to the shape of the molding tool. Therefore, wrinkles  18 C are formed on the surface of the vacuum bag  16  and prevent the inner surface of the vacuum bag  16  from being closely contact with the surface of the composite material  12  and the surface of the molding tool  14 . When the vacuum bag  16  is sealed, the wrinkles  18 C are formed to have a shape for capturing the condensed water. The height of the wrinkles is determined based on the amount of water to be held. 
     The condensed water generated during heating is accumulated on the vacuum bag  16 , and when the amount of the condensed water exceeds a predetermined value, it overflows the ridges of the wrinkles  18 C. Therefore, the thickness of the condensed water is not large and thus the high efficiency of heat transfer during condensation can be maintained. When heat is generated during curing, the condensed water thus held evaporates and an increase in the temperature of the composite material  12  is suppressed. 
       FIG. 6  is a cross-sectional view of the condensed water holder  18  according to another embodiment (a fourth embodiment) of the present invention. 
     The composite material  12  made of a fibrous base material impregnated with a thermosetting resin matrix is sealed together with a molding tool  14  in the vacuum bag  16 . A recessed portion  18 D to serve as the condensed water holder  18  is preformed on a surface (top surface) of the molding tool  14  opposite to a surface on which the composite material  12  is placed. In the present embodiment, the assembly is disposed such that the surface of the composite material  12  faces downward in the gravity direction. 
     The condensed water generated during heating is accumulated in the recessed portion  18 D. When the amount of the condensed water exceeds a predetermined value, it overflows from the recessed portion  18 D. Therefore, the thickness of the condensed water is not large and thus the high efficiency of heat transfer during condensation can be maintained. When heat is generated during curing, the condensed water thus held evaporates, the molding tool  14  is cooled, and an increase in the temperature of the composite material  12  is suppressed by heat transfer. 
     As described above, since the condensed water holder(s)  18  configured to hold the condensed water generated during heating is mounted on at least a portion of the surface of the vacuum bag  16  and/or at least a portion of the surface of the molding tool  14 , a portion of the condensed water generated during heating is held therein. Then, using the subsequently generated curing heat of the composite material  12  for the evaporation of the condensed water, an increase in the temperature of the composite material  12  is suppressed and a composite material  12  molded article having sufficient strength can be obtained. 
     LIST OF REFERENCE NUMERALS 
     
         
           10 : Autoclave molding apparatus 
           12 : Composite material 
           14 : Molding tool 
           16 : Vacuum bag 
           18 : Condensed water holder 
           18 A: Porous material 
           18 B: Mesh material 
           18 C: Wrinkles (of vacuum bag) 
           18 D: Recessed portion 
           20 : Cylindrical vessel 
           22 : Steam supply unit 
           24 : Cooling water supply unit