Abstract:
A method of manufacturing a composite sandwich structure, the method comprising a preform assembly method. The method comprises providing a preform ( 8 ) comprising at least two sheets ( 12, 14 ) connected by a plurality of extensible threads ( 16 ), and affixing to the outer face of each sheet ( 12, 14 ), a face sheet ( 18, 22 ). The adhesion is by a film adhesive ( 20, 24 ) placed over the respective sheets ( 12, 14 ), and the preform ( 8 ) is subjected to pressure or heat to set the film adhesive ( 20, 24 ). The method further comprises a method of molding a composite sandwich structure, the method comprising placing a preform ( 8 ) in a mold ( 30 ), creating pressure differentials between respective chambers ( 40, 42 ) within the mold ( 30 ), feeding a resinous compound into the mold ( 30 ), and allowing the resinous compound to set and cure. The invention also provides composite sandwich structures made in accordance with the methods.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a 371 filing of International Patent Application PCT/AU02/01383, entitled “Method of Manufacturing Composite Sandwich Structures,” filed Oct. 11, 2002, which claims priority from Australian Patent Application PR 8214, filed Oct. 11, 2001, the contents of which are incorporated by reference herein in their entirety. 
     
    
     BACKGROUND  
       [0002]     Three-dimensional textiles for sandwich structures are known. Such textiles have two sheets connected by a plurality of extensible threads. During manufacturing, the extensible threads and the inner surfaces of the sheets are coated with a resin. The sheets are then moved apart by the required distance so that the extensible threads extend generally perpendicularly to both sheets. The resin is then allowed to cure, thereby creating a sandwich structure of relatively high stiffness yet being relatively light weight. Additional load carrying capacity, hence usefulness, may be added to this sandwich structure by adhering face sheets to the outer face of each sheet in a secondary process, the adhesion generally being achieved by the use of film adhesives, normally a thermoset adhesive.  
         [0003]     Difficulty has been experienced in achieving reasonable perpendicularity of the extensible threads and ensuring flat face sheets. The known processes for achieving this are slow and relatively complex, thus making the composite sandwich structure so produced relatively expensive. It also means the process is quite slow, making it difficult for mass production.  
         [0004]     Furthermore, the known manufacturing techniques are not suitable for large or complex shapes, thereby restricting their use.  
         [0005]     It is therefore an object of the present invention to provide a method of manufacturing composite sandwich structures which at least in part addresses the problems of the known methods.  
         [0006]     A further object is to produce a composite sandwich structure from the new method.  
       SUMMARY  
       [0007]     With the above and other objects in mind, the present invention provides a method of manufacturing a composite sandwich structure using a basic preform as its basis, this preform comprising at least two sheets connected by a plurality of extensible threads. The method consists of two stages.  
         [0008]     The first stage, known as the preform assembly method, is a technique to create the preform stacking sequence, and incorporate a bond between individual layers in this stacking sequence sufficient to withstand the loads and chemicals applied during molding, and during the design life of the created structure. A permanent bond is applied between layers incorporated in the final structure. A non-permanent bond is employed between outer face sheets of the final structure and caul plates (if used) or diaphragm, and between the caul plates (if used) and diaphragm.  
         [0009]     The preform assembly method involves affixing to the outer face of a first sheet and/or second sheet a face sheet, the adhesion being by a film adhesive placed over at least substantially all of each outer surface, and subjecting the preform to pressure and/or heat to set the film adhesive.  
         [0010]     Preferably, the film adhesive is one or more of a thermoplastic or thermoset material, a non-porous film, a net- or grid-like open structure, and/or a random filament. The film adhesive may create a permanent or non-permanent bond and will not react with the resin used to impregnate the preform.  
         [0011]     The face sheet may be of any suitable material, and the material of the first face sheet may be different to that of the second face sheet. These variations of these suitable materials may include unimpregnated fabric(s) to be impregnated in-situ with the three-dimensional fabric, or hardened skin(s) such as metals of previously cured composite materials, etc., or caul plate(s) used to give dimensional stability and quality finish to the final molding. Also, no additional face sheets may be incorporated.  
         [0012]     The present invention also includes a composite sandwich structure preform produced by the above method.  
         [0013]     The present invention also provides a second stage in the method of manufacturing a composite sandwich structure, which is a molding method. During the description of this second stage, all sheets assembled against the first sheet during the preform assembly method will be collectively referred to as the first sheet. Likewise, all sheets assembled against the second sheet during the preform assembly method will be collectively referred to as the second sheet.  
         [0014]     The molding method includes the steps of: 
        (a) placing a preform (which may or may not be in accordance with the earlier described method) in a mold, the preform comprising a first sheet and a second sheet, there being a plurality of extensible threads extending between the first sheet and second sheet, the mold comprising a first portion and a second portion, the first sheet being in the first portion, and the second sheet being in the second portion;     (b) placing a diaphragm between the first sheet and the first portion of the mold, the diaphragm extending to at least an outer periphery of the first portion, there being a first chamber between the diaphragm and the first portion, and a second chamber between the diaphragm and the second portion;     (c) non-permanently adhering to the first sheet of the preform to the diaphragm and non-permanently adhering the second sheet to the second mold portion;     (d) creating a pressure differential between the second chamber and the first chamber by reducing the pressure in the second chamber;     (e) using the pressure differential to feed a resinous compound into the second chamber to coat the extensible threads and surfaces of the first sheet and second sheet capable of being infused;     (f) ceasing the feeding of the resinous compound into the second chamber;     (g) ceasing the reduction of the pressure in the second chamber to substantially equalize the pressures in the first and second chambers;     (h) creating a second pressure differential between the second chamber and the first chamber by comprising the pressure in the first chamber below that in the second chamber to thus cause the diaphragm and hence, the first sheet to move away from the second sheet thereby causing the extensible threads to extend;     (i) maintaining the second pressure differential; and     (j) allowing the resinous compound to set and cure.        
 
         [0025]     Preferably, the second pressure differential at step (h) is created by applying sufficient vacuum pressure to the first chamber. Alternatively, or additionally, it may be created by applying a positive pressure in the second chamber. More preferably, the vacuum pressure is created by a first vacuum source.  
         [0026]     Preferably, the first chamber is vented during step (d), the venting ceasing upon the commencement of step (h).  
         [0027]     More preferably, the second chamber is vented during step (h). This venting may be slow, or rapid.  
         [0028]     Advantageously, the reduction of the pressure in step (d) is by means of the application of the vacuum source, preferably to a second vacuum port in the second portion of the mold. Alternatively, a second vacuum source may be used. More preferably the feeding of the resinous compound is by at least one inlet port in the second portion. Alternatively, the at least one inlet port may be between the first and second portions.  
         [0029]     More advantageously, the vacuum port is located at or adjacent to the center of the second portion; the at least one inlet port being located outside the periphery of the preform.  
         [0030]     Preferably, the vacuum source is connected to the vacuum port as well as the first chamber by separate tubes, the separate tubes being separately openable and closable by appropriate devices such as, for example, clamps, valves, or the like. Alternatively, the inlet port may be at a first end of the second portion and the vacuum port at a second end of the second portion, the preform assembly being located between the first end and the second end.  
         [0031]     More preferably, the connection to the first chamber is by means of a first vacuum port in the first portion. Advantageously, the first vacuum port is also substantially centrally located.  
         [0032]     Advantageously, heat is applied for a predetermined time during step (j). Preferably, the heat is applied by heating the first portion and/or the second portion. Alternatively, the heat is applied by introducing heated air into the second chamber. The heated air may be introduced through the at least one inlet port and/or the second vacuum port and/or a further inlet part. Further alternatively, the heat may be applied by placing the mold in a heated oven.  
         [0033]     The first sheet, second sheet and extensible threads form a three-dimensional fabric, which may be of any suitable material such as, for example, glass fibre, carbon fibre, or any other suitable textile material.  
         [0034]     Preferably, a caul plate is releasably adhered to the diaphragm above the first sheet, and to the preform.  
         [0035]     The resinous compound may be fed using a positive pressure. A carrier mesh may be used. If a carrier mesh is used it is preferably attached to the inlet ports and carries the resinous compound to the preform.  
         [0036]     The resinous compound may be of any suitable type such as, for example, a vinylester, phenolin, epoxy, or the like, resin. A carrier may be used, if desired.  
         [0037]     The diaphragm may also extend around the preform, in the form of a bag. In that instance, the vacuum inlet and/or outlet ports may pass through the bag.  
         [0038]     The mold may be any mold which is capable of incorporating a diaphragm and of holding vacuum in a cavity under the diaphragm sufficient to conduct a successful infusion of the preform assembly. This includes a sealed, fully closed mold, a base plate-diaphragm combination, an envelope bag, or any other suitable means.  
         [0039]     The mold preferably incorporates the means for the infusion of the preform assembly with resin.  
         [0040]     The mold advantageously includes means for creating a pressure differential on either side of the diaphragm and is capable of maintaining the pressure differential until the sandwich structure is cured.  
         [0041]     The mold preferably includes means for supporting the predetermined, final component dimensions. This may include: 
        caul plate(s) in the internal cavity (usually limited in separation distances by an external system);     shaped mold(s) external to the diaphragm; and/or     in the case of hardened face sheet(s), an external system to limit the extension between said face sheet(s).        
 
         [0045]     Alternatively, the cavity dimensions of the mold (between the first and second portion) may be mechanically controlled and varied by the use of mechanical devices such as actuators. In this way the first and/or second portions may be relatively moveable to cause the separation of the first sheet and second sheet and thus the extension of the extensible threads. This feature may also be used to control cavity dimensions at the resin injection stage (e). This may result in the non-use of the diaphragm. 
     
    
     FIGURES  
       [0046]     In order that the invention may be fully understood, there shall now be described, by way of non-limiting example only, preferred embodiments of the present invention, with reference to the accompanying illustrative drawings in which:  
         [0047]      FIG. 1  is a schematic side view of a typical preform;  
         [0048]      FIG. 2  is a schematic plan view of the preform assembly of  FIG. 1  during the preform assembly method;  
         [0049]      FIG. 3  is a schematic side view of the preform under vacuum prior to infusion;  
         [0050]      FIG. 4  is a schematic side view of the preform at full height within the mold;  
         [0051]      FIG. 5  is a schematic side view of the mold prior to insertion of preform;  
         [0052]      FIG. 6  is a schematic side view corresponding to  FIG. 5  after insertion of the preform;  
         [0053]      FIG. 7  is a top view corresponding to  FIG. 6 ;  
         [0054]      FIG. 8  is a schematic view of components of the external molding assembly used with the embodiment of  FIG. 11 ;  
         [0055]      FIG. 9  is a schematic side view showing an alternative mold design;  
         [0056]      FIG. 10  is a schematic side view prior to infusion;  
         [0057]      FIG. 11  is a schematic side view of a second embodiment; and  
         [0058]      FIG. 12  is a schematic side view of an alternative preform 
     
    
     DESCRIPTION  
       [0059]     As used in this disclosure, reference to a vacuum source is to be taken to include a source of suction, and includes a suction or vacuum source or sources, or any other source of suction or vacuum.  
         [0060]     To refer to the drawings,  FIG. 1  shows a typical preform assembly  8  created by the Preform Assembly Method (PAM) comprising a distance fabric  10  which has a first sheet  12  and second sheet  14  comprising extensible threads  16  extending therebetween. A first face sheet  18  is attached to first sheet  12  by a first thermoadhesive film  20 ; and a second face sheet  22  is attached to a second sheet  14  by a second thermoadhesive film  24 .  
         [0061]     The attachment of the face sheets  18  and  22  to the distance fabric  10  occurs during PAM. The PAM process depends strictly on the requirements of the adhesive film used to attach the adjacent sheets. The process to be described is for the adhering of the face sheets  18  and  22  to the distance fabric  10  using two thermoplastic adhesive films  20  and  24  requiring heat and pressure for correct application.  
         [0062]      FIG. 2  shows the preform  8  in place on a flat base plate  1 . Over the plate is placed a vacuum diaphragm  2 . This diaphragm  2  extends beyond the perimeter of the flat base plate  1 . Under the diaphragm  2  is placed a vacuum port  36 , breather material  35 , and thermocouple schematically shown as  3 , the thermocouple being to measure the temperature of the adhesive films  20  and  24 . The diaphragm  2  is sealed against the base plate  1  using a sealing bead  4  of mastic. The assembly is then placed in an oven. A vacuum source is applied to the vacuum port  36  to give an appropriate consolidation pressure for the adhesive films  20  and  24 . The oven heats the adhesive films to the appropriate bonding temperature. Alternatively, heated pressure rollers and/or a heated press (with or without pressure) may be used. When the appropriate temperature is shown by the thermocouple  3 , the temperature is maintained for the recommended bond time for the adhesive film. The base plate  1  and preform assembly  8  are then removed from the oven and allowed to cool. Upon cooling, the preform is removed from the base plate  1  and checked for good adhesive film bonding. If the bond is adequate, the preform  8  is ready for infusion.  
         [0063]     During the following description, all sheets assembled against the first sheet  12  during the preform assembly method will collectively be called the first sheet  12 , and all sheets assembled against the second sheet  14  during the preform assembly method will collectively be called the second sheet  14 .  
         [0064]     The preform  8  is located in a second portion  26  of a mold generally designated as  30 , which also has a first portion  28  ( FIG. 4 ). The second portion  26  has a peripheral frame  32 , to which is applied a sealing bead  34 . Between first face sheet  18  and first portion  28  there is located a diaphragm  38 , which extends to and beyond plate  32  and over the sealing bead  34 . The first face sheet  18  is temporarily bonded to the diaphragm  38 . As is shown in  FIGS. 5 and 6 , rather than peripheral frame  32  there may be provided a tapered frame  48  around the periphery of second portion  26  to aid the deformation of the diaphragm  38 . Both frame  32  and frame  48  assist to create a first chamber  40  between diaphragm  38  and first portion  28 , and a second chamber  42  between diaphragm  38  and second potion  26 . Seals  44  may be provided in addition to, or in place of, sealing bead  34 .  
         [0065]     Both first portion  28  and second portion  26  have a vacuum ports  36  which, as shown in  FIG. 5 , may be centrally located in the relevant portions  26 ,  28 . Resin inlet ports  46  are provided in second portion  26 , although they may be in frame  48 , if desired. The resin inlet ports  46  are preferably located between the periphery  50  of preform  8 , and the inner edge  52  of frame  32 , or inner edge  54  of tapered frame  48 . As will be realized from the above description, the frame  48  is used when the frame  32  is not used, and vice versa.  
         [0066]     Therefore, upon first portion  28  engaging with second portion  26 , diaphragm  38  seals on sealing bead  34  and is secured between the peripheries of the first and second portions  26 ,  28 . Suction (vacuum) is then applied to vacuum port  36  in second portion  26 , so that the preform  8  will be securely drawn into second portion  26 , as will be the attached diaphragm  38  ( FIG. 3 ). With an appropriate resinous compound (not shown) being fed into second chamber  42 , the reduced pressure in second chamber  42  will draw the resinous compound into and through the chamber  42  and the preform  8 . The resinous compound may have a positive pressure applied to it, if desired. If a positive pressure is used, a higher positive pressure must be applied in the first chamber  40  to restrict the diaphragm  38  from rising. As diaphragm  38  presses on first face sheet  12 , and as second face sheet  14  presses on second portion  26 , the resinous compound is drawn through the preform  8  and thereby coats the infusible surfaces of those sheets, as well as the extensible threads  16  extending therebetween. Any surplus resinous compound will exit through vacuum port  36 , where it can be recovered in an appropriate trap.  
         [0067]     The feed of the resinous compound continues until all threads  16  and the infusible surfaces of first sheet  12  and second sheet  14  are coated. This time will depend on the nature of the resinous compound, the sheets  12 ,  14  the number and size of threads  16 , and the size of the preform  8 .  
         [0068]     When the resinous compound has fully infused the preform  8 , the resin inlet ports  46  are closed and the vacuum port  36  in second portion  26  is closed. The suction (vacuum) is then applied to vacuum port  36  in first portion  28 . The second chamber  42  is simultaneously or earlier vented to the atmosphere by the vent port  37 , and/or by resin inlet ports  46 , so that second chamber  42  returns to atmospheric pressure. The vent port  37  may, if desired, be through frame  48 . Alternatively, a positive pressure can be applied to second chamber  42  so that the first chamber  40  is at a relatively lower pressure. By virtue of the vacuum applied to vacuum port  36  in first portion  28 , first chamber  40  is of reduced pressure. This therefore creates a pressure differential between first chamber  40  (low pressure) and second chamber  42  (higher pressure), causing diaphragm  38  to be drawn towards first portion  28 , thus drawing first sheet  12  upwardly and away from second sheet  14  which is non-permanently adhered to the second mold portion  26  so that it will not move relative thereto during the molding process. Therefore, the extensible threads  16  are extended. The distance between the first and second face sheets  12 ,  14  is that which is desired, as set by the mold cavity height (the sum of the heights first chamber  40  and second chamber  42 ). This height is usually predetermined by the height of frame  32  or frame  48 . Hence, due to the pressure differential the diaphragm  38  and first sheet  12  are drawn up to the first mold portion. The suction (vacuum) is maintained in the vacuum port  36  in first portion  38 , and second chamber  42  is sealed to allow the resinous compound to set and cure.  
         [0069]     Alternatively, self-foaming resins can be used to infuse the preform. In this case, the pressure differential between the second chamber  42  and the first chamber  40  is created by the foaming of the resin. In this case, after the resinous compound has fully infused the preform  8 , the resin inlet ports  46  are closed and the vacuum port  36  in second portion  26  is closed. The second chamber  42  is then vented to the atmosphere by the vent port  37  and/or by resin inlet ports  46 , so that the second chamber  42  returns to atmospheric pressure. The first chamber  40  remains at atmospheric pressure. The foaming reaction of the resin, initiated chemically or by the application of heat, creates the pressure differential between the first chamber  40  (low pressure) and the second chamber  42  (high pressure), causing diaphragm  38  to be drawn towards the first portion  28 . Thus, the first sheet  12  is drawn upwardly and away from the second sheet  14 . The distance between the first and second face sheets  12 ,  14  is that which is desired, as set by the mold cavity height. The penetration of the first sheet  12  by the foaming resin is resisted by the bonding of the impermeable diaphragm  38  directly on the first sheet  12  during preform assembly. The penetration of the second sheet  14  is similarly resisted by the bonding of the impermeable second portion  26  directly onto the second sheet  14 . When an appropriately engineered foaming resinous compound is used, an excellent quality infusion and finish in the skin in combination with a quality foam core will result. When using this type of foaming resin to create a sandwich structure, the distance fabric  10  may or may not be left out of the preform assembly  8  and, hence, the resulting sandwich structure. In the case of the distance fabric  10  being left out of the preform  8 , the first sheet  12  is bonded to the diaphragm  38  and the second sheet  14  is bonded to the second portion  26 , but they are not bonded to each other. This allows for their separation during the foaming of the resin. Maximum separation distance is again set by the mold cavity.  
         [0070]     If desired, the mold  30  may be heated to assist the setting and curing of the resinous compound. Heating may be by heating elements being placed in mold  30 , or by placing mold  30  in an oven. Alternatively, hot air could be introduced to second chamber  42  through resin inlet ports  46  and/or venting port  37  in second portion  26 .  
         [0071]     Upon setting, and preferably curing, of the resinous compound, the mold  30  is separated, diaphragm  38  removed, and the expanded composite structure removed.  
         [0072]     If desired, external caul plates  56  and  60  ( FIG. 8 ) may be used, particularly for the embodiment of  FIG. 11 . For the embodiment of  FIG. 11  clamping bars  62 , spaced apart by spacer elements  64 , are used to support the separation of the external caul plates  56  and  60 , which in turn give the desired final dimensions of the preform  8  during the later stages of the infusion process and during curing processes. The vacuum ports  36  and/or inlet ports  46  and/or vent port  37  may be beyond the periphery of the preform  8 , if desired. However, the vacuum port  36  and inlet ports  46  should not be adjacent.  
         [0073]     The diaphragm  38  is preferably a non-porous film capable of holding a vacuum of the order of 100 kPa. It may be elastic or semi-elastic. Examples of suitable materials include silicon rubber sheet, latex rubber sheet, and a nylon bagging film, etc. As the resinous compound may contaminate the diaphragm  38  during infusion and/or expansion of the preform  8 , the diaphragm  38  and the resinous compound should be such that there is no chemical interaction between them.  
         [0074]     If desired, a caul plate  59  may be used ( FIG. 6 ). The caul plate  59  is placed between the first sheet  12  and the diaphragm  38  to aid control, and/or to improve the surface quality of first sheet  12 . In addition, the use of caul plate  59  may assist in reducing peeling-off effects as the diaphragm  38  may initially stretch over the entire area of the caul plate  59 . Furthermore, the caul plate  59  may slightly enhance the infusion of the resinous compound as improved flow paths may result. The caul plate  59  is preferably at least as large as the preform  8  and may, if desired, be releasably or securely attached to the diaphragm  38  or first sheet  12  by, for example, double-sided tape. The attachment to the caul plate  59  may also occur during preform assembly method.  
         [0075]     Furthermore, a carrier mesh  58  may also be used to assist the resinous compound to pass from the inlet ports  46  to the preform  8 . The carrier mesh  58  is attached to the interior of second portion  26  at or adjacent the inlet ports  46  and extends to and along the side edges of preform  8 . The carrier mesh  58  may extend totally or partially around the periphery of preform  8 .  
       EXAMPLE  
       [0076]     One example of the production of a composite sandwich structure in accordance with an embodiment of the present invention is described below.  
         [0077]     Preform Assembly Method  
         [0078]     The surface of plate  1  is cleaned with acetone, with the surface being flat and free of debris and lumps.  
         [0079]     A single layer of non-perforated adhesive film is cut to dimensions identical to the preform  8 . This layer  20  is to provide a bond between the outer preform surface  18  and the bagging film  38 . Layers of perforated adhesive film  20  are cut to provide a bonding/interleaf layer between all preform-to-preform surfaces.  
         [0080]     The lower preform surface  22  is placed on plate  1 . Alternate layers of perforated adhesive film  20  and preform fabrics are then placed above lower sheet  22  as required. A layer of the perforated adhesive  20  is placed on all preform-to-preform surfaces to bond the preform surfaces and also allow resin to move between the surfaces. A layer of non-perforated adhesive film is placed on top of preform  8 .  
         [0081]     Mastic tape or other similar sealant is applied to the tooling plate, outside of the perimeter of the preform  8 . A thermocouple  3  is placed on the edge of the preform  8 , such that it is contacting the adhesive film  20 .  
         [0082]     The vacuum source fittings are connected to the vacuum port  36  in plate  1 . A full vacuum (˜100 kPa) is applied to enable checks of vacuum leaks.  
         [0083]     The oven is preheated to the required temperature for adhesive bonding (or just above bonding temperature), and the vacuum is set to the bonding pressure. The plate  1  with preform  8  is placed in the oven and heated until the thermocouple  3  shows that the thermoadhesive film  20 ,  24  has reached its/their bonding temperature. The temperature is held for the appropriate time.  
         [0084]     The plate  1  and preform  8  are removed from the oven and allowed to cool. The vacuum fittings and thermocouple  3  are removed. The bagging/preform is removed from the plate  1  without peeling the bag from the preform  8 . The excess bag/preform is removed to give a net-shape preform with bagging on the skins.  
         [0085]     Liquid Molding Preparation  
         [0000]     All mold surfaces (inside of mold, caul plate) are cleaned with acetone, with there being no remains of resin flash on mold surfaces and resin inlet channels.  
         [0086]     All required hoses are fastened to the appropriate ports in the mold by using compression fittings.  
         [0087]     Assembly  
         [0088]     Strips of double-sided tape are applied to the edges of both sides of the preform  8  and the caul plate  59 . The preform  8  is fixed to the caul plate  59  using this tape. The preform  8  is placed in the center of the second portion  26  of the mold and pressed to achieve a proper bond. The carrier mesh  58  is attached to both inlet ports  46 . Mastic tape is applied to the outside of the mold, using extra strips to seal the corners. The diaphragm  38  is fixed to the mold plate, hence, sealing the mold.  
         [0089]     Process Set-up  
         [0090]     The vacuum and the first venting hose are connected to a resin trap to catch any excess resin. The vacuum source is then attached to the resin trap lid and the resin inlet and the venting hoses are clamped. The lid is closed and securing bars placed in position, and fastening bolts gently tightened. Full vacuum is applied for infusion.  
         [0091]     Liquid Molding Procedure  
         [0092]     An appropriate amount of resin is mixed for a predetermined gel time. The two resin inlet hose(s) are inserted into the cup. The infusion time is recorded. At the first sign of resin in the exit line, the vacuum pressure is reduced to 40 kPa for 3 minutes. The resin inlet lines and exit line are then closed. A full vacuum is then applied to the resin trap, and, after connecting in the vacuum line of the lid directly to the vacuum, full vacuum is applied. The venting hoses are opened and the vacuum in the resin trap reduced to zero at a rate of 2 kPa/sec. The resin lines are removed, and the resin cup and excess hoses also removed. After approximately 2 hours the first venting hose is closed and 5 kPa vacuum applied to the first venting hose. This ventilates the mold and accelerates the curing by replacing the styrene. Curing for several hours is allowed. The clamps are then removed and excess lines cut. The clamping bars are removed from the tool and the lid opened. The diaphragm is removed and the tooling plate released from the expanded structure. The expanded structure is removed from the mold.  
         [0093]     Although the present invention has been discussed in considerable detail with reference to certain preferred embodiments, other embodiments are possible. Therefore, the scope of the appended claims should not be limited to the description of preferred embodiments contained in this disclosure. All references cited herein are incorporated by reference to their entirety.