Patent Publication Number: US-6211497-B1

Title: Induction consolidation system

Description:
The present application is a divisional application based upon U.S. patent application Ser. No. 08/169,655, filed Dec. 16, 1993, which was a continuation-in-part of U.S. patent application Ser. No. 07/777,739, filed Oct. 15, 1991, now U.S. Pat. No. 5,410,132, and of U.S. patent application Ser. No. 08/092,050, filed Jul. 15, 1993, now U.S. Pat. No. 5,410,133, which in turn is a divisional of U.S. patent application Ser. No. 07/681,004, filed Apr. 5, 1991, now U.S. Pat. No. 5,229,562. The benefit of the filing dates of which are claimed under 35 U.S.C. § 120; these applications and patents are incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the consolidation and forming of organic matrix composites, more specifically, the present invention relates to methods and apparatus for inductively heating, forming and consolidating resins to make organic matrix composites. 
     BACKGROUND OF THE INVENTION 
     Fiber-reinforced resin (i.e., organic matrix) composite materials have become widely used, have a high strength-to-weight or high stiffness-to-weight ratio, and desirable fatigue characteristics that make them increasingly popular in weight, strength or fatigue critical applications. 
     Prepregs consisting of continuous, woven, or chopped fibers embedded in an uncured matrix material are cut to the desired shape and then stacked in the desired configuration of the composite part. The prepreg may be placed (laid-up) directly upon a tool or die having a forming surface contoured to the desired shape of the completed part or the prepreg may be laid-up in a flat sheet and the sheet may be draped over a tool or die to form to the contour of the tool., 
     After being laid-up, the prepreg is consolidated (i.e., cured) in a conventional vacuum bag process in an autoclave (i.e., a pressurized oven). The pressure presses the individual layers of prepreg together at the consolidation/curing temperatures that the matrix material flows to eliminate voids and cures, generally through polymerization. 
     In autoclave fabrication, the composite materials must be bagged, placed in the autoclave, and the entire heat mass of the composite material and tooling must be elevated to and held at the consolidation or curing temperature until the part is formed and cured. The formed composite part and tooling must then be cooled, removed from the autoclave, and unbagged. Finally, the composite part must be removed from the tooling. 
     To supply the required consolidation pressures, it is necessary to build a special pressure box within the autoclave or to pressurize the entire autoclave, thus increasing fabrication time and cost, especially for low rate production runs. 
     Autoclave tools upon which composite materials are laid-up are typically formed of metal or a reinforced composite material to insure proper dimensional tolerances and to withstand the high temperature and consolidation forces used to form and cure composite materials. Thus, autoclave tools are generally heavy and have large heat masses. The entire heat mass of the tool must be heated along with the composite material during curing and must be cooled prior to removing the completed composite part. The time required to heat and cool the heat mass of the tools adds substantially to the overall time necessary to fabricate a single composite part. 
     In composite parts requiring close tolerances on both the interior and exterior mold line of the part, matched autoclave tooling must be used. When matched tooling is used, autoclave consolidation pressure is used to force the matched tooling together to consolidate the composite material and achieve proper part dimensions. Matched tooling is more expensive than open faced tooling and must be carefully designed to produce good results, adding to part fabrication costs. 
     An alternative to fabricating composite parts in an autoclave is to use a hot press. In this method, the prepreg is laid-up, bagged (if necessary), and placed between matched metal tools that include forming surfaces that define the internal and external mold lines of the completed part. The tools and composite material are placed within the press and then heated. The press brings the tools together to consolidate and form the composite material into the final shape. Fabricating composite parts in a hot press is also expensive due to the large capital expense and large amounts of energy required operate the press and maintain the tools. 
     Generally, in hot press operations, to obtain close tolerances, the massive, matched tooling is formed from expensive metal alloys having low thermal expansion coefficients. The tooling is a substantial heat sink that takes a large amount of energy and time to heat to composite material consolidation temperatures. After consolidation, the tooling must be cooled to a temperature at which it is safe to remove the formed composite part thus adding to the fabrication time. 
     Another contributor to the cost of fabricating composite parts is the time and manpower necessary to lay up individual layers of prepreg to form a part. Often, the prepreg must be laid up over a tool having fairly complex contours that require each layer of prepreg to be manually placed and oriented. Composite fabrication costs could be reduced if a flat panel could be laid-up flat and then formed into the shape of the part. 
     One method used to reduce the costs of fabricating composite materials is to lay up a flat panel and then place the flat panel between two metal sheets capable of superplastic deformation as described in U.S. Pat. No. 4,657,717. The flat composite panel and metal sheets are then superplastically deformed against a metal die having a surface contoured to the final shape of the part. Typically, the dies used in such superplastic forming operations are formed of stainless steel or other metal alloys capable of withstanding the harsh temperatures and pressures. Such dies have a large thermal mass that takes a significant amount of time and energy to heat up to superplastic forming temperatures and to cool down thereafter. 
     Attempts have been made to reduce composite fabrication times by actively cooling the tools after forming the composite part. These attempts have shortened the time necessary to produce a composite part, however, the time and energy expended in tool heat up and cool down remains a large contributor to overall fabrication costs. 
     The present invention is a method and apparatus for consolidating and forming organic matrix composites that avoid some of the above-identified disadvantages of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for consolidation of organic matrix composites using inductive heating. In the present invention, the dies or tooling for the organic matrix composite parts are made from a material that is not susceptible to inductive heating. Examples of usable tool materials are ceramics or resin composites. The tooling is strengthened and reinforced with fiberglass rods or other appropriate reinforcements to withstand the temperatures and pressures used to form the composite materials. Such materials decrease the cost of tool fabrication and also generally reduce the thermal mass and weight of the tooling. Since the tooling used in the present invention is not susceptible to inductive heating, it is possible to use the tooling in combination with inductive heating elements to heat the composite material. The present invention allows the composite material to be inductively heated without heating the tools significantly. Thus, the present invention can reduce the time and energy required to fabricate a composite part. 
     Graphite or boron reinforced organic matrix composites may be sufficiently susceptible because of their reinforcing fibers that they can be heated directly by induction. Most organic matrix composites require a susceptor in or adjacent to the composite material to achieve the necessary heating. The susceptor is heated inductively and transfers its heat to the composite material. 
     The present invention reduces the time and energy required to consolidate resin composite prepreg lay-ups for a composite. Because induction focuses the heat on the workpiece rather than the tool, there is less mass to heat or cool. Inexpensive composite or ceramic tooling can also be used. The lower operating temperature of the tools decreases problems caused by different coefficients of thermal expansion between the tools and the workpiece in prior art forming systems. The present invention also provides an improved method for fabricating composite parts to close tolerances on both the internal and external mold line of the part. 
     In a method for consolidating/or and forming organic matrix composite materials, an organic matrix composite panel is laid up and then placed adjacent a metal susceptor. The susceptor is inductively heated and then heats the composite panel by thermal conduction. A consolidation and forming pressure is applied to consolidate and form the organic matrix composite panel at its curing temperature. 
     Generally, the composite panel is enclosed between two susceptor sheets that are sealed to form a pressure zone around the composite panel. This pressure zone is evacuated in a manner analogous to convention vacuum bag processes for resin consolidation. The workpiece (the two susceptors and composite panel) is placed in an inductive heating press on the forming surfaces of dies having the desired shape of the molded composite part, and is pressed at elevated temperature and pressure (while maintaining the vacuum in the pressure zone) to consolidate the composite panel into its desired shape. 
     The workpiece may include three susceptors sealed around their periphery to define two pressure zones. The first pressure zone surrounds the composite panel and is evacuated and maintained under vacuum. The second pressure zone is pressurized to help form the composite panel. 
     One preferred apparatus for consolidating and forming the organic matrix composite panels uses ceramic or composite dies. An induction coil is embedded in the dies. When the coil is energized with a time varying current, induction heats the susceptors which in turn heats the composite panel by conduction. Pressure is applied to at least one side of the composite panel to consolidate and form it when it reaches the desired consolidation temperature. 
     It is preferred to use reinforced, cast phenolic or ceramic dies. Reinforcing rods are embedded within the dies to increase their strength by compressing the dies. The phenolic or ceramic dies may be reinforced with chopped reinforcing fibers, with a mat or weave of continuous fibers or with other reinforcements. The die usually includes a cavity adapted to receive a tool insert. The tool insert may include a forming surface that defines the shape of the molded composite part. In this way, different parts can be made simply by changing the insert rather than needing to replace the entire die. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an apparatus, for consolidating and forming organic matrix composite panels; 
     FIG. 2 is a schematic cross-sectional view of the apparatus of FIG. 1; 
     FIG. 3 is a perspective view illustrating the induction coil; 
     FIG. 4 is a cross-sectional view of a flexible coil connector; 
     FIG. 5 is a partially exploded, partially cut away view of a portion of the apparatus of FIG. 1; 
     FIG. 6A is an enlarged cross-sectional view of part of an organic matrix composite panel after it has been placed within the tooling but before consolidation and forming has begun; 
     FIG. 6B is an enlarged cross-sectional view of part of an organic matrix composite panel after consolidation and forming has begun; 
     FIG. 6C is an enlarged cross-sectional view of part of an organic matrix composite panel after consolidation and forming has been completed; 
     FIG. 7 is a flow chart showing an embodiment of the method of consolidation and forming of the present invention; 
     FIG. 8 is a partial perspective view of an organic matrix composite part including a close tolerance attachment section; and 
     FIG. 9 is an enlarged cross-sectional view of tooling for forming a close tolerance section in a part using matched tooling. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, the inductive heating and forming apparatus  10  includes tools or dies  20  and  22  mounted within an upper  24  and a lower  26  strongback, respectively. The strongbacks are each threaded onto four threaded column supports or jackscrews  28 . The jackscrews can be turned using a bellows or other actuation mechanisms to move the upper strongback or lower strongback up or down in relation to each other. 
     Each strongback  24  and  26  provides a rigid, flat backing surface for the upper and lower die  20  and  22  to prevent the dies from bending and cracking during repeated consolidation and forming operations. Preferably, the strongbacks should be capable of holding the dies to a surface tolerance of +/−0.003 inches per square foot of the forming surface in the toolbox. Such tolerances help to insure that proper part tolerances are achieved. The strongbacks may be formed of steel, aluminum, or any other material capable of handling the loads present during forming. However, materials that are nonmagnetic such as aluminum or some steel alloys are preferred to avoid any distortion to the magnetic field produced by the induction coils described below. In some circumstances, the dies may be strong enough without the strongbacks. 
     Both the upper  20  and lower  22  dies hold inserts  46  and  48  (FIGS. 2 and 5) and are reinforced with a plurality of fiberglass rods  32  (FIG. 5) that extend both longitudinally and transversely in a grid through each die. Each die may be attached to its strongback by any suitable fastening devices such as bolting or clamping. In the preferred embodiment, both dies are mounted on support plates  76  (FIG. 5) which are held in place on the respective strongbacks through the use of clamping bars  77  (FIGS.  1  and  5 ). The clamping bars  77  extend around the peripheral edges of the support plates  76  and are bolted to the respective strongbacks through the use of fasteners (not shown). 
     The dies are not susceptible to inductive heating. A composite or ceramic material that has a low coefficient of thermal expansion, good thermal shock resistance, and relatively high compression strength is preferred such as a castable fused silica ceramic. 
     A plurality of induction coils  35  (FIG. 1) extend longitudinally through the length of the upper and lower dies. In the preferred embodiment, four separate induction coils  35  are used, however, other numbers of induction coils could also be used. Each induction coil  35  is formed from a straight tubing section  36  that extends along the length of each die and a flexible coil connector  38  that joins the straight tubing sections  36  in the upper die  20  to the straight tubing sections in the lower die  22 . The induction coils  35  are connected to an external power source or coil driver  50  and to source of coolant by connectors  40  located at the ends of the inductive coils. 
     A composite panel is formed from prepreg laid-up on a contoured surface of a tool and is secured within the upper  20  and lower  22  dies as described in detail below. The upper  24  and lower  26  strongbacks and thus upper  20  and lower  22  dies are then brought together. The composite panel is then inductively heated to the consolidation temperature to promote resin flow and polymerization, as described in greater detail below. 
     Cavities  42  and  44  (FIG.  2 ), respectively, in the dies are sized to hold an upper  46  and a lower  48  tool insert. The upper tool insert  46  includes a contoured forming surface  58  that has a shape corresponding to the desired shape of the outer mold line surface of the composite part to be formed. The lower tool insert determines the inner mold line. The tool inserts generally are not susceptible to inductive heating. In the preferred embodiment, the tool inserts are formed of a castable dielectric phenolic or ceramic. 
     The tool inserts could be formed as an integral part of the dies. The separate die and tool insert configuration shown is preferred because it allows different tool inserts having different forming surfaces to be used in the same dies, simplifying the replacement task for changing the tooling and reducing the tooling costs. 
     Each die surrounds and supports the respective tool insert and holds the straight sections  36  of the induction coils in proper position in relationship to the tool insert. In the preferred embodiment, the interior  70  of the dies is formed of a castable phenolic or ceramic and the exterior sides of the toolboxes are formed from precast composite phenolic resin blocks  72 . In some applications, it is preferable to reinforce the phenolic or ceramic resins with chopped fibers or nonwoven or woven reinforcing mats. 
     To increase the strength of the phenolics or ceramics fiberglass reinforcing rods  32  are used. The rods  32  extend both longitudinally and transversely through the precast blocks  72  and the interior  70  and are then post-tensioned through the use of tensioning nuts  74  after casting the interior  70 . Post-tensioning the reinforcing rods  32  maintains a compressive load on the blocks  72 , interior  70  and the tool inserts to maintain the tolerances of the upper and lower tool inserts and to prevent cracking or damage of the dies are tool inserts during consolidation and forming operations. 
     The straight tubing sections  36  of the induction coils  35  are embedded within the dies, and extend parallel to the bottom surface of the respective tool inserts. The induction coils may be contained within the tool inserts. 
     In FIG. 2, a workpiece  60  including an organic matrix composite panel is shown already laid-up and placed between the upper  46  and lower  48  tool inserts. The detailed structure of the workpiece will be described in detail below. The workpiece  60  is heated to a forming and consolidation temperature by energizing the coils  35 . 
     When the workpiece  60  reaches the consolidation temperature at which the matrix flows, gas pressure is applied to the workpiece by pressure sources  52  and  54 . Pressure source  52  provides a pressure to the upper surface of the workpiece  60  through a conduit  62  that passes through the upper die  20  and upper tool insert  46 , while pressure source  54  applies a pressure to the lower surface of the workpiece  60  through a conduit  64  that passes through the lower die  22  and lower tool insert  48 . 
     In FIG. 2, the composite workpiece  60  is shown partially deformed upwardly toward the forming surface  58 . The pressure applied to the workpiece  60  is maintained until the workpiece has formed to the contour of the forming surface  58  and the matrix resin has consolidated. The pressure sources  52  and  54  may apply an equal or a differential pressure to both the upper and lower surfaces of the workpiece  60 . 
     Pin holes (not shown) may be formed in the upper and lower tool inserts to vent gas trapped between the workpiece  60  and the forming surface  58  as the workpiece deforms. Such pin holes can be coupled to a flow meter to monitor the progress of the workpiece&#39;s deformation. 
     When the workpiece  60  is formed and consolidated, the induction coils  35  are de-energized, and the pressure relieved. The upper  46  and lower  48  tool inserts and upper  20  and lower  22  dies are separated. The composite part is removed. 
     Inductive heating is accomplished by providing an alternating electrical current to the induction coils  35  within which the workpiece is positioned. This alternating current produces an alternating magnetic field in the vicinity of the inductive coils that heats the workpiece via eddy current heating. 
     Curved sections  84  extending between the straight sections  36  (FIG. 4) in the coils  35  are flexible, to accommodate the opening and closing of the upper  20  and lower  22  dies. The curved sections  84  and straight sections  36  are joined at fittings  86  into one or more induction coils or helixes to produce a magnetic field schematically illustrated by field lines  90  in FIG.  3 . Each straight section  36  and curved section  84  preferably comprises a copper tube having an interior longitudinal passage  96  through which a cooling fluid (such as water) may be pumped to cool the coils during operation. 
     FIG. 4 illustrates a preferred construction for each curved section  84 . Each curved section includes a pair of fittings  86 , each of which includes a relatively small diameter section  92  dimensioned to fit snugly within a straight section  36 , and a larger diameter flange  94 . The flange  94  regulates the distance the fittings  86  extend into the straight sections  36 . The passage  96  extends through each fitting  86 , each curved section  84 , and each straight section  36 . 
     A braided flexible conductor  98  extends through passage  96  and is joined between flanges  94  by a suitable method such as brazing or soldering. Finally, a flexible, insulating jacket  100  is placed around the conductor  98  and extends between the flanges  94  to contain the conductors and cooling fluid. One commercial vendor through which a suitable design cable can be obtained is Flex-Cable located at Troy, Michigan. 
     The frequency at which the coil driver  50  (FIG. 2) drives the coils  35  depends upon the nature of the workpiece  60 . Current penetration of copper at 3 kHz. is approximately 0.06 inches, while penetration at 10 kHz. is approximately 0.03 inches. The shape of the coil also has a significant effect upon the magnetic field uniformity. Field uniformity is important because temperature uniformity in the workpiece is directly affected by the uniformity of the magnetic field. Uniform heating insures that different portions of the workpiece will reach the forming and consolidation temperature of the composite material at approximately the same time. Solenoidal type induction coils provide a uniform magnetic field, and are therefore preferred. Greater field uniformity is produced in a workpiece that is symmetric around the center line of the induction coil. The additions of variations, such as series/parallel induction coil combinations, variable turn spacings and distances between the part and the induction coil can be established by standard electrical calculations. 
     Dielectric materials for the tool inserts and dies are generally thermally insulating Thus, the tool inserts and dies tend to trap and contain heat within the workpiece. Since the dies and tool inserts are not inductively heated, and act as insulators maintaining heat within the workpiece, the present invention requires far less energy to form and consolidate the composite panel than conventional autoclave or resistive hot press methods. 
     The forming operation of the present invention also takes much less time than prior art forming operations because time is not expended elevating the large thermal mass of either the dies or tool inserts prior to forming and consolidating the composite panel. Only the workpiece itself is heated by the coils. Thus, forming temperatures are achieved more rapidly and when the driver  50  is de-energized, the dies and the workpiece cool rapidly to a temperature at which the part may be removed, saving time and energy. In addition, the thermal cycle is not limited by the heating and cooling cycle of the equipment and tools so the thermocycle may be better tailored to the material used. 
     In FIG. 7, block  130 , a composite panel  110  is laid-up from individual layers of prepreg. The composite panel  110  is placed between a first sheet  112  and second sheet  114  of a susceptor (block  132 ) to form a workpiece. The susceptors are welded around the periphery thus forming a pressure zone  117  between the susceptors surrounding the composite panel  110 . The resulting workpiece is then placed within the upper  46  and lower  48  tool inserts, block  136 . 
     The periphery of the workpiece, including the area containing the sealing weld  116 , is clamped between the edges of the upper  46  and lower  48  tool inserts, to form a pressure zone  119  between the lower tool insert  48  and the workpiece  60  and a pressure zone  120  between the upper tool insert  46  and the workpiece  60 . 
     In cases where the lower tool insert  48  is formed of a material that is somewhat porous, it is advantageous to place a third susceptor  122  between the lower tool insert  48  and the first sheet  112  to serve as a pressure barrier between the workpiece  60  and the lower tool insert  48 . When a third susceptor  122  is used, it is also advantageous to weld it to the first sheet  112  around the periphery at weld  116 , thus forming the pressure zone  119  between the first sheet  112  and third sheet  122  as opposed to between the first sheet  112  and the lower tool insert  48 . In order to ensure that the periphery of the upper and lower tool inserts are sealed it is also advantageous to use an O-ring seal  124  around the periphery of the upper and lower tool inserts. 
     After placing the workpiece between the upper  46  and lower  48  tool inserts and bringing the tool inserts together to form pressure zones  117 ,  119  and  120 , the air from within the pressure zone  117  surrounding the composite panel  110  is evacuated (block  137 ). Pulling a vacuum around the composite panel  110  helps to reduce voids or in the completed composite part. Pulling a vacuum in the pressure zone  117  also helps to ensure that the first sheet  112  and second sheet  114  remain tightly against the composite panel  110  during consolidation and forming which in turn helps to prevent wrinkles and flaws in the surface of the completed part. 
     After pulling a vacuum in pressure zone  117 , the coils  35  are energized by the coil driver  50  with a time varying electrical field (block  138 ) to heat the susceptors inductively to the forming and consolidation temperature of the composite panel  110 . Heat is transferred by conduction into the composite panel  110 , so it too reaches consolidation temperature. 
     Pressure zone  119  is pressurized (block  140 ) to force the susceptors and composite panel  110  upwardly, as shown in sequential FIGS. 6A-C, until the upper surface of the workpiece conforms to the forming surface  58  of the upper tool insert. The pressure within the pressure zone  119  is maintained until the composite panel  110  has fully formed and consolidated. 
     During forming, it may be advantageous to pressurize the pressure zone  120  between the upper tool insert  46  and the second sheet  114 . Pressurizing pressure zone  120  places a force on the workpiece which helps to consolidate the composite panel  110  and regulates the rate at which the workpiece deforms. In some applications, it may be advantageous to pressurize and maintain pressure zones  119  and  120  at the same pressure for a period of time to help consolidate the composite panel  110  prior to the forming procedure. As the forming procedure begins, the pressure in pressure zone  120  can then be maintained slightly lower than the pressure in pressure zone  119  or can be decreased over time to allow the pressure in pressure zone  119  to deform the workpiece upwardly into contact with the forming surface  58 . In the preferred embodiment, it has been found advantageous to form the susceptors from aluminum or an aluminum alloy. 
     After completing consolidation, the induction coils  35  are shut off and the workpiece and tool inserts are allowed to cool to a temperature at which the formed composite panel  110  may be removed from the tool inserts and first  112  and second  114  sheets (blocks  142 - 146 ). Although there is some heat transfer between the workpiece and the tool inserts, it is insufficient to heat the tool inserts or dies substantially. 
     In one example of composite consolidation and forming in accordance with the present invention, a composite panel formed of 48 layers of thermoplastic PEEK/IM6 prepreg ⅜ inch thick was consolidated and formed Three aluminum sheets having a thickness of {fraction (1/16)} inch were placed around the composite panel and the resulting workpiece was placed in the tool inserts and inductively heated to a temperature of 720° F. by induction heating in five minutes time. The panel was maintained at 720° F. for two minutes and then cooled for twenty minutes. Pressure zone  119  was then pressurized to approximately 250 psi while pressure zone  117  was vented to atmospheric pressure. The pressure zone  120  was not pressurized. The pressure in pressure zone  119  was maintained for 22 minutes to consolidate and cure the composite panel. The times and pressures described above are for representative purposes only and would differ depending upon the composite material used and the thickness and complexity of the formed part. 
     The present invention is applicable to all types of organic matrix composites that is not limited to the example discussed above. For example, the present invention may be used to consolidate/cure both thermal setting and thermoplastic composites including epoxies, bismaleimides and polyimides. 
     Another problem with prior art composite forming and consolidation procedures has been difficulty in forming a close tolerance outer mold line while also maintaining a close tolerance inner mold line on a composite part. In FIG. 8, the tolerances of the outer mold line surface  150  of the wing skin  148  must be closely maintained to ensure that an efficient aerodynamic surface is achieved. The tolerances of the inner mold line surface  152  of the wing skin must also be maintained at a close tolerance in a buildup area  154  where the wing skin is joined to a spar  144  to ensure that the wing skin  148  and spar  144  can be Joined together along joining surface  156  through the use of fasteners  158  without the use of shims. It is not as critical to control the inner mold line surface in areas  160  where the wing skin is not attached to other structures. 
     Composite panel  162  is placed within three sheets  164 ,  166 , and  168  to form a workpiece that is placed within upper and lower tool inserts  170  and  173 , respectively. The composite panel includes a buildup area  174  having additional layers of prepreg so that the buildup area is thicker than the surrounding composite panel. Additional layers of prepreg are used in the buildup area to reinforce the composite panel in the area where the spar  144  (FIG. 8) is to be attached. 
     Similar to the preferred embodiment, the upper tool insert  170  includes a forming surface  172  that is contoured to define the outer mold line of the composite panel  162  after it is formed. The lower tool insert  173  includes a raised portion  180  whose upper surface  182  defines the lower surface of the workpiece after forming. The raised portion  180  is spaced a proper distance from the buildup area  174  to maintain a close tolerance on the interior mold line surface of the composite panel  162  in the buildup area  179  during forming and consolidation. 
     The pressure zone formed between sheets  164  and  166  is evacuated. The induction coils are energized to inductively heat the sheets and, thus, composite panel  162 . The pressure zone  184  formed between sheets  164  and  168  is then pressurized to force the workpiece upwardly into contact with the forming surface  172  and to exert hydrostatic force on the sides  186  of the buildup area  174  to help maintain the required tolerances. 
     To ensure that proper part tolerances are maintained in the buildup area  174 , it may be advantageous to weld sheets  164  and  168  together along the edges of the buildup area  174  along weld line  188 . The weld line  188  help ensure that the pressure within pressure zone  184  does not force sheets  164  and  168  apart in the buildup area  174 . 
     Depending upon the application, it may be advantageous to maintain different pressures in pressure zone  184  at different locations on the composite part. Welding the first sheet  164  and third sheet  168  together along a weld line defines different pressure zones between the sheets that may be pressurized at different pressures. 
     Holes are drilled in the build-up section  174  of the wing skin to receive fasteners  158  to join the spar  144  to the wing skin (FIG.  8 ). 
     While preferred embodiments of the invention has been illustrated and described, those skilled in the art will appreciate that various changes can be made therein without departing from the spirit and scope of the invention.