BACKGROUND OF THE INVENTION Composite and plastic materials are used to fabricate products using various molding techniques and devices. The process of fabricating composite materials usually requires that heat be supplied to the product material which results in that material assuming the form of a mold surface.

In some cases, the heating also provides activation for chemical curing or some other desired chemical or morphological change in the material during the processing. The management of heat flow into the composite material during processing is critical to the forming and/or chemical change of the part and is usually critical to the part quality and application performance.

Often in the thermal processing of composite and plastic materials, the thermal mass of the tooling combined with the inefficiencies of the heating equipment used (e. g., autoclaves and platen presses) results in prolonged cycle times to achieve the required temperature profile to the part being fabricated. As a result, the thermal processing cycles are usually defined by these tooling and equipment limitations rather than the optimum cycle for the material being processed. Further, this prolonged cycle time results in

lower productivity, often requires multiple tool sets, and always results in increased costs.

By controlling the heat so that it predominantly flows into and out of the part being processed, the problems described above can be eliminated, and much more efficient and cost effective processing can be achieved.

Further, for processing materials that require temperatures greater than 500°F, such as thermoplastics, glass, or ceramic matrix composites, vacuum bag processing is often difficult or impossible because high temperature bagging materials and sealants are required. Thermal management into and out of the part can keep all of the heat focused in the part allowing low temperature vacuum bags and sealants to be used for very high temperature (> 2000°F) processes.

These processing improvements are achieved through well controlled, uniform heating and cooling localized at the part being processed along with good thermal management of the tool and the surrounding environment.

SUMMARY OF THE INVENTION The invention provides for a method and apparatus which is capable of heating composite and plastic materials which are formed into parts. The heat is generated from a carbon fabric which is either embedded near the surface of a cast tool base, or supported above a tool base in proximity to the composite material being formed. The tooling using this heating technique can be a closed cavity mold, as is used in resin transfer molding or compression molding, or one-sided tooling, as is used in vacuum bag molding.

In order to focus the thermal energy into the part being processed, the base tooling material is preferably a thermal insulator, causing heat flow into the part rather than into the tool. Similarly, in the one-sided tool approach,

insulation materials are preferably placed on top of the part being processed, causing heat flow into the part rather than the surrounding environment, vacuum bag, or sealant materials.

DESCRIPTION OF THE DRAWINGS Figure 1 illustrates one embodiment of the invention for forming a laminate of ceramic matrix/carbon fiber composite material; Figure 2 is a photomicrograph of the laminate produced by the embodiment of Figure 1; Figure 3 is a photomicrograph of the laminate produced by the embodiment of Figure 1 at an increased magnification; Figure 4 illustrates an embodiment of the invention having a self heated tool comprising a carbon fabric embedded in a castable ceramic; Figure 5 illustrates an embodiment of the invention using multiple heat zones in a vacuum forming operation; Figure 6 illustrates a tooling master for forming a heating tool having a complex heating surface; Figure 7 illustrates the step of applying a release layer to a tooling master of Figure 6; Figure 8 illustrates the step of building a perimeter about the tooling master; Figure 9 illustrates the step of inserting a carbon fabric in the castable ceramic; Figure 10 illustrates adding a layer of cast ceramic on back of carbon fabric; Figure 11 illustrates the step of adding a wax cooling channel which has been preformed in the casting material;

Figure 12 illustrates the step of adding a quantity of castable ceramic material to the back surface of the mold; Figure 13 illustrates the heating tool which has been separated from the tooling master; and Figure 14 illustrates a heating tool in accordance with another embodiment of the invention implemented in a resin transfer mold.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 illustrates one embodiment of the invention for curing high temperature composite materials. The method carried out in accordance with Figure 1 provides a low temperature cure operation using a vacuum bag 13, followed by a high temperature post cure in an inert atmosphere using pressure vessel 14. The initial curing operation and high temperature post cure operation are combined into a single operation using a silicone rubber vacuum bag 13 and a pressure chamber 14.

As a starting material, a preceramic-alumina matrix, available-from Applied Polaramic, Inc., is applied to a T-300 carbon fiber tow on a filament winding mandrel, as is known. The resulting material 16 is placed in a steel pocket die 15, and a steel cal plate 17 is placed over the material 16. A temperature sensor 33 is adhered to the wall of the pocket die 15 to monitor the temperature of the material 16 as it cures. A plain weave carbon fabric 21 is utilized as the source of heat for curing the material 16. The carbon fabric 21 is located between first and second insulation layers 20 and 22, electrically isolating the carbon fabric from the pocket die 15, while maintaining the pocket die 15 in close heat transfer relationship with the carbon fabric 21. A further thermal insulation layer 19 is applied above the pocket die 15 to keep the silicone rubber vacuum bag 13 at a safe operating

temperature. Layer 20 also serves as a thermal barrier directing the heat toward the composite material 16.

The entire vacuum bag 13 assembly is enclosed in a pressure vessel 14, so that an inert gas overpressure can be applied during the processing operation to the vacuum bag 13, and hence to the ceramic matrix composite material 16 being formed by the pocket die 15.

The thermocouple 33 provides a temperature measurement to a computer 29. Computer 29, in turn, is connected through an interface and enables the controller 32 to apply a heating current to the carbon fabric 21.

The computer 29 and thermocouple operate as a thermostat to maintain the temperature of the pocket die 15 at a desired temperature. Variable transformer 25, and step down transformer 30 provide the operating current for the carbon fabric 21 under control of controller 32. Using the foregoing apparatus, a low temperature cure of the preform step of a curing process may be performed. Current is supplied by controller 32 to the carbon fabric 21, and the temperature of the tool is increased to 300°F in a linearly increasing temperature for a two hour period. A compressed air overpressure of 50 psi is applied through inlet 14a of pressure vessel 14, and the vacuum bag internal pressure is set to be approximately 25 inHg (substantially 12 psi subatmospheric pressure). Following the low temperature cure of the preform, the heated ceramic matrix composite 16 conforms under pressure to the surface of the pocket die 15.

A high temperature post-cure step is performed by applying a vacuum through inlet 13a within vacuum bag 13, and the pressure vessel 14 was supplied with argon gas to a pressure of 45 psi. Under the established pressure conditions, the temperature was increased from room temperature to 500°F over a two hour period, and maintained for approximately two hours, and then increased from 500°F to 1400°F over three hours and held

there for two hours. A cooling interval followed, while maintaining a vacuum at 25 inHg during a cool down and simultaneously bleeding off the argon gas pressure from the pressure vessel 14.

The foregoing apparatus and method produces a laminated panel of ceramic matrix carbon fiber reinforced composite having a density, flexural strength and modulus equal to or superior to manufactured laminate using the same matrix material. Table 1 indicates for various temperatures, a comparison between manufactured Alumina-alumina laminates, and a carbon alumina laminate manufactured in accordance with the method and apparatus of Figure 1. The most notable difference is seen in the density and flexural strength of the different materials. The carbon fiber laminate made in accordance with the method and apparatus of Figure 1 is stronger.

The carbon/alumina system using the post-cure process in accordance with Figure 1 had higher strength and modulus at room temperature over the manufactured ceramic matrix precursor.

Table 1. Comparison of properties of two materials and several processes Material/Process Temperature Density Flexural Strength Modulus (deg F) (g/cc) (Ksi) (Msi) Alumina-alumina/mfgr Room 2. 6 26 8 | Alumina-alumina/mfgr 1400 2. 6 38 7.6 Carbon-aluminalfree Room 1. 97 59 11 stand post-cure 75 15 carbon-alumina/Room 1. 99 >32 10.09 overpressure post- (failure in shear) cure carbon-alumina/1400 1. 99 >32.8 9.79 overpressure post->37.8 8. 74 cure in shear)

Figures 2 and 3 are photomicrographs taken at a magnification of 25X and 100X, respectively, of a section of carbon fiber/alumina matrix laminate processed in accordance with the technique of Figure 1. The carbon fibers, represented by white dots in the photomicrograph, are not uniformly distributed in the matrix material which is due primarily to the characteristics of the prepreg not to the application of the technique. Therefore the variation in the flexural strength limits due to shear mode failure and lower modulus values in comparison to free-standing post-cures result from variation in prepreg materials. The Figure 2 illustrates cracking, which was the result of a unidirectional layout of the materials in the pocket die.

Figure 3 further illustrates a material heterogeneity that contributed to the mechanical performance variations.

This embodiment of the described tooling method demonstrates that conventional vacuum bag technology can be used to process ceramic matrix composites at temperatures of 1400°F, and higher, as a final post cure temperature for these materials.

Figure 4 illustrates another embodiment of the foregoing process and apparatus, utilizing a self-heated tool. The self-heated tool of Figure 4, which may be used in place of the separate carbon fabric of Figure 1, provides for greater management of the heat transfer from a heated fabric layer to a thermal processed material used in a molding operation. The self-heated tool shown in Figure 4 comprises a ceramic matrix forming an insulating substrate 34 having a carbon fabric 36 embedded therein. The body of the tool may also be cast from polymer materials such as epoxy, polyester, or polyurethane depending upon the temperature requirements of the process. Additionally, cooling channels 35 may be embedded within the ceramic matrix substrate 34 to provide additional control over the temperature of a heated surface 40. Electrodes 38 and 39 are shown

connected to the ends of the carbon fabric 36. Electrodes 38 and 39 may simply be a continuation of the carbon fabric layer 36, or may include metal electrodes formed in the ceramic matrix substrate 34 which are connected to the ends of the carbon fabric 36. Hollow glass or phenolic microspheres may also be embedded in the ceramic matrix substrate 34 to minimize heat transfer to surface 41. The insulating substrate 34 may also be formed using castable polymers with insulating fillers depending upon the temperature requirements of the process.

Use of the broad layer of woven carbon cloth 36 and the insulating substrate 34 provides for a large uniform heating element which may be used for thermoprocessing of thermoplastic, thermoset, or pre-ceramic materials. The insulating substrate, which may be a ceramic matrix, permits high temperatures to be realized on the surface 40, may also in low temperature applications be made of other materials, including a polyurethane foam. In this case as well, the heat will be transferred preferentially to the surface 40, while maintaining a high measuie of temperature insulation with the opposite surface 41 of the self-heated tool.

Figure 5 is an embodiment of the invention having multiple heat zone configurations in a vacuum forming operation. A self-heated tool 42, in accordance with that of Figure 4 (without cooling channels) heats the base of a thermoplastic material 45. A release layer 46 is shown over which the prepreg composite material 45 may be disposed. The material 45 has an irregular shape supplied by a pair of heated mandrels 47,48. The mandrels 47,48 are used to form reinforcing ribs in the panel 45 having a rectangular cross section. Heated mandrels 47,48 may also be a self heated ceramic tool of the type shown in Figure 4, comprising an insulating substructure 34 containing an embedded carbon fabric 36. In this way, multiple heat sources may be used to form an irreguiarty shaped composite

material 45. Once the composite material 45 has been processed, the mandrels 47 and 48 may be removed from an enclosure of composite material 45.

A release layer 49 is shown, conformally coating the thermopiastic material. A breather layer 50 is also shown which will let gases escape during processing, which are in turn drawn from the vacuum bag 52.

The use of the foregoing heated tool permits application of the heat in the process material with a minimum amount of loss to the surroundings or other parts of the tooling which need not be heated.

The carbon fabric includes distinct advantages as a source of heat for thermoprocessing composite materials. The carbon fabric tends to produce a uniform temperature distribution, and the coefficient of thermal expansion is very low. The thermal conductivity of the fabric tends to diffuse the generated heat minimizing the localized hot spots. The width of commercially available carbon fabric may be as much as 60 inches and has a long life. The fabric is low cost, and is capable of a very high temperature operation when it is protected from oxidation.

The ceramic tooling material compliments the carbon fiber in that it also has a high temperature capability and low coefficient of thermal expansion. A silica-based castable ceramic has thermal expansion properties that match the carbon fabric, as well as the thermal expansion properties of the carbon fiber composites which may be processed thereby reducing the internal stresses generated in the part when it is processed in intimate contact with the heated tooling surface.

The combination of a cast ceramic and carbon fiber fabric provides a wide range of temperature operation and excellent thermal shock capability useful for processing thermoset composites, thermoplastic composites,

glass matrix and ceramic matrix composites, alumina matrix composites and even carbon/carbon composites.

The heated tool may also be fabricated with a heating surface having a complex shape. While the example of Figure 5 provides a technique for manufacturing laminated sheets having a reinforcing rib formed by the mandrels, the entire heating surface of the heating tool may be cast in a form for applying heat to a complementary complex shape.

Figures 6-12 illustrate the process steps for forming the heated tool having a heating surface with a complex shape. Figure 6 illustrates a tooling master 60, having a surface 61 representing the desired shape of a heated tool surface. In Figure 7, a release film is applied to the complex surface 61 which inhibits a subsequently formed layer from sticking to the master 60.

A container 62 is formed in Figure 8 about the perimeter of the tooling master 60 representing the size of the tool to be cast on the complex tooling master 60.

A sufficient quantity of castable tooling material is created. The castable tooling material can be, for instance, castable silica based on a ceramic activator known as RESCOR 750. The casting material is vibrated to remove air bubbles and a thin layer of the castable material 63 is placed over the surface 61 in Figure 9. A precut carbon fabric 64 is layed over the thin casting material 63 and the ends extend to the container 62, where it extends vertically along the container 62 surface. The ends of the carbon fabric 64a and 64b are available to provide electrical connections to the carbon fabric 64. A layer of ceramic 65 is added over the carbon fabric in Figure 10.

Figure 11 illustrates the step of adding a wax cooling channel 66 which has been preformed in the casting material on the back side of the

carbon fabric 64. Carbon fabric 64 is coated with a thin coat of casting material 63 and 65, and the casting material is allowed to set to avoid disturbing the position of the wax cooling channel 66. A quantity of castabie tooling material is poured onto the back surface of the mold in Figure 11, covering the wax channel. A layer of fiberglass cloth or carbon cloth, as mechanical reinforcement, has been placed on top of the castable tooling material 68 in Figure 12. The remaining castable insulation is poured over any reinforcement material to a level determined by a designer to strengthen the tool. The back surface is finished with a trowel to form a smooth contour. Once the cast material has cured, the cast mold is removed as shown in Figure 13 from the master tool. Post cure heating of the mold may be useful as is known to those skilled in the mold making art, to fully remove the residual moisture from the mold.

The finished mold, therefore, contains a surface 69 which is heated from electrical current applied to ends 64a and 64b of the carbon fabric.

Heat from the carbon fabric is transferred to surface 69 where it may be used to heat a complex shape of a thermoplastic material, or a die of a mold supporting the thermoplastic material.

Creating a tool in accordance with the foregoing produces a uniformly heated surface of castable material adjacent to the carbon fabric. In one prototype tool in accordance with the foregoing, having a surface area of approximately 14"x 14", the average temperature measured was 590.9°F, with a coefficient of variation of 1.75% of a sampled area. The tool temperature falls off significantly towards the edge of the tool by virtue of the insulation properties of the ceramic base material. Heat flow through the tool surface facing the component being formed may be further enhanced by using materials having a high thermal conductivity adjacent the heated fabric.

The foregoing integrally heated processing tool may also be used in vacuum bag applications similar to those of Figures 1 and 5. The vacuum bag membrane is insulated from the heated assembly with a flexible insulation layer.

The heated surface generates sufficient heat to form a thermoplastic composite along the molded surface of the tool. The low temperature vacuum bag membrane remains unaffected due to the insulation layer.

Further, the tooling substructure has a ceramic base which insultes the vacuum table supporting the tooling from high temperatures.

Thus, it can be seen that the foregoing tool by controlling the heat flow such that a majority of the heat flow is incident to the component being formed, permits lower temperature environments to be used in the vacuum bag forming process.

The present invention can also be implemented in a resin transfer <BR> <BR> <BR> <BR> mold (RTM) to produce composite components which are formed on a ~ heated surface. Referring now to Figure 14, there is shown a separated two-part RTM mold in accordance with another embodiment of the invention.

The two-part RTM mold applies heat from two directions against a castable material which is injected through an opening in the top half of the mold.

The bottom half of the two-part RTM mold 90 includes a ceramic base 91 material formed by conventional molding techniques, having embedded near a top surface heating fabric 93 for shaping a castable material such as a thermoplastic material. The heating fabric faces 93 on a lower side thereof a cooling channel 94, similar to the ones formed in the embodiment of Figure 4. The ends of the heating fabric are connected to terminals 96,97 on the exterior of the lower mold half and a thermocouple sensor 98 is provided which may be used to control the temperature of the heating fabric 93.

An upper mold half 100 is also provided having an embedded heating fabric 103 adjacent a lower surface thereof. A thermocouple sensor provides an accurate monitoring and control of the temperature. The upper mold half also includes a ceramic base material 99 which reduces the heat flow through the top of the upper mold half, directing the heat flow towards the lower mold half. A cooling channel 104 may also be provided in the upper mold half ceramic base which is opposite the upper side of the heating fabric 103. A pair of terminals 108,109 are provided to supply current to the heating fabric 103.

The two-part resin transfer mold directs heat from two directions against a castable material which has been injected through an opening 106 in the mold, and which extends into the space 101 formed in the lower surface of the upper mold, and upper surface area of the lower mold 92 which are shaped to form the component surface. Thus heat is directed to the material being molded, increasing the efficiency of heat transfer to the component being formed between mold halves.

The foregoing description of the invention illustrates and describes the present invention. Additionally, the disclosure shows and describes only the preferred embodiments of the invention, but as aforementioned, it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings, and/or the skill or knowledge of the relevant art. The embodiments described herein above are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments.