Abstract:
A method of fabricating a composite vessel encompassing rapid manufacturing. The process includes using a liner, of metal or plastic materials, over which a braided or developed preform is wrapped. The dry fiber wrapped liner is placed in a mold and resin injected into the cavity formed between the liner and the mold outer walls. The liner is flooded with heated and/or cooled pressurized fluid thus enabling complete and independent process control from within the liner for both the resin injection and the cure phases. Fiber placement control is determined through combined biaxial and triaxial braid/preform design, and by wrapping at controlled tension onto the supporting liner. Fiber tension control is further enhanced by the methodology of mold loading whereby tensioning forces are enacted during actual load and close. The process may use thermoset or thermoplastic resins and any of a variety fibrous reinforcements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 12/046,716, filed Mar. 12, 2008, which claims from the benefit of Provisional Application Ser. No. 60/906,282, filed Mar. 12, 2007, both of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to the art of fabricating composite hollow structures with internal liners; more particularly to improved methods of manufacture of composite pressure vessels for storage of hydrogen and natural gas, specifically with respect to improvements focused towards high-rate manufacturing methodology. 
       BACKGROUND OF THE INVENTION 
       [0003]    The primary issues associated with implementation of fuel cell vehicles and the like are in manufacturing components at rates compatible with vehicle production, and at costs that can be borne by the consumer and industry. Current high-pressure storage cylinders need to be carbon composite due to the weight penalty associated with metallic tanks. However, current state-of-the-art in composite cylinder production relies on filament winding of either wet fiber tows, or of a pre-preg fiber with pre-impregnated resins (typically epoxy or possibly other thermoset and some thermoplastic resins.) These are wound onto a liner with precise control of fiber orientation to allow optimum stress fields, and the overall wound cylinder is typically placed in an oven or autoclave and cured for some period of time. The typical process sequence can take from 4-8 hours per cylinder, and for example, as referenced by lida et al (U.S. Pat. No. 6,190,481) a cure time of over 6 hours was demonstrated for tank manufacture. The state of the art is further reflected in patent literature such as Goldsworthy et al (U.S. Pat. No. 6,565,793) and more recently Iida et al (U.S. Pat. No. 7,032,769). 
         [0004]    In order to be compatible with vehicle production rates, a process that is approximately 20 minutes or less is desirable. In the above referenced state-of-the-art, from 12 to 24 different tools and process lines are necessary for the parallel processing required to reach 20 minute product cycle times. This is capital, labor, and space intensive, and is not optimum for real-world production requirements. The current status has been verified by recent private communication with a major automotive manufacturer. 
         [0005]    Attempts have been made to modify other composite processes to take advantage of faster cycle time, including using resin transfer molding into a dry-fiber preform which is filament wound onto a cylinder liner. The issues with these approaches have come from holding appropriate tension on the fibers after wrapping, with holding net-shape on the carbon wrapped liner so that mold closure does not wrinkle the carbon fibers and thus knock down the properties to unacceptable levels, and with rapid and repeatable injection processes. The latter is a key factor as the existing systems have typically used heat and pressure transfer via the mold external surface, and utilizing pressure control via injection pressure of the resin. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention utilizes a formed liner which is over-wrapped with a dry fiber fabric preform or braid. Tension control is incorporated through use of a tri-axial form of material such that tension can be developed in the preform without deformation. The wrapped liner is loaded into a mold cavity defining the outer shape of the part to be made. The loading operation incorporates rotation of the liner and fiber preform such that the tension on the preform is improved and is coordinated with tool closing to effectively eliminate any undesirable possibility of wrinkling of the preform. 
         [0007]    The liner is connected to process controls capable of delivering heat and cooling as well as pressure to the inside of the liner via fluid systems. The capability to control the process from the internal liner side is used during the subsequent resin transfer molding operation. Resin is injected into the dry fiber preform occupying the cavity between mold wall and liner. The temperatures and pressures are capable of being set at optimum for the injection cycle, with rapid thermal response from the liner process fluid systems and pressure balance as provided by the liner internal fluid preventing collapse of the liner. Additionally, vibration and other pulse based injection process aids can be accomplished through fluid excitation in the liner. 
         [0008]    Once resin injection is completed at the conditions chosen for resin viscosity and flow control, the liner internal fluid system is switched to provide optimum thermal and pressure settings for cure of the resin. In this manner, complete separation of the resin viscosity profile for injection and for cure are attained, and as evident to any practiced in the art, the overall process cycle can be optimized. Additionally, the pressure conditions developed in the resin can be selected for achieving properties desirable in the final part. On final cure the part may be cooled as desired via internal cooling through the liner, and thus the demolding cycle can be accelerated as well. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]    A brief description of the drawings indicates the process methodology developed herein. 
           [0010]    The cylinder liner, fabricated with double-ended threaded connections, is placed on a mandrel and a direct wrap of fiber is applied,  FIG. 1 . In the first embodiment of the invention, a braided carbon fiber material is used; however either stitched or woven fabric forms may be used as long as the correct fiber architecture can be accommodated. 
           [0011]    In the first embodiment and as applicable to pressure cylinders specifically, the braid is a combination of tri-axial format and bi-axial format as shown in  FIG. 2 . The fiber architecture required for achieving design properties can be developed in this way. The bi-axial fibers conform to the end-dome design and the use of longitudinal fibers allows tension to be held in the fabric without distortion. 
           [0012]      FIG. 3  shows an additional strip of bi-axial fibers stitched onto the wide format braid to allow build-up of additional thickness on the tank ends, as well as to mitigate the effect of changing thickness of the fabric where the uni-directional fibers stop. 
           [0013]      FIG. 4  shows one embodied method of drawing the braid down to the tank ends and using a pick system. 
           [0014]      FIG. 5  shows the schematic for achieving process control over thermal and pressure conditions from the liner outwards during the resin injection and cure process. 
           [0015]      FIG. 6  shows the method of tool load and tension control during the mold closing operation. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0016]    The following discussion of the embodiments of the invention directed to a rapid manufacturing process for a compressed hydrogen tank is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. However, as will be appreciated by those skilled in the art, the process has application to produce tanks other than compressed hydrogen tanks and to produce structures with liners applicable in a number of fields utilizing composites as the preferred material. 
         [0017]    An invention is developed which consists of several process steps, intricately linked, in which no single step is more than 10-20 minutes in total cycle time, thereby resulting in a part fully completed coming off each line every 10-20 minutes. 
         [0018]    A cylinder liner  1  is formed through one of several options, using either superplastic forming of metal pipe with threaded ends, spin forming of cylinders from metal sheet or tubing, or plastic blow-molding or injection molding to form plastic liners. In the case of using plastic liners, an additive may preferably be used to allow higher thermal conductivity than with current plastic systems, and these additives, which are well known in the industry and include nano-particulates of conductive materials, or chain forming additives to preferentially conduct heat. Plastic formulations are incorporated herein that include carbon nanotube modified plastics that have high thermal conductivities as compared to un-modified materials. 
         [0019]    The cylinder liner  1 , fabricated with double-ended threaded connections  2 , is then placed on a mandrel support  3  and direct wraps of fiber  4  are applied. In the first embodiment of the invention, a braided carbon fiber material  5  is used, however either stitched woven fabric forms may be used as long as the correct fiber architecture can be accommodated. The braided material lends itself ideally to this application for high pressure hydrogen storage cylinders as braid fiber architectures have naturally high permeability for the resin transfer molding&#39;s resin injection cycle, and they can be made with the necessary blend of fibers for hoop and axial stresses for the cylinder performance. The braided material is supplied in spools  6  from a common source braider, which allows the braider to be in production full time and operate separately from the tank liner  1  wrapping operation. 
         [0020]    This application is unique to the industry, and the process has several advantages, which include: (A) The braid can be made in a tubular form with bi-axial and unidirectional fibers located in the appropriate places for tank stress design optimization as demonstrated in  5 ; (B) An additional strip of bi-axial fibers  7  can be stitched onto the wide format braid  5  to allow build-up of additional thickness for the tank ends  8 , as well as to mitigate the effect of changing thickness of the fabric where the uni-directional fibers stop  9  (eliminating a step change in thickness that exert effects on stress concentrations in the pressurized tanks); (C) The braided material  5  will effectively place a double layer of material down at once as the braided tubular sock  5  will be layed flat onto the liner  1  thereby improving production rates; (D) The end-contours can be accommodated by the bi-axial braid deformed with fiber shearing through an automatic process using fingers  10 , which are attached to cams  13  to draw down and contour to end-domes, and the fiber ends sealed down by use of a local sealing method, such as adhesive tacking or other process as common in the industrial practice, and this method thereby allows precise end-contouring and thickness control of the material preform; (E) The uni-directional fibers  11  allow the braid to be held under tension during the application. The tension control is effected through tensioning drag devices  12 . Having controlled and precise tension thereby controls the laminate net thickness, and this restraint afforded by the uni-directional fibers  11  prevents the cylinder end wraps  14  from becoming stretched and wrinkling or moving off-plane from the desired fiber application location; (F) The bi-axial fibers can be tailored during the braiding process to optimize the fiber location and orientation, reducing materials application and cost of the tanks; (G) The floor area required for the fiber application process is exceptionally small compared to other filament winding-type operations, and does not rely on movement of either the braider or the liner as is the case of a direct braid over-wrap; (H) The system allows the braid manufacture to be separated from the braid application process, thereby fully utilizing the braider equipment in continuous production of materials and also fully utilizing the liner wrapping equipment in production of over-wrapped liners ready for molding. 
         [0021]    A unique molding system has been developed that is a significant departure from any in the state-of-the-art today. The wrapped dry tank  15  is placed in a cavity mold  16 , and is rotated  17  during mold closing  18  to draw tension in the fabric preform. This allows the part to be enclosed in the tool with no fiber pinching or deformation occurring and develops and retains tension required in the fiber, as the fabric ends  27  are drawn down and into the final cavity shape. The mold segments  25  close sequentially and the tank rotation  17  is made possible as the boss ends  2  of the tank liner  1  are exposed to the outside of the mold  16 . The mold tooling is constructed of several segments  25 , in the first reduction to practice three segments were used, but preferentially four segments are used to optimize mold closing operations. 
         [0022]    The liner of the tank is flooded with heat-transfer fluid  19  through quick-release end connectors  26 . The heat transfer fluid  19  has its temperature optimized for resin injection viscosity control, but still well below the reaction initiation temperature of the resin components as in the case of thermoset resins. The heat-transfer fluid  19  is pressurized  22  with pump  20  to allow use of a thin-wall liner, effectively providing internal support to the liner during resin injection. Resin injection pressure  23  can then be optimized and balanced against collapse or dimensional change of the liner  1  and hence local deformation of the fiber preform, another common problem with resin transfer molding. The fluid  19  and pressure boost pump  20  also allows for application of internal vibration assist during injection, another facet unique to the application in composite storage cylinders. This assist allows for faster injection, more rapid fiber wet-out, and enhanced release of voids in the laminate, and is achieved preferentially as a thin-walled liner is made possible through the invention and the mold does not need to transfer this vibration through the bulk of the metal tool  16 . 
         [0023]    The next stage of the process occurs upon completion of the resin injection  23 . In the case of thermoset resins, a high-temperature, pressurized heat transfer fluid  21  is flooded inside the liner compartment through the same quick release fittings  26 . This hot fluid provides heat to the laminate from within the liner cavity and thereby initiates the reaction of the resin components. Excess heat generated during exothermic curing of the laminate resin will be absorbed back into the heat transfer fluid  21  through the liner  1 , thereby mitigating very high internal temperatures and the possibility of overheating the laminate during cure, and also mitigating some of the thermal stresses typical of cured composites. Similarly, heat loss through system components can be made up by applying external heat  24 , and thus very precise and also rapidly responding process control is made possible. As desired, additional pressure  22  can be applied to the high-temperature heat transfer fluid  21  to ensure hydrostatic pressures internal to the composite laminate are retained. 
         [0024]    On completion of the cure cycle the cold fluid  19  is returned to the liner internal cavity. This cools the now fully cured composite cylinder down to a level whereby the tool opening and part removal are accomplished as rapidly as possible. Once the temperature is reduced to an acceptable level, the part is removed and the tool is ready for the next operation. 
         [0025]    In the case of using a thermoplastic resin, the process is adapted for a different cycle, but principals common to the plastic industry are now modified and applied in a unique environment for achieving rapid processing of fiber-reinforced cylinders. The heat transfer fluid  21  is pumped through quick release fittings  26  into the liner  1  internal cavity. This temperature is set to slightly above the melt temperature of the desired thermoplastic resin. Resin injection  23  of liquid thermoplastic resin occurs and the resin permeates the entirety of the laminate as is common in resin transfer molding, holding the internal pressure  22  and internal temperature of fluid  21  allows the process to be fully executed and the laminate completely saturated with resin. On completion of the injection, cold process fluid  19  is now passed into the liner and serves to solidify the thermoplastic resin and thus consolidate the composite laminate without needing to remove a large quantity of heat through and into the tool  16  walls. As is desired in thermoplastic molding, high pressure can be held on the laminate throughout the cooling cycle via pump  20  pressurizing the cavity  22 . On acceptable cool-down the part can then be demolded and the cycle completed. 
         [0026]    This description of the invention has thus been made clear in illustrative embodiments, and reduced to practice in a composite high-pressure storage tank: comprising four layers of a braided triaxial/biaxial fabric as described herein, coupled with resin transfer molding of a thermoset resin under process control as also described herein. The individual steps of the process have been demonstrated to be practical and complete within 30 minutes, an order of magnitude improvement over the state-of-the-art of filament winding of high. pressure composite overwrapped gas storage tanks. Testing of the completed articles was undertaken and has shown capability to achieve burst pressures of over 4,000 psi in first article demonstration of the process principles. Thus, while there will now be immediately obvious to those skilled in the art many modifications of process, structure, and components as used in the practice of the invention, these invention modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.