Abstract:
The invention is an apparatus for de-bulking a composite material preform laid up in a mold. In detail, the apparatus includes a flexible cover adapted to fit over the mold and preform and attach to the sides of the mold. The cover includes an inlet port located in the cover such that when the cover is fit over the mold and preform, the inlet port positioned over the preform. A stretchable cord seals the peripheral portions of the cover to the sides of the mold forming a chamber over the preform. A hot air pump is coupled to the inlet port of the cover for pumping hot air into the chamber.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of composite structure fabrication, in particular, to an improved system for de-bulking the uncured composite sheets as they are laid up in a mold at the workstation. 
     2. Description of Related Art 
     Typically, composite structures are formed by a process that involves the following steps:
     1. Sheets of filamentary material impregnated with uncured resin are laid up in a mold having the contour of the completed part.   2. After the lay-up is complete, the lay-up is vacuum bagged.   3. Thereafter a vacuum is pulled between the lay-up and mold.   4. The vacuum-bagged lay-up is then placed in an autoclave wherein the temperature is increased to the curing temperature of the resin and the autoclave is pressured until the resin is cured.   5. Thereafter, the cured lay-up and mold are removed from the oven or autoclave and the now cured part is removed from the mold.   

     However, often the number of plies in the lay-up requires multiple de-bulks during the lay-up process that a de-bulking procedure is required. Typically, the lay-up process requires a de-bulk after a certain number of plies of resin impregnated filamentary material have been laid up in the mold. The partially completed lay-up has to be vacuum bagged and a vacuum pulled in order to de-gas and consolidate it if this is not done, there is a high probability that the completed part will have unacceptable voids caused by air pockets. It is preferred that this de-bulking process take place at a temperature at which the resin will flow but not readily cure. Thus the mold with vacuum-bagged partially completed lay-up must be moved into the autoclave. If the part is vary large, thus requiring a large and heavy mold, the process and be difficult and time consuming. 
     Portable oven and autoclave type devices are available. For example, U.S. Pat. No. 5,116,216 “Apparatus For Preparing Thermoplastic Composites” by R. C. Cochran, et al. discloses an apparatus for fabricating composites using thermoplastic matrix materials. A blanket heater is placed over the lay-up is used to supply the necessary heating. An inner flexible vacuum bag is placed over the lay-up and sealed to the mold surface and a ridged outer chamber is placed over the flexible vacuum bag and also sealed to the mold surface. A differential vacuum is drawn from between the flexible vacuum bag and lay-up and between the vacuum bag and ridged outer chamber, with the higher vacuum drawn from under the flexible vacuum bag. The blanket heater is turned on and the lay-up heated until the thermoplastic material has melted. Thereafter, the lay-up is allowed to cool to ambient conditions. The problem with this apparatus is that, its advantage of portability is diminished if the part being formed is large. Secondly, if high compaction pressures were required, an autoclave would still be necessary, even if the apparatus was used for de-bulking. Further the heater system and rigid chamber is part specific and must be fabricated for each tool and lay-up. 
     Thus, it is a primary object of the invention to provide to provide a de-bulking system for the manufacture of composite parts that does not require the partially completed lay-up and mold to be placed in an autoclave. 
     It is another primary object of the invention to provide a de-bulking system for the manufacture of composite parts that is easily transportable. 
     It is a further object of the invention to provide a de-bulking system for the manufacture of composite parts that is inexpensive to manufacture and is can be easily adapted to various size parts. 
     SUMMARY OF THE INVENTION 
     The invention is an apparatus for de-bulking a composite material preform laid up in a mold. In detail, the apparatus includes a flexible cover having a center portion to fit over the mold and preform and a peripheral portion for attaching to the sides of mold. The cover has an inlet port located in the cover such that when the cover is fit over the mold and preform, the inlet is positioned over the preform. Preferably, the cover includes a perforated sheet such that when the cover is installed, the sheet separates the inlet port from the preform. This perforated sheet insures that the hot air is distributed uniformly over the preform. 
     A sealing assembly is provided for sealing the peripheral portion of the cover to the sides of the mold forming a chamber over the preform. The sealing assembly is preferably a stretchable cord tied about the peripheral edge of the cover securing the cover to the sides of the mold. A hot air pump has its output port coupled to the inlet port of the cover. Preferably, the pump produces heated air a sufficient pressure to cause the stretchable cord to expand allowing heated air to escape from the chamber. 
     In another embodiment, the peripheral side portion extends beyond the sealing assembly and the edge of peripheral side portion is secured to the mold to form a second chamber extending about the mold. Alternately, the edge extends to the floor upon which the mold rests also forming a second chamber about the mold. In either instance the second chamber includes an outlet port which is coupled to the inlet port of the pump. Thus hot air can be re-circulated improving heating efficiency. 
     During lay-up of the preform, the process is periodically stopped and the partially completed preform is vacuum bagged and the apparatus installed. A vacuum is pulled from between the vacuum bag and mold, in-situ heated air is applied causing the preform to heat of a temperature that drives off any entrapped gases. After a sufficient time has passed insuring that all entrapped gases have been removed, the apparatus is removed and along with the vacuum bag and the lay-up process continues. 
     The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description in connection with the accompanying drawings in which the presently preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a molding apparatus for the manufacture of composite parts. 
         FIG. 2  is a cross-sectional view of the molding apparatus shown in  FIG. 1  with a first embodiment of the invention installed there on. 
         FIG. 3  is a cross-sectional view of the molding apparatus shown in  FIG. 1  with a second embodiment of the invention installed there on. 
         FIG. 4  is a perspective view of the molding apparatus and second embodiment of the invention shown in  FIG. 3   
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A typical composite fabrication tool is illustrated in  FIG. 1  and generally indicated by numeral  10 , resting on the floor  12 . It includes a mold  14  having a mold surface  16  and side surfaces  18 . The molding surface  16  for purposes of illustration is flat, however, it could, and in most cases would have a complex contour. A release agent  20  is first positioned on the surface  16 . Thereafter, individual plies of pre-impregnated cloth and resin, for example and epoxy impregnated filamentary material is laid up over the release agent  20  on mold surface  16  to form preform  22 . The individual plies could comprise woven material or could be unidirectional plies laid up at plus and minus 90 degrees as well as plus or minus 45 degrees. For purposes of illustration the preform  22  is laid up in three sections  22 A,  22 B and  22 C. After the preform  22  is complete, a release film  24  is placed there over. Finally, an impervious flexible vacuum bag  28 , typically made of nylon or a Silicone based material is placed over the release agent  20  and release film  24  and preform  22  and sealed to the molding surface  16  by means of sealant tape  30  forming a chamber  32 . A vacuum line  34  is provided for drawing a vacuum from the chamber  32 . 
     A vacuum is applied to consolidate and remove entrapped gases from preform  22 . The completed assembly is then moved into an autoclave (not shown) wherein the temperature and pressure are raised, while a vacuum is drawn from the chamber  32 . This will cause the resin within the preform  22  to melt and cure, while consolidating the preform into its final shape. The temperature and pressure are then reduced to ambient and the mold  14  is removed from the autoclave and the now finished part removed therefrom. While the above process has been simplified for purposes of illustration, it does present the basics of composite manufacture. 
     One of the problems that may occur, especially if the preform  22  is made up of a large number of plies, is entrapped gases that will expand during heating may cause voids to be formed in the completed part, greatly reducing its strength. To eliminate this possibility, the prior art method of de-bulking was to only partially complete the preform  22  on the molding surface  16  of the mold  14  and place the assembly in the autoclave. For example section  22 A. The temperature was raised, to a temperature well below the curing temperature of the resin, while vacuum was drawn from the chamber  32 . This allowed any entrapped gases to be removed. This process would be repeated several times, depending upon the complexity of the preform. If the part being formed was large, requiring a corresponding large and heavy tool, the process of de-bulking was time consuming and expensive. The subject invention eliminated this problem. 
     Referring to  FIG. 2 , the subject invention is a portable in-situ heating apparatus, generally designated by numeral  36 , to accomplish de-bulking of the preform as it is assembled. After preform section  22 A has been assembled on the mold surface  16 , the apparatus  36  bought to the mold  14 . The apparatus  36  includes a flexible cover  38 , preferably made of a lightweight Nylon cloth, comprising a center portion  40  for extending over the mold surface  16  and preform section  22 A. The cover  38  also includes a peripheral side portion  42  having an peripheral edge  44 , which is configured to fit about the side surfaces  18 . The cover  38  further includes and inlet port  46  positioned in the center portion  40 . The cover is secured to the mold  12  by means of a stretchable cord (bungee cord)  48  that secures the peripheral side portion  42  of the cover  38  to the side surfaces  18  of the mold  14 . Thus a chamber  50  is created over the mold surface  16 . A suitable cover material is NYLON®, manufactured by the E. I. duPont de Nemours &amp; Company, Delaware. Preferably, Diamond Rip Stop Fabric using Type 6,6 NYLON®, coated with urethane manufactured by Astrostar International Incorporated, Sioux Falls, S. Dak. I 
     The apparatus  36  further includes a portable platform  52  containing a control panel  54 . The platform  52  includes an air pump  56 , having an inlet port  58  and outlet port  60 . The outlet port  60  is coupled to a heater assembly  61 . The heater assembly  61  includes an outlet port  62  that is coupled to inlet port  46  in the cover  38  via a tube  63 . The tube  63  is supported by an over head support beam assembly  64 . Thus in operation, after preform section  22 A of the preform  22  has been assembled and vacuum bagged and the cover  38  installed as illustrated, the pump  56  is turned on. Heated air enters the chamber  50  and heats the partially assembled preform  22 A, to cause any entrapped gases to be expelled and drawn off therefrom. 
     The temperature of the heated air is maintained at a level well below the curing temperature of the resin. The heated air passes over the mold surface  16  and around the side surfaces  18 . Note that the pump  56  is designed to produce sufficient pressure to inflate the flexible cover  38  and forces air pass the bungee cord  48 . This insures that hot air at the required temperature is maintained over the preform section  22 A. Additionally, the center portion  40  includes a perforated partition  65 , which will further insure that hot air is evenly distributed over preform section  22 A. After de-bulking of preform section  22 A has been accomplished the pump  56  is turned off, the cover  38  removed as well as the vacuum bag  28 . Thereafter addition layers of filamentary material is added to the partially assembled preform and the de-bulking process is repeated. 
     In  FIGS. 3 and 4  a second embodiment of the apparatus is illustrated. In this embodiment the peripheral side portion  42  extends passed the stretchable cord  48  and the peripheral edge  44  is either sealed to the floor  12  or the side portion  18  of the mold  14  forming a peripheral plenum  70 . Sealing can be accomplished by a second stretchable cord (not shown) or weights  71 , which seal the plenum  70  to the floor  12 . An outlet port  72  is provided in the plenum  70  that is coupled to the inlet port  58  of the pump  56  via a duct  74 . A Valve  76 , preferably a butterfly valve, is mounted in the duct  74  in proximity to the inlet port  58  of the pump  56 . Thus heated air, now at a reduced temperature, that is expelled passed the stretchable cord  48  is recycled, improving efficiency. The valve  76  is adjusted to insure that the cover  38  is fully inflated and that heated air is circulated over the preform section  22 A. 
     Thus it can be seen that the portable apparatus  36  can be used in place of an oven or autoclave for de-bulking of the preform, thus eliminating the time consuming task of moving a large mold and the expense of operating a large oven and resetting of the ply locater. The de-bulking process is used as many times as necessary. In the example provided this would include a de-bulking step after preform section  22 B of the preform  22  was assembled. A cost benefit analysis has indicated that a significant cost saving is achieved. 
     While the invention has been described with reference to particular embodiments, it should be understood that the embodiments of merely illustrative as there are numerous variations and modifications, which may be made by those skilled in the art. Thus, the invention is to be construed as being limited only by the spirit and scope of the appended claims. 
     INDUSTRIAL APPLICABILITY 
     The invention has applicability to industries manufacturing composite structures.