Abstract:
A pultrusion die is usually fashioned of multiple pieces. One or more die components, and possibly all die components, can be formed by facing less precisely machined and finished tooling materials with highly polished sheet metal. Other possible die components can be formed from conventionally machined and polished steel surfaces. The surfaces that will form the inside of the die are highly polished. The components are fastened together in a pattern that will assure uniform pressure and good heat transfer among the components. The entrance and exit orifices are optionally provided with beveled edges to improve entrance of materials into and release of the finished product from the die.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 60/242,529 Oct. 23, 2000 the entire disclosure of which is incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002] This invention was made in part with United States Government Support under Contract Number F04606-96-C-0086, awarded by the United States Air Force. Therefore, the U.S. Government has certain rights in the invention. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    Pultrusion is a cost effective manufacturing process for producing continuous runs of constant cross section structural members made from fiber reinforced composite material, particularly those made using thermoset and thermoplastic matrix materials. The details of a particular pultrusion process implementation vary depending on the specific materials being converted to useful structures and the shape of the structures being produced. In general, in a typical pultrusion process, a succession of processing operations is arranged one after the other in series and designed to function together as a single, continuously flowing stream, with each step of the process automatically feeding the next with a steady flow of material. In one implementation, dry materials, in the form of individual tows of fibers (i.e., like thread on a spool) and/or fabrics of the same or different fiber on creels are continuously fed into a set of guides that form the materials into the general shape of the finished components. The materials are then fed into a station that completely wets the dry fiber materials with the matrix resin. The wet materials then enter the pultrusion die, in which the resin reacts or cures to a solid material. Curing may continue with additional heaters downstream of the die exit. A pulling mechanism is used to move the material continuously through the process at a steady pace. The production line may end with a cutting mechanism to cut the finished product to predetermined lengths.  
           [0004]    The pultrusion process requires highly polished die surfaces that are often chrome (or other metal) plated in order to provide the combination of low surface friction and wear resistance required to successfully pass composite product through the die without excessive friction or product adhesion to the die surface.  
           [0005]    A pultrusion die is usually made with multiple parts, each part typically being made from a precisely machined and polished, structural, heat transmitting, high quality, costly tooling steel. Usually top and bottom die components are the largest parts of the pultrusion tool, and span a pultrusion machine&#39;s width. The interior surfaces of the die components facing a die cavity generally have a high degree of surface polish, and often a mirror surface. The cost of both the high quality tooling materials and the polishing needed to achieve surfaces with the required surface finish makes conventional pultrusion tooling relatively expensive. Tool cost increases as tool size increases, and can often be prohibitive, particularly for short prototyping runs of large parts.  
           [0006]    One application of the pultrusion process is the production of sandwich panels made with foam core and thin composite skins. In one example of how a sandwich panel might be pultruded, sheets of core, often in the form of a homogeneous closed-cell foam, that have been cut to the proper thickness and width are butted edge-to-edge so that no significant gap exists between the trailing edge of the first-to-be-introduced foam sheet and the leading edge of the next-introduced sheet of foam. These sheets are introduced between upper and lower skins of fiber fabric at any point before the entrance to the pultrusion die. The foam then moves through the process with the skins. The closed cell foam prevents resin impregnation into the cores. The finished part exits the die as two rigid cured composite face sheets laminated to the thicker, lightweight core.  
           [0007]    A limitation to the size of finished parts produced by the pultrusion process is the cost to fabricate large dies. As the components being considered for pultrusion become ever larger, the costs associated with providing such high quality surfaces on large steel plates becomes increasingly larger, due to the quality of steel required and to handling difficulties with the large plates (possibly greater than 10 feet wide and more than 3 inches thick, weighing many thousands of pounds).  
         SUMMARY OF THE INVENTION  
         [0008]    A low cost alternative to conventional pultrusion tooling methods achieves a highly polished pultrusion die surface by replacing the conventional machining and polishing of expensive tooling steel with lining some or all of the die&#39;s interior surfaces with highly polished, low cost, commercially available sheet steel. Using this technique, the component surfaces that do not form the cavity, and possibly the mating surfaces of the die components, are optionally finished to less exacting, less expensive levels. Through holes, tapped holes and tooling pins are drilled in the components so that the die components can be fastened together with the polished surfaces facing the interior and forming the cavity. When the entrance and exit orifices have rounded or beveled edges, the sheeting material conforms to the adjacent plate surfaces, allowing material to feed into the die more smoothly and the finished pultruded product to release from the tool more easily. Heaters and other components needed for the pultrusion process are added to the die before or after it is mounted on the pultrusion machine. Side edges with features such as flanges or recessed regions are formable with the polished sheeting die system. The sides of the cavity can be made using more conventional tooling techniques and integrated with other tooling components lined with polished sheet to form a complete pultrusion die. Alternately, the side pieces of the cavity can be formed less expensively and lined with polished sheet before being integrated with the other components. The technique works well for large, flat panel dies, and can also be used to make tools with rounded surfaces, multiple cavities, multiple curvatures and other complex cross sections. Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0009]    The invention will be understood from the following detailed description in conjunction with the drawings, of which:  
         [0010]    [0010]FIG. 1 is a block diagram of a typical implementation of the pultrusion process as is known in the industry;  
         [0011]    [0011]FIG. 2 is an exploded isometric view of the die according to the invention;  
         [0012]    [0012]FIG. 3 is a front view of the die of FIG. 2;  
         [0013]    [0013]FIG. 4 is a detail of a mounting region of a die according to the invention;  
         [0014]    [0014]FIG. 5 is a side view of the left side of a die producing a straight edge according to the invention;  
         [0015]    [0015]FIG. 6 is a side view of the left side of a die producing an edge with a flange according to the invention;  
         [0016]    [0016]FIG. 7 is a side view of the left side of a die producing an edge with a flange according to the invention;  
         [0017]    [0017]FIG. 8 is a side view of the left side of an alternate implementation of the die of FIG. 7;  
         [0018]    [0018]FIG. 9 is a side view of the left side of a die producing an edge with a recessed section according to the invention;  
         [0019]    [0019]FIG. 10 is a side view of the left side of an alternate implementation of the die of FIG. 9;  
         [0020]    [0020]FIG. 11 is a side view of the left side of a die producing an edge with a flange according to the invention;  
         [0021]    [0021]FIG. 12 is a side view of the left side of a die producing an edge with a recess and a flange according to the invention; and  
         [0022]    [0022]FIG. 13 is a side view of the left side of a die producing a shaped edge according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    A pultrusion die is, according to the invention, usually a multi-part steel tool having a mirror-polished internal cross section of the pultruded composite product machined as a cavity through its length. A pultrusion line incorporating a pultrusion die  20  according to the present invention is shown in FIG. 1. Dry materials  26  and  22  are placed together before being wetout by either a resin bath (not shown) or a resin injection tool  28 . The composite mixture then compresses as it passes through the pultrusion die  20  with distributed heaters, where the resin is activated and starts curing. A puller mechanism  30  grips the product assuring a constant speed through the die  20 . A cut off saw  32  cuts the finished product to length.  
         [0024]    The die  20  comprises a low cost, large panel die cavity formed by surfacing the flat, angled, or curving surfaces of rough-machined metal outer plate components with a highly polished, commercially available sheet metal. This saves considerable expense that would have been otherwise spent polishing the surfaces of a conventionally-built die and allows use of construction grade material, which may include more voids than expensive tool grade material, for the largest component of the die. The polished sheet metal is fastened to the leading edge (entry for uncured materials) and sides of the die  20 . Fastening may be accomplished by screws, bolts or a bonding method such as adhesive or brazing. Conductive grease may be interposed between the plates  40 ,  70  and the sheets  140 ,  142  to improve heat transfer where necessary.  
         [0025]    [0025]FIG. 2 is a simple rectangular die embodiment built using this process. More complex shapes can be similarly made. The die components consist of a bottom plate  40  and a top plate  70 , each faced with a polished sheeting material  140 ,  142 , and side spacers  50 ,  60  that may be made up of multiple parts (not shown). The finished cross section of the cavity  146 , as shown in FIG. 3, is defined by the height of the side spacers  50 ,  60  and the width  74  of the top (or bottom) plate  70  minus the overlap between spacers  50 ,  60  and the plates  40 ,  70  (typically the width  64  of the spacers  50 ,  60 ). The length  42  of the die is selected to allow sufficient heating and cooling. The finished product is often longer than the die length  42 .  
         [0026]    The material for the die components must transmit heat uniformly, not expand excessively at the temperature used to cure the resins, and be able to be manufactured with a highly uniform surface. Typical die component materials are tooling grade steel, although other metals and materials that are sufficient for adequate heat distribution may be readily selected by one skilled in the art. In one implementation, die components that are faced by polished sheet material are produced from construction grade steel, such as A36 steel, finished to between approximately 32-64 micro inch smoothness by a process such as rotary surface grinding. The tolerance for the non-cavity surfaces is determined by the tightness of fit required for product quality and heat transfer. Die components that directly form part of the die cavity are produced from tool grade steel finished to between approximately 8-16 micro inch smoothness. Highly polished sheet steel (with or without a chrome coating that is used for ruggedness) is commercially available finished to approximately 8 micro inch smoothness.  
         [0027]    The width of side  54 ,  64  of the side spacers  50 ,  60  is chosen to allow at least sufficient surface area for the vertical connection of the die components. This connection is typically accomplished with screws or bolts, clamping the surfaces tightly enough that heat is transmitted from the top and bottom plates to the side plates that are not heated. In one implementation, side spacers  50 ,  60  are supplied with multiple sets of through holes to allow some variation in cavity width in a die built with the same side spacers  50 ,  60 .  
         [0028]    The thickness of the top and bottom plates  40 ,  70  is selected to provide a uniform surface with sufficient heating capacity and controlled sag across its width. In one embodiment, the entrance and exit edges  45 ,  75 ,  47 , and  77  of the top and bottom plates  40 ,  70  may be beveled  80 , as illustrated in the detail of FIG. 2, to facilitate lead-in of material and provide an easier surface for wrapping the sheet material around the plate. The side spacers  50 ,  60  are optionally made shorter than the top and bottom plates  40 ,  70  to allow for the beveling of the edges and the addition of auxiliary components to the corners of the die as needed. FIG. 3 is a front view of the die of FIG. 2 illustrating the cavity  146  and that, when a modular spacer  50 ,  60  is used, the outside edges  53 ,  63  of side spacers  50 ,  60  do not necessarily align with the outside edges  43 ,  73  of the top and bottom plates  40 ,  70 .  
         [0029]    One embodiment of die  20  uses a bolt pattern as illustrated in FIG. 4 along the sides of the top and bottom plates  40 ,  70  to fasten the die together. Two staggered rows of bolts  90  clamp the die  20  together. The rows of bolts  90  are configured so those bolts are evenly spaced (approximately every 2 in. for one embodiment) along the length of the die  20 . One line of bolts  92  is positioned close to the inner edge  61  of the spacer  60 , while a second row  94  is further from edge  61  but interposed between the holes of first row  92 . When the spacer  60  is shorter than the top and bottom plates  40 ,  70 , auxiliary components may be affixed to the die using bolt holes  60  in the overhang  63 . Alternate patterns that provide secure fastening may be used. In one embodiment, the holes in the top plate  70  are countersunk to receive the bolt heads and the holes in the bottom plate (not shown) are tapped. The bolt holes in all other components are through holes allowing the bolt to pass therethrough. The bolts are tightened to a high fraction of their yield strength, such as {fraction (2/3)} of the yield strength of the bolt in one implementation.  
         [0030]    An implemented die according to the invention has successfully produced at least 8-foot wide foam-cored and solid composite panels.  
         [0031]    [0031]FIG. 5 shows a left side detail of a basic implementation of this die fabrication method. Thin sheets of highly polished steel  140 ,  142  are sandwiched respectively between structural steel top and bottom plates  40 ,  70  and a side spacer  60  that defines a vertical edge  144  of to the die cavity  146 . All five pieces shown are held together using a fastening pattern as described above. This implementation is suitable for fabricating a rectangular cross section solid or cored panel, whose thickness is determined by the spacer  60  dimensions. The panels&#39; width varies with the top and bottom plates  40 ,  70  and positioning of the spacer edge  144  within the die plates. For a given set of top and bottom die plates  40 ,  70 , spacer  60  is provided with several sets of through holes in some embodiments, allowing some variability of die cavity width without requiring new die plates  40 ,  70 .  
         [0032]    Many applications of pultruded panels require edge details that deviate from the form provided by the simple rectangular cross section of FIG. 5. FIGS.  6 - 13  illustrate several variants of the basic die configuration that accommodate edge details of quite general utility, further variations are within the scope of this disclosure.  
         [0033]    [0033]FIG. 6 illustrates the die for a product having a thin flange  148  extending from one panel edge outward in the same plane as the main panel. This is easily accommodated with the illustrated die fabrication configuration by relieving the spacer  60  by under cutting one surface to form a product extension  148  in the indicated fashion to form the horizontal flange in the desired product. Three spacer surfaces  150  facing the product are finished with a mirror quality surface (˜8 micro inch).  
         [0034]    [0034]FIG. 7 illustrates a die configuration for a panel product in which a flange  152  is desired at the panel edge normal to the plane of the main panel. This is accomplished in the polished sheet die by the indicated multi-piece configuration. The key feature of this die configuration is a strip  154  (in the longitudinal—into the paper—direction), composed of a number of pieces  160 ,  161 ,  163 , that secures an edge  156  of the polished sheet  140  to the top die plate  70 . The strip  154  is secured by a row of bolts  158  as indicated to secure the edge  156  of the polished sheet  140 , and also to secure the die sub-components  160 ,  162  together. FIG. 8 illustrates a modification of FIG. 7 in which a strip  164  is securing an edge  166  of the top polished sheet  140 . A part  167  holds the edge  166  of the polished sheet  140  against the plate  70  extension  163 . This configuration may have some advantages in setting up the die and in finishing the part after it exits the pultrusion machine.  
         [0035]    Another product of interest is a panel with a recessed area  176  along one or both edges. This configuration may be manufactured with the polished sheet die by using a multi-part side spacer  60 ,  174  as shown in FIG. 9. Here, a secondary spacer  174  extends further into the cavity  146  to create the recessed area  176 , which may be implemented on one or both sides of the panel. As before, only the spacer  174 ,  60  surfaces facing the die cavity  146  need to be finished to a high quality surface.  
         [0036]    [0036]FIG. 10 illustrates a second die configuration for creating the recessed panel areas  176 , in which the edge of a secondary spacer  180  is secured to the die plate  70  by a line of threaded fasteners  182 . This configuration allows longer and more complex recessed areas  176  to be incorporated, but requires that a line of holes be drilled in the polished sheet  140 , rendering it less useful for other modularized die configurations.  
         [0037]    Another panel edge flange  184  at an angle to the main panel plane is shown in FIG. 11. In this case, an edge  185  of the polished sheet  140  adjacent to the turned-up flange  184  is secured to a die sub-component  186  next to the flange using an adhesive, soldering, brazing or a welding process to provide somewhat more design flexibility in the panel itself. The securing by adhesive, soldering, brazing or welding is an alternative for securing the polished sheet edge as an alternative to the fastening that provides for more modularity.  
         [0038]    A panel with the recessed area  188  near its outboard edge(s) is shown in FIGS. 12A and 12B, with the die configuration required to produce it. In FIG. 12A, the recess  188  is shown as a convenient way to butt two panels  185 ,  187  into a perpendicular arrangement. This method provides for an overlapping flange  189  that can be used to secure the panels together. In FIG. 12B, a secondary spacer block  190  or strip is secured to die plate  70 , with a line of fasteners  192 , which also penetrate the polished sheet  140 . Both the side edge of spacer  60  and the sides of the secondary spacer block must be finished to mirror polish in this configuration.  
         [0039]    Several different types of edge detail may be machined into the edge  194  of side spacer  60 , as indicated in FIG. 13. For designs of modest physical width and complexity, this method becomes a very simple modification of the die design shown in FIG. 5.  
         [0040]    Having described preferred embodiments of the invention it will now become apparent to those of ordinary skill in the art that other embodiments incorporating these concepts may be used. Accordingly, it is submitted that the invention should not be limited by the described embodiments but rather should only be limited by the spirit and scope of the appended claims.