Method and Apparatus for Forming an Elongated Composite Part

This invention relates to forming an elongated composite material part, and more particularly, to a method and an apparatus for forming an elongated composite material part having a predetermined cross section.

Pultrusion formation of elongated composite material parts is well known to the art. In essence, pultrusion forms parts by pulling uncured laminations of plies of a matrix material (such as graphite fiber cloth) preimpregnated with an adhesive (such as a thermosetting epoxy) through a die having a desired cross section and hardening the resulting elongated composite material part by curing the adhesive. In order to retain the preimpregnated adhesive within the composite material matrix before the adhesive is cured, the preimpregnated plies are enclosed in one or more slip tapes. The slip tape, along with the preimpregnated composite material plies, is pulled through the pultrusion die, which includes a heated punch, and then into an autoclave, wherein the uncured epoxy is hardened. After the epoxy is cured, the slip tape (which does not adhere to the cured epoxy of the part), can be easily removed.

Prior to the development of the present invention, the tools used to wrap the preimpregnated composite material plies with the slip tape provided, at best, a loose fit of the slip tape on the laminations. The loose fit required the additional use of end stops in the slip tape to aid in containing the liquid epoxy along the slip tape edges during the pultrusion process. The loose fit also made the slip tape and preimpregnated plies difficult to pull through the pultrusion die.

It is therefore desirable to provide a tool and method for tightening the wrap of a slip tape around preimpregnated plies of composite material and preforming the wrapped preimpregnated plies into the shape of the desired elongated part.

The present invention provides a method and an apparatus for tightening the wrap of a slip tape around a plurality of plies of uncured, preimpregnated composite material and for preforming the enclosed preimpregnated plies to the desired shape.

It is an object of the present invention to provide a method and an apparatus for progressively tightening a slip tape around a plurality of preimpregnated plies of composite material.

It is another object of the present invention to provide a method and an apparatus for preforming a plurality of preimpregnated composite plies enclosed in a slip tape into a desired cross-sectional shape.

It is a further object of the present invention to provide a method and an apparatus for tightening a slip tape around a plurality of plies of preimpregnated composite material in order to contain the liquid epoxy within the preimpregnated plies.

In one aspect, the present invention provides a tool for tightening a slip tape around a preformed, elongated, composite material part blank, the elongated part having a predetermined desired cross section defined by a perimeter including at least two portions. The tool comprises a form adapted to receive a relatively short section of a length of a slip tape enclosing preimpregnated plies of uncured composite material and to travel along the length of the slip tape. The form has a leading edge and a trailing edge, the edges defining the direction of travel of the form along the length of the slip tape from the leading edge to the trailing edge. A portion of the perimeter of the cross section of the form is complementary to a first of the portions of the perimeter of the elongated composite part. The tool further comprises holding means attachable to the form for forcing the preimpregnated plies of uncured composite material in the elongated slip tape against the perimeter portion of the form to form the first portion of the perimeter of the elongated composite part and for forming a second portion of the perimeter of the elongated part.

In another aspect, the method of the present invention comprises the steps of placing a relatively short portion of the length of a slip tape enclosing preimpregnated plies or uncured composite material in a form having a portion of its cross section complementary to the cross section of the desired elongated composite part, attaching a holding means to the form for forcing the preimpregnated plies of uncured composite material in the elongated slip tape against the perimeter portion of the form to form the first portion of the perimeter of the elongated composite part and for forming a second portion of the perimeter of the elongated part, and causing the form and the holding means to travel the length of the preimpregnated plies of uncured composite material and the slip tape, thereby tightening the slip tape around the preimpregnated plies.

FIG. 1 is an isometric view of preimpregnated plies of composite material enclosed in a slip tape.

FIG. 2 is a closeup cross-sectional view of a portion of FIG. 1.

FIG. 3 is an isometric drawing showing the preimpregnated plies of composite material in the slip tape of FIG. 1 after the plies and slip tape have been formed in the desired shape.

FIG. 4 is an isometric view of one embodiment of the apparatus of the present invention.

FIG. 5 is an edge-on view of the wrapper tool shown in FIG. 4.

FIG. 6 is a cross-sectional view of a portion of the apparatus shown in FIG. 4.

FIG. 7 is a view, in partial cross section, of one face of the tool shown in FIG. 4.

FIG. 8 is an isometric view of a second embodiment of the apparatus of the invention in partial cross section.

FIG. 9 is an edge-on view of a third embodiment of the apparatus of the present invention.

FIG. 10 is an edge-on view of a fourth embodiment of the apparatus of the present invention.

FIG. 1 is an isometric view of a part blank consisting of a plurality of preimpregnated plies 10 of uncured composite material enclosed in a slip tape 12. The plie can be, for example, several lengths of graphite fabric and can be preimpregnated with uncured epoxy. The transverse width of each ply is typically up to six inches, but can be virtually any dimension. The slip tape 12 can be made from any suitable material, such as a nylon fabric having a Teflon.RTM. coating. The slip tape 12 can completely enclose the preimpregnated plies 10 with one or more lengths. In the typical application shown in FIG. 1, slip tape 12 comprises two lengths of Teflon.RTM.-coated nylon fabric. A first length 14 covers substantially the entire width of the bottom surface of preimpregnated plies 10. The second length 16 covers the entire upper surface of preimpregnated plies 10 and overlaps the two edges of the first length 14 on the bottom surface of the preimpregnated plies 10. The details of the overlapping first and second lengths of slip tape can be seen in the closeup cross-sectional view of FIG. 2.

The part blank can be formed to the desired final cross section and wrapped tightly in its slip tape by the method and apparatus of the present invention. A portion of the desired cross section of the elongated part is created by a form (or other forming means) of the apparatus, while the remainder of the cross section is generated by a coacting resilient holding means of the apparatus. Assuming that the part that is to be formed in L-shaped, FIG. 3 shows the shape of the slip tape-wrapped, preimpregnated plies 10 after they have been folded lengthwise to produce a part having the desired cross section.

A first embodiment of the apparatus of the present invention is shown in FIG. 4. Tool 20 consists of a form 22 and a holding means 24. Tool 20 tightens the wrap of the slip tape around the part blank and forms the L-shaped elongated part blank 18 of FIG. 3 (also shown in phantom lines in FIGS. 4, 5, and 7). The concave portion 17 of part blank 18 is placed against the upper surface 26 of form 22. The holding means 24 is resiliently biased against the upper surface of part blank 18 to force the part blank 18 to conform to the shape of the form 22. Holding means 24 also causes slip tape 12 to wrap more tightly around preimpregnated plies 10, as explained in greater detail below.

Aluminum has been found to be a convenient material for making form 22, and it can be formed by bending a plate lengthwise and thereby creating rounded ridge 32. Rounded ridge 32 extends between a leading edge 28 and a trailing edge 30, and separates the two planar portions of upper surface 26. Rounded ridge 32 is slightly tapered adjacent leading edge 28 of form 22 in order to facilitate the shaping of part blank 18 by upper surface 26. Tapered ridge 34 can, for example, taper at an angle of about three degrees with respect to the remainder of the rounded ridge 32. To further facilitate the easy slippage of part blank 18 on tool 20, sections of slip tape fabric can be attached to upper surface 26 by an adhesive.

Holding means 24 can comprise a plurality of separate resilient transverse holding components 36 and 38. Holding components 36 and 38 can be made from, for example, covered elastic straps, such as conventional shock cords. FIGS. 1, 2 and 4 show holding components 36 and 38 being spaced away from form 22 by the thickness of the part blank 18 (indicated in dashed lines). In operation, the part blank is moved along the form 22 from the leading edge 28 to the trailing edge 30. The transverse force produced by holding components 38 and 36 against part blank 18 increases as part blank 18 moves along the tool 20. The increasing tension of the holding components 36 and 38 along the length of the form 22 causes the slip tape 12 to "crawl" and tighten against plies 10 to the required dimensions of part blank 18 before part blank 18 is introduced into the pultrusion die. The tension of the holding components 36, stretching between rounded edges 40 and 42 of side edge 62, thus tighten the slip tape 12 on the part blank 18 and force the part blank 18 to conform to the shape of the form 22. Side edges 62 connect leading and trailing edges 28 and 30 and run substantially parallel to the direction of travel of tool 20. In addition, side edges 62 are located transversely outward from part blank 18. In this way, form 22 further supports uncured part blank 18 until it has been formed by tool 20 and then cured.

Holding components 38 also help to define upper perimeter 44 of the cross section of part blank 18, but, more importantly, define edges 46 of part bank 18. Holding components 38 stretch between rounded edges 48 and 50 of a pair of transversely opposing notches 52, which are formed in side edges 62 of form 22. A holding component 38 must be longer than an adjacent holding component 36 in order to provide the same tensioning force against L-shaped elongated part blank 18. This is because, even though the distance around upper perimeter 44 of L-shaped part blank 18 is shorter between rounded edges 48 and 50 than it is between rounded edges 40 and 42, holding components 38 attach to side edges 62 while holding components 36 attach to the undersurface 66 of form 22. The attachment of holding components 36 and 38 is described subsequently in this application.

The force that is provided by an individual holding component 36 or 38 is determined by the relative change in length of the holding component between its relaxed length and its installed length. The ends of holding components 36 and 38 are defined by knots 54 that engage against pin pairs 56 and 58. Pin pairs 56, placed in holes 60 along side edges 62, define the stretched length of holding components 36. Pin pairs 58, on the other hand, which fit into holes 64 along undersurface 66 of form 22, define the stretched length of holding components 38. The distance between corresponding pin pairs 56 around upper perimeter 44 of part blank 18 is constant, independent of the position of the pin pairs 56 along form 22. Therefore, holding components 36 can provide increasing tension by making them with shorter relaxed lengths the further they are from leading edge 28 and toward trailing edge 30. Similarly, the holding components 38 having shorter relaxed lengths are placed closer to trailing edge 30.

If desired, alternating holding components 36 and 38 can be formed from a single length of shock cord (for example connecting the two adjacent knots 54 shown in FIG. 4). The length can have a series of knots spaced appropriately to define the required changing lengths of the alternating holding components 36 and 38. It may be particularly convenient to attach the knots placed adjacent to pin pairs 56 and 58 along one side of form 22 by an adhesive, such as a small dab of silicone adhesive.

To produce an L-shaped elongated composite part having three-inch legs with a tool that is fifteen inches long, it has been found convenient for holding components 36 and 38 to have relaxed lengths in accordance with the following table:

In this table, position numbers increase from leading edge 28 to trailing edge 30. The positions are spaced one inch apart.

While it is designed primarily to be held in a fixed position with respect to a pultrusion die, the tool 20 can also be adapted for hand use by attaching a handle 68 to the undersurface 66 of form 22. Handle 68 can be attached to form 22 through matching threads formed on the end of handle 68 and in the body of form 22.

The above-described tool 20 produces L-shaped elongated parts 18 having legs whose lengths are retained within a tolerance of ten one-thousandths of an inch and whose angularity is controlled to one-tenth of a degree. In addition, leg thickness can be controlled to three one-thousandths of an inch. In some cases, it may be desirable to further control the uniformity of the legs of an L-shaped part blank 18. This can be accomplished by means of cinch plates 70, as shown in the isometric view of FIG. 8. The undersurface 72 of cinch plate 70 can be made extremely smooth in order to very accurately control the thickness of part blank 18. Typically, undersurface 72 very closely parallels upper surface 26 of form 22. Cinch plate 70 is held in place by a holding component 38, such as knotted shock cord. The knots defining the relaxed length of holding component 38 are caught behind a pin pair 58 and it passes through notch 52 and over cinch plate 70 to a corresponding pin pair 58.

The steps required to produce an L-shaped elongated part blank 18 with the apparatus described above are: first, to place a relatively short portion of the length of slip tape 12 in the form 22. The next step is to attach holding means 24 to the form 22 for forcing the preimpregnated plies 10 of uncured composite material against the perimeter portion of the form 22 to force the part blank 18 to have the desired cross section. The last step is to cause the form 22 and the holding means 24 to travel the length of the predetermined plies 10 and the slip tape 12. This tightens the slip tape 12 around the shape of the preimpregnated plies 10.

The above-described method and apparatus can be be generalized by those skilled in the art to produce elongated parts whose cross sections are more complicated than L-shaped. As shown in the cross-sectional view of a wrapper tool in FIG. 9, a T-shaped elongated part blank 80 can be placed in a form 82 comprising pieces 84-88. The part blank 80 can be formed from preimpregnated plies 10 of a composite material, and fully enclosed in a slip tape 12. Pieces 84 through 88 have internal cross sections, portions of which are complementary to portions of the desired cross section of part blank 80. Force is simultaneously applied to parts 84-88 by holding means 90. Holding means 90 can, for example, be one or more lengths of shock cord. Parts 84-88 can be retained in lengthwise alignment by means of notches 92 which are formed in parts 84-88. The part blank 80 passes through the wrapper tool from a leading edge to a trailing edge. In order ot facilitate the smooth formation of the T-shaped part from part blank 80, the leading edges of the forming means 82-84 can be tapered in a manner similar to the tapered ridge portion 34 of rounded ridge 32 in form 22 of FIGS. 4, 5 and 7. If further desired, the force applied by forming means 82 against part blank 80 can be increased in the direction of travel by decreasing the length of the respective holding means 90 in the direction of travel.

A third embodiment of the tool of the present invention is shown in the cross-sectional view of FIG. 10. In this case, the tool receives an I-shaped elongated part blank 100. Form 102 comprises end plates 104 to define the upper and lower crossbars of the part blank 100, and further comprises forms 106, which define the connecting web between the end pieces of the part blank 100. End plates 104 and forms 106 contain notches 108, which help to retain end plates 104 and forms 106 in correct lengthwise alignment. As disclosed above in connection with FIG. 9, the leading edges of end plates 104 and forms 106 can be tapered in order to facilitate the formation of the I-shaped part. In addition, if it is desirable to impose an increasing transverse force on part blank 100 in the direction of travel, the lengths of holding means 110 can be caused to decrease in the direction of travel.

While the foregoing has described the method and preferred embodiments of the apparatus of the present invention, one skilled in the art will appreciate that a variety of modifications can be made to these embodiments without departing from the spirit and scope of the invention. Accordingly, the spirit and scope of the present invention are to be limited only by the following claims.