Method of manufacturing composite single-tubed structures having ports

A method of manufacturing a structure using a single, hollow primary tube, preferably of a composite material, wherein ports are bonded to the walls of the hollow tube through aligned holes on opposite sides of the hollow tube. The ports improve the stiffness, strength, aerodynamics, and aesthetics of the structure.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing a composite structure for a generic product, and more particularly, where the structure is generally tubular and constructed from a single, hollow tube having at least one, and preferably a series, of ports that extend through the hollow tube. The ports provide specific performance advantages. which may include strength, stiffness, comfort, aerodynamic and aesthetic benefits.

The generic structure thus formed can be any structure having a performance related to weight, balance, strength, stiffness, vibration, aerodynamics, or other performance characteristics. The structure can be solid or hollow, straight or curved. The structure can be used, for example, in sporting goods, tools, automobiles, aerospace, furniture and many other applications.

BACKGROUND OF THE INVENTION

There are numerous examples of existing structures being replaced by lighter weight materials. For example, fiber reinforced resins, also known as composite materials, have replaced wood structures in sporting good applications such as golf clubs, tennis racquets, hockey sticks and baseball bats. Composite materials have also been used to replace metal in similar applications such as golf clubs, tennis racquets, skis, and bicycle frames.

Preferably, the lightest materials and designs are used to achieve the performance goals of the particular structure. The most popular high performance material for modern structure design is carbon fiber reinforced epoxy resin (CFE) because it has the highest strength and stiffness to weight ratio of any realistically affordable material. As a result, CFE can produce a very light weight structure with excellent strength as well as providing a variation of stiffness at various regions along the surface or length of the structure.

However, there are limitations on carbon fiber based materials used for structures when considering strength requirements. For example, a tubular structure made from a carbon fiber composite can be susceptible to catastrophic failure resulting from excessive compressive forces, which can cause buckling of the thin walled tubes. The tubular structure may also be subjected to a multitude of stress conditions, for example, transverse impact loads, torsional loads or vibrational loads. A thin walled tube made of a fiber reinforced composite may not have the strength to withstand various loading condition scenarios.

Also, in the prior art, if holes were required in a hollow structure, for example, to reduce weight or for fastening means or for aerodynamics, the holes would be formed by removing material by cutting or drilling holes in the walls of the structure. This weakens the structure considerably when reinforcing fibers are severed during the cutting of the holes.

Thus, there exists a continuing need for an improved structure that has the combined features of light weight, improved bending, improved stiffness, improved vibration damping, improved aerodynamics, and improved aesthetics.

SUMMARY OF THE INVENTION

The present invention is a structure where at least a portion of the structure is formed of a single, hollow tube having at least one, and preferably a series, of ports that extend through the hollow tube. The ports provide specific performance advantages. Each port has a peripheral wall that extends between opposed, aligned holes in the hollow tube to form the port. The opposite ends of the peripheral wall are bonded to the walls of the tube. The peripheral wall forming the port, which extends between opposite sides of the tube, is preferably elliptically-shaped to form opposing arches, which provide strength, stiffness, comfort, and aesthetic benefits. The ports also provide an aerodynamic advantage because they allow air to pass through the ports, which reduces the aerodynamic drag of the structure.

The present invention applies preferably to composite structures, but will apply to tubular structures of all materials. For the composite structure, the holes to accommodate the ports may be formed in the primary tube prior to molding by punching or other suitable means. Although carbon fibers may be cut in the process, the primary tube retains strength due to the fact that, after molding, the tubular insert members which form the peripheral walls of the ports, are bonded to the hole edges and extend across the primary tube. Alternatively, the holes may be formed by separating fibers in the wall of the structure, in which case fibers will not be cut.

The present invention is designed to provide a combination of tailored stiffness, improved strength, light weight, improved aerodynamics, and improved aesthetics over current prior art structures.

The present invention provides a new and improved structure of durable and reliable construction which may be easily and efficiently manufactured at low cost with regard to both materials and labor. This provides improved aerodynamics, improved strength, improved fatigue resistance, and provides a unique look and improved aesthetics. The invention also allows for specific stiffness zones at various orientations and locations along the length of the structure.

For a better understanding of the invention and its advantages, reference should be made to the accompanying drawings and descriptive matter in which there are illustrated preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference toFIG. 1of the drawings, the present invention is a composite structure10, featuring one or more ports20formed into the walls of the structure for improving the flexibility, strength and other characteristics of the structure. Structure10is preferably fabricated of multiple layers of aligned carbon filaments held together with an epoxy binder. The fibers in the various plies are preferably parallel to one another, but the various plies may have varying fiber orientations. Structure10has a generally hollow configuration.

A plurality of ports20are formed in structure10. Ports20extend between opposed, aligned holes defined in the structure, as described in more detail below. Each port may be of any shape, but is preferably oval in shape, with the long axis of the oval aligned with the longitudinal axis of structure10. Each port20includes a peripheral wall22(seeFIGS. 6-7) that extends, in one embodiment, between opposite faces of the structure. The opposing ends of peripheral wall22are bonded to tubular structure10.

The ports are preferably in the shape of opposing arches which allow the structure to deflect, which deforms the ports, and allows them to return with more resiliency. The ports also allow greater bending flexibility and strength than would traditionally be achieved in a single tube design because internal columns formed by the peripheral walls of the ports help prevent buckling failures of the thin walled tubular structure. If the longitudinal axes of the ports are in line with the direction of travel of the structure, they can also provide an aerodynamic advantage, allowing air to pass through the structure, resulting in higher velocities. Finally, the ports provide the structure with a unique appearance.

The structure is preferably made from a fiber reinforced composite material. Traditional lightweight composite structures have been made by preparing an intermediate material, known as “prepreg”, which will be used to mold the final structure. Prepreg is formed by embedding fibers for, for example, carbon, fiberglass, aramid, boron, liquid crystal polymer, hemp and others, in resin. This is typically done using a prepreg machine, which applies the non-cured resin over the fibers so they are wetted out. The resin is at “B Stage” meaning that only heat and pressure are required to complete the cross linking and to harden and cure the resin. Thermoset resins, like epoxy, are popular because they are available in liquid form at room temperature, which facilitates the embedding process.

A thermoset is created by a chemical reaction of two components, forming a material in a nonreversible process. Usually, the two components are available in liquid form, and after mixing together, will remain as a liquid for a period of time before the cross-linking process begins. It is during this “B Stage” that the prepreg process happens, where the resin coats the fibers. Common thermoset materials are epoxy, polyester, vinyl, phenolic, polyimide, and others. Thermoplastic resins may also be used such as nylon, ABS, PBT and others.

The prepreg sheets are cut and stacked according to a specific sequence, with particular attention given to the fiber orientation of each ply. Each prepreg layer comprises an epoxy resin combined with unidirectional parallel fibers of the type previously mentioned. The prepreg is cut into strips at various angles and laid on a table. The strips are then stacked in an alternating fashion such that the fibers of each layer are oriented differently from the adjacent layers. For example, one layer may be +45 degrees, the next layer −45 degrees. If more bending stiffness is desired, a fiber angle such as zero degrees is used. If more torsional stiffness is desired, a higher proportion of +/−45 degree strips are used. If more bending stiffness is desired, a higher proportion of 0 degree fibers are used. Other fiber angles may also be used. Additionally, the stiffness may be varied in different places along the length of the structure using the method just discussed.

This lay-up, which comprises various strips of prepreg material, is then rolled over an internal mandrel in the shape of a tube. Referring toFIG. 2, according to the preferred embodiment of the invention, a suitable uncured prepreg tube30is formed in the manner just described, with the various composite plies oriented at the desired angles. The internal mandrel is removed following the formation of the prepreg tube.

Although the described method of forming the tubes is the preferred method, other methods could also be used, such as utilizing a wet lay-up, where fibers are impregnated with resin by hand and then rolled up or by resin transfer molding, wherein dry fibers are packed into a mold, the mold is closed, and resin is pumped or drawn by vacuum into the mold to impregnate the fibers.

Next, a one or more pairs of holes32are formed through opposing sides of the wall of the tube, perpendicular to the longitudinal axis of the tube. Holes32may be stamped through the walls, or, preferably, a tool is used to separate the carbon fibers from one another, without cutting the fibers, to form holes32. Holes32, at this stage, need not have the final desired shape.

The tube requires internal support or pressure to force the prepreg material against the surface of the mold. Therefore, an internal support element is necessary. In the preferred method, a inflatable bladders are used for this purpose. A pair of inflatable bladders34,35, preferably made of nylon, is inserted through tube30such that their facing walls36,37are aligned with holes32, as shown inFIG. 2. As discussed below, other types of internal support elements may also be used.

As shown inFIGS. 3-5, after bladders34,35have been inserted, a hollow, tubular plug40is inserted through each of the holes32, between the facing walls36,37of bladders34,35. Thus, as shown inFIG. 4, plugs40separate the two bladders at the points where they are inserted, but otherwise allow the facing walls36,37of bladders34,35to contact each other. Plugs40will form the peripheral walls of ports20.

Plugs40are preferably tubes composed of prepreg material. However, if desired, plugs40may be made of other materials such as metal or plastic. If plugs40are composed of prepreg material, the ends of plugs40will preferably extend beyond the outer surfaces of the prepreg tube30, as shown inFIGS. 4-5.

Finally, as shown inFIG. 5, if plugs40are formed of prepreg material, a mold pin50is inserted through each plug40to form the internal geometry of ports20and to prevent plugs40from deforming during the curing process. This may occur prior to mold packing, or during the mold packing process.

Tube30is then packed into a mold (not shown) which forms the shape of the outer surface of the structure. If the mold and tube are longer than the final desired dimension of the structure, a final cut to length operation can be performed on structure10after molding.

Air fittings are then attached to bladders34,35. The mold is then closed over tube30and placed in a heated platen press. For epoxy resins, the temperature is typically around 350° F. While the mold is being heated, tube30is internally pressurized by inflating bladders34,35, which compresses the prepreg material and forces tube30to assume the shape of the mold. At the same time, the heat cures the epoxy resin. The bladders also compress peripheral walls22of plugs40, so that the inwardly facing surface of each plug40conforms to the shape of mold pin50(which, in the preferred embodiment, is oval). At the same time, the heat and pressure cause the ends of plugs40to bond to the wall of prepreg tube30.

Once cured, the mold is opened in the reverse sequence of packing. Mold pins50are typically removed first, followed by the top portion of the mold. Particular attention is needed if removing the top portion with mold pins50intact to ensure that this is done in a linear fashion. Once mold pins50have been removed from structure10, structure10can be removed from the bottom portion of the mold.

As shown inFIGS. 6-7, after molding, structure10is formed of a primary, hollow, cured tube11, with a plurality of ports20extending through tube11. The ends of port walls54are bonded to the portions of tube11surrounding ports20, and the inwardly facing surfaces22of ports20extend completely through tube11.

In an alternate embodiment of the invention, ports20may be orientated in different directions. For example, alternative ports20may have their longitudinal axes oriented at 90 degrees with respect to each other. Any such arrangement of ports is contemplated to be with the scope of this invention. In such embodiments the manufacturing process is somewhat more complicated and may require the use of multiple bladders instead of two bladders. For example, if it is desired that the ports be oriented at 90 degrees with respect to each other, four bladders will be required, with the interface of the bladders forming a cross shape, where one leg of the cross supports tubular inserts40in one direction and the other leg of the cross supports tubular inserts40in the orthogonal direction. This embodiment will have the advantage of providing the strength improvements regardless of how the structure is swung or used. In addition, it is understood that the size, shape and placement of the holes can vary depending upon the desired performance of the structure. Likewise, more complicated arrangements using any number of tubes may be used.

The above mentioned process describes using internal bladder pressurization for the entire length of the structure as the internal support element. Other materials may also be used for this purpose. An alternative to using internal air pressure is the use of an expanding internal foam core that expands when heated. Another option is a liquid contained in the nylon bladder that turns into a gas when heated to generate internal pressure.

The structure may also be formed by applying external pressure using a rigid material as the core to resist the external pressure. This process is commonly called compression molding, where the pre-form is placed in the mold cavity, and the mold is closed over the perform compressing it. The internal core resists this externally applied pressure and consolidates the prepreg plies in between. Several options exist for the core material, but are not limited to the following examples. A rigid light weight foam can be used which will likely be contained in the part and not removed. If a hollow part is desired, a core made of rigid salt compound can be used and then after molding, the salt can be dissolved using water to create a hollow structure. Another option is to use glass beads contained inside a polymeric bladder to resist the external pressure and following molding, the glass beads can be evacuated from the structure.

In an alternative embodiment, it may be desirable to first mold a traditional portion of the structure without ports, then place this structure in another mold where the bladder molded portion forming the ports would be fused to it.

This alternative process is illustrated inFIGS. 8a-8b. A pre-formed portion10ahas been previously formed by bladder molding or compression molding, or, alternatively, may be composed of an alternate material and has been formed using a process particular to that material. Bladders34a,35amay extend through pre-formed portion10a, if possible, but may also extend only through prepreg portion30a.

Pre-formed portion10ais connected to the prepreg portion30aby means of an overlap joint56. This is to ensure a strong interface between the two portions. Other joining means may be considered. While the mold is being heated, prepreg tube30ais internally pressurized, which compresses the prepreg material and forces tube30ato assume the shape of the mold as well as to bond to pre-formed portion10a.

In yet another embodiment of the invention, the body of the structure may not necessarily be circular in cross sectional shape but, instead, may be elliptical or any other desired shape, including shapes having straight edges and non-symmetrical shapes, such as polygons and teardrops. The cross-sectional shape of structure is determined by the size and shape of the mold which is used to form the outside surface of structure and by the shape of the bladders used to inflate the structure from within.

In yet another embodiment, ports may be grouped in groups running along the lengths of the structure and need not appear as a sequential grouping all in one portion of the structure. Any desired spacing and orientation of the ports is contemplated to eb within the scope of the invention.

The size and spacing of the ports can affect structure stiffness in a desirable way. The ports can direct the flex point of the structure toward a particular region of the structure, if desired. An additional benefit of the ports in the structure is that they improve the durability and strength of the structure. This is because they act as arches to distribute the stress placed on the structure during flexing in a very efficient manner. In addition, the cylindrical internal reinforcements formed by the walls of the ports resist compressive loads, which tend to buckle the thin walls of the tube.

In some embodiments, it may be desirable that the structure have uniform longitudinal or torsional stiffness. In such cases it may be possible to make the structure more stiff at various localized places to compensate for a lack of stiffness that may be caused by a variety of factors. The structure can be made more stiff by adding one or more ridges on the external surface of the structure. For example, the placement of the ports in the structure will tend to decrease the structure stiffness in the areas defining the ports. The stiffness in these areas can be increased by defining ridges in the vicinity of the ports. Such ridges can be longitudinally or circumferentially disposed, and can be of limited length or can run the entire length of the structure. Additionally, the cross-sectional shape of the structure can also affect stiffness, particularly when such cross-sectional shapes define corners, such as with a polygonal or teardrop cross sectional shape. Note that if uniform stiffness is not desired, ridges may be added to increase the stiffness in some areas, while leaving other areas unaltered. Absent any ridges, the stiffness of the structure will be defined by the manner and angle at which the prepreg strips were laid out to form the basic hollow structure, as previously discussed.

In another alternative embodiment, it is also possible to use a metal material for the main structure such as aluminum or steel, and bond composite, metal or plastic cylindrical ports to the aluminum in a similar manner.

With respect to the above description, it is to be realized that the optimum dimensional relationships for the parts of the invention, to include variations in size, materials, shape, form, function and manner of operation, assembly and use, are intended to be with the scope of the invention. Further, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.