WO1996007533A1

WO1996007533A1 - Method of making composite product of tubular structure using clamshell mold

Info

Publication number: WO1996007533A1
Application number: PCT/KR1994/000118
Authority: WO
Inventors: Q. Wayne Lee
Original assignee: Lee Q Wayne
Priority date: 1994-09-02
Filing date: 1994-09-02
Publication date: 1996-03-14

Abstract

Composite tubular structure using clamshell mold made by laying on lower mold one or multiple of prepregs pre-cut larger than the inner surface of mold, inserting a film bladder to the inner space formed by the prepregs; overlapping two ends of the prepreg, closing the mold with upper mold, and heating and pressurizing as applying air into the inner space of the film bladder has superior strength, aesthetic quality and it provides light weight composite tubular structure that exhibits excellent processibility in its manufacture using two-piece mold.

Description

METHOD OF MAKING COMPOSITE PRODUCT OF TUBULAR STRUCTURE USING CLAMSHELL MOLD

BACKGROUND OF THE INVENTION

Field of the invention

The present invention relates to a unique method of making the tubular structure of composite material characterized by its superior strength and aesthetic quality. More particulary, the present invention relates to the manufacture of light weight composite tubular structure that exhibits excellent processibility in its manufacture using two-piece mold that is herein described as upper and lower mold or sometimes called as clamshell mold. The present invention may be used for one-piece bicycle frames which are sometimes called monocogue frames, bicycle forks, tennis rackets, golf shafts, etc, that have tubular shapes.

Description of Related Arts

Composite material has been mostly used by the aerospace and sporting goods industries, and the products made with composite material such as carbon, Kevlar™ (E.I. duPont company), and s-glass fibers are commonly called advanced composite products. Advanced composite products differ from fiber reinforced plastics (FEP) which are made mainly from e-glass fiber. Some fiber reinforced plastics are made with sheet molding compound (SMC) or bulk molding compound (BMC) processes while boats, surf boards, and race cars are built applying wet layup process to cure the product at ambient temperature. A process that is known to manufacture tubular structure using composite materials is the application of pultrusion. This method is to pull and at the same time to extrude the fiber impregnated with resins, and to cure it while passing it through a mold. This method can be applied only to the structures with fiber direction parallel to the machine direction and the same cross section areas. Therefore, this method can not be applied to the structures such as tennis rackets and one-piece bicycle frames in which the cross sections of tubular structures vary and the tubular structures are not straight. Filament winding is yet another techniques that could be used to make tubular structures. However, the technique is not suitable for making complex shapes.

This invention is capable of manufacturing more complex tubular structures with uneven cross section of the tubes, cured in elevated temperature and elevated pressure, thus preventing the presence of air voids.

In recent years, composite fishing rods, golf shafts, ski poles, tennis rackets, and bicycle frames that are tubular structures have been made using carbon, s-glass, Kevlar™ (E.I. duPont company) , Spectra™ (Allied Signal company) fiber, etc. The description of the conventional manufacturing techniques which were applied to manufacturing these is as follows:

To manufacture such rods as fishing poles, golf shafts, and ski poles, prepreg (resins preimpregnated fabric) is cut to the size of the pattern that was predetermined to the dimension of the product pole, and rolled over the mandrel. Next, film tapes such as polypropylene (PP) , nylon, and polyester (PET) are wound over the prepreg layer thus to apply some pressure over the layer, which is then set in an oven in elevated temperature. After the prepreg is cured, mandrel is pushed out, and film tapes are removed, producing the final product.

Such sporting goods as tennis rackets are made by a similar process. The nylon film tube or bladder is rolled over the mandrel, and the pre-cut prepreg is applied over it. Then, mandrel is removed and the prepreg is put in a mold. Through the air inlet tube which is attached at one end of the nylon film tube, pressurized air is applied, and then prepreg is cured in elevated temperature. In order to insert the prepreg in the mold, two parts of mold are necessary, the upper and lower, which are called matched mold or clamshell mold. It is also necessary to attach an air inlet tube at the end of the plastic film tube.

This conventional technique has been commonly used in manufacturing composite fishing poles and tennis rackets. However, this technique can not be used in manufacturing more complex tubular structures which might have various sizes of tubular sections, or uneven cross-sections. An example is a bicycle fork. Furthermore, the technique can not be used when the connectors between the two tubes deviate significantly from the shape of the tubular structures. The lug in one-piece bicycle frame, which connects the two tubes, is one example.

Recently, two techniques have been developed in the production of one-piece bicycle frames and forks using composite materials.

One technique is to produce the tubular section and the lug separately and then join them with adhesives.

This so called "tube and lug" technique is not a true one-piece frame. The second technique, as disclosed in

US Patent No. 4,902,458 by Trimble, is to apply separate layers of pre-cut prepreg 3, 3' in Fig. 1A, to both the upper and lower molds 1, 2 in Fig. 1A. Then the nylon bladder 4 in Fig. 1A, is inserted inside the prepreg layer of lower mold, and pressurized air is applied through an air inlet valve that is attached to the end of the bladder. It is then cured in high temperature, resulting in tubular structure. In order for the prepreg in two molds to join, the pre-cut prepreg in lower mold is made larger than the inner surface of lower mold, then a part of the prepreg laid above the mold edges is joined to the prepreg of the upper mold when the two molds are put together. The overlapping part 5, 5' in Fig. IB will be joined. The width of the overlapping portion is desired to be approximately 1/2 inch according to US patent No. 4,902,458. Fig. 1A and IB illustrate the cross-sectional views of the tubular structures before and after forming the fork. This technique will be called "External Overlap Method" in view of the fact that the top portion of the prepreg of the lower mold is overlapped with the lower part of the prepreg of the upper mold. It is noted that the tubular structure made by this external overlapping method is made by joining the prepregs of less than full circles. These overlapped areas are easily damaged when the tubular structure is compressed.

SUMMARY OF INVENTION

It is an object of the present invention to provide a method of making strong tubular structures with complex shapes in which cross-sections of the tubes deviate from circular shapes and/or they change the dimensions drastically from one section to another.

It is an another object of the present invention to provide a process of the fabrication of tubular structure using composite material that enables to make light weight and complex shapes with superior strength and aesthetic quality.

Also, it is an object of the present invention to provides a unique process that allows for strong products with an improved appearance.

The present invention provides a method of making composite tubular structures using clamshell mold comprising the steps of: laying on lower mold one or multiple of prepregs pre-cut larger than the inner surface of mold; inserting a film bladder to the inner space formed by the prepregs; overlapping two ends of the prepreg; closing the mold with upper mold; and heating and pressurizing as applying air into the inner space of the film bladder.

Also the present invention provides a method of the present invention described above further comprising laying one or multiple layers of prepreg on upper mold before closing mold.

The prepreg layers of the present invention may be made by adjoining the entire areas or a portion of individual plies of prepreg.

And the width of the overlapping portion of the present invention is preferably from 0.1 to 2 inches.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1A is a sectional view of mold before forming the tubular structure according to the conventional method; Fig. IB is a sectional view of mold after forming the tubular structure according to the conventional method;

Fig. 2A is a sectional view of mold before forming the tubular structure according to the present invention;

Fig. 2B is a sectional view of mold after forming the tubular structure according to the present invention;

Fig. 3 is a graph showing viscosity change of epoxy resin system upon heating; and

Fig. 4 is a graph showing UTM measurement of compressive strength.

DETAILED DESCRIPTION OF THE INVENTION

The method of the present invention utilizes two- piece mold of upper and lower mold. Prepreg material is laid up on the lower mold surface with extra prepreg placed over the mold edges. A nylon bladder with an air inlet tube is placed inside the prepreg layer. The mold is closed with the upper mold and pressurized air is applied through the bladder as the mold is heated. The extra prepreg situated over the mold edges expands and adheres to the wall of upper mold. The prepreg makes a full circle, and the ends of the prepreg meet and overlap to make a strong tubular structure.

The present invention relates to the application of prepreg layer 3 in Fig. 2A, to the lower mold which is cut wider than the circumference of the tube, and nylon bladder inside the prepreg layer 4 in Fig. 2A. The portions which are placed above the mold edges are folded inside the mold. The lower and upper mold are closed 1, 2 in Fig. 2B, and pressurized air is introduced through an air inlet tube which is attached to the end of the bladder. The prepreg that was placed above the mold edges are pushed against the wall of upper mold and the rest is overlapped and joined together 6 in Fig. 2B. It is desirable to have the width of the overlap between 0.1 to 2 inches. In this process, one or multiple layers of prepreg can be applied to the upper mold as well before the molds are closed if such is desired. The prepreg layer may be one ply or more. Plies of prepreg may be made by joining the entire areas of prepreg or by joining particular sections of prepreg. Prepreg made of carbon fiber, Kevlar fiber (E.I. duPont company) , fiber glass, Spectra fiber (Allied Signal company) may be used.

Unidirectional prepreg is cut in carbon fiber orientation according to the pattern, and can be applied in several layers. Typical fiber direction may be 0, 30, -30, 45, -45, 60, -60, 90 degrees. Depending on the required strength of the product, various kinds of prepreg, thickness, number of plies or directions may be used. These various options are made possible by this invention.

The novel feature of this invention is that the prepreg which is applied to the lower mold, and extended above the lower mold edge, unfolds and covers the entire wall of upper mold and overlaps to the same piece to join as illustrated in Figure 2. This technique will be called "Internal Overlap Method", since the one-piece will fill both the lower and upper molds, and join to the other end by overlapping. This technique will also be referred Q Method, named after the cross section view that resembles the letter Q, which goes around the circle and has an overlap at the end.

In Internal Overlap Method, one-piece pre-cut prepreg is first applied to the lower mold 2, and when pressurized air is applied, the top portion of the prepreg unfolds and fits into the upper mold, and joins to the other end by overlapping. The cross section view of the tubular section, thus show one smooth circular shape of one-piece prepreg with one overlapping section 6 as illustrated in Fig. 2A and 2B. On the other hand, in External Overlap Method, two separate pieces of prepreg are applied to the upper and lower molds. The two pieces of prepreg are joined by overlapping portions which were extended from the lower mold edge. Thus, the cross-section view shows the two half circles of which the ends are joined at the overlapping areas as illustrated in Fig. IB. It was found that when external force was applied to the tubular samples made by External Overlap Method, it was the overlapped area that failed.

The epoxy resin contained in prepreg is cured when they are subjected to elevated temperature. However, at early stages of heating, the resin has a tendency of losing its viscosity, and gets hardened by gradually increasing heat to exceed the gel point, when ther osetting occurs by crosslinking. During the early stages of heating, when resin viscosity drops, the prepreg becomes pliable and slippery. This condition helps the folded portion of prepreg expand and adhere to the upper mold. Fig. 3 illustrates the viscosity change by controlled heating of prepreg resin system of 250 degrees F cure, made by Sunkyung Industries of Seoul, Korea. The following specific examples are illustrative of the present invention, but are not to be considered limiting thereof in any way.

EXAMPLE 1

In production of the tubular structures by Internal Overlap Method, two kinds of prepreg were used: fabric prepreg (bidirectional prepreg) , and unidirectional prepreg (UD prepreg) . Fabric prepreg was obtained with a constant resin coated on the plain-weave carbon fabrics. Sunkyung Industries was the source of the prepreg. Carbon UD prepreg made by Sunkyung Industries, was used that contains a constant amount of epoxy resin. The resin matrix was cured at 250 degrees F and the prepreg that contains 200 grams per square meter of carbon fiber was used. To enhance the aesthetic quality of the product, one layer of bidirectional prepreg was applied on the mold surfaces. Unidirectional carbon prepreg was cut according to Q method as described in the last sections with carbon fiber orientations of 45, -45, 45, 0 degrees. The prepreg layer was laid up on the lower mold and a considerable portion of the pre-cut prepreg that extends above the mold edge was folded inward. Then the nylon bladder to which an air inlet tube had been attached was inserted, inside the prepreg layer on lower mold. Three plies of prepreg, bidirectional carbon prepreg and 45 and -45 degree unidirectional prepreg, were laid up on the upper mold. The main purpose of inserting prepreg in the upper mold was to make the tubular structure of EXAMPLE 1 as close as possible, with regards to its compositions and total weight to that of COMPARATIVE EXAMPLE 1. The upper mold was placed on top of the lower mold and closed. Then the mold was placed in an oven, and the pressurized air was introduced through an air inlet tube with air pressure of 100 psi at 250 degrees F.

COMPARATIVE EXAMPLE 1

A tubular structure was made according to the general procedure disclosed by U.S. Patent No. 4,902,458. The same two kinds of carbon prepreg as those of EXAMPLE 1 were used for COMPARATIVE EXAMPLE 1. One layer of bidirectional prepreg was cut to the size of the lower mold and laid against it. UD carbon prepregs that were pre-cut with fiber orientations of 45, -45, 45, 0 degrees were laid on the mold. The size of the unidirectional prepreg was such that a portion of prepreg of about 0.5 inches could be extended above the mold edges. A nylon bladder with an air inlet valve was placed on the prepreg. On the upper mold, one layer of fabric prepreg was cut to the size of the mold and laid up and then four plies of 45, -45, 45, 0 degree unidirectional prepreg were laid up. The upper mold was placed on the lower mold and closed. Then the complete mold was placed in an oven heated at 250 degrees F with air pressure of 100 psi.

Testing was performed to compare the strength of the specimens of EXAMPLE 1 and COMPARATIVE EXAMPLE 1. In order to maintain objectivity in testing, tubular cross- sections in ring shape of the same weight were tested. The Universal Testing Machine made by Instron company was used to perform the compressive strength. The cross- head speed was 2 mm per minute, and sampling speed was 2.0 points per second. The results of tests are given to the following TABLE 1. TABLE 1

Load at Max Displacement Stress at Max Kg irrm Kg/mm²

EXAMPLE 1 36.52 6.588 1.230

COMPARATIVE 14.50 6.660 0.4883

EXAMPLE 1

It was observed with COMPARATIVE EXAMPLE 1 that the overlapping portions were breaking apart when the load of 14.50 Kg was applied. EXAMPLE 1 showed no breakage of the ring shape under the same load, and tolerated more than twice the load as COMPARATIVE EXAMPLE 1 did. Fig. 4 illustrates the UTM measurement of compressive strength.

Claims

WHAT IS CLAIMED IS:

1. A method of making composite tubular structures using clamshell mold comprising the steps of: laying on lower mold one or multiple of prepregs pre-cut larger than the inner surface of mold; inserting a film bladder to the inner space formed by said prepregs; overlapping two ends of said prepreg; closing said mold with upper mold; and heating and pressurizing as applying air into the inner space of said film bladder.

2. The method of claim 1 further comprising laying one or multiple layers of prepreg on upper mold before closing mold.

3. The method of claim 1, wherein said prepreg layer(s) are made by adjoining the entire areas or a portion of individual plies of prepreg.

4. The method of claim 1, wherein the width of the overlapping portion is from 0.1 to 2 inches.