WO2017151603A1

WO2017151603A1 - Methods for fabricating preforms for high performance ultra-long fiber reinforced thermoplastic tubing

Info

Publication number: WO2017151603A1
Application number: PCT/US2017/019929
Authority: WO
Inventors: Lee Burch; Sean REYMOND
Original assignee: Impact Composites, Inc.
Priority date: 2016-02-29
Filing date: 2017-02-28
Publication date: 2017-09-08

Abstract

A method for fabricating a preform for hollow parts includes cutting a fiber prepreg material into pieces (1 2) each having an appropriate length; spacing each of the cut pieces (12); orienting each of the cut pieces (12) at an appropriate fiber angle; and securing each of the cut pieces (12) to the other cut pieces (12). A method for fabricating a hollow part includes fabricating a preform for hollow parts and molding the hollow part using the preform.

Description

METHODS FOR FABRICATING PREFORMS FOR HIGH PERFORMANCE ULTRA-LONG FIBER REINFORCED THERMOPLASTIC TUBING

Technical Field

[0001] The present invention relates generally to methods for fabricating performs for hollow parts, and, more specifically, methods for fabricating preforms for high performance ultra-long fiber reinforced thermoplastic tubing.

Background

[0002] Production of hollow thermoplastic composite parts requires placement of fiber materials in various formats such as individual rovings, unidirectional, woven, or non-woven fabrics. Methods of placing those reinforcement fibers and other materials range from manual to the use of highly automated tape laying equipment.

[0003] Traditional continuous fiber reinforced composite parts (e.g., tubing) have an inherent problem of fiber placement inconsistencies in particular where the hollow part has changes in cross section, curvature, or material thickness requirements. A simple example would be on the inside of a curvature where less fiber material is present on the outside of the curvature and greater amounts on the inside of the radius. Changes in shape most often cause displacement of fibers during forming as seen with the use of multiple braid layers, use of prepreg fibers, or the application of fabrics. Drapable fabrics are available to address these issues. However, in many applications, the small dimensions or dramatic geometry prevent accurate and smooth application of these fabrics as a viable solution. Failure of the fiber to be applied evenly can cause a reduction in mechanical properties at fold points with resin rich areas, variances in fiber volume, or voids. The uneven wall thickness and poor or ineffective placement of fibers with shapes other than straight tubing is problematic for processes such as filament winding, semi-automated wrapping, and bladder molding with braided preforms.

[0004] Accordingly, there is a need for improved methods of making continuous fiber reinforced composite parts that address the above drawbacks. Summary

To overcome the drawbacks described above, embodiments of the present invention are directed to methods of arranging fiber reinforcements and preparing a preform that allows the quantity of reinforcement fibers at various areas and sides of a hollow part or tube to be predicted and specified in order to gain the desired best use of materials and design for intended performance. In an embodiment, the size and placement of the fibers used as reinforcement for symmetrical tubing can be accurately controlled on the inside and outside of any given curve. Such control of the characteristics of the reinforcement fibers allows for ultra-long fibers (e.g., having a length of 1 inch or greater) and controlled volume as shape changes are made. This allows for planned and controlled placement of fibers along the composite part in such a way as to benefit from the efficient placement of fibers and the ability to predict volume and orientation all along the composite part geometry.

[0005] Embodiments of the present invention are also directed to methods of calculating and preparing the prepreg fiber material for the preform, which helps ensure an accurate fiber architecture that will optimize performance of the composite structural part. The calculation to determine wall thickness, fiber length, placement, and an exemplary method to prepare the intermediate material are described below. Once the intermediate material is prepared, it is then transferred to the application station where the preform is assembled.

[0006] A method according to the present invention produces high performance thermoplastic composite tubing that can be post shapeable after initial molding. In other words, once the preform is created, an advanced thermoplastic composite process can be employed to convert the thermoplastic polymer and reinforcing fiber preform into an engineered, high performance hollow shape or tube. An

advantageous attribute of processing methods according to the present invention is the ability to produce high performance composite with reduced weight and reduced use of costly materials.

Brief Description of the Figures

[0007] The accompanying figures, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above and the detailed description given below, serve to explain the invention.

[0008] Figs. 1 -3 are views of preform sheets made in accordance with various embodiments of the present invention.

[0009] Fig. 4 is a perspective view of a preform sheet being wound around a preform holder in accordance with an embodiment of the present invention.

[0010] Fig. 5 is a cross-sectional view of a preform wound on a forming cage positioned in a molding tool in accordance with an embodiment of the present invention.

Detailed Description

[0011] Embodiments of the present invention are directed to methods for fabricating preforms for high performance ultra-long fiber reinforced thermoplastic tubing or other hollow parts. An advantage of a method according to an embodiment is the ability to efficiently place continuous and or ultra-long fibers into a preform that, when consolidated, yields a composite part with predicted high performance, which makes the best use of properties available from the selected raw materials. Zero axis fibers for stiffness and selected transverse fibers in appropriate amounts and angle in the cross-section of irregular shaped parts is often a challenge with current alternative methods. Improved placement of fibers and polymers in the preform and thus the final composite part results in weight savings, overall performance benefits, reduced processing time, and cost savings of materials.

[0012] In an embodiment, prepreg material is metered into position by use of an automated process. The layering, spacing, angle variation, and length of the fibers or rovings are controlled to create the desired fiber architecture and design of the preform and, consequently, the composite part. Thus, the fiber characteristics and placement are accurately controlled to allow predictable mechanical properties of the composite part. Careful planning and preparation of the preformed material is of the utmost importance in order to ensure an accurate fiber architecture, which controls the properties and performance of the final consolidated composite part.

[0013] A preferred method of fiber delivery is a fully or partially consolidated roving of sufficient "shape" that can be easily handled and controlled during placement. This is of particular importance for the single roving components of the preform which, on most parts, will typically make up the bulk of the composite part's reinforcement. Using fibers in this form also make it possible to accurately control the fiber / polymer volume fractions. It should also be noted that preparing the reinforcement partially or fully encapsulated in this manner is a cost effective method of metering, handling, and combining the materials used in the composite. Partial or completely consolidated fibers can then be fed into the preform assembly device. Methods of forming a preform and using the preform to make a consolidated composite part are further described below.

Composite Part Design

[0014] In an embodiment, composite part design is the first step to define the composite part and proposed material requirements needed to meet the application requirements. Physical requirements of all aspects of the desired composite part may be considered such as weight, strength, stiffness, conductivity, and any other specific performance requirements, as well as the cost of manufacturing. In some applications where the properties of the composite are critical (e.g., aerospace, medical, etc.), an analysis can be conducted to develop the composite part design. The choice of materials for the composite part and the placement of those materials in forming the preform may be determined.

Prepreg Material

[0015] In an embodiment, the prepreg material includes fiber reinforced resin. The resin and fiber materials may be combined to form the prepreg material in a variety of forms. These forms include, without limitation, unidirectional tapes, partially impregnated tows, and/or comingled ravings. The fiber or fiber materials, such as fabric, must be prepared for handling in an embodiment of the invention. This step includes the introduction of the selected resin material to be used in the final formation of the composite part. In various embodiments, the selected resin may be introduced into or on the choice of fibrous material through a form of extrusion, by introduction of preprocessed films, or by metering powders. The amount of resin must be accurately controlled, which in the completed composite part will yield a material with the proper fiber volume.

[0016] The choice of fibers to reinforce the chosen resin is important to the planned performance of the composite material to be formed. Fibers can be, for example, fiberglass (e.g., E-glass, S-glass), carbon fiber (e.g., standard, intermediate, and high modulus carbon fiber), aramid fiber, or natural cellulosic fibers. The fibers may be in a variety of forms. Fibers can be in the form of fiber bundles called "tows", in woven form, or non-woven fabric. Fibers going into the preform can be a mix of fiber weights and/or a mix of fiber types. For example, in an embodiment, various weights of carbon fiber may be used. A mixture of fiber weights and/or types may be used to achieve accurate fiber loading, variations of performance characteristics in selected areas, or variations of the orientation of fibers in the finished composite part or product. Further embodiments could include the use and mixing of fibers such as glass fiber along with carbon fiber to achieve performance with impact properties, weight savings, variations in stiffness, or cost savings, to name a few reasons.

[0017] Choice of resin is determined by the end use application and not necessarily the process. Thermoplastic polymers are generally suitable for use in a preform, though special consideration and changes to basic elements of the process must be employed when using polymers with high processing temperatures. The thermoplastic resin systems may include, without limitation, polypropylene (PP), various grades of the polyamides (PA6, PA1 1 , PA12, and PA66), polyphthalamide (PPA), polyethylene terephthalate (PETG), polyphenylene sulfide (PPS), polyetherimide (PEI), polyethersulfone (PES), polyaryletherketones (PAEK), and polyether ether ketone (PEEK). The resin may include polymers with high glass transition temperatures (T_g) and chemical resistance.

[0018] It is important to realize that, in composites, thermoplastic polymers have benefits not typically found in thermoset resin based composites. Thermoplastic composites are typically more impact resistant due to molecular structure, are often more chemically resistant, and, importantly, are more easily recycled. The difficulty of working with thermoplastics in composites is the need to process the polymer in a molten state and to hold the composite in position until the composite returns to a frozen state at operational temperatures. Advantageously, methods of the present invention take advantage of this processing requirement in order to produce high performing, cost effective composite materials.

Prepreg Sheet Preparation

[0019] With reference to Fig. 1 , in an embodiment, a prepreg sheet 10 includes a plurality of prepreg pieces or rovings 12 secured in place by a fixture 14. More than one fixture 14 may be used (e.g., as shown in Fig. 4). The fixture 14 acts as a sacrificial transverse thermoplastic "thread" that connects each of the rovings 12. In an embodiment, the assembly thread can be made of the same or similar

thermoplastic material as the base resin used in the prepreg material.

[0020] To create the prepreg sheet 1 0, there are several important considerations and steps required to ensure the material is appropriate for the given application. The prepreg material is cut into rovings 12 having appropriate length(s). Next, the rovings 12 are placed into position and are secured to each other using the fixture 14. The length of the individual rovings 1 2, spacing between the rovings 12, and relative angle between the rovings 12 are controlled during this step according to the predetermined architecture.

[0021] In an embodiment, the rovings 12 are fed out onto an assembly table, cut, and bonded together by the fixture 14. This step in the process serves several functions. It sets the fiber angle, determines spacing of fibers, and controls selective fiber length. The prepreg sheet 10 may then be applied to the preforming assembly device. The prepreg sheet 10 is in a format that will facilitate ease of handling without changes to the desired orientation, metered, and positioning of the prepreg material.

[0022] With reference to Figs. 2 and 3, variations in prepreg sheet attributes are illustrated. All elements of the process must work in conjunction to yield proper coverage in achieving structure, volume fraction, fiber orientation, and polymer selection. This can be complex but, once established, can result in a high

performing cost effective composite structure. Fig. 2 illustrates the fiber angle of the prepreg sheet 10a. In this embodiment, the rovings 12a, which have the same length, are aligned such that the prepreg sheet 10a is at an angle. In this

embodiment, the fixture 14a connects to each of the rovings 12a at about the midpoint of the roving 12a. While the rovings 12a are parallel to each other, the fixture 14a is skewed at an angle that is not perpendicular to the length of the rovings 12a. This wrap angle affects the layering and volume of the prepreg material when the prepreg sheet 10a is helically wrapped around and on to a preform holder. Thus, varying the angle allows control of the prepreg material volume in the final composite part.

[0023] In an embodiment, the spacing between the rovings 1 2a may be uneven. Fig. 2 also illustrates variation in roving spacing. Variation in the roving spacing allows control of the prepreg material volume in the final composite part. The spacing, in conjunction with the angle, yields a prepreg material volume at particular points along the final composite part.

[0024] With reference to Fig. 3, in an embodiment, a prepreg sheet 10b includes a fixture 14b connecting ravings 12b, which have varying lengths. Changes in individual roving length can be accommodated to meet the needs of the preform assembly. A benefit to this process is the volume placement along the primary axis of any given composite part to impart stiffness, which is not easily accomplished with traditional methods such as the use of braids or other fabrics. As an example, in some cases, there could be an interest in composite design to include an angle to the fibers. Once interlayered, an "interlocking" of fibers from layer to layer will provide transverse properties beneficial to the final part. Variation in roving length when the ravings 12b are placed to form a curved composite part may achieve controlled fiber volume from the inside to outside of the radius of the curve. Fiber length can also be a consideration during initial design to later aid in secondary post forming. In general, the overall length of fibers increases the performance and mechanical properties of the composite being produced. The length of the fiber contributes to the stiffness, strength, and toughness of the fiber reinforced resin material. However, if the fiber lengths are significantly long (e.g., approaching 100 mm), the fibers will behave like quasi-continuous fibers.

Forming the Preform and Molding the Composite Part

[0025] The prepreg sheet is used to form the preform. After the preform is created, an advanced thermoplastic composite process is used to convert the preformed assembly into a well-engineered high performance hollow composite. The three most common molding processes used to consolidate continuous fiber composite tubes or other hollow parts using thermoset resin are roll wrapping, bladder molding, and filament winding. The roll wrapping process uses a mandrel to shape the inside of the composite tube and the prepreg material is wrapped around the mandrel. To consolidate the prepreg after it is wrapped around the mandrel, typically a shrink tape is overwrapped on the prepreg to induce pressure on the prepreg as the resin cures in an oven. Bladder molding uses a mold to shape the outside of the part while relying on a pressurized bladder to press and consolidate the assembled composite material against the mold surface. Filament winding typically uses dry fiber running into a resin bath to form a wet layup around a mandrel.

[0026] With reference to Fig. 4, in an embodiment, roll wrapping may be used to create the composite hollow part. An important part of the process is the ability to "step" and space the rovings 12 in preparation for wrapping onto a preform holder. This is accomplished by planned stepping of the rovings 12 as they are wound or "leafed" around the preform holder. Further consideration is made in forming the shaped preform 10 by controlling and variations in length of the rovings 12. There is also the opportunity to mix fibers during this step depending on need. As the assembled prepreg sheets are wrapped, transverse fibers can be selectively included in the wrapping process. These supporting fibers can be the same or different as the rovings 12. The supporting fibers may be selected for performance reasons or may be non-reinforced thermoplastic threads used for only holding the assembled rovings 1 2 in place.

[0027] With reference to Fig. 5, in another embodiment, a bladder molding process is used to mold hollow part using the thermoplastic resin because of its ability to make more varied geometries while producing a smooth exterior finish. The bladder molding process includes tooling designed for the final composite part, which includes a molding tool 16 (or master shape tool) and a preforming forming cage 18 (or preform holder), which is designed to work closely with the molding tool 16. The forming cage 18 follows the shape of the composite part and is designed to mate with the molding tool 16 to form a seal allowing bladder inflation. The shape of the forming cage 18 is near the net shape of the final composite part and is designed to work within the specific part molding tool 16. The forming cage 18 is sized to allow unconsolidated preform materials to be efficiently placed in the molding tool 16 for consolidation. In an embodiment, the forming cage 18 can be flexible and/or segmented in order to be effectively installed in parts that do not accept a straight forming system. Finally, the design of the molding tool 16 and the forming cage 1 8 may take into account equipment considerations for heating, handling, and cycling of the tooling system. For example, in an embodiment, the molding tool 1 6 may include a heating system (not shown).

[0028] In use, a bladder 20 is positioned on the forming cage 18. Bladder materials are selected based on processing temperatures and, to a lesser degree, on pressures needed. The design and method of applying the bladder 20 is also very dependent on shape and complexity of the final part to be consolidated. A preferred method is the use of films applied or wrapped onto the forming cage 18. Bladder materials extend beyond the layup of the prepreg materials and are used as a method to seal the process for final forming inside the tooling. On more complex parts, bladder materials are layered and spiral wrapped in some cases. For those parts requiring polymers of significantly higher processing temperatures, the bladder material may include metallic materials. The forming cage 18 may be selectively perforated to permit proper inflation of the bladder 20 within the molding tool 16.

[0029] After the bladder 20 is positioned on the forming cage 18, the prepreg sheet 10 is typically wrapped or wound on the forming cage 18 or otherwise placed on the forming cage 18 to form the preform. As the prepreg sheet 10 is wrapped around the forming cage 18, the layers of fiber and polymer of the final composite part are arranged. The fixture 14 carries the rovings 12 as they are fed and wound into place onto the forming cage 18. The fixture material is chosen to be compatible with the prepreg resin and is intended to present no resistance to the expansion of the bladder 20 during process consolidation and seating of the rovings 12. It is important to note the fiber layup or architecture might require periodic layers of off axis fibers, braids, or fabrics. These materials can be applied as a step during the process as long as consideration is given to allow for some expansion during processing. It should also be recognized that multiple preforms can be prepared for cycling through final tooling. When the prepreg sheet 10 is placed over the forming cage, the outer surface of the wound prepreg sheet 10 and the inner surface of the molding tool 16 may be spaced apart. Space is allowed but minimized to prevent movement of rovings in the prepreg sheet during pressurization and to prevent excessive uncontrolled movement during composite consolidation.

Final Composite Part Properties

[0030] Methods according to the present invention allow for efficient placement of prepreg material for part performance, cost effective use of materials, better performance to weight results, and increased impact toughness. Composite parts made according to embodiments of the present invention have: superior mechanical properties; excellent abrasion resistance; high impact strength; excellent vibration dampening; and excellent toughness. Additionally, methods of the present invention allow for the production of high quality molded parts that can perform exceptionally well in applications where weight reduction is a concern. Such lightweight structures made of, for example, PEEK and carbon fiber may offer unidirectional properties that are not found in curved tubing that use a textile fabric preform. In addition to improved mechanical properties, it should be stressed that there is also the benefit of using a thermoplastic matrix that is easily recyclable.

[0031] A wide variety of fiber angle configurations may be provided while controlling fiber placement. Additionally, the composite parts may have uniform wall thickness around curvatures. Further, the final composite parts reduce the need for excess trimming when compared to other methods (e.g., where extra material exists in a curved section of the composite part).

[0032] The improved repeatability of the process of forming preforms allows preform assembly to be automated. Further, preforms can be prepared "off line" or in advance, stored, and later consolidated into finished composite parts. An additional benefit is that, if planned and prepared properly, the final consolidated composite part can be post thermo-formed. The accurately made preforms ultimately will reduce cycle time while being consolidated in the molds.

[0033] Preforms made according to embodiments the present invention are suitable for use in manufacturing many small to medium sized hollow shapes such as in specialized tubing required for many applications. For example, in an embodiment, a preform is prepared and used for a curved and tapered strut. Curved tapered struts are beneficial for landing members on Unmanned Air Vehicles (UAV) since they need to be lightweight and be able to absorb the landing forces. This style of part geometry makes it problematic to use traditional preforming methods without getting fiber distortion and applying excess material. This makes it challenging to optimize the fiber architecture so that weight is minimized since the strut is curved, tapered, and has varying wall thickness. Methods of making a preform described above provide a preform that is optimized for the part geometry with reduced costs. For another example, in an embodiment, a preform is prepared and used for tubing on jet engines. These tubes have a wide variety of functions ranging from high pressure to vent tubes. This application requires a composite material with a high temperature resin system that is chemically resistant, while having good mechanical properties so that it can be used to replace the metal tubes to save weight. Since there are many curves to this type of tubing, there is a lot of added weight that is incorporated into the tubes using traditional tubing production. Methods of making a preform described above allow tubes to be made without excessive material on the inside of the bends in the tube and allows for a 0 degree fiber to be used in the material layup.

[0034] The weight savings that can be achieved using this technology varies depending on the part geometry. Table 1 provides theoretical weight savings (%) for 30 different 90 degree elbows, each having a 2.5 mm wall thickness, made according to an embodiment of the present invention compared to filament winding.

Table 1

[0035] While specific embodiments have been described in considerable detail to illustrate the present invention, the description is not intended to restrict or in any way limit the scope of the appended claims to such detail. The various features discussed herein may be used alone or in any combination. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods and illustrative examples shown and described.

Accordingly, departures may be made from such details without departing from the scope of the general inventive concept.

Claims

What is claimed is:

1 . A method for fabricating a preform for hollow parts comprising:

cutting a fiber prepreg material into pieces each having an appropriate length; spacing each of the cut pieces;

orienting each of the cut pieces at an appropriate fiber angle; and

securing each of the cut pieces to the other cut pieces.

2. The method of claim 1 , wherein securing each of the cut pieces includes bonding the cut pieces using together with one or more transverse thermoplastic threads.

3. The method of claim 1 , further comprising:

positioning the secured cut pieces to create a desired preform shape.

4. The method of claim 3, wherein positioning the secured cut pieces includes wrapping the secured cut pieces around a forming cage.

5. The method of claim 4, wherein the forming cage includes a bladder.

6. The method of claim 1 , further comprising:

determining a fiber architecture that includes the appropriate length and the appropriate fiber angle of each of the cut pieces and the spacing between the cut pieces.

7. The method of claim 1 , wherein the fiber prepreg material includes a composite of a resin and a fibrous material.

8. The method of claim 7, wherein the resin is selected from the group consisting of one of polypropylene, polyamide, polyphthalamide, polyethylene terephthalate, polyphenylene sulfide, polyetherimide, polyethersulfone, polyaryletherketone, and polyether ether ketone.

9. The method of claim 7, wherein the fibrous material is selected from the group consisting of fiberglass, carbon fiber, aramid fiber, and natural cellulosic fiber.

10. A method for fabricating a hollow part comprising:

fabricating a preform for hollow parts according to the method of claim 1 ; and molding the hollow part using the preform.

1 1 . The method of claim 10, wherein molding the hollow part includes one of bladder molding, filament winding, and roll wrapping.