Patent Application
20140083609
The present invention relates generally to processing composite material preforms, and more particularly to compacting pre-impregnated composite material preforms prior to curing the compacted composite material preforms.
Composite materials are increasingly being used for the manufacture of a variety of products because of their high strength and durability along with the ability to be formed into a variety of shapes. A longstanding problem for the processing of composite materials is that they are most commonly processed using a vacuum bag. Decades ago this was thought to be an ideal solution because one process (vacuum) would provide two desired effects on the laminated preform, to pull air and volatiles from the laminate while also providing ambient pressure to consolidate the laminate. Augmentation of pressure by autoclave is used routinely for smaller components but is not practical for large parts, for example, boat hulls and wind turbine blades. Resin systems have been developed that enable the curing of pre-impregnated materials without the use of autoclaves specifically to solve the large part problem (boats and wind turbine blades). A single vacuum bag is typically used for curing these systems; however, the resulting void content in the composite is usually too high, which can negatively affect strength and other properties of the composite. The single vacuum bag technique is not considered the most ideal way to make very low void content composite components without the use of an autoclave. The pressure that is applied to the preform inside the vacuum bag actually hinders the removal of bubbles by trapping the bubbles containing air and/or volatiles between the tacky layers of fiber and resin.
One known way to overcome this problem is to use multiple vacuum bags to obviate the need for a large expensive vacuum chamber. U.S. Pat. No. 7,413,694 B2 describes a vacuum bag within a vacuum bag to form an inner and outer chamber. The respective pressures in the inner and outer chambers are regulated to facilitate resin infusion into a dry fibrous preform. The level of independence achievable by this method is limited and not sufficient for pre-impregnated forms of composites. Another patent, U.S. Pat. No. 7,186,367, also describes a double vacuum bag approach. In this approach, the outer bag is constrained by an added rigid component, positioned between the inner and outer vacuum bags, from applying any pressure to the preform while the preform is under a vacuum. Another patent, U.S. Pat. No. 7,935,200 describes a protocol for determining process parameters for debulking composite laminates using a double vacuum debulk process.
However, in addition to previously mentioned limitations, the same apparatus for receiving composite material preforms and the manufacturing methods disclosed in each of the above-mentioned patents further utilize a combination of heat and pressure sufficient to cure the composite material, resulting in the completed composite article that is permanently sized and shaped to conform to the mold or base plate used. In a manufacturing facility in which large composite parts (up to and even exceeding 70 meters in length), such as wind turbine blades are fabricated, operations could be conducted with significantly less expense if a central processing area could be utilized, in which pre-impregnated composite material preforms could be compacted prior to curing, such that an uncured, compacted preform could be transported to and placed on one of a multiple of molds, from which different composite parts could be fabricated from the same compacted preform. Further, by having already compacted the preform prior to reaching the mold in which curing occurs, the molding process itself would be greatly simplified with a significant reduction in the amount of time required to provide a cured (completed) composite article.
Therefore, a method for producing uncured, compacted preforms that do not suffer from one or more of the above drawbacks would be desirable in the art.
According to an exemplary embodiment, a method of making a compacted composite material preform includes providing a pre-impregnated preform comprising a plurality of reinforcing fibers and a polymer matrix and positioning the preform on a base plate. The method includes enclosing the preform inside a vacuum bag defining a first cavity and enclosing the preform and vacuum bag inside a substantially rigid cover defining a second cavity. The method includes drawing a vacuum in the first cavity and drawing a vacuum in the second cavity that is substantially equal or greater than the vacuum in the first cavity to remove air and volatiles from the preform. The method then includes drawing a greater vacuum in the first cavity relative to the second cavity to urge the vacuum bag into compressive contact with the plurality of reinforcing fibers and the polymer matrix to a compacted arrangement.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
Wherever possible, the same reference numbers will be used throughout the drawings to represent the same parts.
An apparatus and method of making composite material articles is described below in detail. The apparatus and method is described in reference to a wind turbine blade, but can be applicable to any composite article. The method utilizes a vacuum bag to provide a first vacuum cavity, and a substantially rigid cover overlying the first vacuum cavity to provide a second vacuum cavity that can be independently controlled relative to the first vacuum cavity. In addition, the method provides for pressures higher than the pressure provided by known vacuum bag processes, which is limited to about 15 pounds per square inch (psi). Utilizing a rigid cover permits operation in the range of known vacuum bag processes of greater than about 0 psi to about 15 psi. An advantage of higher processing vacuum pressure is that the higher vacuum pressure will result in faster and more complete evacuation of air from between the laminated layers of the preform. The pre-impregnated preform materials are typically B-staged (partially cured) into a state of very high viscosity and very low flow of the polymer matrix of the preform. However, for purposes of this disclosure, the term “cure” or variations thereof is intended to mean that the composite material cannot be flexed without risk of structurally damaging the resultant composite article, or reducing the strength properties of the composite part. Stated another way, composite material is still considered a preform, or compacted preform after being subjected to the method of the present disclosure for providing a compacted preform, the compacted preform being capable of being subjected to an amount of flexure in a mold without a significant amount of reduction of strength properties of the resultant cured composite article. The compacted preform is therefore considered to still be in a “pre-cured” or prior to curing” or similarly worded state or condition. With a known vacuum bag pressure only process, hold time to wait for the flow of polymer matrix may be reduced. A higher vacuum pressure provided by the substantially rigid cover can facilitate shortening of the hold time of the preform.
Although heat can be used to quicken the flow, heat will also advance the cure of the polymer matrix, which is not desired at this point in the fabricating process. The goal is to compact the preform without advancing the cure. The compacted preform is then moved to a shaped mold for final shaping and curing inside a vacuum bag. This molding step is where trapped air and gas is typically removed from the wind turbine blade being formed, but not very effectively. Utilizing the apparatus and method of processing the preform, described more completely below, provides for substantially complete removal of voids prior to molding and curing to the final blade geometry. This void reduction drastically reduces the time required in the expensive mold of final geometry. Therefore, process cycle time is improved and fabricating capacity is increased.
Referring to the drawings,
Various components of wind turbine 10, in the exemplary embodiment, are housed in nacelle 16 on top tower 12 of wind turbine 10. The height of tower 12 is selected based upon factors and conditions known in the art. In some configurations, one or more microcontrollers in a control system are used for overall system monitoring and control including pitch and speed regulation, high-speed shaft and yaw brake application, yaw and pump motor application and fault monitoring. Alternative distributed or centralized control architectures are used in alternate embodiments of wind turbine 10. In the exemplary embodiment, the pitches of blades 24 are controlled individually. Hub 22 and blades 24 together form wind turbine rotor 20. Rotation of rotor 20 causes a generator to produce electrical power.
In use, blades 24 are positioned about rotor hub 22 to facilitate rotating rotor 20 to transfer kinetic energy from the wind into usable mechanical energy. As the wind strikes blades 24, and as blades 24 are rotated and subjected to centrifugal forces, blades 24 are subjected to various bending moments. As such, blades 24 deflect and/or rotate from a neutral, or non-deflected, position to a deflected position. Moreover, a pitch angle of blades 24 can be changed by a pitching mechanism to facilitate increasing or decreasing blade 24 speed, and to facilitate reducing tower 12 strike.
The basic configuration of a rotor blade 24 is shown in
As shown in
Preform 42 is formed from a plurality of reinforcing fibers and a polymer matrix by impregnating a web/mat of reinforcing fibers with a polymer. Any suitable reinforcing fiber can be used in preform 42. Examples of suitable reinforcing fibers include, but are not limited to, glass fibers, graphite fibers, carbon fibers, polymeric fibers, ceramic fibers, aramid fibers, kenaf fibers, jute fibers, flax fibers, hemp fibers, cellulosic fibers, sisal fibers, coir fibers and mixtures thereof. Any suitable polymer can be used to form preform 42, including thermosetting polymers and thermoplastic polymers. Examples of suitable thermosetting polymers include, but are not limited to, vinyl ester polymers, epoxy polymers, polyester polymers, polyurethane polymers, and mixtures thereof. Examples of thermoplastic polymers include, but are not limited to, polyolefins, polyamides, polyesters, polysulfones, polyethers, acrylics including methacrylic polymers, polystyrenes, polypropylenes, polyethylenes, polyphenelene sulfones, polyvinyl alcohol, and mixtures thereof
Base plate 44, cover 66 and caul plate 50 can be made from any suitable material, for example, any suitable metal, such as steel. Base plate 44, cover 66 and caul plate 50 can also be composed of any suitable material, for example, cover 66 can be composed of a suitable nonmetal, such as commercially available polyvinyl chloride round stock that is formed into a hemisphere, such as one half of a section of the round stock. Cover 66 can be constructed of non-circular segments, or even a closed geometry into which base plate 44, pre-impregnated preform 42 and vacuum bag 54 may be inserted, if desired, so long as the material and geometry can withstand the range of vacuum pressures. In other embodiments, any one of base plate 44 and caul plate 50 (when used) can be formed from other materials, for example, plastic materials, including fiber reinforced fibers. In an exemplary embodiment, base plate 44 is flat; however, in another embodiment, base plate 44 can have any suitable shape to produce a three dimensional preform 42. In a further embodiment, caul plate 50 may be flexible to facilitate forming shaped articles other than flat shaped preforms. A flexible caul plate 50 may be formed from a flexible plastic or rubber material, with or without fiber reinforcement.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
