Fiber reinforcement forms

Rovings, the most common form of glass, can be chopped, woven or otherwise processed to create secondary fiber forms for composite manufacturing, such as mats, woven fabrics, braids, knitted fabrics and hybrid fabrics. Rovings are supplied by weight, with a specified filament diameter. The term yield is commonly used to indicate the number of yards in each pound of glass fiber rovings.

One interesting area is the growing use of "stretch-broken"carbon fiber tows, commercialized by Hexcel (Dublin, Calif.). The process pulls carbon tow at differential speeds, which causes random breakage of individual filaments, yet leaves the filaments aligned. The breaks make the tow more formable and give it the ability to stretch under load, with greater strength properties than chopped, random fibers.

Mats are nonwoven fabrics made from fibers held together by a chemical binder. They come in two distinct forms: chopped and continuous strand. Chopped mats contain randomly distributed fibers cut to lengths typically ranging from 38 mm to 63.5 mm/1.5 to 2.5 inches. Continuous-strand mat is formed from swirls of continuous fiber strands. Because their fibers are randomly oriented, mats are isotropic, that is, they possess equal strength in all directions. Chopped-strand mats provide low-cost reinforcement primarily in hand layup, continuous laminating and some closed-molding applications. Inherently stronger continuous-strand mat is used primarily in compression molding, resin transfer molding and pultrusion applications, and in the fabrication of preforms and stampable thermoplastics. Certain continuous-strand mats used for pultrusion and needled mats used for sheet molding eliminate the need for creel storage and chopping.

Woven fabrics are made on looms in a wide variety of weights, weaves and widths. Wovens are bidirectional, providing good strength in the directions of yarn or roving axial orientation (0º/90º) and facilitate fast composite fabrication. However, the tensile strength of woven fabrics is compromised to some degree because fibers are crimped as they pass over and under one another during the weaving process. Under tensile loading, these fibers tend to straighten, causing stress within the matrix system.

Varying weaves are used for bidirectional fabrics. In a plain weave, each fill yarn (i.e., the yarn oriented at right angles to the fabric length) alternately crosses over and under each warp yarn (the lengthwise yarn). Other weaves, such as harness, satin and basket weave, allow the yarn or roving to cross over and under multiple warp fibers (e.g., over two, under two). These weaves tend to be more drapable, that is, they are more pliable and conform more easily to curved surfaces than do plain weaves.

Woven roving is relatively thick and is used for heavy reinforcement, especially in hand layup operations and tooling applications. Due to its relatively coarse weave, woven roving wets out quickly and is relatively inexpensive. Exceptionally fine woven fiberglass fabrics can be produced for applications such as reinforced printed circuit boards.

Hybrid fabrics can be constructed with varying fiber types, strand compositions and fabric types. For example, high-strength strands of S-glass or small-diameter filaments may be used in the warp direction, while less-costly strands compose the fill. A hybrid also can be created by stitching woven fabric and nonwoven mat together.

Multiaxials are nonwoven fabrics, made with unidirectional fibers laid atop one another in different orientations, and held together by through-the-thickness stitching, knitting or a chemical binder. The proportion of yarn in any direction can be selected at will. In multiaxial fabrics, the fiber crimp associated with woven fabrics is avoided because the fibers lie on top of each other, rather than crossing over and under. This makes better use of the fibers' inherent strength and creates a fabric that is more pliable than a woven fabric of similar weight. Super-heavyweight nonwovens are available (up to 200 oz/yd2) and can significantly reduce the number of plies required for a layup, making fabrication more cost-effective, especially for large industrial structures. High interest in noncrimp multiaxials has spurred considerable growth in this reinforcement category.

Braided fabrics are generally more expensive than woven fabrics, due to their more complex manufacturing process, but are typically stronger by weight than wovens. The strength comes from intertwining three or more yarns without twisting any two yarns around each other. Braids are continuously woven on the bias and have at least one axial yarn that is not crimped in the weaving process. This arrangement of yarns allows for highly efficient load distribution throughout the braid.

Braids are available in both flat and tubular configurations. Flat braids are used primarily for selective reinforcement, such as to strengthen specific areas in pultruded parts. Tubular braids can be placed over a mandrel to produce hollow parts, such as windsurfing masts, hockey sticks, lamp posts and utility poles. Braid is increasingly competitive with other fabrics, due to declining manufacturing costs.

Preforms are near-net shape reinforcement forms designed for use in the manufacture of particular parts by stacking and shaping layers of chopped, unidirectional, woven, stitched and/or braided fiber into a predetermined three-dimensional form. Complex part shapes may be closely approximated by careful selection and integration of any number of reinforcement layers in varying shapes and orientations. Because of their potential for great processing efficiency and speed, a number of preforming technologies have been developed, with the aid of special binders, heating and consolidation methods and the use of automated methods for spray up, orientation and compaction of chopped fibers.

Resin-impregnated fiber forms, commonly called prepregs, are manufactured by impregnating fibers with a controlled amount of resin (thermoset or thermoplastic), using solvent, hot-melt or powder impregnation technologies. Prepregs can be stored in a "B-stage,"or partially cured state, until they are needed for fabrication. Prepreg tape or fabric is used in hand layup, automated tape laying, fiber placement and in some filament winding operations. Unidirectional tape (all fibers parallel), is the most common prepreg form. Prepregs made with woven fibers and other flat goods offer reinforcement in two dimensions and are typically sold in full rolls, although small quantities are available from some suppliers. Those made by impregnating fiber preforms and braids provide three-dimensional reinforcement.

Prepregs deliver a consistent fiber/resin combination and ensure complete wetout. They also eliminate the need to weigh and mix resin and catalyst for wet layup. For most thermoset prepregs, drape and tack are "processed in"for easy handling, but they must be stored below room temperature and have out-time limitations; that is, they must be used within a given time period after removal from storage to avoid premature cure reaction. Thermoplastic prepregs do not suffer from such limitations, but without special formulation, they lack the tack or drape of thermoset prepregs and, therefore, are more difficult to form.