Fiber Forms in Composites

 

Fibers used to reinforce composites are supplied directly by fiber manufacturers and indirectly by specialized fiber sellers in a variety of different forms that vary depending on the application. A guide is available here.

 

The automated FPP line (left) produces dry unidirectional carbon fiber patches, which are precisely placed on a 6-axis robotic articulated tool (white arm) by a 4-axis motion robot (black armature in center). Cevotec imports digital files prepared for specialized kiteboards into its own software to design local reinforcements that match the kiteboard geometry and manufacturing process requirements and achieve the desired customer-specified performance improvements.

 

Roving and Tow

Roving and tow. Roving is the simplest and most common form of glass fiber. It can be chopped, woven, braided, knitted, and hybrid fabrics used in composite manufacturing. Roving is supplied by weight with a specified filament diameter. The term “yield” is often used to refer to the number of yards per pound of glass fiber roving. Similarly, tow is the basic form of carbon fiber. Typical aerospace-grade tow sizes range from 1K to 24K (K=1000, so 12K means the tow contains 12,000 carbon filaments).

 

PAN and pitch-based 12K carbon fibers are available in medium (33-35Msi), medium (40-50Msi), high (50-70Msi), and ultra-high (70-140Msi) moduli. (Modus is a mathematical value that describes the stiffness of a material by measuring its deflection or change in length under load.) Newer heavy-tow carbon fibers, sometimes referred to as commercial-grade fibers, have filament counts of 48K-320K and are less expensive than aerospace-grade fibers. They typically have a 33-35Msi modulus and 550ksi tensile strength and are used when rapid part assembly is required, most commonly in the recreational, industrial, construction, and automotive markets.

 

Heavy-tow fibers exhibit properties that can approach those of aerospace-grade fibers, but can be manufactured at a lower cost due to precursor and processing differences. (The high cost of carbon fiber and its historically significant fluctuations in supply and demand have long generated a high level of interest in the composites industry regarding the state of the global carbon fiber market.

 

A potentially significant recent change is the development of carbon fiber tows, which feature aligned discontinuous fibers. These tows are produced in special processes that either apply tension to the carbon tow at varying rates, which results in random breakage of the individual filaments, or that otherwise cut or separate the individual filaments so that the start and end points of the filaments are staggered and their relative lengths are roughly uniform, thereby maintaining alignment and the tow retaining its integrity.

 

The breakage allows the filaments to move position with greater independence relative to adjacent filaments, making the tow more formable and enabling it to stretch under load with greater strength properties than chopped random fibers. Fiber forms made from aligned discontinuous tows are more drapable; that is, more flexible than fiber forms made from standard tows. Compared to chopped strands, they are more flexible and therefore conform more easily to curved tool surfaces.

 

Mats

Mats are nonwoven fabrics made by holding the fibers together with a chemical binder. They come in two different forms: chopped strand and continuous strand. Chopped strand mats contain randomly distributed fibers and are typically cut to lengths between 38 mm and 63.5 mm. Continuous fiber mats are formed from swirls of continuous fiber strands. Because their fibers are randomly oriented, mats are isotropic—they have the same strength in all directions.

Chopped strand mats provide low-cost reinforcement primarily in hand layup, continuous lamination, and some closed molding applications. Inherently stronger continuous strand mats are used primarily in compression molding, resin transfer molding, and pultrusion applications, as well as in the manufacture of preforms and compression moldable thermoplastics. Certain continuous strand mats for pultrusion and needle felts for sheet molding eliminate the need for creel storage and shredding.

 

Stitched Mat

Stitiched Mat Bidirectional fabrics are made on a loom and come in a variety of weights, weaves, and widths. Woven fabrics are bidirectional, providing good strength in the axial direction (0º/90º) of the yarn or roving, and facilitate rapid fabrication of composites. However, the tensile strength of woven fabrics is somewhat compromised because the fibers curl as they pass over and under each other during the weaving process. Under tensile loads, these fibers tend to straighten, creating stresses within the matrix system.

 

Bidirectional Fabrics

Bidirectional fabrics use several different types of weaves. In a plain weave, each fill yarn (i.e., yarn that runs at right angles to the length of the fabric) alternately passes over and under each warp yarn (longitudinal yarn). Other weaves, such as slings, satins, and basket weaves, allow the yarn or roving to cross over and under multiple warp fibers (e.g., over two, under two). These weaves tend to drape more easily than regular weaves.

 

Woven Roving

Woven roving Woven roving is relatively thick and is used for heavy-duty reinforcement, especially in hand layup operations and tooling applications. Due to its relatively coarse weave, woven roving wets out quickly and is relatively inexpensive. However, special fine glass fiber fabrics can be used to reinforce applications such as printed circuit boards.

 

Hybrid Fabrics

Hybrid fabrics can be constructed with different fiber types, strand compositions, and fabric types. For example, high-strength strands of S-glass or small-diameter filaments can be used in the warp direction, while lower-cost strands make up the fill. Hybrids can also be created by stitching woven fabrics and nonwoven mats together.

 

Multiaxial Fabrics (Non-crimp fabrics – NCF)

Multiaxial fabrics (Non-crimp fabrics – NCF) are nonwoven fabrics made of layers of unidirectional fibers stacked in different directions and held together by thickness stitching, knitting, or chemical adhesives. The proportion of yarn in any direction can be chosen at will. In multiaxial fabrics, fiber crimp associated with woven fabrics is avoided because the fibers are laid on top of each other rather than crossing over and under each other.

 

This makes better use of the inherent strength of the fiber and creates a fabric that is more flexible than a woven fabric of equal weight. The availability of ultra-heavyweight nonwovens (up to 200 oz/yd²) can significantly reduce the number of layers required for layup, making manufacturing more cost-effective, especially for large industrial structures. High interest in non-crimp multiaxial fabrics (NCF) has stimulated considerable growth in this reinforcement category.

 

Relatively new, thin, biaxially reinforced C-PLY (shown here) is used to form the fuselage of a quarter-scale VX-1 KittyHawk drone, which features wings that blend smoothly into the airfoil fuselage. The top and bottom skins of the VX-1 KittyHawk fuselage use +45°/0° and -45°/0° C-PLY in alternating six-ply stacks (12 sheets per six-ply stack), with two layers each 0.006 inches thick.

 

In 2011, Dr. Stephen Tsai of Stanford University, together with Chomarat (Le Cheylard, France and Anderson, SC, US), developed a new type of multiaxial reinforcement material that orients the fibers at very shallow angles, such as 0°/20°, which could replace quasi-isotropic fiber orientation, resulting in better performance and lower weight. One result is a product called C-PLY, which was recently used by VX Aerospace (Morganton, NC, US) for its quarter-scale VX-1 KittyHawk UAV.

 

With its wings blending smoothly into an airfoil-shaped fuselage, it is the first aircraft to use Tsai’s anisotropic laminates, and a full-scale version of it is intended to serve in an unmanned civilian or military capacity (see left and image). Braided fabrics are woven continuously on a bias and have at least one axial yarn that does not curl during the weaving process. The braid’s strength comes from interweaving three or more yarns together without twisting any two yarns together. This unique structure generally provides greater strength and weight than a fabric.

 

It also has a natural compliance that makes the braid particularly suitable for producing sleeves and preforms because it readily accepts the shape of the part it is reinforcing, eliminating the need for cutting, stitching, or manipulating fiber placement. Braids are also available in flat fabric form. These braids can be produced in a triaxial structure with fibers oriented at 0°, +60°, -60° within one layer. This quasi-isotropic structure within a single layer of braided fabric eliminates the problems associated with multiple 0˚, +45˚, -45˚, and -60˚ orientations.

 

The problems associated with delamination of 90˚ fabrics are eliminated. In addition, quasi-isotropic woven fabrics significantly reduce the tendency for delamination (separation of fiber layers). The 0°, +60°, -60° structure gives the fabric the same mechanical properties in all directions, thus eliminating the possibility of mismatched stiffness between layers.

 

In both the sleeve and flat fabric forms, the fibers are continuous and mechanically interlocked. Since all fibers in the structure participate in the load event, the load is evenly distributed throughout the structure. As a result, the woven fabric absorbs a large amount of energy when it fails. The impact resistance, damage tolerance and fatigue properties of the woven fabric have attracted composite manufacturers to a wide range of applications, from hockey sticks to jet engine fan cases.

 

Preforms are near-net-shape reinforcement forms designed to manufacture specific parts by stacking and forming chopped, unidirectional, woven, stitched and/or braided fiber layers into a predetermined three-dimensional form. Complex part shapes can be approached by carefully selecting and integrating any number of reinforcement layers of different shapes and orientations. Due to its potential for high processing efficiency and speed, many preforming technologies have been developed with the help of special adhesives, heating and consolidation methods, and the use of automated methods to spray, orient and compact the chopped fibers.

 

Recently, Cevotec (Germany) The Fiber Patch Placement (FPP) technology of North Kiteboarding (Oberhaching, Germany) is an automated method of placing preformed “patches” of carbon fiber components into lower-cost glass fiber reinforced kit pieces manufactured by North Kiteboarding (Oberhaching, Germany) as a means of addressing the highly individualistic preferences of kit piece enthusiasts in terms of performance on the kit pieces without significantly increasing the price of the kit pieces (see illustration/photo and description).

 

Prepreg is a resin-impregnated fiber form, manufactured by impregnating the fibers with a controlled amount of resin (thermoset or thermoplastic) using solvent, hot melt or powder infusion techniques. Prepreg can be stored in a “B-stage,” partially cured state, until needed for manufacturing. Prepreg tapes or fabrics are used for hand lay-ups, automated tape laying, fiber placement, and some filament winding operations.

 

Unidirectional tapes (all fibers parallel) are the most common form of prepreg blanks. Prepreg sheets made from woven fibers and other flat items provide reinforcement in two or more dimensions and are usually sold in full rolls, although some suppliers offer them in smaller quantities.

 

Those made by impregnating fiber preforms and braids provide three-dimensional reinforcement. Prepregs provide a consistent fiber/resin combination and ensure complete wetting. They also eliminate the need to weigh and mix resin and catalyst for wet layup. With most thermoset prepregs, drape and tack are “processed” for ease of handling, but they must be stored below room temperature and have time-out restrictions; that is, they must be used within a certain time after being removed from storage to avoid premature cure reactions.

 

Thermoplastic prepregs do not require refrigeration and are not subject to life restrictions, but they lack the tack or drape of thermoset prepregs without special formulations and are therefore more difficult to form. It is undisputed that prepregs produce finished products with the lowest weight, highest mechanical properties, and low void content. However, historically, they have also been the most expensive, in part because they have historically been produced by specialists—the production of prepreg blanks has always been an intermediate, discrete step in the composites supply chain.

 

Recently, efforts have been made to address the inefficiencies and associated costs associated with this additional step. SPE introduced two interesting approaches, in-line processes, at the 2015 Automotive Composites Conference and Exhibition in Detroit, Michigan, U.S. They turn composites manufacturers into prepreg machines, much as the direct long fiber thermoplastic (D-LFT) process did when composites work shifted to manufacturers in the late 1990s/early 2000s. Both new technologies eliminate the previously necessary and expensive steps of freezing and storing prepreg before shipping it to customers, who then must store and thaw it before using it in a molding process, a process that is borne by the processor and the processor’s customers.

 

The closest to commercialization is the in-line prepreg process being developed jointly by Mitsubishi Rayon Co., Ltd. (Tokyo, Japan) and Mitsubishi Rayon Carbon Fiber & Composites Co., Ltd. (Irvine, California, U.S.). Mitsubishi scientists cut costs by applying individual carbon tows directly, calibrating the width, and then rewinding the product onto a spool. An automated filament placement (AFP) system—which Mitsubishi calls Automatic Tow Laying—is then used to lay down the plies to eliminate the labor of hand layup. The stackup is then preformed and formed via the company’s own PCM-prepreg compression molding process.

 

Another approach is the new InPreg-inline-prepreg process developed by the Fraunhofer Institute for Chemical Technology (ICT) (F-ICT, Pfinztal, Germany). Like Mitsubishi’s PCM approach, InPreg prepreg is designed to be formed in a compressor rather than in more exotic equipment, opening the door to laminate composites to a wider range of processors. Both the InPreg preforming and molding steps are done in the compression tool. This not only eliminates the time required to heat, preform, and cool the prepreg blanks, it also eliminates the cost and space of a preforming station. Key to the Inpreg process is a four-part B-stage epoxy system from Huntsman Advanced Materials (Basel, Switzerland) and lower-cost 24-50K tow carbon fiber that forms UD non-crimp fabric (NCF).

 

A spread tow is an individual fiber strand (or untwisted yarn) that is spread out until the individual filaments are aligned side by side, forming an ultra-thin tape. For example, a 12K carbon fiber tow can be expanded from 5mm to 25mm in width, making it 80% thinner. These spread tows can be woven into fabrics, laid down to form multiaxial non-crimp fabrics (NCFs), or receive liquid or powder resin to form spread ribbons or tow prepregs. Using woven spread tow fabrics instead of more traditional reinforcements can save 20-30% of weight in a composite laminate.

 

This is achieved by closing the gaps between the warp and weft yarns, thereby reducing the amount of resin trapped there, but also by reducing fiber curl, resulting in straighter fibers and thus increased strength. As a result, the final composite laminate can use fewer, thinner layers to achieve the same or better performance.

 

Fiber supplier Hexcel claims that with carbon fiber, fabric interstices are reduced by 5-8%, enabling 6K tow performance at 3K tow areal weight, 12K tow characteristics at 6K tow areal weight, and so on.

 

North Thin Ply Technology (NTPT, Penthalad Cossonay, Switzerland) claims that any fiber can be spread, and claims very low areal weights can be achieved: 30 g/m2 for PAN-based carbon fiber and 14-micron diameter quartz fiber, 35 g/m2 for 9-micron diameter glass fiber, 20 g/m2 for aramid fiber, and 30 g/m2 for polybenzoxazole (PBO) and other synthetic fibers. Suppliers of spread tow reinforcement include Hexcel, NTPT, Oxeon (Boras, Sweden), Sigmatex (UK) Ltd. (Runcorn, UK), Chomarat, and FORMAX (Leicester, UK).

 

Applications include bicycles, skis, hockey sticks, rackets, sailboats, racing cars and solar-powered aircraft. Recycled carbon fiber (RCF) reinforcements come in a variety of forms, including chopped fibers cut to specific lengths, chopped fibers compounded into long fiber thermoplastic (LFT) pellets, three-dimensional web preforms and randomly oriented chopped fiber mats – either dry or combined with thermoplastic materials – including polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA or nylon), polyphenylene sulfide (PPS), polyetherimide (PEI), polyetheretherketone (PEEK).

 

Chopped fiber mats can also be processed, such as by carding, to achieve greater fiber alignment and thus better mechanical properties. Such products are available from a range of RCF suppliers worldwide and are recycled through pyrolysis, which burns the resin from waste prepreg and cured structures. Technical Fibre Products Inc. (TFP, Schenectady, NY, US and Burnside, UK) produces RCF veils as light as 2 g/m2.

 

RCF products are also made in-house from dry fiber manufacturing waste.

 

SigmaRF products reuse Sigmatex’s in-house dry manufacturing waste by combining 45 mm to 60 mm carbon fibers with a thermoplastic carrier to form chips for making non-crimp fabrics, such as 220 g/m2 ±45° carbon fiber/PET biaxial NCF. Other variants include RCF/Kevlar/PEI, RCF/PA, and RCF/PES. The Institute for Plastics Processing (IKV) at RWTH Aachen University (Aachen, Germany) chops nascent fibers (a carbon fiber manufacturing waste or byproduct) that are not collected on rollers during the spinning of carbon fiber PAN precursor, carbonizes them, and forms them into a uniform mat using a continuous air path process.

 

New methods for producing continuous recycled fibers have also been developed, including solvolysis using alcohols or other solvents to remove resins without combustion or high temperatures, pyrolysis and unwinding of filament-wound pressure vessels, and the use of epoxy resins to enable the matrix to be recycled as a thermoplastic, such as Recyclamine hardeners from Connora Technologies (Hayward, CA, USA).

 

Molding compounds are another way to incorporate fibers into composites.

 

Traditionally, these were developed by the plastics industry and feature a low weight percentage (5-50%) of short fibers (2-25 mm). Bulk molding compounds (BMC) are used for injection molding, while sheet molding compounds (SMC) are used for larger parts and higher strength requirements, usually in a compression molding process.

 

Glass mat thermoplastic (GMT) is also a compression moldable material with continuous random fiber reinforcement. GMT was developed in the 1960s from short-fiber reinforced nylon. It faces increasing competition from long fiber-reinforced thermoplastics (LFRTs, or LFTs), which are produced by cutting small-diameter pultruded continuous glass fiber rods into pellets. LFTs feature continuous unidirectional fibers running the entire length of the pellet and offer properties between GMTs and short-glass thermoplastics.

 

In the 1990s, machinery OEMs developed inline compounding (ILC) systems that integrated previously separate compounding and molding processes. These direct long fiber thermoplastic (D-LFT) systems combine resin, reinforcements, and additives in the press, delivering measured shots or charges directly to injection or compression molding equipment.

 

This eliminates inventory of pre-mixed products and enables tailored fiber lengths. SMC, BMC, GMT and LFT are used in a wide range of applications requiring complex shapes and molded details, including automotive parts, appliances (washing machine tubs), medical devices, consumer products, electronics, sporting goods, brackets, housings, transportation vehicle parts and electrical applications

 

Low-density SMC: After five years of development at Tier 1 automotive supplier Continental Structural Plastics (CSP, Auburn Hills, MI, US), judges at two recent industry events unanimously recognized CSP’s TCA Ultra Lite sheet SMC (specific gravity 1.2) as a winner, for example, for molding this very complex, one-piece Corvette right front fender. The CAMX 2015 steering committee honored it with its Excellence in Innovation Award at its October meeting in Dallas, Texas, US, and a month later it topped the SPE automotive division’s materials category and took home the grand prize at the 45th SPE Automotive Innovation Awards gala in suburban Detroit.

 

SMC, in particular, offers part consolidation, deep-drawn contours and a host of other advantages over steel and aluminum: It is typically 40% lighter than metal for similar geometry. While it will not rust or corrode and does not require such treatment, it is heat and chemical resistant enough to survive the electrophoretic (e-coat) deposition rust-proofing process that automakers use on metal chassis components, so SMC parts can be attached to the body in white (the preferred assembly method) without the need for special e-coat post-assembly.

 

Until recently, however, SMC has maintained a cost advantage for production runs of 150,000 units or less. However, a new low-density SMC from Continental Structural Plastics (CSP, Auburn Hills, MI, US) called TCA (Tough A-Grade) Ultra-Lite is now available. At a specific gravity (SG) of 1.2, it offers a 28% reduction in mass compared to CSP’s medium-density TCA-Lite (1.6 SG) grade and a 43% reduction in weight compared to conventional 1.9 SG grade SMC.

 

Moreover, it not only offers comparable mechanical properties to TCA-Lite (both have an unsaturated polyester matrix from AOC Resins, Collierville, TN, US), but it also reportedly bonds more effectively to paints and adhesives. Most importantly, a life cycle analysis conducted by CSP reportedly shows that TCA Ultra Lite has a lower cost per part than aluminum, even at production volumes of 350,000-400,000 units per year. Glass fiber is the most common and least expensive reinforcement used in molding compounds, aramid fibers provide abrasion resistance, stainless steel fibers provide both electrostatic dissipation (ESD) and electromagnetic interference (EMI) shielding, and carbon fibers provide higher modulus, lower weight, and ESD performance.

 

Molding compounds reinforced with natural fibers (hemp, flax, sisal, and wood-derived fibers) have also been developed, including. These products are becoming increasingly popular in automotive, sporting goods, and consumer products. Advanced molding compounds are designed to achieve higher performance applications, including aerospace and military parts. These materials use higher-performance resins such as epoxies, phenolics, vinyl esters, bismaleimides (BMIs), and polyimides, with fiber loadings ranging from 45% to 63% by weight. Fibers include carbon fiber and E-glass, but also higher-performance S2 glass.

 

TenCate (Nijverdal, the Netherlands) makes BMCs from 12 to 50 mm in length using epoxy, cyanate ester, nylon, PPS, or PEEK resins and carbon fiber or S2 glass. HexMC is produced by Hexcel using 50-mm-long carbon fiber and epoxy. A variety of other carbon fiber SMC products are available from suppliers including Continental Structural Plastics, Quantum Composites Inc. (Bay City, Michigan, U.S.), and a joint venture between Zoltek Corporation (St. Louis, Missouri, U.S.) and Magna Exteriors (Paris, France).

 

Recently, molding compounds have been able to reinforce products made through so-called additive manufacturing processes, also known as 3D printing. Chopped and short fiber reinforcements can be adapted for a type of 3D printing called fused deposition modeling. Most reinforced plastics can be 3D printed in limited sizes. But at least one recent demonstration project shows that large-format printing is technically practical and economically justifiable: Oak Ridge National Laboratory (Oak Ridge, Tennessee, U.S.) and machinery manufacturer Cincinnati Incorporated (Harrison, Ohio, U.S.) demonstrated large-format printing capabilities using a Big Area Additive Manufacturing (BAAM) system, in partnership with Local Automotive (Chandler, Arizona, U.S.) to produce the world’s first 3D-printed car body.

 

The custom-designed Strati sports car body was printed on the showroom floor in 44 hours at the 2014 IMTS show using a 15% carbon fiber-reinforced acrylonitrile-butadiene-styrene (ABS) compound supplied by Saudi Basic Industries Corporation (Pittsfield, Massachusetts, U.S.).