Introduction to Composites

Introduction to Composites Composites are materials formed from a mixture of two or more components to produce a material with properties or characteristics superior to those of the individual materials.

Most composites are formed of two phases: Matrix and Reinforcement. The matrix is a continuous phase material that is usually less stiff and weaker than the reinforcement. It is used to hold the reinforcement together and distribute the load among the reinforcements. Reinforcements in the form of fibers, fabric, whiskers, or particulates are embedded in the matrix to produce the composite. They are discontinuous, usually stronger and stiffer than the matrix and provide the primary load-carrying capability of the composite.

The shape of the finished part is dependent on a mold, die or other tooling that controls the geometry of the composite during processing. Composites may be thought of as advanced materials, but they mimic the features of living organisms that have existed for millions of years such as the microstructures of wood and bioceramics like mollusk shells.

The fibers and matrix of advanced composites may be combined using a variety of fabrication processes, with the choice depending on the desired alignment of fibers, the number of parts to be produced, the size and complexity of the parts, and so on. Perhaps best known for their use in aerospace applications, advanced composites are also used by the automotive, biomedical, and sporting goods markets.

All of these developments mean a larger and more complicated materials-choice menu. This diversity has made plastics applicable to a broad range of consumer, industrial, automotive, and aerospace products. It has also made the job of selecting the best materials from such a huge array of candidates quite challenging.

Composites fall into the general categories:

1. Polymer-Matrix Composites
• Thermoplastic Composites
• Thermoset Composites
• Laminated Plastics
2. Ceramic-matrix Composites
• Carbon-matrix Composites
3. Metal-matrix Composites

Autoclave Molding

Autoclave molding is a modification of pressure-bag and vacuum-bag molding. This advanced composite process produces denser, void free moldings because higher heat and pressure are used for curing. It is widely used in the aerospace industry to fabricate high strength/weight ratio parts from preimpregnated high strength fibers for aircraft, spacecraft and missiles. Autoclaves are essentially heated pressure vessels usually equipped with vacuum systems into which the bagged lay-up on the mold is taken for the cure cycle. Curing pressures are generally in the range of 50 to 100 psi and cure cycles normally involve many hours. The method accommodates higher temperature matrix resins such as epoxies, having higher properties than conventional resins. Autoclave size limits part size.

Filament Winding

Filament winding consists of winding resin impregnated fibers or rovings of glass, aramid, or carbon on a rotating mandrel in predetermined patterns. The method provides the greatest control over fiber placement and uniformity of structure. In the wet method, the fiber picks up the low viscosity resin either by passing through a trough or from a metered application system. In the dry method, the reinforcement is in the preimpregnated form. After the layers are wound, the component is cured and removed from the mandrel. Traditionally used to produce pressure vessels, rocket motor cases, tanks, ducting, golf club shafts and fishing rods, filament winding technology has been expanded, and noncylindreical, nonspherical composite parts are now commonplace. Typical thermoset resins used in filament wound parts include polyesters, vinyl esters, epoxies, and phenolics.

Pultrusion is a continuous method of manufacturing various reinforced plastic shapes of uniform cross sections. Glass reinforcements, such as unidirectional rovings or multi-directional glass fiber mat, are guided through a liquid resin bath to thoroughly wet every fiber. The reinforcements are then guided and formed, or shaped, into the profile to be produced before entering a die. As the material progresses through the heated die, which is shaped to match the design profile, the resin changes from a liquid to a gel, and finally, into a cured, rigid plastic.

A pulling device grips the cured material and literally pulls the material through the die. Hence, the name pultrusion. It is the power source for the process. After the product passes through the puller, it is sawed into desired lengths. Although pultrusion is ideally suited for custom shapes, some standard products include rods, bars, angles, channels, and I-beams.

Structural Reaction Injection Molding (SRIM)

This process uses two resin components that are combined and mixed together, then injected into a mold cavity containing reinforcement. In the mold cavity, the resin rapidly reacts and cures to form the composite part.

Centrifugal Casting

Centrifugal Casting is used for making cylindrical, hollow shapes such as tanks, pipes and poles. Chopped strand mat is placed into a hollow, cylindrical mold, or continuous roving is chopped and directed onto the inside walls of the mold. Resin is applied to the inside of the rotating mold.

Hand Lay-Up

Hand lay-up is the simplest and oldest open molding method of the composite fabrication processes. It is a low volume, labor-intensive method suited especially for large components, such as boat hulls. Glass or other reinforcing mat or woven fabric or roving is positioned manually in the open mold, and resin is poured, brushed, or sprayed over and into the glass plies. Entrapped air is removed manually with squeegees or rollers to complete the laminates structure. Room temperature curingpolyesters and epoxies are the most commonly used matrix resins. A catalyst initiates curing in the resin system, which hardens the fiber-reinforced resincomposite without external heat. For a high quality part surface, a pigmented gel coat is first applied to the mold surface.

Reaction Injection Molding (RIM)

Reaction injection molding (RIM) is also known as liquid reaction molding or high- pressure impingement mixing. The low-pressure method involves mechanical mixing often with plural component dispensing equipment rather than by impingement and immediately injects the mixture into the closed mold cavity. These reactive polymer resins such as polyols, isocyanates, polyurethanes, and polyamides provide fast molding cycles well suited for automotive and furniture applications. Common RIM parts include automotive bumpers, fender and panel components, appliance housings, and furniture components.

When short fibers or flakes are used to produce a more isotropic product, the process is called reinforced reaction injection molding (RRIM). Many hybrids such as polyurethane/urea, polyurethane/polyester hybrid systems are currently being used as well as those for RIM already mentioned.

Tape / Fiber Placement

Tape placement is an advanced composites process that orients resin preimpregnated high-strength reinforcement fibers in specific directions. Concentrating high strength in only the directions needed minimizes part weight. It is widely used in the fabrication of high strength/weight ratio parts in the aerospace industry such as aircraft wing and body skins,control surfaces, spacecraft and missiles. Automated equipment is predominately used for large structures while hand tape placement remains widely used but usually for smaller parts. Most automated machines are large, sophisticated and represent a major capitol investment. Resin choices for tapes can be grouped by processing characteristics. These are, Tacky, B-staged resins later cured to a thermoset stage, e.g., epoxies. Non-tacky solid resins that melt and flow prior to curing to a thermoset polymer, e.g., bismaleimides and non-tacky thermoplastic resins processed entirely by melting and freezing, e.g., polyetherketone, (PEEK), and polyphenylene sulfide, (PPS).

Compression Molding

Compression molding is done with matched metal molds utilizing sheet-molding compound (SMC), bulk-molding compound (BMC), or preform mat. A weighed charge of SMC or BMC, or a preform of glass reinforcement shaped to the mold is placed on a press ranging in size from 300 to 4,000 tons. Resin is added with the preform while SMC and BMC contain all components including fiber, resin, fillers, catalyst etc. Heat and pressure is applied, with temperature ranges of 225 to 325 oF. and 150 to 1,000 psi pressure required to cure parts. Cycles can range from less than one to five minutes. Typical thermoset resins used in compression molded parts are polyesters, vinyl esters, epoxies, and phenolics. Compression molded products vary from dinnerware, trays, buttons, appliance housings, large containers, electrical, to recreational vehicle body panels such as snow mobiles, and jetskis.

Pressure Bag Molding

Pressure bag molding is similar to the vacuum bag moldimg method except that air pressure, usually 30 to 50 psi, is applied to a rubber bag, or sheet that covers the laid up composite to force out entrapped air and excess resin. Pressurized steam may be used instead, to accelerate the cure. Cores and inserts can be used with the process, and undercuts are practical, but only female and split molds can be used to make items such as tanks, containers, and wind turbine blades.

Resin Transfer Molding

Resin transfer molding (RTM) is a low pressure closed molding process for moderate volume production quantities, filling the gap between the slow, contact molding processes and the faster, compression molding process, which requires higher tooling costs. Continuous strand mats and woven reinforcement is laid up dry in the bottom mold half. Preformed glass reinforcements are often used for complex mold shapes. The mold is closed and clamped, and a low viscosity, catalyzed resin is pumped in, displacing the air through strategically located vents. Metered mixing equipment is used to control resin/catalyst ratios that are mixed through a motionless/static mixer and injected into the mold port. Common matrix resins include polyester, vinyl ester, epoxy, and phenolics. Advantages over contact molding methods are a uniform thickness, two finished sides and low emissions. For optimum surface finish, a gel coat would be applied to the mold surface prior to molding. High quality parts produced by this method include automotive body parts, bathtubs, and containers.

Vacuum Bag Molding

Vacuum bag molding, a refinement of hand lay-up, uses a vacuum to eliminate entrapped air and excess resin. After the lay-up is fabricated on either a male or female mold from precut plies of glass mat or fabric and resin, a nonadhering film of polyvinyl alcohol or nylon is placed over the lay-up and sealed at the mold flange. A vacuum is drawn on the bag formed by the film while the composite is cured at room or elevated temperatures. Compared to hand lay-up, the vacuum method provides higher reinforcement concentrations, better adhesion between layers, and more control over resin/glass ratios. Advanced composite parts utilize this method with preimpregnated fabrics rather than wet lay-up materials and require oven or autoclave cures.

Continuous Lamination

Continuous Lamination makes composites in sheet form such as composite glazing, corrugated or flat construction panels, and electrical insulating materials. Reinforcement is combined with resin and sandwiched between two plastic carrier films. The sheet takes shape under forming rollers, and the resin is cured to form the composite.

Pulshaping

One of the greatest limitations of the pultrusion process is its inability to permit the manufacture of fiber-reinforced plastics of varying cross-sections in the axial direction. Pulshaping is a revolutionary new technique developed by Pultrusion Dynamics, Inc that allows changes in shape in a pultrusion.

Spray-Up

Spray-up is an open mold method that can produce complex parts more economically than hand lay-up. Chopped fiberglass reinforcement and catalyzed resin, and in some cases, filler materials, are deposited on the mold surface from a combination chopper/spray gun. Rollers or squeegees are used to manually remove entrapped air and work the resin into the reinforcements. Woven fabric or woven roving is often added in specific areas for greater strength. General purpose, room temperature cure polyesters are usually used to produce such parts as truck camper shells and tub & shower stalls. As in hand lay-up, gel coats are used to produce a high quality colored part surface.

VacuumInfusion

Vacuum infusion, also called resin infusion, utilizes a vacuum bag to debulk or compact a parts complete laminate ply schedule of reinforcements and or core materials laid onto the mold. After debulking, the resin is allowed to be infused by the vacuum to completely wetout the reinforcements and eliminate all air voids in the laminate structure. High quality composite parts made from a wide range of fiber and resin combinations can be utilized to infuse laminates up to six inches thick. Typical resins used are polyester, vinyl ester, and epoxy with many being UV cure initiated. This process can routinely produce large 2,000 sq. ft. parts such as boat hulls, bus bodies, and railcar panels. This processes added benefits include eliminating weaker secondary bonds and reduced VOC emissions vs. current open molding processes. Pigmented gel coats provide the parts surface finish and often a hand lay-up skin laminate may be fabricated to allow fabricators to walk on gel-coated surface while loading the dry reinforcement laminate ply schedule and vacuum bag.