Chapter 19 Manufacturing with Plastics and CompositesComposites – a judicious combination of two or more materials that produces a synergistic effect. Composites are made up of a fiber and a matrix. Reinforced plastic or simple composite - The fiber in a composite is laid in random directions or they are very shortAdvanced composite - The fibers are long, continuous, and laid in a given direction (Figure 19-1). TYPES OF FABRICS Currently, the most common fibers used in composites are glass, graphite (carbon), and Kevlar.  Graphite has a tensile strength three to five times stronger than steel and has a density that is one-fourth that of steel.  Fiberglass has about five times the tensile strength of steel and has a density of about one third that of steel. Advantage DisadvantageGlass strong and cheap reinforcing fiber twice as heavy as Kevlar and graphitesKevlar good abrasion and puncture resistance as well as tensile strengthhas a relatively low modulus ofelasticity.Graphite best rigidity of any of the fibers expensiveHybrids A hybrid is a combination of fibers within a single matrix.  Intraply hybrids (within) - Alternate strands of different fibers in a single layer or ply (Fig. 19-6).  Interply hybrid (across) – Different plies of different fibers (Figure 19-7). TYPES OF RESINS  Resins may be either thermosetting or thermoplastics, but must be compatible with the fibers.  A thermosetting resin should not be used with a thermoplastic fiber nor should a resin that chemically dissolves the fiber be used. MANUFACTURING TECHNIQUES OF COMPOSITES  The fabric is cut to size Placed in or on a mold Resin is applied and must be cured. Note that resins do not “dry” but react chemically to polymerize to a final product.Hand Layup The form is coated with a resin and then the fabric is pressed into the resin and another coat of resin applied to the outside. Pre-Preg The fabric is pre-saturated with the resin, the excess resin squeezed out by rollers.Filament Winding Can be used if the pattern is a convex shape, having no indentations Pultrusion (Pull-Extrusion)A bundle of arranged fibers is drawn through a resin bath, then pulled through a heated die. The result is a product having the cross-sectional shape of the die. CURING OF COMPOSITES If air bubbles and delamination are to be avoided in composites, they must be cured underpressure. Vacuum Bagging The simplest and cheapest methods of applying pressure.The part is placed in a bag and a vacuum of around 12 psi is applied to squeeze out bubbles.Autoclaving Used for extremely complex parts may require more than one atmosphere of pressure. JOINING OF COMPOSITES Can be joined by most conventional means. ADVANTAGES AND DISADVANTAGES OF PLASTICS AND COMPOSITES Plastics and composites have many advantages over traditional materials. Advantages: They do not fail by fatigue nor do they corrode as do metals.  Composites also have a much higher strength- to-weight ratio than do metals.  Plastics and composites can be machined, drilled, sawed, shaped, or joined by most conventional manufacturing techniques. Disadvantages: Composites can delaminate.  Cutting composites and their reinforcing fibers is difficult. The glass fibers have asurface hardness greater than many steels.  The greatest drawback to the use of composites in present-day products is the curing time  Cost is also a drawback to the use of composites. The raw materials (reinforcing fibers and resins) are