Reinforced Plastic Components

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2 Reinforced Plastic Components Reinforced-plastic components for a Honda motorcycle. The parts shown are front and rear forks, a rear swing arm, a wheel, and brake disks. 2

3 Processing Reinforced Plastics Reinforced plastics A type of composites are among the most important materials can be engineered to meet specific design requirements: high strength-to-weight ratios stiffness-to-weight ratios creep resistance require special methods for shaping into useful products, due to their unique structure. Cost The care required and the several steps involved in manufacturing reinforced plastics make processing costs substantial, and generally are not competitive with processing traditional materials and shapes. 3

4 Processing Reinforced Plastics Requirements This situation has necessitated the careful assessment and integration of the design and manufacturing proceses (concurrent engineering) in order to minimize costs while maintaining product in integrity and production rate. Environmental concern An important environmental concern in reinforced plastics is the dust generated during processing, such as airborne carbon fibers which are known to remain in the work area long after fabrication of parts has been completed. 4

5 Processing Reinforced Plastics The reinforcement may be chopped fibers or continuous lengths of fiber. In order to obtain good bonding between the reinforcing fibers and the polymer matrix as well as protect the fibers during subsequent processing, fibers are surface treated. When the impregnation is done as a separate step, the resulting partially cured sheets are called (a) prepregs, (b) bulk-molding compound (BMC), or (c) sheet-molding compound (SMC), depending on their form. 5 The sheets should be stored at a sufficiently low temperature to delay curing. Alternatively, the resin and the fibers can he mixed together at the time they are placed in the mold. More recently, a thick molding compound (TMC, a trademark) has been developed that combines the characteristics of BMC (lower cost) and SMC (higher strength). These molding compounds are usually injection molded using chopped fibers of various lengths. One application is for electrical components because of the high dielectr c strength of TMC.

6 Processing Reinforced Plastics Commercial reinforced plastics are available as sheet-molding compounds. Continuous strands of reinforcing fiber are chopped into short fibers (Fig ) and deposited over a layer of resin paste, usually a polyester mixture, carried on a polymer film such as polyethylene. A second layer of resin paste is deposited on top, and the sheet is pressed between rollers. The product is gathered into rolls, or placed into containers in layers, and stored until it undergoes a maturation period, reaching the desired molding viscosity. The maturing process is under controlled temperature and humidity, and usually takes one day. The matured SMC, which has a leather like feel and is tack-free, has a shelf life of about 30 days and must be processed within this period. 6

7 Sheet Prepreg Manufacturing process for producing reinforced-plastic sheets. The sheet is still viscous at this stage and can later be shaped into various products. 7

8 Processing Reinforced Plastics A typical procedure for making reinforced plastic prepregs is shown (Fig. a). The continuous fibers are aligned and subjected to surface treatment to enhance adhesion to the polymer matrix. They are then coated by dipping in a resin bath and made into a sheet or tape (Fig. b). Individual pieces of the sheet are then assembled into laminated structures, such as the horizental stabilizer for the F-14 fighter aircraft. Special computer-controlled tape-laying machines have been developed for this purpose. Typical products are flat or corrugated architectural paneling, panels for construction and electric insulation, and structural components of aircraft. 8

9 Prepreg Tape Manufacture (a) Manufacturing process for polymer-matrix composite. (b) Boron epoxy prepreg tape. 9

10 Processing Reinforced Plastics Case study: Tennis rackets made of composite materials In order to impart certain desirable characteristics, such as light weight and stiffness, composite-material tennis rackets are being manufactured with graphite, fiberglass, boron, ceramic (silicon carbide), and Kevlar as the reinforcing fibers. Rackets have a foam core; some have unidirectional and others have braided reinforcement. Rackets with boron fibers have the highest stiffness, followed by graphite (carbon), glass, and Kevlar. The racket with the lowest stiffness has 80 percent fiberglass, whereas the stiffest has 95 percent graphite and 5 percent boron. Thus it has the highest percentage of inexpensive reinforcing fiber and the smallest percentage of the most expensive fiber. 10

11 Molding Compression Molding In compression molding, the material is placed between two molds and pressure is applied. Depending on the material, the molds may be either at room temperature or heated to accelerate hardening. The material may be in bulk form (bulk molding compound), which is a viscous, sticky mixture of polymers, fibers, and additives. It is generally shaped into a log which is cut into the desired mass. Fiber lengths generally range from 3 to 50 mm, although longer fibers (75 mm) may also be used. Sheet molding compounds can also be used in molding. Sheet -molding compound is similar to BMC, except that the resin fiber mixture is laid between plastic sheets to make a sandwich that can be easily handled. The sheets are removed when the SMC is placed in the mold. 11

12 Vacuum-bag molding prepregs are laid in a mild to form the desired (Fig), shape. In this case, the pressure required to form the shape and create good bonding is obtained by covering the lay-up with a plastic bag and creating a vacuum. If additional heat and pressure are desired. In order to prevent the resin from sticking to the vacuum bag and to facilitate removal of excess resin, several sheets of various materials (release cloth, bleeder cloth) are placed on top of the prepreg sheets. The molds can be made of metal, usually aluminum, but more often are made from the same resin (with reinforcement) as the material to be cured. This eliminates any problem with differential thermal expansion between the mold and the part. 12

13 Bag Forming (a) Vacuum-bag forming. (b) Pressure-bag forming. 13

14 Contact molding Contact molding processes use a single male or female mold (Fig) made of materials such as reinforced plastics, wood, or plaster. Contact molding is used in making products with high surface area-to-thickness ratios, such as swimming pools, boats, tub and shower units, and housings. This is a wet method in which the reinforcement is impregnated with the resin at the time of molding. The simplest method is called hand lay-up. The materials are placed and formed in the mold by hand (Fig. a), and the squeezing action expels any trapped air and compacts the part. Molding may also be done by spraying (spray-up; Fig. b). Although spraying can be automated, these processes are relatively slow and labor costs are high. However, they are simple and the tooling is inexpensive. Only the moldside surface of the part is smooth and the choice of materials is limited. Many types of boats are made by this process. 14

15 Hand Lay-up and Spray-up Manual methods of processing reinforced plastics: (a) hand lay-up (b) spray-up. These methods are also called open-mold processing. 15

16 Resin transfer molding is based on transfer molding whereby a resin, mixed with a catalyst, is forced by a piston-type positive displacement pump into the mold cavity filled with fiber reinforcement. The process is a viable alternative to hand lay-up, spray-up, and compression molding for low or intermediate volume production. Most of the exterior body of the new cars are made from resin-transfermolded components. They are 40 percent lighter than similar metal part would be. 16

17 Filament winding, pultrusion, and pulforming Filament winding is a process whereby the resin and fibers are combined at the time of curing. Axisymmetric parts, such as pipes and storage tanks, as well as asymmetric parts are produced on a rotating mandrel. (Fig.) The reinforcing filament, tape, or roving is wrapped continuously around the form. The reinforcements are impregnated by passing them through a polymer bath (Fig. a). The process can be modified by wrapping the mandrel with prepreg material. The products made by filament winding are very strong because of their highly reinforced structure. Filament winding has also been used for strengthening cylindrical or spherical pressure vessels (Fig. b) made of materials such as aluminum and titanium. בּ ל תּ י ח ד יר ( impermeable The presence of a metal inner lining makes the part (לנ וזל ים 17

18 Filament Winding (a) (b) Schematic illustration of the filament winding process. Fiberglass being wound over aluminum liners for slide-raft inflation vessels for the Boeing 767 aircraft. 18

19 Filament winding cont. Filament winding can be used directly over solid-rocket propellant forms. חו מר ה י וצ ר הנעה Seven-axis computer-controlled machines have been developed for making ח ד-כּ וּוּנ י asymmetric parts that automatically dispense several unidirectional prepregs. Typical asymmetric parts made are aircraft engine ducts, fuselages struts. propellers, blades, and,גוף המטוס 19

20 Pultrusion Long shapes with various constant profiles, such as rods, profiles, or tubing (similar to drawn metal products), are made by the pultrusion process. Typical products are golf clubs, drive shafts, and structural members such as ladders, walkways, and handrails. In this process, developed in the early 1950s, the continuous reinforcement (roving or fabric) is pulled through a thermosetting polymer bath, and then through a long heated steel die (Fig). The product is cured during it travel through the die and cut into desired lengths. The most common material used in pultrusion is polyester with glass reinforcements. 20

21 Pultrusion Schematic illustration of the pultrusion process. This figure shows a microwave heating arrangement, although curing can be performed in a heated die. 21

22 Pulforming Continuously reinforced products other than constant cross- sectional profiles are made by pulforming. After being pulled through the polymer bath, the composite is clamped between the two halves of a die and cured into a finished product. The dies recirculate and shape the products successively. Common examples are glass-fiber reinforced hammer handles and curved automotive leaf springs. 22

23 Quality considerations The major quality considerations for the processes : Internal voids and gaps between successive layers of material. Volatile gases developing during processing must be allowed to escape from the lay-up through the vacuum bag to avoid porosity due to trapped gases within the lay-up. Micro-cracks always develop due to improper curing or during transportation and handling. These defects can be detected using ultrasonic scanning and other techniques. 23

24 Processing Metal-Matrix and Ceramic-Matrix Composites Outline of the basic manufacturing processes for composite materials. Important considerations involve the nature and strength of the interface between the matrix and the fibers, as well as uniform distribution of the fibers and the residual stresses developed during processing. Processing metal-matrix composites (MMC). There are three methods of manufacturing these composites into near-net shape parts: 1. Liquid-phase processes 2. Solid-phase processes 3. Two-phase (liquid/solid) processes. 24

25 1. Liquid-phase processing Basically consists of casting the liquid matrix and the solid reinforcement, using either conventional casting processes or pressure infiltration casting. In the latter process, pressurized gas is used to force the liquid matrix metal into a pre-form (usually as sheet or wire) made of the reinforcing fibers. 2. Solid-phase processes Consist basically of powder-metallurgy techniques, including cold and hot isostatic pressing. Proper mixing for homogeneous distribution of the fibers is important. An example of this technique is employed in tungsten-carbide tool and die manufacturing with cobalt as the matrix material. 25

26 In making complex MMC parts with whisker fiber reinforcement, die geometry and control of process variables are very important in ensuring proper distribution and orientation of the fibers within the part VIC parts made by powder-metallurgy processes are generally heat treated for optimum properties. 3. Two-phase processing The techniques used for two-phase processing consist of rheocasting, and spray atomization and deposition. In the latter processes, the reinforcing fibers are mixed with a matrix that contains both liquid and solid phases 26

27 Processing ceramic-matrix composites (CMC). Slurry infiltration The most common process, involves the preparation of a fiber preform that is hot pressed, and then impregnated with a slurry that contains the matrix powder, a carrier liquid, and an organic binder. A further improvement is reaction bonding or reaction sintering of the slurry. High strength, toughness, and uniform structure is obtained by the slurry infiltration process, but the product has limited high-temperature properties due to the low melting temperature of the matrix materials used. 27

28 Processing ceramic-matrix composites (CMC). Chemical synthesis Processes involve the sol-gel and polymer precursor techniques. In the sot-gel process, a sol (a colloidal fluid with the liquid as the continuous phase) containing fibers is converted to a gel, which is then subjected to heat treatment to produce a ceramic-matrix composite. The polymer precursor method is analogous to the process used in making ceramic fibers. In chemical vapor infiltration a porous fiber preform is infiltrated with the matrix phase using the chemical vapor deposition technique. The product has very good high-temperature properties, but the process is costly and time-consuming. 28

29 Manufacturing Honeycomb Materials There are two principal methods of manufacturing honeycomb materials. In the expansion process (Fig) - the most common method sheets are cut from a coil and an adhesive is applied at intervals (node lines). The sheets are stacked and cured in an oven, whereby strong bonds develop at the adhesive joints. The block is then cut into slices of desired dimension and stretched to produce a honeycomb structure. This procedure is similar to expanding folded paper structures into the shape of decorative objects. 29

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31 Manufacturing Honeycomb Materials In the corrugation process (Fig), the sheet passes through a pair of specially designed rolls, producing corrugated sheets, which are then cut into desired lengths. Again, adhesive is applied to the node lines, and the block is cured. Note that no expansion process is involved. The honeycomb material is then made into a sandwich structure (Fig). Face sheets are subsequently joined with adhesives to the top and bottom surfaces. 31

32 Design Considerations Selection of an appropriate material from an extensive list requires consideration of service requirements and possible long-range effects on properties and behavior, such as dimensional stability and wear as well as their ultimate disposal after completion of their life cycle. Compared to metals, plastics have lower strength and stiffness, although the strength-to-weight and stiffness-to-weight ratio for reinforced plastics is higher than for many metals. Section sizes should be selected accordingly, with view to maintaining a sufficiently high section modulus for improved stiffness. 32

33 Design Considerations Cont. One of the major design advantages of reinforced plastics is the directional nature of the strength of the material. Forces applied to I he material are transferred by the resin matrix to the fibers, are much stronger and stiffer than the matrix. When fibers are all orient d in one direction, the resulting material is exceptionally strong in the fiber direction. For strength in two principal directions, the unidirectional materials are often laid at different angles to each other. If strength in the third (thickness) direction is desired, a different type of material is used to form a sandwich structure. Physical properties, especially high coefficient of thermal expansion, and hence contraction, are important. 33

34 Design Considerations Cont. Improper part design or assembly can lead to warping and shrinking (Fig. a). The overall part geometry often determines the particular forming or molding process. The following table is a guide to making this selection. Even after a particular process is selected, the design of the part and die should be such that it will not cause problems concerning shape generation (Fig. b), dimensional control, and surface finish. 34

35 Design Considerations Cont. Large variations in cross-section sizes (Fig. c) and abrupt changes in geometry should be avoided for better product quality and increased product life. Furthermore, contraction in large cross-sections tends to cause porosity in plastic part. Conversely, because of a lack of stiffness, removing thin sections from molds after shaping may be difficult. The low elastic modulus of plastics further requires that shapes be selected properly for improved stiffness of the component (Fig. d), particularly when saving materials is important. 35

36 Design Considerations Cont. The properties of the final product depend on the original mater al and its processing history. Cold working of polymers improves their strength and toughness. On the other hand, because of the nonuniformity of deformation, residual stresses develop in polymers, as they do in metals. Residual stresses can also be generated by thermal cycling of the part. Like in metals, the magnitude and direction of residual stresses, are important factors. These stresses can relax over a period of time and cause distortion of the part during its service life. 36

37 Characteristics of various molding and forming processes for plastics 37

38 The Economics of Forming and Shaping Reinforced materials As in all other processes, design and manufacturing decisions are ultimately based on performance and cost, including the costs of equipment, tooling, and production. The final selection of a process depends greatly on production Volume. High equipment and tooling costs can be acceptable if the production run is large, such as casting and forging. The most expensive are injection-molding machines, with costs being directly proportional to clamping force. A machine with a 2000-kN (225-ton) clamping force costs about $100,000, and one with a 20,000-kN (2250-ton) clamping force costs about $450,

39 The Economics of Forming and Shaping Reinforced materials Cont. The optimum number of cavities in the die for making the product in one cycle is an important consideration, as in die casting. As the number of cavities increases, so does the cost of the die. Larger dies may be considered for larger numbers of cavities, increasing die cost even further. On the other hand, more parts will be produced per machine cycle, thus increasing the production rate. Hence a detailed analysis has to be made to determine the optimum number of cavities, die size, and machine capacity. 39

40 The Economics of Forming and Shaping Reinforced materials Cont. Similar considerations apply to other plastics processing methods. Tables 18.2 and 18.3 are general guides to economical processing of composite materials. For composite materials, equipment and tooling costs for most molding operations are generally high. Production rates and economic production quantities vary widely. 40

41 The Economics of Forming and Shaping Reinforced materials Cont. EQUIPME NT CAPITAL COST PRODU CTION RATE TOOLING COST Machining Med Med Low Compression molding High Med High Transfer molding High Med High Injection molding High High High Extrusion Med High Low * Rotational molding Low Low Low Blow molding Med Med Med Thermoforming Low Low Low TYPICAL PRODUCTION VOLUME, NUMBER OF PARTS Casting Low Very Low low Forging High Low Med Foam molding High Med Med Source: After R.L.E. Brown, Design and Manufacture of Plastic Parts. Copyright 1980 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc. * Continuous process. 41

42 ECONOMIC PRODUCTION QUANTITIES FOR VARIOUS MOLDING METHODS Molding Method Hand lay-up Spray-up Casting Vacuum-bag molding Compressionmolded BMC SMC and preform Pressure-bag molding Centrifugal casting Filamentwinding Pultrusion Rotational molding Injection molding Relative investment required Equipment VL L M M H H H H H H H VH Tooling L L L L VH VH H H H H H VH Relative Production Rate L L L VL H H L M L H L VH Economic production quantity VL L L VL H H L M L H M VH 42 Source: J. G. Bralla ed.), Handbook of Product Design for Manfacturing. New York: McGraw-Hill, VL, very low; L, low; M, medium; H, high; VH, very high.