Review Questions
15.1 What are the principal polymers used in fiber-reinforced polymers?
Answer. Principal polymer matrices in FRPs are unsaturated polyesters and epoxies.
15.2 What is the difference between a roving and a yarn?
Answer. A roving consists of untwisted filaments, while a yarn consists of twisted fibers.
15.3 In the context of fiber reinforcement, what is a mat?
Answer. A mat is a felt consisting of randomly oriented fibers held loosely together in a binder.
15.4 Why are particles and flakes members of the same basic class of reinforcing material?
Answer. Flakes are simply particles that possess very low width-to-thickness ratios.
15.5 What is sheet molding compound (SMC)?
Answer. SMC consists of TS polymer resin, fillers, and chopped glass fibers, all rolled into a sheet of typical thickness = 6.5 mm (0.250 in).
15.6 How is a prepreg different from a molding compound?
Answer. Prepregs have continuous fibers rather than chopped fibers as in molding compounds.
15.7 Why are laminated FRP products made by the spray-up method not as strong as similar products made by hand lay-up?
Answer. Because in hand lay-up, orientation of the fibers is controlled; whereas in spray-up, the fibers in each layer are randomly oriented.
15.8 What is the difference between the wet lay-up approach and the prepreg approach in hand lay-up?
Answer. In wet lay-up, the layer of fiber reinforcement is placed into the mold dry, and the uncured resin is then applied to it to form the composite laminate. In the prepreg approach, layers of fiber preimpregnated with resin are laid into the mold.
15.9 What is an autoclave?
Answer. An autoclave is an enclosed chamber which can supply heat and/or pressure at controlled levels.
15.10 What are some of the advantages of the closed mold processes for PMCs relative to open mold processes?
Answer. The advantages of a closed mold are (1) good finish on all part surfaces, (2) higher production rates, (3) closer control over tolerances, and (4) more complex three-dimensional shapes are possible.
15.11 Identify some of the different forms of polymer matrix composite molding compounds.
Answer. PMC molding compounds include sheet molding compounds, thick molding compounds (a.k.a. dough molding compounds), and bulk molding compounds.
15.12 What is preform molding?
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Answer. Preform molding is a compression molding process in which a precut mat is placed into the lower half of a mold together with a charge of thermosetting resin; the materials are then pressed between heated molds to cure the resin and produce a fiber-reinforced molding.
15.13 Describe reinforced reaction injection molding (RRIM).
Answer. RRIM involves the injection of resins that cure by chemical reaction together with reinforcing fibers into a closed mold. The resulting part is a fiber-reinforced (usually glass fiber) plastic molding.
15.14 What is filament winding?
Answer. Filament winding is a process in which resin-impregnated continuous fibers are wrapped around a rotating mandrel with the internal shape of the FRP product; the resin is cured and the mandrel is removed.
15.15 Describe the pultrusion process.
Answer. Pultrusion is a process in which continuous fibers are dipped into a resin and pulled through a shaping die (somewhat like an extrusion die) where the resin cures. The resulting sections are similar to extruded parts except that they are reinforced with continuous fibers.
15.16 How does pulforming differ from pultrusion?
Answer. Pulforming is pultrusion with the added operation of a shape change in the length (straight length becomes curved) and cross section (different cross sections throughout the length).
15.17 With what kinds of products is tube rolling associated?
Answer. Typical products include bicycle frames and space trusses.
15.18 How are FRPs cut?
Answer. Uncured FRPs are cut by methods that include knives, scissors, power shears, steel-rule blanking dies, laser beam cutting, and water jet cutting. Cured FRPs are cut by cemented carbides and HSS cutting tools, diamond cutting tools, and water jet cutting.
15.20 (Video) According to the video on composites, list the primary purpose of the matrix and the reinforcement in a composite.
Answer: The primary purpose of the matrix is to transfer the load or stress to the reinforcement.
A secondary purpose of the matrix is to protect the reinforcement from the environment. The primary purpose of the reinforcement is to improve the mechanical properties of the composite.
The reinforcement is the main load bearing element in the composite.
15.21 (Video) List the primary methods of fiber reinforced thermoset polymer composite production according to the composite video.
Answer: The methods of fiber-reinforced thermoset polymer composite production identified in the video are (1) manual lay-up, (2) automated lay-up, (3) spray-up, (4) filament winding, (5) pultrusion, and (6) resin transfer molding.
15.22 (Video) What are the advantages and disadvantages of using prepreg material for lay-up of composites according to the composite video?
Answer: The advantages are that prepreg eliminates the separate handling of the matrix and resin, reduces resin consumption, and can improve part quality by providing more consistent proportions of resin and reinforcement content. The disadvantage of prepreg is that it must be stored in a refrigerator so the resin does not cure.
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Multiple Choice Quiz
There are 14 correct answers in the following multiple choice questions (some questions have multiple answers that are correct). To attain a perfect score on the quiz, all correct answers must be given. Each correct answer is worth 1 point. Each omitted answer or wrong answer reduces the score by 1 point, and each additional answer beyond the correct number of answers reduces the score by 1 point. Percentage score on the quiz is based on the total number of correct answers.
15.1 Which one of the following is the most common polymer type in fiber-reinforced polymer composites: (a) elastomers, (b) thermoplastics, or (c) thermosets?
Answer. (c).
15.2 Most rubber products are properly classified into which of the following categories (three best answers): (a) elastomer reinforced with carbon black, (b) fiber-reinforced composite, (c) particle-reinforced composite, (d) polymer matrix composite, (e) pure elastomer, and (f) pure polymer?
Answer. (a), (c), and (d).
15.3 Other names for open mold processes include which of the following (two best answers): (a)
compression molding, (b) contact lamination, (c) contact molding, (d) filament winding, (e) matched die molding, (f) preform molding, and (g) pultrusion?
Answer. (b) and (c).
15.4 Hand lay-up is classified in which of the following general categories of PMC shaping processes (two best answers): (a) closed mold process, (b) compression molding, (c) contact molding, (d) filament winding, or (e) open mold process?
Answer. (c) and (e).
15.5 A positive mold with a smooth surface will produce a good finish on which surface of the laminated product in the hand lay-up method: (a) inside surface or (b) outside surface?
Answer. (a).
15.6 A molding operation that uses sheet-molding compound (SMC) is a form of which one of the following: (a) compression molding, (b) contact molding, (c) injection molding, (d) open mold processing, (e) pultrusion, or (f) transfer molding?
Answer. (a).
15.7 Filament winding involves the use of which one of the following fiber reinforcements: (a) continuous filaments, (b) fabrics, (c) mats, (d) prepregs, (e) short fibers, or (f) woven rovings?
Answer. (a).
15.8 In filament winding, when the continuous filament is wound around the cylindrical mandrel at a helix angle close to 90°, it is called which of the following (one best answer): (a) bi-axial winding, (b) helical winding, (c) hoop winding, (d) perpendicular winding, (e) polar winding, or (f) radial winding?
Answer. (c).
15.9 Pultrusion is most similar to which one of the following plastic shaping processes: (a) blow-molding, (b) extrusion, (c) injection molding, or (d) thermoforming?
Answer. (b).
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15.10 Water jet cutting is one of several ways of cutting or trimming uncured or cured FRPs; in the case of cured FRPs, the process is noted for its reduction of dust and noise: (a) true or (b) false?
Answer. (a).
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