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  1. 1. The word plastics is from the Greek word Plastikos, meaning “able to be shaped and molded”Ken Youssefi Mechanical Engineering 1
  2. 2. Why Design with Plastics?• Light weight, high weight to • Relatively low cost compared strength ratio, particularly when to metals and composites reinforced Density Cost Ken Youssefi Mechanical Engineering 2
  3. 3. Why Design with Plastics?• Corrosion resistance• Low electrical and thermal conductivity, insulator• Easily formed into complex shapes, can be formed, casted and joined.• Wide choice of appearance, colors and transparenciesKen Youssefi Mechanical Engineering 3
  4. 4. Disadvantages of using Plastics• Low strength• Low useful temperature range (up to 600 oF)• Less dimensional stability over period of time (creep effect)• Aging effect, hardens and become brittle over time• Sensitive to environment, moisture and chemicals• Poor machinabilityKen Youssefi Mechanical Engineering 4
  5. 5. Ken Youssefi Mechanical Engineering 5
  6. 6. Mechanical Properties of Various Plastics Steel: 350 to 1900 MPa Brass: 200 to 850 MPa Aluminum: 100 to 550 MPaKen Youssefi Mechanical Engineering 6
  7. 7. Polymers • The earliest synthetic polymer was developed in 1906, called Bakelite. • The development of modern plastics started in 1920s using raw material extracted from coal and petroleum products (Ethylene). Ethylene is called a building block. • Polymers are long-chain molecules and are formed by polymerization process, linking and cross linking a particular building block (monomer, a unit cell). • The term polymer means many units repeated many times in a chainlike structure. • Most monomers are organic materials, atoms are joined in covalent bonds (electron-sharing) with other atoms such as oxygen, nitrogen, hydrogen, sulfur, chlorine,….Ken Youssefi Mechanical Engineering 7
  8. 8. The structure of polymersKen Youssefi Mechanical Engineering 8
  9. 9. Classification of polymersThere are two major classifications of polymersThermoplasticsAs the temperature is raised above the melting point, the secondary bondsweaken, making it easier to form the plastic into any desired shape. Whenpolymer is cooled, it returns to its original strength and hardness. The processis reversible. Polymers that show this behavior are known as thermoplastics.Thermosetting Plastics (thermosets)Thermosetting plastics are cured into permanent shape. Cannot be re-melted tothe flowable state that existed before curing, continued heating for a long timeleads to degradation or decomposition. This curing (cross-linked) reaction isirreversible. Thermosets generally have better mechanical, thermal andchemical properties. They also have better electrical resistance and dimensionalstability than do thermoplastics.Ken Youssefi Mechanical Engineering 9
  10. 10. Polymer’s Structures Bonding – monomers are linked together by covalent bonds, forming a polymer chain (primary bonds). The polymer chains are held together by secondary bonds. The strength of polymers comes in part from the length of polymer chains. The longer the chain, the stronger the polymer. More energy is needed to overcome the secondary bonds. Linear polymers Branched polymersA sequential structure resulting in Side branch chains are attached to thethermoplastics like nylon, acrylic, main chain which interferes with thepolyethylene. A linear polymer relative movement of the molecular chains.may contain some branched and This results in an increase in strength,cross-linked chains resulting in deformation resistance and stress crackingchange in properties. resistance. Lower density than linear chain polymers.Ken Youssefi Mechanical Engineering 10
  11. 11. Polymer’s StructuresCross-linked polymersThree dimensional structure, adjacent chains are linkedby covalent bonds. Polymers with cross-linked chainsare called thermosetting plastics (thermosets), epoxyand Silicones. Cross-linking is responsible for providing hardness, strength, brittleness and better dimensional stability.Network polymersA three dimensional network of three or morecovalent bonds. Thermoplastic polymers that havebeen already formed could be cross-linked toobtain higher strength. Polymers are exposed tohigh-energy radiation. Ken Youssefi Mechanical Engineering 11
  12. 12. Additives in Plastics Additives are added to polymers in order to obtain or improve certain properties such as strength, stiffness, color, resistance to weather and flammability. Plasticizers are added to obtain flexibility and softness, most common use of plasticizers are in PVC. Ultraviolet radiation (sunlight) and oxygen cause polymers to become stiff and brittle, they weaken and break the primary bonds. A typical treatment is to add carbon black (soot) to the polymer, it absorbs radiation. Antioxidants are also added to protect against degradation. Fillers such as fine saw dust, silica flour, calcium carbide are added to reduce the cost and to increase harness, strength, toughness, dimensional stability,…..Ken Youssefi Mechanical Engineering 12
  13. 13. Additives in Plastics • Colorants are added to obtain a variety of colors. Colorants are either organic (dye) or inorganic (pigments). Pigments provide greater resistance to temperature and sunlight. • Flame retardants such as chlorine, phosphorus and bromine, are added to reduce polymer flammability. Teflon does not burn and nylon and vinyl chloride are self-extinguishing. • Lubricants such as mineral oil and waxes are added to reduce friction.Ken Youssefi Mechanical Engineering 13
  14. 14. Applications of Thermoplastics Design requirement: strength Applications: Valves, gears, cams, pistons, fan blades, … Plastics: nylon, acetal (delrin), polycarbonate, phenolic Design requirement: wear resistance Applications: bearings, gears, bushings, wheels, …. Plastics: nylon, acetal (delrin), polyurethane, phenolic, polymideKen Youssefi Mechanical Engineering 14
  15. 15. Applications of ThermoplasticsDesign requirement: functional and decorativeApplications: knobs, handles, cases, moldings, pipe fittings, …Plastics: ABS, acrylic, polyethylene, phenolic, polypropylene, polystyreneDesign requirement: functional and transparentApplications: lens, goggles, signs, food processing equipment, …Plastics: acrylic, polycarbonate, polystyrene, polysulfoneDesign requirement: hollow shapes and housingsApplications: pumps, helmets, power tools, cases, …Plastics: ABS, polyethylene, phenolic, polypropylene, polystyrene, polycarbonateKen Youssefi Mechanical Engineering 15
  16. 16. Popular Plastics Polyethylene (LDPE (low density) and HDPE (high density) Properties: good chemical and electrical properties, strength depends on composition Applications: bottles, garbage cans, housewares, bumpers, toys, luggage Acetal (Delrin) Properties: good strength, good stiffness, good resistance to heat, moisture, abrasion and chemicals Applications: mechanical components; gears, bearings, valves, rollers, bushings, housings ABS Properties: dimensionally stable, good strength, impact and toughness properties, good resistance to abrasion and chemicals Applications: automotive components, helmets, tool handles, appliances, boat hulls, luggage, decorative panelsKen Youssefi Mechanical Engineering 16
  17. 17. Popular Plastics Polycarbonates Properties: very versatile and has dimensional stability, good mechanical and electrical properties, high resistance to impact and chemicals Applications: optical lenses, food processing equipments, electrical components and insulators, medical equipments, windshields, signs, machine components Nylons Properties: good mechanical and abrasion resistance property, self- lubricating, resistant to most chemicals but it absorbs water, increase in dimension is undesirable Applications: mechanical components; gears, bearings, rollers, bushings, fasteners, guides, zippers, surgical equipments,Ken Youssefi Mechanical Engineering 17
  18. 18. Applications of Thermosetting Plastics Epoxies Properties: good dimensional stability, excellent mechanical and electrical properties, good resistance to heat and chemicals Applications: electrical components requiring strength, tools and dies, fiber reinforced epoxies are used in structural components, tanks, pressure vessels, rocket motor casing Phenolics Properties: good dimensional stability, rigid, high resistance to heat, water, electricity, and chemicals Applications: laminated panels, handles, knobs, electrical components; connectors, insulatorsKen Youssefi Mechanical Engineering 18
  19. 19. Applications of Thermosetting Plastics Polyesters (thermosetting, reinforced with glass fibers) Properties: good mechanical, electrical, and chemical properties, good resistance to heat and chemicals Applications: boats, luggage, swimming pools, automotive bodies, chairs Silicones Properties: excellent electrical properties over a wide rang of temperature and humidity, good heat and chemical properties Applications: electrical components requiring strength at high temp., waterproof materials, heat sealsKen Youssefi Mechanical Engineering 19
  20. 20. Website: Plastics Stress vs. Strain curveKen Youssefi Mechanical Engineering 20
  21. 21. Structural and mechanical Appl. Light duty mechanical & decorative Gears, cams, pistons, rollers, fan Handles, knobs, steering wheel, blades, rotors, pump impellers, tool handles, pipe fittings, camera washing machine agitators cases, eyeglass frames ABS X Acetal (Delrin) X Acrylic X Cellulosics X Fluoroplastics Nylon X Thermoplastics Phenylene Oxide Polycarbonate Polyester Polyethylene X Polyimide Polyenylene sulfide Polypropylene X Polystyrene X Polysulfone X Polyurethane Polyvinyl chloride XThermosets Phenolic X X Polyester Polyurethane Ken Youssefi Mechanical Engineering 21
  22. 22. Parts for wear applications Optical and transparent parts Gears, bearings, bushings, Lenses, safety glasses, tracks, wheels, ware strips signs, refrigerator shelves, windshields ABS Acetal (Delrin) X Acrylic X Cellulosics X Fluoroplastics X Nylon X Thermoplastics Phenylene Oxide Polycarbonate X Polyester Polyethylene X Polyimide X Polyenylene sulfide X Polypropylene Polystyrene Polysulfone X Polyurethane X X Polyvinyl chlorideThermosets Phenolic Polyester X Polyurethane X Ken Youssefi Mechanical Engineering 22
  23. 23. Small housing & hollow shapes Large housing & hollow shapes Phone and flashlight cases, Boat hulls, large appliance helmets, housings for power housings, tanks, tubs, tools, pumps, small appliances ducts, refrigerator liners ABS X X Acetal (Delrin) Acrylic Cellulosics X Fluoroplastics Nylon Thermoplastics Phenylene Oxide X X Polycarbonate X Polyester X X Polyethylene X X Polyimide Polyenylene sulfide Polypropylene Polystyrene X X Polysulfone X X Polyurethane Polyvinyl chloride XThermosets Phenolic X Polyester X X Polyurethane X Ken Youssefi Mechanical Engineering 23
  24. 24. Structural & Light Small Large Parts for Optical and Plastic Mechanical duty housing & housing wear transparent mech & hollow & hollow applications parts deco shapes shapes ABS X X X Acetal (Delrin) X X Acrylic X X Cellulosics X X X Fluoroplastics X Nylon X X Thermoplastics Phenylene Oxide X X Polycarbonate X X Polyester X X Polyethylene X X X X Polyimide X Polyenylene sulfide X Polypropylene X Polystyrene X X X Polysulfone X X X X Polyurethane X X Polyvinyl chloride X XThermosets Phenolic X X X Polyester X X X Polyurethane X X Ken Youssefi Mechanical Engineering 24
  25. 25. Manufacturing Processes for PlasticsFabrication of Plastics Injection MoldingEjector pin Molded part Heaters Granular plastic Plunger Torpedo Ken Youssefi Mechanical Engineering 25
  26. 26. Ken Youssefi Mechanical Engineering 26
  27. 27. DFM Design Guidelines Injection MoldingProvide adequate draftangle for easier moldremoval. Minimize section thickness, cooling time is proportional to the square of the thickness, reduce cost by reducing the cooling time.Ken Youssefi Mechanical Engineering 27
  28. 28. DFM Design Guidelines Injection MoldingKeep rib thickness less than60% of the part thickness inorder to prevent voids andsinks. Avoid sharp corners, they produce high stress and obstruct material flow.Ken Youssefi Mechanical Engineering 28
  29. 29. DFM Design Guidelines Injection MoldingProvide smooth transition, Keep section thickness uniformavoid changes in thickness around bosses.when possible.Ken Youssefi Mechanical Engineering 29
  30. 30. DFM Design Guidelines Injection Molding• Use standard general tolerances, do not tolerance; Dimension Tolerance Dimension Tolerance 0 ≤ d ≤ 25 ± 0.5 mm 0 ≤ d ≤ 1.0 ± 0.02 inch 25 ≤ d ≤ 125 ± 0.8 mm 1 ≤ d ≤ 5.0 ± 0.03 inch 125 ≤ d ≤ 300 ± 1.0 mm 5 ≤ d ≤ 12.0 ± 0.04 inch 300 ± 1.5 mm 12.0 ± 0.05 inch• Minimum thickness recommended; .025 inch or .65 mm, up to .125 for large parts.• Round interior and exterior corners to . Standard thickness 01-.015 in radius (min.), prevents an variation. edge from chipping.Ken Youssefi Mechanical Engineering 30
  31. 31. Rotational Molding Rotational molding process consists of six steps • A predetermined amount of plastic, powder or liquid form, is deposited in one half of a mold. • The mold is closed. • The mold is rotated biaxially inside an oven. • The plastics melts and forms a coating over the inside surface of the mold. • The mold is removed from the oven and cooled. • The part is removed from the mold.Ken Youssefi Mechanical Engineering 31
  32. 32. Rotational Molding Machines Vertical wheel machine Turret machine Shuttle machine Rock and roll machineKen Youssefi Mechanical Engineering 32
  33. 33. Rotational Molding Advantages • Molds are relatively inexpensive. • Rotational molding machines are much less expensive than other type of plastic processing equipment. • Different parts can be molded at the same time. • Very large hollow parts can be made. • Parts are stress free. • Very little scrap is producedKen Youssefi Mechanical Engineering 33
  34. 34. Rotational Molding Limitations • Can not make parts with tight tolerance. • Large flat surfaces are difficult to achieve. • Molding cycles are long (10-20 min.) Materials Polyethylene (most common), Polycarbonate (high heat resistance and good impact strength), Nylon (good wear and abrasion resistance, good chemical resistance, good toughness and stiffness).Ken Youssefi Mechanical Engineering 34
  35. 35. Rotational Molding Nominal wall thickness• Polycarbonate wall thickness is typically between .06 to . 375 inches, .125 inch being an ideal thickness.• Polyethylene wall thickness is in the range of .125 to .25 inch, up to 1 inch thick wall is possible.• Nylon wall thickness is in the range of .06 to .75 inch.Ken Youssefi Mechanical Engineering 35
  36. 36. Rotational Molding ExamplesKen Youssefi Mechanical Engineering 36
  37. 37. Rotational Molding ExamplesKen Youssefi Mechanical Engineering 37
  38. 38. Blow Molding Blow molding is generally the same process as glass blowing adapted to polymers. In extrusion blow molding a tube is extruded and clamped in a split mold. Air under pressure (50-100 psi) is injected into the tube blowing the plastic outward to fill the mold cavity.Ken Youssefi Mechanical Engineering 38
  39. 39. Blow Molding • Blow molding is used for medium size, hollow thin-walled shapes; containers, tool cases, hollow structures, …. • Blow molding is limited to thermoplastics such as polyethylene, polycarbonate, ABS. • Wall thickness between .015 - .125 • Maximum tolerance .01 - .04Ken Youssefi Mechanical Engineering 39
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