Plastics (Polymers)

1. Plastics (Polymers) The word plastics is from the Greek wordPlastikos, meaning “able to be shaped and molded” Mechanical Engineering

2. Light weight, high weight to strength ratio, particularly when reinforced • Relatively low cost compared to metals and composites Density Cost Why Design with Plastics? Mechanical Engineering

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 transparencies Mechanical Engineering

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 machinability Mechanical Engineering

5. Mechanical Engineering

6. Mechanical Properties of Various Plastics Brass: 200 to 850 MPa Steel: 350 to 1900 MPa Aluminum: 100 to 550 MPa Mechanical Engineering

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,…. Mechanical Engineering

8. The structure of polymers Mechanical Engineering

9. Classification of polymers There are two major classifications of polymers Thermoplastics As the temperature is raised above the melting point, the secondary bonds weaken, making it easier to form the plastic into any desired shape. When polymer is cooled, it returns to its original strength and hardness. The process is reversible. Polymers that show this behavior are known as thermoplastics. Thermosetting Plastics (thermosets) Thermosetting plastics are cured into permanent shape. Cannot be re-melted to the flowable state that existed before curing, continued heating for a long time leads to degradation or decomposition. This curing (cross-linked) reaction is irreversible. Thermosets generally have better mechanical, thermal and chemical properties. They also have better electrical resistance and dimensional stability than do thermoplastics. Mechanical Engineering

10. Linear polymers Branched polymers A sequential structure resulting in thermoplastics like nylon, acrylic, polyethylene. A linear polymer may contain some branched and cross-linked chains resulting in change in properties. Side branch chains are attached to the main chain which interferes with the relative movement of the molecular chains. This results in an increase in strength, deformation resistance and stress cracking resistance. Lower density than linear chain polymers. 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. Mechanical Engineering

11. Cross-linked polymers Three dimensional structure, adjacent chains are linked by covalent bonds. Polymers with cross-linked chains are called thermosetting plastics (thermosets), epoxy and Silicones. Network polymers A three dimensional network of three or more covalent bonds. Thermoplastic polymers that have been already formed could be cross-linked to obtain higher strength. Polymers are exposed to high-energy radiation. Polymer’s Structures Cross-linking is responsible for providing hardness, strength, brittleness and better dimensional stability. Mechanical Engineering

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,….. Mechanical Engineering

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. Mechanical Engineering

14. Design requirement:wear resistance Applications:bearings, gears, bushings, wheels, …. Plastics:nylon, acetal (delrin), polyurethane, phenolic, polymide Applications of Thermoplastics Design requirement:strength Applications:Valves, gears, cams, pistons, fan blades, … Plastics:nylon, acetal (delrin), polycarbonate, phenolic Mechanical Engineering

15. Design requirement:functional and transparent Applications:lens, goggles, signs, food processing equipment, … Plastics:acrylic, polycarbonate, polystyrene, polysulfone Design requirement:hollow shapes and housings Applications:pumps, helmets, power tools, cases, … Plastics:ABS, polyethylene, phenolic, polypropylene, polystyrene, polycarbonate Applications of Thermoplastics Design requirement:functional and decorative Applications:knobs, handles, cases, moldings, pipe fittings, … Plastics:ABS, acrylic, polyethylene, phenolic, polypropylene, polystyrene Mechanical Engineering

16. 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 panels 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 Mechanical Engineering

17. 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, 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 Mechanical Engineering

18. Phenolics Properties:good dimensional stability, rigid, high resistance to heat, water, electricity, and chemicals Applications:laminated panels, handles, knobs, electrical components; connectors, insulators 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 Mechanical Engineering

19. 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 seals 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 Mechanical Engineering

20. Website: www.ge.com/plastics Plastics Stress vs. Strain curve Mechanical Engineering

21. Structural and mechanical Appl. Light duty mechanical & decorative Handles, knobs, steering wheel, tool handles, pipe fittings, camera cases, eyeglass frames Gears, cams, pistons, rollers, fan blades, rotors, pump impellers, washing machine agitators X ABS Acetal (Delrin) Acrylic Cellulosics Fluoroplastics Nylon Phenylene Oxide Polycarbonate Polyester Polyethylene Polyimide Polyenylene sulfide Polypropylene Polystyrene Polysulfone Polyurethane Polyvinyl chloride Phenolic Polyester Polyurethane X X X X Thermoplastics X X X X X X X Thermosets Mechanical Engineering

22. Parts for wear applications Optical and transparent parts Lenses, safety glasses, signs, refrigerator shelves, windshields Gears, bearings, bushings, tracks, wheels, ware strips ABS Acetal (Delrin) Acrylic Cellulosics Fluoroplastics Nylon Phenylene Oxide Polycarbonate Polyester Polyethylene Polyimide Polyenylene sulfide Polypropylene Polystyrene Polysulfone Polyurethane Polyvinyl chloride Phenolic Polyester Polyurethane X X X X X Thermoplastics X X X X X X X Thermosets X X Mechanical Engineering

23. Small housing & hollow shapes Large housing & hollow shapes Boat hulls, large appliance housings, tanks, tubs, ducts, refrigerator liners Phone and flashlight cases, helmets, housings for power tools, pumps, small appliances X X ABS Acetal (Delrin) Acrylic Cellulosics Fluoroplastics Nylon Phenylene Oxide Polycarbonate Polyester Polyethylene Polyimide Polyenylene sulfide Polypropylene Polystyrene Polysulfone Polyurethane Polyvinyl chloride Phenolic Polyester Polyurethane X X X Thermoplastics X X X X X X X X X X X Thermosets X X X Mechanical Engineering

24. Structural & Mechanical Light duty mech & deco Small housing & hollow shapes Large housing & hollow shapes Parts for wear applications Optical and transparent parts Plastic X X X ABS Acetal (Delrin) Acrylic Cellulosics Fluoroplastics Nylon Phenylene Oxide Polycarbonate Polyester Polyethylene Polyimide Polyenylene sulfide Polypropylene Polystyrene Polysulfone Polyurethane Polyvinyl chloride Phenolic Polyester Polyurethane X X X X X X X X X X X X Thermoplastics X X X X X X X X X X X X X X X X X X X X X X X X X Thermosets X X X X X Mechanical Engineering

25. Manufacturing Processes for Plastics Fabrication of Plastics Injection Molding Molded part Ejector pin Heaters Granular plastic Plunger Torpedo Mechanical Engineering

26. Mechanical Engineering

27. DFM Design GuidelinesInjection Molding Provide adequate draft angle for easier mold removal. Minimize section thickness, cooling time is proportional to the square of the thickness, reduce cost by reducing the cooling time. Mechanical Engineering

28. Avoid sharp corners, they produce high stress and obstruct material flow. DFM Design GuidelinesInjection Molding Keep rib thickness less than 60% of the part thickness in order to prevent voids and sinks. Mechanical Engineering

29. Keep section thickness uniform around bosses. DFM Design GuidelinesInjection Molding Provide smooth transition, avoid changes in thickness when possible. Mechanical Engineering

30. Standard thickness variation. DFM Design GuidelinesInjection 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 .01-.015 in radius (min.), prevents an edge from chipping. Mechanical Engineering

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. Mechanical Engineering

32. Turret machine Shuttle machine Rock and roll machine Rotational Molding Machines Vertical wheel machine Mechanical Engineering

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 produced Mechanical Engineering

34. 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). Rotational Molding Limitations • Can not make parts with tight tolerance. • Large flat surfaces are difficult to achieve. • Molding cycles are long (10-20 min.) Mechanical Engineering

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. Mechanical Engineering

36. Rotational Molding Examples Mechanical Engineering

37. Rotational Molding Examples Mechanical Engineering

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. Mechanical Engineering

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 - .04 Mechanical Engineering