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Ken Youssefi Mechanical Engineering 1
The word plastics is from the Greek word Plastikos,
meaning “able to be shaped and molded”
Ken Youssefi Mechanical Engineering 2
Why Design with Plastics?
Cost
• Relatively low cost compared to metals and composites
Density
• Light weight, high weight to strength ratio, particularly when reinforced
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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
Ken Youssefi 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
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Mechanical Properties of Various Plastics
Steel: 350 to 1900 MPa Brass: 200 to 850 MPa
Aluminum: 100 to 550 MPa
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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,….
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The structure of polymers
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Classification 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.
There are two major classifications of polymers
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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
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.
Branched polymers
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.
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Polymer’s Structures
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.
Cross-linking is responsible for providing hardness, strength, brittleness and better dimensional stability.
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.
Network polymers
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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,…..
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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.
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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, polymide
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Applications of Thermoplastics
Design requirement: functional and decorative
Applications: knobs, handles, cases, moldings, pipe fittings, …
Plastics: ABS, acrylic, polyethylene, phenolic, polypropylene, polystyrene
Design requirement: hollow shapes and housings
Applications: pumps, helmets, power tools, cases, …
Plastics: ABS, polyethylene, phenolic, polypropylene, polystyrene, polycarbonate
Design requirement: functional and transparent
Applications: lens, goggles, signs, food processing equipment, …
Plastics: acrylic, polycarbonate, polystyrene, polysulfone
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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
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
Acetal (Delrin)
Properties: good strength, good stiffness, good resistance to heat, moisture, abrasion and chemicals
Applications: mechanical components; gears, bearings, valves, rollers, bushings, housings
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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,
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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, insulators
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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 seals
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Plastics
Website: www.ge.com/plastics
Stress vs. Strain curve
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ABSAcetal (Delrin)AcrylicCellulosicsFluoroplasticsNylonPhenylene OxidePolycarbonatePolyesterPolyethylenePolyimidePolyenylene sulfidePolypropylenePolystyrenePolysulfonePolyurethanePolyvinyl chloride
PhenolicPolyesterPolyurethane
Th
erm
op
last
ics
Th
erm
ose
tsStructural and mechanical Appl.
Gears, cams, pistons, rollers, fan blades, rotors, pump impellers, washing machine agitators
X
X
X
X
Handles, knobs, steering wheel, tool handles, pipe fittings, camera cases, eyeglass frames
X
XX
X
X
Light duty mechanical & decorative
X
X
X
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ABSAcetal (Delrin)AcrylicCellulosicsFluoroplasticsNylonPhenylene OxidePolycarbonatePolyesterPolyethylenePolyimidePolyenylene sulfidePolypropylenePolystyrenePolysulfonePolyurethanePolyvinyl chloride
PhenolicPolyesterPolyurethane
Th
erm
op
last
ics
Th
erm
ose
tsParts for wear applicationsGears, bearings, bushings, tracks, wheels, ware strips
X
XX
XX
X
X
XX
Lenses, safety glasses, signs, refrigerator shelves, windshields
XX
X
XX
Optical and transparent parts
Ken Youssefi Mechanical Engineering 23
ABSAcetal (Delrin)AcrylicCellulosicsFluoroplasticsNylonPhenylene OxidePolycarbonatePolyesterPolyethylenePolyimidePolyenylene sulfidePolypropylenePolystyrenePolysulfonePolyurethanePolyvinyl chloride
PhenolicPolyesterPolyurethane
Th
erm
op
last
ics
Th
erm
ose
tsSmall housing & hollow shapesPhone and flashlight cases, helmets, housings for power tools, pumps, small appliances
X
X
XXXX
XX
XX
Boat hulls, large appliance housings, tanks, tubs, ducts, refrigerator liners
Large housing & hollow shapes
X
X
XX
XX
X
XX
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ABSAcetal (Delrin)AcrylicCellulosicsFluoroplasticsNylonPhenylene OxidePolycarbonatePolyesterPolyethylenePolyimidePolyenylene sulfidePolypropylenePolystyrenePolysulfonePolyurethanePolyvinyl chloride
PhenolicPolyesterPolyurethane
Th
erm
op
last
ics
Th
erm
ose
tsSmall housing & hollow shapes
X
X
XXXX
XX
XX
Large housing & hollow shapes
X
X
XX
XX
X
XX
Parts for wear applications
X
XX
XX
X
X
XX
XX
X
XX
Optical and transparent parts
Light duty mech & deco
Structural & Mechanical
XX
X
X
X
X
XX
X
X
XX
Plastic
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Manufacturing Processes for Plastics
Injection Molding
Fabrication of Plastics
Molded part
HeatersEjector pin
Granular plastic
TorpedoPlunger
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DFM Design GuidelinesInjection Molding
Minimize section thickness, cooling time is proportional to the square of the thickness, reduce cost by reducing the cooling time.
Provide adequate draft angle for easier mold removal.
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DFM Design GuidelinesInjection Molding
Avoid sharp corners, they produce high stress and obstruct material flow.
Keep rib thickness less than 60% of the part thickness in order to prevent voids and sinks.
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DFM Design GuidelinesInjection Molding
Provide smooth transition, avoid changes in thickness when possible.
Keep section thickness uniform around bosses.
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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
Standard thickness variation.
• 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.
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Rotational Molding
• 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.
Rotational molding process consists of six steps
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Rotational Molding Machines
Rock and roll machine
Vertical wheel machine
Shuttle machine
Turret machine
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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
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Rotational MoldingLimitations
• 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).
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Rotational Molding
• 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.
Nominal wall thickness
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Rotational Molding Examples
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Rotational Molding Examples
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Blow MoldingBlow 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.
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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