Plastics (Polymers)
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Plastics (Polymers). The word plastics is from the Greek word Plastikos , meaning “able to be shaped and molded”. Light weight, high weight to strength ratio, particularly when reinforced. Relatively low cost compared to metals and composites. Density. Cost. Why Design with Plastics?.

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Plastics (Polymers)

The word plastics is from the Greek wordPlastikos, meaning “able to be shaped and molded”

Mechanical Engineering


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  • Relatively low cost compared to metals and composites

Density

Cost

Why Design with Plastics?

Mechanical Engineering


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Why Design with Plastics? when reinforced

  • 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


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Disadvantages of using Plastics when reinforced

  • 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


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Mechanical Engineering when reinforced


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Mechanical Properties of Various Plastics when reinforced

Brass: 200 to 850 MPa

Steel: 350 to 1900 MPa

Aluminum: 100 to 550 MPa

Mechanical Engineering


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Polymers when reinforced

  • 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


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The structure of polymers when reinforced

Mechanical Engineering


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Classification of polymers when reinforced

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


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Linear polymers when reinforced

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


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Cross-linked polymers when reinforced

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


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Additives in Plastics when reinforced

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


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Additives in Plastics when reinforced

  • 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


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Design requirement: when reinforcedwear 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


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Design requirement: when reinforcedfunctional 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


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Acetal (Delrin) when reinforced

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


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Nylons when reinforced

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


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Phenolics when reinforced

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


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Silicones when reinforced

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


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Website: www.ge.com/plastics when reinforced

Plastics

Stress vs. Strain curve

Mechanical Engineering


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Structural and mechanical Appl. when reinforced

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


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Parts for wear applications when reinforced

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


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Small housing & hollow shapes when reinforced

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


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Structural & Mechanical when reinforced

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


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Manufacturing Processes for Plastics when reinforced

Fabrication of Plastics

Injection Molding

Molded part

Ejector pin

Heaters

Granular plastic

Plunger

Torpedo

Mechanical Engineering


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Mechanical Engineering when reinforced


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DFM Design Guidelines when reinforcedInjection 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


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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


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Keep section thickness uniform around bosses. material flow.

DFM Design GuidelinesInjection Molding

Provide smooth transition, avoid changes in thickness when possible.

Mechanical Engineering


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Standard thickness variation. material flow.

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


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Rotational Molding material flow.

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


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Turret machine material flow.

Shuttle machine

Rock and roll machine

Rotational Molding Machines

Vertical wheel machine

Mechanical Engineering


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Rotational Molding material flow.

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


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Materials material flow.

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


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Rotational Molding material flow.

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


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Rotational Molding Examples material flow.

Mechanical Engineering


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Rotational Molding Examples material flow.

Mechanical Engineering


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Blow Molding material flow.

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


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Blow Molding material flow.

  • 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


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