plastics

Ken Youssefi Mechanical Engineering 1

The word plastics is from the Greek word Plastikos,

meaning “able to be shaped and molded”


Why Design with Plastics?

Cost

• Relatively low cost compared to metals and composites

Density

• Light weight, high weight to strength ratio, particularly when reinforced


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


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 Properties of Various Plastics

Steel: 350 to 1900 MPa Brass: 200 to 850 MPa

Aluminum: 100 to 550 MPa


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,….


The structure of polymers


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


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.


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


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,…..


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.


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


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


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


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,


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


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


Plastics

Website: www.ge.com/plastics

Stress vs. Strain curve


ABSAcetal (Delrin)AcrylicCellulosicsFluoroplasticsNylonPhenylene OxidePolycarbonatePolyesterPolyethylenePolyimidePolyenylene sulfidePolypropylenePolystyrenePolysulfonePolyurethanePolyvinyl chloride

PhenolicPolyesterPolyurethane

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Optical and transparent parts




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Boat hulls, large appliance housings, tanks, tubs, ducts, refrigerator liners

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Parts for wear applications

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Light duty mech & deco

Structural & Mechanical

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Plastic


Manufacturing Processes for Plastics

Injection Molding

Fabrication of Plastics

Molded part

HeatersEjector pin

Granular plastic

TorpedoPlunger


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.



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.



Provide smooth transition, avoid changes in thickness when possible.

Keep section thickness uniform around bosses.



• 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.


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


Rotational Molding Machines

Rock and roll machine

Vertical wheel machine

Shuttle machine

Turret machine


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


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).


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


Rotational Molding Examples


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.


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

plastics

Design

strength of polymers

plastics additives

linear chain polymers

word plastics

plastics colorants

thermoplastic polymers

strength ratio

original strength