The word plastics is from the Greek word Plastikos,
meaning “able to be shaped and molded”
Ken Youssefi
Mechanical Engineering
1
Why Design with Plastics?
• Light weight, high weight to
strength ratio, particularly when
reinforced
• Relatively low cost compared
to metals and composites
Density
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Cost
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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
3
Disadvantages of using Plastics
• Low strength
o
• Low useful temperature range (up to 600 F)
• 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 Engineering
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Ken Youssefi
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Mechanical Properties of Various Plastics
Brass: 200 to 850 MPa
Steel: 350 to 1900 MPa
Aluminum: 100 to 550 MPa
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Mechanical Engineering
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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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Mechanical Engineering
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The structure of polymers
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Mechanical Engineering
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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.
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Mechanical Engineering
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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.
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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.
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.
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Mechanical Engineering
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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,…..
Ken Youssefi
Mechanical Engineering
12
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
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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
Ken Youssefi
Mechanical Engineering
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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: 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
Ken Youssefi
Mechanical Engineering
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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
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
Ken Youssefi
Mechanical Engineering
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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, selflubricating, 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
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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
Ken Youssefi
Mechanical Engineering
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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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Mechanical Engineering
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Website: www.ge.com/plastics
Plastics
Stress vs. Strain
curve
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Mechanical Engineering
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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
cases, eyeglass frames
washing machine agitators
ABS
X
Acetal (Delrin)
X
Acrylic
X
Cellulosics
X
Thermosets
Thermoplastics
Fluoroplastics
Nylon
Phenylene Oxide
Polycarbonate
Polyester
Polyethylene
Polyimide
Polyenylene sulfide
Polypropylene
Polystyrene
Polysulfone
Polyurethane
Polyvinyl chloride
Phenolic
Polyester
Polyurethane
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X
X
X
X
X
X
X
X
Mechanical Engineering
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Parts for wear applications
Gears, bearings, bushings,
tracks, wheels, ware strips
Thermosets
Thermoplastics
ABS
Acetal (Delrin)
Acrylic
Cellulosics
Fluoroplastics
Nylon
Phenylene Oxide
Polycarbonate
Polyester
Polyethylene
Polyimide
Polyenylene sulfide
Polypropylene
Polystyrene
Polysulfone
Polyurethane
Polyvinyl chloride
Phenolic
Polyester
Polyurethane
Ken Youssefi
Optical and transparent parts
Lenses, safety glasses,
signs, refrigerator shelves,
windshields
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Mechanical Engineering
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Thermosets
Thermoplastics
Small housing & hollow shapes
Phone and flashlight cases,
helmets, housings for power
tools, pumps, small appliances
X
Ken Youssefi
Large housing & hollow shapes
Boat hulls, large appliance
housings, tanks, tubs,
ducts, refrigerator liners
X
ABS
Acetal (Delrin)
Acrylic
Cellulosics
Fluoroplastics
Nylon
Phenylene Oxide
Polycarbonate
Polyester
Polyethylene
Polyimide
Polyenylene sulfide
Polypropylene
Polystyrene
Polysulfone
Polyurethane
Polyvinyl chloride
X
X
X
X
X
X
X
X
X
Phenolic
Polyester
Polyurethane
X
X
X
X
X
X
X
X
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Thermosets
Thermoplastics
Plastic
Ken Youssefi
Structural & Light
Mechanical duty
mech &
deco
ABS
Acetal (Delrin)
X
Acrylic
Cellulosics
Fluoroplastics
X
Nylon
Phenylene Oxide
Polycarbonate
Polyester
Polyethylene
Polyimide
Polyenylene sulfide
X
Polypropylene
Polystyrene
Polysulfone
Polyurethane
Polyvinyl chloride
X
Phenolic
Polyester
Polyurethane
X
X
Small
housing
& hollow
shapes
X
Large
Parts for
Optical and
housing wear
transparent
& hollow applications parts
shapes
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
X
X
X
X
X
X
X
X
X
Mechanical Engineering
X
X
X
X
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Manufacturing Processes for Plastics
Fabrication of Plastics
Injection Molding
Ejector pin
Molded part
Heaters
Granular
plastic
Plunger
Torpedo
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Mechanical Engineering
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Ken Youssefi
Mechanical Engineering
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DFM Design Guidelines
Injection 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.
Ken Youssefi
Mechanical Engineering
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DFM Design Guidelines
Injection Molding
Keep rib thickness less than
60% of the part thickness in
order to prevent voids and
sinks.
Ken Youssefi
Avoid sharp corners, they
produce high stress and
obstruct material flow.
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DFM Design Guidelines
Injection Molding
Provide smooth transition,
avoid changes in thickness
when possible.
Ken Youssefi
Keep section thickness uniform
around bosses.
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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
5 ≤ d ≤ 12.0
± 0.04 inch
12.0
± 0.05 inch
125 ≤ d ≤ 300 ± 1.0 mm
300
± 1.5 mm
•
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.
Ken Youssefi
Mechanical Engineering
Standard thickness
variation.
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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.
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Mechanical Engineering
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Rotational Molding Machines
Vertical wheel machine
Turret machine
Shuttle machine
Rock and roll machine
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Mechanical Engineering
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Rotational Molding
Advantages
Ken Youssefi
•
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 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
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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
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Rotational Molding Examples
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Rotational Molding Examples
Ken Youssefi
Mechanical Engineering
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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
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
Ken Youssefi
Mechanical Engineering
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