Plastics (Polymers)

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Plastics (Polymers)
The word plastics is from the Greek word
Plastikos, meaning able to be shaped and molded
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Why Design with Plastics?
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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

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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
Brass 200 to 850 MPa
Steel 350 to 1900 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
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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Polymers 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.
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Polymers Structures
Cross-linking is responsible for providing
hardness, strength, brittleness and better
dimensional stability.
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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
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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
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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
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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
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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
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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
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Website www.ge.com/plastics
Plastics
Stress vs. Strain curve
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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
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ABS Acetal (Delrin) Acrylic Cellulosics Fluoroplas
tics Nylon Phenylene Oxide Polycarbonate Polyester
Polyethylene Polyimide Polyenylene
sulfide Polypropylene Polystyrene Polysulfone Poly
urethane Polyvinyl chloride Phenolic Polyester Po
lyurethane
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Thermoplastics
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Thermosets
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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 Fluoroplas
tics Nylon Phenylene Oxide Polycarbonate Polyester
Polyethylene Polyimide Polyenylene
sulfide Polypropylene Polystyrene Polysulfone Poly
urethane Polyvinyl chloride Phenolic Polyester Po
lyurethane
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Thermoplastics
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Thermosets
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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
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ABS Acetal (Delrin) Acrylic Cellulosics Fluoroplas
tics Nylon Phenylene Oxide Polycarbonate Polyester
Polyethylene Polyimide Polyenylene
sulfide Polypropylene Polystyrene Polysulfone Poly
urethane Polyvinyl chloride Phenolic Polyester Po
lyurethane
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Thermoplastics
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Thermosets
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Small housing hollow shapes
Large housing hollow shapes
Parts for wear applications
Optical and transparent parts
Light duty mech deco
Structural Mechanical
Plastic
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ABS Acetal (Delrin) Acrylic Cellulosics Fluoroplas
tics Nylon Phenylene Oxide Polycarbonate Polyester
Polyethylene Polyimide Polyenylene
sulfide Polypropylene Polystyrene Polysulfone Poly
urethane Polyvinyl chloride Phenolic Polyester Po
lyurethane
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Thermoplastics
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Thermosets
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Manufacturing Processes for Plastics
Fabrication of Plastics
Injection Molding
Molded part
Ejector pin
Heaters
Granular plastic
Plunger
Torpedo
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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.
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DFM Design GuidelinesInjection Molding
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.
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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

• 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
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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Rotational Molding Machines
Vertical wheel 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 Molding
Limitations

• Can not make parts with tight tolerance.

• Large flat surfaces are difficult to achieve.

• Molding cycles are long (10-20 min.)

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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.

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Rotational Molding Examples
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Rotational Molding Examples
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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.
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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