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					          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                  • Relatively low cost compared
  strength ratio, particularly when               to metals and composites

                Density                                       Cost
 Ken Youssefi                    Mechanical Engineering                          2
               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

Ken Youssefi            Mechanical Engineering     3
               Disadvantages of using Plastics

• Low strength
• 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

Ken Youssefi               Mechanical Engineering   4
Ken Youssefi   Mechanical Engineering   5
                  Mechanical Properties of Various Plastics

               Steel: 350 to 1900 MPa             Brass: 200 to 850 MPa
                               Aluminum: 100 to 550 MPa
Ken Youssefi                            Mechanical Engineering            6
   •     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,….

Ken Youssefi                          Mechanical Engineering                          7
               The structure of polymers

Ken Youssefi            Mechanical Engineering   8
               Classification of polymers
There are two major classifications of polymers

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.

Ken Youssefi                       Mechanical Engineering                          9
                            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                                  Branched polymers

 A sequential structure resulting in             Side branch chains are attached to the
 thermoplastics like nylon, acrylic,             main chain which interferes with the
 polyethylene. A linear polymer                  relative movement of the molecular chains.
 may contain some branched and                   This results in an increase in strength,
 cross-linked chains resulting in                deformation resistance and stress cracking
 change in properties.                           resistance. Lower density than linear chain
Ken Youssefi                           Mechanical Engineering                          10
                     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.

 Ken Youssefi                       Mechanical Engineering                  11
                              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

               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                  13
               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: 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                    15
                             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

       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                     16
                           Popular Plastics
      Properties: very versatile and has dimensional stability, good
      mechanical and electrical properties, high resistance to impact and
      Applications: optical lenses, food processing equipments, electrical
      components and insulators, medical equipments, windshields, signs,
      machine components

      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,

Ken Youssefi                          Mechanical Engineering                       17
         Applications of Thermosetting Plastics

   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

   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                        18
         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

   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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                                                          Stress vs. Strain

Ken Youssefi                     Mechanical Engineering                     20
                                      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
                                     washing machine agitators          cases, eyeglass frames
                                 ABS                                                  X
                                 Acetal (Delrin)       X
                                 Acrylic                                              X
                                 Cellulosics                                          X
                                 Nylon                 X

                                 Phenylene Oxide
                                 Polyethylene                                       X
                                 Polyenylene sulfide
                                 Polypropylene         X
                                 Polystyrene                                        X
                                 Polysulfone                                        X
                                 Polyvinyl chloride                                 X

                                 Phenolic              X                            X
                  Ken Youssefi                             Mechanical Engineering                21
                                         Parts for wear applications                Optical and transparent parts
                                         Gears, bearings, bushings,                 Lenses, safety glasses,
                                         tracks, wheels, ware strips                signs, refrigerator shelves,
                                 Acetal (Delrin)       X
                                 Acrylic                                                        X
                                 Cellulosics                                                    X
                                 Fluoroplastics        X
                                 Nylon                 X

                                 Phenylene Oxide
                                 Polycarbonate                                                  X
                                 Polyethylene          X
                                 Polyimide             X
                                 Polyenylene sulfide   X
                                 Polysulfone                                                    X
                                 Polyurethane          X                                        X
                                 Polyvinyl chloride

                                 Polyester             X
                                 Polyurethane          X
                  Ken Youssefi                             Mechanical Engineering                            22
                                         Small housing & hollow shapes              Large housing & hollow shapes
                                         Phone and flashlight cases,                Boat hulls, large appliance
                                         helmets, housings for power                housings, tanks, tubs,
                                         tools, pumps, small appliances             ducts, refrigerator liners
                                 ABS                   X                                         X
                                 Acetal (Delrin)
                                 Cellulosics           X

                                 Phenylene Oxide       X                                       X
                                 Polycarbonate         X
                                 Polyester             X                                       X
                                 Polyethylene          X                                       X
                                 Polyenylene sulfide
                                 Polystyrene           X                                       X
                                 Polysulfone           X                                       X
                                 Polyvinyl chloride                                            X

                                 Phenolic              X
                                 Polyester             X                                       X
                                 Polyurethane                                                  X
                  Ken Youssefi                             Mechanical Engineering                          23
                                            Structural & Light       Small          Large    Parts for    Optical and
                                 Plastic    Mechanical duty          housing        housing wear          transparent
                                                         mech &      & hollow       & hollow applications parts
                                                         deco        shapes         shapes
                                 ABS                      X                X          X
                                 Acetal (Delrin)     X                                            X
                                 Acrylic                  X                                                   X
                                 Cellulosics              X                X                                  X
                                 Fluoroplastics                                                   X
                                 Nylon               X                                            X

                                 Phenylene Oxide                           X          X
                                 Polycarbonate                             X                                  X
                                 Polyester                                 X          X
                                 Polyethylene             X                X          X           X
                                 Polyimide                                                        X
                                 Polyenylene sulfide                                              X
                                 Polypropylene       X
                                 Polystyrene              X                X          X
                                 Polysulfone              X                X          X                       X
                                 Polyurethane                                                     X           X
                                 Polyvinyl chloride       X                           X

                                 Phenolic           X     X                X
                                 Polyester                                 X          X           X
                                 Polyurethane                                         X           X
                  Ken Youssefi                             Mechanical Engineering                              24
                Manufacturing Processes for Plastics

Fabrication of Plastics
    Injection Molding

Ejector pin              Molded part
                               Heaters                                Granular


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Ken Youssefi   Mechanical Engineering   26
               DFM Design Guidelines
                 Injection Molding
 Provide adequate draft
 angle for easier mold

                                                   Minimize section thickness,
                                                   cooling time is proportional to
                                                   the square of the thickness,
                                                   reduce cost by reducing the
                                                   cooling time.
Ken Youssefi              Mechanical Engineering                                27
                DFM Design Guidelines
                  Injection Molding

Keep rib thickness less than
60% of the part thickness in
order to prevent voids and
                                                        Avoid sharp corners, they
                                                        produce high stress and
                                                        obstruct material flow.

Ken Youssefi                   Mechanical Engineering                               28
               DFM Design Guidelines
                 Injection Molding

 Provide smooth transition,                      Keep section thickness uniform
 avoid changes in thickness                      around bosses.
 when possible.

Ken Youssefi                  Mechanical Engineering                          29
                       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
          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
 •        Round interior and exterior corners to                Standard thickness
          .01-.015 in radius (min.), prevents an                variation.
          edge from chipping.
Ken Youssefi                         Mechanical Engineering                      30
                    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.

Ken Youssefi                  Mechanical Engineering       31
                    Rotational Molding Machines

      Vertical wheel machine

                                                           Turret machine

               Shuttle machine
                                                          Rock and roll machine
Ken Youssefi                     Mechanical Engineering                           32
                           Rotational Molding

               •   Molds are relatively inexpensive.
               •   Rotational molding machines are much less
                   expensive than other type of plastic processing
               •   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

Ken Youssefi                          Mechanical Engineering         33
                           Rotational Molding

                  •   Can not make parts with tight tolerance.
                  •   Large flat surfaces are difficult to achieve.

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

               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            34
                   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       35
               Rotational Molding Examples

Ken Youssefi             Mechanical Engineering   36
               Rotational Molding Examples

Ken Youssefi             Mechanical Engineering   37
                             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              39

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