Design Realization lecture 12

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Design Realization lecture 12

• John Canny
• 10/2/03

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Last Week

• Introduction to materials physical properties,
  density, strength, stiffness, thermal and
  electrical conduction.
• Metals Steel, Aluminum, Brass.
• Ferromagnetism, solid, flexible and liquid
  magnets.

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This time

• One more metals topic shape memory alloy.
• Introduction to plastics.

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Shape-Memory Alloy

• Two main metal phases are shown below

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Shape-Memory Alloy

• In steel, the martensite/austenite transition is
  influenced by alloying, cold-working etc.
• In shape memory allow, the transition is caused
  by a small change in temperature.
• The best-known shape memory allow is Nitinol NiTi
  (Nickel Titanium).

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Shape-Memory Alloy

• The austenite is stiffer and has lower volume.
• Heating SMA wire causes it to contract with some
  force. Strains of 3-5 are typical.

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Shape-Memory Alloy

• Nitinol has the following attributes

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Plastics

• Plastics exhibit an incredible variety of
  properties due to their rich chemical makeup.
• They are inexpensive to produce, and easy to
  mold, cast, or machine.
• Their properties can be expanded even further in
  composites with other materials.

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Polymer chemistry

• Polymers are chain molecules. They are built up
  from simple units called monomers.
• E.g. polyethylene is built from ethylene units
  which are assembled into long chains

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Polymer structure

• The polymer chain layout determines a lot of
  material properties
• Amorphous
• Crystalline

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Cross-linking

• Generally, amorphous polymers are weak.
• Cross-linking adds strength vulcanized rubber is
  polyisoprene with sulphur cross-links

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Branched polymers

• Polymer chains can branch
• Or the fibers may aligned parallel, as in fibers
  and some plastic sheets.

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Copolymers

• Polymers often have two different monomers along
  the chain they are called copolymers.
• With three different units, we get a terpolymer.
  This gives us an enormous design space

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Glass-rubber-liquid

• Amorphous plastics have a complex thermal profile
  with 3 typical states

Glass phase (hard plastic)
Leathery phase
Log(stiffness)Pa
Rubber phase (elastomer)
Liquid
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Thermoplastics

• Polymers which melt and solidify without chemical
  change are called thermoplastics.
• They support hot-forming methods such as
  injection-molding and importantly for us, FDM.

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Thermoset plastics

• Polymers which irreversibly change when heated
  are called thermosets.
• Most often, the change involves cross-linking
  which strengthens the polymer (setting).
• Thermosets will not melt, and have good heat
  resistance.
• They are often made from multi-part compounds and
  formed before setting (e.g. epoxy resin).
• Setting accelerates with heat, or for some
  polymers with UV light.

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Notable plastics - Polyethylene

• Probably most common plastic glad bags and
  packing material, childrens toys thermoplastic
• Simple formula
• Not quite amorphous! (demo)
• Glass transition -130? to -80? C
• Melting point 130? C
• Tensile yield (strength) 25 MPa
• Tensile modulus (stiffness) 1 GPa (soft)
• Density 0.95

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Notable plastics - Acrylic

• Most common optical plastic - refractive index
  very close to glass (1.5), aka Plexiglas, Lucite
• Full name polymethyl methacrylate (PMMA).
• Also an important fiber, paint.
• Glass transition 110? C
• Melting point 130? C
• Tensile yield 50 MPa
• Tensile modulus 30 GPa
• Density 1.15
• Excellent laser cutter material!

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Notable plastics contd.

• ABS popular construction thermoplastic, used in
  FDM machines.
• PVC plumbing pipes, electrical insulation.
• Nylon most important fiber.
• Polyester 70s disco clothing plastic bottles.
• Polystyrene computer housings, toys, also made
  into foam (Styrofoam).
• Polycarbonate strong, refractive index gt glass,
  eyeglass material. A thermoset plastic.
• Cellulose natural wood fiber.

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Elastomers

• Elastomers are synthetic rubbers E lt 1 GPa
• Polyurethane used in pillows and cushions.
• Silicones used for caulking and Space Shuttle
  heat tiles. Silicones are inorganic with an
  S-O backbone.
• Fluoroelastomers good electrical insulators.

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High performance plastics

• PTFE Polytetrafluoroethylene aka Teflonlong
  name, simple structure
• Exceptional resistance to solvents, great
  lubricant, nothing sticks to it!
• The fluorine-carbon bonds are very strong,
  fluorines protect carbon backbone.
• High melting point 330? C
• High electrical breakdown artificial muscle.
• Technically a thermoplastic, but hard to process.

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High performance plastics

• Kevlar is an aramid polymer
• Chains are stiff and straight.
• Highly crystalline polymer, difficult to process.
  
• Melting temperature 500? C
• Tensile strength 3.6 GPa, about 4x steel!

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High performance plastics

• Epoxy resin is made from the2-part kits.
• Its the basis of composites like fiberglass,
  carbon fiber composites etc.
• Apart from an excellent glue, it is an important
  molding compound for rapid prototyping.
• Tensile strength 60 MPa
• Stiffness 2.6 GPa