Design Realization lecture 12 John Canny 10/2/03

Last Week*  Introduction to materials: physical properties, density, strength, stiffness, thermal and electrical conduction.  Metals: Steel, Aluminum, Brass.  Ferromagnetism, solid, flexible and liquid magnets.

This time  One more metals topic: shape memory alloy.  Introduction to plastics.

Shape-Memory Alloy  Two main metal phases are shown below:

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).

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.

Shape-Memory Alloy  Nitinol has the following attributes: MartensiteAustenite Stiffness GPa2875 Resistivity7682 Transition T

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.

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:

Polymer structure  The polymer chain layout determines a lot of material properties:  Amorphous:  Crystalline:

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

Branched polymers  Polymer chains can branch:  Or the fibers may aligned parallel, as in fibers and some plastic sheets.

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…

Glass-rubber-liquid  Amorphous plastics have a complex thermal profile with 3 typical states: Log(stiffness) Pa Temperature Glass phase (hard plastic) Rubber phase (elastomer) Liquid Leathery phase

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.

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.

Notable plastics - Polyethylene  Probably most common plastic – glad bags and packing material, children’s 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

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!

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 > glass, eyeglass material. A thermoset plastic.  Cellulose – natural wood fiber.

Elastomers  Elastomers are synthetic rubbers E < 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.

High performance plastics  PTFE – Polytetrafluoroethylene – aka Teflon long 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.

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!

High performance plastics  Epoxy resin is made from the 2-part kits.  It’s 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