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:
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…
9 8 7 6 5 4 3 Temperature 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
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 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.
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 the2-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