Design Realization lecture 12

John Canny10/2/03

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:

Martensite Austenite

Stiffness GPa 28 75

Resistivity 76 82

Transition T 62-72 88-98

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

Temperature3

9

6

7

8

4

5

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 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