Polymer - Large molecules, repeating structure, connected largely through covalent bonds. Polymers can be natural or synthetic (common language is synonymous with plastic). Proteins and also polymers.
Amorphous Solids - No long-range order to the position of atoms/molecules within the solid
Thermoplastics - A polymer that liquifies above a certain temperature and returns to a glass state when cooled. It is generally difficult to machine because of low heat conductivity - heat builds up in cutting zone leading to local melting. Keep free of chips and use sharp tools. Watch for absorption of lubrication - can use air cooling.
Glass Transition - Temperature at which an Amorphous Solid becomes brittle on cooling or soft on heating. There’s a decent comparison table at http://en.wikipedia.org/wiki/Glass_transition.
Heat-Deflection Temperature - A tricky measure of the temperature at which a material sample deflects a certain amount - a number of standard setups makes this less useful.
Elastomer - Relatively soft and deformable, these are amorphous polymers existing above their glass-transition temperature. Most are thermosetting, but some are thermoplastics (e.g. polyurethane). Most elastomers can be vulcanized. Elastomers can be quite difficult to machine (liquid CO2 is recommended to chill the part and making it stiffer).
Fluoropolymers - Thermoplastics with multiple carbon-fluorine bonds, generally low friction (fluorine has have high electronegativity), high resistance to solvents, acids, bases, started in 1938 when Dr. Roy Plunkett accidentally polymerized tetrafluoroethylene to make PTFE (Teflon).
Recyclability - Many thermoplastics are easily recycled. The major recyclable classifications (as taken from http://en.wikipedia.org/wiki/Plastic_recycling) are as follows:
1 Polyethylene Terephthalate (PET, Polyester) 2 High-Density Polyethylene (HDPE) 3 Polyvinylchloride (PVC) 4 Low-Density Polyethylene (LDPE) 5 Polypropelene (PP) 6 Polystyrene (PS) 7 Other, often Polycarbonate (PC)
Cross-linking - Bonds linking polymer chains which can be initiated using heat, pressure, or radiation. The process is often irreversible, and is common in thermosetting plastics. In general, the more cross-linking, the stiffer the resulting polymer.
Thermosetting Plastics - Polymers that irreversibly cure-form (by heat, chemical reaction, or irradiation). Typically liquid/malleable prior to curing, transformed by cross-linking, which form larger molecules with a higher melting temp. Thermosets cannot be melted and re-shaped, which implies that they are non-recyclable. They are also generally stronger than thermoplastics. Machining thermosets materials (which typically have a resin binding a ceramic or other hard material) can quickly dull cutting tools.
Vulcanization - Heating of a material, often with chemical additives (most commonly sulfur) to cross-link polymer chains, and thereby harden and create a smooth surface finish that is less gummy.