Thermoplastics

According to Wikipedia, a thermoplastic is a plastic that melts to a liquid when heated and freezes to a brittle, very glassy state when cooled sufficiently.  Thermoplastic polymers differ from thermosetting polymers in that they can, unlike thermosetting polymers, be re-melted and remolded.  Thermoplastic materials generally fall within two classes of molecular arrangement, amorphous and semi-crystalline.  The difference between thermosets and thermoplastics is that thermoset plastics can not be welded or remolded but simply burn in the presence of heat.  On the other hand, once a thermoset is cured it tends to be stronger than a thermoplastic.

Below are some of the most common thermoplastics.  Included in this list are plastics that are often called "engineering plastics".  Some of the most common engineering plastics are ABS, PC, PA, PBT, PET, PPO, PSU, PEK, and PEEK.

PP - Polypropylene

Strengths

Impact and wear resistant.  Flexible, can have very high elongation before breaking. Resistant to acids and bases.

Weaknesses

Sensitive to thick sections in your part. It may have voids, bubbles, or sink.  Poor dimensional accuracy.  Low mechanical and thermal properties.  Price is low to very low.

HDPE - High Density Polyethylene

Strengths
Impact and wear resistant.  Flexible, can have very high elongation before breaking.  Generally good chemical resistance.

Weaknesses

Sensitive to thick sections in your part. It may have voids, bubbles, or sink.  Poor dimensional accuracy.  Low mechanical and thermal properties.  Price is low to very low.

PA66 - Nylon (Polyamide)

Strengths
High stenght. Chemcial resistant except strong acids or bases.  Flows well, good for thin part geometry.

Weaknesses

Parts are more subject to warp due to non-linear shrink.  Molded parts in your application can absorb moisture, changing dimensions and mechanical properties.  Price is moderate to high.

POM - Acetal (polyoxymethylene)

Strengths

High strength, good lubricity and resistance to hydrocarbons and organic solvents.  Acetal machines well.

Weaknesses

Sesitive to thick sections and no-uniform part geometry.  It may have voids, excess shrink or warp.  Poor resistance to acids and bases.  Price is average to high.

PBT - Polybutylene Terephthalate

Strengths
Good electrical properties for power components.  Moderate to high strength depending on glass fill.  Unfilled grades trough and flexible.  Good resistance to organic solvents and fuels.

Weaknesses

Hard to fill thin parts.  Glass filled resins very prone to warp.  Poor resistance to acids, bases and hydrocarbons.  Price is moderate.

ABS - Acrylonitrile Butadiene Styrene

Strengths
Excellent impact resistance. It can have good appearance for cosmetic parts.  Strength is moderate.  Good resistance to acids and bases.

Weaknesses
It is sensitive to thick sections in your part which may cause voids. bubbles, or sink.  Attacked by hydrocarbons and organic solvents.  Heat resistance is low.  Price is low to moderate.

PC - Polycarbonate

Strengths
Excellent impact resistance and strength at low and high temperatures.  Available in visually clear grades.  Good heat strength.

Weaknesses
It is sensitive to thick sections in your part which may cause voids, bubbles or sink.  Fair to poor chemical resistance which can cause stress cracking or opaque dulling of the surface.  Price is moderate to high.