Three classes, rising in price and performance
Technical plastics are usually divided into three levels. The commodity plastics are cheap, chemically resistant and soft: PE, PP and PVC. The engineering plastics carry load and hold their dimensions: POM, PA, PET and PC. The high performance plastics take heat and chemicals that knock out the rest: PEEK, PPS, PEI, PI and the fluoropolymers.
Moving up a level costs money. The price difference between POM and PEEK is a factor of fifty. The difference in performance is rarely that dramatic, so it almost always pays to start low and go up only where the requirement forces you to.
The families, in short
- Polyolefins (PE, PP). Light, chemically resistant, slippery. They take no glue, so you weld them instead. PE-1000 is the wear champion of the family.
- Acetals (POM). Dimensionally stable and easy to machine, with low friction. The standard choice for precision parts and plain bearings.
- Polyamides (PA, nylon). Tough and wear resistant, but they take up moisture and grow with it. Best where the dimensions are allowed to move a little.
- Polyesters (PET, PBT, PCT). Stiff and dimensionally accurate, but sensitive to hot water and steam over time.
- Amorphous transparent plastics (PMMA, PC, PETG). See-through. Acrylic is the hardest and clearest, polycarbonate the toughest, PETG bends cold.
- Sulfones (PSU, PPSU). They take repeated sterilization. Sensitive to certain solvents.
- Aryl ether ketones (PEEK, PEKEKK). The highest mechanical performance in heat. The most expensive in the range after PI.
- Fluoropolymers (PTFE, PVDF, FEP, PFA, ECTFE). Chemically almost untouchable and the slipperiest of them all. Soft, and they carry load poorly.
Grades and additives say more than the family
The family points you in a direction, the grade settles it. PA6 and PA6 C are the same polymer made in different ways, and the cast one comes in heavier dimensions with lower internal stresses. Additives move the properties even further: glass fiber gives stiffness and heat resistance, molybdenum disulfide and oil lower the friction, and flame retardant grades exist in most families.
A cautionary example of how much the designation matters: PPs with a lowercase s is flame retardant polypropylene, while PPS with a capital S is polyphenylene sulfide, a high temperature plastic at many times the price. The same three letters, completely different materials.
How to choose
Start with the hardest constraint, usually temperature or chemistry, and rule out everything that cannot handle it. What is left you rank on mechanics and price. Plastics guide does exactly that sorting for you, and the comparison table shows the numbers side by side.