Overview
ISO 6691:2021 - Thermoplastic polymers for plain bearings - Classification and designation defines a standardized system to classify and designate the most common unfilled thermoplastic polymers used in plain bearings. The third edition (2021) establishes an unambiguous block-style coding scheme that helps designers, material engineers and manufacturers identify material families and key characteristic levels (e.g., viscosity number, density, melt flow) relevant to bearing applications. The standard also outlines typical properties, application guidance and fundamental selection parameters, while noting that identical designations do not guarantee interchangeability in all cases.
Key topics and technical requirements
- Block-based designation system: A Description Block plus an Identity Block; the Identity Block contains an International Standard number and an Individual Item Block broken into four data blocks (1–4).
- Data block 1 (material symbol): Uses chemical symbols in accordance with ISO 1043‑1 (examples: PA, POM, PE‑UHMW, PTFE, PEEK, PVDF, PPS, PAI).
- Data block 2 (intended application / processing & additives):
- Position 1 codes intended processing/application (E = Extrusion, M = Injection moulding, G = General, Q = Compression, R = Rotational, X = No indication).
- Positions 2–4 can indicate key additives or properties (A = processing stabilizer, F = special burning characteristics, H = heat ageing stabilizer, L = light/weather stabilizer, R = release agent, S = slip/lubricant).
- Data block 3 (distinctive properties): Polymer-specific coding of key properties:
- Polyamides: two-digit viscosity number (ISO 307) + three-digit modulus of elasticity code (ISO 527 series) - optional N for rapid-setting products.
- Polyethylenes: two-digit density code (ISO 17855‑1 / ISO 1183) + MFR code (letter for test load D/T/G and three-digit flow rate per ISO 1133‑1).
- Polyalkyleneterephthalates: viscosity number per ISO 20028‑1 / ISO 1628‑5.
- Data block 4 (fillers / reinforcements): Type and content of fillers or reinforcing materials; if omitted, indicated by empty data block separators.
- Important note: The designation system does not include every material property - materials with identical codes might still differ for some applications.
Practical applications and users
- Who uses ISO 6691:2021:
- Plain-bearing designers and mechanical engineers specifying polymer bushings and sliding elements.
- Material and polymer engineers selecting base polymers (PA, PE, POM, PEEK, PTFE, etc.) for tribological performance.
- Manufacturers and quality managers creating material specifications, product data sheets and ordering codes.
- Procurement and standards/technical committees ensuring consistent material identification across suppliers.
- Practical benefits:
- Faster, unambiguous communication of polymer type, processing intent and key properties.
- Improved selection for load, wear and environmental requirements of plain bearings.
- Basis for supplier comparison and product specification templates.
Related standards
ISO 6691:2021 is essential for anyone specifying or sourcing unfilled thermoplastic polymers for plain bearings and complements polymer-specific ISO designation standards and test-method references.
