PTFE Seal Materials — Accofluor Grade Reference

PTFE does not behave like an elastomer. It does not rebound, it creeps under sustained load, and almost every performance property an engineer cares about — wear, extrusion resistance, thermal conductivity, food compliance — is decided by the filler, not by the base polymer. This page sets out the Accofluor PTFE and engineering-plastic grade range Sealparts supplies, and how to choose between the grades.

Why PTFE is specified

PTFE (polytetrafluoroethylene) offers a chemical resistance envelope almost nothing else in sealing can match, a service range that runs from cryogenic temperatures to roughly 250 °C, and the lowest coefficient of friction of any bulk-machinable sealing material. It does not stick, it does not swell in most media, and it will not stick-slip on start-up after a long dwell — the failure mode that ruins elastomeric seals in slow-cycling and intermittently used equipment.

The trade-offs are equally clear. PTFE has no elastic memory, so a PTFE sealing element almost always needs an energiser — an O-ring or a spring — behind it to generate contact stress. It cold-flows under sustained load, so groove design and back-up support matter more than they do with rubber. And unfilled PTFE wears quickly in loaded dynamic duty. Fillers exist to fix the second and third problems without giving away the first.

Accofluor grade range

Grades are identified by an AF number. The tables below reproduce the published Accofluor material list, grouped by filler family. EU-FCM indicates conformity with the EU food-contact materials framework (Regulation 1935/2004 and the plastics regulation 10/2011); FDA indicates the constituents are FDA-listed for food contact. Where a cell is blank in the source list it is shown here as an em dash — that means no food-contact declaration is held for that grade, not that the grade is unsuitable for industrial use.

Virgin and modified PTFE

Unfilled grades for chemical service, hygienic duty and light-load or static sealing. The TFM grades are chemically modified PTFE with a finer particle structure — lower porosity, less cold flow and better weld integrity than conventional PTFE, at the same chemical resistance.

Carbon and graphite filled

The workhorse family for dynamic hydraulic and pneumatic sealing. Carbon raises compressive strength and thermal conductivity so frictional heat leaves the sealing lip; graphite provides dry lubricity for poorly lubricated or intermittently dry running. Carbon-fibre grades are the strongest and the kindest to the counterface.

Glass fibre and mineral filled

Glass is the cost-effective route to creep and extrusion resistance in general hydraulic duty. It is abrasive, so it should be avoided against soft, plated or thinly coated counterfaces, and against stainless steel running dry.

Metal filled

Bronze gives the highest load capacity and thermal conductivity of the common fillers, which suits fast, heavily loaded, well-lubricated service. It is chemically vulnerable — keep it out of water, acids, steam and any food-contact application. Stainless-steel filled PTFE covers the same load case where bronze would corrode.

Polymer and special filled

Ekonol (an aromatic polyester) and the proprietary polymer fills give creep resistance close to glass-filled PTFE while remaining soft on the mating surface. These are the grades to reach for when the counterface is the component you cannot afford to wear.

Engineering plastics

Not everything in a sealing stack is PTFE. Guide rings, wear rings, back-up rings and hard-wearing scrapers are machined from polyester fabric, phenolic, POM, PET, PEEK, PUR and UHMW-PE. Accofluor holds these under the same AF numbering.

What each filler actually changes

Filler choice is a set of trade-offs, not a ranking. Adding filler always costs some chemical resistance and some friction performance; it buys back strength, wear life and dimensional stability.

Choosing a grade

Start from the duty, not the material. The table below gives a defensible first choice for the applications we are asked about most; confirm it against pressure, surface speed, temperature, media and counterface finish before committing to tooling.

Counterface and groove notes

• Target Ra 0.1–0.4 µm on dynamic counterfaces. Too smooth starves the transfer film; too rough abrades the seal.
• A hard, well-finished counterface (typically 55 HRC or better, or a hard chrome / ceramic coating) is worth more to PTFE seal life than any filler upgrade.
• PTFE sealing elements need an energiser. Specify the O-ring or spring material against the same chemistry and temperature as the PTFE.
• Design in extrusion gap control. PTFE will extrude into a clearance that an elastomer would bridge; add a PEEK or PET back-up ring above roughly 250 bar.
• Allow for thermal expansion. PTFE moves an order of magnitude more than steel, and machined dimensions must be quoted at a stated temperature.

Food, pharmaceutical and hygienic duty

Where a seal touches product, the grade must carry a declaration — engineering judgement is not a substitute for documentation. In the Accofluor range AF-1004, AF-1024, AF-2424, AF-2524, AF-4024, AF-7524 (TFM), AF-7720, AF-7724, AF-7734, AF-9324 (PEEK) and ATXC-33 are listed against both EU-FCM and FDA. AF-1086 and AF-1124 carry FDA listing only. Ask us for the current declaration of compliance with each order — the certificate, not the datasheet, is what an auditor accepts.

Supply through Sealparts

Sealparts supplies Accofluor PTFE seals and machined components in the UK. Accofluor is a Danish PTFE specialist producing isostatically moulded PTFE and machined sealing elements — piston seals, rod seals, wipers, guide and back-up rings — in both standard profiles and fully custom geometry. Because the parts are machined rather than moulded, one-off and small-batch sizes are practical: there is no tooling to pay for, and prototypes can be produced in the same grade as the eventual production part.

Send us the groove dimensions, the operating pressure and speed, the temperature range, the medium and the counterface specification, and we will propose a grade and profile. If you are replacing a failed seal, send us the failed part as well — the wear pattern usually tells us more about the grade you need than the original drawing does.