PTFE Glossary
Terms that appear on PTFE drawings, datasheets and RFQs, defined as they are actually used in moulding and machining rather than as dictionary entries.
Material and grades
PTFE — polytetrafluoroethylene. A fluoropolymer with near-universal chemical resistance, the lowest coefficient of friction of the common engineering plastics, and a service band of roughly −200 °C to +260 °C. It does not melt-flow like a thermoplastic, which is why it is compression moulded and sintered rather than injection moulded.
Virgin PTFE — unfilled PTFE, containing no additive. Maximum chemical resistance and lowest friction, but the least resistant to creep and wear.
Filled PTFE — PTFE compounded with a filler to change its mechanical behaviour. Glass, carbon, graphite, bronze, molybdenum disulphide and stainless steel are the common ones. Filling always trades some chemical resistance or non-stick behaviour for load capacity, wear life or conductivity.
Reprocessed PTFE — material made from reground PTFE. Cheaper, with reduced and less predictable properties. Not interchangeable with virgin unless the drawing permits it.
Skived — sheet or tape peeled continuously from a rotating sintered billet, as distinct from moulded sheet. Skiving reaches thin sections that moulding cannot.
Process
Preform — the compacted but unsintered part that comes out of the press. Also called a green preform. It holds together by mechanical interlock only and has very little strength.
Green density — the density of the preform before sintering. Under-compaction here cannot be recovered in the oven, so it is the critical in-process check.
Sintering — heating the preform above about 327 °C so the particles coalesce into a solid. The part shrinks during sintering, and the shrinkage must be allowed for in the tool.
Free sintering — sintering the preform out of the mould, unconstrained. The normal route for PTFE.
Cold flow / creep — the slow permanent deformation of PTFE under sustained load, even at room temperature. The single most important design limitation of the material, and the reason most fillers exist.
Compression ratio — the ratio of loose powder volume to finished part volume, typically in the region of 3:1 to 5:1 for PTFE powders. It determines how deep the die cavity must be for a given part height.
Dwell — the hold period at full pressure during compaction, which lets the powder settle and air escape.
Decompression — the controlled release of pressure after dwell. Stepped rather than instantaneous, so stored strain energy does not split the preform.
Dimensions and inspection
Sphericity — the deviation of a ball from a true sphere. On a check ball this governs shutoff tightness more than diameter does.
Interference — the designed oversize of a seal or ball relative to its seat, which creates the sealing contact force.
Extrusion gap — the clearance a seal can be forced into under pressure. Governs whether a lip survives at working pressure.
PV limit — contact pressure multiplied by surface speed, the standard way of expressing the duty limit of a plain bearing material.
MTC / mill test certificate — the document stating the resin lot, filler content and measured properties for a delivered batch.
Where these terms are applied
Cold flow and filler selection are the two ideas that drive most PTFE specification decisions, and both are worked through in more depth on the grade pages: glass-filled PTFE for creep resistance, bronze-filled PTFE for load and heat, and virgin PTFE where chemical breadth leads.
For the process terms — preform, green density, sintering shrinkage — see PTFE powder compaction behaviour, which explains what the powder demands of the press. The equivalent vocabulary for metal and ceramic powders is in the powder compaction glossary.
If you are specifying a part rather than reading up, send the drawing and duty to HEMSUN and the grade discussion can happen against your actual application.