PTFE Rod Guide
PTFE rod is machining stock. How it was made determines how it machines and how the finished part behaves.
Moulded versus extruded
Compression moulded and free-sintered rod is pressed as a billet and sintered unconstrained. It has the most uniform density through the section and the least internal stress, which is why it is preferred for parts that will be machined to tight tolerance or that must stay dimensionally stable.
Ram-extruded rod is produced continuously and is more economical in smaller diameters, but carries more internal stress and can show a density difference between skin and core on larger sections.
For anything where the finished part must hold tolerance after machining, moulded rod is the safer specification — and it is worth stating on the drawing rather than leaving to the supplier.
Why parts move after machining
PTFE rod contains residual stress from processing. Machining removes material asymmetrically, the stress redistributes, and the part changes shape after it comes off the machine. For close-tolerance work the answer is to rough-machine, stress-relieve by annealing, then finish-machine. Skipping that step is the most common cause of parts that measure correctly at the machine and wrongly the next day.
Machining PTFE
- Use sharp tooling with high positive rake. PTFE smears rather than cutting cleanly with a dull edge.
- Support the work. PTFE is soft and deflects under tool pressure, so a long unsupported section will produce a tapered bore.
- Expect thermal movement. PTFE has a high coefficient of thermal expansion, so a part measured warm off the machine will not measure the same cold. Let it stabilise before final inspection.
- Chips are stringy and do not break. Plan swarf clearance rather than fighting it.
Diameter, tolerance and stock allowance
Rod is supplied nominally oversize to allow for cleaning up the surface. State the finished dimension you need rather than the stock size, and state the tolerance — asking for “50 mm rod” and expecting a finished 50 mm part will not work.
Grade selection for machined parts
Virgin PTFE machines most cleanly and gives the best surface finish. Filled grades are more abrasive to tooling and give a slightly rougher finish, but resist creep far better — which usually matters more for a load-bearing machined part.
Related pages
See PTFE rod for the product itself, and the PTFE machining guide for cutting parameters. Grade behaviour is covered on virgin PTFE and glass-filled PTFE. Terms are defined in the PTFE glossary. Send a drawing to HEMSUN.
