Polyimide Rod - Vespel SP-3, 15% MoS2 Filled

Vespel SP is a polyimide, and it is not a thermoplastic in any useful sense. It has no melt phase at all - it cannot be extruded or injection molded, so stock comes as isostatically molded billet pressed from powder and sintered, much closer to powder metallurgy than to plastics processing. That is also why it has no melting point and no glass transition temperature: a spec table listing either one is invented. It simply keeps working until it degrades.

SP-3 carries 15 percent molybdenum disulfide, a dry solid lubricant, and it exists for one job: running in vacuum and in dry gas where there is no moisture to help a bearing surface along. Graphite-filled grades rely on adsorbed water to lubricate and lose that in vacuum; molybdenum disulfide does not. If the application is a vacuum chamber, dry nitrogen or space, this is the grade. Specific gravity 1.60.

Temperature is the reason to buy it. Continuous service in air runs to about 550F with excursions far higher, and it holds useful mechanical properties over a range from cryogenic up to the point where few other polymers are still solid. There is no softening point to design around, which is the practical difference between polyimide and every melt-processable plastic.

How this is priced, because it is unusual and worth saying plainly. Polyimide rod is sold BY THE INCH, not by the foot - that is normal trade practice for this material and it reflects both the cost and the 38 inch maximum billet length. You buy the inches your part needs. Plate is priced per piece at the 10 by 10 inch size.

Sizes are genuinely limited by the process, so it is worth knowing before you design. The manufacturer publishes a maximum rod of 3.25 inch diameter by 38 inch length, and square plaque to 10 by 10 inches by 2 inches thick - 10 by 10 is the maximum, not a convenient stock size. Larger diameters exist only as short slugs cut from compression-molded disc stock, so do not design around a big diameter in a long length. Rod and square plaque are what reaches distribution; tube, disc and ring generally do not.

A note on color: the manufacturer publishes none for this grade. Trade listings say brown, which may be inherited from the unfilled grade, since 15 percent molybdenum disulfide would be expected to darken it well past amber. Ask us and we will describe the stock in front of us.

Machining notes. It machines like a very hard, very stable plastic - sharp carbide, light cuts, and it holds tolerance unusually well because the billet is essentially stress-free and there is no extrusion stress to relax. Unlike polyamide-imide it needs no post-cure after machining. It does absorb moisture, which matters in two places: a vacuum part needs a bakeout or your pump-down takes much longer than you planned, and a tight-tolerance part moves a little with humidity.

The honest limits. Chemical resistance is good but not universal - polyimides are hydrolytically attacked by strong alkalis and by hot water and steam, and caustic attack visibly changes the surface color. On radiation, the published data covers gamma and electron beam to 10 to the 8 rads, with under 1 percent weight loss and under 6.5 percent tensile loss, but elongation falls about 19 percent so irradiated parts get more brittle. Neutron is the carve-out: tensile strength was reported substantially reduced, so a neutron environment needs a conversation with the manufacturer rather than a blanket radiation claim from us.

One restriction we are obliged to pass on, in the manufacturer's own words: do not use these materials in medical applications involving implantation in the human body, or contact with internal body fluids or tissues, except under a specific written agreement with the manufacturer. No USP Class VI, ISO 10993 or biocompatibility data exists for these grades, and we make no FDA, food-contact or NSF claim for any of them.

Vespel is a registered trademark of DuPont and we are not affiliated with or endorsed by them. The generic name is polyimide, PI.