If PEEK is the workhorse of high-temp plastics, polyimide is the machine you call when PEEK throws in the towel. We machine a lot of PEEK every week — it pays the bills. But maybe 5% of the jobs that come through our shop actually need polyimide, and those jobs are always the ones where failure isn't an option.
What polyimide actually is (and why it costs what it costs)
Polyimide isn't a single material — it's a family. The aromatic imide rings in the polymer backbone are what give it the heat resistance. Unlike PEEK or PEI, most polyimides are thermosets or pseudo-thermoplastics. You can't just melt and remelt them the way you can with PEEK.
The grades we handle most often:
- Vespel SP-1 (unfilled): General-purpose high-temp work. Machines decently with sharp carbide and light cuts. We stock rod and plate in common diameters.
- Vespel SP-21 (15% graphite): For bearings and wear pads. The graphite makes chip formation better and the material less gummy than SP-1. Still abrasive on tooling though.
- Vespel SP-22 (40% graphite): Even lower friction. Less common but serves extreme dry-running bearing applications.
- Vespel SP-211 (15% graphite + 10% PTFE): Lowest friction of the standard grades. The PTFE content makes surface finish a bit more forgiving.
Price-wise, a 1-inch diameter rod of Vespel SP-1 runs about 8-12x the cost of the same size in PEEK. A 2-inch plate can hit four figures. This is not a material you want to scrap parts on.
What happens when you actually machine it
Polyimide machines differently from anything else we cut. It's stiff — really stiff — but simultaneously notch-sensitive in a way that catches people off guard. Put a sharp internal corner on a PI part under load, and it WILL crack. Not maybe. We learned this the expensive way on a customer's aerospace bushing about three years ago.
Tool wear is the biggest day-to-day headache. Standard carbide lasts maybe 20-30 minutes of continuous cutting before the edge is done. For any production quantity, PCD tooling pays for itself within the first batch — we keep a dedicated set of PCD end mills just for PI jobs.
Speeds need to come way down compared to PEEK. We run PI at 150-250 m/min surface speed — roughly half of what we'd use for PEEK. Feed rates: 0.05-0.12 mm/rev for turning, 0.03-0.08 mm/tooth for milling. Trying to rush a polyimide job is how you turn a $500 piece of material into expensive chips.
Coolant-wise, air blast only — no exceptions. Flood coolant on a hot PI part can thermal-shock it and create microcracks you won't find until the part fails in service.
Tolerances: ±0.075 mm is comfortable for most PI grades. With thermal control and staged machining (rough, cool 30+ min, semi-finish, cool again, final finish), we've held ±0.025 mm on SP-1. Beyond that you're looking at grinding, not milling.
Where PI actually earns its price tag
Semiconductor. Etch chamber parts, wafer clamping rings, plasma-resistant fixturing. When the chamber runs at 300°C+ in fluorine plasma, PI is one of the only polymers standing.
Aerospace. High-temperature bushings, seals, thermal isolators. We've machined PI parts for jet engine bleed air systems — operating temps that would turn PEEK into a puddle.
Oil and gas. Compressor rings, valve seats, downhole components. The combination of 250°C+ heat, abrasive flow media, and aggressive chemicals kills most polymers. PI survives.
Nuclear. The extreme end. PI's radiation resistance combined with thermal stability makes it one of very few polymer options for reactor environment components.
Should you even spec polyimide?
Before you write "PI" on the BOM, ask yourself three things:
- Is my continuous temperature genuinely above 260°C? If not, PEEK or PAI probably handles it at a fraction of the cost.
- Am I over-specifying out of habit? We see a lot of drawings where someone defaulted to "the best material" when the second-best would perform identically in the real application.
- Does my shop actually know how to cut this? PI is unforgiving. Wrong tooling, wrong fixturing, wrong speeds — the part is scrap before you know it, and the material cost stings.
We've helped customers migrate from PI to glass-filled PEEK in applications where the real-world temperature never exceeded 250°C. Parts ran fine. Material cost dropped 70%. On the flip side, we've also talked customers out of switching away from PI when the application genuinely demanded it — because one field failure costs more than decades of material savings.
Got a PI application you're working through? Send us the operating parameters. We'll give you an honest read on whether you need it — even if the answer is "you don't need our most expensive material."