PAI Plate - Torlon 4203 / 4203L, Unreinforced

Polyamide-imide, PAI, is the highest-performing melt-processable thermoplastic there is. It holds more strength and stiffness at 500F than most plastics have at room temperature, it creeps almost not at all, and it machines to tight tolerance. It is also expensive and slow to make - every stock shape goes through a multi-week cure cycle before it ships - which is why it is specified only where nothing cheaper survives.

This is the unreinforced grade, and the toughest of the four. Published stock-shape figures: tensile strength 20,000 psi, flexural strength 24,000 psi, flexural and tensile modulus both about 600,000 psi, compressive strength 24,000 psi at 10 percent deformation, notched Izod 2.0 ft-lb/in, Rockwell M120. Elongation at break is published between 10 and 20 percent depending on source - it is the only PAI grade with real ductility.

It is also the electrical grade. Surface resistivity is around 1E16 ohm and dielectric constant about 4.2, so this is the PAI to specify for connectors, insulators, switch and relay parts and standoffs. The graphite-filled 4301 grade is NOT an insulator and must not be substituted for this one.

One naming point: 4203 is the historical designation and 4203L the current one, and the shape makers state the chemistry is identical and the performance equal. The L denoted a lubricated, pigmented version carrying a little PTFE - which is why "unreinforced" is the accurate word here rather than "unfilled". If your drawing says 4203, this is it.

One disclosure about the numbers on this page, because it is the single biggest trap in PAI copy. Polyamide-imide is published twice: once by the resin producer, measured on injection-molded test bars, and once by the shape makers, measured on specimens machined out of actual extruded stock. The two sets do not agree and the shape figures are lower - for the 30 percent glass grade the resin tensile is 32,000 psi against the shape's 23,000, a 39 percent difference. Everything quoted here is the stock-shape figure, because stock shape is what you are buying. If you have been designing to a resin datasheet, check your margins.

Temperature, which is the reason to buy it: glass transition is 527F and it is the same for every grade, because it is a property of the polymer rather than the filler. Long-term continuous service in air is 500F. Heat deflection at 264 psi runs 520 to 532F depending on grade. Note that PAI is amorphous - it does NOT melt at 527F, and a datasheet row reading "melt point 527F" is a mislabeled glass transition.

Post-cure, and this is the thing most buyers are not told. Stock shapes arrive fully cured, with a harder, darker cure skin on the outside. The published rule is that a part with more than 0.030 inch removed from its surface may need re-curing - which is nearly every real machined part, because the bore of a bushing turned from rod is all freshly exposed interior. Re-curing roughly doubles tensile strength against uncured material, raises heat deflection, and keeps improving wear resistance out to about eleven days at 500F. We can arrange post-cure; ask when you order.

On approvals, stated plainly because the industry is loose about it. We make no FDA, food-contact, NSF, USDA, 3A dairy or USP Class VI claim for any PAI grade, filled or unfilled - the stock-shape datasheets answer "no" to every one of those. The resin producer does publish a UL 94 V-0 rating for the base resins at stated thicknesses, and the shape makers label their own V-0 figure an estimate, so the accurate statement is that the resin is rated, not that this shape or your finished part is UL listed or UL recognized. Ask us and we will tell you exactly what the mill certificate covers.

Torlon is a registered trademark of Syensqo and we are not affiliated with or endorsed by them. The generic name is polyamide-imide, PAI - not polyamide, which is nylon, and not polyimide, a different family.