PET-P Rod

PET-P rod in natural and black, cut to length and sold by the foot.

PET-P is the semi-crystalline polyester machining grade - the same polymer family as a drinks bottle, crystallised and cast into stock shapes so it can be turned and milled. It is not PETG sheet and not bottle resin, and it behaves nothing like either.

It competes with acetal, and the trade is a real one. PET-P wins on strength (12,400 psi against 10,000), stiffness (490,000 psi flexural modulus against 420,000), hardness, and two things that matter more than any of those: it moves about half as much with temperature (3.3 against 6.8 millionths per degree F) and takes up a third of the water (0.07 percent against 0.25 at 24 hours). Acetal wins on impact - two to three times the notched Izod - on elongation, on heat deflection, and on being easier to machine.

Against nylon the moisture gap is the whole argument. Extruded nylon 66 takes up about 1.2 percent water in 24 hours and softens and swells as it does; PET-P takes up 0.07 percent, roughly a seventeenth, and its mechanical properties are effectively unchanged wet. If a part has to hold a tolerance in a wash-down environment, that is the reason to specify it.

It runs dry against steel at a coefficient of friction around 0.20 that does not climb as it picks up moisture, which is what nylon does. One honest caveat on bearing duty: plain PET-P carries a limiting PV of about 2,800 psi.fpm, which is barely above acetal and nylon at 2,700. For real bearing loads, the internally lubricated grade more than doubles that.

Three limits worth knowing before you specify it. PET hydrolyses, so it is not for steam, not for hot water above 140F, and not for caustic or high-pH wash-down - on alkali resistance it is the weakest of the common engineering plastics, not a middling one. It is notch sensitive: notched Izod is about 0.5 ft-lb/in against acetal's 1.0 to 1.5, and unnotched-to-notched impact runs about 25 to 1, so inside corners want at least a 0.040 inch radius and holes want a diameter of material between them. And it is not a high-temperature material under load - heat deflection is 240F at 66 psi but only 175F at 264 psi.

Continuous service runs to about 210F in air, with a melting point of 491F and a glass transition near 178F. The low internal stress of cast PET-P stock is what lets it hold a tenth of a millimetre over eight inches through heavy material removal - tighter than nylon or acetal will hold.

It resists weak acids, aromatic and aliphatic hydrocarbons, ketones, esters, ethers, alcohols and salt solutions, and most neutral CIP solutions. Strong acids are limited service. Strong alkalis, steam and chlorinated solvents are not.

Food contact: the plain grades are declared by their manufacturer to meet the compositional requirements of 21 CFR 177.1630 for food types I to IX up to 250F. That is a manufacturer self-declaration of composition, not an FDA approval - there is no such thing for a stock shape - and it does not extend to EU 10/2011 or carry an NSF listing. Alcoholic beverages above 50 percent and infant formula are outside it.

Machine it with carbide rather than high speed steel, climb mill where you can, and keep coolant for drilling and parting off. It is stress relieved as supplied; if you are taking a lot off one face, relieve it again between roughing and finishing. Chamfer sharp corners before they chip.