PET-P Rod, Internally Lubricated Bearing Grade

Internally lubricated PET-P rod in grey, cut to length and sold by the foot.

This is the internally lubricated bearing grade - the same PET-P base with a solid lubricant dispersed through it, which is why it is grey rather than natural. The lubricant is not something its makers publish, so we will not guess at it.

What it buys you, and what it costs you. Limiting PV goes from about 2,800 psi.fpm to about 6,000, and the wear factor drops roughly 40 percent, from 60 to 35 - it outwears plain PET-P, acetal and most nylons. In exchange, tensile strength falls about 15 percent to 10,500 psi and elongation at break drops from 15 or 20 percent to 5 or 7, so it is appreciably more brittle. Note what does NOT change: the coefficient of friction is essentially the same as plain PET-P, around 0.19 against 0.20. This is a wear and load grade, not a low-friction grade, and anyone selling it as the latter is overselling it.

It goes into journal and liner bearings, wear and slide pads, thrust washers, rollers and wheels running without bearings, dynamic seals, valve seats, scraper blades and dosing pistons - particularly against soft metal or plastic mating faces, and where running noise matters.

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.

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.