Machine Design


Polyetherimide (PEI) is an amorphous engineering thermoplastic characterized by high heat resistance, high strength and modLilus, excellent electrical properties that remain stable over a wide range of temperatures and frequencies, and excellent processibility. Unmodified PEI resin is transparent and has inherent flame resistance and low-smoke evolution. The resin is produced by the General Electric Co. under the Ultem trademark.

Polyetherimide resin is available in an unreinforced grade for general-purpose injection molding, blow molding, foam molding, and extrusion, in four glass-fiber-reinforced grades (10, 20, 30, and 40% glass), in bearing grades, and in several high-temperature grades. The unreinforced grade is available as a transparent resin and in standard and custom colors. Also available is an impact-modified Ultem sheet.

The resin can be processed on conventional thermoplastic molding equipment. Melt temperatures of 660 to 800°F are typical for injection-molding applications. Mold temperatures of 150 to 350°F are required.

Polyetherimide is extruded to produce profiles, coated wire, sheet, and film. Film thicknesses as low as 0.25 mil are obtained by solvent-casting techniques. Molded and extruded parts can be machined using either conventional or laser techniques and can be bonded together or to dissimilar materials using ultrasonic, adhesive, or solvent methods.

Properties: The UL continuous-use listing of PEI is 338° F; gIass m transition temperature is 419 ° F, and heat-deflection temperature is 392° F at 264 psi, contributing to its high strength and modulus retention for service under load at elevated temperatures. Unmodified polyetherimide is listed by UL as 94V-0 at 0.016 in. and 94-5V at 0.075 in. without the use of additives. Limiting oxygen index is 47%, one of the highest among the engineering thermoplastics. Smoke evolution, as measured in the NBS chamber test (ASTM E662), is low.

Dielectric constant of PEI remains virtually unchanged between frequencies of 60 10° Hz and temperatures of 73 to insulative capabilities are demonstrated by the high volume resistivity 10"' ohm-cm and dielectric strength ranging from 830 V/mil at 1/16 in. in air to greater than 6,500 V/mil for submil firm thicknesses. Arc resistance exceeds 120 sec, meeting one of the UL electrical requirements for sole support of live parts.

A key feature of polyetherimide is maintenance of properties at elevated temperatures. For example, at 356°F, tensile strength and flexural modulus are 6,000 and 300,000 psi. Moduli and strengths of the glass-reinforced grades are still higher. For example, flexural modulus is 1,300,000 psi and 10 ° Hz 180 ° F.

Polyetherimide has good creep resistance as indicated by its apparent modulus of 350,000 psi after 1,000 hr at 180T under an initial applied load of 5,000 psi. Gardner impact strength is in excess of 320 in.-lb at room temperature. Ile resin is notch sensitive, however, so sharp corners and other stress concentrations should be minimized for maximum impact resistance. Tensile elongation at yield of 8% provides the necessary ductility for snap-fit designs.

The resin resists a broad range of chemicals under varied conditions of stress and temperatures. Compatibility has been demonstrated with aliphatic hydrocarbons and alcohols including gasoline and gasohol, mineral-salt solutions, dilute bases, and fully halogenated hydrocarbons. Resistance to mineral acids is outstanding. The polymer is attacked by partially halogenated solvents such as methylene chloride and trichloroethane and by strong bases.

Resistance to UV radiation is good; change in tensile strength after 1,000 hr of xenon arc exposure is negligible. Resistance to gamma radiation is also good, strength loss is less than 6% after 500 megarads exposure to cobalt 60 at the rate of one Mrad/hr.

Hydrolytic-stability tests show that more than 85% of tensile strength is retained after 10,000 hr of boiling-water immersion. The material is also suitable for applications requiring short-term or repeated steam exposure.

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