Designing with polyetherimide

PEI fits neatly between polysulfone and polyphenylene sulfide while sharing some of the attributes of each.

Sabic Innovative Plastics’ polyetherimide (PEI) was patented by the General Electric Co. in 1973. Over the next 10 years, Joseph Wirth developed a commercially viable material called Ultem. Pilot plant production was initiated and PEI became profitable in 1988.

There is a rumor that GE invested $50 million on PEI. It is difficult to envision a modern corporation investing that much in a material that would take 15 years to turn a profit. This may be why few totally new plastics are being developed.

Design characteristics
Ultem combines inherent flame resistance, outstanding electrical properties, high heat resistance, and mechanical strength in a material that is easier to process than other high-performance plastics.

This is a strong material with a tensile strength of 15,200 psi and a flexural modulus at 480,000 psi. The addition of 30% short-glass-fiber reinforcement increases the tensile and flexural strength to 24,500 psi and 1,300,000 psi respectively.

Notched Izod impact strength is 1.0 for unreinforced and 1.6 ft-lb/in for 30% reinforced grades. Unnotched impact is 25.0 and 8.0 ft-lb/in respectively.

Ultem has a heat deflection temperature of 392°F at a loading of 264 psi. Reinforcing 30% only provides an additional 18 deg F of temperature resistance.

The atmosphere contains an average of 21%-22% oxygen. PEI requires an atmosphere of 47% oxygen in order to burn. This material is inherently nonburning and achieves a UL 94 V-0 rating without halogen additives. The use of this material avoids Europe’s restrictive fire-retarding additives regulations. If forced to burn, PEI generates very little low-density smoke. PEI is a good material for use in confined spaces such as airplanes, submarines, and high-rise office buildings.

This material’s nonburning characteristics, its high heat resistance, and a dielectric strength of 831V combine to account for its wide use in electrical applications.

Ultem exhibits better chemical resistance than many other amorphous thermoplastics. It is unaffected by most hydrocarbons, alcohols, and halogenated solvents. It is resistant to mineral acids and can withstand short-term contact with dilute bases. PEI retains its strength following steam autoclave sterilization or 10,000 hours of immersion in 212°F water.

Ultem is resistant to ultraviolet radiation without stabilizers. This material also exhibits good resistance to gamma radiation. The current list price for PEI is around $8.20/lb in less-than-truckload quantities. It is interesting to note that the cost of this material is such that the addition of glass reinforcement reduces the price per pound of PEI.

Part design tips
Wall thickness. Small 0.015-inch-thick parts have been produced in nonreinforced PEI. A better minimum thickness is 0.060 inch, which can have a flow length of 7 inches. A 20-inch flow length is possible with a 0.125-inch wall. Thickness variations must be smoothly blended and limited to 25% of the wall thickness.

Corner radiuses. PEI is a notch-sensitive material. Inside radiused corners can be up to 25 times stronger than sharp corners. The minimum inside radius is 0.015 inch. Twenty-five percent of the part’s wall thickness would be better and 50% will provide the maximum strength. Radiused corners also improve the flow of material through a cavity.

Draft angles. PEI parts have been successfully molded with draft angles of only ½º/side. Larger draft angles allow shorter cycles and lower costs. A draft of 1º/side is preferred. Textured surfaces require a draft of 1½º/side for each 0.001 inch of texture depth.
Projections. Reinforcing ribs, gussets, and solid bosses are projections off of a part’s nominal wall. The thickness where they join the nominal wall should be limited to 70% of the part’s thickness. Thicker projections encourage sink marks and molded-in stress. Properly proportioned projections will be thinner than a part’s nominal wall. They will be more difficult to fill and eject. A 0.015-inch radius at the junction of the projection and the part’s nominal wall will improve cavity filling. A ¼-½º/side draft angle will reduce the force required to eject the projection from its cavity.

Depressions and holes. High cavity filling forces can bend the core pins that form small holes. The bending of core pins will be minimized if the depth of holes is limited to two to three times the diameter of the core pin. All of the inside corners on irregularly shaped holes should have the standard radius to avoid molded-in stress.

PEI is strong and it shrinks tightly onto core pins. Providing radiuses, draft angles, and smooth surfaces reduces ejection force, which allows shorter cycles and lower cost. The core pins that form holes produce weldlines. Melt flowing around a core pin reunites on the far side, creating a weldline. These weldlines are typically weaker than the surrounding material that doesn’t have a weldline. PEI produces strong weldlines that can retain 95% of the material’s original tensile strength.

Tolerances. One of PEI’s important attributes is its ability to maintain precision dimensions over a wide range of temperatures, repeated sterilization, and submersion in boiling water. A commercial tolerance for a 1-inch-long, 0.125-inch-thick PEI part is ±0.002 inch. A more costly fine tolerance could be ±0.001 inch. This fine tolerance should only be specified when that tolerance is more important than the cost of the part.

Ultem is a good candidate for nonburning, low-smoke-emission applications requiring greater temperature resistance and stiffness than that provided by polycarbonate or polysulfone. 

Typical applications for PEI
Electrical/Electronic: Flame resistance, coupled with high strength, dimensional stability, and excellent electrical properties account for PEI’s usage in all sorts of electrical applications. This combination of attributes produces both flexible circuits and printed circuit boards that withstand vapor-phase and wave soldering. PEI has a higher continuous service temperature than the competitive epoxy circuit boards. Other uses include burn-in sockets, switches, connectors, kitchen appliance components, high-temperature lighting, insulating tapes, and dielectric films.

Transportation: Long-term-creep resistance, high strength retention at elevated temperatures, coupled with good resistance to fuels, lubricants, and coolants account for PEI’s use in under-the-hood automotive applications. Aircraft and aerospace uses include wiring, lighting, seating, structural, and engine components. Interior paneling takes advantage of PEI’s resistance to burning and its low smoke generation, which meet FAA regulations.

Healthcare and food handling: Laboratory ware and healthcare applications rely on PEI for its resistance to chemicals and gamma radiation sterilization. Heat-generating diagnostics instruments rely on PEI’s high-temperature characteristics. The current pressure to reduce healthcare costs requires the sterilization and reuse of an increasing number of products. PEI is finding use as containers for carrying these products through repeated strong alkaline cleaning and autoclaving. The ability to withstand 1000 autoclave cycles further reduces costs.

Food preparation appliances rely upon PEI’s resistance to fats and oils, its heat resistance, and its microwave transparency. This material’s high gloss and ease of coloring add to its appeal in consumer markets.

Other applications: Film and high-strength fiber for industrial filters, papermaking drying screens, and flame-resistant garment fabric for firemen and race car drivers. Fiber optics is another growing market. PEI is also used in hair dryers, steam irons, microwave ovens, and other heat-generating appliances.