By Design: Designing with polysulfone

PSU takes the stage in this continuing series on designing with specific plastics.

Union Carbide introduced poly­sulfone (PSU) in 1965 under the Udel tradename. Carbide sold the PSU business to Amoco in 1986. British Petroleum subsequently purchased Amoco and marketed the material as BP Amoco Polymers. Today that same Udel is available from Solvay Advanced Polymers. Other companies entered this market after the original patents expired. However, Udel is still the largest-volume PSU.

In this recurring column, Glenn Beall of Glenn Beall Plastics Ltd. (Lib­erty­ville, IL) shares his special perspective on issues important to design engineers and the molding industry. You can reach him at glennbeall [email protected].

Udel was the first thermoplastic with a service temperature greater than 300ºF. Prior to that time, polycarbonate dominated the high-temperature thermoplastic market with a continuous service temperature of 250ºF. Udel created excitement among the product design community, which saw PSU opening markets that heretofore were dominated by heat-resistant glass, metal, and thermoset plastics.

Defining characteristics

PSU is a transparent, semicrystalline thermoplastic engineering material. It is known for its high service temperature, low creep, dimensional stability, and good stiffness.

This is a strong material with a tensile strength of 10,200 psi and a flexural modulus at 390,000 psi, which positions it between polyphenylene oxide (PPO) and acetal. The addition of 30% short-glass-fiber reinforcement increases the tensile and flexural strengths to 17,000 and 1,200,000 psi respectively. The high-temperature creep resistance of PSU is among the best of the nonreinforced thermoplastics.

PSU is notch sensitive with a notched Izod impact strength of only 1.3 ft-lbhttps://www.plasticstoday.com/in. Unnotched impact strength is impressive at 60 ft-lbhttps://www.plasticstoday.com/in. Also, this material has a heat deflection temperature of 345ºF at a loading of 264 psi. Adding glass fiber results in only a 20 deg F increase.

This material has a light amber color with 75% light transmission and 5% haze. This is the only plastic material that retains its transparency after prolonged exposure to temperatures of 400ºF. Transparency, coupled with a high service temperature, accounts for many of its applications.

Other applications rely on PSU’s hydrolytic stability, which provides excellent long-term resistance to steam and boiling water. Its ability to be repeatedly exposed to wet-dry and cold-hot environments accounts for its use in the food and healthcare industries. This is one of only two transparent plastics that are suitable for repeated steam autoclave sterilization at temperatures up to 270ºF. Some surgical organizing trays made of PSU have continued to be used following 2000 autoclave cycles.

Some PSUs are self-extinguishing and others enjoy a UL 94-V0 flammability rating. These materials have good chemical resistance, but they are affected by polar organic solvents such as ketones, chlorinated hydrocarbons, and aromatic hydrocarbons.
PSUs absorb ultraviolet energy. Outdoor applications must be UV stabilized, painted, or plated. The average market price for neat PSU is $6.97https://www.plasticstoday.com/lb and $0.31https://www.plasticstoday.com/ in3https://www.plasticstoday.com/truckload.

Part design tips

Wall thickness. The ideal nominal wall thickness is the thinnest wall that will satisfy both the functional and manufacturing requirements. Recommended wall thickness is 0.030-0.250 inch. PSU is a viscous material. Flow length for a 0.030-inch thickness can be up to a maximum of 3 inches. A flow length of 12 inches can be achieved with a thickness of 0.080 inch. When variations in wall thickness cannot be avoided, they should be limited to 25%. These changes in thickness should be smoothly blended in order to avoid molded-in stress, warpage, and loss in impact strength.

Corner radiuses. Rounding the corners of injection molded parts has two benefits. Radiuses eliminate sharp corners, which are stress concentrators. Rounding the corners also makes it easier for plastic to flow through a cavity. This allows cavities to be filled with lower ­injection pressures, which reduce molded-in stress. This is especially important with PSU, which is a relatively hard-flow material that is notch sensitive. These problems are minimized with an inside radius equal to 50% of the part’s thickness. A 75% radius would be even better.

Molding draft angle. Draft angles are tapers on the walls of a part and are perpendicular to a mold’s parting line. These tapers allow parts to be ejected from a mold with reduced force. This, in turn, allows parts to be molded on shorter and less-costly molding cycles. Draft angles of 1⁄2ºhttps://www.plasticstoday.com/side can be used on smoothly polished surfaces. A draft of 1ºhttps://www.plasticstoday.com/side is better. Textured surfaces require a draft of 11⁄2ºhttps://www.plasticstoday.com/side for each 0.001 inch of texture depth.

Projections. Reinforcing ribs, gussets, solid bosses, and pads are projections off of a part’s nominal wall. Their thickness at the junction with the part’s wall should be limited to 66% of that part’s thickness. This thickness can be reduced to 50% of the part’s thickness to eliminate any possibility of sink marks on appearance surfaces. Properly proportioned projections are thinner than the part’s nominal wall. This makes projections more difficult to fill and eject. A radius of 0.010-0.015 inch at the junction of the projection and the part’s nominal wall will improve cavity filling. A molding draft angle of 1⁄4-1⁄2ºhttps://www.plasticstoday.com/side will reduce the force required to eject a projection from its cavity.

Depressions and holes. Weldlines are created when plastic flows around a hole and reunites. Weldlines are normally weaker than the surrounding material. Under ideal molding conditions, PSU weldlines can retain 100% of the material’s original strength.
The high injection pressure used with this viscous material can bend the free-standing core pins that form small holes. Bending will be minimized if the depth of these holes is limited to two to three times the diameter of the core pin.

All inside corners on holes should have the standard radii. This is a strong material that grips the mold very tightly as it shrinks. Providing radii and molding draft with a smooth surface reduces ejection force, which allows shorter molding cycles.

Tolerances. Once molded, PSU parts have the ability to maintain their size over a wide range of temperatures for prolonged periods of time. This dimensional stability accounts for its selection for intricate precision electric and healthcare components. A commercial tolerance for a 1-inch-long, 0.125-inch-thick PSU part is ±0.003 inch. A more costly fine tolerance on that same part is ±0.002 inch. This fine tolerance should only be specified when that dimension is more important than the lowest possible cost.
PSU is worth considering if an application calls for a strong, dimensionally stable, creep-resistant, transparent material with more temperature resistance than polycarbonate.