Case history: Engineered PS replaces polysulfone

Electroscope Inc. has chosen Dow’s Questra syndiotactic polystryene (SPS) to mold a surgical instrument after polysulfone failed to retain the necessary seal between molded plastic and a metal insert.

“The challenge for the plastic was that it needed to mold around the metal insert, achieve part dimensions, and maintain a seal between the metal and plastic through initial autoclave sterilization and subsequent sterilization cycles,” explains Tom Wessel, development leader, Dow Plastics.

“Other plastics, such as polysulfone, which were used for prototyping the application, would allow water to migrate between the metal and plastic interface,” reports Bob Mitchiner, director of engineering, ElectroScope Inc. “As a result of the loss of dimensional integrity, water migrated into the molding and formed a conductive path between the high and low potential areas and caused the instrument to fail.”

The failure rate with polysulfone parts was at least 25 percent after 25 autoclave and cleaning cycles. With Questra 2730, 15 parts were molded in the first trial run and all survived 25 autoclave and cleaning cycles.

The Questra medical grade polymer was able to be used in the existing mold, met the critical part dimensions for assembly, and provided chemical resistance for cleaning and use.