Equipment and design changes in extrusion of foamed fluoropolymer resins

Recent growth of computer networks has increased the market for foamed coaxial cables. To meet this need, the gas injection process for foaming of Teflon* FEP and PFA fluorocarbon resins introduced in 1981 at the 30th Wire and Cable Symposium has been refined. Advances in die and extruder screw design have broadened capability from 500 CATV coaxial cable to miniature wire sizes. Increased processing speeds and higher core quality have been achieved. These coaxial cables have a unique combination of properties that are maintained over a wide range of temperatures and signal frequencies. They have dielectric constants as low as 1.3 and low flame spread and smoke generation as tested to UL 910. These cables are now widely used for high frequency signal transmission in compliance with the National Electric Code for installation without conduit in building air handling plenums. Military applications represent newer areas of interest with weight and space savings and high electrical quality. The characteristics of several coaxial cables are reviewed.