Polyester Web Sling: Model: 1T~50T (SF=6:1) Endless polyester web slings & Reinforced lifting eyes web sling. Slingle or double ply. Polyester Round Sling: Model: SWL=1T~500T (SF=6:1) Round sling is a continuous loop made from polyester yarn encased in a sleeve of polyester or Nylon fabric. The sleeve is so constructed that it protects the internal rows of yarn filament and keeps them in a parallel formation at all times.