Bus Duct Bellows the best materials are typically natural rubber, EPDM Rubber Bellows, Neoprene Rubber Bellows, and sometimes Hypalon Rubber Bellows or Butyl Rubber Bellows, chosen for their flexibility, durability, and temperature resistance. The recommended hardness grades for these bellows usually fall in the range of 40 to 65 shore A, optimizing the balance between flexibility and mechanical strength.