Suitable for direct burial and underground applications. The sheath material contains the following components in parts by weight: 20-50 parts of high density polyethylene (HDPE), 20-30 parts of low density. The HFBR-R/EXXYYYZ series of plastic fiber optic cables are constructed of a single step-index fiber sheathed in a black poly ethy lene jacket. The duplex fiber consists of two simplex fibers joined with a zipcord web. Its MFR and density are outer the traditional range which provide better processability, and also contains appropriate level of additives and well-dispersed carbon. essive shear, is recommended. The best extrusion performance can be obtained by using a PE screw extruder with a length-to-diameter ratio of 25:1 and a compression ratio of about 3.