Wrapping is laid above stranded core for form saving and fastening, and also for mechanical and thermal protection from the next processes: shielding, filler adding, cable sheath laying, cable armouring (hot bitum spraying). Because of wrapping, electric strength of insulated cores increases relative to a metal sheath or a cable screen.
In air-space paper insulation cables it is used as wrapping: 10-100 pairs – two tapes of telephone-cable paper (thickness 0.05 mm); more than 100 pairs – cable paper (thickness 0.12 mm). In cables with polyethylene insulation as a wrapping it is used one or two plastic tapes (depends on cable’s diameter): polyvinilchloride, polyethylene and polyethylene terephthalate. Polyvinilchloride plasticised (PVC) film, polyethylene (PE) or polyethylene terephthalate (PET) film.
Fundamental properties of the films (PVC, PE, PET) are shown in the table below.
Polyethylene film has a good insulation properties, though because of the high softening temperature (108…112 °C) it sweats while passing an extruder orifice in the process of polyethylene cable-sheath laying. The most common is polivinilchloride film. It is possible to use two types of film (for example: PVC and PET).Wrapping is laid spirally, with overlapping by width 15…50%, or lengthwise. A winding angle can change in a wide range: from 80º for the thinnest cables and to 20º for the thickest cables.