| 摘要 |
Cellular sheathing is formed around a cable or similar support being pulled longitudinally through or past the sheathing die. The sheathing compsn. incorporates both a pressurised gas to generate cells and a crosslinking agent which is activated prior to the extrusion outlet to raise the melt viscosity sufficiently highly and rapidly that the blowing agent is not promptly dissipated on emerging from the die. Pref. the sponge leaving the extruder is chilled externally, e.g. by sprays, and heated internally, e.g. by induction heating, though external heating may subsequently be applied to postcure the skin. The crosslinking agent may be a constituent of the sheathing compound o may be injected after generation of the gas in the melt. The gas may be generated by chemical decomposition or physical injection. Pref. method is for mfr. of electrical telecommunication cables, esp. coaxial high frequency cables, with cellular insulation. Gas trapping by viscosity enhancement allows low density shething to be generated more readily and facilitates prodn. of sheathed cables with concentrically located cores. |
