Quality Control In Plastic Cable-Making

Press, Volume XCIX, Issue 29239, 24 June 1960, Page 11

Quality Control In Plastic Cable-Making

is imported as hot rolled rods, which are pickled in dilute sulphuric acid before being drawn down into wires of various sizes, the largest being 0.160 in in diameter, the smallest 0.006 in.

Electrolytically refined copper is the purest commercial grade of this metal. Crude copper is produced after processes of fire refining, and eventually cast into heavy plates approximately 2ft 6in high by 2ft wide by 3in thick. The plates are called anodes. They are suspended in an electro-plating plant, and the pure copper is plated on to a thin copper cathode of high purity, gradually building up by electro-deposition. Impurities in the anodes—some o| these are precious metals such as silver, gold, selenium and tellurium—drop beneath the anodes and termed “anode mud.” They are salvaged for further refining. The pure copper cathodes are melted under carefully controlled furnace conditions and cast into “wire-bars.” The bars are rolled hot through grooved rollers into rods of various sizes.

The most sensitive and by far the quickest method of determination of the purity of copper wire is to measure its electrical resistivity This test is employed at Christchurch, constant checks being taken to ensure that the copper fully complies with the standard laid down by the International Electrotechnical Commission. The instruments used for the purpose are resistance bridges. They are extremely accurate, and provide constant control of the quality of copper being used in the plant.

grade will comply with “enduse” specifications. Approximately 50 per cent, of the wire drawn has to be softened by annealing. This is carried out in a furnace, where the copper is protected by an inert atmosphere to prevent oxidation during heating. The check on the efficiency of annealing is a simple one, that of determining the percentage elongation at fracture. After annealing, a considerable proportion of the annealed wire has to be stranded. This process is carried out on highspeed machines, which are

machine, or a butcher’s sausage-making machine. As during the process of extrusion, however, it is necessary to raise the temperature of the plastic compound being traversed by the screw through the length of the barrel, electric resistance heaters with sensitive temperature controllers are employed to provide the temperature gradient.

A similar process is used for the production of plasticsheathed cables, but in this case immediately -fter leaving the finishing die, and whilst

Five such instruments control temperatures at the various points, and those in use at Christchurch are electronically operated by means of small transistors.

A cross-head is used at the production end of the extruder barrel, and this permits the insertion of the conductor wire or wires, which are immediately surrounded by the hot plastic material under pressure from the screw. The conductor is carefully centred and emerges through the finishing die completely covered with the hot plastic compound.