Common rubber-sheathed cables use Category 5 copper conductors as their cores. During the production process, oxidation discoloration of the copper conductors is common, severely impacting product quality and presenting a significant technical challenge for many domestic cable manufacturers.
Copper, a transition metal, is easily oxidized to black CuO by oxygen in the air under acidic conditions. When water film or droplets form on the copper conductor's surface, atmospheric dust, such as smoke, coal, automobile exhaust, chlorides, and other acidic, alkaline, and salty particles, dissolve in the water film or droplets, forming an electrolyte that exacerbates the oxidative discoloration. This can negatively impact the product's appearance and quality, while more serious consequences can directly affect the copper conductor's performance and result in wasted production costs. Therefore, copper conductors must be properly stored, and extreme care must be taken to prevent oxidative discoloration in rubber-sheathed cables.
In principle, the copper used in wire and cable products is primarily protected by physical methods to isolate the copper conductor from moisture; cathodic protection oxidation-reduction methods prevent oxidation of the copper conductor in rubber-sheathed cables; chemical methods such as forming a passivation film on the copper conductor surface to prevent oxidation; or spraying a special liquid on the conductor surface to provide protection. Let's use the production process of general-purpose rubber-sheathed cables as an example to examine the main anti-oxidation control methods employed in each process.
1. Copper Rod Pre-Factory Transportation, Inspection, and Storage
Most cable companies in China purchase copper from external suppliers. They should select high-quality copper rod and standardize their suppliers' transportation and delivery procedures and systems. Upon arrival, copper rod inspection should be strictly carried out in accordance with GB/T 3048.2 or the cable company's corporate standards. Copper rod can be stored by completely covering it with plastic sheeting or film. This is the simplest physical barrier, preventing it from coming into contact with moisture. When receiving copper rod from the workshop, each coil must be visually inspected for any blackening, ensuring strict control from the source of production.
2. Copper Rod Wire Drawing Process Control
Drawing 0.4mm monofilament generally requires continuous annealing copper large and medium drawing machines, which generally involve various processes, including pay-off, drawing and annealing, cooling, drying, and winding. First, the appropriate die should be selected. It is crucial not to use a too small die, as this will force the copper lattice to change and exacerbate the metal's temperature rise. Second, the pH of the emulsion should be checked before starting the machine to ensure it is alkaline. Antioxidants should also be added to the drawing oil to form a passivation film on the copper conductor surface to prevent oxidation. During pay-off, the tension should be stable and uniform, and excessive vibration should be avoided. During the drawing process, the operator should maintain an appropriate water level to ensure uniform annealing and avoid under- or over-annealing. During winding, there should be no residual liquid on the copper conductor surface. A dry felt can be placed in front of the winding line (remember to change it frequently) to ensure the monofilament is dry. Finally, after the wire is drawn off the coil, it is sealed with transparent plastic film and stored in a dry environment until ready for circulation. Otherwise, the high surface temperature of the conductor may lead to oxidation due to contact with humid air.
3. Copper Wire Twisting (Bundling) and Extrusion of Insulating Rubber and Sheathing Rubber
Take copper wire twisting as an example. During the twisting process, the copper conductor passes through various die presses, which alters the copper metal's lattice structure. Under the influence of strong external forces, the temperature of the copper conductor after die extrusion increases significantly compared to before die extrusion, making the outer layer of the copper conductor susceptible to oxidation. Therefore, during the twisting process, an antioxidant (0.3% benzotriazole alcohol solution) can be dripped into the copper wire using an infusion hose. The ideal amount of dripping should be just enough to wet the surface of the copper wire to avoid insufficient dripping, which may cause localized oxidation, or excessive dripping, which may result in waste. Only start the machine after all anti-oxidation preparations are complete. The take-up reel should be kept dry at the take-up area. Once the reel is full, seal it with transparent plastic film.
Before extruding the insulation, the conductor should be longitudinally or circumferentially wrapped with polyester tape to prevent corrosion from substances in the rubber insulation. During the extrusion process, the insulating and sheathing rubber should be carefully protected from water ingress, which can lead to oxidation and blackening of the wire ends.
Through daily production observations, we have found that by selecting high-quality copper rods, effectively controlling the wire drawing process, the concentration and temperature of the emulsion, the annealing process, the conductor stranding process, or passivating the surface of the stranded copper wire, and preventing water ingress, we have also fostered quality awareness and strengthened professional training for workshop operators, ensuring they are fully aware of the adverse consequences of copper wire oxidation. By strictly controlling quality, we can effectively control the quality of the rubber-sheathed copper conductors in cables, prevent oxidation, significantly improve work efficiency, and reduce the likelihood of rework, ultimately achieving the high-return effect of reducing costs and improving product quality.
