What’s the first step in making enameled wire?

1. Lay out the lines

On a normally operating enameled wire production line, operators spend most of their mental and physical energy on the wire-unwinding process. Switching between unwinding reels requires considerable labor from the operator, and during reel changes, splices are prone to quality issues and operational failures. An effective solution is to use large-capacity unwinding devices. The key to proper unwinding lies in controlling the tension: excessive tension not only thins the conductor, causing the wire surface to lose its luster, but also adversely affects multiple performance characteristics of the enameled wire. From an external perspective, wires that have been excessively stretched exhibit poorer gloss in the finished enameled wire coating. In terms of performance, the enameled wire’s elongation, resilience, flexibility, and thermal shock resistance are all compromised. If the unwinding tension is too low, the wire tends to jump around, leading to wire bunching and contact with the furnace opening. During unwinding, it’s crucial to avoid situations where one half-reel has high tension while the other has low tension—such imbalances not only cause the wire to become loose, tangled, and broken, resulting in section-by-section thinning, but also induce significant wire movement within the oven, leading to wire bunching and short-circuiting faults. Therefore, unwinding tension must be kept uniform and appropriately adjusted. Installing a tension-assisting roller upstream of the annealing furnace greatly aids in tension control. At room temperature, soft copper wire exhibits a non-elongation tensile strength of approximately 15 kg/mm²; at 400°C, this value drops to about 7 kg/mm²; at 460°C, it further decreases to 4 kg/mm²; and at 500°C, it falls to just 2 kg/mm². During normal enameled wire production, the tension applied to the wire should be significantly lower than its non-elongation tensile strength—ideally controlled at around 50% of the non-elongation tensile strength—and the unwinding tension should be maintained at roughly 20% of the non-elongation tensile strength.

Large-specification, high-capacity spools generally use radial rotary pay-off reels; medium-specification wires typically employ over-end or brush-type pay-off reels; and fine-specification wires usually utilize brush-type or double-cone sleeve pay-off reels.

Regardless of the stringing method used, there are strict requirements for the structure and quality of bare copper wire spools.

The surface should be smooth to ensure the wire is not scratched.

The shaft core and the inner and outer sides of the side plates feature R-fillets with radii ranging from 2 to 4 mm, ensuring uniform wire release during the wiring process.

After the spool is machined, it must undergo dynamic and static balance tests.

The spool winder for brush holders requires a shaft core diameter such that the side plate diameter is less than 1:1.7; for end-of-winding applications, the ratio should be less than 1:1.9; otherwise, the wire may break when being wound onto the shaft core.

2. Annealing

The purpose of annealing is to heat the conductor—whose hardness has increased during the die-drawing process due to changes in its crystal lattice—to a specific temperature. This allows the molecular crystal lattice to rearrange itself, restoring the desired softness required for the manufacturing process. At the same time, the annealing process removes any residual lubricants and oil contaminants from the conductor’s surface that may have accumulated during drawing, making it easier to coat the wire and ensuring high-quality enameled wire. Importantly, annealing guarantees that the enameled wire maintains an appropriate level of flexibility and elongation when used as a winding component, while also helping to improve its electrical conductivity.

The greater the degree of deformation of a conductor, the lower its elongation and the higher its tensile strength.

There are three commonly used methods for annealing copper wire: coil annealing, continuous annealing on a wire-drawing machine, and continuous annealing on an enameled wire machine. The first two methods cannot meet the requirements of the enameled wire production process. Coil annealing can only soften the copper wire; however, since the wire becomes softer after annealing, it tends to bend more during unwinding. Although continuous annealing on a wire-drawing machine can soften the copper wire and remove surface oils, the softened wire tends to form numerous bends when wound onto the bobbin after annealing. By performing continuous annealing before coating on an enameled wire machine, not only can we achieve the desired softening and oil removal, but the annealed wire also remains straight, allowing it to be directly fed into the coating device and ensuring a uniform enamel film is applied.

The annealing furnace temperature should be determined based on the furnace length, copper wire specifications, and wire-drawing speed. At the same temperature and speed, the longer the annealing furnace, the more complete the recovery of the conductor’s crystal lattice. At lower annealing temperatures, the higher the furnace temperature, the better the elongation; however, at very high annealing temperatures, the opposite phenomenon occurs: the higher the temperature, the smaller the elongation, and the copper wire surface loses its luster and may even become brittle and prone to fracture.

If the annealing furnace temperature is too high, it not only shortens the furnace’s service life but also increases the likelihood of wire breakage during shutdowns for maintenance or when threading and re-threading wires. Therefore, the temperature of the annealing furnace should be controlled at around 500℃. Adopting a two-stage temperature-control system for the furnace and selecting control points at positions where static and dynamic temperatures are approximately equal proves to be effective. Copper readily oxidizes at high temperatures; the resulting copper oxide is extremely brittle, making it difficult for the varnish coating to adhere firmly to the copper conductor. Moreover, copper oxide accelerates the aging process of the varnish coating and adversely affects the flexibility, thermal shock resistance, and thermal aging performance of enameled wires. To prevent oxidation of copper conductors, it’s essential to keep them from coming into contact with oxygen in the air at high temperatures; hence, a protective gas is required. Most annealing furnaces have one end sealed with water and the other end open. The water in the annealing furnace’s water tank serves three purposes: sealing the furnace opening, cooling the conductors, and generating steam that acts as a protective gas. At the initial stage of operation, since there is very little steam inside the annealing tube, the air cannot be promptly expelled. In such cases, a small amount of alcohol-water solution (1:1) can be injected into the annealing tube. (Please note that pure alcohol must never be used, and the amount used should be carefully controlled.)

The water quality in the annealing tank is critically important. Impurities in the water can leave the wires unclean, affecting the painting process and preventing the formation of a smooth paint film. When using recycled water, the chlorine content must be less than 5 mg/L, and the electrical conductivity must be below 50 μΩ/cm. Over time, chloride ions adhering to the surface of copper wires can corrode both the copper wires and the paint film, resulting in black spots on the wire surfaces within the enameled wire’s paint film. To ensure product quality, the tank should be cleaned regularly.

There are also requirements for the water temperature in the tank. A higher water temperature helps generate steam, which protects the copper wires during annealing; wires exiting the tank will be less likely to carry water, but this can hinder their cooling process. On the other hand, a lower water temperature does promote cooling, yet it leaves the wires heavily coated with water, which is detrimental to the painting process. Typically, thicker wires require a lower water temperature, while thinner wires need a higher one. If you hear a hissing sound as the copper wires leave the water surface—indicating that water is vaporizing and splashing—it means the water temperature is too high. Generally, the water temperature for thick wires should be maintained between 50 and 60°C, for medium wires between 60 and 70°C, and for thin wires between 70 and 80°C. Since thin wires move at a faster speed and tend to carry more water, it’s advisable to use hot-air drying to remove the moisture effectively.