The coating method for enameled wire

Painting is the process of applying a coating of varnish onto a metallic conductor to form a uniform layer of a certain thickness. This process involves several physical phenomena related to liquids as well as various painting techniques.

Viscosity refers to the resistance a liquid exhibits when it flows. As the liquid flows, molecules collide with one another, causing one layer of molecules to carry another layer along with it. At the same time, the mutual forces of interaction cause the subsequent layers of molecules to impede the movement of the preceding layers, thereby manifesting as an active form of internal friction—this is precisely what we call viscosity. Different painting methods and different wire gauges impose varying requirements on the viscosity of the coating. The magnitude of viscosity is primarily determined by the molecular weight of the resin: the higher the molecular weight of the resin, the greater the viscosity of the coating. Coatings made with resins of high molecular weight are particularly suitable for applying thick wires, since coatings derived from such resins exhibit superior mechanical properties. On the other hand, coatings with lower viscosity are ideal for fine wires, as resins with lower molecular weights are easier to apply evenly, resulting in smoother coating films. Surface tension arises because molecules within a liquid are surrounded by other molecules, and the attractive forces among these molecules can reach a temporary equilibrium. However, molecules at the surface of the liquid experience two opposing forces: on one hand, they are attracted inward toward the bulk of the liquid; on the other hand, they are also attracted by gas molecules. Yet, there are fewer gas molecules than liquid molecules, and they are farther away. Consequently, the molecules at the liquid’s surface are subjected to stronger inward attraction from the liquid’s interior, causing the surface to contract as much as possible and assume a spherical shape. Among geometric shapes of the same volume, the sphere has the smallest surface area. Thus, if a liquid were not subject to any external forces, under the influence of surface tension alone, it would always tend to adopt a spherical shape. According to the surface tension acting on the surface of the coating liquid, uneven surfaces have varying curvatures at different points, leading to imbalanced normal pressures at each point. Before entering the varnish-coating oven, the thicker areas of the coating liquid, driven by surface tension, flow toward the thinner areas, thereby achieving a more uniform distribution of the coating. This process is known as leveling. The uniformity of the coating film is influenced not only by the leveling effect but also by gravity; the final uniformity of the coating film results from the combined action of these two forces.

After the lacquered wire passes through the felt, there is a process of rounding it out. This is because, after the wire is coated with lacquer and passes through the felt, the lacquer liquid takes on an olive-shaped form. Under the influence of surface tension, the lacquer liquid overcomes its own viscosity and instantly transforms into a circular shape.

If the wire gauge is smaller, the viscosity of the paint is lower, and less time is required to draw the wire into a round shape. Conversely, as the wire gauge increases, the viscosity of the paint also rises, requiring more time to achieve the desired roundness. In paints with high viscosity, surface tension sometimes fails to overcome the internal friction within the paint liquid, resulting in an uneven paint layer.

After the lacquered wire emerges from the felt, there’s also the issue of gravitational effects during the process of rounding out the lacquer layer. If the rounding action is brief, the sharp corners of the olive-shaped profile will quickly disappear, and the duration of gravitational influence will be short, resulting in a relatively uniform lacquer film on the wire. However, if the rounding action lasts longer, the sharp corners at both ends will persist for a longer time, and the gravitational effect will also last longer. Under these circumstances, the lacquer film at the sharp corners tends to flow downward, causing local areas to become thicker. Meanwhile, surface tension further encourages the lacquer to form spherical droplets, ultimately giving rise to particles. Since gravitational effects become particularly pronounced when the lacquer film is thick, it’s crucial not to apply too much lacquer in each coating pass. This is one of the reasons why “thin coats applied multiple times” are used in the manufacturing of enameled wire.

When applying fine lines, if the coating is too thick, it will contract under the influence of surface tension, forming wavy or bamboo-joint-like threads.

If there are very fine burrs on the wire, under the influence of surface tension, these burrs will not only be difficult to coat with paint but also tend to wear away and become thinner, resulting in pinholes in the enameled wire.

If the round conductor itself is elliptical, under the additional pressure applied during painting, the paint layer will be thinner at the two ends of the ellipse’s major axis and thicker at the two ends of the minor axis, resulting in a pronounced non-uniformity. Therefore, the out-of-roundness of round copper wire used for enameled wire must meet the specified requirements.

When bubbles form in the lacquer, these bubbles are air entrapped in the lacquer during stirring and material addition. Because air is less dense than the lacquer, buoyancy causes it to rise toward the surface. However, due to the surface tension of the lacquer, the air cannot break through the surface and remains trapped within the liquid. When this lacquer containing air bubbles is applied onto the surface of a wire and then fed into the varnish-coating oven, the air inside the bubbles expands rapidly upon heating. As the surface tension of the lacquer decreases with the heat, the air bursts out of the surface, resulting in an uneven and rough finish on the coated wire.