Manufacturing Process of Glass Fiber Covered Wire

The manufacturing of glass fiber covered wire is a precise and multi-step process. The core objective is to wrap the metal conductor evenly, tightly, and without defects with fine glass fibers, and to impart final comprehensive performance through impregnation treatment.

I. Core Raw Materials

✅ Conductor

🔹Material: Usually copper (soft round copper wire) or aluminum. Copper is most commonly used due to its excellent conductivity and ductility.

🔹Grade: Copper conductors generally use T2 or T1 grade pure copper. Requirements for the conductor itself include low resistivity, smooth surface, no burrs, and no oil stains, to avoid affecting the adhesion and performance of the insulation layer.

🔹Specification: Round or flat wires of different diameters according to design requirements.

✅ Insulation Material

Glass Fiber Yarn: Alkali-free glass fiber (E-glass) is used. This is because alkali-containing glass (such as A-glass) has poor electrical insulation properties and high moisture absorption, which does not meet electrical requirements. Alkali-free glass fiber has very high resistivity and heat resistance.

Impregnating Varnish: This is the key material that determines the final performance of the glass fiber covered wire (such as heat resistance class, bond strength, chemical resistance). Common types include:

🔹Polyester: Typically Class B heat resistance (130°C).

🔹Modified Polyester: Offers superior performance.

🔹Polyurethane: Good solderability.

🔹Epoxy: Strong adhesion and good chemical resistance.

🔹Silicone: Heat resistance can reach Class H (180°C) or higher, used for high-temperature environments.

🔹Polyimide: Highest heat resistance, can reach Class C (>220°C).

II. Manufacturing Process Flow

The entire process can be divided into three main stages: conductor preparation, glass fiber covering, and post-treatment.

• Stage 1: Conductor Preparation and Pre-treatment

✅ Wire Drawing: Copper or aluminum rods are cold-drawn through dies to achieve the precise diameter required for the product. This process hardens the metal, necessitating the next step.

✅ Annealing: The cold-drawn hard-state wire is subjected to high-temperature annealing in a protective atmosphere (e.g., nitrogen) to relieve internal stress and restore it to a soft state, becoming “soft round copper wire.” This is crucial for subsequent tight wrapping and customer winding.

✅ Cleaning: Removes grease, dust, and other contaminants from the conductor surface to ensure good adhesion of the glass fiber and impregnating varnish.

• Stage 2: Glass Fiber Covering (Braiding)

This is the core process, performed on specialized glass fiber covering machines.

✅ Pay-off: The spool of processed conductor is placed on the pay-off device, maintaining constant pay-off tension through a tension control system.

✅ Wrapping/Braiding:

Multiple bobbins loaded with glass fiber yarn rotate rapidly around a conductor moving at a constant speed.

Through precise mechanical control, the glass yarn is spirally wrapped around the conductor with a specific pitch (i.e., winding density) and overlap rate (typically 50%-55%, meaning the next layer covers more than half of the previous layer’s width).

Double-layer reverse wrapping (one layer left-hand twist, another layer right-hand twist) is usually adopted to make the covering layer tighter, more uniform, prevent gaps, and improve mechanical strength.

✅ Take-up: The wire covered with glass fibers is wound onto another spool. The product at this stage is called the “green wire” or “bare covered wire,” where the glass fibers are loose and have low strength.

• Stage 3: Post-treatment and Curing

✅ Impregnation:

The wound “green wire” is passed through a varnish tank containing impregnating varnish, allowing the glass fiber layer to fully absorb the resin varnish.

The felt coating method is commonly used, where squeeze felts control the amount of varnish applied, ensuring even penetration without excessive thickness.

✅ Baking / Curing:

The impregnated wire immediately enters a long oven (baking tunnel).

At precisely controlled temperatures, the solvent in the resin varnish volatilizes, while the resin undergoes cross-linking polymerization, completely curing.

This process bonds the loose glass fiber yarn into a solid, smooth, dense integral insulation layer, firmly attached to the conductor.

Cooling & Rewinding: The cured wire is cooled by a cooling device and finally wound neatly onto the finished product spool via a tension control system.

III. Key Quality Control Points

• Visual Inspection: The surface should be smooth, free of bubbles, impurities, yarn breakage, or exposed copper.
• Dimensional Accuracy: Includes conductor diameter, outer diameter of the insulation layer, uniformity, etc.
• Electrical Performance: Breakdown voltage is the core indicator, testing the insulation layer’s ability to withstand high voltage. Insulation resistance tests are also conducted.
• Mechanical Performance: Includes flexibility and adhesion (often checked through wrap tests, sudden pull break tests for cracking or exposed copper), abrasion resistance, and scratch resistance.
• Heat Resistance Class: Determined through thermal aging tests to confirm the product’s temperature class (e.g., B, F, H).
• Chemical Resistance: Tests the stability of the insulation layer after contact with chemicals such as solvents and oils.

IV. Why Choose These Materials and Processes?

Q:Why use alkali-free glass fiber? 

A:Because it offers excellent electrical insulation, high-temperature resistance (softening point up to 860°C), non-flammability, low moisture absorption, and high mechanical strength.

Q:Why impregnate? 

A:Bare glass fibers are porous, unbonded, have poor mechanical strength, and inadequate moisture resistance. The impregnating varnish acts as a binder, sealant, and additional insulating medium, filling the gaps between the fibers to form a continuous, solid, smooth composite insulator.

Q:Why use double-layer reverse wrapping? 

A:Single-layer wrapping inevitably has gaps. Double-layer reverse wrapping allows the layers to cover each other’s gaps, ensuring the continuity of the insulation layer and higher mechanical strength.

Original Source:Magnet Wires for Power Industry