How Fiberglass Covered Wire is Manufactured

I. Introduction

Fiberglass covered wire, also known as glass fiber cable or glass fiber wrapped wire, is a type of winding conductor with glass fiber yarn as the outer insulation material. Due to its excellent high temperature resistance, outstanding mechanical strength, and good chemical stability, this type of product is widely used in high-temperature motors, welding machine cables, large generators, dry-type transformers, and aerospace electrical equipment.

As an important branch of the electrical industry, the manufacturing process of fiberglass covered wire is characterized by high technical barriers, complex procedures, and strict quality control. This article aims to systematically describe the complete manufacturing process of fiberglass covered wire from conductor preparation to finished product delivery, providing professional reference for relevant engineering technicians and purchasing decision-makers.

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II. Basic Structure and Material Composition of Fiberglass Covered Wire

2.1 Basic Structure

Fiberglass covered wire typically consists of the following three layers:

Conductor Layer: High-purity oxygen-free copper or electrical aluminum is used as the conductive matrix, providing excellent conductivity. The conductor shape can be either round wire or flat wire, and the cross-sectional area is determined according to the product design current.

Bottom Insulation: A layer of enameled material (such as polyester imide, polyimide, etc.) is coated on the conductor surface to serve as a basic barrier for electrical insulation and to enhance the adhesion between the fiberglass yarn and the conductor.

Outer Fiberglass Layer: Fiberglass yarn is spirally wrapped around the bottom insulation layer and then impregnated with resin and cured to form a robust composite insulation structure. Single or double layers of fiberglass wrapping can be used depending on the thermal class requirements.

2.2 Main Raw Materials

The quality of fiberglass covered wire largely depends on the selection of raw materials:

Material Category	Specifications Requirements	Main Functions
Oxygen-free copper rod	Purity ≥99.95%, conductivity ≥101% IACS	Conductive matrix
Enamelled insulating varnish	Polyester imide (QZY), polyimide (QY), etc.	Underlying insulation
E-glass glass fiber yarn	Diameter 9μm～24μm, linear density determined according to conductor specifications	Outer insulating skeleton
S-glass glass fiber yarn	High strength models, used in special occasions	High strength outer insulation
Impregnating resin	Organosilicon resin, epoxy modified polyester, etc.	Filling glass fiber gaps, curing reinforcement

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III. Detailed Explanation of Manufacturing Process

3.1 First Stage: Conductor Preparation and Wire Drawing

The manufacturing process begins with the processing of high-purity oxygen-free copper rod.

Copper Rod Pretreatment: The surface of the 8mm diameter oxygen-free copper rod is cleaned to remove the oxide layer and oil impurities, ensuring a smooth subsequent wire drawing process.

Continuous Wire Drawing: The copper rod passes through multiple drawing dies, gradually reducing its diameter to the target size. The wire drawing process is typically divided into three stages: rough drawing, intermediate drawing, and fine drawing:

• Rough Drawing: Drawing the 8mm copper rod to 2.0mm～4.0mm
• Intermediate Drawing: Drawing the 2.0mm～4.0mm copper wire to 0.8mm～2.0mm
• Fine Drawing: Drawing the 0.8mm～2.0mm copper wire to the final size (can be as thin as 0.05mm)

Precise control of the die angle, lubricant temperature, and drawing speed is required during the wire drawing process to ensure consistent conductor surface finish and diameter tolerance within ±0.005mm.

Annealing Treatment: The drawn copper wire undergoes a continuous annealing furnace for softening treatment. The annealing temperature is controlled between 350°C and 500°C, adjusted according to conductor specifications and final product requirements. The elongation of the annealed copper wire should reach 25%–40% to meet the requirements of subsequent wrapping and winding processes.

3.2 Second Stage: Enamelling

Before glass fiber wrapping, a layer of enamel coating must be applied to the conductor surface. This process not only provides basic electrical insulation but also provides a good adhesion surface for the glass fiber yarn.

Enamelling Process: Using a mold coating method or immersion method, the insulating varnish is evenly coated onto the conductor surface. Commonly used paint types include:

• Polyester Imide (EI) paint: Thermal class F (155°C), cost-effective.
• Polyimide (AI) paint: Thermal class H (180°C) and above, suitable for high-temperature applications.
• Polyester (PE) paint: Thermal class B (130°C), suitable for general applications.

Baking and Curing: The coated wire enters the baking tunnel for baking and curing. The baking tunnel is usually divided into several temperature zones:

• Preheating Zone: 150°C～200°C, solvent evaporation.
• Curing Zone: 300°C～450°C, cross-linking and curing of the enamel coating.
• Cooling Zone: Natural cooling to room temperature.

The thickness of each coat is controlled between 0.015mm and 0.050mm, and usually 4 to 8 coats are required to achieve the required insulation thickness. The enamel coating surface should be uniform, smooth, free of pinholes and bubbles.

3.3 Third Stage: Glass Fiber Yarn Wrapping

Glass fiber wrapping is the core process in the manufacturing of glass fiber covered wire.

Glass Fiber Yarn Selection: Select appropriate glass fiber yarn based on conductor specifications and insulation thickness requirements. Commonly used yarn linear densities range from 4.4 tex to 200 tex, corresponding to monofilament diameters of 9 μm to 24 μm.

Wrapping Methods: There are two main methods for glass fiber yarn wrapping:

Spiral Wrapping: The glass fiber yarn is wound spirally around the conductor surface, with a certain gap between adjacent yarns. Spiral wrapping is fast and has high material utilization, making it the most commonly used wrapping method for glass fiber covered wire.

Longitudinal Wrapping: The glass fiber yarn is wrapped parallel to the conductor axis, resulting in smaller interlayer gaps and more uniform insulation. Longitudinal wrapping is slower but offers higher insulation quality, making it suitable for applications with high insulation requirements.

Number of Wrapping Layers: Depending on the product’s thermal class and insulation thickness requirements, single-layer or double-layer wrapping can be used:

• Single-layer Wrapping: Suitable for general applications with thinner insulation thickness.
• Double-layer Wrapping: Two layers of fiberglass yarn are wrapped in opposite directions, with gaps overlapping each other, resulting in more uniform insulation. Suitable for high-temperature or high-insulation-requirement applications.

During the wrapping process, precise control of the wrapping tension (typically 0.5N～3.0N), wrapping angle (15°～45°), and wrapping density (coverage 85%～95%) is necessary to ensure uniform insulation layer coverage.

3.4 Fourth Stage: Impregnation Treatment

After the fiberglass wrapping is completed, the wire needs to undergo impregnation treatment to allow the insulating resin to fully penetrate into the gaps between the fiberglass yarns.

Impregnation Resin Selection: Select the appropriate impregnation resin based on the product’s thermal class and operating environment:

• Silicone Resin: Thermal class H (180°C) and above, excellent high temperature resistance.
• Epoxy Modified Polyester Resin: Thermal class F (155°C), good mechanical strength.
• Polyurethane Resin: Thermal class B (130°C), good flexibility.

Impregnation Process: There are two main impregnation processes:

Dip Impregnation: The wrapped wire is immersed in a resin bath and left for a certain period to allow for full resin penetration. The immersion method is simple to operate and suitable for mass production.

Roll Coating: The wire is passed through a resin coating roller, and the resin is evenly coated onto the glass fiber surface. The roll coating method offers good coating uniformity and controllable resin usage, making it suitable for high-precision products.

After impregnation, the resin content in the gaps between the glass fiber yarns should reach 60%–80% to ensure that the insulation layer is fully filled and without gaps.

3.5 Fifth Stage: Drying and Curing

The impregnated wires are then placed in a drying and curing oven for curing treatment.

Drying and Curing Process: The drying and curing oven is typically divided into multiple temperature zones, with precise control over the temperature and dwell time in each zone:

• Preheating and Evaporation Zone: 100°C～150°C, dwell time 2～5 minutes, to remove excess solvent and moisture.
• Preliminary Curing Zone: 150°C～250°C, dwell time 5～10 minutes, the resin begins to cross-link and cure.
• Complete Curing Zone: 250°C～350°C, dwell time 10～20 minutes, the resin is completely cured, forming a solid insulation layer.
• Cooling Zone: Natural cooling or forced air cooling to room temperature.

The cured insulation layer should possess the following characteristics:

• Smooth, uniform surface, free of cracks and bubbles.
• Insulation thickness meets product standard requirements.
• Electrical strength, dielectric loss, thermal class, and other performance indicators meet standards.

3.6 Sixth Stage: Inspection and Packaging

The finished product must undergo strict quality inspection before leaving the factory.

Routine Testing Items:

Testing Item	Testing Method	Acceptance Standard
Conductor Dimensions	Micrometer Measurement	Meets IEC/GB Standard Tolerances
Insulation Thickness	Microscopic Measurement	Meets Product Specifications Requirements
Electrical Strength	Withstand Voltage Test	Not lower than the standard specified value
Dielectric Loss	tanδ Test	≤0.05 (in oil at 90°C)
Thermal Class	Heat Aging Test	Meets Thermal Class Requirements
Flexibility	Bending Test	No Insulation Layer Cracking
Appearance Quality	Visual Inspection	No Surface Defects

Packaging: Qualified finished products are packaged according to specifications. Common packaging methods include:

• Reel packaging: Small wires are packaged in plastic reels or wooden reels.
• Spool packaging: Large wires are packaged in wooden spools.
• Moisture-proof packaging: Inner layer moisture-proof film + outer layer cardboard/wooden box to prevent moisture damage during transportation.

Each batch of products is accompanied by a factory inspection report, including key parameters such as conductor size, insulation thickness, electrical strength, and thermal class.

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IV. Key Points for Quality Control of Fiberglass Covered Wire

4.1 Conductor Quality Control

Conductor quality is the foundation of fiberglass covered wire quality. The purity of the copper rod should be ≥99.95%, and the conductivity should be ≥101% IACS. During the wire drawing process, strict control of die wear and lubricant quality is required to prevent scratches on the conductor surface or uneven diameter.

4.2 Enamel Coating Quality Control

Enamel coating quality directly affects the electrical performance of the product. The viscosity of the enamel, the coating speed, and the baking temperature must be precisely controlled to ensure uniform enamel coating, free of pinholes and bubbles. The thickness of each enamel coating should be controlled within the process range to avoid being too thin or too thick.

4.3 Glass Fiber Wrapping Quality Control

Glass fiber wrapping is a core process. Wrapping tension, wrapping angle, and wrapping density must be strictly controlled to ensure that the glass fiber yarn evenly covers the conductor surface, without excessive gaps or excessive overlap.

4.4 Impregnation and Curing Quality Control

The viscosity of the impregnation resin, impregnation time, curing temperature, and time must be precisely controlled to ensure that the resin fully penetrates the gaps in the glass fiber yarn and is completely cured. Incomplete curing will lead to insufficient mechanical strength and decreased heat resistance of the insulation layer.

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V. Common Problems and Solutions

5.1 Insulation Layer Bubbles

Phenomenon: Bubbles appear on or inside the insulation layer, affecting electrical insulation performance.

Causes: Incomplete solvent evaporation during impregnation or excessively high curing temperature causing resin boiling.

Solutions: Adjust the impregnation time to ensure sufficient solvent evaporation; lower the curing zone temperature and extend the curing time.

5.2 Glass Fiber Yarn Breakage

Phenomenon: Frequent breakage of glass fiber yarn during the wrapping process affects production efficiency.

Cause: Excessive wrapping tension or substandard glass fiber yarn quality.

Solution: Adjust the wrapping tension to a reasonable range; replace with a qualified glass fiber yarn supplier.

5.3 Insufficient Enamel Adhesion

Phenomenon: Poor adhesion between the glass fiber yarn and the conductor, easily leading to insulation layer peeling.

Cause: Improper surface treatment of the enamel layer or incompatibility between the impregnation resin and the enamel layer.

Solution: Optimize the enamel layer surface treatment process; select an impregnation resin compatible with the enamel layer.

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VI. Conclusion

The manufacturing of glass fiber coated wire is a complex process involving multiple collaborative steps, including conductor drawing, enamel coating, glass fiber wrapping, impregnation treatment, drying and curing, and quality inspection. Each step has a significant impact on the electrical performance, mechanical strength, and thermal class of the final product.

When selecting fiberglass-coated wire, it is recommended to focus on the following factors: conductor purity and dimensional accuracy, enameling quality, fiberglass wrapping uniformity, impregnation and curing integrity, and the supplier’s quality control system. Choosing a supplier with comprehensive testing equipment and a quality management system is crucial to ensuring product quality and lifespan.