Fiberglass Filament Wound Tube Manufacturing: Winding Angles, Curing, Extraction & NDT Inspection

The manufacturing process of filament wound epoxy fiberglass tubes (G10, G11, EPGC) determines their internal mechanical isotropy and dielectric reliability in high-voltage SF₆ switchgear, transformer bushings, and hydraulic cylinders. Unlike continuous pultrusion, multi-axis CNC filament winding allows precise control over glass fiber placement, resin matrix cross-linking, and wall thickness uniformity.

This factory manufacturing guide from RDS Composite provides a step-by-step technical breakdown of filament wound tube production—covering CNC winding angle setup, thermal oven curing, hydraulic mandrel extraction, centerless OD grinding, and non-destructive dielectric inspection.

1. Step 1: CNC Fiber Placement & Winding Angle Control

Continuous non-alkali E-glass rovings are drawn from tensioned creels through a temperature-controlled epoxy resin bath. A multi-axis CNC delivery carriage lays wetted fiber bands onto a rotating precision steel mandrel at programmed helical patterns:

2. Step 2: Rotational Thermal Oven Curing Cycle

Once the target wall thickness is reached, the wound tube assembly is transferred to a computer-controlled thermal convection oven. To prevent liquid resin sagging or thermal eccentricity before gelation, the mandrel rotates continuously at 10 to 15 RPM throughout the heat cycle:

• Ramp-Up & Gelation Phase (80°C – 120°C): Resin viscosity drops briefly, allowing trapped air to release before exothermic cross-linking gels the matrix.
• Full Curing Dwell Phase (140°C – 170°C): Sustained thermal dwell completes polycondensation, establishing high Tg (> 150°C) and cross-link density.
• Controlled Annealing Cooling (< 50°C): Slow cooling prevents internal laminar stress buildup and micro-cracking across the tube wall.

3. Step 3: Hydraulic Mandrel Extraction & OD Grinding

Inner tube dimensions are governed by the mirror-polished steel mandrel, while outer dimensions undergo secondary finishing:

1. Hydraulic Stripping: A specialized high-tonnage hydraulic extractor engages the cured tube end-flange, smoothly pulling the polished steel mandrel out without scoring the tube inner wall (supported by heat-resistant fluoropolymer release agents).
2. Centerless OD Grinding: The raw cured outer surface is passed through multi-stage centerless grinding machines to remove resin ridges and achieve precision outer diameter tolerances (down to ±0.03 mm, ISO h8/h9).

4. Step 4: Quality Inspection: NDT & Dielectric Testing

Before custom CNC slotting or shipment, 100% of high-voltage wound tubes undergo non-destructive testing (NDT) quality checks:

• Ultrasonic Flaw Detection: Scans the tube wall for internal delaminations, dry fiber pockets, or void inclusions.
• Partial Discharge (PD) Testing: High-voltage tubes are stressed under partial discharge equipment to verify low PD activity (≤ 5 pC at rated operating voltage per IEC 60270).
• Power Frequency Withstand Voltage (Hi-Pot): Applies short-time AC high voltage perpendicular to the tube wall (typically ≥ 16–20 kV/mm) to verify dielectric breakdown resistance.

To compare filament winding with pultrusion and roll wrapping, read our master comparison guide: Comprehensive Guide to Fiberglass Tube Manufacturing Methods.