FR4 Tube — Sizes, Properties & Uses for Electrical Insulation
FR4 tube is mandrel-wound glass-epoxy thermoset tube with UL94 V-0 flame retardancy, produced by winding brominated epoxy prepreg continuously around a mandrel and leaving a bore. The hollow cross-section makes it the correct form for coil forms, transformer winding cores, insulating conduit, and sleeve bushings where the bore accommodates a conductor, shaft, or fastener while the tube wall provides electrical isolation and mechanical support. Standard OD range is 0.50" to 12"; wall thickness is specified to order.
At a glance:
- OD range: 0.50" to 12.0" (standard); larger to order
- Wall thickness: 0.062"–0.500" standard; verify with supplier for specific OD
- Lengths: 36"–48" standard
- UL94 rating: V-0
- Tg: 130–140°C (standard); high-Tg grades available
- Hoop strength is high — mandrel winding optimizes for radial compressive loads
- Bore finish: as-wound (rough); machined ID available for tighter tolerance
Standard FR4 Tube Sizes
| OD (in) | Common Wall Thickness | Nominal ID | Primary Use |
|---|---|---|---|
| 0.500" | 0.062"–0.125" | 0.250"–0.376" | Small coil forms, sleeve insulators |
| 0.750" | 0.062"–0.125" | 0.500"–0.626" | Coil bobbins, small conduit |
| 1.000" | 0.062"–0.125" | 0.750"–0.876" | Standard insulating tube |
| 1.250" | 0.093"–0.187" | 0.876"–1.064" | Medium coil forms |
| 1.500" | 0.093"–0.250" | 1.000"–1.314" | Transformer winding tubes |
| 2.000" | 0.125"–0.250" | 1.500"–1.750" | Larger coil forms, conduit |
| 3.000" | 0.125"–0.375" | 2.250"–2.750" | High-voltage insulating sleeves |
| 4.000"–6.000" | 0.187"–0.500" | Custom | HV column insulators, heavy-duty conduit |
Wall thickness and ID tolerances: OD tolerance ±0.020" as-wound; ID tolerance ±0.030" as-wound; finish-machined ID available to ±0.005".
How FR4 Tube Is Made
FR4 tube manufacturing is similar to rod, but the mandrel is sized to the finished bore and is extracted after curing:
- Brominated epoxy glass-fabric prepreg is wound at controlled tension around a steel or release-coated mandrel
- Wound assembly is pressed in a cylindrical fixture and oven-cured at 175°C
- After curing and cooling, the mandrel is extracted (or dissolved if soluble), leaving the hollow tube
- Tube is OD-turned and cut to length; ID machining optional
The continuous circumferential fiber orientation from mandrel winding provides excellent hoop strength (resistance to radial compressive and external pressure loads). Axial bending and tensile loads rely on the resin matrix and are lower than in-plane (hoop) values.
Properties of FR4 Tube
Applications
Transformer and inductor coil forms — FR4 tube is the standard winding form for dry-type power transformer primaries and secondaries, high-frequency inductor bobbins, and flyback transformer tubes. The tube provides the physical winding surface and, after winding, remains in the assembly as structural support for the coil. V-0 flame rating is required by IEC 61558 and UL 506 for transformer components.
Insulating sleeves and conduit — FR4 tube slid over a conductor or copper bus provides a continuous insulating sleeve. Used in motor lead exits, transformer terminal connections, and bus bar runs where flexible sleeve material would not provide adequate mechanical protection.
High-voltage column insulators (standoff tubes) — Hollow FR4 tube used vertically as a standoff insulator provides through-bore access for grounding cables or optical fibers while supporting equipment at line voltage above ground potential. The hollow section reduces material weight compared to solid rod without compromising dielectric length along the tube OD surface.
Sleeve bushings — FR4 tube in small OD sizes provides electrically insulating sleeve bushings for bolted joints in switchgear where metal fasteners must pass through a grounded panel into an energized bus bar block. The tube sleeve isolates the fastener from the panel.
Coil retaining rings and bobbins — In high-voltage solenoid valve actuators and electromagnets, FR4 tube forms the bobbin around which coil wire is wound, with flanges machined from FR4 sheet bonded or press-fit to the tube ends.
Machining FR4 Tube
FR4 tube machines similarly to rod on a lathe:
- OD turning: 200–400 SFM, carbide insert
- ID boring: Carbide boring bar; air blast for chip clearing
- Parting/cutoff: Carbide parting blade; support the bore with a plug or sleeve to prevent collapse under parting forces in thin-wall tube
- Threading: Single-point or tap; carbide preferred; thread engagement ≥ 1.5 D
- Dust controls: Identical to FR4 sheet and rod — dust collection and P100 respirator
Thin-wall FR4 tube (wall < 0.093") is prone to delamination if clamped too aggressively in a chuck. Use expanding mandrel or soft jaws to distribute clamping force.
Order FR4 tube — standard OD and wall sizes, cut to length
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