Alkali-Free Wax-Free Fiberglass Tape
A woven strapping tape made from alkali-free glass yarn with a wax-free finish, used to wrap and band transformer and CT windings. It is a wrap product: no shrink step, no adhesive, the wrap holds by winding tension and the tape's own tensile strength, specified at ≥ 0.7 N/tex. The wax-free finish exists so varnish can wet the fiber during coil impregnation. Tell us the coil dimensions, varnish system and wrap pattern and we confirm the tape width and roll count.
Specification
| Material | Alkali-free fiberglass (woven tape) |
|---|---|
| Shrink behavior | Non-shrink : mechanical wrap |
| Color | White |
| Tensile strength | ≥ 0.7 N/tex |
Construction: glass yarn, weave, finish
Three facts define this tape, and each one matters for a different reason.
- Alkali-free glass yarn. The yarn is drawn from a low-alkali glass composition : the conventional basis for electrical-grade fiberglass textiles, in contrast to soda-lime glass used where electrical properties are not the point.
- Woven tape construction. Warp and weft yarns are woven into a flat tape. The weave is what lets the tape conform around bobbins and corners while staying flat under tension; a slit film or a knit would behave differently on a coil edge.
- Wax-free finish. Fiberglass yarns are sized during spinning so they can be woven without yarn breakage. Common sizings include starch-oil and volatilized systems; wax-containing finishes are used in some textile applications but are avoided on electrical tapes because wax migrates to the fiber surface and spoils varnish adhesion. On this tape the finish is specified wax-free so impregnating varnish can wet and bond to the yarn.
One thing the construction does not include: an adhesive layer, and no shrink or recovery step. Tension does all the holding.
What the tape does on a transformer or CT winding
Between the winding wire and the outside world, a coil usually carries several layers: wire enamel, interwinding insulation, the impregnating varnish, and outer wraps that tie everything together. This tape is one of those outer wraps. Its job is mechanical: hold end-turns down against vibration and handling, band core-and-winding assemblies, secure lead exits, and keep the winding square while it goes through varnish dip and bake. Because the fiberglass weave is open, varnish passes through the wrap and bonds the layers beneath it. Which is exactly why the wax-free finish matters. A waxed tape would sit on top of the impregnation as a poorly bonded, removable layer.
Selection: width, thickness, overlap, tension
- Width. Match the width to the coil feature being wrapped: narrow widths follow small CT bobbins and lead exits; wider tape lays flat across transformer winding faces with fewer turns and less build-up.
- Thickness and layers. Containment comes from layers, not from one heavy pass. If your drawing calls a total build-up, count layers at the overlap: a 50% overlap doubles the build everywhere.
- Overlap. 50% overlap is the common starting point when every turn must be backed by another layer; butt wraps suit faces where build-up is limited. State the pattern on your drawing.
- Tension. The ≥ 0.7 N/tex tensile figure is a yarn-strength property. In practice you pull hard enough to seat each turn without cutting the yarn into your hands or the coil edge : steady, moderate tension to the end of the roll, then secure the tail.
Send the coil drawing and wrap pattern; we return a tape width, an estimated roll count and, if your varnish system is stated, the finish confirmation.
Common failure modes on coil wraps, and prevention
The wrap loosens after impregnation or in transit
Cause: The tail was not secured under tension, so the last turns relaxed; the whole wrap then shifts during handling or varnish immersion.
Prevention: Hold tension through the final turn and secure the tail by tuck or per your process before releasing. A wrap that survives the varnish bath stays put; one released early does not.
Varnish will not wet the tape surface
Cause: The tape surface carries a non-wetting finish, either the wrong sizing on this product or a waxed textile tape substituted in production.
Prevention: Confirm the wax-free designation at order stage and keep the electrical-grade tape separate from textile-grade fiberglass tapes in stores. If wet-out fails on a sample, test the finish before blaming the varnish.
Tape cuts through at a sharp coil corner
Cause: The full wrap tension concentrates on a square edge, and the yarns locally overload.
Prevention: Break the corner with an edge pad or change to a narrower width applied in more passes so the tension spreads over more turns.
Wrong width chosen for the coil geometry
Cause: Wide tape bridges over small radii without contacting, leaving voids; narrow tape on a large flat face wastes turns and adds build-up.
Prevention: Match width to feature: narrow for bobbins and exits, wide for flat faces. A sample wrap on a scrap coil settles it in minutes.
Tape relied on as the dielectric layer
Cause: The wrap was counted as primary electrical insulation in the winding design.
Prevention: Treat the tape as mechanical reinforcement; any dielectric contribution comes from the impregnated system as a whole and must be proven by system-level test, not assumed from the fiberglass weave.
Frequently asked questions
What does alkali-free / wax-free mean for coil impregnation?
Alkali-free refers to the glass composition: the yarn is made from a low-alkali glass rather than soda-lime glass, which is the standard basis for electrical-grade fiberglass textiles. Wax-free refers to the finish: the yarn carries no wax-based sizing that could interfere with varnish or resin wet-out. For impregnation, this means the varnish can bond directly to the fiber surface instead of fighting a wax release layer. Confirm the exact glass grade and finish chemistry against your varnish system with us at order stage.
How should tape width and overlap be selected?
Width follows the coil geometry: narrow widths conform around small CT bobbins, corners and lead exits; wider tape covers flat winding faces faster with fewer turns. Overlap follows the duty: a 50% overlap gives two layers everywhere and is the common starting point for banding; edge-to-edge (butt) wraps are used where build-up must stay minimal. The wrap must be applied with steady tension so each turn seats on the previous one; the tape holds mechanically, there is no adhesive to fall back on.
Is this tape primary electrical insulation or mechanical reinforcement?
On this page it is positioned as mechanical reinforcement and banding for transformer and CT windings: it contains and supports the coil through impregnation, baking and service. An unwaxed woven fiberglass tape can contribute to the insulation system once impregnated, but we do not publish a dielectric or thermal-class value for the tape alone; that value belongs to the complete impregnated insulation system and must come from system-level test data. If your spec needs a dielectric or thermal-class number for the tape, request it and we will confirm what test data exists before you rely on it.
Why does the wrap tension matter if the tape has no adhesive?
There is no adhesive and no shrink step, so every turn is held only by friction under tension and by the tape's own tensile strength, specified at ≥ 0.7 N/tex. If tension is released before the end is secured, turns relax and the wrap loosens in transit or during varnish immersion. Secure the final turn the same way the body of the wrap is held, by tension and tuck or by the banding method your process defines.
Does the fiberglass tape have a temperature or thermal-class rating?
A thermal class (for example 155 or 180) describes an impregnated insulation system, not a bare woven tape, so this page does not publish one for the tape alone. If your winding specification calls out a thermal class, send it with the varnish system and we will confirm what evidence exists for this tape in that system before order stage.
Request a quote for Alkali-Free Wax-Free Fiberglass Tape
Send the coil dimensions, varnish/resin system, required tape width and thickness, wrap pattern and quantity. We return the tape specification, roll count, documentation status and lead time.