Two insulating tubes are often compared for motor and transformer service: the spiral or convolute wound kraft paper tube and the vulcanized fiber (hardened fiber) tube. They look similar in a catalog but are made very differently, and that difference decides where each one belongs.
How Each Tube Is Made
- Kraft paper tube: insulating kraft paper is slit, dipped in insulating resin, and cured by high-temperature spiral or convolute rolling.
- Vulcanized fiber tube: 100 percent cotton-pulp paper is rolled in a zinc chloride solution, then desulfurized for about 25 days, baked, pressed, and cured.
The two processes diverge at the very first step. Kraft paper tubing is a resin-bonded structure, so the paper is a reinforcement and the resin is the binder. Vulcanized fiber is a chemically transformed material, where the zinc chloride treatment gelatinizes the cellulose fibers so they fuse into one dense mass. There is no separate binder to fail, which is why the material behaves like a single body when an arc hits it.
Production Differences
Kraft paper tubes have a short production cycle and a wide range of available thicknesses and bores, which makes them well suited to customized tubes. Hardened fiber tubes require corresponding rolling and pressing molds, so the production cycle and process are more complex.
| Step | Kraft paper tube | Vulcanized fiber tube |
|---|---|---|
| Raw material | Insulating kraft paper | 100 percent cotton-pulp paper |
| Bonding | Resin dip | Zinc chloride gelatinization, no separate binder |
| Forming | Spiral or convolute rolling at temperature | Rolling on a core, then pressing to shape |
| Post-treatment | Cure | About 25 days desulfurization, baking, pressing |
| Cycle time | Short | Long, driven by the desulfurization soak |
| Tooling | Few molds needed, wide size range | Rolling and pressing molds per size |
Performance Comparison
| Property | Kraft paper tube | Vulcanized fiber tube |
|---|---|---|
| Insulation | High quality, widely used in motor bushings and transformer protection | High, plus arc-quenching behavior |
| Arc quenching | No | Yes, releases gas under arc |
| Stiffness | Moderate | Stiffer than kraft paper, phenolic paper, and phenolic cloth |
| Moisture sensitivity | Depends on coating, moderate | Hygroscopic, swells in sustained humidity |
| Machinability | Cut and shaped, but the coating is the insulation | Cut, ground, and drilled with a clean bore finish |
| Relative cost | Lower | Higher, reflecting the long process |
| Typical duty | Motor bushings, transformer insulation | Motor bushings, electrical protection tubes, fuse tubes |
Why Vulcanized Fiber Takes About 25 Days
The desulfurization soak is the step that puzzles most buyers, so it is worth explaining rather than repeating. After the paper is rolled in zinc chloride, residual zinc salts and reaction by-products remain inside the wall. If they stay there, they are conductive and hygroscopic in service, which is the last thing an insulating tube needs. The long soak in the desulfurization pool leaches those residues out and lets the cellulose finish fusing into a dense, uniform wall. Bake, press, and cure follow.
None of it can be compressed on a schedule. Shorten the soak and the tube still looks right coming off the line, but the residue content is high and the dielectric performance degrades over time rather than at test. This is why two apparently identical fiber tubes can behave very differently in the field, and why the process record matters as much as the dimensional certificate.
Choosing on the Line
Choose kraft paper tubing when you need fast turnaround and a custom bore for insulation duty. Choose vulcanized fiber when the application needs arc quenching or higher stiffness, such as a fuse tube or an electrical protection tube. If the part only insulates and never clears a fault, kraft paper is usually the better value. If the part sits inside a protection device, the arc behavior is not optional and the fiber tube is the correct specification.
One further point matters in motor and transformer work. Both tubes are used as shaft spacers, bushings, and barriers, and in those positions the deciding factor is often stiffness rather than insulation, because a bushing that deflects under load can change the air gap. Vulcanized fiber holds that dimension better.
Why Source From a Direct Manufacturer
Tianze Electrical produces both kraft paper and vulcanized fiber tubes, so we can recommend from application data rather than a single product line. Production is ISO 9001 certified, with SGS material testing available. Our hard fiber tubes for fuse or motor article compares the four hard fiber grades, and the motor shaft fiber bushing page shows one of the common motor applications.
Frequently Asked Questions
Q: Can kraft paper replace vulcanized fiber in a fuse?
A: Generally no. Kraft paper lacks the arc-quenching behavior that fuse duty requires.
Q: Can either be machined?
A: Yes. Both can be cut and shaped, with vulcanized fiber offering higher stiffness.
Q: Why is the fiber tube more expensive?
A: The roughly 25 day desulfurization soak and size-specific rolling and pressing molds drive the cost, not the raw paper.
Q: Which is better for a motor shaft bushing?
A: Either can insulate, but vulcanized fiber holds its stiffness under load, which keeps the air gap stable. Confirm the load case with us and we will match the wall and grade to it.
Tell us your application at sale@fusestube.com, or see the vulcanized fiber tube page.
