Spiral Duct Forming Explained

What a spiral tubeformer actually does

A spiral tubeformer takes a narrow slit coil (typically 137 mm wide for most SBKJ machines) and runs it through a forming head where three rollers bend the strip into a helix and lock the edges together with a continuous spiral seam — the familiar spiral lock. The head rotates and the duct emerges straight out the other side like extrusion. Once it reaches the target length, the line cuts it to size and drops it on a run-out table.

Diameter range by model

SBKJ's spiral tubeformer catalog covers essentially every commercial HVAC diameter:

• SBTF-1500 — Φ80 to Φ1,500 mm, saw blade cutting
• SBTF-1602 — Φ80 to Φ1,600 mm, flying shear (faster, cleaner cut)
• SBTF-2020 — up to Φ2,000 mm for industrial and large-building projects

Flying shear vs saw blade — which cut?

Flying shear cuts the duct while it is still moving through the line. No sparks, no swarf, no secondary deburring. This is what SBKJ uses on the SBTF-1602 and it is the preferred choice for high-volume production.

Saw blade stops the duct briefly and cuts with a rotating blade. Slightly slower on short lengths but mechanically simpler, which is why SBKJ keeps it available on the SBTF-1500 for buyers who prioritize maintenance simplicity.

Realistic output

A well-tuned SBTF-1602 can produce roughly 15–25 meters of finished duct per minute on mid-range diameters (Φ300–Φ800 mm). Throughput drops at the extremes — very small or very large diameters take longer per meter. Plan capacity around the mid-range, not the brochure peak.

Material and thickness

Standard SBTF machines run galvanized steel 0.5–1.2 mm. Stainless and aluminium coils are supported with the appropriate roller set — specify up front, because changing rollers on site is a 1–2 day job.

Quality checks the forming head should pass

1. Seam continuity — no gaps, no overlap steps along the full length
2. Straightness — the duct should run true without spiraling out of round
3. Diameter tolerance — typically ±0.5 mm on commercial duct
4. Cut squareness — a flying-shear cut should be perpendicular within 0.5°
5. Burr — minimal burr on the internal edge; excess burr indicates worn tooling

Step‑by‑step: how a coil becomes a duct

Understanding the forming sequence helps when you scope the line and when you troubleshoot it later. Every spiral duct produced on an SBKJ tubeformer follows the same five‑step path from coil to run‑out table.

1. Decoiler — the slit coil is mounted on a powered decoiler that feeds the strip into the line under controlled tension. Tension is critical: too low and the seam loosens; too high and the coil edges scuff the rollers.
2. Edge preparation — the strip passes through a pre‑bender that turns up both edges roughly 90 degrees so the spiral lock seam can engage cleanly when the helix forms.
3. Forming head — three forming rollers wrap the strip into a continuous helix. As each turn meets the previous turn, the pre‑bent edges interlock and the seam is rolled flat by a final pressure roller.
4. Cutting station — once the duct reaches the programmed length, the flying shear (or saw blade) cuts it cleanly. On the SBTF‑1602 the cut happens at full line speed, so there is no machine pause between sections.
5. Run‑out table — the finished duct rolls onto a powered run‑out where it is barcoded, labelled and moved to packing. SBKJ run‑outs include a soft‑stop at the end to prevent end‑of‑line damage.

Material thickness vs achievable diameter

Not every diameter can be produced from every gauge. Spiral duct forming has a sweet spot where the strip is thick enough to hold the seam but thin enough to bend cleanly around the forming head. As a rule of thumb on SBKJ machinery:

• Φ80–Φ300 mm — best at 0.5–0.8 mm; thicker gauges put excess load on the head and shorten roll life
• Φ300–Φ800 mm — runs cleanly at 0.6–1.0 mm in galvanised steel
• Φ800–Φ1,500 mm — typically 0.8–1.2 mm depending on pressure class and reinforcement
• Φ1,500–Φ2,000 mm — only on the SBTF‑2020 platform, 1.0–1.5 mm with auxiliary support stand

Stainless and aluminium spiral duct

SBKJ spiral tubeformers also handle stainless steel and aluminium with the appropriate roller set. Stainless 304 and 316 spiral duct is common in semiconductor fabs, food processing, hospital cleanrooms and marine applications. Aluminium spiral is preferred for short‑run flexible installations and areas where weight matters. Specify the material at order time so the correct roller hardness and surface finish are supplied — changing rollers on site is a 1–2 day job and a clean, dedicated stainless line will outperform a galvanised line that has been retrofitted occasionally for stainless work.

Spiral duct vs round seam‑welded duct

Spiral duct is the dominant choice for commercial HVAC because it is fast, cheap, light, strong and easy to handle. The alternative — longitudinally seam‑welded round duct — is sometimes specified on heavy‑gauge industrial process duct or where the spiral lock seam is not acceptable to the project standard. SBKJ supplies both. For the vast majority of HVAC applications, spiral duct is the right answer; for high‑pressure or process exhaust above roughly 2,500 Pa, ask SBKJ engineers about a welded duct option.

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FAQ

What is the difference between flying shear and saw blade cutting?

Flying shear cuts the spiral duct while it is still moving through the line — no sparks, no swarf, no secondary deburring — and is preferred for high-volume production. Saw blade stops the duct briefly and cuts with a rotating blade; mechanically simpler and easier to maintain.

What diameter range can SBKJ spiral tubeformers produce?

From Φ80 mm up to Φ2,000 mm across the SBTF product line. The SBTF-1602 covers Φ80–Φ1,600 mm with flying shear, and the SBTF-2020 extends up to Φ2,000 mm for large industrial duct.

What output can an SBTF-1602 achieve?

Approximately 15–25 meters of finished spiral duct per minute on mid-range diameters (Φ300–Φ800 mm). Throughput drops at very small or very large diameters.