How HVAC Duct Is Made: The Fabrication Process, Step by Step (2026)

The five stages at a glance

Every piece of HVAC ductwork, round or rectangular, goes through the same sequence. The machine changes with duct shape and material, but the stages do not.

Stage 1 — Cutting

Fabrication starts with mill coil or flat sheet. Slitting cuts a wide coil lengthwise into the narrow strip a spiral tubeformer needs (a 137 mm strip is standard). Shearing cuts flat sheet into square-edged blanks for rectangular duct. Plasma and laser machines make CNC profile cuts — holes, fittings, elbow gores and custom shapes. The operation chosen depends on whether you are making round duct, rectangular duct or fittings. See the cutting machine comparison for thickness and bed-size ranges.

Stage 2 — Forming

Round duct is formed on a spiral tubeformer: a continuous strip is wound helically and lock-seamed into a tube of any length, from Φ80 to Φ2500 mm. It is the fastest, lowest-leakage way to make round duct.

Rectangular duct is formed in sections. An auto duct line takes coil and notches, folds and seams it into four-sided duct in one pass; a lockformer and folders do the same job station by station for lower volumes. This is also where the material’s gauge matters — heavier galvanized needs a machine rated for it (see the gauge & thickness chart).

Stage 3 — Seaming

The longitudinal seam is what makes a formed section rigid and airtight. Round spiral duct closes with a rolled lockseam as it is wound. Rectangular duct closes with a Pittsburgh lock (a mechanically continuous interlocking corner) or a faster snap lock. The seam is folded, not welded, so the zinc coating survives intact. The Pittsburgh vs snap lock guide covers when each is used.

Stage 4 — Welding (where required)

Most low-pressure duct never needs welding — a locked seam is enough. But high-pressure, industrial, kitchen-exhaust or stainless duct calls for a gas-tight weld. A seam welder runs a continuous resistance weld along the lap; spot and stitch welders tack round duct and fittings; a medium-frequency or laser welder handles heavier or stainless work. Welding is the step that turns a mechanically-joined duct into a sealed one.

Stage 5 — Flanging and connection

Finally the duct needs a way to join to the next section on site. The modern standard is the TDF (Transverse Duct Flange) — an integral flange rolled directly onto the duct ends by a TDF flange line, so no separate angle iron is needed. Older or heavier duct uses a bolted angle flange. With the flange formed, the section is complete and ready to ship.

What it’s made of

The default material through all five stages is galvanized steel — zinc-coated for corrosion resistance, usually G90/Z275 in 26–20 gauge (0.55–1.0 mm). It cuts, forms, seams and flanges cleanly at full speed. Stainless or aluminium is reserved for wet, coastal or corrosive air. The whole line — cutting through flanging — is sized to the gauge you run; Taokron machines form 0.4 mm up to 2.0 mm galvanized.

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FAQ

Round vs rectangular — how does fabrication differ?

Round duct is wound continuously on a spiral tubeformer; rectangular duct is folded and seamed in sections on an auto line or lockformer, then TDF-flanged. Round is faster and lower-leakage; rectangular suits tight spaces.

What machines are used to make ductwork?

A duct shop uses cutting machines (coil slitter, shear, plasma or laser), forming machines (spiral tubeformer for round duct, auto duct line and lockformer for rectangular), seam and spot welders for airtight joints, and TDF flange-forming lines for the connection. Folders, roll benders and grooving/beading machines handle ancillary forming.

What material is HVAC duct made from?

Most HVAC duct is galvanized steel — zinc-coated for corrosion resistance, usually G90/Z275 grade in 26 to 20 gauge (0.55-1.0 mm). Stainless steel or aluminium is used in wet, coastal or corrosive environments. The machines form galvanized from 0.4 mm up to 2.0 mm.

What is the difference between round and rectangular duct fabrication?

Round duct is made continuously: a spiral tubeformer winds a steel strip into a lock-seamed tube of any length. Rectangular duct is made in sections: an auto duct line or lockformer folds and seams flat blanks into four-sided duct, then a TDF flange is formed on the ends. Round is faster and lower-leakage; rectangular fits tight spaces and high volumes.

Machinery for this application

The duct fabrication workflows above run on standard Taokron equipment: spiral tubeformers for round duct, auto duct production lines for rectangular duct, and TDF flange and lockformer machines for closure — or browse the full machine catalog.