Longitudinal Seam Welder: TIG vs Plasma for Round Duct Buying Guide (2026)
What a Longitudinal Seam Welder Actually Does
A longitudinal seam welder is a finishing machine for round duct. It takes a rolled duct section — long edges prepared for welding — and fuses the seam into a continuous welded joint as the duct travels through the machine. Where a Pittsburgh lock seam is mechanically closed by roll pressure, a welded seam is fused metal: one continuous, airtight joint with no interlock to leak and no sealant line to trust.
The machine exists because specifications, not preferences, decide seam type. When a drawing calls for a welded joint — stainless kitchen exhaust, clean room and pharmaceutical systems, high-pressure exhaust, fume and dust collection — a mechanically closed lock seam will be rejected at inspection, and the duct has to be re-rolled and re-seamed on site at your cost. The welder moves that joint into your shop, where it is cheap, inspectable and consistent. Published Preda figures for this machine: welding process TIG or plasma, application round duct longitudinal seams, materials steel and stainless steel, automatic operation, list pricing from around $7,200, CE certified, 2-year warranty.
It is not a general-purpose welding robot. It is a purpose-built seam station: duct in, welded duct out, ready for flanging and fitting attachment — typically installed downstream of a spiral tubeformer or plate rolling machine and upstream of a flange line.
Lock Seam vs Welded Seam — Where the Line Is
Both seam types produce a closed round duct. They are not interchangeable, and the difference is specification-driven:
When a lock seam is enough
Low-pressure heating, ventilation and air conditioning supply and return in commercial buildings. A Pittsburgh lock closed on a lock former and finished on a seamer is the industry default — fast, cheap, tooling-light. Sealant covers the leakage allowance of these systems.
When the specification demands a weld
- Stainless steel duct — kitchen exhaust and process duct: inspectors expect fused seams, and sealant lines in hot, grease-laden vapour are a known liability.
- Leakage-classed systems — clean room, pharmaceutical and laboratory duct specified to tight leakage classes; a lock seam's many potential leak paths are hard to certify.
- Pressure and exhaust duty — fume extraction, dust collection and material handling duct that must hold static pressure without seam weep.
- Sanitary and corrosive service — duct that gets washed down or carries aggressive vapour, where sealant degrades quickly.
The Machine at a Glance
Published figures for the Preda longitudinal seam welder, with the practical meaning of each number alongside.
| Parameter | Published figure | What it means on your floor |
|---|---|---|
| Welding process | TIG or plasma | Two configurations; TIG favours thin stainless, plasma favours travel speed on galvanized |
| Application | Round duct longitudinal seams | A seam station for rolled tube — not a fitting welder |
| Material | Steel, stainless steel | Galvanized and stainless duct stock; confirm aluminium at quotation |
| Operation | Automatic travel with welding process control | Constant speed and heat input; bead quality independent of operator skill |
| Construction | Heavy-duty steel frame | Vibration-free running; factory rates 10+ year service life |
| Certification / warranty | CE / 2 years | European safety compliance plus factory warranty and parts support |
| Indicative price | ≈ $7,200 | Class-typical band; final price depends on process option and tooling |
What the spec sheet does not state — and what you must ask for in writing — is the build-dependent set: maximum duct diameter, seam length per cycle, weld travel speed, tracking tolerance and duty cycle. These vary with the configuration you order; treat any number that is not in your quotation as unconfirmed.
TIG vs Plasma — Choosing the Process
The same frame accepts either process. The choice is a trade between bead control and travel speed:
| Criterion | TIG configuration | Plasma configuration |
|---|---|---|
| Best for | Thin stainless, cosmetic seams, autogenous joints | Galvanized steel, higher travel speed, deeper penetration |
| Heat input | Lower and narrower — less distortion on 0.6–1.0 mm sheet | Higher energy density; faster, but watch burn-through on thin stock |
| Filler | Usually autogenous on butt and lap seams | Often run with filler wire for root reinforcement |
| Shielding | Argon or argon-helium cover gas plus trailing shield | Plasma gas plus shielding; fume extraction mandatory on zinc-coated stock |
| Class-typical travel speed | ≈ 400–700 mm/min on 1.0 mm galvanized | ≈ 800–1,500 mm/min on the same gauge — confirm by trial weld |
| Skill demand | Automatic travel removes hand-guided torch variability | Same automatic head; parameter set-up per material and gauge |
Rule of thumb: if most of your welded work is thin stainless in visible locations, specify TIG. If it is galvanized exhaust and dust duct at volume, plasma pays back in cycle time. If the mix is both, order the machine with the process swap in mind — torch station, gas train and power source on a common platform, so changeover is a kit rather than a rebuild.
Matching the Welder to Your Line
A seam welder is only as fast as the station that feeds it. Before ordering, run the numbers:
- Upstream output. A band type tubeformer or rolling machine produces metres per minute; the welder's cycle per duct length must consume that output, not throttle it. If your line runs 10–15 m/min and the welder finishes a 1.2 m section in three minutes, the buffer between them grows all shift.
- Diameter range. The published application is round duct; ask for the fixture set covering your working diameters. Fixtures come per diameter group, not per diameter — but confirm the groups.
- Gauge range. Duct stock for this machine class typically runs 0.5–1.2 mm galvanized and 0.6–1.5 mm stainless. Below 0.5 mm, distortion control dominates the design; above 1.5 mm, ask whether the power source still holds penetration at production travel speed.
- Seam geometry. Butt or lap, and the edge preparation your rolling station actually produces. A welder fed mis-prepared edges tracks poorly no matter how good the head is — edge condition is a line problem before it is a welding problem.
- Floor plan. Infeed and outfeed tables at working height, gas cylinder storage per local code, extraction ducting route. The machine is the small part of the installation.
Factory Audit & Purchasing Checklist
Use this list when you compare suppliers or run a factory audit:
- Weld samples on your stock. Send one metre each of your thinnest and heaviest production material; require welded seam samples back. A shop that cannot produce samples on customer material is guessing.
- Trial weld acceptance criteria. Agree in writing: visual (no undercut, no burn-through, uniform bead width), penetration (root fusion shown by bend test or macro-section), and the travel speed achieved on your gauge.
- Seam tracking. Ask how the head follows the seam when rolling leaves ovality — automatic tracking or manual follow-up, and to what tolerance.
- Gas and electrical scope. Confirm the argon supply interface, cooling method, and plant power (voltage, phases, breaker size) — frequent exclusions in low quotes.
- Duty cycle. A seam station running two shifts needs a stated duty cycle at production current, not peak current.
- CE file and warranty terms. Declaration, manual, wiring diagrams, and the wear-parts list with prices — electrode, nozzle, liner, tracking components.
- Service path. Local agent or remote support; 24-hour response and spare-parts shipping terms should be contractual, not verbal.
Common Faults and What They Tell You
| Symptom | Likely cause | First response |
|---|---|---|
| Porosity in the bead | Contaminated edge, moisture, insufficient cover gas or a draught blowing the shield off | Clean and degrease edges; check gas flow and shield placement; screen the station |
| Burn-through on thin gauge | Travel speed too low or current too high for the gauge | Reduce heat input or raise speed; run the parameter card for that gauge |
| Bead wanders off the seam | Edge prep out of tolerance, seam gap too wide, tracking drift | Check edge squareness and gap upstream; recalibrate tracking |
| Inconsistent penetration | Supply fluctuation, worn consumables, poor ground connection | Check plant supply and ground clamp contact; replace consumables on schedule |
| Excessive distortion | Heat input too high for thin stock, insufficient clamping | Lower heat input, add clamping, weld in sequence rather than one pass |
FAQ
Can a lock seam be welded after closing?
Not practically — once the edges are rolled into an interlock they cannot be fused cleanly. The decision is made upstream, at the forming stage: if the specification says welded, the edges go into the welder prepared, not into a lock former.
How does it compare with buying pre-welded pipe?
Pre-welded pipe removes the welding station but fixes diameters and lengths to the supplier's stock, adds freight on air, and puts you at the mercy of another shop's schedule. The welder keeps diameter and length under your control and pays back against the freight and markup on prefabricated sections.
Will it run with my existing tubeformer and flange line?
That is the normal installation — welder between the forming station and the flanger. Confirm interfacing: duct support height, travel direction, and the control handshake if you want automatic start and stop from the line.
What is the real bottleneck to watch?
Edge preparation. Rolling stations that deliver square, consistent edges make the welder look good; sloppy edge prep makes a good welder look bad. Fix preparation first, then judge the weld.