One joining operation dominates rectangular duct fabrication: fixing the flange rail and the corner pieces to the duct shell. The duct frame — four lengths of flange profile locked together by corner pieces — has to be fastened to the sheet point by point, all the way round. Plenty of workshops still do this with spot welds or rivets, even though the material is galvanised sheet: exactly the case where heat is the wrong tool for the job.
Why galvanised sheet and welding disagree
The zinc layer is thin, and it protects the duct for the whole service life of the installation. A spot weld puts heat straight into it: the coating burns away at and around the spot, leaving a discoloured mark and a heat-affected zone, and the workstation needs extraction and spatter screens. In a duct that later sits in a damp plant room or carries kitchen extract air, corrosion starts at exactly those points. Riveting brings no heat, but it needs a hole, a fastener and all the logistics around it — and a hole in the shell is one more path for moisture.
What clinching gives you on a duct frame
Clinching joins sheet metal cold. The punch presses both layers — the flange rail and the duct shell — into the die; the material flows and forms an interlock. Workshops also call it press joining or clinch fastening; the principle is the same. No welding, no rivets, no screws and no additional fasteners of any kind, which also means no sparks, no fume and no spatter at the bench.
- The galvanised or painted coating is not damaged: there is no heat and no spark, and the material is formed rather than cut.
- The joint has no heat-affected zone, so its fatigue strength is comparable to a spot weld or higher — which matters on ductwork running downstream of a vibrating fan.
- The static strength of a single point is usually lower than that of a spot weld, so points are spaced around the frame at a regular pitch, just as they always have been.
- Inspection is simple: the geometry of the button tells you what the joint is worth, and it can be measured without taking anything apart.
- Dissimilar metals can be joined — aluminium to steel, for example — which spot welding does not allow.
The process itself is described in more detail on our clinching technology page.
Materials and thicknesses in ductwork
Duct sheet is thin: the wall of a rectangular duct rarely exceeds 1 mm, and the flange profile is usually thinner still. Even added together, the two layers sit well inside the working range of the tools in our catalogue — for structural steel up to 400 N/mm² the stack can go to 4.0 mm. Aluminium, used in lighter ducts and in casings, covers the same range up to 4.0 mm. Stainless steel — hoods, kitchen extract, hygienic ductwork — up to 3.0 mm at strengths up to 600 N/mm². In practice, thickness is not what decides the tool: access to the joint and cycle time are. Exact figures for each model are given in the technical data table on its product page.
Clinching needs access from both sides: the punch on one, the die on the other. On duct frames, air handling unit casings, panels and bag filter frames that access comes for free, because the work happens on an open edge — which is why the process settled into HVAC fabrication so easily.
Which tool to choose
Cycle time decides, along with whether the tool goes to the part or the part comes to the tool.
- Manual clinching tool — site work, remedial joints, one-offs; it works with no supply at all.
- Pneumatic clinching tool — repetitive work on frames and duct sections; needs compressed air, typically up to 7.0 bar.
- Pneumatic benchtop press — when the part comes to the tool: corner pieces, filter frames, smaller sub-assemblies.
- Stationary pneumatic press — a series-production station in duct prefabrication.
For any of them the die and punch are chosen to suit the material and the total stack thickness; that catalogue category holds 18 items, including holders, strippers and adjustment shims. The die choice matters more here than the choice of drive — it sets the shape of the button and how firmly it holds.
Send an enquiry
Tell us what you are joining: sheet type (galvanised, stainless, aluminium), wall and profile thickness, flange height, and how many points the station has to make per shift. On that basis we will propose a tool and a die set. Send your enquiry — we usually reply within one business day.
We will match the tool and dies to your ductwork.