When buyers spec a duct line, they spend all their questions on the forming machines and give the cutting station one glance. That is backwards. The blanking method - shear or plasma - decides your edge quality, your noise level, your cost per meter of coil, and half of your maintenance schedule. I have run both setups in the same workshop on the same coil, and the difference shows up in the first hour of a shift. So here is how the two really compare, with the numbers I wish someone had given me before our first purchase.
One clarification first: on modern lines the choice sits between a hydraulic or electric shear (a guillotine that chops the blank in one stroke) and a CNC plasma cutting machine integrated into the line. Both cut the same coil into blanks. Almost nothing else about them is similar.
The shear works like an industrial paper cutter. The blade comes down, the sheet separates in one stroke, done. Cycle time on a 1200 mm blank is 3 to 6 seconds, and the edge is clean - a slight rollover on the top edge and a small burr on the bottom, both acceptable for Pittsburgh lock and TDF work right off the machine.
The strength of the shear is consistency. There is no heat, so no distortion, no oxide, no dross. The weakness is flexibility: every blank length change means moving a back gauge, and the shear only cuts straight lines. If your product ever needs notches, corner cuts, or damper openings cut in the blank, a shear cannot do any of it - those jobs go to a separate machine or to hand tools, and hand tools are where injuries happen.
Noise is the part nobody warns you about. A shear stroke on 1.0 mm galvanized sits around 95 to 105 dB(A) at the operator position. Our people wear protection regardless of local rules, and the workshop walls get a serious knocking test over the years. If your shop shares a building with offices, plan acoustic treatment into the installation.
Plasma cuts with a constricted arc at roughly 20,000 degrees C through the sheet. The gantry follows the programmed path, so a plasma station cuts straight blanks, notches, mitres, and openings with zero mechanical changes between them. Changing product is a software selection, not a back gauge adjustment - on our mixed-order days that alone saves 20 to 30 minutes per shift.
The cost of that flexibility shows up at the edge. Plasma leaves a small heat-affected zone, a thin oxide film on galvanized, and occasionally dross on the bottom edge at the wrong speed or a worn electrode. For lock forming and flanging the edge is still perfectly usable - the Pittsburgh seam covers the cut edge entirely, and TDF flanges fold it away. But for exposed slips and drives, or for anything the customer sees in a plant room, operators deburr or wipe the edge. That is 5 to 10 seconds per blank of extra handling on the critical pieces.
Quality hangs on consumables. A fresh electrode and nozzle at correct standoff gives a squarish, clean edge; an electrode 20 percent past its life gives bevel, dross, and drift. Good shops log every consumable change against arc starts - electrodes typically last 60 to 120 arc starts on thin galvanized, nozzles a bit less. Ignore that discipline and your "plasma vs shear" comparison becomes a comparison between plasma done well and plasma done badly, which tells you nothing.
Numbers below are from 0.5 to 1.2 mm galvanized coil, blanking 1200 mm lengths, and reflect what I measure on the floor rather than brochure peaks:
| Item | Shear blanking | Plasma blanking |
|---|---|---|
| Cycle time per 1200 mm blank | 3 - 6 s | 8 - 15 s |
| Max practical thickness on duct line | up to 1.5 - 2.0 mm | up to 6 mm (with higher amp source) |
| Edge quality out of machine | Clean, small burr | HAZ + oxide, occasional dross |
| Can cut notches / openings | No | Yes, program-based |
| Noise at operator | 95 - 105 dB(A) | 75 - 85 dB(A) |
| Consumables cost per 1000 blanks | Near zero (blade wear only) | Electrodes/nozzles, roughly 20 - 40 units |
| Changeover between blank lengths | 2 - 5 min back gauge | Software selection, seconds |
| Typical annual blade / consumable budget | Blade regrind 1 - 2 times | Electrode, nozzle, shield, swirl ring |
The best shear operators I have worked with do three unglamorous things. They verify the blade gap after every gauge change with a feeler gauge rather than trusting the dial. They watch the first blank of every new batch end to end, looking for the hooked top edge that signals a wide gap. And they keep the hold-down pads clean - a build-up of zinc dust under the pad lets the sheet lift mid-stroke, and a lifted sheet is how you get a burr on 30 blanks before anyone notices. None of this takes more than a minute. All of it shows in the lock former's jam rate that afternoon.
Plasma operators live by standoff and speed. Standoff held at 2 to 3 mm with a steady hand or a good height control gives a square edge; dragging the torch close gives a beveled edge and short nozzle life. Speed matters just as much: cutting 1.0 mm galvanized at 3500 to 4500 mm/min with a 60 A setup gives clean separation, and slowing down does not improve quality - it widens the kerf, piles on dross, and cooks more zinc off the edge. The instinct to "go slow for quality" is exactly backwards with plasma, and correcting it is the first thing I do when training a new operator.
The other habit is logging. A sheet on the wall with electrode changes, arc starts, and air pressure readings costs nothing and turns edge complaints from arguments into data. When a buyer questions an edge sample six months later, the log answers in seconds.
Let me work a real example. Say your workshop blanks 900 meters of coil per day in mixed lengths.
Shear route: electricity per cut is negligible, blade regrind happens twice a year, and the shear motor draws 4 to 7 kW for seconds at a time. Your real costs are labor waiting on the cycle and the back gauge. Plasma route: a 100 A source under real duty cycle pulls 15 to 25 kW including the extraction fan, and consumables run 1.5 to 3 percent of your material cost depending on how well your operators maintain standoff height. On pure electricity and consumables, the shear wins by a wide margin - roughly 8 to 15 times cheaper per blank.
But blanking cost is rarely the whole story. If plasma's software changeovers save 25 minutes per shift and its notching ability removes a separate nibbling operation, the plasma station can come out ahead on total production cost the moment your order book contains variety. For a shop doing nothing but ten thousand identical 400 mm blanks, buy the shear and never look back.
Blade gap setting per gauge is the entire skill. Gap too wide on thin galvanized gives a rolled, hooked edge that jams the lock former; too narrow and the blade edges chip. Check the gap weekly, keep the hold-downs adjusted, and grease the guides per the manual. Blades on a duct line shear last 1 to 3 years between regrinds at our volumes, and a regrind costs a fraction of a new blade set.
Plasma needs air the way the shear needs grease. Oil-free, dry air at 5.5 to 6.5 bar with a proper dryer and filter - moisture in the air line is the number one consumable killer I see in humid coastal workshops. Add daily torch inspection, standoff control, and a leads-slack management habit, and the system runs for years. Skip them and you will buy electrodes weekly while blaming the machine.
Pure straight-blank workshops with thin gauge, standard sizes, tight budgets, and shared buildings should take the shear: cheaper, cleaner edges, simpler to maintain, and its noise can be contained. Workshops with mixed orders, opening-cut requirements, thicker industrial work, or a need to feed blanks to several downstream processes will earn their money back on the plasma station's flexibility - that is the route most of our export customers now choose, and the auto duct line 5 pairs with it naturally. There is also a middle path worth asking about: shear for the standard straight runs plus a standalone plasma for special pieces, which keeps each tool in its sweet spot and spreads the investment.
Whichever you pick, test it before you pay for it: bring your own coil and your own part list to the factory acceptance, blank a full day's worth, and inspect the edges with your own eyes. Ten minutes with a file and a square tells you more than any comparison table - including the one above.
Locally, yes - the cut line loses its zinc layer over a width of 1 to 3 mm, and the exposed edge can show surface oxidation over time. In ductwork this is rarely a problem because cut edges sit inside locks and flanges, and the coating on the faces of the sheet is untouched.
Thin stainless up to about 1.0 mm cuts fine on a duct-line shear with correct blade gap, but work hardening wears blades faster than galvanized does. Above that, or for high volumes, plasma with the right electrode setup handles stainless more economically.
Count arc starts, not hours: 60 to 120 starts on thin galvanized is typical, fewer on thick or dirty coil. Keep a written log on the machine; guessing is how shops end up with a week of bad edges and no idea why.
With fume extraction, eye protection rated for the arc, and dry gloves, yes - it is routine industrial practice. The arc's UV is severe, so never let anyone watch a cut without a proper shade; that rule is non-negotiable in every shop I respect.