At least once a week a buyer sends me the same message: "Line 3 or line 5 - which one should we get?" It is a fair question, and the honest answer is that it depends far more on your order mix than on your budget. Photos of both lines look similar. You see a decoiler, a leveller, a cutting station, a Pittsburgh lock former, a TDF flange former, a folder, and a beading unit. But the way work flows through them is not the same at all, and that difference decides whether the machine pays for itself in eighteen months or sits half idle.
The short version: a auto duct line 3 is a sequential line where one operator stays at the machine and the sheet advances station by station. A auto duct line 5 splits the work into parallel stations - the lock former and the flange former run as independent units, often with a second operator, so two operations happen at the same time instead of one after the other. Everything else in this article follows from that single design choice.
On a line 3 the coil is decolied, leveled, cut to length, and then the blank travels forward into the lock former, then the flange former, then the folder. Each stage waits for the previous one. When the folder is bending the last corner of a 1200 mm long blank, the lock former sits empty. When the lock former is running a 250 mm wide piece, the cutter waits. The line only moves as fast as its slowest current station, and on short pieces that slowest station is usually the handling, not the forming.
In practice a well-run line 3 produces a finished L-shaped or fully formed blank every 20 to 30 seconds on common sizes like 400x300 mm in 0.8 mm galvanized. That works out to roughly 120 to 170 pieces in an eight-hour shift with breaks and coil changes. Not bad. But it is a ceiling you can feel as soon as order volume grows.
The line 5 design separates the lock forming and the flange forming from the main transport line. While the Pittsburgh seam is being closed on one blank, the TDF flange is already being rolled onto the next one, and the folder handles the elbows and squares. The coordination happens through a transfer table and the control system, not through a single operator walking back and forth.
The throughput gain is not the naive "double" that some sales brochures imply. Coils still change, blanks still travel, the folder is still one machine. What I have seen on shop floors is a realistic 1.5 to 1.8 times the output of a line 3 for the same product family - so that 120 pieces per shift becomes 190 to 220. On long straight runs of standard rectangular duct, where changeovers are rare, the gap widens further because both parallel stations stay loaded.
Here is a comparison based on the configurations we ship most often, with 0.5 to 1.2 mm galvanized coil as the reference material:
| Item | Auto Duct Line 3 | Auto Duct Line 5 |
|---|---|---|
| Working mode | Sequential, single operator | Parallel lock + flange stations |
| Sheet thickness range | 0.5 - 1.2 mm | 0.5 - 1.5 mm |
| Blank length range | 300 - 1500 mm | 300 - 2500 mm |
| Cycle time, 400x300 duct, 0.8 mm | 20 - 30 s | 12 - 18 s |
| Realistic output per 8 h shift | 120 - 170 pcs | 190 - 240 pcs |
| Operators | 1 (+1 crane/loader optional) | 2 |
| Installed power | About 12 - 15 kW | About 18 - 24 kW |
| Minimum floor length | 9 - 11 m | 13 - 16 m |
| Changeover between sizes | 3 - 6 min | 5 - 10 min (two stations to reset) |
Two rows in that table deserve a second look. First, changeover: the line 5 is actually slower to switch between duct sizes, because you reset and re-zero two forming stations instead of one. If your week is a patchwork of fifty-piece orders in different sizes, that penalty eats into the throughput advantage quickly. Second, floor length. I have visited workshops that bought a line 5 and then had to install it in an L, which costs transfer time and invites handling damage. Measure your building before you measure your budget.
If you serve mechanical contractors on mid-size projects - a hotel here, a clinic there - your daily reality is dozens of size changes. A line 3 with one sharp operator who knows the size table by heart will beat a bigger machine that spends half the morning resetting. The line 3 also trains easier: one person can learn the full sequence in two weeks and own the whole process, which matters a lot in markets where skilled labor is scarce.
For many workshops the sensible path is a line 3 now and a second shift later. Doubling shifts doubles output without touching the building, and the machine is fully paid for before you even think about more steel.
If your orders are dominated by standard rectangular duct - straight lengths, repeated sizes, project after project of the same 400, 500, 600 mm families - then the parallel stations of a line 5 stop being a burden and become pure profit. Changeovers become rare events, the two operators settle into a rhythm, and the extra 60 to 90 pieces per shift drop straight to the bottom line. Buyers who feed a line 5 from one or two large annual contracts are the ones who never regret the purchase.
There is also the thickness ceiling. If you are asked for 1.2 to 1.5 mm flanged duct for industrial work, a line 5 folder and flange former handle it with more margin than a line 3 that was spec'd for thin commercial gauge. Pushing a line 3 to its upper thickness limit day after day is how you wear out tooling early.
Some buyers eventually ask about the U-shape auto duct line 5, where the line folds back on itself to fit the same work in a shorter room. The production logic is the same as the linear line 5; the trade is 3 to 4 meters of floor length against a slightly more complex transfer and one more alignment point to keep square. In long, narrow rented workshops it is often the only sensible layout, and in purpose-built halls the linear layout is simpler to maintain. Neither choice changes throughput meaningfully - I have timed both.
Let me put rough numbers on it. Assume galvanized at 0.9 mm, a mixed order book, one shift, labor at whatever your local rate is - the ratios matter more than the currency.
On a line 3, one operator producing 140 pieces per shift means labor cost per piece is one operator-shift divided by 140. On a line 5, two operators producing 210 pieces means two operator-shifts divided by 210. Per piece, the line 5 labor is actually slightly higher - about 1.3 times - but your overhead, your rent, and your machine depreciation are spread over 50 percent more pieces. Add the faster delivery promise you can now make, and the line 5 wins on projects where the buyer cares about schedule. It loses on tiny varied orders where its changeover penalty dominates. That is the whole decision in two sentences.
Electricity follows the same logic: the line 5 draws more installed power, roughly 18 to 24 kW against 12 to 15 kW, but most motors on either line run intermittently. Measured consumption per finished piece usually comes out within 10 percent of each other, which surprises people.
Whichever line you buy, budget real time for leveling. Both lines need a flat floor - we ask for level within 3 mm over the line length - and a line 5 with its longer footprint doubles every leveling error. An out-of-level line drifts blanks toward one side, which shows up as flanges formed 1 or 2 mm off square and folders that need re-zeroing every month. Two days of shimming at installation is cheaper than two years of small rework.
Decoiler capacity matters more than most people expect. A 3-ton decoiler holds a coil of 0.8 mm galvanized about 500 meters long; a busy line 3 changes coils two or three times a shift, and a line 5, running faster, changes more. If your building layout allows a coil car or a second decoiler position, the payback in avoided line stops is weeks. Also check your doorway: a 1600 mm wide coil will not enter a 1500 mm door, and I have watched a crew unload a coil with a forklift through a window because nobody measured.
Not really. The parallel stations need their own drives, transfers, and control coordination designed in from the start. What you can do on a line 3 is add a standalone folder or a duplex TDF flange former beside it and run forming as a second step. It works, but it costs you the automation between stations.
Yes, with mixed orders expect 1.5 to 1.8 times. The double figure only appears with long runs of identical sizes where changeovers vanish. Any salesman who promises a clean 2x is quoting you the best case as if it were the average.
Neither line makes round duct. If your market needs round, that is a spiral tube machine and elbow maker - a separate investment and usually a separate corner of the shop. Keep the two product lines apart when you plan your floor.
Lock former rollers and shafts, TDF tooling sets, plasma electrodes and nozzles if fitted, shear blades, and a full set of belts and proximity sensors. One spare parts crate covers 90 percent of first-year stoppages, and the supplier should pack it with the machine before shipment.