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Rectangular Downpipe Machines: Lock-Seam Box Section
Table of Contents
1.Definition
A rectangular downpipe roll forming machine turns coated steel coil into a closed box pipe for rainwater systems. The strip is folded through a series of chain-driven stands into two flat faces and two corners, the two edges meet on one face and are closed into a single lock seam, and the pipe is cut to length with a cold saw.
It is a single-purpose line: coil in, box pipe out. A typical build includes a manual cantilever decoiler, a run-out table, a hydraulic power unit and a PLC cabinet with touchscreen. The section is specified by width, height, corner radius and wall thickness, not by a single diameter.
2.What this is not
● It is not a round downpipe machine. A circle is rolled, not folded, and its seam closes at the top of the section; a box line bends flat faces and corners and closes the seam along a face. The two roll sets cannot be interchanged.
● It is not a square downpipe line either, although the two look alike at a glance. An equal-sided square section needs its own corner sequence and its own rolls; a rectangle is a different drawing and a different roll set.
● It is not a gutter machine. A gutter is an open channel with a bead and a hanger zone, formed on rolls that cannot produce a closed tube, and cut with a different head.
● It is not a welded tube mill. There is no weld, no edge preparation and no post-weld treatment here; the joint is a mechanical lock seam. That is what allows thin coated coil to be used without burning the coating at the joint.
● It is not an elbow machine. Elbows and reduced ends are made on a pipe bender, normally about 2.2 kW and up to roughly 90°, quoted as a separate auxiliary. End caps, outlet funnels and brackets are stamped or folded fittings, not roll formed parts.
3.How it is formed
● The coil is loaded on the decoiler and the strip is fed through a side guide that squares it to the first stand and holds the seam line on the face that will close.
● The first stands bend the two long faces while the strip is still open, so the section looks like a wide channel with a lip on each side.
● Later stands fold the two corners to the radius on the drawing. At this point the section is a nearly closed box, still open along one face where the two edges will meet.
● The closing stations bring those edges together and fold them into a single lock seam. Folding on a flat face rather than on a corner keeps the corner radius from being pulled out of shape, which is why most box downpipes carry the seam on a face.
● The cold saw, held by a clamp, cuts the pipe to the ordered length. A saw is used rather than a shear because a shear can collapse the box at the cut; a saw leaves square corners and a small burr that cleans up with a file.
● Cut pipes run out onto the table and are stacked and bundled. Thin wall box pipe dents at the corners easily, so it is nested rather than dropped.
4.Published numbers on this mill
The figures below belong to this single rectangular downpipe build and should be quoted together, not mixed with other drainage machines.
| Item | Value on this quotation |
|---|---|
| Section | Closed box, single lock seam folded on one face |
| Size class | 76.8 × 105 mm; width, height and corner radius confirmed on the signed drawing |
| Working speed | 8–10 m/min |
| Main motor | 4 kW |
| Hydraulic power | 3 kW |
| Shaft | φ60 / φ70, per quotation |
| Cut | Cold saw with clamp, square end |
| Voltage | 380 V / 60 Hz / 3-phase |
| Control | PLC with touchscreen |
5.FAQ
Is a rectangular downpipe stronger than a round one?
They resist different things. A round pipe resists internal pressure and bending evenly; a rectangular pipe has flat faces that dent more easily but sits flat against a wall and takes less space. The wall thickness matters more than the shape for dent resistance.
Can the same machine make a round pipe?
No. Rounding needs a different roll set and a different closing sequence. A plant that sells both keeps two roll sets or two machines.
Why put the lock seam on a face instead of a corner?
Closing on a flat face does not disturb the corner radius, and the seam ends up on the side that faces the wall. A corner seam is harder to close cleanly and shows on the visible face.
Does a cold saw slow the line down?
The saw cuts in a controlled cycle, so the published 8–10 m/min is the forming speed, not a promise about pieces per hour. Piece rate depends on the cut length.
What has to be sent with an enquiry?
The section drawing with width, height, corner radius, wall thickness and seam position; the material and coating; the ordered lengths; the coil weight and inner diameter; the voltage and frequency at the destination; and whether elbows or reduced ends are needed.
