The short answer
A slitting line is a set of machines that cuts a wide steel coil lengthwise into several narrower strips and winds each strip back into a coil. An uncoiler pays off the coil, a feed guide leads the strip into the slitter, and the slitter cuts it with pairs of circular knives on an upper and a lower arbor, separated by spacers that set the strip widths. A scrap winder takes away the trimmed edges, a tensioning device holds the strips back, and the recoiler winds all the strips at once into slit coils.
Slitting lines are also called coil slitters, slitting machines or simply slitters. They prepare strip for tube mills, roll-forming lines and other processes that need a narrower width than the steel mill supplies. A slitting line changes only the width of the strip, not its thickness.
Why tube and roll-forming plants slit coils
Steel mills usually supply wide coils, while a tube mill needs strip of one exact width for each tube size. For round tube the strip is roughly as wide as the tube's outside perimeter: about OD × 3.14 as a theoretical value. The actual width is set by the roller design and is usually slightly less. A plant making Φ50.8 mm tube therefore needs strip of about 160 mm, and one making Φ76 mm tube about 239 mm. For square and rectangular tube, see our guide ERW Tube Mill Calculations: Strip Width and Weight per Meter.
A slitting line cuts the wide coil into exactly these widths. Plants can also buy strip that a steel service center has already slit; which route fits is discussed at the end of this guide.
The machines on a slitting line, in order
Following the strip, a slitting line has these machines:
- Uncoiler: holds the wide master coil on an expanding mandrel and pays off strip. Wide, heavy coils usually call for a single-head hydraulic uncoiler or a double top-cone uncoiler; see our guide Single- or Double-Head Uncoiler: Coil Weight and Strip Width.
- Feed guide: leads the strip head from the coil into the slitter and keeps the strip centered.
- Slitter: the cutting machine, with circular knives on an upper and a lower arbor.
- Scrap winder: winds up the narrow edge strips trimmed from both sides of the coil.
- Tensioning device: holds the slit strips back so that they wind tightly.
- Recoiler: winds all the strips at once on one mandrel into slit coils, which are then unloaded.
How the slitter cuts
The slitter works like a row of rotary shears. Circular knives are mounted on two parallel arbors, one above the strip and one below. Each upper knife is paired with a lower knife: their edges overlap slightly, with a small side clearance between them. As the strip passes between the turning knives, each pair shears it along a straight line.
The overlap and the side clearance are set to suit the strip thickness and material. If they are wrong, the cut edge can show a large burr or a rough, torn face, and the knives wear faster. Knife settings are therefore part of the setup for every slitting pattern.
Spacers sit on the arbors between the knives. Their combined width fixes the distance between neighboring knives, and so the width of each strip. Changing to a new pattern means taking the knives and spacers off the arbors and stacking them in a new order. Rubber stripper rings are often fitted beside the knives to guide the strips out of the knife gap.
Planning a slitting pattern: a worked example
A slitting pattern lists the strip widths, and how many of each, cut from one coil. All strips plus the edge trim must fit within the usable coil width.
Example, for illustration only: Φ50.8 mm round tube needs strip of about 160 mm. A 1,000 mm wide coil gives six strips of 160 mm, 960 mm in total, leaving about 40 mm for edge trim, or about 20 mm per side. One pattern can also mix widths for two tube sizes, as long as the total fits the coil.
Before the knives are set, check the pattern against the actual roller design, the width tolerance of the incoming coil and the edge trim the line needs. The number of strips per pass also depends on strip thickness and the slitter configuration, and is confirmed for each project.
Edge trim and the scrap winder
The two outermost knife pairs usually trim both edges of the coil. The mill edges of a hot-rolled or cold-rolled coil can be uneven or slightly damaged, and trimming gives every slit strip a clean, defined edge. The trimmed edges come off as thin, continuous strips, and the scrap winder winds them up so that they leave the line in a controlled way. Edge scrap is a material loss to count when you compare slitting in-house with buying slit strip.
Tension and recoiling: why the strips are held back
A wide coil is rarely exactly the same thickness across its width; the middle is often slightly thicker than the edges. After slitting, all strips are wound on one recoiler mandrel, so the slightly thicker strips build up a slightly larger diameter. Without control, some strips wind tight while others go slack, and slack strips wind loosely or drift sideways.
The tensioning device sits between the slitter and the recoiler and holds every strip back with a steady pull, so that each slit coil winds tightly and evenly. On many slitting lines a looping pit before the tensioning device lets the strips hang in slack loops of slightly different depth, which takes up the small length differences between strips.
The recoiler winds all strips at the same time on an expanding mandrel, and separator discs keep neighboring strips apart so that their edges do not overlap. When the coils reach the planned diameter, the line stops, the mandrel collapses and the slit coils are pushed off for storage, packing or the tube mill.
What decides slit-strip quality
Buyers of slit strip usually check five things:
- Width tolerance: each strip is within the agreed width range.
- Burr: the raised lip along the cut edge stays small.
- Camber: the strip runs straight along its length instead of curving sideways.
- Edge condition: no cracks, tears or edge wave.
- Coil winding: tight, even wraps without telescoping, where the wraps slide out sideways.
These depend on knife condition and settings, tension, the material and the incoming coil. Tolerance values are agreed for each project; no single figure applies to every material and thickness.
Slitting line or cut-to-length line?
Both lines start with an uncoiler, but they cut in different directions. A slitting line cuts along the coil and produces narrower coils. A cut-to-length line cuts across the strip and produces flat sheets of a set length. Tube mills and roll-forming lines run from coil, so they are fed by a slitting line, not by a cut-to-length line.
Slit your own strip or buy it slit?
The choice comes down to how many widths you run, how often they change and how much you produce:
- Buying slit strip: no slitting line, floor space or knife tooling is needed. You depend on the widths and lead times your supplier can offer, and the slitting cost is in the strip price. It suits plants that run a few regular widths at moderate volume.
- Slitting in-house: you buy wide coils and cut the widths you need when you need them, with width and edge quality under your own control. The line, floor space, crane capacity, operators, knife and spacer sets and edge scrap are your costs. Tube plants that run many sizes, change sizes often or produce at high volume usually slit in-house.
InGen MillTek FT slitting lines
InGen MillTek supplies slitting lines in five widths. The number in each model is its maximum strip width in millimeters. The values below come from our product list; coil weight and coil diameters are confirmed for each project.
- FT250: strip width up to 250 mm, thickness up to 4 mm, 70 m/min
- FT800: strip width up to 800 mm, thickness up to 4 mm, 70 m/min
- FT1000: strip width up to 1,000 mm; up to 4 mm thick at 100 m/min, or up to 8 mm thick at 30 m/min
- FT1250: strip width up to 1,250 mm; up to 4 mm thick at 100 m/min, or up to 8 mm thick at 30 m/min
- FT1600: strip width up to 1,600 mm; up to 4 mm thick at 80 m/min, or up to 8 mm thick at 30 m/min
The lines slit cold-rolled and hot-rolled carbon steel strip, galvanized strip and zinc-aluminium-magnesium (ZAM) coated strip; stainless and high-strength steel are reviewed for each project. A slitting line can be supplied on its own or together with a tube mill line: in 2018, for example, we supplied an ERW tube mill line and a 1250 slitting line to a tube producer in Central Asia (see our Projects page). Prices are quoted for each configuration.
What to send for a slitting line proposal
- Material grade, with yield and tensile strength
- Incoming coil width and thickness range, coil weight, inside and outside diameter
- The strip widths and number of strips you plan to cut (your main slitting patterns)
- Edge requirements, and the weight or outside diameter of the finished slit coils
- Required line speed or annual output
- Workshop space, crane capacity, and local supply voltage and frequency
Frequently asked questions
Prepared by the InGen Engineering Team. Values are reference values and are confirmed in the technical proposal and technical agreement.
- What is the difference between a slitter and a slitting line? The slitter is the cutting machine that carries the knives. The slitting line is the whole set: uncoiler, feed guide, slitter, scrap winder, tensioning device and recoiler.
- Does slitting change the strip thickness? No. A slitting line changes only the width; the thickness stays as supplied.
- How many strips can one coil make? That depends on the coil width, the strip widths, the edge trim, the strip thickness and the slitter configuration. Send us your slitting patterns and we will confirm them.
- How fast does a slitting line run? Our FT lines run at 70 to 100 m/min for strip up to 4 mm thick, depending on the model, and at 30 m/min for heavy-gauge strip up to 8 mm. The speed you can actually run depends on material and pattern.
- Do I need my own slitting line to run a tube mill? No. Many plants buy slit strip. A slitting line of your own makes most sense when you run many widths, change them often or produce at high volume.
- Can a slitting line cut stainless or high-strength steel? These materials need suitable knives and settings, so we review them for each project.
- What does a slitting line cost? It is configured to coil size, slitting patterns and speed, and quoted individually.
About this article
Prepared by the InGen Engineering Team. This is a general technical guide, not a specification or performance guarantee for any project. Final parameters are set in the project's technical proposal and technical agreement.