Wire breaks. It is the most common problem on a drawing line, and it is also the one where the operator usually gets blamed first.
In most cases the machine is fine and the operator is doing what they were trained to do. The cause sits somewhere else in the setup. Here are the ones we see most often, roughly in the order they are worth checking.
1. The die sequence asks too much of the wire
Every pass reduces the cross section, and every reduction adds work hardening. Published practice puts area reduction at around 15–25% per pass for small wire and 20–45% for larger wire. Working out the right pass schedule for your material comes before anything else on this list. Push past that and the wire surface starts cracking before the die has finished its job.
The tell is where the break happens. Wire that breaks at the last pass or between the last two capstans is usually being asked for more reduction than the material will take at that point in the sequence. Wire that breaks at the first pass is a different problem — usually surface condition or die entry.
A pass schedule that looks fine on paper can still be too aggressive for a particular heat of steel. If your supplier gave you a schedule for the machine but not for your material, the schedule may need adjusting.
2. The incoming wire is not what the specification assumed
Machine data sheets state an inlet strength ceiling, commonly ≤1250 MPa on a general straight-line series. That number is not a suggestion. Wire above it will not run at the published speed, and it will break.
Incoming wire also varies between suppliers and between coils. Two coils stamped with the same grade can differ in surface condition, scale, and residual stress from the rod mill. A line that runs clean on one coil and breaks on the next is usually reacting to that variation rather than to anything the operator did.
If the breaks cluster on certain coils, check the wire before you touch the machine settings.
3. Scale and surface defects reaching the die
Hot rolled wire arrives with scale on it. Drawing that scale through a die does two things: it wears the die bore unevenly, and it drags broken scale into the lubricant where it scratches the wire.
To help you plan for it, the relevant equipment is the descaling, peeling and coating range. Read that section before you accept a quotation that leaves it out. It is one of the most expensive places to save money on a line. A peeling or shelling machine, a sand belt descaler, or a mechanical descaler all do the same job at different levels of capital cost.
If your line has no descaling provision and you are buying broken dies and broken wire, that is where to look first.
4. Lubrication and coating failure
The carrier coating holds lubricant to the wire surface. Boronization and similar coating processes exist because dry lubricant alone does not stay on the wire at the pressure and temperature inside a die.
When the coating is too thin, uneven, or not fully dried, lubricant film breaks down at the die entry. Friction rises, temperature rises, and the wire fails. The same thing happens if the coating bath chemistry drifts and nobody checks it.
Drying temperature and line speed interact here. Run faster than the drying section can handle and the coating is still wet when it reaches the first die. That is a machine setting problem, not a wire problem, but it shows up as broken wire.
5. Die condition and die matching
A worn die no longer has the profile it was made with. The bearing length shortens, the approach angle effectively changes, and the wire is no longer being shaped the way the schedule assumed.
Dies wear at different rates depending on material, reduction, and lubrication. A die that has run its life in one pass position may be fine for a lighter duty elsewhere, but it should not stay in the same position because it still looks serviceable.
Tungsten carbide dies are consumable and are usually quoted separately from the machine. If a quotation includes dies without saying how many or of what grade, that is worth clarifying before you compare prices.
6. Tension and speed mismatch between passes
Multi-pass machines hold tension by running each capstan slightly faster than the one before it. That relationship has to stay correct, and it has to stay correct at every speed.
If one capstan is running too fast relative to its neighbour, it pulls harder than the pass was designed for. If it is too slow, wire accumulates and can cross over itself on the capstan. Both end in a break, and both look like operator error because the machine appears to be running normally right up to the moment it fails.
This is worth checking when breaks happen at a specific line speed but not at others. Variable frequency drives make speed control easy, but the settings still have to match the pass schedule, and they need rechecking after any die change.
What to check before you change anything
When a line starts breaking wire, the useful order is: wire first, dies second, coating third, machine settings last. Machine settings are the easiest thing to change and the easiest thing to change wrongly.
Some specifics worth having on hand:
- The exact inlet diameter and strength of the coil that broke, not just the nominal grade
- Which pass the break occurred at, and whether it was consistent
- Die history — how many tonnes each die has run since it was last changed
- Whether the coating bath is being checked, and against what specification
- Whether the break started at a speed change or during steady running
Most breakage problems resolve once one of these is pinned down. The ones that do not usually turn out to be a pass schedule that was written for a different material and never revisited.
If you are chasing a breakage problem and want a second opinion on the pass schedule, send us the wire specification, the die sequence and where the break happens. We can tell you whether the schedule matches the material you are running.
If the problem turns out to be a machine that was never specified for your material, buying machinery from China covers what should have been agreed before the order was placed.
