What is dross?
Dross, often called slag, is molten metal that re-solidifies on the bottom edge of a cut instead of being blown clear. A little is normal on some materials. Heavy dross usually means the speed, gas pressure, focus or nozzle isn't right for the material and thickness being cut.
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What does dross mean in fabrication?
When a laser, plasma or oxy-fuel torch cuts, it melts a narrow channel of metal and the assist gas blows that molten metal out through the bottom of the sheet. Dross is the part that doesn't make it out. It cools and sticks to the underside as beads, a ridge or a ragged fringe along the edge.
Shops use dross and slag interchangeably for this. In welding, slag also means the crust of flux left on top of a weld, which is a different thing.
Light dross often snaps off with a scraper. Heavy dross is fused on hard and needs grinding.
How does dross come up?
Dross is one of the first things an operator checks when parts come off the bed, because the underside of a cut shows how well the parameters suit the job. Its appearance gives clues:
- Small, loose beads that knock off easily usually mean the cut is close to right.
- Fine, hard dross that clings tightly often points to cutting too fast.
- Large droplets and an overheated, rough edge often point to cutting too slow.
Some materials are more prone to it than others. Molten aluminium is sticky and hangs on to the edge. Thick stainless steel cut with nitrogen tends to leave a fine, burr-like dross. Small parts and tight clusters of holes trap heat, so dross builds up there first.
Why does dross matter?
Dross turns a cut part into a part that needs finishing. Heavy dross has to be ground or deburred before the part can ship, and if the part is going to powder coat, paint or welding, even light dross can cause problems. That finishing is real labour, and it's easy to leave out of a quote when the part looks simple on the drawing.
It also affects fit. A ridge of dross on an edge that has to sit flat, slide into a slot or fold on a press brake can stop the part working.
Reducing dross on the machine
Most dross problems come down to a short list of settings and parts:
- feed rate too fast or too slow for the thickness
- assist gas pressure too low, or the wrong gas for the finish required
- focus position wrong for the material
- a worn, damaged or off-centre nozzle
- a dirty protective window cutting the power that reaches the sheet
- rusty, scaled or painted material
Operators usually start with the nozzle and gas because they're quick to check, then adjust speed and focus. Material choice helps too: pickled and oiled steel or cold-rolled steel cuts cleaner than heavily scaled hot-rolled plate.
Dross and quoting
Dross isn't a line on a quote, but the cleanup is. If a job is likely to need deburring (thick plate, aluminium, or parts going to powder coat), price that finishing as its own step. In Tempus Tools, deburring and other finishing can be added as secondary processes on the same quote as the cutting, so the time gets charged for.
Further reading: Are secondary processes costing you money or adding to your profit?
Related terms
Price the cleanup with the cut
Tempus Tools lets you set up deburring, grinding and other finishing as secondary processes, priced on the same quote as the cutting, so finishing time is charged for instead of given away.
FAQs
What causes dross in laser cutting?
Usually a cutting speed, gas pressure or focus setting that doesn't suit the material and thickness, or a worn or off-centre nozzle. Rusty or heavily scaled material makes it worse.
Is dross the same as slag?
In laser and plasma cutting, yes. Shops use both words for metal that re-solidifies under the cut. In welding, slag also means the crust of flux left on top of a weld.
Is dross the same as a burr?
They're related. Dross is re-solidified metal stuck to the underside of a cut, while a burr is a sharp raised edge. Both are removed by deburring.
How do you remove dross?
Light dross often knocks off with a scraper. Heavier dross is removed by grinding, a deburring machine, or tumbling for small parts. Adjusting the cut so less forms is the better long-term fix.