Cut Quality Problems: Dross / Mill Scale

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Dross — what it is and why it forms

Dross is solidified, oxidized molten metal that remains in the kerf after the laser beam passes through. It is the most common laser-cutting quality problem. Dross can form as:

large bubble-like deposits along the bottom edge of the sheet — low-speed dross;
small solidified strips of uncut material — high-speed dross;
a thin coating on the top surface of the sheet — top spatter.

Dross formation depends on many process variables: cutting head travel speed, nozzle-to-workpiece standoff, gas pressure, voltage, and the condition of the consumables. It is also affected by material properties — thickness, type, grade, chemical composition, surface condition, flatness, and even temperature changes in the material during cutting. The three parameters that influence dross formation most strongly are cutting speed, gas pressure, and nozzle-to-workpiece standoff.

Low-speed dross

If the cutting speed is too low, the laser beam starts to "search" for additional material to cut. Insufficient gas pressure fails to blow molten material out of the kerf. As a result, the melt accumulates along the bottom edge of the sheet as thick spherical deposits — this is low-speed dross. Excessive power means that more energy is deposited in the cutting zone at a given instant. Too high a power level or too short a standoff between the cutting head and the workpiece can also cause low-speed dross.

To eliminate low-speed dross:

Increase the cutting speed.
Increase the nozzle-to-workpiece standoff.
Reduce the power.
If none of these actions improves the cut quality, consider using a smaller nozzle.

High-speed dross

If the cutting speed is too high, the cut begins to lag along the kerf, producing small solidified strips of uncut material or "rolling" dross along the bottom edge of the sheet. High-speed dross is more resistant — its removal usually requires heavy mechanical post-processing. At excessive speeds, cutting becomes unstable: the length of the beam column inside the kerf alternately grows and shrinks, producing a "longitudinal ridge" of sparks and molten material. The beam may fail to pierce the metal through its full thickness, or it may extinguish.

A large nozzle-to-workpiece standoff or low power (for the given material thickness and cutting speed) can also cause high-speed dross.

To eliminate high-speed dross:

Check the nozzle for signs of wear (nicks, oversized hole, or an oval orifice).
Reduce the cutting speed.
Reduce the nozzle-to-workpiece standoff.
Increase the power.

Dross caused by top spatter

Top spatter is an accumulation of solidified metal that is splashed along the top surface of the workpiece. This type of dross is easy to remove. Causes: cutting speed too high, or standoff between the cutting head and the workpiece too large. It is produced by a swirling plasma plume that, at a certain angle of attack, pushes molten material out of the front of the kerf rather than driving it down into the kerf.

To eliminate top spatter:

Check the nozzle for signs of wear.
Reduce the speed.
Reduce the nozzle-to-workpiece standoff.

The dross-free window

Between the high and low cutting speeds at which dross forms, there is a specific range (window) of cutting speeds where dross does not form, or forms only minimally. Finding this window is the central task in minimizing the rework needed for parts cut on laser systems.

This window depends on the gas. For example, nitrogen and air give a relatively narrow dross-free window when cutting mild steel, whereas with oxygen the window is wider. (Oxygen reacts with the mild steel, producing small spatter droplets; each droplet has lower surface tension and is more easily blown out of the kerf.)

The dross-free window also depends on the material type. For example, cutting cold-rolled or pickled steel produces less dross than cutting hot-rolled or non-pickled steel.

To find the optimum cutting speed:

Method 1. Make several consecutive cuts at different speeds and pick the one with the least dross. Drag lines (the small ridges on the cut face) are a good speed indicator. At low speeds the drag lines are vertical and perpendicular to the sheet plane; at high speeds they are inclined, S-shaped, and run parallel to the sheet along the bottom edge. By reading the drag lines, the operator can tell whether to increase or decrease speed in order to find the dross-free window. Many operators habitually slow the machine down as soon as dross appears — yet quite often the correct action is to increase speed.
Method 2. Watch the beam during cutting and vary the speed significantly to obtain the optimum beam-arc characteristics. Observe the angle at which the arc exits the workpiece. If the assist gas is air, the arc should be vertical as it leaves the bottom of the sheet. If the gas is nitrogen, a slight backward lag of the arc is best. For oxygen, the optimum speed is the one at which the arc leads slightly forward.