Dross

Dross is re-solidified oxidized molten metal that is not fully ejected from the kerf during cutting. It is the most common cut quality problem of plasma cutting. Dross may form as a thick bubbly accumulation along the bottom edge of the plate (low speed dross), a small hard bead of uncut material (high speed dross) or a light coating along the top surface of the plate (top spatter).

Dross formation is dependent on many process variables including torch travel speed, standoff distance, amperage, voltage and consumable condition. It is also affected by material variables such as thickness and type of material, grade, chemical composition, surface condition, flatness, and even temperature changes in the material as it is cut. However, the three most critical variables to consider in dross formation are cutting speed, amperage, and standoff distance.

Low speed dross

If the cutting speed is too slow, the plasma jet begins to look for more material to cut. The arc column grows in diameter, widening the kerf to a point where the high velocity portion of the plasma jet no longer ejects the molten material from the cut. As a result, this molten material begins to accumulate along the bottom edge of the plate in a thick globular form. This is called low-speed dross. At extremely low speeds the arc extinguishes because there is not enough metal to sustain a transferred arc. Increasing the amperage or decreasing the standoff (while keeping material thickness and speed constant) have a similar effect on the cut as slowing down the cut speed. Both of these changes cause more energy from the plasma jet to contact a given area of the material in a given period of time. Excessive amperage or low standoff can also cause low-speed dross. (Some low speed dross in the corners of a plasma cut is normal since velocity does not remain constant through a sharp turn.)

To eliminate low speed dross

1. Increasing the cut speed in 5 ipm increments
2. Increase the standoff in 1/16 increments or 5 volt increments
3. Decrease the amperage in 10 amp increments
4. If none of these measures improve the cut, consider a smaller nozzle size

High Speed Dross

If the cutting speed is too fast, the arc begins to lag back in the kerf leaving a small hard bead of uncut material or rollover dross along the bottom of the plate. This high-speed dross is more tenacious and usually requires extensive machining to remove. At extremely high speeds the arc becomes unstable and begins oscillating up and down in the kerf causing a rooster tail of sparks and molten material. At these speeds the arc may fail to penetrate the metal or extinguish. High standoff or low amperage (for a given material thickness and cutting speed) can also cause high speed dross since both of these changes cause a reduction in the energy of the plasma jet.

To eliminate high speed dross

1. Check the nozzle first for signs of wear (gouging, oversize or elliptical orifice)
2. Decrease the cutting speed in 5 ipm increments
3. Decrease the standoff in 1/16 increments or 5 volts increments
4. Increase the amperage (do not exceed 95% of the nozzle orifice rating)