Plasma dross is oxidized molten metal which is still attached to the outer edges of a part during the cutting process. Dross is the result of many factors including cutting speed, arc current (amperage), standoff distance, gas selection, and the metal being cut along with its thickness. Essentially, cutting speeds that are too high or too low cause dross, but there lays a window in between the two extremes for achieving “dross-free” cuts. Finding this window is essential to reducing the need for a secondary process to finish off the plasma cut piece.
Low speed dross forms when the cutting speed is too slow causing the arc column to grow in diameter. The kerf is widened to the point where it gets harder for the pressurized jet to blow away the molten metal. Low speed dross is thick, porous, and very easy to remove. The slow speed puts more heat into the material than necessary creating a larger heat-affected zone and top edge rounding.
In comparison, high speed dross results when using a higher cutting speed without increased amperage. The arc begins to lag back in the kerf, bevelling the cut, and leaving a small portion of uncut metal along the bottom of material. When this occurs, the arc becomes unstable and creates dross that is difficult to remove.
Top spatter is a light layer of dross that forms along the top edge of the cut piece. In addition to the usual factors that create dross, top spatter may be the result of a worn nozzle, extreme cutting speed, or a high standoff.