Plasma Cut Quality - Dross
The following is a critical examination of plasma process variables in relation to the minimization of dross. Secondary operations to remove the dross, such as chipping and sanding, are significant cost centres and as such plasma cutting must be looked at as a process, with the plasma operator aware of the repercussions of the quality of work coming from their machine.
Dross
Dross is the re-solidified molten metal from the kerf that has not been fully ejected. Dross formation is dependent on a number of variable but for the purposes of this discussion we will limit ourselves to cut speed, amperage and cut height.
Low speed dross
When the cutting speed is too slow, the plasma jet ‘looks’ for more material to cut. The arc flame grows in diameter causing a widened kerf. An effect of this is that the high velocity part of the flame does not have the necessary velocity to eject the molten material from the cut and this molten material accumulates along the bottom edge of the plate. It can appear ‘bubbly’ and is usually fairly easy to remove. At extremely slow speed the flame can be extinguished as there is insufficient metal available to sustain a transferred arc. Too low a cut height or amperage has a similar effect and can also produce low speed dross.
To eliminate low speed dross:
aIncreasing the cut speed in 100mm/min increments
aDecrease the amperage in 10 amp increments
aIncrease the standoff in 0.2mm increments or 5 volt increments
High speed dross
When cut speed is too fast the arc lags in the kerf leaving a small hard bead of material on the bottom edge of the plate. This can be distinguished from slow speed dross by its tenacious character. Its removal usually requires intensive machining. High cut height or low amperage can also cause high speed dross. The overriding idea is that at any given point in time there is too little energy from the plasma jet in relation to the material.
To eliminate high speed dross:
aCheck the nozzle first for signs of wear (gouging, oversize or elliptical orifice)
aDecrease the cutting speed in 100mm/min increments
aDecrease the cut height in 0.2 increments or 5 volts increments
Top Spatter Dross
Top spatter caused by the swirling flow of the plasma jet flinging molten metal out in front of the kerf, rather than down through it. It is easy to remove.
To eliminate top spatter dross:
aCheck the nozzle for signs of wear
aDecrease the cutting speed in 100mm/min increments
aDecrease the cut height in 0.2mm increments or 5 volt increments
Additional Indicators Used to Judge the Optimum Cutting Speed:
- Lag lines: these are small ridges in the surface of the cut and are indicative of cutting speed. Vertical lines indicate slow cutting speed. Slanted s-shaped lines running parallel to the plate along the bottom edge indicate cutting speed is too fast.
- Watch the Arc: Observe the angle of the arc as it exits the bottom of the workpiece. With air plasma gas the arc should be vertical as it exits the workpiece. With nitrogen and argon/hydrogen a slightly trailing arc is optimum and with oxygen a slight leading arc.