Cut Quality Problem - Dross in Carbon Steel

When cutting carbon steel up to 2 in. thick using optimum parameters, precision plasma systems, like the Spirit® II and ProLine™, are capable of consistently producing parts that are virtually dross free.  However, it is not uncommon for those who are new to cutting with precision plasma, or even those who have years of experience, to have encountered dross buildup and the many hours of labor expense required to grind or chip away the dross before the parts can move to the next phase of production. The ability to identify the type of dross and understand the possible cause can help the operator reduce or eliminate the problem and, as a result, save their company the time and money associated with secondary processing.

What is dross?

Dross is molten metal that does not blow away during the cutting process and instead adheres to the bottom or top of the part in a re-solidified state.  There are many factors that can contribute to the accumulation of dross.  The most common are: cutting speed, torch standoff height or damaged consumables.  In most instances, dross can be reduced or eliminated completely by adjusting the cutting speed to the optimum condition as prescribed in the operator’s manual.  However, there may be times when a simple speed adjustment is not possible or will have little effect, such as when the thickness of the material or the cutting amperage requires a slow cutting speed.  In this case, dross accumulation is inevitable and cannot be eliminated.  Also, the quality, grade and composition of the material are factors that can increase the likelihood of dross and are outside of the control of cutting parameters. For example, a lower quality sheet of carbon steel may be more susceptible to dross buildup due to the increased level of impurities.  Finally, as the temperature of the plate increases from the plasma cutting process, dross is more likely to stick to the bottom even with optimum parameters.

High Speed Dross

Cause:
When the programmed cutting speed is too fast for the amperage being used or the material thickness being cut, the bottom of the arc will lag behind the top.  When this happens, the high pressure gas found at the orifice of the nozzle is not as effective at material removal, allowing small amounts of dross to form on the bottom of the plate. High speed dross is typically dotted in appearance and cannot be removed easily by scraping with a hand tool.  It must be removed by grinding or machining the finished part.

Solution:
Verify the cutting speed matches that of published cutting charts for the selected amperage, material type and thickness.  If it already does, decrease the cutting speed in small increments (5-10 inches per minute) until the best result is achieved.

Select a lower cutting amperage.  The optimum cutting amperage for a given material thickness ideally is where the thickness is located near the middle of the range in the cutting chart.

Examine the electrode and nozzle for excessive wear and replace as needed. 

Low Speed Dross

Cause:
If the cutting speed is too slow for the material thickness or selected amperage, a solid line of dross that resembles a weld bead will form on the bottom of the part.  To understand why, remember that the plasma cutting process is electrical in nature.  When the torch is moving too slowly, the arc begins to expand in an effort to maintain contact with the edge of the kerf in order to keep its path to ground through the plate.  As the arc widens, the distance from the cut edge to the high pressure section of the plasma jet increases to the point where the gas is no longer able to blow away the material effectively.  Low speed dross is easily removed with a hand scraping tool.

Solution:
Verify the cutting speed matches that of published cutting charts for the selected amperage.  If it already does, increase the cutting speed in small increments (2-5 in. per minute) until the best result is achieved.

Select a higher cutting amperage and adjust the parameters accordingly.

Examine the nozzle and shield cap for damage and replace as needed. 

Corner Dross

Cause:
Dross will intermittently form in the corners of a part due to the speed reduction required for a cutting machine to perform an extreme change in direction, such as a right angle.  The likelihood of this occurring varies depending on the thickness of the material, cutting amperage, and material composition.

Solution:
This is a normal occurrence and cannot be avoided without altering the part drawing to include options such as corner loops.

Fortunately, the amount of dross this condition presents is minimal and is easily removed with a hand scraping tool.  

Top Dross

Cause:
Occasionally, small amounts of dross will form on the top of the part when the programmed cutting speed is too fast or the torch standoff distance is too high.  This is caused by the plasma arc’s inability to blow all the molten metal through the bottom of the kerf when the tip of the nozzle is too high above the plate.  Top dross is normally a very light accumulation that can be removed easily with a scraping tool. 

Solution:
1. Reduce cutting speed in 5 in. per minute increments, while monitoring for the introduction of low speed dross.
2. Lower the arc voltage setting in 2 volt increments.
3. Check the nozzle for damage and replace as needed.