How To Reduce Dross and Slag During CNC Plasma Cutting

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Reducing Dross in Your CNC Plasma Cuts

What is Dross and Slag?

“Dross” refers to the unwanted accumulation of waste and foreign matter resulting from molten metal created during the plasma cut. The term “dross” is often used interchangeably with the words “slag,” and “spatter.” Whichever you choose to call it, excessive buildup of hardened metal on your cut can lead to costly downtime needed for post-cut cleanup. While dross and slag are inevitable in the CNC plasma cutting process, there are several ways to reduce them.

Types of Dross and Their Solutions

1. Top Dross and Slag

In top dross, spatter appears on the top edge of both pieces of the plate. This can be caused by using worn consumables including cutting tips that have outlasted their usefulness. Cutting the piece too fast can also introduce top dross. Incorrect torch height (or standoff) will also result in top dross accumulation.


  • Replace worn cutting tip
  • Decrease the speed of your cut incrementally until dross in minimized
  • Lower the torch height incrementally

  • Lower voltage incrementally

2. High-Speed Dross and Slag

High-speed dross occurs, as its name implies, is when the cutting speed is too high. Higher speeds causes the CNC plasma arc to fall behind the torch, leaving hard globs of uncut metal on the bottom of the plate and requiring tenacious cleanup.


  • Replace worn cutting tip, especially if the orifice shows signs of wear
  • Decrease the speed of your cut incrementally until dross in minimized
  • Lower the torch height incrementally

  • Boost current, being careful not to exceed 95% of your nozzles AMP rating

3. Low-Speed Dross and Slag

When your cutting speed is too slow, the plasma arc becomes like an hungry animal, looking for more metal to keep itself alive. The arc becomes wider, which in turn widens the cut to a point where the gas velocity from the torch can no longer blast the molten metal away. As a result, thick slag that would have been blown away forms on the bottom of the plate.


  • Incrementally increase cut speed until dross in minimized
  • Raise the torch height incrementally
  • Reduce current incrementally

As you can tell, having a worn/ineffective plasma cutting tip is one of the top causes for dross and slag appearing on your plates. Without 5-star equipment, you’ll find this problem to be recurring.

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