Sometimes a sheet or plate-cutting job is outside the capabilities of a laser cutting machine. If your metal fabrication project involves a challenge like that, the answer may well be plasma cutting.

Laser and plasma cutting are complementary technologies: plasma can handle jobs that lasers aren’t suitable for. This blog covers the advantages and disadvantages of plasma cutting, so readers can understand when it’s the right technology to use. It also discusses what you need to know to get the best out of the process, and offers a few safety tips.

First though, an introduction to plasma cutting and its applications.

What is Plasma Cutting?

Physicists get excited about plasma, describing it as the “fourth state of matter”. For the discussion here though, just think of it as a gas that’s hot enough to melt metal.

A plasma cutting head is somewhat similar to a MIG welding torch. An electrode creates an arc, but within the nozzle, it contains an electrode. This arc heats a stream of fast-flowing gas, which is often compressed air, or sometimes nitrogen or other inert gases, depending on the application.

The gas ionizes, essentially freeing the electrons from their atoms and raising its temperature to some 20,000°F. When this plasma touches a conductive workpiece, the current and arc are diverted from the nozzle to the material, which heats and melts it. In addition, the velocity of the gas blows the molten metal away from the melt zone.

Handheld plasma cutters are available for hobbyists, car repair shops, and so on. In the fabrication sector though, we use industrial plasma cutters.

An industrial plasma cutter is very similar to a laser cutter. The cutting head is mounted on a gantry that moves over a metal sheet or plate. With CNC handling x and y-axis motion, the head can cut out complex two-dimensional shapes composed of straight lines and curves.

This raises a question in many people’s minds: since laser and plasma cutting are so similar, why does a fabrication shop like ours need both? We can answer that by looking at the advantages and disadvantages of plasma.

Pros and Cons of Plasma Cutting

Plasma and laser cutting both have strengths and limitations. We can cover these under the headings of:

  • Material type
  • Material thickness
  • Cutting speed
  • Cut width (kerf)
  • Edge quality

Material Type

Plasma cutting needs an electrically conductive workpiece to form an arc from the electrode in the torch and create plasma. This gives rise to the first big con: unlike a laser, it can’t cut plastics, composites, or any other nonconductive material.

However, a big pro comes in terms of the materials it can cut. With laser cutting, highly reflective alloys like aluminum, stainless steel, brass, and copper reflect light away rather than absorbing it to generate heat. In contrast, plasma is indifferent to surface appearance, so it cuts these shiny metals with ease.

Material Thickness

Another con with plasma is that it’s not good with very thin sheets. The plasma is so hot that the head has to move very quickly, and the result is often a messy cut with a poor surface finish. Lasers cut thin material quickly and cleanly but have to run slower as thickness increases. The crossover point between laser and plasma is at around ½”. When the material is thicker than this, plasma is usually going to be faster.

To put this another way, the pro for plasma is that it cuts thick material faster than a laser. In fact, the upper limit, depending on alloy composition, is around 6”.

Cutting Speed

When material is less than ½” thick, laser cutting is usually faster. At greater thicknesses, the plasma cutter wins.

Cut Width (Kerf)

A limitation of plasma cutting is that it has a sizable kerf. This can be 1/8” or more, depending on the diameter of the plasma, which in turn is determined by the size of the orifice in the nozzle the gas passes through. Increasing plasma power, to cut faster and/or thicker material, requires a larger diameter orifice, so kerf width increases at the same time.

Edge Quality

Surface roughness and squareness are determined by the cutting conditions. While careful set up can achieve good levels of both, it won’t be as smooth as laser cutting.

Applications for Plasma Cutting

Plasma occupies the niche that laser cutting can’t fill. It’s used for cutting shapes from thicker material, and from reflective material. If you need blanks cutting from 1” thick stainless, we’ll probably run the job on our plasma machine.

In terms of end uses, plasma-cut material is used for structural elements, heavy-duty brackets, support arms, and frames. It’s used in furniture manufacturing, and artists and sculptors often use it to cut two-dimensional designs from sheets and plates.

Optimization Techniques for Plasma Cutting

The plasma exits the nozzle with a clockwise rotation as a result of the current generating a magnetic field. This should be considered when planning a cutting operation. For best quality on external edges, configure the direction of movement so the torch travels in a clockwise direction, cutting on the right side of the plasma. When cutting holes, the travel should be counterclockwise.

Edge squareness is optimized by remembering that the plasma has a bulb form. Thus the widest part should be positioned at the midpoint of the plate or sheet thickness. A torch that’s too high or too low will put an angle on the edge.

When the travel speed is too high the plasma tends to drag against the surface, producing a characteristic edge pattern. There will also be fine dross on the bottom edge and spatter on the top. If the speed is too low this will be evident by globular dross on the bottom surface.

Arc generation wears the nozzle, and this should be considered a consumable. Always replace nozzles and electrodes as a pair.

Plasma Cutting Machine Safety

Like all industrial equipment, a plasma cutting machine must be guarded and maintained in a safe condition. However, they have two additional characteristics to consider:

  • Electromagnetic radiation: The arc gives off the full spectrum of light, from IR to UV, at very high intensity. It needs shielding, and most importantly, anyone nearby who might be exposed should wear appropriate eye protection.
  • Fumes: Plasma cutting gives off fumes that may be hazardous, depending on the alloy and the condition of the surface. An extraction system is usually a prudent investment.

Comprehensive Capabilities for Your Fabrication Work

Visitors to our fabrication shop are sometimes surprised to see a plasma cutting machine in addition to our lasers. When we explain how the plasma does things the lasers struggle with, we see one of those lightbulb moments where they grasp how it expands the range of work we can take on.

If you’re looking for a fabrication shop that can handle a variety of materials and work, talk to us. We’ll be happy to explain our capabilities and expertise, and of course, we would welcome the opportunity to quote your project.