Saturday, November 26, 2022

Comparison of mechanized thermal cutting technologies

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The various options offered by the three thermal cutting technologies (oxyfuel, laser and plasma) often make it difficult to decide which mechanized process to adopt in practice. There are many variables to take into account, such as the type and thickness of the material and the geometry of the pieces to be cut, the quality and precision required for cutting, the heat input during cutting, special care when subsequent welding must be carried out and obviously the economic aspect.

In many cases, the mechanized plasma cutting It has an advantage over oxyfuel and laser. To understand why and for the purposes of establishing an operational tripod based on quality, productivity and cost, it is important to focus our attention on a series of parameters that we will develop later.

But first, let’s briefly review some fundamental concepts about each of these techniques.

flame cutting

Oxyfuel cutting machines do not require electricity to operate, but instead use chemically generated heat to increase the temperature of the workpiece until it melts; then the molten metal is expelled with the addition of oxygen. The general process can be summarized in the following figure.

What is the Oxycut process like?

The most common gas used is acetylene, but propylene, liquefied petroleum gas (LPG), propane, natural gas and hydrogen can also be used. The oxygen combines with the glowing metal, oxidizing it and turning it into slag. The operator must regulate the gases for each cut, which requires some skill and training. For cutting different materials and/or material thicknesses, various techniques are applied that use different types of gases in their mixtures.

Laser cut

Lasers transmit energy in the form of coherent photons into a chamber with two mirrors at its ends, where the light radiation bounces back and amplifies. The material is vaporized and expelled by a jet of gas, leaving an edge with a high-quality surface finish (see the process schematic in the figure below). The strongly oriented energy of a laser can produce very narrow widths of cut, especially in thin materials.

How is the laser cutting process?

How is the laser cutting process?

Plasma cutting

In these systems, an ideal temperature is used to heat and melt the material by means of a jet of ionized gas (plasma). After melting, this same jet expels that molten material from the bottom. If the nozzle through which the gas current passes is extremely reduced, the so-called “constriction of the arc” originates, with which the resistance will be increasing and causing greater heating of the gas; this allows the achievement of very high temperatures and speeds similar to that of sound. Below is a schematic representation of the plasma cutting process.

What is the Plasma cutting process like?

How is the Plasma cutting process?

Comparison between systems

To evaluate the performance of each mechanized thermal cutting process, experts recommend emphasizing the following highly relevant factors.

to) quality factors

  • Cut quality: How clean and square is the final cut? How much residue is left after cutting? Is any subsequent grinding required?

b) productivity factors

  • Previous work: How much cleaning and pre-work is needed before a material can be cut?
  • Cutting speed: what is the speed of the cutting process?
  • Flexibility: Can you cut different types of materials, perform different types of cuts and different thicknesses of material? Is it a technique that can be applied manually?

c) Operating cost factors

  • Back Finish: What operations are required after cutting is complete and how long do those operations take?
  • Maintenance: Is it difficult to maintain or repair the system? Can this task be carried out by the operator himself or by the internal maintenance team of an industry?
  • Initial cost: How much does it cost to acquire the cutting table and consumables?

Once we set these parameters, we can establish the comparison between oxyfuel, plasma and laser, evaluating how each process behaves against those previously defined parameters.

flame cutting

  • Cut quality
    • Good angularity.
    • Extensive heat-affected zone (HAZ) deformation in thin plates.
    • Slag levels that require a lot of post work.
  • previous work
    • Oxyfuel cutting machines have to preheat the workpiece before cutting it.
    • The cut area must be free of rust, dirt and/or paint before making the cut.
    • The operator must regulate the gas flow for each torch.
  • cutting speed
    • It is slow over a wide range of thicknesses.
    • Preheat time significantly increases piercing times, slowing down the overall speed of the cut.
    • To compensate for lower cutting speeds, oxyfuel tables often have several moving torches.
  • Flexibility
    • Oxyfuel is limited to mild steel and is not effective on stainless steel or aluminum.
  • back finish
    • The operator may need to grind the heat affected zone, which is larger than with other systems; this process can be slow and difficult.
  • Maintenance
    • Simple requirements that can often be performed by company maintenance groups.
  • Initial cost
    • Oxyfuel cutting has the lowest initial cost of all thermal cutting processes.

To be

  • Cut quality
    • Excellent angularity.
    • Small heat affected zone.
    • Virtually no slag.
    • Good to excellent dimensional accuracy on the tightest cuts.
  • previous work
    • For the laser to work, the material has to be clean.
  • cutting speed
    • Very fast on thin material and slower on thicker material.
    • Requires more drilling time in thick material.
  • Flexibility
    • The laser is the best system for cutting thin carbon steel.
    • With a single pass you can produce final cuts in both directions. This reduces or eliminates parts that are scrapped from the metal sheets after parts are cut into them.
    • Manual laser systems do not exist, so the workpiece must be placed horizontally on a table.
    • Cutting reflective material (aluminum) requires prior work to cover the surface of the material.
  • back finish
  • Maintenance
    • Complex tasks that require specialized technicians.
  • Initial cost
    • The laser has the highest initial cost of all thermal cutting systems.
    • There are models of laser cutting tables that cost ten times more than a plasma cutting table.

Plasma

  • Cut quality
    • Good to excellent angularity.
    • Small heat affected zone.
    • Virtually no slag.
    • Good to excellent fine cut characteristics.
  • previous work
    • Little or no preparation.
    • Tolerates paint, dirt, rust and/or oil.
  • cutting speed
    • Fast in a wide range of thicknesses.
  • Flexibility
    • Cuts a wide variety of thicknesses and types of material.
  • back finish
    • Little or no grinding, typically much less than flame cutting.
  • Maintenance
    • Moderate requirements.
    • Many components are repairable by an industry’s internal maintenance team.
  • Initial cost
    • Typically between oxyfuel and a laser.

Conclusions

From the above we can see that, among the mechanized thermal cutting systems, plasma cutting brings together the greatest number of benefits, because:

  • It has the highest cutting speed in any thickness.
  • Although it does not offer optimum cut quality like lasers, it can achieve excellent cuts suitable for a wide variety of applications.
  • Virtually no pre-work is required as the workpiece does not need to be preheated and there is no need to clean it before cutting.
  • Like laser cutting, and unlike flame cutting, it delivers perfectly clean parts that do not require any type of subsequent finishing.
  • The flexibility it offers is unique. For example, in mechanical workshops and applications in the automotive industry in general, where many manual operations are required that do not allow the use of laser cutting and, obviously, combustible processes such as oxyfuel, plasma cutting cannot be used. It is a highly recommended option.
  • Like oxyfuel, the low maintenance requirements of plasma cutting equipment again make it a favored resource, offering better performance than an oxyfuel table for a much lower maintenance and repair cost than a gas table. Laser cut.
  • Consistent with the above, the market price of a CNC plasma cutting table including ancillary equipment such as water filters and power supplies is less than a quarter of what a laser cutting table costs.

For further illustration, we can attend a comparative demonstration of the results of oxyfuel and plasma in the following video.

The multitude of advantages of mechanized plasma cutting equipment They place them in a privileged place. That is why we will advance with more information on the subject, providing, in future articles on welding, a series of guides that will help us select the plasma cutting system, table and software that can guarantee us the most satisfactory results and the increased productivity in our work.

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