Plasma - The Fourth State of Matter
One common description of plasma is to describe it as the fourth state of matter. We
normally think of the three states of matter as solid, liquid and gas. For a common
element, water, these three states are ice, water and steam. The difference between these
states relates to their energy levels. When we add energy in the form of heat to ice, the
ice melts and forms water. When we add more energy, the water vaporizes into hydrogen and
oxygen, in the form of steam. By adding more energy to steam these gases become ionized.
This ionization process causes the gas to become electrically conductive. This
electrically conductive, ionized gas is called a plasma
(see Figure 1).
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Figure 1
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Plasma - The Cutting Process
The plasma cutting process, as used in the cutting of electrically conductive metals,
utilizes this electrically conductive gas to transfer energy from an electrical power
source through a plasma cutting torch to the material being cut.The basic plasma arc cutting system consists of a power supply, an
arc starting circuit and a torch. These system components provide the electrical energy,
ionization capability and process control that is necessary to produce high quality,
highly productive cuts on a variety of different materials.
The power supply is a constant current DC power source. The
open circuit voltage is typically in the range of 240 to 400 VDC The output current
(amperage) of the power supply determines the speed and cut thickness capability of the
system. The main function of the power supply is to provide the correct energy to maintain
the plasma arc
after ionization.
The arc starting circuit is a high frequency generator
circuit that produces an AC voltage
of 5,000 to 10,000 volts at approximately 2 megahertz. This voltage is used to create a
high intensity arc inside the torch to ionize the gas, thereby producing the plasma.
The Torch serves as the holder for the consumable nozzle
and electrode, and provides cooling (either gas or water) to these parts. The nozzle and
electrode constrict and maintain the plasma jet.
Sequence of Operation
The power source and arc starter circuit are connected to the torch via interconnecting
leads and cables. These leads and cables supply the proper gas flow, electrical current
flow and high frequency to the torch to start and maintain the process.
l. A start input signal is sent to the power supply. This
simultaneously activates the open circuit voltage and the gas flow to the torch (see
Figure 2). Open circuit voltage can be measured from the electrode (-) to the nozzle (+).
Notice that the nozzle is connected to positive in the power supply through a resistor and
a relay (pilot arc relay), while the metal to be cut (workpiece) is connected directly to
positive. Gas flows through the nozzle and exits out the orifice. There is no arc at this
time as there is no current path for the DC voltage.
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Figure 2
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2.
After the gas flow stabilizes, the high frequency circuit is activated. The high frequency
breaks down between the electrode and nozzle inside the torch in such a way that the gas
must pass through this arc before exiting the nozzle. Energy transferred from the high
frequency arc to the gas causes the gas to become ionized, therefore electrically
conductive. This electrically conductive gas creates a current path between the electrode
and the nozzle, and a resulting plasma arc is formed. The flow of the gas forces this arc
through the nozzle orifice, creating a pilot arc. (see Figure 3).
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Figure 3
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3.
Assuming that the nozzle is within close proximity to the workpiece, the pilot arc will
attach to the workpiece, as the current path to positive (at the power supply) is not
restricted by a resistance as the positive nozzle connection is. Current flow to the
workpiece is sensed electronically at the power supply. As this current flow is sensed,
the high frequency is disabled and the pilot arc relay is opened. Gas ionization is
maintained with energy from the main DC arc (see Figure 4).
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Figure 4
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4. The
temperature of the plasma arc melts
the metal, pierces through the workpiece
and the high velocity gas flow removes the molten material from the bottom of the cut
kerf. At this time, torch motion is initiated
and the cutting process begins
(see Figure 5).
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Figure 5
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Variations of the Process
1. Conventional Plasma Cutting.
This process generally uses a single gas (usually air or nitrogen) that both cools and
produces the plasma. Most of these systems are rated at under 100 Amps, for cutting
materials under 5/8" thick. Such systems
are primarily used in hand held applications
(see Figure 6).
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Figure 6
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2.
Dual Gas Plasma Cutting.
This process utilizes two gases; one for the plasma and one as a shield gas. The shield
gas is used to shield the cut area from atmosphere, producing a cleaner cut edge.
This is probably the most popular variation,
as many different gas combinations can be used to produce the best possible cut quality on
a given material (see Figure 7).
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Figure 7
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3.
Water Shield Plasma Cutting.
This is a variation of the dual gas process where water is substituted for the shield gas.
It produces improved nozzle and workpiece cooling along with better cut quality on
stainless steel. This process is for
mechanized applications only
(see Figure 8).
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Figure 8
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4.
Water Injection Plasma Cutting.
This process uses a single gas for plasma and utilizes water either radially or swirl
injected directly into the arc to greatly improve arc constriction, therefore arc density
and temperatures increase. This process is used from 260 to 750 amps for high quality
cutting of many materials and thicknesses. Mechanized applications only (see Figure 9).
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Figure 9
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5.
Precision Plasma Cutting.
This process produces superior cut quality on thinner materials, (less than 1/2") at
slower speeds. This improved quality is a result of using the latest technology to super
constrict the arc, dramatically increasing energy density. The slower speeds are required
to allow the motion device to contour more accurately. Mechanized applications only
(see Figure 10).
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Figure 10
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Effect of Various Plasma
Shield Gases
There are many different plasma and shield
gas combinations available. These different combinations can be used to enhance the
cut performance on different materials and applications. Be sure to consult with the
plasma system manufacturer to ensure that the system is compatible with a particular gas
combination. Good cut quality is usually defined as perpendicular, smooth and dross-free
edges.1. Air.
Air is the most widely used plasma gas, probably due to the fact that compressed air is
readily available at virtually any location. This gas is used in most conventional and
dual flow systems under 200 amps. Consumable parts life is acceptable (usually between 100
and 200 starts). Cut quality is acceptable on most materials, although some surface
nitriting can occur on carbon steel, and some oxidation
can occur on aluminum and stainless steel.
2. Nitrogen.
This gas, when used in conjunction with Water Injection, produces the best cut qualities
on aluminum and stainless steel. Good cut quality can be expected with dual gas on
stainless and aluminum. Cut quality on most carbon steels is marginal due to surface
nitriting and dross formation. Consumable parts life is excellent. This gas is used from
20 to 750 amps for cutting gauge to 4" thickness materials.
3. Argon- Hydrogen.
This gas, in a 35% Hydrogen, 65% Argon mixture, is used with dual gas systems to improve
cut quality on stainless steel and aluminum from 3/8" to 2". It is also used
with high power systems 750 to l000 amps
for cutting stainless and aluminum to 6". Consumable life is excellent.
4. Oxygen.
Oxygen is used for best cut quality on carbon steels. The cut edges are nitrite free,
allowing good weldability, formability, and machinability. This gas is used from 15 to 260
amps for cutting gauge to 1" steel. Consumable parts life, until recently, was
marginally acceptable. With the advent of the new LongLife oxygen plasma systems
(Precision, dual gas, and water injected) the consumable parts life
is now excellent.
LongLife Oxygen
Plasma Technology
Oxygen plasma has long been known for its excellent cut quality combined with speeds
approximately six times those of oxyfuel cutting for carbon steel applications. The
drawback with this process has always been relatively short electrode life, therefore high
process (cost per foot) costs. Fortunately, technology advances in the plasma cutting
industry have dramatically improved electrode life without affecting other aspects of the
process. This advancement is called LongLife oxygen plasma. This technology has been
adapted to dual gas, water injected, and HyDefinition plasma
systems.
The LongLife process requires new flow technology at the
torch, along with precise control of the output of the power supply. New power supplies
and flow panels with precise microprocessor based control are required, along with very
precise tolerances in torch
and consumable manufacturing.
The following innovations are included in the LongLife
process:
1. Mixed gases (oxygen and nitrogen) during pilot
arc.
This enhancement allows the two gases to mix at relatively low pressures inside the plasma
chamber of the torch - allowing quicker, easier starting. This gas mixture changes the
chemical properties of the hafnium electrode insert, therefore decreasing the wear rate
during pilot arc.
2. Ramp up at arc transfer.
The microprocessor-based control coordinates the ramp up of both current and gas flow.
This further increases electrode life by reducing thermal shock to the electrode insert
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3.
Ramp down at stop.
It was discovered that during the cut the hafnium insert is actually a molten puddle that
is held in place due to the vortex created by the swirling gas in the plasma chamber. With
a standard oxygen plasma system, a portion
of this molten puddle is ejected through the nozzle orifice and into the cut kerf at the
end
of each cut. In the LongLife process the microprocessor ramps down gas flow and current
simultaneously, allowing the molten hafnium to re-solidify. It is important that this ramp
down is allowed to occur with the arc transferred to the plate, thus allowing the complete
solidification of the hafnium.
(see Figure 11).
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Figure 11
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Consumable Life - LongLife vs Standard Oxygen Plasma
There are many variables involved when calculating the usable life of consumables. These
variables include gas purity, compliance with manufacturers' cut parameters, average
duration of cut, number of pierces and current level. Because it is virtually impossible
to exactly estimate nozzle and electrode wear rates we must speak in terms of average
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Figure 12
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testing shows increases in electrode life of between 4 and 6 times that of standard oxygen
plasma when using LongLife systems. This, of course, is under controlled conditions, and
the data is quite repeatable (see Figure 12). Actual field data has also been collected
from some of the many field installations with LongLife systems. The data from one of
these locations is plotted (see Figure 13), in order to show the effects of the variables.
This plot represents actual electrode life (as determined by the machine operator) of 54
electrodes during normal production. Note that the average (441 starts, 2.6 hours) falls
in line with the graph in Figure 12. |
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Figure 13
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Cost Comparison - LongLife Oxygen Plasma vs Oxyfuel
The deciding factor with any type of cutting system has to be a balance of the following
objectives; cut quality, cut speed, metallurgical effects and environmental concerns. The
sum of all of these objectives is the cost per foot of cut. Since the majority of the
carbon steel cut is under 1" thickness, and the cut quality of oxygen plasma is
comparable to that of the oxyfuel process, then a comparison of the
cost per foot cut would be helpful in choosing
a process.The chart (Figure 14) represents the costs
associated with the oxyfuel process, while Figure l 5 shows costs associated with a
LongLife plasma cutting process (260 amp, water injected).
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