plasma arc maching

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INTRODUCTION:Plasma arc cutting was developed 20 years ago primarily for cutting stainless steel and

aluminum. Although favorable economically, mild steel was seldom cut with this process

because of three fundamental limitations: relatively poor cut quality, equipment

reliability, and inability of the earlier cutting machines to handle plasma cutting speeds.

As a result of these limitations, plasma cutting did not encounter rapid growth until after

Water-injection Plasma Cutting was introduced in 1970. The plasma arc process has

always been seen as an alternative to the oxy-fuel process. In this part of the series the

process fundamentals are described with emphasis being placed on the operating features

and the advantages of the many process variants.

THE basic principle behind plasma arc cutting is that the arc formed between the

electrode and the workpiece is constricted by a fine bore, copper nozzle. This increases

the temperature and velocity of the plasma emanating from the nozzle. The temperature

of the plasma is in excess of 20,000C and the velocity can approach the speed of sound.

When used for cutting, the plasma gas flow is increased so that the deeply penetrating

plasma jet cuts through the material and molten material is removed in the efflux plasma.

The process differs from the oxy-fuel process in that the plasma process operates by

using the arc to melt the metal whereas in the oxy-fuel process, the oxygen oxidises the

metal and the heat from the exothermic reaction melts the metal. Thus, unlike the oxy-

fuel process, the plasma process can be applied to cutting metals which form refractory

oxides such as stainless steel, aluminium, cast iron and non-ferrous alloys.

The Plasma arc cutting process

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A plasma cutter is a relatively easy-to-use tool to cut steel and other electrically-

conductive metals. These cutters work by using a high-voltage electrical arc and a

compressed gas, usually air. An electrical arc generated by an internal electrode ionizes

gas passing through a nozzle, creating a concentrated arc of plasma at the cutter's tip. The

arc's contact with the working surface makes a high heat circuit which melts a section

less than 1/16" (1.6mm) wide. The force of the plasma flow then literally blows out the

molten area on the work piece, creating a fairly clean cut with little or no slag. The

plasma arc travels through the nozzle at a speed of up to 20,000 feet per second, and at

temperatures as high as 30,000 degrees Fahrenheit (16,600 Celsius)!

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PLASMA:In physics and chemistry, plasma is a gas in which a certain portion of the particles are

ionized. The presence of a non-negligible number of charge carriers makes the plasma

electrically conductive so that it responds strongly to electromagnetic fields. Plasma,

therefore, has properties quite unlike those of solids, liquids, or gases and is considered to

be a distinct state of matter. Like gas, plasma does not have a definite shape or a definite

volume unless enclosed in a container; unlike gas, in the influence of a magnetic field, it

may form structures such as filaments, beams and double layers Some common plasmas

are flame, lightning, and the Sun.

Plasma was first identified in a Crookes tube, and so described by Sir William Crookes in

1879 (he called it "radiant matter").[1] The nature of the Crookes tube "cathode ray"

matter was subsequently identified by British physicist Sir J.J. Thomson in 1897,[2] and

dubbed "plasma" by Irving Langmuir in 1928,[3] perhaps because it reminded him of a

blood plasma. Langmuir wrote:

Plasma lamp, illustrating some of the more complex

phenomena of a plasma, including filamentation.

The colors are a result of relaxation of electrons in

excited states to lower energy states after they have

recombined with ions. These processes emit light in

aspectrum characteristic of the gas being excited.

Degree of ionization

For plasma to exist, ionization is necessary. The term "plasma density" by itself usually

refers to the "electron density", that is, the number of free electrons per unit volume. The

degree of ionization of a plasma is the proportion of atoms which have lost (or gained)

electrons, and is controlled mostly by the temperature. Even a partially ionized gas in

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which as little as 1% of the particles are ionized can have the characteristics of a plasma

(i.e., response to magnetic fields and high electrical conductivity). The degree of

ionization, is defined as = ni/(ni + na) where ni is the number density of ions and na is

the number density of neutral atoms. The electron density is related to this by the average

charge state of the ions through ne = ni where ne is the number density of

electrons.

Plasm

a cutters work by sending a pressurized gas, such as nitrogen, argon, or oxygen, through

a small channel. In the center of this channel, you'll find a negatively charged

electrode. When you apply power to the negative electrode, and you touch the tip of the

nozzle to the metal, the connection creates a circuit. A powerful spark is generated

between the electrode and the metal. As the inert gas passes through the channel, the

spark heats the gas until it reaches the fourth state of matter. This reaction creates a

stream of directed plasma, approximately 30,000 F (16,649 C) and moving at 20,000 feet

per second (6,096 m/sec) that reduces metal to molten slag. For example

Whe

n energy, in the form of heat, is applied to ice, the ice melts becoming water. The H2O

transforms from the solid-state, ice, to the liquid state, water.When more heat is appliedto the water, the water vaporizes becoming steam. The H2O transforms from the liquid

state, water, to the gas state, steam (H2& O2).Finally, when additional heat is applied tothe individual gases, the gases ionize. The ionization of the gases is the final change in

states. The gases are now in an electrically conductive state called a plasma.

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PLASMA ARC TERMINOLOGY :

PLASM ARC TORCH TERMINOLOGYA plasma torch works by projecting an electric arc produced by polarized electrodes

through gas that is being forced through a small opening, the nozzle. As the gas becomes

electrically charged its temperature elevates until the gas enters the plasma state. The

electric charge is transferred to the metal causing it to melt, and the high velocity gas cuts

through the molten material. Nitrogen, oxygen and argon can all be used, but the most

popular gas is simply forced air. A plasma torch can cut cheaper, faster and more

accurately than the traditional oxy-acetylene torch.

A plasma torch can be effective for both cutting and welding. As with cutters, plasma

welding torches have high speeds and high quality performance for creating pore-free

welding seams. These tools come with a standard electrical cord, a chamber for the air or

gas, and a nozzle with an electrode placed behind it. A gas source, such as an air

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compressor or bottled air or gas, is attached to the torch. For most metals, compressed air

is sufficient, though nitrogen or argon may be used for cutting stainless steel or other

exotic metals.

The automotive and construction industries use a computer numerical control (CNC)

plasma torch to make custom auto shapes required for chassis or frames, and to cut large

steel beams. The versatility and accuracy of this device makes it a great tool for the

creating metal art, jewelry and ornamental iron work. Many CNC torches are large,

expensive equipment designed to operate on the assembly line of a large shop, such as an

automobile factory; however, smaller and more affordable models are now available for

small shops and individual craftsmen. For applications which do not require CNC

equipment, hand-operated units are available at a relatively inexpensive price. The air

pressure capacity required varies depending upon the size of the plasma torch, so the

purchaser should select the torch prior to determining what size air compressor to buy.

Replacement parts, such as air filters, nozzles and electrodes are available from a number

of sources.

A plasma torch, like any other power tool, can cause serious injury if appropriate safety

measures are not followed. The light from the plasma torch can seriously damage the

naked eye, so safety glasses with side shields should be worn at all times. Gloves and

fire-resistant clothing can protect against sparks. These cutters operate at extremely high

temperatures which can burn through gloves, so hands should be kept away from the

nozzle, arc and metal workpiece. To prevent fires or explosions, the torch should only be

used in well-ventilated areas away from any other flammable material.

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PLASMA ARC CUTTING SYSTEM:

A plasma jet can either be operated in the transferred mode, where the power supply is

connected between the electrode and the workpiece, or in the nontransferred mode

wherethe power supply is connected between the electrode and the nozzle. Both modes of

Operation are illustrated in Figure 2. Although a stream of hot plasma emerges from the

Nozzle in both modes of operation, the transferred mode is always used in plasma cutting

because the usable heat input is most efficiently applied when the arc is in electrical

contact with the workpiece.

Schematic diagram for non transferred and transferred arcs

The thermal efficiency is low for non transferred up to 65-75% while for transferred arcs

it is up to 85-90%.

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CONVENTIONAL PLASMA ARC CUTTING:The plasma jet that is generated by conventional "dry" arc constriction techniques can be

used to sever any metal at relatively high cutting speeds. The thickness of plate can range

from 1/8 inch to a maximum thickness depending on both the current capacity of the

torch and the physical properties of the metal. A heavy duty mechanized torch with a

current capacity of 1000 amps can cut through 5 inch thick stainless steel and 6 inch thick

aluminum. However, in most industrial applications the plate thickness seldom exceeds

1-1/2 inch. In this thickness range, conventional plasma cuts are usually beveled and have

a rounded top edge.

Fig.- Pojitive cut angle

Beveled cuts are a result of an imbalance in heat input into the cut face. As shown in

Figure , a positive cut angle will result if the heat input into the top of the cut exceeds

the heat input into the bottom. One obvious approach to reduce this heat imbalance is to

apply the arc constriction principle described in Figure 1: increased arc constriction will

cause the temperature profile of the plasma jet to become more uniform and,

correspondingly, the cut will become squarer. Unfortunately, the conventional nozzle is

limited by the tendency to establish two arcs in series---electrode to nozzle, and nozzle to

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work. This phenomenon is known as "double arcing" and can damage both the electrode

and nozzle. Conventional plasma cutting can be cumbersome to apply if the user is

cutting a widevariety of metals and plate thickness. For example, if the conventionalplasma process is used to cut stainless steel, mild steel and aluminum, it will benecessary to have three different cutting gases on hand if optimum cut quality is to be

obtained. This requirement not only complicates the process, but necessitates stocking

expensive cutting gases such as 65% argon - 35% hydrogen.

PROCESS VARIENT OR PROCESS REFINEMENT:

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The process variant have principally been designed to improve cut quality and arc

stability, reduce the noise and fume or to increase cutting speed.

DUAL GAS PLASMA ARC:

The same features as conventionalplasma cutting except that a secondary shield gas isadded around the nozzle. Usually the cutting gas is nitrogen and the secondary shielding

gas is selectedaccording to the metal to be cut. Secondary shield gases typically used are:

mild steel--either air or oxygen; stainless steel--CO2; aluminum--argon--hydrogen

mixture. Cutting speeds are slightly better than with conventional cutting on mild steel;

however, cut quality is inadequate for many applications. Cutting speed and quality on

stainless steel and aluminium are essentially the same as with the conventional process.

The major advantage of this approach is that the nozzle can be recessed within a ceramic

shield gas cup as shown in Figure 4, thereby protecting the nozzle from double arcing. If

no shield gas were present, the ceramic shield gas cup could deteriorate because of the

high radiative heat load produced by the plasma jet.

Schemetic diagram of dual gas plasma torch

Air cutting was introduced in the early 1960's for cutting mild steel. The oxygen in the air

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provides additional energy from the exothermic reaction with molten steel. This

additionalenergy increases cutting speeds by about 25%. Although the process can be

used tocut stainless steel and aluminum, the cut surface will be heavily oxidized and

unacceptablefor many applications.

Special electrodes, made of zirconium or hafnium, must be used since tungsten will erode

in seconds if the cutting gas contains oxygen. Even with these special electrodes, the

service life is must less than what can be achieved with the conventional plasma cutting

process.

The beneficial effects of the secondary gas are increased arc constriction and more

effective 'blowing away' of the dross. The plasma

forming gas is normally argon, argon-H2 or

nitrogen and the secondary gas is selected

according to the metal being cut.

Steel

air, oxygen, nitrogen

Stainless steel

nitrogen, argon-H 2, CO 2

Aluminium

DUALGAS TORCH

argon-H2, nitrogen / CO 2

The advantages compared with conventional

plasma are:

Reduced risk of 'double arcing'

Higher cutting speeds

Reduction in top edge rounding

WATER INJECTED PLASMA TORCH:

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Earlier it was stated that the key to achieving improved cut quality is through increasing

arc constriction. In the Water-injection Plasma Cutting process, water is radially injected

into the arc in a uniform manner as shown in Figure 6. The radial impingement of the

water around the arc provides a higher degree of arc constriction than can be achieved by

conventional means. Arc temperatures in this region are estimated to approach 50,000

degrees K, or roughly nine times the surface temperature of the sun. The net result is

improved cut squareness and increased cutting speeds. This radial water-injection

techniquewas developed and patented by Hypertherm, Incorporated.Another approach to

constricting the arc with water is to develop a swirling vortex ofwater around the arc.

This technique does not perform as well as radial injection becausethe degree of arc

constriction is limited by the high swirl velocities needed to produce astable water vortex:

the centrifugal force created by the high swirl velocity tends to flatten the annular film of

water against the inner bore of the nozzle.

Schemetic diagram of water injection plasma torch

Despite the extremely high temperatures generated at the point where the water impinges

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the arc, less than 10% of the water is vaporized. The remaining 90% of the water exits

from the nozzle in the form of a conical spray which cools the top surface of the

workpiece. This additional cooling prevents the formation of oxides on the cut surface.

Little water is evaporated at the arc because an insulating boundary layer of steam forms

between the plasma and the injected water. This steam boundary layer, usually referred to

as a "Lindenfrost Layer", is the same principle that allows a drop of water to dance

around on a hot skillet rather than immediately vaporizing.

Nozzle life is greatly increased with the Water-injection technique because the steam

boundary layer insulates the nozzle from the intense heat of the arc, and the water cools

the nozzle at the point of maximum arc constriction. The protection afforded by the

water-steam boundary layer also allows a unique design innovation: the entire lower

portion of the nozzle can be ceramic. Consequently, double arcing from the nozzle

touching the workpiece--the major cause of nozzle destruction--is virtually eliminated.

Nitrogen is normally used as the plasma gas. Water is injected radially into the plasma

arc,Fig. , to induce a greater degree of constriction. The

temperature is also considerably increased, to as high as

30,000C.

The advantages compared with conventional plasma are:

Improvement in cut quality and squareness of cut

Increased cutting speeds

Less risk of 'double arcing'

Reduction in nozzle erosion

WATER SHROUD PLASMA TORCH:

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The plasma can be operated either with a water shroud, Fig. 2c, or even with the

workpiece submerged some 50 to

75mm below the surface of the water.

Compared with conventional plasma,

the water acts as a barrier to provide

the following advantages:

Fume reduction

Reduction in noise levels

Improved nozzle life

In a typical example of noise levels at

high current levels of 115dB for

conventional plasma, a water shroud was effective in reducing the noise level to about

96dB and cutting under water down to 52 to 85dB.

As the water shroud does not increase the degree of constriction, squareness of the cut

edge and the cutting speed are not noticeably improved.

AIR PLASMA TORCH:

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The inert or unreactive plasma forming gas (argon or nitrogen) can be replaced with air

but this requires a special electrode of hafnium or zirconium mounted in a copper holder,

Fig. The air can also replace water for cooling the torch. The advantage of an air plasma

torch is that it uses air instead of expensive gases.

It should be noted that although the electrode and nozzle are the only consumables,

hafnium tipped electrodes can be expensive compared with tungsten electrodes.

Schematic diagram of air plasma torch

OXYGEN INJECTED PLASMA TORCH:This process refinement circumvented the electrode life problem associated with air

cutting by using nitrogen as the cutting gas and introducing oxygen downstream in the

nozzle bore as shown in Figure

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Schematic diagram of oxygen-injected torch construction

This process is used exclusively on mild steel and increases cutting speed by about 25%

if the optimum gas mixture is used (80% N2 - 20% O2). The major disadvantages are

lack of cut squareness, short nozzle life, and limited versatility (mild steel only).

Although this process is still being used at some locations, it has been almost entirely

displaced by Water-injection cutting.

HIGH TOLERANCE PLASMA:In an attempt to improve cut quality and to

compete with the superior cut quality of laser

systems, High Tolerance Plasma Arc cutting

(HTPAC) systems are available which operate

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with a highly constricted plasma. Focusing of the plasma is effected by forcing the

oxygen generated plasma to swirl as it enters the plasma orifice and a secondary flow of

gas is injected downstream of the plasma nozzle,Fig. 2e. Some systems have a separate

magnetic field surrounding the arc. This stabilises the plasma jet by maintaining the

rotation induced by the swirling gas. The advantages of HTPAC systems are:

Cut quality lies between a conventional plasma arc cut and laser beam cut

Narrow kerf width

Less distortion due to smaller heat affected zone

HTPAC is a mechanised technique requiring precision, high-speed equipment.

The main disadvantages are that the maximum thickness is limited to about 6mm and the

cutting speed is generally lower than conventional plasma processes and approximately

60 to 80% the speed of laser cutting.

NON SHIELDING PART ISSUE:

Double Arcing:

During the pierce, droplets of the molten metal can form a conductive path to the nozzle,

causing the nozzle to be at positive potential.This can cause a path of least

resistancefrom the electrode to the nozzle to the plate known as a double arc. This can

also occur if the nozzle contacts the plate during a cut.

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Nozzle Damage:

Contact to work piece damage and blow-back of spatter during cutting damages the

nozzle by pitting and ovaling the orifice.normalovaling

TORCH SHIELDING TECHNOLOGYShielded Front End Nozzle is protected by an electrically isolated shieldSignificantly reduces double

arcingProlongs parts lifeFacilitates drag cutting and template tracingDramatic

improvement in nozzle life

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ISOLATED SHIELD

Shielded Front End Benefits

During mechanized torch cutting:

Greater nozzle life

Lower cost of operation

Longer service life with consistent cut quality

Thicker Pierce Capacity

Protects the nozzle from occasional contact withplate.

CUTTING TERM:

Kerf:

Opening created by the metal removed

by the plasma arc. The width of the

kerfis determined by:

amperage

gases

nozzle orifice size

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consumable condition

torch-to-work distance Cutting

Dross:

The resolidifiedmetal on the bottom or top of the cut.

Dross formation and its condition is determined by

many factors

:travel speeds

amperage

gases used

type and thickness of metal

torch-to-work distancematerial surface coatings

LAG LINES:These are the ripples on the cut face or surface. The more consistent the power produced

by power supply is, the smoother the cut. Depending on the process, normal lag lines are

curved and slanted at about 15with properspeeds.

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TORCH STATING METHOD:

High Frequency

High voltage (5,000V -10,000V), high

frequency AC forms a spark between

electrode and nozzle

Gas is forced to flow through this spark,

raising it to its ionization temperature

This is an effective starting method, but

generates electrical noise that may affect

sensitive electronicequipment

Contact Starting

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MATERIAL TO BE CUT GAS OR GAS MEXTURE USED

Aluminum Nitrogen,nitrogen-hydrogen.argon-

hydrogen

magnesium Nitrogen,nitrogen-hydrogen.argon-

hydrogen

Stainless steel and some othernon ferrous metals

Nitrogen-hydrogen. argon hydrogen

Carbon and alloy steels, cast

iron

Nitrogen-hydrogen. Compressed air

COMPARISON B/W PAM AND LASER CUTTING:

Fundamental process difference:

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Safety consideration and operating environment :

PLASMA ARC CUTTING ADVANTAGE:

Automated plasma arc cutting systems provide several advantages over other cutting methods

such as oxyfuel and laser.

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Rapid cutting speed:

Plasma arc cutting is faster than oxyfuel for cutting steel up to 2 inches thick and is competitive

for greater thicknesses. Plasma cutting achieves speeds greater than those of laser cutting

systems for thicknesses over 1/8 inch. CNC controls allow speeds of up to 500 inches per minute

(ipm) to be achieved on gauge thicknesses. These fast cutting speeds result in increased

production, enabling systems to pay for themselves in as little as 6 months for smaller units.

Wide range of material thikness

Plasma cutting systems can yield quality cuts on both ferrous and nonferrous metals.

Thicknesses from gauge to 3 inches can be cut effectively.

Easy to use

Plasma cutting requires only minimal operator training. The torch is easy to operate, and new

operators can make excellent cuts almost immediately. Plasma cutting systems are rugged, are

well suitable for production environments, and do not require the potentially complicated

adjustments associated with laser cutting systems.

Economical

Plasma cutting is more economical than oxyfuel for thicknesses under 1 inch, and comparable up

to about 2 inches. For example, for inch steel, plasma cutting costs are about half those of

oxyfuel.

LIMITATION AND CONCERN:

A chief concern about plasma arc technology is ensuring that gaseous emissions are kept

to a minimum and cleaned before being released to the atmosphere.

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Concerns have been raised regarding the reliability of plasma torch technology.

The water-cooled copper torch must be replaced periodically to prevent burn-through at

the attachment point of the arc and a subsequent steam explosion due to rapid heating of

the released cooling water.

other limitation are

Large heat affected zone.

Rough Surfaces

Difficult to produce sharp corners.

Smoke and noise.

Burr often results.

APPLICATION:

Automated plasma cutting systems are being chosen over oxyfuel, hand tools, and laser

cutting in the following areas:

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Sheetmetal

Plasma cutting is commonly used to cut sheet metals from 24 gauge up to 1/8 inch thick

at high speeds on carbon steels, aluminum, and stainless steels.

Plasma cutting is widely used in the transportation industry to form the outer

skins of tractor trailers, buses, and agricultural equipment.

Plasma cutting systems are also used in the heating, ventilating, and air

conditioning industry to cut complex duct work.

PlateThicknesses

Industries involved in cutting plate thicknesses also find many applications for plasma

cutting. Plasma systems cut plate thicknesses from 1/8 to 3 inches, but the most common

applications are for carbon steel plate to inch thick.

Steel service centers cut large plates of steel down to size with plasma.

Makers of large construction machinery, mining equipment, and material

handling equipment utilize plasma cutting to produce cranes, bulldozers, and

other large equipment.

Plasma cutting also produces structural steel framework for railroad cars, trucks,

and other heavy equipment.

Other applications include cutting metal for ship building and the production of

pressure vessels.

OtherApplications

Plasma cutting is not limited to flat sheets of metal. Plasma torches placed on robots are

being used increasingly for contour cutting of pipes and vessels, removal of sprues and

risers from castings, and cutting of formed shapes, angles, and curves in various planes.

BIBLIOGTAPHY:

Advance Machining Process ------ VIJAY K. JAIN

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Modern Machining Process --------- P.C. PANDYE,H.S. SHAN

Plasma Arc Cutting----------------- AHMAD HASSAN SAYED

Plasma Processes of Cutting and Welding------ J.A. Hogan and J.B. Lewis

Plasma Arc Cutting----------------- Bill Lucas in collaboration with DerrickHilton,

Plasma Arc Cutting------------------ WIKIPADIA

Basic Plasma Theory--------------------- HYPERTHERM PRIVATE LIMITED

plasma arc maching

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