Plasma Cutting

Plasma Cutting: 

Plasma Cutting What is Plasma? E.T.Masih

What is Plasma? The Fourth State of Matter: 

What is Plasma? The Fourth State of Matter

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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.

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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. How Plasma Cuts Through Metal 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.

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Plasma Arc Cutting System 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.

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Sequence of Operating a Plasma Cutter 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. 1. 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)

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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 (work piece) 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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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. It is done in such a way that the gas must pass through this arc before exiting the nozzle.

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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.

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3. Assuming that the nozzle is within close proximity to the work piece, the pilot arc will attach to the work piece, the current will start flowing as the current path to positive is not restricted. Current flow to the work piece is sensed electronically now the high frequency is disabled and the pilot arc relay is opened. Gas ionization is maintained with energy from the main DC arc.

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The temperature of the plasma arc melts the metal, pierces through the work piece 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.

Inside a Plasma Cutter : 

Inside a Plasma Cutter Plasma cutters come in all shapes and sizes. Inside a plasma cutter: The electrode is at the center, and the nozzle is just below it. The orange piece is the swirl ring, which causes the plasma to turn rapidly as it passes.

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Plasma 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.

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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. The plasma itself conducts electrical current. The cycle of creating the arc is continuous as long as power is supplied to the electrode and the plasma stays in contact with the metal that is being cut. In order to ensure this contact, protect the cut from oxidation and regulate the unpredictable nature of plasma, the cutter nozzle has a second set of channels. These channels release a constant flow of shielding gas around the cutting area. The pressure of this gas flow effectively controls the radius of the plasma beam.

Variations of the Plasma Cutting Process : 

Variations of the Plasma Cutting Process 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. Primarily used in hand held applications

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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.

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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 work piece cooling along with better cut quality on stainless steel. This process is for mechanized applications only.

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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. This process is for mechanized applications only .

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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 constricts the arc, dramatically increasing energy density. The slower speeds are required to allow the motion device to contour more accurately.

Cutting Terms: 

Cutting Terms The width of the kerf is determined by: Amperage Gases Nozzle Orifice Consumable Conditions Torch to work-distance Kerf: Opening created by the metal removed by the plasma arc. Kerf Dross

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Dross The resolidified metal on the bottom or top of the cut Dross formation and its condition is determined by may factors: Travel speed Amperage Gases used Type & thickness of metal Torch to work-distance Material surface coating Top Dross Top Dross High Speed Dross Low Speed Dross

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Lag Lines These are the ripples on the cut face or surface. Smooth surface Lag line breaks about In middle of cut Dross free 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 15 0 with proper speeds.

Set-Up Procedures : 

Set-Up Procedures · A clean compressed air supply without water or oil is suitable for work. Black burn marks on the plate, may indicate that the air is contaminated. · Correct air pressure - this can be checked by looking at the gauges on the unit · A nozzle and electrode are correctly in place A good connection of the work lead to a clean portion of the work Before you start, check for the following items:

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1. Piercing the Work Many inexperienced users try to pierce the metal by coming straight down, perpendicular (90 degrees) to the work. This results in molten metal being blown back into the torch. A better method is to approach the metal at an angle (60 degrees from horizontal, 30 degrees from vertical) and then rotate the torch to the vertical position. This way, the molten metal is blown away from the torch.

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2. Don't Touch the Nozzle to the Work Piece Do not touch the nozzle to the work when using current levels of 45 amps or more. Doing so will drastically reduce the nozzle life as the cutting will double arc through the nozzle. Double arcing can also occur if the torch is guided by dragging it against a metal template. The result is the same as dragging the nozzle on the work -- prematurely worn nozzles.

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3 . Travel at the Right Speed When moving at the right cutting speed, the molten metal spray will blow out the bottom of the plate at a 15 to 20 degree angle. If you are moving too slowly, you will create slow speed dross, when moving too fast, high-speed dross on the top surface. Traveling too fast or too slow will create a low-quality cut.

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4. Set the Current to Maximum As You Begin When setting the current, put it on the maximum output of the machine, then turn it down as needed. More power is usually better, except when doing precision cutting or when you need to keep a small kerf.

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5. Minimize Pilot Arc Time Because of the wear it creates on the consumables, try to minimize the amount of time spent in pilot arc mode. To do this, position the plasma torch by the edge of the work before starting the arc so you can get right to cutting.

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6. Maintain A Constant Work Distance Optimally, you should maintain a 3/16" to 1/8" distance from the nozzle to the work. Moving the torch in an up and down fashion will only hinder your efforts.

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7. Travel in the Direction that will Give You the Best Finished Work If you are making a circular cut and plan to keep the round piece as your finished work, move in a clockwise direction. If you plan to keep the piece from which the circle was cut, move in a counterclockwise direction. As you push the torch away from you, the better cut will appear on the metal that is on the right hand side, since it will tend to have a better, squarer edge.

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8. End with a Push Angle on Thick Material One trick to use on thicker material is to rotate the torch slightly, increasing the torch orientation to a push, rather than drag angle as you cut through the last section of material. This increase in the push angle at the finish will cut through the bottom first and get rid of the bottom corner that is usually left at the end of thick plate. Never finish a cut by using the torch to hammer away the last corner of the work.