Patented Hypertherm technology

HyDefinition Plasma

How HyDefinition plasma cutting technology works compared with conventional plasma; it's benefits and capabilities.

Conventional Oxygen and Nitrogen Plasma Systems

Figure 1 Conventional Dual Flow Torch

Plasma cutting is a process that utilizes a highly positioned nozzle orifice to constrict a very high temperature, ionized gas so that it can be used to melt and sever sections of electrically conductive metals (Figure 1).

The energy density produced by a plasma torch is determined by the ratio of electrical current flow through the nozzle to the effective area of the nozzle orifice.

This energy density can be measured as amps per square inch. Conventional nitrogen plasma cutting systems have an energy density in the range of 12,000 to 20,000 amps per square inch.

This energy density has typically been determined by economic factors. In other words, if you increase the energy density by changing the amperage to nozzle orifice ratio, the electrode and nozzle (consumables) will wear at an unacceptable rate, thus increasing the cost of the cutting process. Although the higher density produces better cut edge quality, it comes at an unacceptable cost. Because of this, manufacturers of plasma cutting equipment have had to design their plasma systems to operate with an acceptable cut quality, combined with an acceptable level of consumable life.

Figure 2

In the early 1980's oxygen was introduced as a plasma gas for cutting carbon steels with greatly improved cut edge quality, and at power levels that allowed it to compete productively with nitrogen cutting. The oxygen cut edge was squarer, and the dross formation was minimal. Oxygen plasma also provided a metallurgically cleaner edge that allowed for better weldability, formability, and machineability of the cut part (Figure 2).

Unfortunately, this advancement in technology created even shorter consumable parts life due to the reaction of the oxygen on the electrode material inside the torch. The earlier conventional (nitrogen) plasma systems were capable of cutting for up to 500 to 600 starts before the consumable wear would seriously affect cut quality. Oxygen systems, cutting the same material at the same speeds, required new consumables after 100 to 150 starts.

The oxygen systems produced a higher per foot cutting cost, but when the better cut quality, along with lower secondary operation costs were factored in, the oxygen cutting process was an acceptable alternative to the conventional nitrogen systems. Clearly the oxygen plasma process would be well received if consumable parts life could be improved.

Cutting Capabilities of Conventional Plasma Both oxygen and nitrogen plasma systems - when properly operated - are capable of cutting materials (carbon steels, aluminum, and stainless steels) from 3/8" (9.5 mm) through 1" (25 mm) with cut edge squareness in the range of within 1 to 4 degrees of square. Generally the thicker the material cut, the better the edge squareness. When cutting materials thinner than 3/8" (9.5 mm) it is generally expected that the cut angle increases, ranging from 3 to 4 degrees on 3/8" to as much as 15 to 25 degrees on gauge thickness materials. These cut edge angles, while not desired, have generally been accepted by the users of plasma cutting systems, primarily due to the high productivity (linear feet per hour cut) of these systems. Plasma cutting systems could benefit from improvements in edge angularity below 3/8" thickness.

The cutting thickness capability of conventional plasma systems varies depending on system manufacturer and power levels. In general terms, plasma systems are available to cut aluminum to 6" (150 mm) thick, stainless steel to 5" (125 mm) thick, and carbon steels to 1 1/4" (32 mm) thick. It certainly is possible to cut thicker sections of carbon steel, but the process is not economically advantageous when compared to the Oxy-Fuel process above 1 1/4" thickness.

Other Factors Affecting the Plasma Process

Figure 3 Pierce slag contacts the nozzle, making it the same electrical potential as the workpiece. The nozzle is damaged by this "double arc.

Double arcing is a common problem that affects the plasma process. In simple terms, a double arc is an arc that finds it's path from negative (electrode) to positive (workpiece) through the copper portion of the nozzle. One arc occurs from the electrode to the inside of the nozzle, and the second arc from the outside of the nozzle to the workpiece (Figure 3).

When this occurs, the nozzle orifice is usually damaged. This damage seriously affects the arc constriction, causing a reduction in cut quality. It also increases the wear on the electrode necessitating more frequent nozzle replacement.

Double arcing may be caused by several factors: 1.Piercing too close to the workpiece. 2.Dragging the nozzle on the workpiece during the cutting process. 3.Improper gas flow or current settings for the nozzle orifice dimension.

Double arcing must be controlled in order to make any mechanized plasma process economical, repeatable and reliable.

Figure 4 Exaggerated effect of poor electrode to nozzle alignment.

Figure 5 Hypertherm shielded technology

This development, patented by Hypertherm, allows hand-held plasma torches to drag cut with the nozzle directly touching the workpiece. It has also virtually eliminated the double arcing problem that adversely affects nozzle life and cut quality. This technology incorporates a thin, electrically insulated copper shield that isolates the nozzle from the workpiece. This shield technology dramatically increases nozzle life and improves workpiece piercing capacity (Figure 5).

LongLife Oxygen Consumable Technology
This is one of the most important developments in modern plasma cutting, allowing the advantages of oxygen plasma cutting, along with incredibly long consumable parts life. During the oxygen plasma cutting process, the electrode material (hafnium) remains in a molten state. At the end of each cut cycle, a small portion of this molten material ejects from the face of the electrode - part of this depositing on the inner bore of the nozzle, and part exiting through the nozzle orifice. This material loss creates a pit in the end of the electrode that eventually (after 100 to 150 starts) causes a change in the cut quality. The nozzle is also affected by the deposits on the inner bore causing improper gas flow patterns. The LongLife technology patented by Hypertherm incorporates a microprocessor control that simultaneously controls electrical current and gas flow throughout the cutting process.This control minimizes the chemical and thermal shock at the beginning of every cut, and re-solidifies the electrode material at the end of every cut, producing consumable parts life in the range of 600 to 1200 starts with no effect on cut quality.

Torch and Consumables Concentricity

The design of the HyDefinition torch and consumables allows for very accurate, repeatable torch parts alignment. This is accomplished by utilizing the swirl ring as an insulator, alignment fixture, and flow control. This ensures that the cut part angularity will be consistent.

Dross Free Interval

The HyDefinition systems incorporate a gas mixing manifold that creates a shield gas mixture that dramatically increases the dross free cutting speed range. This virtually eliminates dross on most materials and thicknesses.

High Flow Vortex Nozzle

Figure 6 Hypertherm HyFlow vortex increased energy density

It is important to create a strong vortex of gas around the face of the electrode in order to maintain an accurate arc attachment point. This vortex is created by swirling the plasma gas around the electrode, which creates a "tornado like" gas flow pattern. Due to the small orifice diameter required to properly constrict the HyDefinition arc, it proved difficult to create enough swirl strength. There just wasn't enough gas flow. The HyFlow Vortex nozzle was developed to allow relatively high swirl flow, while relieving some pressure inside the plasma plenum before the arc is finally constricted (Figure 6). The HyFlow Vortex nozzle overcame the problem of a constricted gas flow and allowed proper arc attachment to maintain parts life.

Increased Energy Density

The innovations above combined to allow the increasing energy density of the HyDefinition system to the required range of 40,000 to 60,000 amps per square inch. It has also yielded consumable life spans in the range of 600 to 1,200 starts. This process effectively eliminates the cut quality problems that have always plagued plasma cutting processes in the thin (3/8" (10 mm) and below) material range.

Consumable Life

Before production release of HyDefinition systems it was necessary to perform extensive laboratory testing of both consumable life and cut quality.

Typically, consumable wear on conventional plasma systems results in a fairly linear degradation of cut quality to the point where cut quality is no longer acceptable. As the consumables wear, cut angularity becomes less uniform, and more dross tends to form. It may also be necessary to change cutting parameters to compensate for this wear. It is also normal for conventional consumables not to wear evenly-the electrode will usually wear twice as fast as the nozzle in an oxygen system, for instance.

Figure 7 LongLife oxygen plasma provides greatly increased consumables life

Consumable life can be realistically estimated using the graph (Figure 7). As with all plasma systems, consumable life is dependent on the number of starts and the average cut duration. This chart assumes also that the torch is started and stopped always with the workpiece under the torch, to fully utilize the LongLife technology.

Many HyDefinition users have reported much longer life than is stated on this graph, and that is certainly possible. The graph indicates the point where cut quality starts to deteriorate, not the point where the consumables fail to operate.

HyDefinition Cutting Capabilities

HyDefinition, like all plasma systems, produces it's best cut quality within a certain thickness range and material type. It is also important to remember that it cannot possibly produce smooth, high quality, dross free cuts if the motion control device is inadequate.

The motion control must have excellent acceleration/deceleration characteristics, fluidic, vibration free contouring capabilities, along with accuracy and repeatability specifications exceeding those of conventional devices. Parameters such as speed, torch to work distance, gas pressures and purity must be maintained according to specifications in order to fully utilize the cutting capabilities.

Carbon steels in the thickness range of 20 gauge (.9 mm) to 3/8" (9.5 mm) can be cut with the HyDefinition system. Reports from the field indicate that thinner and thicker materials have been successfully cut, although some cut quality and consumable life degradation would be expected.

The best cut quality can be expected when cutting materials at low current ranges. For example, 16 gauge cold rolled steel can be cut at 30 amps and 80 inches per minute with good cut quality. Edge squareness would be in the 1 to 3 degree range. If the same material is cut at 15 amps, 65 inches per minute, edge squareness would typically be closer to 0 to 1.5 degrees, along with a smoother finish. If higher speeds are required and edge squareness is not as critical, this same 16 gauge material can be cut at much higher speeds, at 30 amps 150 inches per minute, or at 70 amps, approximately 250 inches per minute.

The cut appearance on carbon steel thinner than 1/4" is smooth and square, with virtually no top edge rounding. On cold rolled steel virtually no dross is present, while on hot rolled material there may be some easy-to-remove dross. These cuts are very similar in appearance to laser cuts, with the exception of a wider kerf. In this range, cut angularity can easily be held within 0 to 3 degrees.

On carbon steel sections from 1/4" to 3/8" that are cut at the 70 amp range, some top edge rounding is present, with angularity from 1 to 3 degrees.Stainless steel cutting in the thickness range of 20 gauge (.9 mm) to 3/8" (9.5 mm) can also be accomplished with HyDefinition Plasma. For this process, air is used as both the plasma and shield gas up to 10 gauge thickness. Beyond this thickness a small amount of methane is blended into the shield gas to eliminate the oxidation, providing a more weldable edge. On thicknesses less than 10 gauge, a square, somewhat rough edge is present, while the material above 10 gauge has a very smooth, clean edge, with virtually no top edge rounding.

Aluminum cuts very well with the HyDefinition system, using air as the plasma gas. It can be cut with either air or methane as the shield gas. Although it costs more, methane provides for an incredibly smooth cut edge on aluminum.

Care must be used in programming and cutting small holes. Since plasma cutting is a transferred arc process, a small indentation is sometimes produced at the end of a cut at the moment of crossing the lead-in kerf. At this point the center of the hole drops out, and the arc deflects, creating an indentation in the part. This can be minimized through careful programming. A good rule is to limit hole sizes to larger than 3/16" diameter.

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Hypertherm, LongLife and HyDefinition are registered trademarks of Hypertherm, Inc., and may be registered in the United States and/or other countries.