Plasma Cutting Sheet Metal
Plasma cutting is a cutting technique that turns gas and electricity into plasma. Plasma has an extremely high temperature and cutting speed. Temperature may reach 25.000°C. The cutting head can be turned to cut sheets at an angle. Moreover, the plasma cutting technique is practically suitable for thick sheet metal, in materials ranging from mild steel, stainless and aluminium.
Plasma cutting techniques can be used for cutting sheet metal measuring up to 8m x 4m and above. 3D contour cut 450 can be used to accurately cut thick sheet metal as well. Mild steel can be cut up to 80mm in thickness, aluminium up to 100mm and stainless steel up to 120mm. Plasma cutting techniques can be applied to large sheets as well as smaller products. Plasma cutting jobs can be performed by a range of plasma cutting machinery.
Plasma Cutting Machinery
Plasma cutting machines may be fitted with two cutting heads. They are suitable for cutting thick sheet metal, 45° bevel cuts, engraving sheets and inkjet marking. Sheet metal measuring 8 000mm x 4 000mm and above can be cut using plasma cutting techniques.
Plasma refers to gas that has been partly ionized by extreme heat; which is called the fourth state of matter (solid, liquid, gas and plasma). The high energy input causes some gas atoms to loose electrons. The electrons that have been released can move around freely.
Plasma cutting is a cutting technique based on melting. The plasma gas is blown out of a small opening in the torch. Within this opening, there is a negative electrode. The positive product and the negative electrode creates a circuit. This circuit, or plasma arc, is extremely small and stable so the energy density in plasma cutting is high. The temperature and speed of plasma are extremely high. The intense heat makes the metal melt away and thus creates a kerf. The plasma gas then blows the molten material from the kerf. Plasma cutting is a melting technique that is suitable for cutting stainless steel, mild steel and aluminium.
Plasma Cutting Technology
High Definition Plasma Cutting
The difference in cutting quality between laser cutting and plasma cutting used to be considerable. Today, a swirl gas cap can be used for extreme constriction so that the plasma arc can be focused extraordinary well now. HD plasma cutting produces better cutting work, a narrower kerf and a smaller heat-affected zone. A second gas flow provides extra support and protection.
Plasma Cutting Heads
Plasma cutting machines may be fitted with two adjustable cutting heads. These cutting heads can cut at a 45° angle. Cutting at an angle yields more waste metal. This affects the cutting parameters, and the sources can be set to adjust the plasma beam to these parameters. The plasma head consists of several sets of wearing parts for various sheet thicknesses. Different thicknesses require different cutting heads. Moreover, bevel cutting yields a larger cut length and therefore requires a heavier cutting head.
Types of Gas
Two types of gas are used for plasma cutting: cutting gases and protective gases. The cutting gas used are oxygen, nitrogen, argon or an argon-hydrogen mixture. The protective gases are selected according to the type of material. Oxygen and nitrogen are used for mild steel; while oxygen, nitrogen or an argon-hydrogen mixture are used for stainless steel and nitrogen or an argon-hydrogen mixture for aluminium. Higher cutting speeds can be reached by using different combinations of gases to generate higher temperatures which allow higher cutting speeds.
Plasma Cutting with a single gas flow
A common form of plasma cutting is cutting with a single gas flow. The cutting head includes a negative tungsten electrode. The torch is normally cooled with gas; in case of extensive cutting, it is better to use a water-cooled cutting head.
Plasma Cutting with a dual gas flow.
Plasma cutting with a secondary gas flow yields a higher quality cut and higher cutting speed. A secondary gas is added to protect and control the ionizing gas. The secondary gas flow also prevents the formation of oxide layers. The second gas further constricts the ionizing gas, which makes the flow much more powerful.
Plasma Cutting with compressed air
Another plasma cutting technique is cutting with compressed air. Using compressed air as an ionizing gas is cheaper, but it has a number of disadvantages. This technique yields more noise and dust particles. It also requires a special electrode, hafnium or zirconium in a copper holder. Due to the limited heat it is particular suitable for thin air.
Plasma Cutting with water
Plasma cutting with water is a technique used for thicker sheet metal. The water is lead along the cutting head and is then washed away. It constricts the gas and cools the cutting head. Because of the intensive cooling, the sheet metal retains its natural properties.
Fine focus Plasma Cutting
Fine focus plasma cutting, the cutting gas is heavily constricted. Constriction takes place by rotating the arc plasma. A protective gas is added before the plasma leaves the cutting head. Sometimes, a magnet is used o constrict the plasma beam even further. The heavy constriction makes that the cutting quality is close to that of laser cutting; the cut is very narrow, straight and accurate. In addition, the heat is lower, which reduces the risk of warping.
Sheet metal can be marked for identification. The plasma cutting machine has been fitted with two different appliances for this: engraving or printed by inkjet. The product codes are then engraved on the sheet metal or to avoid defacing the product codes can be printed onto the sheet metal with an inkjet.
Hole Diameter Limitations
Plasma cutting is a technique that does not allow holes with a diameter that is smaller than the thickness of the sheet. A 10mm sheet can only have holes cut with a minimum diameter of 10mm. If smaller holes are required, they can be drilled into the sheet. It is not possible to rough cut holes for drilling and tapping. Rough cutting for milling, however, is possible. The same rule for minimum hole diamter applies.
Cutting Edge Quality Limitations
Plasma cutting generates a clean cut compared to other cutting methods. Nevertheless, the rough cut edge, the square edge and the kerf may pose problems.
Heat-Affected Zone Limitations
Plasma cutting is a thermal process that generates lots of heat. This heat causes a heat-affected zone that may be problematic. This is the main limitation of plasma cutting due to the heat-affected zone being much larger than in laser cutting.
Oxidation occurs mostly when using a burning technique. Plasma cutting is a melting technique. Only when the cutting speed is too high and the oxygen flow restricted that an oxide layer can be formed. The oxide layer is normally fairly loose and may be easily removed.
Plasma cutting creates a kerf. The top of the kerf is wider than the bottom. The kerf originates at the plasma focal point. The shape of the cutting head is elliptical. A straight kerf can be cut by adjusting the height of the cutting head. In addition, a bevelled edge may be prevented all together by positioning the torch under a small angle. Straight cut edges are more difficult in plasma cuting than in laser and/or flame cutting.