WHAT YOU MUST KNOW ABOUT PIERCING THICKER PLATE WITH PLASMA
The ability to pierce metal plate is a necessity for many fabricators and steel processing centers. Using plasma, rather than oxyfuel, is most desirable as it means faster piercing times, faster cut speeds, and a cleaner finished product. And speed, as we all know, translates into higher productivity and profitability.
Despite the many benefits of plasma technology, some companies have found piercing thicker material – say anything over 1¼ in – difficult with their older plasma systems. They aren’t alone. Several factors have often left operators with a torch filled with melted consumables, or consumables covered in a quick layer of dross.
Thanks to improvements in plasma torch and consumable design, the piercing capabilities of plasma are now so significantly better that it’s time to revisit the factors that have traditionally impacted piercing, as well as the technological advancements that are making plasma a worthy choice when it comes to piercing thicker material.
THE PIERCING PROCESS
Let’s begin by taking a look at the piercing process. When piercing with a plasma torch, the plasma arc attaches to the top surface of the plate and transfers enough energy to melt the metal near the top.
This molten material must then be removed, usually accomplished with the non-current carrying cold gas and the plasma shielding gas.
As this molten material is removed, the arc transfers the energy to the bottom of the pierce hole and melts deeper into the plate. This process continues as the arc penetrates deeper into the plate until it is through.
Operators of plasma systems employ various techniques, either alone or in combination with other methods, to address the limitations of piercing thick metals:
Stationary Pierce Method
In the stationary pierce process, the torch remains stationary during the entire piercing operation, generally at the manufacturers’ recommended pierce height (Figure 1).
After the plasma arc pierces through the plate, the torch drops to the cut height and begins the cut.
If there is a significant slag puddle on the surface of the plate, the torch can remain at an elevated height until the torch motion has moved beyond the pierce puddle.
This is the simplest and most straightforward piercing technique currently used in the industry. Limitations of this technique include damage to torch and consumables as well as increased operating costs.
Low Transfer/Stretch Arc Method
In the low transfer/stretch arc method, the torch is raised once the plasma arc has transferred to the plate, stretching the arc (Figure 2).
By increasing the distance between the torch and the plate, the torch and shield are somewhat removed from the path of the material being ejected.
A major drawback of this method is that the elevated torch height, when combined with the depth of the hole, can drive voltages to a very high level, thereby increasing the probability that the arc will lose energy and even snap out.
While the impact to the torch and consumables is reduced using higher pierce heights, the resulting pierce times are generally longer and not all lifters and controllers can perform this technique.
Moving Pierce Method
With the moving pierce technique, the torch is positioned over the plate some distance from the desired pierce point. Shortly after the plasma arc has transferred to the plate, the torch motion begins.
As the piercing process takes place and the torch moves, the plasma arc penetration depth increases, creating a trough that directs the molten material and sparks away from the torch, in the opposite direction of motion, instead of directly back at the torch (Figure 3).
Operators using this technique need to ensure their lead-in lengths are long enough to allow full penetration of the material. Operators also need to be extra cautious as a “rooster tail” of sparks can spray off the cutting table and potentially cause a fire.
Double Pierce Method
The double pierce process starts by positioning the torch over the plate at the maximum transfer height of the system.
Continue the same process as with a stationary pierce, until molten material begins to spray back at the torch. This usually occurs when the pierce is about half way through the material.
Shut off the plasma arc, clean the slag from the plate and reposition the torch to the side of the partially pierced hole as though for an edge start. Finish the process by piercing a second time through the plate (Figure 4).
Because the torch is positioned at the edge of the partial hole, the molten material will spray away from the torch.
Although this technique can greatly increase the piercing capability of a plasma system, drawbacks include significantly larger pierce holes, increased material waste and much longer piercing times.
Pages: 1 2