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Top 10 Factors Affecting Plasma Consumable Life

September 23, 2013

Things that can cause poor nozzle and electrode life

There are a lot of things that can cause problems with plasma torch consumable life, and troubleshooting a problem can be complicated and time consuming. So we’ve put together a list of common things that you can look for if you are having problems, or if you just want to make sure you are getting the most from your CNC plasma system.

Here’s 10 things to look for if you are experiencing a reduction in plasma electrode or nozzle life:

1) Low Coolant Flow

The electrode is emitting a lot of power, which means it will generate a lot of heat. If there isn’t plenty of cooling fluid flow to carry away the heat, the electrode will quickly heat up and fail catastrophically. This usually means replacing the electrode, nozzle, and possibly the electrode holder, gas baffle, or other parts that got damaged in the process.

2) Low Gas Flow

Not only does the plasma gas constrict the arc as it goes through the nozzle orifice, but it also cools the nozzle, and a thin boundary layer of cool gas keeps the nozzle from melting rapidly. If there is not enough gas flow, the inside of the nozzle will heat up quickly and start to melt, causing premature failure.

3) Incorrect Pierce Height

When a plasma torch pierces, there can be a large volume of molten pierce spatter that has to go somewhere. If the torch is too close to the plate when piercing, the spatter can contact the shield and/or nozzle and either cause melting or double-arcing. This requires a plasma height control system that will set the correct piercing height. But this can also happen if cutting thin material that springs up after the torch touches the plate, causing the initial height to be lower than expected. Therefore, a height control system that incorporates an electrical through-the-torch contact (ohmic sensor) is best.

4) Running Off the Plate Edge

Under normal circumstances, the plasma shut-down procedure turns off the plasma arc gracefully and safely, minimizing the effect on the consumables. However, if you are manually cutting off scrap, or if an automatic program drives the torch off the edge of the plate, the arc will “snap” off, and several bad things can happen. First, the arc is stretched as the power supply ramps up voltage while trying to maintain the arc. When voltage goes up, power goes up (Power = Amps x Volts). Putting more power through the electrode causes more wear. The second thing that can happen is the arc gets pulled off to the side as the torch moves off the edge of the material. This can cause the arc to contact the nozzle or shield, damaging the orifice. Always try to shut off the plasma in the scrap area, so the proper shut down sequence can occur.

5) Crashing the Torch

Sometimes stuff happens. Sometimes it happens while the plasma torch is cutting. When the plate pops up, a part is tipped up, or there is a ball of slag on top of the plate, the plasma torch can run into these obstructions and cause the torch to “crash”. While the CNC machine should have a system that automatically shuts down the arc when the torch crashes, sometimes it can be too late to avoid damage. If the nozzle contacts the plate while cutting, it can cause double-arcing, or it can damage the orifice.

6) Incorrect Current Setting

Most automated systems these days will set the plasma cutting current automatically. But if you have a manual system where the operator has to set the current by hand, setting the wrong current can take out a nozzle in a heartbeat. Putting 600 Amps through a 200 Amp nozzle just doesn’t work. When that happens, you know right away. But what if you are running a 260 Amp nozzle, but your plasma power source is actually set for 300 Amps? It may not fail right away, but nozzle life is going to be a lot shorter than expected.

7) Contaminated Gas or Water

Impurities in the plasma gas can cause deposits to form inside the nozzle, and can also cause double-arcing, which damages the nozzle. The same is true for cut water when using water injection nozzles.

8) Incorrect Cutting Parameters

Plasma cutting on modern CNC plasma cutting machines can require a lot of settings, including things like cut speed, kerf width, cutting amperage, arc voltage, gas pressure, gas flow rates, pierce time, and many others. To simplify setting all of these parameters, most systems use a database built into the CNC that allows the operator to select a parameter set, and then everything is adjusted automatically based on that parameter set. But what happens if you pick the wrong parameter set for the electrode and nozzle that you just put into the torch? You know, stuff happens.

9) Wrong Consumable Set

Likewise, you might pick the correct parameters for the material you are going to cut, but mistakenly put the wrong nozzle in the torch. This can also cause extremely short consumable life – like 1 pierce if you get it really wrong.

10) Wrong Coolant Type

Just like low coolant flow can reduce electrode life, having the wrong coolant in the system can do the same. Never use regular anti-freeze solutions, such as for an automobile, as the additives will harm the pump and torch. Plasma torch coolant mixes with the highest percentage of pure water will give longer electrode life, as water is actually the best fluid for removing heat from the electrode. Mixes with a higher percentage of glycol will give better freeze protection, so you may have to trade some electrode life in favor of a lower freezing temperature depending on the machine’s environment.   

 

Posted in Cutting Systems , Tagged with Plasma, Process

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