How to control the TORCH nozzle and electrode

[howied]

Dear all,

I would like to know how do you guys control your plasma spraying torch electrode and nozzle; are there any established criteria for electrode and nozzle replacement? Something like to verify nozzle bore diameter against nominal one, if it is oversize (bigger, smaller?) above certain persent, then we shall replace. what about electrode?

How much wear rate of the nozzles electrode, I must replace new.


This is a NADCAP AC 7109/1 requirement.

thanks.
Howied

[Gordon]

Hi Howied

Good question, but probably not easy to answer. I only know of the rather subjective visual examination, water leak test and monitoring of voltage. The problem is erosion/wear of nozzles and electrodes is rarely even, usually it is a localised scar or gouge that ends their lives, which is difficult to quantify. Obviously you want to get the longest life from your nozzles/electrodes, but this needs to be balance with the risks to deterioration of coating quality and also damage to the plasma gun if a nozzle 'blows'. It really comes down to the experience of your people to judge at what point a nozzle/electrode becomes too risky to use.

I know we have a few members that must go through large amounts of nozzle/electrode consumables, and probably have a lot of experience with this problem. Comments and feedback more than welcome

[Alexangel1226]

Howied,

Best practices:

Nozzles:

1. Rotate the nozzle body in quadrant (~90deg) each time you spray 1~2 parts, depend on the spraying hrs too, tried more than 3 parts, the erosion happen too deep in and the rotation will not be effective after that. it seems that the more eroded area gets the attack more frequently. by rotating regularly, you will see an even erosion on ID of nozzle bore.
Well, marking on each rotation may help. Try it out. Also you may wanna develope your own guideline to show your guys on how a well rotated nozzle scrapping condition look like, or some kinda comparator, to show the condition. Theses visual comparator will assit the experience in determining what's good and what's bad.

2. Also to check on your chilled water conductivity, the higher it gets you will notice drop in cooling effect which result in faster wear rate. You will see formation of oxidized or carbon like material on the nozzle body if you were to remove the body from the shell. To change the water regularly once it exceed the limit. Else u will need to remove the nozzle more frequently to remove the carbon like material. But the former is the most recommended and convenient.

For electrode, pls check out the previous link.

hope this help

Alexangel1226

[Gordon]

Hi Howied & Alex

Good advice from Alex . I've always tried to rotate nozzles at regular intervals, believing that to be good practice and it sounds logical. But to be honest I can't claim in my case a big difference, because I hav'nt really been involved much with repetitive production spraying to analyse the effect on nozzle life. I do know that once a nozzle becomes gouged (localised attack) nozzle rotation has little effect, the plasma arc appears to be attacted to this area and will follow any rotation. So my advice would be to make sure you rotate the nozzles regularly from early on and not to wait for the first signs of scarring.

[Alexangel1226]

Gordon,

I believe you will not see much difference if you do not run the parts of same set-up, powder type or production in badge. Different power type spraying will give different wear rate, it's hard to quantify or measure, but by experience we can see it happening. Thus, it will be cost effective (material or man hr) to run all similar parts in lots, or back to back without having to change much powder type.

Then the nozzle or electrode will last longer. Also depending on the number of ignition and duration of spraying does play a main part of the wear rate. The number of ignition seems proportional to the wear rate. Compare operation which need longer spraying time, more frequent strat-up ignited ones though clock same running hrs will have worse wear condition.

Also last thing is we find, using Argon gas as primary process gas compared to nitrogen...is again a difference in nozzle shell life. Seems argon/helium gas is "mild" not attacking the nozzle as fast as nitrogen/Hydrogen. Also increasing 2nd gas will see a faster nozzle wear rate.

Regards,
Alexangel1226

[Gordon]

Hi Alex

I agree with your comments. Also, you made an important observation on the effects of different plasma gas compositions on nozzle wear. To add to this, the presence of oxygen, water or other contaminants can have a very dramatic effect, which leads us to your new thread today: Process Gasses Purity Impact towards Plasma Spray Process, equipment or outcome