Methods for back purging

[dgbingham]

I have to do some sanitary welds in SS. I tried using some aluminum foil tape that I got from Arc Zone to build a tent for purge gas but it melted almost instantly and made a real mess. Has anyone used the insulated tape that they sell? If so how would you use it? By that I mean would you apply it directly to the back of the weld and hope it does not melt instantly and make a really bad mess or use it to make a tent for a gas purge? The reason I am asking these questions is that this tape is quite expensive and I would rather not have to waste a lot it in experiments and the equipment I will be welding is also very expensive so I don't want to experiment on that either. I bought some Solar Flux but I have read that it can be hard to clean up the residue. Precisely what I am doing is welding SS fittings in the side of a SS pot/kettle. I will be using a Dynasty 200 for the welding. I tried this a few years ago and ruined some very expensive pots.

[Rojodiablo]

There are so many options for this. Can you get a line splitter, and run a small second argon feed directly to the area you're welding?? If not, you can use tin foil, but what you may want to do is build it like a pocket or cup, bigger than the area you need to weld. Then, clamp it down where you can get a clamp to hold it. If you can get the edge away from the weld a few inches, you may get tape to work before it heats up too much.

Another way is to use a piece of wood over the edge of the foil; hard to explain other than say the foil is a cup with a flat edge. Your clamp will clamp the wood to the pot. The wood lays over the foil edge, and clamps the foil in place. If you can make that work, then preflow into the weld gap for a second or 3, and go. If the 'cup' you make has to cover a spot on the backside that is solid, then tear a small hole in the top of the foil, put your tip in, and preflow for 3-4 seconds, then weld the other side. It's not a perfect world, but these are some ideas for you to try out.

[dgbingham]

"Can you get a line splitter, and run a small second argon feed directly to the area you're welding??"

Yes, I have a dual regulator set up for that purpose. Is foil the best choice for a tent or is something else better? I have never really seen any pictures of this process except for pipe. Building a tent for backpurging a flat sheet is glossed over in all the texts I have read.

[revhard]

I had to do some small pieces in basically flat shape years ago.
I just quickly built a little box with some thin aluminum I had around, but any metal would work. I just tacked it together, then drilled a hole for the argon feed, and sealed it in with a paper towel(the argon hose end in the hole). It worked great for that job. It was only about 1" deep, and the job required it to be 10" long and 3" wide.
It worked way better than anything I could come up with in foil or tape back then.
Took about 30 mins to make, and I welded over 100 pieces with it.
I wish I had something more I could share, but that's it. Maybe it will help..

[Rojodiablo]

Originally Posted by dgbingham

"Can you get a line splitter, and run a small second argon feed directly to the area you're welding??"

Yes, I have a dual regulator set up for that purpose. Is foil the best choice for a tent or is something else better? I have never really seen any pictures of this process except for pipe. Building a tent for backpurging a flat sheet is glossed over in all the texts I have read.

Maybe make a box out of thin sheetmetal and rest the part on top of it is all I can think of.

[dgbingham]

Thanks, I have some aluminum flashing around here somewhere so I will build a box from it. I have one more question: suppose you were welding a fitting, say a 1.5" long 1/4" pipe nipple, into a sheet with 1/2 of the fitting on either side of the sheet and you put a purge box on one side. Would you just let the purge gas exit through the fitting or plug the fitting and put an exit hole for the gas at the level of the sheet?

[hotrodder]

For one off/odd shaped applications i tend to use aluminium foil for the tent itself, making it big enough to avoid burning the tape used to seal it. The list of various purging methods is huge... a length of Al with a groove ground/milled in it works well for straight seams (makes a good heat sink too). If you work with a lot of small items it's worth making a purge tray- knock up a shallow box, add a fitting for gas inlet and fill part of the volume with stainless wool. A perforated shelf allows argon to flow up over the part being welded. The sides of the box help reduce the amount of argon lost and are easily extended with some tape and card/offcuts.

Originally Posted by dgbingham

<snip>...Would you just let the purge gas exit through the fitting or plug the fitting and put an exit hole for the gas at the level of the sheet?

Depends on the orientation of the part. Given that argon is denser than air, ideally the purge wants to enter at the lowest point with the exit being at the highest. I would be more inclined to use the fittings as a purge entry and arrange the exit at a high point in the 'tent'

[dgbingham]

Thanks for the input, hotrodder. I will try some of those suggetions.