Welding plastic

Has anyone here welded plastic using a hot air "torch," like the one Harbor Freight sells? My first efforts were horrible. It's better, now that I
realize that you don't make a puddle, like you do with O/A. What I'm doing now, which seems to work, is drive the filler into the vee-ed out joint, letting it bend into place, and keep moving. This produces a fairly neat bead. but I'm wondering whether there might be a better technique, strength-wise.
Any help will be appreciated.
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Don't know a damn thing about it, but I'd think cleanliness of materials and ambient temperature may play a large part. Don't know if there's any way to prep the surfaces that would be beneficial. Will have to wait for someone who knows.
Steve
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I have done a little plastic welding and watched a lot of it. It is my impression that "Plastic Welding" has very little relationship to Mild Steel Welding. Len has seen what I recognize as the way things are. The hot air blast softens the base material and the filler rod. they melt together. Penetration as we know it in SMAW (stick welding) does not seem to exist. I have seen experienced welders lay bead after bead to try to achive some strength. Triple beads were not uncommon. I hope this helps.
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SteveB wrote:

I haven't tried it, but I'd expect the most significant factor would be ensuring your filler plastic is the same type as your base plastic.
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I really do not have a clue in that I have never tried it. But if I were trying, I would try using an inert gas to see if it makes any difference. CO2 ought to be inert at the temperatures one uses for plastic. It also sounds as if trying to stir with the rod might help. Probably would look worse, but may have better strength.
Dan
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wrote:

I really do not have a clue in that I have never tried it. But if I were trying, I would try using an inert gas to see if it makes any difference. CO2 ought to be inert at the temperatures one uses for plastic. It also sounds as if trying to stir with the rod might help. Probably would look worse, but may have better strength.
Dan
Doesn't CO2 come out cold?
Steve
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SteveB wrote:

Pretty much all gasses that have been compressed will cool after being decompressed. You just need to run it through enough line for it to warm to ambient temp. That said, I don't think inert gas shielding has any relevance to hot air plastic welding.
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wrote:

I would think so, too, and in the absence of any oxygen at all, may decrease the bonding related to oxygen and nitrogen, which is 4x O2.
Still waiting for someone who really knows this to chime in.
Steve
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it makes any difference. CO2 ought to be inert at the temperatures one uses for plastic. It also sounds as if trying to stir with the rod might help. Probably would look worse, but may have better strength. ^^^^^^^^^^^^^^^^^^^ The welder is designed to be used with an air supply. The "flame" is a stream of hot air which melts the plastic. If I had to do this in a location where I had no air supply, I can see how a small CO2 tank might be substituted, but I doubt that it has anything to do with shielding. There is no evidence that the plastic, as it melts, is oxidizing.
Stirring with the rod: That was my first mistake. The material does ot go liquid, like metal. Once welding temperature is reached, as soon as the rod touches the parent material, it sticks. Then, if you move it, you pull out filaments of plastic, which are no longer part of the weld, and just make a mess.
Thanks for the CO2 idea, though. I may try it to see whether I can dispense with the compressor.
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I would not expect that the plastic oxidizes significantly, but would expect some oxidation. So would only expect a difference to show up in strength, but not visually. And it may not be a factor at all. Just something that I would try it was easy to do. Testing to see if there is a difference in strength might be difficult if the difference is 20 % or less.
Dan
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snipped-for-privacy@krl.org wrote:

The basic assumption is that the "welded" area will only have 70 - 80% of the surrounding materials strength, more from the way the weld occurs and the materials used than from outside factors.
One trick I do use when I really need strength is to use some stainless screening. You heat the plastic and the screen and sink it in like re-bar in concrete. Makes a big difference.
--
Steve W.

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Leo Lichtman wrote:

Bought one myself a few years ago. What you are doing is the basic process. One thing that I do to get better "penetration" is to V the joint but use a ball tip and leave a small lip at the top. Then use the filler rod. Reheat the joint and use a teflon paddle to smooth the bead down into the joint. The paddle I use was made from an old cook pan section, cut and bent. Works better and gives a stronger and nicer looking joint.
The hard part is getting the proper filler rod. Oh if you want a neat portable system grab one of the small airbrush compressors. Allows you to use the system anyplace with a standard electric outlet.
--
Steve W.

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"Steve W." wrote: Bought one myself a few years ago. What you are doing is the basic

^^^^^^^^^^^^^^^^^^^^^ Thanks, Steve. You're just the person I have been looking for. All of your suggestions are bound to improve my welding. Using a Teflon paddle to smooth the weld is something I would never have thought of. I'm going to start shopping for a small airbrush compressor and a Teflon frying pan at the flea markets.
The instructions are very adamant about keeping the airflow going for ten minutes after electric heat is turned off. That seems like an awfully long time--the "torch" seems to cool off much sooner than that. Do you stick to the rules, or do you "cheat" a little like I do?
Thanks for your help
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Leo Lichtman wrote:

I usually leave it running 5-6 minutes or so. Compressor wise the little diaphragm ones work well.
One other add-on with mine is a speed control, it allows me to control the temperature of the element. The lower temps allow use on thinner materials a bit easier.
I also made a small set of legs for mine out of model airplane landing gear. Makes it easier to control because you just guide it. They are just spring wire. I used a mandrel just a bit smaller than the barrel to form a double loop with two legs and small wheels. Squeeze the legs and it slips off. (Larger version of the wire hose clamps in use now)
I looked at the one they have now with the built in blower. It has a thermal switch that controls the blower motor. It turns on/off as needed. Plus it has built in temperature control.
--
Steve W.

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Among many at Youtube. I've got a welding book with a chapter on plastic welding. I recall nitrogen being used on polypropylene

http://www.youtube.com/watch?v=cgTQlID-jms&feature=related


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Even better;
http://www.youtube.com/watch?v=UTssoaX7h5c&feature=related

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Rick wrote:

That makes it clear - it really isn't "welding" at all. Not even brazing, as the plastic never fully melts.
Thanks, Bob
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Bob Engelhardt wrote:

Oh, you can get it hot enough to flow. I will do that if the material is backed up with ceramic (smooth and heat resistant).
--
Steve W.

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It looks like MIG to me. I'm sure the tool melts the corner were the plastic is then flowing - it being melted on-the-fly. The
Something like running a solder iron down a seam adding solder as you run. Takes a big iron or one with a high enough wattage to keep heating more and more. Big Black Beauty. 150 or 200 watt in an iron 2.5" in diameter.
Martin
Steve W. wrote:

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Martin H. Eastburn wrote:

The HF iron is 500 watts.
--
Steve W.

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