Plastic Welding

By TimGary (****)

Date 08-15-2004 13:26

Hi Jerry,

I've been around plastic welding for a few years now. It has many positive benefits. I've seen it used on oil rigs for acid piping systems, high pressure reverse osmosis water purifying systems, square structural tanks for holding acid, and basic water piping systems.
One of the things I like best about it is that if you have a glue bonded joint that leaks, you can repair it with a plastic weld instead of cutting out and replacing the whole leaking joint.
One thing that I found kind of surprising about plastic welding is that if you are performing under the ASME code, the welder qualification requirements are more strict than that for arc welding.
I remeber some plastics discussions on this forum about a year ago. You may find them by using the search function.
What are you using plastic welding for?

Tim

By jfwi

Date 08-20-2004 19:38

It is a 2.67" wall 24" diameter pipe fresh water pipe being pulled through 1600" of directional drilling. The contractor is using the hot plate method for fusion. We have search ASTM for testing methods with D638 as the only real results. The owner and the contractor both asked to bend test mentioned in D2657 and 7 days later we have not found an establish test method other than the tensile.

I see plastics being use more in the construction field as Giovanni mentioned, but other than testing for sheets there is not real test methods for plastic pipe. I found a contact that is using a German standard.

That my update at this point, if anyone has any other information please pass it on.

Jerry

By G.S.Crisi (****)

Date 08-20-2004 20:02

I'll clear a little bit more my first posting.
Plastic behave in a quite different manner than metals do, it's of no meaning to submit them to bending (flexion). Most of them will bend to 180° without breaking.
Welds in plastic pipe are submitted to tensile test. Longitudinal specimens containing the weld in the middle are cut from the pipe, put in the testing machine and pulled until they break. If they break outside the weld, the weld is approved. If they break in the weld, it's rejected.
It's Friday evening now and I'm at Mackenzie. To morrow, at home, I'll search for the construction practices of pipelines I've got at home and Monday I'll come back with some more solid answer.
Now, to be frank, I'm surprised at using a 2,67 inches wall pipe for fresh water, buried píping. Is the pressure that high to need such a thick wall?
Giovanni S. Crisi

By G.S.Crisi (****)

Date 08-30-2004 20:37

Sorry, folks! I couldn't find anything in English language regarding testing of plastic welds.
However, I did find a mention to Italian standard UNI 9737, which deals specifically with plastic pipe welding.
Giovanni S. Crisi

By G.S.Crisi (****)

Date 08-16-2004 20:57

There are three types of plastic welding: hot air, friction and hot plate.
With hot air, the welder holds in one hand a manual nozzle that blows hot air and a plastic rod in the other hand. The plastic rod has the same chemical composition as the base plastic. With the hot air jet, the welder melts the base plastic bevels and the rod tip, in a similar way as is done in oxy-acetylene weld. When they solidify, the weld is done.

With friction welding, a plastic of a round shape is firmly held in position. Then, another plastic of the same shape is pushed firmly against the stationary one and begins to rotate. The friction between the two materials (the stationary and the rotating one) generates a large amount of heat which melts both ends. The rotation is then stopped and both plastic ends cool down and get welded together.

The hot plate method begins with two pieces of plastic of the same shape that are separated by a couple of inches distance. A hot plate is inserted between both and they are pushed against it. The temperature of the plate is sufficient to melt the pieces' ends. Then the two pieces are taken away, the hot plate is removed and both pieces are pushed against each other until the melted material solidifies and the pieces get welded together.

The hot air method is manual, the other two are authomatic. Further information on the machines that are used you'll find in the following site: www.omicronitaly.com

Plastics behave in a quite different manner than metals. They don't cristallize, so there's no grain growth nor structural changes in the heat affected zone. To test the welds, a tensile test is carried out in a specimen. If the specimen breaks outside the weld, it's considered approved. If it breaks within the weld, it's rejected.

Giovanni S. Crisi
Sao Paulo - Brazil