Undoing ultrasonic welds?

[boogyman]

Is it possible to "unweld" ultrasonic welds on plastics?

I do LED brake light conversions on car tail lights as a sort of hobby, and usually will just use a dremel to cut off the rear part of the tail light to do the conversion, and use epoxy resin to seal afterwards.

I know that some people just use a knife tip on a soldering iron and basicaly cut the rear off.

I heard from a friend today that it's actually possible to unweld the ultrasonic welds that bond the tail light cover to the rear housing, so that you get a clean disassembly. He mentioned something about putting some kind of liquid on the weld and that the housing would separate after a certain amount of time.

Has anyone heard of anything like this?

 

[patprimmer]

It sounds lika absolute garbage to me.

If the weld is perfect, the two halves are joined as the materials melt and fuse together as one.

Having said this, the weld will never be perfect and is normally a weak spot that might be more susceptible to being prised apart. Usually the weld is a bit inconsistent and the part mainly breaks on the weld line, but occasionally the break will move away from the weld line and pull some material from either side at random.

I would cut the parts apart with a very sharp serrated knife, then use a cement made of solvent and the same resin as the tail light to glue it together again.

The solvent used will depend on the plastic involved. Most common for the non transparent parts are ABS, PC or PMMA.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dwightdixon]

Methylene Chloride or Ethylene Dichloride are the most common solvent used to make up the cement for polycarbonate lens. Be aware, they are suspected of causing cancer in large dosages. Ethyl acetate is the most commonly used solvent, if the lens are made from acrylic. You can find these solvents at many local hardware stores. Using a Dremel tool would be my suggestion also for cutting the assemblies in half with a predictable to produce a thin bond line for the final product. There are many clear epoxies which also would work to reassemble the two halves, if you do not want to deal with the solvent vapors.

 

[aspearin1]

Good welds create an integral product. It is possible, though, that if the two halves are poorly welded or are of dissimilar material, that the weld is purely mechanical at the joint, perhaps there is a solvent that can infiltrate the porosity at that joint and dissolve the weld before the more dense portions of the whole part. Typically, though, ultrasonic welding is performed on two pieces of the same material, and a good weld would impart significant pressure to limit such porosity.

Aaron A. Spearin
ASQ CSSBB
Engineering Six-S'$

http://www.Engineering6ss.com

"The only constant in life is change." -Bruce Lee

 

[bvanhiel]

Pulycarbonate (which may be what your lenses are) will craze and crack in the presence of stress and tap oil (among other chemicals). It's probably what your friend was referring to. I wouldn't use it to separate the lense. Exposing the rest of the lens to tap oil will damage them, even if the damage is not readily apparent.

 

[Demon3]

I fully agree, inducing environmental stress cracking to separate materials is not a good idea no matter how cool it may look you're in danger of destroying your parts.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell