Welding close to bonded plastic - advice

[Pistom]

I've got a welding job to do which is about 50mm from an epoxy bonded plastic component which is attached to the steel I'm welding.

I'm wondering if MIG welding this is a good idea and how to prevent the heat destroying the epoxy bond? I believe the epoxy is the limiting factor as that can only take up to about 120 to 140 deg C.

Is using a heat blocking putty an idea?

I'm thinking of welding short tacks to minimise heat build up.

[E-bmw]

Don't know how good the heat blocking putty is so can't comment on that specifically.

If it were me & the job had to be welded I would be stitching it & have a very wet rag between the plastic & the weld & use another wet rag on the weld metal between stitches.

[Krikkit]

E-bmw said:

If it were me & the job had to be welded I would be stitching it & have a very wet rag between the plastic & the weld & use another wet rag on the weld metal between stitches.

This ^ and give a good amount of time between stitches - do 10mm, wait for it to cool almost completely, then repeat.

Definitely cover the plastic, for spatter protection and reducing the UV and IR you'll get hitting it.

[JohnMcL]

Can you not cut the epoxy bond, move the plastic component out of the way, do the weld, let cool, redo the epoxy?

[E-bmw]

Krikkit said:

Definitely cover the plastic, for spatter protection and reducing the UV and IR you'll get hitting it.

Good point also.

[Pistom]

JohnMcL said:

Can you not cut the epoxy bond, move the plastic component out of the way, do the weld, let cool, redo the epoxy?

That's a really good idea in principle but the difficulty is cutting the bond. It's very difficult to get at.

I think the best idea is very short stitches which are allowed to cool.

The heat resisting putty won't do any harm.

[Pistom]

Other than an infra red contactless thermometer, I wonder if there's some device I could use to log the temperature the metal gets closest to the bonded part?

[hellorent]

https://www.lidl.co.uk/p/parkside-infrared-thermom...

[GreenV8S]

The putty is only really useful on thin panels and is unlikely to help on anything structural.

You definitely need to do the welds in small sections with plenty of cooling in between. Do you have the option to TIG it? That can be done more delicately.

[Pistom]

Yes, TIG is a good idea.

[donkmeister]

Amateur plumber here, never welded but many times have needed to solder in close proximity to plastic and rubber and needed to mitigate the same issue. I appreciate that the scale is a little different as copper pipe doesn't have the same thermal mass as chunky steel structural elements and soldering is a much lower temperature, however the science is the same - keeping one bit cold while another bit stays hot.

Take lots of old rags, soaked in water, wrap them around where the epoxy bond is, and as close as you can get to the site of the weld without ruining your weld. Also wrap them around the plastic itself - I know it's not a naked flame but if it's hot enough to melt steel then it's hot enough to melt plastic not too far away. The water acts as a heatsink both through getting hotter, and if it gets really hot through vapourising, ensuring that the temperature away from the welding site stays lower.

You CAN get a special gel at plumber's merchants. Never used it myself but it basically evaporates to protect the surface underneath.

If it were me then afterwards I'd be giving the bonded join a thorough knocking about as much as seems reasonable to replicate its service life (up to you to judge how much force that would be!) to check it hasn't weakened the bond - I'd rather break it when the car's in bits than when driving.

If it DOES all go tits up, do you think you'd be able to grind off the epoxy and redo the bond? Epoxy isn't witchcraft; it's simply a different process to follow (and getting the correct materials of course). sorry, just re-read and saw you'd explained it would be a bugger to get to.