Welding spatter

[David Wilson]

Fellow optiboarders,
I have been asked a question by someone not happy with the "Just because it does!" answer. Apparently when I learned this fact I was happy with that answer. Anyway, the question is, why does welding spatter bounce off plastic lenses but burn glass lenses.
My guess is that the greater thermal conductivity of glass causes an instant transfer of heat from the spatter to the glass, causing burning of the surface by a now solidified spatter, whereas the lower conductivity of plastic leaves the spatter as hot liquid, thereby bouncing off the surface.
Another possible answer is that plastic is more hydrophobic than glass, repelling the liquid spatter.
The problem I can see with either of my theories is the time factor. That is, is there time for these effects to take place?
Darryl and Steve, given your intimate knowledge of CR-39 manufacturer, perhaps you've thought about this before. Incidentally, Darryl, I've just read you post about your move. I'm sure that Sola's loss will be some other astute company's or college's gain. It's a real shame that Missouri is not close to Sydney!
Regards
David

[Darryl Meister]

Hi David,

It might have something to do with static electricity/electrostatic charge, too.

Best regards,
Darryl

[harry a saake]

David, i have often wondered about this also, and wonder if it is the conductivity of the material. Just as you find certain metals conduct electric better then others, but there all metals.
.................I also pondered whether or not, it is due to the ambient heat, within the material. I would think the two materials being so vastly different would retain different amounts of heat, due to there molecular structure. Perhaps the one that was hottest would be more prone to sticking.
.................Seems like it would be a good experiment to place the two lenses under different controlled tempertures, and then hit them with welding slag to see what would happen. Anyway just some thoughts and will await replies from you and the meister.

[hcjilson]

I rarely come into this forum alone!

A management professor of mine once said "You don't have to have all the answeres-but you should know where to find them.With that in mind-after reading the question posed by David, I thought my brother in law could shed some light.I emailed him this page and his reply follows.I would like to preface his reply by saying he is a good teacher as well as a physisist, but like many teachers, he won't spoon feed you.Before dismissing his "I dunno!" go back and re-read what he has written.Pay no attention to his Washington statements!:D :D :D
his reply:


> From,
> hcjilson
> Bob this subject just came up and I was wondering if you could shed any light on this phenomena.Welding spatter sticks to
glass...but not plastic lenses.( I say sticks because it feels like whatever it is stuck to the lens)They say "burned" and
maybe they're right.Whatever happens, it doesn't happen with plastic.I thought you might have the answer locked away
somewhere.Tv from harry
>
> OptiBoard Discussion Forums takes no responsibility for messages sent through its bulletin board systems.

Hi Harry-

(Sent directly to you, not to the forum)

Phew: at first I thought the subject was "wedding spLatter".

There seems to be two questions, they appear to ask why spatter
bounces off plastic lenses, but burns glass, and you talk about
"sticks to glass." I guess you feel the answer is "burns because
it sticks."

OK, first answer is that science and physics (my speciality)
gives one some ideas for answering such questions and/or
calculating from first principles the results of real-world
events, but in the end, given some ideas about mechanisms of
causation, only experimentation can confirm them as being correct
or not.

Aristotle held that a heavy object dropped from a height will
fall faster than a light object, which seems intuitively
"obvious". But some 2000 years later Galileo, in his Tower of
Pisa experiment showed that this was completely false. Another
400 years passed before Einstein revealed the reason for
gravitation in a satisfying and consistent theory called the
General Theory of Relativity.

So with this is mind, on the "sticks to" side, surface contact,
wetting, etc. is a very complex and poorly-understood subject.
During my extremely long Ph.D. research (a.k.a., "the
crucifixion"), I needed to solder wires to leads of a envaporated
coating of a metal structure on a glass (Pyrex) plate. To do
this I had to first paint gold-resin electrodes onto the glass
plate and then bake it in an oven to create the gold electrodes.
I then used pure gallium solder and an absolutely brand-new,
clean miniature soldering iron to glob molten gallium onto the
gold electrode, while simultaneously bonding on a very thin wire
of copper. This then worked at liquid helium temperatures.

I "knew" this would succeed because it was part of the
laboriously gathered lore of low temperature physicists, who must
battle extreme materials problems due to thermal contraction and
the failure of substances at very low temperatures near absolute
zero.

Gallim sticking to glass was not intuitive nor could it be looked
up or calculated (as far as I know.)

I would think sticking would have to do with the details of the
chemical and physical nature of the surface, and for sure whether
it could be presumed clean or not. But it would seem only some
experiments would settle matters.

In sum, you see from the above that I am saying, "I dunno". :)

Check back in about 300 hundred years, "they" will probably know
by then. Maybe.

(Given the current know-nothings in Washington and the state of
science knowledge and funding, "maybe never".)

Bob
--
Robert Lynch Berkeley CA USA

[chip anderson]

I think Harry has most of it, also most plastics have some silicate coating. This is very bad for welding fusion.
Chip

[David Wilson]

Thanks, guys.
Useful information . Thanks for chasing up with your physicist friend, Harry. I have also been asking around the halls of academia. I've tried three PhDs, (two in optometry and one in engineering) but without any more definitive answer. It seems that this is something we have all believed but have never known why. But I haven't given up hope yet.
Regards
David

[Darryl Meister]

I also had some mixed theories from a few scientists. A physicist I know suggested that it might have something to do with thermal conductivity, since glass transfers the heat much faster than plastic. However, a chemical engineer I know suggested that the limited contact during the "bounce" would probably be insufficient for the thermal conductivity idea. This actually summarized David's initial thoughts on it. The chemical engineer did suggest that "surface energies" might play a role in the affinity of the glass to the splatter.

Best regards,
Darryl

[Darris Chambless]

Hello there,

You see this is what you get when you have too many people with IQ's higher than the average person getting to gether to solve or explain a simple fact. Sorry guys but let one that welds explain it to you...step aside, listen and learn :)

Think this way, rate of convection and liquification temperatures. Now I'm not going into the scientifics of this, but am going to explain it in laymens terms. Darryl and Harry can get the science for it ;) (Also, Harry, why was your friend trying to make an anode?)

Anyway...Glass has a much higher melting point than plastic. When a material that has a slightly higher melting temperature than glass comes in contact with it it will bond becuase it reaches it's bonding temperature (i.e. welding pool for flow welding two joints together) The slag actually bonds to the glass surface but in most cases falls out because of the rate at which each material cools down and contracts.

Plastic, on the otherhand, has a much lower melt point so when the hot slag hits it the surface will instantly liquify in that spot not allowing the slag any time to bond. Since Plastic will cool almost instantly very little evidence of the slag hitting it is left behind.

If you want two extremes as an example take a twelve inch square piece of wax and a twelve inch square piece of metal and weld next to them. The slag will stick to the metal because the melting temperatures are the same or very close (that's why flow welding and some types of metal bonding of disimilar metals works). The slag will hit and fall off of the wax because it melts instantly and cools almost instantly. No time for runs in the wax surface to be apparent.

In short Slag can't bond to plastic because it melts too fast and very little sign of slag hitting it is left behind because it cools too fast. Not to mention, plastic is a poor conductor of heat which is why the slag doesn't go all the way through the lenses. It only melts on the moment of contact.

I hope that is what you were looking for. Science is great but doesn't always explain it the way practicle application can. Now it's up to Darryl to come up with the science ;)

Take care gang,

Darris C.

PS. One final anology since I'm on a roll here. If you take 1 cubic inch or liquid rock and throw it at a pile of dough you end up with burnt bread with a rock in the middle :)

[David Wilson]

Thanks, Darris,
An excellent explanation. I think that it's fairly obvious that I haven't done any welding. I must say that I was thinking along the same lines as Darryl, and have even been given the last theory you mentioned, Darryl, by someone over here.
As you say, Darris, there is quite a difference between the temperature at which plastic (CR-39) burns, 100 degrees, and glass melts (becomes more viscous), around 700 degrees. Your answer satisfies the problem with all the other theories, the time factor.
Regards
David

[Darryl Meister]

I dunno, Darris... Although it sounded neat, I don't know that it's really possible. You definitely get an "A" for effort though. ;)

As David already pointed out, CR-39 is a thermosetting resin and doesn't really have a melting point. Consequently, it would burn rather than melt and cool rapidly.

However, Darris might have a point in the remainder of his posting that could augment some of the other propositions... Perhaps the welding splatter strikes the glass, which has a melting point and high thermal conductivity, and then rapidly melts into the glass surface. Whereas welding splatter hitting hard resin, which transfers less heat and wouldn't melt anyway, simply bounces off.

Best regards,
Darryl

[Darris Chambless]

Darryl,

You silly, silly boy :) Has anyone ever done a study or a test on the effects of welding slag on plastic lenses during the actual process? I think not. So...here's what we can conclude. The welder (that being Darris :) has decided that his facts are facts and that the surface momentarily melts and instantly cools which leaves little or no trace of the slag. I've also seen the surface of plastic lenses worn during torch welding that were, infact slightly melted (not burn) in small spots from flare up when the torch got too close to the weld pool. The spots looked like smudges but wouldn't come off.

My deduction from practicle application is that CR-39 will liquify under these circumstances on a very minisquel level which is why the slag seems to bouce off of it's surface.

I must now go into the lab and experiment.

Darris C. :)

[Darris Chambless]

As Chad is my witness...:)

Hello,

I'm back. Chad and I both went back into the lab, started up the old soldering torch and did a little experiment. With a monetary shot of blue flame on the surface of a CR-39 plastic lens the surface remained clear but slightly crinkle if you caught it in the right light. Then we held the torch on the surface for two seconds and got a cloudy but not burned spot on the surface. I ran my fingernail across this spot and found it to be waxy feeling (who would have figure ;) Then we held the torch on the surface in another spot for about three seconds and got a much more pronounce indentation and again the cloudy looking spot.

We then took the torch to the outer edge of the lens and held it there and we did see it liquify for split second and then turn chalky and remained slightly pliable while being heated. So Chad and I have done the experiment and deduced that we are correct :)

Take care,

Darris C.

[Darryl Meister]

Hi Darris,

It doesn't really sound like you and Chad actually melted the lens though, does it? It sounds like you may have blistered the surface with the heat, and perhaps burned some stuff either on or off the surface? Maybe affected some of the unreacted monomer? (And this is assuming that the lens didn't have a hard coating.) But it doesn't sound like the lens actually softened throughout its entirety or began to flow in a viscous state or anything. Did the lens continue to feel waxy, even after it had cooled?

Best regards,
Darryl

[Pete Hanlin]

You're ALL wrong! Okay, if you take a stick of butter and fling a hot spoon at an angle precisely 46 degrees...

Gee whiz guys! As someone who has inserted a CR-39 lens into a Kirk Oven (now, how many here can say- er admit- they've actually done this), let me assure you that CR-39 burns (not melts) into a fine powdery substance that gives off a lot of heavy black smoke.

Its hard to believe that enough heat could be transferred in the instance the splatter makes contact with the glass to cause the glass to melt. My instincts (which are inevitably wrong, btw) tell me that the plastic lens may be more hydrophobic than the glass, so the splatter simply bounces off because it "beads." It would be interesting to see if the splatter sticks to an AR coating which does not have a hydrophobic coating (or conversely, to a glass lens with a hydrophobic coating).

Pete "neither a scientist nor a welder, so..." Hanlin

[Darris Chambless]

Pete,

Please stop clouding the issue with facts! Since you're not a welder or a scientist them you need to understand that if I say I'm right and Darryl says he's right, then I'm obviously right. Why because I said so that's why and this is my post so there. Neyah :)

"My instincts (which are inevitably wrong, btw) tell me that the plastic lens may be more hydrophobic than the glass, so the splatter simply bounces off because it "beads." " Pete, what do you have against "sheeting" action? After all it's much better for liquids to "sheet" off of a surface than to "bead" so where does this prejudice come from, huh? Plus I've never met a plastic lens that was afraid of the water, in fact most just jump right in when you toss em. So would you mind explaining yourself you "bead" hater! (but we know that's not true don't we? But that's a different story ;-) :)

Actually Darryl...the plastic did in fact liquify then caught fire and turned chalky looking, but that was with prolonged exposure to the flame itself. The spot that felt waxy is still at this monent waxy, so yes it stayed that way even after cooling down. All in all it was an interestig experiment. Chad and I both got high off the smoke we produced while doing the experiment. I think Chad even saw God and still yells "BIG DOGS!!! BIG DOGS!!! LANDING ON MY FACE!!!" :) He'll be okay though.

The true scientific explaination for this (which I didn't want to give in the beginning because I was having too much fun giving you guys a hard time :) has to do with heat transfer through a liquid or solid. Plastic absorbs heat at a very rapid rate and disipates it at a rapid rate which causes the offending liquid metal to cool instantly, turn into a solid and bounce (yes, I said "bounce") off the plastics surface. Glass obsorbs heat much slower but will retain it much longer giving its surface time to liquify and allow the metal to stick or embed itself in the surface.

With that said, microscopically we may never know the whole truth about the illusive welding spatter. BUT we do know that it sticks to glass and doesn't stick to plastic, thank you Captian Obvious :)

Take care yous guys,

Darris "I can see Nervana from here" C.