Glass lens hardening
OK, Ladies and Gentlemen, I have a young man in the office that tells me that taking a glass lens that has been ALREADY heat or chem treated does NOT loose any of it's impact resistance when it is beveled down afterwards on a hand stone. Would like you common feedback please, for Jr, in the lab. Thanks......
Been a long time since I've worked in a lab with glass, but I remember edging and beveling before heat treatment, because it was unsafe to work on lenses afterwards, If I'm not mistaken, it was against OSHA, all fabricating had to be done before heat treatment. Good Question.
:cheers:
The lenses must be re-tempered/hardened.
It is dangerous to cut on a heat hardened lens, it could explode. Chemed ones you can reshape but should be rehardened.Originally Posted by Lonewolfharley
Explode is a little too drastic a term to use. They do have a tendency to "come apart" though.
Heat treating is a case hardening process where the lens is heated to a critical point slightly cooler than its deformation temperature and then quenched by blowing air simultaneously on both surfaces. The surfaces of the lens contract first resulting in a state of internal tension that makes the lens more resistant to fracture under impact.
Heat treated glass lenses that are scratched should be replaced as they may have lost some of their impact resistance and could come apart spontaniously. Re-edging them is a fools chore.
As rbaker stated, even a scratch on a tempered glass lens looses surface tention and is not as impact resistant. You MUST re-temper if you bevel/edge a tempered lens. (even if chem-tempered).
We do a lot of glass here and I agree completely with everyone. REHARDEN AFTER BEVELING !!
Thank You! everyone for your input, I think Jr, now might possibly learn something, like listening to us that have been around awhile.
The correct answer is that in the US, under FDA requirements, all glass lenses must be drop-balled after processing. That means ALL processing. Period. If it passes drop-ball, then it is OK to dispense. If it doesn't, well--it's broken.
There really shouldn't be any guess work. If you're compliant with FDA regulations, you're drop-balling.
RT
At one time I was casting .025 thick 28x28 inch CR39 flat sheet destined for laser cutting to a size for dosimeter badges. I used 30x30" 3/8" glass plates that were tempered.Originally Posted by gemstone
It is dangerous to cut on a heat hardened lens, it could explode. Chemed ones you can reshape but should be rehardened.
Explode is a little too drastic a term to use. They do have a tendency to "come apart" though.
Heat treating is a case hardening process where the lens is heated to a critical point slightly cooler than its deformation temperature and then quenched by blowing air simultaneously on both surfaces. The surfaces of the lens contract first resulting in a state of internal tension that makes the lens more resistant to fracture under impact.
Heat treated glass lenses that are scratched should be replaced as they may have lost some of their impact resistance and could come apart spontaniously. Re-edging them is a fools chore.
If there was a scratch or a ding on the outer side, there was a possibility of the plate breaking during or after demolding (the inside was very smooth and scratch free, also chem treated to keep the CR39 pure without contamination). I pulled the mold assemblys at 80°c and demolded, the plate was still hot to the touch. It was a muffled explosion but thank goodness the energy level of the "blast" was low. Man does a plate that size make for a lot of small pieces!
More than once I walked into the shop to find a mold from the previous cast cycle had "exploded" overnight, this after cleaning and place back into wooden storage racks. The internal stress and cool down released the energy after we had left the shop. The molds were warm, but not hot when placed into the racks.
When casting CR39 planos with 6 base molds, I had more than one mold come apart on me but never with the excitement generated by a disintegrating a flat plate mold.
BTW, I always use safety eyewear when working in the shop! To many things can happen that could ruin your day. That topic should be another post.
Jim
Jim Schafer
Retired From PPG Industries/
Transitions Optical, Inc.
When you win, say nothing. When you lose, say even less.
Paul Brown
I have personal experience in my young and stupid days of 1977. I reedged a heat treated lens with the diamond handstone. I was holding the lens in my right hand after about 10 seconds, the lens imploded in my hand. It scared the carp out of me but we all got a good laugh afterward and my boss docked me $1.32 for the lens.
Money carefully refunded
I remember being told that a case-hardened lens can be un-tempered
Yes, heat treated lenses can be annealed.
Thereby rendering the lens safe for modification?
Yes and subsequent retempering.
We used to cut a lot of ray ban tinted toughened glass years back
worse thing that happened was they cracked neatly in half in the machine
If you think about it, the very defintion of glass is A LIQUID IN SUSPENSION , this is why glass has to be handled as stated in the aforementioned posts, another consideration is glass because of its inherent nature has tiny imperfections in it known as griffith flaws{thank you darryl}, which is why when these lens are hit repeatly, even though they are treated, will eventually shatter, its all a matter of when.
If you want proof of this take a minus lens and heat treat it like normal, after the proper cooling off time keep drop balling it and watch it break , maybe the second time , maybe the tenth time, but it will break.
Years ago at B@L when i was running these tests, we did not have chem hardening available so i dont have results on that, although i would imagine it would be the same.
If anyone dares to do this or has the time to do it, you might publish the results, as it may also be glass is made better today, all these tests i personally conducted where done in 1970.
I am pretty sure that spectacle crown glass has the same formulation today as it did in the nineteen fifties although there have certainly been improvements in manufacturing methods and process control. Why monkey around with a nearly flawless product when it has been replaced in the market by an inferior one.
.
Never ever heat treat a laminated polarized glass lens.
I hope this advice saves you some smoke damage......I learned the hard way many years ago. I thought it was just gray 3.:(
I would not want to "reprocess" a glass lens. I always felt that even the drop ball weakened the lens everytime I did it. I would not want to go through all that stress again.
The original photogrey FT 25 I used was laminated. Heat treated it just like normal and gunked up the chamber for 20 minutes until the gunk got consumed by the heat, but not as messy as heat treating CR 39. YUCK
Money carefully refunded
I "used" to have a study from the 1970's, conducted by the Army's ophthalmic division, that showed that *any* glass lens's impact resistance was dramatically reduced when it was "drop-ball" tested.
So the FDA mandated, in-the-interest-of-public safety, impact test is *destructive* in some way to the lenses we dispense.
Go figure???
Barry
Barry, partially wrong, one of the other properties of glass that is heat treated is when it shatters, the edges are rounded off and not as likely to do any permanent damage, untreated glass will have razor sharp edges. We use to, as a demonstration take a properly heat treated lens and shatter it, and then roll the broken pieces around in our hands to show that it would not cut. The other fact that you alluded to is true, as i also stated, that constant impacts will weaken the glass, however its a moot point when you think about how easy untreated glass breaks, all in all, i think where better with the law then without it.
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