Does CR39 Shrink?

[Monica]

I everyone. I was hoping you maybe able to help me. I heard a rummor that CR39 shrinks from heat and that in warm climates lenses are falling out of frames because of this shrinkage. Does anyone know if this is true? If it is true then does anyone know where I can find resource that I could find it in?

Please help

Monica

[OPTIDONN]

I know that in heat it can expand. I took A CR-39 lens and measured it with digital calipers then placed it in the hot sun and noticed there was some exmansion. I then placed the same lens in the frezer and measured it and noticed that it shrank a little bit. The expanding and shrinking were very minimal. If fited properly in a frame there should be no real issue. If they wre edged a little big or small that may be differant. Thats why if you ever edge a lens on size then tint it it may be a little big and needs to be cooled off. I hope that this helps.:hammer:

[Joann Raytar]

http://www.visioncareproducts.com/30/product_solu.html

Lens Shrinkage
The CR-39® lens that I recently sent to my local lab for AR coating was initially fitted snugly into a metal frame. When it came back from the coating process it was about .25mm too small for the eyewire. The lab told me that the size reduction was caused by the lens not being cured sufficiently during manufacturing. How could this have happened to a stock lens I bought from a reputable supplier?

Your lab is likely correct about this. When lenses are manufactured, they go through a curing cycle known as “degassing.” In this process, the soft, moldable lens material expels the gases trapped inside and becomes hard.

Manufacturers compensate the lens’s curvature and diameter for this process so the finished product turns out well within power, thickness, and diameter specifications. During preparation for being placed into an AR-coating chamber, lenses are subjected to a number of steps. including an intense cleaning in an ultrasonic cleaner, a drying process, and a curing process. The drying process may include exposure to nitrogen gas vapor (in the range of -320°). This intense drying removes the possibility of minute water vapor on the lens’ surface. The curing process is often done by heating the lens. This step ensures that lens has been completely degassed. Either of these processes can cause a lens to shrink.

Because the lens will be subjected to strong forces in the coating chamber, it is imperative that the lens be ultra clean, super dry, and completely cured. Even a minor flaw in these areas may cause a coating failure.

— William Underwood, ABOM

[drk]

When I started out, I had frequent problems with lenses coming back too loose and falling out, well after dispensed. When I called my (then) lab, the owner got on the line and said "those darn lenses weren't cured properly, blah, blah", and to soak them in very hot water for five minutes. I did, and the lens fit again.

Obviously, I expanded the lens, again, from the heat. But, instead of it being a cheap trick, it seemed to permanently solve the problem, to my knowledge.

Ever since, when this comes up (rarely)I've been heating the lenses in hot water, and I've never had a single person come back and say "that worked for about three days"...it seems to solve it long-term.

Does this make sense to anyone?

[For-Life]

When I started out, I had frequent problems with lenses coming back too loose and falling out, well after dispensed. When I called my (then) lab, the owner got on the line and said "those darn lenses weren't cured properly, blah, blah", and to soak them in very hot water for five minutes. I did, and the lens fit again.

Obviously, I expanded the lens, again, from the heat. But, instead of it being a cheap trick, it seemed to permanently solve the problem, to my knowledge.

Ever since, when this comes up (rarely)I've been heating the lenses in hot water, and I've never had a single person come back and say "that worked for about three days"...it seems to solve it long-term.

Does this make sense to anyone?

What temperature do you soak them at (more of a concern for AR)?

[Spexvet]

Heat may increase the likelihood of warpage, especially if the lens was large to start with. Check the base curve. If it is higher than when the lens was made, the lens has "bowed", reducing the diameter and circumference. You may also notice that in the lensometer, the sphere and/or cyl lines are not crisp, or the power has changed. When I find this, I will heat the lens in the frame warmer, lay it on the bench convex side up, cover it with a towel, and press on the top (front) of the lens. This flattens the lens, back to its original shape (check the base again) and increases the diameter and circumference, so it fits better.

[drk]

...the lens has "bowed", reducing the diameter and circumference.... When I find this, I will heat the lens in the frame warmer, lay it on the bench convex side up, cover it with a towel, and press on the top (front) of the lens. This flattens the lens, back to its original shape (check the base again) and increases the diameter and circumference, so it fits better.

This may be a reason why lenses "shrink"? They don't really contract, but they increase in curvature from heat? Heating them and flattening them puts them back into original form?

I use near-boiling water, but I've never needed to do it on an AR lens. Can't say that I've done it on polycarb, either. This may be significant.

[Jacqui]

We will usually edge CR-39 slightly oversize, dip in hot water until warm, and the rinse in COLD water until cool. This seems to take all the shrink out permanently. For hot water I use the tint unit, I keep one pan filled with clear water just for this.

[Chris Ryser]

We will usually edge CR-39 slightly oversize, dip in hot water until warm, and the rinse in COLD water until cool. This seems to take all the shrink out permanently. For hot water I use the tint unit, I keep one pan filled with clear water just for this.

Being a micro wave fanatic I do all those things in the microwave in seconds............the tint unit is collecting thick dust.

[Monica]

I am trying to write a research paper. Does anyone know where I can find any thing in writing to back this up.

[Jacqui]

Don't know of anything in writing, my post is from practical experiance only. However you may quote me. :)

[Chris Ryser]

I am trying to write a research paper. Does anyone know where I can find any thing in writing to back this up.

if you write a research paper, dont you have to do some research on your own?

[For-Life]

if you write a research paper, dont you have to do some research on your own?

Well she is just looking for some secondary sources. Part of the research is finding it, but the most important part is interpreting from your findings and being able to create something useful from it.

[Monica]

if you write a research paper, dont you have to do some research on your own?

I have already done alot of research. I have not been able to find any resourses that mention anything about the lenses shrinking. My research on the internet led me here and I was hoping somebody here could lead me in the right dirrection.

Trust me I am doing alot of work myself and I am not asking anyone to search for resourses. I just thought that somebody may know of a source offhand.

I really do appreciate all the help I have recieved so far. :o

[Pete Hanlin]

What temperature do you soak them at (more of a concern for AR)?
This is an excellent concern to have. When heated, the surface area of a CR-39 lens is going to expand more than the AR coating. The result will inevitably be crazing of the ARC- and possibly the SRC as well.

The lens swells in hot water because the "pores" of the material open up to allow the influx of water molecules. This is the same principle used for traditional tinting. Ironically, the tint itself doesn't even have to be warm to enter the lens. The lens itself, however, must be warm enough to allow the tint molecules to enter the lens (optimally, about 205F). You should be able to infuse some water into the lens at about 180F or so, but this temperature is not sufficient for reliable tinting. At 180F, the red molecules in a dye will absorb into the lens more efficiently than blue or green dye molecules- resulting in "pinkish" grey lenses.

Of course, after weeks of tinting at this temperature, you'll eventually reach a point where grey lenses actually turn out grey- because the proportion of red dye has been lowered. Naturally, the time required to tint at this point gets rather lengthy, so you change out the tint and end up with pinkish lenses again (sound like a familiar cycle).

Sorry to go off subject with the tinting rant, but its a related subject and one that I run into constantly from a tech services standpoint. One of the side-effects of low tint temperatures is "ghosting" on PAL lenses (when the original ink markings appear on the lenses as a light ghost- dark ghosting also occurs, but the reason is different).

[chip anderson]

Most plastics expand and absorb a little water when place in hot water. I once tried to anneal some PMMA in water and it swole up too big to fit in the collett of a CL lathe, became slightly cloudy. It cleared and went back to size after being heated in a drying oven.
I have seen many pairs of spectacle on which the lenses have shrunken. One of which was lost inside an airconditioned house for six months. New lenses had enough gap to place your fingernail between lenses and frame on each side of lens.
For the most part putting lenses (or any plastic) in hot water does not cure it. Water can be used a a heat diffuser if plastic is sealed in a pressurised mold.


Chip

[Chris Ryser]

At 180F, the red molecules in a dye will absorb into the lens more efficiently than blue or green dye molecules- resulting in "pinkish" grey lenses.

Of course, after weeks of tinting at this temperature, you'll eventually reach a point where grey lenses actually turn out grey- because the proportion of red dye has been lowered. Naturally, the time required to tint at this point gets rather lengthy, so you change out the tint and end up with pinkish lenses again (sound like a familiar cycle).

I should not even get into this discussion as I will get accused of self interest.

There are dye formulas on the market, even for the old smelly dye pots that will NOT do above quoted effect. They will result in a perfect grey color as of day one, (near perfect straight line on the spectrometer) but tinting at a slightly higher temperature. When the red gets used up as per above quote the lens tends to turn towards blue and can be corrected by a quicky dip into red. (does not sound like a familiar cycle?)

[Monica]

Most plastics expand and absorb a little water when place in hot water. I once tried to anneal some PMMA in water and it swole up too big to fit in the collett of a CL lathe, became slightly cloudy. It cleared and went back to size after being heated in a drying oven.
I have seen many pairs of spectacle on which the lenses have shrunken. One of which was lost inside an airconditioned house for six months. New lenses had enough gap to place your fingernail between lenses and frame on each side of lens.
For the most part putting lenses (or any plastic) in hot water does not cure it. Water can be used a a heat diffuser if plastic is sealed in a pressurised mold.


Chip

So Maybe the lenses are shrinking in warm dry climates because of water leaving the lens? :o

[Chris Ryser]

So Maybe the lenses are shrinking in warm dry climates because of water leaving the lens? :o

CR39 does retain humidity. If a surfacing lab has problems hard coating CR39 is that they did not properly dry the lens after washing and cleaning. When heated steam forms and the hard coat will create bubbles while curing in the oven or under a UV lamp.

[Pete Hanlin]

When the red gets used up as per above quote the lens tends to turn towards blue and can be corrected by a quicky dip into red. (does not sound like a familiar cycle?)
When tints are "corrected" via this method; however, the first color to fade from the lens is usually the last one added- this is also sometimes the cause when a grey lens turns purple over time.

In traditional tinting, keeping the pot temperature at 205F is the best way to assure consistent, repeatable tinting. Temperature should be measured in the pot with a reliable digital thermometer (the thermostats on most units are unreliable). Also, there are several single and double pot units that have built in digital thermostats- as well as magnetic operated mixers that circulate the tint.

At 205F, a neutral grey3 solid tint should be attainable in 5-7 minutes for most lenses.

[Chris Ryser]

When tints are "corrected" via this method; however, the first color to fade from the lens is usually the last one added- this is also sometimes the cause when a grey lens turns purple over time.

I beg to differ: WRONG, When a lens turns purple, it still contains red, because red is the base color for purple.

When dyes are mixed for a certain color and there usually is one of the basic colors called the "workhorse" which is used as a basic color, from which one adjust's to the wanted color shade.

When the basic workhorse is red (which is the least fading color) grey lenses will look purple when the blue (the fastest fading color) is fading away.

You can use all the "brain power" you want, but in a black dye red should and has to be the minimal factor. Actually the same goes for brown colors which turn very reddish when tinted to dark shades. There are dye formulas on the market where dark browns show NO red.

In traditional tinting, keeping the pot temperature at 205F is the best way tint.

The hotter the dye the faster it will tint a lens. But if you go any higher as per your quote, in tradiitonal tinting the dye disintegrates within hours and has to be replaced.

The dye manufacturers have never upgraded their technology to modern and newer materials. If your company would have the same way of thinking you would still be selling the same original VARILUX from back in 1955 without any improvements over the years..

At 205F, a neutral grey3 solid tint should be attainable in 5-7 minutes for most lenses.

Actually there are now ways with a newer technology that will allow you to tint hundreds of lenses per hour and thousands per day using conveyors if you need the quantity or you can do a simple one pair in 60 seconds the inexpensive way without having to smell toxic fumes.

[Chris Ryser]

PROPERTIES GLASS PC¹ PMMA² CR39
____________________________________________________________ ______________

Superior optical quality YES NO NO YES
Lightness and strength NO YES YES YES
Scratch resistant YES NO NO YES
Stress cracking resistant NO NO NO YES
Solvents and chemical resistant YES NO NO YES
Welding sparks resistant NO YES NO YES
Plasticizer migration resistant YES NO NO YES
Ageing resistant YES NO NO YES
Gamma radiation resistant YES NO NO YES
Wide range of colour NO YES YES YES
Wide range of U. V. absorbtion NO YES NO YES