Any Trivex lenses being molded in the USA?
I'm curious as to where molded Trivex lenses (stock single vision, semi-finished flat-tops, others?) are being made, and if none in the USA, why not?
I'd like to explore the fabrication methods being used, so any pointers there would also be helpful.
Also, I've always wondered how they manufacture a front surface mold for a flat top, and even an exec for that matter.
The labs will hate me but a Trivex exec intrigues me.
Does AO still have any say in calling them "Executive" multi-focals? Dick???
I have been away from AO for many years but if you want any info contact Dick Whitney as he is the one holding down the fort these days.Originally Posted by Uncle Fester
www.dickwhitney.net/RBWAOHistoryIndex.html
He is the resource for AO history. A wonderful web site.
AO sold their ophthalmic lens business to Zeiss
Last edited by rbaker; 08-26-2015 at 09:47 AM.
PPG makes the monomer. They will probably be delighted to sell you some. They might also have some info on how to mold or cast the stuff.
http://www.ppgoptical.com/Home.aspx
Last edited by rbaker; 08-26-2015 at 10:33 AM.
METHOD MAKING A MOLD AND MOLDING A BiFOCAL LENS
METHOD MAKING A MOLD AND MOLDING A BiFOCAL LENS Jan. 12, 1971 s Sheets-Sheet 2 Filed Dec. 19. 1967 Fig.
ABRAHAM GITSON ATTORNEYS .22?- Figzl I United States Patent 3,555,126 METHOD OF MAKING A MOLD AND MOLDING A BIFOCAL LENS Abraham Gitson, 15705 Van Blvd., Cleveland, Ohio I 44120 Filed Dec. 19, 1967, Ser. No. 691,828 Int. Cl. B29c 1/02'; B29d 11/00 US. Cl. 2641 3 Claims
ABSTRACT OF THE DISCLOSURE In the Whitney Pat. 3,109,696, a method of making plastic lenses is described, in which a mold portion for casting plastic lenses is formed by mounting a first disk of relatively rigid material, such as glass, having a relatively high melting point on a holder, dividing the disk into two section, grinding and polishing a predetermined first surface curvature on the exposed side of the divided disk, mounting a second disk of a rigid material, such as flint or barium crown glass, having a lower melting point than the first disk onto a holder, dividing the second disk into two sections, removing the respective divided disks from their holders, placing the divided edge of a section of the first disk in edge-to-edge relation with the divided edge of a section of the second disk, securing the edges together while in said relation and grinding and polishing a second surface on the assembled sections of a predetermined curvature in overlapping relation with and at such an angle with respect to the first surface formed on the section of the first disk as to position the ultimate optical center of the cast lens resulting from said surface at a desired location and simultaneously controlling the depth of said grinding and polishing of said second surface so as to reduce the focal field resulting from said first surface to the desired size and shape.
The first surface referred to is designed to produce the reading field in the cast lens, while the second surface is designed to produce the distance field in the cast lens.
The edges, in one instance, are secured together by fusion, and while it is stated in the patent that the temperature of fusion is controlled, as is possible by reason of the different softening points of the respective glasses of the segments, so that the previously ground and polished surface of the reading field will not distort during the fusing, but since fusion of the edges implies or necessitates actual melting of the edges, it is virtually impossible, irrespective of the nature of the glasses used for the segments, to avoid substantial distortion of the ground and polished surface of the reading field, which, after the segments are fused together, cannot be reground.
The patent further discloses that although the segments for the reading portion and distance portion of the mold are preferably fused together, that they may also be joined by cementing or the like, but this does not obviate the grinding and polishing of the second surface on the assembled section.
It is further obvious from the disclosure in the aforesaid Whitney patent that in grinding and polishing the second surface, that this surface is ground about a center which lies on a line normal to and which passes through ice the reading portion of the lens at the location desired of the optical center of the reading portion, so that the center of the reading portion of the ultimate lens and the center of the distance portion of such lens coincide, as a result of which these centers, when the lenses are mounted in a frame, have the same interpupillary distance.
The distance portions of the lenses, as made in accordance with the Whitney patent, have little or no correction for distance vision, so that if the optical centers of the reading portions of the lenses are at the required interpupillary distance, it is inconsequential that the distance portions of the lenses are at the same interpupillary distance as the reading portions.
However, if the distance portions of the lenses are designed or required to have substantial correction for distance vision, as, for example, in lenses made for postoperative cataract patients, is is essential that the distant portions of the lenses be at a required interpillary distance or spacing which is different than the interpupillary distance or spacing of the reading portions of the lenses, since otherwise the patient or wearer of the lenses, in looking through the reading portions of the lenses which are of themselves at the proper interpupillary spacing, is subjected to considerable eyestrain by reason of the effect of the distance portions of the lenses on the eye muscles.
continued .....................
see all of it at: ============>
http://www.google.ca/patents/US3555126
There are two ways of manufacturing plastic lens molds, one way involves CNC milling and polishing of metal, the other, much simpler way, is similar to the patent method Chris mentioned above (and for the record, that patent has expired and the information is in the public domain).
Essentially, the process consists of making two glass lenses, a "back surface" mold, which has a polished convex "front side good" surface (back side concave polished for inspection only), and a "front surface" mold, which is polished concave "back side good" surface (front side convex polished for inspection only).
The concave side of the front surface mold is ground and polished with an offset chuck to the power required for the add power (let's say we are working with CR-39, a 6 base lens and a +2.00 add. Using the index of refraction differences, the curve on the offset area would be (2 x 1.06) + 6.25 = 8.37 concave. Then a matching "upper" segment with a convex curve of about 8.40 (a slightly higher curve is used for the segment upper, so that when the fusing process takes place, there is center contact and it fuses outward from there). The segment has a polished cut (this forms the flat top), and it is set in place and held with a fusing cement. The placement is such that the line is (for example) 5 mm down from the mechanical center of the mold, and rotated so that the center of the segment (for example) is 5 mm in. Note that matching right and left molds have to be made.
The lens is then run through a fusing cycle to fuse the upper segment portion to the main mold.
After fusing, the remaining opening of the add area is filled with a stiff pitch material to protect the polished surface. The concave side is then curve generated to the desired base curve (example 6.25), ground and polished. The pitch is removed, and the mold is inspected for defects.
There are other materials in use in place of pitch, epoxies and such, but old school folks like myself still prefer to use pitch.
And to answer the OP's question: X-Cel is casting Trivex in their facility just down the road from me in Sauk Rapids, Minnesota.
Thanks both of you for all that info. I figured it had to be some kind of Rube Goldberg process, and it is, of the elegant variety. One last question, how many lenses can be run off one set of molds, and how many lenses are cured at a time (what is it 7+ hours for Trivex?) or is a continual operation conveyer belt style like in a bakery?
The number of castings that a mold can make varies widely, but overall, the average is about 20 individual castings. Cleanliness of the mold, quality of the monomer/polymer, consistent water temperature in the curing bath, quality of the release coating all have effects that cannot be overlooked.
Back in the 80's when I was making plus cylinder molds for casting finished SV, my customer was selling "do it yourself and save" systems for in-house casting to a variety of optical shops. Their average life was 6 castings, which, for the time, wasn't bad.
Mold failure (again glass), typically comes from heat shock (not getting the mold into the curing bath fast enough -- some of the monomer/polymers are extremely exothermic), micro-fractures from too many castings leading to "pull out's" where the glass mold actually fragments and sticks to the cast lens, and of course, slippery fingers.
Generally speaking, the quantity cast depends on two factors: how many molds you have and how much space you have for curing.
As you say, it is an elegant process. Making aspheric molds (SV) is even more elegant. The simplest way is to slump (soften in a kiln) the mold. For consistency of the finished products, the molds all have to be the same exact thickness -- a usual tolerance is +/- 0.25 mm in thickness. A slumping mold is usually CNC milled, and tiny holes are drilled through the mold and the mold is attached to a vacuum manifold. The mold is sprayed with a release (believe it or not, aluminum chlorhydrate [antiperspirant] is commonly used), and a polished (both sides) lens is placed on the mold. It's put into a kiln, and once it reaches transformation temperature (when the glass changes from a solid to soft state), vacuum is induced, pulling the lens down to the mold body. Because the lens is uniform in shape, the lens matches the shape of the mold. The kiln then goes through an annealing cycle to remove any residual stress from the glass, and when finished, you have an aspheric mold. {Of course, this is just one of the many ways they can be made.}
The above is the exact same way that glass progressive lenses are made, except that the concave side is pulled down onto a convex mold.
Probably more than you wanted to know.
Exec molds are a bit more involved, but the process is essentially the same, two separate pieces of glass with differing curves, one for the base and one for the add power, have a polished chamfered flat, and are fused together across the polished surface. Careful care is taken to ensure that there is an overlap of the base curve, so you don't end up with a center spot with no line. After fusing, the now one-piece mold is centered and edged round to fit the necessary casting ring, and a flat is ground and polished.
That's a simplified version of the process.
Just the right amount of info to keep me wanting to visit one of those factories, like x-cel's to actually see it in action. I got a personal tour of VSPone lab in Folsom (5000+ finished jobs/day) and it was an amazing Willie Wonka style factory. But maybe not, sounds like all those organic chemicals may make it a bit of a toxic environment. Also had a great tour of a brandy making facility and where all the barrels were stored it smelled wonderfully explosive.
sent pm
Last edited by kmcross; 08-27-2015 at 10:18 AM. Reason: sent pm
Thanks for that. I actually loved exec bifocals when they were popular, and still do except the lines have gotten a little sloppy, not the crisp clear lines of glass execs. kind of rolled these days. As for the little noline spot in the center, to me that's perfection, a perfect tangential meeting of differing curves. But it seems to be going away. I really liked the FD and ED trifocals. sigh...
Can't ST45s accomplish most if not all of what an executive did? I haven't found a 45mm seg trivex lens, only cr39
Exec's have no image jump when the patient transitions to the reading area. They also give imo a purely psychological boost to the patient in their field of near vision. They're definitely not for everyone but have a place for some presbyopic patients.
Calling them Executives was marketing genius for AO as it gave them a certain cachet back in the 60's and 70's that any other name would lack.
My fingers are still scared from trying to insert or remove glass exec's from Zylowares nylon Invincible frame if they were edged 1mm large; which was easy to do in those days when all edgers needed patterns.
Last edited by Uncle Fester; 08-27-2015 at 11:52 AM. Reason: another thought...
Ugh. Cutting Execs on a Weco edger... Craziest bevels you'll ever see.
I'm Andrew Hamm and I approve this message.
One reason why I was so happy to go digital, and with the push of a button be able to run the bevel by following the back curve!
I rarely use ft45 because of the huge prism jump at the line, and the thickest part of the line is where the patient's eyes cross every time going from dist to near. As someone said, another name for exec was "no-jump" bifocal due to what I think they called "bicentric grinding", but now that I know how they're made, I won't use that term. Optically very nice.
The ledge on a FT45 protrudes out from the carrier lens significantly, especially with the higher adds.
X-Cel makes a FT35 Trivex. Limited base curves though.
Unless the wearer is monocular, a FT28 offers more than what is necessary for near field of vision, and can be had with optimizations.
I had 6 months cutting and edging glass execs on an AIT MK V, placing the safety bevel by hand with a hand edger that was circa 1933 Chicago World's Fair, heat tempering afterwards. There was some spoilage.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Well, they are "bicentric", but like a flat top, you only have one optical center when finished. If you took two identical semi-finished execs, finished one plano power to match the upper and finished the other plano power to match the lower, you would find that there is a distinct optical center for each section.
The term bi-centric came from the way the glass execs were made, in huge drum grinders & polishers. Because of the design, they could not be front surfaced by small spherical tooling as is traditionally done, but instead, by placing the lens blanks in special jigs and placing them inside of a drum grinder/polisher. The inside of the drum was a truncated sphere which rotated on its main axis and also along the curve radius. If you think about the hole of a donut with the major and minor radius being equal, that is what the drum looked like.
I don't imagine it's practical to do that with any/all of the plastic lenses? Seems that may be why the ledges were always so clean back then...
Yes, that's why the manufacturing process of the molds is different, relying on fusing two segments together, and casting the resulting lens.
I agree with this, but the guys who come asking for Executive Bifocals are, in my humble experience, without a doubt the quirkiest bunch of quirks ever to place their crazy in front of me - often I feel like they want the executive without understanding what they are getting. They are rarely if ever happy and that is why I don't offer them anymore, they can try st28-45D segs and like it, or they can go somewhere else. I've eaten way too many remakes on Executives over the last decade, I won't do it anymore. People are always surprised because I'm known as the shop that will do ANYTHING.
Thanks for the info on the Trivex.
With a high enough add, you could stick some books or knick knacks on top of that ledge.
The reading ADD is, in most cases, on the bottom of the edge of the mounted lens. Therefore, as high add power would keep your toes dry.
Don't clean them and after a week you could grow tomatoes on it, nice terrace...
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