Polycarbonate Lens fractures!

[choppa]

Hi all,

I need some advice, I recently opened a retail location with a small mini lab. We are using a 6E to edge our lens. I have been noticing that some of the poly stock lens we use have been cracking in the middle (where the block is). Usually about 3-6 small, vertical cracks about 2mm in length, grouped in a 2-3mm diameter.

Some information that might be relevant is that the 6e we were using had an AIT chuck, and we switched over to weco vario. We also are trying out a local stock lens distributor. We haven't had this problem before with the 6e, but we were using the AIT blocks back then. Lens power of fractured lens has ranged for 1.5 to -5.00 so far.

I am trying to figure out if it is the edger or the quality of the lens.. I was wondering if anyone has insight or experience on this. If it is the lens I need to return my stock ASAP!

Thank you

[Wes]

Curious. Does this happen with all poly, or just the AR coated poly? If it's AR only, it's most likely crazing (coating cracks). If so, use the AR settings on the machine. The lens may be crazing due to excessive chuck pressure or the angular pressure from cutting too fast.

[choppa]

I have only noticed it on Poly with AR so far.. I use the AR poly setting on the 6e, maybe I need to make some changes to the chuck pressure.

How do I tell if it the AR cracking or the poly itself?

[Wes]

It's a safe bet that it's the AR. It will look superficial, not through and through. Poly usually cracks (when it cracks) from the edges in, most often due to chemical exposure.

[choppa]

thanks for your responses, I haven't noticed this with my other suppliers, is this saying something about the quality of the AR?

[Wes]

Also, make sure you select AR poly before you chuck the lens. If you do it after chucking the lens, it has already chucked with CR-39 pressure and will craze your lens.

[Wes]

It may be saying something about the quality of the AR, the center thickness of the poly, or both. And then there's my previous post to consider.

[choppa]

Thanks wes, I will do a few more tests to figure out whats going on.

[Judy Canty]

There were some problems in the past with stock poly w/AR where the CT was too thin and caused the lens to fracture.

[Sledzinator]

When I was working with the 6e we had to adjust the settings to a really low chuck pressure and slow down the whole edging process but then you risk slipping so high quality pads are a must.

[EyeManFla]

Exactly. I am using a 7e now. We had to do a lot of changes on the old 6es.

[choppa]

What should the minimum CT be for the stock poly?

Did some more tests, problem is still occurring. I did not have problems with chuck pressure when I was using a different supplier.. I think I may have to return everything, good thing I didn't have to pay them yet.. Getting my 7e this week, I will give these lens a try on that as well.

[choppa]

Exactly. I am using a 7e now. We had to do a lot of changes on the old 6es.

What settings did you have to change on the 6e?

[charan]

Oh wow it just happened to me today cutting a poly progressive with ar rx around -4.50, it's got a crack right through the middle.

[Kara]

It's a safe bet that it's the AR. It will look superficial, not through and through. Poly usually cracks (when it cracks) from the edges in, most often due to chemical exposure.

That has been my experience. Often times we get drill mounts that start spider-webbing around the drill holes due to exposed material getting hit with some sort of chemical. The cracks in the center are usually because someone here has flexed the lens just enough to cause the a.r. to craze. I haven't seen it happen on the edger yet, usually just in mounting, but I 'll keep an eye out for any occurrences and try to figure out the underlying cause.