How to prevent crazing

[brandon76]

To members with high experience on dip-hardcoating and ar coating, I hope I could be enlightened on my theory on how to prevent HMC crazing. My theory is that lenses that have been dipped in a primer coating material prior to a varnish coating material would have less chances of crazing than a lens that went straight to varnish coating. The reason why I say this is based on our very very low returns due to crazing for all our lenses(poly,trivex,1.67 and 1.6) that have been dipped in our primer tank. The only thing is that these lenses comprise only less than 15% of our business and thus, a little bit of a low sample to say that my theory is true. Anyway, I'm not sure if primer coating could serve as a cushion to our hardcoat layer to have less stress born out of the varying expansion ratios of the substrate, hardoat, and the AR coat.

Actually, we are looking at applying primer on all our lenses, but I want to make sure that the benefits outweigh the costs.

Thanks a lot for taking time to read my post and thanks in advance for your comments.

Cheers!

[Mauro.Airoldi]

dear Brendon
you're right about thePrimer benefits but remember that the crazing derived from the relationship between tensil and compressive stress between all different layers, so you have to consider lens thermal dilatations, Primer thermal dilatations, Har coat ... , sigle AR layer ....(composed from SiO2 and a Hight index materiale (ZrO2 or Ti2O3 or Ti3 O5....).
So Primer help but the AR must be flexble too.

[brandon76]

Thanks Mauro. We actually tried to make some changes in one of our TIO2 layers to make the AR a bit flexible(softer). However, what is difficult for us is to strike the balance between flexibility and scratch resistance. We do not have a scratch resistance tester and do not know where we could get one that is not too expensive but eeliable,

[uncut]

This discussion interests me....so I will "subscribe" by posting.

I speak purely from a dispensing/fabrication of the end product point-of-view.

I feel that the base lens has a lot to do with coating stability, and I find coating failure(crazing) is magnified by the lens expansion/contraction in various climatic conditions.

[brandon76]

Thanks Uncut. I agree. In the past, just after edging we would already see some micro cracks which we were able to considerably prevent from occurring by softening our AR.