Avoiding AR Fractures During the Edging and Mounting Process

By Scott Pickering
The degree of difficulty for optical labs to process lenses has never been higher. Labs are dealing with lenses that are thinner than ever and anti-reflective (AR) coatings that are more slippery than ever. With these new technologies, eye care professionals (ECPs) are more likely to order the thin, slick lenses edged at the lab rather than ordering uncut lenses. Your lab is producing great AR coated lenses, but is the quality of the coating still intact when it reaches the patient?

As more ECPs order AR lenses, we must take special care with thin lenses and polycarbonate lenses to avoid fractures in the AR coating. Thin lenses are more likely to be flexed during the edging, de-blocking and mounting processes. Especially susceptible are low power polycarbonate lenses, as the soft nature of these lenses makes them very easy to flex during lab processing.

The result of lens flexing is that scratch-resistant coating and/or layers of the AR coating itself may crack or fissure. Depending upon the severity of the flexing, these fissures may or may not be noticeable to the final inspector or the ECP. However, over time, the combination of cleaning, temperature changes and everyday wear causes the AR lenses to deteriorate much more quickly than normal. This obviously leads to patient dissatisfaction with their AR lenses. Patients are less likely to want AR lenses on future eyewear purchases, making it more difficult for the ECP to sell AR lenses to these patients. After several bad patient experiences, the ECP is less likely to offer AR lenses to new patients.

Here are some tips to avoid flexing the lens during processing:

• Ensure edger wheels have regular maintenance and changed on schedule. A good edger wheel cuts cleanly through the lens without creating unnecessary torque on the lens.

• Ensure clamp pressure and feed rate are set to your manufacturer’s recommendation. Clamp pressure must be strong enough to prevent slips but not too strong to flex the lens during clamping. If a center defect appears which looks like parentheses ((( ))), this is an indication that the edger clamping pressure is set too high and must be reduced to the lowest setting possible recommended by the equipment manufacturer without introducing slippage. Slowing the feed rate increases the cycle time, but lessens the torque on the lens.

• Use a “backside” clamp that matches the shape and size of the receptor chuck on the edger. This ensures the pressure is the same on both sides.

• When de-blocking, ensure that all operators are trained to de-block in a manner that puts the least amount of torque on the lens. This is especially crucial when de-blocking manually. De-block carefully with a tool that is designed to work in a twisting motion. • Sizing is crucial. If the lens is even the slightest bit large, tightening the screws on metal frames creates excess force on the lens. Plastic lenses can also put stress on lenses that are sized too large.

Many times, fissures in the AR coating can not be detected in the normal quality assurance process. Spot-checking thin orders with an arc lamp is necessary to detect damage to the AR coating that can occur during processing.

What we do in the lab has a major impact on the quality of AR lenses that reaches the patient, and therefore has an impact on the patient’s satisfaction with their eyewear. If the patient is satisfied with their AR lenses, it makes it easy for the ECP to sell it on the patient’s next purchase. Let’s make sure that the patient’s satisfaction is not diminished by the procedures we use in the lab.

Scott Pickering Vision Council of America AR Committee

CURRENT ISSUE


May/June LabTalk 2017