EYE ON EQUIPMENT
AR is Great But What About Those Edging Challenges?
By Susan P. Tarrant

The Cracking, the Crazing

PROBLEM: AR-coated lenses have certainly presented a challenge to the in-office dispenser as well as the wholesale optical lab. One of the biggest issues with AR coated lenses is the fracturing of the coating which the AR adheres to during the edging cycle. When this occurs, there�s a spider web effect on the lens at the point where the block adheres to the lens.

SOLUTION: This occurs for several reasons. 1) Because of flexing of the lens which has too much chuck pressure; 2) there is excessive torque on the lens during the grinding cycle because of the amount of head pressure; and 3) if the leap pad is left on the lens too long and during the deblocking of the lens, it can harm the AR coating. This occurs quite often on conventional pattern driven edgers and older style patternless edgers that don�t have the capability to control the chuck pressure or head pressure during edging.

There are several ways to handle these problems. The first is to buy a new machine which has the capability to control both head pressure and chuck pressure. If that�s not feasible and your existing machine does not have the capability to control head and chuck pressure, here are some options:

� Cut your lens down slowly in several steps.

� Make sure you are using the appropriate block on the front side of the lens if you are using metal blocks. There are three different base curve metal blocks made by 3M.

� Use a plastic film tape between the leap pad and lens on the frontside as well as on the backside of the lens.

� Try a plastic block on the front side. There are manufacturers that make plastic blocks of all the most popular block styles.

� Deblock your lenses immediately after edging.

� The safest and most reliable way is to sell a longer turnaround time and send it out for AR after edging.

With high-index materials and polycarbonate lenses, an AR coat does not adhere as well as a CR 39 lens and therefore, it requires special handling and caution. When purchasing a new machine, it should be confirmed that it is capable of edging these types of lens es. If you�re using an older edger and are experiencing excessive scrap with AR lenses, the capabilities of a newer edger just may justify its cost.

�Matt Vulich, vice president, sales, AIT Industries

MORE HINTS ON CRAZING

SOLUTIONS: Many people have seen problems with AR coating due to edging. People have seen the AR coating crack or spider crack, or �craze.� This could be due to a few different problems:

� Crazing can sometimes result from the lens �flexing� during the edging process. The AR coating actually acts as a thin, brittle, glass-like coating over a flexible, porous, plastic lens. The coating doesn�t give when the lens gives, and can craze. A way to solve this problem is to equalize the pressure on both sides of the lens during edging.

The left side of the chuck holds the block, and on the right side is the clamp. Any time the clamp is larger or smaller than the diameter of the block, you have the potential for a flexing situation.

�Stuart Watson, director of training, National Optonics

� In addition to the mis-matched block problem, crazing can also be caused when taking the block off the lens. We recommend putting the lens in lukewarm soapy water before taking the block off. It will practically fall off by itself after it�s been in the water for a couple of minutes.

�Clark Smith, Eastern U.S. sales manager, Santinelli International

SCRATCHY PROBLEMS

PROBLEM: How can I avoid scratches when finishing AR-coated lenses?

SOLUTION: The film coating on the surface of AR-coated lenses makes them more vulnerable to scratching. First, it�s important to keep your edger�s finishing chamber clean and free of any debris that might scratch the lens. The best edging systems feature a three-way water delivery system that automatically flushes the edging chamber while edging and when idle to constantly remove any lens fragments or debris.

Second, minimize operator handling. The less a lens is handled, the less chance there is for operator-caused scratches to occur. Handling mishaps can be avoided by using an all-in-one edger that edges, grooves, safety bevels, and polishes without need to remove the lens.

Lastly, it�s important to maintain a clean lab environment because any foreign material on your hands, in the job tray or work area could scratch the lenses.

�Greg Dean, technical training manager, Briot