Crazing - How to Detect It, and Maximize Your Lab

The Merriam-Webster’s Dictionary describes crazing as “minute cracks on the surface.” In our industry, crazing defines a defect where the AR lens looks like it has many fine and random cracks on the surface of the lens. Many lab support staffers, technicians, and managers of AR coating labs are aware of this concern. When your account comes back to you with a craze defect and requests a warranty replacement, several thoughts can go through your mind.

“I will waste time, waste material, have a lower profit, and can damage my reputation for quality.” Sometimes the cause of a craze is outside the control of your lab, but how does one avoid contributing to this issue in your own production? The answers are knowledge, training, and sustaining good lab practices. As an AR lab owner, manager, or operator, you have worked hard to develop your AR product, market it and sell it to your customers. So, arming yourself with these tools can help you sustain and grow your AR business.

The AR lab will first need to be able to recognize crazing (either before it leaves the lab or after it returns from the field), and where on the lens it has occurred. Many equipment and lens manufacturers can supply training and even testing services to allow the AR lab to detect crazing. A common mistake is to interpret an AR lens with multiple scratches to be a crazed lens. A key to detection and identification is having the correct lighting, inspection background, and most importantly, trained eyes.

Although a lens has the same AR technology on both sides of the lens, they may or may not share the same hard coat technology. Some Rx lenses in the U.S. will have a factory applied thermal cured, dip-coat on the front side of the lens, and an independent UV-cured, spin coat on the backside. By “factory” I mean the lens’ hard coat was applied by the lens manufacturer. By “independent” I mean the lens’ hard coat was applied by the ECP, surfacing lab, or by your AR lab with a UV-cured, spin coating of choice.

Crazing can occur on either side and there are several factors that can contribute to crazing. That said, one type of crazing which labs can positively affect, and thereby minimize, involves the back side spin coating applied in the lab. By taking a few simple steps toward good process control, the chances of your spin coating production contributing to a crazing return can be eliminated.

One modus under which crazing can occur in an AR lens is when there is sufficient stress to weaken the bond between the hard coat layer and the AR layer. A back side coating applied outside the manufacturer or coating designer recommendations can have incompatibilities with various AR processes. Check with your suppliers for their recommendations. Two factors that can contribute to crazing are hardcoating layer thickness and proper cure. The typical desired coating thickness of a backside spin coated lens can vary from 2 to 8 microns. Backside crazing can occur when process controls are lax and backside coating thickness gets too thick or too thin Also, if the UV energy is not sufficient, the resulting under-cured coating will not allow the AR process to properly bond to the hard coat, or the hard coat to the lens, and can increase the chances of crazing.

Some reliable and time tested methods are available to ensure your AR lens or hard coating production will not contribute to crazing. For AR labs that utilize spin coating technology, it is important to have a quality assurance program in place to monitor the coating thickness for your spin coater, and the UV output for your cure unit. The most accurate methods are to use a spectrophotometer for measuring coating thickness and to use a calibrated radiometer to measure UV energy inside the cure chamber. Lens manufacturers and equipment suppliers use similar tools to ensure their production machines are working correctly. The cost to have such tools at the AR lab level and train your staff on its proper use is not insignificant. However, if crazing were to run rampant in your AR lab, it would have a much higher financial impact on your business. If every AR lab works diligently to ensure proper operating parameters are maintained, defects like crazing can be kept in line and help ensure continued growth of AR.


Labtalk June 2020