Common Pitfalls & How to Avoid Them - Part 2

By Steve Schneider, Satisloh
In Part 1, which appeared in the March issue of LabTalk, I covered the aspects of surfacing a lens up to the point of generating. In Part 2, I would like to focus on fining and polishing of all lens materials and how to clean the lens after processing.

Now that we have generated our lens we have to smooth the lens and bring it to proper thickness. For every lens cut we have to match it with a lap tool, which provides the same curve as the lens but convexed.

Because today’s generators offer greater accuracy in the curve we generate we need to make sure our lap tools do not decrease that accuracy. Lap tools should be handled with care and inspected for damage periodically. Lap tools are made from several different types of materials including aluminum, plastic and foam. Aluminum provides the best performance and has the most durability. Plastic is okay for the filling in of out of range needs. Foam laps are mainly a one-time use tool and should be used as little as possible because they do not hold curve accuracy past one use. Many times I observe lap tools being tossed into buckets for handing back to the lap racks. Lap tools should be handled with care to prevent damage such as nicks or dents, which can cause poor surface quality. I recommend that the tools be rinsed after every use then set on their sides in a low walled tub. Limit the tub to 10 or 15 tools to prevent carrying excessive weight.

More labs are also utilizing base pads to cover the lap tools for easy removal of fining and polish pads and identification of curve ranges. Base pads must be inspected for damage or wear every time the lap tool is used because it will have a direct effect on the performance of the fining or polishing process.

When preparing your job for fining there are a few basic rules you should follow: first, select the proper pad size to fit your lap tools. This is important because you want to cover as much of the lap tool as possible. We never want the lens to come in direct contact with the lap as it will cause damage to the lens surface. Fining pads are available in three basic sizes 3”, 31/4 and 31/2” which accommodate 99 percent of lap tools today. When applying the fining pad to the lap it is important to place the pad as close to the center of the lap tool as possible, this will allow for the lens to stay in contact during the fining cycle and provide a consistent lens quality and stock removal. The pad should be pressed onto the lap tool making sure it lays flat with no wrinkles or high spots. Most labs today can utilize a one step-fining pad that starts off aggressively to remove the necessary stock and then less aggressive to finish the lens providing a quality lens surface.

Remember, the finish of the lens after fining is critical to the quality of the surface and your final lens quality after polishing. Most backside defects from processing come during the fining of the lens. For proper use of your fining pad you should consult your supplier for important information regarding the cycle time and pressure used when processing. Now that you have fined the lens to proper thickness and surface quality, it is time to polish.

Today’s labs run an assortment of lens materials that use different types of fining pads to smooth the lens but then the same polish and polish pad for the final outcome of the product. Because of this it is very important that you select a polish pad and polish that provides the best quality for all the materials you process. Polish pads come in the same sizes as fining pads to best fit the lap tools you run.

When determining a polish pad, I recommend that you test with a product like polycarbonate, which traditionally is a difficult material to polish. If you can polish a material successfully like polycarbonate your other materials will also have a quality surface.

The critical factors of a good polish pad are the ability to hold polish during the cycle and the minimal loss of flock during the process. Flock is the soft hair-like material on the polish pad. If your pad losses this during the cycle it can cause swirls and scratches on the lens surface and will deplete the quality of the polish over time. This happens because these small hair-like fibers collect and pull the critical particulate from the polish, which decreases the performance. Polish is the other factor that you need to determine based on the lens materials produced in your lab.

There are many polishes to choose from today that offer different levels of quality. It is critical that the polish is filtered and kept at a consistent temperature. Filtering of the polish is critical to surface quality because it removes all the unwanted materials like polish pad flock, lens material and anything else that might enter into the system. Also temperature consistency is important because many of today’s polishes react differently at different temperatures. This information can be obtained from your supplier. Remember, you want to keep the polish and fining water within a few degrees of each other to prevent thermal shocking of the lens. So our job is almost finished. We are at the point where we remove the lens from the block to clean and prepare for the next process. When removing the lens from the block it is important that we do not damage the surfaces. I recommend that if you use the shock method for deblocking that a hard surface is used. This will allow for the lens to deblock with only minimal down force to help prevent damage or stress to the lens. After the lens is off the block it needs to be washed to remove any polish or dirt that could scratch the lens. Washing should be done in a two-part system: first rinse the lens in warm soapy water to remove dirt and debris, and then in warm clean rinse water to remove soap. You should replace the water often to provide best results. Now that you have successfully surfaced your lens remember that your goal is to repeat this process the same way every time. Consistency proves to provide the best quality of lens time and time again. Take a look in your lab today for these common mistakes and be more successful in the future with better lenses overall.

Steve Schneider has been a part of the optical industry for 19 years. Steve joined LOH Optical Machinery in February 1993 and became a product manager for Rx and finishing consumables in 2001. in 2004 LOH merged with Satis and his role changed again to sales manager of consumable products.

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Labtalk November/December 2018