Why should we calibrate?
I would hope that at some point during your life, you will have the opportunity to witness the “Changing of the Guard” at Arlington National Cemetery in Washington D.C. The first time for me was as a small child in the ‘60s and I vividly remember how precise it was. Since that time, I have been honored to witness this two more times. It is still perfect and precise. No need to change a thing.
You may be asking yourself what this has to do with our ophthalmic industry. It is all about our willingness to change what we do in our laboratories to achieve different results. Our marketplace has been experiencing dramatic changes at a furious pace over the last several years. I fully expect the landscape to continue changing. New materials and technologies are being introduced as never before. Are you keeping up and changing your practices and methods to achieve the best results?
Why should we calibrate? I believe we should invest our energy into doing things right the first time. Stop trying to fix what’s broken after it already broke. We have to believe that lens designs, equipment, processing systems and supplies are actually intended to work. We do have time, REALLY. As many say, “it ain’t rocket science.” we just need to have a good plan.
Layout & Blocking
Let’s look at first things first. How about the basics of surface layout and blocking? Your layout and blocking devices need to be checked daily for accuracy and proper function. If we layout or block the lens incorrectly to start with, everything else we do to that lens during surfacing will be at risk for coming out as desired for optics and cosmetics. If you are using pin style blocks, check that the centers are not worn causing a poor interaction between the pins and the block during fining and polishing which can lead to aberrations, waves and surface deformation. For reception style blocking, make sure that the reference surfaces are not nicked or damaged in such a way that would prevent them from seating properly in the reception chuck. This is so fundamental, yet often overlooked in many laboratories. Treat your blocks with care and replace the centers as needed. If the block is bad, replace it.
When blocking with wax mediums, allow the lens and block to cool and cure for a minimum of 30 minutes prior to generating. This is especially critical when working with thin centered lenses such as high index and polycarbonate. With alloy blocking, the desired cool and cure time should be a minimum of 30 to 45 minutes. As with wax, this is very important with thin lenses.
It is recommended to perform a curve, thickness and prism check twice per 8 hour shift. This should be conducted prior to the start of the day and then again after the lunch break. Cut a minus 10.00 diopter sphere and measure with a good sag gauge and center thickness device. Measure the curve at the center and off center as noted in the diagram. The measurements at all three points should be the same and actually what you intended to cut. Measure the lenses directly from the generator. Adjust the pic rate if necessary for this check.
-10.00 diopter curve
Temperature control is another element that needs to be considered during generating. If you use wet cutting, the coolant should be controlled from 55 to 60 degrees Fahrenheit for alloy blocking and 60 to 65 degrees Fahrenheit for wax medium.
The most overlooked item in the surfacing area are laps / tools which ultimately control the final power of the lens. Very often these are not taken care of properly and end up with dents and dings from tossing into buckets, which is just careless handling. Your lens surface quality will never exceed your lap surface quality. Don’t forget to check the bottom references as well to insure that the tool seats properly on the lap table. Aluminum is still the preferred material as it will help remove heat from the fining and polishing process. The trend over the last several years is to have a set cut in tenth increments, 1.600 index with zero pad compensation. If you are looking at this investment in the near future, speak with your supply company for the best recommendations for your application.
Are you using the right fining consumables for the increasing variety of lens materials and indices? There have been major advancements in abrasive and backing technology over the last several few years that will give you more consistent stock removals and surface quality finish. Speak with your supply company for the best products and process recommendations.
Do you recirculate, chill and filter your fining water? This is the preferred way to go for laboratories of all sizes. By doing so, you can control your temperatures, thus controlling your process. Your lenses will come out better plus the cost savings of your water and sewage bill would pay for this relatively inexpensive investment. As in wet generating, you should control your temperature between 55 and 60 degrees for alloy blocking and 60 to 65 degrees for wax style blocking. This virtually eliminates the possibility of thermal shock.
Each day after conducting the generator curve check, a short fining test should be run to validate the correct fining evolution. Below is an illustration for the patterns in resin lens materials. Fine your test 10.00 diopter curve on a known good tool for 5 to 7 seconds and inspect.
Polish & Polishing
As in fining, are you using the right polish pad and polish chemistry for the materials you process? Here too, there have been significant advancements in polishing materials to help you achieve superior surface quality which is especially critical with the growth of AR coatings.
In order to continue with excellent process control, the use of a central slurry system with chilling and filtering is critical. Not only will the lenses come out better for optics and cosmetics, it is possible to extend the normal polish life. Make sure to match the temperatures in polishing to your wet generating and fining areas. Six minutes is still the industry standard for all resin materials.
So how long will my polish last? There are some trusted “rules of thumb” that are very good. On average, 1 gallon of polish will process approximately 350 to 400 lenses before it is spent. This number goes down with higher mixes of polycarbonate. Be sure to monitor temperature and Baume as this is a good indicator of the condition of your polish. Use a nylon mesh filter bag in the appropriate size rating for the type of polishing chemistry that you use. This is a must to remove lens swarf, pad nap and any other contaminants that may be introduced into the polish during the production day. Many labs try to run the polish longer than it is designed resulting in poor optics and degraded surface quality.
In summary, it is critical to understand all of the interactions within the surfacing department. Checking these areas infrequently will likely create escalating breakage, rework and reject percentages. Check daily and check again, “ONCE IS NOT ENOUGH.”
– Al Bednar
Director of Technical Service and Sales