SPECTACLE LENS PROCESSING
Processing Poly...a Look at Problems and Solutions
By Liz Martinez DeFranco, A.B.O.C., N.C.L.C.

Processing polycarbonate is hardly the challenge it used to be. In fact, several finishing equipment suppliers say the number of problems reported to their technical support staffs have decreased dramatically. Like any material, however, dispensers still have some questions.

Here are five reader queries about polycarbonate and the answers given to them by executives at several equipment companies.

VISUAL ACUITY

PROBLEM

"A lot of our patients prefer polycarbonate lenses because they are less expensive than high index, but they still offer thinner edges and lighter weight. The problem is that we get occasional returns from patients who aren't satisfied with the visual acuity their poly lenses provide."

SOLUTION

"The quality of the optics in polycarbonate has certainly improved," acknowledges Jay Lake, director of technical services at Briot Edgers Inc. "In fact, now it has become the alternative high index material."

However, because poly's abbe value is not as good as that of other materials, some patients will have difficulty adapting, especially when there is cylinder correction. "When there is more than 1D of cylinder, measure heights for the optical centers, just as you would for progressive or aspheric lenses," advises Lake.

EDGER SWARF

PROBLEM

"We process a lot of polycarbonate, and I notice that after we edge several lenses, the edger gets gummed up. I wonder why other materials don't cause the same problem."

SOLUTION

"Polycarbonate material has static properties," says Bill Galindo, president of ODI/Topcon. "These properties cause the 'swarf' to stick to the metal parts of the edger." According to Galindo, the carriage, floating head, shaft, and bearings of older edgers attract the most swarf because they were designed to process materials that do not have static properties, such as CR 39, high-index, and glass.

Briot's Lake adds that swarf tends to build up on edgers because the coating on polycarbonate lenses is different from that applied to other plastics. "Polycarbonate hard-coat becomes stringy when cut," Lake explains.

"When poly is edged wet, the material is peeled away rather than cut," adds Rick Noonkester, Western regional manager at Santinelli International.

"Wet" edgers require a lot of cleaning and maintenance, especially when polycarbonate lenses are being processed, Lake explains. "The grinding chamber needs to be cleaned regularly--usually after every 50 pairs," he adds.

PROBLEM EDGER

PROBLEM

"We started getting in so many polycarbonate jobs that we had one of our edgers modified to cut poly. But that machine always needs more attention than the edgers we use for processing CR 39 and high-index."

SOLUTION

"A lot of dispensers keep their equipment for an average of seven or eight years, and these older edgers were not designed to run polycarbonate, whereas newer edgers are built poly-capable," adds Jay Little, vice president of sales at DAC Vision.

There are several steps in the automated edging process that are designed for poly. "Poly has to be run dry, using a more abrasive wheel," says Gerber Coburn's Product Manager Michael Urban. "Also, more torque, or pressure, is required at the roughing stage."

"Older edgers that are retrofitted to process poly may apply an incorrect amount of weight to the lens, which can warp the lens or craze the coating," warns ODI/Topcon's Galindo. "In addition, edgers designed for CR 39 and HI materials are set up so that the wheel and the lens both rotate in only one direction. Poly edgers are made so that they rotate in opposing directions."

Joe Meaney, LOH Optical's finishing systems product manager, also points out that it's OK if the water coolant cycle resumes to finish poly wet. "The roughing cycle and the first and second beveling cycles must be dry, but the coolant can come on after that," he says. If the machine has been retrofitted, this may mean controlling the water supply manually rather than having the water automatically come on at the right time.

EDGE POLISH

PROBLEM

"We're having a difficult time using a manual edge-polish machine for polycarbonate lenses, but we don't want to avoid polishing the edges because they look so unattractive. Suggestions?"

SOLUTION

Urban from Gerber Coburn says that newer edgers have built-in edge-polish cycles to solve this problem and shorten processing time. "The blade on a dry-cut edger typically leaves 'stutter marks' that make the edges appear ugly," he says. "Auto-polishing is a good alternative to a separate machine because it requires no additional skill and reduces the chances that the lens surface will be ruined during polishing."

AR COATINGS

PROBLEM

Our stock anti-reflective coated polycarbonate lenses seem to craze more frequently than AR-coated CR 39 or HI lenses do.

SOLUTION

"Anti-reflection coating on top of polycarbonate material is a more fragile surface than AR on other lens substrates, so it fractures more easily," says Matt Vulich, vice president of sales of A.I.T. Industries. To avoid this difficulty, Vulich advises that the pressure of the chucking system as well as the head pressure be checked to ensure that they do not create unnecessary stress on the lens.

"Use plastic blocks that conform to the base curve of the lens better than metal blocks," Vulich says. "It's also a good idea to place a lens chip protector, or a slip-stop guard, between the lens and the leap pad as well on the backside of the lens where the chuck will make contact," he adds. These clear round disks will help to prevent scratching in the centers of the lenses.

DRILLING THE LENS

PROBLEM

"When I drill a polycarbonate lens, the hole seems to 'bubble' out the back side of the lens. What can I do to prevent this?"

SOLUTION

"If you try to drill or groove a poly lens too quickly, the lens material actually burns," says Stuart Watson, director of training at National Optronics. To avoid ruining the lens in this manner, Watson suggests that holes be drilled only .5 mm at a time. "Back the bit out, allow the material to cool for a few seconds, then drill another .5 mm," he advises.

To successfully groove a polycarbonate lens, Watson's approach is to groove the lens in two passes instead of one. "Run the lens through the cycle twice," he says. "The first time, groove to half the depth; the second time to the full depth."

DYING POLY

PROBLEM

"Dying poly dark and evenly is always difficult. I'm afraid to leave the lens in the dye pot too long because it might warp, but it seems to take forever to achieve a sunglass tint."

SOLUTION

According to National Optronics's Watson, poly lenses won't dye to a deep sunglass shade because of the nature of the polycarbonate hard coats. "You can't really get darker than a '2-1/2' shade," he admits. A full sunglass tint is a '3'.

To help poly lenses absorb the maximum amount of dye, he suggests that lenses be dipped into a lens prep solution or into hot water in a separate pot in the tint unit. "The lens prep helps remove some surface dirt for more even dye application," Watson says. "Both lens prep and hot water open the pores of the lens and help it accept the dye better." EB