The craftsmanship required for finishing a pair of spectacle lenses means paying attention to detail throughout several labor-intensive steps. In particular, finishing premium lenses can improve a bottom line or wreck it if spoilage is high.
The following tech tips are designed to help you prevent spoilage and improve efficiency at the various stages in the lens finishing process.
- Cleanliness in the finishing lab is critical. Dirt, debris and particles will not only affect equipment, but also cause scratches. This is not just true for the edger, but the overall lab. Keep floors, walls, counter tops and tools clean. Clean the overall lab daily and keep logs of when cleanings are done. AR coatings are especially susceptible to scratching from particles that may not affect uncoated lenses.
- Batch the jobs. This may be easier said than done, especially in a retail setting where fast turn around time and emergency rush jobs are critical factors for dispensing success. However, as much as possible, group the finishing jobs by material. Most edgers can cut more than one lens material, but unless the edger is thoroughly cleaned, this can expose the lenses to scratching. Glass and polycarbonate, lab managers agree, can be particularly messy.
- Even lenses edged in the most sophisticated patternless edger will sometimes require touch up work with a hand stone. Follow manufacturer guidelines keeping the stone sharp and the bearings in the machine oiled. Trying to cut a lens down a size is a procedure best done in the edger and not by the hand stone. Use the hand stone for touch up work only. Chipping and pitting may indicate that too much pressure by the technician is being placed on the lens, which can also result in cracking.
- The safety bevel is also placed on a lens by a hand stone, which basically blunts the edge of the lens so it will not cut the patient. The hardest lenses to safety bevel are high-plus lenses—these edges tend to be the sharpest, likewise they’re the thinnest and easiest to break. Edge chipping or cracking, or a bevel becoming misaligned, usually indicates too much pressure from the technician on the hand stone.
- For nylon and wire rimless frames, a hand groover is necessary. Keep the groover clean, as well as your hands. This is a very labor-intensive procedure, during which lenses can easily be scratched.
- Drill mountings for rimless frames use a guide or pattern to ensure the holes are drilled in the proper place. If not used, the holes can be misaligned. Also, if the temporal and nasal edges are not the proper thickness, the drill can crack or chip the lens, a common problem for high-plus lenses.
- Lens mounting, the last step, requires skill and care. Scratches are the big concern here due to the intensive handling required to put the lens in the frame. Make sure the mounting area and technician’s hands. Another common pitfall occurs when a technician attempts to mount a lens that is too big for a frame. Usually, touching up the lens on the hand stone will correct the lens sizing. However, technicians may try to manipulate the frame.
With zyl frames, where a heated salt pan is used to warm the frames, too much exposure can warp the lens. “Salt pan” is actually a holdover term from previous generations and today’s salt pans are actually filled with glass beads. Although the beads can last a very long time, they do have to be replaced and dirty beads can affect a job. A common problem is that the beads can also be caught inside the eyewire, which can cause pitting and scratches, but always results in sloppy work.
- With metal frames, trying to force the frame around the lens can lead to bending, even cracking the lens or the frame. In both instance, the frame itself can lose its original shape, spoiling both the frame and the lens. Check sizing before mounting and use as gentle a “touch” as possible.
- Most tinting and UV treatment errors can be easily bleached out and although extra time will be added to the job, the lens is not spoiled. However, a few simple procedures must be kept in mind so the worst doesn’t happen. Edge the lens before tinting it. Don’t add UV to high-index plastic or polycarbonate. These lenses have inherent UV inhibiting capabilities and the treatment may craze the scratch coating. Also, maintain consistent temperatures in the tint baths, between 200 to 210 degrees Fahrenheit. Exceeding this temperature range can warp lenses.
- Change edger coolant regularly—at least weekly for most small edging labs—if more than two dozen pairs of lenses are edged by the machine within a week, change the coolant twice a week or more. Some technicians recommend using two coolant buckets, one for glass and one for plastic, and switching them when the operator changes materials. Dirty coolant can lead to numerous forms of spoilage, including chipping and scratches, as well as knock out of alignment water nozzles in the edger, causing the cutting point to change, cutting lenses off-size. If your edger uses re-circulated water (as opposed to fresh water), most lab managers recommend changing the water approximately every 15 to 20 jobs.
For approximately every 100 to 150 jobs, run warm soapy water through the machine to clean out any lingering particles and debris. Labs with smaller volumes do this procedure more frequently. Running warm soapy water through the machine is also a good idea for edgers using fresh water, except it doesn’t need to be done as frequently.
- Know what you are finishing. When lens and frame manufacturers introduce new products, inquire as to the finishing specification, including chuck pressure, cycle times and mounting techniques. Do not introduce new lenses or frames without knowing if the lens can be edged, or if the frame can be traced, or if the pattern for that frame is accurate. Before introducing a new brand of lenses to your finishing lab, check with the lens manufacturer for finishing specifications and recommendations. Most lens manufacturers will supply labs with demo lenses to run through the finishing system.
- Technicians should be aware that extra careful handling is required when edging AR lenses. The coating can be susceptible to scratches and crazes. Some lab managers recommend protecting AR lenses on both sides with protective edging tape or blocking pads. If using blocking pads, do not leave pads on lenses overnight or else the lenses can be permanently marked. Remove the lens from the chuck carefully and peel edging tape off gently. When mounting an AR lens into a frame, avoid excessive pressure on the center of the lens when inserting the lens into the frame. If using axis alignment pliers, carefully rotate and do not twist or torque the lens.
Finishing different types of lens materials and treatment requires a few adjustments in finishing technique. Here are some basic techniques to use when edging polarized, polycarbonate, high-index plastic and progressive lenses.
FINISHING POLARIZED LENSES
Polarized lenses require special handling at several steps in the finishing process to minimize spoilage and enhance performance, optics and cosmetics. Polarized lenses are twice as expensive as their non-polarized counterparts. Therefore, pay careful attention when processing the product.
- Polarized lenses feature a “polarizing filter”—a film or laminate that filters out reflected glare. In glass polarized lenses, the polarization filter is sandwiched between the front and back sides of the lens, but the technology that produces polarized polycarbonate and plastic lenses moves that laminate closer to the front surface of the lens. And with poly, the laminate is actually on the front surface of the lens. With plastic and high-index plastic, the laminate is less than a millimeter beneath the front surface. On these jobs, caution is key.
Even though the filter is closer to the center of the lens, lab techs say glass polarized lenses need to be handled with extra care when being edged. While most glass lenses can withstand some unwanted heat build-up in an edger, polarized glass lenses are comparatively delicate. Techs advise making sure the edger is operating up-to-par prior to cutting a glass polarized lens.
Lens slippage in the edger, which can be caused by incorrect chuck pressure, clogged wheels and inaccurately calibrated edges, will knock the lens off the axis. One way to ensure the lens is on axis after edging is to check the performance of the polarization. Some dispensers keep a marked polarized lens blank in the lab. After the polarized lens is edged, place one lens over the other, turning one 90 degrees. If the lens is finished for optimum polarized performance, it should block out the light at the correct axis line.
FINISHING HIGH-INDEX PLASTIC
Remember: High-index plastic does not finish like conventional plastic. It requires special handling because it is softer and can flex during processing. It is also, like poly, more sensitive to scratches and heat.
Unlike polycarbonate, however, high-index plastic is a more diverse lens category. Terms such as mid-index and “higher-index” differentiate the number of indices now available (in general, from 1.53 to 1.74). Retailers selling high-index plastic typically use a variety of manufacturers—their 1.60 supplier is very likely a different manufacturer than their 1.66 vendor. Each substrate finishes a little different, experienced dispensers say.
FINISHING AND FITTING PALS
- To avoid warpage and scratching, all finishing lab equipment and the general lab area must be kept clean when processing high-index plastic. Dust and grit can damage the lens and excessive debris build-up can also generate heat, potentially hot enough to warp high-index plastic. Experienced dispensers suggest wiping the lens off before and after edging, using a clean towel—not one lying around the lab.
Successful fitting and finishing of progressives is related. The key to both is the proper positioning of the “fitting cross.”
The fitting cross, which is marked by the manufacturer on the lens, is the major reference point of the lens and indicates the optical center as well as the placement of the distance, intermediate and near powers. The fitting cross must be positioned so that the patient’s pupil is centrally located behind it. Different brands of progressives place the fitting cross in slightly different areas because of the specifics of their respective designs.
- A patient’s pupillary distance and monocular height determine the position of the fitting cross in a finished progressive. For finishing, the lens must be “decentered.” In other words, the fitting cross must be properly aligned on the layout blocker so the optical center of the lens (i.e., intersection of the fitting cross) will be positioned over the center of the patient’s pupil when the lens is edged and mounted.
To assist in-office lab techs and dispensers, contemporary layout blockers now use computer monitors that show operators the correct position of the fitting cross during blocking. In addition, on-board software calculates pupillary distance and monocular height for proper decentration. With today’s patternless edgers, operators follow computer prompts and align the fitting cross with the image or guides on the system’s layout screen. In the past, these calculations were done by hand.
- Sometimes the manufacturer’s factory markings can come off during surfacing. To deal with this, some wholesale labs will use either a marker or verification sticker to indicate the fitting cross. A mild acetone should be used to remove any markings in order to prevent coating damage. Remove these verification stickers before blocking the lens. If left on, they can cause slippage and clog the edger, jeopardizing edge integrity and possibly damaging the machine itself.
Because polycarbonate is susceptible to warping and scratching during processing, proper handling and equipment maintenance are key. If a poly lens is damaged during processing, dispensers should check all equipment at each and every stage of the process to track and address the problem. Too much pressure in the edger’s chuck, which clamps the lens as it is being edged, is a common source of finishing scratching with polycarbonate.
- Cutting polycarbonate generally takes between 30 seconds and two minutes longer than conventional plastic, depending on the edger. The lens stock removed is not fine or powdery like plastic. It comes off in spirals, making the swarf “chunkier,” which in turn can “gum” up the cutting devices in the edger. Therefore, edging polycarbonate requires more cleaning and general maintenance of the equipment due to the distinctive texture of its swarf.