Veteran lab managers estimate approximately two-thirds of lens spoilage occurs during the finishing process. Lab technicians being poorly trained, jobs being rushed through and insufficient attention to detail are the most common causes of spoilage.
The most effective ways to prevent and reduce finishing spoilage are basic and obvious—proper training and adherence to set protocols. Here are some basic protocols that will help prevent spoilage at the various finishing stages.
Do Your Homework
Know what you are finishing. When lens and frame manufacturers introduce new products, inquire about 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, 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.
Lens Inspection Prior to Edging
All lenses should be inspected with a lensometer (or vertexmeter) prior to layout. This is especially critical for retail labs. If the retail lab gets a lens from a wholesale lab and, after finishing it finds out that the prescription is off or some other surfacing-caused error was not previously detected, the surfacing lab is no longer liable for that lens. The unwritten rule is “If you finish it, you bought it,” so make sure you are finishing what you ordered.
It’s also important to inspect a lens for sizing immediately after edging it. Trying to mount a lens that still requires time in the edger or touch up from a hand stone, means needless handling and increased potential for spoilage.
Keep it Clean
Dirt, debris and lens particles are leading causes for lens scratches. Ironically, if these substances are not removed in the correct manner from the substrate, the act of cleaning will cause scratches.
To clean a lens, submerse it in warm water, then wipe it off with a clean cloth. Of course, submerging after each finishing step is not feasible and wiping it will suffice, but the cloth must be clean. Do not let the cleaning cloth come in contact with any surface in the finishing lab. Instead of placing the cloth on a table where particles can attach themselves to it and then scratch the lens, drape the cloth over your shoulder or hang it on a hook. Don’t try to clean an entire day’s worth of lens jobs with one cloth. Change cloths frequently. Some labs have switched to disposable cloths, others use one cloth per lens and some have even taken a more preventative measure of having workers wear rubber gloves or finger cots for lens handling, particularly for AR jobs. Clean the lens prior to layout and immediately after edging. Do not use the hand stone or groover on a dirty lens. And, clean the lens prior to mounting, which is when the lens is handled the most, thus creating a potential for scratching.
It’s important to keep the lab clean as well. Keep floors, walls, counter tops and tools clean. Clean the lab daily and keep logs of when cleanings are done. AR lenses are especially susceptible to scratching from particles that may not affect uncoated 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 scratching, as well as knocking the water nozzles in the edger out of alignment causing the cutting point to change, thus cutting lenses off-size. If your edger uses re-circulated water (as opposed to fresh water), most lab managers recommending 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.
When using a hand stone to touch up lenses after edging, be sure to follow manufacturer guidelines keeping the stone sharp and the bearings in the machine oiled.
When using a hand groover for nylon or wire rimless frames, make sure the groover is clean, as well as your hands. This is a very labor-intensive procedure, during which lenses can easily be scratched.
Wholesale labs batch jobs according to lens material. It’s a good idea for in-office finishing labs to do it too. This may be easier said than done, however, especially in a retail setting where fast turnaround time and emergency rush jobs are critical.
It’s important to note that with many of the mid- and high-index materials now available finishing specifications are no longer uniform. Different brands of high-index plastic may require different wheels and different settings. It’s best to check with the lens manufacturer for specific recommendations on how to edge their lenses.
To edge polycarbonate and high-index plastic effectively, you will need an edger with good regulation of water and coolant. Some materials work better with a “drier cut” while others require more lubricant. If your edger cannot switch easily between these settings, the effectiveness of edging these materials will be compromised.
Technicians should be sensitive to the fact that when edging AR lenses extra careful handling is required. The coating may be susceptible to scratches and crazes. Some lab managers recommend protecting AR lenses on both sides with edging tape or blocking pads. When 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 the edging tape off gently. When mounting an AR lens into a frame, avoid excessive pressure on the center of the lens during insertion. If using axis alignment pliers, carefully rotate, do not twist or torque the lens.
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. Do not 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.
Lens mounting requires skill and care. Scratches are a big concern due to the intensive handling required to put the lens in the frame. Make sure the mounting area and your hands are clean.
A 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. One common problem is the beads can also be caught inside the eyewire, which can cause pitting and scratches, and 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 instances, the frame itself can lose its original shape, spoiling both the frame and lens. Check sizing before mounting and use as gentle a “touch” as possible.
After the lens is edged and mounted, the entire piece of eyewear must be inspected, which means not just the placements of the optical centers, but also the alignment of the frame. In addition to using a lensometer for prescription and curve verification, carefully look at the lens for any hairline scratches and minuscule defects that will affect vision. The inspector must also carefully look over the eyewear, checking the edge of the lens, the quality of the safety bevel or any gaps. It’s a good idea to have one person finish and another person inspect.