Edging Polycarbonate & High-Index Lenses
Objective. Students will edge polycarbonate and high-index lenses using material-specific wheel types and speeds, producing finished lenses free of crazing, delamination, or stress fractures.
This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.
VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.
Why Polycarbonate and High-Index Lenses Need Special Edging Attention
Polycarbonate and high-index plastics (such as 1.60, 1.67, and 1.74 index materials) have become standard in modern dispensing because they offer thinner profiles, lighter weight, and, in the case of polycarbonate, superior impact resistance. However, both materials behave very differently from CR-39 during edging. Using the wrong wheel, speed, or coolant flow can produce crazing, stress fractures, or delamination that ruin the lens and, more importantly, compromise patient safety.
How Polycarbonate Differs from CR-39
CR-39 is a thermoset plastic that machines cleanly with moderate wheel pressure and standard coolant flow. Polycarbonate is a thermoplastic with a much lower heat tolerance. Friction generated during edging can raise surface temperature enough to cause crazing—a network of fine surface cracks that scatter light and weaken the lens structurally. Polycarbonate is also highly birefringent, meaning it already shows internal stress patterns under a polariscope; additional stress from edging can create visible stress fractures.
Key differences when edging polycarbonate versus CR-39:
- Diamond wheel hardness and bond: Use wheels rated specifically for polycarbonate. These wheels typically have a softer resin bond that releases abrasive particles more readily, reducing heat buildup.
- Edge speed (chuck/wheel RPM): Run at a lower speed than CR-39. Higher speeds generate friction heat faster than coolant can remove it.
- Coolant (water) flow rate: Increase coolant flow to maximize heat dissipation. Never edge polycarbonate dry or with reduced water.
- Cycle time and pressure: Use lighter tracing pressure and allow the edger's automatic cycle to complete without manual overrides that rush the cut.
Always consult the edger manufacturer's material guide—specific settings vary by machine model and wheel brand.
High-Index Materials: Additional Concerns
High-index lenses (1.60 and above) are often made from proprietary thermoset or thermoplastic resins and frequently include anti-reflective (AR) coatings applied at the factory. These coatings bond to the front surface but do not extend to the uncut edge, which is where edging occurs. Even so, aggressive edging vibration can cause edge delamination where the AR layer meets the lens body near the bevel.
Practical points for high-index edging:
- Use a high-index-rated wheel or a wheel the manufacturer specifies for hard materials; some high-index resins are harder than polycarbonate.
- Maintain adequate coolant flow similar to polycarbonate protocols.
- Avoid regrinding the bevel multiple times on the same lens; repeated passes generate cumulative heat and stress.
- Handle with care after edging: High-index lenses are thinner and more brittle at the edge; dropping or squeezing during insertion can chip them.
The Production Log and Edger Settings Reference Card
Every lens pair you edge in this lab must be documented on the production log: material type, index, wheel type, RPM or speed setting, coolant rate, cycle time, and any observations during edging. This habit mirrors professional lab practice and creates an audit trail if a lens fails inspection. Your edger settings reference card provides the starting-point parameters for each material; adjust only within the ranges shown and record any deviation.
Inspection: What to Look For
After edging, inspect every lens before fitting:
1. Under the inspection loupe or polariscope: Look for crazing (fine surface crackle), stress marks (dark bands visible under polarized light), chips, or rough edges.
2. Edge profile: The bevel should be centered or positioned as needed for the frame, smooth, and continuous with no flat spots.
3. Frame fit: The lens should seat into the frame groove without rocking, gapping, or requiring excessive force. Forcing an oversized lens can crack polycarbonate.
4. Optical center verification on the lensometer: Place the edged lens in the lensometer and confirm the optical center (or prescribed prism reference point) is within the tolerance specified on your check sheet.
Safety Points
- Eye protection: Wear safety glasses or goggles whenever the edger is running. Coolant spray and lens chips are genuine hazards.
- Wet hands / electrical: Keep hands clear of wheel mechanisms; dry hands before adjusting controls.
- Do not bypass the automatic cycle: Manual overrides can stall the wheel against the lens, causing sudden heat spikes.
- Defective lenses must be remade—never dispensed. A crazed or stress-fractured polycarbonate lens may fail under impact, defeating its primary safety purpose.
- Scope of practice: Lens fabrication decisions that affect the prescription must not be altered without authorization from the supervising optician or prescriber.
Analyzing Failures Before Remaking
If a lens fails inspection, stop and diagnose before remaking it. Common causes:
| Defect | Likely Cause |
|---|---|
| Crazing | Excessive heat: speed too high, coolant too low |
| Stress fractures | Wrong wheel bond, excessive pressure, dry edging |
| Chips at edge | Wheel worn, speed too high for material hardness |
| Delamination near bevel | Multiple regrind passes, coolant failure |
Document the defect, the likely cause, and the corrective settings on your production log before remaking.
Key terms8
Worked examples3
- Identify the CR-39 setting: 2,000 RPM.
- Calculate 20% of 2,000: 2,000 × 0.20 = 400 RPM.
- Subtract the reduction from the CR-39 setting: 2,000 − 400 = 1,600 RPM.
- Note the measured deviation: 2.5 mm nasal.
- Note the allowable tolerance: ±1 mm (i.e., any deviation greater than 1 mm in any direction fails).
- Compare: 2.5 mm > 1 mm.
- Determine result: The lens fails optical center placement by 1.5 mm beyond the tolerance limit.
- Identify the CR-39 flow rate: 80 mL/min.
- Calculate 25% of 80: 80 × 0.25 = 20 mL/min.
- Add the increase: 80 + 20 = 100 mL/min.
The math in this lesson1
Materials: Automatic edger with polycarbonate-rated diamond wheels, polycarbonate lens blanks (4 pairs), high-index 1.60 blanks (4 pairs), polariscope or inspection loupe, frame set, lensometer, production log forms, edger settings reference card
Check: No crazing, chips, or stress fractures visible under loupe on any lens; lens fits frame without rocking; optical center within ±1 mm of prescription; instructor signs off each pair before student advances to safety beveling lab.
Practice check8
Answer each question, then check it. Nothing is recorded; this is your own practice.
