Glass Quality Inspection

GLASS QUALITY INSPECTION
Zebra Marks After Toughening
Roller Wave Measurement Instruments
Methods to Check Roller Wave & Edge Lift
A Comprehensive Technical Reference for Glass Industry Professionals
Standards: EN 12150 | EN 1863 | ASTM C1048 | ISO 12543

1. INTRODUCTION TO GLASS QUALITY INSPECTION

Glass quality inspection is a critical process in the manufacturing of toughened (tempered) and heat-strengthened glass. Visual and instrumental methods are used to evaluate optical distortion, surface irregularities, and edge quality. Among the most important quality indicators are:

• Zebra marks (anisotropy)

• Roller wave distortion

• Edge lift

Key Standards Covered:

• EN 12150-1 – Toughened soda lime silicate glass

• EN 1863-1 – Heat strengthened glass

• ASTM C1048 – Heat-treated flat glass

• ISO 12543 – Laminated glass

• EN ISO 14438 – Energy balance determination

2. ZEBRA MARKS (ANISOTROPY)

Zebra marks are optical patterns visible in toughened glass under polarized light. They appear as alternating dark and light bands.

Causes:

• Non-uniform quenching

• Nozzle misalignment

• Glass thickness variation

• Furnace roller marks

• Edge stress concentration

• Tin bath residue

Inspection Method:

• Use Zebra Board (anisotropy checker)

• Inspect at 45° angle

• Rotate glass 90°

• Use polarized light

• Check zones (Central, Peripheral, Edge)

Acceptable Distortion (EN 12150):

• Zone A (>100mm from edge): ≤ 2mm/m

• Zone B (50–100mm): ≤ 3mm/m

3. ROLLER WAVE IN TOUGHENED GLASS

Roller wave is periodic surface distortion caused by sagging between furnace rollers during heating (~640°C).

Key Influencing Factors:

• Glass thickness (3–19mm)

• Furnace temperature (620–660°C)

• Roller spacing (100–150mm)

• Travel speed (1–6 m/min)

• Roller diameter (50–80mm)

Measurement Methods:

• Straight edge + feeler gauge

• Bow gauge / dial indicator

• Laser profilometer

• Optical deflectometry

• Moiré fringe analysis

• Structured light scanners

Straight Edge Method Steps:

1. Place glass on flat table

2. Place 1000mm straight edge perpendicular to roller direction

3. Insert feeler gauge at max gap

4. Record 5 readings

5. Calculate: Wave (mm/m) = Max Gap / 1m

6. Compare with EN 12150 limits

Acceptance Limits:

• Standard toughened: ≤ 0.5 mm/m

• Architectural: ≤ 0.3 mm/m

• Heat strengthened: ≤ 0.5 mm/m

• Laminated toughened: ≤ 0.3 mm/m

4. EDGE LIFT (EDGE CURL / SHELL BOW)

Edge lift is when the edges curve upward/downward relative to the center.

Causes:

• Thermal gradient

• Edge quench overcooling

• Poor edge cutting

• Roller pressure variation

• Thin glass (<5mm)

Measurement:

• 300mm straight edge + feeler gauge

• 3-point gauge

• Digital height gauge

• Laser flatness scanner

• CMM measurement

Typical Limit:

• ≤ 1.0 mm / 300mm

5. ADVANCED MEASUREMENT SYSTEMS

Modern systems:

• Structured light deflectometry

• Polarized imaging

• CCD scanners

• Laser triangulation

• Interferometry

Outputs include:

• 3D surface maps

• Stress maps

• Defect logs

• PDF reports

6. BEST PRACTICES

• Calibrate instruments every shift

• Measure after glass cools 30 minutes

• Mark roller direction before testing

• Use SPC charts

• Maintain furnace rollers weekly

• Replace rollers >0.1mm runout

7. DEFECT SUMMARY

8. CONCLUSION

Glass quality inspection combines:

• Visual inspection

• Precision measurement

• Statistical process control

• Proper furnace maintenance