Abstract

TEA CO₂ and modulated c.w. CO₂ lasers are now commonly used to mark packaging materials by stencil mode projection and dot writing, respectively. In particular, glass containers can be marked directly by a combination of surface crazing and material removal. In order to help optimise this process and evaluate the possible debris, we have analysed the stress-stain state of laser irradiated glass using thermoelastic equations.¹ Frozen-in stress caused by the thermal cycling of the material through a low-viscosity regime has been identified as the most likely cause of laser induced microcracking (Fig. 1). Studies of the laser irradiated area by x-ray fluorescence and emission spectroscopy of the luminous plume also indicate a loss of, in particular, sodium from soda-lime glass. This change in stoichiometry may be a complementary mechanism for the precipitation of surface crazing. Observed time delays before cracking begins are broadly consistent with the typical cooling time of the glass and are of the order ≈10⁻³ s. Similarly, surface features recorded by scanning electron microscopy suggest that crazing occurs after the viscosity of the glass has returned to its room temperature value. However, cracking may not occur at this time but may only happen when initiated by stress enhancement (surface defects, handling, etc.). Soda-lime glass has been observed to craze consistently at fluences ≥1 Jcm⁻² when irradiated by TEA CO₂ lasers.

© 1994 IEEE