A numerical simulation of machining glass by dual CO₂-laser beams

Abstract

In the flat panel display (FPD) industry, lasers may be used to cut glass plates. In order to reduce the possibility of fracture in the process of cutting glass by lasers, the thermal stress has to be less than the critical rupture strength. In this paper, a dual-laser-beam method is proposed, where an off-focus CO₂-laser beam was used to preheat the glass sample to reduce the thermal gradients and a focused CO₂-laser beam was used to machine the glass. The distribution of the thermal stress and the temperature was simulated by using finite element analysis software, Ansys. The thermal stress was studied both when the glass sample was machined by a single CO₂-laser beam and by dual CO₂-laser beams. It was concluded that the thermal stress can be reduced by means of the dual-laser-beam method.

Introduction

With the development of laser technology and flat panel display (FPD) technology, many studies have been carried out to investigate the methods of cutting glass using lasers [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. J.F. Li et al. [3] put forward a mathematical model to explain the heat transfer of glass heated by lasers and to analyze the differences of the effect on the thermal behavior of glass between the application of lasers as a volumetric heating source and that of a surface heating source. C.Y. Wei [4] and W.X. Tian [5] investigated the thermal behavior of glass heated by a CO₂-laser beam numerically, and concluded that the resulting temperature distribution was strongly dependent on the speed of the moving laser beam and the laser parameters, i.e., the size of laser beam and the power of the laser. Chwan-Huei Tsai et al. [6] studied the thermal stress of alumina ceramic substrates irradiated by a moving laser beam. Meanwhile, experiments were carried out to investigate how the crack propagation was influenced by laser power, cutting speed, and specimen geometry. R.M. Lumley [7] used a controlled fracture technique to cut brittle materials such as ceramic and glass for the first time in 1969, and a 7 mm thick alumina substrate was separated successfully under a speed of 50 mm/s. With this controlled fracture technique, a liquid crystal display (LCD) glass substrate was also cut successfully by S.L. Ye et al. [8]. Several patents for glass laser cutting are also available, such as those by W. Hafner [9], Willy Verheyen et al. [10], and Kazuyuli Komagata et al. [11].

When a laser beam irradiates on the glass surface, part of the laser energy is absorbed and conducted into the material, and thermal stress is produced as a result of thermal expansion. If the stress exceeds the critical value, the fracture will be induced undesirably. Lots of work has been done previously to search for a suitable method to reduce the thermal stress and avoid fracture when glass is machined by lasers. Chui [12] preheated glass to the annealing temperature (510–590 °C) in a high furnace before laser cutting. L.Bradley et al. [13] used a flame-assisted method in the laser surface treatment in order to avoid fracture. It has been shown that glass should be machined in a high-temperature condition for desired results. But, the methods mentioned before have proved to be impractical due to the heavy pollution and low efficiency caused.

A dual-laser-beam method has been applied to manufacture brittle materials in recent years, where an off-focused laser beam is used to control thermal stress. In this way, the defects of using the high furnace and the flame-assisted method can be eliminated. D. Triantafyllidis et al. [14] used two kinds of laser sources (a CO₂-laser and a diode laser) to control the thermal gradients in the surface modification of alumina ceramics. An active-stressing technique was used by R. Akarapu et al. [15] to cut alumina, and an off-focus laser beam was applied to control (delay) the fractures. Meanwhile, how the fracture propagation is affected by the laser parameters and the relative location of two laser beams was also numerically studied by him. In most of the previous studies on machining brittle materials with the dual-laser-beam method, the focused beam was first applied to the brittle materials, followed by the off-focused beam for reducing the cooling rate and controlling the fracture propagation [7], [8], [14], [15]. In this study, an off-focused CO₂-laser beam was applied ahead of the focused CO₂-laser beam to preheat the glass so that the thermal gradients in the machining process are reduced.

In the present study, glass was machined both with a single CO₂-laser beam and with a dual CO₂-laser beam. Meanwhile, the distribution of the temperature and the thermal stress was simulated using Finete element analysis (FEA) software, Ansys. The calculated results showed that the thermal stress could be reduced with the dual-laser-beam method.