The crack as a tool

Glass is a versatile, popular material for a wide range of applications. Cutting the glass is key to obtaining high-quality products. In the conventional process used to cut flat glass, a small cutter wheel scores a line into the glass. Pressure is then applied to the glass along this line so that it breaks.

Unfortunately, glass splinters may come off in the process, producing defects known as micro-cracks. The glass consequently needs to be reworked by grinding and polishing, which costs time and money. Nonetheless, damages may remain in the glass that reduce its strength.

A great deal of time and money, limited design options – reason enough to develop a better, more effective process for separating glass. In a project funded by the Federal Ministry of Education and Research (BMBF), Dr. Rainer Kübler has been working with his five-strong team on a laser-induced stress separation process for flat glass that causes minimal damage. Dr. Rainer Kübler has been awarded the Joseph-von-Fraunhofer Prize 2008 for his work.

So what do the Fraunhofer scientists do that is different? Instead of scoring the glass mechanically, they do it by applying stress. “We have to heat the glass along the required separation line without damaging it,” explains Dr. Kübler. “We do it with a CO2 laser.” The second part of the secret is to shock-cool the glass by means of a cooling nozzle following right behind the laser beam, blowing cold air onto a specific area of the glass. The temperature difference creates a stress field and, in turn, a crack. Then the thermal crack introduced into the surface by this process is opened by bending the glase plate until it separates. Extensive experience and numerical simulation have helped to manage the process and particularly the crack – to produce the crack in a controlled manner and use it as a tool. “Our process has enabled us to produce extremely high-quality glass edges. And flawless, smooth edges mean firmer glass,” says Dr. Kübler. The stability of the edges determines the strength of the entire pane.

All of this opens up entirely new applications for the use of glass panes in architecture. Thanks to the flawless edges, the installed glass panes can be made thinner without sacrificing any of their reliability. And Grenzebach, the development partner, is a global player in glass production technology, providing the new process with ready access to a global market.