Veröffentlichungen:
Ostendorf, A.; Stute, U.; Büsching, C.: EU-Projekt MATRA: Machining of Transparent materials by Multiple Absorption – Inhalt und Ergebnisse. In: Lasertechnologien für die Glasbearbeitung – Laser Technologies for Glass Processing; 11. November. Düsseldorf, 2004
Lasertechnologien für die Glasbearbeitung – Laser Technologies for Glass Processing
Summary
The goal of the European Union project MATRA is the development of an industrial separation process for glass materials, based on thermal tension. This process should also compensate the present disadvantages of conventional separation methods. Laser radiation was selected for the separation process because automation process with the laser are not difficult, and it has a defined heat source. For the manufacturing of complex 2-D geometries and the separation of rotationally symmetric products, conventional methods require complex grinding and polishing procedures. By using laser technology, which is non-tactile and has a high reproducibility level, it is possible to combine these processing steps into one step. The attainable quality of the separation edges is comparable with that of conventionally polished edges.
1 Motivation and Basics
In Europe, approximately 28,9 million tons of glass are manufactured annually. Container glass and float glass compose 30,4 % of this amount. Finishing container glass and refining flat glass to high quality products takes a leading position in the glass processing industry /CPI04/. New, innovative and modern procedures for the separation of glass materials have a special potential for increasing economical aspects in the glass industry. Flat glass is predominantly cut by scratching and breaking. Bottle glass products are predominantly separated by thermal bursting using gas flames and metallic or gas-cooling. These processes produce splinters or glass fragments, and micro- cracks along the separation line, respectively distortions of the separation edges. In many cases, micro-cracks are starting points for further damage of the product and eventually its failure. Glass fragments can cause scratches on glass surfaces or coatings. These influences can reduce the quality of the product to the point of product waste. Therefore, in many cases, the edges are improved by additional work, such as grinding and polishing. These processes call for further work steps, and require cost and time-intensive cleaning of the product from the grinding/polishing agents. A one-step separation process resulting in high-quality edges is the goal of the MATRA project, the laser Full Body Cut.
2 Project Contents
In the context of the MATRA project, flat glass and hollow wares are to be separated with high quality edges. The machine prototype was designed and manufactured according to the process requirements demanded by the consortium. The final users among the project partners have supplied industrially relevant product geometries and tolerances as well as quality specifications which should be fulfilled by the laser separation process. Process development is aided by FEM analyses and simulations.
3 The Process
The process is based on the multiple reflection of Nd:YAG laser radiation (Multiple Laser Beam Absorption, MLBA). This procedure uses an Nd:YAG laser, whose radiation, depending on glass thickness, is transmitted up to 85 % by the glass. Ferric oxide is mainly responsible for the absorption of Nd:YAG laser radiation in glass /Bue94 /, /Sch88/. The multiple reflections of the beam through the product increases total absorption, and makes it possible to induce thermal tensions over the entire glass plate thickness. These tensions are necessary to separate the glass. The crack caused by the thermal tensions perpendicularly to the main stress level spreads, based on an initial crack. Due to the locally limited and temporally and locally variable, precisely adjustable heat fields, the crack formation can be controlled by the laser beam, and can be guided through the product. Breaking, grinding and cleaning the work pieces are no longer necessary.
4 The Prototype
The development of a prototype began by converting a conventional machine used for cutting laminated glass. This station was provided with a mount for the laser processing head and the lower reflector. This machine demonstrated a linear separation process at the Glasstec 2002. The second prototype concept developed by the consortium was changed so that the entire working table could be used as a reflector. This ensures a constant beam configuration over the entire working field, and simplifies the structure of the machine for 2-D cutt. Linear direct drives can be used to move at slow and high feed rates, without oscillations caused by the drive unit. Further development will lead to an integrated working head, which consists of beam-shaping optics and a temperature control unit.
5 Results / Practical Applications
The surfaces of the separated edges after using the MLBA procedure exceed the quality of mechanically polished surfaces, and additionally increase the firmness of the products. Thus, thermal downstream processes without reworking the glass edge are possible /Hes03/. Advantages from the emission-free process promise successful use in the manufacturing of high-tech products. Glass fragments, abrasive dust and cleaning processes can be avoided for the processing of coated glass, which is not only an economic argument for the laser process. Non-tactile separation in one working step without auxiliary materials is made possible for the first time using this technology. Due to the volume absorption, several stacked glass plates can be separated in one work procedure. Also bonded glass, for example laminated glass, can be cut in a defined manner. Further high potentials for the laser beam-based separation process can be seen in the production of glass tubes and hollow ware. Economically, the total time for conventional separation processes, with grinding and polishing processes, cleaning and transport of the glasses, exceed the processing time of the MLBA process. The final users within the field of flat glass are represented by the companies Schott Spezial Glas (Germany) and Thales Optics (England). Both partners manufacture high-tech glass products, which characterize the area of application of the new development. Within the field of hollow ware, the company ARC International (France) manufactures high-quality products. The edges require complex reworking after being separated from blowing caps. Laser beam supported separation of hollow ware for use in serial production is the goal of the French project partner. Further products which can be manufactured economically with this technology are, for example, display glasses, automotive mirrors (fig. 6) and solar cell cover plates, as well as design glass products. The prototype for the process was manufactured by one of the largest glass processing machine manufacturers of the world, Bottero (Italy). A goal of the Italian project partner is the conversion of MLBA technology to machine concepts for industrial use in the high-tech area.
6 Conclusions
The laser beam as a tool for separating glass materials has already been introduced in modern industrial processes. Examples are laser beam scratching of display substrates as well as laser beam blasting of glass tubes. Laser processes have become economically interesting in mass production as well as for manufacturing high-tech products. Due to the shortened process chain, investments in the laser source amortize in a short time. The laser possesses the potential to replace the cutting wheels within wider areas of high-tech glass product manufacturing.
7 Acknowledgements
This work has been done within the scope of the European research project MATRA „Machining of Transparent Materials by Multiple Absorption“(EU-Project number: G1RD-CT-2002-00746). The authors would like to thank the partners of the consortium: Schott Spezialglas GmbH (D), Bottero S.p.A. (I), ARC International (F), Thales Optics Ltd. (GB).