is the temperature difference caused by laser heating. During
the cutting process, locally limited and precisely
adjustable heat fields are created and controlled by the laser beam. The heat
fields lead the crack, through the product. Veröffentlichungen:
Ostendorf, A.; Stute, U.; Büsching, C.: Cutting of Flat Glass and Laminated Glass by Solid-State Lasers. In: Glass Processing Days, 17.–20. Juni 2005, Tampere
Keywords:
1=Laser 2=Glass 3=Cutting 4=MLBA
Abstract
The method of cutting and breaking in order to separate glass is as old as the material itself. This separation method, which is still predominantly used today, underwent a revolution at the beginning of the 20th century, when cutting wheels were invented. The next relevant evolutional step in glass separation was the use of laser radiation to cut glass products. Laser processing of glass shows, especially in the area of the high-tech products, clear advantages compared to separation by cutting wheels.
In this paper, the principle and results of using NIR-Laser radiation for separating flat glass and laminated glass will be shown. The delineated characteristics of the process lead to the presented prototype machine. This laser method enables to cut without mechanical forces and with high reproducibility. Complex 2D geometries, which can be conventionally manufactured only by using time intensive grinding and polishing procedures, can be realized in one processing step with the help of laser technology. Laser cut edges are shown attaining a quality comparable with those of conventionally polished edges. The laser separated edges are convincing due to a high mechanical stability and thermal shock resistance. Thus, lasers are one of the most interesting high potential tools which can be easily automated in high-tech glass processing.
Introduction
The ongoing competition between car manufacturers is followed by a drop off in prices for the suppliers. At the same time in the area of separating 2-D contours in flat glass, increasing complexity and functionality of modern glass products for automotive industry require higher demands on glass finishing processes. Thinner wall thicknesses or sensitive function coatings on the glass surfaces necessitate sensitive manufacturing processes. Therefore effective production methods are of high interest espacially for automotive glass suppliers.
Flat glass is cut predominantly by scratching and breaking. This process produces splinters of glass fragments and micro-cracks along the separation line, respectively distortions of the separation edges. Micro cracks are in many cases starting points for further damages of the product and its failure. Glass fragments can cause scratches on glass surfaces or already applied coatings. These influences can reduce the quality of the product up to the uselessness of the product [1]. To remove micro cracks in many cases the edges in additional work through to grinding and polishing have to be improved. These processes mean a further post processing and require further cost and time-intensive cleaning of the product of working debris. The process of separation with high-quality edges can be reduced to one process step with the Multiple Laser Beam Absorption method (MLBA), the full body laser cut.
The Main Text begins here
The Process
Nearly all cracks in glass are induced by tensile stress at the glass surface. Due to this effect the cooling of glass is more critical than heating up. Using this effect, a laser based glass cutting technology has been developed to guide thermal induced cracks through the glass. The MLBA process is based on the multiple reflection of laser radiation. Within this process, NIR-lasers in the high transmissive range of glass smaller than l=2.000 nm are used. The best results are realised with conventional industrial lasers emitting radiation of the wavelength l=1.030 µm or 1.064 µm. The radiation of this type of laser, depending upon glass thickness and type, is transmitted up to 90 %. In the glass present ferric oxide is considerably responsible for the absorption of NIR-laser radiation [7]. Regarding the effect that nearly 4 % of the radiation is reflected at each glass surface, the total absorbtion can be calculated by the following formula.
100%=Transmission + Reflection + Absorption (Formula 1)
In contrast to the CO2 laser glass cutting methods, where the radiation is nearly totally absorped at the glass surface, the NIR-Laser causes due to the low absorption an increased temperature over the whole treated glass volume. Multiple reflection of the beam through the product provides a sufficient absorption and good efficiency [2,3,4].
One special effect by laser heating of glass is the locally variation between the laser cross section and the point of highest temperature at the glass surface. This distance is variating by the feed rate of the laser beam and causes special solutions in controlling the maximum glass temperature during complex cutting operations. For one value of velocity and laser power the effect is shown in the simulation below.
Figure1: temperature simulation of MLBA cutting
The crack results from stress differences in the glass material and the surfaces. In the heating phase, the laser beam creates compressing stress over the whole glass thickness. Cooling the surfaces by free convection to air causes a tensile-compression-tensile stress profile without using any cooling fluids. The crack caused by the thermal tensions perpendicularly to the main stress level propagates beginning at an initial crack [5]. The stress levels can be calculated as follows:
s= DT a E/(1-m) (Formular 2)
Where E is Young´s modulus,
a
is the thermal extension coefficient and µ is
the
Poisson's
ratio. is the temperature difference caused by laser heating. During
the cutting process, locally limited and precisely
adjustable heat fields are created and controlled by the laser beam. The heat
fields lead the crack, through the product.
The process parameters for cutting depend on several factors. The geometry of the raw material and of the laser path are relevant for the needed mechanical forces to separate the products. Glass parameters as the absorption are relevant for the neccessary laser power. Further the laser beam caustic and mode are relevant settings for the energy density and distribution [6].
The shape of the raw material has a substantial influence on the maximum separation speed. With linear, symmetrical separation processes the relationship from sample width to sample length is crucial. The needed tensile stress to induce the crack is depending on the mechanical stiffness of the work piece. For example, by using 8 mm thick glass with always the same length and increasing the relation of length to width by the eight fold, the maximum separation speed can be increased by 28%. The more narrow the raw material is, as faster it can be cut. This measured variable is also crucial for the separation of complex outlines, where the relationship of the sample dimensions for the momentary separation process can vary strongly with its position.
For cutting laminated glass, the optimal deposition of energy per distance is very important. Too much energy causes an uncontrolled cracking of the Glass plates because of the stress in the foil, induced by the laminating process. Insufficient energy is responsible for the crack stopping on its way through the glass. Parameters are also different for certain materials, glass thicknesses and coatings.
One factor influencing the maximum cutting speed is the used energy per section. Following by figure 1 a delay between the initial heating of the glass due to the laser propagation and the maximum glass temperature can be realised. Thereby it was necessary to evaluate if this delay has an influence on an increasing cutting speed. The following diagram shows, that the use of more laser energy is inducing a higher maximum cutting speed. Because of the linearity of this effect the energy per section for reaching the maximum cutting speed is constant. Following from this it can be declared, that in the evaluated scope the delay in heating up the glass is not a relevant factor for an increasing cutting speed by using a higher power level. The curve is limited in the maximum laser power because of back reflection. It is liable that the cutting speed increases by the use of higher laser power.
Figure 2: MLBA cutting speed in relation to used laser power
The surfaces of the separated edges with MLBA technology exceed the quality of mechanically polished surfaces and increase additionally the firmness of the products. The roughness of a laser cut edge can be found in a range of Rz DIN = 0.034 µm to 0.038 µm according to measurements with ATOS PLµ. Bending tests showed that laser cut glass products had an up to 2 time’s higher firmness than conventionally cut samples. Thereby thermal hardening processes without rework of the glass edge are possible [1]. From the emission-free process arise advantages, which promise a successful use in the manufacturing of high-tech products. Glass fragments, abrasive dust and the necessary cleaning processes are to be avoided by the processing of coated glasses, which is not only an economic argument for the laser process. The contactless separation in one working step without auxiliary materials in an industrial standard is made possible for the first time with this technology.
The Prototype
The machine needed for this process was conceived and manufactured particularly according to the requirements of the process by the consortium of the EU funded project called MATRA. The development of prototypes began with the conversion of a conventional machine for cutting laminated glass. This station was provided with a holder for laser head and lower reflector and realized already on the Glasstec 2002 a first linear separation process. The second prototype concept developed by the consortium was changed in the way that the entire working table was used as a reflector. This ensures a constant beam figuration over the entire working field and simplifies the structure of the machine for the application of 2D-cutting (fig. 3). Linear direct drives make it possible to move at slow and high feed rates without oscillations caused by the drive unit. The further development will lead to an integrated working head, which unites a figuration optic for laser radiation and a temperature control unit. Standard industrial NIR laser sources are used, which can also use optical fibres for beam guidance. Thus, flexible integration into industry production areas is possible.
Figure 3: Prototype machine for laser glass cutting on Glasstec 2004
Practical Applications
Due to the volume absorption several stapled glass plates can be separated in one processing step without the usual breaking process. Also bonds, like for example laminated glass, can be cut in a defined way. Further selective cutting of individual, single glass sheets in a multi layer stack, as for example contact steps in an electrical heated windscreen, is possible. The process can be used for all soda-lime glass, including coated rear view mirrors and ITO-coated glass.
The economical view shows that the total process times of conventional separation processes in connection with grinding and polishing processes, cleaning and transport of the glasses, exceed the operating time of the MLBA process. Product samples which have already been manufactured with this technology are tachograph coversheets, rearview mirrors and sidewindow samples. Further products, which can be manufactured economically with this technology, are for example display glasses, solar cells cover plates, as well as design glass products.
Figur 4: Laser cut laminated glass Figure 5: Laser cut rear view mirrors and tachograph coversheets
In the area of automotive glass processing the cutting of windsceens could be an interesting application. Windscreens and side windows more and more move to complex 3-D construction parts. Because of the increasing area of glass in modern car bodys the windows have to fullfill bracing tasks. The processing of these products could be done with the use of robots for a flexible production. This configuration is not stated by defined mirrors, but can be programmed to cut different contours in one line. One robot for handling the glass sheet and one for moving the laser head relative to the product.
Figure 6: Robot Solution for separating windshields
Conclusions
The laser beam as a tool for separating glass materials is already introduced in modern industrial processes. Examples are laser beam scratching of display substrates as well as the laser beam blasting of glass tubes. Laser processes become economically interesting in mass production as well as for manufacturing high tech products. Over the shortened process chain investments in the laser source amortize at a short time. The laser possesses the potential to replace the cutting wheels within wider areas of manufacturing high tech glass products.
Acknowledgements
This work has been carried out within the scope of the European research project named MATRA „Machining of Transparent Materials by Multiple Absorption“ (EU-Project number: G1RD-CT-2002-00746; MATRA-Homepage: http://matra.laser-zentrum-hannover.de). The authors would like to thank the partners of the consortium: Schott AG (D), Bottero S.p.A. (I), ARC International (F).