Fusing and Forming of glass tubes with CO2 laser radiation
Abstract
The traditional gas flame technique used for fusing and forming glass tubes is becoming inefficient for today’s small-scale manufacturing companies. Due to the manual production of technical glass tubes the quality and process speed is depending on the glass blower. It requires trained specialists who are becoming increasingly difficult to find. Thus, the goal is to replace and automate the manual gasflame process with a laser beam supported process. An analysis of conventional techniques and solutions to associated problems will be performed. With the aid of modern CNC machines and high power lasers, a prototype machine will be developed. With this handling system a laser based process developed for optimal fusing and forming of glass tubes is available. For simplicity, only simple geometrical shapes will be analyzed. The outcome of this project is to develop an economically feasible solution to automate the fusing and forming process of glass tubes. The aspects of high quality and precision will both be emphasized throughout the development.
Keywords: glass, glass tubes, fusing, joining, forming, laser
1. Introduction
For nearly 2.000 years glass blowing by hand was the main method of forming glass articles. The last few years of the 19th century saw the beginnings of blowing glass by compressed air. The 20th century brought in the revolution of mechanisation, although glass blowing is still carried out by craftsmen today. /Bri04/
Two methods of glass processing are known as glass blowing. First the processing of molten glass gobs with an iron pipe, which is used today mostly for artistic work. The second is the processing of glass tubes with gas flames to get more technical parts for thermometers and aerometers as well as parts for chemical laboratories.
This paper will describe the processing of glass tubes for the manufacturing of lamps, thermometers and aerometers.
2. State of the art
Fusing and forming of glass tubes for lamps, thermometers and aerometers is traditionally done by hand working with gas flames. The process is controlled by well skilled glassblowers, who have experience choosing the right glass temperature and following from this the right glass viscosity. Tolerances in tube diameters and straightness through the tube manufacturing process are adjusted by the glass workers.
Joining of Glass tubes
One significant application for joining of glass tubes in the thermometer and aerometer industry is the welding of two tubes with different diameters as shown below.
Manual welding of glass and can be divided in three parts:
1. Preheating:
The Temperature gradient by heating with gas flames goes from the outside inward and lengthwise increasing to the joining area. The temperature gradient should be so small that no crack is initiated. The reach of the Fusing temperature can be realised by the glass surface starting to glow in a bright yellowish-red. /war04/
2. Joining:
The main conditions for the joining of two glass pieces are the right temperature and viscosity of the glass. The fusing process itself is realised at temperatures of nearly t = 800 °C. It consists of a solution process at a viscosity of nearly η=106 dPa·s for at least one of the partners. During the fusing procedure deformation in the joining area occurs because of surface tension, air pressure differences, mechanical pressure as well as centrifugal and gravitational forces. Termination of the adding procedure occurs only once no more material accumulations are present. The evaluation of successful joining is done by the glass blower who observes the light reflexes. Each joining process causes temporary and permanent tensions in the glass. Permanent tensions occur when cooling from the relaxation range is too fast. The tensions can be released only at same or higher temperatures, at which the tensions were brought in.
3. Cooling:
Cooling is done in three steps: cooling fast, until the temperature range of the relaxation is reached, slow cooling up to 50 to 100° under transformation temperature and fast cooling up to ambient temperature. Temporary tensions only occur within the last range.
The durability of the connection depends on several factors. The similarity of the coefficients of expansion at temperatures below the glass relaxation temperature gives advantages because of simple cooling. Different coefficients of expansion have to be considered and the effects have to be compensated by differential cooling. The organization and construction of the connection should be realised so that compressive stresses develop at the surface of the glass work pieces. To avoid stress in the finished product it’s very important that the two glass pieces cool down very slow if they have very differently coefficients of expansion.
The processing of glasses with large thermal expansion coefficients and thick tube walls require smaller temperature gradients. During local thermal treatment temperature gradients at the edge of the softening glass zone result in plastic deformations caused by internal mechanical forces. These can lead to tension in the glass upon cooling.
Forming of Glass tubes
Forming of glass can be done in two ways. The first way is casting a present carbon form. This forming method is used for complex forms which require high repetition accuracy.
The second way, which is pursued here, is the free forming of the glass without any auxiliary tools. The conventional forming process is done by warming up the glass tube until the necessary viscosity is reached and then blowing, pulling or pushing and rotating the tube in or outside the gas flame. Therefore the glassblower is controlling the viscosity by warming the area to form in the gas flame or cooling it outside. The process is observed by eye and controlled by hand.
The most significant applications for forming glass tubes in the thermometer and aerometer industry are shown below.
To extend the diameter of the tube the worker blows up the glass tube. This process is usually done outside the gas flame, because the glass blower can’t rotate the tube and blow it up at the same time. This would result in overheating one area of the forming zone. To keep the wall thickness equal for the whole product, the glassblower pushes the tubes, to accumulate material in the lower viscosity area.
The main area of applications for manual glass blowing can be found in production of lamps, thermometers and aerometers. Most parts are joined and formed by hand. Some partly automated processes can be done on machines called glass lathe. Here the glasses are formed mostly by casting a present carbon form.
3. Emphasis of this work / Experimental
For fusing and forming of glass tubes the glass blower has all degrees of freedom when he is working manually. Automating this process causes the difficulties of reducing the processes to lower degrees of freedom by just rotating and pulling and pushing the tubes coaxial to the tube axis. Further the process controlling which is done visually for manual work has to be replaced by physical values.
In future tubes should be formed just by variation of rotation speed, laser power and differences of pressure inside and outside the tube.
Process strategies
The development of joining and forming processes for tubing glasses will follow conventional methods. Here the gas-flame-based joining and forming process with consideration of the laser-specific process conditions are copied. The analysis of the conventional joining and forming methods by means of gas flame shows that the processes consist of the combination of different process steps.
Joining processes are always combined with forming processes, since fusing glass contains a deformation of the joint. The process of joining tubes can be divided into three process steps. The two tube ends which are to be fused, are preheated by even rotating in a gas flame. In the next step adding takes place by bringing together the two pieces under even rotation in the gas flame. Since the pipes must be joined over the entire cross section of the wall thickness, they are pressed together such that a small bulge develops. In the third step this material accumulation is again eliminated for the achievement of even wall thicknesses by changing the distances of the joint without further energy entry. The process steps for adding glass tubes by means of laser radiation can be derived thus directly from the conventional procedures. Process parameters, such as the rotating speed, must be adapted to the laser beam-supported procedure. An increase of the numbers of revolutions is necessary as the substantially more concentrated energy is input by laser radiation.
The form processes are likewise copied from the conventional procedures. For warming up the glass tubes to the warm-soft viscosity they are moved into the flame in rotation. For the manual controlling of the glass viscosity, the glass construction units are led out from the flame when sufficient energy was brought in. The deformation steps without influence of the gas flame accomplished around the toughly viscous condition before that to fall below the glass transformation temperature "to freeze". When the glass cools down during the forming process or when the process is not finished, the workers heat it up in the flame again. The temperature control can be substantially automated simply by the use of laser radiation. Blowing the warm-soft glass tubes is controlled via proportional valves, which adjust the demanded tubing internal pressure. During the manual process, the softened glass is not subjected to pressure, as long as it is in the flame. The validity of this procedure for laser beam processes is to be examined in separate investigations. During the forming process the glassblowers often blow the air into the glass tubes using pulses rather than a constant pressure.
Annealing
Whenever glass has been altered or shaped by heat exposure, stresses are introduced. The glassware can have some of the stresses relieved by annealing. During manual processing a soft gas flame is used to relieve stress areas located in and around the area of forming and fusing during conventional processing. Using Laser radiation this has to be adapted by controlled reducing of laser power after processing the work piece.
4. Practical applications / Results
Glass Type
The favourite glass type for the explained applications is AR-glass. This type of glass is a weather-resistant soda lime glass. As shown in the following diagram it is not transparent for CO2-Laser radiation. Above a wavelength of nearly l = 4.5 µm the radiation is absorbed in the glass surface because of SiO2-lattice vibrations. This causes a high efficiency of the used laser radiation. /sch99/
Laser source
For processing of glass several types of laser are industrial used today. For this work a CO2 -laser with a maximum output power level of Pmax = 250 W has been used. This laser has a wavelength of l = 10.6 µm and is established in industrial use. The laser source is a modular, RF excited, sealed industrial CO2-laser system which consists of a laser head, RF amplifier, and digital interface.
For beam forming a cylindrical lens is used, to form a line spot. This causes a homogeneous heating without extreme temperature gradients.
Machine construction
To join and form glass tubes a prototype has been developed, which allows rotational and lateral movement of the glass tubes relative to each other and to the laser beam.
The clamping system is build to fix tubes of different wall thicknesses and diameters. The lateral movement is realised by linear direct drives to have exact movement and further a high acceleration. This allows to push and to contract the tubes during processing.
Blowing up the joining zone can be realised by a nozzle, which is connected to the rotating tubes. Using proportional valves allow to control the pressure inside of the tube.
The glass temperature is controlled contact less by an IR-pyrometer. This measuring tool is connected to the steering system of the setup to observe the temperature and regulate the laser power in a closed loop control. The pyrometer is build to observe a temperature range of t = 500 – 2500 °C.
One application to manufacture glass products is the reduction from one diameter to a different one. To manufacture durable glass products its important to guarantee the constancy of the wall thickness.
By pulling the heated glass tube, the area of the lowest viscosity will stretch and get thinner. By reducing or accumulating glass there will be stress in the product because of different cooling of thinner and thicker areas. Therefore it is necessary to heat up the area to form homogeneously. When the glass thickness is reduced, it is possible to increase it by tossing the tube again.
Joining
The joining of glass tubes with laser radiation requires very exact planarity of the to join tube edges. The conventional joining process allows to correct inexact planarity by the glassblower due to bending and rotating the joining area. This is not possible with the automated process. Therefore the joining process has to be adjusted very exact, because correction of defects is not possible in an automated industrial process.
For joining process, the glass tubes are positioned in with a distance of a = 2 mm to each other and rotate with n = 30 rpm. The joining temperature is controlled to t = 1400 °C. For connecting the tubes they are move x1 = 2.5 mm together and then x2 = 0.5 mm apart. As shown on the figure below it can be realised that the joining process causes nearly constant tube diameters, but a welding zone is still visible. The visible lines on the tube are accumulated and thinned out areas. They are caused by surface tensions of the liquid glass during the welding process. As explained, the joining process is completed, when no fusing area is visible any more. The glass blower can correct this defect by observing the process and bending and rotating the tube in the gas flame. This is not possible with the automated system, so that some new strategies to avoid these effects have to be developed.
Forming of glass tubes
In first tests to generate a constriction in a glass tube, the glass has been heated up over the transition temperature and then stretched until the target diameter was reached. Micrographs of the forming zone showed, that the wall thickness of the tubes where to thin. As described a constant wall thickness is necessary for durable products.
Therefore the tubes have been modelled and out of the volume difference of formed and unformed tube of the same length and wall thickness, the stretching parameters of the machine have been calculated. This lead to a new strategy, where the stretching length is constant and the processing time has been varied until the target diameter was reached. This strategy lead to glass tubes with a constant wall thickness and defined diameters.
Forming of flat bottoms
The forming of flat bottoms is realised by heating up the glass tube over glass transformation temperature. In the next step the tube is stretched, until it closes at one side. The small heating area causes a sharp 90° angle for the Bottom. The product is formed by surface tensions of the liquid glass.
Forming of round glass bottoms
The forming of round glass bottoms is done nearly the same way as the forming of flat bottoms. The difference is that one side of the glass tube has been closed before. The forming process causes the heating of the glass material and his heats up the air inside the glass tube. After closing the tube on the second side, the air inside of the tube extends further on and supports the forming of the round bottom. Constant wall thicknesses depend on variating the tube position under the laser beam during the forming process.
5. Summary / Discussion
Fusing and Forming of glass tubes can be done automated by CO2-laser radiation. The movement of the tube, laser power, air pressure and processing time have to be adjusted to glass tube diameters and wall thicknesses. It is very important to use the exact moving and laser power parameters for the process to generate products with a constant glass thickness.
In relation to a conventional gas flame the energy is brought in very concentrated by laser radiation. This causes a higher thermal gradient which requires defined and controlled heating and cooling.
The forming of constrictions and joining areas by compressed air has not been used until now. This will add a further degree of freedom to the process.
For further investigations it is thinkable to use Nd:YAG – laser radiation to heat up the volume of the glass instead of the surface. This may lead to a more stable process because of homogeneous heating and therefore less stress into the glass.