Patent Application
20070266734
The invention relates generally to the production of glass ceramic, in particular to measures for reducing the surface roughness.
Glass ceramic plates are widely used for applications such as hot plates or fireproof glass, for example as a window or viewing window for stoves. In particular for use as a window, it is desirable for a glass ceramic plate to be as transparent as possible. Apart from the internal properties of the glass ceramic, the roughness of the surface is of particular importance here.
The hot forming of glass ceramic plates usually takes place from a green glass, which is formed by a rolling process. In this case, a certain roughness is imparted to the surface of the green glass, but may become smoother directly after rolling on account of the high temperature of the glass. However, the given temperature-time profile is not adequate to achieve complete smoothing. Typical roughness values give an average roughness of 0.1 to 0.2 μm.
The invention is therefore based on the object of providing glass ceramic articles with an improved surface with regard to roughness. This object is already achieved in a very surprisingly simple way by the subject matter of the independent claims. Advantageous refinements and developments are specified in the dependent claims.
Accordingly, the invention provides a method for producing glass ceramic panels in which a green glass panel or plate is produced by hot forming and the green glass panel is subsequently ceramized to form a glass ceramic, the panel being fire-polished on at least one surface in an intermediate step.
The green glass panel may in this case also be a continuous ribbon, which is later divided up into individual panels. Accordingly, one embodiment of the invention envisages carrying out the fire polishing directly on a continuous green glass ribbon produced by hot forming.
The fire polishing may in this case be performed on the green glass panel before the ceramizing, during the ceramizing as well as after the ceramizing.
In a further refinement of the invention, the fire polishing is performed before the introduction into an annealing furnace, in which the hot-formed glass panel is cooled in a controlled manner. This is of advantage, since the heating of the panel that in any case goes back to the hot forming process can also be used here for the fire polishing. In the case of a corresponding apparatus for producing an intermediate product for the production of glass ceramic panels or an apparatus for producing glass ceramic panels, an annealing furnace arranged downstream of the hot forming apparatus is accordingly provided, the device for fire polishing being designed for the fire polishing of at least one surface of panels formed with the hot forming apparatus before the introduction into the annealing furnace. The annealing furnace serves for a controlled cooling of the panel to avoid stresses. If the hot forming apparatus is set up for the production of a continuous green glass ribbon and the device for fire polishing is set up for the fire polishing of the green glass ribbon, the device for fire polishing may be arranged in particular between the hot forming apparatus and the annealing furnace, or else be integrated in the annealing furnace.
It has been found to be particularly advantageous if the green glass panel is fire-polished during the cooling that follows the hot forming before a temperature at which nucleation commences is reached. This permits the production of clear, unclouded glass ceramic with a fire-polished surface. If fire polishing is otherwise carried out only after cooling is performed to below this temperature, for example in the annealing furnace to room temperature, the associated renewed heating leads to additional nucleation. The additional nuclei may then cause clouding of the glass ceramic on account of the crystals forming around the nuclei during the ceramizing. A clear or unclouded glass ceramic is in this case understood as meaning not only colorlessly transparent but also colored glass ceramic, such as for instance the bulk-colored glass ceramic with a red to red-brown tint that is often used.
However, the possibility of also using the invention to polish panels that are cooler than the lower temperature at which nucleation commences is not ruled out. This is conceivable for example whenever a clouded, opaque appearance is especially desired, or if the heating is performed so quickly that nucleation is suppressed.
An apparatus according to the invention for producing glass ceramic panels that are suitable in particular for carrying out the present method comprises a hot forming apparatus for producing green glass panels and a ceramizing furnace for ceramizing the green glass panels, as well as a device for fire polishing at least one surface of green glass panels formed with the hot forming apparatus.
In a way corresponding to the particularly preferred refinement of the method according to the invention, in the apparatus the device for fire polishing is preferably arranged so close to the hot forming apparatus along the transporting path of the green glass panels that the glass panels are still hotter than the temperature at which nucleation commences during cooling.
Green glass production and ceramizing do not necessarily have to be performed in one and the same installation. Accordingly, the invention also relates to a method and an apparatus for producing an intermediate product for glass ceramic production in which a green glass panel is produced by hot forming and the green glass panel is fire-polished on at least one surface, and to an intermediate product that can be produced in this way for the production of glass ceramic panels, comprising a green glass panel fire-polished on at least one side. An apparatus for producing an intermediate product for the production of glass ceramic panels, in particular by the method according to the invention, comprises for this purpose a hot forming apparatus for producing green glass panels and a device for fire polishing at least one surface of green glass panels formed with the hot forming apparatus.
Hot forming, and a hot forming method, are understood for the purposes of the invention as meaning all methods in which the form, or the surface, of a glass melt or of a glass body is changed at a temperature that is so high that softening of the glass adequate to permit the changes in shape takes place. A hot forming apparatus accordingly comprises the devices required for these methods.
It has surprisingly been found with respect to the invention that the smoother surface produced by the fire polishing is also retained in the course of ceramizing, so that the fire polishing can already be performed before the ceramizing, at least before its completion.
The device for fire polishing is preferably designed in such a way that the surface to be polished of the green glass panel becomes hotter than internal regions of the green glass panel. In order to heat the green glass panel during fire polishing in such a way that the surface to be polished of the green glass panel becomes hotter than internal regions of the green glass panel, heating methods which as far as possible heat only the surface of the green glass are therefore preferred. In particular, it is preferred that the heating during the fire polishing is carried out in such a way that the green glass panel remains dimensionally stable. Consequently, subsequent deformation is avoided. In particular, it is then also possible to dispense with laborious aging of the green glass panel to avoid deformations.
Particularly surprisingly, it has been found that an improvement of the surface with regard to roughness is also retained even if the green glass plate is mounted with a fire-polished surface on an underlying support during ceramizing. Although the glass ceramic becomes very soft during the ceramizing, and one would expect the roughness of the underlying support to be transferred to the glass ceramic panel, it is found that a reduced roughness is obtained if the supported side has been fire-polished. Accordingly, it is also envisaged in a development of the invention to fire-polish the green glass panel on both sides.
Heating the surface to be polished with a gas burner is particularly suitable for the fire polishing. Direct contact with the flames can then be advantageously avoided by the use of a porous burner.
Heating the surface to be polished with an electrically heated emitter can also be used for the fire polishing. In particular, the aforementioned methods, and corresponding devices, for heating may also be combined with one another to achieve fire polishing. For example, preheating may be performed with a gas burner, the superficial melting finally being performed with an infrared emitter. In this way it is possible, for example, to use an emitter of lower output or, with a given output, to increase the surface area treated per unit of time, in order to lower the production costs.
Generally, the fire polishing can accordingly comprise the superficial melting of a preheated green glass panel. The green glass panel is in this case preferably preheated to at least 500° C., preferably at least 600° C. This is not only favorable for achieving quicker melting for the fire polishing, but the risk of stress cracks occurring is also reduced.
If the green glass panel has not already been preheated by the hot forming process, the apparatus according to the invention may also be provided with an additional device for preheating the green glass panel, in particular for preheating to a temperature of at least 500° C.
In order to achieve fire polishing, the green glass panel, in particular in the already preheated state for fire polishing, may be superficially heated to at least 900° C., with preference above 1000° C.
With particular preference, the hot forming of the green glass panel also comprises the rolling of a green glass panel, the fire polishing being carried out after the rolling. In the case of an apparatus according to this embodiment of the invention, the hot forming apparatus accordingly comprises a rolling device. As already mentioned at the beginning, rolling is the standard method for producing green glass panels. In this way, the invention can be easily integrated in existing installations. In the production of glass ceramic, rolling offers particular advantages over other hot forming methods, such as floating for example. Although panels produced by floating generally have a smoother surface, only certain glass compositions that can also be ceramized are suitable for this method. By contrast, virtually any desired type of glass can be hot-formed by rolling, so that there are scarcely any restrictions regarding desired glass composition. It is therefore also possible by means of the invention to produce rolled glass ceramic plates of a very high surface quality.
According to one embodiment of the invention, the green glass panel is fire-polished before the introduction into a ceramizing furnace. For this purpose, a device for fire polishing is accordingly arranged downstream of the hot forming zone and arranged upstream of the ceramizing furnace.
According to a further embodiment of the invention, the fire polishing may also be carried out at least partially in a ceramizing furnace. For this purpose, a device for fire polishing the green glass panel is accordingly provided in the ceramizing furnace. For example, an infrared emitter may be made to pass over the surface to be polished of the green glass through a window or an opening. Then, the fire polishing may advantageously also be carried out at least partially during the ceramizing. This is with the idea in mind of carrying out the fire polishing before the nucleation temperature is reached.
A glass ceramic panel that can be produced by means of a method according to the invention or a corresponding apparatus is distinguished in comparison with the panels obtained by previously used production methods by lower roughness of the glass ceramic surface. If, for example, the method according to the invention or the apparatus is used for producing fire-resistant glazing, in particular a stove viewing window or fireproof glazing, these products are distinguished by better transparency. However, improved surface quality also yields various esthetic advantages when the invention is used for producing glass ceramic hot plates. In this way, hot plates with a gleaming appearance can be produced.
The invention is explained in more explicit detail below on the basis of preferred embodiments and with reference to the accompanying drawings, in which the same designations refer to the same or equivalent parts and in which:
In the case of the example shown in
Subsequently, controlled cooling of the green glass ribbon 5 is performed in the annealing furnace 20, allowing subsequent separation into individual green glass panels 10 by means of a separating device 7. In the case of the example shown in
Consequently, an intermediate product for the production of glass ceramic panels is obtained in the form of a green glass panel 10 fire-polished on at least one side, in this case on both sides, on opposite surfaces 11, 12. In the case of the example shown in
In an alternative refinement, a ceramizing furnace may also be provided instead of the annealing furnace 20. The fire polishing is then carried out before the introduction of the ribbon 5 or individually separated green glass panels 10 into the ceramizing furnace.
The burners 30, 31 emit infrared radiation at a wavelength of over 10 μm. In this spectral range, the green glass is highly absorbent. Accordingly, the green glass panel is heated during the fire polishing by the burners 30, 31 in such a way that the surface 11 to be polished of the green glass panel becomes hotter than the internal regions of the green glass panel. In particular, virtually nothing but superficial heating is achieved, so that the green glass panel only melts superficially. In this case, the green glass panel 10 is superficially heated to at least 900° C., with preference above 1000° C.
In order to facilitate the melting for the fire polishing, it is of advantage if the fire polishing thereby comprises the superficial melting of a preheated green glass panel 10. Preheating may in particular constitute the heat remaining from the hot forming process. However, an additional preheating device 45 may also be optionally provided. The superficial heating has the effect that the green glass panels 10 also remain dimensionally stable. Preferably, the fire polishing at a temperature of at least 500° C. is performed by the preheating, for example by remaining heating provided by the hot forming process and/or by a separate preheating device 45. Consequently, excessive temperature gradients within the green glass panel 10 are also avoided.
As a difference from the way that is shown in
Heating that is substantially restricted to the surface to be polished, as can be achieved with the burners 30, 31, can only be achieved with difficulty with thermal emitters. In this respect, reference is made to the diagram of
With hotter emitters, most of the output is given off at lower wavelengths. If an emitter with a temperature of 1200° C. (curve 24) is used, the maximum of the radiation output is at a wavelength of approximately 2.5 micrometers. In this range, however, the green glass is very transparent, so that regions of the green glass panel that lie further below the surface are also heated up. However, thermal emitters, such as for example electrically heated emitters, may nevertheless be used for example as a preheating device 45, to assist another device for the fire polishing.
As a difference from the way that is shown in
In order to avoid direct contact of the green glass panel 10 with the flames of a gas burner, a porous burner may advantageously also be used as a device for indirect heating. An exemplary embodiment of a device for fire polishing with a porous burner as a variant of the exemplary embodiment represented in
The porous burner 50 comprises a gas supply, with which fuel gas is supplied to a fine-pored material 52. In this material, a preheating of the fuel gas takes place. Subsequently, the fuel gas flows through a coarse-pored material 53, in which the combustion takes place. The coarse-pored material 53 gives off combustion heat to the green glass panel 10. The simultaneous heat transfer along the porous material 53 at the same time has the effect that the reaction zone is stabilized and spreads along this material. Consequently, an extensive heating area is achieved for the heating of the green glass panel 10. A porous glass ceramic comes into consideration for example as the coarse-pored material 53. The maximum achievable temperature is generally approximately 1400° C. Achievable thermal outputs per unit area of such a device lie in the range around 1000 kW/m2. Consequently, adequate heating of the surface 11 can be achieved, since it is substantially the case with a porous burner that the heat cannot flow laterally along the surface 11 in the green glass panel on account of the extensive heating area. Consequently, the heat can substantially only flow away perpendicularly to the surface 11 into the panel 10. Moreover, preheating at a high temperature on account of the preceding hot forming process can be used.
It is evident to a person skilled in the art that the invention is not restricted to the embodiments described above by way of example, but rather can be varied in many diverse ways. In particular, the features of the individual exemplary embodiments can also be combined with one another.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 023078.7 | May 2006 | DE | national |
