Abstract:
The method produces a reshaped glass-ceramic article by forced reshaping of a flat green glass part during a ceramicizing process with temporarily lowered viscosity due to crystallization heat. To perform the forced reshaping economically the forced reshaping takes place in a continuous oven for ceramicizing and in an oven section in which the viscosity of the green glass part is temporarily lowered as a result of crystallization heat. An apparatus for performing the process is provided in the continuous oven including different active reshaping devices and/or a hollow mold. The method produces glass-ceramic articles with undamaged surfaces corresponding to surfaces produced during the making of the green glass part (smooth or structured, e.g. knobbed).

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to methods of making reshaped flat glass-ceramic parts or articles by forced reshaping of a corresponding green glass part during the ceramicizing process. The reshaping can be partial or full surface. The invention also relates to an apparatus for performing the method and to the glass-ceramic articles made by the method.  
         [0003]     2. Related Art  
         [0004]     Glass-ceramic parts or articles in the form of glass-ceramic plates are typically used to provide cooking surfaces in modern cooking ranges and as viewing windowpanes for oven, range and fireplace.  
         [0005]     For many years cooking ranges with glass-ceramic cooking surfaces and/or covers have been known in the prior art. The cooking ranges with the glass-ceramic cooking surfaces have been marketed because of their specific properties. They have long-lasting resistance to high temperature differences, surface strength and friendliness to cleaning materials, in addition to their desirable aesthetic properties. Typically the glass-ceramic cooking surfaces are used for heating cooking zones and the like with electric cooking elements and gas burners, including both atmospheric gas burners set in the cooking surfaces, and also with radiant burners under the cooking surface. In more recent times also more and more inductively heated cooking devices have been marketed.  
         [0006]     It is also possible to prepare food directly on a glass-ceramic cooking surface, e.g. by grilling, besides using these glass-ceramic cooking surfaces for indirect food preparation, i.e. for heating of food in a cooking vessel or container. The glass-ceramic plates for the above-mentioned cooking surfaces are usually made in a continuous melt process, in which a starting glass is first rolled to its nominal thickness and is then cut to the appropriate cooking surface size. In this state the starting material is called green glass or pre-courser glass. In a second manufacturing step additional edge working occurs, the drilling and decorating, and subsequently thermal treatment, namely the ceramicizing. These processing steps produce the glass-ceramic with its characteristic properties (high temperature resistance and low thermal expansion).  
         [0007]     The cooking ranges found on the market with glass-ceramic plates providing cooking surfaces (with in gas cooking ranges with atmospheric burners a cover can be provided) typically have continuous plane surfaces. However it has been shown that many special cooking unit embodiments can only be realized with considerable extra expenses or effort or for limited applications. A series of proposals has been made for improvements in cooking surfaces, especially by partial reshaping. Glass-ceramic plates, especially with partially reshaped regions, thus attain ever-stronger significance on the market.  
         [0008]     Edges of holes for receiving open, i.e. atmospheric, gas burners in cooking surfaces typically pose a problem. This sort of cooking range typically must be protected with an additional sealing collar around the edge of each hole for overflowing cooking materials or cleaning liquids. It has also been known to bend up the edges of the holes in an expensive bending process as a substituted for the sealing collars. That means that a reshaped raised region is provided around the edge of each hole, as e.g. described in DE 198 13 691 C1.  
         [0009]     Furthermore it has been suggested in DE 43 33 334 C2 to fold or bend the region around the hole for the atmospheric gas burner up from the main plane of the cooking surface to form a truncated conical, spherical or circular conical section or to from a prism, tetrahedron, pyramid and/or other section. An arrangement of this sort has, among others, the advantage that scarcely any further measures are required for sealing of the gas burner from the cooking surface (e.g. as a protection against overflowing cooking materials) and also the cleaning of the cooking surface at the transition region from the cooking surface to burner is essentially simplified.  
         [0010]     Further desirable shaping of glass-ceramic cooking surfaces results, for example, from the needs to fix the cooking grates over the gas burners and to protect switch surfaces from overflowing cooking material or from unintended impacts due to sliding of pots and other cooking vessels according to DE 199 06 911 C1.  
         [0011]     Also shaping and reshaping can have functional and aesthetic advantages for viewing windowpanes used in ovens, ranges and fireplaces. For this purpose these panes are already reshaped currently by sinking into a shape under their own weight during a ceramicizing process or before that in a green glass condition by bending or folding or vacuum deep drawing with rapid heating by means of a gas burner or infrared heating. A series of methods for making partial-surface or full-surface glass-ceramic plates are known, of which two typical methods are described next.  
         [0012]     WO 97/00407 (=EP 0 834 044 B1) describes a method of reshaping a glass-ceramic plate, especially providing a glass-ceramic cooking surface. For this purpose a glass plate is made in a plastic state with a width, which is larger than that of the cooking surface to be formed, and which has a viscosity of 1000 to 5000 Pas, with the help of a melted precursor (green glass) mass for the glass ceramic by rolling the previously melted glass between rollers. Then the pasty glass sheet or disk is conveyed to a surface of a mold, which is equipped with vacuum devices, so that this glass disk covers the above-described shaping surface, which contains a number of impressions of bulges. The glass disk is then pulled against the surface of the mold by the suction or vacuum produced by a suction apparatus. The parts of the glass disk, which protrude through the surface of the mold, are cut away so that a shaped disk of the green glass is obtained. Subsequently the shaped glass plate or disk is subjected to a heat treatment for ceramicizing it and is converted to a shaped glass-ceramic plate.  
         [0013]     This known manufacturing process has the disadvantage that the melt vessel, the rollers and the shaping parts can only be used for this shaping process. The special and very expensive mold permits economical production of only a very large number of individual products.  
         [0014]     An additional disadvantage is the visible damage of the plate underside, i.e. the surface in contact with the mold, produced by the method. This surface is covered by a leathery structure formed by shrinkage and flow waves, depending on the existing temperature conditions after the rolling process. This not only interferes with the usually desired aesthetics of the cooking surface, but it also causes strength reductions.  
         [0015]     EP 0 879 797 describes a method for introduction of raised portions and depressions in a glass-ceramic cooking surface, which comprises a partial sinking of a bar or cylindrical disk supported on a partially underlying supporting mold during the ceramicizing process, but only because of the action of gravity. A reshaping process occurring because of the force of gravity alone is also disclosed in DE 101 02 576 A1.  
         [0016]     In this known reshaping method by sinking due to gravity during ceramicizing a partial reshaping is possible, but only with very great limitations. Only very “soft” reshaping with very large tapering radii can occur. Small local borders, like for example are required for control functions or centering mushrooms for pot support fixtures, cannot be produced with this method. Because of the large tapering radii it is not possible to use this method for electric cooking surfaces, since the planarity of the cooking zones is negatively influenced, which could lead to a reduction in the performance of the cooking apparatus.  
         [0017]     DE 100 47 576 A1 discloses a method for reshaping a “glass-ceramic blank”, i.e. a green glass body, during its ceramicizing by a forced reshaping. The forced reshaping can occur by vacuum sinking, by means of a pressing tool or by blowing pressurized air into the respective hollow mold. The green glass body is placed in an IR radiating body equipped with an IR radiator and is heated there to the required temperatures and reshaped. This reference describes a discontinuous process, in which the individual treatments of the green glass body are performed in separate processing steps. The plane green glass plate is put in an IR radiating chamber as a single piece. After that the radiating chamber is heated by means of the IR radiator, the reshaping takes place, and subsequently ceramicizing is performed. Then the process concludes by cooling down the radiating chamber and removing the ceramic plate. This process is comparatively uneconomical because of the discontinuous individual treatment of the green glass plate to be ceramicized.  
       SUMMARY OF THE INVENTION  
       [0018]     It is an object of the present invention to provide a flexible economical method for reshaping, even of different regions in small scale production, a glass-ceramic article, especially a glass-ceramic cooking panel or a viewing window pane for an oven, range and fireplace, wherein the shaping can be locally limited and must have no large transitional radii and further the glass-ceramic plate underside can be either smooth or structured (e.g. knobbed) without damage.  
         [0019]     This object and others, which will be made more apparent hereinafter, is attained in method of making a reshaped glass-ceramic article by forced reshaping of a corresponding flat green glass part during a ceramicizing process.  
         [0020]     According to the invention the method comprises forced reshaping of a softened green glass part in a continuous furnace for ceramicizing the green glass part and in a section of this furnace, in which a viscosity of the green glass part is lowered temporarily on account of crystallization heat produced during ceramicizing.  
         [0021]     In contrast to the above-described DE 101 02 576 A1 forced reshaping occurs in the case of the present invention in a continuous furnace, as is used for ceramicizing green glass plates. A special reshaping section is added to the furnace for this purpose. This continuous oven operates continuously in an economical manner.  
         [0022]     The plates are thus inserted in the continuous furnace on one side, heated reshaped and ceramicized, and then taken out on the other side as a finished product, without the need to turn on and turn off the oven again.  
         [0023]     The process according to the invention is similar to the known process according to the previously described EP 0 879 797, which is based on the situation that there is a definite temporary lowering of the viscosity during the ceramicizing program. This viscosity lowering occurs immediately after the start of the crystal growth process. The viscosity reaches values of about 10 9  to 10 12  dPas. In contrast to the process according to the previously described EP 0 879 797 the conversion process does not occur because of the action of gravity alone, but also because of the targeted forced reshaping.  
         [0024]     In contrast to the methods in the prior art with the method according to the invention it is possible to shape or mold different regions of glass-ceramic plates, especially glass-ceramic cooking surfaces or viewing windows for ovens, ranges and fireplaces, in many different ways in a very economical manner both in large scale production and also for special products, which are produced in small numbers. The shaping can be both locally limited and full-surface and have tight radii or other transitional radii, according to the forced reshaping tool action. Since the glass-ceramic plate underside can be smooth or structured (e.g. knobbed). The glass-ceramic parts to be reshaped can be fed through the continuously operating ceramicizing furnace in any arbitrary sequence, even alternating with smooth glass parts.  
         [0025]     The forced reshaping can be performed in different ways.  
         [0026]     According to a first embodiment of the invention the method comprises placing the flat green glass part in a hollow mold and drawing it into the hollow mold by producing a low pressure or vacuum in the hollow mold when its viscosity is lowered, assisted by the force of gravity.  
         [0027]     This first embodiment of the method is preferred in large-scale shaping.  
         [0028]     According to a second embodiment of the invention the method comprises placing the flat green glass part in at least one reshaping device provided with movable reshaping tools and activating the tools by means of low pressure or an overpressure when the viscosity of the flat green glass part is lowered locally so that a surface of the softened glass is forced out to form a raised region.  
         [0029]     This second embodiment of the method is preferred for locally limited shaping.  
         [0030]     The invention also includes an apparatus for performing these methods, which comprises a continuous furnace for performing the ceramicizing process and a hollow mold or a reshaping device, on which the flat green glass part is placed, which has at least one activating opening. The continuous furnace is provided with at least one lateral opening for temporarily inserting at least one activating line for connection or docking with the hollow mold or the reshaping device in the at least one activating opening in a sealed manner, so that the hollow mold can be evacuated or pressurized.  
         [0031]     The invention also includes reshaped glass-ceramic articles made by a method comprising forced reshaping of flat green glass parts during ceramicizing of these green glass parts. The reshaping takes place according to the methods claimed in the claims appended below and described above. These reshaped glass-ceramic articles have surfaces corresponding without damage to the surfaces formed during making of the flat green glass parts. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0032]     The objects, features and advantages of the invention will now be described in more detail with the aid of the following description of the preferred embodiments, with reference to the accompanying figures in which:  
         [0033]      FIG. 1A  is a cross-sectional view through a first embodiment of an apparatus according to the invention for performing the method according to the invention, which comprises a continuous ceramicizing furnace and a hollow mold, in which a flat green glass plate to be ceramicized is placed, which is transported through the furnace according to a ceramicizing program and in which a low pressure is produced in a section of the furnace, in which exothermic crystal growth takes place;  
         [0034]      FIG. 1B  is a cross-sectional view through the first embodiment of the apparatus shown in  FIG. 1A , showing the reshaped green glass article resulting from the production of the low pressure during ceramicizing in the continuous furnace;  
         [0035]      FIGS. 2A and 2B  are cross-sectional views of an alternative embodiment of an apparatus for performing the method according to the invention, which is similar to that shown in  FIGS. 1A and 1B , in which additional lateral openings are provided through which pressing and hold-down tools are inserted; and  
         [0036]      FIGS. 3A and 3B  are cross-sectional views of another embodiment of an apparatus for performing the method according to the invention, which is similar to that shown in  FIGS. 1A and 1B , with a shaping device for forced reshaping, which has two shaping tools moved through vacuum or pressurized air, which locally reshape the green glass softened by the crystallization heat in areas to be raised or embossed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]      FIGS. 1A, 1B  and  2 A,  2 B show a continuous ceramicizing furnace  3 , which can be in the form of a grating furnace or roller furnace. A hollow mold  2  is introduced into the continuous ceramicizing furnace  3 . The green glass article or plate  1  to be ceramicized is placed in the hollow mold  2 . The green glass plate  1  can be made by rolling or according to a float process. The hollow mold  2  has a molding surface  2   a  with a shape that corresponds to or is suitable for the desired reshaping.  
         [0038]     The hollow mold  2  with the green glass plate  1  is conveyed in a known manner for thermal ceramicizing treatment through the continuous ceramicizing furnace  3  by means of a symbolically represented conveyor belt  3   b  or the like. The hollow mold  2  with the green glass plate  1  is stopped in that region of the continuously ceramicizing furnace  3 , in which the exothermic crystal growth occurs. Because of that the green glass plate  1  experiences a temporary viscosity lowering. Movable vacuum lines or conduits  5  are pushed through lateral openings  4  in a furnace section, in which exothermic crystal growth occurs, into previously prepared openings  6  in the hollow mold  2 . The hollow mold  2  is evacuated by these vacuum lines  5  and because of that causes the softened glass on the mold to be drawn into the mold, assisted by the force of gravity. The vacuum connectors  5   a  on the tube ends of the vacuum lines  5  are formed (e.g. cone or ball shaped), so that they automatically center themselves in the openings  6  fitting their shape. Because of that a sufficient vacuum-tight connection arises for evacuation. The state of the reshaped glass and/or now glass-ceramic plate  1 ′ is shown in  FIG. 1B .  
         [0039]     The stop or halt of the hollow mold  2  required for docking of the vacuum conduits or lines  5  in the hollow mold  2  is suitably achieved when the hollow mold is accelerated in an furnace section upstream of a reshaping region RR so that subsequently sufficient time for a stop or halt for a short time interval results and the hollow mold  2  can be synchronized again to the normal furnace speed after the stopping stage. The continuous ceramicizing furnace  3  is preferably equipped with automatic slide gates upstream and downstream of the reshaping region RR, which can be moved into and moved out of the continuous ceramicizing furnace  3  to help maintain a uniform temperature distribution in this region.  
         [0040]     Furthermore the continuous ceramicizing furnace  3  is equipped with an air circulation system ACS for keeping the temperatures more uniform within the reshaping region RR, as shown in  FIG. 2B .  
         [0041]     An alternative embodiment to that shown in  FIGS. 1A and 1B  is shown in  FIGS. 2A and 2B , in which pressing and hold-down tools  7  can be inserted temporarily through the additional lateral openings  3   a  in the furnace section for reshaping, which cause reshaping alone or with additional assistance of an applied vacuum, especially at the edge regions of the glass-ceramic plate or which prevent an unwanted bulges of the edge regions, which results from the sinking of the center portion into the mold due to the vacuum and/or the force of gravity.  
         [0042]     Pressurized air lines or conduits are temporarily inserted through the additional lateral openings  3   a  in the furnace section in the reshaping zone, as described in  FIGS. 1A and 1B  for the vacuum lines, so that reshaping tools integrated in the mold, e.g. pressing elements or centering bolts, can be moved into the mold (not shown) alone and/or additionally to the action of the applied vacuum and/or the pressing and hold-down tools  7 .  
         [0043]     The reshaping process is preferably observed in the furnace section in which reshaping occurs or in the reshaping region RR by means of a furnace probe or a viewing window VP.  
         [0044]     Another embodiment besides that shown in  FIGS. 1A and 1B  is shown in  FIGS. 3A and 3B , in which the apparatus for performing the method has a reshaping device  2 ′ with reshaping tools  9 , e.g. two pressing elements, instead of the hollow mold  2 . The green glass plate  1  to be reshaped is placed in the reshaping device  2 ′ and transported through the furnace  3 . The pressing elements are to be understood as only examples of movable reshaping tools  9  for forced reshaping, whose number is determined by the number of reshaping events performed on the green glass plate  1 .  
         [0045]     The reshaping device  2 ′ has two activating openings  6 , at which activating lines  8  are connected after insertion through openings  4  in the furnace section, in which the crystal growth occurs. The green glass plate  1  can be fixed or held by a vacuum in this reshaping device  2 ′ on it. Hold-downs  7  according to  FIGS. 2A and 2B  can also be used for this purpose.  
         [0046]     The reshaping tools  9  can be moved vertically by the activating lines  5  by means of a vacuum or low pressure or pressurized air, which forces an area of the surface of the softened green glass plate  1  out locally out to form a desired raised region, e.g. an operators&#39; panel for a cooking surface.  
         [0047]     Further description regarding this third embodiment would be the same as for the embodiments shown in  FIGS. 1A and 1B  and  2 A and  2 B and has therefore been omitted here.  
         [0048]     The reshaped glass-ceramic part  1 ′ can be the desired product, but it can also be divided up into a number of individual desired products.  
         [0049]     The reshaped glass-ceramic parts are preferably used as cooking panels, especially for glass-ceramic cooking ranges, and for fireplace viewing windows or similar applications. The glass-ceramic parts made according to the invention have the advantages of improved aesthetic properties and/or increased strength because of the method of reshaping green glass parts made by rolling and float processes according to the invention, because of their undamaged smooth or structured surfaces, for example a knobbed lower or upper side.  
         [0050]     The disclosure in German Patent Application 10 2004 008 595.1-45 of Feb. 21, 2004 is incorporated here by reference. This German Patent Application describes the invention described hereinabove and claimed in the claims appended hereinbelow and provides the basis for a claim of priority for the instant invention under 35 U.S.C. 119.  
         [0051]     While the invention has been illustrated and described as embodied in a method for making reshaped glass-ceramic articles, apparatus for performing the method and glass-ceramic articles made thereby, it is not intended to be limited to the details shown, since various modifications and changes may be made without departing in any way from the spirit of the present invention.  
         [0052]     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.  
         [0053]     What is claimed is new and is set forth in the following appended claims.