Abstract:
A microwave oven for the thermal treatment of at least one dental restoration part comprises a firing chamber inside which the dental restoration part as well as at least one susceptor that may be moved or rotated by means of a drive motor and that substantially is disc-shaped, are disposed, and a microwave radiation source that indirectly heats the dental restoration part with the aid of the susceptor, wherein the susceptor at least partially is microwave-tight and wherein the microwave radiation source is arranged below the susceptor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of European Patent Application No. 10 169 856.1 filed Jul. 16, 2010, which is hereby incorporated by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The invention relates to a microwave oven for the thermal treatment of at least one dental restoration part. 
       BACKGROUND 
       [0003]    Microwave ovens have been known for a long time. For heating the dental prostheses for example, it is known from DE 41 02 129 A1, which is hereby incorporated by reference, to provide a microwave oven with a rotary table and to introduce the dental prosthesis therein for hardening. In a manner known per se homogenization of the introduced microwave radiation is to be achieved. 
         [0004]    Whereas this approach relates to the polymerization of dental prostheses, it has also become known in the dental field to combine a rotary table with a susceptor, for which reason reference is made to WO 96/41500, which is hereby incorporated by reference. 
         [0005]    Further, it is also known from EP 1 060 713 B1, which is hereby incorporated by reference, to use a rotary table in combination with a susceptor element for the firing of dental ceramics. This approach indeed is a good thing and the use of microwave radiation for the heating of the dental ceramic part represents a significant progress. Despite the use of the rotary table, it would be favorable, however, to still improve the heating curve of the dental ceramic part, in particular in view of the short cycle time that is desired in dental laboratories. 
       SUMMARY 
       [0006]    The invention is based on the object of producing a microwave oven having an improved heating cycle without affecting the heating uniformity. 
         [0007]    This object is surprisingly solved by providing a microwave oven having a firing chamber in which a susceptor is disposed, the susceptor may be rotated by a drive motor. The microwave radiation source indirectly heats the dental restoration part with the aid of the susceptor, wherein the susceptor at least partially is microwave-tight and the microwave radiation source is arranged below the susceptor. All the claims are hereby incorporated by reference in the specification. 
         [0008]    According to the invention, the measure of arranging the microwave radiation source below the at least partially microwave-tight susceptor, surprisingly results in a considerable improvement of the coupling of the microwave source to the susceptor and thus indirectly to the dental material. The susceptor is disc-shaped or, if applicable, comprises a disc having a raised edge. By means of the arrangement of the microwave source below the susceptor, it may be ensured that the emitted radiation first encounters the susceptor and not directly the dental ceramic material. In this way, the preferred indirect heating is ensured in a surprisingly easy manner. The susceptor then heats the dental material that is supported thereon in an extensive manner and in a good thermal connection therewith, the dental material forming the dental restoration part, wherein according to the invention a particularly good field homogenization of the microwave radiation is achieved. The susceptor at the same time inventively acts as a mode stirrer. 
         [0009]    The high-temperature resistant receptacle comprises at least one through-hole or a lateral recess through which the temperature measuring element may sense the temperature of the susceptor. 
         [0010]    It is to be understood that the coupling is especially favorable in as much as the microwave radiation source is arranged exactly below the susceptor. The radiation source in this connection refers to the outlet of the respective waveguide. However, it is also possible to effect a certain lateral offset so that the radiation source lies below the plane of the susceptor, however slightly radially outside the disc-shaped susceptor. An angle of 45 degrees to the vertical, viewed as from the radial end of the susceptor, readily has the desired effects. 
         [0011]    An advantageous design provides that the center of an area of the susceptor relative to the axis of rotation is arranged in a radially offset manner. In this way, the heating uniformity is surprisingly improved, as areas that absorb radiation with different intensity, are periodically exposed to the microwave radiation during rotation, so that partially more intensive and partially less intensive microwave radiation may be conveniently used. In this connection it is to be understood that it is favorable to arrange the microwave radiation source somewhat outside the axis of rotation of the rotary table. 
         [0012]    In a modified embodiment, the susceptor substantially has the shape of a pot and thus is at least partially closed. With this solution, the laterally reflected microwave radiation from the susceptor is absorbed and thus serves to heat the dental restoration part before it directly couples to the microwave radiation at an increased temperature of for example 700 or 800° C. 
         [0013]    It is to be understood that either a microwave permeable and thin rotary table that for example is made from high-temperature resistant glass, is used, or that the susceptor itself may be formed as a rotary table. The susceptor, for example, may be formed of silicon carbide having a sufficient strength, wherein it is favorable that it also forms a basic load for the microwave impingement of the microwave oven in case of a non-existent dental restoration part. 
         [0014]    According to an advantageous embodiment it is provided that the center of an area of the susceptor relative to the axis of rotation of the susceptor is arranged in a radially offset manner thereto. 
         [0015]    According to an advantageous embodiment it is provided that the center of an area of the susceptor coincides with the axis of rotation of the susceptor. 
         [0016]    According to an advantageous embodiment it is provided that the susceptor is formed as a ring or a disc and/or at least comprises one bearing surface for the dental restoration part. 
         [0017]    According to an advantageous embodiment it is provided that the susceptor comprises a circular outer contour or an outer contour that deviates from the circular shape. 
         [0018]    According to an advantageous embodiment it is provided that the susceptor comprises at least one susceptor element that extends in parallel to the axis of rotation of the susceptor and that in particular is formed as a ring section. 
         [0019]    According to an advantageous embodiment it is provided that the susceptor is formed as a container that in particular is closed at least partially. 
         [0020]    According to an advantageous embodiment it is provided that the working temperature range of the microwave oven that in particular exclusively works with microwave radiation, is between room temperature and 2200° C. 
         [0021]    According to an advantageous embodiment it is provided that the interior of the microwave oven is sealed in a gas-tight manner and that the microwave oven may be filled with a given gas selected by the user and/or may be evacuated. 
         [0022]    According to an advantageous embodiment it is provided that the one or more susceptors is/are firmly connected to the rotary table and even in case of a nonexistent dental restoration part represent a base load for applying microwaves to the microwave oven. 
         [0023]    According to an advantageous embodiment it is provided that the susceptor that is connected to the rotary table acts as a mode stirrer. 
         [0024]    According to an advantageous embodiment it is provided that the rotary table at the same time is formed as a mode stirrer and is fixedly connected with the susceptor by having the shape of a crucible or bowl for receiving the dental restoration part. 
         [0025]    According to an advantageous embodiment it is provided that at least one high temperature resistant contact or support surface is disposed on or within the susceptor, said support surface serving to receive the dental restoration part. 
         [0026]    According to an advantageous embodiment it is provided that at least one high temperature resistant support surface that is fixedly arranged within the firing chamber, is disposed above the susceptor, said support surface serving to receive the dental restoration part. 
         [0027]    According to an advantageous embodiment it is provided that the susceptor and/or the high temperature resistant support surface and/or the dental restoration part interact with a temperature sensing element, in particular with an optical temperature sensing element, and that in particular the susceptor and/or the drive motor interact with a measuring device for sensing the microwave power. 
         [0028]    According to an advantageous embodiment it is provided that the optical temperature sensing element is directed towards the surface of the susceptor and/or the high temperature resistant support surface and/or the surface of the dental restoration part and that the focus of the detecting area of the temperature sensing element lies outside the rotational axis of the drive motor. 
         [0029]    According to an advantageous embodiment it is provided that the position of the susceptor is variable by the drive motor in the direction of rotation and/or in the vertical direction in order to minimize the reflecting microwave radiation within the measuring device. 
         [0030]    According to an advantageous embodiment it is provided that the temperature sensing element senses the temperature at a specific location or area of the susceptor and/or the high temperature resistant support surface and/or the dental restoration part, and that the drive motor causes the susceptor to rotate at least a few degrees if a deviation from a given set temperature is determined. 
         [0031]    According to an advantageous embodiment it is provided that the susceptor remains constant or may be rotated in the same direction or alternately in different directions a limited period of time or at intervals, and/or that the susceptor in particular during its rotation may be vertically adjusted. 
         [0032]    According to an advantageous embodiment it is provided that the susceptor has a thermal conductivity of more than 20 W/mK and at least partially comprises a wall thickness of at least 0.5 mm. 
         [0033]    According to an advantageous embodiment it is provided that the susceptor in particular is formed in a multi-part design as a bowl or disc or crucible. 
         [0034]    According to an advantageous embodiment it is provided that the susceptor is formed to have a disc-shaped structure which is comprised of at least one large-scale or extensive disc and/or several smaller disc-shaped, ring-shaped, rod-shaped or ball-shaped elements that are arranged side by side and/or one above the other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    Embodiments of the present invention will be more fully understood and appreciated by the following Detailed Description in conjunction with the accompanying drawings, in which: 
           [0036]      FIG. 1  shows a schematic representation of an inventive microwave oven in a first embodiment; 
           [0037]      FIG. 2  shows a schematic representation of a further embodiment of microwave oven according to the invention; 
           [0038]      FIG. 3  shows a schematic view of a third embodiment of a microwave oven according to the invention; 
           [0039]      FIG. 4  shows a schematic view of a fourth embodiment of a microwave oven according to the invention; 
           [0040]      FIG. 5  shows a schematic view of a fifth embodiment of a microwave oven according to the invention; 
           [0041]      FIG. 6  shows a schematic view of a sixth embodiment of a microwave oven according to the invention; 
           [0042]      FIG. 7  shows a schematic view of a seventh embodiment of a microwave oven according to the invention; 
           [0043]      FIG. 8  shows a schematic view of an embodiment in which the susceptor for a microwave oven according to the invention has a circular symmetric design relative to the firing chamber and to the axis of rotation; 
           [0044]      FIG. 9  shows a view according to  FIG. 8 , wherein the susceptor, however, is supported in an asymmetric manner relative to the axis of rotation; 
           [0045]      FIG. 10  shows a further embodiment in the representation of  FIG. 8 , wherein a susceptor is employed that is square-shaped in the top view; 
           [0046]      FIG. 11  shows a further embodiment of the susceptor in the representation of  FIG. 8 , wherein the susceptor substantially is dumbbell-shaped, however is supported in a circularly symmetric manner; 
           [0047]      FIG. 12  shows a further embodiment of a susceptor in the representation of  FIG. 8 ; 
           [0048]      FIG. 13  shows a further embodiment of a ring-shaped susceptor in the representation of  FIG. 8 ; and 
           [0049]      FIG. 14  shows a susceptor arrangement in a different embodiment comprising four individual susceptors, and apart from that in the representation according to  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION 
       [0050]    The microwave oven  10  represented in  FIG. 1  comprises a firing chamber  12  that is surrounded by heat-insulating material  14 . The microwave oven  10  is provided with a drive motor (not shown) that rotates a susceptor  16  about an axis of rotation  18 . For this, the drive motor may be arranged on the drive shaft  20 , but may also be arranged laterally alongside the drive shaft  20  via a transmission. In the shown example, an air ventilation is provided coaxially with the drive shaft  20 , which provides for the supply of air  22  via recesses (not shown) in the material  14 , wherein the air ventilation  24  in the drawing according to  FIG. 1  is represented schematically. The ventilation  24  is in particularly switched on, in order to provide for a faster cooling after termination of the firing phase, as required, or in order to provide the firing chamber with desired gaseous media during the heat treatment processes. 
         [0051]    In the represented exemplary embodiment, the susceptor  16  comprises the shape of a disc or a flat pot. It serves to receive the dental restoration parts  26  schematically represented in  FIG. 1 . In this exemplary embodiment, the susceptor  16  is arranged on the rotary table  28  in a surface-mounted manner. It is to be understood, however, that the susceptor may also be fixed directly on the drive shaft  20 . 
         [0052]    According to the invention a microwave radiation source  30  is arranged below the susceptor  16 . It comprises a waveguide output  32  and thus applies microwave radiation to the susceptor  16  from below, that is to say from the side, thus preventing the microwave radiation from directly striking the dental restoration parts  26 . 
         [0053]    In the represented embodiment, there is a slot  34  between the susceptor  16  and the wall of the firing chamber  12 . The width of the slot  34  is selected so that a given portion of the microwave radiation may also laterally pass through there, the main portion, however, in any case is forwarded to the susceptor  16 . 
         [0054]    In a manner known per se, a temperature sensor  36  is provided, for example a pyrometer that measures the temperature of the firing chamber and/or the temperature of the dental restoration parts. 
         [0055]    According to the invention it is particularly favorable that also the speed of rotation of the susceptor  16  may be set in any suitable manner. For example, the speed of rotation may amount to between 0, 5 and 70 rotations per minute, and it is also possible to implement a non-constant or clocked speed of rotation that may also be provided in both directions of rotation. 
         [0056]    The maximum temperature for firing the dental ceramic material that may be reached in this way, amounts to 2200° C. It is also particularly favorable that the rotary table  28  is formed in such a way that is acts as a mode stirrer for the supplied microwaves. The susceptor, on the other hand, preferably comprises a very high thermal conductivity of at least 20 W/mK and preferably consists of silicon carbide and/or zirconium oxide and/or a mixture of SiC and SiNi. 
         [0057]    In order to minimize the reflected microwave energy (P reflected) relative to the supplied microwave energy (P source), the position of the susceptor may be varied in the direction of rotation and/or in the vertical direction. 
         [0058]    The further embodiments according to  FIGS. 2 to 7  substantially comprise the same basic design compared to the embodiment of  FIG. 1 , wherein the same reference numerals refer to the same components. 
         [0059]    The second embodiment according to  FIG. 2  comprises a flat, disk-shaped rotary table  28  on which a susceptor  16  is mounted that is as well flat and disk-shaped. 
         [0060]    The rotary table  28  is supported on the drive shaft  20  that in this exemplary embodiment may also be displaced in vertical direction apart from its rotability about the axis thereof. 
         [0061]    In this arrangement, the susceptor  16  that forms a high-temperature stable receiving section, comprises at least one through-opening  28 , and the dental restoration parts  26  are supported next to the through-opening  38  on the high-temperature-stable receiving section. 
         [0062]    In contrast thereto, in the representation of  FIG. 3  it is provided that the susceptor  16  is formed in two parts. A first part is of planar or flat configuration and is received within a recess of the rotary table  28 . It is covered by a separate high-temperature stable receiving section  40  on which the dental restoration parts  26  are arranged. 
         [0063]    In addition to the pot-shaped design of the susceptor  16  according to  FIG. 1 ,  FIG. 3  provides a susceptor cap  42  that is flush with the susceptor element  16  and together with it surround a space that is destined to receive the dental restoration parts  26 . 
         [0064]    The fourth embodiment of  FIG. 4  also comprises a planar or flat susceptor  16 . In this embodiment, the susceptor  16  is received within the rotary table  28  in a recessed manner. On the edge of the rotary table  28  the high-temperature stable receiving section  40  that receives the dental restoration parts  26 , is supported so as to leave an air space  44  towards the susceptor  16 . 
         [0065]    The fifth embodiment of  FIG. 5  differs from the fourth embodiment in that the susceptor  16  that is arranged in a recessed manner as well, is provided with an additional susceptor  48  that extends below the susceptor  16  and consists of individual susceptor structures such as discs, rings, rods etc., wherein it is also possible to employ a susceptor granulate. 
         [0066]    An asymmetric embodiment of the susceptor  16  on the rotary table  28  becomes apparent from the sixth embodiment according to  FIG. 6 . In this embodiment, the susceptor is received within the rotary table  28  in a recessed manner, wherein the surfaces of the rotary table and the susceptor are flush. Merely in the middle portion of the susceptor, a high-temperature stable receiving section  40  is provided that is destined to receive the dental restoration parts. 
         [0067]    The embodiment according to  FIG. 7  differs from the remaining embodiments in that the dental restoration parts are supported on a support member  50  fixed to the oven, said support member  50  extending over the rotary table  28  and the susceptor  16  laterally alongside the heat insulation material  14  of the firing chamber  12 . The extension takes place as far as possible, and in the represented exemplary embodiment in an asymmetric manner over somewhat more than half of the diameter of the rotary table  28 . 
         [0068]    From  FIG. 8  it becomes apparent, in which manner the susceptor  16 , for example in the embodiments according to  FIG. 1  or  3 , may extend in the firing chamber  12 . In this embodiment, the susceptor in the top view is a circular disk that circulates or rotates around the axis of rotation  18 . 
         [0069]    According to  FIG. 9  a further embodiment of a susceptor  16  is shown which has an asymmetrically mounted configuration of the susceptor  16 . The susceptor  16  in this embodiment extends to the inner diameter of the firing chamber  12  and for example comprises a diameter of ⅗ of the inner diameter of the firing chamber  12 . 
         [0070]    A symmetric configuration of a susceptor  16  becomes apparent from the embodiment of  FIG. 10 . In this embodiment, the susceptor  16  is square-shaped and supported on the axis of rotation in a symmetric manner. 
         [0071]    The susceptor  16  according to  FIG. 11  is also supported in a symmetric manner. This susceptor is formed according to the type of a propeller so that its rotary motion relative to the restoration part situated thereabove alternately generates a shadowing effect and exposure. 
         [0072]    According to  FIG. 12 , a plurality of through-openings  38  that also may be shaped differently, is provided that extends over the susceptor  16  in a distributed manner, wherein the susceptor by the way is formed in a circular manner and is symmetrically supported on the axis of rotation  18 . 
         [0073]    According to  FIG. 13 , the susceptor  16  is formed so as to be ring-shaped and extends along the oven wall of the firing chamber  12 , but is supported symmetrically about the axis of rotation. 
         [0074]    The embodiment according to  FIG. 14  shows a multiple arrangement of susceptors  16  that rest on the rotary table  28 . The arrangement is symmetrical about the axis of rotation  18 , and the susceptors  16  are formed in the represented embodiment substantially according to the type of a four-leaf clover. 
         [0075]    Although preferred embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the claims which follow.