Dental Furnace And Process For Operating A Dental Furnace

Abstract

The invention relates to a dental furnace, in particular a high-temperature dental furnace for oxide ceramics such as zirconium dioxide having sintering temperatures of between 1300 and 1850 C., comprising a heating element (10) which is intended to give off heating energy to the firing chamber. It is provided that the heating element (10) comprises at least two heating element sections (48, 50) adjoining one another at a transition area (34) which is not current-carrying and/or which extends away laterally, that the transition area (34) is supported on a position, in particular on the free end, spaced apart from the electrical connections (16, 18) on the dental furnace and carries at least the two adjoining parts of heating element sections (48, 50).

Claims

1. A dental furnace comprising a heating element arrangement (10) which is intended to give off heating energy to the firing chamber, wherein the heating element arrangement (10) has at least a first electrical connection (16) and a second electrical connection (18), each at least first and second electrical connections connected to a configuration of current-carrying heating elements, wherein the at least first and second electrical connections (16) (18) are supported on the dental furnace outside of the firing chamber, wherein the configuration of current-carrying heating elements is adjoined to a transition area element (34) which does not carry a current and/or which extends away from the configuration of current-carrying heating elements.

2. The dental furnace as claimed in claim 1 wherein the transition area element (34) supports the configuration of current-carrying heating elements at at least two adjoining heating element sections (48, 50) of the configuration of current-carrying heating elements, and wherein the transition area element (34) comprises a free end supported on the dental furnace.

3. The dental furnace as claimed in claim 1, wherein the transition area element (34) comprises a plurality of transition area elements (32, 34, 36, 38, 40), wherein every heating element section (48, 50) of the configuration of current-carrying heating elements is suspended or supported on one end by one of the at least first and second electrical connections (16) (18) and on a second end by one of the plurality of the transition area elements (32, 34, 36, 38, 40).

4. The dental furnace as claimed in claim 1, wherein a free end of the transition area element penetrates into the thermal insulation (64) and wherein the electrical connections (16, 18) are disposed beyond the thermal insulation (64) on a side opposite from the firing chamber and from the heating elements.

5. The dental furnace as claimed in claim 1, wherein the transition area element (34) divides the configuration of current-carrying heating elements into the heating element sections (48, 50).

6. The dental furnace as claimed in claim 1, comprising a plurality of transition area elements (32, 34, 36, 38, 40) disposed at a plurality of locations on the configuration of current-carrying heating elements.

7. The dental furnace as claimed in claim 1, comprising wherein the transition area element is fabricated as a single unit and is fabricated of metal.

8. The dental furnace as claimed in claims 1, wherein the transition area element is welded to the configuration of current-carrying heating elements, wherein the transition area element is mounted in or on thermal insulation, wherein the configuration of current-carrying heating elements extend from the thermal insulation on a side opposite the transition area element.

9. The dental furnace as claimed in claim 1, wherein the configuration of current-carrying heating elements extend from a bottom side of thermal insulation (64) in a way such that the thermal insulation (64) forms a support for the transition area element on a top side of the thermal insulation (64).

10. The dental furnace as claimed in claim 1, wherein the dental furnace heats the configuration of current-carrying heating elements to an element temperature of 1850 C.

11. The dental furnace as claimed in claim 1, wherein the dental furnace comprises thermal insulation (64) on which the transition area element (34) is supported or with which the transition area element is in contact, wherein the thermal insulation (34) cools and stabilizes the configuration of current-carrying heating elements by thermal contact and thermal conduction through the transition area element.

12. The dental furnace as claimed in claim 1, wherein the configuration of current-carrying heating elements comprises an oxide layer having thermomechanical properties, and wherein the transition area element comprises an oxide layer having thermomechanical properties which are more resilient then the thermomechanical properties of the oxide layer of the current-carrying elements.

13. The dental furnace as claimed in claim 1, wherein, adjacent to the transition area element, the largest temperature gradient in the configuration of current carrying heating elements is present, as viewed along a path that carries current.

14. The dental furnace as claimed in claim 1, wherein the configuration of current-carrying heating elements comprises molybdenum disilicide, and wherein the dental furnace heats the molybdenum disilicide to a temperature at which a mechanical tensile strength of the molybdenum disilicide is reduced to less than half of the tensile strength of molybdenum at room temperature.

15. The dental furnace as claimed in claim 14, wherein the tensile strength is reduced to a strength of less than 100 MPa at a temperature of 1600 C.

16. The dental furnace as claimed in claim 1, wherein the configuration of current-carrying heating elements is configured as an induction heating element, and wherein the transition area element supports and holds the configuration of current-carrying heating elements by metallic contact.

17. The dental furnace as claimed in claim 1, wherein the configuration of current-carrying heating elements is not in contact with the thermal insulation (64) and other parts of the furnace, and wherein the transition area element which contacts the thermal insulation (64) is mounted movably on the thermal insulation in order to compensate for temperature-related changes in size of the configuration of current-carrying heating elements.

18. The dental furnace as claimed in claim 1, wherein the transition area element is U-shaped, T-shaped, L-shaped, bar shaped, rod-shaped, closed or open circular, closed or open oval-, closed or open squaree, or closed or open rectangular-shaped.

19. The dental furnace as claimed in claim 1, wherein the transition area element is fabricated of metal, metal alloys, ceramic materials, metal-ceramic materials (eg., MoSi) or a mixture thereof.

20. A process for operating a dental furnace comprising a heating element arrangement (10) which comprises molybdenum disilicide, said process comprising subjecting the heating element arrangement (10) to regeneration firing at 1300-1800 C., wherein in the regeneration firing process the dental furnace is brought to an oxidation temperature of the molybdenum disilicide without the placement of dental restoration parts.

21. The process as claimed in claim 20, wherein the heating element arrangement comprises a configuration of current-carrying heating elements adjoined to a transition area element, wherein no current flows through the transition area element, wherein the transition area element extends transversely to and away from the configuration of current-carrying heating elements, and wherein the transition area element is supported on the dental furnace.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Further advantages, details and features may be taken from the following description of several exemplary embodiments of the invention in conjunction with the drawings.

[0067] FIG. 1 shows a schematic perspective illustration of a prior art known dental furnace, namely of the heating element;

[0068] FIG. 2a shows a perspective view of an inventive embodiment of the heating element;

[0069] FIG. 2b shows a changed embodiment of the inventive heating element, also in a perspective implementation;

[0070] FIG. 3a shows a perspective view of a changed embodiment of the heating element;

[0071] FIG. 3b shows an even more changed embodiment of the inventive heating element, also in a perspective implementation;

[0072] FIG. 4a shows a perspective view of a changed embodiment of the heating element;

[0073] FIG. 4b shows an even more changed embodiment of the inventive heating element, also in a perspective implementation;

[0074] FIG. 5 shows a side view of a further embodiment of an inventive heating element;

[0075] FIG. 6 shows a side view of a further embodiment of an inventive heating element;

[0076] FIG. 7 shows a side view of a further embodiment of an inventive heating element;

[0077] FIG. 8 shows a perspective view of an inventive heating element in a further implementation;

[0078] FIG. 9 shows a further embodiment of an inventive heating element;

[0079] FIG. 10 shows a further embodiment of an inventive heating element; and

[0080] FIG. 11 shows a further embodiment of an inventive heating element.

DETAILED DESCRIPTION

[0081] FIG. 1 shows a prior art heating element arrangement 10 as part of an inventive furnace, in particular dental furnace, wherein the heating element comprises thickened regions 12, 14. At the lower end of the thickened regions are electrical connections 16, 18.

[0082] The heating element 10 extends from the thickened regions to the top and comprises vertical legs 20, 22. It is disposed in a circular fashion in the actual heating area 24as viewed in the top viewand surrounds a firing chamber, which is not illustrated. It is received in a thermal insulation in an annular recess which is open towards the inside. There, the heating element 10 is supported on its bottom end, namely on three positions in contact with the insulation in the embodiment illustrated. For this purpose, the heating element is virtually configured in the shape of a meander such that three heating element loops which are open towards the top and three heating element loops which are open towards the bottom are formed. The heating element loops 26, 28 and 30 which are open towards the top are rounded at the bottom, and the heating element is supported in the heating area 24 thereat.

[0083] The support at this position has the disadvantage that mechanical contact between the hot heating element at this position and the comparatively cooler thermal insulation is realized. Additionally, the heating element expands due to the heating such that a relative movement between the thermal insulation and the heating element occurs. The SiO.sub.2 layer is damaged and a possible discoloration of the dental restoration is the result. Without this contact, the mechanical strength of the heating element is not sufficient at too high temperatures, and consequently this will lead to deformation and in the end to contact with the thermal insulation surrounding the heating element nevertheless.

[0084] The invention avoids these disadvantages of the prior art. For this purpose, in the embodiment according to FIG. 2a, heating element arrangement 10 includes a configuration of current-carrying elements 11 which includes transition areas 34, 36, 38 and 40. The transition areas comprise metallic elements which are intensively connected with the heating element 10, in particular by welding. In the heating area 24, the metallic elements which may also be referred to as support sections extend away from the heating element laterally as viewed along its extension, respectively.

[0085] Here, the support section refers to the metallic element which extends away transversely to the main direction of extension of the heating element 10. It forms the transition area together with the welded joint or any other metallic connection with the adjacent heating sections. Every transition area adjoins two heating sections, as is obvious from the figures.

[0086] In the first embodiment of the invention illustrated herein the free ends of the transition areas and naturally their associated support sections extend into recesses of the thermal insulation 64 of furnace 100, which is illustrated in FIG. 11. They are supported thereat, namely vertically with respect to the metal element or support section 40 extending downwards, and laterally with respect to the metal elements or support sections 32 to 38 extending upwards.

[0087] In this configuration, the heating element 10 is carried by the free end 42 of the support section 40 and the vertical legs 20 and 22 of the heating element. They are disposed diametrically opposed to one another with regard to the circle of the heating element.

[0088] In this embodiment, the support sections 32 to 40 are configured as U-shaped arches whose center leg is welded to the heating element. Then, the open end of the U-shaped arch extends away from the open end of the adjacent heating element loops 26 to 30, respectively. Although shown as U-shaped, the support sections may be any shape including but not limited to U-shaped, T-shaped, L-shaped, bar-shaped, rod-shaped, closed or open circular, closed or open oval, closed or open square, closed or open rectangular, and the like.

[0089] It is preferable that the support sections are fabricated of, but not limited to, metal, metal alloys, ceramic materials, metal-ceramic materials (eg., MoSi) and the like.

[0090] Regions of the heating element adjacent to the transition area, which are referred to as heating element sections herein, are cooled by it and thus stabilized. While the entire heating element 10 carries current, at least the free ends 42 of the transition area 30 to 40 considered respectively, or rather of the respective metal element, are free and no heating current flows through them. This means that they are automatically cooler.

[0091] This can be considered more closely with respect to FIG. 2b.

[0092] FIG. 2b shows a further configuration of an inventive heating element 10. Here, just like in the further figures, the same or respective parts are provided with the same reference numerals.

[0093] Here, the metal element 34 is also configured in the shape of a U, but with a width of the arch corresponding to the width of the heating element loops provided in each case. It is welded to the heating element 10 along a length L. Starting from this point, two free ends 42 and 44 of the support section 34 extend away from the heating element 10, to the top in the support section illustrated, as the heating element 10 extends to the bottom starting from the length L in a curved fashion.

[0094] The support section 34 is in intensive metallic contact with the heating element 10 along the length L. Heating element sections 48 and 50 adjoin this contact area. They are not in contact with the support section 34.

[0095] Due to the cooling effect of the support section 34 along the length L, the temperature at the transition between the heating element section 48 or rather the heating element section 50 and the section L is relatively low, for instance 1200 or 1300 C. It increases along the heating element section 48 or 50 starting therefrom, as electric current flows through the heating element 10 uniformly. The temperature reaches its maximum value of 1850 C. in the vertical legs 52 of the heating element 10. The inventive heating element withstands this temperature as it consists of MoSi.sub.2.

[0096] Accordingly, with this solution, the hot and ductile area between the upper and lower heating element sections is comparatively short in the entire heating area 24, which is for the benefit of mechanical stability, such that the heating element can be operated up to the limits of the ductile range.

[0097] A further modified embodiment of the invention is apparent from FIG. 3a. There, the transition areas are configured as support sections 32, 34, 36 and 38 which extend exclusively at the top of the heating element. Every metal element is, in turn, bent in the shape of a U wherein the direction of extension of the open Us is radially towards the outside such that the metal elements extend horizontally.

[0098] With this solution, the heating element loops 26, 28, 30 are carried in a hanging fashion by the support sections 32 to 38 adjacent to one another, respectively. In addition, the vertical legs 20 and 22 of the heating element 10 are carried by the adjacent support sections 36 and 38 and supported by the thickened regions 12 and 14.

[0099] A further embodiment which is slightly modified with respect to the embodiment in FIG. 3a is apparent from FIG. 3b. Here, the U-shaped support sections 32 and 34 are wider, but extend in turn radially towards the outside and horizontally.

[0100] A further modified configuration of the inventive heating element 10 is apparent from FIG. 4a. With this solution, the support sections 32, 34, 36 and 38 are configured as bars each which extend radially towards the outside to form the cold ends and which are each welded to the respective heating element loops at the top. Here, too, the support is realized in bores or recesses of the thermal insulation; the heating element loops 26, 28 and 30 are sagging starting therefrom to the bottom.

[0101] The embodiment according to FIG. 4a is apparent in FIG. 4b in a slightly modified fashion in another perspective and in another illustration. In this embodiment, the cold ends or support sections 32 to 38 are slightly longer and accordingly slightly colder than in FIG. 4a.

[0102] A further embodiment of an inventive heating element is apparent from FIG. 5. The embodiments according to FIGS. 5 to 7 are illustrated in an unrolled fashion, which means that they extend circularly in principle but that they are illustrated in a linear fashion here.

[0103] It is to be understood that in case of other embodiments of the inventive heating element a linear configuration may be realized indeed which may comprise a substantially larger number of transition areas and heating element loops, by all means.

[0104] In the configuration according to FIG. 5 heating element connections 60 and 62 are provided. They are supported on a thermal insulation layer 64. The heating element 10 passes through the thermal insulation layer 64, wherein three heating element loops 26, 28 and 30 are configured whose upper ends each in turn pass through the thermal insulation 64.

[0105] Two transition areas 34 and 36 are configured which extend on this side of the thermal insulation, as it were, and are cooler in this respect due to the larger cross-section of the lines.

[0106] In the configuration according to FIG. 6, electrical connections 60 and 62 are provided in turn which extend away from the heating element 10 laterally on this side of the thermal insulation. The transition areas 34 and 36 are also formed on this side of the thermal insulation 64, namely by a fastening element which is supported on the thermal insulation 64. The heating element loops 26 and 28 merge with one another on the other side of the transition area 34; likewise, this holds true for the heating element loops 28 and 30 with respect to the transition area 36.

[0107] In this embodiment, the flow of current does not pass through the transition areas 34 and 36 on this side of the thermal insulation 64 but only on the other side of the thermal insulation 64.

[0108] The flow of current forms a region providing for heat output, the heat handling region, in which the heating elements 10 extend or at least partially extend.

[0109] This also holds true for the configuration according to FIG. 7. Here, the transition areas are configured by support sections 34 and 36 whose cold free ends are supported on the thermal insulation 64 and welded to the heating element 10. This solution allows for a production that saves material and bends.

[0110] A further configuration of the invention is apparent from FIG. 8. In this solution, the cold ends or transition areas 34, 36, 38 and 40 extend vertically, wherein the respective heating element loops 26, 28 and 30 are suspended by way of this solution. For this purpose, the support sections 34 to 40 are clamped to the thermal insulation.

[0111] A solution with equal parts is apparent from FIG. 9 which may be produced relatively cost-effectively accordingly. The equal parts are welded to one another wherein a short cold end 66 is combined with a long cold end 68 at every transition area, which are welded to one another along the length of the short cold end 66.

[0112] Then, the long cold ends 68 are each received in recesses of the thermal insulation 64 such that the heating element is supported thereon.

[0113] FIG. 10 shows a further embodiment of an inventive heating element 10.

[0114] An induction heating process is realized in this embodiment. For this purpose, the heating element comprises closed electric circuits of which the electric circuits 70 and 72 are designated in FIG. 10. An electromagnetic alternating field induces currents in the electric circuits of the heating element 10 in a way known per se. They and their internal resistance lead to a drop in voltage, i.e. power loss, respectively, which is used as heating energy. In this respect, here, too, the heating element is a resistance heating element.

[0115] Horizontal legs 74 and 76 of the heating element 10 belong to the electric circuits. By heating, they lose part of their strength such that they sag in the hot state.

[0116] They are suspended on posts 78, 80 and 82. Their lower support sections 84, 86 and 88 do not carry a current such that they remain cold and strong, and at the same time cool surrounding regions of the electric circuits 70, 72, the transition areas 34, 36, and slightly increase the strength thereat.

[0117] These support sections are mounted in or on the thermal insulation of the furnace not illustrated herein.

[0118] It is to be understood that it is possible to form any desired other electric circuits without leaving the scope of the invention. In this way, particularly with the inductive heating, the horizontal legs may be configured to be shorter than the vertical legs in order to further reduce sagging. It is also possible to realize a hanging support at the posts 78, 80, 82.

[0119] Alternatively, with the inductive configuration of the heating it is also possible to use a considerably higher number of support sections and/or also lateral support sections, that means support sections which extend transversely to the current-carrying sections of the heating element, namely horizontally or transversely to the top or bottom.

[0120] Free ends of support sections of this type are each mounted in or on the thermal insulation, too.