Dental furnace

Abstract

The invention relates to a dental furnace wherein a firing chamber is heated up in a first heating-up period at a first heating-up rate of more than 501 K/min, in particular more than 1001 K/min, which heats the furnace to at least 10001 C, in particular to 1100-12501 C. The first heating-up period is followed by an intermediate heating period, which is at least five minutes long, in particular at least ten minutes long, the gradient or heating-up rate of which is adapted to the material to be sintered in the dental furnace (10), and wherein this is followed by an end heating-up period (44) during which heating up is effected at a heating-up rate of more than 301 K/min, in particular approximately 501 K/min, and wherein during this the furnace temperature is held for at least five minutes, in particular for at least 25 minutes, above the temperature toward the end of the first heating-up period, and wherein forced cooling of the furnace (10) is performed after this.

Claims

1. A dental furnace for sintering oxide-ceramic materials comprising a heating chamber surrounded by heating elements and a thermal insulation layer, wherein the dental furnace is configured such that the heating chamber is heated up in a first heating-up period at a first heating-up rate of more than 50 K/min, which heats the furnace to at least 1000 C., the first heating-up period is followed by an intermediate heating period having an intermediate heating-up rate which is at least five minutes long, wherein during the intermediate heating period about 90 percent of final density of the oxide-ceramic material being sintered is achieved, the intermediate heating period is followed by an end heating-up period during which end heating-up is effected at a heating-up rate of more than 30 K/min, during the end heating-up period the furnace temperature is held for at least five minutes, above a temperature that has been measured at the end of the first heating-up period, and wherein forced cooling of the furnace is performed after the furnace temperature is held for at least five minutes, and the end heating-up period has a rate that is slower than the first heating-up rate.

2. The dental furnace as claimed in claim 1, wherein the dental furnace is further configured such that the maximum temperature in the heating chamber is approximately 1600 C. and the furnace can be heated up to 1600 C. proceeding from room temperature in its heating chamber, within less than 30 minutes.

3. The dental furnace as claimed in claim 1, wherein the dental furnace is further configured such that the intermediate heating-up rate is lower than the initial heating-up rate by approximately a power of 10.

4. The dental furnace as claimed in claim 1, wherein the heating chamber of the furnace is surrounded by a heat-resistant insulation comprising a pressed shaped part composed of fiber having a wall thickness between 15 and 25 mm.

5. The dental furnace as claimed in claim 1, wherein the dental furnace is further configured such that the intermediate heating period is chosen in terms of a temperature and/or a time such that it covers an intermediate stage of a sintering process of the dental material to be sintered, in which a sintering rate, plotted against the temperature/time, is the highest.

6. The dental furnace as claimed in claim 1, wherein the dental material to be fired is presintered and/or precompacted and prior to sintering has a strength which is lower than the final strength of the sintering material.

7. The dental furnace as claimed in claim 1, wherein the initial heating-up rate is configured such that it corresponds to a maximum heating-up rate at which no overshoot arises upon the transition from the first heating-up period to the intermediate period, but is at least 50 K/min.

8. The dental furnace as claimed in claim 1, wherein the dental furnace is further configured such that after the holding time has elapsed, the furnace cools down at a first cooling-down rate, which is less than the heating-up rate of the first heating-up period and greater than the heating-up rate of the intermediate heating-up period, and wherein a higher, second cooling-down rate is set after this.

9. The dental furnace as claimed in claim 1, wherein the dental furnace is a microwave furnace.

10. The dental furnace as claimed in claim 1, wherein the dental furnace is further configured such that the heating chamber is heated up in the first heating-up period at the first heating-up rate of more 100 K/min, which heats the furnace to at least 1100 C., the intermediate heating period is at least ten minutes long, the end heating-up period is effected at a heating-up rate of more than 50 K/min, and during the end heating-up period the furnace temperature is held for at least 25 minutes above the temperature toward the end of the first heating-up period.

11. The dental furnace as claimed in claim 4, wherein the dental furnace is further configured such that the intermediate heating-up rate is lower than the initial heating-up rate by a factor of 10 to 50.

12. The dental furnace as claimed in claim 6, wherein the strength of the presintered and/or precompacted dental material prior to sintering the dental material to be fired is less than half the magnitude of the final strength of the sintering material.

13. A dental furnace for sintering oxide-ceramic materials, wherein the dental furnace is configured such that a sintering material to be sintered is heated up in a first heating-up period at a heating-up rate of more than 50 K/min, between an end of the first heating-up period and a beginning of an end heating period there is an intermediate heating period at a heating-up rate lower than 50 K/min, the first heating-up period and the end heating period are set in a material-independent manner, the intermediate heating period is defined, with regard to its length and its heating-up rate, in a manner dependent on the material to be sintered, during the intermediate heating period about 90 percent of final density of the dental material being sintered is achieved, and the end heating-up period has a rate that is slower than the first heating-up rate.

14. The dental furnace of claim 13, wherein the dental furnace is further configured such that the heating-up rate of the intermediate heating period is lower than 10 K/min.

15. A total firing method for sintering a dental material comprising: heating a heating chamber in a first heating-up period at a first heating-up rate of at least 50 K/min, which heats up the furnace to at least 1000 C., continuing to heat the chamber in an intermediated heating period, wherein the first heating-up period is followed by the intermediate heating period, which is at least five minutes long, and continuing to heat the chamber in an end heating-up period, wherein the intermediate heating period is followed by the end heating-up period, during which heating-up is effected at a heating-up rate of more than 20 K/min, wherein the end heating-up period has a rate that is equal to or slower than the first heating-up rate, and holding the temperature in the end heating period for at least five minutes, above a temperature that has been measured at the end of the first heating-up period, and force cooling the furnace after the end heating period for a cooling down of the dental material to a removal temperature of approximately 400 C., in less than or equal to 60 minutes, wherein the total firing method takes place over a time span from a start of the first heating-up period at room temperature to the cooling down of the heating chamber to room temperature, wherein the heating chamber has a temperature higher than 1100 C. at least 68% of the time span of the total firing method, wherein during the intermediate heating period about 90 percent of final density of the dental material being sintered is achieved.

16. The method as claimed in claim 15, wherein the dental material comprises a dental tooth replacement material and is sintered in dry fashion or in liquid sintering, wherein the dental material comprises an oxide ceramic which is composed of ZrO.sub.2, of Al.sub.2O.sub.3 and compositions thereof and comprises a doping auxiliary.

17. The method according to claim 15, wherein the heating-up rate of the intermediate heating period is less than 10 K/min.

18. The method according to claim 15, wherein the cooling of the furnace is performed in 20 to 60 minutes.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a schematic and partially cut-away view of a dental furnace according to the invention; and

(2) FIG. 2 shows two exemplary embodiments of firing curves for the dental furnace according to the invention.

DETAILED DESCRIPTION

(3) The dental furnace 10 illustrated in FIG. 1 has a furnace hood 12, which is mounted on a furnace lower part 16 by means of a pivoting articulated joint 14. The lower part 16 has on its top side a bearing surface 18, which is intended for receiving the dental material to be fired. A firing chamber 20 is provided in the furnace hood 12, and it extends in the manner of a rather flat cylinder and, with the furnace hood 12 closed, is closed off at the bottom by the bearing surface 18, such that the bearing surface 18 forms the bottom of the firing chamber 20.

(4) The firing chamber 20 is surrounded annularly or spirally by heating elements 22.

(5) According to the invention, particularly powerful heating elements are provided, which are designed such that they are fundamentally able to heat up the furnace from room temperature to 1600 C. within approximately a quarter of an hour. The dental furnace accordingly has a max. temperature gradient of 120 K/min.

(6) The heat capacity of the firing chamber 20 and of the parts surrounding the firing chamber 20 is low.

(7) The heating elements 22 are additionally significantly rear-ventilated. An air space 28 is provided for this purpose, said air space surrounding the heating elements 22 and thus the firing space 20 on all sides. The air space 28 is extremely large and takes up a considerable part of the interior of the furnace hood 12. The furnace hood 12 has a thermal insulation layer 30 surrounding the air space 28, which layereven though this cannot be seen in FIG. 1can also have perforations forming air ducts in order to facilitate the air flow via air outlets 32 in the upper region of the furnace hood 12.

(8) The dimensioning both of the air space 12 and of the thermal insulation layer 30, can be adapted to the requirements within wide ranges, it also being possible to work with an extremely thin thermal insulation layer of just 15 mm, for example.

(9) The dental material preferably provided is applied to the bearing surface 18 according to the invention. After the furnace hood 12 has been closed, the heating element 22 is switched on with max. power, such that the firing chamber 20 is heated extremely rapidly to 1200 C., for example. This temperature may substantially correspond to the presintering temperature. After this, during an intermediate heating period, the temperature is increased with a small temperature gradient until a temperature of approximately 100 C. below the final temperature has been reached. After this, the temperature is increased extremely rapidly again to the final temperature and after this is held for a predetermined time duration, wherein the holding time may depend both on the applied amount of dental material and on further parameters.

(10) After this, the temperature is reduced, to be precise preferably firstly without active cooling, wherein the active cooling is switched on when the presintering temperature has been reached again, such that the cooling proceeds more rapidly starting from said temperature until room temperature is reached.

(11) As an alternative, in an even more highly accelerated firing cycle, the cooling down can take place directly after the holding time with active cooling, such that the cooling-down period overall is shortened further.

(12) An initial heating-up period 40, which is ended at approximately 1100 C. in accordance with curve 2, is followed by an intermediate heating period 42, which performs heating up to approximately 1350 C.

(13) After this, an end heating-up period 44 is provided, which increases the temperature to 1500 C., which final temperature is reached 100 min after the beginning of the firing cycle in the case of curve 2.

(14) During the holding time 46 of approximately 30 min, the temperature is held at 1500 C. and, during the initial cooling-down period 48, the temperature is lowered to 1100 C. within less than 30 min.

(15) After this, the end cooling period 50 is provided, by means of which the temperature is lowered to room temperature within likewise somewhat less than half an hour.

(16) This firing cycle according to the invention is illustrated in two embodiments in FIG. 2. Accordingly, the following firing curve results for the curve designated as curve 2:

(17) TABLE-US-00001 Firing curve 2 Temperature/ Rate/ Time/ Total/ Ramp C. K min.sup.1 min min 0 25 1 1100 100 10.75 10.75 2 1350 3 83.33 94.08 3 1500 50 3.00 97.08 4 1500 0 30.00 127.08 5 1100 15 26.67 153.75 6 200 50 18.00 171.75

(18) TABLE-US-00002 Firing curve 1 Temperature/ Rate/ Time/ Total/ Ramp C. K min.sup.1 min min 0 25 1 1250 100 12.25 12.25 2 1350 5 20.00 32.25 3 1500 50 3.00 35.25 4 1500 0 25.00 60.25 5 1100 25 16.00 76.25 6 200 46.6 19.31 95.56

(19) The total firing curve is reduced even further in the modified embodiment in accordance with curve 1 to approximately 95 min, wherein a substantially trapezoidal curve profile is provided in both cases, each having a high initial heating-up rate and an equally or almost equally high end cooling-down rate. This is also evident from the table above.

(20) In contrast to the firing cycle in accordance with curve 2, an overshoot of the heating power to a temperature of, for example, 50 C. above the temperature of the holding time 46 is provided in the case of curve 1.

(21) Surprisingly, the strength is increased by the rapid heating-up to the presintering temperature, or alternatively to 1250 C., while there is no measurable influence on the accuracy of fit. By contrast, the accuracy of fit and hence the distortion are improved by the slow heating up during the intermediate heating period, while the strength is not adversely affected. By contrast, the end heating up, for example by 150 C., to the end heating up temperature, which may lie between 1500 C. and 1600 C., has no particular influence on the strength and no influence at all on the accuracy of fit.

(22) By contrast, the comparatively long holding time has a very great influence on the strength and in particular also on the final density, and the relatively slow cooling-down to the presintering temperature as provided in accordance with curve 2 also has a measurable influence on the strength, while the subsequent cooling down to room temperature has practically no further influence on the strength or the density.

(23) According to the invention, it is therefore possible to realize a dental furnace with a short firing cycle, yet particularly attractive firing results.

(24) While a preferred form of this invention has been described above and shown in the accompanying drawings, it should be understood that applicant does not intend to be limited to the particular details described above and illustrated in the accompanying drawings, but intends to be limited only to the scope of the invention as defined by the following claims. In this regard, the terms as used in the claims are intended to include not only the designs illustrated in the drawings of this application and the equivalent designs discussed in the text, but are also intended to cover other equivalents now known to those skilled in the art, or those equivalents which may become known to those skilled in the art in the future.