Process for producing a blank, and a blank

Abstract

The invention relates to a blank for producing a dental molded part such as an inlay, onlay, crown or bridge, and to a method for producing the blank. To be able to machine a dental molded part, in particular one having thin wall thicknesses, from the blank without difficulty, the blank is designed to consist of a glass ceramic having a density of between 30 and 60% of theoretical density, and of glass-ceramic powder particles with a particle size distribution d.sub.90≦80 μm, lithium silicate crystals being present in an amount of 10 to 90% by volume.

Claims

1. A blank for producing a dental molded part, the blank comprising: pre-crystalized glass-ceramic powder particles having lithium silicate crystals; and wherein the blank has a density between 30% and 60% of a theoretical density of a fully-sintered blank; and wherein the blank is formed from the pre-crystalized glass-ceramic powder particles having a grain size distribution d.sub.50≦25 μm; and wherein a fraction of lithium silicate crystals is between 10% by volume and 90% by volume and wherein: the blank possesses a disk-, cube-, or rod-shaped geometry, and means for arrangement in a milling machine originating from the circumferential surface of the blank and extending diametrically relative to the center of gravity.

2. The blank of claim 1, wherein: the blank possesses an open porosity between 5% by volume and 60% by volume.

3. A blank for producing a dental molded part, the blank comprising: pre-crystalized glass-ceramic powder particles having lithium silicate crystals; wherein the blank has a density between 30% and 60% of a theoretical density of a fully-sintered blank; and the pre-crystalized glass-ceramic powder particles having a grain size distribution d.sub.90≦80 μm; wherein a fraction of lithium silicate crystals is between 10% by volume and 90% by volume and wherein the pre-crystalized glass-ceramic powder particles possess a composition in % by weight: SiO.sub.2 46.0-72.0; Li.sub.2O 10.0-25.0; ZrO.sub.2 6.5-14.0; P.sub.2O.sub.5 1.0-10.0; Al.sub.2O.sub.3 0.1-8.0; K.sub.2O 0.1-5.0; CeO.sub.2 0.1-4.0; B.sub.2O.sub.3 0.0-4.0; Na.sub.2O 0.0-4.0; Tb.sub.4O.sub.7 0.0-2.5; and 0.0 to 4.0 of at least one additive.

4. The blank of claim 1, wherein the pre-crystalized glass-ceramic powder particles possess a composition in % by weight: SiO.sub.2 49.0-69.0; Li.sub.2O 11.5-24.0; ZrO.sub.2 7.0-13.5; P.sub.2O.sub.5 1.5-9.0; Al.sub.2O.sub.3 0.2-7.5; K.sub.2O 0.2-4.5; CeO.sub.2 0.2-3.5; B.sub.2O.sub.3 0.0-3.5; Na.sub.2O 0.0-3.5; Tb.sub.4O.sub.7 0.0-2.0; and 0.0 to 4.0 of at least one additive.

5. The blank of claim 4, wherein the pre-crystalized glass-ceramic powder particles possess a composition in % by weight: SiO.sub.2 52.0-66.0; Li.sub.2O 12.0-22.5; ZrO.sub.2 7.5-13.0; P.sub.2O.sub.5 2.0-8.5; Al.sub.2O.sub.3 0.3-7.0; K.sub.2O 0.3-4.0; CeO.sub.2 0.3-3.5; B.sub.2O.sub.3 0.0-3.0; Na.sub.2O 0.0-3.0; Tb.sub.4O.sub.7 0.0-2.0; and 0.0 to 4.0 of at least one additive.

6. The blank of claim 3, wherein the pre-crystalized glass-ceramic powder particles possess a composition in % by weight of: SiO.sub.2 55.0-63.0; Li.sub.2O 12.5-21.5; ZrO.sub.2 8.0-12.0; P.sub.2O.sub.5 2.5-8.0; Al.sub.2O.sub.3 0.4-6.5; K.sub.2O 0.4-4.0; CeO.sub.2 0.5-3.0; B.sub.2O.sub.3 0.0-3.0; Na.sub.2O 0.0-3.0; Tb.sub.4O.sub.7 0.0-2.0; and 0.0 to 4.0 of at least one additive.

7. The blank of claim 1, wherein the pre-crystalized glass-ceramic powder particles possess a composition in % by weight: SiO.sub.2 58-60; Li.sub.2O 13.5-20.5; ZrO.sub.2 8.5-11.5; P.sub.2O.sub.5 3.0-7.5; Al.sub.2O.sub.3 0.5-6.0; K.sub.2O 0.5-3.5; CeO.sub.2 0.5-2.5; B.sub.2O.sub.3 0-3; Na.sub.2O 0-3; Tb.sub.4O.sub.7 0-1.5; and 0.0 to 4.0 of at least one additive.

8. The blank of claim 3, wherein: the additive is selected from the group consisting of: color pigment, and fluorescent agent.

9. The blank of claim 3, wherein: the additive comprises at least one oxide selected from the group consisting of BaO, CaO, MgO, MnO, Er.sub.2O.sub.3, Gd.sub.2O.sub.3, Pr.sub.6O.sub.11, Sm.sub.2O.sub.3, TiO.sub.2, V.sub.2O.sub.5, and Y.sub.2O.sub.3.

10. The blank of claim 1, wherein: the fraction of lithium silicate crystals is between 40% by volume and 60% by volume.

11. A method for producing a dental molded part, the method comprising the steps of: producing a molten mass with a composition (in % by weight): SiO.sub.2 46.0-72.0; Li.sub.2O 10.0-25.0; ZrO.sub.2 6.5-14.0; P.sub.2O.sub.5 1.0-10.0; Al.sub.2O.sub.3 0.1-8.0; K.sub.2O 0.1-5.0; CeO.sub.2 0.1-4.0; B.sub.2O.sub.3 0.0-4.0; Na.sub.2O 0.0-4.0; Tb.sub.4O.sub.7 0.0-2.5; and 0.0 to 4.0 of at least one additive, producing a glass frit by atomizing the molten mass and quenching the molten mass in a medium, generating glass powder particles from the glass frit with a grain size distribution d.sub.90<80 μm, crystallizing lithium silicate crystals with a volume fraction between 10% and 90% by a first thermal treatment of either the glass frit or the glass powder particles in a first temperature range at a temperature T.sub.1 with 500° C.≦T.sub.1≦750° C. for a duration t.sub.1 with 5 min≦t.sub.1≦120 min, whereby, glass-ceramic powder particles with a grain size distribution d.sub.90≦80 μm are produced from a heat-treated glass frit, pressing the glass-ceramic powder particles to form a blank, machining the blank by milling to produce a pre-form part corresponding to the dental molded part, taking into consideration the shrinkage characteristics of the blank, and sintering the pre-form part at a temperature T.sub.2 with 800° C.≦T.sub.2≦1050° C. for a duration t.sub.2 with 5 min≦t.sub.2≦60 min.

12. The method of claim 11, wherein the molten mass comprises (in % by weight): SiO.sub.2 49.0-69.0; Li.sub.2O 11.5-24.0; ZrO.sub.2 7.0-13.5; P.sub.2O.sub.5 1.5-9.0; Al.sub.2O.sub.3 0.2-7.5; K.sub.2O 0.2-4.5; CeO.sub.2 0.2-3.5; B.sub.2O.sub.3 0.0-3.5; Na.sub.2O 0.0-3.5; Tb.sub.4O.sub.7 0.0-2.0; and 0.0 to 4.0 of at least one additive.

13. The method of claim 11, wherein the molten mass comprises (in % by weight): SiO.sub.2 52.0-66.0; Li.sub.2O 12.0-22.5; ZrO.sub.2 7.5-13.0; P.sub.2O.sub.5 2.0-8.5; Al.sub.2O.sub.3 0.3-7.0; K.sub.2O 0.3-4.0; CeO.sub.2 0.3-3.5; B.sub.2O.sub.3 0.0-3.0; Na.sub.2O 0.0-3.0; Tb.sub.4O.sub.7 0.0-2.0; and 0.0 to 4.0 of at least one additive.

14. The method of claim 11, wherein the molten mass comprises (in % by weight): SiO.sub.2 55.0-63.0; Li.sub.2O 12.5-21.5; ZrO.sub.2 8.0-12.0; P.sub.2O.sub.5 2.5-8.0; Al.sub.2O.sub.3 0.4-6.5; K.sub.2O 0.4-4.0; CeO.sub.2 0.5-3.0; B.sub.2O.sub.3 0.0-3.0; Na.sub.2O 0.0-3.0; Tb.sub.4O.sub.7 0.0-2.0; and 0.0 to 4.0 of at least one additive.

15. The method of claim 11, wherein the molten mass comprises (in % by weight): SiO.sub.2 58-60; Li.sub.2O 13.5-20.5; ZrO.sub.2 8.5-11.5; P.sub.2O.sub.5 3.0-7.5; Al.sub.2O.sub.3 0.5-6.0; K.sub.2O 0.5-3.5; CeO.sub.2 0.5-2.5; B.sub.2O.sub.3 0-3; Na.sub.2O 0-3; Tb.sub.4O.sub.7 0-1.5; and 0.0 to 4.0 of at least one additive.

16. The method of claim 11, wherein: prior to machining and after the first thermal treatment, the blank is tempered at a temperature T.sub.3 with 750° C.≦T.sub.3≦900° C. for a duration t.sub.3 with 5 min≦t.sub.3≦30 min.

17. The method of claim 11, wherein: to produce a blank with a disk geometry, the glass-ceramic powder particles are first axial pressed and subsequently, upon insertion into an encasing element such as a pouch coated with polyethylene on its inside, are subjected to isostatic re-compression, whereby the re-compression in particular is performed at a pressure p.sub.n with 250 MPa≦p.sub.n≦350 MPa for a time t.sub.4 with 5 sec≦t.sub.4≦30 sec.

18. The method of claim 11, wherein: for the production of a blank with a cube geometry, the glass-ceramic powder particles are axially pressed successively and in particular continuously with increasing pressure for a duration t.sub.5 with 10 sec≦t.sub.5≦20 sec, whereby the maximum pressure is p.sub.5, with 50 MPa≦p.sub.5≦400 MPa.

19. The method of claim 11, wherein: for the production of a rod-shaped blank, the glass-ceramic powder is introduced into a tubular press mould and subsequently is subjected to quasi-isostatic pressing.

20. The method of claim 11, wherein: the blank is subjected to at least a coarse machining by milling and subsequent precision machining, whereby milling parameters for the coarse machining comprise: Cutter diameter: 2 to 5 mm, Feed: 500 to 4000 mm/min, Lateral feed ae: 0.2 to 3 mm, Depth feed ap: 0.1 to 2 mm, Cutter speed: 10.000 to 50.000 1/min, the milling parameters for the precision machining comprise: Cutter diameter: 0.3 to 1.5 mm, Feed: 300 to 2000 mm/min, Lateral feed ae: 0.2 to 0.6 mm, Depth feed ap: 0.05 to 0.3 mm, Cutter speed: 20,000 to 60,000 1/min.

21. The method of claim 20, wherein: the cutter is a radius cutter with the following cutting edge angles: Cutting angle: 0° to −13°, Clearance angle: 0° to 15°, Wedge angle: Results from: 90° minus clearance angle minus cutting angle.

22. The method of claim 11, wherein: the blank is immersed in silicic acid or in an alkali silicate solution, is dried, and subsequently is machined by dry milling, or in that the blank is machined by milling and subsequently, prior to the sintering to the final density, is immersed in silicic acid or alkali silicate solution and subsequently dried.

23. A monolithic dental molded part manufactured in accordance with the method of claim 11 using a blank comprising lithium silicate crystals and one or more glass ceramics with a density between 30% and 60% of a theoretical density of a fully-sintered blank, wherein the blank is formed from glass-ceramic powder particles with a grain size distribution d.sub.90≦80 μm and wherein a fraction of lithium silicate crystals is between 10% by volume and 90% by volume; and wherein: the dental molded part comprises a crown and possesses a crown margin with a thickness D.sub.R of 0.05 mm≦D.sub.R≦0.4 mm.

24. The monolithic dental molded part of claim 23, wherein: the molded part has a thermal expansion coefficient WAK with WAK≦12.5×10.sup.−6 1/K.

Description

(1) Further details, advantages, and features of the invention are not only found in the claims and the characteristic features described therein—on their own and/or in combination—but also in the following exemplary embodiments.

(2) FIG. 1 shows a graph of pressure versus time during the pressing of a blank.

(3) In accordance with the invention, a blank consisting of pressed glass-ceramic powder is used to produce a dental molded part. To make the glass-ceramic powder, one at first melts a powder and uses the molten mass to produces a glass fit, which can possess the following preferred composition:

(4) TABLE-US-00011 SiO2 49.0-69.0 Li2O 11.5-24.0 ZrO2  7.0-13.5 P2O5 1.5-9.0 Al2O3 0.2-7.5 K2O 0.2-4.5 CeO2 0.2-3.5 B2O3 0.0-3.5 Na2O 0.0-3.5 Tb4O7 0.0-2.0 as well as 0.0 to 4.0 of at least one additive.

(5) In particular it is intended that the molten glass has a composition of (in % by weight):

(6) TABLE-US-00012 SiO2 49.0-69.0 Li2O 11.5-24.0 ZrO2  7.0-13.5 P2O5 1.5-9.0 A12O3 0.2-7.5 K2O 0.2-4.5 CeO2 0.2-3.5 B2O3 0.0-3.5 Na2O 0.0-3.5 Tb4O7 0.0-2.0 as well as 0.0 to 4.0 of at least one additive.

(7) Preferably the molten glass has a composition of (in % by weight):

(8) TABLE-US-00013 SiO2 52.0-66.0 Li2O 12.0-22.5 ZrO2  7.5-13.0 P2O5 2.0-8.5 A12O3 0.3-7.0 K2O 0.3-4.0 CeO2 0.3-3.5 B2O3 0.0-3.0 Na2O 0.0-3.0 Tb4O7 0.0-2.0 as well as 0.0 to 4.0 of at least one additive.

(9) Especially emphasized is a composition of the molten glass with a composition (in % by weight) of:

(10) TABLE-US-00014 SiO2 55.0-63.0 Li2O 12.5-21.5 ZrO2  8.0-12.0 P2O5 2.5-8.0 A12O3 0.4-6.5 K2O 0.4-4.0 CeO2 0.5-3.0 B2O3 0.0-3.0 Na2O 0.0-3.0 Tb4O7 0.0-2.0 as well as 0.0 to 4.0 of at least one additive.

(11) Preferably it is intended that the molten glass has a composition (in % by weight) of:

(12) TABLE-US-00015 SiO2 58.0-60.0 Li2O 13.5-20.5 ZrO2  8.5-11.5 P2O5 3.0-7.5 A12O3 0.5-6.0 K2O 0.5-3.5 CeO2 0.5-2.5 B2O3 0.0-3.0 Na2O 0.0-3.0 Tb4O7 0.0-1.5 as well as 0.0 to 4.0 of at least one additive.

(13) The at least one additive is at least one additive selected from the group composed of colour pigment, fluorescent agent. In particular it is intended that the additive is at least one oxide chosen from the group of BaO, CaO, MgO, MnO, Er2O3, Gd2O3, Pr6O11, Sm2O3, TiO2, V2O5, Y2O3 or contains such an oxide.

(14) The corresponding mixture of starting materials, e.g. in form of oxides and carbonates, subsequently is melted in a suitable crucible of refractory material or a noble metal alloy at a temperature between 1350° C. and 1600° C. for a time period between 1 h and 10 h, in particular for a time of 4 h to 7 h at a temperature of 1540° C. Homogenization is achieved, e.g. by stirring, at the same time or subsequently. The liquid glass produced in this manner subsequently is fed to a nozzle, which preferably has been caused to oscillate, and which itself is set to a temperature in the region between 1250° C. and 1450° C., in particular to 1310° C. The nozzle may possess a diameter between 1 mm and 2 mm. The oscillation frequency of the nozzle may be in the range between 40 Hz and 60 Hz, in particular in the region of 50 Hz. Subsequently the liquid glass is quenched in a suitable medium, such as water for liquids or high-temperature insulation wool. The glass frit produced and quenched in this manner is then dried. This is followed by grinding e.g. in a ball mill. A subsequent sifting stage can use a screen with a mesh width between 50 μm and 500 μm. If required, a further grinding can be performed, e.g. using a jet mill or an attrition mill.

(15) From the glass- or glass-particle powder produced in this manner, one in particular selects those that correspond to a grain size distribution of d.sub.90≦80 μm, in particular 10 μm≦d.sub.50≦60 μm. d.sub.90 and d.sub.50 indicate that 90% or 50%, respectively, of the particles present possess a diameter that is smaller than the specified value or that is in that particular region.

(16) In order to facilitate easy machining of the blank, without risking any instabilities during the final sintering of the molded part produced from blank, one subjects either the frit obtained after melting or the pre-ground or completely ground powder to a crystallization step. In this, one subjects the frit or the powder in a first thermal treatment step to a temperature T.sub.1 between 500° C. and 750° C. for a duration t.sub.1 between 5 min and 120 min. The first thermal treatment step may also be implemented as a two-stage process, i.e. first thermal treatment step 640° C., preferably 660° C. for 60 min and 750° C. for 40 min.

(17) Preferably this is followed by a further thermal treatment in form of tempering, whereby the temperature T.sub.3 to be selected should be between 750° C. and 900° C. This tempering step is performed for a duration t.sub.3, in particular between 5 min and 30 min.

(18) Subsequently the glass-ceramic particles are pressed, where in dependence on the geometry to be produced, one uses suitable pressing methods, in particular an axial or isostatic pressing or combinations of these. The compressing is carried out to such a degree that the density of the blank corresponds to 30% to 60% of the theoretical density of the blank material of approximately 2.64 g/cm.sup.3. In particular, the blank should possess a density corresponding to approximately 50% of the theoretical density.

(19) During the pressing of the glass-ceramic powder, the latter preferably is subjected to a pressure between 50 MPa and 400 MPA, in particular between 100 MPa and 200 MPa.

(20) FIG. 1 shows as an example a graph of pressure versus time during the pressing of a blank. In a first phase P1 the pressure is increased from a starting value of 0 with a pressure build-up of for example 15 MPa/sec to a pressure of for example 30 MPa. In a second phase P2 the pressure is increased from 30 MPa using a pressure build-up of 100 MPa/sec to a pressure of approximately 200 MPa. In a third phase P3 the pressure is kept constant at a value of approximately 200 MPa for a hold time of approximately 10 sec. A fourth phase preferably contains a two-stage pressure reduction, whereby in a phase P4a the pressure is reduced from approximately 200 MPa to approximately 20 MPa with a pressure-reduction of 40 MPa/sec and in a phase P4b the pressure is reduced from 20 MPa to 0 MPa excess pressure with a pressure reduction rate of approximately 10 MPa/sec.

(21) The pressing is followed by machining by means of milling, whereby it is possible to at first perform a coarse machining, to be followed by precision machining. The machining may be performed without cooling, which allows dry machining.

(22) The following milling parameters should be taken into account for the coarse machining:

(23) Cutter diameter: 1 to 5 mm, in particular 2 to 3 mm

(24) Feed: 500 to 4000 mm/min, in particular 2000 to 3000 mm/min

(25) Lateral feed ae: 0.2 to 3 mm, in particular 1 mm to 2 mm

(26) Depth feed ap; 0.1 to 2 mm, in particular 0.5 mm to 1 mm

(27) Cutter speed: 10,000 to 50,000 l/min, in particular 10,000 to 20,000 l/min

(28) In particular, the milling tool should be a carbide cutter.

(29) Milling parameters to be considered for the precision machining:

(30) Cutter diameter: 0.3 to 1.5 mm, in particular 0.5 to 1.0 mm

(31) Feed: 300 to 2000 mm/min, in particular 800 to 1500 mm/min

(32) Lateral feed ae: 0.2 to 0.6 mm, in particular 0.1 mm to 0.2 mm

(33) Depth feed ap: 0.05 to 0.3 mm, in particular 0.1 mm to 0.15 mm

(34) Cutter speed: 20,000 to 60,000 l/min, in particular 25,000 to 35,000 l/min

(35) In particular, the milling tool should be a carbide cutter.

(36) Preferably one uses a radius cutter of carbide that may be coated with titanium nitride. In this, the following cutting edge angles represent preferred values:

(37) Cutting angle: 0° to −13°, in particular −9° to −11°

(38) Clearance angle: 0° to 15°, in particular 11° to 13°

(39) Wedge angle: results from: 90° minus clearance angle minus cutting angle

(40) Because of the density of the blank and the crystalline fraction, it becomes possible to easily produce dental molded parts with filigree edges. For crowns in particular, it has been found that this results in stably extending edge thicknesses between 0.05 mm and 0.4 mm.

(41) After the cutting work, the molded part created from the blank should be referred to as pre-form part, since it exhibits an oversize compared to the dental molded part after complete sintering in accordance with the shrinkage characteristic of the blank material. The oversize is calculated in dependence on the density of the blank, in order to provide a high-precision dental prostheses after the final sintering.

(42) The sintering to final density takes place at a temperature T.sub.2 between 800° C. and 1050° C. for the duration of a holding time t.sub.2 between 5 min and 60 min. Holding time in this regard means that the blank is kept at this temperature during the final sintering stage.

(43) For the final sintering, the pre-form part is arranged on a fire-proof base, such as firing pads, or on free-of-scale metal layers. Support structures are not required, since the dimensional stability is guaranteed by the preceding crystallisation of the original powder material.

(44) The following exemplary embodiments illustrate further characteristic features of the invention, whereby the listed parameters are of particular significance on their own but not necessarily in combination:

(45) 1. Producing a Disk-Shaped Blank

(46) A quantity of 230 g pre-crystallized glass-ceramic powder, which also contains lithium silicate crystal, with a composition (in % by weight):

(47) TABLE-US-00016 SiO2 58-60 Li2O 13.5-20.5 ZrO2  8.5-11.5 P2O5 3.0-7.5 A12O3 0.5-6.0 K2O 0.5-3.5 CeO2 0.5-2.5 B2O3 0-3 Na2O 0-3 Tb4O7 .sup. 0-1.5 as well as 0 to 4 of at least one additive,
with a grain size distribution of d.sub.50=18.7 μm are pre-compacted at a pressure of 50 MPa by means of a tool with a diameter of 105 mm using a hydraulic press. Subsequently the pellet is introduced into a PE-coated pouch, which is evacuated and sealed watertight. The pellet is subjected to an isostatic re-compression at 290 MPa for 10 sec in a water-oil emulsion. The unpacking is followed by a thermal treatment and a partial sintering at 650° C. The blank density is 1.88 g/cm.sup.3.

(48) The final geometry of the blank is created by lathing to an outside diameter of 98.5 mm. A recess is lathed on each of the two front ends to facilitate acceptance into a milling machine.

(49) Into the blank surface with a circular geometry one nests dental molded parts with an appropriate sintering oversize. If crowns are the chosen molded part, they exhibit an excellent and fine crown margin and an outstanding milling surface.

(50) Sintering takes place in a dental furnace on Al.sub.2O.sub.3 firing pads with a multi-step sintering program over a total duration of 60 min. A multi-stage sintering program in this context means that holding times are provided for at least two different temperatures, so that these temperatures are maintained constant for the duration of the respective holding times. The maximum sintering temperature was 950° C., and was kept for a duration of 10 min. The subsequent evaluation of the crowns revealed an aesthetic visual appearance with a good dental fit.

(51) 2. Producing a Cuboid Blank

(52) A quantity of 9.6 g of pre-crystallized glass-ceramic powder with a composition (in % by weight):

(53) TABLE-US-00017 SiO2 58-60 Li2O 13.5-20.5 ZrO2  8.5-11.5 P2O5 3.0-7.5 A12O3 0.5-6.0 K2O 0.5-3.5 CeO2 0.5-2.5 B2O3 0-3 Na2O 0-3 Tb4O7 .sup. 0-1.5 as well as 0 to 4 of at least one additive,
with a grain size distribution of d.sub.50=21.3 μm is axially compressed under continuously rising pressure up to 120 MPa using a hydraulic press in a carbide press mould and is demolded under a suitable load of preferably 5 MPa. The resulting pellet possesses dimensions of 20.2×19.1×15.9 mm and a density of 1.56 g/cm.sup.3. Subsequently the pellet is subjected to two-stage thermal treatment at 630° C. and 700° C. in an elevator furnace. The blank density after the thermal treatment rose to 1.75 g/cm.sup.3.

(54) A mushroom-shaped adapter is glued to the narrow side of the blank to facilitate acceptance into a processing machine. The carving work on the dental crown that was oversized to compensate for sintering shrinkage took place using a special speed milling operation with significantly reduced cutting time using a cutting feed of up to 2000 mm/min. This represents a significant shortening of the cutting time in comparison to the part produced in example 1. The crown exhibited a smooth exterior and the crown margin was free from break-outs. The sintering took place on Al.sub.2O.sub.3 firing pads in a dental furnace with a stepped cycle with a total duration of 65 min and a maximum sintering temperature of 950° C. for 10 min. A subsequent evaluation of the crown revealed an aesthetic colour and a good dental fit.

(55) 3. Producing a Rod-Shaped Blank

(56) A quantity of 210 g of pre-crystallized glass-ceramic powder with a composition (in % by weight) of:

(57) TABLE-US-00018 SiO2 58-60 Li2O 13.5-20.5 ZrO2  8.5-11.5 P2O5 3.0-7.5 A12O3 0.5-6.0 K2O 0.5-3.5 CeO2 0.5-2.5 B2O3 0-3 Na2O 0-3 Tb4O7 .sup. 0-1.5 as well as 0 to 4 of at least one additive,
with a grain size distribution of d.sub.50=19.1 μm is compressed using a wet-bag press at a quasi-isostatic pressure of 195 MPa in a tubular polyurethane mould. The demolding is followed by a thermal treatment for additional crystallization at 620° C. and pre-sintering at 680° C. The final blank geometry is created by lathing to an outside diameter of 25 mm and a length of 198 mm. The blank possesses a density of 1.81 g/cm.sup.3

(58) From the face of rod-shaped glass-ceramic blanks one cuts dental crowns with an appropriate sintering oversize. The crowns possess a narrow crown margin free of break-outs and a good cutting surface. Sintering takes place in a small batch furnace on trays with Al.sub.2O.sub.3 firing pads. One employs a sintering program with an overall cycle time of 45 min. The maximum temperature of the sintering treatment is 980° C. The blank was kept at this temperature for 5 min. The completed crowns exhibit an aesthetic visual appearance and a good dental fit.