CA1292608C

CA1292608C - Process for producing a metallic dental prosthesis

Info

Publication number: CA1292608C
Application number: CA000517935A
Authority: CA
Inventors: Werner Groll; Josef Rothaut; Angela Klaus; Rudi Steinke
Original assignee: Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 1985-09-11
Filing date: 1986-09-10
Publication date: 1991-12-03
Anticipated expiration: 2008-12-03
Also published as: US4689197A; EP0214341A2; AU580687B2; JPS6260551A; AU6242486A; JPH0728879B2; EP0214341B1; DE3585536D1; DE3532331C2; DE3532331A1; EP0214341A3; ATE72964T1

Abstract

ABSTRACT OF THE DISCLOSURE
Faceable dental prostheses with metallic structural matrix may be produced by powder metallurgy without shrink-age and without pores when multimodal metal powder mixtures, when required with the addition of glass and ceramics powders, are converted with water into a slip, the dental prosthesis is moulded therewith and the slip mass is sintered at a tempera-ture at which the solidus temperature of at least one powder component is exceeded.

Description

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The present invention relates to a process for the powder-metallur~ical production of a dental prosthes1s face-able with ceramics or plastics and having a metallic structural matrix from a mixture of metal powders and, when required, glass or ceramic powders, which is prepared with a mixing liquid to a spreadable mass with which the dental prosthesis is moulded from ceramics by means of the technology conventionally used for dental ceramics on a mould of the teeth to be treated and serv-ing as a burn-on support and subsequently sintered on said mould.
The production of metallic prostheses for the treat-ment of dental disease or upon loss of one or several teeth, as for example, inlays, crowns faceable or non-faceable with cera-mics or plastics and bridges is usually carried out with the so~called "wax-coating process", a high-quality casting tech-nique which assures a high degree of accuracy to size.
Apart from the accuracy to the size, the advantages of the crowns and bridges thus produced lie primarily in the strength and the existing ductility which must be guaranteed for large bridgework construction to avoid stress fractures in case of overload. However, the process per se is very time-consuming and requires a lot of material and many instruments. The neces-sity of using cast-on ducts and casting cones causes a dist-inctly increased use of material as compared with the weight of the casting object. The repeated reuse of the material can result in changes in the properties of the alloy and if it is not reused it remains as scrap. A further disadvantage of this technique lies in that in the case of flaws in the casting object repairs are not possible and the entire production pro-cess beginning with the wax moulding must be repeated.

Another proces~ for producing jacket crowns reinforced ; with metal caps is described in EP-OS 0104320. A pre-shaped cap ~g ()8 provided with pleats and consisting of a metal foil which pre-ferably is constructed of several layers of diEferent metals is put on the mould of the prepared tooth stump and rotated thereto with a suitable tool. When annealing with a Bunsen burner the superposed pleats are welded and a metal cap having a wall thickness of approximately 100 ~m is obtained and this cap can then be faced with dental ceramics. As compared with the wax-melting process the expenditure of work and the costs of instru-ments actually are distinctly reduced but the dental prosthesis thus produced does not nearly attain the strength properties of a cast dental prosthesis so that the production of bridges by means of this process is not possible. Furthermore, in case of intensely worn stumps and in the case of large teeth, parti-cularly molars, the always identical thickness of the metal foil requires a very thick ceramic mould so that the danger of ceramic fracture, particularly in the region of adjacent teeth is very great.
A conventional method for producing fully ceramic crowns is the jacket-crown technique in which an aluminium-oxide-containing ceramics mass is coated on a Pt foil pre-moulded according to the shape of the total stump and sintered.
The crown is exposed and moulded by hand so that the entire equipment required for producing cast crowns is not needed. The moulding properties of the ceramics mass permit an exact repro-duction of the most complex tooth shapes. A substantial dis-advantage of this type of dental prosthesis is the friability of the ceramic material which results in catastrophic fractures in the case of momentary overload. The strength is not adequate either for producing thick-walled crowns and large bridges.
DE-OS 19 15 977 describes a process for producing dental prostheses which s~tarts from a metal or alloy powder having a particle size of 2 to 25 ~m. The powder is stirred l~Z~8 in-to a paste with the aid of a binder that is volatile below the sintering temperature. This paste is freely moulded with a spatula on a mould reproducing the tooth stump accurately to size and serving as a burn-on support, wherepuon the binder is expelled at elevated temperature and the metal particles are sintered together. However, this process has the disadvant-age that a high degree of compression of the metal powder mass is not attainable with the paste so that on sintering there results a relatively intense shrinkage. Therefore, the high degree of accuracy of fit required for a dental prosthesis cannot be attained with this process, not even when using very fine spherical powders which can be produced only with low yields and thus with high costs.
Therefore, the present invention provides a process for the production by sintering of a dental prosthesis faceable with ceramics or plastics and having a metallic structural matrix from a mixture of metal powders and, when required, glass or ceramics powders, which is prepared with a mixing liquid to form a spreadable mass with which the dental prosthesis is moulded from ceramics on a mould of the teeth to be treated and which serves as a burn-on support, using the conventional tech-nology of dental ceramics, and is subsequently sintered on the mould. In this process the shrinkage during the sintering pro-cess should be kept as small as possible in order to obtain an accurate fitting dental prosthesis which is substantially free from open pores and can be produced at a favourable cost.
According to the present invention this is achieved by the powder mixture having a multimodal particle size distri-bution of coarse and fine fractions, the particle size of the coarsest fraction not exceeding 100 ~m, this powder mixture being converted with wat~er into a slip and the sintering temperature of the slip mass is so selected that it exceeds the solidus temperature of at least one component of the powder mixture and in the case of intended facing with ceramics it lies at least 50C above the burn-on temperatures of the ceramics.
In the process according to the present invention a mixture of metal powder of the elements required for the desired alloy composition in the corresponding relative amount and/or alloy powder and, when required, ceramics powder, is mixed with water to form a slip consistency and moulding properties corres-pond to those of conventional dental and facing ceramics. To attain a gross density as high as possible and correspondingly a low degree of shrinkage it has been found that the use of powder mixtures with multimodal particle size distribution of the metal and ceramics powders is important and that powders having par-ticle sizes smaller than 100 ~m must be used. The proportion of ceramics powder can only be so selected that a metallic structural matrix is always assured. The slip stirred into a paste is moulded on a mould of the teeth to be treated - which is true to size - with the aid of conventional instruments and techniques used in dental ceramics and compressed by means of conventional techniques used in the production of ceramics teeth and ceramics facings (for example, by jarring with the grooved portion of a moulding instrument). During the compression process liquid is expelled whereby the powder particles are rearranged into more favourable positions and can draw together more closely. The mould used which consists of refractory ceramics is advantageously enlarged with suitable materials prior to the moulding process corresponding to the known shrinkage of the alloy and insulated against excessive moisture absorption.

On optimal gross density of the slip masses and a slight shrinkage on sintering is assured by these measures. In order to attain a high si~nter density, a sintering process in 6()~

which the sintering temperature lies above the solidus tem-perature of at least one component of the powder mixture but care must be taken that in the case of an intended ccramics facing the sintering temperature lies at least 50C above the burning temperature of the ceramics. This last condition must be satisfied in order to avoid a deformation of the metallic structure when burning on the ceramics. Depending on the alloy composition the sintering in air (for example, noble metals) is carried out under protective gas or vacuum. On completing the sintering process a sufficiently dense dental prosthesis with metallic structural matrix is obtained.
Multimodal means that powder mixtures having a par-ticle size distribution with several maxima at various particle sizes.
For attaining a high gross density of the slip masses powder mixtures whose coarse fractions are in the range of bet-ween 30 and 100~ m with a proportion by volume of 30 to 90% of the entire powder mixture and which have primarily the spherical or globular shape are particularly suitable. The shape of the fine particles (~50~ m) is optional per se but spherical or globular on platelet-like powders are preferred.
The powder component whose solidus temperature is higher than the sintering temperature of the slip mass are preferably added as coarse fractions while the powder com-ponents whose solidus temperature is lower that the sintering temperature of the slip mass are added as fine fractions. For example, when adding the higher melting component as the fine fraction then the formation of a rigid structure can result due to sintering the powder particles together on drying or on heating to the burning temperature. In that case a compression by regrouping the particles when sintering in the liquid phase can no longer be attained. The liquid phase forming on excee-lZ~2ti(~8 ding the liquidus temperature of the low-melting powder compo-nent penetrates the porous structure of the high-melting com-ponent so that the places previously occupied by it remain as pores.
Favourable sintering temperature for the production by sintering of dental prostheses are temperatures in the range of between the solidus temperature of the sintered alloy TSolidus and TSolidus minus 200C, but the limiting conditions that at least one powder component must have a solidus temperature lower 10 than the sintering temperature and that in the case of facing with ceramics the sintering temperature must be 50C above the burn-on temperature of the ceramics must be taken into account.
During the process the liquid phase can be entirely or partially consumed because of the occurring formation of the alloy. For the use of the described sintering temperatures in the range of between T lid and TSolidus minus 200 C it is a prerequisite that the pwoder mixture consists of at least metal or alloy powders having different solidus temperatures.
For powder mixtures whose different fractions con-20 sist of only one alloy a sintering temperature of betweenT and T . can be used with advantage. A portion of solldus llquldus the alloy then is present in the form of à molten phase corres-ponding to the phase relation solid/liquid. The liquid phase can then be present only to an extent such that the inherent stabi-lity of the sintering body is maintained during the sintering process.
Finished parts, as for example, wire sections or metal teeth, can be used for the productlon of bridges. When moulding the crown caps these finished parts can be slipped in and sintered thereto or soldered to the sintered mould caps.

Because of this measure ~a better accuracy of fit is attained since the intermediate members do not shrink on sintering. A

ti08 fur~her possibility of producing bridges lies in that the indi-vidual teeth - as well as the intermediate members - are pro-duced by means of the process according to the present invention and then soldered. In order to improve the accuracy of fit of the dental prothesis the moulded stump can be coated with a material burning without residue, as for example, wax, the thickness of the coating being so selected that the peripheral extension corresponds to the expected shrinkage of the slip mass on sintering. The slip is coated on this coating and compres-10 sed. The material burning~without residue is then burned out ata suitable temperature. In the sintering process the moulded body (for example, a crown) shrinks on the stump so that its shape is exactly reproduced.
Furthermore, prior to applying the slip the burn-on support can be coated with a metal whose melting point is higher than that of the alloy to be sintered. The slip is then coated on said mould stump thus prepared and compressed by liquid-phase sintering. The liquid phase wets the metallized mould stump and assures that the alloy adapts to the shape in the 20 region of the ceramics stump.
For producing the slip the metal powder mixtures are mixed with water, which preferably contains electrolytes, as for example, soda, sodium hydroxide or strontium chloride. Mono- or polyhydric alcohols can also be added to the water.
For the production of an accurately fitting dental prosthesis by sintering means, a green density of the slip mass as high as possible prior to sintering is important in order to minimize the shrinkage due to sintering. This is accomplished in that powder mixture of one or several metals or metal alloys 30 with bi- or multimodal particle size distribution are used, spherical globular platelet-like particles or particles of other shape can be used. A number of examples of powder combinations ; 7 -1;~9~08 have been listed in Table 1. Gold, platinum and palladium powders having varlous particle size distributions and particle shapes were used as mould powders. Purely sptlerical powders ~material 1) result in a higher green density than purely platelet-shaped powders (material 6). By adding further powders having a smaller particle size to multimodal powder mixtures a distinct increase in the green density is attainable. Slip masses whose coarse fraction consists of spherical particles (material 2 to 5) yield the best results.

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l~Ztj()8 Apar~ from the particle size distribution, the size and the shape oE the powders used the sintering tem~erature also is oE decisive importance for the attainable density and strength. The properties of the sintered alloy Au 50 Pt 35 Pd 15 when using various starting powder mixtures with multimodal particle size distribution after the sintering process have been listed in Table 2. The attainable density values and 0.2% ex-tension limits are favourable for use as crown and bridge material. The 0.2~ extension limits of conventional cast dental and bridge alloys also lie in the range of from 450 to 600 MPa.
The alloys thus produced show a closed porosity; this is impor-tant for avoiding plaque deposits and points of attack of cor-rosion. For the alloys 5 and 7 three-point bending tests were carried out on test specimens having a length of 35 mm and a cross section of 3 x 3 sq. mm, the extension limits obtained in the bending test corresponding to those of the pressure tests.
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6()~3 The following Examples will illustrate th~ process accordlng to the present invention:
Example 1 From a master mould a duplicate is produced from a high temperature-resistant, castable ceramics. A wax cap whose wall thickness is approximately 0.3 mm is moulded on the mould stump. The wax satisfies the function of insulation against the mould stump on the one hand and serves for enlarging the mould stump to compensate for shrinkage during sintering on the other.
The wax cap is shaped from a wax tablet (thickness 0.3 mm) or by using a dipping wax.
A slip containing 10~ by volume of TiO2 and 90% of a metal powder mixture is spread on the mould stump thus prepared.
The powder mixture consists of 74.4~ by weight of Au powder (spherical) ~ 90 ~m, 18.6~ of Au powder~platelet-shaped) ~ 10 ~ m and 7~ by weight of Pt powder (platelet-shaped) --15 ~m.
Water with 0.5 g/litre of strontium chloride issued as the mix-ing fluid. This slip has properties corresponding to those of dental ceramics slips. With the technique conventionally applied to dental ceramics and with the instruments used for this purpose ~brush, spatula, grooving part, etc.) a faceable crown cap is constructed with the slip. On completing the moulding process the entire assembly is kept in a waxing oven at 200C for 30 minutes. During this time the wax burns out free from residue. The dewaxed assembly is then put into the drying chamber of a ceramics-burn-on oven and dried at 600C for 15 minutes, whereupon it is transferred to the combustion chamber preheated to 1200C, where it is sintered for 15 minutes.
After the sintering process the crown cap cools in air and can then be removed from the burn-on support. The ceramics facing is applied directly to the sintered cap in a conventional 6()8 manner without intermediate treatment. 'I'he crown thus produced has a metallic matrix and a high accuracy of fit associated with a high degree of strength.
Example 2 The production and preparation of the mould stump are carrled out as in Example 1. The slip stirred with water into a paste also consists of 10~ by volume of TiO2 and 90% by volume of a metal powder mixture which is in turn composed of 65.1~ by weight of Au powder (spherical) of the fraction 36-25 ~, 27.9%
weight of Au powder (platelet-shaped)~25 ,~m and 7~ by weight of Pt powder (platelet-shaped) C 15 ~m. A crown with bite sur-face is moulded with this slip, using the conventional technique of ceramics facing. Because of the outstanding mouldability of the slip finenesses of the occlusal surfaces can be perfected.
The operating steps, dewaxing, drying and sintering are carried out as in Example 1. On completion of the sintering process the crown is removed from the burn-on support, the surface is ground until it is smooth and then polished. This crown also has a high accuracy of fit. No pores can be detected.

Claims

1. In a process for the production of a denture, having a metallic microstructure, using sintering technology, from a mixture of metal powders, which mixture is treated with a mixing fluid to form a spreadable mass with which the denture is modelled on a ceramic model of the prepared tooth which is used as bat and then sintered comprising employing a metal powder mixture having a multimodal size distribution of coarse and fine fractions in which the particle size of the coarsest fraction does not exceed 100 microns; converting this powder mixture with water to a slip sintering the slip and selecting the sintering temperature of the slip mass such that it exceeds the solidus temperature of at least one component of the powder mixture and in the case of planned veneering with ceramic exceeds the baking-on temperature of the ceramic by at least 50°C.

2. A process according to claim 1 wherein the denture is veneered with a ceramic or plastic.

3. A process according to claim 1 wherein the powder mixture consists of metal powder.

4. A process according to claim 1 wherein the powder mixture in addition to metal powder contains a minor amount by weight of glass or ceramic powder.

5. A process according to claim 1 wherein th coarse fraction has a particle size between 30 and 100 microns and is of spherical shape and the fine fraction has a particle size below 50 microns and is of any shape, the percent by volume of the coarse fraction being 30 to 90%.

6. A process according to claim 5 comprising adding the powder component having a solidus temperature below the sintering temperature of the slip composition as a fine fraction.

7. A process according to claim 1 comprising adding the powder component having a solidus temperature below the sintering temperature of the slip composition as a fine fraction.

8. A process according to claim 7 wherein the slip composition contains powders of a plurality of metals and the sintering temperature lies in the range between TSolidus minus 200°C. and TSolidus °f the sintered metal but is also above the solidus temperature of a least one of the powder components.

9. A process according to claim 6 wherein the slip composition contains powders of a plurality of metals and the sintering temperature lies in the range between TSolidus minus 200°C. and TSolidus of the sintered metal but is also above the solidus temperature of at least one of the powder components.

10. A process according to claim 5 wherein the slip composition contains powders of a plurality of metals and the sintering temperature lies in the range between TSolidus minus 200 C. and Tsolidus of the sintered metal but is also above the solidus temperature of at least one of the powder components.

11. A process according to claim 1 wherein the slip composition contains powders of a plurality of metals and the sintering temperature lies in the range between TSolidus minus 200 C. and Tsolidus °f the sintered metal but is also above the solidus temperature of at least one of the powder components.

12. A process according to claim 7 wherein the slip composition contains powders of only one alloy and sintering temperature lies between the solidus and liquidus temperature of the sintered metal.

13. A process according to claim 6 wherein the slip composition contains powders of only one alloy and sintering temperature lies between the solidus and liquidus temperature of the sintered metal.

14. A process according to claim 5 wherein the slip composition contains powder of only one alloy and sintering temperature lies between the solidus and liquidus temperature of the sintered metal.

15. A process according to claim 1 wherein the slip composition contains powders of only one alloy and sintering temperature lies between the solidus and liquidus temperature of sintered metal.

16. A process according to claim 13 comprising sintering a low melting metal or a low melting alloy for finishing a surface and closing pores onto the surface of the sintered slip composition.

17. A process according to claim 9 comprising sintering a low melting metal or low melting alloy for finishing a surface and closing pores onto the surface of the sintered slip composition.

18. A process according to claim 1 comprising sintering a low melting metal or a low melting alloy for finishing a surface and closing pores onto the surface of the sintered slip composition.

19. A process according to claim 17 comprising applying a layer consisting essentially of a material which burns without residue to the model used as a bat according to shrinkage during sintering.

20. A process according to claim 13 comprising applying a layer consisting essentially of a material which burns without residue to the model used as a bat according to shrinkage during sintering.

21. A process according to claim 1 comprising applying a layer consisting essentially of a material which burns without residue to the model used as a bat according to shrinkage during sintering.

22. A process according to claim 1 wherein the metal is an alloy.