Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Making composite or hollow article
Patent
1998-05-11
1999-06-01
Mai, Ngoclan
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Making composite or hollow article
419 27, B22F 700
Patent
active
059096128
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for manufacturing a dental restoration, such as a crown or a bridge. In particular, the invention relates to properly fitting, functional metallic restorations manufactured by sintering metal powders on a refractory material. In such methods it is usual that first a chin layer consisting of a mixture of fine metal powder and a glass fraction is applied to the refractory base. Then a composition of a desired metal alloy in powdered form and a thermoplastic binder is applied, e.g. using a heated wax molding instrument. Finally, a number of heating steps is carried out, resulting in a dental restoration at least partially coated with an alloy of the metal powder.
The conventional method commonly used method heretofore for manufacturing dental restorations from metal, which may optionally be coated with plastic or porcelain, comprises casting a dental alloy according to the so-called "lost wax method". In particular, this method, in which an alloy is poured into a refractory die which has the shape of a burned-out wax model, is characterized by the following steps. In the first step, an impression is taken of the preparation in the mouth using a molding mass, usually consisting of silicone material. In the second step, a positive cast in gypsum is made from this impression. The third step comprises molding a wax model on the gypsum model, which wax model is provided in a fourth step with supply ducts of wax and is embedded in a refractory mass. This die of refractory material is heated to a temperature of 600-1000.degree. C. in step 5. Step 6 consists of pouring a liquid metal into the cavity formed. During cooling, the cast suffers thermic shrinkage, which must be compensated for by the refractory mass, by expansion occurring during heating of the die. Finally, the cast workpiece is finished off in step 7 with ceramically bound flints or hard metal cutters, optionally followed by coating the workpiece with plastic or porcelain.
In more recent developments, which will hereinafter be discussed more extensively, process steps 2-7 can advantageously be replaced by the following three steps.
The positive may be cast in refractory material, instead of in gypsum, and the model can then be burned out at a temperature of 1000-1200.degree. C. The refractory material may consist of a mixture of quartz sand with a given granule distribution and magnesium oxide and biammonium phosphate as binders. This powdered refractory material mixture is mixed with collodial silica to form a slurry, which slurry is poured into the impression of the above step 1, where the slurry binds to form a hard mass within 5-10 minutes.
Now a powdered metal is applied to the resulting refractory model in the locations where such is considered necessary for reasons of reinforcement.
Finally, the powder applied is sintered at a temperature of 1000-1300.degree. C. to form a solid metal mass.
To prevent oxidation, the last step is carried out under a vacuum, which can be carried out only in very special oven designed for the purpose, as described in U.S. Pat. No. 4,702,696. The special oven is so expensive that the method described is not economically feasible for many dental laboratories; the high costs make the process unattractive for implementation in laboratories.
U.S. Pat. No. 3,502,466 describes the making of dental crowns using metal powders and in particular precious metal powders. The metal powder used is first mixed with a binder to form a paste or putty. The binder to be used consists of an adhesive, such as ethyl celluose, and a solvent, e.g. propylene glycol. This binder is liquid which renders molding laborious and the chances of residual undesirable porosity in the final product are substantial.
U.S. Pat. No. 4,661,071 describes a procedure using powders or mixtures of powders that are used according to the more recently developed process for manufacturing dental restorations. The process described has a number of disadvantages. Thus, for instance, a costly high-vacuum oven must be used, which is not
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Elephant Edelmetaal B.V.
Mai Ngoclan
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