Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1999-11-22
2002-05-14
Elve, M. Alexandra (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S116000, C164S062000, C164S457000, C164S155400
Reexamination Certificate
active
06386265
ABSTRACT:
The present invention relates to a method of manufacturing dental prostheses, such as an inlay, crown, base, implant, and upper part of implant, from precious metals or nonprecious metals by pressure casting. The present invention also relates to an apparatus for casting dental prostheses.
BACKGROUND OF THE INVENTION
If a person loses a part or whole of the teeth as a result of caries (tooth decay), periodontal disease or the like, the person suffers not only functional declination in speech and chewing and/or a change in the facial appearance, but also the health of the whole body is influenced badly thereby. It is therefore important to undergo a treatment as soon as possible to restore the missing tooth (or teeth). According to one of the known restoration methods, a metallic casting is put in place of the missing part of the teeth. Dental prostheses for restoring missing parts of teeth, however, cannot be mass-produced because there is a significant individual difference in the shape of teeth, and the size and shape of the missing part differs depending on the case. Therefore, it is necessary to manufacture a prosthesis having a particular shape depending on the case of each patient. Also, it is necessary to manufacture the prosthesis with a high degree of accuracy in order to provide a correct occlusion. Thus, in the field of dental casting, the lost wax process, which is known for providing a high degree of accuracy of casting, is generally used for obtaining castings that meet the above demands.
FIG. 6
is a flow chart showing the steps of dental casting according to the lost wax process. Referring to this
FIG. 6
, the steps of manufacturing a prosthesis used for dental treatment is described. At first, a dentist takes a negative impression model of the mouth and teeth around the object part of a patient (Step S
1
). A dental technician pours modeling material, such as gypsum, into the negative impression, and solidifies the material to produce a positive model (Step S
2
). The dental technician forms a casting model of the object part such as an inlay or crown using wax or resin on the positive model (Step S
3
). A sprue wire for forming a sprue runner is attached to an appropriate part of the casting model with wax or the like (Step S
4
). After that, the casting model is detached from the positive model, and the free end of the sprue wire is stabbed on a crucible former (Step S
5
).
FIG. 7
is a front view of a casting model mounted on a commonly used crucible former. The crucible former
60
has a conical base
61
formed at its center, and a hole
62
for inserting the sprue wire
64
is formed on the top of the conical base
61
. The hole
62
is filled with softened wax and the free end of the sprue wire
64
(to which the casting model
63
is attached) is inserted in the soft wax. When the wax solidifies, the casting model
63
is fixed on the top of the conical base
61
with the sprue wire
64
.
A metallic cylinder (not shown) is fit on the crucible former
60
so that the casting model
63
is surrounded by the cylinder, and investment material such as gypsum or phosphate is poured into the cylinder to conceal the casting model
63
(Step S
6
). After the investment material is solidified, the crucible former
60
is removed, and the investment material is heated (Step S
7
). By heating, the wax inside is burned off, leaving a cavity corresponding to the sprue wire
64
and the casting model
63
. Thus, a mold is obtained.
Heating the mold to a preset temperature, molten metal is poured into a reservoir at the top of the mold, which is a conical depression having a shape corresponding to the conical base of the crucible former, and the molten metal flows into the cavity through the sprue runner (Step S
8
). After the poured metal has cooled down and solidified, the mold is broken to take out the casting inside (Step S
9
). Then, unnecessary parts such as fringe metals along the sprue runner is removed from the casting, and after-treatments such as sanding of the surface is carried out (Step S
10
). Thus, a prosthesis is completed.
For assisting molten metal to flow smoothly into the cavity in Step S
8
, one of the following three methods is generally used. The first is the centrifugal casting wherein the mold is revolved around an axis so that the molten metal is pressurized into the mold by the effect of the centrifugal force. The second is the pressure casting wherein the molten metal is at first poured into the reservoir under a vacuum, and the pressure is then increased so that the molten metal is forced into the cavity due to the pressure difference between the cavity and the outside (this process is referred to as “pressurizing operation” in this specification). The third is the vacuum casting wherein the molten metal poured into the reservoir is introduced into the cavity by evacuating the cavity from the other side.
Japanese Unexamined Patent Publication Nos. H06-126422 and H07-132364 disclose conventional casting apparatuses utilizing the pressure casting. The apparatus disclosed therein has a casting chamber in which a crucible and a mold are oppositely positioned across a horizontal axis so that the top of the crucible and the reservoir of the mold face each other. The casting chamber is rotatable about the horizontal axis to turn the posture of the casting chamber, or the relative position of the crucible and the mold, upside-down. At first the casting chamber is held in a position where the open top of the crucible is directed upwards, alloy ingots are put in the crucible, and the crucible is heated to melt the alloy. The mold located above the crucible is heated to a temperature of about 800-900 C (degrees Celsius), and the casting chamber is evacuated with a vacuum pump. At a preset timing when the pressure in the cavity inside the mold decreases adequately, the casting chamber is turned upside-down. The open top of the crucible is now directed downwards, and the molten metal in the crucible is poured into the reservoir of the mold. The molten metal closes the mouth of the sprue runner in an airtight manner. After that, the evacuation of the casting chamber is stopped, and the pressure in the casting chamber is increased by supplying gas such as pressurized air or inert gas, while the pressure in the cavity of the mold is maintained low. Due to the pressure difference, the molten metal is forced to flow through the sprue runner into the cavity of the mold. Thus, the pouring of the molten metal is completed.
In the above-described pressure casting process, if the pressurizing operation is started before an appropriate amount of molten metal is poured in the reservoir, the gas used for increasing the pressure in the chamber intrudes into the cavity, which results in defects in the casting. For preventing this, it is desirable to start the pressurizing operation after an adequate amount of molten metal is poured in the reservoir.
When an alloy having a high melting point (usually over 1000 C), such as nickel-chromium alloy or cobalt-chromium alloy, is used as the casting material, the following problem must be considered. Even when a mold is made of an investment material durable to high temperature casting (e.g. material of the phosphate group), the highest allowable temperature is about 900 C. This means that, in casting, the temperature of the mold is normally lower than the melting point of the molten metal. Therefore, it is probable that the molten metal solidifies before the pressurizing operation starts since the temperature of the molten metal decreases rapidly after the molten metal is poured into the mold. Even if the molten metal does not solidify, casting defects due to an inadequate pouring is likely to occur since a lowered fluidity of the molten metal prevents it from flowing smoothly.
When the temperature of the molten metal is set higher, the time required for the solidification of the molten metal poured into the mold becomes longer, and the casting workability is enhanced. When, however, the temperature is too high, the molte
Denken Co., Ltd.
Elve M. Alexandra
Oliff & Berridg,e PLC
Tran Len
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