Metal founding – Means to apply vacuum directly to work or to hold or... – Means applying vacuum or suction directly to molten casting...
Reexamination Certificate
2001-08-08
2002-12-03
Lin, Kuang Y. (Department: 1725)
Metal founding
Means to apply vacuum directly to work or to hold or...
Means applying vacuum or suction directly to molten casting...
C164S259000, C164S338100
Reexamination Certificate
active
06488074
ABSTRACT:
The present invention relates to a casting apparatus for manufacturing dental prostheses, such as inlay, crown, base, implant, and upper parts of implants, from precious metals or non-precious metals.
BACKGROUND OF THE INVENTION
A loss of some or all of the teeth as a result of caries (tooth decay), periodontal disease or the like, causes not only functional declination in speech and chewing and/or a change in the facial appearance but also a bad influence on the health of the whole body. It is therefore important to undergo treatment as soon as possible in order 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 the teeth shape is unique for each patient and further the size and shape of the missing part differs depending on the case. Therefore, it is necessary to manufacture prosthesis having a particular shape depending on the case of each patient. Also, the prosthesis must be manufactured with a high degree of accuracy 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. 19
is a flow chart showing the process of dental casting according to the lost wax process. Referring to
FIG. 19
, the steps of manufacturing prosthesis used for dental treatment are described. 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 desired type of casting model, such as an inlay or crown, using wax or resin for 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 pushed into a crucible former made of rubber (Step S
5
).
FIG. 20
is a front view of a casting model mounted on a commonly used crucible former. The crucible former
90
has a conical base
91
formed at its center, and a hole
92
for inserting a sprue wire
94
is formed on the top of the conical base
91
. The hole
92
is filled with softened wax, and the free end of the sprue wire
94
(to which the casting model
93
is attached) is inserted in the soft wax. When the wax solidifies, the casting model
93
is fixed on the top of the conical base
91
with the sprue wire
94
.
A metallic cylindrical ring (not shown) is fitted onto the crucible former
90
so that the casting model
93
is surrounded by the ring, and investment material such as gypsum or phosphate is poured into the metallic ring to conceal the casting model
93
(Step S
6
).
After the investment material is solidified, the crucible former
90
is removed, the sprue wire
94
is pulled out, and the ring is heated to a high temperature. By heating, the wax inside is burned off, leaving a cavity corresponding to the sprue wire
94
and the casting model
93
. Thus, a mold is obtained (Step S
7
).
When the mold is heated 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. The molten metal flows into the cavity through the sprue runner. This is so-called pouring work (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 are removed from the casting, and after-treatments such as sanding the surface of the cast are carried out (Step S
10
). Thus, the prosthesis is completed.
In general, the above-described manufacturing work is carried out by a dental technician. Conventionally, in the work of Step S
7
, an electrical furnace, called a “ring furnace”, is used to heat the ring to burn off the wax and to heat the mold to a preset temperature. The pouring work of Step S
8
is carried out, for example, using a pressure casting apparatus with an inverting casting chamber. An example of this type of casting apparatus is disclosed in Japanese Unexamined Patent Publication No. 2000-176629.
The above type of casting apparatus includes a chamber rotatable within 180 degrees, in which a crucible and a mold are oppositely positioned so that the top of the crucible and the reservoir of the mold face each other. With this casting apparatus, the pouring work is carried out as outlined below. First, posing the crucible with its open top directed upwards, alloy ingots are put in the crucible, and the crucible is heated to melt the alloy. Next, the mold, having been heated to about 800-900° C. (degrees Celsius), is placed above the crucible so that the reservoir is directed downwards. Then, the chamber is closed tightly and evacuated with a vacuum pump. As a result, the pressure in the cavity of the mold decreases. After that, the casting chamber is turned upside-down at a preset timing so that the open top of the crucible is directed downwards. Then, the molten metal in the crucible is poured into the reservoir of the mold. The molten metal closes the sprue gate of the sprue runner, so that the pressure in the cavity of the mold is kept low. After that, the evacuation of the casting chamber is stopped, and pressurized air, inert gas or another kind of gas is supplied to increase the pressure in the chamber. The pressure difference between the chamber and the cavity of the mold forces the molten metal to flow through the sprue runner into the cavity of the mold. Thus, the pouring of the molten metal is completed.
With the above type of casting apparatus, it is necessary to carry out the pouring work immediately after the alloy in the crucible has been melted. This is because keeping the metal in the molten state over an excessive length of time will cause oxidization or other quality changes in the metal, which will make the product defective. Therefore, after the metal has been melted, it is necessary to take out the heated mold from the ring furnace and set it in the casting chamber as soon as possible to start the pouring work as described above. As for the ring furnace, the burning needs to be carried out for one to a few hours at about 700-800° C. of temperature. Therefore, it is necessary to put the ring in the furnace to start the burning at an appropriate timing, calculating back from the timing of pouring the molten metal.
Thus, being required to manufacture various forms of prostheses, dental technicians spend much time and labor carrying out the casting work as described above.
Further, the temperatures and time periods for burning the ring and melting the metal must be appropriately determined depending on the selection of investment material and alloy material, because a desired quality of prosthesis cannot be obtained when the settings are inappropriate. Conventionally, however, not a few pieces of prosthesis result in being defective (e.g. missing a part) as a result of inappropriately setting the apparatuses, wrong order of work and/or accidental omission of work. This is inevitable when the casting work is manually carried out as described above.
To solve the above problems, the present invention proposes an apparatus for casting dental prosthesis, which greatly improves the efficiency of the casting work while preventing wrong operations in the pouring work.
SUMMARY OF THE INVENTION
Thus the apparatus for casting dental prosthesis according to the present invention includes the following elements:
a) a ring-placing platform on which a cylindrical ring with a casting model of a thermally subliming material concealed inside may be placed;
b)
Denken Co., Ltd.
Lin Kuang Y.
Oliff & Berridg,e PLC
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