Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
METHOD FOR PRODUCING CASTING MOLDS
FIELD OF THE INVENTION
The present invention relates to the production of custom-
made casting molds. More specifically, the present invention is concerned
with a method used to cast molds for dental prostheses and to such
molds.
BACKGROUND OF THE INVENTION
The conventional approach to produce custom casting
molds has traditionally been the lost wax casting process. This process
is based on the following principle:
~ A model or pattern of the desired finished product is
made in wax;
~ This pattern is mounted with sprues and reservoirs to
support the pattern in the desired casting position, provide passageways
for wax elimination and form lines through which molten metal enters the
mold;
~ This pattern is surrounded by a creamy investment
plaster that hardens to form a mold;
~ This mold is heated to melt away the wax pattern that is
then "lost";
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~ Metal is then cast into the cavity left by the "lost wax",
thus duplicating the original wax pattern (the quantity of metal is calculated
based on measurements of either the weight or the volume of the wax
pattern);
~ The mold is then destroyed to recover the casting.
This process dates back in time beyond Egypt's pyramids.
Examples of castings include the Shang Dynasty in China, 16th Century
Europe and the Aztecs of Mexico, to name a few, are monuments to the
abilities of casters down through the ages. Today, the process is
becoming even more popular because of the need for a near-finished
casting.
Refinements of this process were developed by a dentist in
1907, and applied to the casting of gold inlays and dental bridgework.
Today's craftsmans apply the very same techniques in making parts for
models, fishing lures, specialized machine components and endless
pieces of jewelry.
Since every detail created in the surface of the finished wax
20 model will ultimately be re-created in metal when the casting is complete,
special care taken in making the wax pattern can minimize the finishing
and polishing steps later.
To produce dental prostheses, the dental laboratory
technician typically duplicates the patient teeth in plaster from an
impression provided by a dentist, then uses the lost-wax method to realize
a casting mold, then invests the mold in the casting oven filling it with
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metal in fusion to build the substructure components of dental prostheses,
then porcelain is applied and fused on the metal substructure to complete
the prosthetic work. The casting oven either uses the centrifuge effect
principle or a vacuum chamber.
The lost-wax method to realize a casting mold therefore
generally consists of the following steps:
1. Apply die spacer on the surface of the plaster die to
emulate the cement space;
2. Realize the prosthesis out of wax (by dipping and wax-up
technique);
3. Add wax pouring sprue (off-the-shelf item) on top of the
wax prosthesis;
4. Add several prosthetic elements with pouring sprues on
top of an investment tree (off-the-shelf element or not) with an adequate
casting reservoir;
5. Add wax cooling sprues and vent sprues (off-the-shelf
items) when desirable;
6. Install the wax ensemble in an investment cylinder;
20 7. Invest the ensemble with a refractory material (heating
will eliminate the wax and result in a negative mold of the ensemble:
casting reservoir, tree, sprues, prosthesis).
However, the lost-wax method is labor-intensive and also
25 requires dexterity to manipulate this fragile wax structure without
breaking
it, twisting it or otherwise distorting it. Furthermore, the use of out-the-
shelf items constrains the design of the casting mold.
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The computer-aided design and manufacturing of the
prosthesis is an alternative method for producing dental prosthesis. This
method is typically used in conjunction with computer-controlled milling
machines. Blanks are milled into prostheses. However, with this process,
a large proportion of the blank material is rejected. When the prosthesis
substructure is composed of a precious alloy, the scrap metal represents
more value than the prosthesis itself.
OBJECTS OF THE INVENTION
An object of the present invention is therefore to provide
an improved wax casting mold and method for making same.
Other objects, advantages and features of the present
invention will become more apparent upon reading of the following non-
restrictive description of preferred embodiments thereof, given by way of
example only with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the appended drawings:
Figure 1 is a perspective view of three wax models
according to the present invention used to produce casting molds;
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Figure 2 is a side elevational view of a wax model
according to the present invention, including several prosthesis; and
Figure 3 is a bottom plan view of the wax model of Figure 2.
5 DESCRIPTION OF THE PREFERRED EMBODIMENT
According to the present invention, there is provided a
method to design and manufacture casting mold for patterns, particularly
dental prostheses and to wax models to create such casting molds. This
method will be referred herein as the WaxproT"" system.
Generally stated, the method of the present invention
comprises following steps:
1. Designing the prosthesis using computer-aided design
(CAD) tools such as Cynovad's Pro 50T"" scanner and CAD software;
2. Designing the conduits and reservoirs configuration as
well as the mold container using CAD tools to support multiple patterns in
the desired casting position, provide passageways for wax elimination and
form lines through which molten metal enters the mold;
3. Realizing the designed structure using a low fusion
temperature material such as wax and a rapid prototyping machine such
as that of ThermojetT"" by 3D System Inc.;
4. Invest the ensemble with a refractory material (heating
will eliminate the wax and result in a negative mold of the ensemble:
casting reservoir, tree, sprues, prosthesis);
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Each of these steps will now be described in more detail.
In step 1, quantitative data on shape of the patient's teeth is
5 advantageously acquired by a digital 3D scanner either directly in the
patient's mouth or on a model made from an impression made by the
dentist; then the morphology of the prosthesis is advantageously produced
using CAD software. In particular, the regularity of the coping thickness
obtained with machine precision ensures that structural specifications are
met with the minimal quantity of metal and no rework.
The digital measurements of the shape of teeth, such as
those obtained by devices and methods described in United States
Patents Nos 4,611,288, 4,663,720, 4,742,464, 4,952,149, 5,092,022 and
5,237,998which are herein included by reference. It is to be noted that
other devices may also be used without departing from the spirit and
nature of the present invention.
In step 2, the positioning of the prosthetic elements, the
design of the conduits and reservoirs configuration given the custom-made
prostheses to invest, and the casting mold external shape is
advantageously produced using a computer-aided design software tool.
The conduits and reservoirs are custom shaped and positioned to
minimize the quantity of metal needed while improving the invested metal
flow in order to fill completely the prostheses and allowing the
crystallization to begin simultaneously where the highest precision is
needed, thus ensuring the quality of the prosthetic work. The design is
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optimized given the fluid dynamics properties of the chosen metal alloy.
The computer simulation of casting process provides the preferred
analysis tool. The manufacturing of the ensemble (the pattern, the wax
conduit and reservoir elements, the mold external wall) in a single piece
allows for the precise placement of tiny fragile vent sprues close to the hot
metal mass and cooling sprues close to the mold external wall.
Furthermore, the pouring sprue leading to each prosthetic element can
automatically be marked to identify it, as well as the casting cone and
mold external wall. Finally, the mold external wall can be shaped and/or
marked to indicate the optimal orientation when place in the casting oven.
In step 3, the designed structure combining the prostheses,
the conduits and reservoirs configuration, and the external wall of the mold
is advantageously realized using a process such as the rapid prototyping
15 machine of 3D System Inc.. It is to be noted that other devices may also
be used without departing from the spirit and nature of the present
invention.
Step 4 is the same as the typical approach used by dental
20 technician to produce a casting mold that is investing the designed
structure with a refractory material. Vibration is used to facilitate the
flow.
Heating eventually eliminates the wax and result in a negative mold of the
casting reservoir, sprues and prosthesis.
25 The appended figures illustrate realizations of wax
models for dental prosthesis according to the present invention.
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More specifically, Figure 1 illustrates three wax models
constructed according to the principle of the method of the present
invention in perspective view. These wax models are intended to be used
to create casting molds.
5 Figures 2 and 3 respectively illustrate side elevation and
bottom plan views of a wax model 10 comprising a plurality of prostheses
12 interconnected by connectors 14. The prostheses 12 are mounted on
individual sprues 16 connected to a reservoir 18 itself connected to a
master inlet 20 via primary sprues 22.
10 As will easily be appreciated by one skilled in the art, the
use of a computer modelling tool allows the design of custom investment
tree suited for the number, dimensions and position of the dental
prosthesis. Furthermore, it allows a perfect positioning of the prostheses
with respect to the reservoir to thereby improve the success rate of the
15 casting steps.
An advantage of the WaxProT"" system compared to the
traditional way is the time saving linked with the automation of many steps
involved in the process that are carried in parallel: for example, it is
20 possible to start scanning the next batch of prostheses and creating the
CAD models while the waxing machine is making previous ones. A waxing
machine, like the ones used in the rapid prototyping field, is also capable
to make several ensembles (conduits, reservoirs, and prostheses) in a
single session.
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Another advantage of the WaxProT"" system is that it does
not require the dental laboratory technician to go through a long and
tedious manual procedure in which the manipulation of fragile wax pieces
can be awkward and yield a significant percentage of costly remakes. The
5 computer-aided design of the conduits and reservoirs ensemble minimizes
the quantity of precious alloy needed, thus cost, and it prevents cavities
resulting from bubble formation during the casting step thus avoiding
another cause of remakes. The CAD system automatically provides the
precise quantity of metal needed, time is saved. Yet another advantage
of the WaxproT"~ system is the referencing of the different prosthetic
elements and the different casting mold; this feature also to save time
after and ease quality control.
Finally, this system gives a more controlled result in, for
15 example, the regularity of the thickness of the prosthesis obtained. The
resulting quality saves time and precious metal after by avoiding lengthy
polishing step before applying the porcelain.
To design the wax model, a digital library of predefined
investment trees is used. Of course, the system allows the user to create
his own tree via a "tree edition software" where the user can change
different parameters of dimension, diameter of the different parts of the
tree, size of the reservoir, so it is customized every time depending on the
shape of the parts (simple copings, bridges, etc... ) and the metal that will
be used for casting. An expert system may also be used to determine the
optimum dimensions of these elements according to the prosthesis to be
made.
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Although the present invention has been described
hereinabove by way of preferred embodiments thereof, it can be modified,
without departing from the spirit and nature of the subject invention as
defined in the appended claims.
