Note: Descriptions are shown in the official language in which they were submitted.
212~880
TITLE OF THE lNv~N.ION
Injection molding method and apparatus for
preparing dental prosthesis and medical devices.
FIELD OF THE lNv~NlION
The present invention relates to a transfer
molding station and diaphragm cartridge for use in
preparing dental prosthesis and medical devices in
general. In addition, the invention includes a method
for preparing the prosthesis.
BACRGROUND OF THE lNv~N.ION
Conventional denture manufacturing devices and
methods suffer from inadequacies and drawbacks.
Conventional materials are made by manually mixing a
powder of polymethylmethacrylate with liquified
methylmethacrylate monomer in order to obtain a plastic
(dough) material. These materials can be processed using
compression and/or injection molding systems. Various
methods can be used to cure the material: self-curing,
heat-curing in hot water, light-curing and microwave
curing.
The disadvantages of these systems are due to,
among other things, curing shrinkage of the molding
material which occurs during the molding process, the
presence of residual monomer which is toxic, a high
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porosity and a high water absorption leading to poor
dimensional accuracy.
Acrylic dentures tend to shrink during the
curing process, and, as a result, conventional molding
machines overcharge or overpack the mold to compensate.
The pressure must be maintained during the curing phase
and this overcharging is usually inadequate and is
difficult to control.
Some denture manufacturing devices are based on
the injection of a thermoplastic material. These devices
are actionned using an air-injection mechanism and
injection pressures are relatively low. So, these
methods are limited to non-viscous polymers having poor
mechanical properties such as thermoplastic acrylics.
Injection molding equipments suffer from the
drawback of inadequate spruing of the thermoplastic
material into the mold. If the sprues do not enter the
mold cavity at the proper location, strains in the
denture result which may also tend to lift the dentures
away from the mold and also to give a poor dimensional
accuracy.
If the flask halves are improperly closed or if
the flask cannot resist to the injection force, a change
in the position of the teeth relative to one another may
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occur, which can cause an adverse effect on the
articulation of teeth in the patient's mouth and alter
the patient's jaw position, increasing discomfort.
Furthermore, conventional thermoplastic
injection devices are complicated and require significant
operator adjustment and time, particularly for the
cleaning of the machine.
OBJECTS OF THE lNV~ lON
According to the foregoing, it is an object of
the present invention to provide an apparatus and method
for producing dentures in which a cartridge containing
thermoplastic resin is employed, wherein the cartridge
has a unique design to eliminate the cleaning of the
machine and facilitate the flow of resins in the cavity.
Another object of the present invention is to
provide an injection molding apparatus with a tubular
heating chamber to melt uniformly the thermoplastic resin
included in the cartridge.
Another object of the present invention is to
provide an apparatus and method in which the flask and
the injection apparatus are self-aligning and auto-
clamping.
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Another object of the present invention is to
provide an injection molding apparatus with a tubular
heating chamber which can be closed safely and resist to
the force generated by the mechanical power unit and
supply the clamping force to keep the flask closed.
Another object of the present invention is to
provide a flask and a method which can be reversible to
allow the making of a dental prosthesis using a direct or
in inverted injection and improve the fit of the
prosthesis on the model.
Another object of the present invention is to
provide a molding machine and method which reduces
strains on the dentures by improving the spruing of the
plastic into the mold.
SUNMARY OF THE lNv~.,ION
The foregoing and other objects of the present
invention are achieved by providing an apparatus for
preparing injection molded prosthesis comprising:
a cartridge containing resin to form a
prosthesis, the cartridge having an upper surface with an
opening and a lower surface;
a plunger contacting the lower surface;
a flask containing a mold to form the
prosthesis, the flask having an opening for alignment
with the opening in the cartridge;
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heating means disposed adjacent the cartridge
to heat the resin prior to injecting the resin into the
flask; and
means applying pressure to the lower surface of
the cartridge so that heated resin is ejected from the
cartridge through the opening in the cartridge and the
opening in the flask to permit the resin to enter the
flask and form the prosthesis.
In one form of the invention, the apparatus
comprises a compression plate which is located between
the cartridge and the flask; the plate has a lower
surface having a shape corresponding to that of the upper
surface of the cartridge and an upper face having an
opening for alignment with the opening of the cartridge
and the opening of the flask.
In another preferred form of the invention, the
opening of the cartridge and the opening of the plate are
conical in shape.
In another form of the invention the heating
means consist of a pair of heating elements, one being
disposed in the plunger and the other being disposed
circumferentially to the cartridge.
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The present invention also relates to a method
for producing injection molded prosthesiss which
comprises the steps of:
inserting a resin-containing cartridge into a
molding apparatus;
applying heat to the cartridge to heat the
resin;
applying pressure to the cartridge so that the
cartridge, injection molding apparatus and a flask
containing a mold to form the prosthesis align with one
another.
Other objects and further scope of
applicability of the present invention will become
apparent from the detailed description given hereinafter.
It should be understood, however, that this detailed
description, while indicating preferred embodiments of
the invention, is given by way of illustration only,
since various changes and modifications within the spirit
and scope of the invention will become apparent to those
skilled in the art.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is an elevational view of the
apparatus of the present invention;
Figure 2 is a cross-sectional elevation of the
upper part of the apparatus;
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Figure 3 is a cross-sectional view of a flask;
and
Figure 4 is a cross-sectional view of another
embodiment of a cartridge used with the apparatus of the
present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to figure 1, there is shown an
apparatus, generally designated 10, for preparing
injection molded prosthesis. It comprises a housing 12
in which is mounted a DC motor 14 that drives a belt 16
mounted to a pair of pulleys 18 and 20. Pulley 20,
through a speed reducing element 22, serves to actuate a
ball bearing screw jack 24.
Referring to figure 2, to the top end 25 of the
jack is secured an adaptor 27 to which is mounted a
piston or plunger 26 having a cover 28.
A tubular casing 30 surrounds the adaptor and
the piston and has its upper peripheral side wall
threadedly engaged to a head 32. Casing 30 is fixed to
a base 31 of the housing 12.
Shown on the top face of the plunger cover 28
is a cartridge 34 disposed at the center of the injection
apparatus. The cartridge is a substantially flat
cylinder, containing resin 35 or other suitable
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thermoplastic or thermoset material, and has a conical
opening 36 in its upper surface 38. However, opening 36
may be closed prior to being used in the apparatus of the
present invention.
A compression plate 40 is supported onto the
cylindrical inner side wall of the casing 30 over the
cartridge 34. The upper face of the plate has an annular
inverted V-shaped ridge 41 while its lower surface 42 has
a central portion 44 corresponding in shape to the upper
surface 38 of the cartridge. It also includes a conical
opening 46 in which is received the conical portion 36 of
the cartridge.
Resting on the upper face of the compression
plate 40 is a flask shown schematically in figure 2 as
consisting of two halves 48 and 50. The lower face of
the bottom flask half 50 has an annular inverted V-shaped
recess to correspondingly receive the annular ridge 41 of
the plate thereby ensuring proper alignment of the flask
onto the compression plate. A flat cover 52 surmounts
the upper flask half 48.
The compression plate comprises a pair of
handles 51 and 53 which will allow the plate with its
flask placed thereon to be lowered into position in the
chamber of casing 30 over the piston cover 28.
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In the center of casing 30, there is provided
a radial ring 54 within which is received a
circumferential heating element 56 and over which extends
a covering ring 58. A heat insulating element 60
separates the casing 30 from the heating ring 54 as well
as from the heated lower chamber 62 in which is housed
the heat plunger 26. An insulating sleeve 63 surrounds
chamber 62.
The plunger 26 and its cover 28 are further
heated by means of an element 64 controlled by a thermo-
couple 66. An insulation element 68 extends between the
screw jack adaptor 27 and cover 28 of the piston 26.
Figure 3 shows a cross-section of the flask
halves 48 and 50, both filled with plaster 70 and
defining centrally a cavity 72 conforming to a dental
prosthesis to be made and including an opening 74 and a
channel 76 for receiving injected thermoplastic resin.
In operation, a cartridge 34 is positioned on
the piston cover 28 and confined within the tubular
heating ring 54. The resin in the cartridge is heated by
elements 56, 64. The compression plate 40 with a flask
positioned on its top face is then lowered into the
chamber and is suspended over the cartridge with its
peripheral edge resting on a shoulder on the interior
wall of the chamber. The lowering of the compression
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plate is accomplished by using handles 51 and 53. Cover
32 is screwed on tightly thus closing the chamber. When
motor 14 is activated, the screw jack 24 causes the
piston to be raised causing the cartridge to come into
contact with the lower face of the compression plate with
opening 36 fitting into the conical opening 46.
As the plunger applies upward pressure through
its cover 28 onto the cartridge 34 bearing against plate
40, the plastic resin is ejected from opening 36 into
opening 74 of the flask. As the plunger continues to
apply pressure, the cartridge is deformed as the flat
bottom face 39 of the cartridge is pushed inwardly; the
volume of the cartridge decreases. The cartridge is made
from steel; however, other pliable, but pressure
resistant, material, such as an alloy of aluminum and
steel can also be used.
The two parts of the flask are automatically
clamped together without the use of clamping devices due
to the tight securement of the casing head 32 against
which bears the flask cover 52.
The entire device is an automatic self-aligning
device requiring no guiding elements due to the fact that
the cartridge opening fits into the opening in the bottom
of the compression plate and that the flask opening 74 is
properly located over the openings 36 and 46 following
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the engagement of the annular ridge 41 in the annular
groove 49.
The entire device is either single stand alone
or desk top unit which is controlled through a keyboard
(not shown). Once the device is activated, the heating
and plunger movement steps are all automatically
performed by the station, thus eliminating any further
user input. The speed and amount of pressure are all
controlled and automated by the device which may be
manual of computer controlled.
Figure 4 illustrates another embodiment of a
cartridge which may be used with the present invention.
The cartridge 134 in which is included a thermoplastic
resin 135 is formed of an upper face 138 having an
opening 136 and a lower face 139 which is recessed with
respect to the cylindrical side wall of the cartridge.
This recess is shaped to correspondly receive the piston
cover of the plunger. It is also formed of pliable and
pressure resistant material so that this surface 139 will
progressively be pushed inwardly towards the top surface
138.
This embodiment of the invention also
eliminates any possibility of deformation or shrinkage.
Since the cartridge fits into a recess in the bottom of
the compression plate, and since the bottom of the
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cartridge fits on the top of the plunger and in the
center of the peripheral heating element, the entire
device is an automatic self-aligning device which
requires no guiding elements. The pressure from the
plunger deforms the bottom plate portion of the
cartridge, forcing the material out of the opening at the
center of the top portion. As the plunger applies more
and more pressure, the volume of the cartridge shrinks as
the bottom diaphragm of the cartridge is deformed.
A key advantage to the present apparatus is the
elimination of additional clamping devices to avoid resin
leakage from the cartridge. Compressive tightening force
is applied automatically at the interface of the
components (the cartridge, heat chamber, plate and flask)
by the ascending force of the plunger, which applies a
uni-axial force on the cartridge which, in turn,
transmits this same force to the upper plate, which in
turn transfers it to the flask, which transfers the force
to the head, thereby completing the auto-clamping
process. The only element which is fixed during the
whole process is the head 32. The flat shape of the
cartridge makes the process possible. Since the
cartridge has an active surface larger than that of the
flask cavity, it is possible to obtain contact forces on
the flask greater than the hydrostatic force generated by
the injection. Consequently, the force of the process
(hydrostatic force of the plastic) inside the cavity is
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less than the clamping force of the flask, avoiding any
resin leakage.
Since the cartridge is sized so that it
contains more thermoplastic resin than is required to
fill the mold cavity contained by the flask, the pressure
in the cavity is maintained while the resin solidifies.
This back pressure permits good dimensional stability by
significantly decreasing any possibility of deformation
or shrinkage, and thereby greatly decreases the
possibility that the resin will be flowing out of the
flask and resulting in a less than optimum prosthesis.
The heating elements heat the cartridge at
approximately 400 centrigrade. The insulation and
covers are selected so that uniform temperature
distribution is achieved and plastic degradation is
prevented. A precision of +/- 5C on the cartridge
envelope is necessary to assure reproducible injections.
The flask is designed reversible to accept a
direct or an inverted denture model. Inverted injection
is used for upper prosthesis whereas direct injection is
used for lower denture. In an inverted injection, the
channel delivering the plastic melt is located at the
rear of the palatal region denture and directed on the
edge of the impression side. Inverted injection gives a
better fit of the denture with respect to the original
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model by providing a mechanical anchoring and by reducing
the thermal shrinkage of the resin during cooling.
Although the invention has been described above
with respect to specific forms, it will be evident to a
person skilled in the art that it may be modified and
refined in various ways. It is therefore wished to have
it understood that the present invention should not be
limited in scope, except by the terms of the following
claims.
