Note: Descriptions are shown in the official language in which they were submitted.
2034893
Attorney Dock~t No. 80460
M~OD FOR ~H~ .ÇON~ROL OF A ~y~NACE
BACKGROUND OF THE INVENTION
The invention relates to a method and a furnace for
molding and hardening dental compositions.
Methods or ~urnaces for the production of dental
prostheses are known from German Patent 26 32 846 and European
Patent Al 231 773. There, dental prostheses are produced in a
furnace in which the desired dental prosthesis material, for
example, dental ceramics, is fired in a mold.
To produce the mold, a wax model is first prepared.
In a method known from German Patent 664 133 or AT Patent 157
210, a temperature-resistant mold insert is set up with a
pouring channel left open above the wax body with the help of
either a wax body or by separately forming it. After hardening
the material which forms the mold insert and, for example,
placing it in a mold housing, the wax is removed with heat so
that a molding cavity or space remains. The pouring channel,
which serves as a premolding space in the aforementionsd state
of the art, adjoins this space.
The dental ceramics, for example, i~ inserted into
this premolding space in the form of an unfinished piece or
blank and softened by h~ating so that it can be introduced into
the molding cavity and where it as~umes the shape of the
desired dental pro~thesis.
To avoid dangerous air inclusions and to prevent
shrinkage of the material, pressure is quickly applied to it
with a weighted piston as can be seen for example from Austrian
Patent 157 210. This method of applying pressure remained
basically unchanged although generating the pressure with a
pneumatic actuator is also known from European patent Al 231
773.
However, there is the problem that dental prostheses
must be produced in different shapes, from different materials
and with differing degrees of complexity. For cost saving
2 Z034893
reasons, the same furnace is always used. To take different
requirements into account, programs based on data gathered from
past experience have been set up to eætablish a suitable turn-
off time for the furnace.
The previously Xnown methods have certain
shortcomings however. For example, with the known methods or
furnaces, an accurate form is not always guaranteed since the
heating or molding time is somewhat arbitrarily established.
Given the relatively large flow resistance ~or ceramics, for
example when molding delicate crowns, a complete filling of the
mold cavity is not assured so that the crown produced in this
manner may not be usable.
~ f course, in order to guarantee the complete
filling, one can suitably postpone the turn-off time for the
furnace. However, this prolongs the production cycle, which is
undesirable. Moreover, if the heating and molding time is
excessively long, the material might become overheated which
can impair its quality.
On the other hand, a relatively short heating and
molding time can cause problems since the production of an
unusable crown becomes known only after it has been finished.
Other requirements for dental prostheses might not be
met: for example, air bubbles may appear in the ceramic crowns,
or the compression strength of a dental prosthesis finished in
this manner might be unsatisfactory.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to
create a furnace-controlling method and a furnace in which the
production cycle for the dental materials is shortened without
compromising the quality of such materials as dental ceramics
even when different quantities are being produced.
This object is attained in accordance with the
invention defined by claim 1 or 11. Advantageous further
developments are set forth in the subclaims.
A particularly advantageous aspect of th~ invention
is that the furnace can be left open after the molding cycle
without danger by determining the change of the piston speed
2(:)3~893
with surprisingly simple measures and at the same time
extrapolating the temperature in the interior of the mold
housi~g with the aid of a control unit provided by the present
invention. Thus, in a very simple manner it i8 possible to
replace the heretofore required temperature sensor, which, if
used, would have to be connected separately to permit the
removal of the housing together with the dental ceramics.
Furthermore, it is possible to optimize the filling
of the mold cavity in accordance with the present invention
independently of the shape and size of the mold and ths dental
prosthesis to be produced therewith.
Surprisingly, it turns out that, independent of the
shape of the dental prosthesis, at a temperature lower than the
desired one, the viscosity of the dental ceramics was
considerably higher. This is attributable to the fact that the
unfinished piece then forms a plug which resists the pressure
generated by the piston. With appropriate control measures it
is possible, for example, to further reduce the required
pressure while assuring a uniform filling of the cavity after
increasing the temperature. In this way, high quality,
reproducible dental prostheses, such as crowns made of dental
ceramics, or of metal alloys and even plastic prostheses, can
be produced because even for these an exact temperature control
assures good results.
The preeent invention recognizes that as a result of
the increase in the flow resistance during the latter stages of
the injection process, the piston speed declines somewhat at
first -- assuming a constant molding pressure -- but thereafter
declines greatly as soon as the mold is 100% filled so that the
flow and the downward movement of the piston ceases altogether.
Taking into consideration an extended molding time,
if appropriate, the furnace can then be turned off which,
because of the stored heat, leads to no or no notable immediate
temperature change in the interior of the mold.
It is particularly advantageous to maintain the
piston pressurized so that even after the flow has stopped,
secondary compaction takes place during this extended molding
("hold-over") time.
203~893
The hold-over time begins when a change in thQ piston
movement is detected, i.e. when the constant piston movement is
over, and ends with when the furnace is turned off. The time
when a change in the piston movement is detected can be defined
alternately as the time at which the piston speed falls below
the previously established threshold, or as the time when the
piston speed becomes zero.
At the end of the hold-over time, when the pressure
is released, a guide rod for the piston can be retracted into
its upper position and the mold housing can be removed so that
the next dental ceramic molding can begin immediately
thereafter.
It is particularly advantageous, however, to
calculate the time, including the hold-over time, required for
molding the article in question. Where the mass of the
unfinished blank and crown are known, the travel of the piston
can be used to determine the degree to which the mold space is
filled. With a suitable control unit, the measured values can
be automatically processed further to thereby establish a
completely automatic control.
The piston speed can also be controlled with the
pressure. Should the piston speed increase excessively at a
preselected pressure -- which can lead to excessive turbulence
and the formation of bubbles -- the pressure and heat input can
be appropriately reduced with immediate effect. The reduction
of the heat input also reduces the piston speed although a time
delay resulting from the heat stored in the mold must be taken
into account.
It is particularly advantageous that the complete
filling can be detected in a simple manner by determining when
the piston speed has dropped to zero, or almost zero.
Furthermore, it is particularly advantageous that the mass to
be molded can be calculated exactly although it is possible to
use uniform or standardized blanks even when the volumes of the
dental replacement parts (crowns, prosthesss etc.) differ.
It is particularly advantageous that the resistance
against deformation of the mass i~ readily determined on the
basis of the piston speed in relation to the force acting on
~03~893
the piston. If very differently ~haped dental prostheses are
to be produced, it may be advantageou~ to ~el~ct a smaller
molding force for the production of, for example, a single
delicately shaped crown. It i8 also possible to make the time
of the change in piston movement dependent on the increase of
the shaping resistance since it greatly increases when the mold
cavity becomes completely filled.
Furthermore, it i8 advantageous that the furnace
temperature at the beginning of the molding cycle is
automatically taken into consideration. For example, when the
insertion of a cool mold housing lowers the temperature of
furnace significantly, the plasticizing and thus the piston
movement is simply delayed.
Other details, advantages and features of the
invention are explained in more detail in the following
description with the aid of the drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic view in cross section of a mold
in a mold housing constructed in accordance with the
invention;;
Fig. 2 shows in cross section a raw blank of the mass
or material used in the practice of the present invention;
Fig. 3 shows the raw blank of Fig. 2 inserted in the
mold housing shown in Fig. 1, and the cooperating pressure
activated piston;
Fig. 4, an enlarged, schematic, cross-sectional view
of a mold housing disposed in a furnace including a pressure
activated piston for practicing the method of the present
invention; and,
Fig. 5, a schematic representation of a control
system used for practicing the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Fig. 1 shows mold housing 10 in cross section for
practicing the method of the present invention into which a
mold insert 12 is placed. The mold insert 12 defines a mold
cavity 14 and a premolding space 16. The mold cavity 14 is
2034893
formed in a well known manner with a model of the dental
prosthesis to be produced, such as the crown. The mold insert
12 is made of a temperature-resistant material.
The premolding space 16 has a cylindrical form and
communicates with the mold cavity 14. Premolding space 16 i8
defined by the molding insert 12 and has the same smooth
surface as the molding cavity 14.
Fig. 2 shows a raw blank 18, shaped as a solid
cylinder and having a diameter selected so that it is readily
introduced into the premolding space 16 as illustrated in
Fig. 1. The volume of blank 18 slightly exceeds th~ volume of
the mold cavity 14 and thus of the dental prosthesis to be
produced.
The raw blank 18 is made of a dental material such as
a premolded dental ceramic, a metal alloy or a dental plastic.
In a modified form of the invention several crowns are produced
simultaneously. A plurality of mold cavities 14 are suitably
connected with premolding space 16 via appropriate channels and
the site of the raw blank 18 is correspondingly larger. When
the blank is made of a dental ceramic, it iB preferable to mold
it in a vacuum and subsequently sinter it so that it is non-
porous.
According to Fig. 3, the raw blank 18 has been
advanced into the premolding space 16 60 that it is contiguous
with a casting channel 19 which is formed either by the
material of the molding insert 12 or, as is shown in Fig. 4, as
a separate insert. After the blank has been positioned a
piston 20 i8 entered into the premolding space 16. Its
diameter is chosen so that it effectively seals with respect to
the walls of the premolding space 12 while being readily
reciprocable therein. If desired, a known æuitably
temperature-resistant, special seal can be employed.
As can be seen from Fig. 4, a pressure actuator
drives the piston 20 downwardly. In the depicted embodiment,
the pressure actuator acts on piston 20 via a piston rod 24.
Thus, the unit consisting of housing 10, mold insert 12 and
piston 20 is easily removable from the furnace. At its upper
end, the piston rod 24 is attached to a piston 26 of the
Z0~4893
actuator; its diameter corresponds to the diameter of the
pressure cylinder 22. Pressure conduits 28 or 30 are
communicate with pressure cylinder 22 below and above piston 26
so that the drive piston 26 and the piston rod 24 can be
lowered and raised. By pressurizing conduit 28 piston 26
applies pressure to blank 18 so that it can be deformed and
pressed into the mold cavity 14 after it has softened. Since
piston rod 24 merely abuts piston 20, a precise alignment of
the mold insert 12 is not mandatory. Moreover, no lateral
forces are generated because only vertically acting forces can
be transferred.
Piston rod 24 extends slidably through the top of a
furnace 33 and seal 32 is provided so that a vacuum can be
maintained in an inner chamber 36 of the furnace which receives
15 housing 10. Furnace 33 includes a heater 34, e.g., a spiral
heater. The furnace 33 also has a base. A furnace hood is
defined by the top wall and the side walls of the furnace. The
hood can be raised or tilted off the base 38. The separation
between the base and the hood is sealed.
A horizontal control plate 10 is affixed to the
piston rod and engages the lower end of a spring-loaded
feeler 42 which movably ext~nds into a sensor 44 that i~ in
turn attached to pressure cylinder 22. Thus, the relative
position of the unit consisting of piston 26, piston rod 24 and
piston 20 can be precisely detected by sensor 44.
The lowering of piston 20 is controlled with a
control unit 47 shown in Fig. 5. The sensor 44 i8 suitably
constructed. In the embodiment shown in Fig. 5 it is a
potentiometer 46. Its resistance value is determined via the
control plate 40 by the position of the piston 20. The
potentiometer 46 has a very accurate linearity so that the
detected resistance value corresponds precisely to the position
of the piston.
It should be understood that instead of the depicted
potentiometer 46, other types of sensors 44 can be provided,
such as, for example, an optoencoder, which exhibits a better
linearity than a potentiometer but at increased cost.
8 2~)3~393
Sensor 44 is connected with control unit 47. Th~
control unit is also connected to heater 34 via an amplifier 48
such as a thyristor or a relay. The other side of the heater
is connected to ground. Thus, unit 47 directly control~
heater 34 of furnace 33.
Control unit 47 i8 further connected to an input-
output console 50. With it the desired heating times, the
molding materials used, the desired type of operation
(automatic or manual), and other parameters are entered.
The invention further provides a valve arrangement 52
which, in the illustrated embodiment, includes a lift valve 54
and a lowering valve 56. Each valve i8 electrically connected
with the control unit and has the required magnets 58 or 60.
In the illustrated embodiment the lowering valve 56
is a flow control valve which can be regulated so that the
pressure in the pressure cylinder 22 can be adjusted in
accordance with the setting of the control magnet 60, which
preferentially has two coils. In this way, the force applied
to piston 20 can be controlled with control unit 47 both over
wide ranges as well as in small increments. Pressure
conduit 28 connects lowering valve 56 with pressure
cylinder 22, whereas pressure conduit 30 connects lift valve 54
with the pressure cylinder 22. In addition, the two pressure
conduits 28 and 30 are each connected with a throttle 61 or 62
to provide à relatively inexpensive pressure control for the
pressure conduits.
Lift valve 54 is constructed so that when control
magnet 58 is activated, pressure is applied to conduit 30,
thereby raising piston 20. This occurs between the melting
cycles, for example, when the housing 10 is to be removed.
The valve arrangement 52 is connected with a pressure
vessel 64, which is fed by a pump 66 to assure that there is
always sufficient pressure for the valve arrangement 52 to
initiate the desired control steps.
It is to be understood that the type of pneumatic
control can be carried out in many other ways. For example,
the lift valve 54 can be replaced by a two-way valve, which
brings about a release of pressure when it is not activated so
9 203~39
that the throttle point 62 can be omitted an~ presæure
conduit 30 is closed at that point. Also, the function o~
throttle 61 can be advanta~eously integrated into lowering
valve 56. Furthermore, motor-controlled valves can be used for
the valve arrangement 52.
In accordance with another embodiment of the
invention, the entire pneumatic system can be replaced by an
electric drive as a substitute for the pressure cylinder 22.
Piston rod 24 is replaced by a corresponding drive shaft for
the piston 20. The current flow through the motor for the
drive shaft can be used as an alternative or additional sensor
to the control plate 40 for determining the position and
controlling the movement of the drive shaft. Pre~sure can be
applied to piston 20 in any other manner.
In accordance with another embodiment of the
invention, sensor 44 is a pressure ~ensor. With this
modification it is also possible to detect a movement parameter
of the piston 20 by monitoring the increase in pressure just
before the mold cavity 14 is completely filled with the molding
material. This pressure increase corresponds to the previously
described decline in the forward speed, and the output signal
of the pressure sensor 44 is also fed to control unit 47. This
embodiment, which can be combined with position measurement or
distance-time measurement devices, makes it possible to also
take into consideration the effect of the mold cavity shape, or
of variations on the amount of material that i~ being molded,
on temperature and absolute pressure, and the changes in the
resulting counterpressure which acts on the piston.
It is un~erstood that the evacuation of he
interior 36 of the furnace 33 (see Fig. 4) can also be included
in the automatic control. Also, the desired hold-over time
and/or additional heating time can be automatically set, on the
basis of the measured movement of the control plate, which in
turn depends on the volume of mold cavity 14, or they can be
preselected with the input/output console.
In accordance with another embodiment of he
inven~ion, piston 20 is equipped with a head of sufficient
weight ~o that it exerts pressure on the molded article even
X0~89~
,
after housing 10 ha~ been removed from furnace 33. In
addition, the head then has an enlarged frontal surface for
engaging piston rod 24 so that the alignment of the housing 10
in furnace 33 becomes even le85 critlcal.
