AU617064B2

AU617064B2 - Method for the control of a furnace

Info

Publication number: AU617064B2
Application number: AU69424/91A
Authority: AU
Inventors: Gerhard Beham; Viktor Moldaschl; Volker Rheinberger
Original assignee: Ivoclar AG
Current assignee: Ivoclar AG
Priority date: 1990-01-26
Filing date: 1991-01-17
Publication date: 1991-11-14
Anticipated expiration: 2011-01-17
Also published as: EP0438802A1; DE59004425D1; AU6942491A; ATE100694T1; EP0438802B1; CA2034893A1; DE4002358C1

Abstract

The raw material (18) is pressed into the shaping space (14) by a ram (20) driven by the pressure cylinder (22). It is guided by a rod (24) into the interior (36) of the oven which is heated by a spiral element (34). A seal (32) enables a partial vacuum to be produced in the oven. The position of the driving piston (26) in its cylinder (22) is detected by a probe (44) mounted alongside it on a control plate (40) supported by the guiding rod. An analogue measurement pick-up, e.g. a potentiometer, can register the speed of the piston for use as a PI regulator. An opto-encode can be used as the pick-up with a control unit in the form of a micro-controller for piston drive and oven heating.

Description

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AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Int. Class 617064 Priority .,Related Art: 6ir

S

o e .,Applicant(s): Ivoclar AG Bendererstrasse 2, FL-9494 Schaan, LIECHTENSTEIN Address for Service is: c PHILLIPS ORMOND FITZPATRICK c Patent and Trade Mark Attorneys 367 Collins Street c Melbourne 3000 AUSTRALIA Complete Specification for the invention entitled: METHOD FOR THE CONTROL OF A FURNACE t c S Our Ref 203067 SPOF Code: 1502/77261 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): 6006 o;

PATENT

Attorney Docket No. 80460 METHOD FOR THE CONTROL OF A FURNACE BACKGROUND OF THE INVENTION The invention relates to a method and a furnace for molding and hardening dental compositions.

Methods or furnaces 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 0*e 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.

oo 15 In a method known from German Patent 664 133 or AT Patent 157 e 0 210, 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, 20 placing it in a mold housing, the wax is removed with heat so gee...

%oo: that a molding cavity or space remains. The pouring channel, tee.

.o o which serves as a premolding space in the aforementioned state of the art, adjoins this space.

,ooooe S•The dental ceramics, for example, is inserted into this premolding space in the form of an unfinished piece or blank and softened by heating so that it can be introduced into the molding cavity and where it assumes the shape of the desired dental prosthesis.

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 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 establish a suitable turnoff time for the furnace.

The previously known 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 for ceramics, for example when molding delicate crowns, a complete filling of the mold cavity is not assured so that the crown produced in this 0o00 0 manner may not be usable.

0 ".Of course, in order to guarantee the complete 90 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 X o~i 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 V unusable crown becomes known only after it has been finished.

b 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 the invention is that the furnace can be left open after the molding cycle without danger by determining the change of the piston speed 3 with surprisingly simple measures and at the same time extrapolating the temperature in the interior of the mold housing with the aid of a control unit provided by the present invention. Thus, in a very simple manner it is 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 the dental prosthesis to be produced therewith.

,,Surprisingly, it turns out that, independent of the S shape of the dental prosthesis, at a temperature lower than the desired one, the viscosity of the dental ceramics was aet, ,,15 considerably higher. rhis 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 Z0 increasing the temperature. In this way, high quality, reproducible dental prostheses, such as crowns made of dental r as ceramics, or of metal alloys and even plastic prostheses, can be produced because even for these an exact temperature control assures good results.

The present invention recognizes that as a result of S 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 o100% filled so that the flow and the downward movement of the piston ceases altogether.

Taking into consideration an extended molding time, Sif appropriate, the furnace can then be turned off which, Sbecause 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, i secondary compaction takes place during this extended molding ("hold-over") time.

The hold-over time begins when a change in the 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 jental ceramic molding can begin immediately thereafter.

0 It is particularly advantageous, however, to calculate the time, including the hold-over time, required for .g O 1 5 molding the article in question. Where the mass of the unfinished blank and crown are known, the travel of the piston oo 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.

o 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 I 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, prostheses etc.) differ.

It is particularly aavantageous that the resistance against deformation of the mass is readily determined on the basis of the piston speed in relation to the force acting on the piston. If very differently shaped dental prostheses are to be produced, it may be advantageous to select a smaller molding force for the production of, for example, a single delicately shaped crown. It is 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 is advantageous that the furnace temperature at the beginning of the molding cycle is automaticaly 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.

According to another advantageous embodiment of the invention the mold is disposed within a housing preheated to a temperature of 700-900 0 C and the material is thereafter brought to its plasticizing temperature.

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 y Fig. 1 is a schematic view in the cross section of a mold in a mold housing constructed in accordance with the T '25 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 Smold housing shown in Fig. i, 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.

0i U Ui 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 1e..

ft *i *r 1 -~r 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 is 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 the volume of Q.r. 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 is preferable to mold S* it in a vacuum and subsequently sinter it so that it is nonf porous.

According to Fig. 3, the raw blank 18 has been t advanced into the premolding space 16 so 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 is entered into the premolding space 16. Its fdiameter 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 suitably 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 -7 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 *0 furnace 33 and seal 32 is provided so that a vacuum can be maintained in an inner chamber 36 of the furnace which receives oono 15 housing 10. Furnace 33 includes a heater 34, a spiral *0 a 0 heater. The furnace 33 also has a base. A furnace hood is 0 so *.06 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.

0 A horizontal control plate 40 is affixed to the 000000 piston rod and engages the lower end of a spring-loaded o. feeler 42 which movably extends into a sensor 44 that is in turn attached to pressure cylinder 22. Thus, the relative S• 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 is suitably 0 °mo 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 Sensor 44 is connected with control unit 47. The 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 controls heater 34 of furnace 33.

Control unit 47 is further connected to an inputoutput 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 S and a lowering valve 56. Each valve is electrically connected S with the control unit and has the required magnets 58 or ~In the illustrated embodiment the lowering valve 56 4 1 5 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 *444.. 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 a relatively inexpensive pressure control for the I pressure conduits.

Lift valve 54 is constructed so that when control magnet 58 is activated, pressure is applied to conduit 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 f: 9 that the throttle point 62 can be omitted and pressure conduit 30 is closed at that point. Also, the function of throttle 61 can be advantageously 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 4. controlling the movement of the drive shaft. Pressure can be 04 applied to piston 20 in any other manner.

~4*4 In accordance with another embodiment of the a.

S invention, sensor 44 is a pressure sensor. 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 o20 material. This pressure increase corresponds to the previously 4&4094 described decline in the forward speed, and the output signal rroo S, of the pressure sensor 44 is also fed to control unit 47. This embodiment, which can be combined with position measurement or orr4 distance-time measurement devices, makes it possible to also take into consideration the effect of the mold cavity shape, or i of variations on the amount: of material that is being molded, 0 si on temperature and absolute pressure, and the changes in the resulting counterpressure which acts on the piston.

It is understood that the evacuation of the 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 the invention, piston 20 is equipped with a head of sufficient weight so that it exerts pressure on the molded article even after housing 10 has been removed from furnace 33. addition, the head then has an enlarged frontal surface for engaging piston rod 24 so that the alignment of the housing in furnace 33 becomes even less critical.

-0I 4 0 o @0 o f I 4 I I tl t O i x i

Claims

2. Method according to claim i, including the step I ,of applying pressure to the piston for the molding of the material for a hold-over time period which extends beyond the time when the change in the piston movement was detected and until after the turn-off time has elapsed.
3. Method according to claim 1 including the step *944 of sensing at least one of the position or the time at or by which the deformation of the material begins. t lr b
4. Method according to claim 2 wherein the t monitoring step includes monitoring at least one of the 4 movement time and the movement path of the piston from the time f deformation of the material begins until the change in piston movement rate is detected, and including the step of establishing the hold-over time period as a function of the deforming characteristic of the material to be formed into an article. Method according to claim 1 including the step of monitoring the speed of the piston while the material is Sbeing deformed, and setting a desired speed for the piston by adjusting at least one of the pressure applied to the piston Si, and the temperature of the furnace. 12
6. Method according to claim 5 wherein the step of monitoring the piston speed includes monitoring the speed with an analog instrument such as a potentiometer which records the movement of the piston, and using a PI controller as a regulating element.
7. Method according to claim 5 including an optoencoder for monitoring the piston movement, and a control device including a microcontroller operatively coupled with a drive for the piston and a heater for the furnace, the control ~device determining the resistance of the material against S: deformation as a function of the piston speed produced by a given piston driving force in relation to the temperature of 4~4,5 the material as calculated from the heat output of the furnace and subjecting the piston to a desired, pre-set pressure. Sa, ,.ii i i:i ii 1:i ti M a i.i ::i 4r 4 @114 a 44t
8. Method according to claim 7 including the step of determining when the mold cavity is filled with the material q0 on the basis of the deformation resistance calculated from the piston speed, establishing a temperature dependent threshold value for the deformation resistance for the material, and assuming that the mold cavity is completely filled by the material when the threshold value is exceeded.
9. Method according to claim 1 wherein the mold cavity is disposed within a housing preheated to a temperature of ab.i-700-90 0 C and including the step of thereafter bringing the material to its plasticizing temperature.
10. Method according to claim 1 including the step of providing a temperature regulating device and therewith maintaining for at least a predetermined length of time a temperature corresponding to the plasticizing temperature for the material.
11. Furnace for the shaping and hardening of a ,-ndental material comprising a mold having a premolding space and 13 an adjacent mold cavity, the material being initially disposed in the premolding space, a movable piston formed and constructed to act on the material in the premolding space, a heater, sensing means for the detection of movement parameters of the piston, and a control unit operatively coupled with the sensing means for activating the piston.
12. Furnace according to claim 11 wherein the sensing means comprises a position sensor capable of detecting the position of the piston, and including a pressure cylinder for activating the piston. 0 13. Furnace according to claim 12 wherein the sensing means comprises a pressure sensor capable of determining the increasing resistance generated by the material as it fills the molding space.
14. Furnace according to claim 11 wherein the control unit includes switching means for adapting the movement I parameters for the piston to the material being used, such as, Sc for example, dental ceramic, plastic or a metal alloy, and/or 4 to the amount of material that is present. S* 15. Furnace according to one of claim 11 wherein the piston includes a piston rod, the piston and the rod forming t tindependent parts having cooperating, each-other-opposing frontal surfaces. DATED: 10 January 1990 PHILLIPS ORMONDE FITZPATRICK Attorneys for: IVOCLAR AG fc^-