CA2034893A1 - Method for the control of a furnace - Google Patents
Method for the control of a furnaceInfo
- Publication number
- CA2034893A1 CA2034893A1 CA002034893A CA2034893A CA2034893A1 CA 2034893 A1 CA2034893 A1 CA 2034893A1 CA 002034893 A CA002034893 A CA 002034893A CA 2034893 A CA2034893 A CA 2034893A CA 2034893 A1 CA2034893 A1 CA 2034893A1
- Authority
- CA
- Canada
- Prior art keywords
- piston
- furnace
- movement
- space
- mold
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61C—DENTISTRY; APPARATUS OR METHODS FOR ORAL OR DENTAL HYGIENE
- A61C13/00—Dental prostheses; Making same
- A61C13/20—Methods or devices for soldering, casting, moulding or melting
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61C—DENTISTRY; APPARATUS OR METHODS FOR ORAL OR DENTAL HYGIENE
- A61C13/00—Dental prostheses; Making same
- A61C13/20—Methods or devices for soldering, casting, moulding or melting
- A61C13/206—Injection moulding
Abstract
METHOD FOR THE CONTROL OF A FURNACE
ABSTRACT OF THE DISCLOSURE
In a method to control a furnace for deformable, hardenable dental materials, the degree of filling of a molding cavity is ascertained by sensing the movement of a piston and a control is effected with a control unit via certain preadjusted parameters.
ABSTRACT OF THE DISCLOSURE
In a method to control a furnace for deformable, hardenable dental materials, the degree of filling of a molding cavity is ascertained by sensing the movement of a piston and a control is effected with a control unit via certain preadjusted parameters.
Description
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 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 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.
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 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 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.
Claims (15)
1. Method for controlling a furnace for deforming and hardening a deformable dental material in a mold, comprising the steps of introducing the material into a premolding space of the mold, the space being connected to a mold cavity, deforming the material by pressing it into and conforming it to the mold cavity under the influence of heat with a piston extending into the premolding space, monitoring movement parameters of the piston, detecting a change in a piston movement parameter indicating that the material fills the mold cavity, and, upon detecting such a change, setting a time for turning off the furnace.
2. Method according to claim 1, including the step 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 of sensing at least one of the position or the time at or by which the deformation of the material begins.
4. Method according to claim 2 wherein the monitoring step includes monitoring at least one of the movement time and the movement path of the piston from the time 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.
5. Method according to claim 1 including the step of monitoring the speed of the piston while the material is being deformed, and setting a desired speed for the piston by adjusting at least one of the pressure applied to the piston and the temperature of the furnace.
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 deformation as a function of the piston speed produced by a given piston driving force in relation to the temperature of the material as calculated from the heat output of the furnace and subjecting the piston to a desired, pre-set pressure.
8. Method according to claim 7 including the step of determining when the mold cavity is filled with the material 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 about 700-900°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 dental material comprising a mold having a premolding space and 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.
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 parameters for the piston to the material being used, such as, for example, dental ceramic, plastic or a metal alloy, and/or to the amount of material that is present.
15. Furnace according to one of claim 11 wherein the piston includes a piston rod, the piston and the rod forming independent parts having cooperating, each-other-opposing frontal surfaces.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DEP4002358.3 | 1990-01-26 | ||
| DE4002358A DE4002358C1 (en) | 1990-01-26 | 1990-01-26 | Oven control for dental material pressing and hardening - detects degree of filling of shaping space by speed of movement of piston in pressure cylinder |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2034893A1 true CA2034893A1 (en) | 1991-07-27 |
Family
ID=6398874
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002034893A Abandoned CA2034893A1 (en) | 1990-01-26 | 1991-01-24 | Method for the control of a furnace |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0438802B1 (en) |
| AT (1) | ATE100694T1 (en) |
| AU (1) | AU617064B2 (en) |
| CA (1) | CA2034893A1 (en) |
| DE (2) | DE4002358C1 (en) |
Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6252202B1 (en) | 1998-02-10 | 2001-06-26 | Jeneric/Pentron, Inc. | Furnace for heat treatment of dental materials |
| JP2008132323A (en) * | 2006-10-27 | 2008-06-12 | Ivoclar Vivadent Ag | Operation method of pressure furnace and pressure furnace |
| JP2008253770A (en) * | 2007-03-30 | 2008-10-23 | Ivoclar Vivadent Ag | Muffle detection |
| US20160184062A1 (en) * | 2013-12-20 | 2016-06-30 | Ivoclar Vivadent Ag | Method For Processing a Dental Material, Regulating Device For A Dental Furnace, And Dental Furnace |
| JP2018521718A (en) * | 2015-05-22 | 2018-08-09 | イボクラール ビバデント アクチェンゲゼルシャフト | Dental kiln |
| US11376104B2 (en) | 2013-12-20 | 2022-07-05 | Ivoclar Vivadent Ag | Method for processing a dental material and a dental furnace |
| US11808520B2 (en) | 2016-11-15 | 2023-11-07 | Ivoclar Vivadent Ag | Arrangement of a furnace and of bulk material of glass particles as well as method for operating a furnace |
Families Citing this family (14)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE4340200C2 (en) * | 1993-11-25 | 1998-07-23 | Dekema Dental Keramikoefen Gmb | Arrangement for the heat treatment of objects, in particular dentures or partial dentures |
| JPH11226976A (en) * | 1998-02-10 | 1999-08-24 | Yamato Scient Co Ltd | Pressure molding apparatus |
| DE19844136B4 (en) * | 1998-09-25 | 2005-08-11 | Ivoclar Vivadent Ag | Oven for the production of dental prostheses |
| US6303059B1 (en) | 1999-03-26 | 2001-10-16 | Ivoclar A.G. | Method of controlling an oven |
| DE19913891C2 (en) * | 1999-03-26 | 2003-03-27 | Ivoclar Vivadent Ag | Process for the production of dentures |
| JP2001137263A (en) * | 1999-11-10 | 2001-05-22 | Gc Corp | Method for fabrication of dental prosthesis |
| DE10208801C1 (en) * | 2002-03-01 | 2003-06-12 | Horst-Juergen Heidenegger | Vacuum furnace firing pressed ceramic false teeth, includes upper piston drive unit connected to vacuum pump and main chamber |
| DE10257208B4 (en) * | 2002-12-06 | 2008-02-21 | Ivoclar Vivadent Ag | Ring system |
| DE10324404A1 (en) * | 2003-05-28 | 2004-12-30 | Dekema Dental-Keramiköfen GmbH | Forming process for dental teeth uses a press with controlled heating while controlled pressure is applied |
| DE10327892B3 (en) * | 2003-06-20 | 2004-12-02 | Volker Merten | Device for pressing dental ceramic compositions used in the production of dental prostheses such as crowns, bridges and inlays comprises an upper part, a lower part, and a piston sliding a plunger into a muffle having a negative mold |
| DE202006004578U1 (en) * | 2006-03-22 | 2006-06-01 | Dekema Dental-Keramiköfen GmbH | Press oven for dentures or tooth replacement |
| DE202007004265U1 (en) * | 2007-03-20 | 2007-05-24 | Wegold Edelmetalle Ag | Device for pressing a dental material, especially a dental plastic, used in the production of a denture comprises a pressing unit with a pneumatic drive |
| DE102010053873A1 (en) * | 2010-12-09 | 2012-06-14 | Dekema Dental-Keramiköfen GmbH | Press oven for dentures or tooth replacement |
| EP2551620B1 (en) | 2011-07-25 | 2013-09-11 | Ivoclar Vivadent AG | Dental oven |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2264247A1 (en) * | 1972-12-30 | 1974-07-11 | Krupp Gmbh | METHOD FOR MANUFACTURING MOLDINGS |
| AT352863B (en) * | 1975-07-31 | 1979-10-10 | Pfaffenbauer Ludwig Ing | VACUUM FURNACE, IN PARTICULAR FOR DENTAL CERAMIC PURPOSES |
| CH668699A5 (en) * | 1986-01-17 | 1989-01-31 | Sonja Wohlwend Erne | METHOD FOR PRODUCING DENTAL SPARE PARTS. |
| DE3831539C3 (en) * | 1988-09-16 | 2001-06-13 | Kaltenbach & Voigt | Control arrangement for a dental furnace, in particular a microprocessor-controlled preheating furnace |
-
1990
- 1990-01-26 DE DE4002358A patent/DE4002358C1/en not_active Expired - Fee Related
- 1990-12-28 EP EP90125698A patent/EP0438802B1/en not_active Expired - Lifetime
- 1990-12-28 AT AT90125698T patent/ATE100694T1/en not_active IP Right Cessation
- 1990-12-28 DE DE90125698T patent/DE59004425D1/en not_active Expired - Lifetime
-
1991
- 1991-01-17 AU AU69424/91A patent/AU617064B2/en not_active Ceased
- 1991-01-24 CA CA002034893A patent/CA2034893A1/en not_active Abandoned
Cited By (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6252202B1 (en) | 1998-02-10 | 2001-06-26 | Jeneric/Pentron, Inc. | Furnace for heat treatment of dental materials |
| JP2008132323A (en) * | 2006-10-27 | 2008-06-12 | Ivoclar Vivadent Ag | Operation method of pressure furnace and pressure furnace |
| JP2008253770A (en) * | 2007-03-30 | 2008-10-23 | Ivoclar Vivadent Ag | Muffle detection |
| US8232506B2 (en) | 2007-03-30 | 2012-07-31 | Ivoclar Vivadent Ag | Dental firing furnace |
| US20160184062A1 (en) * | 2013-12-20 | 2016-06-30 | Ivoclar Vivadent Ag | Method For Processing a Dental Material, Regulating Device For A Dental Furnace, And Dental Furnace |
| US10398536B2 (en) | 2013-12-20 | 2019-09-03 | Ivoclar Vivadent Ag | Method for processing a dental material, regulating device for a dental furnace, and dental furnace |
| US11376104B2 (en) | 2013-12-20 | 2022-07-05 | Ivoclar Vivadent Ag | Method for processing a dental material and a dental furnace |
| JP2018521718A (en) * | 2015-05-22 | 2018-08-09 | イボクラール ビバデント アクチェンゲゼルシャフト | Dental kiln |
| US10441390B2 (en) | 2015-05-22 | 2019-10-15 | Ivoclar Vivadent Ag | Dental press furnace |
| US11808520B2 (en) | 2016-11-15 | 2023-11-07 | Ivoclar Vivadent Ag | Arrangement of a furnace and of bulk material of glass particles as well as method for operating a furnace |
Also Published As
| Publication number | Publication date |
|---|---|
| EP0438802A1 (en) | 1991-07-31 |
| AU617064B2 (en) | 1991-11-14 |
| DE4002358C1 (en) | 1991-10-24 |
| EP0438802B1 (en) | 1994-01-26 |
| ATE100694T1 (en) | 1994-02-15 |
| AU6942491A (en) | 1991-08-01 |
| DE59004425D1 (en) | 1994-03-10 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Discontinued |