US3128326A - Electrically heated vacuum furnace - Google Patents

Description

A ril 7, 1964 K. J. HINTENBERGER 3,128,326

v ELECTRICAL-LY HEATED VACUUM FURNACE Filed Feb. 28, 1961 3 Sheets-Sheet l H u INVENTOR lllll ATTORNEY April 7, 1964 Y K. J. HINTENBERGER 3,123,326

ELECTRICALLY HEATED VACUUM FURNACE Filed Feb. 28, 1961 3 Sheets-Sheet 2 I N VENTOR Km; Jam flnvmvamsre ATTORNEY United States Patent r 3,128,326 ELETRICALLY HEATED VACUUM FURNACE Karl Josef Hintenherger, Hochstrasse 39, Frankfurt am Main, Germany Filed Feb. 28, 1961, Ser. No. 92,370 Claims. (Cl. 13--31) This invention relates to an improved electrically heated vacuum furnace used particularly in denture ceramics, tor instance, to bake, fire and glaze artificial teeth, crowns, and the like, and generally, where the items to be processed in the furnace are made of ceramics and are fused, or dried, at reduced atmospheric pressure in a chamber accommodating the heater mufiie.

More specifically, the invention relates to assemblies comprising an instrument case and a furnace mounted thereon, where the instrument case contains a vacuum pump and driving means for the same as well as switching and control devices for all power-consuming units while the furnace comprises a vacuum chamber mounted in spaced relationship to and above the chest-like instrument case, the furnace chamber being connected by a conduit or pipe to a vacuum pump. The furnace comprises a vertically extending inverted cup-shaped chamber heat-insulated or shielded at the sides and the top, but open in the lower face which is nearest the instrument case. A charge elevator also is supported by the instrument case for vertical movement toward and from the top so that vthe furnace charge placed on a horizontal table of the elevator is moved from a loading position into the furnace, the table also having means to seal the furnace aperture.

7 Vacuum furnaces of the type above described have several advantages over those where the charge is inserted into the furnace from the top or from one side, these being, (1) the loss caused by heat radiation is reduced due to the fact that the furnace top has no aperture at its top side, and (2) the operator is not discomforted by the heat radiated from the furnace charging opening.

Another advantage of the type of vacuum furnace comprising the invention is that the charges placed within the furnace chamber always move identical short distances and in a uniform, smooth manner, resulting in uniform products, the homogeneity being enhanced as a result of the elimination of the conventional necessity of removing and inserting, by tweezers, shielding bricks and heat insulators into and from the furnace, respectively.

Itis an object of this invention to provide an improved vacuum furnace in which the operations are automatically controlled and especially the manual work is simplified over that required in conventional porcelain firing furnaces so as to eliminate human errors and irregularities resulting from manual handling, as well as [faulty operation particularly under conditions where a substantial series of successive firings are required.

Another object of this invention is to eliminate some of the control functions previously required to be performed by human operators, thus simplifying the mode of operation, reducing manual work to a minimum and providing greater convenience and reliability.

Still another object of this invention is to provide electrical circuits and electrical devices adapted to provide proper timing and control of individual operations.

A further object of this invention is to provide cooperative functioning of the vacuum and pressure ends of the vacuum pump by means of electrical and pneumatic mechanism so as to conveniently and automatically control and perform mechanical operations.

Another object of this invention is to provide a vacuum furnace with adjustable means to control the various mechanical, electrical and pneumatic means which may 3,128,326 Patented Apr. '7, 1 964 ICC be regulated and interrelated without interference or impedance.

Details of the foregoing objects, and others, are set forth in the following specification and are illustrated in the accompanying drawings forming a part thereof.

In the drawings:

FiG. l is a front elevation of a vacuum furnace according to the present invention and showing the elevating apparatus in its lowered position, prior to being elevated.

FIG. 2. is a schematic side elevation of a control system according to the invention for moving the elevating apparatus up and down by means of the vacuum and pressure produced by the vacuum pump of the device.

FIG. 3 is a circuit diagram of the instruments and controls of the preferred embodiment of the invention.

FIG. 4 is a vertical sectional view, partially in perspective, of the rfiurnace and especially showing details of the insulated firing chamber, including the columns upon which the furnace is supported and details of the connection between the furnace and the instrument casing, and

FIG. 5 is a fragmentary vertical elevation, partially sectioned, showing details of the detachable connection between the columns carrying the furnace and the instrument casing, on a larger scale than in FIG. 4.

Referring to FIG. 1-, there is shown a supporting base comprising a housing or case 1 containing the instruments, and a furnace 2. The furnace 2 has an internal firing or baking chamber, which is an inverted cup, supported on columns 3 a predetermined distance above case 1. The front side of case 1 contains the dials of the various switching and control units, such as the operating handles and dials of two timing switches 4 and 5, a voltmeter 6, a transformer 7, a toggle type main switch 8, which is manually operated, bywhich the entire circuit is controlled, and two pilot lamps 9 and '16]. The furnace is provided with a vacuum gage 11. Hose 12' is connected, at one end, to a vacuum pump 20, see FIG. 2, arranged inside of the instrument casing, and, at its other end, to a control valve, not shown, mounted at the rear of the furnace Z.

The elevating apparatus 13 is supported by casing 1 and consists of a pneumatic cylinder 26 and piston 27:, and table 14 connected to the upper end of piston 27, said table also serving to hermetically close the opening in the bottom of the furnace chamber. For this purpose, a seal 15 of silicone rubber is arranged on the, perimeter of table 14;. Concentrically arranged on the table 14 within seal 15 is an asbestos washer 16 which abuts against the rim of the opening in the furnace. A cylindrical charge carrier base 17, which carries three columns supporting an upper disc portion, is mounted coaxially on the table 14. This charge carrier base 17 is formed of ceramic material such as fire clay. A charge carrier 18' is provided which may consist of a bowl and supporting members, if desired. A charge to be fired is placed with in the carrier 18 and may, for example, consist of a ceramic artificial tooth or crown. I

To perform the firing process according to the present invention, the elevating apparatus 13 is operated to elevate table 14 by means of pneumatic piston 27, after the charge has been inserted into the carrier 18. Although this movement may be manually performed by a suitable handle rod 19, the invention contemplates prefer-ably that the piston 27 be moved automatically by means of air' pressure from the pressure side of the vacuum pump 20. Cylinder 26, as shown in FIG. 2, communicates at its bottom with a pipe in which a ball check valve 34 is mounted below the bottom of the cylinder, and a vent hole 35 is formed in saidpipe between the cylinder and the ball check valve, said vent hole having a diameter substantially smaller than that of the pipe. Said pipe is.

connected by means of another pipe 25 to the pressure side of the vacuum pump it and has a valve block 28 connected therein which is provided with an opening having a valve seat 29. Similarly, the suction pipe 24 connected to the suction side of the vacuum pump 26 leading to the furnace has a valve block 24a having an opening also provided with a valve seat. B- oth valve blocks 24a and 29 are closed or opened by electrical means provided for that purpose, to be described.

Referring now to FIG. 2, the vacuum; pump 23 is shown having a suction pipe 21 provided with a hose, connector 22 to which the hose 12 shown in FIG. 1 is connected. Furthermore, a connecting nipple 23 is provided at the pressure side of the pump. Air pressure line 25 is connected to the connector 23 at the pressure side of the pump 20 and leads to the hollow cylinder 26 in which piston 27 is closely slidable for moving the lifting apparatus. A double headed valve member 31 is movable reciprocably in a guide member 30 between valve seat 24a which controls flow to suction line 24 and valve seat 29 which controls flow to pressure line 25. Movement of valve member 31 into seat 24a is caused by means of a retracting spring 32. Upon energization of an electrical relay 33, described hereinbelow, the double-seat valve member 31 is moved away from valve seat 24 a, against the action of the retracting spring 32, and is pressed against the valve seat 29 of the valve block 28. p The arrangement which has been described above is used to produce the following results relative to the firing furnace comprising the invention. When the vacuum pump of the system is started, preferably after the furnace 2 has been heated to firing temperature, the double head valve 31 is moved against the valve seat 2? of the valve block 28 through energizati-on of relay 33, whereby the vacuum pipe 24 is opened to atmosphere, and air under pressure from nipple 23 is transmitted through line 25, through ball check valve 34, thereby elevating the ball of said valve and acting on the piston 27 to elevate it and the table 14 toward the entrance opening in the lower end of the furnace 2. In this manner it is possible, smoothly, and without jamming, to move the charge carrier together with the charge into the baking chamber, in a highly improved manner over previously used manual insertion.

Upon de-energization of the relay 33, spring 32. retracts the double seat valve to its normal seat position against seat 24a, whereby the vent line of the vacuum pipe is closed and vacuum is produced in the interior of the furnace 2 to draw table 14 tightly against the furnace opening, since the vacuum rapidly increases to accomplish this. Although pressure line 25 is opened by the aforementioned shifting of valve member 32, table 14 will not drop quickly in any event, prior to the action of the vacuum, because of the limited air discharge through small vent hole 35 and the action of ball check valve 34 which is in its seat under such conditions. Relay 33 which can for example be of the electromagnet type, may be replaced by any other suitable mechanism to reverse the double headed valve member 31. It is also possible to use other types of electrically controlled devices, such as slide valves, to alternately close and open the valves in the valve blocks 24a and 29.

Reversal of the double headed valve member 31 can be further improved by a time switch, such as a timing apparatus connected in conventional manner to a suitable electric switch, the time switch being connected in series with the circuit which supplies current to the motor of the vacuum pump and to other components, if necessary, and being adapted to reverse the movement of the double headed valve member rather than by using the above mentioned relay or other electric switch. As the circuit of the motor for the vacuum pump, to which other devices such -as the furnace heating elements and its controls are connected, is closed by actuating a main switch 8 which is provided on the instrument casing and is connected to a suitable circuit of a source of electrical energy, through the means of an electric plug 36 as shown in FIG. 3, for example, and the timing switch 4 has been set for a desired period of vacuum firing time, produces a desired degree of vacuum in the furnace, the relay 33 will be energized by timing switch 4 to connect pressure pipe 25 to cylinder 26 and thereby cause the elevating apparatus to move table 14 upwards. Such vacuum atmosphere in furnace 2 is maintained until the expiration of the time as set by the timing switch 4, whereupon the circuit to the vacuum pump motor is interrupted, so that the vacuum pump is stopped, and the elevating apparatus gradually will be lowered as the air from cylinder bleeds through hole 35, as described.

The foregoing description refers to the time switch 4 in the circuit of the vacuum pump and the other aforementioned units by which the circuit thereto is closed at the instant the time switch is closed and interrupted after a predetermined period of time. Further advantages re salt from the inclusion of a second timing switch 5 which is connected in series with this first timing switch, the contacts of the second timing switch being arranged so that a circuit leading from said second timing switch and through the control relay 33 is opened at the instant when the switch starts to run, and is closed upon termination of the running time of the switch. Relay 41 is arranged to reverse the double headed valve member 31 in the manner described above, as will be seen from FIG. 3 of the drawings.

In FIG. 3, there is also illustrated an electric plug 36 which is on a flexible electric cord extending from the instrument casing 1 and is connected in the circuit with main switch 8 mounted on the instrument casing and also with a time delay timer switch 37 conveniently mounted on case 1. The main switch 8 serves to close the main circuit 38 which controls the operation of the motor of the vacuum pump 21), and the muflle heater 40 within the furnace 2 for heating the chamber thereof, and another relay 41 which functions as a temperature-sensitive switch of conventional type and is adapted to interrupt the current supply to the furnace heater 4t) automatically upon the chamber reaching a preselected maximum temerature. To achieve such interruption of the current, a pyrometer 52 is mounted in the furnace chamber and is connected in the heater circuit. The pyrometer is connected to the relay 41 by leads 43 and 44 shown in FIG. 3.

The variable transformer '7 serves to infinitely vary the heater voltage and is provided with scale divisions which are identical with those on voltmeter 6. Excessive or under voltages are compensated for by transformer 7. Pilot lamp 9 of a certain color serves to indicate that the circuit to the furnace heater 40 is ON, as controlled by the action of the relay 41. Another pilot lamp 10 of a different color indicates that the circuit of the entire system is energized.

The contacts of timing switch 5 are arranged so that circuit 38 as well as conduit 50, which is connected with in said circuit between timing switch 4 and relay 33 (see FIG. 2), will be closed when timing switch 5 starts to run. After expiration of a predetermined running time, time switch 5 interrupts the entire circuit 38. The contacts of the second time switch 4 are so arranged that conduit 50, which is connected with relay 33, will be interrupted when the switch 4 starts to run, and will be closed again after expiration of a predetermined running time for which time switch 4 is set.

This circuit arrangement permits the device to operate as follows: When the furnace has been put into operation by closing switch 8 and has been suitably heated by the heater 4%, and the charge has been placed on the charge carrier table 14 of the lifting apparatus, time switch 5 is set for the total desired heating time for furnace 2. Since the device, as described above, provides for automatic control of the entire process commencing from the insertion of the charge until the removal of the finished material, and also including in the time cycle control of the operation time and duration of the vacuum atmosphere within the furnace, followed by restoration of atmospheric pressure during the remaining firing period, the second time switch 4 is preset for a given time interval to control the vacuum firing period duration.

Under the foregoing circumstances, the circuit for heating the furnace chamber 2 includes the variable transformer 7 and the motor for the vacuum pump is started directly upon closing timer switch 5. Relay 41 has a rocking mercury switch which is ready to operate. By starting pump 20, the valve seat 29 of valve block 28 of the pressure line is maintained closed by the double headed valve member 31 as long as time switch 4 is not actuated. Piston 27 now moves upward to elevate the charge into the furnace. Upon time switch 4 closing, pressure line 25 is opened by movement of valve member 31 away from seat 29. However, the elevating apparatus remains in its upper position, since it is held in place by the suction action of the vacuum within the furnace chamber. Upon expiration of the predetermined running time of switch 4, the circuit leading from this timing switch to relay 33 is again closed so that the lifting apparatus will remain in its upper position because of the application of pressure to piston 27. By reversing the double headed valve member 31, the furnace is opened to atmospheric air, whereby the further firing is accomplished under atmospheric pressure until expiration of the running time of timing switch 5, whereupon the current supply to the vacuum pump is interrupted. The lifting apparatus then slowly lowers and permits the removal ofthe baked charge.

A bypassing switch 42 serves to bridge the timing switch 5 past one of its poles when the furnace is to be heated up to a predetermined temperature. By this bridging arrangement, it also is possible to dispense with the automatic interruption of the heating circuit. In such case, which is of practical importance in performing several successive firings, the muffle remains constantly heated, it being controlled only by the relay 41.

The invention also permits firing under atmospheric pressure alone, without including vacuum, by using only time switch 5, whereby the pump 20 and the relay 33 are energized so that the elevating apparatus raises table 14. Upon expiration of the running time of time switch 5, both the pump and the relay are de-energized, thus causing the table 14 to move downward.

For accomplishing the entire baking process under vacuum it will be necessary first to activate timing switch 5 and shortly afterwards, switch 4. Both switches must be set for the same period of elapsed time under such circumstances. However, provision should be made that time switch 4 is actuated a little earlier than switch 5 so that relay 33 will reverse when intended and, hence, insure elevation of the table 14. This small time differential is necessary, since, if time switch 5 should open earlier, the valve controlled by relay 33 would remain closed against its seat.

FIG. 4 and the enlarged partial view of FIG. 5 illustrate how the space between the instrument case 1 and the furnace can be easily adjusted to set the proper stroke length for piston 27 which is very important relative to automatic control of the elevating apparatus to insure that the table 14 may positively engage the rim of the furnace opening. FIG. 4 also illustrates certain details of the design of the furnace 2. The metal casing 53 of furnace 2 is open at its upper end and is closed by a cover which is partly broken away in the illustration. Heat-insulating bricks 56 are arranged in such manner that the heater 40 is held in place therein within helical grooves which are disposed at a certain distance from the bottom entrance opening of the firing chamber 57.

A tube projecting from the pyrometer extends through an opening in the bricks 59 which line the firing chamber 57. The circuit leads 43 and 44 of the pyrometer 52 are connected to the relay 41 (FIG. 3), as described hereinbefore. A cap brick 60, which is shown partly broken away, is formed of ceramic insulating material and surrounds the bricks 59 through which the pyrometer extends. The casing 53 has a bottom flange which is secured to columns 3 upon which collars 62 are mounted. Cover 54 of the casing is securely connected to an upper flange of the casing by knurled cap nuts 63 which engage the upper threaded ends of upper extensions on the columns 3 which extend through openings in the flange and the cover 54. I

The lower ends of the columns 3 preferably are inserted into guide means 64 (FIG. 4) in the instrument case 1 and are provided with threaded end portions 65 (FIG. 5) having supporting nuts 66 thereon. By adjusting the nuts, columns 3 can be adjusted vertically relative to nuts 66 and the guide means 64 in general.

The columns 3, or at least one of them, should preferably be hollow in order to receive electric leads 69 for connection of electrically energized items in the furnace 2 with the controls in the instrument case 1. For this purpose, the columns 3 are preferably of the plugtype so that the entire furnace can be easily removed from the instrument casing. These plug-type columns have moreover the advantage that, when the furnace is placed upon the instrument case, the electrical connection is made automatically also. For this purpose, the lower open ends of the columns 3 are provided with insulators 67 which are locked in the columns by set screws 68. Moreover, the leads 69 extending through the columns are provided with split, elastic metallic plugs 70. The cover of the instrument case also contains guide members for the insulators, each guide member consisting of an insulating ring 71 secured to the cover from below, and a bottom portion 72 of the same insulating material, both parts of which are secured to the cover by screws 73. A metal bushing 74 in the bottom plate '72 serves to connect conduit 75, comprising part of circuit 38, with conduit, 69 within the column.

I claim:

1. An electrically heated vacuum furnace particularly for fusing dental ceramics and comprising in combination, a base housing, an inverted cup-shaped furnace chamber supported above said base with the entrance facing said base, a vacuum pump connected by conduit means to the interior of said furnace chamber, the sides and the top of the furnace chamber being insulated for heat retention, a charge elevator movable in opposite vertical directions above said base housing toward and from the entrance in the bottom of the furnace chamber and provided at its upper end with a charge carrying horizontal table adapted to fit against the entrance of the furnace vessel to seal the entrance opening tightly when the table is lifted into engagement with the said aperture, the lowerend of said charge elevator having a piston slidably movable within a vertical cylinder for movement in opposite directions, an air conduit connected to the lower end of the said cylinder, a ball check valve mounted on said conduit, said conduit having between this valve and the said cylinder a bleeder hole of a diameter substantially less than that of said conduit, the other end of said air conduit being connected to the pressure side of the said vacuum pump, a conduit extending from said air conduit to receive air under pressure therefrom and having a valve seat at the outer end thereof, another branch conduit extending from said vacuum conduit interconnecting the vacuum pump and the said furnace vessel and having a valve seat at the free end thereof, and valve means operable relative to said valve seats successively first to close said valve seat in said first-mentioned branch conduit to cause air pressure to raise said piston and charge-carrying table against said entrance of said furnace to close it and said valve means then being movable to close said valve seat in said second mentioned branch conduit to close said vacuum conduit to said furnace chamber to maintain said table against said furnace entrance while said air pressure is released from said piston.

2. The furnace according to claim 1 further including a solenoid connected to said valve means and operable to actuate the same to close said air pressure conduit, and spring means connected to said valve means to move it to close said vacuum conduit.

3. The furnace according to claim 1 further characterized by said valve means comprising a valve member reciprocable in opposite directions between said valve seats, means to reciprocate said valve member in the sequence aforesaid, and timer means operable automatically to control the interval between such reciprocating movements of said valve member first to close the furnace and next to apply vacuum to the furnace.

4. The furnace according to claim 3 further including electric heating means for said furnace, circuit means to connect the same to a source of current, and a second timer switch connected in said circuit in series with said a timer switch for said valve member, said second timer switch being operable to control the period of time the furnace is heated.

5. The furnace according to claim 4 further including an electric motor to drive said pump and said motor being connected in the circuit of said electric heating means for said furnace and controlled by the timer thereof, whereby when said heating means is shut off by said timer therefor said pump will stop and permit said chargecarrying table to descend by the dispelling of air from said cylinder thereof through said bleeder hole in said air pressure conduit.

References Cited in the file of this patent UNITED STATES PATENTS 1,617,359 Westberg Feb. 15, 1927 1,678,875 Rohn July 31, 1928 1,994,917 McGregor Mar. 19, 1935 2,966,537 Witucki et a1 Dec. 27, 1960 3,075,263 Iuckniess et al Jan. 29, 1963

Claims (1)

1. AN ELECTRICALLY HEATED VACUUM FURNACE PARTICULARLY FOR FUSING DENTAL CERAMICS AND COMPRISING IN COMBINATION, A BASE HOUSING, AN INVERTED CUP-SHAPED FURNACE CHAMBER SUPPORTED ABOVE SAID BASE WITH THE ENTRANCE FACING SAID BASE, A VACUUM PUMP CONNECTED BY CONDUIT MEANS TO THE INTERIOR OF SAID FURNACE CHAMBER, THE SIDES AND THE TOP OF THE FURNACE CHAMBER BEING INSULATED FOR HEAT RETENTION, A CHARGE ELEVATOR MOVABLE IN OPPOSITE VERTICAL DIRECTIONS ABOVE SAID BASE HOUSING TOWARD AND FROM THE ENTRANCE IN THE BOTTOM OF THE FURNACE CHAMBER AND PROVIDED AT ITS UPPER END WITH A CHARGE CARRYING HORIZONTAL TABLE ADAPTED TO FIT AGAINST THE ENTRANCE OF THE FURNACE VESSEL TO SEAL THE ENTRANCE OPENING TIGHTLY WHEN THE TABLE IS LIFTED INTO ENGAGEMENT WITH THE SAID APERTURE, THE LOWEREND OF SAID CHARGE ELEVATOR HAVING A PISTON SLIDABLY MOVABLE WITHIN A VERTICAL CYLINDER FOR MOVEMENT IN OPPOSITE DIRECTIONS, AN AIR CONDUTI CONNECTED TO THE LOWER END OF THE SAID CYLINDER, A BALL CHECK VALVE MOUNTED ON SAID CONDUIT, SAID CONDUIT HAVING BETWEEN THIS VALVE AND THE SAID CYLINDER A BLEEDER HOLE OF A DIAMETER SUBSTANTIALLY LESS THAN THAT OF SAID CONDUIT, THE OTHER END OF SAID AIR CONDUIT BEING CONNECTED TO THE PRESSURE SIDE OF THE SAID VACUUM PUMP, A CONDUTI EXTENDING FROM SAID AIR CONDUIT TO RECEIVE AIR UNDER PRESSURE THEREFROM AND HAVING A VALVE SEAT AT THE OUTER END THEREOF, ANOTHE BRANCE CONDUIT EXTENDING FROM SAID VACUUM CONDUIT INTERCONNECTING THE VACUUM PUMP AND THE SAID FURNACE VESSEL AND HAVING A VALVE SEAT AT THE FREE END THEREOF, AND VALVE MEANS OPERABLE RELATIVE TO SAID VALVE SAEATS CUSSESSIVELY FIRST TO CLOSE SAID VALVE SEAT IN SAID FIRST-MENTIONED BRANCH CONDUIT TO CAUSE AIR PRESSURE TO RAISE SAID PISTON AND CHARGE-CARRYING TABLE AGAINST SAID ENTRANCE OF SAID FURNANCE TO CLOSE IT AND SAID VALVE MEANS THEN BEING MOVABLE TO CLOSE SAID VALVE SEAT IN SAID SECOND MENTIONED BRANCH CONDUIT TO CLOSE SAID VACUUM CONDUIT TO SAID FURNACE CHAMBER TO MAINTAIN SAID TABLE AGAINST SAID FURNACE ENTRANCE WHIE SAID AIR PRESSURE IS RELEASED FROM SAID PISTON.

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