Furnace Closure

Abstract

A closure for a high-temperature furnace such as a muffle kiln is disclosed. The closure is formed of a hollow, elongated member, the rearward end of which supports a high-temperature resistant window such as a quartz panel, and the forward end of which forms a closure seal with the outer surface of the muffle while permitting visual monitoring of the interior of the furnace. The recess of the window decreases the effective volume of the furnace which permits more rapid heating of the furnace to processing temperature and more even control of the furnace at peak temperature.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a closure for a high-temperature furnace and, more particularly, to a closure permitting inspection of the interior of the furnace during processing of ceramic and metal articles such as dental prosthetics.

2. Description of the Prior Art

The preparation of dental teeth prosthetics and, particularly, porcelain-to-gold restorations involves a complicated interplay and highly intimate and critical relationship between the gold substracture and bonding porcelain, materials of entirely different physical structure and properties. Even when using materials of the highest compatibility, the criticality of the end point of the particular pyrochemical reaction in each step makes it difficult to assure proper processing in each stage.

The difficulty in assuring the proper end point is accentuated since the heat flux within the muffle furnace depends on the total mass of material within the furnace and the individual mass shape and curvature of each separate restoration. The end point may not be the same at each point of the surface but may start at certain localities and develop incrementally across the surface. If the heat treatment is terminated too early, that is underfired, it requires short, incremental heat treatments to finish the processing step. Each time the processing is resumed, it requires waiting to bring the furnace back to temperature. If the heat treatment is carried on too long, i.e. overfired, the restoration may be ruined, involving a loss of the time, effort and material in processing it to that stage or it may require a laborious effort to mechanically or chemically remove the overfired layer.

For example, the typical gold-alloy coating for a porcelain substructure is fired in air at 1,950.degree.F to a warm, bright gold color end point. An underfired coating will have a dull appearance and will not properly bond to porcelain. If overfired, some of the gold may burn off. During application of a bisquit bake, the restoration is again fired at 1,800.degree.F under vacuum for about five minutes and then in air to mature and vitrify the coating. Underfiring results in a grainy surface; overfiring can cause melting of the substrate resulting in loss of necessary contours. However, the correct end point can be visually determined. Similarly, a gold alloy metal seal coat may be applied to the gold surface to prevent gassing. Correct firing is again essential and the end point can be visually determined.

Another sensitive processing step relates to the soldering of pieces after procelaininzing. A torch cannot be used since the procelain would crack. Therefore, soldering is accomplished in the furnace. If the treatment is terminated before the solder flows, it must be repeated. If continued too long, the solder acts as a flux and dissolves the adjacent metal. Again the solder flow end point is visual.

These operations are conducted in muffle furnaces that can be operated in a vacuum, inert gas or air fired mode. Vacuum or inert gas operations must be conducted with the outer door closed and sealed to ambient. The air fired modes can be performed with the outer door open. However, the front of the muffle area must be faced with an insulated closure to prevent heat loses to the ambient and to maintain a uniform temperature within the muffle. The closures typically utilized are opaque metal and/or refractory plates that fit into the muffle opening.

The furnace is calibrated before a particular step. If it is attempted to follow a visual end point by removing the plate closure, the calibration is rendered useless and heat is lost and the temperature drops. Moreover, the excessive heat flux makes it difficult to look directly into the muffle.

It also has been attempted to place a transparent quartz plate in front of the muffle opening. The heat loss is significant making it difficult to calibrate and maintain an even temperature. Again, the intense heat flux at the surface of the quartz plate makes it difficult and hazardous to closely approach the plate and peer into the muffle area to view the visual end point.

SUMMARY OF THE INVENTION

The improved furnace closure of the invention permits direct, visual monitoring of the distinct and characteristic end points experienced during processing of dental prosthetics, while accommodating for heat loss through a transparent window. The furnace closure of the invention is formed of a high-temperature insulator. The closure comprises a front sealing section adapted to overlap the front of the muffle opening and a rear hollow viewing tunnel section, having a length equal to at least one-third the length of the furnace and preferably at least one-half the length thereof and terminating in a frame for receiving a transparent, high-temperature panel such as quartz.

The rearward disposition of the viewing panel permits effective, direct viewing by the human eye without danger of burning the observer. The rearward disposition of the panel decreases the effective volume of the muffle and, thus, permits the temperature to be maintained at an even, high temperature. Calibration is accomplished in an easier manner and is now more useful and reliable during processing. The provision of visual viewing of the end points reduces the criticality and sensitivity of the processing steps and now allows the processing to be run by less-skilled technicians, without danger of destroying the substructure.

These and other attendant advantages of this invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic side view of a vacuum muffle furnace showing the emplacement of the furnace closure of this invention;

FIG. 2 is a front elevational view of the furnace closure of the invention;

FIG. 3 is a cross-sectional view taken along the line 3--3 of FIG. 2; and

FIG. 4 is a top elevational view of the furnace closure of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, the vacuum muffle furnace 10 generally includes an outer housing 12 connected to a vacuum pump, not shown, and having a central vacuum chamber 14 in which a muffle furnace 16 is supported.

The front panel 18 of the housing includes a recessed portion 20 for forming a gas-tight seal when engaged by a door, not shown. The muffle furnace 16 is generally an elongated rectangular member formed of a high-temperature refractory such as silica or alumina in which is embedded electrical resistance heating rods 22 connected to a variable electrical power source, not shown. The furnace 10 usually also includes temperature sensing means such as a pyrometer, and a vacuum gauge.

The front panel 24 of the muffle is provided with a rectangular opening 26 to provide access to the interior chamber 27 of the furnace. The restorations 28 to be processed are usually supported on the pins of trays 30 which rest on the surface 32 of the base of the interior 27 of the muffle furnace. The furnace closure 34 of the invention is utilized by inserting it into the opening 26 in abutment with the adjacent portions of panel 24 such that it extends partially into the interior 27 of the muffle furnace.

Referring further now to FIGS. 2, 3 and 4, the furnace closure 34 is formed by a rearward, hollow viewing tunnel section 36 and an annular front lip section 38, which together form an open, elongated viewing barrel 29. The tunnel section 36 is formed of a pair of side walls 40, 42, a top wall 44 and a bottom wall 46. The exterior dimensions of the tunnel section 36 are adapted to be sized substantially to the dimensions of the interior dimensions of the muffle opening 26 so as to be slidingly received therein. The length of the walls 40, 42, 44, 46 are selected to be at least one-third the length of the interior of the muffle furnace, preferably at least one-half said length but no more than three-fourths of said length. The closure is formed of a material capable of withstanding 3,000.degree.F temperatures for sustained periods, suitably silica or alumina.

A transparent panel 48 capable of withstanding said temperatures is supported adjacent the rear end of the tunnel 36. The panel is suitably formed of quartz. The panel may be permanently secured in the tunnel. However, quartz has a tendency to darken after prolonged service at high temperature, is susceptible to the condensation of various materials on the surface of the panel 48, and quartz panels are fragile and can be cracked or broken on impact. For these reasons, it is preferable to make provision for ready removability of the panel 48.

A preferred manner of supporting the panel is to provide a sliding frame for the panel 48 composed of grooves 50, 52 formed in the side walls 40, 42 of the tunnel, a groove 54 formed in the bottom wall 46 and a slot 56 formed in the top wall 44 of the tunnel section 36. In this manner a quartz panel 48 can readily be slidingly received in the frame and can be removed whenever it is necessary to be replaced.

The front section 38 is formed of a series of interconnecting side members 58, 60, a top member 62 and a bottom member 64. These members form a lip which engages the front panel 24 of the muffle furnace 16 and serves as a stop when inserting the closure 34 and aids in insulating the closure and retaining heat within the interior 27 of the furnace. Removability is provided by forming a gripping bar 66 in the front surface of the top member 62 by recessing a cavity 70 and a cutout 68 on each side of the gripping bar 66. Thus the bar 66 may be gripped with a tool such as a pair of tongs and utilized to insert or remove the closure member 34 from the furnace 16.

It is to be understood that only preferred embodiments of the invention have been described, and that numerous substitutions, alterations and modifications are all permissible without departing from the spirit and scope of the invention as defined in the following claims.

Claims

What is claimed is:

1. A closure for a high-temperature furnace having an elongated furnace chamber comprising:

an annular, front lip section adapted to overlap the opening of said furnace chamber;

an elongated, hollow tunnel section having a length of at least one-third and no more than three-fourths the length of said furnace chamber having a rearward end adapted to be slidingly received within said chamber and having a forward end sealingly attached to said lip section and adapted to abut the front face of the opening to said chamber; and

frame means formed adjacent said rearward end for receiving a transparent panel, said closure being formed of refractory material capable of withstanding a temperture of 3,000.degree.F.

2. A closure according to claim 1 in which said chamber has a rectangular cross section, and the outer surface of said tunnel section has a correspondingly configured rectangular cross-section formed of a pair of side walls, a bottom wall and a top wall.

3. A closure in accordance with claim 1 in which the length of said tunnel section is at least one-half and no more than three-fourths the length of the length of said furnace chamber.

4. A closure according to claim 2 in which said frame means comprises a set of opposing grooves formed in the interior surface of the side and bottom walls, a groove in the bottom wall joining said side wall grooves and a slot in the top wall joining the upper ends of said side wall grooves forming a frame for slidingly receiving said panel.

5. A closure according to claim 5 in which said panel is formed of quartz.

6. A closure for a high-temperature furnace having an elongated furnace chamber comprising:

an annular, front lip section adapted to overlap the opening of said furnace chamber;

an elongated, hollow tunnel section having a rearward end adapted to be slidingly received within said chamber and having a forward end sealingly attached to said lip section and adapted to abut the front face of the opening to said chamber;

frame means formed adjacent said rearward end for receiving a transparent panel; and

means for gripping said closure.

7. A closure according to claim 6 in which the gripping means includes a recess formed adjacent an edge of the inner opening of the front lip forming between said edge and the recess a gripping bar for engagement with a clamp of a carrying tool.

8. A closure according to claim 6 in which a cutout is formed in said edge portion opposite said recess for securing said clamp from sideward movement when engaging said gripping bar.