US3260783A - Vacuum oven - Google Patents

Description

July 12, 1966 F. A. POTENZO ETAL 3,260,783

VACUUM OVEN Original Filed Feb. 17, 1961 K36 INVENTORS FRANK A. POTENZO ATTORNEYS United States Patent 3,260,783 VACUUM OVEN Frank A. Potenzo and Raymond L. Shearer, Kennebunlr,

Maine, assignorsto The'Baker Company, Inc., Biddeford, Maine, a corporation of Maine Original application Feb. 17, 1961, Ser. No. 90,090, now Patent No. 3,160,404, dated Dec. 8, 1964. Divided and this applicationOct. 23, 1964, Ser'. No. 411,666

2 Claims. (Cl. 13--31)v This is a division of copending application Serial No. 90.090,fil d'Fe-bruaryl7, 1961, now'Patent No. 3,160,404, dated'December 8, 1964.

This invention relates to industrial ovens, or moreparticularly, to'a' novetimprov'ement in what are generally referred to'aslvacuum ovens.

lncertain fields, and. particularly'the fieldof semiconductor manufacture, there has developed a need for relatively smallovens which can be arranged in conjunction withali'ne' of sealed work spaces which are referredto as'dry boxes. A controlled environment is maintained in the dry boxes. Gas lock installations between the dry boxes allow the parts to be transferred from one station to another without danger o-fftransferring contaminants. The dry boxes are fitted with hand holes which accommodate surgicalr'ubbe-r gloves or the like whereby, a workman can handl-epartswithin the dry box from externally of thedry box'withou't destroying the integrity of the box.

Vacuumovens may be placed in the line of'dry boxes so thatwork can enter an oven from one'dry box and be removedifrorn the oven into another dry box. In some instances, a vacuum'oven'may bemounted at the end of a line so th'at parts may be placedin the oven through one door therein andthen removed from another'door communicating with one of the dry boxes. The vacuum oven derives its namefrorn the factthatthe oven is evacuated to a relatively'lo'w pressure during the heattre'at-ment of the partsiin the oven. The ovens are usually fitted with sh'elves'arranged vertically in theoven and on which parts may be placed forheat treatment.

One of the problems which has. arisen in connection with vacuum ovens is that of heat'distribution in the oven. Forexample, inthe manufacture of transistors, it is import-antthat-th'e temperature at which the parts are treated is maintained very closely. With many small parts spread over aplurality, of vertically arranged shelves, it is important that'the heat distribution within the oven,,both in a vertical "and ina horizontal direction, be substantially uniformso as to assure that each of the parts is exposed to the same desired temperature. Another problem which has arisen in connection with the-vacuum oven is concerned with the sealing around the doors ofthe oven. Because it'is more resistant to high temperature, silicon rubber is a preferred material for use as a door seal. However, when thesilicon mhber is exposed tohigh temperature and 'at'the same time, is exposed to very low pressures,

the volatiles in the silicon rubber tend to boil off readily. These volatiles may contaminate the parts in the oven. Further, the rubber quickly, becomes hard and brittle and deteriorates into the condition where-it no longer performs the desired function.

It isacc-ordingly an objectof this invent-ion to provide a vacuum oven of the type described which will provide ice The invention accordingly consists in the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction hereafter set forth and the scope of the application of which will be indicated in the appended claims.

In the drawings:

FIG. 1 is a cross sectional view of a vacuum oven incorporating'the present invention; and

FIG. 2 is an enlarged fragmentary cross sectional view of theportion of theoven of FIG. 1 enclosed by brokenlines 2.

With reference to the drawings, a vacuum oven ofatype with which this invention is concerned generally comprises a stainless steel mufile 10 which, in the specific embodiment shown, is an elongated hollow member having a generally rectangular cross section and being open at its opposite ends. The mufile. is contained within a housing which comprises a pair of metal flanges 12 (only one of which is shown). which are welded to the nrufiie adjacent the opposite ends thereof. The flanges 12 extend completely around the motile andextend outwardly at right angles therefrom. A sheet metal shell 14 is suitably mounted .on the outer ends of the flanges 12 so as to surround the mufiie in spaced relation and complete the housing for the muffle. The oven is adapted to be mountedin an aperture in a wall or frame 16. To facilitate mounting of the oven, a plurality of studs are welded to the flanges or end plates 12. As shown in FIG. 1, the studs extend through a gasket lfidisposed between the end plates 12 and'wall16, and a plurality of nutsretain the parts in assembly. This manner of mounting the oven is, of course, only shown by way of example, and other suitablemeans could be used.

The oven further comprises apair of hinged doors 2t and 22 which form endwalls for the'opposite ends of the back fill pipe 26 also extends from the bottom wall of l themufileand through the shell14. The back fill pipe is intended to be valved in order to provide means for breaking the vacuum in the oven and permit the pressure within the mutile to return to atmospheric pressure prior to opening the doors. A plurality of vertically arranged shelves 27 are provided within the muffle to accommodate parts to be treated within the oven. The shelves are fabricated from a ferrous metal, preferably stainless steel. The shelves are perforated to facilitate heat transfer vertically within the muffle and are preferably slidably mounted for ready removal from the mufile.

In order to provide a heat source for'the oven, a plurality of' electrical resistance heaters 28" are arranged aroundthe top, bottom and side Walls of the muffle. In the specific embodiment, these heaters are ofa type generally referred to as tubular heaters. More specifically,

as shown in FIG. 1, a plurality of such heaters are arranged fle. The sandwiching of the heaters between the plates 30' and 32 facilitatesmounting of the heaters on the mufiie. Further, as can be seen from FIG. 1, each plate assembly is ofrelatively large planar area and covers the majority of the particular wall of the mufile. It is preferred that the plates have a relatively high thermal conductivity so that the heat provided by the heaters will be substantially uniformly distributed over the areas covered by the respective heater plate assembly. In this manner, the tendency of the heaters to provide hot spots within the mufile in the immediate area of the heaters will be substantially eliminated, and there will tend to be uniform heat distribution within the muffle.

As noted above, one of the problems in vacuum ovens of the type shown is that of providing optimum uniform heat distribution within the mufile. In order to provide more uniform heat distribution both vertically and horizontally within the oven, the inner walls of the muflle, the end walls formed by the doors 20, 22, the shelves 27 and the inside of the vacuum draw pipe 24 and back fill pipe 26 are Black Oxide coated. Thus, all of the surfaces exposed to or seeing the inside of the muflle are Black Oxide coated. In accordance with the invention, the treatment provided serves to increase the resistance to oxidizing of the surfaces treated, While at the same time provides the inner surfaces of the mufile with a much higher thermal emissivity than would be provided by the usually highly reflective surface provided by stainless steel. The problem of heat distribution in an oven of the type described is complicated by the fact that the heat treatment is conducted under a partial vacuum, for example at pressures from l l to 1X10" millimeters of mercury. At this low pressure, heat transfer within the mufile is substantially entirely by means of radiation rather than by convection. The Black Oxide coating provided in accordance with this invention will increase the radiation heat transfer from the mufi le walls to improve the efiiciency of the oven. Also, the particular treatment utilized provides a coloration and finish which is quite uniform so as to provide that the thermal emissivity of the inner surfaces of the mufile will be uniform and there will be no reflective portions which could cause hot spots. The Black Oxide treatment specifically contemplated involves heating of the mufiie at a relatively high temperature in an alkaline bath. Such treatments are normally of a proprietary nature. A specific one of such treatments contemplated is provided by the Rust Proofing and Metal Finishing Corporation of Cambridge, Massachusetts, under the name of Black Oxide No. 160. It will be understood that where the term Black Oxide is used herein, the term refers to a treatment such as that just identified.

As noted above, another problem in the operation of vacuum ovens is the rapid deterioration of the door seals because of the exposure of the seals to relatively high temperatures and low pressures. These environmental conditions tend to cause any volatiles in the seal material to boil off leaving the seal in a hard brittle condition which will seriously reduce its utility for the purpose intended. With reference to FIGS. 1 and 2, the door sealing arrangement is identical with respect to the doors 20 and 22. Accordingly, in the interest of brevity, only the details of construction of the door 20 and its sealing arrangement have been shown and will be described. The door 20 comprises a sheet metal shell 36 of rectangular cup-like configuration. A plate 38 is disposed within the door shell 36 and is fixed to the upstanding side walls of the door shell intermediate the ends of the walls so that the plate 38 extends parallel to the general plane of the door in spaced relation generally midway between the ends of the side walls of the door shell 36. The door further includes a stainless steel pressure plate 40 forming an inner wall for the door and an end wall for the muffle. The pressure plate 40 extends parallel to the general plane of the door and is mounted for movement toward and away from the plate 38. More specifically, a plurality of pins 42 are mounted on the pressure plate and extend at right angles thereto and inwardly of the door. The pins 42 extend through apertures in the plate 38 with the bordering edge portions of the apertures supporting the pins for sliding movement. A plurality of coil springs 44 are arranged over the pins 42 and engaged at their opposite ends between the pressure plate 40 and plate 38 for the purpose of urging the pressure plate away from the plate 38. The free ends of the pins 42 are provided with radially extending stops 46 to limit movement of pins, and thus the pressure plate, away from the plate 38. When the door is open, the springs 44 will urge the pressure plate outwardly of the door until the stops 46 engage the plate 38. When the door is closed, the pressure plate 40 engaged against a rectangular cross sectioned silicon rubber seal 48 arranged around the outside of the mufile. When the door is securely locked in the closed position, the seal 48- engages the pressure plate 40 and will cause the pressure plate 48 to be moved inwardly of the door against the force of the springs, whereby the pressure plate will be maintained in firm engagement with the seal 48.

The seal 48 extends entirely around the periphery of the muffle and, as can be seen in FIG. 2, the seal projects beyond the end of the muffle. In order to provide cooling for the seal 48, a cooling ring 50 is mounted on the end wall 12 of the oven housing. The cooling ring 50 is an elongated, rectangular cross sectional, hollow member extending completely around the seal 48 in contact with the side thereof opposite the muffle. A plurality of fittings 52 are provided on the cooling ring. These fittings extend through apertures in the end wall 12 of the oven housing and are adapted to be connected to a source of cooling liquid, such at water, as well as to a drain, whereby the cooling liquid may be continuously circulating through the cooling ring. As will be seen in FIG. 2, the wall of the cooling ring next adjacent the door is spaced from the pressure plate 40 when the door is closed. The flat engaged surfaces of the seal and cooling ring assure good heat transfer between these members inasmuch as these surfaces conform to each other and the over-lying surface of the ring is generally coextensive with the underlying surface of the seal. The cooling of the seal in the manner provided tends to alleviate the problems of the boiling off of the volatiles in the seal when the oven is operated at relatively high temperatures, for example 250 C. and relatively low pressures specified above. However, as noted above, the exposure of the seal to high temperature is only a portion of the problem; the other being the exposure of the seal to low pressures. Also, as noted above, the seal 48 projects beyond the end of the muifie so that, when the door is closed, the pressure plate 40 will be spaced from. the end of the mufile. This assures that firm sealing will be provided between the pressure plate and silicon seal, and that there will be no metal to metal contact between the pressure plate and the mufile. However, the spacing of the ends of the muflie from the pressure plate is particularly important with regard to another aspect of the invention.

More specifically, the end of the muflie is spaced from the pressure plate a distance selected to provide what will be referred to as an optical seal between the interior of the mufile and the silicon seal, whereby the lowest pressure to which the seal 48 will be exposed will be greater than the lowest pressure achieved within the interior of the mufile. The term optical seal connotes a seal whereby the silicon seal 48 does not see the pressure within the muflie. In the embodiment of FIG. 2, the space 54 between the muffle and pressure plate is approximately &2 of an inch, which dimension is quite small as compared to the overall dimensions of the oven. In the specific embodiment, the oven is approximately 10 /2 inches in its cross sectional diameter and approximately 14 inches long. It will be apparent from FIG. 2 that the only portion of the seal directly exposed to the interior of the mufile is that portion which projects beyond the ends of the muffle. When the oven is evacuated, the air at the outer end portions of the gap or space 54 can be drawn out only by passing through the very narrow passage provided by the space 54 and into the interior of the muffle. As the air is drawn from the muflle, there will be no flow of air through the passage 54 which will readily sweep the molecules of air from the. passage 54. This is particularly true as the pressure within the muffle is substantially reduced. Rather, the molecules of air in the passage 54 will tend to move between the opposing surfaces of the end of the muffle and pressure plate 40 substantially at right angles to these surfaces instead of moving parallel to these surfaces. This action materially retards the evacuation of the space 54 with the result that a higher pressure will be present within the passage 54 than will be present within the main body of the muffle. This will be particularly true in the area of the passage immediately adjacent the seal 48. It can be seen that the molecules of air in the passage 54, and particularly at the end thereof adjacent the seal 48, follows what amounts to a tortuous path into the interior of the muflle. Admittedly, their path is not as tortuous as would be the case if the passage 54 were a labyrinth passage. However, the configuration shown has been found to be sufficient for the purpose intended. It is to be understood, however, that a more complex optical seal is within the scope of this invention. Admittedly, there Will be a tendency of the air molecules within the passage 54 to diffuse into the main body of the muffle until the pressure within the passage 54 is equal to the pressure within the muffle. However, such diffusion will require a substantial length of time to occur. In practice, the oven will normally be opened every three or four hours, and in this short length of time, the advantages gained by the optical seal will not be lost to any substantial extent by diffusion.

Accordingly, it can be seen that by the door sealing arrangement of this invention the silicon rubber seal is both exposed to temperatures considerably less than that within the muflle and is exposed to pressures substantially greater than the pressure within the muflle. By this means, the service life of the seal has been materially improved. The specific arrangement of the cooling ring and rubber seal results in the cooling ring being disposed remotely from the muffle so as not to cause cooling of the ends of the muffle with an attendant impairment of the uniformity of heating in this area of the muffle. Also, the cooling ring is not in contact with the pressure plate so as to preclude undesired cooling of the end wall of the mufile formed by the pressure plate.

Inasmuch as changes could be made in the above construction and apparently different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the language in the following claims is intended to cover all the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having thus described our invention, we claim:

1. In a vacuum oven, a ferrous metal muffle having side, top and bottom walls and having end walls, electrical resistance heating means arranged in intimate heat trans fer relation over the majority of the area of the outer surface of each of said side, top and bottom walls of the muffle, at least one of said end walls being formed. by a door which closes one end of the muflle and which includes a ferrous metal plate-like member extending across said one end of the muffle in sealed relationthereto, a Black Oxide coating on the inner surfaces of all of the aforementioned walls of the muflle, said Black Oxide coating being evenly distributed over the entire area of the muffle walls and materially improving the thermal emissivity of said top, bottom, and side walls and materially improving the uniformity of heat distribution both vertically and horizontally through the interior of the muffle when the over is operated at a vacuum, and means for drawing a vacuum on the interior of the muffle.

2. In a vacuum oven a stainless steel muffle having side, top and bottom walls and having end walls, electrical resistance heating means arranged in intimate heat transfer relation over the majority of the area of the outer surface of each of said side, top and bottom walls of the muffle, at least one of said end walls being formed by a door which closes one end of the muflle and which includes a stainless steel plate extending across said' one end of the muffle in sealed relation thereto, a plurality of stainless steel shelves arranged vertically within the oven, a Black Oxide coating on the inner surfaces of all of the aforementioned walls of the muffle and on said shelves, said Black Oxide coating being evenly distributed over the entire area of the muflle Walls. and shelves to materially improve the thermal emissivity of said top, bottom and side walls and materially improve the uniformity of heat distribution both vertically and horizontally through the interior of the muflle when the oven is operated at the vacuum, and means for drawing a vacuum on the interior of the muffle.

References Cited by the Examiner UNITED STATES PATENTS 1,127,021 2/1915 Klett 219-407 2,035,757 3/1936 Ottenstein 219-407 2,078,840 4/1937 Dorl et al. 219-406 2,085,450 6/1937 Rohn 2257 2,156,845 5/1939 Gentile 3473 2,251,380 8/1941 Warrender 13--3l 3,028,476 4/1962 Hug 219201 X 3,118,042 1/1964 Parker 219406 OTHER REFERENCES Westinghouse Engineer, Control of Radiant Heat by Surface Finish, July, 1954, pp. 147-151.

RICHARD M. WOOD, Primary Examiner.

V. Y. MAYEWSKY, Assistant Examiner.

Claims (1)

1. IN A VACCUM OVEN, A FERROUS METAL MUFFLE HAVING SIDE, TOP AND BOTTOM WALLS AND HAVING END WALLS, ELECTRICAL RESITANCE HEATING MEANS ARRANGED IN INTIMATE HEAT TRANSFER RELATION OVER THE MAJORITY OF THE AREA OF THE OUTER SURFACE OF EACH OF SAID SIDE, TOP AND BOTTOM WALLS OF THE MUFFLE, AT LEAST ONE OF SAID END WALLS BEING FORMED BY A DOOR WHICH CLOSES ONE END OF THE MUFFLE AND WHICH INCLUDES A FERROUS METAL PLATE-LIKE MEMBER EXTENDING ACROSS SAID ONE END OF THE MUFFLE IN SEALED RELATION THERETO, A BLACK OXIDE COATING ON THE INNER SURFACES OF ALL OF THE AFOREMENTIONED WALLS OF THE MUFFLE, SAID BLACK OXIDE COATING BEING EVENLY DISTRIBUTED OVER THE ENTIRE AREA OF THE MUFFLE WALLS AND MATERIALLY IMPROVING THE THERMAL EMISSIVITY OF SAID TOP, BOTTOM AND SIDE WALLS AND MATERIALLY IMPROVING THE UNIFORMITY OF HEAT DISTRIBUTION BOTH VERTICALLY AND HORIZONTALLY THROUGH THE INTERIOR OF THE MUFFLE WHEN THE OVER IS OPERATED AT A VACCUM, AND MEANS FOR DRAWING A VACCUM ON THE INTERIOR OF THE MUFFLE.

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