US2097093A - Furnace - Google Patents

Description

Oct. 26,y 1937. E. H. BERRYMAN 2,097,093

ELMER h'. BERRVMAN.

A TTORNE Y Oct. 26, 1937,

E'. H. BERRYMAN FURNAGE Filed om. 1a, 1933 2 sheets-sheet 2 INVENTORr ELMER H. BERRY/1MM A-TTORNE Y Patented Oct. 26, 1937 UNITED STATES PATENT OFFICE 7 Claims.

This invention relates in general to casting furnaces and in particular to a pneumatically operated ldental casting furnace. f

To make a casting that is exactly complementary to the mold from which it is made the casting material should of course be maintained in a molten state until it has completely filled the mold. This requires that the temperature of the casting material be maintained above its melting point during the entire process and that a su'ostantially constant and uniform pressure be applied to it during the casting operation. Since most materials contract upon cooling and solidify- 1 ing it is essential in making a Vdental casting that the casting material cools and solidifies progressively from the point furthest away from the sprue hole to the entrance of the sprue hole. I have found that a closed air tight muflie forms the most convenient equipment in which a technique of this kind can be carried out.

In this method of making castings the investment ring is subjected to a more continuous application of elevated temperatures and consequently some means must be resorted to to protect the ring as much as possible.

The length of time required to melt the casting material preparatory to casting depends not only upon the temperature used but also upon the amount of material and its melting point. The simplest method of determining when the casting material has reached a molten state is to look at it periodically. The light given off from a molten metal is very intense and I have therefore found it desirable to view the material i through a colored sight glass or window.

In general the object of my invention is the provision of a pneumatically operated casting furnace by which the casting material can be maintained above the melting point during the entire operation and subjected to a substantially constant and uniform pressure during the actual casting period.

Another object of my invention is the provision of a pneumatically operated casting furnace the muiile of which has a substantially negligible volumetric gas capacity in comparison to the gas receiver which serves as the source of pressure.

A further object of my inventionV is the provision of a furnace having a colored Window so that the action of the molten metal within the mule may be watched.

Another object of my invention is the provision of an investment ring having a set of bayonet pins at each end so that the ends are interchange- (Cl. ,Z2-57.1)

able, one set of pins being used to lock the investment ring in place whiie the other set serves as a means by which the ring may be gripped by a pair of tongs having apertured ends adapted to engage the pins.

Still another object of my invention is the provision of a sprue holder and Crucible former to be used in conjunction with an investment ring to form a protective layer of investment material over that end of the ring which is subjected to 10 the most intense heat.

Another object of my invention is the provision intermediate the furnace and the source of air pressure of a receiver having a volumetric capacity such that the volumetric capacity of the muiiie 15 is negligible in comparison to it.

The invention possesses other advantageous features, some of which with the foregoing, will be set forth at length in the following description where I shall outline in full the form of my in- 20 vention which I have selected for illustration in the drawings accompanying and forming a part of the present specification. In said drawings,

I have shown one form of my invention, but it is to be understood that I do not limit myself to 25 such form, since the invention as set forth in the claims, may be embodied in a plurality of forms.

Referring to the drawings:

Figure 1 is a vertical mid-section of the casting furnace taken on the broken line l--I of Fig- 30" ure 2.

Figure 2 is a horizontal section taken on the line 2-2 of Figure 1.

Figure 3 is a vertical mid-section taken through the investment ring and its associated sprue hold- 35 er and crucible former.

Figure 4 is a diagrammatic representation of the furnace in combination with a source of high pressure and an intermediate gas receiver.

Figure 5 is a vertical midsection of a larger investment ring of modified construction.

Figure 6 is a section on the line 6-6 of Figure 4.

As best shown in Figure l the furnace comprises a generally cylindrical metal casing i 45 provided at its base with inwardly and outwardly extending anges 2 and 3 respectively and bolted to the outwardly extending flange of a depending generally semi-circular carriage or apron 4. If desired a gasket 4 may be placed 50 between the casing I and the apron 4 to decrease the transfer of heat between these two elements. Supported on the upper face of the flange 2 is a hollow inverted funnel shaped muiile 5 having an upwardly extending stem 6. Forming part 55 of the muifle 5 is an annular plate l the periphery of which forms a channel 8 which accommodates a gasket 9. The peripheral edge of the muffle 5 rests on the gasket 9 and its lower surface overlies an annular refractory plate lil. The muiiie 5 and its associated plate l are made of quartz or other refractory material. `Fused to the upper end of the stem 5 is a colored quartz window l l.

Diametrically opposed pockets i2 and i3 formed on the outer surface of the casing l accommodate electric contact members lf3 and l5 which are bolted in place as clearly shown in Figure l. The contact members and the bolts by which they are to be secured to the casing may be keyed to prevent any relative rotation and are insulated from the casing by means of insulating washers l and mica tubes l5. The free ends of the contact members lli and i5 are forked or bifurcated and are formed with curved surfaces i'l. As shown in Figure 2 two sets of spaced contact members are provided.V Spaced electric heating elements i8 and i9 are symmetrically disposed within the mule 5, one on either side of the muie stem To the ends of these heating elements are sweated Ascaloy caps 29 provided with outwardly extending vertical webs 25 and with curved shoulders 2i complementary to the curved surfaces il of the contact members. The curved tines of the contact members straddle the webs 25 and are resiliently seated on the complementary curved shoulders 2i. From this construction it will be noted that contact is always maintained between the contact members and the heating elements over the entire surface of the curved shoulders 2l regardless of the expansion 'or contraction of the heating elements due to changes in temperature.

As shown in Figure l the heating'elements i8 and i9 pass through apertures formed in the casing l and in the mufile 5, and it is to be particularly noted that the muiiie 5 is only deep enough to accommodate the heating elements, and therefore has what may be termed a minimum air or gas capacity. This feature is of considerable importance as will be elxplained later.

The top of the casing l is covered by means of a disc 22 supported on the shoulder '23 formed on the inner` surface of the casing i. The muile stem 6 is sealed to the disc 22 by means of a boss 2li ldepending from the plate 22.

The entire cylindrical casing i is enclosed by meansof a bell 25, the. lower peripheral edge of which is supported on a gasket 26, carried in a groove formed in the outwardly extending flange 3 of the casing l. Screws 2l secure the bell 25 to the casing l and hold its peripheral edge sealed against the gasket 26. Gasket washers 28 disposed beneath the heads of the screws 2 prevent the leakage of air or gas outwardly through the bell. The disc 22 and the bell '25 are provided with central apertures 2S and 3l adapted to register with the mufle stem 5, and' consequently an unobstructed view into the center of the muflle is permitted. A short stem 32 is provided on the bell 25 and is internally threaded to accommodate a threaded plug 33. Engaged by the piug etV between twoy gaskets 3d and 35 is a second transparent quartz or glass window 35 which may be either colored or clear.

As shown in Figure 2 the mufle 5 is of a substantially rectangular shape. The stem 6 how- Y ever is of circular cross-section at its upper end.

but merges with the mule in an ellipse. This construction therefore gives an extended unobstructed view into the mulile between the heating elements and permits a greater area to be viewed within the muiiie than would be permitted by a cylindrical stem.

The casing l is provided with passageways 3l' and 38 which establish communication between the interior of the 5 and a gas receiver which will be explained later in connection with Figure 4l. The passageways 3l and 33 are formed in part (as shown in Figure l) by registering apertures 35i, (il, ft2, and i3 formed in the plate 'l and in the refractory disc i6.

To the lower end of the carriage or apron 4 is secured a plate. 45, by any suitable means, such as screws It@ and threaded to this plate is a screw al the lower end of which is provided with a crank LES, while to its upper end is swiveled a plate 5. A cup 5l provided with a iiange 52 and formed with counterbores and 53 is secured to the plate i9 by means of screws 51%. |ihe ange is formed with diametrically opposed notches 55 adapted to straddle ribs 55 and 5l formed on the interior surface of the apron 5. It will therefore be seen that since by this construction the rotational movement of the cup 5l is prevented, the cup of necessity must move in a vertical direction rotation of the crank 8. The upper face of the flange 52 is recessed to accommodate a gasket 5S, and alternate rings 5S and 6i are formed on the floor of this recess and on the lower of the inwardly extending flange 2 for the purpose of biting into the gasket 53 to thereby form a tighter seal between the cup 5i and the flange 2 when the cup is in its uppermost position. If desired the gasket 58 may be held in place by small pins 52 secured to the ange 52, and having their heads bent over into the gasket.

Secured'within the cup 5l is a sleeve provided with diametrically opposed bayonet slots Gi. Disposed within the cup 5l is an investment ring which is adapted to be locked within the cup by the engagement of the bayonet pins S5 within the bayonet slots l. The lower end of the investment ring 55 is sealed against an annular gasket 5i' placed on the floor of the cup 5l and a screen 53 accommodated in the counterbore 53 prevents foreign matter such as particles of investment from plugging the gas outlet 55 provided in the counterbore 53'. The gas outlet 59 communicates through a valve 'il as shown in Figure 4, either with the atmosphere or with a source of vacuum. The contact members id and l5 may be connected to a source of electric current by leads is provided with two sets of bayonet pins, one

set 55, and another set l2 at its opposite end. As has already been explained one set is used for locking the ring within the cup 5l. The other set serves as a means. by which a pair of tongs having apertured ends may be used to lock high pressure tank 85.

the investment ring in place or to dsengage it. This feature is of importance, for it has been found that considerable force is sometimes required to disengage the investment ring after it has been subjected to the heat within the muffle, and the application of the ordinary tongs to the periphery of the ring scores the ring within a short time. By making both ends of the ring identical and providing each end with a set of bayonet pins, either end can be made to serve as the bottom or the top and no damage is done to the ring in handling it.

The upper end of the ring is shown as capped by a sprue holder and crucible former 13. This member is used only when the mold within the investment ring is being formed and serves three purposes. A central convex boss 14 forms a concave depression or crucible in the investment material for the metal button used in making the casting. A small hole 15 formed in the boss 14 holds the sprue in place and the sloping walls 18 form an annular shoulder of investment material which completely overhangs or covers the upper peripheral edge of the investment ring. The purpose of this overhanging shoulder is to protect the upper edge of the investment ring against the intense heat to which it would otherwise be directly exposed when within the muiiie. As shown in Figures l and 4, pipes 8| communicate with the passageways 31 and 38 leading to the interior of the muiile and are connected by means of a valve 82 with a gas receiver 83. The gas receiver in turn communicates through a pressure reducing valve 84 with a The volumetric air or gas capacity of the mufile should be negligible in comparison with the volumetric capacity of the gas receiver 83.

To fully appreciate this relationship some consideration should be given to the operation of the furnace. After the mold for the casting has been prepared in the investment ring, a button of metal placed in the crucible formed in the upper part of the ring, the investment ring locked in the cup 5| and the cup 5| locked in its uppermost position so as to form a closed chamber with the muiie 5, the heating elements |8 are connected with the source of current. By looking through the wind-ows and 38 which are in axial alignment with the investment ring it can easily be determined when the metal has been heated to a molten state. Previously, the valves 1| and 82 have been closed and the pressure reducing Valve 84 adjusted to maintain an operating pressure of say 30 pounds within the gas receiver 83. The pressure within the tank 85 may be of course run as high as 1500 pounds. When the metal within the muille has been completely melted, valve 82 is opened thereby sub- 1, jecting the interior of the muflie and the molten metal to substantially the pressure of the gas receiver 83. The molten metal is forced down through the sprue hole into the mold completely lling it and displacing the air which was previously contained therein. This displaced air is driven downwardly through the porous investment material into the bores 53 and 53 of the cup 5|. The volumetric capacity of the bores 53 and 53' is of course somewhat greater than the capacity of the mold and sprue hole. The

temperature within the munie is maintained throughout the period that the casting is being made and consequently the metal within the mold tends to solidify from the bottom upwardly instead of in the reverse direction as would be the case if the heat were discontinued. By having the volumetric capacity of the mufile negligible as compared with the capacity of the gas receiver 83 the molten metal is subjected to a substantially constant pressure. For let V1 represent the volume of the receiver 83, P1 its initial pressure, V2 the combined volume of the mufi'le and the receiver and Pz the nal equalized pressure within the muflle and the receiver after the valve 82 has been opened. By applying the well known law ViXPi and bearing in mind that in the present instance V2 is substantially equal to Vi, it will be seen that P2 is substantially equal to P1. It is therefore desirable to have a relatively large volume of gas available at substantially the operating pressure and an intermediate receiver answers this requirement most conveniently. In the absence of the gas receiver 83 the gas pressure within the pipes leading from the pressure reducing valve 84 would be materially reduced when the valve 82 is opened for the small quantity of gas contained in this pipe would be expanded into the mulfle. By maintaining the capacity of the mufile and the counterbores in the cup 5| as small as possible and using a relatively large gas receiver there is substantially no drop in pressure and consequently a denser and better defined casting can be made.

The design of the investment ring permits of its being manipulated without injury and the shoulder of investment material formed over its free edge protects it from the intense heat to which it would otherwise be subjected. The use of a, colored window in the mufile stem reduces the intensity of the light emitted by the molten metal to such an extent that it can be comfortably watched to determine its condition. The complementary curved contacting surfaces of the heating elements and the contact members insures a good electrical connection between these members regardless of any changes in the length of the heating elements due to temperature variations. Furthermore 'because of its particular design the furnace above described can be easily constructed, operated, inspected and repaired. To disassemble the muflie and replace the heating elements it is only necessary to remove the bell 25 by unscrewing the screws 21. The cup 5| and its operating screw are readily removed by simply unscrewing the screws 4S and the cup itself can be removed by means of the screws 54. The casing and the depending carriage or apron 4 have been made as two distinct elements largely to diminish the transfer of heat from the former to the latter.

For the purpose of making large castings I have shown in Figure 5 a large investment ring which although of a somewhat diiferent construction than the ring above described can be operated in exactly the same manner. As shown the ring has a cylindrical base 9| and tapered open neck 92 which merges with a shoulder 93. The diameter of the base 9| is such that the base can be supported on the gasket 58 of the cup 5| and the tapered neck 92 is small enough to be accommodated within the opening formed at the center of the inwardly extending ange 2. An annular gasket 94 is adapted to be disposed between the shoulder 93 and the lower face of the flange 2 and effects a seal between these two members. This sealing action is aided by the beads or rings 5i formed on the ange 2 and beads or rings formed of the shoulder 93. When this investment ring is sealed in this way between the cup 5| and the ange 2 the casting operation can be carried out under pressure in exactly the same manner as when a small ring is used and with the same advantages.

The screen 66 may be replaced by an inverted cup (not shown) provided with a perforated bottom and disposed within the cup 5| with its bottom (inverted) slightly below the level of the top of the cup 5l. In effect this merely raises the level of the screen 68 and decreases the volumetric capacity of the cup 5I when the large investment ring is being used.

I claim:

1. A furnace comprising a casing provided with a depending apron; a shallow inverted funnel shaped mule supported within said casing; a heating element disposed within said muflie at right angles to the axis thereof and a cup slidably associated with said apron and arranged in its uppermost position to close the lower open side of the muille.

2. A furnace comprising a casing provided with a depending generally semi-cylindrical apron; a shallow inverted funnel shaped mufe supported within said casing; a heating element disposed within said muiile at right angles to the axis thereof and a cup slidably disposed within said apron and arranged in its uppermost position to close the lower open side of said muiile.

3. A furnace comprising a casing provided with a depending apron; a shallow inverted funnel shaped muie supported within said casing; a counterbored cup slidably associated with said apron and arranged in its uppermost position to close the lower open side of said muflle; a gas line communicating with the interior of the muiile and a gas outlet provided in the counterbore of the cup. 1

4. A furnace comprising a casing provided with a depending apron and an inwardly extending flange; a muiiie supported on the upper face of said ange and a cup slidably associated with said apron and adapted in its uppermost position to engage the lower face of said ange and thereby form a closed chamber with said muile.

5. A furnace comprising a casing provided with a depending apron and internal and external flanges; a muiile supported on the upper face of said internal flange; a cup slidably associated with said apron and -adapted in its uppermost position to engage the lower face of said internal flange; and a bell inclosing said cylindrical casing and supported on its external flange.

6. A furnace comprising a casing having inwardly and outwardly extending flanges and a depending carriage; a shallow inverted funnel shaped muffle supported on the upper face of said inwardly extending flange; a counterbored work retaining cup slidably associated with said depending carriage and arranged in its uppermost position to close the lower open face of said muile, a cover disposed over the upper end of the casing; diametrically opposed recesses formed on the outer surface of the casing and electric contact members supported therein; electric heating elements secured to said contact members and extending through the casing into the muffle; a bell enclosing said casing and supported on said outwardly extending flange; a gas inlet associated wtih the muiiie and a gas outlet associated with said work retaining cup. Y

7. A furnace comprising'a casing provided with a depending apron; a mufile having an open lower face supported within said casing; a screw associated with said apron; and a cup swiveled to the upper end of said screw and adapted in the extended position of the screw to close the open face of the muilie.

ELMER H. BERRYMAN.

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