US 2932864 A
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Description (OCR text may contain errors)
April 19, 1960 E:. .1. MELLEN, JR., ErAL 2,932,864 METHOD oF MAKING AND DRYING SHELL-TYPE REFRACTORY MoLDs Filed June 17, 1958 5 Sheets-Sheet l INVENTORS 'l .Edward JlMellen Jr.
Robert deFass elle Joh/n M. Webb B 7;; ATET'LNEYS April 19, 1960 E. J. MELLEN, JR., Em 2,932,864
METHOD OF MAKING AND DRYING SHELL-TYPE REFRACTORY MOLDS Filed June 17, 1958 5 Sheets-Sheet 2 INVENToRS ATT/62e Evs .Edward Mellen Jr. Robert .LoZeFvz/,Sselle John M. Webb BY@ 7 April 19, 1960 E. J. MELLEN, JR., ETAL 2,932,864
METHOD OF MAKING AND DRYING SHELL-TYPE REFRACTORY MOLDS Filed June 17. 1958 `5 Sheets-Sheet 3 u INVENTORS EdWaraZJMellen Jr.
Robert J- oZeFaSselle John M. Webb April 19 1960 E. J. MELLEN, JR., ETAL 2,932,864
METHOD OF' MAKING AND DRYING SHELL-TYPE REF'RACTORY MOLDS Filed June 17, 1958 5 Sheets-Sheet 4 IIII "\ u E /v"\ Lm* INVENToRs f Edward Mellen Jr.
Robert 1. deFa/,sselle Jahn M- Webb g F1 7. 5 BMMFOW April 19 1960 E. .1. MELLEN, JR., ETAL 2,932,864
METHOD OF MAKING AND DRYING SHELL-TYPE REFRACTORY MOLDS Filed June 17, 1958 5 Sheets-Sheet 5 United States Patent METHOD OF MAKING AND DRYING SHELL-TYPE REFRACTORY MOLDS Edward J. Mellen, Jr., Shaker Heights, Robert l. de Fasselle, Willoughby, and John M. Webb, (lhagrin Falls, Ghio; said De Fasselle and said Webb assignors to said Mellen Application June 17, 1958, Serial No. 742,554
11 Claims. (Cl. 22--196) This invention relates to a method for rapid production. of thin-walled or shell molds of refractory material for precision casting of metal articles. These articles can range in size from a few ounces to several hundred pounds. Molds are formed by the dipping of a destructible pattern in a refractory slurry, drying the refractory materials on the patterns, repeating the process to build up a suitable thickness of mold, and then destroying or removing the pattern material and firing the mold.
The invention relates to the rapid production of uniform thin walled molds with uniform permeability utilizing destructible pattern materials which are solid at normal room temperatures. A major feature of this invention relates to the rapid drying of the molds with closely limited and controlled temperature changes to provide dimensional accuracy.
This great advance in the art of precision casting is made possible largely by a fast, rapid production method of building up the shell mold over destructible patterns in which a multi-layer shell mold can be built up and the mold completed, fired and ready for pouring in a period of three hours. This is comparable to present known methods of building up an investment mold or shell type mold in a minimum of 24 hours. Accompanying this rapid production is the fact that molds are produced equal to or better than prior methods. An improved finish and a controlled grain size of the casting is also obtained.
One object of the present invention is to provide a method and apparatus for building up and drying a plurality of refractory layers on a destructible pattern to make a mold suitable for precision casting within as short a time as 3 hours or even less.
Another object is to provide a method and apparatus for making a shell mold for precision casting which is satisfactory and practical in every Way and gives tolerances and other physical characteristics which are at least equal,
to those obtained by the frozen mercury casting process.
Still other objects, uses and advantages of the present invention will become apparent to those skilled in the art from the following description and claims and from the drawings in which:
Figure l is a plan view of an apparatus suitable for applying wet refractory coatings around a pattern and drying the same in accordance with the present invention;
Fig. 2 is a fragmentary elevational view taken along the lines indicated at 2--2 in Figure 1 and showing the dipping enclosure and drying apparatus of Fig. 1 with parts broken away and shown in section;
Fig. 3 is a fragmentary sectional view taken along the lines indicated at 3 3 in Fig. 2;
Fig. 4 is a fragmentary sectional view on an enlarged scale of the exhaust damper system of the drying apparatus of Fig. l;
Fig. 5 is a fragmentary sectional view taken along the lines indicated at 5 5 in Fig. 3;
Fig. 6 is a fragmentary view on an enlarged scale Showing a ceramic mold formed over a pattern `and suspended from a continuous conveyor which carries the -molds in and out of drying tunnels;
Fig. 7 is a front elevational view with parts broken Fig. 1l is a fragmentary elevational view of the drying apparatus of Fig. 10 with parts broken away;
Fig. 12 is a fragmentary side view of the drying ap paratus of Fig. 10; and
Fig. 13 is a sectional view taken along the lines indicated at 13-13 in Fig. 12.
According to the present invention, the temperature of the pattern material is held substantially constant from the time the pattern is formed until the pattern is removed from the shell mold. A room temperature in the range of 70 to 80 F. is selected as the pattern temperature and this temperature held even though the processing is continuous and fast. The wet ceramic coatings are quickly yet safely dried using a high velocity air stream which has a wet bulb temperature equal to that temperature originally selected for the pattern. The dry bulb temperature of the air stream may be maintained at any point from the wet bulb temperature upward, say generally about to 110 F. using a Wet bulb of 75 F. The dry bulb temperature used is dependent upon the number of wet layers to be dried, the nature of the ceramics and pattern materials and the configuration of the mold.
A plan view of apparatus for veution is shown in Fig. l. apparatus A which comprises a series of connected tunnels 5, 6, 7, 8, 9, 10 and 11 which are U-shaped in tunnels or where molds may be stored overnight WithoutV harm.
The mold and pattern assembly and the means of handling the same is shown in Fig. 6. A handle or eye '4l is molded into the pattern 38 and is placed over conveyor hook S8 to suspend the pattern from the conveyor. The pattern may be referred to as a cluster, each cluster containing a multiplicity of parts such as turbine blades and buckets. The mold is formed by dipping the pattern in a slurry of ceramic material to deposit a thin layer,
such as layer 39, over the pattern. A series of ceramic layers deposited in this way forms the mold.
yThe molds are hung on a continuous conveyor chain 12 which transports them in and out of the drying tunt nels. Before following the path of a mold in and out of the drying tunnels of the drying apparatus A,I the means of transporting the molds will be discussed in further detail. As best seen in Fig. 6, the conveyor means comprises a hollow rolled metal tubing 52 with a 1ongitudinal slot 53 in the bottom and a set of generally parallel spaced rollers 54 which ride on the inside of the hollow tube adjacent the slot. A stem 55 is rigidly connected to a shaft 56 upon which the rollers are rotatably mounted and extends down through the slot to swivel "joint 57. Hook 58 connects to swivel joint 57 and into Patented Apr. 1s, leso.`
practicing the present in- A This consists of a drying escasas eye 41 molded into the pattern .38. The swivel 57 is preferably provided with a rubber roller 59 which rolls along on a rail or shoe 60 which is mounted on one side of channel space 61. Thus the clusters are rotated to insure even drying as they pass through the drying tunnels.
The pattern is first exposed to controlled temperature conditions in enclosure E until it reaches the control temperature, which is usually about 75 F. The pattern may also be brought to the control temperature Vby being kept in a separate room or by other suitable means. Once the pattern has reached the control temperature, it is dipped in the first dip tank 18. The dip tank contains a ceramic slurry and deposits awet layer of ceramic on the pattern.
The path of a wet pattern through drying apparatus A starts as a cluster is suspended on one of the hooks S' of the conveyor means. The pattern with the wet coating thereon is transported by the conveyor intoincoming leg 1S of drying tunnel S through central portion 17, and thereafter out of the tunnel through outgoing leg 16.
The cluster or mold travels through the remainder of drying apparatus A by going through drying tunnels 6, 7, 8, 9, and 1l. After leaving outgoing leg 16 of tunnel 5 and before entering tunnel 6, the dried mold is dipped in dip tank 19. Dipping is done manually by a Worker stationed at the tunnel exit as shown in Fig. 3. The worker unhooks the pattern from hook 58, dips it, dusts it ifV called for, and places it back Von the hook. In a similar manner, the mold is dipped and `dried as each successive ceramic coat is built up. Thus, the mold -passes by dip tanks 19 through 24 inclusive during its travel in and out of the tunnels 7 through 1i). The mold emerges from tunnel 11 with an adiabatically dried coating comprising seven ceramic layers.
In accordance with the present invention, the incoming and outgoing legs of each drying tunnel are contoured and shaped to maintain the passage of an air stream at high velocity past the wet molds. For this reason, mold and other air ilow restricting objects such as the. hook 58 take up generally at least 1/s and preferably at least l of the cross sectional area of the contoured tunnel. =For example, Fig. 6 shows a section of incoming leg 25 of tunnel 10 having side walls 66 and 67. The width of the cluster is equal to at least 1/1 the distance between the side walls 66 and 67.
The cross-sectional outline of each of the legs ofthe other tunnel are preferably the same as leg Z5 of tunnel 10 for the best drying results. The shape of the tunnel legs can be varied according to the size and shape of the molds being dried so that the outer surfaces of theV wet molds are subjected to a rapidly moving air stream which provides a high lm coeliicient as previously described.
.Fig. 3 shows the first dip tank 18 at which a man stands and manually removes each pattern from the conveyor, dips it into the dip tank and returns it to the conveyor. After the first dip, the wet ceramic coating 39 next is dusted with ceramic particles. As best shown in Figs. 7, 8 and 9, the dusting apparatus 35 comprises a hollow cylindrical dusting chamber 43 and a suction fan 44, mounted on one end of the cylinder or drum 43 which draws just enough air out of the dusting drum to prevent the dusty air from escaping into enclosure B. The wet ceramic coatings` are dusted by merely holding and rotating by hand a cluster of molds inside the drum through an open end 45. The mold hook rests on notch 37'of support 36. The drum is rotated by a pair of driving rollers 46 and a curtain or shower of fine refractory particles is provided by this rotary action in conjunction with a plurality of buckets 47 which are attached along the inner circumference of the drum. The sand is continuously in motion, being scooped from bottom of the drum by the rotating buckets Iand in turn released from the buckets by the force of gravity when the buckets reach the top position.
Fines are carried into theifan 44 from the drum by means of conduit 48. The ines are transported out-bythe fan 44 by means of `conduit `i9 and into a header conlivered to the drying tunnel 1) which has incoming leg 25 and outgoing leg 26, is supplied by air conditioning means comprising a blower 69. The blower 69 has a conduit 'Til connected to its suction side and conduit 71 connected on its positive side. Blower 69 also supplies air to tunnel 11 5 which has incoming leg 2S and outgoing leg 29 as best seen in Fig, 3. Located in conduit 71 are heating means comprising a hot water heating coil 72 having a plurality of metal tins 73, the coil being heated by hot Water which enters the coil through water inlet 74 and leaves the-coil through water drain pipe 75. Also located in conduit 71 is a means for supplying water to the air stream comprising an atomizing steam nozzle 76 which delivers steam into the heated air stream.
After the air has been properly treated by heating and humidifying operations the air is blown into header conduit 77 which may feed air into one or two or even more of the tunnels. For example the air stream blownk into header conduit 77 enters central portion 27 which houses the U-shaped portions of both tunnels 10 and 11. The stream of air coming intocentral portion 27 dividespart going down tunnel 1u and part down tunnel 11. 1
The air stream entering tunnel 10, also divides, part going down leg 25 past wet molds Aand another part going past wet molds down outgoing leg 26 of tunnel i6, In a simi lar manner, tunnels 6, 7, 8, 9 rand 10 are supplied with rapidly moving conditioned yair.
Returning again to the path of the mold in drying tunnel 5, it i s noted the ceramic coating forming the mold becomesv gradually drier as it continues down incoming leg 15, around central portion '17 .and down outgoing leg 16. Care should be exercised during the travel of the mold inthe outgoing leg 16 since the ceramic coating might become over-stressed if the mold isl allowed to re main in the drying tunnel too long `after becoming dry. In production, preferably about 50-80% of water is removed from the molds in -leg 15 and the molds are preferably completely dry just 'before leaving tunnel 5 whereupon they are immediately dipped into `a second dip tank 19 located just outside of drying tunnel 6. Thus -a second layer of ceramic material is Iapplied over the pattern, the mold again suspended from the conveyor and carried in and out'of tunnel 6 where the first and second layers are dried. The process steps of applying successive layers of ceramic material over the patternand adiabatic drying are repeated in the drying tunnels 7, S, 9, 10 and 11. While it is not necessary to dust after each Clip, the procedure of dusting each wet layer after it is dipped may be preferred. When the wet ceramic layers are dusted after each dip, the mold is built up faster, and its permeability is easier controlled and its strength is increased.
Referring to Fig. 3, `it can be seen that one end of suction conduit 70 is located near the exit and entrances of the tunnel so that room air used for diluting the moistureladen air coming down the drying tunnel does not pass over the molds during their adiabatic drying step. Referring to Fig. 2, to further describe the path of the molds through drying apparatus A, the builtin shell molds leave drying tunnel 11 and are carried outside the drying apparatus A and enclosure B on the conveyor chain which is moved by driving means 78.
The dried molds are removed from the conveyor chain after being conducted past driving means 78 and control panel 79 which may be used to house temperature and humidity controllers. The diluting air from the outside atmosphere preferably enters the drying chamber from openings provided in the dryer for ingress and egress of the molds. The amount of air entering the system from the outside is controlled by venting some of the recycling air to the atmosphere. -It can be seen that whatever amount of air is vented only an equal amount of diluting air will be able to enter the system.
In `accordance with the above considerations, we have found that an excellent drying apparatus can be made by locating the suction side of a `blower near the entrance and exit openings of the. dryer. The discharge of the blower is directed through a conduit over the air conditioning means and enters the drying tunnels -at the vertex of the Us such as the central portion 1'7 of drying tunnel 5. The exhaust is taken ot said conduit preferably between the fan and the conditioning means so that energy is not expended in conditioning exhaust air. The sampling elements of the control means which modulate the heating and humidifying mechanisms yare located ata point in the lower conduit sutiiciently removed from the conditioning means to assure complete mixing of air at this control point.
At the point of entry of the air `from the tunnel into the suction side of the blower a plenum 82. is provided with baie `83 and water pan 83a to collect iinely dispersed particles of ceramic which may be picked up from the mold and prevent their recirculation. The exhaust of this apparatus is recirculated into the dip dust enclosure B to provide humidity as required for the processes taking place in this area. A cooling coil 102, more fully described hereinafter, is provided to prevent excessive temperatures in this area. The quantity of exhaust air which is sent to this area and the amount it is cooled is controlled from elements located Within the drying apparatus enclosure B. It is possibie in application where desired, to utilize this exhaust air as the basic conditioning means for a complete assembly building or plant. An annular' area 153 is provided at the point where the exit air enters the cooling coil so that when the total quantity of air required by the enclosure B and drying apparatus A exceeds that being supplied from the exhaust system, the proper additional quantity of room or outside 'air will he drawn across the cooling coil. Suitable additional cooling and/ or heating means might be required in this instance.
Referring to Fig. 3 to illustrate the above comments, means of venting the moisture laden air from air conditioning unit 68 is shown comprising an exhaust stack Si) and a damper 81. As shown in the accompanying draw ings, air conditioning units 84, 85 and 816 which feed air into tunnels 5, `6, 7, 8 and 9 respectively, may be vented in a manner similar to air conditioning unit 68. The units 84,185 and 86 are vented through dryer exhaust stacks 87, 88 and y89 respectively, which in turn along with dryer stack 80 of air conditioning unit 63, are headered into a main stack 90 by means of two horizontal iiues 91.
As best shown in Figs. 3 and 4, the moisture laden air exhausted through stack 80 may be used to air condition and humidity enclosure B. A relatively large conduit 93 joints the main stack 90 at the juncture of main stack 90 and exhaust stack 80. Humidity laden exhaust air from the air conditioning units can be diverted to enclosure B through conduit '93 and duct 94 by means of main stack damper 95 and damper 96 which controls the flow of moisture-laden air into enclosure B through conduit 93 and duct 94. The dampers, as shown in Fig. 3, are connected by linkage 97 which holds the dampers in a fixed position relative to each other. The dampers are opened and closed when the position of the linkage is changed by air controller 98. YAs seen in Fig. 3, the damper plates 95 and 96 are partly open. As seen in Fig. l0, when the main stack damper `95 is closed, the damper 96 is full open.
As previously indicated, when damper 96 is open, humidiiied air ilows through conduit 93 and into duct 94 where it passes over a cooling coil 102 to reduce the air temperature to about room temperature before reaching enclosure B; Outside air enters the otherwise closed circuit from an annular opening C103 between conduit 93 and duct 94. Conduit 93 is smaller in cross sectional area than duct 94 so that make-up air can enter the system around the duct 93. From duct 93 the make-up air passes through enclosure B and into the air conditioning units through the entrances and exits to the U-shaped tunnels.
A low volume form of a drying apparatus C is shown in Figs. l() to 13 which can be used for batch-type adiabatic drying of ceramic mold clusters in accordance with the present invention. The generally rectangular shaped dryer C has three compartments '105, 106, 107 used to house clusters 108 of wet molds during drying. The fourth compartment on tunnel 109 contains an air conditioning unit `110 to supply treated air to adiabatically dry the wet ceramic coatings. Compartments 105 and 106 are upper drying tunnels while 107 is a lower drying tunnel. The mold clusters 10S do not circulate through the drying compartments, remaining stationary once in the compartments while the treated air stream passes rapidly by the clusters.
The air conditioning unit 1110 has a fan 111, a steam nozzle 1'12 and a hot water heating coil 113. The conditioned air is delivered by fan 111 into a feed header 114 from Where it blows down the 3 drying compartments into return header 115 where it is returned once again to fan in tunnel |109. Louvres 116 and 117 may be adjusted to exhaust on a suitable amount of wet air and admit fresh air into the units. The mold clusters 108 are conveniently placed in the drying compartments by hanging them from a long slidable hanger or a T-shaped rail 118. Fig. 13 shows how molds may be suspended from rail 118 of compartment 106 by means of hooks H9. As noted in detail in Fig. 13, the rail 118 is T-shaped in cross section and may be pulled out as illustrated in dotdash lines in Fig. l2 so that the mold clusters 198 can be suspended from the rail conveniently by a worker. A rail 118 is located in each of channel spaces 105er, 106a, 107a and 108a which are dead air spaces over compartments 105, `166, 107 and 10S respectively. A rail 118 used in channel space 106e over compartment N6 rides on a pair of front rollers 121 which are rotatably mounted on U-shaped housing 123 and has a pair of rear rollers 122 which are rotatably mounted to the rail 118 itself.
As also seen in Fig. l2, a door 126 for compartment 106, is rigidly connected to one end of rail 118, and the door can be opened conveniently by handle 127. Thus the door and rail both slide out to a lirst position for mounting of the clusters and both slide shut to a second position where the molds are dried adiabatically. The refraetory coatings are dried in accordance with the adiabatically drying principles already discussed for drying apparatus A. After the prime coating has been dried, at say 75 F. wet bulb and 90 F. dry bulb, the temperature of the drying unit C may be increased say to 95 F. dry bulb for the next coating while still maintaining the 75 F. wet bulb temperature. Thus the clusters are dried quickly in a manner similar to that already discussed in which elevated temperatures are used efficiently and safely. The clusters are removed from the oven, dipped and dusted if desirable and then returned to the oven for the subsequent drying operation. The cycle is repeated for each layer. As the mold layers are built-up with each cycle, the dry bulb temperature of the drying air may be increased while the same wet bulb temperature is maintained.
It is important that the temperature of the pattern material be held constant from the time the pattern first enters the drying chamber, until the time that the pattern material is removed from the shell mold. Room temperature, generally 75 or 80 F. is generally selected as the control temperature Ior pattern material temperature since it is convenient to store the patterns and apply the ceramic coatings to the molds at room temperature. The pattern assegnata itself shouldbe stored at the predetermined temperaturel for preferablyseveral hours before starting the processing. This procedure of always maintaining the pattern material at a substantially constant temperature insures a subsequently produced mold that is dimensionally correct.
In the present invention, the wet ceramic coating is dried rapidly yet safely by moving air of controlled quality and velocity past the shell mold coatings which may be suspended from a moving chain in aV drying tunnel for eliicient processing.
We have found that the temperature of the wax pattern material or other pattern material remains constant during the initial drying phase where the coating contains at least some water. As long as the coating has some water in it, the coating and the pattern material maintain a constant temperature, namely, the wet bulb temperature of drying air. Even after the outer surface of the ceramic coating becomes bone-dry in the drying process and its outer surface temperature approaches the dry-bulb temperature, the ceramic coating only a few thousandths of an inch below the surface apparently can be relatively cool for a `short period of time. Thus in accordance with the present invention, the heat transfer characteristics ofthe ceramic material itself is utilized.
Thus, while care must be exercised during the drying of the completed green shell mold, apparently the completely built-up green mold doeshave more strength than the only partially completed shell mold. This additional strength, even though it be small, is enough to generally enable the green mold to withstand a very small change of pattern temperature without undue stress on the green mold. While it is preferred that the pattern temperature may not vary more than plus or minus F. even after the green mold is built-up, the temperature may change as much as 10 F. in some cases without objectionable stress on the shell mold.
As aforementioned, the wet bulb temperature selected is equal to the dry bulb temperature of the working room. For economic considerations, dry bulb temperatures then will be determined not only on the basis of the drying rate, but so as to give a moisture level at the wet bulb selected which is also above the maximum ambient absolute humidity expected in the particular area where the `oven is located. By this means no expensive dehurnidiiication means will be required.
The wet bulb temperature is selected to equal the room temperature and the slurry temperature and a maximum dry bulb temperature varying from about 5 and upward above the wet bulb is provided. This dry bulb temperature is established in accordance with the strength of the mold at the particular stage of coating, the maximum allowable being used to accelerate drying. As successive layers are applied, the mold gains strength, permitting greater dry bulb temperature levels which gives an accompanying lowering of the air steam vapor pressure with an accompanying increase in the drying rate. This is desirable because each successive layer gives a green mold with a higher water content. Thus this mechanism provides equal drying times for each coating.
For example, when drying wet ceramic coatings over a destructible pattern and using air with a wet bulb temperature of 75 F. throughout, the prime coat may be quickly, yet safely dried by air having a dry bulb ternperature in the range of 90 F. The second and third coats may be advantageously dried with a dry bulb temperature of 95 F. the fourth and fifth coats with a dry bulb temperature of 100 F. and the sixth and seventh coats with a dry bulb temperature of 105 F. In other words, as the mold is built up over the pattern, the diy bulb temperature of the fast moving drying air is increased successively, giving in each step conditions of reduced humidity and air stream vapor pressure. The increments of increased dry bulb temperature as shown before have been about 5 F. per step higher. Increments. may be required and/orV desirable in certaincircumstances depending; upon the nature of the ceramic and.v pattern material used and the configuration of the mold.
Once the wet bulb temperature has been selected, a maximum dry bulb temperature of about 5 to 35 F.
above the wet-bulb temperature has proved to provide fast economical drying of the shell molds. The dry bulb temperatures that can be used with a wet bulb temperature of 75 F. are about 80 F. to 110 F. depending upon the amount of build-up of the shell mold; generally only temperatures from 5 P. up to about 20 F. over the wet bulb temperature being tolerable for accelerated drying of the irst refractory layer without cracking the layer and temperatures up to 40 or 50 F. over the wet bulb being tolerable only after the final layer has been applied to the pattern.
The dry bulb temperature used to dry the layer applied after the prime layer may be the samevas that for the prime layer or may be increased slightly, for` example 2 to 3 F. and not more than 5 F. It is then increased in increment of 2 to 7 F. with each successive layer until the dry bulb temperature used to dry the last layer is from 20 to 35 F. above the wet bulb temperature. Larger temperature increments of up to 10 F. may be used in certain circumstances when the pattern and Ceramic permit.
The accelerated adiabatic drying is accomplished by selection and maintenance of a certain wet bulb temperature for the drying air, as previously indicated,` and by drying with gradually decreasing relative humidity as the mold layers are built up. For example, when drying wet ceramic coatings over a wax pattern and using air with a wet bulb temperature of 75 F. the prime coat and the second coat may be quickly yet safely dried by air having a relative humidity of l5-55%. The third and fourth coats may be advantageously dried with a relative humidity of 35-45 percent, the fifth and sixth coats with a relative humidity of 25-30 percent and the last coat with a relative humidity of l525 percent. In other words, as the mold is built-up over the pattern, the relative humidity of the drying air may be gradually reduced as much as 3 to 5 percent per coat depending on the speciiic nature of the ceramic and pattern materials.
In order -to accomplish accelerated adiabatic drying with minimum temperature and, vapor pressure diierences, it is important that the velocity of the drying air stream be the maximum allowable consistent with the characteristics of the mold. Experience has shown that velocities in the 4range of 2000 ft. per minute can be used with molds of reasonable large surfaces and small mass without danger of breakage or blowup of ceramic particles. Molds of greater mass might tolerate substantially higher velocities, there having been established no denite upper limit to this.
A prime consideration is, of course, to contour on a lixed or adjustable basis the tunnel cross-sections so that a minimum area is always provided and maximum velocity will be obtained for the least quantity of air moved.
The specific drying units referred to herein have had chambers with cross-sectional areas in the range of 3 to 4 sq. ft. and the quantities of air have ranged from 10,000 to 15,000 cu. ft. per minute per chamber although air volume as low as 4,000 cu. ft. per minute could be used. In any event a minimum velocity of at least 800-1000 ft. per minute should be used to obtain the benefits of the present invention. In the present process, one method of making a shell mold, of good strength and resistance to thermal shock, is to apply the first layer so that Yit is of a thickness of .005 to .020" and while it is still wet it is advantageous to dust the prime `layer with fine refractofry particle using suthcient force to embed the particle therein. A method of dusting or sanding is used that provides a dense uniform cloud of tine particles that strike the wet coating with substantial impact force. The
force is, of course, not so great that it breaks or knocks off the wet prime layer fromr the pattern. The above 9 process is repeated until a multiplicity oflayers; is obtained, the thickness of the layers generally being about from about .005I to .02" although in some layers the thickness may reach about 0.2 inch particularly when only 2 or 3 layers are used.
Also, in the present invention, the nal layer need not be dusted with refractory particles, satisfactory results being obtained without the dusting step. This makes the subsequent handling of the molds easier and eliminates chances of the small refractory particles from falling into the casting.
The invention is further illustrated by the following example:
EXAMPLE A destructible pattern, having a temperature equal to 75 F., of a turbine bucket for a jet engine similar to those shown in Fig. 6 of the drawing was dipped in an aqueous ceramic slurry having a temperature of 75 F. to form a prime refractory layer about .005 inch thick. The formula of the ceramic slurry is shown below:
Ceramic slurry-Ingredients High and low temperature binder, colloidal silica litres-.. l
Thickener, methyl cellulose grams-- Ceramic material, zirconium lbs-.. 100
The wet prime layer formed from the above slurry was promptly dusted with a dense uniform cloud of Vycor (hereinafter dened) particles of about 40 to 200 mesh particle size. The dusted wet refractory layer over the destructible pattern was then suspended on a conveyor chain moving about 18 inches per minute through a drying oven having a controlled humidity and temperature similar to drying apparatus A shown in the drawings. Air was blown by the mold at a velocity of about 3000 ft./min. in the drying tunnel which has a contoured area of about 2% square feet for fast, yet safe drying. The air had a dry bulb temperature of 90 and a Wet bulb temperature of 75 F.
The steps of dipping, dusting and adiabatic drying were repeated using drying atmospheres for each successive layer as shown below. The above ceramic slurry formula is for the first or prime layer dip. The slurry should preferably be diluted with water or other liquid for the remaining dips by adding approximately seven liters of water to the above formula.
Thereafter the pattern and mold was placed in a tiring furnace atmosphere of 1500 to 1900 F. where the skin layer of the pattern next to the shell mold quickly melted and the pattern material flowed easily and promptly from the green shell mold. Some of the melted pattern escaped from the interior of the mold by flowing through the very porous mold walls. Within 5 minutes the pattern was removed from the mold and also taken out of the furnace by means of a removable false bottom. The mold was then fired at 1500 to 1900 F. for an additional 15 minutes, there being no need to remove the mold from the furnace between the operations of melting the pattern and ring the shell mold.
Thereafter the hard smooth permeable ceramic shell mold thus produced was cooled to room temperature and stored. These cooled molds were placed in individual pre-heated furnaces and prior to casting were preheated to the predetermined temperature. The molds were not supported or invested prior to this preheat and pouring.
If desired, the mold may be cast immediately after firing thereby eliminating much of the time involved in cooling and preheating the molds.
The shell molds of the present invention are porous in nature and less than 3/s in thickness, generally being about l/s toli" in thickness. The porosity or permeability caused by the deterioration and burning out of low temperature binders during the firing of the green molds, greatly aids in removal of the destructible pattern since some of the pattern material actually drains through the Y walls of the mold. ln the above examples, other refractory coating formulations may be substituted for the composition of the example. The water used to prepare the coating slurry may be substituted for by other liquid dispersing media such as Water-alcohol mixtures, and alcohol, although the required properties of the drying atmosphere previously mentioned must be changed to adjust to the various liquids in order to obtain adiabatic drying.
Suitable refractory materials in accordance with the present invention are quartz, alumina, silicon carbide, graphite, aluminum silicate, 4llint and zirconium silicate. Another suitable refractory material is Vycor, a nely ground high silicon oxide glass supplied by Corning Glass Works, Corning, New York. It is made by smelting ceramic materials to form a molten glass which is thereafter cooled to' the solid state. About one-third of the weight of glass is chemically leached away and the remaining material is refired, cooled, dried and powdered. The material is supplied by Corning Glass Works, Corning, New York. it comprises about 96% silica and a small quantity of boric acid (about 2.5 percent B203) together with traces of aluminum, sodium, iron :and arsenic. The high silica glass has a specific gravity just under 2.18 and a softening point of 2730 F. The glass has a vitreous rather than crystalline structure and a low uniform thermal expansion. Generally the particle size of the refractorymaterials should be 100 to 400 mesh. However, for dusting purposes, the particle size may be: larger, say of 5 to 100 mesh.
While the colloidal silica of the example may be substituted for by other binders such as ethyl silicate, the preferred binder is colloidal silica such as Nalcoag (35% solids by weight) sold by National Aluminate Corporation, Chicago, Illinois. Colloidal silica is used in the form of a colloidal aqueous solution resulting from the decomposition of sodium silicate solutions by acid. Accordingly, an excellent colloidal silica may be formed by reacting a water solution of sodium silicate and a dilute aqueous solution of phosphoric acid. The acid and silicate salt reacts to form monosodium phosphate and colloidal silicic acid, the phosphate salt generally thereafter being removed from the mixture to produce colloidal sil icic acid solutions up to 30% in strength. The preparation of colloidal solutions of silica are described in U.S. Patents Nos. 2,515,960, 2,375,738 and 2,285,449.
Any of the commercially available investment casting waxes may be used as the pattern material in addition to plastic materials having melting ponits of about F. to 240 F. It is noted however, that generally the pattern temperature and drying conditions may be changed to fit each pattern material; Patterns may be molded of polystyrene or acrylic o'r methacrylic or copolymer mixtures thereof or other suitable fusible, vaporizable, cornbustible or otherwise destructible materials.
It is well understood that, in accordance with the provisions of the patent statues, Variations and modifications of the specific invention may be made without changing the spirit thereof.
What We claim is:
1. The method of preparing refractory molds having a wall thickness of less than one-half inch for casting metals therein comprising the steps of (l) dipping a destructible pattern in an aqueous slurry containing refractory materials and a binder therefor to form a first coating of refractory materials on said pattern, said pattern and said slurry being at about room temperature, (2) adibatically drying said first coating so that the temperature of the pattern remains substantially constant, said drying being achieved by the use of a volume of air of controlled humidity that is moved rapidly past said pattern with said first coating thereon, said air containing sufficient moisture to maintain a substantially constant wet bulb temperature which is substantially the same as the initial temperature of said pattern and having a dry bulb temperature which is at least 10 F. higher than said Wet bulb temperature, (3) dipping said pattern in an aqueous refractory slurry to form additional coatings on said pattern,'and (4) adiabatically drying each successive coating in the manner described for said first coating While maintaining the temperature of said pattern substantially constant, destroying said pattern, and (6) firing said built-up coatings to form a refractory mold.
2. The method o'f preparing thin-Walled refractory molds for casting metals therein comprising the steps of (l) dipping a Wax pattern in an aqueous slurry containing refractory materials and a binder to form a first coating of refractory materials on said pattern, said pattern and said aqueous slurry being at room temperature, (2) adiabatically drying said first coating so that the temperature of the Wax pattern remains substantially constant, said drying being achieved by the use of a volume of air of controlled humidity that is moved rapidly past said pattern, said air containing sufficient moisture to maintain a Wet bulb temperature substantially the same as the initial temperature of said pattern and having a relative huidity less than 75 percent, (3) repeatedly dipping said pattern in aqueous slurries to form additional coatings on said wax pattern, (4) adiabatically drying each successive coating in the manner described for said rst co'ating While maintaining the temperature of said pattern substantially constant by gradually decreasing the relative humidity of the air to a minimum relative humidity of about 2,0 percent, (5) thereafter destroying said pattern,
and (6) firing said built-up coatings to form a refractory mold.
3. In a method of preparing a thin-walled refractory mold from a destructible pattern, the steps of forming the, pattern and then holding it at room temperature until it has uniformly assumed that temperature, dipping the pattern in a slurry of refractory material at room temperature to form thereon a refractory layer, removing the pattern from the slurry and subjecting it to an airk stream which has a wet bulb temperature which is substantially the same as room temperature and a dry bulb temperature which is at least 5 F. greater than room temperature, removing the pattern from the air stream after the refractory layer has substantially dried and before the temperature of the pattern adjacent the refractory layer has increased substantially above room temperature, promptly dipping the pattern in a refractory slurry at room temperature, and repeating the process until a suitable number of refractory layers have been fomed on the pattern.
4. The method of claim 3 in which the pattern is dusted with refractory particles after each dip inthe 4refractory slurry except the last dip.
5. The method of claim 3 in which the dry bulb temperature of the air stream is increased for each successive drying operation while the yvvet bulb temperature in each drying operation is held substantially at room temperature.
6. The method of claim 3 in which the dry bulb temperature of the air stream is `from 5 to 50 F. greater than the Wet bulb temperature ofthe air stream and thel humidity level ofthe air stream is always greater than that of the normal room atmosphere.
7. The method of claim 3 in which the air stream hasV a velocity of at least 800 feet per minute and a relative humidity inthe range of to 20 percent.
8. The process of forming a ceramic shell mold frame aV destructible pattern comprising the steps of bringing the pattern substantially to a control temperature, coating the pattern by dipping it into a slurry of ceramic materials that is maintained at substantially the same temperature as the control temperature, drying the coating on the pattern in a rapidly moving atmosphere having a dry bulb temperature that is at least 5 F. higher than the control temperature and having a Wet bulb temperature that is substantially the same as the control temperature, as soon as the coating on the pattern is dry and before there is any substantial change in the temperature of the pattern, again dipping the pattern into a slurry of cermic materials maintained at ay temperature substantially the same as the control temperature, successively drying and dipping the patternV in like manner until the desired thickn ness of coating is obtained, destroying and removing the vceramic shell mold, the slurry temperature, Wet bulb temperature of the drying atmosphere, and initial temperature of the patterntall being substantially the same as the control temperature.
9. A process of manufacturing a thin-Walled refractory mold from a destructible pattern comprising the steps of bringing the pattern substantially to a control temperature, providing the pattern with a coating of refractory materials in a liquid medium, said coating as deposited on the pattern being at the control temperature, drying the coating on the pattern in a rapidly moving atmosphere having a dry bulb temperature that is at least 5 F. higher than the control temperature and having a Wet bulb temperature that is substantially the same as the control temperature so that the temperature of the pattern remains substantially constant during the drying operation, as soon as the coating on the pattern is dry and before there is any substantial change in the temperature of the pattern, again providing the pattern With a coating of refractory materials at the control temperature, successively drying and coating the pattern in like manner until the desired thickness of coating is ob- Y tained, removing the pattern from within its dried coating of refractory materials, and firing said coating to convert said coating into a shell mold.
10. The process of claim 9 in which the dry bulb temperature of the air stream is increased for each successive drying operation While the wet bulb temperature is held substantially constant at the control temperature.
1l. The process of claim .9 in whichthe slurry is an aqueous slurry and the pattern is 'dusted with refractory materials after at least one coating exclusive of the last coating.
References Cited in the file of this patent UNITED STATES PATENTS