US 2756475 A
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y 1956 D. K. HANINK ET AL INVESTMENT MOLD AND coRE ASSEMBLY Filed Feb. 24, 1953 Attorneys United States Patent INVES'IWNT MOLD AND CORE ASSEMBLY Dean K. Hanink, Birmingham, and Robert E. Clague,
Berkley, Mich assignors to General Motors Corporalion, Detroit, Mich, a. corporation of Delaware Application February 24, 1953, Serial N 0. 338,334
13 Claims. (Cl. 22196) This invention relates to investment molding and particularly to a process for forming an investment mold and core assembly which is especially suitable for precision casting operations.
Inasmuch as it is exceedingly difficult to maintain close dimensional tolerances in precision casting operations using mold investments, it is desirable that any internal refractory core be located and supported in such a manner so that suitable space is provided at the core print locations to allow for difierential thermal expansion and contraction of the mold and core. This differential movement between the core and investment material can be caused by non-uniform heating of the entire mold assembly from the exterior to the interior, localized shock heating generated by the molten metal during the pouring operation, or simply by the use of dissimilar materials in the investment and core. In the absence of such a provision for differential expansion of the mold and core, stresses will be set up within thecore which result in upsetting and subsequent failure of the core. Heretofore this problem has not been satisfactorily solved, and methods previously employed for the production of precision castings requiring the use of cores, such as hollow turbine buckets, have involved excessively high costs.
A principal object of the present invention, therefore, is to provide an investment mold and core assembly for precision casting operations which provides for differential expansion and contraction of the assembly parts. Precision castings which necessitate the use of cores thus may be accurately produced in accordance with the invention because this assembly eliminates core upsetting and breakage due to thermal stresses. A further object of this invention is to provide a process for forming a mold and core assembly which eliminates the v disadvantages of high cost inherent in precision casting methods heretofore used by employing common foundry materials which are both inexpensive and adaptable to quantity production with the use of standard foundry equipment.
These and other objects are attained in accordance with the present invention by an improved investment mold and core assembly, and a process for forming the same, for the precision casting of metal parts, particularly hollow parts requiring the use of cores. This is accomplished by investing a sand core and a destructible pattern Within a refractory composition in a manner so as to provide for differential expansion and contraction between the core and the investment material.
In accordance with the invention, an improved investment mold and core assembly possessing the above desirable characteristics may be formed from common foundry materials and equipment by a process in which an internal core is first embedded within a destructible pattern, the core print projections subsequently coated with wax or other equivalent destructible material to provide for the aforementioned differential thermal expansion and contraction, and the formed core and pattern assembly 2,756,475 Patented July 31, 1956 ice 2 thereafter invested in a conventional manner in a refractory mold.
Other objects and advantages of this invention will more fully appear from the following detailed description of the invention, reference being made to'the accompanying drawing which shows a somewhat schematic sectional view of a sand core and a destructible pattern invested in a refractory mold in accordance with the invention.
Referring more particularly to the drawing, an internal sand core 10, which preferably has been previously baked, is positioned in an injection mold. Subsequently, wax or other destructible pattern material is injected into the mold under pressure in a conventionalmanner. Upon solidification of the wax, the core, together with the surrounding wax pattern 12, is removed from the mold.
Although the pattern 12 is generally described herein as being a wax pattern, it also may be formed of any other suitable low fusing substance, such as a thermoplastic resinous material or any other vaporizable, fusible, combustible or otherwise destructible material. In fact, plastic patterns are frequently employed to provide optimum results. Among the plastic patterns which have been found to be satisfactory are those formed of polystyrene, although other thermoplastic pattern materials, such as resinous polymerized derivatives of acrylic acid and resinous polymerized derivatives of methacrylic acid, may be used.
After removal of the formed core and pattern subassembly from the injection mold, the pattern is preferably cleaned with an alcohol solution and air dried. Wax 14 is then applied over the entire surfaces of the core print projections or locators 16 and 18 to a thickness which will subsequently permit the satisfactory dilferential expansion and contraction of the parts of the formed investment mold assembly. We have found that the thickness of this wax coating 14 preferably should not be greater than approximately one-half the dimensional tolerance of the casting wall thickness. For example, in the case of a turbine bucket, if the tolerance of the adjacent opening formed by the core is 0.012 inches on the diameter, the wax layer should not'be thicker fl'ian 0.003 inches. While the thickness of the wax coating is somewhat exaggerated in the drawingfor pur- Following the application of wax to the core print' projections 16 and 18, a suitable gating portion 20 having a riser and pouring basin part 22 at its outer end is attached to the pattern in which the core. 10 is embedded.
The portions 20 and 22 may be formed of wax, plastic or other appropriate destructible material similar-to that used to form the pattern. Of course, if convenient for the particular application, all or part of the gates, risers and/or headers may be formed integral with the pattern in the injection molding machine. Likewise, although side gating is shown as being used in the drawing to cast a hollow turbine bucket, end gating may be alternatively employed if the bucket design so warrants.
The surfaces of the pattern are next coated with a ceramic wash or coating material 24 which is to provide the smooth casting surface of the refractory mold to be formed. This coating material comprises an aqueous,
Coating of the pattern with the ceramic wash is pref-.
erably accomplished by dipping the pattern in the coating solution. Although in some instances the ceramic coating may also be applied by spraying or painting it on the pattern or in any other suitable manner, dipping is preferred because it assures moreuniform coating of all the pattern surfaces and is the simplest method of application. J i i The dip coat slurry is preferably kept in constant motion by stirring means except during the actual-dipping operation. However, the mixing action should not be such as to unnecessarily introduce air into the 'slurry, and care should be exercised in immersing the pattern in the slurry to prevent air entrapment on the pattern. Normally the dip coat solution is retained at room temperature during the dipping operation because excessive heat can result in distortion of the plastic or'wax' pattern. The excess coating material is permitted to drain olf prior to subsequent treatment and investment.
After the pattern has been completely coated with the dip coat slurry, it may be sanded or stuccoed to provide a rough surface on the coating, thus insuring greater adhesion between the principal refractory portion of the mold and the dip coat on the pattern. This sanding may be accomplished by merely screening or otherwise applying silica sand or other suitable refractory materials in known manner to the outer coated surface of the destructible pattern. When all the molding surfaces of the pattern have been effectively covered with sand, the pattern and embedded core should be air dried.
Following the formation and treatment of the core and pattern assembly, suitable investment material 26, usually containing a coarser refractory material, is formed about the pattern and sprue, the latter being permitted to extend through the wall of the resultant refractory mold so as to permit the escape of the destructible pattern and to form an ingate for the fluid casting meta-h This refractory mold may be formed about the pattern in any suitable manner, but the following preferred procedure provides excellent results. A base plate 28 is preferably first sprayed or otherwise coated with molten wax so as to form a thin film of wax over its upper surface. Before the wax has completely solidified, the pattern and core assembly to be invested is positioned on the plate 28 with the gate and pouring basin portions 20 and 22 thereof extending downwardly and seated firmly in the wax'film. A sleeve or flask 30, preferably of steel, is then placed around the pattern and pressed lightly into the wax layer. In order to completely seal the flask '30 to the plate 28, it is preferable to again spray molten wax around the outer surfaces of these parts at their junction. This wax should then be permitted to thoroughlysolidify. Y
The refractory material is mixed with a predetermined quantity of liquid binder, and the resultant slurry ;is poured into the sleeve or flask 30, which is preferably vibrated during this pouring operation, the mold 26 then being permitted to set. The mold body 26 maybe formed 'of a conventional silica investment having an ethyl silicate Y binder or may be formed of any other suitable investment material. An example of an investment dry mix or grog which may be used is one comprisingmajor proportions ofafinely ground, dead burned fire clay and silica flour and minor proportions of magnesium 'oxide and hora-x glass. The binder for this grog may include anaqueous solution of condensed ethyl silicate, ethyl alcohol and bydrochloric acid. 7
When the mold body 26 has solidified or set to a sulfic'ient extent, the base plate 28 is removed "from beneath the mold and heat is applied to melt the destructib'le pattern 12 and wax coating 14 on the core print projections. It'is necessary to apply sufficient heat to raise the mold temperature above the fusing points of these materials, thus permitting the molten pattern and coating Ito-escape through the gate .and sprue passage in the mold formed by the .removed gating and riser portions-l0 .22, irespectively. When a wax pattern and coating are employed, a temperature of approximately 400 F. has been found to be satisfactory. In this manner the dip coat which had covered the pattern tightly adheres to the remainder of the mold and provides the casting cavity with a smooth coating. It is also possible to vaporize the pattern and coating, if vaporizable materials are used, by heating the mold rapidly to a high temperature.
After removal of the pattern from the mold in the foregoing manner, the core 10 remains within the mold with its body portion extending through the formed mold cavity. The core print projections 16 and 18 are still positioned in the core prints 32 and 34 which were previously formed in the investment slurry by the wax-coated core print projections. The mold assembly is then burned out to remove substantially all the volatile matter. This assembly is preferably preheated to the desired-temperature, and the molten casting metal is poured or otherwise introduced into the mold cavity formed around the core by the fusible pattern. In the majority of instances it is necessary to pour the casting metal while the mold is still hot. After the molten metal has been poured and the casting has solidified, the refractory mold body 26 and the adhering coating 24 may be broken to permit the removal of the casting.
After the wax layer 14 has been melted off or otherwise removed from between the core prints 32 and 34 and the core locators 16 and 18, the core is free to expand independently of the mold proper to a limited extent. Hence, during the pouring operation this core is not subjected to the thermal stresses which would otherwise result in upsetting and failure of the core. The degree of accuracy of castings obtained by this arrangement, of course, is dependent upon the space provided at the core prints for dilferential movement between the core and mold. It will be noted, therefore, that the thickness of the layer of wax applied to the core print locations determines the accuracy of the final location.
Although the spacing layers 14 are herein generally referred to as formed of wax, it will be appreciated that other destructible materials, such as those referred to as appropriate pattern materials, may also be employed under many conditions to coat the core print projections.
Hence, any suitable substance, suchas for example, resinin the resulting arrangement is that the core is not rigidly retained in its initial position by the investment or any other mold component which is fixed or fastened tothe investment. In the selection of a wax, resin or any other destructible material to be used as the spacing coating,
it is important that such materials will melt, volatilize or burn attemperatures not in excess of approximately 1800 F. and preferably at considerably lower temperatures.
The core and mold assembly formed in the abovedescribed manner maybe used without limitation as to the typeof metal to be-cast. Hence, this assembly maybe successfullyemployedwith, for example, both low-melting ferrous metals and high-melting cobalt-based-super alloys, the lattercurrently frequently being used to form turbine buckets. The process for forming the mold assembly does not adversely affect core properties, the cores retaining their desirable permeability and collapsing characteristicswhen used in this manner. 7
While .the present invention has been described by means of certain specific examples, it will be understoodthat the scope of the invention is not to be limited thereby except as defined in'the following :claims. i
We claimf .1. A .method of forming an investment .mold and core assembly which comprises embeddin a core :within a fusiblepattern, coatingza'core-print projectionon said core with a layer -.of fusible material .tor ov e for expansion and contraction of said core, thereafter investing said core and pattern in a refiactory mold, and subsequently removing the pattern and material from the refractory mold.
2. A method of forming an investment mold and core assembly which provides for differential expansion and contraction between the core and investment material, said method comprising embedding an internal core within a fusible pattern, coating core print projections extending from said core through said pattern with a layer of fusible material, thereafter introducing a refractory molding composition around said core and pattern, permitting said molding composition to solidify, and finally removing the fusible pattern and layer of fusible material from the formed investment mold and core assembly.
3. A method of forming an investment mold and core assembly which comprises molding a fusible material around a sand core into the form of a pattern, applying a layer of fusible material to the surfaces of core print projections on said core, subsequently applying a thin refractory coating to the surfaces of said pattern, thereafter investing the core and coated pattern assembly in a refractory investment material, and finally removing the pattern and fusible material from within said investment material to provide a casting cavity within the formed investment mold and around said core.
4. A method of forming a refractory mold and core assembly which provides for difierential thermal expansion of the core and mold, said method comprising inserting a refractory core in an injection mold, injecting a combustible material selected from the class consisting of wax and plastic at least partially around said core and molding said material into the shape of a pattern for the casting to be formed, coating the surfaces of projections on said core with a layer of combustible material, said layer having a thickness not greater than one-half the dimensional tolerance of the casting wall thickness, thereafter applying a thin coating of a fine refractory mixture to the surfaces of said pattern, investing said coated pattern and core in a refractory mold, and thereafter eliminatmg the pattern and layer of combustible material around said core print projections from said mold, whereby said refractory coating adheres to the refractory mold and the core remains positioned within said cavity.
5. The process of forming a refractory mold and core assembly in which a core is positioned within a mold cavity and has core print projections spatially separated from core prints formed in said mold, said process comprising forming a fusible pattern around a refractory core, coatmg core print projections formed on said core with a thin layer of fusible material, thereafter investing said coated pattern and core in a body of investment material, setting the body of investment material, and thereafter melting and removing the pattern and layer of fusible material, whereby the core print projections become spatially separated from the core prints formed in said refractory mold to permit thermal expansion and contraction of said core relative to said mold.
6. A method of forming an investment mold and core assembly which comprises positioning a sand core in a Wax injection mold, injecting wax into the mold to form a pattern around said core, thereafter removing the core and surrounding Wax pattern from said mold, applying a layer of wax to the surfaces of core print projections formed on the core, assembling and attaching destructible gates and risers to the pattern and sand core assembly, subsequently coating the pattern with a ceramic wash, thereafter stuccoing said wash with silica sand and drying the coated and stuccoed pattern and core assembly, investing said assembly within a refractory molding composition, thereafter elimina g the Wax Pattern and Wax layer by heating the assembly to a temperature above the melting point of the wax, and subsequently preheating the mold to the desired temp Prior to casting- 7. A process for forming an investment mold and core assembly which provides for differential expansion and contraction of the core and investment material, said process comprising positioning a sand core in a wax injection mold, injecting a fusible material into the mold into the form of a pattern around said core, thereafter removing the core and surrounding wax pattern from said mold, applying a layer of a fusible material over the entire surfaces of core print projections formed on said core, said layer having a thickness not greater than approximately one-half the dimensional tolerance of the casting wall thickness, assembling and attaching fusible gates and risers to the pattern and sand core assembly, subsequently coating the surfaces of the pattern with a ceramic wash, thereafter applying a small amount of silica sand to the surfaces of said wash, thereafter drying the coated and sanded pattern and core assembly, introducing a refractory molding composition around said assembly, permitting said molding composition to solidify, removing the pattern and layer of fusible material by heating the formed mold and core assembly to a temperature of at least approximately 400 F., and subsequently preheating the mold and core assembly to the desired temperature prior to castmg.
8. A refractory mold assembly comprising a body of refractory material provided with a casting cavity, and a core positioned in said cavity and extending into a core print in the casting cavity walls of said body of refractory material, said core being generally separated from the walls of said core print an average distance equal to approximately one-half the dimensional tolerance of the casting wall thickness to permit differential expansion of said core and body of refractory material.
9. A refractory mold and core assembly comprising a body of refractory material provided with a casting cavity therein, and a core positioned in said cavity and having core locators projecting into core prints in the casting cavity Walls of said body of refractory material, said core locators being generally separated from the walls of said recesses a distance sufiicient to permit differential thermal expansion and contraction between said core and body of refractory material but not appreciably greater than approximately one-half the dimensional tolerance of the casting wall thickness.
10. An investment mold and core assembly for precision casting operations comprising a body of refractory material provided with casting cavity walls having a smooth refractory coating thereon, said walls having recesses formed therein, a refractory core positioned within said mold cavity and provided with core locators engaging said recesses, said core locators being generally separated from the Walls of said recesses by a thin layer of combustible material which is removable to allow subsequent differential thermal expansion and contraction of said core and mold.
11. An investment mold and core assembly for precision casting of hollow turbine buckets, said assembly comprising a mold formed of a comminuted refractory material provided with a casting cavity therein, a baked sand core positioned within said cavity and having locating portions extending into core prints formed in said mold, a destructible pattern at least partially surrounding said core, and a thin layer of a fusible material interjacent the mating surfaces of the core prints and the locating portions of said core, the surfaces of said pattern being provided with a smooth refractory coating.
12. A refractory mold assembly comprising a body of refractory material provided with a casting cavity, a core positioned in said cavity and extending into a core print in the casting cavity walls of said body of refractory material, and a layer of fusible material interjacent the mating surface of the core print and the core positioned therein.
13. An investment mold and core assembly for precision casting operations comprising a body of refractory material rovided with .a casting cavity therein, a bak d san core positioned within sai a i y and h ng loca ng portions extending into core prints formed in said mold, a fusible pattern selected from the class consisting of wax and plastic at least partially surrounding said core, and a thin layer of wax provided between the mating surfaces of the core prints and the locating portions of said core.
References Cited in the file of this patent UNITED STATES PATENTS ea nm-a- ,--V-.- ay 1948