US2752257A - Investment molding - Google Patents

Description

June 26, 1956 1 p BRADLEY ET AL 2,752,257

INVESTMENT MOLDING Filed Nov. l5, 1952 INVENTORS MM5/@ATTORNEYS lNVESTMENT MOLDING James P. Bradley and Robert R. Dohrmann, Bedford, Ind., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application November 15, 1952, Serial No. 320,736

3 Claims. (Cl. 1045-3822) This invention relates to refractory molds for casting metal and particularly to an improvement in the coating of the casting surfaces of such molds.

In precision casting metallic articles by means of refractory molds it is frequently necessary that the casting surfaces of the molds possess high strength and exceptional smoothness. Thus, in investment molding it is usually desirable that the invested pattern be coated with material which will tightly adhere to the principal refractory portion of the mold and provide the mold with a smooth casting surface upon removal of the destructible pattern. Moreover, such a coating must also have suiiicient strength and rigidity so as to withstand the pressure of the molten casting metal.

Accordingly, a principal object of the present invention is to provide an improved coating for the casting surfaces of refractory molds. A further object of the invention is to provide a dip coat composition and a process for `applying the same wherein the resulting smooth molding surface possesses a sufficient degree of hardness and eX- cellent adherence to the principal refractory portion of the mold. Such a coating layer is sufficiently strong and rigid to prevent warping under the localized pressure caused by the molten metal and also is not subject to rippling or cracking.

The above objects are obtained in accordance with the subject invention by the provision of a coating mixture comprising a high temperature cement binder, a finely comminuted refractory material, defoaming and wetting agents, a gelatine and water. An acid is preferably also added to regulate the pH of the mixture.

Other objects and advantages of this invention will more fully appear from the following detailed description of the invention in conjunction with the accompanying drawing, which contains a somewhat schematic sectional view of a destructible pattern invested in a refractory mold and provided with a dip coat composition in accordance with the invention.

Referring more particularly to the drawing, a pattern of a turbine bucket to be cast is shown invested in a refractory mold 12 within a metallic container or flask 14. The pattern l@ is preferably formed of a low fusing substance, such as wax or a thermoplastic material, or any other vaporizable, fusible, combustible or otherwise destructible material. However, plastic patterns are preferably employed in order to obtain optimum results with the coating embodying the present invention. 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.

The pattern may be initially cast under pressure in a conventional manner. A gating portion 16 having a pouring basin part 18 at its outer end is attached to the pattern, the portions ld and 16 usually being formed of a destructible material similar to that of the pattern. After the pattern is formed, it is preferably cleaned with an ares Patent alcohol solution and air dried prior to application yof the coating solution. Before the pattern is invested within the mold, it is coated with the coating material embodying the invention and which is to form the casting surface 20 of the refractory mold.

Coating of the pattern is preferably accomplished by dipping the pattern in the coating solution. Although in some instances the 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 more uniform coating of all the pattern surfaces and is the simplest method of application.

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 12 of the mold and the dip coat 20 on the pattern. This sanding may be accomplished by merely screening or otherwise applying silica 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 should be air dried.

Following the formation of the pattern, a suitable mold 12, usually containing a coarser refractory material, is formed about the pattern and sprue, the latter being permitted to extend through` the wall of the refractory mold so as to permit the escape of the destructible pattern material `and to form an ingate for the fluid casting metal. This main refractory mold may be formed about the pattern in any suitable manner, and hence the procedure for forming the mold will not be described in detail. Among the procedures for forming the body of the mold 12, however, is that of mixing the refractory mixture with a definite quantity of liquid binder, pouring it into the sleeve or flask 14, which is preferably vibrated during this pouring operation, and then allowing the mold to set. The mold body 12 may be formed of a conventional silica investment having an ethyl silicate binder or may be formed of any other suitable investment material.

When the mold body has solidified or set to a sufliicient extent, the base plate 22 is removed from beneath the mold and heat is applied to melt the pattern. It is necessary to apply sufficient heat to raise the mold temperature above the fusing point of the material, thus permitting the molten pattern to escape through the gate and sprue opening in the mold formed by the pattern portions 16 and 18. 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, if a vaporizable material is used, by heating the mold rapidly to a high temperature.

Upon removal of the pattern from the mold in the foregoing manner, the molten casting metal is poured or otherwise introduced into the mold cavity formed by the 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 12 and the adhering coating 20 may be broken to permit the removal of the casting. As a result of using the improved coating layer 20 in accordance with the present invention, the finished casting possesses excellent surface smoothness and detail and requires little, if any, finishing operations.

In accordance with the invention, a dip coat containing an air-setting silicate cement and gelatine is employed for coating destructible patterns used for forming turbine buckets or other precision cast parts. 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 introduce air into the slurry. The pattern is immersed in the dip coat slurry preferably to within 1/2 to 3%; inch of the end of the gating portion of the pattern. Care should be taken 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 a distortion of the plastic pattern. After dipping, the excess coating material is permitted to drain olf prior to subsequent treatment and investment.

The dip coat composition embodying the invention generally comprises an aqueous dispersion of a high temperature cement binder, a finely comminuted refractory material, a defoaming agent, a wetting agent and a gelatine. The amount of water in the mixture should be suflicient to provide the coating with the proper dipping consistency if it is to be applied as a dip coat. An acid, preferably hydrochloric acid, may advantageously be employed to regulate the pH of the slurry. Gctyl alcohol has proved to be a satisfactory defoaming agent, while silica our is the preferred fine refractory material. Other refractory powders, such as zirconium silicate our, may be used, however. Generally it is desirable to use a silica fluor of sucient neness so that at least 99% of it will pass through a 140 mesh screen. Glycerine may also be included in the mix, but satisfactory results are obtained when it is omitted.

Accordingly, an example of a dip coat composition which provides excellent results is one consisting essentially of approximately 71/2 to 8%/2 pounds of a refractory cement, 17 to 20 pounds silica flour, 3500 to 4500 cc. water, 180 to 220 cc. of a solution of a wetting agent, 50 to 150 cc. octyl alcohol, glycerine not in excess of 150 cc., 22 to 26 grams of gelatine and 15 to 22 cc. of concentrated hydrochloric acid. An aqueous wetting agent solution having a concentration of approximately ounces of a wetting agent in powdered form per 2700 cc. of water has proved to be satisfactory.

Converting the above composition to percentages by weight and adding the water supplied with the wetting agent to the water separately added, this mixture consists of approximately 20% to 25% of the cement, 43 to 51% of the silica iiour, 20% to 31% water, 0.1% to 0.2% wetting agent, 0.2% to 0.8% octyl alcohol, 0 to 1.3% glycerine, 0.15% to 0.2% gelatine and 0.1% to 0.2% concentrated hydrochloric acid. These exact percentages are not critical in all instances, however; and a mixture comprising t0 30% of the cement, 35% to 70% of the fine refractory filler material and 15% to 35% water has proved to be effective to provide a coating layer of satisfactory strength for refractory molds when small but effective amounts of a wetting agent, a defoaming agent and gelatine are included in the mixture.

rl`he dry wetting agent normally should be present in amounts of at least 0.1% of the weight of the total mix and may be used in quantities as large as 0.3%. Likewise, if octyl alcohol is employed as a defoaming agent, it may constitute between 0.1% and 1% of the weight of the slurry. As hereinbefore stated, it is not necessary to include glycerine in the mixture for all applications, but it may be beneficially added in amounts as high as 2% by weight. Under particular circumstances the gelatine range may vary from as low as 0.1% to as high as 0.4%, while the acid is preferably and normally present in an amount suti'icient to make the slurry distinctly acidic prior to the cement addition. Hence, a hydrochloric acid content between 0.05% and 0.3% by weight is beneficial.

The silica iiour, of course, functions as a refractory filler while the refractory cement is a high temperature binder. Among the cements which may be used are air-setting cements having sodium silicate binders and recommended service temperatures of approximately 2700 F. The sodium silicate binder serves as the only high-temperature binder in the dip coat. The clay body of this cement, which may be an aluminum silicate clay, is a highly important constituent in the resultant mixture inasmuch as it provides the dip coat mixture with the ability to maintain the silica flour in suspension. In the absence of such a clay in the cement the silica flour settles out of the solution very rapidly, but when the cement is present the silica flour remains in suspension for several days. This cement, which may contain calcium aluminum silicate, preferably should have a low organic content in order to provide the coating with the desired strength. An example of this type of cement is Norvit cement, manufactured by Johns-Manville Corporation and identified as No. 20 refractory cement. This material is commercially available in paste form.

When exceptionally high strength is desired in the coating for particular applications and where it is not necessary to coat sharply concave pattern surfaces, sodium silicate may be separately added in amounts up to approximately 18% by weight, based upon a waterglass solution of at least approximately 30 Baume. It will be appreciated that the water content in such solutions is the sum of water of hydration and any additional water which may be present in commercial waterglass. Accordingly, either pure or commercially pure solid sodium silicate may also be employed in equivalent amounts. If anhydrous silicate is used, it therefore normally should not be included in amounts in excess of approximately 7% of the total weight of the coating slurry.

When waterglass is included in amounts approaching the aforementioned maximum quantity, however, the water and silica iiour contents should be decreased proportionately to obtain the desired consistency for dipping. Hence, when the above maximum amount of waterglass is used, the additional water content may be decreased to as low as 12% and the silica flour content may be reduced to as low as 30% by weight. Such compositions, however, should not be used on patterns having concave surfaces with exceptionally short radii of curvature since this extremely high strength coating has a tendency to pull away from such surfaces during solidiiication of the coating.

If waterglass is to be added in the foregoing manner, sodium silicate is preferably used because it is very inexpensive. However, it should be understood that the coating mixture is not limited to that particular type of waterglass. By the term waterglass we wish to include not only sodium silicate and its water solutions, but also potassium silicate and mixtures of these two silicates, the latter generally being referred to as double waterglass. Moreover, the use of sodium tetrasilicate and similar silicates is also meant to be included. Accordingly, statements herein made with respect to waterglass or sodium silicate compositions are intended to also encompass the use of the above other silicates.

As hereinbefore indicated, glycerine may be included in the coating slurry and prevents rapid drying of the coating due to its hygroscopic nature. In this manner glycerine aids in preventing cracking and peeling of the coating from the pattern. Moreover, the glycerine is also believed to improve the dipping qualities of the dip coat.

A wetting agent is included in the coating mixture to aid in providing the coating with a smooth surface upon solidiiication. The wetting agent should not be present in amounts greater than those indicated above since it otherwise would cause bubbles in the coating composition, thus resulting in castings having rough surfaces. Among the wetting agents which have been found satisfactory are sulfatate, a hydrocarbon sulfonate; sodium dioctyl sulfo-cuccinate or various sulphates of higher alcohols. Modied sulphated fatty acid esters, such as WetanoL which is commercially supplied by the Glyco Products Company, Inc., are generally preferred as wetting agents.

The defoaming agent, preferably octyl alcohol, is added to the mix in order to minimize bubble formation in the mixture and in the coating on the pattern. Excellent results have been obtained with an octyl alcohol consisting of approximately 55% primary octanol, 33% secondary octanol and 12% water. Other defoaming agents, such as Fomax, which is a mixture of aliphatic esters, may be used, however. Likewise, although hydrochloric acid is indicated as the preferred acidic constituent to be added to the dip coat slurry, various other acids such as sulphuric, phosphoric and lactic acil frequently may be substituted for hydrochloric acid.

The gelatine to be employed should be a relatively pure gelatine of an organic nitrogenous colloidal substance of the protein or albumen class. This gelatine normally has a pH value of between and 6 and is commercially obtainable in sheet form. Inasmuch as this type of animal gelatine is usually fairly hard and brittle at room temperature and is almost completely insoluble in cold water, it should first be dissolved in hot water. 'Ihe rate at which this gelatine may be dissolved may be further increased by making the hot water acidic. The gelatine serves to increase the green strength of the coating and improve the adherence of the coating to the pattern. Moreover, it tends to properly seal the coating and reduce shrinkage of the coating during drying.

The above-described coating composition is preferably mixed in the following manner. First, approximately 1/s of the total water required is heated to about 160 F. and the hydrochloric acid is added thereto. The gelatine is subsequently introduced into the acid solution and the mixture stirred until the gelatine is completely dissolved. The aqueous solution containing the dissolved gelatine has a pH of approximately l due to the presence of the hydrochloric acid. After dissolution of the gelatine the remainder of the water is added, thereby raising the pH of the solution to approximately 2. Following the water addition the glycerine, wetting agent, and octyl alcohol are successively added and the solution thoroughly stirred. Next, the cement is introduced and mixed thoroughly with the other constituents. After the cement addition, the silica flour is slowly added to the slurry while the mixture is being stirred. The addition of the cement lowers the pH of the mixture to approximately 11 and this degree of alkalinity is retained after the silica our addition. Upon complete mixing of the above constituents, the slurry is transferred to a dip tank, if a dipping process is to be employed.

lf waterglass is separately added to the coating mixture in order to provide additional strength, it should be introduced after the water addition and before the wetting and defoaming agents are introduced. Likewise, under these circumstances the defoaming agent is preferably added before the glycerine and wetting agent additions. These constituents should be added slowly while the mixture is being stirred.

Any suitable mixture of materials may be used as the investment material to form the body or principal portion 12 of the mold. An example of an investment dry mix or grog which may be used is one comprising major constituents of a finely ground, dead burned fire clay and silica iiour and minor proportions of magnesium oxide and borax glass. The binder for this grog may include an aqueous solution of condensed ethyl silicate, ethyl alcohol and hydrochloric acid.

The above-described coating for destructible patterns to be invested iu refractory molds adheres tightly even to concave surfaces and therefore is particularly adapted for use in casting articles having such concave surfaces, such as turbine buckets. Furthermore, this coating does not react with nickel-base alloys, a material used in cast turbine buckets, and hence has no adverse effect on the surface qualities of such buckets. As a result, the dip coat and process embodying the present invention produces an average of two and one-half to three times the percentage of sound casting as compared with dip coatings heretofore used for casting turbine buckets of nickelbase alloys.

While the present invention has been described by means of certain specific examples, it is to be understood that the scope of the invention is not to be limited thereby except as defined in the following claims.

We claim:

1. A dip coat for application to a destructible pattern to be invested in a refractory mold, said dip coat consisting essentially, by weight, of approximately 15% to 30% of au air-setting silicate cement, 35% to 70% silica our, 15% to 35 water, sodium silicate not in excess of 7%, 0.1% to 0.3% dry wetting agent, 0.1% to 1% octyl alcohol, glycerine not in excess of 2%, 0.1% to 0.4% animal gelatine, and an acid not in excess of 0.3%.

2. A refractory mold having a smooth coating covering the walls of its casting cavity, said coating resulting from applying to said walls a mixture consisting essentially, by weight, of approximately 15% to 30% airsetting refractory cement, 35 to 70% silica flour, 15% to 35% Water, 0.1% to 0.3% wetting agent, 0.1% to 1% defoaming agent, and 0.1% to 0.4% gelatine.

3. A refractory mold comprising a principal body portion and a thin air-dried coating layer covering the casting surfaces thereof, said layer resulting from applying to said casting surfaces a slurry consisting, by weight, essentially of 20% to 25% of air-setting cement having a sodium silicate binder and a high service temperature, 43% to 51% silica liour, 20% to 31% water, 0.1%l to 0.2% wetting agent, 0.2% to 0.8% octyl alcohol, 0 to 1.3% glycerine, 0.15% to 0.2% animal gelatine, and 0.1% to 0.2% concentrated hydrochloric acid.

References Cited in the tile of this patent UNITED STATES PATENTS 1,561,956 Thomas Nov. 17, 1925 1,914,532 Saeger June 20, 1933 2,145,317 Salzberg Ian. 31, 1939 2,159,952 .Tones et al. May 23, 1939 2,441,695 Feagin et al. May 18, 1948

Claims (1)

1. A DIP COAT FOR APPLICATION TO A DESTRUCTIBLE PATTERN TO BE INVESTED IN A REFRACTORY MOLD, SAID DIP COAT CONSISTING ESSENTIALLY, BY WEIGHT, OF APPROXIMATELY 15% TO 30% OF AN AIRSETTING SILICATE CEMENT, 35% TO 70% SILICA FLOUR, 15% TO 35% WATER, SODIUM SILICATE NOT IN EXCESS OF 7%, 0.1% TO 0.3% DRY WETTING AGENT, 0.1% TO 1% OCTYL ALCOHOL, GLYCERINE NOT IN EXCESS OF 2%, 0.1% TO 0.4% ANIMAL GELATINE, AND AN ACID NOT IN EXCESS OF 0.3%.

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