US2558402A - Mold composition - Google Patents

Description

Patented June 26, 1951 MOLD COMPOSITION Burgess P. Wallace, Brooklyn, N. Y., assignor to Whitehead Brothers Company, New York, N. Y., a corporation of New Jersey No Drawing. Application October 23, 1948, Serial No. 56,263

Claims.

This invention relates to the casting of ferrous and non-ferrous metals and alloys thereof but more especially ferrous metals and aluminum and has for its especial objects the production in green sand molds of castings of such ferrous and nonferrous metals from which the sand layer, constituting the mold-metal interface, can be readily peeled or stripped without resorting to prolonged and expensive mechanical cleaning operations such as are at the present time commonly employed, whereby remarkably smooth, clean pattern-true castings are obtainable that are characterized by their almost complete freedom fromburnt-on sand grains. Other objects of the invention are the expeditious and economical production of a synthetic molding sand from. which semi-permanent molds can be prepared wherein such improved castings can be obtained and which molding sand can be repeatedly re-used.

Heretofore, in an attempt to improve the character of the castings obtained from green sand molds and to minimize the amount of mechanical cleaning operations which otherwise would be required in order to obtain smooth clean cast ings, it has been a common practice in foundries to apply so-called facings as, for example, powdered or ground graphite or coke, the same being either dusted, brushed, slicked or sprayed onto the surface of the finished mold. Also, in lieu of employing such facings, the facing sand itself has been mechanically mixed with ground carboniferous materials as sea coal (a bituminous coal of unusually high volatiles content), pitch, gilsonite (a natural resin) and the like in order to produce a special facing sand that was applied as a layer of substantial thickness to the inner surfaces of a green sand mold whose backing consisted of ordinary molding sand. These materials suffer very serious disadvantages when used in ground dry powder form. For example, ground bituminous coal of the ordinary variety when added to the molding sand in powder form of about 40 to 200 mesh size, possess no bonding qualities in the green state, that is to say, the coal cuts the bonding qualities of the clay substance by mere dilution thereof. It also creates pinholes or gas pooh-marking of the casting surface due to the relatively coarse size of the coal particle which is in contact with the metal surface. The coal also, when in coarse ground form, does not act to give high hot strength to the molding sand as the particles are too coarse to knit the sand particles together when the coal is acted on by the heat of the metal. Still another disadvantage is the fact that such coal particles,

due to their large particle size, are acted upon only very slowly by the heat of the incoming metal, as it thus takes the heat an appreciable length of time to penetrate the coal particle and to drive out its volatile gases which act to face the mold.

The foregoing arguments apply with equal force to the solid pyrobitumens, such as the asphalts, coal tar pitch or giisonite, when added in dry, ground form to molding sand. There is also a further serious disadvantage in using solid particles of any of the aforesaid materials and this lies in the fact that such solid particles, being relatively large, will melt and run over the adjacent sand grains (under the heat of pouring) and thus coat the same with a very heavy film, rendering them substantially completely waterproof so that, as the sand is used over and over, the Whole molding sand heap will gradually become waterproof and become repellent of the tempering water. Many foundries, using coal tar pitch and/or gilsonite, have encountered serious trouble on this account and have been forced to discard large quantities of the resultant waterproof sand.

My investigations have lead to the discovery that both bituminous coal (of a certain character hereinafter described) and coal tar pitch and/or asphalt from petroleum, distillation, as well as gilsonite, can be utilized to advantage if these materials are combined in a special way and then put in a colloidal state and suspended as a permanent dispersion in water. I have discovered that the activity of both bituminous coal and asphalt, coal tar pitch, gilsonite, etc, can be enormously increased as a molding medium for iron and steel casting in sand molds, by first grinding a bituminous coal, of predominantly anthraxylon or attritus character, to a very fine powder of finer than 325 mesh or, in other words, sub-sieve fineness, and then adsorbing on the surface of these very fine coal particles either fluid coal tar pitch, melted hard petroleum asphalt or melted gilsonite, while constantly agitating the mass so as to thoroughly adsorb such hydrocarbons on the enormous surface area of the coal, whereby the coal tar pitch, petroleum asphalt, etc., are also dispersed in extremely finely divided form by being so adsorbed. This mixture or composition is then dispersed in water with the aid of a wetting agent, and is then run through a colloid mill With plate setting of not more than about which the particle size is of the millimicron order with oversize particles ranging from 2 to 5 microns. After passing through the colloid mill, the dispersion of the solids will show a very strong Brownian movement. The colloid mill thus takes particles of about 325 mesh, i. e., about 44 microns, and reduces their size to about 2 to 100 millimicrons. After passing through the colloid mill the dispersion is so iine that no suspending agent whatsoever is necessary to maintain the solid particles in suspension in the water phase of the dispersion for a period of even four or live days, but if the material is to stand longer than that, it is advisable to add a suspending or stabilizing agent of the nature of methyl cellulose or sodium cellulose glycollate.

In the production of my improved molding medium, I preferably proceed as follows:

A predominantly anthraxylon or attritus bituminous coal is first ground in the usual method to a powder of a fineness of about 325- mesh. Such powder is then introduced into a suitable type of agitating mixer equipped with provision for heating. In a separate heated kettle, the coal tar pitch, petroleum asphalt, gilsonite, etc., is melted. Then, to 1,000 pounds of the finely divided coal, is added 500 pounds of the molten asphalt, etc., and the mass constantly agitated while such addition is being made and also the heat on the entire mass is kept up to a point above the melting point of the asphalt or other hydrocarbon used (usually about TKO-230 F.). After the hydrocarbon is thoroughly dispersed on the surface of the iinely ground coal, the mass 'is allowed to cool to normal room temperature,

while continuing the agitation so as to prevent the mass from caking. This resultant mixture is then added to water or other suitable noninfia-mmable liquid containing about to 2% of a wetting agent such as alkyl aryl sulphonate, in the proportion of 46% solids to 54% liquid and stirred until a soft thin slurry is obtained. This slurry is then passed through a colloid mill, twice, first with a setting of .007 between the mill plates and then with a setting'of between .002" and .005. rather mobile soft-flowing slurry of colloidal particles of heavy hydrocarbons adsorbed on coal. The nature of the coal is materially changed by this treatment since, instead of being a suspension of a black color it is of a reddish brown color, this being due. to the fact that the colloid mill has broken down the anthraxylon coal into thin colloidal platelets which by transmitted light show a very distinct red color and by reflected light a brown color. Furthermore, the suspension so prepared is miscible with water in all proportions.

The resultant product when prepared as aforesaid is then ready to use in molding sand, either naturally bonded or synthetic. It has several very distinct advantages in the physical state alone. The first of these is that it is condensed; that is, the bulk is cut to a minimum due to the solids being dispersed in water and therefore not so bulky as they would be in dry powdered form. Also the fire or explosion hazard is entirely eliminated, due to the product being in dispersion in water or other non-inflammable liquid. Ease of handling is also accomplished in introducing the facing material in molding sand, as it is merely necessary to either pour the same on the sand heap and cut thereinto in the regular manner or else it can be poured directly into the muller, all without dust loss of any kind. Due to its extremely fine character it disperses in the sand The product so obtained is a with a mini-mum of effort or mechanical mixing. It is also free from the usual dust hazard on introducing dry powdered coal.

The chemical advantages possessed by my im proved molding medium are that all the sand particles are coated with a film of colloidal coal alone, or an admixture of same with heavy hydrocarbons such as aforesaid, something that is impossible when employing ordinary ground bituminous coal. Due to the extreme state of subdivision of the colloidal coal, its chemical action in the mold is'entirely different from ordinary ground coal. As soon as the metal is poured into a sand mold containing my improved colloidal coal product, the heat of the metal acts instantly on all the colloidal sized particles and releases all the volatiles present therein. This action is very evident to the naked eye, for as soon as the mold is poured a very heavy yellow smoke issues from the parting line or the mold, thus showing a complete breakdown of the coal particles. This condition is never evident in the usual sea coal admixtures with molding sand. The release of all the volatile matter from the coal serves to create a large volume of reducing hydrocarbon vapors in both the mold cavity and in the interstices of the molding sand itself and these vapors are burned in the limited supply of air present to form large volumes of soot which act to separate the sand surface from the metal. The residual coke film from the fine coal particles is in direct contact with the sand grains (as the grains were thoroughly coated with the colloidal coal) and this cokefilm will cement the sand grains together and'form a mold surface of exceptional hot strength, which hot strength resists the washing and cutting action of the incoming metal. The coke film also, under the action of heat on ferrous metal and in the presence of silicon dioxide will partly react to form a highly refractory graphitic layer over such grains and will therefore serve to insulate the silica grains from the metal.

' As stated above, it is preferable to use a bituminous coal of high attritus or anthraxylon character as contrasted to bright or splint coal. The reason for this lies in the fact that coal of an attritus or anthraxylon character has more humic matter and the cell wall degradation thereof is well advanced, which means that the coal will lend itself to very fine disintegration as it is softer and the cell walls will break down more easily under the action of the hydraulic shear of a colloid mill.

The reason for sometimes employing coal in combination with coal tar pitch, petroleum asphalt and like hydrocarbons, resides in the fact that in certain cases, as for example in the production of heavy castings of thick sections, it has been determined that the coal alone will not give sufficient volatile matter and the asphalt or pitch alone has insufficient fixed carbon content. Thus the coal will supply the highly refractory fixed carbon residue to coat the sand grain and the pitch or asphalt will supply the high volatiles content possessed by high molecular weight hydrocarbons.

In practice, it is desirable to add the above colloidal product prepared as aforesaid to molding sand in the proportions of from 2 to 10% for the initial treatment of the sand; then it can be asphalt is adsorbed on the coal particle and cannot melt or run over the sand grain.

For producing castings of light or medium section, colloidal coal alone, in suspension in water, without the addition of coal tar pitch or other heavy hydrocarbons, has been found to be quite satisfactory when admixed in the above described manner with the molding sand. For example, a molding composition which is adapted to produce a semi-permanent mold in which several casting operations can be successively performed with out destroying the same, can be prepared as follows:

A mixture is prepared by mulling together 200 parts of sharp silica sand of a grain size suitable for the production of the desired type of castings with a colloidal coal dispersion such as aforesaid, having a solid content of about 16 parts, by weight, of such sand and. of an average particlesize of not in excess of about 5 microns. After the mulling has continued for several minutes, 8 parts of powdered bentonite or other suitable bonding clay are added, while the mulling is continued and then sufficient water is added to bring up the moisture content to about 3.2%. The resultant molding composition is then utilized to form the desired mold. For example, in order to produce a 4 -bend, soil pipe fitting, the cope and drag are formed by riddling the molding composition, in the usual manner, on to the selected pattern and then jolting, say about 50 times, to insure a hard solid mold. A normal green sand core, previously formed on an arbor is then placed in the drag before the cope is closed. In the production of castings with the aforesaid mold, the same is poured in the customary manner and allowed to stand for but four or five minutes and the resultant casting is then removed. At that time, if casting ferrous metal the sprues will still have appreciable red color (the cope sprue should have the shape of a cone so that the smallest part of the opening will be uppermost, thus rendering the removal of the cope possible). By rapping the sprue lightly and lifting the cope after pouring, the castings can be readily extracted from the drag. The drag is then immediately blown out with air and fresh green sand cores are inserted. The cope is then again closed and the mold immediately poured a second time, before it has had an opportunity to cool and contract to any objectionable extent, thereby producing another perfectly clean set of castings from the same mold. When the metal is first introduced to the newly made mold, the heat of the metal brings about a coking action of the colloidal coal that results in the formation of a very thick and extremely hard cemented mold face to a depth of approximately 1 This depth of action will vary proportionately with the section of metal cast in it. Such newly formed hard mold face has the appearance of a good grade of metallurgical coke, the grains being cemented to one another to such an extent as not to allow any of the sand to adhere to the casting.

Also, when casting ferrous metals, the coke will, under the action of the high heat of the molten metal and in the presence of the sand (silicon dioxide), partially react to form a highly refractory graphitic enveloping layer over such grains which will serve to insulate the sand grains from the metal. A graphitic coating will likewise be deposited on the metal casting. In the case of non-ferrous metals, such as aluminum for example, which is cast at temperatures between about 1250 and 1425 F., the temperature prevailing in the mold is insuificient to form such graphitic layer or coating, and the surfaces of the mold contacting the casting will become blackened due to the coking or decomposition of the colloidal coal particles.

The first two or three ferrous metal castings produced with colloidal coal in the aforesaid semi-permanent mold will be very clean and the first one, especially, will have a continuous layer of graphite on its surfaces. This action is unique when using coal, considering the amount of graphite produced and is believed to be largely caused bythe extremely fine particle size of the coal that is subjected to the heat of the molten iron. The speed of action of the colloidal coal and the simultaneous release of all the volatile constituents thereof, not only induces the heavy deposition of graphite, but also effects said cementing action of the sand grains which results from the coking of the coal, that is uniformly and evenly coated on the sand grains. Thus the mold surface is ideally prepared for a second cast.

When casting thick sections or special alloys, it is not feasible to employ sufiicient colloidal coal alone to furnish the required amount of volatiles, as such a large amount would be required that the cementing action of the colloidal coal particles would be excessive and the retained strength of the mold would be such that it could not be broken down for reuse of the mold composition after a few castings had been made therein.

While water, because of its extreme cheapness and other highly desirable properties, is preferred as the medium in which the colloidal coal is dispersed, other non-inflammable liquids capable of substitution therefor, such for example as diethylene glycol, may be substituted therefor.

The speed and the nature of the reaction that occurs during the formation of the aforesaid semi-permanent mold during the formation of the first casting is such that a very firm layer is produced at the mold metal interface, which layer is resistant to scabbing, buckling and normal defects due to expansion of the sand facing.

Having thus described the invention, I claim:

1. A molding composition for metal-casting purposes, the same consisting essentially of sharp silica sand Whose grains are evenly coated with reddish brown platelets of colloidal bituminous coal of a particle size in the millimicron class, and between two and one hundred millimicrons, and said composition containing a modicum of a bonding clay and a small amount of temper ing water.

2. An intermediate product comprising a liquid coating for use in making a molding composition for metal-casting purposes, the same consisting essentially of reddish brown platelets of colloidal bituminous coal in dispersion in water, said coal particles having a heavy hydrocarbon adsorbed thereon and being of a particle size in the millimicron class, and between two and one hundred millimicrons.

3. A molding composition for metal casting purposes, the same consisting essentially of silica sand of a selected grain size suitable for producing the desired casting, the grains of such sand being coated with reddish brown platelets of colloidal bituminous coal of a particle size between about two and one hundred millimicrons admixed with a heavy hydrocarbon and said mold:

7 ing' composition also containing an amount of a bonding clay suflicient to impart desirable molding characteristics thereto.

4. A molding composition for metal-casting purposes, the same consisting essentially of molding sand the grains of which are coated with reddish brown platelets of colloidal bituminous coal of a particle size in the millimicron class, and between two and one hundred millimicrons.

5. A molding composition for metal-casting purposes, the same consisting essentially of molding sand the grains of which are coated with reddish brown platelets of colloidal bituminous coal of a particle size in the millimicron class, and between two and one hundred millimicrons, on which latter is adsorbed a heavy hydrocarbon.

BURGESS P. WALLACE.

' REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,221,259 Woddrop Apr. 3, 1917 1,673,356 Hanley w June 12, 1928 1,777,998 Dent Oct. 7, 1930 1,959,179 Snell May 15, 1934 2,159,952 Jones May 23, 1939 2,348,155 Shanley May 2, 1944 2,398,047 Schmidt Apr. 9, 1946 FOREIGN PATENTS Number Country 7 Date 15,619 Great Britain of 1898 248,289 Great Britain Mar. 4, 1926 426,077 Great Britain Mar. 27, 1935

Claims (1)

1. 3. A MOLDING COMPOSITION FOR METAL CASTING PURPOSES, THE SAME CONSISTING ESSENTIALLY OF SILICA SAND OF A SELECTED GRAIN SIZE SUITABLE FOR PRODUCING THE DESIRED CASTING, THE GRAINS OF SUCH SAND BEING COATED WITH REDDISH BROWN PLATELETS OF COLLOIDAL BITUMINOUS COAL OF A PARTICLE SIZE BETWEEN ABOUT TWO AND ONE HUNDRED MILLIMICRONS ADMIXED WITH A HEAVY HYDROCARBON AND SAID MOLDING COMPOSITION ALSO CONTAININ AN AMOUNT OF A BONDING CLAY SUFFICIENT TO IMPART DESIRABLE MOLDING CHARACTERISTICS THERETO.