US2818620A - Method of improving foundry sand cores - Google Patents

Description

Jan. 7, 1958 Filed Nov. 20, 1953 W. H. MOORE METHOD OF IMPROVING FOUNDRY SAND CORES I5 Sheets-Sheet 1 HOT TOUGHNESS OF AN OIL CORE SAND 10o BASE SAND EQUALS1- SILICA sANo-----9s.4% CEREAL- L894, LINSEED OIL- |.a%

I CRYOLITE BARIUM FLUORIDE I00 BASE SAND MINUTES SOAK AT 2000 F I INVENTdR.

WILLIAM H. M0 0RE Jan. 7, 1958 Filed Nov. 20, 1953 W. H. MOORE METHOD OF IMPROVING FOUNDRY SAND CORES 3 Sheets-Sheet 2 INVENTOR WILLIAM H. MOORE WW/(17%; WWW? ATTORNEY Jan. 7, 1958 W. H. MOORE METHOD OF IMPROVING FOUNDRY SAND CORES 3 Sheets-Sheet 3 Filed Nov. 20, 1953 ON 2 w E N 0- w w Oom- CONN DOWN OOON v t xi 8. 323 2.39 5

INVENTOR. WILLIAM H. MOORE OO- OON 00m 00* 00m 00m szunmw s a o00oz slmn SSBNHSFIOJ. aaoow V M United States Patent 0 lVIETHOD OF IMPROVING FOUNDRY SAND CORES William H. Moore, Larchmont, N. Y., assignor to Meekanite Metal Corporation, a corporation of Tennessee Application November 20, 1953, Serial No. 393,275

2 Claims. (Cl. 22-194) This invention relates to the improvement of molding sands and core sands used in the manufacture of castings and more particularly to the use of certain ingredients in these sands.

Foundry sands when subjected to the heat of molten metals undergo a number of changes. These changes often result in cracking and rupture of the molds and cores. This deterioration of the foundry sand under heat leads to a number of well known casting defects, examples of which are buckles, rat tails, scabs, veins, burn-on and penetration.

It has been recognized by those skilled in the art that casting defects of this type are due primarily to the effect of differential expansion which occurs when a sand is heated. Such differential expansion leads to severe forces within the sand body and when sufficiently severe these forces will rupture the sand body thereby disturbing the continuity and quality of the mold surface.

Many notable advances have been made in the direction of eliminating casting defects that originate with the inability of a sand mold to retain its dimension and form under the action of heat. Examples of such advances are found in the use of cushioning agents such as cereal binders and sawdust which provide a degree of porosity which allows movement of sand grains under heat thereby reducing the extent of the expansion forces which are generally agreed as being the forces which cause mold rupture.

Examples of other advances are found in the careful balancing of grain size distribution to minimize these forces and in the use of materials such as zirconite, forsterite, mullite and sillimanite which do not exhibit the volume inversion under heat that silica sand does and, therefore, are not as subject to the effects of differential expansion.

Up to now, however, no universally acceptable method plastic flow effectively dissipates the expansion stresses thereby preventing deterioration of the mold surface.

It is among the objects of the present invention to pro-- vide a means of eliminating casting defects due to differential expansion of silica molding sands.

A further object of this invention is to provide an improved sandmix which does not disintegrate or rupture under the effect of heat from molten metal.

A further object of this invention is to provide an improved sand mix which has a'high strength under the action of heat and will, therefore, hold its shape during the pouring of a casting.

Other objects and advantages of the present invention will be apparent to those skilled in the art from the following description taken in conjunction with the figures in which:

Figure l graphically illustrates the effect of time of heating on the hot toughness of an oil sand core mix and shows the effect of 1.0% of cryolite and 1.0% of barium fluoride in the same base mix;

Figure 2 is a photograph showing heat cracking in an oil sand core mix;

Figure 3 is a photograph showing freedom from heat cracks in the same mix with 1.0% of lithium fluoride;

Figure 4- graphically illustrates the progressive increase in hot toughness produced by additions of cryolite to a sand mix; and

Figure 5 graphically illustrates the effect of ingredients having various fusion points on the hot toughness of a sand mix.

I have found that certain materials having a relatively low fusion point (that is, relatively low when compared to the fusion point of silica sand or clay which is normally contained in a foundry sand mix) when included in a sand mix will provide a degree of plastic flow in the sand mix when it is subjected to the high heat of casting, thereby effectively preventing rupture of the mold surface and effectively increasing the strength of the mold surface so that it remains essentially stable during the process of casting.

To be particularly effective the additive materials besides having a suitable fusion point should be of such a chemical nature that they do not materially affect the normal room temperature properties of the sand mix.

For example, it has been known for some time that materials such as borax, sodium carbonate, sodium silicate and the like may be used effectively as washes on a mold surface thereby partially preventing surface cracking of the molds. However, it has also been recognized that such washes are only partially effective in that their action is confined to the extreme surface of the mold. I have also found that these materials have an alkaline or an acid reaction in the sand so that they very seriously affect the green properties and the baked properties of any sand mix. For this reason it has not been desirable to incor-' porate materials of this type directly into a sand mix, but when they are used, they are preferably used as Washes on the mold surface.

In carrying out the process of the invention, I select additive materials which have limited solubility in water but more particularly materials which do not have a pronounced alkaline or acid reaction when they are added to the molding sand. The absence of such reaction usually ensures that the normal working properties of the sand mix are not adversely affected in any way.

Examples of materials which may be used successfully in carrying out the process of my invention are the fluorides of metals or alkaline earths, the oxides of metals and alkaline earths, certain fluoborates and fiuosilicates, certain carbonates and certain chlorides. I particularly prefer to use fluorides such as calcium fluoride, barium fluoride, sodium fluoride, potassium fluoride, sodium aluminum fluoride, lithium fluoride and magnesium fluoride. A more comprehensive list is shown hereinafter. These materials have the desired attributes and are available on a commercial scale not being subject tosevere cost limitations.

The process of this invention is best illustrated by a series of examples of laboratory tests conducted particularly to illustrate the effect of these aforementioned additive materials.

Referring to the figures and illustrations, Figure 1 shows how the hot toughness of a simple oil sand mix is increased first by a 1% addition of barium fluoride and secondly by v a 1.0% addition of cryolite (sodium aluminum fluoride).

Fatented Jan. 7, 1958 3 the method outlined in United States Patent No. 2,491,512.

In general, I have found that the increase in hot toughness of a sand mix by means of additives of the type employed in carrying out the process of my invention is related to some extent to the fusion point of the additive material and to the amount of the material added to the sand.

These additive materials may be incorporated into the sand by any of the methods well known to those skilled in the art. Examples of such methods are mixing by hand or by mechanical means, mulling or by hydraulic means where the material is added in the form of a slurry. In any case they should be sufliciently fine to be capable of intimate blending. The progressive increase in strength according to the amount of additive used is illustrated in Figure 4. In this figure, cryolite has been used as an example of an additive material and an increase in the amount of cryolite added has resulted in a progressive increase in the hot toughness of the sand mix.

In general, I have found that very small additions, viz., as low as one-eighth percent are measurably effective in increasing hot strength and that very high additions, viz., as high as 10.0% are also effective but not proportionately as effective. Very large additions of 5.0% or more may be considered as wasteful as an addition of 2.0% or possibly 3.0% usually gives all the increase in strength that is desired. I prefer to use additions ranging from one-eighth percent by weight to 3.0% by weight in carrying out the process of my invention.

The effect of the fusing temperature of the additive material on the increase in strength brought about by the material has been illustrated in Figure 5. The data for this Figure 5 was obtained at 2000 F. but a similar relationship has been found to exist at both lower and higher temperatures.

In this figure various materialshave been selected to give a range of fusion points and additions of 1.0% by weight were made to a simple sand mix. In the graph the increase in strength has been expressed as a ratio. Thus, the figure 20 means the sand was 20 times as strong as it was without any additive material.

Very little strength increase is obtained with additive materials which fuse above 2500 F. and the strength increase is more pronounced as the fusion point of the additive becomes less.

In actual practice extremely low fusion point materials are not desirable as the prolonged action of molten metal will eventually destroy them. However, with low fusion point metals such as non-ferrous alloys, this danger of premature destruction of the additive material is not as prevalent.

In general for the casting of steel, I prefer to use additive materials that fuse in the range of 2000 F. to 2600" F. For the casting of gray cast iron and other ferrous metals, I prefer to use additive materials that fuse in the range 1500" F. to 2500 F. whereas for non-ferrous alloys, I prefer to use additives that fuse in the range 1000 F. to 1800 F.

The selection of the additive of the most desirable fusion point can be accomplished in a number of ways. As an example, the selection may be made on the basis of laboratory tests at any chosen temperature corresponding to or bearing a relationship to the fusion point of the metal to be cast. The amount of the addition may be established according to the increase in toughness desired.

As a further example the additive material may be selected according to the results obtained with a test casting. This method of selection is well known to those skilled in the art.

Additive materials in general should be selected because of their .inertness at the temperatures to which they are to be exposed. Thus, additives which decompose chemically to evolve gases and the like are not suitable as they may present problems in casting. I prefer to use additives which fuse and flow but which do not decompose chemically with the formation of gases.

I have discovered that these additive materials which fuse and tend to flow at the temperatures to which they are exposed in use, in addition to increasing the strength of the sand mix to which they have been added, also have the property of effectively preventing surface cracking of the molds made from the sand containing the additives.

This property has been illustrated in Figure 3, where lithium fluoride has been used by way of example, as compared with Figure 2, where no additive was used. In general, I have found that cracking of the sand under heat is almost completely prevented when the fusion point of the additive material is below 2000" F.

When the fusion point of the additive material is higher than 2000 F., viz., between 2000 F. and 2500 F. the surface cracking may only be partially eliminated.

In actual practice, I have used many additive materials in carrying out the process of my invention, but I have also found that fluorides in particular have the most desirable characteristics in that they have well defined fusion points, they are relatively inert, they are not extremely acidic or basic in nature, they are relatively inexpensive and are commercially available.

Examples of materials which I have successfully used as additive materials in carrying out by invention are:

Some of these materials, for example sodium borate, potassium silicate and sodium carbonate, affect the normal properties of the sand to which they are added but where this effect is not serious, they are nevertheless useful because of their pronounced effect on the properties of the sand under the action of casting heat. In many cases I have used mixtures of some of these materials. A typical example of a sand mix which I have used to overcome the problem of veining which is caused by surface cracking is as follows:

Percent by weight Silica sand 96.0

Cereal binder 1.5 Linseed oil 1.5 Powdered cryolite 1.0

A similar mix which proved equally effective in preventing veining which I have used successfully is:

Percent by weight Silica sand- 96.5 Linseed nil 2.0 Western bentonite 0.5 Calcium fluoride 0.7 Sodium fluoride 0,3

Percent by weight Silica sand 73.5 Natural bonded sa 15.0 Silica flour 7.0 Pine resin binder 3.0 Calcium fluoride 1.0 Cryolite 0.5

A mix which I have used where high hot toughness is necessary for a successful casting is:

Percent by weight Silica sand- 94.0 Linseed nil 2.0 Cereal binder 2.0 Basic g 2.0

The basic slag used in this mix is the well known falling slag which is familiar to those skilled in the art of basic steel melting.

I have used these additive materials in many sand mixes and have always been successful in reducing or eliminating casting defects due to the failure of the sand under the action of the heat from molten metal.

I have attributed this success to the fusing and flowing of the additive material under the action of heat in such a way that the plastic flow prevents build up of expansion stresses and the cohesiveness of the molten additive imparted a high degree of hot toughness to the sand.

It is, therefore, a feature of this invention that the additive material has a fusion point which is lower than the casting temperature of the molten metal that is to be cast. In general, I prefer to have the additive material fusing at a temperature at least 50 F. below the casting temperature of the molten metal.

In actual foundry practice a mold made of the sand of this invention will be made at a temperature below the fusing temperature of the additive, and will be heated by the molten metal poured into the mold. There will, therefore, be a temperature gradient from the metal cavity to the exterior of the mold. The additive may actually be heated beyond the point of mere fusion at the cavity surface, but a slight distance inwardly the temperature will be such that the additive will be more plastic in nature. This stage gives the desired hot toughness. Further, this particular phase will be located on a moving frontier as time elapses and the heat penetrates outwardly. Thus, the stresses will be progressively relieved.

I have described this invention with a certain degree of particularity but it is understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such variations and modifications apparent to those skilled in the art are considered to be within the purview and scope of the invention and the appended claims.

What is claimed is:

l. The method of improving the hot strength of a sand core adapted for use in the casting of ferrous metals comprising adding to the sand mixture used for making said core, a preselected compound from the group consisting of an alkaline earth, and alkali metal fluoride in the range of from one-eighth percent by weight to three percent by weight, said sand mixture consisting essentially of sand and a binder from the group composed of cereal, oil, natural resin and clay, molding said core and heating said core to a temperature suflicient to render said binders effective but insufi'icient to render said fluoride effective.

2. The method of improving the hot strength of a sand core adapted for use in the casting of ferrous metals comprising adding to the sand mixture used for making said core, a preselected compound from the group consisting of an alkaline earth, and alkali metal fluoride in the range of from one-eighth percent by weight to three percent by weight, said fluoride compound having a fusion point approximately 50 F. lower than the casting temperature of said ferrous metal, said sand mixture consisting essentially of sand and a binder from the group composed of cereal, oil, natural resin and clay, molding said core and heating said core to a temperature sufi'icient to render said binders effective but insuflicient to render said fluoride efiective.

References Cited in the file of this patent UNITED STATES PATENTS 1,898,437 Gann Feb. 21, 1933 2,137,715 Erdmann et a1 Nov. 22, 1938 2,322,667 Seastone et al June 22, 1943 2,682,692 Kohl July 6, 1954 2,683,296 Drumm et a1 July 13, 1954 2,753,608 Nolan July 10, 1956 FOREIGN PATENTS 74 Great Britain of 1906 678,798 Great Britain Sept. 10, 1953 826,334 Germany Dec. 27, 1951

Claims (1)

1. THE METHOD OF IMPROVING THE HOT STRENGTH OF A SAND CORE ADAPTED FOR USE IN THE CASTING OF FERROUS METALS COMPRISING ADDING TO THE SAND MIXTURE USED FOR MAKING SAID CORE, A PRESELECTED COMPOUND FORM THE GROUP CONSISTING OF AN ALKALINE EARTH, AND ALKALI METAL FLUORIDE IN THE RANGE OF FROM ONE-EIGHTH PERCENT BY WEIGHT TO THREE PERCENT BY WEIGHT, SAID SAND MIXTURE CONSISTING ESSENTIALLY OF SAND AND A BINDER FROM THE GROUP COMPOSEED OF CEREAL, OIL, NATURAL RESIN AND CLAY, MOLDING SAID CORE AND HEATING SAID CORE TO A TEMPERATURE SUFFICIENT TO RENDER SAID BINDERS EFFECTIVE BUT INSUFFICIENT TO RENDER SAID FLUORIDE EFFECTIVE.

Images