US2381734A - Mold chill - Google Patents

Description

Patented Aug. 7, 1945 MOLD CHILL Marvin E. Gantz, Lakewood, Ohio, asslgnor to Aluminum Company of America, Pittsburgh, Pa, a corporation of Pennsylvania No Drawing. Application March 31, 1942, Serial No. 437,089

3 Claims. (Cl. 22-188) This invention relates to the art of making molds, or parts thereof, of sand or similar material that will make a frangible mold or mold part, and it is especially concerned with providin chills for said molds or parts.

In making castings in sand molds or molds containing sand cores, it is well recognized that the molten metal freezes relatively slowly in such molds as compared to the rate of freezing in an all metal mold. In making some castings the rate of freezing is important, and the slow freezing which normally occurs in a sand mold or in contact with a sand core must be increased in order to avoid shrinkage, reduce the grain size, and/or to increase the surface hardness of the casting. To thus hasten the freezing rate in portions of the mold, it has been acommon practice to embed blocks of metal in the sand mold or core. Such blocks of metal are referred to in the art as chills. The quantity cf'heat that any chill will absorb and the rate at which the heat is absorbed constitute what is herein referred to as chilling effect.

in certain parts, it is frequently necessary to provide specially shaped chills to conform to the contour of the casting. Such special chills are obviously expensive to make and maintain, to say nothing of the labor involved in positioning them in the mold. In some cases where chills have been desired, it has been physically impossible to use blocks because of the size or location of the chill. This situation has led me to devise a new type of chili which can be molded to fit any desired contour or location as the mold or core is being made. the principal object of my invention being the provision of a chillwhich is not only initially moldable but which will provide a chill that performs the same function as the conventional type of metal block chills used heretofore. A particular object is to provide a moldable chill mixture which is adapted for use in green sand molds and may or may not be, hardenable at the discretion of the foundryman. Another object is to provide a chill for use in making light alloy castings.

My invention is predicated upon the discovery that a composition comprising an intimate mixture of small metal pellets and a suitable binder is readily moldable, when properly tempered, prior to placement in a mold or core made of sand or similar material and yet can be hard ened, if desired, at room temperature or at an elevated temperature, depending upon the character ofathe binder, with the result thata firm When castings are made that have a complicated shape and must be chilled chill body is produced which will not crumble incontact with the molten metal in the mold. The benefit derived from the use of such chills is reflected in a reduction in grain size and shrinkage and/or an increase in surface hardness of the casting as compared to the same casting made without the chills. B means of such a composition it is possible to use chills of varying size at any place in a sand mold or core, particularly in locations where block chills cannot be employed such as in thin sections or tortuous passages, and also to make the chill body conform to the shape of the casting to be made in the mold. In some instances it may even be desirable to make an entire core of the chill mixture. In no event, however, does moldingsand form a part of the mixture because it reduces the chilling effect too greatly. The selection of the correct amount of chill for a given portion of a casting can be readily determined by the foundryman. After the casting has been formed in the mold, the chill, together with the sand, can be readily broken away in the usual manner. This is especially important in the removal of sand cores. The moldable mixture containing metal pellets, binder, and tempering medium may be referred to for purposes of convenience as a green chill mixture while if the binder is of the hardening type, the resulting chill body may be referred to as, a hardened chill mixture. The improved chill which I have devised may be said to be characterized by its initial moldability, i. e., its adaptability to being shaped during the usual molding operations, its capability of being hardened, if desired, and a chilling effect similar to that of the metal blocks heretofore employed.

By a chilling effect similar to that of metal block chills, I mean that the chill composition herein described and claimed reduces the grain size and shrinkage of the casting in the region where the chili is employed as compared to the condition prevailing when no 'chill is used. In some cases the chilling may also cause an increase in the surface hardness of the casting.

The term, harden, as used herein refer to the loss of moldability and setting of a green chill mixture through evaporation of all or part of the tempering medium or through a change in the binder, or both, which occurs naturally on standing exposed to the atmosphere or by exposure to an elevated temperature prior to the time that the casting is poured in the mold. The expression, chill mixture, without reference to greenness or hardness covers the chill mixture in both conditions.

Although the chill mixture herein described is adapted for use in molds and cores employed in the casting of a large variety of metals and alloys, I have found it to be especially useful in making castings of light metals, particularly those of magnesium and magnesium base alloys. The molds and cores where the chill mixture is used are generally made of sand or similar material. The molds may be eith'erof the green or dry typ but the cores are usually baked prior to use. The baked cores may be used in so-called permanent molds'as well as in sand molds. The term, sand,

as used herein, is intended to cover not only sand itself, but also its equivalents.

The metal portion of the chill mixture should consist of metal pellets which will not ruse at the temperature of the molten metal being cast in the mold. A variety of metals may generally be so employed, but for practical purposes a cheap metal having a high and rapid heat absorbing capacity will generally be preferred, such as iron or copper. Another consideration which affects the selection of the kind of metal pellets employed in certain instances is the readiness with which the pellets will alloy with the molten metal in the mold if the two come into metal-to-metal contact. Such alloying is to be avoided or minimized.

The metal portion of the chill mixture constitutes the largest part thereof, at least 85 per cent of the total weight being made up of metal pellets, but at least 90 per cent is employed in my preferred practice. The size of the metal pellets has an important bearing upon their utility. I have found that if the pellets are larger than 30 mesh, they are not satisfactorily held in place by the usual binders, and they also tend to produce a rough surface on the casting. On the other hand, if the pellets are smaller than 150 mesh, a large part of the chilling capacity of the mixture is lost and the disadvantages of metal dust or powder are encountered. I have also found that the best results are obtained by using substantially spherically shaped bodies of metals. Pellets oi the foregoing size and shape are frequently referred to as shot. The term, pellet, as herein employed, however, is intended to describe rounded and substantially sphericai'bodies and includes metal particles of 30 to 150 mesh size known in the trade as grit.

Although pellets of a single mesh size may be used in a chill mixture, better results are obtained if two or more mesh sizes are employed. For examplaa combination of 40 and 60 mesh pellets are better than 40 or 60 mesh pellets used separately. In some instances it is advantageous to use three mesh sizes of pellets, for example, 40, 60, and 90.

The binder portion of the mixture may consist of any of the common green or dry binder materials on the market so long as the chill mixture has the desired degree of cohesion and resistance to crumbling when exposed to the molten metal in the-mold. A binder that is particularly useful where chills are used in green sand molds is a bonding clay such as bentonite. This type of binder generally does not harden and hence a chill mixture containing such a binder is in a green condition when used as a chill. Chill mixtures containing this binding material can be rammed in the molds in the same manner as molding sand, and the resulting product is then handled as any other green sand mold. A further restriction on the choice of binder is that in cores it should not yield enough g s during the casting operation to form blow holes. Although thermosetting synthetic resins find some application in Q green sand molds, they are especially useful as binders in my chill mixture when used in baked sand cores because a smaller quantity is needed to hold the metal pellets in place, than where other types of core binders are used, and they produce but a very small amount of gas in the mold. This relative freedom from gas production is highly important in making castings of metals which are sensitive to gas, for example, magnesium base alloys. In general, from 0.5 to 3 per cent by weight of the chill mixture may consist of the synthetic resin. A large number of thermoset'ting resins are on the market which are satisfactory for this purpose, but I prefer to use at least one substance of the group consisting of the urea formaldehyde and phenol formaldehyde types of resins. Where other types of binders are used in either green sand molds or cores, from 0.5 to as much as 5 or 6 per cent may be required to obtain the necessary bond.

In making up a green chill mixture the pellets and binding material are mixed together so that there is a uniform distribution of binder throughout the metal mass. The mixing may be conveniently accomplished in a muller. Enough water or other aqueous or non-aqueous tempering medium should be added to provide the right degree of moldability. In making green chill mixtures containing iron pellets, it may be desirable to use a dilute aqueous solution of l per cent potassium dichromate or potassium permanganate, as the tempering medium, to prevent rusting. Other rust or corrosion inhibitors may be employed, of course, the choice in any case depending on the metal composing the pellets and the conditions favoring corrosion.

Once the green chill mixture has been prepared, it can be placed in the mold next to a pattern or in the core box and the remaining sand which constitutes the mold or core can be placed behind the chill and the whole mass rammed in the usual manner. In cases where dried sand core are to be made, the same molding procedure may be followed, but the green cores are baked in order to give them the required strength and freedom from moisture in the final mold. The green chill mixture herein described has been found to be particularly useful in green sand mold in which aluminum base alloys are cast. The reference herein to light alloys is intended to include both aluminum and magnesium and the alloys in which these metals predominate.

Minor amounts of other substances than those enumerated hereinabove may be present in the chill mixture either as impurities or as intentionally added components. For example, where magnesium alloys are being cast, especially in green sand molds, it is necessary to include well known oxidation inhibitors-such as boric acid and/or diethylene glycol in the mixture in amounts, for example, of 0.5 to 3 per cent by weight of the mixture. The added components should not, in any event, materially reduce the chilling effect of the mixture, nor should they adversely affect the moldability of the green mixture. Molding sand, however, should be excluded from the chill mixture because of its adverse influence upon the chilling effect. In referring to the chill mixture inthe appended claims as being substantially composed of metal pellets, a binder, and a tempering medium, it is intended that other substances of the kind Just enume ate shall not be excluded so long as the essential characteristics of the mixture are retained.

An example of a chill mixture which gives satisfactory results in casting light alloys is one consisting of 150 pounds of 60 mesh iron pellets, 150 pounds of 90 mesh iron pellets, 6 pounds of bentonite, 6 pounds of cornstarch and enough 1 per cent potassium dichromate aqueous solution to temper the mass. useful for chills in green sand molds. Another mixture which is particularly useful in baked sand cores is one consisting of 150 pounds of 60 mesh iron pellets, 150 pounds of 90 mesh iron pellets, 1.75 pounds of urea formaldehyde, 1.75 pounds of phenol formaldehyde, and enough 1 per cent potassium dichromate aqueous solution to temper the mass. The water in the mixture is of course removed by the baking, thus leaving a hard chill mass.

I claim:

1. A chill mixture containing atleast 85 per cent by weight of 30 to 150 mesh size metal pel- This mixture is especially lets, a binding material, and a tempering, medium containing a corrosion inhibitor, said mixture casting in the region of the chill as compared to the condition prevailing in the absence of said chill. Y

2. A chill mixture for use in green sand molds for the-casting of magnesium base alloys consisting of at least 85 per cent by weight of to 150 mesh size iron pellets, 0.5 to6 percent by weight of bentonite, 0.5 to 3 percent by weight of an oxidation inhibitor, and a tempering medium'containing an iron rust inhibitor, said chill mixture being characterized by being moldable and having a chilling efiect suflicient to reduce the grain size and shrinkage in the casting in the region of the chill as compared to the condition prevailin in the absence ofsaid chill.

A chill mixture for use in green sand molds consisting of at least per cent by weight of at least two different sizes of 30 to mesh size iron pellets, 0.5 to 6 per cent bentonite, and a tempering medium containing an iron rust inhibitor, said chill mixture being characterized .by being moldable and having a chilling effect sufllcient to reduce the grain ize and shrinkage in the casting in the region oflthe chill as compared to the condition prevailing in the absence of said chill.

' MARVIN E. GANTZ.