US2426987A - Mold coating - Google Patents

Description

' a casting process,

Patented Sept. 9, 1947 MOLD COATING Walter A. Dean, Lakewood, Ohio, asaignor to Aluminum Pa",

Company of America. Pittsburgh, a corporation of Pennsylvania No Drawing. Application November 13, 1942, Serial No. 405,483.

This invention relates to the casting of magnesium and magnesium base alloy and is directed to the problems encountered when that molten metal, or such alloys, is poured into contact with coatings provided on the surfaces of a, mold cavity. While the invention has useful aspects in connection with any mold where such coatings are desirable, it is specifically concerned with the problem of molds of the permanent or semi-permanent type in which at least a portion of the surfaces of the mold cavity are composed of a relatively permanent material such as, for example, a metal. Such surfaces are usually composed Of mold wall, cores and, perhaps, chills cooperating to form and define a cavity in the mold.

Coatings are usually applied to the walls of a mold cavity, Prior to casting molten metal therein, to prevent a sudden chilling of that portion of the metal which contacts the surfaces of the mold cavity, to. prevent reaction between mold material and molten metal, to prevent cavity wall erosion and sticking of the metal casting to the mold wall, and to serve other purposes, all of which are well known in the foundry art. For successful operation the coating should have the required thermal characteristics, should provide a surface from which a casting of smooth clean surface may be obtained, should promote ready and exact filling of the mold cavity by the molten metal and should be resistant to the erosive or abrasive action of the incoming metal, as wellas capable of preserving its physical integrity under the varying thermal conditions to which it is necessarily subjected during repeated casting operations. It is desirable that a balance of all of these qualities be obtained with the end in view of producing a sound satisfactory casting without constant expensive interruption of the'casting process to renew the coating on the casting cavity.

- magnesium as lend to the The object of this invention is to provide a method of coating molds which will meet the peculiar problems encountered in the casting of magnesium and magnesium base alloy.

A further object of the invention is to provide coated molds and methods of casting which will improve commercial casting conditions in the casting of light metals such as aluminum and its alloys and magnesium and its alloys.

The coatings of this invention are comprised of two general elements-binder and insulator or body material. The coatings are applied to the mold cavity surfaces by means of a carrier, liquid Heretofore in the casting of magnesium and magnesium base alloys thereof the desirable qualities above mentioned have not been achieved in the mold coatings used. Magnesium is, in molten form, a highly reactive metal, and the mold coatings with which it comes in contact -ften fall after just a few castings are made in the mold cavity. This necessitates interruption of the casting procedure while the coating is renewed and this adds materially to the cost of particularly where a perma nent orv semi-permanent mold is being used and costs are based upon the number of castings which can be produced from said mold in a given period. Moreover,

in character, other liquids may be used. The proportions or concentration of this carrier with respect to the coating will depend upon the operators-needs and the manner in which the coating is to be applied, for instance, by spraying, brushing or the like. The insulator or body material may be one or more of many inorganic materials which are used to perform this function in mold coatings. Their specific selection may depend upon several factors, including their relative chemical inertness, the temperatures involved, the thermal characteristics required, the nature of the desired surface of the casings to be. produced (smooth or rough) and, with regard to obtaining optimum coating quality and performance under particular molding conditions, the binder employed. As is later specifically noted, colloidal graphite, furnace slag and vermiculite are particular insulators which are adapted to achieve optimum benefits and specific results in the practice of this invention, but other inorganic materials such as, for instance, other undue reaction between mold forms of graphite, magnesium oxide, diatomaceous silica, asbestos, soapstone, china clays, and the like, may also be used toward the obtainment of the general objects above stated. Any one or more of these and other insulators may be used in one coating. The proportion-of insulator to binder in the final coating is governed by considerations specific and immediate to the which is usually water, although foundry conditions faced by the operator, and selection of specific proportions is governed by the intricacy and size of the mold cavity, the particular alloy being handled, the relative binding quality of the binder, the bulk and covering power of the insulator, the thickness of coating desired and other well known factors. In any event, absolute proportions are a matter of simple and routine-selection. r I 1 The binder promotes adherence of the particles of the insulator or body material to the mold wall and forms a matrix which gives continuity to the coating and may also serve to protect the insulator particles against erosion or attack of the molten metal. Binderspreviously used for thisruption of the casting process in order to renew the coating.

According to this invention I providea binder selected from the class of soluble sulphates consisting of magnesium sulphate and the alkali metal sulphates. Where, as in the case of sodium,

more than one soluble sulphate exists, I intend to include in such group all such sulphates of that metal which are soluble in the water, or other carrier. It will be apparent that once the coating is upon the cavity surface solubility of the sulphate becomes unimportant, and indeed the action of the heat of casting upon the coating may alter the solubility of the sulphate of the coating. Of the sulphates in the class just described I prefer to use magnesium sulphate (MgSOc'ZHzO) but all such sulphates have the common property of withstanding the action of the molten magnesium or magnesium base alloy and of also performing the other functions desirablein a binder to produce with the insulator material a relatively hard, adherent and continuous coating upon the mold cavity surfaces. The practice of this inventionwill therefore comprise the use of a coating containing at least one of these sulphate binders, and insulator or body material. While the insulator or body material may be chosen from the wide range of materials which are available, I have found that certain of these cooperate with the named sulphate binders to form coatings which are particularly superior and, therefore, I prefer to use these substances, which are black iron oxide, vermiculite, and furnace slag.

Examples of mold coatings compounded in accordance with the principles of this invention are the following:.

Coating A Magnesium sulphate (MgSOc'II-IcO) grams 25 Iron oxide (F6304) do--.... 25

a Carrier. 100 cubic centimeters of water.

The proportions of binder and insulator above given form no part of this invention and will be varied by the operator to meet specific conditions. Usually I prefer to add the sulfate binder in an amount not less than about percent by weight of the total weight of binder and insulator, butpercentages expressed by weight are not necessarilysigniflcant since some insulators, such as asbestos,-diato'maceous silica such as Silocel, and others, have a low weight per unit volume. Simple trial will indicate the amount of the described sulphate desired. F1 The mo'idcoatings herein described may be applieddi'rectly to the surfaces of the mold cavity.

and its alloys. The- -In some instances it may be desirable to first wash the mold cavity surfaces with a preliminary wash that highly reactive or'coating'to obtain a surface to which the mold coating"= will more readily adhere. Such preliminary washes or base coatings are well known.

- Their-use is, of course, no part of this invention,

and since they are-completely covered by the mold coating applied thereon, they 'do not contact the molten metal poured into the mold cavity. Other similar"-- steps such as are "normally used in foundry practice may be undertaken in connection with the practice of the present invention.

While the desirable property of these sulphate bound coatings is their relative'permanence as compared with coatings previously used in the casting of magnesium. neither these new coatings, nor the previously used coatings, are, in any relative sense, as permanent as many of the mold cavity surfaces to which they are applied. In

a large modern foundries permanent mold casting is a relatively continuous operation in which costs depend, to a substantial extent, upon lack of interruption of the casting routine. The molds used are, through one or more production days, per manent, and a continuous supply of molten metal to such molds is merely a matter of arrangement. Given good foundry conditions the casting routine can therefore proceed uninterrupted provided the coating on the mold cavity does not fail and thus necessitate interruption of foundry procedure in order that it may be renewed.

Another aspect of my invention is based on my discovery that in the casting of light metals,

such as aluminum and its alloys and magnesium and its alloys, the longevity of any mold coating may be increased if there is provided on the mold coating a superficial layer selected from the class consisting of soluble magnesium sulphate and soluble alkali metal sulphates. Such a, layer, applied in any convenient manner as by spraying in solution forms a hard layer on the coating which adds to the effective life of the mold coating without materially altering the functional characteristics thereof. This layer prevents severe erosion, minimizes reaction and generally increases the longevity of the coating to which it is applied. In applying such a superficial layer, neither the manner of application nor the concentration of sulphate compound is a critical factor. The concentration employed will govern the applications necessary to produce a superficial layer of a given thickness. I have used, with good results, a solution containing 25 per cent .by weight of magnesium sulphate (MgSO4.7H2O) and applied this solution by spraying. The desirable result is a well-dried continuous superficial hard layer of magnesium sulphate or alkali metal sulphate imposed on the mold coating.

, necessary to bind the insulator to the mold and to produce the coating continuity Where the mold cavity is,'at least in part, defined by a non-permanent material, such as a sand core, no problem of permanence arises since the cavity. is, to that exent, destroyed after each casting operation. However, the value of the superficial layer in decreasing the reaction of the molten metal on the core coating and in preventing erosion during casting is sufficiently great as to justify its use in many cases.

While certain preferred embodiments of the invention, and specific examples thereof, have been described, it will be understood that such are by way of example only and that the invention may be otherwise practiced within the scope of the appended claims. It will also be understood that the mold cavity "surfaces" referred to in the appended claims may be originally bare or originally p ovided with a preliminary or base coating.

I claim:

1. In the method of casting light metal into 20 mold cavities at least a portion of the surfaces of which are provided with a coating containing a binder and inorganic insulator material, the improvement consisting in facing the coating with a layer of a compound consisting of magnesium sulphate and alkali metal sulphate.

2. In the method of mold cavities at least which casting light metal into a portion of the surfaces of are provided with a coating containing selected from the class.

a binder and inorganic insulator material, the improvement consisting in facing the coating with a layer of magnesium sulphate.

3. In the method of casting magnesium and magnesium base alloy in molds having mold cavity surfaces coated with a coating containing a binder and inorganic insulator material, the improvement consisting in forming the coating with a binder which is composed of a substance selected from the class consisting of magnesium sulphate and alkali metal sulphate and in facing said coating with a layer of a substance selected from said class. 1 4

WALTER A. DEAN.

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

UNITE!) STATES PATENTS Certificate of Correction Patent No. 2,426,987. September 9, 1947.

WALTER A. DEAN I It is hereby certified that the error appears in the printed specification of the above numbered patent requiring correction as follows: Column 2, line 39, for casings read casting; and that the said Letters Patent should be read with this correction therem that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 25th day of November, A. D. 1947.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.