US2618530A - Mold coating - Google Patents

Description

Patented Nov. 18, 1952 MOLD coA'rmo George R. Gardner, Berea, Ohio, assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application June 18, 1949, Serial No. 100,083

Claims.

This invention relates to washes employed in the coating of the interior walls of molds in con-' tact with molten metal, especially the mold cavity surfaces in the type of molds known as permanent molds as distinguished from molds which are destroyed at the end of each casting operaprevent sticking of the hot body to the mold.

They also serve other purposes recognized by those who make cast products. In order to serve the foregoing purposes, the washes or coatings should have the ability of being easily applied to the desired mold surface, of firmly adhering thereto, of being non-reactive with the metal or other body introduced into the mold, and resisting spelling or cracking under changing thermal conditions. The coatings used heretofore have had some or all of the foregoing properties in varying degrees, but where used on metal molds or others of relatively permanent character they have lacked the capacity for easy removal after having served their purpose on the mold. The coatings normally become dried and hardened through continued use, and in some cases there may even be some chemical change in one or more of the components of the coating. As a result, it has been difficult to remove the coatings for repair or replacement. Ordinarily, a stiff wire bfush or even a knife or similar tool is necessary to clean a previously coated mold surface. Removal operations such as these not only consume time, but may actually lead to a change in dimensions of the mold in critical parts where any scraping may occur.

My invention is directed to overcoming the foregoing disadvantages and in providing a coating which has all of the desirable properties enumerated above, and in addition can be removed easily from the mold walls. In achieving these objectives the invention provides coatings, methods of coating, and coated molds which are especially adapted to producing castings of aluminum and magnesium and alloys in which they predominate.

I have discovered that highly satisfactory mold coatings can be made by applying to either a part or all of the surface of a mold cavity a mixture containing an insulating body and a binder,-

here designated as a metal borophosphate, in a liquid vehicle such as water and drying the coating preparatory to use of the mold. The concentration of the vehicle with respect to the other substances is dependent upon the manner in which the coating is to be applied, whether by spraying, brushing, dipping, orthe like, and can be readily adjusted by the operator to suit his particular needs. In any event, the vehicle is evaporated from the coating before the mold is used, leaving the desired deposit of insulating material and binder.

Insulating material employed in the mold coating may be one or more well known inorganic substances, the selection being a matter of choice as influenced by the conditions encountered in making any given cast body. Such factors as the temperature of the hot body in the mold,

the thermal characteristics of the insulating material, the character of the surface desired on the casting, and cost, are considered in making such a selection. The insulating materials which have been commonly used for this purpose and which are suitable for the coatings I have described are: graphite, magnesium oxide, diatomaceous silica, asbestos, soapstone, china clays, furnace slag, and the like. One or more of these insulating materials may be employed in making a given wash or coating.

The adherence of the wash or coating to the mold surface and the bond between the particles of the insulating material are dependent upon the binder. The binder also serves to protect the coating against erosion or attack by the molten metal or other body cast in the mold. Although both organic and inorganic binders have been employed heretofore, especially alkali metal silicates, yet they have not been entirely satisfactory in that they become insoluble or relatively so, after being dried and used and hence it is difficult to remove the coating from the mold surface. Furthermore, coatings containing this type of binder are often brittle to the extent that the coatings easily crack or spall. I have found that a non-crystalline glassy substance referred to herein as a metal borophosphate forms a superior binder. The borophosphate is formed by combining within certain proportions a substance from the group consisting of soluble metal tetraborates and orthoboric acid and a substance from the group consisting of soluble metal meta-, pyroor orthophosphates, and orthophosphoric acid, but not the two acids by themselves, and the pyrophosphate only in combination with orthoboric acid. This means that the tetraborates may be combined with the metaand orthophosphates or orthophosphoric acid while the orthoboric acid may be combined Under some conditions it may be desirable to include a soluble metal hydroxide as mentioned hereinbelow. This borophosphate possesses an exceptionally high adhesive quality both in liquid and in dry conditions. The term borophosphate as here used is not to be understood as necessarily referring to a single compound since the chemical relationship between the components is not exactly known, but it 'does include the combination of substances herein defined that yield a non-crystalline glassy product in the dry condition. In this condition the borophosphate forms a hard bound coating on the mold surface easily removed with water and gentle brushing. Although the coatings containing this binder are very resistant to erosion by virtue ,of their hardness and adherence, yet the binder possesses sufficient plasticity to minimize or eliminate the cracking or spalling often observed where sodium silicate is used, for example. The improved plasticity also makes it possible to repair or replace a portion of the coating without damaging the remaining portion. In all these respects the metal borophosphate binder is a distinct improvement over any of the materials used to form the borophosphate when employed as coating binders. It has been found that the metal borophosphate binder is substantially non-reactive with molten magnesium, and, hence. is well suited for making coatings for molds in which this metal is cast.

The binder may be prepared in advance of its addition to the other coating ingredients, or the borate and phosphate components may be mixed directly with the insulating material and water or other solvent added to produce the desired consistency for application to the mold surface. As mentioned above. the metal borophosphate is made by combining certain soluble metal borates or orthoboric acid and certain soluble metal phosphates or orthophosphoric acid in the presence of a solvent in such proportions that when the solvent is evaporated a clear or opaque glassy residue remains which is substantially non-crystalline, that is, the residue is entirely amorphous or if a small amount of crystalline material is present its effect is negligible as far as the desired qualities in the binder are concerned. By non-crystalline is meant that diffraction X-ray examination reveals little if any substance. For this reason the residue is here considered to be non-crystalline and the term will be used in that sense. It has been found that unless the residue is of such a character the adherence and bonding power of the product is greatly diminished. In other words, a crystalline or powdery residue is distinctly inferior as a binder.

The exact amounts of borate and phosphate components will vary somewhat with the particular compounds chosen, but the test of the product in every instance will be the same, namely, its non-crystalline glassy character. The combination of orthoboric and orthophosphoric acids alone does not produce the desired glassy residue and hence if both are used a third soluble component capable of supplying a metal oxide must be added,- such as an easily soluble hydroxide, preferably an alkali metal hydroxide, in suitable proportion. lFurther, the hydroxide with the meta-, pyroand orthophosphates.

to which it is applied and yet the coating can be evidence of a crystalline structure in the dry does not appear to be beneficial in making the pyrophosphates useful but it may be present if it does not interfere with the combination ofother substances with the pyrophosphate. The addition of an hydroxide may be needed, however, where an insufilcient amount of metal'oxide is provided by the borate or phosphate components to yield a glassy residue or to increase the metal oxide content to change or improve some other property of the binder than non-crystallinity. When sodium tetraborate and the proper sodium phosphates are used, supplemented, if desired, by orthoboric or orthophosphorlc acids, and possibly sodium hydroxide, or sodium pyrophosphate and boric acid are employed, the proportions of these compounds in terms of their oxides should be as follows: 5 to 65% B203, 3 to 68% P205 and 12 to 68% NazO. Sodium borophosphatemay therefore be considered to be formed in suilicient quantity for binder purposes when these oxides are present in the indicated proportions for within these ranges a clear glassy or an opaque glassy residue is obtained. Careful tests, employing NazB4O-110H2O and have shown that an optimum condition exists when these salts are combined in the ratio of one molecular weight of the former to two molecular weights of the latter. When calculated on a weight basis and in terms of NazO, B203 and P205, these molecular proportions amount to 30.6% NazO, 34.4% B203 and 35.0% P205. By using the above proportions of the two salts, a maximum solubility in water was attained. An excess of either salt tended to lower the solubility somewhat but the glassy residue referred to herein was still obtained which possessed adequate bonding power and solubility in water to serve very satisfactorily as a binder for mold coatings.

Translated into specific compounds, the binder can be made from any of the following combinations.

. Percent 1. Na:B4O1.10Hz0 10-90 NaHzPO4.HzO 10-90 2. HJBOi 30-80 NMPIOI 20-70 3. NazB4O1.10H:O 10-50 HaPO4 50-90 4. NBz'BsOmlOHzO 10-60 NaaPO4.12I-L-O 40-90 The combination of the following sodium and potassium salts is also useful:

Percent N82B40'L10H20 20-80 K HzPO4 20-80 An example of a mixture requiring the presence of an hydroxide to produce a glassy non-crystalline residue is that made from 64% NaaBsomloHzo,

wash to be applied to the molds. For most foundry operatic however, it is more convenient to mix the phosphate or orthophosphoric acid, tetraborate or orthoboric acid, and the insulat ing material and add a sumcient amount 01' water to provide the desired consistency in the mold wash.

The non-crystalline glassy metal borophosphates are highly soluble in water in all proportions, and this solubility is not substantially altered by the drying of the mold coating preparatory to use and by continued use. This is of great advantage in renewing the coating on permanent mold surfaces. The solubility of the metal borophosphates in water greatly exceeds that of most 01' the crystalline borates and phosphates from which the borophosphates are made.

The metal tetraborate or orthoboric acid components must be soluble in the vaporizable vehicle used for making up the wash. For purposes of producing a metal borophosphate the soluble metal tetraborates and orthoboric acid are equivalent in B203, and hence either one or .both may be used as stated above. 01 the soluble metal tetraborates, the alkali borates are preferred.

that both supply the oxide,

In regard to the phosphate and phosphoric,

acid components required to form a metal borophosphate binder it is necessary that they also be soluble in the vaporizable vehicle. These components, as previously mentioned, may consist of metal orthophosphates or orthophosphoric acid but the metaand pyrophosphates may also be utilized providing the conditions in the solution or the treatment during mixing with the other components to form a metal borophosphate are such as to permit at least partial conversion to the orthophosphate. Thus, the pyrophosphates can only be employed in combination with orthoboric acid. It is to be understood, therefore, that the presence of the orthophosphate form is necessary at the time it joins with the borate component.

The best proportions of borophosphate to insulating material vary considerably with the mold and casting, and the former may range from about 10 to about 500 per cent of the weight of the insulating material. In making light metal castings, it has been found that diatomaceous silicia and china clays, either separately or in combination, provide highly desirable mold coatings.

While the invention has been desired particularly with respect to coating permanent molds, it will be appreciated that sand cores used in semi-permanent molds, as well as sand molds themselves, may be coated with the wash where a hard erosion-resistant mold surface is desired. In these instances, however, the solubility of the coating is not of so great importance as in the case of permanent molds, although this quality may be of value in shaking out the castings from the molds or in disintegrating the cores within the cast bodies.

As examples of mold coatings made in accordance with the principles of this invention, the

following may be cited:

Coating A Grams Water 160 China clay s 36 Na2B407.10H20 19 15 NaH2PO4.H2O L to provide a preliminary coating,

8 CoatingB Grams Water' 100 Diatomaceous silica 6.3 N8:B4O7.10H20 12 NaHzPOnHzO 9 Coating C Grams Water 100 China clay 4.5 Talc 13.6 HaBOs 9.5 C'oatingD Grams Water 100 Diatomaceous silica 7.5 China cl'ay 30.0 Na:B40-:.10H:O 15.0 NaHmOal-lzo 10.0

The mold coatings herein described may be applied directly to the surfaces 01 the mold cavity. In some cases, however, it may be desirable such preliminary coatings being well known in the art. However, this procedure is generally unnecessary in view 01' the excellent adhesiveness or the metal borophosphate binder to the mold surface.

Reference has been made hereinabove to the casting of light metals, and it will be understood that this term applies to both aluminum and magnesium and the alloys in which these metals predominate. It is also to be understood that the use of the coating is not confined to the casting of the foregoing metals; that it may be employed in other places where a soluble coating is desired.

While certain preferred embodiments of the invention have been described above, it will be understood that they are shown only by way of example, and that the scope of my invention is comprehended by the appended claims.

This application is a continuation-in-part of my co-pending application Serial No. 621,167, filed October 8, 1945, now abandoned.

I claim:

1. A mold wash consisting essentially of an insulating material, a vaporizable vehicle, a substance from the group consisting oi. soluble metal tetraborates and orthoboric acid'and a substance from the group consisting of soluble metal metaand orthophosphates and orthophosphoric acid where the said tetraborates are employed and a substance from the group consisting of soluble metal meta-, pyroand orthophosphates where orthoboric acid is employed, the proportions of the boron and phosphorus-containing compounds being such that when the said vehicle is evaporated a non-crystalline glassy residue remains.

2. A mold wash consisting essentially of an insulating material, a vaporizable vehicle, a substance from the group consisting oi! soluble metal v compounds and alkali metal hydroxide being such that upon vaporization of said vehicle a noncrystalline glassy residue remains.

3. A mold having a cavity therein for receiving molten metal and a coating covering at least aportion of the wall surface 01' said cavity consisting essentially-of insulating material and a metal borophosphate binder composed 01' a nonconsisting of soluble metal metaand orthophosphates and orthophosphoric acid where said tetraborates are employed and a substance from the group consisting of soluble metal meta-, pyro and orthophosphates where said orthoboric acid is employed, said substances being present' 10 within the proportions necessary to form said non-crystalline glassy reaction product.

4. A mold having a cavity therein for receiving molten metal and a coating covering at least a portion of the wall surface of said cavity consisting essentially of insulating material and a metal borophosphate binder composed of a noncrystalline glassy reaction product resulting from the combination of a substance from the group consisting of soluble metal tetraborates and orthoboric acid and a substance from the group consisting of soluble metaand orthophosphates and orthophosphoric acid and a soluble metal hydroxide, said substances being present within the proportions necessary to form said non-crystalline glassy reaction product.

5. A mold having a cavity therein for receiving molten metal and a coating covering at least a portion of the wall surface of said cavity consisting essentially of insulating material and a 3 metal borophosphate binder composed of a noncrystalline glassy reaction product resulting from the combination of a substance from the group consisting of soluble alkali metal tetraborates and orthoboric acid and a substance from the group consisting of soluble alkali metal metaand orthophosphates and orthophosphoric acid where said tetraborates are employed and a substance from the group consisting of soluble alkali metal meta-, pyro and orthophosphates where orthoboric acid is employed, said substances being present within the proportions necessary to form said non-crystalline glassy reaction product.

6. Aimold having a cavity therein for receiving moltenimetal and a coating covering at least a portion{ of the wall surface of said cavity consisting essentially of insulating material and a non-crystalline glassy binder whose entire composition, expressed in terms of oxide components,

is 5 to 65% B203, 3 to 68% P205 and 12- to 68% N820 by weight; said oxide components being supplied by the combination of a substance from the group consisting of sodium tetraborate and orthoboric acid and a substance from the group consisting of sodium metaand orthophosphates and orthophosphoric acid where said tetraborate is employed and a substance from the' group consisting of sodium meta-, pyro and orthophosphates where said orthoboric acid is employed.

'7. A mold having a cavity therein for receivingmolten metal and a coating covering at least a portion of the wall surface or said cavity consisting essentially of insulating material and a binder composed of a non-crystalline glassy reaction product resulting from the combination of 10 to 90% by weight of NaflOmlOI-hO and 10 to 90% by weight of NaHaPOaHaO.

8. A mold having a cavity therein for receiving molten metal and a coating covering at least a portion of the wall surface of said cavity consisting essentially of insulating material and a binder composed of a non-crystalline glassy reaction product resulting from combining one molecular weight of Nil-2B4O1JOH2O with two molecular weights of NaHzPO4.H2O.

9. A mold having a cavity therein for receiving molten metal and a coating covering at least a portion of the wall surface of said cavity consisting essentially of insulating material and a binder composed of a non-crystalline glassy reaction product resulting from the combination of 10 to 50% by weight of NB,2B40'1.10H2O and 50 to 90% by weight of orthophosphoric acid.

10. A mold having a cavity therein for receiving molten metal and a coating covering at least a portion of the wall surface of said cavity consisting essentially of insulating material and a binder composed of a non-crystalline glassy reaction product resulting from the combination of 10 to by weight of Na2B40'L10HzO and 40 to by weight of NaaPO4.12H2O.

GEORGE R. GARDNER.

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

UNITED STATES PATENTS Number Name Date 1,868,457 Wood July 19, 1932 2,004,030 Boughton June 4, 1935 2,426,988 Dean Sept. 9, 1947 FOREIGN PATENTS Number Country Date 303,858 Great Britain Nov. 21, 1939 526,740 Germany June 10, 1931

Claims (1)

1. A MOLD WASH CONSISTING ESSENTIALLY OF AN INSULATING MATERIAL, A VAPORIZABLE VEHICLE, A SUBSTANCE FROM THE GROUP CONSISTING OF SOLUBLE METAL TETRABORATES AND ORTHOBORIC ACID AND A SUBSTANCE FROM THE GROUP CONSISTING OF SOLUBLE METAL METAAND ORTHOPHOSPHATES AND ORTHOPHOSPHORIC ACID WHERE THE SAID TETRABORATES ARE EMPLOYED AND A SUBSTANCE FROM THE GROUP CONSISTING OF SOLUBLE METAL META-,PYRO- AND ORTHOPHOSPHATES WHERE ORTHOBORIC ACID IS EMPLOYED, THE PROPORTIONS OF THE BORON AND PHOSPHORUS-CONTAINING COMPOUNDS BEING SUCH THAT WHEN THE SAID VEHICLE IS EVAPORATED A NON-CRYSTALLINE GLASSY RESIDUE REMAINS.