US1794536A - Coated mold and method of preparing same - Google Patents

Description

Patented Mar. 3, 1931 UNITED STATES PATENT OFFICE NELsoN .1. QUINN, or TOLEDO, OHIO, AssIeNon 'ro THE METAL CASTINGS nonnme CORZPOBATIOTT, OF CLEVELAND, OHIO, A CORPORATION OF DELAWARE COATED MOLD AND METHOD OF PREPARING SAME No Drawing. Application filed July 28, 1927, Serial No. 209,159. Renewed liay 17, 1980.

order to improve the condition of the castings secured therefrom, and in order, in fact, to secure castings which are acceptable and usable Without any further treatment, such for example as annealing, which is the case with all of the coatings now in use. Various methods of coating and treating surfaces of permanent molds have been attempted, but none have been found successful and none have gone into general use, except in certain cases where all of the castings produced in such molds are annealed in order to soften them and remove the inevitable hard spots which'appear in various points in the casting.

The present invention relates to a mold provided with a coating and to a method of preparing such coating which is of such a character that it produces uniformly soft, finegrained castings which may be used without further treatment of any character, and which are equal or superior to castings formed -in sand molds.

To the accomplishment of the foregoing and related ends, said'invention, then, consists of the means hereinafter fully described and particularly pointed out in the claims; the following description setting forth in detail one method and one product exemplifying my invention, such disclosed procedure and product constituting, however, but one of various applications of the principle of my invention. I

Refractory materials, either alone or combined with various binders or carbonaceous materials, have been used in the treatment of metal molds, but without any impression upon the general commercial article, and so far as I am aware no such processes for coating are today in any general use, or have been found capable of producing uniformly soft, fine-grained and superior castings. I have found, however, that a coating of suitable refractory materials may. be applied to the matrix surfaces of a metal mold to produce a relatively thin fixed coating thereon which i is capable of producing satisfactory soft castlngs provided the coating possesses certain characteristics as to permeability or porosity and capacity to withstand both the physicalwash of the molten metal cast thereagainst and the high, temperature of such metal, together with the proper insulating properties to prevent the too rapid transmission of heat through the coating to the metal mold, although the latter property is to some extent identical with' that initially named as permeability or porosity. The coating whichI have found to be successful is formed with a multiplicity of cells or passages therein, the porosity thus determined being obviousl variable, depending upon the nature 0 the article which is being formed and increasing from a given minimum porosity suitable for the castmg of articles having thick sections to a maximum porosity which is essential for articles having very thin sections, or so formed that they cool or permit the flow of heat rapidly.

The present invention is based upon the observation, which is of course known to the entire foundry trade, that sand molds are suc cessful because of the permeabilityor porosity of the sand, and more and more attention is being given to the maintenance of the permeability of sand within close limits when used in sand molds. Starting with this observation I have found materials which lend themselves particularly well to the formation of coatings of high porosity and to methods of applying these materials to the matrix surfaces of metal molds, in such a way as to produce a resulting coating of high porosity. These coatings I have found satisfactory for producing uniformly 'soft castings and this is universally regarded as'the one and only test of the success of a coating for a metal mold. For obvious reasons the coating aplied to a metal mold must be relatively thin and the high conductivit of the'mold itself compared with the con uctivity of a sand mold must be prevented from conducting away the heat of the cast metal with too great rapidity, and this result can be effected, so far as I, know, only by the interposition .are, first, the porosity, for the reason stated,

then, equally important, a suitable resistance to the temperature of the molten metal cast thereagainst, and suficient physical strength to withstand the wash of metal as it is poured against the coating. Obviously the degree in which the above properties should be present in the coating will vary with the type of article which is being formed. Thus castings in which large masses of metal, as for example, as a typical illustration, an iron ball, requires less porosity in the coating since the total heat units in the molten metal prevent,

to a large extent, any sudden chilling with resultant hard spots adjacent the exterior of the ball, and for such articles a coating having a considerably less porosity can be used than that which is required for articles having thin sections, such, for example, as a small piston, in which the section of the wall or skirt of the piston is relatively'thin, and in which there is a very much greater danger of chilling and hard spots. Similarly, the temperature of the metal which is being cast will permit a selection from among the materials which have the other necessary properties, certain materials which are resistant to the temperatures at which alloyed metals are cast being entirely insufficient for withstanding temperatures of steel or similar metal. In each case it is possible for a certain selection of materials, to be used and I will therefore describe for convenience various materials and methods which may be employed under varying conditions.

The method of treating the mold, that is,

of applying the coating to the matrix surface,

which forms a part of the present invention,

' consists, briefly stated, in suspending in a suitable liquid, such as Water, a quantity of finely divided refractory material, and then spraying this emulsion against the matrix face of the metal mold, while also applying to the deposited emulsion sufficient heat to evaporate the liquid at a rate, and in a way, which permits the suspended solid matter to deposit and build up on the mold surface in a cellular or porous layer. One method of securing this desired rate of evaporation to produce a coating of suitable porosity to effect the results desired is to maintain the metal mold, during the spraying of the liquid and refractory material, at a temperature above the boiling point of the liquid. In'this way heat is applied to the la er of liquid and solid material from the sur ace of the mold. causing an evaporation of the particles of mouse liquid in contact with the surface. These particles when evaporating bubble through the liquid coating above them, producing a cellular or porous condition of the refractory deposit which is desired. At the same time heat must not be applied to the liquid coating on the mold surface too fast or at such a rate that it will evaporate the liquid before depositing, as in this case the solid deposit which will remain will not be cellular, but will be substantially solid and willhave no materially better insulating properties than the material itself, while it is an essential feature of my process and an essential to the final mold structure to have an increased insulat ing value in the coating applied to the mold in order to prevent the rapid transferrence of heat from the molten metal to the mold it self, and in this way to prevent the bad effects on the metal structure which always follow too rapid chilling of any parts of the casting.

In the continuous production of castings it may be necessary to cool the mold between castings in order to prevent the temperature of the mold rising to such a point that it will too rapidly evaporate the coating when sprayed on, or rather actually prevent the depositing of the liquid, and this may be done by any suitable means, such as an air blast directed against the mold or otherwise.

The molds may be made of various metals or alloys of metals, provided the particular metal used has a melting point which is high enough to withstand the heat of the metal being cast, and for ordinary purposes I use common cast iron molds which will handle temperatures in the molten metal of 3000 If. or over, it being understood of course that the refractory coatings protects the mold so that molten metals with temperatures considerably in excess of the melting point of the metal of the mold may be applied.

Various refractorymaterials may be employed, all of which should be in a finely ground condition and capable of being readily suspended in a liquid vehicle for convenient spraying. Any suitable liquid may be used, although on account of its low cost and low boiling point I prefer water, to which may be added if desired a small amount of viscous material or binder, such as syrup, molasses, glucose or similar material in order to increase the adherence of the matrix coating to the surface of the mold and to prevent the rapid scaling off of this coating when the castings are withdrawn, necessitating a second application of the coating, where otherwise several castings might be made before the mold surface would necd to he recoated.

Of the various materials which may be used extremely porous coatings can be formed of magnesium oxide, chalk, talc. diatomaceous earths, other materials high is required.

areaaae in magnesium content and certain metallic oxides and silicates, all of which should be.

finely divided and capable of remaining in suspension in the liquid used, and particularly in water. Less porous coatings, but coatings which are entirely satisfactory for castings having thick sections can be made from various refractory clays, carbon in the form of graphite, and finely divided carbide. lit will be understood of course that all of the materials which may be used must have sufficient structural strength to withstand the wash of the'molten metal thereagainst, and sufficient refractoriness to withstand unchanged the temperature of the molten metal. By testing the porosity of coatings which have been found satisfactory for the production of thin sectioned castings and also for castings having thick sections i have determined that the porosity must he between 22% and 86%, but it is obviously impossible to define the exact porosity for all conditions as this quality varies directly with the temperature of the metal being cast, the section of the metal being cast, and possibly also with the conductivity of the metalrnold itself, governedby the thickness and the heat of the mold. For the production of automobile pistons of some three inches in diam eter and having a relatively thin section in the skirt l have found that a porosity lying between 50% and 86% is required. The porosity was determined by first measuring the volume of a portion of a coating applied to a mold, then compressing the material comprising the coating under a pressure of 2000 pounds per square inch, measuring the new volume and taking the ratio between the -initial and final volumes.

The mixture of liquid and suspended refractory material may be sprayed onto the tation of the mixture during the spraying process. lln applying the spray to the mold the point of issuance of the material from the spray nozzle should be brought sufficiently close to the face of the mold so that the mixture will spread rapidly over the mold and build up a layer of coating of sufficient thickness to force the articles of liquid next to the face of the mol which are first evaporated, to bubble through the coating in order to produce the cellular porous structure that causes the mixture to carry an excess of air which assists in building up the porous structure.

When'par-ticularly smooth castings are "desired the treatment of the mold must be completed by directing the spray at the molten surface from a greater distance than when first applied. and either the same or a different mixture may be employed at this stage. By thus withdrawing the point of The operation of spraying,

issuance of the spray from the matrix face of the mold the deposit of refractory material will be very much thinner over the surface of the mold, and the liquid will evapoproducing rate practically instantaneouslg,i

e' thickness a surface layer of no apprecia which is fine-grained and acts to close the cells or pores in the coating proper.

l have secured very satisfactory results and perfectl uniform and finely grained castings wit the use of molds treated in accordancev'vith the present invention, and castings in which a metal structure of very closely knit formation with an even distribution of carbon is secured. As an illustration,

l have made castings consisting of so-called open skirt automobile pistons which are extremely difiicult articles to produce, since they have a heavy section in the bosses, a second heavy section in the head, and an extremely thin section, as thin as in some cases, in the skirt. l have produced such castings by the use of my improved molds which have a uniform metal structure notwithstanding the great dilference in the thickness of this structure at various points,

and in which the metal is uniformly soft throughout the entire casting. Such a structure has generally been considered as impossible to successfully make in metal molds.

Other forms may be employed embodying the features of my invention instead of the one herein explained, change being made in the form or construction, provided the ele-.

ments stated by any of the following claims or the equivalent of such stated elements be employed, whether produced by my preferred method or by others embodying steps equivalent to those stated in the following claims.

I therefore particularly point out and distinctly claim as my invention 1. In a method of applying a coating of a liquid containing refractory material onto the matrix surface of a heated metal mold, the

steps which consist in sprayin said liquid by pressure from a spray nozz e when held in close proximity to the surface to build up a relatively thick coating rapidly, and then removing said spray nozzle to a greater distance from said surface to produce a surface coating of .a different character. 7

2. A coating for a metal mold consisting of a refractory material having a multiplicity of air cells therein acting to retard the transfer of heat through said coating and having a smooth coating pf the same refractory material of slight thickness overlying one surace of the said first-named coating and int'egrally joined thereto";

3. A. coating for a metal mold consisting of a magnesium containing compound havin suflicient strength to withstand the wash an d pressure of molten metal thereagainst and a porosity of not less than 25% nor more than 86%.

4. A coating for a metal mold consisting of a magnesium containing compound hav- 5 ing a porosity of not less than 50 per cent.,

nor more than 86 per cent.

.5. A coating for a metal mold consisting of a refractory material having a lightness approximating that of magnesium oxide and having a porosity of not less than 50% and not more than 86%.

6. A coating for a metal mold consisting of a refractory material having a lightness approximating that of magnesium oxide and 5 a porosity of not less than and not more than 86%. Y

7. A coating for a metal mold consisting of magnesium oxide of a high degree of porosity..

2o 8. A coating for a metal mold for the production of uniformly soft machinable castings consisting of a refractory material having sufficient strength to withstand the-Wash of the molten metal thereagainst and a po- 25 rosity of between 40% and 86%.

9. A coating for a metal mold consisting of a refractory material having a multiplicity of air cells therein acting to retard the transfer of heat through said coating and having a fine grained coating of lesser porosity of the same refractory material of slight thickness overlying one surface of said first named coating and integrally joined thereto.

10. A permanent mold for the production of uniformly soft machinable castings having a surface layer of refractory. material of a porosity of not less than nor more than 40 11. A coat-ing for a metal mold consisting of a refractory material sprayed onto a heated mold surface, said material initially having properties in a loose, finely divided condition, of lightness and suspensibility in water approximating those of magnesium oxide, and said ,coating having a porosity of from 40% to 86% and other properties approximating those of a coating of magnesium oxide similarly applied.

Signed by me, this 22nd day of July, 1927.

NELSON J. QUINN.