US2220703A - Process of casting metal, mold, therefor, and method of making same - Google Patents

Description

Nov. 5, 1940. M. BEAN 2,220,703

PROCESS OF CASTING METAL, MOLD THEREFOR, AND METHOD OF MAKING SAME Filed June 16, 1936 2 Sheets-Sheet 2 Patented Nov. 5, 1940 UNITED STATES PATENT yOFFICE PROCESS OF CASTING METAL,

MOLD

THEREFOB, AND METHOD OF MAKING SAME 18 Claims.

This invention relates to permeable materials, to articles made thereof and to methods of making and using the same. More particularly the invention relates to permeable plasters and like materials, and especially to molds of plaster and the like for use in the casting of metals and other fusible or thermoplastic materials, to the process of making such plaster and molds and to a process of casting metals and other fusible or thermoplasticmaterials.

In many cases in the technical arts it is desirable to have a material which, although presenting a smooth fine grained surface and substantial strength is, nevertheless, highly permeable to the passage of fluid. This is particularly true in the casting of metals and the molding of other materials which require a relatively high molding temperature, since at these temperatures gases are frequently formed within the mold either by de- U composition of some substance in the mold or by release of chemically or physically bound gases which, unless they are permitted quickly to escape from the mold space may seriously interfere with the faithful reproduction of the mold shape 25 or the quality of the metal in the casting, or, as in the case of magnesium alloys, it may be advisable to supply a. gas to the mold during casting to protect the metal against oxidation or otherwise to modify the conditions of the casting. In such 30 case there mustbe sulcient provision in the mold for venting the gas when the cavity is filled with metal. i

Plaster of Paris has been known prior to my invention to be one of the most desirable mate- 35 rials for fine casting molds because plaster casts will give a smooth line grained surface accurately reproducing a desired shape and yet the material, asis we ll known, is porous. However, even plaster of Paris has not been wholly satisfactory because,

40 even though the porosity may be so high that 50% of the apparent volume may be air-space, yet because of its internal structure the material offers a high resistance to the passage of gases and thus frequently does not allow suiliciently 45 rapid escape of the gases formed in the mold.

l, This property of permitting passage of gases I shall refer to as permeability as distinguished from porosity, which I shall use only to define the relation between the volume of solid material 50 and the volume of voids.

APlaster molds used for the casting of metals before my invention, have always been subjected to a special baking treatment to drive oil the water of crystallization and it has been univer- 5.- sally accepted in the art that no satifactory cast.

ing could be made in a plaster mold until after such dehydration.

I'he dehydrated calcium sulphate, however, yis unstable and tends always to partial rehydration by absorption of water from the atmosphere. It is necessary, therefore, when using such molds to take special precautions to prevent rehydration of the calcium sulphate.

Among the most important properties to be sought in plaster molds to' be used for casting 10 metals as shown by my'experience and study are fluidity in pouring so that the plaster reproduces accurately the fine detail of the pattern, freedom from excessive shrinkage and preferably the quality of expanding slightly, smooth ne tex- 15 tured surface, stability at room temperature in the open atmosphere, permeability Y to gases, strength during handling, and low strength after casting so that the mold can be readily removed from the casting. Prior to my invention numerous efforts and extended` research have been devoted to improving these qualities individually, but it has always been found that one could only be improved at the expense of another and consequently the qualities of any given mold have represented necessarily a compromise. By my present invention I have shown for the first time how these qualities can be simultaneously improved, although, as will be understood from the following specification, I am still able to emphasize one or another of these various qualities by varying the composition or the treatment or by various means for emphasizing the particular properties which are already known in the prior art.

Accordingly, one object of my present invention is to provide a material which) will present a smooth, fine grained surface and yet Will have a permeability to gases comparable only to the coarser materials, such as sand molds.

Another object of my invention is to provide a 40 material which, while having a relatively high strength en masse, may be of a granular nature so that it can be readily disintegrated and removed fromdelicate molded surfaces without injury thereto, and especially one which may be weakened by exposure to the hot cast metal.

Another object of my invention is to provide a mold material adapted at the same time to carry a relatively large amount of bound water, the evaporation of which at the surface will chill the material being molded thereby helping to simplify the procedure of molding and to produce a perfect and smooth surface on the molded article, and at the same time will have sufficiently high permeability to gases so that the water vapor 'y may pass off without interfering with the ow and molding of a fused or thermoplastic material Within the mold.

Another object of my invention is to provide plaster molds for the casting of fusible or thermoplastic material which are stable so that they may be stored and handled in the open atmosphere Without special precaution to protect them against absorption of water.

Another object of my invention is to provide molds and a method of molding in which the contraction of the molded article during and after the process of molding is compensated for by an expansion in the material of the mold itself whereby the finished molded article may reproduce substantially the dimensions of the pattern from which it is made.

Another object of the invention is to provide a mold material which is adapted to undergo an endo-thermic change by which the metal is rapidly chilled at the surface of the mold to avoid objectionable "shrinks and leakage of the liquid metal from cracks in the mold.

Another object of the invention is to provide a material suitable for structural purposes,

for thermal insulating purposes, for acoustical purposes, for' filtration and other uses which may be chemically homogeneous, vbut has a granular structure, permeability, strength and shrinkage properties which may be'controllcd to accord with the requirements of the particular use for which the material is intended, and may have different physical characteristics at the surface than in the interior.

These objects I achieve according to my present invention by the formation of compact granules in the material, and advantageously with larger grains in the internal structure of the material than at the surface. I have found plaster of Paris particularly suited for the purposes of my invention because I am able to form such granules rapidly in situ by changing the calcium sulphate to the hemi-hydrate and back to di-hydrate after the plaster has set; and it will, therefore, be used as an example for purposes of description.

The plaster of Paris or other material used may first be cast or molded to give a fine grained porous product having the desired surface shape and characteristics. Recrystallization 'is then effected e. g., in the case of plaster of Paris, first by partial dehydration resulting in a change in the crystal form of the calcium sulphate in the plaster and then by a rehydration with the pres. ence of excess water at least within the body of the article. As a consequence of this treatment the surface structure remains substantially in the same fine grained condition, whereas within the body of the article the recrystallization is accompanied by a growth of the larger crystalline grains at the expense of the smaller.

'I'he material which is most suited for the practice of my invention is a plaster of Paris plastic and preferably one which contains not less than -15 nor more than 90% of plaster of Paris mixed with some finely divided or granular refractory material and especially a fibrous or foliated material such as talc, asbestos, etc. Other refractory materials, especially granular materials, as for example silica, may-also be used in addition to or instead of the fibrous or follated material. Very finely divided or impalpable powdery materials, such as finely divided clays, are not recommended, however, since many of these tend to prevent the formation i the de.-

-sired physical properties. Certain asbestos,

especially when not freed from impalpable powdery material, also tends to inhibit the granule formation. Fibrcus talc and especially when classiied, e. g., by an air flotation process to give fairly uniform ber length has been found especially suitable for this purpose. Such a talc incr-eases the rate at which the desired recrystallization takes place. In general, when brous materials are used, best results arc obtained when the ber length is of the same order of magnitude as the diameter of the granues, especially if more than one half and less than five times the granule diameter.

In the prior use of plaster of Paris compositions for casting molds it has been necessary to restrict the amount of water used in the casting composition in order to avoid excessive shrinkage upon dehydration after setting and drying of the plaster. Since, according to my present invention, I am able to compensate for such shrinkage by an expansion in the mold material due to recrystallization, my invention now makes it possible to use much more uid mixes with a much higher proportion of water, and to use other ingredients which would otherwise tend to accentuate shrinkage. Thus I can obtain in products of highest quality many desired properties which could only be had before at the expense of excessive shrinkage. However, although it is an advantage of my invention that it makes possible the use of more fluid plaster compositions and of other ingredients which I find advantageous it should be understood that my invention does not require the use of such large proportions of water or of these other ingredients. In some cases it may be desirable to use the invention with relatively stii plastic composition, using only enough water to make the composition moldable, and in some cases it is desirable to use plaster of Paris with no other solid ingredients.

The following are a few representative compositions which I have used to advantage according to my invention:

Parts I. Water 84 Plaster of Paris 60 Silicavl 25 I Talc.

Parts II. Water 100 Plaster of Paris 80 Talc Parts III. Water 46 Plaster of Paris 40 Silica 60 Parts IV. Water 80 Plaster of Paris 90 Talc l0 Parts V. Water 65 Plaster of Paris 100 In all of these compositions the plaster referred to is ordinary commercial calcined gypsum plaster, e. g., such as has been commonly used heretofore for the making of plaster of Paris casting molds.

These ingredients may be combined in the ordinary way by simply mixing the constituents in water until there is uniform distribution of the dry material in the liquid.

annonce I'he casting of the material over the pattern to form a mold may also be done according yto known and commonly practiced procedure. As already stated above, my invention makes possible the use of more fluid compositions., The material when cast is preferably sufliciently fluid so that it flows freely around the pattern, filling every minute detail so as to assume a faithfully exact reverse of the pattern.

The mold as thus formed is allowed to set in the ordinary way and may be air-dried by leaving it in an ordinary room atmosphere, or by placing it in a stream of warm air. After setting, the calcium sulphate of the plaster is in the form of the ,di-hydrate, and the material of the mold is, therefore, stable and can be kept without special precautions for an indeinite period.

The air-drying step is not essential, but I have found that it is desirable and tends to a better surface of the finished mold resultingvafter the other steps of its preparation; 'I'his drying step may, for example, be carried to such an extent that the total weight of the mold after molding the plaster is reduced about -'20% by the evaporation of water.

After the setting of the mold, it is, of course, also necessary to remove the pattern. In the ordinary sectional mold process the plaster mold thus formed will be taken apart and removed from the pattern in sections.

My invention is also applicable to the lost wax process, in which case the mold will pass to the next step directly with the wax pattern still within the mold.

I have found that if the mold is closed during the recrystallization step and unless special precautions are taken to separate the adjoining faces of the sectional parts of the mold, e. g., by

'- oiling these faces, or by separating during the subsequent treatment, they may be united by the recrystallization.

As already stated above, my invention contemplates especially the growth of the larger grains within the body of the mold at the expense of the smaller. The principles of such growth and conditions favoring it are matters which have received and will continue to receive detailed study by many scientists, so that, given this conception, others skilled in the art will have little difficulty in obtaining the result in various ways. From a commercial point of view, however, it is important that these crystalline changes should occur with sufficient rapidity so' that the process can be completed without substantially increasing the cost of handling and storing the molds during processing. A very important feature of the described embodiment of my present invention, therefore, is the inclusion of the step of dehydrating and rehydrating the calcium sulphate by which a rapid recrystallization is enforced. The desired crystal growth according to the present embodiment of my invention thus is quickly effected as an incident of this recrystallization.

This recrystallization step ofthe process in the preferred example is effected by: first, a controlled heat treatment for uniform conversion of the calcium sulphate dihydrate to the hemi-hydrate. Secondly, a saturation of the body of the mold with moisture sufllcient not only to re-convert the calcium sulphate to the di-hydrate but to provide excess water at the interfaces in which the recrystallization can occur. Thirdly, where a fine-grained surface is desired the removal of excess water from a thin layer at the surface i of the mold. Fourthly, the maintenance of such conditions favorable to the recrystallization until the desired redistribution into larger grains has occurred; and iinally, the termination of such conditions, e. g., by drying, to prevent any further objectionable change in the granular structure.

The most practical and satisfactory method of securing the desired uniform dehydration is to subject the entire mold to anv atmosphere of saturated steam in an autoclave at a temperature sufficient to eiect the desireddehydration; The

dehydration begins at only a few pounds of steam pressure, but with very low pressures itis too slow to be practicable. Thus the temperature should ordinarily be above about 110 C., i. e., a pressure above about 6 pounds gauge, and preferably a pressure of between 8 and 15 pounds gauge should be used. For example, I have obtained uniformly good results by subjectingvthe mold to an atmosphere of saturated water vapor under 9 pounds gauge pressure for a period of about 9 hours, more or less depending upon the size of the mold. With a higher steam pressure the period of exposure can be decreased and with lower pressure the period must be substantiallyA increased. Furthermore, with different mold compositions somewhat different times will be required to give satisfactory results. Thus, as already mentioned above, the use of talc hastens the treatment to some extent.

I have found that it is desirable to allow,a small amount'of steam to pass through the autoclave in order to carry off any air which may be trapped in or about the mold.

In the preferred practice of my invention this autoclave treatment is regulated so that approximately three-fourths of the water of crystallization of the said gypsum plaster is liberated, i. e., the di-hydrate is converted to hemi-hydrate; but the liberated water is not evaporated because the treatment takes place in an atmosphere saturated with water vapor. Upon the release of pressure from the autoclave the mold is wet through and recrystallization of the calcium sulphate to di-hydrate begins immediately. The

surface of the mold is, therefore, exposed to the air or to other material adapted to remove free water from the surface, and thereby to prevent the crystal growth in the thin layer at the surface of the mold. The simplest procedure for this purpose is simply to remove the molds from the autoclave and expose them to the open atmosphere in a room where the air is substantially below 100% humidity. I have.found, however, that even if the molds are packed in wet sand or sawdust and are left in this wet sand after the autoclave treatment they will still exhibit the fine grained surface. This I believe is due to the difference in capillarity between the relatively coarse sand or sawdust and the ne grained plaster, as a result of which the free water is drawn away from the surface and toward the interior of the plaster. A good procedure is to pack the molds in dry sand or sawdust when put into the a autoclave. The steam will moisten the pack and the mold may be left in it during the reorystallization step.

These damp molds may be left, e. g., in a normal atmosphere at room temperature for from 3 to 16 hours. The exact time required will depend partly upon the manner in which the mold has been treated previously, partly upon the type of mold material which is being used and partly upon the granular structure desired. During this period some evaporation of water from the mold may occur, but they must not be dried out so long as further grain growth within the material is desired. It is during this period that the redistribution of thecrystals Within the body of the mold mainly occurs, and the degree of this redistribution is controlled by the amount of excess water present within the body of the mold material, by the temperature, and by the length of time during which these conditions are allowed to continue. i

The greatest improvement in the properties of my materials takes place during this setting-up interval after lthe material has been autoclaved. This apparently results from the growth of crystals in bunches forming little bundles of very closely packed crystals. These bundles or granules give the material tensile and compressive strength, but leave between them relatively large interstices, which give high permeability. The structure of the material after this treatment is very similar to oolitic rock, with the granules bonded together only at the points of contact, whereas the structure of the original plaster cast might be thought of as analogous to finely divided fibers packed together.

Tests of the materials during this period indicate that the rehydration of the hemi-hydrate to the di-hydrate and the recrystallization are not complete for at least three hours in the most rapid setting samples tested and that in other samples a much longer time is required up to as much as 16 hours.

The best explanation which I have for the formation of the granular structure during this step of the process is that during the long period of setting there is a slow solution and redeposition of calcium sulphate in and from the water at the crystal faces. This growth of the crystal grains does not take place at the surface of the mold probably because during this recrystallization period there is not sucient water present on the surface to permit this process of solution and redeposition to proceed, or perhaps because thesurface is at a lower temperature than the interior during most of the recrystallization period. The surface, like the interior, is changed from the di-hydrate to hemi-hydrate and back again to the di-hydrate during .the autoclaving and subsequent recrystallization step, but the crystal sizes and therefore the character of the surface remain substantially the same throughout these steps. In fact, after this treatment is complete the material may be put into an oven and baked at temperatures above 300 C. to remove all the water of hydration Without substantially changing the character of the granular structure within the interior and the smooth fine grain structure at the surface.

It is my opinion also that the growth of the crystal grains, as outlined in the preceding paragraph, accounts for the reduction in shrinkage or the actual expansion of the material when it is dehydrated. The growth of these grains apparently is accompanied by internal forces which are able to expand the material and thus further to enlarge the interstices and increase the permeability.

This phenomenon is of extraordinary importance in molding, since it makes possible a substantially accurate reproduction of the pattern without the necessity for allowing for shrinkage. Thus, for example, in dental casting the size of the impression made in the mouth of the patient can be reproduced substantially perfectly by using a composition and a recrystallization treatment such that the expansion in the mold material after it is formed over the pattern is equal to the shrinkage of the metal from its casting temperature to the ordinary mouth temperature, or if a slight increase in size is desired in order to assure a tight iit or a slight decrease in size to allow for cement is required these can be achieved by a suitable adjustment of the recrystallizaton treatment.

When the desired internal change has taken place in the mold it is put into a stream of Warm air or into a very dry atmosphere where it remains until the free water has substantially completely evaporated. The mold in this condition is ready for use in the casting of metals or the molding of thermoplastics, etc.

While I have referred to the use of saturated steam in the dehydration step I have found that it is advantageous to have the steam actually very slightly unsaturated. This is ordinarily ac-v complished by using steam at a somewhat higher pressure to feed into the autoclave. The unsaturation preferably is only sufficient to assure that condensation Water will not collect on the surface of the mold.

Instead of the autoclave treatment with saturated steam one may subject the mold to a dry heat of carefully regulated temperature and thereby produce the desired dehydration. Such a dry dehydration step, however, is much harder to control and requires as a subsequent step that the body of the material be thoroughly dampened in order to provide the water necessary for rehydration and recrystallization.

Instead of completely dehydrating the calcium sulphate dai-hydrate to the henri-hydrate. the damp material may be held for a period of several hours at a temperature at which only a partial dehydration occurs, with an equilibrium between the di-hydrate and the hemi-hydrate. In this condition the molecules of the heini-hydrate being slightly more soluble, tend to go into solution; and the molecules of di-hydrate to crystallize out. The crystallization tends to occur most at the largest nuclei, whereas the solution may occur from all the minute scattered crystals. Thus the larger nuclei grow to the desired grain size at the expense of the smaller crystals. This process can be carried out by heating the damp plaster in a moist atmosphere at a temperature between 90-l00 C. The heat and moisture may be supplied by free steam; or the wet molds after pouring of the plaster or after it has set can be placed in a closed chamber provided with a heater to maintain the temperature, and the humidity may be maintained therein by evaporation of excess water from the successive molds treated in the chamber.

After the molds have been heated throughout to a higher temperature they may be left in still air at room temperature where the thermal insulation afforded by the plaster itself will keep the interior of the plaster mass within the desired temperature range for several hours. This is especially true after a dehydration treatment as in the preferred example, because the rehydration is accompanied by evolution of chemical heat which serves to keep the mold at the higher temperature.

When the lost wax process is used the temperature of the dehydration step is chosen sufliciently high so that the wax will leave the mold with no residue during the autoclaving process.

I have found that the re-forming of the internal structure begins to occur during the autoclave treatment. Thus. for example, if the molds immediately after removal from the auto- .clave are placed in a furnace and maintained at a temperature between 125 C. and 400 C. the resulting molds may have a permeability three or four times greater than would have been the case if the molds had been dehydrated simply by being heated in an oven according to the usual practice. This, however, compares with permeabilities which may be ten times as great as the ordinary molds made according to prior practice if the recrystallization period is allowed after the autoclave treatment. However, where a very high permeability is not required there may be a. saving of time by following this procedure, or other qualities that go with the lower permeability may be of advantage. An intermediate structure may be obtained by 1151118 a shorter or longer period of recrystallization following the autoclave treatment before the i drying or the dehydration step. Intermediate effects may also be obtained by using llers such, for example, as clays and certain types of asbestos, as mentioned above, or by ingredients which retard the recrystallization.

The growth of the large dense grains within the body oi' the mold. which is apparently responsible for the great advantages of my invention, may be stimulated in other ways. In the initial setting of the plaster composition the size of the grains and how rapidly the material sets depends to a considerable degree upon the presence of retarders in the solution, such as sodium citrate and the presence of accelerators such as finely divided set plaster 'of Paris or potassium sulphate, etc.

I have found that by using both an accelerator and a retarder in the same composition the permeability can be increased. In this case the crystals seem to grow around nuclei and they grow to comparatively large diameters with short lengths. This, therefore, affords a supplemental method and to some extent an alternative for promoting the crystal growth instead of or in addition to the dehydration and rehydration as already described. The various materials which may be added in this way give somewhat different effects and modified crystal structures. Therefore, it will sometimes be desirable to use such additional materials in the composition even though the set plaster is to be subjected to the autoclaving and recrystallization processl specifically described above. It should be understood, however, that the present invention does not require the addition of retarders or accelerators or any other special materials to the plaster composition.

After a granule-forming treatment as described above, the plaster in the completed mold is ordinarily in the form of calcium sulphate di-hydrate. The material of the mold is, therefore, stable and the molds can be freely stored in the open atmosphere for any length of time.

Ordinary plaster of Paris molds cannot be used for casting metals or other high temperature fusible materials or thermoplastlcs while the plaster remains in the di-hydrate condition but must first be subjected to a baking treatment to remove the water of crystallization. Without this. water vapor would be released from the plaster by the hot metal in such amounts as to prevent any technically useful result. The presenceof such water vapor has always been considered wholly undesirable and has been avoided at substantial expense by baking out the molds.

Nevertheless, I have found that the presence of water vaporizable by the metal actually helpe to produce improved results and to avoid some of the present diillculties experienced in ordinary casting practice. Taking advantage of the very great increase in premeabillty resulting from the treatment according to my invention. which permits such water vapor easily and promptly to escape from the mold during the casting step, I intentionally utilize the didrate, which, due to the presence of its water of crystallization at the surface of the mold, serves, at the instant the metal or other material being molded strikes the surface of the mold, to withdraw heat from the material being molded and thus to congeal it upon its reaching the mold surface. 'I'hus a more perfect casting is obtained by the use of the mold embodying my invention than has been possible heretofore with plaster molds. g

The low compressive strength of the molds is also of importance in the casting of metals, especially in that it avoids producing strains in the cast metal which otherwise might result from shrinkage due` to cooling of the casting where projecting parts of the casting would be held against such shrinkage by the mold, and in that it permitsv removal of the mold from the ca sting without straining even the delicate parts of the casting.

Another very important advantage of my invention when used for making molds for casting of metals is that due to their high permeability the tendency for air or water vapor or other gases to bubble into the material being cast is substantially overcome; and consequently the castings can be made entirely free from defects due to such bubbles and can be heat treated, as is important for many purposes; whereas castings made by die casting, as known prior to my invention, were often weak and could not be heat treated without blistering, because of inclusion of gases within the casting.

If for any reason it is desired to completely dehydrate the mold material, as is desirable for the casting of certain metals where the presence of the water vapor would be objectionable, it can be done e. g., in accordance with prior practice with molds embodying my invention even more readily than with previously known plaster molds; and the dehydration does not harm the structure of the mold. The mold after its removal from the furnace can be handled without danger of breakage and retains the extraordinarily high permeability characteristic of my invention,

Any desired mold treating materials can be used in my invention as in other plaster molds by including them in the plaster mix or by applying to the surface. Forexample, in the casting of magnesium alloys, I may include in the mix a few percent of sulphur or boric acid or a fluoride, or I may paint or dust these materials onto the surface.

The castings made in this manner are far superior to any made in sand or plaster molds prior to my invention and substantially equal in surface texture and quality to the best die castings.

In the following tables I have given comparative values on physical properties of various compositions when given varying treatments during the autoclave step and following the autoclave step. Table I is a comparison of various different of warmed air.

'I'he permeability figures given in these tables represent the volume of air in cubic centimeters 5 at one pound per square inch pressure which passes per second through a cylindrical test block shrinkage represents the percent change in distrength testing machine using the two inch cylinder block already described. The tensile l5 strength also has the usual meaning and was measured on a tensile strength testing machine using a dumb-bell block having at its neck a cross section of one square inch.

Table DI is a similar comparison two inches in diameter and two inches high. The

Sets Nos. 36, 37, 38, 39

compositions, each treated for 3, 6, 9 and` 12 sure and were then allowed to stand for 16 hours hours respectively in the autoclave at 9 pounds in an open room and then air-dried in a stream The samples represented by column III were autoclaved for 8 hours at 17 pounds gauge presgauge pressure. Table II gives a similar comparison but in this case all samples were treated ior 5 six hours at 9 pounds gauge pressure in an autoclave and were then allowed to stand under recrystallizing conditions for 6, 12, 18 and 24 hours respectively. for various compositions treated rst as in the l0 prior art and then according to this invention. ymension of the same two inch test block from its 1o The samples represented by the column I were original casting to the conclusion of the treatoven baked at 300 C. after setting in accordance ment. The compressive strength has the usual with the usual practice without the autoclaving meaning and was measured on a compressive treatment. The samples represented by column l5 II, were autoclaved for 8 hours at 1'7 pounds gauge pressure and then oven baked at 300 C. immediately.

- son to more complicated shapes.

` Comparative table of physical properties of mould materials shrinkage per- Tensile Compressive Pmgbmty cent' strength strength Composition I II III I nml nIIIIIIIII 1o 4:l2} .ss 5.16 .s4 .oe-+1.51 14 7.o 22 so so 91 eo 44g .so 9.09 20.8 .sa .a1+1.4o 24 s as 153.---157 so 7o 12.5 29.6 278.0 1.16 .42 +.e5 14 s s 45 29 2s .12o

a5 s5}s.o9 26.1 71.1 .4s .17+1.71 2s 1s 5s 74 45 17s 10o ao 13.5 11o .1a +.64 17 15..... 37 75 sa No. I Samples-oven baked.

No. II Sam les-autoclaved and oven baked immediately. No. III Senliples-autoclaved, allowed to reset and change internally, and air dried. means expansion.

'I'he plaster used in the above compositions had been treated with a small percentage of deliquescent calcium chloride substantially in accordance with the U. S. Patent No. 1,370,581, and I believe that the presence of this deliquescent salt assists the recrystallization and the development of the improved properties according to my invention.

Although the permeability figures given in the above tables are strictly accurate only for the cylindrical test block as described above, they can be applied for purposes of practical compari- Thus, for example, with a given mold having a more or less intricate surface the sides of the mold can be confined in a rubber tube which is tightly pressed against them so as to prevent any leakage around the sides and the passage of air through the mold at one pound per square inch air pressure can be determined. The figure thus obtained can then be converted to the same basis as the figure given in the above table by dividing the volume of air per second by a factor which is equal to the cross sectional area within the tube around the mold multiplied by the average thickness of the material in the bold and dibided by 4f. The resulting figure will be somewhat larger than the permeability figures given above depending upon the ratio between the actual surface of the mold to the cross sectionalarea within the tube which surrounds the mold; but for practical purposes it will be comparable.

In the accompanying drawings I have illustrated the preferred embodiment of my invention as applied to the casting of metals.

Fig. 1 is a perspective view showing a pattern placed in a shell used for casting the plaster molds;

Fig. 2 is a cross section through the shell and pattern assembled ready for making a mold;

Fig. 3 is a perspective view of the bottom of the mold as formed in the pattern and shell shown in Figs. 1 and 2;

Fig. 4 is a view in elevation of the interior structure exposed by cutting transversely the mold part shown in Fig. 3;

Fig. 5 is a perspective view showing the mold assembled on a rack ready for the autoclaving process; and

Fig. 6 is a perspective view showing the mold assembled on the bottom half of a flask for casting.

Referring first to Fig. 1, I have shown there part of a shell adapted to give exterior form to the mold, i. e., to serve as part of the pattern mold in which the mold for casting a metal is to be formed. 'I'his shell consists of a bottom part I Il, ends or partitions Il and I2 and a top i3. These may be made of plaster, or metal, or rubber, or other suitable material and are used repeatedly where many plaster casts of a similar nature are to be made. Onto this shell I0 is placed the pattern I4. Where a single casting is to be made the pattern may advantageously be of wax or clay, whereas if a great many castings are to be made from the same pattern it should preferably be of more durable material, such as metal or rubber or other rubbery plastic. I have found the latter to be of greatest advantage because its flexibility and capacity for stretch allows it to be removed from undercut portions without injury to the mold, whereas a similar pattern in metal would have to be made in several sections in order to be removed from the mold at all. One material which I have used for this purpose with very satisfactory results is the rubbery thermoplastic material known as Korogel, a vinyl chloride resin plasticized to an elastic gel with tricresylphosphate. (See Jour. Ind. and Eng. Chem., vol. 27 (June, 1935) pp. 667-672) 'I'his material has a very high shrinkage from the temperature at which it is molded, and I have found it necessary therefore to use some means to confine its shrinkage to a. part awayfrom the surface of the pattern. To this end I make at least a part of the pattern mold movable to follow the contraction, e. g., by covering one part, preferably not on the face of the pattern, with metal foil which is free to pull away from the mold face.

In the molding of the Korogel on a plaster pattern mold, there is some tendency for the material to adhere too stronglyv to the plaster of the mold; and in order to prevent this I preferably coat the plaster of the pattern mold with a varnish which is not affected by the hot Korogel plastic. For this purpose I have found a phenolformaldehyde condensation resin, such as Bakelite varnish, to be the most satisfactory material, since it gives a mild adhesion at the pattern face sumcient to prevent the Korogel from drawing away from the pattern face while it is congealing, and yet is readily free from the Korogel after it has congealed. Other coatings or lubricating materials can be used on the face of the pattern mold instead of the varnish. For example, the molds may be dusted with graphite, talc or bronzing powder. Such lubricants can also be used on the surface ofthe mold which forms the back of the pattern, instead of the foil as already described to prevent adhesion of the Korogel or other pattern material.

In any case, it is preferable to have a considerably stronger adhesion at the face of the pattern than at its back, so that the face may remain in contact with the mold throughout the congealing period. This is more fully set forth and claimed in my co-pending application Serial No. 349,032, filed August 1, 1940.

With the parts I0, II, l2 and i4 in place as shown, the top I3 of the shell may be fitted over them to complete the enclosure of the space into which the plastic mold is to be cast.

The mold plastic, for example a plastic composition as set forth above, is then poured into the shell over the pattern, allowed to set, and removed. 'Ihe resulting cast i6 will be substantially as shown in Fig. 3. A complementary cast I1 is then formed in a similar manner which is shaped to fit over the mold as shown in Fig. 5 and mold the back of the casting poured thereon. 'I'he complementary casts will be provided with a suitable gate for pouring the molten metal. These two complementary parts i 6 and I1 are assembled on a rack I8 as shown in Fig. 5, and are then ready for the autoclaving step. After completion of the dehydration treatment in the autoclave the rack with the mold in position as shown is removed from the autoclave and left in still air at room temperature for a time as already described, after which period a stream of warm dry air is blown over the mold until it is fully dried. The mold is then moved to a flask i9 as shown in Fig. 6 and is provided with a pouring funnel 20 over -the gate of the mold. A complementary section of the iiask (not shown) is fitted over the flask section I 9, as will be obvious to those skilled in the art, and the metal then poured into the mold.

In Fig. 4, I have shown the interior body of the mold as exposed by cutting transversely through the bottom of the mold shown in Fig. 3. As will be observed from this figure, and especially by comparison with Fig. 3, there is a very thin layer of fine grained plaster at the surface which is supported by the main body of the mold made up entirely of large stubby granules. These are best referred to as spherical because they are of substantially uniform dimensions on various axes so that when broken apart and placed on an inclined piane they Will roll down the plane. By use of this word, however, I do not mean to imply that their surfaces are smooth or geometrically spherical in the strict sense.

In size these grains vary from about one millimeter or even'larger down and very nearly the entire mass is made up of grains varying between one millimeter and one-tenth millimeter in diameter, but with different composition and/ or treatment may be made substantially larger or smaller. These are, of course, homogeneous with thesurface and are integrally joined together and to the liner grained surface structure.

Although'these granules are so interlociked as to give relatively high strength to the body as a whole the individual granules are not strongly bonded to one another and the mass can, therefore, be readily disintegrated, thus permitting removal of the mold material from delicate detail parts of a casting without injury to the casting. Furthermore, the sudden heating of the surface by the molten metal weakens the plaster and makes it easier to remove.

In the above I have discussed particularly the application of my invention in cases where a fine grained surface is desired. In some cases, however, it may be desirable to have the greater porosity which results from exposing the coarser granular material at the surface. This may be accomplished according to my invention by maintaining excess water at the surface during the recrystallization period or by grinding away the fine grained surface to expose the coarser material beneath, or a granular gypsum material can be formed, for example by disintegrating the coarse granular plaster, made according to my invention, or by breaking up other forms of gypsum into particles of the desired size and these may be moistened and molded into a desired form about a pattern, as for exam le in the case of sand molding, after which these particles may be bonded together into an integral structure by recrystallization, e. g., by dehydration and rehydration, as already described above.

Still another method of forming such a permeable material is by forming the plastic into small balls, e. g., by passing a spray of water or preferably of thin plaster mix through a cloud of finely divided dry plaster suspended in air, and then applying onto a pattern the resultant balls so that they leave open interstices between them. It is also possible to spray a plastic mix which is sufiiciently thick so that the spray globules will not completely run together but will retain open interstices between them; and in this case the application of dry plaster to the sprayed globules can be dispensed with. Ordinarily, however, it will be found simpler to spray the plaster in a thinner consistency and to thicken it by the application of dry plaster to the globules or evaporation of water or partial setting after they are sprayed.

It should also be understood that my invention is applicable to other plastics than gypsum plaster. For example, ordinary lime putty, the oxychloride cements, and cements of the type represented by zinc oxide-phosphoric acid cornpositions may be treated to effect growth of crystal aggregates at the expense 0f single crystals according to my invention for the production of highly permeable materials and to obtain certain of the other advantages of my invention where their chemical and physical properties are not otherwise objectionable for particular uses; but I have found that the gypsum plaster as described in the preferred examples given above is possessed of great advantage over any of these other materials.

Although in the above I have described my invention with particular reference to molds for casting and molding of plastic materials because I have found in this application of my invention the greatest practical advantage, nevertheless, my invention is applicable and is possessed of great advantage for other purposes. For example, excellent insulating and acoustical materials can be made in this manner.I The smooth surface gives a desirable wall or ceiling texture, whereas the open granular structure behind provides an excellent sound deadening and heat insulating body. The eiciency for this purpose may be increased by punching small holes through the surface at denite intervals as is now well understood in the art. The material also is particularly adapted for ltering, since by a suitable regulationof the recrystallization treatment almost any desired ineness can be obtained at the surface and this thin permeable surface layer can be supported on a much more highly permeable granular body which permits the ready passage of the fluid. This isparticularly suitable for the filtering of gases.

In general for any purpose requiring a highly permeable material having substantial tensile and compressive strength and especially where a ner grained structure is required at the surface than at the interior, my invention will be found of great advantage and may be applied for the production of the desired permeability in numerous articles and various materials. Y

In the above specification I have described in detail a preferred embodiment of my invention and have suggested various modifications and alternatives and set forth various theories which I have found helpful in developing and applying my invention. These are not intended to be exhaustive and on the contrary it is well understood that the invention may be applied in other ways than set forth above and that some of the theories may prove to be incorrect. These are given for the purpose of explaining the invention and assisting others to apply it in the ways best suited to the requirements of each application, and they are not intended and should not be taken as in any way limiting of the invention.

By the phrase plaster mold" as used in the .following claims, I refer to a mold made of plaster rather than to one in which plaster is to be molded. I have used the words "molding" and casting to described the shaping of the plaster without regard to whether it is a plastic or a fluid poured into a mold.

I have used the term plaster in the accompanying claims broadly to include various plaster compositions as well as plaster of Paris.

What I claim is:

1. A plaster mold comprising a thin, fine grained permeable surface layer representing the surface to be reproduced and composed of crystal grains integrally joined to give strength and resistance to disintegration, and a homogeneous granular interior structure integral with the surface in which the grains are much larger than those occurring in said surface and having between them interstices larger than those occurring at said surface, whereby to give high permeability and relatively low resistance to disintegration, such surface layer and interior structure'merging gradually each into the other with no sharp cleavage between.

2. A mold as defined in claim 1 which comprises a body of set plaster of uniform composition throughout, the crystal grains of which in the surface layer are in approximately the condition and size attained on the original setting whereas the crystal grains beneath the surface are many times larger in size and have intersticial spacing many times greater than in the surface layer.

3. A mold ready for casting as dened in claim 1 in which the material at the surface of the mold contains bound water the evaporation of which by heat absorbed from the cast material hastens the solidification of the cast material at the mold surface.

4. A stable plaster mold capable of standing in the open atmosphere without objectionable changes, which comprises a coarse granular structure of calcium sulphate dihydrate crystals having channel formed by interstices between the grains thereof shuilicient to carry oif through the mold during casting water vapor freed therefrom by heat of molten material cast in the mold and said grains being composed predominantly of nucleated crystal groups of at least about 0.1 mm. average diameter.

5. A plaster body comprising an interior structure composed of roughly spherical plaster granules of diameter of the order of at least about 0.1 mm. integrally cemented together in said body but individually removable therefrom upon rubbing an exposed part of said interior structure, and a surface crust thereon of chemically similar material but of finer structure and resistant to disintegration by rubbing, said surface crust and interior structurebeing integral and merging gradually each into the other with no sharp cleavage between.

6. A plaster body as dened in claim 5 which comprises 15 to 90% calcium sulphate and at least 10% of a fibrous material, the ber length being approximateLv one-half to iive times the diameter of the granules.

7. A molded body of cementitious material comprising a thin fine-grained permeable surface layer composed of crystal grains integrally joined to give strength and resistance to disintegration, and a granular interior structure integral with the surface in which the grains are much larger than those occurring at said surface, whereby to give high permeability, such surface layer and interior structure merging gradually each into the other with no sharp cleavage between.

8. A stable permeable mold of cementitious material capable of standing in the open atmosphere without objectionable changes.,which comprises a coarse granular structure of hydrated cementitious material, the grains of which are composed predominantly of nucleated crystal groups of at least about 0.1 mm. average diameter, and having channels formed by interstices between the grains sufficient to carry oi through the mold water of hydration freed as vapor by heat of molten material cast in the mold.

9. The method of making a plaster product of high permeability ulrich comprises molding the plaster, allowing the molded product to set, dehydrating calcium sulfate di-hydrate of the `set plaster to calcium sulfate hemi-hydrate by heatlng above the initial dehydration temperature of calcium sulfate di-hydrate causing free water to be distributed through the plaster in excess of the amount theoretically necessary for re-hydration of the hemi-hydrate to di-hydrate, cooling said plaster in the presence of free water, maintaining free water in said plaster for a time sufficient to effect recrystallization with growth of grain size and, when a desired permeability has been attained, terminating the grain growth by drying out free water from the plaster.

10. The method as defined in claim 9 ln which the dehydration of the calcium sulphate is effected before the molded product has been dried.

11. 'I'he method as defined in claim 9 in which the plaster is molded as a thin fluid.

12. The method of controlling the size of a molded plaster article which comprises forming a negative impression of a pattern by molding plaster thereon, removing the resulting plaster mold from said pattern, dehydrating the set plaster by heating above the initial dehydration temperature of the plaster, causing free water to be distributed through the plaster in excess of .theamou'nt theoretically necessary for rehydration, cooling the plaster mold in the presence of free water and malntainingsaid fr ee water in said plaster for a time suillcient to effect recrystallization with growth of grain size and expansion' of the mold body. and when said expansion has reached an amount adapted to give-a desired correction -for dimensional changes occurring on A which comprises forming a mold the body of which consists of a granular cementitious material having relatively large interstices oiIering low resistance to passage of gases therethrough and which includes a substance close to the mold surface which yields water of hydration endothermically and withoutphysical disintegration at temperatures reached during casting, and casting the molten material into contact with said surface layer. whereby the molten material is chilled by dehydration and given a ne grained surface corresponding to the surface of said mold.

14A. The method of casting non-ferrous metals which comprises preparing a plaster mold comprising hydrated calcium sulfate said mold having a high permeability to carry on water vapor released by the heatoi molten metal cast therein and casting molten metal into said mold while at least a part of said calcium sulfate near the mold surface is present as the di-hydrate, whereby the molten material is chilled by heat absorbed for dehydration thereof.

15. The method as dened in claim 9, in which, during lthe re-hydration treatment, the molded 4o product is exposed to a ow of glass adapted to remove tree water from the surface of the set plaster whereby 'to prevent the grain growth at said surface of the product.

16. The method of preparing a mold for casting of non-ferrous metals which comprises molding a fluid plaster mix containing calcium sulphate, allowing the resulting plaster body to set, air drying said body to remove water to the extent of about l0-20% of the original weight of the molded plaster, treating said body with steam at a gauge pressure between 8 and 15 pounds for a period of at least 9 hours, removing the body from the steam bath and keeping the plaster moist for from 3 to 16 hours, thereafter drying the plaster by a stream of warm air at a temperature not exceeding about C.

17. The method of preparing a mold for casting of non-ferrous metals which comprises moldingv a uid plaster mix containing calcium sulphate. allowing the resulting plaster body to set. air drying said body to remove water to the extent of about 10-20% of the original weight of the molded plaster, treating said body with steam at a gauge pressure between 8 and 15 pounds for a period of at least 9 hours. removing the body from the steam bath and keeping the plaster moist for from 3 to 16 hours, and thereafter heating the plaster at a temperature sumcient to remove combined water.

18. 'I'he method of making a mold member which consists in giving a desired mold shape to a quantity of mixed plaster containing calcium sulfate, setting the so-shaped plaster and then sub- Jecting it to a temperature above about C. for a period sufficient to dehydrate calcium sulfate` xii-hydrate substantially, and thereafter subjecting it to the action of free water within the mass of the plaster at a temperature below about 100 C. for a period suillcient to eilect substantial growth of crystal grains within the body of the mold.

MORRIS BEAN.

OERTIFI'OATE OF CORRECTION. Patent NO.- 2,220,705'. November 5, 19`ho.

MORRIS BEAN.

`ItJ1s hereby certified that error appears 1n the printed specification of the ebove numbered patent requiring correction as follows: Page 5, second column, lne8, for "premeablity" read `permeab1lityg page Y, first comm, line 5o, for "dibided" read -d1v1ded; page 8, first column, line 22, and line 25 second occurrence, for "plastic" read --pla'ster-q page 9,

first column, line )4.0, for described read --descr1be-; and second column, line 6, claim h., for "shufficient" read suff1c1ent; page l0, first co1- umn, line 14.0, claim l5, for glass read `-gas; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case 1n the Patent Office. Y

Signed and sealed this 214th day of December, A. D. 19).;0.

i Henry Van"Arsdale, (Seal) Acting Commissioner of Patents.

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