US1351599A - Art or method of casting articles of clay - Google Patents

Description

F. J. WOLFF.

ART 0R METHOD OF CASTING ARTICLES 0F CLAY. APPLICATION FILED lungs, 1918.

Patented Aug. 31, 1920.

FRANKLYN J. WOLFE, OF TRENTON, NEW JERSEY.

ART 03 METHOD OF CASTING ARTICLES OF CLAY.

Specification of Letters Patent.

Patented Aug. 31, 1920.

Application filed. July 25, 1918. Serial No. 246,733.

T 0 all whom it may concern Be it known that I, FRANKLYN J. \VoLFF, a citizen of the United States, residing at Trenton, in the county of Mercer and State of New Jersey, have invented new and useful Improvements in the Art or Methods of Casting Articles of Clay, of which the following its a specification.

In the art of casting or molding clay articles the methods which have been pursued, more generally if not exclusively, in the past, are known as the pressing process wherein the plastic material is applied to the surface of a molding or shaping member or element by means of a trowel or equivalent tool; and the casting process in which the mixture of clay and water, or equivalent liquid, either with or without supplemental agents, constituting the slip or batch, is poured into a coreless mold of absorbent material which absorbs sufficient moisture from the slip adjacent the walls of the mold to form a shell of comparatively solid material having the shape of the interior of the mold. 'hen this shell is of the desired thickness the liquid slip is poured out of the mold while the shell is permitted to remain therein until it has dried sufficiently hard to permit its removal.

Each of these well known methods is open to objections from a practical or commercial standpoint. For example, the second named method is relatively slow as to results, re-' quires the exercise of a high order of skill and care, is defective in that the product lacks uniformity in the matter of thickness and density between different portions of the mass,.and these irregularities result in unequal strains throughout the article dur ing the firing process with the consequent formation of scams or cracks in the struc ture. To minimize these disadvantages v: rious expedients have been adopted such as auxiliarymeans of maintaining moisture in the upper portionsof the molded article during the earlier stages of the setting or drying step, as by applying moistened cloths to the upper edges thereof so asto supply moisture to take the place of that which descends through the texture of the cast by gravital action, with the object of preserving as far as possible a uniformity in the degree of moisture throughout the body of the article. Or. as another means of avoiding the disadvantages incident to unequal strains or tensions in the firing process the plan of placing the article after 'it has been dried in a moisture charged receptacle or room, has been tried, the object being to charge the article throughout its mass with an equal quantity of moisture. Neither of these methods, however, has resulted in materially reducing the waste or loss due to defective products, and as a consequence an economical production has been impossible.

In the pressing process it has also been found that air pockets and blow holes are of frequent occurrence with the result that when the articleis subjected to the firing heat the expansion of the pocketed air re sults in fractures and imperfections.

The existing process commonly known in the art as the. pouring process, is open to the serious objection that an initial shrinkage of the outer surface of the article takes place immediately after the material has been introduced into the mold, due to the immediate absorption of a portion of the moisture of the slip, while what will be the inner surface of the article is still in a liquid state, and this partial setting of one surface while the opposite surface is still in liquid form results in variations in cubical dimensions of different portions of the article; variations of the moisture content of the different portions; an unequal degree of dryness in different portions atany given stage in the setting step; difficulty in removing the article from the mold; unequal strains during firing, and other minor disadvantages, all of which tend to increase the loss by breakage, cracks and other imperfections.

In an effort to overcome these disadvantages it has been found necessary to use a mold having a core or cores which with the outer mold member provides a receptacle for the slip of the shape and dimensions of the article to be molded; and to use a slip or batch which is of a relatively high degree of fluency in order to insure the complete filling of the molding space and thc exclusion of all air therefrom so that the cast may accurately take the shape of the interior surfaces of the molding or casting members or elements and at the same time to provide for the supplemental filling or charging of the mold after what has been referred to hereinbefore as the initial shrinkage, so that there may be no possibility of irregularity in the continuity of the surface of the cast. In this connection it has been found that the first effect of drying after placing a slip in the mold is the mechanical removal of what for. the lack of a better name maybe regarded as the free water or moisture which occupies the spaces between the particles or elements of the molecular structure of the clay andwhich renders the material mobile or fluid 'so that it adapts itself to the interior form of the mold, and it is this free water which later is affected gravitally and tends to sink so as to establish a pronounced difference between the moisture content of the lower portion of the article as distinguished from the upper portion thereof, and which so called free water is distinguished from water or moisture which is absorbed in the mixture by the molecular structure itself and which has the effect of enlarging or swelling the particles or molecules of the clay as distinguished from the separation of the molecules or particles due to the pres' ence of the free water or moisture. This deduction with reference to the presence of moisture in the slip is based, in part at least, upon the. fact that the earlier effect of the drying or setting of the material in a mold appears in a bodily contraction or condensation of the mass in so far as its cubical dimensions are concerned, with the result, as for example in molding a tubular or cylindrical article or an article which being arranged within a mold, for instance, surrounds a core, that there is a tendency to withdraw the surface of the molded article from the interior surfaces of the mold. or from the surfaces of both. the mold and the core so that vthe core is free to be withdrawn contraction which is far more pronounced lengthwise of the article or in the plane of its exposed surfaces, so that with a tubular or cylindrical article a contraction with reference to the core occurs, frequently resulting in the cracking of the article if the core is permitted to remain, and rendering the removal of the core difficult or impossible should the article stand the strain due to this contraction. This second stage in the drying process is believed to be due to the extraction or removal of the moisture from the molecular structure of the clay and which results in the contraction of the particles or molecules of the material without disturbing the relative arrangement of the previous removal of the free of particleseparating water or moisture, as above noted.

In other words there are two distinct stages of moisture removal in the course of drying a mass of slip in a mold, which may for convenience be referred to as the first stage consisting in the removal of the free water by which the particles have been sepparated to a greater or less extent to produce the required fluidity or mobility to adapt it to properly fill the mold, such removal resulting in the condensation of the mass by permitting the particles or molecules to come together or approach each other bodily, and a second stage resulting in the removal of the moisture which has been absorbed, sponge-like, by the particles or molecules of the clay, and which results in the contraction of such particles or molecules individually, and obviously the contraction I face cracks or seams but from'any -tendency to warp or twist during either the drying or thefiring steps, it has been found necessary to produce an abstraction or removal of moisture from the mass under such conditions that at any given time in the course of the procedure the liquid content shall be substantially the same in all portions of the article, not only with reference to its area but also and more particularly with reference to its thickness, and to this end to withdraw or remove the moisture from opposite surfaces of the article with substantially equal activity and to substantially the same extent, and to maintain this coi'idition of equilibrium 01' equality of action throughout the area of the article even though the thickness oil the wall of the article which is being formed may vary at different points to correspond with the interval between the opposed walls of the mold as of the flask and core, whether this balance of action is dependent upon the absorptive capacity of the opposed media or other qualifications incident to a removal of the moisture from the material occupying the molding space. I

Thus it is the object of the invention to provide a method of and means for molding or casting clay articles which shall contemplate the mechanical removal of the free water or moisture from all portions of the mass in substantially the same time and to substantially the same degree, so that at any given time during the progress of the operation the moisture content of the mass shall be the same in all portions thereof, to the end that the condensation and contraction of the mass in all directions shall be equal and inequality of strains in firing or burning shall be eliminated.

The invention contemplates the use of a -mold of plaster of Paris or equivalent moisplates, further, the procedure with reference .to the molding of tubular, cylindrical, or

other core enveloping or surrounding objects, of removing the core after the initial bodily shrinkage or condensation of said object due to the removal of the free water during the first stage of the drying or moisture removing operation and prior to the contraction incident to the molecular drying of the mass incident to the succeeding stage of said operation, and to these ends a suitable apparatus is illustrated 1n the accompanying drawings, wherein:

Figurel is a sectional view of a mold embodying an outer member and a core and related elements arranged in operative positions.'

Fig. 2 is a plan view partly in section of the same to illustrate a condition wherein an article of cylindrical form is provided with a wall embodying variations in thick ness.

The illustrated apparatus consists of an outer member 10 and an inner element or core 11 of plaster of laris or like material, the mold having a plurality ofgates or inlet openings 12 and 13 in communication with the molding space or the interior of the apparatus for the reception of the slip which preferably is introduced by means of a funnel as shown at 14. It has been found in practice that this material may be introduced through one of the funnels or through one of the gates so as to utilize the other gate as an air vent, and after the mold has been charged, with a small reserve remaining in the inlet' funnel. the material if allowed to stand for a short while will settle due to free water of the slip having been absorbed by the walls of the mold. Further material should then be introduced through both funn'els until the mold is completely filled and all of the air has been excluded therefrom,

whereupon the walls of the mold continue to absorb the moisture laterally or in directions perpendicular to the planes of the surfaces of the molded article represented at 15 and if the opposite portions of the walls of the mold are of the same gradeof material and density and hence'of the same porosity, and thus have the same co-etlicient of absorption per cubic unit. they will have absorptive capacities for all equal thicknesses and hence will act equally in opposite directions in extracting or mechanically removing free water or moisture from the slip or clay mixture, so that the degree of dryness or of remaining moisture content of the molded article at any point or portion thereof at any given time or stage of the drying or moisture extracting operation will be uniform throughout the thickness and extent of the article. In the event that thereshould be variations in the quality of the material or in the density or the porosity of the mold elements, or in the relative thicknesses of different portions of the molded article or of the molding space between the walls of the "mold elements or members, compensation for the same should be made by relative varia tions in thickness or otherwise of the walls of said elements so as to maintain a balance or substantial equality of the opposite absorptive forces or influences. This element of approximate equality in absorptive capacity and activity between opposite portions of the Walls of the mold, or of the media employed as a moisture withdrawing agency should bemaintained with regard to all parts of the area of the molded article. ample, when one portion of the interposed article is thinner or thicker than another a corresponding variation in the absorptive ca 'pacities or co-eflicients of absorption of the portions of the mold walls or media opposite and adjacent or proximate to such thinner or thicker portions of the area of the article should be made, so that not only is For ex the moisture extraction uniform, inwardly and outwardly or in opposite directions throughout the thickness of an article at any given point of its area, but is uniform throughout the area of the article to the end that, cubically considered, the reduction of water content in the clay mixture constituting the mold occupying article shall be uniform throughout,-to result, necessarily in the uniform shrinkage in bulk of the molded article. i

As above suggested, in the course of this initial or primary stage in the removal of moisture from the molded or cast article the shrinkage of condensation serves to withdraw the surface of the article from the surface of both of the mold elements whether they are of flat or cylindrical contour, and

as the article is at the conclusion of this be regarded as a set condition, the core, if a tubular or cylindrical article is being formed, should be removed, so that the subsequent stage of the drying operation, consisting in the removel of the particle or molecule investing moisture, resulting in a bodily contraction of the article, and a diametrical reduction thereof, may be accomplished without subjecting the article to a circumferential tensile strain.

\Vhilethe method described is thus peculiarly adapted for use in connection with the casting of clay articles wherein the slip or fluent batch is poured into the mold, it is also of value under conditions involving, for example, the placing of the batch or material in a container such as an outer mold element or member and forcing the core or inner member as a plunger into the mass for the purpose of spreading the plastic material into the mold, in thatby preserving .the described relation ofmoisture absorptive characteristics as beetween the inner and outer mold members a substantially equal and uniform reduction of the moisture content throughout the mass may be secured, to the end that unequal strains in shrinkage, and more especially during the firing or burning operation may be avoided with an incident increase in the percentage of commercial products without resort to supple-- mental means or steps in the process of production. A

\Vha-t is claimed is:

l. The method of molding clay articles which contemplates the exposure of opposite surfaces of the molded mass to the simul taneous effect of media having moisture absorbent characteristics of substantially equal capacity throughout the area of said surfaces.

2. The method of casting clay articles which consists in exposing the opposite surfaces ofthc mass to moisture absorptive media having substantially equal moisture absorbent capacities of substantially equal activity throughout the area of said surfaces.

3. The method of casting clay articles which consists in exposing the opposite surfaces of the mass to moisture absorptive media of graduated. thickness corresponding With the cubical dimensions of respectively adjacent portions of the article.

4. The method of casting clay articles which consists in exposing the opposite surfaces of the mass to moisture removing media having substantially equal co efficients of absorptive capacity.

5. The method of casting clay articles which consists in'exposing the opposite surfaces of the. mass to elements of a moisture absorptive medium paralleling said surfaces and having a co-eflicienet of absorption corresponding throughout with the cubical dimensions of the interposed portions of the mass.

6. The method of casting articles of clay which consists in introducing a fluent slip into a mold having elements of porous liquid absorptive capacity proportionate'to the cubical dimensions of the portions of the molding space respectively adjacent thereto, to effect a uniform reduction of the moisture content of the several portions of the mold.

7 The method of casting ,clay articles which consists in employing a mold of liquid absorptive material of which the several wall portions have absorptive capacities proportionate respectively to the proximate portions of the clay receiving space.

8. A step in the method of casting clay articles which consists in primarily and uniformly removing the free water of the mix-

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