US2861893A - Foundry cores - Google Patents

Description

Unit

Patented Nov. 25, 1958 FOUNDRY CORES Frank S. Brewster, Lynwood, Califl, assignor to Brumley- Donaidson 130., Huntington Park, Calif., a corporation of California No Drawing. Application May 25, 1956 Serial No. 587,223

2 Claims. (Cl. 10638.35)

The present invention relates to a binder composition particularly adapted for use in forming molded foundry cores from molding sand, the resulting cores being characterized by unusually high strength and stability in a dried state and at casting temperatures up to about 2000 F. but being readily desintegratable at higher temperatures.

Foundry cores must have unusual dimensional and chemical stability and strength during the early stages of metal casting operations and before the hot metal starts to solidify. The dry molded walls of the cores must be smooth, remain completely rigid and true to pattern and not crumble. Shrinkage, collapse or breakdown of the core cannot be tolerated. The core composition must be relatively free from moisture or compounds which might release gases at casting temperatures and thereby cause excessive porosity of the cast object. Strength and resistance to deformation must remain uniformly high over a wide range of temperatures up to about 2000 F. In contrast, the core must be capable of readily disintegrating at temperatures above about 2000 F. (when the metal being cast has started to solidify) to permit its ready removal from the casting. or collapsed state the core should be capable of flowing out of the cast cavity without tendency to adhere to the inner walls of the castings.

The recent development of the carbon dioxide process of hardening foundry cores has greatly simplified and speeded the production and molding of such cores. In this process, cores and molds may be hardened in a few seconds by injecting carbon dioxide gas into a core or mold containing sand mixed with a certain percent of a binder based on water-glass (Na SiO Apparently the CO precipitates a gelatinous silicic acid out of the silicate solution which acts to cement the individual sand grains together during handling and the initial stages of the casting operation. While the CO process and binder solution has been widely used, the dry strength of foundry cores produced by this technique has heretofore been low, rarely exceeding about 150 p. s. i. in compression, for example. Hot strength has been also below desired values and the degree of expansion at high temperatures (on the order of 1500 F.) has been greater than desired.

The present invention provides a binder solution for use in making molded foundry cores by which dry strength of the core can be almost doubled, hot strength and high temperature stability increased by as much as 50%, tially reduced. Moreover, the invention has application to cores produced from any green dry sand and permits the use of lower grade sands of larger grain size without substantial sacrifice in desired properties.

It has been determined that a small amount of sugar added to a binder composition used in forming foundry cores, will materially alter the strength of the bond formed between individual sand particles of the mold, particularly at casting temperatures, without detracting from the desired friability of the mold at higher tempera- In its disintegrated and expansion at elevated temperatures substant tures. The present invention utilizes this property to provide molding cores capable of withstanding the pressure and temperature conditions imposed upon them by flowing hot metal, without deformation, and which actually increase in strength while absorbing heat from the metal. After the temperature of the core has exceeded 2000 F. and the metal being cast has begun to solidify, the cores lose their strength and can be readily disintegrated (as by vibration) to permit their ready removal from the casting. Moreover, the presence of the sugar in the binder does not decrease fiowability of the initial molding composition and therefore permits ready tamping or ramming of the molding sands into mold forms. Nor does the presence of the sugar in the binder materially or even appreciably reduce the permeability of the core to gases, such as the carbon dioxide gas used in the mold hardening process.

It is an object of the present invention therefore to disclose and provide a binder for molding sands, containing sugar, that materially improves the dry and hot strength of foundry mold cores at temperatures up to 2000 F. without reducing the friability and tendency of the core to disintegrate at higher temperatures.

Another object of the invention is to disclose and provide a binder composition for use with molding core sands and adapted to binding the sand into a foundry core without baking, as upon exposure of the core to CO gas.

Another object of the invention is to provide a binder for molding core sands which appreciably reduces the tendency towards hot confined expansion of the core during casting operations.

Other objects and advantages of the invention will be apparent from the following description and from the examples enumerated therein.

The present invention has application to any molding sand or similar high melting, chemically inert, granular material which is adapted to being held together by a binder. For example, molding sands having grain sizes ranging from to mesh, or finer, down to sands of 60 to 80 mesh, and coarser sands, have proved particularly adaptable. According to the invention, a quantity of such sand is mixed with from 3% to 7% of a binder comprising from 80% to 87% of a sodium silicate solution, such silicate solution containing between 40% and 60% of a hydrous water dispersible sodium silicate, and correspondingly from 13% to 20% of sugar dissolved in the silicate solution. As used herein, the term sugar means sucrose or cane or beet sugar in either refined or unrefined grannular form, as white or brown sugar. Preferably the composition of the silicate. in the solution should be of a high silica type, that is, when using sodium silicate the ratio of SiO to Na O should be within the range from 2.0 to 3.22.

The binder resulting from mixing the sodium silicate solution and sugar in water is a thick viscous translucent fluid having a viscosity resembling that of a heavy syrup and is therefore readily mixed with the molding sand in conventional manner, for example,.wheel type muller mixers or paddle mixers may be used. Mixing is complete when the sand feels smooth and no soft lumps of binder concentration can be felt.

After the binder is mixed with the sand in the portions stated, the mixed sand and binder is poured into a foundry mold of desired size and shape. The binder containing sugar does not decrease flowability of the sand and consequently permits the sand to be readily tamped or rammed into the mold, with but a few strokes and at normal pressures per stroke. The core may now be hardened by exposure to carbon dioxide gas, the presence of the binder not materially or even appreciably reducing the permeability of the core to gases.

With the molding sands of the invention, the entire procedure of pouring, ramming the core and gassing need not take morethan about 4 to 5 minutes. The hardened cores are now in readiness for subsequent casting operations.

, Dried cores produced in the above manner have unusually high strength and resistance to deformation or crumbling and consequently may be readily handled and positioned within the casting molds without fear of breakage. For example, cores produced by the invention have a dry compression strength on the order of 250 or 300 p. s. i., and higher, nearly twice that of similar cores not containing sugar in the binder. Upon subsequent exposure to flowing hot metal in a foundry or casting mold, the cores are capable of withstanding the pres sures and temperatures imposed upon them without the loss of shape or accuracy of dimension. Moreover, the cores actually increase in strength as they absorb heat from the metal. For example, at 1500 F., the cores exhibit hot strength in compression as high as 700 p. s. i. and higher. However, at temperatures in excess of about 2000 F. strength and stability rapidly fall off so that the cores may be easily broken up and disintegrated by a sharp blow or vibration of the casting mold. As a result, the molding core sand literally pours out of the cast cavity without the need of special tools or effort.

Table 1 exemplifies the exceptional results obtained using binder compositions of the invention in producing foundry cores of unusual strength and stability. The two compositions reported in the table were prepared and tested under identical conditions, using the same sand (95% Juniata Bank and 5% Volclay Bentonite). The formulations expressed in weight percent were as follows:

The following are illustrative of results obtained in tests on cores molded from the above formulations:

Table 1 Properties tested A Moisture, percent 4. 4 4. 4 Green permeability 75. 3 74. 7 Green compression, p. s. 1. 5. 7 5. 8 Green deformation, in./in. .0168 .0163 Dry compression, p. s. i 151.0 277. 6 Flowability, percent:

Dietert 80 78 74. 3 71. 8

. 001 0013 .0035 .0027 2000 F. 015 012 Hollow-confined expansion, in./in. at 1500 F 022 .0145

4 From the above illustrative data, it will be apparent that the presence of sugar in the binder results in a substantial increase in both dry and hot strength of molded cores, actually reduces the rate and extent of 5 hot deformation of such cores, and substantially reduces the values for hollow confined expansion. These superior properties are achieved without any appreciable loss in permeability of the core to gases or in flowability of the initial molding sands.

To those skilled in the art to which this invention relates, widely differing procedures, embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the invention. The disclosures and the description herein are purely illustrative and are not intended to be in any sense limiting.

I claim:

1. A binder for making stable foundry cores, comprising: 80% to 87% by weight of a hydrous, water dispersible sodium silicate solution and 13% to 20% of sucrose dissolved in said silicate solution, said silicate solution containing between 40% and 60% of a sodium silicate having one part Na O to each 2.0 to 3.22 parts SiO said binder, upon being added to foundry sand in the proportion of from 3% to 7% ofsaid binder to 93% to 97% of foundry core sand, producing a composition which readily pours and flows into molds, hardens into stable cores having a compression strength of over 250 p. s. i. upon exposure to carbon dioxide and without baking, and such hardened cores maintaining high strength and stability at temperatures up to about 2000 F. and then become readily disintegratable.

2. A composition for making foundry cores, said composition readily pouring and flowing into molds and hardening upon exposure to carbon dioxide to form stable cores having a compression strength of over 250 p. s. i. without baking of the cores, said cores maintaining high strength and stability at temperatures up to about 2000 F. and then becoming readily disintegratable, comprising: from 93% to 97% foundry core sand and from 3% to 7% of a binder, said binder comprising from 80% to 87% by weight of a 40% to 60% solution of hydrous water dispersible sodium silicate having one part of Na O to each 2.0 to 3.22 parts of SiO and from 13% to 20% of sucrose dissolved in said silicate solution.

References Cited in the file of this patent UNITED STATES PATENTS 1,091,690 Moore Mar. 31, 1914 1,247,595 Tompkins r Nov. 20, 1917 2,143,930 Anderson Jan. 17, 1939 2,748,435 Hackett June 5, 1956 FOREIGN PATENTS 912,875 France Mar. 5, 1945 710,099 Great Britain June 9, 1954 745,402 Great Britain Feb. 22, 1956

Claims (1)

1. A BINDER FOR MAKING STABLE FOUNDRY CORES, COMPRISING: 80% TO 87% BY WEIGHT OF A HYDROUS, WATER DISPERSIBLE SODIUM SILICATE SOLUTION AND 13% TO 20% OF SUCROSE DISSOLVED IN SAID SILICATE SOLUTION, SAID SILICATE SOLUTION CONTAINING BETWEEN 40% AND 60% OF A SODIUM SILICATE HAVING ONE PART NA2O TO EACH 2.0 TO 3.22 PARTS SIO2; SAID BINDER, UPON BEING ADDED TO FOUNDRY SAND IN THE PROPORTION OF FROM 3% TO 7% OF SAID BINDER TO 93% TO 97% OF FOUNDRY CORE SAND, PRODUCING A COMPOSITION WHICH READILY POURS AND FLOWS INTO MOLDS, HARDENS INTO STABLE CORES HAVING A COMPRESSION STRENGTH OF OVER 250 P. S. I. UPON EXPOSURE TO CARBON DIOXIDE AND WITHOUT BAKING, AND SUCH HARDENED CORES MAINTAINING HIGH STRENGTH AND STABILITY AT TEMPERATURES UP TO ABOUT 2000* F. AND THEN BECOME READILY DISINTEGRATABLE.