US3037252A - Core vents made from expanded thermoplastic resinous material - Google Patents

Description

June 5, 1962 J. G. HOUSE ET AL 3,037,252

com: VENTS MADE FROM EXPANDED THERMOPLASTIC RESINOUS MATERIAL Filed Sept. 28, 1959 IN VEN TORS. James 6. House BY Robe/f M TheobO// REE/VT atent 3,037,252 Patented June 5, 1962 filice 3,937,252 CORE VENTS MADE FROM EXPANDEQTHERMO- PLASTIC RESIN OUS MATERIAL James G. House, Bay City, and Robert W. Theobald,

Midland, Mich, assignors to The Dow Chemical Company, Midland, Mich, a corporation of Delaware Filed Sept. 28, 1959, Ser. No. 842,686 12. Claims. (Cl. 22-171) This invention relates to foundry cores and a process for preparing vented foundry cores and, more particularly, is concerned with foundry cores which advantageously contain a core vent of a shrinkable material. The invention also pertains to a method of preparing vent holes in such cores.

It is well-known in foundry art to prepare core vents by employing a removable, generally rod-like article. Wire or similar material is often inserted into the mold cavity before the core material is rammed. Frequently vents are prepared by forcing a rod-like article into the green core and immediately removing it therefrom. The vent-forming material may be alternately positioned in suitable grooves if the core is prepared by joining together two or more pieces. The rod-like article may be withdrawn to leave an exit route for gases evolved during casting.

Various attempts have been made to provide a suitable material for use in curved or complex shaped core vents. Among such approaches which have been made to the problem are use of such items as oxidizable core materials that are adapted to be burned out of the core after it has been formed and baked.

Another expedient is to employ a wax composition, generally in the form of flexible cylindrical tube or fragile rod-shaped vent material which may be readily formed into a desired configuration. On baking or heating, the vent material is raised above the melting point of the wax composition and the resulting molten fluid is adsorbed into the interstitial voids between the core sand grains. Unfortunately, however, the wax composition often tends to plug the interstitial voids in the sand after heating. This causes substantial undesirable resistance to the flow of gases therethrough. Incomplete venting of the core frequently results in blow holes and similar defects caused by the entrapped gas remaining in a casting.

Permeable sleeves of ceramic or like, characteristically rigid, synthetic materials are also used as vent materials when shaped into porous cylinders or similar forms. These tubular core vents are quite prone to becoming plugged when the pattern is rammed in the course of normal fabrication techniques. Furthermore, besides being relatively uneconomic, permeable tubular vents of this type are additionally disadvantageous when sharp bends or changes of direction must be made with them. Under such circumstances, the ceramic or hard tubes become restricted and may collapse or rupture.

It would be beneficial if a green core containing a corevent material were available which, on baking, would ensure the presence of a permeable vent hole.

It would be also beneficial if the core-vent material contained in such a core, on baking or heating, would shrink in cross section by a factor of two or greater.

It would be correspondingly advantageous if a method were available to prepare core vents, including, in particular, those of complex shape, which, after baking of the mold, would ensure the presence of a permeable vent hole without encountering any appreciable restriction in the core vent.

It would also be advantageous if a method for preparation of core vents were available that would avoid the possibility of the core vent being plugged when the mold is rammed.

It would be of further advantage if such a method would readily permit and accommodate the formation of core vents having complicated shapes and configurations while involving only minimum mechanical manipulation for fabrication.

To the achievement of the indicated ends and realization of corollary benefits and advantages, vent holes in cores may expeditiously be made by practice in accordance with the present invention wherein, in fundamental essence, there is employed as a core-vent material for cores an expanded or foamed, thermoplastic, synthetic, resinous (i.e., plastic) material, said material being shrinkable on being subjected to heat at elevated temperatures.

The Vent holes are simply prepared by positioning the expanded, plastic material of appropriate diameter, in the sand core during its fabrication; surrounding at least a portion of said material with the particulate, refractory core (or mold)-forming composition (generally sand); then baking or heating said mold-forming composition until said expanded, plastic core material shrinks or collapses to a sufficient extent to leave an efiicient and effective vent in the configuration as established by the original presence of the core-vent material. Generally, the normal baking operation on the core is adequate to accomplish the desired vent formation the plastic core-vent material.

The method of the invention may be readily employed with any of the particulate mold or core-making compositions that are adapted to be used for mold and core manufacture in the foundry.

Further features and advantages of the invention are manifest in the ensuing description and specification, taken in connection with the accompanying drawing, wherein;

FIGURE 1 is a coss-sectional view of a complex core fitted with an expanded plastic core-vent material for pro vision of a vent in accordance with the invention; and

FIGURE 2 is a cross-sectional view of the core of FIGURE 1 illustrating the vent formed by the expanded plastic core-vent material after baking.

With initial reference to FIGURE 1, there is depicted a core 10 having a core-vent material 12 of expanded thermoplastic, heat-shrinkable, resinous material placed appropriately therein prior to baking or heating for provision of a suitable vent.

In FIGURE 2 of the drawing the core 10 is illustrated after baking or heating. The core vent 14 is formed to provide adequate venting passageway for the core. The residue of the core-vent material or string 12 remains in the vent 14 and is indicated by reference numeral 16 as the shrunken (or collapsed) plastic string.

Core vents may be readily formed in accordance with the invention by employing the expanded plastic corevent material pursuant to the ordinary foundry practice. When the mold has been prepared for the core, the expanded, plastic, core-vent material is inserted or positioned in the desired place in the conventional manner.

When a core vent of relatively simple geometry is prepared, the expanded, plastic, core string may be anchored in the walls of the mold after having been pre formed by hand, after which the core-forming composition is rammed into the mold cavity.

Many of a wide variety of thermoplastic, resinous materials of the type capable of being expanded into thermally shrinkable foams may be employed with benefit in the practice of the invention. These include various polyolefins, such as polyethylene, polypropylene, copolymers of ethylene and propylene, and the like; thermoplastic rubbers; vinyl aromatic resins such as expanded polystyrene, polyvinyltoluene, copolymers of styrene and vinyl toluene, and the like; and such halohydrocarbons as polyvinylchloride, copolymers of vinyl chloride and monomers such as vinyl acetate, vinylidene chloride, diethyl maleate,

ethyl acrylate, and the like. Other suitable types of resins include thermoplastic cellulose esters such as cellulose acetate, polyurethanes, acrylate, and acrylate copolymers, cellulose ethers such as ethyl cellulose and the like.

Usual foundry practice requires baking or heating of cores at a temperature within the range of about 300 F. to about 600 F. to ensure a relatively moisture free core. Expanded thermoplastics having a softening point below the baking temperature required for the core are satisfactory for the practice of the invention. Expanded polyolefins, which usually have a softening point within the range of 90 C. to 130 C., are particularly well adapted for the practice of the invention. Core-vent materials prepared from such polymers have a convenient softening range.

In many instances such core-vent materials are also sufficiently flexible to permit their being formed into relatively complex shapes without rupture. In any event, they are readily fabricated into any desired or necessary configuration. In addition, there is generally no problem of storage and shipping with such materials. As an illustration, polyolefin foams or expanded polyolefins are generally flexible enough to be stored on spools if a string form is employed. The polyolefin material is less prone to damage by handling and shipping.

The advantages and benefits of the invention may be obtained by employing either an open or a closed cell structure in the expanded core-vent materials. In vent materials prepared from any given expanded resins in some particular range of bulk density, a closed cell structure tends to offer superior characteristics of compression strength as compared to open cell structure. In contrast, an open cell structure frequently allows packaging advantages since such a form of most expanded, thermoplastic materials may be compressed to a relatively smaller volume.

Suitable expanded, plastic, core-vent material may comprise a resinous substance that has been foamed or expanded by the action of suitable blowing agents to an expanded volume that is at least twice its original unexpanded volume. Advantageously, the expanded plastic may be foamed up to about '70 and even more times its original unexpanded volume. In applications where substantial physical strength is required in the core-vent material, an expanded, resinous substance having a foamed or expanded volume that is between about 2 and times its original unexpanded volume may be advantageously employed. If a core-vent material of relatively low physical strength can be tolerated, use of more highly expanded foams is desirable and may frequently be of considerable benefit, particularly where a relatively small collapsed volume is desired. Expanded plastics foamed to a volume between about 10 and about 70 times their original, unexpanded volume of the resins are ordinarily suitable for use as core-vent materials of lower physical strength.

When an expanded, thermoplastic, resinous material is heated above its softening range, the surface tension of the polymer is sufficient to cause the cell or passageway walls of the foam to rupture and/or contract. The expanded, resinous body will increase in bulk density and decrease in volume until the bulk density of the heated material approaches the density of the thermoplastic resins from which the body was prepared. Thus, a thermoplastic resin that has been expanded about 10 times its original volume will shrink to about its expanded volume on heating above its softening range. The density of such a heat shrunken thermoplastic foam will, in general, be from about 90 percent to 100 percent of the density of the unexpanded resin composition. The time required to collapse, by heating, of expanded thermoplastic material varies, but at conventional core-heating o baking temperatures is substantially less than the minimum time required to dry and/ or cure the core. Consequently, variations in shrinkage rate of the expanded core-vent material are not significant.

In order to obtain suitable venting of cores according to the invention the expanded, thermoplastic, core-vent material, on heating, must shrink to about /2 of its original expanded volume and, advantageously, to about to or even a smaller fraction of its original volume.

Core-vent materials in accordance with the invention may be fabricated by conventional fabricating techniques employed in conjunction with synthetic resins. Expanded core-vent materials may be extruded, molded, cast, or shaped by cutting from a larger mass of expanded synthetic resinous substance. Blowing or expanding agents may be incorporated into a resinous mass prior to heat shaping such a molding or extruding, or a shaped corevent material may be prepared and impregnated with a suitable blowing agent and expanded by heating to a suitable temperature. Molding is particularly advantage ous when the geometry of the vent string or material is complex.

If particularly complex vent forms or relatively long passageways are involved, it is frequently advantageous to insert a relatively thin wire into the expanded, plastic, core-vent material or string. This will render the core string self-supporting. Such a reinforcing wire is best inserted so as to be in a generally coaxial disposition in the string.

In cases where a plurality of elongated core vents are to be provided, suitable continuity of the vent holes may be had by placing the surface of one of the expanded, plastic, core-vent strings in contiguity with the surface of another. Alternately, one core string may be knotted about the other. Frequently, the knotting of one corevent string about another may be used to provide better physical support for core-vent strings when they are positioned within the mold cavity. Adhesives or heat sealing may advantageously be employed to connect the corevent materials together.

The invention is further illustrated in and by the following representative examples which are intended to be demonstrative but in no way limiting thereof.

Example I A core similar to that shown in the drawing was prepared, employing the following recipe for the core material:

(a) Urea formaldehyde sand: Percent by weight Gratiot bank sand (AFS No. 65) 94.5 Water 1.5 Cereal 0.3 Resin (50-50 urea-fOrmaldehyde-H O mixture) 2.0 Glycol (diethylene) 0.4 Ammonia (dilute soln.) 0.l Sulfur 0.6 Boric acid 0.2 KBF 0.2 Urea 0.2

Prepared according to United States Letters Patent A closed, cell, expanded, polyethylene, core-vent string having a bulk density of about 3 pounds per cubic foot was employed. This core-vent material collapsed and released its entrapped gas at a temperature of about 220 F. The core pattern and the core-vent strings were first arranged substantially as illustrated in the drawing, after which the core-vent material was rammed. After separating the core from the pattern, the resulting core was baked at about 325 F. for about one hour. Upon examination, a free passageway was found to exist within the core. On dissection, the walls of the core passageway were found to be free of interstitial plugging.

Example 11 The procedure and core-vent, string material of Example I was employed using the following recipe for the core material:

(b) Oil sand: Percent by weight On dissection, the core vent hole was found to be free of appreciable obstruction. It showed excellent gas permeability through the entire walls of the passageway.

Excellent results are also obtained when the above procedures are duplicated with the exception that as corevent materials are employed expanded polystyrene, expanded polyvinyl chloride, and expanded polypropylene, and foams of other expandable, thermoplastic, resinous materials of the above-indicated type are within the mentioned scope.

As is apparent, the method of the invention is susceptible of being embodied with various alterations and modifications from that which has been described in the preceding description and specification. For this reason, as has been indicated, it is to be fully understood that all of the foregoing is merely intended to be illustrative and is not to be construed or interpreted as being restrictive or otherwise limiting of the present invention excepting as it is set forth and defined in the appended claims.

What is claimed is:

1. The method of forming a vent passageway in foundry cores comprising: incorporating a core string in a foundry core prior to baking, said core string comprising an expanded heat shrinkable thermoplastic material, said core string being positioned within said unbaked core in a location at which a vent passage is desired and subsequently baking said core at a temperature sufficient to cause said core string to shrink and form a gas passage within said core.

2. The method of claim 1, wherein said expanded thermoplastic resinous material is polyethylene.

3. The method of claim 1, wherein said expanded thermoplastic resinous material is polystyrene.

4. The method of claim 1, wherein said core is heated in a temperature range of 300 to 600 Fahrenheit until said core string collapses.

5. The method of claim 1, wherein said expanded thermoplastic resinous material is expanded from about 2 to about times the unexpanded volume of the resinous material.

6. The method of claim 1, wherein said expanded thermoplastic resinous material is expanded from about 10 to about 70 times the unexpanded volume of the resinous material.

7. As an article of manufacture, a foundry core comprising a compacted particulate core forming material having disposed therein a core string, said core string comprising an expanded thermoplastic resinous material, said expanded resinous material being shrinkable when subjected to heat at core baking temperatures.

8. The article of claim 7, wherein the expanded thermoplastic resinous material is polyethylene.

9. The article of claim 7, wherein the expanded thermoplastic resinous material is polystyrene.

10. The article of claim 7, wherein said expanded thermoplastic resinous material is expanded from about 2 to about 70 times the unexpanded volume of said resinous material.

11. The article of claim 7, wherein said expanded thermoplastic resinous material is expanded from about 10 to about 70 times the unexpanded volume of said resinous material.

12. As an article of manufacture a foundry core comprising a baked particulate core forming material said core having at least one vent formed therein, said vent containing the collapsed residue of a heat shrunken, expanded thermoplastic resinous material.

References Cited in the file of this patent UNITED STATES PATENTS 1,837,877 McDowall Dec. 22, 1931 1,864,107 Walter June 21, 1932 2,045,556 Almen June 23, 1936 2,424,047 Morin July 15, 1947 2,474,186 Marks June 21, 1949

Claims (1)

1. 7. AS AN ARTICLE OF MANUFACTURE, A FOUNDRY CORE COMPRISING A COMPACTED PARTICULATE CORE FORMING MATERIAL HAVING DISPOSED THEREIN A CORE STRING, SAID CORE STRING COMPRISING AN EXPANDED THERMOPLASTIC RESINOUS MATERIAL, SAID EXPANDED RESINOUS MATERIAL BEING SHRINKABLE WHEN SUBJECTED TO HEAT AT CORE BAKING TEMPERATURES.

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