US2907084A

US2907084A - Hollow cores for making castings

Info

Publication number: US2907084A
Application number: US574270A
Authority: US
Inventors: Robert T Wood
Original assignee: Aluminum Company of America
Current assignee: Alcoa Corp
Priority date: 1956-03-27
Filing date: 1956-03-27
Publication date: 1959-10-06
Anticipated expiration: 1976-10-06

Description

Oct. 6, 1959 R. 'r. WOOD HOLLOW CORES FOR MAKING CASTINGS Filed March 27, 1956 lNV ENTOR Roberi 7," Wood ATTORNEY United States Patent HOLLOW CORES FOR MAKING CASTINGS Robert T. Wood, Pittsburgh, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Application March 27, 1956, Serial No. 574,270

11 Claims. (Cl. 22-165) This invention relates to the production of hollow collapsible cores, especially those of tubular shape, which are adapted to form passageways of relatively small cross section in metal castings. It is particularly concerned with providing long collapsible hollow cores which can be readily bent into desired shape for insertion in the mold and which can be easily removed from the subsequent casting. The collapsible cores are adapted to be used With metals whose pouring temperatures do not exceed the softening point of the structural component of the cores, that is, the point at which the structural component sags and loses its ability to support itself. The cores are especially useful in the production of light metal castings, i.e. those composed of aluminum, magnesium and alloys which contain more than 50% by weight of these elements.

Metal castings are finding increased use in applications involving service at elevated temperatures but to gain the highest efiiciency it is sometimes necessary to provide ducts within the body of the castings through which a liquid coolant can be circulated. Where such ducts are straight and relatively short, the castings can be drilled from the exterior, but this cannot be done where the ducts are curved or are of considerable length.

Conventional sand cores cannot be employed to form such ducts because of the fragility of long, thin cores,

the diificulty of supporting them in a mold to prevent displacement by the molten metal entering the mold and the trouble encountered in removing all of the sand from the ducts in the cast product. To avoid the problems associated with sand cores, it has been proposed that small metal tubes of the desired shape be embedded in light metal castings. However, blows are apt to occur at the tube surface as the molten metal fills the mold and surrounds the tube. The blows cause voids between the tube and the casting with resultant reduction in capacity to transfer heat from the casting to the tube and cooling liquid passing through it. In addition, the use of another metal than that of which the casting is made may present corrosion problems and otter some difiicult y in securing a firm bond between the tube and the light metal. In view of the shortcomings of the prior methods, I have been led to devise cores which permit the formation of small passageways of considerable length in light metal castings.

One of the objects of my invention is to provide a method of making accurate hollow, thin-walled collapsible cores which can be easily removed from the casting in' which they are embedded. Another object is to provide long hollow collapsible cores having a relatively small transverse dimension which can be bent and yet will retain their shape when placed in molds. Still another object is to provide long tubular collapsible cores of'small diameter which can be readily extracted from the castings and leave clean cored passageways. These and other objects are achieved as more particularly pointed out in the following description and claims taken in conjunction with the accompanying figures wherein:

Fig. 1 shows in enlarged view a side elevation of a woven fiber glass tube which has been treated with an organic heat hardening substance;

Fig. 2 taken on line IIII of Fig. 1 illustrates a cross section of the core filled with loose sand;

Fig. 3 is an enlarged view of a side elevation of a woven fiber glass tube treated with an organic heat hardening substance and having a metal wire concentrically disposed therein with ends projecting beyond the ends of the tube; and

Fig. 4 taken on line IV-IV of Fig. 3 is a cross section of the tube with loose sand surrounding the central wire. I have found that relatively long, accurate, clean passageways or ducts can be provided in castings by employing a yieldable, thin-walled, collapsible, porous hollow core made from woven fiber glass treated with a substance which though solid at room temperature decomposes at least in part upon contact with the molten metal with the evolution of little or no gas. The treating substance serves to stiffen the hollow core but does not render it impervious to the passage of gas nor does it create a degree of rigidity which results in cracking of the core when bent or conformed to a desired shape. Upon cooling of the casting, the woven fiber glass core can be easily collapsed and withdrawn from the passageway or duct in substantially intact form since all or a substantial part of the treating substance will have disappeared. Inasmuch as the hollow core loses its stiffness during the casting operation, becomes loosened from the wall of the cored opening and can be pulled out of the passageway it is refered to herein as being collapsible. Moreover, the length of the core body considerably exceeds the transverse dimension.

The woven fiber glass constitutes the yieldable but relatively permanent structural component of the core. The glass used may be of the common type generally consisting of about 60 to SiO 3 to 20% A1 0 7 to 20% CaO and/or MgO and from 0 to 10% of Na O. The glass, in any case, must have a higher melting point than the metal being cast, generally above 1800 F., and should not soften to any appreciable extent during the pouring of the casting. Also, the glass should not decompose or react with the metal and hence is regarded as being a relatively permanent component of the core. Other glass compositions than the one named above may be employed providing they can be formed into strands and meet the other requirements.

The strands of glass fibers may be woven or braided in any suitable manner which will allow suificient space between the strands to allow the treating material to pass through and not completely fill the interstices. The unfilled space between the strands must be small enough to prevent any metal from leaking through or penetrating to the extent of leaving a rough surface on the wall of the passageway. A very suitable form of woven fiber glass product is the braided sleeve insulation employed for covering electrical conductors and which ranges from about to 1 inch in diameter. It is generally most convenient to use such a continuous sleeve instead of forming the hollow core from one or more woven strips of fiber glass and overlapping or bonding the edges together. A commercial woven fiber glass sleeve or strip which has a thickness of 0.006 to 0.10 inch is very satisfactory. The choice of a particular thickness within this range depends upon the size of core desired and the degree of stiffness required.

The stiifening material employed to coat the strands of fiber glass may be any one of several organic heat hardening compositions or combinations thereof which decompose, at least in part, at the temperature of the molten metal surrounding the core. For example tung oil, the common foundry core oils, linseed oil, phenolic or urea formaldehyde resins, acrylic acid or a mixture of glycerine and an oxide of a metal such as calcium or magnesium may be used. Those compositions which yield only a small amount of gas or no gas in contact with the molten metal are preferred. The stiffening materials are generally applied to the glass fiber product in liquid form or suspended or dissolved in a liquid'carrier and the coated article subsequently dried and baked to harden the organic composition. The resultant baked article should be stiff enough to retain its original shape or to remain in the shape imparted by bending to a desired contour. The conditions under which the heat hardening substance can be dried and baked varies with the substance selected, the time and temperature for effecting hardening being well known in the art, the time ranging from several minutes to a few hours depending upon the temperature. A very suitable material for most purposes is acrylic acid which yields very little gas upon contact with molten metal and which can be baked in a relatively short time at a low temperature. It has been found, for example, that baking acrylic acid treated woven fiber glass for one hour at 350 to 450 F. provides a stiff, easily handled product.

A convenient and effective manner of applying the heat hardening substance to the Woven fiber glass is to dip the the glass product in the liquid substance or a suspension or solution thereof, or to brush or spray the liquid onto the glass product and then mount it upon a glass or metal surface, or in the case of a sleeve to pull it over a glass or metal rod. Alternatively, the'woven fiber glass may be first mounted on the glass or metal base and the heat hardening substance applied thereto in liquid form as just described. In any event, it is essential that at least the strands of fiber glass on the external surface of the product be coated with the liquid, and preferably that all of the strands be so coated but an excess is to be avoided, especially such a quantity as will completely fill all the meshes of the woven article. It is imperative that the final product shall be porous to permit escape of any gas. The freshly coated fiber glass may be allowed to dry in air or in a heated oven. Following this, the dried product is heated to a suitable baking temperature to effect the desired hardening of the organic substance.

After the baking and upon cooling to room temperature, the stiffened fiber glass is stripped from the glass or metal base. The thickness of the core wall is substantially the same as that of the original woven fiber glass pro-duct and hence is relatively thin. On the other hand the hollow or tubular core is stiff enough to withstand normal handling and yet yieldable enough to permit bending and retention of that bend when placed in a mold.

In order to insure a smooth bending of the hollow core without buckling and to provide additional support for the core walls during pouring of the molten metal, it is advantageous to fill the core with dry sand or similar granular refractory before the core body is bent and placed in the mold. To retain the dry sand in place it may be desirable to plug the ends of the core with a material such as bonded sand or a common metal core box vent which can be easily removed.

The granular refractory material used to fill the hollow core may consist of the usual silica sand employed in foundries or it may consist of other granular refractory materials such as Carborundum, ground firebrick, etc. It is important in any case that the material be dry, that it retain its granular easy flowing property following the casting operation so that it may be conveniently poured out or blown out from the cored passageway, and that it be free from lumps or large particles which might interfere with introducing the granular material to the core and removing it therefrom.

Where it is desirable to provide still further stiffness to the hollow core, especially where it is of considerable length and it is not supported except at the ends or over a long span, a metal wire or cord may be inserted in the hollow core body and, if space permits, to surround it with dry sand or a similar material such as described above. At least one end of the wire should project beyond the core to facilitate removal. The sand protects the metal wire from excessive heat. The term wire as used herein not only refers to a single strand or piece but includes cords or other forms of woven or twisted products made of very small wires.

The metal wire which may be employed to stiffen the core may be of any suitable metal having a sufficiently high melting point to remain intact during pouring of molten metal around the core in the mold and yet which is soft enough to take any bonds or other shaping incident to producing the desired core contour. Generally, the wire should have a melting point no lower than that of the metal being cast. Aluminum or aluminum alloy wire in an annealed condition is satisfactory as is soft iron wire. The size of the wire depends upon the cross section of the core, generally a wire of 1 to lg inch in diameter is satisfactory. Although a single wire or multiple wire product may be employed, it may also be desirable under some circumstances to use two or more wires in a core, especially if they are small.

Where an exceptionally smooth core surface is desired, it may be desirable to coat the baked woven fiber glass core with a very thin layer of conventional mold wash containing as the major constituent at least one inorganic parting substance selected from the group composed of graphite, talc, lime, French chalk and the like and a suitable binder, if necessary. This wash coating must be thin and not of such a density as to make the core wall impervious to the passage of gas therethrough. The wash may be applied in the form of a suspension and subsequently dried to remove the volatile vehicle.

Referring to the figures, the woven fiber glass tube 1 is composed of fiber strands 2 coated with a thin layer of a heat hardening substance 3. A space 4 is left between the coated strands to provide a porous structure. The hollow tube may be filled with a granular refractory 5 as seen in Fig. 2. To add stiffness to the hollow core, a wire 6 may be inserted as seen in Figs. 3 and 4, the ends of the wire projecting beyond the ends of the core. Sand or similar refractory material 5 fills the space between wire 6 and tube 1 as seen in Fig. 4 thereby keeping the wire in substantially concentric position.

To utilize the hardened woven fiber glass core it is shaped to the desired contour and placed in a mold in the same manner as conventional sand cores except that the conventional chaplets can be omitted unless the core is of considerable length. The mold may be of the conventional sand or permanent mold type. The molten metal is introduced in the conventional manner through gates and freezes upon the core. After the casting has cooled to room temperature the loose material is poured out or blown out of the passageway and the Wire withdrawn. Finally, the woven fiber glass itself can be easily extracted from the cored opening since it collapses and can be pulled out in substantially intact form because of its strength in the lengthwise direction. In many cases the fiber glass sleeving is so little affected by the procedure described that it may be again coated and used as a core if desired.

Having thus described my invention and certain embodiments thereof, I claim:

1. A tubular porous woven fiber glass core adapted to form a passageway in a metal casting, said core comprising a collapsible woven fabric tube of glass fibers, the glass having a melting point above that of the metal poured around the core, said woven fiber glass core being stiffened with a baked heat hardenable organic coating on at least those fibers at the external surface of the core, said stiffened core being yieldable and capable of being shaped to a desired contour, said heat hardenable organic substance being decomposable at least in part at the temperature of the molten metal surrounding the core whereby the fiber glass fabric is rendered collapsible for withdrawal from the casting in intact form, said stiflened fiber glass core being sufficiently porous to permit the passage therethrough of mold gases.

2. A tubular porous woven fiber glass core according to claim 1 wherein the heat hardenable coating is acrylic acid.

3. A tubular porous woven fiber glass core according to claim 1 wherein the heat hardenable coating is tung oil.

4. A tubular porous woven fiber glass core according to claim 1 wherein the heat hardenable coating is a phenolic resin.

5. A tubular porous woven fiber glass core adapted to form a passageway in ametal casting, said core comprising a collapsible woven fabric tube of glass fibers, the glass having a melting point above that of the metal poured around the core, said woven fiber glass core being stifiened with a baked heat hardenable organic coating on at least those fibers at the external surface of the core, said stiffened core being yieldable and capable of being shaped to a desired contour, said heat hardenable organic substance being decomposable at least in part at the temperature of the molten metal surrounding the core, whereby the fiber glass fabric is rendered collapsible for Withdrawal from the casting in intact form, said stiffened fiber glass core being sufficiently porous to permit the passage therethrough of mold gases, and loose granular refractory material filling said tubular core which retains its granular form at the maximum temperature attained by the core in the mold.

6. A tubular porous woven fiber glass core adapted to form a passageway in a metal casting, said core comprising a collapsible woven fabric tube of glass fibers, the glass having a melting point above that of the metal poured around the core, said woven fiber glass core being stiffened with a baked heat hardenable organic coating on at least those fibers at the external surface of the core, said stiffened core being yieldable and capable of being shaped to a desired contour, said heat hardenable organic substance being decomposable at least in part at the temperature of the molten metal surrounding the core whereby the fiber glass fabric is rendered collapsible for withdrawal from the casting in intact form, said stifiened fiber glass core being sufiiciently porous to permit the passage therethrough of mold gases, a metal wire of smaller diameter than the woven fiber glass core disposed within said core but spaced from the wall thereof and extending throughout the length of the core and loose granular refractory material filling the space between said wire and the core wall, said refractory material being capable of retaining its granular form at the maximum temperature attained by the core.

7. A tubular porous woven fiber glass core adapted to form a passageway in a metal casting, said core comprising a collapsible woven fabric tube of glass fibers, the glass having a melting point above that of the metal poured around the core, said woven fiber glass core being stiffened with a baked heat hardenable organic coating on at least those fibers at the external surface of the core, said stiffened core being yieldable and capable of being shaped to a desired contour, said heat hardenable organic substance being decomposable at least in part at the temperature of the molten metal surrounding the core whereby the fiber glass fabric is rendered collapsible for withdrawal from the casting in intact form, said stiffened fiber glass core being sufficiently porous to permit the passage therethrough of mold gases, said stiffened fiber glass core being further coated on its external surface with a wash containing as a major constituent at least one inorganic parting substance selected from the group consisting of graphite, talc, lime and French chalk.

8. A mold for the production of metal castings which have passageways formed therein comprising in combination a mold body with a cavity therein for receiving molten metal and a tubular porous woven fiber glass,

core, and at least those fibers at the external surface ofthe core being coated with the heat hardenable organic substance, said heat hardenable substance being decomposable at least in part at the temperature of the molten metal surrounding the core whereby the woven fiber glass core is rendered collapsible for withdrawal in intact form from the casting, the stiffened core being sufiiciently porous to permit passage of mold gases therethrough.

9. The combination of a mold and core according to claim 8 wherein the heat hardenable organic substance is acrylic acid.

10. A method of providing elongated passageways in metal castings comprising coating at least the external fibers of a woven fiber glass tube with a heat hardenable organic substance decomposable at least in part at the temperature of the molten metal surrounding the core, baking said coated core, the baked core having sufficient porosity to permit the passage therethrough of mold Igases, positioning said coated core in the mold cavity of a mold body, pouring molten metal into said cavity and effecting removal of the organic substance from at least the surface of the core by decomposition of the organic substance whereby the fiber glass tube is rendered collapsible, cooling the casting with the core therein, collapsing the core and finally withdrawing it from the casting in substantially intact form.

11. The method according to claim 7 wherein the heat hardenable organic substance is acrylic acid.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Foundry Core Practice (Dietert), Published by Americar; Foundrymens Society, Chicago, Ill. (page 335 relied on Bacon: Abstract of application Serial'Number 610,- 591, published Sept. 20, 1949, 626 CG. 3.