US3429359A

US3429359A - Method and apparatus for blowing cores using microwave energy

Info

Publication number: US3429359A
Application number: US457747A
Authority: US
Inventors: Ashley James Hollingsworth
Original assignee: Litton Precision Products Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1965-05-21
Filing date: 1965-05-21
Publication date: 1969-02-25
Anticipated expiration: 1986-02-25
Also published as: DE1508676B1; FR1478864A; GB1132373A; BE680552A; NL6606859A

Description

Feb. 25; 1969 A. J. HOLLINGSWORTH 3,429,359

METHOD AND APPARATUS FOR BLOWING CORES USING MICROWAVE ENERGY Sheet F'iled May 21, 1955 IINVEN'I'OR.

ASHLEY .1 HOLL/A/GSWOETH ATTORMEV F 25, 96 A. .1; HOLLIN GSWORTH 3,429,359

METHOD AND APPARATUS FOR BLOW ING CORES. USING MICROWAVE ENERGY Filed May 21. 1965 Sheet 2 of 2 INVENIORI. v ASHLEY J. HOLLJIUGSWOQT'H United States Patent M 3,429,359 METHOD AND APPARATUS FOR BLOWING CORES USING MICROWAVE ENERGY Ashley James Hollingsworth, Atherton, Calif., assignor to Litton Precision Products, Inc., San Carlos, Calif. Filed May 21, 1965, Ser. No. 457,747

US. Cl. 164-37 16 Claims Int. Cl. B22c 23/00; B22d 27/02 ABSTRACT OF THE DISCLOSURE Apparatus and method for blowing cores in which a core is formed within a core box that is transparent to microwave energy. The core box is then supported in a microwave heating cavity and the core is cured by the application of microwave energy.

This invention relates to core blowing devices and processes and more particularly to such devices and processes in which the core is cured'by the application of high frequency electromagnetic wave energy.

Millions of articles are cast from molten metal by foundries every year. One of the major problems of foundries is to provide molds for the articles being cast. Experience has shown that the most economical way to provide these molds is to make the mold of as cheap a material as possible which can withstand the heat of the molten metal and which will retain its shape long enough for the molten metal to solidify. After solidification of the metal, the mold is then broken and the cast article removed. Such destructible molds are not only economically feasible but absolutely necessary when a hollow object is being cast in which the cross sectional area at some point in the hollow part is greater than the cross sectional area at the entrance to the hollow part. In such application, the only way to remove the center part of the mold, which center part is known as the core, is to break the core and remove its pieces through the smaller cross sectional openings.

The most widely used material for constructing molds and cores in the foundry industry is sand to which has been added a suitable binder material to cause the mold or core to retain its shape until the metal has been poured and solidified. When sand is so used in a foundry, the core and mold are formed by a suitable process, several of which will be later described, the core is placed within the mold and molten metal is poured into the mold. When the metal has solidified, the outer mold is broken off and the metal object is usually vibrated to break up the sand in the core and the sand is poured out of the openings of the hollow cast device. The sand from the mold and core can then be recovered and used over and over.

The sand mold is traditionally made from so-called green sand, which is merely damp sand, packed around a model or pattern of the object to be cast. This mold is usually made in two halves, each half being pressed around opposite halves of the model, thereby forming a two-piece hollow mold the inner surfaces of which reproduce the outer surface of the object to be cast.

It is recognized by those skilled in the art that the core must be of sturdier construction than the mold since the core, after being formed, must be placed within the two halves of the mold prior to the insertion of the molten metal. Since the core must be so handled, it would probably be broken if it were made of simple damp sand in the manner described above.

One of the early ways of making suitable cores was to form a two-piece mold, the inner surfaces of which conform to the outer surfaces of the core which it is desired to make, to place sand saturated with oil or serial 3,429,359 Patented Feb. 25, 1969 binders in the mold and then to heat this in a suitable furnace, baking the oil and sand mixture and causing it to adhere together reasonably well. Such a mold from which a core is made is known in the art as'a core box. Such core boxes are traditionally made of iron and steel. It is necessary that the inner dimensions of the core box be extremely accurate. Accordingly, such core boxes are traditionally made by hand by skilled die makers. Thus, if a foundry is producing a large number of different pieces, it is necessary to have many such core boxes on hand, with these core boxes representing a substantial investment to the foundry.

A later improvement on this method of making cores was the introduction of the so-called core blower. This is a rather complex machine which utilizes core boxes such as was described above. The core blower supports the core box in a suitable manner and blows the mixture of said and oil into the core box through a suitable opening therein. The blowing opeartion both filled the box and compacted the sand mixture in the core box. The filled core box is then removed from the core blower and placed in a furnace for baking as before.

A still later improvement in the art was to substitute a thermosetting resin in place of the oil as a binder. The resin is mixed with the sand and the mixture is blown into the core box by the core blower, as before. The core box is kept hot in a conventional manner, either by electric heating elements or by gas burners, and heat is transferred by conduction from the core box to the resin, setting the resin and curing the core.

From the above discussion is can be seen that even using the more modern methods currently available, forming and curing cores is one of the most expensive operations in a typical foundry, with this operation requiring a large investment in expensive core boxes. Thus the more advanced core blowing techniques described above are economically feasible only if a large number of identical cores are to be blown, thus amortizing the cost of the core box over many cores.

It is accordingly an object of the present invention to provide an improved means and method for forming cores.

It is yet another object of the present invention to provide an improved device for forming cores which utilizes an inexpensive and easily made core box.

It is yet another object of the present invention to provide an improved means and method for forming cores in which high frequency electromagnetic wave energy is used to cure the core.

Briefly stated and in accordance with one embodiment of the present invention a core blowing device is provided which includes a microwave cavity. A core box is provided which is constructed from a cheap easily worked material which is transparent to microwave energy. The core blowing device also includes a hopper or other suitable container which contains a mixture of sand and thermosetting resin as a binder. Suitable metering means measure a quantity of the mixture of said and resin into a blower chamber from which the mixutre is blown into the core box by compressed air. Microwave energy is then applied to the cavity to cause the resin to set, thereby curing the core. The cavity is then opened, the core box separated and the core removed from the core box, after which the operation can then be repeated until the desired number of cores is formed.

For a complete understanding of the invention together with other objects and advantages thereof, reference may be had to the accompanying drawings in which:

FIGURE 1 shows a front view of a microwave core blower embodying the present invention;

FIGURE 2 shows a side view of the device of FIG- URE l;

FIGURE 3 shows a cross sectional view of the blower chamber and upper platen of the device of FIGURES 1 and 2;

FIGURE 4 shows a cross sectional view of the curing chamber of the device of FIGURES 1 and 2, with a core box in position; and

FIGURE 5 shows a cross sectional view of the curing chamber of the device of FIGURES 1 and 2 which incorporates a second embodiment of the present invention.

Referring now to FIGURES 1 and 2, therein are shown front and side views respectively of a microwave core blower device embodying the present invention. The device includes a frame which supports most of the components of the core blower and a hopper 12 into which is placed a mixture of sand and a thermosetting resin, which mixture may be the same as is used in conventional core blowing machines such as was described above. The hopper 12 is supported from frame 10 on rubber pads 14 and is connected at its bottom to the rest of the device through a large diameter rubber hose 16. This arrangement allows the hopper 12 to be vibrated, if desired, to keep the sand and resin mixture free flowing until the mixture is fed into the device. Since the hopper 12 is rubber mounted at all points of contact with the rest of the device, the vibration does not substantially affect any other components of the device.

Beneath rubber hose 16 is the blower chamber 18, with these sections being separated by a valve plate 20 such as will be described in greater detail later. The bottom section 22 of blower chamber 18 has a trapezoidal cross section, as shown in FIGURES 1 and 2, and is closed on its bottom side by a plate 24.

The curing chamber 26 is formed of two halves or platens, an upper platen 28 and a lower platen 30. The upper platen 28 is supported from cross arms 32 of frame 10. Plate 24 is supported on the top of platen 28 and thus the cross arms 32 also support the weight of blower chamber 18. As will later be described in more detail, matching holes are drilled in plate 24 and the horizontal surfaces of platen 28 and metallic tubes are provided to form a passageway for the mixture of sand and resin to travel from blower chamber 18 to the inside of a core box when the device is being used.

The weight of lower platen 30 is supported on shaft 34 which is in turn connected to the piston of a hydraulic cylinder 36. Cylinder 36 is supported from a mounting plate 38 which is in turn supported from cross arms 40 of frame 10. Lower plate 38 is also positioned by guide shafts 42 which are secured between

cross arms

32 and 44 of frame 10.

Microwave energy is supplied to curing chamber 26 through a wave guide 46 which connects the device to a console 48 which includes a microwave transmitter, and which may also include the control elements for the hydraulic and pneumatic portions of the device which are to be later described in more detail. For example, the microwave transmitter might include a magnetron which operates at a frequency of 2450 megacycles per second and a suitable power supply for the magnetron. Such microwave transmitters are well known to those skilled in the art and are available from a number of commercial sources, such as Litton Industries, Atherton Division, 974 Commercial St., Palo Alto, Calif. Accordingly, details of the microwave transmitter will not be given herein.

FIGURE 3 shows a cross sectional view of the blower chamber 18 and the upper half platen 2'8 and shows details of how the sand and resin mixture is blown from the blower chamber 28 into the curing chamber 26. As shown therein, plate 24 and the top wall of upper platen 28 have a plurality of aligned holes through which are fitted conductive tubes 50. These tubes serve as conduits for the sand and resin mixture when the mixture is blown into the core box, as will be seen in more detail in connection with the description of FIGURES 4 and 5 below.

FIGURE 3 also shows how wave guide 46 is connected through an opening 52 in upper platen 28 so that microwave energy may be coupled from wave guide 46 into curing chamber 26 when the microwave transmitter in console 48 is energized. Of course, it will be obvious to those skilled in the art that any other form of coupling of microwave energy from wave guide 46 into curing chamber 26 may also be used with the invention.

FIGURE 4 shows a view, partially in cross section, of the upper and

lower platens

28 and 30 and lower chamber 18 and also shows a core box 54 positioned in the curing chamber 26.

Core box 54 is formed from any suitable material which is essentially transparent or lossless to microwave energy. In the embodiment shown, the outer dimensions of core box 54 are such that it completely fills the curing chamber 26 and is hollowed such that the surfaces of its inner chamber 56 reproduce the surface of the core it is desired to produce. It is not necessary to the present invention that the core box completely fill the curing cavity. It is sufficient that the core box be supported in the cavity sufficiently to withstand the internal pressure of the blowing operation, to be later described in detail. Thus the core box could be constructed of mating sections which do not completely fill the curing cavity but which are supported from the top and bottom walls of the cavity and which, because of mating portions such as alignment pins, remain firmly together during the blowing operation.

Core box 54 is made in two halves, with the upper half having openings through which conductive tubes 50 extend into the chamber 56. The upper half of core box 54 is positioned in the upper platen 28 and the lower half of core box 54 in lower platen 30. If it is desired to reproduce a large number of identical cores, the two halves of core box 54 may be temporarily secured to their respective platens by any known expedient such as by being bolted to the platen.

Core box 54 may be formed from lossless materials such as plaster, polypropylene, silicone rubber or the like. Each half may conveniently be formed by making a model of the respective half of the core to be blown from a material such as wood and by casting the plaster, polypropylene, or silicone rubber around the wood model. Suitable holes for conductive tubes 50 may have been cast into the core box originally or may be drilled into the upper half thereof after the core box is formed.

Still referring to FIGURE 4, cores corresponding to chamber 56 are produced in the following manner: As will later be described in more detail, a mixture of sand and resin is blown by compressed air through the tubes 50 into chamber 56, such that the chamber 56 is completely filled with the mixture of sand and resin and the mixture is suitably compacted. Microwave energy is applied through the coupling opening 52 to the curing chamber 26. Since the material from which core box 54 is constructed istransparent to microwave energy, all of the microwave energy is converted into heat in the sand and resin mixture. Since sand is also essentially lossless, this conversion may be effected in either of two ways. First, a resin such as a urea resin or furan resin which is lossy may be selected and the microwave energy is converted into heat in the resin itself. However if for some reason it is desired to use a relatively lossless resin, such as a phenolic resin, a small amount of moisture may be included in the sand and resin mixture and the microwave energy is converted into heat in the moisture. This heat then triggers the .thermosetting reaction of the curing resin.

The tubes 50 are made from a conductive material, such as copper or any other suitable metal, and have an interior diameter less than one-quarter wave length of the frequency of microwave energy being used in the microwave core blower. Thus, the conductive walls of the tubing shield the sand and resin mixture in the tubing from the microwave energy and the small diameter prevents any microwave energy from entering the lower ends of the tube. This prevents the sand and resin mixture from being cured in the tubes and clogging the tubes.

FIGURE 5 shows a cross sectional view of the curing chamber similar to that shown in FIGURE 4 but illustrates another embodiment of the present invention. In this embodiment, the two halves 58 and 60 of the core box are formed from a lossless flexible material such as silicone rubber and, as shown, the two halves of the core box are hollow or ballon-like having inflatable compartments in their walls. Piping 62 is provided to communicate with the interior of the two halves of the core box 58 and 60. In this embodiment, the walls of the core box 58 and 60 may be reinforced with a material such as fiberglass to prevent the walls from stretching, and a lossless fluid is introduced under pressure through piping 62 into the interior of the core box halves 58 and 60. This fluid under pressure causes the halves 58 and 60 to become rigid and to retain their shape when the sand and resin mixture is blown under pressure into the inner chamber 56. The fluid may be either a liquid or air under sufficient pressure. After microwave energy has been applied to curing chamber 26 and the core cured, the fluid may be pumped out through piping 62, thus causing the ballon-like core box halves 58 and 60 to collapse whereby the cured core may be easily removed from the core box.

It will be noted that, as is illustrated in FIGURE 5, this embodiment enables an operator to blow cores which have undercut portions such as is shown in each corner of the chamber 56 in FIGURE 5. Using the metal core boxes of the prior art, it has been possible to blow cores having undercut surfaces only by using extremely expensive sectionalized core boxes. The present invention enables an operator to easily blow undercut cores while still meeting the advantages of a cheap core box, quick curing, and no intermediate handling.

Having thus described the details of the curing chamber 26, the core box 54 and the application of microwave energy to the core box, let us now consider how the sand and resin mixture is blown into the core box. Referring again to FIGURE 2, the mixture of sand and resin is placed in hopper 12. Valve plate normally closes the bottom of hopper 12. Valve plate 20 is connected through a suitable linkage 64 to the piston in pneumatic cylinder 66. Pneumatic cylinder 66 is connected to a source of compressed air 68.

At this point it is noted that the entire pneumatic system and its controls, together with the hydraulic system of cylinder .36 to be later described, are shown only schematically since these systems are quite conventional and well known to those skilled in the art.

When it is desired to form a core, and thus to blow sand into the curing chamber, valve plate 26 is opened by opening valve 70, thus allowing compressed air into cylinder 66 which moves its piston to the right and opens valve plate 20, allowing a quantity of the sand and resin mixture to fall into the blowing chamber 28. Valve 70 is then closed and valve 72 is opened, thus moving the piston in cylinder 66 to its other extreme position and closing valve plate 20 again. Now valve 74 is opened and the compressed air supply 68 is connected to blowing chamber 18, in which chamber the compressed air forces the sand and resin mixture through the tubes 50 to the chamber 56 of core box 54. Next the microwave transmitter in console 48 is energized, microwave energy is supplied to the curing cavity 26, and, as was previously described, the core is cured. Valve 74 is now closed and the hydraulic system 76 is activated so as to supply hydraulic fluid under pressure into line 78, thus forcing the piston in hydraulic cylinder 36 into its lower position thereby lowering lower platen 26 and enabling an operator to remove the cured core from the lower half of core box 54. The hydraulic system 76 is now activated to supply fluid under pressure into line 80 to raise the piston of hydraulic cylinder 36 to its upper position, thereby closing lower platen 30 against upper platen 28 and the device is now ready to repeat the cycle and blow and cure another core.

The controls for the hydraulic and pneumatic portions of the device may conveniently be housed in console 48 along with the microwave transmitter therein. Indeed, in fully automated systems the above described cycle may be programmed into console 48, as is well known to those skilled in the control art, and the entire cycle may be effected merely by pushing a start button. Experience has shown that if a five kilowatt microwave transmitter is used, cores may be cured to a suitable degree of hardness to withstand normal handling with an application of about 20 seconds of microwave energy. Thus a single operator using the device as described can produce several cores a minute if the operating cycle is properly programmed. It is then only necessary for the operator to push a start button and to remove the cured core when the lower platen 30 is in its lower position. As a safety factor, it is desirable to have the cycle begin with the platen in its lower position. When the start button is pushed, the platen is then raised, the sand and resin mixture blown into the core box, the microwave energy applied and the platen then lowered so that the operator may remove the cured core. Thus the operators hands are not placed between the platens during a portion of the cycle in which the platens are separated for only a fixed period of time and there is little likelihood that the operators hands will be caught between the closing platens.

In an improved embodiment of the present invention, in which the apparatus utilized appears essentially the same as that described above, the inner surfaces of the portions of the core box may be coated with any suitable ferrite material such as Mn Zn Fe O CuFe O or Ferrite materials, as is well known, exhibit lossiness because of their magnetic properties and thus when the sand and resin mixture is blown into a core box so coated the ferrite coating shields the sand and resin mixture from the microwave energy but is itself heated quite hot by the energy. This heat is then transferred by conduction to the outer surfaces of the sand and resin mixture and rapidly cures this outer surface. Since the cured outer shell, even though it might have a thickness of only a sixteenth to an eighth of an inch has sufficient strength to enable the core to be handled, it is then possible to remove the core from the core box even though the curing has not completed in the inner portions of the core. Indeed, it may sometimes be desirable to so remove the core and to shake the re mainder of the sand and resin mixture out of the center of the core, leaving only a cured shell core.

One advantage of this embodiment is that the inner surface of the core box is maintained at a uniform temperature, thus assuring uniform curing of the core. In the prior art metal core boxes, difliculty is frequently experienced when heat is unevenly drawn from the core box, thereby causing uneven temperature and inconsistent curing.

Another advantage of this embodiment is that when the ferrite material is heated to its Curie temperature point, it ceases to have magnetic properties and thus becomes essentially lossless to microwave energy. At this time, the surface of the core continues to receive heat by conduction from the hot ferrite coating but the microwave energy now passes through the ferrite coating and is converted directly into heat in the sand and resin mixture in a manner as was originally described. Thus, the core is no longer shielded by the ferrite lining and maximum rapid heating and curing of the core is effected. Also, a ferrite may be selected having a Curie temperature which is the optimum curing temperature of the particular resin being used.

In another variation on this latest described embodiment, the ferrite material may be mixed directly with the sand and resin mixture instead of being coated on the inner surface of the core box. This renders the mixture extremely lossy and causes rapid heating and thus curing of the core when microwave energy is applied to the mixture in the core box. For this embodiment, after the core has been used in making a casting and has been broken to allow the core to be removed from the casting, the ferrite material may be removed from the broken sand mixture by a simple magnetic separator and may be used again in the cycle.

A particular advantage of all of the shown embodiments of the invention which will be appreciated by those skilled in the art comes from the use of relatively cool core boxes. Since the core boxes are lossless, they will heat only by conduction from the curing core, and will not even approach the high temperatures at which the prior art metal boxes are maintained. Thus, cores having thinner sections can be formed, since the resin will not begin curing on contact with the core box, as in the prior art. :Instead curing will not begin until the blowing is completed and the microwave transmitter is energized.

It is to be understood that the above described arrangements are merely illustrative of the principles of the present invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention. Thus by way of example, and not of limitation, frequencies other than 2450 megacycles may be used and material other than those mentioned but having the same physical properties may be used. Also the curing chamber 26 could be bounded by several moving sides instead of being formed from two relatively moveable platens as shown and the core box could be formed in more than two pieces. Proper selection of the number of moving sides and core box pieces enables blowing of more complex shaped cores. Accordingly, it is understood that the present invention is limited only by the spirit and scope of the appended claims.

What is claimed is:

1. In combination, a core box made from a material which is transparent to microwave energy, a microwave heating cavity, means for supporting said core box in said cavity, means for blowing a core-forming material into said core box, and means for applying microwave energy to said cavity.

.2. In combination, a core box made from a material which is transparent to microwave energy, a microwave heating cavity, means for supporting said core box in said cavity, means for blowing a mixture of sand and a thermosetting resin into said core box, and means for applying microwave energy to said cavity.

3. In combination, a core box made from silicone rubber, a microwave heating cavity, means for supporting said core box in said cavity, means for blowing a mixture of sand and thermosetting resin into said core box, means for applying microwave energy to said cavity, and means for opening said cavity and core box to remove the cured core from within said core box.

4. In combination, a core box made from a material which is transparent to microwave energy, a ferrite lining on the interior surface of said core box, a microwave heating cavity, means for supporting said core box in said cavity, means for blowing a core-forming material into said core box, and means for applying microwave energy to said cavity.

5. In combination, a core box made from a material which is transparent to microwave energy, a ferrite lining on the interior surface of said core box, a microwave heating cavity, means for supporting said core box in said cavity, means for blowing a mixture of sand and thermosetting resin into said core box, and means for applying microwave energy to said cavity.

6. In combination, a core box made from a material which is transparent to microwave energy, a microwave heating cavity, means for supporting said core box in said cavity, means for blowing a mixture of sand, thermosetting resin and ferrite material into said core box, and means for applying microwave energy to said cavity.

7. A core blowing device which'comprises, in combination, a hopper for receiving a mixture of sand and thermosetting resin, a blowing chamber, valve means connecting said hopper and blowing chamber, a microwave cavity, a lossless core box adapted to be supported in said microwave cavity, at least one conductive tube from said blowing chamber to the interior of said core box, means for applying air under pressure to said blowing chamber to force said mixture through said tube into the interior of said core box, means for applying microwave energy to said cavity, and means for opening said cavity and core box to remove the cured core from within said core box.

8. A core blowing device which comprises, in combination, a hopper for receiving a mixture of sand and thermosetting resin, a blowing chamber, valve means connecting said hopper and blowing chamber, a microwave cavity bounded by a plurality of relatively moveable sections, a lossless core box formed from an equal plurality of mating sections adapted to be supported in said microwave cavity, at least one conductive tube from said blowing chamber to the interior of said core box, means for applying air under pressure to said blowing chamber to force said mixture through said tube into the interior of said core box, means for applying microwave energy to said cavity, and means for opening said cavity and core box to remove the cured core from within said core box.

9. A core blowing device which comprises in combination a hopper for receiving a mixture of sand and thermosetting resin, a blowing chamber, valve means connecting said hopper and blowing chamber, a microwave cavity bounded by a plurality of relatively moveable sections, a lossless core box formed from an equal plurality of mating sections adapted to be supported in said microwave cavity, each of said mating sections being attached to a respective one of said sections bounding said cavity, at least one conductive tube from said blowing chamber to the interior of said core box, means for applying air under pressure to said blowing chamber to force said mixture through said tube into the interior of said core box, means for applying microwave energy to said cavity, and means for separating said sections bounding said cavity, thereby opening said core box, to remove the cured core from within said core box.

10. The device of claim 9 in which the interior surface of said core is coated with a lining of ferrite material.

11. A core blowing device which comprises in combination a hopper for receiving a mixture of sand, thermosetting resin and ferrite material, a blowing chamber, valve means connecting said hopper and blowing chamber, a microwave cavity bounded by a plurality of relatively moveable sections, a lossless core box formed from an equal plurality of mating sections adapted to be supported in said microwave cavity, each of said mating sections being attached to a respective one of said sections of said cavity, at least one conductive tube from said blowing chamber to the interior of said core box, means for applying air under pressure to said blowing chamber to force said mixture through said tube into the interior of said core box, means for applying microwave energy to said cavity, and means for separating said sections bounding said cavity, thereby opening said core box, to remove the cured core from within said core box.

12. A process for making cores which comprises the steps of forming a core box from a material which is lossless to microwave energy, blowing a lossy coreforrning material into said core box, and applying microwave energy to said core box.

13. A process for making cores which comprises the steps of forming a core box from a material which is lossless to microwave energy, blowing a mixture of sand and thermosetting resin into said core box, and applying microwave energy to said core box.

14. A process for making cores which comprises the steps of forming a core box from a material which is lossless to microwave energy, blowing a mixture of sand and thermosetting resin into said core box, applying microwave energy to said core box, and removing the cured core from said core box.

15. A process for making cores which comprises the steps of forming a core box from a material which is lossless to microwave energy, coating the interior surface of said core box with a lining of ferrite material, blowing a lossy core-forming material into said core box, and applying microwave energy to said core box.

16. A process for making cores which comprises the steps of forming a core box from a material which is los'slss to microwave energy, coating the interior surface of said core box with a lining of ferrite material, blowing a mixture of sand and thermosetting resin into said core box, and applying microwave energy to said core box.

References Cited UNITED STATES PATENTS 3,259,947 7/1966 Knight 164-43 Hoke 16449 McAvoy et a1 21910.55

Luebkeman 164119 X Jeppson 26422 X Shallenberger et al. 164-165 Bodine 26423 FOREIGN PATENTS Germany.

I. SPENCER OVERHOLSER, Primary Examiner.

EUGENE MAR, Assistant Examiner.

U.S. Cl. X.R.