US3501320A

US3501320A - Die casting core

Info

Publication number: US3501320A
Application number: US684257A
Authority: US
Inventors: Robert J Pietryka; Raymond S Amala
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1967-11-20
Filing date: 1967-11-20
Publication date: 1970-03-17
Anticipated expiration: 1987-03-17

Description

March 17, 1970 R. J. PIETRYKA ETAL 3,501,320

DIE CASTING CORE Filed Nov. 20, 19s? MIXING CORE MATERIAL WITH BINDER FORMING THE CORE SHAPE CURING THE CORE PRE- FIRING THE com:

IMMERSING THE CORE IN SALT BATH I NVE NTORS United States Patent 3,501,320 DIE CASTING CORE Robert J. Pietryka, Warren, and Raymond S. Amala, Oak Park, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Nov. 20, 1967, Ser. No. 684,257 Int. Cl. B28b 7/34; B22d 17/20 US. Cl. 106-3827 6 Claims ABSTRACT OF THE DISCLOSURE A leachable core body suitable for use in high pressure metal casting operations and a method of preparing the same is disclosed. The outer surface portion of a sand core is impregnated with a water soluble inorganic sealing salt having a high melting point such as a sodium chloride-potassium chloride mixture. The sealing salt impregnated outer sand core layer prevents molten metal from penetrating the core body during high pressure metal casting. This sealing salt mixture has a melting point higher than the temperature of the metal during the'metal casting operation. The core is bonded with a water soluble inorganic binder such as sodium silicate which has a melting point higher than the melting point of the sealing salt.

This invention relates to die casting cores, and more particularly to die casting cores adapted for use in high pressure metal casting operations and including a method of forming such cores.

Conventional sand cores used in gravity cast molds are not suitable for pressure die casting of metals because the surface of such cores is penetrated by the metal under the high pressures imposed in the process. Attempts to reduce the metal penetration by the introduction of fine material such as silica powder into the core mix has not been satisfactory since it increases the required binder content thereby resulting in a core having reduced solubility characteristics which is ditficult to remove from the casting.

It is a primary object of this invention to provide a leachable core adapted to withstand the pressure of the metal in high pressure metal casting operations. It is still another object of this invention to provide a method of forming a high strength, pressure resistant core which can be readily leached.

These and other objects are accomplished by a sand core having a water soluble inorganic binder dispersed throughout and the outer surface portion thereof impregnated with a water soluble inorganic sealing salt. The melting point of the binder is higher than the melting point of the sealing salt. The sealing salt has a melting point higher than the temperature of the metal in the metal casting operation. A specific example is a sand core bonded with 4% by weight of the binder sodium silicate. The outer layer of the sand core is sealed to a depth of about 4 inch with a sodium chloride-potassium chloride sealing salt mixture.

The core is formed by mixing the core material, such as sand, with a water soluble inorganic binder, for example, sodium silicate. The resultant mixture is formed into a core of the desired shape by molding. The core is then cured in a microwave oven or hot cured by heating in an oven. The cured core is prefired in an oven ice to remove residual binder water. The prefired core is then immersed in an inorganic salt bath, such as a molten sodium chloride-potassium chloride mixture. The outer surface portion of the sand core absorbs the molten salt thereby sealing the core. The core is then removed from the bath and cooled to yield a strong core body having an outer salt impregnated surface layer resistant to metal penetration in metal casting operations. The resultant core body can be readily leached with hot water.

Other objects and advantages of this invention will be apparent from the following detailed description, reference being made to the accompanying drawing wherein a flow diagram depicts the subject process.

The expendable cores are made with readily available foundry sands such as the Muskegeon Lake sand and the Hemlock bank sand. The Muskegeon Lake sand contains less than 0.2% clay and has a particle size that will pass through a 50 mesh US. Standard screen sieve. It has an American Foundry Society designation of AFS 46-56 particle size. The Hemlock bank sand contains 0.2 to 0.4% clay. This sand passes through a 55 mesh screen and has an AFS rating of 4665. Although sand is the preferred core material, other ceramic powdered materials such as alumina and the like may be used.

The foundry sand is mixed with a water soluble inorganic binder. The primary purpose of the inorganic binder is to afford sufiicient cohesive strength to sustain the core body intact while the core body is immersed subsequently in a molten sealing salt bath as will be hereinafter fully described. In order to hold the core body intact while immersed in the molten sealing salt bath, the binder should have, preferably, a melting point higher than the temperature of the molten sealing salt bath. The inorganic binder should have a high solubility in water so that it can be readily removed by conventional water leaching techniques. The binder should not dissolve appreciably in or react with the sealing salt. Sodium silicates, which have a melting point range of 1700-1900" F., are appreciably higher than the temperature of the molten salt bath and are the preferred binders. The concentration range of the sodium silicate is from about 3 to 10 weight percent with the preferred concentration being 3 to 5 weight percent. Cured Muskegeon Lake sand core bodies containing 3 weight percent and .5 weight percent sodium silicate had a tensile strength of 315 to 330 and 345 to 366 p.s.i. respectively. Cured Hemlock bank sand core bodies containing 3 weight percent sodium silicate had a tensile strength of 255 to 285 p.s.i. This strength was sufficient to hold the core together during the subsequent immersion step in the molten sealing salt. Binder concentrations at the low end of the 3 to 10 weight percent range yield cores which are more soluble whereas high binder concentrations yield cores having higher strength. The concentration of the binder is selected depending upon the degree of solubility and the strength desired in the core body. Other suitable water soluble inorganic binders which may be used include potassium silicate and lithium silicate. These alkali metal silicates have a melting point range (17901900 F.) higher than the temperature of the molten sealing salt bath, they do not react with the sealing salt and they provide sufiicient cohesive strength to the core body.

The water soluble binders are mixed with the sand either in the dry condition followed by the addition of water, or by dissolving the binder in water prior to the addition of the sand. The binder-sand mixture is placed in a core box and formed into cores of the desired shape by ramming a rod with a pressure of about 20 p.s.i. The core may be formed by conventional blow investing techniques which are widely used.

After the core shape is formed, the core is subjected to a curing step. The core may be cured by any one of several methods. One method which is preferred because of its speed involves exposing the core for 2 minutes in a microwave oven. Another widely used curing method is the hot cure procedure which involves baking the core in an oven having a temperature of about 350 F. for a period of 20 minutes. Another curing method is the conventional carbon dioxide gas curing procedure in which carbon dioxide is passed through the core body while the core is still in the core box. The curing step yields a core body having sufiicient cohesive strength to be handled in normal operations.

After the core is cured, the core is subjected to a prefiiring step in order to remove the residual binder water. One method of removing the residual water is by baking the core in an oven at a temperature of from about 400 to 1300 F. High prefiring temperatures with this temperature range remove the volatiles more rapidly and more completely than the low temperatures within this range but have the disadvantage of causing a considerable reduction in core mechanical strength. The preferred temperature range is about 400 to 500 F. for a period of about 1 hour. At a temperature of 400 to 500 F. heating 1 inch thick cores for a period of 1 hour adequately removes the residual water and no serious loss of mechanical strength is encountered.

After the core has been cured and been subjected to a prefiring treatment it is immersed in a molten inorganic sealing salt. The water soluble sealing salt has, preferably, a melting point higher than the die casting temperature range for the particular metal alloy which is cast. For example, the casting temperature of certain aluminum alloys is in the range of 1200 to 1250 and for this temperature range a potassium chloride-sodium chloride mixture having a melting point of 1220 to 1280" F. is satisfactory. Salts having melting points lower than the die casting temperature range may be used as sealants if the heat capacity and heat of fusion of the salts are great enough to prevent melting during the limited time of contact between the molten metal and the core surfaces. The preferred sealing salts, however, have a melting point higher than the metal die casting temperature range. For the die casting of aluminum alloys having a temperature casting range of 1200 to 1250 F. a sealing salt composition formed by mixing 0 parts by weight sodium chloride and 50 parts by weight potassium chloride is the preferred sealing salt. The sodium chloride-potassium chloride sealing salt mixture has a melting point in the range of 1220 to 1280 F. and possesses a sufficiently low viscosity at a temperature of 1350 to 1400 F. to allow the absorption of the sealing salt into the interstices of the outer core surface. The molten sealing salt is absorbed through the pores of the sand cores to a depth of about A; of an inch to /2 of an inch from the outer core surface during the immersion in the sealing salt bath. It has been determined that a minimum sealing salt impregnation depth of /s of an inch is required to effectively seal the core from penetration by the metal during the die casting operation. The preferred depth is /8 of an inch to A of an inch. Cores made from Muskegon Lake sand are successfully sealed in a sodium chloride-potassium chloride molten salt bath by immersing the cores for 2 minutes at a bath temperature of 1400 F. Cores made from Hemlock bank sand which contain a small percentage of clay require a longer immersion time, in the range of 3 /2 to 4 minutes and at 1400 F. Calcium chloride, which has a melting point of about 1422 F., may be used as the sealing salt. However, since calcium chloride is hygroscopic, the resultant core has a tendency to absorb water thereby requiring that cores sealed with calcium chloride be stored under anhydrous conditions. The sealing salts should be soluble in water and preferably have a solubility of at least 30 grams per milliliters of cold water. The following table lists a group of salts suitable for sealing the core used in zinc die casting and a second group of salts suitable for either aluminum or zinc die casting.

TABLEFOR ZINC DIE CASTING Solubility Melting point, (glIlS./1OO ml.

Salt F. cold water) Cadmium bromide. 1,052 57 Cadmium chloride 1, 054 Calcium iodine. 1, 067 66 Calcium nitrate- 1, 042 102 Cupric bromide. 928 V.S. Cupric chloride... 928 71 Ferrous chloride. 1, 237 64 Manganous chloride 1, 102 62 Potassium lead chloride. 915 Soluble Sodium iodine 1, 202 159 Strontium bromide.. 1, 190 85 Strontium nitrate 1, 058 40 V.S.-Very soluble.

FOR ALUMINUM O R ZINC CASTING Solubility Melting point, (gms./100 ml.

Salt F. cold water) Barium bromide 1, 557 98 Barium iodide 1, 364 Calcium bromide.- 1, 410 125 Calcium chl0ride.- 1, 422 60 Magnesium bromide l, 292 102 Magnesium chloride 1, 307 54 Manganous sulfate... 1, 292 52 Potassium bromide 1, 3 54 Potassium calcium chloride... 1, 390 Soluble Potassium carbonate 1, 637 112 Potassium chloride. 1, 430 35 Potassium iodide. 1, 332 128 Potassium molybdate 1, 684 Potassium sulfide 1, 544 Soluble Potassium tungstate 1, G92 52 Sodium bromide- 1, 392 80 Sodium chloride.. 1, 472 36 Sodium molybdate. 1, 270 Soluble Sodium sulfate 1, 622 Soluble Sodium tungstate- 1, 290 41 Strontium chloride 1, 602 44 Mixtures of these salts may also be used.

The salt seal cores described above retain their shape and resist metal penetration during the high pressure die casting of aluminum alloys. The core is removed from the aluminum casting by immersing the castings in warm water which dissolves the core.

The following examples show different embodiments of this invention which yield a Water soluble core suitable for use in high pressure metal casting.

EXAMPLE N0. 1

Fifteen pounds of Muskegon Lake foundry sand was mixed with 68 grams of water for one minute. Then 340 grams of sodium silicate was mixed with the wet sand for four minutes to form a uniform mixture containing 5 weight percent sodium silicate. The sodium silicate used contained 14.7% Na O and 29.4% .SiO The sand was placed in a core box and a pressure of about 20 p.s.i. was applied with a ramming rod by hand to compact the core. The core was cured (hardened) by baking in a microwave oven for a period of about 2 minutes. The tensile strength of the cured core was 345 to 366 p.s.i. as measured by the Dietert core sand tensile tester. After curing, the core would collapse into loose sand when when immersed for two to five minutes in warm Water having a temperature 120150 F. The cured core Was then subjected to a prefiring treatment by heating in an oven having a temperature of about 400-500 F. for a period of about one hour. The core was immersed in a sealing salt bath having a. temperature of 1400 F. and consisting essentially of 50 parts by weight potassium chloride and 50 parts by Weight sodium chloride. The core was immersed in the bath for. two to three minutes during which time the sealing salt penetrated the outer surface portion of the core to a depth of about inch. The core was then removed from the bath and allowed to cool. The resultant core had a tensile strength of 50-100 p.s.i. The salt seal core softened up sufficiently to break up after one half to one hour immersion in warm water having a temperature of 12 150 F. The core was used in die casting an aluminum alloy part in a high pressure die casting operation where the temperature of the aluminum metal was 1220 F. and the pressure of the die casting operation was 150 p.s.i. During this metal casting operation the core remained intact and no aluminum metal penetrated the sealed core.

of, said binder having a melting point higher than said sealing salt, said sealing salt having a melting point higher than the temperature of said metal in said metal casting operation.

2. A core as described in claim 1 wherein said inorganic binder is sodium silicate.

3. A core as described in claim 1 wherein said sealing salt is a mixture containing equal weights of potassium chloride and sodium chloride.

4. A core as described in claim 1 wherein said sealer The following table lists Examples 1 through in is calcium chloride. which different sand core compositions were sealed with 5. A water leachable unitary sand core for use in a molten sodium chloride-potassium chloride mixtures or high pressure aluminum metal casting operation compriswith molten calcium chloride. ing a sand core formed from a foundry sand composi- Composition,wt. percent Prefiring Salt bath Cured core Sealed core Example Sodium baked tensile Tex n Time, Temp., Immersion baked tensile No. Sand silicate strength,p.s.1. min. Composition F. time, min. strength,p.s.1.

Muskegeonnu 3 315-330 400-500 50 NaClKCl 1, 400 2-3 50-100 Hemlock- 3 255-285 400500 50 1, 400 4-5 50-100 Muskegeon 5 345-366 400-500 60 1,400 2-3 50-100 "Mao 1 50-00 ..400-500 00 1, 400 2-3 5 d0 4 (320340) 1,300 30 C3012 1,500 2 50-100) Example No. 2 listed in the table containing 3% tion containing 3 to 10 weight percent sodium silicate, sodium silicate and Hemlock bank foundry sand rethe outer surface portion of said core being impregnated quired a longer immersion time in the molten sodium to a depth of at least Ms inch with a sealing salt mixture chloride-potassium chloride salt bath to obtain the decontaining equal weights of sodium chloride and posired salt penetration in the outer surface portion of tassium chloride. the core. Examples 1, 3 and 4 indicate that a concen- 6. A process for forming water leachable sand cores tration of 3-5 weight percent sodium silicate has the adapted to be used for high pressure metal casting operpreferred tensile strength for the cured core whereas 1% ations comprising the steps of mixing an inorganic binder silicate yields a core with insufiicient tensile strength. All taken from the group consisting of sodium silicate, poof the sealed cores described above softened sufficiently tassium silicate and lithium silicate with a foundry core to break up after immersion of one half to one hour in said to form a mixture containing 3 to 10 weight percent warm water. of said binder, forming said mixture into a core shape, While the invention has been described in terms of curing said core shape, prefiring said cured core shape at a specific examples, it is to be understood that the scope temperature between 400 F. and 1300 F. for at least of the invention is not limited thereto except as de- 2 hours to remove the residual binder water, and imfined in the following claims. mersing said prefired core in a molten sealing salt solu- What is claimed is: tion containing equal weights of sodium chloride and po- 1. A water leachable unitary sand core for use in a tassium chloride for a time suflicient for said sealing salt high pressure metal casting operation comprising a core to be drawn into the interstices of the outer surface mahaving 3 to 10 weight percent water soluble inorganic terial by capillary action to a depth of at least its inch. binder taken from the group of alkali metal silicates con- 45, sisting of sodium silicate, potassium silicate and lithium References Cited silicate distributed uniformly throughout, the outer Slilf- UNITED STATES PATENTS face portion of said core impregnated with a water so uble sealing salt taken from the group consisting of cad- I889O07 11/1932 Wallace 106*383 mium bromide, cadmium chloride, calcium iodide, cal- 2322667 6/1943 'seastone at 1O6' 38'3 XR cium nitrate, cupric bromide, cupric chloride, ferrous 3,113,360 12/1963 Nefi et 1175-2 XR hloride, manganous chloride, potassium lead chloride, 3,121,269 2/1964 Nefi 1O638-22 sodium iodide, strontium bromide, strontium nitrate, barium bromide, barium iodide, calcium bromide, cal- FOREIGN T F cium chloride, magnesium bromide, magnesium chloride, 170,677 10/1921 Great Brltallliinanganous sulfate, potassium bromide, potassium calcium chloride, potassium carbonate, potassium chloride, JULIUS F ROME Pnmary Exammer potassium iodide, potassium molybdate, potassium sul- LOREN B- HAYES, Assistant Examiner tte odiu bromide sodiu hl fide, potassium tungs a s m m c o US. Cl XR.

ride, sodium molybdate, sodium sulfate, sodium tungstate, strontium chloride and compatible mixtures there- (22 33 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 0 0 2 Dated March 17, 1970 Inventor(s) Robert J. Pietryka and Raymond S. Amala and that said Letters Patent are It is certified that error appears in the above-identified patent hereby corrected as shown below:

Colugn 2. 51m 59, "(179o-19oo 1a)" should read (1790 -1900 +F.) column 4, in Table For zinc Die Casting "iodine", each occurrence, should read iodide Column 5, line 7, "150 p.s.i." should read 1500 p.a.i. --1 line 32, after "1%" insert sodium SIGNED AND SEALED JUL 281970 Anest:

Edward M. Fletcher, 11%

11mm Gemissioner or Pat n