US3549736A - Process for forming sintered leachable objects of various shapes - Google Patents

Description

Dec. 22,-;1970

PARAFF I N PROCESS FOR FORMINGSINTERED LEACHABLE OBJECTS A IWAUGI-I-I 3,549,736

.OF VARIOUS SHAPES Filed Sept. 2. 1966 REFRACTORY MATERIAL MIXING DEFLOCCULANT MOLDING COOLING PACKING AND VIBRATING IN CRUCIBLE VAPORIZING PARAFFIN AN D DEFLOCCULANT SINTERING United States Patent flice 3,549,736 Patented Dec. 22, 1970 3,549,736 PROCESS FOR FORMING SINTERED LEACHABLE OBJECTS OF VARIOUS SHAPES Arthur Waugh, Brookline, Mass., assignor to Lexington Laboratories, Inc., Cambridge, Mass., a corporation of Massachusetts Filed Sept. 2, 1966, Ser. No. 576,851 Int. Cl. B29d 27/08; C04b 21/06, 35/04, 35/1] U.S. Cl. 26444 1 Claim ABSTRACT on THE DISCLOSURE A method of making leachable refractory casting cores suitable for use in investment casting. The practice of this method allows the production of cores having relatively intricate shapes with minimal amounts of shrinkage and distortion occurring during processing thereof.

This invention relates to sintered refractory casting cores and more particularly pertains to easily leachable cores and a method of making the same.

The characteristics of products which are investment casted depend in large measure upon the composition, permeability, dimensional stability or surface finish of the casting core used, and any casting defects are often traceable to the unsuitability of the commercial core for a specific job. Because of the high permeability of the heretofore available commercial cores, one major problem encountered by the investment caster has been pitting of the surface of the cast product by gas escaping from the per meable core, and, in extreme instances, the occurrence of gas holes in the interior of the cast product. Similarly, because of the dimensional instability of the heretofore available commercial cores, cast products have been found to be off-specification in dimension, apparently due to a core shrinkage occurring when the core is surrounded by the poured metal blowthrough, A prime factor in investcasting methods. To obtain precise dimensions, the castcore as such leachability affords a preferred method of separating the casting core and cast product.

Casting cores are conventionally produced by causing a fine powder of a refractory material to be consolidated into a coherent mass of putty-like consistency, pressing the material into a form or die to cause the material to be molded into a piece of approximately the desired finished shape, causing the piece to harden to a state where it can be ground or otherwise machined to its final configuration, and heating the green piece at a sufficiently high temperature to cause the grains of powder to fuse together. The last step, called firing, increases the density of the piece because of the reduction in volume occurring as the powder grains sinter or fuse together. In essence, the core is a precision refractory casting.

Due to wear of the die or form by the refractory powders and because of uncertain shrinkage of the green piece during the firing process, refractory cast cores of highly precise dimensions cannot be produced by conventional casting methods. To obtain precise dimensions, the casting produced by conventional methods is usually deliber ately made oversize and then ground to the desired size. Such grinding of the casting is not only expensive, but difficult, as refractory materials tend to be extremely hard and brittle. Furthermore, core shapes having sharp corners or portions of small cross-section cannot be made by conventional casting methods as strains, introduced into the green piece during molding, tend to cause cracks to appear when the casting is fired into its final form. To produce by conventional casting methods cast articles having irregular contours, under-cuts, internal holes, sharp corners or severe bends, expensive and inconvenient machining operations must be performed upon the green piece.

Accordingly, it is a primary object of the present invention to provide a process for making leachable refractory casting cores suitable for use in investment casting.

One aspect of the present invention resides in providing leachable casting cores composed of mixtures of zircon and either silica or frits. Another aspect of the present invention resides in a process for forming leachable casting cores which, without machining of either the green or finished pieces, permits intricately shaped sintered articles of highly precise dimensions to be produced,

In the novel process, the refractory material, in the form of fine powder or granular material, is mixed with a vaporizable molten binder, for example a Wax such as paraffin, and a miscible vaporizable liquid defiocculant or suspending agent such as oleic acid to form a slurry. In order to maintain the mixture as a slurry in which the powdered refractory material is dispersed by the deflocculant so that the grains are distributed homogeneously throughout the binder, the mixture is kept at a temperature where the binder is liquid. The slurry is then caused to assume a predetermined configuration, for example by introducing it into a mold or die. Afterwards, the binder is permitted to solidify by cooling so that a green piece of uniform density is formed. The slurry, being a liquid suspension, conforms closely to the configuration of the mold, so that the green piece can have more intricate shapes and sharper corners and bends then are possible with a process in which the slurry is merely poured into the mold.

Absent the binder, the green piece would have no strength and could not be handled. It is necessary, therefore, prior to driving off the binder, to put the green piece in an inert refractory packing, SlJlCh as alumina powder, which does not react with the casting, which is composed of grains of such size as to give a desirable surface finish to the casting, and which has a sintering temperature above that of the material of the casting.

Accordingly, after removal from the mold, the green piece is packed in alumina and then heated slowly to a temperature at which the binder and defiocculant vaporize and are driven off. A slow temperature rise is employed to prevent the binder from vaporizing so rapidly as to weaken or rupture the casting. After a length of time sufficient to drive off all the binder, the temperature is raised to a higher level at which sintering of the molded refractory material occurs While the casting remains packed in the alumina powder. Such sintering alfords sufficient strength to the casting to permit it to be removed from the alumina packing and dusted to re move any adhering alumina. The sintered casting may be used as is or further fired; that is, heated to a firing temperature and there maintained for a period of time determined by the desired density of the finished article.

An alternative procedure may be followed in order to facilitate removal of adhering alumina packing grains from the casting, and also in order to minimize the casting distortion caused by shrinkage of the green piece. In this alternative procedure, the steps are identical with those indicated above for the main procedure until after the binder has been removed. Upon removal of the binder, the temperature is raised only to a level where partial sintering or presintering occurs, this partial sintering affording sufficient strength to the casting to permit it to be removed from the alumina packing. The presintered casting is then removed from the packing, dusted to remove any adhering packing grains, and then heated to sintering or firing temperature, apart from the packing material and preferably in an inert or reducing atmosphere. The presintered casting is maintained at this firing temperature for a period of time determined by the desired density of the finished article. In view of the relatively lower temperatures used prior to removal of the alumina packing from the presintered casting, this removal is relatively easy to accomplish. It will also be noted that, according to this procedure, the packing material need only have a presintering temperature higher than that of the material of the casing. Furthermore, as only limited shrinkage of the green piece had been caused at the presintering temperature, the distorting effect of the incompressible packing material about which the green piece attempts to shrink is minimized. Accordingly this alternative procedure is especially useful when substantial shrinkage is expected to occur during firing or the green piece is of complex shape.

Cores and other castings having intricate shapes may be made by the novel process here described and irregular contours, undercuts, internal bores, and threads can be produced. Virtually any shape and size can be reproduced, depending principally upon the skill of the mold maker. Further, because the powder is homogeneously dispersed in the binder the resultant green casting is of uniform density so that its shrinkage from the green state to the size of the finished casting does not result in stresses which crack the casting. Furthermore, the final size of the finished or fired product can be known beforehand with a large degree of certainty as the shrinkage of the sintered casting is, in many instances, less than one percent of its volume in the green state.

The invention will now be described in detail with general reference to the drawing, the single figure of which constitutes a flow chart displaying in sequence the significant steps of the process.

More specifically, and with general reference to the drawing throughout, the binder is a material that is a solid at room temperature and is characterized by a low melting point, a low viscosity when molten, and a vaporization point below the presintering temperature of the refractory powder used. The binder is preferably chosen from the waxes or paraflins, and wherever the term paraffin is employed herein, a binder having the foregoing characteristics is intended. The deflocculant is a dispersing agent which is miscible with the binder and which acts to provide complete and uniform wetting of the powder grains. It is preferred that the deflocculant, like the binder, have a vaporization point below the presintering temperature of the refractory powder used.

The amount of paraflin or other binder used in the slurry is generally such as to fill the interstices of the powdered refractory material, the volume ratio ef refractory powder to binder being of major importance. The ideal ratio is one where, when the particles of powder are in contact with each other, the binder is just sufficient to occupy the interstitial spaces. If the ratio of powder to binder is too low, then the powder particles will not be in contact so that movement of the powder grains will occur when the binder is driven off, thereby causing the green piece to distort and lose its dimensions. If the ratio of powder to binder is too high, not enough binder is present to completely fill the interstices of the powder so that, when the slurry within the mold solidifies, the green piece will not be of uniform density. Furthermore, when the ratio is too high, the slurry tends to be viscous and, therefore, may not easily conform to the shape of the mold.

The relative quantities of the slurry components to be used are determined first by measuring the volume and weight of the predetermined quantity of refractory powder. An amount of dispersing agent is used which has a weight equal to about 1 to 1.5% of the powders weight. An amount of binder is used which has a volume in its solid form equal to about the volume of the interstices of the powdered refractory material. When in the molten state, the volume of binder will be sufficient to permit adequate dispersal of the grains so that the viscosity of the mixture is such that the slurry can be poured freely. It has been found that if the volume of binder is less than this proportion, the viscosity of the slurry at the melting point of the paraffin is too high for easy flow; and if the volume of binder is significantly higher than this proportion, large shrinkage occurs upon driving off the binder.

The temperature of the slurry is kept between the melting and vaporization points of the binder, generally between 250 and 300 F., and the slurry is constantly agitated so as to maintain turbulence at least for a time sufficient for the slurry to outgas. A period of thirty minutes was generally found to be adequate for outgassing the slurry. The slurry temperature should not be allowed to exceed temperature at which vaporization of the binder begins (about 300 F. for paraifin). Following outgassing of the slurry, it may be permitted to solidify in ingots or bars until needed for introduction into the mold.

Prior to its introduction into the mold, the ingot material is heated with agitation until it again becomes a slurry. The slurry is then introduced into a mold which has been coated with silicone grease or some other parting agent. Once in the mold, the slurry flOWs and assumes the mold configuration, after which it is permitted to cool and solidify.

After removal from the mold, the solid green casting is trimmed of any parting lines and placed in a crucible or moly boat. A fine grain refractory packing material, such as 325 mesh alumina packing sand, is poured into the crucible about the sample, and the crucible is then vibrated for about 10 seconds to cause the settling of the packing sand. A second packing sand layer is next introduced, vibrated, and the procedure repeated as necessary. Afterwards, the crucible containing the packed sample is slowly heated to a temperature sufficient to drive off substantially all of the binder and defiocculant (about 400 C. for paralfin and oleic acid). It should be noted that both the binder and the deflocculant vaporize without leaving any appreciable residue, the packing being vented to permit the vapors to escape. The temperature of the package is raised slowly so that rapid vaporization does not occur, a rate of temperature increase of 1.5 C. per minute generally being found satisfactory.

Once the binder and deflocculant are driven off, the temperature is raised (for example, from 400 C. to 1320 C. in about eight hours). After firing, the package is allowed to cool to room temperature at a natural rate (about four hours), after which the fired casting is removed from the packing sand and scoured with a wire wheel to remove any adhering packing sand. Minor difiiculties may on occasion he encountered in removing tightly adherent packing sand, and in particular instances the samples may become somewhat distorted in configuration as a result of their having been forced to shrink during the firing step about the incompressible packing sand. In the alternative procedure, after all the binder and defiocculant are driven off, the temperature is raised to the presintering temperature and then maintained at the presintering temperature for a period of time sufficient to impart enough rigidity to the casting to permit it to be handled (generally about three to four hours at 1000 C.). Subsequent to presintering, the casting is removed from the packing sand and, after being cleaned or dusted to remove any adhering packing sand particles, is fired in an oxidizing atmosphere at the sintering temperature for a time determined by the desired density of the finished casting, for example, at 1320" C. for four hours. Little shrinkage, and hence little distortion, occurs between the molding and presintering temperatures, and the distortion resulting from the firing step is generally less than that which would have resulted if the shrinking green piece had been supported by an incompressible alumina packing sand.

EXAMPLE The general procedure described above was utilized in the preparation and testing of the sintered ceramic products described in this example. A predetermined amount 6 bar furnace with programmed temperature control. The temperature of the green piece was slowly increased from room temperature to 400 C. (at about a 50 C. increase per hour) in order to allow the wax to burn out gradually without disturbing the solids. After the wax was substanof a binder such as wax was melted in a heated container. 5 To assist the binder in the later formation of a ceramiccompletely burned out, the perature Was quickly a binder slurry, a suitable quantity of a deflocculant or diselevated to smtenng temperature about a persing agent, such as oleic acid, was mixed into the 2 b i remwtl from the packmg matebinder. Calculated amounts of granular solid materials f j place. r g i as to remove any (for example, ceramic materials such as zircon and mixati 2 g a f i t d f tures of zircon with silica or frits) were then added in 6 sm are places. us Olme .Y en tes e small increments to the molten binder base, with exten- Slinnkage urface fimsh permiablhty and leacimblhty' sive mixing to insure complete suspension and outgasing Lmear Shrinkage. Y determmed averaguig the of the slurry. After addition of all the solids, the product measured Slze variatlons between the green "i Smiered mixture was again thoroughly mixed and the resultant 15 pieces. Surface finish was graded. on the basis of visual liquid slurry injected through a small mold orifice into a g compargtlvehpilmeablhty niasurements mold of predetermined configuration at a selected temwere ma 6 y metsurmg t e lme.reqmre or a Water perature and'pressure. In some cases a hand pump was 9 to 92 through the smtereg i rela used to inject the slurry into the mold; in others, a Small tive permeability being measured as the tlme requlred for wax injection pump with a modified nozzle and Valve the water column to fall between two bench marks so assembly the Passages of the nozzle being enlarged and a that longer time 1nd1cated low permeability. Permeab lity solenoid-operated air valve being installed to allow full was also evaluated by metal around mlectlon air pressure to be either applied to the Wax or vented to molded cores and deten.mmng the presence or absence of the atmosphere to eliminate excessive nozzle leakage. blow 1 gas $3 d 1 t d b After cooling of the injected mixture, the green piece of Leac 1 my e Smtere. plecesd g gf predetermined configuration was checked for complete die tests; (1) W at er a 9 lmmrse m ea f eac fill, straightness, the presence of air holes, cracks, and the mg solutlon completely dssolved m 15 mmutes and (2) like whether a core was completely leached from a metal cast- The green piece was then placed in a layer of packing ing in 15 minutes. It will be noted that in test (2) the total material, such as fine grains of aluminum oxide, located urface area of the Core exposed to the leaFhmg soluilon on the bottom of a crucible. The crucible was vibrated to 15 redmfed by the P e of h metal i A sodium cause settling of the grains, the green piece covered with llydroxlde-based leachlng sqllltlon at 1200 was P additional grains, and the crucible vibrated again. Addi- In all cases; namely Scaling Bath commerclany tional layers of packing material were added, as required, available from Investment Supply House. depending on the amount of settling and one the size of The significant process conditions and the test propthe crucible and the green piece. The green piece was then erties of the resultant casting cores are summarized in the fired for a predetermined firing cycle in a resistance glotable below.

TABLE Binder 0212220; Injectionmolding v wt. Pres- Bindor percent percent Temp., sure Batch wax of solids of solids Solids wt. percent F. p.s.i. Properties Die release 16---" Esso 38 1.0 100% Zircon 220 90 Good SiglEly t 27 do. 21.2 1.5 20% $102 Zircon 250 8 Excellent... 8 10 152 5.... 30 Pamfiin 30 1. 5 2.5% Frit 97.5% Zircorn- 250 8 Good. Do 34 do 32 1. 5 7.5% Frit 92.5% Zi1'con 250 80 Excellent.. 22 do 32.5 1.5 10% Frit Zircon 250 80 Do Do.

1 Oleic acid. 1 In 320 mesh alumina (Norton 38 Alundum X) from Norton Company. 3 Esso Mikrovan 1600 wax from Humble Oil & Refining Company. 4 325 Mesh amorphous silica from Glasrock Products. 5 P-786 Fritz from Glidden Company. Samples soft.

Average variation 1n Removal Smtering sinteretl Perme- Leachabllity of temperature dunensions, ability, Surface packing C. 111. sec. finish Test (1) Test (2) grains Batch:

16 1,300 18 Good Pass Fail-.."

22 1,000 .110 .do Pass Easy.

1,300 24 Excellent do do Do.

a4 1,000 Very good .410 .-d0. Easy 1, 300 533,5 3, .-do .dc ..do. Do.

It was found that by increasing the binder concentration in steps of 2% there finally appeared a transition from the no injection state to the easy injection state. For example, the slurry of batch 34 ingredients at a 30% binder concentration would not inject to give anything approximating complete die-fill at 80 p.s.i. and 250 F., but the same ingredients at a 32% binder concentration easily injected to give complete die-fill. The transition occurred at a point when the total binder volume, in solid form, completely filled the interstices of the granular material. Further additions of binder permitted injection of the slurry at somewhat lesser pressures, but also increased shrinkage and caused distortion of the piece during removal.

The different binder percentages used in the several compositions reflect the different shapes and sizes of the several ceramic particle types used. It will be noted, however, that all of the zircon mixture slurries in the example were injectable at pressures not exceeding 80 p.s.i. and temperatures not exceeding 250 F.

Optimum suspension of the granular material in the slurry was achieved with monodisperse systems of granular material of 200 mesh or less, although mixed systems of course and fine grains also dispersed well, especially if the finer grains were first added to the wax base. Parafiin and oleic acid were found to constitute a preferred binder and deflocculant combination with about 1.5 parts of oleic acid per 100 parts by weight of granular material being the optimum deflocculant ratio.

The zircon-frit green pieces matured or achieved sintered strengths at considerably lower temperatures (about 1000 C.) than zircon itself or zircon-silicon mixtures (about 1300 C.) because of the formation of interparticulate bonds caused by the molten frits. As a result of the lower sintering temperatures required for these zircon-frit green pieces, subsequent removal of the packing grain was greatly facilitated without the usual loss in the strength of the sintered product resulting from the use of a lower sintering temperature to facilitate removal of the packing.

Although injection molded cores of 100% zircon were leached completely in a few minutes, the presence of metal castings around these cores apparently so reduced the areas exposed to the leaching solution that leaching was not complete even after an hour. On the other hand, injection molded cores of a zircon mixture having about 20% by weight of silica or from 2.5 to by weight of frits were completely leached from metal castings in minutes.

Injection molded-cores sintered in 60, 100 and 200 mesh grain alumina packing were found to have very poor surface finishes, the best surface finishes being obtained by use of a finer 320 mesh grain packing. Castings made from injection molded cores sintered in finer grain packing were generally found to produce metal castings having satisfactory surface finishes.

Batch 24 featured a superior surface finish and easy removability from the packing grains whether sintered at 1000 C. or 1300 C.

Batch 22 displayed excellent dimensional stability as well as high strengths, good surface finish, easy removability from the packing grains, and a low sintering temperature (1000 C.). Batches 25, 26 and 27 also displayed superior properties.

While the above processes and products were presently deemed the preferred embodiments of the present invention, it will be obvious to those skilled in the art that there are other changes and modifications which can be made therein without departing from the inventive concept. Accordingly, the appended claims are intended to cover all such changes and modifications that fall within the true spirit and scope of the invention therein defined.

What is claimed is:

1. A process for forming sintered objects of various shapes and configurations which are readily leachable in a basic solution, said process comprising the steps of:

mixing a molten paraffin binder, a liquid deflocculant and a granular material comprising a mixture of non-metallic refractory material and non-refractory frit together to form a flowable slurry; said binder and said defiocculant being vaporizable at a temperature below the sintering temperature of said frit, the volume of said binder when in a solid state being substantially equal to the volume of the interstices of said granular material and when molten being increased to thereby separate the grains of said granular material; said frit being sinterable at a temperature substantially below the sintering temperature of said refractory material; said granular material comprising a mixture of 97.5% by weight zircon and 10-2.5% by weight frit, respectively;

maintaining said slurry at a temperature sufiicient to retain said binder in liquid form;

introducing said slurry in a flowable state into a mold;

cooling said slurry to cause it to solidify, thereby forming a solid object having substantially the configuration of said mold; packing said object in a crucible containing an inert refractory powder having a particle size of substantially 320 mesh and a sintering temperature higher than that of said granular material, said object being completely surrounded by said refractory powder;

compressing said refractory powder closely about said object; slowly heating said object while packed in said refractory powder to a first temperature sufiicient to vaporize and drive off said binder and said defiocculant while leaving said granular material;

thereafter further heating said object while packed in said refractory powder at a relatively rapid rate to a second temperature for a sufficient period of time so that said object is sintered together by formation of interparticulate bonds between the grains of said granular material and said frit;

cooling said object; and

removing said sintered object from said refractory packing powder.

References Cited UNITED STATES PATENTS 1,394,034 10/1921 McVickar 264-328X 1,458,376 6/1923 Anderson 26444 2,422,809 6/ 1947 Stupakotf et al 26463X 2,515,790 7/1950 Navias 264-63X 2,593,507 4/1952 Wainer 264-44 2,889,229 6/1959 Steinhotf 106-57 3,051,566 8/1962 Schwartz 26463X 3,351,688 11/1967 Kingery et al 10657X FOREIGN PATENTS 137,807 1961 U.S.S.R 264--63 PHILIP E. ANDERSON, Primary Examiner US. Cl. X.R.

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