Self-lubricating shell molds
US 2923989 A
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Description (OCR text may contain errors)
United States Patent 2, 98 SELF-LUBRICATING SHELL MOLDS Robert F. Thomson, Grosse Pointe Woods, Mich., assignor to General Motors Corporation, Detroit, Micln, a corporation of Delaware Nq lra a. Appl c t o p mb r 1958 lfial .N 0, 55.
12 Claims. (61.22-193) Th in ntion lates t9 ha l me in n a l;- la'rly to a shell mold and method of forming the same fromajself-lubricating molding mixture which prevents the mold from adhering to the' hot patterns on which it isformed. The present patent application is a continuamar a-Part of. p i ation. Serial 9- 542,792, which wa l ca October, .1 w a n n As is now well known, the shell molding process involves the formation and use of thin-walled dispensable molds and cores composed of sand and resinousbinders. This process can be used to produce precision castings in a w e v ie 'Qfm t s- In shell molding, the advantages obtained by the use ofthin-walled sand-resin molds and cores in casting metal parts to close dimensional tolerances result principally from the fact that these castings have very smooth as-cast surfaces. Accordingly, it is imperative in the formation of the molds that there is, a clean separation of the sand 11 th ur asss t e P t s a In he P some d fi'r qulty had. been as uatsted n tr p in the mold, item the pattern because of the inadequacy of the parting material used, thereby frequently partially breaking or tearing the mold edges anddestroying mold details.
Mold release agents, usually in liquid form, heretofore generally have been applied directly to the hot pattern surfaces. However, the spraying of moldrelease agents or lubricating materials on the surfaces of the pattern results in an uneven distribution of the lubricating material: on the pattern because the recesses formed therein tend to accumulate this material, while the vertical surfaces tend to be inadequately lubricated. This uneven distributiomin turn, results in an excessive lubricant build-up on the patterns, thereby interfering with the precise and accurate formation of molds. These CQIL: ditions are aggravated. by the factthat such mold release agents generally must be applied to the pattern surfaces each time the pattern is used, the lubricating prcperlies of these agents not being satisfactorily retained on the pattern for more than one molding cycle. Hence, spraying a mold release agent on a pattern also is a relatively expensive procedure, particularly since a considerable proportion of the sprayed lubricant is usually lost to the atmosphere. The problems involved in spraying a lubria cantare especially troublesome with automatic equipment where the spray heads are located in fixed positions, thus. increasing the difficulty of uniformly and adequately covering all pattern surfaces.
Accordingly a principal object of the present inventionis to provide a shell molding mixture which does not require the application of an external mold release agent tn the pattern to which the mixture is applied. A further object of this invention is to form a self-lubricating shell mold from a molding mixture having high green strength andcontaining a mold release agent which prevents the molding mix from too strongly adhering to the pattern. A still further object of the invention is to provide a methodzof forming shell molds which elimi' hates the necessity of spraying a lubricant on the pattern ii teate. Feb- 2. .26.9-
2 and at the same time provides a more even distribution of the lubricating material on thepatternisurfaces, t by substantially reducing the amountof residue bu' These and other objects and advantages are attained n c r n t m a ntisa. b th us o am l a mix ich, ontains a sm ll butef setiy ams of high me a in olu is ia a r r nriate .qr ol nt Suc a me n m xtur inT h ssr be wax is preferably used as the mold nelease ag si y a d n x ens P epared; in e. he-wazsfs a rea y p in Warm th hthala a Th at, this internal lubricant the molding per its ac; curate reproduction of pattern details and e11 the @aa i n p e iable a u t of he. n 1. ki t h Pa e du t e St p n qpst ti u th rm r t e r e s f. h t d es 2 m rially reduce the strength of the formed shell mo 4 while the use of solvents hereinafter describedappears. to, actually increase both the green strength of the mold mix and the strength of the cured mold. A mold mixture of this type functions equally well when app} to cast iron, aluminum and other pattern andeorehox materials.
Previously it has been proposed to use carnauba. wa t as an internal lubricant in a shell molding Such a, molding mixture and process for using the same, are disclosed in co-pending patent application S.N. 349,110, filed on April 13, 1953, in the name of Joseph C. firemen and owned by the assignee of the present applicatien. This mix proved to be commercially successful, hut o of its principal disadvantages is that'it generally isn a to Purc a mme ia fia ew t and a quently reduce it into smaller particles, such as by atom F ing, chilling and sizing. A procedureof this, type so times causes appreciable losses of the rather experts carnauba wax. Hence the present invention serves o reduce the cost of a self-lubricating shell molding by preventing loss of the wax and by permitting more rapid production use of the resultant mix.
It will be understood that the term mold, as used herein, is generally applied in its generic sense to mean a casting form which includes both molds and cores, this invention notbeing limited to the former. Similarly, the word pattern is used herein as including both mold patternsand core boxes.
' Essentially the shell molding process consists ofv using, a thermosetting plastic or resin as a binden for-thefgrain's' of sand or other refractory material to form rigid molds having high gas permeability, good surface smoothness, and dimensional stability. The molding material, which is generally a mixture of a major proportion of silica, sand and a minor proportion of resinous binder, is nor-. mally used in dry form with no water being added. For. some applications it is desirable to coat the'sand particles, with the thermosetting resin before application to the pattern, while in other instances the resin is merely mixed with the sand in powdered form. Such sand-resin mold; ing mixes usually contain between 3% and 15% by weight of binder although in some applications the ,binder content may be as low as 1%, particularly if pre-coated sands are used. For most production work a phenol formaldehyde binder content of 2% to 7% by weight is preferred. Of course, it is frequently advantageous to also include in the mix an appropriate small amount of accelerator or condensation agent, such as hexamethylenee tetramine or paraformaldehyde. i
Among other thermosetting binders which may be satisfactorily used are melamine formaldehyde, phenol furfural, urea formaldehyde and furfuryl alcohol resins, as well as mixtures thereof. Under particular conditions: it also maybe desirable to include inthe molding mix other resinous binder constituents, such as the thermos I. messes setting and thermoplastic resins disclosed in United States Patent No. 2,751,650 and co-pending patent application Serial No. 364,202, both filed in the name of Charles F. Froberger and owned by the assignee of the present application. Thermoplastic lignin-type resinous materials" derived from wood and containing both metho'xyl and hydroxyl groups are examples of such binder constituents. If a thermoplastic material is mixed with a the'rmosetting resin in any appreciable quantity; it ispreferre'd that the total amount of binder constituteat lea'st 3% of the weight of the molding mix.
Itfis desirable touse sand which is free of metal oxides, clayf moisture' and organic matter. Silica flour or other finely'comminuted refractory facing materials also may be ineluded inthemolding' mix to provide the molds with exceptionally smooth working surfaces.
These sand-resin molds are prepared by allowing the sand and resin to come into contact with a hot pattern for a' short period of time. A generally uniform layer of the mix adheres to the pattern surfaces due to the melting of the resin which bonds the sand with which it is intimately mixed, thereby accurately reproducing pattern details. The half patterns, gates and runners usually are permanently fixed on metal plates. Metal patterns normally must be used because they are subjected to elevated temperatures. Pattern temperatures between 350 F.
and 500 F. are preferred, but temperatures as low as 250 F. or lower and as high as 700 F. or even higher may be advantageously employed under particular conditions.
The pattern temperature, the characteristics of the resin and sand used, and the length of time the molding'material is allowed to remain in contact with the hot pattern's'urface determine the thickness of the mold. Mold build-up times ranging from a few seconds to approximately one minute are appropriate for various applications. After this short time interval any excess dry sand and resin are removed, and the closely adhering sand-resin layer is' preferably cured while in contact with the pattern by subjecting it to a temperature within the range of approximately 300 F. to 1500 F. The curing or baking time is relatively short, usually from a few seconds to five minutes. This curing operation results in the conversion of the resinous material to a hard, insoluble binder which securely bonds the sand grains together. After curing of the mold, it is stripped from the pattern and is ready for use. The formed molds are, in effect, thin shells which have sufi'icient strength and stillness to make them suitable for many casting operations.
As hereinbefore explained, in the past partial breaking or tearing of the mold edges often has been experienced in the stripping operation because of the unavailability of molding mixtures which produce shell molds having satisfactory parting or mold-releasing properties. In accordance with my invention, however, I have found that a shell molding mixture which contains a small amount of a solution of high melting point wax and an appropriate organic solvent evenly distributed throughout the mix prevents the adhesion of the resultant molds to pattern surfaces. Certain aromatic esters such as dialkyl phthalates and tricresyl phosphate have been foun'dto be suitable solvents for the wax. Among the dialkyl phthalates those having 1 to 5 carbon atoms in each alkyl group are most practical since they are commercially available and are liquids at room temperature. While the use of dibutyl phthalate is preferred in practicing the invention, dimethyl phthalate, diethyl phthalate, and diamyl phthalate are other examples of such liquid dialkyl phthalates which are highly satisfactory.
Itshould be noted that each of these organic compounds not only is a solvent for the wax but also functions as a dust suppressant for the sand-resin mix. The presence of these solvents in appropriate amounts in the molding mix likewise improves the green strength of;
stitutes between approximately 1% and 3% of. the total the mix. Moreover, the strength of the resultant cured shell mold also appears to be increased by the elfect of the solvent on the thermosetting resin binder. It is believed that the solvent acts as a wetting agent and improves the flowability of the molten resin when the molding mix is in contact with the hot pattern. As a result, the bonding action of the resin is improved.
With regard .to the wax which should be used,-a hard,
high melting point natural wax, such as carnaubawax, provides excellent results with a minimumof expense; The'amount of wax whichsho'uldbe employed may be as small as 0.02% of the total weight of the molding mixture for some applications, but generally it is desirable that at least 0.05% by weighto'f wax be present in order to completely eliminate adhesion of the mixture to the pattern. Only a small amount of wax is necessary because the wax melts upon contact with the hot pattern and normally tends to flow toward the mold-defining surface of the pattern, thus frequently causing the mold to have its greatest concentration of wax adjacent its molding surface. The wax content of the molding mix will vary with the type of pattern being used, of course, as well as with the type of sand employed and quantity of resin in the mixture. Normally the maximum amount of wax which should be used is approximately 1% by weight since a wax content in excess of 1% weakens the resultant mold shell to too greatan extent. Thisweakening effect is due to the fact that carnauba wax will soften and melt 'at approximately. 190 F., while the resin binders will normally retain their rigidity at about 450 F. or higher; Although the resin binders begin to decompose at this latter'temperature, the softening of the wax at substantially lower temperatures, if it is present in excessive amounts, will lower the overall strength of the mold. Moreover, it becomes uneconomical to use amounts of wax greater than 1%, and in most instances amounts appreciably less than this provide optimum results. In view of these considerations, therefore, it is normally preferable to use a sand-resin molding mix containing 0.1% to 0.3% by weight of wax, although generally a wax content between approximately 0.1% and 0.5% is completely satisfactory.
In order to dissolve the wax in the organic solvent to a sufficient extent, it is normally desirable to use an amount of the latter constituent which is appreciably greater than the wax content. However, depending on the amount of wax contained in the molding mix, this mix may initially contain about 0.2% to 7% by weight of the solvent. When the aforementioned preferred amount of wax is used, however, it is desirable that the organic solvent content be between 0.5%' and 2.5% by weight. Thus a useful wax-solvent solution is one comprising approximately 5% to 50% by weight of carnauba wax and 50% to by weight of the organic solvent. For most applications, however, a solution consisting of about 10% to 25% by weight of carnauba wax and 75% to 90% by weightof the solvent is recommended. Such a solution is preferably added in an amount which conweight of the molding mix, although'for some'pui'poses this amount may vary from about 0.3% to 7.5%. If
excessive solvent is used, the molding mix 'becomes'too wet and is difiicult to handle.
The above-described internal lubricant can, be introduced into the shell molding mix in a simple and highly efficient manner. Refined carnauba wax and dibutyl phthalate, for example, may be heated together until the wax dissolves in the dibutyl phthalate. This may be conveniently done at a temperature of approximately F. or above. solution is mulled with silica sand, such as Juniata sand, until it is uniformly distributed in the molding .mix. A mulling time of about seven to eight minutes has proved to be satisfactory for average size mixes. After this mull ing operation powd cred resin-bind y be added to Thereafter the hot. wax-phthalate the mixtureof sand and wax-phthalate solution. The resultant mix should be further mulled for a short period of time, a, mulling period of approximately eight minutes being typical. Of course, if resin-coated sand is used, the hot wax-phthalate solutionis addeddirectly to the coated sand and only one mulling period is required.
When mixing the wax-solvent solution with the other constituents of the shell molding mix, any conventional mulling equipment may be employed. lf'an emulsifier isused, itshould be. added to the mix either immediately before or at the same time the wax solvent solution is introduced into the. sand.
When the molding mix isplaced in contact with the hot metal pattern during themold forming operation, a high percentage of the solvent volatilizes. Hence it does not reduce the strength of the final shell mold. At the same.
time, the wax melts and flows to the pattern surface to the necessary extent, thus preventing the resin from adhering to the pattern.
In the above-described molding mixture the basic and preferred constituent which effects the ready. release of the shell mold from the pattern is carnauba wax, ahard natural wax having a high melting point. Such a wax is not easily vaporized upon contact with a metallic pattern which is maintained at proper temperature, thuspermitting the formation of a substantially unbroken film of wax on the hot pattern surface. Highly satisfactory results also can be obtained when less expensive candelilla wax, another hard natural wax, is substituted for a portion of carnauba wax in the aforementioned parting mixture. Where these two waxes are mixed together, it is preferable to use at least by weight of carnauba wax; and for optimum resultsthe wax should contain at least 50% carnauba wax.
- Under some circumstances it also may be desirable to substitute synthetic waxes for part or all of the carnauba wax. It is essential, however, that such synthetic waxes have physical properties which do not a preciably lower the green strength and tensile strength of the shell mold. Polyethylene types of waxes, such as ethylene polymers having a molecular weight between 2000 and 6000'and having free acid groups built into the polymer molecule, have proved to be satisfactory. Normally such waxes have an ASTM ring and ball softening point of about 210,F. to 230 F. and a Brookfield viscosity at 250 F. between 1700 cps. and 2000 cps. Hardy high melting point waxes of this type usually are readily soluble in the aforementioned phthalates at temperatures above 180" F. Examples of suitable synthetic waxes are Epolene N, a polyethylene wax manufactured by Eastman Chemical Products, Inc. and Acrawax, a prodnot of Glyco Products Company. Each of these waxes is compatible with the above-described phthalates and forms a clear solution with them at 250 F. It should be noted, however, that shell molds containing carnauba wax are generally more easily removed from the patterns than similar molds containing synthetic waxes.
Sand-resin molds may be easily and completely removed from the most intricate pattern equipment when the abovedescribed molding mixture is employed. Furthermore, inasmuch as the use of a molding mix containing an internal lubricant in accordance with my invention causes a negligible amount of residue build-up on the pattern equipment, there results no deviation from desired dimensions in the cured mold as would otherwise be the case. Since excessive residue build-up also increases the sticking tendency of the mold on the pattern, the subject shell mold composition is particularly advantageous. After use of a pattern for a considerable period of time, if the residue build-up should ever become greater than would normally be desirable for precision casting operations, the residue resulting from the formation of these shell molds is of such a nature as to be easily, quickly and completely burned off by merely heating the pa'tternto a temperature'above thatinitiallyiusedinform;
ing the molds.
Before commencing the first mold-forming cycle. after.
burning any. such residue off the pattern, it is beneficial;
to condition the pattern by applying an external mold It is preferable] binder to essentially carbon. The gaseswhich are-gen.- erated readily escape through thehighly-perrneable; sand l resin shell. As a result of the plastic breakdown, the. shake-out is easily accomplished.
The efliciency of a shell molding mix containingthe aboveedescribed internal lubricant is demonstrated by'ithe fact that such a mix allows a substantial increase in the number of shell molds produced per unit time. For example, it has been found that the use of'an internal 1ubricant in accordance with the present invention permits,
onthe average, approximately twice as many. molds to be produced per hour by shell mold blowing equipment as compared with spraying thev same patterns with. con-I:
ventional mold release agents, such as silicones.
Thus it will be seen that the use of a molding mixture in accordance with my invention increases the value of the shell molding process for precision casting operations. This mix provides. molds which faithfully reproduce pattern details, maintain good dimensional tolerance and possess excellent surface qualities. As a result, the castings produced have veryv smooth and clean.
surfaces, true dimensions, and a minimum of fin at the parting line. residual mold material, thereby eliminating the necessity of shot blasting.
While my. invention has been described by means o-f. certain specific examples, it is to be understood that the scope of my invention is not to be limited thereby except as defined in the following claims.
1. In a molding composition for forming shell molds consisting essentially of a major proportion ofrefr'actoryfiller and a minor proportion of potentially thermosetting binder, the improvement which consists of including therewith as a mold release agent a solution of a high melting point wax and at least one organic solvent for said Wax selected from the group consisting of dimethyl phthalate, diethyl phthalate, dibutyl phthalate, diamyl phthalate and tricresyl phosphate in an amount such that the wax and solvent constitute approximately 0.02% to 1% and 0.2% to 7%, respectively, of the total weight of the molding composition. l I 2. In a molding mix for application to a heated pat tern to form shell molds, said molding mix consisting essentially of a major proportion of sand and a minor proportion of thermosetting binder, the improvement which consists in including therewith 0.3% to 7.5% by weight of a solution of a hard, high melting point wax and at least one member of the group consisting of dimethyl phthalate, diethyl phthalate, dibutyl phthalate, diamyl phthalate and tricresyl phosphate.
3. A molding mixture to be placed in contact with a heated pattern to form a thin-walled sand-resin mold, said molding mixture consisting essentially, by weight, of approximately 1% to 15% resinous binder, 0.1% to 0.5% high melting point wax, 0.2% to 7% of a dialkyl phthalate and the balance substantially all sand.
4. A molding mixture to be placed in contact with a heated pattern to form shell molds, said mixture consisting essentially of a major proportion of sand, .a,
The surfaces of these castings are free of minor proportion of potentially thermosetting resin binder, and a mold release agent consisting essentially of a solution of carnauba wax and dibutyl phthalate in an amount'such that the wax and dibutyl phthalate conheated pattern, to form shell molds, said molding mixture consisting essentially of a major proportion of silica sand,.a minor proportion of thermosetting-resin binder, 1% to. 2.5% byweightsof an organic solvent from the group consisting of dimethyl phthalate, diethyl phthalate, dibutyl phthalate, diamyl phthalate and trieresyl phosphate,.and 0.1% to 0.5% by weight of: a. mixture of carnauba wax' and candelilla wax in solution'in said solvent,-the amount of carnauba wax constituting-at least 25% .by'weight of the total wax content in the molding mixture.
6. A shell mold formed by applying to a heated pattern a molding mix consisting essentially of 1% to 7%- by weight of thermosetting binder, a small but effective amount of accelerator for said binder, a small but effective amount not in excess. of approximately 1% by weight of a hard, high melting point wax in solution in a dialkyl phthalate having 1 to 5 carbon atoms in each alkyl group, and the balance substantially all refractory filler material.
7. A shell mold which is readily released from a hot pattern on which it is formed, said mold being formed from a mix corisistingfe'ssentially of a major proportion of sand, a minor proportion of thermosetting binder, and 0.3% to, 7.5% by weight of. amold release agent intimately mixed with said sand and binder, said mold release agent comprising a solution of 5% to 50% by weight of carnauba wax and 50% to 95% by weight of an organic ester selected from the group consisting of dimethyl phthalate, diethyl phthalate, dibutyl phthalate, diamyl phthalate and tricresylphosphate, said mold having its greatest concentration of wax adjacent its molding surface.
8. A molding mix for application to a heated metallic pattern to form shell molds, said molding mixture consisting essentially of 1% to 15% by weight of a resinous binder, a small amount of an accelerator for the binder, 1% to 3% by Weight of a liquid mold release agent consisting essentially of 75% to 90% by weight of dibutyl phthalate and to 25% by weight of carnauba wax, and the balance substantially all sand.
9. In the processof forming a shell mold by mixing a major proportion of sand with a minor proportion of a thermosetting binder and thereafter placing said mixture in contact with a heated pattern for a period of time sufiicient to bond a substantial portion of the sand particles together, the improvement which consists in adding a solution of carnauba wax and an organic solvent to the molding mixture as a mold release agent prior to contacting the pattern with the mixture, said solvent being selected from the group consisting of dimethyl phthalate, diethyl phthalate, dibutyl phthalate, diamyl phthalate and tricresyl phosphate, said solution being of such a composition and added in such an amount so that the organic solvent and wax constitute 0.2% to 7% and 0.05% to 1%, respectively, of the total weight of the mixture.
10."l'n the process'of forming a thin-walled sand-resin mold including the steps of mixing a major proportion of sand with a minor proportion of thermosetting resin binder and thereafter placing the mixture into contact with a hot metallic pattern for a short period of time, the improvement which consists in thoroughly mixing 0.3% to 7.5% by weight of a solution of wax and a 'dialkyl phthalate with the molding mixture prior to contacting the pattern with said mixture, said solution comprising 5% to 50% by weight'o'fwax and 50% to by weight of the clialkyl phthalate'. Y
11. A method of forming a thin-walled sand-resin mold'by the shell molding process which comprises mixing together carnauba wax-and a dialkyl phthalate having 1 to 5 carbonatoms in each alkyl group, heating saidwax and phthalate'until the wax dissolves in the dialkyl phthalate, mixing the resultant hot solution with sand and mullingthe said mixture until said solution is generally uniformly dispersed throughout the sand, thereafter adding thermosetting resin binder to the mixture of sand and wax-phthalate solution, mulling the 'mixture so produced for an additional period of time to intimately mix the various constituents, the dialkyl phthalate and carnauba wax constituting approximately 0.5% to 7% and 0.1% to 0.5 respectively, of the total weight of said mixture, placing the molding mixture so formed in contact with a heated metallic pattern for an interval at least sufiicient to bind together a substantial portion of said particles, removing any excess molding material,
subsequently curing the resultant mold layer by baking while in contact with the pattern, and thereafter strippingthe formed mold shell from said pattern.
12. A method of preparing a shell molding mixture which eliminates the necessity of applying an external lubricant to a pattern on which the mixture is placed, said method comprising thoroughly mixing together 5% to 50%-by weight of carnauba wax weight of dibutyl phthalate, heating said mixture at a temperature of at least approximately F. until the wax dissolves in the dibutyl phthalate, adding the hot Waxphthalate solution to silica sand in an amount sufiicient to produce a wax content and a dibutyl phthalate content in the final molding mix of about 0.1% to 0.3% and 0.5% to 2.5%, respectively, thereafter mulling the resultant mixture until said solution is uniformly dispersed throughout the sand, subsequently adding a small amount of thermosetting resin binder to the mixture of sand and Wax-phthalate solution, and mulling the mixture so produced for an additional period of time to thoroughly mix the various constituents.
References Cited in the file of this patent UNITED STATES PATENTS Great Britain Aug. 17, 1955 and 50% to 95% by