US3177537A - Methods and apparatus for forming investment molds and mold produced thereby - Google Patents

Description

April 13, 1965 R. A. HORTON 3,177,537

METHODS AND APPARATUS FOR FORMING INVESTMENT MOLDS AND MOLD PRODUCED THEREBY Filed Dec. 2'7, 1962 2 Sheets-Sheet 1 ;27 27 12 ml #l 28 29% 30 28 2 INVENTOR.

ROBERT A. HORTON April 13, 1965 R. A. HORTON 3,177,537

METHODS AND APPARATUS FOR FORMING INVESTMENT MOLDS AND MOLD PRODUCED THEREBY Filed Dec. 27, 1962 2 Sheets-Sheet 2 F jg 2 INVENTOR.

ROBERT A. HORTON United States Patent 3,177,537 METHODS AND APPARATUS FOR FORMING INVESTMENT MOLDS AND MOLD PRO- DUCED THEREBY Robert A. Horton, Chesterland, Ohio, assignor to Precision Metalsmiths, Inc. Filed Dec. 27, 1962, Ser. No. 247,600 Claims. (Cl. 22-130) This application is a continuation-in-part of application Serial No. 51,072, filed August 22, 1960 and entitled Method of Form Removal From Precision Casting Shells, now Patent No. 3,094,751, issued June 25, 1963.

This invention relates generally to the art of investment casting, and more specifically to new and improved methods and apparatus useful in forming molds for the socalled lost-wax processes of investment casting.

In general, investment casting processes involve the use of two types of molds, namely, bulk investment molds and ceramic shell molds. Both types of molds are usually prepared using patterns which are replicas of the parts to be cast, including the necessary gates and risers, and which are formed of an expendable material, such as wax or a suitable synthetic resin. The patterns are attached to a sprue form to constitute what is commonly known as a set-up or tree. After the formation of the set-up or tree, a suitable refractory mold is formed around the patterns which are then removed from the mold so that metal or other molten material can be poured into the mold and allowed to solidify. The mold material is subsequently removed to obtain castings which are reproductions of the original patterns.

Conventional bulk investment molds are produced by dipping the pattern set-up in a refractory slurry which hardens to form a smooth coating that serves as a mold face. After this pre-coat or dip coat has dried, the coated pattern set-up is placed in a suitable heat-resistant flask and the flask is filled with a cementitious refractory material, known as the investment, which hardens at room temperature to form the bulk of the mold.

The pre-coating of the patterns usually is carried out by dipping the set-up into a suspension of a fine refractory powder in a suitable liquid binder that is capable of hardening by drying at room conditions. Following dipping, the excess slurry is drained from the patterns and the coating is stuccoed while wet with coarser refractory particles which help to set the coating and to facilitate bonding between the coating and the back-up investment material. Under certain conditions, particularly in nonferrous casting operations, the precoat can be omitted,

however, a better surface inside the mold usually is obtained if the coating is used.

Ceramic shell molds are usually prepared by repeating the dipping and stuccoing operations described above until arefractory shell having a sufiicient thickness to resist the stresses occurring in subsequent operations is built up around the set-up. After each dipping step, it is the customary practice to drain excess slurry from the patterns so as to attain uniform coverage. The draining operations can be facilitated by rotating the set-up to throw off excess slurry from within the pattern cavities and recesses. The stuccoing operations may be carried out conveniently by plunging the set-ups into an air- I fluidized bed of the refractory particles. The usual thickness of the shell which is formed by repeating these operations is in the range of from about A; of an inch to about /2 of an inch, although thinner or heavier shells may be formed for special situations.

Depending upon the pattern material, different pat- When using wax patterns and certain recently developed pattern material blends, the patterns can be removed successfully from the molds by a procedure known as flash de-waxing. According to this procedure, the molds are placed directly into a furnace operating in the range of from about 1600 F. to about 2300 F. The high temperature results in a surface layer of the pattern material melting before the bulk of the patterns can heat up enough to crack the molds. The molten pattern material is either absorbed into the mold or flows out of the mold to create a space sufficient to accommodate subsequent expansion of the patterns.

Attempts to apply the flash de-waxing concept to the removal of plastic patterns from ceramic shell molds have n t been successful, however. Apparently, the conventional synthetic resins soften very gradually without significant melting at the mold inner-face. As a result, plastic patterns will expand and crack the shells before becoming fluid enough either to be absorbed into the mold or to squeeze out of the mold openings. Further, the conventionally used plastics, especially polystyrene, tend to decompose rapidly under flash de-waxing conditions, and it is believed that the vapors which are evolved become trapped in the remaining viscous plastic to produce a foamy mass which further increases the cracking pressure within the mold.

The only known pattern removal technique heretofore developed which permits the use of conventional plastic patterns and ceramic shell molds involves the steps of at least partially dissolving the plastic patterns by the application of a liquid solvent and thereafter heating the molds to volatize any remaining pattern material. Usually the solvent is applied at room temperature by soaking the molds in a solvent bath. After the plastic patterns have been substantially completely dissolved by the solvent, the molds can then be heated to burn off any remaining vestiges of pattern material and to me pare the molds for casting.

As described in the above-identified, copending application, Serial No. 51,072, now Patent No. 3,094,751, the solvent removal technique can be facilitated by preparing the pattern-mold assembly so that the root end portions of the patterns can be directly attacked by the liquid solvent. For example, it has been found that in certain situations the progressively softening plastic tends to develop fluid pressure in the individual pattern cavities when the solvent is allowed to primarily attack the patterns by penetrating the shells. In these situations, the combined fluid pressures in a large number of pattern cavities has actually caused the entire side wall of the shell to break loose and to destroy the entire tree assembly.

The problem of developing fluid pressure in the pattern cavities of shell molds is substantially eliminated by initially exposing the root end portions of the patterns to the solvent. When this is done, the solvent immediately commences to attack the patterns at their exposed ends and continues to work inwardly into the cavities until the patterns are substantially completely softened or dissolved. In this manner, the plastic material is allowed to continually drain from the cavities without developing detrimental fluid pressures. T Y

The present invention is concerned with improvements in the construction of the center tree or sprue form to which the expendable patterns are attached in making the set-ups. As will hereinafter be made apparent, these improvements facilitate the formation of the pattern set-ups and the handling of the set-ups when making both bulk investment molds and ceramic shell molds. Another important advantage is that the invention makes it possible to carry out the steps of pattern removal in an improved manner. More specifically, pattern removal operations casting. a V I a A more specific object of the invention isyto provide an improved, structurally strong centertree to which a the like which is" suitably secured over the open end 12.

handle; and, q

FIGURE 2 is a longitudinal sectional viewpf 'a pat-1' 'co atingmaterialp v I I r 7L For the purposes ofidisclosure, the invention isgshown' involving the application of heat can be accomplished faster and with less damage to the molds than whenusing conventionally formed, wax center trees'which require substantial'heating of the molds.

sprue form of the inventionalso provides a,convenient way of exposing the root end-portions. of the plastic patpatterns to expand and crack the shells.

According to the preferred construct1on,thesprueiform In carrying out the. solvent removal techniques described'above, the novel terns without damaging the shell molds and, again, with i cut requiring substantial heating such as might cause the" in such molds, it is to be understood that this disclosure is not limiting of the invention. As pointed out above,

the invention also has advantages when used in conjunction with Wax patterns and in thepr'oduction of bulk inof the inventioncornprises a hollow metalform which is 1 closed at one end. 'The opposite, open end of the hollow form is preferably shaped to produce a'pouring basin in 1 the mold and is constructed so that a handle can .be removably attachedthereto. When making both bulk investment molds and ceramic shell molds, the pattern setups are prepared using themetal formats the center tree:

The handles which are attached to the metal forms or center trees make. it easy-to handle the' set-ups when dipping them intothe refractory slurry, draining and spin,-

' ning the set-ups after each dipping operation, and subse quently placing the set-ups in the air-fluidized bed of recan be suspended from conveyors and other equipment which may be employed'in the mold forming and casting operations.

Following the formation. of a mold, the'metalforr'n is quickly removable by applying heat'to loosen acoating which is placed on the form when preparing the set-up :vestment molds',.because of the improved manner in which'the patt ern set-ups can he handled and the patterns removed from thermclds,

Referring now to FIG. 1, the preferred hollow metal sprue form .of the illustrated assembly is designated, by reference numeral 10." The sprue:form 10 is closed at one end. by a relatively "thick, integral end wall 11 and his preferably tapered from'the end wall 11.to the op posite, largeropenend 12, inorde'r' tofacilitate removal of the .form from the mold... The outer shape of the sprueform'may be, generally frusto-conical as shown for it maybe {of'rectangularor other desired cross-sectional' configuration. '-'In the-preferredembodiment, the

v a i 'open end portion 12 is enlarged in the area 13 to provide fractory-materiaL- The handles also provide aeonven ient means by which the set-ups and the formed molds;

a pouring 'basi'n'forrner. The sprue form 10 is desirably constructed of aluminum'to provide lightness and good thermal conductivity.

' The end wall'11 is provided jwith, a threaded hole andthen sliding the form from thc'mold." The time interval during whichthe mold is in actual contact with a the heated metal form'is very short, usually no Imo're than from 10 to 30 seconds; This is of particular advantage in shell molding operations, since substantial heating of the molds, such 'as might cause the patterns to expand and crack the m'olds, is avoided. Further, in; operations involving the use of plastic patterns atidthe removal of the. patterns by solvent attack, the removal 7 which is adapted to receive one'end of a'center rod 20 which extends" axially within the. sprueform' 10. The rod 20'is shown as being threadedon .both ends so that one endcan' be screwed into the hole in the end wall 11.

I The opposite threaded end of th'e' rod 20'terminates short of the open, endp12 ofthe sprue form and provides a support to, which an end closing plateor disc 21 can be connected. j The plate21, in turn, provides a support to [Qwhi'ch the handle 22 canbe -connected,.andalso serves of the metal form is efiectiveto expose the rootend por-f tions of the. patterns.

Although the use of hollow wax center trees jhasi been proposed in the past as minimize'tl ie.problems'associated with patternremoval, suchtrees are not strong enough to permit a handle to be attached for the dipping, spinplastic trees does'not'facilitate the'solvent removal operation in'which it is desired to initially expose the root ends of the patterns directly to the solvent.

'ning, draining and. other mold-forming operations, p

Moreover, the use'of the conventional hollow 'wax or to seal the pouring basin'former 13 'against the entrance of *mold material during the dipping and stuccoing steps of forming'a refractory moldr 1 I In the preferred 'embodimerihthe plate or disc 21 is constructed ,to have a thickened center portion 23 through which is formed a threaded hole 24. 'The periphery. of

the disc hasa fiangezs'which is adapted to closely conform to the adjacent inner-wall surface of the hollow sprue form 10. ,In use, the disc 21 is. threaded onto the "end of the rod 20 'so. that the outer surface of the disc v is approximately flush fwith' the end 12 of the sprue form, andthe peripheral space. between theflange 25 and the innerspruewall is. preferably. sealed, as by way 26 or ,Accordingly, an object-Tot the invention is toprovide v improvements in the formation of pattern set-ups which facilitate handling of the set-ups, the making of both" bulk investment moldsv and ceramic shell molds, and the removal of the handle can be attached for holdinga pattern set-up durforming operations.

' Other objects and advantages of the invention will beother suitable means, to prevent" entry of the dipping slurry. As shown, the disc 21 also is provided witha pair of'holes 27 which facilitate mounting and removal 'aof the disc from the rod 20in. a manner to bemore 3 specifically described; These holes '27' may be closed 5 5. patterns from the' 'rnold preparatory to 5 during the m old-forming. operations by plugs 28 or by other suitable rnean s such as wax, modeling clay, or the like. Theillustrated construction of the disc, 21 may be made of meta1, plastic,'lor other. suitable material, In an 1 Thefillustrated handle 22 is tubular and may be constructed of wood, metal or other material. The end of the come apparent from the following detailed description:

when taken in conjunction with the, accompanying; drawings. I

In the drawings:

FIGURE ljis a longitudinalsectionatview o fian. as sembly of the preferred; sprue form andan attached,

handle which connects to the disc 21 is provided with an extending threaded stud 29 which may be screwed into 3 the hole24 of the disc. A lock nut 30 is shown as being tern assembly, includingithe preferred sprue form of the invention, which has been dipped a refractorylshell' f formingoperations.

iprovided on the stud 29 between the disc and the adjacent end of the handle. {Theoppositee'nd of the handle 22 is a v preferably provided with a hook 31 by which the assembly can be connected a a conveyor, aspinning apparatus for rotating th'e patternset-up after theapplication of slurry, andto other apparatus which'may/be used in the rnold- 7 In useythe, several'components are first assembled as illustratedand described infconjunction with FIG. 1.

and ceramic shell molds.

After assembly, a low melting wax coating 40 (FIG. 2) is formed on the outside surface of the sprue form by dipping it one or more times into a molten wax bath. Generally, a coating of approximately 5 of an inch is satisfactory. After the wax coating 40 has cooled sufliciently, the root end portions 41 of the patterns 42,

which may be formed of wax, plastic or other expendable material, are attached to the wax coated form by the usual soldering techniques to complete the pattern set-up.

The ceramic shell molding steps which may be followed in building up a shell mold around the completed pattern assembly are generally described above and are the same as when using conventional wax trees. In summary, the set-up is immersed in a suitable refractory slurry, removed, and the excess slurry allowed to drain from the patterns. The set-up is usually rotated to facilitate the draining operation. While still wet, the slurry coating is stuccoed with refractory material, as by dipping the set-up into an air-fluidized bed of refractory particles, and the stuccoed coating is then allowed to dry. The dipping, stuccoing, and drying operations are repeated until a coating of the desired thickness has been built up. This resulting coating constitutes the shell molding shell 43 which is shown in FIG. 2.

When the required thickness of shell has been built up around the pattern set-up, the hollow metal form 10 is removed from the mold so that it can undergo further processing. This is accomplished by first removing the disc 21 from the opening 12 of the metal form. The disc 21 can be removed in a number of ways depending upon its construction. With the preferred construction illustrated in FIG. 1, the disc 21 is easily detached by removing the plugs or corks 28 from the holes 27 and inserting the teeth of a spanner wrench or similarly constructed jig into the holes. In this manner, the disc can be twisted to break the seal 26 and to unscrew the disc from the rod 20.

. drain out the liquified wax that had formed the coating on the metal form 10. As noted above, the time interval during which the mold is in actual contact with the heated metal form is sufiiciently short so that the patterns are not heated to such an extent as would cause them to expand and crack the mold.

After the foregoing steps have been completed, the patterns 42 can be removed from the mold by any suitable procedure which is compatible with the pattern material. In general, these pattern removal operations can be accomplished faster and with less damage to the mold, since there is no large wax center tree which must be removed by substantial heating of the mold. Further, in applications in which plastic patterns are used, it will be seen that removal of the sprue form 10 and the incidental melting of the wax coating 40 serves to expose the root end portions 41 of the patterns. This permits the shell mold to be filled with a liquid solvent which may unidirectionally attack the patterns so as to avoid the possible creation of fluid pressures in the several pattern cavities.

It will also be apparent from the foregoing description that the novel sprue form assembly of the invention is useful in making both conventional bulk investment molds In either case, the sprue form 10 provides a structurally strong center tree which permits the pattern set-up conveniently to be handled during the mold-forming operations. 1

The terms expendable pattern and expendable material are used herein in accordance with acceptance terminology of the investment casting art to designate the class of patterns and pattern materials suitable for use in lost-pattern processes of precision casting wherein a refractory mold is formed around a pattern having the shape of the article to be cast and the pattern is then dissolved, melted, vaporized and/or burned from the mold leaving substantially no residue. The term heat dispos able as used herein specifically dmignates those expendable patterns or materials which can be melted, vaporized, or burned from the refractory molds under elevated temperature conditions.

Many modifications and variations of the invention will be apparent to those skilled in the art in view of the foregoing detailed disclosure. For example, it is possible to make the sprue forms solid instead of hollow and to attach the handle directly to the solid form. Therefore, it is to be understood that, within the scope of the claims, the invention can be practiced otherwise than as specifically shown and described.

What is claimed is:

l. A shell mo'l-d assembly comprising a hollow metal form, said form having wax coating on the outer surface thereof, said coating having a thickness of about onesixteenth of an inch, at least one pattern formed of an expendable material, said pattern having a portion thereof embedded in said wax, and a refractory shell mold suitable for casting metal surrounding said pattern and said waxcoated form.

2. The mold assembly as claimed in claim 1 wherein said expendable pattern material is wax.

3. The mold assembly as claimed in claim 1 wherein said expendable pattern material is a soluble synthetic resin, said pattern being removable from said mold by first destroying said Wax coating, removing said metal form from said mold to expose said ipattern portion, and thereafter applying a solvent for the synthetic resin to said exposed pattern portion.

4. The mold assembly as claimed in claim 1 wherein said hollow metal form has an open end at a surface of said mold, and wherein said form is enlarged adjacent said open end to define a pouring basin former.

5. A process of producing a refractory mold comprising the steps of providing a metal form, coating said form with a heat-disposable material, said coating having a thickness of about one-sixteenth of an inch, attaching to said coating a portion of an expendable pattern, forming a refractory mold around said coated metal form and said pattern, removing said metal form from said mold, and thereafter removing said pattern from said mold.

6. A process of forming a refractory mold comprising the steps of providing a hollow metal form, coating said form with wax, attaching a plurality of expendable pat-, terns to said coating, forming a refractory mold around said coated metal form and said patterns, heating said hollow metal form internally to melt said coating, removing the heated metal form and said coating from said mold, and thereafter removing said patterns from said mold.

7. A sprue form assembly adapted for use in forming a pattern set-up comprising an elongated hollow portion, end wall forming means at one end of said portion, a removable disc closing the opposite end of said portion, means forming an enlargement adjacent the opposite end of said elongated portion and defining a pouring basin former, a rod extending between the said end wall forming means and said disc, said rod being connected to said disc, and means defining an external handle adjacent said opposite end of said portion and extending from said disc.

8. A sprue form assembly adapted to form the center tree of a pattern set-up for use in producing a refractory mold, said assembly comprising an elongated, hollow metal portion, said portion having a closed end and an opposite end, means defining an enlarged portion adjacent said opposite end, said enlarged portion having an external surface off-set from the external surface of said elongated portion thereby to define a pouring basin former, removmetal form with a heat-disposable material, attachingto; 10

a refractory mold around said coatedflmetal form and a a a a able means closing said elongated portion to prevent entry References Cited by theExa l niner of mold-forming materialwhen producing a mo1d,fsaid I a UNITED E PATENTS removable means beingthereafter removable to permit o i v r s heating of said metal'portion, and a handle secured to said 11 5 3 Schmldt, '.'*"'V".' assembly and extending from said-enlarged portion. 2,155,800 9; t 5 9. The assembly as claimed ill-claim 8 wherein said" 5 a 77/56 Harrmk'et 22196 'means fdefining said; enlarged portion is an integral a;- 12/57 T m -1 tension of the wall of said elongated'portion. q a 310634137 11/62 opsrhal'l V- "1 1 0. A process of producing'a refractory moldcompris- 3,094,751 HOYWH 'r 7 ing the steps of providing a metal form, coating said r 3,139,656, 64 Watts et 3 -1-- 2 51 a FOREIGN/PATENTS said coating a portion oftaniexp'enda ble pattern, forming 1 5 559" Frances a .said pattern, heating said metal fornito melt said :coat- J EN O Examiner ing and then sliding the'form from said 'mold, whereby a I V v v v 7 said pattern portion is exposed, and thereafter removing MARCUS, LYONS, MICHAEL B L saidpattern f rom saidmold. j 7 a r v '1 Ea cqminers.

Claims (2)

1. A SHELL MOLD ASSEMBLY COMPRISING A HOLLOW METAL FORM, SAID FORM HAVING WAX COATING ONTHE OUTER SURFACE THEREOF, SAID COATING HAVING A THICKNESS OF ABOUT ONESIXTEENTH OF AN INCH, AT LEAST ONE PATTERN FORMED OF AN EXPENDABLE MATERIAL, SAID PATTERN HAVING A PORTION THEREOF EMBEDDED IN SAID WAX, AND A REFRACTORY SHELL MOLD SUITABLE FOR CASTING METAL SURROUNDING SAID PATTERN AND SAID WAXCOATED FORM.

10. A PROCESS OF PRODUCILNG A REFRACTORY MOLD COMPRISING THE STEPS OF PROVIDING A METAL FORM, COATING SAID METAL FORM WITH A HEAT-DISPOSABLE MATERIAL, ATTACHING TO SAID COATING A PORTION OF AN EXPENDABLE PATTERN, FORMING A REFRACTORY MOLD AROUND SAID COATED METAL FORM AND SAID PATTERN, HEATING SAID METAL FORM TO MELT SAID COATING AND THEN SLIDING THE FORM FROM SAID MOLD, WHEREBY SAID PATTERN PORTION IS EXPOSED, AND THEREAFTER REMOVING SAID PATTERN FROM SAID MOLD.

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