METHOD OF USING LOST METAL PATTERNS TO FORM CERAMIC MOLDS

Abstract: A method for forming a ceramic mold comprises the step of placing a pattern (1) having critical pattern surfaces (13) in a flask (3) having an open end. The critical pattern surfaces face upward toward the open end. Successive steps include adding a heat reversible liquid metal (5) to the flask (3) to cover the critical pattern surfaces, and cooling the liquid metal to form a solid metal mold. The metal mold (7) has critical metal mold (7) surfaces inverse to the critical pattern surfaces (10). Further steps include removing the pattern from the metal mold (7), casting a ceramic mold (11) around the metal mold, and liquifying the metal mold to remove it from the ceramic mold (11). The ceramic mold (11) has critical ceramic surfaces (10) inverse to the critical metal mold surfaces, thereby accurately replicating the critical pattern surfaces.

Description: .METHOD OF USING LOST .METAL PATTERNS TO FORM CERAMIC MOLDS FIELD OF THE INVENTION This invention relates to a method for preparing an accurate ceramic mold by using a heat reversible metal to make an intermediate mold of a pattern and then using the intermediate mold for casting the ceramic mold. The ceramic mold can then be used to create a more durable metal mold for casting multiple plastic parts similar to the original pattern. BACKGROUND OF THE INVENTION Getting new products to the market faster than one's competition is recognized as a key to gaining a large market share. One area of product development having a significant impact on overall market timing is the making of product and package prototypes for market testing. Such testing usually requires multiple look-like, feel-like, and function-like prototypes for consumers to examine or use. Package components generally involve plastic parts made in very expensive, multiple cavity, steel molds. For example, most bottles are blow-molded and most bottle closures are injection molded. It usually takes large production quantities to justify the cost of a production mold with many cavities. For smaller markets, or for making only a few hundred test parts, single cavity molds or prototype molds are created. Prototype molds provide important learning on whether the part can be made consistently, as well as to provide a tool that can be used to make test parts. In the case of making molds from production class metals such as P20 or HI 3 steel, often electro discharge machining (EDM) is used. EDM electrodes are shaped to generate reverse patterns in the metal they are applied to. Such electrodes are typically machined from copper or graphite. These electrode materials break down and wear in the EDM process and replacement electrodes must be machined and substituted in order to complete the EDM process. Increased wear resistance materials for EDM electrodes are now available, but they cannot be easily machined. What is needed is a method of making high wear resistant electrodes without having to machine them. One method of rapidly prototyping containers or parts is investment casting using patterns generated by rapid prototyping systems instead of traditional injection molded wax patterns. example of such a pattern is a QUICKCAST™ pattern, a Trademark of 3D Systems, Inc. of Valencia, CA. A hollow plastic pattern is coated with a thin ceramic shell usually by a dipping process. The plastic is burned out of the ceramic shell leaving minimal amounts of ash residue behind. Molten metal is then poured into the ceramic shell to cast a metal part or metal mold for a plastic part. Because the shell has only a small hole for admitting molten metal, it is difficult to inspect the critical surfaces for ash residue. y ash residue on a critical surface will potentially ruin the metal casting. The molten metal cools and shrinks such that critical surfaces are not reproduced accurately. The larger the parts, the greater the inaccuracy. .An improved method of constructing a fully dense mold is disclosed in U.S. 5,507,336 issued to Tobin, April, 1996. The method comprises placing a pattern within a tube which has a melting point greater than that of the infiltration material which will be used in making the metal mold. A ceramic member is cast between the pattern surfaces and the open end of the tube to transfer the critical pattern surfaces to the ceramic member. The ceramic surfaces are inverse to the pattern surfaces. The pattern is burned out and the ceramic surfaces remains in the tube. The ceramic is then covered with metal powder and an infiltration material from the other end of the tube, and the tube is placed in a furnace to foim the metal part over the ceramic surfaces. The metal part has surfaces inverse to the ceramic surfaces. A metal mold results when the ceramic piece is removed. The metal mold has the same shape as the pattern, and is useful for molding plastic parts having an inverse shape. This is an ideal process for parts having exterior critical surfaces. Tobin's process destroys the pattern from which the ceramic mold is created. A process for quickly forming a ceramic mold pattern which does not destroy the pattern, but which is accurate, is needed. .Also, it is often necessary to provide a mating metal mold for plastic part molding. In order to do this, the metal mold may require a shape which is the inverse of the pattern. Thus, the ceramic mold needs to have the same shape as the pattern, and therefore requires an intermediate mold be produced between the ceramic mold and the pattern. As with Tobin's earlier process, any ceramic mold should not be contaminated on its surface so that the resulting metal mold is accurate. In order to avoid destroying the pattern, it is desirable to use an intermediate mold made of a material which can be discarded or reused as needed to transfer the critical pattern surfaces to the ceramic mold. Wax and silicone rubbers have been used for these purposes. Wax (which is heat reversible) has the disadvantage of being brittle and when removed from the pattern can cause small pieces to break off especially where undercuts and thin features are involved. It also can expand and crack the ceramic when heated. Silicone rubbers need to be cured, and when the ceramic releases heat as it "sets", the silicone rubber can distort and cause inaccuracies to develop in the ceramic pattern. .Also, silicone rubber has to be removed from the pattern by air injection or other means which forces the silicone from the ceramic. This can cause the ceramic mold to break especially where thin features are involved. It is therefore an object of the present invention to provide a process for making a ceramic mold having the same shape as a pattern, which produces accurate reproductions of a pattern of any size, within a tolerance of ±0.005 inches and which does not leave an ash or other residue on the ceramic mold. It is also an object of the present invention to provide a process which uses a heat reversible metal to make an inverse intermediate mold of a pattern and which is not distorted during the forming of a ceramic mold therefrom, but which can be removed easily from the ceramic mold without destroying the delicate features of the ceramic mold. These and other objects will be evident from the description herein. SUMMARY OF THE INVENTION In one aspect of the present invention a method of forming a ceramic mold comprises the step of placing a pattern having critical pattern surfaces in a flask having an open end. The critical pattern surfaces face upward toward the open end. Successive steps include adding a liquid metal to the flask to cover the critical pattern surfaces, and cooling the liquid metal to foirn a solid metal mold. The metal mold has critical metal mold surfaces inverse to the critical pattern surfaces. Further steps include removing the pattern from the metal mold, casting a ceramic mold around the metal mold, and liquifying the metal mold via heating for removal from the ceramic mold. The ceramic mold has critical ceramic surfaces inverse to the critical metal mold surfaces, thereby accurately replicating the critical pattern surfaces. The method may further comprise the step of degassing the liquid metal as it is cooled to form the solid metal mold. BRIEF DESCRIPTION OF THE DRAWINGS While the specification concludes with claims which particularly point out and distinctly claim the present invention, it is believed that the present invention will be better understood from the following description of preferred embodiments, taken in conjunction with the accompanying drawings, in which like reference numerals identify identical elements and wherein: Figure 1 is a sectioned front elevational view of a pattern 1, having critical pattern surfaces 13, positioned inside a first flask 3. Figure 2 is a sectioned front elevational view of pattern 1 inside the first flask 3 into which a liquid metal 5 has been poured. Figure 3 is a sectioned front elevational view of a solidified metal mold 7, having critical metal mold surfaces 10 transferred from critical pattern surfaces 13, positioned inside a second flask 8 with an annular space 12 between second flask 8 and solidified metal mold 7. Figure 4 is a sectioned front elevational view of second flask 8 having a plaster or ceramic solution 9 poured over the solidified metal mold 7 and into annular space 12 and covering critical metal mold surfaces 10. Figure 5 is a sectioned front elevational view of a solidified plaster mold 11 from which has been removed second flask 8 and metal mold 7, exposing critical ceramic surfaces 14, which transferred from critical metal mold surfaces 10 and which accurately replicate critical pattern surfaces 13. DETAILED DESCRD7TION OF THE INVENTION As used herein, the term "ceramic" refers to a material such as plaster, clay, silica or other nonmetallic material wWch can be fired to create a hardened product. As used herein, the term "metal" refers to any metal, metal alloy, or metal alloy composite that melts below a temperature at which damage to a pattern may occur, or which could cause substantial thermal expansion of the pattern. This metal, metal alloy, or metal alloy composite does not substantially expand or shrink upon solidification. It also freezes above a temperature that could cause substantial thermal contraction of the pattern. Preferably, it solidifies at about 3° C above room temperature. The metal forms a solid which has minimal expansion or shrinkage when exposed to a range of temperatures associated with pouring a ceramic that sets into a solid shape. As used herein, the term "heat reversible" refers to a metal which solidifies at a temperature below about 35°C and which melts or liquifies at temperatures above about 45°C. .Although there are non-metal thermally reversible materials available, such as gelatins, a low melting point metal is believed fastest to melt because of its high thermal conductivity. .Also, a low melting point metal has a lower viscosity than gelatin doesn't have a s much tendency to wash away ceramic surface particles when removed from a ceramic mold. Such fine ceramic particles are key to maintaining a good surface finish of parts cast therefrom. Figure 1 illustrates a pattern 1 which fits tightly against an internal surface of a flask. Pattern 1 is a representation of the exterior of a bottle closure. Pattern 1 has critical pattern surfaces, 13, which represent the detail on the outside of the bottle closure. The pattern is preferably made by a stereolithography process, well known in the prototyping art, in which an electronic file describing the pattern is rapidly fabricated by laser curing of a polymer. The pattern is placed in the flask with critical pattern surfaces facing upward toward the open end of the flask. A liquid metal is poured over the pattern. The metal mold is intended to be an intermediate mold which transfers the critical pattern surfaces to a ceramic mold. A ceramic solution is similarly poured over the metal mold in an open flask and allowed to harden. However, the ceramic material typically generates heat in an exothermic binding reaction. The metal mold must eventually be removed from the ceramic mold by melting the metal. Where there are thin sections melting the metal away avoids damage to the brittle ceramic mold. The reliquified metal is easily removed from a ceramic mold by pouring it out. The exothermic reaction of the ceramic typically melts the metal adjacent to it so that surface distortions do not occur as the ceramic hardens. The resulting ceramic mold can be washed with molten brazing flux to remove any residue before firing the ceramic mold to harden it. The preferred metal is CERROLOW® 117, a Trademark of Cerro Corporation, and is available from Cerro Metal Products of Bellefonte, PA. Fibers or other structural materials can be dispersed in the metal. These will add strength and can be easily removed with the melted metal from the ceramic mold. The metal is poured over the pattern in an open ended flask, as shown in Figure 2. The metal casting may be done in multiple pours, depending on the size of the part. The first pour of a multiple pour is preferably allowed to form a skin before the next pour so that air bubbles will not penetrate the first pour. The flask is refrigerated or allowed to cool in ambient room temperature until the metal has solidified. Depending on the size of the pattern, and the depth of the metal layer, from about 2 to about 8 hours are required to solidify the metal. The depth of the metal pour will depend upon the pattern and the size that is desired for the ceramic mold. One skilled in the art can easily determine this without undue experimentation. Typically, a minimum metal thickness of about 2.5 cm is desired above each critical pattern surface. The solidified intermediate metal mold is then pulled from the pattern. In a preferred embodiment, the flask is built with easily removable sides which are then pulled off the metal mold and the metal mold is then pulled off the pattern. The metal mold retains the inverse replications of the critical surfaces of the pattern without distortion, even when thin features are involved. It is important that the pattern have smooth surfaces with no undercuts. Undercuts and rough surfaces can prohibit removal of the solidified metal from the pattern. Figure 3 discloses the metal mold placed in a second flask to which a plaster or ceramic solution will be added. The metal mold is placed with the critical metal mold surfaces facing upward toward the open end of the second flask. Preferably, sufficient space is allowed between the second flask and the metal mold so that ceramic will be formed around the metal mold in that space. The ceramic mold made therefrom will have a continuous annular ceramic rim surrounding the critical ceramic surfaces so that the ceramic mold may be readily used for casting purposes without the need for another flask. Plaster or other ceramic material is poured into the second flask to a depth above the metal mold. Preferably, the depth is from about 1 cm to about 5 cm above the metal mold. The poured ceramic material is preferably degassed under vacuum to remove any air which could affect the final ceramic mold formation. The plaster or ceramic material first "sets" or takes a solid shape and then completely solidifies. During the binding process, an exothermic reaction takes place in the plaster which melts the surrounding metal. The flask is preferably coated with a release agent so that the flask may be easily removed from the ceramic mold. The preferred exothermic ceramic material is Cl-Core Mix, available from Ranson & Randolph of Maumee, Ohio. It is a mixture of fused silica, zirconium silicate, ammonium phosphate, silica (cristobalitc) and magnesium oxide. Core hardener 2000, also available from Ranson & Randolph, can be used. It contains amorphous silica and dipotassium-6-hydroxy -3- oxo -9- xanthene -0- benzoate. .After the ceramic is set, the ceramic mold and remaining metal can be heated in an oven or by a heat gun to completely melt the metal for easy removal. The temperature of the oven should be about 200°C to about 500°C to insure the melting of the metal. The open end of the ceramic mold, which corresponds to the bottom end of the second flask, allows easy access to pour the melted or liquid metal from the ceramic mold. Also, critical ceramic surfaces may be easily inspected from the open end to see that all metal and any residue are removed. Placing the ceramic mold in a furnace and heating it to at least 1100°F (990°C) for at least 3 hours fully sets the plaster for fiirther processing. A hydrogen atmosphere can be used as there is no residue remaining on the ceramic which needs to be burned off. This lack of residue is an important distinction when compared to ceramic mold making processes using epoxies and waxes. While particular embodiments of the present invention have been illustrated and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention, and it is intended to cover in the appended claims all such modifications that are within the scope of the invention.

Claims: What is claimed is: 1. A method of foπning a ceramic mold characterized by the steps of: a) placing a pattern having critical pattern surfaces in a flask having an open end, said critical pattern surfaces facing upward toward said open end; b) covering said critical pattern surfaces with a heat reversible liquid metal added to said flask; c) cooling said metal solution to form a solid metal mold, said metal mold having critical metal mold surfaces transferred from said critical pattern surfaces which are inverse to said critical pattern surfaces; d) removing said flask and said pattern from said metal mold; and e) casting a ceramic mold around said solid metal mold, said ceramic mold having critical ceramic surfaces transferred from said critical metal mold surfaces which are inverse to said critical metal mold surfaces, said critical ceramic surfaces thereby accurately replicating said critical pattern surfaces; and f) liquifying said metal mold for removal from said ceramic mold. 2. The method according to Claim 1, further characterized in that said metal is CERROLOW® 117. 3. The method according to Claim 1, further characterized by the step of degassing said liquid metal as it is cooled to form a solid metal mold. 4. The method according to Claim 1, further characterized in that said liquid metal further comprises fibers or other thickeners. 5. A method for forming a ceramic mold characterized by the steps of: a) placing a pattern having critical pattern surfaces in a first flask having an open end, said critical pattern surfaces facing upward toward said open end; b) covering said critical pattern surfaces with a heat reversible liquid metal added to said first flask and; c) cooling said liquid metal while degassing said liquid metal to foim a solid metal mold, said metal mold having critical metal mold surfaces transferred from said critical pattern surfaces which are inverse to said critical pattern surfaces; d) removing said pattern and said first flask from said metal mold and placing said metal mold in a second flask with said critical metal mold surfaces facing upward toward an open end of said second flask; e) covering said critical metal mold surfaces with a ceramic solution added to said second flask while degassing said ceramic solution, said ceramic solution solidifying and then exothermically binding to form a ceramic mold around said metal mold, said ceramic mold having critical ceramic surfaces transferred from said critical metal mold surfaces which are inverse to said critical metal mold surfaces, said ceramic critical surfaces thereby accurately replicating said critical pattern surfaces; and f) liquifying said metal mold via heating to remove said metal from said ceramic mold and removing said second flask from said ceramic mold. 6. The method according to Claim 6, further characterized in that said liquid metal further comprises fibers or other thickeners. 7. A method for forming a ceramic mold characterized by the steps of: a) placing a pattern having critical pattern surfaces in a first flask having an open end, said critical pattern surfaces facing upward toward said open end; b) covering said critical pattern surfaces with a metal solution added to said first flask and; c) cooling said metal solution while degassing said metal solution to form an elastic solid metal mold, said metal mold having critical metal mold surfaces transferred from said critical pattern surfaces which are inverse to said critical pattern surfaces; d) removing said pattern and said first flask from said metal mold and placing said metal mold in a second flask with said critical metal mold surfaces facing upward toward an open end of said second flask, said second flask dimensioned to provide an annular space around said metal mold; e) filling said annular space with a first ceramic solution added to said second flask while degassing said first ceramic solution, said first ceramic solution solidifying without generating heat to form a first ceramic mold in order to anchor said metal mold in place and to form a continuous annular rim surrounding said critical metal mold surfaces; f) covering said first ceramic mold and said metal mold with a second ceramic solution added to said second flask, said second ceramic solution exothermically binding to form a second ceramic mold bonded to said first ceramic mold, said second ceramic mold having critical ceramic surfaces transferred from said critical metal mold surfaces which are inverse to said critical metal mold surfaces, said critical ceramic surfaces thereby accurately replicating said critical pattern surfaces; and g) liquifying said metal mold via heating to remove said metal from said first and second ceramic molds and removing said second flask from said first and second ceramic molds.

Inventors: TOBIN JAMES ROBERT, TOBIN, JAMES, ROBERT
Applicants: PROCTER & GAMBLE, THE PROCTER & GAMBLE COMPANY
Application year: 1998
Main classification: B22C7
IPC codes: B22C7, B22C9, B29C33