MXPA00005633A

MXPA00005633A - Method of using lost metal patterns to form ceramic molds

Info

Publication number: MXPA00005633A
Application number: MXPA/A/2000/005633A
Authority: MX
Inventors: James Robert Tobin
Original assignee: The Procter & Gamble Company
Priority date: 1997-12-08
Filing date: 2000-06-07
Publication date: 2002-02-26

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 FOR USING LOOSE METAL PATTERNS TO FORM CERAMIC MOLDS FIELD OF THE INVENTION This invention relates to a method for preparing an exact ceramic mold by using a heat-reversible metal to make an intermediate mold of a pattern and subsequently using the intermediate mold to cast the ceramic mold. The ceramic mold can then be used to create a more durable metal mold to cast multiple plastic parts similar to the original pattern.

BACKGROUND OF THE INVENTION Launching new products to the market faster than the competition is recognized as a key to winning a large market share. One area of product development that has a major impact on market opportunities in general is the manufacture of a product and packaging prototypes for market research: Such a study generally requires consumers to examine or use multiple prototypes of appearance type, sensation and function. The packaging components generally comprise plastic parts made in steel molds with multiple cavities of very high cost. For example, most bottles are blow molded and most bottle closures are injection molded. Generally large quantities of production are needed to justify the cost of a production mold with many cavities. For small markets, or to make only a few test parts, single-cavity molds or prototype molds are created. The prototype molds provide important knowledge about how the part can be done consistently, and the provision of a tool that can be used to make test parts. In the case of making molds from metals of production class such as P20 or H13 steel, electro-discharge machining (EDM) is often used. The EDM electrodes are formed to generate inverted patterns in the metal to which they are applied. Said electrodes are typically machined from copper or graphite. These electrode materials are broken and worn out in the EDM procedure and the replacement electrodes must be machined and replaced to complete the EDM procedure. Materials of increased wear resistance for EDM electrodes are now available, but can not be easily machined. What is needed is a method to make electrodes highly resistant to wear without having to machine them. One method for rapidly prototyping containers or parts is by melting the lost wax using patterns generated by quick prototyping systems instead of traditional injection molded wax patterns. An example of such pattern is a QUICKCAST ™ pattern, a registered trademark of 3D Systemas, Inc. of Valencia, CA. A hollow plastic pattern is coated with a thin ceramic cover generally by an immersion method. The plastic burns outside the ceramic cover leaving minimal amounts of slag. The molten metal is subsequently poured into the ceramic cover to cast a metal part or metal mold for a plastic part. Since the cover has only a small hole to allow the entrance of the molten metal, it is difficult to inspect the critical surfaces for slag residues. Any slag residue on a critical surface will potentially ruin metal casting. The molten metal cools and shrinks so that critical surfaces do not reproduce accurately. The larger the parties, the greater the inaccuracy. An improved method for building a fully compact mold is described in the U.S.A. 5,507,336 issued to Tobin, in April 1996. The method comprises placing a pattern inside a tube that has a melting point higher than the infiltration material that will be used to make the metal mold. A ceramic element is cast between the pattern surfaces and the open end of the tube to transfer the critical surfaces of the pattern to the ceramic element. The ceramic surfaces are inverted to the surfaces of the pattern. The pattern burns and the ceramic surfaces remain in the tube. Then the ceramic is covered with metal powder and an infiltration material from the other end of the tube, and the tube is placed in an oven to form the metal part on the ceramic surfaces. The metal part has inverted surfaces to the ceramic surfaces. A metal mold is formed when the ceramic piece is removed. The metal mold has the same shape as the pattern, and is useful for molding plastic parts that have an inverted shape. This is an ideal procedure for parts that have critical exterior surfaces. The Tobin procedure destroys the pattern from which the ceramic mold was created. A procedure is needed to quickly form a pattern of the ceramic mold that does not destroy the pattern but is accurate. Also, it is often necessary to provide a matching metal mold for molding the plastic part. To do this, the metal mold requires a shape that is inverted to the pattern. Thus, the ceramic mold needs to have the same shape as the pattern, and therefore requires that an intermediate mold be produced between the ceramic mold and the pattern. As with the previous Tobin procedure, any ceramic mold must not be contaminated on its surface so that the resulting metal mold is accurate. To avoid destroying the pattern, it is desired to use an intermediate mold made of material that can be discarded or reused, as needed, to transfer the critical surfaces of the pattern to the ceramic mold. The wax and silicone rubber has been used for these purposes. The wax (which is reversible to heat) has the disadvantage of being fragile and when removed from the pattern can cause small pieces to break, especially in the thin and recessed parts. It can also expand and crack the ceramic when heated. The silicone rubber needs to be cured, and when the ceramic releases the heat as it "sets", the silicone rubber can deform and cause imperfections to develop in the ceramic pattern. Also, the silicone rubber has to be removed from the pattern by injection by air or other means that forces the silicone of the ceramic.

This can cause the ceramic mold to break, especially where thin parts are found. It is therefore an object of the present invention to provide a method for making a ceramic mold having the same shape as a pattern, which produces exact reproductions of a pattern of any size, within a tolerance of + 0.0127 cm and which does not leave scum or other residue in the ceramic mold. It is also an object of the present invention to provide a method that uses a heat-reversible metal to make an intermediate mold inverted from a pattern and that does not deform during the formation of a ceramic mold thereof, but can be easily removed from the mold ceramic without destroying the delicate characteristics of the ceramic mold. These and other objects will be evident from the description presented in this document.

BRIEF DESCRIPTION OF THE INVENTION In one aspect of the present invention a method for forming a ceramic mold comprises the step of placing a pattern having critical surfaces of the pattern in a flask having an open end. The critical surfaces of the pattern face up toward the open end. Successive steps include adding a liquid metal to the flask to cover the critical surfaces of the pattern, and cooling the liquid metal to form a solid metal mold. The metal mold has critical surfaces of the metal mold that are inverted to the critical surfaces of the pattern. In addition the steps include removing the pattern from the metal mold, casting a ceramic mold around the metal mold, and liquefying the metal mold by means of heating to remove the ceramic mold. The ceramic mold has critical ceramic surfaces that are inverted to the critical surfaces of the metal mold, thereby reproducing the critical surfaces of the pattern. The method may further comprise the step of degassing the liquid metal while cooling to form the solid metal mold.

BRIEF DESCRIPTION OF THE DRAWINGS Although the specification concludes with the claims that particularly indicate and claim the present invention, it is believed that the present invention will be better understood from the following description of the preferred embodiments, which are taken together with the accompanying drawings, in which the numbers reference identifies the identical elements where: Figure 1 is a sectional front elevational view of a pattern 1, having critical surfaces of the pattern 13, placed on the inside of a first flask 3. Figure 2 is a view in frontal elevation in section of the pattern 1 within the first flask 3 into which a liquid metal has been poured 5. Figure 3 is a front elevation view in section of a solidified metal mold 7, having critical surfaces of the metal mold that are transferred from the critical surfaces of the pattern 13, placed inside a second flask 8 with an annular space 12 between the second flask 8 and the solidified metal mold 7. Figure 4 is a front elevational view in section of the second flask 8 having a plaster or ceramic solution 9 poured on the solidified metal mold 7 and in an annular space 12 and covering the critical surfaces of metal 10. Figure 5 is a front elevation view in section of a solidified gypsum mold 11 from which the second flask 8 and the metal mold 7 have been removed, exposing the critical ceramic surfaces 14, which are transferred from the critical surfaces of the metal mold 10 and which reproduce exactly critical patterned surfaces 13 DETAILED DESCRIPTION OF THE INVENTION As used herein, the term "ceramic" refers to a material such as gypsum, clay, silica or other non-metallic material that can be burned to create a hardened product. As used herein, the term "metal" refers to any metal, metal alloy, or mixed metal alloy material that melts below a temperature at which the standard may be damaged, or which may cause substantial thermal expansion of the pattern. This metal, metal alloy, or mixed metal alloy material does not substantially expand or coalesce after solidification. It also freezes above a temperature which can cause substantial thermal shrinkage of the pattern. Preferably, it solidifies at about 3 ° C above room temperature. The metal forms a solid that has minimal expansion or shrinkage when exposed to a temperature scale related to the pouring of a ceramic that sets in a solid form. As used herein, the term "heat-reversible" refers to a metal that solidifies at a temperature below about 35 ° C and that melts or liquefies at temperatures above about 45 ° C. . It is believed that although available thermally reversible non-metallic materials, such as gelatins, are available, a low-melting metal melts faster due to its high thermal conductivity. Also, a low melting point metal has a lower viscosity than gelatin and does not have much tendency to drag particles from the ceramic surface when it is removed from a ceramic mold. Said fine ceramic particles are the key to maintaining a good surface finish of the castings thereof. Figure 1 illustrates a pattern 1 that fits tightly against an inner surface of a flask. Pattern 1 is a representation of the exterior of a bottle closure. Pattern 1 has critical surfaces of pattern 13, which represent the detail on the outside of the closure of the bottle. The pattern is preferably made by a stereolithography method, well known in the prototype technique, in which an electronic file describing the pattern is manufactured rapidly by laser curing of a polymer. The pattern is placed in the flask with the critical surfaces of the pattern facing up towards the open end of the flask. A liquid metal is poured over the pattern. It is intended that the metal mold be an intermediate mold that transfers the critical surfaces of the pattern to a ceramic mold. A ceramic solution is similarly poured over the metal mold into an open flask and allowed to harden. However, ceramic material typically generates heat in an exothermic bonding reaction. The metal mold must finally be removed from the ceramic mold by melting the metal. Where there are thin sections, fusing the metal prevents damage to the fragile ceramic mold. The metal that was re liquefied 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 while the ceramic hardens. The resulting ceramic mold can be washed with melt flow in the molten state to remove any residue before burning 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 molten metal from the ceramic mold. The metal is poured over the pattern into an open-ended flask, as shown in Figure 2. Metal casting can be done with multiple spills, depending on the size of the part. The first pouring of a multiple pour is allowed to form a layer before the next pour so that the air bubbles do not penetrate, at the first pour. The flask is cooled or allowed to cool to room temperature until the metal solidifies. Depending on the size of the pattern, and the depth of the metal layer, it takes about 2 to about 8 hours, to solidify the metal. The depth of the metal pour will depend on the pattern and size desired for the ceramic mold. Those skilled in the art can easily determine this without much experimentation. Typically, a minimum metal thickness of about 2.5 cm above each critical surface of the pattern is desired. The subsequently solidified intermediate metal mold is pushed from the pattern. In a preferred embodiment, the flask is constructed with easily removable sides which are then pushed out of the metal mold and subsequently the metal mold is pushed out of the pattern. The metal mold retains the inverted replicas of the critical surfaces of the pattern without distortion, especially in the thin parts. It is important that the pattern has smooth surfaces with no recessed parts. The lowered and rough surfaces can prevent the removal of the solidified metal from the pattern. Figure 3 describes the metal mold placed in a second flask to which the gypsum or ceramic solution is to be added. The metal mold is placed with the critical surfaces of the metal mold facing up towards the open end of the second flask. Preferably, sufficient space is allowed between the second flask and the metal mold so that the ceramic is formed around the metal mold in that space. The ceramic mold made of the same, will have a continuous annular ceramic ring surrounding the critical ceramic surfaces so that the ceramic mold can be easily used for casting purposes without the need for another flask. The 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 air that could affect the formation of the final ceramic mold. The plaster or ceramic material first "sets" or takes a solid form and then solidifies completely. During the bonding process, an exothermic reaction is carried out in the gypsum that fuses the metal that surrounds it. The flask is preferably coated with a release agent so that the flask can be easily removed from the ceramic mold. The preferred exothermic ceramic material is C1-Core Mix, available from Ranson & Randolph of Maumee, Ohio. It is a mixture of silica, zirconium, silicate, ammonium phosphate, silica (cristobalitic) and fused magnesium oxide. You can also use Core hardener 2000, also available from Ranson & Randolph. It contains amorphous silica and dipotasium 6-hydroxy-3-oxo-9-xanten-O-benzoate. After the ceramic is set, the ceramic mold and the remaining metal can be calcined in an oven or by a hot air gun to completely fuse the metal for easy removal. The oven temperature should be from about 200 ° C to about 500 ° C to ensure metal melting. The open end of the ceramic mold, corresponding to the lower end of the second flask, allows easy access to pour liquid or molten metal from the ceramic mold. Also, critical ceramic surfaces can be easily inspected from the open end to see that all metal and any residue is removed. Place the ceramic mold in an oven and burn at at least 900 ° C for at least 3 hours completely forges the plaster for further processing. A hydrogen atmosphere can be used since there is no residue in the ceramic that needs to be burned. This lack of waste is an important distinction when compared to the procedures for making ceramic molds that use epoxy materials and waxes. Although particular embodiments of the present invention have been illustrated and described, it will be apparent 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 modifications they are within the scope of the invention.

Claims

NOVELTY OF THE INVENTION CLAIMS

1. - A method for forming a ceramic mold characterized by the steps of: a) placing a pattern with critical surfaces of the pattern in a flask having an open end, said critical surfaces of the pattern facing up towards said open end; b) covering said critical surfaces of the pattern with a liquid metal reversible to the heat added to said flask; c) cooling said metal solution to form a solid metal mold, said metal mold having critical surfaces of the metal mold transferred from the critical surfaces of the pattern which are inverted to said critical surfaces of the pattern; d) removing said flask and said pattern from the metal mold; and e) casting a ceramic mold around said solid metal mold, said ceramic mold having critical ceramic surfaces transferred from the critical surfaces of the metal mold which are inverted to said critical surfaces of the metal mold, said critical surfaces. ceramic in this way duplicating precisely said critical surfaces of the pattern; and f) liquefying said metal mold for removal of 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 while cooling 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 further characterized by the steps of: a) placing a pattern with critical surfaces of the pattern in a first flask with an open end, said critical surfaces of the pattern facing up towards said open end; b) covering said critical surfaces of the pattern with a heat-recoverable liquid metal added to said flask and; c) cooling said liquid metal while degassing said liquid metal to form a solid metal mold, said metal mold having critical surfaces of the metal mold transferred from said critical surfaces of the pattern which are inverted to said critical surfaces of the pattern; d) removing said pattern and said first flask from said metal mold and placing said metal mold in a second flask with said critical surfaces of the metal mold facing up towards an open end of said second flask; e) covering said critical surfaces of the metal mold with a ceramic solution added to said second flask while said ceramic solution is degassed, said ceramic solution solidifying and subsequently agglutinating exothermically to form a ceramic mold around said metal mold, said ceramic mold having critical ceramic surfaces transferred from the critical surfaces of the metal mold which are inverted to said critical surfaces of the metal mold, said ceramic critical surfaces thereby reproducing exactly said critical surfaces of the pattern; and f) liquefying said metal mold by means of heating to remove said metal from the 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 further characterized by the steps of: a) placing a pattern with critical surfaces of the pattern in a first flask having an open end, said critical surfaces of the pattern facing up toward the open end; b) covering said critical surfaces of the pattern 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 surfaces of the pattern that are inverted to said critical surfaces of the pattern; d) removing said pattern and said first flask from the metal mold and placing said metal mold in a second flask with said critical surfaces of the metal mold facing up towards an open end of said second flask, said second flask sized 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 said ceramic solution is degassed, said first ceramic solution solidifying without generating heat to form a first ceramic mold to fix said metal mold and form a continuous annular ring surrounding said critical surfaces of the metal mold; f) covering said first ceramic mold and said metal mold with a second ceramic solution added to said second flask, said second ceramic solution agglutinating exothermically to form a second ceramic mold joined to the first ceramic mold, said second mold of ceramics having critical ceramic surfaces transferred from the critical surfaces of the metal mold that are inverted to said critical surfaces of the metal mold, said ceramic critical surfaces thereby reproducing exactly said critical surfaces of the pattern; and g) liquefying said metal mold by means of heat to remove said metal from said first and second ceramic molds and removing said second flask from said first and second ceramic molds.