Investment casting method yielding cast articles with enhanced surface finish

Abstract: A modified investment casting process is provided which allows cast articles having a smooth surface finish to be obtained while using casting patterns made of a photocurable liquid polymer by a stereolithography process, or by molded thermoplastic materials. Moreover, the process of the invention eliminates or minimizes the incidence of mold cracking due to expansion of the casting pattern. The process involves first preparing a casting pattern (10) through a stereolithography process or through molding a thermoplastic material. Thereafter, a wax or wax like coating (14) is applied to the casting pattern and an investment assembly is prepared. A plurality of casting patterns (10) are assembled by their gate areas (16) to a riser system (18) to form a cluster (20). The cluster is then coated with one or more layers (22) of a refractory material. Following drying, an investment assembly (22) is formed which comprises cluster (20) encased in a ceramic shell (24). A mold is obtained from the investment assembly by first subjecting the investment assembly (22) to a first temperature range sufficient to melt and extract the wax or wax like material (14), leaving behind the casting pattern. Thereafter, the investment assembly is subjected to an elevated temperature in a second temperature range that is sufficient to melt and extract the casting pattern.

Description: BACKGROUND OF THE INVENTION The invention relates to methods for obtaining an improved surface finish on cast articles. The invention is particularly applicable to investment casting processes that use patterns formed of photocurable polymers and moldable thermoplastics. Various metal casting processes are well known. Investment casting (or lost wax casting) is commonly used to fabricate a variety of metal parts. This process requires several steps, the first of which is to fabricate patterns that are used to form a master mold. The master mold is then used to form the cast articles. Typically, several wax patterns are joined together on a wax tree to enable the simultaneous manufacture of several parts. The wax tree is a solid wax tube that has side walls upon which a stem of the wax patterns is joined to form a cluster. Once all wax patterns are joined to the wax tree, the cluster is coated with one or more coats of a refractory to form an investment assembly. After drying, the assembly is heated and fired and the solid wax patterns and the wax tree are extracted, yielding a shell. Molten metal is then pored into the shell so that it fills each of the cavities formerly occupied by the wax patterns and the wax tree. Upon cooling, the shell is fractured and removed, and the cast metal parts are severed from the metal tree. Japanese patent application JP-A-01 018 546 (upon which the preamble of claim 1 is based) describes a method for forming a sublimation pattern for producing a casting. A wax layer is provided on the surface of a polystyrene mold having the desired sublimation pattern. Recently it has become possible to use stereolithography techniques to manufacture one or more patterns to be used in a casting process. Stereolithography involves the application of energy, such as UV radiation, to a photocurable liquid polymer. Exposure of selected regions of the polymer to the energy source causes curing and solidification of the polymer. Through these techniques, three dimensional parts can be constructed in a stepwise process. Polymeric patterns prepared by a stereolithography process do not have as fine a surface finish as do wax patterns that are typically employed in investment casting processes. Stereolithography-produced patterns are prone to surface imperfections, particularly on angular surfaces, when compared to conventional wax pattern materials used in precision investment casting. As a result, during the application of a refractory material to the pattern, the surface irregularities in the pattern may also be incorporated into the resulting ceramic mold. This will cause the surface irregularities present in the pattern to be carried over to the resulting cast articles. Polymers used to form patterns prepared by stereolithography processes, as well as by other processes such as injection molding, tend to have a higher coefficient of thermal expansion than do conventional casting waxes. Investment casting processes all require the application of some degree of heat to extract the heat disposable patterns from within the ceramic mold. In some instances the thermal expansion of the polymer can cause the ceramic mold to fracture. Patterns formed by stereolithography may include hollow regions. It is possible for the refractory to seep into the hollow regions within a pattern and result in ceramic inclusions and/or voids being formed within the casting. Accordingly, the use of photocurable polymers and moldable thermoplastics to form casting patterns provides certain advantages, such as by enabling relatively easy and time efficient manufacture of patterns. However, the use of such materials to form casting patterns also presents drawbacks, such as a less fine surface finish on cast articles, the potential of mold cracking due to the higher thermal expansion of the polymeric patterns, and the possibility of introducing ceramic inclusions and/or voids in the casting due to the seepage of ceramic slurry into an interior, hollow portion of a pattern. Thus, new techniques are necessary to enable the use of patterns formed by photocurable polymers and other polymers, while eliminating the drawbacks of using such materials to form casting patterns. It is thus an object of the invention to provide a casting process that enables the use of photocurable polymers or other polymers to produce cast articles having a smooth surface finish. Another object is to provide a casting method that enables the use of casting patterns manufactured from polymers having higher coefficients of thermal expansion, while minimizing the potential of mold cracking. A further object is to provide a casting method that minimizes the possibility for producing cast articles having voids and/or ceramic inclusions. These and other objects will be apparent to those of ordinary skill in the art upon reading the following disclosure. SUMMARY OF THE INVENTION The present invention is directed to a casting method which utilizes casting patterns formed of a photocurable polymer or a moldable thermoplastic, while still obtaining cast articles having a smooth surface finish. The casting process of the invention comprises the steps of first forming from a photocurable polymer or moldable thermoplastic one or more solid, heat disposable patterns of an article to be cast. Thereafter, a layer of a casting wax material, or a similar low melting compound, is applied to the patterns to coat the exterior surface of the patterns, thus yielding coated patterns. A ceramic shell is then built up around the coated patterns by applying one or more coatings of a refractory material to the patterns which, upon drying, forms an investment assembly. The investment assembly is then subjected to heat at a first temperature range that is sufficient to melt and extract the layer of casting wax, but which is insufficient to melt or distort the patterns. The investment assembly is then subjected to heating at a second temperature range that is sufficient to melt and extract the patterns, leaving a shell having one or more cavities. The shell can then be fired to strengthen the shell and to burn off any residue remaining from the patterns or the layer of casting wax. The shell is then filled with a molten casting material such that the molten casting material fills the cavities in the shell, forming, upon cooling, one or more cast articles. The shell is then fractured and the cast articles are removed. As noted above, the casting process of the invention is advantageous because the resulting cast articles have an improved surface finish as compared to cast articles prepared by typically known processes using a casting pattern formed of a photocurable polymer or a moldable thermoplastic. In addition, there is a reduced potential of mold fracture during the stage of the process in which the heat disposable solid patterns are extracted from within the investment assembly because the initial extraction of the casting wax layer provides an expansion zone to accommodate thermal expansion of the polymeric patterns. The present invention also enables hollow, or partially hollow, patterns to be used with a reduced risk that the refractory material will seep into interior portions of the pattern, thus creating ceramic inclusions and/or voids in the resulting cast article. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view of a casting pattern. Figure 2 is a schematic view of a casting pattern having a wax coating applied thereto according to the present invention. Figure 3 is a schematic view of a cluster in which a plurality of casting patterns of the type shown in Figure 2 are assembled to a riser system. Figure 4 is a schematic view of the cluster assembly of Figure 3, encased in a ceramic shell, forming an investment assembly. Figure 5 is a schematic view of the investment assembly of Figure 4 in which the wax layer has been extracted from the investment assembly. Figure 6 is a schematic view of a ceramic shell formed by extracting the casting patterns from the investment assembly shown in Figure 5. Figure 7 is a flow chart illustrating the overall process of the present invention. DETAILED DESCRIPTION OF THE INVENTION The invention provides a casting process that is better able to accommodate the use of casting patterns formed of photocurable polymers or moldable thermoplastics. As noted above, cast patterns formed of photocurable polymers and moldable thermoplastics offer many advantages, however, their use can also be disadvantageous as resulting cast articles may often have a lower quality surface finish. Mold cracking is possible due to higher thermal expansion of these polymeric materials, and the use of hollow patterns can result in seepage of refractory slurry into the pattern, generating ceramic inclusions and/or voids in the resulting cast articles. The cast articles that result in the method of the present invention have a smooth surface finish, and the possibility of mold cracking, ceramic inclusion, and void formation in the cast article is minimized. The present invention involves a casting process, such as known investment casting processes, modified to take advantage of and enhance the use of photocurable polymers and moldable thermoplastics used as casting patterns. With reference to Figures 1-7, one or more casting patterns 10 are fabricated in a desired shape that corresponds to the shape desired of an article to be cast. The exterior surfaces 12 of casting patterns 10 are then coated with a layer 14 of a wax material, such as a casting wax or a similar low melting material, to form a coated pattern 15. Thereafter, typical casting procedures are followed. That is, a plurality of casting patterns 10 are assembled by their gate areas 16 to a riser system 18 to form a cluster 20, as shown in Figure 3. The cluster is then coated with one or more layers 22 of a refractory material, while allowing drying between the sequential applications of the refractory. Following drying, an investment assembly 22 is formed which comprises cluster 20 encased in a ceramic shell 24. Following the formation of the investment assembly 22, the layer 14 of wax material is extracted without affecting patterns 10. This procedure can be accomplished by subjecting the investment assembly to an elevated temperature at a first temperature range that is sufficient to melt the wax layer 12, but which is insufficient to melt or cause any significant thermal expansion of patterns 10. The first temperature range preferably is about 21 to 105°C (70 to 220°F), and more preferably about 24 to 55°C (75 to 130°F). This step can be conducted using procedures well known in the art to extract solid, heat disposable patterns from an investment assembly during an investment casting process. As shown in Figure 5, extraction of the wax layer 14 creates within investment assembly 22 an expansion gap 26. Expansion gap 26 accommodates the thermal expansion of patterns 10 during subsequent heat extraction of these patterns. Following the extraction of the wax layer 14, the investment assembly 22 is subjected to an elevated temperature at a second temperature range that is sufficient to melt and extract casting patterns 10. In the course of heating the investment assembly to extract the patterns, thermal expansion of patterns occurs. Expansion gap 26 enables these patterns to expand without exerting a force on the mold sufficient to result in mold cracking. The second temperature range should be sufficiently high to melt and extract the casting patterns 10, and thus the temperature range will depend to a large extent upon the physical properties of the polymer that is used to form pattern 10. Nevertheless, the second temperature range typically is from about 57 to 1093°C (135 to 2000°F) and more preferably from about 593 to 1093°C (1100 to 2000°F). As noted above, this heat extraction step can be conducted using procedures well known in the art to extract solid, heat disposable patterns during an investment casting process. Once patterns 10 are extracted from the investment assembly, a ceramic mold 28, having a plurality of cavities 30, remains as shown in Figure 6. Mold 28 can then be heated and fired to impart increased strength to the mold, and to remove any residual polymeric matter used to form casting patterns 10. Mold 28 preferably is fired at a temperature in the range of about 649 to 1093°C (1200°F to 2000°F) for approximately 1/2 hour or more. The mold 28 is then ready to accept molten casting material, which is poured into the mold by techniques known in the art, to form solid, cast articles. After the cast articles solidify and cool, the ceramic shell 24, which forms mold 28, can be fractured and removed, by known techniques to recover the solid, cast articles. The patterns used in the process of the invention can be formed by a variety of known photocurable polymers or moldable thermoplastics. In a preferred embodiment of the invention, photocurable polymers can be used to form casting patterns through known stereolithography techniques in which a reservoir of a liquid, photocurable polymer is exposed, in selected regions, to an energy source, e.g., ultraviolet light, to cure (solidify) the polymer. Successive exposures of the polymer to the energy source in a defined pattern achieves a solid, three dimensional object of a desired shape. Casting patterns can also be manufactured using moldable thermoplastics by a variety of known procedures, including injection molding and reaction injection molding. Among the preferred liquid, photocurable chemicals are those that possess rapid curing properties when subjected to energy sources such as ultraviolet light. Another requirement of suitable liquid, photocurable polymers is that they be somewhat adhesive so that successive layers of a pattern to be formed will adhere to one another. The viscosity of these materials should be low enough so that additional reactive photocurable polymer will flow across the surface of the partially formed object upon movement of the part being formed. Preferably, the liquid, photocurable polymer will absorb light (e.g., UV light) so that a reasonably thin layer of material is formed. The polymer should also be soluble in a suitable solvent in its liquid state while being insoluble in the same solvent in its solid state. This enables the object to be washed free of remaining liquid, photocurable polymer once the object has been formed. Useful reactive chemicals must also be heat destructible in their solid state. Preferred materials are those that melt or destruct in the range of about 57 to 316°C (135 to 600°F) or higher. This heat destructibility is essential as the objects formed through the stereolithography process are positive models of articles to be cast. During the casting process, as noted above, the models are encased in a ceramic forming binder material and once the binder solidifies, heat is applied to melt and extract the models leaving behind cavities that represent negative images of the articles to be cast. An exemplary reactive chemical useful as a liquid, photocurable polymer to form a cast article is Potting Compound 363, a modified acrylate made by Locktite Corporation of Newington, Connecticut. A process useful to make a typical UV curable material is described in U.S. Patent No. 4,100,141, which is hereby incorporated by reference. Other examples of useful liquid, photocurable polymers are blends of epoxy resin and acrylate ester such as CIBATOOL® SL5081-1, SL5131, SL5139, SL5149, SL5154, SL5170, SL5170, SL5177, SL5180, all of which. are available from Ciba-Geigy Corporation, Tooling Systems, East Lansing, Michigan. Other suitable photocurable polymers will be readily apparent to those of ordinary skill in the art. A variety of moldable thermoplastics can also be used to form the casting patterns. These materials include those that melt (and/or expand) in the range of about 79 to 330°C (175 to 625°F), or higher. Examples of such materials include polystyrene, polyethylene, nylons, ethyl cellulose, and cellulose acetate. These and other moldable thermoplastics are well known to those of ordinary skill in the art and are available from a variety of manufacturers. Suitable photocurable polymers and moldable thermoplastics should also be able to be extracted from the mold by heating and/or flash firing in the range of about 593 to 1093°C (1100°F to 2000°F) to rapidly vaporize the solid polymer. A variety of wax materials that are well known in the art and which are typically used in investment casting processes may be used to form the wax layer 14. Suitable casting waxes tend to melt in the range of about 24 to 127°C (75 to 260°F), and more preferably from about 38 to 79°C (100 to 175°F). These waxes can include a variety of vegetable waxes, animal waxes, mineral and synthetic waxes, or mixtures thereof. A variety of useful materials are well known in the art and are readily available from a variety of sources, including CERITA waxes available for Argueso & Company, Mamaroneck, New York. The wax layer 14 can be applied to the casting patterns by a number of techniques, including spraying and dipping. The wax layer 14 can also be formed by injecting the layer around the pattern. Preferably, the thickness of the wax layer 12 is slightly greater than the maximum anticipated expansion of the cast pattern. Preferably the thickness of this wax layer is in the range of about 0.076 to 12.7 mm (0.003 to 0.5 inch). The foregoing description of the method of manufacture and the illustrative embodiments is presented to indicate the range of constructions to which the invention applies. Variations in the materials to be used in conducting the process of the invention, temperature ranges and the like will be readily apparent to those having ordinary skill in the art. Such variations are considered to be within the scope of the invention in which patent rights are asserted, as set forth in the claims appended hereto.

Claims: An investment casting process, comprising the steps of: forming one or more solid, heat disposable patterns (10) of an article to be cast; applying a layer of wax material (14) to each pattern (10) to coat exterior surfaces (12) of each pattern and yield one or more coated patterns (15); building a ceramic shell (24) around each coated pattern (15) by applying thereto one or more coatings of a refractory to form, upon drying, an investment assembly (22); characterised by extracting the layer of wax material (14) from within the investment by subjecting the investment assembly (22) to a first elevated temperature that is sufficient to melt and extract the layer of wax material (14) but which is insufficient to melt or disorder each pattern (10); extracting each pattern (10) from within the investment assembly (22) by subjecting the investment assembly to a second elevated temperature sufficient to melt and extract each pattern (10), leaving a shell (24) having one or more cavities (30); and filling the shell (24) and the one or more cavities (30) within the shell (24) with a molten casting such that the molten casting material fills the one or more cavities (30) in the shell (24) to form, upon cooling, one or more cast articles. The process of claim 1, which further includes the steps of: providing a hollow riser tube (18); and before the shell is built, mounting on the riser tube (18) said one or more coated patterns (10) of an article to be cast to form a cluster (20), each coated pattern (15) being mounted by adhering at least one gate (16) on each solid pattern (10) to the riser tube (18), wherein the shell (24) is filled through said riser tube (18). The method of claim 1 or claim 2 wherein the patterns are formed from a photocurable polymer or a moldable thermoplastic. The process of claim 3 wherein the photocurable polymer has a melting temperature in the range of about 57 to 316°C (135 to 600°F) and preferably is selected from the group consisting of epoxy resins and blends of epoxy resins and acrylate esters. The process of claim 3 wherein the moldable thermoplastic has a melting temperature in the range of about 79 to 330°C (175 to 625°F) and preferably is selected from the group consisting of polystyrene, polyethylene, nylons, ethylcellulose, and cellulose acetate. The process of any one of claims 1 to 5 wherein the casting wax has a melting temperature in the range of about 21 to 105°C (70 to 220°F). The process of any one of claims 1 to 6 wherein the wax material (14) is selected from the group of consisting of vegetable waxes, animal waxes, mineral waxes, synthetic waxes, and mixtures thereof. The process of any one of claims 1 to 7 wherein the ceramic shell (24) is applied by dipping the coated patterns (15) in the refractory or by spraying the refractory on the coated patterns (15). The process of any one of claims 1 to 8 wherein the first elevated temperature is in the range of about 24 to 55°C (75 to 130°F). The process of any one of claims 1 to 9 wherein the second elevated temperature is in the range of about 57 to 1093°C (135 to 2000°F).

Inventors: LASALLE DAVID, OTI JAMES, LASALLE, DAVID, OTI, JAMES
Applicants: JOHNSON & JOHNSON PROFESSIONAL, JOHNSON & JOHNSON PROFESSIONAL INC.
Application year: 1996
Main classification: B22C7
IPC codes: B22D23, C08F299, B22C9, C08F290, B22C7