Investment casting

Abstract: Methods are provided for producing investment cast articles, such as orthopedic implants, or portions thereof, having at least a partially textured surface that is formed during casting of the article. In an exemplary method, a textured metal casting is produced by creating a heat destructible pattern and spraying the pattern with a texturing material to cause the texturing material to form a textured surface on at least a portion of the pattern. In another embodiment, a textured template is pressed against a heat softenable pattern to provide a textured pattern. With respect to each of these methods, a shell is the created around the textured pattern to form a mold, and the pattern is removed from the shell. Molten metal is introduced into the mold and allowed to harden, after which the mold is removed. In yet another embodiment a textured pattern for investment casting is provided by creating a textured model and enveloping it with a resilient material to create a resilient mold. The textured model is removed from the resilient mold and the mold is filled with investment casting wax to create a textured pattern.

Description: Field of the Invention The present invention relates to investment casting, and more particularly to the manufacture of investment cast articles, such as orthopedic implants, with a textured surface. Background of the Invention Various metal casting processes, such as investment (or "lost wax") casting are well known for the fabrication of metal objects. This process requires several steps, the first of which is to create or provide a pattern or shape to be replicated. The pattern, often made of wax, is used to make a mold that is then used to form cast metal articles. Typically, several wax patterns are joined together on a wax "tree" to enable the simultaneous manufacture of several parts. The tree is a solid wax tube that has side walls to which a stem of each wax pattern is joined to form a cluster. The wax tree defines what will become a gate leading to passages for allowing molten metal to travel through the mold to each cluster and part pattern. Once all of the wax patterns are joined to the wax tree, the cluster is coated with one or more coats of a refractory by dipping the wax pattern-tree assembly in a ceramic slurry. After the slurry dries, fabrication of a shell or mold is completed by heating the slurry coated wax to cure or harden the ceramic, and to melt out the solid wax patterns and the wax tree. Molten metal is then poured into the shell so that it fills each of the cavities formerly occupied by the wax patterns and the wax tree. After the metal has cooled and hardened, the shell is fractured and removed, and the cast metal parts are severed from the metal tree. The cast parts are then subjected to post-machining, grinding off the gates, bead blasting, and polishing, as required. With With respect to medical implants, such as joint prosthesis components it has been discovered that texturing or roughening the surface of a cast metal implant can improve the interface and fixation between the implant and the bone, with or without bone cement. The creation of a roughened surface on an implant, whether it was investment cast or forged, is typically one of the last steps in the manufacturing process of the component. Known methods for obtaining a roughened surface include grit-blasting, grinding, direct machining, laser etching, and sintering of beads to the surface of the implant. Known surface texturing techniques, however, have severe short-comings with respect to manufacturing speed, efficiency, and cost, as well as structural limitations and deficiencies. For example, if the surface of the implant is improperly roughened, the near finished implant must be discarded. Depending on the alloy used to fabricate the implant and the size of the implant, the creation of unacceptable texturing can significantly increase manufacturing cost. The creation of a textured surface requires a skilled craftsperson. Therefore, for other than simple, uniform texturing, precise replication of a particular configuration is difficult to achieve in even limited production quantities. Furthermore, known surface texturing techniques are incapable of creating certain complex shapes and patterns, such as undercutting, which foster bone ingrowth and more secure fixation. Yet another disadvantage of known techniques for texturing a previously-cast implant is that the application of texturing materials, such as with a plasma-spray process results in an imperfect bond between the sprayed-on material and the implant which can lead to abrasive detachment and weak substrate coating interfaces. Similarly, sintering beads on a surface causes loss of favorable mechanical properties of the implant. EP-A1-0 668 062 makes known the use of stereolithography as a step in making a prosthesis having a macrotextured outer surface. Stereolithography is used to form a solid, heat destructible model having a macrotextured outer surface. The model is then encased in a ceramic refractory material to form an investment assembly. After hardening of the refractory material, the investment assembly is heated to discharge the model. The refractory material is then fired to form a shell having a negative impression of the prosthesis. The prosthesis is then cast in the shell. In "Role of Rapid Prototyping in Preoperative Planning and Patient-Specific Implant Generation", Proceedings of the 1996 15th Southern Biomedical Engineering Conference, Dayton, Ohio, USA, IEEE, Anderson, D. L., it is made known to make models of bone through stereolithography. The bone being modelled is scanned and a stereolithographic model formed from the scan data. A resilient mold is then formed about the model. The bone model is then cast in the resilient mold from a pourable foam material. This paper also makes known using stereolithography to form a model of an implant and to cast the implant from that model. Summary of the Invention The present invention overcomes the disadvantages of known surface texturing techniques by providing a method as set forth in claim 1 appended hereto. The textured pattern made by the method is used to create an investment casting mold which in turn is used to create the cast article. This technique allows very specific and/or complex patterns to be integrally formed with the implant surface as cast. In one way of creating the investment casting mold, a shell is created around the textured pattern to form the mold, and the pattern is removed from the mold. Molten metal is then introduced into the mold and allowed to harden, after which the mold is removed from the textured casting. The textured model can be fabricated with a rapid prototyping machine that performs stereolithographic operations, which enable the model to include very complex geometries, as well as undercuts, that are particularly well suited for bone ingrowth. Brief Description of the Drawings A more complete understanding of the present invention and the attendant advantages and features thereof will be more readily understood by reference to the following detailed description when it is considered in conjunction with the accompanying drawings wherein: FIG. 1 is a schematic illustration of the creation of a model using a computer-assisted design system; FIG. 2 is a schematic illustration of a rapid prototyping machine making an object corresponding to a computer-assisted design model of FIG. 1; FIG. 3 is an illustration of the object of FIG. 2 encased in a resilient material; and FIG. 4 is a perspective view of one-half of a soft mold. Detailed Description of the Invention An embodiment of the invention is illustrated by FIGS. 1-4, wherein a textured pattern for investment casting is provided by creating a textured model and enveloping it with a resilient material to create a resilient mold. The method begins by creating a textured model with a rapid prototyping machine. This is particularly advantageous as it permits fabrication of configurations and textures not obtainable by conventional methods. Referring now to FIG. 1, an input device 40, such as a keyboard, mouse, light pen, and the like, is associated with a computer 42 and a display unit 44, to facilitate the creation of a data file that defines a three-dimensional article 46 representative of at least a portion of an orthopaedic implant. The data file can be created by any of a number of drawing, graphic, design, or solid modeling programs known to those skilled in the art. An exemplary computer aided design program (CAD) is "Pro Engineer," available from Parametric Technology Corporation. The data file can also define bleeders and gates used in casting as required. By initially creating the article 46 as a CAD model, the article can be reviewed "on screen" or as computer generated drawings by a surgeon/engineer and modified as required prior to actually creating a mold or an implant. To further improve the review and design process, a three-dimensional model can be fabricated to assess potential modifications. Furthermore, the complexity and subtlety of the model is greatly enhanced. Special contoured surfaces, undercuts, internal cores, or typically machined features, as well as other features that are not machineable, can be easily incorporated into a CAD model and built with a rapid prototyping machine due to its layered fabrication capability. In a subsequent step, a data file 48 that has been created as described above is provided to, or accessed by, a rapid prototyping machine 50 capable of transforming the data into a tangible object 52. Exemplary rapid prototyping machines include: selective laser sintering, solid ground curing, fused deposition modeling, StereoLithography, LOM, and three-dimensional printing. The data defines at least a portion of an orthopaedic implant having a surface configuration that promotes bone ingrowth. For example, the article includes several discrete layers each having a different configuration and/or porosity. These layers can have a 10% to 20% porosity difference from layer to layer. Alternately, or additionally, the article includes different regions, wherein the porosity is different in each region. Although there are many ways to configure the surface to make it porous, an exemplary implant is defined to include generally round or oval pores that provide openings in the implant surface. The pores are in the range of 50 to 400 microns in diameter and have a depth in the range of 175 to 1,000 microns; and the implant surface can have an open porosity in the range of 30% to 70% of its total surface area. After the textured article or model 52 has been fabricated, by whatever method, a resilient material 54, such as room temperature vulcanization (RPV) silicone is applied to the model to envelop it, thus creating a resilient mold of the textured model as shown in FIG. 3. Prior to envelopment, one or more dividers 56 or partitions can be placed around the model to create one or more parting lines in the mold in a manner known to those skilled in the art, which allows the mold to be separated into two or more portions. Once the portions of the resilient material 54 are separated, the textured model 52 is removed from the textured material. As shown in FIG. 4, the resilient material 54, defining half of a mold, can be a thin layer inlaid in a support structure 56 which reinforces the resilient material around and behind the cavity 58 defined by the mold. In other embodiments, the resilient material 54 has a thickness sufficient to provide adequate stability for molding without requiring a supplemental support structure. The mold is filled with investment casting wax, using techniques known to those skilled in the art. When cured or hard, the textured wax pattern can be removed from the resilient mold by flexing the mold or peeling it away from the wax model, depending on the thickness and properties of the resilient material. The flexible mold can then be filled with wax again and again to replicate the model. Additional molds are readily fabricated using the original rapid prototype model. The textured wax models or patterns are subsequently subjected to known investment casting operations to make textured metal castings. In an exemplary method, the pattern is coated with particles of a refractory material by spraying or dipping to create a shell around the textured pattern. The textured pattern is removed from the shell by the application of heat which causes the pattern, to melt, burn, or vaporize, allowing it to be drained or exhausted from an opening in the shell. Heating, either in this or a subsequent step, cures the refractory material to complete the mold making process. Subsequent casting steps are not discussed in great detail, as they are well known to those skilled in the art of metal casting. These steps include introducing molten metal, such as Cr-Co-Mo alloy (ASTM F75) through one or more gates in the mold. The mold and the metal are allowed to cool, and the mold is removed from the hardened metal. The textured implant can then be subjected to post-machining, grinding off of gates, bead blasting, and polishing, as required. Yet another way to impart surface texturing to the wax model, in addition to the above described techniques, is to place one or more forms 60 having a textured face onto the surface of the resilient material 54 that defines a mold, textured face down toward the surface of the resilient material. When the mold is filled with wax, the textured form 60 or forms bond with the wax and thereby define a portion of the outer surface of the wax model. This technique is particularly well suited for creating a model with undercuts, cores, and other complex configurations. In an exemplary embodiment, the form 60 are forms are thin ceramic plaques having a selected pattern or texture. As used herein, a "pattern" is a full-scale representation or model of an orthopaedic implant or portion thereof that is either machine or hand made. Although the invention has been shown and described with respect to exemplary embodiments thereof, various other changes, omissions and additions in form and detail thereof may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims: A method of making a heat destructible, textured pattern for investment casting of at least a portion of an orthopaedic implant comprising the steps of: creating a data file (48) that defines a three-dimensional shape (46) of the at least a portion of an orthopaedic implant which includes (i) a plurality of layers each having a different porosity, and/or (ii) a plurality of surface regions each having a different porosity, and/or (iii) a plurality of layers each having a different configuration; providing the data file to a rapid prototyping machine (50); creating a textured model (52) representative of the data file with the rapid prototyping machine; enveloping the textured model with a resilient material (54) to create a resilient mold of the textured model; removing the textured model from the resilient mold; and filling the resilient mold with investment casting wax to create the heat destructible, textured pattern. The method of claim 1, wherein the three-dimensional shape is configured to promote bone ingrowth. The method of claim 1, wherein the step of creating a data file includes accessing a computer assisted design program. The method of claim 1, wherein the rapid prototyping machine creates a solid textured object (52). The method of claim 1, wherein the resilient material includes silicone. The method of claim 1, wherein the step of enveloping the textured model includes the step of defining a parting line. The method of claim 1, wherein the step of removing the textured model from the resilient mold includes the step of dividing the resilient mold into two pieces. The method of claim 1, wherein the resilient mold is joined to a rigid support (56). The method of claim 1, wherein the textured model is removed from the resilient mold by flexing the resilient mold. The method of claim 1, further comprising the step of placing a textured form (60) on the surface of the resilient mold that defines the textured pattern, before the step of filling the resilient mold with investment casting wax. The method of claim 1, wherein the step of creating a data file includes the step of defining pores in the at least a portion of an orthopedic implant, wherein the pores define openings in the surface of the at least a portion of an implant than are in the range of 50 to 400 microns in diameter and wherein the pores have a depth in the range of 175 to 1000 microns. The method of claim 11, wherein the step of creating a data file further includes the step of defining an implant surface having an open porosity in the range of 30% to 70%. The method of claim 1, wherein the difference in porosity ranges from 10 to 20 percent.

Inventors: FIFOLT DOUGLAS A, MASTRORIO BROOKE W, FIFOLT, DOUGLAS A., MASTRORIO, BROOKE W.
Applicants: JOHNSON & JOHNSON PROFESSIONAL, JOHNSON & JOHNSON PROFESSIONAL INC.
Application year: 1997
Main classification: B22C7
IPC codes: B22C7, B22C9, B29C43, A61F2, B29C67