Path: bloom-picayune.mit.edu!snorkelwacker.mit.edu!americast.com!americast.com\!americast-post Newsgroups: americast.mech From: americast-post@AmeriCast.Com Organization: American Cybercasting Approved: americast-post@AmeriCast.com Subject: Rapid Mold-Making For Investment Casting Date: Fri, 13 Nov 92 21:37:04 EST Message-ID: Rapid Mold-Making For Investment Casting A small tart-up company is developing a faster, simpler, and less costly method to produce complex ceramic molds for investment casting of metal parts, including blanks for plastic-injection molding. By: Steven Ashley Associate Editor Rapid prototyping, as a concept, is great. It's certainly true that engineers need a quick and easy way to verify CAD designs and fix any problems before they proceed with tooling and production, said Yehoram Uziel. But as we all know, most rapid prototyping systems really aren't very rapid and they don't produce real prototypes. At best, current machines make plastic models. Engineers want something that can make real parts right now. Uziel should know. Until last year, he was vice president of engineering for 3D Systems Inc. (Valencia, Calif.), the leading maker of photolithographic rapid prototyping systems. Now, as president and chief executive officer of Soligen Inc., a start-up company in Northridge, Calif., Uziel is heading a year-long effort to commercialize three-dimensional printing, an alternative prototyping technology invented at the Massachusetts Institute of Technology (Cambridge). MIT's three-dimensional printing process is a direct free-form fabrication technique in which arrays of ink-jet printing heads selectively deposit liquid binders onto layers of metal or ceramic powders. The solid objects that are formed consist of sequentially printed cross sections derived from computer-aided design data (see Special Report: Rapid Prototyping Systems, April 1991). Last year, Soligen obtained the exclusive license to develop the MIT process for production of ceramic molds for investment casting of metal parts. Its new direct shell production casting (DSPC) process can automatically fabricate complex molds directly from an image on a computer screen. The process bypasses tooling fabrication and other slow and labor-intensive steps in traditional investment (lost-wax) casting foundry operations, currently a $5 billion a-year business worldwide. We believe that this technology will have a significant impact on the practice of casting, said Emanuel Sachs, MIT professor of mechanical engineering and coinventor of the process. Sachs, two other MIT faculty members, and a former graduate student have applied for patents on the technology. Sachs said the MIT researchers are continuing to develop the technology for the direct printing of metal and ceramic parts in a powder metal-type process that manufactures usable parts and tooling. Parts of almost any shape can be cast in metals such as steel, aluminum, and nickel-based superalloys less than a week after the design is completed, Uziel said. This turnaround time is much faster than conventional investment-casting processes in which a designer can wait many months for delivery of a cast part. Because it requires no tooling or patterns, DSPC is a totally flexible and highly automated manufacturing process for metal parts, Uziel said. Thus, the process can dramatically reduce time to market for new products while lowering development and manufacturing costs, he said. The DSPC molding shells can include several cavities, allowing more than one copy of a part to be made at a time. The shells may also contain integral ceramic cores, allowing hollow parts to be made. Uziel said plastic-injection molders are very interested in proprietary methods under development by MIT and Soligen researchers, through which the DSPC process could be used to fabricate blanks for molds with better and more efficient cooling passages that could greatly improve thermal management. Turbine makers, for example, could use the process to fabricate temperature-resistant turbine blades with integral cooling cores, he said. Other applications include custom orthopedic implants and preforms for metal-matrix composites. Metal Parts Making For fabrication of most metal parts, engineers have machining and casting as alternatives. Though extremely useful for many jobs, computer numerical control (CNC) machining systems have limitations in machining complex shapes and hard metals. In addition, CNC machining requires complicated tool-path programming and manual fixturing, which can waste valuable material while cutting chips. On the other hand, conventional casting techniques would be attractive for many metal parts if it were not for the costly, time-consuming, and labor-intensive methods required to build tooling. Soligen's DSPC process removes many of conventional casting's drawbacks, Uziel claimed. 3-D Printing The DSPC process is as follows: A CAD model of the desired part is converted on screen into a digital model of a shell by the system's Shell Design Unit (SDU), a powerful graphics workstation running specialized object-oriented software that allows the geometry of a casting shell to be generated quickly. The digital model is then processed using the proprietary software to generate a series of cross-sectional slices. The system also accepts industry-standard .STL files. The SDU can also run a simulation of the casting process for verification. The other major component of the system is the Shell Production Unit (SPU), which includes a bin containing alumina ceramic powder. The bin is fitted with a piston that can be moved vertically in precise increments under computer control. Above the piston on a gantry is a hopper of fine ceramic powder. A roller located at the upper edge of the bin rotates as it traverses the piston, spreading a thin layer of the alumina powder. Also above the piston is an ink-jet printhead that is supplied with liquid binder (colloidal silica). The printhead moves across the piston under computer control in raster fashion, ejecting tiny drops of binder onto the powder layer in a pattern corresponding to the layer's cross section, selectively gluing the particles together. In a continuous-jet ink-jet head, a pressurized stream of binder is broken into droplets when it is vibrated by a piezoelectric ceramic as it exits an orifice. The droplets are charged as they pass through a capacitor and then deflected to the correct location by an electric field. In regions contacted by the binder, the alumina powder hardens into a solid ceramic body. The surrounding unbound powder serves to provide support layers to be glued later. The CAD model is again sectioned at a slightly higher position and the process is repeated until all cross sections have been printed one on top of another. After firing, the unbound powder is removed and recycled, yielding a three-dimensional ceramic shell. The shell is then ready to accept molten metal for casting. In investment-casting processes, a gating or plumbing system must be created to distribute molten metal from a central pouring cup to the mold cavities. In traditional lost-wax casting operations, a complex wax structure called a tree or cluster must be fabricated. This procedure requires the design and fabrication of the wax tooling, which must then be dipped in an alumina ceramic slurry to form a shell. The wax in the shell is then melted and poured out to provide void space. Cores to produce hollows in the final part must also be built. In the new process, no wax is needed because the tree is constructed on the computer screen. Internal mold surface finish is still a concern for the Soligen and MIT developers. As the DSPC machine's resolution improves, the final part finish will improve, but the researchers are looking into methods by which the inside mold surfaces could be smoothed or coated to achieve improved part surface quality. New Machines Soligen, a company based in a new 10,000-square-foot manufacturing facility, will deliver first-generation alpha (early production) DSPC machines to United Technologies Pratt & Whitney Group, Johnson & Johnson Orthopedics division, and Sandia National Laboratories by the end of this year, Uziel said. The build-space volume for the alpha version is 8 by 12 by 8 inches. Its raster resolution and layer thickness are each 0.007 inch. We re excited about the direct shell production process because of its potential to compress lead time for investment castings, said Dick Aubin, project manager at Pratt & Whitney (East Hartford, Conn.). This is one of the few technologies that extends current manufacturing capabilities to fabricate parts and tooling that are not possible with conventional manufacturing methods. It is conceivable that with additional development, this technology will dramatically change the way production parts are fabricated. The start-up company plans to ship preproduction beta machines in late 1993, Uziel said. Each turnkey machine, valued at about $250,000, will feature a 20- by 20- by 20-inch workspace volume and 0.002-inch resolution. Uziel said that the build rate for the entire workspace volume will vary from 9 to 20 hours, depending on the complexity of the part design. He estimated the cost per molding shell to be between $115 and $250. Soligen's management envisions selling DSPC machines to qualified customers and establishing service bureaus under a franchise arrangement. Members of the industrial consortium originally established by MIT to develop the three-dimensional printing technology have first rights to the new products. In addition to United Technologies, Johnson & Johnson, and Sandia, the consortium includes General Motors Corp., Howmet Corp., AMP Inc., 3M, Proctor & Gamble Co., and the National Center for Manufacturing Sciences (Ann Arbor, Mich.). To guide the development of the product, Soligen's management has established a technology advisory committee consisting of foundry industry experts from companies such as United Technologies, Johnson & Johnson, Sandia, Howmet, and Precision Castparts Corp. Members help to develop goals for the machines performance and advise on the development of user interfaces and features of future machines. Copyright 1992, Mechanical Engineering. For more information, send-email to American Cybercasting Corporation (usa@AmeriCast.COM)