S. DhaliwalS. K. Gupta1J. HuangM. KumarMechanical Engineering Department andInstitute for Systems Research,University of Maryland,College Park, MD 20742A Feature-Based Approach toAutomated Design of Multi-PieceSacrificial MoldsThis paper describes a feature-based algorithm for automated design of multi-piece sac-rificial molds. Our mold design algorithm consists of the following three steps. First, thedesired gross mold shape is created based on the feature-based description of the partgeometry. Second, if the desired gross mold shape is not machinable as a single compo-nent, then the gross mold shape is decomposed into simpler geometric components tomake sure that each component is machinable using 3-axis CNC machining. The decom-position is performed to ensure that each component is accessible to end-milling tools,and decomposed components can be assembled together to form the gross mold shape.Finally, assembly features are added to mold components to eliminate unnecessary de-grees of freedom from the final mold assembly to facilitate molding.关DOI: 10.1115/1.1419195兴1 IntroductionMolded parts are used extensively because they produce net-shape parts that require minimal secondary operations. On thebasis of the number of parting surfaces in a mold, molds can bedivided into two-piece molds and multi-piece molds. Multi-piecemolds refer to molds having more than one parting surface andhence more than two components. These molds can produce com-plex parts that cannot be made using two-piece molds. They en-able the use of molding for making parts that were previouslymanufactured using other processes. Since they have more thanone parting surface, they can be decomposed along different di-rections and thus can be used to make geometrically complexparts.Sacrificial molds refer to molds that can be destroyed after thepart has been produced. They are generally made of low meltingpoint materials such as wax and are typically destroyed by heatingthe mold-part assembly. Moreover, the wax molds can be easilymachined making them very easy to manufacture at high produc-tion rates. Therefore, sacrificial molds can be used to circumventthe disassembly problems that arise in permanent mold casting.Sacrificial multi-piece molds find use in a number of manufac-turing domains. Examples include:1 Manufacture of polymer parts: Polymer parts made up ofmaterials such as polyurethanes solidify at room temperatures dueto chemical curing. Since the quality of the parts is not dependenton the material properties of the mold 共e.g., porosity of mold兲,sacrificial molds made of wax provide an alternative to traditionalpermanent molds for making polyurethane parts in small batches.2 Gelcasting of ceramic parts: Gelcasting is emerging as apopular method for making high performance ceramic parts for awide variety of aerospace, automotive, and industrial applications关1兴. Gelcasting can be used to produce geometrically complexparts. The process is simple and uses conventional equipment.Low pouring temperatures in gelcasting enable use of sacrificialmolds. Furthermore, green parts need not be extracted from sac-rificial molds before sintering. Sacrificial molds containing greenparts can be directly put into sintering ovens. The sintering pro-cess melts the sacrificial molds and sinters the green part, andtherefore eliminates potential problems that result from handlinggreen ceramic parts.Unlike permanent molds where disassembly considerationsdrive the mold decomposition, in case of sacrificial molds primaryconsiderations that drive the mold decomposition are manufactur-ability of individual mold components. We currently use 3-axisCNC machining for making individual mold components. CNCmachining provides very good surface finish on the mold compo-nents without requiring any post-processing. Unfortunately forwax, electro-discharge machining does not work due to poor con-ductivity of these materials. Therefore, we use milling and drillingprocesses for making individual mold components. In order to bemachinable, the boundary of mold components needs to be acces-sible to cutting tool. Therefore, accessibility of mold componentsdrives the mold decomposition process in design of sacrificialmolds.This paper describes a feature-based approach to automated de-sign of multi-piece sacrificial molds. The basic idea behind ourmold decomposition algorithm is as following. We first form thedesired gross mold shape based on the feature-based descriptionof the part geometry. If the desired gross mold shape is not manu-facturable as a single piece, we decompose the gross mold shapeinto simpler shapes to make sure that each component is machin-able using 3-axis CNC machining. During the decomposition step,we account for tool accessibility to make sure that 共1兲 each com-ponent is manufacturable, and 共2兲 components can be assembledtogether to form the gross mold shape. Finally, we add assemblyfeatures to mold components to eliminate unnecessary degrees offreedom from the final mold assembly to facilitate molding.Our approach has the following advantages. First, by usingmulti-piece molds we can create geometrically complex objectsthat are impossible to create using traditional two-piece molds.Second, we make use of sacrificial molds. Therefore, using multi-piece sacrificial molds, we can create parts that pose disassemblyproblems for permanent molds. Third, mold design steps are sig-nificantly automated in our methodology. Therefore, we can createthe functional part from the CAD model of the part in a matter ofdays and so our approach can be used in small batch manufactur-ing environments.We believe that our research will be useful in small batch pro-duction of geometrically complex gelcast ceramic parts and poly-mer parts. Once the CNC code has been generated for machiningindividual mold components, cost of machining wax molds isrelatively small due to low cost of wax stock and very high feed-1Corresponding authorContributed by the Computer Aided Product Development 共CAPD兲 Committeefor publication in the JOURNAL OF COMPUTING AND INFORMATION SCIENCE INENGINEERING. Manuscript received July 2000; revised manuscript receivedSept. 2001. Associate Editor: D. Rosen.Journal of Computing and Information Science in Engineering SEPTEMBER 2001, Vol. 1 Õ 225Copyright © 2001 by ASMErates used in machining of wax stocks. Therefore, sacrificialmolds produced by CNC machining are an attractive alternative topermanent molds and