A geometric algorithm for automated design of multi-piece sacrificial molds

Abstract

Molds are required in a large number of manufacturing operations such as metal casting, die making, injection molding, ceramic and polymer processing etc. Molded and cast parts are used extensively because they produce net-shape parts that require minimal secondary operations. On the basis of the number of pieces in a mold, molds can be divided into two piece molds and multi-piece molds. Multi-piece molds refer to molds having more than two pieces. These molds can produce complex parts that cannot be made using two-piece molds. They enable the use of molding for making parts that were previously manufactured using other processes. Since they have more than two pieces, multi-piece molds have more than one parting surface. This enables the mold to be decomposed along different directions and thus can be used to make geometrically complex parts. Sacrificial molds refer to molds that can be destroyed after the part has been produced. They are generally made of low melting point materials such as wax or ABS and are typically destroyed by heating the mold-part assembly. Moreover, the wax molds can be easily machined making them very easy to manufacture at high production rates. Therefore, sacrificial molds can be used to circumvent the disassembly problems that arise in permanent mold casting. Sacrificial multi-piece molds find use in several manufacturing domains. Examples include manufacture of polymer parts and gelcasting of ceramic parts. Our algorithm for automated design of multi-piece sacrificial molds uses a three-step approach. The gross mold is created by subtracting the part from a large rectangular block that completely encloses the part. The three steps of the mold design algorithm are listed below. Decomposition to Solve Accessibility Problems: First, a feature-based decomposition of the mold is done to generate individual mold components for each of the primitives constituting the part. All decompositions are performed along planar faces. Second, once the feature-based decomposition is completed, some of the individual mold components are further decomposed to eliminate concave edges that are not accessible to non-zero diameter milling tools. Combining Mold Components to Reduce Manufacturing Cost: Once the decomposition has been completed, some of the individual mold components may be combined if the resulting mold component is completely accessible. The list of candidate combinations consists of all pairs of mold components that share a common planar face. Among them, only valid combinations are performed. The validation check is guided by a set of rules to ensure the accessibility of the composite mold components resulting from combinations. Addition of Assembly Features: Once the mold combination is completed, assembly features are added to the mold components in the mold assembly. There are a number of potential benefits of automating the design of multi-piece molds. The principal benefits are enumerated below. Mold design is a laborious process that requires significant time from the mold designer. This is aggravated in the case of multi-piece molds. Automated mold design significantly reduces the mold design time. This approach allows us to manufacture parts that could not be produced earlier using two-piece molds. Thus it expands the design space for parts that can be produced using casting processes such as gelcasting and polyurethane manufacturing. Since this approach automatically produces solid models of mold components, it can be integrated with CAM systems to generate the cutter path plans for manufacturing the individual mold components. Thus an integrated system can be developed that can simultaneously design and generate the cutter path plans for manufacturing the individual mold components in a mold assembly.