Machinery's Handbook, 31st Edition
USING COMPUTER MODELING 1519 Modeling Optimization at Design Conception.—Some foundries become involved in the design of cast components at conception. In such cases, the foundry is able to op- timize a cast component’s structure and manufacturability from the beginning of the process, instead of reengineering later in the process. Two computer-aided engineering analysis methods used to optimize the structure of cast components and process of metal casting from conception are topology optimization and multidisciplinary response sur- face optimization. Both methods utilize casting modeling as the foundation for design iterations. Topology Optimization: This method of layout optimization is most effectively em- ployed in early stages of the design process, ideally at the start. In topology, the only input required is a layout of the package space, a definition of the loads and constraints on the structure, and a mass target. The optimization process starts with a uniform topology (uniform mass distribution) and then modifies the topology to minimize the compliance of the structure. In other words, the mass is moved around in the package structure to achieve the stiffest design for a given mass target. Multiple loading conditions can be input, as well as weighing factors. Topology optimization can quickly provide information to the design engineer as to the optimal layout of the mass, before concept designs are even developed. This can reduce the number of iterations significantly during design, since optimal part configuration will be known in the beginning. If used on existing designs, the process may be used to reduce structural weight and optimize performance while maintaining mass. Multidisciplinary Response Surface Optimization: This optimization method is used in the conceptual design stage to ensure manufacturability of design. Using solidification modeling, it can increase the quality of casting and productivity of foundries. The steps involved are: 1) Set up the optimization problem, including the definition of the baseline problem and a description of the component geometry (finite element, finite difference mesh, material properties, etc.). 2) Define process variables and constraints, including fill rates, mold temperatures, etc. 3) Define shape variables and constraints. 4) Define the objective function (such as minimizing porosity or solidification time)— the key to optimization. 5) Set the input parameters, including a solidification modeling input deck. 6) Submit the analysis to the solver system (software) and perform an initial run. 7) Compare the results of this analysis with the objective function. In the last step, if the convergence criteria for the objective function are met, the result will be an optimized process. If the criteria are not met, the process will repeat itself. The optimization software then will reconfigure the input and launch a new analysis. The soft - ware controls the input of the analysis each time after checking the results of the previous run. The greater the number of variables, the greater number of iterations are required. The goal of computer modeling is to make the casting right the first time. By first making design changes on a computer, foundries save time and production costs. As the demand for shorter lead times continues, foundries taking the “cast and see” approach without computer modeling analysis will find it difficult to compete. Extrusion of Metals Extrusion is a metalworking process used to produce long, straight semifinished products such as bars, tubes, solid and hollow sections, wire and strips by squeezing a solid slug of metal, either cast or wrought, from a closed container through a die. An analogy to the process is the dispensing of toothpaste from a collapsible tube. During extrusion, compressive and shear, but no tensile, forces are developed in the stock, thus allowing the material to be heavily deformed without fracturing. The extrusion
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