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Groover: Fundamentals of Modern Manufacturing, 5e Case Study by Daniel Waldorf, California Polytechnic State University Casting Case Study: Kevin Working in Detroit Kevin was very excited about his new engineering job near Detroit. He was finally going to be able to contribute to the next generation of automobile design for one of the world’s largest carmakers. Even better, his first project was in development of a new-model hydrogen-fueled sports car. The assignment involved product and process design for three large frame-type structural components for the front half of the car. Since the parts would eventually be needed in fairly high volume, Kevin figured a net shape process such as casting would be the only economical approach for production. Although cast iron is definitely the best structural material for casting, the demands of the part required the strength and toughness of steel. Kevin’s boss agreed and told Kevin to get started on completing the remaining design details for the parts and getting the plans for production rolling. As for production, the parts would be cast at their usual foundry, located over the border in Canada. Both to save costs and to maintain control over the geometry, Kevin and his colleagues decided to produce the original patterns for the castings for later shipment to the foundry. They decided to have each pattern machined out of aluminum. One of the key decisions Kevin had to make involved the shrinkage allowance. The direction and uniformity of shrinkage in a casting often depends on the geometry and part features, though in this case he decided they could use a linear shrink rate in all directions. Kevin’s last task for the new parts was to get an estimate of production cost. For the casting, Kevin consulted the foundry and they told him that the cost mainly depended on two quantities: the heat energy (and thus time) needed to melt the material for each part and the cycle time needed for solidification of each part in the mold. For the first part design, Kevin computed a 3.5 minute melting time based on the heat properties of steel and a 1000 kW electric-arc furnace which operates at 80% efficiency (i.e., 20% of the heat energy from the furnace is lost to the environment). Solidification trials on a simple 2-inch diameter, 4-inch long cylinder took 4.0 minutes, so Kevin calculated a 16 minute time for solidification of his first part based on its volume and surface area. WATCH THE VIDEO: Casting Sand Mold Casting 1. Sand casting molds include risers that attach to the part cavity but are away from the pouring channels and gates. What is a riser’s purpose? 2. Is it likely that some features on Kevin’s parts will need to be machined after they come back from sand casting? Explain. 3. Kevin decided to use a traditional sand casting process for the components partly due to their large size. What other part design considerations can make investment casting a more attractive option? 4. When wax or plastic patterns are dipped in ceramic slurry for the first time in investment casting, special attention is paid to the composition of the slurry. What characteristics should the slurry have and why? GO TO THE TEXT: Chapter 11 5. What are “no-bake” molds, and how do they compare to green sand molds? See Section 11.1. Groover: Fundamentals of Modern Manufacturing, 5e Case Study by Daniel Waldorf, California Polytechnic State University 6. How are the expanded polystyrene foam patterns made for the lost foam casting process? See Section 11.2. 7. Which sand casting defects in Section 11.5 are due to the release of gases or from moisture in the sand molds? 8. Why is steel so much harder to cast than cast iron? See Section 11.6. 9. What typical tolerances can Kevin and his colleagues expect out of the sand casting process on their large, steel parts? See Section 11.7. SOLVE 10. Although Section 10.3.3 in the text shows a more complicated picture of shrinkage, it is still common for practical casting operations to assume a consistent linear shrink rate. If Kevin’s part design calls for a length of 38 inches and a width of 22 inches, compute the length and width for the pattern to accommodate a linear shrinkage value of 1.8%. 11. Use values from Tables 3.10 and 3.11 and Equation 10.1 to estimate the part volume corresponding to Kevin’s computation of 3.5 minutes for melting time. Assume the specific heat of the liquid metal steel is 20% smaller than that of the solid, and the heat of fusion is 120 J/g. The steel melts at 1530ºC and is to be poured at 100ºC higher. 12. Use the part volume from Question 11 and the Solidification time method in Section 10.3.2 (with the results of Kevin’s solidification trials) to estimate the surface area for Kevin’s part corresponding to his 16-minute calculation.