Groover/Introduction to ManufacturingProcesses

Casting Case Study: Kevin Working inDetroit

Kevin was very excited about his new engineering job nearDetroit. He was finally going to be able to contribute to the nextgeneration of automobile design for one of the world’s largestcarmakers. Even better, his first project was in development of anew-model hydrogen-fueled sports car. The assignment involvedproduct and process design for three large frame-type structuralcomponents for the front half of the car.

Since the parts would eventually be needed in fairly highvolume, Kevin figured a net shape process such as casting would bethe only economical approach for production. Although cast iron isdefinitely the best structural material for casting, the demands ofthe part required the strength and toughness of steel. Kevin’s bossagreed and told Kevin to get started on completing the remainingdesign details for the parts and getting the plans for productionrolling.

As for production, the parts would be cast at their usualfoundry, located over the border in Canada. Both to save costs andto maintain control over the geometry, Kevin and his colleaguesdecided to produce the original patterns for the castings for latershipment to the foundry. They decided to have each pattern machinedout of aluminum. One of the key decisions Kevin had to makeinvolved the shrinkage allowance. The direction and uniformity ofshrinkage in a casting often depends on the geometry and partfeatures, though in this case he decided they could use a linearshrink rate in all directions.

Kevin’s last task for the new parts was to get an estimate ofproduction cost. For the casting, Kevin consulted the foundry andthey told him that the cost mainly depended on two quantities: theheat energy (and thus time) needed to melt the material for eachpart and the cycle time needed for solidification of each part inthe mold. For the first part design, Kevin computed a 3.5 minutemelting time based on the heat properties of steel and a 1000 kWelectric-arc furnace which operates at 80% efficiency (i.e, 20% ofthe heat energy from the furnace is lost to the environment).Solidification trials on a simple 2-inch diameter, 4-inch longcylinder took 4.0 minutes, so Kevin calculated a 16 minute time forsolidification of his first part based on its volume and surfacearea.

Question 1

GO TO THE TEXT: Chapter 10 (Groover/Introduction toManufacturing Processes)

a) What are “no-bake” molds, and how do they compare to greensand molds? How are the expanded polystyrene foam patterns made forthe lost foam casting process?

b) Which sand casting defects are due to the release of gases orfrom moisture in the sand molds?

c) Why is steel so much harder to cast than cast iron?

d) What typical tolerances can Kevin and his colleagues expectout of the sand casting process on their large, steel parts?

e) Although Section 10.3.3 in the textbook shows a morecomplicated picture of shrinkage, it is still common for practicalcasting operations to assume a consistent linear shrink rate. IfKevin’s part design calls for a length of 38 mm and a width of 22mm, calculate the length and width for the pattern to accommodate alinear shrinkage value of 1.8%.

f) Use values from Table 4.1 and 4.2 and equation 10.1 toestimate the part volume corresponding to Kevin’s computation of3.5 minutes for melting time. Assume the specific heat of theliquid metal steel is 20% smaller than that of the solid, and theheat of fusion is 120 J/g.   The steel melts at 1530ºCand is to be poured at 100ºC higher.

g) Use the part volume from Question 11 and the Solidificationtime method in Section 10.3.2 (with the results of Kevin’ssolidification trials) to estimate the surface area for Kevin’spart corresponding to his 16-minute calculation.

Answer must be typed and presentedclearly..