Question

Groover/Introduction to Manufacturing Processes

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.

Question 1

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

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

b) Which sand casting defects are due to the release of gases or from 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 expect out of the sand casting process on their large, steel parts?

e) Although Section 10.3.3 in the textbook 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 mm and a width of 22 mm, calculate the length and width for the pattern to accommodate a linear shrinkage value of 1.8%.

f) Use values from Table 4.1 and 4.2 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.

g) 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.

Answer must be typed and presented clearly..

Answer 1

Question 1

(a) 'No-Bake' Molds are the molds in which, the sand is mixed with the Chemical binders such as Urethane or Furan. That mixture is to be filled into each molding box and the two pattern halves are to be placed on their respective positions. That sand turns rigid with the help of catalysts when kept in the room temperature. Then, the two pattern halves are removed and the mold boxes are joined together to form a complete mold. Whereas, In Green Sand Molding, the pattern haves are kept on the floor and the mold boxes are to filled with moist sand. The sand in both the mold boxes is compressed by jolting or squeezing the sand (Latest technologies replaced the Man power compression with hydraulics and other explosive methods). The boxes are to be inverted and the patterns halves are to be removed to assemble the boxes. Lost Foam Pattern is a revolutionary method of consisting a Polystyrene foam pattern and the mold doesn't require a Cavity. The foam pattern is later replaced by the molten metal. Expanded Polystyrene Foam Patterns consists of 97.5% of Air and 2.5 % of Polystyrene, approximately. That foam pattern is coated with a refractory material (Ceramic Investment) which acts as a barrier to the Sand Erosion due to the molten metal. The coating also acts as permeable membrane to allow gases to escape through it and then from the sand. After the coating is dried, the molten metal is to be poured and the pattern is replaced.

?(b) Casting defects can be defined as the irregularities in the final metal product. The defects are of many types namely, Blow holes, Pin holes, Misrun, Drop, Swell, Metal Penetration, Shift or Mismatch, Hot tears, cold shut etc.,
When the molten metal consists of dissolved gases, which forms bubbles when the metal cools. Those bubbles cause pores in and outside the product which are known as blow holes and pin holes. This defect is also caused due to the high moisture content in the molding sand. These defects reduce the strength of the Final product. These defects can be reduced by melting the metal at Vacuum conditions, increasing the permeability of the sand.

(c) In bigger view, both the Cast Iron and the Steel are ferrous. But, they differ much when seen in detail. The cast iron is mostly used in casting because, the Cast Iron melts at 2300degree Fahrenheit whereas the Steel melts at 2600degree Fahrenheit. The Cast Iron shrinks lesser than the Steel which represents the requirement of lesser molten Cast Iron than the Steel. The pouring of Cast Iron is easier than the steel. Becuase, the molten cast iron is more fluidic in nature. The Steel reacts with the mold material and damages it. The machining of the Cast iron is easy than the machining of the steel. The cost of Steel is higher when compared to that of the Cast iron. The Cast iron is best material in the terms of vibration damping and corrosion resistance when compared to that of the steel. Cast Iron has better Impact and compressive strengths than the Steel. Hence, the Steel is Harder to cast than the Cast Iron.

(d) The Typical tolerances which, Kevin and his Colleagues expect out of the sand casting process on their large, steel parts are

• The Time required for the casting of steel.
• The Shrinkage allowance for the molten steel.
• The Cost of the steel.
• They decided to prepare the original patterns instead of miniature. So, they are going to tolerate maintenance control
• Kevin and his colleagues should cross the tunnel which is approximately one mile long, 75 feet under the Detroit river. They should face security check every time they cross the tunnel.
• The 20% Energy loss (heat energy) to the environment from the furnace.
  

(e) To accommodate the linear Shrinkage value of 1.8% for which, the Kevin's part dimensions are 38 mm long and 22 mm wide, the cavity should be made of the following dimensions:

By substituting the Length and Width individually for 0.018 Shrink rate, the Cavity Length would be 37. 982 mm and the width would be 21.982 mm.