Machinery's Handbook, 31st Edition
SAND CASTING 1501 10) The mold is closed by placing the cope on top of the drag and securing the assembly with pins. The flasks are subjected to pressure to counteract buoyant forces in the liquid, which might lift the cope. 11) Molten metal is poured through the riser into the mold cavity. 12) After pouring and solidification, the part is removed with the required pattern shape. 13) The sprue and risers are cut off and recycled. 14) The casting is cleaned, inspected, and heat treated (when necessary). 15) The final casting is inspected using nondestructive testing and destructive methods in accordance with standards. Rammed Graphite Molding.—A rammed graphite mold typically is used for large in- dustrial casting for reactive metals such as titanium and zirconium. It uses graphite in- stead of sand in a process similar to sand casting. Traditionally, a mixture of properly size- fractioned graphite powder, pitch, corn syrup, and water are rammed against a wooden or fiberglass pattern to form each mold section. The mold sections are air dried, baked at 350°F (177°C), and then fired in a furnace for 24 hours at 1877°F (1025°C), causing the mold to carbonize and harden. Rammed graphite molds must be stored under controlled humidity and temperature. Mold ramming is a labor-intensive process that cannot be easily mechanized, and the graphite mold is so hard that it must be chiseled off the cast parts. Castings made with this process usually are cleaned in an acid bath, followed, if necessary, by chemical milling, to remove any reaction zone; weld-repaired as needed; and then sandblasted for a good surface appearance. Shell Molding Shell molding is a foundry process in which the molds are made in the form of thin shells. This process can produce many types of castings with close dimensional tolerance and good surface finish at relatively low cost. Shell molding applications include small mechanical parts requiring high precision, including gear housings, cylinder heads, con- necting rods, and close-tolerance molding cores. Advantages of Shell Molding: Rigidly bonded sand provides great reproducibility and produces castings near to net shape with intricate detail and high dimensional accuracy of ± 0.010 in. (± 0.25 mm). Castings can range from 1 oz to 25 lb (28 g to 11 kg). The resin- bond strength of the mold allows for smaller draft angles, deep draws, and built-in mold locators that prevent mold shift mismatch. Because mold shells are thin, permeability for gas escape is increased, allowing use of finer sands. Finer sand and excellent flowability produce dense mold surfaces and con - tribute to producing complex casting with high-quality finishes with a roughness average of 50 µ in (1.25 µ m) Ra. The virtual absence of moisture eliminates moisture-related de- fects, and the burning resin provides a favorable anti-oxidizing atmosphere for the casting surface. In addition, heat from burning slows the casting-cooling rate, yielding a more machinable structure. Disadvantages of Shell Molding: Since the tooling requires heat to cure the mold, pattern costs and pattern wear can be high. Material costs are higher than for green-sand molding, and energy costs are higher than for other processes. Shell molds are made in the following operations: 1) A pattern made of a ferrous metal or aluminum is heated to 347–698°F (175–370°C). 2) The pattern is coated with a parting agent, such as silicone. 3) It is clamped to a box or chamber that contains fine sand, mixed with 2.5–4% thermosetting resin binder (such as phenol-formaldehyde) that coats the sand particles. 4) Either the box/chamber is rotated upside down, or the sand mixture is blown over the pattern to coat it.
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