Machinery's Handbook, 31st Edition
PLASTER MOLD CASTING 1503 This process is useful since it is very cheap, can produce complex geometry, and yields good surface finish. There are no runners, risers, gating, or parting lines. Thus, the design process is simplified. This method is used to manufacture crankshafts for engines, alumi - num engine blocks, manifolds, and other parts. Plaster Mold Casting Plaster mold casting produces parts with good surface finish and dimensional accuracy, and provide the capability to make parts with thin cross sections. The mold is made by mixing plaster of Paris or simple “plaster” (CaSO 4 ) with talc and silica flour. This pro - duces a fine white powder that, when mixed with water, forms a clay-like consistency and can be shaped around pattern halves in a flask and allowed to set. The mold halves are extracted carefully from the pattern and then dried in an oven at a temperature range of 248–500°F (120–260°C) to remove moisture. The mold halves are carefully assembled to form the mold cavity and used for the casting process as follows: 1) The mold is preheated to about 248°F (120°C). 2) Molten metal is poured in the mold cavity. 3) After the metal is solidified and cooled, the plaster mold is broken away from the finished casting. Plaster casts are relatively soft and not strong enough at temperatures above 2192°F (1200°C). So this method is mainly used to make castings from nonferrous metals, such as aluminum, magnesium, zinc, and copper-based alloys. It cannot be used to cast ferrous materials, because the sulfur in gypsum slowly reacts with iron. Since plaster has lower thermal conductivity, the casting cools slowly, and therefore has more uniform grain structure, with the advantages of less warpage and less residual stresses. Ceramic Mold Casting Ceramic mold casting also is called cope-and-drag investment casting . It is similar to the plaster mold process, and the metal is cast the same way, but it uses refractory materi- als suitable for high-temperature applications. Ceramic mold casting uses a permanent pattern made of plastic, wood, or metal. To make the slurries for molding, fine-grained zircon (ZrSiO 4 ), aluminum oxide, and fused silica are mixed with bonding agents and poured over the pattern, which has been placed in a flask. With this process, parts may be cast that weigh as much as 1,540 lb (700 kg). Typically, it is used for impellers, cutters for machining operations, dies for metalworking, and molds for making plastic and rubber components. The sequence of operations in making a ceramic mold is: 1) A pattern is placed in a flask. 2) A slurry is poured over the pattern to create the mold. 3) It is dried in a low temperature oven, and the pattern is removed. 4) The mold is backed by clay for strength and baked in a high-temperature oven to burn off any volatile substances. 5) The all-ceramic mold is firmly clamped for use in casting. The metal is cast the same way as in plaster casting. Although this process is somewhat expensive, it is used because castings can be made that feature good dimensional accuracy and surface finish over a wide range of sizes and intricate shapes. It can be used to make very good quality castings of steel or even stainless steel and parts such as impeller blades for turbines or pumps and rotors for motorboats.
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