PowerPoint Slideshow about 'CHAPTER SEVEN CASTING PROCESSES' - eyal
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Casting process is one of the earliest metal shaping techniques known to human being. Casting process consists of pouring molten metal into a refractory mold cavity and allows it to solidify. The mold cavity needs to retain its shape until the metal has solidified and the casting is removed from the mold.
Patterns are made to produce the proper sizes and shapes of cavities in the sand molds. Except for some very simple shapes, usually those that are circular, a pattern is necessary if the mold cavity is to have the correct shape and dimensions. Patterns are typically made from wood, plaster, plastics, various metals, and, for lost foam castings, from polystyrene foam.For investment castings, patterns are made from wax or plastics.
Sharp corners and abrupt changes in at any section or portion in gating system should be avoided for suppressing turbulence and gas entrapment.
(ii) The most important characteristics of gating system besides sprue are the shape, location and dimensions of runners and type of flow. It is also important to determine the position at which the molten metal enters the mould cavity.
(iii) Gating ratio should reveal that the total cross-section of sprue, runner and gate decreases towards the mold cavity which provides a choke effect.
This process is commonly known as permanent mold casting in U.S.A and gravity die casting in England. A permanent mold casting makes use of a mold or metallic die which is permanent. A typical permanent mold is shown in Fig. Molten metal is poured into the mold under gravity only and no external pressure is applied to force the liquid metal into the mold cavity.
This process uses a one-piece mold made of ceramic material, the same material that is used in ceramic casting. The mold is made by surrounding an expendable wax or plastic pattern with the ceramic material in slurry form. When the mold material solidifies it is heated and the wax or plastic replica of the part is melted out, leaving a cavity corresponding to the shape of the desired part. The mold is baked to remove all residues of the pattern and to fuse the ceramic. Molten metal is poured into this cavity. When the metal solidifies, the ceramic mold is broken free and removed from the casting.