Wednesday, January 4, 2012

शेल्लिंग इन काट

Sand casting

Sand casting has many defects that can occur due to the mold failing. The mold usually fails because of one of two reasons: the wrong material is used or it is improperly rammed.[22]

The first type is mold erosion, which is the wearing away of the mold as the liquid metal fills the mold. This type of defect usually only occurs in sand castings because most other casting processes have more robust molds. The castings produced have rough spots and excess material. The molding sand becomes incorporated into the casting metal and decreases the ductility, fatigue strength, and fracture toughness of the casting. This can be caused by a sand with too little strength or a pouring velocity that is too fast. The pouring velocity can be reduced by redesigning the gating system to use larger runners or multiple gates.[22][23] A related source of defects are drops, in which part of the molding sand from the cope drops into the casting while it is still a liquid. This also occurs when the mold is not properly rammed.[24]

The second type of defect is metal penetration, which is when the liquid metal penetrates into the molding sand. This causes a rough surface finish. This caused by sand particles that are too coarse, lack of mold wash, or pouring temperatures that are too high.[24]

If the pouring temperature is too high or a sand of low melting point is used then the sand can fuse to the casting. When this happens the surface of the casting produced has a brittle, glassy appearance.[24]

A run out is when the liquid metal leaks out of the mold because of a faulty mold or flask.[24]

Scabs are a thin layer of metal that sits proud of the casting. They are easy to remove and always reveal a buckle underneath, which is an indentation in the casting surface. Rattails are similar to buckles, except they are thin line indentations and not associated with scabs. Another similar defect is a pulldowns, which are buckles that occur in the cope of sand castings. All of these defects are visual in nature and no reason to scrap the workpiece.[25] These defects are caused by overly high pouring temperatures or deficiencies of carbonaceous material.[24]

A swell occurs when the mold wall gives way across a whole face, and is caused by an improperly rammed mold.[24]

Burn-on occurs when metallic oxides interact with impurities in silica sands. The result is sand particles embedded in the surface of the finished casting. This defect can be avoided by reducing the temperature of the liquid metal, by using a mold wash, and by using various additives in the sand mixture.[26]

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