Answer:

Cold shut is a defect in casting due to sand sliding from the cope surface.

Explanation:

Cold shut is a common defect in casting that occurs when the molten metal does not properly fuse together during solidification. This results in a discontinuity or incomplete fusion in the casting, which weakens the structure and compromises its integrity.

Causes of cold shut:

There are several factors that can lead to the formation of cold shuts in castings. However, in this specific question, the correct answer is option 'A', which states that cold shut is a defect in casting due to sand sliding from the cope surface.

Explanation of option A:

When molten metal is poured into a mold, it comes into contact with the sand surface of the mold cavity. The sand acts as a refractory material, providing insulation and preventing the molten metal from solidifying too quickly. However, during the solidification process, the weight and pressure of the molten metal can cause the sand to shift or slide from the cope surface (the top surface of the mold). This movement of sand can result in a lack of contact between the molten metal streams, leading to incomplete fusion and the formation of cold shuts.

Other causes of cold shut:

While the correct answer for this particular question is option 'A', it is important to note that cold shuts can also be caused by other factors such as:

- Internal voids or surface depressions due to excessive gas trapped: If there is excessive gas trapped in the molten metal, it can create voids or depressions in the casting, leading to cold shuts.

- Discontinuity resulting from hundred contraction: During the solidification process, metal undergoes contraction as it cools and solidifies. If there is a significant difference in the cooling rates or the rate of solidification between different sections of the casting, it can result in a discontinuity and the formation of cold shuts.

- Two streams of material that are too cold to fuse properly: If two streams of molten metal come into contact with each other but are too cold to fuse properly, a cold shut can occur.

In conclusion, cold shut is a defect in casting that occurs when the molten metal does not properly fuse together during solidification. In this specific question, the correct answer is option 'A', which explains that cold shut is a defect in casting due to sand sliding from the cope surface.