Shrinkage allowance in casting is the amount of extra material added to the pattern to compensate for the shrinkage of the metal during solidification. Different materials have different shrinkage rates, and the shrinkage allowance required for each material varies accordingly. Among the given materials, Steel requires the highest shrinkage allowance, and the reasons are explained below.

Factors affecting Shrinkage Allowance:

1. Type of Material: Different materials have different shrinkage rates, which are affected by their composition, structure, and cooling rate. Materials with high carbon content, such as gray iron, tend to have higher shrinkage than low carbon steel or aluminum.

2. Casting Method: The casting method used also affects the shrinkage allowance. The sand casting process has more shrinkage compared to investment casting, due to the higher cooling rate of sand molds.

3. Shape and Size of Casting: The shape and size of the casting also influence the shrinkage allowance. Thick and complex sections tend to shrink more than thin and simple sections.

Reasons for Steel requiring the highest shrinkage allowance:

1. High Carbon Content: Steel contains a higher percentage of carbon compared to other materials, which results in higher shrinkage during solidification.

2. Cooling Rate: Steel has a slower cooling rate compared to other materials, which increases the shrinkage allowance required.

3. Casting Method: Steel is often cast using sand casting, which has a higher shrinkage compared to other casting methods.

Conclusion:

In conclusion, among the given materials, Steel requires the highest shrinkage allowance due to its high carbon content, slower cooling rate, and casting method. It is essential to consider the shrinkage allowance while designing the pattern and selecting the casting method to ensure the final product's dimensional accuracy.