What are two mandatory requirements for casting operation forms?

The correct answer highlights the importance of ensuring that casting operation forms are waterproof and leak-free as essential requirements. This characteristic is critical because it prevents any undesirable materials or fluids from escaping during the casting process, which could compromise the integrity of the final product or lead to contamination of the surrounding area.

Waterproof and leak-free forms help maintain the exact dimensions and shape of the cast as they retain the necessary materials in a controlled manner. Additionally, these forms contribute to the smoothness and quality of the cast surface by minimizing the risk of water damage and material loss, which can occur if the form allows leakage. Properly designed forms guarantee that the casting materials remain contained, ultimately ensuring a successful casting operation.

The other options do not capture the fundamental attributes necessary for effective casting forms. While adjustable and varied size may be beneficial for versatility, and portability and ease of assembly could add convenience, they do not address the critical aspect of material integrity during the casting process. Heavy-duty and lightweight might be useful in determining the durability of the forms but do not necessarily guarantee the essential properties of being waterproof and leak-free.