Instructions: Answer all questions.

The required properties of good molding sand are essential for producing sound castings. These properties ensure that the mold can withstand the casting process and produce a defect-free product. Key properties include:

• Permeability: The ability of the sand to allow gases (like moisture vapor and combustion products) to escape from the mold cavity during pouring. High permeability is crucial to prevent gas defects such as blowholes and porosity. This is influenced by grain size, shape, and packing density.
• Green Strength: The strength of the sand mixture when it is in a damp or 'green' state (containing moisture). This property allows the mold to be handled and closed without collapsing. It is primarily developed by the moisture content and binding agents.
• Dry Strength: The strength of the sand after it has been dried. This is important for molds that are baked or dried before pouring, providing structural integrity.
• Hot Strength: The strength of the sand at elevated temperatures encountered during and immediately after pouring molten metal. This prevents the mold from sagging or eroding under the thermal load.
• Collapsibility: The ability of the sand to easily break down or collapse after the casting has solidified. This is vital for easy removal of the casting and to prevent cracks due to contraction stresses.
• Refractoriness: The ability of the sand to withstand high temperatures without melting or fusing. This ensures the mold face does not fuse with the molten metal, preventing surface defects.
• Adhesiveness: The ability of the sand to adhere to the pattern, which helps in clean withdrawal of the pattern without damaging the mold cavity.
• Cohesiveness: The tendency of sand grains to stick together, contributing to the overall strength of the mold.
• Flowability: The ability of the sand to flow into all parts of the mold and around the pattern, ensuring accurate reproduction of the pattern's shape.
• Low Expansion: The sand should have minimal expansion when heated to prevent casting defects like 'scabbing' or 'rat tails'.

The solidification of pure metals in casting is a fundamental process involving the transition from liquid to solid state. For pure metals, this occurs at a constant temperature, known as the freezing point or melting point. The process can be described in stages:

1. Cooling to Freezing Point: Initially, the molten metal is heated above its melting point and then poured into the mold. As it enters the mold, it begins to cool. For a pure metal, the temperature drops linearly until it reaches the freezing point.
2. Nucleation: At the freezing point, the solidification process begins. Solidification starts with the formation of tiny, stable solid particles called nuclei within the liquid metal. This nucleation can be homogeneous (spontaneous formation within the liquid) or heterogeneous (occurring on existing surfaces like mold walls or impurities). Heterogeneous nucleation is more common in practical casting.
3. Crystal Growth: Once nuclei are formed, they grow into larger solid crystals. This growth occurs by the addition of atoms from the liquid metal onto the surface of the solid nuclei. For pure metals, this growth happens at a constant temperature (the freezing point) as latent heat of fusion is released. The interface between the liquid and solid phases is planar or slightly curved.
4. Latent Heat Release: As the liquid metal transforms into solid, it releases a significant amount of energy known as the latent heat of fusion. This heat must be removed from the metal for solidification to continue. In a casting, this heat is transferred through the solidifying metal, mold walls, and eventually to the surroundings.
5. Complete Solidification: The process continues until all the liquid metal has transformed into a solid structure. The resulting solid structure consists of an aggregate of crystals, often referred to as a dendritic structure, especially if growth is directional.

A key characteristic of pure metal solidification is the absence of a mushy zone. The transition from liquid to solid is sharp and occurs at a single temperature. This contrasts with alloys, which solidify over a range of temperatures.