Manufacturing Practices (BT-106) - Important Questions

State Chvorinov's rule for solidification time and derive the relation $$t = C \left( \frac{V}{A} \right)^2$$ starting from the basic heat transfer assumptions. Explain the physical meaning of each term and how the mould constant $C$ depends on material and mould properties. Finally, using Chvorinov's rule, determine the diameter of a cylindrical riser required to feed a casting of volume $V_c = 250\,\text{cm}^3$ and surface area $A_c = 150\,\text{cm}^2$ if the riser must solidify $1.5$ times slower than the casting. Assume the riser is a cylinder with height equal to its diameter.

Explain the objectives and functions of a gating system in casting. Describe the design considerations for a gating system and discuss how the gating ratio influences metal flow and defect prevention. Include sketches to illustrate a typical gating system with parts labeled.

List and describe the common casting defects. For each defect give typical causes and suggest remedial measures to eliminate or reduce that defect.

Define pattern allowances used in sand casting. Explain the different types of allowances such as shrinkage allowance, machining allowance, draft allowance and shake allowance. Calculate the required pattern dimensions for a component whose finished dimension is $100\,\text{mm}$ in length when the material has an expected solidification linear shrinkage of $1.2\%$, machining allowance of $1.0\,\text{mm}$ and draft of $1.5\,\text{mm}$ per side.

Compare the commonly used moulding materials for sand casting: natural sand, synthetic sand, and green sand. Discuss the required properties of moulding sands and the role of binders and additives in achieving these properties.

Describe the principles for selection of riser type and proper placement of risers in a casting. Explain the concept of a hot spot and how riser location can prevent shrinkage cavities. Provide examples of riser types and their typical applications.

Sketch and explain a typical cooling curve for a pure metal and for a hypoeutectic alloy during solidification. Discuss the significance of the liquidus and solidus temperatures and the formation of dendritic microstructures during solidification.

A casting requires a molten metal velocity of $0.6\,\text{m/s}$ at the entrance to the mould cavity. If the cross-sectional area of the sprue is $200\,\text{mm}^2$ and the pouring rate is constant, calculate the required cross-sectional area of the ingate assuming incompressible steady flow. State the assumptions made and explain how turbulence and gating geometry would affect the actual design.

Compare investment casting and shell moulding processes in terms of process steps, advantages, limitations, typical applications and surface finish achievable. Explain why each process is chosen for particular components.

Explain the causes of gas porosity in sand castings. Describe the methods and process controls used to minimize gas-related defects including venting, use of dry sands, control of moisture and use of degassing techniques.