Question

The shrinkage allowance has been provided on the pattern to compensate the shrinkage when:

• A. The temperature of liquid metal drops from pouring to freezing temperature
• B. The metal changes from liquid to solid state at freezing temperature
• C. The temperature of solid phase drops from freezing to room temperature
• D. The temperature of metal drops from pouring to room temperature

Hint:

Remember the compensation mechanism dividing line: - Liquid and Solidification shrinkage $\rightarrow$ Compensated by the Riser. - Solid shrinkage $\rightarrow$ Compensated by the Pattern Allowance.

Answer 1

The Correct Option is C

Concept: When molten metal transitions from its liquid pouring state down to a room temperature solid casting, it undergoes thermal contraction across three distinct stages:

• Liquid Shrinkage: Contraction occurring during the drop from the pouring temperature to the liquidus/freezing temperature.

• Solidification Shrinkage: Contraction occurring while the metal undergoes a phase change from liquid to solid at a constant freezing temperature.

• Solid Shrinkage: Contraction occurring as the temperature of the fully solid casting drops from the freezing point down to the ambient room temperature.

Step 1: Distinguish how each contraction stage is compensated.

• Both

Liquid Shrinkage and

Solidification Shrinkage result in a volumetric deficit within the liquid pool, forming voids or pipes. To overcome this, a

Riser (or feeder head) is incorporated into the mold design to continuously supply extra molten metal into the cavity until solidification finishes.

• On the other hand,

Solid Shrinkage occurs after the metal has completely frozen into its geometric shape. As it cools down to room temperature, the overall linear dimensions reduce. To ensure that the final cooled casting matches the blueprint design specifications, the physical dimensions of the

Pattern must be made deliberately larger. This dimensional enlargement is known as the Pattern Shrinkage Allowance.

Step 2: Correlate with the options. The reduction in dimensions in the solid state is compensated solely by making the pattern oversized. Thus, the shrinkage allowance compensates for the contraction when the temperature of the solid phase drops from freezing to room temperature.