The hardness of green sand molds is an important factor in the casting process. It determines the ability of the mold to withstand the pressure and forces exerted during casting. The hardness of the mold affects the quality of the casting produced.


- Moisture content refers to the amount of water present in the green sand mold.
- The hardness of the green sand mold increases with an increase in moisture content up to a certain point.
- This is because the presence of water helps in binding the sand particles together, increasing the strength and hardness of the mold.
- However, beyond a certain moisture content (usually around 6 percent), the excess water starts to weaken the mold by increasing its porosity.
- The excess water creates steam during the casting process, which leads to the formation of blowholes and other defects in the casting.
- Therefore, an increase in moisture content beyond 6 percent decreases the hardness of the green sand mold.


- Permeability refers to the ability of the mold to allow gases and steam to escape during the casting process.
- A higher permeability means that the mold has better venting capabilities, allowing for the efficient escape of gases.
- An increase in permeability improves the hardness of the green sand mold.
- This is because better venting reduces the pressure build-up inside the mold, preventing blowholes and other defects in the casting.
- On the other hand, a decrease in permeability restricts the escape of gases, leading to higher pressure inside the mold and decreased hardness.


- Based on the above explanations, it can be concluded that the hardness of green sand mold increases with a decrease in permeability.
- Option 'C' is the correct answer as it states that a decrease in permeability leads to an increase in hardness.
- The other options, such as an increase in moisture content beyond 6 percent (option 'A'), an increase in permeability (option 'B'), and an increase in both moisture content and permeability (option 'D'), are incorrect as they do not align with the explanation provided above.