Match the following sand mold casting defects with their respective causes.

Defect		Cause
P	Blow hole	1	Poor collapsibility
Q	Misrun	2	Mold erosion
R	Hot tearing	3	Poor permeability
S	Wash	4	Insufficient fluidity

The correct option is P-3, Q-4, R-1, S-2.

To understand the correct matching, let us analyze each sand mold casting defect along with its underlying physical cause:

1. Blow Hole (P matches with 3):
Blow holes are smooth, round or oval-shaped cavities produced by entrapped gases in the casting. During pouring, moisture in the sand mold vaporizes, and gases are generated. Permeability is the physical property of the molding sand that allows these gases and steam to escape easily from the mold cavity. If the mold has poor permeability, the gases cannot escape and remain trapped within the molten metal, leading to the formation of blow holes.

2. Misrun (Q matches with 4):
A misrun occurs when the molten metal fails to completely fill the mold cavity before it solidifies, leaving an incomplete casting. Fluidity refers to the ability of the molten metal to flow and fill all corners of the mold cavity. Insufficient fluidity, often caused by low pouring temperatures or composition, prevents the metal from reaching the extremities of the mold before freezing, resulting in a misrun.

3. Hot Tearing (R matches with 1):
Hot tearing (or hot cracking) is the formation of cracks in the casting during solidification and cooling. As the metal solidifies, it undergoes shrinkage. Collapsibility is the ability of the mold to shrink or collapse under the contracting force of the cooling casting. If the mold has poor collapsibility, it rigidly resists the natural contraction of the metal, generating high tensile stresses that cause the casting to rupture or tear at weak points.

4. Wash (S matches with 2):
A wash is a casting defect that appears as an excess projection of metal on the surface. This happens when the flowing molten metal wears away or washes off sand from the gating system or the mold cavity surface. This process of wearing away is referred to as mold erosion. The eroded sand is then trapped in the casting, and the space left behind by the eroded sand is filled with metal.