________ is/are caused due to the release of gases during pouring which consists of small gas cavities.

Explanation:

Some of the defects encountered in casting are mentioned below.

Blow holes:

• Blow holes, gas holes, or gas cavities are well-rounded cavities having a clean and smooth surface. They appear either on the casting surface or in the body of a casting.

These defects are caused by: 

1. Excessive moisture content
2. Inadequate gas permeability of the moulding sand
3. Low pouring temperature

Shrinkage:

• Volumetric contraction both in a solid and liquid state. Shrinkage due to thermal contraction of a liquid convert to solid-state.

Porosity or Permeability:

• Permeability or porosity of the moulding sand is the measure of its ability to permit air to flow through it. Gases evolving from the molten metal and generated from the mould may have to go through the core to escape out of the mould. Hence cores are required to have higher permeability.

Inclusions:

• Particles of slag, refractory materials, sand, or deoxidation products are trapped in the casting during pouring upon solidification. The provision of choke in the gating system and the pouring basin at the top of the mould can prevent this defect.

Pin-holes or gas porosity:

• It is sub-surface porosity caused by gases absorbed by molten metal, particularly hydrogen.
• Pinholes are caused due to the release of gases during pouring which consists of small gas cavities.

The various defects of the casting are as follows: