Explanation:

Improper Riser:
- In the context of casting, a riser is a reservoir of molten metal that is connected to the casting cavity.
- It helps to compensate for the shrinkage that occurs during solidification and prevents the formation of shrinkage cavities or voids in the casting.
- An improper riser refers to a riser that is not designed or placed correctly.

Shrinkage Cavity:
- Shrinkage cavity is a common casting defect that occurs due to the insufficient supply of molten metal during solidification.
- It appears as a depression or void in the casting surface.
- This defect is caused by the shrinkage of the metal as it cools and solidifies, resulting in a cavity formation.

Causes of Shrinkage Cavity:
- Insufficient feeding of molten metal during solidification.
- Inadequate riser design or placement.
- Improper gating system that restricts the flow of molten metal.
- High solidification rate.

Relation to Improper Riser:
- An improper riser can lead to the formation of a shrinkage cavity defect in the casting.
- If the riser is not properly designed or positioned, it may not provide sufficient molten metal to compensate for the shrinkage.
- As a result, the metal in the casting cools and solidifies without being adequately replenished, leading to the formation of shrinkage cavities.

Other Casting Defects:
- Shift: It refers to the misalignment or displacement of the mold halves, resulting in an improper casting shape.
- Penetration: It occurs when the molten metal penetrates the mold cavity, causing rough or uneven surfaces in the casting.
- Misrun: It happens when the molten metal fails to completely fill the mold cavity, resulting in an incomplete casting.

Conclusion:
- An improper riser can contribute to the formation of a shrinkage cavity defect in the casting.
- It is essential to design and position the riser correctly to ensure proper feeding of molten metal and prevent the occurrence of shrinkage cavities.