Casting defects are common issues that can occur during the casting process. These defects can result in poor quality castings and may lead to the failure of the final product. One of these defects is metal penetration, which is not caused by the high pouring temperature of the melt.

Metal penetration occurs when the molten metal seeps into the mold cavity beyond the desired limits. This can result in a rough surface finish, dimensional inaccuracies, and reduced mechanical properties of the casting. Metal penetration is generally caused by factors such as low viscosity of the molten metal, low permeability of the mold material, and improper gating system design.

Here is a detailed explanation of the factors that contribute to metal penetration:

1. Low viscosity of the molten metal:
- When the molten metal has a low viscosity, it can easily flow into small gaps and cavities in the mold.
- This can result in the metal penetrating into areas where it is not intended to be, leading to defects in the casting.

2. Low permeability of the mold material:
- The permeability of the mold material determines how easily gases can escape from the mold cavity during the casting process.
- If the permeability is low, gases may become trapped within the mold, causing pressure build-up.
- This pressure can force the molten metal to penetrate into the mold cavity, resulting in defects.

3. Improper gating system design:
- The gating system is responsible for controlling the flow of molten metal into the mold cavity.
- If the design of the gating system is not proper, it can lead to uneven flow of metal or excessive turbulence.
- This can cause the molten metal to penetrate into areas where it is not desired, resulting in defects.

4. Inadequate mold venting:
- Proper venting is essential to allow gases to escape from the mold cavity during the casting process.
- If the mold venting is inadequate, gases may become trapped, leading to pressure build-up.
- This pressure can force the molten metal to penetrate into the mold cavity, causing defects.

In conclusion, metal penetration is a casting defect that can occur due to factors such as low viscosity of the molten metal, low permeability of the mold material, improper gating system design, and inadequate mold venting. While high pouring temperature can cause other defects such as cuts, fusion, and rat tails, it is not the cause of metal penetration.