What type of material is primarily used in the core of a fire door?

The primary material used in the core of a fire door is typically incombustible material, which is designed to resist fire and prevent its spread. This type of core can be made from various substances, including mineral-based materials, concrete, or other composite materials that are specifically engineered for fire resistance.

Incombustible cores are critical because fire doors must meet stringent fire safety standards. Their core composition contributes to the door's ability to maintain its integrity under high temperatures, which is essential for providing the necessary time for occupants to evacuate and for controlling the movement of smoke and flames during a fire.

The other options, while they may be components of different door types or used in specific situations, do not provide the necessary fire-resistant properties required of a fire door's core. For instance, plastic is generally not suitable as it can melt and release toxic fumes in a fire. Wood, while used in some fire doors, does typically not meet the same level of fire resistance as incombustible materials unless treated. Metal, although it can be used in fire doors, often serves more as a cover rather than the core material that directly contributes to the door's fire resistance.