What material is generally NOT recommended for constructing a sledgehammer's head?

The material that is generally not recommended for constructing a sledgehammer's head is plastic. Sledgehammers are designed to deliver powerful blows and need to be made from materials that can withstand significant impact and stress.

Steel is commonly used for sledgehammer heads due to its durability and ability to handle heavy-duty tasks without deforming. Wood is sometimes used in lighter hammers, typically for less intensive applications, but it generally does not provide the same level of impact resistance as metal. Rubber may be used for certain specialized applications, such as for striking softer materials or where a non-marring surface is needed, but it cannot provide the necessary weight and hardness for general use sledgehammers.

Plastic, while lightweight and resistant to rust, lacks the density and strength required for a sledgehammer, making it unsuitable for heavy-duty work where high-impact force is involved. A sledgehammer needs a head that can deliver and absorb energy efficiently, and plastic simply does not meet those requirements.