The mold material used for plastic injection molds is “steel”. “Steel” contains carbon. Containing more carbon makes the material harder but the lower the strength. Conversely, containing less carbon makes more tenacious but softer. In general, materials called “steel” basically have a carbon content of 2% or less. Some of the materials contain very low, less than 0.3% (stainless steel). The characteristics of this “steel” include high elasticity, high toughness, excellent dynamic strength and good heat conduction. Due to their properties as metal materials, they can be said to be very suitable for mold materials for injection molding.