In molds or plastic materials, the physical phenomenon of processing or deforming is done by putting in or taking out heat by heating or cooling. The property of conveying heat is determined by the structure of the material. Thermal conductivity is one of the properties as being easy to convey (or hard to convey) heat through the material. Thermal conductivity is an index which expresses how much quantity of heat is conveyed within a unit time, over a unit length, and with a unit temperature change. The most commonly used symbol for this is λ (lambda). The unit is kcal/m• h• °C.