Thermoregulation by diathermic oil
With this type of thermal control, the die is brought to the required temperature by circulating heated diathermic oil into the die channels.
There is a difference between mineral oils, obtained by distillation from crude oil, and synthetic fluids: both are mainly composed by hydrocarbons, but the latter have better ageing properties.
In fact, fluid decomposition produces volatile substances, called “low boilers” and high viscosity polymers called “high boilers”. The former, in both cases, must be vented out regularly to avoid pump cavitation, vapour locks or mechanical damage, while the latter are soluble only in synthetic fluids up to a given ratio (10-15%).
Due to its high boiling point, diathermic fluid allows die preheating to temperatures up to 300°C.
On the contrary, it should be stressed that when temperature increases, thermal conductivity (W/m*°K) decreases and specific heat (Kcal/Kg*°K) increases, thus inhibiting heat transmission.