Heat transfer is one of the main functions of a mould. However, many times neglected.
The three mechanism of heat transfer, conductivity, convection and radiation, are applied when cooling down a mould.
Q — amount of heat transfer in watt (W)
k — thermal conductivity of the material (W/mK)
A — surface of exposed area (m2)
∆T — temperature differential between the two points or zones being considered in kelvin (K)
∆x — distance between the two points or zones.
A mould is in fact a heat exchanger.
It receives the injected polymer in liquid state at high temperature (say 200ºC), and removes the necessary heat to solidify the plastic rapidly (say at about 40ºC). Heat comes both from the injected plastic mass at high temperature and from the liquid / solid state phase change (internal heat of solidification).
There are essentially three possible active types of temperature control in a mould:
- Cooling (usually with water less than 18ºC);
- Regulation (above 20ºC and can exceed 100ºC - oil);
- Hot runner system temperature control.