An arrangement to assist in heat transfer through a mold for applications such as injection molding of plastics material including at least one closed chamber within the mold, the chamber being only partially filled with liquid and a remainder of the chamber being filled with substantially only vapor of the liquid within the chamber, at least a portion of the chamber being positioned to transmit heat from a targeted location of the mold into liquid within the chamber, and condensing means adapted by reason of heat exchange to effect condensation of vapor within the chamber, the mold being arranged in that the liquid such as water is arranged to be held in the chamber in such a way that the liquid will be distributed to reach or be held at different heights within the chamber. This is achieved in one case by the addition of a foaming agent in the liquid. In another case by having a flock attached to the inner surface of the chamber. In a further example there are separate tiers of reservoirs holding the liquid above a base level of the liquid.