A thermal transfer fluid circulator system for a mold, die or barrel of a molding machine, the system comprising: a fluid control loop comprising a fluid supply line (12, 27) configured for fluid communication with a respective thermal transfer fluid supply manifold (14, 19) on said molding machine, and thereby with at least one cooling cavity (15, 16) proximate a said mold, die or barrel and hence with a fluid return manifold (20, 26) on said molding machine, and a respective fluid return line (11, 28); the system further comprising: a fixed displacement pump disposed in said control loop whereby the thermal transfer fluid is volumetrically advanced from said fluid return line (11, 28) into said fluid supply line (12, 27), said thermal transfer fluid having a first temperature in the fluid supply line (12, 27) and a second temperature in the fluid return line (11, 28); a servo motor by which said fixed displacement pump is driven; and characterized by a control system by which a rate of advancing said thermal transfer fluid that transfers a desired quantity of heat energy per unit time to the mold, die, or injection barrel is calculated based on differences in said first temperature and said second temperature, and said servo motor is controlled to advance the thermal transfer fluid at the calculated rate. a fluid control loop comprising a fluid supply line (12, 27) configured for fluid communication with a respective thermal transfer fluid supply manifold (14, 19) on said molding machine, and thereby with at least one cooling cavity (15, 16) proximate a said mold, die or barrel and hence with a fluid return manifold (20, 26) on said molding machine, and a respective fluid return line (11, 28); the system further comprising: a fixed displacement pump disposed in said control loop whereby the thermal transfer fluid is volumetrically advanced from said fluid return line (11, 28) into said fluid supply line (12, 27), said thermal transfer fluid having a first temperature in the fluid supply line (12, 27) and a second temperature in the fluid return line (11, 28); a servo motor by which said fixed displacement pump is driven; and characterized by a control system by which a rate of advancing said thermal transfer fluid that transfers a desired quantity of heat energy per unit time to the mold, die, or injection barrel is calculated based on differences in said first temperature and said second temperature, and said servo motor is controlled to advance the thermal transfer fluid at the calculated rate. a fixed displacement pump disposed in said control loop whereby the thermal transfer fluid is volumetrically advanced from said fluid return line (11, 28) into said fluid supply line (12, 27), said thermal transfer fluid having a first temperature in the fluid supply line (12, 27) and a second temperature in the fluid return line (11, 28); a servo motor by which said fixed displacement pump is driven; and characterized by a control system by which a rate of advancing said thermal transfer fluid that transfers a desired quantity of heat energy per unit time to the mold, die, or injection barrel is calculated based on differences in said first temperature and said second temperature, and said servo motor is controlled to advance the thermal transfer fluid at the calculated rate.