Passive mold cooling and heating method

The method of modifying the temperature of a mold for molding articles of plastic or like materials, the mold including a plurality of vertically extending internal passages for a heat transfer fluid, the method comprising:

• A. connecting the internal passages of the mold into a closed loop with a heat exchanger, the relative elevations of the mold and the heat exchanger being such that a heat transfer fluid tends to flow from the mold passages to the heat exchanger in response to the effect of gravity when the fluid is in a given physical state;
• B. filling the closed loop, including the mold passages and the heat exchanger, with a heat transfer fluid that changes from an initial physical state to the given physical state in response to thermal conditions occurring in critical portions of the mold passages during operation of the mold, the fluid in the mold passages being in the initial physical state and the heat exchanger containing fluid in both physical states when filling is completed;
• C. Establishing a predetermined operating pressure in the closed loop;
• and
• D. controlling operation of the heat exchanger to maintain a controlled operating pressure in the closed loop and to change fluid in the heat exchanger from the given physical state back to the initial physical state at approximately the same rate as the fluid flows from the mold to the heat exchanger, thus achieving basic circulation of heat transfer fluid in the closed loop through the effect of gravity based upon a change of state of the heat transfer fluid.