Cycle time: Science vs. Rules of thumb

Mike Sepe describes an understanding of the how the modulus of a polymer increases as it cools in the mold. As the polymer cools in the mold, the reverse process takes place. The molten material entering the mold begins to build modulus rapidly as it cools through this temperature interval. The layers of flowing material that are in direct contact with the mold will undergo this change at the most rapid rate. Interior layers will cool more slowly, due to their distance from the cavity and core surfaces and the fact that polymers are relatively poor conductors of heat. But it is a given that the temperature of the mold must be set below the glass-transition temperature of an amorphous polymer to ensure that the material sets up. The Vicat softening temperature and the HDT (or DTUL), at the two most commonly used stresses of 66 and 264 psi are shown on the modulus curve for the PC. It should be obvious that at the Vicat softening temperature there is little chance that the material will achieve an ejectable modulus. Assuming that the innermost layers of the part have reached this temperature, the part has nearly all of the properties it will have when it has cooled completely.