Excessive Shrinkage, Cracking or Warping in Parts

Molds usually allow for the shrinkage which occurs in an elastomer when the prepolymer and curative react. Typical shrinkage rates are 1-2% under normal processing conditions. Excessive shrinkage, however, will cause warping or unnatural stresses in the part. Cracking, which can also result, will vary from small internal cracks, to more obvious surface cracks.

Potential Causes

Incompatible Resin and Mold Temperatures
As a prepolymer/curative mixture begins to react, it generates heat which increases the temperature of the mixture. If there is a substantial difference between the reaction temperature and the mold temperature, there will be stresses placed on the system as it tries to shrink or expand during the reaction. The critical time for these two temperatures to be approximately in balance is when the mixture goes through its "green stage," which is when it changes from a liquid to a solid. Too low of a mold temperature relative to the mixture temperature will cause the gelling reactants to shrink excessively, sometimes to the point of cracking. Too high of a mold temperature is generally less of an issue, though this could lead to excessive warping.

Localized Temperature Variation in the Mold
Localized hot and cold spots within a mold can also cause internal stresses which may lead to shrinkage, cracking and warping.

Low Stoichiometry
Low stoichiometry leaves unreacted isocyanate which will increase crosslinking of the system. This additional crosslinking will tighten the elastomer and lead to increased cracking and shrinkage.

Solutions

Balance the Exotherm and Mold Temperatures
Every polyurethane system will have a different peak exotherm, which is the highest reaction temperature reached by the mixture. For a given system, this is a function of the individual components as well as the reaction mass. Larger mass mixtures will have a higher peak exotherm than smaller masses. The ideal case is to balance the mold temperature to within ±5°C of the peak exotherm.

Heat the Mold Adequately
Ensure that the oven is circulating air properly, so that there is a uniform temperature throughout the oven. Also, the mold should stay in the oven long enough to come to a constant, uniform temperature.

Monitor Raw Material Temperatures
Raw materials can lose heat as they are degassed and mixed, or while they sit before mixing, and therefore, they may need to be reheated in order to maintain the proper temperature. If the temperature of the system is too high, cracking may result. Lower system temperatures and higher mold temperatures are generally recommended.