Pressure reaction injection molding process for making molded bodies of thermosets optionally containing filler and/or reinforcing material

In a pressure reaction injection molding process for manufacturing a molded body of thermoset material by introducing into a mold a reactive mixture of thermosetting pre-polymer material, hardening agent and from 0% by weight to an effective property enhancing amount of filler, heating the material in the mold to hardening temperature, and adding fresh material to the mass during the hardening phase thereof,

• the improvement comprising carrying out the aforesaid steps in a mold consisting essentially of an upper half and a lower half and an inlet opening downwardly through the upper half into the and, during the hardening phase, while introducing said fresh material under pressure into the mold through said inlet, maintaining the temperature of the upper half substantially below the hardening temperature of the reactive mixture and maintaining the temperature of the lower half at least at such hardening temperature, thereby compensating practically completely for any loss of volume in the molded body caused by shrinkage thereof during the hardening phase.