The curing of unsaturated polyester resin is an exothermic reaction and can be divided into three stages:

Gelation stage: The time from when the accelerator is added to when the unsaturated polyester resin turns into a gel state is the most important stage of the curing process. Factors that affect the gel time: the amount of polymerization inhibitor, initiator and accelerator added, the evaporation loss of the crosslinking agent, ambient temperature and humidity, etc.

Hardening stage: the time from when the resin starts to gel to a certain degree of hardness and the product can be removed from the mold.

Full curing stage: usually carried out at room temperature, it may take several days to several weeks.

Source:https://www.up-resin.com/