Curing is a chemical reaction — and reaction rates respond to heat
The free-radical cure reaction that hardens unsaturated polyester resin, like most chemical reactions, proceeds faster at higher temperatures and slower at lower temperatures. That means the same resin, catalyst and accelerator dosage that gels in a predictable time on a warm afternoon can behave noticeably differently on a cold morning or in an unheated winter workshop.
This is one of the most common reasons a fabricator sees "the same mix" behave inconsistently across a day or a season — the chemistry hasn't changed, but the ambient conditions it's reacting under have.
How production planning accounts for it
Workshops that see meaningful seasonal temperature swings often adjust accelerator or catalyst dosage between summer and winter to keep gel time within the range their process needs, rather than using one fixed recipe year-round. Mould and resin temperature at the point of use — not just the room's air temperature — also matter, since a cold mould can locally slow the cure of the resin in contact with it.
Because dosage adjustment is resin- and product-specific, it should be planned against the manufacturer's technical data sheet rather than by informal trial and error, particularly since going too far in either direction has its own risks — see the troubleshooting guide for what happens when cure runs too fast or too slow.