Technically, there are three main components used to cast polyester resin:
- Resin - uncured polyester macro-molecules dissolved in styrene,
- Promoter - without which the resin will take a very long time to set, and
- Hardener - which is used to initiate the curing process.
Note that there is no "catalyst". It is a common misnomer to call the hardener a catalyst.
Silmar 41 (the most commonly used PR casting resin for pen blanks) comes pre-promoted with Cobalt Naphthenate. Because of that, it has a limited shelf life and it will turn solid all by itself in time. The crystal it makes, however, is not as strong as one cast with hardener. The most commonly used hardener is Methyl-Ethyl-Ketone-Peroxide (MEKP). It initiates the reaction and provides oxygen molecules that cross-link the polyester chains.
So, to cast the strongest, least brittle blanks, it is important to use fresh resin containing the right type and amount of promoter, and use the right amount of hardener given the ambient temperature, thickness of cast, and other variables. Insufficient hardener will starve the crystal of the oxygen it needs for strength. Too much and the cure will be too hot - resulting in high shrinkage and leading to a brittle blank.
I hope that makes sense,
Eric