Reasons - Epoxy cracking before and during
1) Proper Temperature (For best results, the product should be used in conditions between 75° F to 85° F.) Extreme temperatures can affect the epoxy as it is plastic when it hardens so it is possible that it can warp or crack in excessive heat or cold climates.
2) Insufficient mixing of resin and hardener mixture or incorrect mix ratio
3) Vast variation in epoxy wall thickness in the casting
Different heat capacity of internal components resulting in different rates of gelation(when used in an art format and many kinds of materials are incorporated there may be something perhaps breathing with temperature fluctuations causing the epoxy to crack)
TIP: Before using epoxy, some elements may need to be properly sealed.