To tight... Six bolts create massive pressure, even when you think they sit loose. The problem is: You can't tighten them bolts, since one bolt will become loose the moment you tighten up the one next to it. So you keep going in circles and always think: They still sit loose.
The entire wheel is flexible!
The bolts should not protrude the inner surface of the pulley! Flush is already quite tight.
I barley tighten them M4 bolts. Just very little. The teething of the pulley interlocks with the urethane to build up the friction. This works even when being very careful with bolt tension.
Ted managed to do the same to ABEC 11 wheels btw. All spokes split. Its not the plastic, its the pressure from the bolts. Low temperatures ad to the problem. Winter and plastic is sometimes an issue.
There is a manual online. Just browse to any E-STREET CARVER and hit the manual tab (desktop site).