It's probably not the milk carton itself; I've used them many times without any problems, other than tearing them off the soap in some cases (until it occurred to me I could cut the opening along the long end making it easier to remove the soap.)
It's the formula + time in the carton & how long to the cut & the cutting implement. All play a role.
It's the formula + time in the carton & how long to the cut & the cutting implement. All play a role.