crack in production

Hi everyone...
we have an issue in SAN cylindrical housing used for the first stage of water filter ..
there are a crack appear on 10% of the production after the injection molding process completed, may be after one day to one month without any load.
i.e. when we store the production without any load we find around 10% of the production has a crack
we think that crack due to residual stress....if that right how we reduce this problem.
thanks for help