A spool cartridge for use in a cutting head of a string trimmer, in which the cutting head includes a locking arm having an engagement end and a balance end. The spool cartridge includes a spool rotatably mounted within a spool cassette housing and has cutting line wound thereon. The spool cassette housing includes one or more openings for the cutting line. An internal gear is formed in the spool and is in geared engagement with a locking assembly. The locking assembly is arranged to be engaged by the engagement end of the locking arm to automatically lock and unlock the spool with respect to the spool cassette housing.
