a thin , flat package of heat sealed thermoplastic material is filled with plastic string trimmer line by inserting a mandrel into an opening in the rear wall of the package to receive the free end of a length of string trimmer line inserted through a slot in the periphery of the package to engage the mandrel . the mandrel is rotated while the package is held in a relatively fixed position to wind line onto the mandrel and , therefore , into the package , until the package is filled . the mandrel then is withdrawn , leaving the free end of the line accessible through the opening in the rear wall , and the other end of the line extends from the slot in the periphery of the package .