a method for manufacturing reclosable plastic bags or packages includes the step of presoftening the complete length of the zipper before it is introduced into the space between two plies of plastic sheet material , which form the sides of the plastic bag or package . the presoftening enables the zipper , which may comprise interlocked male and female profiles , to be fused together and completely flattened when a seal bar clamps thereacross to form a side seal area transverse to the zipper .