Abstract: A heat sealed butt joint for plastic parts such as a battery formed by at least a two step force application. The adjacent portions of the butt joint of the battery cover and case are preheated to a near liquid state causing rollover on the sides of the joint. The first step force application is of a lighter amount and is held for a predetermined time which causes the heated rollover on the sides of the battery to solidify. The second step force is of a heavier amount and is held for a predetermined time which causes the molten polymers of the heated portions of the battery cover and case to mix and bond together while the solidified rollover from the first step acts as a dam to prevent further rollover during the second step. The formed sealed battery has improved bursting strength and the process can be used for joining various other butting plastic parts to improve the tensile strength of the bond between the parts.