The inventive drinking vessels are prepared by stretch blow-molding a preform and exhibit increased Rigidity as well as elevated, preferably relatively uniform crystallinity. The heated air andor steam vented to the duct is accelerated by the airflow. A press plate holds the substrate, which is held by the transfer robot, by suction. After being unblocked from the pivoting member, the spring holder is pivoted to hold a top of the ceiling in completion of the mounting of the speaker mount. The substitute idle task may have a low priority, such as that of the conventional idle task, which it replaces. The invention provides a spring balance comprising a top assembly which is securable to a frame member, a bottom assembly which is securable to a sliding sash, a tension spring connected to the top assembly and to the bottom assembly, a torsion spring, a spiral rod, and a follower bush interconnecting the spiral rod and the torsion spring.