Nutrunner driver for a plastic bottle capping machine

A method is provided for joining a flexible container to a threaded closure. According to the method, a flexible container is inflated with pressurized gas before applying torque to the container. A base of the flexible container is engaged in a container receptacle of a capping apparatus, then the capping apparatus is operated to cause a neck of the flexible container to engage a threaded opening in the closure. The bottle is then inflated and the capping apparatus is operated to thread the neck of the flexible container into the opening in the closure by causing relative rotation between the flexible container and the closure. The pressurized gas stiffens the flexible container and prevents it from buckling or twisting under torque loads applied to screw the container into the closure.