Disclosed is a mechanism, associated with a thermoplastic bag making machine, for preparing a strip of thermoplastic to produce bags having the upper edges of the bag mouth formed in a sinusodial shape such that the peak of such shape is substantially equi-distant relative to the side edges of the bag. While the disclosed bag machine is of the type that advances the web strip incrementally, shaping of the web strip in the sinusodial shape occurs while a portion of the web strip is advanced at substantially constant velocity. A mechanism is provided for maintaining constant tension in the web despite the constant and intermittent feeding of the web.