The molecular movement process has been developed according to previous study on the novel ultrasonic welding method of inserting interposed sheet and its apparatus Based on the classical reptation theory, a hauling theory of interdiffusion has been proposed which can accelerate molecular movement and interpenetration surprisingly Hauling effect can promote the interdiffusion but has no effect on the interpenetration length We involve a coefficient k(u) to modify the relationship between the reptation time before and diffusion time, which can be related to the relative displacement and friction coefficient among the molecular in terms of the equations The surface roughness, welding time, and welding force have been experimented and resulted uniformly in which the diffusion has been accelerated from our experiments POLYM ENG SCI, 51 103-108, 2011 (C) 2010 Society of Plastics Engineers