The definition of resistance welding is:
That being said the heat is generated per "JOULES LAW"
In order to make a seam weld current must flow through the desired joint area. As far as I know plastic is an electrical insulator and not able to pass current. Therefore is not a candidate for resistance welding.
To my knowledge plastic is not being resistance welded.
I am aware of ultrasonic welding of plastics. This is frictional heat formed by ultrasonic movement and pressure.
Reference: RWMA Resistance Welding Manual 4th Edition