An outer clamp ring is non-rotating, pressing down to secure the top sheet to its substrate. A shoulder, which is also a ring, and a probe, which is a cylinder within the shoulder, rotate together and move down flush with the surface. The shoulder and probe spin and push to generate frictional heat, whereupon the shoulder descends into the sheet while the probe retracts.
This combined vertical motion allows plasticized material to be displaced from the shoulder area into the volume created by the probe. Once the shoulder is at full depth, which approximates the top sheet thickness, the top and bottom sheet materials become mixed together. Then, the process reverses the shoulder, and the probe returns to a position flush with the top surface. What results is a top and bottom sheet that are joined by a circle the size of the shoulder outer diameter and a smooth top surface.