In the solid friction phase, the heat generated due to frictional energy
between the two surfaces (frequency of vibration, amplitude, and pressure)
causes the material to heat up and melt.
In the second phase the molten polymer layer increases due to shear heating in
the viscous (melt) phase. Heating decreases as the thickness of the viscous
In the third phase, the rate at which melt is formed becomes equal to the
outward flow rate (film drainage) and this comes to a steady state (the
thickness of the molten layer becomes constant). Vibrations are stopped at
The polymer melt starts to cool, the cooling phase, and solidification results
at the interface of the joint. Film drainage will continue while the joint is
held under pressure. When solidification is complete the pressure is withdrawn
and the joint is formed.