friction welding is also used with thermoplastics, which act in a fashion analogous to metals under heat and pressure. the heat and pressure used on these materials is much lower than metals, but the technique can be used to join metals to plastics with the metal interface being machined. for instance, the technique can be used to join eyeglass frames to the pins in their hinges. the lower energies and pressures used allows for a wider variety of techniques to be used.