Only certain polymers which contain dipoles can be heated by RF waves, in particular polymers with high loss power. Among these, PVC, polyamides (PA) and acetates are commonly welded with this technology. In practice, two pieces of material are placed on a table press that applies pressure to both surface areas. Dies are used to direct the welding process. When the press comes together, high frequency waves (usually 27.120 MHz) are passed through the small area between the die and the table where the weld takes place. This high frequency (radio frequency) heats the plastic which welds under pressure, taking the shape of the die.