In this welding method the two sheets to be joined are clamped between two metal plates and a high frequency field is passed between the plates. If the dielectric loss of the plastic is high, sufficient heat can be generated to melt the plastics at their interface. By applying pressure through the metal plates the plastic sheets can then be welded. This technique is most widely used for PVC film. Other plastics can also be used, but if their dielectric loss is low then very high frequencies and hence more expensive equipment must be used. Normally the frequency used is in the band 27.12 MHz +0.6 percent. The platens, which can be large (2m x 1.5m) must be parallel to quite close tolerances and capable of withstanding pressures upto 1.2 GPa. Platens are usually pneumatically or hydraulically operated with facilities to control the applied pressure, the stroke, and the weld time.